藏南马拉山高钙二云母花岗岩的年代学特征及其形成机制

Age and formation mechanism of the Malashan high-Ca two-mica granite within the Northern Himalayan Gneiss Domes,southern Tibet

马拉山片麻岩穹窿位于特提斯喜马拉雅带内,由马拉山二云母花岗岩、错布二云母花岗岩和派枯错复合淡色花岗岩组成。马拉山二云母花岗岩东西展布约10km,锆石U-Pb分析表明,马拉山二云母花岗岩的结晶时间较长,从17.6Ma到16.9Ma,或者至少是两次深熔作用的产物,分别发生在17.6Ma和16.9Ma。全岩主量元素、微量元素和Sr、Nd、Hf同位素分析表明马拉山二云母花岗岩是一个较均一岩体,具有以下特征:(1)高SiO2,Al2O3和相对较高的CaO(1.2%～2.0%);(2)较高的Sr,较低的Rb和Rb/Sr比值(<1.3),且随着Ba浓度的增加,Rb/Sr比值保持不变;(3)高度变化的Zr/Hf比值(25.9～39.9);(4)富集轻稀土,亏损重稀土,几乎无或弱的负Eu异常;(5)较一致的Sr和Nd同位素组成;(6)锆石岩浆增生边和继承性锆石的Hf同位素比值高度变化,εHf(t)分别为-20.4～-8.0和-27.2～-9.5。这些特征暗示马拉山二云母花岗岩是变泥质岩在较高温压条件下水致白云母部分熔融的产物,与藏南裂谷系的东西向伸展作用密切相关。

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.