Chapter 3

GEOCHEMISTRY AND AGE DETERMINATION OF VOLCANO-PLUTONIC ROCKS

3.1 Introduction

Volcanic and plutonic rocks are thought to be formed by partial melting of mantle and lower crustal rocks above subduction zones and mantle plumes, during rifting or during other tectonic processes (Campbell et al., 1974). This magmatism tends to produce hydrothermal fluids either through circulation of pre-existing water through heated rock or directly through the expulsion of magmatic fluids during magmatic crystallization. Most metals are soluble in these hot fluids but precipitate to form ore deposit when these fluids undergo major physical or chemical changes, e.g. porphyry copper, and volcanic hosted massive sulphide (Burnham, 1979; Tatsumi and Eggins, 1993). In Chapter 2 it was demonstrated that the occurrences of major gold and iron-gold provinces in Thailand were closely associated with the occurrences of volcano-plutonic rocks indicating a hydrothermal origin for these deposits. In this chapter the link amongst the magmatism, the tectonics and the ore deposit will be explored by undertaking a brief study of the geochemistry and age of mineralisation. The occurrences of major gold and iron-gold deposits in northern and northeastern Thailand are restricted to two orogenic belts (Jungyusuk and Khositanont, 1992); the Sukhothai located to the west of the Nan-Uttaradit suture and the Loei- Phetchabun Fold Belt, to the east (Fig. 3.1). With notable exceptions, previous tectonic reconstructions of Thailand and mainland Southeast Asia were based mostly on sedimentary basin analysis and paleobiogeographic reconstruction but tended to ignore the geochemistry of volcanic and plutonic rocks. This chapter, therefore, examines the major elements, trace elements and age of the igneous and volcaniclastic rocks within the Sukhothai and 22

Figure 3.1 Geological map of Thailand showing approximate distribution of Sukhothai and Leoi – Phetchabun Fold Belts, and distribution of major rock units (modified from 1:2,500,000 Geological Map of Thailand, Department of Mineral Resources, 1999) 23

Loei – Phetchabun Fold Belts (Fig. 3.2). The geochemistry is then combined with the field observations in order to characterise the tectonic setting and evolution of the Sukhothai and Loei-Phetchabun orogenic belts.

3.2 Methodology

3.2.1 Major, Trace Elements and REE Analysis Igneous rocks erupted in different tectonic settings tend to have different major, and trace element characteristics (Rollinson, 1993). By comparing the geochemistry of the rocks from the Sukhothai and Loei Phetchabun Fold Belts to those from modern tectonic settings using well characterised geochemical diagrams, this study will test the tectonic models for these areas determined by previous studies.

3.2.1.1 Sample preparation

Selected samples were crushed into 10 cm size using a rock splitter, cleaned and crushed into 1 cm pieces using Jaw Crusher and then finally crushed to fine powder using both W carbide and Cr-steel ring mills. Fusion discs and pellets were prepared for XRF major elements analysis, whereas sample solutions were prepared for ICP- MS REE analysis.

3.2.1.2 Analytical Techniques

Major and trace elements were analysed by a PANalytical (Philips) PW 1480 X- Ray Fluorescence (XRF) spectrometer installed at the CODES ARC Center of Excellent in Ore Deposit, University of Tasmania, Australia. Major elements were measured from fusion discs, which were prepared at 1,100 °C in 5%Au/95%Pt crucibles using 0.500g of sample, 4.500g of 12-22 Flux (lithium tetraborate- metaborate mix) and 0.0606g of LiNO3, following the technique described in Robinson (2003). Loss on ignition (LOI) for each sample was determined by heating 24

Figure 3.2 Map of Thailand showing sample locations (open stars), the Nan- Uttaradit suture and the Sukhothai and Loei-Phetchabun Fold Belts 25

1-2 g of sample at 1,000 ºC for 12 hours and reweighing. Pressed powder pellets for trace element analysis were prepared using 10 g of sample and PVPMC (Polyvinyl- pyrrolidone-Methylcellulose) as a binder. The samples were pressed to 3.5 tonnes cm-2 within a mold with a diameter of 32 mm. A 3kW (maximum) ScMo anode X-Ray tube and 3kW (maximum) Au anode X-Ray tube were used in order to measure trace elements. For the low-abundance trace elements and rare earth elements (REE), 16 volcanic and plutonic rocks were selected and analysed using a HP4500 Inductively Coupled Plasma Mass Spectrometer (ICP-MS). Sample solutions were prepared using

PicoTrace® high pressure acid (HF/H2SO4) digestion. Aliquots (100 mg) of powdered sample were weighed into 30 ml PTFE digestion containers. A few drops of ultra-pure water were added into samples after weighting and adding 0.1 ml of µg g -1 indium

solution to each digestion container, 3 ml HF and 3 ml H2SO4 were slowly added. After shaking a few times for thorough mixing, the PTFE containers were left in the digestion block at 180 °C for 16 hours. The digestion mixture was then left over in the

evaporation block for four days at 180 ºC. HClO4 (1 ml) was then added to the residue

and dried before adding the final 2 ml HNO3 and 1 ml HCl. The residue was then dissolved by warming the solution in the digestion block at 60-70 ºC for ~ 1 hour. Finally, it was transferred into a polypropylene bottle and diluted to 100 ml after the solution became clear (Yu and Robinson, 2003), and then analysed. The results obtained from XRF analysis and from solution ICP-MS analysis show that the concentrations of individual elements, which were analysed by both techniques, are less than 5% differences.

3.2.2 U-Pb zircon age determination

Zircon is an accessory mineral that generally forms simultaneously with crystallisation of intermediate to acid igneous rocks. Since, the blocking temperature of the zircon is as high as 800 ºC. It is, therefore, resistant to physical and chemical changes. In other words, the isotopic values are not easily changed and can be used to determine the age of the igneous host rocks. The age of rock is determined by determining the amount of U, which has decayed to Pb after crystallization of the zircon. 26

Zircons generally separate non-radiogenic Pb from their crystal lattice during crystallisation and therefore the age of rock can be determined directly from the U-Pb decay constants.

3.2.2.1 Sample preparation . Zircons were separated from 100 to 200g of rock by crushing to < 400 microns in a mortar and pestle or a Cr-steel ring mill, depending on sample hardness. Heavy minerals were then separated using a combination of mechanised panning device (superpan) and a hand pan. The heavy mineral residue was then dried. Magnetic and paramagnetic minerals were removed using a hand magnet and a Franz magnetic separator. Approximately 20 to 30 zircons were picked from the non-magnetic heavy mineral separate using a single hair from a fine artist’s paint brush, and mounted on double sided sticky tape. Epoxy glue was then poured into a 2.5 cm diameter mould on top of zircon grains. The mount was dried for 12 hours and polished using clean sandpaper and a clean polishing lap. The samples were then washed in distilled water in an ultrasonic bath prior to analysis.

3.2.2.2 Analytical techniques

Zircons were ablated in a He atmosphere in a custom-made chamber with the laser pulsing at 5 Hz and a 30 micron diameter beam delivering ~ 12 J/cm2 and drilling at approximately 1 micron/s. A total of 11 masses were counted (96Zr, 146Nd, 178Hf, 202Hg, 204Pb, 206Pb, 207Pb, 208Pb, 232Th , 238U), with longer counting times on Pb isotopes giving a total quadrupole cycling rate of 0.2 second. Each analysis began with a 30 seconds analysis of background gas followed by 30 seconds with the laser switched on. Four primary (Temora zircons of Black et al., 2004) and 2 secondary standards 91500 were analysed both before and after every 12 zircon analyses to correct for mass bias, machine drift and down hole fractionation. Repeated monitoring of U/Pb mass fractionation during drilling showed an average fractionation of U/Pb varying from 0.050 at the start of a 30 second analysis to 0.053 at the deepest level of laser ablation. Monitoring of international standards showed a reproducible error of approximately ± 2 Ma at 200 Ma. 27

3.3 Volcano-plutonic rock in the Lampang – Phrae volcanic belt

The formation of Sukhothai orogenic belt was especially closely associated with N-S arcuate trending volcano-plutonic rocks, which have been known as Lampang- Phrae volcanic belt (Jungyusuk and Khositanont, 1992). The Lampang – Phrae volcanic rocks can be geographically subdivided into three major belts. Doi Ton Belt is located in the western flank of the Sukhothai Fold Belt. The majority of Lampang – Phrae volcanic belt is occupied by Doi Luang Belt, which forms a N-S trending arcuate S-shape (Fig. 3.3) in the central Sukhothail Fold Belt, whereas Den Chai Belt formed complex structure next to the eastern part of Doi Luang Belt in the eastern Sukhothai Fold Belt. Previous study showed that Doi Luang Belt was formed at 240 ± 1 Ma (Barr et al., 2000). Since volcano-plutonic rocks in the Doi Luang Belt are closely associated with gold and iron-gold mineralisation (e.g. Huai Kham On, Mae Mok, and Mae Bo Thong Deposits), this project extends more detailed studies on geochemical characteristics and U-Pb zircon age determination of the Doi Luang Central Belt of the Lampang-Phrae volcano-plutonic rocks in order to obtain better understanding of gold and iron-gold mineralisation in relation to tectonic setting and evolution of the Sukhothai Fold Belt.

