Granitic gneiss(orthogneiss)and Himalayan leucogranite are widely distributed in the Himalayan orogen,but whether or not the granitic gneiss made a contribution to the Himalayan leucogranite remains unclear.In this st...Granitic gneiss(orthogneiss)and Himalayan leucogranite are widely distributed in the Himalayan orogen,but whether or not the granitic gneiss made a contribution to the Himalayan leucogranite remains unclear.In this study,we present the petrological,geochronological and geochemical results for orthogneisses and leucogranites from the Zhada area,Western Himalayas.Zhada orthogneiss is composed mainly of quartz,plagioclase,K-feldspar,biotite and muscovite,with accessory zircon and apatite.Orthogneiss zircon cathodoluminescence(CL)images show that most grains contain a core with oscillatory zoning,which indicates an igneous origin.Sensitive high-resolution ion microprobe(SHRIMP)U-Pb dating of the zircon cores in the orthogneiss shows a weighted ^(206)Pb/^(238)U age of 515±4 Ma(early Paleozoic),with sponge-like zircon rims of 17.9±0.5 Ma(Miocene).Zhada leucogranite shows^(206)Pb/^(238)U ages ranging from 19.0±0.4 Ma to 12.4±0.2 Ma,the weighted average age being 16.2±0.4 Ma.The leucogranites have a low Ca content(<1 wt%),FeOt content(<1 wt%),Rb content(67.0-402 ppm),Sr content(<56.6 ppm),Ba content(3.35-238 ppm)and Rb/Sr ratio(0.5-14.7),which are similar to the geochemical characteristics of the Himalayan leucogranite derived from muscovite dehydration partial melting of metasediments and representative of most Himalayan leucogranites.The highly variable Na_(2)O+K_(2)O(4.33 wt%-9.13 wt%),Al_(2)O_(3)(8.44 wt%-13.51 wt%),ΣREE(40.2-191.0 ppm),Rb(67.0-402 ppm)and Nb(8.23-26.4 ppm)contents,^(87)Sr/^(86)Sr(t)ratios(0.7445-0.8605)andεNd(t)values(−3.6 to−8.2)indicate that the leucogranite is derived from a heterogenetic source.The nonradiogenic Nd isotope values of the studied Zhada leucogranite and orthogneiss range from−8.2 to−3.6 and from−8.7 to−4.1,respectively.Therefore,the general mixing equation was used to perform the Sr and Nd isotope mixing calculations.The results indicate that the heterogenetic source was the Tethyan Himalayan Sequence(THS)/Higher Himalayan Crystalline(HHC)metasediments and Zhada orthogneiss.The Zhada area experienced crustal anatexis during the Miocene and the heterogenetic source of the orthogneiss and metasediment may have experienced crustal anatexis controlled by muscovite dehydration.The Zhada leucogranite inherited not only the geochemical characteristics of the Himalayan metasediment(muscovite dehydration melting),but also the trace elements and Sr-Nd isotopic characteristics of the Zhada orthogneiss.These results indicate that the Paleozoic Zhada orthogneiss was involved in crustal anatexis at 17.9±0.5 Ma(Miocene)and that the muscovite dehydration of the metasediments in the heterogenetic source produced fluid,which may have caused the orthogneiss solidus lines to decline,triggering a partial melting of the Zhada orthogneiss.It is therefore proposed that Himalayan leucogranite is a crust-derived granite rather than a S-type granite,as previously hypothesized.展开更多
Objective In recent years,hydrous silicate melts by dehydrationdriven in situ partial melting constrained from experiments and natural rocks have been increasingly recognized in UHP rocks,indicating partial melting of...Objective In recent years,hydrous silicate melts by dehydrationdriven in situ partial melting constrained from experiments and natural rocks have been increasingly recognized in UHP rocks,indicating partial melting of UHP slab.Partial melting of UHP metamorphic rocks can dramatically affect the rheology of deeply subducted crust and thus play a crucial role in accelerating the exhumation of UHP slabs.展开更多
The Berere HTHP Complex belt in Maevatanana area of north–central Madagascar formed in the^2.5 Ga orogeny and underwent high temperature(up to 1050℃)and high pressure(up to 11.5 kbar)granulite facies metamorphism.Th...The Berere HTHP Complex belt in Maevatanana area of north–central Madagascar formed in the^2.5 Ga orogeny and underwent high temperature(up to 1050℃)and high pressure(up to 11.5 kbar)granulite facies metamorphism.Then a widespread anatexis took place and numerous widely distributed felsic leucosomes formed.The majority of these leucosomes are parallel to the schistosity of the complex or are present as stockworks,as thin layers,or as lenses at different scales in the host rocks.Here,we report new petrographic data,zircon LA-ICP-MS U-Pb ages,and Lu–Hf isotopic data for felsic leucosomes within this complex.Anatexis,as identified by the petrological study of felsic leucosomes in the field and in thin sections,involved initial ternary feldspar exsolving to produce antiperthite and a quartz+plagioclase±K-feldspar+sericite mineral assemblage around feldspar grain boundaries.Dissolution is apparent along muscovite grain boundaries,and residual sericite is present around the margins of feldspar and quartz,all suggesting that anatexis was driven by reactions involving muscovite.Zircon U–Pb dating indicates that the felsic leucosomes within the complex formed at 2467–2369 Ma.The majority of samples have positiveεHf(t)values,although a few have negative values,suggesting their formation from magmas predominantly sourced from the depleted mantle,possibly with the involvement of minor amounts of crustal materials.Two-stage Hf model ages andεHf(t)values for these samples are consistent with those for gneisses of the basement,indicating that the felsic leucosomes were formed by the anatexis of gneisses and both of their protolith formed during the formation of continental crust in Meso-Neoarchean(ca.3.1–2.7 Ga).As such,the crystallization age of the felsic leucosome(~2.4 Ga)represents the timing of regional anatexis and a change to post-orogenic tectonism.And this anatexis is also corresponds to the thermal event in Dharwar craton in India which has a pronounced similar Precambrian geology with Madagascar,providing an important constraints on the correlation of the two continental fragments.展开更多
‘Single shot'laser-ablation split-stream(SS-LASS)technique analyzing unpolished zircon grains makes their thin rims tenable for determination,which thus offers great potential in deciphering the timing of multipl...‘Single shot'laser-ablation split-stream(SS-LASS)technique analyzing unpolished zircon grains makes their thin rims tenable for determination,which thus offers great potential in deciphering the timing of multiple and short-lived episodes of anatexis and metamorphism in deeplysubducted continental crusts.Dominated granitic gneisses in the deeply subducted continental crust undergoing considerable fluid-melt activities persist multistage growth of zircon.Therefore,a comparative study of SS-LASS and laser ablation inductively coupled plasma mass spectrometer(LA-ICP-MS)zircon dating was conducted on the granitic gneisses from the Sulu belt in this study.Zircons mostly show a core-mantle-rim structure with CL-bright rims thinner than 5μm.For LA-ICP-MS dating,relict magmatic zircon cores yield protolith ages of ca.756-747 Ma;whereas the dark mantles record synexhumation anatexis at ca.214 Ma.By contrast,according to the U-Pb dates,trace element features,zircon crystallization temperatures and geological context,SS-LASS zircon petrochronology deciphers three episodes of anatectic events,as follows:(i)the first episode of anatexis at ca.218-217 Ma dominated by phengite-breakdown melting,likely facilitating the exhumation of the UHP slice from mantle depth;(ii)the second episode of anatexis at ca.193–191 Ma indicating part of northern Dabie-Sulu belt was still“hot”because of buried in the thickened orogenic crust at that time;(iii)the third episode of anatexis(ca.162–161 Ma)consistent with the intrusion ages(ca.161–141 Ma)of the Jurassic to Cretaceous granitoids in this orogen,suggesting the initial collapse of the orogenic root of the Sulu belt occurred at Late Jurassic due to the Izanagi plate initially subducting beneath the margin of Eastern Asia.This study sheds new light upon the utilization of SS-LASS petrochronology deciphering multiple anatectic events in the deeply-subducted continental crust and supports us in better understanding the tectonic evolution of Dabie-Sulu Orogen.展开更多
