Carlos W. Rapela

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

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.

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.

The Famatinian magmatic arc in the central Sierras Pampeanas: an Early to Mid-Ordovician continental arc on the Gondwana margin

Abstract A new multi-disciplinary study of the central Sierras Pampeanas encompasses fieldwork, petrography, metamorphic and micro-structural analysis, geochemistry and geochronology. Remnants of a low-to-medium grade metasedimentary sequence, which also occurs in the Sierras de Córdoba to the east, are considered regionally equivalent to the Puncoviscana Formation; a ?mid-Cambrian Rb-Sr whole-rock isochron of 513 ± 31 Ma probably dates their main metamorphism. The predominant granitoids of the Los Llanos-Ulapes batholith constitute a calc-alkaline suite representative of the Famatinian subduction-related magmatic arc. The main granodiorite phase of the batholith is associated with an S2 fabric and shear zone formation, and was emplaced late during the deformational history of the metasediments. Conventional and SHRIMP U-Pb zircon dating yielded a combined age of 490 ± 5 Ma. Younger monzogranites gave Rb-Sr whole-rock ages of 470–450 Ma, typical of granites in the Sierra de Famatina, but geochemical continuity with the main granodiorite suite raises the possibility that these are partially reset ages. A minor cordierite granite phase is ascribed to local anatexis caused by heat from the granodiorites. All the calc-alkaline rocks of the Los Llanos-Ulapes batholith have high initial 87Sr/86Sr (0.7075–0.7105) and low ɛNdt (−4.6 to −6.3), inherited from lower crust. Sm-Nd model ages of 1600–1700 Ma indicate that the underlying crust is identical to that beneath the foreland to the east. This part of the Famatinian arc was thus a continental magmatic arc and was established significantly before the arrival of the allochthonous Precordillera terrane in mid-Ordovician times.

Age and origin of coeval TTG, I- and S-type granites in the Famatinian belt of NW Argentina

Three granitoid types are recognised in the Famatinian magmatic belt of NW Argentina, based on lithology and new geochemical data: (a) a minor trondhjemite–tonalite–granodiorite (TTG) group, (b) a metaluminous I-type gabbro-monzogranite suite, and (c) S-type granites. The latter occur as small cordieritic intrusions associated with 1-type granodiorites and as abundant cordierite-bearing facies in large batholithic masses. Twelve new SHRIMP U-Pb zircon ages establish the contemporaneity of all three types in Early Ordovician times (mainly 470-490 Ma ago). Sr- and Nd-isotopic data suggest that, apart from some TTG plutons of asthenospheric origin, the remaining magmas were derived from a Proterozoic crust-lithospheric mantle section. Trace element modelling suggests that the TTG originated by variable melting of a depleted gabbroid source at 10-12kbar, and the I-type tonalite-granodiorite suite by melting of a more enriched lithospheric source at c. 5 kbar. The voluminous intermediate and acidic I-types involved hybridisation with lower and middle crustal melts. The highly peraluminous S-type granites have isotopic and inherited zircon patterns similar to those of Cambrian supracrustal metasedimentary rocks deposited in the Pampean cycle, and were derived from them by local anatexis. Other major components of the S-type batholiths involved melting of deep crust and mixing with the I-type magmas, leading to an isotopic and geochemical continuum.

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.

Involvement of the Argentine Precordillera terrane in the Famatinian mobile belt: U-Pb SHRIMP and metamorphic evidence from the Sierra de Pie de Palo

New data suggest that the eastern margin of the Argentine Precordillera terrane comprises Grenvillian basement and a sedimentary cover derived from it that were together affected by Middle Ordovician deformation and metamorphism during accretion to the Gondwana margin. The basement first underwent low pressure/temperature ( P/T ) type metamorphism, reaching high-grade migmatitic conditions in places (686 ± 40 MPa, 790 ± 17 °C), comparable to the Grenvillian M2 metamorphism of the supposed Laurentian counterpart of the terrane. The second metamorphism, recognized in the cover sequence, is of Famatinian age and took place under higher P/T conditions, following a clockwise P-T path (baric peak: 1300 ± 100 Mpa, 600 ± 50 °C). Low-U zircon overgrew detrital Grenvillian cores as pressure fell from its peak, and yields U-Pb SHRIMP ages of ca. 460 Ma. This is interpreted as the age of ductile thrusting coincident with early uplift; initial accretion to Gondwana must have occurred before this. The absence of late Neoproterozoic detrital zircons is consistent with a Laurentian origin of the Argentine Precordillera terrane.

