Burial Metamorphism in the Hamersley Basin, Western Australia

Abstract

The low-grade metamorphic minerals prehnite, pumpellyite, epidote and actinolite in rocks of basic and intermediate composition have a broad, systematic distribution in the Hamersley Basin. Assemblages of these minerals are wisespread in the Fortescue Group, the lowermost group in the Hamersley Basin. Because of sunsuitability of rock type no relevant mineral assemblages were observed in samples from the Hamersley Group. However, metamorphism of this group can be implied from mineral assemblages in the younger Turee Creek Group, and because the Hamersley Group conformably overlies the metamorphosed Fortescue Group. Unfolded stratigraphic cross sections show that depth of burial was the dominant control of increase in metamorphic grade. Four metamorphic zones are defined over a relative depth of burial of 9 km. From lowest grade to highest these are: Zone I (ZI) prehnite–pumpellyite zone; ZII, prehnite–pumpellyite–epidote zone; ZIII, prehnite–pumpellyite–epidote–actinolite zone; and ZIV, (prehnite–epidote–actinolite zone. Laumontite, definitive of the zeolite fades is absent but that part of the sequence may coincide with rocks of unsuitable composition, or may have been removed by erosion. A large area of prehnite–pumpellyite fades (ZI and ZII) dominates the north side of the basin, while greenschist fades (ZIV) dominates the south. Separating the two is a curved central strip of pumpellyite-actinolite facies (ZIII). Microprobe data of pumpellyites from the three pumpellyite–bearing zones, ZI, II and III, show two systematic trends: extensive variation in Al/Fe ratios at any one grade, and a general decrease of Mg with increasing metamorphism. Consideration of the compositions of the most abundant pumpellyites in the metabasic rocks shows that these two trends spread about a more fundamental linear trend towards AJ-enrichment with increasing metamorphism as total Fe and Mg decrease. Epidote shows a wide range in Fe content in ZII and ZIII (Ps15 to Ps40) crossing the miscibility gap proposed by Raith (1976). In ZIV epidote compositions are more aluminous and restricted in composition (Ps11 to Ps20). Magnesium has entered the epidote lattice in ZII and ZIII (up to 0–17 ions Mg where £ cations = 8) but to only half this in ZIV. Synthesis of the burial model with published experimental work puts constraints on the ancient thermal gradient that existed during burial metamorphism. For the peak of metamorphic adjustment fluid pressure appears to have been equal to load pressure. A relatively high gradient of 80 to 100 deg;C/km seems likely for the shallow part of the sequence, with a gradient of 40 deg;C/km for the deeper part of the sequence, the change being at about 2–5 km. The prehnite-pumpellyite facies corresponds to a fluid pressure of 0–5 to 1 kilobar and a temperature range of about 100 to 300 deg;C. The prehnite-bearing pumpellyite-actinolite facies is interpreted to have developed at about 1–5 kb over a temperature range of 300 to 360 deg;C. This facies is probably a low pressure subfacies of the pumpellyite-actinolite fades of Hashimoto (1966).