Zirconium is one of high field strength elements but its isotope behavior during geochemical processes is still uncertain because of the limited database.While Zr isotopes in magmatic rocks are often used to trace the...Zirconium is one of high field strength elements but its isotope behavior during geochemical processes is still uncertain because of the limited database.While Zr isotopes in magmatic rocks are often used to trace the evolution of magmas through fractional crystallization,it is intriguing how highly heterogeneous Zr isotopes were produced by the growth of zircon during crustal anatexis.We address this issue by in-situ zircon Zr isotope analyses of migmatites from two high-temperature metamorphic terranes in the South Lhasa zone and the North Dabie zone,respectively,in China.The results show highly variable δ^(94)Zr values from-0.30‰ to +0.81‰ and from-0.58‰ to +0.49‰,respectively.In addition to the relict zircon of magmatic origin,two types of newly-grown zircons were identified in terms of their occurrences,trace elements and δ^(94)Zr values.The peritectic zircon,mainly occurring in the in-situ leucosomes,exhibits the highest Nb-Ta-Hf-U contents and variably higher δ^(94)Zr values than those of the relict zircon.The anatectic zircon,mainly occurring in the leucocratic veins,shows higher Nb-Ta-Hf-U contents than and similar δ^(94)Zr values to those of the relict zircon.Model calculations demonstrate that the variable Zr isotope compositions of newly-grown zircons would result from decoupled release of Zr from zircon and non-zircon phases.The Zr supply of the peritectic zircon is mainly derived from the decomposition of Zr-bearing minerals in the in-situ anatectic melt(the non-zircon effect),whereas the Zr supply of the anatectic zircon is mainly from the dissolution of pre-existing zircons in the evolved melt(the zircon effect).The significant Zr isotope variations in the migmatites well illustrate the generation,migration and accumulation of the anatectic melts during the partial melting.Therefore,Zr isotopes can be used as a powerful means for distinguishing between the peritectic and anatectic zircons during crustal anatexis.展开更多
Migmatization in Higher Himalayan Crystallines (HHC) results from anatexis. The widely distributed migmatites in HHC are an important clue to investigate the relationship be- tween anatexis and the origins of Higher H...Migmatization in Higher Himalayan Crystallines (HHC) results from anatexis. The widely distributed migmatites in HHC are an important clue to investigate the relationship be- tween anatexis and the origins of Higher Himalayan leucogranites (HHL), and to understand the effect of anatexis on crustal evolution during the post-collision period. We studied in detail the chemical features of three basic constituent parts of the migmatites, i.e. leucosome, mesosome and melanosome, and determined the K-Ar ages of leucosomes. Our studies indicate that type-I leucosome is the product of crystallization of melt generated by partial melting of mesosome at source region, but type-II leucosome and HHL probably underwent crystallization differentiation of plagioclase during melt aggregation and migration. The age of 22.67 Ma of Type-I leucosome, which is a little older than the beginning of MCT movement, indicates that anatexis may have played an important role in the formation of MCT. That the ages of type-II leucosome (ranging from 14.82 to 18.37 Ma) are consistent with that of HHL provides new chronological evidence for the relationship between migmatization and HHL. We obtained a very young age of 6.23 Ma of Type-II leucosome that provides new time constraint on magma activity in the central segment of Higher Himalayas.展开更多
The Higher Himalayan Crystallines(HHC), in western Garhwal, Uttarakhand are located in a regionalscale intracontinental ductile shear zone(15-20 km wide) bounded by the Main Central Thrust at the base, and the South T...The Higher Himalayan Crystallines(HHC), in western Garhwal, Uttarakhand are located in a regionalscale intracontinental ductile shear zone(15-20 km wide) bounded by the Main Central Thrust at the base, and the South Tibetan Detachment System at the top. The migmatite zone in the centre has the highest grade of metamorphism in the NW Himalayas and show evidence of flowage. Zircons extracted from samples of metasediment, migmatite, biotite granite and in situ partial melt(tourmaline-bearing leucogranite) along the Bhagirathi Valley, preserve U-Pb isotopic evidence of magmatic history, magma source and effects of the Himalayan orogeny in the region. Three distinct periods of zircon growth in the leucogranite record the episodic influx of magma between 46 Ma and 20 Ma indicating a time span of more than 25 Ma between the onset of fluid-fluxed partial melting in the mid-crustal intracontinental shear zone and the emplacement of the magma into the upper crust in a post-collisional extensional setting. Metamorphic zircon growth was initiated about 46 Ma, when the partial melts were generated as the migmatite zone was exhumed.展开更多
The albite rim is present in most felsic gneisses of the Fuping Complex. The presence of the rim indicates the coexistence of plagioclase and K-feldspar in the rock. The rim is formed immediately after the myrmekite, ...The albite rim is present in most felsic gneisses of the Fuping Complex. The presence of the rim indicates the coexistence of plagioclase and K-feldspar in the rock. The rim is formed immediately after the myrmekite, and both textures were derived from the alteration of K-feldspar. The difference is that that there is no quartz present in the rim, and the rim is nearly albite and the anorthite content of the rim plagioclase is substantially lower than that of the myrmekite plagioclase. Formed at 400- 500~C the albite rim was derived from the K-feldspar composition adjustment in the late or post- magmatism stage. As the temperature decreased, the equilibrium between K-feldspar and plagioclase could be maintained, and reactions between the minerals occurred. The leucocratic veins in the complex show distinguished magma or migmatitic characteristics. The rim might form in the late magma or deuteric stage. The formation of the rim implies obvious granitic magmaor melt-injection activity. Typical metamorphic rocks cannot produce the rims. Anatexis after medium-high grade metamorphism might be subordinate. If present, the anatexis is water-present, but the rim texture cannot be taken as the symbol of anatexis.展开更多
Fluid-absent and fluid-fluxed melting of muscovite in metasedimentary sources are two types of crustal anatexis to produce the Himalaya Cenozoic leucogranites.Apatite grains separated from melts derived from the two t...Fluid-absent and fluid-fluxed melting of muscovite in metasedimentary sources are two types of crustal anatexis to produce the Himalaya Cenozoic leucogranites.Apatite grains separated from melts derived from the two types of parting melting have different geochemical compositions.The leucogranites derived from fluid-fluxed melting have relict apatite grains and magmatic crystallized apatite grains,by contrast,there are only crystallized apatite grains in the leucogranites derived from fluid-absent melting.Moreover,apatite grains crystallized from fluid-fluxed melting of muscovite contain higher Sr,but lower Th and LREE than those from fluid-absent melting of muscovite,which could be controlled by the distribution of partitioning coefficient(D_(Ap/Melt))between apatite and leucogranite.D_(Ap/Melt) in granites derived from fluidabsent melting is higher than those from fluid-fluxed melting.So,not only SiO_(2) and A/CNK,but also types of crustal anatexis are sensitive to trace element partition coefficients for apatite.In addition,due to being not susceptible to alteration,apatite has a high potential to yield information about petrogenetic processes that are invisible at the whole-rock scale and thus is a useful tool as a petrogenetic indicator.展开更多
