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The Famatinian arc formed around the South Iapetus rim during the Ordovician, when oceanic lithosphere subducted beneath the West Gondwana margin. We present combined in situ U–Th–Pb and Lu–Hf isotope analyses for zircon to gain insights into the origin and evolution of Famatinian magmatism. Zircon crystals sampled from four intermediate and silicic plutonic rocks confirm previous observations showing that voluminous magmatism took place during a relatively short pulse between the Early and Middle Ordovician (472–465 Ma). The entire zircon population for the four plutonic rocks yields coherent εHf negative values and spreads over several ranges of initial εHf (t) units (−0.3 to −8.0). The range of εHf units in detrital zircons of Famatinian metasedimentary rocks reflects a prolonged history of the cratonic sources during the Proterozoic to the earliest Phanerozoic. Typical tonalites and granodiorites that contain zircons with evolved Hf isotopic compositions formed upon incorporating (meta)sedimentary materials into calc–alkaline metaluminous magmas. The evolved Hf isotope ratios of zircons in the subduction related plutonic rocks strongly reflect the Hf isotopic character of the metasedimentary contaminant, even though the linked differentiation and growth of the Famatinian arc crust was driven by ascending and evolving mantle magmas. Geochronology and Hf isotope systematics in plutonic zircons allow us understanding the petrogenesis of igneous series and the provenance of magma sources. However, these data could be inadequate for computing model ages and supporting models of crustal evolution. [ABSTRACT FROM AUTHOR]

Trace element abundances in constituent minerals from mafic and ultramafic rocks of a deep arc crustal section are studied to observe their abundance and distribution. Five cumulate rocks were selected from a sequence that consists of pyroxene hornblende peridotite, olivine hornblende gabbronorite, pyroxene hornblende gabbronorite, hornblende gabbronorite, gabbro and anorthosite. Solid/liquid partition coefficients calculated with the equilibrium distribution model indicate that Cr, Ni, Co, Zn and V are highly compatible with an olivine-dominated mineral assemblage from the mafic and ultramafic cumulates. The compatibility of Ti and Sc would be dependent on the stability of clinopyroxene, amphibole and oxides in the magmatic system. With few exceptions, the other trace elements are incompatible with the minerals that form the cumulate. At most half the mass of Sr carried by a primitive arc magma could be stored in anorthite-rich plagioclase from the mafic cumulates. Magmatic amphibole fractionates Y, middle and heavy REE from other incompatible elements during crystal accumulation of mafic cumulates. In contrast, late magmatic to subsolidus amphibole has no effect on the differentiation of a primitive arc magma. In fact, the trace element variability of amphibole and clinopyroxene is easily related to different stages of magmatic evolution within the cumulate pile. The results presented here are helpful for modelling other cases of arc magma petrogenesis where the cumulate rocks with dominant mafic assemblages are not exposed. [ABSTRACT FROM AUTHOR]

Abstract: The crustal structure of the Famatinian paleoarc is reconstructed by determining the metamorphic crystallization P–T conditions from metasedimentary rocks at various structural levels in the Valle Fértil section. The bulk section exhibits a 15-km-thick arc crustal section. Thermobarometry shows that nested tonalitic and granodioritic plutons constructed the arc crust at depths <20km. Dioritic and tonalitic bodies dominated between 20 and 25km. The deepest exposed paleodepths are formed by cumulate and non-cumulate gabbroic rocks with interlayered quartz diorites that crystallized below 25km. The boundary between the crust and the lithospheric mantle is not observed and would have been underneath a depth of 27km. P–T estimates throughout the section reveals high geothermal gradients ranging from 25°C/km to 35°C/km. The thermal regime inferred for middle crustal levels is observed in active and ancient magmatic arcs. Thermophysical models predicting the array of retrieved P–T estimates require heat advection conducted by mafic magmas which either emplaced in the lower crust and/or intruded into middle crustal levels. Calculated seismic wave velocities of plutonic rocks dominating progressively deeper paleodepths are used to deduce the internal architecture of the Valle Fértil section. This result indicates that the Famatinian arc had a middle crustal structure very similar to that of the ancient Talkeetna arc; however a rapid increase of seismic wave velocities from ~6.3km/s to >6.6km/s is located at deeper depths in the Famatinian arc than in Talkeetna arc. The thickness of a crustal layer dominated by plutonic rocks with low seismic wave velocities (<6.2km/s) is 10km thinner than the crustal layer with similar physical properties in the Sierra Nevada batholith. A putative model for the whole Famatinian arc suggests a total crustal thickness between 30 and 35km with three distinct layers. [Copyright &y& Elsevier]

The Famatinian arc, over central-western Argentina, is one of the few examples of exposed crustal arc cross-section in the world. This Paleozoic magmatic arc and orogenic system on the Earth's surface offers unique insights into the nature of whole-arc processes, continental crust generation, and preservation. A detailed account of birth, growth, and closure is integrated throughout the region, and the entire geodynamic history of the Famatinian arc is presented. The Famatinian magmatic arc grew during a single-cycle episode that spanned a few tens of million years. Upon waning of the Cambrian Pampean orogen, an Upper Cambrian – Lower Ordovician marginal and open sea basin developed mostly on the recently stabilized Cambrian crystalline crust and was flooded by a turbidite wedge. Typical subduction zone magmatism resumed outboard of the Pampean orogen in the Lower Ordovician. After about 20 My of magmatism driven by subduction zone dynamics at plate scale, magmatism waned and stopped with the entry of a Laurentia-rifted continental microplate in the subduction zone. A full phase of a mountain-building process accompanied the continent-arc collision. The orogenic collapse occurred during the Devonian, ending the Famatinian system after nearly 150 My of magmatic arc, plate convergence, and continent-arc collision development on the proto-Andean Gondwana margin. • Geology shows a well-preserved Paleozoic magmatic arc and orogenic system on the Earth's surface. • A full plutonic crustal section formed in about 20 My of subduction-related magmatism. • A segment of Famatinian arc ended sandwiched between a Laurentian microplate and Gondwanan margin. • The subducting continental crust merged the overriding plutonic arc crust to make stable Continental Crust. [ABSTRACT FROM AUTHOR]

