PLATINUM-GROUP ELEMENTS AS PETROLOGICAL INDICATORS IN MAFIC-ULTRAMAFIC COMPLEXES OF THE CENTRAL AND SOUTHERN URALS - PRELIMINARY-RESULTS

Platinum-group elements (PGE) and gold were analyzed by ICP-MS after t he Ni-sulphide fire assay pre-concentration step in 44 rocks from 5 or ogenic mafic-ultramafic massifs of the central and southern Urals. The y comprise: (a) ring-zoned, Ural-Alaskan type bodies of the 'Pt-bearin g belt' (Kachkanar, Tagil, Uktus); and (b) lherzolite-harzburgite comp lexes overlain by layered wehrlite, gabbro, and diorite (Nurali, Mindy ak) supposed to be ophiolite-type suites. Total PGE and Au contents ar e generally higher in Ural-Alaskan type complexes compared with lherzo lite-harzburgite complexes. Mantle-normalized PGE patterns range from nearly unfractionated in the dunites (Pd/Ir = 0.57-3.68) to mildly and strongly fractionated in wehrlite, clinopyroxenite, and gabbro (Pd/Ir = 9.1-187). Exceptionally high values of Pd/Ir (761-2622) are observe d in amphibole-rich lithologies at Kachkanar, possibly indicating a re lationship between fractionation of the less refractory Pd and increas ed fluid activity in the parental magma. Pronounced, positive Pt-anoma lies, up to 20 times the mantle estimate, distinguishes the Ural-Alask an type dunites, and progressively reduces in pyroxenites and gabbros. In the lherzolite-harzburgite complexes of Mindyak and Nurali, layere d wehrlites and pyroxenites, as well as rocks from the gabbro-diorite units have fractionated PGE patterns (Pd/Ir = 2.94-92.39) and positive Pt-anomaly as in the Ural-Alaskan complexes, Conversely, the Pt-anoma ly is negative in the peridotites, and increases from lherzolites and harzburgites, to dunites, similarly to residual mantle from alpine-typ e orogenic massifs, The Pd/Ir (0.71-3.7) is higher than in ophiolites, and resembles that of Iherzolite massifs of the western Mediterranean , representing sub-continental mantle.Variation of the Pt-anomaly vers us Pd/Ir in Ural-Alaskan type complexes and in the layered sequences o verlaying lherzolite-harzburgite mantle suites is consistent with magm atic fractionation, in which Pt behaves as a compatible element. It re flects a progressive depletion by partial melting in mantle peridotite s, the Pt becoming incompatible in the last stage of melting in which Pt-poor residual dunites are formed. The positive Pt-anomaly in Ural-A laskan type complexes is striking, and would appear as a distinctive f eature of the parent magma, probably inherited from its mantle source.