The anatomy of a batholith

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Abstract
The Mesozoic-Cenozoic Coastal Batholith of Peru is a multiple intrusion of gabbro, tonalite and granite occupying the core of the Western Cordillera over a length of 1600 km. Its structure and composition are described within the context of an intracratonic Andean zone in which vertical movements were dominant. The emplacement was controlled by growth-fractures on all scales and the magmas were channelled to high levels in the crust along a single mega-lineament, to be intruded finally as hundreds of separate plutons. Discussions follow on the space problem, on the assembly (episodic or continuous) of this immense body during 70 Ma, on the extent to which the magmas of the associated volcanic piles were vented to the surface via the subvolcanic ring-complexes, and on the physical nature of the magmas. The rocks of the batholith can be assigned to distinct plutonic units, consanguineous sequences of which form super-units. The outcrop of these along the batholith reveals a compositional segmentation which may correspond with structural and metallogenic segmentation of the Andes as a whole. Further, the super-units represent temporally distinct rhythms of magma generation and differentiation, one following the other, and with increasing overall acidity and decreasing volume. The way in which each separate melt was produced, particularly the triggering role of the gabbros, is discussed in the light of a rather 'uncomfortable' model of subduction beneath progressively thickening continental crust.