Geochemical and O–C–Sr–Nd Isotopic Constraints on the Petrogenetic Link between Aillikites and Carbonatites in the Tarim Large Igneous Province

Aillikites are carbonate-rich ultramafic lamprophyres often associated with carbonatites. Despite their common field relationships, the petrogenetic links, if any, between aillikites and carbonatites remain controversial. To address this question, this study reports the results of a detailed geochemical and isotopic examination of the Permian Wajilitag aillikites in the northwestern Tarim large igneous province, including bulk-rock major- and trace-element and Sr–Nd isotope compositions, olivine major- and trace-element and (in situ secondary ion mass spectrometry) oxygen isotope compositions, oxygen isotope data for clinopyroxene separates, and bulk-carbonate C–O isotopic analyses. Olivine in the aillikites occurs in two textural types: (1) microcrysts, 0·3–5 mm; (2) macrocrysts, 0·5–2·5 cm. The microcrysts exhibit well-defined linear correlations between Fo (79–89) and minor and trace elements (e.g. Ni = 1304–3764 μg g–1 and Mn = 1363–3042 μg g–1). In contrast, the olivine macrocrysts show low Fo (79–81), Ni (5·3–442 μg g–1) and Ca (477–1018 μg g–1) and very high Mn (3418–5123 μg g–1) contents, and are displaced from the compositional trend of the microcrysts. The microcrysts are phenocrysts crystallized from the host aillikite magmas. Conversely, the lack of mantle-derived xenoliths in these aillikites suggests that the macrocrysts probably represent cognate crystals (i.e. antecrysts) that formed from earlier, evolved aillikite melts. Olivine phenocrysts in the more primitive aillikite dykes (Dyke 1) have relatively higher Fo (Fo82–89) and mantle-like oxygen isotope values, whereas those in the more evolved dykes (Dykes 2 and 3) exhibit lower Fo (Fo79–86) and oxygen isotope values that trend toward lower than mantle δ18O values. The decreasing δ13C values of carbonate from Dyke 1 to Dykes 2 and 3, coupled with the indistinguishable Sr–Nd isotopes of these dykes, suggest that the low δ18O values of olivine phenocrysts in Dykes 2 and 3 resulted from carbonate melt/fluid exsolution from a common progenitor melt. These lines of evidence combined with the overlapping emplacement ages and Sr–Nd isotope compositions of the aillikites and carbonatites in this area suggest that these exsolved carbonate melts probably contributed to the formation of the Tarim carbonatites, thus supporting a close petrogenetic relationship between aillikites and carbonatites.