The role of the Cretaceous Caribbean Oceanic Plateau in the genesis of late cretaceous arc magmatism in Ecuador

*Allibon, J., **Monjoie, P., **Lapierre, H., **Jaillard, E., *Bussy, F. & ***Bosch, D.

* Institut de Mineralogie et de Geochimie, Université de Lausanne, Suisse ** Laboratoire de Géodynamique des Chaines Alpines, UJF-Grenoble1 *** Laboratoire Tectonophysique, UMR-CNRS 5568, cc049 Université Montpellier II

The eastern part of the Western Cordillera of Ecua- volcanics. The subduction zone that generated the dor comprises thick buoyant oceanic plateaus asso- late Cretaceous arcs occurred far from the conti- ciated to island arc-tholeiites and subduction related nental margin, in an oceanic environment (Jaillard et calc-alkaline series, accreted to the continental mar- al., 1995). This implies that no terrigenous detrital gin of Ecuador from late Cretaceous to Eocene times sediments have interacted with the source at this pe- (Kerr et al. 2002, Mamberti et al. 2003). Two oceanic riod. Thus, the EM2 component can only result from plateau sequences have been identified: the San the melting of subducted pelagic sediments. The Juan oceanic plateau dated to 123 Ma (Lapierre et CCOP that supports these arc sequences is char- al. 2000) and the Guaranda oceanic plateau (90-86 acterised by a HIMU component (Révillon et al. Ma, Mamberti et al. 2001) considered as remnant of 1999, Mamberti et al. 2001) that could have been the Cretaceous Caribbean Oceanic Province assimilated by the island arc lavas. (CCOP). Mamberti et al. (2003) suggest that this Mixing models reveal that proportions of 20% of plateau is radiogenically enriched in 206Pb/204Pb and 207 204 the HIMU component and 20% of the pelagic sedi- Pb/ Pb and contains a HIMU component similar ment end-member are needed to explain sample to that observed in the Gorgona basalts and Gala- chemistry. These surprisingly high proportions can pagos lavas. Guaranda plateau Pb isotopes plot in a be explained by the young age of the CCOP (5 to 10 restricted field overlapping those of Pacific MORB. Ma) when the Late Cretaceous arcs developed. The Samples studied here were taken from four Upper CCOP, basement of these arc sequences, was Cretaceous arc-sequences in the northern part of the probably still hot and easily assimilated by the island Western Cordillera of Ecuador (Rio Cala, Macuchi) arc lavas. and in the Chogòn-Colonche Cordillera (Las Or- quideas and Cayo). These four island arcs rest on the CCOP. They consist predominantly of clinopy- roxene-bearing basalts and andesites. The complete REFERENCES petrological and geochemical study of these rocks reveals that some have a primitive island-arc nature Hauff, F., Hoernle, K. and Schmidt Angelika. (2003). Sr-Nd-Pb (MgO values range from 6 to 11 wt%). Their arc- composition of Mesozoic Pacific oceanic crust (site 1149 and 801, ODP Leg 185): Implications for alteration of ocean affinity is shown by the Nb, Ta and Ti negative crust and the input into the Izu-Bonin-Mariana subduction anomalies. These rocks are LREE-enriched and system. Geochem., Geophys., Geosyst. V.4, N°8. their bearing clinopyroxenes show a tholeiitic affinity Jaillard, E., Ordoñez, O., Suárez, J., Toro, J., Iza, D., Lugo, W., (FeOt-TiO2 enrichment and CaO depletion from core 2004. Stratigraphy of the Late Cretaceous-Paleogene de- to rim within a single crystal and the whole sample). posits of the Western Cordillera of Central Ecuador: Geo- dynamic implications. J. South Am. Earth Sci., 17, 49-58. Initial Nd, and Pb isotope ratios are very homoge- Kerr, A.C., Aspden, J.A., Tarney, J. and Pilatasig, L.F., 2002. neous and suggest that these rocks result from mix- The nature and provenance of accreted terranes in West- ing of three components: an E-Pacific MORB mantle ern Ecuador: Geochemical and tectonic constraints. Jour- nal of the Geological Society, London, 159, 577-594. (Hauff et al. 2003), an EM2 enriched component, Mamberti, M. (2001). Origin and evolution of two Cretaceous and a HIMU (Zindler & Hart. 1986) component. oceanic plateaus accreted in Western Ecuador (South Characterising the EM2 and HIMU components is America), evidenced by petrology, geochemistry and iso- important to constrain the genesis of these arc- topic chemistry. Thèse univ. Lausanne-Grenoble, Mamberti, M., Lapierre, H., Bosch, D., Ethien, R., Jaillard, É., Hernandez, J., Polvé, M. (2003). Accreted fragments of the Late Cretaceous Caribbean-Colombian Plateau in Ecuador. Lithos, 66, 173-199. Révillon, S., Arndt, N.T., Hallot, E., Kerr, A.C., Tarney, J., 1999. Petrogenesis of picrites from the Caribbean Plateau and the North Atlantic magmatic province. Lithos, 49, 1-21. Zindler, a. & Hart, S.R. (1986).Chemical geodynamics. Ann. Rev. Earth Planet. Sci. Lett., 14: 493-571. Consequences of Mai 1960 major subduction earthquake in the Andes and on lacustrine and marine sedimentation of Northern Patagonia (Chile, Argentina)

a Chapron, E., b Ariztegui, D., c Mulsow, S., d Villarosa, G., c Pino, M., d Outes, V., e Charlet, F. & f Juviginié, E.

a Geological Institute, ETH Zentrum, Zurich; Switzerland, b Institute F.A. Forel and Department of Geology and Paleontology, University of Geneva, Switzerland; c Instituto de Geociencias, Universidad Austral de Chile, Valdivia, Chile; d Centro Regional Universitario Bariloche, Universidad Nacional del Comahue, Bariloche, Argentina; e Renard Centre of Marine Geology, Gent University, Belgium; f Physical Geography, Université de Liège, Belgium. b In this Glaciologypart of Chile and Geomorphodynamicsand Argentina located Group, at Department 40- fissure of Geography, eruption University was triggered of Zurich, alongSwitzerland the Liquiñe- 42°S, the active subduction setting of the Nazca Ofqui Fault Zone (LOFZ), a major active intra-arc *Remote Sensing Laboratories, Department of Geography, University of Zurich, Switzerland plate beneath the South America plate, the melting shear zone in this part of the Andes. Thousands of of the Patagonian Ice Sheet during the Late glacial landslides were also triggered in the Andes along the and the concomitant growth of large stratocones LOFZ, during the multiple seismic shocks of the over the most active volcanoes of the Americas, re- major 21-22 Mai 1960 subduction earthquakes (Mw sulted in a complex Late Quaternary geomorphol- 9.5). ogy. Some of these well-documented landslides were Northern Patagonia is characterized by strong catastrophic and deeply affected the drainage basins precipitations driven by the Westerlies over the SE of most of the glacial lakes from the Chilean Lake Pacific and is including many sub-aqueous environ- District. In Argentina earthquake-triggered landslides ments that are sensitive natural archives of past en- were also documented, but the most striking histori- vironmental changes: lakes of glacial, tectonic or cal chronicles are the descriptions of violent lake volcanic origin, but also fjords and bays that were water oscillations or destructive waves triggered by flooded by the postglacial sea level rise. Strong the main seismic shock, in some of the largest Ar- westerly winds and intense volcanic activity in the gentinean glacial lakes between 39.5 and 46°S. study area result in the formation of thick and unsta- Our study shows that while this catastrophe in- ble andosoils (soils developed on volcanic ash) duced a major hyperpycnal flood deposit of ca. 3.106 draping the steep morphologies of the Andes. Such m3 in the proximal basin of Lago Puyehue, it only climate and soils favours the development of a very triggered and unusual organic rich layer in the dense vegetation cover consisting of a temperate proximal basin of Lago Frías, but destructive waves evergreen rain forest. and a large sub aqueous slide in the distal basin of In this study, the recent sedimentation processes Lago Nahuel Huapi. A very recent megaturbidite in in four contrasting lacustrine and marine basins of the two distal basins of Reloncavi fjord (Chile) lo- Northern Patagonia are documented by high- cated close to the LOFZ is also suggesting that 1960 resolution seismic reflection profiling (3.5 kHz) and co-seismic movements in this area may have trig- short cores at selected sites in deep lacustrine ba- gered the remobilization of ca. 187.106 m3 of marine sins. The regional correlation of the cores is provided sediments. Sub bottom profiling in these contrasting by the combination of 137Cs dating in lakes Puyehue lacustrine and marine basins at both sides of the (Chile) and Frías (Argentina), and by the identifica- Andes are highlighting the recurrent incidence of tion of the Cordon Caulle 1921-22 and 1960 tephras major sedimentary events during the Late Holocene. in lakes Puyehue and Nahuel Huapi (Argentina) and In order to confirm these reconstructions on the im- in their catchment areas. This event stratigraphy al- pact of 1960 earthquakes in Northern Patagonia, lows correlating across the Andes the formation of further studies should include dense grids of high- striking sedimentary events in these basins with the resolution seismic profiling and detailed studies of consequences of May-June 1960 earthquakes and sediment cores. Moreover, these deep lacutrine and the well-documented induced Cordon Caulle erup- marine basins have the potential to document the tion next to the Puyehue volcano; only 38 hours after recurrence of major subduction earthquakes in this the main shock offshore Valdivia. This rhyodacitic part of South America over several millennia. The interandean Quindío-Risaralda basin in Central Colombia and its Pleistocene infill by stacked volcaniclastic mass flows derived from the Central Cordillera

*Guarin, F., *Gorin, G. & **Espinosa, A.

