The Journal of Geology

doi: 10.1086/629020pmid: N/A

The Indian Precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes. The bulk of this crust was formed prior to 2600 m.y. ago and remobilized at least twice between 2600-2000 m.y. ago (early Proterozoic Mobile Belt, EPMB) and 2000-1500 m.y. ago (middle Proterozoic Mobile Belt MPMB). Three early Precambrian nucleii: Karnataka (KN), Jeypore-Bastar (JBN), and Singhbhum (SN) appear to have survived in the craton and are characterized by low-grade supracrustals and tonalitic trondhjemite gneisses, formed 3800-2600 m.y. ago. The EPMB event involved sedimentation, amphibolite-granulite facies metamorphism, and $$CO_{2}-K$$ metasomatism and produced amphibolite facies rocks and K-granites in the north, and charnockite and other granulite facies rocks in the south. Gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the EPMB event. K-granites form a garland around the central Dharwar craton, suggesting some type of collision between two blocks. The compressional stress directions in the craton and the surrounding mobile belts were EW, producing almost identical structures in all the regions. The supracrustals of the Indian Archean are broadly divisible into an older and a younger sequence. Older belts are characterized by argillites and chemogenic sediments of high Mg, Fe, Al, Cr, and Ni abundances, while younger belts are characterized by graywacke shale suites with abundant Na, K, Rb, and Sr. The REE, U, and Th abundance patterns of the two groups show significant differences. The small amount of ultramafic rocks in the Indian Precambrian necessitates alternative sources for the high Ni and Cr contents in the supracrustals. Cr and Ni contents are high even in gneisses of this region. The available data provide constraints for a model which suggests that older schist belts were developed in shallow water basins on a simatic crust. On the other hand, the platformal components of the younger greenstone belts were laid down in rifted basins on a sialic basement. Crustal deformation and thickening gave rise to the EPMB. At 2000-1500 m.y. ago, another intensive mobile belt event occurred in which subduction and flexure at the eastern and northern margins of the Dharwar-Singhbhum Protocontinent gave rise to Proterozoic sedimentary basins, rift valleys, and igneous and metamorphic suites. Plate tectonic regimes had clearly set in by 2000 m.y. ago; the middle Proterozoic orogeny shows clear evidence of modern-style collision tectonics.

doi: 10.1086/629021pmid: N/A

High-grade and low-grade supracrustal belts (Sargur and Dharwar) set in a matrix of gneisses dotted with granitic plutons of different ages make up the Dharwar craton. The overall structural pattern is one of subparallel linear belts and stringers whose regional trend changes from NW-SE in the northern part to N-S in the south. Both the high-grade and low-grade belts show three principal episodes of folding. However, the correlation between the deformation episodes in the Sargur and the Dharwar rocks is controversial, and the structural relations between them are not unequivocally established: parallelism of the first phase structures in the Sargur and the Dharwar rocks can be interpreted either as synchronous development or rotation of the earlier Sargur structures into parallelism with the younger Dharwar structures during later deformation. The extension of the supracrustal belts is parallel to the $$F_{2}$$ axial traces of the second deformation. Granite diapirism and basement uplift further modified the structural pattern in the Dharwar belts. In the Sargur Group the peak of metamorphism (upper amphibolite to lower granulite facies) was attained during the first deformation, which may be older than 3000-3100 Ma. There is no consensus regarding the age of the first deformation of the Dharwars; it is either coeval with the Sargur first deformation or younger. The closing stage of the second deformation in the Dharwars is no younger than 2500-2600 Ma. The climax of Dharwar metamorphism (greenschist or lower amphibolite facies) is post-tectonic with respect to the first deformation. The relation of the oldest component (3400 Ma) of the polyphase Peninsular Gneiss to the Sargurs is not established. Another major component was emplaced at 3000-3100 Ma, syn- to post-tectonic with respect to the first phase Sargur deformation. A third component was emplaced at 2500-2600 Ma at the late- to post-tectonic stage of the second phase Dharwar deformation. Ductile deformation style and polyphase migmatization, interspersed with folding episodes, characterize the Peninsular Gneiss. The gneisses exhibit a complicated deformation history: no comprehensive regional analysis is currently possible. The high-grade granulite terrain in southern Karnataka evolved at about 2500-2700 Ma under essentially static conditions. Much charnockite formation was definitely later than the formation of the gneissic foliation in the Peninsular Gneiss, and the structures in the granulites and in the low- to medium-grade terrains are essentially the same. It remains to be seen whether this is a mimetic preservation. It is possible that the high-grade terrain contains even older granulites, perhaps resulting from a metamorphism at 3000-3100 Ma ago. The youngest deformation features are the E-W shear systems in southern Karnataka and northern Tamil Nadu, which are probably Proterozoic in age and correlate with the N-S thrust system in the Eastern Ghats. These have caused retrogression of the granulites and development of mylonites and augen gneisses.

