Silicic calderas can evacuate 100 to >1000 km3 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–Verbano Zone of 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 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 ∼260 km3 composite silicic intrusion ranging from quartz-monzonite to high-silica leucogranite (∼67–77 wt% SiO2), which intrudes into roughly coeval rhyolitic products of the >15 km diameter Sesia Caldera. In the caldera field, the emplacement of a large, crystal-rich rhyolite ignimbrite(s) (>400 km3) is followed by eruption of minor volumes (1–10 km3) 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 °C higher than in cores (780–820 versus ∼720 °C), 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. 101–102 years). A protracted cooling history is instead recorded in the diffusion timescales of quartz from the silicic cumulate units (c. 104–106 years). 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 Arbansky massif (~70 km^, thickness 1 km) is located in 150 km to northwest from Lake Baikal in the bordering Prisayanian uplift of the Siberian platform Precambrian basement.The uplift formed by 2 large structures: Sharyzhalgay block which formed by the early proterozoic (2.5-2.4Ga) granulite complex, and Onotsky graben -fragment of the late archean granite-greenstone terrane (fig.1).

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 ≈ 0.708 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 ≈ 30% of partially depleted crust and ≈ 15%–30% gabbro fractionation. Trace elements of the deep crustal gabbro cumulates require a further ≈ 60% fractionation of the andesitic basalt and loss of ≈ 40% 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 model which links the production of the two granitic components to the evolution of the Mafic 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 residual melt. 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 famous deep crustal section of the Ivrea-Verbano Zone (IVZ, western Alps, Fig. A) has received enormous attention over the last three decades as one of the best examples of continental "magmatic underplating". Recent investigations, comprising structural, petrochemical and geochronological data, point to the occurrence of a "Sesia-type IVZ" (i.e. central IVZ) and a "Finero-type IVZ" (i.e. northern IVZ), which underwent different magmatic and tectonic evolution. In the Sesia area (box a in Fig. A), the Permian gabbroic pluton known as the mafic complex (reaching thicknesses > 8 km) intruded the deepest rocks of the crustal section, comprising amphibolite to granulite-facies paragneiss and interlayered mantle peridotite bodies, while they were resident in the deep crust. The broader magmatic context of this voluminous intrusion remained unclear until Quick et al. (2009) demonstrated that the emplacement of the mafic complex was coeval to the activity of a mainly silicic volcanic field, including extensive caldera deposits, and to the growth of silicic plutons in the upper crust of the adjacent "Serie dei Laghi". The Sesia magmatic system constitutes an exposure of the plumbing system of a caldera from the surface to a depth of about 25 km (Quick et al., 2009). In this framework, the mafic complex records processes occurring in the deep crust beneath the caldera.The onset of volcanic activity correlates strictly with the climax of the growth of the upper mafic complex, when the mafic intrusion invaded fertile crustal levels and the crust was pervasively heated. In the upper crust, igneous activity was dominated by hybrid silicic melts produced by anatexis in the deep crust, but including significant amounts of mantle component. During the life of the volcanic activity, the mafic complex grew from a relatively small but continuously fed magma chamber according to the "gabbro glacier" process. The excursion will transect the entire igneous system, starting from the deepest exposures of the mafic complex up to reach the outcrops of megabreccia within the caldera fill.The Finero area (box b in Fig. A) is characterized by the occurrence of a pervasively-metasomatised mantle unit made by phlogopite-bearing ultramafic rocks (i.e. spinel-facies harzburgites, dunites and pyroxenites). These rock types were produced by several episodes of pervasive-to-channeled porous flow migration of K-LILE-Mgenriched hydrous melts containing large amount of crustal components. The mantle unit is surrounded by a layered mafic-ultramafic intrusion, i.e. the Finero mafic complex, comprising garnet hornblendites, cumulus amphibole peridotites, amphibole gabbros and diorites, with tholeiitic to transitional geochemical affinity.Recent U-Pb zircon data point to a Middle-Triassic intrusion age for the Finero mafic complex, which may thus represent the deep-crustal counterparts of the Middle-Upper Triassic volcanism widespread throughout the Southern Alps. In any case, the Finero mafic complex can no longer be considered as part of the Permian mafic complex exposed in the Sesia area. Instead, U-Pb ages of zircons from massive chromitites of the mantle unit are Lower Jurassic. The marked age span of the Finero mafic complex and the associated mantle unit suggests that they experienced different evolutions until Lower Jurassic, and were subsequently tectonically juxtaposed during the opening of the Jurassic Neo-Tethys or later. The geodynamic setting related to the intrusion of the Finero mafic complex, the sources, the age and geodynamic environment of the metasomatism of the mantle unit and the age of the emplacement of the latter in contact with the crustal rocks are some of the issues that will be discussed in this guide.This excursion aims to illustrate the Sesia-type IVZ as a complete sequence of a section from the mantle to a supervolcano and show the different geochronological, petrographic and geochemical characteristics between the Sesia-type and the Finero-type IVZ in the light of the results of latest research.

