Bryan Krapez

Sequence stratigraphy of the Archaean supracrustal belts of the Pilbara Block, Western Australia

Abstract The stratigraphic succession of the 3580-2770 m.y. old granite-greenstone terrain of the Pilbara Block is re-assessed by the first- and second-order sequence-stratigraphic techniques that are the standard for Phanerozic terrains, with the view to testing the conformity of stratigraphic cause and response. A holistic tectonostratigraphic framework is presented that pairs the supracrustal and crustal rock records. The resulting sequence scheme is tested against the cause of first-order sequence cycles, viz. global sea-level responses to supercontinent breakup and assembly. Three, first-order, inter-regional unconformities divide the stratigraphic succession of the supracrustal belts into four units of first-order, megasequence status. In order of decreasing age, these are named the Warrawoona, Gorge Creek, Roebourne, and Mount Negri Megasequences. A total of 17 second-order supersequences are also named, although some may represent supersequence sets. The megasequences, and/or their genetic crustal equivalents, represent the preserved rock record of four megacycles of forearc, arc, and/or back-arc geotectonic evolution associated with the convergent margins of a Pilbara Continent. Individual supersequences are the rock record of separate second-order basins or basin phases, and reflect the different geotectonic components of each megasequence. The four megacycles had respective longevities, from oldest to youngest, of 150 m.y., 230 m.y., 160 m.y. and 180 m.y. They are identical in their stratigraphic successions and predicted first-order sea-level cycles to Phanerozoic first-order tectono-eustatic cycles, and are interpreted as the same response to global tectonic change. The Warrawoona and Gorge Creek Megacycles, and the Roebourne and Mount Negri Megacycles are tectonogenetic pairs of the East Pilbara and the West Pilbara Megacycle Sets, respectively. The East Pilbara Megacycle Set evolved over a period of 380 m.y. from 3490 Ma to 3110 Ma, whereas the West Pilbara Megacycle Set evolved over a period of 340 m.y. from 3110 Ma to 2770 Ma. The two megacycle sets are identical in their longevity, geotectonic components, and predicted sea-level cycles to convergent-margin Wilson and Supercontinent Cycles, and are thereby interpreted individually as the oldest established records of the Supercontinent Cycle. The implication of this interpretation is that a steady-state global tectonic regime has been in operation from early in Earths history, which has important consequences for theoretical modelling of the Earths early tectonic style and secular changes in its geochemical regime.

The Late Archaean bonanza: metallogenic and environmental consequences of the interaction between mantle plumes, lithospheric tectonics and global cyclicity

Abstract The Late Archaean records periods of intense magmatism and the development of prodigious metallogenic provinces of Ni, Fe, Cu Zn and Au deposits. In particular, the period 2.74-2.66 Ga represents one of the most widespread episodes of ultrabasic and basic volcanism preserved in the geological record, as well as anomalously widespread granitoid magmatism. Extensive assemblages of this age, which comprise komatiites and komatiitic basalts derived from mantle plumes, together with tholeiites and calc-alkalic volcanic rocks, are preserved on most Late Archaean cratons. Intense submarine volcanism in plume-like environments resulted in rich komatiite-hosted Ni mineralization in continental-margin basins, and Cu Zn sulphide mineralization in extensional volcanic arcs. Peak submarine magmatism was accompanied by marine transgression and thereby flooding of previously exposed continental crust. Elevated hydrothermal activity and widespread su☐ic conditions in submarine basins are reflected by sulphide-rich carbonaceous sedimentary rocks, that contain the organic remains of bacterial communities, and banded iron formations (BIF). A major episode of mesothermal gold mineralization accompanied accretionary tectonics, as metal-and carbon-rich submarine volcanic and sedimentary successions were subducted or incorporated into nascent continental crust by 2.59 Ga. Comparison with Neoproterozoic and Phanerozoic tectonic and metallogenic patterns indicates that the period 2.78-2.59 Ga represents the first half of an ∼ 360 m.y. global tectonic cycle. This period records the breakup of a supercontinent and the opening and closing of marginal basins along long-lived convergent margins of the external ocean to that supercontinent. Enhanced magmatic events between 2.74 and 2.66 Ga were most likely the result of intrabasinal mantle plumes and a subsequent global plume-breakout event. Together, the plume events were responsible for the extreme environmental conditions during the Late Archaean relative to both the preceding and succeeding periods of Earth history. Interactions between mantle plumes and long-lived convergent margins of a Pacific-type ocean were responsible for the prodigious metal inventory of Late Archaean granitoid-greenstone terranes. Extensive convergent-margin and plume magmatism during that period, coupled with episodic periods of low-angle subduction underplating by oceanic lithosphere, may also have been the cause of development of the buoyant, continental mantle lithosphere that is responsible for the preservation of these highly mineralized cratons. It is also likely that the bonanza metallogenic provinces in the Witwatersrand basin and Paleoproterozoic orogens of West Africa and Laurentia-Baltica reflect interactions of mantle plumes with long-lived convergent margins of the external ocean.

