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

Localised low-strain domains in the similar to 3.75 Ga Isua Greenstone Belt , West Greenland, contain a wealth of well-preserved primary structures. Th ese include pillow lavas with dark, fine-grained cooling rims, pillow brecc ias, debris flows and conglomerates. Here we describe quartz globules from a relatively undeformed pillow breccia, which contains unstrained quartz. M ost 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 represen t the early stage of a prograde event at 460-480 degreesC and similar to 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 similar to 3.75 Ga. T he globules, which are interpreted as former gas vesicles filled with quart z (and some carbonate), are embedded in an altered basaltic matrix now comp rising biotite, muscovite and quartz +/- tourmaline and carbonate. Silica-f illing in the vesicles was contemporaneous with the formation of an intrica te hydrothermal vein system. During deformation, the strain was partitioned into the mica-rich rock matrix, when the vesicles behaved as competent obj ects, eventually cutting or deforming the thinner veins. Amphibolite-facies metamorphic overprint involved a limited flattening of some vesicles and t he development of a very faint, sporadic schistosity, preserving most of th e pre-metamorphic hydrothermal features. Major metamorphic structures corre spond to quartz annealing, yielding curved grain boundaries and triple poin t junctions. We report rare primary fluid inclusions from quartz in the ves icles. The inclusions contain remnants of two independent fluid/mineral sys tems comprising pure methane and highly saline aqueous fluids (about 25 wt. % NaCl equivalent), and co-precipitating carbonates (calcite). Especially f or the aqueous system, these fluids bear a strong resemblance to present-da y sea-floor hydrothermal fluids. They are interpreted as relics of the sea- floor hydrothermal fluid, responsible for the alteration of the pillow frag ments and co-precipitation of quartz and carbonate. (C) 2001 Elsevier Scien ce B.V. All rights reserved.