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Geology and Tectonic Implications of Tourmaline Bearing Leucogranite of Bastipadu, Kurnool, Andhra Pradesh, India

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Geology and Tectonic Implications of Tourmaline Bearing Leucogranite of Bastipadu, Kurnool, Andhra Pradesh, India J. Earth Syst. Sci. (2018) 127:87 c Indian Academy of Sciences https://doi.org/10.1007/s12040-018-0990-8 Geology and tectonic implications of tourmaline bearing leucogranite of Bastipadu, Kurnool, Andhra Pradesh, India Kiran Jyoti Mishra, Santanu Bhattacharjee* , M S Reddy, M N Praveen, A D Bhimte and N Mahanta Geological Survey of India, Southern Region, Hyderabad 500 068, India. *Corresponding author. e-mail: [email protected] MS received 17 October 2017; revised 2 January 2018; accepted 19 January 2018; published online 2 August 2018 Tourmaline bearing leucogranite occurs as a pluton with pegmatitic veins intruding the Archaean granodiorite in the Bastipadu area, Kurnool district of Andhra Pradesh. We present field and petrographic relations, mineral chemistry and geochemical data for the leucogranite. It is essentially a two-mica granite, composed of quartz, perthite, microcline, albite, tourmaline and muscovite along with minor biotite and titanite. The euhedral tourmalines are regularly distributed in the rock. The geochemical studies show that the leucogranite is calc-alkaline, peraluminous to metaluminous which formed in a syn-collisional to volcanic arc-related setting. It displays strong ‘S’ type signatures with high K/Na ratios, moderately fractionated light rare earth elements, relatively flat heavy rare earth elements with [Ce/Yb]N ≤ 27.8 and a strong negative Eu anomaly. The geochemical characteristics indicate that the leucogranite melt might have been generated from partial melting of metasediments. Electron probe microanalyser data show the presence of alkali group tourmaline in leucogranite represented by schorl and dravite. Tourmaline compositions plot in the Li-poor granitoids and associated pegmatites and aplites and metapelites/metasammites fields. Partial melting of boron-enriched source rocks is linked with the development of tourmalines in the leucogranite. Keywords. Bastipadu; tourmaline; borosilicate; schorl; leucogranite. 1. Introduction are used as semiprecious stones for making Leucogranite is one of the occasionally reported ornaments (Pezzotta and Laurs 2011). The study and least informative rocks in the granitoids (Moyen area, around Bastipadu, is spotted with numer- et al. 2003). It is associated with tin–tungsten min- ous outcrops of leucogranite. It is located 10 eralisation as reported from Sewaria-Govindgarh km SSW of Kurnool in Andhra Pradesh and areas of Rajasthan (Pandian and Dutta 2000). belongs to the Gadwal schist belt of the East- The leucogranites from the higher Himalayan belt ern Dharwar Craton (EDC), lying adjacent to are better known for their tourmaline content the Proterozoic Cuddapah basin (figure 1). Basti- showing variable abundances from pluton to plu- padu is an important sector of the Bastipadu– ton and within a single pluton (Guillot and Le Chetlamallapuram–Nayakallu corridor as it hosts Fort 1995). Jowhar (2010) reported schorl, an Fe–Cu±Au–Nb–Ta–REEs mineralisation in a geo- Fe-rich variant of tourmaline from the Gangotri logical setting similar to ‘Iron Oxide Copper Gold granite in the Garhwal Himalaya. Tourmalines (IOCG) type’ (Mishra and Bhattacharjee 2015, 1 0123456789().,--: vol V 87 Page 2 of 18 J. Earth Syst. Sci. (2018) 127:87 Figure 1. Location and simplified geological map of the study area (after Mishra and Bhattacharjee 2017). 2017; Bhattacharjee et al. 2016; Mishra et al. for tin and tungsten in particular (Charoy 1982; 2016). Reddy (1994) reported consistent anoma- Grew and Anovitz 1996; Henry and Dutrow 1996; lous values of W (up to 100 ppm) and B (up London et al. 1996; Slack 1996; Fareeduddin et al. to 1086 ppm) from the geochemical sample in 2010). In many Archaean orogenic and Protero- the Bastipadu area. Bhattacharjee et al. (1999) zoic gold deposits, tourmaline forms an important reported the presence of B (up to 1170 ppm) and mineral constituent and economically the gold– W (up to 197 ppm) north of Chetlamallapuram. tourmaline veins form some of the world’s largest Mishra and Bhattacharjee (2015) reported for the and richest gold deposits (Slack 1996). In India, first time the presence of columbite and tantalite tourmaline is found in the Kolar gold deposit, (Col–Tan), allanite and euxenite bearing mineral Dharwar Craton (Siva Siddaiah and Rajamani phases from the Chetlamallapuram–Bastipadu sec- 1989), Honnamaradi gold mineralisation in the tor. These were recorded from the pegmatites Chitradurga greenstone belt (Mohakul and Babu that intrude the country rocks in the study area. 2001), in the outer fringes of the hydrothermal Later on, Mishra et al. (2016) reported rare metal gold mineralised