Metamorphism of Proterozoic agpaitic nepheline syenite gneiss from North Singhbhum Mobile Belt, eastern India

Magmatic, hydrothermal and subsolidus evolution of the agpaitic nepheline syenites of the Sushina Hill Complex, India: implications for the metamorphism of peralkaline syenites

“…Other minerals found within the eudialyte decomposition assemblage include: cerianite-(Ce); pyrochlore; perraultite; ceriothorianite, together with rare wadeite and celsian (Mitchell and Chakrabarty, 2012;. Goswami and Basu (2013) reported the occurrence of cancrinite, piemontite and epidote from this variety of agpaitic gneiss, although we did not find any of these phases either as discrete grains or within the alteration assemblages associated with nepheline, albite or eudialyte. Sporadic occurrences of Mn-rich spessartine garnet were also noticed (Table 10) and have not been reported previously from these gneisses.…”

“…The entire P-T window starting from the early-to late-magmatic through subsolidus deuteric stages overlap with the greenschist-lower-amphibolite facies of metamorphism and consanguineous deformational event in the agpaitic rocks of the Sushina Hill Complex. This further implies that the incorporation of the jadeiitic component in pyroxene could be related to an extended subsolidus process at very low temperature (up to 348°C) rather than as a metamorphic process as described earlier (Goswami and Basu, 2013). It is most probable that during such a relatively low-temperature process, any accompanying deformation could result in preferential alignment of the mafic ( pyroxene) and felsic minerals…”

“…The mafic minerals coalesce and cluster, giving rise to an incipient gneissic texture as well as a 'knotted' or 'mottled' appearance. Various textural features such as unusually elongated albite, nepheline and pyroxene along the gneissic trend, ovoid nature of some felsic minerals with their long axes parallel to the gneissic trend are suggestive of a metamorphic overprint on the precursor eudialyte syenite (Goswami and Basu, 2013). However, it must be noted that similar textural features found in the Norra Kärr agpaitic complex have been attributed entirely to magmatic processes (Adamson, 1944;von Eckermann, 1968).…”

“…It is now well-established that the rocks of the complex have undergone extensive postintrusion metasomatism together with multiple phases of metamorphism during Palaeo-Mesoproterozoic times . The majority of recent work on the peralkaline rocks of Sushina Hill have been restricted to describing eudialyte and rinkite and their decomposition to diverse REE-bearing phases such as cerianite-(Ce), hingganite-(Ce), catapleiite, hilairite (Chakrabarty et al, 2011Mitchell and Chakrabarty, 2011;Goswami and Basu, 2013), and little attention has been given to the major rock-forming minerals. Goswami and Basu (2013) reported a metamorphic assemblage of epidote, scapolite, piemontite and natrolite from these agpaitic gneisses and suggested the metamorphic grade was within greenschist-to-amphibolite facies.…”

“…The majority of recent work on the peralkaline rocks of Sushina Hill have been restricted to describing eudialyte and rinkite and their decomposition to diverse REE-bearing phases such as cerianite-(Ce), hingganite-(Ce), catapleiite, hilairite (Chakrabarty et al, 2011Mitchell and Chakrabarty, 2011;Goswami and Basu, 2013), and little attention has been given to the major rock-forming minerals. Goswami and Basu (2013) reported a metamorphic assemblage of epidote, scapolite, piemontite and natrolite from these agpaitic gneisses and suggested the metamorphic grade was within greenschist-to-amphibolite facies. These authors also suggested that pure end-member compositions of nepheline, albite and orthoclase together with the development of gneissosity are suggestive of metamorphic origin for these agpaitic rocks.…”