ORIGINAL PAPER

Estimation of depth extent of Gangam-Peruru complex of EasternDharwar Craton (EDC) from aeromagnetic data

K. Satish Kumar & R. K. Kishore & Parveen Begum &

D. Seshu & Rama Rao Ch.

Received: 3 December 2013 /Accepted: 17 March 2014# Saudi Society for Geosciences 2014

Abstract A high-intensity aeromagnetic anomaly zone locat-ed in the region of Gangampalli-Peruru with an amplituderange of 500–1000 nT in the midst of peninsular granite-gneissic complex in a part of the Eastern Dharwar Craton(EDC) is associated with quartz monzodiorite with abundanttitanite, Zr, and REE elements. This zone is situated betweenthe eastern part of the Closepet granite (2513Ma) and westernpart of the Ramagiri schist belt (2614 Ma). Spectral analysisand modeling of the aeromagnetic anomaly field of theGangampalli-Peruru complex has been carried out to estimatethe thickness of the complex. The analysis has yielded anaverage thickness ranging from 1700–2250 m.

Keywords Aeromagnetic . REE elements . Titanite . EasternDharwar Craton (EDC) . Power spectrum and Ramagiri schistbelt

Introduction

Gangam-Peruru granodiorite complex is located in parts ofEastern Dharwar Craton (EDC), bounded between theClosepet granite in the west and Ramagiri schist belt in theeast (Fig. 1). The Gangam complex is mainly associated withthe quartz monzodiorite with abundant titanite consisting ofplagioclase, quartz, and mafic minerals like biotite and horn-blende. Geological and geochemical studies of Zachariah et al.(1996), Anand and Balakrishnan (2011) indicated the abun-dance of rare earth element (REE) associated with the titanitesof granodiorite and monzodiorite in this complex. Gromet and

Silver (1983) noticed the importance of REE abundant titanitein granitoid rocks. The presence of titanite indirectly revealsthe conditions of metamorphism and/or migmatization in aregion (Franz and Spear 1985; Tucker et al. 1987; Deer et al.1992; Giere 1992; Tiepolo et al. 2002). REE abundant titaniteis useful for dating Sm-Nd isotope system. The use of REEs inglass polishing industry, in high-performance magnets, highcatalysts, electronics, glass, ceramics, and alloys is wellknown.

In general, the analysis of aeromagnetic anomalies ofmetamorphic basement complexes reveals variations inlithological diversities of the crustal rocks both in lateraland vertical extents. The magnetic anomaly fabric overregions of granite-gneissic basement is characterized,generally, by short wavelength magnetic anomaliessuperimposed on broad long wavelength anomaly pat-terns. The amplitudes of the anomalies vary in a widerange and orient along structural features like fault/fracture systems and shears invaded by dykes, quartzreefs, etc. Zones of migmatization, emplacement ofgranites, gabbros, and granodiorite complexes give char-acteristic magnetic anomaly patterns.

In view of the abundance of REE in the titanites associatedwith host rock, aeromagnetic data was relooked over thiscomplex to understand the nature of the magnetic anomaliesand to estimate the thickness of the Gangam complex.

Geological setting of the study area

The study area forms a part of the EDC and is located onthe granite-greenstone terrain of the Peninsular gneissiccomplex. The area is bounded by 14° 15′ to 14° 30′ Nlatitude and 77° 15′ to 77° 30′ E longitude (Fig. 1b) and isconfined between the western part of the Ramgiri schistbelt (2614 Ma) and the eastern part of the Closepet

K. S. Kumar (*) : R. K. Kishore : P. Begum :D. Seshu : R. R. Ch.National Geophysical Research Institute (CSIR), Uppal Road,Hyderabad 500007, Indiae-mail: satish_marine777@yahoo.co.in

Arab J GeosciDOI 10.1007/s12517-014-1383-1

granite (2513 Ma) GSI (1993). The western margin ofthe Ramgiri schist belt, in the study area, is boarded bynorth–south trending granite-granodiorite intrusives. Thisnorth–south trending granodiorite suite shows a break inthe region between 14° 16′ and 14° 25′ N. The Gangamcomplex, with intense conspicuous east–west arcuate-shaped magnetic anomaly zone, is located within thisbreak zone. This high-intensity anomaly zone is inferredto be associated with a possible north plungingmigmatitic fold zone. This fold zone seems to havebeen dissected by NNE–SSW shear/fault zone in theeastern part and by NNW–SSE trending fault/shear inthe western part. The western boundary of the Gangamcomplex is boarded by the Closepet granite (Fig. 1b).The Gangam complex is mainly associated with thequartz monzodiorite with abundant titanite consisting

of plagioclase, quartz, and mafic minerals like biotiteand hornblende. The granodiorite shows abundantquartz, plagioclase, and biotite with accessory titaniteand zircon (Anand and Balakrishnan 2011). The com-plex is associated with massive granitic rocks withoccasional narrow shear zones. Zachariah et al. (1996)observed three phases of granitic rocks consisting ofmesocratic medium-grained quartz monzodiorite with abun-dant titanite, a coarse-grained granodiorite with megacrysts ofalkali feldspar, and a leucocratic and medium-grained granite.Zachariah et al. (1996) observed the inclusions of most prim-itive granitic rocks of quartz monzodiorite composition withsanukitoid affinity. The total REE content in the titanites in thequartz monzodiorite and granodiorite suits of Gangam com-plex show, respectively, 13,756 and 9,375 μg/g of titanite(Anand and Balakrishnan 2011).

