Petrochemical study of post-Triassic basalts from the Nan Suture, northern Thailand

Journal of Southeast Asian Earth Sciences, Vol. 8, Nos 1-4, pp. 147-158, 1993 Printed in Great Britain

0743-9547/93 $6.00 + 0.00 © 1993 Pergamon Press Ltd

Petrochemical study of post-Triassic basalts from the Nan Suture, northern Thailand Y. PANJASAWATWONG a n d W . YAOWANOIYOTHIN Department of Geological Sciences, Chiang Mai University, Chiang Mai 50002, Thailand

Abstract--Mafi~ultramafic rocks along the Nan Suture, a continental suture between Shan-Thai and Indo-China cratons, include ocean-island basalts, backarc basin basalts and andesites, island-arc basalts and andesites, supra-subduction cumulates, and continental intraplate basalts. The first four compositional groups formed in the Carboniferous to Permo-Triassic, prior to the Late Triassic continental collision, whereas the other erupted in post-Triassic time, possibly the Cenozoic. The post-Triassic lavas form a discontinuous narrow belt, disconformably overlying metagabbros/amphibolites and serpentinite melange. They are much less deformed and altered relative to the older rocks. The post-Triassic lavas are evolved mildly alkalic rocks, characterized by SiO2=51.2-58.0wt%, MgO = 3.445.3 wt%, Nb/Y = 0.8-1.0, Ti/V > 50, (La/Yb)n = 7.4-11.6, and FeO*, TiO 2 and V depletion with progressive fractionation. They may be classified as hawaiite, mugearite and benmoreite with Na20/K20 = 1.8 2.4. They are phyric with olivine (Fo68.0_83.6), calcic clinopyroxene (mg# = 0.70-0.86), plagioclase, magnetite and ilmenite phenocrysts and microphenocrysts embedded in fine-grained matrix made up mainly of plagioclase, clinopyroxene and olivine. Orthopyroxene (mg # = 0.65-0.70) occurs as an additional microphenocryst phase in the highly evolved samples. These basalts are chemically broadly comparable with alkalic basalts such as those erupted in the postshield stages of Haleakala in the Hawaiian chain, and are interpreted to have erupted in continental environment as the Late Cenozoic basalts in mainland SE Asia.

INTRODUCTION THE TECTONIC history of mainland SE Asia has been extensively studied during the last decade. A number of workers have paid attention to the Nan-Uttaradit mafic-ultramafic belt in the eastern part of northern Thailand that extends discontinuously in a NE-SW direction for over 150 km (Fig. 1). The mafic-ultramafic belt is generally considered to mark a continental suture, formed by amalgamation of Indo-China (to the present east) and Shan-Thai (to the present west) cratons (Gatinsky et al., 1978; Beckinsale et al., 1979; Bunopas and Vella, 1978, 1983; Burton, 1984, Sengor, 1984; Hahn, 1985; Hahn et al., 1986; Barr and MacDonald, 1987, 1991; Hayashi, 1989; Cooper et al., 1989; Barr et al., 1990; Piyasin, 1991). This suture (herein the Nan Suture) trends northward into Laos and may correlate with the mafic-ultramafic belt in southeastern Thailand, and the Bentong-Raub ophiolite line in Malaysia (Gatinsky et al., 1978; Hutchison, 1983; Bunopas and Vella, 1983; Burton, 1984; Barr and MacDonald, 1987). The Nan Suture is an extensive melange zone, made up importantly of variably sized igneous-dominated blocks in foliated serpentinite matrix (serpentinite melange). These igneous lithologies have experienced variable degrees of deformation and low- to mediumgrade metamorphism. Recent petrochemical studies (Panjasawatwong, 1991) have shown that they may be separated into ocean-island basalts, backarc basin basalts and andesites, island-arc basalts and andesites, and supra-subduction mafic-ultramafic cumulates.

Although timing of magmatism and continental-continent collision is poorly constrained, available data suggest that these magmas formed during Carboniferous to Permo-Triassic times, and that plate suturing commenced in the Middle Permian and ended in the Late Triassic. In addition to the above magmatic groups, there is younger basaltic flows that form a discontinuous narrow belt, superimposing on the Nan Suture (Fig. 2). They unconformably overlie metagabbros/amphibolites and serpentinite melange; baked contact between metagabbro/amphibolite and lava flows is present at the road-cut approximately 500 m north of Ban Ngom Tham. These lavas are much less deformed and altered relative to the older igneous rocks. Accordingly, they are assigned to be post-Triassic basalts erupted in continental setting. The purpose of this paper is to present petrochemical characteristics of the least altered post-Triassic basaltic lavas, collected from locations approximately 700 m NE and 250 m west of Ban Ngom Tham (Fig. 2) to clarify their tectonic setting.

