Chapter-3 Petrography of Basement samples

Chapter-3 Petrography of Basement samples 3.1 Introduction Petrographic characters of the rock reflect its chemical composition and cooling history. To obtain an authentic petrogenetic model, detailed petrographical, mineralogical and textural studies on carefully chosen rock samples are necessary. The overall mineralogy and texture of rock gives an insight into the nature, composition and to a great extent, the processes involved during magmatic crystallization. For the present study, we have chosen forty three basement cores from KLR-1 borehole, which vary in depth range from 351.50 m to 616.80 m as stated earlier. In hand specimen, these rocks range in color from dark to light grey, few of them showing secondary quartz and calcite veins. For each rock samples, one to three thin sections were prepared and studied using optical microscope to decipher detailed textural, mineralogical and deformational characters. Based on detailed geochemical data and Total Alkali Silica (TAS) diagram (as discussed later), theses samples are divided into three groups: basic, Intermediate and acid, which contain 6, 27 and 10 samples respectively. Petrographical character of each sample is discussed below. 3.2 Petrography 3.2.1 Basic group Basic group of rocks includes six samples as discussed earlier. This group is mainly made up of medium to coarse grain holocrystalline rock which dominantly contains ferromagnesian minerals such as amphibole, biotite, clinopyroxene, relict orthopyroxene, plagioclase and quartz which occur as major constituents. Secondary alteration products and accessory opaques are also present as clusters, represented mainly by magnetite and minor pyrite. 23

KIL-34, KIL-35 and KIL -37: Coarse to medium grained schistose amphibolite rock which contains plagioclase feldspar, hornblende, biotite, quartz as major constituent and titanite, apatite as accessory. Most of the feldspar grains are sausurritized (Fig.3.1a) and sericitized. Boundaries of the feldspar and amphibole grains are curved. Chloritization (Fig.3.1b) and sausurritization suggest hydrothermal alteration during the course of retrogression. KIL-39: Medium grained basic rock which contains hornblende, plagioclase, quartz and biotite as major mineral and opaque (magnetite), epidote, titanite, present as accessories (Fig 3.1c). Uralatisation and saussuritization are also clearly seen. In these samples, two phases of rocks can be observed, one portion is quartzofeldspathic rich and the other, rich in ferromagnesian minerals. KIL-41: Coarse grained amphibolite rock constituting plagioclase feldspar, biotite, hornblende and quartz as major minerals. Here also, cluster of opaque minerals (mainly magnetite) (Figs.3.1d, e) can be seen at the contacts of titanite and hornblende (Fig.3.1f). Myrmekite texture (quartz-plagioclase intergrowth) is also present at the contact of K-feldspar and plagioclase. Majority of the plagioclase grains are sausuritized. KIL-46: Medium grained amphibolite rock, which is mainly composed of biotite, hornblende, plagioclase, quartz and clusters of opaque. Segregation of opaques are noticed here. 3.2.2 Intermediate group This group consists twenty seven samples, mainly containing hornblende, plagioclase, biotite, clinopyroxene, orthopyroxene (minor), quartz and k-feldspar as major constituents and titanite, opaques, apatite, chalcopyrite as accessories. This group indicate maximum variation in litho-units, containing three types of rocks, mainly amphibolite but also granulite, tonalite and granodiorite. 24

Fig.3.1 Photomicrographs taken from the thin sections showing sausuritized plagioclase and hornblende in KIL-34 (Fig.a), process of chloritization in KIL-34 (Fig.b), hornblende, opaque and plagioclase in KIL-39 (Fig.c), opaque and biotite in KIL-41 (Fig.d), hornblende, plagioclase, and opaques in KIL-41 (Fig.e), reaction between opaques, titanite and amphibole in KIL-41 (Fig.f). Hbl: Hornblende, Plg: Plagioclase, Bt: Biotite, Chl: Chlorite, Tnt: Titanite, Opq: Opaque 25

