Multistage Magmatism in Ophiolites and Associated Metavolcanites of the Ulan-Sar’dag Mélange (East Sayan, Russia)

Abstract

We present new whole-rock major and trace element, mineral chemistry, and U-Pb isotope data for the Ulan-Sar’dag mélange, including different lithostratigraphic units: Ophiolitic, mafic rocks and metavolcanites. The Ulan-Sar’dag mélange comprises of a seafloor and island-arc system of remnants of the Paleo-Asian Ocean. Detailed studies on the magmatic rocks led to the discovery of a rock association that possesses differing geochemical signatures within the studied area. The Ulan-Sar’dag mélange includes blocks of mantle peridotite, podiform chromitite, cumulate rocks, deep-water siliceous chert, and metavolcanic rocks of the Ilchir suite. The ophiolitic unit shows overturned pseudostratigraphy. The nappe of mantle tectonites is thrusted over the volcanic-sedimentary sequence of the Ilchir suite. The metavolcanic series consist of basic, intermediate, and alkaline rocks. The mantle peridotite and cumulate rocks formed in a supra-subduction zone environment. The mafic and metavolcanic rocks belong to the following geochemical types: (1) Ensimatic island-arc boninites; (2) island-arc calc-alkaline andesitic basalts, andesites, and dacites; (3) tholeiitic basalts of mid-ocean ridges; and (4) oceanic island basalts. U–Pb dating of zircons from the trachyandesite, belonging to the second geochemical type, yielded a date of 833 ± 4 Ma which is interpreted as the crystallization age during mature island-arc and intra-arc rifting stages. The possible influence of later plume magmatic-hydrothermal activities led to the appearance of moderately alkaline igneous rocks (monzogabbro, trachybasalt, trachyandesite, subalkaline gabbro, and metasomatized peridotites) with a significant subduction geochemical fingerprint.

Keywords

• ophiolite
• volcanic rocks
• geochemistry
• subduction
• plume magmatic-hydrothermal activity