3.3.1 Geology of Lampang-Phrae volcano-plutonic rocks

As mentioned earlier, the majority of Lampang-Phrae volcanic belt is occupied by the Doi Luang volcano-plutonic belt. This study, therefore, focuses mainly on the Doi Luang Belt. Major, trace and rare earth elements analyses were conducted to obtained the geochemical characteristics, and the U-Pb zircon age determination of selectively representative volcano-plutonic rocks, regarding to special association with gold and iron-gold mineralisation in the Sukhothai Fold Belt, were performed. The volcano-plutonic rocks in the Doi Luang Belt are divided into three geographic zones as Western Zone, Central Zone and Eastern Zone. The representative volcano-plutonic rocks of the Western Zone are located at the southwest corner of the Doi Luang Belt, including Mae Prik volcaniclastic unit, Mae Pa volcaniclastic unit and Mae Mok granite unit. The Central Zone is located in the middle of Doi Luang Belt. The representative volcano-plutonic rocks were collected 28 from Huai Kham On volcaniclastic unit and Mae Khaem granite unit. The representative volcanic rock units in the Eastern Zone are located in the northeastern part of the Doi Luang Belt, including Kaeng Luang volcaniclastic unit. Sample collection was carried out on the most abundant rock type of each unit. In addition, Mae Bo Thong meta-volcanic rock was collected for U-Pb age determination and REEs analysis.

3.3.1.1 Western Zone

Three representative rock units in the western zone, Mae Prik volcaniclastic unit, Mae Pa volcaniclastic unit and Mae Mok granite unit (Fig. 3.3), are presented in this project.

(1) Mae Prik volcaniclastic unit

The outcrops of Mae Prik volcaniclastic unit are exposed along the road-cuts on Highway No. 1 between Mae Prik District and Thoen District, Lampang Province. They form relatively small hills, scattering in the flood plain area. Mae Prik volcaniclastic unit is composed mainly of reddish brown volcaniclastic rocks. Due to the scarcity occurrence of the coherent facies, two volcaniclastic samples were collected to represent the Mae Prik volcaniclastic (ST-1 and ST-2). ST-1 sample is tuff, with a reddish brown colour (pale brownish grey on weathering surfaces). This rock sample is occasionally slightly foliated. Micorscopic observation shows that this rock is composed mostly of fine ash grains, with minor coarse ash grains (Fig. 3.4). The coarse ash grains, which are the minor constituents, have occasionally been completely altered to sericite. The fine ash grains are completely altered to quartz and sericite, with abundant remnants of glass shards fragments. It is noticeable that this rock sample also contains abundant of opaque minerals including magnetite, hematite and iron-hydroxide. ST-2 sample is reddish brown in colour (light brownish grey on weathering surfaces). It is generally fine grained tuff in hand specimen with sparse rock fragments. Microscopic observation shows that it is composed mostly of fine ash grains with minor 29

coarse ash grains (Fig. 3.5). The coarse ash grains are composed of quartz crystals, with rounded edges, carbonates, and fragments of fiamme, pumice, tuff and some welded tuff or rhyolitic to dacitic glass. Some fine quartz fragments of coarse ash grains are occasionally micropoikilitic suggesting that they were formed from high temperature divitrification prior to the formation of the Mae Prik volcaniclastic unit. The fine ash grains, which are the most abundant constituents in this sample was replaced by fine- grained quartz and sericite aggregates. Opaque minerals including magnetite, hematite and iron hydroxide were also present in this rock in minor amount.

(2) Mae Pa volcaniclastic unit

The volcaniclastic rocks in Mae Pa volcaniclastic unit are exposed as road-cut outcrops along the Thoen-Thung Saliam road, adjacent to Ban Mae Pa, Thoen District. The volcaniclastic rocks as well as country rocks form continuously, moderately high elevated mountain against the flood plain. The Mae Pa volcaniclastic unit is characterised by reddish brown lapilli tuff (ST-4). It contains coarse ash grains, with minor lapilli and fine ash grains. (Fig. 3.6). Microscopic observation shows that the lapilli grains are composed mostly of quartz, plagioclase, K-feldspar, carbonates and minor welded tuff and tuff fragments. The rock also contains trace amounts of euhedral to subhedral quartz, micropoikilitic quartz and devitrified glassy rock fragments. The fine ash grains have been mostly undergone low-temperature recrystallisation to fine-grained quartz and feldspar aggregate. The feldspar grains have been replaced by sericite. The coarse ash grains also contain rock fragments, which are compositionally similar to those of lapilli. This rock also contains magnetite, hematite and iron-hydroxide in minor amount.

(3) Mae Mok granite unit

The outcrops of Mae Mok granite are continuously exposed as road-cut exposures along Thoen-Thung Saliam main road, adjacent to Ban Mae Mok road junction, next to the Mae Pa volcaniclastic unit.

30

Figure 3.3 Geological map of Doi Luang belt in the Lampang – Phrae volcanic belt showing distribution of the volcano-plutonic rock units (modified from 1:250,000 geological map of Thailand, Uttaradit sheet, Department of Mineral Resources, compiled by Piyasin, 1974).

31

Figure 3.4 Photomicrographs of Mae Prik volcaniclastic (ST-1) showing quartz (Qtz) and feldspar (Feld) fragments, with rounded edges, and altered glass shards. The top is in plane polarised light and the bottom is in crossed polarised light 32

Figure 3.5 Photomicrographs of Mae Prik volcaniclastic (ST-2) showing of quartz (Qtz) and carbonates (Cc) in highly sericitised matrix. The top is in plane polarised light and the bottom is in crossed polarised light. 33

Figure 3.6 Photomicrographs of Mae Pa lapilli tuff (ST-4) showing fragments of altered feldspar (Feld) and quartz (Qtz) in low-temperature recrystallised matrix. The top is in plane polarised light and the bottom is in crossed polarised light 34

The Mae Mok granite unit is characterised by medium to coarse-grained, non- foliated porphyritic biotite granite (ST-3, Fig. 3.7). K-feldspar is present as sparsely megacrysts and a groundmass constituent. The rocks are pink in colour, caused by major distributions of pink K-feldspar megacrysts and groundmass K-feldspar. The K-feldspar megacrysts are subhedral, and have equant to short tabular forms. Of the groundmass, biotite crystals are a minor constituent (<10%). They generally occur as anhedral flakes. Groundmass plagioclase crystals are a subordinate constituent and generally show anhedral to subhedral outlines, with equant to short tabular forms. Groundmass K-feldspar crystals are generally subhedral, with short tabular forms. Quartz crystals are the most abundant constituent, which are mostly polycrystalline anhedral clusters with non-sutured grain boundaries. They are also present as isolated crystals in K- feldspar and plagioclase grains. These quartz crystals have a secondary origin. It is also noticeable that a micrographic texture of quartz and K-feldspar has occasionally been observed under the microscopic.

3.3.1.2 Central Zone

The Central Zone volcano-plutonic rocks are especially located in the middle between the eastern and western flanks of Lampang–Phrae volcanic belt. Two representative rock units, Huai Kham On volcaniclastic unit and Mae Khaem granite unit were selected for petrographic study.

(1) Huai Kham On volcaniclastic unit

Huai Kham On volcaniclastic unit forms low elevated, small hills in the Huai Kham On water shade area, northwest of Ban Mae Kratom, Wang Chin District, Phrae Province. The Huai Kham On volcaniclastic unit is tuff characterised by matrix support, poorly sorted, subangular to subrounded polymictic clasts, and moderately sorted fine-grained andesitic matrix (ST-5). Microscopic observation shows that the clasts are composed of great varieties of rock fragments including basalt or andesite, quartz, plagioclase, tuff, sandstone, siltstone and carbonates (Fig. 3.8). The basic rocks 35

Figure 3.7 Photomicrographs of Mae Mok granite (ST-3) showing coarse-grained K- feldspar megacrysts and replacement quartz. The top is in plane polarised light and the bottom is in crossed polarised light

36

Figure 3.8 Photomicrographs of Huai Kham On andesitic tuff (ST-5) showing fragments of quartz (Qtz) and feldspar (Feld) fragments. The top is in plane polarised light and the bottom is in crossed polarised light 37

fragments have been strongly silicified and mostly replaced by quartz, magnetite, hematite, iron hydroxide and sphene or leucoxene. It is also noticeable that few of large zircon grains have been observed under microscope.

