Using Mineral Chemistry and Texture to Track the Transition from Subduction to Collision

Metamorphic terranes containing juxtaposed high-pressure/low-temperature and medium-pressure/medium-temperature (Barrovian) rocks in apparent structural continuity occur in orogenic belts worldwide (e.g. Naxos, Greece; Seward Peninsula, Alaska, USA; Sivrihisar, Turkey). The composition and textures of metamorphic minerals can be used to understand the processes and rates of subduction to collision transitions. Blueschist-to-Barrovian terranes are characterized by (1) apparent structural continuity between the HP-LT and MP-MT domains; (2) similar protoliths in both domains (typically passive margin sedimentary rocks); (3) HP metamorphic/cooling ages that are 10's of millions of years older than the age of Barrovian metamorphism; and (4) collapsed (closely spaced) isograds in the Barrovian sequence; e.g. the total present-day structural thicknesses for Barrovian sequences ranging from the lowest-grade to sillimanite/Sil-Kfs zones range from 1-4 km.

The Sivrihisar Massif is one of the few lawsonite eclogite localities in the world and is dominated by HP rocks, but in the southern part of the massif, HP/LT rocks grade into higher-T rocks. The transition zone is characterized by progressive recrystallization of HP phases, the presence of pseudomorph textures, and complex Ar spectra for white mica. The Sivrihisar rocks record a difference in metamorphic P-T conditions of 20 kbar (25-5 kbar) and 300 C (400-700 C) from the eclogite facies rocks to the highest-grade Barrovian domain. Barrovian zones include chloritoid, garnet, staurolite, kyanite, and sillimanite. Mica-rich quartzite in the Sil zone contains andalusite + kyanite + prismatic sillimanite, in which Sil is the texturally latest polymorph. Intrusion of a granitic pluton at the late stages of Barrovian metamorphism resulted in an overprinting zone of contact metamorphism. The metamorphic history of the entire sequence, from subduction to Barrovian metamorphism to late orogenic magmatism and cooling, occurred within 30 million years.