Conditions of crystallization of dunite from the Nizhnii Tagil platinum-bearing ultrabasic massif (Urals)

Abstract

Platinumbearing dunite–clinopyroxenite–gabbro complexes of the Urals have attracted the attentionof researchers for a long time and are currently beingstudied in detail. In addition, despite the increasingvolume of information, the conditions of petrogenesisof these associations are still debatable. Of specialimportance are the problems of the formation of dunitecontrolling chromite–platinum mineralization [1–4].The problems of genesis of these ultrabasic rocks wereconsidered in numerous papers; most of them containdata on the significant role of magmatic processes incrystallization of dunite [3, 5–8]. At the same time,most published papers providing evidence for themagmatic nature of dunites from zoned platinumbearing complexes of the Urals practically do not contain direct proofs of the participation of hightemperature silicate melts.Investigations of basic–ultrabasic complexes ofSiberia, the Urals, the Far East, and the Central Atlantic demonstrate that analysis of chrome spinellids andmelt inclusions in them is the most promising way toobtain direct information on the genesis of ultrabasicrocks [9–13]. This approach was applied for study ofthe condition of crystallization of dunite from theNizhnii Tagil ultrabasic massif (Platinumbearing Beltof the Urals).Information on the geological structure of theNizhnii Tagil massif, petrography, mineralogy, and orepotential has been discussed in many publications [1–4, 6, 7, 14]. It forms an ellipselike body in plan with thecentral part composed of a large dunite core surrounded by a clinopyroxenite rim of complex morphology. Petrostructural investigations allowed theauthors of [7] to distinguish protogranular, porphyroclastic, and mosaic–granoblastic types of microtextures. The first type is considered as primary corresponding to the class of protomagmatic adcumulatetextures. The two other types are formed later in theprocess of hightemperature plastic deformation andsyntectonic recrystallization.Investigation of dunite samples from the NizhniiTagil massif mainly corresponding to the protogranular (primary) type allowed us to discover and studymelt inclusions in accessory chrome spinellids. Theconsidered chromites are almost intransparent and donot provide direct observations during heating;because of this feature, a special methodology [10, 12]was applied. Chrome spinellid grains were heated upto 1320–1330°C in graphite microcontainers of a special microthermocamera in the course of hightemperature experiments. After being kept for 10 min,they were quenched in water in order to obtain glass ininclusions. Glasses of quenched inclusions andchrome spinellids were studied on a CamebaxMicroXray microanalyzer and on a LEO 1430 VP scanningelectron microscope at the Institute of Geology andMineralogy, Siberian Branch, Russian Academy ofSciences (Novosibirsk).According to the values of the chromium and magnesium mole fractions, chrome spinellids (with meltinclusions) from dunite of the Nizhnii Tagil massif arepractically consistent with chromites from dunite ofthe platinumbearing Konder and Inagli massifs(Siberian Platform). The studied chrome spinellidsdiffer from minerals of ultrabasic rocks from modernand ancient (ophiolites) oceanic structures. As awhole, chrome spinellids from platinumbearing massifs are characterized by a high chromium mole fraction (Cr# = 78–89%). Chromites from oceanic complexes provide a single trend of Cr# decrease (from65 to 45%) at a significant decrease in Mg# (from 78to 28%).The studied primary melt inclusions (10–40 µm)are regularly located in grains of accessory chromite.Inclusions have round morphology, being in equilibrium with host chromite, and are often slightly faceted. Before experiments they contained a set of vari