Pampean Orogeny Research Articles

The Río de la Plata craton and the assembly of SW Gondwana

The extent and nature of the Archaean–Palaeoproterozoic craton of Río de la Plata of southern South America, a major but poorly understood crustal component in Neoproterozoic plate reconstructions, as well as the depositional, metamorphic and magmatic history of the surrounding orogenic belts, are reviewed and reassessed, in part through the analysis of material recovered from deep boreholes in western Argentina that penetrated Palaeozoic cover into basement. U–Pb SHRIMP zircon ages for these samples of 2162 ± 6 Ma (diorite), 2189 ± 14 Ma (amphibolitic schist) and 2088 ± 6 Ma (granite) encompass the range of ages determined for the major Palaeoproterozoic orogenic events in the exposed parts of the craton close to the Atlantic coasts of Uruguay and Argentina. Taken together with the geochemical and Nd-isotope characteristics of these samples and an olivine gabbro from a further borehole that failed to yield zircon, these results strongly suggest that the Río de la Plata craton is extremely uniform in its dominant chrono-tectonic and lithological make-up, and that it extends westwards as far as the 535–520 Ma Pampean orogenic belt, against which it probably has a fault contact. U–Pb SHRIMP zircon detrital age patterns are presented for representative metasedimentary samples from the craton cover in the Tandilia belt of eastern Argentina, and from the Pampean (Cambrian) and Famatinian (Ordovician) belts to the west of the craton. Whereas the oldest cover rocks in Tandilia clearly show material derived from the underlying craton, such detritus only appears in the younger (Ordovician) units to the west. Sedimentary protoliths in the Pampean belt were dominated by Neoproterozoic (broadly ∼ 600 Ma) and late Mesoproterozoic (broadly ∼ 1100 Ma) provenance, and derivation from the Río de la Plata craton is highly unlikely. Regional considerations, including previously published zircon data, palaeocurrent and structural data, suggest that these rocks must have had an origin within Gondwana-forming blocks, for which the closest identifiable sources are ‘Brazilian’ and ‘African’ (Namaqua–Natal). Consequently, the preferred model for the Pampean orogeny is that the Río de la Plata craton reached its present position by large-scale dextral strike-slip movement against fore-arc sedimentary sequences that had developed on the southern and western margins of the Kalahari craton during the Early Cambrian. In the final stage the displaced sedimentary sequences outboard of the RPC collided with the Mesoproterozoic Western Sierras Pampeanas terrane, which was at the time attached to the large Amazonia craton and other smaller continental blocks, such as Arequipa–Antofalla and Río Apa. Protracted relative displacement of the RPC after the Pampean Orogeny led to its final position.

Cambrian initiation of the Las Pirquitas thrust of the western Sierras Pampeanas, Argentina: Implications for the tectonic evolution of the proto-Andean margin of South America

The 40 Ar/ 39 Ar hornblende crystallization ages from the Las Pirquitas thrust of the Sierra de Pie de Palo, Argentina, indicate that imbrication of the proto-Andean margin had initiated by ca. 515 Ma. The ages do not permit the Pie de Palo block to represent basement of the adjacent Argentine Precordillera terrane, but rather require it to be a separate fragment not juxtaposed with the Precordillera prior to the Ordovician. The Pie de Palo block was likely autochthonous to the Famatina arc of the proto-Andean margin in the Early Cambrian. When combined with existing regional ages of magmatism, metamorphism, and deformation, the ages suggest that east-dipping subduction beneath the Famatina arc initiated by the Early to Middle Cambrian. The newly established convergent margin resulted from a westward shift of the plate boundary at the end of the Pampean orogeny in the eastern Sierras Pampeanas. The Famatina arc persisted as an Andean-type active margin until the collision and accretion of the Precordillera terrane in the Middle Ordovician.

Deformation of Gondwana margin turbidites during the Pampean orogeny, north-central Argentina

Basement uplifts in the Eastern Sierras Pampeanas of north-central Argentina expose sedimentary rocks that originated on the Neoproterozoic to early Cambrian margin of Gondwana for over 1000 km along strike. A series of transects through unmetamorphosed sections of the Vendian-aged Puncoviscana Formation and metasedimentary equivalent rocks from deeper structural levels demonstrates that an early stage compaction cleavage has been folded into chevron structures, with a concomitant axial planar tectonic cleavage, prior to erosion and deposition of the overlying Middle to Late Cambrian Meson Group. Changes in structural style are shown to be gradational; all mesoscopic ductile fold structures in the region can be explained with one major deformation episode. Late-stage Andean brittle folds and thrusts are more noticeable in the northern sections, but do not significantly alter the overall structural style or geometry of the basement rocks. A static metamorphic overprint in the Ordovician with associated granitic intrusions only affects fabrics and mesoscale structures adjacent to plutons. Published age constraints indicate an overlap between the timing of sedimentary deposition and that of structural and metamorphic events at depth. We develop a tectonic model for the Pampean orogeny that involves the buttressing of off-scraped sedimentary rocks above a subducting slab beneath the Gondwana margin. The collision of outboard terranes did not occur in this area until at least the mid-Ordovician Famatinian orogeny.

Cambrian to Devonian Geologic Evolution of the Sierra de Comechingones, Eastern Sierras Pampeanas, Argentina: Evidence for the Development and Exhumation of Continental Crust on the Proto-Pacific Margin of Gondwana

Crystalline rocks from the Sierra de Comechingones, eastern Sierras Pampeanas, evolved through three distinct orogenic cycles during the Eopalaeozoic: (1) the first tectono-thermal event named Pampean orogeny (550 to 505 Ma), which peaked in the Early Cambrian, was responsible for extensive metamorphism, partial melting, juvenile magmatism, rapid decompression, and persistent tectonic activity. Large part of the crustal section that was residing at middle levels (c. 27 km) was heated above 800 °C during the thermal peak stage of the Pampean orogeny; decompression of the Pampean orogen's core took place at this high temperature. The exhumation mechanism that assisted rapid uplifting combined the effects of ongoing tectonic forces with a buoyant instability created by a large amount of anatectic magmas in the middle to lower crust. (2) Beginning at the Early Ordovician, the Famatinian orogeny produced an overall shortening, causing pervasive textural reworking of the Cambrian metamorphic sequences under a high-strain regime. By being adjacent to the Famatinian magmatic arc, the western border of the Cambrian crystalline package absorbed imposed deformation along a crustal scale ductile shear zone. Within this zone, the high-grade metamorphic rocks were reworked and re-hydrated to lower temperature assemblages (<600°C and 3–6 kbar). Early Ordovician subduction-related igneous activity, even though manifested as small plutons, intruded Cambrian crystalline sequences, and experienced textural reworking during Late Famatinian tectonic exhumation. Late Famatinian convergence resulted in west-vergent ductile shear zones that placed Cambrian onto Ordovician crystalline sequences. (3) During post-Famatinian times (360–400 Ma) enduring crustal perturbation produced intra-crustal-derived granitic magmatism. West- to northwest-directed thrusting was concentrated in belts nucleated along crustal-scale tectonic boundaries formed between older tectono-stratigraphic units. As a result, Devonian anatectic granites were formed and tectonically extruded among Pampean and Famatinian crystalline sequences. The post-Famatinian event is also characterised by the intrusion of batholith-scale monzogranites into Pampean and Famatinian crystalline sequences residing in the upper crust. Crystalline rocks currently exposed in the Sierra de Comechingones show that they crystallised and were exhumed in a setting where tectono-thermal activity lasted, even though it might have waned, until the Middle Palaeozoic. From the latest Neoproterozoic (c. 550 Ma) until the Late Devonian (c. 360 Ma) tectonic activity was intermittently acting, indicating continuous convergence along the proto-Pacific margin of Gondwana.

Crustal Evolution along the Early Ordovician Proto‐Andean Margin of Gondwana: Trace Element and Isotope Evidence from the Complejo Igneo Pocitos (Northwest Argentina)

The Paleozoic geodynamic evolution of the proto‐Andean margin of Gondwana was characterized by extensive subduction processes associated with the docking of several terranes. The major and trace element as well as Sr‐Nd‐Pb isotope composition of plutonic rocks from the Complejo Igneo Pocitos (CIP) are used here to constrain the processes of magma genesis in the Early Ordovician continental arc along the proto‐Andean margin of Gondwana in northern Chile and northwestern Argentina. U‐Pb sphene ages of $$476\pm 2$$ Ma for the CIP date the time of intrusion. The CIP comprises two magmatic suites, each with a distinct and homogeneous isotope composition that was acquired by magmatic processes in the deep crust or mantle. Both suites are strongly enriched in incompatible trace elements and display a Sr‐Nd isotope composition intermediate between mid‐ocean ridge basalt (MORB) and Andean continental crust. The petrological, geochemical, and isotopic characteristics are best modeled by generation of the CIP melts in a subduction setting. Multicomponent modeling shows that the trace element and isotope inventory of group 1 samples can be generated by admixture of ∼0.5% melts from subducted sediments and 1%–2% of MORB‐derived fluids to the mantle source. Constant initial Sr and Nd isotope compositions at varying SiO2 contents for both suites exclude significant modification of the melts by assimilation and fractional crystallization processes in upper‐crustal levels. Assimilation of group 1 parental magmas in the deep crustal MASH (melting, assimilation, storage, and homogenization) zone with 20%–25% crustal material, however, produced the group 2 parental magmas. The CIP can be considered to be the northern continuation of the Early Ordovician Famatinian arc known from central Argentina. As indicated by the Nd‐Pb isotope composition of the CIP, this arc was built on continental crust of Middle Proterozoic age. Similar protolith ages have been reported for the Famatinian arc in the southern Andes and for the Late Precambrian–Early Paleozoic metamorphic basement of the central Andes. The identification of MASH processes requires a thick continental crust in order to enable melting and assimilation at the base of the crust. Thickening of the crust can be related to the Pampean orogeny at ∼500 Ma. Magmatic rocks from the Famatinian arc in the southern Andes display a more crustlike Sr‐Nd isotope signature compared with the CIP, suggesting that crustal thicknesses in the Early Ordovician continental arc decreased from south to north.

Paleozoic deformation in the Sierras de Cordoba and Sierra de Las Minas, eastern Sierras Pampeanas, Argentina

Basement orthogneisses, paragneisses, and migmatites in the Sierras de Córdoba of the eastern Sierras Pampeanas of central Argentina represent remnants of a Cambrian arc and accretionary prism that initially formed above a subduction zone along the early Cambrian margin of Gondwana. These basement rocks contain many high strain zones that record major events in the tectonic evolution of the western margin of Gondwana during the latest Proterozoic–middle Paleozoic. Initial orthogonal shortening and chevron folding of the accretionary prism rocks occurred prior to a high temperature/low pressure, relatively static metamorphism and migmatization event during 509–525 Ma, simultaneous with the Pampean orogeny. Localized deformation along a narrow zone of dextral transpression occurred late in the Pampean cycle. After peak metamorphism, the gneisses and migmatites deformed on east-dipping, greenschist-grade, mylonite-, ultramylonite-, and pseudotachylyte-bearing zones. Kinematic data from a selection of these shear zones include field data, microstructural shear sense indicators, and quartz C-axis fabric asymmetry. Almost all show west-directed, dip-slip reverse fault movement, but most do not achieve major crustal shortening. One major ultramylonite zone in western Sierras de Cordoba may represent a major tectonic boundary with the adjacent Sierra de San Luis rocks. This high strain zone is intruded by the Devonian-aged Achala batholith. Other age constraints include a pseudotachylyte vein that has been dated by 40Ar/ 39Ar method as mid-Silurian and Ordovician I-type plutonic rocks in the Sierra de Las Minas to the west that are deformed into greenschist-grade protomylonite and pseudotachylyte-bearing zones. Our data are consistent with late Ordovician to mid-Devonian orthogonal deformation throughout the Sierras de Córdoba, related to the emplacement of the Precordillera and Chilenia terranes to the west.

