Petrology and Geochemistry of Mafic Dyke
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J. Earth Syst. Sci. (2019) 128:116 c Indian Academy of Sciences https://doi.org/10.1007/s12040-019-1102-0 Petrology and geochemistry of mafic dyke and sills in Cumbum Formation, of the Proterozoic Nallamalai fold belt, Rajampet area, Andhra Pradesh, India Sankha Das* and Munmun Chakraborty Geological Survey of India, SU: Andra Pradesh and Telangana, Southern Region, Hyderabad, India. *Corresponding author. e-mail: [email protected] MS received 28 March 2018; revised 26 July 2018; accepted 27 July 2018; published online 8 May 2019 Intensely deformed Proterozoic Nallamalai fold belt (NFB) occupies the eastern part of undeformed, Proterozoic Cuddapah basin and is separated from Cuddapah basin by easterly dipping Rudravaram thrust line. Igneous activity in NFB is represented by intrusive syenite and lamproite dykes. This study brings out the petrological and geochemical character of unreported intrusive dolerite dykes and sills within the Cumbum Formation of NFB. The dykes are undeformed, whereas the sills suffered D1 phase of deformation, although both are petrologically and geochemically similar. The rocks are highly fractionated sub-alkaline tholeiite, consist mainly of clino-pyroxene and plagioclase. Chondrite- normalised rare earth element (REE) plot shows enriched light rare earth element (LREE) and flat heavy rare earth element (HREE). A primitive mantle-normalised multi-element spider diagram shows troughs in Nb, Ta, Ti and Zr indicating subduction zone character. Tectonic discrimination plots indicate both within-plate and subduction zone environment of formation. Geochemical modelling also indicates enriched spinel peridotite as a probable source for dykes and sills. We suggest that the lithospheric mantle beneath NFB got enriched by subduction-derived fluid around 1.6 Ga, and in later stage, these dykes and sills were generated by extension of the enriched lithosphere at different phases in turn inheriting the subduction zone geochemical characters. Keywords. Nallamalai fold belt; Cumbum Formation; mafic dyke and sills. 1. Introduction dykes of Bastar and Dharwar cratons (Cuddapah basin). Majority of the dykes from the Bastar and Variably oriented dyke swarms are common Cuddapah basins show age around 1.9 Ga, and features of the Archaean cratons in India (Sri- this resemblance in age has been postulated as a vastava and Singh 2004). Study of Archaean/ result of widespread extensional tectonics around Proterozoic mafic dykes is important to under- 1.9 Ga resulting in mafic/ultramafic magmatism stand the geodynamic processes and mantle con- in the southeastern part of Dharwar protoconti- dition prevailing at that time. The general trend nent and has been informally called as Bastar– of dykes in and around Cuddapah basin is NE– Cuddapah Large Igneous Province. Such type of SW, although some N–S and E–W trending dykes mafic/ultramafic magmatism around 1.9 Ga is also are also present (Srivastava et al. 2014). French found in Superior craton and Kalahari craton of et al. (2008) have done a detailed analysis of the Africa. This widespread synchronous magmatism 1 0123456789().,--: vol V 116 Page 2 of 22 J. Earth Syst. Sci. (2019) 128:116 in Bastar–Cuddapah–Superior and Kalahari basin. King (1872) carried out pioneering geological cratons has been linked to the existence of the old- work in the Cuddapah basin and recorded his est supercontinent/craton Ur (French et al. 2008). observations in his classic memoir ‘On the Study of these dyke swarms provides an under- Kadapah–Kurnool Formations in the Madras Pres- standing and record of the large igneous provinces idency’. King suggested that the Kadapah and (LIPs), which might help in palaeo-tectonic recon- Kurnool are possibly of Palaeozoic age and have structions by precise geo-chronological and geo- been deposited over crystalline basement. He sub- magnetic dating (Srivastava et al. 2014). LIPs divided the rocks of Cuddapah ‘System’ into four represented by giant dyke swarms, sill provinces ‘series’, viz., Papaghni, Cheyair, Nallamalai and and layered intrusions are examples of deep-seated Krishna, in ascending order. He named the rocks plumbing systems, which might help in locating occurring around Pullampet–Rajampet area under ancient plumes and in turn locating breakup cen- Pullampet ‘stage’ of the Cheyair ‘Series’ of the tres of ancient supercontinent (Srivastava 2011). Cuddapah ‘System’ and described shale, slate, Halls (1982) has dealt in detail regarding the quartzite, limestone and dolomite. For the first importance of study of dyke swarms and has time, he identified the presence of acid volcanic opined that though dyke swarms often represent (quartzofeldspathic) rocks in the Tadpatri shale monotonous lithology, their