Geochemistry of Late Triassic Weak Peraluminous A-Type Karimun Granite, Karimun Regency, Riau Islands Province

DOI: 10.17014/ijog.4.1.21-37Karimun is an island with various geological deposits: tin, granite, sand, and others. The tin deposit in Karimun is related to the granitoid tin belt which extends from Myanmar to western Indonesia. Late Triassic Karimun Granite is composed of quartz, K-feldspar, plagioclase, biotite, and/or muscovite with small amounts of accessory minerals. The granitoid unit is different with other felsic intrusive rocks in Malay Peninsular because of its A-type affinity although it is classified as part of the Tin Islands. All eight samples can be classified as altered rocks since the occurrence of secondary chlorite was identifed both macroscopically and microscopically. Petrography was used to describe the minerals that form the samples, whereas XRF and ICP-MS were used to study Karimun Granite from the geochemical point of view. Harker diagrams confirm that the granites show similarity to other granite units in Malaysia except for CaO. Whalen diagrams indicate the granite as A-type as well as the SiO2, REE, and LILE amounts. REE content in the weak peraluminous granitoid ranges from 183 to 3,296 ppm with Eu and Ce show negative anomalies in the REE spider diagram. Negative anomalies of Eu, Ba, Sr, P, and Ti in normalized spider plot also conclude that the studied granitoid indicates A-type.


Background
The continental collision between the Sibumasu and Indochina in Malay Peninsular in latest Permian to Late Triassic period produced wide-spread magmatic activity. This collision happened after a prolonged subduction of the Devonian-Triassic Palaeo-Tethys Ocean (Sone and Metcalfe, 2008;Faure et al., 2014). Both intrusive and extrusive rocks were emplaced before, after, and during the collision of the pa-leocontinents. The development of well known three parallel belts of Southeast Asia: the Eastern Belt, the Central Belt, and the Western Belt are related to the tectonic event. These three northsouth-trending zones were classified based on differences of stratigraphy, mineralization, and structure (Metcalfe, 2000). Researches were conducted to study about granitoids with relation to the tectonic pattern of Malay Peninsular. For example, granitic rocks from Endau Rompin (Ghani et al., 2013) and Muncung Granite (Irzon, 2015) were emplaced due to the collision of two I J O G blocks and are called as syn-collision granites. On the other hand, the main peak of post-collisional plutonism took place in 220-200 Ma, the period of stanniferous granites in Sumatra and Malay Peninsular were generated (Barber et al., 2005).
Karimun Regency of Riau Islands Province consists of three main islands: Karimun, Kundur, and Moro. Pulunggono and Cameron (1984) proposed that southwards extension of the Bentong-Raub Line traversed between the Kundur and Karimun Islands. Karimun Island is specific for containing many deposits such as tin, granite, sand, and others. The soils as the products of weathering mostly covers all the rocks, less fertile for cultivation. Karimun Granite, that is also known as Karimun Besar Granite, is considered as one of the best quality granitic rocks in the world for its content. PT. Karimun Granite is the largest company that runs the granite mining in the regency, and it began to be export the granite to Singapore in 1972 about 170,000 tons/month. Along with the end of Singkep mining, tin mining in the regency started to decline since 1991 (Diantoro, 2014). The tin deposit in Karimun is related to the granitoid tin belt which extends from Myanmar to Billiton Island. Cobbing et al. (1992) classified the Karimun Granite as part of the Main Range Granite Province, while Kurniawan (2014) considered it as part of the Eastern Granite Province.
This paper deals essentially with the geochemistry characteristic of the Karimun Granite: major, trace, and rare earth elements. The geochemical crarateristics are then compared to other granite intrusions in Peninsular Malaysia. The Karimun Granite has not been dated radiometrically, but Cameron et al. (1982) suggested a date of emplacement between Mid and Late Triassic (Carnian-Norian). Previous authors proposed that the Karimun Granit was an A-type granitoid which covered Malarco Formation by contact metamorphism (Cobbing et al., 1992).
The studied location is generally made up of hills as remnants of an erosion. The hill slopes of Karimun are normally steeper than 45% and are covered by a primary forest. The temperature range is 23.1º -33.2ºC with humidity between 60% -98% (Djunaedi et al., 2005). Tanjung Balai, located in Karimun Island, is the capital city of the Karimun Regency.
Karimun Island is included in the Geologic Map of Bengkalis Quadrangle (Cameron et al., 1982). Merah Complex outcrops in the southeastern area of Karimun Besar Island and principally consists of nonfoliated, variegated hornblende metagabbro. The mafic rock unit was probably emplaced in Early Devonian (Cameron et al., 1982).

