Diverse models have been proposed for the role of the Tarim Craton within the Paleoproterozoic Columbia supercontinent assembly. Here we report a suite of-1.71 Ga Nb-enriched meta-gabbro lenses in the eastern Quanji M...Diverse models have been proposed for the role of the Tarim Craton within the Paleoproterozoic Columbia supercontinent assembly. Here we report a suite of-1.71 Ga Nb-enriched meta-gabbro lenses in the eastern Quanji Massif, within the Tarim Craton in NW China. The meta-gabbroic rocks have Nb contents of 11.5-16.4 ppm with Nb/La ratios varying from 0.84 to 1.02((Nb/La)_N = 0.81-0.98) and Nb/U ratios from 38.0 to 47.2. They show low SiO_2(45.1-48.5 wt.%) and MgO(5.96-6.81 wt.%) and Mg#(Mg# = Mg/(Mg + Fe) = 43.5-47.7), high FeO^t(13.0-15.7 wt.%) and moderate Ti02(1.70-2.51 wt.%).with tholeiitic affinities. These rocks possess low fractionated REE patterns without obvious Eu anomalies(Eu/Eu~* = 0.87-1.02). Their primitive mantle-normalized elements patterns display significant Zr-Hf troughs, positive Nb anomalies, weak negative Ti and P anomalies, and high contents of Rb and Ba,resembling Nb-enriched basalts generated in arc-related tectonic settings. Their arc-like geochemical signatures together with whole rock εNd(t) values of 0.4-2.1 and corresponding old T_(DM)(2.22-2.37 Ga)as well as(^(143)Nd/^(144)Nd)_t and(^(87)Sr/^(86)Sr)t(t = 1712 Ma) values of 0.5104-0.5105 and 0.7030-0.7058,respectively, suggest that their precursor magma originated from mantle wedge peridotite metasomatised by subduction-derived melts. The results from our study reveal subduction along the eastern periphery of the Tarim Craton and marginal outgrowth continuing to ~1.7 Ga within the Columbia supercontinent.展开更多
A series ophiolitic massifs in the northeastern part of Cuba are generally considered to be relics of proto-Caribbean or Caribbean lithosphere, emplaced during Paleogene collision of the NW Caribbean and North America...A series ophiolitic massifs in the northeastern part of Cuba are generally considered to be relics of proto-Caribbean or Caribbean lithosphere, emplaced during Paleogene collision of the NW Caribbean and North American plates(Iturralde-Vinent et al., 2016). However, neither their age nor their geodynamic settings are well constrained. They are generally considered to be Creataceous in age but they have been interpreted as having formed in a back-arc basin, forearc basin or plume setting(e.g. Giunta et al., 2009;Marchesi et al., 2016;Kerr et al., 1999). The Moa-Baracoa ophiolitic massif consists of mantle harzburgite, a Moho transition zone, layered and isotropic gabbro, and pillow basalts. The isotropic gabbros, collected west of Moa(20°35′17.99″N, 75°02′56.58″W), are composed mainly of subhedral plagioclase and subhedral to euhedral clinopyroxene with minor primary brown amphibole. The plagioclase grains are fresh, whereas the clinopyroxene very weakly altered and partly replaced by secondary amphibole. Cathodoluminescence(CL) images of zircon grains separated from isotropic gabbro display oscillatory zoning and broad zoning, analogous to typical igneous zircon in gabbro. Ten zircon grains with oscillatory zoning in CL images yielded a mean U-Pb SHRIMP age of 136.7±1.8 Ma. Isotropic gabbro samples have low SiO2(47.22 wt%–49.25 wt%), TiO2(0.13 wt%–0.26 wt%) and K2O(0.04 wt%–0.08 wt%) and high MgO(8.22 wt%–16.72 wt%), Cr(1337–2065 μg/g) and Ni(192–507 μg/g) contents. Their CaO, Al2O3 and Na2O contents vary from 13.56 wt% to 15.78 wt%, 14.98 wt % to 21.98 wt %, and 0.71 wt % to 1.87 wt %, respectively. In the SiO2 versus Zr/TiO2*0.0001 diagram, the samples plot in the sub-alkaline basalt field but show calcalkaline affinities in the FeOT/MgO vs. SiO2 diagram. The gabbros contain low REE contents(∑REE = 4.71–7.71), and their chondrite normalized REE patterns show very weak depletion in light REE(LREE,(La/Yb)N = 0.34–0.50) and flat heavy REE(HREE,(Gd/Yb)N = 1.02–1.15) profiles, with slightly positive Eu anomalies(Eu/Eu* = 1.55–1.76) suggesting magmatic accumulation of plagioclase. Furthermore, all samples are depleted in Nb and Th, and enriched in Ba, Pb and Sr in the primitive mantle-normalized trace element diagram. Geochemical features, such as high values of Ba/La(30–38) and low Th/Yb(0.02–0.03), medium values of(Ta/La)PM(0.30–0.35)and(Hf/Sm)PM(0.58–0.80), indicate enrichment by slab-derived fluids and subduction-related metasomatism. These isotropic gabbros are geochemically comparable with those of the Kermanshah and Neyriz ophiolites of Iran(Monsef et al., 2018) that are thought to have originated in a SSZ environment. Therefore, we suggest that the isotropic gabbros in the MoaBaracoa massif crystallized from early Cretaceous SSZ magmas that were most likely derived from a mantle source metasomatized by slab fluids in a suprasubduction zone environment.展开更多
The Rif Mountain located in Northern Morocco represents the westernmost extremity of the Peri-Mediterranean Alpine orogenic system.This belt is classically subdivided into:1)Internal zones(or Internal Rif which is an ...The Rif Mountain located in Northern Morocco represents the westernmost extremity of the Peri-Mediterranean Alpine orogenic system.This belt is classically subdivided into:1)Internal zones(or Internal Rif which is an allochthonous metamorphic domain known worldwide thanks to the presence of large peridotite outcrops);2)flysch zones(sediments corresponding to the original cover of the Maghrebian Tethys);and 3)External zones(or External Rif representing the North African margin remnants).The External Rif is itself subdivided into Prerif,Mesorif and Intrarif domains from the South to the North and is composed by Upper Triassic to Cenozoic sediments structured in a nappe stack.Mafic rock intrusions were reported in this External Rif since the preliminary geological studies even if they were wrongly mapped as"granites".Later careful fieldwork and mapping studies specified nevertheless their gabbroic nature.A more recent petrological and geochemical study of a few outcrops of those gabbroic intrusions conducted the corresponding authors,encouraged by the presence of ultramafic rocks(Beni Malek serpentines)in the eastern part of the External Rif to consider those gabbros as oceanic and so,associated to"an ophiolitic suture".Nevertheless,a recent and extended survey on the architecture and the rifting history of the Mesozoic margin of the Maghrebian Tethys focusing mainly on the Mesorif domain favor another interpretation.The detailed petrography of those mafic intrusions(mainly gabbros+dolerites and rarely extrusive rocks);their geochemical fingerprints and mainly their geochronological data(ages around 200 Ma based on U-Pb zircon LA-ICP dating)all together indicate clearly that those mafic intrusions are rather a"newly discovered"manifestation of the Central Atlantic Magmatic Province activity"CAMP"in the External Rif belt.The CAMP magmatism has been largely well described and characterized in all the other geological domains of Morocco(Anti-Atlas,High Atlas,Middle Atlas and Meseta)but never described until the date in the Rif belt which highlights the importance of the results reported here.展开更多
The present work is a petrological study of the gabbroic rocks of wadi Shianite Southeastern Desert of Egypt. Chemical analyses for major and trace elements showed that there are 3 main gabbro types. These are: 1)?pyr...The present work is a petrological study of the gabbroic rocks of wadi Shianite Southeastern Desert of Egypt. Chemical analyses for major and trace elements showed that there are 3 main gabbro types. These are: 1)?pyroxene hornblende gabbronorite;?2) hornblende gabbro;?and 3) anorthosite.?The opaque minerals study of the gabbroic rocks showed that they composed mainly of ilmenite, magnetite and sulphides. The present gabbroic rocks work are related to calc-alkaline magma type, similar to the younger gabbros in other areas in the Eastern Desert.展开更多
In order to investigate the evolution of Shiquanhe-Yongzhu-Jiali ophiolitic melange belt, the gabbros from new discovered Zhongcang ophiolitic melange are studied through petrology, whole-rock geochemistry, zircon U-P...In order to investigate the evolution of Shiquanhe-Yongzhu-Jiali ophiolitic melange belt, the gabbros from new discovered Zhongcang ophiolitic melange are studied through petrology, whole-rock geochemistry, zircon U-Pb dating and Lu-Hf isotope. The gabbros investigated in this paper contain cumulate gabbro and gabbro dike, and they have undergone greenschist-amphibolite facies metamorphism. The chondrite normalized rare earth element （REE） patterns of most of these rocks show flat types with slightly light REE （LREE） depletion and the N-MORB normalized incompatible elements diagrams indicate depletion in high field strength elements （HFSE） （Nb, Ta） and enrichment in large ion lithophile elements （LILE）. These gabbros have island arc and mid-ocean ridge basalt af- finities, suggesting that they were originated in an oceanic back arc basin. Whole rock geochemistry and high positive εNd（t） values show that these gabbros were derived from -30% partial melting of a spinel lherzolite mantle, which was enriched by interaction with slab-derived fluids and melts from sediment. U-Pb analyses of zircons from cumulate gabbro yield a weighted mean age of 114.3±1.4 Ma. Based on our data and previous studies, we propose that an intra-oceanic subduction system and back arc basin operated in the Neo-Tethy Ocean of central Tibet during Middle Jurassic and Early Creta- ceous, resembling modern active intra-oceanic subduction systems in the western Pacific.展开更多
