SYNTECTONIC TONALITE MAGMA EMPLACEMENT DURING CRUSTAL THICKENING - THE BELL ISLAND PLUTON, SE ALASKA

The 1,300 km2 Bell Island pluton dated at 90-89 Ma represents a large volume of magma emplaced in a short period during convergent deformation and crustal thickening. The pluton consists of intermediate to felsic hornblende-biotite and biotite tonalite, mafic at the margins and more leucocratic with mafic enclaves in the interior. In the country rock metamorphic aureole, defined by the presence of Al-silicate and staurolite-bearing assemblages, there is a clear thermal gradient toward the pluton identified by a staurolite-out isograd and by the occurrence of sillimanite rather than kyanite. At several sites, pre-existing andalusite was replaced by kyanite+staurolite or by sillimanite pseudomorphs. The presence of these pseudomorphs documents a pressure increase associated with pluton emplacement. Structural data summarized below record W to WNW transport during pluton emplacement overprinted by structures ascribed to a WSW-ENE directed regional compression. The margins of the pluton show strong solid state deformation that gradually fades toward the pluton interior where aligned minerals define an E-W to NW-SE trending magmatic linear flow fabric. At the margins, late felsic melt veins and melt-filled shears as well as the orientation of pegmatite dikes within the pluton document that the steeply dipping solid state fabric formed prior to full crystallization of the pluton. Reclined and sheath folds having hinges parallel to the linear fabric in the pluton, shear bands, and other kinematic indicators in the aureole, record top to the west transport along the southern and western pluton margins. Close to the pluton, this fabric is reoriented by tight upright folds with south-southeast plunging hinges. Granodiorite dikes are common close to the western pluton margin; two of these were dated at 89.5+/-1Ma and 88+/-5Ma. Some of these dikes are undeformed, others show foliation and boudinage that documents that they were flattened and stretched by the same WSW-ENE compression that refolded the country rocks. Regional foliation trends in the pluton suggest that the pluton was also affected by that compression. Data presented by Cook and Crawford (1994) document that the crustal block that contains the Bell Island pluton was tilted down to the west by approximately 10 degrees at about 55-50 Ma.