Geochemistry of Plutons in Central Sumatra and their Correlation to Southeast Asia Tectonic History

Previous investigations of plutons in Sumatra were focused on age dating with minimum geochemistry composition analysis. The purpose of this study is to define the geochemistry classification of the intrusions in central Sumatra on describing the emplacement mechanism associated with Southeast Asia tectonics. The rocks composed of quartz, K-feldspar, and plagioclase with amphibole, biotite, epidote, and zircon as accessory minerals. Six of seven studied plutons range from monzodiorite to granite with metaluminous-slight peraluminous, medium to very high-K calc-alkaline, magnesian, calcic to calc-alkalic affinities. The studied samples depict a wide range of total REE composition of 39-1,369ppm. Most of the rocks denote Sr, Ti, Y, Ce, and Eu anomalies on the primitive-mantle and chondrite normalized diagram. LREE are more enriched in comparison to HREE with (La/ Sm) N = 1.71-18.75 and (Gd/Lu) N = 0.15-2.59. Most of the studied plutons are classified in the I-type according to the A/CNK value, negative SiO 2 to P 2 O 5 correlation with magnesian and arc-associated character. A-type nature of Sijunjung Granite is displayed on its high silica and REE content with ferroan, calc-alkalic, and within-plate affinities. The existence of A-type intrusion implies an extensional setting during long time subduction episodes, which triggered I-type magmatism since Late Permian to Neogene in Sumatra.

INTRODUCTION triggered the development of the Eastern granite province whilst the later collision of Sibumasu and Indochina-East Malaya generated the Main Range province (Barber et al., 2005;Gasparon and Varne, 1995;Irzon et al., 2020;Metcalfe, 2013;Ng et al., 2017). Within the Malay Peninsula, the Eastern province was further subdivided into the Central and Eastern Belts while Main Range Belt corresponds to the Main Range province (Cottam et al., 2013;Nguyen et al., 2019). Ceno-Tethys subduction-related Western Province extends mainly from Upper Peninsula Thailand into Myanmar (i.e. Hazad et al., 2019;Jiang et al., 2017). In addition, Clark and Beddoe-Stephens (1987) proposed to include the Hatapang Granite in northern Sumatra as a part of the Western Province, on the basis of its geochemical affinities. Based on the investigation along the suture between Mergui Microplate and the Permian Volcanic arc in Central Sumatra, another granitic belt was identified (McCourt et al., 1996). The subduction-related magmatism formed small batholiths emplaced nearby the continental margin of Sundaland to the southwest on West Sumatra, which was amalgamated to Sibumasu in the Early Triassic (McCourt et al., 1996). Other studies interpreted these plutons as a volcanic arc suite nearby the Bukit Barisan Mountains with composition ranging from gabbro to monzogranite (Cobbing, 2005;Irzon et al., 2018).
Most of the Mesozoic intrusions in Sumatra are I-type, subduction-associated, and situated in the Barisan Mountains (Barber, 2000;McCourt et al., 1996;Zhang et al., 2018). However, several studies notified the presence of anorogenic A-type intrusions, namely Sibolga Granite (Setiawan et al., 2017;Zhang et al., 2020) and Sijunjung Granite  in the central Sumatra region. Therefore, more complete petrography and geochemistry data on these plutons are required to explain the tectonic  Central Sumatra plutons geochemistry with Southeast Asia tectonics 3 history of Sumatra. The aim of this paper is to establish the classification of plutons from central Sumatra on drawing their correlation with Southeast Asia tectonic history.

