Petrogenesis and mode of emplacement of a Neoarchean tonalite-trondhjemite-diorite (TTD) suite: the Eau Jaune Complex, Abitibi greenstone belt

Magmatism during the maturation phase of Archean greenstone belts produced voluminous tonalite-trondhjemite-granodiorite (TTG) suites, as well as a lesser amount of tonalite-trondhjemite-diorite (TTD) suites. Such TTD suites have recently been recognized in the Archean Abitibi greenstone belt, on the southern flank of the Superior Craton, Canada, but their source(s), differentiation processes and depths of emplacement remain poorly constrained. The Neoarchean Eau Jaune Complex (EJC) lies in the northeastern corner of the Abitibi greenstone belt and represents one of the most voluminous tonalite-dominated and diorite-bearing intrusive suites of the Chibougamau region. This TTD suite comprises six intrusive phases with distinct petrology and chemistry. All units were emplaced as laccolith-like intrusions injected along discontinuities within the volcanic succession at ca. 2724 Ma (U-Pb zircon dating), during the synvolcanic interval (i.e., construction and maturation phase), at a depth of approximately 7–8 km. The most HREE-depleted phases (granodiorite, tonalite and trondhjemite) correspond to magmas that fractionated amphibole and were likely produced by partial melting of a garnet- and titanate-bearing amphibolite, akin to TTG magmas. The least HREE-depleted phases are dioritic in composition and correspond to mantle-derived magmas that may have interacted with TTG melts. This indicates interaction between coeval mantle-derived and crustal melts during the maturation phase of the Abitibi greenstone belt. Models formulated to address the geodynamic evolution of greenstone belts must account for the coeval production of basalt-derived (TTG suites) and mantle-derived (tholeiitic magmatism) melts occasionally interacting to form TTD suites.

Congo-São Francisco at 1.11 Ga and the megacontinent Umkondia

<p>Currently three supercontinent cycles have been identified and existed supercontinents named from youngest to oldest: Pangea, Rodinia and Nuna/Columbia. Recently Wang et al. (2020) suggested that supercontinent amalgamation were each preceded by ~200 Myr by the assembly of long-lasting <em>megacontinent</em> aking to Gondwana.</p><p>The Congo-São Francisco (C/SF) craton is a main building block in Gondwana due to its central location, but its participation to Rodinia is controversial. Salminen et al. (2018) presented 1.11 Ga paleomagnetic and geochronological data from a prominent Epembe-Huila swarm of gabbronoritic dykes in the southern part of the Congo craton in Namibia and in Angola. This paleomagnetic pole yields a relatively low paleolatitude for the C/SF craton at ca. 1.11 Ga and permits a direct connection between Congo and Kalahari cratons. This connection supports an earlier qualitative comparison (Ernst et al., 2013), that the mafic Epembe-Huila swarm was an integral component of the Umkondo Large Igneous Province (LIP). The 1.11 Ga Umkondo LIP is widespread across Kalahari craton, and coeval mafic magmatism has been identified in several of the world’s other late Mesoproterozoic cratons: Laurentia, India, Amazonia, and Antarctica (Grunehogna). Were these coeval provinces spatially linked at the time of emplacement during the amalgamation of Rodinia? Robust paleomagnetic and geochronological data from Laurentia and Kalahari have demonstrated substantial separation between those two blocks at 1.11 Ga (Swanson-Hysell et al., 2015). However, based on similar tholeiitic magmatism Choudhary et al. (2019) proposed that Kalahari and C/SF together with Amazonia and northern India constituted “Umkondia” at 1.11 Ga. It has been proposed that Umkondia occupied an intermediary “megacontinental” role in the Nuna-Rodinia transition analogous to Gondwana in Rodinia-Pangea evolution (Wang et al., 2020). Contradicting Gondwana the proposed Umkondia was not long-lasting, since it has been proposed that Kalahari and Congo separated after 1.10 Ga to form a vast ocean (ca. 6000 km) during the formation of Rodinia and widespread juvenile intra-oceanic magmatism along the present-day central Brazil indicates a large ca. 0.94 Ga ocean between C/SF and Amazonia (Cordani et al., 2003).</p><p> </p><p>Choudhary et al. 2019. Precambrian Research 332, 105382.</p><p>Cordani et al. 2003. Gondwana Research 6, 275-283.</p><p>Ernst et al. 2003. Lithos 174 1-14.</p><p>Salminen et al. 2018. Geology 46, 1011-1014.</p><p>Swanson-Hysell et al. 2015. Geophysical Journal International 203, 2237-2247.</p><p>Wang et al. 2020. Geology 49, https://doi.org/10.1130/G47988.1</p><p> </p>