KIL-1: A medium grained plagioclase dominated amphibolite rock containing hornblende, clinopyroxene, k-feldspar, biotite, epidote as major constituents and titanite, ilmenite as accessories. Saussuritized and sericitized plagioclase grains are also seen which are altered to, sericite (Fig.3.2a) and muscovite (Fig.3.2b), exhibiting hydrothermal alteration during the course of retrogression. Clinopyroxene grains appear to be marginally replaced by hornblende showing uralitisation (Fig. 3.2c). Antiperthite texture which is also one of the characteristic features of granulite facies metamorphism (particularly in charnockitic roks), can also be seen here (Fig. 3.2d). It could have been developed both by exsolution and replacement during active deformation and shifting from higher P and T to lower P and T conditions. This rock contains abundant inclusions of ilmenite in brown hornblende (Fig.3.2e) as well as titanite (Fig. 3.2f), latter indicating possible reversal in P&T conditions. Recrystallized and curved boundary showing shearing of biotite with released iron oxide, can also be clearly seen in the sample (Fig.3.3a). KIL-2: Medium grained plagioclase dominated amphibolite rock, which exhibit similar characteristics as KIL-1. It consists large grains of saussuritized plagioclase megacryst (Fig. 3.3b) and deformed quartz. KIL-3: A medium grained amphibolite rock containing plagioclase, hornblende, biotite, k- feldspar, quartz and epidote as major minerals and titanite as accessory. Opaques such as ilmenite, pyrite and chalcopyrite are also present in minor quantities, which are usually associated with medium to high grade amphibolites. KIL-8: It is deformed and medium grained amphibolite rock, containing hornblende, clinopyroxene (augite), epidote, chlorite, saussuritized plagioclase and biotite as major constituent. Minor grains of orthopyroxene are also present. Most of orthopyroxene grains have been converted to horneblende (pseudomorh after orthopyroxene). Titanite is present as accessory. Carbonate stringer is seen passing through this rock, which indicates secondary vein activity (Fig.3.3c). It contains 26

Fig.3.2 Photomicrographs taken from the thin section of the sample KIL-1, showing, sericitization, in which plagioclase is altered into sericite (Fig. a) and muscovite (Fig. b), uralitization wherein clinopyroxene is replaced by hornblende (Fig. c), antiperthite texture in plagioclase grain (Fig. d), inclusion of ilemenite in brown hornblende (Fig. e), release of Fe from titanite (Fig.f). Plg: Plagioclase, Ser: Sericite, Ms: Muscovite, Cpx: Clinopyroxene, kfs: K-feldspar. 27

calcite, epidote, and chlorite besides saussuritized plagioclase as secondary alteration product. Calcites are primary and appear to be formed by action of volatiles (CO 2 ). Preferred alignment of ferromagnesian minerals like hornblende and biotite (Fig. 3.3d) is also seen, which may have been caused due to intense stress related deformation. It is pierced by a carbonate stringer, suggesting hydrothermal fluid interaction. KIL-9: Holocrystalline coarse grained altered granulitic rock showing pronounced metasomatic alterations. It contains plagioclase, hornblende, clinopyroxene, altered or relict orthopyroxene, quartz and biotite as major constituents and titanite as secondary. Most of the plagioclase is found to be saussuritized due to intense fluid interaction. Secondary released iron oxide is also present at the contact of titanite and biotite mineral. Antiperthite texture and deformed quartz vein can be clearly seen. KIL-10: Coarse grained rock mainly composed of plagioclase, hornblende, biotite as major constituents and titanite as accessory. This sample exhibit highly altered orthoclase and mafic minerals with released iron. Clinopyroxene grains appeared degenerated due to recrysatallization. Mafic minerals are in abundance, which are often altered to chlorite, clinopyroxene and hornblende minerals. KIL-11: Schistose amphibolite rock which contains hornblende, plagioclase, biotite, quartz, chlorite as major minerals and epidote, titanite, apatite, opaques as accessories. Here schistocity is defined by alignment of hornblende and biotite minerals. KIL-12: Medium grained partially retrograded deformed granulite rock, containing clinopyroxene, orthopyroxene and hornblende (Fig.3.3e), besides, calcic plagioclase, biotite as major constituents and titanite as accessory. Some grains of orthopyroxene are highly pleochroic. Clinopyroxenes grains are marginally altered to hornblende during the process of uralitisation (Fig.3.3f). Secondary saussuritization and biotitisation are also seen, which may indicate retrograde overprinting. 28