Field Description
Totally, eight granitoid samples were collected for this study. The studied granitoids are generally composed of quartz, K-feldspar, plagioclase, biotite, and or muscovite with small amounts of accessory minerals. Note that no hornblende can be detected macroscopically. Sample from Meral (GKR 37) was taken from an outcrop forming a hill of granite with intergrowth fractures. GKR 39 is an oxidized rock in an active iron mining area as the colour is not as bright as other samples. Just nearby the outside the mining location, another light grey granitoid outcrop was identified (GKR 58). GKR 40 and GKR 42 are granitoid boulders from Lembah Permai, which are light grey in colour, holocrystalline-phaneritic of medium grain. A big size of greenish mineral, about 2 mm, in a granitoid outcrop was detected near the west coast of the island (GKR 41). Two medium grain granitoids (GKR 43 and GKR 55) located in Guntung Puna area showing oxidized layers, were taken from five meters high outcrops. Chlorite as an alteration mineral was clearly detected macroscopically in all these samples. Some field conditions are shown in Figure 2.

Analytical Method
Both petrography and geochemistry analyses were done in the geological laboratory of the Centre for Geological Survey in Bandung. The whole granitoid samples were dried at the room temperature for one day in minimum. Dried samples were then crushed with jaw crusher to gain the particle size of 200 mesh and were grounded using a ball mill. Pressed pellets were analyzed with the Advant XP X-ray fluorescence method (XRF) for the major oxides (SiO 2 , TiO 2 , Al 2 O 3 , Fe 2 O 3T , MnO, MgO, CaO, Na 2 O, K 2 O, P 2 O 5 , SO 3 ) and lost of ignition (LOI). Four certified reference material pellets: STSD 1, STSD 2, STSD 3, STSD 4, and LKSD 1-4 were measured using the same procedure as the samples and were used in data calibration.
For totally 23 trace and rare earth element analyses, 100 mg of sample powder were weighted carefully, then placed in the crucible. The samples were dissolved with three acids leach: using nitric acid (HNO 3 , ultrapure grade), formic acid (HF, ultrapure grade), and perchloric acid (HClO 4 , pro analysis grade) at 120 o C until all the acids were evaporated. Ultrapure grade water was used instead of normal boiled water for rinsing or downgrading acid concentration. Nitric acid was added to the digested samples and

I J O
G stored as a 'mother' solution. Few hours before ICP-MS measurement, an aliquot of the 'mother' solution was diluted with ultrapure water. GBW 7103, AGV 2, and GBW 07110 were the certified reference materials for ICP-MS analysis. Sample preparation, ICP-MS set up procedure, and certified reference evaluation are based on the study of Irzon and Permanadewi (2010).

Petrography
Megascopically, rock samples are generally light grey, holocrystalline, and medium grain granitoids. Under a microscope, the dominant minerals in decreasing abundance are quartz (22 -45%), K-feldspar (20 -45%), plagioclase (<25%), and biotite (<15%). Quartz crystals are anhedral, showing irregular cracks, and filling interstices among other minerals. The majority of alkali feldspar phenocrysts falls between orthoclase and sanidine series. The grain size of K-feldspars ranges from 1 to 4,3 mm. The euhedral and subhedral plagioclases may show oscillatory zoning and albite twinning as in GKR 40, GKR 43, and GKR 48. Muscovite occurs in minor amounts in GKR 39 (20%), whilst biotite as the only mafic mineral in selected samples is detected in the other seven samples. The green chlorite as biotite replace-

I J O
G ment is the most abundant alteration mineral (7 -20%). The petrographic analysis confirmed that the previous mentioned big green mineral in GKR 41 was as chlorite. The porosity of selected samples ranges from 1 to 5%. The petrographic analysis of representative samples from Karimun Granite is presented in Table 1. The modal data of petrographic analyses were then transformed into QAPF diagram of Streckeisen (1976) in Figure 3.
Half of the all selected samples was classified as alkali-feldspar-granite, three samples as syenogranite (GKR 40, GKR 43, and GKR 58), and one sample as monzo-granite (GKR 41). Microscopic pictures of some selected samples of the Karimun Granite are shown in Figure 4.

