Geochemistry of amphibolites associated with ophiolite massifs of the Urals (on the example of Rai-Is and Middle Kraka) as a key to unravelling of their origin

Abstract

Introduction: amphibolites often occurs within the framework of ophiolite massifs, but their relationship is not always clear. In order to clarify their source and conditions of formation, we carried out mineralogical and geochemical studies of amphibolites from the Kaershor (Polar Urals) and Sukholyad (Southern Urals) complexes, confined to the Rai-Iz and Middle Kraka ophiolite massifs, respectively.

Methodology: petrographic and mineralogical studies were carried out using optical (Altami Polar 3) and scanning electron (Tescan Vega Compact) microscopy. The chemical composition of the rocks was determined using X-ray fluorescence analysis (X-Calibur) and inductively coupled plasma mass spectrometry (Agilent 7500cx).

Results and discussion: amphibolites are most characterized by a gneissic structure and nematogranoblastic texture, and the main mineral amphibole corresponds to magnesiohornblendite, sadanagaite and pargasite. The amphibolites of the Kaershor complex are relatively homogeneous in geochemistry and according to geothermobarometry of amphibole and plagioclase were formed at T = 673–581 °C and P = 7.7–4.9 kbar. The amphibolites of the Sukholyad complex were formed at =776–681 °C and P=4.9–2.6 kbar; they are diverse in geochemical features, which made it possible to suggest three types of protolith.

Conclusions: the geochemical features of the Kaershor amphibolites indicate their origin from a basic source similar to the island arc calc-alkaline basalt. The following types of protoliths have been proposed for the Sukholyad amphibolites: 1) calc-alkaline island arc basites, 2) island arc tholeiitic basites, and 3) volcanic-sedimentary rocks of the island arc. The totality of the obtained mineralogical and geochemical data indicates that amphibolites may be parts of ophiolite associations of the suprasubduction (SSZ) type.