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 Lu zon are with Asia, northern Taiwan and the offshore islets are marked by a series of latest Pliocene-Quaternary volcanoes whose eruptions have been co nventionally ascribed to westward propagation of the Ryukyu volcanic are. O n the basis of new geochemical data, along with supporting geologic and geo physical evidence, we propose instead that this young volcanism resulted fr om post-collisional lithospheric extension in the northern Taiwan mountain belt and the mantle source regions involved in the melt generation have bee n significantly modified by the nearby Ryukyu subduction-related processes. Magmas thus produced through the northeast to the southwest in the Norther n Taiwan Volcanic Zone (NTVZ) vary from low-K to calc-alkaline and then sho shonitic compositions. Such a spatial geochemical variation, characterized by southwestward increase in enrichments of potassium and incompatible trac e elements, runs subparallel to the southwestern part of the present-day Ry ukyu Trench. The geochemical variation that can be explained by southwestwa rd decrease in degrees of partial melting of the mantle sources is reconcil ed 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 develop ment of the Ryukyu subduction zone west of similar to 124 degrees E. This n ascent 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 ha d been previously alleged, but represents an 'atypical' back-are basin whic h developed broadly synchronously or prior to its are-trench system in this particular collision/extension/subduction tectonic environment. (C) 1999 E lsevier Science B.V. All rights reserved.