The Ivrea-Verbano Zone of northern Italy provides an opportunity to study directly the effects of intrusion of large volumes of mantle-derived melts into the lower continental crust. Alpine uplift has exposed a complex of mafic to intermediate plutonic rocks, which had intruded the lower continental crust during the Permian. Field mapping reveals a gross arcuate structure within the complex that has survived tilting and uplift. In terms of the orientation during intrusion, banding and foliation are subhorizontal near the base of the complex and steepen gradually up section. Outcrop relations indicate that the cumulates were profoundly affected by extensional deformation while they still contained interstitial melts. The intensity of this hypersolidus deformation increases downward in the complex. The above characteristics are explained in terms of a model in which a huge volume of cumulates (8 km thick and >20 km long) crystallized from a small (<1 km thick and <4 km wide), periodically refilled magma chamber which remained at a relatively fixed position through time. Large-scale ductile deformation of the complex and transport of cumulates downward and outward from the magma chamber occurred as a consequence of extensional tectonics. This process is capable of generating large sheets of gabbro in the lower crust. Gabbro complexes formed in this manner would be characterized by (1) widespread evidence of synmagmatic deformation dominated by stretching, (2) downward increasing intensity of deformation, (3) upward steepening layering and foliation that is concave toward the center of spreading, and (4) a scarcity of crosscutting intrusive relationships.
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.
JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Geology. A B S T R A C TThe southern Ivrea-Verbano Zone of the western Italian Alps contains a huge complex of mafic to intermediate plutonic rocks that intruded the lower continental crust during the Permian. Recent geologic mapping of the complex has characterized the effects of Alpine deformation and has demonstrated that the complex contains an arcuate internal structure. Building on this geologic foundation, we report a thermobarometric study of the plutonic and associated metamorphic rocks that was performed to better constrain the depth and orientation of the complex at the time of intrusion. The results demonstrate a continuous increase in equilibration pressure from 5 Ϯ 1 kb along the eastern intrusive contact of the complex to 8 Ϯ 1 kb near the western limit of the complex. After correcting for the effects of Alpine faulting, the observed pressure gradient ranges from 0.32 to 0.38 kb/km. Given the large uncertainties inherent in the geobarometric calculations relative to the narrow pressure range recorded in the complex, we conclude that the observed gradient is indistinguishable from a normal, vertical pressure gradient in the lower crust. It appears that, following intrusion and equilibration at a depth of 15 to 25 km, the complex was uplifted and rotated approximately 90°. The data also demonstrate that the internal arcuate structure formed in the complex before the observed pressure gradient was established. This result reinforces models for the growth of the complex by synmagmatic deformation and large-scale necking during crustal extension and excludes the possibility that the arcuate structure was produced during the more recent Alpine uplift.
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