Fig 1 a Location map of the study area in India. b Geology of the study area showing Gangampalli-Peruru complex, gap region between thegranodiorite (after GSI 1993)

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Fig 2 a Total aeromagnetic intensity anomaly map of Cuddapah basinand part of Eastern Dharwar Craton; b total aeromagnetic intensityanomaly map of Gangampalli-Peruru complex with three principle

profiles, AA, BB, and CC near Gangampalli, Peruru, and Rala Anathpurvillages, respectively. An arcuate-shaped fold zone is marked in red colorline

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Aeromagnetic anomaly map/image of the study area

Data treatment

The National Geophysical Research Institute (CSIR-NGRI)has collected total intensity aeromagnetic data over an area of35,000 km2 in parts of EDC and CB during 1980–1982 fieldseasons. The surveywas flown at a line spacing of 1 kmwith aflight elevation of 150 m along N–S direction. A tail boommounted rubidium-vapor magnetometer of 1 nT sensitivity isused in the data collection. The data has been corrected for thetransient geo-magnetic variations (diurnal) and InternationalGeomagnetic Reference Field (IGRF) of the Epoch 1981. Theaeromagnetic image is presented in Fig. 2a in which the studyarea forms a part of the image.

Appraisal of the aeromagnetic image of the study area

High-frequency, short wavelength magnetic anomalies domi-nate the western part of the Cuddapah Basin. The range ofmagnetic anomalies vary between 50 and 2000 nTrepresenting the varied nature of the lithological unitsconsisting of migmatites, granodiorites, hornblende and bio-tite gneiss, granite intrusions, and N–S trending green stonebelts that include parts of Hagari Ramgiri, Kadiri, Veligallu,and Jonnagiri schist belts in the gneissic basement complex(Fig. 2a). Babu Rao et al. (1987, 1991), Bhaskara Rao et al.(1989), Rama Rao et al. (2011), and Kishore and Rama Rao(2004) have extensively studied various aspects of the aero-magnetic anomaly patterns in relation to the crustal evolution

of this part of the EDC. The aeromagnetic anomaly image ofthe CB part is associated with a smoothly varying long wave-length anomaly field superimposed by short wavelengthanomalies due to the sills and flows that have invaded theCuddapah sediments in six episodes of igneous activity duringthe Papaghni and Tadipatri times (Nagaraja Rao et al. 1987).The south western part of the CB is associated with a high-intensity anomaly related to a 4-km thick norite-gabbro bodyfed by a series of feeder dykes below the Cuddapah sediments.The estimated thickness of the Cuddapah sediments is around6.5 km in the present part of the CB (Babu Rao 1991). Thepositive anomaly trends in the region are generally associatedwith fault/shear zones intruded, at some places, by quartz reefsand at some places by dolerite dykes in different directionslike ENE–WSW, E–W, and NE–SW and NW–SE directions.

Fig 3 Log normalized radiallyaverage spectrum of the entirestudy area. Depth is computed by1/4π*(ΔE/ΔN)

Table 1 Body parameters of the three profiles AA (near Gangampalli),BB (near Peruru), and CC (near Rala Anathpur)

Body parameters Profile AA Profile BB Profile CC

Depth 150 m 450 m 500 m

Thickness 3,650 m 1,700 m 1,390 m

Dip 49° 35° 46°

Susceptibility 0.0064 c.g.s 0.025 c.g.s 0.016 c.g.s

Position 4,460 m 2,350 m 1,430 m

Dip Direction 0° 180° 0°

Base Level 167 nT −112 nT −166 nT

Base Slope 0.173 nT/km 0.03 nT/km 0.04 nT/km

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The magnetic anomaly feature presented here (Fig. 2b; inthe present study) is a conspicuous high-intensity (500–1200 nT), arcuate-shaped, east–west trending anomaly spreadover an area of around 300 km2 located on the hornblende-biotite gneiss/migmatite gneiss. This anomaly zone is boardedon the west by the Closepet granitic suite and in the east by theRamgiri schist belt. The other part of the gneissic basementsurrounding this zone does not show such an intense anomalypattern. The eastern edge of the anomaly zone with an ampli-tude of around 800 nT is associated with the Gangam complex(2526±1 Ma). The middle part of the anomaly zone with anamplitude of 500 nT, around Peruru, is located in the penin-sular gneissic terrain, and no surface exposures are seen in thispart. The anomaly with maximum amplitude of around1000 nT in the western edge, near the Closepet granite, isassociated with an exposure of metapyroxenite/metagabbrosuite. The Chenna gneissic complex (2545 Ma) located to-wards east of the Ramagiri schist belt does not show any highamplitude magnetic anomalies unlike the Gangam complex.