Fig. 3.3 Photomicrographs taken from the thin sections showing recrystallized boundary of biotite with released iron oxide in KIL-1 (Fig.a). saussuritization of plagioclase in KIL-2 (Fig.b), typical vein containing calcite in KIL-8 (Fig.c), preferred alignment of hornblende and plagioclase feldspar in KIL-8 (Fig.d), orthopyroxene and clinopyroxene grains in KIL-12 (Fig.e), saussuritized plagioclase with altered clinopyroxene in KIL-12 (Fig. f). Plg: Plagioclase, Hbl: Hornblende, Opx: Orthopyroxene, Cpx: Clinopyroxene, Bt: Biotite. 29

KIL-13: It is a medium-grained amphibolite rock containing altered hornblende, plagioclase feldspar, clinopyroxene altered to green and brownish green amphiboles, minor apatite, pyrite and chalcopyrite with secondary carbonate veins. Presence of pyrite and chalcopyrite inclusions possibly indicate role of high grade metamorphic fluids. In most oxidized granulites, pyrite is a dominant sulphide with chalcopyrite as accessory. Further, hornblende and plagioclase are aligned in a particular direction (Fig.3.4a), indicating stress related deformation in which alignment and elongation of minerals usually takes place in the direction of least stress. Compositionally, this rock is amphibolite but it exhibits granulitic textures. Clear granular texture is also present which is one of the distinct metamorphic texure. KIL-14: Medium grained amphibolite rock containing hornblende, plagioclase, clinopyroxene, biotite, zoned orthoclase and quartz as major and titanite and pyrite as secondary minerals. Clinopyroxene are partially retrograded to hornblende with sausuritized plagioclase (Fig.3.4b). This sample exhibits feeble foliation, defined by parallel alignment of partially to completely altered hornblende and plagioclase (Fig.3.4c), the later showing partial sericitization (clay making process) with recrystallization at curved boundaries of the hornblende (Fig.3.4d). KIL-18: Inequigranular coarse grained rock which is mainly composed of plagioclase feldspar, biotite, hornblende, k-feldspar and quartz. Volumetric percentage of plagioclase is more than alkali feldspar. Medium to fine grained recrystallised quartzo-feldspathic groundmass contains partial chloritized biotite. Minor apatite and magnetite are also present. Rock is deformed as evidenced by undulose extinction in quartz. This sample shows development of foliation planes as, defined by preferred alignment of hornblende, biotite and plagioclase. KIL-19: Coarse grained rock, with major constituents as plagioclase feldspar, biotite, quartz, hornblende, clinopyroxene, muscovite, chlorite, orthoclase, albite. Opaque minerals are present as accessory, which are found at the boundary of biotite (Figs. 3.4e, f). Flattening of grains can also be seen in biotite and plagioclase. 30

Clinopyroxene grains are seen converting into hornblende, indicating uralitization during retrogression. Here also, most of the plagioclase grains are sausuritized. Rock is deformed and in photomicrograph, shearing effect can be seen. This rock is well banded, which is represented by quartzo-feldspathic and ferromagnesium rich layer. KIL-21: A medium to coarse grained altered granulite rock that exhibits porphyritic texture and contain quartz, calcic plagioclase, biotite and hornblende as major minerals and titanite and magnetite as a secondary. Plagioclase phenocrysts are zoned and sausurritized. Margins of plagioclase and hornblende grains are curved. Alteration is evident here due to the presence of abundant clay minerals and muscovite. KIL-24: Coarse grained foliated amphibolite rock, containing plagioclase, hornblende, biotite, quartz, k-feldspar as major constituents and titanite as secondary mineral. Intense saussuritization, biotitization and uralitisation suggest retrogression. Most of the amphibole and plagioclase grains are recrystallized and curved at the boundary. KIL-25: Coarse grained quartzo-feldspathic rock. It contains abundant zoned plagioclase feldspar, biotite, amphibole, and quartz. Biotitization is also seen where hornblende is being replaced by biotite (Fig.3.5a). Quartz is deformed showing undulose extinction (Fig.3.5b). Cluster of opaque minerals can also be seen at the contact with biotite mineral (Fig.3.5c), which is formed due to breakdown of biotite during the late-stage of magma crystallisation. Presence of subgrain formation (polygonisation) in quartz together with biotitization (Fig.3.5d), indicate deformation and retrograde metamorphism. Perthitic intergrowth can also be seen here (Fig.3.5e). Myremekite texture consisting of irregular wormy blebs of quartz in a plagioclase host, adjacent to alkali feldspar grain is also noticed (Fig.3.5f). KIL-26: It is an inequigranular coarse grained granulitic rock containing calcic plagioclase, clinopyroxene, relict orthopyroxene, hornblende, biotite, rutile, microcline as major and zoisite, clinozoisite, titanite, magnetite, pyrite as accessories. 31

