Remnants of an Early Archaean (>3.75 Ga) sea-floor, hydrothermal system in the Isua Greenstone Belt

Abstract

Localised low-strain domains in the ∼3.75 Ga Isua Greenstone Belt, West Greenland, contain a wealth of well-preserved primary structures. These include pillow lavas with dark, fine-grained cooling rims, pillow breccias, debris flows and conglomerates. Here we describe quartz globules from a relatively undeformed pillow breccia, which contains unstrained quartz. Most quartz globules contain only few quartz grains, but some of the larger ones preserve a subidiomorphic rim reminiscent of former geodic lining. The globules are associated with a complex system of quartz veins. Metamorphic conditions recorded in pelites associated with the pillow breccia represent the early stage of a prograde event at 460–480 °C and ∼4 kb. Both globules and quartz veins pre-date the metamorphic fabric, and are interpreted as remnants of a sea-floor type hydrothermal system, operating at the time of lava eruption and pillow formation at ∼3.75 Ga. The globules, which are interpreted as former gas vesicles filled with quartz (and some carbonate), are embedded in an altered basaltic matrix now comprising biotite, muscovite and quartz±tourmaline and carbonate. Silica-filling in the vesicles was contemporaneous with the formation of an intricate hydrothermal vein system. During deformation, the strain was partitioned into the mica-rich rock matrix, when the vesicles behaved as competent objects, eventually cutting or deforming the thinner veins. Amphibolite-facies metamorphic overprint involved a limited flattening of some vesicles and the development of a very faint, sporadic schistosity, preserving most of the pre-metamorphic hydrothermal features. Major metamorphic structures correspond to quartz annealing, yielding curved grain boundaries and triple point junctions. We report rare primary fluid inclusions from quartz in the vesicles. The inclusions contain remnants of two independent fluid/mineral systems comprising pure methane and highly saline aqueous fluids (about 25 wt.% NaCl equivalent), and co-precipitating carbonates (calcite). Especially for the aqueous system, these fluids bear a strong resemblance to present-day sea-floor hydrothermal fluids. They are interpreted as relics of the sea-floor hydrothermal fluid, responsible for the alteration of the pillow fragments and co-precipitation of quartz and carbonate.