TECTONO-MAGMATIC EVOLUTION, AGE AND EMPLACEMENT OF THE AGARDAGH TES-CHEM OPHIOLITE IN TUVA, CENTRAL ASIA: CRUSTAL GROWTH BY ISLAND ARC ACCRETION

Abstract

The Agardagh Tes-Chem ophiolite in Tuva, Central Asia, is part of the Central Asian Mobile Belt which formed during subduction-accretion processes lasting from the early Neoproterozoic to the late Palaeozoic. The ophiolite was obducted onto the Tuva-Mongolian Massif (microcontinent?) in the early Palaeozoic towards the SE along N-and NW-dipping faults and is embedded within a tectonic mélange and thus is part of an ac-cretionary wedge. Dating of three small zircon fractions from a plagiogranite by the evap-oration technique yielded a 207 Pb/ 206 Pb age of 569.6 ± 1.7 Ma, which reflects the crystal-lization age of the plutonic section of the ophiolite. Geochemical data reveal an island arc-related origin for the ophiolite, where typical island arc volcanic rocks predominate over MORB-like pillow lavas. In contrast to the highly incompatible element-enriched volcanic rocks, all plutonic rocks of the ophiolite are depleted, and mineral compositions of ultra-mafic cumulates indicate the presence of boninitic parental melts. The ophiolite therefore consists of an association of island arc and back-arc related sequences that have been amal-gamated during subduction-accretion and collisional obduction. Isotopic and trace element data reveal the existence of a depleted and refractory mantle source beneath Central Asia, from which the volcanic and plutonic rocks of the ophiolite were formed. However, source contamination took place by sediment subduction before the parental melts of the island arc volcanic rocks were formed.