Palaeoproterozoic arc related supracrustal units from the Tasiilaq Region, SE Greenland: Insights into the convergence of the Rae and North Atlantic Cratons (original) (raw)
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Lithos, 2023
The Tasiilaq region in SE Greenland records the suturing of two major cratons during the 1.9 Ga Nagssugtoqidian orogeny, with the Rae Craton to the north subducting beneath the North Atlantic Craton (NAC) to the south. Supracrustal rocks that were deposited prior to the orogeny, and record it, can be used to put constraints on the composition and geodynamic relationships between these two cratons. Here, we present a detailed study of the pressure-temperature conditions and metamorphic history of the Kuummiut Terrane (Rae craton) using geothermobarometry and pseudosection modelling and compare it to the adjacent Schweizerland (Rae) and Isertoq terranes (NAC). We show that the supracrustal rocks of the Tasiilaq region record an orogenic pressure temperature path, typical of an accretionary wedge, similar to what is seen in modern orogenies, including the Variscides. The Kuummiut Terrane contains two peak mineral assemblages with distinct peak pressure and temperature at 8.7 to 9.8 kbar at 636 to 680 • C in the west and ~ 8.2 kbar at ~735 • C in the east. The suture zone between the Isertoq and Kuummiut terranes was intruded by the Ammassalik Intrusive Complex resulting in the formation of anatectic melts at ~790 • C and 6.3-8.0 kbar at 1910-1870 Ma, syn-to late-tectonically. This was followed by strike slip, crustal thickening, and regional shearing at 614 to 713 • C and 4.3 kbar to 7.4 kbar. This is the first event to affect all three terranes and juxtaposes blocks with differing peak metamorphism, bringing the Isertoq Supracrustal rocks to the same crustal level as the Kuummiut Supracrustal rocks and thrusting the Schweizerland Terrane on top of the Kuummiut Terrane. The nappe stacking kept upper to mid crustal rocks at close to peak temperatures, resulting in extensive melting, and the formation of pegmatites throughout the region. Post tectonic collapse began as early as 1820 Ma, which marks the end of decompression, with the intrusion of gabbro and granitic complexes at ~8 km depth. Finally, circulation of hydrothermal fluids at temperatures of <400 • C resulted in region wide retrogression.
A Geochemical Overview of Mid-Archaean Metavolcanic Rocks from Southwest Greenland
The present contribution reviews bulk-rock geochemical data for mid-Archaean (ca. 3075–2840 Ma) metavolcanic rocks from the North Atlantic Craton of southwest Greenland. The data set includes the most recent high quality major and trace element geochemical analyses for ten different supracrustal/greenstone belts in the region. When distilling the data set to only include the least altered metavolcanic rocks, by filtering out obviously altered samples, mafic/ultramafic cumulate rocks, late-stage intrusive sheets (dolerites) and migmatites, the remaining data (N = 427) reveal two fundamentally distinct geochemical suites. The contrasting trends that emerge from the filtered geochemical data set, which best represents the melt compositions for these mid-Archaean metavolcanic rocks are: (1) tholeiitic (mainly basaltic) versus (2) calc-alkaline (mainly andesitic). These two rock suites are effectively separated by their La/Sm ratios (below or above three, respectively). It is demonstrated by geochemical modelling that the two contrasting suites cannot be related by either fractional crystallization or crustal assimilation processes, despite occurring within the same metavolcanic sequences. The tholeiitic basaltic rocks were directly mantle-derived, whereas the petrogenesis of the calc-alkaline andesitic rocks involve a significant (>50%) felsic component. The felsic contribution in the calc-alkaline suite could either represent slab-melt metasomatism of their mantle source, mafic-felsic magma mixing, or very large degrees of partial melting of mafic lower crust. At face value, the occurrence of andesites, and the negative Nb-Ta-Ti-anomalies of both suites, is consistent with a subduction zone setting for the origin of these metavolcanic rocks. However, the latter geochemical feature is inherent to processes involving crustal partial melts, and therefore independent lines of evidence are needed to substantiate the hypothesis that plate tectonic processes were already operating by the mid-Archaean.
Contributions to Mineralogy and Petrology, 2019
Ultramafic rocks found within the ~ 3.81 Ga Itsaq Gneiss Complex (IGC) have some mantle-like geochemical characteristics that have led to them being used to directly constrain the nature of the Eoarchean mantle. The discrimination of mantle perido-tites that are the residues of partial melting, from cumulate peridotites generated by crystal accumulation from mantle-derived magmas can be difficult in ancient, altered ultramafic rocks whose field relations have been obscured by multiple tectonic episodes. Hence it is important to scrutinize significant individual occurrences of Eoarchean ultramafic rocks in some detail prior to using them to constrain the nature of Earth's early mantle. Here we present mineral chemistry, whole rock major-, trace-, and platinum-group-element abundances, and Re-Os isotope compositions of a previously unstudied large ultramafic enclave in the IGC-the Tussaap Ultramafic Complex (TUC)-with the aim of documenting its origin. High FeO contents of up to 15.5 wt% and correlations between MgO and Os provide strong evidence that the TUC evolved through fractional crystallization rather than partial melt extraction. In addition, co-variations of major elements in the TUC lithologies can be modeled via fractional crystallization of picritic basalts using MELTS. Later alteration and metasomatism of these ultra-mafic rocks has largely overprinted primary mineral chemistry and resulted in a redistribution of light rare earth elements, rendering these tools ineffective for ascertaining the origin of the TUC or quantifying some of the petrogenetic processes that formed the body. In addition, it is clear that many geochemical features used to identify residual mantle peridotites can also be produced by cumulate or alteration processes, such as some variations in olivine and chromite chemistry, whole rock Al/Si vs Mg/Si systematics, and trace and platinum group element patterns. Finally, combined discrimination diagrams for high field strength elements and moderately high 187 Os/ 188 Os ratios suggest the parental melt of the TUC partially assimilated basaltic crust prior to precipitating the TUC cumulates. As such, these rocks represent a variably obscured record of Eoar-chean crystal fractionation from mantle-derived melts. Despite not being prima facie mantle rocks, it is possible that such early formed ultramafic cumulates in nascent continents found their way into the later-stabilized roots of Archean cratons, helping to explain the high compositional variability of cratonic peridotites.
