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A Petrochemical study of the Late Cenozoic Granitic Rock from Sulawesi Indonesia

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  • Research Subject Categories::Natural Sciences

Abstract

Sulawesi, with its unusual K-shape consisting of four arms that merge in the central part of the island, is composed of an intricate collage of metamorphic complexes, ophiolite terranes, volcanic arcs, granitoid belts, and sedimentary basins. This represents a complex history of subduction, accretion, ophiolite obduction and collision. Together they form four major litho-tectonic units comprising the Eastern, Northern and Western Sulawesi Provinces and the Banggai-Sula Microcontinent. One of the most widely distributed rock units in this island are granitic rocks, which cover almost 20% of the island stretching from the southern part of the Western Sulawesi Province to the central part of the Northern Sulawesi Province. Some granitic rocks have been identified on the basis of geochemical characteristics, but to date no detailed examinations of the granitic rock units have been undertaken. This dissertation is based on field sampling of granitic rocks from 11 areas in the Western and Northern Sulawesi Provinces. Over 110 granitic samples including some enclaves were systematically collected from various localities in the studied areas. Of these, 80 thin sections were prepared and studied petrographically to determine the rock types, mineral assemblages, fabric and textural relations. Concentrations of major elements of 84 fresh samples were analyzed using the X-ray fluorescence spectrometer (XRF), whereas trace and rare earth elements were determined by the inductively couple mass spectrometer (ICP-MS) method. Isotopic ratios of Sr, Nd, Pb and O were determined for 12 samples using a Thermofisher Triton RPQ multicollector mass spectrometer (for radiogenic isotope) and VG Optima mass spectrometer (for oxygen isotope). In order to get the cooling age, the argon-argon dating method was used for geochronologic analyses of 3 samples. Mineral chemistry analysis was conducted on the basis of electron microprobe analysis (EPMA) using a JEOL JXA-8800R to determine the mineral composition used for geotermobarometry calculation. Magnetic susceptibility of the rocks was measured in the field with a portable KT-10 Magnetic Susceptibility Meter of Terraplus. XRD analysis was conducted in order to determine the mineral composition and clay mineral in the weathered samples. Petrographical and geochemical study shows that the granitic rocks have a considerable compositional range from granodiorite through quartz monzonite, monzonite, monzodiorite, syenite to granite with enclaves of diorite. Major element composition (SiO2 and K2O) indicates that the plutons can be classified as high-K or shoshonitic (HK) which are concentrated in the southern and central-western (CW) part of the Western Sulawesi Province, high-K calc-alkaline (CAK) which are found in the central and north-western (NW) part of the province and low-K to tholeiitic series which are dominated in the central part of the Northern Sulawesi Province. However, some samples from Masamba and Mamasa Pluton which are located in the CW part of the Western Sulawesi Province show a low-K to tholeiitic affinity. Most of the granitic rocks are metaluminous I-type granitic rocks. With an exception of tonalitic rocks in Gorontalo, all granitic samples resemble the upper continental crust pattern in their trace and rare earth element patterns. Enrichment of large ion lithophile elements (Rb and Sr) and depletion of high field strength elements (especially Nb and Ta) suggests an arc magma affinity. Negative Eu anomaly in most of the samples shows plagioclase fractionation during magmatic differentiation. Most of the samples show a high 87Sr/86Sr values but a low 143Nd/144Nd, suggesting a strong upper crustal component sources. In addition, they have high 206Pb, 207Pb and 208Pb isotope ratios. However, microdioritic enclaves and tonalitic rocks from Gorontalo Pluton in the Northern Sulawesi Province show lower 87Sr/86Sr values but higher 143Nd/144Nd and relatively higher 206Pb, 207Pb and 208Pb values, suggesting a more basic source. Whole-rock ??18O values from the granitic rocks are in the range of +5.7 to +9.6 permil (outlier three samples lower than +5.1 permil and two samples higher than +12 permil). The low ??18O value can be attributed to the introduction of meteoric hydrothermal alteration whereas the higher ??18O value indicates the significant involvement of high ??18O metasedimentary rocks in the melting process. 40Ar/39Ar ages of hornblende and biotite separated from the granitic rocks in Sulawesi range between 9.5 and 3.12 Ma suggesting that the cooling occurred during the Late Miocene to Late Pliocene. The magnetic susceptibility of the granitic rocks varies between 0.08 x 10-3 SI to 18.5 x 10-3 SI, corresponding respectively to ilmenite-series (< 3x 10-3 SI; reduced type) and magnetite-series (> 3x 10-3 SI; oxidized type) granite. Geothermobarometry condition was calculated for 5 different plutons; including the Mamasa and the Masamba Plutons from CW of Western Sulawesi Province; the Lalos-Toli and the Sony Pluton from NW of Western Sulawesi Province and the Gorontalo Pluton from Northern Sulawesi Province. Pressure and temperature of crystallization estimation were based on the Al-in-hornblende geobarometry whereas the temperature of formation were calculated using the hornblende and plagioclase geothermometry. The results show that the Mamasa and Masamba