Blueschist facies overprint of late Triassic Tethyan oceanic crust in a subduction-accretion complex in north-central Anatolia, Turkey

摘要

The late Cretaceous accretionary complex along the I.zmir-Ankara-Erzincan suture zone, northern Turkey, includes various types of metamorphic rock together with radiolarite, sandstone, mudstone, serpentinite, basalt and limestone. Meta-plagiogranite blocks (up to 5 m in diameter) and ameta-gabbro slice (800 x 500 m) cross-cut by meta-plagiogranite are observed in a matrix of serpentinite, mudstone and radiolarian chert. These meta-gabbros and meta-plagiogranites show subduction-related geochemical characteristics. Laser ablation inductively coupled plasma mass spectrometry U-Pb dating of zircons from two meta-plagiogranites yielded 222.3 +/- 1.5 and 227.2 +/- 1.6 Ma crystallization ages. This study shows that the meta-gabbro and metaplagiogranite from the accretionary complex are remnants of the metamorphosed equivalents of the late Triassic lower crustal rocks of the subducted Tethyan oceanic crust. They are the oldest lower crustal rocks observed in the I.zmir-Ankara-Erzincan suture zone. Both the meta-gabbros and meta-plagiogranites have Na-Ca amphiboles, indicating blueschist facies metamorphism. However, phengites from a blueschist facies rock yielded an Ar-40/Ar-39 age of 104.3 +/- 0.7 Ma, indicating that blueschist metamorphism occurred during the Albian. All these data suggest that the oceanic crust formed during the Norian, metamorphosed during the Jurassic to early Cretaceous and then dismembered during accretionary complex formation in the late Cretaceous.