Tectonic Evolution Of Nan-Uttaradit And Loei Suture Zones, Thailand And Lao P.D.R.

Southeast Asia, locate in the Eastern Tethyan Tectonic Domain, is the junction area of Gondwana to south and Laurasia to north. This area, including northeast Thailand and northwest Laos, is composed of five slices of micro-continental blocks, separated by suture zones or faults. The five micro-continents, Sibumasu block, Inthanon terrane, Sukhothai terrane, Nakhon Thai terrane and the Indochina block, from west to east, were separated by Mae Yuam/Mae Hong Son Fault, Chiang Rai-Chiang Mai suture zone, Nan-Uttaradit suture and Loei suture zone, respectively. The four suture zones and faults are proximally parallel to each other, representing the ancient oceanic basins. The tectonic evolution and dynamics of the Nan-Uttaradit suture zone and Loei suture zone has not been resolved due to the complex geological phenomena.In this paper, an alternative model for the tectonic evolution of Nan-Uttaradit and Loei suture zones was proposed. The reconstruction is based on the radiolarian fauna, magmatic zircon U-Pb geochronology and geochemical study of cherts, greywackes and igneous rocks.Nan-Uttaradit radiolarian cherts are of Middle Permian to Middle Triassic (Ladinian). The cherts from Nan River section yield radiolarian Follicucullus porrectus with age of latest Middle Permian to earliest Late Permian. Radiolarians are rather abundant in occurrence in chert of the Phrae section. The thin-bedded cherts yield radiolarian fossils Muelleritortis cochleata (Nakaseko and Nishimura), Cryptostephanidium cornigerum Dumitrica, Pseudogodia sp., Cenosphaera sp., Pentaspongodiscus sp., Eptingium nakasekoi Kozur and Mostler, Dumitricasphaera sp., Paroertlispongus sp. cf. P. hermi (Lahm), Spongostyus tricostatus Kozur, Archaeospongoprunum sp., Pseudostylosphaera compacta (Nakaseko and Nishimura), Pseudostylosphaera nazarovi (Kozur and Mostler), Triassocampe scalaris Dumitrica, Kozur and Mostler, Triassocampe coronata Bragin, Triassocampe sulovensis Kozur and Mostler, Triassocampe deweveri (Nakaseko and Nishimura), Triassocampe diordinis Bragin, Annulotriassocampe sp., Yeharaia sp.. The presence of Permian-Middle Triassic radiolarian fauna imply the occurrence of deep water basin in that period, and also indicate that the closure of the Nan-Uttaradit occurred at least after Middle Triassic.Permian radiolarian cherts have high SiO2 (>92%) content, low Al/(Al+Fe+Mn) ratio (0.51 on average). The NASC-normalized REE patterns are relatively flat with slightly positive Ce anomaly (Ce/Ce*=1.14, on average). These geochemical characteristics mentioned above indicate the continental marginal depositional environment.Nan ophiolite section in Nan area is composed of cumulate gabbros, Mid-Oceanic tholeiites, oceanic island basalts and oceanic island andesites. The cumulate gabbro zircon grains give 206Pb/238U ages of 351 to 411 Ma with a weighted mean age of 377.1±9.1 Ma (n=12,2σ, MSWD=0.82). Cumulate gabbro is believed to originate from a spreading ridge magma chamber, and the crystallization age of cumulate gabbro could represent the age of oceanic crust. The Mid-Oceanic-Ridge tholeiite zircon grains give coherent 206Pb/238U age of 296 to 330 Ma with a weighted mean age of 315.8±2.5 Ma (n=24, 2σ, MSWD=0.93). The Mid-Oceanic-Ridge basalts have low Ti, P, K and total trace element contents. Chondrite-normalized REE pattern show LREE enrichment over HREE. Oceanic-Island basalts are characterized by high Ti, P, K contents, LREE enrichment and HREE depletion. The Oceanic-Island basalts also have high abundances of large ion lithophile elements and high field strength elements with K-Ti peak in MORB-normalized spider diagrams. The Island-Arc volcanic rocks are of Permian age. They are characterized by high Na and Al, low Ti and K. The Island-Arc volcanic rocks have high K, Rb, Ba, U, Th, LREE enrichment and Nb, Ta, Zr, Hf depletion.The Permian greywacke from Nam phang section, Nan-Uttaradit suture zone were selected for geochemical analysis. The geochemical characteristics of the greywacks indicate the provenance were median-basic volcanic rocks. Chondrite-normalized REE pattern show LREE enrichment over HREE. Provenance and tectonic setting discrimination using geochemical data reflect the continental island arc environment.Well-preserved radiolarian fossils were extracted from Xanakhan area, Vientiane, Laos. More than twenty species were indentified, including Albaillella cartalla Ormiston and Lane、Albaillella sp. cf. Al. furcata Won、Albaillella undula Deflandre、Albaillella paradoxa Deflandre、?Popofskyellium undulatum Deflandre、Archocyrtium callimorphum Braun、Archocyrtium sp.