Magmatic Conditions as Indicated by Petrological and Mineralogical Aspects in the Tertiary Basaltic Lava, Southeastern Bali, Indonesia

Penulis: Dr. I.G.B. Eddy Sucipta
Lembaga: Fakultas Ilmu dan Teknologi Kebumian, Institut Teknologi Bandung

Lava basaltik Tersier di bagian tenggara Pulau Bali hadir sebagai lava bantal dan berasosiasi dengan rangkaian magma kalk-alkali K tinggi di busur pulau sistem vulkanik. Komposisi mineral batuan ini tersusun atas olivin dan klinopiroksen sebagai fenokris utama dengan fenokris plagioklas dan magnetit sebagai kelimpahan minor. Masa dasarnya mengandung plagioklas, mineral buram, piroksen, dan kaca vulkanik yang membentuk tekstur intersertal. Olivin memiliki komposisi forsterit (Fo63-92) dengan tekstur zoning normal. Piroksen memiliki komposisi augit dan diopside dengan zonasi normal dan terbalik (reverse). Zonasi terbalik pada fenokris piroksen memiliki kandungan Mg-, Cr-, Ti- dan Al yang sangat bervariasi. Beberapa zona telah diamati bahwa zonasi terbalik terbentuk dari inti ke tepi pertama, dan zonasi normal ke tepi berikutnya. Plagioklas dalam massa dasar memiliki komposisi yang berkisar dari bitownit hingga albit K tinggi. Geotermobarometri klinopirokses menunjukkan bahwa lava basaltik Tersier di tenggara Pulau Bali terbentuk oleh magma kogenetik dengan suhu 1100o-1300o C dan tekanan 3-9 kbar. Pencampuran magma adalah proses penting sebelum lava basaltik meletus di lingkungan laut.

