Arthur H. Hickman

REE geochemistry and isotopic data of Archean silicic volcanics and granitoids from the Pilbara Block, Western Australia: implications for the early crustal evolution

Eighteen silicic volcanic rocks of the Warrawoona Group and ten associated plutonic rocks from the Pilbara Block, Western Australia, have been chosen for geochemical and isotopic studies. Silicic volcanics of the UNSB (Upper member of North Star Basalt) are dated at 3.56—3.57 , by both the Rb-Sr and the Sm-Nd methods. The respective 1 (initial isotopic composition) values are 0.7005 ± 5 (Sr) and 0.50810 ± 39 (Nd). This age is consistent with the stratigraphic interpretation that the TalgaTalga Subgroup, in which the North Star Basalt occupies the lowermost position, is overlain by the Duffer Formation, whose age was earlier established at 3.45 by the zircon U-Pb method. The new Rb-Sr data on six silicic lava samples from the Duffer Formation yield an isochron of 3.23 ± 0.28 (2v). Though imprecise, this age agrees with the zircon age within error limits. Rb-Sr ages of 2.3–2.4. obtained for the ‘Panorama’ rocks and the Wyman Formation do not correspond to their initial eruption ages. Chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2.3–2.4 ago. Geochemically, most of these analyzed rocks (volcanic and plutonic) are of tonalite-trondhjemitegranodiorite (TTG) composition, a typical feature found in many other Archean terrains. They generally show fractionated REE patterns, except the Panorama Formation rocks. Furthermore, the Wyman Formation rhyolites and the post-tectonic adamellites show significant negative Eu anomalies, suggesting a similar mode of magma generation and a probable genetic link. Theoretical considerations suggest that most of these TTG rocks could have been generated by partial melting of amphibolitic or basaltic sources, followed by fractional crystallization. Although the Archean granitic gneisses often possess mantle-like Isr values, the trace element data indicate that they could not have been derived by direct melting of upper mantle materials. The immediate tectonic implication is that in any Archean terrain, the formation of Na-rich continental crust of TTG suite must be preceded by the presence of basaltic crust. The occurrence of this basaltic crust is a matter of controversy. Such crust might have been totally destroyed by repeated melting processes, or its remnants are now represented by some of the mafic-ultramafic enclaves within the tonalite-trondhjemite batholiths.

UPb zircon geochronology of Archaean felsic units in the Marble Bar region, Pilbara Craton, Western Australia☆

Abstract Archaean supracrustal rocks that are well exposed in the Marble Bar region, Pilbara Craton, have been assigned to the Warrawoona Group and younger sequences. The predominantly volcanic Warrawoona Group, previously dated at 3300 to 3500 Ma, is largely basaltic with locally intercalated thick felsic volcanic units. The supracrustal rocks have been folded and are distributed around the margins of large, roughly circular to ovoid, “granitic-gneissic” batholiths. A UPb zircon geochronology study was undertaken to obtain precise age constraints for some of the ore deposits in the area, especially the Big Stubby, North Pole (barite), and Miralga Creek (ZnPbCuAu) deposits, in support of efforts to improve Archaean lead isotope models. The results also help significantly in interpreting stratigraphic relationships and crustal evolution in the area. The new conventional zircon UPb data indicate that most of the Warrawoona Group (from the Duffer Formation upwards) was deposited over a period of ∼20 Ma, from ∼3470 Ma to 3450 Ma. The age of the Duffer Formation is closely constrained by results of 3471±5 Ma and 3465±3 Ma, and the age of the upper Salgash Subgroup appears to be established at 3454±1 Ma. The Panorama Formation at the southern margin of the North Pole Dome is 3458±1.9 Ma; unless the Duffer Formation was deposited over a period of at least 10 Ma, this casts doubt on a previously suggested Panorama-Duffer correlation. The age of the lower part of the North Pole succession is greater than ∼3458 Ma, consistent with the correlation of the basal greenstones at North Pole with the Talga Talga Subgroup. The North Pole chert-barite unit, which is well known for its preservation of Early Archean stromatolites, microfossils, and evaporite sediments, is clearly older than 3458 Ma. The 3325 Ma age now established for the Wyman Formation shows that this unit must be excluded from the Warrawoona Group. The age of the Talga Talga Subgroup remains uncertain; a felsic schist in the Mount Ada Basalt, which has been dated at 3449±3 Ma, is interpreted to be a sill, probably related to the 3450 Ma granitoid bodies. An age > 3724 Ma for a zircon xenocryst in the Panorama Formation is the oldest obtained so far for zircon from the Pilbara granite-greenstone terrane, and indicates the existence of pre-Pilbara Supergroup sialic basement. Ages of 3465±3 Ma and 3449±2 Ma applied to previous Pb isotope data for the Big Stubby and Miralga Creek deposits, respectively, in combination with comparable data for other syngenetic Archaean sulphide deposits, yield a general single-stage Pb evolution model with the parameters T0=4560 Ma, A0=9.0818, B0=9.9002 and C0=29.343. This gives model ages in accord with known constraints for many Archean deposits. A model lead age of 3490 Ma for the North Pole deposit, which is older than 3457 Ma, may be too great, but attempts to directly date this deposit have not yet succeeded. There are striking parallels between the Warrawoona Group ages reported here and those recently obtained for the Barberton region, Kaapvaal Craton, South Africa. The Talga Talga Subgroup and Lower Onverwacht (Tjakastad Subgroup) sequences have approximately equivalent constraints, and an age for the upper Salgash Subgroup agrees closely with those for the Hoogenoeg Formation of the upper Onverwacht Group (Geluk Subgroup).

