中部地壳剪切带花岗质岩石变形及其对于构造-岩浆关系的约束:以滇西点苍山为例

Deformation characteristics of the granitic rocks from the midcrustal shear zone and constraints on the tectono-magmatic relationship: Exemplified by the Diancang Shan complex,western Yunnan

哀牢山-红河剪切带剪切时间的厘定至今存在较多的争议,争议的本质更确切地说是对于剪切带内及其附近剪切作用与岩浆作用认识的差异。点苍山杂岩是位于哀牢山-红河剪切带北端的一个杂岩体,由西部的深变质岩、东部的叠加退变质带和不同时代的花岗质岩石组成。为了正确厘定剪切前、剪切期(包括剪切前期和剪切后期)、剪切后花岗质岩脉,剪切作用过程中构造变形与岩浆作用之间的关系,同时对哀牢山-红河剪切带剪切时间进行新的约束,本文围绕点苍山杂岩开展详细的宏观构造解析、显微构造观察和组构分析,以查明岩体(脉)岩浆流动构造和晶质塑性变形构造,同时对同剪切岩脉开展锆石U-Pb学分析。结果显示,剪切不同阶段就位的花岗质岩石具有不同的宏观构造、显微构造与组构特征。现今所见岩石变形构造、显微构造和组构特点一方面取决于递进剪切变形作用过程中的时间早晚,另一方面还与它们在递进剪切变形作用过程中的构造位置有密切联系。获得两组同剪切年龄为28.54±0.15Ma和27.31±0.23Ma,其中前者为剪切早期阶段同就位的花岗岩脉,后者为剪切稍晚阶段同就位的花岗岩。研究揭示出就位较晚的岩脉(即后者)因其位于递进剪切变形的高应变带而具有较为复杂的变形构造、显微构造和组构型式,就位较早的岩脉(前者)因在递进剪切变形作用过程中位于低应变带而具有简单的变形构造、显微构造和组构型式,并仍然保留有岩浆结晶组构特点。哀牢山-红河剪切带在28.54~27.31Ma间持续发生着从早期阶段较高温(~700℃),向中期阶段中高温(550~600℃)和晚期阶段低温(低于400℃)环境的递进剪切变形作用。

The shearing duration of the Ailao Shan-Red River（ ASRR） shear zone has been debated over the years. The essence of this controversy is rooted in the relationship between the shearing and the magmatism in or near the shear zone. Diancang Shan complex lies in the north part of the ASRR shear zone,and is composed of three parts,i. e.,the western high-grade metamorphic rocks,the eastern superimposed retrogressive metamorphism belt,and varying ages of magmatic rocks. In order to better constraint pre-,syn-and post-shearing granitic veins,the relationship between deformation and magmatism during shearing,and the initiation time of the shear zone,this study has carried out detailed macro-and micro-structural analyses and fabric study,which are helpful to reveal the flow structure and crystal plastic deformation structure,and also zircon U-Pb dating of granitic veins of syn-shearing. The results show that different stages of granitic rocks have different macro-and micro-structure and fabric features. Two zircon U-Pb ages of syn-shearing at28. 54 ± 0. 15 Ma and 27. 31 ± 0. 23 Ma are dated. 28. 54 ± 0. 15 Ma represents the early shearing time,and 27. 31 ± 0. 23 Ma represents the late shearing age. This study reveals that the late shearing vein（ the latter） is located in the high strain zone of progressive shearing and possesses more complex deformation structure,microstructure and fabric type,however,the early shearing vein（ the former） in the low strain zone of progressive shearing has a simple structure,microstructure and fabric type,and still has the characteristics of magma crystallization fabric. ASRR shear zone experienced a progressive shearing of a higher temperature deformation in the early stage（ - 700℃）,medium-high temperature deformation in the middle stage（ 550 - 600℃） and low temperature deformation in the late stage（ lower than 400℃） from 28. 54 Ma to 27. 31 Ma.