Production of hybrid granitic magma at the advancing front of basaltic underplating: Inferences from the Sesia Magmatic System (south-western Alps, Italy)

The Permian Sesia Magmatic System of the southwestern Alps displays the plumbing system beneath a Permian caldera, including a deep crustal gabbroic complex, upper crustal granite plutons and a bimodal volcanic field dominated by rhyolitic tuff filling the caldera. Isotopic compositions of the deep crustal gabbro overlap those of coeval andesitic basalts, whereas granites define a distinct, more radiogenic cluster (Sri 480.708 and 0.710, respectively). AFC computations starting from the best mafic candidate for a starting melt show that Nd and Sr isotopic compositions and trace elements of andesitic basalts may be modeled by reactive bulk assimilation of 4830% of partially depleted crust and 4815%\u201330% gabbro fractionation. Trace elements of the deep crustal gabbro cumulates require a further 4860% fractionation of the andesitic basalt and loss of 4840% of silica-rich residual melt. The composition of the granite plutons is consistent with a mixture of relatively constant proportions of residual melt delivered from the gabbro and anatectic melt. Chemical and field evidence leads to a conceptual modelwhich links the production of the two granitic components to the evolution of theMafic Complex. During the growth of the Mafic Complex, progressive incorporation of packages of crustal rocks resulted in a roughly steady state rate of assimilation. Anatectic granite originates in the hot zone of melting crust located above the advancing mafic intrusion. Upward segregation of anatectic melts facilitates the assimilation of the partially depleted restite by stoping. At each cycle of mafic intrusion and incorporation, residual and anatectic melts are produced in roughly constant proportions, because the amount of anatectic melt produced at the roof is a function of volume and latent heat of crystallization of the underplated mafic melt which in turn produces proportional amounts of hybrid gabbro cumulates and residualmelt. Such a process can explain the restricted range in isotopic compositions of most rhyolitic and granitic rocks of the Permo-Carboniferous province of Europe and elsewhere

The growth of large mafic intrusions: Comparing Niquelandia and Ivrea igneous complexes

The Niquelandia Complex, Brazil, is one of the world's largest mafic-ultramafic plutonic complexes. Like the Mafic Complex of the Ivrea-Verbano Zone, it is affected by a pervasive high-T foliation and shows hypersolidus deformation structures, contains significant inclusions of country-rock paragneiss, and is subdivided into a Lower and an Upper Complex. In this paper, we present new SHRIMP U-Pb zircon ages that provide compelling evidence that the Upper and the Lower Niquelandia Complexes formed during the same igneous event at ca. 790 Ma. Coexistence of syn-magmatic and high-T subsolidus deformation structures indicates that both complexes grew incrementally as large crystal mush bodies which were continuously stretched while fed by pulses of fresh magma. Syn-magmatic recrystallization during this deformation resulted in textures and structures which, although appearing metamorphic, are not ascribable to post-magmatic metamorphic event(s), but are instead characteristic of the growth process in huge and deep mafic intrusions such as both the Niquelandia and Ivrea Complexes. Melting of incorporated country-rock paragneiss continued producing hybrid rocks during the last, vanishing stages of magmatic crystallization. This resulted in the formation of minor, late-stage hybrid rocks, whose presence obscures the record of the main processes of interaction between mantle magmas and crustal components, which may be active at the peak of the igneous events and lead to the generation of eruptible hybrid magmas. (C) 2012 Elsevier B.V. All rights reserved.Research Support Foundation of the State of Sao Paulo (FAPESP)Brazilian National Research Council (CNPq

An oxygen isotope and chemical study of the Ivrea Zone, Italy: interaction between mafic magmas and lower crustal metamorphic rocks

The Ivrea Zone is arguably the best example of an exposed tectonic slice through middle to lowermost continental crust. The study area is dominated by a large, composite, ultramafic-mafic intrusion termed the Mafic Complex, which was emplaced during the late Hercynian (~260-300 Ma) into a middle amphibolite- to granulite-facies, metavolcanic-sedimentary sequence. Oxygen isotope analyses of ~130 samples from the Mafic Complex comprise the most complete data base presently available for underplated lower continental crust

SHORT MOBILIZATION AND ERUPTION TIMESCALES RECORDED IN THE QUARTZ CRYSTALS OF A FOSSIL CALDERA PLUMBING SYSTEM, SESIA MAGMATIC SYSTEM, SOUTHERN ALPS

