Archean ocean-floor metamorphism in the North Pole area, Pilbara Craton, Western Australia

doi:10.1016/S0301-9268(03)00186-4

Precambrian Research

Volume 127, Issues 1–3, 10 November 2003, Pages 167-180

https://doi.org/10.1016/S0301-9268(03)00186-4 Get rights and content

Abstract

Archean greenstones (ca. 3.49–3.46 Ga) in the North Pole Dome, Pilbara Craton, Western Australia, have undergone prograde metamorphism from prehnite-pumpellyite facies to greenschist-amphibolite facies. Observations of ∼3000 thin sections shows that a metamorphic aureole was restricted to within a few kilometers of the contact with the 3459 Ma North Pole Monzogranite. Three types of alteration were observed; carbonate, silicic and chlorite types. The distribution of the three types of alteration suggests that secondary chemical migration of elements occurred in the whole area, but that some samples were unaffected by this alteration. The southern North Pole area is outside of the contact metamorphic aureole, and is composed of five units from north (unit I) to south (unit V), dipping south about 30–70°. Metamorphism of the North Pole greenstones corresponds to the low-pressure type, which is broadly comparable to that inferred for ocean-floor metamorphism. Prehnite-pumpellyite (PP), transitional (TR), and greenschist (GS) facies prograde metamorphic zones occur from structural top to bottom within each of units. Metamorphic grade increases downward through the units and metamorphic isograds never cut unit boundaries.

Introduction

The North Pole area of the Pilbara Craton, Western Australia is known as one of the best regions to investigate early life and ancient environments in the Archean (Buick and Dunlop, 1990, Nijman et al., 1999, Van Kranendonk, 2000). Hickman, 1983, Hickman, 1984 suggested that Pilbara greenstones have been affected by contact metamorphism associated with intrusion of granites during diapirism. This model has been supported by more recent studies (Collins, 1989, Delor et al., 1991, Collins and Van Kranendonk, 1999, Van Kranendonk et al., 2002). Smith et al. (1982) proposed that deposition of the overlying Hamersley Group caused low-grade burial metamorphism of Pilbara greenstones. Van Kranendonk et al. (2002) proposed five deformation/metamorphic events prior to deposition of the Fortescue Group at 2.77 Ga.

Field mapping of the North Pole area was conducted for two months in each of 1991, 1994, 1995, 1996, by about 10 geologists, covering an area greater than $10 km ×30$ km. A new lithotectonic map at 1:5000 scale was completed for the North Pole area (Kabashima et al., in prepration). The southern North Pole study area is outside of the contact metamorphic aureole. Secondary chemical migration of elements is developed in the study area, but more than 200 samples avoided this alteration. This paper summarizes the mode of occurrence and mineral paragenesis of metabasites, and P–T conditions for the formation of index assemblages in those unaltered samples. These data are synthesized in order to speculate on a model for ocean-floor metamorphism in the North Pole area.

Section snippets

Geological setting

The North Pole area of the east Pilbara is located about 160 km southeast of Port Hedland and 50 km west of Marble Bar (Fig. 1). The area is underlain by the Warrawoona Group (Hickman, 1983), and is characterized by a shallow to steeply dipping chert-greenstone sequence that was folded by the North Pole Dome (Fig. 1). No shears were found along the contact between the intrusive North Pole Monzogranite in the core of the dome and the greenstones. Several high-angle normal faults due to the

Alteration of greenstones

Secondary chemical migration of elements is developed in the study area. Alteration is most intense in the greenstones just below the thick chert sequence of the Dresser Formation in unit I. Even in strongly altered greenstones, however, pillow shapes, way-up structures, vesicles, amygdules and interpillow material are well preserved.

