Pressure-Temperature history of the 1.9 Ga Nagssugtoqidian Orogeny in the Tasiilaq Region, South-East Greenland: Amphibolite facies metamorphism of a Palaeoproterozoic accretionary prism (original) (raw)
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Precambrian Research, 2022
The Tasiilaq region in SE Greenland contains Palaeoproterozoic supracrustal rocks, which record the collision and suturing of the Rae Craton and North Atlantic Craton during the 1.9 Ga Nagssugtoqidian orogeny. The original mineralogy and textures of the supracrustal rocks have been largely overprinted by amphibolite facies metamorphism, obscuring the original protoliths and complicating the interpretation of the early Earth surface environment in which these rocks formed. Here, we present new major and trace element data for the Schweizerland, Kuummiut and Isertoq Terranes, with the former two representing the Rae craton, and the latter the northern margin of the North Atlantic Craton. Supracrustal lithologies include metapelites, marbles, calcsilicate rocks and mafic to felsic volcanic rocks and/or shallow intrusive rocks. Pelites normalised to Post Archaean Australian shales have (sub)horizontal rare earth element patterns, with La/Sm CN : 1.61-23.91 and Eu/Eu* 0.34-1.83 and Th/Sc 1. The volcanic rocks are enriched relative to the primitive mantle (PM) with parallel to sub parallel PM-normalised heavy rare earth elements. They have a suprasubduction zone signature with distinct Nb-Ta troughs. The amphibolites can be further divided into those that interacted with the crust; back-arc basalts with subtle Nb-Ta troughs; and fore-arc basalts with distinct Nb-Ta and Zr-Hf troughs and negative Ti anomalies. The marbles consist of a Mg-rich dolomite group with δ 13 C 0.48 to 0.91 ‰ and δ 18 O − 6.13 to − 7.57 ‰; and Mg-poor calcitic carbonates with δ 13 C − 0.74 to 0.17 ‰ and δ 18 O − 10.81 to − 14.87 ‰. This isotopic composition suggests formation at ~ 2050 Ma during the low δ 13 C excursion following the Great Oxidation Event. We propose that the supracrustals of the Tasiilaq region were deposited at 2050 Ma in a series of interconnected basins during dual subduction convergence of the North Atlantic and Rae cratons, with development of an island arc with back arc basin in the north, and a continental arc in the south. These basins were inverted during continental collision, thrust, and folded into the Archaean Kuummiut and Isertoq TTGdominated terrane basement, before being subjected to the region-wide amphibolite facies metamorphism at ~ 1840 Ma during the Nagssugtoqidian orogeny.
The Tartoq greenstone belt of southwest Greenland represents a well-preserved section through >3 Ga old oceanic crust and has the potential to provide important constraints on the composition and geodynamics of the Archaean crust. Based on a detailed structural examination, it has been proposed that the belt records an early style of horizontal convergent plate tectonics where elevated temperatures, compared to the modern-day, led to repeated aborted subduction and tonalite–trondhjemite–granodiorite (TTG) type melt formation. This interpretation hinges on pressure–temperature (P–T) constraints for the belt, for which only preliminary estimates are currently available. Here, we present a detailed study of the pressure–temperature conditions and metamorphic histories for rocks from all fragments of the Tartoq belt using pseudosection modelling and geothermobarometry. We show that peak conditions are predominantly amphibolite facies, but range from 450 to 800 • C at up to 7.5 kbar; reaching anatexis with formation of TTG-type partial melts in the Bikuben segment. Emplacement of the Tartoq segments into the host TTG gneisses took place at approximately 3 Ga at 450–500 • C and 4 kbar as constrained from actinolite–chlorite–epidote–titanite–quartz parageneses, and was followed by extensive hydrothermal retrogression related to formation of shear zone-hosted gold mineralisation. Tourmaline thermometry and retrograde assemblages in mafic and ultramafic lithologies constrain this event to 380 ± 50 • C at a pressure below 1 kbar. Our results show that the convergent tectonics recorded by the Tartoq belt took place at a P–T gradient markedly shallower than that of modern-day subduction, resulting in a hot, weak and buoyant slab unable to generate and transfer 'slab pull', nor sustain a single continuous downgoing slab. The Tartoq belt suggests that convergence was instead accomplished by under-stacking of slabs from repeated aborted subduction. The shallow P–T path combined with thermal relaxation following subduction stalling subsequently resulted in partial melting and formation of TTG melts.
