Gerhard Wörner

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

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.

Ridge collision, slab-window formation, and the flux of Pacific asthenosphere into the Caribbean realm

Mantle wedge–derived arc volcanism ceased in southern Costa Rica after ca. 8 Ma because of subduction of the aseismic Cocos Ridge beneath the Central American arc and the subsequent opening of a slab window. Geochemical and isotopic compositions of small volumes of adakitic and alkalic backarc lavas erupted between 5.8 and 2 Ma identify a source derived from the Galapagos plume. The presence of this source is explained by an influx of Pacific upper mantle into the Caribbean mantle wedge through a slab window, where the alkalic rocks form by melting of the upwelling mantle and the adakites result from melting of the leading edge of the subducted Cocos Ridge. By using geochemical and isotopic signatures, we trace this upper mantle flow beneath Central America from southern Costa Rica northward at a rate of 40 mm/yr.

Magmatic history and evolution of the Central American Land Bridge in Panama since Cretaceous times

Chemical compositions for 310 igneous rocks from the Cordillera de Panama and the Sona and Azuero peninsulas were supplemented by 40 Ar/ 39 Ar dating and Sr-, Nd-, Pb-, and O-isotope analysis to determine the magmatic evolution and oceanic plate interactions over the past 100 Ma in western Panama. An initial phase of intraplate magmatism, having geochemical characteristics of the Galapagos hotspot, formed the oceanic basement of the Caribbean large igneous province from 139 to 69 Ma. Younger accreted terranes with enriched trace element patterns (accreted ocean island basalt [OIB]) were amalgamated between 70 and 20 Ma. A second magmatic phase in the Azuero and Sona peninsulas has trace element patterns (Sona-Azuero arc) suggesting the initiation of subduction at 71–69 Ma. Arc magmatism continued in the Chagres basin region (Chagres-Bayano arc) from 68 to 40 Ma. A third phase formed discrete volcanic centers across the Cordillera de Panama (Cordilleran arc) from 19 to 5 Ma. The youngest phase consists of isolated volcanic centers of adakitic composition (Adakite suite) in the Cordillera de Panama that developed over the past 2 million years. Initiation of arc magmatism at 71 Ma coincides with the cessation of Galapagos plateau formation, suggesting a causal link. The transition from intraplate to arc magmatism occurred relatively quickly and introduced a new enriched mantle source. The arc magmatism involved progressive transition to more homogeneous intermediate mantle wedge compositions through mixing and homogenization of subarc magma sources through time and/or the replacement of the mantle wedge by a homogeneous, relatively undeleted asthenospheric mantle. Adakite volcanism started after a magmatic gap, enabled by the formation of a slab window.

Hydrated sub-arc mantle: a source for the Kluchevskoy volcano, Kamchatka/Russia

Oxygen isotope ratios of olivine and clinopyroxene phenocrysts from the Kluchevskoy volcano in Kamchatka have been studied by CO2 and ArF laser techniques. Measured δ18O values of 5.8–7.1‰ for olivine and 6.2–7.5‰ for clinopyroxene are significantly heavier than typical mantle values and cannot be explained by crustal assimilation or a contribution of oceanic sediments. Positive correlations between δ18O and fluid-mobile elements (Cs, Li, Sr, Rb, Ba, Th, U, LREE, K) and a lack of correlation with fluid-immobile elements (HFSE, HREE) suggest that 18O was introduced into the mantle source by a fluid from subducted altered oceanic basalt. This conclusion is supported by radiogenic isotopes (Sr, Nd, Pb). Mass balance excludes simple fluid-induced mantle melting. Instead, our observations are consistent with melting a mantle wedge which has been hydrated by 18O-rich fluids percolating through the mantle wedge. 18O-enriched fluids are derived from the subducted oceanic crust and the Emperor seamount chain, which is responsible for a particularly high fluid flux. This hydrated mantle wedge was subsequently involved in arc magmatism beneath Kluchevskoy by active intra-arc rifting.

