Structure and petrology of the La Perouse gabbro intrusion, Fairweather Range, southeastern Alaska

The middle Tertiary La Perouse gabbro intrusion occurs in a Mesozoic metamorphic terrane (Chugach terrane) in the Fairweather Range, southeastern Alaska. The intrusion is 12 km wide and 27 km long, and has an exposed cumulate layering thickness of about 6000 m. The contact consists of biotite and hornblende gneiss of the granulite facies, which forms a discontinuous belt as much as 200 m wide. The outer contact of the zone is a fault, along which it is in contact with regionally metamorphosed biotite schist and amphibolite of the amphibolite facies. This fault seems to be a peripheral fault along which the intrusion was uplifted to its present level. The layering in the intrusion forms an asymmetrical funnel that is elongate northwestward and is structurally deepest at the southeast end near Mount La Perouse where the layering has inward dips of as much as 85°.

Although the region was probably under compression at the time the gabbro was intruded, there is evidence that the intrusion's extremely sunken form is due not to tectonism but instead to subsidence from magmatic loading at high temperatures that continued after solidification. This tectonism was probably related to the interaction between the north-moving Pacific oceanic plates and the North American continent during the Tertiary. This interaction, which involved long-term right-lateral movement with a compressional component, and possibly minor subduction, on the site of the Fairweather fault, may have controlled the mode of magmatic intrusion and possibly the production of magma at depth, but does not appear to have significantly deformed the intrusion.