Melting History of an Ultrahigh-pressure Paragneiss Revealed by Multiphase Solid Inclusions in Garnet, Kokchetav Massif, Kazakhstan

Abstract

Abundant multiphase solid inclusions (MSI) were found in garnet in an ultrahigh-pressure (UHP) paragneiss from the Kokchetav complex, Kazakhstan. The MSI are composed of mineral associations that include rock-forming and accessory minerals, which crystallized during exhumation. We present experimental and analytical protocols for how such inclusions can be homogenized to glass and analysed for major and trace elements. After homogenization we identified two types of glass. One type is present in garnet porphyroblasts in the melanocratic part of the sample and represents a high-pressure melt formed close to peak conditions of >45 kbar, 1000°C. These inclusions are characterized by high concentrations of light rare earth elements (LREE), Th and U. Extraction of these melts resulted in a pronounced depletion of the Kokchetav gneisses in those elements. Measured partition coefficients of large ion lithophile elements (LILE) between phengite inclusions and melt inclusions are D Rb = 1·9–2·5, D Ba = 1·1–6·9 and D Cs = 0·6–0·8, resulting in limited depletion of these elements during partial melting in the presence of phengite. The Nb concentration in melts (27 ppm) is about double that in the restite (15 ppm), indicating slightly incompatible behaviour during UHP anatexis, despite the presence of residual accessory rutile and phengite. A second type of inclusion occurs in garnet from the leucocratic part of the rock and represents a late-stage melt formed during exhumation at 650–750°C and crustal pressures. These inclusions are characterized by low LREE and Nb and high U. Zircon domains formed during high-temperature melting are characterized by high Ti content (100–300 ppm) and unfractionated Th/U (0·4–0·8), whereas the low-temperature domains display low Ti (10 ppm) and Th/U (0·08). The composition of UHP melts with moderate enrichment in LILE, no depletion in Nb and extreme enrichment in LREE and Th is remarkably different from the trace element signature of arc basalts, arguing against involvement of this type of melting in the generation of arc crust. The composition of the UHP melt inclusions is similar to that of melt inclusions from HP crustal xenoliths from Pamir and also to some shoshonites from Tibet. UHP anatexis, as observed in the Kokchetav massif, might be related to the formation of shoshonitic alkaline igneous rocks, which are common in collisional settings.