(2) Mae Khaem granite unit

Mae Khaem granite unit is exposed as road-cut outcrops along Highway No. 11 north of Ban Mae Khaem adjacent to road junction to Long District. However, the more abundant outcrops are widely exposed at Ban Pan Jaen, Wang Chin District. The Mae Khaem is characterised by non-foliated, medium-grained, sparsely K- feldspar megacrystic hornblende-biotite granite (ST-6, Fig. 3.9). K-feldspar crystals are pink in colour, which are present as phenocrysts/microphenocrysts and a groundmass constituent. The K-feldspar phenocrysts/microphenocrysts are subhedral, with short tabular forms. Groundmass hornblende crystals are occasionally present as a trace constituent in this rock type. Groundmass biotite crystals have commonly been observed as a trace constituent and show anhedral flakes to subhedral books. Groundmass plagioclase crystals are a minor constituent, and generally show subhedral and short tabular forms. Groundmass K-feldspar grains are subhedral, with a short tabular forms. It is also noticeable that a granophyric texture has commonly been observed under the microscope.

3.3.1.3 Eastern Zone

The Eastern Zone volcano-plutonic rocks are assigned to volcanic and volcaniclastic rocks that are composed mainly of andesite, basaltic andesite, andesitic volcaniclastic and pyroclastic breccia (Osataporn, 2007). They are widely distributed in the northern flank of Lampang-Phrae volcanic belt and made up largely of volcaniclastic rocks. The selected representative rock types are volcaniclastic rocks that are exposed as road-cut outcrops around Ban Kaeng Luang road junction on Highway No. 11 (Den Chai-Lampang), Long District. It is noticeable that the exposure of volcanic rocks associated plutonic rocks, which are found in the northern part of the Lampang-Phrae volcanic belt, is not shown in the map (Fig. 3.3). 38

Figure 3.9 Photomicrographs of Mae Khaem granite (ST-6) showing a granophyric texture. The top is in plane polarised light and the bottom is in crossed polarised light 39

(1) Kaeng Luang volcaniclastic unit

The Kaeng Luang volcaniclastic unit is characterised by coarse tuff (ST-7b) and minor lapilli tuff (ST-7a). The lapilli tuff is dark greenish grey in colour and reddish brown on weathering surfaces. Microscopic observation of ST-7a sample shows that the lapilli rock fragments are made of pyroclasts including perlitic, pumiceous, fiamme breccia and carbonate fragments (Fig. 3.10). Of the coarse ash matrix, it is composed mainly of plagioclase, with subordinate K-feldspar and minor polycrystalline quartz clusters. The polycrystalline quartz clusters generally show sutured grain boundaries and spherulites, which have been resulted from high temperature devitrification. The intercalated volcaniclastic rocks (ST-7b) are coarse tuff. They are pale greenish grey in colour, and has a brownish grey on weathering surfaces. The coarse tuff has grain sizes 1 mm on average. Microscopic observation shows that the intercalated volcaniclastic rock is composed of rhyolitic tuff, pumiceous, perlitic and silicified rock fragments sitting in fine ash groundmass (Fig. 3.11). Quartz fragments are the most abundant constituent of mineral fragments, whereas plagioclase and K- feldspar fragments are subordinate constituents. Basic igneous rock fragments are mostly altered to chlorite, sericite and clay minerals.

(2) Mae Bo Thong metavolcanic rock unit

Mae Bo Thong meta-volcanic rocks occur in the eastern flank of the Lampang- Phrae volcanic belt at Ban Mae Bo Thong, Thung Saliam District in Sukhothai Province, as a southerly extension of the Kaeng Luang andesite and volcaniclastic unit. The Mae Bo Thong meta-volcanic rock unit is characterised by porphyroclasts in mylonitised, fine-grained, weakly foliated, greenish grey matrix (Fig. 3.12). The porphyroclasts are mostly composed of coarse-grained quartz and a minor rock fragments. The mylonite is composed of foliated very-fine-grained quartz and mica. The porphyroclasts are commonly surrounded by the foliation of very fine-grained clayey quartz-micaceous matrix. 40

Figure 3.10 Photomicrographs of Kaeng Luang lapilli tuff (ST-7a) showing a pumiceous clast. The top is in plane polarised light and the bottom is in crossed polarised light 41

Figure 3.11 Photomicrographs of Kaeng Luang coarse tuff (ST-7b) showing perlitic volcanic fragments in devitrified fine-grained groundmass. The top is in plane polarised light and the bottom is in crossed polarised light 42

Figure 3.12 Photomicrographs of Mae Bo Thong meta-volcanic unit showing quartz porphyroclasts wrapped around by foliated fine-grained quartz-mica mylonite matrix. The top is in plane polarised light and the bottom is in crossed polarised light 43

3.3.2 Geochemical characteristic of the Lampang-Phrae volcano-plutonic rocks

The results of major and trace elements and REEs analyses for the Lampang- Phrae volcano-plutonic rocks are shown in Tables 1 and 2. The relation of alkali and silica is shown in total alkali versus silica (TAS) diagram (LeBas et al., 1986) signifying that the volcanic rocks and granite in Western Zone and Central Zone are chemically rhyolitic, whereas those of Eastern Zone are chemically andesitic (Fig. 3.13).

Similar results were obtained from the Zr/TiO2 and Nb/Y petrology classification diagram (Winchester and Floyd, 1977), i.e. the Western Zone and Central Zone volcano-plutonic rocks are chemically rhyolitic, whereas the Eastern Zone volcano-plutonic rocks are chemically andesitic (Fig. 3.14). The relations of Rb versus Y+Nb and Nb versus Y on tectonic discrimination of granite diagrams show that the volcano-plutonic rocks in the Lampang-Phrae volcanic belt are plotted in volcanic arc and syn-collision granite fields (Figs. 3.15 and 3.16). The rare earth elements were normalised to chondrite using chondrite rare-earth- element values of Sun and McDonough (1989). The REE patterns of the Mae Prik rhyolitic tuff shows that the slopes of heavy rare-earth-element patterns are relatively flat (Fig. 3.17) with chondrite-normalised Sm/Yb = 3.44, and La/Sm = 3.64 (ST-1), and Sm/Yb = 3.10 and La/Sm = 3.42 (ST-2). The REE pattern of the Mae Mok granite has chondrite-normalised Sm/Yb = 1.77 and La/Sm = 3.80 (ST-3), whereas the REE pattern of the Mae Pa tuff has chondrite-normalised Sm/Yb = 2.44 and La/Sm = 5.29 (ST-4). In addition, similar REE patterns were obtained from the Central Zone and Eastern Zone volcano-plutonic rocks (Fig. 3.18) with chondrite-normalised Sm/Yb = 2.67 and La/Sm = 3.94 (ST-5), and Sm/Yb = 1.78 and La/Sm = 4.62 (ST-6), Sm/Yb = 2.08 and La/Sm = 2.48 (ST-7a), and Sm/Yb = 2.03 and La/Sm = 2.55 (ST-7b). These REE patterns signify that the Lampang-Phrae volcano-plutonic rocks are chemically calc-alkalic.

3.3.3 U-Pb zircon age of volcanic and plutonic rocks in Lampang-Phrae volcanic belt

The U-Pb zircon age determination was carried out for selectively representative samples of the Lampang-Phrae volcanic and plutonic rocks. Seven representative samples 44

Table 3.1 Major and trace elements analyses for Lampang – Phrae volcanic – plutonic rocks

Western Zone Central Zone Eastern Zone Huai Mae Kaeng Luang Kaeng Luang Mae Prik Mae Mok Mae Pa Kham On Khaem lapilli tuff coarse tuff Mapor (%) ST2 ST3 ST4 ST5 ST6 ST7A ST7B

SiO2 74.27 76.80 71.54 69.63 75.00 60.11 60.27 TiO2 0.12 0.09 0.30 0.62 0.16 0.78 0.87 Al2O3 12.40 12.34 14.07 12.85 13.19 12.82 14.51 Fe2O3 1.77 1.23 2.27 5.25 1.46 6.25 7.03 MnO 0.01 0.03 0.07 0.03 0.04 0.16 0.17 MgO 0.39 0.18 0.64 1.82 0.17 2.60 3.22 CaO 2.75 0.11 0.62 1.48 0.12 5.39 3.72

