Apatite triple dating and white mica 40Ar/ 39Ar thermochronology of syntectonic detritus in the Central Andes: A multiphase tectonothermal history

B. Carrapa1, P.G. DeCelles2, P.W. Reiners2, G.E. Gehrels2, and M. Sudo3 1Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA 2Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA 3Universität Potsdam, Institut für Geowissenschaften, 14476 Golm, Germany

ABSTRACT We applied apatite U-Pb, fi ssion track, and (U-Th)/He triple dating and white mica 40Ar/39Ar thermochronology to syntectonic sedimentary rocks from the central Andean Puna plateau in order to determine the source-area geochronology and source sedimentary basin thermal his- tories, and ultimately the timing of multiple tectonothermal events in the Central Andes. Apa- tite triple dating of samples from the Geste Formation in the Salar de Pastos Grandes basin shows late – apatite U-Pb crystallization ages, Eocene apatite fi s- sion track (AFT), and Eocene– (U-Th)/He (ca. 8–47 Ma) cooling ages. Double dating of cobbles from equivalent strata in the Arizaro basin documents early Eocene (46.2 ± 3.9 Ma) and (107.6 ± 7.6, 109.5 ± 7.7 Ma) AFT and Eocene–Oligocene (ca. 55–30 Ma) (U-Th)/He ages. Thermal modeling suggests relatively rapid cooling between ca. 80 and 50 Ma and reheating and subsequent diachronous basin exhumation between ca. 30 Ma and 5 Ma. The 40Ar/39Ar white mica ages from the same samples in the Salar de Pastos Grandes area are mainly 400–350 Ma, younger than apatite U-Pb ages, suggesting source-terrane cooling and exhumation during the Devonian–early . Together these data reveal multiple phases of mountain building in the and Cenozoic. Basin burial temperatures within the plateau were limited to <80 °C and incision occurred diachronously during the Cenozoic.

INTRODUCTION ern using the apatite U-Pb, apatite fi s- answering the following questions, which have The provenance, geochronology, and thermal sion track (AFT), and apatite (U-Th)/He meth- implications for paleogeographic reconstruc- history of syntectonic sedimentary rocks pro- ods. We also apply 40Ar/39Ar thermochronology tions and tectonic models of Andean evolution. vide valuable information about the location, on detrital white micas from the same samples in (1) What is the geochronological source-terrane age, and exhumation history of source terranes order to determine the mid-temperature cooling signature of Eocene sedimentary rocks? (2) Are and dynamics of orogenic processes (e.g., Ber- history of the detritus. The closure temperatures Eocene AFT ages widespread within the pla- net et al., 2001; Najman et al., 2001; Carrapa et of these systems are ~450–550 °C for apatite teau, and do they represent regional exhuma- al., 2003; Hodges et al., 2005). Although much U-Pb (e.g., Flowers et al., 2007), ~350 °C for tion, rather than magmatic input? (3) What is the progress has been made during the past decade white mica 40Ar/39Ar, ~120–60 °C for AFT (e.g., magnitude of basin burial (heating) and subse- toward routine detrital thermochronology, the Green et al., 1989), and ~80–60 °C for apatite quent exhumation, and is basin exhumation syn- combination of multiple thermochronological (U-Th)/He (e.g., Farley, 2000). chronous within the plateau? (4) Did the Central and geochronological methods on individual The Central Andes have been the site of arc- Andes undergo pre-Cenozoic exhumation and, detrital grains (multidating) is still in its infancy. related and foreland basin deposition since the if so, when, and what were the magnitude and The advantage of multidating is that a high-tem- Paleozoic, and therefore are an ideal place in cause? In order to answer all these questions, a perature method can reveal the crystallization which to investigate the thermal effects of multi- multidating approach covering a large tempera- age and a lower-temperature method can reveal ple orogenic phases. In the Central Andes, docu- ture window (~500–60 °C) is necessary. the cooling and exhumation age of a grain, thus mented Cenozoic exhumation rates are ~0.2 mm/ providing valuable information about source and yr (e.g., Carrapa et al., 2005, 2006; Deeken et al., GEOLOGICAL BACKGROUND basin histories. Although several studies using 2006; Coutand et al., 2006). However, recently The region defi ned as the Puna Altiplano, multiple chronometers on individual zircon published AFT detrital thermochronologic data or central Andean plateau, is characterized by grains have been published (Rahl et al., 2003; document relatively rapid exhumation rates (0.5 high mean elevation (>3500 m), internal drain- Campbell et al., 2005; Reiners et al., 2005; Ber- to >1 mm/yr) during Paleocene–Eocene time age, and aridity resulting from geodynamic net et al., 2006; van der Beek et al., 2006), this is (Carrapa and DeCelles, 2008), coeval with con- and surfi cial processes related to convergence the fi rst work using three methods on individual tractional deformation. It remains unknown if between the Nazca and South American plates detrital apatite grains. the Central Andes underwent earlier phases of since the mid-Cretaceous (e.g., Isacks, 1988; We present triple dating of detrital apatite grains rapid exhumation, because 40Ar/39Ar ages do not Allmendinger et al., 1997; Strecker et al., 2007). from Eocene syntectonic sedimentary rocks of record early Cenozoic signals. Rocks now in the central Andean plateau record the Geste Formation in the Pastos Grandes and With this study we demonstrate the unique deposition in a backarc basin during the Cam- Arizaro basins in the Puna plateau of northwest- power of the detrital multidating approach by brian–, a foreland basin during the

