藏东贡觉—白玉地区金沙江构造混杂岩带物质组成的厘定

Determination of material composition of Jinshajiang tectonic mélange belt in Gonjo-Baiyu area, eastern Tibet

关键词:  藏东  金沙江构造混杂岩带  古特提斯  构造岩块  榴辉岩

Key words:  Eastern Tibet  Jinshajiang tectonic mélange belt  Paleo Tethys  tectonic blocks  eclogite

摘要:

金沙江构造混杂岩带保存了大量与古特提斯洋的形成与扩张、洋-陆俯冲、洋盆闭合和陆-陆碰撞等地质过程相关的变形、变质和岩浆作用的重要信息，是研究碰撞造山作用和板块构造的重要视窗。本文主要基于对藏东贡觉—白玉地区开展的1∶5万区域地质调查工作，结合室内薄片鉴定、岩石地球化学分析和年代学测试等结果，对金沙江构造混杂岩带的物质组成和构造变形特征进行了精细厘定。藏东贡觉—白玉地区金沙江构造混杂岩带主要由一系列规模不等、岩性不一和变形程度不同的构造岩块及强变形的基质组成；各岩块之间由诸多近平行于构造带走向的韧性-脆韧性断层呈网结状连接，表现出局部有序、整体无序的构造混杂岩特征。构造岩块主要包括：超基性岩-基性岩、大理岩、大理岩+变质基性岩、榴闪岩或榴辉岩、变质硅-灰-泥质岩等。基质物质主要包括含石榴子石片岩和不含石榴子石片岩两类。岩石地球化学分析数据揭示出基性岩岩块原岩具有富集型洋中脊或正常型洋中脊基性岩(E-MORB、N-MORB)的特征；洋岛-海山岩块中的变质基性火山岩具有洋岛玄武岩特征(OIB)；榴辉岩的原岩具有洋岛型玄武岩(OIB)或正常型洋中脊玄武岩(N-MORB)特征。结合前人研究成果，可以精细划分出金沙江古特提斯构造带经历了裂(陷)谷盆地(D)、洋盆扩张(C₁—P₁)、洋壳俯冲消减(P₂—P₃)和弧陆碰撞造山(T₁—T₂)、上叠火山裂谷盆地阶段(T₂²—T₃¹)5个主要演化阶段。

Abstract:

The Jinshajiang tectonic mélange belt has preserved a large amount of important information on deformation, metamorphism and magmatism related to the formation and expansion of the paleo Tethys, ocean-continent subduction, ocean basin closure and continent-continent collision, and is an important window to study collision orogeny and plate tectonics. Based on the 1∶50000 regional geological survey of Gonjo-Baiyu area in eastern Tibet, combined with the results of indoor thin section identification, rock geochemical analysis and chronological test, this paper makes a detailed determination of the material composition and structural deformation characteristics of Jinshajiang tectonic mélange belt. The Jinshajiang tectonic mélange belt in Gonjo-Baiyu area, eastern Tibet is mainly composed of a series of tectonic blocks with different scales, lithology and deformation degree and strongly deformed matrix. The rock blocks are connected in a network by many ductile brittle ductile faults nearly parallel to the strike of the junction zone, showing the characteristics of locally ordered and globally disordered tectonic mélange. Tectonic blocks mainly include: ultrabasic-basic rock, marble, marble + metamorphic basic rock, eclogite or eclogite, metamorphic silica-limestone-argillaceous rock, etc. Matrix materials mainly include garnet bearing schist and schist. The geochemical analysis data of rocks reveal that the original rocks of basic rock blocks have the characteristics of enriched mid ocean ridge or normal mid ocean ridge basic rocks (E-MORB, N-MORB). The metamorphic basic volcanic rocks in the ocean island seamount block have the characteristics of ocean island basalt (OIB); The protolith of eclogite has the characteristics of ocean island basalt (OIB) or normal mid ocean ridge basalt (N-MORB). Combined with previous research results, it can be finely divided into five main evolution stages: rift valley basin (D), ocean basin expansion (C₁-P₁), ocean crust subduction (P₂-P₃), arc continent collision orogeny (T₁-T₂) and superimposed volcanic rift basin (T₂²-T₃¹).