Petrophysical properties of the Himalayan granitoids: Implication oncomposition and source
Ruchika Sharma, Vikram Gupta ⁎, Baldev R. Arora, Kaushik SenWadia Institute of Himalayan Geology, 33 Genaral Mahadeo Singh Road, Dehra Dun, 248 001, India
Article history:Received 9 March 2010Received in revised form 20 August 2010Accepted 21 October 2010Available online 31 October 2010
Keywords:Petrophysical propertiesHimalayan granitoidsSeismic wave velocityMagnetic susceptibilityHornblende–biotite series
The paper reports the characterization of density, magnetic susceptibility, magnetic anisotropy, seismic wavevelocities, attenuation as well as mineralogy and major element chemistry of the four generation of granitoidsfrom the Indian Himalaya. Based on these petrophysical properties, only the Cretaceous granitoids of the Trans-Himalayan region by virtue of their mantle affinity and domination of magnetite and/or magnetite–ilmeniteseries qualify to be the I-type granitoid. On the other hand, rest of the 3 suites of granitoids have a crustal affinityand can be categorized as S-type granitoids enriched with ilmenite and/or hemo-ilmenite series. Beside thisgeneral classification, some anomalous petrophysical properties can be related to distinctive mineralogy, stagesofmagmatic crystallization, and intensity of deformation indifferent class of granitoids. For example; (i) presenceof heavy minerals like hornblende and magnetite accounts for the significantly high density and seismic wavevelocity of the Cretaceous granitoids; (ii) fractional crystallization of mantle melts leads to hornblende-richgranitoids (rich inmagnetite) in the earlier stage where biotite-rich granitoids (lowmagnetite) crystallize in thelater stage, thus explainingbimodal distribution ofmagnetic susceptibility inCretaceousgranitoids; (iii) in S-typegranitoids, high quartz content (45%) account for the lowest density recorded in Saruna Proterozoic granitoidswhereas high content of micaceous minerals reduce the seismic wave and are responsible for the lowest S-wavevelocity in the Early Palaeozoic Mandi granitoids; (iv) further, the effect of texture is seen as varying attenuationcharacter of P- and S-waves on grain size. In general, the higher the grain size, the greater the attenuation. Onceagain Cretaceous granitoids negate this well established relation. Incorporation of this anomalous dependence ofphysical properties onmineralogical, tectonic fracturing, texture will help the translation of geophysical maps tomore a realistic region specific crustal tectonic evolution models.
Geophysical imaging by active and passive seismology (e.g.Fountain et al., 1992; Christensen and Mooney, 1995) as well asinterpretation of satellite mapped long wavelength magnetic anoma-lies (e.g. Langel and Hinze, 1998) contain rich information on thenature and composition of continental middle and lower crust.However, translation of deduced seismic velocity/magnetizationdistribution to litho compatible composition model requires a-prioriknowledge of the physical properties of rocks at different depths. Thelaboratory measurements of physical properties on samples fromigneous and high-grade metamorphic rocks (Ishihara and Sasaki,1989; Burke and Fountain, 1990; Zappone et al., 2000; Gregorovaet al., 2003; Kitamura et al., 2003; Aydin et al., 2007) or xenoliths(Parsons et al., 1995; Jackson and Arculus, 1984; Berg et al., 2005;Nishimoto et al., 2005) form primary source on the petrophysical
characterization of deep crustal rocks. These studies have shown thatintrinsic properties of a rock sample like density, mineralogy,microfractures and porosity greatly control seismic wave propagationcharacteristics and allied physical properties.
Beside the exhumed deep crustal sections or xenoliths, the activeorogenic belts, like Himalaya, provide a unique setting to study deepcrustal and mantle derived lithologies (granitoids) as they have beenbrought to the surface by tectonic uplift or emplaced at shallowdepths by underplating, obduction, etc. The Tibet Plateau has longbeen a hot-spot for geophysical investigations (Nelson et al., 1996;Kind et al., 2002) and in recent years the investigations haveexpanded to cover entire collision zone including the tectonicallyactive Himalaya (Schulte-Pelkum et al., 2005; Rai et al., 2006; Aroraet al., 2007). However, very limited literature is available on thepetrophysical properties of different suites of lithologies within thismountain chain. Gogte and Ramana (1982) studied the elasticproperties and petrological characteristics of few granitoid bodiesrelated to major tectonic zones of the Himalaya and concluded thatthe Trans-Himalayan granitoids exhibit higher density and differentseismic wave characteristics than the granitoids from the Lesser andCentral Himalayas.
