Burial Metamorphism in the Hamersley Basin, Western Australia |
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| Authors: | SMITH RAYMOND E; PERDRIX J L; PARKS T C |
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| Institution: | CSIRO, Institute of Earth Resources, Division of Mineralogy, Private Bag, P. O., Wembley, W.A. Australia, 6014 |
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| Abstract: | The low-grade metamorphic minerals prehnite, pumpellyite, epidoteand actinolite in rocks of basic and intermediate compositionhave a broad, systematic distribution in the Hamersley Basin.Assemblages of these minerals are wisespread in the FortescueGroup, the lowermost group in the Hamersley Basin. Because ofsunsuitability of rock type no relevant mineral assemblageswere observed in samples from the Hamersley Group. However,metamorphism of this group can be implied from mineral assemblagesin the younger Turee Creek Group, and because the HamersleyGroup conformably overlies the metamorphosed Fortescue Group. Unfolded stratigraphic cross sections show that depth of burialwas the dominant control of increase in metamorphic grade. Fourmetamorphic zones are defined over a relative depth of burialof 9 km. From lowest grade to highest these are: Zone I (ZI)prehnite–pumpellyite zone; ZII, prehnite–pumpellyite–epidotezone; ZIII, prehnite–pumpellyite–epidote–actinolitezone; and ZIV, (prehnite–epidote–actinolite zone.Laumontite, definitive of the zeolite fades is absent but thatpart of the sequence may coincide with rocks of unsuitable composition,or may have been removed by erosion. A large area of prehnite–pumpellyitefades (ZI and ZII) dominates the north side of the basin, whilegreenschist fades (ZIV) dominates the south. Separating thetwo is a curved central strip of pumpellyite-actinolite facies(ZIII). Microprobe data of pumpellyites from the three pumpellyite–bearingzones, ZI, II and III, show two systematic trends: extensivevariation in Al/Fe ratios at any one grade, and a general decreaseof Mg with increasing metamorphism. Consideration of the compositionsof the most abundant pumpellyites in the metabasic rocks showsthat these two trends spread about a more fundamental lineartrend towards AJ-enrichment with increasing metamorphism astotal Fe and Mg decrease. Epidote shows a wide range in Fe content in ZII and ZIII (Ps15to Ps40) crossing the miscibility gap proposed by Raith (1976).In ZIV epidote compositions are more aluminous and restrictedin composition (Ps11 to Ps20). Magnesium has entered the epidotelattice 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 workputs constraints on the ancient thermal gradient that existedduring burial metamorphism. For the peak of metamorphic adjustmentfluid pressure appears to have been equal to load pressure.A relatively high gradient of 80 to 100 deg;C/km seems likelyfor the shallow part of the sequence, with a gradient of 40deg;C/km for the deeper part of the sequence, the change beingat about 2–5 km. The prehnite-pumpellyite facies correspondsto a fluid pressure of 0–5 to 1 kilobar and a temperaturerange of about 100 to 300 deg;C. The prehnite-bearing pumpellyite-actinolitefacies is interpreted to have developed at about 1–5 kbover a temperature range of 300 to 360 deg;C. This facies isprobably a low pressure subfacies of the pumpellyite-actinolitefades of Hashimoto (1966). |
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