特提斯喜马拉雅马拉山花岗岩的年代学、地球化学特征及成因机制

马拉山花岗岩位于特提斯喜马拉雅的西部,其主要矿物组成为石英、钾长石、白云母和黑云母。锆石LA-MC-ICP-MS U-Pb定年表明,花岗岩的发育记录了(28.0±0.5)Ma和(18.4±0.3)Ma两期深熔作用,(18.4±0.3)Ma代表了最终的结晶时间。全岩地球化学分析结果显示,样品具有高的SiO2(72.36%~72.51%)、Al2O3(15.22%~15.37%)和CaO(1.64%~1.66%)含量,高的K2O/Na2O值(0.97~1.05)和A/CNK值(1.15~1.20),显示高钾钙碱性过铝质的特征;岩石富集Rb、Th、U和K,亏损Ba、Nb、Sr和Zr,Eu负异常不明显(δEu=0.80~0.89),轻重稀土分馏较强[(La/Yb)N=7.09~19.68]。马拉山花岗岩具有较低的Rb/Sr值(0.90~1.10)和较高的CaO/Na2O值(0.44~0.46),指示岩浆源区物质成分可能以页岩为主;样品(87Sr/86Sr)i和εNd(t)分别为0.742522~0.744097和-14.5~-13.7,与大喜马拉雅结晶杂岩中变质沉积岩成分一致,表明其来自变质沉积岩的部分熔融。岩石具有较低的(87Sr/86Sr)i和较高的Sr含量,且随着Ba含量的增加,Rb/Sr值基本不变,表明马拉山花岗岩是水致白云母部分熔融的产物,部分熔融可能与南北向裂谷的东西向伸展关系密切。

Geochronology,Geochemistry and Formation Mechanism of Malashan Granite in Tethyan Himalaya

The Malashan granite was exposed in the west of the Tethyan Himalayan sedimentary sequence(THS).It is characterized by gneissic texture and the assemblage of quartz, K-feldspar, muscovite and biotite.LA-MC-ICP-MS zircon U-Pb dating results indicated that the granite recorded two episodes of anatexis at(28.0±0.5) Ma and(18.4±0.3) Ma, respectively.The age of(18.4±0.3) Ma represents the final crystallized time of Malashan granite.Geochemical data show that these samples are characterized by high SiO2(72.36%-72.51%), Al2O3(15.22%-15.37%), CaO(1.64%-1.66%) and high value of K2O/Na2O(0.97-1.05) and A/CNK(1.15-1.20), and the enrichment in Rb, Th, U and K, the depletion in Ba, Nb, Sr and Zr, weak negative Eu anomalies(δEu=0.80-0.89), and strong fractionation between LREE and HREE((La/Yb)N=7.09-19.68).These features suggest that they are high potassium calc-alkaline and peraluminous granites.The relatively low Rb/Sr ratios(0.90-1.10)and high CaO/Na2O ratios(0.44-0.46)imply that the magma source was probably psammitolite.The(87Sr/86Sr)I(0.742522-0.744097) and εNd(t)(-14.5--13.7) can compare well with those of the metasedimentary rocks in the Greater Himalaya Crystalline complex(GHC), so these granite wasgenerated from partial melting of the GHC metasedimentary rocks.The features of relatively low(87Sr/86Sr)i and high Sr content,and the constant Rb/Sr ratios relative to large variations in Ba concentrations approve that, the Malashangranite wasderived from fluxed melting of the GHC metasedimentary rocks,possibly associated tightly with the E-W extension along the North-South trend rift system(NSTR).