* Department of Geology-Paleontology, University of Geneva, Switzerland, [email protected]; [email protected] ** Faculdad de Ingeneria, University of Quindío, Armenia, Colombia, [email protected]

The Quindío-Risaralda basin lies west of the Central than one million years for the latter formation. Cordillera. It is limited westward by the Tertiary Therefore, the largest part of the mass-flow fans are sediments of the Serranía de Sta Barbara, which of Pleistocene age (Suter et al., this symposium). were folded during the late Tertiary Andean tectonic So far, the origin of subsidence in the Quindío- phase and separate the the basin from the Cauca Risaralda basin cannot be clearly explained. Never- Valley (Fig. 1). Several SSW-NNE trending major theless, a clear relation exists between the active fault lineaments dissect the area, particularly the fault pattern, present-day drainage pattern and dis- Romeral Fault System (Fig. 1B). The latter is associ- tribution of volcaniclastic units. The basin bears the ated with the activity of the Ruiz-Tolima volcanic expression of three major transpressional fault system, which produced the material deposited in trends observed at a more regional scale: the N-S the fans. Following the dramatic earthquake of Ar- trending Cauca-Romeral System, the E-W trending menia in 1999, sedimentological, geomorphological Salento System and the SW-NE trending Palestina and tectonic studies have been undertaken to better System (Fig. 2). The present-day drainage pattern of understand the geology of this area and ultimately the fans has been heavily influenced by the vertical unravel the mechanisms associated with the Pleisto- throws along the fault systems. This multiple active cene geological history of this zone (Gorin et al., this fault system led to the formation of localized pull- symposium). apart basins that became depositional lows for the Geographically, the sedimentary infill of the volcaniclastic units. The study of the Quindío- Quindío-Risaralda basin can be subdivided from Risaralda basin is integrated with that of the Zarzal south to north into the Quindío, Pereira and Cartago Formation at the western edge of the volcaniclastic Fans (Fig. 1C). Several individual units can be dis- fans and in the Cauca Valley (Suter et al., this Sym- tinguished within these deposits according to their posium). It is hoped to come up with an integrated stratigraphic succession, lateral continuity, genesis interpretation of the dynamics of this interandean re- and sedimentological and petrographic parameters, gion. (Guarin et al., 2005). Sedimentology shows a transi- tion with the increasing water/sediment ratio from This research is supported by the Swiss National debris avalanches to debris flows, transitional flows, Science Foundation (grant no. 21-67080.01). hyperconcentrated flows and finally normal stream- flows. This gradation permits the subdivision of the fans into proximal, intermediate and distal parts (Fig. REFERENCES 1C). The imbrication of the different units permits their relative dating. Thicknesses vary from more Gorin, G., Guarin, F., Neuwerth, R., Suter, F., Espinosa, A. & than 200 meters in the proximal part to less than 50 Guzman, C. (2005): Contribution of Quaternary sediments to the understanding of the tectonic history in Central Co- meters in the most distal parts. In the distal part of lombia: the volcaniclastic fans in Quindío-Risaralda and the the fans, particularly along the La Vieja River and in Zarzal Formation in the Cauca Valley. 3rd Swiss Geo- the Cartago Fan in the north (Fig. 1C), volcaniclastic science Meeting, Zurich 2005. mass flows are interbedded with fluvio-lacustrine Guarin, F., Gorin, G.& Espinosa, A. (2005): A Pleistocene sediments of the Zarzal Formation (Neuwerth et al., stacked succession of volcanic mass flows in Central Co- 2005). Palynological data indicate an age of less lombia: the Quindío-Risaralda Fan. Acta Vulcanologica (in press). .Neuwerth, R., Suter, F., Guzman, C. & Gorin, G. (2005): Soft- bia) and its stratigraphic relationship with the volcaniclastic sediment deformations in a tectonically active area : the mass flows derived from the Central Cordillera. 3rd Swiss Pleistocene Zarzal Formation in the Cauca Valley (Western Geoscience Meeting, Zurich 2005 Colombia). Sedimentary Geology (in press). . Suter, F., Neuwerth, R., Guzman, C. & Gorin, G. (2005): Depo- sitional model for the Quaternary Zarzal Formation (Colom-

Figure 1. A) Location of study area; B) Geological cross-section AA’ through the Quindío-Risaralda basin; see Fig. 1C for location. C) Simplified geological map. The fluvio-volcanic infill of the Quindío-Risaralda basin is illustrated by a Digital Elevation Model (DEM). The zone framed in white underwent detailed field studies. The profile below the map shows the present-day surface slope of the Quindío Fan up to the Quindío volcano.

Figure 2. Interpretation of active faults and drainage patterns in the Quindío Fan (i.e., the lower part of Figure 1C). The limits of drainage compartments are shown as dotted lines on the interpretation at the right handside and as white lines on the DEM. The three major fault trends observed both at local and regional scales are shown at the right handside. Healed microfractures and mineral or fluid inclusions in olivine xenocrysts and in xenoliths from Tatara San Pedro complex, Central Chile: Evidence for subsolidus and remobilisation history

Ginibre, C. & Dungan, M. A.

Department of Mineralogy, University of Geneva, Switzerland [email protected], [email protected]

Phenocryst proportions in many mafic lavas of the The HMF’s are show two types of characteristic Quaternary Tatara-San Pedro complex (TSPC; features: 1) many of the HMF’s are marked by thin 36ES, Chilean Andes) are minor compared to modal linear low-Fo zones (_Fo = 1 to 5; width= 3 to 10 abundances of coarse olivine, clinopyroxene, and µm) along the healed microfracture. 2) various kinds plagioclase xenocrysts derived from disaggregated of fluid and mineral inclusions were trapped after mafic-ultramafic xenoliths. The xenocrystic nature of healing of the fracture. We have investigate the the olivine crystals is indicated by the presence of chemical variations in olivine adjacent to the HMF embayments, melt channels with mineral composi- using back scattered electron images and electron tion different from the groundmass, and an abun- microprobe In order to assess the extent of modifi- dance of healed microfractures (HMF, Dungan & cation that the HMF have undergone following incor- Davidson, 2004). Modelling of iron diffusion in olivine poration of host in magma. Some of the HMF par- shows that the residence time of these crystals in the tially retain subsolidus histories in the form of fluid basalt is on the order of a few years (Costa & Dun- and or mineral inclusions. We use Raman spec- gan, 2005). trometry to identify these phases. Other units, in particular a Holocene dacite flow Back-scattered electron images (Fig 1a) and from Volcan San Pedro contain abundant xenoliths electron microprobe analyses delineate narrow of amphibole- and phlogopite-bearing gabbros zones of high Fe contents along the HMF. Because (Costa et al., 2002.). Most minerals in these xeno- of the rapid diffusion of iron in olivine, the fact that liths, and in particular the olivines, also contain these features have not been erased shows that abundant HMF. The dacitic composition of the host they are relatively late and the shape of the Fe pro- lava implies moderate temperatures (<980°C), and file can be modelled in terms of residence time, as- therefore less thermal impact following incorporation suming the temperature known. in the host magma on the observed features. This is In the olivine xenocrysts from the basalts, the iron- consistent with observations suggesting limited re- rich zone is usually narrow (<5 µm) with a sharp action / melting of the xenoliths with the host boundary, and (isothermal) iron diffusion modelling magma. gives time since crystallisation of a few hours at We are investigating these HMF and their inclu- magmatic temperatures, indicating that these frac- sions in both types of rocks. Because these features tures were probably open upon eruption and that the are found in volcanic rocks but originally formed in crystallisation of the iron rich zone is mainly post- plutonic rocks, they provide two types of information: eruptive. Some longer diffusion times (a few days) 1) The original fractures are remnants from a sub- suggest possible crystallisation during ascent. In solidus history, indicating the extent of remobilisation both cases most information (fluids) from the sub- of plutonic roots of the arc. 2) The history of remobi- solidus stage will have been lost. The large effect of lisation has reheated the xenocrysts in a basaltic and remobilisation is consistent with the high tempera- dacitic environment for various amounts of time. ture of the host lavas and seems to be correlated with the residence time of the olivine in the lava. Figure 2. HMF in olivine: HMF1 (showing Fe rich line in BSE image) is empty whereas HMF2 inclusions, partially decripi- tated, still contain CO2, and N2 as well as magnesite.

The moist common fluid inclusions are rich in CO2 (gaseous state) with or without significant amount of N2. (Fig 2). The inclusions exhibit partial decrepitation features, and, in some cases, magne- site as a reaction product of the CO2 with olivine. No fluid water was observed, but a few inclusions seem to contain antigorite, a reaction product of water with olivine. We also found anhydrite crystals in some in- clusions suggesting very oxidizing conditions. A more detailed Raman study is under way, in or- der to characterise the fluids phases. The xenoliths Figure 1. HMF in olivine xenocrysts from TSPC basalt. a) and xenocrysts are thought to come from the plu- Backscattered electron image showing Fe rich zone (lighter tonic root of the arc at relatively shallow depth. The grey); b) Transmitted light microscopy. inclusions present in the olivines have, therefore, re- corded fluids flowing through the crust. Although In contrast the olivines from the xenoliths en- modified by the post incorporation heating, the inclu- trained in the Holocene dacite, rarely show the iron sions contained in xenocysts and xenoliths have the rich zone along the HMF axis, although such healed potential to reveal the nature of the crust sampled fractures are especially abundant. When present, the and assimilated by the magma, especially in terms of iron-rich zones have a much more diffuse profile volatile elements and fluid phases. similar to those near olivine rims. The time given by iron diffusion at temperatures between 925 and 525°C is a few years to over 1 Myrs. This seems too REFERENCES long for post-eruptive crystallisation event at mag- matic temperatures. Therefore, these iron variations Costa, F. Dungan, M.A. & Dinger B.S.(2002): Hornblende- and probably represent either a crystallisation in the re- phlogopite-bearing gabbroic xenoliths from Volcan San Pedro (36°S), Chilean Andes: Evidence for melt and fluid opened HMF immediately after xenolith incorporation migration and reactions in subduction-related plutons. or a low temperature subsolidus history. Journal of Petrology, 43(2) : 219-241. First results of Raman spectroscopy analyses for Dungan, M.A. & Davidson J. (2004): Partial assimilative recy- cling of the mafic plutonic roots of arc volcanoes: An exam- the identification of fluid and mineral inclusions show ple from the Chilean Andes. Geology 32(9): 773-776. following features : Costa, F. & Dungan, M. A. (2005): Short timer scales of mag- Whereas most of the HMF’s that show recent re- matic assimilation from diffusion modelling of multiple ele- opening, in the form of Fe-rich zones, seem, as ex- ments in olivine Geology, in press. pected, to have lost their trapped fluids (or contain low pressure fluids), some others still retain some of the original fluids / minerals, modified to an unknown extent following incorporation. Phlogopite-bearing absarokites and basic andesites from the Ecuadorian rear-arc area: Genesis and evolution

1Hoffer, G., 1,2Eissen, J.-P., 3Beate, B., 4Bourdon, E., 5Fornari, M., 1Laporte, D., 1Martin, H., 6Samaniego, P. & 7Cotten, J.

