Preliminary Geochemical Study of Volcanic Rocks in the Pang Mayao Area, Phrao, Chiang Mai, Northern Thailand: Tectonic Setting of Formation

Preliminary geochemical study of volcanic rocks in the Pang Mayao area, Phrao, Chiang Mai, northern Thailand: tectonic setting of formation

Burapha Phajuy*, Yuenyong Panjasawatwong, Pukpong Osataporn

Department of Geological Sciences, Chiang Mai University, Chiang Mai 50200, Thailand

Received 1 March 2003; accepted 14 June 2004

Abstract

The least-altered, Permian mafic volcanic rocks from the Pang Mayao area, Phrao District, Chiang Mai Province, part of Chiang Rai– Chiang Mai volcanic belt, have been analyzed and are found to be mid-ocean ridge and ocean–island basalts. The mid-ocean ridge basalts occur as lava flows or dike rocks. They are equigranular, fine- to medium-grained and consist largely of plagioclase, clinopyroxene and olivine. These basalt samples are tholeiitic, and have compositions very similar to T-MORB from the region where the Du Toit Fracture Zone intersects the Southwest Indian Ridge. The ocean–island basalt occurs as pillow breccia, and lava flows or dike rocks. They are slightly to moderately porphyritic, with phenocrysts/microphenocrysts of clinopyroxene, olivine, plagioclase and/or Fe–Ti oxide. The groundmass is very fine-grained, and made up largely of felty plagioclase laths with subordinate clinopyroxene. These basalt samples are alkalic, and chemically analogous to those from Haleakala Volcano, Maui, Hawaiian Chain. These mafic volcanic rocks may have been formed in a major ocean basin rather than in a mature back-arc basin. q 2004 Elsevier Ltd. All rights reserved.

Keywords: Mid-ocean ridge basalt; Ocean–island basalt; REE; Paleo-Tethys; Shan-Thai

1. Introduction Bunopas and Vella, 1983; Panjasawatwong, 1999). It has also been claimed that volcanic rocks in the Chiang Rai The Permian mafic volcanic rocks discussed in this area were erupted in a subduction environment (Macdo- study are part of the westernmost Chiang Rai–Chiang Mai nald and Barr, 1978; Barr et al., 1990), while the volcanic Volcanic Belt (Fig. 1). They occur to the south and east of rocks in the Chiang Mai and Lamphun areas were erupted Pang Mayao Village, Phrao District, Chiang Mai Pro- in a continental within-plate environment (Barr et al., vince, northern Thailand (Fig. 2). The Chiang Rai–Chiang 1990). On the other hand, Panjasawatwong et al. (1995) Mai Volcanic Belt is composed of mafic lavas, hyalo- and Panjasawatwong (1999) believed that those in clastites, pillow breccias, and mafic dikes. These rocks the Chiang Mai and Lamphun areas formed in an were classified chemically by Macdonald and Barr (1978), oceanic within-plate environment as ocean islands and Barr et al. (1990), Panjasawatwong et al. (1995) and seamounts in either a major ocean basin or a mature Panjasawatwong (1999) as tholeiitic basalt and/or transi- back-arc basin. A major ocean basin environment is tional tholeiitic basalt. Previous workers suggested that consistent with the interpretations of Caridroit (1993) and the basalts were erupted in the Carboniferous (e.g. Baum Metcalfe (2002). and Hahn, 1977; Macdonald and Barr, 1978; Hess and This preliminary study has been undertaken in order to Koch, 1979; Barr et al., 1990) or the Permian to Permo- characterize the least-altered, Permian mafic volcanic rocks Triassic (e.g. Chuaviroj et al., 1980; Bunopas, 1981; in the Pang Mayao area in terms of occurrence, petrography and chemistry. Particular attention is paid to least-mobile * Corresponding author. Tel.: C66-53-942032; fax: C66-53-892261. elements to clarify the tectonic settings for the formation of E-mail address: [email protected] (B. Phajuy). these mafic volcanic rocks.

Fig. 1. Distribution of the Pre-Jurassic volcanic rocks in Thailand (modiﬁed from Panjasawatwong et al., 1997) and the study area. Abbreviation: NCZ - Nan- Chanthaburi suture zone.

