Sandra M. Barr

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.

Depositional and tectonic setting of the Paleoproterozoic Lower Aillik Group, Makkovik Province, Canada: evolution of a passive margin-foredeep sequence based on petrochemistry and U–Pb (TIMS and LAM-ICP-MS) geochronology

Abstract The Paleoproterozoic Lower Aillik Group is a deformed metasedimentary–metavolcanic succession located in the Makkovik Province of Labrador, eastern Canada. The group is situated near the boundary between reworked Archaean gneiss of the Nain (North Atlantic) craton and juvenile Paleoproterozoic crust that was both tectonically accreted and formed on or adjacent to this craton during the ca. 1.9–1.78 Ma Makkovikian orogeny. The Lower Aillik Group is structurally underlain by Archaean gneiss and structurally overlain by ca. 1860–1807 Ma bimodal, dominantly felsic volcanic and volcaniclastic rocks of the Upper Aillik Group. We present geochemical data from metavolcanic rocks and U–Pb geochronological data from several units of the Lower Aillik Group in order to address the depositional and tectonic history of this group. U–Pb data were obtained using both thermal ionization mass spectrometry (TIMS) and laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS). Two quartzite units near the structural base of the Lower Aillik Group contain detrital zircons only of Archaean age, and are interpreted to have been deposited on the Nain craton during post-2235 Ma rifting and initiation of a passive continental margin. Overlying mafic metavolcanic rocks contain thin horizons of intermediate tuff, one of which is dated at 2178±4 Ma. This relatively old age, and an inferred stratigraphic relationship with underlying sedimentary units, suggest that the volcanic rocks represent transitional oceanic crust, consistent with their geochemical similarity to tholeiitic rifted margin sequences of Mesozoic age in eastern North America. A package of interlayered psammitic and semipelitic metasedimentary rocks that appears to stratigraphically overlie the mafic volcanic unit is dominated by Paleoproterozoic detrital zircons but also contains Archaean grains. This package was deposited after 2013 Ma, the age of the youngest concordant zircon. The U–Pb data imply a minimum 165 m.y. time gap between mafic volcanism and sedimentation, and are consistent with deposition of the psammite–semipelite unit in an evolving foredeep that heralded the approach of a Paleoproterozoic arc terrane. Accretion of this terrane to the Nain cratonic margin at ca. 1.9 Ga initiated the Makkovikian orogeny. Although the Lower Aillik Group is highly deformed and may contain internal tectonic boundaries or be incomplete, the U–Pb and geochemical data allow quantitative assessment of a prolonged rift-drift-basin closure cycle that characterized the Early Paleoproterozoic evolution of the southern Nain cratonic margin.

Zircon and monazite UPb dating of the Doi Inthanon core complex, northern Thailand: implications for extension within the Indosinian Orogen

Abstract The Doi Inthanon metamorphic core complex occupies a prominent central position within the northwestern Thai gneiss belt which has been inferred to be as old as Precambrian and as young as Permo-Triassic. UPb dating of zircon from core orthogneisses of the complex suggests that the orthogneisses were derived from granitic protoliths with essentially the same Late Triassic-Early Jurassic age as the Mae Cham pluton (205 + 3 / −4 Ma) to the west of the Doi Inthanon complex. Monazite ages from the orthogneisses indicate that low-pressure and high-temperature metamorphism and migmatitization occurred in the Late Cretaceous (84-72 Ma); limited published radiometric age data from elsewhere in the migmatitic gneiss belt suggest that widespread, minor plutonism accompanied this thermal event. A younger UPb data of 26.8 ± 0.5 Ma for zircon and monazite from the Mae Klang granite which intruded mylonitic gneisses on the eastern side of the Doi Inthanon complex indicates that there was also significant plutonism in the late Oligocene. These UPb ages constrain the development of the complex, and hence of the migmatitic gneiss belt as a whole, to an interval between Late Cretaceous and Early Miocene. Ductile shearing, uplift, and tectonic denudation via detachment faulting chronologically overlap the initiation of extensional basin development to the east and southeast of the gneiss belt and imply a genetic connection. Thus Oligocene-Early Miocene thermal rifting from the Gulf of Thailand northward into mainland Thailand may have been the direct product of crustal extension rather than the transtensional product of regional strike-slip faulting.

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.

Tectono-stratigraphic terranes in Cape Breton Island, Nova Scotia: Implications for the configuration of the northern Appalachian orogen

Cape Breton Island is divided into four terranes on the basis of contrasts in pre-Carboniferous geology. The Blair River Complex in the north is an exposure of North American Grenvillian basement, analogous to the Humber zone basement in Newfoundland. Ordovician to Devonian metavolcanic, metasedimentary, gneissic, and granitic rocks of the Aspy terrane are correlative with parts of the Gander terrane of Newfoundland and New Brunswick. The Bras d9Or terrane, characterized by low-pressure gneisses, a carbonate-clastic platform sequence, and late Precambrian-Early Cambrian plutons, may be correlative with units previously included in the Gander terrane in southern Newfoundland and the Avalon terrane in southern New Brunswick. The Mira terrane in southeastern Cape Breton Island, including late Precambrian to Early Cambrian volcanic and sedimentary sequences and fossiliferous Cambrian-Ordovician units, is clearly part of the Avalon terrane. Therefore, with the exception of the Dunnage terrane, which is not represented in Cape Breton Island, the terranes of Newfoundland continue through Cape Breton Island. They are offset to the northwest to the mainland part of the Appalachian orogen in New Brunswick.

Provenance of the Meguma terrane, Nova Scotia: rifted margin of early Paleozoic Gondwana

Detrital zircon ages from the lower part of the Late Proterozoic(?) to Middle Cambrian Goldenville Group in the Meguma terrane of Nova Scotia suggest derivation from local sources in the Avalonian and Pan-African orogens on the margins of Early Cambrian Gondwana. Samples from near the top of the group show a broader distribution, including ages back to Archean. The eNd data show a corresponding trend, from slightly positive in the lower Goldenville Group to highly negative in the upper Goldenville Group and overlying Upper Cambrian to Lower Ordovician Halifax Group. The trends are consistent with deposition of the lower part of the Meguma succession in a rift, in which uplifted rift-flanks were the main source of the early basin fill, whereas subsequent thermal subsidence of rift margins allowed for more widespread sediment sourcing in younger units. The rift was possibly located between Gondwana and Avalonia, and may have been the locus for separation of Avalonia from Gondwana to form part of the Rheic O...

Provenance and tectonic evolution of Ganderia: Constraints on the evolution of the Iapetus and Rheic oceans

We provide estimates for the width, timing, and rates of opening and closing of the Iapetus and Rheic oceans, the evolution of which profoundly infl uenced Paleozoic global paleogeography. These estimates are primarily derived from the transfer of Ganderia and Avalonia from Gondwana to Laurentia, which led to closure of the Iapetus Ocean and opening of the Rheic Ocean. Ganderia, a long-lived arc terrane, separated from the paleo-Caribbean margin of Amazonia at 505 Ma with a latitudinal speed of ~9 cm/a northward, initiating the Rheic Ocean as a backarc basin. Ganderia’s trailing edge was reduced to ~5 cm/a following opening of a 600‐800-km-wide backarc basin within Ganderia at 475 Ma. Opening and closing of the Iapetus Ocean was largely driven by far-fi eld stresses, slab pull in some places and slab rollback in others.

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.