1 Laboratoire Magmas et Volcans, UMR 6524, Université Blaise Pascal, Clermont-Ferrand, France 2 Institut de Recherche pour le Développement (IRD), URM 163, Clermont-Ferrand, France 3 Departamento de Geologia, Escuela Politecnica Nacional, Quito, Ecuador 4 Institut de Géologie, Université de Neuchâtel, Suisse 5 Instituto Geofisico, Escuela Politecnica Nacional, Quito, Ecuador 6 Institut de Recherche pour le Développement (IRD), UMR 6526, Géosciences Azur, Université de Nice, France 7 Domaines Océaniques, UMR 6538, Université de Bretagne Occidentale, France

The subduction of the buoyant aseismic Carnegie The recent characterization of two new volcanic Ridge beneath the active margin of Ecuador, since formations, the Puyo scoria cones and the Mera lava at least 6 Ma, results in several modifications in the flows, wich outcrop around the Puyo city (latitude magmatic activity of this region, namely the widening 1°S), allow to extend the Ecuadorian rear-arc of the volcanic arc and noticeable changes of the province some 100 km south of the three previously geochemical composition of the erupted lavas, due known large stratovolcanoes (Sumaco, Pan de to sources and P-T conditions variations. Till today, Azucar, Yanaurcu). Ecuadorian rear-arc volcanism was known through Petrographical and geochemical data obtained on previous studies focused on the quaternary Sumaco these poorly known edifices provide new constraints volcano (0.5°S-77.6°W), which erupted hauyne on the volcanism of this area and preliminary phenocrysts-bearing undersaturated basalts, with modellings about the petrogenetical origin of rear-arc alkaline affinities (Colony et Sinclair 1928; Barragan magmas are also presented. et al. 1998; Bourdon et al. 2003). On one hand, the Puyo cones represent nine scoria cones, with several associated lava flows, aligned along a NW-SE fissure. They sit over the Mera-Upano detritic formation, whose erosion surface was dated at about 40 kaBP (Bès de Berc et al. 2005). Consequently, these young scoria cones were probably emplaced during a single fissural eruptive event of late Pleistocene to possible Holocene age. The scoriae and lavas coming from these small formations consist of olivine + clinopyroxene ± phlogopite phenocrysts-bearing absarokites. On the other hand, the Mera lavas are represented by several isolated thick lava flows, whose emission centre as well as their age (probably upper Pleistocene) still remain unknown. These lavas range in composition from medium-K basalts to high-K andesites, bearing olivine and clinopyroxene phenocrysts. Figure 1. Geodynamical simplified map of the Northern Andes The enrichment of the rear-arc lavas in all (modified from Gutscher et al. 1999). Grey arrows correspond incompatible elements, particularly in niobium and in to the direction of subduction. Filled triangles represent the rear-arc edifices. potassium, like their low content in SiO2, give them an atypical character. Indeed, these compositions Subsequently, an experimental approach is differ noticeably from fore arc or main arc lavas, wich envisaged, in order to improve these preliminary are mainly andesitic. Although only few isotopical results. Low degree partial melting experiments of a data are available, the 87Sr/86Sr (0.7041-0.7042) and peridotite, enriched by an adakitic liquid 143Nd/144Nd (0.5128) isotopic ratios, obtained on (Quimsacocha adakite, Ecuador), will be carried out three Mera samples, show that extremely limited in a piston-cylinder apparatus, for the purpose of continental contamination might have affected these evidence the importance of the hydrated phases magmas during their ascent towards the surface, (e.g. phlogopite or pargasite) during the melting. despite the presence of an old cratonic basement Effectively, the presence of phlogopite in the mantle (Guyana shield) and their similarities with the fore source, as well as its residual behaviour during arc lavas. partial melting, is a key point of the genesis of such undersaturated incompatible rich melts in the rear- arc area of the Ecuadorian subduction zone.

REFERENCES

Bès de Berc, S., Soula, J.C., Baby , P., Souris, M., Christophoul, F. & Rosero, J. (2005): Geomorphic evidence of active deformation and uplift in a modern continental wedge-top-foredeep transition: example of the eastern Ecuadorian Andes. Tectonophysics 399: 351-380. Barragan, R., Geist, D., Hall., M.L., Larson, P. & Kurz, M. (1998): Subduction controls on the compositions of lavas from the Ecuadorian Andes. Earth and Planetary Science Letters 154: 153-166. Figure 2. K2O vs. SiO2 diagram showing Puyo cones and Mera lavas. Field for Sumaco (rear-arc stratovolcano) and Altar Bourdon, E., Eissen, J.-P., Gutscher, M.-A., Monzier, M., Hall, volcano (edifice belonging to the main arc) come from Bourdon M.L. & Cotton, J. (2003): Magmatic response to early et al. 2003 and from IRD geochemical data base. aseismic ridge subduction: the Ecuadorian margin case (South America). Earth and Planetary Science Letters 205: 12-138. The genesis of the rear-arc magma is also Colony, R.J. & Sinclair J.H. (1928): The lavas of the volcano investigated, as for the chemical composition and Sumaco, Eastern Ecuador, South America. American the mineralogy of the mantellic source, the residual Journal of Science 216: 299-312. Hesse, M. & Grove, T.L. (2003): Absarokites from the western assemblage, the degree of partial melting and the P- Mexican Volcanic Belt: constraints on mantle wedge T conditions of the mantle wedge during the melting. conditions. Contributions to Mineralogy and Petrology 146: Petrogenetical modellings, constrained with major 10-27. and trace elements, allow a preliminary estimation of these parameters. The peridotitic mantle, previously enriched by about 3% of slab-melts, undergoes a very low partial melting event (1-1.5%). The residual assemblage contains always olivine, orthopyroxene and phlogopite. Nevertheless, depending on the modellings, clinopyroxene and garnet might be present in the residue. All the lavas of the Puyo cones series and most of the Mera lavas can be explained only by a slight increase of the degree of partial melting (from 1% to 2%), linked with the disappearance of the residual phlogopite. However, the two most differentiated Mera lavas (>55% SiO2) require additional fractional crystallization, leaving a cumulate with olivine, clinopyroxene, plagioclase, titano-magnetite and some apatite. Surface processes and associated timescales: Cosmogenic nuclide and sediment yield data from the Central Andes of northern Chile

a Kober, F., b Ivy-Ochs, S., c Schlunegger, F., d Zeilinger, G., e Kubik, P.W., f Baur, H. & f Wieler, R.

a Institute of Geology & Institute of Isotope Geology, ETH Zurich,, CH-8092 Zurich, Switzerland [email protected], Tel.: ++41-44-6323637 b Institute of Particle Physics, ETH, CH-8093 Zurich & Institute of Geography, University of Zurich, CH-8057 Zurich Switzerland; c Institute of Geology, University of Bern, CH-3012 Bern, Switzerland; d Institute of Geosciences, University of Potsdam,, D-14476 Golm/Potsdam, Germany; e PSI/c/o Institute of Particle Physics, ETH Hoenggerberg,CH-8093 Zurich, Switzerland; f Institute of Isotope Geology, ETH Zurich,, CH-8092 Zurich, Switzerland

The desert parts of the Andes of northern Chile are Escarpment) but suggest a coupling between hill- regarded as being one of the oldest landscapes on slope and channel processes. Earth. Therefore, landscape forming processes must Erosion and denudation rates positively correlate act at very slow rates. These slow rates have pro- with elevation and the historical precipitation record, moted controversial ideas on the evolution of the suggesting a coupling between climate and erosion. central Andean mountain chain and discussions In addition, it is suggested that the very old land- whether climatic or tectonic forces predominate the scapes could be preserved in the western Central geodynamic evolution of the Andes (see: Lamb and Andes thanks to low tectonic activity and the pre- Davis, 2003; Hartley, 2005). vailing dry climate during the late Cenozoic. In order to quantify the rates of various landscape The analyses of multiple terrestrial cosmogenic forming processes we analyzed erosion rates of hill- nuclides and the use of various “erosion-island” dia- slope interfluves across the slope of the western grams allowed the identification of system states Central Andes (Arica area, northern Chile) in a tran- (disequilibrium, transient, steady state) of the nuclide sect from the Coastal Cordillera to the Western Es- system as well as possible complex exposure histo- carpment into the Western Cordillera. The data con- ries. Complex exposure histories for samples ana- sist of the analysis of several long-lived terrestrial 10 21 26 lysed by cosmogenic nuclides were identified for cosmogenic nuclides ( Be, Ne, Al - mostly in non-bedrock samples, such as boulders or amalga- quartz of the Oxaya-ignimbrites) forming bedrock mated clast samples (disequilibrium state). Cos- and preliminary data from catchment wide denuda- 21 mogenic nuclide concentrations from bedrock sam- tion rates derived from cosmogenic Ne in river ples of the lower Western Escarpment, however, sediments. These long-lived cosmogenic nuclides imply near steady-state or transient states over mil- are associated with timescales of millions of years, lion year timescales likely caused by processes such depending on the erosion rate. Furthermore, we as episodic bedrock spalling in the cm-scale. analysed sediment yield data from river gauging sta- tions representative for the last decade. Erosion Landscape processes studied by morphometric rates determined by the cosmogenic nuclide analysis analysis suggest near-steady state conditions for are estimated back into the middle to late Miocene most of the western slope of the Andes (Kober et and rates are on the order of 10-100cm/My at the al., 2005, in press; Kober et al., subm. 2005). Al- hyperarid Western Escarpment (Atacama Desert) though the surfaces have demonstrably exposed and the Costal Cordillera. In contrast, bedrock ero- since the Miocene, timescales to achieve cos- sion rates for the semiarid Western Cordillera are up mogenic nuclide saturation (dynamic equilibrium) to >3000cm/My, at least back into the Holocene/late and landscape steady states may not be necessarily Pleistocene. Likewise, catchment wide erosion rates the same. Nevertheless, landscape forming proc- of the Lluta-drainage system yield similar orders of esses during the late Cenozoic act with very low magnitudes. Sediment yield data obtained on a rates and relief modification is therefore almost neg- decadal scale indicate denudation rates of - again - a ligible. similar order of magnitude. These landscapes form- ing processes rates are one to two orders of magni- tude higher compared to the desert parts (Western REFERENCES

Hartley, A.J., 2005. What caused Andean uplift? In: 6th ISAG. IRD, Barcelona, pp. 824-827. Kober, F., Ivy-Ochs, S., Schlunegger, F., Baur, H., Kubik, P.W. and Wieler, R., subm. 2005. Denudation rates and a topog- raphy-driven precipitation threshold in northern Chile: mul- tiple cosmogenic nuclide data and sediment yield budgets. Geomorphology. Kober, F., Schlunegger, F., Zeilinger, G. and Schneider, H., 2005, in press. Surface uplift and climate change: The geomorphic evolution of at the Western Escarpment of the Andes of northern Chile between the Miocene and present. In: S. Willet, N. Hovius, D. Fisher and M. Brandon (Editors), Tectonics, Climate and Landscape evolution. GSA Special Paper. Lamb, S. and Davis, P., 2003. Cenozoic climate change as possible cause of the rise of the Andes. Nature, 425: 792- 797. The Torres del Paine laccolith, S-Chile

*Michel, J., *Baumgartner, L.P, **Malthe-Sørenssen, A., *Darbelly, B., ***Oberhänsli, R., *Putlitz, B., & *Robyr, M.

