Dacitic lapilli metatuff (dacitic meta-lapillistone) from the Precambrian of Minnesota, USA (cut surface). (public display, Soudan Underground Mine State Park visitor center, Soudan, Minnesota, USA)

This is a metamorphosed volcanic tuff. Volcanic tuff is a clastic-textured, extrusive igneous rock that forms by explosive volcanic eruptions. The fragments generated by an eruption are deposited by ash fall or ash flow (pyroclastic flow), then buried and lithified. Lapilli are fragments between 2 and 64 mm in size (= granule- and pebble-sized in sediment classification). Fragments smaller than 2 mm are referred to as volcanic ash and volcanic dust (= sand-sized & silt-sized & clay-sized grains in sediment classification). The rock shown above has abundant lapilli-sized fragments, and is thus a lapilli tuff. The composition of the lapilli is dacitic. Dacite is an extrusive igneous rock that is common at subduction zone stratovolcanoes. It is a high-silica intermediate rock or a low-silica felsic rock, depending on which definition of the felsic-intermediate boundary one uses. This rock has been metamorphosed, as has much of Minnesota's Precambrian rock record, so a full lithologic name is dacitic lapilli metatuff. The nature and size of the lapilli are still readily apparent in the rock, despite metamorphic alteration.

The lapilli metatuff succession from which this rock derives is closely associated with the famous Soudan Iron-Formation (see: <a href="http://www.flickr.com/photos/jsjgeology/albums/72157652553006284">www.flickr.com/photos/jsjgeology/albums/72157652553006284</a>). A description of the lapilli tuff succession is given by Vallowe et al. (2010): "Dirty white to light grey and in areas, sulfide stained, poorly bedded to massive. Fragments locallly consist of light to dark chert and at times small egg shaped concretions. North and east of the Soudan Mine in Section 27, the unit consists of 1 x 3 cm domains of cherty tuff (± sericite) in, and separated by, a coarse anastamosing sericite-chlorite foliation with pyrite clots common. May have local massive beds nor oriented parallel to foliation. Immediately north of the historic workings of the Soudan Mine, the unit is virtually a sericite-quartz schist with foliation wrapping around 2 to 5 m angular to subrounded cherty tuff clasts."

Stratigraphy: "upper sequence" of Vallowe et al. (2010), above the Soudan Iron-Formation & below the Lake Vermilion Formation, Neoarchean, ~2.7 to 2.72 Ga

Locality: undisclosed locality near the Soudan Mine, Soudan Underground Mine State Park, Soudan, northeastern Minnesota, USA

Site-specific geologic info. synthesized from: