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Petrography, Geochemistry And U-pb Detrital Zircon Dating Of The Phu Khat Formation In Nakhon Thai R

Phu Khat Formation,Provenance,U-Pb detrital zircon dating,geochemistry

The clastic Phu Khat Formation is the topmost unit of the red bed sequence (mainly Khorat Group and overlying salt formation) in the Indochina Block (Heggemann, 1994). It is well exposed in the Nakhon Thai region which is located between the two tectonic terranes, the composite Nan-Uttaradit Suture and Sukhothai Zone to the west and the Loei-Phetchabun Fold Belt and Indochina Block to the east. The formation was interpreted to have been derived from both western and eastern thrust fault blocks in the Latest Cretaceous-Early Tertiary (Heggemann, 1994). However, this interpretation is principally based on surface mapping without detailed study. Therefore, the proposed of this study is to determine the provenance and tectonic setting of the Phu Khat Formation using petrography and whole-rock geochemistry integrated with U-Pb detrital zircon dating.
The alluvial fan facies of the Phu Khat Formation is underlain unconformably by aeolian sandstone of the Khao Ya Puk Formation (or Phu Tok Formation). The sandstone of the Phu Khat Formation is chiefly characterized by unsorted texture and high unstable volcanic lithic fragments. Geochemically, the tectonic setting discrimination including plot of K2O/Na2O ratio against SiO2 (Roser and Korsch, 1986), Al2O3/SiO2 versus (Fe2O3 + MgO), discriminant function plot of major elements (Bhatia,1983) and Th-Sc-Zr/10 triangular plot (Bhatia and Crook, 1986) indicate that the Phu Khat Formation was accumulated in the passive margin tectonic setting. While the provenance discrimination comprising discriminant function plot of major elements (Roser and Korsch, 1988) and plot of La/Th ratio against Hf (Floyd and Leveridge, 1987) collaborated with plot of Th/Sc against Zr/Sc ratios (McLennan et al., 1993) and the variation of Eu anomaly value (Eu/Eu* 0.42 to 0.74) reveal that the provenance of the Phu Khat Formation consists primarily of sedimentary rocks associated with continental volcanic arc rocks in the uplifted either the western or the eastern continental terranes or both. However, the U-Pb detrital zircon dating provides clear evidence that the provenance of the Phu Khat Formation was uniquely from the western terrane with igneous activity predominantly occurred in the Middle to Late Triassic time (Shichan, 2008: 2009; Khositanont, 2008; Barr et al., 2000:2006; Qian et al., 2013).
The result indicates that while the Phu Khat Formation was accumulated in Nakhon Thai region, the western terrane was uplifted by reactivation of the pre-existing structure probably since the Maastrichian time (Ahrendt et al., 1993) to be the source area of sediments. Meanwhile, the eastern terrane (mainly Loei-Phetchabun Fold Belt) had not been uplifted probably until the accumulation of the Phu Khat Formation terminated. Subsequently, the whole region began to uplift forming a high mountainous area since the Ypresian time (Racey et al., 1997) when the Greater India collided with the Eurasia.