The early Paleozoic tectonic evolution of the Xing'an-Mongolian Orogenic Belt is dominated by two oceanic basins on the northwestern and southeastern sides of the Xing'an Block,i.e.,the Xinlin-Xiguitu Ocean an...The early Paleozoic tectonic evolution of the Xing'an-Mongolian Orogenic Belt is dominated by two oceanic basins on the northwestern and southeastern sides of the Xing'an Block,i.e.,the Xinlin-Xiguitu Ocean and the Nenjiang Ocean.However,the early development of the Nenjiang Ocean remains unclear.Here,we present zircon U-Pb geochronology and whole-rock elemental and Sr-Nd isotopic data on the gabbros in the Xinglong area together with andesitic tuffs and basalts in the Duobaoshan area.LA-ICP-MS zircon U-Pb dating of gabbros and andesitic tuffs yielded crystallization ages of 443-436 Ma and 452-451 Ma,respectively.The Early Silurian Xinglong gabbros show calc-alkaline and E-MORB affinities but they are enriched in LILEs,and depleted in HFSEs,with relatively low U/Th ratios of 0.18-0.36 andεNd(t)values of-1.6 to+0.5.These geochemical features suggest that the gabbros might originate from a mantle wedge modified by pelagic sediment-derived melts,consistent with a back-arc basin setting.By contrast,the andesitic tuffs are characterized by high MgO(>5 wt.%),Cr(138-200 ppm),and Ni(65-110 ppm)contents,and can be termed as high-Mg andesites.Their low Sr/Y ratios of 15.98-17.15 and U/Th values of 0.24-0.25 and moderate(La/Sm)_n values of 3.07-3.26 are similar to those from the Setouchi Volcanic Belt(SW Japan),and are thought to be derived from partial melting of subducted sediments,and subsequent melt-mantle interaction.The Duobaoshan basalts have high Nb(8.44-10.30 ppm)and TiO2 contents(1.17-1.60 wt.%),typical of Nb-enriched basalts.They are slightly younger than regional adakitic rocks and have positiveεNd(t)values of+5.2 to+5.7 and are interpreted to be generated by partial melting of a depleted mantle source metasomatized by earlier adakitic melts.Synthesized with coeval arc-related igneous rocks from the southeastern Xing'an Block,we propose that the Duobaoshan high-Mg andesitic tuffs and Nbenriched basalts are parts of the Late Ordovician and Silurian Sonid Zuoqi-Duobaoshan arc belt,and they were formed by the northwestern subduction of the Nenjiang Ocean.Such a subduction beneath the integrated Xing'an-Erguna Block also gave rise to the East Ujimqin-Xinglong igneous belt in a continental back-arc basin setting.Our new data support an early Paleozoic arc-back-arc model in the northern Great Xing'an Range.展开更多
The Yesilova ophiolite located in the Alpine zone. This work deals with differentiation mechanism of ultramafic cumulate in ophiolite. Generally, the sequence consists of gabbro and plagiogranite. The petrographic and...The Yesilova ophiolite located in the Alpine zone. This work deals with differentiation mechanism of ultramafic cumulate in ophiolite. Generally, the sequence consists of gabbro and plagiogranite. The petrographic and petrolgichal properties of it show that the layering in gabbros are products of a differentiation by fractional crystallization insitu. Because it has ferrous compounds （magnetite, hematite） by means of volatiles （H2O, CO2） that evidence magma at high temperatures （〉700 ℃）. Ferrous liquids are compatible with fractional crystallization through olivine, plagioclase, clinopyroxene removal; whereas the evolved gabbros represent clinopyroxene, plagioclase cumulates from ferrous liquids with little amounts of trapped melt. Furthermore, cathodo luminesans image of zircons shows chemichal characteristic of oceanic plagiogranit （such as Fe2O3/MgO, Rb, Sr, Zr, TiO2） that these characters can be explained by fractional crystallization processes in the late stage of magma generation. Furthermore, all plagiogranites have small positive Eu anomalies indicating the significant role of plagioclase in the fractional crystallization. So, the Yesilova ophiolite ultramafic cumulates are the most probably related to crystal-liquid fractionation process of the oceanic crust of the Alpine belt. The plagiogranite is differentiation products of crystal-liquid insitu ofa mafic magma in the magma chamber.展开更多
Abundant small mafic intrusions occur associated with granitoids along the Gangdise^ magmatic belt. In addition to many discrete gabbro bodies within the granitoid plutons, a gabbro-pyroxenite zone occurs along the so...Abundant small mafic intrusions occur associated with granitoids along the Gangdise^ magmatic belt. In addition to many discrete gabbro bodies within the granitoid plutons, a gabbro-pyroxenite zone occurs along the southern margin of the Gangdise^ belt to the north of the Yarlung Zangbo suture. The mafic intrusion zone spatially corresponds to a strong aeromagnetic anomaly, which extends -1400 km. The mafic intrusions consist of intermittently distributed small bodies and dikes of gabbro and dolerite with accumulates of pyroxenite, olivine pyroxenite, pegmatitic pyroxenite and amphibolite. Much evidence indicates that the Gangdise^ gabbro-pyroxenite assemblage is most likely a result of underplating of mantle-derived magma. Detailed field investigation and systematic sampling of the mafic rocks was conducted at six locations along the Lhasa-Xigaze^ segment of the mafic intrusive zone, and was followed by zircon SHRIMP Ⅱ U-Pb dating. In addition to the ages of two samples previously published （47.0±1 Ma and 48.9±1.1 Ma）, the isotopic ages of the remaining four gabbro samples are 51.6±1.3Ma, 52.5±3.0 Ma, 50.2±4.2Ma and 49.9±1.1Ma. The range of these ages （47-52.5 Ma） provide geochronologic constraints on the Eocene timing of magma underplating beneath the Gangdise^ belt at ca. 50 Ma. This underplating event post-dated the initiation of the India-Eurasia continental collision by 15 million years and was contemporaneous with a process of magma mixing. The SHRIMP Ⅱ U-Pb isotopic analysis also found several old ages from a few zircon grains, mostly in a range of 479-526 Ma （weighted average age 503±10 Ma）, thus yielding information about the pre-existing lower crust when underplating of mafic magma took place. It is believed that magma underplating was one of the major mechanisms for crustal growth during the Indian-Eurasia collision, possibly corresponding in time to the formation of the 14-16 km-thick ＂crnst-mantle transitional zone＂ characterized by Vp=6.85-6.9 km/s.展开更多
The Weiya gabbro in eastern Tianshan was formed during the early Indosinian. This rock, with low ratios of Ce/Pb （5.74-10.16）, is notably characterized by enrichment in large ion lithophile elements （LILE）, such a...The Weiya gabbro in eastern Tianshan was formed during the early Indosinian. This rock, with low ratios of Ce/Pb （5.74-10.16）, is notably characterized by enrichment in large ion lithophile elements （LILE）, such as Rb, K, Ba and Pb, and in high field strength elements （HSFE）, such as U and Th, but depletion in Nb and Ta. All samples of the Weiya gabbro display similar chondrite-normalized patterns with moderate enrichment in LREE （72.58-135.61ppm）, moderate depletion in HREE （15.26-25.31ppm） and mild fractionation between LREE and HREE （L/ H=4.09-5.98）. The average initial Sr value of the rock is 0.7069, and δ18O values of the rock range from 5.67‰-8.04‰. In terms of Nd isotope ratios, the Weiya gabbro is characterized by positive eNd（t） values （0.52-0.76）. All these characteristics indicate that the source region of the Weiya gabbro was metasomatized by fluids released from subducted young continental crust, with limited crustal contamination during magma ascent and emplacement. Continental （A-type） subduction was induced by northward subduction of the Paleo-Tethyan oceanic plate during the latest Permian to Triassic. From this point of view, it is supposed that tectonic conversion from the Paleo-Asian to the Paleo-Tethys regime occurred during the latest Permian or earliest Triassic.展开更多
Field geological investigation and geochemical analysis are carried out on Baya＇ertuhushuo Gabbro in South Great Xing＇an Range. Field investigation reveals that the gabbro is a magmatic intrusion rather than a compo...Field geological investigation and geochemical analysis are carried out on Baya＇ertuhushuo Gabbro in South Great Xing＇an Range. Field investigation reveals that the gabbro is a magmatic intrusion rather than a component of an ophiolite suite as previously thought. Zircon laser ablation inductively coupled plasma mass spectroscopy （LA-ICP-MS） U-Pb dating indicates the gabbro was formed in 274-275 Ma, just as the widespread volcanic rocks of Dashizhai Formation （P1d）, monzogranites and miarolitic alkali-feldspar granites in the study area. The gabbro has SiO2 content between 47.23 wt% and 50.17 wt%, high MgO and FeOT contents of 6.95-11.29 wt% and 7.32- 12.24wt%, respectively, and it belongs to low-K tholeiitic series in the SiO2-K2O diagram. The Chondrite-normalized rare earth element （REE） patterns and primitive mantle-normalized spider diagrams of the gabbro are similar to those of Normal Mid-Ocean Ridge Basalt （N-MORB） except for the enrichment of large ion lithophfle elements （LILE）, such as Rb, Ba and K. In trace element tectonic discriminative diagrams, the samples are mainly plotted in the N-MORB field, and Zircon in situ Lu-Hf isotopic analysis also indicates the gabbro originated from depleted mantle. Through synthetic studies of the geochemical characteristics and petrogenesis of Baya＇ertuhushuo gabbro, volcanic rocks of Dashizhai Formation and granitoids in the area, it is suggested that the early Permian magmatism in the Xilinhot-Xiwuqi area formed in the tectonic setting of asthenosphere upwelling, which was caused by breaking-off of the subducted Paleo-Asian Ocean slab.展开更多
