Contrasting Two Facies of Muncung Granite in Lingga Regency Using Major, Trace, and Rare Earth Element Geochemistry

DOI:10.17014/ijog.2.1.23-33Lingga Regency is located in the main range of the famous Southeast Asia granitic belt related to tin resources. There are two granitic units in this region: the S-type Muncung Granite and I-type Tanjungbuku Granite. XRF and ICP-MS were used to measure the major, trace, and rare earth elements of nine Muncung Granite samples. Two different patterns were identified from major data plotting on Harker variation diagram. Granitic rocks from Lingga and Selayar Islands are classified as A facies while others from Singkep Island is B facies. This paper used graphs and variation diagrams to reveal the differences of those two facies. Thus, REE correlation to SiO2, trace element spider diagram, and REE spider diagram show more contrasts correlation. However, both facies are syn-collisional and High-K calc-alkaline granites. Some identical characters with other granitic units in Peninsular Malaysia were also detected in this work.


Background
Tin was produced massively in Lingga Regency for more than four decades and was one of the three main resource locations in Indonesia besides Bangka and Belitung. This resource is related to granitic rock of the area. There are two granitic units in Lingga region such as Muncung and Tanjungbuku. Cobbing et al. (1992) classified the Muncung Granite as stanniferous S-type while Tanjungbuku Granite as I-type. The Centre for Geological Survey of Indonesia conducted a research regarding rock types and chemical composition of Lingga granite. The research area (Lingga) is part of the main range of granite province in Indonesian Tin Islands (Barber et al., 2005). Formerly, only low level of connec-tion could be observed about geochemistry data from plutonic samples. Better correlations obtained after splitting the samples into two facies although they just come from one unit of rock, The Muncung Granite.
The aim of this research is to identify characteristic of two facies of the S-type Muncung Granite based on major, trace, and rare earth element data. Various geochemistry diagrams about plutonic rock classification, correlation, and tectonic setting are used to portray the divergence.

Geological Setting
Lingga is a regency in Kepulauan Riau Province, located to the east of Sumatra and northwest of Bangka Island (Figure 1). The survey of island toponymy in Lingga Regency has been I J O G identified a total of 455 islands (Yulius, 2009).
Lingga, Singkep, and Selayar are the three main islands in the regency. The general geology of this region consists of five sequences (Sutisna et al., 1994): (1)

Analytical Method
Four granitoid samples were collected from both Singkep and Lingga Islands and only one from Selayar Island. The chemical composition of the nine samples were measured using X-ray fluorescence analyzer (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). The chemical data were then compared to petrography analysis of the samples. Both the preparation and instrumental analysis were conducted at Geo-logical Laboratory of The Centre for Geological Survey of Indonesia in Bandung.
After being dried at the room temperature, the samples were crushed by a jaw crusher to 200 mesh and were ground by a mill. Major and trace elements were analyzed with Advant XP XRF while REE were measured using The X Series Thermo ICP-MS. Before ICP-MS measurement, rock samples were first dissolved with three acids leach using nitric acid (ultrapure grade), formic acid (ultrapure grade), and perchloric acid (pro analysis grade). AGV-2 and GBW 07110 andesites were also measured as calibration materials for ICP-MS method, while GBW 7103 for XRF. Sample preparation, ICP-MS set up procedure, and certified reference evaluation are based on study of Irzon and Permanadewi (2010).

Petrology
Nine granitic rock samples in this study representing the Triassic Muncung Granite comprise granite and diorite (Sutisna et al., 2004). Samples are generally holocrystalline, medium-grained, phaneritic, and composed of quartz, K-feldspar, and plagioclase. Granitoids from Singkep Island are lighter in colour than others which is confirmed with quartz composition from petrographic data (Tabel 1). Plagioclase and biotite may partly

Geochemistry
Geochemistry data of the nine granitoid samples are described in Tabel 2. A number of schemes based on chemical composition have been applied for the classification and nomenclature of igneous rocks. Granitic rocks from Lingga Regency are classified based on Middlemost (1985) using total alkali and silica data. All granitoids from Singkep Island belong to granite suites, only one from Lingga Island is granodiorite ( Figure 2). This result confirms the previous study of Muncung Granite that consists of granite and granodiorite (Sutisna et al., 1994). The diagram indicates that all plutons are acid igneous rocks.