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The Himalayan terrain hosts four generations of granitoids ofvarying sources (Sharma, 1983). Thus establishing relationship ofpetrophysical parameters with petrological and mineralogical com-position for each category of granitoids based on their varying origin,age could be critical in constraining the crustal evolution model forthe mega collision zone. The objective of this study is to quantify theseismic and other physical properties of different suites of theHimalayan granitoids to provide an experimental database toconstrain and guide the transformation of geophysical anomalies tocrustal models consistent with petrology and chemical composition.
2. Brief overview of the Himalayan granitoids
The Himalaya contains numerous granitoids bodies which arewidely distributed in space and time (Fig. 1). Based on their tectonicplacement and radiometric ages, these granitoids are broadly classifiedinto four categories (Sharma, 1983). These are (i) Proterozoic granitoids(2200–1800 Ma and 1400–1200 Ma), (ii) Early Palaeozoic granitoids(600–500 Ma), (iii) Cretaceous granitoids (99–105 Ma) includingyounger phase of magmatism (42–30 Ma), and (iv) Tertiary granitoids(30–12 Ma). There have been various studies to understand theevolution processes of each Himalayan granitoid bodies. Islam et al.(2005) synthesized the characteristic tectonomagmatic history of eachcategory of granitoids. The Si and K2O enriched Proterozoic granitoidswith presence of normative corundum were assumed to derived asbasement slivers frommiddle crustal level and were originally granitesduring Proterozoic but they have been deformed during Himalayanorogeny in granitic gneiss while the production of Early Palaeozoicgranitoids is proposed either due to the Pan African orogeny or by themantle involvement. The Cretaceous granitoids with the presence ofcharacteristics trace element (positive Sr anomaly) indicates theirgeneration and evolution in subduction related magmatism. The laterevent of dehydration and incipient melting of the subducted oceaniccrust serves as a secondary source material for granitoid. The proposedtheory of the source of Tertiary leucogranites is the liberation of largequantity of fluids generated due to the inverted metamorphism of theunderthrusting Indian slab along the Main Central Thrust (MCT) atcontinental scale.
Fig. 1. The spatial distribution of granitoids of the Proterozoic, Early Palaeozoic, Cretaceous astudy are also indicated. Inset shows the location of Himalayan arc on Asia map.
3. Sampling and methodology
Twenty cylindrical cores of Proterozoic granitoids from Ghuttu,Bhatwari, Junana and Saruna, five cores of Early Palaeozoic granitoidsfrom Mandi, sixteen cores of Cretaceous granitoids from Ladakh andfive cores of Tertiary granitoids from Gangotri were taken for thepresent study. The locations of sampling sites are marked in Fig. 1. Thelaboratory measurements, especially of seismic velocity and aniso-tropy, are sensitive to the density and geometry of pores andmicrocracks (Faccenda et al., 2007). Therefore, extraction of intrinsicvalues of elastic properties, representative of composition, requiremeasurements above the confining pressure (~200 MPa) at whichpores and microcracks close irreversibly (Ji et al., 2007). In theabsence of high pressure laboratory facilities at the up-coming rockphysics laboratory at the host Institute, the velocity measurements inthe present study have been carried at ambient pressure conditions. Inorder tominimize the biases thatmay result due to the likely presenceof pores and microcracks, numbers of precautionary steps wereundertaken. In the first instance rock samples for laboratorymeasurements were cut from the section of the core free from surfaceweathering. Next we measured the porosities of representativesamples from different locations. The measured porosities except insamples from Saruna were less than 1% indicating that indexproperties of the studied granitoids are not likely to be affectedstrongly by cracks and fracture matrix. Further, detailed petrographicexamination in conjunction with magnetic fabric measurements areused to estimate degree of deformation different generations ofgranitoids have witnessed. Finally, only those measured values thatcompare well with the worldwide accepted values or are validated bygenetically related factors were considered for tectonophysicalinterpretation. In this background, the cylindrical cores of 2.54 cmin diameter and with length-diameter ratio of 1:1 were used fordifferent measurements. Both end surfaces of the cores were madeparallel and were polished sufficiently smooth to provide goodcoupling.