2. Geologic setting limestone and Quaternary alluvial sediments (Fig. 2). In addition, an outcrop of cumulus dunite was recently A detailed geologic map of the study area at a scale of discovered in the volcanic terrain, to the northeast of Pang 1:10,000 (Wongko et al., 2000) shows four rock units. From Mayao Village. The relationship between maﬁc volcanic older to younger, these are Middle–Upper Carboniferous rocks and ultramaﬁc cumulates is, however, not known due sedimentary rocks, Permian tuff and basalt, Permian to poor exposure. The Middle–Upper Carboniferous B. Phajuy et al. / Journal of Asian Earth Sciences 24 (2005) 765–776 767

the Permian limestone. The Permian tuff and basalt, and the Permian limestone units are overlain unconformably by Quaternary alluvial sediments, including gravel, sand, silt and clay.

3. Methodology

The Permian volcanic rocks in the Pang Mayao area have experienced varying degrees of alteration so that their chemical composition may have been modiﬁed. However, it has been well documented that altered volcanic rocks can be informative with regard to their primary afﬁnities, if due care is taken with the selection of samples, and with the elements and element ratios used in a diagnostic study.

3.1. Sample selection

Thirty-five mafic volcanic samples were collected from outcrop and float in the Pang Mayao area. The mafic volcanic rocks are either hydroclastic or non-fragmental. The collected samples were re-examined petrographically to obtain least-altered samples without secondary minerals (e.g. quartz, epidote, chlorite, amphibole and albite), without a well-developed foliation, with no vesicles/amyg- dales, xenocrysts or xenoliths or veinlets or patches of quartz, epidote and/or calcite totalling more than 5 modal%. After examination the 12 least-altered volcanic samples (sample Nos. A-1, A-2, A-3, B-1, B-2, C-1, D-1, E-1, F-1, G-1, H-1 and H-2) were chosen as representative of the magma prior to solidification. The location of these samples Fig. 2. Geologic map of the study area (modified from Wongko et al., are shown in Fig. 2. 2000). Solid circles are sample locations of subalkalic basalt and diamonds are those of alkalic basalt (solid symbolZnon-ophitic/subophitic-textured sample and open symbolZophitic/subophitic-textured sample). 3.2. Chemical analysis

