[SMP32-P03] Long-lived (>90 Myr) granulite-facies metamorphism in the Trivandrum Block, southern India.
Keywords:Trivandrum Block, P–T–t evolution, Ultrahigh-temperature metamorphism, Phase equilibria modeling, U–Pb geochronology
Phase equilibria modeling of the khondalite from Elavinmoodu quarry located in the western part of the Trivandrum Block indicates peak P-T condition of 920-1030 °C and 6.0-7.6 kbar, suggesting UHT metamorphism. Prograde and retrograde P-T conditions of ~750 °C/~7 kbar and ~750 °C/~4 kbar, respectively, were also obtained, based on which a clockwise P-T path with geotherm-parallel slow cooling is inferred. Consistent P-T conditions are obtained from khondalite exposed at Kakkod quarry where incipient charnockite patches within khondalite and foliation-parallel bleached layers within massive charnockite are preserved. Phase equilibria modeling of the incipient charnockite and the bleached rock indicate metamorphic P-T conditions of 820-950 °C/ 5-10 kbar and 800-1000 °C/~6 kbar, respectively. These data suggest that the bleaching of charnockite occurred during high-grade (or UHT) metamorphism, possibly because of fluid flow around the peak metamorphism.
Zircon and monazite U-Pb geochronology for a khondalite sample from Elavinmoodu quarry suggests that prograde, peak, and retrograde metamorphisms took place at around 582±17 Ma, 555.1±8.1 Ma, and 527.3±8.0 to 501.9±8.5 Ma, respectively. Later infiltration of hydrous fluid and hydration of garnet to form biotite occurred at around 489±12 Ma. Our results suggest that high-grade metamorphism continued at least 90 Myr, from 582 Ma to 489 Ma, suggesting a long-lived thermal event possibly related to the input of radiogenic heat from the crust and/or magmatic heat from syn- to post-tectonic intrusions.
The results of this study suggest single metamorphic event along a clockwise P-T path. The duration of high-grade metamorphism (>90 Myr) is nearly consistent with that of metasediments from the Wanni Complex (>50 Myr; Hirayama et al., 2018), but it is shorter than that of long-lived high-grade metamorphism of the Highland Complex (~180 Myr; He et al., 2018). Although previous studies suggested that the supracrustal units of the Trivandrum Block could be a continuation of the Highland Complex (e.g., Dharmapriya et al., 2016; Takamura et al., 2018), the high-grade metamorphism of the Highland Complex might have started earlier than that of the Trivandrum Block possibly due to complex and multi-stage collisional processes related to the amalgamation of Gondwana Supercontinent.