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Correct Nomenclature for the Angadimogar Pluton, Kerala, Southwestern India

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Correct Nomenclature for the Angadimogar Pluton, Kerala, Southwestern India Correct nomenclature for the Angadimogar pluton, Kerala, southwestern India HMRajesh Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japan. e-mail: [email protected] The proper usage of modal composition and geochemical classification of granitoids is discussed for assigning a proper nomenclature for the Angadimogar pluton, Kerala, southwestern India. This discussion is mainly aimed at addressing questions concerning the nomenclature of Angadimogar pluton (syenite vs. granite). Modal composition and whole-rock XRD data clearly show that the pluton exposed near Angadimogar is a quartz-syenite and its geochemistry is typical of a ferroan, metaluminous, alkali (A-type) granitoid. 1. Introduction suggest that these bodies are granites based on their normative compositions. They apparently Classification of granitoid rocks involving various question the nomenclature of Angadimogar plu- criteria has long been a subject of frequent debate ton (syenite vs. granite) used by Santosh and Nair and voluminous literature. It can be a confusing (1986). Rajesh (1999) carried out a detailed field, exercise involving the minerals present and their petrographic, geochemical, and isotopic character- relative proportion in the rock (depending largely ization of the Angadimogar pluton. The present on the chemical composition of the magma), the paper is based on this work and intends to demon- texture of the rock (depending largely on the cool- strate that the pluton exposed near Angadimogar ing history of the magma), the colour of the rock is a quartz-syenite. (depending largely on the minerals present and on their grain size), and the chemical composi- tion of the rock. Granitoid classification schemes 2. Study area and field relations have evolved from being genetic (e.g., Chappell and White 1974) and/or tectonic (e.g., Pearce et al The Neoproterozoic granulite-facies terrain of 1984) in nature to non-genetic and non-tectonic southwestern India is intruded by a suite of high-K in nature (e.g., Frost et al 2001). Controversies calc-alkalic plutons (metaluminous to slightly per- surrounding the classification of granitoid rocks aluminous biotite granites and hornblende-biotite are usually due to the usage of different schemes granites) with subsequent widespread alkaline of classification by different workers. Such contro- (A-type) plutons (including quartz syenites) and versies rarely arise from the quality of geochemi- minor garnet-bearing leucogranites (Rajesh 1999, cal data used. This study however, illustrates an 2000, 2003, 2004). These granitoids, emplaced example of misclassification of a granitoid rock between ∼ 640 and 500 Ma (figure 1a), were due to the incorrect application of poor-quality generated in environments of high heat flow geochemical data. and volatile activity, correlative with an exten- Anil Kumar et al (2005) reported geochem- sional tectonic regime (Rajesh 1999, 2003, 2004). ical data from two plutons (Angadimogar and The pluton exposed near Angadimogar (fig- Kumbdaje) from Kerala, southwestern India, and ure 1a), northern Kerala, belongs to this suite, Keywords. Nomenclature; modal composition; whole-rock XRD; geochemistry; quartz-syenite; A-type granitoid. J. Earth Syst. Sci. 115, No. 2, April 2006, pp. 239–248 © Printed in India. 239 240 HMRajesh Figure 1. (a) Generalized geologic map of Kerala, southwestern India, showing the approximate location of granitoids and anorthositic gabbro mentioned in the text. Available ages of granitoids are also given. Inset is a shaded relief image of southern India showing the states of Kerala and Tamil Nadu. (b) Photograph illustrating the pink syenite exposed near Angadimogar, northern Kerala. (c) Modal composition (this study) and normative compositions (reproduced from Anil Kumar et al 2005) of the Angadimogar samples in a QAP plot. The three components, Q (quartz), A (alkali (Na–K) feldspar), and P (plagioclase), were recalculated from the modal amounts to sum to 100 per cent (as recommended by IUGS). The plagioclase ratio (100 × P/(A + P )) is also useful to name the rock, as the non-horizontal divisions in the QAP diagram are lines of constant plagioclase ratio. and is a quartz-syenite composed dominantly minor quartz, to a medium- to coarse-grained of medium- to coarse-grained K-feldspar, with rock rich in pink K-feldspar, with fine-grained minor amounts of quartz, plagioclase and uni- hornblende, to a medium-grained rock rich in formly distributed ferromagnesian minerals (fig- grey feldspar, with finer grained hornblende. The ure 1b). Greenish amphibole constitutes the syenite is locally cut by thin (< 2 cm) amphibole- principal mafic constituent with