An extensive, hot, vapor-charged rhyodacite flow, Baja California, Mexico

The Providencia rhyodacite lava flow of southern Baja California is an unusually extensive salic extrusion. Remnants of the flow overlie lower to middle Miocene volcanic rocks and occur in a 27-km-long belt near the city of La Paz. Isopachs of the flow show a maximum thickness of 120 m and indicate a minimum volume of 8.6 km 3. Persistent flow bands are closely spaced and parallel the base of the flow. These flow bands are thin, planar lithophysal cavities that give the rock a distinct parting. In the upper part of the flow the banding is strongly deformed into isoclinal to open folds. Flow directions, developed from fold axial information, together with the isopach data, suggest that the rhyodacite flowed at least 23 km north-northwest from its source south of La Paz. The Providencia rhyodacite (68–72.5% SiO 2, 3.8% Na 2O, and 4.5% K 2O) contains about 5% phenocrysts (plag > opx > Fe-Ti oxides) set in a devitrified groundmass of fine-grained alkali feldspar and tridymite ?). Lithophysal planar cavities are lined with large (as long as 3 mm) vapor-phase crystals whose paragenetic relationships define a crystallization order from oldest to youngest: (1) fayalite + thick laths of brown horn-blende, (2) α-quartz, (3) hematite, and (4) tridymite + apatite + rare biotite + rare fibrous green hornblende. Field evidence suggests, but does not prove, that the Providencia rhyodacite is a primary lava flow rather than a remobilized pyroclastic flow. A high volatile content together with a high eruption temperature acted in concert to maintain a low viscosity, a fact that probably facilitated flow of the lava to great distances.