ABSTRACT The Providencia island group comprises an extinct Miocene stratovolcano located on a shallow submarine bank astride the Lower Nicaraguan Rise in the western Caribbean. We report here on the geology, geochemistry, petrology, and isotopic ages of the rocks within the Providencia island group, using newly collected as well as previously published results to unravel the complex history of Providencia. The volcano is made up of eight stratigraphic units, including three major units: (1) the Mafic unit, (2) the Breccia unit, (3) the Felsic unit, and five minor units: (4) the Trachyandesite unit, (5) the Conglomerate unit, (6) the Pumice unit, (7) the Intrusive unit, and (8) the Limestone unit. The Mafic unit is the oldest and forms the foundation of the island, consisting of both subaerial and subaqueous lava flows and pyroclastic deposits of alkali basalt and trachybasalt. Overlying the Mafic unit, there is a thin, minor unit of trachyandesite lava flows (Trachyandesite unit). The Breccia unit unconformably overlies the older rocks and consists of crudely stratified breccias (block flows/block-and-ash flows) of vitrophyric dacite, which represent subaerial near-vent facies formed by gravitational and/or explosive dome collapse. The breccias commonly contain clasts of alkali basalt, indicating the nature of the underlying substrate. The Felsic unit comprises the central part of the island, composed of rhyolite lava flows and domes, separated from the rocks of the Breccia unit by a flat-lying unconformity. Following a quiescent period, limited felsic pyroclastic activity produced minor valley-fill ignimbrites (Pumice unit). The rocks of Providencia can be geochemically and stratigraphically subdivided into an older alkaline suite of alkali basalts, trachybasalts, and trachyandesites, and a younger subalkaline suite composed dominantly of dacites and rhyolites. Isotopically, the alkali basalts together with the proposed tholeiitic parent magmas for the dacites and rhyolites indicate an origin by varying degrees of partial melting of a metasomatized ocean-island basalt–type mantle that had been modified by interaction with the Galapagos plume. The dacites are the only phenocryst-rich rocks on the island and have a very small compositional range. We infer that they formed by the mixing of basalt and rhyolite magmas in a lower oceanic crustal “hot zone.” The rhyolites of the Felsic unit, as well as the rhyolitic magmas contributing to dacite formation, are interpreted as being the products of partial melting of the thickened lower oceanic crust beneath Providencia. U-Pb dating of zircons in the Providencia volcanic rocks has yielded Oligocene and Miocene ages, corresponding to the ages of the volcanism. In addition, some zircon crystals in the same rocks have yielded both Proterozoic and Paleozoic ages ranging between 1661 and 454 Ma. The lack of any evidence of continental crust beneath Providencia suggests that these old zircons are xenocrysts from the upper mantle beneath the Lower Nicaraguan Rise. A comparison of the volcanic rocks from Providencia with similar rocks that comprise the Western Caribbean alkaline province indicates that while the Providencia alkaline suite is similar to other alkaline suites previously defined within this province, the Providencia subalkaline suite is unique, having no equivalent rocks within the Western Caribbean alkaline province.

Providencia is the only example of subaerial volcanism on the Lower Nicaraguan Rise. In this volume, the authors examine this volcanism and the geological history of the western Caribbean and the Lower Nicaraguan Rise, whose origin and role in the development of the Caribbean plate has been described as enigmatic and poorly understood. While the Providencia alkaline suite is similar to others within the Western Caribbean Alkaline Province, its subalkaline suite is unique, having no equivalent within the province. In order to unravel its complex history and evolution, this volume presents new and previously published results for the geology, geochemistry, petrology, and isotopic ages from the Providencia island group.

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 2 O, and 4.5% K 2 O) 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.