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Metasomatic Alteration Associated with Regional Metamorphism: an Example from the Willyama Supergroup, South Australia

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Metasomatic Alteration Associated with Regional Metamorphism: an Example from the Willyama Supergroup, South Australia Lithos 54Ž. 2000 33±62 www.elsevier.nlrlocaterlithos Metasomatic alteration associated with regional metamorphism: an example from the Willyama Supergroup, South Australia A.J.R. Kent a,),1, P.M. Ashley a,2, C.M. Fanning b,3 a DiÕision of Earth Sciences, UniÕersity of New England, Armidale, NSW, 2351, Australia b Research School of Earth Sciences, The Australian National UniÕersity, Canberra, ACT, 0200, Australia Received 20 October 1998; accepted 12 May 2000 Abstract The Olary Domain, part of the Curnamona Province, a major Proterozoic terrane located within eastern South Australia and western New South Wales, Australia, is an excellent example of geological region that has been significantly altered by metasomatic mass-transfer processes associated with regional metamorphism. Examples of metasomatically altered rocks in the Olary Domain are ubiquitous and include garnet±epidote-rich alteration zones, clinopyroxene- and actinolite-matrix breccias, replacement ironstones and albite-rich alteration zones in quartzofeldspathic metasediments and intrusive rocks. Metasomatism is typically associated with formation of calcic, sodic andror iron-rich alteration zones and development of oxidised mineral assemblages containing one or more of the following: quartz, albite, actinolite±hornblende, andradite-rich garnet, epidote, magnetite, hematite and aegerine-bearing clinopyroxene. Detailed study of one widespread style of metasomatic alteration, garnet±epidote-rich alteration zones in calc-silicate host rocks, provides detailed information on the timing of metasomatism, the conditions under which alteration occurred, and the nature and origin of the metasomatic fluids. Garnet±epidote-bearing zones exhibit features such as breccias, veins, fracture-controlled alteration, open space fillings and massive replacement of pre-existing calc-silicate rock consistent with formation at locally high fluid pressures and fluidrrock ratios. Metasomatism of the host calc-silicate rocks occurred at temperatures between ;4008C and 6508C, and involved loss of Na, Mg, Rb and Fe2q, gain of Ca, Mn, Cu and Fe3q and mild enrichment of Pb, Zn and U. The hydrothermal fluids responsible for the formation of garnet±epidote-rich assemblages, as well as those involved in the formation of other examples of metasomatic alteration in the Olary Domain, were 3q 2y hypersaline, oxidised, and chemically complex, containing Na, Ca, Fe , Cl, and SO4 . Sm±Nd geochronology indicates that the majority of garnet±epidote alteration occurred at 1575"26 Ma, consistent with field and petrographic observations that suggest that metasomatism occurred during the retrograde stages of a major amphibolite-grade regional metamorphic event, and prior to the latter stages of regional-scale intrusion of S-type granites at ) Corresponding author. Present address: Danish Lithosphere Centre, éster Volgade 10, 1350 Copenhagen K, Denmark. Fax: q45-38-14-2667. E-mail address: [email protected]Ž. A.J.R. Kent . 1 Fax: q61-2-6773-3300. 2 Fax: q61-2-6773-3300. 3 Fax: q61-2-6249-4835. 0024-4937r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0024-4937Ž. 00 00021-9 34 A.J.R. Kent et al.rLithos 54() 2000 33±62 1600"20 Ma. The fluids responsible for metasomatism within the Olary Domain are inferred to have been derived from devolatilisation of a rift-related volcano-sedimentary sequence, perhaps containing oxidised and evaporitic source rocks at deeper structural levels, during regional metamorphism, deformation and intrusion of granites. At the present structural level, there is no unequivocal evidence for the fluids to have been directly sourced from granites. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Proterozoic; Willyama Supergroup; Calc-silicate; Metasomatism; Geochemistry; Sm±Nd dating 1. Introduction examples of regional and local scale metasomatic alteration phenomenon are numerous and widespread Rocks that have experienced metamorphism com- Že.g. Cook and Ashley, 1992; Ashley et al. 1998a,b; prise a large proportion of the continental regions Skirrow and Ashley, 1999. The metasomatic fea- and thus an understanding of the changes that are tures of rocks from the Olary Domain also have associated with metamorphic activity is critical for strong analogies with alteration phenomena that have gauging the chemical and mineralogical evolution of been documented in other Proterozoic terranesŽ both the continental crust. Traditional studies of metamor- elsewhere in Australia and in other parts of the phic phenomena have emphasised the isochemical world. , some of which are associated with Cu, Au, mineralogical changes