Pegmatoid and Gabbroid Layers in Jurassic Preakness and Hook Mountain Basalts, Newark Basin, New Jersey

摘要

Coarse-grained segregations are found in several subaerial flows of Jurassic flood basalts in the Watchung Mountains, New Jersey. They are particularly common within a 140-180-m-thick quartz-tholeiitic Preakness flow, the thickest of the Watchung flows. The segregations are enriched in iron and depleted in aluminum compared with the enclosing basaltic rock and are composed of pyroxene, plagioclase, and magnetite in a dark fine-grained interstice. Most of the segregations are characterized by a 0.1-0.6-cm (gabbroid) crystal size range, but a few lenses are characterized by a 0.4-2-cm (pegmatoid) size range. Three types of coarse-grained segregations are found within the thickest Preakness flow: type 1, vesicular horizontal sheets (<2 m, typically 5-20 cm thick) within the upper 40 m of the flow; type 2, a thick (10-30 m) mappable but discontinuous nonvesicular layer located at the contact of the colonnade with the overlying entablature; and type 3, very thick (up to 96 m) lens-shaped accumulations within thickened portions of the flow. No single mechanism is capable of generating all three types. Our evidence indicates that most type 1 segregations were transported from the top of the lower crystallization front to the base of the upper crystallization front as vesiculated diapirs of residual melt, and type 2 segregations were filter pressed out of thickened lower colonnade during the collapse of crystal networks. At two locations, some type 2 residual melt drained into depressions caused by local subsidence, resulting in type 3 accumulations.