Geochemical Signatures of the potassic to sodic Adang Volcanics, Western Sulawesi : Implications for their tectonic setting and origin

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The Adang Volcanics represent a series of (ultra-)potassic to sodic lavas and tuffaceous rocks of predominantly trachytic composition, which forms the part of a sequence of Late Cenozoic high-K volcanic and associated intrusive rocks occurring
  Indonesian Journal on Geoscience Vol. 3 No. 3 December 2016: 195-214 (Original Draft)  Adang Volcanics, Page : 1/19 Geochemical Signatures of the potassic to sodic Adang Volcanics, Western Sulawesi : Implications for their tectonic setting and srcin Godang Shaban 1* , Fadlin 2 , Bambang Priadi 3   1: Geochemist, Rare Minerals & REEs Researcher, 2: Geological Engineering, Jenderal Soedirman University (UNSOED), 3: Department of Geology, Institute of Technology Bandung (ITB), Corresponding author: [email protected]  Abstract The Adang Volcanics represent a series of (ultra-)potassic to sodic lavas and tuffaceous rocks of  predominantly trachytic composition, which forms the part of a sequence of Late Cenozoic high-K volcanic and associated intrusive rocks occurring extensively throughout Western Sulawesi. The tectonic setting and srcin of these high-K rocks have been the subject of considerable debate. The Adang Volcanics have mafic to mafitic-intermediate characteristics (SiO2: 46–56 wt%) and a wide range of high alkaline contents (K2O: 0.80–9.08 %; Na2O: 0.90–7.21 %) with Total Alkali 6.67–12.60 %. Al2O3 values are relatively low (10.63–13.21 %) and TiO2 values relatively high (1.27–1.91 %). Zr and REE concentrations are also relatively high (Zr: 1154–2340 ppm; Total REE (TREY = TRE): 899.20–1256.50 ppm; TRExOy: 1079.76– 1507.97 ppm), with an average Zr/TRE ratio of ~ 1.39. Major rock forming minerals are leucite/pseudoleucite, diopside/aegirine, and high temperature phlogopite. Geochemical plots (major oxides and trace elements) using various diagrams suggest the Adang Volcanics formed in a post-subduction, within-plate continental extension/initial rift tectonic setting. It is further suggested magma was generated  by minor (<0.1%) partial melting of depleted MORB mantle material (garnet-lherzolite) with the silicate melt having undergone strong metasomatism. Melt enrichment is reflected in the alkaline nature of the rocks and geochemical signatures such as Nb/Zr > 0.0627 and (Hf/Sm) PM  > 1.23. A comparison with the Vulsini ultrapotassic volcanics from the Roman Province in Italy shows both similarities (spidergram  pattern indicating affinity with Group III ultrapotassics volcanics) and differences (nature of mantle metasomatism).  Keywords : Adang Volcanics, sodic and potassic/ultrapotassic, within-plate continental extension/initial rift, metasomatized silicate melts, leucite/pseudoleucite. 01.   Introduction. Large parts of the Western Sulawesi Province are covered by thick (up to 5000 m) piles of Upper Cenozoic shoshonitic to ultra-potassic and subordinate sodic volcanic rocks together with associated intrusives and volcaniclastics. The volcanic rocks occurring in the central part of the province have been subdivided into four units, these are Sekala Formation, Sesean, Adang, and Talaya Volcanics (Ratman and Atmawinata, 1993). The Adang Volcanics are the subject of this paper. Their location is shown in Figure 1. The unit consists of a sequence about 400 m thick of poorly bedded leucite-bearing lapilli tuff, volcanic  breccias consisting of leucite basalt fragments embedded in leucite-bearing tuffaceous matrix and leucite  basalt flows (Ratman and Atmawinata, 1993). Major element analyses carried out previously on a number of samples suggest the Adang Volcanics are mafic to intermediate in composition, ranging from trachyte to tephrite and phonolite (Sukadana et al., 2015). Waele and Muharam (2014) describe the rocks as  phlogopitic and leucitic volcanic rocks The Adang Volcanics interfinger with the marine sedimentary Mamuju formation, which has a latest Miocene to Early Pliocene age (Ratman and Atmawinata, 1993), and have yielded K/Ar ages of 5.4 and 2.4 Ma (Bergman et al., 1996). The morphology of the volcanics is youthful, showing a volcanic center and several domal structures (Sukadana et al., 2015). Opinions differ regarding the srcin of these and other high-K rocks in the province: 1) Magmatism took place in a post-subduction, continental-margin rift setting with the source mantle having been metasomatized by previous subduction processes (e.g. Yuwono et al., 1985; Leterrier et al., 1990);  Indonesian Journal on Geoscience Vol. 3 No. 3 December 2016: 195-214 (Original Draft)  Adang Volcanics, Page : 2/19 N o r t h  N o r t h  S u l a w e s i S u l a w e s i T r e n c h T r e n c h       P    a     l    u     P    a     l    u   -   -      K    o    r    o      K    o    r    o      F    a    u     l     t      F    a    u     l     t    S  a   n  g     i    h  e     T   r  e   n  c    h   S  a   n  g     i    h  e    T   r  e   n  c    h Ba tui Tru s tBa tui Tru s t Matano Fault Matano Fault  W         a      l         a      n      a      e      F         a      u      l         t        W         a      l         a      n      a      e      F         a      u      l         t                        T          o               l          o               T           r          e           n          c               h               T          o               l          o                T           r          e           n          c               h 124124 oo EE120120 oo EE22 oo NN22 oo SS66 oo SS 100 Km100 KmNN Neogenesedimentary rocksPaleogenesedimentary rocksMesozoicsedimentary rocksPliocene-RecentvolcanicsNeogenecalc-alkalinevolcanicsNeogenepotassivevolcanicsPaleogenevolcanicsIntrusive rocksMetamoorphicrocksOphioliteMajor thrustMajor strike-slipfault 9,680,0009,680,0009,685,0009,685,0009,690,0009,690,0009,695,0009,695,0009,700,0009,700,0009,705,0009,705,0009,710,0009,710,0009,715,0009,715,000695,000695,000700,000700,000705,000705,000710,000710,000715,000715,000720,000720,000725,000725,000730,000730,000735,000735,000 Mamujucity      R    i    f    t    i   n   g     f   a   u    l    t Tmtv Tmtv ( ( TalayaTalayaVolcanic)Volcanic) Kls(Latimojong formation)Tmpi (Granitoid complex)    M  a   k  a  s     s  a   r   M  a   k  a  s  s  a   r S   t   r  a   i   t   S   t   r  a   i   t SW SW CW CW NW NW  E     a   s   t     e   r    n    E     a   s   t     e   r    n    S    u    l     a   w    e   s   i     S    u    l     a   w    e   s   i      N o r t h e r n  N o r t h e r n  S u l a w e s i S u l a w e s i  TmaTma( (  Adang  Adang VolcanicsVolcanics ), ),(Mid (Mid - - Late Miocene)Late Miocene) (1b)(1b)(1a)(1a) 2) The volcanic rocks were formed in an Active Continental Margin (ACM) setting in which magma was generated from mantle melting in the final stage of the subduction process (Puspita et al., 2005), which involved subduction of micro-continental crust (Sukadana et al., 2015); 3) The volcanics developed as the result of collision between the Banggai-Sula fragment and western Sulawesi (Bergman et al., 1996). The goal of our research is to deduce from the magmatic typology of the Adang Volcanics their tectonic setting and the nature of sub-mantle enrichment, using the geochemical approach. The study area is defined by the following coordinates 118°45'11.10"E - 119°5'13.41"E, 2°33'28.27"S - 2°59'34.52"S, covering the Sub-regencies of Kalukku, Mamuju, Tabulahan, Simboro, Tapalang and Malunda. 