Whole rock and mineral chemistry of ultramafic-mafic cumulates from the Orhaneli (Bursa) ophiolites, NW Anatolia

Abstract

The Orhaneli ophiolite, situated in the western part of the İzmir-Ankara-Erzincan Suture Zone (İAESZ) and south of Bursa, exposes cumulates belonging to the mantle-crust transition zone. The Orhaneli ophiolite consists mainly of ultramafic cumulates and subordinate mafic cumulates. The basal cumulate sequence comprises dunites locally interlayered with chromitite levels. Above, dunitic cumulates, commonly interbanded with cumulate wehrlites, grade up into two pyroxene peridotites (cumulate lherzolites showing some transition to cumulate harzburgites in places). The observation of cumulate gabbroic thin layers intercalated with ultramafic cumulates at least three times suggests that the magma chamber was periodically replenished by basaltic liquids. Petrographic studies show that the crystallization orders in the mafic-ultramafic cumulates follow two lines: (1) sp-ol-cpx-plag (chromitite, dunite, wehrlite, pyroxenite, gabbro), (2) sp-ol-cpx-opx-plag (chromitite, dunite, wehrlite, lherzolite, harzburgite, pyroxenite, gabbro). Geoc.hemical analysis has shown that Th enrichment within the LILE (K, Sr, Rb, Ba) group and the depletion of HFSE (Ti, Nb, Zr, Y) relative to MORB indicate a suprasubduction zone tectonic setting for the formation of the Orhaneli ophiolite. The cumulate gabbroic rocks show low Ti (<1%) and Zr (<50 ppm) and very low Ti (<0.2%) and Zr (<8 ppm) while isolated diabases have low TiO2 (0.72-0.87%) and Zr (<55 ppm). Mineral compositions, notably olivine (Fo81-93), clinopyroxene (Mg#83-94), orthopyroxene (Mg#77-92), plagioclase (An86-99) and Cr-spinel (Cr#0.40-0.'7'3) also indicate that the Orhaneli ophiolite is a suprasubduction zone (SSZ) type. Field and petrochemical studies suggest that the Orhaneli ophiolite formed as a product of island arc tholeiitic (IAT) and/or boninitelike magmatism in an intraoceanic suprasubduction zone system, similar to other Neotethyan Ophiolites, within the northern branch of the Neotethys Ocean, probably in the forearc region. Copyright © TÜBİTAK.