Cambrian Garnet Sm-Nd Isotopic Ages from the Polydeformed Bolangir Anorthosite Complex, Eastern Ghats Belt, India: Implications for Intraplate Orogeny Coeval with Kuunga Orogeny during Gondwana Assembly Available to Purchase

Terminal Gondwana assembly occurred along the Cambrian Kuunga suture with the collision of the late Neoproterozoic Greater India–Enderbia plate and the combined North Australian–Mawson-Crohn cratonic block, supported by paleomagnetic latitudinal positions from India between 750 and 500 Ma. As the scale of the collision would have resulted in far-field intraplate orogenesis, one likely locale to be affected is the 990–940 Ma Eastern Ghats Belt (EGB) in southeastern India, exposing reworked granulitic middle to lower crust. The EGB (conjugate to the Rayner Complex, East Antarctica) is a collage of Archean and Paleoproterozoic crustal provinces separated by high-grade ductile shear zones. From the ∼933 Ma polydeformed Bolangir anorthosite in a Paleoproterozoic EGB province, an attempt was made to constrain the time of the amphibolite facies reworking to test possible Cambrian synchronicity of exhumation of middle to lower crust with the 530–490 Ma Kuunga orogeny. Anorthosites have Paleoproterozoic depleted-mantle model ages (∼1.9 Ga, with $εNd(930 Ma)=−3.6$ and −4.9) implying significant crustal assimilation. Combined anorthosite-garnet isochrons of samples PN 581e and PN 604 yielded an age of $495±9$ Ma, with Nd_i of $0.51150±0.00003$ and MSWD of 2.9 (⁠$n=4$⁠). The isochron age from ferrodiorite PN 581e and PN 589 garnet fraction b and its leachate was $481±12$ Ma, with Nd_i of $0.51155±0.00005$ and MSWD of 0.02 (⁠$n=3$⁠), whereas this age from multigrain garnet fraction c and its leachate was $475±16$ Ma, with Nd_i of $0.51180±0.00006$⁠. The ∼500 Ma garnet ages from the anorthosite and ferrodiorite are interpreted as a regional upper amphibolite facies tectonothermal event. Localization, strain partitioning, and tectonic exhumation of middle to lower crust during Cambrian intraplate orogeny were due to (1) interaction of the in-plane stress field with the heterogeneous terranes comprising the Tonian-age EGB–Rayner Complex orogeny, (2) juxtaposition with a rheologically contrasted cratonic domain, and (3) thermal weakening due to the high-crustal-heat-flow regime.