02. Materials and Methods. For this purpose, six outcrop samples (locations shown on Figures 1a and 2) were subjected to major oxide, trace element, and rare earth element (REE) analyses (Tables 1 and 2). The rocks also studied in  petrographically in order to support the interpretation of the geochemical data. Geochemical analyses involved XRF and four acids digest (ICP-OES/MS, ICP-REE) assay methods, which were carried out at Intertek Laboratories in Jakarta and Perth. In addition, the trace element data have been plotted on various diagrams previously obtained from 31 company drill core samples, and major oxide data for 36 previously analyzed outcrop samples for which LOI contents were not determined. These samples were also analyzed in the Intertek Laboratories. Figure : (1a). Geological map of central Western Sulawesi (Ratman and Atmawinata, 1993) showing the location of the research area and the six samples described in the text. Coordinate system UTM Zone 50S; (1b). Simplified geological map of Sulawesi (modified after Sukamto, 1975b; Hamilton, 1979; Silver et al., 1983; Parkinson, 1991)(in Van Leeuwen and Pieters, 2011). Western Sulawesi (dashed line) subdivided into NW (Northwest; north part of Western Sulawesi), CW (central-west) and SW (southwest) Sulawesi. 03. Field Relationships The field and petrologic observations indicate the Adang Volcanics, covering an area of ~820 Km2 (Ratman and Atmawinata, 1993) are composed predominantly of leucite/pseudoleucite-bearing trachytic tuff, lapilli-tuff, agglomerate, volcanic breccia, volcanic-sedimentary products (volcaniclatics consisting of trachytic weathering residue, trachytic fragments), volcaniclastics and lava intercalations of mafic/mafitic-intermediate composition (consisting of leucite/pseudoleucite, diopside/aegirine and high temperature  phlogopite). Other rocks observed locally include peralkaline dyke, a mafic trachyte body, multiple dykes of mafic and leucitic intrusions, subvolcanic sanidine trachyte intrusion, intercalations of crystalline limestone, and other carbonate rocks. The limestone intercalations and the presence of pillow lavas indicate the Adang Volcanics were deposited in a marine environment. Figures 3 and 4 show how the volcanics occur in the field.  Indonesian Journal on Geoscience Vol. 3 No. 3 December 2016: 195-214 (Original Draft)  Adang Volcanics, Page : 3/19 Scale 1 : 180,000Scale 1 : 180,000 Mamuju Mamuju city city      R    i    f    t    i   n   g      f   a   u    l    t    R    i    f    t    i   n   g      f   a   u    l    t TmaTmaTmtv Tmtv KlsKls Tmpi Tmpi  Sample for Sample for Petrographic Petrographic (GS1(GS1- - 0016 0016 - - 17)17)Sample for Sample for Petrographic Petrographic (GS1(GS1- - 0607 0607 - - 10)10)Sample for Sample for Petrographic Petrographic (GS1(GS1- - 0926 0926 - - 28)28) 9,680,0009,680,0009,685,0009,685,0009,690,0009,690,0009,695,0009,695,0009,700,0009,700,0009,705,0009,705,0009,710,0009,710,0009,715,0009,715,000695,000695,000700,000700,000705,000705,000710,000710,000715,000715,000720,000720,000725,000725,000730,000730,000735,000735,000   Figure 2. Image of DEM-SRTM (ASTER 30m) and the position of sampling points. 04. Analytical Results   The mineralogy of three samples in thin secton is shown in Figures 5-7. Moreover, major and trace element results for the six samples presented in Tables 1 and 2 respectively have a Mafic Index of Alteration (MIA(o)) ranging from 31.6 to 45.9 (Fig. 8). This suggest that all six samples were fresh (after    Nesbitt and Wilson, 1992, including two samples (No.82 and No.1002) with relatively high LOI contents (4.3–5.7 wt%). This is supported by petrographic observations (Fig. 7). The Adang Volcanics samples are mafic/mafitic-intermediate in composition with relatively low SiO2 contents (46–56 wt%). Na2O and K2O contents show a wide range of values, 0.90–7.21 wt% and 0.80–9.08 wt%, respectively. Four of the samples have high K2O contents (7.86 and 9.08 wt%), and Na2O