09:30 〜 09:45
[SMP22-03] Crustal evolution of olivine-charnockite from south-eastern Madurai Block, Indiaand Wanni Complex, Sri Lanka
キーワード：fayalite-bearing charnockite , Madurai Block, India, Wanni complex, Sri Lanka, Crustal evolution
Olivine (fayalite)-bearing granitic rocks are not common. Oyawoye (1964) reported a rare fayalite-orthopyroxene-bearing quartz monzonite from Nigeria named Bauchite. Subsequently fayalite-bearing, felsic plutonic rocks are occasionally reported worldwide. Frost and Frost (2008) and Touret and Huizenga (2012) mentioned the rare occurrence of olivine-bearing charnockite. The present study identified a peculiar fayalite-bearing charnockite from the south-eastern Madurai Block, India. The primary rock type in the region is hornblende-rich charnockite, intercalated with thin, disrupted layers of pyroxenite and garnet-rich felsic rocks. Fayalite-bearing charnockite is exposed in a rock quarry at Paneerkulam village and nearby areas, hosted within hornblende-rich charnockite. Geochemically, olivine-bearing charnockite and surrounding rocks have a similar bulk chemical composition (slightly low SiO2content). Similar olivine bearing granitic rocks are also identified from Wanni complex, Sri Lanka.
The prominent reaction texture is coarse-grained fayalite and clinopyroxene rimmed by orthopyroxene coronas. The clinopyroxene grains have crisscrossing exsolution lamellae of orthopyroxene oriented in at least three directions, a texture indicating that the primary clinopyroxene was pigeonite. In a few domains, fayalite is associated with magnetite and minor quartz. The matrix assemblage is mostly plagioclase, mesoperthite and minor quartz. Zircon is associated with both the mafic and felsic matrix mineral assemblages. Zircon grains associated with olivine or clinopyroxene are rounded, with a concentrically zoned CL-bright core rimmed by a CL-dark mantle overprinted by a thin grey rim. Zircon grains associated with mesoperthite and plagioclase are more elongate, with a similar zoning pattern but a thick outer rim. Temperature estimates based on the composition of clinopyroxene and integrated mesoperthite are ca. 850–890°C. Based onP-Testimates from an isochemical phase diagram, the primary minerals were formed at high-temperature (ca. 850°C) at a pressure of ca. 7.5 kbar. These mineral assemblages were later overprinted by orthopyroxene at a similar temperature (ca. 800°C) but lower pressure (ca. 6 kbar). Sri Lankan sample also have similar mineral assemblages however they are texturally different with pyroxene exsolution and mesoperthite was not identified in these samples.
Zircon U-Pb dating gave a mean core age of ca. 800 Ma and a rim age of ca. 520 Ma. There is a small cluster of analyses (n= 4) at ca. 670 Ma. Based on the textural relations and dating, the primary magma was emplaced at ca. 800 Ma, possibly in connection with the break-up of the Rodinia supercontinent. Primary zircon was later overgrown by ca. 520 Ma rims during metamorphism and orthopyroxene formation associated with the amalgamation of Gondwana. The significantly negative εNd(t)and the 1000 to 1800 Ma TDM model ages suggest that the melting of an existing lower crust could have formed these rocks during Rodinia break-up. The primary crystallization of fayalite, along with clinopyroxene, falls below the fayalite-magnetite-quartz buffer, indicating low activity of oxygen and volatiles such as H2O. This conclusion is supported by the absence of hydrous phases such as hornblende or biotite in any of the observed fayalite-bearing charnockite samples. These results indicate that during Rodinia break-up, the lower crust of south-eastern India was subject to a high-temperature heat pulse under highly reducing conditions.
Reference
Frost, B.R. and Frost, C.D. (2008) On charnockites Gondwana Research 13, 30–44.
Oyawoye, M.O. (1964) Bauchite: A New Variety in the Quartz Monzonitic Series. Nature, P689.
Touret, J.L.R. and Huizenga, J. M. (2012) Charnockite microstructures: From magmatic to metamorphic. Geoscience Frontiers 3, 745-753.
The prominent reaction texture is coarse-grained fayalite and clinopyroxene rimmed by orthopyroxene coronas. The clinopyroxene grains have crisscrossing exsolution lamellae of orthopyroxene oriented in at least three directions, a texture indicating that the primary clinopyroxene was pigeonite. In a few domains, fayalite is associated with magnetite and minor quartz. The matrix assemblage is mostly plagioclase, mesoperthite and minor quartz. Zircon is associated with both the mafic and felsic matrix mineral assemblages. Zircon grains associated with olivine or clinopyroxene are rounded, with a concentrically zoned CL-bright core rimmed by a CL-dark mantle overprinted by a thin grey rim. Zircon grains associated with mesoperthite and plagioclase are more elongate, with a similar zoning pattern but a thick outer rim. Temperature estimates based on the composition of clinopyroxene and integrated mesoperthite are ca. 850–890°C. Based onP-Testimates from an isochemical phase diagram, the primary minerals were formed at high-temperature (ca. 850°C) at a pressure of ca. 7.5 kbar. These mineral assemblages were later overprinted by orthopyroxene at a similar temperature (ca. 800°C) but lower pressure (ca. 6 kbar). Sri Lankan sample also have similar mineral assemblages however they are texturally different with pyroxene exsolution and mesoperthite was not identified in these samples.
Zircon U-Pb dating gave a mean core age of ca. 800 Ma and a rim age of ca. 520 Ma. There is a small cluster of analyses (n= 4) at ca. 670 Ma. Based on the textural relations and dating, the primary magma was emplaced at ca. 800 Ma, possibly in connection with the break-up of the Rodinia supercontinent. Primary zircon was later overgrown by ca. 520 Ma rims during metamorphism and orthopyroxene formation associated with the amalgamation of Gondwana. The significantly negative εNd(t)and the 1000 to 1800 Ma TDM model ages suggest that the melting of an existing lower crust could have formed these rocks during Rodinia break-up. The primary crystallization of fayalite, along with clinopyroxene, falls below the fayalite-magnetite-quartz buffer, indicating low activity of oxygen and volatiles such as H2O. This conclusion is supported by the absence of hydrous phases such as hornblende or biotite in any of the observed fayalite-bearing charnockite samples. These results indicate that during Rodinia break-up, the lower crust of south-eastern India was subject to a high-temperature heat pulse under highly reducing conditions.
Reference
Frost, B.R. and Frost, C.D. (2008) On charnockites Gondwana Research 13, 30–44.
Oyawoye, M.O. (1964) Bauchite: A New Variety in the Quartz Monzonitic Series. Nature, P689.
Touret, J.L.R. and Huizenga, J. M. (2012) Charnockite microstructures: From magmatic to metamorphic. Geoscience Frontiers 3, 745-753.