Proterozoic low‐pressure/high‐temperature metamorphism and an anticlockwise P–T–t path for the Hazeldene area, Mount Isa Inlier, Queensland, Australia

Abstract

In the Hazeldene area, situated in the Mount Isa Inlier, Queensland, the metamorphic grade changes from chlorite zone, through biotite and cordierite zones, to sillimanite/K-feldspar zone.Microstructural studies of rocks near the sillimanite isograd demonstrate that cordierite grew early during the development of a steep foliation (S2), was replaced by biotite, andalusite and sillimanite at the metamorphic peak late in S2, and in turn by kyanite + chlorite adjacent to localized small post-D2 shear zones. Although the anticlockwise P-T-t path is well defined, the precise P-T conditions are uncertain because of problems with experimental and thermodynamic data. The best estimate for the metamorphic peak for rocks close to the sillimanite isograd is around 600" C at 4 kbar.The metamorphism has been dated at 1544 Ma, and was synchronous with a major crustal shortening event. Because proposed extensional events occurred more than 60 Ma earlier, their contribution to the peak metamorphic thermal perturbation would have been insignificant. The syn-metamorphic Mica Creek Pegmatites, the abundance of high heat-producing elements in the nearby pre-D2 Sybella Granite, and advective heat by fluids which caused considerable metasomatism in the Hazeldene area, may have each contributed to the thermal budget. However, the metamorphic thermal gradient may be 80" C km-' or higher, strongly suggesting a local magmatic control. As none are known in the area, such synmetamorphic plutons would have to lie beneath the exposed high-grade rocks.

Isograd pattern and regional low-pressure, high-temperature metamorphism of pelitic, mafic and calc-silicate rocks along an east – west section through the Mt Isa Inlier

“…Kyanite + chlorite occurs locally and is indicative of anticlockwise P -T -t paths where this retrograde assemblage is superimposed on cordierite -andalusite -sillimanite assemblages. Previous workers have defined anticlockwise P -T -t paths (or segments of paths) in all three belts in the Mt Isa Inlier (Reinhardt 1992a, b;Rubenach 1992;Lally 1997;Rubenach & Lewthwaite 2002). Schematic P -T -t paths defined for the Western Fold Belt, Mary Kathleen Fold Belt, and the Snake Creek Anticline in the Eastern Fold Belt are shown in Figure 3.…”

“…Differences in the tectonothermal history of the Eastern Fold Belt compared with the Western Fold Belt have been suggested by many workers (Holmes 1992;New 1993;Lally 1997;Adshead-Bell 1998;Mares 1998;Rubenach & Barker 1998;Rubenach & Lewthwaite 2002). Until recently, most workers correlated the metamorphic peak with D 2 , which they argued occurred ca 1550 -1530 Ma (Green 1975;Page 1983;Page & Bell 1986;Connors & Page 1995;Pollard & Perkins 1997;Oliver et al 1991Oliver et al , 1999Perkins et al 1999;Reinhardt 1992a, b;Rubenach 1992;Mark et al 1998;Rubenach & Barker 1998;Foster & Rubenach 2001). For example, Foster and Rubenach (2000) proposed that the metamorphic peak occurred during the second major metamorphic episode at ca 1550 -1530 Ma, based on zircon ages in the Mt Isa area (Page & Bell 1986;Connors & Page 1995) and U -Pb SHRIMP ages of 1547 -1545 Ma for igneous zircons in foliated early granite phases in the Eastern Fold Belt, interpreted to be pre-to synregional D 2 (Blake et al 1984;Williams & Phillips 1992;de Jong 1995).…”

“…A variety of processes have been proposed to explain the high geothermal gradients necessary for this style of metamorphism. Mechanisms suggested include: compression of previously extended continental crust (Etheridge et al 1987;Oliver et al 1991); mantle delamination (Loosveld & Etheridge 1990); heat advection via intrusions due to high mantle heat flow (Vernon et al 1990;Rubenach 1992;Sandiford et al 1995;Rubenach & Barker 1998); radiogenic heating due to enrichment of heat-forming elements in the upper crust (Mildren & Sandiford 1995;Sandiford et al 1998;McLaren et al 1999); and the burial of heat-producing stratigraphic sequences (Hand & Rubatto 2002).…”

Stepping stones and pitfalls in the determination of an anticlockwise P‐T‐t‐deformation path: the low‐P, high‐T Cooma Complex, Australia

“…Phillips & Wall, 1981;Hobbs el al., 1984;Clarke et al, 1987;Reinhardt & Rubenach, 1989;Vernon et al, 1990;Rubenach, 1992). Most of these areas shown limited information on prograde and commonly more detailed information on regrograde histories.…”

Structural cross-section based on the Mt Isa deep seismic transect

“…During shortening, the region around the Sybella Granite experienced low-pressure metamorphism reaching upper amphibolite facies Rubenach 1992). …”