Powdered samples for whole-rock chemical analyses sedimentary rocks are made up largely of black chert and were prepared by splitting the samples into convenient- black shale with minor gray siltstone and gray limestone sized fragments with a hydraulic splitter and then sawing off interbeds. This rock unit is cross cut by basaltic dikes that the weathering surfaces. The sample fragments were then might have served as feeders for the Permian volcanic rocks. crushed with a steel jaw crusher and cleaned with an air The Permian tuff and basalt unit was previously inferred to hose. 30–50 g aliquots of the crushed fragments without overlie the Carboniferous sedimentary rocks unconform- oxidation surfaces, veining, amygdale minerals, xenoliths or ably. However, an alternative interpretation is put forward steel smeared from the jaw crusher were pulverized for a in this study is that they are juxtaposed by thrust faults. few minutes in a Rocklabs tungsten–carbide ring mill. The Permian tuff and basalt unit is composed of green The powdered samples were analyzed for major oxides tuff, gray to dark green basaltic flows, hyaloclastite and (SiO2, TiO2,Al2O3, total iron oxide as Fe2O3, MgO, MnO, pillow breccia. Possibly reworked fusulinid fragments of CaO, Na2O, K2O, P2O5 and loss on ignition) and some trace Upper Carboniferous to lower Middle Permian (R. Ingavat- elements (Rb, Sr, Ba, Nb, Zr, Y, Th, Ni, Cr, V and Sc). Four Helmcke, personal communication, 2004) were found in the representatives of the least-altered samples were analyzed carbonate cement of pillow breccias. Permian limestone, for rare-earth elements (REE: La, Ce, Pr, Nd, Sm, Eu, Gd, inferred to rest unconformably on the Permian tuff and Tb, Dy, Ho, Er and Yb). Almost all major-oxide and trace- basalt, and thrust over the Middle–Upper Carboniferous element analyses were carried out by a wavelength sedimentary units, is composed principally of light gray to dispersive X-ray fluorescence technique, whereas REE black limestone with rare brown shale intercalations. The analysis was done by high resolution inductively coupled limestone includes bioclastic, crystalline and oolitic lime- plasma mass spectrometer (HR-ICP-MS). Loss on ignition stones. Brachiopods, corals, and foraminifera were found in was determined via a gravimetric method. All the analyses 768 B. Phajuy et al. / Journal of Asian Earth Sciences 24 (2005) 765–776 were performed at the School of Earth Sciences, University The subalkalic basalts are sample Nos. E-1, H-1 and H-2, of Tasmania. whereas the alkalic basalt includes sample Nos. A-1, A-2, A-3, B-1, B-2, C-1, D-1, F-1 and G-1. 3.3. Least-mobile elements 4.1. Subalkalic basalt It is well documented that high field strength elements (e.g. Ti, Zr, Y, Nb, P and Th) and transitional elements (e.g. Ni, Cr, 4.1.1. Occurrence and petrography V and Sc) remain relatively immobile during alteration and Subalkalic basalts were collected from outcrop and float low-grade metamorphism of basaltic and more evolved rocks in a stream due north of road No. 1090 (Fig. 2). The (Pearce and Cann, 1973; Pearce et al., 1975; Coish, 1977; outcrops of subalkalic basalt are massive, and could Floyd and Winchester, 1975; Winchester and Floyd, 1977; possibly be subaerial lava flows or dike rocks. Samples Shervais, 1982; Holm, 1985). Furthermore, although are bluish gray and dark greenish gray, and are fine- to occasional reports have appeared of REE-, especially light medium-grained, with averaged grain sizes up to 2 mm REE (herein LREE), mobility during hydrothermal alteration across. and low-grade metamorphism (Frey et al., 1974; Humphris et Under the microscope, these rocks are composed largely al., 1978; Whitford et al., 1988), the overwhelming consensus of plagioclase, clinopyroxene and olivine with trace of opinion is that the REE patterns of carefully selected secondary patches of chlorite, pyrite, and Fe–Ti oxide altered igneous rocks are probably little removed from their (partly replaced by titanite/leucoxene). In sample No. H-1 primary patterns. Limited LREE mobility or parallel dilution/ xenoliths of microgabbro are seen, with a coarser grain size enrichment (upward and downward) trends of REE patterns than the host. Biotite may be present sporadically. are certainly unlikely to lead to a different petrogenetic Plagioclase is subhedral to euhedral, with sizes up to interpretation than that gleaned from primary patterns (see 0.4 mm across. It is slightly to moderately replaced by Whitford et al., 1988). Accordingly, in attempting to chlorite, titanite/leucoxene, calcite, sericite, quartz and/or determine the geochemical affinities and tectonic signifi- Fe–Ti oxide. Clinopyroxene has anhedral to subhedral cance of the altered igneous suites in the Phrao area, concentration has focused on the relatively immobile elements, namely the HFSE, REE and transition elements.

4. Magmatic groups

The studied volcanic samples may be separated into two magmatic groups: subalkalic basalt and alkalic basalt on the basis of the Zr/TiO2 against Nb/Y diagram (Fig. 3).

Fig. 3. Plot of Zr/TiO2 against Nb/Y for the studied least-altered maﬁc volcanic samples (symbols are as in Fig. 3). Field boundaries for different Fig. 4. Zirconia variation diagrams for least-mobile major oxides in the magma types are taken from Winchester and Floyd (1977). studied maﬁc volcanic rocks (symbols are as in Fig. 3). B. Phajuy et al. / Journal of Asian Earth Sciences 24 (2005) 765–776 769

Fig. 5. Zirconia variation diagrams for least-mobile trace elements in the studied maﬁc volcanic rocks (symbols are as in Fig. 3). 770 B. Phajuy et al. / Journal of Asian Earth Sciences 24 (2005) 765–776 outlines, with grains up to 4 mm across, and is ophitic/ subophitic to plagioclase laths. Clinopyroxene is slightly replaced by titanite/leucoxene, Fe–Ti oxide, quartz and/or chlorite. Olivine is anhedral to euhedral, and is completely replaced, largely by chlorite/serpentine with trace calcite, titanite/leucoxene, Fe–Ti oxide and/or pyrite. Brown subhedral biotite forms grains up to 2 mm across, and is partly altered to chlorite, titanite/leucoxene and/or Fe–Ti oxide.

