Mafic and ultramafic pods with eclogitic relics from the Proterozoic Nagssugtoqidian mobile belt of East Greenland

doi:10.1016/0024-4937(90)90009-P

Lithos

Volume 25, Issues 1–3, 16 November 1990, Pages 101-118

https://doi.org/10.1016/0024-4937(90)90009-P Get rights and content

Abstract

The cores of amphibolitic and ultramafic pods within the basement complex of the Nagssugtoqidian mobile belt of East Greenland preserve metastable relics of older high-pressure assemblages. The bulkrock geochemistry of amphibolites indicates that their protoliths derive from low-pressure crystal fractionation (mainly controlled by plagioclase, pyroxene and olivine) of tholeiitic melts of probable MORB affinity. Amphibolites from the cores of the pods are corona-textured and provide microtextural evidence of a polyphase retrogression subsequent to a high-T eclogitic event, producing omphacite-garnet assemblages. Symplectitic intergrowths of Ca-clinopyroxene and plagioclase are the result of the unmixing of the older omphacite, whilst garnet is partly replaced by a fine-grained pseudomorph of orthopyroxene, anorthitic plagioclase and magnetite. This suggests subsequent partial re-equilibration in the intermediate-pressure granulite facies, producing clinopyroxene-orthopyroxene-plagioclase-magnetite assemblages. The final event in the amphibolite facies was controlled by the influx of dominantly hydrous fluids. According to kinetic constraints, the amphibolites developed coronitic or granoblastic textures. The coronas consist of plagioclase and hornblende between remaining garnet and unmixed clinopyroxene. Re-crystallized granoblastic amphibolites do not retain relics; according to the bulk rock chemistry, plagioclase and hornblende coexist with either garnet or clinopyroxene. Adjacent ultramafic rocks conform with this metamorphic evolution, being retrogressed from the spinel ±amphibole lherzolite facies to the tremolite-chlorite peridotite facies. Estimated equilibrium conditions for the final amphibolite facies event are T = 600 + 70°C and P = 5.2 ± 1 kbar. Mafic and ultramafic pods from the basement complex of the Nagssugtoqidian mobile belt of East Greenland probably represent fragments of an oceanic crust which was subjected to a subduction event of Proterozoic age.

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Cited by (16)