About 600 samples were collected from the study area. Three types of alteration were observed; carbonate (modal calcite >33.3%), silicic (modal quartz >50.0%) and

Mineral zones

The North Pole greenstones are weakly metamorphosed to yield secondary minerals such as prehnite, pumpellyite, epidote, Ca-amphibole, chlorite, quartz, plagioclase, calcite and phengitic muscovite. Under such low-grade metamorphic conditions, original igneous textures and minerals are well preserved. Coarse-grained samples tend to preserve more igneous minerals than fine-grained ones. Most volcanic samples are fine-grained. In the groundmass of samples with intergranular, intersertal, or

Mineral chemistry

Mineral compositions were analyzed with a JEOL electron-probe microanalyzer JXA-8800R with a wavelength-dispersive system at Naruto University of Education. Accelerating voltage, beam current and beam diameter for quantitative analyses were kept at 15 kV, 15 nA on the Faraday cup and 3 μm, respectively. Detailed tabulated mineral compositions are available from the first author on request.

Metamorphic conditions

The observed changes of mineral assemblages and mineral compositions with stratigraphic depth within each unit indicate that the North Pole greenstones have undergone prograde metamorphism from prehnite-pumpellyite facies to greenschist facies. The highest-grade portion in unit III reaches the transitional states between the greenschist-amphibolite facies. The mineral assemblage epidote+actinolite±prehnite±pumpellyite+chlorite+quartz assemblage is transitional from the prehnite-pumpellyite to

Acknowledgements

We are deeply indebted to A.H. Hickman and W. Powell for their critical reviews and constructive comments. We are grateful to M.J. Van Kranendonk for his encouragement to improve the manuscript. Field survey and collection of rock samples used in this study were carried out with S. Maruyama, Y. Isozaki, Y. Kato, T. Kabashima, H. Masago and K. Kitajima, who are gratefully acknowledged. Collaboration in the field with A. Thorne and A. Hickman was helpful. Dr. T. Ota thanked for critical reading

References (34)

W.J. Collins
Polydiapirism of the Archean Mt. Edgar batholith, Pilbara block, Western Australia
Precambrian Res.
(1989)
W. Nijman et al.
Growth fault control of Early Archaean cherts, barite mounds and chert-barite veins, North Pole Dome, Eastern Pilbara, Western Australia
Precambrian Res.
(1999)
R.I. Thorpe et al.
U–Pb zircon geochronology of Archean felsic units in the Marble Bar region, Pilbara Craton, Western Australia
Precambrian Res.
(1992)
J.C. Alt et al.
Alteration of the upper oceanic crust: mineralogy and processes in Deep Sea Drilling Project Hole 504B, Leg 83
Initial Rep. Deep Sea Drilling Project
(1982)
Buick, R., Dunlop, J.S.R., 1990. Evaporitic sediments of early Archean age from the Warrawoona Group, North Pole, W.A....
M. Cho et al.
Prehnite-pumpellyite to greenschist facies transition in the Karmutsen metabasites, Vancouver Island, BC
J. Petrol.
(1987)
W.J. Collins et al.
Model for the development of kyanite during partial convective overturn of Archean granite-greenstone terranes: the Pilbara Craton, Australia
J. Metamorphic Geol.
(1999)
D.S. Coombs et al.
Pumpellyite-actinolite facies schists of the Taveyanne Formation near Loeche, Valais, Switzerland
J. Petrol.
(1976)
C.A. Delor et al.
Relations diapirisme-metamorphisme dans la province du Pilbara (Australia-occidentale): implications pour les régimes thermiques et tectoniques àla Archéen
Comptes Rendus de l’Academie des Sciences, Paris
(1991)
A.F. Hallimond
On the graphical representation of the calciferous amphiboles
Am. Mineralogist
(1943)

M.H. Hey

A new review of the chlorites

Mineralogical Mag.

(1954)

A.H. Hickman

Geology of the Pilbara block and its environs

Geol. Surv. Aust. Bull.

(1983)

Hickman, A.H., 1984. Archean diapirism in the Pilbara block, Western Australia. In: Kröner, A., Greiling, R. (Eds.),...

A. Ishiwatari

Granulite-facies metacumulates of the Ykuno ophiolite, Japan: evidence for unusually thick oceanic crust

J. Petrol.

(1985)

H. Ishizuka

Prograde metamorphism of the Horokanai ophiolite in the Kamuikotan Zone, Hokkaido, Japan

J. Petrol.

(1985)

Kabashima, T., Kitajima, K., Isozaki, Y., Maruyama, S., Terabayashi, M., in prepration. The Archean greenstone/chert...

