Quartz exsolution topotaxy in clinopyroxene from the UHP eclogite of Weihai, China

Abstract

Abundant oriented silica precipitates of α-quartz (4.0±1.0vol.%), in part coexisting with calcic amphiboles (<0.2vol.%), have been identified in clinopyroxenes of eclogites from the Weihai area, Sulu ultrahigh pressure terrane, eastern China. Electron backscatter diffraction (EBSD) analyses demonstrate that the majority (97%) of quartz precipitates have topotactic relationships with their host clinopyroxenes. Three types of crystallographic topotactic relationship have been identified between quartz and host clinopyroxene: (1) 52% quartz precipitates share the same orientation for the c-axes with [0001]qz//[001]cpx; (2) 34% quartz precipitates share the same orientation for the a-axes with [112¯0]qz//[001]cpx; and (3) 11% quartz precipitates share the same orientation for the s-planes with (112¯1)qz//(100)cpx. Other quartz axes and planes disperse in large or small girdles around the shared axes or planes. Many quartz rods/needles are elongated parallel to the [001]cpx with the long axes of quartz being either [0001]qz or [112¯0]qz. Amphibole precipitates have also a strong crystallographic relationship with host clinopyroxene, i.e., (100)amp//(100)cpx, [010]amp//[010]cpx, and [001]amp//[001]cpx. These results provide quantitative microstructural evidence supporting an exsolution origin for oriented quartz needles/rods in clinopyroxene and demonstrate that the exsolution of quartz from clinopyroxene occurred within the stability field of α-quartz rather than coesite. The oriented precipitates of α-quartz, in part coexisting with calcic amphiboles, in host clinopyroxene are probably promoted by supercritical fluid or partial melting during the early exhumation of eclogites. Our results suggest that oriented quartz precipitates in clinopyroxene cannot be used as an indisputable UHP-indicator.