eCite Digital Repository

Geochemistry of ultrahigh-pressure anatexis: fractionation of elements in the Kokchetav gneisses during melting at diamond-facies conditions

Citation

Stepanov, AS and Hermann, J and Korsakov, AV and Rubatto, D, Geochemistry of ultrahigh-pressure anatexis: fractionation of elements in the Kokchetav gneisses during melting at diamond-facies conditions, Contributions to Mineralogy and Petrology, 167, (3) Article 1002. ISSN 0010-7999 (2014) [Refereed Article]

Copyright Statement

Copyright 2014 Springer-Verlag Berlin Heidelberg

DOI: doi:10.1007/s00410-014-1002-x

Abstract

The Kokchetav complex in Kazakhstan contains garnet-bearing gneisses that formed by partial melting of metasedimentary rocks at ultrahigh-pressure (UHP) conditions. Partial melting and melt extraction from these rocks is documented by a decrease in K2O and an increase in FeO + MgO in the restites. The most characteristic trace element feature of the Kokchetav UHP restites is a strong depletion in light rare earth elements (LREE), Th and U. This is attributed to complete dissolution of monazite/allanite in the melt and variable degree of melt extraction. In contrast, Zr concentrations remain approximately constant in all gneisses. Using experimentally determined solubilities of LREE and Zr in high-pressure melts, these data constrain the temperature of melting to ~1,000 °C. Large ion lithophile elements (LILE) are only moderately depleted in the samples that have the lowest U, Th and LREE contents, indicating that phengite retains some LILE in the residue. Some restites display an increase in Nb/Ta with respect to the protolith. This further suggests the presence of phengite, which, in contrast to rutile, preferentially incorporates Nb over Ta. The trace element fractionation observed during UHP anatexis in the Kokchetav gneisses is significantly different from depletions reported in low-pressure restites, where generally no LREE and Th depletion occurs. Melting at UHP conditions resulted in an increase in the Sm/Nd ratio and a decoupling of the Sm–Nd and Lu–Hf systems in the restite. Further subduction of such restites and mixing with mantle rocks might thus lead to a distinct isotopic reservoir different from the bulk continental crust.

Item Details

Item Type:Refereed Article
Keywords:anatexis, UHP metamorphism, REE, accessory minerals
Research Division:Earth Sciences
Research Group:Geology
Research Field:Igneous and Metamorphic Petrology
Objective Division:Expanding Knowledge
Objective Group:Expanding Knowledge
Objective Field:Expanding Knowledge in the Earth Sciences
Author:Stepanov, AS (Mr Sasha Stepanov)
ID Code:94698
Year Published:2014
Web of Science® Times Cited:9
Deposited By:Centre for Ore Deposit Research - CODES CoE
Deposited On:2014-09-16
Last Modified:2015-03-05
Downloads:0

Repository Staff Only: item control page