INVESTIGADORES
GONZALEZ GUILLOT mauricio Alberto
congresos y reuniones científicas
Título:
Whole rock and mineral chemistry of mafic dykes from Beauvoir range, Tierra del Fuego.
Autor/es:
ACEVEDO, R.D.;; VALÍN-ALBERDI, M.L; GONZÁLEZ GUILLOT, M.; ELSZTEIN, C.
Lugar:
Santiago de Chile
Reunión:
Congreso; GeoSur; 2007
Resumen:
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.