HORNBLENDE GABBRO IN THE URALS: TYPES, GEOCHEMISTRY, AND PETROGENESIS

Abstract

Hornblende gabbroids, which are widespread in the Urals, are products of hydrous basic magmatism, mainly subduction-related. Two groups of these rocks are recognized: (1) pre-Middle Devonian (older than 390 Ma) gabbroic rocks associated with ultrarnafics and (2) Late Devonian-Permian (360 Ma and younger) rocks somehow related to granitoids. The rocks of these two groups systematically differ from one another by the contents of most major and trace elements. These differences display, first of all, compositional variations of the magma sources, primarily the metasomatized mantle wedge doped with components of a hydrated slab with fluid. The postophiolitic hornblende gabbro and its varieties associated with granitoids reveal a notable crustal contamination. The hornblende gabbroids are characterized by deformation structures related to the motion of the incompletely consolidated, fluid-rich mass and to the effects of partial melting, locally imparting a migmatite appearance (automagmatic migmatization). This process was caused by the enrichment of the residual basic melt in water and gave rise to the formation of granitoid magma. The products of granitoid magma crystallization make up large tonalite-granodiorite plutons at continental margins. The same process is responsible for the origin of anorthosite-plagiogranite series in the Platinum Belt of the Urals.