CHAPTER 3 METHODS 3.1 PREPARATION AND SAMPLING METHODS

Transcription

1 CHAPTER 3 METHODS 3.1 PREPARATION AND SAMPLING METHODS Two field seasons were conducted in mid-may 2007 and during December 2008 in the Bantimala and Barru blocks, south Sulawesi. These provided samples for the ensuing petrology, geochemistry and metamorphic evolution investigation of the basement complexes. Samples collected during these sessions are summarised in Table A1 and A2 of Appendix 1. Preliminary reconnaissance was undertaken with the aid of topographic maps and aerial photographs, which were then used to delineate the distribution of the basement rocks on a topographic base map. Regional geological mapping, supported by older geological maps from previous workers, was conducted to outline the geology of these areas on the scale of 1: Measured sections were used as the main sampling method, however given the high level of alteration and high rate of erosion shown by some of the outcrops, some samples were taken from fresh boulders in the rivers. The measured section transects were mainly in areas where the outcrops of the basement rock are well exposed, especially along rivers. Lithology, mineral assemblages and geological relationships were mapped and described. Some locations of sample collection were specifically chosen in order to ensure representative sample collection from the Bantimala and Barru blocks (Fig. 3.1 and 3.2). The sample locations were focused on areas where the outcrops of the basement rock are well exposed especially in the river section and at the mountain flanks. In the Bantimala block, site 1 was at the junction of the Cempaga River and Pateteyang River near Mangilu village, and can be reached only on foot, with no possible access for a vehicle. This site is located approximately 7 km west of Pangkajene, the capital of Pangkep Regency. Site 2 was taken in the Bantimala River, north of Bantimala village and approximately 3 km north of Mangilu village. Access to this site is by either small 31

2 Fig.3.1 Sample locations in the Bantimala Block. 32

3 4-wheeled vehicle or motorcycle parallel to the river. However, the only access to the river is walking, since the topography is very steep. Sites 3 and 4 were in the Batupute River and on Batugarencing Hill, north of Bantimala village, respectively. These sites can be reached by small vehicle (including motorcycle) right up to the outcrops. Site #5 was on Moreno Hill, approximately 20 km north from Bantimala village; this can also be reached by small vehicles. In the Barru block, the sample locations were distributed in the Dengenge River, Kamara village, Camming village, and Lasitae Mountain. Site 1 was in the Dengenge River, approximately 5 km west of Barru, the capital of Barru Regency. This site can be reached by driving to the closest village, Anabanua village, and then walking across the hillside. Site 2 was taken in Kamara village, which is located near Barru and can be reached by vehicle. Site 3 was in Camming village, approximately 2 km west of Barru, whereas site 4 was on Lasitae Mountain, which is located between the Dengenge River and Camming village. The outcrops in the Camming area are freshly exposed near the main road, whereas the only access to the Lasitae Mountain site is by walking through the rain forest from either Camming or Anabanua village. Site 5 was in the Sabangnairi Hill, approximately 1 km westward of Batubessi Villages and can be easily reached by all vehicles. 3.2 ANALYTICAL METHODS Altogether, 72 samples were collected during the field seasons. These included metabasites, metapelitic, metavolcanic and ultramafic rocks as well as metasediments from the two blocks. Of these, 60 thin sections were made and studied petrographically to determine the rock types, mineral assemblages, fabric and textural relations. Optical petrography was undertaken manually by using a Nikon petrographic microscope with 10 eyepieces and 5, 10, 20 and 40 objective lenses, equipped with a Nikon E4500 camera attached to the trinocular port for micrography. 33

4 Fig.3.2 Sample locations in the Barru Block. Two samples of mudstone containing radiolarians were examined for microfossil identification and age determination by Dr. Chris Hollis, Paleontology & Environmental Change, GNS Science, New Zealand. Both samples were processed by cleaning in heated hydrogen peroxide, flowed by leaching in 5% hydrofluoric acid (HF) for several hours. HF treatment was repeated two times. Residues were sieved through a 45 µm screen and examined under a stereomicroscope. Representative specimens were picked and mounted on a numbered cavity slide. Identifications and age determinations 34

