Whole rock and mineral chemistry of mafic dykes from Beauvoir range, Tierra del Fuego

ACEVEDO, R.D.; VALÍN-ALBERDI, M.L.; GONZÁLEZ GUILLOT, M. Y ELSZTEIN, C.

Santiago de Chile

Congreso; GeoSur 2007; 2007

Pontificia Universidad Católica de Chile

Several hydrothermally altered mafic dikes crop out in the Beauvoir range, at the Rodríguez Mt. area, in the Argentinian Fuegian Andes (AFA). They are intruded in the Beauvoir Fm (Lower Cretaceous). A whole rock K/Ar age of 104.0 ± 4.0 Ma was obtained from one dyke. The dikes exhibit a porphyritic (lamprophyre-like) texture with visible hornblende and pyroxene phenocrysts. The groundmass has a fluidal microporphyritic texture containing plagioclase and interstitial quartz. The accessory minerals are apatite and opaques. Occasionally, hornblende is altered to mica. Plagioclase was extensively transformed to albite (Ab98.05), chlorite (brunsvigite-pycnoclorite), epidote, calcite and quartz. Major elements indicate an alkaline geochemistry in a TAS diagram (SiO2: 47.04%, Na2O: 5.17%, K2O: 0.17%), with low K2O content (given the altered nature of the dykes -LOI: 7.4%-, these data must be taken with caution). Trace elements show low values (except for Sr, 1061 ppm) of Zr (95 ppm), Nb (<10 ppm) and Ba (155 ppm). Phenocrysts are represented mainly by potassic-magnesiohastingsite (IMA2004-027b) with relatively high K2O content (1.49-1.53%) and structural formulae Na0.533 K0.296 (Ca1.926 Na0.075) (Mg2.802 Fe1.125 Mn0.027 Fe3+0.535 AlVI0.243 Ti 0.266) Si5.934 Al2.066 O22 (OH)2, and lesser amounts of diopside (Wo47-49 En29-42 Fs10-2). On an attempt to correlate these dykes with other igneous rocks of the area, we must consider the plutonic bodies cropping out in the AFA (115-91 Ma), and the lamprophyric dykes cutting them. The plutons show alkaline affinity with high K2O content (shoshonites), with hornblendes also rich in this oxide (1.13-2.28%). Contrarily, the lamprophyric dykes that cut the shoshonitic plutons show a calc-alkaline trend and yielded whole rock K/Ar ages between 88-87 Ma (at Estancia Túnel pluton). The geochemical data of the basic dykes of Beauvoir range do not favour any correlation with the gabbros of the shoshonitic plutons and the lamprophyres that cut them. Their basic chemistry and lamprophyric texture may be compared to both units. The low content in some trace elements (K, Zr, Nb, Ba), although, preclude a clasiffication as true lamprophyres. Additionally, the K/Ar age of the Beauvoir dykes is clearly older than that of the lamprophyres. Hornblende chemistry, on the other hand, suggest a link to the gabbros of the shoshonitic plutons (with high K2O content). The recently discovered Mt. Kranck pluton (this volume) ~5 km southward, shows large exposures of gabbros with petrographic similarities (lamprophyric texture with hornblende and pyroxene phenocrysts), feature not seen in gabbros of the other plutons of the area. In addition with the previous data, the dykes of the Beauvoir range may represent magmatic equivalents to the Mte. Kranck gabbro.