4.1.2. Chemistry Subalkalic basalts have Zr/TiO2 and Nb/Y ratios in ranges of 0.005–0.006 and 0.167–0.514, respectively (Fig. 3). The samples appear to form the same compositional trends as the alkalic basalt samples but their TiO2, P2O5, Nb, Y, Zr and V are lower (Figs. 4 and 5). Only sample No. H-2 was analyzed for REE (Table 2). The chondrite-normalized multi-element pattern for sample No. H-2 is relatively flat, with a chondrite-normalized La/Yb ratio of 1.4 (Fig. 6a). This signifies that the subalkalic basalts have a tholeiitic affinity. A mature back-arc basin or a major ocean basin tectonic environment of is well supported by the tectonomagmatic discrimination diagrams: TiO2–Y/Nb (Fig. 7a), Ti–V (Fig. 7b), Ti/Y–Nb/Y (Fig. 7c), TiO2–Zr (Fig. 7d), Zr/Y–Zr (Fig. 7e), Cr–Y (Fig. 7f), Ti–Zr–Y (Fig. 8a) and Nb–Zr–Y (Fig. 8b). The studied tholeiitic basalts are chemically analogous to the transitional mid-ocean ridge basalt (herein T-MORB) from the area where Du Toit Fracture Zone intersects Southwest Indian Ridge (Le Roex et al., 1989), especially in terms of REE and N-MORB normalized multi-element patterns (Fig. 9).

4.2. Alkalic basalt

4.2.1. Occurrence and petrography Fig. 6. Chondrite-normalized rare-earth-element patterns (a) and N-MORB Alkalic basalt samples were also collected from both normalized multi-element patterns (b) for the representatives of subalkalic outcrop and float rock in streams. They are texturally basalt (sample No. H-2), ophitic/subophitic-textured, alkalic basalt (sample divided into two groups: non-ophitic/subophitic-textured No. A-1 and G-1), and non-ophitic/subophitic-textured, alkalic basalt basalt (sample Nos. A-1, A-2, A-3, B-1, B-2, C-1 and D-1) (sample No. A-2). The normalizing values used in (a) and (b) are those of and ophitic/subophitic-textured basalt (sample Nos. F-1 and Taylor and Gorton (1977) and Sun and Macdonough (1989), respectively. G-1). The former occurs to the south, whereas the latter occurs north of road No. 1090 (Fig. 2). The outcrops to the south of road show a hydroclastic texture, consisting mainly and/or plagioclase and minor Fe–Ti oxide. The groundmass of basaltic blocks embedded in a foliated, fine-grained is made up largely of felted plagioclase laths with matrix, whereas those to the north are massive and may be subordinate clinopyroxene and chlorite patches, trace part of thick lava flows or dike rocks. The collected float calcite, titanite/leucoxene and/or Fe–Ti oxide (partly samples to the south of road were certainly derived from replaced by hematite/iron hydroxide). Amygdale minerals basaltic blocks in a pillow breccia. are chlorite, epidote, quartz, Fe–Ti oxide and titanite/ The non-ophitic/subophitic basalt has dark greenish leucoxene, whereas vein minerals are brucite (?) and zeolite. black, olive greenish black, and dark greenish gray colors. Clinopyroxene phenocrysts/microphenocrysts are anhedral It ranges texturally from slightly porphyritic to moderately to euhedral, and show rounded edges, embayed outlines porphyritic. The phenocrysts (sizes up to 3 mm across) sit in and/or ophitic/subophitic features. Groundmass clinopyr- the fine-grained groundmass (grain size less than 0.5 mm oxene grains are intergranular to plagioclase laths. The across). Some samples contain sporadic amygdale and/or alteration products of clinopyroxene are chlorite, calcite, vein minerals. Under the microscope, phenocrysts and quartz, titanite/leucoxene, hematite/iron hydroxide and microphenocrysts include abundant clinopyroxene, olivine Fe–Ti oxide. Olivine phenocrysts/microphenocrysts are B. Phajuy et al. / Journal of Asian Earth Sciences 24 (2005) 765–776 771

