Örenli metamorphics (Biga Peninsula, NW Anatolia): evidence for Triassic active continental margin of Paleo-Tethys Ocean.

The pre-Liassic basement of the Biga Peninsula, located in the westernmost part of the Sakarya Zone, consists of metamorphic assemblages that were considered to represent different continental basements in the north and south, and the suture (ophiolitic/mélange) zone(s) between them. The Çetmi mélange, defined as between these continental fragments (the Kazdağ Massif in the south and the Çamlıca Massif in the north) was evaluated as a subduction accretionary mélange of the Intra-Pontide Ocean and it was assumed that a suture passed through this part of the peninsula. This study was carried out on the most widely distributed outcrops of the Çetmi mélange(?) on the northwest flank of the Kazdağ Massif. These units, which are named the Örenli metamorphics, are represented by a succession of low-grade metamorphic volcanic-volcaniclastic rocks and alternating metasedimentary rocks and recrystallized limestones. The tectonostratigraphically lowermost levels of the Örenli metamorphics are a blocky-chaotic assemblage, whereas the uppermost levels are a regular metavolcanic-metapelite succession, which passes upward into the Late Triassic recrystallized limestones, and is cut by very low-grade metamorphic diabase dykes of 165.81 ± 1.55 Ma age. Furthermore, metavolcanic rocks of this succession show a calc-alkaline association of Basalt-Andesite-Dacite-Rhyolite (BADR) pattern in the chemical classification diagram, which is the signature volcanic rock suite of convergent margins. In addition, all the metavolcanic rock samples show a significant enrichment in terms of incompatible elements and LIL elements and negative Nb and Ti anomalies on the MORB normalized multielement spider diagram, which indicates that these rocks characterize high-K calc-alkaline volcanism at the active continental margins. Field relationships, zircon U-Pb geochronology, and geochemical data support that these units, outcropping in the study area, represent a Triassic active continental margin, most probably a fore-arc setting, related to the Paleo-Tethyan subduction, but not the subduction mélange of the Intra-Pontide Ocean. [ABSTRACT FROM AUTHOR]