Evolution of the West Andean Escarpment at 18°S (N. Chile) during the last 25 Ma: uplift, erosion and collapse through time

Abstract The geological record of the Western Andean Escarpment (WARP) reveals episodes of uplift, erosion, volcanism and sedimentation. The lithological sequence at 18°S comprises a thick pile of Azapa Conglomerates (25–19 Ma), an overlying series of widespread rhyodacitic Oxaya Ignimbrites (up to 900 m thick, ca. 19 Ma), which are in turn covered by a series of mafic andesite shield volcanoes. Between 19 and 12 Ma, the surface of the Oxaya Ignimbrites evolved into a large monocline on the western slope of the Andes. A giant antithetically rotated block (Oxaya Block, 80 km×20 km) formed on this slope at about 10–12 Ma and resulted in an easterly dip and a reversed drainage on the blocks surface. Morphology, topography and stratigraphic observations argue for a gravitational cause of this rotation. A “secondary” gravitational collapse (50 km3), extending 25 km to the west occurred on the steep western front of the Oxaya Block. Alluvial and fluvial sediments (11–2.7 Ma) accumulated in a half graben to the east of the tilted block and were later thrust over by the rocks of the escarpment wall, indicating further shortening between 8 and 6 Ma. Flatlying Upper Miocene sediments (

Pb isotopes define basement domains of the Altiplano, central Andes

Detailed Pb isotopic maps of the central Andes, based on 345 (163 previously published, 182 new) analyses of ores, volcanic rocks, and their host rocks, elucidate the gross structure of the basement and reveal that several isotopically distinct basement domains are juxtaposed in this region. The data clearly show that most of the Pb in central Andean igneous and ore samples is derived from the local basement, including Pb in ore deposits of the Bolivian tin belt. Some of the isotopic domain boundaries correspond to geologic structures and the residual gravity pattern, as well as to metallogenic boundaries such as the western edge of the Bolivian tin belt.

Volcano evolution and eruptive flux on the thick crust of the Andean Central Volcanic Zone: 40Ar/39Ar constraints from Volcán Parinacota, Chile

The 163 k.y. history as well as the chemi- cal and 46 km 3 volumetric evolution of Vol- can Parinacota are described in detail by new mapping, stratigraphy, and 57 40 Ar/ 39 Ar ages determined from groundmass or sani- dine crystals in basaltic andesitic to rhyolitic lavas. A more precise chronology of eruptions and associated eruptive volumes of this cen- tral Andean volcano, which was built upon 70-km-thick crust, provides a more com- plete view of how quickly volcanic edifi ces are built in this setting and how their mag- matic systems evolve during their lifetime. Development of the complex involved initial eruption of andesitic lava fl ows (163-117 ka) followed by a rhyodacite dome plateau (47- 40 ka) synchronous with the onset of the building of a stratocone (52-20 ka), which was later destroyed by a debris avalanche ~3 times larger than that at Mount St. Helens in 1980. Dome plateau emplacement occurred faster and later than has previously been published, implying a compressed duration of cone building and introducing a preced- ing 65 k.y. hiatus. Debris avalanche timing is refi ned here to be older than 10 but younger than 20 ka. Rapid postcollapse rebuilding of the volcanic edifi ce is delineated by 16 groundmass and whole-rock 40 Ar/ 39 Ar ages, which include some of the youngest lava fl ows dated by this method. Increase in cone- building rate and a continued trend toward more mafi c compositions following collapse imply an inter-relationship between the pres- ence of the edifi ce and fl ux of magma from the feeding reservoir. Cone-building rates at Parinacota are similar to those at other well- dated volcanoes on thinner crust; however, the distributed basaltic volcanism prevalent in those other arcs is virtually absent both at Parinacota and elsewhere in the Central Volcanic Zone. This suggests that while the hydrous, calc-alkaline magmas that make up the central volcanoes are not signifi cantly retarded by thick crust, primitive, dry basalts might be.

Precambrian and Early Paleozoic evolution of the Andean basement at Belen (northern Chile) and Cerro Uyarani (western Bolivia Altiplano)

Abstract Exposures of metamorphic basement in the Central Andes are scarce and reconstructions of the history of the Pacific margin of Gondwanaland must rely on a few isolated outcrops. We studied two areas of exposed basement in northernmost Chile (Belen) and westernmost Bolivia (Cerro Uyarani). The Belen metamorphic complex has been known for some time and consists of fault-bounded amphibolites, gneisses, schists, and minor quartzites overlain by folded Mesozoic to Cenozoic strata. The Cerro Uyarani is the only basement outcrop on the Bolivian Altiplano and has only recently been found and studied by geological reconnaissance. It consists of foliated mafic and felsic granulites, charnockites, and amphibolites. How do these basement occurrences compare and how do they relate to the other Precambrian crustal domains in the Central Andes? To answer these questions, we used geothermobarometers to reconstruct the P–T conditions of metamorphism, as well as geochemical analyses and petrological methods to study these rocks. The two basement blocks were found to have distinct geological histories and are probably separated by a major crustal domain boundary. Isotopic fingerprinting by Pb-isotopes clearly exclude Laurentian crustal components either in the protoliths or as reworked material. This signature is quite distinct from basement rocks farther south in Chile and northwestern Argentina.

