Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Abstract

The ultrabasic rocks and norites of Manbhurn are not to be included in the charnockite series. They belong to the earlier sedimentary-igneous sequence that underwent intense deformation and recrystallization before the charnockites (sensu stricto) came into being. The basic charnockites (pyroxene granulites) are derived as a result of metamorphic convergence at granulite facies from cafemic sediments and norites. These are isofacial with diopside-calc-granulite, khondalite and leptynite. Influx of granitic material converted the pyroxene granulites to a series of rocks of intermediate composition and ultimately to a granite. A part of all these varieties are hypersthene bearing with the features characteristic of the group.

The charnockite habit, including the index mineral, was impressed on the rocks by the pre-granite metamorphism, retained in the successive products of granitization, and obtained by the intrusive granite by crystallization under identical PT environment.

All hypersthene granites in this typical milieu, and the granulite facies rocks are charnockitic, and have a dry mineral assemblage association together with other members of the charnockite series. In the term 'charnockite' is to stay, it should be used as a synonym of hypersthene granite in the typical set-up and association. The term 'charnockite series' should be dropped.

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Abstract

In the Aravalli Range and its environs, the Precambrians consist of the Delhi and the Aravalli systems with the underlying Banded Gneissic Complex (BGC) regarded as the 'Basement Complex' of the region and correlated with the Bundelkhand Gneisses. The unconformity below the Delhis is described as a 'spectacular hiatus', a fact not apparent in the region under review. Portions of the unit mapped as basal conglomerate have been found to be feldspathized mylonites, though there are stretches of basal grits and conglomerates, especially along the northwestern border with the Pre-Delhis. BGC occurs in four patches: (1) N. of the Aravalli metasediments (BGC I), (2) S. of the Aravallis (BGC II), (3) E. of the Aravallis (Berach Granite), and (4) in an oval inlier below the Aravallis, around Serera. The Aravalli-BGC I boundary is diffuse and indefinite, and was obviously unrecognizable north of latitude 25⁰40'. The boundaries of the Aravallis with the BGC II and the Berach Granite are unconformable, so also is the Aravalli-BGC boundary in the Serera inlier. The correlation of Berach Granite and Bundelkhand Gneiss, though deeply entrenched in literature, is only of a tentative nature. The existence of the Raialos seems doubtful.

The Delhis show three sets of superposed folds: F₁ represented by megascopic folds mainly, and in the complex map pattern at times, is rotated by the immediately following F₂ along a coincident or nearly parallel axis; a comparatively open and broad F₃ cuts them at high angles producing simple interference in the regional pattern. The nature of the fault between the Delhis and the Pre-Delhis along the eastern border is not clear. No structures could be recognized in the small areas of Aravallis and BGC country studied, that could be regarded as diagnostic of the systems. They show the same structures in the same sequence as found in the Delhis. Search for a diagnostic orogenic trend' in small sample areas is beset with difficulties and may be positively dangerous. Only a detailed and extensive mapping could establish such a feature.

The radiometric ages available for rocks of this region are not many. At times contradictory, it is not clear in most of the cases if these 'ages' really date an event to be correlated with the relevant orogeny. A scheme of Precambrian stratigraphy of Rajasthan is proposed, based mainly on the field observations of the author and his associates, and on analysis of published data:

                                      Delhi System

                                 (1100± to ?750m.y.)

                               Aravalli System ≡ BGCI

                                  (I590± to 1000± m.y.)

                      BGC II ≡ BGC (Serera) ≡ ? Berach Granite

An attempt has also been made to fit this scheme with the recently proposed classifications of the Precambrians.

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