Alan S. Macdonald

Geochemistry and geochronology of late Cenozoic basalts of Southeast Asia: Summary

Geological maps of Southeast Asia (Asia, 1971; Vietnam, 1971; Thailand, 1969) show numerous occurrences of basalt, which are considered to be late Cenozoic in age. Occurrences in Thailand and western Kampuchea are generally small and scattered; those in eastern Kampuchea, southern Laos, and Vietnam are larger and more extensive (Fig. 1). Basalt bodies of similar age also occur in Malaysia and southern China (Fig. 1). These basalts form a late Cenozoic continental volcanic province comparable to those in eastern Australia (Irving and Green, 1976) and the western United States (Christiansen and Lipman, 1972; Leeman and Rogers, 1970). Basalts in Southeast Asia are especially intriguing as they are a major source of gem-quality ruby, sapphire, and zircon (Fontaine and Workman, 1978; Berrange and Jobbins, 1976). The purpose of this paper is to describe the distribution, geochemistry, and age of basalts in Southeast Asia. Available geochemical and age data are compiled and combined with new data from previously unstudied areas. This is the first known regional synthesis that has been attempted for this large but relatively undocumented basaltic province.

Nan River suture zone, northern Thailand

The Nan River suture zone consists of a belt of ophiolitic mafic and ultramafic rocks formed in a back-arc or inter-arc setting, and metasedimentary rocks including epidote-crossite blueschists that may represent an inner-trench sequence. These rocks are probably pre-Permian in age. The Nan River belt forms part of the suture between the Indosinian and Shan-Thai cratonic blocks and is inferred to correlate with the Changning-Shuangjiang suture zone in southern China.

Petrochemistry, U–Pb (zircon) age, and palaeotectonic setting of the Lampang volcanic belt, northern Thailand

Two NE‐trending belts of mainly subaerial dacitic to rhyolitic flows and tuffs occur in the area between the towns of Lampang and Denchai in northern Thailand. In the western belt (Doi Ton), the rocks have been pervasively altered to quartz keratophyre; rocks in the eastern belt (Doi Luang) are generally less altered. Mobile chemical components such as Na2O and K2O show wide variation, particularly in samples from the Doi Ton belt. However, low Zr/TiO2 ratios and low Nb and Ta contents support an origin at a convergent plate margin. A positive epsilon Nd value of +4.9 for rhyolite from the Doi Luang belt supports derivation from a primitive crustal source. A rhyolite sample from the Doi Luang belt yielded a U–Pb zircon age of 240 ± 1 Ma (early Mid‐Triassic). The Doi Ton and Doi Luang belts are part of the Lampang volcanic belt, which can be traced to the north into the Lincang–Jinghong volcanic belt in southern China. Comparison with published petrological data from the Lincang–Jinghong belt shows strong similarity, including the widespread development of keratophyric mineralogy and chemistry. The Lampang–Lincang–Jinghong belt formed at an early Mid‐Triassic convergent plate margin, and is similar in age and tectonic setting to the more mafic Phetchabun volcanic belt on the east side of the Nan River suture zone. These data constrain the timing of final amalgamation between the Indochina and Shan‐Thai terranes to Mid‐Triassic or younger.

Petrology and tectonic implications of Upper Paleozoic volcanic rocks of the Chiang Mai belt, northern Thailand

Abstract The Chiang Mai volcanic belt in north-central Thailand consists of small areas of mainly basaltic to a plate margin (subduction) environmental. This is not consistent with orevious tectonic interpretations of the that the volcanic rocks formed mainly in an extensional continental setting, with no apparent direct relationship and pyroxene-rich ultramafic rocks occur locally. Pyroxene compositions and whole-rock chemical data indicate that the volcanic rocks formed mainly in an extensional continental setting, with no apparent direct relationship to a plate margin (subduction) environment. This is not consistent with previous tectonic interpretations of the area which have suggested that these rocks represent a plate suture zone. However, rocks from the northeastern part of the Chiang Mai belt appear to differ in chemical character from the rest of the belt and may belong to an older, subduction-related volcanic sequence.

The Doi Inthanon metamorphic core complex in NW Thailand: age and tectonic significance

Abstract The Doi Inthanon complex occupies a prominent central position in the belt of high-grade gneissic rocks which extends 400 km along the western mountain ranges of Thailand. These rocks have generally been inferred to be a Precambrian basement. Mapping shows that the complex displays many of the features of a Cordilleran-type metamorphic core complex. A dome-shaped orthogneissic core is mantled by mylonitic paragneisses that are separated by detachment surfaces from a cover of low-grade to unmetamorphosed sedimentary rocks of mainly Early Paleozoic age. Mineral assemblages in the gneisses are indicative of metamorphism to upper amphibolite facies under low-pressure conditions. Porphyritic biotite granites have intruded the cover rocks west of Doi Inthanon whereas to the east, a macrogranitic pluton has intruded the orthogneiss and paragneiss, and has been overprinted locally with a mylonitic fabric. A preliminary UPb age for zircon from core orthogneiss suggests that it was derived from a Late Triassic-Early Jurassic granitic protolith, and the UPb monozite age suggests that high-grade metamorphism occurred in the Late Cretaceous. Zircon from the Mae Chaem pluton to the west of Doi Inthanon yields a similar Triassic-Jurassic crystallization age. Monazite from the Mae Klang microgranite to the east gives a Late Oligocene age. Thus, development of the Doi Inthanon complex appears to have occurred between Late Cretaceous and Miocene. Widespread mylonitization and detachment faulting associated with the complex indicate that this development was in response to crustal thinning on a major scale.

Age, tectonic setting and regional implications of the Chiang Khong volcanic suite, northern Thailand

The Chiang Khong volcanic suite forms the northeastern part of the Tak–Chiang Khong volcanic belt in the Sukhothai terrane of northern Thailand. It consists mainly of subaerial andesitic to rhyolitic tuffaceous rocks. A rhyolitic tuff yielded a Middle Triassic U–Pb (zircon) age of 232.9 ± 0.4 Ma. The suite displays calc-alkalic chemical trends and Nb, P, and Ti depletion characteristic of subduction-related magmatism. Based on petrological similarities, the suite is correlated with the c. 240 Ma Lampang volcanic rocks located farther to the SW in the Tak–Chiang Khong belt, and with volcanic rocks of similar age in the Lincang–Jinghong belt in Yunnan, in the western part of the Simao block. These correlations support the placement of the Palaeo-Tethyan suture to the west of the Sukhothai terrane.

Palaeomagnetism, age, and geochemistry of the Denchai Basalt, northern Thailand

Abstract The Denchai Basalt of northern Thailand is part of a large continental alkalic basaltic province extending through Thailand into Laos, Kampuchea, and Vietnam. It covers an area of about 70 km 2 and consists of seven flows, all with reversed magnetic polarity. The uppermost flow has yielded an age of 5.64 ± 0.28 Ma by the K-Ar method. During the igneous activity, magmas were apparently erupted from only one vent area, but changed in chemical composition from “hawaiite” (transitional to continental tholeiite) to true hawaiite and then to basanite. The basanite is the probable source of gem-quality zircon and sapphire mined from placer deposits associated with the basalts.