Long-lasting influence of the Discovery plume on tholeiitic magmatism in the South Atlantic: data on basalts recovered by hole 513a, dsdp leg 71

The paper presents the very first data on concentrations of major and trace elements; Sr, Nd, and Pb isotopic ratios of rocks; and the composition of olivine phenocrysts of 38-Ma basalts recovered by Hole 513a (DSDP Leg 71) in the South Atlantic. The bulk-rock samples and the chilled glasses are mildly magnesian (7–8 wt % MgO) and bear elevated FeO and low Na2O concentrations, as is typical of MORB of the TOR-1 type. Olivine phenocrysts (Fo84.5–88) in these rocks contain concentrations of trace elements (Ni, Mn, Cr, and Zn) that are typical of classic MORB, which are produced by partial melting mantle peridotite. The rocks are strongly depleted in incompatible elements [(La/Sm)n ~ 0.6] but have elevated Ba/Nb, K/Nb, and Pb/Ce ratios and Cu, Ag, and Au concentrations that are 1.5–4 times higher than in typical depleted MORB (N-MORB) and in most rift basalts in the South Atlantic. Isotope compositions of the basalts (average ratios 206Pb/204Pb ~ 18.0; 207Pb/204Pb ~ 15.6, 208Pb/204Pb ~ 38.0, 143Nd/144 Nd ~ 0.5130, and 87Sr/86Sr ~ 0.7040) are close to those in modern tholeiites from the southern MAR segment (SMAR) north of the Agulhas Fracture Zone. The data indicate that the magmas were derived from a strongly depleted mantle source that contained a minor (~3%) admixture of an enriched component, which is discernible in the magmas of the Discovery hotspot. The composition of the source, which is more depleted than DM, and the high degrees of melting of this source explain why the basalts from DSDP Hole 513a are enriched in chalcophile elements. It is believed that spreading magmatism at 45°–48° S in SMAR as far back as 40 Ma was already affected by the Discovery hotspot. This hotspot might be related to the Tristan plume system, and its origin and long-lasting influence on spreading magmatism in the South Atlantic are regarded as evidence of the extensive effect of the Tristan plume.

The petrogenesis of tholeiitic diabases in eastern Parnaíba Basin: evidence for geochemical heterogeneities in the subcontinental lithospheric mantle in NE Brazil

ABSTRACT: The sedimentary rocks within the Paleozoic Parnaiba basin in NE Brazil were intruded by voluminous tholeiitic diabase sills and covered by coeval basaltic flows. This paper presents lithogeochemical data of borehole samples obtained from wells located in the eastern portion of the Parnaiba basin. The diabases are subalkaline tholeiitc rocks comprising three high-TiO2 and three low-TiO2 suites that are unrelated by differentiation processes. Fractional crystallization of olivine and augite was the predominantly evolutionary processes within individual high- and low-TiO2 suites as depicted by trace element geochemical modelling, exception being made for one low-TiO2 suite that evolved by AFC. Parental compositions for both low- and high-TiO2 suites are related with variably enriched, spinel harzburgitic sources likely to represent the heterogeneous subcontinental lithospheric mantle underneath the sedimentary basin. The geochemical provinciality of the Parnaiba tholeiitic magmatism seems unrelated with the Transbrasiliano Lineament but may be due to lithospheric mantle amalgamation and remobilization occurred during previous tectonic events.