CHEMICAL EVOLUTION OF A LARGE MAFIC INTRUSION IN THE LOWER CRUST, IVREA-VERBANO ZONE, NORTHERN ITALY

The Ivrea-Verbano and adjacent Strona-Ceneri zones have been described collectively as a section through the continental crust. While reside nt in the lower crust, amphibolite- to granulite-facies paragneiss of the Ivrea-Verbano Zone was intruded by huge volumes of mafic to interm ediate plutonic rocks grouped as the Mafic Complex. Growth of the Mafi c Complex involved hypersolidus deformation in an extensional environm ent. Isotopic and trace element variations close to the axis of this s tructure indicate crystallization from mantle-derived melts that were extensively contaminated by crustal material. Previous investigations determined that the contaminant was fingerprinted by Sr-87/Sr-86 > 0.7 1, delta(18)O = 10-12.5%, and a positive Eu anomaly. In the present st udy, contaminant is also shown to have been enriched in Ba with respec t to Rb and K. Charnockites associated with paragneiss septa in the lo wer part of the Mafic Complex have the appropriate chemistry to be sam ples of the contaminating material. These chemical features can be exp lained by melting of granulite-facies paragneiss, which had previously been depleted in K and RB by an earlier melting event. The Ba enrichm ent in the core of the Mafic Complex can be modeled by a replenishment -tapping-fractional-crystallization (RTF) process operating within a s mall magma chamber is repeatedly replenished by mantle melts and conta minated by Ba-rich charnockite. Very high Ba/K in the lower part of th e complex are tentatively attributed to chemical exchange between the cumulate framework and infiltrating anatectic melts from underlying pa ragneiss septa. In contrast to the Mafic Complex, the chemistry of coe val granites in the adjacent Strona-Ceneri zone reflect a component de rived 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 simila r to model compositions for the lower and upper crust, respectively.