Granulite facies metacarbonates from La Huerta Range, Province of San Juan, NW-Argentina

GALLIEN, FLORIAN; MOGESSIE, ABERRA; SCHNEIDER, ISABELLA; CASTRO DE MACHUCA, BRÍGIDA; BJERG, ERNESTO; DELPINO, SERGIO; PREVILEY, LORENA; PONTORIERO, SANDRA; MEISSL, ESTELA; KOSTADINOFF, JOSÉ

Tagung, Hannover

Workshop; DMG 2006 (Deutschen Mineralogischen Ges.); 2006

German Mineralogical Society

A preliminary investigation of several marble and calcsilicate outcrops in the southern part of the Sierra de Valle Fértil including the Sierra de La Huerta have been made. The metacarbonate units represent a sequence of impure marbles with partially preserved primary layering. In some places, basement rocks are found included and syndeformed within the calcsilicate and marble layers. Based on field observation, macroscopic and microscopic investigations, the metacarbonate units are classified into four groups: (1) white coarse grained marbles, with a small amount of clinopyroxene + quartz + tremolite. The clinopyroxene has an XMg = 0.86 – 0.94; (2) thin calcsilicate reaction rims, composed of wollastonite + grossular + epidote-calcite-quartz-symplectites + scapolite + clinopyroxene are formed at the immediate contact of the marble with the amphibolite and Bt-Grt gneiss host rocks. Assuming internal buffered prograde decarbonatization reactions, the paragenesis wollastonite + scapolite + clinopyroxene indicates minimum temperature of about 860°C and an intermediate X-CO2 of 0.5. Clinopyroxene from these reaction rims is slightly enriched in Fe2+ with an XMg = 0.57 – 0.69. Grossular represents a solid solution of andradite and grossular with a XAdr = 0.53 – 0.78, the spessartine component is negligible. During retrogression grossular was consumed to form symplectites of zoisite + quartz + calcite at low X-CO2 ~ 0.0 – 0.18 and temperatures below 730°C. Scapolite with an XMeionite= 0.72 – 0.78 is supposed to be derived from the reaction anorthite+calcite = meionite; (3) preserved primary layering of siliceous impurities in the marbles formed centimetre thick layers of serpentinized forsterite rich bands and clinopyroxene bands during metamorphism. Brucite which may imply the presence of periclase as a precursor phase was found as inclusions in calcite; (4) marble outcrops in contact with metaigneous rocks (tonalite to gabbro) contain typical granulite facies paragenesis represented by sapphirine + spinel + corundum with clinochlore as matrix phase. This paragenesis is confined to peraluminous pockets about 400µm diameter and can be seen as a fairly closed system with no chemical interaction between the marble and the host rocks. Geothermobarometry on related gneisses gives high-grade amphibolite facies conditions of about 7.5 ± 1kbar and 650 - 750°C and 780 - 820°C for the granulites. Hence similar metamorphic conditions or higher must have overprinted the marble units.