The coastal Batholith of central Peru

A sector of the Peruvian Coastal Batholith 120 km long has been mapped in detail. Little altered volcanics of Cretaceous and Lower Tertiary age make up the bulk of the country rocks and these and their structures are flagrantly cross-cut by a great composite intrusion in which some 16 distinct petrographic units are distributed in many time separated plutons. Though tonalites bulk large, substantial volumes of basic and acid rocks are involved in a clearly marked basic to acid sequence, a primary rhythm, in the distribution of which there is often a marked symmetry: gabbros on the flanks, tonalites and quartz diorites internally and in a medial position there are centred complexes in which adamellites are particularly important. Each of the main petrographic units involved a secondary rhythm of in situ, high level, differentiation in their emplacement. The adamellites have followed earlier arcuate basic intrusions and the resulting associations of multiple plutons and ring dykes are thought to represent the basal wrecks of volcanoes which were active over the entire intrusion history of the batholith. The space problem has not been fully solved and the resolution of this problem will depend on structural analysis of the envelope over a wide area. It has however been demonstrated that stoping and cauldron subsidence were important and because the attitude of many of the main contacts was controlled by contemporaneous fracturing it seems that a general uplift on faults may have provided some of the space for intrusion.