Nellore Schist Belt Research Articles

Intracontinental thrusts and inclined transpression along eastern margin of the East Dharwar craton, India

Recent works suggest Proterozoic plate convergence along the southeastern margin of India which led to amalgamation of the high grade Eastern Ghats belt (EGB) and adjoining fold-and-thrust belts to the East Dhrawar craton. Two major thrusts namely the Vellikonda thrust at the western margin of the Nellore Schist belt (NSB) and the Maidukuru thrust at the western margin of the Nallamalai fold belt (NFB) accommodate significant upper crustal shortening, which is indicated by juxtaposition of geological terranes with distinct tectonostratigraphy, varying deformation intensity, structural styles and metamorphic grade. Kinematic analysis of structures and fabric of the fault zone rocks in these intracontinental thrust zones and the hanging wall and footwall rocks suggest spatially heterogeneous partitioning of strain into various combinations of E-W shortening, top-to-west shear on stratum parallel subhorizontal detachments or on easterly dipping thrusts, and a strike slip component. Although relatively less prominent than the other two components of the strain triangle, non-orthogonal slickenfibres associated with flexural slip folds and mylonitic foliation-stretching lineation orientation geometry within the arcuate NSB and NFB indicate left lateral strike slip subparallel to the overall N-S trend. On the whole an inclined transpression is inferred to have controlled the spatially heterogeneous development of thrust related fabric in the terrane between the Eastern Ghats belt south of the Godavari graben and the East Dharwar craton.

Sheeted dykes in Kandra ophiolite complex, Nellore schist belt, Andhra Pradesh — Vestiges of Precambrian oceanic crust

Extensive field and petrological studies carried out in Kandra ophiolite complex (KOC) in the southern part of Nellore schist belt (NSB) revealed the existence of sheeted dykes in Kandra- Kondakuru-Gollapalli section. The sheeted dykes occur in the northern part of the complex and compositionally vary from medium-grained dolerites showing typical sub-ophitic texture to diabase dykes that are extremely fine grained aphanitic to fine-grained cryptocrystalline parallel basic dykes (varying in width from 4 cm to 1.25 m). These dykes show chilled margins on either side with the host dykes into which these are intruded. Veins of oceanic plagiogranite (0.5 × 5 m) are noticed in a major sheeted dyke. Occurrence of oceanic plagiogranite as ovoid / semicircular patches of 6 cm diameter resembling “immiscible droplets” within basic dykes is also noticed. The southern part of the complex is made of a major NW-SE trending cumulus gabbro that grades into olivine gabbronorite in its eastern part. The southern gabbros represent the plutonic component and form the deeper part of the complex. In this paper an attempt has been made to present the field, petrographic and mineral chemistry account of various lithounits that constitute the Kandra ophiolite complex in a succinct manner.

Petrological and geochemical characterization of Proterozoic ophiolitic mélange, Nellore–Khammam schist belt, SE India

The ophiolitic mélange occurs close to a major fault within the metavolcanics of Kanigiri area in the western margin of the Khammam–Nellore schist belt, Andhra Pradesh, India. The unique feature of Kanigiri mélange is the occurrence of ophiolitic bodies of various sizes, most of which display petrological and geochemical characteristics typical of supra-subduction zone oceanic crust. The ophiolitic mélange is composed mainly of blocks of variably deformed and metamorphosed pillow basalts, serpentinized ultramafic rocks, meta-gabbros, cherts and pelagic sediments in a pervasive scaly argillite matrix. The mélange has a tectonic contact with a granite intrusion and numerous thrust faults cut across the mélange exposures. Primary magmatic features such as drainage cavities in pillow basalts, magmatic layering in gabbros, and clinopyroxene cumulates in ultramafic rocks are well preserved in low-strain domains. Mafic components enclosed in the mélange are characterized by low pressure secondary mineral assemblages retrograded from amphibolite down to greenschist facies as commonly observed in subducted and exhumed orogens. All amphiboles in the studied fragments are calcic and cluster in the range from winchite, actinolite to hornblende. Amphibole compositions are characterized by Al 2O 3 (2.4–13.8%), FeO (5–13%), K 2O (0.04–0.7%) and X Mg (0.67–0.77). The metabasalt and wherlite samples exhibit light rare earth element (LREE) – enriched patterns and negative high field strength element (HFSE: Zr and Ti) consistent with a suprasubduction zone geodynamic setting. A notable feature of both metabasalt and wherlite samples is their pronounced negative Ce anomaly and a positive Eu anomaly as observed in a number of modern island arc suites. As the petrological features and the geochemical characteristics of the remnant fragments we analyzed in this study are reminiscent of their Phanerozoic ophiolite counterparts, we suggest that these rocks were derived from a Proterozoic oceanic plate belonging to a suprasubduction ophiolite assemblage. Subduction zone geochemical signatures and ophiolite-like crustal lithologies in the study region are consistent with the accretion of juvenile oceanic fragments to the pre-existing continental blocks through a forearc-continent collision.