Major Oxides
Chemical analyses of representative samples from Karimun Granite are given in Table 2. As felsic plutonic rocks, SiO 2 content of the studied samples is high (70.56-73.39%) and their potassic character is indicated by K 2 O values which are greater than Na 2 O (K 2 O/Na 2 O ratios >1.3). In Harker variation diagrams, TiO 2 , Al 2 O 3 , MgO, P 2 O 5 , and Na 2 O show descending trends versus SiO 2 . On the other hand, K 2 O, CaO, and Fe 2 O 3T indicate positive tendencies with the silicon dioxide escalation ( Figure 5). Compared to other plutons in Malay Peninsula these trends show a similarity with Boundary Range Batholith (Ahmad et al., 2002), Endau Rompin Granite (Ghani et al., 2013), and Muncung Granite (Irzon, 2015) except for Fe 2 O 3T and CaO versus SiO 2 . Positive correlation of CaO to SiO 2 is unusual because the calcium oxides would decrease through magma differentiation. Plagioclase is the only Ca-bearing mineral of the selected samples. GKR 41, GKR 58, GKR 40, and GKR 43 are the four samples with plagioclase content of 25%, 15%, 13%, and 10% respectively. However, the geochemistry value does not reflect the previous petrographic data: GKR 40 (13% plagioclase) is the sample with highest CaO content (1.16%), whilst GKR 41 (25% plagioclase) only contains 0.69% of CaO. Moreover, the decrease of CaO with increasing silica in this study only shows a weak correlation coefficient with r<<0.9 (r = 0.63). After all, the calcium content in this study should be rechecked using different method to gain more calibrated data.
All rock samples have high alkaline (Na 2 O+K 2 O) contents ranging from 8.44 to 9.24%. Plots of Na 2 O+K 2 O versus SiO 2 which is known as total alkali silica (TAS) diagram (Middlemost, 1985) show that all samples lie within granite field (Figure 6a). TiO 2 , MnO, MgO, and P 2 O 5 are low (0.09 -0.11%, 0.004 -0.02%, 0.06 -0.27%, ad 0.01 -0.015%, respectively). Molar A/CNK ranges between 1.14 and 1.24      (Figure 6c). The 10,000×Ga/Al ratios in the Karimun Granite are 1.73 to 3.14, with the average value of 2.18. Although the average value is relatively below the global average of 3.75 for A-type granites (Whalen et al., 1987), the samples fall into the A-type granite field in 10,000×Ga/Al vs. Na 2 O+K 2 O, (Na 2 O+K 2 O)/CaO and K 2 O/MgO diagrams ( Figure  7). The diagrams agree with the previous author conclusions that A-type granites have also been identified in some of Tin Islands such as Karimun (Cobing et al., 1992;Barber et al., 2005). The Karimun Granite contains high SiO 2 (>68%) and total alkaline as another major oxide character of A-type granitoid (Christiansen and Keith, 1996;Kebede et al., 1999;Ahmad and Chaudhry, 2008).

Trace and rare earth elements
Fractional crystalization in magma can be observed based on trace and/or rare earth element ratios. Ba/Sr ratio ranges from 1.72 to 3.67 (2.89 in average) whereas Rb/Sr with significant variation from 5.85 to 13.28 (10.65 on the average). High Rb/Sr, Ba/Sr ratios, high K% suggest that the Karimun Granite is primarily derived from a   Figure 6. a) All granitoid samples from Karimun Island are classified as granite using TAS diagram of Middlemost (1985); b) A/NK vs. A/CNK plot, for the Karimun Granite (Shand, 1943); c) shoshonite character of studied samples (Peccerilo and Taylor, 1976).

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G felsic source (Ray et al., 2011). On Ba versus Sr diagram, the positive correlation between Ba and Sr ( Figure 5i) suggests that K-feldspar, biotite, and plagioclase are being removed in the granite differentiation (Ghani, 2005 (Table 3) (2015) consistent with plagioclase fractionation (Nagudi, 2003). Chemically, transition elements are described as elements in the d-block of the periodic table.

