Indonesian Journal on Geoscience Vol. 3 No. 2 August 2016: 107-126 INDONESIAN JOURNAL ON GEOSCIENCE Geological Agency Ministry of Energy and Mineral Resources Journal homepage: hp://ijog.geologi.esdm.go.id ISSN 2355-9314, e-ISSN 2355-9306 Petrogenesis of Rinjani Post-1257-Caldera-Forming-Eruption Lava Flows Heryadi Rachmat1,2, Mega Fatimah Rosana1, A. Djumarma Wirakusumah3, and Gamma Abdul Jabbar4 1Faculty of Geology, Padjadjaran University Jln. Raya Bandung - Sumedang Km. 21, Jatinangor, Sumedang, Indonesia 2Geological Agency Jln. Diponegoro No. 57, Bandung, Indonesia 3Energy and Mineral Institute Jln. Gajah Mada, Karangboyo, Cepu, Kabupaten Blora, Indonesia 4Hokkaido University, Kita 10, Nishi 8, Sapporo, Japan Corresponding author: [email protected] Manuscript received: March 7, 2016; revised: May 17, 2016; approved: June 29, 2016; available online: August 2, 2016 Abstract - After the catastrophic 1257 caldera-forming eruption, a new chapter of Old Rinjani volcanic activity began with the appearance of Rombongan and Barujari Volcanoes within the caldera. However, no published petrogenetic study focuses mainly on these products. The Rombongan eruption in 1944 and Barujari eruptions in pre-1944, 1966, 1994, 2004, and 2009 produced basaltic andesite pyroclastic materials and lava flows. A total of thirty-one samples were analyzed, including six samples for each period of eruption except from 2004 (only one sample). The samples were used for petrography, whole-rock geochemistry, and trace and rare earth element analyses. The Rombongan and Barujari lavas are composed of calc-alkaline and high K calc-alkaline porphyritic basaltic andesite. The magma shows narrow variation of SiO2 content that implies small changes during its generation. The magma that formed Rombongan and Barujari lavas is island-arc alkaline basalt. Generally, data show that the rocks are enriched in Large Ion Lithophile Elements (LILE: K, Rb, Ba, Sr, and Ba) and depleted in High Field Strength Elements (HFSE: Y, Ti, and Nb) which are typically a suite from a subduction zone. The pattern shows a medium enrichment in Light REE and relatively depleted in Heavy REE. The processes are dominantly controlled by fractional crystallization and magma mixing. All of the Barujari and Rombongan lavas would have been produced by the same source of magma with little variation in composition caused by host rock filter process. New flux of magma would likely have occurred from pre-1944 until 2009 period that indicates slightly decrease and increase of SiO2 content. The Rombongan and Barujari lava generations show an arc magma differentiation trend. Keywords: post-caldera, lava, basaltic-andesite, fractional crystallization, magma mixing © IJOG - 2016, All right reserved How to cite this article: Rachmat, H., Rosana, M.F., Wirakusumah, A.D., and Jabbar, G.A., 2016. Petrogenesis of Rinjani Post-1257-Caldera- Forming-Eruption LavaIJOG Flows. Indonesian Journal on Geoscience, 3 (2), p.107-126. DOI: 10.17014/ijog.3.2.107-126 Introduction beneath Eurasian Plate. The crustal thickness is about 20 km (Curray et al., 1977). The Benioff- Rinjani volcano complex lies in Lombok Is- Wadati zone lies about 164 km beneath Rinjani land, West Nusa Tenggara Province, Indonesia (Nasution et al., 2010). The Rinjani calc-alkaline (Figure 1). The Lombok Island is tectonically suite probably originated by partial melting of located in the west of eastern most Sunda Arc the peridotite mantle wedge overlying the ac- where Australian Continental Plate subductes tive part of Benioff Zone beneath Lombok with 107 Indonesian Journal on Geoscience, Vol. 3 No. 2 August 2016: 107-126 100o E 105 o 110 o 115o 120 o 125 o 130o 135o 140o E IndoChina Plate South China Philippines Plate o Philippines o 10 N Trough 10 N Plate Malaysia Basin South China Sea 5o 5o Medan North Sulawesi Trough Pacific Ocean SINGAPORE o HALMAHERA 0 0o Sunda Plate KALIMANTAN Balikpapan SUMATRA SULAWESI SERAM Jayapura o o 5 Banda Sea 5 Java Sea PAPUA Fore arc Basin Jakarta JAVA FLORES Arafuru Sea Hindia Ocean Plate LOMBOK BALI SUMBAWA Merauke 10o SUMBA TIMOR 10o Sunda Arc Hindia Plate Java Trough Darwin N o 15 S Australia Plate 15o S 0 250 500 1000 1500 km 0 600 miles AUSTRALIA 100o E 105 o 110 o 115o 120 o 125 o 130o 135o 140o E Figure 1. Map of Indonesia Archipelago and Lombok Island (the study area is indicated by the red square). existence of variation in primary melts (Foden Selayar, Kalibalak, and Lekopiko overlying the and Varne, 1981). older formation. The Lombok volcanic rock The aims of this paper is to discuss petro- group is unconformably overlain by Quaternary genesis of lava from Rinjani post-caldera activ- Undifferentiated Volcanic Rock that presumably ity. The purpose of the paper is to complete our came from Pusuk, Nangi, and Rinjani Volcanoes. understanding of how