Geochemical Classification and Determination of Maturity Source Weathering in Beach Sands of Eastern San ’ in Coast , Tango Peninsula , and Wakasa Bay , Japan

Geochemistry of beach sand sediments collected from the Eastern San’in coast (n=17), Tango Peninsula (n=14) and Wakasa Bay (n=7) shorelines were investigated using XRF analyses for major and trace elements to characterize their composition, classification, maturity, provenance, tectonic setting and degree of weathering in source areas. Investigated sands from all sites were very similar showing depletion in all elements except SiO2, K2O and As relative to the UCCN and JUCN, suggesting a moderate geochemical maturation. Beach sand sediments from these locations can be classified as arkose, subarkose and litharenite that are chemically immature and formed under arid/semi-arid conditions with a tendency towards increasing chemical maturity suggesting that they are from multiple sources. The relatively low to moderate values of weathering indices of Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA) and Chemical Index of Weathering (CIW), the beach sands from all sites in the source area have undergone low to moderate degree of chemical weathering. A-CN-K and A-CNK-FM plots, which suggest a granitic source composition, also confirm that the sand samples from these sites have undergone low to moderate degree of chemical weathering in consistent with CIA, PIA and CIW values. A plot of the analyzed beach sands data on the provenance discriminating function F1/F2 showed that most of the investigated beach sand sediments in all locations fall within mafic to intermediate ocean island arc source; similar to the tectonic setting discrimination diagrams based on major elements suggesting a passive margin.


Introduction
The composition of coastal sediments is influenced by numerous components and processes, which contain important information about geochemical composition, weathering conditions and tectonic settings of the provenance and associated depositional basins.Beach sands are composed mainly of quartz, feldspar and other minerals resistant to wave abrasion and are products of a combination of weathering, fragmentation and degradation (Pettijohn et al., 1987).The geochemistry of clastic sediments can be effectively utilized for the evaluation of tectonic setting and provenance determination (Bhatia, 1983;Roser & Korsch, 1986, 1988;Condie et al., 1992).Tectonic setting discrimination diagrams based on major elements proposed by Roser & Korsch (1988, 1988) has been applied to this study to estimate the provenance and its tectonic setting.Chemical weathering indices such as Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA) and Chemical Index of Weathering (CIW) are commonly used for characterizing weathering profiles.These indices incorporate the bulk of major element oxide chemistry into a single value for each sample.In order to evaluate the chemical weathering intensity, the investigated beach sands from the Eastern San'in coast, Tango Peninsula, and Wakasa Bay were subjected to weathering indexes such as CIA, PIA and CIW.For further interpretation, a variety of triangular and scatter plots were also constructed from the geochemical data obtained.Other important reference compositions such as average Upper Continental Crust (UCC) (Rudnick & Gao, 2005) and average Japan arc Upper Crust (JUC) (Togashi & Imai, 2000) estimated from the representative surface rocks were also included for comparison.
The present study examines the geochemistry of sand samples collected from beaches on the Eastern San'in coast, Tango Peninsula and Wakasa Bay.The main purpose of the present is to determine their classification, maturity, sediment source area weathering, provenance and tectonic settings and to shed light on the source area paleo-weathering conditions in these sites.These factors were evaluated using elemental abundances, weathering indices and elemental ratios in comparison to the average Upper Continental Crust (UCC) and average Japan arc Upper Crust (JUC) as estimated from the representative surface rocks.This study summarizes the results of an investigation on beach sand samples from Eastern San'in coast (Hyogo and Kyoto Prefecture), Tango Peninsula (Kyoto Prefecture) and Wakasa Bay (Fukui Prefecture), southwest Japan.

