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A Post-Ordovician Juxtaposition of the Cuyania Terrane

A Post-Ordovician Juxtaposition of the Cuyania Terrane

U N I V E R S I D A D D E C O N C E P C I Ó N DEPARTAMENTO DE CIENCIAS DE LA TIERRA 10° CONGRESO GEOLÓGICO CHILENO 2003

POST- JUXTAPOSITION OF THE AND THE FAMATINIAN MAGMATIC ARC

FINNEY, S., GLEASON, J., GEHRELS, G., and PERALTA, S.

Department of Geological Sciences, California State University at Long Beach, Long Beach, CA 90840 USA; [email protected] Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109 USA; [email protected] Department of Geosciences, University of Arizona, Tucson, AZ 85721 USA; [email protected] CONICET, Universidad nacional de San Juan, 5400 Rivadavia, San Juan, Argentina; [email protected]

INTRODUCTION With its thick Cambro-Ordovician carbonate platform succession that contains a “Pacific” olenellid trilobite fauna and warm-water conodont and shelly faunas, the Precordillera of western Argentina is considered by many to be part of a , the Cuyania terrane, that was derived from . Evidence indicates that it had arrived at its present position adjacent to and outboard of the Famatinian magmatic arc before Carboniferous time. Several models have been proposed to explain the transfer of the Cuyania terrane from Laurentia to with the most widely accepted being the microcontinent model of Thomas and Astini (1996; Astini et al, 1995). In this model, the Cuyania terrane rifted from the Ouachita embayment of Laurentia in the Early , drifted across the Iapetus as a microcontinent until the Mid Ordovician when it docked against the proto-Andean margin of Gondwana. of the leading edge of the plate bearing the Cuyania terrane generated the Early Ordovician magmatic activity of the Famatinian arc against which the Cuyania terrane subsequently docked. Other proposed models that incorporate a Laurentian origin of the Cuyania terrane are the - continent collision of Dalla Salda et al. (1992a, 1992b), the Texas Plateau model of Dalziel (1997), and the microcontinent model of Keller (1999). None of these is accepted widely. In the Dalla Salda and Dalziel models, the Cuyania terrane was juxtaposed against the Famatinian arc in the Mid Ordovician. Keller, on the other hand, argued for final separation from Laurentia in Mid to Late Ordovician time and for docking against Gondwana in to Carboniferous time.

Soon after the exotic and allochthonous nature of the Precordillera was first fully elucidated (Ramos et al., 1986), Baldis et al. (1989) presented an alternative, para-autochthonous model, in which the Precordillera terrane was always of Gondwanan affinity, having been displaced by transcurrent motion along the proto-Andean margin from a position far to the southeast (present coordinates) of its present location. From stratigraphic relations, Baldis et al. (1989) proposed that migration commenced in the Mid Ordovician, and that the Precordillera terrane reached a position adjacent to the Famatinian magmatic belt in Devonian time. Although this model was more-or-less ignored during the wave of excitement generated by the Laurentian microcontinent model in the 1990s (Dalziel et al., 1996), Aceñolaza and Toselli (2000) and Aceñolaza et al.

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(2002) revived it with their proposal that the Precordillera (or Cuyania) terrane originated from a location in southern Gondwana (present coordinates) between the southern parts of , , and . This early Gondwanan affinity for the Cuyania terrane received further support from provenance analysis of upper Lower Cambrian and lower Upper Ordovician beds in the Precordillera. U/Pb age populations of detrital zircons from these sandstone beds led Finney et al (2003) to conclude that 1) the provenance of the Cambrian sandstone bed is Gondwanan and thus the Cuyania terrane was part of Gondwana in the late Early Cambrian, and 2) the Upper Ordovician sandstone bed lacks zircons that are coeval in age with Famatinian magmatism and thus the Cuyania terrane was not adjacent to the Famatinian arc in the early Late Ordovician. These conclusions are consistent with the para-autochthonous model, in which the Cuyania terrane rifted from a location in southern Gondwana in the Mid to early Late Ordovician, was displaced to the northeast along transcurrent faults, and arrived at its present position adjacent to the Famatinian arc during the latest Ordovician time or possibly later. The purpose of this paper is to further argue that the Cuyania terrane was not adjacent to the Famatinian arc in the early Late Ordovician on the bases of U/Pb geochronology of detrital zircons from lower Upper Ordovician and the occurrence of K-bentonites interbedded with Middle Ordovician strata of the Precordillera.

DETRITAL ZIRCONS FROM UPPER ORDOVICIAN SANDSTONE BEDS IN THE PRECORDILLERA The Argentine Precordillera makes up a significant part of the greater Cuyania terrane. Although basement rocks are not exposed in the Precordillera, the Cambrian to Carboniferous stratigraphic succession is more-or-less completely and continuously exposed. In eastern and central thrust belts in the central and northern part of the Precordillera, the Cambrian to Mid Ordovician carbonate succession is overlain diachronously by black shale of the Gualcamayo Formation, which, in turn, is overlain locally by thick silciclastic sequences of coarse conglomerate and sandstone deposits and fine-grained strata, referred to the Las Plantas, Las Vacas, Trapiche, and La Cantera formations (Astini et al., 1995; Astini, 1998). In central thrust belts in the southern part of the Precordillera, near Mendoza, the stratigraphic succession represents a platform margin. The Middle Ordovician Los Sombreros Formation is composed of shale that forms the matrix for a variety of conglomerate and sandstone beds and huge olistoliths composed of Cambrian and Lower Ordovician carbonate rocks. The Los Sombreros Formation is overlain by the Upper Ordovician Empozada Formation, which is composed of sandstone and siltstone that accumulated in a shelf environment. The rather dramatic change in the Middle to Upper Ordovician stratigraphy from platform carbonates to black shale and overlying coarse siliciclastic strata reflects a major tectonic event in the history of the Cuyania terrane. Proponents of the Laurentian microcontinent model (Astini et al, 1995; Astini, 1998; Astini and Thomas, 1999; Thomas et al., 2002; Thomas and Astini, in press) interpret this event as the docking of the Cuyania terrane against the proto-Andean margin of Gondwana. They conclude: 1) that the thick successions of coarse siliciclastic strata represent a synorogenic clastic wedge that developed in response to the of the Cuyania terrane to the margin of Gondwana; and 2) that the source area for much of the clastic wedge was the Famatinian arc, an eroding topographic high immediately to the east.

Detrital zircon samples were taken from sandstone beds in the Las Vacas, lower and upper La Cantera, and Empozada formations (Finney et al., 2003, in press). All are of early Late

Ordovician age. U/Pb age populations are predominately of Mesoproterozoic age (Fig. 1). Those of the Las Vacas and La Cantera formations are very similar to the Mesoproterozoic age population of the La Cébila sample of Finney et al. (2003), which is taken as an unmistakable record of Gondwanan provenance. Zircons of Neoproterozoic and early Paleozoic age are rare in the Las Vacas, La Cantera, and Empozada samples. Two grains in the upper La Cantera sample have ages (642.2±5.9 and 613.8±4.6 Ma) coeval with Brasiliano events, such as Damara orogenesis, that are older than the Pampean . The Empozada sample has one zircon with an age (528.8±7 Ma) that is coeval with the and one grain with an age (470.0±22.7) that is within the age range of Famatinian magmatism.

Zircons with ages of magmatic units in the Famatinian magmatic belt (490-470 Ma, Pankhurst et al., 2000) and the Pampean magmatic belt (535-520 Ma, Rapela et al., 1998) are virtually absent from the Las Vacas, La Cantera, and Empozada samples. Out of 328 zircons in the four samples, only two grains may have been derived from these two extensive magmatic belts. From this, we conclude that the basins in which the Las Vacas, La Cantera, and Empozada formations accumulated during early Late Ordovician time (460-455 Ma) received negligible from the Famatinian arc. This is surprising given: 1) the proximity of the Las Vacas and La Cantera depositional basins to the Famatinian magmatic belt, in particular the Sierra de Famatina, both today and, according to the Thomas/Astini model in the early Late Ordovician, 2) the abundance of Famatinian-age zircons in the La Cébila sample (11 grains with ages of 515-480 Ma), which is evidence of surface exposure and of Famatinian granitoids or volcanics in the Early Ordovician, 3) the interpretation of Astini et al. (1995) and Astini (1998) that the depositional basins of the Las Vacas and La Cantera formations formed by extension after collision of the Cuyania terrane against the Famatinian magmatic arc, which stood as a topographic high immediately to the east, and 4) the interpretation of Thomas and Astini (in press) that granitoid clasts in the Las Vacas Formation and equivalent formations were derived from the Famatinian magmatic belt.

Detrital zircon age populations in the Las Vacas, La Cantera, and Empozada samples are restricted to a relatively narrow range, especially in comparison to the range of populations in the La Cébila sample. This indicates that the immediate provenances of these samples were of limited variability. The Mesoproterozoic grains in the La Cantera and Las Vacas samples were possibly derived from the same source area as the Mesoproterozoic grains in the La Cébila sample, given their similar plots. This source area must have been the Gondwanan continent, and possibly the Sunsas (1.0-1.3 Ga) and Rondonian-San Ignácio (1.3-1.55 Ga) geochronological provinces of the Amazonian (Tassinari and Macambira, 1999). The provenance of the Empozada sample was more restricted with the unimodal zircon-age population of the sample correlating with the Rondonian-San Ignácio province. The sampled sandstone beds in the Las Vacas, La Cantera, and Empozada formations contain appreciable feldspar and lithic fragments and are texturally submature. This, together with the relatively unimodel zircon-age populations, indicates that most of the zircon grains are not recycled, but instead were delivered directly from the Mesoproterozoic source area to the depositional basins.

