Trace element composition of parental magmas from mafic–ultramafic cumulates determined by in situ mineral analyses: The Juquiá mafic–ultramafic alkaline–carbonatite massif, SE Brazil

Abstract

Major and trace element analyses of relict cores of cumulus minerals (olivine and clinopyroxene) from primitive rocks of the Juquiá mafic–ultramafic alkaline–carbonatite complex (Early Cretaceous) in the Ponta Grossa Arch Alkaline Province, southeastern Brazil, were used to calculate the equilibrium melt compositions. Olivine relict cores are compositionally restricted to the Fo83–74 interval, and they exhibit significant concentrations of Mn (2220–3001 ppm), Ni (1188–2327 ppm), Ca (175–649 ppm), Co (169–216 ppm), Zn (115–215 ppm), Ti (55–305 ppm), Cr (4–320 ppm) and P (31–154 ppm). Clinopyroxene relict cores are essentially diopside, with minor amounts of hedenbergite and tschermakite components. The trace element contents found in clinopyroxene cores are Ni (196–339 ppm), V (99–318 ppm), Sr (110–260 ppm), Zr (12–163 ppm), Sc (50–78 ppm), P (16–118 ppm), Zn (16–48 ppm) and Co (25–43 ppm). The rare earth element (REE) concentrations are variable (28–240 ppm), with the lighter elements present in the greatest abundance, particularly Ce (up to 83 ppm).The melt compositions in equilibrium with olivine and clinopyroxene cores from different samples suggest that distinct basanite magma batches have played a role in the formation of the Juquiá cumulate rocks. These calculated liquids have compositions that are quite similar to those of the nearby lamprophyre dikes. The CaO/Al2O3 values (>0.8) from the lamprophyre dikes and the high La/Zr and low Ti/Eu values from the calculated liquids point to a previous carbonatite metasomatic event in the magmatic source that lead to the formation of wehrlite veins in a peridotitic mantle. The variation in the Hf/Y ratio implies differences in cpx/gt modal proportions in the source. Non-modal mantle batch-melting models indicate that a homogeneous source with little variation in the degree of partial melting cannot explain the trace element differences among the calculated liquids. The models suggest that the geochemical differences represent (1) mixing between metasomatic vein partial melt and garnet lherzolite partial melt and (2) clinopyroxene/garnet modal ratios of the residual mantle.