Deformation Pattern in the Kurnool and Nallarnalai Groups in the Northeastern Part (Palnad Area) of the Cuddapah Basin, South India and its Implication on Rodinia/Gondwana Tectonics

The Proterozoic basins of India adjoining the Eastern Ghats Granulite Belt (EGGB) in eastern and southern India contain both Mesproterozoic and Neoproterozoic successions. The intracratonic set-up and contractional deformation fo the Neoproterozoc successions in the Paland sub-basin in the northeastern part of Cuddapah basin and similar crustal shortening in contemporaneous successions lying west of the EGGB and Nellore Schist Belt (NSB) are considered in relation to the proposed geodynamic evolution of the the Rodinia and Gondwana supercontinents. Tectonic shortening in the Palnad sub-basin (northeast Cuddapah), partitioned into top-to-westnorthwest thrust shear, flexural folds and cleavage development under overall E-W contraction, suggests foreland style continental shortening within an intracratonic set-up. A thrust sheet containing the Nallamalai rocks and overlying the Kurnool rocks in the northeastern part of Palnad sub-basin exhibits early tight to isoclinal folds and slaty (phylllitic) cleavage, which can be correlated with early Mesoproterozoic deformation structures in the nothern Nallamalai Fold Belt (NFB). NNE-SSW trending folds and cleavage affect the Kurnool Group and overprint earlier structures in the thrust sheet. Thrusting of the Nallamalai rocks and the later structures may have been related to convergence of the Eastern Ghats terrane and the East-Dharwar-Bastar craton during Early Neoproterozoic (Greenvillian) and/or later rejuvenation related to Pan-African amalgamation of East and West Gondwana.

Multi-Stage Deformation in the Nallamalai Fold Belt, Cuddapah Basin, South India - Implications for Mesoproterozoic Tectonism Along Southeastern Margin of India

The common occurrence of small-scale refolded folds, deformed lineations and overprinting of foliation in the northern Nallamalai Fold Belt (NFB) within the intracratonic Cuddapah Basin, South India, suggest three phases of deformation, D 1, D 2 and D 3 during the late Paleoproterozoic to Neoproterozoic. D 1 structures are represented by tight to isoclinal folds, a slaty cleavage and local development of mylonites. D 2 structures include NE trending tight to open folds (F 2) with variable plunge indicating control of large domal structures and a steep crenulation cleavage developed in phyllites and other schistose rocks. The above structures are overprinted by E-W trending D 3 folds and cleavage, which affect Mesoproterozoic rocks of the Srisailam Formation (uppermost Nallamalai Group) in the E-W trending Vami Konda range and metasediments of the Palnad area, thought to be equivalent of the Neoproterozoic Kurnool Group. Taking into account the ages of intrusive Vellaturu Granite (∼1575 Ma) which is post-D 2 deformation in the NFB and the Chelima Lamproite (∼1400 Ma) which intrudes the folded Nallamalai rocks, we suggest that the earlier episodes of multiple deformation in the NFB represent either late Paleoproterozoic or early Mesoproterozoic NW-SE compression in this part of the East Gondwana. The E-W fold-thrust structures affecting the Srisailam Quartzite and adjacent metasediments in the Palnad area represent late Mesoproterozoic and/or Neoproterozoic event. The granite gneisses and schists of the Nellore Schist Belt, bordering the NFB are affected by both early and later events and thrust over rocks of the Nallamalai Group. We propose that early deformation in the NFB lying southwest of the Eastern Ghats and roughly parallel to the present southeastern coast of India, was unrelated to, and occurred prior to the main deformation in the Eastern Ghats, which is generally correlated with the ∼1000 Ma old Grenville event. Alternatively, in view of the complex, protracted evolution of the Eastern Ghats and published geochronological data supporting pre-Grenvillian tectonism particularly in the Western Charnockite Zone of the Eastern Ghats, D 1 and D 2 deformation in the NFB may be linked to this older (1600-1400 Ma) tectonic disturbance in the Eastern Ghats.

