The nature and age of the Suroyam clinopyroxenite massif (Middle Urals)

Abstract

The Suroyam alkaline ultrabasitesyenite (mainly pyroxenite) massif, including the Suroyam apatitetita� nomagnetite deposit, is a part of the Nyazepetrovsk structural-formational zone located on the western slope of the Middle Urals, within the Ufimskii promon� tory of the East European Platform. The Nyazepetrovsk complex tectonically overlaps the sedimentary shelf and bathyal deposits of the mar� gin of the Paleozoic continent (Fig. 1) and is the uppermost tectonic sheet developed here. The com� plex is made of volcanogenic-sedimentary and intru� sive rocks of Ordovician-Early Devonian age, approximately correlated with the Early and Late islandarc oceanic formations of the Tagil zone (1). According to the chemical composition and metallo� genic features, the Suroyam massif located in the area of these formations can be compared with massifs of a palladiumbearing "branch" of the Platinum belt (Ural PGEbearing belt) in the Urals. The Rb-Sr iso� chrone age of the massif is 415.2 ± 1.1 Ma. Recent data obtained provide confirmation of the allochthonous origin of the Suroyam massif and its positional con� nection with the Tagil zone. The stratigraphic section of the Nyazepetrovsk zone is represented by volcanic and volcanogenic- sedimentary formations with subordinate siliceous rocks, siltstone, and sandstones. The section is distin� guished into three formations (from ancient to young): basalt, basalt-andesitebasalt, and trachybasalttra� chyte. Based on rare findings of radiolarians, con� odonts, and macrofauna, the age of the formations is regarded as Ordovician-Early Devonian. The Suroyam massif is located in the southern area of the axial part of the Nyazepetrovsk zone, and it is mainly composed of apatitetitanomagnetite clinopy� roxenites, subjected to strong superimposed potassium and sodium metasomatism, connected with manifes� tations of alkaline magmatism (2). The nature of the massif is under discussion. There were ideas expressed in (3, 4) about its platform origin, but more convincing evidence is needed. We have identified, however, sim� ilarities of the Suroyam massif with some specific mas� sifs of the Platinum belt with titanomagnetite mineral� ization. In particular, among the most widespread types in the Suroyam massif (Fig. 2) are highferrugi� nous clinopyroxenites, green and dark green rocks, characterized by fine to coarse grained structure, as well as pegmatoid structure. The main rockforming minerals are clinopyroxene, titanomagnetite, apatite, and phlogopite. The quantitative ratios of these miner� als vary greatly. The central part of the Suroyam massif is composed of coarsegrained titanomagnetite cli� nopyroxenites (like kosvites) with a titanomagnetite content of 6-25%; the marginal part is characterized mainly by apatitetitanomagnetite types of rocks, con� taining 2.9-19.5% of apatite. The undeformed rocks have sideronitic and granoblastic texture. Clinopyroxene forms short and long prismatic grains. According to the chemical composition, cli� nopyroxene is represented by diopsidehedenbergite, containing 25-30% of the hedenbergite molecule. The iron content in pyroxene ranges from 14 to 26% (20 at. % on average). There was noted a high content of alumina (2.49-5.05%) with variations in the augite content from 6 to 11%. The titanomagnetite is lowtitanium, and it is rep� resented by xenomorphic grains with sideronitic tex� ture. In addition it forms finegrained, platy, den� dritic, and dropshaped inclusions in clinopyroxene. In general, titanomagnetite is distributed uniformly in the rock. However, there are sections with a banded distribution of ore minerals. In titanomagnetite there are inclusions of xenomorphic clinopyroxene and prismatic apatite. Sometimes, the ore mineral forms