Many igneous rocks distribute in Gejiu tin polymetallic ore-field at Yunnan province, rocks including basalt, gabbro, mafic microgranular enclaves, granites （porphyritic granite and equigranular granite） and akaline...Many igneous rocks distribute in Gejiu tin polymetallic ore-field at Yunnan province, rocks including basalt, gabbro, mafic microgranular enclaves, granites （porphyritic granite and equigranular granite） and akaline rocks. The ages of the granites and akaline rocks which are considered to have genetic connecting with the mineralization have been comfirmed, but the gabbro- mafic microgranular enclaves-granite assemblage＇s ages are still unknown. By means of LA-ICP-MS zircon U-Pb dating, the data of Shenxianshui equigranular granite, the mafic microgranular enclave in Jiasha area, the host rock of the mafic microgranular enclaves and the Jiasha gabbro are around ~80 Ma. Besides the above mentioned data, a group of new ages at ~30 Ma were discovered in this study, which is from gabbro and mafic microgranular enclaves. Based on the previous data and the new data gained this time, we suggest the major geochronology framework of the magmatism and mineralization events in Gejiu area is ~80 Ma, which is consistent with the Late Cretaceous magmatism and mineralization events in the whole southeast Yunnan and west Guangxi area and they were suggested to belong to the same geotectonic setting in late Yenshannian. And the new ages of the ~30 Ma obtained in this study is considered to represent a responding to the complicate tectonic evolution history of the Tibetan orogenic events in Cenozoic.展开更多
Objective The Bayanhua Nb-enriched gabbro is newly discovered in the Diyanmiao-Meilaotewula SSZ-type ophiolitic m61ange belt of the Hegenshan suture zone, Inner Mongolia. Nb-enriched arc gabbros are usually believed ...Objective The Bayanhua Nb-enriched gabbro is newly discovered in the Diyanmiao-Meilaotewula SSZ-type ophiolitic m61ange belt of the Hegenshan suture zone, Inner Mongolia. Nb-enriched arc gabbros are usually believed to result from partial melting of the mantle wedge peridotites metasomatized by slab melts derived from the subducting oceanic slab, which represent arc magmatic markers of the oceanic subduction zone. However, whether the Hegenshan ocean basin of the Paleo-Asian Ocean was in its subduction stage in the Early Permian requires further study for a final conclusion, and what is the evolution process of the oceanic subduction and lithospheric mantle of the Hegenshan suture zone remains speculative for the lack of further definitely petrological and chronological evidence and constraints. Therefore, this study carried out zircon LA-ICP-MS U-Pb geochronology and geochemistry on the Bayanhua Nb-enriched gabbro to discuss its origin, in order to provide new evidence for the tectonic evolution of the Hegenshan suture zone of the eastern Central Asian Orogenic Belt.展开更多
The giant Central Asian Orogenic Belt is an extensive accretionary orogen,of which the Solonker suture,as a major regional suture,coincides closely with an early Permian paleobiogeographical boundary.This suture is co...The giant Central Asian Orogenic Belt is an extensive accretionary orogen,of which the Solonker suture,as a major regional suture,coincides closely with an early Permian paleobiogeographical boundary.This suture is considered to mark the location of the final closure of the Paleo-Asian Ocean between the North China Craton and the Mongolian Terrane.Although the closure time of the Paleo-Asian Ocean along the Solonker suture has generally been regarded as Late Permian-Early Triassic,uncertainty remains because of a lack of typical collisionrelated features(e.g.,high-grade regional metamorphism and well-developed fold-thrust structures)and a scarcity of outcrops.The present study reports Early Permian foliated gabbros and dikes(288-275 Ma)and Middle-Late Permian undeformed layered gabbros,strongly peraluminous granites,and I-type granites(265-254 Ma)in the Xinhure area along the northern margin of the North China Craton.The Early Permian foliated intrusions have a subduction-related geochemical signature and were derived from partial melting of lithospheric mantle modified by subduction-related melts or fluids at the active margin of the North China Craton.In contrast,the Late Permian undeformed layered gabbros and strongly peraluminous granites were derived from partial melting of lithospheric mantle and middle-upper crust,respectively,triggered by asthenospheric upwelling.Therefore,a transition from an end-compressional to an extensional environment according to a transition from collision termination to postcollision of the North China Craton and Mongolian Terrane may have occurred between 275 Ma and 262 Ma.This time span can be considered as the final closure time of the Paleo-Asian Ocean along the Solonker suture.展开更多
The Dangqiong ophiolite, the largest in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)ophiolite belt in southern Tibet, consists of discontinuous mantle peridotite and intrusive mafic rocks. The former is...The Dangqiong ophiolite, the largest in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)ophiolite belt in southern Tibet, consists of discontinuous mantle peridotite and intrusive mafic rocks. The former is composed dominantly of harzburgite, with minor dunite, locally lherzolite and some dunite containing lenses and veins of chromitite. The latter, mafic dykes(gabbro and diabase dykes), occur mainly in the southern part. This study carried out geochemical analysis on both rocks. The results show that the mantle peridotite has Fo values in olivine from 89.92 to 91.63 and is characterized by low aluminum contents(1.5–4.66 wt%) and high Mg# values(91.06–94.53) of clinopyroxene. Most spinels in the Dangqiong peridotites have typical Mg# values ranging from 61.07 to 72.52, with corresponding Cr# values ranging from 17.67 to 31.66, and have TiO2 contents from 0 to 0.09%, indicating only a low degree of partial melting(10–15%). The olivine-spinel equilibrium and spinel chemistry of the Dangqiong peridotites suggest that they originated deeper mantle(>20 kbar). The gabbro dykes show N-MORB-type patterns of REE and trace elements. The presence of amphibole in the Dangqiong gabbro suggests the late-stage alteration of subduction-derived fluids. All the lherzolites and harzburgites in Dangqiong have similar distribution patterns of REE and trace elements, the mineral chemistry in the harzburgites and lherzolites indicates compositions similar to those of abyssal and forearc peridotites, suggesting that the ophiolite in Dangqiong formed in a MOR environment and then was modified by late-stage melts and fluids in a suprasubduction zone(SSZ) setting. This formation process is consistent with that of the Luobusa ophiolite in the eastern Yarlung-Zangbo Suture Zone and Purang ophiolite in the western Yarlung-Zangbo Suture Zone.展开更多
The mafic intrusive Moskin and Parvaneh regions in the northwest and south of Urmia were investigated for their geochemistry and petro-physical properties such as size,shape,and spatial distribution of their crystals....The mafic intrusive Moskin and Parvaneh regions in the northwest and south of Urmia were investigated for their geochemistry and petro-physical properties such as size,shape,and spatial distribution of their crystals.This type of study can be a powerful tool for analyzing the history of magma crystallization,along with geochemistry examinations.The gabbroic calc-alkaline rocks,with the Early Cenozoic is the relative age,consist of coarse plagioclase and amphibole with rare clinopyroxene and olivine in the Sanandaj-Sirjan zone at sedimentary-structural subdivisions of Iran.The depletion of HFSEs and enrichment of LILEs as arc-related rocks,as well as lower crust contamination(Nb/Th and K/Ce)symptoms,show that these gabbros formed at the continental arc setting from an enriched source.Considering the geographical position and relative age of the gabbros,they possibly outcropped during the Neo-Tethys subduction processes after the enrichment by considerably lower crust effects.The amphibole and plagioclase crystals with considerable abundance show linear crystal size distribution(CSD)patterns with a slope of(-1/Gt)-3 to-4 for amphiboles and plagioclase except for Parvaneh plagioclase which shows-16.The annealing(Ostwald ripening)caused the convex CSD on the small grains part at Parvaneh gabbro.Therefore,the ratio of nucleation to growth rates(J/G)for these minerals is 5-6 times,only for Parvaneh plagioclase is ten times which numerous nuclei were made by a significant degree of overstepping in the rocks.There is no magma mixing evidence because of the linear(one population of crystals)CSD patterns.The shape of amphibole crystals in gabbros is bladed(with S/I=0.4-0.5 and I/L=0.1-0.3)and bladed to tabular for plagioclase(S/I=0.4 and I/L=0.6-0.7).The spatial distribution patterns(SDPs)are calculated from a big R-value based on the nearest neighbor’s theory for amphibole and plagioclase crystals,confirming clustered SDP for all of them.Also,the dihedral angle measurements(between amphibole and plagioclase grains)display that solid-state equilibrium is prevailing in the rocks.In this study,we tried to establish a relationship between geochemistry and petro-physical results and investigate the evolution of igneous rocks more accurately and all-inclusive.We should not neglect the importance of quantifying the texture of rocks.展开更多
