越南北部紅河剪切帶自中生代以來變質史之探討

Abstract

紅河剪切帶在越南北部包含了象背山變質帶以及延伸到紅河平原上的一小部分，探討此區的變質作用可以讓我們了解紅河剪切帶在南段所經歷的變質史。本研究利用微構造的判斷，結合前人的研究結果，歸納出此區至少記錄了三期塑性變形作用。這三期變形產生的三期葉理上所生長的礦物種類變化：由第一期葉理到第二期葉理顯示環境為前進變質，且第二期葉理上分布著此區最高變質度的藍晶石及矽線石；由第二期葉理到第三期葉理則顯示為退向變質。將第二期左旋變形作用與紅河大規模左移剪切事件作對應，則第一期變形作用可能是受印支期的活動所影響而留下的證據，此外，第三期右旋變形則可能是約12Ma時鮮水河斷裂帶的活動造成西藏東邊地塊被迫向東南逃逸而在紅河剪切帶有右旋的記錄。由地質溫度壓力計的計算，得知此區石榴子石在同第一期變形作用 (syn-D1) 前已深埋至約21公里深處，而到第二期變形作用前 (pre-D2) 岩石從約21公里深處深埋至約23公里深處，記錄了前進變質；且在第二期變形作用前到達本區的峰期變質，溫度及壓力為800±107℃ 以及 9.1±2.2 kbar，可達角閃石相-麻粒岩相交界處附近。

This study focuses on the southern part of the Red River Shear Zone (RRSZ), i.e. the southern most part of the metamoophic belt lies in northern Vietnam which includes Day Nui Con Voi metamorphic dome and an outcrop on the Red River delta plain. This study tried to verify the metamorphic history of the southern part of the ASRR for more detailed information about the whole shear zone. Combined with previous studies, it’s possible to conclude that this region has recorded at least three ductile deformation stages (D1, D2, and D3). Fromvariations of mineral assemblages on the foliation of three stages, it’s possible to interpret a prograde metamorphism from the first to the second deformation stage and a retrograde metamorphism from the second to the third deformation stage. Correlating the second stage of deformation to the strong, dominant left-lateral shear, the first stage of foliation may caused by Indosinian orogeny, and the third stage of foliation may be due to the change in slip sense of the RRSZ from left-lateral to right-lateral because of the activation of Xianshui He Fault. Thermobarometric data shows the rock of Day Nui Con Voi has already been under about 21 km depth before the first deformation stage, and then gone to about 23 km depth before the second deformation stage, thus recorded a prograde metamorphism from syn-D1 to pre-D2. The peak metamorphism took place approximately under the boundary of amphibolite to granulite facies conditions of 800±107℃ and 9.1±2.2 kbar.