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.     

Amphibolite to granulite transition in aluminous greywackes from the Sierra de Comechingones, Córdoba, Argentina

We discuss upper-amphibolite to granulite facies, early Palaeozoic metamorphism and partial melting of aluminous greywackes from the Sierra de Comechingones, SE Sierras Pampeanas of Central Argentina. Consistent P–T estimates, obtained from equilibria involving Al and Ti exchange components in biotite and from more traditional thermobarometric equilibria, suggest that peak metamorphism of the exposed section took place at an essentially constant pressure of 7–8 kbar, and at temperatures ranging from 650 to 950 °C. Mineral compositions record an initial decompression, after peak metamorphism, of c. 1.5 kbar, which was accompanied by a cooling of c. 100 °C. Upper-amphibolite facies gneisses consist of the assemblage Qtz+Pl+Bt+Grt+Rt/Ilm. The transition to the granulite facies is marked by the simultaneous appearance of the assemblage Kfs+Sil and of migmatitic structures, suggesting that the amphibolite to granulite transition in the Sierra de Comechingones corresponds to the beginning of melting. Rocks with structural and/or chemical manifestations of partial melting range from metatexites, to diatexites, to melt-depleted granulites, consisting of the assemblage Grt+Crd+Pl+Qtz+Ilm±Ath. The melting stage overlapped at least partially with decompression, as suggested by the occurrence of cordierite, in both the migmatites and the residual granulites, of two distinct textural types: idiomorphic porphyroblasts (probably representing peritectic cordierite) and garnet-rimming coronas. Metapelitic rocks are unknown in the Sierra de Comechingones. Therefore, it appears most likely that the Al-rich residual assemblages found in the migmatites and residual granulites were formed by partial melting of muscovite- and sillimanite-undersaturated metagreywackes. We propose a mechanism for this that relies on the sub-solidus stabilization of garnet and the ensuing changes in the octahedral Al content of biotite with pressure and temperature.     

Geology and petrology of a deep crustal zone from the Famatinian paleo-arc, Sierras de Valle Fértil and La Huerta, San Juan, Argentina

The ranges of the Sierras Valle Fértil-La Huerta expose natural cross sections through a paleo-arc crust that formed in the Late Cambrian - Early Ordovician Famatinian magmatic arc, northwestern Argentina. Thick mafic sequences of amphibole gabbronorites to orthopyroxene-amphibole-biotite diorites form the lower levels of the exposed paleo-arc section. This mafic unit includes lens-shaped bodies of olivine-bearing cumulate rocks and tabular-shaped sill/dike intrusions of fine-grained chilled amphibole gabbro. The mafic magmas were emplaced into regional metasedimentary sequences at lower crustal levels, corresponding to pressure from 5 to 7 kbar. Gabbronorites likely representing the parental magmas that fluxed into the exposed paleo-arc crust differ from primitive magmatic arc rocks in having somewhat lower Mg-number (ca. 0.60) and compatible (Cr and Ni) trace element contents, and slightly higher Al₂O₃ contents. This difference is taken to indicate that a pyroxene-rich olivine-bearing assemblage with a bulk high Mg/Fe ratio and low Al₂O₃ content crystallized from mantle-derived melts before mafic magmas reached the crustal levels currently exhumed. However, some gabbronorites have incompatible trace element signatures typical of primitive mafic arc magmatism. Igneous rocks to some extent more evolved than those of the mafic unit make up a tonalite-dominated intermediate unit. The intermediate unit consists of a heterogeneous suite that ranges from orthopyroxene-bearing amphibole-rich diorites to biotite-rich amphibole-poor tonalites. Within the intermediate unit, chilled mafic rocks are found as a network of dikes, whereas metasedimentary migmatites appear interlayered as m-wide septa and km-long strips. The tonalite-dominated intermediate unit passes into a granodiorite batholith through a transitional zone that is up to 2-km wide. The boundary zone separating the tonalite-dominated and granodiorite-dominated units is characterized by mingling of tonalitic and leucogranitic magmas, which together appear multiply-intruded by mafic sill/dike bodies. Within the tonalite- and granodiorite-dominated units, the less evolved mafic rocks occur as: (1) bodies tens of meters long, (2) chilled dikes and sills, and (3) microgranular inclusions (enclaves), supporting the inference that mafic magmatism was the main source for generating a vast volume of intermediate and silicic igneous rocks. Mass balance calculations and trace element systematics are combined to demonstrate that tonalites and granodiorites formed by concurrent closed-system fractional crystallization and open-system incorporation of paragneissic migmatites and/or anatectic leucogranites into the evolving igneous sequence. This study argues that the sequence of igneous rocks from Valle Fértil-La Huerta was formed as the result of complementary petrogenetic processes that operated concurrently at different levels of the Famatinian arc crust.     