Granitic intrusions are not considered the ideal target for the study of short-lived, transient processes associated with remobilization and eruption of highly crystalline silicic magmas. However, crystal zoning preserved in phenocrysts from fossil upper crustal crystal mushes can retain information on the timescales and reactivation dynamics of silicic magma chambers. In the Southern Alps, the plumbing system of a Permian rhyolitic caldera is exposed to a depth of about 25 Km in tilted crustal blocks. The mid- to upper-crustal segment of this magmatic system (a.k.a. Sesia Magmatic System) is represented by a monzogranitic intrusion ( 4867 to 77 wt% SiO ), the Valle Mosso pluton (VMP), which intrudes cogenetic rhyolitic products of the >15 km diameter Sesia caldera. Field and petrographic evidence suggest that a significant portion of the VMP (ca.15% of the intrusion volume) underwent one or more rejuvenation and mobilization episodes. Titanium (Ti) in quartz content in grains from granitic and volcanic units of the Sesia Magmatic System has been investigated through cathodoluminescence (CL) imaging and microprobe (EPMA) analyses. Sharp contrast in concentration between Ti-poor cores and Ti-rich rims is observed in most of the granitic and volcanic quartz grains. Application of TitaniQ thermometer indicates sharp temperature increase across core-rim boundaries (\u394T of min 50 \ub0C to max 100 \ub0C) assuming uniform a TiO2 and pressure at the time of crystallization. Furthermore, one-dimensional modeling of Ti diffusion core-rime interfaces indicate short elapsed time (10s of years) between crystallization of the high-T rim and cooling of the system below magmatic temperatures, with striking similar results obtained for quartz grains from rejuvenated portion of VMP and volcanic products. These results suggest that a short-lived thermal flare-up, possibly related to mixing with a batch of hotter, more mafic magma, interested the upper portion of the Sesia Magmatic System during its upper crustal residence as a crystal mush, triggering remobilization and eruption of portions of the magma chamber. Such short timescales, typical of explosive eruptive processes, have never been identified before in fossil magma chambers, making this discovery relevant in the framework of the ongoing volcano-plutonic connection debate

Mantle versus crustal contributions in crustal‐scale magmatic systems (Sesia Magmatic System, northern Italy) from coupling Hf isotopes and numerical modelling

The growth and evolution of crustal-scale magmatic systems play a key role in the generation of the continental crust, the largest eruptions on Earth, and the formation of metal resources vital to our society. However, such systems are rarely exposed on the Earth’s surface, limiting our knowledge about the magmatic processes occurring throughout the crust to indirect geo- chemical and petrographic data obtained from the shallowest part of the system. The Hf isotopic composition of accessory zircon is widely used to quantify crust-mantle evolution and mass transfers to and within the crust. Here we combine single- grain zircon Hf isotopic analysis by LA-MC-ICP-MS with thermal modelling to one of the best-studied crustal-scale igneous systems (Sesia Magmatic System, northern Italy), to quantify the relative contribution of crustal- and mantle-derived magmas in the entire system. Zircons from the deep gabbroic units define a tight range of εHf (−2.5 ± 1.5). Granites and rhyolites overlap with this range but tail towards significantly more negative values (down to −9.5). This confirms that the entire system consists of hybrid magmas that stem from both differentiation of mantle-derived magmas and melting of the crust. Thermal modelling suggests that crustal melting and assimilation predominantly occurs during emplacement and evolution of magmas in the lower crust, although melt production is heterogeneous within the bodies both spatially and temporally. The spatial and temporal heterogeneity resolved by the thermal model is consistent with the observed Hf isotope variations within and between samples, and in agreement with published bulk-rock Sr–Nd isotopic data. On average, the crustal con- tribution to the entire system determined by mixing calculations based on Hf isotopic data range between 10 and 40%, even with conservative assumptions, whereas the thermal model suggests that this space- and time-averaged contribution does not exceed 20%. However, spatial and temporal variations in the crustal melt proportion (from 0 up to 80% as observed in the thermal model) may impart significant isotopic variability to different batches of magma observed on the outcrop scale, emphasizing the need to consider a magmatic system as a whole, i.e., by integrating all spatial and temporal scales, to more precisely quantify crustal growth vs. reworking

Timescales and Mechanisms of Crystal-mush Rejuvenation and Melt Extraction Recorded in Permian Plutonic and Volcanic Rocks of the Sesia Magmatic System (Southern Alps, Italy)

