Abstract
Ultramafic-mafic complexes are abundant in the Paleoproterozoic Marowijne Greenstone Belt (ca. 2.2-2 Ga) on the Guiana shield. For many of these the petrogenesis is not yet understood, except for a single potential komatiite flow facies in the belt. The Bemau ultramafic complex (BUC) in Suriname has dunite-wehrlite-clinoyroxenite and gabbro intercalated
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with fine-grained ultramafic schists. Earlier work with a limited dataset suggested that the BUC is an Alaskan-type complex associated to possibly arc-related andesites in the metabasaltic Paramaka formation. The petrogenesis of these complexes thus directly ties into the question whether modern style plate-tectonics was active in greenstone belts. Fieldwork was conducted to describe and sample ultramafic-mafic schists in the complex. Whole-rock data and geochemistry of cumulate phases from drill core samples were used to constrain parental magma composition. With mineral-melt partition coefficients for a range of conditions and compositions, melt trace element contents were calculated from Laser-Ablation Inductively Coupled Mass Spectrometry analyses of clinopyroxene. The origin of an unusual corundum-spinellite was constrained by corundum and spinel geochemistry combined with isocon analysis. Field and core samples reveal that interbedded samples are sheared alteration zones after the cumulates. Whole rock geochemistry shows that cumulates are related to another by fractional crystallization processes. Separated clinopyroxene and olivine generally have basaltic Mg#, while high Mg# are interpreted as derived from magma mixing. Enrichment of LILE, LREE, Pb & Cs and strong depletion of Nb-Ta, Zr-Hf & Ti in the calculated melts shows these are arc-like magmas sourced from hydrated mantle. Th/Yb vs Nb/Yb systematics suggests that BUC melts derived from a variably enriched and depleted mantle column, while high La/Sm with low Ba/Th relate to a dominant sediment melting component. High LILE & LREE enrichment and potentially igneous phlogopite + pargasite indicates an alkaline parent magma formed by melting of alkali-rich metasomatized mantle and sediments. Low-Ca, high-Fo olivine is igneous, at odds with the commonly held belief that such olivine must derive from the mantle. Although in principle vertical tectonic models allows introduction of hydrated mafic crust and sediments into the mantle, the transition of back-arc basin basalts to calc-alkaline andesites in the Paramaka is readily explained by to arc evolution. Serpentinization is the earliest metamorphic event in the BUC. Subsequent isochemical alteration to Tlc+/-Tr+/-Chl+/-Serp, potassic metasomatism, carbonation, steatization and chloritization were facilitated by magmatic fluids from TTGs intruded late in the arc’s history. Intense metasomatism by desilicification of such acidic fluids, favoured above subduction zones, may have produced the corundum-spinel assemblage from a cumulate protolith.
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