Other
Scientific paper
Apr 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60.1341b&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 60, Issue 8, p. 1341-1354.
Other
21
Scientific paper
The amphibolite to granulite facies transition has been studied in a high-K calc-alkaline, hornblende-biotite, K-feldspar megacrystic augen gneiss series from the Rogaland-Vest-Agder (Rog-VA) sector of the Sveconorwegian province (southwest Norway). Hornblende begins to break down mainly to clinopyroxene at the Cpx-in isograd and biotite mainly to orthopyroxene at the Opx-in isograd (abbreviations for minerals following Kretz, 1983). The magmatic accessory mineral association comprises titanite and allanite, which begin to break down before the Cpx-in isograd. Titanite is preserved as relict inclusions in other minerals at higher grade. Monazite and thorite formed in the breakdown of allanite. Monazite abundance reaches a maximum between the Cpx-in and Opx-in isograds. The middle and heavy rare earth element (M-HREEs, except Eu) content of apatite steeply increases with increasing metamorphic grade and is directly correlated to the decrease of the modal abundances of titanite, hornblende, and biotite. The U contents of apatite are low and do not increase with metamorphic grade. The light rare earth elements (LREEs) and Th content of apatite are not correlated to the breakdown of allanite around the Cpx-in isograd but increase around the Opx-in isograd. A simplified equation is proposed for monazite crystallisation in the vicinity of the Cpx-in isograd that accounts for the allanite, titanite, and hornblende breakdowns and the M-HREE substitution in apatite: 3 (M-HREE)2O3in hornblende and litanite+3(LREE)2in allaniteO3+2Ca5(PO4)3apatite(F,OH)+6SiO2quartz⇔6(LREE)PmonaziteO4+2Ca2(M-HREE)3(SiO4)3(F,OH)lessingite in apatite+6CaOin plagioclase. At the Opx-in isograd, the increase of LREEs and Th content of apatite results either from the breakdown of some monazite: 3(LREE)PO4monazite+3SiO2quartz+4CaOin plagioclase+(F2,H2O)fluid⇔Ca5(PO4)3(F,OH)+apatiteCa2(LREE)3(SiO4)3(F,OH)less ingite in apatite. or the breakdown of the remaining allanite: 3(LREE)2O3in allanite+6SiO2quartz+4CaOin plagioclase+(F2,H2O)fluid⇔2Ca2(LREE)3(SiO4)3(F,OH)lessingite in apatite. The release of fluorine from the breakdown of biotite at the Opx-in isograd may increase apatite stability relatively to monazite in the granulite facies. when compared to amphibolite facies, the granulite facies augen gneisses do not show any U or Th depletion. This means that changes in the accessory mineral associations and the simultaneous breakdown of hydrous minerals at the amphibolite-granulite facies transition do not inevitably result in depleted granulite facies rocks but rather to an isochemical element redistribution. The coexistence of small amounts of metamorphic monazite and of relict inclusions of titanite in upper amphibolite facies augen gneisses suggests that (high-K) calc-alkaline orthogneisses are a suitable material to date (with the U-Pb method) prograde path of amphibolite facies regional metamorphism on monazite and the cooling path on titanite.
Bingen Bernard
Demaiffe Daniel
Hertogen Jan
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