Fig.3.4 Photomicrographs taken from the thin sections showing preferred alignment in hornblende in KIL-13 (Fig.a), saussuritization of plagioclase in KIL-14 (Fig.b), alignment in amphibole and plagioclase grains in KIL-14 (Fig.c), alignment of hornblende and plagioclase grains with curved boundary in KIL-14 (Fig.d), biotite, opaques and plagioclase in KIL-19 (Fig.e), opaque, biotite and quartz in KIL-19 (Fig.f). Hbl: Hornblende, Qtz: Quartz, Plg: Plagioclase, Opq: Opaque, Bt: Biotite, Cpx: Clinopyroxene. 32

Pleochroic haloes due to inclusions and antiperthite texture (Fig. 3.6a), as well as myrmekite textures are also noticed. Most of the titanites are associated with mafic hornblende and indicate released iron. Zoned plagioclase rims have been saussuritized and shows development of zoisite (Fig.3.6b). Besides, plagioclase grains show partial albitization. Occurrence of zoned saussuritized, sericitized and albitized plagioclase, together with biotitization (Fig. 3.6c) indicates retrograde overprinting. KIL-27: Amphibolite rock that contains hornblende, plagioclase, biotite and relict clinopyroxene as major minerals and quartz as minor. Small amount of titanite and pyrite are also present. This rock is highly biotitized (Fig.3.6d), where hornblende has been replaced by biotites. Calcic plagioclase is saussuritized and completely altered into secondary minerals like calcite and clays. At the boundary of hornblende, titanite grains can also be observed (Fig.3.6e). KIL-28 and KIL-29: Coarse grained holocrystalline quartzo-feldspathic rock which contains plagioclase, k-feldspar, quartz, amphibole, biotite as major minerals and titanite, apatite as accessories. Large grains of zoned plagioclase indicate disequilibrium conditions. Zoisite, clinozoisite and epidote minerals are present as secondary alteration product. KIL-31: Medium grained highly altered quartzo-feldspathic rock containing hornblende, plagioclase, epidote, chlorite, quartz as major constituents and ilmenite, magnetite, pyrite, titanite and clinozoisite are present in minor amounts. Quartz grains are deformed. Abundant carbonate-rich stringers are seen passing through the minerals. This rock also shows plagioclase segregation at some places. During the saussuritization of plagioclase, calcium is released which formed epidote, besides chlorite and clinozoisite (Fig.3.6f). Hornblende shows reaction rims and indicates primary alteration. Chloritization and biotitization are also commonly seen in this sample. 33

Fig.3.5 Photomicrographs taken from the thin section of the sample KIL-25 showing biotitization (Fig.a), undulose extiction in quartz (Fig.b), hornblende and opaque minerals (Fig.c), subgrain formation in quartz (Fig.d), perthite texture (Fig.e), myrmekite texture (Fig.f). Bt: biotite, Hbl: Hornblende, Opq: Opaque, Qtz: Quartz, Ser: Sericite, Bt: Biotite, Kfs: K-feldspar, Tnt: Titanite. 34

KIL-36: Coarse grained altered amphibolite rock which contains plagioclase feldspar, amphibole, biotite, quartz as major constituent. Opaque minerals, formed at the contact of biotite and plagioclase during the breakdown of biotite, are present as accessories. The plagioclases are coarse-grained and show deformed bent twin lamellae with marginal recrystallisation. Grain elongation and shearing effect can also be seen (Fig.3.7a). KIL-38: Coarse grained holocrystalline quartzo-feldspathic rock which contains altered plagioclase, quartz, k-feldspar, amphiboles, biotite as major mineral and titanite, opaque are present as accessories. KIL-40: Coarse grained amphibolite rock showing hornblende with sausuritized plagioclase (Fig.3.7b). It shows mineralogical similarity with KIL-39. KIL-42: Medium to coarse grained altered rock which constitutes plagioclase feldspar, biotite, amphibole, quartz as major minerals. This sample shows weakly developed foliation, characterised by parallel alignment of partially to completely altered plagioclase, quartz, biotite, opaque and amphibole. Quartz and plagioclase grains are elongated, deformed and marginally recrystallized. Cluster of opaques minerals can be seen (Fig.3.7c, d) at the boundary of biotite and quartz grains, which show preferred alignment. In this sample biotite is almost invariably associated with opaque minerals, characteristically forming rims around the magnetite and ilmenite, especially at the contact with plagioclase. KIL-44 and 45: Coarse grained holocrystalline rock which is dominantly composed of plagioclase, amphiboles, k-feldspar, quartz and biotite as major minerals. It also contains abundant titanite as accessory mineral. KIL-47: Medium to coarse grained holocrystalline rock which exhibits similar mineralogy and texture to sample KIL-44. Preferred alignment of plagioclase, hornblende and chlorite is seen along with titanites (Figs.3.7e, f). 35

