HR: 1340h
AN: V13E-0596    [Abstracts]
TI: Xenolith incorporation, distribution, and dissemination in a mid-crustal granodiorite, Vega pluton, central Norway
AU: * Marko, W
EM: wayne.t.marko@ttu.edu
AF: Geoscience Department, Texas Tech University, Lubbock, tx 79409 United States
AU: Barnes, M
V13E-0596 AF: Geoscience Department, Texas Tech University, Lubbock, tx 79409 United States
AU: Vietti, L
V13E-0596 AF: Department of Geosciences, University of Wyoming, Laramie, wy 82071 United States
AU: McCulloch, L
V13E-0596 AF: Geoscience Department, Texas Tech University, Lubbock, tx 79409 United States
AU: Anderson, H
V13E-0596 AF: Geoscience Department, Texas Tech University, Lubbock, tx 79409 United States
AU: Barnes, C
V13E-0596 AF: Geoscience Department, Texas Tech University, Lubbock, tx 79409 United States
AU: Yoshinobu, A
V13E-0596 AF: Geoscience Department, Texas Tech University, Lubbock, tx 79409 United States
AB: A wide compositional variety of enclaves have been observed in intrusions of various tectonic settings and emplacement histories. Some enclaves may have been incorporated as the result of magma mingling (e.g. mafic magmatic enclaves) while others (xenoliths or residuum) are the product of processes acting to disaggregate solid host or source rocks. These processes may include stoping, diking, partial melting and dissolution. The Vega intrusion in central Norway contains numerous xenoliths ranging in size from sub-mm to sub-km in map extent. These xenoliths occur in a heterogeneous and in some cases strongly peraluminous granodiorite host and appear to have been distributed throughout most of the intrusion. Xenolith compositions include marble, utramafic, gneiss, biotite garnet schist, calc-silicate, quartzite, and meta- and diatexite. Additionally a number of xenocrystic minerals in the host granodiorite may include feldspar, quartz, cordierite, and biotite. Calc-silicate, gneissic and schistose xenoliths are the most abundant. Calc-silicate xenoliths tend to exhibit an angular geometry and may be partially surrounded by leucogranitic magmas. Gneissic and schistose xenoliths are commonly less angular and may exhibit reactions rims in the form of grain coarsening and/or changes in mineralogy. Migmatitic xenoliths have lobate or circular 2-D contacts. In some cases xenoliths appear composite in composition with calc-silicate apparently folded around diatexitic cores that are enclosed by host granodiorite. Xenoliths >= 5 cm² (short axis) have average population densities on the order of 2/m² to 4/m² of host and some show a statistical alignment of long axis at the outcrop scale in measured areas up to 100 m². Assuming elliptical 2-D area from maximum measured axial ratios the xenolith areas range from 0.1 m² to 0.3 m² per m² of host. Area-frequency (size) distributions for enclaves >= 5 cm² are positively skewed for both composite calc-silicate xenolith populations and populations of primarily gneissic and/or schistose xenoliths. We suggest that dissemination of xenoliths throughout the intrusion may have resulted from disaggregation and incorporation of the residuum from the source region, as well as dyking and stoping of host rocks incorporated during magma migration. Mechanical rock fragmentation studies suggest that particle size - frequency distributions commonly display log-linear relationships. The apparent absence of such a distribution suggests that thermally induced fracturing (stoping) may not be the only processes controlling xenolith size distribution. Furthermore, intrusion mineralogies appear consistent with possible dehydration melting reactions in diatexitic components of the intrusion. Host rocks do not appear to have been involved in melting reactions and have structures, which are both discordant and concordant to the pluton host rock contact. Several map scale xenoliths of calc-silicate and quartzite are also included in the intrusion and dykes are observed at several scales within the host rocks. The absence of stock work or net-veining in the Vega host rocks argues to limit a wide range of xenolith size contributions to the magma via dyking. Furthermore, small xenoliths (xenocrysts) size-frequency distributions may be governed in part by thermodynamic and chemical processes including dissolution, crystallization, and melting of smaller crystals and aggregates.
DE: 3642 Intrusive structures and rocks
DE: 3652 Pressure-temperature-time paths
DE: 8108 Continental tectonics: compressional
DE: 8145 Physics of magma and magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005