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

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.