Estimation of thickness of the Gangam complex

Quantitative analysis of aeromagnetic data provides detailsabout the vertical extent of the sources in addition to the depth,altitude, susceptibility, and other size and shape parametersprovided that the anomalies are isolated and well-definedunaffected by the surrounding sources. Generally, in aeromag-netic data sets connected with the metamorphic basementcomplexes, occurrence of individual isolated anomalies israre. In such a situation, the computation of 2D power spec-trum helps in estimating an ensemble average depth to differ-ent magnetic horizons/markers which have distinct change inmagnetic properties (Spector and Grant 1970; Naidu 1970).This method is most dependable and finds its usefulness whensegregation of individual anomalies becomes difficult due to

overlapping. The ensemble average depth to a distinct mag-netic marker/horizon is estimated with the following relation.

Depth=1/4π*(ΔE/ΔN).Where,ΔE/ΔN is the slope of each segment.ΔE is the log energy.ΔN is the wave number increment.Generally, the depth estimated from the straight line seg-

ment from the long wavelength component is related to theregional sources and the later segment is attributed to theresidual sources. However, in both the cases, estimated depthsare average ensemble depths to the sources. So it is preferable,in addition to the spectral depth estimates, to analyze theprincipal magnetic profiles also to estimate the exact sizeand shape parameters of the sources.

2D power spectrum of the magnetic field of the study area

Figure 3 is the log normalized radially averaged power spec-trum computed for the aeromagnetic anomaly field of the studyarea using the Geosoft Oasis Montaj (2006) software. Thecalculated depth in the region of low-frequency band is around1.7 km. This depth can be inferred to be the average depthto a definite magnetic marker horizon above which theGangam complex is located or the depth extent of theGangam complex.

Quantitative estimates of three anomalies

Three principal magnetic anomaly profiles (AA, BB, and CC)were taken to estimate the depth, half width, dip, magnetiza-tion, depth extent/thickness, etc. for the magnetic sources. Inthis study, we used the MAGMOD software of Paterson,Grant, and Watson, PGW (1982) Ltd., Canada. The software

Fig 4 a Derived depth model of AA profile near Gangampalli village, b derived depth model of BB profile near Peruru village, and c derived depthmodel of CC profile near Rala Anathpur village

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iteratively adjusts various size and shape parameters, with agiven set of initial starting parameters for a given geometricmodel. (2D, 2½ D, Ribbon, Fault/contact, Prism) and finallygives the best fit model.

In the present analysis, we have chosen three principalmagnetic profiles AA, BB, and CC (Fig. 2b) and chose 2½D model for inversion. The magnetic profile CC with anamplitude of 1000 nT is located very near to the north–southtrending Closepet granite (2513 Ma). The profile AAwith anamplitude of 800 nT is located near to the Ramagiri schist beltin the hornblende biotite gneissic complex (2614 Ma). As thelocation of the anomalies AA and CC are represented in timeby approximately 100Ma, the anomalies assume significance.Thus, it can be construed that the present study area might beunder intense metamorphic activity giving rise to magneticanomalies with an amplitude range 500–1000 nT. This meta-morphic activity might have caused the enrichment of REEelements in this region. So, based on the above facts, weanalyzed the magnetic profiles in this region to estimate apossible thickness of Gangampalli-Peruru complex. The re-sults are presented in Table 1, and observed and calculatedprofiles are shown in Fig. 4a–c.

Profile AA (Fig. 4a) is taken from the Gangampalli arealocated in the gap region of granodiorite, west of Ramagirischist belt (Fig. 1b). An amplitude of 800 nT is observed overthis zone with a dominant low. The Gangampalli area consistsof Hornblende gneiss/migmatite gneiss. The estimated depthis around 150 m with a thickness of 3650 m.

Profile BB (Fig. 4b) is taken from Peruru area in the middleof the anomaly zone and having an amplitude of 500 nT, and itis associated with Hornblende gneiss/migmatite gneiss. Thedepth of the source is 450 m below the ground and having athickness of 1700 m.

Profile CC (Fig. 4c) with an amplitude of 1000 nT is takenfrom south of Ralla-Anantapur and is located very near to theClosepet granite over a meta gabbro exposure. The depth ofthe source is around 500 m having a thickness of 1400 m. Theaverage thickness for the sources of the anomaly zone worksout to be around 2250 m.

Conclusions

The conspicuous east–west trending arcuate-shaped high-intensity anomaly zone with an amplitude range of 500–1000 nT is associated with quartz monzodiorite designatedas Gangam complex. This complex is associated with abun-dances of titanite and REE elements. Spectral analysis andmodeling of the aeromagnetic profiles of this region has beencarried out to estimate the thickness of this important highabundant REE element source region. The estimated thicknessof complex is in the range of 1700–2200 m within the granite

gneissic terrain of the EDC and is inferred to be related tohighly migmatized rock suits.

Acknowledgments The authors are thankful to Dr. Y.J. Bhaskar Rao,Acting Director NGRI for granting permission to publish this paper.

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