A geochemical and Nd, Sr and stable Ca isotopic study of carbonatites and associated silicate rocks from the similar to 65 Ma old Ambadongar carbonatite complex and the Phenai Mata igneous complex, Gujarat, India: Implications for crustal contamination, carbonate recycling, hydrothermal alteration and source-mantle mineralogy

摘要

Major, trace element concentrations and Nd, Sr and Ca stable isotopic compositions (delta Ca-44/40 and delta Ca-44/42 w.r.t. NIST SRM915a) of carbonatites and associated igneous silicate rocks from the -65 Ma old Ambadongar carbonatite complex and the surrounding Phenai Mata igneous complex of western India are reported. Samples of fluorspar from Ambadongar and the Bagh Limestone and Sandstone, which are part of the country rocks at Ambadongar, have also been analysed. The Ambadongar carbonatites are primarily calcio- and ferro-carbonatites while the silicate rocks from these two complexes are alkaline and tholeiitic in composition. The delta Ca-44/40 values of the carbonatites (0.58-1.1 parts per thousand, n = 7) and the associated igneous silicate rocks (0.50-0.92 parts per thousand, n = 14) show a broad range. The low K/Ca values of the carbonatites (0.2) and silicate rocks (2) along with their young eruption age (similar to 65 Ma) rule out any effect of radiogenic Ca-40 ingrowth due to decay of K-40 on the delta Ca-44/40 values. The lack of correlations between delta Ca-44/40 and Mg# as well as La/Yb-(N) values suggest that the variability in delta Ca-44/40 is not controlled by the degree of partial melting. The delta Ca-44/40 values of the carbonatites (0.58-1.1%0) overlap with that of the upper mantle/Bulk Silicate Earth and is mostly higher than the delta Ca-44/40 value of the Bagh Limestone (0.66 parts per thousand) suggesting that assimilation of these crustal limestones by the magma is unlikely to have caused the variability in delta Ca-44/40 of the carbonatites. In plots of delta Ca-44/40 versus epsilon Nd-(t) and Sr-87/Sr-86((t)), the igneous silicate rocks from the Ambadongar and Phenai Mata complexes plot on a mixing trend between a primitive (plume) mantle source and the continental crustal basement suggesting the role of continental crustal contamination during eruption of the Reunion plume. While simple binary mixing calculations yield unrealistically high amounts of crustal contamination (40%), assimilation and fractional crystallization (AFC) models suggest up to 20% contribution from a heterogeneous basement for these igneous silicate rocks. The role of continental crustal contamination in the genesis of the igneous silicate rocks is further supported by their unradiogenic epsilon Nd-(t), radiogenic Sr-87/Sr-86((t)) and low Ce/Pb values. In contrast, carbonatites plot away from the mixing trend between a primitive mantle (plume) source and continental crust in Ca-Sr-Nd isotopic diagrams suggesting that the Ca isotopic variability of carbonatites is not caused by continental crustal contamination. In contrast, the isotopic composition of the carbonatites can be explained by mixing of the plume end-member with up to 20% of similar to 160 Ma-old recycled carbonates suggesting their derivation from a highly heterogeneous, recycled carbonate-bearing plume mantle source.