Evidence for Extremely Shallow-Level Assimilation of Phonolitic Pumice By Basaltic Magma, Diego Hernandez Formation, Tenerife, Canary Islands, Spain

A 25 meter-wide basaltic dike crosscuts some 120 m of phonolitic pumice deposits in the Diego Hernandez Formation (DHF) in the Las Cañadas caldera complex on Tenerife. Direct field observation of fingers and lenses of basaltic magma irregularly radiating from the main dike indicates intrusion into unconsolidated pumice, while fragments and aggregates of pumice up to several cm in size have been mechanically incorporated by the basaltic magma. Entrained phonolitic pumice fragments, originally glassy, are now crystalline and resemble microsyenites. The basalt includes numerous resorbed feldspar, pyroxene, and biotite phenocrysts inherited from melted pumice. Major and trace element data constrain some of the entrained pumices to have originated from layers in the DHF section that occur no more than 80 m below where they are found, and 100 m below the paleosurface onto which the dike erupted. These observations show that shallow mafic magmas on Tenerife, within a few hundred meters of the surface, are capable of assimilating phonolitic pumices. Geochemical data from mafic lavas within the DHF section show that contamination by phonolite has been an important process in their petrogenesis, but do not allow identification of the physical nature of the phonolitic-composition contaminant. The evidence from the dike is that at least some of this contamination may have occurred very near the surface. Assimilation of pumice in a near surface environment, where thermal barriers would seem formidable, is facilitated by its glassy nature. This may be an important, overlooked process is central volcanoes where mafic magmas erupt through thick felsic pyroclastic formations.