Age and temperature-time evolution of retrogressed eclogite-facies rocks in the Paleoproterozoic Nagssugtoqidian Orogen, South-East Greenland: Constrained from U-Pb dating of zircon, monazite, titanite and rutile
2018, Precambrian Research
LA-ICP-MS U-Pb dating of zircon, monazite, titanite and rutile was carried out to investigate the temperature-time evolution of eclogite-facies rocks in the Kuummiut Terrane of the Paleoproterozoic Nagssugtoqidian Orogen in South-East Greenland. The terrane is dominated by Archean TTG gneiss and a variety of supracrustal rocks; basic dykes intruded the gneiss during Paleoproterozoic deformation. Detrital zircon in garnet-kyanite schist gives Archean to Paleoproterozoic dates and confines the maximum deposition of the metasediment precursor to 2107 ± 21 Ma. Intrusion of the metabasic dykes occurred at 2146 ± 63 and 2092 ± 22 Ma, within error of the detrital zircon date, possibly indicating near-contemporaneous dyke emplacement and sedimentation. About 200 m.y. after dyke emplacement, the Kuummiut Terrane underwent a clockwise PT-evolution, involving eclogite-facies metamorphism and subsequent exhumation into the mid crust. The majority of zircon, monazite and titanite give metamorphic dates between 1891 ± 10 and 1882 ± 3 Ma. Although the REE patterns in metamorphic zircon reflect growth at eclogite-facies conditions, the zircons are associated with retrograde mineral assemblages and their dates are indistinguishable from amphibolite-facies titanite. This may be interpreted to indicate that the timing of eclogite- and high-pressure amphibolite-facies metamorphism overlap within error, consistent with rapid and tectonically-controlled exhumation. However, previous studies have shown that the REE and U-Pb systematics in zircon may be decoupled during retrograde metamorphism, and the range in dates is thus best interpreted to reflect mineral growth and recrystallization during high-pressure amphibolite-facies retrogression. Monazite and titanite dates between 1872 ± 70 and 1821 ± 31 Ma reflect regional medium-pressure amphibolite-facies metamorphism and mark the final stages of compressional deformation in the Kuummiut Terrane. The subsequent thermal evolution was associated with titanite growth until 1738 ± 61 Ma, a time where the majority of rutile cooled below its closure temperature. The youngest rutile dates at 1645 ± 63 and 1617 ± 91 Ma correlate with the emplacement of post-tectonic intrusive complexes. Collectively, the data show that after an initial tectonically-controlled exhumation, the Kuummiut Terrane experienced relatively slow, erosion-controlled cooling with only minor thermal perturbations during the waning stages of metamorphic and magmatic activity.
Mineral textural evolution and PT-path of relict eclogite-facies rocks in the Paleoproterozoic Nagssugtoqidian Orogen, South-East Greenland
2018, Lithos
Citation Excerpt :
However, retrogressed eclogites represent samples in which reactions at amphibolite-facies conditions ceased prior to completion (i.e. due to limited fluid availability). In these samples, the hornblende-plagioclase coronas around garnet effectively shield the two domains from each other and hinder the transport of fluid and chemical components, which drive the coupled replacement reactions, across the domains (Messiga et al., 1990). This results in the formation of domainal equilibration volumes (as described by Zhang et al., 2000; O'Brien and Rötzler, 2003; Scott et al., 2013), with different mineral compositions between the two domains (see also Tubia and Gil Ibarguchi, 1991; Dachs and Proyer, 2001; Baldwin et al., 2004; Štípská and Powell, 2005; Sajeev et al., 2010b; Doukkari et al., 2014) and locally (as observed for garnet, hornblende and plagioclase) between domains of the same type.
The Nagssugtoqidian Orogen in South-East Greenland is a deeply eroded, Paleoproterozoic collision orogen. It consists of a variety of Archean and Paleoproterozoic rocks, most notably TTG gneiss, a variety of supracrustal rocks and basic dykes. This study aims at providing new insight into the geodynamic processes and subduction depth of this orogen by investigating the metamorphic evolution of garnet pyroxenite, retrogressed eclogite and amphibolite-facies rocks that are exposed within the Kuummiut Terrane of the Nagssugtoqidian Orogen. The garnet-pyroxenite has a dominant mineral assemblage of garnet, orthopyroxene, clinopyroxene and hornblende, while garnet-amphibolite and garnet-kyanite schist are made up of garnet, hornblende, plagioclase and quartz, and garnet, kyanite, biotite and quartz, respectively. Relicts of, and pseudomorphs after, eclogite-facies mineral assemblages are frequently found within basic metavolcanic rocks and Paleoproterozoic discordant basic dykes. In the retrogressed eclogite, the retrograde mineral reactions ceased prior to completion, resulting in the formation of two domains. A clinopyroxene domain consists of diopside-plagioclase symplectites, which are interpreted to have grown at the expense of omphacite. The symplectites are surrounded and partly replaced by hornblende and plagioclase. Omphacite (X_Jd 25–42) is preserved in a Na-rich sample, where it occurs in the core of large clinopyroxene and as inclusion in garnet and hornblende. In a garnet domain, garnet is variably replaced by an inner corona of plagioclase and an outer corona of amphibole +/− orthopyroxene and clinopyroxene. The degree of retrogression as well as the type of the retrograde assemblage in both domains appears to be dependent on fluid activity. Large garnet grains preserve Ca-rich cores, interpreted as prograde in origin, while Mg-rich garnet rims formed during eclogite-facies metamorphism and later re-equilibration. Pseudosection modelling combined with conventional geothermobarometry reveals a clockwise PT-evolution, involving eclogite-facies conditions of 17–19 kbar and 740–810 °C, followed by near-isothermal decompression to medium-pressure granulite-facies conditions (13.8–15.4 kbar, 760–880 °C) and subsequent decompression with minor cooling to high-pressure amphibolite-facies grades (8.8–10.9 kbar, 660–840 °C). These data show that rocks of the Kuummiut Terrane were exhumed from 70 to about 30 km into the mid- and lower crust. The PT-path implies that exhumation initially was rapid and tectonically-controlled.
