Middle Paleozoic greenstones of the Hangay region, central Mongolia: Remnants of an accreted oceanic plateau and forearc magmatism

Abstract

The Hangay-Hentey belt in Central Mongolia constitutes a Pacific-type accretionary orogen that formed through the evolution and closure of the Hangay-Hentey paleo-ocean during the Early Paleozoic to Early Mesozoic. From this belt, new geochemical and petrological results are presented for greenstones from the Erdenetsogt Formation hosted by the Tsetserleg accretionary terrane in the Hangay region, with particular emphasis on newly found picritic and andesitic rocks. These rocks occur mostly in the lower portion of the Erdenetsogt Formation as massive lavas, sills, and dikes closely associated with varicolored bedded ribbon cherts and siltstones. The protoliths of the studied greenstones comprise (1) enriched, plume-derived tholeiitic greenstones including picrites and ferrobasalts with oceanic plateau basalt affinity, (2) non-enriched, plume-derived tholeiitic basalts with E-MORB affinity, and (3) arc-derived high-Mg andesites (HMAs). The plume-derived rocks are characterized by chemical signatures such as slight LREE enrichment similar to that of tholeiitic OIB and the existence of ferropicrite with high FeO* (>14 wt%) and MgO (12-22 wt%), which is characteristic of large igneous provinces (LIPs), including oceanic plateaus. Their tholeiitic composition and high-Fe and -Ti contents require melting of the source mantle peridotite with addition of some recycled Fe- and Ti-rich basaltic material. The non-enriched basalts may have been generated by a higher degree of melting of the same source mantle. The HMAs are characterized by glassy texture, high MgO content (up to 7 wt%), and significant LREE enrichment with depletion in Nb and resemble sanukite of the Setouchi volcanic belt, SW Japan. We infer that the Hangay tholeiitic greenstones probably represent an accreted upper section of an oceanic plateau that developed in the deep-water region of the Hangay-Hentey paleo-ocean in the Devonian. The Hangay HMAs may have been produced by subduction of young oceanic plate after an oceanward back-stepping of the subduction zone that was a result of the collision during the Carboniferous of the oceanic plateau and the active continental margin of the Central Mongolian Massif.