Silicic calderas can evacuate 100 to >1000\u2009km3 of rhyolitic products in a matter of days to months, leading to questions on pre-eruptive melt generation and accumulation. Whereas silicic plutonic units may provide information on the igneous evolution of crystal-mush bodies, their connection with volcanic units remains enigmatic. In the Ivrea\u2013Verbano Zone of the southern Alps, the plumbing system of a Permian rhyolitic caldera is exposed to a depth of about 25\u2009km in tilted crustal blocks. The upper-crustal segment of this magmatic system (also known as the Sesia Magmatic System) is represented by the Valle Mosso pluton (VMP). The VMP is an 3c260\u2009km3 composite silicic intrusion ranging from quartz-monzonite to high-silica leucogranite ( 3c67\u201377\u2009wt% SiO2), which intrudes into roughly coeval rhyolitic products of the >15\u2009km diameter Sesia Caldera. In the caldera field, the emplacement of a large, crystal-rich rhyolite ignimbrite(s) (>400\u2009km3) is followed by eruption of minor volumes (1\u201310\u2009km3) of crystal-poor rhyolite. Here, we compare silicic plutonic and volcanic units of the Sesia Magmatic System through a combination of geochemical (X-ray fluorescence, inductively coupled plasma mass spectrometry and electron microprobe analyses) and petrological (rhyolite-MELTS, trace element and diffusion modeling) tools to explore their connection. Textural and compositional features shared by both VMP and crystal-rich ignimbrites imply thermal rejuvenation of crystal-mush as the mechanism to create large volumes of eruptible rhyolitic magma. Bulk-rock composition of crystal-rich rhyolite erupted during the caldera collapse overlaps that of the bulk VMP. Quartz and plagioclase from these two units show resorbed cores and inverse zoning, with Ti- and anorthite-rich rims, respectively. This indicates crystallization temperatures in rims >60\u2009\ub0C higher than in cores (780\u2013820 versus 3c720\u2009\ub0C), if temperature is the sole parameter responsible for zonation, suggesting heating and partial dissolution of the crystal-framework. Decrease in crystallinity associated with thermal energy input was calculated through rhyolite-MELTS and indicates lowering of the mush crystal fraction below the rheological lock-up threshold, which probably promoted eruptive activity. Also, after the climatic eruption, Si-rich melts in the Sesia Magmatic System were produced by extraction of interstitial melt from un-erupted, largely crystalline mush. Regarding both textures and chemical variations, we interpret the deep quartz-monzonite unit of the VMP as a compacted silicic cumulate. Fractionated melts extracted from this unit were emplaced as a leucogranite cupola atop the VMP, generating the final internal architecture of the silicic intrusion, or alternatively erupted as minor post-caldera, crystal-poor rhyolite. Ti-in-quartz diffusion profiles in thermally rejuvenated units of the Sesia Magmatic System demonstrate that the process of reheating, mobilization and eruption of crystal-mush took place rapidly (c. 101\u2013102\u2009years). A protracted cooling history is instead recorded in the diffusion timescales of quartz from the silicic cumulate units (c. 104\u2013106\u2009years). These longer timescales encompass the duration of evolved melt extraction from the cumulate residue. We argue that the VMP preserves a complex record of pre-eruptive processes, which span mechanisms and timescales universally identified in volcanic systems and are consistent with recently proposed numerical models

The growth and contamination mechanism of the Cana Brava layered mafic-ultramafic complex: new field and geochemical evidences

The Cana Brava complex is the northernmost of three layered complexes outcropping in the Goiás state (central Brasil). New field and geochemical evidences suggest that Cana Brava underwent hyper- to subsolidus deformation during its growth, acquiring a high-temperature foliation that is generally interpreted as the result of a granulite- facies metamorphic event. The increase along the stratigraphy of the incompatible elements abundances (LREE, Rb, Ba) and of the Sr isotopic composition, coupled with a decrease in εNd(790), indicate that the complex was contaminated by the embedded xenoliths from the Palmeirópolis Sequence. The geochemical data suggest that the contamination occurred along the entire magma column during the crystallization of the Upper Mafic Zone, with in situ variations determined by the abundance and composition of the xenoliths. These features of the Cana Brava complex point to an extremely similarity with the Lower Sequence of the most known Niquelândia intrusion (the central of the three complexes). This, together with the evidences that the two complexes have the same age (c.a. 790 Ma) and their thickness and units decrease northwards suggests that Cana Brava and Niquelândia are part of a single giant Brasilia body grown through several melt impulses