* Institute of Mineralogy and Geochemistry, University of Lausanne, Switzerland ** Physics of Geological Processes, University of Oslo, Norway *** Institute of Geoscience, University of Potsdam, Germany

The Torres del Paine Laccolith (TPL) in Patago- Field evidence shows the intrusion of the granite nia/Chile is part of a chain of isolated Miocene intru- after the PMC and a feeder-zone is located at the sions, which intruded into the eastern foothills of the western end, in the Lago Grey area. Here PMC- southernmost Andes of Chile and Argentina. The rocks are vertically cut by the granite. Here no fluid- TPL has been dated by Halpern (1973) at 12±2 Ma saturation is evident at the contact of PMC and (Rb/Sr model) and13±1 Ma (K-Ar biotite), respec- granite. Further east granite overlays the PMC with tively. mostly sharp contacts. Edges of the PMC are par- The laccolith intruded at a shallow level (2-4 km) tially broken and intruded by the granite. between mudstones, sandstones and conglomerates Sediments south of the intrusion have been re- of the Cretaceous Punta Barrosa and Cerro Torre gionally deformed showing horizontal N-S trending Formation. It consists of a basal part with layered fold-axes, being tighter in the western part. Ap- gabbroic and minor dioritic and granitic rocks (Paine- proaching the intrusion the fold-axes begin to dip Mafic-Complex PMC, Michael 1984). southwards with increasing angle closer to the gran- The main i-type granite is peraluminous and can ite. In the west structures indicate the ascending limb be subdivided into an alkali-feldspar porphyritic me- of an anticline, which is underlain by the PMC and dium grained granite. A fluid-saturated miarolitic gra- the granite. Similar features, with inverse dip rela- nophyric alkali-granite is found towards the host- tions, can be observed at the western contact. rock. Miaroles contains mainly quartz and feldspar, Vertical emplacement of the granite most likely with some biotite, tourmaline, sphene, pyrite and occurred at the level of PMC resulting in uplift of fayalite. Granitic phases with miaroles, inside the sedimentary strata. main-granite body, bordered by biotite-Schlieren in- We present models using the discrete element dicate transport and ascent of fluid-saturated magma model (DEM),(Malthe-Sorenssen et al. 2004), for through the crystallizing mush. different scenarios of emplacement for the granitic Major element chemistry indicates a development laccolith to reproduce the field observations. towards evolved granites from centre to the rim of the intrusion. Contact between granite and host-rock is sharp REFERENCES and with minor or no stoping in the roof areas. Strik- ing features are multi-phase dykes, oriented perpen- Halpern, M. (1973): Regional geochronology of Chile South of dicular to the host rock contact. They continue for 50°S Latitude. Geological Society of America, Bulletin, 84, p. 2407-2422. several tens of meters into the host rock. Late basal- Michael, P.J. (1984): Chemical differentiation of the Cordillera tic, rhyolitic and composite dykes crosscut the intru- del Paine granite (southern Chile) by in situ fractional crys- sion and the host rocks. tallization. Contributions to Mineralogy and Petrology, Vol. 87, p. 179-195. A. Malte-Sørenssen et.al. (2004): Formation of saucer-shaped sills, in: Physical Geology of High-Level Magmatic Systems (F. Breitkreuz, C. Petford, eds), Geological Society, Lon- don, Special Publications, 234, 215-227. The Paleozoic-Mesozoic geodynamic transition along the Western Gondwanan margin – Geochemical and chronometric constraints from the Eastern Peruvian Cordillera

Miskovic, A., Schaltegger, U. & Chew, D.

Département de Minéralogie ; Université de Genève 13 Rue des Maraîchers, 1205 Genève, Switzerland [email protected]

The Eastern Cordillera of Peru represents a major, bimodal, calc-alkaline to tholeiitic lavas of the Mitu yet relatively unstudied part of the proto-Andean Group and are characterized by restricted bimodal continental margin. Paleozoic to early Mesozoic compositional range (66-72 wt. % SiO2), Fe batholiths that span its length exhibit profound and enrichment, lack of Nb anomalies, Ba depletions systematic variations in the chemistry and timing of relative to Th and Rb and higher Ga/Al ratios, all of emplacement from north to south (Mégard, 1978; which are associated with the transitional post- Soler, 1991; Vidal et al., 1995; Jacay et al., 1999). orogenic to within-plate granitoid suites; As products of long-lived magmatic episodes, these (3) Late Triassic-Early Jurassic peralkaline, A-type plutonic belts mark loci of active lithospheric plutons of the southern Cordillera de Carabaya boundaries between the western Amazonian Craton intrude alkaline Mitu Gr. basalts. They are nepheline and variable Neoproterozoic to Paleozoic crustal normative, and characterized by highly elevated HFS domains during the final assembly and ultimate elements (ZR, Ti, and P). break-up of Pangea. Recognizing variations in their geochemical signature through time and space Combined 87Sr/86Sr, 143Nd/144Nd isotopic ratios as places constraints on the type of tectonism along the well as various Pb isotope systematics from the paleo-margin, the composition and provenance of three intrusive provinces however lack systematic crustal members involved, as well as the nature of variations, and suggest uniformly large degrees of the underlying lithospheric mantle. assimilation of the Proterozoic Amazonian basement throughout, thus constraining their paleo-geographic Here, a new data set from plutonic rocks of the position proximal to or within the Gondwana craton. Eastern Cordillera is integrated with the existing geochemical, chronometric and isotopic Interestingly, high precision U/Pb (zircon) and characterizations of the Peruvian landmass and a 39Ar/40Ar (mica, hbd.) geochronometry reveal a provisional geodynamic model is proposed for the general younging-southward trend. A ~ 20 Ma long Late Devonian - Early Jurassic evolution of this magmatism associated with the formation of a segment of the western Gondwana. Mississippian continental arc in the north-central Cordillera Oriental culminated between 336-325 Ma, A striking relationship exists between the three and was followed by c.a. 40 Ma hiatus, briefly principal plutonic belts of eastern Peruvian punctuated during a 314-312 Ma episode of orogenic Cordillera: Au-Ag mineralization and a 307-305 Ma, S-type (1) Mississippian to Pennsylvanian I-type magmatic pulse, both interpreted to reflect an metaluminous to peraluminous, hornblende- episode of tectonic uplift of the convergent margin dominated granitoids are restricted to the segment (Haeberlin et al., 2002). o o north of 11 S (dominantly north of 9 S), and display Resumption of Permo-Triassic magmatism (279- calc-alkaline evolutionary trends with elevated 230 Ma) initially saw deposition of the bimodal calc- LILE/HFSE ratios characteristic of continental alkaline to tholeiitic volcanics of the Mitu Group subduction zones; contemporaneously with the emplacement of the (2) Mid-Permian to Early Triassic peraluminous, S to post-collisional S-type plutons in the south-central I-type, mica-rich granitoids of the (south) central Eastern Cordillera (Soler, 1991). The magmatic Peru, are comagmatic with the compositionally activity throughout Triassic was marked by eruption of progressively more mafic and alkalic Mitu lavas The proposed change in strike of the subduction and initiation of the A-type plutonism (sensu stricto) could have resulted in transport of a buoyant that peaked between 216-205 Ma in the segment of oceanic crust (island arc root / plateau), southernmost Carabaya Batholith (Kontak et al., which plugged the subduction zone and resulted in 1990). an ocean-ward trench migration coupled with an Complementarity of the arc and rift-related initial margin uplift and subsequent fore-arc plutonic belts in the eastern Peruvian Andes points extension. This scenario explains the “craton- free” to a major tectono-magmatic change that took place basement underlying the Western Cordillera of along this segment of the proto-Andean margin of northern Peru as well as development of ubiquitous Gondwana during the late Paleozoic. ensialic basins filled with the Mitu Gr. Molasses and bimodal volcanics, following the termination of arc- Any self-consistent tectonic model for the region related magmatism in Pennsylvanian. must take into account the following: Continued oblique subduction of oceanic crust in (1) an apparent absence of the cratonic crust under Permian generated incipient S-type melts within the most of the Western Peruvian Cordillera north of 13o thickened crust of the central Peru, while progressive S as inferred from isotopic (Mukasa and Tilton, strike-slip duplexing resulted in formation of 1984) and gravimetric surveys (Polliand et al, 2005); transtenisonal basins filled with the Permian rift- (2) Existence of a constructive continental margin as related magmas further south during Triassic. We inferred from the subduction-related plutonism exclude the possibility of extending the Arequipa restricted to the northern Eastern Cordillera of Peru terrane north of its present isotopic borders during during mid-to-late Mississippian. The activity this time and consider it either non-existent, or resumed 25 Ma later along the Chilean Frontal reserve its removal from the Peruvian segment of Cordillera (Mpodozis and Kay, 1992); the Gondwanan margin before Carboniferous. (3) Purely Gondwanan Pb isotopic signature of both the Carboniferous and Permo-Triassic plutonic rocks (Macfarlane, 1999); REFERENCES (4) A north-to-south transition from subduction- Haeberlin, Y., 2002. Ph.D. thesis, Department of Mineralogy, related I-type through the S-type, post-orogenic University of Geneva, Terre et Environement, v. 36, 182 p. leucogranitoids into the rift-associated A-type Jacay, J., Sempere, T., Carlier, G., and Carlotto, V., 1999. 4th ISAG Conference Extended Abstracts, Göttingen, plutons, and Germany, p. 358- 362. (5) A diachronous onset of the Permo-Triassic rift- Kontak, D.J., Clark, A.J., Farrar, E., and Strong, D.F., 1985. In: related magmatism in the central and southern Peru Pitcher, W.S., Atherton, M.P., Cobbing, J., and Beckinsale, R.D., (Eds.), Magmatism at a plate edge: the Peruvian with a younging-southward trend (Sempere et al., Andes. London, Blackie & Son, p. 36-44. 2002). Macfarlane, A.W., Tosdal, R.M., Vidal, C.E., and Paredes, J., 1999. In: Skinner, B.J., ed., Geology and ore deposits of The aforementioned geochemical and tectonic the Central Andes: Economic Geology Special Publication evidence can be integrated in a geodynamic model Series, v. 7, p. 267-279. in which an originally orthogonal eastward Mégard, F., 1978. Travaux et Documents de l’ORSTOM, Paris, subduction of the paleo-Pacific crust below the v. 86, 310 p. western Gondwana during the Late Devonian to Mpodozis, C., and Kay, S. M., 1992. Geological Society of Early Carboniferous became strongly oblique America Bulletin, v. 104, p. 999-1014. Mukasa, S. B., and Tilton, G. R., 1985. In: Pitcher, W.S., towards south-east thus imposing a sinistral strike- Atherton, M.P., Cobbing, E.J., and Beckinsale, R.D., (Eds.), slip stress regime on the Gondwanan margin and Magmatism at a plate edge; the Peruvian Andes: London, induced a counter-clockwise rotation of the northern Blackie & Son, p. 203-207 edge of the Arequipa terrane (Figure 4). Polliand, M., Schaltegger, U., Frank, M., and Fontbote, L. 2005. International Journal of Earth Sciences (Geol. Rund.), v. 94, p. 231-242. Sempere, T., Carlier, G., Soler, P., Fornari, M., Calotto, V., Jacay, J., Arispe, O., Neraudeau, D., Rosas, S., and Jimenez, N., 2002. Tectonophysics, v. 345, p. 153-181. Soler, P., 1991. Thèse de doctorat d’Etat, Université Pierre-et - Marie-Curie (Paris VI), 950 p. Vidal, C.E., Paredes, J., Macfarlane, A.W., and Tosdal, R.M., 1995. Sociedad Geológica del Perú, Lima, volumen jubilar A., p. 351- 377. Nevado de Longaví volcano (Chilean Andes, 36.2 ºS): Adakitic magmas by fractional crystallization from hydrous mafic melts

1Rodríguez, C., 1Sellés, D., 1Dungan, M., 2Langmuir, C. & 3Leeman, W.

1 Université de Genève, Section des Sciences de la Terre, Département de Minéralogie. 13 Rue des Maraîchers. 1205 Geneva, SWITZERLAND. [email protected] 2 Department of Earth Science MS-126, Rice University, 6100 Main St., Houston, TX, 77005, U. S. A. 3 Department of Earth and Planetary Sciences, Harvard University. 20 Oxford Street, Cambridge, MA 02138. U. S. A.

