Ultrabasic stocks of the Khudolaz differentiated complex (Southern Urals) as evidence of magma differentiation in intermediate chambers

Abstract

Introduction: The Khudolaz differentiated complex of the Southern Urals is characterized by a complex geological structure and a wide variety of intrusions in terms of morphology, petrography and mineralogy. It includes a belt of ultrabasic stocks containing sulfide-platinum-metal mineralization. The issues of origin of the complex are debatable, therefore, the mineralogical and geochemical study of ultrabasic rocks makes it possible to better understand the problem of the Khudolaz complex genesis. Methodology: The chemical composition of the rocks was determined using an X-Calibur X-ray fluorescence spectrometer and an Agilent 7500cx mass spectrometer. The study of the Sm-Nd isotope system was carried out on a seven-channel Finnigan-MAT 262 (RPQ) mass spectrometer, the Rb-Sr isotope system was studied using an MI-1201-T mass spectrometer. Results and discussion: The main rock type is schriesheimite having a cumulative structure characterized by the presence of olivine phenocrysts with a variable amount of plagioclase and intercumulus amphibole. It is shown that variations in the chemical composition of rocks are associated with trends in the fractionation of the three main minerals of rocks - olivine, amphibole and plagioclase. In general, the geochemical features of the rocks are close to the average composition of island arc basalts. A model for the formation of schriesheimite stocks is proposed, which includes the differentiation of the initial melt in intermediate chambers followed by the migration of partially crystallized magma from the cumulus part of the chamber. At the place of intrusion clinopyroxene was replaced by amphibole due to interaction with a water-saturated melt/fluid. An additional source of water could be the crust, as indicated by a sharp positive anomaly of cesium in the rocks. Conclusion: The Sr-Nd isotope data indicates an insignificant contribution of the upper crust material to the source of the melt. The source of parental magma was a component obtained by the interaction of the spinel-peridotite mantle and the lower part of the island-arc crust.