Thomas P. Frost

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

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.

Evidence from the Lamarck Granodiorite for Rapid Late Cretaceous Crust Formation in California

Strontium and neodymium isotopic data for rocks from the voluminous 90-million-year-old Lamarck intrusive suite in the Sierra Nevada batholith, California, show little variation across a compositional range from gabbro to granite. Data for three different gabbro intrusions within the suite are identical within analytical error and are consistent with derivation from an enriched mantle source. Recognition of local involvement of enriched mantle during generation of the Sierran batholith modifies estimates of crustal growth rates in the United States. These data indicate that parts of the Sierra Nevada batholith may consist almost entirely of juvenile crust added during Cretaceous magmatism.

MAGMIX: a BASIC program to calculate viscosities of interacting magmas of differing composition, temperature, and water content

Abstract MAGMIX is a BASIC program designed to predict viscosities at thermal equilibrium of interacting magmas of differing compositions, initial temperatures, crystallinities, crystal sizes, and water content for any mixing proportion between end members. From the viscosities of the end members at thermal equilibrium, it is possible to predict the styles of magma interaction expected for different initial conditions. The program is designed for modeling the type of magma interaction between hypersthenenormative magmas at upper crustal conditions. Utilization of the program to model magma interaction at pressures higher than 200 MPa would require modification of the program to account for the effects of pressure on heat of fusion and magma density.

Volcanic ash in surficial sediments of the Kodiak shelf — An indicator of sediment dispersal patterns

Abstract Surficial sediments of the Kodiak shelf, Gulf of Alaska, contain various amounts of volcanic ash whose physical properties indicate that it originated from the 1912 Katmai eruption. The distribution of ash is related to the shelf physiography and represents redistribution by oceanic circulation rather than the original depositional pattern from the volcanic event. The ash distribution can be used, in conjunction with the distribution of grain sizes, as an indicator of present-day sediment dispersal patterns on the shelf. No significant modern input of sediment is occurring on the Kodiak shelf, which is mostly covered by Pleistocene glacial deposits. Coarse-grained sediments on flat portions of shallow banks apparently are being winnowed, with the removed ash-rich fine material being deposited in shallow depressions on the banks and in three of the four major troughs that cut transversely across the shelf. The other major trough seems to be experiencing a relatively high-energy current regime, with little deposition of fine material.