Nature and origin of A-type granites with particular reference to southeastern Australia |
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| Authors: | W J Collins S D Beams A J R White B W Chappell |
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| Institution: | (1) La Trobe University, Bundoora, Victoria, Australia;(2) Australian National University, Canberra, Australia |
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| Abstract: | In the Lachlan Fold Belt of southeastern Australia, Upper Devonian A-type granite suites were emplaced after the Lower Devonian
I-type granites of the Bega Batholith. Individual plutons of two A-type suites are homogeneous and the granites are characterized
by late interstitial annite. Chemically they are distinguished from I-type granites with similar SiO2 contents of the Bega Batholith, by higher abundances of large highly charged cations such as Nb, Ga, Y, and the REE and lower
Al, Mg and Ca: high Ga/Al is diagnostic. These A-type suites are metaluminous, but peralkaline and peraluminous A-type granites
also occur in Australia and elsewhere.
Partial melting of felsic granulite is the preferred genetic model. This source rock is the residue remaining in the lower
crust after production of a previous granite. High temperature, vapour-absent melting of the granulitic source generates a
low viscosity, relatively anhydrous melt containing F and possibly Cl. The framework structure of this melt is considerably
distorted by the presence of these dissolved halides allowing the large highly charged cations to form stable high co-ordination
structures. The high concentration of Zr and probably other elements such as the REE in peralkaline or near peralkaline A-type
melts is a result of the counter ion effect where excess alkali cations stabilize structures in the melt such as alkali-zircono-silicates.
The melt structure determines the trace element composition of the granite.
Separation of a fluid phase from an A-type magma results in destabilization of co-ordination complexes and in the formation
of rare-metal deposits commonly associated with fluorite. At this stage the role of Cl in metal transport is considered more
important than F. |
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