CHAPTER 1 GRANITOIDS - A REVIEW

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1 CHAPTER 1 GRANITOIDS - A REVIEW

2 CHAPTER 1 GRANITOIDS - A REVIEW Granitoid rocks constitute a significant part of the continental areas especially the Precambrian shields and many younger orogenic belts. They occur both in continents and oceanic areas as well, in different geologic period and tectonic milieu, although with different abundances. Hence, the study of granitoid rocks of different geologic ages occurring in different tectonic milieu is essential for understanding the continental growth through the geological past. Granites and the related rocks, hence potential to contribute to our understanding of the crustal growth trends and development processes, hence attracted the attention of a large number of investigators since the beginning of this century. This has resulted in voluminous publications on granitoid rocks -like classification, origin, their tectonic implications, mineralization potential, implications on crustal evolution trends etc. 1.1 HISTORICAL REVIEW The historical review on the study of granitoid rocks can be see in the works of Didier and Barbarin (1991), Clarke (1992), Atherton (1993) and Pitcher (1997). The early history of study of granitic rocks began during the later half of 18th century in the backdrop of 'Neptunian' theory that, granite was chemical precipitate formed in water. This theory was later gradually replaced by the 'Plutonian'

3 2 concept (that the granites crystallized from subterranean lava with great slowness) through the nineteenth century into the twentieth centuiy. By then another controversy started taking roots leading to a protracted debate between the 'transformationists' and magmatists. The initial ideas on the granitisation (transformation of preexisting solid rock by permeating granitising material) started during the early 19th century and took a shape during the late nineteenth century and early twentieth centuiy- Tectonism, metamorphism, transformation (metasomatism), anatexis and even bulk mobilization and assimilation were seen to be interrelated by the early workers. Johannes Sederholm followed by Pentti, Eskola and Cesare Eugen Wegmenn in the beginning of the present century advocated high-grade metamorphism and anatexis as the main processes involved in the formation of granite. Sederholm advocated lit-par-lit injections and transformation; Eskola emphasized the importance of partial melting and Wegmann envisaged regional metamorphism due to migration of elements through the crust in the form of waves or fronts. The concept of granitization or metasomatic replacement was popularized by Read (1957). Among the metasomatists, two schools of thoughts developed, one envisaging transformation of any rock into granite through displacement of elements by large scale ionic diffusion (dry metasomatism) and the other favouring a major role for fluids during transformation (wet metasomatism).

4 3 Right from the Lyell s time, the magmatists continued to believe that granites resulted from crystallisation of magma, which existed below the surface of the earth. Magmatists were also divided into two groups; one led earlier by Rosenbusch and later by Bowen (1958) advocating the existence of a primordial basaltic magma which on extreme fractional crystallisation produced granitic magma. The other opinion initiated by Bowen (1958) and later expanded by Reginald Daly, envisages the existence of two separate primary magmas viz., basaltic (basic) and granitic (acidic). During the later period, the advances made in the physical sciences and their increasing influence led to the modem studies in geology since the early part of the present century. 1.2 MODERN APPROACH TO THE STUDY OF GRANITIC ROCKS The experimental petrologists mainly led by Bowen (1958) put forth arguments against granitization, and favored magmatic origin that the granites crystallized from melt. With the advent of new techniques in conducting melting experiments, the contributions in the field of experimental petrology during 1950"s and 60s brought about a sea change in the development of concepts on the origin of granites. The classical work of Tuttle and Bowen (1958) on the haplogranite system was a significant contribution in this direction. It was followed by many other works notable among them are that of Luth et al. (1964), Luth and Tuttle (1969) and James and Hamilton (1969). These experimental results were supported by the studies on the trace element distribution in relation to the major element chemistry of

5 4 granites and some of their minerals. The fundamental contributions among them are by Marker (see Pitcher, 1997 for review) on major element distribution and Nockolds and Allen (1953) on major and trace elements, which were followed by other workers. Parallel to the above developments, the common observation that high-grade metamorphic rocks occur in close spatial association with granites led to the development of the concept of migmatization or partial melting and anatexis. It was supported by the melting experiments on metamorphic rocks (Winkler, 1965; Wyllie, 1977; Winkler, 1979; Wyllie, 1983). This concept also addressed the difficulty in envisaging the formation of large volumes of granitic magma by fractional crystallization of basaltic parent. The experimental work and studies in geochemistry which gave strong support to the magmatic theory were unmatched by similar studies in support of the metasomatic transformation. The other main and difficult question that went against the theory of magmatic intrusion - i.e., the room problem has successfully been addressed by the modem structural studies, beginning with the monumental work of Cloos and Balk (see Pitcher, 1997 for review) on granite tectonics. The recognition and study of the primaiy magmatic structures such as flow foliation and lineation and their interrelationship with the secondary structures in the surrounding country rocks enabled a better understanding of the interplay of the granite intrusion and regional tectonics (Buddington.1959; Viljoen and Viljoen, 1969; Clliford, 1972; Fyson, 1975, Brun and Pons, 1981;

6 5 Bateman, 1984; Hollister and Crawford, 1986; Mutton, 1988; Brun et al., 1991; Patterson and Vemnon, 1995). Thus, the metasomatic theory has taken a back seat (Clarke, 1992; Pitcher, 1997). In addition, there have also been attempts to link the plate tectonics, the source rock with the type of granite magma generated, taking advantage of the advancement in trace element and isotope geochemistry during and after 1970s (Chappel and White, 1974; White and Chappel, 1977; Pitcher, 1983; Pearce et al., 1984; Harris et al., 1986; Whalen et al., 1987; Maniar and Piccoli.1989; Barbarin, 1990; Pitcner, 1997; Moghazi et al., 1998). The geochemical and isotope studies in conjunction with the fundamental field and mineralogical observations have currently become the mainstay in understanding the origin of granitic magma its source, the processes of magma generation and evolution, crystallization history, physical nature, emplacement mechanism and tectonic environment. The present day approach is of interdisciplinary nature combining the geological, geochemical, and geochronological studies in understanding the role of crustal rock 'granite' in the global tectonics through geological time. The ultimate aim of all these studies is to relate granite formation and emplacement with metallogeny. 1.3 NOMENCLATURES AND CLASSIFICATION The term granite is a latinized version of the ancient Welsh expression, gwenith faen meaning a grinding stone for making wheat flour (Pitcher, 1997). For a layman it means any hard granular rock,

7 6 which is used as -building stone. During the early years of geological studies, the term was used to include all the quartz-feldspar bearing igneous and "igneous looking" rocks. Later on, it also acquired a more restricted meaning applied to rocks with sub -equal amounts of quartz, plagioclase feldspar and potash feldspar. Other names such as adamellite, granodiorite, tonalite, and quartz monzonite have been used with different meanings. Streckeisen (1976) as Chairman of the IUGS Sub-commission on the Systematics of Igneous rocks had streamlined the terminology and recommended the classification and nomenclature of granites and related rocks which find wide acceptance. Adopting the terminology strictly based on the modal mineral proportions of quartz (Q), alkaii feldspar (A) and plagioclase feldspar (P), the quartz rich plutonic felsic igneous rocks (>20% quartz) have been classified into tonalite. granodiorite, monzogranite, syenogranite and alkali feldspar granite with increasing ratio between alkali feldspar and plagioclase feldspar. Other related rocks on QAP diagram are diorite, monzodiorite, monzonite, syenite and alkali feldspar syenite, corresponding to the above five types of rocks in that order but with less than 5% quartz. Similarly, there are five other types of rocks transitional between the above two series and containing between 5% and 20% quartz. They are named as quartz diorite, quartz monzodiorite, quartz monzonite, quartz syenite and quartz alkali feldspar syenite with increasing alkali feldspar content. When the mafic minerals are present in considerable amount (>10%), such names as hornblende tonalite, biotite

8 7 granodiorite etc. have been recommended. The terminology based on the QAP proportion is simple to follow and forms a fundamental and descriptive terminology without going into the genetic aspects of granites and related rocks (Pitcher, 1997). Other classifications have also been proposed which are based on modal mineralogy and chemical parameters, separately or in combination. Using Streckeisen s modal diagram, the granitic rocks are categorized into 9 rock series, which include tholeiilte, cald-alkaline trondhjemite, calc-alkaline granodiorite, monzonitic and alkaline series (Lameyre and Bowden, 1982). There are quite a few classifications based entirely on chemistry of granites. Shand (1947) used alumina saturation expressed as the ratio of molar AI2O3 to the molar Ca0+Na20+K20 or A/CNK. Rocks with A/CNK more than one are peraluminous and those with less than one, mataluminous. When A Is less than N + K, they are peralkahne. Debon and Le Forte (1982) based their classification on the cations and proposed a chemicalmineralogical categorization of rocks. O'Connor (1965), Barker (1979). and Streckeisen and Le Maitre (1979) have used the CIPW norm based classification of granites. The norm-based classifications adopt the QAP names while the cation based one uses both the QAP names and the rock (magma) series names. There are other classification schemes, which either denote the tectonic setting or suggest the type of source rock. The granite types indicating tectonic setting include those using the criteria of chemical composition (Pearce et al,, 1984; Whalen et al., 1987; Rogers and

9 8 Greenberg, 1990) or chemistry in combination with modal mineralogy (Maniar and Piccoli, 1989). They categories the granites into orogenic and anorogenic types and their sub-types belonging to different plate tectonic settings. Classification suggestive of the type and nature of source rock uses the terms I, S and M - type granite, corresponding to infracrustal igneous source, supracrustal sedimentary source and the mantle source respectively (Chappel and White, 1974; White, 1979; Pitcher, 1982; White and Chappel, 1983; Chappel and Stephens, 1988; White and Chappel, 1988; Chappel and White, 1992). Another popular category is the A-type granite, which has the characters similar to the other three in one way or other and have variable source rocks but has a definite tectonic significance. A-type granite indicates anorogenic granitoid found in the stable cratons and rift zones (Loislle and Wones, 1979; Collins et al., 1982; Creaser et al.,1991). However,, the apparently simple, alphabetical types of granites are noc necessarily well defined and the pigeon-holing the granites into these categories is not always possible as the transitional types with overlapping mineralogical and chemical characteristics are found to occur in different tectonic environments (Clarke, 1992; Pitcher, 1997). Despite the uncertainties such categorization is still adopted while studying the petrological evolution of different granites (e.g. Landenberger and Collins, 1996: Petford and Atherton, 1996; Whalen et al., 1996; King et al., 1997; Mass et al., 1997; Shannon et al., 1997). Some genetic classifications have also been proposed integrating the tectonic setting and the source characteristics, taking care of the

10 9 mixed and transitional nature of the granitic magma (Barbarin, 1990; Pitcher, 1997). As the objective of this study is to understand the petrogenesis, their tectonic setting and source characteristics, the study is focused on the gecochemical front supported by mineralogical and field setting. Therefore normative based classification is adopted.

Classification and Origin of Granites. A Multi-faceted Question

Classification and Origin of Granites A Multi-faceted Question What is a granite? IUGS classification Based on Modal Mineralogy Plutonic rock with less than 90% mafic minerals Alkali Granite Granite Quartz