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1、Precambrian Research 155 (2007) 229250 A non-marine depositional setting for the northern Fortescue Group, Pilbara Craton, inferred from trace element geochemistry of stromatolitic carbonates Robert Bolhara, Martin J. Van Kranendonkb a University of Queensland, Brisbane, Qld 4072, Australia b Geolog
2、ical Survey of Western Australia, 100 Plain St., East Perth, WA 6004, Australia Received 21 December 2005; received in revised form 11 February 2007; accepted 15 February 2007 Abstract The depositional environment of sedimentary rocks from the Neoarchaean Fortescue Group (2.782.63Ga) in the Hamersle
3、y Basin, Pilbara Craton, Western Australia, is controversial, with both lacustrine and shallow-marine settings proposed. Stromatolitic carbonates occur throughout the stratigraphic section at various levels, and their trace element geochemistry is used herein to examineambientwaterfromwhichthesecarb
4、onatesprecipitated.Specifi cally,rareearthelements(REE)andyttriumwereanalysed to understand whether: (i) Fortescue carbonates display a marine or lacustrine geochemical signature, and (ii) to identify temporal trends in the geochemistry consistent with a transition in the depositional environment. D
5、etailedinspectionrevealsthatFortescuestromatolitesfromthenorthernpartoftheHamersleyBasinaredevoidoftraceelement compositionsthatarecharacteristicofmarinecarbonates.Forinstance,thecarbonateslackdistinctLa(expressedasLa/3Pr2Ndin shale-normalised diagrams) and Gd (Gd/(2TbDy)shale) anomalies and supra-c
6、hondritic Y/Ho ratios, which are well-developed in seawater-derivedcarbonatesformedthroughoutEarthhistory.TheNeoarchaeanFortescueGroupstromatolitesalsolackdepletionof the light REE relative to the middle and heavy REE when shale-normalised, in contrast to modern seawater and marine carbonates, which
7、 are typically HREE-enriched. Importantly, the absence of a clear temporal trend in the geochemistry suggests that the depositional environment remained unchanged from 2.8 to 2.6Ga. It is concluded that Neoarchaean stromatolitic carbonates throughout the Fortescue Group in the northern part of the H
8、amersley Basin were deposited in a lacustrine environment, or a very shallow lagoonal environment dominated by freshwater infl ux from rivers. Our study demonstrates that trace element geochemistry is a powerful tool to constrain depositional environments of ancient carbonate rocks. This study is th
9、e fi rst to document the geochemical characteristics of carbonates formed through microbial activity in an ancient lake, or shallow lagoonal, setting. 2007 Elsevier B.V. All rights reserved. Keywords: Archaean; Carbonate; Fortescue Group; Geochemistry; Lacustrine; Pilbara Craton; Stromatolite; Rare
10、earth elements Corresponding author. Present address: Department of Geological Sciences, University of Canterbury, Christchurch, New Zealand. Tel.: +64 3 364 2987x7683; fax: +64 3 364 2769. E-mail address: robert.bolharcanterbury.ac.nz (R. Bolhar). 1. Introduction The interpreted depositional settin
11、g of some sedi- mentary rocks of the Neoarchaean Fortescue Group, Pilbara Craton, is controversial, including models of either lacustrine (Awramik and Buchheim, 2001; Awramik, 1992; Buick, 1992; Walter and Bauld, 1983) 0301-9268/$ see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/
12、j.precamres.2007.02.002 230R. Bolhar, M.J. Van Kranendonk / Precambrian Research 155 (2007) 229250 or shallow-marine settings for stromatolitic carbon- ates of the Tumbiana Formation (Sakurai et al., 2005; Thorne and Trendall, 2001). Resolution of this con- troversy is important because possibly the
13、 largest and most diverse assemblage of ancient, bona fi de stromatolites occur in carbonates of the Tumbiana FormationoftheFortescueGroup(Walter,1983).Refi n- ing knowledge of the depositional setting will help to better understand how life evolved on Earth and what geological factors controlled su
14、itable habitats for early life to cause the build-up of atmospheric oxy- gen. 2. Geological setting The Neoarchaean Fortescue Group was deposited in the Hamersley Basin from 2.78 to 2.63Ga, as a platformal succession of fl ood basalts and interbedded sedimentaryformations,upto7kmthick,onabasement of
15、 3.72.83Ga granite-greenstone crust of the Pilbara Craton (Fig. 1). The Fortescue Group forms the lower part of the Mount Bruce Supergroup, which includes overlyingsedimentaryandvolcanicrocksoftheHamers- leyandTureeCreekGroups,withminimumdepositional ages of 2.45Ga (Trendall et al., 2004). Sedimenta
16、ry faciesanalysisindicatesanoveralldeepeningofthebasin to the south across what is known as the Yule-Sylvania High (Blake, 1993), from predominantly subaerial in the north, to deeper marine in the south (Thorne and Trendall, 2001). The stratigraphic framework of the Fortescue Group forboththenorther
17、nandsouthernpartofthePilbaraCra- ton is shown in Fig. 2, along with stratigraphic positions where the samples were obtained. The group consists of three main fl ood basalt sequences that include, from base to top, the 2.78Ga Mount Roe Basalt (MR), ca. 2.74Ga basalts of the Kylena Formation (Ky), and
18、 2.72Ga basalts and tuffaceous rocks of the Maddina Formation (Ma). Interbedded with these dominantly basaltic formations are a variety of siliciclastic and carbonate rocks belonging to the 2.76Ga Hardey For- mation (Ha) and 2.72Ga Tumbiana Formation (Tu). Carbonate rocks also occur in the Mopoke Me
19、mber of the Kylena Formation, and in the ca. 2.70Ga Jeeri- nah Formation (Je) at the top of the group (Fig. 2). Depositional age data were reported by Blake et al. (2004). Basaltic rocks of the Mount Roe Basalt, and the Kylena and Maddina Formations were predominantly erupted under subaerial conditi
20、ons, as indicated by stacked sequences of thick, homogeneous fl ows with scoriaceous fl owtops, and the local presence of pahoe- hoe textures, large, irregular gas cavities and pipe vesicles (Fig. 3a). Pillow structures are locally devel- oped at the base of the Mount Roe Basalt and Kylena Formation
21、, but are overlain by, and/or occur interbedded with, subaerial fl ows that are much more widespread and voluminous (Fig. 3b; Hickman and Van Kranendonk, 2007; Thorne and Trendall, 2001). Conglomerate and sandstone that underlie the Mount Roe Basalt were deposited in braided fl uviatile and lacustri
22、ne settings (Blake, 1993; Thorne and Trendall, 2001). TheHardeyFormationconsistspredominantlyofsili- ciclasticrocksdepositedoverabroadareaofcontinental sedimentation, with alluvial fan, braided fl uvial facies, and lacustrine facies. Geochemical analysis of a shale facies within the formation indica
23、ted a lacustrine set- ting (Hickman and de Laeter, 1977). Further south, the upper part of the formation is interpreted to have been deposited under deltaic to shoreline conditions (Thorne and Trendall, 2001). Deposition of the Hardey Forma- tion occurred at 27568Ma, the age of a widespread felsic v
24、olcanic unit in the formation (Arndt et al., 1991). The Tumbiana Formation extends for over 680km across the northern exposure of the Fortescue Group (Fig. 1). The formation consists of 45, La/La shale1, LREE/HREEshale45). This variability implies some frac- tionation of Y/Ho in the freshwater envir
25、onment. Y/Ho ratios for Fortescue Group stromatolites range from 20 to 35, overlapping the river water values from Australia and the value for the freshwater stromatolite from the Green River Formation (Y/Ho=29). From variable Eu anomaliesandfractionatedREE+Ypatternsoftheriver waters (when normalise
26、d to local catchment composi- tions), Lawrence et al. (2006) inferred that weathering reactionsofindividualmineralphasesandwaterpHexert a stronger infl uence than bedrock make-up on aqueous REE+Y patterns, in agreement with Elderfi eld et al. (1990). 6.7. Fractionation in the estuarine environment E
27、stuaries represent the physical link between fresh- waterpathwaysandtheopenmarineenvironment,where complex (bio)geochemical reactions infl uence the fl ux and composition of trace elements delivered into the oceans. Chief processes include salt-induced coagula- tion of FeMn-REE colloids, ad/desorpti
28、on onto/from suspended particles and bottom sediments, as well as re-suspension of bottom sediments and diagenesis (e.g. Elderfi eld et al., 1990; Sholkovitz, 1992; Sholkovitz et al., 1994). During coagulation in the low salinity region, LREE are preferentially removed from solution, while HREE are
29、preferentially released back into solu- tion as carbonate complexes in the mid to high salinity region of estuaries. The net effect is that river water becomes more LREE-depleted and HREE-enriched as estuarinemixingprogresses,producingtheREE+Ysig- natures commonly observed in open ocean water (e.g.
