The Rearing Pond intrusion is a small basic body that displays a range of composition and textures illustrating its cooling history. Modal and mineralogical data indicate that it is a single differentiated unit or a cyclic unit in a larger poorly exposed intrusion.

Fractionation has resulted in a succession that varies from olivine cumulates through olivine-plagioclase-clinopyroxene cumulates to plagioclase clinopyroxene cumulates. Compositional variation of phases is small, with iron enrichment about 10 mole percent in the mafic phases and with plagioclase varying from An 80 to An 60. Some zoning of plagioclase is apparent, becoming stronger in the later units.

Primary clinopyroxene first appears as large oikocrysts in the olivine-plagioclase cumulates, and later changes to a granular habit in the olivine-free rocks. This is interpreted as a change in the ratio of growth to nucleation rates of the pyroxene corresponding to the cessation of olivine crystallization as the magma composition leaves the olivine field. This occurs at the quaternary invariant point, Fo-An-En-Di-Liquid in the system An,Di,Fo,SiO2. The habit change occurs at this point as a result of the change in degree of supersaturation as the liquid fractionates past the invariant point as olivine is consumed. This is either a kinetic effect or a compositional one due to the differences in competition for components on opposite sides of the invariant point. Evidence suggests that both nucleation and growth accelerated in the latest rocks, as grain size is diminished and zoning becomes more pronounced.

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