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

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

Santiago de Chile

Congreso; GeoSur; 2007

Several hydrothermally altered mafic dikes crop out in the Monte Rodríguez area, Sierra Beauvoir, in the Argentinian Fuegian Andes. They are intruded into the Beauvoir Fm (Lower Cretaceous). A whole rock K/Ar age of 104.0 ± 4.0 Ma was obtained from one dyke. The dykes 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), 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 of Zr (95 ppm), Nb (<10 ppm) and Ba (155 ppm), but Sr content is high (1061 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). In attempting to correlate these dykes with other igneous rocks of the area, we must consider the plutonic bodies cropping out in the Argentine Fuegian Andes (115-91 Ma), and the lamprophyric dykes cutting them. The plutons show alkaline affinity with high K2O content (shoshonites), and with hornblendes also rich in this oxide (1.13-2.28%). In contrast, the lamprophyric dykes that cut the shoshonitic plutons show a calc-alkaline trend and yielded whole rock K/Ar ages between 88-87 Ma (for the Estancia Túnel pluton). The geochemical data of the basic dykes of Sierra Beauvoir do not support correlation with the gabbros of the shoshonitic plutons and the lamprophyres that cut them. Their basic chemistry and lamprophyric texture are comparable, but low content in some trace elements (K, Zr, Nb, Ba), 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, suggests a link to the gabbros of the shoshonitic plutons (with high K2O content). The recently discovered Monte Kranck pluton (this volume) ~5 km to the south, has 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 to the previous data, the dykes of the Sierra Beauvoir may represent magmatic equivalents to the Monte Kranck gabbro.