Sampling and Analytical Methods
Locations of sampled sites are given in Figure 1.
Figure 1.Location of sampled beaches on the Eastern San'in coast, Tango Peninsula, and Wakasa Bay shorelines, Japan Sampling sites were selected based on accessibility and the character of the beach.Sampling was carried out at moderate to low tides, based on tidal information available from the Japan Meteorological Agency and beach sand samples were collected using a stainless steel scoop from the uppermost centimeters in selected locations.
Approximately 200 g of sand samples was collected from the foreshore of thirty-eight beaches along the Japan Sea coast.These included Eastern San'in coast (n=17), Tango Peninsula (n=14) and Wakasa Bay (n=7) (Figure 1).The sand samples were oven-dried at 110°C for 24 hours then crushed in an automatic agate mortar and pestle grinder.Contents of major (SiO 2 , TiO 2 , Al 2 O 3 , Fe 2 O 3 * , MnO, MgO, CaO, Na 2 O, K 2 O, and P 2 O 5 ) and trace (As, Pb, Zn, Cu, Ni, Cr, V, Sc, Y, Nb, Zr, Th, and Sr) elements were determined by X-ray fluorescence (XRF) at Shimane University using RIX-2000 Spectrometer (Rigaku Denki Co. Ltd) following the Ogasawara (1987) method.Contents of major elements were obtained on fused glass discs by the pre-ignited materials with an alkali flux involving 80% lithium tetraborate (Li 2 B 4 O 7 ) and 20% lithium metaborate (LiBO 2 ).Trace element abundances were determined by pressing prior powdered sand samples into a 40 mm diameter plastic rings using 200 kN force for one minute in an automatic pellet press (E-30 T.M Maekawa) Average errors for the analyzed elements are less than ±10%).

Major and Trace Elements Geochemistry
Data on geochemical composition of sand samples from the studied sites is shown in Table 1.
For comparison with the Upper Continental Crust (UCC) (Rudnick & Gao, 2005) and Japan arc Upper Crust (JUC) (Togashi & Imai, 2000) estimated from the representative surface rocks, means of major elements (Table 2a) and trace elements (Table 2b) are also shown.
Table 1.Major (wt%) and trace element (ppm) composition of beach sands from the Eastern San'in coast, Tango Peninsula and Wakasa Bay, Japan Results showed that SiO 2 , dominated the analyzed sand samples averaging 78.02wt% (Eastern San'in coast sands), 81.02wt% (Tango Peninsula sands) and 84.83wt% (Wakasa Bay sands) (Table 2a).Except for the samples from the Shibayama, Kirihama, and Takeno beaches with (66.82wt%; 69.06wt%; and 41.89wt %) respectively, SiO 2 contentswere relatively high in other sites.Contrary to their lower SiO 2 contents, the beach sands from Shibayama, Kirihama and Takeno had distinctively high CaO contents of 11.21wt%; 13.77wt% and 43.35wt% respectively (Table 1).The high SiO 2 concentrations result in low contents of other elements, with Al 2 O 3 contents of between 5.30 to 11.86wt% in Eastern San'in coast sands, 7.60 to 12.44wt% in the Tango Peninsula and 5.55to 10.27wt% Wakasa Bay sands (Table 2a).Average Al 2 O 3 contents are 8.45wt% in the Eastern San'in coast sands, 9.92wt% for the Tango Peninsula sands and 7.95wt% in the Wakasa Bay sands (Table 2a).Concentrations of the remaining major elements that are present in abundant amounts (TiO 2 , Fe 2 O 3 * , MnO, MgO, Na 2 O, K 2 O and P 2 O 5 ) were generally <5wt% and often <1wt% (Table 2a).Among the trace elements, the contents of ferromagnesian elements (Ni, Cr, V and Sc) and large cations (Y, Nb, Zr, Th and Sr) tended to be less abundant in contrast with average values of the Upper Continental Crust (UCC) and the Japan arc Upper Crust (JUC) (   (Rudnick & Gao, 2005) and JUC (Togashi & Imai, 2000) Table 2b.Summary statistics of trace element abundances in beach sands from the Eastern San'in coast, Tango Peninsula and Wakasa Bay, Japan compared to UCC (Rudnick & Gao, 2005) and JUC (Togashi & Imai, 2000)

Geochemical Classification
The geochemical classification of beach sand samples of Eastern San'in coast, Tango Peninsula and Wakasa Bay was investigated using the log ratios of Na 2 O/K 2 O plotted against the log ratios of SiO 2 /Al 2 O 3 (Pettijohn & Potter, 1972) and the log ratios of Fe 2 O 3 * /K 2 O plotted against the log ratios of SiO 2 /Al 2 O 3 (Herron, 1988).The SiO 2 content and SiO 2 /Al 2 O 3 ratio are the most commonly used geochemical criteria for differentiating mature and immature sediments (Potter, 1978); which also reflect the abundance of quartz, feldspar and clay contents.Herron, (1988)  ) < 0, lithicarenite can be confused with greywacke).The most peculiar characteristic in the geochemical classification of the beach sands from Eastern San'in coast, Tango Peninsula and Wakasa Bay is in their corresponding compositions and all these samples are scattered mostly around arkose, subarkose and litharenite (Figures 3a and b).Arkoses are immature sandstones with abundant feldspars; both potassic feldspar and plagioclasean indication ofrelatively immature beach sand sediments with low to moderate weathering.

