| Popis: |
The Northern Arran Granite is regarded as an example of an upper-crustal granite diapir due to its sub circular outcrop and deformed aureole. However diapiric emplacement to shallow levels in the crust is physically difficult and unambiguous evidence for shouldering aside of country rocks by a diapir as opposed to shouldering due to in situ expansion of a laccolith or ballooning pluton is difficult to find. The key is in finding evidence for vertical ascent of a diapiric body where a diapir should preserve vertical stretching either around the periphery or in central portions. A laccolith on the other hand is unlikely to have consistently vertical or steep lineations but contain mainly gently plunging lineations or evidence of multiple horizontal pulses or lobes. Therefore evidence for diapiric ascent and emplacement should be dominated by vertical kinematic indicators. These structures may however be too subtle if preserved within the granite, and those in the aureole may become overprinted by late stage insitu expansion. To test whether the internal structure of the Northern Arran Granite is consistent with diapiric or laccolithic emplacement we have measured the anisotropy of magnetic susceptibility from oriented block samples from the Northern Arran Granite to determine if there are subtle or weak fabrics that will support either diapirism with vertically oriented stretching, or laccolithic emplacement where lateral emplacement is dominant and vertical motion only restricted to vertical thickening of an initially thin sheet. Our results reveal concordant dome shaped planar fabrics with mainly gently plunging or horizontal lineation, i.e. an absence of vertical stretching or flow, and possible evidence of partial lobes in the north western margins and Inner Granite. We interpret these fabrics and the deformation of the aureole in terms of dome-shaped expansion consistent with the latter model. In more detail the lineation pattern indicates convergence toward a south or south eastern point, consistent with the deepest part of the pluton (from previously published gravity modelling). Our model suggests that there may be some link to a major crustal structure (the Highland Boundary Fault) providing insight into the ascent route of the magma and possible influence of the Highland Boundary fault zone. This model also suggests that the magmatism on Arran including the central complex and sill complexes in the south of the island may have been supplied by a long lived, deeply penetrating feeder zone controlled by this major tectonic structure. |
