Metamorphic and tectonic evolution of a structurally continuous blueschist-to-Barrovian terrane, Sivrihisar Massif, Turkey

Metamorphic terranes comprised of blueschist facies and regional metamorphic (Barrovian) rocks in apparent structural continuity may represent subduction complexes that were partially overprinted during syn- to post-subduction heating or may be comprised of unrelated tectonic slices. An excellent example of a composite blueschist-to-Barrovian terrane is the southern Sivrihisar Massif, Turkey. Late Cretaceous blueschist facies rocks are dominated by marble characterized by rod-shaped calcite pseudomorphs after aragonite and interlayered with blueschist that contains eclogite and quartzite pods. Barrovian rocks, which have ⁴⁰Ar/³⁹Ar white mica ages that are >20Myr younger than those of the blueschists, are also dominated by marble, but rod-shaped calcite has been progressively recrystallized into massive marble within a ~200-m transition zone. Barrovian marble is interlayered with quartzite and schist in which isograds are closely spaced and metamorphic conditions range from chlorite to sillimanite zone over ~1km present-day structural thickness. Andalusite, kyanite and prismatic sillimanite are present in muscovite-rich quartzite; in one location, all three are in the same rock. Andalusite pre-dates Barrovian metamorphism, kyanite is both pre- and syn-Barrovian and sillimanite is entirely Barrovian. Muscovite with phengitic cores and relict kyanite in quartzite below the staurolite-in isograd are evidence for pre-Barrovian subduction metamorphism preserved at the low-T end of the Barrovian domain; above the staurolite isograd, all evidence for subduction metamorphism has been erased. Some regional metamorphism may have occurred during exhumation, as indicated by syn-kinematic high-T minerals defining the fabric of L-tectonite. Quartz microstructures in lineated quartzite reveal a strong constrictional fabric that may have formed in a transtensional bend in the plate boundary. Transtension accounts for the closely spaced isograds and development of a strong constrictional fabric during exhumation.