Geological Setting
Sumatra is situated at the western edge of Indonesian archipelago and at the northeastern margin of Indian Ocean. The island is presently constituted of two major continental blocks: the Cathaysian West Sumatra and the northern Gondwana-origin Sibumasu. Cathaysia which is formerly consisted of Indochina, South China, North Qiantang-Qamdao-Simao, West Burma, and West Sumatra, possibly began to move away from Gondwana during Devonian across the Palaeo-Tethys Ocean (Barber and Crow, 2009;Hall and Sevasjanova, 2012;Metcalfe, 2011Metcalfe, , 2013. West Sumatra and West Burma detached from the Cathaysia since Permian and inserted at the west margin of Sibumasu during Triassic (Barber et al., 2005;Barber and Crow, 2009). The two blocks were separated by the Andaman Sea since the Miocene. The contact of the West Sumatra and the Sibumasu blocks is marked by the Medial Sumatra Tectonic Zone (MSTZ), which extends from the Andaman Sea to southern Sumatra (Barber and Crow, 2009). The studied intrusions are situated in a volcanic arc suite on the West Sumatra continental region ( Figure 2). The plutonic rocks are located in the proximity of the volcanic Bukit Barisan Mountains, which extends almost 1,700km from north to South of Sumatra.
Volcanic rocks were emplaced in Sumatra episodically since pre-Triassic. Silungkang and Palepat Formations are the two Permian extrusive rock units located in central Sumatra region. The Palepat Formation which comprises basic to intermediate lava lies near Tanjung Gadang while the Silungkang Formation that is situated to the southeast of Lake Singkarak and at the north of Ombilin Granite mainly consists of andesite and meta-andesite (Silitonga and Kastowo, 1995). Most of the Tertiary volcanic rocks in Central Sumatra are situated less than 10km before the west coast of Sumatra. Quaternary extrusives that occupy large area in the studied location are easily found near mountains, lakes, and sometimes near pluton bodies ( Figure  2). Metamorphic rocks in Sumatra generally resulted from contact effect of intrusion and subsequently by shearing (Barber and Crow, 2009  Central Sumatra plutons geochemistry with Southeast Asia tectonics 4 SAMPLE DESCRIPTIONS

Sijunjung Granite
The Triassic Sijunjung Granite (SG) is exposed in the northern part of the studied region with a dimension of about 50×10km 2 (Silitonga and Kastowo, 1995) as shown in Figure 2. Major geochemistry composition of two samples from this pluton were provided by Sato (1991) and Sutanto (2005). The pluton was crystallized in the Triassic according to K-Ar biotite and hornblende datings (Sato, 1991;Silitonga and Kastowo, 1995). This study analyzed five SG samples (SJ-30, SJ-31, SJ-87, SJ-89 and SJ 90). SJ-30 and SJ-31 were collected from the northwest whilst the others from the south of the granite body. The specimens are fresh, pinkish, coarse-grained, porphyritic with a mineral assemblage formed by quartz, K-feldspars (megacrystal), biotite, and plagioclase. Hornblends are clearly detected in a couple of samples from the south of this intrusion.

Balimbing Pluton
The 16km 2 Balimbing Pluton is situated to the east of Singkarak Lake ( Figure 2). OB-26 and OB-27 are the fresh light grey and medium-grained rocks of Balimbing Pluton (BP). Some very fine-grained aplites were identified at the OB-26 outcrop. The samples are mainly composed of quartz, K-feldspar, biotite, and hornblende.

Sulit Air Granite
The Triassic Sulit Air Granitoid (SAG) is located to the east of the Balimbing Pluton and to the southwest of the Sijunjung Granite. Cu mineralization is detected in the 60km 2 pluton. Imtihanah (2005) described major geochemistry of two samples from SAG. This study collected four more samples from the pluton: SA-34, SA-54, SA-56 A and SA-56 B. The first two are relatively unaltered, light grey, and medium-grained rocks. SA-56 A is a dull, medium-grained, and chloritized intrusion while SA-56 B is a malachite ore with high Cu composition.

Tanjung Gadang Granite
The 60x35km 2 large Cretaceous Tanjung Gadang Granite (TGG) body is placed to the southwest from SG and to the west from Lassi Pluton (LaP). Intrusive rocks with their weathering horizons were found at TG-02 and TG-05 stations on Tanjung Gadang Granite domain. TG-02 might have been influenced by a higher degree of weathering than TG-05 regarding its colour. The specimens are majorly built of quartz, plagioclase, K-feldspar with minor biotite.

Lassi Pluton
The Lassi Pluton (LaP) crops out in three different bodies ( Figure 2) with a northwest elongate exposure of about 38x9km 2 . Sato (1991), Imtihanah (2005) and Sutanto (2005) discussed a total of six samples of the Eocene pluton regarding its petrography and major geochemistry points of view. Ten LaP rocks are described and are analyzed in this paper. LS-06, LS-07 A, LS-07 B, LS-08, LS-15 and LS-16 were sampled from the northern body of Lassi Pluton whilst LS-12, LS-20 and LS-62 from the southwestern ones. Six of the samples are categorized as fresh, light grey, mediumgrained rocks whilst LS-06, LS-07 A, LS-07 B, LS-08 and LS-15 might have experienced some degree of alteration regarding foliations, color, and texture. The alteration might be explained by their location close to the Great Sumatra Fault (Figure 2).

