Chemical evolution of a large mafic intrusion in the lower crust, Ivrea‐Verbano Zone, northern Italy

Abstract

The Ivrea‐Verbano and adjacent Strona‐Ceneri zones have been described collectively as a section through the continental crust. While resident in the lower crust, amphibolite to granulite‐facies paragneiss of the Ivrea‐Verbano Zone was intruded by huge volumes of mafic to intermediate plutonic rocks grouped as the Mafic Complex. Growth of the Mafic Complex involved hypersolidus deformation in an extensional environment. Isotopic and trace element variations close to the axis of this structure indicate crystallization from mantle‐derived melts that were extensively contaminated by crustal material. Previous investigations determined that the contaminant was fingerprinted by 87Sr/86Sr > 0.71, δ18O = 10–12.5‰, and a positive Eu anomaly. In the present study, the contaminant is also shown to have been enriched in Ba with respect to Rb and K. Charnockites associated with paragneiss septa in the lower part of the Mafic Complex have the appropriate chemistry to be samples of the contaminating material. These chemical features can be explained by melting of granulite‐facies paragneiss, which had previously been depleted in K and Rb by an earlier melting event. The Ba enrichment in the core of the Mafic Complex can be modeled by a replenishment‐tapping‐fractional‐crystallization (RTF) process operating within a small magma chamber is repeatedly replenished by mantle melts and contaminated by Ba‐rich charnockite. Very high Ba/K in the lower part of the complex are tentatively attributed to chemical exchange between the cumulate framework and infiltrating anatectic melts from underlying paragneiss septa. In contrast to the Mafic Complex, the chemistry of coeval granites in the adjacent Strona‐Ceneri zone reflect a component derived from crustal rocks that had not been significantly depleted by a previous melting event. Significantly, the incompatible trace element abundances in the Mafic Complex and Strona‐Ceneri granites are similar to model compositions for the lower and upper crust, respectively.

Crustal-scale magmatic systems during intracontinental strike-slip tectonics: U, Pb and Hf isotopic constraints from Permian magmatic rocks of the Southern Alps

“…A large volume of mafic intrusives known as the ''Mafic Formation'' dominates the southwestern IvreaVerbano Zone and comprises a variety of partly layered gabbros, ultramafic rocks, and diorites (Rivalenti et al 1984;Sinigoi et al 1994;Quick et al 2003). A UPb zircon age of 285 +7/-5 Ma for a diorite (Pin 1986) and SHRIMP U-Pb-ages of 285-287 Ma for the same and other lithologies of the ''Mafic Formation'' (Quick et al 2003) suggest an Early Permian age for the emplacement of diorite magmas in the originally, structurally highest levels in the eastern part of the ''Mafic Formation''.…”

“…Rutter et al 1993;Quick et al 1994) as well as for the entire crustal-scale magmatism (e.g. Handy and Zingg 1991;Sinigoi et al 1994;Snoke et al 1999), despite the poor geochronological database. Melt ascent was proposed to occur through a network of granite sheets (Snoke et al 1999) or through conduits primarily along crustscale transtensional, oblique-slip faults (Handy and Streit 1999;Mulch et al 2002).…”

Contrasting magma types and timing of intrusion in the Permian layered mafic complex of Mont Collon (Western Alps, Valais, Switzerland): evidence from U/Pb zircon and 40Ar/39Ar amphibole dating

“…For the Mont Collon complex, Monjoie (2004) favored a metasomatized lithospheric mantle to account for relatively low εNd280 Ma values between +3.2 and -0.6. In the Ivrea gabbroic complex, the upper part exhibits initial whole-rock isotopic compositions indicating an origin from crustally contaminated liquids (δ 18 O values up to 9.5; +2.9 < εNd290 Ma < +6.4 and 0.7070 < ( 87 Sr/ 86 Sr)290 Ma < 0.7086 ;Pin 1990;Voshage et al 1990;Sinigoi et al 1994). On the other hand, the lower part of the same complex is characterized by isotopic values close to the depleted mantle.…”

Late orogenic magmatism and sedimentation within Late Carboniferous to Early Permian basins in the Balkan terrane (Bulgaria): geodynamic implications

“…An extensional post-orogenic geodynamic regime unrelated to subduction has been stressed by several authors (e.g. Boriani et al 1992;Sinigoi et al 1994). In southwestern Europe, as well as in the studied southeastern Balkan sequences, the surface geodynamic pattern can hardly be related to subduction.…”