Abstract

Geochemical and structural aspects of the Josephine ophiolite indicate that it formed by alternating cycles of magmatic and structural (amagmatic) extension. Episodic magma chambers are indicated by the presence of very primitive lavas in the lower pillow lavas which would have mixed into a magma chamber. Fe-Ti basalts that occur in the uppermost extrusive rocks and plagiogranites that occur along the sheeted-dike/gabbro contact represent highly fractionated magmas produced during the last stages of crystallization of a magma chamber. During periods of amagmatic extension, breccias accumulated and the entire crustal sequence was tilted >50°, probably by faulting above an upper-mantle detachment. More than 1000 m of uplift and down-dropping between magmatic cycles is suggested by the presence of out-of-place screens in the sheeted-dike complex.

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