caused by metamorphic re- Fe and U mineral depositsŽ Kalsbeek, 1992; Frietsch equilibration under differing pressure and tempera- et al., 1997; Oliver et al., 1998; Williams, 1998. ture regimes. However, metasomatic mass-transfer of In this study, the major styles of metasomatic chemical components is increasingly recognised as alteration in the Olary Domain are documented and an important process accompanying regional meta- described; to do this we both present new informa- morphismŽ e.g., Chinner, 1967; Yardley and Bal- tion and review results of earlier studies in the tatzis, 1985; Ferry, 1992; Ague, 1994a,b, 1997; region. Further, in order to constrain the timing and Oliver et al., 1998. Metasomatic redistribution of nature of metasomatic alteration, and to investigate volatile and fluid-mobile non-volatile chemical com- the composition of the responsible fluids, a detailed ponents during the prograde and retrograde phases of study has been undertaken on a specific type of regional metamorphism can profoundly influence the metasomatic rock, viz. skarn-like garnet±epidote- final chemical and mineralogical status of a meta- bearing alteration zones within laminated calc-sili- morphosed terraneŽ. Ague, 1997 . Such changes must cate rocks. This style of metasomatic alteration, be quantified in order to understand the effects that which occurs throughout the Olary DomainŽ. Fig. 1 , metamorphic and related metasomatic processes can is a manifestation of intense mineralogical and chem- produce on rock masses. ical change resulting from focused fluid passage, and In this study we have investigated the role of therefore provides an opportunity to investigate the metasomatism in the formation and evolution of nature, origin and effects of the metasomatising flu- rocks from the Proterozoic Willyama Supergroup in ids. In addition, as these rocks are suitable for Sm±Nd the Olary Domain of eastern South Australia. The isotopic dating studies, they allow important con- Olary Domain, part of the Curnamona Province, a straints to be placed on the timing of metasomatic major Proterozoic terrane located within eastern activity. South Australia and western New South Wales, Aus- Directly after the attainment of peak regional traliaŽ. Fig. 1 , provides an excellent example of a metamorphic conditions, the Olary Domain experi- geological terrane that has been significantly effected enced regionally extensive episodes of the passage of by metasomatic processesŽ e.g. Cook and Ashley, hot, saline and oxidised aqueous fluids. The fluids 1992; Ashley et al., 1998a,b. Within the Olary responsible for metasomatic alteration were probably Domain, the chemical and mineralogical composi- derived from metamorphic devolatilisation of crustal tions of rocks within the Willyama Supergroup have rocks, largely a sedimentaryŽ. ±felsic volcanic se- been strongly altered by metasomatic processes, and quence. Importantly, although we suggest that intru- A.J.R. Kent et al.rLithos 54() 2000 33±62 35 Fig. 1. Map of the Olary Domain showing locations mentioned in the text and locations of garnet±epidote replacement zones and clinopyroxene- and actinolite-matrix breccias. Bold dashed line represents the location of the boundary between metamorphic zones IIA Ž.Ž.Ž.andalusite±chloritoid and IIB andalusite±sillimanite of Clarke et al. 1987 . The approximate position of 1600"20 Ma granitoids is also shown. sion of granitoid rocks may have been an important tions, Sm±Nd isotopic composition and fluid inclu- factor in promoting devolatilisation reactions in the sions are given in Appendix A. surrounding wallrocks, there is no clear evidence for the direct contribution of water derived from crys- 2.1. Rock and mineral analysis tallising granitoids to the metasomatising fluids. Samples of altered and unaltered calc-silicate rocks were analysed for major and trace elements by 2. Analytical methods X-ray fluorescence at the University of Melbourne and University of New England, Armidale, Aus- Descriptions and locations for all samples anal- tralia, using Siemens SRS-300 instruments. Mineral ysed for whole rock and mineral chemical composi- compositions were measured using a JEOL 5800 36 A.J.R. Kent et al.rLithos 54() 2000 33±62 scanning electron microscope run in EDS mode at a epidote, garnet, actinolite and quartz. , and not older beam current of 25 nA at the University of New metamorphic mineralsŽ see discussion below on the England and using a range of natural standards for effect of this on isochron calculations. Samples for calibration of X-ray intensities. Mineral composi- analysis were weighed into dissolution vessels, spiked tions, for phases containing Fe2q and Fe3q were with a mixed 146 Ndr150Sm solution and dissolved calculated assuming stoichiometry. using HF±HNO3 ±HCl acid digestion. Sm and Nd were separated and purified using 3g cation ex- change and HDEHP-teflon columns using
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