Tectonic evolution of the Karakoram metamorphic complex(NW Himalayas)reflected in the 3D structures of spiral garnets:Insights from X-ray computed micro-tomography  

作　　者：M.Sayab D.Aerden J.Kuva W.U.Hassan 

机构地区：[1]Geological Survey of Finland,P.O.Box 96,FI-02151 Espoo,Finland [2]Departamento de Geodinámica and IACT-CSIC,Universidad de Granada,Granada 18002,Spain [3]National Centre of Excellence in Geology,University of Peshawar,Peshawar 25130,Pakistan

出　　处：《Geoscience Frontiers》2021年第3期528-540,共13页地学前缘（英文版）

摘　　要：Spiral garnet porphyroblasts are known to record lengthy periods of deformation and metamorphism by preserving single or multiple FIAs(Foliation Intersection Axis)formed normal to tectonic shortening directions.Thanks to technological advances in X-ray computed micro-tomography(XCMT),FIAs can now be readily determined in relatively large samples in contrast to previous methods that require the preparation of a set of radial vertical and horizontal thin sections of samples.XCMT scanning not only alleviates tedious thin section based procedures but also illuminates the complete internal architecture of a rock sample allowing three-dimensional(3D)quantitative shape analysis of an individual porphyroblast as well as precise measurement of FIAs.We applied the technique to a sample from the Hunza Valley in the Karakoram metamorphic complex(KMC),NW Himalayas,containing numerous garnet porphyroblasts with spiral-shaped inclusion trails.The XCMT imaging reveals an E-W trending FIA within the sample,which is consistent with orthogonal N-S collision of the India-Kohistan Island Arc with Asia.Garnet long axes(X_(GT))have variable plunges that define a broad sub-vertical maximum and a small sub-horizontal maximum.The X_(GT) principle maxima lie at N-090 and N-120.Smaller maxima lie at N-020 and N-340.Geometric relationships between X_(GT) axes and FIA orientation in the sample suggest that porphyroblast shapes are controlled by the geometry of the lens-shaped microlithons in which they tend to nucleate and grow.The orientation of inclusion trails and matrix foliations in the sample are correlated with three discrete tectono-metamorphic events that respectively produced andalusite,sillimanite and kyanite in the KMC.Late staurolite growth in the sample reveals how the rocks extruded to the surface via a significant role of roll-on tectonics,which can be correlated with the Central Himalayas.

关 键 词：PORPHYROBLAST Garnet FIA X-ray tomography KARAKORAM HIMALAYAS 

分 类 号：P588.3[天文地球—岩石学] P542[天文地球—地质学]