INVESTIGADORES
COLOMBO Fernando
artículos
Título:
A-type magmatism in the sierras of Maz and Espinal: A new record of Rodinia break-up in the Western Sierras Pampeanas of Argentina
Autor/es:
COLOMBO, FERNANDO; BALDO, EDGARDO; CASQUET, CÉSAR; PANKHURST, ROBERT; GALINDO, CARMEN; RAPELA, CARLOS W.; DAHLQUIST, JUAN A.; FANNING, C. MARK
Revista:
PRECAMBRIAN RESEARCH
Editorial:
ELSEVIER SCIENCE BV
Referencias:
Lugar: Londres; Año: 2009 vol. 175 p. 77 - 86
ISSN:
0301-9268
Resumen:
Two
orthogneisses have been recognized in the sierras of Espinal and Maz (Western
Sierras Pampeanas, NW Argentina) that were emplaced within a Grenvillian
metasedimentary sequence. Microcline, plagioclase and quartz are the main
rock-forming minerals, with accessory zircon, apatite-(CaF), magnetite, biotite
(Fe/(Fe + Mg) = 0.880.91), ferropargasite (Fetotal/(Fetotal + Mg) = 0.880.89),
titanite (with up to 1.61 wt% Y2O3) and an REE-rich
epidote. REE-poor epidote and zoned garnet (Ca and Fe3+-rich) are
metamorphic minerals, while muscovite, carbonates and chlorite are secondary
phases. Texture is mylonitic. Two representative samples are classified as
granite (from Sierra de Espinal) and granodiorite/tonalite (from Sierra de Maz)
on the grounds of immobile trace elements. Some trace element contents are
rather high (Zr: 603 and 891 ppm, Y: 44 and 76 ppm, 10,000×Ga/Al: 2.393.89)
and indicate an affiliation with A-type granites (more specifically, the A2 group).
Both samples plot in the field of within-plate granites according to their Y
and Nb contents.
Concordant
crystallization ages (zircon UPb SHRIMP) are 842±5 and 846±6 Ma, respectively.
87Sr/86Sri (845) ratios are 0.70681 and
0.70666; eNdi (845) values are −1.5
and +0.3 and depleted-mantle Nd model ages (2TDM*) are 1.59 and 1.45 Ga,
respectively. These values indicate the involvement of an isotopically evolved
source. 2TDM* values are compatible with the presence of inherited zircon
crystals of up to 1480Ma in one of the rocks, thus implying that magmas
incorporated material from Mesoproterozoic continental source. This is also
indicated by the relatively high contents of Y, Ga, Nb and Ce compared to magmas
derived from sources similar to those of oceanic-island basalts.
These
orthogneisses represent a period of extension at ca. 845Ma affecting the
Western Sierras Pampeanas continental crust that was already consolidated after
the Grenvillian orogeny (1.21.0 Ga). They are thus a record of the early
stages of Rodinia break-up. Metamorphic conditions during the subsequent Famatinian
orogenic cycle (ca. 420 Ma, SHRIMP U-Pb on zircon) attained 7.7±1.2 kbar and
664±70 ºC.