minor biotite. bearing veinlets. Pegmatites and aplites cut across This is in contrast to other syenites in south- the syenite exposures and attest to the presence of western India, where pyroxene constitutes the late-stage fluids. main mafic mineral. The syenitic body shows The syenite shows an intrusive relationship with variations from a coarse-grained rock rich in the surrounding light coloured, medium- to coarse- pink feldspar, with large hornblende grains and grained (sometimes migmatitic) hornblende-biotite Correct nomenclature for Angadimogar pluton 241 gneiss/biotite-hornblende gneiss, often carrying Rajesh 2005), with a dominant peak for feldspar boudins and bands (concordant as well as dis- (figure 2). The syenite shows a general allotrio- cordant) of amphibolites. These ortho-gneisses morphic texture and consists predominantly of range in composition from alkali-feldspar-granite perthitic orthoclase (68–75%), normally zoned pla- with less than 6% modal biotite to tonalite with gioclase (6–9%), and a variety of mafic minerals: up to 10% biotite. Hornblende-rich granitoid is amphibole (potassian-ferro edenitic hornblende to much more common than biotite-rich granitoid. ferro edenite to ferro actinolite (in the nomencla- Dark greenish-grey coloured, medium grained, ture of Giret et al 1980)), clinopyroxene (diop- orthopyroxene-bearing quartzo-feldspathic charno- side (in the nomenclature of Morimoto 1988)), ckite is exposed in the northwestern and southeast- and biotite (annite) (Rajesh 1999). In some sam- ern parts of the study area. This rock also contains ples acmite was found. Quartz occurs as intersti- hornblende and biotite, with occasional garnet. tial grains. Magnetite is the main opaque phase and is generally intergrown with ilmenite. Other accessory phases include zircon, allanite, apatite, 3. Petrography titanite, calcite, and epidote. For plutonic rocks with less than 90% (by volume) mafic (dark) minerals, the rock name is determined 4. Geochemistry from the relative proportions of quartz, alkali feldspars, plagioclase feldspars, and felspathoids Santosh and Nair (1986), Rajesh (1999) and (nepheline, leucite, etc.). In any classification Anil Kumar et al (2005) presented geochemical scheme, boundaries between classes are set arbi- data on the Angadimogar pluton. Representative trarily; however, if the boundaries can be placed major element data from these studies are given in close to natural divisions or gaps between classes, table 1. The syenite has high K2O (up to 6 wt%). they will seem less random and subjective, and In terms of aluminium saturation index (ASI; such standards can facilitate universal understand- Shand 1943) (which is better than A/CNK (molar ing. The most commonly used scheme was devised Al2O3/(CaO + Na2O+K2O)) in defining peralka- by the International Union of Geological Sciences line/metaluminous/peraluminous compositions, as (IUGS) and is based on Streckeisen (1976) (fig- it takes into account the presence of apatite, and ure 1c). IUGS recommends that if the mineral is expressed as the molecular ratio of Al, Ca, P, Na mode cannot be determined (or difficult to deter- and K), Angadimogar syenite samples are clearly mine) as is often the case for volcanic rocks, then a metaluminous (ASI < 1 and molecular Na + K < chemical classification (e.g., total alkali versus sil- molecular Al) similar to quartz-syenites worldwide ica diagram of Le Bas et al (1986)) is better. More and different from alkali-feldspar-syenites which discussions on this topic can be found in Le Bas are mostly peralkaline (e.g., Eby 1990; N´ed´elec et al and Streckeisen (1991) and Le Maitre et al (2002). 1995; Rajesh 2003). In such rocks (where ASI < 1 Modal analyses (obtained by point count- and molecular Na + K < molecular Al) there is ing) of representative samples from the Angadi- likely to be excess Ca after aluminium has been mogar pluton plotted on a Streckeisen (1976) accommodated in the feldspars. As a result, meta- QAP plot indicates that they range in composi- luminous rocks contain calcic phases such as horn- tion from quartz-syenite to quartz-alkali-feldspar- blende and augite but lack either muscovite or syenite (figure 1c). Whole rock XRD analyses were sodic ferromagnesian phases. This clearly accounts carried out as part of this study on powdered for the mineralogy of the Angadimogar syenite. rock samples from six representative plutons (three On the other hand, if ASI > 1, then the granites, one anorthositic gabbro, and two syen- rock is corundum-normative and peraluminous. ites, including the Angadimogar pluton) across This means that the rock has more Al than southwestern India, using a Philips PW3020 model can be accommodated in feldspars and that it X-ray diffractometer (housed at Fukuoka Univer- must have another aluminium phase present. For sity of Education, Japan), using
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