Cauldron Subsidence and Fluidization: Mechanisms of Intrusion of the Coastal Batholith of Peru into Its Own Volcanic Ejecta JOHN S. MYERS Geological Survey of Greenland, 0stervoldgade 10, Copenhagen, Denmark ABSTRACT 10 to 60 km inland, and has subvertical walls and a flat roof. It was emplaced be- The Coastal Batholith of Peru shows how tween Late Albian and Eocene time (about diorite, tonalite, granodiorite, and granite 100 to 50 m.y. ago) into two groups of magmas rose through their last few volcanic rocks which were erupted from the kilometers by a process of magmatic stop- rising batholith. ing by their fluidized upper surface. The Structures and features relating to intru- magmas successively displaced both older, sive mechanisms are described together, more basic plutons and their own volcanic drawing on examples from different units debris downward and canje to rest within 3 of the batholith because the structure and km of the Earth's surface after loosing vol- intrusion mechanism of most plutons are atiles by eruptions through fissures and cal- broadly similar. A more complete account deras. There was a continuous association of the batholith, the volcanic rocks, and the between volcanic eruptions and plutonic in- regional geology is in press (Myers, in trusions from at least 100 to 30 m.y. ago in press). Plutonic rock names follow the a narrow belt parallel with the continental margin. The batholith is 50 km wide, more than 1,100 km long, and probably 15 km or less thick. It has steep walls and an extensively preserved flat roof exposed in mountainous desert with relief of 4,500 m. It consists of plutons and sheets which were intruded in five distinct episodes into the same tabular belt by repeated cauldron subsidence. Each subsidence was preceded by the formation Figure 1. Map showing the extent of the of small shear zones which were fractured Coastal Batholith of Peru (after Bellido and and fluidized, and accompanied by the rise others, 1972) and locations of the area described and of section A-B of Figure 12. of corrosive, turbulent gas-liquid-solid mix- tures. Individual plutons form relatively thin tabular bodies with flat roofs and relief of more than 4,500 m. This paper de- floors and steep walls which pass down- scribes the structure of the batholith and ward into ring dikes and upward into ring evidence of the mechanism of its intrusion. dikes and calderas. Key words: structural Earlier descriptions of other parts of the geology, igneous rocks, batholiths, vol- Coastal Batholith were made by Jenks canism. (1948), Cossio (1964), Cossio and Jaén (1967), and Stewart (in Garcia, 1968). Part INTRODUCTION of the batholith lying immediately south of this area was described by Cobbing and The Coastal Batholith of Peru is one of Pitcher (1972) and Cobbing (1973a). These the largest and best exposed of a chain of studies showed the batholith to consist of a Mesozoic and Tertiary batholiths which number of plutons of which tonalite, occur along the western margin of the granodiorite, and adamellite far exceed feftvj Coastal Batholith American continent. It forms an almost basic rocks and granite in total volume. The continuous outcrop 1,100 km long and 50 earliest intrusions are more basic than I" Calipuy Voleantes km wide on the western flank of the Andes younger ones and local basic to acid dif- j v v | Casma Voleantes between latitudes 8° and 16° south (Fig. 1). ferentiation sequences occur within some The 80-km-long section of it which lies be- composite plutons. K-Ar minimum ages, • ' Crttacvous shtlf stdiments tween latitudes 10° and 10° 30' south was mostly of biotite and hornblende obtained Pr«-Crttac«ous rocks mapped on the scale of 1:50,000 during 12 from a wide area of the Coastal Batholith, Figure 2. Part of northern Peru showing the months spent in the field in 1968-1970. range from about 100 to 10 m.y. (Stewart close association of the Casma and Calipuy The shapes of the intrusions and the struc- and others, 1974). Groups of volcanic rocks with the Coastal ture of their country rocks can be clearly Between latitudes 10° and 10° 30' south, Batholith (after Cobbing, 1973b); the area de- observed here in mountainous desert with the batholith lies parallel with the coastline. scribed is outlined. Geological Society of America Bulletin, v. 86, p. 1209-1220, 12 figs., September 1975, Doc. no. 50903. 