Metamorphic evolution of the contact aureole of the Jhirgadandi pluton, Sonbhadra district, Mahakoshal mobile belt, central India SPSingh1,∗, Anand K Srivastava 2, Gopendra Kumar3 and SBDwivedi4 1Department of Geology, Institute of Earth Science, Bundelkhand University, Jhansi 284 128, India. 2Formerly, Department of Geology, Lucknow University, 124 Kashipur House, Aryanagar, Lucknow 226 004, India. 3Formerly, Geological Survey of India, B58, Kendriya Vihar Apartment, Sector 6, Jankipuram Vistar, Lucknow 226 021, India. 4Department of Civil Engineering, Indian Institute of Technology (BHU), Varanasi 221 005, India. ∗Corresponding author. e-mail: spsinghbu@rediffmail.com The metamorphic evolution of the contact aureole around the Late Paleoproterozoic Jhirgadandi pluton in the eastern part of Parsoi Formation of Mahakoshal terrain, central India represents three distinct metamorphic zones, characterized by definite mineral assemblages. The contact-metamorphic event pro- duced the peak-metamorphic mineral assemblages Bt + Qtz + Alb + Sil ± Cd ± Grt ± Mus ± Kfs in the metapelites of inner aureole, Bt + Qtz + And + Mus + Kfs + Plag ± Cd ± Chl in middle aureole and Chl + Mus + Bt ± And + Alb + Qtz ± Ep + Mt ± tourmaline in the outer aureole. The estimated P–T conditions based on detailed geothermobarometric calculations in the thermal metamorphosed rocks are 690◦C/3.4 kbar, 580 ± 15◦C and 487 ± 30◦C in inner aureole, middle aureole and outer aureole, respec- tively. The variation in metamorphic condition suggests that the shallow crustal level emplacement of Jhirgadandi pluton is responsible for the overprinting of contact metamorphic assemblages (M2) in the low grade metapelites (regional metamorphism M1) of Mahakoshal Group. 1. Introduction et al. 2007). The occurrences of well-developed contact aureoles and their systematic petrologi- The emplacement of granitic pluton at shallow cal studies have been reported from several locali- depth forms a thermal aureole which is associated ties (Kerrick 1987; Buntebarth 1991; Symmes and with textural, chemical and mineralogical changes Ferry 1995; Kusunose et al. 2003; Brocker and in wall rock. Such mineralogical and textural trans- Franz 2000; Buri´anek and Doln´ıˇcek 2011), which formations are functions of a distance from the con- lead to better understanding the processes with tact with the pluton, chemical composition of the respect to space and time. Although few studies country rock, temperature of the intruding magma, on thermal aureoles in Paleozoic granitoids of the composition of coexisting fluid phases and perme- Himalayan fold belt, particularly from the Tethys ability of the wall rock (Norton and Knight 1977; zone (Thakur 1993; Pognante et al. 1990; Steck Hanson 1995; Cartwright and Buick 1996; Brocker et al. 1998;Vijanet al. 2003) and from the Lesser and Franz 2000; Kusunose et al. 2003; Rossetti Himalaya of Kumaun region (Das 1985) have been Keywords. Contact aureole; geothermobarometry; hornfels texture; Jhirgadandi pluton; superimposed metamorphism. J. Earth Syst. Sci. 122, No. 3, June 2013, pp. 743–757 c Indian Academy of Sciences 743 744 S P Singh et al. carried out, contact aureoles from the peninsular Mahakoshal Group constitutes volcanosedimentary part of Indian subcontinents are relatively least rock sequences of Paleoproterozoic and developed studied. A number of granitic intrusions associ- between the Bundelkhand Craton in the north ated with contact metamorphism have also been and Chhotanagpur Gneissic Complex of peninsu- reported from the Mahakoshal belt of central India lar Indian shield in the south. According to Nair (Srivastava 1969; Nair et al. 1995;Royet al. 2002; et al. (1995) and Srivastava et al. (2000a), the Kumar 2005) however, none of the works brought Mahakoshal Group comprises of Chitrangi, Agori details on thermal aureoles and systematic docu- and Parsoi Formations in ascending order. Subse- mentation of their geothermobarometry. Through quently, Kumar (2005) separated out Parsoi For- detailed petrographic, mineral chemistry and P–T mation from the Mahakoshal Group on the basis conditions, we try to fill this gap by providing of record of polymictic conglomerate (Roy and new constraints on the metamorphic evolution of Devarajan 2000) at its base and considered the the thermal aureole of Jhirgadandi pluton in the Chitrangi and Agori Formations to be part of the eastern part of the Mahakoshal Belt. Mahakoshal Group of Archaean age. The north- ern boundary of Mahakoshal belt has sheared rela- tionship with the granite-gneisses of Sidhi block 2. Geological setting and as faulted contact and at some places angular unconformable contact with the Vindhyan Super- The arc-shaped supracrustal belt constituting the group (figure 1). The southern part