青藏高原古特提斯洋早石炭世弧后拉张:来自A型花岗岩的证据  

A back-arc extensional environment of the Early Carboniferous Paleo-Tethys Ocean in Tibetan Plateau: Evidences from A-type granites

作　　者：胡培远[1] 李才[2] 吴彦旺[2] 解超明[2] 王明[2] 李娇[3] 

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]吉林大学地球科学学院,长春130061 [3]中国科学院地质与地球物理研究所,北京100029

出　　处：《岩石学报》2016年第4期1219-1231,共13页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41522204;41502216);中国地质调查局项目(12120113093900;12120115026901);中国地质科学院地质研究所基本科研业务费项目(J1524)联合资助

摘　　要：本文报道了羌塘中部冈玛错钾长花岗岩的锆石U-Pb定年、岩石地球化学和锆石Hf同位素分析结果。钾长花岗岩中的锆石具岩浆生长环带,未见继承的老核,并且锆石Th/U比值大于0.5(0.58~1.05),显示出典型岩浆成因的锆石特征。锆石LA-ICP-MS定年结果为352.4±1.9Ma,表明其形成时代为早石炭世。钾长花岗岩富硅(Si O_2=74.17%~77.88%),低铝(Al2O3=10.50%~11.98%),贫镁(Mg O=0.23%~0.36%),富碱(Na_2O+K_2O=5.74%~7.24%),Na_2O>K_2O,K_2O/Na_2O=0.53~0.71,A/CNK=0.87~1.06,富集轻稀土元素和Zr、Hf、Rb、Th和U等元素,亏损Sr、Eu、P和Ti等元素,10000Ga/Al=3.12~4.14,显示出A2型花岗岩的地球化学特征。钾长花岗岩中锆石的εHf(t)值和Hf同位素两阶段模式年龄分别变化于+4.40^+12.14和549~985Ma,显示出正的、不均一的同位素组成,可能形成于壳-幔混合作用,其中幔源端元应当是伸展环境下上涌的地幔岩浆,而壳源端元则可能是扬子板块新元古代的新生地壳部分熔融形成的长英质岩浆。结合区域地质资料,认为该花岗岩可能形成于古特提斯洋壳对羌北-昌都板块北向俯冲引起的陆缘弧后拉张环境。This paper reports new zircon U-Pb age and Hf-isotope,and whole-rock major and trace element data from K-feldspar granites located in Gangmacuo area of Longmu Co-Shuanghu-Lancang River suture zone,central Qiangtang,northern Tibetan Plateau.Combined with high Th/U ratios（ 0. 58 - 1. 05）,the zircons from K-feldspar granites show no euhedral crystals and have clear oscillatory zones,indicating a magmatic origin. Zircon LA-ICP-MS dating for K-feldspar granites yields a weighted mean age of 352. 4± 2. 4Ma,suggesting that the crystallization age of Gangmacuo K-feldspar granites is Early Carboniferous. Petrological and geochemical study show that the intrusion is characterized by high silicon（ Si O2= 74. 17% - 77. 88%） and low aluminum（ Al2O3= 10. 50% -11. 98%）,depleted magnesium（ Mg O = 0. 23% - 0. 36%） and abundant alkali（ Na2 O ＋ K2 O = 5. 74% - 7. 24%,Na2 O K2 O,K2O/Na2 O = 0. 53 - 0. 71,A/CNK = 0. 87 - 1. 06）. Enrichment of Zr,Hf,Rb,Th,U and REE,depletion of Sr,Eu,P and Ti with high 10000 Ga/Al（ 3. 12 - 4. 14）,indicate that the K-feldspar granites are aluminous A-type granite and further classified to A2 type.Their zircons have positive εHf（ t） values（ ＋ 4. 40 - ＋ 12. 14） and old second stage Hf mode ages（ tDM2= 549 - 981Ma）,indicating that they were generated by mixing of crust and mantle. Gangmacuo K-feldspar granites were formed in a back-arc extensional environment of an active continental margin which was resulted from the subduction of Paleo-Tethys Plate beneath the Qiangbei-Qamdo plate.

关 键 词：青藏高原 羌塘中部 古特提斯洋 A型花岗岩 锆石U-PB定年 

分 类 号：P588.1[天文地球—岩石学]