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

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

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SiO_2(72.36%~72.51%)、Al_2O_3(15.22%~15.37%)和CaO(1.64%~1.66%)含量,高的K_2O/Na_2O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/Na_2O值(0.44~0.46),指示岩浆源区物质成分可能以页岩为主;样品(^(87)Sr/^(86)Sr)_i和ε_(Nd)(t)分别为0.742 522~0.744 097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(^(87)Sr/^(86)Sr);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。

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%）,A12O3（15.22%-15.37%）,CaO（1.64%-l.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（87^Sr//86^Sr）1（0.742 522-0.744 097） 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（87^Sr/86^Sr,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）.