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Influence of stretching and density contrasts on the chemical evolution of continental magmas: an example from the Ivrea-Verbano Zone
No full textThe role of crustal fertility in the generation of large silicic magmatic systems triggered by intrusion of mantle magma in the deepcrust
No full textThe 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
Magmatic underplating and incremental growth of a granite pluton in the Sesia Magmatic System
No full textDevelopment of s deep crustal shear zone in response to syntectonic intrusion of mafic magma into the lower crust, Ivrea-Verbano Zone, Italy
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Emplacement and rejuvenation of a granitic batholith: the Valle Mosso plutob (Sesia Magmatic System)
No full textThe Lower Permian Valle Mosso pluton (VMP) is a granitic body intruded at upper crustal levels in the rocks of the pre-Alpine basement of the Ivrea-Verbano and Serie dei Laghi units, respectively. The VMP has been recognized as an integral part of a magmatic system termed Sesia magmatic System (Quick et al., 2009), which during the Lower Permian developed through the continental crust up to the surface and caused explosive rhyolitic volcanism that eventually led to the formation of a > 15 km diameter rhyolitic caldera. Fieldwork helped in constraining the VMP internal geometry, with the distinction of several laccolithshaped intrusive units noticeable for their modal abundances and textural features. Equigranular coarsegrained granite constitutes the bulk of the intrusion, together with subordinate volumes of porphyric granite and numerous small fine-grained two-micas intrusive bodies. A small body (roughly 0.5 by 1 km) of porphyry occurs within the porphyric granite and presents irregular to gradational contacts to its host rock. Major and trace-element and isotopic composition of this porphyry is equal to that of the surrounding granite. However, porphyry matrix and phenocrysts present microtextural features that indicate resorption and undercooling. These features (sieve textures of plagioclase, quartz and biotite resorption) paired with indications from Ti-in- Qz geothermometer on porphyry samples (Wark & Watson, 2006) are descriptive of temperature fluctuations within a crystal mush. Field evidence indicate that the Valle Mosso pluton experienced numerous episodes of mafic melt injections during different stages of its incremental growth. These mafic melts produced significant effects on the thermal budget of the granitic intrusion, rejuvenating and possibly mobilizing batches of the VMP. Based on similar whole-rock composition and reabsorption texture observed in coeval granite porphyry and volcanic products of the Sesia Caldera, a possible link between rejuvenated granitic melts and eruptive products has been postulated. The VMP may provide an insight into the storage and remobilization process that drive the eruption of large amount of melts in caldera-forming volcanic systems
Hybrid granitic magma originated at the advancing front of basaltic underplating: inferences from the Sesia Magmatic System (Wesyern Alps)
No full textMost rhyolitic and granitic rocks of the large Permo-Carboniferous province of Europe show a restricted range in isotopic compositions, intermediate between mantle and crustal values. We propose an explanationto the relative homogeneity of these hybrid granitic magmas based on geochemistry and field observationsof the Sesia Magmatic System, which includes a deep crustal gabbroic complex, upper crustal granite plutons and a volcanic field dominated by rhyolitic caldera fill tuff (Quick et al., 2009). Isotopic compositionsof the deep crustal gabbro overlap those of coeval andesitic basalts, whereas coeval granites define adistinct, more radiogenic cluster (Sri ≈ 0.708 and 0.710, respectively). AFC computations starting from thebest mafic candidate for a starting melt show that isotopic compositions and trace elements of andesitic
basalts may be modelled by assimilation of about 30% of partially depleted crust and about 15-29% fractionation. Trace elements of the deep crustal gabbro cumulates require a further ≈ 60% fractionation of the andesitic basalt and loss of about 40% of silica-rich residual melt (Sinigoi et al., 2016). The compositionof the granite pluton is consistent with a mixture of almost equal parts of residual melt delivered from the gabbro and the anatectic melt. Chemical and field evidence lead to infer a conceptual model which links the production of the two granitic components to the evolution of the gabbroic complex. During the growth of the gabbroic complex, progressive incorporation of packages of crustal rocks resulted in a roughly steady-state rate of assimilation. Upwards segregation of anatectic melts delivered from the hot zone above the
advancing mafic intrusion facilitates reactive bulk assimilation of the restite by density-driven stoping. At each cycle of mafic intrusion and incorporation of roof layers, residual and anatectic melts are produced in more or less constant proportions, because the amount of anatectic melt produced at the provisional roof is a function of volume and latent heat of crystallization of the intruded 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
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