contents varying between 0.90 and 3.52 wt%, which classifies these rocks as ultrapotassic (Foley et al., 1987). The other two samples have high  Na2O contents (5.52 and 7.21 wt%) and low K2O values, indicating they have a sodic composition. It is these two samples that also have high LOI contents, possibly indicating the presence of primary volatiles. Al2O3 contents are relatively low (10.63–13.21 wt%) and TiO2 contents relatively high (1.27–1.91 wt%) (typical of non-subduction volcanics {TiO2 > (-1.161 + 0.1935 x Al2O3), (in wt%)}(Fig. 9)(Muller & Groves, 1993 and 2000). Zircon concentrations are high (1154–2340 ppm), the Total REE (TREY = TRE) ranges between 899.20 and 1256.50 ppm, and TRExOy between 1079.76 and 1507.97 ppm (Table 2). The percentage of LREE is 88.23% (La--Sm) and that of HREE 11.77% (Eu--Y). The Th/U ratio is ~ 4.32/1, indicating a “Non-Uranium anomaly”, based on a Th/U ratio of ~ 3/1 {normal abundance of the earth crust : Th (6 ppm), U (1.8 ppm)} ( Relatively high radiometric signatures (200–419 cps) may be related to relatively high contents of Zr, K, Th, U and Ba (Fig. 3g).  Indonesian Journal on Geoscience Vol. 3 No. 3 December 2016: 195-214 (Original Draft)  Adang Volcanics, Page : 4/19 No287582991211002Sample_codeGS1-0016-17GS1-0481-85GS1-0533-36GS1-0607-10GS1-0280-86GS1-0926-28Rock_typesFine TrachyteMafic TrachyteLeucitic TrachyteMafic TrachyteMafic TrachyteFine TrachyteAreaSalu DangoTakandeangAmpalasBottengBebangaTapalangSiO2 55.4849.2854.8650.7949.1553.74 TiO2 1.911.771.271.451.701.70 Al2O3 10.9110.9213.2110.6311.8912.10 Fe2O3 11.7810.807.509.8212.8011.42 FeO MnO MgO 2.545.873.006.193.532.11 CaO 2.036.983.837.254.573.32 Na2O 3.520.905.521.401.897.21 K2O LOI 0.901.325.701.211.204.32 P2O5 0.220.860.550.560.460.22 Total100.7699.9699.5499.3499.2198.56TA=Na2O+K2O12.609.936.679.2610.058.01K2O/Na2O2.5810.030.215.614.320.11Cr2O3 0.0060.01<0.0050.040.04<0.005 Radiometric345 cps255 cps419 cps200 cps250 cps210 cps No287582991211002Sample_codeGS1-0016-17GS1-0481-85GS1-0533-36GS1-0607-10GS1-0280-86GS1-0926-28Rock_typesFine TrachyteMafic TrachyteLeucitic TrachyteMafic TrachyteMafic TrachyteFine TrachyteCr  5.0056.0030.00197.005.005.00 Cu 128.00144.00122.00104.00199.0096.00 Mn 1,600.001,320.00831.00965.001,150.00838.00 Ni 14.0046.0035.00116.0017.0011.00 P 900.003,740.002,410.002,510.002,020.00870.00 S 90.00<501,330.00160.001,150.0060.00 Sc 7.0021.009.0023.0011.008.00 Ti 11,300.0011,000.008,030.009,180.0010,100.009,650.00 V 254.00218.00177.00210.00350.00319.00 Zn 162.00136.00130.00133.00180.00101.00 Ag <0.10.300.300.300.500.40 As 17.006.0029.005.0015.004.00 Ba >50002,450.004,970.004,270.00>5000>5000 Be 34.2019.8028.4025.1028.3039.00 Bi 1.571.530.720.892.490.95 Cd 1.360.750.750.831.101.06 Co 22.0033.0023.0030.0028.0017.00 Cs 32.3057.7050.6068.2031.60214.00 Ga 21.4020.2023.7020.1024.3023.00 Ge 0.400.600.400.800.200.60 Hf  61.5036.6035.2031.9045.3047.20 In Li 61.5033.1033.3030.8052.0012.60 Mo 0.300.301.800.400.300.20 Nb 192.00102.00117.0094.70121.00104.00 Pb 330.00143.00293.00147.00313.00203.00 Rb 336.00636.00614.002,126.26498.001,310.00 Sb 0.600.401.400.700.800.70 Re <0.05<0.05<0.05<0.05<0.05<0.05 Se <1<1<1<1<1<1 Sn 29.8018.1024.0015.6025.7031.00 Sr  699.001,350.00930.001,300.001,410.00769.00 Ta 10.005.836.384.827.308.44 Te <0.1<0.1<0.1<0.10.10<0.1 Th 310.00154.00 266.80 167.80 230.30 324.10  Tl 4.842.661.483.435.5212.90 Table 1 : Major Oxides (wt%) of six samples (Adang Volcanics). Table 2: Trace element concentrations (ppm) in six samples (Adang Volcanics).  