Discontinuous ophiolite suites along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet representing remnants of the Neo-Tethyan oceanic lithosphere are considered to be formed between Late Jurassic and Early Creta...Discontinuous ophiolite suites along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet representing remnants of the Neo-Tethyan oceanic lithosphere are considered to be formed between Late Jurassic and Early Cretaceous.Older ophiolite or ophiolitic mélange(e.g.Triassic)to reveal the initial evolution the Neo-Tethyan ocean within the YZSZ have rarely been documented so far.The western YZSZ extending from the Saga to Ladakh area are composed of the northern ophiolitic subbelt,the Zhongba terrane and the southern ophiolitic sub-belt.In this study,we document structural,petrological and geochronological data of mafic intrusions from the Mayoumu massif in the southern ophiolitic sub-belt of the western YZSZ.Two lithos-tectonic sub-units,the southern ophiolitic complex and the northern ophiolitic mélange,are recognized in the Mayoumu massif in terms of structural pattern and petrology.LA-ICP-MS zircon U-Pb dating of gabbro from the ophiolitic mélange yields an age of 243±1 Ma with zirconεHf(t)values of+7.9 to+13.2.Two samples of diabase samples from the ophiolite yield ages of 131±1 Ma and 124±1 Ma with zirconεHf(t)values ranging from+10.8 to+15.0 and+12.3 to+15.4,respectively.Geochemically,rare earth elements(REEs)patterns show that these mafic intrusions are similar to those of the normal mid-ocean-ridge basalt(N-MORB).Enrichment of fluidmobile elements(e.g.Rb and Ba)and depletion of Nb,and Ta suggest that these intrusions were possibly originated from melting of a depleted mantle source influenced by subducted slab.Our data strongly confirms that the Neo-Tethyan ocean between the Lhasa terrane and Gondwana had been existed since the Middle Triassic at least.The ages of the two diabases from the ophiolitic complex reveal that mafic magma activities within the Neo-Tethyan ocean could be as a continuing process during 120–130 Ma.Emplacement and preservation of older ophiolitic recorder during evolution of the Neo-Tethyan Ocean may be closely related to the occurrence of the Zhongba micro-terrane within the western YZSZ.展开更多
Based on the detailed petrographic study,the characteristics of source region of the Weiya gabbro and tectonic implications were studied.The results show that the gabbroic rock consists mainly of gabbro,with less amou...Based on the detailed petrographic study,the characteristics of source region of the Weiya gabbro and tectonic implications were studied.The results show that the gabbroic rock consists mainly of gabbro,with less amount of ultra-mafic rocks.The ultra-mafic rocks show cumulate texture and are gradually transitional contact with gabbro,indicating that they are cumulate products of parental magma.The ultra-mafic rocks consist mainly of spinel periodite and spinel clinopyroxenite.The former is mainly composed of olivine(65%-70%),spinel(10%-15%),hornblende(5%-10%) and phlogopite(5%-10%);the latter consists mainly of clinopyroxene(70%-80%),spinel(15%-20%) and phlogopite(0-10%),with minor amounts of carbonate(0-2%).No olivine or orthopyroxene is found.The gabbro is composed mainly of clinopyroxene(35%-40%),plagioclase(An 55-65,40%-45%),hornblende(5%-15%),with variable amounts of carbonate(0-5%).Petrographic observations show that the source region of the Weiya gabbroic rock is water-rich due to intensive intra-continental A-type subduction occurring in this region during late Permian to early Triassic.展开更多
Objective Lhasa terrane has recorded the geologic history conceming the formation and evolution of Paleo-Tethys and the intra-continental convergence in Qinghai-Tibet Plateau （Yin and Harrison, 2000）. Previous inve...Objective Lhasa terrane has recorded the geologic history conceming the formation and evolution of Paleo-Tethys and the intra-continental convergence in Qinghai-Tibet Plateau （Yin and Harrison, 2000）. Previous investigations have focused on the initial timing of the India-Asia collision and the Cretaceous-Cenozoic magmatism and sedimentation （Wang Tianyang et al., 2017）, however, there are only few studies concerning the Paleozoic evolution in Lhasa terrane. This paper mainly reports a new discovery of 341 Ma gabbro, which is located in the Xiongcun copper-gold district at the southern margin of Lhasa terrane, Tibet. As an important magmatic record during the Paleozoic evolution process of Lhasa terrane, the Early Carboniferous Xiongcun gabbro will provide new constraints for the tectonic evolution of Paleo-Tethys Ocean in the Late Devonian to Early Carboniferous.展开更多
Gabbro is selected as a sample for experimental deformation to investigate and validate the migration of elements during rock deformation. Samples are deformed for 3 h under a strain of about 5 % at T =700 ℃, ...Gabbro is selected as a sample for experimental deformation to investigate and validate the migration of elements during rock deformation. Samples are deformed for 3 h under a strain of about 5 % at T =700 ℃, p =100 MPa, σ =50 MPa. It is shown that there are 4 areas with different colors in the section of the samples parallel to σ 1: the extensional, contractional, and strongly compressional areas and a ductile shear zone, respectively on the basis of the stress states and the direction of material movement. The chemical components such as K, Na, Al and Fe from materials in different areas have changed. The four elements mentioned above in pyroxene grains decrease in content from the extensional area through the ductile shear zone, the contractional area, to the strongly compressional area. The contents of the same elements in feldspar grain vary in a reverse direction.展开更多
This paper reports stick-slip behaviors of Indian gabbro as studied using a new large-scale biaxial friction apparatus, built in the National Research Institute for Earth Science and Disaster Prevention （NIED）, Tsuk...This paper reports stick-slip behaviors of Indian gabbro as studied using a new large-scale biaxial friction apparatus, built in the National Research Institute for Earth Science and Disaster Prevention （NIED）, Tsukuba, Japan. The apparatus consists of the existing shaking table as the shear-loading device up to 3,600 kN, the main frame for holding two large rectangular prismatic specimens with a sliding area of 0.75 m^2 and for applying normal stresses an up to 1.33 MPa, and a reaction force unit holding the stationary specimen to the ground. The shaking table can produce loading rates v up to 1.0 m/s, accelerations up to 9.4 m/s^2, and displacements d up to 0.44 m, using four servocontrolled actuators. We report results from eight preliminary experiments conducted with room humidity on the same gabbro specimens at v = 0.1-100 mm/s and an = 0.66-1.33 MPa, and with d of about 0.39 m. The peak and steady-state friction coefficients were about 0.8 and 0.6, respectively, consistent with the Byerlee friction. The axial force drop or shear stress drop during an abrupt slip is linearly proportional to the amount of displacement, and the slope of this rela- tionship determines the stiffness of the apparatus as 1.15 × 10^8 N/m or 153 MPa/m for the specimens we used. This low stiffness makes fault motion very unstable and the overshooting of shear stress to a negative value was recognized in some violent stick-slip events. An abrupt slip occurred in a constant rise time of 16-18 ms despite wide variation of the stress drop, and an average velocity during an abrupt slip is linearly proportional to the stress drop. The use of a large-scale shaking table has a great potential in increasing the slip rate and total displacement in biaxial friction experiments with large specimens.展开更多
基金supported by the National Science Foundation of ChinaNSFC grants(Grant Nos.41602056,41372075,41172069)+1 种基金the China Postdoctoral Science Foundation(Grant No.2016M590729)the Fundamental Research Funds for National Universities,China University of Geosciences(Wuhan)(Grant No.CUG160846)
文摘Diverse models have been proposed for the role of the Tarim Craton within the Paleoproterozoic Columbia supercontinent assembly. Here we report a suite of-1.71 Ga Nb-enriched meta-gabbro lenses in the eastern Quanji Massif, within the Tarim Craton in NW China. The meta-gabbroic rocks have Nb contents of 11.5-16.4 ppm with Nb/La ratios varying from 0.84 to 1.02((Nb/La)_N = 0.81-0.98) and Nb/U ratios from 38.0 to 47.2. They show low SiO_2(45.1-48.5 wt.%) and MgO(5.96-6.81 wt.%) and Mg#(Mg# = Mg/(Mg + Fe) = 43.5-47.7), high FeO^t(13.0-15.7 wt.%) and moderate Ti02(1.70-2.51 wt.%).with tholeiitic affinities. These rocks possess low fractionated REE patterns without obvious Eu anomalies(Eu/Eu~* = 0.87-1.02). Their primitive mantle-normalized elements patterns display significant Zr-Hf troughs, positive Nb anomalies, weak negative Ti and P anomalies, and high contents of Rb and Ba,resembling Nb-enriched basalts generated in arc-related tectonic settings. Their arc-like geochemical signatures together with whole rock εNd(t) values of 0.4-2.1 and corresponding old T_(DM)(2.22-2.37 Ga)as well as(^(143)Nd/^(144)Nd)_t and(^(87)Sr/^(86)Sr)t(t = 1712 Ma) values of 0.5104-0.5105 and 0.7030-0.7058,respectively, suggest that their precursor magma originated from mantle wedge peridotite metasomatised by subduction-derived melts. The results from our study reveal subduction along the eastern periphery of the Tarim Craton and marginal outgrowth continuing to ~1.7 Ga within the Columbia supercontinent.