Preface Comparative tectonic and dynamic analysis of cratons, orogens, basins, and metallogeny: A special volume to honor the career of Brian F. Windley 1. Preface Cratons, orogens, and basins of the world each show a distinctive pattern of evolution and metallogeny, and relationships to superconti-nent cycles. Some aspects of these histories have remained similar through time, yet others have changed with Earth's changing biota, heat production and flow, and atmospheric composition and temperature. To understand the similarities and differences between these cratons, orogens, and basins through time, we need systematic comparative tectonic analyses between these different elements from similar and widely different ages. One of the pioneers in comparative tectonic analysis is Brian F. Windley, whose research has spanned all of these topics, and more, for more than five decades. This special issue is a tribute to his career. Brian's career began with graduation from Liverpool in 1960, after which he went to Exeter University for his PhD studies under Ken Coe, working on the Precambrian rocks of SW Greenland, which led to a six year contract working for the Geological Survey of Greenland mapping most of the Precambrian of western Greenland. Brian enjoyed this time, but did not want to be constrained to only mapping the geology of Greenland, so moved on to Leicester University, from where he based the rest of his career. During this career Brian visited many universities around the world (see review of Brian's career in Kusky et al., 2010), and worked on problems in tectonics, mineralogy, geochemistry, geochro-nology, and metallogeny from every continent (and the moon). Brian has not slowed down in his career and is currently active in many projects in the Central Asian Orogenic Belt, India, Madagascar, China, Scotland, and other places and is currently working on a new edition of his book "The Evolving Continents" with his colleague T. Kusky. We have collected 29 papers from Brian's peers and colleagues along these themes for this special issue of Tectonophysics and summarize them from "snippits" of the papers here. The papers are organized into five themes, namely Archean Tectonics, Proterozoic Tectonics, Structural and Metamorphic Evolution of Orogens, Tethyan and Paleo-Asian Tectonics and the Central Asian Orogenic Belt, and the Supercontinent Cycle and Earth History. We hope readers enjoy reading these papers as much as the authors have enjoyed working with Brian and doing the science to prepare them. 2. Archean tectonics Five papers focus on different aspects of Archean tectonics. Peng et al. (2015) present a study of the Qingyuan high-grade granite-greenstone terrane in the North China craton, consisting of circa 2510 Ma ultramafic-mafic-felsic rocks, 2570-2510 Ma quartz diorite, a similar aged TTG suite, and a 2510-2490 Ma quartz monzonite. Using mainly petrologic and geochemical methods, Peng et al. (2015) model the petrogenesis to be related to Archean subduction in a mantle wedge-absent flat-hot-subduction system, with considerable vertical accretion of magmatic rocks. The ultramafic-mafic rocks are interpreted to be derived from the primitive mantle, whereas the meta-dacite-rhyolite originated from eclogite facies overriding crust. The quartz dio-rite is interpreted by Peng et al. (2015) as a mixture of melts from mantle and the overriding crust, the TTG was derived from the subducting slab under amphibolite to amphibole-bearing eclogite facies conditions, and the quartz monzodiorite was derived from the overriding middle-lower crust. The Qingyuan terrain represents the root of a Neoarchaean arc and its anticlockwise P-T-path records the initiation of the arc at 2570 Ma to its cession and exhumation/cratonization at~2480 Ma. Arai et al. (2015) document intermediate P/T type regional meta-morphism of the 3.7-3.8 Ga Isua Supracrustal Belt from southwest Greenland. Using a group of metabasites that range in metamorphic grade from greenschist to amphibolite facies, and they model isochemical phase diagrams with the Perple_X software. The metamor-phic pressure and temperature both increase systematically to the south from 3 kbar and 380°C to 6 kbar and 560°C. The monotonous metamor-phic P-T change suggests that the northeastern part of the ISB preserves regional metamorphism resulting from the subduction of an accretion-ary complex, although there are also Neoarchean metamorphic over-prints. Both the presence of the regional metamorphism and an accretionary complex having originating at a subduction zone suggest that the ISB may be one of the oldest Pacific-type orogenic belts in the world (see also Komiya et al., 2015, for an example of an Eoarchean (N3.95 Ga) Pacific-type orogenic belt). The metamorphic conditions are similar to extant zones of subducting young oceanic crust such as the Philippine Sea Plate in SW Japan. Since this type of metamorphism is characteristic of most Archean terranes, Arai et al. (2015) suggest that subduction of young micro-plates was common in the Archean. Polat et al. (2015) describe the Eoarchean to Neoarchean tectonic zonation of West Greenland and interpret these as a series of accreted accretionary prisms, island arcs, and fore-arcs. They compare the structures , particularly those of high-grade migmatites with those recently documented by Wang et al. (2014) from the Mesozoic Sulu orogen of China. Polat et al. (2015) document that melting of the metavolcanic rocks during tectonic thickening of arcs during collision is important in the generation of TTG's, and that the style of deformation along with partial melting in the Archean of Greenland is similar to that of the Sulu orogen that formed by subduction processes, suggesting that subduction and collision, and hence plate tectonics, also operated in the early Archean. Tectonophysics 662 (2015) 1-6 http://dx.