越南西北部Phan Si Pan带太古宙Cavinh杂岩的岩石学研究及其构造意义

Archean is the important period in the evolution history of the earth. Studying on the residues of the Archean rocks is the most direct and effective way to know the origin and the early evolution of the continental crust, which is of great significance to reconstruct the evolution history of the earth. The Cavinh complex is the only Archean rock found in Southeast Asia. It is distributed in Phan Si Pan belt of northwestern Vietnam which is the tectonic junction of the South China and Indochina blocks. The degree of geological research in this area is very low. With the exception of a few simple dating works for the Cavinh complex, petrogeochemical and isotope geological research is lacking. This project, based on precious studies, intends to conduct systematic geochronological, petrological, petrogeochemical and isotope geological researches on the Archean Cavinh complex. The aims of the proposed project are to confirm the rock assemblage and formation age of the Cavinh complex, to identify the petrogenesis and tectonic setting, and to discuss evolution history of the Precambrian crust in study area. This work will help us to deeper understand the composition of crust, the mechanism of tectonic-magmatic activity and its evolution law with time and space during the Archean in Southeast Asia. Moreover, this project hopes to confirm the tectonic affinity of Phan Si Pan belt by comparing with the Archean basement in Yangtze and other blocks, which can provide new evidence to constrain the boundary between South China and Indochina blocks.

太古宙是地球演化历史中的重要阶段。对现今残留的太古宙岩石的研究，是了解大陆地壳起源与早期演化最直接有效的方法，对重建地球演化历史具有非常重要的意义。分布于华南与印支板块交汇处的越南西北部Phan Si Pan带中的Cavinh杂岩是目前东南亚地区发现的唯一一个太古宙岩体，前人对其的研究程度很低，除少数简单的定年工作外，缺少岩石地球化学以及同位素地质学的研究。本项目拟在以往的研究基础上，对Cavinh杂岩展开系统的年代学、岩石学、岩石地球化学以及同位素地质学研究，旨在精确厘定Cavinh杂岩具体岩石组合和形成时代，查明Cavinh杂岩的岩石成因和构造背景等，探讨研究区前寒武纪地壳的演化历史，从而更深入了解东南亚太古宙时期的物质组成、构造-岩浆活动机制和时空演化规律。对比扬子地块和其他陆块的太古宙组成，确定Phan Si Pan带的构造归属，为限定华南与印支板块的分界线提供重要限制条件。

越北地区位于扬子地块、华夏地块以及印支板块的交汇地带。前寒武纪变质基底主要分布在越西北的Phan Si Pan地区以及Nam Co地区。由于前人对该区域的前寒武纪基底的关注度较低，该区域的构造属性仍存在争议。. 本项目对Phan Si Pan地区发现的唯一一个太古宙岩体—Cavinh杂岩的样品进行系统的年代学、岩石学、岩石地球化学及同位素地质学研究。结果表明Phan Si Pan地区主要有2期太古宙岩浆事件: 约3.0 Ga的岩浆作用主要形成TTG岩系，这一时期主要存在地壳的生长过程; 2.9-2.8 Ga主要形成斜长角闪岩以及富钾花岗岩，地幔新生物质和古老地壳物质均被卷入岩浆形成过程。与分布在华南以及印支板块内的太古宙基底进行对比研究表明，Phan Si Pan地区的太古宙基底可能是扬子地块太古宙基底的延伸。. 此外,本项目还对越西北部Phan Si Pan带以西的Nam Co地区 Nam Co组样品进行研究，结果表明，Nam Co组为一套中新元古代（821-721 Ma）沉积序列。Nam Co组碎屑锆石年龄谱特征以及它们的 Hf 同位素组成相似于扬子地块南缘新元古代沉积岩。Nam Co 组沉积物的源区主要来自扬子地块南缘 (江南造山带)，少数来源于扬子地块西缘攀西带和邻近的哀牢山-红河变质带以及越南西北部的 Phan Si Pan 带。通过对比研究，越西北Nam Co地区的构造属性应该也归属于扬子地块，且位于扬子地块的西南角，而Nam Co 地区西南侧的马江断裂才更可能作为华南与印支板块的分界线。.