Mineral compositions and petrogenetic evolution of the ultramafic-alkaline - carbonatitic complex of sung valley, northeastern india

Abstract

The Sung Valley alkaline complex is a relatively small intrusion of Lower Cretaceous age emplaced slightly before or during the India-Antarctica break-up. It consists of ultramafic rocks (dunites, wehrlites, clinopyroxenites, uncompahgrites), mafic rocks (ijolites sensu lato), felsic rocks (nepheline syenites) and carbonatites. The chemical composition of the mafic minerals indicates the expected enrichment in iron toward the felsic rocks. On the other hand, carbonatites feature very Mg-rich minerals, generally Cr-rich, indicating that their genesis is completely unrelated to that of mafic and felsic rocks (ijolites and nepheline syenites). The parageneses indicate that this complex was formed by batches of primitive magmas with a distinct magmatic affinity (olivine melilitites and olivine nephelinites, basanites, and possibly also carbonatites) which evolved independently, generating the observed spectrum of intrusive rocks. Clinopyroxenites have interstitial alkali feldspar and titanite, indicating that they formed from evolved feldspar-normative (phonotephritic, tephriphonolitic) magmas. The sequence perovskite-titanite and titanite-garnet noted in some ijolitic rocks indicates changes in the chemical composition of coexisting silicate melts and, most likely, an increasing ƒ(O2). The trace-element profiles of coexisting phases in interesting associations in a sample of ijolite were documented by means of LA-ICP-MS analyses.