Major Elements Variations
ratios of the samples are more than 1.10. In the A/CNK versus A/NK (Al 2 O 3 /Na 2 O+K 2 O) diagram (Shand, 1943;Figure. 3) these rocks were plotted into the peraluminous domain, hence the rocks are S-type in the sense of Chappell and White (1974). The presence of two subgroups within Muncung Granite is apparent on this ratio. Samples from Singkep Island are more peraluminous than others because of the higher degree of A/CNK ratio. The peraluminous nature of the granitic rocks is evident from major cation parameters of Debon and Le Fort (1983), which essentially consist of muscovite and biotite ( Figure 3b) and is confirmed with petrographic data (Tabel 1). The main compositional trends of the intrusive rocks are tried to be correlated using Harker variation diagrams. After splitting the data into two facies, correlation coefficients of SiO 2 versus major oxides (Table 3) are close to 1, pointing to strong degree of relationship (Taylor, 1990 (Ghani et al., 2013) and Machang plus Kerai batholith (Ahmad et al., 2002) Figure 4). A little iron depletion in A facies during fractionation suggests their calc alkaline nature (Zaw et al., 2011) which is confirmed by SiO 2 -K 2 O diagram ( Figure 5).

Trace and Rare Earth Elements Variations
A sample with full set of trace and rare earth elements is of greater help in determining the nature of the source material, and in constructing the tectonic setting of origin. HFSE (High Field Strength Elements) in both groups show a positive correlation to SiO 2 without significant difference on HFSE enrichment (Figure 6a). Relative enrichment of HFSE suggests that the granitic rocks are primarily derived from a felsic source (Ray et al., 2011). All granitoid samples from Lingga Regency show a negative anomaly of Ba, Nb, P, and Tl in     fractional crystallization process with plagioclase as the major precipitating felsic phase (Atherton, 1993;Cid et al., 2001;Ahmad et al., 2002;Ghani et al., 2013;Ray et al., 2011). The negative correlations of CaO and Al 2 O 3 to SiO 2 are relevant with the fractional crystallization (Sun et al., 2010). This also means that granitic rocks of A facies is not highly differentiated compared with B facies (Sanematsu et al., 2009).  Batchelor and Bowden (1985) showed a bivariate graph using the plotting parameters R1 [(4Si − 11 (Na + K) − 2(Fe+Ti)] and R2 (Al + 2Mg + 6Ca) to discriminate five granitic groups related to the tectono-magmatic divisions. The granitoid samples are plotted in the syn-collisional field (Figure 8). This result agrees with the age of granite tectonic scenario of Peninsular Malaysia (Ghani et al., 2013). The subduction of the Sibumasu eastward beneath the Indochina Blocks in Peninsular Malaysia during Permian to Triassic produced volcanic and granitic magmatisms broadly known as East Malaya Volcanic Arc and Eastern Belt Granite respectively (Metcalfe 2000).
Large contrast between the two facies could be observed in Rare Earth Elements (REE) plot versus SiO 2 . REE classification to Light-REE (LREE: La, Ce, Pr, Nd, and Pm), Medium-REE (MREE: Sm, Eu, and Gd), and Heavy-REE (HREE: Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y) of Koltun and Tharumarajah (2014) is used in this study. There is an enrichment trend of all  group of REE from A facies, but a depletion of LREE and slightly enrichment of MREE from B facies. The total REE in granitic rocks from A facies raises with the increase of SiO 2 , but no significant correlation is identified in B facies ( Figure 9). The REE pattern in B facies is opposite to granitoids from Laos that are depleted in HREE and enriched in LREE (Sanematsu et al., 2009). REE Spider diagram clarifies the diversity of the plutons in this study. The A facies generally contains more REE than B facies which may correlate to peraluminous level ( Figure 10). From A/CNK versus A/NK diagram, B facies is more peraluminous than A facies, confirming that it also possesses more S-type character. S-type granite could be identified from its low content of REE (Christiansen and Keith, 1996)

Conclusions
On the basis of geochemistry, the Muncung Granite could be divided into two facies. Aluminium Saturation Index of granitic rock samples from Singkep Island (B facies) is higher than others from Lingga and Selayar Island (A facies). The greater level of peraluminous of facies B could be the reason respecting its lower REE contents. The two groups reveal more differences on their relation to SiO 2 , where B facies has an identic character with granitoid from Peninsular Malaysia. The difference of A facies to granitoid from Peninsular Malaysia is the rise of Al 2 O 3 with increasing SiO 2 . The importance of fractional crystallization process with plagioclase as the major precipitating felsic phase is a B facies character. The REE pattern of A facies demonstrates similarity with granites from Laos. The REE pattern in B facies is opposite to granitoids from Laos that are depleted in HREE and enriched in LREE.
Although trace element spider diagram tells slight contrast, however, both facies are syn-collisional and High-K calc-alkaline granites.