study may bring out which is considered to be time equivalent of the the details of Archaean craton assembly and the lithounits of the Pullampet Formation. Nagaraja stress pattern prevailing at the time of intrusion in Rao et al. (1987) recognised four sub-basins within the area. Study of dyke swarms can eventually lead the Cuddapah basin, namely Papaghni, Kurnool, to identification of mantle plumes (Halls 1982). Srisailam and the Palnad sub-basins. The Cud- This paper brings out the petrology and geochem- dapah supergroup is divided into three groups, istry of unreported dolerite dyke and sills, intrusive Papaghni, Chitravati and Nallamalai from bottom within the sediments of Cumbum Formation of to top (table 1). The relatively undeformed Cud- Nallamalai fold belt (NFB) and its significance dapah basin is overlain by intensely deformed and with respect to the adjoining terrains and evolu- folded NFB. tion of NFB. The NFB consists of intensely deformed rocks belonging to the Nallamalai group. The Nallamalai 2. Geological setting group comprises quartzites (Bairenkonda/Nagari Formation) overlain by argillaceous sediments with Undeformed Cuddapah basin (figure 1) represents bands of dolomite and quartzite (Pullampet/ a classic example of intra-cratonic sedimentary Cumbum Formation). In the east central part Figure 1. Generalised geological map of the Cuddapah basin and Nellore Schist belt showing study area. J. Earth Syst. Sci. (2019) 128:116 Page 3 of 22 116 Table 1. Stratigraphy of Cuddapah supergroup. Group Formation Thickness (m) Lithology Age of intrusive Kurnool group Nandyal shale 50 ± 100 Shale Koilkuntla limestone 15 ± 50 Limestone Paniam quartzite 10 ± 35 Quartzite Owk shale 10 ± 15 Shale-ocherous Narji limestone 100 ± 200 Limestone Banaganapalli quartzite 10 ± 50 Conglomerate, quartzite ......................................................................................Unconformity....................................................................................... Srisailam quartzite 300 Quartzites and shale ......................................................................................Unconformity....................................................................................... Nallamalai group Cumbum: phyllite, slate, quartzite, dolomite Cumbum (Pullampet) 2000 Pullampet: shale, Racherla syenite: Formation dolomite, quartzite 1326 ± 73 Ma, Chelima lamproite: Bairenkonda (Nagari) 1500 ± 4000 Bairenkonda: quartzite 1328 ± 52 Ma, quartzites and shale Zangamrajupalle lamproite: Nagari: conglomerate, 1090 Ma quartzites and shales with intrusives ................................................................................Angular unconformity................................................................................ Chitravati group Gandikota quartzite 300 Quartzites and shale Tadpatri Formation 4600 Shale, ash fall tuffs, Mafic sill: 1899 ± 20 Ma quartzite, dolomite with intrusives Pulivendla quartzite 1 ± 75 Conglomerates and quartzite .....................................................................................Disconformity........................................................................................ Papaghni group Vempalli Formation 1900 Stromatolitic dolomite, dolomite mudstone, chert breccia and quartzite with basic flows and intrusives Gulcheru quartzites 28 ± 210 Conglomerate arkose, quartzite and shale Non-conformity ...............................................................................Archaean and Dharwar.............................................................................. of the NFB, both the Bairenkonda Formation Rudravaram thrust line. The eastern boundary and Cumbum Formation have been divided (Saha of NFB is separated from Nellore Schist belt and Tripathy 2012) into three members, viz., (NSB), along east dipping Velikonda Thrust Front. Bairenkonda quartzite consisting of Tekurupeta NFB is further divided into four different sub- quartz-phyllite member, Porumamilla quartzite basins from south to north, viz., Pullampet/ member and Kalavakunta member and Cumbum Rajampet, Zangamrajupalle, Markapur and Formation consisting of Chintalapalle slate mem- Agnigundla. ber, Sonam Cherevu phyllite member and Stratigraphically, NSB (located at the eastern Pandurangapuram quartzite member, from part of NFB) is divided into a western Udaya- bottom to top. NFB is separated from the unde- giri group and eastern Vinjamuru group (Ramam formed Cuddapah basin along east dipping and Murty 1997). Dobmeier and Raith (2003) have 116 Page 4 of 22 J. Earth Syst. Sci. (2019) 128:116 divided NSB into eastern Vinjamuru domain and local intra-formational oligomictic quartz-pebble western Udayagiri domain. However, subsequently, conglomerates. The Vinjamuru group is domi- NSB has been divided into four tectonic units,