基金supported by grants the Natural Science Foundation of China(Grant Nos.41802055 and 41802034)the International Geoscience Programme(Grant No.IGCP649,2015–2020)+2 种基金the Natural Science Foundation of Jiangsu(Grant No.BK20180349)the Research Founding from Chinese Academy of Geological Sciences(J1903)the fund from the Key Laboratory of DeepEarth Dynamics of Ministry of Natural Resources
文摘A series ophiolitic massifs in the northeastern part of Cuba are generally considered to be relics of proto-Caribbean or Caribbean lithosphere, emplaced during Paleogene collision of the NW Caribbean and North American plates(Iturralde-Vinent et al., 2016). However, neither their age nor their geodynamic settings are well constrained. They are generally considered to be Creataceous in age but they have been interpreted as having formed in a back-arc basin, forearc basin or plume setting(e.g. Giunta et al., 2009;Marchesi et al., 2016;Kerr et al., 1999). The Moa-Baracoa ophiolitic massif consists of mantle harzburgite, a Moho transition zone, layered and isotropic gabbro, and pillow basalts. The isotropic gabbros, collected west of Moa(20°35′17.99″N, 75°02′56.58″W), are composed mainly of subhedral plagioclase and subhedral to euhedral clinopyroxene with minor primary brown amphibole. The plagioclase grains are fresh, whereas the clinopyroxene very weakly altered and partly replaced by secondary amphibole. Cathodoluminescence(CL) images of zircon grains separated from isotropic gabbro display oscillatory zoning and broad zoning, analogous to typical igneous zircon in gabbro. Ten zircon grains with oscillatory zoning in CL images yielded a mean U-Pb SHRIMP age of 136.7±1.8 Ma. Isotropic gabbro samples have low SiO2(47.22 wt%–49.25 wt%), TiO2(0.13 wt%–0.26 wt%) and K2O(0.04 wt%–0.08 wt%) and high MgO(8.22 wt%–16.72 wt%), Cr(1337–2065 μg/g) and Ni(192–507 μg/g) contents. Their CaO, Al2O3 and Na2O contents vary from 13.56 wt% to 15.78 wt%, 14.98 wt % to 21.98 wt %, and 0.71 wt % to 1.87 wt %, respectively. In the SiO2 versus Zr/TiO2*0.0001 diagram, the samples plot in the sub-alkaline basalt field but show calcalkaline affinities in the FeOT/MgO vs. SiO2 diagram. The gabbros contain low REE contents(∑REE = 4.71–7.71), and their chondrite normalized REE patterns show very weak depletion in light REE(LREE,(La/Yb)N = 0.34–0.50) and flat heavy REE(HREE,(Gd/Yb)N = 1.02–1.15) profiles, with slightly positive Eu anomalies(Eu/Eu* = 1.55–1.76) suggesting magmatic accumulation of plagioclase. Furthermore, all samples are depleted in Nb and Th, and enriched in Ba, Pb and Sr in the primitive mantle-normalized trace element diagram. Geochemical features, such as high values of Ba/La(30–38) and low Th/Yb(0.02–0.03), medium values of(Ta/La)PM(0.30–0.35)and(Hf/Sm)PM(0.58–0.80), indicate enrichment by slab-derived fluids and subduction-related metasomatism. These isotropic gabbros are geochemically comparable with those of the Kermanshah and Neyriz ophiolites of Iran(Monsef et al., 2018) that are thought to have originated in a SSZ environment. Therefore, we suggest that the isotropic gabbros in the MoaBaracoa massif crystallized from early Cretaceous SSZ magmas that were most likely derived from a mantle source metasomatized by slab fluids in a suprasubduction zone environment.
文摘The Rif Mountain located in Northern Morocco represents the westernmost extremity of the Peri-Mediterranean Alpine orogenic system.This belt is classically subdivided into:1)Internal zones(or Internal Rif which is an allochthonous metamorphic domain known worldwide thanks to the presence of large peridotite outcrops);2)flysch zones(sediments corresponding to the original cover of the Maghrebian Tethys);and 3)External zones(or External Rif representing the North African margin remnants).The External Rif is itself subdivided into Prerif,Mesorif and Intrarif domains from the South to the North and is composed by Upper Triassic to Cenozoic sediments structured in a nappe stack.Mafic rock intrusions were reported in this External Rif since the preliminary geological studies even if they were wrongly mapped as"granites".Later careful fieldwork and mapping studies specified nevertheless their gabbroic nature.A more recent petrological and geochemical study of a few outcrops of those gabbroic intrusions conducted the corresponding authors,encouraged by the presence of ultramafic rocks(Beni Malek serpentines)in the eastern part of the External Rif to consider those gabbros as oceanic and so,associated to"an ophiolitic suture".Nevertheless,a recent and extended survey on the architecture and the rifting history of the Mesozoic margin of the Maghrebian Tethys focusing mainly on the Mesorif domain favor another interpretation.The detailed petrography of those mafic intrusions(mainly gabbros+dolerites and rarely extrusive rocks);their geochemical fingerprints and mainly their geochronological data(ages around 200 Ma based on U-Pb zircon LA-ICP dating)all together indicate clearly that those mafic intrusions are rather a"newly discovered"manifestation of the Central Atlantic Magmatic Province activity"CAMP"in the External Rif belt.The CAMP magmatism has been largely well described and characterized in all the other geological domains of Morocco(Anti-Atlas,High Atlas,Middle Atlas and Meseta)but never described until the date in the Rif belt which highlights the importance of the results reported here.
文摘The present work is a petrological study of the gabbroic rocks of wadi Shianite Southeastern Desert of Egypt. Chemical analyses for major and trace elements showed that there are 3 main gabbro types. These are: 1)?pyroxene hornblende gabbronorite;?2) hornblende gabbro;?and 3) anorthosite.?The opaque minerals study of the gabbroic rocks showed that they composed mainly of ilmenite, magnetite and sulphides. The present gabbroic rocks work are related to calc-alkaline magma type, similar to the younger gabbros in other areas in the Eastern Desert.