Plutons were crystallized at 0.91 to 1.2 kbar with the formation temperature of 677 to 729 ??C and 2.3 to 2.8 kbar with temperature range of 756 to 774??C, respectively. The Lalos-Toli and Sony Plutons were crystallized at 3.1 to 3.3 and 3.2 to 3.4 kbar at temperature of 731 to 736??C and 601 to 609??C, respectively whereas the Gorontalo Pluton were crystallized at pressure of 2.6 to 2.7 kbar and temperatures of 662 to 668??C. The crystallization depths were estimated from the pressure of crystallization and the results coupled with the Ar-Ar age were used to calculate the tentative exhumation rate. Crystallization depth of 3.2 to 4.3 km and 8.2 to 10 km were estimated from the Mamasa and Masamba Pluton respectively whereas the Lalos-Toli and Sony Plutons show deeper crystallization depth (11.3 and 11.6 km, respectively). The Gorontalo Pluton shows an average emplacement depth of 9.3 km. The exhumation rate estimation shows that the Mamasa and Masamba Plutons were exhumed at a rate of 0.39 and 1.68 mm/year respectively, whilst Lalos-Toli and Sony Plutons at 1.69 and 2.69 mm/year, respectively and Gorontalo Pluton was exhumed at 0.51 mm/year. The rapid exhumation rate of the Sony Pluton is attributed to the active vertical movement of the Palu-Koro Fault Zone. The oxygen fugacity calculation showed that the Mamasa, Masamba and Lalos-Toli Pluton are classified as reduced- and contaminated- I type features consistent with their magnetic susceptibility values, whereas the Sony and Gorontalo Pluton show a normal I type rocks. The occurrence of reduced (ilmenite-series) - I-type granitic rocks were resulted from reduced magmas related to carbon-bearing metasedimentary rocks. The exhumations of the granitic rocks in the Western Sulawesi Province were mainly triggered by the collision of the Banggai-Sula microcontinent with eastern Sulawesi in the Late Miocene to Pliocene whereas the exhumation of the granitic rocks in the Northern Sulawesi Province was attributed to the subduction of Celebes Sea Basin. The crystallization depth estimates refute the low-angle extensional type of emplacement model. Study on geochemistry of REE in the weathering crust from granitic rocks, particularly from Mamasa and Palu region shows that the weathered crusts can be divided into horizon A (lateritic profile) and B (weathered horizon) in Mamasa and horizon C (weathering front) in Palu region. Quartz, kaolinite, halloysite and montmorillonite prevail in the weathered crust and weathering front. Both weathered profiles show that the total REE increased from the parent rocks to horizon B, but significantly decrease towards the upper part (horizon A). The total REE content of the weathered crust are relatively elevated compared to the parent rocks, particularly in the lower part of horizon B in the Mamasa profile and in horizon C in Palu profile. The mass transfer illustration using isocon diagram shows a different transfer trend from Mamasa and Palu weathering profiles. The positive Ce anomaly in the horizon A of Mamasa profile indicated that Ce is rapidly precipitated during weathering and is retained at the upper soil horizon. Geochemical data shows that the petrogenesis of the granitic rocks was controlled not only by fractional crystallization processes but also by crustal contamination, particularly for the HK and CAK granitic rocks in the Western Sulawesi Province. Magnetic susceptibility suggests that country rocks bearing reduced organic carbon may have played an important role in the reduction of normal I-type granitic rocks to reduced- I-type. Radiogenic isotopic data suggests that the HK and CAK granitic rocks were derived from partial melting of lower crustal sources with an arc signature. Low-K to tholeiitic series magma in the Gorontalo have originated from amphibolite in the lower to mid crust which were partially melted and mixed with a crustal source producing low Sr and high Nd isotopic values. The HK and CAK granitic rock occurrences were linked to the geodynamic setting of collision to subduction between microcontinent derived from Australia and Sundaland (Eastern Sulawesi) particularly in the CW and NW Western Sulawesi Province from Late Miocene to Late Pliocene. The low-K to tholeiitic granitic rocks in Masamba and Mamasa plutons share a similarity with the Lamasi Complex which originated from oceanic floor basalt. The low-K to tholeiitic series in the Gorontalo Pluton suggests an origin from subduction of lower crustal segment in the Celebes Sea. Although regionally, the granitic rocks from Sulawesi are dominated by ilmenite-series granites, the ratio of ilmenite/magnetite series granites decreases substantially from the southern part to the northern part of the island, indicating various magmatic processes and sources. The occurrence of the ilmenite-series with I-type character (reduce I-type) in the southern and CW part of the Western Sulawesi Province may be explained by an assimilation process between magma and crustal materials containing a variable quantity of reduced C- and S-bearing sediments whereas the occurrences of the normal- I type magnetite-series in the NW part of the West Sulawesi Province and in the Northern Sulawesi Province resulted by contamination of more basic sources. The differences in oxygen fugacity, source rock composition, country rocks, petrogenesis and mineral chemistry play a key part in the genesis of mineralization in Sulawesi. It is suggested from this study that both granitic series are associated with Au ??? Cu and base metal (Pb, Zn and Cu) mineralization.

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