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The Uralian orogen has abundant Hercynian granitic rocks, organized in medium-sized batholiths with a section either roughly circular or, more commonly, N-S elongated. Urals granitoids show strong W-E polarity, from subduction-related batholiths in the accreted terrains in the West, to continental-type batholiths in the East (Fershtater et al., 1994; Fershtater et al., 1997). Subduction-related batholiths are formed by a wide variety of granitoid facies which have extraordinarily complex field relationships due to repeated episodes of melting and intrusion. The existence of multiple intrusion pulses clearly separated in time, together with the close spatial relationship with the Main Uralian and Serov-Mauk faults (sutures) as well as with other large-scale shear zones (Fig. 1) make these batholiths potentially useful markers for unravelling the tectono-magmatic evolution of the Urals. A presentation (Bea et al., 1997) at the Europrobe Uralides-Variscides meeting in Granada (April 1996) described the petrogenesis of Verkhisetsk, the largest and one of the most complex subduction-related batholiths in the Central Urals, as a tool to obtain insight into the tectono-magmatic evolution of the Urals collisional orogen. The most important conclusions are summarized here.
The Verkhisetsk batholith, c. 100 km long and c. 25 km wide, is elongated N-S ( Fig. 2), and located between the Serov-Mauk (in the west) and Ekaterinburg (in the east) shear zones; it intrudes middle Palaeozoic metavolcanic rocks. In general, the Verkhisetsk batholith comprises an outer envelope - composed of older, coarse-grained, strongly deformed tonalites, trondhjemites and granodiorites with Rb-Sr ages of 315-320 Ma - intruded by an inner core composed of younger (Rb-Sr ages of 285-275 Ma), fine-grained, undeformed granodiorites, adamellites and granites. The younger intrusions frequently contain rounded and partially assimilated enclaves of older rocks and usually have sharp intrusive contacts with the latter, although gradational contacts and migmatite-like relationships are also common. Older intrusions crystallized at c. 6 kb and have a chemical composition similar to that of high-Al TTG/adakite, with positive etchur(Nd) and initial 87Sr/86Sr ratios c. 0.7043; these characteristics suggest that they crystallized from magmas produced by partial melting of metabasalts at a depth of c. 50 km. They show a marked west to east magmatic polarity, with an increase of Mg number, Cr and Ni contents (Fig. 3) and LREE/HREE fractionation (Fig. 4). This asymmetric zoning, as well as the close spatial relationship to the Serov-Mauk suture, clearly favours the idea that they were derived from a subducted slab of oceanic lithosphere.
The pronounced lateral zoning of the Verkhisetsk batholith gives further information about melting conditions. The low LREE/HREE fractionation of western magmas suggest that they did not leave garnet as a residual phase, whereas eastern magmas did; therefore, the garnet-in univariant marked the boundary between their respective generation loci within the subducted slab. Recent experimental work on dehydration melting of metabasalt (Rapp and Watson, 1995; Wyllie and Wolf, 1993) (Fig. 5) indicates that at 1000 øC the garnet-in univariant is placed at 13 kb approx., and that just below this univariant and inside the amphibole stability field, the melt fraction at 1000-1050 øC is as high as 25-30%, sufficient to permit melt segregation (Rushmer, 1995; Wolf and Wyllie, 1995). We therefore suggest that the melting conditions for the "Western" intrusions were near 1000-1050 C and 12-13 kb, at slightly lower pressure than the garnet-in univariant (Fig. 5). Given the close spatial relationships and similar age, the "Eastern" magmas were obviously generated on the same geotherm, but at slightly higher pressure than the garnet-in univariant, near 1050-1100 øC and 13-14 kb (Fig. 5). The differences in Cr, Ni, and Mg contents suggest that the intensity of the magma-mantle interaction increased eastwards (see Drummond et al., 1996), thus indicating that the temperature of the asthenospheric wedge above the melting slab also increased, which is consistent with the hypothesis that the depth of magma generation increases towards the east. The younger intrusions of the Verkhisetsk batholit were equilibrated at c. 4 kb. The existence of migmatite-like structures as well as the abundance of enclaves of older rocks with different stages of assimilation indicate that these intrusions were produced by anatexis of older rocks. Major-, trace- element, and isotope geochemistry also support this interpretation. The composition of the less silicic younger intrusions is indistinguishable from that of older suite with similar silica content, probably due to insignificant melt-restite segregation. The composition of the more silicic younger intrusions, however, becomes increasingly potassic and peraluminous as silica increases. This effect could be due either to low-pressure magmatic differentiation of anatectic melts, involving amphibole fractionation, or by the segregation of low melt-fraction magmas with Q-Or-Ab compositions near the "ternary minimum", these being enriched in alumina by the effect of reequilibration of restitic amphibole at lower pressure. The great abundance of dykes and the homogeneous undeformed fabric of the younger intrusions indicate that they were generated during an extensional episode.