Giant iron-ore deposits of the Hamersley province related to the breakup of Paleoproterozoic Australia: New insights from in situ SHRIMP dating of baddeleyite from mafic intrusions

Banded iron formations of the ca. 2770–2405 Ma Hamersley province of Western Australia were locally upgraded to high-grade hematite ores during the Early Paleoproterozoic by a combination of hypogene and supergene processes after the initial rise of atmospheric oxygen. Ore genesis was associated with the stratigraphic break between the Lower and Upper Wyloo Groups of the Ashburton province, and has been variously linked to the Ophthalmian orogeny, late-orogenic extensional collapse, and anorogenic continental extension. Small-spot in situ Pb/Pb dating of baddeleyite by sensitive high-resolution ion microprobe (SHRIMP) has resolved the ages of two key suites of mafic intrusions, constraining for the first time the tectonic evolution of the Ashburton province and the age and setting of iron-ore formation. Mafic sills dated as ca. 2208 Ma were folded during the Ophthalmian orogeny and then cut by the unconformity at the base of the Lower Wyloo Group. A mafic dike swarm that intrudes the Lower Wyloo Group and has a close genetic relationship to iron ore is ca. 2008 Ma, slightly younger than a new syneruptive 2031 ± 6 Ma zircon age for the Lower Wyloo Group. These new ages constrain the Ophthalmian orogeny to the period between ca. 2208 and 2031 Ma, before Lower Wyloo Group extension, sedimentation, and flood-basalt volcanism. The ca. 2008 Ma dikes pre s ent a new maximum age for iron-ore genesis and deposition of the Upper Wyloo Group, thereby linking ore genesis to a ca. 2050–2000 Ma period of continental extension similarly recorded by Paleoproterozoic terrains worldwide well after the initial oxidation of the atmosphere by ca. 2320 Ma.

Tectonic and geochronological constraints on late Archaean and Palaeoproterozoic stratigraphic correlation within and between the Kaapvaal and Pilbara Cratons

Recent radiometric dating of carbonates and banded iron-formations on the Kaapvaal Craton, southern Africa, has suggested that late Archaean and Palaeoproterozoic carbonate sedimentation was diachronous across the craton. We present new SHRIMP U–Pb zircon ages of 2583±5 Ma and 2588±7 Ma for two samples of a tuff bed, 320 km apart in the upper Oak Tree Formation, Transvaal Supergroup. These ages, in conjunction with published data, support previously established sequence stratigraphic correlations that show the temporal equivalence of late Archaean carbonates in the region. A review of the sequence stratigraphy and geochronology of the Mount Bruce Megasequence Set on the Pilbara Craton, Western Australia, and the newly named Highveld Megasequence Set on the Kaapvaal Craton, indicates a remarkably similar geohistory. Both cratons are characterized by a divergent megacycle (Hamersley Range and Vaal Megasequences) followed by a convergent megacycle (Chichester Range and Drakensberg Megasequences). Our data show that the megasequence boundary is marked by drowning of both cratons at approximately the same time (c. 2590 Ma), which permits more meaningful geotectonic correlations between them.

Tectonic settings of Archaean (3325-2775 Ma) crustal-supracrustal belts in the West Pilbara Block

