Identifying the crust-mantle interactions in association with the evolution of the Precambrian microcontinents provides critical constraints on the accretionary evolution in the Central Asian Orogenic Belt(CAOB).The B...Identifying the crust-mantle interactions in association with the evolution of the Precambrian microcontinents provides critical constraints on the accretionary evolution in the Central Asian Orogenic Belt(CAOB).The Bainaimiao arc terrane(BAT)is one of the most important Precambrian microcontinents in southeastern CAOB,however,few studies have paid attention to the types and the evolving processes of the crust-mantle interactions that occurred before its final accretion onto the northern North China Craton.This study presents an integrated study of geochronology,zircon Hf isotope and whole-rock geochemistry on the latest Neoproterozoic diabases and the Early Paleozoic arc intrusions in the western BAT.The latest Neoproterozoic(ca.546 Ma)diabases display low SiO2(46.52-49.24 wt.%)with high MgO(8.23-14.41 wt.%),Cr(66-542 ppm)and Ni(50-129 ppm),consisting with mantle origin.Their highly negative zirconεHf(t)(-12.0 to-24.7)and high Fe/Mn ratios(62.1-81.7)further indicate a significantly enriched mantle source.Considering that the BAT maybe initially separated from the Tarim Craton with a thickened crustal root,we propose that these diabases were generated through partial melting of an enriched lithospheric mantle source that had been hybridized by lower-crustal eclogites during foundering of the BAT lower crust.The Early Paleozoic(ca.475-417 Ma)arc intrusions in western BAT can be divided into PeriodsⅠandⅡat approximately 450 Ma.The PeriodⅠ(>450 Ma)intrusions contain abundant mafic minerals like hornblende and pyroxene,and show positive zirconεHf(t)(+1.5 to+10.9).They are predominantly medium-K calc-alkaline with broad correlations of SiO2 versus various major and trace elements,which correlate well with the experimental melts produced by the fractional crystallization of primitive hydrous arc magmas at 7 kbar.We assume they were formed through mid-crustal differentiation of the mantle wedge-derived hydrous basaltic melts.By contrast,the PeriodⅡ(≤450 Ma)intrusions are characterized by variable zircon eHf(t)(-15.0 to+11.5)with irregular variations in most major and trace elements,which are more akin to the arc magmas generated in an open system.The general occurrence of elder inherited zircons,along with the relatively high Mg#(>45)of some samples,call upon a derivation from the reworking of the previously subduction-modified BAT lower crust with the input of mantle-derived mafic components.In combination with the Early Paleozoic tectonic melanges flanking western BAT,we infer that the compositional transition from PeriodⅠtoⅡcan be attributed to the tectonic transition from south-dipping subduction of Solonker ocean to north-dipping subduction of South Bainaimiao ocean in southeastern CAOB.The above results shed light not only on the latest Neoproterozoic to Early Paleozoic multiple crust-mantle interactions in western BAT,but also on the associated crustal construction processes before the final arc-continent accretion.展开更多
Late Paleozoic to Early Mesozoic granites are widespread in the southern Qaidam Basin, northern margin of the eastern Kunlun orogenic belt. Their petrogenesis can provide us insights into the tectonic evolution and cr...Late Paleozoic to Early Mesozoic granites are widespread in the southern Qaidam Basin, northern margin of the eastern Kunlun orogenic belt. Their petrogenesis can provide us insights into the tectonic evolution and crustal growth process in the Qaidam Basin. This paper reports Permian–Triassic granites from the Kunbei area, southwestern Qaidam Basin. Detailed zircon LA-ICP MS U-Pb dating reveals that the granites from the four drilling cores（q404, q406, q1612-8, q1613-8） have identical ages of 251±3, 256±4, 247±2, and 251±6 Ma, respectively, these ages are identical with the Permian–Triassic granites from the eastern Qaidam Basin. Detailed geochemical analyses indicate that these granites display typical affinities of highly-fractionated I-type granites：（1） they have high SiO_2（up to 76.5 wt.%）, Na_2O＋K_2O（7.91 wt.% to 9.48 wt.%） contents and high FeO^T/MgO values of 4.7 to 9.3, suggesting significant fractional crystallization;（2） their low A/CNK values of 0.54 to 1.03, no normative Al-rich minerals, inconsistent with the per-aluminous S-type granites;（3） their low Ga（14.5 ppm to 20.7 ppm） and 10 000×Ga/Al（2.23 to 3.03, most of them 〈2.6） values are inconsistent with the A-type granites;（4） the high Rb（191 ppm to 406 ppm） contents and Rb/Sr（2.1 to 13.4） ratios, as well as the significant negative Eu anomalies（0.10 to 0.42） also indicate significant fractional crystallization of feldspars;（5） their low P_2O_5 contents（0.02 wt.% to 0.10 wt.%） suggest the limited solubility of phosphorus in primitive metaluminous melts. In combination with the geological background, we propose that the Permian–Triassic highly-fractionated I-type granites resulted from partial melting of intra-crustal mafic rocks, and the primitive I-type granitic melts underwent significant fractional crystallization of feldspars. The occurrence of highly-fractionated I-type granites in the southwestern Qaidam Basin suggests a Permian–Triassic active continental margin in the northern margin of the East Kunlun orogenic belt.展开更多
