Taiwan is an active mountain belt created by the collision between the Luzon arc and the Asian continent since the late Miocene (Ho, 1986). This collision, related to northwestward motion of the Philippine Sea plate, ceased the lithosphere stretching in the continental margin of Southeast China and intraplate basalt eruptions around the Taiwan Strait. It may have also induced westward propagation of the Ryukyu subduction system that resulted in emplacement of young arc volcanics (2.8-0.3 Ma) in the northern tip of Taiwan and its offshore region. Subsequently, as a response to southwestward migration of opening of the Okinawa Trough, compressional tectonism diminished recently in the northern part of the Taiwan orogen. A post-collisional extension regime thus initiated gave rise to generation of highly magnesian potassic magmas, termed as absarokites, in northwestern Taiwan (Chung et al., 1995).
The absarokites (¾0.2 Ma) show characteristic major element compositions, with SiO2×50%, MgO×15%, K2O×5% and K2O/Na2O>2. In addition, they are highly enriched in the large ion lithophile elements [in particular, Cs around 200 ppm and Rb up to 1000 ppm; i.e., (Cs)N×6000, (Rb)N×1500; normalized to the primitive mantle values], moderate enriched in the light rare earth elements [(La)N×30-40], and depleted in the high field strength elements [(Nb/La)N×0.6] and the heavy rare earth elements [(Yb)N×2-3]. Such elemental features are distinct from those of the Miocene intraplate basalts around the Taiwan Strait (Chung et al., 1994) and the young Ryukyu arc volcanics in northern Taiwan and offshore islets (Chen et al., 1996). When compared to well-documented orogenic potassic lavas from the Italian and Indonesian volcanic provinces, the magmas in northwestern Taiwan possess corresponding incompatible trace element patterns marked by an extreme enrichment in the cesium and rubidium abundances. Whilst the absarokites have relatively higher Sr and lower Nd isotope ratios (87Sr/86Sr×0.7055; eNd × 0), their Pb isotope compositions are similar to those of the northern Taiwan arc rocks (206Pb/204Pb×18.44-18.62; 207Pb/204Pb×15.56-15.65; 208Pb/204Pb×38.50-38.86) (Chen et al., 1996; Sun, 1980); the latter are generally attributed to involvement of an EM2-type magma source by the Ryukyu subduction zone processes.
Diverse geological processes, e.g., crustal contamination, crystal fractionation, sediment subduction and melting of ancient subcontinental mantle, have been considered responsible for the petrogenesis of potassic lavas in various tectonic settings (Rogers et al., 1987; Foley and Peccerilo, 1992). The generation of highly magnesian potassic magmas in the arc-continent collision zone of Taiwan therefore facilitate not only understanding dynamic mantle processes during a drastic change of the tectonic environment but also evaluating different petrogenetic models for orogenic potassic rocks. The overall geochemical features of the absarokites in northwestern Taiwan are consistent with an origin from a relatively refractory source region in the continental lithospheric mantle that had been metasomatized by a subduction-related enrichment particularly in the large ion lithophile elements. Consequently, it could have major and trace element compositions resembling the phlogopite-bearing harzburgite xenoliths in the Batan island of the Luzon are system (Maury et al., 1992). We suggest the metasomatic agent to be hydrous fluids derived from the nearby Ryukyu subduction zone because experiments (Tatsumi et al., 1986) have demonstrated that the mobility of incompatible elements transported by dehydration in the subducting regime increases with ionic radii, thereby the enrichment factors being Cs>Rb>K for the alkalis. A small degree melting of such a metasomatized refractory mantle source resulting from onset of a post-collisional extension domain in the northern Taiwan orogen formed the nascent absarokites with unique geochemical characteristics.
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