Masatoshi Sone

Tectonic evolution of the Sibumasu-Indochina terrane collision zone in Thailand and Malaysia: constraints from new U-Pb zircon chronology of SE Asian tin granitoids

Three principal granite provinces are defined across SE Asia, as follows. (1) The Western Thailand–Myanmar/Burma province consists of hornblende–biotite I-type granodiorite–granites and felsic biotite–K-feldspar (± garnet ± tourmaline) granites associated with abundant tin mineralization in greisen-type veins. New ion microprobe U–Pb dating results from Phuket Island show zircon core ages of 212 ± 2 and 214 ± 2 Ma and a thermal overprint with rims of 81.2 ± 1.2 and 85–75 Ma. (2) The North Thailand–West Malaya Main Range province has mainly S-type biotite granites and abundant tin mineralization resulting from crustal thickening following collision of the Sibumasu plate with Indochina during the Mid-Triassic. Biotite granites around Kuala Lumpur contain extremely U-rich zircons (up to 38000 ppm) that yield ages of 215 ± 7 and 210 ± 7 Ma. (3) The East Malaya province consists of dominantly Permian–Triassic I-type hornblende–biotite granites but with subordinate S-type plutons and A-type syenite–gabbros. Biotite–K-feldspar granites from Tioman Island off the east coast of Malaysia also yield a zircon age of 80 ± 1 Ma, showing Cretaceous magmatism in common with province 1. Geological and U–Pb geochronological data suggest that two east-dipping (in present-day coordinates) subduction zones are required during the Triassic, one along the Bentong–Raub Palaeo-Tethyan suture, and the other west of the Phuket–Burma province 1 belt. Supplementary material: A full description of U–Pb analytical methods used and data tables are available at www.geolsoc.org.uk/SUP18523.

Petrogenesis of Malaysian granitoids in the Southeast Asian tin belt: Part 2. U-Pb zircon geochronology and tectonic model

In our complementary geochemical study (Part 1), the Malaysian granitoids of the Southeast Asian tin belt were divided into a Middle Permian to Late Triassic I-type–dominated Eastern province (Indochina terrane) and a Triassic to Early Jurassic transitional I/S-type Main Range province (Sibumasu terrane), separated by the Bentong-Raub suture zone which closed in the Late Triassic. Previous geochronology has relied on only a few U-Pb zircon ages together with K-Ar and whole rock Rb-Sr ages that may not accurately record true magmatic ages. We pre sent 39 new high-precision U-Pb zircon ion microprobe ages from granitoids and vol canics across the Malay Peninsula. Our results show that ages from the Eastern province granitoids span 289–220 Ma, with those from the Main Range province granitoids being entirely Late Triassic, spanning 227–201 Ma. A general westerly younging magmatic trend across the Malay Peninsula is considered to refl ect steepening and roll-back of the Bentong-Raub subduction zone during progressive closure of Paleo-Tethys. The youngest ages of subduction-related granites in the Eastern province roughly coincide with the youngest ages of marine sedimentary rocks along the Paleo-Tethyan suture zone. Our petrogenetic and U-Pb zircon age data support models that relate the Eastern province granites to pre-collisional Andean-type magmatism and the western Main Range province granites to syn- and post-collisional crustal melting of Sibumasu crust during the Late Triassic. Tin mineralization was mainly associated with the latter phase of magmatism. Two alternative tectonic models are discussed to explain the Triassic evolution of the Malay Peninsula. The fi rst involves a second Late Triassic to Jurassic or Early Cretaceous east-dipping subduction zone west of Sibumasu where subduction-related hornblende and biotite–bearing granites along Sibumasu are paired with Main Range crustal-melt tinbearing granites, analogous to the Bolivia Cordilleran tin-bearing granite belt. The second model involves westward underthrusting of Indochina beneath the West Malaya Main Range province, resulting in crustal thickening and formation of tin-bearing granites of the Main Ranges. Cretaceous granitoids are also present locally in Singapore (Ubin diorite), on Tioman Island, in the Noring pluton, of the Stong complex (Eastern Province), and along the Sibumasu terrane in southwest Thailand and Burma (Myanmar), refl ecting localized crustal melting.