The Neoarchean charnockites of North margain of North China Craton(NCC) has become a hot topic into understanding the Early Precambrian basement. Although there is a broad consensus that charnockite is usually related...The Neoarchean charnockites of North margain of North China Craton(NCC) has become a hot topic into understanding the Early Precambrian basement. Although there is a broad consensus that charnockite is usually related to granulite facies metamorphism, whether its petrogenesis and tectonics characteristics remains controversial. Inclusions within hypersthene and garnet in charnockite are used to identify the peak granulite facies mineral assemblage, with the formation of Magnesian-charnockite attributed to anatexis of the protolith associated with this granulite facies metamorphism. The distribution of major and trace elements in charnockite is very uneven, significant depleted in LILEs(eg. Cs, U, Th) and HFSEs(eg. Nb, Ta, P and Ti), riched in Sr. Raising to the coexistence of Eu-enrichment and Eu-depletion type of REE patterns that influenced by the content of plagioclase and the remnants minerals of zircon and apatite. Comparative the petrography, geochemistry and geochronology data of Magnesian-charnockite indicate that the ratios of mafic pellites and basalts involved in anatectic melting are different by the upwelling of mantle magma, also resulting in the Eu anormals characteristics. The formation of the Magnesian-charnockite is closely connected with the subduction of the NCC oceanic crust(About ~2.5 Ga). However, Ferroan-charnockite may be the formed by the crystallization differentiation of the upwelling of mantle-derived shoshonitic magma(About ~2.45 Ga), with the lower crust material addition.展开更多
Tonalite-trondhjemite-granodiorite(TTG) suites constitute a large proportion of the Archean geological record;however,the geodynamic processes that generated them,and Archean continental crust in general,remain a subj...Tonalite-trondhjemite-granodiorite(TTG) suites constitute a large proportion of the Archean geological record;however,the geodynamic processes that generated them,and Archean continental crust in general,remain a subject of debate.The concentrations and ratios of Sr,Y,La,Yb,Nb,and Ta in TTGs are commonly used to determine the depth of melting of their metabasic sources.The trace element composition of melt produced by metabasic source rocks during anatexis is strongly affected by the presence and abundance of pressure-sensitive minerals,such as plagioclase(Sr-bearing),garnet(Y-and HREE-bearing),and rutile(Nb-and Ta-bearing).Elevated Sr/Y and La/Yb ratios and low concentrations of Nb and Ta in TTGs are generally considered to indicate melting at high pressures(≥2.0 GPa).The depth of melting is a key factor in determining the origin of TTGs as this provides critical information on the tectonic setting of their generation.We use phase equilibrium and trace element modelling to explore the effects of three potential influences on TTG trace element compositions:fractionation of trace elements into peritectic garnet cores,progressive melt loss from the source,and source bulk composition.We model three different compositions of Archean basalts along thermal gradients of 500℃/GPa,750℃/GPa,and 1000℃/GPa.The models produce maj or and trace element melt compositions that are generally consistent with measured compositions of TTGs.Although Sr/Y,La/Yb,Nb,and Ta exhibit pressure-dependent behaviour,other factors also affect these values.Garnet fractionation causes Sr/Y and La/Yb to reach much greater values and in this scenario,the values also increase with increasing temperature.Source bulk composition has an effect in all scenarios and most strongly influences La/Yb,Nb,and Ta.Overall,these results show that Sr/Y,La/Yb,Nb,and Ta can reach values generally considered to be indicative of high pressure melting at a range of P-T conditions including P <2.0 GPa.Consequently,trace element compositions of TTGs alone may provide a misleading impression of the depth of melting of metabasites and the geodynamic environment of Archean crustal growth and reworking.展开更多
Partially migmatized rocks, banded migmatites, augen-banded migmatites and gneissic migmatites are developed successively from Xindong to Yunlu, Gaozhou, Guangdong Province in the Yunkai Caledonian orogenic belt at th...Partially migmatized rocks, banded migmatites, augen-banded migmatites and gneissic migmatites are developed successively from Xindong to Yunlu, Gaozhou, Guangdong Province in the Yunkai Caledonian orogenic belt at the border between Guangdong and Guangxi. Mass-balance calculations, statistical analysis of the textural relations and mineralogical and geochemical studies of the migmatites and the study of the metamorphlc setting of the Yunlu area indicate that the migmatites in the study area were primarily formed by anatexis without remarkable introduction of foreign components such as K, Na and Si and removal of Ca, Fe, Mg, etc.展开更多
Higher Himalayan Crystalline（HHC） complex of the Sikkim Himalaya predominantly consists of high-grade pelitic migmatites.In this study,reaction textures,mineral/bulk rare earth elements （REE）,trace element partiti...Higher Himalayan Crystalline（HHC） complex of the Sikkim Himalaya predominantly consists of high-grade pelitic migmatites.In this study,reaction textures,mineral/bulk rare earth elements （REE）,trace element partition coefficients and trace element zoning profiles in garnet are used to demonstrate a complex petrogenetic process during crustal anatexis.With the help of equilibrium REE and trace element partitioning model,it is shown that strong enrichment of Effective Bulk Composition（EBC） is responsible for the zoning in garnet in these rocks.The data strongly support disequilibrium element partitioning and suggest that the anatectic melts associated with mafic selvedges are likely produced by disequilibrium melting because of fast melt segregation process.展开更多
A popular hypothesis of in situ transformation of amphibolite facies gneisses to patchy charnockites by CO2 influx from mantle was proposed primarily from the Kabbaldurga quarries in South Karnataka and subsequently r...A popular hypothesis of in situ transformation of amphibolite facies gneisses to patchy charnockites by CO2 influx from mantle was proposed primarily from the Kabbaldurga quarries in South Karnataka and subsequently reported from several south Indian localities. However, presence of abundant mafic granulite enclaves in Kabbaldurga and its neighborhood and its implications in relation to patchy charnockite genesis were not discussed. In these quarries patchy charnockites occur in various modes and associations. Some of these patches do occupy structural weak zones, such as shear bands and fold noses in the migmatitic gneisses, but many of the patchy charnockite bodies occur as branching veins transecting the gneissic foliation and hence do not account for fluid pathways. Most importantly, charnockitic leucosomes at margins of mafic granulite enclaves and charnockitic veins within some mafic granulite enclaves indicate a close genetic link between them via dehydration partial melting. This is further corroborated by trace element distribution between them. Dehydration partial melting in mafic rocks in a migmatite terrain such as Kabbaldurga, can explain all the different modes of the patchy charnockites as various stages of segregation and mobility relative to deformation. Abundant mafic granulite enclaves and field features suggesting a relatively late origin of the patchy charnockites, are compelling evidence against the notion of a transition zone. Mantle derivation age of the mafic source rocks (protoliths of mafic granulites) at Kabbaldurga at 3.08 ± 0.08 Ga with small positive ? values is virtually identical to the source of the massive charnockite of Karnataka craton at 3.08 Ga. This could imply a widespread mafic magmatism in South India around 3.0 Ga.展开更多