Constraining a Precambrian Wilson Cycle lifespan: An example from the ca. 1.8 Ga Nagssugtoqidian Orogen, Southeastern Greenland
2018, Lithos
In the Phanerozoic, plate tectonic processes involve the fragmentation of the continental mass, extension and spreading of oceanic domains, subduction of the oceanic lithosphere and lateral shortening that culminate with continental collision (i.e. Wilson cycle). Unlike modern orogenic settings and despite the collection of evidence in the geological record, we lack information to identify such a sequence of events in the Precambrian. This is why it is particularly difficult to track plate tectonics back to 2.0 Ga and beyond. In this study, we aim to show that a multidisciplinary approach on a selected set of samples from a given orogeny can be used to place constraints on crustal evolution within a P-T-t-d-X space. We combine field geology, petrological observations, thermodynamic modelling (Theriak-Domino) and radiogenic (U-Pb, Lu-Hf) and stable isotopes (δ¹⁸O) to quantify the duration of the different steps of a Wilson cycle. For the purpose of this study, we focus on the Proterozoic Nagssugtoqidian Orogenic Belt (NOB), in the Tasiilaq area, South-East Greenland.
Our study reveals that the Nagssugtoqidian Orogen was the result of a complete three stages juvenile crust production (X_juv) – recycling/reworking sequence: (I) During the 2.60–2.95 Ga period, the Neoarchean Skjoldungen Orogen remobilised basement lithologies formed at T_DM 2.91 Ga with progressive increase of the discharge of reworked material (X_juv from 75% to 50%; δ¹⁸O: 4–8.5‰). (II) After a period of crustal stabilization (2.35–2.60 Ga), discrete juvenile material inputs (δ¹⁸O: 5–6‰) at T_DM 2.35 Ga argue for the formation of an oceanic lithosphere and seafloor spreading over a period of ~ 0.2 Ga (X_juv from < 25% to 70%). Lateral shortening is set to have started at ca. 2.05 Ga with the accretion of volcanic/magmatic arcs (i.e. Ammassalik Intrusive Complex) and by subduction of small oceanic domains (M1: 520 ± 60 °C at 6.6 ± 1.4 kbar). (III) Continental collision between the North Atlantic Craton and the Rae Craton occurred at 1.84–1.89 Ga. Crustal thickening of ~ 25 km was accompanied by regional metamorphism M2 (690 ± 20 °C at 6.25 ± 0.25 kbar) and remobilization of pre-existing supracrustal lithologies (X_juv ~ 40%; δ¹⁸O: 5–10.5‰).
Rates and durations obtained for seafloor spreading (175 ± 25 Ma), subduction (125 ± 75 Ma) and continental collision (ca. 60 Ma) are similar to those observed in Phanerozoic Wilson Cycle but differ from what was estimated for Archean terrains. Therefore, timespans of the different steps of a Wilson cycle might have progressively changed over time as a response to the progressive cratonization of the lithosphere.
Identifying weathering sources and processes in an outlet glacier of the Greenland Ice Sheet using Ca and Sr isotope ratios
2014, Geochimica et Cosmochimica Acta
Chemical and isotope data (ε⁴⁰Ca, δ^44/42Ca, ⁸⁷Sr/⁸⁶Sr, δ¹⁸O) of river water samples were collected twice daily for 28 days in 2009 from the outlet river of Leverett Glacier, West Greenland. The water chemistry data was combined with detailed geochemical analysis and petrography of bulk rock, mineral separates and sediment samples in order to constrain the mineral weathering sources to the river. The average isotopic compositions measured in the river, with 2SD of all the values measured, were ε⁴⁰Ca = +4.0 ± 1.4, δ^44/42Ca = +0.60 ± 0.10‰ and ⁸⁷Sr/⁸⁶Sr = 0.74243 ± 0.00327. Based on changes in bulk meltwater discharge, the hydrochemical data was divided into three hydrological periods. The first period was marked by the tail-end of an outburst event and was characterised by water with decreasing suspended sediment concentrations (SSC), ion concentrations and pH. During the second hydrological period, discharge increased whilst ⁸⁷Sr/⁸⁶Sr decreased from 0.74550 to 0.74164. Based on binary mixing diagrams using ⁸⁷Sr/⁸⁶Sr with Na/Sr, Ca/Sr and ε⁴⁰Ca, this is interpreted to reflect an increase in reactive mineral weathering, in particular epidote, as the water residence time decreases. The decrease in water residence time is a result of the evolution from a distributed (long water residence time) to a channelised (short water residence time) subglacial drainage network. The third hydrological period was defined as the period when overall discharge was decreasing. This hydrological period was marked by prominent diurnal cycles in discharge. During this period, significant correlations between δ^44/42Ca and SSC and δ¹⁸O were observed which are suggestive of fractionation during adsorption. This study demonstrates the potential of radiogenic Ca to both identify temporally changing mineral sources in conjunction with ⁸⁷Sr/⁸⁶Sr values and to separate source and fractionation effects in δ^44/42Ca values.
High-temperature, high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian, NW Scotland: Crustal-scale subduction in the Neoarchaean
2013, Gondwana Research
Citation Excerpt :
