XENOLITH INCORPORATION IN PLUTONS: IMPLICATIONS TO STOPING, ASSIMILATION, AND INCREMENTAL PLUTON CONSTRUCTION, ANDALSHATTEN PLUTON, NORWEGIAN CALEDONIDES

Two hypotheses for xenolith incorporation into a magma include: 1) stoping, in which xenoliths represent blocks removed from the intrusion's host rocks, during which physical and/or chemical interaction may occur with the magma; and 2) in situ isolation of host rock screens by successive magma emplacement, during which no significant chemical or physical interactions occur.

The Andalshatten pluton is a mid-crustal, megacrystic K-feldspar granodiorite and intrudes the lithologically distinct Helgeland Nappe Complex. The pluton consists of at least four magmatic pulses revealed by sharp and gradational contacts. Chemical abrasion (CA)-TIMS ²⁰⁶Pb/²³⁸U age dates of 442.66 ± 0.18 Ma and 442.86 ± 0.20 Ma were determined for samples within the central K-feldspar-megacrystic phase. Based on a 480.1 ± 2.8 Ma detrital zircon age, host rocks are determined to be Ordovician in age and include medium- to low-grade pelitic, calcareous and migmatitic rocks. Within the pluton, volumetrically significant amounts of metasedimentary xenoliths, mafic magmatic enclaves and a moderately strong N-NW-trending magmatic fabric are preserved. Xenoliths range from the centimeter to kilometer-scale and display similar lithologies to that of the adjacent host rocks.

Based on abundant preservation of xenoliths and well-defined host rock tectonostratigraphy, the Andalshatten pluton is an excellent natural laboratory in which to evaluate xenolith incorporation/interaction models. Mapping of the pluton, xenoliths and host rocks is consistent with incremental emplacement during which the largest xenoliths were preserved in situ and thus retain relict host rock stratigraphy. However, U-Pb data indicate that large batches of magma were fed into the chamber in short intervals. Evidence for fragmentation, rotation and translation of xenoliths and local hybrid granite adjacent to xenoliths suggest that both physical and chemical interaction has occurred with the magma. This data reveals that both models for xenolith incorporation/interaction has occurred and indicates that different mechanisms may be operating at different scales. Additionally, structural data reveal that xenoliths, magmatic foliations and host rock markers are broadly sub-parallel, suggesting regional deformation may have also occurred during emplacement.