HR: 1340h
AN: V33C-1530    [Abstracts]
TI: The role and fate of peritectic garnet in S-type granite Petrogenesis: The example of the Cape Granite Suite (South Africa)
AU: * Villaros, A
EM: arnaud@sun.ac.za
AF: Centre for Crustal Petrology, Stellenbosch University Private Bag X1, Stellenbosch, 7602, South Africa
AU: Stevens, G
EM: gs@sun.ac.za
AF: Centre for Crustal Petrology, Stellenbosch University Private Bag X1, Stellenbosch, 7602, South Africa
AU: Buick, I
EM: ian.buick@anu.edu.au
AF: Research School of Earth Sciences, Australian National University Bldg 61 Mills Road, Acton, ACT 0200, Australia
AB: The 560-530 Ma S-type components of the Cape Granite Suite (CGS) vary in composition from granodiorite to leucogranite. In places these rocks contain an abundance of xenoliths, including rare granulite facies metasediments and metabasic rocks. Thermobarometry applied to these assemblages gives consistent results of 850 ± 45 °C and 9 ± 1 kbar. These conditions are interpreted to reflect those of the source area at the time of magma genesis and lie within the experimentally determined interval for biotite fluid-absent melting, which, under these conditions, produces garnet as the dominant peritectic phase. Entrainment of different proportions of this generation of garnet (up to 20wt%) into the experimentally determined leucocratic melt compositions produces magma compositions that match well with the major element bulk composition of the CGS S-type granites. Zr and Fe + Mg are strongly positively correlated in the CGS S-types, and in many rocks occur in concentrations exceeding those possible in pure melts. This indicates the co-entrainment of peritectic garnet, derived from biotite breakdown, and, and zircon, typically hosted as an inclusion within biotite prior to anatexis. A model for the trace element composition of the granites, that is a near perfect fit with the natural rocks, can be created by applying a partial disequilibrium melting model. In this model, elements contained within pre- existing metamorphic garnet and the refractory fraction of zircon are not available for sequestration into the melt and melt trace element abundance is a function of the trace element composition of the accessible portion of the source, the fraction of zircon that dissolves and partitioning between melt and the peritectic garnet and other residual minerals. Magma trace element compositions are a function of the melt composition and the compositions of the entrained garnet, zircon and other accessory minerals such as monazite. Ce concentration in the granites over that which occurs in the compositions identified as close to pure melts is taken to be a proxy for monazite entrainment. Garnet in the CGS rocks is strongly zoned from relatively homogenous Mn-poor interiors to Mn-enriched rims some 100 μm in width. Pseudosections created for the more mafic varieties of S-type granite indicate that the garnet interiors equilibrated at close to 750 °C and 5 kbar and the rims at 730 °C and 3 kbar. This is interpreted to reflect re-equilibration and then partial re-equilibration of the entrained peritectic garnet during the ascent of the magma.
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3651 Thermobarometry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting