Andean plutonism in Peru and its relationship to volcanism and metallogenesis at a segmented plate edge

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Abstract
Segmentation of both the Lower Paleozoic and the Mesozoic orogenic belts of Peru is indicated by rapid variation in thickness and lithology of stratigraphic units and by the presence of lateral structures. Some of these segmental boundaries are common to both the Paleozoic and the Mesozoic fold belts and are probably old structures which have been episodically rejuvenated. The Coastal Batholith is also segmented compositionally, and some of the segment boundaries coincide with the orogenic segmentation. Others, however, do not. Those which do are probably deep-reaching, crustal-mantle faults which have penetrated sufficiently deeply into the source region of the granitoid magmas to affect both the composition of the magmas and the timing of magma generation within adjacent segments. A suite of early gabbros is common to the batholith as a whole, but the batholithic segments comprise granitoid super-units which are specific to the segments. Each super-unit may range from diorite to monzogranite, and the differences are established on the basis of textural features, the order of emplacement, and geochronology. Two batholithic segments have been mapped in detail. These two segments are compared, and it is shown that the Lima segment is the more complex, having no more than ten units and super-units which were emplaced between 100 and 30 Ma. The Arequipa segment consists of only 5 super-units which were emplaced between 100 and 80 Ma. A belt of Tertiary intrusives has also been distinguished to the east of the batholith which consists of numerous, small, scattered, plutons. The mainly Cretaceous plutonism of the batholith is spatially associated with thick sequences of pre-orogenic submarine lavas and volcaniclastic rocks which are the products of a volcanic arc constructed on thinned continental crust, whereas the Tertiary intrusives are associated with post-orogenic terrestrial plateau volcanics which were deposited on thickened crust. There is some evidence that the Cretaceous and Tertiary lavas are isotopically different. In terms of metal association, the Cretaceous belt is a copper, molybdenum, and gold metallogenic province, whereas the Tertiary belt is polymetallic, producing mainly copper, lead, zinc, and silver.

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