Fig. 7. Plots of (a) TiO2–Y/Nb (after Floyd and Winchester, 1975), (b) V–Ti/1000 (after Shervais, 1982), (c) Ti/Y–Nb/Y (after Pearce, 1982), (d) TiO2–Zr (after Pearce and Cann, 1973), (e) Zr/Y–Zr (after Pearce and Norry, 1979) and (f) Cr–Y (after Pearce, 1982) for the studied maﬁc volcanic rocks. HAZ Hawaiian alkalic basalt, HTZHawaiian tholeiite, MORBZmid-ocean ridge basalt, BABBZback-arc basin basalt, IATZisland-arc tholeiite, WPBZwithin- plate basalt, TholZTholeiitic basalt, TranZtransitional basalt, and AlkZalkalic basalt. Symbols are as in Fig. 3. subhedral to euhedral, and are almost totally replaced by largely subhedral to euhedral; some crystals may have chlorite/serpentine with small amounts of Fe–Ti oxide, rounded edges. The plagioclase grains are variably replaced titanite/leucoxene, hematite/iron hydroxide, pumpellyite, by chlorite, titanite/leucoxene, calcite, sericite, pumpellyite, brucite (?), quartz, calcite and zeolite. All plagioclase is hematite/iron hydroxide, quartz and/or Fe–Ti oxide. 772 B. Phajuy et al. / Journal of Asian Earth Sciences 24 (2005) 765–776

Fig. 8. Ternary discrimination diagrams, (a) Ti–Zr–Y (after Pearce and Cann, 1973)(AZwithin-plate tholeiitic basalt, BZMORB, island-arc tholeiite, and calc-alkalic basalt, CZcalc-alkalic basalt, and DZisland-arc tholeiite), and (b) Nb–Zr–Y (after Meschede, 1986) (AIZwithin-plate alkalic basalt, AIIZwithin-plate alkalic basalt and within-plate tholeiite, BZE-type MORB, CZwithin-plate tholeiite and volcanic arc basalt, DZ Fig. 9. Comparative diagrams, in terms of (a) REE and (b) N-MORB N-type MORB and volcanic arc basalt), for the studied mafic volcanic normalized multi-element patterns, between the studied tholeiitic basalt and rocks. Symbols are as in Fig. 3. T-MORB from the area where the Du Toit Fracture Zone intersects the Southwest Indian Ridge (sample No. P20-33) (Le Roex et al., 1989). All phases of primary Fe–Ti oxide show subhedral to Chondrite- and N-MORB normalizing values used are the same as those in euhedral outlines and are replaced by titanite/leucoxene and Fig. 6. hematite/iron hydroxide to varying extents. The ophitic/subophitic basalt is fine-grained, with a to reddish brown and has anhedral to subhedral outlines. It greenish gray color, and may contain rare veinlets. Under is partly replaced by chlorite, titanite/leucoxene, calcite, the microscope, the rocks have a seriate texture and are pyrite and hematite/iron hydroxide. Quartz is interstitial to composed mainly of plagioclase with subordinate clin- plagioclase laths. opyroxene, and small amounts of biotite, Fe–Ti oxide, quartz, Fe sulfide, chlorite, calcite and/or titanite/leucox- 4.2.2. Chemistry ene. The vein minerals are chlorite, quartz, zeolite, calcite The studied alkalic basalt samples have higher values for and hematite/iron hydroxide. Plagioclase is largely sub- Zr/TiO2 (0.007–0.011) and Nb/Y (0.742–1.255) relative to hedral to euhedral, and is slightly to moderately altered to the subalkalic basalt (Fig. 3). They also have higher TiO2 chlorite, titanite/leucoxene, calcite, sericite, hematite/iron (2.7–3.9 wt%), P2O5 (0.4–0.6 wt%), Nb (23–45 ppm), Y hydroxide, pyrite and/or Fe–Ti oxide. Clinopyroxene (30–44 ppm), Zr (180–343 ppm) and V (318–383 ppm) than commonly shows anhedral to subhedral outlines and is the subalkalic basalt (Table 1). The ophitic/subophitic- ophitic/subophitic to plagioclase laths. It is slightly altered textured basalt has lower values for Zr (Figs. 4 and 5), TiO2 to chlorite, calcite, titanite/leucoxene, hematite/iron (Fig. 4), Nb/Y (Fig. 3) and Zr/Y (Fig. 5) than the non- hydroxide, Fe–Ti oxide and/or quartz. Biotite is brown ophitic/subophitic-textured basalt. The non-ophitic/ Table 1 Whole-rock XRF analyses of the studied mafic volcanic samples

