Compositional variations of Central Andean subduction-related igneous rocks reflect the plate-tectonic evolution of this active continental margin through time and space. In order to address the effect on magmatism of changing subduction geometry and crustal evolution of the upper continental plate during the Andean orogeny, we compiled more than 1500 major- and trace-element data points, and 650 Sr-, 610 Nd-, and 570 Pb-isotopic analyses of Mesozoic-Cenozoic (190–0 Ma) magmatic rocks in southern Peru and northern Chile (Central Andean orocline), mostly from new data and the literature. This data set documents compositional variations of magmas since Jurassic time, with a focus on the Neogene period, when major crustal thickening developed and its influence on magma composition was most pronounced. We relate the observed variations in Sr/Y, La/Yb, La/Sm, Sm/Yb, and Dy/Yb ratios, as well as in Sr-, Nd-, and Pb-isotopic ratios, to the crustal structure and evolution of the Central Andean orocline. In particular, the evolution of Dy/Yb and Sm/Yb ratios, which track the presence of the higher-pressure minerals amphibole and garnet, respectively, in the lower crust, documents that crustal thickness has grown through time. Spatial variations in trace elements and isotopic ratios further suggest that crustal domains of distinct composition and age have influenced magma composition through some assimilation. The crustal input in Quaternary magmas is quantified to have been between 7% and 18% by simple two-components mixing. When comparing our geochemical data set to the geological record of uplift and crustal thickening, we observe a correlation between the composition of magmatic rocks and the progression of Andean orogeny. In particular, our results support the interpretation that major crustal thickening and uplift were initiated in the mid-Oligocene (30 Ma) and that crustal thickness has kept increasing until present day. Our data do not support delamination as a general cause for major late Miocene uplift in the Central Andes and instead favor continued crustal thickening.

Geochemical variations in igneous rocks of the Central Andean orocline (13°S to 18°S): Tracing crustal thickening and magma generation through time and space

We quantitatively analyse the spatial pattern of deformation partitioning and of temporal accumulation of deformation in the Central Andes (15–26° S) with the aim of identifying those mechanisms responsible for initiating and controlling Cenozoic plateau evolution in this region. Our results show that the differential velocity between upper plate velocity and oceanic plate slab rollback velocity is crucial for determining the amount and rate of shortening, as well as their lateral variability at the leading edge of the upper plate. This primary control is modulated by factors affecting the strength balance between the upper plate lithosphere and the Nazca/South American Plate interface. These factors particularly include a stage of reduced slab dip (33 to 20 Ma) that accelerated shortening and an earlier phase (45 to 33 Ma) of higher trenchward sediment flux that reduced coupling at the plate interface, resulting in slowed shortening and enhanced slab rollback. Because high sediment flux and transfer of convergence into upper plate shortening constitute a negative feedback, we suggest that interruption of this feedback is critical for sustaining high shortening transfer, as observed for the Andes. Although we show that climate trends have no influence on the evolution of the Central Andes, the position of this region in the global arid belt in a low erosion regime is the key that provides this interruption; it inhibits high sediment flux into the trench despite the formation of relief from ongoing shortening. Along-strike variations in Andean shortening are clearly related to changes of the above factors. The spatial pattern of distribution of deformation in the Central Andes, as well as the synchronization of fault systems and the total magnitude of shortening, was mainly controlled by large-scale, inherited upper plate features that constitute zones of weakness in the upper plate leading edge. In summary, only a very particular combination of parameters appears to be able to trigger plateau-style deformation at a convergent continental margin. The combination of these parameters (in particular, differential trench-upper plate velocity evolution, high plate interface coupling from low trench infill, and the lateral distribution of weak zones in the upper plate leading edge) was highly uncommon during the Phanerozoic. This led to very few plateau-style orogens at convergent margins like the Cenozoic Central Andes in South America or, possibly, the Laramide North American Cordillera.

Deformation of the Central Andean Upper Plate System — Facts, Fiction, and Constraints for Plateau Models

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

Lithospheric evolution of the Andean fold-thrust belt, Bolivia, and the origin of the central Andean plateau

Sedimentary record of Andean mountain building

Late Permian–Middle Jurassic lithospheric thinning in Peru and Bolivia, and its bearing on Andean-age tectonics

(1) IRD (ex-ORSTOM), apartado postal 18-1209. Lima 18, Peru (ird@chavin.rcp.neLpe) (2) Universidad Nacional de San Antonio Abad dei Cusco, Cusco, Peru (carlo!to@chaski.unsaac.edu.pe) (3) Universidad Nacional Mayor de San Marcos, apartado postal 3973, Lima 100, Peru (4) Universidad Mayor de San Andres, casilla 6568, La Paz, Bolivia (nesj im@kolla.net) (5) Sociedad Geol6gica dei Per u, Arnaldo Marquez 2277, Lima Il , Peru (6) IRD (ex-ORSTOM), 211 rue Lafayette, 75480 Paris cedex 10, France (soler @paris .orstom.fr)

