[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
The Southern Granulite Terrane in India is composed of various Archean to Neoproterozoic continental blocks, magmatic arcs, and supracrustal units amalgamated through complex subduction-accretion-collision event and high-grade metamorphism during the Latest Neoproterozoic to Cambrian Gondwana amalgamation. The Trivandrum Block corresponds to a unit in the southern part of the terrane with abundant metasediments and orthogneisses. The pressure-temperature (P-T) evolution of the high-grade metamorphism and its time scale are important for unraveling metamorphic processes during the collisional event. The P-T evolution recorded in various lithologies in the Trivandrum Block has been argued in numerous previous studies, and it is a subject of ongoing debate. The peak metamorphic ages reported in previous studies from the Trivandrum Block also show a wide range of ca. 600-510 Ma. It is therefore still not known whether the Trivandrum Block underwent single metamorphism with P-T variations within the block, or it experienced two discrete metamorphic events at 800-900 °C and >950 °C. In this study, we present new petrological, geothermobarometric, and geochronological data from khondalites, charnockites, bleached charnockites in the western part of the Trivandrum Block and discuss pressure-temperature-time (P–T–t) evolution of the block for unraveling the duration and heat source of high-grade metamorphism.
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.
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.