New data on the composition and age of complexes in the pre-Paleozoic basement of the Tagil Paleo-island arc system in the Northern Urals (original) (raw)

ABSTRACT The Late Ordovician–Devonian intrusive, volcaa nic, and volcanoosedimentary complexes of the Tagil paleooisland arc system constitute the synonymous meridional megablock extending for over 700 km along the eastern slope of the Middle and North Urals. In some recent publications, their authors mention preePaleozoic igneous and metamorphic rocks in diff ferent parts of the megablock, which likely constituted the basement of the Paleozoic island arc [2, 5, 7–9]. The formation of complexes constituting the Tagil structure on the preePaleozoic basement was also assumed, based on the geochemical properties of voll canics, in earlier works [3, 4, 6, 10, 11, and others]. Three groups of preePaleozoic rocks forming tectonic blocks and slices in the western and eastern parts of the Tagil megablock are definable in the Middle Urals: (1) dunite–clinopyroxenite–tylaite association [2, 9] and olivine gabbro [5, 7], which represent constituents of polyformation mafic–ultramafic massifs in the Platii nummbearing belt (PBB) of the Urals; (2) metamorr phosed gabbro of the ophiolite type [8]; (3) Belaya Gora gneiss–amphibolite complex defined in strucc tures surrounding the Kytlym pluton [7]. The available Sm–Nd isochron dates indicate that all these rocks are Vendian in age (542–574 Ma). Our investigations revealed the same three Vendian complexes that were studied and dated in the Tagil megablock of the North Urals. The aboveementioned rocks constitute isolated tectonic blocks or large xenoliths among Silurian gabb broids. Taking into consideration the data available in [7, 8], it may be concluded that preePaleozoic comm plexes are widespread in the basement of the Ordovii cian–Devonian Tagil paleooisland arc. When carrying out investigations in the Northern Urals, we received new data on the composition and age of rock associations constituting the basement of the Tagil paleooisland arc. In this large region, we have defined and dated all three aboveementioned Paleoo zoic rock associations: (1) olivine gabbro in the Yalpp ingn'er massif; (2) amphibolites and gneisses of the Belaya Gora Complex; and (3) amphibole–sossurite metagabbro of the ophiolite type that constitute screens in the complex of parallel dolerite dikes. Olivine gabbro forms an irregularly shaped block 1.8 × 3.5 km in size in structures surrounding amphibb ole gabbro in the northern part of the Yalpingn'er mass sif (Khuntyn'ya and Arbyn'ya river interfluve). Their contacts with host rocks are unobservable. Olivine gabbro are mediumm to coarseegrained locally banded gray–green rocks composed of anorthite (up to 50%), clinopyroxene (20–30%), olivine (up to 10–15%), orthopyroxene (up to 15–20%), pargasite, green spinel, VVbearing (1.1–1.2%) magnetite with ingrowths of Mn ilmenite (2.0–2.8%), saussurite, zoisite, epidote, and serpentine. Gneisses and amphibolites of the Belaya Gora Complex occur in tectonic slices in the zone of the Main Uralian Fault west of the Yalpingn'er massif (Fig. 1). The complex includes garnettbearing biotite–amphibole and bimicaceous gneisses, garnet– muscovite–quartz crystalline schists, and plagioo clase–hornblende amphibolites. The latter represent dark gray to gray finee to mediummgrained rocks comm posed of Mg hornblende and plagioclase. Gneisses are composed of almandine garnet, plagioclase (albite– oligoclase), amphibole (ferrochermakite, ferroparr gasite), biotites, phengite (Na 2 O, 1.9–2.1%), and quartz. Mineral parageneses imply that the temperaa tures and pressures of the formation of these metaa morphic rocks were 450–500°C and up to 8–9 kbar, respectively. Metamorphosed gabbro of the ophiolite type occurs in the form of lenticular blocks (screens) and small residual massifs among the complex of parallel dolerite dikes most widespread in the Vizhai and Ivdel