Plagiogranite magmatism in the Mesozoic island-arc structure of the Pekulney Ridge, Chukotka Peninsula, NE Russia

Tectonic setting, geochemistry, and petrogenesis of low-K granitoid magmatism in the fore-arc of Late Jurassic-Early Cretaceous Pekulney-Zolotogorskaya fossil arc are addressed using a case study consisting in two types of Neocomian plagiogranite complexes from the eastern Pekulney Ridge (Central Chukotka). These plagiogranites outcrop as small dyke-like bodies and are confined to a zone of strong deformation related to an olistostrome development at the boundary between allochthonous Middle Jurassic-Early Cretaceous oceanic volcanics and cherts and parautochthonous Late Jurassic-Early Cretaceous island arc complexes. The geochemical features of plagiogranites [HFSE depletion and LILE enrichment, Nb- and Ta-negative anomalies in ocean ridge granite (ORG)-normalized plots, and Rb vs. Nb+Y and Zr vs. (Nb/Zr)n correlations] indicate that these rocks are likely to have originated in suprasubduction zone setting. The parameters of tectonic structure and the morphology of the Pekulney fossil arc segment in Neocomian time, structural position of plagiogranites, and their REE patterns militate in favour of the following models for plagiogranite origin: (1) migration of tholeiite-basalt melts from mantle wedge into the fore-arc of the island arc and their subsequent fractionation, (2) partial melting of metabasite crustal source in the fore-arc and subsequent migration of felsic melts. Geochemical modelling shows that one type of plagiogranites could have originated by 10-15% partial melting of metabasite source in a metabasite-gabbroic complex of pre-Late Mesozoic basement of the island arc pile and that the other type could be generated by progressive fractional crystallization of low-K tholeiite from the Late Jurassic-Early Cretaceous island arc complex.