西藏措勤麦嘎岩基的锆石U-Pb年代学、地球化学和锆石Hf同位素:对中部拉萨地块早白垩世花岗岩类岩石成因的约束

Geochemistry,zircon U-Pb geochronology and in-situ Hf isotope of the Maiga batholith in Coqen,Tibet:Constraints on the petrogenesis of the Early Cretaceous granitoids in the central Lhasa Terrane

西藏中部拉萨地块大规模早白垩世花岗岩类的岩浆源区和岩石成因迄今尚未得到很好约束,对这些问题的深入理解将有助于揭示拉萨地块白垩纪时期的岩浆作用过程及成矿背景。本文报道了中部拉萨地块代表性花岗岩基——措勤麦嘎岩基的锆石U-Pb年代学、全岩元素地球化学、Sr-Nd同位素和锆石Hf同位素数据。本文锆石U-Pb定年结果表明,麦嘎岩基花岗质岩主要侵位于122±1Ma和113±2Ma,闪长质包体与后者同期(113±2Ma)。122±1Ma花岗质岩属I型弱过铝质高钾钙碱性系列,(87Sr/86Sr)i值高(0.7147),全岩εNd(t)(-12.0)和锆石εHf(t)(-15.7～-11.1)为较大的负值,表明其很可能来源于古老下地壳物质的重熔。113±2Ma寄主花岗质岩为I型偏铝质-弱过铝质高钾钙碱性系列,相对于122±1Ma花岗质岩石,其(87Sr/86Sr)i比值偏低(0.7094～0.7156)、全岩εNd(t)值(-12.1～-7.3)和锆石εHf(t)值(-11.1～0.1)较高,很可能来源于古老下地壳物质的部分熔融,并含有更多幔源物质。闪长质包体(113±2Ma)为偏铝质中-高钾钙碱性系列,以变化范围大的(87Sr/86Sr)i(0.7058～0.7105)、负的全岩εNd(t)值(-10.7～-9.8)及负的锆石εHf(t)值(-14.0～-5.6)为特征,可能是古老富集岩石圈地幔物质部分熔融的产物或亏损地幔物质经历强烈地壳混染作用的结果。在目前已有资料条件下(缺乏同期基性岩石的相关数据),本文暂将麦嘎岩基113±2Ma寄主花岗质岩及同期闪长质包体解释为镁铁质岩浆与长英质岩浆发生不同程度岩浆混合作用的产物,这一解释可能对中部拉萨地块同期花岗类的岩石成因具普遍意义。麦嘎岩基及中部拉萨地块同期岩浆岩约113Ma幔源物质增加现象,可能是南向俯冲的班公湖-怒江洋壳岩石圈板片断离的结果。

The magma source and petrogenesis of the Early Cretaceous granitoids,which widely exposed in the central Lhasa Terrane,Tibet,remain unconstrained so far.A better understanding of such issues will help us to reveal the magmatic processes and mineralization setting of the Lhasa Terrane during the Cretaceous.This paper reports zircon U-Pb age,whole-rock geochemistry,Sr-Nd isotopic data and zircon Hf-isotope data from the Maiga batholith,which is one of the representative Early Cretaceous batholiths in the central Lhasa Terrane.Zircon U-Pb dating indicates that the granitic rocks in the Maiga batholith were emplaced at 122 ± 1Ma and 113 ± 2Ma,respectively,in which the late phase is also the emplacement timing of the dioritic enclaves（113 ± 2Ma） within this batholith.The ca.122Ma granitic rocks are slightly peraluminous and high K calc-alkaline I-type granites and are characterized by high（87Sr/86Sr） i（0.7147） ratio,negative whole-rock εNd（t）（-12.0） value,and negative zircon εHf（t）（-15.7 ～-11.1） values,indicating that they were derived from antaxis of ancient lower crust materials.The 113 ± 2Ma host granitoids are metaluminous to slightly peraluminous and high K calc-alkaline I-type granite and exhibit low（87 Sr/86 Sr） i（0.7094 ～ 0.7156）,enhanced whole-rock εNd（t）（-12.1 ～-7.3）,and zircon εHf（t）（-11.1 ～ 0.1） in relative to the 122 ± 1Ma granitic rocks,suggesting that they were generated by partial melting of ancient lower crust with increased constributions from mantle-derived magmas.The 113 ± 2Ma dioritic enclaves are metaluminous and medium-high K calc-alkaline and are characterized by a wide range of（87Sr/86Sr） i（0.7058 ～ 0.7105） ratios,negative whole-rock εNd（t）（-10.7 ～-9.8） values,and zircon εHf（t）（-14.0 ～-5.6） values,indicating that these enclaves were derived from either partial melting of ancient enrichment lithospheric mantle materials or depleted mantle-derived melts that have mixed signicantly with mature continental ctustal materials.Considering the data currently available（e.g.,the absence of data of coeval mafic rocks）,the 113 ± 2Ma granitoids（including host granites and coeval dioritic enclaves） in the Maiga batholith are tentatively interpreted as the consequences of varying extents of mixing between the continental crust-derived silicic melts and mantlederived melts.Such interpretation may be applicable to the petrogenesis of the coeval granitoids in the central Lhasa Terrane.Theincreased contributions of mantle-derived materials at about 113Ma observed in the Maiga batholith and granitoids from other areas in the central Lhasa Terrane can be attributed to the slab break-off of the southward subduction of the Bangong-Nujiang Ocean seafloor.