A geochemical traverse across the North Chilean Andes: evidence for crust generation from the mantle wedge

The address is a wide-ranging commentary on the physical aspects of granitic magma and deals especially with the mode of emplacement. Recommendations on the nomenclature of plutons and batholiths involve both a description of the actual shape and their modeling by experimental and salt-diapir analogues. The nature of granitic crystal-mushes, wet or dry, is discussed in the light of the special properties of pseudoplastics, particularly the influence of the physical state on the sinking of crystals and xenoliths, on magma mobility in zoned plutons, and on the final rock structure. The longevity of plutonic events and their episodic character are commented on in relation to the subduction process. The generation of granitic magmas, their upwelling and the structural control of their intrusion, whether forceful or permissive, are discussed. The structural form of batholiths in depth is described in terms of a batholith denudation series particularly in the circum-Pacific context. Finally, two main orogenic environments of granites are contrasted in terms of the Hercynotype and Andinotype, and composition and structure are interrelated in a proposed general model for the emplacement of batholiths.

The nature, ascent and emplacement of granitic magmas

Iron oxide-copper-gold (IOCG) deposits, defined primarily by their elevated magnetite and/or hematite contents, constitute a broad, ill-defined clan related to a variety of tectono-magmatic settings. The youngest and, therefore, most readily understandable IOCG belt is located in the Coastal Cordillera of northern Chile and southern Peru, where it is part of a volcano-plutonic arc of Jurassic through Early Cretaceous age. The arc is characterised by voluminous tholeiitic to calc-alkaline plutonic complexes of gabbro through granodiorite composition and primitive, mantle-derived parentage. Major arc-parallel fault systems developed in response to extension and transtension induced by subduction roll-back at the retreating convergent margin. The arc crust was attenuated and subjected to high heat flow. IOCG deposits share the arc with massive magnetite deposits, the copper-deficient end-members of the IOCG clan, as well as with manto-type copper and small porphyry copper deposits to create a distinctive metallogenic signature. The IOCG deposits display close relations to the plutonic complexes and broadly coeval fault systems. Based on deposit morphology and dictated in part by lithological and structural parameters, they can be separated into several styles: veins, hydrothermal breccias, replacement mantos, calcic skarns and composite deposits that combine all or many of the preceding types. The vein deposits tend to be hosted by intrusive rocks, especially equigranular gabbrodiorite and diorite, whereas the larger, composite deposits (e.g. Candelaria-Punta del Cobre) occur within volcano-sedimentary sequences up to 2 km from pluton contacts and in intimate association with major orogen-parallel fault systems. Structurally localised IOCG deposits normally share faults and fractures with pre-mineral mafic dykes, many of dioritic composition, thereby further emphasising the close connection with mafic magmatism. The deposits formed in association with sodic, calcic and potassic alteration, either alone or in some combination, reveal evidence of an upward and outward zonation from magnetite-actinolite-apatite to specular hematite-chlorite-sericite and possess a Cu-Au-Co-Ni-As-Mo-U-(LREE) (light rare earth element) signature reminiscent of some calcic iron skarns around diorite intrusions. Scant observations suggest that massive calcite veins and, at shallower palaeodepths, extensive zones of barren pyritic feldspar-destructive alteration may be indicators of concealed IOCG deposits. The balance of evidence strongly supports a genetic connection of the central Andean IOCG deposits with gabbrodiorite to diorite magmas from which the ore fluid may have been channelled by major ductile to brittle fault systems for several kilometres vertically or perhaps even laterally. The large, composite IOCG deposits originated by ingress of the ore fluid to relatively permeable volcano-sedimentary sequences. The mafic magma may form entire plutons or, alternatively, may underplate more felsic intrusions, as witnessed by the ore-related diorite dykes, but in either case the origin of the ore fluid at greater, unobserved depths may be inferred. It is concluded that external 'basinal' fluids were not a requirement for IOCG formation in the central Andes, although metamorphic, seawater, evaporitic or meteoric fluids may have fortuitously contaminated the magmatic ore fluid locally. The proposed linkage of central Andean and probably some other IOCG deposits to oxidised dioritic magmas may be compared with the well-documented dependency of several other magmatic-hydrothermal deposit types on igneous petrochemistry. The affiliation of a spectrum of base-metal poor gold-(Bi-W-Mo) deposit styles to relatively reduced monzogranite-granodiorite intrusions may be considered as a closely analogous example.