5 are based on Pessagno (1977), Sanfilippo and Riedel (1985), O Dogherty (1994) and Hollis and Kimura (2001). In order to obtain quantitative compositional data for the minerals, 45 sections were examined in a JEOL 6400 scanning electron microscope, equipped with an Oxford Instruments light element dispersive spectrometer (EDS) detector and Link ISIS analytical software. Operating conditions for the energy-dispersive X-ray analyses (EDXA) were 15 kv accelerating voltage, 1 na beam current, and a range of beam diameters (higher current, focused beam for garnet; lower current, beam defocused to 5 µm for micas and plagioclase). Natural mineral standards and the ZAF matrix correction routine were used. The following standard were used: sanidine for Si and K, albite for Na and Al, diopside for Ca, TiO 2 and pure Ti for Ti, Fe 2 O 3 for Fe, Cr 2 O 3 for Cr, MgO for Mg, pure Mn for Mn, pure apatite for P, zircon for Zr and Hf, calcite for Ca, pyrite for S, chalcopyrite for Cu, pure Co for Co, pure nickel for Ni, and baryte for Ba. All samples were polished with 1 µm diamond paste and carbon-coated to approximately 20 nm thickness. In addition to spot analyses, the SEM was used to construct X-ray maps for Fe, Mn, Mg,Ca, and either Al or Si by using a beam current of 100 na, 50 ms dwell time, and 5 9 mm scanned area. These facilitated the identification of minerals in backscattered electron images, and the location of uncommon accessory minerals. Garnets were quantitatively analysed along traverses from core to rim to obtain zoning data. SEM analyses and carbon coating were carried out at the Electron Microscopy Unit, RSBS, at The ANU. Experimental data were combined with published phase equilibrium data and calibrations for geothermometers and geobarometers (Appendix 2) to determine quantitative and qualitative metamorphic conditions experienced by the metamorphic rocks. 41 samples were crushed and milled to obtain the whole rock and trace element compositions. Whole-rock major elements were analysed by X-ray fluorescence analyses (XRF), and whole-rock and individual mineral trace element analyses by laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). Major elements Na, Mg, Al, Si, P, S, K, Ca, Ti, Mn, Fe, F and Cl were assessed by XRF with a Phillips (now Panalytical) PW2400 wavelength-dispersive X-ray fluorescence spectrometer. Lithium borate discs were prepared by fusion of 0.27g of dried sample powder and 35

6 1.72g of eutectic lithium metaborate-lithium tetraborate at 1010 C for 10 minutes in a rocker furnace. The major elements were calibrated against a set of 28 international standard rock powders. Trace elements analyses were obtained by LA-ICP-MS at the Research School of Earth Sciences, ANU. Trace elements concentration were determined on glasses made from rock powders fused with lithium borate flux (1: 3 mass ratio). The LA ICP- MS employs an ArF + (193 nm) excimer laser and a Hewlett Packard Agilent 7500 ICP- MS. Laser sampling was performed in an Ar-He atmosphere using a spot size between 80 and 100 µm. The counting time was 20 seconds for the background and 60 seconds for sample analyses. The external standard for calibration was NIST 612 glass, using the standard reference values of Pearce et al. (1979). Si was employed as the internal standard, employing the SiO 2 concentration previously measured by XRF. All the results are displayed in Table 4-1 (Appendix 4). Loss-on-ignition (LOI) values were calculated from the mass differences in approximately 2 grams of powdered sample after heating to 1010 C in the furnace for one hour. Mineral abbreviations used in the discussion were after Kretz (1983); namely Grt = Garnet, Alm = Almandine, Grs = Grossular, Prp = Pyrope, Sps = Spessartine, Cpx = Ca clinopyroxene, Opx = orthopyroxene, Omp = Omphacite, Di = Diopside, Aug = Augite, Ens = Enstatite, Wo = Wollastonite, Fs = Ferrosilite, Ms = Muscovite, Pg = Paragonite, Bt = Biotite, Gln = Glaucophane, Act = Actinolite, Tr = Tremolite, Hbl = Hornblende, Ath = Anthophyllite, Pl = Plagioclase, Ab = Albite, Or = Orthoclase, Kfs = K feldspar, Ep = Epidote, Chl = Chlorite, Qtz = Quartz, Rt = Rutile, Ttn = Titanite, Ilm = Ilmenite, Zrn = Zircon, Zo = Zoisite, Czo = Clinozoisite, Cal = Calcite, Ap = Apatite, Ol = Olivine, Srp = Serpentinite, Py = Pyrite, Ccp = Chalcopyrite, Brn = Bornite, Pn = Pentlandite, Mag = Magnesite. Cr-spl and Fe-Cr are also used for Chromium spinel and Fe 3+ rich spinel, respectively and Phe for Phengite, Barr for Barroisite, Winc for Winchite and Brt for Baryte. 36