Linking zircon formation to partial melting events in Kerala Khondalite using nanogranitoids

The Kerala Khondalite Belt – part of the Southern Granulite Belt of SW India – is well known for its excellent outcrops of metapelites which underwent partial melting during the Panafrican orogeny at lower crustal conditions with temperatures up to 1000°C. Several groups have worked on the metamorphism and petrology of this area which is now complemented by our study of nanogranitoids, i.e., unmodified melt inclusions (MI) of anatectic origin, in zircons from Khondalite. The investigated samples originate from the locations Koliakkode and Pottangodu quarries. Whereas nanogranitoids in peritectic garnets from Koliakkode quarry have already been investigated (Cesare et al., 2009; Ferrero et al., 2012), in Pottangodu we report the first finding of such inclusions.

Preliminary results from optical microscopy investigation reveal that the inclusions are always primary, with an azonal distribution in zircon and a cluster distribution in the garnets from both localities. MicroRaman spectroscopy shows the occurrence of a constant phase assemblage in MI in zircon consisting mainly of kokchetavite, white mica, cristobalite/quartz ± kumdykolite. Phase 430 and phase 412 (both new phases and currently under investigation), carbonate and graphite may be present, in some cases also CO₂ and N₂. The MI hosted by garnet are quite similar but here we also find biotite in the main assemblage. Additionally, the MI in garnet may contain osumilite, andalusite, apatite and rutile.

In numerous studies by our group we have demonstrated that the identified metastable polymorphs, like kokchetavite (hexagonal k-feldspar), kumdykolite (orthorhombic albite) and cristobalite, are an excellent indicator that melt inclusions are pristine i.e. unadulterated due to decrepitation or by post entrapment infiltration of material along cracks. In order to obtain new data on the pristine melt compositions these nanogranitoids will be then re-homogenized via piston cylinder experiments.

The detailed investigation of these inclusions, coupled with geochronological studies on the host zircon from both localities will unravel the microchemistry of the original unmodified anatectic melt preserved in inclusions, as well as its evolution over time at the host rock scale. In particular, these data will clarify 1) which zircon domains originated before, during and after melt production, as well as 2) the age of the melting event(s?) at HT-UHT conditions. Moreover, to date this represents one of the few examples of anatectic MI in zircons from metamorphic rocks to be investigated in detail via in situ MicroRaman spectroscopy.

References:

Cesare, B., Ferrero, S., Salvioli-Mariani, E., Pedron, D. & Cavallo, A. 2009. Nanogranite and glassy inclusions: the anatectic melt in migmatites and granulites. Geology, 37, 627–630, https://doi.org/10.1130/G25759A.1

Ferrero, S., Bartoli, O., Cesare, B., Salvioli-Mariani, E., Acosta-Vigil, A., Cavallo, A., Groppo, C., Battiston, S., 2012. Microstructures of melt inclusions in anatectic metasedimentary rocks. Journal of Metamorphic Geology, 30, 303–322, https://doi.org/10.1111/j.1525-1314.2011.00968.x