Na2O 0.20 3.69 3.69 2.60 3.81 2.82 2.95 K2O 2.90 4.59 5.34 2.60 4.85 2.40 1.64 P2O5 0.04 0.02 0.08 0.11 0.04 0.33 0.27 Loss on ignition 4.71 0.67 1.24 3.11 0.97 6.33 5.22 Total 99.56 99.74 99.88 100.09 99.81 99.99 99.87 S Bdl 0.01 Bdl 0.13 Bdl 0.01 0.04

Trace (ppm) Nb 7 22 13 18 16 7 8 Zr 97 102 183 247 176 123 136 Sr 201 42 221 163 51 184 240 Cr 5 2 6 16 1 8 9 Ba 625 172 1229 448 283 316 191 Sc 7 Bdl 4 10 Bdl 19 21 V 11 Bdl 23 61 4 93 125 La 26 26 46 38 33 16 20 Ce 55 55 85 76 65 38 38 Nd 24 24 29 33 25 21 19 Y 20 38 19 29 17 24 25 U 3 16 10 4 13 2 Bdl Rb 120 347 245 96 221 130 88 Th 15 50 45 13 52 6 7 Pb 23 27 19 6 13 12 12 As 5 Bdl Bdl 5 Bdl 26 15 Bi Bdl Bdl Bdl Bdl Bdl Bdl Bdl Zn 27 17 70 74 8 93 94 Cu 6 3 4 7 19 11 17 Ni 3 5 4 5 3 4 4

Bdl = below detection limit

45

Table 3.2 Rare earth elements analyses for Lampang – Phrae volcanic – plutonic rocks

Element ST1 ST2 ST3 ST4 ST5 ST6 ST7a ST7b 7 Li 11.7 6.05 3.28 15.2 13.4 3.42 50.8 79.7 9 Be 2.12 1.93 4.05 2.82 1.92 2.90 1.15 1.37 45 Sc 9.37 5.94 1.80 3.72 11.3 2.19 19.0 20.1 47 Ti 1061 719 538 1729 4262 981 4896 5310 51 V 22.8 11.8 2.8 20.9 60.0 5.5 88.1 110 55 Mn 260 133 237 514 216 334 1118 1354 60 Ni 2.65 2.83 5.29 4.57 5.94 3.72 3.67 3.67 63 Cu 9.60 6.41 5.57 3.22 7.01 20.3 32.6? 18.4 Zn (XRF) 41 27 17 70 74 8 93 94 71 Ga 15.3 14.2 14.9 14.7 15.8 14.6 14.0 17.1 As (XRF) 38 5 Bdl Bdl 5 Bdl 26 15 85 Rb 149 117 332 245 111 221 131 83.8 88 Sr 102 210 40 213 172 48 188 238 Y (XRF) 45 20 38 19 29 17 24 25 Zr (XRF) 200 97 102 183 247 176 123 136 93 Nb 8.39 6.40 20.3 13.0 19.0 16.4 6.70 7.32 95 Mo 0.96 0.36 1.96 0.92 0.72 2.09 0.32 0.26 107 Ag Bdl 0.05 0.05 0.05 0.11 Bdl 0.10 0.06 111 Cd Bdl Bdl Bdl Bdl 0.1 Bdl 0.15 Bdl 118 Sn 0.49 2.87 2.71 3.25 3.52 3.74 1.69 1.78 121 Sb 3.99 4.99 0.43 1.27 1.37 0.39 3.17 3.00 125 Te Bdl Bdl Bdl Bdl Bdl Bdl Bdl Bdl 133 Cs 18.3 14.7 3.14 9.82 6.73 3.37 25.4 19.4 Ba (XRF) 1547 625 172 1229 448 283 316 191 139 La 71.4 29.0 37.6 41.3 46.4 27.5 18.2 18.3 140 Ce 108 58.8 79.2 78.4 89.6 55.5 38.3 38.3 141 Pr 16.6 6.91 8.85 8.42 10.5 5.89 4.85 4.79 146 Nd 62.7 26.0 30.2 28.5 38.8 19.8 20.2 19.9 147 Sm 11.93 5.16 6.02 4.76 7.17 3.63 4.47 4.37 151 Eu 1.83 0.61 0.18 0.79 1.44 0.35 1.11 1.21 157 Gd 10.1 3.73 4.98 3.63 6.09 2.74 4.22 4.33 159 Tb 1.56 0.55 0.89 0.57 0.96 0.45 0.69 0.69 163 Dy 8.61 3.10 5.23 3.21 5.50 2.74 4.07 4.10 165 Ho 1.56 0.59 1.06 0.64 1.05 0.56 0.82 0.83 166 Er 4.40 1.77 3.28 1.93 3.09 1.81 2.46 2.48 169 Tm 0.63 0.27 0.55 0.31 0.46 0.31 0.37 0.36 172 Yb 3.76 1.80 3.68 2.11 2.91 2.21 2.32 2.33 175 Lu 0.55 0.28 0.58 0.33 0.46 0.35 0.37 0.36 181 Ta 0.88 0.80 2.60 1.50 1.65 2.26 0.55 0.66 205 Tl 0.67 0.59 1.94 1.64 0.61 1.24 0.62 0.38 208 Pb 24.7 24.5 26.7 16.6 6.9 12.5 11.9 11.7 209 Bi Bdl 0.09 0.29 0.15 0.24 0.08 0.21 0.22 232 Th 12.5 14.8 48.6 40.0 12.6 54.8 5.99 6.35 238 U 3.51 2.46 13.6 8.84 3.29 12.68 1.60 1.84 Bdl = Below detection limit The numbers in front of the alphabets are isotope number commonly found in nature 46

Figure 3.13 Total alkali versus silica petrology discrimination diagram (after LeBas et al., 1986) for Lampang-Phrae igneous rocks. Solid symbols represent granite.

Figure 3.14 Trace element relation in the Zr/TiO2-Nb/Y petrology discrimination diagram (after Winchester and Floyd, 1977) for Lampang-Phrae igneous rocks. The numbers on the data points represent the sample numbers. 47

Figure 3.15 Granite tectonic discrimination diagram (after Pearce, et al., 1984) for the Lampang-Phrae granite. ORG = Oceanic Ridge granite, VAG = Volcanic Arc granite, Syn-COLG = Syn-collision granite and WPG = Within Plate granite.

Figure 3.16 Granite tectonic discrimination diagram (after Pearce, et al., 1984) for the Lampang-Phrae granite (VAG = Volcanic arc granite, Syn-COLG = Syn-collision granite and WPG = Within Plate granite). 48

Figure 3.17 Chondrite normalised plots for Mae Prik tuff (ST-1 and ST-2), Mae Pa lapilli tuff (ST-4) and Mae Mok granite (ST-3) units using chondrite normalising values of Sun and McDonough (1989).

Figure 3.18 Chondrite normalised plots for Huai Kham On tuff (ST-5), Mae Khaem granite (ST-6) Kaeng Luang lapilli tuff (ST-7a) and Kaeng Luang coarse tuff (ST-b) using chondrite normalizing values of Sun and McDonough (1989). 49 were selected for zircon extraction, followed by U-Pb isotopic measurement using the LA ICP-MS analytical technique as described earlier (Appendix 1).

3.3.3.1 The ages of Western Zone volcanic and plutonic rocks

Zircon grains were extracted from Mae Prik volcaniclastic unit, Mae Pa volcaniclastic unit and Mae Mok granite unit, and measured by LA ICP-MS technique. The U-Pb values of zircon from Mae Prik tuff were plotted on Concordia plots (Fig. 3.19). The data points are scattered on the Concordia, which yield a zircon age of 231 ± 3 Ma. The U-Pb isotope values of Mae Pa lapilli tuff are mainly clustered on Concordia (Fig. 3.20). The averaged zircon age was calculated to be 224 ± 4 Ma. The U-Pb values of Mae Mok granite are clustered on Concordia plots (Fig. 3.21), which yield a zircon age of 228 ± 3 Ma close to those of Mae Prik tuff and Mae Pa lapilli tuff.

3.3.3.2 The ages of Central Zone volcanic and plutonic rocks

Zircon grains were extracted from Huai Kham On volcaniclastics and Mae Khaem granite for zircon U-Pb isotopic measurement. The U-Pb values of zircon grains in the Huai Kham On tuff show widely scattering on Concordia plots (Fig. 3.22). Calculation for U-Pb zircon age determination yields the result of 247 ± 5 Ma. The zircon U-Pb values of Mae Khaem Mae Khaem granite are regularly scattered on Concordia plots (Fig. 3.23) indicating that the age of crystallisation is 224 ± 4 Ma.