The Dabie-Sulu orogenic belt was formed by the Triassic continental collision between the South China Block and the North China Block. There is a large area of Mesozoic magmatic rocks along this orogenic belt, with em...The Dabie-Sulu orogenic belt was formed by the Triassic continental collision between the South China Block and the North China Block. There is a large area of Mesozoic magmatic rocks along this orogenic belt, with emplacement ages mainly at Late Triassic, Late Jurassic and Early Cretaceous. The Late Triassic alkaline rocks and the Late Jurassic granitoids only crop out in the eastern part of the Sulu orogen, whereas the Early Cretaceous magmatic rocks occur as massive granitoids, sporadic intermedi- ate-mafic intrusive and volcanic rocks throughout the Dabie-Sulu orogenic belt. Despite the different ages for their emplacement, the Mesozoic magmatic rocks are all characterized not only by enrichment of LREE and LILE but depletion of HFSE, but also by high initial Sr isotope ratios, low εNd(t) values and low radiogeneic Pb isotope compositions. Some zircons from the Jurassic and Cretaceous granitoids contain inherited magmatic cores with Neoprotozoic and Triassic U-Pb ages. Most of the Cretaceous mafic rocks have zircon δ18O values and whole-rock δ13C values lower than those for the normal mantle. A systematic comparison with adjacent UHP metaigneous rocks shows that the Mesozoic granitoids and mafic rocks have elemental and isotopic features similar to the UHP metagranite and metabasite, respectively. This indicates that these magmatic and metamorphic rocks share the diagnostic features of lithospheric source that has tectonic affinity to the northern edge of the South China Block. Their precursors underwent the UHP metamorphism and the post-collisional anatexis, respectively at different times and depths. Therefore, the Mesozoic magmatic rocks were derived from anatexis of the subducted continental lithosphere itself beneath the collision-thickened orogen; the geodynamic mechanism of the post-collisional magmatisms is tectonic collapse of orogenic roots in response to lithospheric extension.展开更多
A set of granitic and amphibole mylonite are exposed in the Altyn Tagh fault zone. The preliminary study shows that these rocks are the product of the syntectonic anatexis in the process of the left-lat- eral strike-s...A set of granitic and amphibole mylonite are exposed in the Altyn Tagh fault zone. The preliminary study shows that these rocks are the product of the syntectonic anatexis in the process of the left-lat- eral strike-slip shear, and are the result of the ductile transpression. There are two types of zircon sorted from the mylonite formed with syn-shear anatexis. Among them, one is the anatectic long co-lumnar zircon and another is the residual metamorphic sub-rounded columnar zircon. Two groups of age for single zircon measured by ion microprobe (SHRIMP) are obtained: one is 461-547 Ma for the sub-rounded columnar residual metamorphic zircon, and the other is 239-244 Ma for the long-columnar anatectic zircon. This type of zircon is direc-tionally spread in rock, and the long axis direction of its crystal is identical to that of stretching lineation, represent-ing the direction of tectonic stress in the process of the strike-slip. 40Ar-39Ar age of the directional growth horn-blende in the same mylonite展开更多
It is generally believed that trondhjemitic rock, an important component of TTG rocks, is the anatectic product of mafic rocks. However, in many TTG gneiss terranes, for instance, the granulite facies terrane in Easte...It is generally believed that trondhjemitic rock, an important component of TTG rocks, is the anatectic product of mafic rocks. However, in many TTG gneiss terranes, for instance, the granulite facies terrane in Eastern Hebei, trondhjemites occur as small dikes, intrusions or leucosomes in tonalitic gneisses, suggesting their origin of in-situ partial melting. Based on the petrological analysis of a tonalitic gneiss sample from Eastern Hebei, in combination with zircon U-Pb dating, we investigated the petrogenesis of trondhjemite through simulating anatectic reactions and the major and trace element characteristics of the product melt at different pressures(0.7, 1.0 and 2.0 GPa). The results indicate that hornblende dehydration melting in a tonalitic gneiss at 0.9–1.1 GPa and 800–850°C, corresponding to the high-T granulite facies, with melting degrees of 5–10wt.% and a residual assemblage containing 5–10wt.% garnet, can produce felsic melts with a great similarity, for instance of high La/Yb ratios and low Yb contents to the trondhjemitic rocks from Eastern Hebei. However, the modelled melts exhibit relatively higher K2 O, and lower CaO and Mg~# than those in the trondhjemitic dikes and leucosomes from Eastern Hebei, suggesting that the leucosomes may not only contain some residual minerals but also be subjected to the effect of crystal fractionation. The zircon U-Pb dating for the tonalitic and trondhjemitic rocks in the Eastern Hebei yields a protolith age of 2518±12 Ma and a metamorphic age of 2505±19 Ma for the tonalitic gneiss. The latter age is consistent with a crystallization age of 2506±6 Ma for the trondhjemitic rock, confirming a close petrogenetic relation between them.展开更多
Dehydration melting of metasupracrustal rocks at mid-to deep-crustal levels can generate water undersaturated granitic melt.In this study,we evaluate the potential of~1.89–1.88 Ga metasupracrustal rocks of the Precam...Dehydration melting of metasupracrustal rocks at mid-to deep-crustal levels can generate water undersaturated granitic melt.In this study,we evaluate the potential of~1.89–1.88 Ga metasupracrustal rocks of the Precambrian of southern Finland as source rocks for the 1.86–1.79 Ga late-orogenic leucogranites in the region,using the Rhyolite-MELTS approach.Melt close in composition to leucogranite is produced over a range of realistic pressures(5 to 8 kbar)and temperatures(800 to 850℃),at 20%–30%of partial melting,allowing separation of melt from unmelted residue.The solid residue is a dry,enderbitic to charnoenderbitic ganulite depleted in incompatible components,and will only yield further melt above 1000–1050℃,when rapidly increasing fractions of increasingly calcic(granodioritic to tonalitic)melts are formed.The solid residue after melt extraction is incapable of producing syenogranitic magmas similar to the Mid-Proterozoic,A-type rapakivi granites on further heating.The granitic fraction of the syenogranitic rapakivi complexes must thus have been formed by a different chain of processes,involving mantle-derived mafic melts and melts from crustal rock types not conditioned by the preceding late-orogenic Svecofennian anatexis.展开更多
基金This work was supported by the National Natural Science Foundation of China(Grant Nos.42072114,41503006)the Deep Resources Exploration and Mining,the National Key R&D Program of China(Grant No.2018YFC0604106)+1 种基金the Chinese Geological Survey(Grant No.DD20190167)the China Scholarship Council(Grant No.201808110080).