SAMPLE SELECTION AND ANALYTICAL PROCEDURE Sample selection

Samples were carefully selected to eliminate those displaying: (1) a well-developed foliation; (2) abundant vesicles, amygdale minerals, xenocrysts and xenoliths; and

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(3) quartz, epidote or calcite veining or patches totailing more than approximately 5 modal %. Consequently, seven least altered samples were analysed for major elements (Si, A1, Fe, Mg, Ca, Na, K, Ti, P, Mn and ignition loss) and a range of trace elements (Nb, Zr, Y, Sr, Rb, Ni, Cr, V, Ba and Sc); two of these were analysed for rare earth-elements (herein REE: La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er and Yb). In addition, analyses of clinopyroxene, orthopyroxene and olivine phenocrysts and microphenocrysts were carried out.

Sample preparation The selected samples were prepared for wholerock analyses by splitting into conveniently sized fragments using a hydraulic splitter, and discarding those with weathered surfaces• The fragments were then crushed with a steel jaw crusher and cleaned with an air hose; 30-50 g aliquots of the crushed fragments showing no signs of veined and amygdale minerals, xenoliths and steel smeared from the jaw crusher were pulverized for 2 min in a tungsten-carbide Tema swing mill.

Wholerock analyses Wholerock analyses, except for ignition loss, were performed on an automated Philips PW 1410 X-ray

fluorescence (XRF) spectrometer in the Geology Department, University of Tasmania. Major elements were measured from fusion discs prepared with 1.5 g lithium borate flux, 0.02 g lithium nitrate and 0.28g sample powder. Trace elements were determined on pressed powder pills backed by boric acid. Mass absorption coefficients calculated from major-element compositions were used for the determinations of trace elements• The analytical methods for major and trace elements are outlined in Norrish and Hutton (1969) and Norrish and Chappell (1967). REE analyses were performed via a combined ion exchange-XRF techniques described by Robinson et al. (1986). Samples are decomposed using a sodium peroxide sinter, followed by two-stage ion-exchange procedure using hydrochloric and nitric acids; REE are adsorbed on ion-exchange papers for XRF analysis• Ignition loss was gravimetrically determined by heating approximately 1 g of sample at 1000°C for 12 h.

Electron microprobe analysis All mineral analyses were carried out by an energy dispersive system attached to a JEOL-50A scanning electron microprobe in the Central Science Laboratory at the University of Tasmania. The beam current used was 7× 10 ~°amp at 15kV. Spectra between 0 and

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Post-Triassic basalts from the Nan Suture, northern Thailand Table 2. Electron microprobe analyses of orthopyroxene microphenoerysts 59- I 59-3 60- ! 1

2

SiO2 53.19 TiO2 0.22 AI203 1.06 Fe20 ~ 1.18 FeO 18.30 MnO 0.57 MgO 24.20 CaO 1.40 Total 100.13 Cations per 6 oxygens Si 1.955 iVAi 0.045 *iAl 0.001 Ti 0.006 Fe3+ 0.033 Fe2+ 0.563 Mn 0.018 Mg Ca

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1.960 0.040 0.016 0.006 0.011 0.467 0.014

1.957 0.043 0.006 0.006 0.025 0.661 0.021

1.949 0.051 0.004 0.009 0.029 0.571 0.015

1.964 0.036 0.008 0.005 0.019 0.586 0.036

1.430 0.055

1.229 0.053

1.340 0.033

1.292 0.056

Sum 4.000 4.000 4.000 4.000 4.000 mg# 0.70 0.75 0.65 0.70 0.69 Wo 2.83 2.83 2.72 1.69 2.89 En 68.22 73.23 63.28 68.93 66.81 Fs 28.95 23.94 34.00 29.37 30.31 Fe203 and FeO calculated by assuming stoichiometry on the basis of 6 oxygens and 4 cations. End-members in terms of wollastonite (Wo), enstatite (En) and ferrosilite (Fs) are calculated according to the method recommended by the Subcommittee on Pyroxenes, IMA (1988). tr = trace, mg# = Mg/(Mg + total Fe as Fe~+). 20 keV were counted for 60 s and data reduction was done on-line by a c o m p u t e r p r o g r a m T A S - S U E D S (Griffin, 1979).

