INGEOSUR   20376
INSTITUTO GEOLOGICO DEL SUR
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Characterization of an ultrabasic lamprophyre (evolved damtjernite) in Tandilia basement, southernmost Río de la Plata Craton, Argentina.
Autor/es:
DRISTAS A. JORGE; MARTÍNEZ JUAN CRUZ; MASSONNE, H-J; WEMMER, K
Lugar:
Mar del Plata
Reunión:
Congreso; 10th International Geological Congress on the Southern Hemisphere (GEOSUR); 2010
Institución organizadora:
Istituto Nazionale di Oceanografia e di Geofisica Sperimentale
Resumen:
A small dyke of an ultrabasic lamprophyre was recently found in the Sierra Alta de Vela (SAV), east of the town of Tandil, where basement of the southernmost Río de la Plata Craton is exposed. The wall rock of this dyke is variably deformed meta-igneous rock of granodioritic composition. The ultrabasic dyke, which is also partially deformed, consist mainly of zoned clinopyroxene (Cr-free diopside) and phlogopite (Cr-free, Ti-poor and Al-rich) phenocrysts. A scarce oxide phase is chromite. Three mentioned minerals are considered to be primary. The groundmass is also dominated by Cr-free diopside, which occurs as micro-phenocrysts with aegerine-augite rims, and subordinate interstitial phlogopite altered to chlorite. Additional phases are calcite, chlorite, sericite, andradite-rich garnet, pure albite, epidote and apatite forming a partial “oscellar” interstitial texture. According to the bulk rock chemistry, the dyke is a damtjernite (SiO2 : 44.70 - 46.45 wt. %). The Cr (550 ppm) and Ni (160 ppm) contents as well the Mg number (64) point to a moderately evolved rock, though the Ce to Pb ratio was found to be 44 (>25), but the contents of this rather mobile elements could have been changed during the secondary processes. The Ti and K contents of the SAV damtjernite are low compared to typical damtjernites, but CO2, Ca, Al and LaN/TbN fit well world-wide occurrences of damtjernites (see Tappe et al., 2006 Journal of Petrology, 47 (7): 1261-1315). Also the Zr/Nb ratio of 7 satisfies the higher values for typical damtjernites compared to other ultrabasic dykes. The positive anomaly of Dy in the REE pattern of the SAV-damtjernite may be due to the dominant presence of clinopyroxene in the rock, which enhances the fractionation (higher mineral/melt partition  coefficient) of Dy from basic melts according to Fujimaki et al. (1984, J. Geophys. Res., 89, suppl. B662-B672). The oscillatory zoning of andradite-rich garnets reflects mainly the Fe3+ - Al3+ substitution. This phase probably formed at upper crustal levels. The corrosion of garnet (atoll texture) is also due to late-stage processes. The absence of large quantities of Ti in andradite also points to a secondary origin of this phase. We will continue our work in the Sierra Alta de Vela area studying the probable presence of other lithological variety of this type of diatremites outcropping there and in other areas of Tandilia.