The end-Permian to Early–Middle Triassic magmatic rocks in Inner Mongolia can provide valuable insights into the relationships between the collisional processes and the magmatic responses during the final orogenic ev...The end-Permian to Early–Middle Triassic magmatic rocks in Inner Mongolia can provide valuable insights into the relationships between the collisional processes and the magmatic responses during the final orogenic evolution of Xing-Meng orogenic belt(XMOB). This paper presents zircon U-Pb ages and Hf isotopes, whole rock geochemical and Sr-Nd-Pb isotopic data for the Early–Middle Triassic diabases and monzogranites from the Langshan area, southwestern XMOB. Our results suggest that the studied diabases and monzogranites were respectively formed during Early Triassic and Middle Triassic. The Early Triassic diabases are characterized by "arc-like" geochemical signatures, including enrichment in Rb, U and K, and depletion in Nb, Ta, P and Ti. They have negative to weak positive εNd(t) values(-3.1 to +1.5) and relatively high initial ratios of 208 Pb/204 Pb(35.968–37.346), 207 Pb/204 Pb(15.448–15.508) and 206 Pb/204 Pb(16.280–17.492), indicating a subduction-metasomatized enriched lithospheric mantle source. Their low Ba/Rb(2.72–6.56), Ce/Y(0.97–1.39) and(Tb/Yb)N ratios(1.31–1.45) suggest that the parental magma was likely originated from low degree partial melting of the phlogopite-bearing lherzolite in a spinel-stability field. The Middle Triassic monzogranites show high Sr/Y ratios, low Mg O, Cr and Ni contents, high Zr/Sm ratios(40–64), negative zircon εHf(t) values(-25.8 to-8.8), as well as relatively flat heavy rare earth element patterns. They were likely derived from low degree partial melting of a moderately thickened ancient lower crust. The diabases and the slightly postdated high Sr/Y granites in this study represent the magmatic responses to the final orogenic evolution in the southwestern XMOB. Together with regional works, we propose that the slab break-off of the Paleo-Asian oceanic lithosphere following the terminal collision between the North China Craton and the South Mongolia terranes triggered asthenospheric upwelling, and the ongoing convergence further initiated moderately crustal thickening and uplift in the XMOB.展开更多
基金financially supported by the China Geological Survey(1212011085490 and 1212011220465)the National Natural Science Foundation of China(41421002)。
文摘Identifying the crust-mantle interactions in association with the evolution of the Precambrian microcontinents provides critical constraints on the accretionary evolution in the Central Asian Orogenic Belt(CAOB).The Bainaimiao arc terrane(BAT)is one of the most important Precambrian microcontinents in southeastern CAOB,however,few studies have paid attention to the types and the evolving processes of the crust-mantle interactions that occurred before its final accretion onto the northern North China Craton.This study presents an integrated study of geochronology,zircon Hf isotope and whole-rock geochemistry on the latest Neoproterozoic diabases and the Early Paleozoic arc intrusions in the western BAT.The latest Neoproterozoic(ca.546 Ma)diabases display low SiO2(46.52-49.24 wt.%)with high MgO(8.23-14.41 wt.%),Cr(66-542 ppm)and Ni(50-129 ppm),consisting with mantle origin.Their highly negative zirconεHf(t)(-12.0 to-24.7)and high Fe/Mn ratios(62.1-81.7)further indicate a significantly enriched mantle source.Considering that the BAT maybe initially separated from the Tarim Craton with a thickened crustal root,we propose that these diabases were generated through partial melting of an enriched lithospheric mantle source that had been hybridized by lower-crustal eclogites during foundering of the BAT lower crust.The Early Paleozoic(ca.475-417 Ma)arc intrusions in western BAT can be divided into PeriodsⅠandⅡat approximately 450 Ma.The PeriodⅠ(>450 Ma)intrusions contain abundant mafic minerals like hornblende and pyroxene,and show positive zirconεHf(t)(+1.5 to+10.9).They are predominantly medium-K calc-alkaline with broad correlations of SiO2 versus various major and trace elements,which correlate well with the experimental melts produced by the fractional crystallization of primitive hydrous arc magmas at 7 kbar.We assume they were formed through mid-crustal differentiation of the mantle wedge-derived hydrous basaltic melts.By contrast,the PeriodⅡ(≤450 Ma)intrusions are characterized by variable zircon eHf(t)(-15.0 to+11.5)with irregular variations in most major and trace elements,which are more akin to the arc magmas generated in an open system.The general occurrence of elder inherited zircons,along with the relatively high Mg#(>45)of some samples,call upon a derivation from the reworking of the previously subduction-modified BAT lower crust with the input of mantle-derived mafic components.In combination with the Early Paleozoic tectonic melanges flanking western BAT,we infer that the compositional transition from PeriodⅠtoⅡcan be attributed to the tectonic transition from south-dipping subduction of Solonker ocean to north-dipping subduction of South Bainaimiao ocean in southeastern CAOB.The above results shed light not only on the latest Neoproterozoic to Early Paleozoic multiple crust-mantle interactions in western BAT,but also on the associated crustal construction processes before the final arc-continent accretion.