Until recently, models for the formation of adakitic respects unlike other SVZ mafic magmas. They have magmas have focused on partial melting of oceanic relatively low contents of many incompatible ele- lithosphere in regions where young, hot slabs are ments, notably Th, U, Zr, Nb, Hf and REE, in combi- subducted. Nevertheless, adakitic-like rocks also oc- nation with high B (19-25 ppm), Be, Cs, and Li con- cur in continental arcs related to subduction of colder tents and high Ba/Th, Ba/Zr, Pb/Th ratios. These oceanic lithosphere, where they have been ex- features are consistent with these mafic magmas plained in terms of remelting of basaltic material un- being derived from high degrees of melting of the derplated at the base of thickened orogenic crust or mantle source as a consequence of being fluxed by as the result of modification of the sub-arc mantle anomalously high amounts of slab-derived fluids. related to tectonic erosion of the forearc crust. Ne- This highly wet character of enclaves is put in evi- vado de Longaví volcano (NLV; 36°12’S – 71°10’ W) dence by an amphibole rich (30 vol %) mineralogy located just to the south of the region that has been and a notably oxidized (NNO+2) character. strongly affected by Tertiary crustal shortening and The low incompatible element contents of NLV thickening and associated eastward arc migration, is dacites are inconsistent with assimilation of upper the only occurrence of Quaternary magmas with an crustal rocks as well as with fractionation of typical unequivocal adakitic signature in the accessible part anhydrous pyroxene+plagioclase dominated assem- of the Andean Southern Volcanic Zone (SVZ: 33-41° blages that are proposed for the rest of the arc. Nev- S). In this contribution we propose a fractional crys- ertheless, the similarities between adakitic dacites tallization model to explain the occurrence of ada- and mafic quenched enclaves (low incompatible kites at NLV related to highly hydrous mafic melts. element contents, oxidized, water-rich), suggest a NLV is a mainly andesitic late Quaternary edifice possible cogenetic relation between them. In order to whose magmatic suite progressively evolves from evaluate the feasibility of fractional crystallization to early basalts and basaltic andesites towards dacites produce the NLV adakitic melts from the wet mafic by a trend characterized by a low increase rate of melts, we have developed a major elements mass- K2O and other incompatible trace elements com- balance model combined with Rayleigh fractional pared to the rest of SVZ volcanoes, and decreasing crystallization that considers 50% fractionation of an concentrations of Y and HREE, contrary to the ob- assemblage composed of: 0.5 Hbl + 0.37 Plag + served trend for the other volcanoes in the arc. 0.07 Opx + 0.03 Aug + 0.03 Mgt + 0.007 Ap + 0.02 These features are most extreme in the Holocene Gt. This model successfully reproduce the major and dacitic products (63-65 wt% SiO2) which are char- trace element characteristics of NLV dacites and is acterized by high Sr and low incompatible element supported by abundant amphibole-bearing cumu- contents (especially K, Y and HREE), a mineral as- lates whose mineralogy agrees with the assemblage semblage with amphibole as the main mafic phase, considered (except for garnet). This combination of an unusually high fO2 (NNO+1.7) and elevated water fractionating phases also explains the observed contents of 5-6 wt % H2O (as inferred from experi- Y+HREE depletions in NLV andesites and dacites mental results on closely comparable Pinatubo 1991 relative to mafic magmas, as well as minimal en- dacite; Scaillet and Evans 1999). On the other hand, richments in elements that are incompatible relative mafic magmas preserved as enclaves on these to anhydrous silicates. dacites (53-56 wt% SiO2; MgO <6 wt%) are in many The proposed model requires high water contents REFERENCES in the mafic melts in order to stabilize early amphi- bole instead of anhydrous mafic phases and to re- Grove, T., Elkins-Tanton, L., Parman, S., Chatterjee, N., duce the stability field of plagioclase (e.g. Grove et Müntener, O. & Gaetani, G. (2003): Fractional crystalliza- tion and mantle-melting controls on calc-alkaline differen- al. 2003). Highly wet melts also permit the crystalli- tiation trends. Contributions to Mineralogy and Petrology, zation of garnet at crustal pressures similar to those Vol. 145, No. 5, p.515-533. expected in the lower crust under NLV (35-40 km) Müntener, O., Kelemen, P. & Grove, T. (2001): The role of H2O (Müntener et al. 2001; Ulmer et al. 2003). The pro- during crystallization of primitive arc magmas under up- jection of the Mocha fracture zone (Eocene-age permost mantle conditions and genesis of igneous pyrox- enites: an experimental study. Contributions to Mineralogy Nazca plate) under NLV is our favored candidate to and Petrology, Vol.141, p.643-658. explain the occurrence of the unusually wet mafic Scaillet, B. & Evans, B. (1999): The June 15, 1991 eruption of melts at NLV (Sellés et al. 2004). This feature of the Mount Pinatubo. I. Phase equilibria and pre-eruption of P- slab probably contains serpentinized bodies that de- T-fO2-H2O conditions of the dacite magma. Journal of Pe- hydrate during subduction and release large trology, Vol.40, p. 381-411. amounts of fluid to the mantle, resulting in high de- Sellés, D., Rodríguez, C., Dungan, M., Naranjo, J. & Gardeweg, M. (2004) : Geochemistry of Nevado de Lon- grees of melting and the generation of water-rich gaví volcano (36.2°S): a compositionally atypical arc vol- mafic melts. This also explains the extremely local cano in the Southern Volcanic Zone of the Andes. Revista occurrence of the NLV adakites in the geodynamic Geológica de Chile, Vol.31, No.2, p.293-315. context of SVZ. Ulmer, P., Müntener, O. & Alonso-Pérez, R. (2003): Potencial role of garnet fractionation in H2O-undersaturated andesite We propose that adakites in cold subduction liquids at high pressure: an experimental study and a com- zones can alternatively be formed by fractional parison with the Kohistan arc. Geophysical Research Ab- crystallization of amphibole-rich assemblages from stract, Vol.5, 08308. hydrous mafic melts. NLV constitutes a case of study in which lack of evidence for upper crustal assimila- tion, hornblende-bearing cumulates throughout the volcano, increasing modal abundances of horn- blende toward Holocene magmas, and the unusually incompatible element-depleted character of the melts appear to be ultimately related to an excep- tionally high fluid-flux from the subducted Mocha Fracture Zone which projects beneath NLV, and which is inferred to have generated water-rich but in- compatible element-poor basalts through flux- melting. Lake sediments and Medieval Climate Anomaly (MCA) and Little Ice Age Type Events (LIATES) in the South-Central Andes of Chile

*Salvetti, Ch, *von Gunten, L. & **Grosjean, M.

* Institute of Geography, University of Bern, Hallerstr. 12, CH-3012 Bern ** NCCR Climate, Erlachstr. 9a, CH-3012 Bern [email protected]; [email protected]; [email protected]

THE PROJECT THE RESEARCH PLAN

In order to understand the interhemispheric telecon- During the field campaign in March 2004 short sedi- nections and global climatic change, the lack of pa- ment cores (38-48 cm) from several potential study leoclimate data from the Southern Hemisphere has lakes at altitudes between 2500 and 3400 m a.s.l. to be overcome. South-Central Chile is located at the were retrieved. Preliminary sediment analysis show windward side of the Andes and at the northern limit in some cores structures at the (sub-) cm scale. On of the Southern Westerlies influence, with dry sum- the basis of these results, we select the 3-4 most mers and humid winters. This area is, therefore, ex- promising lakes for further investigations including a pected to be sensitive to climatic changes (Bertrand second field campaign in October 2005. et al. 2005). In a next step these lakes, their catchments and Our project aims at producing a (sub)decadal, additional sediment cores will be analysed using se- multi-proxy paleoclimate reconstruction for the last lected geochemical and sedimentological standard ca. 1000 years based on sediments from high- methods, dating methods and digital image proc- elevation pro-glacial lakes in the Central Andes of essing. Chile (33°S). The study of this period of time can provide information that may answer the question to what extent the 20th century is unusual in the light of PRELIMINARY RESULTS the recent past. Lake catchment analysis. Data from SRTM (Shut- tle Radar Topography Mission) were obtained in or- der to compute digital elevation models for the lake catchments. Orthofotographies are calculated based on topographical maps (1:50’000) and aerial views. This information will serve as a basis for detailed geomorphological maps and description of surface processes within the lake catchments.

Geochemical and sedimentological analysis. Our data set contains the following parameters for all sediment cores: magnetic susceptibility, density (_- ray absorption), C, N measurement and biogenic silica measurements, grain size, thin section and smear-slide analysis.

Dating. Radiogenic Pb and Cs were measured and show that dating methods work in this environment. Additional 14C Dating was carried out. Fig. 1: Overview of the study site cored lakes Table 1. Preliminary results from geochemical and For selected windows of time during the last 1000 sedimentological analysis years (Medieval Climate Anomaly, Late Maunder Minimum and Dalton Minimum) our data set will be Laguna El Laguna del Laguna Ne- Laguna del compared with data derived from GCM ensemble Ocho Encañado gra Inca runs carried out within the framework of NCCR Cli- C/N ratio 6.10 11.01 6.64 6.24 mate (e.g. Raible et al., 2005). biSi [%] 12.71 1.53 2.38 2.48 Ø grain size 6.11 11.01 13.72 31.25 [_m] REFERENCES

Bertrand, S. et al. (2005): Temporal evolution of sediment sup- ply in Lago Puyehue (Southern Chile) during the last 600 yr RESEARCH QUESTIONS and its climatic significance. Quaternary Research, 64, 163- 175. 1. Which climate parameters and processes are Luckmann, B., Villalba, R. (2001): Assessing the Synchroneity recorded today in the lake sediments? of Glacier Fluctuations in the Western Cordillera of the Americas during the last Millennium. In: Markgraf, V. (Ed) 2. Can this information be used for downcore ex- Interhemispheric Climate Linkages, Academic Press, San trapolation for the last ca. 1000 years and cli- Diego. mate reconstruction? Raible, C.C, Stocker, T.F., Yoshimori, M., Renold, M., Beyerle, 3. How do the sediment records compare with C., Casty, C., Luterbacher, J. (2005): Northern hemispheric other regional climate reconstructions in adja- trends of pressure indices and atmospheric circulation pat- cent areas (Luckmann & Villalba 2001; Villalba terns on observations, reconstructions and coupled GCM simulations. Journal of Climatology, in press. et al. 2003) and inter-hemispheric, circum- Villalba, R. et al. (2003): Large-scale temperature changes Pacific or global phenomena for the recent across the southern Andes: 20th century variations in the past? context of the past 400 years. Climatic Change, 59, 177- 232. Garnet phenocrysts in Early Miocene intrusives in Central Chile. Evidence for a crystal fractionation origin of adakite-like magmas

Sellés, D., Dungan, M., *Gana, P. & Rodríguez, C.