Normalized Compositions
Average compositions of sands from Eastern San'in coast, Tango Peninsula and Wakasa Bay beaches were compared by normalization against average UCC and JUC.Results showed that the Eastern San'in coastal sands are strongly depleted in all elements except SiO 2 , CaO, As and Sr relative to both UCC N and JUC N (Figure 4).The enrichment of CaO relative to the UCC N and JUC N , is in consistent with the variable enrichment of carbonate compositions in the beach sands from Shibayama, Kirihama, and Takeno (Table 1).The similarity in normalized patterns observed in UCC N , JUC N as well as the patterns for the sands from Eastern San'in coast, Tango Peninsula and Wakasa Bay beaches further suggest moderate geochemical maturation.

Weathering Intensity and Chemical Maturity
As the degree of chemical weathering is a function of climate and rates of tectonic uplift (Wronkiewicz & Condie, 1987), the raising chemical weathering intensity suggests the decrease in tectonic activity and/or the change of climate towards warm and humid conditions which are more favorable to chemical weathering in the source region (Jacobson et al., 2003).Weathering indices of sedimentary rocks can therefore provide useful information of tectonic activity and climatic conditions in the source area.

Chemical Index of Alteration
The Chemical Index of Alteration (CIA) of Nesbitt andYoung (1982, 1984) was used to evaluate the degree of weathering.This index measures the extent to which feldspar has been converted to aluminous weathering products.CIA ratios in feldspar and fresh source rocks are typically ~50, whereas those in residual weathering products such as kaolinite and gibbsite can reach 100.This index can be calculated using molecular proportions, from the formula: Where CaO * represents the amount in silicates only.
In this study, CaO was corrected with subsequent methodology proposed by McLennan et al., (1993) (Nesbitt and Young, 1982).
The computed CIA values of the Eastern San'in coast sands ranged between 40 and 55 with an average of 50 an indication that no weathering has virtually occurred.The average CIA values for Tango Peninsula and Wakasa Bay sands were 54 and 55; ranging from 48 to 62 and 51 to 61 respectively.However, the values are moderately high relative to the ranges of Eastern San'in coast sands (Table 2a).These CIA values indicate that the Tango Peninsula and Wakasa Bay sands have undergone weak to intermediate chemical weathering.Overall, the relatively low to intermediate CIA beach sand values across the three study areas indicate low degree of chemical weathering, which may reflect cold and/or arid climate conditions in the source area (Nesbitt & Young, 1982).

Chemical Index of Weathering
Weathering effects can also be evaluated using the Chemical Index of Weathering (CIW) in molecular proportions) identical to the CIA, from the formula: Where CaO* is the CaO residing only in the silicate fraction.
This equation is more appropriate in understanding the extent of plagioclase alteration alone since K 2 O is subtracted from Al 2 O 3 in the numerator and denominator of the CIA equation.However, Fedo et al., (1995) argued that the use of CIW calculation to quantify chemical weathering intensity is inappropriate and should be used with caution since the equation provides values of 80 for unweathered potassic granite and values close to 100 for clay minerals such as kaolinite, illite and gibbsite, similar to values found for residual products of CIW for smectite, 80; kaolinite, illite and gibbsite, 100.The CIA and CIW are interpreted in similar way with a value of 50 for unweathered upper continental crust and about 100 for highly weathered materials with complete removal of alkali and alkaline-earth elements (McLennan et al., 1983;McLennan, 1993;Mongelli et al., 1996).
CIW values of beach sands from the Eastern San'in coast, Tango Peninsula and Wakasa Bay ranged from 51 to 72, 56 to 71 and from 62 to 82 respectively.The average CIW for Wakasa Bay and Tango Peninsula sands (60 and 56) were slightly higher compared to those of the Eastern San'in coast (50) (Table 2a).The CIW index values are higher than CIA values for the analyzed samples, due to the exclusion of K 2 O from the index.On the basis of CIW, the weathering intensity of the investigated beach sands from Eastern San'in coast, Tango Peninsula and Wakasa Bay maybe interpreted to show low to moderate weathering.

