Detrital-zircon geochronology and provenance of the Ocloyic synorogenic clastic wedge, and Ordovician accretion of the Argentine Precordillera terrane

Abstract

The Precordillera terrane in northwestern Argentina is interpreted to be an exotic (Laurentian) continental fragment that was accreted to western Gondwana during the Ordovician. One prominent manifestation of the subduction and collision process is a Middle-Upper Ordovician clastic wedge, which overlies a passive-margin carbonate-platform succession in the Precordillera. U/Pb ages of detrital zircons from sandstones within the clastic wedge, as well as zircons from clasts within conglomerates, provide documentation for the composition of the sediment provenance. The ages of detrital zircons are consistent vertically through the succession, as well as laterally along and across strike of the Precordillera, indicating a single, persistent sediment source throughout deposition of the clastic wedge. The dominant mode (~1350-1000 Ma) of the detrital-zircon ages corresponds to the ages of basement rocks in the Western Sierras Pampeanas along the eastern side of the Precordillera. A secondary mode (1500-1350 Ma) corresponds in age to the Granite-Rhyolite province of Laurentia, an age range which is not known in ages of basement rocks of the Western Sierras Pampeanas; however, detritus from Granite-Rhyolite-age rocks in the basement of the Precordillera was available through recycling of synrift and passive-margin cover strata. Igneous clasts in the conglomerates have ages (647-614 Ma) that correspond to the ages of minor synrift igneous rocks in the nearby basement massifs; the same ages are represented in a minor mode (~750-570 Ma) of detrital-zircon ages. A quartzite clast in a conglomerate, as well as parts of the population of detrital zircons, indicates the importance of a source in the metasedimentary cover of the leading edge of the Precordillera. The Famatina continental-margin magmatic arc reflects pre-collision subduction of Precordillera lithosphere beneath the western Gondwana margin; however, no detrital zircons have ages that correspond to Famatina arc magmatism, indicating that sedimentary detritus from the arc may have been trapped in a forearc basin and did not reach the foreland. The indicators of sedimentary provenance for the foreland deposits are consistent with subduction of the Precordillera beneath western Gondwana, imbrication of basement rocks from either the Precordillera or Gondwana into an accretionary complex, and recycling of deformed Precordillera cover rocks.

Tips on the SW-Gondwana margin: Ordovician conodont-graptolite biostratigraphy of allochthonous blocks in the Rinconada mélange, Argentine Precordillera

“…The record of middle Darriwilian graptolites (H. spinosus Zone) and conodonts (L. variabilis-Y. crassus zones) in large blocks of quartzarenites (phantom stratigraphic units) is consistent with cannibalization of the Ordovician synorogenic clastic wedge that was generated during the accretion of the Precordillera terrane to Gondwana (e.g., Thomas et al, 2015). Moreover, the repeated occurrence of km-scale blocks of the San Juan Formation in the Rinconada mélange may represent either a series of major sliding events or the sliding of an Ordovician nappe stacking during the Silurian -Early Devonian, subsequent to the accretion of the Precordillera terrane to SW Gondwana.…”

“…5). Accordingly, the middle Darriwilian quartz-arenite blocks of the Rinconada mélange would represent part of the synorogenic clastic wedge that was generated during the accretion of the Precordillera to Gondwana (Astini et al, 1995;Thomas et al, 2015). In addition, the Holmograptus spinosus Zone (sample RIN16) has been recorded in calcareous nodules from the top of the Gualcamayo Formation in the Villicum Range (Kaufmann and Ortega, 2016), but is absent in the Central Precordillera due to a hiatus (Ortega et al, 2007).…”

Detrital zircon record of Phanerozoic magmatism in the southern Central Andes

“…Key phases include the Pampean (555-515 Ma) and Famatinian (495-460 Ma) magmatic arcs, generated during east-dipping subduction (Ramos et al, 1986;Bahlburg et al, 2009). The Precambrian to Ordovician history included accretion of Laurentian terranes (including Cuyania) to the Gondwana margin, followed by the poorly constrained accretion of the Chilenia terrane (Ramos et al, 1984;Thomas et al, 2015;Rapela et al, 2016;Martin et al, 2019). Isolated granitic plutons of Devonian age across the Sierras Pampeanas region are interpreted to have been emplaced during extensional conditions (Dahlquist et al, 2013;Moreno et al, 2020).…”

“…440 Ma suggest arc cessation occurred immediately following accretion of the Cuyania terrane (Fig. 8A; Thomas et al, 2015;Rapela et al, 2016;Martin et al, 2019). Low relative proportions of Silurian-Devonian (435-385 Ma) zircon grains are associated with a magmatic lull and formation of metamorphic zircon supported by relatively enriched εHf values and low (<0.1) Th/U ratios.…”

Neogene Retroarc Foreland Basin Evolution, Sediment Provenance, and Magmatism in Response to Flat Slab Subduction, Western Argentina

“…This broken foreland province is composed of metamorphic and igneous rocks exposed along the Sierra de Valle Fértil and Sierra Pie de Palo at 30-32°S ( Figure 2a; Mulcahy et al, 2014;Ortiz et al, 2015). Precambrian-Ordovician igneous and metamorphic rocks are preserved across the Sierras Pampeanas and attributed to Pampean (555-515 Ma) and Famatinian (495-460 Ma) orogenesis, during accretion of Laurentian (i.e., Cuyania, and Pampia/MARA) terranes to the Gondwanan margin (Ducea et al, 2010;Ramos, 2004;Rapela et al, 2016;Schwartz et al, 2008;Thomas et al, 2015). On the eastern flank of the Sierra Pie de Palo, complex west vergent upper-crust structures kinematically linked to deeper east vergent basement structures (Bellahsen et al, 2016) expose Neogene basin fill that is capped by modern range-front alluvial fans ( Figure 2b; Cuerda et al, 1983).…”