3.3.3.3 The ages of Eastern Zone volcanic rocks

The U-Pb isotopic values were measured from zircon grains, which were extracted from Kaeng Luang lapilli tuff and Mae Bo Thong meta-volcanic rock. The U-Pb values for the Kaeng Luang lapilli tuff are regularly scattered on Concordia plots (Fig. 3.24). The results from calculation yield the age of 219 ± 4 Ma. The U-Pb values of Mae Bo Thong meta-volcanic rock are slightly scattered on Concordia plots (Fig. 3.25). The calculation of U-Pb isotopic ratio yields the Mae Bo Thong meta-volcanic zircon age of 232 ± 4 Ma. 50

Figure 3.19 U-Pb Concordia plots for Mae Prik volcaniclastic unit (ST-2).

Figure 3.20 U-Pb Concordia plots for ST-4 Mae Pa volcaniclastic unit (ST-4) 51

Figure 3.21 U-Pb Concordia plots for Mae Mok granite unit (ST-3)

Figure 3.22 U-Pb Concordia plots for Huai Kham On volcaniclastic unit (ST-5) 52

Figure 3.23 U-Pb Concordia plots for Mae Khaem granite unit (ST-6)

Figure 3.24 U-Pb Concordia plots for zircon in Kaeng Luang volcaniclastic unit (ST-7a). 53

Figure 3.25 U-Pb Concordia plots for zircon in Mae Bo Thong meta-volcanic rock unit (KZ07-01)

3.3.4 Summary on geochemistry and ages of Lampang-Phrae volcanic and plutonic rocks

The major and trace elements characteristics of the Lampang-Phrae volcano- plutonic rocks signify that the plutonic rocks are chemically rhyolitic, whereas the volcanic rocks are chemically andesitic to rhyolitic. The trace elements characteristics of plutonic rocks are similar to those of volcanic arc and syn-collision granite. The REE patterns signify that the volcanic and plutonic rocks in the Lampang-Phrae volcanic belt are chemically calc-alkalic. The ages of volcanic and plutonic rocks from 219 Ma to 247 Ma obtained from U-Pb zircon age determination suggest that the Lampang-Phrae volcanic and plutonic rocks were crystallised in the Middle to Late Triassic. However, the majority of them were crystallised in the Middle Triassic (224 Ma to 232 Ma). It is noticeable that the formation of Lampang-Phrae volcanic belt has been interpreted as a convergent plate margin based on petrochemical characteristics of Doi 54

Luang (240 ± 1 Ma) and Chiang Khong (232.9 ± 4 Ma) volcanics (Barr et al., 2000, 2006; Panjasawatwong et al., 2003; and Osataporn 2007). However, the post collision model for the formation of the Lampang-Phrae volcanic belt in Middle Triassic was proposed by recent study of Chiang Khong volcanics (Srichan, et al., in press). Therefore, further detailed study is required in order to clarify the tectonic setting of the Lampang-Phrae volcanic belt, which is the major component of the Sukhothai Fold Belt.

3.4 The volcano-plutonic rocks in Loei-Phetchabun Fold Belts

The Loei-Phetchabun volcanic belt covers a very large area and generally shows complexly relationship to the sedimentary host rocks (Fig. 3.1). For the sake of convenience, the study area will be separately discussed, based on regional geographical settings, i.e. the Loei area to the north, and the Phetchabun area to the south.

3.4.1 Geology of Loei-Phetchabun volcano-plutonic rocks

The volcanic rocks in the Loei area, which were collected for this study, have been previously assigned to Silurian-Devonian volcanic and meta-volcanic basement rocks (Fig. 3.26). The Silurian-Devonian volcanic rocks are occasionally overlain by Silurian-Devonian metamorphic rocks and Carboniferous sedimentary rocks. The Lower to Middle Palaeozoic successions were locally intruded by Carboniferous granitoids. Carboniferous basalt and basaltic andesite overlie the Carboniferous sedimentary sequences. The Silurian-Devonian and Carboniferous sequences are overlain by Permian clastic rocks and limestone, which were intruded by Triassic granite. The Silurian-Devonian and Triassic volcano-plutonic rocks, which have not been dated, were selected for this project in order to constrain on the age of volcanism and plutonism, and to compare with the vicinity rocks. Similarly to the Loei-Phetchabun volcanic belt, the geochemistry and U-Pb zircon age determination of the volcano-plutonic rocks in the Loei-Phetchabun belt were also separated into the Loei area (Fig. 3.26) and the Phetchabun area (Fig. 3.27). 55

Figure 3.26 Geological map of the northern Loei area showing distribution of Ban Na Ko, Ban Na Ngiew and Ban That rock units (Salyapong and Khositanont, in preparation) 56

Figure 3.27 Geological map of Khao Lek and Khao Mae Kae areas showing distribution of volcanic and plutonic rocks in the Khao Lek and Ban Thung Faeg area (prepared by Khositanont, 2008) 57

However, the intrusive rocks were grouped into one unit due to their similarities in lithology and significant structural features. It is noticeable that samples were collected from the most abundant rock type of each unit.

3.4.1.1 Loei volcanic rocks

Loei rhyolite is located in the northern part of Loei Province and in the western part of Nong Kai Province (Fig. 3.26). Two sub-parallel volcanic belts form approximately N-S trending from the Mae Khong River to Ban Na Ngiew, Sang Kom District in the east and to Ban Na Ko, Pak Chom District in the west. Two volcaniclastic facies in the northern Loei Province were selected for this study, i.e. Ban Na Ngiew volcaniclastic unit in the eastern belt and Ban Na Ko volcaniclastic unit in the western belt.

(1) Ban Na Ngiew volcaniclastic unit

The outcrops of Ban Na Ngiew volcaniclastic unit have originally been found at Ban Na Ngiew, Sang Khom District and along the Mae Khong River banks. The volcaniclastic rock is tuff, characterised by abundant fine ash grains, a small amount of coarse ash grains, and trace lapilli (T-8554, Fig. 3.28). The lapilli are composed mainly tuff and welded tuff rock fragments. The coarse ash grains are quartz fragments, plagioclase fragments, fiamme fragments, siltstone fragments and high- temperature devitrified glassy rock fragments, which may have been originally derived from glassy or welded tuff rock fragments. Accessory opaque minerals are commonly present in the rock. They are composed of magnetite, hematite and leucoxene. The fine ash grains are composed of glass shards, fiamme, quartz and fine white mica (sericite). The glass shards have been replaced by clay minerals impregnated with hematite/iron hydroxide.

(2) Ban Na Ko volcaniclastic unit

The outcrops of Ban Na Ko volcaniclastic unit are well exposed at Ban Na Ko barite mine, 16 km SE of Pak Chom District. It is characterised by welded tuff that is 58 composed mostly of fine ash grains, with a small amount of coarse ash grains and trace lapilli (T-8570, Fig. 3.29). The lapilli are mostly tuff and welded tuff. The coarse ash grains are composed mainly of quartz, plagioclase, fiamme, glassy rock fragments with low-temperature recrystallisation, and high-temperature devitrification evidenced by the presence of micropoikilitic quartz and spherulite. Minor K-feldspar fragments are also present as a coarse ash grain. Quartz and epidote are secondary minerals.

3.4.1.2 Loei plutonic rocks

Loei plutonic rocks comprise Ban That unit and Phu Tham Phra unit. Petrographic modal analysis suggests that the Loei plutonic rocks are composed of 15-20 % K-feldspar, 75-70% plagioclase and 10% secondary quartz, which are equivalent to monzodiorite.

(1) Ban That monzodiorite unit

The outcrops of Ban That monzodiorite unit are widely exposed at Ban That quarry, 20 km south of Chiang Khan District. The Ban That monzodiorite is medium- grained, sparsely K-feldspar megacrystic, amphibole-biotite monzodiorite. K-feldspar phenocrysts are generally euhedral to subhedral, with tabular forms. Groundmass amphibole grains are euhedral, with prismatic forms, and are up to 2 mm across (T-8584, Fig. 3.30). Groundmass biotite grains are anhedral flakes and books, with sizes up to 3 mm across. Groundmass plagioclase crystals are the most abundant constituent and generally show subhedral to euhedral outlines, with short tabular forms. Quartz crystals occur as irregular replacement patches.

