Geochemical constraints for the Genesis of post-collisional magmatism and the geodynamic evolution of the northern Taiwan region

Taiwan is an active mountain belt created by the oblique collision of the northern Luzon arc with Asia. Late Pliocene extensional collapse of the northern Taiwan mountain belt (NTMB) was accompanied by magmatism that formed the Northern Taiwan Volcanic Zone (NTVZ; 2·8-0·2 Ma). The geochemical characteristics of the NTVZ magmas can thus provide constraints both for the mantle source composition and the geodynamic processes operating in the late orogenic stage of the region. The NTVZ volcanic rocks consist dominantly of calc-alkaline andesites and basalts, along with subordinate but heterogeneous lavas including low-K, shoshonitic and ultrapotassic magmas. From the NE to the SW in the NTVZ the magmas show systematic compositional variations from low-K to calc-alkaline and then shoshonitic. This spatial geochemical variation, characterized by southwesterly increase in potassium and incompatible trace elements, appears to be subparallel to the southwestern part of the modern Ryukyu subduction system. Sr-Nd isotope ratios of the NTVZ volcanic rocks (⁸⁷ Sr/⁸⁶ Sr ≈ 0·70376-0·70551; ¹⁴³ Nd/¹⁴⁴ Nd ≈ 0·51259-0·51301) suggest that two mantle source components are involved in the magma generation, the asthenosphere and metasomatized subcontinental lithospheric mantle. These two components are represented by the 2·6 Ma Mienhuayu high-Mg basaltic andesites and the 0·2 Ma Tsaolingshan high-Mg potassic lavas, respectively. The latter are interpreted to be the products of small-degree melting of a phlogopite-bearing, harzburgite lithospheric mantle source metasomatized recently by the nearby Ryukyu subduction zone processes. The Sr-Nd-Pb isotope systematics and specific trace-element ratios of the NTVZ volcanic rocks suggest that melts derived from subducted sediments and fluids released from slab dehydration reactions were both involved in metasomatizing this mantle source. The unique spatial geochemical variation of the NTVZ volcanic rocks can be successfully modelled using variable degrees of partial melting of the mantle source regions, coupled with mixing of different melt components from depleted asthenospheric and metasomatized lithospheric mantle components beneath individual volcanic fields. It is inferred that mixing of melts from specific mantle components and the degree of partial melting are spatially and temporally related to the tectonic evolution of the northern Taiwan region, and not simply due directly to subduction zone processes. The overall NTVZ geochemical characteristics can be explained by various degrees of melting within an ascending region of the asthenospheric mantle, triggered by extensional collapse of the NTMB, and interaction of these melts with overlying fluid- and sediment-modified lithospheric mantle.