Petrology and Geochemistry of Neogene Continental Basalts and Related Rocks in Northern Taiwan (iii): Alkali Basalts and Tholeiites from Shiting-yinko Area

Basaltic rocks exposed in the Shiting to Yinko area in northern Taiwan are considered to represent a member of the products of the Neogene intraplate volcanism in the Fujian-Taiwan region of the SE China continental margin. This paper presents new age dating, petrographic (including mineral chemistry) and geochemical data which suggest the existence of two groups of the basaltic rocks: the early Miocene (23 to 20 Ma) alkali basalts and the late Miocene (12 to 9 Ma) tholeiites. The former group distributes mainly in the Chingshuikeng anticlinal area with small outcrops at Shiting and Taliaoti, while the latter group occurs as volcanic bodies widespread in the vicinity of Yinko, including Chienshan, Hengchi, Chulun and Tzelichiao. Compared with similar volcanic rocks from the KuanhsiChutung area and Penghu Islands, alkali basalts in the ShitingYinko area generally have lower Mg-values as well as lower Ni and Cr concentrations, indicative of higher degrees of crystal fractionation involved in the petrogenesis. On the other hand, tholeiites in Shiting-Yinko, as those in the two other regions, have uniform chemical compositions. We propose that tholeiites in the Shiting-Yinko area are the northeastern extension of the KuanhsiChutung volcanics because of their contemporaneous eruption ages and the close spatial relationship and elemental similarities. However, isotope data of these tholeiites (ε Nd around +5) suggest involvement of different magma sources from the KuanhsiChutung volcanics (ε Nd=+3.5~-1.4) whose generation requires significant contribution by an EM-1 type reservoir in the 20 Western Pacific Earth Sciences, Vol.1, No.1 lithospheric mantle (Chung et al., 1995). In the Shiting-Yinko area, as both the older alkali basalt and younger tholeiites have isotopic compositions similar to those of common Fujian-Taiwan Neogene intraplate basalts, the magma generation may be explained by a plum-pudding asthenosphere model proposed in the SE China continental margin.