An assembly of the Indian Shield at c. 1.0 Ga and shearing at c. 876–784 Ma in Eastern India: Insights from contrasting P-T paths, and burial and exhumation rates of metapelitic granulites

Abstract

Recent studies indicate that the Indian Shield formed contemporaneously but separately from Rodinia in Late Mesoproterozoic-Early Neoproterozoic. However, assembly of the eastern Indian Shield is poorly constrained. The dominant WSW-ENE ∼1.0 Ga foliation in Central Indian Tectonic Zone (CITZ) including Chotanagpur Gneissic Complex (CGC) in Eastern India indicates a north-south collision between the Northern and Southern Indian Blocks (NIB and SIB). But in northeastern CGC, the dominant foliation is north-south and parallel to the Eastern Indian Tectonic Zone (EITZ), a linear N-S to NNE-SSW belt associated with sinistral shearing at ∼876–784 Ma along the eastern margin of CGC, in Elan Bank of Kerguelen Plateau, and possibly in East Antarctica. The youngest dates in the metapelitic granulites from EITZ and NE-CGC (>13 km west of EITZ) are ∼876–784 Ma and ∼1.0 Ga, respectively. But monazite crystallization depends on bulk composition, and it is unclear whether the N-S foliation in NE-CGC metapelites originated at ∼1.0 Ga or ∼876–784 Ma. Pseudosection analysis in the MnO-CaO-Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O-P2O5-Y2O3-Ce2O3-F system shows contrasting P-T paths for the metapelites of NE-CGC (clockwise with <7 kbar/700 °C to ∼14 kbar/825 °C prograde conditions) and EITZ (counterclockwise with 8.93 kbar/794 °C to 8.80 kbar/809 °C prograde conditions, loading to 10.22 kbar/777 °C, and isothermal decompression to 6.83 kbar/787 °C). Furthermore, garnet diffusion modeling indicates that the burial rate (3.5–8.4 km/Ma) was similar to the exhumation rate (2.2–7.2 km/Ma) in NE-CGC, whereas exhumation (15–27 km/Ma) was ∼8–25 times faster than burial (1.1–2.0 km/Ma) in EITZ. The N-S foliation in NE-CGC is a manifestation of a ∼1.0 Ga collision between the CGC and Dinajpur Block along the EITZ, and is similar to the N-S trend in the NIB-Marwar Block collision zone in northwestern India. Evidently, SIB collided northward with NIB, NIB collided westward with Marwar Block, and Dinajpur Block collided westward with NIB simultaneously at ∼1.0 Ga. The EITZ was reactivated by sinistral shearing at ∼876–784 Ma, when the Dinajpur Block moved northward. In Cretaceous, dextral strike-slip movement on the E-W Dauki Fault split the Dinajpur Block into its present configuration. The northern parts of NIB and Dinajpur Block collided with Asia in Cenozoic during Himalayan Orogeny.