Y. Kawachi

Pumpellyite-actinolite and contiguous facies metamorphism of the upper Wakatipu district, South Island, New Zealand

N. Z. J. Geol. Geophys.

(1975)

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Carbonaceous matter (CM) in Archean rocks represents a valuable archive for the reconstruction of early life. Here we investigate the nature of CM preserved in ∼ 3.5 Ga old black bedded barites from the Dresser Formation (Pilbara Carton, Western Australia). Using light microscopy and high-resolution Raman mapping, three populations of CM were recognized: (i) CM at the edges of single growth bands of barite crystals (most frequent), (ii) CM within the barite matrix, and (iii) CM in 50–300 µm wide secondary quartz veins that cross-cut the black bedded barite. Raman spectra of CM inside black bedded barite indicated peak metamorphic temperatures of ∼ 350 °C, consistent with those reached during the main metamorphic event in the area ∼ 3.3 Ga ago. By contrast, CM in quartz veins yielded much lower temperatures of ∼ 220 °C, suggesting that quartz-vein associated CM entered the barite after 3.3 Ga. Near edge X-ray absorption fine structure (NEXAFS) and solid-state nuclear magnetic resonance (NMR) revealed a highly aromatic nature of the CM with a lower aliphatic content, which is in line with the relatively elevated thermal maturity. Catalytic hydropyrolysis (HyPy) did not yield any hydrocarbons detectable with gas chromatography–mass spectrometry (GC–MS). Secondary ion mass spectrometry (SIMS) based δ¹³C values of individual CM particles ranged from − 33.4 ± 1.2 ‰ to − 16.5 ± 0.6 ‰ and are thus in accordance with a biogenic origin, which is also consistent with stromatolitic microbialites associated with the black bedded barite. Based on these results we conclude that CM at growth bands and inside the barite matrix is syngenetic and only the CM inside quartz veins, which represents a minor portion of the total CM, is a later addition to the system. Furthermore, we discuss different pathways for the input of CM into the barite-forming environment, including the cycling of biological organic material within the hydrothermal system.
Interstitial carbonates in pillowed metabasaltic rocks from the Pilbara Craton, Western Australia: A vestige of Archean seawater chemistry and seawater-rock interactions
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Calcites hosted in the interpillow void spaces of extremely well preserved, 3.47–3.12 Ga pillow lavas of the Archean Pilbara Craton, Australia, provide new geochemical insights into the composition of Archean seawater and its interaction with basaltic crust. We present a comprehensive dataset of major and trace elements, radiogenic ¹⁴⁷Sm-¹⁴³Nd, ⁸⁷Rb-⁸⁷Sr, and stable C-O isotopes for these calcites.
Based on their elemental composition and Post-Archean Australian Shale-normalized rare earth element and yttrium (REY_PAAS) patterns, two types of calcites can be distinguished. Type I (n = 9) are coarse-grained calcites with elevated Mn concentrations (478–14,790 ppm) and high REY concentrations that match the basaltic host rock compositions. Petrographic textures in combination with geochemical data indicate precipitation from boiling seawater trapped between pillows during basalt eruption. Small light-REY_PAAS depletions, only slightly super-chondritic Y/Ho (34.5–39.1), and low δ¹⁸O_(VSMOW2) (9.25–16.66 ‰) suggest relatively high fluid-rock interactions of boiling seawater with the basaltic host rock. These characteristics are similar to those of interstitial carbonates found in modern mid-ocean ridge basalts. Type II carbonates (n = 6) are fine-grained, low-Mn (13.7–420 ppm) calcites that exhibit comparably high Sr concentrations (825–2,516 ppm) and anoxic modern seawater-like REY_PAAS patterns, strong super-chondritic Y/Ho (39.1–55.8) and δ¹⁸O_(VSMOW2) values of 9.10–13.18 ‰. These features hint to direct calcite precipitation from seawater within the degassing space with little fluid-rock interaction.
Almost all (Type I and Type II) of the calcites investigated here, combined with their respective host rocks, yield well-defined Sm-Nd isochron ages for the Apex (3,466 ± 23 Ma), Euro (3,365 ± 43 Ma), Honeyeater (3,187 ± 33 Ma), and Bradley (3,131 ± 24 Ma) formations. Only calcites from the Mt. Ada Basalt and their associated host rocks show a ∼ 100 Ma younger Sm-Nd isochron age (3,361 ± 22 Ma) compared to the U-Pb zircon age of formation.
Highly radiogenic initial Sr isotope compositions of the high-Mn calcites (Type I) and low Sr concentrations, suggest Sr remobilization for these calcites. In contrast, the low-Mn calcite (Type II) exhibit higher Sr concentrations and lower ⁸⁷Sr/⁸⁶Sr_(i) of 0.7010–0.7011 at 3.34 Ga and 0.70137 at 3.12 Ga, suggesting increasing influence of crustal weathering on the composition of Paleoarchean seawater through time, and progressive decoupling from the Archean mantle (⁸⁷Sr/⁸⁶Sr_(i) = 0.7003–0.7007).
