NATURE OF A FLOOD-BASALT-MAGMA RESERVOIR BASED ON THE COMPOSITIONAL VARIATION IN A SINGLE FLOOD-BASALT FLOW AND ITS FEEDER DIKE IN THE MESOZOIC HARTFORD BASIN, CONNECTICUT

The erosional remains of the Mesozoic Holyoke basalt in the Hartford, Pomperaug, and Deerfield basins of Connecticut and Massachusetts indic ate an original how volume of > 1200 km(3). Its feeder dike, which is about 50 m wide and 160 km long, can be traced down through 2 km of Me sozoic sediments and, as a result of faulting associated with basin fo rmation, through an additional 6 km of Paleozoic metamorphic rocks. Ch emical profiles through the distal and proximal parts of the flow and through the dike at depths of 2, 4, and 8 km provide sequential sample s of the magma that rose during this rone eruptive event. The flow and dike have restricted compositions that indicate saturation with olivi ne, augite, and plagioclase at depth. The flow consisted largely of a liquid at the pigeonite reaction point. Dike compositions can be model ed as mixtures of this liquid with up to 24% crystals of plagioclase, augite, and olivine. The dike compositions indicate equilibration with these minerals at 3.8 kbar. This pressure corresponds to a depth of 1 2.2 km, which is believed to have been the depth of the brittle/ductil e transition in the crust at the time. This transition appears to be t he only reasonable barrier that could have caused pending of the magma at the mid-crustal level. The Holyoke liquid is interpreted to have s egregated from a compacting crystal mush following 30% crystallization of the magma in this mid-crustal reservoir. Eruption of the basalt ex hausted the supply of segregated liquid, and when the remaining crysta l mush began to rise in the dike, the average density of the magma col umn increased until it matched the average density of the intruded cru st, and the eruption ended. By analogy with the differentiation that t ook place in the solidifying Holyoke flow on the surface, the mid-crus tal magma reservoir is estimated to have had a volume of at least 12,0 00 km(3). The magma in this chamber must have come from a still deeper chamber, because it was too fractionated to have come directly from a mantle source.