Research Updates

New Lithos ScreenshotThe charnockite suites of rocks possess greater significance in addressing the processes operating at lower continental crust and, thereby, provide ample evidence to decipher the petro-tectonic evolution of the Earth’s crust. The new research findings of Ravindra Kumar and Sreejith, published in ‘Lithos’ provide a comprehensive understanding on the lower crustal processes leading to protolith diversification on the genesis of different charnockite suites of southern India. The study distinguishes three different suites of charnockites, viz., tonalitic, granitic, and augen suites based on their petrological and geochemical attributes. Further, the study envisages four-stage magmatic crustal evolution model for the Kerala Khondalite Belt (KKB), spanning from Meso- to Neoarchaean up to Mesoproterozoic. The authors propose that the onset of juvenile magmatism in the KKB was initiated by the formation of Meso- to Neoarchaean basaltic crust in an oceanic lithosphere, which underwent melting due to basaltic underplating leading to the formation of TTGs. Subsequent intra-crustal melting during a stage of arc accretion initiated differentiation of the TTG crust into tonalitic and granitic magmas during Palaeoproterozoic. The fourth stage of crustal evolution is correlated with the Mesoproterozoic emplacement of megacrystic K-feldspar granites. This novel research contribution offers an insight into the long ignored aspect of origin and evolution of orthopyroxene-bearing, felsic ortho-granulites (charnockite) of KKB and their geodynamic setting. The study also establishes fairly–well correlation between the magmatic episodes of KKB and prominent crustal growth events recorded globally. The full-article can be read online at “Lithos” journal website hosted by SciVerse ScienceDirect.
Bibliographic Info: Ravindra Kumar, G.R. and Sreejith, C., 2016. Petrology and geochemistry of charnockites (felsic ortho-granulites) from the Kerala Khondalite Belt, Southern India: Evidence for intra-crustal melting, magmatic differentiation and episodic crustal growth. Lithos, vol. 262, pp. 334-354. doi: 10.1016/j.lithos.2016.07.009

New Evidence for High GradeIn-situ zircon U-Pb isotopic ages obtained on a garnet + cordierite bearing leucosome in migmatitic paragneiss at Kanjampara in the Trivandrum Block, demonstrate zircon crystallisation from melt at 1.92 ± 0.04 Ga. The unique trace element chemistry of the zircon, including high U/Yb, low Th/U, and flat MREE-HREE patterns with pronounced negative Eu anomalies, coupled with the presence of inclusions of sillimanite, apatite, quartz, feldspar and rare biotite in the zircon, confirms its formation as a result of anatexis associated with a high-T metamorphism at 1.92 Ga. This conclusion is supported by the local preservation of Palaeoproterozoic chemical age domains (820-860ºC. The strongly REE-P zoned garnet present in the Kanjampara leucosome, which is demonstrated to not be in HREE or Eu equilibrium with the 1.92 Ga zircon, formed as a result of anatexis and melt interaction during this Neoproterozoic-Cambrian tectonothermal event. Th-U-Pb chemical age data indicates that monazite was either formed or extensively recrystallized at during this anatexis at 565 ± 6 Ma, undergoing further modification to form high-Th cuspate rims at 517 ± 15 Ma. The age of the final monazite chemical modification is equivalent to the lower intercept age of 528 ± 18 Ma for extensive Pb-loss from the highly discordant but otherwise well-preserved Palaeoproterozoic (1.92 Ga) zircon. The growth or recrystallisation of monazite at ca. 565 Ma and its further modification at ca. 520 Ma indicates that the high-T metamorphism in the Pan-African was long-lived, with a duration of at least 45 Myr. The new age-event results from Kanjampara confirm that at least some of the metasedimentary paragneisses in the Trivandrum Block are polymetamorphic, initially metamorphosed in the Palaeoproterozoic in an event older than the 1.89-1.85 Ga granitic orthogneisses recognised from the region.

nandakumarFluid inclusions (FIs) are microsamples of fluids trapped in minerals during mineral growth and/or as in subsequent modifications. FI technology is greatly diverse in its application in oil exploration and invaluable in the reconstruction of fluid flow histories in reservoir rocks, indirectly yielding information about the temperature, salinity, and composition of fluids that migrated through the basin in the geological past. Hence, FIs represent samples of reservoir fluids of certain geologic time bands. In petroliferous basins, FIs, mostly in the size range of 5−20 μm, are either polyphased or biphased (containing oil, gas,