Fig.3.6 Photomicrographs taken from the thin sections showing, antiperthite texure in KIL-26 (Fig.a), occurrence of zoisite and saussuritized plagioclase phenocryst in KIL-26 (Fig.b), process of biotitization where hornblende is seen converted into biotite in KIL-26 (Fig.c), biotitization in KIL-26 (Fig.d) and hornblende with titanite grain in KIL-27 (Fig.e), formation of chlorite and epidote due to saussuritization in KIL-31 (Fig.f). Bt: biotite, Hbl: Hornblende, Plg: Plagioclase, Qtz: Quartz, Kfs: K-feldspar, Tnt: Titanite. 36

Fig.3.7 Photomicrographs taken from the thin sections of the samples showing, shearing effect in KIL-36 (Fig.a), plagioclase, k-feldspar and hornblende in KIL- 40 (Fig.b). elongated deformed quartz and clusters of opaques in KIL-42 (Fig.c), opaque with hornblende and quartz in KIL-42 (Fig.d), altered plagioclase with hornblende and titanite in KIL-47 (Fig.e), plagioclase, hornblende and chlorite with preferred alignment in KIL-47 (Fig.f). Hbl: Hornblende, Plg: Plagioclase, Qtz: Quartz, Kfs: K-feldspar, Tnt: Titanite, Opq: Opaque, Chl: Chlorite. 37

3.2.3 Acid group This group consists of ten samples and characterised by salic minerals. Percentages of femic minerals are comparatively less in comparison to basic and intermediate group. Here, large porphyroblast grains of plagioclase and k-feldspar can be clearly seen. It dominantly contains quartz and plagioclase. Few samples are characterized by quartzo-feldspathic matrix, with relatively high volumetric percentage of quartz. KIL-15: Plagioclase and hornblende dominating coarse to medium grained holocrystalline amphibolite rock, which contains saussuritized plagioclases and recrystallized hornblende with curved boundary. KIL-16 and 17: Holocrystalline coarse grained amphibolite rock exhibiting plagioclase, hornblende, biotite, quartz as major constituents and titanite and apatite as accessories. Plagioclase gains are mostly sausuuritized. Small scale biotitisation is also seen. Mineralogicaly, this rock is metagabbroic in nature having suffered amphibolite facies metamorphism. KIL-20: It is a deformed quartzo-feldspathic rock, containing coarse-grained zoned phenocrysts showing porphyritic microstructure. Magnetite, pyrite and chalcopyrite are present in minor quantities together with secondary carbonates. The volumetric percentage of plagioclase is more than 50%. Myrmekitic texture is visible along the margins, which show exsolution of k-feldspar lamellae (Fig.3.8 a). Quartz grains exhibit deformed bands showing undulose extinction. Few grains of biotites are kinked (Fig.3.8 b), indicating that the rock has undergone substantial deformation, which usually occurs at high shock pressure conditions. KIL-22: Coarse grained holocrystalline porphyritic rock with plagioclase phenocryst and quartzo-feldspathic groundmass. In this rock, plagioclase, k-feldspar, biotite and hornblende is present as major constituents and titanite and apatite as accesories. Undulose extinction in quartz indicates deformation. Presence of biotitization and saussuritization indicate hydrothermal alteration. 38

