Geochemical discrimination of metabasalt rocks of the Fan-Karategin transitional blueschist/greenschist belt, South Tianshan, Tajikistan: seamount volcanism and accretionary tectonics

The Fan-Karategin metamorphic belt, South Tianshan, Tajikistan, is regarded to be an ancient subduction-accretionary complex and is composed of three tectonostratigraphic units which display lithologies consistent with differ ent tectonic settings. The mafic schists, which make up the major part of t he alder unit of the belt, contain both alkali and tholeiitic metabasalts. On the basis of rare-earth and other immobile element characteristics, the alkali metabasalts are akin to within-plate ocean island basalts, whereas t he tholeiitic metabasalts resemble E-type MORE. The association is interpre ted to have been formed on seamount-like structures under a within-plate pl ume. Bedded cherts and marbles in the unit are regarded as ancient pelagic sediments and carbonate caps developed upon basaltic seamounts, respectivel y. Dismemberment of the seamount-related basalts and pelagic sediments and the high-P/low-T prograde metamorphism of the unit rocks up to transitional blueschist/greenschist facies was the result of paleoseamount submergence into a subduction zone. This unit is tectonically overlain by are-derived m etavolcanic unit and a disrupted, mainly clastic unit of Upper Ordovician-L offer Silurian age. Metavolcanic and metasedimentary rocks of the two upper units have geochemical characteristics compatible with subduction-related origin. The lithological assemblages of the individual units and their juxt aposition suggest an origin involving collision-accretionary processes. The Fan-Karategin belt is a subduction-accretionary complex which formed durin g subduction of oceanic crust under a volcanic are and was subjected to tec tonic juxtaposition and imbrication of seamount, deep-sea, trench and volca nic are sequences. (C) 1999 Elsevier Science B.V. All rights reserved.