基金supported by the National Nature Science Foundation of China (No. 41272240)the Project of China Geological Survey (No. 1212011121248)
文摘In order to investigate the evolution of Shiquanhe-Yongzhu-Jiali ophiolitic melange belt, the gabbros from new discovered Zhongcang ophiolitic melange are studied through petrology, whole-rock geochemistry, zircon U-Pb dating and Lu-Hf isotope. The gabbros investigated in this paper contain cumulate gabbro and gabbro dike, and they have undergone greenschist-amphibolite facies metamorphism. The chondrite normalized rare earth element （REE） patterns of most of these rocks show flat types with slightly light REE （LREE） depletion and the N-MORB normalized incompatible elements diagrams indicate depletion in high field strength elements （HFSE） （Nb, Ta） and enrichment in large ion lithophile elements （LILE）. These gabbros have island arc and mid-ocean ridge basalt af- finities, suggesting that they were originated in an oceanic back arc basin. Whole rock geochemistry and high positive εNd（t） values show that these gabbros were derived from -30% partial melting of a spinel lherzolite mantle, which was enriched by interaction with slab-derived fluids and melts from sediment. U-Pb analyses of zircons from cumulate gabbro yield a weighted mean age of 114.3±1.4 Ma. Based on our data and previous studies, we propose that an intra-oceanic subduction system and back arc basin operated in the Neo-Tethy Ocean of central Tibet during Middle Jurassic and Early Creta- ceous, resembling modern active intra-oceanic subduction systems in the western Pacific.
基金supported by National Natural Science Foundation of China(41802236)“the Fundamental Research Funds for the Central universities”(N170103013,N2001004 and N170104022)Opening Foundation of Key Laboratory of Mineral Resources Evaluation in Northeast Asia,Ministry of Natural Resources(DBY-KF-18-05)。
文摘The early Paleozoic tectonic evolution of the Xing'an-Mongolian Orogenic Belt is dominated by two oceanic basins on the northwestern and southeastern sides of the Xing'an Block,i.e.,the Xinlin-Xiguitu Ocean and the Nenjiang Ocean.However,the early development of the Nenjiang Ocean remains unclear.Here,we present zircon U-Pb geochronology and whole-rock elemental and Sr-Nd isotopic data on the gabbros in the Xinglong area together with andesitic tuffs and basalts in the Duobaoshan area.LA-ICP-MS zircon U-Pb dating of gabbros and andesitic tuffs yielded crystallization ages of 443-436 Ma and 452-451 Ma,respectively.The Early Silurian Xinglong gabbros show calc-alkaline and E-MORB affinities but they are enriched in LILEs,and depleted in HFSEs,with relatively low U/Th ratios of 0.18-0.36 andεNd(t)values of-1.6 to+0.5.These geochemical features suggest that the gabbros might originate from a mantle wedge modified by pelagic sediment-derived melts,consistent with a back-arc basin setting.By contrast,the andesitic tuffs are characterized by high MgO(>5 wt.%),Cr(138-200 ppm),and Ni(65-110 ppm)contents,and can be termed as high-Mg andesites.Their low Sr/Y ratios of 15.98-17.15 and U/Th values of 0.24-0.25 and moderate(La/Sm)_n values of 3.07-3.26 are similar to those from the Setouchi Volcanic Belt(SW Japan),and are thought to be derived from partial melting of subducted sediments,and subsequent melt-mantle interaction.The Duobaoshan basalts have high Nb(8.44-10.30 ppm)and TiO2 contents(1.17-1.60 wt.%),typical of Nb-enriched basalts.They are slightly younger than regional adakitic rocks and have positiveεNd(t)values of+5.2 to+5.7 and are interpreted to be generated by partial melting of a depleted mantle source metasomatized by earlier adakitic melts.Synthesized with coeval arc-related igneous rocks from the southeastern Xing'an Block,we propose that the Duobaoshan high-Mg andesitic tuffs and Nbenriched basalts are parts of the Late Ordovician and Silurian Sonid Zuoqi-Duobaoshan arc belt,and they were formed by the northwestern subduction of the Nenjiang Ocean.Such a subduction beneath the integrated Xing'an-Erguna Block also gave rise to the East Ujimqin-Xinglong igneous belt in a continental back-arc basin setting.Our new data support an early Paleozoic arc-back-arc model in the northern Great Xing'an Range.
文摘The Yesilova ophiolite located in the Alpine zone. This work deals with differentiation mechanism of ultramafic cumulate in ophiolite. Generally, the sequence consists of gabbro and plagiogranite. The petrographic and petrolgichal properties of it show that the layering in gabbros are products of a differentiation by fractional crystallization insitu. Because it has ferrous compounds （magnetite, hematite） by means of volatiles （H2O, CO2） that evidence magma at high temperatures （〉700 ℃）. Ferrous liquids are compatible with fractional crystallization through olivine, plagioclase, clinopyroxene removal; whereas the evolved gabbros represent clinopyroxene, plagioclase cumulates from ferrous liquids with little amounts of trapped melt. Furthermore, cathodo luminesans image of zircons shows chemichal characteristic of oceanic plagiogranit （such as Fe2O3/MgO, Rb, Sr, Zr, TiO2） that these characters can be explained by fractional crystallization processes in the late stage of magma generation. Furthermore, all plagiogranites have small positive Eu anomalies indicating the significant role of plagioclase in the fractional crystallization. So, the Yesilova ophiolite ultramafic cumulates are the most probably related to crystal-liquid fractionation process of the oceanic crust of the Alpine belt. The plagiogranite is differentiation products of crystal-liquid insitu ofa mafic magma in the magma chamber.
文摘Abundant small mafic intrusions occur associated with granitoids along the Gangdise^ magmatic belt. In addition to many discrete gabbro bodies within the granitoid plutons, a gabbro-pyroxenite zone occurs along the southern margin of the Gangdise^ belt to the north of the Yarlung Zangbo suture. The mafic intrusion zone spatially corresponds to a strong aeromagnetic anomaly, which extends -1400 km. The mafic intrusions consist of intermittently distributed small bodies and dikes of gabbro and dolerite with accumulates of pyroxenite, olivine pyroxenite, pegmatitic pyroxenite and amphibolite. Much evidence indicates that the Gangdise^ gabbro-pyroxenite assemblage is most likely a result of underplating of mantle-derived magma. Detailed field investigation and systematic sampling of the mafic rocks was conducted at six locations along the Lhasa-Xigaze^ segment of the mafic intrusive zone, and was followed by zircon SHRIMP Ⅱ U-Pb dating. In addition to the ages of two samples previously published （47.0±1 Ma and 48.9±1.1 Ma）, the isotopic ages of the remaining four gabbro samples are 51.6±1.3Ma, 52.5±3.0 Ma, 50.2±4.2Ma and 49.9±1.1Ma. The range of these ages （47-52.5 Ma） provide geochronologic constraints on the Eocene timing of magma underplating beneath the Gangdise^ belt at ca. 50 Ma. This underplating event post-dated the initiation of the India-Eurasia continental collision by 15 million years and was contemporaneous with a process of magma mixing. The SHRIMP Ⅱ U-Pb isotopic analysis also found several old ages from a few zircon grains, mostly in a range of 479-526 Ma （weighted average age 503±10 Ma）, thus yielding information about the pre-existing lower crust when underplating of mafic magma took place. It is believed that magma underplating was one of the major mechanisms for crustal growth during the Indian-Eurasia collision, possibly corresponding in time to the formation of the 14-16 km-thick ＂crnst-mantle transitional zone＂ characterized by Vp=6.85-6.9 km/s.
基金This study was jointly supported by National Natural Science Foundation of China （Nos. 40672040, 40472042 and 40603008）；National Basic Research Program of China （2001CB409802）；China Postdoctoral Science Foundation （No. 2005038237） ；Laboratory for Radiogenic Isotope Geochemistry, Institute of Geology and Geophysics, Chinese Academy of Sciences.
文摘The Weiya gabbro in eastern Tianshan was formed during the early Indosinian. This rock, with low ratios of Ce/Pb （5.74-10.16）, is notably characterized by enrichment in large ion lithophile elements （LILE）, such as Rb, K, Ba and Pb, and in high field strength elements （HSFE）, such as U and Th, but depletion in Nb and Ta. All samples of the Weiya gabbro display similar chondrite-normalized patterns with moderate enrichment in LREE （72.58-135.61ppm）, moderate depletion in HREE （15.26-25.31ppm） and mild fractionation between LREE and HREE （L/ H=4.09-5.98）. The average initial Sr value of the rock is 0.7069, and δ18O values of the rock range from 5.67‰-8.04‰. In terms of Nd isotope ratios, the Weiya gabbro is characterized by positive eNd（t） values （0.52-0.76）. All these characteristics indicate that the source region of the Weiya gabbro was metasomatized by fluids released from subducted young continental crust, with limited crustal contamination during magma ascent and emplacement. Continental （A-type） subduction was induced by northward subduction of the Paleo-Tethyan oceanic plate during the latest Permian to Triassic. From this point of view, it is supposed that tectonic conversion from the Paleo-Asian to the Paleo-Tethys regime occurred during the latest Permian or earliest Triassic.