Rinjani Volcano complex Alluvium appears as the youngest formation evolves, especially in post-caldera activity. spreading around the shore. Geology of Lombok Island Volcanic Activities of Rinjani Volcano Com- Geologically, Lombok Island has been plex mapped by Mangga et al. (1994). Generally, the The Rinjani Volcano activity can be divided geological history of Lombok Island started from into Pre-Stratocone Building stage, Stratocone Tertiary (Early Miocene) until Holocene. Late Building stage, Low-Activity Stage, Syn-Caldera Oligocene-Early Miocene Pengulung Forma- Stage, and Post-Caldera Stage (Nasution et al., tion is the oldest formation overlain by Middle 2004). The Pre-Stratocone Building stage leads Miocene Kawangan Formation. Both formations to the formation of Old Rinjani or Samalas (Lavi- are intruded by MiddleIJOG Miocene dacite and ba- gne et al., 2013) that is much older than 12,000 salt that lead to the alteration of rock from those years B.P. formations. Ekas Formations of Late Miocene in The Stratocone Building stage produced age overlies the previous older formations. Pen- Young Rinjani formed in the eastern flank of the gulung, Kawangan, and Ekas Formations form a Old Rinjani. Nasution et al. (2004) argued that hilly area in southern Lombok. the Stratocone Building stage had ages ranging Lombok volcanic rock group overlies those from 11,940 ± 40 until 5990 ± 50 years B.P. The older formations. This rock group comprises Low-Activity stage focused its activity in the Kali Palung Formation which is a member of Young Rinjani with the age of 2550±50 years B.P. 108 Petrogenesis of Rinjani Post-1257-Caldera-Forming-Eruption Lava Flows (H. Rachmat et al.) The Syn-Caldera stage relates to the cata- products are mainly characterized by dacite with strophic eruption that destroyed the Old Rinjani SiO2 content ranges from 62 to 63 wt % (Nasu- and led to the formation of the so-called Segara tion et al., 2004). Anak Caldera. The tephra are pumice fall depos- The catastrophic eruption in 1257 AD (Lavigne its, pumiceous pyroclastic-flow deposits, and de - et al., 2013) led to the formation of a 6 km x 7 km bris-flow deposits. Nasution et al. (2004) argued caldera. This makes Rinjani one of great caldera- that this catastrophic eruption occurred between forming volcanoes in Indonesia. After this cata- AD 1210 and 1300 AD, whereas Lavigne et al. strophic caldera-forming eruption, there was an (2013) suggested that the eruption occurred be- eruption from Young Rinjani that formed Segara tween May and October 1257 AD, corresponding Muncar Crater. Then, a new chapter of Rinjani to the so-called “1258 mystery eruption” found volcanic activity began with the appearance of in ice cores from both Greenland and Antarctica Rombongan and Barujari Volcanoes within the (Oppenheimer, 2003; Sigl et al., 2014, 2015). The caldera (Rachmat, 2016) (Figures 2 and 3). W E Stage I (Pre-Caldera) During the Pleistocene, the Old Rinjani or Samalas initially reach ~4000 m asl height. It was formed far before 12,000 years B.P. W E Stage II (Pre-Caldera) The Young Rinjani was formed in the eastern flank of the old Rinjani. between 11,940 ± 40 until 5990 ± 50 years B.P. W E Stage III (Syn-Caldera) There was a paroxysmal eruption from Old Rinjani in 1257 that was followed by the formation of 7.5 x 6 km Rinjani Caldera W E Stage IV (Post-Caldera) After paroxysm eruption in 1257 there was an IJOGeruption from Young Rinjani W E Stage V (Post-Caldera) Mt. Rombongan The Rinjani caldera was filled with (+ 2110 m) Mt. Barujari (+2376) water and formed Segara Anak Lake. Mt. Anak Barujari Within the caldera, the volcanic (+2112 m) activity continues with Segara Anak Lake Mt. Rinjani (+3726) Water Surface (+ 2000 m) the formation of Barujari, Rombongan, and Anak Barujari Volcano. The last eruption occurred in December 2015. Figure 2. Evolution history of Rinjani Volcanic Complex (Rachmat, 2016; not to scale). 109 Indonesian Journal on Geoscience, Vol. 3 No. 2 August 2016: 107-126 N N A D Lava 1944 Lava 2009 Lava Lake Lava 1994 2004 Segara Anak A' Lava 1966 Lava Pra 1944 Lava Pra 1944 B 0 1970 m C 0 1970 m ‘94 Volcanic Neck Volcanic Neck A Pyroclastic (Fine sand - bom) B C D 2500 Pyroclastic 2500 Crater of Barujari Peak 2004 Lava covered ‘44 Lava Flow Barujari Peak Entering ‘44 Lava 3 by 2009 lava ‘66 Lava 3 the lake 2009 lava 2250 2250 4 ‘44 Lava 2 ‘44 Lava Pra 4 5 2 5 1 6 2000 1 2000 0 250 1000 2000 3000 3750 m 0 250 1000 2000 3000 m A’ Pre ‘44 Lava Flank crater of 2004 and 2009 lava flows Legend: Legend: Pyroclastic ‘94 lava ‘44 lava Pyroclastic 2004 lava Pre ‘44 lava ‘94 Volcanic neck ‘66 lava Pre ‘44 lava 2009 lava ‘94 Volcanic neck Figure 3. An aerial view of Rinjani Caldera (top image source: Google, left; CRISP, right)) and cross-section of Rombongan and Barujari Volcanoes (bottom, not to scale).