KIL-23: Coarse grained, holocrystalline porphyritic rock containing plagioclase porphyroblast. KIL-30 and 32: Medium grained rock composed of plagioclase, hornblende, quartz and biotite as major constituents and titanite as accessories. It dominantly consists of sausuritized and sericitized plagioclase, together with secondary alteration product such as, zoisite, clinozoisite, epidote, sericite. KIL-33: Medium to coarse grained granodiorite rock which is dominantly rich in quartzo-feldspathic material. Coarse grained holocrystalline, anhedral to subhedral grains exhibit hypidiomorphic texture. It is composed of quartz, orthoclase, plagioclase, biotite and hornblende as major minerals and titanite as accessory. Suturing in quartz grain boundary (Fig.3.8.c) and undulose extinction with elongated quartz grains (Fig.3.8.d) suggest intense deformation. KIL-43: Holocrystalline, coarse grained quartzo-feldspathic rock composed of sausuritized and sericitized plagioclase, recrystallized hornblende, biotite, k-feldspar and quartz. Titanite is present in minor quantity. 3.3 Deformation signature Most of the samples from Killari region show deformational textures such as kinking in biotite, subgrain formation in quartz, shearing effect, undulose extinction in quartz, flame perthites and elongation in quartz grain, quartz ribbons etc. Preferred alignments of hornblende, biotite, quartz and plagioclase can be seen in samples KIL-8, 11 and 13. Deeply sutured boundary in quartz is noticed in sample KIL-33. Most of the quartz grains are characterized by dusty appearance, elongation in grain size and undulose extinction. The deformed quartz grains have sub parallel alignment of long axes, defining thereby a crude foliation along with sub parallel orientation of amphiboles and biotite. Almost all the basement samples exhibit weak to intense deformation which seems to have been formed during the event of metamorphism. 39

Fig.3.8 Photomicrographs taken from the thin sections showing, myrmekitic texture in contact with plagioclase and perthetic feldspar in KIL-20 (Fig.a), kinked biotite in KIL- 20 (Fig.b), suturing in quartz grain in KIL-33 (Fig.c), elongaion in quartz grains in KIL- 33 (Fig.d). Plg: Plagioclase, Qtz: Quartz, Kfs: K-feldspar, Bt: Biotite. 3.4 Modal analysis Mode refers to actual minerals present in the rock and it can be determined quantativaly to express the abundances of the mineral constituents in volume per cent. Representative modal percentages of samples are summarized in Table 3.1. Petrological studies have indicated that the basement samples are altered and deformed. In such cases, modal analysis does not provide accurate results. It only gives rough idea about volumetric percentage of major minerals. However, as 40

we are dealing with plagioclase and amphibole dominating rocks, we have carried out modal analysis on representative samples using standard point counting method on thin sections. The dominant mineral hornblende is found to vary between 20 % and 50%, followed by plagioclase from 10% to 80 %. In the case of tonalite, plagioclase percentage is more than k-feldspars exceeding 50%. Minerals which are altered and recrystallized and showing complete replacement are not included in counting. Petrographically these samples are showing large variation. So on the basis of modal analysis, we have classified these samples in QAP diagram (Fig.3.9), where we find that maximum samples fall in the field of quartz gabbro and gabbro, but few samples also confirm the tonalite and granodiorite affinity, which is also reflected in the petrographic study on these samples. 3.5 Scanning Electron Microscope (SEM) studies Since clustering of opaque minerals is a characteristic feature, observed in many thin sections, we studied their nature under the Scanning Electron Microscope (SEM), wherein Hitachi, S-3400N model was used to characterise the opaque minerals. Most of the opaque minerals are represented by magnetite. A few grains of pyrite and chlacopyrite were also detected. Most of the magnetite and pyrite grains appear to be in the contact with amphibole and plagioclase grains. Grains of magnetite have shown high peak of iron with little amount of silica and oxide. Magnetite grains also show presence of other oxides such as MgO, Al 2 O 3, and SiO 2 which suggest reaction with other minerals within the grains through cracks and other weak zones. BSE images of the magnetite grains are shown in Fig.3.10. Spot analyses were performed using energy-dispersive X-ray spectroscopy (EDS), to estimate the qualitative chemical composition of the observed phases. The qualitative relative elemental peaks are also shown along with the BSE images, which inturn shows characteristic elements in the respective mineral grains. 41

