GENERATION AND EVOLUTION OF SUBDUCTION-RELATED BATHOLITHS FROM THE CENTRAL URALS - CONSTRAINTS ON THE P-T HISTORY OF THE URALIAN OROGEN

Batholiths from the accreted terranes in the Urals were generated by r epeated episodes of melting and intrusion. Verkhisetsk, the largest an d one of the most complex subduction-related batholiths from the Urals , comprises an outer envelope of older tonalites, trondhjemites and gr anodiorites dated at 315-320 Ma and equilibrated at 6 kbar, intruded b y an inner core of younger granodiorites, adamellites and granites dat ed at 275-290 Ma and equilibrated at 4 kbar. Older rocks have a high-A l TTD/adakite chemistry, epsilon(chur)(320Ma)(Nd) approximate to 2-5, and initial Sr-87/Sr-86 approximate to 0.7043, with pronounced lateral zoning marked by the increase of LREE/HREE, Cr, Ni, and Mg eastwards. They were originated by melting of metabasalts in a subducted slab of young hot oceanic lithosphere with a temperature/depth trajectory tha t intersected the garnet-in univariant at approximate to 1050 degrees C and 13 kbar, thus causing the lateral zoning. We believe that such a 'warm' trajectory can be attributed to the oblique subduction of a yo ung lithosphere. Younger rocks range from Na-rich metaluminous granodi orites to K-rich peraluminous two-mica granites with relict epidote cr ystals, and have almost the same isotopic signature as older rocks, fr om which they were generated by anatexis as a consequence of a melting event that occurred throughout the Urals at 275-290 Ma and produced m ost of the huge continental-type batholiths on the eastern side. This melting event is tentatively supposed to be the result of underplating by mafic magmas, which also caused the growth of the Urals crust from the Moho downwards until it reached the increased thickness revealed by seismological studies.