. (1999). Petrology and geochemistry of late-stage intrusions of the a-type, mid-Proterozoic pikes peak batholith (central Colorado, USA): Implications for petrogenetic models. Precambrian Research,, 271-305. doi:10.1016/ S0301-9268(99) Gabbros and mafic dikes associated with the sodic granitoids have ߳ Nd (1.08 Ga) of −3.0 to +3.5, which are lower than depleted mantle at 1.08 Ga, and their trace element characteristics suggest derivation from mantle sources that were previously affected by subduction-related processes. However, it is difficult to characterize the mantle component in these magmas, because assimilation of crust during magma ascent could also result in their observed geochemical features.The Pikes Peak batholith is composed of at least two petrogenetically different granite types, both of which exhibit geochemical characteristics typical of A-type granites. Models proposed for the petrogenesis of the granitoids imply the existence of mafic rocks at depth and addition of juvenile material to the crust in central Colorado at ~1.1 Ga.3

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The Silurian Cadillac Mountain intrusive complex occurs along the coast of Maine and consists of the Cadillac Mountain granite (CMG), a gabbro-diorite unit, and three smaller granitic units that were emplaced at a shallow depth into coeval volcanic rocks. Multiple infusions of basaltic magma were injected into and ponded on the floor of the CMG magma chamber, producing the gabbro-diorite unit. Fractionation and mixing of these infusions generated layers of mafic to intermediate magma enriched in incompatible elements that were trapped beneath silicic magma. Granitic rocks that crystallized before the basaltic infusions (lower CMG) are relatively hydrous with two feldspars, and early hornblende and biotite (I-type). Granitic rocks that crystallized after the basaltic infusions (upper CMG) are hypersolvus with late interstitial hornblende, greatly enriched in high-field-strength elements, and have higher Ga/Al (A-type). Mafic enclaves in the upper CMG originated from the incompatible-rich intermediate magma that developed at the base of the chamber. The CMG provides a record of the thermal and chemical transformation of resident granitic magma from I-type to A-type due to interactions between resident silicic magma and basaltic infusions. It demonstrates that A-type characteristics can be produced by processes in high-level magma chambers and need not reflect the original conditions of melting or the chemical character of the source rocks for the granitic magma. The processes that operated in the CMG may be important in the generation of other A-type granites that occur in areas of bimodal magmatism.