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Trace and Rare-Earth Element Characteristics of Acidic Tuffs from Southern Peru and Northern Bolivia and a Fission-Track Age for the Sillar of Arequipa

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Trace and Rare-Earth Element Characteristics of Acidic Tuffs from Southern Peru and Northern Bolivia and a Fission-Track Age for the Sillar of Arequipa Journal ofSouth Amrrican Earth ScirncrJ, Vu!. 9, Nus 112, pp. 91-109, 1996 ~ Pergarnon Copyrighl () 1996 Elscvicr Science Lld & Earth Sciences & Re.~ourccs Institute Printed in Great Britain. All righlS reserved PH: S0895-9811(96)OO030-2 OR95-9R111'J6 115.m + O.lXI Trace and rare-earth element characteristics of acidic tuffs from Southern Peru and Northern Bolivia and a fission-track age for the Sillar of Arequipa IN. VATIN-PERIGNON, lG. POUPEAU, 2RA OLIVER, 3A. LAVENU, lE. LABRIN, IF. KELLER AND 1.2L. BELLOT-GURLET ILaboratoire de Geologie, URA 69 CNRS, Universite Joseph Fourier de Grenoble, 15, rue Maurice Gignoux, 38031 Grenoble Cedex, France 2Institut Laiie-Langevin, BP 156, Avenue des Martyrs, 38042 Grenoble Cedex, France ~ 30RSTOM, UR IH, 213 rue La Fayette, 75480 Paris Cedex 10, France et Laboratoire de geodynamique et modelisation des bassins sedimentaires, IPRA, Universite de Pau et des Pays de o I'Adour, avenue de l'Universite, 64000 Pau, France 0::: Abstract - Trace-element and REE data of glass and purnices of acidic tuffs and related fall deposits erupted in southern Peru >< and nonhern Bolivia between 20 and 0.36 Ma display typical characteristics of subduction related continental arc rnagmatism of W the CYZ with strong LILFlHFSE enrichment and non enrichment of HREE and Y. Geochernical variations of these tuffs are linked to subduction processes and controlled by changes in tectonic regimes which occured with each Quechua tectonic pulse and affected the astenospheric wedge and both the dowgoing and the overriding Iithospheres. During Neogene - Pleistocene times. tuffs erupted in nonhern Bolivia are typically enriched in Zr. Hf. Th. Ba. LREEs and other incompatible elements and incompat­ ible Nb ratios are much higher relative to those erupted from southern Peru. at a given Si02 content (6S--{i7 wt. for dacites. 72-73 wt. % for rhyolites). ZrlHf ratios increase eastward from 27 to 30 and CelYbN ratios from II to 19 reflecting the variation of degree of wedge contribution. Fractionation of the LREE over the HREE and fractionation of incompatible elements may be due to their heterogeneous distribution in the magma source. More highly fractionated REE patterns of Bolivian tuffs than Peruvian tuffs are attributed to variable amounts of contamination of magmas by lower crust. After the Quechua compressional event at -7 Ma. Sr/Y ratios of tuffs of the same age. erupted at IS0-2S0 km or 2S0-400 km from the Peru-Chile trench. increase from southern Peru -_It) to nonhern Bolivia. These differences may be attributed to the subduction of a swarm oceanic lithosphere under the Bolivian Alti­ --0 piano. leading to panial melting of the sudbucted lithosphere. New FT dating of obsidian fragments of the siliar of Arequipa at -"_N 2.42± 0.11 Ma. This tuff dates the last Quechua compressional upper Pliocene phase (-2.S Ma) and confirms that the siliar is not contemporaneous with the Toba 76 tuff or the Perez ignimbrite of nonhern Bolivia. Geochemical characteristics of tuffs erupted -" (-7 -~_0 before and after this last compressional phase remained the same and provide evidence that the upper Miocene Ma) compres­ sional deformations played the most imponant role on the variability of the geochernical characteristics of the southern Peruvian • and northern Bolivian tuft's. Copyright © 1996 Elsevier Science Ltd & Eanh Sciences & Resources Institute INTRODUCTION (CVZ, Thorpe et al., 1984) between 15°S and 18°S. In the Central Andes, large volumes of Neogene acidic These tuffs were emplaced during Neogene and early tuffs cover large areas of the Altiplano of Bolivia and the Quaternary times (Early Miocene to Pleistocene) in the Puna of northern Chile and Argentina. These have been Pacific piedmont and the south of the western Cordillera extensively studied by numerous workers (see Schmitt­ in Peru and in the northern part of the Altiplano and the Riegraf and Pichler, 1988; de Silva, 1989, de Silva and eastern Cordillera piedmont (La Paz and Titicaca basin) Francis, 1989 and refs. therein). Magmas of these volumi­ in Bolivia. A detailed discussion of the geochronological nous silicic pyroclastic flow deposits are considered as evolution of this part of the Bolivian Central Andes based the response to uplift and thickening of the crust during on new K-Ar ages from volcanics including some of these different tectonic pulses (Isacks, 1988; de Silva, 1989, de tuffs is given in a previous work (Lavenu et al., 1989). The Silva and Francis, 1991). A variety of models have been specific goals of the present study are to provide a descrip­ proposed for the genesis of these silicic magmas, includ­ tion of the physical and chemical characteristics of these