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V. Natarajan ¹, S. Rajagopalan Nair ², J. K. Nanda ³, H. Sarvothaman ⁴

Affiliations
1 3-4-756 A/I, Barkatpura, Hyderabad, IN
2 10-2-317/22, Vijayanagar Colony, Hyderabad, IN
3 12-2-111/A, Muradnagar, Mehdipatnam, Hyderabad, IN
4 3-4-448,2/1, Narayanguda, Hyderabad, IN

Abstract

A number of laterite cappings have been demarcated over the Deccan Trap basaltic rocks in parts of Medak and Nizamabad districts of Andhra Pradesh. Favourable lithology of traps and a geomorphic surface with almost gentle unidirectional slope from south to north appear to have strongly influenced the lateritisation process in the area that resulted in thick lateritic cappings.

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H. Sarvothaman ¹, C. Leelanandam ²

Affiliations
1 Geological Survey of India, Samchi, Bhutan, IN
2 Department of Geology, Osmania University, Hyderabad 500 007, IN

Abstract

The model compositions of the Archaean granitic (s.1.) rocks of the Medak area in Andhra Pradesh correspond to tonalite, quartz diorite, granodiorite. adamellite, quartz monzodiorite, granite (s.s.) and alkali feldspar granite. The granitic units show retrograde effects such as albitic rims around plagioclase at K-feldspar contacts, and inclusions of chlorite, epidote and ores in hornblende; anti-rapakivi mantling of plagioclase by K-feldspar is common; K-feldspars show incipient microclinisation. The diagnostic mineral parageneses of the granitic units indicate an overall metamorphism under greenschist facies conditions though locally culminating in amphibolite facies.

The major oxide chemistry reveals that the granitic rocks are divisible into three types. (i) tonalite-quartz diorite suite; (ii) calc-alkaline granites; and, (iii) potassic granites. Tonalites and quartz diorites seem to have formed from a melt which is produced by partial melting from a potassium-depleted sub-crustal (or a mantle) source; the melts of calc-alkaline and potassic granites are probably produced by repeated anatexis from an early sialic crustal source.

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H. Sarvothaman ¹, C. Leelanandam ²

Affiliations
1 Precambrian Geology Division, Geological Survey of India, Bandlaguda, P.O : Saroornagar, Hyderabad 500 660, IN
2 Department of Geology, Osmania University, Hyderabad 500007, IN

Abstract

Predominant rocks in the Medak-Nizamabad segment in Andhra Pradesh are of a granodiorite-adamellite-granite (GAG) suite; the Alumina Saturation Indices or the molar Al₂O₂/(CaO + Na₂O + K₂O) of these units indicate that they arc metaluminous. This suite contains inclusions of a peraluminous tonalite-trondhjemite (IT) suite. A younger peraluminous suite of K-rich granite, granite (s.s.) and adamellite (KGA) intrudes the metaluminous GAG suite and the peraluminous IT suite; a body of alkaline granite intrudes the peraluminous granites.

The tonalite-trondhjemite protoliths of the ca. 3000 Ma old gneisses of Kamataka are peraluminous, and the granites associated with them metaluminous. The Closepet granite activity began with the emplacement of mctaluminous granites, followed by the intrusion of peraluminous granites.

Partial melting (at lower to middle crustal levels) of charnockites, mafic granulites and basic lithologies may have generated the melts of metaluminous and alkaline granites. The ca. 3000 Ma old peraluminous granites are suggested to have formed by partial melting of basic rocks such as basalt and amphibolite under water-excess condition at lower crustal levels. Metaluminous granites, greenstone lithologies and the ca. 3000 Ma old peraluminous gneisses are the precursors for the peraluminous components of the Closepet granite and the KGA suite of Andhra Pradesh. The alkaline granite of Medak-Nizamabad is post-collisional and is emplaced in a rift-free setting.

Keywords

Granites, Peraluminous, Metaluminous. Alkaline, A/CNK, Andhra Pradesh, Karnataka, Dharwar Craton.

