【作者】 张晓倩；

【导师】 朱弟成； 赵志丹；

【作者基本信息】 中国地质大学(北京) ， 矿物学、岩石学、矿床学， 2012， 硕士

【副题名】对中部拉萨地块早白垩世花岗岩类岩石成因的约束

【摘要】 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可靠地约束拉萨地块白垩纪时期的岩浆作用过程及成矿地质背景。本文报道了中部拉萨地块西部措勤地区代表性花岗岩基——措勤麦嘎岩基的锆石U-Pb年代学、元素地球化学、Sr-Nd同位素和锆石Hf同位素数据。这些新数据，结合西部措勤尼雄岩体、中部申扎岩体和东部桑巴岩体数据，讨论了中部拉萨地块早白垩世大规模花岗岩类的岩浆起源、岩石成因及其对拉萨地块白垩纪成矿地质背景的约束。麦嘎岩基由二长花岗岩、花岗闪长岩和大量闪长质包体组成，其中的花岗质岩石主要侵位于122±1Ma和113±2Ma，闪长质包体与后者同期（113±2Ma）。122±1Ma花岗质岩以富硅、高钾和较高铝饱和指数为特征，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。113±2Ma寄主花岗质岩为偏铝质－弱过铝质高钾钙碱性I型花岗岩，相对于122±1Ma花岗质岩石，其(⁸⁷Sr/⁸⁶Sr)i偏低（0.7094⁰.7156）、全岩εNd（t）（-12.1^-7.3）和锆石εHf（t）（-11.1⁰.1）较高。闪长质包体（113±2Ma）为偏铝质中－高钾钙碱性系列，具变化范围大的(⁸⁷Sr/⁸⁶Sr)i（0.7058⁰.7105）、负的全岩εNd（t）（-10.7^-9.8）及负的锆石εHf（t）（-14.0^-5.6）。本论文的综合分析表明，从中部拉萨地块西部、中部到东部，广泛发育122±3Ma和113±3Ma的岩浆活动，并且从西至东，SiO2、K2O及A/CNK含量逐渐增加，Al2O3、TFe2O3、MgO、CaO以及Mg#逐渐降低；轻重稀土分馏程度及Eu负异常程度也逐渐升高。西部措勤和中部申扎地区以I型花岗岩类为主，很可能来自古老中下地壳物质的部分熔融，与壳－幔岩浆不同比例混合有关；东部桑巴地区以S型花岗岩为主，来自加厚地壳的部分熔融，幔源物质的贡献量相对较小。西部与东部这种差异，很可能指示其基底性质和岩石圈结构有所不同，这种不同可能正是现今拉萨地块地质、地球物理和成矿特征存在东西差异的原因。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，进一步证实南向俯冲的班公湖－怒江洋壳岩石圈板片发生了断离。更多还原

【Abstract】 The magma source and petrogenesis of Early Cretaceous granitoids whichwidely exposed in the Central Lhasa Terrane, Tibet, remain unconstrainted so far. Abetter understanding of these issues will help to reveal magmatic processes andmineralization background of the Lhasa Terrane during the Cretaceous. This thesisreports zircon U-Pb age, bulk-rock geochemical, Sr-Nd isotope data, and zircon Hfisotope data from the Maiga batholith, one of the representative Early Cretaceousbatholiths in the west areas of the Central Lhasa Terrane. These new data, incombination with the data of the Nixiong pluton in the west, Xainza pluton in themiddle, and Sangba pluton in the east of the Central Lhasa Terrane, the origin andpetrogenesis of these plutons and their constraints on the mineralization backgroundof the Lhasa Terrane have been explored.The Maiga batholith is mainly composed of monzogranite and granodiorite, withabundant dioritic enclaves. Zircon U-Pb dating for the Maiga batholith yieldsemplacement ages of122±1Ma and113±2Ma for granitic rocks. The dioriticenclaves have been dated to be113±2Ma. Granitoids of122±1Ma arecharacterized by high-Si, high-K, and high aluminum saturation index （A/CNK）, andare slightly peraluminous and high-K calc-alkaline I-type granites. These rocksexhibit high （87Sr/86Sr）i（ca.0.7147） values, low negative bulk-rock εNd（t）（ca.-12.0）and zircon εHf（t）（-15.7^-11.1）. Host granitoids of113±2Ma are metaluminous andhigh-K calc-alkaline I-type granite. Compared to granitoids of122±1Ma, they showrelatively low （87Sr/86Sr）i（0.7094⁰.7156） and enhanced bulk-rock εNd（t）（-12.1^-7.3） and zircon εHf（t）（-11.1⁰.1）. Dioritic enclaves （113±2Ma） are metaluminousand medium-to high-K calc-alkaline, and are characterized by varying （87Sr/86Sr）i（0.7058⁰.7105）, negative bulk-rock εNd（t）（-10.7^-9.8）, and zircon εHf（t）（-14.0^-5.6）.A synthetical compiliation for existing data indicate that the122±3Ma and113±3Ma magmatic rocks are widely exposed in the Central Lhasa Terrane from thewest, via middle to the east of this terrane. We found that SiO₂, K₂O₃, A/CNK, and the degrees of REE fractionation and Eu anomaly increase from west to east, while Al₂O₃,TFe₂O₃, MgO, CaO, and Mg decrease from west to east. Existing data reveal that thegranitoids from Coqen in the west and Xainza in the middle of the Central LhasaTerrane are mainly composed of I-type granites, which can be accounted for byanatexis of an ancient middle-lower crust with varying contributions of basaltic melts.However, the coeval granitoids from Sangba in the east of the Central Lhasa Terraneare mainly composed of S-type granites derived from partial melting of a thickenedcrust, with insignificant input of basaltic melts. Thesse differences are likelycontrolled by the different nature of basement and lithospheric architecture beneaththe Central Lhasa Terrane, possibly resulting in the differences of geological,geophysical, and metallogenic features observed in the present-day Central LhasaTerrane from west to east. The increased contributions of basaltic melts at about113Ma observed in the Maiga batholith and the other coeval batholiths in the CentralLhasa Terrane further verify the model of slab break-off of the southward subductionof the Bangong Nujiang Ocean seafloor at that time.更多还原