This work carried out systematic geological field investigation, petrography observation, zircon geochronology and whole rock geochemistry on Late Paleozoic intrusions in the Xingxingxia region near the Xinjiang-Gansu...This work carried out systematic geological field investigation, petrography observation, zircon geochronology and whole rock geochemistry on Late Paleozoic intrusions in the Xingxingxia region near the Xinjiang-Gansu provincial boundary, western China, aiming to constrain the Late Paleozoic tectonic framework of the Xingxingxia region and the final closure time of South Tianshan Ocean in the East Tianshan. The Xingxingxia area is located in the east part of the Tianshan orogen, and adjacent to the north of the Tarim Basin. The Late Paleozoic magma activities in the Xingxingxia region can be mainly divided into three stages. The first stage includes intrusive magma activities under a collision setting between Late Ordovician to the Late Devonian. The second stage is intrusive magma activities under a subduction setting during（304±3）–（278±3） Ma, and the third stage involves intrusive magma activities under a collision and post-collision setting during（268±5）–（259.9±2.6） Ma. The final suture zone of South Tianshan Ocean should be between the Central Tianshan Block and South Tianshan accretionary complex. Based on previous work, both the first stage magma activities（i.e., intrusive magmatic activities between the Late Ordovician to Late Devonian） and the Hongliuhe ophiolitic complex indicate a close event between Central Tianshan Block and South Tianshan Accretionary Complex. The 304±3 Ma dioritic metamorphic gneiss of the XingX ingxia complex and the 278±3 Ma diorite are all island arc calc-alkaline rocks, the 289±3 Ma gabbro is island arc tholeiitic gabbro formed by magma from metasomatic enrichment mantle. All these results indicate that the second stage of magmatic activities is under a subduction setting. The third stage magma activities i.e. the granitic magma activities of（268±5）–（259.9±2.6） Ma occurred at a transitional setting from compressional to post-collision extensional tectonic setting. Thus, around（268±5）–（260±3） Ma, the final closure of the South Tianshan Ocean occurred and the Tianshan orogen shifted into the intracontinental evolution stage. During and after the closure process, a wide range of metamorphism and large dextral strike-slip faults developed.展开更多
The Triassic granitoids in Central Tianshan play a key role in determining the petrogenesis and tectonic evolution on the southern margin of the Central Asian orogenic belt.In this study,we present SHRIMP zircon U-Pb ...The Triassic granitoids in Central Tianshan play a key role in determining the petrogenesis and tectonic evolution on the southern margin of the Central Asian orogenic belt.In this study,we present SHRIMP zircon U-Pb ages,Hf isotopic and geochemical data on the Xingxingxia biotite granite,amazonite granite and granitic pegmatite in Central Tianshan,NW China.Zircon U-Pb dating yielded formation ages of 242 Ma for the biotite granite and 240 Ma for the amazonite granite.These granitoid rocks have high K_(2)O with low MgO and CaO contents.They are enriched in Nb,Ta,Hf and Y,while being depleted in Ba and Sr,showing flat HREE patterns and negative Eu anomalies.They have typical A-type granite geochemical signatures with high Ga/A_(1)(8–13)and TFeO/(TFeO+MgO)ratios,showing an A_(2) affinity for biotite granite and an A_(1) affinity for amazonite granite and granitic pegmatite.Zircon ε_(Hf)(t)values of the granitoids are 0.45–2.66,with Hf model ages of 0.99–1.17 Ga.This suggests that these A-type granites originated from partial melting of the lower crust.We propose that Xingxingxia Triassic A-type granites formed under lithospheric extension from post-orogenic to anorogenic intraplate settings and NE-trending regional strike-slip fault-controlled magma emplacement in the upper crust.展开更多
Metapelites from the Mesoproterozoic Xingxingxia group in the Eastern Segment of the Central Tianshan,Northwestern China,were analyzed for major and trace elements,includ-ing rare earth elements.Compared with post-Arc...Metapelites from the Mesoproterozoic Xingxingxia group in the Eastern Segment of the Central Tianshan,Northwestern China,were analyzed for major and trace elements,includ-ing rare earth elements.Compared with post-Archean shales,the metapelites are enriched in Sr,Hf and Zr.The other elements are similar to the compositions of the Phanerozoic North American Shale Composite and the Post-Archean Australian average Shale(PAAS).The characteristics of Al_(2)O_(3)/TiO_(2),Cr/Zr,Cr/Th,Th/Sc and high Zr concentration suggest that these pelites could be derived from a significant proportion of felsic and a few proportion of participation of mafic mate-rials in nature.These rocks display highly fractionated REE patterns,and average(La/Yb)_(n)=18.6.Several samples have slightly negative Eu anomalies(Eu/Eu^(*)=0.7―0.84)in comparison with PAAS(Eu/Eu^(*)=0.65),indicating contribution from young immature materials.The low K_(2)O/Al_(2)O_(3) values of these metapelites suggest that their source materials could contain minimal alkali feldspar.In the A-CN-K triangular diagram,the sample data array intersects the feldspar join,indicating that the source had a plagioclase:K-feldspar ratio of approximately 5:1,repre-senting weathered products from components that could approximately correspond to granodio-rites and tonalites.The low Chemical Index of Alteration(CIA)and high Index of Compositional Variability(ICV)values suggest low degrees of weathering of the source and low compositional maturity of the sediments,respectively.The geochemical characteristics of the metapelites from the Xingxingxia group also demonstrate that these sediments were deposited in a back-arc background within a continental-arc system.展开更多
