Abstract

The allochthonous Coastal Range ophiolite suite of Taiwan is sporadically exposed within a sequence which includes Miocene chaotic turbidites and Miocene-Pliocene andesitic agglomerates. Unmetamorphosed basaltic rocks, directly resting on peridotite-gabbro masses, occur consistently in regular order from massive flows upward through brecciated flows to compact pillow lavas which in turn are interbedded with manganiferous shale. The glassy basalts at the top of the pillowed sheet are remarkably fresh except for minor palagonite on the outer surface and locally along small cracks. Some basalts contain as much as 98 volume percent glass with small euhedral olivine microphenocrysts (OL-bearing tholeiite). Others are typified by the intergrowth of olivine and plagioclase and spherulites of plagioclase + pyroxene + olivine (PL-bearing tholeiite). Delicate supercooling textures are well preserved, and all crystal morphologies of olivine and plagioclase described from oceanic tholeiites of the Mid-Atlantic Ridge are also common in the samples studied.

Electron-microprobe analyses indicate that, except for spinel and palagonite, the chemical compositions of olivine, plagioclase, and glass in eight glassy basalts are very homogeneous. The glassy basalts are remarkably constant in bulk composition as regards K2O, TiO2, Na2O, and probably Fe2O3 and H2O concentrations. The K2O concentrations are very low. The constancy holds both between different parts of a sample and between different samples. The slight variations in chemical composition, texture, and mineral content between the OL-bearing tholeiite and PL-bearing tholeiite are probably related to differences in the degree of differentiation of a single parent liquid. Olivine (Fo87) was first to crystallize, later joined by calcic plagioclase (An72) along the cotectic curve, and finally by spherulites of pyroxene + plagioclase + olivine, during rapid chilling.

The field relations and all available petrological, textural, and chemical evidence suggest that the glassy basalts from Taiwan more nearly resemble recent oceanic tholeiite than any other modern volcanic rock type. It appears that the Coastal Range ophiolites in Taiwan are fragments of oceanic crust and upper mantle materials which were exposed by thrusting of the Philippine Sea plate over the continental crust.

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