Evidence for HFSE and REE mobility during calc-silicate metasomatism, Mesoarchean (∼3075 Ma) Ivisaartoq greenstone belt, southern West Greenland

Abstract

The Mesoarchean ( ca. 3075 Ma) Ivisaartoq greenstone belt consists dominantly of variably deformed and metamorphosed mafic to ultramafic flows (basalts to picrites). In addition, there are minor serpentinites, gabbros, clinopyroxene-rich cumulates, and volcaniclastic and siliciclastic sedimentary rocks. Primary volcanic structures such as pillows and volcanic breccia are well preserved in low strained zones. The belt underwent intense metasomatic hydrothermal alteration between 3075 and 2961 Ma. An early stage of calc-silicate metasomatism (stage-I) resulted in the formation of epidosites in pillow cores, pillow interstitials, and intrusive margins of gabbroic dykes. This alteration appears to have resulted from sea-floor hydrothermal alteration. A second stage of calc-silicate metasomatism (stage-II) formed centimeter- to meter-scale layers concordant to discordant relative to the regional foliation. These layers consist of diopside + garnet + epidote + plagioclase + quartz + titanite ± vesuvianite ± calcite ± actinolite ± scheelite. The second stage calc-silicate rocks are associated with shear zones and their origin is attributed to interaction of reactive fluids with mafic and ultramafic rocks during regional metamorphism. Both stages of calc-silicate metasomatism caused significant mobilization of major elements (Si, Mg, Ca, Fe, Na, K, and Mn), LILE (Rb, Cs, Sr, Ba, Pb), and LREE. Heavy rare earth elements (HREE) and HFSE (Th, Nb, Ta, Zr, Ti) remained relatively immobile during stage-I metasomatic alteration but were variably disturbed during the stage-II metasomatism. Accordingly, the stage-II calc-silicate rocks exhibit evidence for significant mobility of Th, Nb, and Ta on a local scale (1–4 m), whereas Zr and Ti were not significantly mobilized. This resulted in calc-silicates with complementary trace element patterns displaying extreme negative to positive Nb-anomalies (Nb/Nb * = 0.19–7.28). Transition metals Ni, V, Co, Cr, and Sc were relatively immobile during both metasomatic events. Amphibolites spatially related to the second stage calc-silicate rocks preserve mineralogical and major element characteristics consistent with a mafic to ultramafic composition. However, their near-primary magmatic trace element compositions were variably modified due to strong REE mobility during the stage-II metasomatic event. This gave rise to various REE characteristics including LREE-enriched, LREE-depleted, and U-shape REE patterns. Some of these element patterns resemble those shown by modern boninites and mid-ocean ridge basalts (N-MORB). In contrast, the least altered amphibolites exhibit similar trace element patterns to those displayed by the least deformed non-metasomatized mafic and ultramafic pillow flows. They possess relatively flat trace element patterns with negative Nb–Ta anomalies indicating a subduction zone geochemical signature.