Transcript

1. MAGMATIC CONDITIONS AS INDICATED BY PETROLOGICAL AND MINERALOGICAL ASPECTS IN THE TERTIARY BASALTIC LAVA, SOUTHEASTERN OF BALI, INDONESIA I G.B. Eddy Sucipta 1, Muhammad Taufik Abdullah 2, Purnama Sendjaja3 1Department of Geological Engineering, Institut Teknologi Bandung 2Laboratory of SEM-EDS UPP Chevron – Institut Teknologi Bandung 3Center of Geological Survey, Geological Agency, Ministry of Energy and Mineral Resources of Republic Indonesia Corresponding author: sucipta@itb.ac.id
2. 2 Index map of Bali Island (after Self and King, 1996) Research area is shown by a red box (after Google Maps, accessed on November 26, 2019) Geological map and stratigraphy of Bali Island (after Sutawidjaja, 2009) Outcrop of pillow lava in Gua Lawah Area, Southeastern Bali The purpose of research: 1. To know about petrology, mineralogy, geochemical characteristics of Tertiary basaltic lava in Southeastern Bali. 2. To determine their magmatic condition before eruption in marine environment. Introduction
3. 3 Petrology and Mineralogy Basalt outcrop of LEBU-1K (Margenta, 2015) Basalt outcrop of ABAH 1 (Margenta, 2015) Micro-photograph of thin section LEBU-1K Micro-photograph of thin section ABAH 1 1 mm 1 mm 1 mm 1 mm PPL PPL XPL XPL
4. 4 Mineral Chemistry of Phenocrysts Olivine have normal zoning with Fosterite contents (Fo%) of Fo63-92 Olivine Pyroxene Plagioclase (+ groundmass) Pyroxene are classified as augite – diopside (clinopyroxene) with normal and reverse zoning Plagioclase are classified as bytownite – high K albite with Anorthite contents (An%) of An3-89 Diagram of Morimoto et al. (1988) Diagram of Arem (2011)
5. 5 Thermobarometer of Clinopyroxene Formula after Putirka (2008)
6. 6 Whole Rock Chemistry (Major Elements) Formation Sample no. RM 13-03 BL-3 MG-4 ABAH 1 LEBU - 1K LEBU - 1M Fossil age (Blow, 1969) Rock name Basalt Basalt Basalt Basalt Basalt Basalt Clinopyroxene (Cpx) 30 30 20 30 27 30 Orthopyroxene (Opx) 3 Olivine (Ol) 20 15 15 20 10 10 Plagioclase (Plg) 25 45 30 25 25 20 Opaq mineral (Opq) 10 10 30 10 5 8 Volcanic glass 10 5 5 10 5 17 Secondary minerals 5 5 25 15 Major elements (weight %) : SiO2 47.260 45.970 45.880 48.140 46.350 46.130 TiO2 0.525 0.506 0.550 0.601 0.495 0.494 Al2 O3 13.370 12.280 11.860 15.180 12.410 12.940 Fe2 O3 13.630 14.950 15.010 13.060 12.170 12.490 MnO 0.209 0.194 0.203 0.171 0.138 0.158 MgO 8.890 11.050 10.430 6.130 11.110 11.700 CaO 10.750 11.450 12.010 9.790 8.940 8.340 Na2 O 1.660 1.430 1.280 1.990 1.480 1.540 K2 O 2.680 2.160 2.220 2.930 1.430 1.510 P2 O5 0.476 0.336 0.360 0.479 0.286 0.302 LOI 1.470 1.560 2.140 1.760 4.850 4.230 Total 100.920 101.886 101.943 100.231 99.659 99.834 K2 O/Na2 O 1.61 1.51 1.73 1.47 0.97 0.98 Na2 O + K2 O 4.34 3.59 3.50 4.92 2.91 3.05 FeO* 12.26 13.45 13.51 11.75 10.95 11.24 FeO*/MgO 1.38 1.22 1.30 1.92 0.99 0.96 CaO/Al2 O3 0.80 0.93 1.01 0.64 0.72 0.64 Mg# 56.4 61.9 60.5 50.8 66.8 67.3 Ni (ppm) 101 208 175 69 151 163 Cr (ppm) 289 562 497 214 425 355 Normative minerals (weight %) : Plagioclase (Pl) 31.16 25.60 24.31 40.58 37.88 39.13 Orthoclase (Or) 16.13 12.88 13.30 17.79 9.04 9.46 Nepheline (Ne) 2.50 4.09 3.84 0.67 Diopside (Di) 24.39 28.04 30.96 18.49 16.92 13.32 Hypersthene (Hy) 15.53 13.34 Olivine (Ol) 21.66 25.44 23.47 18.22 17.04 21.12 Ilmenite (Ilm) 1.01 0.97 1.06 1.18 1.01 0.99 Magnetite (Mt) 2.02 2.19 2.20 1.94 1.88 1.91 Apatite (Ap) 1.11 0.79 0.86 1.14 0.72 0.74 Total 99.98 100.00 100.00 100.01 100.02 100.01 Diopside (Di)/Olivine (Ol) 1.13 1.10 1.32 1.01 0.99 0.63 Fe2 O3 * (total Fe as Fe2 O3 ); FeO* (total Fe as FeO); CIPW norm calculated with 10% total iron as Fe2 O3 , 90% as FeO Mineralogy and major elements compositions of volcanic rocks at Southeastern Bali Island Mineralogy (modal %) N19-21 (Pliocene) Ulakan Formation TAS diagram (Le Bas et al., 1986) Peccerillo and Taylor (1976)
7. 7 Conclusions 1 The Tertiary basaltic lava in southeastern of Bali Island present as pillow lava. They are classified as basalt with porphyritic texture of pyroxene and olivine as main phenocrysts with minor abundants of plagioclase phenocrysts, in the groundmass of fine grains plagioclase, opaque minerals, pyroxene, and volcanic glass that they formed the intersertal texture. Olivine have the forsterite composition (Fo63-92 ) with normal zoning texture. Pyroxene have augite and diopside composition with normal and reverse zoning. Plagioclase in groundmass have range composition of bytownite to high-K albite with anortite composition of An3-89 . In geochemically, their basalt have magma series of high-K calc-alkaline - alkaline basalt in the island arc of volcanic system and they can devided as nepheline normative mineral basalt (Ne-norm basalt) and hypersthene normative mineral basalt (Hy-norm basalt). The magmatic condition before eruption in marine environment have temperature of 1160-12900C dan pressure of 3-9 kbar, the crystal fractionation of olivine, clinopyroxene, minor plagioclase, and magma mixing with injection of new basaltic magma as important process in their magmatic differentiation before the basaltic lava erupted in the marine environment. 2 Thank you for your attention