Random Harvest (Biographical Sketches)

This is a companion volume to the Anthology of Editorials by Dr. B.P. Radhakrishna brought out as Memoir 5 1 of the Society in 2003. The current volume deals exclusively with biographical sketches and tributes to renowned earth scientists and scientists of India and other countries, sketches of public figures and benefactors with path-breaking contributions to our society and portraits of personal friends, colleagues and associates with exceptional qualities.This compilation of portraits of distinguished men is not only an essential reading for all geologists to recapitulate a part of the history of our science but also to learn a little more about the character and qualities that moulded the lives of some of our illustrious predecessors. All young scientists irrespective of their discipline, stand to+ benefit from going through BPR's writings to find inspiration in their own quest for excellence and originality.Even for the layman, these essays are of value and relevance in highlighting some of the major issues in geology in simple terms, and in acquainting them with some of the best minds that have shaped geological science in India and abroad as a part of the larger human heritage

Age of younger tonalitic magmatism and granulitic metamorphism in the South Indian transition zone (Krishnagiri area); comparison with older Peninsular gneisses from the Gorur-Hassan area

A major episode of continental crust formation, associated with granulite facies metamorphism, occurred at 2.55-2.51 Ga and was related to accretional processes of juvenile crust. Dating of tonalitic-trondhjemitic, granitic gneisses and charnockites from the Krishnagiri area of South India indicates that magmatic protoliths are 2550-2530+/-5Ma, as shown by both U-Pb and Pb-207/Pb-206 single zircon methods. Monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510+/-10 Ma. These data provide precise time constraints and Sr-Nd isotopes confirm the existence of late tonalitic-granodioritic juvenile gneisses at 2550 Ma. Pb single zircon ages from the older Peninsular gneisses (Gorur-Hassan area) are in agreement with some previous Sr ages and range between 3200+/-20 and 3328+/-10 Ma. These gneisses were derived from a 3.3-3.5-Ga mantle source as indicated from Nd isotopes. They did not participate significantly in the genesis of the 2.55-Ga juvenile magmas. All these data, together with previous work, suggest that the 2.51-Ga granulite facies metamorphism occurred near the contact of the ancient Peninsular gneisses and the 2.55-2.52-Ga 'juvenile' tonalitic-trondhjemitic terranes during synaccretional processes (subduction, mantle plume?). Rb-Sr biotite ages between 2060 and 2340 Ma indicate late cooling probably related to the dextral major east-west shearing which displaced the 2.5-Ga juvenile terranes toward the west

The geology and petrogenesis of the southern closepet granite

The Archaean Closepet Granite is a polyphase body intruding the Peninsular Gneiss Complex and the associated supracrustal rocks. The granite out-crop runs for nearly 500 km with an approximate width of 20 to 25 km and cut across the regional metamorphic structure passing from granulite facies in the South and green schist facies in the north. In the amphibolite-granulite facies transition zone the granite is intimately mixed with migmatites and charnockite. Field observations suggests that anatexis of Peninsular gneisses led to the formation of granite melt, and there is a space relationship between migmatite formation, charnockite development and production and emplacement of granite magma. Based on texture and cross cutting relationships four major granite phases are recognized: (1) Pyroxene bearing dark grey granite; (2) Porphyritec granite; (3) Equigranular grey granite; and (4) Equigranular pink granite. The granite is medium to coarse grained and exhibit hypidiomorphic granular to porphyritic texture. The modal composition varies from granite granodiorite to quartz monzonite. Geochemical variation of the granite suite is consistent with either fractional crystallization or partial melting, but in both the cases biotite plus feldspar must be involved as fractionating or residual phases during melting to account trace element chemistry. The trace element data has been plotted on discriminant diagrams, where majority of samples plot in volcanic arc and within plate, tectonic environments. The granite show distinct REE patterns with variable total REE content. The REE patterns and overall abundances suggests that the granite suite represents a product of partial melting of crustal source in which fractional crystallization operated in a limited number of cases

Late Archean crust-​mantle interactions: geochemistry of LREE-​enriched mantle derived magmas. Example of the Closepet batholith, southern India