doi: 10.1086/629025pmid: N/A

The major rock type of Peninsular India consists of deformed, sialic rocks broadly termed "Peninsular Gneiss." Suites investigated in the western part of the Dharwar craton have a tonalitic/trondhjemitic composition and have isotopic ages generally in the range of 3,400 to 3,000 m.y. The last event recorded by numerous whole-rock isotopic systems was 3,000 m.y. ago, coincident with the emplacement of diapiric trondhjemite plutons. The gneissic suite appears to consist of a large number of separate magmatic bodies, some of which were equilibrated at very low pressures and may be extrusive. Low initial $$^{87}Sr/^{86}$$Sr ratios indicate derivation of the melts from the mantle or from materials that had acquired crustal Rb/Sr ratios only a short time prior to partial melting. Some of the ages represent metamorphism of rocks emplaced not long before the metamorphic event. The gneisses contain very little sedimentary component, as shown by the absence of rounded zircons, low Cr contents, and low (or zero) normative corundum contents. Some compositional modification occurred after initial emplacement of the gneisses. Considerable addition of U, and some Th, took place about 3,000 m.y. ago. Other lithophile elements may have been mobilized at the same time. There is no direct evidence for metasomatic addition of elements to the gneisses during the granulite-facies metamorphism that occurred in deep levels of the crust about 2,500 m.y. ago, despite depletion of the highest-grade charnockites in lithophile components.

doi: 10.1086/629026pmid: N/A

The amphibolite-facies to granulite-facies transition at the southern margin of the Dharwar craton has been studied in the Krishnagiri, Satnur-Halaguru, and Kushalnagar areas of southern India. In all three areas the transition appears to be a progressive metamorphic overprint on cratonal gneisses and their mafic and metasedimentary enclaves, without major structural interruption. In the Kabbal-Satnur-BR Hills section of Karnataka, a high-grade charnockite massif with pronounced Rb depletion is the culmination of an apparently continuous increase of metamorphic grade southward. In this and the Kushalnagar areas, increase of paleopressure from near 6 to near 8 kbar with increasing grade indicates a depth-zone relationship of the amphibolite and granulite facies. Incipient charnockite replacing Peninsular Gneiss first appears along N-S shears parallel to the regional grain of the craton. Low-P($$H_{2}O$$), high $$CO_{2}$$ vapors were instrumental in the creation of orthopyroxene. Introduction from a deep source, either a decarbonating mantle, basaltic underplate, or deeply buried sediments, was facilitated by the N-S deformation system. A deformed continental margin or infracontinental basin in the latest Archean is a plausible setting for the metamorphism. Great crustal thickening, perhaps with entrainment of shelf or basin sediments, was effected by overthrusting, the record of which may be preserved in an early isoclinal foliation and in fold-interference patterns in the southern cratonal margin. Subsequent transcurrent shearing facilitated outgas-sing of the deep crust and upper mantle, and rising vapors transported heat, $$H_{2}O$$, and K to middle levels of a thickened crust, which resulted in anatectic melting. Eventual uplift and erosion exposed a 6-8 kbar paleopressure surface at the southern cratonal fringe. The weakened and segmented continental shelf or platform south of the present craton was prone to later remobilization and retrogression, in the manner of a typical "mobile belt." This view of the relationship of the Dharwar craton to the southern high-grade terrain provides no support for the concept that granulite facies terrains of the general aspect of the South Indian massifs everywhere underlie the interior of the craton. The 2-3 kbar terrain of the cratonal interior was not greatly thickened by overthrusting nor magmatic underplating during the 2.6 Ga high-grade event that affected the southern cratonal margin. It is, however, possible that granulite-grade roots of an older metamorphic cycle underlie the cratonal interior.

