Estelar. Introduction

Transcription

1 Chapter-1 Granitic magmas only occur in volume where a thick crust, old or new, can provide a fertile source, a thermal blanket and sufficient space for differentiation processes to operate. 1.1 Wallace S. Pitcher (1993); p293 Nature and Origin of granites (sensu lato) have been intriguing to the geologist since more than two centuries contemporary to the active discussion on the evolutionary theory of the Earth. Wallace S. Pitcher s (1993) strong and intelligent perception on development of science of granites and his critical evaluation of past and present theories on granitogeny based on spectacular field, petrographic and experimental observations have provided future direction of researches on granites and related rocks. In the beginning of 18 th century secular explanations for the origin of granite was considered as chemical precipitate from a primordial universal ocean. Neptunian propounded the formation of granite by the consolidation of heat-induced matter made fluids. A philosophical triumph of James Hutton in the late 18 th century gave the birth of intensive debate between Neptunists and Plutonists but it was not clear whether granite was formed by fusion of originally stratified materials or melt body transferred from subterraneous regions and then invaded the strata. Intrusive relation of granite was established based on spectacular features of irregularities developed in limestone country with Fleet granite. A general consensus arrived in 1830s that all granites were formed at depth from slow cooling lavas of igneous origin. First scientific basis was introduced that magma of imperfect crystals enveloped in a liquid with vapour content and viscosity controlled by composition. There were then continued discussion on room problem for more than two decades since the introduction of concept of metamorphism, which later invoked process of granitification (granitization) or metasomatism for the origin

2 of granites which was inspiring to transformists. Magmatists introduced the concept of forceful magma intrusion and resolved that space was created by stopping and assimilation of country-rocks. Meanwhile a group of experimental geologists forwarded the theory of co-existing felsic and mafic lavas derived from depth, which were mixed to form rocks of intermediate composition. Such finding was a major boost to magmatic petrologists and settled the granite controversy in favour of plutonists. Science of granites scaled one step up with the debate on the origin of K- feldspar bearing granitic lits of gneisses formed from granite mass or from countryrocks, which led to introduce the concept of anatexis and migmatization, and then genetic series of granites in the context of orogenic time viz. syn-kinematic, latekinematic and post-kinematic groups of granites were developed. H. H. Read (1957) with regard to existence of granite series put forwarded a theory that not only were there granites and granites but all were genetically related in time and place (Fig. 1.1) Place Origin Intrusive granites Para-autochthonous Autochthonous granites, migmatites, metamorphites Plutons Times Figure 1.1 The granite series according to H. H. Read (1956) (after Pitcher, 1993). With the advent of rock thin section technique, concept of petrographic province was introduced based on the fact that there exists intimate connectivity between magma composition and geological event in space and time. Importance of eutectic in granite was realized in the petrographic world, which indeed explained the 2

3 origin of graphic granite, and developed a sustenance concept that most igneous rocks correspond to eutectic mixtures. At the end of 19 th century hypothesis of single differentiating magma was published. N. L. Bowen in 1910 based on experimental studies provided a widely accepted model that igneous rocks are formed from a common magma through some processes of differentiation. Several other magmatic processes such as liquid immiscibility, crystal settling, magma mixing and filter pressing (squeezed out from crystal + melt mush) were favoured by many workers. Frank Tuttle and N. L. Bowen in 1958 carried out bench mark experimental work in the system albite orthoclase - silica - H 2 O and threw light on the origin of granites. Series of water-present melting experiments on natural and synthetic materials have constrained the physico-chemical controlling factors on the genesis of granitic melts. Detailed field investigation demanded mechanical explanation for the granite magma ascent and emplacement from source to sink. This was later realized that structural counter between deformational domains and country-rocks provide clues to the emplacement issue. Diapirisim as a mechanism of granite melt ascent and transport was highly questioned and an alternative model involving dyking and network style of magma ascent was developed. Plate tectonic theory overpowered the conventional theory of granitization and gave a more holistic approach and direction to the origin of granites in diverse tectonic regimes. Rheological and mathematical formulations of fluid dynamics have explained movement of magma and crystal accumulation operating in deep magma chamber. Today s most fascinating aspects are mineralogical, geochemical and isotopic aspects of granites which made us realized that different types of granite are indeed images of their sources and likely tectonic environment acted upon them. Researches on granites are now directed to understand the nature of lithosphere and reworking (inheritance) of ancient and juvenile crustal materials in the genesis of granitic rocks and their tectonic environs. 1.2 Granite Types Granites (sensu lato) are commonly produced from melting of different crustal sources or formed as late felsic derivatives of mantle-derived magma. Field, 3