It is interesting that, according to Snoeyenbos et al. (1995), the oldest reliably dated HP metamorphic rocks at that time were the 1100 Ma eclogitic rocks at Glenelg in NW Scotland (see later). However, there are a number of recently reported and reliably dated Proterozoic, partly retrogressed eclogites or HP granulites (O'Brien and Rötzler, 2003) in deep crustal gneisses that formed above 10 kbar: The Sittampundi Complex, southern India (2541 ± 138 Ma magmatic age and 2487–2461 Ma metamorphic age; Bhaskar Rao et al., 1996; Ram Mohan et al., 2012) at 20 kbar, 1020 °C (Sajeev et al., 2009); eclogites in the Usagaran belt, Tanzania at ca. 2000 Ma, 18 kbar and 750 °C (Möller et al., 1995; Collins et al., 2004); mafic granulites, Snowbird zone, Canada at 1900 Ma, 19–13 kbar and 960–890 °C (Baldwin et al., 2003); garnet websterites in Siberia at 2480–2410 Ma, 12 kbar and 540 °C (Ota et al., 2004); eclogites in the Belomoride belt of Russia at 2120–1860 Ma, 10.0 ± 0.5 kbar and 700 ± 40 °C (Kozlovskii and Aranovich, 2010), and at 2450–2120 Ma 13.5–15 kbar, 805 °C (Travin and Kozlova, 2009); eclogites, Aldan Shield, Siberia at < 2400 Ma, 10–15 kbar, uncertain temperature (Smelov and Beryozkin, 1993); eclogites in the North China Craton at 1860–1820 Ma (or 2500 Ma or 2000–1900 Ma), 14–12 kbar, and ca. 800 °C (O'Brien et al., 2005; Kröner et al., 2006; Zhai et al., 2010; Zhai and Santosh, 2011); eclogites, Nagssugtoqidian, E. Greenland at ca. 1850 Ma, 5.2 ± 1 kbar and 600 ± 70 °C (Messiga et al., 1990); and eclogites at Glenelg in NW Scotland at ca. 1100 Ma, 25–18 kbar and 1000–850 °C (Brewer et al., 2003; Storey et al., 2005; Sajeev et al., 2010b). However, a minimum pressure of 10 kbar for HP metamorphism may be deemed as surprisingly low, because many Precambrian granulite terrains have peak pressures of ~ 10 kbar, in which case HP metamorphism should be regarded as the norm for lower (or middle) crustal rocks, a concept that has as yet not been considered.
Eclogites and associated high-pressure (HP) rocks in collisional and accretionary orogenic belts preserve a record of subduction and exhumation, and provide a key constraint on the tectonic evolution of the continents. Most eclogites that formed at high pressures but low temperatures at > 10–11 kbar and 450–650 °C can be interpreted as a result of subduction of cold oceanic lithosphere. A new class of high-temperature (HT) eclogites that formed above 900 °C and at 14 to 30 kbar occurs in the deep continental crust, but their geodynamic significance and processes of formation are poorly understood. Here we show that Neoarchaean mafic–ultramafic complexes in the central granulite facies region of the Lewisian in NW Scotland contain HP/HT garnet-bearing granulites (retrogressed eclogites), gabbros, lherzolites, and websterites, and that the HP granulites have garnets that contain inclusions of omphacite. From thermodynamic modeling and compositional isopleths we calculate that peak eclogite-facies metamorphism took place at 24–22 kbar and 1060–1040 °C. The geochemical signature of one (G-21) of the samples shows a strong depletion of Eu indicating magma fractionation at a crustal level. The Sm–Nd isochron ages of HP phases record different cooling ages of ca. 2480 and 2330 Ma. We suggest that the layered mafic–ultramafic complexes, which may have formed in an oceanic environment, were subducted to eclogite depths, and exhumed as HP garnet-bearing orogenic peridotites. The layered complexes were engulfed by widespread orthogneisses of tonalite–trondhjemite–granodiorite (TTG) composition with granulite facies assemblages. We propose two possible tectonic models: (1) the fact that the relicts of eclogitic complexes are so widespread in the Scourian can be taken as evidence that a > 90 km × 40 km-size slab of continental crust containing mafic–ultramafic complexes was subducted to at least 70 km depth in the late Archaean. During exhumation the gneiss protoliths were retrogressed to granulite facies assemblages, but the mafic–ultramafic rocks resisted retrogression. (2) The layered complexes of mafic and ultramafic rocks were subducted to eclogite-facies depths and during exhumation under crustal conditions they were intruded by the orthogneiss protoliths (TTG) that were metamorphosed in the granulite facies. Apart from poorly defined UHP metamorphic rocks in Norway, the retrogressed eclogites in the central granulite/retrogressed eclogite facies Lewisian region, NW Scotland have the highest crustal pressures so far reported for Archaean rocks, and demonstrate that lithospheric subduction was transporting crustal rocks to HP depths in the Neoarchaean.
Late Paleozoic retrograded eclogites from within the northern margin of the North China Craton: Evidence for subduction of the Paleo-Asian ocean
2006, Gondwana Research
Retrograded eclogites from the central part of the northern margin of the North China Craton, Hebei Province, China occur as separate tectonic lenses or boundins within garnet–biotite–plagioclase gneisses of the Paleoproterozoic Hongqiyingzi Complex characterized by amphibolite facies paragneisses. The petrographic features and mineralogical compositions represent three main metamorphic stages: (1) the peak eclogite facies stage (P > 1.40–1.50 GPa, T = 680–730 °C), (2) the granulite facies stage and (3) the amphibolite facies stage (P = 0.67–0.81 GPa, T = 530–610 °C) formed during decompression. The major and trace element and Sm–Nd isotopic data suggest that most of the retrograded eclogite samples had protoliths of tholeiitic oceanic crust with geochemical characteristics of mid-ocean ridge basalt (MORB) or island arc tholeiite (IAT) environment, and were contaminated by crustal components during subsequent subduction. Zircon SHRIMP isotopic dating of two different textural varieties of retrograded eclogite defines a weighted mean age of ∼325 Ma, which is interpreted as the peak metamorphic age of the eclogites and reflects the occurrence of eclogite facies metamorphism related to subduction of Paleo-Asian Oceanic crust beneath the North China Craton during the Late Paleozoic. Finally, we show that the retrograded eclogite from Hebei Province is not related to the Baimashi retrograded eclogite at the northern foot of the Heng Mountains, approximately, 300 km to the southwest.

