Prograde hbl- and bio-breakdown reactions at high temperatures: A natural example from the Wilmington Complex, PA-DE
- West Chester Univ., West Chester, PA (United States). Dept. of Geology and Astronomy
Breakdown of hornblende (hbl) and biotite (bio) in the deep crust can be a source of fluids and silicic melts. In the Wilmington Complex, prograde reaction stages at P = 600 MPa are preserved in granulite-facies gneisses near the margin of an intrusive gabbro stock. Away from intrusive plutons, the low-K, mafic and felsic gneisses contain opx-cpx-hbl-plag(An40-55)-mt [+-] qtz [+-] bio [+-] ilm. At 50--100 m from the contact with the intrusive gabbro, hbl is partly rimmed by symplectic intergrowths of opx-cpx-plag; individual grains are several [mu]ms in any dimension. At 15 m from the contact, the reaction texture is less obvious, but small grains of opx and cpx occur along hbl grain boundaries. Modal hbl and qtz decrease toward the contact; samples within a few m of the contact contain no hbl or bio. Plag composition is zoned in the matrix and is variable within symplectites. An overall reaction can be written using compositions of matrix hbl and symplectite prxs: 1 Hbl + 2.3 Qtz = 1.6 Opx + 1.2 Cpx + 1.3 Plag (An58), (+fluid+K). Any supercritical fluid or silicate melt produced migrated out of the volume of gneiss samples. Two-prx thermometry indicates that maximum temperatures reached at least 850 C, 50 m from the contact. The preservation of fine-scale disequilibrium features suggests that high-T metamorphism was not followed by retrogression or deformation. This work supports previous work on associated aluminous gneisses, in which relict bio and sill co-exist in the same thin section with restite assemblages of cordierite, spinel, corundum, opx, and gar. Dehydration partial melting of bio produced low-K silicic melts.
- OSTI ID:
- 5589858
- Report Number(s):
- CONF-921058-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 24:7; Conference: 1992 annual meeting of the Geological Society of America (GSA), Cincinnati, OH (United States), 26-29 Oct 1992; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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