Abstract The Archaean Pilbara Block comprises six fault-bounded tectonostratigraphic domains. The West Pilbara Block contains, from east to west, Domains 4, 5 and 6. Review of stratigraphic, magmatic, structural and geochronologic data, and new field mapping, indicates that the crustal and supracrustal rock records correlate to three megacycles. Megacycle IVU (3325-3135 Ma) is recorded by an intra-arc basin and arc granitoids in Domain 6, and a back-arc basin in Domains 4 and 5. The domains were an arc-continent orogen from ∼ 3200-3155 Ma, during which the outboard are setting was accreted to the back-arc continental margin. Although Domain 6 is allochthonous with respect to Domains 5 and 4, it was not exotic with respect to the tectonic settings of the Pilbara continent. Megacycle VL (3135-2955 Ma) is recorded by, in decreasing age: (i) a back-arc basin and arc granitoids in Domain 5: (ii) a deep-marine, retroarc or remnant-ocean basin in Domain 4 and a plutonic belt in Domain 5: (iii) a plutonic belt in Domains 5 and 6, and possibly also in Domain 4: and (iv) craton-wide deformation belts, coeval with the Whim Creek basin and granitoid plutons in Domain 5. The Whim Creek basin was a half-graben that developed at a releasing bend (or overstep) along the boundary fault to Domains 4 and 5, and, to the northeast. Domains 4 and 6. The basin was closed by sinistral transpression as the boundary fault between Domains 4 and 6 propagated to the southwest from the releasing bend, and thereby generating a restraining bend geometry. Fundamental strike-slip faults, which are interpreted to have been tectonic-escape responses to a cause external to the preserved Pilbara Block, juxtaposed domains craton-wide. The initial stage of Megacycle VU (2955-2775 Ma) is recorded by the eroded remnants of a rift province. Preserved elements include the Negri basin (Domain 5 only), mafic-ultramafic sub-volcanic layered intrusions, and granitic plutons and related high-level sills. Two periods of granitoid intrusion are preserved. Base-metal sulphide deposits, hosted by the Whim Creek basin, were probably sourced from extensional geofluids in the rift province. A plume origin is likely for the komatiitic parent magmas to sub-volcanic intrusions, and to basalts of the Negri basin. Plume magmatism is interpreted to have been coincident with, but not the cause of, lithospheric extension because plumerelated volcanism post-dates the first period of plutonic intrusion by ∼ 25 million years. Extension was followed by orogenic deformation that converted Domains 4, 5 and 6 to a hinterland fold-thrust belt. Deformation is interpreted to have been caused by collision orogeny during terrane accretion along the western Pilbara margin. Collision orogeny was the last event of a long history of Pacific-type tectonics along the margins of the Archaean Pilbara continent.

Archean Oil: Evidence for Extensive Hydrocarbon Generation and Migration 2.5-3.5 Ga

Archean sedimentary rocks from the Pilbara Craton, Australia, contain evidence for petroleum generation and migration in the form of bitumen nodules produced by radiogenic immobilization of fluid hydrocarbons around detrital uraninite, thorite, and monazite grains. The nodules are preserved in sandstones at several stratigraphic levels in the Fortescue Group (~2.75 Ga) and Lalla Rookh Formation (~3.0 Ga), both nonmarine successions, and in deltaic sediments of the Mosquito Creek Formation (~3.25 Ga). The most ancient evidence comes from the Warrawoona Group (>3.46 Ga), where hydrocarbon droplets were apparently formed in situ from kerogenous sediments by flash maturation during early hydrothermal silicification. Bituminous relics of petroleum are also commonly preserved in shallow-marine sandstones of the Black Reef Formation (~2.59 Ga) and the Witwatersrand Supergroup (~2.85 Ga) from the Kaapvaal Craton, South Africa, along with subeconomic methane accumulations. In all cases, the petroleum was apparently sourced from Archean shales, generated during the Archean, and migrated before the late Archean or early Early Proterozoic metamorphism occluded fluid pathways. Given this widespread and abundant evidence for hydrocarbon generation and migration in Archean depositional basins, it seems that primordial bacterial biomass, producing labile type I kerogen, was often buried in sufficient quantities to successfully generate and expel petroleum. Depositional basins on ancient cratons clearly contained permeable rocks amenable to the migration, and probably to the accumulation, of petroleum. Thus, the main factors precluding the discovery of economically exploitable hydrocarbon accumulations in Archean basins are the subsequent destructive effects of deformation and metamorphism, which causes trap breaching, imperfect sealing, or thermal obliteration. However, there are ancient stable cratons where such disruption may not have occurred, and so petroleum explorers may wish to reassess the possibility of finding valuable hydrocarbon resources in Archean rocks.

1.2 Ga thermal metamorphism in the Albany–Fraser Orogen of Western Australia: consequence of collision or regional heating by dyke swarms?

