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

Rogelio Daniel Acevedo

doi:10.1186/s40064-016-3059-7

Abstract

At the southernmost part of Tierra del Fuego a few outcrops and erratic boulders of alkali basaltic rocks with ultramafic enclaves have been studied. Alkali basalt plugs or pipes hitherto identified are scarce, and host rocks are constituted by slates that belong to Mesozoic deposition. The petrography, texture and composition of the basalt and xenoliths were investigated by petrographic microscope and electron microprobe analysis. Xenocrysts of amphibole and alkali feldspar, phenocrysts of nepheline, olivine, spinel, phlogopite and Fe–Ti minerals (10 %) and a diversity of xenoliths, mainly lherzolitic, pyroxenite and wehrlitic nodules (15 %), but also from metamorphic rocks provenance, are contained in the basalt groundmass (75 %). This finer-grained material is made up of laths or needles of plagioclase, pyroxene, opaque minerals, apatite and glass, with intersertal, hyalopilitic and pilotaxitic. Locally, rock has an even granoblastic 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 an alkali basalt too and plots over the TAS diagram fall in the foidite (Na-rich or nephelinite) and basanite fields. The REE patterns are fractionated (La/Yb primitive mantle normalized is approximately 30). The K–Ar isotopic technique on 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 derived material thus, based on the position in the actual tectonic setting, indicate that the basalt is older than its isotopic age.

Highlights

Until its recent discovery, the dense sub Antarctic forest had hidden some small outcrops of a dark brecciated basalt-like rocks, irregular and amygdaloidal, containing olivine, pyroxene, amphibole, feldspar and various lithic fragments, specially ultramafic nodules. These rocks, with their inclusions and nodules, appear spread at the foothills of the Susana Mount, West of Ushuaia city, and they are the first rocks of this type ever reported for the Isla Grande de Tierra del Fuego
Description of the outcrops The best exposition of the studied rocks can be observed at the base of the SW hillside of the Susana Mount, on the coast of Canal Beagle in front of the Estorbo Island, next to the Beatriz Mine, a lens of quartz and sulphides known since long time ago (Fig. 1)
The basalt has small and varied inclusions and its host rocks are constituted by banded gray slates which strike N75°E and dip 50/60° SE

Summary

Introduction

Background Until its recent discovery, the dense sub Antarctic forest had hidden some small outcrops of a dark brecciated basalt-like rocks, irregular and amygdaloidal, containing olivine, pyroxene, amphibole, feldspar and various lithic fragments, specially ultramafic nodules. They are isolated, without visible contact with any surrounding rock and they contain inclusions of a larger size and macrocrysts of amphibole (sample A2) and feldspar (sample A3).

Results

Conclusion