Field, chemical, and physical constraints on mafic-felsic magma interaction in the Lamarck Granodiorite, Sierra Nevada, California

Abstract

Evidence of magma interactions resulting in both hybridization and mingling are preserved in a diverse suite of gabbroic to intermediate rocks associated with the compositionally zoned hornblende-biotite Lamarck Granodiorite of the eastern Sierra Nevada, California. Ellipsoidal mafic enclaves were formed by quenching of small amounts of high-alumina basaltic magma upon injection into and dispersal through granodiorite magma early in its crystallization. Synplutonic intrusions of hornblende gabbro through hybridized mafic granodiorite represent injection of mafic magma at a later stage of crystallization of the granodiorite, as they crosscut regional trends in foliation and compositional zoning in the host pluton. Where compositional contrasts between intrusion and host granodiorite are large, contacts are sharp and abundant enclaves derived from the mafic intrusion are present in the granodiorite. Where the host is relatively mafic or where the local-scale proportion of mafic magma is large, contacts are zones of extensive hybridization that contain both enclaves and hybrid schlieren. Uncontaminated mafic intrusions have high-alumina basaltic compositions, whereas hybridized intrusions have silica contents as high as 63.5%. Mafic intrusions locally contain coarse-grained cumulus gabbro inclusions. Mafic schlieren in granodiorite far from mafic intrusions represent localized accumulations of hornblende, Fe-Ti oxides, and biotite from the granodiorite. Intrusion of late mafic dikes mobilized and entrained granitic residue from the granodiorite and formed composite dikes of aplite and pillowed diorite. Whether interacting magmas mix or mingle is a function of the heat contents and mass fractions of the end members. Calculations that account for compositions, heats of fusion, heat capacities of liquids and crystals, and a range of initial temperatures, crystallinities, crystal sizes, and magma water contents indicate that in most circumstances the basalt end member quenches; the resulting large viscosity contrast between the end members prevents hybridization. Homogenization is likely only if the compositional difference between host and injected mafic magma is less than 10% SiO2 or if the mass fraction of mafic magma is greater than 0.5. Resulting mixtures have the composition of tonalite or mafic granodiorite; thus, the more silicic rocks of the granodiorite pluton must represent differentiation products rather than direct hybrids of mafic or intermediate magma and felsic magma.