Composition and structural control of crustal domains in the central Andes

The development of large seismic arrays and the large throughput of MC-ICP-MS are providing new impetus to the integration of seismic tomography data (VP, VS, attenuation, shear-wave splitting), geophysical maps (heat flow), and geochemical maps with geology. Synoptic representation of geochemical data started nearly 50 years ago with Hurley and others, who demonstrated that time-integrated parent/daughter ratios (Rb/Sr, U/Pb, Sm/Nd) and apparent crustal residence times inferred from the isotope compositions of radiogenic elements in felsic magmas and metamorphic rocks could be used to identify tectonic provinces. Geochemical parameters derived from such long-lived radioactive isotopic systems are far less noisy than raw trace element ratios in the same rocks: for example, measured uranium concentrations are severely biased by the transit of samples through the water table during erosive exhumation. The U-Th-Pb isotope system is particularly powerful. However, georeferenced geochemical databases are still incomplete. We recently began to map at the continental scale the Pb isotope compositions into axes with geologically informative content: the two-stage Pb model age, which dates the closure of the U-Pb chronometer, and the time-integrated U/Pb (mu) and Th/U (kappa) ratios of the Pb source [1]. Maps of model ages essentially depict the maximum extension of the ~600°C isotherm. Because rocks from granulite facies terranes tend to have Th/U higher than the planetary value of 3.88, maps of kappa ratios track the rise of lower crustal material and its melts. We compiled maps of the above-mentioned Pb parameters from the archeological OXALID database (Western Europe) [2] and localized its samples and from Mamani et al.s [3] database for the Central Andes. For the Western US, we used NAVDAT ( http://www.navdat.org/ ) for Cenozoic continental felsic igneous rocks and added ore data from the literature (Bouchet et al., this meeting). In all cases, we found that the regional consistency of model ages on the one hand and kappa values on the other hand is very strong and in particular highlights the existence of well-defined Pb isotope provinces. We also found some striking similarity between these provinces and prominent seismological features, such as Moho depth and VP/VS. The understanding of the nature, age, and dynamic role of the units identified by seismic tomography will clearly benefit from incorporating geochemical and geophysical spatial information into surface geology. [1] Albarede F. et al. (2012) Archaeom. 54, 853-867 [2] Stos-Gale, Z.A. and Gale, N. (2009) Archaeol. Anthropol. Sci. (2009) 1, 195-213. [3] Mamani M. et al. (2008) G-cubed 9

Palaeoclimatic implications of Mio–Pliocene sedimentation in the high-altitude intra-arc Lauca Basin of northern Chile

Abstract This paper examines the Late Miocene to Pliocene sedimentary succession in the high-altitude, intra-arc Lauca Basin of northern Chile, and discusses the palaeoclimatic evidence recorded within the basin fill. Sedimentological and geochronological data allow the reconstruction of four stages of the fluvial to lacustrine basin fill from the Late Miocene (>6.4 Ma) to the Late Pleistocene. We interpret the sedimentary evolution to reflect changes in precipitation during a time interval of general aridity. Climatic changes exerted the dominant control on sediment accumulation. Volcanism, tectonism, changes in morphology, and sediment supply were factors of subordinate importance. Stage 1 comprises a clastic red bed sequence developed during the Late Miocene under relatively humid conditions. An abrupt change in depositional style occurred around 6.4 Ma, when ephemeral saline lake sedimentation prevailed (stage 2). This change in depositional style is considered to document the begin of a phase of desiccation, that lasted until the Early Pliocene (3.7 Ma), and which was succeeded by less arid climatic conditions during the Late Pliocene and Pleistocene (stages 3 and 4). The transition between stages 1 and 2 and the subsequent dry period is assumed to coincide with a global cooling event during the latest Miocene to Early Pliocene. Comparable sedimentary responses to this climatic change can be expected in adjacent basins of the Central Andes.