The Valle Fértil-La Huerta batholith is a differentially tilted, compositionally stacked, calc-alkaline plutonic sequence in the central Famatinian arc of Argentina. It consists of two major lithologic units: these are (1) an intermediate unit largely dominated by amphibole- and biotite-bearing tonalites but encompassing from gabbro to leucogranite; and (2) an overlying silicic unit that shows the same petrological diversity as the intermediate unit, but granodiorite and monzogranite are its prevailing lithologies. The silicic unit is separated into four subunits using lithological changes at regional mapping scale. All the boundaries are gradational among lithologic units and subunits. The internal fabric along the entire batholith is either magmatic or submagmatic and it was developed during the arc formation. The original magmatic foliation is north–south striking and steeply-dipping. The magmatic foliation crosses over and transposes the gradational contacts among lithologic units. Petrological architecture, structural geology, geobarometric estimates and geochronology show that the entire batholith is a comagmatic system exposing more than 13 km of a middle arc crust. Crystallization-fractionation models computed using whole rock and mineral chemistry account for the mineralogy of plutonic rocks and coincide with the results of experimental petrology. Mass-balanced modeling predicts that the unexposed cumulate roots of the Famatinian arc should have been about 1.3 times larger than the intermediate and silicic plutonic batholith. However, the mass of calculated cumulate would diminish significantly if a portion of the host metasedimentary material were consumed in constructing the batholith. The batholith grew progressing upward from the lowest levels and acquired a stratified compositional organization. The driving mechanism was continuous influxes of mantle-derived mafic magmas that made up the lower crust, caused crustal-level melting and magma mixing, and formed intermediate – silicic rocks at increasingly shallower depths. • The plutonic middle crust of the Famatinian arc was at least 13 km thick. • The plutonic crust grew progressing upward and acquiring a stratified organization. • Pre-arc metasedimentary sequences play a role in fertilizing the magmatic system. • Modeling yields a 1.3:1 ratio of cumulates over derivatives to make the magmatic column. • A few tens of kilometers of cogenetic cumulates are lost in the Famatinian arc. [ABSTRACT FROM AUTHOR]

The Chuschín formation is a sedimentary-volcanic succession of the Famatina System subduction-related arc magmatism. This formation developed along the western margin of Gondwana during the early and middle Ordovician. The interpretation of the Chuschín formation is significant for the understanding of the Famatinian arc. This study provides a detailed facies analysis, which allows characterizing the deposits as well as defining some of the characteristics of the volcanism and sedimentation. Coherent, autoclastic, peperitic, pyroclastic and volcaniclastic facies were recognized between the different volcanic facies. The characteristics and relationships between these facies indicate events of effusive subaqueous volcanism as well as explosive volcanism showing predominantly high temperature features. Both types of volcanism are interbedded with volcanogenic deposits and epiclastic sediment that are associated mainly with turbidity currents. The magmatic and volcanic rocks are dominantly subalkaline, weakly peraluminous and silicic. Two new U-Pb zircon crystallization ages on volcanic rocks from the Chuschín Formation show that this section of the Famatinian arc formed between 463 and 469 Ma (Middle Ordovician). [ABSTRACT FROM AUTHOR]

The Ordovician rocks of Sierra Las Planchadas are an exceptionally preserved natural example of a plutonic-volcanic connection. Located in northwestern Argentina, the Las Planchadas Formation consists of a monzogranitic batholith that displays a window into the source of large subvolcanic rhyolitic rocks, cryptodomes and volcanic feeder-dykes. Mafic intrusions and felsic dykes occur cutting the plutonic rocks. Grain-scale relationships, whole-rock compositions and mineral chemistry indicate that monzogranites and rhyolites are two comagmatic members which differentiated within upper crust magma reservoirs. Monzogranites accumulated some plagioclase and trapped a large volume of melt, denoted by a high modal percentage (~64–71%) of near-solidus minerals, comprised of alkali feldspar, albite-rich plagioclase (An <30), and late crystallized quartz. The formation of felsic dykes and rhyolitic cryptodomes is the result of a late melt extraction stage facilitated by the injection of hot mafic magmas from deeper in the system. The mafic-induced thermal reactivation increased the amount of melt-filled pore stored within a highly-crystallized monzogranite reservoir. The breakdown of the low-temperature phases overpressurized the mush and opened a network of microfractures into which residual melts, depleted in feldspar-compatible trace elements (Ba, Sr, and Eu), infiltrated. The draining system involved thin interconnected vein-like channels that merged into wider sheet-like conduits. Our results suggest that channelized segregation of late-stage melts in the subvolcanic environment is a mechanism for triggering rhyolitic eruptions. • Las Planchadas Formation exposes a monzogranitic-rhyolitic stratified magma chamber of the Famatinian magmatic arc. • The plutonic-volcanic construction involved low-pressure crystal fractionation within a single upper-crustal magmatic system. • Thermal reactivation drove late remobilization of residual melts depleted in feldspar-controlled trace elements. • Channelling of felsic melt through a highly crystallized reservoir acted as a potential volcanic feeding mechanism. [ABSTRACT FROM AUTHOR]