© 2009 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, May May 2009; 2009 v. 37; no. 5; p. 407–410; doi: 10.1130/G25698A.1; 3 fi gures; Data Repository item 2009103. 407 68°W 67° 66° 24°S ple 2SP38 because of its abundant high-quality B apatites and lowest (i.e., deepest) stratigraphic C position, which provides the best constraints on Silt Sand Pebble Boulder basin burial and exhumation history. 2000 Three cobbles from an ~500-m-thick sec-

Salar Pocitos 4SP700 tion of fl uvial, eolian, and alluvial fan deposits of equivalent Geste Formation in the Arizaro

Salar de basin (Fig. 1), farther to the west, were selected,

Pastos Grandes Salar de Pastos Grandes Macon Range and apatites were analyzed for fi ssion track 1500 and (U-Th)/He ages (Table DR2). We also ana- 40 39 Nevados de Palermo lyzed detrital white micas for Ar/ Ar ther- 25° mochronology from two samples, one from the lowest part (1SP32) and one from the top LEGEND A Peru 3SP431 Bolivia Mean paleocurrent (4SP700) of our measured sections in the Salar Salar de direction 16 S de Pastos Grandes basin (Fig. 3C). We picked Hombre Muerto Conglomerate 1000 > 3 km those two samples to check for possible strati- elevation Eolian 20 S 40 39 Sandstone graphic shifts in the detrital Ar/ Ar signatures. e l i 24 S h Partially covered We analyzed 19 grains from sample 1SP32 and C Study Siltstone area 25 grains from sample 4SP700 by single fusion Phyllite 28 S Argentina Pacific 3SP431 Sample site 2SP277 analysis (Fig. 2C). One grain from each sample 0 20 2SP238 Ocean 500 500 was selected for step-heating analysis (Table km 74 W 66 W 26° TG408 DR3). Multikinetic inverse thermal modeling Holocene salt lakes and Neogene 2SP38 salt flats plutonic rocks sedimentary rocks of fi ssion track and (U-Th)/He ages was applied Alluvium undifferentiated volcanic rocks Paleozoic plutonic rocks to the three cobbles from the Arizaro basin and Upper Miocene-Pliocene Undifferentiated Ordovician: sedimentary TG190 andesites, dacites and basalts Cenozoic rocks, local volcanic rocks one sandstone from equivalent strata in the Salar

Cretaceous Precambrian/Cambrian Geste Formation Salar de Arizaro

Neogene ignimbrites Geste Formation sedimentary rocks plutonic rocks TG41 de Pastos Grandes basin (Figs. 1 and 3). (For Carboniferous 1SP32 Neogene basalts Precambrian/Cambrian 0m 0m sedimentary rocks sedimentary and igneous Cambrian- rocks 1SP0 details regarding analytical methods, see the Pleistocene-Holocene Late Paleozoic Precambrian undifferentiated volcanic rocks plutonic rocks Faults Study areas granites Ordovician Data Repository.)

