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Tectonic relationships between high pressure subduction complex assemblages and co-existing granitoids, Sivrihisar Massif, Turkey

Timothy A. Shin

Tim Shin (left) and ? Kose (right)
From left to right: Nilay Yağcıoğlu , Tim Shin, Gökhan Köse. Field photo of Sivrihisar granitoids.

My undergraduate honors research was a field- and laboratory-based study that developed a model to explain a very high pressure/low-temperature exhumed metamorphic accretionary subduction complex intruded by granitoid plutons in the Sivrihisar Massif in western Turkey. The Sivrihisar Massif is a portion of the Tavşanlı Zone metamorphic subduction complex which (in conjunction with the Izmir-Ankara-Erzincan ophiolite suite) is a remnant of the Late Cretaceous to Eocene closure of the northern branch of the NeoTethyan Ocean. High pressures and very low temperatures have been reported for rocks from the area, and are thought to be one of the deepest portions of a subduction zone ever exhumed. The relationship of these granitoid plutons to the subduction complex is not well-known. This study used techniques never before applied to the Sivrihisar area rocks, including in situ U-Pb zircon ion microprobe dating with Secondary Ion Mass Spectrometer and relative barometry of Na and Ca amphibole analysis via electron microprobe. These two techniques are combined with major and trace element geochemistry and textural analysis via back-scattered electron and cathodoluminescence imagery to gain new insights on the region’s geologic history.

Major and trace element geochemistry and zircon U-Pb ages from two granitoid samples from the Sivrihisar Pluton and one granitoid sample from the Kaymaz Pluton and zircon U-Pb ages from one chlorite-schist sample associated with the high-pressure/low-temperature metamorphic terrane suggest calc-alkalic granitoid crystallization occurred from Late Cretaceous (79.9±8.6 Ma) to Early Oligocene (33.3±2.0 Ma) sourced from a subducting slab and later mixed with syn-collisional and within-plate granitoid magmas while undergoing fractional crystallization. Subduction of the Tavşanlı Zone may have been occurring as early as the Cambrian (564.8±32.1 Ma) or Carboniferous (310.8±6.0 Ma) with metamorphism of detrital zircons from the Paleoproterozoic (1870±102 Ma and 1646±81 Ma). Na and Al contents in amphiboles from three samples of the high-pressure/low-temperature metamorphic rocks reveal relative pressures higher than that observed in subduction zones like the Franciscan, Sanbagawa, and very high pressure zones like that of the Qiantang Plateau in Tibet.

high-pressure lawsonite-blueschis
Field photo of high-pressure lawsonite-blueschist and possible eclogite metamorphic pod.

The new tectonic model developed in this study explains granitoid intrusion into the metamorphic complex of the subduction margin that created the Tavşanlı Zone by melts derived from the younger Afyon subduction zone to the south. Rapid exhumation of the metamorphic terrane and entrainment of the granitoids is accommodated in a multistage poly-subduction zone environment that progressively transitions to collision from north to south over time. The Sivrihisar and Kaymaz Plutons are a part of a suite of Afyon slab generated arc magmatic rocks that were originally intrusive to the Tavşanlı Zone but later became syn-collisional. Alternatively, the granitoids are sourced from subducting slab breakoff.

Advisor: Elizabeth Catlos