Abstract
Locally along their strike, several mafic dikes from the Bighorn Mountains contain leopard rock interiors which consist of closely-spaced, commonly ellipsoidal or subspherical, cracked and resorbed plagioclase (An 64–75) phenocrysts typically 5–8 cm in maximum dimensions. The leopard rock groundmass is petrographically and mineralogically identical to the nonporphyritic tholeiitic dolerite which occurs as uniformly thick tabular bodies on both flanks of the leopard rock interior. The boundary between the flanking and interior facies is marked by a rapid but gradational increase in phenocryst concentration toward the dike center. The data indicate that leopard rock is formed by axial segregation of calcic plagioclase phenocrysts within a single body of ascending phenocryst-rich magma via flow differentiation. Flow differentiation is probably related. to the creation of a grain dispersive pressure due to mechanical interactions between phenocrysts.
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