zones, Majjur, Gadag schist belt, bearing leucogranite–pegmatites from the adjacent Karnataka (Sarma et al. 2004), inner and distal areas. hydrothermal alteration zones of Hira Buddini gold The leucogranite mapped in and around Basti- deposit in the Archean Hutti-Maski greenstone padu area is marked with intense occurrence of belt of the EDC (Krienitz et al. 2008; Hazarika tourmaline which is found ubiquitously from mil- et al. 2015), G.R. Halli gold deposits in the Chi- limetre to centimetre scale and at places also found tradurga greenstone belt of the Western Dharwar as quartz–tourmaline rocks within the pegmatitic Craton (Gupta et al. 2014). Tourmaline is also phases of leucogranite. The presence of tourmaline helpful in understanding the physical and chemical in the leucogranite is linked with exploration work environments where it forms and retains the chem- in hydrothermal mineral systems in general and ical signature through geologic time(London and J. Earth Syst. Sci. (2018) 127:87 Page 3 of 18 87 Manning 1995; Keller et al. 1999; Dutrow and The granitoids intrude the schist belt components. Henry 2011). The main role played behind the These are in turn intruded by pegmatites (both scene is ‘boron’, an indispensable element for the K-feldspar rich and albite rich), gabbro/dolerite formation of tourmaline (Dutrow and Henry 2011). dyke and quartz reefs along the large-scale faults. The availability of boron in the source is linked The large-scale potash metasomatism along the to the tourmaline development in the granitic fault planes resulted in the formation of K-feldspar rock such as the higher Himalayan leucogranites bearing pegmatites. These pegmatites host the (Dutrow and Henry 2011). Fe–Cu minerals in a geological setting similar to Tourmaline-rich leucogranite is exposed to a the‘IOCG’type(Mishra and Bhattacharjee 2015, considerable portion in and around the study area. 2017; Bhattacharjee et al. 2016; Mishra et al. 2016). No attempt has been made so far for the detailed The albite-rich leucopegmatite is associated with geological and geochemical study of this litho unit Nb–Ta mineralisation in the area (Mishra et al. and also to classify the associated tourmaline. In 2016). The older units are unconformably over- the present work, attempt has been made to docu- lain by the Cuddapah sediments including the ment mineralogy, chemical composition and classi- Gulcheru conglomerate and quartzite of the Cud- fication of tourmaline associated with leucogranite. dapah Supergroup. Detailed petrographic studies, mineral chemistry and geochemical characterisation of leucogranite 3. Tourmaline bearing leucogranite have also been carried out with an attempt to deduce the tectonic setting. 3.1 Field relation and host rock description The leucogranite intrudes into the Gadwal schist 2. Geological setting belt and older granitoids. It is found over large areas in Bastipadu, Chetlamallapuram, Chinnako- The study area forms a part of EDC which is a tala, Ulindakonda, Erradoddi, Laddagiri, etc. (fig- typical Archaean-granite-greenstone terrain with ure 1) and occurs as small plugs within the older sediments of Meso- to Neoproterozoic intracra- litho units. The exposures are cropped out in tonic Cuddapah Basin unconformably resting over and around Bastipadu (figure 2a) where it mostly it. It exposes rocks of Gadwal greenstone belt of occurs as small hills/mounds, isolated patches and Archaean age and younger granitoids of Archaean bouldery outcrops. The leucogranite is medium to Paleoproterozoic age along with Cuddapah sed- to coarse grained, light or milky white coloured. iments of Mesoproterozoic age (table 1 and fig- The colour changes locally to light grey and pink ure 1). The Gadwal litho are represented by quartz due to large-scale tourmalinisation and potash ± chlorite ± sericite ± actinolite schist, agglom- metasomatism (Mishra et al. 2016; Mishra and erate, metaandesite and dacite to rhyo-dacite, Bhattacharjee 2017). The constituent mineralogy metarhyolite and banded iron formation (Srini- includes quartz, albite, microcline, muscovite and vasan and Nagaraja Rao 1992). The granitoids tourmaline on millimetre to centimetre scale. The in the study area belong to tonalite–granodiorite– rock becomes locally very coarse grained showing monzogranite (TGM) suite, represented by por- the pegmatitic texture with the size of individual phyritic granite/granodiorite and the younger grain exceeding 1 cm (figure 2b). The pegmatite monzogranite–syenogranite (MS) suite, represented occurs as a sheet within the main leucogran- by biotite and tourmaline bearing leucogranite. ite, where the size of tourmaline crystal reaches up to 11 cm in length (figure 2c). Tourmalines Table 1. Stratigraphy worked out for the study area (after within leucogranite host occur as fine-to-coarse Mishra et al. 2016). disseminations, clusters/aggregations (figure 2a) and vein fillings/crude
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