Premium
Subduction initiation in the Neo‐Tethys: constraints from counterclockwise P–T paths in amphibolite rocks of the Nagaland Ophiolite Complex, India
Author(s) -
Bhowmik S. K.,
Ao A.
Publication year - 2016
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/jmg.12169
Subject(s) - blueschist , greenschist , geology , metamorphism , metamorphic facies , geochemistry , metamorphic rock , subduction , ophiolite , eclogite , petrology , facies , paleontology , tectonics , structural basin
High‐ P metamorphic rocks that are formed at the onset of oceanic subduction usually record a single cycle of subduction and exhumation along counterclockwise (CCW) P–T paths. Conceptual and thermo‐mechanical models, however, predict multiple burial–exhumation cycles, but direct observations of these from natural rocks are rare. In this study, we provide a new insight into this complexity of subduction channel dynamics from a fragment of Middle‐Late Jurassic Neo‐Tethys in the Nagaland Ophiolite Complex, northeastern India. Based on integrated textural, mineral compositional, metamorphic reaction history and geothermobarometric studies of a medium‐grade amphibolite tectonic unit within a serpentinite mélange, we establish two overprinting metamorphic cycles (M 1 –M 2 ). These cycles with CCW P–T trajectories are part of a single tectonothermal event. We relate the M 1 metamorphic sequence to prograde burial and heating through greenschist and epidote blueschist facies to peak metamorphism, transitional between amphibolite and hornblende‐eclogite facies at 13.8 ± 2.6 kbar, 625 ± 45 °C (error 2 σ values) and subsequent cooling and partial exhumation to greenschist facies. The M 2 metamorphic cycle reflects epidote blueschist facies prograde re‐burial of the partially exhumed M 1 cycle rocks to peak metamorphism at 14.4 ± 2 kbar, 540 ± 35 °C and their final exhumation to greenschist facies along a relatively cooler exhumation path. We interpret the M 1 metamorphism as the first evidence for initiation of subduction of the Neo‐Tethys from the eastern segment of the Indus‐Tsangpo suture zone. Reburial and final exhumation during M 2 are explained in terms of material transport in a large‐scale convective circulation system in the subduction channel as the latter evolves from a warm nascent to a cold and more mature stage of subduction. This Neo‐Tethys example suggests that multiple burial and exhumation cycles involving the first subducted oceanic crust may be more common than presently known.