A-type magmatism in the sierras of Maz and Espinal: A new record of Rodinia break-up in the Western Sierras Pampeanas of Argentina



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Colombo, F. and Baldo, Edgardo G. and Casquet, César and Pankhurst, R.J. and Galindo Francisco, Mª del Carmen and Rapela, Carlos W. and Dahlquist, Juan A. and Fanning, C.M. (2009) A-type magmatism in the sierras of Maz and Espinal: A new record of Rodinia break-up in the Western Sierras Pampeanas of Argentina. Precambrian research, 175 . pp. 77-86. ISSN 0301-9268

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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) = 0.88–0.91), ferropargasite (Fetotal/(Fetotal + Mg) = 0.88–0.89), 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: 2.39–3.89) 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 U–Pb SHRIMP) are 842±5 and 846±6 Ma, respectively.
87Sr/86Sri (845) ratios are 0.70681 and 0.70666; εNdi (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 (1.2–1.0 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 ◦C.

Item Type:Article
Uncontrolled Keywords:Rodinia; U-Pb SHRIMP dating; Anorogenic magmatism; Mesoproterozoic Western Sierras Pampeanas Argentina
Subjects:Sciences > Geology > Petrology
ID Code:12137
Deposited On:02 Feb 2011 10:11
Last Modified:02 Feb 2011 10:11

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