PETROGRAPHY The basaltic lavas presented are microporphyritic to porphyritic with fine-grained groundmass made up largely o f plagioclase, and m i n o r olivine, clinopyroxene, F e - T i oxides and devitrified glass in various proportions. They contain variable a m o u n t s and sizes o f phenocrysts and microphenocrysts including plagioclase, olivine, clinopyroxene and Fe-Ti oxide; plagioclase and olivine are the most predominant microphenocryst and phenocryst phases. Orthopyroxene 0.20,

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is sparsely present as microphenocrysts in a few samples. These phenocrysts and microphenocrysts occasionally form monomineralic and polymineralic clots. A m y g d a l e minerals, such as quartz, epidote, calcite and chlorite, basaltic xenoliths and calcite veinlets m a y rarely exist. Olivine phenocrysts and microphenocrysts show idiomorphic to hypidiomorphic outlines with corroded features in some cases, whereas g r o u n d m a s s olivines are xenomorphic granular. They are partially or totally replaced by serpentine/chlorite, Fe oxides a n d / o r calcite. Plagioclases, as microphenocryst and phenocryst phases, are generally subhedral and embayed. They c o m m o n l y exhibit oscillatory zoning and contain numerous small inclusions o f groundmass constituents in the form o f sieved texture in the areas adjacent to rims. G r o u n d m a s s plagioclase laths are subparallel, and wrap a r o u n d phenocryst/microphenocryst phases in ocellar style. All plagioclases are slightly p s e u d o m o r p h e d by sericite, calcite and Fe oxides. Pyroxene phenocrysts and microphenocrysts are mainly subhedral, and scarcely display sieved textures. C o r o n a texture made up o f olivine core mantled by orthopyroxene and then clinopyroxene rim has been occasionally detected. G r o u n d m a s s clinopyroxenes are anhedral granular. Primary Fe-Ti oxides (magnetite and ilmenite) occur either as microphenocryst or as groundmass phases. They are largely euhedral to subhedral and evenly distributed t h r o u g h o u t the studied samples.

152

Y. PANJASAWATWONGand W. YAOWANOIYOTHIN MINERAL CHEMISTRY

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Probe microanalyses have been carried out only on clinopyroxene, orthopyroxene and olivine phenocrysts and microphenocrysts. Analyses of these clinopyroxenes, orthopyroxenes and olivines are reported in Tables I, 2 and 3, respectively. Clinopyroxenes, as phenocrysts and microphenocrysts, are diopsidic and augitic according to the Subcommittee on Pyroxene, IMA (1988). They have a limited range of Mg/(Mg + total Fe as Fe 2+ ) (herein m g # ) ranging from 0.70 to 0.86, and appear to be transitional between subalkalic and alkalic clinopyroxenes (Figs 3 and 4). Their V~Al and Ti contents bear positive relationships to ~VA1in the approximate ratios viA1/iVA1= 0.52 + 0.22 and Ti/~VAl = 0.22 _ 0.10, respectively (Fig. 5), indicating multiple coupled substitutions dominated by ViAl-i~Al and (Ti4+/2)-i~AI. Following the terminology of the Subcommittee on Pyroxenes, IMA (1988), orthopyroxene microphenocrysts are enstatites with mg # varying from 0.65 to 0.70. They form a linear trend superimposing on the subsolidus orthopyroxene trend of the Skaergaard Intrusion in the conventional pyroxene quadrilateral (Fig. 3). In terms of "Others" components, they have low contents of Ai203 (l.0-1.3wt%) and TiO2 (0.2-0.3 wt%). Olivine phenocrysts and microphenocrysts range compositionally from Fo68 to Fo84 (Fig. 3) and contain small amounts of MnO (0-0.6wt%) and CaO (0-0.3 wt%).

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Igneous rocks are susceptible to alteration and metamorphism, resulting in addition and subtraction of element abundances during element redistribution. Accordingly, their present compositions are unlikely to be primary. To test whether any element is mobile, both major and trace elements of the carefully selected samples (Table 4) are plotted against Zr which is generally accepted as the least mobile incompatible element (Figs 6 and 7). They show regular trends of changing element concentration although a restricted range of Zr (159-201 ppm), implying that all the elements have not been significantly mobilized by post-igneous processes. It is impossible to prove the mobility of REE as a limited number of analysed samples. However, the overwhelming consensus of opinion is that the REE patterns of carefully selected samples are probably little removed from their primary patterns. Limited REE mobility or parallel dilution/enrichment trends of REE patterns are certainly unlikely to lead to a different petrogenetic interpretation than that gleaned from primary patterns (see Whitford et al., 1988). Therefore, all the elements analysed are considered to represent the compositions of magma prior to consolidation.