Age of the Archean Talga-Talga Subgroup, Pilbara Block, Western Australia, and early evolution of the mantle: new SmNd isotopic evidence

Archean komatiites, high-Mg basalts and tholeiites from the North Star Basalt and the Mount Ada Basalt formations of the Talga-Talga Subgroup, Warrawoona Group, Pilbara Block, Western Australia, define a linear correlation on the normal143Nd/144Nd vs.147Sm/144Nd isochron plot. The data give an age of 3712 ± 98 Ma and initialeNd(T) of +1.64 ± 0.40. The 3712 ± 98 Ma date is consistent with the regional stratigraphic sequence and available age data and the SmNd linear array may be interpreted as an isochron giving the eruption age of the Talga-Talga Subgroup. An alternative interpretation is that the isochron represents a mixing line giving a pre-volcanism age for the Subgroup. Consideration of geochemical and isotopic data indicates that the true eruptive age of the Talga-Talga Subgroup is possibly closer to about 3500 Ma. Regardless of the age interpretation, the new Nd isotopic data support an existence of ancient LREE-depleted reservoirs in the early Archean mantle, and further suggest that source regions for the Pilbara volcanic rocks were isotopically heterogeneous, witheNd(T) values ranging from at least 0 to +4.0.

SmNd geochronology of greenstone belts in the Yilgarn Block, Western Australia

Abstract SmNd geochronology has been used to date 3 widely spaced metavolcanic (greenstone) belts in the Yilgarn Block. The measured isochron ages are 2.78 ± 0.03 Ga (initial ϵNd = 2.5 ± 0.3) for greenstones at Kanowna in the Eastern Goldfields Province, 3.05 ± 0.10 Ga (initial ϵNd = 0.9 ± 0.7) for Diemals-Marda in the Southern Cross Province and 2.98 ± 0.12 Ga (initial ϵNd = 0.7 ± 1.2) for the Warriedar fold belt in the Murchison Province. These data show that substantial crustal components of age ∼3.0 Ga exist in the Southern Cross and Murchison Provinces, and support published evidence that the Eastern Goldfields Province contains no crustal material ⩾ 2.8 Ga. These ages are all significantly younger than ages commonly observed in the Western Gneiss Terrain and so support the suggestion of a progressive crustal age trend across the Yilgarn Block. The measured age values depend strongly on the inclusion of data for felsic components of the greenstone belts; data for mafic and ultramafic units allow that eruption in the 3 localities could have been contemporaneous at ∼ 2.8 Ga. The age range of metavolcanic rocks within each province is possibly comparable to the maximum possible age difference between study areas (∼ 0.2 Ga), so the documented lithological variations between the greenstone successions of the 3 provinces cannot be attributed to depositional age differences. Leaching experiments on altered Warriedar samples indicate that both alteration and isotopic homogenisation occurred early in the evolution of the system, probably prior to the ∼ 2.6 Ga regional metamorphic event. The initial ϵNd values do not conform to previously suggested mantle depletion curves. When considered with published data for greenstone belts they suggest complex mantle heterogeneity, with eNd diverging over time to both positive and negative values.