、Archocyrtium riedeli Deflandre、Archocyrtium diductum Deflandre、Archocyrtium lagabriellei Gourmelon、Polyentactinia polygonia Foreman、Astroentactinia biaciculata Nazarov、Trilonche foremanae Ormiston and Lane、Polyentactinia aranea Gourmelon、Astroentactinia multispinosa (Won)、Trilonche palimbola (Foreman). These radiolarians fauna are of mid-Visean age. The radiolarian-bearing cherts have high SiO2 (>85%), various A12O3 (0.5-8.32%), low Al/(Al+Fe+Mn) ratio(0.23-0.66). These cherts are characterized by low∑REE (9.34-96.69 ppm), no NASC normalized Ce anomaly and negative Chondrite normalized Eu anomaly. All these geochemical characteristics imply the continental margin depositional environment of the cherts.The zircon U-Pb isotopic dating results show the Early Carboniferous age for the volcanic rocks from Loei suture zone.206Pb/238U weighted mean ages of dacite, tuff and basalt from Ban Baham area, Vientiane are 334.9±1.7 Ma (2σ, MSWD=0.99)、349.6±1.7 Ma (n=21, 2σ, MSWD=1.17)、330.4±2.2Ma (n=22,2a, MSWD=2.5), respectively. The formation ages of basalt and andesite from Paklay is also Early Carboniferous, but slightly younger than that from Vientiane area. The 206Pb/238U weighted mean ages of basalt and andesite from Paklay are 315.4±3.8Ma (n=9,2σ, MSWD=1.6) 314.6±2.7Ma (n=9,2σ, MSWD=2.1) respectively. The Early Carboniferous volcanic rocks from Vientiane and Paklay and Permian andesite from Phetchabun have similar geochemical characteristics. They are enrich in K, Rb, Ba and HREE, while depletion in Nb, Ta, Zr, Hf and LREE. All the geochemical characters imply the oceanic island arc of these volcanic rocks.Loei basin should be a Proto-Tethys remnant basin, while Nan-Uttaradit basin is a branch of Paleo-Tethys Ocean basin. The tectonic evolution of the Nan-Uttaradit suture zone and Loei suture zone can be divided into four stages:1) Late Devonian-Early Carboniferous period with the subduction of Loei basin and spreading of Nan-Uttaradit basin; 2) Late Carboniferous period with the closure of Loei basin and spreading of Nan-Uttaradit basin; 3) Permian-Middle Triassic period with the subduction of Nan-Uttaradit suture zone; 4) Late Triassic period with the closure of Nan-Uttaradit basin.1. Late Devonian-Early CarboniferousThe Loei basin (Proto-Tethys Ocean), separating Nakhon Thai to the west and Indochina to the east, began to subduct during this period. Mid-Oceanic-Ridge basalt and arc volcanics (rhyolite, dacite, andesite and basalt) were exposed in Loei suture zone. The strata in Loei suture zone are Late Devonian limestone, radiolarian chert, Late Devonian-Early Carboniferous sandstone, siltstone, mudstone, shale with interbedded limestone. The Upper Devonian-Lower Carboniferous can be divided into three sedimentary styles:1) oceanic island sequence, consist of Lower Carboniferous limestone and underlying Upper Devonian volcanics; 2) oceanic basin sequence composed of Upper Devonian and Lower Carboniferous pelagic radiolarian chert; 3) continental marginal sequence, Upper Devonian-Lower Carboniferous turbidite. The Nan-Uttaradit basin, to the west of Loei basin, started spreading activity.2. Late CarboniferousThe spreading of Nan-Uttaradit basin lasted to Late Carboniferous. Loei basin was completely closed in this stage. Upper Carboniferous is shallow water deposits, and can be divided into three styles:coal-bearing clastic sediments, gypsum-bearing limestone, and coarse clastic rocks with chert pebble. They were in angular unconformity with underlying strata.3 Permian-Middle TriassicThe Nan-Uttaradit basin started to subduct toward its both sides in Permian. There were Middle Triassic arc volcanic rocks (rhyolite) and remnant basin sediments with radiolarian-bearing chert and mudstones in Phrae, west side of the basin. To the east side of basin, there was also an arc volcanic belt in Phetchabun and Loei area. Nam Phang area was in forearc basin or interarc basin, its sediments have provenance of media-basic volcanic rocks.4. Late TriassicUpper Triassic in Nan-Uttaradit suture and its surroundings were molasse, Nam Pat Group to the east and Lampang Group to the west, respectively. They were covered unconformably by Lower Jurassic red bed. The basin represented by Nan-Uttaradit suture zone was completely closed in Late Triassic.