Cordieritite and Leucogranite Formation during Emplacement of Highly Peraluminous Magma: the El Pilon Granite Complex (Sierras Pampeanas, Argentina)

Cordieritites and highly peraluminous granites within the El Pilon granite complex, Sierras Pampeanas, Argentina, were emplaced during a medium-P, high-T metamorphic event during the initial decompression of a Cambrian orogen along the southwestern margin of Gondwana. Very fresh orbicular and massive cordieritite bodies with up to 90% cordieritite are genetically associated with a cordierite monzogranite pluton and a larger body of porphyritic granodiorite. The petrogenesis of this association has been studied using petrographical, mineralogical, thermobarometric, geochemical, geochronological and isotope methods. The granitic magmas were formed by anatexis of mid-crustal metamorphic rocks formed earlier in the Pampean orogeny. The cordieritites appear at the top of feeder conduits that connected the source region located at ∼6 kbar with the pluton emplacement level at 3·7 ± 0·3 kbar. A fall in the liquidus temperature of the melt during emplacement was produced by a marked increase in fluid activity owing to rapid decompression and assimilation of surrounding hydrous metapelitic schists, followed by isobaric crystallization. High-Mg cordierite crystallized early on biotite–sillimanite restitic mineral assemblages of the assimilated schists or at the wall of the feeder conduits. Strong convection in the small magma chamber caused flow segregation of cumulate cordierite and restite, developing leucogranites and highly evolved pegmatoids that are in isotopic equilibrium. Rapid ascent of highly peraluminous magmas might explain why emplacement of these granites was simultaneous with the metamorphic peak registered in neighbouring high-grade migmatite and granulite terranes.

The Pampean Orogeny of the southern proto-Andes: Cambrian continental collision in the Sierras de Córdoba

Abstract A detailed study of the pre-Silurian geology of the Sierras de Córdoba, Eastern Sierras Pampeanas, is used to define the sequence of magmatic and metamorphic events during the Pampean orogeny. This primarily involved Early to Mid-Cambrian subduction and terrane collision at the western margin of Gondwana during the amalgamation of the super-continent. Evidence for this is based principally on new information concerning (a) regional mapping and field relations, (b) analysis of the structures, deformational history and meta-morphic evolution and (c) geochronology and geochemistry of the igneous and metamorphic rocks. The main events recognized are (1) Late Proterozoic break-up of Rodinia (Nd model ages of 1500 ± 200 Ma, inherited zircons 800–1400 Ma), (2) development of an Early Cam-brian passive margin sequence (Puncoviscana Formation and equivalents), (3) emplacement of metaluminous calc-alkaline granitoids (G1a, dated at 530 ± 3 Ma) as a result of NE-directed subduction, (4) crustal thickening, ophiolite obduction, compression and high-grade metamorphism (M2: 8.6±0.8 kbar, 810 ± 50°C, c. 525 Ma) related to collision, and culmina-ting in (5) isothermal uplift and widespread low- P anatexis (M3, 4.0±0.5 kbar, 715 ± 15°C, c. 520 Ma). The last event is responsible for the linked generation of highly peraluminous granites (G1b) and cordieritites. Subsequent emplacement into the accreted terrane of Ordovician trondhjemite-tonalites (500-470 Ma) and dextral wrench shear are interpreted as inner cordilleran counterparts of the Famatinian arc, which developed to the west along the newly-formed proto-Andean margin.

U-Pb, Th-Pb and Ar-Ar geochronology from the southern Sierras Pampeanas, Argentina: implications for the Palaeozoic tectonic evolution of the western Gondwana margin

Abstract New SHRIMP zircon and monazite 206 Pb/ 238 U and 208 Pb/ 232 Th ages on structurally controlled units and 40 Ar- 39 Ar step-heating ages from shear fabrics, define three distinct regional tectonic events in the southern Sierras Pampeanas. The first, the Pampean orogeny, involved closure of a late Neoproterozoic basin on the western margin of Gondwana. New rims on detrital zircons and concurrent monazite growth suggest that the metamorphic peak was attained by c. 530 Ma. The second event, the Famatinian orogeny, marks the initiation of eastward-dipping subduction on the western Gondwana margin, and may represent a continuation of the earlier Pampean event. Metasedimentary rocks from the Sierras de San Luis have zircons with a predominantly Early Cambrian detrital age, indicating a Pampean source. The metamorphic peak in these rocks was contemporaneous with the emplacement of felsic, mafic and ultramafic rocks at c. 480 Ma in a collisional setting. Monazite ages and limited new zircon growth in the metasedimentary rocks suggest that the Famatinian orogeny had ceased by about 450 Ma. This correlates well with a 450–460 Ma Ar-Ar age for late shearing in the southern sierras of La Rioja province. The third tectonic event, the Achalian orogeny, involved W-directed compression and emplacement of multiple, voluminous, granite intrusions. Deformation during this event was partitioned between discrete shear-zones and regions of open to tight folding. The shear zones alternate between W-directed thrusts and NNW-trending, sinistral shear-zones. Ar-Ar data from the low-grade shear fabrics indicate that transpressional deformation continued through most of the Devonian.

The proto-Andean margin of Gondwana: an introduction

Abstract A basic background is presented for the discussion of the Early Palaeozoic geology of western Argentina covered by this book. This includes the definition and terminology of orogenic cycles on this part of the Gondwana margin, represented by the Eastern Sierras Pampeanas. The Pampean orogeny (Early Cambrian) relates to an intense but short-lived period of terrane collision predating the rifting of the Precordillera terrane from Laurentia. The Famatinian cycle is predominantly represented by intense subductionrelated magmatism of Early-Middle Ordovician age, developed on the continental margin of Gondwana during the rifting and drifting of the Precordillera terrane. The Grenvillian basement of the latter is further exemplified by a new Rb-Sr whole-rock isochron age of 1021 ± 12 Ma for orthogneisses from the Western Sierras Pampeanas. A mid-Ordovician granite in this area (dated at 481 ± 6 Ma by U-Pb ion microprobe data) may be related to rifting while the Precordillera terrane was still attached to Laurentia. A divergence of opinion is pointed out between some authors in this book who favour mid-Ordovician collision of the Precordillera with Gondwana, and others who place it much latter, in Silurian or Devonian times.

Early evolution of the Proto-Andean margin of South America

From a detailed study of a 500 km transect in the Sierras Pampeanas, central-west Argentina, two pre-Silurian tectono-magmatic episodes are recognized and defined, each culminating in micro-continental collisions against the proto-Andean margin of Gondwana. The Pampean orogeny started in Early Cambrian time with short-lived subduction, indicated by ca. 535 Ma calc-alkaline granitoids. Following Pampean terrane collision, burial to granulite facies conditions (ca. 9 kbar) generated widespread migmatites and ca. 520 Ma highly peraluminous granites in the Eastern Sierras Pampeanas. After brief quiescence, a second major episode, the Famatinian orogeny, started with subduction ca. 490 Ma, forming a wide continental arc and ensialic backarc basin. This heralded the approach of Laurentia to Gondwana, during which the Precordillera terrane separated from the southern Appalachian region, finally colliding with Gondwana in Silurian–Devonian time.

Evolution of the Sierras de Córdoba, Argentina

Located in the southeastern part of the Sierras Pampeanas (Argentina), the Pampean Ranges of Córdoba show the evolution at the western margin of the Rio de la Plata craton, from extensional stages during Neoproterozoic times to compressive stages during the Pampean orogeny. The Cumbre de Gaspar Group represents distal deep-sea clastic sedimentation deposited in a passive margin environment, very possibly during a time of incipient oceanization. The Santa Sabina Group, representing shelf sedimentation, rests conformably on the Cumbre de Gaspar Group. Stocks of granites and granodiorites of calc-alkaline affinity intruded the Cumbre de Gaspar Group during the early Pampean or earlier. The polyphase Pampean orogeny involves several distinct metamorphic and orogenic events. Metamorphism is discussed using the chemical composition of minerals. The major effect of the Pampean orogeny was the formation of a pile of westward thrust blocks. The Rio de la Plata craton, thrust above its marginal basin, forms the uppermost unit. The intrusion of the El Pilon granite (dated at 520 ± 5 Ma) post-dates these tectonic and metamorphic events. After this main orogenic event, the domain remained permanently located on the cratonic side of younger Famatinian and Andean orogenic events. The Mermela Group probably corresponds to the molasse of the Pampean orogeny and to the base of a new sedimentary cycle. It is difficult to distinguish Famatinian tectonics in the Sierras de Córdoba because the main orogeny occurred further west. However, this orogenic cycle is marked by the intrusion of the large Achala batholith. During the late Palaeozoic and Mesozoic the Sierras de Córdoba remained a stable platform. The whole region was rejuvenated by the Andean orogeny, which induced tilted blocks in its foreland and the Pocho volcanics of shoshonitic affinities.