Explosive subaqueous pyroclastic deposits associated with a mafic summit caldera: an Archean analogue

The 2706-2696 Ma Blake River megacaldera complex of the Archean Abitibi greenstone belt is composed of (1) the E-W, 40x80 km, ellipsoid, Misema caldera (MC) of predominantly tholeiitic basaltic composition, and (2) the NW-striking, 14x35 km, New Senator caldera (NSC) of both tholeiitic and calc-alkaline composition, and (3) the felsic, ENE-striking, 15x20k km, Noranda caldera (NC). The subaqueous Misema caldera, the size of Toba, hosts the two nested graben calderas NSC and NC. This megacaldera complex represents a world class gold-massive sulfide mining camp with abundant rock-Archean seawater interaction, km-scale hydrothermal alteration, and a recently discovered site of inferred early life in basalts. Between the inner and outer ring faults of the MC are circular to ellipsoid 2-12 km-scale gabbro to quartz diorite ring dyke complexes that represent the roots of summit calderas. The studied mafic summit calderas, comparable to the present day Axial Seamount, are centres of extensive subaqueous pyroclastic deposits and their reworked counterparts. The Montsabrais and Reneault volcanic centres (RVC) have 200-1000 m-thick volcaniclastic units in the core of the ring dyke complexes, and extensive km-long outflow facies. The 80-100 m-thick Jevis South Tuff and the Kino North Tuff are related to the RVC. The pyroclastic density current deposits are traceable on outcrop 500 metres along strike. Pyroclastic deposits are interstratified with pillowed mafic flows and black shale suggesting a calm, deep-water, subaqueous environment. Individual flow units are 5-50 m-thick and display fining-upward. Internal deposit geometry shows 3 main divisions: A) massive, matrix or clast supported, lapilli tuff breccia (2 - 25 m-thick); B) lapilli tuff with normal or inverse grading (1 - 10 m-thick); C-1) coarse- to fine-grained tuff with parallel to wavy stratification and low-angle crossbeds with erosive bases (0.5 - 15 m-thick), or C-2) beds of fine- to very fine-grained tuff with parallel and wavy laminations (0.1 - 35 m-thick). Heat retention structures were not observed. Division A suggests laminar mass flow conditions, probably in which water is excluded from the transport process. Divisions B and C-1 represent low-density, turbulent turbiditic flows with ingestion of water during transport causing density segregation. Flow unsteadiness is responsible for crossbed formation and erosive processes. Division C-2 corresponds to suspension fallout deposition. A pyroclastic origin is indicated by massive to vesicular juvenile clasts, pumice and liberated to broken feldspar crystals. A major subaqueous eruption column is required to produce these deposits. Such columns are common to modern felsic submarine calderas (e.g. Myojin Knoll, Izu Bonin arc) but novel to mafic systems.