Figure 1. A: Map of central South America showing location of study area in northwestern RESULTS Argentina. B: Geological map of southern Central Andes modifi ed after Reutter et al. (1994). Apatite U-Pb ages of P1 and P2 grains are C: Stratigraphic column of Geste Formation in the Salar de Pastos Grandes (modifi ed after DeCelles et al., 2007) and Arizaro basins. New paleocurrent data, from this study, are shown almost exclusively between 500 Ma and 1000 to the right of the Arizaro stratigraphic column. Ma (Fig. 2A; Table DR1). Only a single grain from P1 yielded a Cenozoic age, but this age is much younger than the depositional age, sug- Early Devonian, a continental rift during the north-south–trending Macon Range, which is gesting signifi cant Pb loss. These late Precam- Early Cretaceous, and a foreland basin again composed of Cambrian and Precambrian grani- brian and early Paleozoic U-Pb apatite ages are during the Cenozoic (Jordan and Alonso, 1987; toid rocks (Fig. 1). slightly younger than zircon U-Pb ages from Isaacson and Díaz-Martínez, 1995; Rapela et the same samples (DeCelles et al., 2007), as al., 1998; Sempere, 1995; DeCelles and Horton, METHODS expected for a lower closure temperature. Dat- 2003; Carrapa and DeCelles, 2008). We selected 76 apatites, belonging to AFT ing of the same apatites by (U-Th)/He reveals The Salar de Pastos Grandes basin, in the populations P1 and P2 (43.7 ± 3.2 and 56.2 ± Eocene–late Miocene ages. The youngest central Puna plateau (Fig. 1), contains ~3.5 km 2.7 Ma, respectively), and 1 grain belonging to (U-Th)/He ages range between ca. 15 and ca. 8 of Cenozoic syntectonic fl uvial-alluvial depos- P3 (Carrapa and DeCelles, 2008), from the AFT Ma (Table DR1), which together with low eU its, including 2 km of the Eocene Geste Forma- mount (of 100 AFT dated grains) of sample (effective uranium) are consistent with a higher tion (Alonso, 1992). Provenance data document 2SP38 from the lower part of the Geste Forma- sensitivity to postburial heating and resetting Ordovician quartzite and minor phyllite and tion, and analyzed them by laser-ablation–mul- (Shuster et al., 2006; Flowers et al., 2007), com- schist as source rocks located to the west (Car- ticollector inductively coupled plasma–mass pared with grains having higher eU values and rapa and DeCelles, 2008). Detrital zircon U-Pb spectrometry (Fig. 2A). Of the same grains, older detrital ages. ages from both the Geste Formation and underly- 13 were subsequently extracted from the grain Detrital 40Ar/39Ar analyses of 44 white micas ing Ordovician quartzite cluster in the 900–1200 mount and analyzed by (U-Th)/He thermo- from the Geste detritus in the Salar de Pastos Ma (Grenville, Sunsás) and late Precambrian– chronology (Fig. 2B; GSA Data Repository Grandes basin show Devonian–early Carbon- Cambrian (Panafrican, Pampean) ranges. Late Table DR11). Apatites were selected on the basis iferous cooling ages (Fig. 2C). Step-heating Eocene (ca. 37–34 Ma) grains are also present of their AFT age (P1 and P2) and homogeneity analyses show plateau ages of 393.8 ± 1 Ma and and document limited volcanic input (DeCelles (inclusion and zonation free). We selected sam- 396.0 ± 1.4 Ma on samples 1SP32 and 4SP700, et al., 2007). Detrital AFT data show dominance respectively (Table DR3), indicating that the of grains with Eocene–Paleocene ages requiring single fusion ages are most likely undisturbed. rapid (>~1 mm/yr) source-terrane exhumation 1GSA Data Repository item 2009103, data tables, Three cobbles (TG41, TG190, and TG408) during that time (Carrapa and DeCelles, 2008). geochronology, thermochronology, and thermal from the Geste Formation in the Arizaro basin, modeling, is available online at www.geosociety. In the Arizaro basin (Jordan and Mpodozis, org/pubs/ft2009.htm, or on request from editing@ derived from the Macon Range (Fig. 1C), were 2006), equivalent coarse-grained conglomer- geosociety.org or Documents Secretary, GSA, P.O. analyzed for AFT and (U-Th)/He ages. TG41 ates have been reported along both fl anks of the Box 9140, Boulder, CO 80301, USA. produced an AFT age of 107.6 ± 7.6 Ma and