4 petrography, mineralogical, chemical and isotopic criterions have been commonly used for genetic and tectonic discrimination (e.g. Pitcher, 1993; Barbarin, 1999). Granites are usually bimodal recognized based on geological, geochemical and geophysical criteria, and collectively form a strong basis to delineating the petrographic and metallogenic provinces. Two-fold classification schemes of granites have been proposed, primarily based on alumina saturation index (ASI = molar Al 2 O 3 / CaO + Na 2 O + K 2 O) I-type (metaluminous, ASI < 1.05) and S-type (peraluminous, ASI > 1.05) (Chappell and White, 1974), granites formed by the melting of igneous or metaigneous sources and sedimentary protoliths respectively. Later more alphabets were used to describe genetic variety of granite types viz. hybrid (H-type, Castro et al., 1991), mixed (M-type, Didier, 1987), mantle (M-type, Pitcher, 1983), and anorogenic (A-type, Whalen, 1987) granites. Magnetic susceptibility (MS) values ( 10-3 SI) of granitoids were used to classify magnetite series (MS > SI unit) and ilmenite series (MS SI) granites (Kanaya and Ishihara, 1973; Ishihara, 1977; Takagi, 2004) corresponding to oxidized-type and reduced-type granites respectively (Takagi and Tsukimura, 1997). In principle, magnetite series and ilmenite series granites should correspond to I-type and S-type granites respectively but in nature S-type magnetite series and I-type ilmenite series granites also occur (e.g. Takahasi et al., 1980), which most likely represent a modified composition because of assimilation enroute, magma mixing, and/or later tectonic processes acted upon them significantly affecting their original magnetic property (e.g. Takagi, 2004; Kumar et al., 2006; Singh and Kumar, 2005; Kumar and Singh, 2008, Kumar and Pathak, 2009). MS values combined with whole rock geochemical features of anorogenic (A-type) granites have been used to make further distinction between oxidized A-type magnetite series and reduced A-type ilmenite series granites (Dall Agnol et al., 2007). 1.3 Spatial and Temporal Distribution and Magmeto-tectonothermal Evolution of Granites in India The Indian subcontinent is characterized by spatial distribution of felsic magmatic bodies from south to north which are temporally varying from Archaean to Tertiary, related with varied tectonic settings, where Indian subcontinent thought to have played a role in a number of supercontinental cycles including (from oldest to youngest) Ur, Columbia, Rodinia, Gondwana and Pangea (Meert et al., 2010). The 4