基金supported by the State Key Program of National Natural Science of China(grant no.40739905)Special Projects of Investigation and Evaluation of Countrywide Strategic Petroleum Area Selection(grant no.XQ-2007-07)+1 种基金Science and Technology Project of Sinopec(grant no.GO800-06)the Fund for Basic Scientific Research of the Institute of Geology,Chinese Academy of Geological Sciences(grant no.J0920)
文摘Field geological investigation and geochemical analysis are carried out on Baya＇ertuhushuo Gabbro in South Great Xing＇an Range. Field investigation reveals that the gabbro is a magmatic intrusion rather than a component of an ophiolite suite as previously thought. Zircon laser ablation inductively coupled plasma mass spectroscopy （LA-ICP-MS） U-Pb dating indicates the gabbro was formed in 274-275 Ma, just as the widespread volcanic rocks of Dashizhai Formation （P1d）, monzogranites and miarolitic alkali-feldspar granites in the study area. The gabbro has SiO2 content between 47.23 wt% and 50.17 wt%, high MgO and FeOT contents of 6.95-11.29 wt% and 7.32- 12.24wt%, respectively, and it belongs to low-K tholeiitic series in the SiO2-K2O diagram. The Chondrite-normalized rare earth element （REE） patterns and primitive mantle-normalized spider diagrams of the gabbro are similar to those of Normal Mid-Ocean Ridge Basalt （N-MORB） except for the enrichment of large ion lithophfle elements （LILE）, such as Rb, Ba and K. In trace element tectonic discriminative diagrams, the samples are mainly plotted in the N-MORB field, and Zircon in situ Lu-Hf isotopic analysis also indicates the gabbro originated from depleted mantle. Through synthetic studies of the geochemical characteristics and petrogenesis of Baya＇ertuhushuo gabbro, volcanic rocks of Dashizhai Formation and granitoids in the area, it is suggested that the early Permian magmatism in the Xilinhot-Xiwuqi area formed in the tectonic setting of asthenosphere upwelling, which was caused by breaking-off of the subducted Paleo-Asian Ocean slab.
基金supported by the National Science Foundation of China(40930419)Special Research Funding for the Public Benefits Sponsored by MLR (200911007–12)+2 种基金Research Program of Yunnan Tin Group (2010–04A)Geological Investigation Program by CGS (1212011120994)the Fundamental Research Funds for the Central Universities(2–9–2010–21)
文摘Many igneous rocks distribute in Gejiu tin polymetallic ore-field at Yunnan province, rocks including basalt, gabbro, mafic microgranular enclaves, granites （porphyritic granite and equigranular granite） and akaline rocks. The ages of the granites and akaline rocks which are considered to have genetic connecting with the mineralization have been comfirmed, but the gabbro- mafic microgranular enclaves-granite assemblage＇s ages are still unknown. By means of LA-ICP-MS zircon U-Pb dating, the data of Shenxianshui equigranular granite, the mafic microgranular enclave in Jiasha area, the host rock of the mafic microgranular enclaves and the Jiasha gabbro are around ~80 Ma. Besides the above mentioned data, a group of new ages at ~30 Ma were discovered in this study, which is from gabbro and mafic microgranular enclaves. Based on the previous data and the new data gained this time, we suggest the major geochronology framework of the magmatism and mineralization events in Gejiu area is ~80 Ma, which is consistent with the Late Cretaceous magmatism and mineralization events in the whole southeast Yunnan and west Guangxi area and they were suggested to belong to the same geotectonic setting in late Yenshannian. And the new ages of the ~30 Ma obtained in this study is considered to represent a responding to the complicate tectonic evolution history of the Tibetan orogenic events in Cenozoic.
基金funded by the National Natural Science Foundation of China (grant No.41502211)the China Geological Survey (grants No.1212011120701 and 1212011120711)Hebei Province Education Department (grant No.ZC20165013)
文摘Objective The Bayanhua Nb-enriched gabbro is newly discovered in the Diyanmiao-Meilaotewula SSZ-type ophiolitic m61ange belt of the Hegenshan suture zone, Inner Mongolia. Nb-enriched arc gabbros are usually believed to result from partial melting of the mantle wedge peridotites metasomatized by slab melts derived from the subducting oceanic slab, which represent arc magmatic markers of the oceanic subduction zone. However, whether the Hegenshan ocean basin of the Paleo-Asian Ocean was in its subduction stage in the Early Permian requires further study for a final conclusion, and what is the evolution process of the oceanic subduction and lithospheric mantle of the Hegenshan suture zone remains speculative for the lack of further definitely petrological and chronological evidence and constraints. Therefore, this study carried out zircon LA-ICP-MS U-Pb geochronology and geochemistry on the Bayanhua Nb-enriched gabbro to discuss its origin, in order to provide new evidence for the tectonic evolution of the Hegenshan suture zone of the eastern Central Asian Orogenic Belt.
基金supported by the National Natural Science Foundation of China(Grant Nos.41402042,41002064)Fundamental Research Funds for the Central Universities(Grant Nos.310827172006,300102278402)Geological investigation work project of China Geological Survey(No.12120115069701)。
文摘The giant Central Asian Orogenic Belt is an extensive accretionary orogen,of which the Solonker suture,as a major regional suture,coincides closely with an early Permian paleobiogeographical boundary.This suture is considered to mark the location of the final closure of the Paleo-Asian Ocean between the North China Craton and the Mongolian Terrane.Although the closure time of the Paleo-Asian Ocean along the Solonker suture has generally been regarded as Late Permian-Early Triassic,uncertainty remains because of a lack of typical collisionrelated features(e.g.,high-grade regional metamorphism and well-developed fold-thrust structures)and a scarcity of outcrops.The present study reports Early Permian foliated gabbros and dikes(288-275 Ma)and Middle-Late Permian undeformed layered gabbros,strongly peraluminous granites,and I-type granites(265-254 Ma)in the Xinhure area along the northern margin of the North China Craton.The Early Permian foliated intrusions have a subduction-related geochemical signature and were derived from partial melting of lithospheric mantle modified by subduction-related melts or fluids at the active margin of the North China Craton.In contrast,the Late Permian undeformed layered gabbros and strongly peraluminous granites were derived from partial melting of lithospheric mantle and middle-upper crust,respectively,triggered by asthenospheric upwelling.Therefore,a transition from an end-compressional to an extensional environment according to a transition from collision termination to postcollision of the North China Craton and Mongolian Terrane may have occurred between 275 Ma and 262 Ma.This time span can be considered as the final closure time of the Paleo-Asian Ocean along the Solonker suture.
基金funded by grants from the NSF China (No. 41672046, 41720104009)the China Geological Survey (DD20160023-01)+1 种基金the Ministry of Science and Technology (201511022)IGCP (International Geoscience Programme) Project 649
文摘The Dangqiong ophiolite, the largest in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)ophiolite belt in southern Tibet, consists of discontinuous mantle peridotite and intrusive mafic rocks. The former is composed dominantly of harzburgite, with minor dunite, locally lherzolite and some dunite containing lenses and veins of chromitite. The latter, mafic dykes(gabbro and diabase dykes), occur mainly in the southern part. This study carried out geochemical analysis on both rocks. The results show that the mantle peridotite has Fo values in olivine from 89.92 to 91.63 and is characterized by low aluminum contents(1.5–4.66 wt%) and high Mg# values(91.06–94.53) of clinopyroxene. Most spinels in the Dangqiong peridotites have typical Mg# values ranging from 61.07 to 72.52, with corresponding Cr# values ranging from 17.67 to 31.66, and have TiO2 contents from 0 to 0.09%, indicating only a low degree of partial melting(10–15%). The olivine-spinel equilibrium and spinel chemistry of the Dangqiong peridotites suggest that they originated deeper mantle(>20 kbar). The gabbro dykes show N-MORB-type patterns of REE and trace elements. The presence of amphibole in the Dangqiong gabbro suggests the late-stage alteration of subduction-derived fluids. All the lherzolites and harzburgites in Dangqiong have similar distribution patterns of REE and trace elements, the mineral chemistry in the harzburgites and lherzolites indicates compositions similar to those of abyssal and forearc peridotites, suggesting that the ophiolite in Dangqiong formed in a MOR environment and then was modified by late-stage melts and fluids in a suprasubduction zone(SSZ) setting. This formation process is consistent with that of the Luobusa ophiolite in the eastern Yarlung-Zangbo Suture Zone and Purang ophiolite in the western Yarlung-Zangbo Suture Zone.
