西藏拉萨地块盐湖石英闪长岩成因:锆石SHRIMP U-Pb年代学、地球化学及Sr-Nd-Pb-Hf同位素的制约

Petrogenesis of the quartz diorite in the Yanhu area of Lhasa terrance,Tibet:constraints from zircon SHRIMP U-Pb geochronology,geochemistry and Sr-Nd-Pb-Hf isotopes

西藏阿里盐湖地区发育不同类型的火成岩,通常被认为形成于班公湖-怒江特提斯洋南向俯冲板片发生断离的构造背景。本文报道了盐湖石英闪长岩的锆石SHRIMP U-Pb年龄,全岩地球化学以及Sr-Nd-Pb-Hf同位素数据。盐湖石英闪长岩的锆石^(206)Pb/^(238)U加权平均年龄为118.5±1.9Ma,属早白垩世晚期。石英闪长岩样品主量元素表现为富钠的钙碱性岩石系列,A/CNK值介于0.84～0.89之间,属准铝质。微量元素富集Rb、K、Th、U等大离子亲石元素(LILE)和轻稀土元素(LREE),强烈亏损Nb、Ta、Zr、Hf、P、Ti等高场强元素(HFSE)和重稀土元素(HREE),无明显的Eu异常(δEu=0.81～0.98),无白云母和碱性暗色矿物,属准铝质未分异的I型花岗岩。盐湖石英闪长岩的(^(87)Sr/^(86)Sr)_i值为0.7045～0.7048,ε_(Nd)(t)值介于+0.5～+2.1之间,ε_(Hf)(t)值介于+7.9～+13之间,Hf同位素模式年龄t_(DM2)变化于283～674Ma之间。基于同位素以及岩石地球化学数据,表明盐湖石英闪长岩很可能是具亏损地幔印记的新生下地壳在角闪岩相发生部分熔融作用形成的。结合本文以及区域上的研究资料,盐湖石英闪长岩的形成时代和岩石地球化学特征与盐湖复式岩体中的花岗岩存在较大差异,具较低的锆石饱和温度(596～614℃),表明该岩体的形成尚未受到幔源物质上涌带来的热作用影响,很可能形成于板片断离前,班公湖-怒江特提斯洋持续南向俯冲的板片发生折返,俯冲角度变陡的深俯冲背景。

The compositional diversity of rock types in the Yanhu area is usually considered as the tectonic setting that southward subduction of the Bangong-Nujiang Tethyan Ocean lithosphere broken off. In this paper, we report for the first time the zircon SHRIMP U-Pb age and Hf isotopic composition data, whole-rock major and trace element composition data, and Sr-Nd-Pb-Hf isotopic data from Yanhu quartz diorite. SHRIMP U-Pb dating yields a weighted mean of 118.5±1.9 Ma,indicating that Yanhu quartz diorite belongs to late Early Cretaceous. Geochemically, the quartz diorite from Yanhu belongs to Na-rich calc-alkali series, A/CNK=0.84~0.89 and are metaluminous. The quartz diorite enriched in Rb、K、Th、U and light rare earth elements(LREEs), relatively depleted in Nb、Ta、Zr、Hf、P、Ti and heavy rare earth elements(HREEs), without Eu anomalies(δEu=0.81~0.98). No alkaline mafic minerals and muscovite were observed duringoptical microscopy. All these features above suggest that quartz diorite belongs to metaluminous unfractionated I-type granite. The quartz diorite yielded whole-rock εNd(t) of +0.5^+2.1 and(87Sr/86Sr)i 0.7045~0.7048, the zircons εHf(t) values are +7.9^+13, and the model ages(tDM2) range 283 Ma to 674 Ma. Based on integrated isotopic and geochemical data, we infer that the quartz diorite formed by the partial melting of juvenile lower crustal material derived from depleted mantle under amphibolite facies. Based on the above data, combined with the regional geological background, we conclude that the quartz diorite are differernt from granites in Yanhu composite body in age and geochemical characteristics, have relative low zircon saturated temperature(596~614℃), suggesting mantle-derived materials had not involved in the petrogenesis of the Yanhu quartz diorite, the parent magma may formed before the slab broke off, sustainable subducted slab of Bangong-Nujiang Tethyan ocean had returned and the subduction angle becomes steep, enter geodynamic movement of deep subduction.