Detrital zircon U-Pb ages provide provenance and chronostratigraphic information from synorogenic deposits in northwestern

P.G. DeCelles Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA B. Carrapa Universität Potsdam, Institut für Geowissenschaften, 14476 Golm, Germany G.E. Gehrels Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA

ABSTRACT progressively more abundant in the upper part Paleogene clastic sedimentary rocks in the Puna plateau of northwestern Argentina contain of the formation (Fig. 2; Carrapa and DeCelles, valuable information about the timing and location of early mountain building in the central 2006). Conglomerates in the Geste Formation Andes. Because these rocks generally lack tuffaceous facies, only paleontological ages have been contain clasts of vein quartz, quartzite, and available. We present U-Pb ages from detrital zircons in the conglomeratic Eocene Geste Forma- phyllite that are identical to rocks in the under- tion of the central Puna plateau. The zircon ages indicate that the Geste Formation was derived lying Copalayo Formation (Azeño- from nearby high-relief ranges composed of Ordovician metasedimentary rocks. A small popu- laza, 1973). Paleocurrent data indicate chiefl y lation of ca. 37–35 Ma grains also confi rms the late Eocene stratigraphic age of the Geste Forma- eastward paleofl ow directions (Carrapa and tion, and suggests that U-Pb detrital zircon ages may provide a new tool for determining deposi- DeCelles, 2006). Middle to upper Eocene fos- tional ages and provenance of widespread Paleogene deposits in the central Andes. sils have been recovered from the middle part of the Geste Formation (, notoungulates, Keywords: Andes, Argentina, syntectonic sedimentation, U-Pb geochronology. and ; Pascual, 1983; Alonso, 1992). The ranges to the east of the Salar de INTRODUCTION samples of the Geste Formation in the Salar de Pastos Grandes are composed of Ordovician– Acquisition of U-Pb ages of detrital zircons Pastos Grandes area in the central Puna plateau, granitoid rocks and pegmatites, and from clastic sedimentary rocks is revolution- we discovered a small but systematic group of upper metasedimentary rocks izing the discipline of sedimentary petrology Eocene grains in several samples. These data (Puncoviscana Formation, mainly phyllite by providing information about both prov- are consistent with previous vertebrate biostrati- and quartzite), gneisses, and orthogneisses. enance and, in some cases, chronostratigraphy graphic ages (Pascual, 1983; Alonso, 1992), and Pliocene andesites form the >6 km peaks of (Gray and Zeitler, 1997; DeCelles et al., 1998, provide some of the best constraints yet avail- the Nevados de Pastos Grandes, directly north 2004; Gehrels et al., 1999, 2000; DeGraaff- able on the exact ages of these strata. Moreover, of the study area (Fig. 1B). Surpless et al., 2002; Dickinson and Gehrels, our data raise the possibility that U-Pb ages of 2003; Amidon et al., 2005; Link et al., 2005; detrital zircons in general may provide useful METHODS Surpless et al., 2006). Although there have chronostratigraphic information in retroarc set- Six samples of medium- to coarse-grained been detrital zircon geochronological studies tings such as the Andean orogenic belt, where from the Geste Formation and one of the pre-Cenozoic rocks of the southern and comagmatic zircons may be prevalent even in sample of medium-grained quartzite from the central Andes (Hervé et al., 2003; Augustsson apparently ash-free sediments. underlying Ordovician Copalayo Formation were et al., 2003; Finney et al., 2003, 2005; Fanning collected and processed by standard methods et al., 2004; Thomas et al., 2004; Gelcich et al., GEOLOGICAL SETTING AND for separating zircons. U-Pb geochronology of 2005; Do Campo and Ribeiro Guevara, 2005), STRATIGRAPHY zircons was conducted by laser-ablation–multi- no attempt has been made to apply the method The Geste Formation is one of many local collector inductively coupled plasma–mass spec- to Cenozoic strata. Widespread Paleogene strata accumulations of coarse-grained Cenozoic sedi- trometry at the University of Arizona LaserChron of the central Andes (Fig. 1; Jordan and Alonso, mentary rocks of generally pre-Neogene age in Center. Details of the method were described in 1987; Alonso, 1992) are especially attractive northwestern Argentina and southern Bolivia Gehrels et al. (2000, 2006). targets for detrital U-Pb geochronology because (Fig. 1; Jordan and Alonso, 1987; Sempere et al., Analyses that yielded isotopic data of accept- these sparsely dated, coarse-grained clastic 1997; Horton, 1998). The unit is ~2 km thick able discordance, in-run fractionation, and pre- strata potentially contain information about the (Fig. 2) and is exposed in the Salar de Pastos cision are in GSA Data Repository Table DR1.1 timing and location of early mountain building Grandes area of the Puna plateau, a vast high- Because 206Pb/238U ages are generally more in the central Andes, a topic that generates vigor- elevation (>4 km average elevation) region in precise for younger ages, whereas 206Pb/207Pb ous debate (Allmendinger et al., 1997; Sempere the interior of the central Andean orogenic sys- ages are more precise for older ages, we rely on et al., 1997; Lamb and Hoke, 1997; Kley et al., tem. The Geste Formation unconformably over- 206Pb/238U ages up to 1000 Ma and 206Pb/207Pb 1997; Jordan et al., 1997, 2001; Coutand et al., lies Middle to Upper Ordovician phyllite and ages if the 206Pb/238U ages are older than 1000 2001; Horton et al., 2002; DeCelles and Horton, quartzite of the Copalayo (or Colquena) Forma- Ma (Gehrels et al., 2006). We report here a total 2003; Elger et al., 2005; Carrapa et al., 2005; tion (Fig. 1; Alonso, 1992; Blasco et al., 1995), of 606 zircon ages. These analyses are plotted Arriagada et al., 2006). Historically, the ages of which forms the bulk of the range directly to the on relative age-probability diagrams (Fig. 3), Paleogene strata in the central Andes have been west of our investigated section. which represent a sum of the probability distri- diffi cult to document owing to a paucity of dat- The Geste Formation consists of an overall butions of all analyses from a sample, normal- able volcanogenic sediment. upward-coarsening succession of sandstone, We set out to document U-Pb ages of detri- conglomerate, and subordinate mudstone (Fig. 2; 1GSA Data Repository item 2007079, Table DR1, U-Pb geochronologic analyses, is available online at tal zircons in Cenozoic basin fi lls of the central Alonso, 1992). The lower part of the Geste www.geosociety.org/pubs/ft2007.htm, or on request Andes of northwestern Argentina primarily as a Formation consists of exclusively fl uvial facies, from [email protected] or Documents Secre- provenance tool. In the course of our work on and coarse-grained alluvial-fan deposits become tary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

