Why is lawsonite eclogite so rare? Metamorphism and preservation of lawsonite eclogite, Sivrihisar, Turkey

摘要

A rare occurrence of lawsonite eclogite crops out in a belt of high-pressure rocks in the Sivrihisar Massif, Turkey. Although lawsonite eclogite is predicted to be common at depths of similar to 45-300 km in subduction zones, lawsonite seldom survives exhumation. In the Sivrihisar Massif, lawsonite eclogite occurs as 5-cm-long to 3.5-m-long pods in lawsonite blueschist and blueschist facies quartzite and marble. We have identified > 70 eclogite pods within an similar to 14 km(2) area; most of them are lawsonite bearing. Phase diagrams calculated for Sivrihisar eclogite bulk compositions indicate metamorphic conditions of 21-24 kbar, similar to 422-580 degrees C. High-pressure minerals (omphacite, lawsonite) are synkinematic with respect to the main fabric, and therefore preserve chemical and structural features of high-pressure subduction metamorphism. The presence of both pristine and retrogressed lawsonite eclogite within meters of each other suggests that lawsonite eclogite preservation is related to rapid exhumation and other factors. For example, the effects of exhumation-related deformation +/- fluid infiltration may locally overprint some rocks, producing clinozoisite and/or epidote, whereas rocks that escape these effects retain primary lawsonite.