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

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 rockforming
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,
10000*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; Ndi (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 1480 Ma 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. 845 Ma 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  70ºC.