408 GEOLOGY, May 2009 He ages of 55.0 ± 5.0 and 52.8 ± 3.2; TG190 yielded an AFT age of 109.5 ± 7.7 Ma and a He 1200 A 2SP38 age of 47.5 ± 2.8; and TG408 produced an AFT (AFT populations calculated on 100 grains) P1: 43.7 ± 3.2 age of 46.2 ± 3.9 Ma and He ages of 50.1 ± 2.7 1000 P2: 56.2 ± 2.7 and 29.6 ± 2.6 (Table DR2). Cretaceous ages are consistent with data reported for the northern 800 Macon Range (Deeken et al., 2006). In order to test the hypothesis that the young- est (U-Th)/He ages are geologically meaningful 600 we modeled the youngest He ages, with the low- est eU content, of AFT age population P2 from 400 samples 2SP38 (8.02 ± 0.28 Ma) from the Salar Apatite U/Pb age (Ma) de Pastos Grandes basin, and TG41 (52.8 ± 3.21 200 Ma) and TG408 (29.6 ± 2.56 Ma) from equiva- lent strata in the Arizaro basin. The best mod- 0 eling tests show monotonic and slow cooling B 20 40 60 80 100 120 140 160 between ca. 160 Ma and 60 Ma, relatively rapid 1 50 C 1SP32 cooling between ca. 80 and 50 Ma, and reheat- 0.8 (n=19)(n = 19) ing and subsequent cooling between ca. 30 Ma 40

y and 5 Ma (Fig. 3; see Data Repository). 0.6 30 4SP7004SP700 0.4 (n(n=25) = 25) DISCUSSION

(U-Th)/He age (Ma) 20 Probabilit Probability 0.2 Apatite U-Pb data show Cambrian–Pre- 0 cambrian apatite crystallization ages of source 10 0 100 200 300 400 500 600 terranes west of the Salar de Pastos Grandes Apatite 40Ar/ 39 39 Ar Ar age age (Ma) (Ma) basin, mainly corresponding to Ordovician 0 rocks, as indicated by similar Precambrian– 0 20406080100 Cambrian zircon U-Pb ages (DeCelles et al., AFT age (Ma) 2007). These results further support the inter- Figure 2. A: Apatite U-Pb versus apatite fi ssion track (AFT) ages for 75 double dated apatites; pretation that Paleocene–Eocene AFT ages note that populations P1 and P2 are calculated on 100 AFT dated grains. B: AFT versus (U- from the same grains represent true exhuma- Th)/He ages for 13 triple dated apatites (Table DR2). C: Probability density diagrams of white 40 39 tion ages rather than magmatic input (Carrapa mica Ar/ Ar detrital ages from samples 1SP32 and 4SP700 (Table DR3). and DeCelles, 2008). The youngest He ages of selected grains, rang- ing from ca. 15 to 8 Ma (Tables DR1 and DR2), 0 mean L: C are interpreted as the result of cooling during TG 408 0.4 14.3± 0.7 Good fit 2SP38 basin exhumation. Also, the fact that only a few Best fit Acceptable fit 0.2 apatites are fully reset for He and none for AFT TG 41 Best fit Acceptable fit constrains the maximum heating temperature to 0

Stratigraphic order Best fit <~80 °C, as supported by modeling results. AFT mean L: B and (U-Th)/He ages from equivalent units in the 13.0± 1.5 100 0.2 Arizaro basin document mainly Eocene and Arizaro basin limited Cretaceous cooling ages. Modeling of (°C) T AFT and (U-Th)/He ages of samples from both 0 the Arizaro and Salar de Pastos Grandes basins mean L: A 13.1± 1.1 suggests diachronous basin exhumation and late Salar de Pastos Grandes 0.2 Miocene out of sequence deformation within basin the plateau after the orogenic front had already

