The past decades have seen tremendous advances in analytical capabilities regarding the sensitivity, spatial selectivity, and instrumental precision of U-Th-Pb zircon geochronology. Along with improved zircon pretreatment to mitigate the effects of Pb-loss, these advancements have resulted in the emergence of U-Th-Pb dating as the most widely used geochronometer. In parallel, it became increasingly obvious that modern analytical techniques can resolve zircon age dispersal beyond instrumental uncertainties and that this dispersion cannot be attributed to Pb-loss or inheritance. Hence, there is a pressing need to refine statistical procedures for displaying and interpreting dispersed age data from volcanic and plutonic rocks, where zircon ages were traditionally assigned to the quasi-instantaneous events of eruption and magma emplacement, respectively. The ability to resolve zircon age spectra, which often range over timescales of 103–106 years, also offers new opportunities to monitor magmatic processes, because zircon crystallization directly relates to the temperature and composition of its host melt. This relation is, at least for typical subalkaline melt compositions, well calibrated by multiple zircon saturation experiments, although absolute saturation temperatures derived from them can vary by tens of degrees. Moreover, zircon saturation thermometry is supported by the trace element and isotopic inventory of zircon, which records the thermochemical and compositional evolution of melts at high fidelity. Here, we first review the properties of true zircon age spectra that are defined by a statistically robust overdispersion relative to analytical uncertainties. Secondly, we evaluate existing models and present new models that aim to quantitatively translate the properties of zircon age spectra into parameters controlling the longevity and thermal evolution of crustal magma bodies such as magma recharge flux and duration. These developing approaches, which aspire to capture all processes that affect the formation and dispersal of zircon in dynamic crustal magma systems, have the potential to foster an improved understanding of magmatism with implications for volcanic hazard assessment, geothermal energy uses, and the origins of ore deposits.

Includes database with matched compositions for bulk plutonic rocks and associated minerals; detailed tutorial for using iSpectra®; and more exhaustive set of simulations.

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.

Mafic and ultramafic plutonic rocks are often considered to be crystal cumulates (i.e., they are melt-depleted), but such a classification is much more contentious for intermediate to silicic granitoids (e.g., tonalite, granodiorite, granite, and syenite). Whether or not a given plutonic rock has lost melt to feed shallower subvolcanic intrusive bodies or volcanic edifices has key implications for understanding igneous processes occurring within the crust throughout the evolution of the Earth. We use statistical analyses of a global volcanic and plutonic rock database to show that most mafic to felsic plutonic rock compositions can be interpreted as melt-depleted (i.e., most of the minerals analyzed are more evolved than their bulk-rock compositions would allow). To illustrate the application of the method to natural samples (from the Tertiary Adamello Batholith in the southern Alps), we estimate the degree of melt depletion using a combination of magmatic textures, bulk-rock chemistry, modal mineralogy, distributions of plagioclase composition (using scanning electron microscope phase mapping/electron microprobe analyses), and thermodynamic modeling. We find that melt depletion correlates with the magmatic foliation and is accompanied by bulk depletion in incompatible elements, low amounts of near-solidus minerals, and mineral compositions that are too evolved (i.e., depleted in Ca or Mg, depending on the mineral) to be in equilibrium with their bulk-rock chemistry. The analytical and modeling workflow proposed in this study provides a path to quantifying melt depletion in any plutonic samples.

From their source in the mantle to eruption at the surface, convergent margin magmas follow a complex path involving the recycling of pre-existing crust either at mantle depths (“source” contamination) and/or en-route to the surface (“path” contamination). These two mechanisms have fundamentally different implications for crustal growth versus recycling in subduction zones. However, due to limited access to mantle and deep crustal exposures, the roles of “path” and “source” contamination are usually interpreted on the basis of indirect, often ambiguous geochemical data from volcanic and/or shallow plutonic rocks. Here, we present a case of “source” contamination from an Ordovician magmatic series, the Famatinian continental paleo-arc (Argentina), sampled at all paleo-crustal depths and distributed along a ~600km N-S transect that formed during an 8 Myr long peak magmatic event (474-466Myrs; from high-precision U/Pb zircon dating). Bulk-rock and zircon geochemistry show that all samples belong to a cogenetic differentiation suite. However, zircon Hf-O isotope ratios vary from mantle-to crustal-dominated along the arc transect. These variations cannot be explained by “path” contamination as the nature of the local crust does not change along strike and the most crustal isotopic ratios are unexpectedly carried by the deepest and less differentiated rocks. Rather, the isotopic variations correlate with the thickness of sediments entrained in the source with the paleo-slab at the different latitudes. We propose that overlooking

The presence of a thick continental crust makes Earth a unique planet in the solar system. During post-Archaean times, with the onset of plate tectonics, processes by which continents form is a complex function of juvenile growth and recycling of pre-existing crust. Indeed, post-Archean mantle-derived magmas commonly intrude pre-existing, felsic continental crust. As a result, the origin of upper crustal granitoids, the most accessible products of planetary differentiation, is either accounted for by the melting of the pre-existing mid- to lower crust or the differentiation of mantle-derived mafic magmas. It is therefore critical to identify the relative contribution of these two different granite-forming processes in a given magmatic province, as well as how this relative contribution evolves over time, to assess crustal growth and/or recycling. To shed some light on this question, we used the combination of oxygen, hafnium and uranium-lead isotopic systems in zircons from granitoids of the Ordovician Famatinian Arc (Argentina) representing a typical crust-forming geotectonic setting. While the lower crustal section of Valle Fertíl, representing the basal level of the Famatinian crust, is already well studied, little is known on the timing and nature of igneous processes that built up the mid- and upper crust. From our study, we observe a systematic co-variation of the O and Hf isotopic signatures of zircon in the mid- to upper crustal rocks, from a clearly crustal footprint (granodiorites with zircon δ18O of ca. +8 ‰; εHft of ca. –3) to a mantle-like signature (granites and rhyolites: zircon δ18O of ca. +5 ‰; εHft of ca. +5). Moreover, the high-precision (ID-TIMS) U-Pb dating obtained from the same zircons seem to record a progressive building of the Ordovician continental crust lasting for ca. 13Myrs from 483 to 470 Myrs ago. The results overlap with published ID-TIMS U-Pb data for the Famatinian lower crust, clustering at 470 Myrs, which confirms that the Famatinian Arc was a transcrustal magmatic system ultimately fed by mantle-derived magmas. In details, the oldest granitoids (483 Myrs) show the strongest crustal Hf-O isotopic fingerprint while the younger ones define a continuous range from this end-member towards the mantle signature. These results could be explained by (i) continuous ingrowth and “self-shielding” of lower crustal mafic intrusions progressively decreasing crustal melting or contamination of ascending mafic magma from a homogenous mantle source; (ii) progressive defertilization of an enriched lithospheric mantle or a strongly slab-enriched mantle wedge. The fact that the earliest (483 Myr-old) granitoids also show a more significant crustal contribution (ASI >1.1, inherited zircon cores) supports the first scenario. In this case, the combination of Hf-O isotopic studies as well as high precision U-Pb dating for the Famatinian arc comply with a progressive building of a magmatic column where a certain amount of time is needed for the system to mature and eventually reach mantle dominated processes in the formation of granites and so, new continental crust.

