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The thermal structure of continental crust in active orogens: insight from Miocene eclogite and granulite xenoliths of the Pamir Mountains
Author(s) -
GORDON S. M.,
LUFFI P.,
HACKER B.,
VALLEY J.,
SPICUZZA M.,
KOZDON R.,
KELEMEN P.,
RATSHBACHER L.,
MINAEV V.
Publication year - 2012
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.2012.00973.x
Subject(s) - geology , eclogite , granulite , xenolith , geochemistry , subduction , mantle (geology) , crust , continental crust , partial melting , peridotite , kimberlite , petrology , tectonics , geomorphology , seismology , facies , structural basin
Rare ultrahigh‐temperature–(near)ultrahigh‐pressure (UHT–near‐UHP) crustal xenoliths erupted at 11 Ma in the Pamir Mountains, southeastern Tajikistan, preserve a compositional and thermal record at mantle depths of crustal material subducted beneath the largest collisional orogen on Earth. A combination of oxygen‐isotope thermometry, major‐element thermobarometry and pseudosection analysis reveals that, prior to eruption, the xenoliths partially equilibrated at conditions ranging from 815 °C at 19 kbar to 1100 °C at 27 kbar for eclogites and granulites, and 884 °C at 20 kbar to 1012 °C at 33 kbar for garnet–phlogopite websterites. To reach these conditions, the eclogites and granulites must have undergone mica‐dehydration melting. The extraction depths exceed the present‐day Pamir Moho at ∼65 km depth and suggest an average thermal gradient of ∼12–13 °C km −1 . The relatively cold geotherm implies the introduction of these rocks to mantle depths by subduction or gravitational foundering (transient crustal drip). The xenoliths provide a window into a part of the orogenic history in which crustal material reached UHT–(U)HP conditions, partially melted, and then decompressed, without being overprinted by the later post‐thermal relaxation history.