Petrogenesis of Mafic Inclusions in Granitoids of the Adamello Massif, Italy

Abstract

ABSTRACT The Tertiary Adamello calc-alkaline batholith in the Italian Alps is characterized by tonalite and granodiorite plutons associated with small mafic/ultramafic intrusions, syn-plutonic mafic dykes and sills, and ubiquitous mafic inclusions. In the southernmost Val Fredda Complex, syn-plutonic hornblende-gabbro and diorite sheets pass laterally into swarms of mafic inclusions intermingled with tonalite. Petrological and geochemical data show that the mafic sheets represent hydrous mafic magmas derived by fractional crystallization from parental hydrous basalt and picro-basalt. The fractionation process is recorded by inclusions of spinel, olivine, and pyroxenes in the cores of hornblende phenocrysts and by the widespread occurrence of calcic plagioclase. Fractionation occurred at high pressure (Ptoul = 8–10 kb) before intrusion at shallow depths (Ptotal≈ 2 kb). Geothermometry and melting experiments at PH2O= 1 kb, combined with textural evidence, indicate that the mafic sheets were emplaced at temperatures of 1050–1100°C into hot, but consolidated, granitoid host rocks. Transfer of heat and hydrous fluids from the sheets remobilized the host rocks into crystal-mush, which in turn disrupted the sheet margins to form mafic inclusions. Dynamic crystallization experiments indicate that the mafic inclusions and sheet margins were quenched to temperatures below 970 C, resulting in the failure of the high-temperature liquidus phases olivine and clinopyroxene to nucleate and the formation of acicular hornblende and plagioclase. Several other Adamello plutons display syn-plutonic intrusions and mafic inclusions with comparable features to the Val Fredda Complex. The Adamello mafic inclusions show pronounced enrichments in certain trace elements compared with values expected by fractional crystallization and magma mixing. K, Rb, Ba, Y, heavy REE, Mn, and Nb have absolute abundances in the inclusions greater than the interiors of neighbouring mafic sheets and, in some cases, than the host granitoids. Many inclusions also display leucocratic haloes, margins rich in ferromagnesian minerals and abundant groundmass biotite. These features are interpreted in terms of a three-stage evolution. (1) A blob of mafic magma is quenched by the felsic host to form a rigid crystal-rich inclusion containing an interstitial melt phase. Leucocratic haloes and crenulate margins to the inclusions form as a result of volume contraction on cooling. (2) The more mobile elements (notably the alkalis and H2O) diffuse between the melt phases of host and inclusion. Using published experimental data on the variation of melt fraction with temperature in hydrous basic and acid magmas, it is argued that the observed diffusion of K from host to inclusion requires interaction temperatures of >900°C. Reaction of K-enriched melt with existing hornblende in the inclusion forms biotite, which sequesters and concentrates further K2O and other alkaline elements. (3) During protracted cooling the mafic inclusions equilibrate with interstitial melt in the host granitoid. Equilibrium partitioning of heavy REE and Y into the mafic minerals in the inclusion results in the observed enrichments. Magnetite likewise concentrates Nb and Mn. It is proposed that mafic inclusions form in the waning stage of pluton evolution when the granitoid magma is sufficiently consolidated to allow the penetration of mafic intrusions, but sufficiently hot to be readily remobilized and disrupt these intrusions to form mafic inclusions. Subsequent chemical equilibration of mafic inclusions with their host can have a marked impact on the trace element chemistry of both rock types. Granitoids which have experienced extensive interaction with mafic inclusion-forming magmas may undergo significant depletion in those trace elements which partition strongly into the minerals of the mafic inclusion.