Geochemical constraints on the petrogenesis of high-Mg basaltic andesites from the Northern Taiwan Volcanic Zone

The Northern Taiwan Volcanic Zone (NTVZ) is a Late Pliocene-Quaternary volcanic field that occurred as a result of extensional collapse of the northern Taiwan mountain belt. We report here mineral compositions, major and trace element and Sr/Nd isotope data of high-Mg basaltic andesites from the Mienhuayu, a volcanic islet formed at similar to 2.6 Ma in the central part of the NTVZ. The rocks are hypocrystalline, showing porphyritic texture with Ma-rich olivine (Fo = 81-80), bronzite (En = 82-79) and plagioclase (An 66-58) as major phenocryst phases. They have uniform whole-rock compositions, marked by high magnesium (MgO 5.9-8.1 wt.%, Mg value approximate to 0.6) relative to accompanying silica contents (SiO(2) = 52.8-54.5 wt.%). The high-Mg basaltic andesites contain the highest TiO(2)(similar to 1.5 wt.%) and lowest K(2)O (similar to 0.4 wt.%) among the NTVZ volcanic rocks. In the incompatible element variation diagram, these Mienhuayu magmas exhibit mild enrichments in large ion lithophile (LILE) and light rare earth elements (LREE), coupled with an apparent Pb-positive spike. They do not display depletions in high field strength elements (HFSE), a feature observed universally in the other NTVZ volcanics. The high-Mg basaltic andesites have rather unradiogenic Nd (epsilonNd = + 5.1-7.2) but apparently elevated Sr ((87)Sr/(86)Sr = 0.70435-0.70543; leached values) isotope ratios. Their overall geochemical and isotopic characteristics are similar to mid-Miocene ( similar to 13 Ma) high-Mg andesites from the Iriomote-jima, southern Ryukyus, Japan. Despite these magmas have lower LILE and LREE enrichments and Pb positive spike, their "intraplate-type" incompatible element variation patterns are comparable to those of extension-induced Miocene intraplate basalts emplaced in the Taiwan-Fujian region. Therefore, we interpret the Mienhuayu magmas as silica-saturated melts derived from decompression melting of the ascended asthenosphere that had been subtly affected by the adjacent Ryukyu subduction zone processes. This interpretation is consistent with the notion that in the northern Taiwan mountain belt post-orogenic lithospheric extension started in Plio-Pleistocene time. (C) 2002 Elsevier Science B.V. All rights reserved.