COEXISTING ANDALUSITE, KYANITE, AND SILLIMANITE IN A BLUESCHIST TERRAIN, CENTRAL TURKEY

Muscovite-bearing quartzites from the southern part of the Sivrihisar massif, Turkey, contain coexisting andalusite, kyanite, and sillimanite, and are interlayered with metasedimentary and meta-igneous rocks containing sodic amphibole + epidote. In the quartzites, staurolite occurs as partial pseudomorphs after kyanite, and as intergrowths with prismatic sillimanite. Textures suggest the crystallization sequence andalusite + chlorite, kyanite + garnet, then sillimanite + staurolite. Although most sillimanite crystals are aligned with the other polymorphs to define a lineation, and some crystals are slightly deformed (bent), undeformed sillimanite that grew at a high angle to the lineation also occurs.

The initial part of the path (andalusite to kyanite) represents subduction of the continental margin and metamorphism in the epidote-blueschist facies. Collision followed subduction. During collision and subsequent decompression accompanied by magmatism, the rocks passed through the sillimanite zone. Estimates of peak temperatures are 540-560 C. Intrusion of a monzonite may have driven growth of late sillimanite and staurolite in the quartzite, but contact effects clearly post-dated regional metamorphism that produced lineation-defining andalusite, kyanite, and sillimanite.

The northern part of the Sivrihisar massif is comprised of similar protoliths but records higher pressure blueschist facies conditions, including garnet + lawsonite-bearing blueschists containing eclogite lenses. The unusual association of three aluminum silicate polymorphs and epidote blueschists in the southern part of the massif likely represents the effects of collision and collision-related magmatism following subduction, whereas metamorphism and deformation of rocks in the northern massif were primarily driven by subduction, in addition to tectonic emplacement of a large ultramafic massif. Ongoing work involves comparing the P-T-t-d evolution of the low- and high-P blueschist facies rocks to understand the chemical/mechanical processes involved in subduction of continental crust.