Published online September 25, 2009; doi:10.1130/B26538.1 Geochemical variations in igneous rocks of the Central Andean orocline (13°S to 18°S): Tracing crustal thickening and magma generation through time and space Mirian Mamani1,†, Gerhard Wörner1, and Thierry Sempere2 1Abteilung Geochemie, Geowissenschaftlichen Zentrum der Universität Göttingen, Goldschmidtstrasse 1, D-37077 Göttingen, Germany 2Institut de Recherche pour le Développement and Université de Toulouse Paul Sabatier (SVT-OMP), Laboratoire Mécanismes de Transfert en Géologie, 14 avenue Edouard Belin, F-31400 Toulouse, France ABSTRACT data set to the geological record of uplift and Peru, Bolivia , northern Chile, and northwestern crustal thickening, we observe a correlation Argen tina, covering an area of ~1,300,000 km2. Compositional variations of Central between the composition of magmatic rocks Its width, between the subduction trench and the Andean subduction-related igneous rocks re- and the progression of Andean orogeny. In sub-Andean front, is locally >850 km, and its fl ect the plate-tectonic evolution of this active particular, our results support the interpreta- crustal thickness reaches values >70 km, par- continental margin through time and space. tion that major crustal thickening and uplift ticularly along the main magmatic arc (James, In order to address the effect on magmatism were initiated in the mid-Oligocene (30 Ma) 1971a, 1971b; Kono et al., 1989; Beck et al., of changing subduction geometry and crustal and that crustal thickness has kept increasing 1996; Yuan et al., 2002). The Central Andean evolution of the upper continental plate dur- until present day. Our data do not support de- orocline thus appears to be an extreme case of ing the Andean orogeny, we compiled more lamination as a general cause for major late crustal thickening among the various arc oro- than 1500 major- and trace-element data Miocene uplift in the Central Andes and in- gens of the Pacifi c Ocean margins. Because points, and 650 Sr-, 610 Nd-, and 570 Pb- stead favor continued crustal thickening. this segment does not result from continent- isotopic analyses of Mesozoic-Cenozoic continent collision and displays signifi cant tec- (190–0 Ma) magmatic rocks in southern Peru INTRODUCTION tonic shortening mainly along its eastern half, and northern Chile (Central Andean oro- the origin of its anomalous crustal thickness cline), mostly from new data and the litera- The Andean Cordillera is a classic example remains uncertain (Sempere et al., 2008). The ture. This data set documents compositional of continental subduction-related magmatism. fact that the crust is thickest along the main arc variations of magmas since Jurassic time, These magmas intrude and traverse a continen- (and in some other volcanic areas) is intriguing with a focus on the Neogene period, when tal crust up to 70 km thick en route toward the and has suggested that Central Andean magma- major crustal thickening developed and its surface. Because the composition of these mag- tism participated in thickening the crust (James, infl uence on magma composition was most mas may be strongly affected by interaction 1971b; Kono et al., 1989; James and Sacks, pronounced. We relate the observed varia- with crustal material and because the thickness 1999). Crustal thickening in the Central An- tions in Sr/Y, La/Yb, La/Sm, Sm/Yb, and of the crust has changed through time, the Cen- dean orocline, however, is dominantly believed Dy/Yb ratios, as well as in Sr-, Nd-, and Pb- tral Andes are an excellent natural observatory to have developed since ca. 30 Ma in associa- isotopic ratios, to the crustal structure and to study the interaction between crustal evolu- tion with tectonic shortening (e.g., Isacks, 1988; evolution of the Central Andean orocline. In tion and magma genesis. While the Andes span Sempere et al., 1990, 2008; Gregory-Wodzicki, particular, the evolution of Dy/Yb and Sm/Yb the entire length of the Pacifi c coast of South 2000; Garzione et al., 2008). Magmatic addi- ratios, which track the presence of the higher- America, the Central Andes stand out as the tion to the arc has apparently remained largely pressure minerals amphibole and garnet, re- segment of greatest orogenic volume (Fig. 1). constant, and available estimates are too low to spectively, in the lower crust, documents that Such a contrasted segmentation refl ects the fact explain the observed crustal thickness (Wörner crustal thickness has grown through time. that the Andes have been built by a variety of et al., 2000, 2002; Trumbull et al., 2006). Spatial variations in trace elements and iso- processes that have changed along and across The boundaries of the active Central vol- topic ratios further suggest that crustal do- strike in nature, time, and intensity (Sempere canic zone coincide with those of the Central mains of distinct composition and age have et al., 2008). The Central Andes are themselves Andean orocline. The northern boundary also infl uenced magma composition through some segmented into the northern Central Andes apparently coincides with the edge of the sub- assimilation. The crustal input in Quaternary (5°30′S–~13°S), the Central Andean orocline ducted Nazca Ridge and thus of the present-day magmas is quantifi ed to have been between (~13°S–28°S), and the southern Central Andes “fl at-slab” subduction (Hampel, 2002; Fig. 2). 