of Mahakoshal Mahakoshal Group (Nair et al. 1995), earlier known Group also has a sheared relationship with granite as Bijawar Group (Mathur and Narain 1981; gneisses of Chhotanagpur block. The Parsoi For- Iqballudin and Mohgni 1981), is developed along mation comprises low grade pelitic metasediments, the southern margin of Bundelkhand Craton. bluish grey or green in colour with silky lusture The ‘Mahakoshal Group’ represents a rift basin with banded or ribbon-like structures, and some- developed during the Paleoproterozoic in central times with varve-like alternating light and dark India (Srivastava et al. 2000a, b;Roy2008). The gray bands. The arenaceous sediments occur as belt is about 500 km in length and extends from lenticles and laminae in the main phyllitic mass northeast corner of Jabalpur district of Madhya (Srivastava 1969). Pradesh along the Son Valley to the border of The present study area forms a part of the Palamau, in Jharkhand (figure 1). This belt forms eastern extremity of the Parsoi Formation of the northern part of the Central Indian Tectonic Mahakoshal belt. The Jhirgadandi pluton is Zone (CITZ) (Radhakrishna 1989; Acharyya and lensoidal in shape, trending in the E–W direction Roy 2000) and constitutes a part of Son Narmada with an area extension of ca 20 km2. It is exposed (SONATA) zone (Ravi Shanker 1993). The near the confluence of Kanhar River with Son River Figure 1. Regional geological map showing the arch shaped Mahakoshal Group bounded between Son–Narmada south fault (SNFS) and Son–Narmada north fault (SNNF). The abbreviations are: CITZ=Central Indian Tectonic Zone, CIS=Central Indian Suture (after Roy et al. 2002). Metamorphic evolution of the contact aureole of the Jhirgadandi pluton 745 (figure 2a). The thermal aureole has been studied The granitoid of Jhirgadandi is interpreted to be in the southern part only whereas its northern part hybrid type post-orogenic granitoid, highly metalu- is faulted by north Son–Narmada Fault (Srivastava minous, transitional between sub-alkaline and calc- 1969). The Jhirgadandi aureole has average uni- alkaline with low initial Sr ratios (Nair et al. 1995; form width of 600 m and the main part of aureole Srivastava et al. 2000a;Roy2008; Srivastava and is situated between the villages Parach and Nigai Kumar 2010). Based on the model composition, it where it has maximum width ca 800 m (figure 2b). is suggested to be composite in character, having (a) (b) Figure 2. (a) Geological map of the eastern part of Parsoi Formation of Mahakoshal Group showing series of granitoid intrusives, marked by numbers. 1=Jhirgadandi, 2=Baghisoti, 3=Tumia, 4=Rohiniyan, 5=Kumba Khurd, 6=Harnakachar (Kon road) and 7=Harnakachar. (b) Geological map around Jhirgadandi showing the three zones of contact aureole at the southern part of Jhirgadandi pluton (modified after Srivastava 1977). 746 S P Singh et al. a granodioritic core fringed by quartz-monzonite, the inner aureole and occasionally preserved in the with gradational contact (Srivastava 1977). The middle and frequently observed in outer aureole. less precise Rb–Sr whole rock radiometric ages (1869 ± 180 and 1813 ± 65 Ma, Pande et al. 1996;Sarkaret al. 1998) and 1753 ± 9 U–Pb zir- 3.1 Country rocks outside the contact aureole con SHRIMP geochronological data (Sita Bora and In general, the Parsoi Formation is characterized Kumar 2012) indicate a predominant Late Paleo- by low-grade tuffaceous phyllite and metasedi- proterozoic intrusion event for the Jhirgadandi ments of green-schist facies, occupying the large area pluton. between Jhirgadandi to Renukoot of Son valley (Nair et al. 1995;Royet al. 2002). Metasediments contain white mica (sericite), chlorite, quartz, 3. Architecture of the Jhirgadandi aureole albite and sometimes graphite. A metamorphic mineral association of metavolcanics consists The thermal aureole of the Jhirgadandi pluton is chlorite + actinolite + albite + epidote ± quartz ± about 600 m wide (figure 3a). The metamorphic white mica ± prehnite ± calcite ± biotite ± titan- rocks in the aureole are mainly metapelites with ite ± ilmenite ± magnetite (Nair et al. 1995). layers of metapsammites of several mm to cm thick- Some times the chlorite porphyroblasts (up to ness. Based on the textural and mineralogical asso- 0.9 mm across) are arranged parallel or subparallel ciations the aureole has been divided into three to the axial planes of crenulation folds devel- parts, viz., inner, middle and outer aureoles/zones. oped which is defined by the mineral assemblage Relics of sedimentary textures are nearly absent in actinolite+epidote+feldspar+quartz+chlorite. (a) (b) (c) (d) Figure 3. (a) A view of inner and middle aureoles developed near Jhirgadandi village. The granite is light colour while the dark shade represents