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Journal of Petrology Advance Access published online on April 15, 2005
Journal of Petrology, doi:10.1093/petrology/egi030
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© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oupjournals.org
Received June 24, 2004
Accepted February 16, 2005

Article

Near-Ultrahigh Pressure Processing of Continental Crust: Miocene Crustal Xenoliths from the Pamir

BRADLEY HACKER 1*, PETER LUFFI 2, VALERY LUTKOV 3, VLADISLAV MINAEV 3, LOTHAR RATSCHBACHER 4, TERRY PLANK 5, MIHAI DUCEA 6, ALBERTO PATIÑO-DOUCE 7, MICHAEL McWILLIAMS 8, and JIM METCALF 8

1 DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF CALIFORNIA, SANTA BARBARA, CA 93106-9630, USA
2 DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF BUCHAREST, BUCHAREST, 70139, ROMANIA
3 GEOLOGICAL INSTITUTE OF THE TAJIK ACADEMY OF SCIENCE, 734063, DUSHANBE, TAJIKISTAN
4 INSTITUT FÜR GEOWISSENSCHAFTEN, TECHNISCHE UNIVERSITÄT BERGAKADEMIE FREIBERG, 09599 FREIBERG, GERMANY
5 DEPARTMENT OF EARTH SCIENCES, 685 COMMONWEALTH AVENUE, BOSTON UNIVERSITY, BOSTON, MA 02215, USA
6 DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, TUCSON, AZ 85721, USA
7 DEPARTMENT OF GEOLOGY, UNIVERSITY OF GEORGIA, ATHENS, GA 30602, USA
8 GEOLOGICAL AND ENVIRONMENTAL SCIENCES, STANFORD UNIVERSITY, STANFORD, CA 94305-2115, USA

* To whom correspondence should be addressed.
BRADLEY HACKER, E-mail: hacker{at}geol.ucsb.edu


  Abstract

Xenoliths of subducted crustal origin hosted by Miocene ultrapotassic igneous rocks in the southern Pamir provide important new information regarding the geological processes accompanying tectonism during the Indo-Eurasian collision. Four types have been studied: sanidine eclogites (omphacite, garnet, sanidine, quartz, biotite, kyanite), felsic granulites (garnet, quartz, sanidine and kyanite), basaltic eclogites (omphacite and garnet), and a glimmerite (biotite, clinopyroxene and sanidine). Apatite, rutile and carbonate are the most abundant minor phases. Hydrous phases (biotite and phengite in felsic granulites and basaltic eclogites, amphiboles in mafic and sanidine eclogites) and plagioclase form minor inclusions in garnet or kyanite. Solid-phase thermobarometry reveals recrystallization at mainly ultrahigh temperatures of 1000-1100°C and near-ultrahigh pressures of 2·5-2·8 GPa. Textures, parageneses and mineral compositions suggest derivation of the xenoliths from subducted basaltic, tonalitic and pelitic crust that experienced high-pressure dehydration melting, K-rich metasomatism, and solid-state re-equilibration. The timing of these processes is constrained by zircon ages from the xenoliths and 40Ar/39Ar ages of the host volcanic rocks to 57-11 Ma. These xenoliths reveal that deeply subducted crust may undergo extensive dehydration-driven partial melting, density-driven differentiation and disaggregation, and sequestration within the mantle. These processes may also contribute to the alkaline volcanism observed in continent-collision zones.

Keywords: xenolith; high-pressure; subduction; Pamir; Tibet.
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