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Science 21 April 1989:
Vol. 244. no. 4902, pp. 326 - 329
DOI: 10.1126/science.244.4902.326
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Articles

Origin of Granulite Terranes and the Formation of the Lowermost Continental Crust

S. R. BOHLEN 1 and K. MEZGER 2

1 U.S. Geological Survey, Mail Stop 910, 345 Middlefield Road, Menlo Park, CA 94025.
2 Department of Earth and Space Sciences, State University of New York, Stony Brook, NY 11794.

Differences in composition and pressures of equilibration between exposed, regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust, as had been thought, but rather middle and lower-middle crustal levels. Application of well-calibrated barometers indicate that exposed granulites record equilibration pressures of 0.6 to 0.8 gigapascal (20 to 30 kilometers depth of burial), whereas granulite xenoliths, which also tend to be more mafic, record pressures of at least 1.0 to 1.5 gigapascals (35 to 50 kilometers depth of burial). Thickening of the crust by the crystallization of mafic magmas at the crust-mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep-seated mafic crust represented by many xenolith suites.

Submitted on December 28, 1988
Accepted on March 10, 1989


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