The density (ρ) data were obtained from themeasurement of mass(m) and bulk volume (ν) of each oven-dried test samples using theformula; (ρ)=(m)/(ν). The seismic wave velocity measurementswere done using ‘ultrasonic pulse transmission technique’ as first
nd Tertiary age in the Himalaya. Sampling locations of the granitoids used in the present
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detailed in Birch (1960). The travel time (tp and ts) and the pulsewidth (Δtp and Δts) of the first received pulse were calculated usingthe time cursor of the oscilloscope as described in Rao et al. (2006).The velocities were calculated from the core length and the traveltime using the formula,
Velocity vð Þ = Length of the test sampleTravel time
Theattenuation (α) of P- andS-waveshasalso been computedusing:
α =8:686 πf
VQ
where
f is frequency of pulser which is 1 MHz in the present study,V is velocity in cm/s andQ is Quality Factor (dimensionless quantity) obtained by the
following formula.
Q =L
VΔt
where
L is length (cm) of the core sample,V is velocity in cm/s andΔt is the change in pulse width in microseconds.
Magnetic susceptibility was measured by Bartington Susceptibilitymeter using 0.47 Hz frequency for six orthogonal directions of eachcylindrical core sample. In addition to magnetic susceptibility, theAnisotropy of Magnetic Susceptibility (AMS) was measured usingKLY-3S Kappabridge (Agico, Czech Republic). The ‘SUSAR’ softwaresupplied with KLY-3S Kappabridge was used to obtain the magneticfabric parameters; namelymeanmagnetic susceptibility (Km), degreeof magnetic anisotropy (P′) and the magnetic lineation (L).
For geochemical study, one representative core sample of differentgranitoids was grounded to−200-mesh powder by use of a tungsten-carbide mill and was dried at 110 °C. The powder was then pressedusing few drops of polyvinyl alcohol into pellets and analyzed formajor oxides by wavelength dispersive X-ray fluorescence spectrom-etry (Siemens SRS 3000). The precision and accuracy of thepreparation and instrumental performances were checked usinginternational reference samples, GA, GH, GSN, MA-N (CRPG, France),G-2, GSP-1, RGM-1, AGV-1 (USGS, USA) and JG-2, JG1-a and JA-2 (GSJ,Japan). The accuracy of measurements is b5%. Precision in terms ofmaximum observed relative standard deviation on repeated mea-surements is normally b2%.
The semi quantitative analysis of mineral phases of the rocks wasstudied by means of X-Pert PRO X-ray diffraction system. Quantifi-cation of present major phases was done by using tools provided inthe Xpert high score plus software by RIR method.
4. Petrographical and mineralogical characteristics
The main petrographic features of all the granitoids were studiedin thin sections under plane polarized light and crossed nicols.According to their tectonic setting each has its own petrographiccharacteristics, which are described in the next discussion.
4.1. Proterozoic granitoids (from the Higher Himalaya)
These are fine to medium grained, inequigranular and consistmainly of quartz and feldspar along with mica, chlorite, apatite andopaque as accessory minerals. All these granitoids shows intense
deformation manifested by feldspar mineral that shows kinking anddeformation twins (Fig. 2a). Quartz is stretched to form ‘ribbons’(Fig. 2b). Saruna granitic gneiss are quartz-rich, highly cracked whichare filled with secondary minerals (Fig. 2c) and contain both biotiteand muscovite in equal proportion.
4.2. Early Palaeozoic granitoids (from the Lesser Himalaya,Mandi — granite)
These are exceptionally coarse grained granite composed ofquartz, K-feldspar, plagioclase, biotite and muscovite. The amount ofbiotite is more than muscovite. They show deformation features likewarping of mica folia around feldspar porphyroclasts (Fig. 2d) anddynamic recrystallization of feldspar and quartz (Fig. 2e) in someplaces, indicating their slightly deformed nature.
4.3. Cretaceous granitoids (from the Trans-Himalaya — Ladakh)
These are coarse grained and essentially contain quartz, plagioclase,K-feldspar, biotite and hornblende. These are undeformed in nature andexhibit hypidiomorphic texture. These have been differentiated intobiotite-rich and hornblende-rich granitoids. The percentage of opaqueminerals (magnetite) in the biotite-rich granitoids is less than thehornblende rich granitoids. (Fig. 2f and g) (Table 1).
4.4. Tertiary leucogranite (from the Higher Himalaya)
These are equigranular,fine tomediumgrained andmainly composedof quartz, feldspar, biotite and muscovite with tourmaline as accessorymineral (Fig. 2h). Plagioclase is more abundant than K-feldspar.
