15:30 〜 15:45
[SMP24-07] A comparison between carbon isotope thermometry and Raman spectra of carbonaceous material (RSCM) thermometry in low-medium grade carbonate rocks from the Chitradurga Schist Belt, Dharwar Craton
キーワード：Chitradurga Schist Belt, Carbon isotope thermometry, Raman Spectra of Carbonaceous Material thermometry
The Dharwar Craton (DC), southern India, is a much-discussed Archean terrain that encompasses rocks spanning from approximately Meso to Neoarchean. Generally, the DC is divided into the Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC) based on the abundance of greenstone belts, volume of granitoids and grade of metamorphism (Swami Nath and Ramakrishnan, 1981; Chadwick et al., 2000). The major lithological units are the Basement Gneiss (Tonalite Tronjhemite Granodiorite gneiss), two set of volcano-sedimentary sequences and younger granitic intrusions (Jayananda et al., 2018). The oldest stratigraphic unit in the DC, the Sargur Group, occurs as enclaves in Basement Gneiss. The younger volcano-sedimentary sequence is the Dharwar Supergroup consists of lower Bababudan Group and upper Chitradurga Group. The Chitradurga Schist Belt (CSB) in the WDC is one of the granite-greenstone belts that preserves an almost complete stratigraphic section of the DC and undergone low to medium grade metamorphism. However, the metamorphic temperature within CSB using mineral thermometry (Hokada et al., 2013) is not well established due to low grade conditions. To resolve this issue, we attempted to estimate the regional-scale metamorphic temperature conditions of the CSB using carbon isotope thermometry and Raman spectra of carbonaceous material (RSCM) in the meta-carbonate rocks.
The weakly metamorphosed organic materials included in sedimentary rocks are termed as Carbonaceous Material (CM). The CM undergoes thermal maturation and subsequent recrystallisation during metamorphism is known as graphitization. The irreversible and progressive graphitization with increasing temperature forms the basis for metamorphic geothermometer such as carbon isotope thermometry in carbonate rocks and RSCM in metapelitic rocks (Beyssac et al., 2002). Exchange of isotopes between the lighter carbon (12C) from CM and heavier carbon (13C) from carbonate minerals during prograde metamorphism in equilibrium condition is the basis of carbon isotope thermometry (Satish-Kumar et al., 2002). Moreover, RSCM exhibits a systematic change in the crystallinity with metamorphic grade (Wopenka & Pasteris, 1993) and metamorphic temperature can be obtained using spectral features such as intensity (height) ratio R1, area (integrated intensity) ratio R2, and width (full width at half maximum; FWHM) between several prominent peaks (D1 and G peaks).
The CMs are found mainly in associated with carbonate minerals as well as in quartz as inclusions. The CM which are isolated by the silica phase were avoided for isotopic analysis, since they were not in isotopic equilibrium with the carbonate minerals. The carbon isotope thermometry results yield a metamorphic temperature of range from 340 - 560°C estimated by using Wada and Suzuki (1983). Meanwhile, RSCM gave a temperature range from 400 - 570°C estimated using R2 ratio (area ratio) following the calibration by Beyssac et al., (2002). Both thermometric results are corresponding to each other and observed a progression in the metamorphic temperature from upper green schist facies to lower amphibolite facies from north to south of CSB respectively. We compare the morphological, Raman spectral and carbon isotopic characteristics of CMs in low grade metamorphic rocks to evaluate the lower limit of temperature estimation using RSCM and carbon isotope thermometry. This study also provides the first report of regional metamorphic temperature gradient in the Chitradurga Schist Belt, in the Western Dharwar Craton.
References:
Swami Nath & Ramakrishnan (1981) Geological Survey of India Memoir, 112, 23-38 Chadwick et al (2000) Precambrian Research, 99, 91-111 Jayananda et al (2018) Earth-Science Reviews, 181, 12-42 Hokada et al (2013) Precambrian Research, 227, 99-199 Beyssac et al (2002) Journal of Metamorphic Geology, 20, 859-871 Wopenka & Pasteris (1993) American Mineralogist, 78, 553-557 Satish-Kumar et al (2002) Journal of Metamorphic Geology, 20, 335-350. Wada & Suzuki (1983) Geochimica et Cosmochimica Acta, 47, 697-706
The weakly metamorphosed organic materials included in sedimentary rocks are termed as Carbonaceous Material (CM). The CM undergoes thermal maturation and subsequent recrystallisation during metamorphism is known as graphitization. The irreversible and progressive graphitization with increasing temperature forms the basis for metamorphic geothermometer such as carbon isotope thermometry in carbonate rocks and RSCM in metapelitic rocks (Beyssac et al., 2002). Exchange of isotopes between the lighter carbon (12C) from CM and heavier carbon (13C) from carbonate minerals during prograde metamorphism in equilibrium condition is the basis of carbon isotope thermometry (Satish-Kumar et al., 2002). Moreover, RSCM exhibits a systematic change in the crystallinity with metamorphic grade (Wopenka & Pasteris, 1993) and metamorphic temperature can be obtained using spectral features such as intensity (height) ratio R1, area (integrated intensity) ratio R2, and width (full width at half maximum; FWHM) between several prominent peaks (D1 and G peaks).
The CMs are found mainly in associated with carbonate minerals as well as in quartz as inclusions. The CM which are isolated by the silica phase were avoided for isotopic analysis, since they were not in isotopic equilibrium with the carbonate minerals. The carbon isotope thermometry results yield a metamorphic temperature of range from 340 - 560°C estimated by using Wada and Suzuki (1983). Meanwhile, RSCM gave a temperature range from 400 - 570°C estimated using R2 ratio (area ratio) following the calibration by Beyssac et al., (2002). Both thermometric results are corresponding to each other and observed a progression in the metamorphic temperature from upper green schist facies to lower amphibolite facies from north to south of CSB respectively. We compare the morphological, Raman spectral and carbon isotopic characteristics of CMs in low grade metamorphic rocks to evaluate the lower limit of temperature estimation using RSCM and carbon isotope thermometry. This study also provides the first report of regional metamorphic temperature gradient in the Chitradurga Schist Belt, in the Western Dharwar Craton.
References:
Swami Nath & Ramakrishnan (1981) Geological Survey of India Memoir, 112, 23-38 Chadwick et al (2000) Precambrian Research, 99, 91-111 Jayananda et al (2018) Earth-Science Reviews, 181, 12-42 Hokada et al (2013) Precambrian Research, 227, 99-199 Beyssac et al (2002) Journal of Metamorphic Geology, 20, 859-871 Wopenka & Pasteris (1993) American Mineralogist, 78, 553-557 Satish-Kumar et al (2002) Journal of Metamorphic Geology, 20, 335-350. Wada & Suzuki (1983) Geochimica et Cosmochimica Acta, 47, 697-706