The Ivrea–Verbano Zone and Serie dei Laghi of northern Italy constitute a virtually complete section of the Adriatic continental crust, which was intruded at different levels by coeval magmas of Permo-Carboniferous age, collectively referred to as the “Sesia Magmatic System”. At the deepest levels of the section, the relationship between magmatic underplating and granulite-facies metamorphism in the Ivrea–Verbano Zone may be studied. We present new zircon U–Pb age data, which significantly extend the record of igneous activity in this terrane, and which demonstrate that igneous pulses began intruding the deep and middle crust no later than 314 Ma, an age corresponding to that inferred for regional granulite-facies metamorphism and predating by more than 20 m.y. the well-documented, main magmatic pulse at about 292 to 282 Ma. A gap in age information from 316 ± 3 Ma to 276 ± 4 Ma, observed in the deep-crustal granulite-facies paragneiss of the Ivrea–Verbano Zone overlaps a gap from circa 314 to 283 Ma between igneous and reset zircons in a closely associated mafic–ultramafic sill. However, zircon ages demonstrate intrusive activity in the middle and upper crust during this time gap. We infer that prolonged maintenance of temperatures above zircon saturation in the deep-crustal paragneiss could explain the gap in zircon ages in those rocks. Viewed in this light, a gap in a geochronological record within high-grade terranes might not necessarily mean an absence of events, but instead may record the occurrence and duration of a thermal peak.

New mapping, geochemistry, and 17 U-Pb SHRIMP zircon ages from rocks of the Sirwa, Bou Azzer-El Graara, and Jebel Saghro inliers constrain the Neoproterozoic evolution of the eastern Anti-Atlas during Pan-African orogenesis. In the Sirwa inlier, Tonian quartzite from the pre Pan-African passive margin deposits of the Mimount Formation contains detrital zircon derived entirely from the West African Craton (WAC), with most grains yielding Eburnean Paleoproterozoic ages of about 2050 Ma. Cryogenian Pan-African orogenic activity (PA1) from about 760 to 660 Ma included northward-dipping subduction to produce a volcanic arc, followed by ophiolite obduction onto the WAC. In the Bou Azzer-El Graara inlier, calc-alkaline granodiorite and quartz diorite, dated at 650-646 Ma, are syn- to post-tectonic with respect to the second period of Pan-African orogenesis (PA2), arc-continent accretion, and related greenschist facies metamorphism. Slab break-off and lithospheric delimination may have provided the source for the supra-subduction calc-alkaline plutons. At about 646 Ma, quartz diorite intruded the Tiddiline formation placing an upper limit on molassic deposition. Widespread Ediacaran high-K calc-alkaline to shoshonitic plutonism and volcanism during the final stage of Pan-African orogenesis (PA3) occurred in a setting related to either modification of the margin of the WAC or formation of a continental volcanic arc above a short-lived southward-dipping subduction zone. In the Saghro inlier, eight plutonic rocks yield ages ranging from about 588 to 556 Ma. Sampled plutonic rocks previously considered to be Cryogenian yielded Ediacaran ages. Peraluminous rhyolitic volcanic rocks in the lower part of the Ouarzazate Supergroup, including ash-flow tuffs of the Oued Dar'a caldera, yield ages between about 574 and 571 Ma. The Oued Dar'a caldera developed in a pull-apart graben produced by a left-step in a northeast-trending, left-lateral strike-slip fault zone, and much of the lower Ouarzazate Supergroup volcanic rocks in the area are probably related to caldera out-flow facies and collapse. Late stage PA3 intrusive rocks include the Bouskour-Sidi Flah and Timijt rhyolitic dike swarms at about 563 Ma, the voluminous pink Isk-n-Alla granite (559 +/- 5 Ma), and volumetrically minor gabbro of Tagmout (556 +/- 5 Ma). Rhyolite flows from the upper part of the Ouarzazate Supergroup, above a regional angular unconformity, yielded ages of 558 +/- 4 and 556 +/- 4 Ma. The youngest ages place an upper limit on block faulting and weak folding during latest Pan-African tectonic activity (PA3), coincident with the departure of the Cadomian crustal fragment from the northern margin of the WAC. Published by Elsevier B.V.