Post-Triassic basaits from the Nan Suture, northern Thailand

153

Table 4. WholerockXRF and REE analyses of the studied basaltic lavas. Major elements are normalized to 100% based on loss on ignition free 59-I SiO2 57.84 TiO2 1.19 AlzO3 16.55 FeO* 6.94 MnO 0.11 MgO 3.64 CaO 6.71 Na20 4.34 K20 2.29 P2Os 0.38 LOI 2.48 FeO*/MgO 1.91 Nb 24 Zr 192 Y 25 Sr 699 Rb 32 Ni 42 Cr 69 V 124 Ba 596 Sc 16 La 38.0 Ce 78.2 Pr 8.46 Nd 32.5 Sm 5.57 Eu 1.78 Gd 5.02 Dy 4.69 Er 2.82 Yb 2.16 Ti/V 58 Nb/Y 0.96 Zr/Nb 8.00 Nb/Rb 0.75 Rb/K 0.0017 Ba/K 0.0313 FeO* = Total iron as FeO, nd=

59-2 59-3 57.80 58.11 1.24 1.25 16.86 16.78 6.48 6.40 0.10 0.10 3.51 3.42 6.86 6.80 4.47 4.35 2.28 2.40 0.39 0.39 2.49 1.95 1.85 1.87 25 24 197 201 24 24 694 675 33 37 42 40 65 60 126 119 610 633 20 17 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 59 63 1.04 1.00 7.88 8.37 0.76 0.65 0.0017 0.0019 0.0322 0.0318 not determined.

Classification The studied basaltic lavas are quite evolved, being characterized by relatively high SiO2 (51.2-58.0wt%) and low MgO (3.4-6.3 wt%). They form a well-defined trend ranging from hawaiite via mugearite to benmoreite on the total alkali-silica diagram (Fig. 8a) and have Na20/K20 in the range of 1.8-2.4. Their Nb/Y (0.8-1.0) (Fig. 8b) and Ti/V ( > 50) ratios are typical of alkalic basalts (Winchester and Floyd, 1977; Shervais, 1982). Accordingly, they are best classified as evolved mildly alkalic lavas. This deduction is consistent with the presence of modal olivines both as groundmass and as phenocryst/microphenocryst phases, and the occurrence of modal orthopyroxene in the most fractionated samples.

Chemical variations The zirconia variation diagrams for major- and traceelements of the mildly alkalic basaltic rocks (Figs 6 and 7) show that SiO2, Na20, K20, Nb, Rb and Ba are enriched, whereas AI203,FeO, MnO, MgO, CaO, P205, Y, Sr, Ni, Cr, Sc and V are depleted with increasing fractionation. This implies that the liquid line of descent is probably controlled by olivine, pyroxenes, plagioclase,

60-1 55.69 1.40 17.05 7.14 0.16 3.83 7.62 4.54 2.14 0.43 1.75 1.86 24 181 23 726 17 45 72 156 563 20 nd nd nd nd nd nd nd nd nd nd 54 1.04 7.54 1.41 0.0010 0.0317

61-1 51.62 1.60 17.12 9.09 0.15 5.78 8.56 3.96 1.63 0.49 2.37 1.57 23 165 27 728 20 72 133 184 408 26 nd nd nd nd nd nd nd nd nd nd 52 0.85 7.17 1.15 0.0015 0.0302

61-2 51.24 1.61 16.87 9.24 0.15 6.33 8.83 3.65 1.58 0.49 2.03 1.46 22 159 28 737 18 63 133 183 399 26 28.2 62.3 7.36 30. i 6.23 2.36 6.02 5.37 2.32 2.50 53 0.79 7.23 1.22 0.0014 0.0304

62-t 54.63 t.47 i 6.97 7.65 0.15 4.46 7.97 4.22 2.00 0.48 1.93 1.72 24 183 25 756 19 54 101 169 538 24 nd nd nd nd nd nd nd nd nd nd 52 0.96 7.62 1.26 0.0011 0.0324

Fe-Ti oxides and apatite that formed earlier in cooling history. The appearance of orthopyroxene microphenocrysts in the most fractionated samples precludes the significance of orthopyroxene fractionation. In this magmatic suite, only Nb, Rb and Ba behave as incompatible elements in addition to Zr. These evolved basaltic lavas contain fairly consistent incompatible element ratios, such as Rb/K (average 0.001), Ba/K (average 0.031 +0.001), Zr/Nb (average 7.7 + 0.4) and N b / R b (average 1.05 + 0.33), indicating that these incompatible element pairs form linear arrays which can be traced back to the origin, i.e. they are essentially comagmatic. REE analyses for two of these alkalic basaltic lavas are presented in Table 4, and their chondrite-normalized values are plotted in Fig. 9a. These basalts exhibit L R E E enrichment with (La/Sm)n (chondrite-normalized La/Sm) ranging from 7.4 to 11.6. Their La and Yb abundances vary from 89.5 to 120.6, and 10.4 to 12.0 times chondritic values, respectively.