Lolo Pluton
Alike the northern part of LaP, the Miocene Lolo pluton (LoP) is also emplaced just near the Great Sumatra Fault and to the west of Mount Runcing ( Figure 2). Imtihanah (2005) analyzed four samples from the 45km 2 pluton geochemically but with incomplete REE composition. This work investigated new three samples of LoP, namely LL-58 A, LL-58 B and LL-59. The rocks are light grey, medium grain, and relatively fresh with hornblende crystals detected megascopically. LL-58 might be located towards the edge of the intrusion because hornfels were also discovered near the sample location.

Tarusan Pluton
The Tarusan Pluton (TP) is located near the west coast of Sumatra about 20km to the south from Padang. No previous investigation ever discussed on the approximately 8km 2 rock body. Two samples were attained from this location, namely TR-35 A and TR-35 B. TR-35 A is a pinkish, phaneritic, coarse grain, and massive rock whereas TR-35 B is relatively more brownish because of weathering. Both of the specimens are composed of quartz, K-feldspar, and plagioclase without hornblende observed.

Petrographic Characteristics
The selected plutonic rocks are generally medium to coarse grain, having a porphyritic texture, and poor alteration tendency. Quartz (0.1-1.5mm) is the most predominant mineral in the range of 28-46% modal composition. The euhedral and transparent phase form interlocking texture is detected in a few samples. Although most of the quartz show well-developed crystal faces, some of them have round embayments. The selected samples are built of 7-21% modal composition transparent plagioclase. Some of the subhedral minerals are characterized by albite and Carlsbad-albite twinning. Alkali feldspar of the samples is subhedral and transparent in the range of 5-24% modal composition. Except for the SG and TP, hornblende and biotite are found in most of the microscopic analysis (3-15 modal %). Some plagioclase and alkali feldspar are weakly altered to sericite as shown in LL-59 from LoP. Some of the biotite and plagioclase grains are to chlorite and epidote.
SG is special for the presence of alkali feldspar megacrystals. Some K-feldspars are covered by plagioclase with rapakivi texture. Mica muscovite (5-7 modal %) and zircon are only detected in this Triassic intrusion. Muscovite rarely occurs as separate crystal associated with biotite and some is also identified in feldspar as secondary inclusions. In SG samples, very fine grain zircon and apatite show euhedral to subhedral crystals and commonly as inclusion in quartz. The modal microscopic analysis results of selected rocks are given in Table 1. In the QAP ternary plot, the LaP, LoP, TP and SG samples fall within monzogranitegranodiorite, granodiorite, syenogranite-monzogranite, and syenogranite fields, respectively ( Figure 3).

ANALYTICAL METHODS
Major oxides analysis of 29 samples was conducted using Advant X-ray fluorescence (XRF) while trace elements and the full suite rare earth elements compositions were measured using the quadrupole X-Series Thermo Inductively coupled plasma-mass spectrometry (ICP-MS). Both geochemistry preparation and analysis were performed in the Center for Geological Survey Laboratory in Bandung. Loss on ignition (LOI) measurements were carried out by heating the samples at 1000ºC for an hour to determine their relative weight loss. AGV-2, GBW 7110 and GBW 7113 were the certified reference materials used in this study in verifying measurement accuracy. The detail of the preparation procedure, the analytical instrument setting, and data verification protocols followed the previous studies of Irzon (2017) and Irzon (2018). Data from previous studies are also incorporated in this paper. Whole rock geochemical composition of this study is shown in Table  I, Appendix I.

Major Elements Classification
Some of the studied samples are altered based on the LOI values >2%, so their composition has been recalculated to 100% before plotting in geochemistry-based diagrams. According to the major oxides classification of Middlemost (1994), the samples plot in the monzodiorite (1 sample), diorite (7 samples), granodiorite (3 samples), the granite (9 samples) fields as shown in Figure 4A. The high felsic character (SiO 2 ≥70%) is denoted on SG, TP and TG Granite.   Central Sumatra plutons geochemistry with Southeast Asia tectonics 6 The Alumina Saturation Index (ASI) or molar A/CNK (Al 2 O 23 /CaO+Na 2 O+K 2 O) of the plutons from central Sumatra are mostly <1.1. SAP, TP and LoP are clearly metaluminous with the molar A/CNK <1 ( Figure 4B). On the other hand, the SG and LaP are in metaluminous to slight peraluminous array. The peraluminous character of some samples might result from partial melting of metasedimentary rocks and mafic source rocks or amphibolites (Chappel et al., 2012;Nguyen et al., 2019;Sarjoughian and Kananian, 2017).
The intrusions from central Sumatra show markedly different array as seen in the SiO 2 versus K 2 O chart. LoP, LaP, SAG, and BP show calc-alkaline to high-K calcalkaline tendency whereas TP and TGG are high-K calc-alkaline. SG depicts the richest potassium which is classified in Shohonite Series ( Figure 5A). Moreover, the SG is ferroan with high Fe* (FeO T /(FeO T +MgO)) values ( Figure 5B) which implies strong iron enrichment than the other magnesian plutons. On the basis of Modified Akaline-Lime Index (MALI), both ferroan and magnesian rocks can be further classified into calcic, calc-alkalic, alkali-calcic and alkalic. The SG samples fall in alkali-calcic and alkalic fields whereas the others mostly range in calcic and calcalkaline ( Figure 5C).