Subalkalic basalt Alkalic basalt

E-1 H-1 H-2 A-1 A-2 A-3 B-1 B-2 C-1 D-1 F-1 G-1 765–776 (2005) 24 Sciences Earth Asian of Journal / al. et Phajuy B. Major oxide (wt%) SiO2 48.28 47.41 47.65 48.40 49.80 49.04 51.35 49.65 50.30 48.04 49.90 49.79 TiO2 1.40 1.18 1.13 3.66 3.14 3.94 3.21 3.68 3.79 3.60 2.75 2.67 Al2O3 16.76 16.58 16.93 14.67 17.00 15.82 14.77 15.07 15.23 15.08 14.60 14.48 FeO* 9.59 10.10 9.86 12.84 10.30 12.26 10.94 11.70 11.46 12.36 12.10 12.04 MnO 0.18 0.18 0.18 0.23 0.16 0.23 0.16 0.17 0.21 0.18 0.21 0.20 MgO 7.64 9.78 9.89 6.55 6.26 7.43 5.85 7.39 6.39 6.03 6.59 6.22 CaO 12.33 11.54 11.04 9.90 7.64 7.49 9.29 8.53 8.17 10.02 9.37 9.48 Na2O 2.95 2.72 2.75 2.81 4.41 2.64 3.36 2.61 3.31 4.09 3.48 3.85 K2O 0.64 0.38 0.42 0.41 0.92 0.60 0.58 0.67 0.60 0.13 0.58 0.86 P2O5 0.23 0.13 0.14 0.51 0.37 0.55 0.49 0.53 0.53 0.49 0.42 0.41 LOI 3.94 4.54 4.48 2.95 2.67 3.74 2.55 3.37 2.84 3.25 2.06 2.61 Trace element (ppm) Rb 11.2 9.8 11.3 3 12.6 6.2 5.4 5.7 5.5 1 13.3 22.7 Sr 240 207 269 606 638 545 799 533 608 561 346 307 Ba 123 86 69 212 320 184 205 227 186 59 209 207 Nb 11.3 3.5 4.3 42.0 27.8 40.1 41.4 44.7 38.7 35.9 23 23.2 Zr 90 57 72 315 225 311 338 343 298 270 182 180 Y 222123343044333633333130 Th !2 !2 !2324534443 Ni 119 224 206 93 258 79 101 104 81 95 97 94 Cr 240 513 433 102 345 65 148 132 67 109 154 156 V 260 263 244 362 372 383 318 356 360 370 331 325 Sc 35 33 33 26 32 30 26 28 28 30 30 29

Major oxides are normalized to 100 wt% on the basis of loss on ignition free. Also reported are original sum and loss on ignition. FeO*Ztotal iron as FeO; LOIZloss on ignition. Sample Nos. A-1, A-2, A-3, B-1, B-2, C-1 and D-1 are non-ophitic/subophitic textured, whereas sample Nos. F-1 and G-1 are ophitic/subophitic textured. 773 774 B. Phajuy et al. / Journal of Asian Earth Sciences 24 (2005) 765–776 subophitic basalts forms coherent trends on zirconia variation diagrams (Figs. 4 and 5), signifying that they are co-magmatic. They may have been formed either by different degrees of partial melting of the same source rock, or by different degrees of crystal fractionation of the same parental magma. However, the former is not supported by the compositional trends, for almost all incompatible elements (Ti, P, Y and V) cannot be traced back to their origins. The decrease in Ni, Cr and Sc with increasing Zr values (Fig. 5) suggests olivine, clinopyroxene and/or chrome spinel fractionation. The positive correlation of TiO2 and V with Zr (Figs. 4 and 5) in the early stage, and the negative correlation of TiO2 and V with Zr in the later stage suggest that Fe–Ti oxide suppression did not operate in an early stage, but became involved in a later stage. Two representative samples (sample Nos. A-1 and A-2) of non-ophitic/subophitic-textured basalt and one sample of ophitic/subophitic-textured basalt (G-1) were analyzed for REE (Table 2), and their chondrite-normalized multi- element patterns were drawn (Fig. 6a). All the REE patterns show light REE enrichment and heavy REE depletion, with chondrite-normalized La/Yb values varying from 6 to 9. These are typical of alkalic and transitional tholeiitic volcanic rocks. The positions of these rocks on TiO2–Y/Nb (Fig. 7a), Ti–V (Fig. 7b), Ti/Y–Nb/Y (Fig. 7c) and Nb–Zr– Y(Fig. 8b) diagrams support the alkalic and transitional tholeiitic nature. The N-MORB normalized multi-element plot for the representative samples show step-like patterns without negative Nb anomalies (Fig. 6b). This implies that they are within-plate basalt. The positions of alkalic basalt in Ti–V (Fig. 7b), Ti/Y–Nb/Y (Fig. 7c) and Nb–Zr–Y (Fig. 8b) diagrams support such an interpretation.