I J O G
The group is different from other elements because that in a given inner orbital, it has less than a full quota of electrons which cause the formation in many oxydation states (White, 2013). The amount of some transition elements: Sc, V, and Nb of the studied samples are categorized low at up to 1.4, 5.3 -10.1, and 0.7 -8.8 ppm respectively. Contrastively, other transition elements are high: Y (53-654 ppm), Th (29 -45 ppm), and U (11 -19 ppm). REEs are classified as transition elements. These samples contain moderate to high and variable REE concentration of 183 I J O G to 3,296 ppm, 632 ppm on the average ( Table  2). The high amount of REE exhibits the A-type granite character (Whalen et al., 1987;Goodge and Vervoot, 2006;Shellnutt and Zhou, 2007;Singh and Vallinayagam, 2012).
Primitive Mantle value of McDonough and Sun (1995) is used to normalized both trace and rare earth elements data of this study. The normalized REE patterns relatively show a stronger fractionation of light rare earth elements (LREE) (1.70<(La/Sm) N <2.77) than that of heavy rare earth elements (HREE) (0.75<(Ga/Yb) N <3.1) (Figure 8a). It has been observed that granitic rocks contain higher amounts of Th, U, and light rare earth elements (REEs) compared to other igneous rocks such as basalt and andesite (Sahoo et al., 2011) that can explain the Th and U amounts and REE pattern of these samples. Europium and cerium negative anomalies were detected: Eu/ Eu* of 0.034 to 0.068 and Ce/Ce* of 0.62 to 0.85. Both anomalies are used as indicators of magma evolution. The pronounced negative Eu anomaly may be correlated to plagioclase remaining in the source after partial melting generating the granite, where as cerium negative anomaly probably due to high oxygen fugacity at the source of the magma that cause some of trivalent ceriums were oxydized to tetravalent cerium (Salem et al., 2001;Jurvanen et al., 2005;Papangelakis and Moldoveanu, 2014). Compared to four granitoid units in Malay Peninsular, the average total REE value of Karimun Granite is just smaller than Lingga Granite (Figure 8b). The biggest negative Eu anomaly of the group is shown by Karimun Granite whilst Endau Rompin Granite by the smallest anomaly.
A group of elements that is unsuitable in size and/or charge to the cation sites of the minerals is called incompatible element. The partition coefficient between rock-forming minerals and liquid phases of this group is much smaller than 1. This group is then divided into two subgroups: large-ion lithophile elements (LILE) and high field strength elements (HFSE). K, Rb, Cs, Sr, Ba, U, Th are elements having large ionic radius well known as LILE. On the other hand, HFSE is a group of elements of large ionic valences or high charges: Zr, Nb, Hf, Ta, Sc, Y, and the REE. The selected granitoid samples show high level of LILE at 4,643 -5,256 ppm on the average of 4,846 ppm. This high LILE content and previous high alkali character of Karimun Granite fit the statement that A type granitoid may represent products of mantle-derived magmas with or without crustal contamination through extensive fractional crystalization, common contemporaneity with mafic syenite plutons and characterized by high alkali, LILE, and HFSE contents (Goodge and Vervoot, 2006;Dwivedi et al., 2011).
On a primitive mantle normalized (Mc-Donough and Sun, 1995) extended spider diagram, the granitoid shows positive anomalies of U, La, Nd, Dy and negative anomalies of Ba, Ce, Sr, P, Y, and Ti ( Figure 9). The anomalies indicate fractionation by mineral phases containing these elements at some stage in the history of the source of the magma. As Eu negative anomaly, both Ba and Sr depletions can be related to the fractionation feldspar, especially plagioclase, in the magma chamber (Mao et al., 2011;Keshavarzi et al., 2014). Fractionation of plagioclase would increase NK/A molar (Na 2 O + K 2 O)/Al 2 O 3 ) ratios and negative Sr-Eu anomalies, while separation of K-feldspar would lead to increasing Na 2 O/K 2 O ratios and negative Eu-Ba anomalies. Furthermore, fractionation of K-feldspar usually follows that of plagioclase (Zhang et al., 2012). Selected granitoid samples display negative anomalies of Eu, Ba, and Sr ( Figure 8 and Figure 9), while Na 2 O/K 2 O ratios decrease with increasing SiO 2 (Figure 2j), which favours fractional crystallization of both plagioclase and K-feldspar. Other authors also conclude that the negative anomalies shown by Eu, Ba, Sr, P, and Ti are typical of A-type granitoids (Ahmad and Chaudhry, 2008;Tong et al., 2012;Zhang et al., 2012). Negative Ti anomaly can be interpreted as reflecting ilmenite fractionation. The sharp negative Nb anomaly suggests that the granite melts originated from partial melting of subduction zone environment  (Tong et al., 2012). On the other hand, positive anomalies would indicate element accumulation or minor fractionation of mineral related elements in differentiaton process.

Conclusions
Karimun Granites are composed mainly of quartz, K-feldspar, plagioclase, and biotite. A I J O G Figure 9. Extended spider-diagram of the studied samples normalized to primitive mantle. Normalization values from Sun and McDonough (1995).