1209 1210 J. S. MYERS definitions recommended by the Interna- with amphibolite facies metamorphism. Dikes of microdiorite were intruded be- tional Union of Geological Sciences (Streck- Biotite forms a prominent schistosity, tween and during the emplacement of all eisen, 1973). The term "host rock" is used hornblende crystals are elongate parallel the complexes. In the Sayán area (Fig. 1), to include any rock into which a pluton, with fold axes, and cordierite contains the dikes intruded between the Santa Rosa dike, or sheet was intruded, whereas the spiral inclusion trails of opaque minerals. and Puscao — San Jeronimo Complexes give term "country rock" is restricted to non- The Tapacocha fold belt is an anticlinorium K-Ar whole-rock ages of 72 to 68 m.y. A plutonic rocks, mostly volcanic, into which of isoclinal folds associated with green- fifth group of plutons at Sayán, younger the batholith was emplaced. schist facies metamorphism. than the Puscao — San Jeronimo Com- plexes, give K-Ar mineral ages of ~30 m.y. VOLCANIC COUNTRY ROCKS Calipuy Group (P. A. Wilson, 1974, personal commun.). The Coastal Batholith was intruded into The Calipuy Group is a sequence of ter- Major Structures the Casma Group of volcanic rocks which restrial andesite, dacite and rhyolite, lava, are chiefly marine and Middle to Upper and pyroclastic flows which extends for at The batholith has steep walls, subparallel Albian, and the Calipuy Group of terrestri- least 600 km along the western cordillera of with the coast and continental margin, and al volcanic rocks which unconformably northern Peru (Fig. 2). The rocks were first a flat roof. The roof is extensively preserved overlie them and are Late Cretaceous to described in detail by Cossio (1964) and in the northeastern part of the area (Fig. 3). early Tertiary in age (Fig. 2). Cossio and Jaén (1967) and are extensively Individual plutons have similar flat roofs intruded by the Coastal Batholith. In the and steep outward-dipping or vertical walls Casma Group northeast part of the area described, they (Figs. 4, 5, and 6). The plutons generally overlie the Tapacocha fold belt with form rectangular bodies with long sides The Casma Group is a sequence of ande- marked unconformity (Fig. 3) and their av- parallel to the batholith and short sides site lava and pyroclastic flows, andesite pil- erage thickness is in the order of 2 km. The normal to it [for example, the Huampi low lava, tuff, dikes, and sills, with smaller rocks are probably Late Cretaceous to early Piruroc granodiorite (Fig. 4); the Con- amounts of dacitic pyroclastic flows, chert, Tertiary in age, and they formed the land taderas, Llagumpe, and Maria Cristina plu- and volcaniclastic sediments. The sequence surface during intrusion of most of the tons of the Puscao unit, and northernmost extends for at least 500 km along the batholith. San Jeronimo pluton (Fig. 5)]. The transi- coastal region of northern Peru and inland tion from steep walls to flat roof generally to just beyond the eastern margin of the COASTAL BATHOLITH occurs within a vertical distance of 50 m Coastal Batholith (Fig. 2). In the area de- (Fig. 4, localities B and D). This relation- scribed here, it comprises six formations The batholith is a heterogeneous mass of ship is most clearly seen on a large scale in with a total thickness of about 6.5 km, al- coarse-grained igneous rocks which was in- the dome-shaped outcrops of the Puca Jirca though the actual thickness in any one place truded as numerous plutons and sheets. and Chasquitambo plutons of the Puscao is probably in the order of 3 to 4 km. The Each pluton is a distinct mappable intrusive unit. The Puca Jirca pluton forms a subcir- stratigraphic sequence is summarized in the body with steep walls and a flat roof. Many cular dome-shaped outcrop, 5 km in diame- legend of Figure 3 and described in detail in plutons are composed of identical rock ter, through which a valley cuts a section Myers (1974, and in press). Fossils indicate types, and each major mappable rock type 700 m deep (Fig. 5). At the highest levels that most of the lower part of the volcanic is called a magma unit. This unit is the fun- exposed, the contacts dip outward at 5°; sequence is late Middle Albian in age. In the damental division of the batholith and is whereas in the valley floor, they dip at 30° northeast corner of the area, the Casma analogous to the lithostratigraphic term to 40° outward. The Chasquitambo pluton Group conformably overlies a thick se- "formation." In the same way, a "com- crops out over an area of 300 sq km be- quence of shale and siltstone of probable plex" (or super-unit of Cobbing and tween altitudes of 400 and 2,400 m (Fig. 5). Hauterivian to Middle Albian age (Huayl- Pitcher, 1972) is a group of related units, At the highest topographic levels, its con- lapainpa Group). and a "subunit" is a division of a unit, tact dips at less than 10° outward; at the analogous to the lithostratigraphic terms lowest levels exposed, its contact dips Regional Deformation and Metamorphism "group" and "member." steeply Outward.