文摘Granitic gneiss(orthogneiss)and Himalayan leucogranite are widely distributed in the Himalayan orogen,but whether or not the granitic gneiss made a contribution to the Himalayan leucogranite remains unclear.In this study,we present the petrological,geochronological and geochemical results for orthogneisses and leucogranites from the Zhada area,Western Himalayas.Zhada orthogneiss is composed mainly of quartz,plagioclase,K-feldspar,biotite and muscovite,with accessory zircon and apatite.Orthogneiss zircon cathodoluminescence(CL)images show that most grains contain a core with oscillatory zoning,which indicates an igneous origin.Sensitive high-resolution ion microprobe(SHRIMP)U-Pb dating of the zircon cores in the orthogneiss shows a weighted ^(206)Pb/^(238)U age of 515±4 Ma(early Paleozoic),with sponge-like zircon rims of 17.9±0.5 Ma(Miocene).Zhada leucogranite shows^(206)Pb/^(238)U ages ranging from 19.0±0.4 Ma to 12.4±0.2 Ma,the weighted average age being 16.2±0.4 Ma.The leucogranites have a low Ca content(<1 wt%),FeOt content(<1 wt%),Rb content(67.0-402 ppm),Sr content(<56.6 ppm),Ba content(3.35-238 ppm)and Rb/Sr ratio(0.5-14.7),which are similar to the geochemical characteristics of the Himalayan leucogranite derived from muscovite dehydration partial melting of metasediments and representative of most Himalayan leucogranites.The highly variable Na_(2)O+K_(2)O(4.33 wt%-9.13 wt%),Al_(2)O_(3)(8.44 wt%-13.51 wt%),ΣREE(40.2-191.0 ppm),Rb(67.0-402 ppm)and Nb(8.23-26.4 ppm)contents,^(87)Sr/^(86)Sr(t)ratios(0.7445-0.8605)andεNd(t)values(−3.6 to−8.2)indicate that the leucogranite is derived from a heterogenetic source.The nonradiogenic Nd isotope values of the studied Zhada leucogranite and orthogneiss range from−8.2 to−3.6 and from−8.7 to−4.1,respectively.Therefore,the general mixing equation was used to perform the Sr and Nd isotope mixing calculations.The results indicate that the heterogenetic source was the Tethyan Himalayan Sequence(THS)/Higher Himalayan Crystalline(HHC)metasediments and Zhada orthogneiss.The Zhada area experienced crustal anatexis during the Miocene and the heterogenetic source of the orthogneiss and metasediment may have experienced crustal anatexis controlled by muscovite dehydration.The Zhada leucogranite inherited not only the geochemical characteristics of the Himalayan metasediment(muscovite dehydration melting),but also the trace elements and Sr-Nd isotopic characteristics of the Zhada orthogneiss.These results indicate that the Paleozoic Zhada orthogneiss was involved in crustal anatexis at 17.9±0.5 Ma(Miocene)and that the muscovite dehydration of the metasediments in the heterogenetic source produced fluid,which may have caused the orthogneiss solidus lines to decline,triggering a partial melting of the Zhada orthogneiss.It is therefore proposed that Himalayan leucogranite is a crust-derived granite rather than a S-type granite,as previously hypothesized.
基金financially supported by the National Nature Science Foundation of China (grant No.41572053)
文摘Objective In recent years,hydrous silicate melts by dehydrationdriven in situ partial melting constrained from experiments and natural rocks have been increasingly recognized in UHP rocks,indicating partial melting of UHP slab.Partial melting of UHP metamorphic rocks can dramatically affect the rheology of deeply subducted crust and thus play a crucial role in accelerating the exhumation of UHP slabs.
基金funded by the National Key R&D Program of China(grant numbers 2019YFC0605202 and 2019YFC0605203)Geological Survey Project grants from the China Geological Survey(grant numbers 12120113102100,DD20160056)+3 种基金Research Program of Department of Land and Resources of Hunan Province(grant number 2018-02)the Chinese National Non-Profit Institute Research Grant of CAGS-IMR(grant number IMRKK1927)the National Natural Science Foundation of China(41872096)the Chinese National Non-profit Institute Research Grant of CAGS(JYYWF201814)。
文摘The Berere HTHP Complex belt in Maevatanana area of north–central Madagascar formed in the^2.5 Ga orogeny and underwent high temperature(up to 1050℃)and high pressure(up to 11.5 kbar)granulite facies metamorphism.Then a widespread anatexis took place and numerous widely distributed felsic leucosomes formed.The majority of these leucosomes are parallel to the schistosity of the complex or are present as stockworks,as thin layers,or as lenses at different scales in the host rocks.Here,we report new petrographic data,zircon LA-ICP-MS U-Pb ages,and Lu–Hf isotopic data for felsic leucosomes within this complex.Anatexis,as identified by the petrological study of felsic leucosomes in the field and in thin sections,involved initial ternary feldspar exsolving to produce antiperthite and a quartz+plagioclase±K-feldspar+sericite mineral assemblage around feldspar grain boundaries.Dissolution is apparent along muscovite grain boundaries,and residual sericite is present around the margins of feldspar and quartz,all suggesting that anatexis was driven by reactions involving muscovite.Zircon U–Pb dating indicates that the felsic leucosomes within the complex formed at 2467–2369 Ma.The majority of samples have positiveεHf(t)values,although a few have negative values,suggesting their formation from magmas predominantly sourced from the depleted mantle,possibly with the involvement of minor amounts of crustal materials.Two-stage Hf model ages andεHf(t)values for these samples are consistent with those for gneisses of the basement,indicating that the felsic leucosomes were formed by the anatexis of gneisses and both of their protolith formed during the formation of continental crust in Meso-Neoarchean(ca.3.1–2.7 Ga).As such,the crystallization age of the felsic leucosome(~2.4 Ga)represents the timing of regional anatexis and a change to post-orogenic tectonism.And this anatexis is also corresponds to the thermal event in Dharwar craton in India which has a pronounced similar Precambrian geology with Madagascar,providing an important constraints on the correlation of the two continental fragments.