Plagioclase Index of Alteration
The degree of the chemical weathering can be estimated using the Plagioclase Index of Alteration modified from the CIA equation to monitor plagioclase (Fedo et al., 1995).The plagioclase index of alteration (PIA) is calculated according to the following equation in molecular proportions: Where CaO* is the CaO residing only in the silicate fraction.

A-CN-K Diagram
The geochemical compositions of investigated beach sands from the Eastern San'in coast, Tango Peninsula and Wakasa Bay were plotted on a ternary Al 2 O 3 , CaO * +Na 2 O, and K 2 O, (A-CN-K) diagram as shown in Figure 5a.Generally, Al 2 O 3 (A) is plotted at the top apex, CaO * +Na 2 O (CN) at the bottom left and K 2 O (K) at the bottom right and helps in understanding their weathering trends and mineralogical compositions (Nesbitt & Young (1984, 1989).Plagioclase and K-feldspar plot at 50% Al 2 O 3 on the left and right boundaries, respectively to form the (feldspar join).The clay mineral groups, kaolin, chlorites and gibbsite plot at the A apex (100% Al 2 O 3 ).The initial weathering trends of igneous rocks are sub-parallel to CN-A.Calcite plots at the CN apex.Illite and smectites plot on the diagram at 70% and 85% Al 2 O 3 .As weathering progresses, clay minerals are produced at the expense of feldspars and bulk composition of soil/sediments samples evolve up the diagram towards A apex, along the weathering trend.The most intensely weathered samples will therefore, plot highest on the diagram, reflecting the preponderance of aluminous clay minerals.The weathering trend intersects then A-K boundary once all plagioclase is weathered and then is redirected towards the A apex because K is extracted from the residues in preference to Al (Nesbitt et al., 1996).
As shown in the ternary A-CN-K plot in Figure 5a, the CIA ratios of investigated beach sands from the Eastern San'in coast, Tango Peninsula and Wakasa Bay are generally low (<60), indicating minimal weathering.In A-CN-K ternary plot (Figure 5a), majority of the investigated beach sands from the Eastern San'in coast, Tango Peninsula and Wakasa Bay occupy the central part of triangle, more towards the A-CN line and plot close to the plagioclase and K-feldspar lines, suggesting poor weathering conditions.Most of beach sands from the Eastern San'in coast group are around the weathering trend of granites and felsic volcanic rock near the plagioclase-K-feldspar line.Some beach sand samples from the Eastern San'in coast and Tango Peninsula scattering below the feldspar line but close to the A-CN side of the diagram, attest to the fact that due to high contents of CaO they possess low CIA values.

A-CNK-FM Diagram
Weathering trends can also be observed in molar proportions of Al 2 O 3 -CaO * +Na 2 O+K 2 O -FeO * +MgO (A-CNK-FM) (Nesbitt & Young, 1984, 1989).CaO * , Na 2 O and K 2 O (CNK) are plotted at the lower apex, Al 2 O 3 (A apex) at the top and FeO* (total iron as FeO * ) and MgO are summed to form the third variable (Figure 5b).

Chemical Maturity
The major element compositions of investigated beach sands from the Eastern San'in coast, Tango Peninsulaand Wakasa Bay are shown in the ternary plot Al 2 O 3 ×5-SiO 2 -CaO×2 (Figure 6).This illustration (Brumsack, 1989) is based on the assumption that marine sediments may be regarded as mixtures of alumosilicates (expressed by Al 2 O 3 and SiO 2 content), biogenic silica (partly represented by the SiO 2 content) and biogenic carbonate (largely amounted to the CaO content).Apparently, the investigated beach sands present mixtures of terrigenous detrital material and biogenic silica with minor amounts of carbonate.
Figure 7a presents the bivariate plot of SiO 2 (reflective of quartz content) against Al 2 O 3 * +K 2 O+Na 2 O (reflective of feldspar content) representing chemical maturity trend as function of climate as proposed by Suttner & Dutta (1986).The plotted samples revealed semi-arid to semi humid climatic conditions in the area from various sources tending towards increasing chemical maturity.Overall, the investigated beach sands from the Eastern San'in coast, Tango Peninsula and Wakasa Bay showed variable degrees of chemical maturity extending from low to intermediate levels.The beach sands plotted in the semi-arid region may have experienced little or no chemical weathering and are far less mature than those plotted in the semi humid area.The K 2 O/Na 2 O ratio discrimination diagram (Figure 7b) after Crook, (1974) and SiO 2 contents, show that the majority of the investigated beach sands are predominantly quartz-intermediate type.

