Petrology of ijolite xenoliths entrained in a nephelinite dyke from the Kamthai area, Late Cretaceous polychronous Sarnu-Dandali alkaline complex, North-West India: Evidence for recurrent magmatic pulses and magma mixing

Abstract

The Late Cretaceous (89–65 Ma) Sarnu-Dandali alkaline complex is intrusive into the Neoproterozoic (∼750 Ma) Malani Igneous Suite of rocks in North-Western India. This complex represents a polychronous setup owing to the available age groups of varied rock types involving nephelinites, alkali pyroxenites, syenites, phonolites, ijolites, carbonatites and lamprophyres. In this study, we report xenoliths of ijolite up to 2 mm in size, comprising minerals such as aegirine, nepheline, sodalite, apatite, sphene, fluorite, Ti-rich andradite, and perovskite entrained in the host nephelinite from this complex. Pyroxene thermobarometry for both host and xenoliths reveals that the xenoliths crystallized around 876–1114°C; 4.6–18 kbar (Kdcpx−melt ∼0.164), whereas the host magma crystallized around 800–1165°C and 4.5–17.9 kbar (Kdcpx−melt ∼0.44). The redox conditions of the magma are also estimated from Fe-Nb oxy barometry of perovskite and the log oxygen fugacity varies from -2.5 to -3.7 (ΔNNO), showing a resemblance with that of the world-wide kimberlites derived from the cratonic mantle. Furthermore, the composition as well as the barometric study of clinopyroxenes from both the host and the xenoliths, suggest multiple shallower magma chambers composed of ijolite and nephelinite magmas which were activated by different magmatic pulses, thereby forming a complex magmatic plumbing system.