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H. Sarvothaman ¹

Affiliations
1 Precambrian Geology Division, Geological Survey of India, Bandlaguda, Hyderabad 500 660, IN

Abstract

The quartzo-feldspathic protoliths associated with the Central Crystalline gneisses of the western Bhutan-eastern Sikkim Himalayan segment are of peraluminous adamellite, granite (s.s.) and trondhjemite compositions. Peraluminous granite melt of first event is generated coeval to the kyanite or sillimanite grade metamorphism and it has syntectonically intruded a metapelite-quartzite sequence. These granites, along with the mctasedimentaries. have undergone partial melting under amphibolite facies conditions resulting in the generation of the peraluminous granite melt of second generation. which has syn- to late-tectonically intruded the pelitic sehist-quartzite-peraluminous gneiss assemblage. Approximately 20 Ma old post-tectonic peraluminous granites (s.s.) of the third event have intruded the Central Crystalline. The Central Crystalline adamellite-granite-trondhjemite suite is of 'non-minimum' melt compositions in contrast to the 'minimum melts' of the ≤20 Ma old leucogranites (such as the Manaslu granite of Nepal), which show unifonn granite (s.s.) compositions.

Keywords

Bhutan Himalaya, Central Ctystalline, Peraluminous Granite, Leucogranite, Petrogenesis.

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H. Sarvothaman ¹

Affiliations
1 Precambrian Geology Division, Geological Survey of India, Bandiaguda, Hyderabad 500 660, IN

Abstract

The plots of leucocratic, soda-rich siliceous rocks on K₂O-SiO₂ and Rb-Sr diagrams often provide tenuous conclusions with regard to the discrimination of oceanic plagiogranites from continental trondhjemites. Analytical methods for the generation of REE and Sr-isotope data, the effective constraints for discrimination of the above-mentioned lithotypes, are confined to specialized and sophisticated laboratories. The specifically calculated molar Al₂O₃/(CaO+Na₂O+K₂O) ratios (AlCNK), coupled with the SiO₂/Al₂O₃ ratios, of a large number of proven oceanic plagiogranites and continental trondhjemites are assessed to test whether these ratios can be applied as effective discriminant constraints. The major finding of this work is that oceanic plagiogranites are essentially low-alumina and metaluminous types. while continental trondhjemites are exclusively high-alumina and peraluminous types; the separation between the two groups is distinct and clean.

Keywords

Geochemistry, Plagiogranites, Trondhjemites.

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H. Sarvothaman ¹

Affiliations
1 Geological Survey of India Bandlaguda, Hyderabad 500 068, IN

Abstract

No Abstract.

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H. Sarvothaman ¹

Affiliations
1 Geological Survey of India, Bandlaguda, Hyderabad 500 068, IN

Abstract

A few xenoliths of high-Mg granitoids occur in the Archaean granodiorite and granite of Eastern Dharwar craton. They are composed of quartz, plagioclase, K-Feldspar, clinopyroxene, biotite, ore and occasionally orthopyroxene. They show wide variation in silica content from 61.4 to 73 wt.%. The Mg-Numbers in the units which contain 61.4 to 63.5 wt.% silica vary from 0.44 to 0.57; in the Total Alkalis-Silica diagram, these units plot in the fields of andesite, dacite and trachy-dacite, and in the QAP diagram they show quartz-Monzodiorite composition. In the units which contain >65 wt.% silica, the Mg-Numbers range from 0.43 to 0.51 and, in the normative Ab-An-Or diagram, they show granodiorite and tonalite compositions. These high-Mg granitoids show enrichment of Ba and Cr, and their chondrite-Normalised REE patterns display moderate slopes indicating LREE enrichment and HREE depletion. Quartz-Monzodiorite reveals sanukitoidal geochemistry and it is likely to have been derived from a source which is LREE-Rich and having high Mg-Number such as the products of mantle peridotite. Fractional crystallisation of sanukitoid would have generated high-Mg granodiorite and tonalite.

Keywords

Petrology, Geochemistry, High-Mg Granitoid, Sanukitoid, Dharwar Craton, Andhra Pradesh.

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H. Sarvothaman ¹, K. N. Srinivasan ¹, G. Suresh ¹

Affiliations
1 Geological Survey of India, Bandlaguda, Hyderabad, IN

Abstract

Two dykes of albitite intruding a trondhjemite-A-type granite suite in the Prakasam alkaline province in Andhra Pradesh contain >90% modal plagioclase (albite), and accessory quantities of hornblende, apatite, sphene, anatase and opaque ores. The normative 100·Ab/(Ab + An) is 96 to 100, indicating pure albitic nature of the plagioclase. A two-stage 'branching' differentiation is suggested for this petrological association.

Keywords

Petrology, Albitite, Trondhjemite, Prakasam Alkaline Province, Andhra Pradesh.

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H. Sarvothaman ¹

Affiliations
1 Geological Survey of India, Bhubaneswar, IN

Abstract

No Abstract.

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