5 present day nature of Indian plate is the result of various orogenic events it faced and got evolved in space and time. Broadly the granitic magmatism in India can be grouped in two time period i.e. Precambrian magmatism (Archaen to neoprpterozoic) and younger intrusions, where the Precambrian magmatism is characterized by the period related with craton formation and their stabilization (> 2500 Ma) and other are post stabilization magmatic events (<2500 ma) triggered by younger orogenesis. Physiographically India is divided into three parts, viz. peninsula which is collectively formed of Aravalli Bundelkhand Cratons, Singhbhum and Bastar Cratons, the Western and Eastern Dharwar Cratons and the Southern Granulite Province; extra peninsular, characterized by mighty Himalaya, which is the youngest mountain chain comprised of reworked Precambrian crust and younger intrusives; and recent alluvial-filled Indogangatic plain. Table 1.1 gives a comprehensive detail of Precambrian granite magmatism in India mostly extracted from Meert et al (2010) and updated with ages of Himalayan granitoids and some more plutons like Mikir hills and Shillong plateau of northeastern India, and intrusive granite in central Indian tectonic zone (CITZ) from Kumar et al (2011) and Kumar et al, (unpublished data). The central and southern part of the India is characterized by cratonic regions which are amalgamation of Archaen-Neoarchaen (~ Ma) proto-continents (Naqvi, et al 1974, Meert et al., 2010) mostly formed and stabilized by subduction related calc-alkaline plutons. The north Indian shield regions are comprised of Bundelkhand - Aravalli cratons, which are considered extending further north beneath the present Gangatic plain and Himalayan mountain belt (Ghose, 2000; Sharma, 1998). The granitic magmatism in the Bundelkhand and western India spans between 2.5 and 2.2 Ga (Mondal et al Kumar et al., 2011) of subduction related calc-alkaline affinity. The northern peripheral part of Bundelkhand-Aravalli craton has however witnessed the magmatic events Ma (Kaur et al., 2007) and Ma (Biju Sekhar et al., 2003) occurring as granitic intrusives are found in northern part of Delhi Fold Belt. The Palaeoproterozoic granitic magmatism may be the result of tectonothermal perturbation in northern craton after stabilization of cratonic nucleuses of India at ~ 2500 Ma, which can be correlated well with the pulses of metamorphism (between 1725 and 1621 Ma; Roy et al., 2005) in Aravalli craton at the onset of Delhi Orogenic Cycle with granulites of Sandmata complex (1720 Ma; Buick et al., 2006). 5

6 The northeastern Precambrian shield of India is represented by Shillong pleateau comprised of Pan-African granites plutons, which induced the Paleoproterozoic basement gneissic rocks. The Mikir Hills of Nagaland are made up of foliated granites, granite gneiss and undeformed granites which are ranging in age from 1700 Ma to 1100 Ma (Table 1.1). Table 1.1 Precambrian granite magmatism in India. S.No. Craton / Pluton /Terrain Metamorphic events Igneous events 1 Aravalli ~2.0 Ga (t) Ga (t) Ma (g) Ma (g, d) Ma (s) Ma (t) 2 Bundelkhand 3.3 Ga (t) 2,7 Ga (t) 2.5 Ga (t) 2.15 Ga (d) 2.00 Ga (d) 1.1 Ga (k) 2.55 Ga (g) * 2.55 Ga (v) * 3 Singhbhum 3.3 Ga (g) 3.1 Ga (g) 3.5 Ga (v) 2.1 Ga (d) 1.5 Ga (d) 1.1 Ga (d) 900 Ma (v) 4 Bastar 2.5 Ga (t) 2.3 Ga (t) 2.5 Ga (v) 1.9 Ga (d) 1.1 Ga (t) 5 E. Dharwar Ga (t) 2.5 Ga (g) 2.4 Ga (d) 2.2 Ga (d) 1.9 Ga (d) 1.2 Ga (d) 1.1 Ga (k, l) 1.0 Ga (d) 6 W. Dharwar 3.3 Ga (t) 3.1 Ga (t) 3.35 Ga (v) 2.6 Ga (g) 2.5 Ga (g) 7 S. Granulite ~2.5 Ga (t) 500 Ma (g) Ma (t, s) Ma (t, s) 8 W. Himalaya 8.5 Ma (t) 1.85 Ga (g) 1 1. Kumaun and Gharwal thrust sheet 11.5 Ma (t) 2. Wangtu granite gneiss 2.06 Ga (g) 2 9 C. Himalaya 1. Ulleri Gneiss 1.83 Ga (g) 3 10 E. Himalaya 1. Lingtse gneiss, Sikkim 1.79 Ga (g) 4 2 Daling granite gneiss, Bhutan 1.76 Ga (g) 5 3. Bomdila granite gneiss, W Arunachal Lesser Himalaya 1.74 Ga (g) # 4. Salari granite, W. Arunachal Lesser Himalaya 1.74 Ga (g) # 5. Tawang Gneiss, W. Arunachal Higher Himalaya 500 Ma (g) # 6. Hbl-Bt granite gneiss, W. Arunachal Higher Himalaya 500 Ma (g) # 7. Leucogranite, W. Arunachal Higher Himalaya 23 Ma (g) # 11 Shillong Pleteau 1. Plutons 2. Basement gneiss 1600 Ma (g) 1100 Ma (g) 12 Mikir Hills, Nagaland 500 Ma (g) Abbrevations: (t) = tectonothermal; (s) shear; (d) dyke intrusion (k) kimberlite/lamproite intrusion (g) granitic intrusion and; (v) volcanism. * Kumar et al., 2011; # Kumar et al., (unpublished data), 1- Célérrier et al., 2009; 2.- Singh et al., 2005; 3- DeCellas et al., 2001 ; 4- Paul et al., 1982 ; 5 - Daniel et al.,

