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. 1367-9120/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2004.06.001 766 B. Phajuy et al. / Journal of Asian Earth Sciences 24 (2005) 765–776 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).