© 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, April April 2007; 2007 v. 35; no. 4; p. 323–326; doi: 10.1130/G23322A.1; 3 fi gures; Data Repository item 2007079. 323 68° 67° 66° 24° Salars/ Sijes Fm. B C Queva peaks 6130m ndes Pozuelos Fm.

a Cover Pleistocene upper

Tertiary middle Eocene Pastos Gr Volcanics lower Geste Fm. Salar Pocitos Pliocene Oire Fm. Upper Capalayo Fm.

mo Tertiary Measured sect. Ordovician

¥Nevados de Santa Rosa

Salar de Arizaro dos de Paler de los 27'54''S Pastos Grandes °

Neva 24 25° A Pe r u Bol i vi a Pucara 16?S Salar de Pastos Grandes >3km Salar de elevation Hombre Muerto 20?S SIJES

e l i ? h 24 S C Study

area os 28?S Argentina s je El Morro Paci f i c i

Pozuel S Ocean 0 20 km e 74?W 66?W d 26° a Elevation

Quaternary salt lakes and Neogene Upper Salar de salt flats plutonic rocks plutonic rocks r r 6500 Quaternary continental palayo Neogene e sedimentary rocks volcanic rocks sedimentary rocks i Co S Neogene/Quaternary Tertiary Paleozoic 0 3 km volcanic rocks plutonic rocks 45'54''S sedimentary rocks ° io Neogene/Quaternary Paleogene Ordovician sedimentary r

a 24 n ignimbrites sedimentary rocks and local volcanic rocks e t n Quaternary, locally Plio- Precambrian/Cambrian e 1000 C plutonic rocks . Pleistocene basalt sedimentary rocks S Quaternary, locally Plio- Precambrian/Cambrian sedimentary rocks Pleistocene volcanic rocks sedimentary rocks 66°42'18W'' 66°27'54W'' Faults

