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Developing an inverted Barrovian sequence; insights from monazite petrochronology

Mottram, Catherine M.; Warren, Clare J.; Regis, Daniele; Roberts, Nick M. W.; Harris, Nigel B. W.; Argles, Tom W. and Parrish, Randall R. (2014). Developing an inverted Barrovian sequence; insights from monazite petrochronology. Earth and Planetary Science Letters, 403 pp. 418–431.

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In the Himalayan region of Sikkim, the well-developed inverted metamorphic sequence of the Main Central Thrust (MCT) zone is folded, thus exposing several transects through the structure that reached similar metamorphic grades at different times. In-situ LA-ICP-MS U–Th–Pb monazite ages, linked to pressure–temperature conditions via trace-element reaction fingerprints, allow key aspects of the evolution of the thrust zone to be understood for the first time. The ages show that peak metamorphic conditions were reached earliest in the structurally highest part of the inverted metamorphic sequence, in the Greater Himalayan Sequence (GHS) in the hanging wall of the MCT. Monazite in this unit grew over a prolonged period between ~37 and 16 Ma in the southerly leading-edge of the thrust zone and between ~37 and 14.5 Ma in the northern rear-edge of the thrust zone, at peak metamorphic conditions of ~790 ◦C and 10 kbar. Monazite ages in Lesser Himalayan Sequence (LHS) footwall rocks show that identical metamorphic conditions were reached ~4–6 Ma apart along the ~60 km separating samples along the MCT transport direction. Upper LHS footwall rocks reached peak metamorphic conditions of ~655 ◦C and 9 kbar between ~21 and 16 Ma in the more southerly-exposed transect and ~14.5–12 Ma in the northern transect. Similarly, lower LHS footwall rocks reached peak metamorphic conditions of ~580 ◦C and 8.5 kbar at ~16 Ma in the south, and 9–10 Ma in the north. In the southern transect, the timing of partial melting in the GHS hanging wall (~23–19.5 Ma) overlaps with the timing of prograde metamorphism (~21 Ma) in the LHS footwall, confirming that the hanging wall may have provided the heat necessary for the metamorphism of the footwall.

Overall, the data provide robust evidence for progressively downwards-penetrating deformation and accretion of original LHS footwall material to the GHS hanging wall over a period of ~5 Ma. These processes appear to have occurred several times during the prolonged ductile evolution of the thrust. The preserved inverted metamorphic sequence therefore documents the formation of sequential ‘paleothrusts’ through time, cutting down from the original locus of MCT movement at the LHS–GHS protolith boundary and forming at successively lower pressure and temperature conditions. The petrochronologic methods applied here constrain a complex temporal and thermal deformation history, and demonstrate that inverted metamorphic sequences can preserve a rich record of the duration of progressive ductile thrusting.

Item Type: Journal Item
Copyright Holders: 2014 The Authors
ISSN: 0012-821X
Project Funding Details:
Funded Project NameProject IDFunding Body
Testing models for mountain building in the northeastern HimalayaNot SetRoyal Society
An Integrated Metamorphic and Isotopic Study of Crustal Extrusion Along the Main Central Thrust, Sikkim HimalayaNE/1528018/1NERC (Natural Environment Research Council)
From subduction to sand: Quantifying the balance between tectonic and surface processes during early continental collision and UHP rock exhumationNE/H016279/1NERC (Natural Environment Research Council)
Not SetIP-1129-0511NERC (Natural Environment Research Council)
Keywords: inverted metamorphism; ductile thrusting; petrochronology; monazite geochronology
Academic Unit/School: Faculty of Science, Technology, Engineering and Mathematics (STEM) > Environment, Earth and Ecosystem Sciences
Faculty of Science, Technology, Engineering and Mathematics (STEM)
Item ID: 40695
Depositing User: Nigel Harris
Date Deposited: 11 Aug 2014 10:09
Last Modified: 15 Sep 2019 07:24
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