Trace and Rare Earth Elements Characters
Total REE content in the studied rocks is in the range of 39ppm (LS-16, LaP) to 1,369ppm (SJ-30, SG). SG rocks show higher Ga, Rb, Y and Ba, but lower Sc concentration relative to the other six intrusions. Moreover, this Triassic pluton has the highest REE content of averagely 466 ppm. The normalized trace elements in most studied rock suites, display comparable distribution patterns with negative anomalies for Sr, Ti and Y ( Figure 6). However, the rocks from TP denote positive Y anomaly while the southern group of LaP without Y anomaly. Sr and Ti negative anomalies should have been generated from plagioclase and titanite removal, respectively (Perez-Soba and Villaseca, 2010; Sarjoughian and Kananian, 2017).  Central Sumatra plutons geochemistry with Southeast Asia tectonics 7 Negative Ce anomaly in igneous rocks explains that the source material exposed just below the surface or as a result of post-magmatic supergene processes under oxidizing conditions (i.e. Gazel et al., 2006;Lee et al., 2013). The strong positive correlation of Ce anomaly to Eu anomaly on SG, the northern group of LaP and LoP with correlation coefficient of 0.81, 0.86, 0.98, respectively might imply a secondary oxidizing conditions after emplacement, rather than their nature during magmatic differentiation. However, the number of samples from the other plutons is not enough to establish such conclusion.

Alphabetic Classification
The most commonly used plutonic rocks classification is the alphabetic-SIAM scheme. The igneous (I-type) and supracrustal (S-types) granites were proposed based on the research in Lachland Fold Belt (Chappel and White, 1974). The anorogenic or rift-related A-type rock was originally explained by Loiselle and Wones (1979) whereas the description of the subducted oceanic crust derived  l o g i c a A c t a , 1 9 . 9 , 1 -1 Whalen (1987). The classification of this alphabetic scheme was determined on both minerals and geochemistry characteristics.

M-type intrusion was suggested by
The predominance of hornblende and biotite combined with a lack of muscovite and monazite minerals is a feature of I-type intrusive rocks (i.e. Irzon et al., 2020;Sarjoughian and Kananian, 2017). It is generally accepted that I-type pluton is metaluminous to slight peraluminous with A/ CNK<1.1 whilst S-type granites is characterized by a strong peraluminous affinity (i.e. Charusiri et al., 1993;Irzon, 2016;Irzon et al., 2020;Nguyen et al., 2019). In several cases, peraluminous nature could also result from highly fractionated I-type granites (Jamil et al., 2016;Perez-Soba and Villaseca, 2010;Pollard et al., 1995). SiO 2 to P 2 O 5 correlation is useful for distinguishing the I-and S-type granite, in which I-type granites show a negative correlation while S-type rocks show a positive one (i.e. Ghani et al., 2013;Nguyen et al., 2019). The most unique feature of I-type granites is the magnesian composition whatever their affinity to Peacock index series, namely calcic, calc-alkalic, alkali-calcic and alkalic (Castro, 2019). Most plutons in central Sumatra are classified as I-type based on the metaluminous to slight peraluminous and magnesian features with negative SiO 2 -P 2 O 5 correlation. Moreover, these plutons fall within Volcanic Arc Granites (VAG) field on the Y+Nb versus Rb and Y versus Nb plots (Figure 7). SG denotes different characteristics with the other studied plutons in this paper. The Al 2 O 3 and CaO contents of the Triassic intrusion is low but with high SiO 2 , Nb, Na 2 O+K 2 O, and total REE abundances (Table  II, Appendix I) as the indications of A-type granite (Loiselle & Wones, 1979;White & Chappell, 1983). Those characters combined with the distinctly high TiO 2 / MgO and K 2 O/Na 2 O ratios (Figure 8) imply shallow crust origin of the rock (Patiño Douce, 1997). The ferroan, calc-alkalic, metaluminous-peraluminous, and high-silica content (SiO 2 ≥70%) of Sijunjung Granite is coeval with the A-type plutons widespread in Burma (Jiang et al., 2017), Namibia (Stammeier et al., 2015), southwestern United States and Amazonia (Frost and Frost, 2011). Frost et al. (2016) argued that this kind of silica-rich intrusion results from partial melting of tonalite or granodiorite. In the tectonic discriminant diagram of Pearce et al. (1984), the Sijunjung Granite tends to be in the field of the A-type within-plate granites (Figure 7).

