The low-Ti high-temperature dacitic volcanism of the southern Parana-Etendeka LIP: Geochemistry, implications for trans-Atlantic correlations and comparison with other Phanerozoic LIPs

摘要

The low-Ti silicic volcanic units of the Parana-Etendeka Large Igneous Province (LIP) have a high volume (similar to 20,000 km(3)) and are widespread in South America and Africa. In this paper we present a geochemical investigation of conduit- and lava flow-related metaluminous dacitic volcanic rocks from three areas outcropping in southern Brazil. Trace-element abundances are close to those observed from the low-Ti basaltic parental melts and interbedded basaltic andesites. Assimilation and fractional crystallization models suggest significant fractionation (>60%) from basaltic melts with assimilation involving sources with variable large ion lithophiles (LIL) and high field strength (HFS) elements. Variation in Zr/Nb ratios of the silicic volcanic rocks is interpreted to be related to the source characteristics. An effective magma drainage system through structurally controlled conduits was sustained at high magmatic temperatures (similar to 1000-1100 degrees C), low viscosities of similar to 10(6)-10(4) Pa s and water contents from 0.5-1.3 wt%. Melt structures with RAI < 1 are in agreement with field descriptions of an effusive emplacement, erupting domes, coulees, lobes, and extensive SR-type lavas. Trans-Atlantic correlations indicate some compositional overlaps with the Grootberg and Wereldsend quartz latites of Namibia, but the much higher Pb contents of the Namibian quartz latites, five to ten times greater than for the Brazilian dacites, and higher Pb/Cu ratios, suggest a greater degree of assimilation/fractionation of Pb-rich material, such as shale, slate or schist, for the Namibian silicic rocks. After comparing Phanerozoic silicic units related to LIPs, we propose they can be separated into three main types: (1) large volume, high-temperature, water-poor metaluminous rocks, (2) small volume, lower temperature, more hydrous peraluminous to peralkaline rocks; and (3) high temperature peraluminous rocks with cordierite and garnet, derived from partial melting of metapelitic basement. (C) 2019 Elsevier B.V. All rights reserved.