The results of X-ray diffractometry (XRD) on the whole rockpowder are presented in Table 1. It has been noted that Ghuttugranitoids (Proterozoic age) comprises of 19% quartz, 17% K-feldsparand 64% muscovite with no biotite, whereas Bhatwari granitoidcomprises of about 18% quartz, 40% plagioclase, 37% K-feldspar andabout 5% biotite. Junana granitoid comprises of 23% quartz, 17%plagioclase, 36% K-feldspar and 25% fine grained variety of mica(sericite). The Saruna granitoid contains highest proportion of quartz(~45%) followed by feldspar (26%) and biotite and muscovite in equalproportion (~14%). The Mandi granitoid (Early Palaeozoic age)comprises of about 13% quartz, 18% plagioclase, 23% K-feldspar, 21%muscovite and about 25% biotite. Ladakh granitoids (Cretaceous age)can be differentiated into biotite-rich and hornblende rich varieties.The former contains about 18–22% quartz, 37–42% plagioclase, 25–29% K-feldspar, and 10–15% biotite, whereas later contains about 12–14% quartz, 44–49% plagioclase, 23–28% K-feldspar, 3–4% biotite and9–10% hornblende. The Gangotri granitoids (Tertiary age) contains17% quartz, 13% plagioclase, 43% K-feldspar and about 27% micaceousminerals.
The XRF data (Table 2) reveals that the Cretaceous granitoids arethe least fractionated of all with least SiO2 content and highest Fe–Mgcontent. The Tertiary granitoids are the most fractionated withhighest SiO2 content and low Fe–Mg content. The Proterozoicgranitoids are more fractionated than the Palaeozoic Mandi granite.
5. Petrophysical properties characteristics
The results of the various laboratory measurements of petrophys-ical properties on different category of granitoids have been presentedin Table 3 and are discussed in the succeeding sections.
5.1. Density characteristics
The comparison of density for different types of granitoids indicatesthat the Cretaceous granitoids have the highest density ranging from2.60 g/cc to 2.66 g/cc for biotite-rich granitoids and 2.79 g/cc to 3.04 g/cc
Fig. 2. Photomicrographs of different suites of the Himalayan granitoids (a) Ghuttu granitoid (Proterozoic) exhibiting kinking and deformation twins in the feldspar grain(b) Bhatwari granitoid (Proterozoic) exhibiting ribbon structure (c) Saruna granitoid (Proterozoic) exhibiting number of cracks filled with secondary minerals (d) Mandi (EarlyPalaeozoic) granite exhibiting warping of mica minerals around the feldspar grains (e) Mandi (Early Palaeozoic) granite exhibiting recrystallisation of quartz and feldspar (f) Ladakh(Cretaceous) granitoid exhibiting biotite and magnetite (g) Ladakh (Cretaceous) granitoid exhibiting hornblende, biotite and magnetite (h) Gangotri (Tertiary) granitoid exhibitingtourmaline and biotite.
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for hornblende-rich granitoids with an average value of 2.62 and 2.93respectively (Fig. 3a). This is in contrast to Proterozoic, Early Palaeozoicand Tertiary granitoids having an average density of 2.54 g/cc, 2.58 g/ccand 2.57 g/cc respectively (Fig. 3a).
5.2. Velocity characteristics
Velocity data through different Himalayan granitoids indicate thatthe biotite-rich Cretaceous granitoids exhibit the highest Vp with anaverage value of 5850±267 m/s whereas hornblende rich Cretaceousgranitoids exhibit Vpof 5252±267 m/s. (Fig. 3b)Among theProterozoicgranitoids, Saruna granitoids exhibit the lowest Vp (3185±313 m/s),whereas Ghuttu, Bhatwari and Januna granitoids exhibit an average Vpof 4918±308 m/s, 5400±128 m/s, 5335±98 m/s respectively(Fig. 3b). The Early Palaeozoic and the Tertiary granitoids exhibit anaverage Vp of 4425±202 m/s and 4652±29 m/s respectively (Fig. 3b).
It has been observed that all the Himalayan granitoids exhibit widevariability in Vs. Ghuttu, Bhatwari, Junana and Saruna granitoids ofProterozoic age exhibit an averageVsof 2581±334 m/s, 2751±269m/s,2895±10 m/s, 2190±76 m/s respectively. Early Palaeozoic granitoidsexhibit an average Vs of 2040±339m/s. Biotite- and Hornblende-richCretaceous granitoids exhibit an average Vs of 2260±358 m/s, 2748±407m/s respectively, whereas the Tertiary granitoids exhibit an averageVs of 2657±73m/s (Fig. 3b).