Tectonic Implication
Studies explain that I-type magmatism is generally triggered by subduction in an active continental margin of igneous derived material while S-type is mostly emplaced in continental collisional and intraplate orogenies because of crustal thickening causing melting of lower crustal metasedimentary rocks (i.e. Jiang et al., 2017;Liu et al., 2020;Oliver et al., 2014;Sarjoughian and Kananian, 2017;Setiawan et al., 2017). Castro (2019) argued that a fluidfluxed melting of igneous rocks in the continental crust in short time span (<30Myr) also generates secondary I-type granites and is not directly associated with a subduction setting. The later mechanism of I-type intrusion is not common in Southeast Asia due to the presence of long  Central Sumatra plutons geochemistry with Southeast Asia tectonics 10 life subduction both below Indochina-East Malaya resulting the Eastern Province (Late Permian-Triassic) and beneath West Burma-West Sumatra that generated the Western Province (Triassic-Miocene). In most cases, A-type granite is emplaced in extensional tectonic environments and do not appear to be associated with plate boundaries (Amiruddin, 2011;Searle et al., 2012;Setiawan et al., 2017). Recent Sumatra is formed after a long period of plates detachment, rotation, and accretion/ amalgamation involving continent-ocean and continentcontinent convergencies in Southeast Asia region. Long term intrusions in Sumatra are evidenced from the Late Permian Ombilin Granite to the Miocene Lolo Pluton (Barber et al., 2005). Almost all of the studied plutons are categorized as arc-associated I-type intrusions except for the Sijunjung Granite with anorogenic A-type characteristics as discussed above. The presence of the A-type intrusion explains an extension setting during long time subduction episodes in Sumatra that also happened in Sibolga (Setiawan et al., 2017;Zhang et al., 2020).
Palaeo-Tethys was opened during the Permian at the time when the composite Cathaysia separated from Gondwana. The subduction of the ancient ocean below Indochina should have begun in the Permian (Hutchinson, 2014;Metcalfe, 2013). Meanwhile, some part of Cathaysia, especially West Sumatra and West Burma drifted away since the Late Permian. The Late Permian OG (Koning and Aulia, 1985 in Barber et al., 2005;Silitonga and Kastowo, 1995) most possibly intruded before West Sumatra and West Burma separated from Cathaysia. Sibumasu started to drift away from Gondwana since the Early Triassic that opened the Meso-Tethys but narrowed the Palaeo-Tethys (Metcalfe, 2011). On the other side, West Sumatra and West Burma rotation finished and the plates accreted to Sibumasu. No convergency happened during that time to postulate the anarogenic origin of the Late Permian-Early Triassic SG (Sato, 1991;Silitonga and Kastowo, 1995). The crystallization of SG is coeval with Sibolga Granite, which also indicates several anarogenic facies (Setiawan et al., 2017). The Palaeo-Tethys totally closed in the Middle Triassic.
Sibumasu collision to Indochina-East Malaya is not correlated to the studied intrusions but might triggered plutonisms in Riau Islands and Bangka-Belitong. Meso-Tethys subduction under West Sumatra should have initiated in the Jurassic SAG (Sato, 1991) and the Late Jurassic-Early Cretaceous TGG (Koning and Aulia, 1985 in Barber et al., 2005;Pulunggono and Cameron, 1984). After the closure of the Meso-Tethys, the Indo-Australia Ocean underwent subduction below Sumatra until now and triggered the Cenozoic LaP and LoP (Imtihanah, 2005;Sato, 1991). However, no previous age dating describes the emplacement of the nearshore TP and west Ombilin-situated BP. The emplacement mechanism of studied plutons with their association to Southeast Asia tectonic activities is simplified in Figure 9.        Central Sumatra plutons geochemistry with Southeast Asia tectonics II 2 n.a. 14.3 n.a. n.a. n.a. n.a.