基金supported by National Natural Science Foundation of China(Nos.42072228,42102060,41902036,41572182)China Postdoctoral Science Foundation(No.2021M692983)+4 种基金the Chinese Ministry of Education(No.BP0719022)the Most Special Fund(Nos.MSFGPMR02-3,MSFGPMR30)Open funds from the State Key Laboratory of Geological Processes and Mineral ResourcesChina University of Geosciences(Nos.GPMR201703,GPMR201704 and GPMR201903)the Fundamental Research Funds for National University(No.CUG-G1323511572)。
文摘‘Single shot'laser-ablation split-stream(SS-LASS)technique analyzing unpolished zircon grains makes their thin rims tenable for determination,which thus offers great potential in deciphering the timing of multiple and short-lived episodes of anatexis and metamorphism in deeplysubducted continental crusts.Dominated granitic gneisses in the deeply subducted continental crust undergoing considerable fluid-melt activities persist multistage growth of zircon.Therefore,a comparative study of SS-LASS and laser ablation inductively coupled plasma mass spectrometer(LA-ICP-MS)zircon dating was conducted on the granitic gneisses from the Sulu belt in this study.Zircons mostly show a core-mantle-rim structure with CL-bright rims thinner than 5μm.For LA-ICP-MS dating,relict magmatic zircon cores yield protolith ages of ca.756-747 Ma;whereas the dark mantles record synexhumation anatexis at ca.214 Ma.By contrast,according to the U-Pb dates,trace element features,zircon crystallization temperatures and geological context,SS-LASS zircon petrochronology deciphers three episodes of anatectic events,as follows:(i)the first episode of anatexis at ca.218-217 Ma dominated by phengite-breakdown melting,likely facilitating the exhumation of the UHP slice from mantle depth;(ii)the second episode of anatexis at ca.193–191 Ma indicating part of northern Dabie-Sulu belt was still“hot”because of buried in the thickened orogenic crust at that time;(iii)the third episode of anatexis(ca.162–161 Ma)consistent with the intrusion ages(ca.161–141 Ma)of the Jurassic to Cretaceous granitoids in this orogen,suggesting the initial collapse of the orogenic root of the Sulu belt occurred at Late Jurassic due to the Izanagi plate initially subducting beneath the margin of Eastern Asia.This study sheds new light upon the utilization of SS-LASS petrochronology deciphering multiple anatectic events in the deeply-subducted continental crust and supports us in better understanding the tectonic evolution of Dabie-Sulu Orogen.
基金supported by the B-type Strategic Priority Program of the Chinese Academy of Sciences (Grant No.XDB41000000)the National Natural Science Foundation of China (Grant Nos.92155306,41822201)。
文摘Zirconium is one of high field strength elements but its isotope behavior during geochemical processes is still uncertain because of the limited database.While Zr isotopes in magmatic rocks are often used to trace the evolution of magmas through fractional crystallization,it is intriguing how highly heterogeneous Zr isotopes were produced by the growth of zircon during crustal anatexis.We address this issue by in-situ zircon Zr isotope analyses of migmatites from two high-temperature metamorphic terranes in the South Lhasa zone and the North Dabie zone,respectively,in China.The results show highly variable δ^(94)Zr values from-0.30‰ to +0.81‰ and from-0.58‰ to +0.49‰,respectively.In addition to the relict zircon of magmatic origin,two types of newly-grown zircons were identified in terms of their occurrences,trace elements and δ^(94)Zr values.The peritectic zircon,mainly occurring in the in-situ leucosomes,exhibits the highest Nb-Ta-Hf-U contents and variably higher δ^(94)Zr values than those of the relict zircon.The anatectic zircon,mainly occurring in the leucocratic veins,shows higher Nb-Ta-Hf-U contents than and similar δ^(94)Zr values to those of the relict zircon.Model calculations demonstrate that the variable Zr isotope compositions of newly-grown zircons would result from decoupled release of Zr from zircon and non-zircon phases.The Zr supply of the peritectic zircon is mainly derived from the decomposition of Zr-bearing minerals in the in-situ anatectic melt(the non-zircon effect),whereas the Zr supply of the anatectic zircon is mainly from the dissolution of pre-existing zircons in the evolved melt(the zircon effect).The significant Zr isotope variations in the migmatites well illustrate the generation,migration and accumulation of the anatectic melts during the partial melting.Therefore,Zr isotopes can be used as a powerful means for distinguishing between the peritectic and anatectic zircons during crustal anatexis.
基金supports from the National Natural Science Foundation of China(Grant No.40472113)the National Key Basic Research and Development Program of China(Grant No.2004CB418405)the Foundation of Earthquake Sciences of China Earthquake Administration.
文摘Migmatization in Higher Himalayan Crystallines (HHC) results from anatexis. The widely distributed migmatites in HHC are an important clue to investigate the relationship be- tween anatexis and the origins of Higher Himalayan leucogranites (HHL), and to understand the effect of anatexis on crustal evolution during the post-collision period. We studied in detail the chemical features of three basic constituent parts of the migmatites, i.e. leucosome, mesosome and melanosome, and determined the K-Ar ages of leucosomes. Our studies indicate that type-I leucosome is the product of crystallization of melt generated by partial melting of mesosome at source region, but type-II leucosome and HHL probably underwent crystallization differentiation of plagioclase during melt aggregation and migration. The age of 22.67 Ma of Type-I leucosome, which is a little older than the beginning of MCT movement, indicates that anatexis may have played an important role in the formation of MCT. That the ages of type-II leucosome (ranging from 14.82 to 18.37 Ma) are consistent with that of HHL provides new chronological evidence for the relationship between migmatization and HHL. We obtained a very young age of 6.23 Ma of Type-II leucosome that provides new time constraint on magma activity in the central segment of Higher Himalayas.
文摘The Higher Himalayan Crystallines(HHC), in western Garhwal, Uttarakhand are located in a regionalscale intracontinental ductile shear zone(15-20 km wide) bounded by the Main Central Thrust at the base, and the South Tibetan Detachment System at the top. The migmatite zone in the centre has the highest grade of metamorphism in the NW Himalayas and show evidence of flowage. Zircons extracted from samples of metasediment, migmatite, biotite granite and in situ partial melt(tourmaline-bearing leucogranite) along the Bhagirathi Valley, preserve U-Pb isotopic evidence of magmatic history, magma source and effects of the Himalayan orogeny in the region. Three distinct periods of zircon growth in the leucogranite record the episodic influx of magma between 46 Ma and 20 Ma indicating a time span of more than 25 Ma between the onset of fluid-fluxed partial melting in the mid-crustal intracontinental shear zone and the emplacement of the magma into the upper crust in a post-collisional extensional setting. Metamorphic zircon growth was initiated about 46 Ma, when the partial melts were generated as the migmatite zone was exhumed.
基金supported by the China Geological Survey (nos. 1212011120129,1212010811033,and 1212011120152)the National Natural Science Foundation of China (no. 41072053)
文摘The albite rim is present in most felsic gneisses of the Fuping Complex. The presence of the rim indicates the coexistence of plagioclase and K-feldspar in the rock. The rim is formed immediately after the myrmekite, and both textures were derived from the alteration of K-feldspar. The difference is that that there is no quartz present in the rim, and the rim is nearly albite and the anorthite content of the rim plagioclase is substantially lower than that of the myrmekite plagioclase. Formed at 400- 500~C the albite rim was derived from the K-feldspar composition adjustment in the late or post- magmatism stage. As the temperature decreased, the equilibrium between K-feldspar and plagioclase could be maintained, and reactions between the minerals occurred. The leucocratic veins in the complex show distinguished magma or migmatitic characteristics. The rim might form in the late magma or deuteric stage. The formation of the rim implies obvious granitic magmaor melt-injection activity. Typical metamorphic rocks cannot produce the rims. Anatexis after medium-high grade metamorphism might be subordinate. If present, the anatexis is water-present, but the rim texture cannot be taken as the symbol of anatexis.
