西藏冈底斯带东段早侏罗世英云闪长岩的岩浆起源及其对拉萨地体地壳演化的意义

Magma Origin of Early Jurassic Tonalites in the Eastern Gangdese Magmatic Belt,Southern Tibet and Its Implications for the Crustal Evolution of the Lhasa Terrane

西藏冈底斯岩浆岩带除了大范围分布的晚白垩世以来的花岗岩类以外，还发育少量晚三叠世-侏罗纪花岗岩，目前关于其岩石成因还存在很多争议。为了进一步认识冈底斯早侏罗世花岗岩的岩浆起源，本文选择代表性的加查县香木村英云闪长岩开展了岩相学、岩石地球化学、锆石U-Pb年代学和Hf同位素研究。所研究的英云闪长岩主要由斜长石、石英、黑云母和少量的角闪石组成。LA-ICP-MS锆石U-Pb定年结果表明其结晶年龄为184±4Ma。锆石εHf（t）值均为正值，变化范围为7.9～10.3。在全岩地球化学组成上，英云闪长岩为中钾钙碱性、准铝质，并具有埃达克质岩石特征：高SiO2、Al2O3、Na2O/K2O（〉1）和Sr/Y（35.5～106），低Y（4.46×10-6～12.9×10-6）和Yb（0.51×10-6～1.39×10-6），富集Rb、Ba、K、Pb等大离子亲石元素，亏损Nb、Ta、Ti等高场强元素。在稀土元素配分图中呈现轻稀土元素富集、中稀土亏损的右倾“凹”型的式样，无明显Eu异常。岩石地球化学和锆石Hf同位素特征表明其岩浆可能起源于下地壳基性物质的部分熔融，残留相主要为角闪石、石榴子石，无斜长石。综合前人资料，冈底斯晚三叠世—早侏罗世花岗岩的锆石Hf同位素具有由西向东、由南向北逐渐富集的规律，反映古老地壳组分的贡献逐渐增多，这暗示了南冈底斯带东部也可能存在古老的地壳基底。

Besides the widespread granitoids formed during Late Cretaceous, the late Triassic to Jurassic granites are also known to occur in the Gangdese magmatic belt. However, their petrogenesis remains a controversial issue. In this study, we conducted petrological, geochemical, zircon U-Pb dating and Hf isotopic studies for the Xiangmucun tonalites from the eastern parts of southern Gangdese magmatic belt. The tonalites consist mainly of plagioclase, quartz, biotite with minor amphibole. LA-ICP-MS zircon U-Pb dating yielded a crystallization age of ca. 184±4 Ma for the tonalites. They have positive εHf（t） values ranging from 7.90 to 10.26. The geochemical data further indicate that the Xiangmucun tonalites are middle K calc-alkaline and metaluminous, and generally show characteristics of adakitic rocks, i.e., the high SiO2, Al2O3, Nae2O/K2O （〉1） and Sr/Y （35.5～105.6）, low Y （4.46×10-6～12.9×10-6） and Yb （0.51×10-6～1.39×10-6）, and enrichment in large ion lithophile elements （LILE） of Rb, Ba and K, and depletion in high field strength elements （HFSE） of Nb, Ta, Ti and P. They exhibit concave-upward LREE-riched and MREE-depleted patterns with negligible Eu anomalies. We suggest that the Xiangmucun tonalites were derived from the partial melting of mafic lower crust, with amphibole and garnet as the main residual phases and free of plagioclase. Combined with previous data, the zircon Hf isotopes of the Gangdese Late Triassic-Jurassic granites show a gradual enrichment trend from west to east and from south to north, which indicates that the contribution of ancient crustal components gradually increased and the ancient crust basement may exist in the eastern parts of the southern Gangdese belt.