基金Financial support for this study was jointly provided by the National Natural Science Foundation of China (No.41102037)the foundation for the author of National Excellent Doctoral Dissertation of China (No.201324)+1 种基金the Foundation of Young Excellent Scientists of the Shaanxi Province (No.2014KJXX-60)the MOST Special Fund from the State Key Laboratory of Continental Dynamics
文摘Late Paleozoic to Early Mesozoic granites are widespread in the southern Qaidam Basin, northern margin of the eastern Kunlun orogenic belt. Their petrogenesis can provide us insights into the tectonic evolution and crustal growth process in the Qaidam Basin. This paper reports Permian–Triassic granites from the Kunbei area, southwestern Qaidam Basin. Detailed zircon LA-ICP MS U-Pb dating reveals that the granites from the four drilling cores（q404, q406, q1612-8, q1613-8） have identical ages of 251±3, 256±4, 247±2, and 251±6 Ma, respectively, these ages are identical with the Permian–Triassic granites from the eastern Qaidam Basin. Detailed geochemical analyses indicate that these granites display typical affinities of highly-fractionated I-type granites：（1） they have high SiO_2（up to 76.5 wt.%）, Na_2O＋K_2O（7.91 wt.% to 9.48 wt.%） contents and high FeO^T/MgO values of 4.7 to 9.3, suggesting significant fractional crystallization;（2） their low A/CNK values of 0.54 to 1.03, no normative Al-rich minerals, inconsistent with the per-aluminous S-type granites;（3） their low Ga（14.5 ppm to 20.7 ppm） and 10 000×Ga/Al（2.23 to 3.03, most of them 〈2.6） values are inconsistent with the A-type granites;（4） the high Rb（191 ppm to 406 ppm） contents and Rb/Sr（2.1 to 13.4） ratios, as well as the significant negative Eu anomalies（0.10 to 0.42） also indicate significant fractional crystallization of feldspars;（5） their low P_2O_5 contents（0.02 wt.% to 0.10 wt.%） suggest the limited solubility of phosphorus in primitive metaluminous melts. In combination with the geological background, we propose that the Permian–Triassic highly-fractionated I-type granites resulted from partial melting of intra-crustal mafic rocks, and the primitive I-type granitic melts underwent significant fractional crystallization of feldspars. The occurrence of highly-fractionated I-type granites in the southwestern Qaidam Basin suggests a Permian–Triassic active continental margin in the northern margin of the East Kunlun orogenic belt.
基金supported by the Geological Survey of China (No. 1212011085490)the National Natural Science Foundation of China (No. 41421002)
文摘The end-Permian to Early–Middle Triassic magmatic rocks in Inner Mongolia can provide valuable insights into the relationships between the collisional processes and the magmatic responses during the final orogenic evolution of Xing-Meng orogenic belt(XMOB). This paper presents zircon U-Pb ages and Hf isotopes, whole rock geochemical and Sr-Nd-Pb isotopic data for the Early–Middle Triassic diabases and monzogranites from the Langshan area, southwestern XMOB. Our results suggest that the studied diabases and monzogranites were respectively formed during Early Triassic and Middle Triassic. The Early Triassic diabases are characterized by "arc-like" geochemical signatures, including enrichment in Rb, U and K, and depletion in Nb, Ta, P and Ti. They have negative to weak positive εNd(t) values(-3.1 to +1.5) and relatively high initial ratios of 208 Pb/204 Pb(35.968–37.346), 207 Pb/204 Pb(15.448–15.508) and 206 Pb/204 Pb(16.280–17.492), indicating a subduction-metasomatized enriched lithospheric mantle source. Their low Ba/Rb(2.72–6.56), Ce/Y(0.97–1.39) and(Tb/Yb)N ratios(1.31–1.45) suggest that the parental magma was likely originated from low degree partial melting of the phlogopite-bearing lherzolite in a spinel-stability field. The Middle Triassic monzogranites show high Sr/Y ratios, low Mg O, Cr and Ni contents, high Zr/Sm ratios(40–64), negative zircon εHf(t) values(-25.8 to-8.8), as well as relatively flat heavy rare earth element patterns. They were likely derived from low degree partial melting of a moderately thickened ancient lower crust. The diabases and the slightly postdated high Sr/Y granites in this study represent the magmatic responses to the final orogenic evolution in the southwestern XMOB. Together with regional works, we propose that the slab break-off of the Paleo-Asian oceanic lithosphere following the terminal collision between the North China Craton and the South Mongolia terranes triggered asthenospheric upwelling, and the ongoing convergence further initiated moderately crustal thickening and uplift in the XMOB.
