Paper No. 13
Presentation Time: 4:30 PM
SOLVING CCW GRANULITE–UHT METAMORPHISM: A MODEL FOR THE TECTONIC EVOLUTION OF THE EASTERN GHATS PROVINCE, INDIA–RAYNER PROVINCE, EAST ANTARCTICA
Some Proterozoic orogens record CCW heating and thickening to peak G-UHTM conditions before close-to-isobaric cooling; the tectonics of such orogens, particularly the heating mechanism, are challenging. Here, we propose a model for the late Mesoproterozoic–early Neoproterozoic Eastern Ghats Province (EGP)–Rayner Province (RP) orogen. In the central EGP, in multiple samples Crd predates Crd + Kfs (± Qtz, Bt, Sil, Pl) intergrowths interpreted to represent Osm that reacted with melt around the metamorphic peak. This interpretation indicates a common prograde evolution to T=945–955°C at P=0.78–0.82 GPa (1130–970 Ma) followed by a slight increase in P before development of retrograde mineral assemblages that record slow close-to-isobaric cooling (980–930 Ma) to elevated solidi at T=940–900°C and P=0.85–0.83 GPa. There is no imprint of this event on the Bastar craton. Based on literature data for the RP, reworked Napier basement records CW P–T–t paths that peaked at T=850–900°C and P=~1.0 GPa in the interval 930–890 Ma (Oygardens) and at T=~900°C and P=0.85 GPa in the interval 940–915 Ma (Stillwell Hills). In MacRobertson Land, supracrustal rocks record CCW P–T–t paths that peaked at T=850–900°C at P=~0.60 GPa in the interval 970–910 Ma. Critical features to be explained in any tectonic model are: a cryptic suture and an apparent absence of eclogite; similar isotopic signatures (TNdDM and THf(c)DM) in EGP and RP; continental basement (no evidence of oceanic basement); G-UHTM with CCW P–T–t paths in supracrustal rocks, and CW P–T–t paths in reworked Napier basement; pervasive ductile strain; absence of reworking of the Bastar craton; and, a slow rate of cooling. We propose that subduction of a restricted ocean basin beneath the EGP brought basement of the central sector of East Antarctica (excluding Napier, part of ‘India’ at this time) to the trench where shallow slab break-off led to extension of the overriding lithosphere and formation of a truncated hot collision (Sizova et al., 2013, GR). Continued convergence during amalgamation of Rodinia shortened and thickened the thinned and weakened lithosphere and HHP supracrustal sequence (CCW P–T–t paths, G-UHTM), which over thrust Napier basement (CW P–T–t paths, granulite facies). The low rate of cooling was due to low rates of erosion, which may reflect a hinterland plateau.