Inclusion patterns in zoned garnets from Mager by, north Norway
ABSTRACT. Garnet porphyroblasts in metasediments from Mageroy crystallized uring static metamorphism. They display three optical zones, each having character-istic inclusions and chemistry. The compositional zoning is related to prograde metamorphism (an inner zone, 1, and a graphite-bearing zone) a...
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| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.630.4573 http://www.minersoc.org/pages/Archive-MM/Volume_48/48-346-21.pdf |
| Summary: | ABSTRACT. Garnet porphyroblasts in metasediments from Mageroy crystallized uring static metamorphism. They display three optical zones, each having character-istic inclusions and chemistry. The compositional zoning is related to prograde metamorphism (an inner zone, 1, and a graphite-bearing zone) and retrogression (the outer zone, 2). Inclusions of two types are present in zone 1: type 1 are equidimensional remnants of the matrix, preferen-tially included along planes of rapid growth; type 2 are tubular and represent recrystallized quartz grains concen-trated along defects in the garnet lattice. The defects are lineage boundaries between growth segments related to screw dislocations on crystal faces. Crystal growth developed at relatively high degrees of supersaturation, but below the supersaturation required for the develop-ment of dendrites. The inclusions uggest rapid growth of zone 1, caused by heat flow from an adjacent interkine-matic mafic/ultramafic intrusive complex. The graphite-bearing zone crystallized at the metamorphic peak, while the inclusion-free idioblasfic rim probably developed during retrograde metamorphism. G A R N E T S from Mageroy display similar chemical zonation to that documented in a number of recent papers. In addition, they are optically zoned and contain a complex pattern of inclusions. Chemical zoning in garnet porphyroblasts has been extensively studied and the fractionation-depletion model suggested by Harte and Henley (1966), furthered by Hollister (1966), is widely accepted. Alternative and additional models to explain the common bell-shaped chemical profiles as well as irregular patterns have, however, been suggested, involving continuous and discontinuous reactions and diffusion (Anderson and Buckley, 1973; Tracy et al., 1976; Yardley, 1977). Several authors have utilized the geometry of inclusions in metamorphic minerals for elucidat-ing the conditions under which porphyroblasts nucleated and grew. Garnets have received parti-cular attention and their inclusion ... |
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