Agglutinated volcanic bomb in basaltic lapillistone in the Precambrian of Ontario, Canada.

These unusual-looking rocks are part of the Late Archean-aged Kidd-Munro Assemblage, which consists of ultramafic and mafic volcanic rocks intruded by mafic to ultramafic dikes and sill-like bodies. Minor felsic volcanic rocks are also present. Volcanogenic massive sulfide deposits occur in the Kidd-Munro Assemblage - they have been mined at the Texas Gulf Mine and the Potter Mine.

This outcrop consists of clast-supported, coarse-grained, fragmental tholeiitic basalt - it's a tholeiitic basalt breccia (it's also been characterized as a hyaloclastite). The fragments vary in shape from angular to agglutinated to convoluted, including delicate shapes. This indicates no fragment transport. Fragments vary significantly in size, but most are lapilli-sized (= volcanic fragments between 2 and 64 millimeters). This rock has been interpreted as a proximal vent facies/spatter rampart deposit. An underwater, volcanic fissure eruption occurred here 2.7 billion years ago, resulting in subaqueous fire fountaining. This was the site of an ancient volcanic edifice.

The large clast at center is a volcanic bomb. Click on the photo to zoom in - notice that the bomb is composed of numerous smaller pieces that have molded together. Such objects are called agglutinated bombs.

Description of site from Houlé et al. (2010) [sans parenthetical citations]:

Approximately 500 meters east of the Potter Mine area, the basaltic volcaniclastic rocks are massive to poorly bedded, unsorted, and consist of globular, irregular "moulded" lapilli that resemble agglutinate, fluidal bombs, cored bombs, blocks, and armoured lapilli, with less than 10% matrix containing fine, plate-like hyaloclastite shards and lapilli. Accessory fragments of chert, carbonaceous mudstone, argillaceous mudstone, and massive sulfide account for less than 1% of the breccia.

The basaltic volcaniclastic rocks were previously interpreted to have been derived through quench fragmentation and autobrecciation of basaltic flows. However, the lack of basalt flows and in situ hyaloclastite, the abundance of fluidal and cored bombs, armoured lapilli, globular lapilli, and the shear volume of breccia led Gibson and Gamble (2000) to propose an origin through "explosive fragmentation" rather than autobrecciation. The production of large volumes of lapilli-size granules is interpreted to be a product of the rapid eruption of low-viscosity mafic magma into a water column, where the magma was jetted into the water column, torn apart, and quench fragmented. In this model, massive, poorly sorted tuff breccia containing globular, lapilli-size agglutinate and fluidal bombs are interpreted to represent vent-proximal deposits, similar to subaerial fire fountain and spatter rampart deposits.

Stratigraphy: Middle Tholeiitic Unit, Kidd-Munro Assemblage, Abitibi Greenstone Belt, lower Neoarchean, 2.711-2.719 Ga