The initial isotopic composition of strontium of the Early Jurassic North Mountain Basalts was determined for two of three flow units: the lower unit and upper unit. Each unit was sampled along the outcropping basalts over a distance of 170 km. The initial 87Sr/86Sr ratios are remarkably constant for each unit. For the lower unit, the initial ratios range from 0.70591 to 0.70609; for the upper unit, the range is 0.70675–0.70687. The relatively high ratio could represent tapping of a magma source in an isotopically heterogeneous mantle. Alternatively, the magma could have resided for a time in the lower crust and assimilated crustal material, thereby increasing the radiogenic 87Sr content. Mixing had to have been very efficient, as indicated by similar initial ratios over considerable distances. With the extrusion of the upper unit, an isotopically different part of the mantle was tapped, or, more likely, the magma was retained within the lower crust long enough to assimilate additional crustal material and mix sufficiently well to yield the consistently higher initial ratio observed throughout the upper unit.

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