Recognizable primary volcanic and sedimentary features in a low‐strain domain of the highly deformed, oldest known (≈ 3.7–3.8 Gyr) Greenstone Belt, Isua, West Greenland

Abstract

A low‐strain domain has been identified in the metamorphosed, mostly highly deformed volcanic and sedimentary rocks of the early Archaean Isua supracrustal belt. This domain contains well‐preserved volcanic and sedimentary features, including basaltic pillow lavas, pillow breccia, heterogeneous volcanic breccia, amygdules in metabasalt, and polymict conglomerate dominated by recrystallized chert and volcanic clasts. The low‐strain domain is bounded by highly deformed rocks mostly derived from basalt, chert, and banded iron formation. These discoveries demonstrate that some primary features have escaped the pervasive metasomatism dominant in other parts of the belt and, furthermore, strengthen the characterization of the Isua supracrustals as a typical greenstone belt.

Length of the day and evolution of the Earth's core in the geological past

“…In fact, even with ancient coastlines enhancing the O 1 tide and reducing the M 2 tide, it seems extremely difficult to account over long time intervals for a tidal despinning rate much below 1 ms/cy (= 2.8 hr/Ga), except perhaps if there were no oceans. However, the existence of marine sediments at least 3.8×10 9 years old prove the existence of an Archaean ocean 3.8 Ga ago (Appel et al 1998), and the discovery of detrital zircons aged 4.4×10 9 years in Archaean sediments of the Jack Hills, Northwestern Australia, even seems to indicate that there were oceans already in the Hadean, very soon after the Earth's formation (Wilde et al 2001). We therefore argue here that the discrepancy between the minimum tidal LOD lengthening rate and the observed lengthening rate, i.e., 0.4 ± 0.3 ms/cy, could perhaps be attributed to significant core formation during the Proterozoic.…”

Metamorphic fluid flow in the northeastern part of the 3.8–3.7Ga Isua Greenstone Belt (SW Greenland): A re-evalution of fluid inclusion evidence for early Archean seafloor-hydrothermal systems

“…pillow breccia occurrence, Fig. 2) are well preserved (Appel et al, 1998). For the pillow breccia outcrop, Appel et al (2001) calculated peak-metamorphic temperatures and pressures ranging between 430 and 500°C, at 3.6 to 4.4 kbar, based on mineral composition and garnet-biotite geothermometry.…”

“…The field occurrence of the weakly deformed pillow breccia has been described by Appel et al (1998Appel et al ( , 2001 and a detailed petrographic description of the rocks and their different quartz segregations was given by Appel et al (2001) and Touret (2003). Fig.…”

“…However, greenstone belts older then $3.5 Ga have all been subjected to high or intermediate grade metamorphism, complicating the interpretation of fluid inclusion data. Appel et al (1998Appel et al ( , 2001 and Touret (2003) described a well-preserved occurrence of pillow lavas and pillow lava breccia in a low-strain zone in the northeastern IGB, and provided first results of a fluid inclusion study on quartz ''vesicles'' in pillow fragments. In this paper, the non-genetic term ''globules'' is preferred to ''vesicles'', though the globules studied here correspond with the ''vesicles'' described by Appel et al (2001) and Touret (2003).…”

In-situ iron isotope analysis of pyrites in ~ 3.7 Ga sedimentary protoliths from the Isua supracrustal belt, southern West Greenland

“…2), contains one of the oldest volcanic and sedimentary successions on Earth (Moorbath et al, 1973). The ubiquitous preservation of primary signatures in the least deformed rocks of the ISB enables us to understand their precursor materials (Nutman, 1986;Komiya and Maruyama, 1995;Appel et al, 1998;Komiya et al, 1999). The ISB and the surrounding Itsaq gneiss complex are composed of tectonic slices bounded by folded and metamorphosed faults (Nutman, 1986;Appel et al, 1998;Komiya et al, 1999;Myers, 2001;Hanmer and Greene, 2002;Rollinson, 2002Rollinson, , 2003Crowley, 2003;.…”