Department of Mineralogy, Earth Sciences Section, University of Geneva, Switzerland * National Geology and Mining Survey, SERNAGEOMIN, Chile

The Oligocene magmatism of Central Chile consti- ritic in texture, with plagioclase and amphibole as the tutes a tholeiitic suite generated during a period of main phenocrysts. They have a distinctive chemistry crustal thinning and mantle upwelling. MORB-like relative to other magmas of the area in having low 87 86 REE patterns together with low Sr/ Sri ratios and incompatible element contents (K, Rb, Zr, etc.), high high e-Nd values are consistent with relatively dry Sr and positive Eu anomalies, and high La/Yb and mantle-derived magmas that did not assimilate im- Ce/Y ratios. Compared to the modern SVZ, Man- portant amounts of crustal material and that evolved quehue-type stocks have very modest increases in dominantly by low-pressure fractional crystallization. incompatible elements relative to silica, similar to the In the area of Santiago (33-34°S), the volcanic se- trend depicted by Nevado de Longaví volcano low- quence is cross-cut by coeval to slightly younger Rb magmas (Sellés et al. 2004). Both Manquehue subvolcanic intrusive bodies, most of which are iso- and Longaví differ from Quaternary volcanics of the topically and chemically similar to the effusive rocks northern Southern Volcanic Zone (SVZ) in that high and can be considered the plutonic roots of the Oli- La/Yb ratios are not accompanied by substantial en- gocene arc. However, a subset of slightly younger richments of incompatible elements, which argues intrusive bodies (20-15 Ma; Manquehue-type stocks) against crustal sources for the garnet signature. In- shows a distinctive adakitic chemical signature that terestingly, the less evolved compositions are is not observed in previous or subsequent volcanics. broadly similar to mafic magmas from the SVZ, sug- The front of the volcanic activity shifted eastwards gesting a common mantle source but different evolu- immediately after or simultaneously with intrusion of tionary paths. The absence of significant crustal these adakitic stocks, coinciding with the beginning contamination is further demonstrated by Sr and Nd of a period of increased convergence rate and pro- isotopic ratios. e-Nd values of Manquehue-type gressive crustal thickening and uplift. Compared to stocks are within the range of Oligocene lavas (+6 to the Oligocene lavas, the Miocene arc magmas are +5), although initial 87Sr/86Sr ratios are slightly higher wetter and they incorporated higher proportions of (0.7038-0.7042). Sr and Nd isotopic compositions crustal material, and the residual mineral assem- are moreover uncorrelated with increasing La/Yb blage suggests higher pressures of fractionation with (Figure 1). the involvement of some amphibole but no garnet. At Although garnet is often inferred to make part of the latitude of Santiago, no other Neogene magmatic residual assemblages of continental arc magmas, it unit is known to show a residual garnet signature, is rarely actually observed because it tends to frac- and it is only in Quaternary times that the volcanic tionate from the host magma or is resorbed at low arc, located above a ~60 km thick crust, indicates pressure conditions. One of the Manquehue-type again garnet involvement coupled to crustal con- stocks preserves euhedral to subhedral garnet tamination. In the El Teniente Area (34°S), however, crystals up to 2 mm in diameter. The host rock is al- intrusive units related to porphyry copper mineraliza- most aphanitic in texture, the only phenocrysts being tion show garnet signature during the latest Miocene ~1% garnet and ~5% plagioclase (An40-35). Despite to Pliocene (6-4 Ma; Kay et al. 2004). this particular mineralogy, this body is chemically The adakitic Manquehue-type stocks in the area similar to the amphibole-bearing stocks. Garnet of Santiago are low- to medium-K andesites to crystals are unzoned, almandine-rich (Al73-Py11-Sp6- dacites (56-72% SiO2). They are commonly porphy- Gr10), and contain abundant randomly oriented inclu- sions, mainly needle-like apatite, iron-rich orthopy- Previous interpretations on the petrogenesis of these roxene, Fe-Ti oxides and minor amphibole. Garnets stocks have assumed that garnet was a restitic of similar composition have been interpreted else- phase, either from a subducted slab source or from where as a primary phase crystallizing from hydrous crustal slivers dragged down by subduction erosion mantle-derived calc-alkaline melts. Recent experi- (Kay et al. 2004). Newly acquired isotopic data and ments have proven that garnet can crystallize at petrologic observations suggest that garnet was a moderately high pressures (≥0.8 GPa) from hydrous primary phase, crystallizing from intermediate hy- and oxidized andesitic magmas (e.g. Müntener et al. drous mantle melts evolved at lower to middle 2004). crustal pressures. The hydrous character of Man- quehue-type magmas is suggested by the abun- dance of amphibole and paucity of pyroxene phe- nocrysts. Also, high Al2O3, Sr and positive Eu anomalies indicate that plagioclase did not thor- oughly fractionate, which is also expected to happen in hydrous melts. Moreover, enrichments of fluid- mobile trace elements over less mobile ones sug- gest important participation of slab-derived fluids. In the Quaternary Nevado de Longaví volcano (Sellés et al. 2004), similar characteristics are interpreted to be consequence of the subduction of an oceanic fracture zone that can host serpentinized bodies, potentially efficient water carriers to the subarc man- tle. Fluids released upon serpentine breakdown should have the isotopic composition of sea water, which could explain the relatively elevated Sr iso- topic ratios. Elevated fluid flux to the mantle gener- ates high-degree hydrous melts that evolve mainly by fractionation of amphibole, keeping incompatible element contents low. The recognition of two independent cases of wa- ter-rich, mantle-derived adakitic magmas in the An- dean context opens new perspectives in the inter- pretation of high-La/Yb arc magmas. Figure 1. Sr and Nd isotopic composition of Manquehue-type stocks compared SVZ magmas (top) and Cenozoic magmatic units at 34°S (bottom, modified from Kay et al. 2004). REFERENCES

Kay, S.M., Godoy, E., Kurtz, A. (2004): Episodic arc migration, crustal thickening, subduction erosion, and magmatism in the south-central Andes. GSA Bulletin, v.116, no. 11/12. DOI: 10.1130/B25431.1. Müntener, O., Perez-Alonso, R. & Ulmer, P. (2004): Phase re- lations of garnet, amphibole and plagioclase in H2O under- saturated andesite liquids at high pressure and implications for the genesis of lower arc crust. Geophysical Research Abstracts, v. 6: 05183. Sellés, D., Rodríguez, A.C., Dungan, M.A., Naranjo, J.A., Gardeweg, M. (2004): Geochemistry of Nevado de Longaví volcano (36.2°S): a compositionally atypical arc volcano in the Southern Volcanic Zone of the Andes. Revista Geológica de Chile, v. 31, No. 2, p. 293-315. Controls of gravitational mass wasting on the geomorphic evolution of headwaters: The ‘Lluta collapse’, northern Chile

*Strasser, M., & **Schlunegger, F.

* Geological Institute, ETH Zurich, Switzerland, [email protected] ** Institute of Geological Sciences, University of Bern, Switzerland, [email protected]

The ‘Lluta collapse’ is a prominent geomorphic fea- The source area of the ‘Lluta collapse’ is bor- ture in the landscape of the western escarpment of dered by an amphitheater-shaped scarp. This scarp- the Andes of northern Chile that resulted from line – corresponding to the first-order geometric probably one of the oldest recognizable landslide (> length-scale in the landscape – is made up of coa- 2.5 Ma) in a continental setting, and from subse- lescing units of lower-ordered length-scales that also quent modification of the landslide scar by backward display concave geometries, and that have resulted erosion. The combination of geomorphic and geo- from hillslope mass wasting. It appears, therefore, logical information from the ‘Lluta collapse’ and that the geomorphic evolution of the ‘Lluta collapse’ sedimentological observations from the landslide and the establishment of a dendritic geometry in the deposits imply that a total of 25 km3 of mass was headwaters have been governed to large extents by displaced by landsliding. Subsequent modification of mass wasting processes of different length-scales. In the landslide scar occurred by backward erosion, re- contrast, high-concentrated flows have controlled the sulting in the establishment of a dendritic drainage export of mass, which, in turn, has allowed the base- network and the removal of an additional ca. 24 km3 level to lower to sufficiently low magnitudes to initiate of material. It appears that this mass was produced further landslides. Hence, the data suggest that by mass wasting in the headwaters, and exported by whereas the geometrical development of the ‘Lluta high-concentrated debris flows in channels. collapse’ has been controlled by gravitational mass wasting, the rates of the development of this geo- morphic unit have been limited by the export rates of mass and hence by the transport capacity of the flows. Depositional model for the Quaternary Zarzal Formation (Colombia) and its stratigraphic relationship with the volcaniclastic mass flows derived from the Central Cordillera

*Suter, F., *Neuwerth, R., **Guzman, C. & *Gorin, G.

* Department of Geology-Paleontology, University of Geneva, Switzerland [email protected], [email protected], [email protected]

** Department of Geological Sciences, University of Caldas, Manizales, Colombia [email protected]

The interandean Cauca depression corresponds to The Cauca Depression has been exposed to a a deep sedimentary basin (Fig. 1) filled by regional oblique compressive tectonic regime, approximately 6000 meters of sediments ranging in which has generated different pull-apart basins age from Early Tertiary to recent. This research along the Cauca-Romeral fault system. The latter attempts to establish the mechanisms, which may defines the boundary between continental and have led to the opening of this basin by studying oceanic basements. From west to east, the studied the fluvio-lacustrine Zarzal Formation (Neuwerth et area comprises the foothills of the Western al., 2005). This formation is associated with the Cordillera, the Cauca River valley, the folded latest distensive tectonic phase and related with the Tertiary sediments of the Serranía de Sta Barbara volcaniclastic mass flows derived from the Central Cordillera, which form the volcaniclastic fans of and the La Vieja River valley (Fig. 1B and C). The ta Quindío, Pereira and Cartago (Fig. 1C; Gorin et al. Serranía de S Barbara forms a natural barrier this symposium; Guarin et al., 2005). between the western (Valle del Cauca) and eastern (Quindío) parts of the basin, which coalesce in the Cartago Fan to the north.

In the eastern part of the basin and in the Cartago Fan (Fig. 1C), the fluvio-lacustrine sediments of the Zarzal Formation have a clear volcanic origin and are interbedded with volcanic mass flows. In the western part of the basin, the Zarzal Figure 1. A) Location of study area. B) Digital Elevation Formation is sourced from the Tertiary sediments ta Model (DEM) of the studied area and its surroundings. of the Serranía de S Barbara and from the Location of profiles AA' and BB' shown in Figure 2. C) Western Cordillera (Fig. 1C). Preliminary Simplified geological map of studied area. palynological investigations have yielded an age of less than 1 m.y. Sediments of the Zarzal Fm exhibit numerous soft-sediment deformations interpreted as seismites (Neuwerth et al., 2005), which prove the important seismic activity of this area. Figure 2. Relation between field observations and the proposed depositional model (see figure 1B for location of profiles). Gravitational flows are represented in grey and black, fluvio-lacustrine sedi- ments in white. This model does not take into account the tectonic activity. More- over, the frequency of gravi- tational flows underesti- mated.