P–T estimates from the Nellore schist belt (India) and evidence for superimposed metamorphic events

The Nellore schist belt (NSB) is one of the prominent schist belts of southern India. It is thrust over an unmetamorphosed Proterozoic sedimentary sequence (Cuddapah Basin) in the west and in turn is overthrust by the Eastern Ghats Granulite Terrain (EGGT) in the east. Metamorphic grade has been considered to be either greenschist to amphibolite facies or else to show high- and low-grade groups of rocks. Detailed mineralogical and P, T studies on representative rock types across the high- to low-grade groups (metapelite and metadacite from the high- and low-grade groups respectively) near Vinjamuru reveal that there are high (M1) and medium (M2) grade metamorphic events. While metapelite documents both M1 and M2 events, metadacite shows only the M2 event, thus indicating the presence of high- and medium-grade groups of rocks in the NSB. The stable mineral assemblage during the early prograde part of M1 was Qtz+St1+Ms1+Pl1+Bt±Grt, which subsequently suffered high-grade metamorphic conditions (T∽715–765°C and P∽8·6–9·2 kbar) leading to the demise of St1 and melting of Ms1. In contrast, medium-grade metamorphism (M2) is characterized by the stable association of St2+Qz. Quantitative geothermobarometry suggests T=520–570°C and P=6·1-6·8 kbar, and these results show good consistency with the average P, T estimates obtained from the THERMOCALC program. This metamorphic episode is probably coeval with the 900 Ma granulite facies metamorphic event in the EGGT. The superimposition of M2 over M1 probably has led to widespread retrogression of the high-grade rocks to medium grade, thereby giving the general impression of a greenschist to amphibolite facies gradation. © 1998 John Wiley & Sons, Ltd.

P–T estimates from the Nellore schist belt (India) and evidence for superimposed metamorphic events

P–T estimates from the Nellore schist belt (India) and evidence for superimposed metamorphic events

A bedded deposit of anthophyllite schist in the Precambrian belt of Nellore, South India

SummaryA pure anthophyllite schist occurs as a bedded deposit conformable with garnet-bearing quartz—muscovite, and quartz—chlorite—biotite schists and diopside-bearing caic-silicate rocks in the north-central part of the Nellore Schist Belt. Field and petrochemical studies show that anthophyllite schist has formed from regional metamorphism of Fe-, Mg-rich aluminous sediments aided by metamorphic differentiation due to local kinematic action under amphibolite facies conditions which is the general grade of metamorphism of the Nellore Schist Belt. A pure anthophyllite schist of sedimentary origin reported here is the first of its kind described from any terrain.

Geochronology of the Indian Precambrian

Precambrian geochronology of India is now known with greater precision because of the dating of samples carefully se lected , in respect of geological consideration and set up, from several areas in the pas t decade. Vestiges of older nuclei dating more than 3000 m.y. have been identified in southern, eastern and western India. The Charnockite-Khonda­ lite supergroup (c.2900-2600 m.y.) is now considered to be older than the Dharwar super­ group; the Eastern Ghats (sensu stricto) rocks are now considered belonging to the Charnoc­ kite-Khondalite supergroup proper but with impress of event s at 1650 m.y. and 1400 m.y . a t about the same time as events in Cuddapah Basin, Nellore Schist Belt and Amgaon group. The Aravalli, Bailadila, Bengpal and Iron Ore groups are correlated wi th the Dharwar and range in age from c.2500-<:.2000 m.y. The Delhi supergroup appears to be coeval with the lavas of the Gwalior 'series'. Vindhyan sedimentation which started at c.1400 m.y. continued into Cambrian (?) un­ like the Cuddapah supergroup where sedimentation ended by about 1300 m.y. The success­ ively younger dates for the mobile belts from south to north, with respect to the southern nucleus, indicates a migration of geosynclin es in the same direction. The Satpura 'group' despite valid e.vidences for it s aboliti on as a supergroup appears to indicate the reactivation of the Narmada-Son lineament at 950/ 1000 m.y. in part or eastern India. The manifestation of carbonatite and anorogenic granites dating 725 m.y. in southern India, high-level granites in western India, dating 730 m.y., Mylliem granite at 765 m.y. in eastern India and carbona­ tite in Rajasthan at 960 m.y. appear to be rel ated to reacti vat ion of lineaments in these areas at these dates. The overprint of minera I ages at c.500 m.y. along th e east coast is also suggest­ ive of activity of lineaments but further studies are necessa ry to confirm this view.