阿尔金构造带西段前寒武纪花岗质片麻岩的地球化学特征及其构造背景

Geochemical Characteristics and Tectonic Setting of Precambrian Granitic Gneiss in the Western Segment of Altyn Tagh Tectonic Belt

阿尔金构造带西段花岗质片麻岩侵入到下元古界阿尔金群和长城系巴什库尔干群、蓟县系塔昔达坂群中,主要由黑云二长—钾长花岗质片麻岩组成。其主量元素特征表明,岩石具有富硅和富铝的特征,w(Al2O3)、w(TFeO)、w(TiO2)、w(MgO)、w(CaO)、w(P2O5)与w(SiO2)呈明显的负相关关系,而w(K2O)与w(SiO2)呈正相关关系,反映它们为同来源和同时代的产物。其w(K2O)/w(Na2O)=1.16～2.65,A/NCK大部分为1.05～1.18,里特曼指数δ=1.31～2.01,表明属高钾钙碱性系列。其稀土元素球粒陨石标准化曲线总体呈右倾斜的"V"型特点;洋中脊花岗岩(ORG)标准化蛛网图上显示出强烈富集大离子亲石元素(Rb、Th),而高场强元素Hf、Ta、Nb和Zr则具不同程度的贫化现象,显示出同碰撞花岗岩的特征;在R1-R2构造环境判别图上样品点落于同碰撞花岗岩区。其Rb-Sr全岩等时线年龄为(1035±77)Ma,表明阿尔金山地区存在有1000Ma左右格林威尔(Grenville)期碰撞造山作用的岩浆活动记录,说明塔里木板块和柴达木微板块之间在新元古代早期曾经存在板块的汇聚碰撞,这对于研究阿尔金地区乃至整个西北地区中元古代到新元古代的构造演化具有重要的意义。

Precambrian granitic gneiss in the western segment of Ahyn Tagh tectonic belt intruded in Paleoproterozoic Ahyn Tagh group, Changchengian System Bashenkuergan Group and Jixianian System Taxidaban Group, is mainly consisted of biotite monzonitic granitic gneiss and biotite syenogranitic gneiss. The contents of major elements indicate that this granitic gneiss is of peraluminous type characterized by rich Si and Al, and the contents of Al2O3, TFeO, TiO2, MgO, CaO and P2O5 is negatively correlated to that of SiO2, and K2O is positively correlated to SiO2, which indicates that they are homology and contemporaneousness. The results of K2O/Na2O =1.16 -2. 65, A/NCK = 1.05 - 1.18, and δ = 1.31 -2. 01 reveal that the granitic gneiss belongs to high-K-calc-alkalic type. The chondrite-normalized REE patterns are right-inclined, with a clear V trough ; the ocean ridge granites-normalized trace element spidergrams are characterized by enriched LILE （ such as Rb and Th） and poor HFSE（ such as Hf, Ta, Nb and Zr） , belonging to syn-collision granites. On the R1-R2 tectonic discrimination diagram the samples are plotted in the typical continent-continent syn-collisional granite field. The Rb-Sr isotopic data from granitic gneiss that yields an whole rock isochron age of （1,035±77） Ma, indicate the existence of Grenville period（1,000 Ma ± ） magmatic activity in Altyn Tagh area, it means that a collision and convergence between the Qaidam microplate and the Tarim plate has taken place during the Neoproterozoic time. This study has important significance to the study of the tectonic evolution of the ancient continents in Altyn Tagh area or even northwestern China during Mesoproterozoic Era to Neoproterozoic Era.