Resumen: Fluorite mineralization at the La Nueva and Bubu mines yields Sm-Nd ages of 131 ñ 22 and 117 ñ 26 Ma, respectively. Thus, the mineralization most probably is related to a late Gondwanian (Lower Cretaceous) extensional and magmatic event that affected the Sierras Pampeanas basement during
the opening of the Atlantic Ocean. Hydrothermal fluids involved in the formation of the fluorite probably were of meteoric origin, their isotopic composition (Sr and Nd) resulting largely from the incongruent dissolution of feldspars in the host porphyritic granites.
Palabras clave: Fluorite mineralization; Sierra Pampeana; Argentina
Resumen: The fluorite mineralization from the La Nueva Mine, has a Sm-Nd age of 130 ñ 19 Ma, and thus must
be related to the Lower Cretaceous late-Gondwanic extensional and magmatic event that affected the
Sierras Pampeanas Basement of Argentina. Hydrothermal fluids involved in the formation of fluorite
were probably derived by
mixing of two fluids, an ascending high 87Sr/86Sr one, probably equilibrated
with basement metamorphics, as suggested by the very negative ENd(130) values, and a second, "descending",
with a low 87Sr/86Sr value.
Resumen: The role played by Paleoproterozoic cratons in southern South America from the Mesoproterozoic
to the Early Cambrian is reconsidered here. This period involved protracted continental amalgamation
that led to formation of the supercontinent Rodinia, followed by Neoproterozoic continental
break-up,
with the consequent opening of Clymene and Iapetus oceans, and finally continental
re-assembly as Gondwana through complex oblique collisions in the Late Neoproterozoic to Early
Cambrian. The evidence for this is based mainly on a combination of precise U-Pb SHRMP dating
and radiogenic isotope data for igneous and metamorphic rocks from a large area extending from the
Rio de la Plata craton in the east to the Argentine Precordillera in the west and as far north as Arequipa
in Peru. Our interpretation of the paleogeographical and geodynamic evolution invokes a hypothetical
Paleoproterozoic block (MARA) embracing basement ultimately older than 1.7 Ga in the Western Sierras
Resumen: The La Costa pluton in the Sierra de Velasco
(NW Argentina) consists of S-type granitoids that can be
grouped into three igneous facies: the alkali-rich Santa
Cruz facies (SCF, SiO2 *67 wt%) distinguished by the
presence of andalusite and Na- and Li-rich cordierite
(Na2O = 1.55–1.77 wt% and Li2O = 0.14–0.66
wt%), the
Anillaco facies (SiO2 *74 wt%) with a significant proportion
of Mn-rich garnet, and the Anjullo´n facies (SiO2
*75 wt%) with abundant albitic plagioclase. The petrography,
mineral chemistry and whole-rock geochemistry of
the SCF are compatible with magmatic crystallization of
Na- and Li-rich cordierite, andalusite and muscovite from
the peraluminous magma under moderate P–T conditions
(*1.9 kbar and ca. 735C). The high Li content of cordierite
in the SCF is unusual for granitic rocks of intermediate
composition.
Palabras clave: Andalusite Na- and Li-rich cordierite
S-type granite La Costa pluton Sierras Pampeanas
Resumen: El granate es un mineral accesorio poco común en rocas ígneas y constituye un tema relevante de la petrología ígnea. Estudios combinados de petrología,
química mineral, geoquímica de roca total y geoquímica isotópica
en el cerro Asperecito indican que los granitoides con granate que forman el Granito
Peñón Rosado (GPR) fueron producidos por la fusión parcial de rocas metasedimentarias con subsecuente diferenciación durante su emplazamiento en el
Ordovícico temprano-medio. En este sentido, la facies GPR1 representa un cristalizado temprano, la facies GPR2 representa un fundido diferenciado, y la
facies GPR3 un fundido residual. El Granito Peñón Rosado, fue emplazado en condiciones de alta temperatura (785úC) y presión media (4,4 kb) en profundidades
de la corteza media ( 15 km). El emplazamiento casi sincrónico en la corteza media de una abundante suite metaluminosa durante el Ordovícico
inferior-medio (468 Ma en el cerro Asperecito) fue suficiente para inducir anatexis local de los metasedimentos circundantes. Los contenidos de elementos
mayoritarios (CaO, Na2O) y elementos traza (Rb, Sr, Ba) en el Granito Peñón Rosado sugieren anatexis a partir de un material inmaduro acumulado en un
plataforma continental. Así, el Granito Peñón Rosado es un típico granitoide tipo-S con granate y el modelo establecido aquí puede contribuir a un mejor
entendimiento acerca de la cristalización del granate en rocas graníticas.
Resumen: Massif-type anorthosites are igneous complexes that consist for the most part of anorthosite, a plagioclase-
rich plutonic rock. They are particularly abundant within or near the worldwide Grenvillian mobile
belt (1.0-1.2Ga), which resulted from the assembly of the Mesoproterozoic supercontinent of Rodinia.
In
two of the most western Sierras Pampeanas (Sierras of Maz and Espinal, NW of Argentina), two
anorthositic igneous complexes have been discovered recently (Casquet et al. 2005a). Coarse-grained
anorthosites is the predominant rock type in these complexes. The age of crystallization is of 1070 ñ 41
Ma (U-Pb SHRIMP, in zircons). These rocks have a superposed regional metamorphism of Ordovician age
(431ñ30 Ma; U-Pb SHRIMP, in zircons). These complexes are the only ones mentioned in Argentina and
are the first of recognized Grenvillian age in Sudamerica.
Nd and Sr isotope composition suggest that primary magmas derived from a depleted source. Moreover
moderate contamination with an enriched continental component is also inferred. Nd model ages
suggest that the depleted source formed coeval with the anorthosite magmatism, which is compatible with
a mantle plume probably related to the early break up of Rodinia.
Resumen: We report the discovery of massif-type anorthosites
in the Andean basement of the Western
Sierras Pampeanas of Argentina. U–Pb zircon
dating (by sensitive high-resolution ion
microprobe) of a cogenetic gabbronorite dyke yields ages
of
1070 ñ 41 Ma for igneous emplacement and 431 ñ 40 Ma for
metamorphism. These anorthosites are petrologically and
geochemically comparable with those of the Grenville province
of Laurentia. Palaeogeographical reconstructions of
Rodinia at 1.0–1.1 Ga suggest that the Sierras Pampeanas
anorthosites were part of a large anorthosite province in the
late Mesoproterozoic.
