Metamorphic zoning and behaviour of an underthrusting continental plate

Abstract

Lower continental plates of collision zones are normally inaccessible and their properties inferred by geophysical surveying. By contrast, the eastern margin of the deeply eroded 1‐billion‐year‐old Sveconorwegian Orogen in Scandinavia exposes an underthrusting lower continental plate that records the metamorphic evolution and behaviour during collisional orogeny. A 200 km wide structurally coherent belt of the mid‐ to upper levels of the underthrusting lower plate, composed of Proterozoic igneous protoliths, exposes the gradual transition from unmetamorphosed to high‐P granulite facies. Metamorphic assemblages in Fe–Ti‐rich metagabbro in a continuous ~120 km wide transect across the lower continental plate define a regional metamorphic zoning by eight zones. Significant metamorphic recrystallization is first recorded at conditions of lower to middle amphibolite facies (550–600°C), progressing successively towards the contact with the upper plate to high‐P granulite facies at ~850°C and 10–11 kbar, the latter corresponding to ~40 km depth. An eclogite‐bearing tectonic terrane, detached from the leading edge of the underthrusting plate and extruded back along the interface with the overlying crust, occurs just beneath the lithotectonic contact of the overriding plate. The gradual metamorphic transition demonstrates that, with the exception of the eclogite‐bearing terrane, the underthrusting plate remained essentially structurally coherent despite highly ductile deformation and partial melting. The metamorphic rocks record that, by and large, the underthrusting plate underwent short‐lived metamorphic recrystallization and deformation that started first at an advanced stage of collision, at c. 980–970 Ma; this is in contrast to the eclogite‐bearing terrane which records more protracted metamorphic recrystallization both through prograde metamorphism and decompression. The investigated section is the first described in situ exposure of a coherent underthrusting lower continental plate, and defines a metamorphic field gradient at higher temperatures and P/T than the classical Barrovian style. The exposed section provides potential for increasing understanding of the thermal evolution, rheology, petrophysical properties and distribution of volatiles in an elsewhere inaccessible part of Earth's collisional systems. The data demonstrate that the underthrusting lower continental plate differs in metamorphic character from other parts of orogenic systems in that the crust is relatively dry, competent and responds by significant metamorphic recrystallization first at elevated temperatures (middle amphibolite facies) and that metamorphic recrystallization is largely coupled to deformation that facilitates the entry of hydrous fluids into the crust.

Prograde metamorphic zircon formation in gabbroic rocks: The tale of microtextures

“…Vindøldalen is a Caledonian 0.42 Ga gabbro‐to‐eclogite transition, situated in an allochthonous sheet in the central Scandinavian Caledonides (Figure ). Four metagabbro bodies are from the Sveconorwegian Province, were metamorphosed at 0.98 Ga, and record increasing temperatures and pressures towards the west (Möller & Andersson, ). Of these four, Hägghult and Görbjörnarp are situated in the heterogeneously deformed easternmost part of the Sveconorwegian Province.…”

The early-Sveconorwegian orogeny in southern Norway: Tectonic model involving delamination of the sub-continental lithospheric mantle

“…The later orogenic evolution is well summarized by three distinct orogenic phases (Bingen et al, 2008a,c), the main Sveconorwegian Agder phase (1050 980 Ma), the Falkenberg phase (990 970 Ma) and, finally, the Dalane phase (970 900 Ma). Metamorphism in the orogen has a high pressure sig nature towards the east (Möller and Andersson, 2018;Möller et al, 2015;Söderlund et al, 2008), contrasting with a high tempera ture low pressure signature in the west (Blereau et al, 2017;Laurent et al, 2016). The orogeny ended with pulses of granite and anorthosite mangerite charnockite (AMC) plutonism around 930 Ma (Vander Auwera et al, 2011) and of pegmatite intrusion around 910 Ma (Müller et al, 2017).…”

“…It was affected by an early orogenic event, known as Hallandian or Danopo lonian, between 1465 and 1370 Ma (Brander and Söderlund, 2009;Cecys and Benn, 2007;Ulmius et al, 2015). Sveconorwegian deformation and metamorphism increases westwards in the Eastern Segment (Möller and Andersson, 2018;Möller et al, 2015;Piñán Llamas et al, 2015). Metamorphism peaked in eclogite facies condi tions at c. 990 Ma.…”

“…Metamorphism peaked in eclogite facies condi tions at c. 990 Ma. Retro eclogite bodies are preserved in a recumbent fold, c. 70 km wide, in the footwall of the Mylonite Zone (Möller and Andersson, 2018;Möller et al, 2015).…”

Tracking the prograde P–T path of Precambrian eclogite using Ti-in-quartz and Zr-in-rutile geothermobarometry

“…1b; Möller et al 2015). The nappe represents a part of the Baltica continental crust that experienced westward tectonic burial at eclogite-facies conditions beneath the western Sveconorwegian terranes, followed by eastward, foreland-directed, tectonic exhumation at granulite-facies temperatures (Möller et al 2015;Möller and Andersson 2018;Tual et al 2015Tual et al , 2017. The timing of eclogite metamorphism and exhumation at 40 km depth is bracketed between 0.99 and 0.97 Ga (U-Pb ages of zircon; Johansson et al 2001;Möller et al 2015).…”