Field data and palynological results allow the This research is supported by the Swiss National proposal of a depositional model (Fig. 2). The Science Foundation (grant no. 21-67080.01). Part folding of the Serranía de Sta Barbara, subdivided of the field work was supported by the Swiss the basin into two parts. While the western part Academy of Sciences. was infilled by the fluvial sediments of a paleo- Cauca River, volcanic mass flows derived from the Central Cordillera accumulated in the eastern part. REFERENCE Each of these mass flow unit behaved like a topographical barrier temporarily damming the Gorin, G., Guarin, F., Neuwerth, R., Suter, F., Espinosa, A. & Guzman, C. (2005): Contribution of Quaternary sediments basin and creating a lake. Following the erosion of to the understanding of the tectonic history in Central the damming mass flow, the lake dried out and Colombia: the volcaniclastic fans in Quindío-Risaralda turned into a floodplain. This cycle was repeated and the Zarzal Formation in the Cauca Valley. 3rd Swiss Geosc. Meeting Zurich 2005. each time a new significant mass flow dammed the Guarin, F., Gorin, G.& Espinosa, A. (2005): A Pleistocene valley. The eastern basin continued to infill until the stacked succession of volcanic mass flows in Central volcanic mass flows could flow over the lower- Colombia: the Quindío-Risaralda Fan. Acta Vulcanologica relief, northern part of the Serranía de Sta Barbara (in press). Guarin, F., Gorin, G.& Espinosa, A. (2005): The interandean and spread into the Cauca Valley to form the Quindío-Risaralda basin in Central Colombia and its Cartago Fan (Fig. 1C). Subsidence rates in the Pleistocene infill by stacked volcaniclastic mass flows Cauca Valley seem to have been higher than in the derived from the Central Cordillera. 3rd Swiss Geosc. eastern part. Meeting, Zurich 2005. Neuwerth, R., Suter, F., Guzman, C. & Gorin, G. (2005): Soft- These preliminary results establish the sediment deformations in a tectonically active area : the synchroneity of the deposition of the Zarzal Fm Pleistocene Zarzal Formation in the Cauca Valley with that of the volcaniclastic fans. The Zarzal Fm (Western Colombia). Sedimentary Geology (in press). bears the imprint of an intense tectonic activity partly related with the deposition of the fans (Guarin et al., 2005 and this symposium). These joint studies will help to refine the dynamic interpretation of this basin. Nature and origin of the Interandean Depression in Ecuador

*Villagomez, D., *Spikings, R., **Winkler W. & *Gorin G.

* Section des Sciences de la Terre, Université de Genève, Rue des Maraîchers 13, CH-1211 Genève 4, Switzerland [email protected] ** Geologisches Institut, ETH-Zentrum, Zurich CH-8092, Switzerland, [email protected]

The Ecuadorian Andean mountains comprise the to- We performed geochemical and isotopic analyses pographically distinct Eastern Cordillera and West- of various ultramafic and mafic rocks that comprise ern Cordillera. These topographic ridges are sepa- the basement of the IAD within the depression and rated by the Interandean Depression (IAD), which is along its bounding faults (Pallatanga, Rio Cala and an elongate, tectonic structure that has been active Peltetec Units). 40Ar/39Ar analyses of stratified vol- since the Late Miocene, resulting in an extensive to- canic tuffs exposed in intermontane basins in the pographic depression, and the formation of isolated IAD are in progress. intermontane basins. The IAD straddles the dis- membered Late Cretaceous suture between al- lochthonous oceanic rocks exposed in the Western RESULTS - Mafic basement Cordillera and continental crust in the Eastern Cor- Basement rocks exposed in the northern IAD are dillera. Within Ecuador, the origin and composition of geochemically similar to the Rio Cala arc lavas, and the basement of the IAD is mainly unknown because are characterised by high LILE/HFSE ratios, nega- of the extremely restricted extent of basement inliers tive primitive mantle normalised Nb-Ta anomalies within the post-Oligocene volcanic cover. and LREE enrichment relative to HREE’s ((La/Yb)n This contribution aims to determine the tectonic ~6.) The IAD basement in the south is geochemically provenance and the age of the basement of the IAD similar to the Pallatanga Unit, and is characterised and to reconstruct the post-Late Miocene tectonic by enriched LILE’s and HFSE’s relative to primitive history of the IAD structure, based on field observa- mantle and flat REE patterns ((La/Yb)n ~0.9). Mafic tions and radiometric ages of stratified volcanoclastic rocks of the Peltetec Unit yield both plateau-like rocks. ((La/Yb)n 1.3 – 1.8) and subduction related ((La/Yb)n >2.4) characteristics. These preliminary data reveal broad geochemical BACKGROUND INFORMATION similarities between the basement of the IAD and allochthonous, Late Cretaceous mafic rocks of the The crystalline Cretaceous basement of the Western Western Cordillera, implying that the plateau and Cordillera partly comprises Late Cretaceous oceanic subduction derived basement units of the Western plateau basalts (Pallatanga Terrane), which are in Cordillera extend beneath the IAD. Furthermore, the tectonic contact with a Late Cretaceous, tholeiitic is- same Late Cretaceous basement units may also land arc sequence (Rio Cala Arc). These units ac- crop-out along the western flank of the Eastern Cor- creted against the continental margin during the Late dillera, where they either partly or completely com- Cretaceous (Hughes and Pilatasig, 2002; Jaillard et prise the Peltetec Unit. Consequently, pending im- al. 2004; Spikings et al., 2005). The suture is partly minent geochronological (40Ar/39Ar) analyses of the represented by the Calacali-Pallatanga Fault, which undated Peltetec Unit, there is no evidence for the defines the western border of the IAD. Metamor- existence of a previously proposed (Litherland et al., phosed continental rocks comprise the Eastern Cor- 1994) Early Cretaceous terrane beneath the IAD. dillera, which is juxtaposed against the IAD via the The Peltetec fault may also represent part of the Peltetec Fault. Undated slivers of mafic rocks crop- Late Cretaceous, ocean-continent suture in Ecuador. out in anastomosed zones along the Peltetec Fault. RESULTS - Post-Late Miocene history of the IAD 2005). Syn-sedimentary deformation prevailed dur- Radiometric analyses of basal volcanic deposits ing most of the life-span of the IAD (Spikings and show that the intermontane basins within the IAD Crowhurst 2004). young from ~ 6 Ma in the north to ~ 3 Ma in the south (Winkler et al. 2005). Prior to 5.5 Ma, there ACKNOWLEDGEMENTS was no IAD, implying that one cordillera existed in Ecuador (Spikings and Crowhurst 2004, Spikings et DV was supported by a Swiss Federal Government al. 2005). The basement of the IAD probably shares Grant (2004-2005). the same origin as that exposed in the Western Cor- dillera, although it has since been segmented by fault activity. The complicated and dense fault array REFERENCES within the IAD and its basement has resulted in a segmented morphology and has controlled the sites Jaillard E., Ordoñez M., Suárez J., Toro J., Iza D. Lugo W. of volcanic emplacement. (2004): Stratigraphy of the late Cretaceous–Paleogene de- posits of the cordillera occidental of central ecuador: geo- dynamic implications. Journal of South American Earth CONCLUSIONS Sciences 17: 49-58. Spikings, R., Crowhurst, P. (2004): (U-Th)/He thermochrono- - The basement of the IAD is probably an extension metric constraints on the Late Miocene – Pliocene tectonic of allochthonous, Late Cretaceous accreted oceanic development of the northern Cordillera Real and the Inter- rocks exposed in the WC. andean Depression, Ecuador Journal of South American Earth Sciences 17: 239 – 251. - The undated Peltetec Unit shows broad geochemi- Spikings, R.A., Winkler, W., Hughes, R.A., Handler, R. (2005): cal similarities with Late Cretaceous accreted mafic Thermochronology of the Cordillera Occidental and the Amotape Complex, Ecuador: unravelling the accretionary rocks that are currently exposed in the WC, and and post-accretionary history of the Northern Andes. Tec- hence may also comprise the same Late Cretaceous tonophysics 399: 195 – 220. basement units. Litherland M., Aspden J., Jemielita R. (1994): The metamorphic belts of Ecuador. British Geological Survey, Quito, Over- - Previously identified Late Cretaceous suture zones seas Memoir 11: 147 pp. (e.g. the Calacali-Pallatanga fault zone) may be Mio- Winkler W., Villagomez D., Spikings R., Abegglen P., Tobler S., cene or younger structures and the Late Cretaceous Eguez A. (2005): The Chota basin and its significance for ocean-continent suture is probably located further to the inception and tectonic setting of the Inter-Andean De- the east, partly comprising the Peltetec Fault. pression in Ecuador. Journal of South American Earth Sci- ences 19: 5-19. - The IAD began to open at ca. 6-5 Ma in northern Ecuador and propagated southward (Winkler et al. Constraints from fission track analysis on the evolution of the Rio Tinguiririca valley area in the Main Cordillera of the Andes, Central Chile

*Waite, K., **Fügenschuh, B. & ***Schmidt, S.

* Institute of Mineralogy and Petrology, University of Basel, Bernoullistrasse 30, 4056 Basel, Switzerland, [email protected] ** Institute of Geology and Paleontology, University of Innsbruck, Austria *** Department of Mineralogy, University of Geneva, Switzerland

In the Rio Tinguiririca valley in the Main Cordillera of metamorphic conditions at different times in different the Andes of central Chile, 35° south, parts of a parts of the study area. stratigraphic section ranging from the late Jurassic to A new model is proposed for the tectonic evolu- the Quaternary are exposed. Fission track analysis tion of the study area. Fission track analysis of de- was carried out on samples from all the stratigraphic trital zircons from the Brownish-Red Clastic Unit units exposed in the area in order to gain information shows that the unit must have been deposited during on the low-grade metamorphic history of the Central the latest Cretaceous (Maastrichtian) and that it is Andes and to test older models for the metamorphic certainly younger than the White Tuff. and tectonic evolution of the area. Thermal modeling shows that considerable ex- The sequence exposed in the Rio Tinguiririca humation of the lower part of the Rio Damas Forma- valley is characterised by several distinctive features tion occurred during the Late Cretaceous to Early not found in other localities in the Central Andes Tertiary. This exhumation is thought to be connected (Charrier et al., 1996); the deposits of the middle to tilting and erosion of the Mesozoic units in the Cretaceous (Aptian to Albian) Colimapu Formation area prior to the formation of an extensional basin in and of the middle to late Micoene Farellones Forma- the Late Eocene. Data from the Eocene to Miocene tion are completely missing in the study area. In- Coya Machali Formation imply that sedimentation stead, a volcanic tuff layer, the White Tuff, and a unit within the Tertiary basin continued somewhat longer consisting of fan deposits and alluvial plane depos- than hitherto supposed. its, the Brownish-Red Clastic Unit, unconformably overlie the Late Jurassic deposits of the Baños del Flaco Formation. REFERENCE The fission track data give some indications of the style and timing of metamorphic events in the Charrier, R., Wyss, A.R., Flynn, J.J., Swisher III, C.C., Norell, study area, enable more accurate constraint of the M.A., Zapatta, F., McKenna, M.C., and Novacek, M.J. age of the Brownish-Red Clastic Unit and allow (1996): New evidence for Late Mesozoic-Early Cenozoic some statements on the tectonic evolution of the evolution of the Chilean Andes in the Upper Tinguiririca study area from the Late Jurassic to present. Valley (35[deg]S), Central Chile: Journal of South American Earth Sciences 9: 393-422. Burial metamorphism has been proposed by various authors as the main mechanism to produce large suites of rocks altered at low grades in the Central Andes. The results of this study indicate that, on the contrary, hydrothermal alteration connected to magmatic and/or volcanic activity was the main cause of alteration of the rocks and that burial metamorphism played at most a very minor role. Pulses of hydrothermal activity appear to have oc- curred from Cretaceous to almost recent times and led to alteration of the rocks at slightly varying The Andean Cordillera of Ecuador: timing and mode of orogenic growth as revealed from sediments in the Amazon Basin (heavy minerals and detrital zircon fission-tracks)

Winkler, W., Seward, D., Ruiz, G.M.H. & Martin-Gombojav, N.

