THE HOLYOKE BASALT, ITS SOURCE AND DIFFERENTIATION IN A THICK FLOOD-BASALT FLOW

The lower Jurassic Holyoke basalt is one of the world’s largest flood-basalt flows, with a volume in excess of 1200 km³. It must have erupted rapidly, because it solidified as a single cooling unit, which is up to 200 m thick against the eastern border fault of the Mesozoic Hartford Basin. It was fed by the 50 m-wide Buttress diabase dike, which can be traced through the crystalline basement rocks of Connecticut and Massachusetts on either side of the Hartford Basin and up through the Mesozoic sedimentary rocks of the basin, almost to the point of contact with the flow.

The composition of the Holyoke basalt indicates that it would have been in equilibrium with plagioclase, augite, and olivine at a pressure of 3.8 kbar. The immediate source of the magma is therefore thought to have been a mid-crustal reservoir at a depth of 12 km, possibly near the brittle-ductile transition in the crust. Magma would have risen to the surface from this depth through the 50 m-wide dike in only minutes.

Upon eruption, the Holyoke basalt formed a thick lava lake (sea), which would have taken approximately 100 years to solidify. During this time, it differentiated to produce thick sheets (<10 m) of ferrodiorite, and thin (~1 cm) sheets of granophyre in the central part of the flow, where it is more than 100 m thick. Quantitative measurements of textural anisotropy show that this differentiation resulted from compaction of crystal mush in the lower part of the flow, with upward expulsion of a residual ferrodiorite liquid, which dilated and eventually ruptured the crystal mush beneath the downward solidifying roof of the magma sheet. The granophyre separated and rose toward the top of the ferrodiorite sheets as an immiscible liquid, droplets of which are preserved as silica-rich glassy spheres in iron-rich glass in the rapidly cooled upper part of the flow.

The differentiation that took place in this thick flow provides a model of differentiation that is likely to occur in more slowly cooled plutonic bodies. The differentiation products have compositional gaps between basalt, ferrodiorite, and granophyre. The basalt-ferrodiorite gap is a result of the degree of crystallization (one-third) necessary to produce a continuous 3-D network of crystals in the mush that can undergo compaction, and the ferrodiorite-granophyre gap is due to liquid immiscibility.