Within the northern Famatina system, the Sierras Narvaez and Las Planchadas expose an almost complete series of plutonic, volcanic-subvolcanic, and sedimentary rocks. On the basis of petrologic, stratigraphic, and geochemistry characteristics are distinguished three lithostratigraphic units, which according to previous studies are named Narvaez Formation, Suri Formation including two members I-Vuelta de las Tolas, II-Loma del Kilometro/Punta Petrea), and Las Planchadas Formation with two members I-Pastos Largos, and II-Punta Colorada. The detailed geological mapping and the examination of the geological relationships among the three lithostratigraphic units allow defining a high rank entity in the northern Famatina system named Chaschuil Group. The Narvaez Formation (482-485 Ma) corresponds to the first stages of the Famatinian magmatic arc between latitudes 27° and 28° South. The later development of peri-volcanic basins filled by volcanogenic apron and shelf volcano-sedimentary deposits correspond of Suri Formation, recognized to be of Ordovician age. The last stage of Famatinian magmatism comprises the Las Planchadas Formation (468-471 Ma). During Las Planchadas evolution, plutonic batholiths, subvolcanic bodies, and lava flows of rhyolitic, dacitic and basaltic composition intruded into, assimilated, and deformed the volcano-sedimentary successions of the Suri Formation. The three lithostratigraphic units of Chaschuil Group temporally and lithologically correlate with ordovician sequences of the Puna Occidental, the Cordon de Lila, and the central Famatina System. Overall, it is the geological record of sedimentation and magmatism over the Famatinian arc.

Abstract The magmatic processes occurring in the lowermost arc crust play a major role in the evolution of mantle-wedge-derived melt. Geological evidence indicates that mantle-derived magmas and in-situ products of lower crust partial melting are reacting in a pervasive melt system and are eventually extracted towards higher levels of the crust. Resolving the relative contribution of mantle-derived magma and partial melting products of pre-existing crust is essential to: (1) quantify crustal growth rate; (2) better understand the compositional range of arc magmatic series; and (3) constrain the chemical differentiation of the lower crust. In this study, we present STyx, a new modelling tool, coupling melt and heat flow with petrology to explore the dynamics of storage, transfer and hybridization of melts in complex liquid/rock systems. We perform three models representing a magmatic event affecting an amphibolitic lower arc crust in order to quantify the relative contribution between partial melting of the pre-existing crust and fractional crystallization from mantle-derived hydrous-magma. Our models demonstrate that most of the differentiated arc crust is juvenile, deriving from the differentiation of mantle melts, and that pre-existing crust does not significantly contribute to the total thickness of magmatic products.

Trace element analysis of rocks and minerals can provide valuable insight in the petrogenesis of the crust and therefore may offer fundamental constraints into geodynamical processes. But crustal building involves a multitude of magmatic/metamorphic processes, and the trace element behaviour during each of these processes is complex, which makes the interpretation of trace element composition of the final crustal product non-unique. Therefore, a thorough investigation of trace element systematics between mineral and melt is required to trace the implication of important stable phases in crustal building processes. This can be given using forward numerical modelling and trace element modelling. However, while numerical models simulating thermodynamic reactions are available, a numerical tool for the prediction of trace element behaviour is missing. Such tool would be of key interest to capture the subtle exchanges during magmatic reactions. Therefore, this thesis presents the building of a novel numerical tool able to predict the variation in trace element partition coefficients between 6 mafic minerals and melt (e.g., garnet, olivine, pyroxene, plagioclase and amphibole). To develop such tool, the state-of-the-art predictive models available in the literature are compiled. These predictive models are of two different kinds. First, the lattice strain model (LSM) is based on the energy exchange of trace elements between mineral and melt. Second, multiple regressions analyses are based on the most accurate fit against various chemical/physical parameters and natural data. In addition, this thesis builds predictive models by applying statistical regressions on large dataset of trace element partition coefficients in the case no models are available in the literature. While the stand-alone version of this tool reproduces well the experimental data, coupling with the numerical code Perple_X, which calculates stable mineral assemblages using Gibbs free energy minimization, allows calculation of a full P-T catalogue of trace element behaviour in the partial melting of an amphibolite. The catalogue gives insights in the assimilation processes in a lower crust in arc settings. This coupling offers a new perspective for future, more complex coupling with numerical codes of melt genesis, transfer and emplacement.