It is difficult to reconcile the detrital zircon age populations of the Upper Ordovician sandstone samples with the Laurentian microcontinent model in which the Cuyania terrane was docked against the Famatinian magmatic belt in the early Late Ordovician. The evidence indicates that the zircons were delivered directly from an interior craton of western Gondwana, probably the . However, to reach the depositional basins, the sediment had to be transported across the intervening Pampean and Famatinian magmatic belts with negligible sediment input from these extensive belts that, respectively, were deeply eroded and topographically high and shedding voluminous sediment. On the other hand, the Cuyania terrane may have been in the early Late Ordovician in a different location, perhaps in southern Gondwana, where it received sediment directly from inner without it crossing the Pampean and Famatinian belts.

MIDDLE ORDOVICIAN K-BENTONITES IN THE STRATIGRAPHIC SUCCESSION OF THE PRECORDILLERA The occurrence of numerous K-bentonite beds in the upper San Juan Limestone and overlying Gualcamayo Shale (Huff et al., 1998) is regularly cited as evidence for the Laurentian microcontinent model in which the Cuyania terrane was approaching and in proximity to the Famatinian magmatic arc in Mid Ordovician time (Astini, 1998; Astini and Thomas, 1999; Thomas et al., 2002; Thomas and Astini, in press). The age of the K-bentonite beds (~ 475-465 Ma) overlaps with that (~490-470 Ma) of Famatinian granitoids, and the geochemistry of the K- bentonites is consistent with their origin from subduction-related explosive volcanism (Huff et al., 1998). Thus, it is probable that the K-bentonites in the Precordillera succession represent ashes erupted from volcanoes of the Famatinian arc. However, contrary to most interpretations, this does not necessarily support the Laurentian microcontinent model in which the Cuyania terrane was approaching the Famatinian arc from the west (present coordinates).

In recent paleogeographic reconstructions for Early and Mid Ordovician time (Dalziel, 1997, figs. 16; Cocks and Torsvik, 2002, figs. 4 & 5), the Famatinian arc faces to the east and is located between the equator and 30o south latitude. Assuming the same controls and general patterns of atmospheric circulation as those operating today, the prevailing winds at this paleo-latitude, the southeast trade winds, would have blown and thus carried volcanic ashes to the northwest (Ordovician coordinates) (Fig. 2). This direction is opposite that necessary to disperse the ashes to the Cuyania terrane if it was approaching from the east (Ordovician coordinates) as required by the Laurentian microcontinent model. Instead, the ashes would have been blown to that part of Gondwana represented today by southeastern South America and southern Africa, where Finney et al. (2003, fig. 3) placed the Cuyania terrane.

CONCLUSIONS AND IMPLICATIONS The virtual absence in lower Upper Ordovician sandstone beds of detrital zircons from the Famatinian arc indicates that the Cuyania terrane did not converge on and collide with the Famatinian arc in the Mid to Late Ordovician as proposed in the Laurentian microcontinent model. The Mesoproterozoic-age populations of zircons from the sandstone beds are evidence that the Cuyania terrane was in a position in the early Late Ordovician to receive sediment dispersed directly from exposed cratons within Gondwana. The presence of K-bentonites in the Middle Ordovician strata of the Precordillera indicates that the Cuyania terrane was located in part of Gondwana far to the southeast (present coordinates) of the Famatinian arc and of its present-day location, i.e, in a direction opposite that required by the Laurentian microcontinent model. This location is consistent with the conclusion of Finney et al. (2003), which was based on evidence from geochronology of detrital zircons from Cambrian sandstone and on re- interpretation of paleobiogeography. Accordingly, the abrupt change in Middle to Upper Ordovician stratigraphy in the Precordillera from platform carbonate to Gualcamayo Shale to coarse siliciclastic deposits is not a record of the Cuyania terrane approaching and colliding with the proto-Andean margin of Gondwana. Instead, the stratigraphic succession can be interpreted to record a rifting event as the Cuyania terrane began to be displaced by transcurrent faulting from its original location on the southern margin of Gondwana to its final position on the western margin. Given the time of initiation and the potential distance of this movement, it seems unlikely that it was completed before the end of the Ordovician Period. Perhaps, it was not completed until Devonian time as proposed by Baldis et al. (1989).

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