Indonesian Journal on Geoscience Vol. 3 No. 3 December 2016: 195-214 (Original Draft)  Adang Volcanics, Page : 5/19 NoLaCePrNdSmEuGdTbDyHoErTmYbLuYTRE28 277.00 552.00 59.60194.0032.305.8021.703.1016.302.507.100.905.700.7067.60 1,246.3075 230.60445.4052.90 196.6032.80 6.10 21.80 3.00 13.10 2.10 5.40 0.80 4.20 0.30 61.90  1,077.00 82 234.20429.7047.50 166.7027.40 4.90 20.20 2.70 12.60 2.00 5.40 0.90 4.60 0.60 63.20  1,022.60 99 194.20371.9044.30 159.7026.20 4.70 18.10 2.40 10.90 1.70 4.90 0.60 3.70 0.60 55.30  899.20 121 263.60513.7058.20 191.2033.70 6.50 24.40 3.20 15.20 2.60 6.40 0.90 5.40 0.80 69.40  1,195.20 1002 292.30469.0063.10 211.0037.70 7.20 30.40 4.00 19.20 3.40 8.90 1.20 7.40 1.00 100.70 1,256.50 avg248.65 463.62 54.27 186.53 31.68 5.87 22.77 3.07 14.55 2.38 6.35 0.88 5.17 0.67 69.68 1,116.13  %22.28%41.54%4.86%16.71%2.84%0.53%2.04%0.27%1.30%0.21%0.57%0.08%0.46%0.06%6.24%100.00%LREE = 88.23%HREE = 11.77% No287582991211002Sample_codeGS1-0016-17GS1-0481-85GS1-0533-36GS1-0607-10GS1-0280-86GS1-0926-28Rock_typesFine TrachyteMafic TrachyteLeucitic TrachyteMafic TrachyteMafic TrachyteFine TrachyteU 62.8032.6068.1045.8051.3075.50 W 3.004.5018.403.208.602.30 Zr  2,340.001,384.00 1,269.00 1,154.00 1,549.00 1,651.00  La 277.00230.60 234.20 194.20 263.60 292.30  Ce 552.00445.40 429.70 371.90 513.70 469.00  Pr  59.6052.90 47.50 44.30 58.20 63.10  Nd 194.00196.60 166.70 159.70 191.20 211.00  Sm 32.3032.80 27.40 26.20 33.70 37.70  Eu 5.806.10 4.90 4.70 6.50 7.20  Gd 21.7021.80 20.20 18.10 24.40 30.40  Tb 3.103.00 2.70 2.40 3.20 4.00  Dy 16.3013.10 12.60 10.90 15.20 19.20  Ho 2.502.10 2.00 1.70 2.60 3.40  Er  7.105.40 5.40 4.90 6.40 8.90  Tm 0.900.80 0.90 0.60 0.90 1.20   Yb 5.704.20 4.60 3.70 5.40 7.40  Lu 0.700.30 0.60 0.60 0.80 1.00   Y 67.6061.90 63.20 55.30 69.40 100.70  TRE1,246.301,077.00 1,022.60 899.20 1,195.20 1,256.50  TREO 1,391.461,202.53 1,142.18 1,004.26 1,334.62 1,404.64  TRExOy1,497.891,292.80 1,228.36 1,079.76 1,435.94 1,507.97  TRE2O3 1,464.031,265.29 1,201.96 1,056.78 1,404.32 1,478.70  dEu 0.670.70 0.64 0.66 0.69 0.65  * average values of Th (242.17 ppm) and U (56.02 ppm); average Th/U ratio = 4.32/1, Zr/TRE (avg) = 1.39 05. Interpretation of results. The analytical results on various diagrams have been plotted in a diagram to classify the Adang Volcanics, and to elucidate their tectonic setting and the processes in the source mantle that led to their formation. On the TAS diagram of Le Bas et al. (1986) most samples (including the 36 other samples) plot in the trachybasalt and basaltic-trachyandesite fields (Fig. 10). The two sodic samples fall under the basaltic-trachyandesite field, while the remaining “other samples” are in the tephrite and alkali-basalt fields. On the K2O vs SiO2 diagram (after Peccerillo & Taylor, 1976) (Fig. 11) the Adang Volcanics are represented by three series: potassic alkaline, alkaline, and subalkaline. If we use the recently developed Godangs Trapezoid Geochemistry Diagram (Godang, 2016), which also takes Al2O3 and NaO contents into consideration (Fig. 12), most of the 43 samples plot in the sodic-series and shoshonitic-series fields, and some in the ultrapotassic-series field. A plot on the Rock Classification Diagram of Pearce (1996), which is  based on relatively immobile trace elements, indicates that the rocks are of trachyte affinity (Fig. 13), consistent with the TAS results. As a comparison, data for a leucite phonolite have been plotted from the Vulsini Volcano, Roman Province, Italy (Santii et al., 2003) on Fig. 11, which shows the rock belongs to the ultrapotassic sub- alkaline series (alkaline-calcic character, alumina oversaturated). It should be noted that its alumina content (Al2O3 : 19.97–20.37 %) is significantly higher than that of the Adang Volcanics (Al2O3 : 10.63–13.21 %). Using the diagram designed by Muller & Groves (1994) to discriminate the tectonic settings of potassic to ultrapotassic rocks, the plotting result shows that the Adang Volcanics samples fall within the anorogenic within-plate field (Figures 9 and 14). In contrast, the Vulsini Volcanics appear to have formed in an arc-
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