GEOCHEMICAL EVIDENCE FOR DEHYDRATION MELTING IN GRANULITE-FACIES MIGMATITES DURING CRUSTAL EXTENSION IN THE ALBANY-FRASER OROGEN, SOUTHWESTERN AUSTRALIA

Multiple generations of leucosomes in granulite-facies migmatites of the Biranup Complex, Albany-Fraser Orogen, Bremer Bay, southwestern Australia, are associated with structures formed during bidirectional extension in an overall contractional regime, attributed to slab breakoff and asthenospheric upwelling, and include (i) foliation-parallel, (ii) cross-cutting, (iii) boudin neck and (iv) channelway leucosomes; all cut by late pegmatites. Geochemical data provide evidence for partial melting reactions with and without fluid, synchronous with deformation throughout the orogenic history.

Earliest leucosomes parallel the dominant migmatitic foliation, have the lowest SiO₂ and highest CaO, FeO and MgO contents of all leucosomes. Channelway leucosomes have a narrow range in SiO₂ content, moderate to high CaO and low K₂O contents. Foliation-parallel and channelway leucosomes may contain accumulations of plagioclase and plot within the field of experimental melts formed by fluid-present amphibole-dehydration. Boudin-neck leucosomes have high SiO₂ and K₂O contents and plot within the field of experimental melts produced during fluid-absent biotite-dehydration. Cross-cutting leucosomes range in composition, from samples with low SiO₂ and K₂O and high CaO to high SiO₂ and K₂O and low CaO, and likely formed from different dehydration melting reactions involving fluid-present amphibole-dehydration or fluid-absent biotite-dehydration. The varied geochemistry of the cross-cutting leucosomes is inferred to represent local (amphibole-dehydration melting) and distal (biotite-dehydration melting) source rocks. Pegmatites cross-cut all other structures and range from plagioclase-rich to K-feldspar-rich and can be separated into high and low Ba groups. The high Ba samples have a positive correlation with K₂O content and based on field relationships formed earlier than the low Ba samples. Foliation-parallel, channelway and some cross-cutting leucosomes likely formed early, during prograde metamorphism during the transition from amphibolite-granulite facies metamorphism. Boudin-neck and the remaining cross-cutting leucosomes formed later during extension, and lastly two generations of geochemically distinct pegmatites formed, cross-cutting all previous structures.