Geochemistry and neodymium model ages of Precambrian charnockites, Southern Granulite Terrain, India: Constraints on terrain assembly

• Published in: Precambrian research 2013-04, Vol.227, p.295-315
• Main Authors: Tomson, J.K., Bhaskar Rao, Y.J., Vijaya Kumar, T., Choudhary, A.K.
• Format: Article
• Language: English
• Subjects: Charnockite; Geochemistry; Gondwana; Neoproterozoic; Sm–Nd model ages; Southern India
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/j.precamres.2012.06.014

► New geochemical and Sr–Nd isotopic data for charnockites from the Southern Granulite Terrain (SGT), India are reported. ► A first ever comprehensive Sm–Nd model age map is presented for the SGT. ► Four distinct Nd-isotopic provinces within the SGT are identified with model ages in the range ∼3.5–3.2Ga, 3.2–2.8Ga, 2.8–2.0Ga and 2.0–1.2Ga, which young from NW to SE. ► The study reveals distinct source compositions and tectonic settings for charnockites genesis in the Archean and Neoproterozoic domains of the SGT. ► The Neoproterozoic crust of the SGT can be modeled as a subduction-collision setting at an Archean cratonic margin where crustal reworking involving older continental crust is predominant. The Southern Granulite Terrain (SGT) of southern India comprises a collage of Archean and Neoproterozoic high-grade metamorphic terrains, where the regional tectonic and high-grade metamorphic events culminated at ca.2.52Ga in the northern granulite blocks and ca.0.55Ga in the Neoproterozoic Madurai and Trivandrum Blocks. These domains were presumed to be separated either by the crustal-scale Cauvery shear zone system (CSZ) or the less known Karur–Kambam–Painavu–Trichur shear zone (KKPT). The Neoproterozoic granulite domains comprise mainly the Madurai Block (MB) to the north and Trivandrum Block (TB) to the south separated by the Achenkovil shear zone (ASZ) system. The SGT has been central to several studies focusing on the amalgamation of Gondwana supercontinent. A favored view has been that the Neoproterozoic terrains of the SGT represent segments of orogen(s) that accreted to an Archean continental margin during the early Cambrian although whether the tectonic regime was collisional, accretionary or intracratonic remains enigmatic. This article presents a large data set of major, trace element, Rb–Sr and Sm–Nd isotopic compositions focusing on Sm–Nd model ages (TDM) signifying the crustal residence age of charnockitic orthogneisses from both Archean and Neoproterozoic domains of the SGT, especially from the data deficient MB and TB domains. We present a regional Sm–Nd model age map for the SGT that delineates four distinct isotopic provinces with ages in the range of ∼3.5–3.2Ga, 3.2–2.8Ga, 2.8–2.0Ga and 2.0–1.2Ga. Clearly, the KKPT shear zone separates two isotopic provinces, extending the Archean terranes further south across the CSZ. The Archean and Neoproterozoic charnockites show distinct compositional characteristics indicating contrasting conditions