The thermal regime, inferred above for the generation of the older magmatic suite, implies a temperature of c. 1000 øC at c. 50 km depth, corresponding to a "warm" geotherm of c. 20 øC/km. Several authors have shown that the temperature/depth trajectory in a subduction zone depends mainly on the age of the incoming lithosphere, the amount of previously subducted lithosphere, the vigour of convection in the mantle wedge induced by the subduction slab, the convergence rate and, to a lesser extent, the existence of high rates of shear stress (Peacock, 1990; Cloos, 1993; Peacock et al., 1994). According to these authors, such a "warm" thermal regime may be caused by two mechanisms that are not mutually exclusive: the subduction of young lithosphere, or highly oblique convergence involving slow subduction and high shear stresses. The geometry of large-scale shear zones in the central Urals (Fig. 1) suggests that oblique convergence existed but, according to available numerical modelling of shear heating (Peacock et al., 1994), it is unlikely to have produced temperatures of c. 1000 øC at 12 kb by itself. We therefore suggest that the inferred thermal regime for the Verkhisetsk massif also required the subduction of a young lithosphere, which might have been generated by back-arc spreading during the early Carboniferous stages of the Urals collision (Fershtater and Bea, 1996).
Granites with an age of 275-290 Ma are extraordinary abundant throughout the Urals. In the hinterland, most major batholiths appear to be of this age. 207Pb/206Pb single zircon grain dating of the Dzyabyk massif, for example, gives an age of 292 Ma (unpublished). Reported Rb-Sr ages for Mochagui, Murcinka, etc. are in this range (see compilation in Fershtater et al., 1994). In the western part of the massif, where subduction-related batholiths predominate, the melting event at 275-290 Ma is represented by the partial melting of older batholiths and the appearance of younger rocks as described above for Verkhisetsk. The interpretation of this melting event is not clear. On the one hand, the small pressure variation recorded by Verkhisetsk rocks (from 6 kb to 4 kb) precludes decompressional melting. On the other hand, the low amount of heat-producing elements in the Urals crust (Fershtater et al., 1997, and unpublished data) also precludes melting related to the accumulation of radiogenic heat in an overthickened crust. It seems therefore that the melting event at 275-290 Ma required an input of energy from below. One key point for understanding in what way the heat for melting was supplied to the lower crust could come from the fact that the depth of the Moho beneath the Urals is currently placed at c. 50 km (Thouvenot et al., 1995). Since we cannot expect the Urals island-arc crust to be significantly thicker than recent island-arcs (20-30 km, Tanimoto, 1995), it is logical to suppose that the Urals crust grew from the Moho downwards, with the most feasible mechanism being repeated underplating by mafic magmas. Once heat was available, the fertile nature of the eastern crust - with a significant quartzo-feldspathic component - produced abundant continental-type granite batholiths. In the west, the low fertility of the western crust - mostly formed by metabasalts - restricted the generation of younger granites to the reactivation of former batholiths.
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Last updated: December 29, 1999 Your comments are welcome! © Irina Artemieva, Department
of Earth Sciences, Uppsala University, Sweden Some icons and ideas have been taken from: El Pais Digital, Spektrum
der Wissenschaft (Scientific American), |