The mobilization of boron and lithium in the hydrothermal system of the ∼3.48 Ga Dresser caldera: A stable isotope perspective
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Voluminous hydrothermal circulation in the ∼3.48 Ga Dresser caldera produced zoned alteration haloes around major fluid pathways. Specifically, in the North Star Basalt (the footwall) hydrothermal alteration decreases with increasing distance from the margins of hydrothermal silica±barite veins, changing from argillic (i.e., kaolinite-quartz) to phyllic (i.e., illite-quartz), and then to either propylitic (i.e., chlorite-albite-epidote) or actinolitic (i.e., actinolite-albite-chlorite-epidote) assemblages at distal positions. This alteration series developed through hydrolysis reactions at decreasing acidic conditions, which promoted variable degrees of mobilization of major and trace elements in the hydrothermal fluids.
In this study, we characterize the B and Li concentrations and the relative stable isotope ratios of the altered North Star Basalt to provide new insights into the hydrothermal processes that generated these complex alteration patterns. In particular, we focus on the magnitude and timing of the inputs from different fluid reservoirs, the mobilization of B and Li in the hydrothermal system of the Dresser caldera and their influence on the near-surface environment where ancient stromatolites were forming.
Actinolitic and propylitic samples have homogeneous δ¹¹B and δ⁷Li values (−17 to −15‰ and +1 to +3‰, respectively) that are associated with lower B and Li concentrations in actinolitic (4 to 11 ppm and 46 to 128 ppm, respectively) relative to propylitic samples (9 to 25 ppm and 176 to 305 ppm for B and Li, respectively). This pattern suggests that at least the propylitic assemblage interacted with alteration fluids of largely magmatic origin, thus excluding the possibility of an early seawater contributions to the hydrothermal system. Conversely, phyllic samples have distinctly higher B contents (57 to 99 ppm) and δ⁷Li values (+7 to +8‰), but have much lower δ¹¹B values (−19 to −28‰) and Li contents (5 to 6 ppm) relative to actinolitic and propylitic samples. We argue that these striking differences are attributable to mineralogical and temperature controls that promoted the preferential removal of ⁶Li during chlorite dissolution and incorporation of ¹⁰B into illite. Furthermore, the highly negative δ¹¹B values in phyllic samples suggest that the additional B incorporated into the altered North Star Basalt originated from the magma chamber underlying the Dresser caldera rather than from seawater or other crustal sources. The most intensely altered (argillic) samples have moderate B and Li contents (2 to 43 ppm and 41 to 71 ppm, respectively) and highly variable δ¹¹B and δ⁷Li values (−24 to −0.4‰ and −10 to +11‰, respectively). These signatures are interpreted to represent the input of ‘external’ fluids into the upper portions of the hydrothermal system during periodic crack-seal events that allowed the influx of restricted amounts of seawater and, possibly, meteoric fluids.
Overall, this study provides a novel non-traditional stable isotope perspective on the evolution of the hydrothermal system of the Dresser caldera that adds richness to our understanding of this complex environment. Both B and Li stable isotopes support a prominent magmatic contribution to the hydrothermal fluid budget, reinforcing the interpretation of a hot-spring origin of the tourmaline-bearing layers associated with the stromatolites of the North Pole Chert.
Correlating trace element compositions, petrology, and Raman spectroscopy data in the ∼3.46 Ga Apex chert, Pilbara Craton, Australia
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The Apex chert unit (∼3.46 Ga, Pilbara Craton, Australia) constitutes one of the oldest sedimentary units on Earth in which putative carbonaceous microfossils have been reported. The source of carbonaceous matter (CM) in this unit, however, is hotly debated. Hydrothermal fluids have circulated through the underlying crust and up into the bedded unit; these fluids could have remobilized sedimentary microbial biomass, generated abiological hydrocarbons, or harbored in situ chemolithoautotrophic microbial communities. Are there parts of the unit where microfossils might be best preserved? A potential fossil microbiota – if present – would probably be best preserved in the stratiform portion of the unit where hydrothermal influence seems to have been lowest. In order to shed light on the history of hydrothermal overprinting and the source of carbonaceous fractions in the Apex chert, we correlate here at a high spatial resolution petrographic observations and trace element analyses over a transect from the dyke where putative microfossils were found to the stratiform part where remnants of microbial mats were found. The layered, stratiform part of the unit has positive La anomalies up to 1.7, and Light Rare Earth Element