The Western Sierras Pampeanas: Protracted Grenville-age history (1330–1030 Ma) of intra-oceanic arcs, subduction–accretion at continental-edge and AMCG intraplate magmatism



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Rapela, Carlos W. and Pankhurst, R.J. and Casquet, César and Baldo, Edgardo G. and Galindo Francisco, Mª del Carmen and Fanning, C.M. and Dahlquist, Juan A. (2010) The Western Sierras Pampeanas: Protracted Grenville-age history (1330–1030 Ma) of intra-oceanic arcs, subduction–accretion at continental-edge and AMCG intraplate magmatism. Journal of South American earth sciences, 20 . pp. 105-127. ISSN 0895-9811

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New U–Pb SHRIMP zircon ages combined with geochemical and isotope investigation in the Sierra de
Maz and Sierra de Pie de Palo and a xenolith of the Precordillera basement (Ullún), provides insight into
the identification of major Grenville-age tectonomagmatic events and their timing in the Western Sierras
Pampeanas. The study reveals two contrasting scenarios that evolved separately during the 300 Ma long
history: Sierra de Maz, which was always part of a continental crust, and the juvenile oceanic arc and
back-arc sector of Sierra de Pie de Palo and Ullún. The oldest rocks are the Andino-type granitic orthogneisses
of Sierra de Maz (1330–1260 Ma) and associated subalkaline basic rocks, that were part of an
active continental margin developed in a Paleoproterozoic crust. Amphibolite facies metamorphism
affected the orthogneisses at ca. 1175 Ma, while granulite facies was attained in neighbouring meta-sediments
and basic granulites. Interruption of continental-edge magmatism and high-grade metamorphism
is interpreted as related to an arc–continental collision dated by zircon overgrowths at 1170–1230 Ma.
The next event consisted of massif-type anorthosites and related meta-jotunites, meta-mangerites
(1092 ± 6 Ma) and meta-granites (1086 ± 10 Ma) that define an AMCG complex in Sierra de Maz. The
emplacement of these mantle-derived magmas during an extensional episode produced a widespread
thermal overprint at ca. 1095 Ma in neighbouring country rocks. In constrast, juvenile oceanic arc and
back-arc complexes dominated the Sierra de Pie de Palo–Ullún sector, that was fully developed ca.
1200 Ma (1196 ± 8 Ma metagabbro). A new episode of oceanic arc magmatism at 1165 Ma was roughly
coeval with the amphibolite high-grade metamorphism of Sierra de Maz, indicating that these two sectors
underwent independent geodynamic scenarios at this age. Two more episodes of arc subduction are
registered in the Pie de Palo–Ullún sector: (i) 1110 ± 10 Ma orthogneisses and basic amphibolites with
geochemical fingerprints of emplacement in a more mature crust, and (ii) a 1027 ± 17 Ma TTG juvenile
suite, which is the youngest Grenville-age magmatic event registered in the Western Sierras Pampeanas.
The geodynamic history in both study areas reveals a complex orogenic evolution, dominated by convergent
tectonics and accretion of juvenile oceanic arcs to the continent.

Item Type:Article
Uncontrolled Keywords:Sierras Pampeanas; Grenville-age orogeny; Intra-oceanic arc; Arc–continent collision; AMCG complex; U–Pb SHRIMP dating
Subjects:Sciences > Geology > Petrology
Sciences > Geology > Geodynamics
ID Code:11878
Deposited On:13 Jan 2011 13:20
Last Modified:01 Sep 2011 11:45

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