Petrology of mafic and ultramafic layered rocks from the Jaboncillo Valley,Sierra de Valle Fértil,Argentina: Implications for the evolution of magmas in the lower crust of the Famatinian arc

This work presents the field setting, petrography, mineralogy and geochemistry of a gabbroic and peridotitic layered body that is lens-shaped and surrounded by gabbronorites, diorites, and metasedimentary migmatites. This body exposed at Jaboncillo Valley is one among several examples of mafic and ultramafic layered sequences in the Sierras Valle Fértil and La Huerta, which formed as part of the lower crust of the Ordovician Famatinian magmatic arc in central-western Argentina. The layered sequence grew at deep crustal levels (20–25 km) within a mafic lower crust. The base of the layered body was detached during the tectonic uplift of the Famatinian lower crust, whereas the roof of the layered body is exposed in the eastern zone. In the inferred roof, olivine-bearing rocks vanish, cumulate textures are less frequent, and the igneous sequence becomes dominated by massive or thinly banded gabbronorites. Mainly based on the petrographic relationships, the inferred order of crystallization in the gabbroic and peridotitic layered sequence is: (1) Cr–Al-spinel + olivine, (2) Cr–Al-spinel + olivine + clinopyroxene + magnetite, (3) Cr–Al-spinel + olivine + plagioclase + magnetite ± orthopyroxene, and (4) Al-spinel + orthopyroxene + amphibole. A strong linear negative correlation between olivine and plagioclase modal proportions combined with field, petrographic and geochemical observations are used to demonstrate that the physical separation of olivine and plagioclase results in rock diversity at scales of a few centimeters to tens of meters. However, the composition of olivine (Fo ～ 0.81) and plagioclase (An > 94%) remains similar throughout the layered sequence. Spinels are restricted to olivine-bearing assemblages, and display chemical trends characteristic of spinels found in arc-related cumulates. Gabbroic and peridotitic layered rocks have trace element concentrations reflecting cumulates of early crystallizing minerals. The trace element patterns still retain the typical features of subduction-related arc magmatism, showing that the process of cumulate formation did not obscure the trace element signature of the parental magma. Using the composition of cumulus minerals and whole-rock chemical trends, we show that the parental magma was mafic (SiO₂ ～ 48 wt.%) with Mg-number around 0.6, and hydrous. The oxygen fugacity (fO₂) of the parental magma estimated between +0.8 and ?0.6 log fO₂ units around the fayalite–magnetite–quartz (FMQ) buffer is also characteristic of primitive hydrous arc magmas. The initially high water content of the parental magma allowed amphibole to crystallize as an interstitial phase all over the crystallization evolution of the layered sequence. Amphibole crystallization in the inter-cumulus assemblage gives rise to the retention of many trace elements which would otherwise be incompatible with the mineral assemblage of mafic–ultramafic cumulates. This study shows that there exist strongly mafic and primitive magmas that are both generated and emplaced within the lower crustal levels of subduction-related magmatic arc. Our findings together with previous studies suggest that the Early Ordovician magmatic paleo-arc from central-northwestern Argentina cannot be regarded as a typical Andean-type tectono-magmatic setting.     

Early Paleozoic accretionary orogens along the Western Gondwana margin

Early Paleozoic accretionary orogens dominated the Western Gondwana margin and were characterized by nearly continuous subduction associated with crustal extension and back-arc basin development.The southwestern margin is represented by Famatinian and Pampean basement realms exposed in South America,both related to the protracted Paleozoic evolution of the Terra Australis Orogen,whereas the northwestern margin is mainly recorded in Cadomian domains of Europe and adjacent regions.However,no clear relationships between these regions were so far established.Based on a compilation and reevaluation of geological,paleomagnetic,petrological,geochronological and isotopic evidence,this contribution focuses on crustal-scale tectonic and geodynamic processes occurring in Western Gondwana accretionary orogens,aiming at disentangling their common Early Paleozoic evolution.Data show that accretionary orogens were dominated by high-temperature/lowpressure metamorphism and relatively high geothermal gradients,resulting from the development of extended/hyperextended margins and bulk transtensional deformation.In this sense,retreating-mode accretionary orogens characterized the Early Paleozoic Gondwana margin,though short-lived pulses of compression/transpression also occurred.The existence of retreating subduction zones favoured mantle-derived magmatism and mixing with relatively young(meta)sedimentary sources in a thin continental crust.Crustal reworking of previous forearc sequences due to trenchward arc migration thus took place through assimilation and anatexis in the arc/back-arc regions.Therefore,retreating-mode accretionary orogens were the locus of Early Paleozoic crustal growth in Western Gondwana,intimately associated with major flare-up events,such as those related to the Cadomian and Famatian arcs.Slab roll back,probably resulting from decreasing convergence rates and plate velocities after Gondwana assembly,was a key factor for orogen-scale geodynamic processes.Coupled with synchronous oblique subduction and crustal-scale dextral deformation,slab roll back might trigger toroidal mantle flow,thus accounting for bulk dextral transtension,back-arc extension/transtension and a large-scale anticlockwise rotation of Gondwana mainland.