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Mafic and ultramafic pods with eclogitic relics from the Proterozoic Nagssugtoqidian mobile belt of East Greenland

Abstract

Lithos

Lithos

Geochim. Cosmochim. Acta

Lithos

Earth Planet. Sci. Lett.

Chem. Geol.

Tectonophysics

Chem. Geol.

Lithos

Geochim. Cosmochim. Acta

The Precambrian of South-East Greenland

Eclogitic amphibolites from the Grampian Moines

Mineral. Mag.

Mechanisms of exsolution in omphacites from high temperature, type B, eclogites

Phys. Chem. Miner.

Nagssugtoqidian mobile belt in East Greenland

Outline of the Nagssugtoqidian mobile belt of East Greenland

Rapp. Gronlands Geol. Unders.

Mechanisms of exsolution in sodic pyroxenes

Contrib. Mineral. Petrol.

A proposed subdivision of the granulite facies

Am. J. Sci.

An experimental study of the effects of Ca upon garnet-clinopyroxene FeMg exchange equilibria

Contrib. Mineral. Petrol.

Nagssugtoqidian mobile belt in West Greenland

Experimental calibration of the partitioning of Fe and Mg between biotite and garnet

Contrib. Mineral. Petrol.

Large-ion lithophile element characteristics of an amphibolite facies to granulite facies transition at Gruinard Bay, North-west Scotland

J. Metamorph. Geol.

Revisione di una metodologia analitica per fluorescenza-X, basata sulla correzione completa degli effetti di matrice

Rend. Soc. Ital. Mineral. Petrol.

Fully automated dissolution and separation methods for inductively coupled plasma atomic emission spectrometry rock analysis. Application to the determination of rare earth elements. Plenary lecture

J. Anal. At. Spectrom.

A garnet hornblende geothermometer: calibration, testing, and application to the Pelona Schist, Southern California

J. Metamorph. Geol.

A new review of the chlorites

Mineral. Mag.

Stability of andalusite and the aluminium silicate phase diagram

Am. J. Sci.

The experimental determination of activities in disordered and short-range ordered jadeitic pyroxenes

Contrib. Mineral. Petrol.

Nagssutoqidian mobile belt of West Greenland: a cryptic 1850 Ma suture between two Archaean continents — chemical and isotopic evidence

Earth Planet. Sci. Lett.