Browsing by Author "M. Raith"

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Regional geothermobarometry in the granulite facies terrane of South India
(1983) M. Raith; P. Raase; D. Ackermand; R.K. Lal
In the southern part of the Archaean craton of South India, an approximately 3.4–2.9 b.y. old migmatite–gneiss terrane (Peninsular gneiss complex) has been subjected to granulite facies metamorphism about 2.6 b.y. ago. During this event, the extensive charnockite-khondalite zone of southern India developed. A younger metamorphism (Proterozoic?) led to retrogression of the charnockites and khondalites, mainly under the conditions of the amphibolite facies. The physical conditions of metamorphism have been evaluated by applying methods of geothermobarometry to the widespread charnockitic assemblages with garnet, orthopyroxene, clinopyroxene, plagioclase, and quartz. The interpretation of the P–T estimates includes a critical discussion of potential error sources, e.g. errors of the analytical data and the calibrations of the models, and takes into account the complex metamorphic history of the rocks and the kinetics of the mineral equilibria. P-T estimates were obtained for seven subareas from the rim compositions of the coexisting minerals: Shevaroy Hills 680±55°C—7·4±1 kb; Kollaimalai area 680±40°C—8·6± 1 kb; Nilgiri Hills 680±90°C—6·6±0.8kb (upland massif) and 705±60°C—9·3±0.8 kb (northern margin); Bhavani Sagar area 650±50°C—7·2± 1 kb; Sargur-Mysore area 690±60°C—7·6 kb; Bangalore-Kunigal-Satnur area 760±50°C—6 kb. Except for the last subarea, the P-T model data reflect the conditions of a late annealing stage probably related to the retrogressive metamorphism. Conditions near the peak of granulite facies metamorphism (730–800°C—6·5–9·5 kb) are recorded by the core compositions of the minerals. Although a rather uniform cooling history of the main part of the charnockite-khondalite terrane is suggested from the temperature data, differential uplift of smaller blocks is indicated by the regional variation of the pressure data. © 1983, Royal Society of Edinburgh. All rights reserved.
The Archean Craton of Southern India: Metamorphic evolution and P-T conditions
(Springer-Verlag, 1982) M. Raith; P. Raase; D. Ackermand; R.K. Lal
The Archaean craton of southern India is characterized by a highly complicated and not yet fully understood geological history comprizing several cycles of sedimentation and volcanism, deformation and metamorphism in the span between about 3400 and 2500 m. y. The large scale regional variation in metamorphic grade observed today is essentially related to a metamorphic event at about 2600 m. y. ago which affected an older migmatite, gneiss-greenstone terrain (2900-3400 m. y.). The southern area is characterized by granulite facies (700-750° C/8-10 kb). An extensive charnockite-khondalite belt has been generated by atectonic transformation of the migmatite-gneiss terrain through the influx of a CO2-rich fluid. Towards the north of the terrain the metamorphic grade decreases to amphibolite facies (600° C/6-8 kb) and to greenschist facies (400° C) which is restricted to the Dharwar greenstone belts. Metamorphism related to younger shear zones in the southern part of the craton led to retrogression of the charnockite-khondalite series under conditions of amphibolite to greenschist facies. The P-T conditions of metamorphism have been evaluated applying mineral stability data and methods of geothermometry and geobarometry in an area between Shimoga-Chitradurga (North) and Coimbatore-Karur (South). © 1982 Ferdinand Enke Verlag Stuttgart.