(2) Phu Tham Phra monzodiorite unit

The outcrops of Phu Tham Phra monzodiorite unit are well exposed at Phu Tham Phra hill, approximately 10 km southeast of Loei District. It is characterised by medium-grained, equigranular to porphyritic amphibole-biotite monzodiorite (T-8606, Fig. 3.31). Sparse K- feldspar megacrysts are subhedral to euhedral with tabular forms. 59

Figure 3.28 Photomicrographs of Ban Na Ngiew tuff (T8554) showing ash grains of glass shards and sub-angular quartz fragments. The top is in plane polarised light and the bottom is in crossed polarised light

60

Figure 3.29 Photomicrographs of Ban Na Ko welded tuff (T8570) showing coarse ash grains of feldspar and quartz floating in the devitrified matrix. The top is in plane polarised light and the bottom is in crossed polarised light

61

Figure 3.30 Photomicrographs of Ban That monzodiorite (T8584) showing plagioclase (Plag) interlocking, epidote (Epi) pseudomorphs after amphibole (Amp), and secondary quartz (Qtz) patches. The top is in plane polarised light and the bottom is in crossed polarised light 62

Figure 3.31 Photomicrographs of Phu Tham Phra monzodiorite (T8606) showing amphibole (Amp) and plagioclase (Plag) with secondary quartz (Qtz) patches. The top is in plane polarised light and the bottom is in crossed polarised light 63

Microscopic observation shows that amphibole crystals have rarely been observed as a groundmass constituent and generally show subhedral, with a prismatic form. Groundmass biotite grains are the most abundant mafic mineral, which is present as anhedral flakes. Groundmass plagioclase crystals are subhedral, with short tabular forms. Groundmass K-feldspar crystals are anhedral to subhedral, with short tabular forms. Quartz grains are of secondary origin and form as irregular patches.

3.4.1.3 Phetchabun volcanic and plutonic suites

The majority of volcanic rocks in the Phetchabun area are composed of intermediate to basic rocks, which are extensively distributed in the Phetchabun and Phichit Provinces. Two volcanic units as well as two plutonic units were selected to be the representatives of volcanic and plutonic rocks in the Phetchabun area, including Thung Faeg volcaniclastic, Sap Samran basalt and microgabbro, Khao Lek diorite, and Khao Mae Kae diorite units (Fig. 3.28).

(1) Thung Faeg volcaniclastic unit

Thung Faeg volcaniclastic unit forms a ring shape surrounding the diorite intrusion at Ban Thung Faeg and Ban Khao Mae Kae, Bung Samphan, District, Phetchabun Province. It is characterised by dark greenish grey lapilli tuff, and coarse tuff (Fig. 3.32). Microscopic observation shows that it is composed mainly of lapilli and subordinate coarse ash grains. The lapilli are composed mostly of plagioclase, clinopyroxene (aegirine augite) crystal fragments and basaltic or andesite rock fragments. Coarse ash grains are compositionally similar to those of lapilli.

(2) Sap Samran basalt and microgabbro unit

Sap Samran basalt and micrograbbro unit occurs as an arcuate belt in the western part of Thung Faeg basalt unit and forms a ring shape similar to the Thung Faeg volcaniclastic unit. The Sap Samran basalt outcrops are widely distributed as a low land plain, whereas the microgabbro outcrops are present as small hills surrounding Khao Lek hill, west of Ban Khao Mae Kae (Fig. 3.27). The microgabbro is a dark greenish 64

Figure 3.32 Photomicrographs of Ban Thung Faeg tuff (KP02-6) showing a portion of basaltic clast with plagioclase phenocrysts in the finer-grained groundmass (Plag = plagioclase, and Cpx = clinopyroxene). The top is in plane polarised light and the bottom is in crossed polarised light 65

Figure 3.33 Photomicrographs of Sap Samran microgabbro (KP02-19) showing crystals of clinopyroxene (Cpx) and plagioclase (Plag). The top is in plane polarised light and the bottom is in crossed polarised light. 66

grey, with fine to medium-grained seriate texture (Fig. 3.33). Microscopic shows that plagioclase and clinopyroxene are the most abundant components in this rock. The plagioclase grains are subhedral to euhedral, with short prismatic forms. The clinopyroxene grains are mostly subhedral. Of the basaltic rock, plagioclase is the most abundant, whereas clinopyroxene and amphibole have been rarely observed. The plagioclase is generally subhedral, with short tabular forms, whereas clinopyroxene and amphibole are much finer grains and, generally have anhedral crystals.

(3) Khao Mae Khae diorite unit

Khao Mae Kae unit is exposed as a core of the Thung Faeg basalt and Sap Samran basalt and microgabbro units (Fig. 3.27). It is also noticeable that the Khao Mae Kae diorite intruded into the Thung Faeg basalt and Sap Samran basalt. It is grey to pale grey in colour and medium- to coarse-grained, with sparse plagioclase phenocrysts, and inequigranular to porphyritic textures (Fig. 3.34). Plagioclase phenocrysts are 1-2 cm across, pale grey and subhedral to euhedral, and have short tabular forms. They locally float on the finer-grained groundmass. Of the groundmass constituents, microscopic observation shows that amphibole grains are 1-2 mm across, and are up to 5 percent. They are subhedral crystals, with stubby forms. Biotite grains are mostly flakes, with sizes 1-2 mm across, and are up to 3 percent. Plagioclase crystals are the most abundant. They occur as subhedral to anhedral short tabular crystals, with size 1-5 mm across. K-feldspar crystals are up to 10%, with sizes generally 1-5 mm across. They are subhedral to anhedral, and have tabular outlines. It is noticeable that the plagioclase phenocrysts and groundmass plagioclase grains have been locally replaced by quartz at rims.

(4) Khao Lek diorite unit

Khao Lek diorite unit is exposed as a small exposure between Sap Samran basalt and Khao Mae Kae diorite. It is dark grey and has a porphyritic texture, with sparse plagioclase phenocrysts and medium-grained groundmass. Microscopic observation shows that the plagioclase phenocrysts generally form short tabular crystals, floating in finer-grained groundmass constituents (Fig. 3.35). Groundmass amphibole grains are 67

Figure 3.34 Photomicrographs of Khao Mae Kae diorite (KP02-12) showing plagioclase phenocrysts (Plag) and finer amphibole (Amp) and biotite (Bi) grains. The top is in plane polarised light and the bottom is in crossed polarised light. 68

Figure 3.35 Photomicrographs of Khao Lek diorite (KP02-11) showing amphibole (Amp), plagioclase (Plag) and quartz (Qtz). The top is in plane polarised light and the bottom is in crossed polarised light

69

1-2 mm across, with subhedral stubby outlines, and are up to 5%. The groundmass amphibole grains have been undergone epidote (pumpellyite) and chlorite alteration. Plagioclase grains are the most abundant felsic mineral constituent. They generally have euhedral to subhedral outlines, with short tabular forms. The anorthite content in the plagioclase microphenocrysts, measured on the basis of petrographic technique are in the ranges of 30-50. Anhedral K-feldspar is rarely present in this rock. Anhedral quartz crystals are interstitial to plagioclase crystals, suggesting they were crystallised later than groundmass plagioclase grains.

3.4.3 Geochemical characteristics of the Loei-Phetchabun volcano-plutonic rocks

The results of major, trace element and REEs analyses of the Loei-Phetchabun volcanic and plutonic rocks are shown in Tables 3 and 4. The total alkali versus silica petrology discrimination diagram (LeBas et al., 1986) signify that Sap Samran volcanics are chemically basalt, Ban Thung Faeg volcaniclastics rocks are chemically basaltic andesite, Ban Na Ko and Ban Na Ngiew volcaniclastics are chemically rhyolite, Ban That, Phu Tham Phra and Khao Mae Kae plutonics are chemically andesite, and Khao Lek plutonics are chemically basalt (Fig. 3.36).

These are well supported by their positions on the Zr/TiO2-Nb/Y discrimination diagram (Fig. 3.37) of Winchester and Floyd (1977). The Ti/100-Zr-Y*3 diagram (after Pearce and Cann, 1973) for basaltic rocks signifies that the Sap Samran basalt has similar chemical composition to those of ocean floor basalt (Fig. 3.38). The Khao Lek diorite basalt is chemically within-plate basalt. The Sap Samran microgabbro and Khao Mae Kae diorite are plotted outside the fields. The Ti-V discrimination diagram (after Shervais, 1982) shows that the Sap Samran basalt and microgabbro, Khao Lek diorite, and Khao Mae Kae diorite have magmatic affinitieswith those formed in mid-ocean ridge, back-arc basin and arc environments (Fig. 3.39). The back-arc basin environment is strongly supported by an arc signature in N-MORB nomalised multi-element patterns (Fig. 3.40), and oceanic within-plate and MORB signatures in REE patterns (Fig. 3.41). 70

Table 3.3 Major and trace elements form Loei-Phetchabun volcanic and plutonic rocks

Thung Khao Mae Faeg Khao Lek Kae Sap Samran Sap Samran Thung Faeg Major (%) KPO2-6 KPO2-11 KPO2-12 KPO2-19 KPO2-24 KPO2-69

