Crystallographic evidence for simultaneous growth in graphic granite

Abstract

Deciphering the graphic texture is important for understanding its origin and the crystallization process of granitic rocks. We present here investigations on petrography, crystallographic relationship, and three-dimensional (3D) relations between quartz and alkali-feldspar in graphic granite from the Fangshan Pluton, north China. Under the petrographic microscope, multiple domains of quartz grains exhibit a nearly simultaneous extinction within a single crystal of feldspar. Electron back scattered diffraction (EBSD) analyses indicate that majority of quartz (Qz) grains are in a topotaxic relationship with the host K-feldspar (Kfs) with 112¯3Qz//[001]Kfs, 101¯0Qz//(110)Kfs, and either 101¯1Qz or 011¯1Qz parallel to (110)Kfs. Moreover, essentially all quartz grains in the sample are crystallographically related by either the Dauphiné or the Japan twin laws. Synchrotron X-ray computed micro-tomography demonstrated that majority of the quartz grains are in the form of sub-parallel long rods with D-, L-, and U-shaped cross sections. These results provide new quantitative microstructural evidence of simultaneous growth of quartz and feldspar in graphic granite.