基金financial support from the Natural Science Foundation of Urmia University。
文摘The mafic intrusive Moskin and Parvaneh regions in the northwest and south of Urmia were investigated for their geochemistry and petro-physical properties such as size,shape,and spatial distribution of their crystals.This type of study can be a powerful tool for analyzing the history of magma crystallization,along with geochemistry examinations.The gabbroic calc-alkaline rocks,with the Early Cenozoic is the relative age,consist of coarse plagioclase and amphibole with rare clinopyroxene and olivine in the Sanandaj-Sirjan zone at sedimentary-structural subdivisions of Iran.The depletion of HFSEs and enrichment of LILEs as arc-related rocks,as well as lower crust contamination(Nb/Th and K/Ce)symptoms,show that these gabbros formed at the continental arc setting from an enriched source.Considering the geographical position and relative age of the gabbros,they possibly outcropped during the Neo-Tethys subduction processes after the enrichment by considerably lower crust effects.The amphibole and plagioclase crystals with considerable abundance show linear crystal size distribution(CSD)patterns with a slope of(-1/Gt)-3 to-4 for amphiboles and plagioclase except for Parvaneh plagioclase which shows-16.The annealing(Ostwald ripening)caused the convex CSD on the small grains part at Parvaneh gabbro.Therefore,the ratio of nucleation to growth rates(J/G)for these minerals is 5-6 times,only for Parvaneh plagioclase is ten times which numerous nuclei were made by a significant degree of overstepping in the rocks.There is no magma mixing evidence because of the linear(one population of crystals)CSD patterns.The shape of amphibole crystals in gabbros is bladed(with S/I=0.4-0.5 and I/L=0.1-0.3)and bladed to tabular for plagioclase(S/I=0.4 and I/L=0.6-0.7).The spatial distribution patterns(SDPs)are calculated from a big R-value based on the nearest neighbor’s theory for amphibole and plagioclase crystals,confirming clustered SDP for all of them.Also,the dihedral angle measurements(between amphibole and plagioclase grains)display that solid-state equilibrium is prevailing in the rocks.In this study,we tried to establish a relationship between geochemistry and petro-physical results and investigate the evolution of igneous rocks more accurately and all-inclusive.We should not neglect the importance of quantifying the texture of rocks.
基金granted by the National Key Basic Research Program from the Chinese Ministry of Science and Technology(Grant No.2015CB856101)Foundation of Research Center for Tibetan Plateau Geology(21201010000150014-29)the China Geological Survey(No.1212011221066)
文摘Discontinuous ophiolite suites along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet representing remnants of the Neo-Tethyan oceanic lithosphere are considered to be formed between Late Jurassic and Early Cretaceous.Older ophiolite or ophiolitic mélange(e.g.Triassic)to reveal the initial evolution the Neo-Tethyan ocean within the YZSZ have rarely been documented so far.The western YZSZ extending from the Saga to Ladakh area are composed of the northern ophiolitic subbelt,the Zhongba terrane and the southern ophiolitic sub-belt.In this study,we document structural,petrological and geochronological data of mafic intrusions from the Mayoumu massif in the southern ophiolitic sub-belt of the western YZSZ.Two lithos-tectonic sub-units,the southern ophiolitic complex and the northern ophiolitic mélange,are recognized in the Mayoumu massif in terms of structural pattern and petrology.LA-ICP-MS zircon U-Pb dating of gabbro from the ophiolitic mélange yields an age of 243±1 Ma with zirconεHf(t)values of+7.9 to+13.2.Two samples of diabase samples from the ophiolite yield ages of 131±1 Ma and 124±1 Ma with zirconεHf(t)values ranging from+10.8 to+15.0 and+12.3 to+15.4,respectively.Geochemically,rare earth elements(REEs)patterns show that these mafic intrusions are similar to those of the normal mid-ocean-ridge basalt(N-MORB).Enrichment of fluidmobile elements(e.g.Rb and Ba)and depletion of Nb,and Ta suggest that these intrusions were possibly originated from melting of a depleted mantle source influenced by subducted slab.Our data strongly confirms that the Neo-Tethyan ocean between the Lhasa terrane and Gondwana had been existed since the Middle Triassic at least.The ages of the two diabases from the ophiolitic complex reveal that mafic magma activities within the Neo-Tethyan ocean could be as a continuing process during 120–130 Ma.Emplacement and preservation of older ophiolitic recorder during evolution of the Neo-Tethyan Ocean may be closely related to the occurrence of the Zhongba micro-terrane within the western YZSZ.
基金Projects(40672040,40472042 and 40603008) supported by the National Natural Science Foundation of ChinaProject(2001CB409802) supported by the Major State Basic Research Development Program of China+1 种基金 Project(2005038237) supported by the Postdoctoral Science Foundation of China Project supported by the Opening Foundation of State Key Laboratory of Continental Dynamics,Northwest University
文摘Based on the detailed petrographic study,the characteristics of source region of the Weiya gabbro and tectonic implications were studied.The results show that the gabbroic rock consists mainly of gabbro,with less amount of ultra-mafic rocks.The ultra-mafic rocks show cumulate texture and are gradually transitional contact with gabbro,indicating that they are cumulate products of parental magma.The ultra-mafic rocks consist mainly of spinel periodite and spinel clinopyroxenite.The former is mainly composed of olivine(65%-70%),spinel(10%-15%),hornblende(5%-10%) and phlogopite(5%-10%);the latter consists mainly of clinopyroxene(70%-80%),spinel(15%-20%) and phlogopite(0-10%),with minor amounts of carbonate(0-2%).No olivine or orthopyroxene is found.The gabbro is composed mainly of clinopyroxene(35%-40%),plagioclase(An 55-65,40%-45%),hornblende(5%-15%),with variable amounts of carbonate(0-5%).Petrographic observations show that the source region of the Weiya gabbroic rock is water-rich due to intensive intra-continental A-type subduction occurring in this region during late Permian to early Triassic.
基金financially supported by the National Natural Science Foundation of China(grant No.41502079)China Geological Survey(grants No.[2012]03-002-055 and [2014]01-028-042)
文摘Objective Lhasa terrane has recorded the geologic history conceming the formation and evolution of Paleo-Tethys and the intra-continental convergence in Qinghai-Tibet Plateau （Yin and Harrison, 2000）. Previous investigations have focused on the initial timing of the India-Asia collision and the Cretaceous-Cenozoic magmatism and sedimentation （Wang Tianyang et al., 2017）, however, there are only few studies concerning the Paleozoic evolution in Lhasa terrane. This paper mainly reports a new discovery of 341 Ma gabbro, which is located in the Xiongcun copper-gold district at the southern margin of Lhasa terrane, Tibet. As an important magmatic record during the Paleozoic evolution process of Lhasa terrane, the Early Carboniferous Xiongcun gabbro will provide new constraints for the tectonic evolution of Paleo-Tethys Ocean in the Late Devonian to Early Carboniferous.
文摘Gabbro is selected as a sample for experimental deformation to investigate and validate the migration of elements during rock deformation. Samples are deformed for 3 h under a strain of about 5 % at T =700 ℃, p =100 MPa, σ =50 MPa. It is shown that there are 4 areas with different colors in the section of the samples parallel to σ 1: the extensional, contractional, and strongly compressional areas and a ductile shear zone, respectively on the basis of the stress states and the direction of material movement. The chemical components such as K, Na, Al and Fe from materials in different areas have changed. The four elements mentioned above in pyroxene grains decrease in content from the extensional area through the ductile shear zone, the contractional area, to the strongly compressional area. The contents of the same elements in feldspar grain vary in a reverse direction.
基金supported by the NIED research project titled‘‘Development of the Earthquake Activity Monitoring and Forecasting,’’the JSPS KAKENHI Grant No.23340131the State Key Laboratory of Earthquake Dynamics,Institute of Geology,CEA(LED2014A06)
文摘This paper reports stick-slip behaviors of Indian gabbro as studied using a new large-scale biaxial friction apparatus, built in the National Research Institute for Earth Science and Disaster Prevention （NIED）, Tsukuba, Japan. The apparatus consists of the existing shaking table as the shear-loading device up to 3,600 kN, the main frame for holding two large rectangular prismatic specimens with a sliding area of 0.75 m^2 and for applying normal stresses an up to 1.33 MPa, and a reaction force unit holding the stationary specimen to the ground. The shaking table can produce loading rates v up to 1.0 m/s, accelerations up to 9.4 m/s^2, and displacements d up to 0.44 m, using four servocontrolled actuators. We report results from eight preliminary experiments conducted with room humidity on the same gabbro specimens at v = 0.1-100 mm/s and an = 0.66-1.33 MPa, and with d of about 0.39 m. The peak and steady-state friction coefficients were about 0.8 and 0.6, respectively, consistent with the Byerlee friction. The axial force drop or shear stress drop during an abrupt slip is linearly proportional to the amount of displacement, and the slope of this rela- tionship determines the stiffness of the apparatus as 1.15 × 10^8 N/m or 153 MPa/m for the specimens we used. This low stiffness makes fault motion very unstable and the overshooting of shear stress to a negative value was recognized in some violent stick-slip events. An abrupt slip occurred in a constant rise time of 16-18 ms despite wide variation of the stress drop, and an average velocity during an abrupt slip is linearly proportional to the stress drop. The use of a large-scale shaking table has a great potential in increasing the slip rate and total displacement in biaxial friction experiments with large specimens.
