Age and formation mechanism of the Qiamangba two-mica granitein northern Himalaya
Wang Xiaoxian1,2, Zhang Jinjiang2, Yan Shuyu2, Liu Jiang3, Wang Jiamin2, Guo Lei3
1. Key Laboratory of Crustal Dynamics, Institute of Crustal Dynamics, China Earthquake Administration, Beijing 100085;
2. Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871;
3. State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037
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 εNd(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.