Partial melting has been proposed as a mechanism for producing the low <I>a</I>H<SUB>2</SUB>O conditions characteristic of granulites, but the extent to which melts produced by water-undersaturated partial melting can be removed from granulite-facies migmatite complexes is less well understood. Three leucogneiss suites produced by water-undersaturated partial melting under high temperature, moderate pressure (6Kbar, 860°C), granulite-facies conditions have been distinguished on the basis of geochemistry and petrography. Type 1 Leucogneisses from the Brattstrand Bluffs coastline, eastern Antarctica, have near minimum-melt major element compositions (high SiO<SUB>2</SUB>, low Fe<SUB>2</SUB>O<SUB>3</SUB>, MgO) low Zr, Th and LREE concentrations, positive Eu anomalies and limited entrained residual material, suggesting efficient segregation of small melt volumes before equilibration with the source. The limited dissolution of accessory phases in water-undersaturated melts has led to a concentration of monazite and zircon and high LREE concentrations in melanosomes. Melting depleted the plagioclase component of the metapelite source rocks almost completely, producing melts with high Eu concentrations. Granitic melts formed under vapour-present conditions are unlikely to show such extreme LREE and HREE depletion or positive Eu anomalies, even at high degrees of partial melting. Type 2 Leucogneisses have high Fe<SUB>2</SUB>O<SUB>3</SUB>, MgO and TiO<SUB>2</SUB> contents and were formed at higher degrees of melting than the Type 1 Leucogneisses. Consequently they were more viscous, less extractable and more likely to entrain restitic and peritectic phases as the metapelitic source rocks lost their coherence. Type 3 Leucogneisses have strongly enriched Zr, Th and LREE abundances, negative Eu anomalies, and Zr/Zr<SUP>*</SUP> and LREE<SUB>t</SUB>/LREE<SUB>t</SUB><SUP>*</SUP> >1 resulting from preferential accessory phase entrainment.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:663560 |
| Date | January 1993 |
| Creators | Watt, Gordon Richard |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/11526 |
Page generated in 0.002 seconds