5.3. Attenuation characteristics
Fig. 3c plots the attenuation of P- and S-waves for all categories ofHimalayan granitoids. Among the Proterozoic granitoids, Ghuttu,Bhatwari, Junana and Saruna region exhibit an average P- and S-waveattenuation characteristics of 0.92 db/cm and 8.80 db/cm; 0.52 db/cmand 8.91 db/cm; 0.74 db/cm and 7.96 db/cm and 1.23 db/cm and10.91 db/cm, respectively. Early Palaeozoic granitoids exhibit thehighest attenuation characteristics with 2.00 db/cm for P-wave and12.22 db/cm for S-wave, whereas Cretaceous granitoids exhibit thelowest attenuation characteristics with biotite- and hornblende-richgranitoids exhibiting 0.24 db/cm and 0.13 db/cm for P-waves and0.80 db/cm and 0.86 db/cm for S-waves. Tertiary granitoids exhibitingan average value of 1.11 db/cm and 10.83 db/cm for P- and S-waveattenuation characteristics.
Table 2XRF data of the Proterozoic, Early Palaeozoic, Cretaceous and Tertiary granitoids from the H
Fig. 3d depicts the magnetic susceptibility of all the Himalayangranitoids. It has been noted that magnetic susceptibility of all theHimalayan granitoids display bi-modal distribution with Ghuttu,Bhatwari, Junana and Saruna granitoids of the Proterozoic age, Mandigranitoids of Early Palaeozoic age and Gangotri granitoids of theTertiary age exhibiting magnetic susceptibility of the order of 10−5
[SI], whereas Ladakh granitoids of the Cretaceous age exhibitingmagnetic susceptibility of the order of 10−2–10−3 [SI]. The formercategory falls within the domain of the paramagnetic granitoids,whereas, the later are ferromagnetic granitoids. It has further beennoted that biotite-rich Ladakh granitoids display a lower magneticsusceptibility in comparison to the hornblende-rich granitoids.
5.5. Magnetic anisotropy and deformation
Noting strong dependence of seismic wave propagation andmagnetic anisotropy on the lattice preferred orientation (LPO) ofminerals, the use of these intrinsic properties have emerged as apotential tool to investigate the petrofabric of natural rocks with anoverall objective to estimate intensity of deformation (Tarling andHrouda, 1993; Brosch et al., 2000; Bascou et al., 2001; Punturo et al.,2005; Feinberg et al., 2006). However, more recent work of Ji et al.(2007) show thatmeasurements of seismic wave velocities and seismicanisotropy at low pressure (b100 MPa) are sensitive to the degree andalignment of microcracks patterns. Therefore, their applications as apetrofabric indicator require characterization of theseparameters aboveconfining pressures at which pores and microcracks close (N200 MPa).In contrast, the AMS measurements on surface samples have provedvalid tool for fabric analysis, especially for paramagnetic granites (Kmb500 μSI) whose fabric is controlled by the minerals like biotite,muscovite, hornblende and chlorite (Bouchez, 1997; Mamtani andGreiling, 2005; Sen and Mamtani, 2006). Recognising this merit, in aseparate follow up study, Sen et al. (2010) have established the AMSfabric parameters for granitoids used in the present analysis. The studywas confined to Proterozoic and Palaeozoic granitoids as their magneticproperties are determined by paramagnetic minerals and only thesegranites show varying degree of deformation. In agreement withpetrographic evidences, the degree of magnetic anisotropy (P′)
Table 3Laboratory measured petrophysical properties; viz density, Vp, Vs and, magnetic susceptibility and seismic attenuation in the Proterozoic, Early Palaeozoic, Cretaceous and Tertiarygranitoids from the Himalaya.
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increases with intensity of deformation. The mean value of P′ isrelatively higher for Proterozoic granitoids (1.1342±0.0402) in respectto less deformed Palaeozoic granitoids (1.07±0.0415). Thecorrespondingmean values ofmagnetic lineation (L) for the two classesof granitoids are 1.074±0.0237 and 1.011±0.005.
5.6. Mineral constituents–velocity inter-relations
The quartz and feldspar are the major rock forming minerals in thegranitoids. It has been noted in Fig. 4 that with the increase in feldsparconstituents in rocks, Vp increases, whereas the presence of quartz andmica decrease the compression wave velocity. However no relation hasbeen observed for Vs with the major rock forming minerals.