Chapter 4.1 Paleoarchean Development of a Continental Nucleus: the East Pilbara Terrane of the Pilbara Craton, Western Australia

Publisher Summary This chapter describes the lithostratigraphy, geochemistry, and structural and metamorphic geology of the ancient, eastern nucleus of the Pilbara Craton. The 3.53–2.83 Ga Pilbara Craton of Western Australia is one of only two areas on the Earth that contain large, well-exposed areas of little deformed, low-grade Paleoarchean rocks and the other being the Kaapvaal Craton in southern Africa—and as such is important for understanding the early Earth and the processes involved in the formation of continental crust. The Pilbara Craton is famous for its well preserved Paleoarchean volcano-sedimentary succession that includes evidence of the oldest life on the Earth, and for its classic dome-and-keel geometry of ovoid, domical granitic complexes, and flanking arcuate, synclinal greenstone belts. The 3.53–2.83 Ga Pilbara Craton is a nearly circular piece of crust in the northwestern part of Western Australia whose boundaries are defined by aeromagnetic and gravity anomalies, and by orogenic belts. A significant result of the extensive SHRIMP geochronology of volcanic and sedimentary rocks from the Pilbara Supergroup is the discovery of abundant inherited zircons that are older than the oldest dated supracrustal rocks.

Making it thick: a volcanic plateau origin of Palaeoarchean continental lithosphere of the Pilbara and Kaapvaal cratons

Abstract How and when continents grew and plate tectonics started on Earth remain poorly constrained. Most researchers apply the modern plate tectonic paradigm to problems of ancient crustal formation, but these are unsatisfactory because diagnostic criteria and actualistic plate configurations are lacking. Here, we show that 3.5–3.2 Ga continental nuclei in the Pilbara Craton, Australia, and the eastern Kaapvaal Craton, southern Africa, formed as thick volcanic plateaux built on a substrate of older continental lithosphere and did not accrete through horizontal tectonic processes. These nuclei survived because of the contemporaneous development of buoyant, non-subductable mantle roots. This plateau-type of Archean continental crust is distinct from, but complementary to, Archean gneiss terranes formed over shallowly dipping zones of intraoceanic underplating (proto-subduction) on a vigorously convecting early Earth with smaller plates and primitive plate tectonics.

Gold mineralization of the Chencai-Suichang uplift and tectonic evolution of Zhejiang Province, southeast China

Abstract A metallogenic model is proposed in which gold and base metal mineralization in SE China is related to plate interactions along the Pacific margin with Southeast Asia. From about 900 to 70 Ma, all depositional, orogenic, and metallogenic processes can be explained by subduction events along northeast-trending zones. With progressive accretion and cratonization along the continental margin, these subduction zones moved southeast for a distance of less than 150 km. As a result, the area is one of crustal reworking in which the rocks and mineral deposits of earlier events have been repeatedly modified. The Chencai-Suichang Uplift is an uplifted crustal block in central Zhejiang Province of mainland China, in which the Proterozoic basement is exposed as tectonic windows in overlying Mesozoic volcanics. Several gold and base metal deposits are located in these Proterozoic windows as well as in the younger volcanics. Most economic deposits were formed during the last major tectono-magmatic event affecting the area, the Yanshanian (160–170 Ma) event, although there is evidence for earlier mineralization that has been remobilized during the Yanshanian. It is suggested that crustal reworking was particularly important in remobilising and upgrading gold mineralization. The model presented is supported not only by local geology and isotopic data, but also by the regional distribution of gold deposits, which are arranged in a 50-km-wide belt parallel to the Yanshanian subduction zone. Three major episodes of mineralization occurred during: (1) a subduction-related period of metamorphism, partial melting, and mesothermal fluid generation and mineralization in the lower crust during the Caledonian; (2) subduction-related volcano-plutonic activity with epithermal mineralization during the early Yanshanian; and (3) hydrothermal mineralization during late Yanshanian tectono-magmatic event. In the third, two main categories of metallogenic control are recognized, namely: (1) hydrothermal fluid generation in the lower crust and (2) upper crust structural and lithological controls on mineralization.