基金supported by National Natural Science Foundation of China(41873023,41425010 and 92055202)The Second Tibetan Plateau Scientific Expedition and Research program(2019QZKK070203)+1 种基金National Key Research and Development Project of China(2016YFC0600304)China Geological Survey(DD20190057).
文摘Fluid-absent and fluid-fluxed melting of muscovite in metasedimentary sources are two types of crustal anatexis to produce the Himalaya Cenozoic leucogranites.Apatite grains separated from melts derived from the two types of parting melting have different geochemical compositions.The leucogranites derived from fluid-fluxed melting have relict apatite grains and magmatic crystallized apatite grains,by contrast,there are only crystallized apatite grains in the leucogranites derived from fluid-absent melting.Moreover,apatite grains crystallized from fluid-fluxed melting of muscovite contain higher Sr,but lower Th and LREE than those from fluid-absent melting of muscovite,which could be controlled by the distribution of partitioning coefficient(D_(Ap/Melt))between apatite and leucogranite.D_(Ap/Melt) in granites derived from fluidabsent melting is higher than those from fluid-fluxed melting.So,not only SiO_(2) and A/CNK,but also types of crustal anatexis are sensitive to trace element partition coefficients for apatite.In addition,due to being not susceptible to alteration,apatite has a high potential to yield information about petrogenetic processes that are invisible at the whole-rock scale and thus is a useful tool as a petrogenetic indicator.
基金financially supported by the National Natural Science Foundation of China (41872194, 41872203)
文摘The Neoarchean charnockites of North margain of North China Craton(NCC) has become a hot topic into understanding the Early Precambrian basement. Although there is a broad consensus that charnockite is usually related to granulite facies metamorphism, whether its petrogenesis and tectonics characteristics remains controversial. Inclusions within hypersthene and garnet in charnockite are used to identify the peak granulite facies mineral assemblage, with the formation of Magnesian-charnockite attributed to anatexis of the protolith associated with this granulite facies metamorphism. The distribution of major and trace elements in charnockite is very uneven, significant depleted in LILEs(eg. Cs, U, Th) and HFSEs(eg. Nb, Ta, P and Ti), riched in Sr. Raising to the coexistence of Eu-enrichment and Eu-depletion type of REE patterns that influenced by the content of plagioclase and the remnants minerals of zircon and apatite. Comparative the petrography, geochemistry and geochronology data of Magnesian-charnockite indicate that the ratios of mafic pellites and basalts involved in anatectic melting are different by the upwelling of mantle magma, also resulting in the Eu anormals characteristics. The formation of the Magnesian-charnockite is closely connected with the subduction of the NCC oceanic crust(About ~2.5 Ga). However, Ferroan-charnockite may be the formed by the crystallization differentiation of the upwelling of mantle-derived shoshonitic magma(About ~2.45 Ga), with the lower crust material addition.
基金funded by a National Sciences and Engineering Research Council of Canada Discovery grant to CYsupported by a Vanier Canada Graduate Scholarship。
文摘Tonalite-trondhjemite-granodiorite(TTG) suites constitute a large proportion of the Archean geological record;however,the geodynamic processes that generated them,and Archean continental crust in general,remain a subject of debate.The concentrations and ratios of Sr,Y,La,Yb,Nb,and Ta in TTGs are commonly used to determine the depth of melting of their metabasic sources.The trace element composition of melt produced by metabasic source rocks during anatexis is strongly affected by the presence and abundance of pressure-sensitive minerals,such as plagioclase(Sr-bearing),garnet(Y-and HREE-bearing),and rutile(Nb-and Ta-bearing).Elevated Sr/Y and La/Yb ratios and low concentrations of Nb and Ta in TTGs are generally considered to indicate melting at high pressures(≥2.0 GPa).The depth of melting is a key factor in determining the origin of TTGs as this provides critical information on the tectonic setting of their generation.We use phase equilibrium and trace element modelling to explore the effects of three potential influences on TTG trace element compositions:fractionation of trace elements into peritectic garnet cores,progressive melt loss from the source,and source bulk composition.We model three different compositions of Archean basalts along thermal gradients of 500℃/GPa,750℃/GPa,and 1000℃/GPa.The models produce maj or and trace element melt compositions that are generally consistent with measured compositions of TTGs.Although Sr/Y,La/Yb,Nb,and Ta exhibit pressure-dependent behaviour,other factors also affect these values.Garnet fractionation causes Sr/Y and La/Yb to reach much greater values and in this scenario,the values also increase with increasing temperature.Source bulk composition has an effect in all scenarios and most strongly influences La/Yb,Nb,and Ta.Overall,these results show that Sr/Y,La/Yb,Nb,and Ta can reach values generally considered to be indicative of high pressure melting at a range of P-T conditions including P <2.0 GPa.Consequently,trace element compositions of TTGs alone may provide a misleading impression of the depth of melting of metabasites and the geodynamic environment of Archean crustal growth and reworking.
文摘Partially migmatized rocks, banded migmatites, augen-banded migmatites and gneissic migmatites are developed successively from Xindong to Yunlu, Gaozhou, Guangdong Province in the Yunkai Caledonian orogenic belt at the border between Guangdong and Guangxi. Mass-balance calculations, statistical analysis of the textural relations and mineralogical and geochemical studies of the migmatites and the study of the metamorphlc setting of the Yunlu area indicate that the migmatites in the study area were primarily formed by anatexis without remarkable introduction of foreign components such as K, Na and Si and removal of Ca, Fe, Mg, etc.
文摘Higher Himalayan Crystalline（HHC） complex of the Sikkim Himalaya predominantly consists of high-grade pelitic migmatites.In this study,reaction textures,mineral/bulk rare earth elements （REE）,trace element partition coefficients and trace element zoning profiles in garnet are used to demonstrate a complex petrogenetic process during crustal anatexis.With the help of equilibrium REE and trace element partitioning model,it is shown that strong enrichment of Effective Bulk Composition（EBC） is responsible for the zoning in garnet in these rocks.The data strongly support disequilibrium element partitioning and suggest that the anatectic melts associated with mafic selvedges are likely produced by disequilibrium melting because of fast melt segregation process.