7% and 18% by simple two-components (28°S–37°S). Among these, the Central Andean Previous studies of Central Andean magma- mixing. When comparing our geochemical orocline is characterized by the thickest con- tism have concluded that the thickened crust has tinental crust of any subduction zone world- strongly affected Andean arc magmas through †E-mail: [email protected] wide (Fig. 1), and it extends over southern crustal contamination (e.g., Wörner et al., 1988; GSA Bulletin; January/February 2010; v. 122; no. 1/2; p. 162–182; doi: 10.1130/B26538.1; 14 fi gures; Data Repository item 2009161. 162 For permission to copy, contact [email protected] © 2009 Geological Society of America Geochemical variations in igneous rocks of the Central Andean orocline GEOLOGIC EVOLUTION OF SOUTHERN PERU AND ndes A ADJACENT AREAS n r NVZ he rt It is widely accepted that the Mesozoic- o Figure 1. Segmentation of the N Cenozoic magmatic and tectonic evolution of nor the Central Andean margin has strongly de- Andes Cordillera (modified t h e C pended on a variety of subduction parameters. after Sempere et al., 2002). The r n e C These include the rate and obliquity of plate Central Andean orocline is by en n tr t a l r convergence, the dip and rollback of the sub- far the segment of largest oro- A a n d l ducting plate, and their variations in time and genic volume, which decreases Fig.2 e a n space, as well as the existence of large geologi- northward and southward. o A r CVZ o cal, topographic, and/or thermal heterogeneities Note that the boundaries of the c n l i n subducted with the oceanic plate. In addition, the Central volcanic zone coincide e Fig.3 d structural and morphological heterogeneities of with those of the Central An- Fig.4A e both plates, the geometrical and thermal evolu- dean orocline. Areas covered by s Figures 2, 3, and 4A are indi- tion of the mantle wedge (e.g., Russo and Silver, n r cated by boxes. Black triangles: e 1996; Heuret et al., 2007; Schellart et al., 2007), h t u and the growing thickness of the continental active Andean volcanic zones o s (NVZ, CVZ, SVZ, AVZ—North, crust (Isacks, 1988; Somoza, 1998; Angermann et al., 1999; Oncken et al., 2006; Sempere et al., Central, South, and Austral vol- SVZ s e 2008) have all played important roles. canic zones, respectively). Topo- d n In southern Peru and northern Chile, arc graphic image is from http:// A n photojournal.jpl.nasa.gov/ r magmatism migrated toward the continent at e h least from the Jurassic to the Paleogene (Pitcher catalog/PIA03388. t u o et al., 1985; Clark et al., 1990; Trumbull et al., S 2006). However, arc-related plutons, such as AVZ those forming the Coastal Batholith, are gener- 500 km ally >1-km-thick tabular bodies (Haederle and Atherton, 2002; Pino et al., 2004; Jacay and Sempere, 2008) and, because of tectonic tilt- ing, their present location may not refl ect the position of the arc at the time of their Davidson , 1991; Kay et al., 1994, 1999; Kay and compositions to the evolution of the crust, we emplacement. Mpodozis, 2001). In particular, lavas younger provide a consistent stratigraphic nomenclature In this section, we summarize, with empha- than 5 Ma from the central and southern Central based on absolute ages and correlations. This sis on the magmatic arc, current knowledge volcanic zone have compositional characteristics is needed because previous stratigraphic sys- regarding the geologic evolution of the study that suggest a higher degree of crustal contami- tems have relied on local and multiple names area, and we review Andean magmatic rocks nation compared with similar andesitic magmas for units, which were often poorly dated and before and during crustal thickening in southern erupted in regions where, or times when, the correlated (e.g., Instituto Geológico Minero Peru (Fig. 2). Information on the type, volumes, crust was thinner (Davidson, 1991; Haschke et y Metalúrgico Bulletins; Tosdal et al., 1981; location, and timing of the magmatic arcs in al., 2002). However, alternative mechanisms Kaneoka and Guevara , 1984; Klinck et al., northern Chile was mostly taken from Trumbull have been proposed, such as “source contamina- 1986; Mukasa, 1986a, 1986b; Clark et al., 1990; et al. (2006) and our own unpublished data.