The Pliny complex in northern New Hampshire, a part of the White Mountain Magma Series, is composed of eight distinct intrusions. Previous work has indicated chemical variations that cannot be explained by fractional crystallization and has led to the supposition that the diverse rock suite may have formed in episodes at different times. The 40Ar/39Ar method was applied to biotite and amphibole separates from seven units in order to evaluate the time span of emplacement and to accurately establish the age. The analyses reveal internally and externally concordant ages for both amphibole and biotite in all of the units. The ages of individual units cannot be resolved from the mean age, indicating a time span of emplacement of ∼1 m.y. or less. The intrusives were emplaced in rapid succession and cooled quickly 183.2±1.8 m.y. ago. Chemical and isotopic variations among the rocks point to a complex origin involving extensive interaction of the primary magma with older crust, all of which apparently required only a short period of time.

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