Isla Isabel (Nayarit, México): Quaternary alkalic basalts with mantle xenoliths erupted in the mouth of the Gulf of California

Abstract

Isla Isabel is a small island (~ 1.06 km 2) off the coast of Nayarit where intraplate-type alkali-basaltic volcanic rocks erupted during the Quaternary, and one of two Mexican islands (along with Isla Guadalupe) where it is known that the alkali basalts have transported peridotite xenoliths to the surface. The volume of the island is estimated at approximately 4.2 km 3; the volume above sea level is estimated to be approximately 0.03 km 3. The dominant volcanic rock type exposed above sea level is analcime-cemented basaltic tuff-breccia, the rapidly lithified products of hydrovolcanic eruptions; however, subaerial scoria-fall deposits and lava flows are also present. Ten analyzed Isla Isabel volcanic rocks are relatively homogeneous alkali basalts with 4.5–6.4 wt.% nepheline in the CIPW norm. Compared to other analyzed Mexican intraplate-type volcanic rocks from oceanic and continental localities, at the same MgO content, the Isabel samples are depleted in TiO 2, FeO total, and Na 2O, and moderately incompatible trace elements (e.g., Sm, Eu, Tb, and Zr). On the other hand, they are enriched in CaO and Al 2O 3, and relatively compatible trace elements (e.g., Lu and Sc). It is suggested that relative to other Mexican intraplate alkaline lavas the Isabel lavas were derived by higher degrees of melting at lower pressures of mantle that is characterized by ancient HIMU-like isotope systematics. Isabel olivine phenocrysts and their spinel inclusions range to Mg#s that overlap with the peridotite minerals in the accompanying mantle xenoliths, which has not been reported at other Mexican xenolith localities. Olivine-hosted glass inclusions in rapidly quenched scoria yielded the first estimates for volatile contents in Mexican intraplate-type mafic melts; maximum values are: 1.3 wt.% H 2O, 2241 ppm CO 2, 1500 ppm F, 1300 ppm S, and 1200 ppm Cl. Isabel peridotites (Ol ± Hy ± Di + Sp ± Pl) include dunites, harzburgites, and rare plagioclase–harzburgite, and are depleted in clinopyroxene in comparison to other Mexican peridotite localities where lherzolites dominate; Isabel peridotite clinopyroxenes are also depleted in Na 2O compared to other Mexican localities, another reflection of their refractory nature. Pressure estimates for Isabel xenoliths based on olivine–clinopyroxene Ca exchange indicate that they equilibrated at relatively low pressures (5.9–15.5 kb) compared to other Mexican peridotites (~ 10–20 kb), which is consistent with the presence of a single plagioclase–harzburgite xenolith. The refractory character of the Isabel xenoliths is consistent with the results of the Alarcon segment of the PESCADOR seismic experiment, and Lizarralde et al.'s [Lizarralde, D., Axen, G.J., Brown, H.E., Fletcher, J.M., Gonzáles-Fernández, A., Harding, A.J., Holbrook, W.S., Kent, G.M., Paramo, P., Sutherland, F., Umhoefer, P.J., 2007. Variation in styles of rifting in the Gulf of California. Nature 448: 466–469] hypothesis that wide, magma-poor rifted margins result from rifting of depleted mantle.