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.

Ignimbrites are deposits resulting from the eruption of volatile-rich, silicic magma. Few geochemical or petrological studies have been done concerning the Tertiary Ignimbrite Province of Central America in Honduras and Nicaragua. Previous work, using ages and geographical proximity, suggested that tephra layers recovered during Ocean Drilling Program (ODP) Leg 165 in the western Caribbean Sea were associated with deposits from explosive eruptions in Central America. A total of 112 marine tephra and 79 terrestrial samples from Nicaragua and Honduras were analyzed during this study. An electron microprobe was used for major oxides, and laser inductively coupled plasma–mass spectrometry (ICP-MS) was used for trace elements. The rare earth elements (REEs), due to their resistance to weathering, were used to correlate the marine tephra with the terrestrial samples. Cluster analysis resulted in the division of the samples into 6 geochemical groups. Visual inspection of these groups resulted in the reclassification of these 6 into 14 distinct geochemical groups. Each one of these geochemical groups displays unique REE trends relative to each other. The trends vary, relative to enriched mid-ocean-ridge basalt (EMORB), from strongly light (L) REE enriched with moderate to large, negative Eu anomalies, to nearly equal EMORB values without any Eu anomalies, to enriched REE with positive Eu anomalies. All but two groups consist of both marine and terrestrial samples. The geochemical correlation is strengthened using factor analysis, in which the REE values for each sample were reduced to two factors and replotted. Each group plots in a distinct field. The Lesser Antilles can be ruled out as contributing to the western Caribbean tephra due to the lack of any large-volume ignimbrite sheets within the arc, as well as the distinctly different REE trends of the magmas. The Sierra Madre Igneous Province in México is also ruled out as a potential source, due mainly to a significant age difference between Sierra Madre ignimbrites and the Caribbean Sea tephra layers. Besides the strong similarities in REE patterns, several of the groups and subgroups are geographically limited in extent, which might imply a specific group source location.