2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 11
Presentation Time: 8:00 AM-6:00 PM

Magma Emplacement and the Development of Ghost Stratigraphy in the Andalshatten Pluton, Central Norway

ANDERSON, Heather S., Department of Geosciences, Texas Tech University, Lubbock, TX 79409, YOSHINOBU, Aaron S., Dept. of Geosciences, Texas Tech Univ, Lubbock, TX 79409-1053, CHAMBERLAIN, Kevin, Dept. of Geology and Geophysics, Univ. of Wyoming, Laramie, WY 82071, NORDGULEN, Oystein, Norwegian Geol Survey, Trondheim, N-7491, Norway and BARNES, Calvin G., Dept. of Geosciences, Texas Tech University, Lubbock, TX 79409-1053, aaron.yoshinobu@ttu.edu

The Andalshatten pluton (AP), central Norway, is an 18 x 35 km mid-crustal (670 MPa) intrusion and consists of at least three pulses of magma, including an early eastern gneissic to schlieren-banded granodiorite and co-magmatic megacrystic granodiorite, dated at 442.67 ± 0.14 Ma and 441.53 ± 0.40 Ma (CA-TIMS 206Pb/238U zircon dating), and diorite. Crystallization ages span 600 kyr to 1.7 my. Elongate mafic magmatic enclaves swarms are subparallel to the magmatic fabric observed throughout the pluton. Sub-planar isolation of kilometer-scale xenoliths preserves a remnant host rock stratigraphy, while smaller-scale xenoliths show evidence for deformation in the magma.

The AP intruded four N-trending nappes within the Helgeland Nappe Complex (HNC), the highest tectonostratigraphic unit of the Norwegian Caledonides. Host rocks are Neo-Proterozoic to Ordovician medium to low-grade pelitic, calcareous, calc-silicate, and migmatitic rocks. Cross-cutting relations reveal at least two phases of host rock deformation prior to final pluton construction at 442 Ma. Tightened and refolded folds within the ~ 1 km-wide aureole are deformed about axial planes that mimic the lobate shape of the western pluton margin. The NW margin of the pluton is overprinted by a pervasive protomylonitic fabric defined by recrystallized potassium feldspar and top-to-the-west, thrust-related shearing.

These data are compatible with the following: a) magma emplacement by elongate pulses of kilometric scale at 442 Ma; b) the largest screens reflect stranded host rock ghost stratigraphy; c) pluton construction was facilitated by stoping and lateral/downward ductile displacement of host rocks; d) magmatic  fabric formation likely occurred after emplacement of the diorite, relatively late in the emplacement process, and; d) shearing along the northwestern and eastern pluton margin occurred synchronous with emplacement and may be the result of regional 442 Ma deformation in the HNC.