5.7. Density–velocity inter-relations
Fig. 5 exhibits the inter-relationship of density–velocity fordifferent categories of granitoids of the Himalaya. It has been notedthat both Vp and Vs increase with the increase in density for all types
of granitoids except for the Cretaceous granitoids where P-wavevelocity decreases with the increase of density.
5.8. AMS–velocity inter-relations
Given the success of AMS as a marker of deformation, relation ofmagnetic anisotropy parameters (Km, P′ and L) with Vp and Vs areexamined to infer any possible control of the magnetic fabric andmineralogy on seismic propagation in Himalayan granitoids. Fig. 6shows the plot of Vp and Vs with different magnetic parameters (km,P′and L) and strength of relationship between various pair ofparameters are quantified by correlation coefficient (R). The Vpshows a strong positive correlation with degree of magneticanisotropy (P′) as well as with magnetic lineation (L) — (Fig. 6c ande respectively). Marginally diffused correlation is seen for Vs (Fig. 6dand f). The former relationship implies that Vp is influenced by thealignment of paramagnetic minerals and their magnetic fabricwhereas propagation of seismic waves is controlled by the intensityof deformation these granitoids have suffered as velocities increasewith P′. Further, both Vp and Vs are controlled by paramagnetic
Fig. 4. Plot showing linear relationshipof Vpwith feldspar (solid square)andquarts+micacontent (open square), the major rock forming minerals in the Himalayan granitoids. R isthe correlation coefficient.
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mineralogy is manifested by the decrease of Vp and Vswith increasingKm (Fig. 6a and b).
6. Discussions
The detailed characterisation of petrophysical, petrographic andcompositional properties of four generations of granitoids from theHimalaya has helped to arrive at unified picture of the likely sourcesand origin of granitoids. The control of compositional constituents indetermining the petrophysical properties is demonstrated by theincrease of Vp with increasing content of feldspar and decreasingtrend in Vp with increasing fraction of quartz and mica (Fig. 4).Consistent with the Birch law, various categories of Himalayangranitoids dominated by silicate and oxide mineralogy (Table 1)show increase in seismic velocities, both Vp and Vs, with density(Fig. 5). Only notable exception being the Cretaceous granitoidswhere Vp decreases with the increase of density (Fig. 5).
Once again in agreement with the worldwide occurrences ofgranitoids, the magnetic characterization of Himalayan granitoidsexhibit bi-model distribution and can be used to classify Himalayangranitoids into two main types: I-type and S-type. I-type granitoidsrelated to subduction are enriched in magnetite, hornblende–biotitewith susceptibility values in the high range of 10−3–10−2 [SI](Barbarib, 1999). On the other hand, S-type granitoids formed bymelting of middle crust correspond to 2micas with very small amountof magnetite (Pitcher, 1983). S-type granitoids show magneticsusceptibility values in lower range of 10−5–10−4 [SI]. In this globalclassification, only the Cretaceous granitoids in the Ladakh Batholithqualify to be I-type granitoids and are characterized by high density,seismic wave velocities and magnetic susceptibility. On the otherhand, Proterozoic, Palaeozoic and Tertiary suites irrespective of theirprobable differences in evolution history are dominated by biotite,muscovite as primary magnetic carrier to be clubbed as S-typegranitoids. Given that magnetic properties of I-type and S-typegranitoids are respectively controlled by ferromagnetic and paramag-netic mineralogy, they are referred to as ferromagnetic and paramag-netic granitoids as well.
Fig. 3. Dot plot showing sample values for a range of petrophysical properties of theHimalayan granitoids (a) density; (b) Vp (green square) and Vs (red square); (c) seismicattenuation in P- (green square) and S- (red square) waves (d) magnetic susceptibility.The horizontal bars and number on side indicate locality averages. The different localitiescovered correspond to four generations of granitoids of Proterozoic, Early Palaeozoic,Cretaceous and Tertiary ages.
Fig. 5. Plot showing linear dependence of Vp (solid square) and Vs (open square) on the density of the Proterozoic, Early Palaeozoic, Cretaceous and TertiaryHimalayan granitoids. R is thecorrelation coefficient.