Provenance and Tectonic Setting
The percentage weight (wt%) of major elements shown in Table 1 was used to discriminate the tectonic setting of the investigated beach sands from the Eastern San'in coast, Tango Peninsula and Wakasa Bay.The discriminant functions of the diagram use Al 2 O 3 , TiO 2 , Fe 2 O 3 * , MgO, CaO, Na 2 O and K 2 O contents as variables and classifies samples of unknown provenance in one of four categories (P1-P4) according to their discriminant scores (F1, F2) (Roser & Korsch, 1988).This function discriminate among four sedimentary provenances: mafic, (P1) ocean island arc; intermediate, P2; mature island arc; felsic, (P3) active continental margin; and recycled, (P4) granitic-gneissic or sedimentary source.However, provenance discrimination diagrams based only on major elements are somewhat unreliable because of the mobilization of these components during weathering and alteration (Roser & Korsch 1988).The majority of beach sand samples of East San'in district, Tango peninsula and Wakasa bay plot on the P1 and P2 fields of mafic and ocean island arc (Figure 8a).

Figure 2 .
Figure 2. Boxplots showing the summary of geochemical compositions of beach sands from the Eastern San'in coast, Tango Peninsula and Wakasa Bay, Japan Note.Vertical lines give the range, excluding outliers (circles); boxes enclose 50% of the data and illustrate the 25% quartile, median (horizontal bar), and 75% quartile.Outliers are defined as the upper or lower quartile ±1.5 times the interquartile difference.

Figure 4 .
Figure 4. Average normalized major and trace element plots of beach sands from the Eastern San'in coast, Tango Peninsula and Wakasa Bay, Japan.a) Average normalized samples compared to average Upper Continental Crust (UCC) estimated from the representative surface rocks by Togashi & Imai, (2000).b) Average normalized samples compared to average Japan arc Upper Crust (JUC) normalized values from Rudnick & Gao, (2005).The major element values were normalized as wt% and trace elements as ppm Plagioclase plus K-feldspar (Fel) plot on the left hand boundary at 50% Al 2 O 3 .Illite plots on the left boundary at approximately 75% (and greater) Al 2 O 3 and kaolin and gibbsite plot at the A apex.Biotite plots three quarters of the way along the line between feldspars and FM apex.Chlorite plots on the right hand boundary as a solid solution ranging from approximately 15% to 25% Al 2 O 3 .Most of the investigated beach sands as well as average Upper Continental Crust (UCC) and Japan arc Upper Crust (JUC) plot close to the feldspar composition on the line joining Fel point on the A-CNK boundary to the FM apex.

/ Location SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 ＊ MnO MgO CaO Na 2 O K 2 O P 2 O 5 As Pb Zn Cu Ni Sc V Sr Y Nb Zr Th Cr East San'in coasts (n=17)
Table 2b).Box plots for major elements composition in beach sand samples of Eastern San'in coast, Tango Peninsula and Wakasa Bay are illustrated in Figure 2. Sand samples from Wakasa Bay have the highest mean SiO 2 content and the highest median K 2 O content, but contained the lowest mean and median concentrations of Al 2 O 3 , MnO, MgO, CaO, and Na 2 O.

Table 2a .
Summary statistics of major element abundances in beach sands from the Eastern San'in coast, Tango Peninsula and Wakasa Bay, Japan compared to UCC

Sam ple/ Location SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 ＊ MnO MgO CaO Na 2 O K 2 O P 2 O 5 CIA CIW PIA East San'in coasts (n=17)
, in which CaO values are accepted only if CaO<Na 2 O and when CaO>Na 2 O, it is assumed that the concentration of CaO equals that of Na 2 O. High CIA values reflect the removal of mobile or unstable cations (Ca, Na, K) relative to highly immobile or stable residual constituents (Al, Ti) during weathering PIA values (>84) would indicate intense chemical weathering while lower values (~50) is characteristic of unweathered or fresh rock samples.Post-Archean Australian Shales (PAAS) have PIA value of 79.The PIA values of beach sands from the Eastern San'in coast ranged from 31 to 60 with a mean of 50.The mean PIA values were 56 for Tango Peninsula beach sands and 60 for Wakasa Bay sands.The PIA values ranged from 46 to 67 and from 51 to 65 in Tango Peninsula and Wakasa Bay (Table