Alkali basalts and enclosed ultramafic xenoliths near Ushuaia, Tierra del Fuego, Argentina.

ACEVEDO, R.D.

SpringerPlus

SPRINGER

Año: 2016 vol. 5 p. 1 - 5

At the southernmost part of Tierra del Fuego a few outcrops and erraticboulders of alkalibasaltic rocks with ultramafic enclaves have been studied.Alkali basalt plugs or pipes hitherto identified are scarce, and host rocks are constituted by slatesthat belong to Mesozoic deposition. The petrography,texture and composition of the basalt and xenoliths were investigated bypetrographic microscope and electron microprobe analysis. Xenocrysts ofamphibole and alkali feldspar, phenocrysts of nepheline, olivine, spinel, phlogopiteand Fe-Ti minerals (10%) and a diversity of xenoliths, mainly lherzolitic,pyroxenite and wehrlitic nodules (15%), but also from metamorphic rocksprovenance, are contained in the basalt groundmass (75%). This finer-grained material is made up of laths orneedles of plagioclase, pyroxene, opaque minerals, apatite and glass, withintersertal, hyalopilitic and pilotaxitic. Locally, rock has an evengranoblastic texture. Former amygdules are filled by analcite, zeolites,sodalite and calcite. The normative classification, based on nepheline content,conclude that this rock is an alkali basalt. The chemical classification,considering immobile elements as Zr/TiO2 versus Nb/Y indicate analkali basalt too and plots over the TAS diagram fall in the foidite (Na-richor nephelinite) and basanite fields. The REE patterns are fractionated (La/Ybprimitive mantle normalized is approximately 30). The K-Ar isotopic techniqueon individual macrocrysts gave ages of 146 ±5 Ma (amphibole) and 127 ±4 Ma(alkali feldspar); and K-Ar whole rock datum reported 8.3 ±0.3 Ma.Nevertheless, fertile samples show geochemical features typical of deep derivedmaterial thus, based on the position in the actual tectonic setting, indicatethat the basalt is older than its isotopic age.