SiO2 55.10 49.78 61.83 48.18 55.87 55.38 TiO2 0.92 0.96 0.72 0.73 1.01 0.82 Al2O3 16.00 17.26 16.81 23.09 18.80 17.61 Fe2O3 10.52 10.60 5.10 8.69 8.73 9.93 MnO 0.12 0.10 0.08 0.16 0.13 0.21 MgO 4.61 4.24 2.82 2.96 2.91 4.38 CaO 6.54 12.61 5.51 11.48 6.05 8.72

Na2O 4.36 2.94 4.32 3.33 4.78 1.44 K2O 1.26 0.62 1.58 0.27 0.58 0.32 P2O5 0.15 0.12 0.17 0.13 0.19 0.15 Loss on ignition 0.68 0.24 0.74 0.47 1.33 1.30 Total 100.27 99.47 99.66 99.50 100.37 100.26

Trace ppm Nb 3.4 1.9 5.3 Bdl 3.8 Bdl Zr 77 62 189 9 81 42 Sr 231 581 512 533 434 200 Cr 48 106 43 6 3 240 Ba 145 138 304 80 135 193 Sc 38 21 17 29 30 32 V 300 295 112 255 155 195 La 5 5 11 3 6 2 Ce 12 18 27 7 14 6 Nd 7 13 17 5 11 6 Y 21 14 18 9 30 12 U Bdl Bdl Bdl Bdl Bdl Bdl Rb 32 17 44 3 11 65 Th Bdl Bdl 3 Bdl Bdl Bdl Pb Bdl 5 4 6 2 6 As 4 4 Bdl Bdl 7 4 Bi Bdl Bdl Bdl Bdl Bdl Bdl Zn 57 75 48 130 50 158 Cu 17 74 46 27 65 67 Ni 14 35 24 7 4 33

Bdl = Below detection limit 71

Table 3.3 Continued

Phu Tham Thung Faeg Thung Faeg Na Ngiew Na Ko Ban That Phra Major (%) KPO2-46 KPO2-88 T8554 T8570 T8584 T8606

SiO2 51.26 51.27 73.58 75.80 61.44 60.47 TiO2 0.63 0.86 0.32 0.20 0.67 0.64 Al2O3 18.28 16.49 12.76 12.84 17.69 16.32 Fe2O3 8.20 6.94 2.91 1.80 4.85 5.91 MnO 0.13 0.12 0.05 0.06 0.07 0.10 MgO 1.72 4.62 0.72 0.60 2.56 2.65 CaO 15.11 6.97 0.63 0.81 5.55 5.52

Na2O 0.84 3.80 4.84 3.98 4.55 3.21 K2O 3.53 0.81 2.19 3.35 1.66 2.15 P2O5 0.15 0.20 0.07 0.05 0.16 0.21 Loss on ignition 0.10 7.41 1.36 0.91 0.76 2.33 Total 99.94 99.50 99.42 100.39 99.96 99.50

Trace ppm Nb 2.7 3.8 7.2 7.7 4.2 6.4 Zr 66 130 200 137 182 144 Sr 357 653 116 116 536 542 Cr 10 69 3 2 42 19 Ba 78 929 594 596 316 546 Sc 27 25 10 6 15 16 V 212 201 16 11 105 116 La 6 8 19 20 9 19 Ce 16 25 36 44 25 35 Nd 10 16 23 21 16 20 Y 19 18 35 31 16 17 U Bdl Bdl 2 3 Bdl Bdl Rb 6 17 48 98 46 55 Th Bdl Bdl 7 9 2 5 Pb 4 Bdl 7 7 4 10 As Bdl Bdl 8 Bdl Bdl Bdl Bi Bdl Bdl Bdl Bdl Bdl Bdl Zn 89 65 50 38 46 66 Cu 29 73 3 3 56 31 Ni 8 30 2 2 21 14

Bdl = Below detection limit 72

Table 3.4 Rare earth elements analyses for the Loei-Phetchabun volcanic and plutonic rocks

Elements T8554 T8570 T8584 T8606 KP02-11 KP02-12 KP02-19 KZ05-01 7 Li 10.3 9.15 11.5 21.3 6.19 12.0 7.65 19.4 9 Be 1.36 1.39 0.90 0.95 0.65 0.87 0.37 2.03 45 Sc 10.3 6.79 11.7 13.1 17.8 12.9 26.7 7.41 47 Ti 1949 1177 4164 3891 5989 4323 4276 993 51 V 14.5 10.1 110 114 297 112 259 14.1 55 Mn 397 486 557 767 732 599 1243 510 60 Ni 1.61 1.79 22.3 11.0 39.4 24.1 8.63 2.62 63 Cu 3.76 3.2 55.3 29.5 64.9 45.8 21.3 3.25 Zn (XRF) 50 38 46 66 75 48 130 63 71 Ga 12.4 13.0 18.7 17.0 20.3 18.0 20.6 19.1 As (XRF) 48.5 96.7 45.4 53.3 16.1 44.0 3.07 215 85 Rb 114 114 536 547 577 512 537 39 88 Sr 35 31 16 17 14 18 9 31 Y (XRF) 200 137 182 144 62 189 9 148 Zr (XRF) 5.92 5.99 3.76 5.85 1.11 3.57 0.30 14.3 93 Nb 0.26 0.34 0.36 0.48 0.45 0.88 0.19 0.45 95 Mo Bdl Bdl Bdl 0.09 0 Bdl 0.07 Bdl 107 Ag Bdl Bdl Bdl 0.1 Bdl Bdl 0.19 Bdl 111 Cd 1.69 2.00 1.00 0.94 1.11 1.09 0.37 5.92 118 Sn 0.49 1.48 0.19 Bdl 0.18 0.19 0.12 1.22 121 Sb Bdl Bdl Bdl Bdl Bdl Bdl Bdl Bdl 125 Te 1.80 1.90 1.87 0.77 0.84 1.80 0.81 7.20 133 Cs 594 596 316 546 138 304 80 516 Ba (XRF) 19.7 25.1 13.1 19.4 6.4 13.9 2.61 21.6 139 La 37.7 49.6 28.8 38.6 16.0 30.9 5.76 48.1 140 Ce 5.24 5.87 3.83 4.70 2.42 4.19 0.87 6.12 141 Pr 22.0 22.3 16.4 18.7 11.7 17.6 4.37 23.8 146 Nd 5.20 4.73 3.62 3.66 3.11 3.96 1.24 5.81 147 Sm 1.30 0.67 1.00 1.07 0.80 0.99 0.72 0.62 151 Eu 5.65 4.64 3.30 3.27 3.03 3.59 1.44 5.19 157 Gd 0.99 0.80 0.49 0.52 0.48 0.55 0.25 0.86 159 Tb 6.02 4.93 2.78 2.96 2.77 3.13 1.56 5.13 163 Dy 1.20 1.01 0.52 0.59 0.51 0.58 0.30 1.03 165 Ho 3.62 3.12 1.48 1.69 1.39 1.65 0.91 3.20 166 Er 0.54 0.50 0.21 0.25 0.19 0.23 0.12 0.51 169 Tm 3.55 3.29 1.32 1.66 1.20 1.46 0.82 3.45 172 Yb 0.55 0.54 0.20 0.26 0.18 0.21 0.12 0.53 175 Lu 0.72 0.91 0.59 0.72 0.39 0.65 0.24 1.55 181 Ta 0.27 0.63 0.15 0.27 Bdl 0.15 Bdl 1.30 205 Tl 7.89 7.90 3.88 9.16 3.90 3.67 5.29 9.04 208 Pb Bdl 0.15 Bdl Bdl Bdl Bdl Bdl 0.27 209 Bi 7.12 9.33 3.26 5.06 1.19 3.36 0.057 20.2 232 Th 2.09 2.30 0.90 1.65 0.38 0.88 0.016 3.80

Bdl = Below detection limit 73

Figure 3.36 Total alkali – silica petrology discrimination diagram of Loei- Phetchabun volcanic and plutonic rocks (after LeBas et al., 1986). Solid diamonds are Loei plutonic rocks, pink diamonds are Phetchabun plutonic rocks, and open diamonds are Loei-Phetchabun volcaniclastic.

Figure 3.37 Trace element relation in the Zr/TiO2-Nb/Y petrology discrimination diagram (after Winchester and Floyd, 1977) for Loei-Phetchabun igneous rocks. The numbers on the data points represent the sample numbers. 74

Figure 3.38 Ti-Zr-Y discrimination diagram (after Pearce and Cann, 1973) for the Sap Samran basalt and microgabbro , Khao Lek diorite and Khao Mae Kae diorite.

Figure 3.39 Ti-V discrimination diagram (after Shervais, 1982) for the Sap Samran basalt and microgabbro, and Khao Lek diorite and Khao Mae Kae diorite. 75

Figure 3.40 N-Type MORB trace elements spider plots (after Sun and McDonough, 1989) for the Sap Samran basalt and microgabbro, and Khao Lek diorite and Khao Mae Kae diorite.