depletions, indicating a seawater source. However, as far as 300 m from the dyke, the stratiform part also shows hydrothermal brecciation, high Eu anomalies (2–12; µ = 4.2) and chondritic Y/Ho ratios (24.3–30.3; µ = 27.0), indicating that hydrothermal fluids have laterally infiltrated over large distances. Overall, the pervasive influence of hydrothermal fluids throughout the entire unit and the presence of carbonaceous matter both in the sedimentary part and the hydrothermal dyke is consistent with a ‘hydrothermal pump’ model that was earlier proposed for the nearby Dresser Formation. In this model, organic matter from surface environments is circulated along with hydrothermal fluids and redistributed in the crust and overlying sediments, therefore complicating paleobiological interpretations. Raman measurements show that most of the CM experienced temperatures of ∼350 °C, while some samples contain CM with a variable, but markedly lower maturity (temperature ranging from 200 to 350 °C). Correlation to texture points out a potential mixing of pre-metamorphic CM with post-metamorphic CM during late hydrothermal events.
Archean dome-and-basin style structures form during growth and death of intraoceanic and continental margin arcs in accretionary orogens
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Determining whether plate tectonics or some other mode of planetary dynamics operated in the early Archean is one of the most contentious and debated areas of Earth Sciences today. The Paleo- Mesoarchean dome-and-basin structures of the Eastern Pilbara craton are widely used as an example of an early Archean terrane supposedly unlike any produced by plate tectonics in the current mode of active-lid plate tectonics on Earth. In contrast, we produce a synthesis of the structural, magmatic and sedimentological development of the Eastern Pilbara craton from 3600 to 2800 Ma, and through comparative tectonic analysis, show that the craton developed following a typical orogenic sequence from an immature oceanic arc-dominated accretionary orogen, with the oldest rocks of the craton represented by slabs of primitive circa 3590 Ma gabbro - anorthosite - ultramafic rocks, and 3522–3426 Ma oceanic crust and overlying dominantly hydrothermal deep-water cherts imbricated in thrust piles and giant recumbent nappes. These were intruded by juvenile arc magmas including gabbros, diorites, and TTG suites, most of which intruded as sills into structurally favorable sites between 3484 and 3416 Ma. These oceanic crust and overlying sedimentary litho-tectonic assemblages of gabbro/ basalt/ komatiite/ chert, and TTG-diorite-dominated plutonic rocks were deformed into imbricate and antiformal thrust stacks intruded by suites of sheet-like TTG magmas during late regional shortening-related deformation at 3318–3290 Ma, then soon-after, folded by upright folds during continued contractional deformation. The intrusive style, compositions, and relationships to structures suggest that the late magmatic suites represent a massive slab-failure magmatic event similar to those formed during arc accretion and slab failure events in the North and South American Cordilleras, and older orogens of all ages. Late-orogenic shortening deformed these sheet intrusions into large domal structures, synchronous with or soon-after late- to post-orogenic cross-cutting steep-walled circa 3274–3223 Ma plutons vertically intruded the cores of some of the domes, forming nested plutons akin to the Cretaceous Sierra Crest Suite of the Sierra Nevada batholith, and deformed equivalents in Phanerozoic orogens. In a much younger magmatic event, a wide swath of the craton was affected by circa 2851–2831 monzogranite intrusions, after which the magmatic events of the Archean of the Eastern Pilbara were terminated by the circa 2772 Ma Black Range dolerite dike swarm, that preserves evidence for rapid APW drift during its intrusion.
The Eastern Pilbara represents only a very small preserved Paleo-Mesoarchean crustal remnant, measuring a mere 200 × 200 km², yet has been frequently used to model the presumed tectonic behavior of the entire planet for much of the Archean. In this synthesis, we show that the scale of the craton renders its significance in this literature greatly exaggerated. The Eastern Pilbara is only 1/3 the size of just the Sierra Nevada batholith. Considering the entire Eastern, Central and Western Pilbara belts, the scale and tectonic zonation of lithotectonic assemblages is remarkably similar to that of the California section of the North American Cordillera, from arc root (Eastern Pilbara. cf. Sierra Nevada), to fore-arc overlap basin (de Gray Basin, cf. Great Central Valley Sequence), to an accretionary complex (Western Pilbara, cf. Franciscan). The duration of different magmatic and deformational events, confined to three main pulses in 300 Ma, including about six total magmatic suites over 500 Ma, is similar to that of different pulses and/or accretionary events in the western American Cordilleran examples, such as the peri-Gondwanan Famatinian (Cambro-Ordovician) and Pampean (Cambrian) arcs which were later affected by Permian and Mesozoic intrusives from the Peruvian coastal batholith, and form part of the present day active Andean margin of South America.
From this analysis, we firmly conclude from four degrees of similarity of the full data set of field, structural, temporal, and compositional data that the evolution of the craton is readily and rather simply explained by the plate tectonic paradigm. Thus there is no scientific justification to suggest that the geological characteristics of the Eastern Pilbara require any different imaginative or fantastic type of planetary heat loss mode during the interval of its formation from 3600 to 2800 Ma. The Eastern Pilbara is simply an exceptionally well-preserved small fragment of a poly-phase root of a continental margin accretionary orogen that evolved from accretion and magmatism from some of Earth oldest telescoped oceans and included arcs, in a regime of accretionary-style plate tectonics. Comparison with other well-preserved Eo-Mesoarchean terranes shows a common trait, that of derivation from immature introceanic accretionary orogens, building Earth's first continents.
The role of magmatic fluids in the ~3.48 Ga Dresser Caldera, Pilbara Craton: New insights from the geochemical investigation of hydrothermal alteration
2021, Precambrian Research
Citation Excerpt :
Despite the difficulties in accurately estimating the composition of Paleoarchean seawater interacting with the Dresser Caldera, the zoned mineral assemblages developed in the Mount Ada Basalt (Units III, IV, and V of Terabayashi et al., 2003) can be utilized as a reliable reference for Paleoarchean seawater–dominated hydrothermal alteration (Terabayashi et al., 2003). The alteration assemblages in the Mount Ada Basalt are not related to hydrothermal silica veins, and are largely carbonate–bearing reflecting high CO2 concentrations in the Paleoarchean atmosphere and oceans (Grotzinger and Kasting, 1993; Rouchon and Orberger, 2008; Terabayashi et al., 2003). Furthermore, the Mount Ada alteration assemblages form stratigraphically–related patterns that are remarkably similar to those from modern mid–ocean ridges.
Hydrothermal fluids played a key role in establishing the environmental conditions in which ancient stromatolites grew within the North Pole Chert of the ~3.48 Ga Dresser Formation (Pilbara Craton, Western Australia). However, there has been uncertainty as to the physicochemical conditions of the hydrothermal system in relation to (i) the distribution of hydrothermal alteration, (ii) the relative contribution of seawater and/or magmatic volatiles to the hydrothermal fluids, and (iii) the origin of some of the major elements mobilized in the hydrothermal fluids.
This study examines the hydrothermal alteration of the underlying North Star Basalt in order to better understand the nature of the circulating fluids and to determine the processes responsible for the transport and accumulation of metals and metalloids to the near surface environment. Detailed geological mapping reveals a complex distribution of alteration mineral assemblages that is controlled at all stratigraphic depths by the distance to the major fluid pathways that are now represented by hydrothermal silica veins. With increasing distance from the vein margins, alteration assemblages change from argillic (kaolinite–quartz) to phyllic (illite–quartz), and then to propylitic (chlorite–albite–epidote) and actinolitic (actinolite–albite–chlorite–epidote) at more distal positions. Illite Ar–Ar dating of argillic–altered basalt proximal to major hydrothermal veins immediately below the North Pole Chert confirms a syn–depositional hydrothermal origin of alteration, and demonstrates that the mineralogical and chemical features developed through the circulation of hydrothermal fluids were largely preserved after subsequent thermal overprints at 3.25, 3.06, and 2.29 Ga.
The spatial distribution of the alteration mineral assemblages indicates that the fluids circulating in the hydrothermal system were highly acidic (pH < 3) for at least some time during the evolution of the Dresser Caldera. Such highly acidic fluid conditions were likely promoted by the input of magmatic volatile phases, such as HCl, SO₂, H₂S, and F. Bulk geochemical analyses of altered basalts reveal that large amount of metals, including Fe, Mg, Ni, and Zn, were leached from the North Star Basalt during hydrothermal alteration and delivered to the surface. Furthermore, our data indicate that K and Ba were introduced into the hydrothermal system from external reservoir(s). Although the contribution of K–rich seawater cannot be completely discounted, we argue that the bulk of K and Ba was sourced from an underlying magma chamber undergoing fractional crystallization of a melt with TTG–like composition.

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Archean ocean-floor metamorphism in the North Pole area, Pilbara Craton, Western Australia

Abstract

Introduction

Section snippets

Geological setting

Alteration of greenstones

Mineral zones

Mineral chemistry

Metamorphic conditions

Acknowledgements

Precambrian Res.

Precambrian Res.

Precambrian Res.

Alteration of the upper oceanic crust: mineralogy and processes in Deep Sea Drilling Project Hole 504B, Leg 83

Initial Rep. Deep Sea Drilling Project

Prehnite-pumpellyite to greenschist facies transition in the Karmutsen metabasites, Vancouver Island, BC

J. Petrol.

Model for the development of kyanite during partial convective overturn of Archean granite-greenstone terranes: the Pilbara Craton, Australia

J. Metamorphic Geol.

Pumpellyite-actinolite facies schists of the Taveyanne Formation near Loeche, Valais, Switzerland

J. Petrol.

Relations diapirisme-metamorphisme dans la province du Pilbara (Australia-occidentale): implications pour les régimes thermiques et tectoniques àla Archéen

Comptes Rendus de l’Academie des Sciences, Paris

On the graphical representation of the calciferous amphiboles

Am. Mineralogist

A new review of the chlorites

Mineralogical Mag.

Geology of the Pilbara block and its environs

Geol. Surv. Aust. Bull.

Granulite-facies metacumulates of the Ykuno ophiolite, Japan: evidence for unusually thick oceanic crust

J. Petrol.

Prograde metamorphism of the Horokanai ophiolite in the Kamuikotan Zone, Hokkaido, Japan

J. Petrol.

Pumpellyite-actinolite and contiguous facies metamorphism of the upper Wakatipu district, South Island, New Zealand

N. Z. J. Geol. Geophys.