TEXTURAL ANALYSIS OF A FLOW DIFFERENTIATED, PRE-PENOKEAN MAFIC DIKE IN THE UPPER PENINSULA, MICHIGAN

Magma transport through the crust is primarily via crack transport, i.e. through dikes. Magma commonly carries entrained crystals, and within a flowing dike, these crystal can be sorted by size and relative density compared to the carrying magma. This process of flow differentiation can result in more crystals towards the interior of the dike, coarser crystals towards the interior of the dike, and alignment of crystals. In the upper peninsula of Michigan, regional extension resulted in emplacement of a series of parallel trending doleritic dikes into Archean gneiss. These dikes are ~0.5 m in width, and some carried with them a fairly coarse load of plagioclase crystals on the order of a centimeter in length. This crystal load has been visibly concentrated towards the center of the dike. However, it is unclear from visual inspection whether the crystals are flow aligned, or sorted by size. Using scaled photos of one dike, the crystal load is traced and analyzed. Analyses reveal that the concentration of the crystal load increases towards the center of the dike, but the evidence is not suggestive of size-sorting. A possibility is that flow in the dike was low, such that the grain-dispersive force was not strong enough for crystal size sorting. Additionally, the concentration of crystals within the dike does not follow a symmetric pattern, which may be reflective of a multiple pulse growth history, with subsequent pulses containing variable crystal loads. Lastly, the crystals in transport are oriented with a preference, with long axis perpendicular to the dike-host rock contact. These findings show that flow differentiation within dikes can act to sort crystals by concentration, but not necessarily by size.