The Sesia magmatic system of northwest Italy allows direct study of the links between silicic plutonism and volcanism in the upper crust and the coeval interaction of mafic intrusions with the deep crust. In this paper, we focus on the chemical stratigraphy of the pre-intrusion crust, which can be inferred from the compositions of crustal-contaminated mafic plutonic rocks, restitic crustal material incorporated by the complex, and granitic rocks crystallized from anatectic melts. These data sources independently indicate that the crust was compositionally stratified prior to the intrusion of an 8-km-thick gabbroic to dioritic body known as the Mafic Complex, with mica and K-feldspar abundance decreasing with depth and increasing metamorphic grade. Reconsideration of published zircon age data suggest that the igneous evolution initiated with sporadic pulses at around 295 Ma, when mafic sills intruded deep granulites which provided a minor amount of depleted crustal contaminant, very poor in LIL elements. With accelerated rates of the intrusion, between 292 and 286 m.y, mafic magmas invaded significantly more fertile, amphibolite-facies paragneisses, resulting in increased contamination and generating hybrid rocks with distinct chemistry. At this point, increased anatexis produced a large amount of silicic hybrid melts that fed the incremental growth of upper-crustal plutons and volcanic activity, while the disaggregated restite was largely assimilated once ingested by the growing Mafic Complex. This "igneous climax" was coincident with an increasing rate of intrusion, when the upper Mafic Complex began growing according to the "gabbro glacier" model and, at about the same time, volcanic activity initiated. Cooling lasted millions of years. In the coupled magmatic evolution of the deep and upper crust, the Mafic Complex should be considered more as a large reservoir of heat rather than a source of upper-crustal magma, while the fertility of "under/intra-plated" crust plays a crucial role in governing the generation of large volumes of continental silicic magmas.

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We report evidence for a >= 13-km-diameter Permian caldera associated with a renowned section through the continental crust comprising the Ivrea-Verbano Zone and Serie dei Laghi of northwest Italy. Correlation of ages of volcanic and middle to deep crustal plutonic rocks suggests that they constitute an unprecedented exposure of a subcaldera magmatic plumbing system to a depth of 25 km, and points to a cause and effect link between intrusion of mantle-derived basalt in the deep crust, and large-scale, silicic volcanism.

During the ongoing eruption at Mount St. Helens, Washington, lava has extruded continuously at a rate that decreased from ~7-9 m3 /s in October 2004 to 1-2 m3 /s by December 2005. The volume loss in the magma reservoir estimated from the geodetic data, 1.6-2.7×10 7 m3 , is only a few tens of percent of the 7.5×10 7 m3 volume that had erupted by the end of 2005.

The Ivrea-Verbano Zone in the western Italian Alps contains one of the worlds classic examples of ponding of mantle-derived, mafic magma in the deep crust. Within it, a voluminous, composite mafic pluton, the Mafic Complex, intruded lower-crustal, high-grade paragneiss of the Kinzigite Formation during Permian-Carboniferous time, and is now exposed in cross-section as a result of Alpine uplift. The age of the intrusion is still debated because the results of geochronological studies in the last three decades on different rock types and with various dating techniques range from 250 to about 300 Ma. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon age determinations on 12 samples from several locations within the Mafic Complex were performed to better constrain the age of the igneous event. The results indicate a long history of magma emplacement and cooling, which reconciles the spread in previously published ages. The main intrusive phase took place at 288 +/- 4 Ma, causing a perturbation of the deep-crustal geotherm, which relaxed to the Sm-Nd closure temperature in garnet-free inafic rocks after about 15-20 Myr of sub-solidus cooling at c. 270 Ma. These results suggest that large, deep crustal plutons, such as those identified geophysically at depths of 10-20 km within extended continental crust (e.g.) Yellowstone, Rio Grande Rift, Basin and Range) may have formed rapids but induced a prolonged thermal perturbation. In addition, the data indicate that a significant thermal event affected the country rock of the Mafic Complex at about 310 Ma. The occurrence of an upper amphibolite- to granulite-facies thermal event in the Kinzigite Formation prior to the main intrusive phase of the Mafic Complex has been postulated by several workers, and is corroborated by other geochronological investigations. However, it remains uncertain whether this event (1) was part of a prolonged perturbation of the deep-crustal geotherm, which started long before the onset of intrusion of the Mafic Complex, or (2) corresponded to the intrusion of the first sills of the Mafic Complex, or (3) was related to an earlier, independent thermal pulse.