ACCRETIONARY OROGENS IN THE NORTHERN CRUSTAL SEGMENTS OF THE PROTEROZOIC CENTRAL INDIAN TECTONIC ZONE: CONSTRAINTS FROM A PETROCHRONOLOGICAL STUDY

Abstract

The present dissertation is based on comprehensive field, petrological, geochemical, and geochronological data from the Bhopal Basin along the southeastern margin of the Bundelkhand Craton, Mahakoshal Belt around Jabalpur, and the Betul Belt along the northern segment of the Central Indian Tectonic Zone (CITZ). The geochemical variation diagrams imply that sandstone units in the Bhopal Basin was deposited in rift-induced passive margin tectonic settings. The detrital zircons from sandstones of the basin are fragmentary and polymodal in size. The detrital zircons from the Bhopal basin yield three distinct concordant ages of 2511 ± 5, 1694 ± 6, and 1355±9 Ma (U-Pb SHRIMP ages). The presence of ~2540 Ma concordant zircon population with concentric zonation in the sandstone of Bhopal Basin suggests their derivation from the granite of similar age. Therefore, an extension of Bundelkhand Craton granite below the Bhopal Basin is suggested. The 1355 Ma ages from the Bhopal Basin, respectively, are concluded as the maximum depositional age (MDA) of the lowermost stratigraphic units within the basin. The MDA is concluded to be the timing of passive margin basin formations along margins of the Bundelkhand Craton during the Nuna or Columbia disintegration. Multiple tectonic discrimination diagrams incorporating major and trace element concentrations of the mica schists from the Mahakoshal Belt in Jabalpur have constrained back arc settings for the deposition of the sedimentary units. S1MB foliation formation marks the beginning of the M1MB metamorphic stage. In contrast, the later M2MB metamorphic event resulted in garnet and andalusite formation during heating at 2-3 kbar, followed by staurolite sillimanite, garnet-staurolite assemblages at peak P-T conditions of 5.4 kbar, 550-600°C. Textural evidence suggests that M2MB is pre-syn tectonic to D2MB deformation, resulting in the formation of the NE-SW trending S2MB axial planar foliation. From the isopleth thermobarometry, a clockwise P-T path with near-isothermal decompression has been determined for M2MB. Similar results have been obtained from conventional thermo-barometry performed on multiple samples. Monazite dating (EPMA) of one of the samples suggests a 1.9 Ga, age for the D1MB-M1MB event. The M2MB is correlated with the younger age population of monazites yielding 1.5 Ga. A clockwise P-T path constrained for M2MB suggests collisional tectonics along the northern margin of the CITZ. The present study from the Betul Belt focusses on two rock types, the mica schists and the associated gneisses. Geochemical analyses from the gneisses establish that they are peraluminous S-type granites formed in a collisional tectonic setting by partial melting of metasedimentary rocks. U-Pb isotope ages retrieved from the zircons of the granite gneisses iv constrain their emplacement age at ~1639 Ma. The pervasive NW-SW trending S2BT foliation recorded from the two lithological units was syntectonic with the second metamorphic event M2BT that led to the garnet formation in the mica schists. P-T conditions of the M2BT event have been determined to be ~6 kbar, 570-610°C from isopleth thermobarometry (by pseudosection analyses) and conventional geothermobarometry. Metamorphic monazites from the mica schists and the gneisses constrain the timing of the M2BT event between 968-901 Ma (EPMA chemical ages), and more precisely at ~936 Ma (U-Pb isotope age). The gamut of similar early Neoproterozic ages from different crustal domains of the CITZ, implies that M2BT in the Betul Belt occurred during the accretion of the North India Block (NIB) and South India Block (SIB) during the Rodinia formation. A significant overlap of the older age groups (~1592 Ma, ~1445 1440 Ma, ~1273-1220 Ma) retrieved from the metamorphic monazites of the mica schist and the emplacement age of the gneisses (~1639 Ma) with the Mesoproterzoic tectonic events in the North India Block (NIB) is noted. It is therefore suggested that the Betul Belt evolved with the NIB before its accretion with the CITZ during the Rodinia formation.