We present bulk-rock geochemical and U-Pb zircon age constraints on a ∼580 m thick sequence of Mesoarchean metavolcanic rocks from SW Greenland. The rocks were arguably deformed into a tight synform and meta-morphosed under amphibolite facies conditions, where relict volcanic structures testify to their igneous origins. The sequence includes picrites, tholeiitic basalts and calc-alkaline andesitic to dacitic schists, interbedded with syn-volcanic mafic and felsic feeder intrusions. Amongst late aplitic intrusions, four conform to a minimum U-Pb age for the entire succession of 2929 ± 5 Ma, which is slightly older than a protolith intrusion age of 2902 ± 4 Ma for the regional grey orthogneisses. The metavolcanic section describes an overall inward increase in SiO 2 and Al 2 O 3 , coupled with a decrease in MgO, total iron (FeO T) and CaO, across a major discontinuity that separates: (1) A tholeiitic suite with high FeO T , low Al 2 O 3 /TiO 2 coupled with flat REE-patterns, interpreted as mantle-derived signatures; and (2) a more evolved calc-alkaline suite with relatively low FeO T , high Al 2 O 3 /TiO 2 and steep REE-patterns, yet relatively high Mg, Ni and Cr, which resemble adakites (or high-Mg andesites). A systematic greater negative Nb-anomaly (Nb/Nb *) and LREE-enrichment (La/Sm) N up through the metavolcanic section is consistent with an increasing subduction zone signature, and thereby a likely arc setting for all rocks. The overall field relationships argue strongly for the deposition of (i) enriched tholeiites (including basal picrites that could be either lavas or cumulates), (ii) more depleted tholeiites that also intruded the enriched tholeiites, before (iii) an abrupt transition into distinctly different calc-alkaline and adakite-like volcanics that evolved from andesites to dacites. Thus, the Nigerlikasik section records tholeiitic magmatism, succeeded by calc-alkaline adakitic magmatism during the evolution of an Archean island arc. In our single arc model, we discuss the possibilities of the early tholeiitic suite being formed through progressively more hydrous melting of a juvenile mantle wedge and subsequent low-P fractionation of plagioclase-bearing cumulates; whereas the adakitic suite more likely formed through high-P hornblende + garnet fractionation of an enigmatic parent that was possibly sourced from a crustally contaminated mantle.
Recent geological investigations of the Isua Supracrustal Belt (3.8 Ga), southern West Greenland, have suggested that it is the oldest accretionary complex on earth, defined by an oceanic plate-type stratigraphy and a duplex structure. Plate history from mid-oceanic ridge through plume magmatism to subduction zone has been postulated from analysis of the reconstructed oceanic plate stratigraphy in the accretionary complex. Comparison between field occurrence of greenstones in modern and ancient accretionary complexes reveals that two types of tholeiitic basalt from different tectonic settings, mid-oceanic ridge basalt (MORB) and oceanic island basalt (OIB), occur. This work presents major, trace and rare earth element (REE) compositions of greenstones derived from Isua MORB and OIB, and of extremely rare relict igneous clinopyroxene in Isua MORB. The Isua clinopyroxenes (Cpx) have compositional variations equivalent to those of Cpx in modern MORB; in particular, low TiO 2 and Na 2 O contents. The Isua Cpx show slightly light (L)REE-depleted REE patterns, and the calculated REE pattern of the host magma is in agreement with that of Isua MORB. Analyses of 49 least-altered greenstones carefully selected from approximately 1200 samples indicate that Isua MORB are enriched in Al 2 O 3 , and depleted in TiO 2 , FeO*, Y and Zr at the given MgO content, compared with Isua OIB. In addition, Isua MORB show an LREE-depleted pattern, whereas Isua OIB forms a flat REE pattern. Such differences suggest that the Early Archean mantle had already become heterogeneous, depending on the tectonic environment. Isua MORB are enriched in FeO compared with modern MORB. Comparison of Isua MORB with recent melting experiments shows that the source mantle had 85–87 in Mg# and was enriched in FeO. Potential mantle temperature is estimated to be approximately 1480 ∞ C, indicating that the Early Archean mantle was hotter by at most approximately 150 ∞ C than the modern mantle.