北喜马拉雅恰芒巴二云母花岗岩的年龄及形成机制

恰芒巴二云母花岗岩体位于特提斯喜马拉雅的西部, 岩石发育片麻状构造, 主要矿物组成为石英、钾长石、白云母和黑云母。LA-MC-ICP-MS U-Pb定年显示, 锆石年龄分布范围为35.1~17.3 Ma, 暗示较长时间的深熔作用过程, 其中最年轻的年龄(18.1±0.4 Ma)代表了花岗岩的最终结晶年龄。地球化学分析表明, 岩石具有高的SiO₂(73.06%~73.79%)、Al₂O₃(14.73%~15.06%)和CaO(1.18%~1.24%)含量, 以及高的K₂O/Na₂O值(1.16~1.25)和A/CNK值(1.16~1.20), 属于高钾钙碱性过铝质花岗岩。岩石强烈富集Rb、Th、U和K, 而亏损Ba、Nb、Sr和Zr, 轻重稀土分馏较强(La/Yb)_N=9.98~11.35, 并显示较弱的负Eu异常(δEu=0.70~0.74)。(⁸⁷Sr/⁸⁶Sr)_i和ε_Nd(t)值分别为0.742 298~0.743 092和-14.1~-14.0, 可与大喜马拉雅结晶杂岩(GHC)中变质沉积岩对比, 推测源岩为GHC变质沉积岩或与之成分相当的岩石。岩石(⁸⁷Sr/⁸⁶Sr)_i值较低而Sr浓度较高, 随着Ba浓度的增加, Rb/Sr值基本不变, 与水致白云母部分熔融的趋势一致, 推测恰芒巴二云母花岗岩可能是水致白云母部分熔融的产物, 部分熔融作用可能与藏南拆离系的活动密切相关。

Age and formation mechanism of the Qiamangba two-mica granite in northern Himalaya

The Qiamangba two-mica granites, located in the west of the Tethyan Himalayan sedimentary sequence(THS), are characterized by gneissic texture and the assemblage of quartz, K-feldspar, muscovite and biotite. LA-MC-ICP-MS zircon U-Pb dating reveals that the Qiamangba two-mica granites were crystallized over relatively long time span from 35.1~17.3 Ma and the youngest age of 18.1±0.4 Ma probably represented the final crystallized age. Bulk-rock major and trace elements analyses indicate that the granites are characterized by high SiO₂(73.06%~73.79%), Al₂O₃(14.73%~15.06%), CaO(1.18%~1.24%)and high value of K₂O/Na₂O(1.16~1.25), A/CNK(1.16~1.20), and enrichment in Rb, Th, U, K, depletion in Ba, Nb, Sr, Zr and weak negative Eu anomalies(δEu=0.70~0.74), and strong fractionation between LREE and HREE(La/Yb)_N=9.98~11.35. These features demonstrate they are high potassium calc-alkaline and peraluminous granites. The values of(⁸⁷Sr/⁸⁶ Sr)_i and ε_Nd(t)range from 0.742 298 to 0.743 092 and from -14.1 to -14.0, and can compare well with those of the metasedimentary rocks in the Greater Himalaya crystalline complex(GHC). We speculated the two-mica granites were generated from partial melting of the GHC metasedimentary rocks or other rocks with the similar geochemical compositions. The granites show relatively high Sr, but low Rb and Rb/Sr which are nearly constant relative to large variations in Ba concentrations. These features are concordant with the trend of fluxed muscovite partial melting. Based on the above evidences, we suggest the Qiamangba two-mica granites were derived from fluxed partial melting of the GHC metasedimentary rocks, possibly related tightly with the top-down-to-north detachment along the south Tibet detachment systems.