doi: 10.1086/629027pmid: N/A

In the Archean Dharwar craton of southern India N-S trending belts of metabasic rocks are exposed which underwent regional metamorphism at about 2.5 Ga ago. The progressive changes in the assemblages and mineral chemistry of metabasites was studied in a N-S traverse covering the Chitradurga and Nagaman-gala belts, the Sargur area, and the Nilgiri Hills. Towards the south with increasing metamorphic grade greenschists ($$chl + act + ab + ep \pm carb$$, qtz) give way to amphibolites ($$hbl + plag \pm qtz$$, gar, cumm) and mafic granulites ($$pyx + plag + gar \pm hbl$$, qtz). The amphibole composition changes from actinolite in the greenschist zone to tschermakitic hornblende in the amphibolite zone and pargasitic-hastingsitic hornblende in the granulite zone. The Ti content of the amphiboles systematically increases in this direction. The plagioclase composition changes from albite in the greenschist zone to oligoclase/andesine in the amphibolite zone and andesine/labradorite in the granulite zone. Almandine-rich garnets first occur in the high-grade amphibolite zone whereas garnets in the granulite zone are higher in pyrope and grossularite components. Pyroxenes, which occur only in the granulite zone, are hypersthenes and salites with distinctly lower tschermak components in the pyroxene-granulite zone than in the hornblende-granulite zone, suggesting higher pressures of metamorphism in the latter zone. Geothermobarometry, mineral stability data, and mineral isograds demonstrate that P-T conditions along the N-S traverse increase continuously from about 500°C/4-5 kbars in the greenschist zone to about 600°C/5-7 kbars in the amphibolite zone. Further south there is a more pronounced temperature increase to about 700-750°C/8 kbars in the hornblende-granulite facies zone. This transition virtually coincides with the change from Dharwar to Sargur metabasites. South of the Moyar shear zone, which is characterized by higher pressures (9.5 kbars), metamorphic conditions of 700-750°C/6-7 kbars have been determined in the pyroxene-granulite zone of the Nilgiri Hills. The increase of metamorphic grade toward the south is also documented by the systematic partitioning of Na between plagioclase and the A-site of coexisting amphibole. The progressive development of the metabasites not only reflects increasing temperature and pressure of metamorphism but also a change in the fluid regime from hydrous in the greenschist and amphibolite zones to water-deficient in the granulite zone.

doi: 10.1086/629028pmid: N/A

The polyphase Closepet batholith in South India intrudes Archean tonalitic and granitic gneisses in a terrane that grades from amphibolite to granulite grade at the southern end of the batholith. Two granite and two granodioritic suites are recognized in the batholith. One granite suite from the southern terminus of the batholith has high Sr, Ba, La/Yb, and Eu/Eu*, and low Sc, Y, Nb, Ta, and total REE and is similar to other granites from along the amphibolite-granulite transition zone that have the characteristics of a fractionally crystallized cumulate. The other granite suite has lower Sr and Ba, higher Sc and Y, and no distinctive Eu anomaly. This latter group, which is widespread in the southern Closepet batholith, can be derived by 20% equilibrium partial melting of a tonalite source (containing hornblende and garnet) that is compositionally similar to South India tonalites near the amphibolite-granulite transition. The two granite groups may have been produced by separate episodes of $$CO_{2}$$ metamorphism. The granodiorites appear to have formed by mixing, in different proportions, of tonalitic and granitic gneisses within and around the southeastern margin of the batholith.