Developing an inverted Barrovian sequence; insights from monazite petrochronology

Abstract

Editor: T.M. Harrison Keywords: inverted metamorphism ductile thrusting petrochronology monazite geochronologyIn 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.

The Late Mesoproterozoic Sirdal Magmatic Belt, SW Norway: Relationships between magmatism and metamorphism and implications for Sveconorwegian orogenesis

“…Sorting these events from each other will require detailed mapping to reveal cross-cutting relationships, and detailed, in situ petrochronology where ages can be linked to particular textures and reactions (e.g. Regis et al, 2014;Mottram et al, 2014).…”

Rapid high‐T decompression recorded by Archean granulites in the northern Wyoming Province: Insights from petrological modelling

“…Modern‐day plate tectonic processes may generate HT metamorphic rocks in a variety of geodynamic environments, with the depths, temperatures, and P–T–t paths recorded by such lithologies providing insight into the geological processes that operate in these tectonic settings (e.g. Kohn, ; Mottram et al., ; Pownall, Hall, Armstrong, & Forster, ; Stevens, Baldwin, Cottle, & Kylander‐Clark, ). Likewise, the granulites found in Archean terranes may preserve a record of the processes responsible for heating of Earth's early continental crust, and whether such processes resemble those associated with modern plate tectonics.…”

Lateral extrusion, underplating, and out-of-sequence thrusting within the Himalayan metamorphic core, Kanchenjunga, Nepal

“…Until the recognition of these structures, most models for burial and exhumation have focused on the two fault systems that bound the Himalayan metamorphic core: the South Tibetan detachment system above, and the Main Central thrust below (Beaumont et al, 2001Jamieson et al, 2004;Webb et al, 2007Webb et al, , 2011. Increasing chronologic and thermobarometric data from across the Himalaya, however, indicate that significant amounts of horizontal shortening and vertical thickening were accommodated along structures within the Himalayan metamorphic core (e.g., Jain and Manickavasagam, 1993;Goscombe et al, 2006;Groppo et al, 2009;Carosi et al, 2010;Imayama et al, 2010;Larson et al, 2013;Larson and Cottle, 2014;Mottram et al, 2014;Warren et al, 2014). These structures have only a cryptic surface expression and are often recognized by abrupt spatial breaks in P- T-t(-deformation [D]) paths (e.g., overall shape, timing, peak conditions).…”