THE A-TYPE MOUNT SCOTT GRANITE SHEET - IMPORTANCE OF CRUSTAL MAGMA TRAPS

The presence of rapakivi feldspar and of distinctive porphyritic textu re of Mount Scott Granite indicates a period of crystallization prior to final emplacement beneath an extensive penecontemporaneous rhyolite volcanic pile. Final crystallization conditions are interpreted to ha ve been < 50 MPa at depths < < 1.4 km based on stratigraphic constrain ts. However, geobarometry based on the Al content of amphibole phenocr ysts and comparison of granite compositions with phase relations in th e SiO2-NaAlSi3O8-KAlSi3O8 ternary system both yield pressure estimates of approximate to 200 MPa. These pressure estimates are interpreted a s plumbing the depth of a temporary storage chamber at approximate to 7-8 km. This depth coincides, in this case, both with the probable Pro terozoic basement-cover contact and with the calculated brittle-ductil e transition at time of ascent of Mount Scott magma. Although rising m agma that fed the preceeding voluminous Carlton Rhyolite apparently ro se unimpeded past these horizontal anisotropies, rising magma that for med Mount Scott Granite temporarily paused at this depth. Based on mag mastatic calculations, we suggest that horizontal anisotropies (e.g., brittle-ductile transition) become crustal magma traps where the magma driving pressure exceeds the lithostatic load when the anisotropy is encountered. During rifting, initial large influxes of magma may proce ed passed crustal anisotropies but have the effect of increasing the r elative magma driving pressure through reducing horizontal stress. Thu s, magma driving pressure may eventually exceed the lithostatic load a t the depth of the brittle-ductile transition thereby activating this crustal magma trap. Pending of magma at the brittle-ductile transition chokes the eruption. Such a pause in magma supply rate may permit a r eturn to initial stress conditions and deactivate the crustal magma tr ap. Once again magma will rise to the surface initiating a new magmati c cycle.