Figure 3.41 REE/Chondrite normalised patterns for Phetchabun volcanic rock (KP02-19) and plutonic rocks (KP02-11 and KP02-12) using chondrite- nomalising values of Sun and McDonough (1989). 76

The REE pattern of the Khao Lek diorite (KP02-11), using chondrite-nomalised values of Sun and McDonough (1989), shows relatively a slight increase in light rare- earth-elements with chondrite-normalised Sm/Yb = 2.81 and La/Sm = 1.26 respectively, which is similar to those of ocean-island tholeiitic basalts. The Khao Mae Kae diorite (KP02-12) has a similar REE patterns to those of Khao Lek diorite, with chondrite-normalised Sm/Yb = 2.94 and La/Sm = 2.13, signifying that it is chemically tholeiitic. On the other hand, the REE pattern of the Sap Samran microgabbro (KP02-19) typically shows a slight increase in light and heavy rare- earth-elements with chondrite-normalised Sm/Yb = 1.64 and La/Sm = 1.28 respectively, which is similar to those of mid-oceanic ridge basalt (E-Type MORB). The REE patterns for the Loei volcanic and plutonic rocks show that the Ban That (T-8584) and Phu Tham Phra monzodiorite (T-8606) have flat slopes for heavy rare-earth elements and are relatively steep slope for light rare-earth elements, with chondrite-normalised with Sm/Yb = 2.97 and La/Sm = 2.20, and Sm/Yb = 2.38 and La/Sm = 3.22, signifying that they are chemically calc-alkalic (Fig. 3.42). The Ban Na Ngiew and Ban Na Ko volcaniclastics have similar REE patterns to those of plutonic rocks, with chondrite-normalised Sm/Yb = 1.58 and La/Sm = 2.31 (T-8554), and Sm/Yb = 1.55 La/Sm = 3.23 (T-8570).

3.4.4 U-Pb zircon age determination

The volcanic and plutonic rocks in Loei – Phetchabun volcanic belt were randomly collected for U-Pb zircon age determination using LA ICP-MS analytical technique (Appendix 1).

3.4.4.1 The ages of Loei volcanic and plutonic rocks

The U-Pb isotopic values in zircon of Ban Na Ko and Ban Na Ngiew volcaniclastics are scattered on Concordia plots (Figs. 3.43 and 3.44). The U-Pb isotopic ratios suggest that the zircon were crystallised at 433 ± 4 Ma for Ban Na Ngiew tuff, and 425 ± 7 Ma for Ban Na Ko welded tuff. 77

Figure 3.42 Chondrite normalised patterns for Loei volcanic rocks (T8554 and T8570) and plutonic rocks (T85854 and T8606) using chondrite- nomalising values of Sun and McDonough (1989).

Figure 3.43 U-Pb Concordia plots for zircon from Ban Na Ngiew tuff (T-8554). 78

Figure 3.44 U-Pb Concordia plots for zircon from Ban Na Ko welded tuff (T-8570).

The U-Pb isotopic values of zircon in Ban That granodiorite are also scattered on Concordia plots (Fig. 3.45). The U-Pb isotopic values suggest that that the zircon grains of Ban That granodiorite were crystallised at 230 ± 4 Ma. It is noticeable that the same results were obtained from zircon grains of granodiorite at Phu Tham Phra (Khin Zaw, pers. comm).

3.4.4.2 The age of Phetchabun volcanic and plutonic rocks

The U-Pb isotopic values on Concordia plots for zircon from Khao Lek diorite are widely spread out and yield the age of 254 ± 10 Ma (Fig. 3.46). The U-Pb isotopic values of Khao Mae Kae granodiorite are clustered on Concordia curved (Fig. 3.47), which yield the result of 250 ± 5 Ma for the age of Khao Mae Kae granodiorite crystallisation.

79

Figure 3.45 U-Pn Concordia plots for zircon from Ban That monzodiorite (T-8584)

Figure 3.46 U-Pb Concordia plots for zircon from Khao Lek diorite (KP02-11). 80

Figure 3.47 U-Pb Concordia plots for zircon from Khao Mae Kae diorite (KPO2-12).

3.4.5 Summary on geochemistry and ages of Loei – Phetchabun volcanic and plutonic rocks

The major and trace elements characteristics show that the volcano-plutonic rocks in the Loei-Phetchabun Fold belt have great variation in chemical composition. The volcaniclastic rocks in the northern Loei area have rhyolitic composition, whereas the plutonic rocks have monzodioritic composition. The volcanic and volcaniclastic rocks in the Phetchabun area have basaltic and andesitic compositions, whereas three plutonic rocks have andesitic composition and one (Khao Lek diorite) has basaltic composition. The Khao Mae Kae diorite has rhyodacite and dacite compositions, whereas Khao Lek diorite has andesite or basalt field. The Sap Samran basalt has basalt and andesite compositions, whereas the Sap Samran microgabbro has subalkaline basalt composition. The Sap Samran basalt and microgabbro, and Khao Lek and Khao Mae Kae diorite have back-arc affinities. The Ban Na Ngiew and Ban Na Ko tuffs, and Ban That and Phu Tham Phra monzodiorite have calc-alkalic affinities. 81

The U-Pb zircon ages vary from Silurian-Devonian for Loei volcaniclastics to Middle Triassic. It is noticeable that volcanic and plutonic rocks of Devonian, Carboniferous and Permian ages have been previously observed in the Loei – Phetchabun volcanic belt (Khin Zaw, pers. com.). The Loei-Phetchabun Fold Belt has been interpreted as ocean floor in Palaeozoic and volcanic arc in Upper Palaeozoic to Lower Mesozoic (Panjasawatwong et al., 2006). This study also shows that the back arc was developed in Permo-Triassic. The final collision of Shan-Thai and Indochina may have taken place in the Late Triassic (Bunopas, 1981).

3.5 Comparison of tectonic setting of volcano-plutonic rocks in Sukhothai and Loei - Phetchabun Fold Belts

The volcano-plutonic rocks involved in the evolution of Sukhothai Fold Belt are the products of Triassic igneous activities. The presence of Middle Triassic ammonite in the Lampang Group sedimentary rocks (Chonglakmani, 1989) and Middle Triassic radiolarian chert along the Nan suture zone (Caridroit and Wonganan, 2008) signifies an open sea environment at that time. The occurrence of widespread molasses-type sedimentary rocks of Upper Triassic Khorat Group implies that the Shan-Thai and Indochina were completely amalgamated in the Late Triassic. The ages and chemical characteristics of volcano-plutonic rocks in the Lampang-Phrae volcanic belt show that a volcano-plutonic arc was still active during the Triassic time. The existence of magmatic arc prior to the amalgamation of Shan-Thai and Indochina was also already mentioned by Barr et al., (2000, 2006), Panjasawatwong et al. (2003) and Osataporn (2007). According to the chemical similarities of arc lavas and post-orogenic lavas, the Late Triassic magmatic activity may also have been occurred as post-orogenic magma as mentioned by Srichan et al. (2007, inpress). On the other hand, the chemical characteristics of major elements, trace elements and REEs of the Loei-Phetchabun volcano-plutonic rocks show great difference through time. The Loei rhyolite suites were formed from calc-alkalic affinity magma in the Silurian-Devonian ages. These zircon ages are slightly older than those obtained from basaltic rocks (361 - 374 Ma) in the adjacent area which 82

have been interpreted as the remnants of ocean floor (Intasopa, 1993 and Panjasawatwong, et al., 2006). In addition, the Late Carboniferous to Early Permian volcanic rocks, which have been reported by Khin Zaw and Meffre (pers. comm.), are widely exposed in the Phetchabun areas. The presence of Sap Samran basalt and microgabbro, and Khao Lek and Khao Mae Kae diorite in the back arc basin may be related to the Permian volcanism. The calc-alkalic and back-arc affinities Permo-Triassic diorite (230 – 255 Ma) in conjunction with the widely distribution of Late Carboniferous to Early Permian calc- alkalic affinities volcanic and plutonic rocks in the Loei and Phetchabun areas and vicinities (Khin Zaw and Meffre, pers. comm.) may represent the subduction related magmatism in the Loei-Phetchabun Fold Belt. Summary of tectonic setting and evolution of the Sukhothai and Loei-Phetchabun Fold Belts are illustrated in Figure 3.48. 83

Figure 3.48 Plate-tectonics model showing evolution of the Sukhothai and Loei- Phetchabun Fold Belts (modified from Bunopas et al., 1983, and Intasopa et al., 1993).