The implication of Grenville orogeny on REE mineralization in migmatites and gneisses of Son Valley Crystallines Sonbhadra, Uttar Pradesh, North India

The Son Valley Crystallines (SVC) of the Precambrian Chhotanagpur granite-gneiss complex (CGGC) of the Sonbhadra district Uttar Pradesh contain gneisses and migmatites which are highly deformed and metamorphosed. The study area encompasses different parts of SVC which are Arangapani, Kudri, Anjangira, Jarha, Nawatola, and Naktu. Here we found gneiss and migmatite except Nawatola and Naktu where we found felsic breccia. The foliation in migmatite is E-W but at places it changes to ENE-WSW. The leucosome part of the migmatite occasionally shows pinching and swelling and ptygmatic folding structures. Petrography of the leucosome of this gneiss and migmatite shows the dominance of microcline, orthoclase, quartz, and rare plagioclase with a minor amount of biotite and amphibole whereas the melanosome is dominated by biotite, rarely with amphibole and minor quartz and, feldspars. These rocks contain a higher amount of REE-bearing phases viz. zircon, garnet, apatite, titanite, monazite, and xenotime. The REE-bearing phases particularly zircon, monazite, and xenotime are found as inclusions mostly within biotite and occasionally in feldspars whereas apatite and titanite were found as inclusions exclusively within feldspars. Few zircon and monazite are found in intergranular space between different mineral grains. REE-bearing phases were analyzed in EPMA for total rare earth oxide (TREO) quantification and it was observed that TREO is 0.84 - 1.91 wt% (avg. 1.37 wt%) for apatite and titanite, and for monazite, it is 44.54 - 68.51 wt% (avg. 58.98 wt%). The chemical ages of monazite from Kudri migmatite is 1166 - 748 Ma (avg. 936 Ma; n=10) and Jarha gneiss is 1339 - 837 Ma (avg. 1088 Ma; n=13) these ages coincide with Grenville orogeny (c.1250 - 980 Ma). Hence, REE mineralization in the study area may be the result of the Grenville orogenic event. Most monazite grains are anhedral to elongate in shape and they are found as inclusion in biotite and feldspar however, some of the monazite grains are found in the intergranular space. These two contrasting modes of occurrences of monazite suggest an earlier dominantly of syngenetic (orogenic) mineralization and later with an epigenetic overprint. The geochemical analysis of these rocks shows high SiO₂ and low MgO, FeO, and CaO contents indicating a felsic source which is also supported by the TAS diagram in which most of the samples fall in the granite region. The A/NK vs A/CNK plot shows the rocks are peraluminous in nature. The tectonic discrimination diagram of Y vs Nb, Ta+Nb vs Rb, and Yb vs Ta suggests that most of the rock samples fall within the plate field with few in the volcanic arc and few in the syn-collisional setting which indicates the complexity of the terrain.