基金the Program of China Geological Survey(grant No.1212011220649)
文摘This work carried out systematic geological field investigation, petrography observation, zircon geochronology and whole rock geochemistry on Late Paleozoic intrusions in the Xingxingxia region near the Xinjiang-Gansu provincial boundary, western China, aiming to constrain the Late Paleozoic tectonic framework of the Xingxingxia region and the final closure time of South Tianshan Ocean in the East Tianshan. The Xingxingxia area is located in the east part of the Tianshan orogen, and adjacent to the north of the Tarim Basin. The Late Paleozoic magma activities in the Xingxingxia region can be mainly divided into three stages. The first stage includes intrusive magma activities under a collision setting between Late Ordovician to the Late Devonian. The second stage is intrusive magma activities under a subduction setting during（304±3）–（278±3） Ma, and the third stage involves intrusive magma activities under a collision and post-collision setting during（268±5）–（259.9±2.6） Ma. The final suture zone of South Tianshan Ocean should be between the Central Tianshan Block and South Tianshan accretionary complex. Based on previous work, both the first stage magma activities（i.e., intrusive magmatic activities between the Late Ordovician to Late Devonian） and the Hongliuhe ophiolitic complex indicate a close event between Central Tianshan Block and South Tianshan Accretionary Complex. The 304±3 Ma dioritic metamorphic gneiss of the XingX ingxia complex and the 278±3 Ma diorite are all island arc calc-alkaline rocks, the 289±3 Ma gabbro is island arc tholeiitic gabbro formed by magma from metasomatic enrichment mantle. All these results indicate that the second stage of magmatic activities is under a subduction setting. The third stage magma activities i.e. the granitic magma activities of（268±5）–（259.9±2.6） Ma occurred at a transitional setting from compressional to post-collision extensional tectonic setting. Thus, around（268±5）–（260±3） Ma, the final closure of the South Tianshan Ocean occurred and the Tianshan orogen shifted into the intracontinental evolution stage. During and after the closure process, a wide range of metamorphism and large dextral strike-slip faults developed.
文摘The Triassic granitoids in Central Tianshan play a key role in determining the petrogenesis and tectonic evolution on the southern margin of the Central Asian orogenic belt.In this study,we present SHRIMP zircon U-Pb ages,Hf isotopic and geochemical data on the Xingxingxia biotite granite,amazonite granite and granitic pegmatite in Central Tianshan,NW China.Zircon U-Pb dating yielded formation ages of 242 Ma for the biotite granite and 240 Ma for the amazonite granite.These granitoid rocks have high K_(2)O with low MgO and CaO contents.They are enriched in Nb,Ta,Hf and Y,while being depleted in Ba and Sr,showing flat HREE patterns and negative Eu anomalies.They have typical A-type granite geochemical signatures with high Ga/A_(1)(8–13)and TFeO/(TFeO+MgO)ratios,showing an A_(2) affinity for biotite granite and an A_(1) affinity for amazonite granite and granitic pegmatite.Zircon ε_(Hf)(t)values of the granitoids are 0.45–2.66,with Hf model ages of 0.99–1.17 Ga.This suggests that these A-type granites originated from partial melting of the lower crust.We propose that Xingxingxia Triassic A-type granites formed under lithospheric extension from post-orogenic to anorogenic intraplate settings and NE-trending regional strike-slip fault-controlled magma emplacement in the upper crust.
基金supported by the National Natural Science Foundation of China(Grant No.40072065).
文摘Metapelites from the Mesoproterozoic Xingxingxia group in the Eastern Segment of the Central Tianshan,Northwestern China,were analyzed for major and trace elements,includ-ing rare earth elements.Compared with post-Archean shales,the metapelites are enriched in Sr,Hf and Zr.The other elements are similar to the compositions of the Phanerozoic North American Shale Composite and the Post-Archean Australian average Shale(PAAS).The characteristics of Al_(2)O_(3)/TiO_(2),Cr/Zr,Cr/Th,Th/Sc and high Zr concentration suggest that these pelites could be derived from a significant proportion of felsic and a few proportion of participation of mafic mate-rials in nature.These rocks display highly fractionated REE patterns,and average(La/Yb)_(n)=18.6.Several samples have slightly negative Eu anomalies(Eu/Eu^(*)=0.7―0.84)in comparison with PAAS(Eu/Eu^(*)=0.65),indicating contribution from young immature materials.The low K_(2)O/Al_(2)O_(3) values of these metapelites suggest that their source materials could contain minimal alkali feldspar.In the A-CN-K triangular diagram,the sample data array intersects the feldspar join,indicating that the source had a plagioclase:K-feldspar ratio of approximately 5:1,repre-senting weathered products from components that could approximately correspond to granodio-rites and tonalites.The low Chemical Index of Alteration(CIA)and high Index of Compositional Variability(ICV)values suggest low degrees of weathering of the source and low compositional maturity of the sediments,respectively.The geochemical characteristics of the metapelites from the Xingxingxia group also demonstrate that these sediments were deposited in a back-arc background within a continental-arc system.
