LREE-rich hibonite in ultrapotassic rocks in southern India

Abstract

We describe a rare terrestrial occurrence of hibonite (CaAl 12TiO 19) from the ultrapotassic rocks which occur as veinlets in graphite-bearing spinellite in the Achankovil Shear Zone (ACSZ), southern India. The ultrapotassic rocks are characterized by an unusual mineral assemblage, kalsilite + leucite + corundum + spinel + perovskite + apatite + hibonite ± phlogopite, along with mantle-derived graphite and Fe-sulfides. Three types of hibonite were identified in terms of chemical zoning as observed in back scattered images: (i) unzoned hibonite, (ii) zoned hibonite with brighter core and dark rim, and (iii) zoned hibonite with brighter core, a less bright intermediate zone and a dark outer rim corresponding to the variations in the distribution of LREEs (light rare earth elements), Ca, Al and Ti in the hibonite structure. The core regions of hibonite are highly enriched in LREEs and Sr, and depleted in Ca, Ti and Al, compared to the rim areas. Chondrite-normalized REE distribution patterns for hibonite exhibit extreme LREE enrichment (La N/Yb N = 763 to 374,494) with a negative Eu anomaly. Apatite displays MREE-enriched patterns. Partitioning of LREEs in hibonite and apatite crystallized from the melts is chiefly determined by their preference for hibonite > apatite. We propose a model that considers the formation of the LREE-rich Ti-depleted hibonite from ultrapotassic silica-undersaturated melts. We envisage that the melts were generated by low degree partial melting of a phlogopite-bearing peridotite that was fertilized in LREEs. The ultrapotassic rock that occurs as veins along the ACSZ represents a melt influx associated with deep-seated extension after the final amalgamation of the Gondwana supercontinent.