East Pekulnei Oceanic Crust Terrane (Northeasternmost Asia, Russia): A Fragment of the Late Jurassic-Early Cretaceous Sliding Plate Boundary between the Paleo-Pacific and Chukotka Microcontinent

The West Pekulnei marginal continental arc terrane adjoins the East Pekulnei oceanic crust terrane, the siliceous-volcanic strata of which (Pekulneiveem Formation, PV) are subdivided into two complexes. The first PV-1 complex comprises a dike series; pillow basalts with elevated Nb and TiO2 contents and FeO*/MgO ratios; and siliceous-argillaceous deposits dated as Bajocian-Kimmeridgian. The second PV-2 complex consists of low-Ti, N-MORB-type pillow basalts with a low Nb content, combined with magnesian basalts, picrite-basalts, and mafic tuffaceous rocks. The siliceous-argillaceous deposits of the PV-2 complex are dated as Late Jurassic-Early Cretaceous based on radiolarian assemblages and contain Valanginian macrofauna; the admixture of terrigenous material and the detritus of intermediate and felsic volcanic rocks suggest the PV-2 complex generation in the vicinity of a source of allothigenic material. New data on the geology and geochemical composition of the pillow basalt sequence suggest the formation of the PV-1 complex in a Paleo-Pacific spreading center. The composition and datings of the rocks of the PV-2 complex indicate its formation during the Tithonian-Valanginian in the junction zone of the oceanic plate and the Pekulnei- Zolotogorsk margin of the Chukotka continent. The formation of the PV-2 complex immediately after the convergence of the Paleo-Pacific plate (PV-1) with the continental margin is explained by the young PV-2 oceanic crust generation in a spreading center, which emerged in a transform fault zone. The Pekulnei arc formed during the Tithonian-Valanginian at the Pacific margin of Eastern Chukotka; the magmatic complexes of this arc, besides the TAT and CA series, include low-Ti MORB-type basalts, similar in composition to a certain part of the PV-2 basalts. Presumably, a single tectonic structure existed in the Tithonian- Valanginian that controlled the basaltic magmatism of both terranes; this structure could be a transtensional segment of the transform fault zone. The synchronous development of the Pekulnei arc and PV-2 complexes and the geochemical correlation between the basalts suggest that the Tithonian-Valanginian subduction within the Pekulnei segment was induced by the opening of the PV-2 basin in the transform fault zone and the subduction of the young oceanic crust beneath the margin of Eastern Chukotka.