Basin exhumation 200 0 swept through the plateau in Eocene time (Car- 6 10 14 18 200150 100 50 0 Length (µm) rapa and DeCelles, 2008). Age (Ma) The 40Ar/39Ar white mica detrital ages from equivalent Eocene units in the Salar de Pastos Figure 3. Inverse thermal modeling (T—temperature; L—length) for Geste Formation sam- ples from Arizaro and Salar de Pastos Grandes basins. A: Sample TG 41 (granitic cobble), Grandes basin show Devonian–early Carbon- Arizaro basin. B: Sample TG 408 (granitic cobble), Arizaro basin. C: Sample 2SP38 (detrital iferous ages. Assuming the same source for population: P2), Salar de Pastos Grandes basin. Lengths are c-axis corrected. mica and apatite and given that the apatite U-Pb crystallization ages are generally much older than the 40Ar/39Ar ages, it is plausible that resentative of exhumation of the contractional interpretation. Overall, the Precambrian apatite the mica ages represent exhumation rather than Paleozoic orogenic system. In addition, the U-Pb ages indicate that the maximum tempera- crystallization ages. Considering that a fore- lack of Devonian–early Carboniferous U-Pb ture during Paleozoic tectonism was <450–550 land basin was in place during the Early Devo- ages is consistent with the absence of plutons °C, and Paleozoic 40Ar/39Ar ages indicate that nian (Isaacson and Díaz-Martínez, 1995), we of that age range in the region west of the Salar the maximum temperature during Mesozoic interpret the Paleozoic 40Ar/39Ar ages as rep- de Pastos Grandes basin and supports our and Cenozoic tectonism was <350 °C.

