The Nellore Schist Belt (NSB) is a curvilinear Archaean schist belt, approximately 350 km long and 8-50 km wide. The Nellore Schist Belt is considered to be equivalent of the Sargur Group with a protolith age of 3.3-2.5Ga. Stratigraphically NSB is classified as the upper western Udayagiri Domain (dominated by metasediments) and lower eastern Vinjamuru Domain (dominated by metabasalts). Metabasalts of Vinjamuru Domain have been dated as of 2654 ± 100 Ma age, but so far no dates are available for the Udayagiri Domain. During recent mapping, in the central western part of NSB, west of Pamuru town, in a metapellite of Udayagiri Domain (consisting mainly of quartz, muscovite, paragonite, Spessartine garnet, chlorite, chloritoid and magnetite as accessory mineral) four small grains of Xenotime has been found, among which three grains have been dated. Xenotime is a rare earth phosphate mineral whose major is Yttrium orthophosphate (YPO4). Xenotime preferentially incorporates smaller HREEs and Yitrium. Xenotime has high U content and very low initial Pb concentration, so it is ideal for U-Pb geochronology. Xenotime can be of different origin such as detrital, diagenetic, hydrothermal and metamorphic. The age calculated from xenotime with the Age Quant of SX-Peak sight software gives an average age of1921±120 Ma. This age matches well with CHIME age of 1929±130 Ma of xenotime. All the textural and geochemical characteristics of the grains suggest that the xenotime is of retrograde metamorphic origin which occurs after D1 phase of deformation, hence constraining the upper age limit of the Udayagiri Domain to be 1929±130 Ma.

The Nellore Schist Belt (NSB) is a curvilinear Archaean schist belt, approximately 350 km long and 8-50 km wide. The Nellore Schist Belt is considered to be Neoarchean in age and stratigraphically NSB is classified as the western Udayagiri group (dominated by metasediments) and underlying eastern Vinjamuru group (dominated by metabasalts). There is a long controversy regarding the contact relationship between Udayagiri and Vinjamuru groups. Earlier researchers regarded the contact between two groups as tectonic on the basis of metamorphism. A shear zone and a possible thrust contact between the two groups have also been reported. On the basis of present study, an NNW-SSE trending, westerly dipping inclined transpressional zone is found at the contact between Udayagiri and Vinjamuru groups in the central western part of the NSB. Kinematic analysis of both the hanging wall and foot wall of the westerly dipping thrust zone shows presence of strong S1 schistosity, shear bands and S-C fabric in both strike and dip section along with east-verging overturned fold, westerly dipping inverted beds, suggesting partitioning of non-coaxial deformation in strike-slip and dip-slip component along with a pure shear component. Strike-slip is more prominent in the northern part of the contact than the southern part. The presence of steep to moderate northerly plunging non-orthogonal stretching/mineral elongation lineation all along the contact and clockwise shift of plot of the same in stereo net from its orthogonal position and presence of other kinematic indicators in plan suggests a right lateral strike-slip component. As a whole, it is suggested that Udayagiri group is thrusted over Vinjamuru group along a westerly dipping thrust plane with a right lateral strike-slip motion and simultaneous E-W contraction.