Compressive fabrics in the Late Palaeoproterozoic Mount Barren Group of the Albany–Fraser Orogen, southwestern Australia, record Mesoproterozoic collision between proto-Australia and proto-Antarctica. Petrographical evidence establishes that peak thermal metamorphism produced largely random growth of kyanite, staurolite, biotite, monazite and xenotime that overprinted those fabrics. SHRIMP U–Pb geochronology of xenotime and monazite yields an average age of 1205 ± 10 Ma. Thermal metamorphism therefore occurred at least 45 Ma after fabric formation, and was unlikely to have been caused by collision. Rather, thermal metamorphism overlapped with the emplacement of 1215–1202 Ma dyke swarms into the Orogen and the adjacent Yilgarn Craton, and was followed by emplacement of 1200–1180 Ma granites. Regional heating associated with mafic magmatism was the probable cause of thermal metamorphism, but previous proposals that the dyke swarms were the consequence of collision or extensional orogenic collapse cannot be substantiated. A regional thermal anomaly, craton-scale extension and adiabatic decompression melting of the asthenosphere are implied, but causal mechanisms such as a mantle plume or intracontinental rifting require substantiation from other parts of East Gondwana. The significant time gap between orogenic deformation and thermal metamorphism implies that metamorphism in many other orogens may not necessarily be due to compressive tectonics.

The Late Archaean Melita Complex, Eastern Goldfields, Western Australia: shallow submarine bimodal volcanism in a rifted arc environment

Abstract The Melita Volcanic Complex is a Late Archaean bimodal rhyolite/basalt volcanic succession within the Gindalbie Terrane in the Eastern Goldfields Province of the Yilgarn Craton. The Melita Complex has been dated by ion probe at 2683±3 Ma (95%) and forms part of a distinctive 2681–2692-Ma volcanic association that records bimodal (basalt/rhyolite) and calc–alkaline intermediate-silicic volcanism at several discrete volcanic centres, and which locally hosts volcanic massive sulphide mineralisation (Teutonic Bore). Approximately 3 km of stratigraphic thickness is exposed in the Melita area. The upper 1–1.5 km of the exposed succession is dominated by subaqueously resedimented volcaniclastic sandstones and breccias, rhyolite flows and sills. Primary subaerial pyroclastic deposits including ignimbrites have not been identified in this study, although subaerial explosive activity is indicated by the occurrence of accretionary lapilli and the abundance of vitric material (shards) and pumice fragments in resedimented deposits. The lower part of the succession is dominated by pillowed to massive basalt lavas, and in situ and resedimented mafic hyaloclastites. Mafic extrusive and intrusive rocks are tholeiitic with trace element concentrations similar to modern arc tholeiites. Felsic volcanic rocks at Melita are dacite to high-silica rhyolite. They are highly enriched in incompatible elements (particularly high field strength element-enriched), compared to other felsic associations in the Eastern Goldfields Province, representing evolved partial melts of heterogeneous intermediate arc-type crust. The volcanic facies and geochemistry of volcanic rocks at Melita are consistent with those observed in modern intra-arc or arc-rift settings, and the succession is interpreted to represent the initial stages of back-arc rifting within a complex convergent margin.

Evidence of hydrocarbon and metalliferous fluid migration in the Palaeoproterozoic Earaheedy Basin of Western Australia

Dolomitized limestones of the Earaheedy Basin contain solid bitumen and metal‐sulphides, sulpharsenides and arsenides within a network of fractures which acted as pathways for migrating fluids. The solid bitumen formed from residual oil that was thermally altered within those fractures. Immobilization of oil was postdated by precipitation of fracture‐filling calcite and later replacive dolomite and quartz cements, as well as of magnesian chlorite. Solid bitumen has high concentrations of Fe‐, Zn‐, Pb‐, Ni‐ and FeCu‐bearing sulphides, sulpharsenides and arsenides in minute inclusions that are randomly distributed throughout its groundmass. Galena and pyrite also fill cracks in the bitumen, whereas pyrite, arsenopyrite, galena, gersdorffite, enargite? and chalcopyrite fill contraction vugs. Small crystals of cassiterite are associated with cavity‐filling magnesian chlorite. Multiple phases of base‐metal precipitation are apparent. The first, indicated by the inclusions, probably formed during mixing of hydrocarbon and metal‐rich fluids. Later phases resulted from the filling of bitumen contraction structures that formed during and after thermal alteration and degassing processes, and which imply the migration of metal‐rich fluids after oil immobilization. The association of metal‐rich minerals and fracture‐filling bitumen, in this non‐ore setting, is a guide to the possible existence of ore‐grade mineralization elsewhere in the basin.