ing crustal anatexis and combined assimilation-fractional tuffs and to interpret their geochemical variations in tenns crystallization processes and (e.g. Thorpe et al., 1979, of the source region and their tectonomagmatic signifi­ Francis et al., 1980, Hawkesworth et al., 1982, James, cance in the context of changes in tectonic regimes of the 1984, Hannon et al., 1984), melting, assimilation, frac­ southern Peruvian and northern Bolivian Andes between tionation and homogenization (MASH) processes (David­ 20 - 0.3 Ma. In addition, new fission track (Ff) ages of son et al., 1990. Davidson and de Silva, 1992) and large three obsidian fragments from the sillar of Arequipa are scale of melting of middle to lower continental crust, per­ presented and discussed here, because these tuffs have haps in association with lithospheric thinning (Hawkes­ important geotectonic implications. worth and Clarke, 1994, Francis and Ha~kesworth, 1994). GEOLOGICAL BACKGROUND Our study is focused on some tuffs and fall deposits related to the subduction of the Nazca plate beneath west­ Tuffs and related fall deposits of the CVZ, between ern South America in the Andean Central Volcanic Zone 15°S and 18°S, are found along a west-east geographic 91 I 92 N. VATIN-PERIGNON et al. traverse beginning in the Arequipa Province and continu­ ered as a nonwelded ignimbritic tuff (Jenks and Goldich, ing into the northern Bolivian Altiplano (Fig. I). These 1956). This tuff infilled the valleys and was also depos­ tuffs are divided in four groups belonging to the main ited on relative flat surfaces. It is one of the most inter­ units of the morphostructural zoning of this part of the esting tuffs of the Arequipa Province due to its use as a Central Andes. These paleogeographic domains are, from building stone. Lefevre (197~) recognized the high con­ west to east, the Pacific piedmont (coastal range) and the tent of juvenile fragments and the widespread vertical (up western Cordillera (high volcanic arc) in southern Peru, to 200 m thick) and lateral area of dispersal of the SA tuff the Altiplano and the eastern Cordillera in northern and observed the presence, but not everywhere, of com­ Bolivia (La Paz and Titicaca basin on the western flank of paction and welding of pumiceous fragments. A quarry the eastern Cordillera). The study area is located ISO to located along the Quebrada Arias Huayco at the north­ 300 km from the Peru-Chile trench axis over a moder­ western edge of the town of Arequipa shows a partial ately dipping segment of the subducted Nazca plate (aver­ section of the SA tuff. The base is not exposed and the age 25°_30° E from 25- to lOO-km depth) and about lOO­ visible section consists at least of two ash-flow units with 130 km above the top of the seismic zone. The crustal a total thickness of about 10 m. The working of the thickness is about 70 km beneath the western Cordillera quarry aids the recognition of a fine-grained layer at the and the western part of the Altiplano and 50-55 km base of the upper flow unit suggesting different eruptive beneath the eastern Cordillera. The subducted lithosphere pulses (Sparks et al., 1973) difficult. The SA tuff could is separated from the continental crust by a well­ have been an -emplacement during a single volcanic epi­ developed wedge of astenospheric mantle (Grange et al. sode or a series of events in rapid succession (Fig. 2A 1984) and B). A compilation of available K-Ar dates has provided The lower flow unit (3-4 m thick) is a lithic-rich and a geochronology upon which the tectonic evolution of well indurated tuff. At the base, both white and oxidized these regions has been assessed by various authors (e.g. fibreous and vesicular glassy pumices and glass fragments Tosdal, 1981; Bellon and Lefevre, 1976; Noble et al., all tend to be small. Towards the top of this unit, the 1985). More recently, new KJAr dates on undeformed pumices increase in size ( ~ 10-15 cm) and relative abun­ or slightly folded tuffs from northern Bolivia after 10 dance. Lithic clasts (up to 25%) are concentrated particu­ Ma and their lithological position was discussed in larly at the base of the unit and consist mainly of detail by Lavenu (1986) and Lavenu et al., (1989) and subrounded oxidized andesite fragments (~5 cm) of the a revised stratigraphy for the SW Peru has been estab­ underlying lower Barroso formations. Isolated crystals of lished by Sebrier et al., (1988). Four phases of explo­ plagioclase, biotite and magnetite are abundant in the sive volcanism can be considered, using the geological fine- to medium-grained pumiceous and ashy matrix. time scale of Odin (1994): I) the early Miocene sequences 20.3 - 15.8 Ma) from the Pacific piedmont The upper flow unit (6 m exposure in the quarry) is an and the western Cordillera, 2) the middle to late indurated, homogeneous, pumice-rich (30%) rhyolitic tuff Miocene sequences (9.1 - 5.5 Ma) from the western which shows large columnar joints indicating emplace­ Cordillera, the Altiplano especially with the well­ ment at a high temperature.
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