GEOLOGY, May 2009 409 SUMMARY AND CONCLUSIONS tal zircon fi ssion-track and U/Pb analysis of detrital minerals: Reviews in Mineralogy and Apatite triple dating, coupled with 40Ar/39Ar Siwalik sediments, western Nepal: Basin Re- Geochemistry, v. 58, p. 239–257, doi: 10.2138/ detrital thermochronology, can provide critical search, v. 18, p. 393–412, doi: 10.1111/j.1365- rmg.2005.58.9. 2117.2006.00303.x. Isaacson, P.E., and Díaz-Martínez, E., 1995, Evidence information about provenance, depositional, Campbell, I.H., Reiners, P.W., Allen, C.M., Nicolescu, for a middle-late Paleozoic foreland basin and and postdepositional histories in sedimentary S., and Upadhyay, R., 2005, He-Pb double dat- signifi cant paleolatitudinal shift, Central Andes, units. In particular, apatite triple dating can ing of detrital zircons from the Ganges and Indus in Tankard, A.J., et al., eds., Petroleum basins of link multiple source and postdepositional sig- Rivers: Implications for sediment recycling and South America: American Association of Petro- provenance studies: Earth and Planetary Sci- leum Geologists Memoir 62, p. 231–249. natures through single detrital grains, obviat- ence Letters, v. 237, p. 402–432, doi: 10.1016/ Isacks, B., 1988, Uplift of the central Andean plateau ing assumptions about source rocks of differ- j.epsl.2005.06.043. and bending of the Bolivian orocline: Journal of ent minerals. Also, because of the contrasting Carrapa, B., and DeCelles, P.G., 2008, Eocene ex- Geophysical Research, v. 93, p. 3211–3231, doi: closure temperatures of the apatite (U-Th)/He, humation and basin development in the Puna 10.1029/JB093iB04p03211. fi ssion track, and U-Pb systems, triple dating of northwestern Argentina: Tectonics, v. 27, Jordan, T.E., and Alonso, R.N., 1987, Cenozoic TC1015, doi: 10. 1029/2007TC002127. stratigraphy and basin tectonics of the Andes has unique potential to unravel source-to-basin Carrapa, B., Wijbrans, J., and Bertotti, G., 2003, Epi- Mountains, 20°-28° south latitude: American thermal histories. Aside from the methodologi- sodic exhumation in the Western Alps: Geology, Association of Petroleum Geologists Bulletin, cal signifi cance, our data provide important v. 31, p. 601–604, doi: 10.1130/0091-7613(2003) v. 71, p. 49–64. geological information. 031<0601:EEITWA>2.0.CO;2. Jordan, T.E., and Mpodozis, C., 2006, Estratigrafi a y Carrapa, B., Adelmann, D., Hilley, G., Mortimer, E., evolucion tectonics de la cuenca paleogena de 1. Precambrian and early Paleozoic apatite Strecker, M.R., and Sobel, E.R., 2005, Oligo- Arizaro-Pocitos, Puna Occidental (24°-25°S), U-Pb ages document detrital source rocks of cene uplift, establishment of internal drainage XI Congreso Geologico Chileno, Volume 2: likely Ordovician age in the Central Andes. The and development of plateau morphology in Antofagasta, Chile, p. 57–60. implication is that Paleozoic source rocks were the southern Central Andes: Tectonics, v. 24, Najman, Y., Pringle, M., Godin, L., and Grahame, O., 2001, Dating of the oldest continental sediments deformed and exhumed during the Eocene, as TC4011, doi: 10.1029/2004TC001762. 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Devonian–early Carboniferous (400–350 Evidence from rock provenance and apatite Rahl, J., Reiners, P.R., Campbell, I.H., Nicolescu, S., Ma) mica ages are younger than apatite U-Pb fi ssion track thermochronology along the south- and Allen, C.M., 2003, Combined single-grain ages, suggesting cooling and exhumation during ernmost Puna Plateau margin (NW Argentina): (U-Th) and U/Pb dating of detrital zircons from Earth and Planetary Science Letters, v. 247, the Navajo Sandstone, Utah: Geology, v. 31, mid-Paleozoic orogenic growth along the Gond- p. 82–100, doi: 10.1016/j.epsl.2006.04.010. p. 761–764, doi: 10.1130/G19653.1. wana convergent margin at rates possibly faster Coutand, I., Carrapa, B., Deeken, A., Schmitt, A.K., Rapela, C.W., Pankhurst, R.J., Casquet, C., Baldo, than any event recorded during the Cenozoic. Sobel, E., and Strecker, M.R., 2006, Orogenic E., Saavedra, J., and Galindo, C., 1998, Early 3. Modeling of AFT and (U-Th)/He ages plateau formation and lateral growth of compres- evolution of the Proto-Andean margin of South America: Geology, v. 26, p. 707–710, doi: 10. indicates monotonic source-terrane cooling dur- sional basins and ranges: Insights from sandstone petrography and detrital apatite fi ssion-track ther- 1130/0091-7613(1998)026<0707:EEOTPA>2. ing the early-middle Cretaceous, rapid cooling mochronology in the Angastaco Basin, NW Ar- 3.CO;2. between ca. 80 and 50 Ma, limited Cenozoic gentina: Basin Research, v. 18, p. 1–26. Reiners, P.R., Campbell, I.H., Nicolescu, S., Allen, basin burial, and subsequent exhumation con- DeCelles, P., and Horton, B.K., 2003, Early to middle C.M., Garver, J.I., Mattinson, J.M., and Cowan, D.S., 2005, (U-Pb)/(He-Pb) double dating of sistent with out of sequence deformation. Tertiary foreland basin development and the his- tory of Andean crustal shortening in Bolivia: detrital zircons: American Journal of Science, Geological Society of America Bulletin, v. 115, v. 305, p. 259–311, doi: 10.2475/ajs.305.4.259. ACKNOWLEDGMENTS p. 58–77, doi: 10.1130/0016-7606(2003)115 Reutter, K.J., Döbel, R., Bogdanic, T., and Kley, J., This research was funded by Deutsche Forschungs- <0058:ETMTFB>2.0.CO;2. compilers, 1994, Geological map of the Central gemeinschaft (CA 481/5-1 to Carrapa); National Sci- DeCelles, P.G., Carrapa, B., and Gehrels, G., 2007, Andes between 20° and 26° S, in Reutter, K.J., ence Foundation (NSF) grant EAR-0710724 to Car- Detrital zircon U-Pb ages provide provenance et al., eds., Tectonics of the Southern Andes: rapa and DeCelles; NSF grants EAR-0443387 and and chronostratigraphic information from Eo- Heidelberg, Berlin, Springer, scale 1:1.000.000. EAR-0732436 for support of the Arizona LaserChron cene synorogenic deposits in northwestern Sempere, T., 1995, Phanerozoic evolution of Bolivia, Center; and by ExxonMobil Corporation. We thank Argentina: Geology, v. 35, p. 323–326, doi: in Tankard, A.J., et al., eds., Petroleum basins of Abir Biswas, Stefan Nicolescu, Victor Valencia, and 10.1130/G23322A.1. South America: American Association of Petro- Scott Johnston for analytical assistance, and Shari Deeken, A., Sobel, E.R., Coutand, I., Haschke, M., leum Geologists Memoir 62, p. 207–230. 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410 GEOLOGY, May 2009