Quartz Diorites of the Volkovsky Ore-Bearing Massif (Middle Urals, Russia): U–Pb Age, Nd–Sr–Pb Isotope Systematics, Geochemical Features, Petrogenetic, and Geodynamic Consequences

Abstract

The paper reports the results of U–Pb dating, Nd–Sr–Pb isotope systematics, and geochemical study of quartz diorites associated with ore-bearing (Cu–Pd–Au–Ag) gabbro of the Volkovsky massif, which is localized within the Urals Platinum Belt, at its eastern border with the Silurian–Devonian volcanogenic complexes of the Tagil megazone. The age of the quartz diorites according to U–Pb (TIMS) dating is 429 ± 9 Ma, MSWD = 0.009. Sr–Nd–Pb isotope characteristics (ε_Nd(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 Ma. The obtained data fall within values typical of basalts of ensimatic island arcs. The geochemical features of the granitoids of the Volkovsky massif (low REE concentration, differentiated REE pattern (La/Yb = 8–14) with a weak positive Eu anomaly (Eu/Eu* = 0.9–1.4)) are consistent with the characteristics of melts obtained in experiments on hydrous melting of mafic rocks in equilibrium with amphibole–pyroxene residue. The anomalous concentration of strontium (more than 1000 ppm) in the quartz diorites is due to the high content of this element in a source. Such source could be the oldest rocks of the Urals Platinum Belt—olivine gabbro and metamorphic rocks of their surroundings. The identical ages of the quartz diorites of the Volkovsky massif and monzonitoids of the Kushvinsky massif allow us to consider these rocks as the result of the almost simultaneous mantle and crust melting at the final stage of the formation of the Tagil island-arc system.