Field occurence, ⁴⁰ Ar/³⁹ Ar Daring and petrochemical features of volcanic rocks in Mienhuayu off ne Taiwan

Mienhuayu, a volcanic islet off the NE Taiwan, is located in the central part of the Northern Taiwan Volcanic Zone (NTVZ). In this study, we first conducted a detailed field investigation which reveals that the islet is composed dominantly of subaerial mafic lava flows and scoria deposits. ⁴⁰ Ar/³⁹ Ar dating on a whole-rock sample from a lava flow indicates that the volcanic activity started as early as 2.6 Ma, in good accordance with the time proposed for the onset of extensional collapse of the northern Taiwan mountain belt. Volcanic rocks from the Mienhuayu are generally hypocrystalline, showing porphyritic texture with high-Mg olivine (Fo ≈ 80∼81), bronzite (En ≈79 ∼82) and labradorite (An ≈ 58∼66) as major phenocryst phases in a glassy matrix. They are uniform in the whole-rock composition marked by apparently higher magnesium (i,e., MgO ≈ 5.9∼8.1 wt.%, Mg-value ≈ 56∼62) relative to silica contents (SiO₂ ≈ 52.7∼54.5 wt.%), a feature that may be observed in either silica-saturated melts from the intraplate extension environment or high andesites commonly from the fore-arc settings. In the incompatible element variation diagram, although the Mienhuayu volcanic rocks exhibit moderate enrichments in the large ion lithophile and light rare earth elements and lead, they do not display depletions in the high field strength elements as other NTVZ volcanic rocks. The Mienhuayu volcanic rocks, moreover, show geochemical affinites to the Miocene (∼23-9 Ma) intraplate tholeiitic basalts from NW Taiwan and contemporaneous (∼13 Ma), extension-related high-Mg andesites from the Iriomote-jima, southern Ryukyu. We therefore suggest that, in comparison to other NTVZ volcanics whose generation requires a subduction-modified, lithospheric mantle source, the Mienhuayu magmas originated from an ascenced asthenospheric mantle which had been subtly affected by adjacent Ryukyu subduction zone processes. This asthenosphere upwelling could have been achieved in the beginning of post-collisional lithospheric extension in the northern Taiwan mountain belt during Plio-Pleistocene time.