1.2 Ga thermal metamorphism in the Albany–Fraser Orogen of Western Australia: consequence of collision or regional heating by dyke swarms?

Abstract

Compressive fabrics in the Late Palaeoproterozoic Mount Barren Group of the Albany–Fraser Orogen, southwestern Australia, record Mesoproterozoic collision between proto-Australia and proto-Antarctica. Petrographical evidence establishes that peak thermal metamorphism produced largely random growth of kyanite, staurolite, biotite, monazite and xenotime that overprinted those fabrics. SHRIMP U–Pb geochronology of xenotime and monazite yields an average age of 1205 ± 10 Ma. Thermal metamorphism therefore occurred at least 45 Ma after fabric formation, and was unlikely to have been caused by collision. Rather, thermal metamorphism overlapped with the emplacement of 1215–1202 Ma dyke swarms into the Orogen and the adjacent Yilgarn Craton, and was followed by emplacement of 1200–1180 Ma granites. Regional heating associated with mafic magmatism was the probable cause of thermal metamorphism, but previous proposals that the dyke swarms were the consequence of collision or extensional orogenic collapse cannot be substantiated. A regional thermal anomaly, craton-scale extension and adiabatic decompression melting of the asthenosphere are implied, but causal mechanisms such as a mantle plume or intracontinental rifting require substantiation from other parts of East Gondwana. The significant time gap between orogenic deformation and thermal metamorphism implies that metamorphism in many other orogens may not necessarily be due to compressive tectonics.

In situ SHRIMP U–Pb dating of monazite integrated with petrology and textures: Does bulk composition control whether monazite forms in low-Ca pelitic rocks during amphibolite facies metamorphism?

“…The relationship between deformation and metamorphism in the Mount Barren Group has been described in previous studies (Hollingsworth, 1996;Stephens, 1996;Witt, 1997Witt, , 1998Wetherley, 1998;Dawson, 2000;Dawson et al, 2003). The earlier of the two main foliations (S 2 ) is typically defined by parallel crystals of quartz, muscovite, ilmenite/magnetite and biotite, and is locally crenulated by S 4 .…”

“…Two dominant fabrics were recognized by Wetherley (1998); (S 2 and S 4 , notation used in this paper) and others (Thom, 1977;Hollingsworth, 1996;Witt, 1997Witt, , 1998Stephens, 1996;Dawson, 2000;Dawson et al, 2003), which have been correlated across the Mount Barren Group (Wetherley, 1998). The fabrics are axial planar foliations associated with isoclinal folds that are attributed to early N-S and later NW-SE compression (Wetherley, 1998).…”

“…Stage III involved amphibolite facies metamorphism and northwest-directed contraction, consistent with a period of crustal thickening (Fitzsimons and Buchan, 2005), and was synchronous with the last stage of a thermotectonic event preserved in the Pinjarra Orogen to the west. The absence of 1030 Ma dates in previous geochronological studies of pelitic schists from the same locality (e.g., Dawson, 2000;Dawson et al, 2003) was attributed to the formation of monazite during peak metamorphism by a specific reaction among silicate minerals in a very restricted bulk composition (Fitzsimons et al, 2004. The lack of 1030 Ma dates in the rest of the AlbanyFraser Orogen, was explained by the recrystallization and dehydration of basement rocks at high metamorphic grades during Stages I and II (Fitzsimons and Buchan, 2005).…”

“…Stage II coincided with the emplacement of $1210 Ma dyke swarms along the craton margin and within the Albany-Fraser Orogen (Evans, 1999;Wingate et al, , 2005Rasmussen and Fletcher, 2004). Stage II involved widespread metamorphism (1215-1180 Ma) ranging from greenschist-amphibolite facies Dawson et al, 2003) to granulite facies (Clark et al, 2000;Bodorkos and Clark, 2004), extensive deformation and the intrusion of late-to post-tectonic granites. This stage of tectonism has been linked to intracratonic reactivation (Clark et al, 2000).…”

“…Multiple phases of deformation have been identified in the Mount Barren Group (Thom and Chin, 1984;Witt, 1997;Wetherley, 1998). Five phases of folding were mapped by Wetherley (1998): F 1 -F 4 structures were interpreted to represent stages in a single tectonic event with peak metamorphism occurring during or shortly after F 4 (Hollingsworth, 1996;Stephens, 1996;Dawson, 2000;Dawson et al, 2003;. F 5 structures were related to a second deformational event linked to a phase of retrogression at shallow crustal levels, and comprise upright kinks (F 5 ) and a weak crenulation cleavage.…”