Archean ocean-floor metamorphism in the North Pole area, Pilbara Craton, Western Australia
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 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.
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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
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2021, Precambrian ResearchCitation 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.