Table-3.1 Modal composition and microstructural characteristics of crystalline basement rocks from Killari borehole (KLR-1) drilled in 1993 Latur earthquake epicentral region of Maharashtra (India). Sample no. Density (g/cm 3 ) Primary phases (Vol. %) Accessory Micro-structure Basic rocks KIL-41 3.07 Plag (30), Qtz (6), Amph (25), Bt (25) Kfsp(2) Opq, Tnt, Ap Medium-grained amphibolite with clusters of opaques KIL-46 3.08 Plag (23), Qtz (1), Amph (45), Bt (26) Kfsp(0.5) Opq, Tnt, Ap Medium-grained amphibolite with clusters of opaques Intermediate rocks KIL-1 2.81 Plag (50),Amph (31,) Kfsp (6), Qtz (6), Bt (4)Cpx (1), Ep (1) Tnt, Opq, Ap Plagioclase abundant rock with granoblastic texture KIL-2 2.85 Plag (48), Amph (36), Bt (8), Qtz (3), Kfsp (2) Tnt, Opq, Ap Medium-grained amphibolite KIL-3 2.8 Plag (57), Amph (32), Bt (6), Qtz (3), Ep(1) Opq Medium-grained amphibolite KIL-8 2.95 Plag (45), Amph (41), Bt (8), Qtz (3) Ep (3) Tnt Medium-grained deformed rock showing metamorphic textures and sauss.plag. KIL-11 2.92 Amph (56), Plag (28), Bt (8), Ep (8) Qtz(2) Opq, Tnt, Ap Medium-grained amphibolite KIL-12 2.82 Amph (46), Plag (29), Bt (22), Qtz (1) Cpx (1) Tnt Medium-grained granulite rock. KIL-13 2.87 Amph (42), Plag (48), Qtz (6), Ep (3) Opq, Tnt, Ap Medium-grained amphibolite KIL-14 2.97 Amph (48), Plag (45), Qtz (3), Cpx (3) Opq, Tnt Medium-grained rock containing partially sauss.plag and cpx partially replaced by hb. KIL-18 2.83 Plag (64), Bt (27), Qtz (6), Amph (2) Opq, Tnt, Ap Inequigranular rock with coarse grained porphyroblast KIL-19 2.84 Plag (34), Qtz (18), Amph (15), Bt (18) Opq, Tnt Plagioclase dominating rock with lots of opaque and deformed quartz KIL-21 2.9 Plag (44), Amph (12), Qtz (11), Bt (33) Opq, Tnt Medium-grained granulite rock. KIL-24 2.94 Plag (41), Amph (43), Bt (6), Cpx (3), Ep (2), Kfsp (4) Tnt Medium-grained amphibolite KIL-25 2.76 Plag (34), Qtz (28), Amph (18), Bt (8) Kfsp(10) Opq, Tnt Medium grained deformed rock with quartz subgrains and biotitization KIL-26 2.76 Plag (50), Amph (26), Bt (8), Qtz (7), Kfsp (6), Ep (2) Opq, Tnt Medium-grained granulite rock. KIL-27 3.06 Amph (56), Plag (18), Bt (21), Qtz (1), Ep (3) Opq, Ap, Tnt Biotite-rich medium-grained rock KIL-31 2.79 Plag (80), Qtz (3), Amph (1), Cpx (3), Ep (8), Kfsp (4) Tnt, Opq Highly altered medium-grained rock. KIL-36 2.75 Amph (45), Plag (35), Qtz (6), Bt (12) Kfsp (2) Opq, Tnt Medium grained amphibolites with shearing effect and clusters of magnetite KIL-42 2.92 Plag (23), Qtz (20), Amph (17), Bt (14) Kfsp (2) Opq Amphibole and plagioclase dominating rock with clusters of opaques Acid rocks KIL-20 2.75 Plag (72), Qtz (12), Bt (10), Kfsp (5) Opq Coarse-grained deformed rock showing porphyritic texture. KIL-33 2.69 Plag (39), Qtz (45), Amph (10), Bt (6) Tnt Coarse to medium-grained felsic rock. Amph: amphibole; Plag: plagioclase; Bt: biotite; Qtz: quartz; Cpx: clinopyroxene; Ep: epidote; Kfsp: Kfeldspar; Tnt: titanite; Ap: apatite; Opq: opaque; Sauss: Saussurite. 42

Fig.3.9 QAP classification of coarse grained crystalline rocks (IUGS-BGS) (Upper Ternary) using Modal percentage.q: Quartz, A: Alkali feldspar, P: Plagioclase feldspar. 43