Late Permian-early Mesozoic rifts in Peru and Bolivia, and their bearing on Andean-age tectonics ; GÃ©odynamique andine : rÃ©sumÃ©s Ã©tendus = Andean geodynamics : extended abstracts = Geodinamica andina : resumenes expandidos

/pdf/late-permian-early-mesozoic-rifts-in-peru-and-bolivia-and-3n9319qogo.pdf

Late Permian-early Mesozoic rifts in Peru and Bolivia, and their bearing on Andean-age tectonics

Estiramiento litosférico del Paleozoico Superior al Cretáceo Medio en el Perúy Bolivia

Integrated studies and revisions of Neopaleozoic-Mesozoic sedimentary basins and associated magmatism in the Andes of Pero and Bolivia (8°S-22°S) show that the southwestern margin of Gondwana was areally submitted to several episodes oflithospherie thinning during the Late Paleozoic Middle Cretaceous interval The southernmost Peruvian margin underwent pronounced stretching starting in the Carboniferous; the sedimentary and magmatie records in the Tacna region suggest that a marginal-type basin was formed, where considerable volumes of basie volcanic rocks were erupted during the Pennsylvanian-Triassie interval In the Arequipa basin, whieh resulted from this evolution, the sedimentary record indicates that the Iithospherie thinning culminated in the Middle Jurassic, after having produced considerable subsidence More to the east, rifting processes started in central Peru in the Late Permian and propagated southwards into Bolivia until the Middle Jurassic, along an axis that coincides with the present Cordillera Oriental area1 n the Late Permian Triassic, rifting produced subsident grabens that were filled by red alluvial deposits and volcanie rocks (Mitu Group) Initiation of rifting was diachronie, propagating from north to south: isotopie ages otained on the Mitu magmatism c1early tend to be older (Late Permian) in the north, although Permian ages (280-260 Ma) are also reported from west and south of Lake Titicaca Mitu syn-rift strata were apparently deposited earlicr in the north than in the south, where they overlie a partly marine unit of Late Permian Early Triassie age that was not sedimented in a rift context Deposition of the post-rift carbonates of the Pucara Group progressed from north to south along the axis of the Mitu rift, but did not penetrate southeast of Cusco Tithonian Iithospheric thinning is recorded in western central Peru by the abrupt downwarping of the Chieama basin Termination ofrifting in the Cordillera Oriental was foUowed by the creation ofgentle reliefs: a slight inversion of the Cordillera OrientaI rift system in the late Jurassie-earuest Cretaceous is recorded b)' coeval westerly progadation of locall)' coarse detritus and by a pre-Cretaceous erosional surface developed on uplifted areas that had been previously located within the Late Permian-Triassic rift axis Along the present-day coastal region, ie west of ail the areas previously mentioned, Middle Cretaceous Iithospheric thinning is attested by considerable subsidence and submarine volcanism in the elongated Huarmey basin The evolution of the central Andean margin was thus dominated by Iithospherie thinning during -250 Myr, and ended -93-89 Ma with the onset of Andean crustal thiekening The major episodes of lithospheric stretching (Carboniferous [southern coast] or La te Permian [Cordillera Oriental] to Middle Jurassic; Middle Cretaceous [central Peru)) possibly reflect an evolution ofasthenospheric flow patterns produced by subduction and large-scale mantle convection Logieally, the lithospheric heterogeneities produced by these processes are Iikely to have largely influenced the distribution and modalities of younger deformations In particular, the Altiplano plateau corresponded to a paleotectonic domain of "normal" lithospheric thickness that was bounded by two elongated areas underlain by thinned Iithospheres The high Cordillera Oriental of Peru and Bolivia seems to have resulted from Late OligoceneNeogene intense inversion of the easternmost thinned area

/pdf/estiramiento-litosferico-del-paleozoico-superior-al-cretaceo-1j0eq90wi2.pdf

Silvia Rosas

Papers

Late Permian–Middle Jurassic lithospheric thinning in Peru and Bolivia, and its bearing on Andean-age tectonics

Late Permian-early Mesozoic rifts in Peru and Bolivia, and their bearing on Andean-age tectonics ; GÃ©odynamique andine : rÃ©sumÃ©s Ã©tendus = Andean geodynamics : extended abstracts = Geodinamica andina : resumenes expandidos

Late Permian-early Mesozoic rifts in Peru and Bolivia, and their bearing on Andean-age tectonics

Estiramiento litosférico del Paleozoico Superior al Cretáceo Medio en el Perúy Bolivia

Estiramiento litosférico del paleozoico superior al cretâceo medio en el perd y bolivia