Age and Formation Mechanism of the Malashan High-Ca Two-Mica Granite within the Northern Himalayan Gneiss Domes, Southern Tibet
详细信息
- 责任者：Gao Lei (State Key Laboratory of Continental Tectonics and Dynamics ; Institute of Geology ; Chinese Academy of Geological Sciences ; Beijing 100037 ; China) ; Zeng Lingsen
- 刊名：Acta Petrologica Sinica
- 出版年：2013
- 卷期：29(6)
- 关键词：二云母花岗岩 ; two-mica granite ; 西藏 ; Tibet
- 页码：p.1995-2012
- 图表：illus., 3 tables, 67 refs.
- 分类号：0801
- issnNo：ISSN1000-0569
- isbnNo：CN11-1922/P
摘要
The Malashan gneissdome,one of the prominent domes within the Tethyan Himalaya,consists of Malashan two-mica granite,Cuobu two-mica granite and the Paiku composite leucogranite. The nearly E-W trending Malashan pluton distributes over a length more than～10km. Zircon U-Pb analyses demonstrate that the Malashan two-mica granites were crystallized over relatively long time span from 17. 6Ma to 16. 9Ma,or experienced at least two episodes of anatexis at 17. 6Ma and 16. 9Ma,respectively. Bulk-rock major and trace element and radiogenic isotope ( Sr,Nd and Hf) analyses indicate that the Malashan two-mica granite is a relatively uniform pluton and characterized by: ( 1) high SiO2 ,Al203 ,and relatively high CaO ( 1.2% ～2. 0%) ; (2) relatively high Sr,but low Rb and Rb/Sr ratios ( < 1.3)which are nearly constant relative to large variations in Ba concentrations; (3) highly variable Zr/Hf ratios (25. 9 ～39. 9 ) ; (4 ) enrichment in LREE,depletion in HREE,and no or weak negative Eu anomalies; ( 5 ) relatively homogeneous but low initial 87 Sr/86 Sr ratios and unradiogenic Nd isotope compositions; ( 6 ) highly heterogeneous Hf isotope compositions in the magmatic rims ( ε Hf ( t) = -20.4～-8. 0) as well as inherited cores ( ε Hf ( t) = -27.2～-9. 5) of zircon grains, respectively. These characteristics imply that the two-mica granites were derived from fluxed melting of metapelite at relatively high pressures and temperature,closely related to the E-W extension along the southern Tibetan rift system.
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