Petrogenesis of Mafic Inclusions in Granitoids of the Adamello Massif, Italy |
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| Authors: | BLUNDY J D; SPARKS R S J |
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| Institution: | Department of Geology, Wills Memorial Building, University of Bristol Bristol BS8 1RJ, UK |
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| Abstract: | ABSTRACT The Tertiary Adamello calc-alkaline batholith in the ItalianAlps is characterized by tonalite and granodiorite plutons associatedwith small mafic/ultramafic intrusions, syn-plutonic mafic dykesand sills, and ubiquitous mafic inclusions. In the southernmostVal Fredda Complex, syn-plutonic hornblende-gabbro and dioritesheets pass laterally into swarms of mafic inclusions intermingledwith tonalite. Petrological and geochemical data show that themafic sheets represent hydrous mafic magmas derived by fractionalcrystallization from parental hydrous basalt and picro-basalt.The fractionation process is recorded by inclusions of spinel,olivine, and pyroxenes in the cores of hornblende phenocrystsand by the widespread occurrence of calcic plagioclase. Fractionationoccurred at high pressure (Ptoul = 8–10 kb) before intrusionat shallow depths (Ptotal 2 kb). Geothermometry and meltingexperiments at PH2O= 1 kb, combined with textural evidence,indicate that the mafic sheets were emplaced at temperaturesof 1050–1100C into hot, but consolidated, granitoid hostrocks. Transfer of heat and hydrous fluids from the sheets remobilizedthe host rocks into crystal-mush, which in turn disrupted thesheet margins to form mafic inclusions. Dynamic crystallizationexperiments indicate that the mafic inclusions and sheet marginswere quenched to temperatures below 970 C, resulting in thefailure of the high-temperature liquidus phases olivine andclinopyroxene to nucleate and the formation of acicular hornblendeand plagioclase. Several other Adamello plutons display syn-plutonicintrusions and mafic inclusions with comparable features tothe Val Fredda Complex. The Adamello mafic inclusions show pronounced enrichments incertain trace elements compared with values expected by fractionalcrystallization and magma mixing. K, Rb, Ba, Y, heavy REE, Mn,and Nb have absolute abundances in the inclusions greater thanthe interiors of neighbouring mafic sheets and, in some cases,than the host granitoids. Many inclusions also display leucocratichaloes, margins rich in ferromagnesian minerals and abundantgroundmass biotite. These features are interpreted in termsof a three-stage evolution. (1) A blob of mafic magma is quenchedby the felsic host to form a rigid crystal-rich inclusion containingan interstitial melt phase. Leucocratic haloes and crenulatemargins to the inclusions form as a result of volume contractionon cooling. (2) The more mobile elements (notably the alkalisand H2O) diffuse between the melt phases of host and inclusion.Using published experimental data on the variation of melt fractionwith temperature in hydrous basic and acid magmas, it is arguedthat the observed diffusion of K from host to inclusion requiresinteraction temperatures of >900C. Reaction of K-enrichedmelt with existing hornblende in the inclusion forms biotite,which sequesters and concentrates further K2O and other alkalineelements. (3) During protracted cooling the mafic inclusionsequilibrate with interstitial melt in the host granitoid. Equilibriumpartitioning of heavy REE and Y into the mafic minerals in theinclusion results in the observed enrichments. Magnetite likewiseconcentrates Nb and Mn. It is proposed that mafic inclusions form in the waning stageof pluton evolution when the granitoid magma is sufficientlyconsolidated to allow the penetration of mafic intrusions, butsufficiently hot to be readily remobilized and disrupt theseintrusions to form mafic inclusions. Subsequent chemical equilibrationof mafic inclusions with their host can have a marked impacton the trace element chemistry of both rock types. Granitoidswhich have experienced extensive interaction with mafic inclusion-formingmagmas may undergo significant depletion in those trace elementswhich partition strongly into the minerals of the mafic inclusion. |
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