7 The accreted terrenes forming the Himalayan belt lying between the Main Boundary Thrust (MBT) in the south and the Indus Tsangpo Suture Zone (ITSZ) in the north encompass mostly reactivated / deformed Precambrian crust representing the peripheral part of the north Indian craton. Table 1.1 shows the geochronological details of vast stretch of granite-gneiss thrust sheets through out the length and breadth of the Lesser Himalaya representing the granitic magmatic activity during Ma, which were later intruded by the granitic bodies related to Pan African thermal event. The granodiorite to granitic bodies (500±25 Ma) are very common lithounit in the inner segments of Lesser and Higher Himalaya. The tourmaline-bearing two-mica leucogranite (23-20 Ma) of Tertiary age in the form of intrusive dykes and sill occupies the Higher Himalaya formed by collision-related igneous activities during Himalayan orogeny. 1.4 Researches on western Arunachal Himalaya Eastern Himalayan range mostly occupying the Arunachal Pradesh in the northeastern region of India is geologically least studied segments as compared to western and central parts of Himalayan belt. The preliminary geological investigations of eastern Himalaya were carried out more than a century ago by Godwin-Auston (1875) around Dafla hills of western Arunachal Pradesh. In late 19 th and early 20 th centuries the Geological Survey of India (LaTouche, 1886; MacLaren, 1904; Brown, 1912) paid attention to the northeastern region of India and carried out the geological investigations mostly around upper Assam and major river valleys of Arunachal Pradesh. However, this geologically important region remained unexplored till mid sixties of last century. The last years were crucial for Himalayan geology in which intensive research work had been carried out covering the geological, geodynamic and petrogenetic issues of Himalayan orogenic mountain belt by various group of scientists from all over the world. However, the inaccessibility and lack of proper communication in northeastern region were major set-back to geological studies of eastern Himalayan terrain. During last four decades, a series of systematic geological studies was carried out in Arunachal Pradesh covering the aspects of general stratigraphy, field relation and tectonic evolution of the northeastern Himalaya, (Jain et al., 1974; Thakur and Jain, 1974; Verma and Tandon 1976; Thakur, 1986; Singh and Chowdhary, 1990; Bhushan, 7

