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

JOURNAL OF PETROLOGY Journal of Petrology, 2016, Vol. 57, No. 8, 1531–1554 doi: 10.1093/petrology/egw049 Original Article Melting History of an Ultrahigh-pressure Paragneiss Revealed by Multiphase Solid Inclusions in Garnet, Kokchetav Massif, Kazakhstan Aleksandr S. Stepanov1,2*, Joerg Hermann1,3, Daniela Rubatto1,3, Andrey V. Korsakov4,5 and Leonid V. Danyushevsky2 1 Research School of Earth Sciences, the Australian National University, Canberra, ACT 0200, Australia; 2ARC Centre of Excellence in Ore Deposits (CODES), School of Physical Sciences, University of Tasmania, Private Bag 79, Hobart, TAS 7001, Australia; 3Institute of Geological Sciences, University of Bern, Baltzerstrasse 3, 3012 Bern, Switzerland; 4V. S. Sobolev Institute of Geology and Mineralogy of Siberian Branch of Russian Academy of Sciences, Koptyug Pr. 3, Novosibirsk 630090, Russia and 5Novosibirsk State University, 2 Pirogova Str., Novosibirsk-90 630090, Russia *Corresponding author. Present address: ARC Centre of Excellence in Ore Deposits (CODES), School of Physical Sciences, University of Tasmania, Private Bag 79, Hobart, TAS 7001, Australia. Telephone: þ61 3 6226 1027. Fax: þ61 3 6226 7662. E-mail: Received April 21, 2015; Accepted July 29, 2016 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 DRb ¼ 19–25, DBa ¼ 11–69 and DCs ¼ 06–08, 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 (04–08), whereas the low-temperature domains display low Ti (10 ppm) and Th/U (008). 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. C The Author 2016. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: V 1531 1532 Journal of Petrology, 2016, Vol. 57, No. 8 Key words: anatexis; Kokchetav complex; melt inclusions; ultrahigh pressure; subduction zone; collision; shoshonite INTRODUCTION Partial melting of deeply subducted crust is a key process for element recycling through subduction zones and a major process for the chemical differentiation of Earth (e.g. Hermann & Rubatto, 2014). To understand this element recycling, it is crucial to know the composition of melts generated during ultrahigh-pressure (UHP) anatexis of crustal rocks. There are only a few places in the world where such processes can be studied. The Kokchetav complex, situated in the Central Asia Fold Belt of Kazakhstan, experienced some of the highest peak metamorphic conditions recorded in crustal rocks (>45 kbar, 950–1000 C; Sobolev & Shatsky, 1990; Schertl & Sobolev, 2013). Partial melting of gneisses at UHP conditions has been proposed based on bulk-rock variations (Shatsky et al., 1999; Stepanov et al., 2014b) as well as zircon characteristics (Hermann et al., 2001). Fluid and melt inclusions found in carbonate and silicate rocks from the Kokchetav complex record the unusual composition of fluids produced by UHP metamorphism (Dobrzhinetskaya et al., 2005; Hwang et al., 2005, 2006; Korsakov & Hermann, 2006). However, the majority of these findings were made in carbonate and calc-silicate rocks of uncommon composition that may not be representative of the majority of subducted crust. UHP gneisses within the Kokchetav complex derive from metamorphism of normal terrigenous sediments and they are characterized by depletion in a number of trace elements, particularly light rare earth elements (LREE), Th and U (Shatsky et al., 1999; Stepanov et al., 2014b). This depletion has been related to the extraction of melt at UHP conditions. The composition of melts at UHP conditions has been deduced by comparing restites with inferred protolith compositions (Behn et al., 2011; Stepanov et al., 2014b), but no direct constraint on the composition of such UHP melt has been obtained so far. Behn et al. (2011) suggested that melting of sediment diapirs rising in subduction zones can be compared with UHP melting, as inferred in the Kokchetav massif, and that such melts contribute to the characteristic enrichment of large ion lithophile elements (LILE) and LREE in arc basalts. Other researchers have proposed that some syn- to post-collisional magmas in orogenic belts might be derived from melting of subducted continental crust at UHP conditions (Massonne, 2009; Hermann & Rubatto, 2014). Partial melts derived from UHP gneisses might also interact with the mantle wedge, resulting in a range of igneous rocks with affinities to subducted continental crust (Campbell et al., 2014; Zheng & Hermann, 2014). In this study we provide a detailed description of a sample of Kokchetav UHP gneiss with abundant multiphase solid inclusions (MSI) and document hightemperature, high-pressure experiments that achieved homogenization of the inclusions to glasses. The major and trace element compositions of the homogenized inclusions are complemented by trace element analysis of rock-forming and accessory minerals. The results rev (...truncated)