文摘A popular hypothesis of in situ transformation of amphibolite facies gneisses to patchy charnockites by CO2 influx from mantle was proposed primarily from the Kabbaldurga quarries in South Karnataka and subsequently reported from several south Indian localities. However, presence of abundant mafic granulite enclaves in Kabbaldurga and its neighborhood and its implications in relation to patchy charnockite genesis were not discussed. In these quarries patchy charnockites occur in various modes and associations. Some of these patches do occupy structural weak zones, such as shear bands and fold noses in the migmatitic gneisses, but many of the patchy charnockite bodies occur as branching veins transecting the gneissic foliation and hence do not account for fluid pathways. Most importantly, charnockitic leucosomes at margins of mafic granulite enclaves and charnockitic veins within some mafic granulite enclaves indicate a close genetic link between them via dehydration partial melting. This is further corroborated by trace element distribution between them. Dehydration partial melting in mafic rocks in a migmatite terrain such as Kabbaldurga, can explain all the different modes of the patchy charnockites as various stages of segregation and mobility relative to deformation. Abundant mafic granulite enclaves and field features suggesting a relatively late origin of the patchy charnockites, are compelling evidence against the notion of a transition zone. Mantle derivation age of the mafic source rocks (protoliths of mafic granulites) at Kabbaldurga at 3.08 ± 0.08 Ga with small positive ? values is virtually identical to the source of the massive charnockite of Karnataka craton at 3.08 Ga. This could imply a widespread mafic magmatism in South India around 3.0 Ga.
基金Supported by the Chinese Academy of Sciences (Grant No. KZCX2-YW-131)the Chinese Ministry of Science and Technology (Grant No. 2009CB825004)National Natural Science Foundation of China (Grant No. 40673009)
文摘The Dabie-Sulu orogenic belt was formed by the Triassic continental collision between the South China Block and the North China Block. There is a large area of Mesozoic magmatic rocks along this orogenic belt, with emplacement ages mainly at Late Triassic, Late Jurassic and Early Cretaceous. The Late Triassic alkaline rocks and the Late Jurassic granitoids only crop out in the eastern part of the Sulu orogen, whereas the Early Cretaceous magmatic rocks occur as massive granitoids, sporadic intermedi- ate-mafic intrusive and volcanic rocks throughout the Dabie-Sulu orogenic belt. Despite the different ages for their emplacement, the Mesozoic magmatic rocks are all characterized not only by enrichment of LREE and LILE but depletion of HFSE, but also by high initial Sr isotope ratios, low εNd(t) values and low radiogeneic Pb isotope compositions. Some zircons from the Jurassic and Cretaceous granitoids contain inherited magmatic cores with Neoprotozoic and Triassic U-Pb ages. Most of the Cretaceous mafic rocks have zircon δ18O values and whole-rock δ13C values lower than those for the normal mantle. A systematic comparison with adjacent UHP metaigneous rocks shows that the Mesozoic granitoids and mafic rocks have elemental and isotopic features similar to the UHP metagranite and metabasite, respectively. This indicates that these magmatic and metamorphic rocks share the diagnostic features of lithospheric source that has tectonic affinity to the northern edge of the South China Block. Their precursors underwent the UHP metamorphism and the post-collisional anatexis, respectively at different times and depths. Therefore, the Mesozoic magmatic rocks were derived from anatexis of the subducted continental lithosphere itself beneath the collision-thickened orogen; the geodynamic mechanism of the post-collisional magmatisms is tectonic collapse of orogenic roots in response to lithospheric extension.
基金The work was supported by the National Key Basic Research Project (Grant No. G1998040800) Sino-France Collaborative Project of "Altun Shan-Qilianshan" Tectonic Evolution and Lithosphere Shearing.
文摘A set of granitic and amphibole mylonite are exposed in the Altyn Tagh fault zone. The preliminary study shows that these rocks are the product of the syntectonic anatexis in the process of the left-lat- eral strike-slip shear, and are the result of the ductile transpression. There are two types of zircon sorted from the mylonite formed with syn-shear anatexis. Among them, one is the anatectic long co-lumnar zircon and another is the residual metamorphic sub-rounded columnar zircon. Two groups of age for single zircon measured by ion microprobe (SHRIMP) are obtained: one is 461-547 Ma for the sub-rounded columnar residual metamorphic zircon, and the other is 239-244 Ma for the long-columnar anatectic zircon. This type of zircon is direc-tionally spread in rock, and the long axis direction of its crystal is identical to that of stretching lineation, represent-ing the direction of tectonic stress in the process of the strike-slip. 40Ar-39Ar age of the directional growth horn-blende in the same mylonite
基金supported by the National Natural Science Foundation of China (Grant No. 41430207)
文摘It is generally believed that trondhjemitic rock, an important component of TTG rocks, is the anatectic product of mafic rocks. However, in many TTG gneiss terranes, for instance, the granulite facies terrane in Eastern Hebei, trondhjemites occur as small dikes, intrusions or leucosomes in tonalitic gneisses, suggesting their origin of in-situ partial melting. Based on the petrological analysis of a tonalitic gneiss sample from Eastern Hebei, in combination with zircon U-Pb dating, we investigated the petrogenesis of trondhjemite through simulating anatectic reactions and the major and trace element characteristics of the product melt at different pressures(0.7, 1.0 and 2.0 GPa). The results indicate that hornblende dehydration melting in a tonalitic gneiss at 0.9–1.1 GPa and 800–850°C, corresponding to the high-T granulite facies, with melting degrees of 5–10wt.% and a residual assemblage containing 5–10wt.% garnet, can produce felsic melts with a great similarity, for instance of high La/Yb ratios and low Yb contents to the trondhjemitic rocks from Eastern Hebei. However, the modelled melts exhibit relatively higher K2 O, and lower CaO and Mg~# than those in the trondhjemitic dikes and leucosomes from Eastern Hebei, suggesting that the leucosomes may not only contain some residual minerals but also be subjected to the effect of crystal fractionation. The zircon U-Pb dating for the tonalitic and trondhjemitic rocks in the Eastern Hebei yields a protolith age of 2518±12 Ma and a metamorphic age of 2505±19 Ma for the tonalitic gneiss. The latter age is consistent with a crystallization age of 2506±6 Ma for the trondhjemitic rock, confirming a close petrogenetic relation between them.
文摘Dehydration melting of metasupracrustal rocks at mid-to deep-crustal levels can generate water undersaturated granitic melt.In this study,we evaluate the potential of~1.89–1.88 Ga metasupracrustal rocks of the Precambrian of southern Finland as source rocks for the 1.86–1.79 Ga late-orogenic leucogranites in the region,using the Rhyolite-MELTS approach.Melt close in composition to leucogranite is produced over a range of realistic pressures(5 to 8 kbar)and temperatures(800 to 850℃),at 20%–30%of partial melting,allowing separation of melt from unmelted residue.The solid residue is a dry,enderbitic to charnoenderbitic ganulite depleted in incompatible components,and will only yield further melt above 1000–1050℃,when rapidly increasing fractions of increasingly calcic(granodioritic to tonalitic)melts are formed.The solid residue after melt extraction is incapable of producing syenogranitic magmas similar to the Mid-Proterozoic,A-type rapakivi granites on further heating.The granitic fraction of the syenogranitic rapakivi complexes must thus have been formed by a different chain of processes,involving mantle-derived mafic melts and melts from crustal rock types not conditioned by the preceding late-orogenic Svecofennian anatexis.