Figure 1. A: General location map of central South America showing location of study area in northwestern Argentina. Shaded region rep- resents area in which elevation exceeds 3 km. B: Geological map of central Andes (modifi ed after Reutter et al., 1994). Inset shows location of area within context of broader Andean orogenic belt; rectangle corresponds to C. C: Geological map of Salar de Pastos Grandes area (modifi ed after Alonso, 1992) draped on digital elevation model. Fm.—formation.

ized such that the areas beneath the probability ± 1.5 Ma to 35.4 ± 0.55 Ma (Fig. 3). The pres- these rocks should help to constrain the timing curves are equal for all samples. Age peaks on ence of previously reported Eocene of initial crustal thickening and uplift in the Puna these diagrams are considered robust if defi ned fossils (Pascual, 1983) in the Geste Formation plateau. In this case, our data require proximal by several analyses. suggests that the Eocene zircon grains are close sediment sources in high relief, presumably tec- to the depositional age of the Geste Formation. tonically active terranes directly to the west of RESULTS The fact that the mean ages of the Eocene grains the present Geste Formation outcrops. It is clear Sample 1SP0 is a quartzite from the Ordovi- decrease systematically upsection also gives that the Ordovician rocks in the ranges directly cian Copalayo Formation that crops out directly credence to the idea that they are essentially the west of the Salar de Pastos Grandes were uplifted west of and stratigraphically below the Geste same age as the depositional ages of the host and widely exposed during the Eocene. Our data Formation. It is characterized by age probabil- sediments. This suggests that the Eocene zir- provide no support for the hypothesis that nearby ity peaks at ca. 521 Ma, 796 Ma, 1045 Ma, and cons are volcanogenic. ranges farther east (e.g., the Nevados de Palermo) 2130 Ma (Fig. 3). The majority of the grains provided sediment to the Geste Formation in this from this sample are clustered in the ca. 490– IMPLICATIONS AND DISCUSSION region. Eocene uplift in the central Puna plateau 700 Ma and ca. 1000–1200 Ma ranges. The Our detrital zircon data provide new informa- is consistent with recent sedimentological and six Geste Formation samples exhibit grain-age tion about the provenance of the Geste Formation apatite fi ssion-track thermochronological stud- populations very similar to those from the Ordo- and establish a maximum depositional age of late ies that suggest deformation and exhumation of vician sample, suggesting that the Geste zircons Eocene (ca. 37 Ma). Most signifi cant, our data ranges in the central and southern Puna and along were derived locally from Ordovician rocks. In suggest that U-Pb ages of detrital zircons in Ter- its eastern margin during Eocene–Oligocene time four of the Geste samples, we also found Eocene tiary clastic sedimentary units of the central Andes (Kraemer et al., 1999; Coutand et al., 2001, 2006; grains, which tend to be more angular and may provide a powerful new tool for chronostrati- Ege et al., 2003; Carrapa et al., 2005). euhedral than the older detrital grains. The mean graphic studies. The method may be particularly The presence of Eocene grains in the Geste ages of these grains for each sample in which useful in the sparsely dated, nontuffaceous, lower Formation also indicates that a coeval magmatic more than one grain was found range from 37.3 Tertiary rocks of the central Andes. The ages of source of zircons must have existed in the region.

