Metamorphic and tectonic evolution of a subduction complex: Insights from petrology and fabrics of metagabbro (Sivrihisar massif, Turkey)

Patricia Kang, University of Minnesota
Donna Whitney, University of Minnesota

Blueschist and eclogite belts mark the locations of subduction zones. These high-pressure/low-temperature (HP/LT) belts, however, are typically affected by subsequent collision, thermal overprinting, and/or fluid infiltration, all of which disrupt the high-pressure physical and chemical features of the subducted plate. The Sivrihisar massif of the Tavşanlı Zone in Turkey is one of the rare localities in the world where HP/LT rocks are well exposed. It consists of lawsonite eclogite and blueschist facies metabasalts and metasedimentary rocks (marble, quartz+mica- and calc- schists). The HP/LT unit has fault contacts with metaperidotite and metagabbro/diabase to the north and metaperidotite to the south. The fine-grained HP/LT rocks with basalt protoliths have been well studied, but less attention has been paid to metagabbro, which occurs as rare pods within blueschist, calc-schist, and quartzite. Ongoing study of the HP/LT metagabbro includes new petrographic and microstructural data and P-T estimates, and comparison with previous results from mebasalts and metasedimentary units to understand the deformation and metamorphic evolutions in the context of subduction zone dynamics.

Sivrihisar metagabbro is dominated by lawsonites with variable grain size, ranging from 30 µm to 1800 µm long. Fine-grained lawsonite is located in the matrix and defines a preferred orientation, whereas coarser grains are present as inclusions in amphibole and deformed, porphyroblastic clinopyroxene. Some of the coarse-grained lawsonite has replaced clinopyroxene. Some lawsonite and clinopyroxene exhibit partial alteration to chlorite. Metabasaltic layers associated with the metagabbro pods are interlayered with marble and quartzite, and are characterized by millimeter-scale alternations of blueschist (chloritized glauchophane + recrystallized, fine-grained lawsonite (60 µm)) and eclogite (fine-grained lawsonite + partially chloritized, coarse-grained omphacite with inclusions of glaucophane + garnet with inclusions of lawsonite, clinopyroxene and glaucophane).

Mineral and bulk-rock compositions will be used to estimate P-T conditions at the time of peak assemblage equilibration. Estimated P-T conditions will be compared with the previous results of lawsonite-eclogite (~26 kbar, 500°C) and lawsonite blueschist (12 kbar, 380°C). Results will be integrated with previous P-T-d studies, which indicate that metabasaltic and metasedimentary layers record part of the exhumation history. Our hypothesis is that metagabbro likely records a similar P-T path, as indicated by textural similarities with metabasalt, such as the presence of lawsonite ± glaucophane inclusions in clinopyroxene porphyroblasts. However, the presence of garnet in metabasaltic layers and absence of garnet in metagabbro may indicate some differences; alternatively, metagabbro and metabasalt may have different bulk compositions, accounting for different mineral assemblage.


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