Quartz diorites of the volkovskiy ore-bearing massif (middle Urals, Russia): U–PB age, Nd–Sr–Pb isotope systematics, geochemical features, petrogenetic and geodynamic implications

The results of U–Pb age determination, Nd–Sr–Pb isotope systematics and geochemical study of quartz diorites associated with ore-bearing (Cu–Pd–Au–Ag) gabbro of the Volkovskiy massif localized within the Ural Platinum Belt, at its eastern border with Silurian-Devonian volcanogenic complexes of the Tagil megazone are presented. The age of quartz diorites by U–Pb (TIMS) age determination is 429±9 mln years, MSWD = 0.009. Sr–Nd–Pb isotope characteristics (ƐNd(T) = +5.5 ÷ +6.7; (87Sr/86Sr)t= 0.70382−0.70392;206Pb/204Pb = 18.38−18.57;207Pb/204Pb = 15.56−15.58;208Pb/204Pb = 38.14−38.30) indicate a juvenile source with a model age of 570−760 mln years. The obtained data do not go beyond the values typical for basalts of enzymatic island arcs. The geochemical features of the granitoids of the Volkovskiy massif (low REE concentration, differentiation of their spectrum (La/Yb = 8−14) with weak positive Eu-anomaly (Eu/Eu* = 0.9−1.4)) are consistent with the characteristics of melts obtained in water melting experiments for mafic rocks in equilibrium with amphibole-pyroxene restite. The anomalous strontium concentration (more than 1000 g/t) in quartz diorites is due to the high content of this element in the source. Such source could be the earliest rocks of the Ural Platinum Belt – olivine gabbro and surrounding metamorphic rocks. The same age of quartz diorites of the Volkovskiy massif and monzonitoids of the Kushvinskiy massif allows us to consider these rocks as a result of the mantle and crust melting converged in time at the final stage of the Tagil island-arc system formation.