The Closepet batholith in South India is generally considered as a typical crustal granite emplaced 2.5 Gyr ago and derived through partial melting of the surrounding Peninsular Gneisses (3.3-​3.0 Gyr)​. In the field, it appears as a composite batholith made up of at least two groups of intrusions. An early SiO2-​poor group (clinopyroxene quartz-​monzonite and porphyritic monzogranite) is located in the central part of the batholith. These rocks display a narrow range in both initial 87Sr​/86Sr ratio (Sri, 0.7017-​0.7035) and εNd (-​0.9 to -​4.1)​. A later SiO2-​rich group (equigranular gray and pink granites) is located along the interface between the SiO2-​poor group and the Peninsular Gneisses. They progressively grade into migmatized Peninsular Gneisses, thus indicating their anatectic derivation. Their isotopic characteristics vary over a wide range (Sri = 0.7028-​0.7336 and εNd values from -​2.7 to -​8.3, at 2.52 Gyr)​. Field and geochronol. evidence shows that the two groups are broadly contemporaneous (2.518-​2.513 Gyr) and mech. mixed. This observation is supported by the chem. data that display well defined mixing trends in the εSr vs εNd and elemental variation diagrams. The continuous chem. variation of the two magmatic bodies is interpreted in terms of interaction and mixing of two unrelated end-​members derived from different source regions (enriched peridotitic mantle and Peninsular Gneisses)​. It is proposed that the intrusion of mantle-​derived magmas into mid-​crustal levels occurred along a transcurrent shear zone; these magmas supplied addnl. heat and fluids that initiated anatexis of the surrounding crust. During this event, large-​scale mixing occurred between mantle and crustal melts, thus generating the composite Closepet batholith. The mantle-​derived magmatism is clearly assocd. with granulite-​facies metamorphism 2.51 ± 0.01 Gyr ago. Both are interpreted as resulting from a major crustal accretion event, possibly related to mantle plume activity

Petrogenesis of archaean grey gneisses from the amphibolite-granulite transition zone of southern Karnataka, India

This article does not have an abstract

A note on the occurrence of syenite porphyry near Kundur, Mandya district, Karnataka [India]

The occurrence of syenite from this area is noted. The syenite constitutes the elongated narrow bouldery outcrop and occurs in contact with coarse-​grained nonporphyritic pink granite. The tablets of feldspars are arranged in parallel rows imparting a flow structure to the rock. There is a variation in grain size and texture from margin to the center. The essential components are alkali pyroxene, alkali amphibole, K-​feldspar, and plagioclase; the accessories are magnetite, apatite, and sphene. The optical properties of the alkali pyroxene indicate that it is a sodian augite. The alkali amphibole is a greenish yellow variety and is strongly pleochroic. The optical properties indicate that it is probably arfevedonite. The modal and the chem. compn. of the rock is given

Mineral chemistry of the silicate mineral phases of Banded Iron- Formation of high-grade region, Karnataka.

The silicate minerals present in this BIF are ferrohypersthene, ferro-augite, almandine, lamellar pyroxene, pyroxferroite, grunerite, edenite, ferroan pargasitic hornblende and biotite, all of which crystallized in a metamorphic environment. P-T conditions were estimated at 7-8 kbar and 600-650oC. Chemical variations are attributed to P-T conditions, bulk composition and fO2.-R.E.S

Preliminary result for the Rb-Sr whole-rock age and initial Nd isotope ratios from the Closepet granite in Karnataka, Southern India

The Rb-Sr whole-rock isochron age was determined for granitic rocks from the Closepet granite, southern India. Porphyritic monzogranite, clinopyroxene quartzmonzonite, pink granite and grey granite, four main representative granitic rocks, gave an Rb-Sr age of 2669±125 Ma with an initial ^Sr/^Sr ratio of 0.70193±0.00088. This Rb-Sr whole-rock isochron age indicates a major period of equilibration of the isotopic system on a whole- rock scale at 2669 Ma. Initial εNd values normalized to 2669 Ma for individual granitic rocks range from -4.4 to -7.2. The large initial εNd values and low Sr initial ratios suggest that the Closepet granite has complicated history such as mixing