Iron oxide-copper-gold deposits: an Andean view

Laurentia, the rift-bounded Precambrian nucleus of North America, may have broken out from a Neoproterozoic supercontinent between East and West Gondwana. Several lines of evidence suggest that the Appalachian margin of Laurentia subsequently collided with the proto-Andean margin of the amalgamated Gondwana supercontinent in different relative positions during early and mid-Paleozoic time, in route to final docking against northwest Africa to complete the assembly of Pangea. Hence the Appalachian and Andean orogens may have originated as a single mountain system. The overall hypothesis retains the same paleomagnetic and paleobiogeographic controls as previous global reconstructions for the Paleozoic Era. Laurentia-Gondwana collisions may help to explain contemporaneous unconformities in the Paleozoic sedimentary cover of the Laurentian, Gondwanan, and Baltic cratons.

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Paleozoic Laurentia-Gondwana interaction and the origin of the Appalachian-Andean mountain system

Neogene tectonic segmentation of the Andean Cordillera in northwestern Argentina is a function of both the dip of the subducted plate and of major paleogeographic structures and sedimentary basins. Vergence, fault dip, and lateral continuity of Neogene structures can be related to the character and pre-existing geometries and structures of the materials in which they form. South of 27°S the thick-skinned Sierras Pampeanas, which occur mostly over a flat segment of the subducted Nazca Plate, formed where little sedimentary cover overlay crystalline basement. North of 24°S, the thin-skinned Subandean belt, overlying an inclined slab segment, formed within a thick, elongate, eastward-tapering trough of Paleozoic and Mesozoic sediments that may represent a complex of pre-Andean foreland basin deposits. The transitional region between 24°–27°S (the Santa Barbara System and Cordillera Oriental) is characterized paleogeographically by complex cross-trending structures and irregular basin geometries, and the Neogene structures are equally complex and variable. The paleogeographic features of NW Argentina are a product of compressional tectonism during much of the Paleozoic that resulted in an orogenic belt oriented obliquely to the modern Andes. This earlier orogen was disrupted by several zones of transverse structures and by Cretaceous extensional or pull-apart basins. These features clearly influence the geometry of the superposed Andean structures. More intriguing, however, is the possibility that preexisting structures of the continental lithosphere may in some way help to modify the geometry of the subducted plate.

Paleogeography and Andean structural geometry, northwest Argentina

The Arequipa Massif, between the Andes and the Pacific, is an extensive pre-Devonian metamorphic complex. The sequence of deformations, metamorphisms and magmatism in this complex has been established. Mollendo, Atico and Marcona events are distinguished by structural and metamorphic methods and dated by Rb-Sr whole-rock isochrons, at about 1918, 440 and 392 Ma respectively. The Mollendo event led to partial melting, followed by granulite-facies metamorphism, in sediments buried to about 30 km. Further NW, sillimanite-bearing migmatites and staurolite-andalusite schists are thought to represent the same event. The tectonic trend is uncertain but the structures and metamorphism suggest a collision orogeny which probably pre-dated the Pacific Ocean. The early Caledonian Atico and Marcona events are associated with coast-parallel batholiths, amphibolite- to greenschist-facies metamorphism and penetrative deformations. The Atico and Marcona events are separated by the deposition of the Marcona Formation, which is therefore thought to be Lower Palaeozoic (between about 440 and 392 Ma). The early Caledonian deformations are attributed to a subduction zone near the present Pacific margin. There is no penetrative Hercynian or Andean deformation in the Arequipa Massif. Palaeomagnetic study of Jurassic andesites and dykes suggests that there has been no latitudinal motion of the Arequipa Massif relative to the Brazilian shield during the evolution of the Andes.

P. R. Cobbold

Papers

Structure, metamorphism an geochronology of the Arequipa Massif of coastal Peru