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Stowell, H.H., 1987

Sphalerite geobarometry in metamorphic rocks and the tectonic history of the Coast Ranges near Holkham Bay, southeastern Alaska

Bibliographic Reference

Stowell, H.H., 1987, Sphalerite geobarometry in metamorphic rocks and the tectonic history of the Coast Ranges near Holkham Bay, southeastern Alaska: Princeton, New Jersey, Princeton University, Ph.D. dissertation, 276 p., illust., maps.

Abstract

The Holkham Bay area in southeastern Alaska is underlain by variably metamorphosed supracrustal rocks to the southwest (Western Metamorphic belt - WMB) and gneisses mixed with granodiorites to the northeast (Carlson Creek Pluton and Coast Plutonic Complex - CPC). A major structure, the Coast Range megalineament (CRM), marks a structural and metamorphic discontinuity with high-pressure greenschist facies rocks to the west and moderate-pressure amphibolite facies rocks to the east. The CRM contains asymmetric folds and coexisting foliations that indicate subvertical shearing and right lateral transcurrent motion, respectively. The composition of sphalerite coexisting with pyrite and pyrrhotite in three metamorphosed sulfide deposits in the WMB have been used to calculate the pressures of equilibration using experimentally determined and thermodynamically calculated calibration curves. A pressure of 4.8 +/- 0.7 kb was determined for the sphalerite from the amphibolite facies rocks. This is indistinguishable from a pressure of 4.6 +/- 1.0 kb at 670 degrees C determined from silicate minerals in this part of the WMB. Sphalerite compositions in the greenschist-facies terrane are varied and give a range of pressures. However, the composition of grains in direct contact with pyrite and pyrrhotite, or enclosed by pyrite, give consistent pressures of 8.1 +/- 0.7 kb and 6.8 +/- 0.7 kb, respectively. These results, based on the experimental calibration, are compatible with a pressure of about 9 kb at 490 degrees C calculated from silicate mineral compositions in the greenschist-facies rocks. These data suggest that sphalerite compositions are not temperature dependent and support the experimental calibration of the barometer. The thermo-barometric, structural, and geochronological data indicate two possible tectonic scenarios. Metamorphism of the entire WMB may have occurred at 6-10 kb, followed by cooling of the greenschist facies rocks during the Late Cretaceous. This was accompanied by slower cooling or reheating to the east, resulting in re-equilibration of the amphibolite facies rocks when the Carlson Creek Pluton intruded during the Paleocene. Alternatively, the amphibolite-facies terrane may never have experienced metamorphic pressures greater than 4-6 kb.

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