摘要
In the middle Okinawa Trough （MOT）,rhyolites have been typically considered as products ofcrystallization differentiation of basaltic magma as afeature of bimodal volcanism. However, the evidence isinsufficient. This paper compared chemical trends ofvolcanic rocks from the MOT with fractional crystal-lization simulation models and experimental results andutilized trace element modeling combined with Rayleighfractionation calculations to re-examine fractional crystal-lization processes in generating rhyolites. Both qualitativeand quantitative studies indicate that andesites, rather thanrhyolites, originate by fractional crystallization frombasalts in the MOT. Furthermore, we established twobatch-melting models for the MOT rhyolites and proposedthat type 1 rhyolites are produced by remelting of andesiteswith amphiboles in the residue, while type 2 rhyolites arederived from remelting of andesites without residualamphiboles. It is difficult to produce melts with a SiO2content ranging from 62% to 68% either by magmaticdifferentiation from basalts or by remelting of andesites,and this difficulty might help account for the compositionalgap （Daly gap） for bimodal volcanism in the OkinawaTrough.
In the middle Okinawa Trough （MOT）,rhyolites have been typically considered as products ofcrystallization differentiation of basaltic magma as afeature of bimodal volcanism. However, the evidence isinsufficient. This paper compared chemical trends ofvolcanic rocks from the MOT with fractional crystal-lization simulation models and experimental results andutilized trace element modeling combined with Rayleighfractionation calculations to re-examine fractional crystal-lization processes in generating rhyolites. Both qualitativeand quantitative studies indicate that andesites, rather thanrhyolites, originate by fractional crystallization frombasalts in the MOT. Furthermore, we established twobatch-melting models for the MOT rhyolites and proposedthat type 1 rhyolites are produced by remelting of andesiteswith amphiboles in the residue, while type 2 rhyolites arederived from remelting of andesites without residualamphiboles. It is difficult to produce melts with a SiO2content ranging from 62% to 68% either by magmaticdifferentiation from basalts or by remelting of andesites,and this difficulty might help account for the compositionalgap （Daly gap） for bimodal volcanism in the OkinawaTrough.
