The Coastal Batholith of central Peru

Abstract

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 flagrandy 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 rhytiim, 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.

Geophysical and geologic studies in southern Mecklenburg County and vicinity, North Carolina and South Carolina

“…Another interpretation of these magma groups is that they are part of the normal development of a comp'osite batholith similar to the development of the Coastal Batholith of Peru described by Pitcher (1978), Cobbing and others (1977), and Cobbing and Pitcher (1972). The Coastal Batholith is 120 km long and is composed of hundreds of plutons which represent discrete pulses of magma..…”

“…Pitcher (1977) attributes the emplacement of the Peru Batholith to cauldron subsidence; Cobbing and Pitcher (1972) use the space mechanism of up-doming and erosion; and White and others (1974) believe that stoping and cauldron subsidence are not a primary mechanism of emplacement but a local phenomenon occurring near the top of the pluton. Marsh (1979) explains pluton migration by stoping and viscous diapirism.…”

Ductile shearing and migmatization at mid‐crustal levels in an Archaean high‐grade gneiss belt, northern Gallatin Range, Montana, USA

“…Glikson, 1979), as well as Cordilleran-type batholiths (Cobbing & Pitcher, 1972), are trondhjemitic or tonalitic in composition (trondhjemites predominate in the Archaean). Supracrustal rocks are important, but volumetrically subordinate, in the mid-crustal environment.…”

The Northern Andes: A Review of the Ecuadorian Pacific Margin

“…3), which has contact metamorphosed limestones of presumed Eocene age (Guzman, 1980). The postEocene plutons of the Western Cordillera of Ecuador are quite unlike the composite coastal batholith of Peru (Cobbing and Pitcher, 1972;Cobbing et al, 1981;and Cobbing, Chapter 6 …”