8 1991; Acharyya, 1994, 2007; Kumar, 1997; Bhattacharjee and Nandy, 2007 and Kesari, 2010) and correlated the output with equivalent lithounits and tectonic framework of Himalaya. Bakliwal and Das ( ) brought out the stratigraphic sequences of the area between Bhalukpong and Sela pass, and divided the Precambrian rocks of the area into Sela Group of Higher Himalaya occurring north of the Main Central Thrust (MCT) and south of it Bomdila Group of Lesser Himalaya. Verma and Tandon (1976) described Tenga Formation within the Bomdila Group, and modified the earlier concept of Bomdila Gneiss as basement of the metasedimentary sequences. Later Jain et al. (1979) argued that the Bomdila Gneiss is the basement for Dirang schist. Kaura and Roy (1982) divided the rocks into two major group viz. Bomdila and Tenga Groups, the former divisible into Bomdila Gneiss and Dirang Formation and the later into Tenga, Miri and Buxa Formations. Tripathi et al (1982) considered Bomdila Group as part of Early Palaeozoic and Precambrian whereas Tenga Group has been considered part of Early Palaeozoic. Bhusan et al. (1991) classified Bomdila Group into three Formations, viz. Lower Tenga, Middle Dedza and Upper Dirang Formations. Granitic gneiss constitutes the Bomdila Formation as older felsic magmatic lithounits whereas and tourmaline-bearing leucogranite is related to Himalayan magmatic activity. Kumar and Singh (1992) and Kumar (1997) found that the Proterozoic rocks occupy the major part of the Arunachal Himalaya. They grouped them into Supersequence-I, comprising the Sela Group assigned to Early Paleoproterozoic, Supersequence-II, constituting the Bomdila Group of Middle to Late Paleoproterozoic, and Supersequence-III made up of Dirang and Lum La Formations possibly of Mesoproterozoic age. The magnetic susceptibility (MS) mapping of the granitoids has been carried out in parts of west Kameng and Tawang districts in western Arunachal Himalaya by Kumar and Pathak (2009) where five different types of granitic bodies viz. ms-bt Tawang granite gneiss (TGN), hbl-bt granite gneiss (HBGGn), leucogranite (TLG), ms-bt Bomdila granite gneiss (GGn), and hbl-bt Salari granite (HBG) have been identified based on filed relation, modal mineralogy and magnetic susceptibility. First three are part of Higher Himalaya and the last two are the intrusives of Lesser Himalaya. The structure, geochronology and tectonics of the western Arunachal Himalaya has been discussed in Yin et al (2006, 2010) where the Lesser Himalayan Palaeoproterozoic metasediments along with the intrusive granite gneiss (1743±4 Ma) 8

9 have been considered as basement involved thin skinned multiple thrust sheets related to Himalayan orogeny. Dikhshitulu et al (1995) identified GGn and HBG as Palaeoproterozoic and Mesoproterozoic igneous intrusives of Bondila and Salari Groups respectively derived from crustal protolith based on whole-rock Rb-Sr isotopic study. Kumar and Pathak (2008) gave a brief account of the geochemistry of all five granitic bodies in western Arunachal Himlaya and found the enigmatic nature of HBG in the present tectonic setting. Rashid and Islam (2009) and Singh (2010) have studied the geochemical aspect of the GGn and HBG of Kameng district, and classified them as peraluminous and metaluminous type shallow-level granitoids respectively. Srivastava et al., (2009) reported mafic dykes and plugs into Se La Group in rocks of Higher Himalaya. Mineral chemical and physical conditions of granite magmatism are scarce (e.g. Kumar and Pathak, 2010 and Singh, 2010). Based on biotite chemistry Goswami et al (2009) discussed existence of inverted metamorphism in the study area. Singh (2012) recently reported occurrence of hornfelsic textures in the granitoids of Ziro valley as a signature of pre-himalayan contact metamorphism. 1.5 Aim and Scope of the Present Study The following objectives are framed to achieve systematically through field and laboratory investigations of felsic magmatic rocks from the Kameng district of western Arunachal Himalaya as a part of research works leading to Ph.D. degree. To establish the field relationships between the granitoids and associated lithounits, and to document the geological, lithological, textural and petrological variations in the granitoids of study area, and to sample representative rocktypes for further laboratory treatment. To record magnetic susceptibility values ( 10-3 SI units) and further to evaluate redox nature of granitoids of study area. To document the petrographic features of genetic significance and to depict paragenetic sequence of granitoids. To recognize the igneous series of granitoids based on nomenclature of individual rock as per IUGS recommended parameters. To recognize the mineral-chemical evolutionary path and nature of parental melts, and further to estimate the intensive variables (P, T, fo 2 ) of granite melt evolution. 9

10 To infer the magmatic processes and probable tectonic environment in the evolution of granitoid bodies, and to assess the protolith involved in the generation of granitoid melts To propose a viable petrogenetic and tectonic setup suitable in Kameng district for the origin of granitoids of western Arunachal Himalaya. 10