30、Sholkovitz and Szymczak, 2000). REE+Y patterns displayed by Fortescue Group car- bonates may refl ect either the composition of the source material or the fractionation processes that acted prior to precipitation, or a combination of both. Judging from thefl attoonlymildlyfractionatedtraceelementpat
31、terns (Fig. 4), fractionation processes appear insignifi cant. Sholkovitz and Szymczak (2000) argued that resupply of REE+Y from bottom sediments may be minimal in cases where estuaries have a great water depth. This scenario is consistent with the largely unfractionated REE+Y patterns observed in t
32、his study. On the other hand,agreatwaterdepthisdiffi culttoreconcilewiththe shallow water deposition of several of the stromatolitic carbonates, as inferred from sedimentology. Fractiona- tion of REE+Y has been also invoked due to biological uptake during algae blooming (Nozaki et al., 2000), in whi
33、ch case LREE would become depleted relative to MREE,andHREE-enrichedrelativetoMREE.Thisfea- tureisonlydevelopedinonecarbonatesample(169217). Negative Ce anomalies are common in estuarine waters and are caused by preferential scavenging of the tetravalent species. For instance, Nozaki et al. (2000) h
34、ave documented a progressively stronger depletion of Ce relative to its neighbours as a function of salinity in a modern estuarine (Thailand), and concluded that defi - cientCeisonlypartlyduetomixingbetweenfreshwater and seawater. In their study, relative depletion of Ce is already present in waters
35、 with lowest salinity, indicat- ing that fractionation may have occurred early during riverinetransport(seealsoLawrenceetal.,2006)ordur- ing weathering of source material. With respect to the Fortescue Group carbonate samples, Ce anomalies are not developed, consistent with low levels of free oxygen
36、 in the Archaean hydrosphere. In summary, HREE enrichment over the LREE and MREE, typically found in modern estuarine waters, is not clearly developed in Fortescue Group carbonates. It therefore appears that these Archaean chemical sedi- ments were not formed in an estuarine setting. 6.8. Fractionat
37、ion in the lacustrine environment Geochemical studies of modern and ancient lake deposits are scarce. A detailed understanding of the pro- cesses and factors that infl uence the composition of lake waters must take into account hydrological information and knowledge of physico-chemical parameters. I
38、n the 244R. Bolhar, M.J. Van Kranendonk / Precambrian Research 155 (2007) 229250 absence of such information an exact interpretation of possiblelakedepositsisinherentlydiffi cult.Ofrelevance to this study is the documentation of lake deposits with MREE-enriched shale-normalised patterns, because the
39、 FortescueGroupcarbonatesarealsovariablyenrichedin theMREE(Fig.4).RelativeMREEenrichmenthasbeen documented in various terrestrial waters including some river waters (Hannigan and Sholkovitz, 2001), as well as hypersaline and freshwater acid lakes (Johannesson et al., 1996; Johannesson and Zhou, 1999
40、). Johannesson et al. (1996) noted that MREE-enriched patterns are not documented for high-pH systems (i.e. modern seawa- ter) or alkaline lakes (Moller and Bau, 1993), but are mainly a feature of low-pH waters. Processes involved in MREE enrichment are manifold and include: frac- tionation by collo
41、ids (e.g. Elderfi eld et al., 1990), particle/mineral-liquidinteraction(e.g.Sholkovitzetal., 1994), and dissolution of MREE-enriched mineral sur- face coatings within aquifer materials by (acid) waters and sulfate complexation (e.g Johannesson and Zhou, 1999). An assessment of whether any of the abo
42、ve pro- cesses may be invoked to explain MREE enrichment in some of the Fortescue Group carbonates is diffi - cult due to the limited geochemical data set. However, because of the similarity to documented lake deposits, MREEenrichmentmaybetakenassupporting,although circumstantial, evidence that the
43、Fortescue Group stromatolites were deposited in a lacustrine environ- ment. 6.9. Comparison with Archaean chemical sedimentary rocks PositiveLaanomaliesinchemicalsedimentaryrocks, calculated from shale-normalised REE abundances, can be considered a reliable indicator for derivation of the REE from s
44、eawater (e.g. Bau and Dulski, 1996; Kamber and Webb, 2001). A positive La anomaly can be quan- tifi ed from the overabundance of La relative to Ce in samples that lack Ce anomalies (i.e. Pr/Pr*1). Due to low concentrations of free oxygen in the Archaean environment, Archaean sediments are devoid of
45、nega- tive Ce anomalies, and the extent of the La anomaly can be quantifi ed through comparison of Ce/Ce*and Pr/Pr*. In the case of post-Archaean chemical sedi- ments, the presence of negative Ce anomalies does not allow direct quantifi cation of La anomalies. Fig. 8A reveals a distinct difference b
46、etween Archaean (Camp- bellrand, Strelley Pool, Mushandike, and this study) and Phanerozoic microbial carbonates (Great Barrier Reef, Canning Basin), the latter with Pr/Pr*1 and Ce/Ce*0.9). In terms of Y/Ho (Fig. 8B), the carbonates from this study show no overlap with Archaean and post- Archaean mi
47、crobialites that were deposited in a marine setting (Y/Ho?40). Modern seawater has Y/Ho45, substantially higher than the chondritic value of 2528. Elevated Y/Ho ratios are due to differences in the com- plexation behaviour of both elements that are otherwise geochemically similar. The latter aspect
48、is refl ected by the chondritic ratios of the vast majority of geological materials, including all volcanic and clastic sediments. It is striking that the Fortescue Group carbonates plot welloffthecompositionalspectrumdefi nedbychemical sedimentary rocks (Y/Ho40100), with the average value for the f
49、ormer also being substantially lower than the range displayed by modern marine proxies (4474; Nozaki et al., 1997). For Gd anomalies, Fortescue Group carbonates have values of 10.1 and occupy a compositional fi eld that overlaps with other bona fi de marine precipitates. Kamber et al. (2004) noted that the general trend from high Y/Ho with elevated Gd/Gd*ratios towards chon- dritic values (cross-hair in Fig. 8B) may refl ect variable degrees of detrital contamination. However, as has been shown above, detrital contamination can be ruled out in this study, consiste