Journal of Earth Science, Vol. 27, No. 3, p. 391–402, June 2016 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-016-0671-y Zhao, T. Y., Qian, X., Feng, Q. L., 2016. Geochemistry, Zircon U-Pb Age and Hf Isotopic Constraints on the Petrogenesis of the Silu- rian Rhyolites in the Loei Fold Belt and Their Tectonic Implications. Journal of Earth Science, 27(3): 391–402. doi:10.1007/s12583-016-0671-y. http://en.earth-science.net Geochemistry, Zircon U-Pb Age and Hf Isotopic Constraints on the Petrogenesis of the Silurian Rhyolites in the Loei Fold Belt and Their Tectonic Implications Tianyu Zhao, Xin Qian, Qinglai Feng* State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China; School of Earth Sciences, China University of Geosciences, Wuhan 430074, China ABSTRACT：Zircon U-Pb dating, Lu-Hf isotopic and geochemical data for the Silurian rhyolites from the Loei fold belt are presented to constrain their petrogenesis and tectonic settings. The rhyolites give a weighted mean 206 Pb/ 238 U age of 423.7±2.7 Ma, and are characterized by high SiO 2 , Al 2 O 3 , K 2 O and low MnO, MgO and P 2 O 5 . All samples are enriched in LILEs (e.g., Ba, K, Pb) and LREEs and depleted in HFSEs (e.g., Nb, Ta, Ti) with obvious negative Eu-anomalies (δEu=0.56–0.63). The calc-alkaline rhyo- lites are typical arc-related rocks. The Loei rhyolites have high A/CNK ratios (1.19–1.34) and positive ε Hf (t) (4.03–5.38), which can be interpreted as partial melting of juvenile crustal materials followed by multistage melting and differentiation, similar to highly fractional I-type rocks. Combined with region- al geological surveys, the Loei rhyolites should be formed in a volcanic arc environment and may be in contact with the Truong Son fold belt during the Early Paleozoic. Moreover, the Simao Block might be in contiguity with the Indochina Block during Silurian. KEY WORDS: I-type rhyolite, zircon U-Pb dating, Hf isotopic composition, geochemical characteristics, Loei fold belt; Indochina Block. 0 INTRODUCTION The Early Paleozoic was significant for the tectonic evolu- tion of the Gondwana, in combination with the time of the col- lage of various terranes/blocks and subduction zones along the margin of Gondwana (Zhu et al., 2012). Thailand and its adja- cent area comprises numerous terranes/blocks, including South China, Simao, Indochina, Sibumasu, Sukhothai, etc. (Fig. 1). These terranes/blocks are separated by several volcanic belts, including Loei fold belt, Truong Son fold belt, Nan suture and Chiang Mai suture, etc. (Fig. 1) (Qian et al., 2015; Panjasa- watwong et al., 2006). The tectonic settings of these volcanic belts are hotly debated. Volcanic rocks play an essential role in flourishing tectonic evolution and mineral resources. The west- ern margin of the Indochina Block may contain multiple suture zones including the Nan suture and Loei fold belt (Fig. 1) (Chonglakmani and Helmcke, 2001). The Loei fold belt has been intensely studied regarding the controversy over its re- gional tectonic evolution (Vivatpinyo et al., 2014; Udchachon et al., 2011; Panjasawatwong et al., 2006; Intasopa and Dunn, 1994). The volcanic rocks in the Loei fold belt can be separated into eastern, central and western sub-belts (Boonsoong et al., *Corresponding author: qinglaifeng@cug.edu.cn © China University of Geosciences and Springer-Verlag Berlin Heidelberg 2016 Manuscript received June 11, 2015. Manuscript accepted December 12, 2015. 2011; Panjasawatwong et al., 2006) (Fig. 2). The volcanic rocks of the eastern sub-belt are mainly rhyolitic rocks. The central sub-belt is composed mainly of pillow basalts and hya- loclastites (Udchachon et al., 2011; Khositanont et al., 2008; Intasopa and Dunn, 1994). The western sub-belt is dominated by andesites which have been interpreted as products of arc volcanism active during the the Permo–Triassic; volcanic rocks are also found in the Phetchabun belt, which is the southern extension of the western Loei sub-belt (Kamvong et al., 2014; Salam et al., 2014; Boonsoong et al., 2011; Panjasawatwong et al., 2006; Intasopa and Dunn, 1994; Jungyusuk and Khosita- nont, 1992; Bunopas and Vella, 1983). Intasopa and Dunn (1994) firstly suggested that these rhyolites formed in an arc setting during the subduction of the Cathaysia Block and yielded a whole-rock Rb-Sr isochron age of 374±33 Ma (Late Devonian). Khositanont et al. (2008) reported U-Pb zircon ages of 425±7 and 433±4 Ma (Early Silurian) for the volca- niclastics in the eastern sub-belt. However, the tectonic evo- lution and magmatic processes of rhyolites in the eastern Loei sub-belt have been poorly studied and its northern extension is still debated. Because of these different views and problems, we carried out a geological survey in the Loei fold belt. The purposes of this paper are to present a new LA-ICP-MS U-Pb zircon age, zircon Hf isotopic compositions and geochemical data of the rhyolites from the eastern sub-belt in northeastern (NE) Thail- and. The aims of this study include: (1) to constrain the forma- tion time and geochemical characteristics of the rhyolites;