内蒙古锡林郭勒杂岩中残留基底的岩石学、地球化学及年代学研究

The Xilin Gol Complex, consisting of strongly deformed and metamorphosed rocks, is exposed as a large tectonic unit within the Central Asian Orogenic Belt (CAOB). There are two interpretations of the age and origin of the complex: one suggests it is a Late Archaean - Late Proterozoic terrane that underwent primary metamorphism in the end of Late Proterozoic or the end of Early Paleozoic; the second suggests it is Paleozoic forearc sediments in the northern margin of the North China Plate and subsequently metamorphosed in the Late Paleozoic. As an old and large geological unit within the CAOB, the genesis and tectonic origin of the complex are crucial to understanding the attributes of Precambrian blocks, whose existence or not determine the model of a single arc chain by successive accretion at the southern margin of Siberia Plate or of multiple subduction zones with different subduction polarities of arc terranes and microcontinents that rifted off the margins of Gondwana. On the basis of detailed field work, we disintegrate the Xilin Gol Complex, choose residual basements in it to study their characteristics of petrology, geochemistry and geochronology, and reconstruct the metamorphic P-T-t path of residual basements by software Perple_X to determine their age, attribute and metramorphic history. The data lead to new information on times and attributes of Precambrian microcontinent in the Xilin Gol area, with implications for the tectonic style of the CAOB.

锡林郭勒杂岩是中亚造山带内出露面积较大的变形变质地层单位，对其时代和属性存在两种认识：一种认为它是晚太古代-晚元古代之间的古老地体，变质作用发生在晚元古代末期或早古生代后期；另一种认为它是华北板块北缘一套经历过强变形变质的古生代弧前浊积岩建造。查明其原岩性质和变质过程对于认识中亚造山带内前寒武纪地体的属性具有关键意义，而前寒武纪地体的属性决定了中亚造山带是形成于西伯利亚板块南缘一个长期存在的单一的俯冲增生体系，还是由冈瓦纳古陆边缘裂解出来的众多不相关的岛弧地体和微陆块拼贴而成。本课题通过详细的野外考察对锡林郭勒杂岩中的各地质体进行解体，并以其中的残留基底为对象进行岩石学、地球化学和年代学研究，应用Perple_X软件建立基底变质演化的P-T-t轨迹，以查明锡林郭勒杂岩中残留基底的时代、属性和变质变形历史，进而确定锡林浩特地区前寒武纪地块的时代和属性，为中亚造山带构造样式的确定提供依据。

锡林郭勒杂岩是中亚造山带内出露面积较大的变形变质地层单位，对其时代和属性存在两种认识：一种认为它是晚太古代-晚元古代之间的古老地体，变质作用发生在晚元古代末期或早古生代后期；另一种认为它是华北板块北缘一套经历过强变形变质的古生代弧前浊积岩建造。查明其原岩性质和变质过程对于认识中亚造山带内前寒武纪地体的属性具有关键意义。.本课题通过详细的野外考察对锡林郭勒杂岩进行了解体，期望得到杂岩中保留的前寒武纪残留基底的信息。研究发现，锡林郭勒杂岩主要由长英质片麻岩及磁铁石英岩组成，夹有透镜状及似层状的角闪岩，同时被后期大量的岩浆岩脉体侵入。片麻岩的原岩为晚石炭-早二叠世在南蒙微陆块南缘弧前盆地内形成的一套富铁砂页岩，据其中最年轻的碎屑锆石判断其固结成岩的时限应晚于302Ma。片麻岩内包含有大量的晚寒武-早泥盆世岩浆锆石（~494-402Ma，εHf(t)=-12.82~+7.72，TDM2=2252~960Ma）及前寒武纪碎屑锆石（~3141-837Ma，εHf(t)=-17.84~+10.57，TDM2=3209~1389Ma），分别来源于中亚造山带北向俯冲带内的宝力道弧岩浆岩带和南蒙微陆块基底，而无明显的华北板块的物质记录，表明索伦缝合带在晚石炭-早二叠世已发展成为阻隔北向俯冲带和南向俯冲带的屏障。片麻岩中黑云母40Ar-39Ar年龄介于301-282Ma，反映其原岩发生角闪岩相变质的年龄。杂岩中角闪岩的原岩为辉长-辉长闪长岩，据其锆石U-Pb年龄可分为两期，分别为晚泥盆世早期（~382Ma，εHf(t)=-1.06~+1.25，TDM1=1044~958Ma）和早石炭世晚期-晚石炭世早期（~340-321Ma，εHf(t)=+2.89~+12.98，TDM1=455~855Ma）。两期岩浆均来源于俯冲板片交代过的富集地幔。角闪岩中角闪石及黑云母40Ar-39Ar年龄介于306-265Ma，与片麻岩变质作用时代相当。.综合认为锡林郭勒杂岩不能代表前寒武纪残留基底，而是古亚洲洋洋壳向南蒙微陆块底部俯冲过程中形成的一套岛弧杂岩。杂岩中无明确的信息可以指示锡林浩特地区前寒武纪基底的存在。其中的碎屑物质主要来源于南蒙微陆块及其活动陆缘，指示晚古生代索伦缝合带已初步形成并阻断了北向俯冲带和南向俯冲带。该缝合带最终形成时代为中二叠世-中三叠世（~263-228Ma）。