TECTONO-THERMAL EVENTS RECORDED IN A METAMORPHOSED OPHIOLITIC MELANGE, N MENDERES MASSIF, NW TURKEY

Abstract

The Izmir-Ankara-Erzincan Suture Zone represents the relicts of the Late Mesozoic-Early Cenozoic Neotethyan Ocean in N Turkey. This suture zone marks the boundary of amalgamated S Eurasian and N Gondwanan continents and is characterised by the presence of ophiolitic melanges and ophiolites. Two different ophiolitic melanges may be distinguished in this zone based on their metamorphic tirade. Unmetamorphosed melanges are widespread along the suture zone whereas metamorphic melanges occur only at a structurally lower position as thin tectonic wedges. One of such tectonic slices of metamorphosed ophiolitic melange is exposed along a low angle detachment fault in N Menderes Massif, to the S of Simav Graben. The melange comprises serpentinite, metabasite and marble blocks set in a metapelitic and metapsammitic matrix and display a penetrative mylonitic fabric. A detailed microstructural study of the melange has revealed four deformation phases. The folded relict fabric seen in the staurolite and kyanite porphyroblasts represent D1 and D2 deformation phases. The coexistence of staurolite, kyanite and garnet porphyroblasts indicate a regional metamorphism (M1) at upper-amphibolite facies conditions. The M2 metamorphism is represented by the growth of the sillimanite from kyanite and point out a progressive metamorphism at conditions between 670-680 degrees C and 7-9 kbar, corresponding to a burial depth of 21 km. Andalusite porphyroblasts may have formed in response to contact metamorphism (M3?) of an Early Miocene granite. The first two deformations (D1 and D2) were obliterated by the penetratively developed D3 deformation. D3 is a top-to-the-northeast ductile mylonitic deformation during which NNE-SSW trending mineral stretching lineation was formed on a south dipping mylonitic foliation. Retrograde metamorphism (M4) at greenschist facies conditions was associated with the exhumation of high-grade rocks (i.e. the Menderes Massif) during the D3 event. The latest deformation (D4) is brittle deformation represented by high-angle normal faulting of post-Pliocene age, accommodating the N-S extension of western Anatolia.