Chronological study of the pre-Permian basement rocks of southern Patagonia

Small outcrops of the basement of the Deseado Massif, consisting of weathered, altered granitoids and their metasedimentary host rocks, were dated by the U–Pb zircon method using thermal ionization mass spectrometry and ion microprobe (SHRIMP). The provenance ages of detrital zircon in the metasediments are typical of material available in the adjacent regions of the Gondwana margin, with prominent components at 1000–1100 and 580±6 Ma, the latter probably approximating the age of sedimentation (latest Neoproterozoic). SHRIMP data for pre-Jurassic granodiorite from a borehole in the Magallanes basin in Tierra del Fuego give a Cambrian age of 521±4 Ma, which confirms published conventional U–Pb dating from the same borehole. SHRIMP ages of 476±4 and 472±5 Ma for granitic cobbles in a Permian conglomerate (La Golondrina Formation) are evidence of nearby Ordovician (Famatinian) intrusive activity. In situ granitoids indicate late Silurian to early Carboniferous ages (422±2, 395±3, and 346±4 Ma). All these events are recognised in the evolution of adjacent South America and the Antarctic Peninsula suggesting that these parts of the Gondwana margin were contiguous throughout their Cambrian to Jurassic history.

New SHRIMP U-Pb data from the Famatina Complex: constraining Early-Mid Ordovician Famatinian magmatism in the Sierras Pampeanas, Argentina

New SHRIMP U-Pb zircon ages are reported for igneous and sedimentary rocks of the Famatina Complex, constraining the age of the magmatism and the ensialic basins. Together with whole-rock and isotope geochemistry for the igneous rocks from the complex, these ages indicate that the voluminous parental magmas of metaluminous composition were derived by partial melting of an older lithosphere without significant asthenospheric contribution. This magmatism was initiated in the Early Ordovician (481 Ma). During the Mid-Late Ordovician, the magmatism ceased (463 Ma), resulting in a short-lived (no more than ~20 Ma) and relatively narrow (~100-150 km) magmatic belt, in contrast to the long-lived cordilleran magmatism of the Andes. The exhumation rate of the Famatina Complex was considerably high and the erosional stripping and deposition of Ordovician sediments occurred soon after of the emplacement of the igneous source rocks during the Early to mid-Ordovician. During the upper Mid Ordovician the clastic contribution was mainly derived from plutonic rocks. Magmatism was completely extinguished in the Mid Ordovician and the sedimentary basins closed in the early Late Ordovician.

Temporal evolution and spatial variation of early tertiary volcanism in the Patagonian Andes (40°S–42°30′S)

Early to mid-Tertiary igneous activity in the Cordilleran Series (CS) of the Patagonian Andes between 40°S–42°30′S shows spatial variation and temporal trends that can be correlated with crustal thickness and slab depth. Volcanism in this region is concentrated in two sub-parallel arcs, the Pilcaniyeu Belt to the east and the El Maiten Belt to the west. Compilation of available K-Ar data and paleogeographic constructions suggests three major periods of volcanic activity. The first is a Paleocene-Eocene (60-42 Ma) event which developed primarily in the Pilcaniyeu belt; the second (Oligocene, 33-23 Ma) event is now exposed primarily along the El Maiten Belt. Miocene volcanics (16-11 Ma) have also been found in the northern sector of the El Maiten Belt. After an overall decline in the magnitude of volcanism, the third period of volcanic activity reached its maximum during the Pliocene-Pleistocene as large stratovolcanoes were built along the North Patagonian Cordillera. Temporal variations in the lower Tertiary CS, determined by superposition in selected cross-sections, indicate that the major episodes begin with silicic associations (ignimbritic, plinian, and obsidian rhyolitic facies) and end with intermediate and basic lava flows (stratovolcanoes and monogenetic cones). K2O and total alkalis decrease southward for a given silica content in the Pilcaniyeu and El Maiten Belts. The calc-alkaline andesites of the northern sector (39°30′S–41°30′S) are similar to many other Central Andes series, but towards the south (Cholila) there are also sequences with mild tholeiitic affinities. Spatial variations in major element composition seem to be related to the southward decrease in thickness of the underlying sialic crust. Variations in subduction geometry during the Cenozoic possible correlate with the compositional recurrence observed in the Cordilleran Series.

Basement evolution of the Sierra de la Ventana Fold Belt: new evidence for Cambrian continental rifting along the southern margin of Gondwana

U–Pb sensitive high-resolution ion microprobe data together with geochemical and Nd isotope analyses obtained in the basement complex of the Sierra de la Ventana Fold Belt indicate that the Early Palaeozoic passive margin history of the basin followed Cambrian magmatism related to rifting in a 600 Ma Neoproterozoic crust. The Cambrian episode started with intrusion of 531 ± 4 and 524 ± 5 Ma A- and I-type granites derived from a dehydrated infracrustal source (εNd530 −3.1 to −5.9), and culminated with eruption of high-Zr peralkaline spherulitic rhyolites derived from an undepleted lithospheric mantle (509 ± 5 Ma; εNd509 +0.5 to +1.0). These rift-related magmatic rocks were covered by shelf sediments deposited along a once-continuous passive margin, encompassing the Sierra de la Ventana Fold Belt, the Cape Fold Belt, the Falkland/Malvinas microplate and the Ellsworth Mountains block in Antarctica. The Cambrian rifting event defined the outline shape of the southern part of Gondwana, and can be regarded as the initiation of the supercontinent stage, which lasted until Jurassic break-up. The conjugate continental fragments separated from Gondwana during the Cambrian rifting could be the source for microcontinents with c. 1000 Ma basement rocks that collided with the proto-Andean margin during Ordovician–Silurian times.