Post-collisional magmatism around northern Taiwan and its relation with opening of the Okinawa Trough

Being part of an active mountain belt formed by oblique collision of the Luzon arc with Asia, northern Taiwan and the offshore islets are marked by a series of latest Pliocene-Quaternary volcanoes whose eruptions have been conventionally ascribed to westward propagation of the Ryukyu volcanic arc. On the basis of new geochemical data, along with supporting geologic and geophysical evidence, we propose instead that this young volcanism resulted from post-collisional lithospheric extension in the northern Taiwan mountain belt and the mantle source regions involved in the melt generation have been significantly modified by the nearby Ryukyu subduction-related processes. Magmas thus produced through the northeast to the southwest in the Northern Taiwan Volcanic Zone (NTVZ) vary from low-K to calc-alkaline and then shoshonitic compositions. Such a spatial geochemical variation, characterized by southwestward increase in enrichments of potassium and incompatible trace elements, runs subparallel to the southwestern part of the present-day Ryukyu Trench. The geochemical variation that can be explained by southwestward decrease in degrees of partial melting of the mantle sources is reconciled with a southwestward-weakening extensional regime observed in the NTVZ. The post-collisional extension in northern Taiwan, furthermore, might have played a role in reactivation of the opening in the middle Okinawa Trough, and gave way to its rapid southwestward propagation with associated development of the Ryukyu subduction zone west of ~124°E. This nascent subduction, in turn, resulted in abundant submarine volcanoes which delineate an embryo volcanic front along the southern margin of the trough. Therefore, the southwestern Okinawa Trough is not a 'fore-arc' basin as had been previously alleged, but represents an 'atypical' back-arc basin which developed broadly synchronously or prior to its arc-trench system in this particular collision/extension/subduction tectonic environment.