Table 2 Representative rare-earth-element analyses and some selected ratios of the studied maﬁc volcanic samples

Sample no. Subalkalic Alkalic basalt Fig. 10. Comparative diagrams for (a) REE and (b) N-MORB normalized basalt multi-element patterns, between the studied alkalic basalt and the averaged H-2 A-1 A-2 G-1 alkalic basalt from Haleakala Volcano (sample Nos. H85-7, H85-8, and H85-10 of Hana series and H65-8, H65-9, and H65-10 of Kula series), La 4.3 35.4 21.2 24.9 Maui, Hawaiian Chain (Chen et al., 1990). Chondrite- and N-MORB Ce 10.1 78.4 55.2 56.3 normalizing values used are the same as those in Fig. 6. Pr 1.54 10.41 7.45 7.46 Nd 7.92 43.3 31 30.9 Sm 2.62 9.77 7.1 6.6 The studied samples are chemically similar to alkalic basalt Eu 1.07 3.21 2.43 2.16 Gd 3.6 9.2 7.08 6.56 from Haleakala Volcano (sample Nos. H85-7, H85-8 and Tb 0.65 1.37 1.11 1.04 H85-10 of the Hana series and H65-8, H65-9 and H65-10 of Dy 4.07 7.18 6.07 5.87 the Kula series), Maui, Hawaii (Chen et al., 1990), that were Ho 0.89 1.33 1.15 1.18 erupted after the formation of the tholeiitic shield volcano, Er 2.46 3.3 2.93 3.14 especially in terms of REE and N-MORB normalized multi- Yb 2.17 2.44 2.33 2.61 element patterns (Fig. 10b). Selected ratios [La/Sm]cn 1.08 2.39 1.97 2.49 [Sm/Yb]cn 1.21 4.00 3.05 2.53 [La/Yb]cn 1.31 9.58 6.01 6.30 5. Discussion and conclusions cnZchondrite normalized. Sample Nos. A-1 and A-2 are non-ophitic/ subophitic textured, whereas sample number G-1 is ophitic/subophitic Least-altered, Permian maﬁc volcanic rocks were textured. collected from the Pang Mayao area, Phrao District, Chiang B. Phajuy et al. / Journal of Asian Earth Sciences 24 (2005) 765–776 775

Mai Province. The rocks are part of Chiang Rai–Chiang Mai Acknowledgements Volcanic Belt and can be classified chemically into two main magmatic groups on the basis of the least mobile The authors are thankful to Mr Komson Prompakorn for elements as subalkalic basalt and alkalic basalt. In terms of his help during field work. Mr Zongshou Yu, Mr Phil REE and N-MORB normalized multi-element patterns, the Robinson (School of Earth Sciences, University of Tasma- subalkalic basalt is very similar to T-MORB from the area nia), and Mr Chantip Panthusa (Department of Geological where Du Toit Fracture Zone intersects Southwest Indian Sciences, Faculty of Science, Chiang Mai University) Ridge (Le Roex et al., 1989), whereas the alkalic basalt is helped with whole-rock major-oxide, trace-element, and analogous to alkalic basalt from Haleakala Volcano, Maui, REE analyses, and preparing thin sections, respectively. The Hawaii (Chen et al., 1990) and might have formed in first two authors acknowledge financial help from Faculty of an island arc, a back-arc basin or a major ocean basin. Science, Chiang Mai University through Igneous Rocks and The absence of a negative Nb anomaly in a N-MORB Related Ore Deposits Research Unit, Department of normalized multi-element pattern (Fig. 6b) makes the Geological Sciences, Faculty of Science, Chiang Mai island-arc and immature back-arc basin environments University. The authors are further indebted to Prof. John unlikely. Accordingly, the studied mafic volcanic rocks Winchester and Dr Tony Barber for reviewing and are interpreted as having formed in a mature backarc improving the manuscript. basin or a major ocean basin. A major ocean basin environment is consistent with the tectonic interpretations of Caridroit (1993), Sashida and Igo (1999) and Metcalfe (2000, 2002). 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