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Igneous intrusive sheets are conspicuous features of ophiolites formed at oceanic spreading centers. These include volcanic, subvolcanic, and plutonic dikes and sills and their subtypes. Attention is focused here on the subvolcanic sheets that separate the volcanic and plutonic members of ophiolites: the sheeted dikes, nonsheeted dikes, and the subvolcanic sheeted sills.

The sheeted dikes mark former crustal fissures that channeled magma to seafloor lava flows. They provide a record of continuous upper crustal extensional fracturing and coeval magmatism, characteristic of oceanic spreading centers. But some ophiolites have non-sheeted subvolcanic dikes instead. Those dikes are spaced apart, the intervening crustal rocks having once hosted ephemeral fissures that had opened and later closed without magma moving through them. Thus, while continuous sheeted dikes mark a sustained balance between upper crustal extension and magma supply, the nonsheeted subvolcanic dikes point to a fluctuating magma supply during crustal extension and rifting.

The subvolcanic sheeted sills record magma movements within the oceanic crust during and following its construction, but they rarely fed flows. They record the incremental growth of an inflating crustal melt lens during periods of enhanced magma supply at a mid-ocean ridge (MOR) spreading center. They occur with, but cut across, nonsheeted dikes that mark crustal extension. Thus, the sheeted sills are as much a hallmark of oceanic rifting and spreading as sheeted dikes, but they record different conditions, i.e., an imbalance between crustal extension and magma supply.

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