Petrogenesis of Malaysian granitoids in the Southeast Asian tin belt: Part 1. Geochemical and Sr-Nd isotopic characteristics

The Malaysian granitoids of the Southeast Asian tin belt have been traditionally divided into a Permian to Late Triassic “I-type”–dominated arc-related Eastern province (Indochina terrane) and a Late Triassic “S-type”–dominated collision-related Main Range province (Sibumasu terrane), separated by the Bentong-Raub Paleo-Tethyan suture that closed in the Late Triassic. The present study, however, shows that this model is oversimplified and that the direct application of Chappell and White’s (1974) I- and S-type classification cannot account for many of the characteristics shared by Malaysian granitoids. Despite being commonly hornblende bearing, as is typical for I-type granites, the roof zones of the Eastern province granites are hornblende free. In addition, the Main Range province granitoids contain insignificant primary muscovite, and are dominated by biotite granites, mineralogically similar to many of the plutons of the Eastern province. In general, the Malaysian granitoids from both provinces are more enriched in high field strength elements than typical Cordilleran I- and S-type granitoids. The mineralogy and geochemistry of the Eastern province granitoids, and their relationship with contemporaneous volcanics, confirm their I-type nature. The bulk liquid lines of descent of both granitic provinces largely overlap with one another. Sr-Nd isotopic data further demonstrate that the Malaysian granitoids, especially those of the Main Range, were hybridized melts derived from two “end-member” source regions, one of which is isotopically similar to the Kontum orthoamphibolites and the other akin to the Kontum paragneisses of the Indochina block. However, there are differences in the source rocks for the two provinces, and it is suggested in this paper that these are related to differing proportions of igneous and sedimentary protoliths. The incorporation of sedimentary-sourced melts in the Eastern province is insignificant, which allowed the granites in this belt to maintain their I-type nature. The presence of minor primary tin mineralization in the Eastern province compared to the much more significant tin endowment in the Main Range is considered to reflect the incorporation of a smaller proportion of sedimentary protolith in the melt products of the former.

Middle Permian brachiopods from central Peninsular Malaysia — faunal affinities between Malaysia and west Cambodia

A moderately diverse Permian brachiopod fauna is described from a new rock unit, the Bera Formation, in the Bera District, central Pahang, Peninsular Malaysia. The fauna consists of 19 taxa, including 14 genera and 17 (both identified and unidentified) typically Tethyan species. The fauna appears to be correlative on the basis of brachiopods with the Neoschwagerina-Yabeina fusulinid Zones in Indochina and South China. In particular, it has strong linkage to Member C (Yabeina beds) of the Sisophon Limestone, west Cambodia. This is indicated by three of the Bera species — Urushtenoidea chaoi (Ching), Spyridiophora gubleri Termier and Termier, and Transennatia termierorum sp. nov., being shared with the Cambodian fauna. A possible early Capitanian (Middle Permian) age is proposed for the Bera brachiopod fauna.

Middle Permian cephalopods from central Peninsular Malaysia: implications for faunal migration through the southern Tethys

Abstract A Wordian (Middle Permian) cephalopod fauna consisting of four ammonoid species, Tauroceras aff. scrobiculatum (Gemmellaro), Agathiceras sp., Bamyaniceras orientale n. sp. and Pronoritidae gen. and sp. indet., and two nautiloid species, Tainoceras sp. and Orthocerida fam. indet., were recovered from the Bera South area, southern Pahang, Peninsular Malaysia. The outcrop represents the southernmost extent of the Bera Formation. Bamyaniceras and Tainoceras are recorded in Malaysia for the first time. The presence of T. aff. scrobiculatum suggests a regional correlation with an ammonoid-bearing bed of Sungai Cheroh, western Pahang, and permits global correlations with strata of northeastern Iraq, northern Oman and Sicily (Italy). A possible eastwards faunal migration through the southern Tethys is suggested.

Actinocerid Cephalopods from the Ordovician of Myanmar, and their Paleobiogeographic Implications for Northern Gondwana

Abstract. This paper describes a new actinocerid cephalopod fauna from the western part of the Shan Plateau in Myanmar (Sibumasu Block), and discusses its significance. The cephalopod fossils are preserved in shallow marine limestones of the Wunbye Formation (Pindaya Group) and its equivalent strata. The fauna consists of Ordosoceras theini sp. nov. (Floian or Dapingian, late Early or early Middle Ordovician age), Armenoceras myanmarense sp. nov. (Darriwilian, late Middle Ordovician), Paratunkuskoceras sp. (Darriwilian), and Wutinoceras moeseini (Floian, late Early Ordovician). The presence of Ordosoceras, which was previously known only in the North China Block, and specific characters in the actinocerid fauna of Sibumasu indicate a strong linkage with that of North China during Early—Middle Ordovician time; in contrast, Sibumasus affinity to the coeval Australian fauna is less definable. It is possible that an actinocerid faunal exchange took place between North China and Sibumasu over the shelf sea environment open to the Prototethys Ocean but this did not happen to the inland seas of Australian Gondwana during the period.