324 GEOLOGY, April 2007 4SP700 silt sand pebble boulder n = 79 2000

4SP700 Mean = 35.4 ± 0.55 Ma [1.6%] 95% conf. MSWD = 0.76 (error bars = 2σ) 3SP431 39 n = 82 37 35 1500 33

2SP277 U Age (Ma) n = 92 238 38 Pb/ 3SP431 206 36

1000 34 Mean = 35.9 ± 6.4 Ma [18.0%] 95% conf. 2SP238 MSWD = 1.9 (error bars = 2σ) n = 92

2SP277 LEGEND Eocene Geste Formation 2SP238 Conglomerate 500 2SP38 Sandstone n = 94 39 Partially covered 2SP38 Siltstone U Age (Ma) 37 Phyllite 238 35 3SP431 Sample site Pb/ Mean = 37.3 ± 1.5 Ma [4.0%] 95% conf. 1SP32 206 MSWD = 6.2 (error bars = 2σ) 1SP32 Base of Geste Formation n = 85 0 1SP0 Ordovician

Figure 2. Generalized log of measured strati- graphic section through Geste Formation in Salar de Pastos Grandes area. 1SP0 n = 82

However, no Eocene igneous rocks are present in Figure 3. Age probability plots of U-Pb ages this region, and the Geste Formation lacks tuffa-

Ordovician of detrital zircons from Salar de Pastos

ceous material. Moreover, the absence of any zir- Copalaya Formation 0 0.5 1.0 1.5 2.0 2.5 3.0 Grandes area. MSWD—mean square of cons in the Geste Formation with ages between Age (Ga) weighted deviates; conf.—confi dence. Eocene and early Paleozoic suggests that the Eocene grains did not enter the Geste depocenter by overland travel, because zircons of post–early the Geste Formation (Pascual, 1983). Together REFERENCES CITED Paleozoic age (e.g., derived from Mesozoic igne- these data indicate that local uplift in the cen- Allmendinger, R.W., Jordan, T.E., Kay, S.M., ous rocks that are present in the central Andes tral Puna was under way by late Eocene time. and Isacks, B.L., 1997, The evolution of the to the west of our study area) would also be The presence of Eocene zircons in the Geste Altiplano-Puna plateau of the Central Andes: Annual Review of Earth and Planetary Sci- expected. Therefore, the Eocene zircons proba- Formation also suggests that magmatic sources ences, v. 25, p. 139–174, doi: 10.1146/annurev. bly were provided as air fall from distant eruptive were active during Eocene time. In general, the earth.25.1.139. centers in an active Andean magmatic arc. U-Pb detrital zircon method may be a power- Alonso, R.N., 1992, Estratigrafía del Cenozoico de ful tool for determining ages of apparently ash- la cuenca de Pastos Grandes (Puna Salteña) con énfasis en la Formación Sijes y sus boratos: CONCLUSIONS free Paleogene strata in the Andes, as well as Revista de la Asociación Geológica Argentina, The U-Pb ages from detrital zircons in clastic strata in other retroarc settings where v. 47, p. 189–199. Cenozoic deposits of the central Andean oro- syndepositional arc magmatism is likely. Amidon, W.H., Burbank, D.W., and Gehrels, G.E., genic belt show a close match with U-Pb ages 2005, U-Pb zircon ages as a sediment mixing from zircons in probable Ordovician source ACKNOWLEDGMENTS tracer in the Nepal Himalaya: Earth and Plan- This research was supported by the Exxon Mobil etary Science Letters, v. 235, p. 244–260, doi: rocks. Coupled with the coarse grain size, east- Corporation and the German Science Foundation 10.1016/j.epsl.2005.03.019. ward paleocurrent indicators, and conglomer- (Deutsche Forschungsgemeinschaft project CA481/ Arriagada, C., Cobbold, P.R., and Roperch, P., 2006, ate compositions, the zircon data demonstrate 5-1 to Carrapa and the Leibniz Prize to Manfred Salar de Atacama basin: A record of compres- that local high-relief uplifts in the central Puna Strecker). We thank Ricardo Alonso for logistical sup- sional tectonics in the central Andes since the port and for sharing his knowledge of Paleogene stra- mid-Cretaceous: Tectonics, v. 25, TC1008, doi: plateau were responsible for generating the tigraphy in Argentina. Jim Schmitt, Tim Lawton, and 10.1029/2004TC001770. Geste Formation. The detrital zircon ages (37– Amy Weislogel provided constructive reviews that Augustsson, C., Bahlburg, H., Fanning, M., and 35 Ma) overlap with biostratigraphic ages from helped us to improve the paper. Münker, C., 2003, Detrital zircons as a key to

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Printed in USA

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