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Use of AMS as a marker of deformation is validated by petrographicfeatures. Degree of magnetic anisotropy indicated that Proterozoicgranitoids have suffered relatively higher deformation (~13%) incomparison to Palaeozoic granitoids (~7%). Given this success of AMSparameters in petrofabric analysis, relation of magnetic anisotropyparameters (Km, P′ and L) with Vp and Vs permits to infer thatpropagation of seismic waves is controlled by the intensity ofdeformation as well as by the preferred orientation of magneticminerals. These velocity-AMS relations are developed better for Vpthan Vs though dependence of seismic wave velocity on Km is strongerfor Vs. Although basic cause of this varying relationship of Vp and Vs toAMS parameter is not understood, it can be surmised tentatively thatwhileVp is sensitive to intensity of deformation, preferredorientationofminerals as well as the mineralogy, the Vs seems to be primarilycontrolled by the mineralogy alone. In the paramagnetic granitoids, theincrease in magnetic susceptibility means increase in paramagneticminerals (e.g. muscovite, biotite etc.) and decrease in silica content. Inthis scenario, the observed negative dependence of Vs on Kmmay onlybe proxy to the progressive reduction of SiO2 content with increasingmagnetic susceptibility. The balancing role of magnetic and silicatemineralogy in controlling the pseudo velocity-susceptibility inter-linkage is evident from Fig. 7b. In this plot for Cretaceous granitoid,one notes Vs increases with increase in Km rather than negativedependence seen in respect of Proterozoic and Palaeozoic granitoids(Fig. 6a and b). Perhaps, the cumulative effect of increasing magnetitewith Km in this type of ferromagnetic granitoid is more than offset bydecreasing silica content to account for the varying relation between Vsand Km in two types of granitoids. The presence of minor mineralconstituent influences significantly thebulkpropertyof the rocks is seenin Fig. 7a. Here the increasing content of magnetite with increasing Km,although in small fraction, produces substantial increase in density byvirtue of its extremely high density.
From previous discussion, it can be surmised that on first approxi-mationpetrophysical properties of granitoids canbeagoodproxyof theirmineralogy and composition. Beside this general characterisation, someanomalous petrophysical properties can be related to distinctivemineralogy, stages of magmatic crystallization, texture, and intensity of
deformation in different class of granitoids. For example, in the presentstudy, Ladakh Pluton of Cretaceous age itself displays bimodal distribu-tion in magnetic susceptibility (Fig. 3d). One group has higher magneticsusceptibilityof theorderof 10−2 [SI],whereas theothergrouphas lowermagnetic susceptibility of the order of 10−3 [SI]. The former group hasbeen classified as hornblende-rich Cretaceous granitoids, while the lateris biotite-richCretaceous granitoids. This bimodal distributionmaybe theresult of fractional crystallization where hornblende-rich granitoidscrystallize in the earlier stage and biotite-rich granitoids crystallize in thelater stage ofmagmatic differentiation.During the later stage of fractionalcrystallization, depletion of FeO in magmatic melt produce unfavorableoxidizing condition and hence result in magnetite depleted granitoidwith lower magnetic susceptibility.
Among the Cretaceous granitoids, it has further been noted thathornblende-rich granitoids have the higher density than the biotite-richgranitoids (Fig. 3a). The high density of hornblende-rich granitoids canbe ascribed to the presence of heavy minerals like hornblende andmagnetite. The single crystal of hornblende and magnetite respectivelyhave density of 3.15 g/cc and 5.08 g/cc, which is much higher than thedensity of single crystal of quartz (2.65 g/cc), feldspar (2.67 g/cc) andmica (2.79 g/cc), the most abundant minerals found in the granitoidrocks. Using the modal composition data of hornblende-rich Ladakhgranitoids from Table 1 and adopting mineral densities of quartz,feldspars, mica, hornblende and magnetite from Ji et al. (2002), thelargest density calculated is 2.8 g/cc. Sampleswith densities higher thanthis in Table 1 (LH1, LH2, LH3, LH4, LD2, LD3, and LD4) should containmore magnetite and/or other heavy minerals. The high magnetitecontent is also supported by the significant positive correlation betweendensity andmagnetic susceptibility (Fig. 7a). Therefore, the presence ofheavyminerals, even in a small quantity, greatly increases the density ofthe Cretaceous granitoids.
Among the four suites of granitoids studied, Saruna granitoids ofProterozoic age exhibit the lowest density (Fig. 3a) and lowestVp (Fig. 3b)due to the high quartz (45%) and mica (29%) contents. The influence ofhigh content of micaceous minerals (46%) in lowering the seismic wavevelocity is also evident in the Mandi granitoids of Early Palaeozoic age,which exhibit the lowest Vs (Fig. 3b). The Saruna granitoids of Proterozoic
Fig. 6. Plots showing linear relation of AMS parameters with seismic wave velocities for the Proterozoic and Palaeozoic granitoids in Himalaya; (a) and (b) relation of Vp and Vs withmagnetic susceptibility, (c) and (d) relation of Vp and Vs with degree of magnetic anisotropy; and (d) and (f) relation of Vp and Vs with magnetic lineation. R is the correlationcoefficient.
