Crust–mantle interaction in the genesis of siliceous high magnesian basalts: evidence from the Early Proterozoic Dongargarh Supergroup, India

Abstract Siliceous high magnesian basalts (SHMB) represent a rock type chemically distinct from other common volcanic rocks. The petrogenesis of this magma, reported at or near the Archean–Proteorzoic transition across the world, is controversial. In this paper, we present chemical (XRF, INAA) and mineralogical data on a SHMB suite from the Early Proterozoic (2.1–2.5 Ga) Dongargarh Supergroup, Central India, the first of its kind reported from the Indian Precambrian. This suite of basaltic rocks is unusually high in SiO 2 (54 wt.%) and enriched in incompatible elements. The SHMB melts discussed here can neither have formed by partial melting of the Earth's mantle nor by fractional crystallisation of a mantle-derived melt. It is shown here that excess SiO 2 and incompatible elements in SHMB are supplied by a crustal component to a basaltic komatiitic parent magma of mantle origin. Major and trace elements abundances and geochemical mass balance calculations suggest that a basaltic komatiite melt assimilated 15–20% of acid volcanics, the immediately underlying unit to these rocks in Dongargarh, before erupting as SHMB. Mantle-derived rocks have Ta/Th ratios of around 0.5 whereas crustal rocks have ratios of about 0.2. The Ta/Th ratios of Dongargarh SHMB and acid volcanics are nearly identical but both unusually low ( 2 in SHMB. It is also shown that there is an overall compositional similarity of such temporarily unique but spatially unrelated SHMB magmas occurring in different continents including the Dongargarh SHMB.