T E C T O N I C S E T T I N G OF E R U P T I O N Many empirical diagrams for discriminating tectonic settings of basaltic eruption have appeared in the literature. However, several recent studies, e.g. Holm (1982),

i 54

Y. PANJASAWATWONG

and W. YAOWANOIYOTHIN

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Post-Triassic basalts from the Nan Suture, northern Thailand

155

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Post-Triassic basalts from the Nan Suture, northern Thailand Prestvik (1982), Duncan (1987) and Myers and Breitkopf (1989) have demonstrated that these diagrams may often fail to unequivocally classify tectonic settings of eruption. According to this reason and the evolved nature of the presented basaltic lavas, an extensive search for the modern suites chemically analogous to the studied basaltic lavas has been conducted. The studied basalts are generally similar in chemical compositions to the mildly alkalic basalts which formed in alkalic capping stages of Haleakala Volcano (Hana and Kula series), east Maui (Chen and Frey, 1985), particularly in terms of chondrite and N-MORB normalized patterns (Fig. 9). Field data, however, indicate that they have erupted in a continental environment. The discrepancies between the two lines of evidence are attributed to the scarcity of informative chemical data for modern intraplate continental basalts and the similarity in chemical compositions of intraplate basalts generated in both continental and oceanic environments. Consequently, it can be inferred that the basaltic lavas presented in this study are intraplate continental basalts as the Late Cenozoic basalts in mainland SE Asia (Barr and MacDonald, 1981; Barr and James, 1990).

CONCLUSION It has been shown recently that the Nan Suture, a continental suture between Shan-Thai and Indo-China cratons, is an extensive serpentinite melange zone. Mafic-ultramafic igneous rocks as blocks in the melange are made up of ocean-island basalts, backarc basin basalts and andesites, island-arc basalts and andesites, and supra-subduction mafic-ultramafic cumulates generated in Carboniferous to Permo-Triassic times, prior to the Late Triassic amalgamation of the Shan-Thai and Indo-China cratons. This melange zone is partly disconformably overlain by a discontinuous narrow belt of much less deformed basaltic lavas which erupted in post-Triassic (possibly Cenozoic) time. The least altered post-Triassic basaltic samples studied are microporphyritic to porphyritic with fine-grained groundmass made up mainly of plagioclase, and minor olivine, clinopyroxene, Fe-Ti oxides and devitrified glass in different proportions. Phenocryst and microphenocryst phases include plagioclase, olivine (Fo6s.0_83.6), calcic clinopyroxene (mg# =0.70-0.86), magnetite and ilmenite. Orthopyroxene (mg # = 0.65-0.70) occasionally occurs as an additional microphenocryst phase in the highly evolved samples. Chemically, the post-Triassic basaltic lavas are evolved mildly alkalic rocks, characterized by SiO2 = 51.2-58.0wt%, MgO = 3.4-6.3 wt%, Nb/Y = 0.8-1.0, Ti/V>50, (La/Yb)cn=7.4-11.6, and FeO, TiO2 and V depletion with progressive fractionation. They may be classified as hawaiite, mugearite and benmoreite with Na20/K~O = 1.8-2.4. Their general chemical characteristics are comparable to those of the mildly alkalic basalts which formed in alkalic capping

157

stages of Haleakala Volcano (Hana and Kula series), east Maui. On the basis of field evidence and their modern analogs, i.e. intraplate basalts, the post-Triassic basaltic lavas are interpreted to have erupted in intraplate continental setting as the Late Cenozoic basalts in mainland SE Asia. Acknowledgements--The first author is very grateful to the Australian International Development Assistance Bureau (AIDAB) for financial support to work in Australia from May, 1987 to January, 1991. Microprobe work was performed in the Central Science Laboratory of the University of Tasmania, with valuable assistance from Wieslaw Jablonski. Major, trace and rare-earth element XRF analyses were done in the Geology Department of the University of Tasmania, with valuable assistance from Phil Robinson. Simon Stevens (University of Tasmania) and Pipop Robpairi (Chiang Mai University) helped with making polished thin sections. Finally, we thank CIG Co., Ltd for providing accommodation, transportation and field assistants during fieldwork.

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