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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]

Subduction zone magmatism plays a crucial role in driving the exchange of chemical elements between Earth's interior and surface. This survey focuses on the compositional analysis of primitive mafic rocks within an ancient Early Palaeozoic subduction-related magmatic belt. The primary objective is to assess the petrological processes involved in the cycling of chemical elements from the Earth's surface, through the slab and mantle reservoirs, to the crustal arc magmatic systems. By examining the composition of primitive mafic rocks and utilizing inverse modelling techniques, we estimate the composition of the ambient mantle source, which appears to have undergone depletion due to melt extraction before being metasomatized by slab agents. The inverse approach reveals that fluid-mobile elements (Th, U, La, Ce, and Sr) were primarily derived from the slab contribution and suggests that subduction had influenced the abundances of Nb and Ta in the modified mantle source. Conversely, Zr, Hf, and Ti were sourced from the ambient mantle. However, the available mineral/melt partition coefficients used in inverse modelling may overestimate the contribution of Sr and Nb from the slab. Instead, forward modelling is limited by the incomplete preservation of ancient volcanic arc geology. The forward mixing models that consider the interaction between slab-released sedimentary-derived melts and the ambient depleted mantle can explain the observed variations in Nd and Sr isotopic ratios in primitive mafic rocks. Two-component mixing shows that a small mass fraction (≤1.5 wt%) of average Cambrian continental sediments subducted beneath the Early Ordovician arc into a MORB-depleted mantle source account for the abundance of sedimentary-supplied elements such as Th, La, and Ba, as well as radiogenic isotope ratios of 143Nd/144N and 87Sr/86Sr in the primitive mafic magmas. The chemical and isotopic composition of deep-seated primitive mafic rocks reveals a decoupling between incompatible trace elements, which display minimal variation, and radiogenic isotopic ratios, which exhibit a wide range and reflect contributions from continental crustal reservoirs. To explain the observed variations in trace elements and isotopes, we propose a simple model involving simultaneous bulk assimilation (A) and imperfect fractional crystallization (IFC) in an open igneous system, where crystal accumulation and assimilation occur concurrently. Especially, deep crustal physicochemical interaction of primitive arc magmas with supracrustal host rocks may mask the expected clear signature of subarc mantle petrological processes. Our assessment of the Famatinian-Puna arc, an ancient subduction zone, concurs with estimates from active volcanic arcs, underscoring the general applicability of this study case to subduction zone magmatism. [Display omitted] • The study stresses underlying phenomena complicating the geochemical interpretation. • The pre-arc continental crust is an essential part of the subduction factory. • Crustal contamination could mask the signature of the subducting sedimentary layer. • Lithophile trace elements influx into the mantle wedge comes from subducted sediments. [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]