Abstract
The major-trace element and C-O-Sr isotopic composition of calcite constitute important proxies for reconstructing the magmatic and hydrothermal evolution of carbonatites. This study reports new 87Sr/86Sr isotope data measured in-situ from six textural types of calcite in carbonatite and fenitized phonolites of the Kamthai carbonatite complex in western India. Textural relations, along with major-trace element chemistry, and Sr isotope composition helps to discriminate four types of calcite in carbonatites—magmatic (CalM), partially reequilibrated magmatic (CalPR), and two varieties of secondary (CalSK and CalS). CalM has high concentrations of ƩREE + Y, Sr, Ba and Mn, restricted 87Sr/86Sr (0.70437 ± 0.00005), and lacks Ce or Y anomaly. It crystallized from late-stage brine-melt together with primary carbocernaite after prolonged fractional crystallization of the carbonatite magma. CalPR has identical 87Sr/86Sr (0.70425 ± 0.00024) as CalM, and formed by its partial reequilibration, accompanied by significant loss of REE, and Sr during interaction with synmagmatic fluid. The two secondary varieties of calcite in the carbonatites have overlapping but marginally higher 87Sr/86Sr (CalSK = 0.70469 ± 0.00041; CalS = 0.70478 ± 0.00025) compared to CalM, which indicates that they were altered by syn– to para– magmatic fluids with minor contribution from post-magmatic fluid (e.g., external crust-derived fluid). Both types are highly porous and have low-Z contrast in back scattered electron images, which is the result of expulsion of most of its original Sr and REE budget during fluid-induced reequilibration. Two types of secondary calcite (CalS1 and CalS2) are identified in three veins within fenitized phonolite. These contain the lowest abundance of ΣHREE + Y, Sr, Ba and Mn. One type (CalS2) occurs in calcite-quartz-pyrite veins and is characterized by LREE-depleted REE patterns and igneous-calcite-like 87Sr/86Sr (0.70434 ± 0.00073). It possibly crystallized from evolved late-stage para-magmatic fluids from the carbonatite magma after the brine-melt stage of primary LREE mineralization. The other type of vein calcite (CalS1) is characterized by strong negative Ce (δCe*: ≪0.034) anomalies, which indicates that it crystallized from oxidized fluids from which Ce was removed as Ce (IV). The 87Sr/86Sr of some of these calcites are significantly more radiogenic (0.70768 ± 0.00011) than magmatic calcite. This indicates that post-magmatic fluid played a significant role in their formation. Two component mixing calculations show that up to 40–70 % post-magmatic fluid mixed with syn-magmatic fluid can account for their Sr isotope composition. Magmatic calcite and carbocernaite were the major sources of REE and Sr during secondary redistribution.
Published Version
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