Generation and evolution of subduction-related batholiths from the central Urals: constraints on the P-T history of the Uralian orogen

Abstract

Batholiths from the accreted terranes in the Urals were generated by repeated episodes of melting and intrusion. Verkhisetsk, the largest and one of the most complex subduction-related batholiths from the Urals, comprises an outer envelope of older tonalites, trondhjemites and granodiorites dated at 315-320 Ma and equilibrated at 6 kbar, intruded by an inner core of younger granodiorites, adamellites and granites dated at 275-290 Ma and equilibrated at 4 kbar. Older rocks have a high-A1 "VI'D/adakite chemistry, _320Ma/~r.~x ~ 2--5, and initial 87Sr/86Sr ~,~ 0.7043, with pronounced lateral ~chur ~I~U) 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 that intersected the garnet-in univariant at ~1050°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 young lithosphere. Younger rocks range from Na-rich metaluminous granodiorites to K-rich peraluminous two-mica granites with relict epidote crystals, and have almost the same isotopic signature as older rocks, from which they were generated by anatexis as a consequence of a melting event that occurred throughout the Urals at 275-290 Ma and produced most 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.

Peraluminous granites frequently with mantle-like isotope compositions: the continental-type Murzinka and Dzhabyk batholiths of the eastern Urals

“…K 2 O/Na 2 O>1, ASI~1.0±1.25, elevated Rb, U, Th and LREE and low Sr/Ba ratios) that clearly differentiate them from West Uralian subduction-related granitoids, formed by partial melting of an oceanic source (e.g. Bea et al 1997). They are comparable in many regards to certain post-collisional Variscan granitoids of western Europe that were produced predominantly by crustal melting (e.g.…”

“…Steer et al 1998;Brown et al 1998) indicate that late-orogenic extension was very limited, so that no substantial amount of heat could have been conducted from below. Bea et al (1997) have attributed the lower Permian remelting of older rocks from West Uralian subduction-related batholiths to underplating by mafic magmas, and have invoked the same mechanism for East Uralian granites due to the similar ages; however, there is no direct evidence for such a mechanism either in Murzinka or in Dzhabyk, except, perhaps, the existence of the younger mafic Kozubaeevsk intrusion in the latter case.…”

“…The western Urals contain abundant lower-to middle-Carboniferous batholiths composed of metaluminous tonalites, trondhjemites, granodiorites and granites intruded into middle-Paleozoic accreted terranes that consist of oceanic and island-arc volcanic rocks (e.g. Bea et al 1997). The eastern Urals, in contrast, contain upperCarboniferous to Permian batholiths that comprise mainly peraluminous K-rich granites and granodiorites emplaced in a basement of Proterozoic and Paleozoic gneisses and volcano-sedimentary units (e.g.…”

Paleozoic amphibolite-granulite facies magmatic complexes in the hinterland of the Uralide Orogen

“…ca. 320 Ma) are 10±20 Ma older than the Rb±Sr and Bea et al (1997) for similar rocks further south. This may imply a northwards migration of magmatism with time or reflect the differences in closing temperatures.…”

“…Bea et al (1997) demonstrate from field relationships, together with isotope and trace element studies of the Verkhisetsk batholith (Fig. 2), that the Permian granitoids are derived by partial melting of the TTG intrusions and that this occurred at moderate crustal levels (P " 4 kbar).…”