Department of Earth Sciences, Geological Institute, ETH Zurich, Switzerland

The Ecuadorian Andean Cordillera represents a the clastic supply since the Late Oligocene (post- double-vergent orogenic belt, which generally com- dating a major accretionary event in the forearc). prises accreted oceanic plateau and arc series in the From other circumstantial arguments, it can be sug- Cordillera Occidental (CO), and metamorphic conti- gested that since the Late Miocene an orographic nental basement and sedimentary series in the Cor- situation similar to today has existed. dillera Real (CR) (e.g. Spikings et al. 2001, 2005). The exhumation age of the detrital zircons was The Andean Amazon Basin (AAB) of Ecuador devel- measured in the same sandstones as used for the oped to the east of the evolving Andean chain and to heavy mineral analysis in the northern Subandean the west of the Amazon craton (Guyana Shield) from Zone (e.g. Ruiz et al. 2004). Various populations of the mid-Cretaceous to the Recent. At least since the detrital zircon FT ages were discriminated by statisti- Maastrichtian (Tena Fm.), the basin shows the typi- cal methods in the sandstone samples. The calcu- cal characteristics of a variable, very shallow marine lated lagtime (cooling/closure age minus depositional to continental facies retro-arc foreland basin with re- age), in assuming negligible time of transport of the spect to the growing orogen. Detrital provenance grains into the basin, describes the time necessary analyses (heavy minerals), and fission track (FT) for exhuming the source rocks from depth (ca thermochronology of detrital zircons (source rock 260oC) to the surface with subsequent re- exhumation ages) are used for monitoring the long sedimentation. Typical lagtimes are in the range lasting history of the supplying Andean chain and between 0 My to 400 My (Fig. 1). The absence of adjacent regions. The investigated samples are de- lagtimes shorter than depositional ages proves that rived from the northern and southern Subandean no post-depositional heating at the zircon FT an- zone (including the Napo and Tena uplift areas, re- nealing temperature has occurred. spectively) and proximal parts of the Oriente Basin. The zircon FT age populations describe distinct In the mid- to Late Cretaceous sediments (Hollin population paths in time (D1-n in Fig. 1); the young- and Napo fms.), zircon-tourmaline-rutile dominated est population path (D1) is believed to portray heavy mineral spectra (ZTR association) imply that closely the tectonic activity in the sediment source the basin was supplied from non- and very low-grade terranes. This is corroborated by the combined metamorphic granitic source rocks (referred to as heavy mineral analysis. In considering two extreme shallow continental crust provenance). Since the cases: (1) decreasing lagtime, going even to zero is Maastrichtian (Tena Fm.) and during the Paleogene, associated with increasing supply from metamorphic an increasing amount of detrital medium-grade source rocks, and (2) increase of lagtime correlates metamorphic grains (epidote-group, chloritoid and with change to a new source blocks, which have ex- garnet) is observed. This is culminating with the fur- perienced earlier cooling. However, a steady lagtime ther occurrence of high-grade metamorphic grains at frequent changes of sources is observed during (kyanite, sillimanite) during the Neogene. This trend the last 10-15 Ma of the orogenesis (Fig. 1). shows the continuous exhumation of deeper crustal levels in the supplying CR. The additional appear- The orogenic growth of the Andean cordillera in ance of mafic minerals (augite, hypersthene, diop- Ecuador is summarized as follows: The Early Creta- side, chromian spinel, olivine) infers that oceanic ceous Peltetec compressive tectonic event has given basement rocks in the CO started to contributed to rise of deep erosion of the earlier Misahualli volcanic arc, and the creation of a primordial CR. The subse- ACKNOWLEDGEMENTS quently forming Amazon Basin (Hollin and Napo fms.) was supplied from the very slowly exhuming This work was supported by the Swiss Science Amazon craton (D1), from Paleozoic-Early Mesozoic Foundation Grants No. 21-050844.97 and 20- magmatic/volcanic and sedimentary rocks (D2), and 056794-99. from the moderately exhuming CR (D3) (Fig. 1). De- creasing lagtimes to zero during upper Napo and Tena time (Santonian-Maastrichtian) correlate with starting exhumation of medium-grade metamorphic REFERENCES source rocks in the rapidly exhuming CR, and with the vanishing of the Amazon cratonic source to the Spikings, R.A., Winkler, W, Seward, D. & Handler, R. (2001): east, i.e. the CR became the main source of detrital Along strike variations in the thermal and tectonic response of the continental Ecuadorian Andes to the collision with material in proximal parts of the AAB. The corre- heterogeneous oceanic crust. Earth and Planetary Science sponding decrease of lagtimes in path D2 corrobo- Letters 186: 57-73. rates that these supplying rocks also were situated in Ruiz, G. M. H., Seward, D. & Winkler W. (2004): Detrital ther- the rapidly exhuming CR. From Eocen until Oligo- mochronology – a new perspective on hinterland tectonics, cene, the exhumation of very high-grade meta- an example from the Andean Amazon Basin, Ecuador. Ba- morphics, the frequent changes of source sin Research 16: 413-430. Spikings, R.A., Winkler, W., Hughes, R.A., Handler, R. (2005): rocks/drainage, respectively, and the radical de- Thermochronology of the Cordillera Occidental and the crease of lagtimes in path D2 infers a major phase of Amotape Complex, Ecuador: unravelling the accretionary orogenic growth in the CR. Since ca 15-20 Ma (Mio- and post-accretionary history of the Northern Andes. Tec- cene) the constant lagtime (30-40 My) associated tonophysics: 399, 195-220. with frequent change of source rocks represents preferential erosion of different blocks, that were being exhumed, but not sufficiently for the reset zir- cons to reach the surface. The relatively long lag- time, also seen in the modern river sediment, is pos- Figure 1. Detrital zircon fission track age populations correlated sibly due to a regional exhumation event in the Late with heavy mineral variations in the proximal Andean Amazon Eocene-Early Oligocene. Hence, the younger history Basin, including the interpretation of the active source areas of the Andean chain is rather characterized by mod- and dynamics. 1/1 represents the stratigraphic correlation line, D1-D3 are population paths of the detrital zircon FT ages in erate, but generalised uplift. time. Modified from Ruiz et al. (2004). Recent hydrological changes in subtropical Argentina, east of the Andes: the sedimentary record of Salina de Ambargasta

*Zanor, G., *Piovano, E. & **Ariztegui, D.

* CIGES, Universidad Nacional de Córdoba, Argentina [email protected] ** Institute Forel and Department of Geology and Paleontology, University of Geneva, Switzerland

Salina de Ambargasta is a seasonal playa system mudflat boundary (CM/DM) were cored and studied located in subtropical South America (29°S). To- using a quantitative multiproxy approach including gether with other saline environments, Ambargasta petrophysical properties, microstratigrapy and both occupies a topographically closed depression known organic and inorganic geochemistry. Ongoing as Cuenca Saliniana (Álvarez et al., 1990) in the investigations include mineralogy analyses, stable broken foreland of the Sierras Pampeanas of Argen- isotope geochemistry and dating. Density variations tina. This type of broken foreland basin is character- can be clearly identified in both cored sites, however ized by successive basement uplifts produced by strong fluctuations mostly characterize the CM/DM thick-skinned deformation throughout the Tertiary environment. Water content correlates well with period (Jordan & Allmendinger, 1986 and Allmend- density probably caused by a variable content of inger et al., 1997). Regional and local tectonic set- evaporites. Sedimentary cores in both SM and tings play a main role generating orographic rainfall CM/DM areas show very high magnetic susceptibility and, thus, contributing to the prevalence of a truly (MS) values with sharp fluctuations and a decreasing semiarid climate in the region (~500 mm/year). Pre- trend throughout depth. The sedimentary sequence sent average low precipitation rates generate a consists mainly of brownish red and red massive mudflat-ephemeral lake complex. The sedimentary clays, alternating with gray silty clay beds and yellow record of Ambargasta, therefore, is monitoring recent black mineral-bearing sands. Some levels are environmental changes providing a unique archive of evaporite-rich (probably gypsum), either sand or the dominant climatic conditions that have fluctuated clay-sized sediments. The high values in MS could throughout the Quaternary, in the eastern Andes of be related to the reddish colour of the sediments central Argentina. indicating the presence of abundant iron-minerals. Combining geomorphology, instrumental data and Organic matter and carbonates contents in SM and satellite images analyses allowed the characteriza- CM/DM environments are quite small although a tion of the different modern environments and its dy- relative enrichment can be observed in the CM/DM namics. At present, this multicomponent system core. shows major seasonal changes in the dynamics of These preliminary data indicate substantial the subenvironments that are ruled by the regional changes in the hydrological budget that are shown hydrology and climate. Dry mudflats (DM) occupy by a conspicuous response of the sedimentary fa- the highest western portions of the playa whereas cies throughout the Late Quaternary record, which the eastern low areas include ephemeral and inter- range from more clastic-dominated mudflats to mittent lakes, ringed by clastic and saline mudflats ephemeral lake sediments. The results of ongoing (CM and SM). These lakes fill with brine during the mineralogical and geochemical analyses of the Am- early austral summer (December to March) and be- bargasta sedimentary record integrated within a well gin shrinking by evaporation by late summer where constrained chronological framework will allow us to the subenvironment switch to CM and SM surfaces unravel the environmental history of this system until the next rainy season. during the Late Quaternary. The further combination The integrated study of the modern system pro- of this record with results from similar studies vided an analogue to investigate older sediments. steaming from this region of South America (e.g. Thus, the saline mudflat (SM) and the clastic/dry Laguna Mar Chiquita, Piovano et al., 2002) will help to clarify the still controversial role of tropical regions REFERENCES during intervals of global reorganization in the cli- mate system. Allmendinger, R.W., Jordan, T.E., Kay, S.M. & Isacks, B.L. (1997): The evolution of the Altiplano-Puna plateau of the Central Andes. Annual Rev. Earth Planet. Science, 25 (1): 39-74. Álvarez, L.A., Fernández Seveso, F., Pérez, M.A. & Bolatti, N. D. (1989): Interpretación del subsuelo en los bolsones de Sierras Pampeanas en base a la información Geofísica disponible y Geología de superficie. Inédito YPF, Buenos Aires. Jordan, T.E. & Allmendinger, R.W. (1986): The Sierras Pam- peanas of Argentina: a modern analogue of Rocky Moun- tain foreland deformation. American Journal of Science 286: 737-764. Piovano, E., Ariztegui, D. & Damatto Moreiras, S. (2002): Re- cent environmental changes in Laguna Mar Chiquita (Cen- tral Argentina): A sedimentary model for a highly variable saline lake. Sedimentology 49: 1371-1384