Hickman Crater, Ophthalmia Range, Western Australia: Evidence supporting a meteorite impact origin

A newly discovered, morphologically well-preserved crater with a mean diameter of 260 m is reported from the Ophthalmia Range, Western Australia. The crater is located in hilly terrain ∼36 km north of Newman, and is situated in the Paleoproterozoic Woongarra Rhyolite and the overlying Boolgeeda Iron Formation. The morphometry of the crater is consistent with features characteristic of small meteorite impact craters. The rhyolite of the craters rim exhibits widespread shatter features injected by veins of goethite bound by sharply defined zones of hydrous alteration. The alteration zones contain micro-fractures injected by goethite, which also fills cavities in the rhyolite. The goethite veins are interpreted in terms of forceful injection of aqueous iron-rich solutions, probably reflecting high-pressure hydrothermal activity by heated iron-rich ground water. None of these features are present in the Woongarra Rhyolite outside the immediate area of the crater. Petrography of the rhyolite indicates possible incipient intracrystalline dislocations in quartz. The Boolgeeda Iron Formation, which crops out only on the southern rim of the crater, displays brecciation and mega-brecciation superposed on fold structures typical of the banded iron-formations in the region. Geochemical analysis of two goethite veins discloses no siderophile element (Ni and PGE) anomalies, negating any contribution of material from an exploding meteorite. Instead, the strong iron-enrichment of the fractured rhyolite is attributed to a hydrothermal system affecting both the Boolgeeda Iron Formation and the Woongarra Rhyolite, and localised to the area of the crater. An absence of young fragmental volcanic material younger than the Woongarra Rhyolite is inconsistent with an explosive diatreme, leading us to a preferred interpretation in terms of an original impact crater about 80 m deep excavated by a ∼10 m-diameter projectile and accompanied by hydrothermal activity. A minor north–south asymmetry of the crater, and an abundance of ejecta north, up to about 300 m northwest and northeast of the crater, suggest high-angle impact from the south. A youthful age of the structure, probably Late Pleistocene (104–105 years old), is indicated by damming of the drainage of a south-southeast-flowing creek by the southern crater rim.

Comment on “Evidence for multiphase deformation in the Archaean basal Warrawoona group in the Marble Bar area, East Pilbara, Western Australia” by van Haaften, W.M., White, S.H., 1998: Precambrian Research 88, 53–66

van Haaften and White (1998) presented kine-matic data from shear zones within the TalgaTalga Anticline of the Marble Bar belt to in-voke a regional thrust-accretion tectonic originfor the East Pilbara Craton. By examination ofsmall-scale, local faults, they inferred five phasesof regional deformation, of which an early ESE-directed thrusting event which reactivated ca.3450 Ma normal faults, and a NE-directedtransport event at Further, van Haaften and White (1998) use3300 Ma, were interpretedto be the most important. van Haaften andWhite (1998) state that a lack of radially plung-ing lineations in the Talga Talga Anticline andthe presence of thrust-sense kinematics in bed-ding-parallel shear zones indicate that the TalgaTalga Anticline could not have formed throughthe solid-state diapiric rise of the Mount EdgarBatholith as suggested by Hickman (1984) andCollins (1989). Rather, they interpret the TalgaTalga Anticline to be a thrust culminationformed as a result of three phases of east- tonorth-eastern directed crustal shortening. Theearly thrusting was interpreted to have occurredduring the accretion of a so-called ‘East Pilbaraterrain’, as originally suggested by Krapez(1993).the presence of thrusts, and a re-interpretationof the previous geochronology, to suggest thatthe volcanic stratigraphy of the Marble Bar beltrepresent an imbricate lithotectonic sequence.They conclude that ‘the Talga Talga Anticline isan unsuitable area for the type section of thelower Warrawoona Group’, that the con-

Integration of observational and analytical methodologies to characterize organic matter in early archaean Rocks: Distinguishing biological from abiotically synthesized carbonaceous matter structures

Transmission Electron Microscopy (TEM) was applied to observe and characterize carbonaceous materials (CM) extracted from black cherts and argillite in drill core from the Warrawoona Group of the Pilbara Craton, Western Australia. The black chert came from a ‘white smoker type’ seafloor deposit in the ca. 3.49 Ga Dresser Formation, whereas the black argillites were obtained from the 3.46 Ga Apex Basalt. The samples were observed and analyzed in TEM combined with electron dispersive spectral analysis (EDS), high resolution TEM (HRTEM) to determine molecular ordering, and C-isotope geochemistry. The TEM and HRTEM observations revealed significant morphological and structural differences between the carbonaceous materials of the Dresser and Apex samples enabling interpretations in terms of primary and secondary origins, as well as metamorphic history.