31R. Sharma et al. / Tectonophysics 497 (2011) 23–33
age are marked by wide range of fractures implanted by post-emplacement tectonic deformation during the Himalayan orogeny. Thefractured nature of Saruna granitoids, by way of the nature of fill, in thepresent case low density chlorite, may be additional mechanism inlowering both of density and seismic wave velocity. It may further benoted that the lowest standard deviation in the Tertiary granitoidsmaybedue to its homogenous and equigranular nature (Fig. 2h).
In addition to thepresence of cracks, the texture by virtue of its highersensitivity to grain size and surface irregularities control the physicalproperties of rocks, as demonstrated by the attenuation character of P-and S-waves. It had been noted that the fine grained rocks exhibit higherVp and Vs and lower attenuation than the coarse grained rocks (Lamaand Vutukuri, 1978; Rao et al., 2006). Among all the granitoids studiedhere, the Early Palaeozoic granitoids exhibit the highest P- and S-waves
attenuation (Fig. 3c). This is mainly because of the coarser nature ofgrains (Fig. 2e). Contrary to this, the Cretaceous granitoids exhibit lowerattenuation than the remaining three suites of granitoids viz. Proterozoic,Early Palaeozoic and Tertiary granitoids, even though it is coarse grained.Here the factors controlling the attenuation of different granitoids rocksare still not resolved. It might be possible that combination of diversemineralogy, textures and the deformation combine in differentmodes todetermine the exact attenuation characteristics of rocks, require furtherquantification by specific studies.
7. Conclusions
The present study is unique example where petrophysical, petro-graphic and compositional characterisation of four different generations
Fig. 7. (a) Line graph showing relationship betweenmagnetic susceptibility and densityof the Cretaceous granitoids (b) Line graph showing relationship of magneticsusceptibility with Vp (solid square) and S (open square) for the Cretaceous granitoidsof Ladakh Batholith.
32 R. Sharma et al. / Tectonophysics 497 (2011) 23–33
of granitoids from a single orogenic belt, i.e., the Himalaya arequantified. The most coherent properties such as bi-model distributionof magnetic susceptibility, characteristic density and seismic velocityranges match well with those recorded in association with theworldwide occurrences of granitoids. These properties are usefulproxy to the composition and guide to locate the source region ofgranitoids either to mantle (I-type) ormiddle-lower crust (S-type). It isfound only Trans-Himalayn Cretaceous granitoid related to subductionprocesses correspond to I-typegranitoids. Other Proterozoic, Palaeozoic,and Tertiary granitoids with paramagnetic mineralogy and petrophys-ical properties belong to S-type of granitoids. Some region specificproperties, such as bi-model distribution of density, seismic wavevelocities, magnetic susceptibility in Cretaceous granitoids related todepth-dependent phases of magmatic differentiation, and depletion ofmica and quartz content in Proterozoic, Palaeozoic granitoids can beuseful inputs to develop or refine evolution model for the origin of thegranitoids in the collision associated tectonics. Certain anomalouspetrophysical properties noted in Cretaceous granitoids evidenced insite specific inter-relations, e.g. Vp-density, magnetic susceptibility-Vs,density-magnetic susceptibility require further confirmation with largedata base. A special campaign in Ladakh Pluton to characterize thepetrophysical properties of granitoids and other rock types would beuseful to constrain further effects of collision tectonics on themodulations of compositional and petrophysical properties. Inductionof measurements at high confining pressure and temperature would bestep in right direction.
Acknowledgements
The present study has been supported by DST funded projectentitled “Rock Properties Laboratory — A National Facility” under thescheme Deep Continental Studies (DCS) in India. Authors acknowl-edge with gratitude advise, encouragement and support received
from the Program Advisory & Monitoring Committee and MemberSecretary, Dr Ch Sivaji. Authors also extend their gratitude to ProfRajendra Prasad, Andhra University M.V.M.S. Rao, NGRI and ManikaPrasad for unflinching support on the technical and design parametersof the laboratory facilities developed under the project. Thanks aredue to S.S. Bhakuni, R. Islam and Kavita Tripathi for providing thegranitoid samples for the present study. Critical constructivecomments from Ali Aydin, M. Faccenda and an anonymous reviewerare acknowledged with gratitude.
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