Palaeobiogeographic implications of Middle Permian brachiopods from Johore (Peninsular Malaysia)

A new Middle Permian locality in northern Johore, Peninsular Malaysia, yields a small-sized, but compositionally unique, brachiopod fauna consisting of eight species: Pseudoleptodus sp., Caricula cf. salebrosa Grant, Neochonetes (Nongtaia) aff. arabicus (Hudson & Sudbury), Karavankina sp., Transennatia cf. insculpta (Grant), Hustedia sp., Orthothetina sp., and martiniid indet. The first four genera are new records for Malaysia; in particular, the rare taxa Pseudoleptodus and Caricula characterize the fauna. The brachiopods occur together with the ammonoid Agathiceras sp., the nautiloid Foordiceras ? sp., bivalves, and crinoid stems. The locality belongs to the East Malaya terrane of the Cathaysian biotic region, but some affinities to species of the Sibumasu province are recognized. The Malaysian forms of Pseudoleptodus, Caricula and Transennatia are similar to those of the Ratburi Limestone (southern Thailand). A Roadian–early Wordian age is interpreted for the Johore fauna. The similarity of brachiopods reported here with those from the Ratburi Limestone suggests that there was species interchange or one-way migration between shallow waters of East Malaya and Sibumasu across the main Palaeo-Tethys. The Tethyan seaway between the two terranes must have been narrower than previously interpreted by some authors to allow such faunal traffic during the Roadian–Wordian time period.

A new species of the rare neritopsid gastropod Magnicapitatus from the Guadalupian (Middle Permian) of East Thailand (the Indochina Terrane)

A new species of the rare neritopsid gastropod genus Magnicapitatus is documented from a fusulinoid-rich limestone of the Khao Taa Ngog Formation (Capitanian, Middle Permian) at Khao Makha in East Thailand near the Cambodian border (in the Indochina Terrane). This represents the first record of this Permo-Triassic genus outside South China and from the Capitanian. Magnicapitatus is one of the typical molluscan Lazarus taxa that apparently disappeared at the end of the Permian but reappeared after the Early Triassic.

Triassic nautiloid Sibyllonautilus from Gua Bama, Peninsular Malaysia and its regional stratigraphic implications

Coiled nautiloid Shells referred to Sibyllonautilus bamaensis Sone sp. nov. are reported from the top of the Gua Bama limestone hill in Pahang, Peninsular Malaysia. This is the first record of the genus in Southeast Asia; a pre-Ladinian, Triassic age is indicated for the occurrence. Based on the presence of Sibyllonautilus and previously reported Late Permian (Lopingian) foraminifers and algae, the Gua Bama limestones are interpreted to range from the Late Permian to the Triassic. It further seems plausible that some parts of Gua Bama are stratigraphically correlated to those of the nearby Gua Sei limestone hill, which has yielded basal Triassic conodonts, and that either or both the Gua Bama and Gua Sei hills may contain yet unconfirmed successions of the Permian-Triassic boundary.

Gondwanan nautiloid cephalopods from the Ordovician of Myanmar

Abstract. Two species of Darriwilian (late Middle Ordovician) nautiloid cephalopods are described from the Wunbye Formation and its equivalent strata in the Shan Plateau of Myanmar (Sibumasu Block). They are the orthocerid Sibumasuoceras langkawiense (Kobayashi) and the discosorid Tasmanoceras sp. First, Sibumasuoceras is proposed for a new genus of the cayutoceratin pseudorthoceratids. Sibumasuoceras langkawiense [originally Ormoceras langkawiense, the type species of the genus] was previously assigned either to the Actinocerida or the Discosorida. However, the present investigations reveal that it possesses thin connecting rings and differentiated endosiphuncular deposits and lacks a detailed endosiphuncular canal system, all suggestive of a relationship to the Orthocerida. Sibumasuoceras is known to occur so far from Malaysia and Myanmar of the Sibumasu Block, which was part of northern Gondwana during the early Palaeozoic. Second, the rare genus Tasmanoceras, which was previously known only in Tasmania, is confirmed in Southeast Asia for the first time; this implies an Ordovician marine biotic linkage between Sibumasu and Tasmania over northern Gondwana.