CEOSEJl ' 98 Proc ee ()ill.tjJ, Ce oL. Soc. 1 /I/aLaYJia BIILL. 4 5, De cemb er 1999; pp. 115-129 Ninth Regional Congress on Geology, Mineral and Energy Resources of Southeast Asia - GEOSEA '98 GEOSEA '98 17 - 19 August 1998 • Shangri-La Hotel, Kuala Lumpur, Malaysia Provenance and tectonic setting of deposition of metagreywackes in the Nan River Suture, Northern Thailand SAMPAN SINGHARAJWARAPAN Department of Geological Sciences Chiang Mai University Chiang Mai, 50200, Thailand Abstract: Provenance and tectonic setting of deposition of metagreywackes from the Sirikit Dam area in northern Thailand which covers part of the Nan River Suture were reconstructed on the basis of geochemical and petrographic data. The metagreywackes belong to the Pha Som Metamorphic Complex that consists of a coherent unit of multiply-deformed pumpellyite - actinolite facies metasedimentary rocks and tectonic slices of ophiolitic mafic-ultramafic rocks. The lithology, deformation history, and metamorphism ofthe Pha Som Metamorphic Complex are consistent w ith those observed in ancient and modern accretionary complexes in many parts of the world. Geochemically, the metagreywackes are divided into two categories: the PSM-l and PSM-2 groups . The PSM-l group is characterised by relatively low average concentrations of TiO z ' Al Z03' Fe 2 0 / , and V, a low ZrlTh ratio, relatively hi gh abundances of Pb , Th , light rare-earth el ements (La, Ce, and Nd ), and high ThlSc and CeN ratios. The PSM-2 group is characterised by relatively high average concentrations of TiO z ' AlP a' and Fe 2 0 a *, a high ZrlTh ratio, relatively low abundances of Pb, Th , light rare-earth el ements (La , Ce, and Nd), and low ThlSc and CeN ratios. Immobile trace element characteristics su ggest that the PSM-l metagreywackes were derived from a continenta l island arc source and were probably deposited in a submarine fan setting at a subduction zone. In contrast , major and trace element characteristics suggest that the PSM-2 metagreywackes represent sed im ents derived mainly from an oceanic island arc source. On the basis of modal compositions ofthe framework grains, the metagreywackes can be divided into quartz - rich greywackes, which are comparable to the sandstone of modern and ancient accretionary complexes, and quartz -poor greywackes, which have a transitional magmatic arc source . This study further supports the existence of an anci ent accretionary complex prior to the amalgamation of the Shan-Thai and Indochina terranes in Late Triassic-Early Jurassic time. INTRODUCTION It has long been known that sandstone compositions are influenced by the character of the sedimentary provenance, the nature of the sedimentary processes within the depositional basin, and the kind of dispersal paths that link the provenance to the basin (Dickinson and Suczek, 1979). The provenance of a particular sedimentary suite includes all aspects of the source area, such as source rocks, climate , and relief (Pettijohn et al ., 1972). In areas of intense tectonic and/or magmatic activity, source-rock type has a stronger influence upon sediment compositions than climate and relief (Dickinson, 1970; Dickinson and Suczek, 1979). The compositional and chemical var i ations of sandstone have been utilised to determine of the provenance of sedimentary suites and in palaeotectonic and palaeogeographic reconstruction. For sandstone that retains its origina l sedimentary texture, the use of a modal composition as an indicator of the provenance type has long been established (Dickinson, 1970; Crook, 1974; Dickinson and Suczek , 1979; Ingersoll and Suczek, 1979; Valloni and Maynard, 1981; Dickinson et al., 1983; Marsaglia and Ingersoll, 1992). Later, geochemistry of clastic sediments was used for a similar purpose (Bhatia and Taylor, 1981; Maynard et al., 1982; Bhatia, 1983) and its use became increasingly prominent during the last decade (Bhatia and Crook, 1986; Roser and Korsch, 1988; McLennan and Taylor, 1991; Mortimer and Roser, 1992). On the basis of major element geochemistry, several discrimination diagrams were derived for determining provenance of sandstones (Bhatia, 1983; Roser and Korsch, 1988). Some workers (Bhatia and Taylor, 1981; Bhatia and Crook, 1986)