44

Fig. 3.10 Scanning Electron Microscopic images of opaque minerals. Back-scattered electron (BSE) photograph with spectrum image showing large grain of magnetite mineral with inclusion of other silicates in KIL-19 (Fig.a). BSE photograph with spectrum image showing opaque mineral magnetite in contact with amphibole in KIL-25 (Fig.b). BSE photograph with spectrum image showing opaque mineral magnetite in contact with plagioclase and amphibole in KIL-41 (Fig.c). BSE photograph with spectrum image showing opaque mineral pyrite in contact with amphibole in KIL-41 (Fig.d). BSE photograph with spectrum image showing irregular shape magnetite mineral with inclusion of other silicates in KIL-42 (Fig.e). 45

3.6 Summary As mentioned in the foregoing sections, petrologically, the basement rocks can be classified as meatasomatised, deformed and retrogressed amphibolite to granulite facies transional metabasic rocks conforming to midcrustal lithology. However, some samples correspond to granodiorite and tonalite in texture and composition. Petrologically, the amphibolite to granulite facies rocks are found to predominantly contain plagioclase + amphibole + biotite ± cpx ± opx + k-feldspar + quartz± chlorite as major constituents and titanite + epidote + chalcopyrite ± pyrite ± magnetite ± illmenite + apatite + zoisite + clinozoisite as accessories. These are medium to coarse grain rocks, which exhibit holocrystalline to hypidiomorphic textures. A crude, weakly developed foliation, defined by parallel alignment of partially to completely altered ferromagnesian minerals like hornblende, biotite, chlorite and relict orthopyroxene and elongated, deformed, marginally recrystallized quartz, characterize these rocks. Matrix assemblages are either coarse grained or fine grained. Coarse grained assemblages typically contain zoned plagioclase minerals, whose rims were affected by chemical re-equilibrium in response to changing physical conditions (pressure, temperature, fluid activity). Large grains of sausurritized and sericitized plagioclase porphyroblast, resulted into secondary alteration products such as epidote, sericite, zoisite, clinozoisite. Development of epidote and secondary minerals, may have been formed through the hydration reaction of plagioclase during post-crystalline deformation. Most of the plagioclase grains are altered and recrystallized which, show bent lamellae and mortar structures. Majority of such grains are free of inclusions, but at places, abundant epidote is found at their boundaries. Composition of plagioclase varies from albite to anorthite. Potash feldspar is generally perthitic in nature. Hornblende crystals are medium to coarse grained with the pleochroism which indicate schiller structure due to the presence of opaque dust and streaks along the cleavage traces. Some amphibole grains show inclusions of quartz, ilmenite, titanite, magnetite, epidote and plagioclase. Hornblende grains commonly vary from green to brown in color, which 46

is a typical feature of upper amphibolite grade assemblages. Sometimes such grains are altered, showing recrystallized, curved boundary. Amphibole grains also exhibit biotite and chlorite formation along the cleavage plane, which are of secondary origin. Biotite has partially to completely replaced the amphibole or orthopyroxene in some samples. Quartz is generally more sutured and shattered. Likewise tonalite to granodiorite rocks exhibit plagioclase + amphibole + biotite ± cpx + k-feldspar + quartz ± chlorite as major constituents and titanite + epidote + pyrite ± magnetite ± illmenite + apatite + zoisite as accessories. In these rocks, volumetric percentage of plagioclase is more, varying from 50 to 60% and percentage of k-feldspar is less. They do not reveal any metamorphic equilibration texture. In QAP diagram these samples plot in the field of quartz gabbro, gabbro, tonalite and granodiorite. Some of the samples contain opaque clusters. SEM studies suggest that they are mainly magnetite with a few grain of pyrite, indicating their derivation from melts rich in iron oxides. Magnetite grains are mostly seen formed at the boundary of biotite and amphiboles. Few grains of titanite have also released iron oxide. Deformation signatures such as, preferred alignment of flaky minerals of biotites and amphiboles and kinked biotite, undulose extinction in quartz, sutured quartz, polygonisation, elongation in grain size and flame perthite are also present in these rocks. Various intergrowth textures such as perthite, antiperthite and myremekite are also seen, which are related to retrogression processes at the time of cooling and falling of temperature and pressure. 47