Massimo Matteini

High spatial resolution analysis of Pb and U isotopes for geochronology by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS)

A determinacao de idades atraves do decaimento dos isotopos radioativos 235U e 238U para os isotopos radiogenicos 207Pb e 206Pb, respectivamente, utilizando o mineral zircao (ZrSiO4), e amplamente aplicada para decifrar processos geologicos. Um novo metodo tem sido desenvolvido nos ultimos anos, a ablacao a laser multi-coletor espectrometria de massas com plasma indutivamente acoplado (LA-MC-ICP-MS), superando o laborioso trabalho anteriormente necessario em outros metodos, na preparacao de amostras, e permite obtencao de razoes isotopicas com alta resolucao espacial de micrometros. O presente estudo descreve os procedimentos analiticos e os metodos usados na reducao de dados que estao sendo aplicados no Laboratorio de Geocronologia da Universidade de Brasilia. Explora-se a precisao e exatidao do metodo atraves da analise de tres padroes internacionais de zircao. Observa-se uma precisao entre 1,9 a 3,7% (2σ desvio padrao) e uma exatidao de 0,6 a 3,8% (2σ desvio padrao) para as razoes isotopicas de Pb e U dos padroes. Tambem foram obtidas idades pelo metodo LA-ICP-MS de duas amostras de zircoes naturais, que ja foram datadas anteriormente por outros metodos analiticos. A comparacao dos resultados mostra uma boa conformidade das idades obtidas, dentro dos limites de erro.Os dados demonstram o grande potencial do metodo analitico para analises isotopicas rapidas, precisas e exatas de U-Pb, numa escala de micrometros.

Geodynamical evolution of Central Andes at 24°S as inferred by magma composition along the Calama-Olacapato-El Toro transversal volcanic belt

Abstract Miocene to Recent volcanism on the Puna plateau (Central Andes) developed in three geological settings: (a) volcanic arc in the Western Cordillera (Miocene–Recent); (b) trans-arc along the main NW–SE transverse fault systems (Miocene); and (c) back-arc, mainly monogenic volcanic centres (Pliocene–Quaternary). We have studied the evolution of the arc–trans-arc volcanism along one of the most extensive transverse structures of Central Andes, the Calama–Olacapato–El Toro, at 24°S. Compositional variations from arc to trans-arc volcanism provide insights into petrogenesis and magma source regions. Puntas Negras and Rincon volcanic centres are arc-type and have typical calc-alkaline geochemical and Sr–Nd–Pb isotopic characteristics. East of the arc, lavas of the Tul-Tul, Del Medio and Pocitos complexes (TUMEPO) are heavy rare earth element-depleted and could be derived from 20–30% of partial melting of a lower crustal garnet-bearing metabasite. These liquids could be variably mixed with arc magmas at the base of the crust (MASH). This suggests important contributions from lower crustal sources to TUMEPO centres. Products at the Quevar and Aguas Calientes volcanic complexes to the east of TUMEPO show a prominent upper crustal signature (high 86Sr/87Sr, low 143Nd/144Nd) and could represent mixtures of 20–30% TUMEPO-type liquids with up to 70–80% of upper crustal melts. We propose a geodynamic model to explain geochemical variations for the arc–trans-arc transverse volcanism from the Upper Miocene to Recent. In our model, arc volcanism is linked to dehydration of the subducting Nazca plate, which produces typical calc-alkaline compositions. During the Upper Miocene (10–5 Ma), lithospheric evolution in the Puna plateau was dominated by thickening of ductile lower crust and thinning of the lithosphere. Lower crustal melting was promoted by concomitant asthenospheric upwelling and water release from the amphibolite–eclogite transformation, yielding TUMEPO magmas with lower crustal signatures. At the same time, the eastern sector of the Puna plateau experienced westward underthrusting of the Brazilian shield and upper crustal brittle deformation. Partial melts from underthrust upper crustal wedges mixed with lower crustal magmas to produce lavas at Aguas Calientes and Quevar. During Pliocene to Quaternary, delamination of the lithosphere and lowermost crust promoted a widespread monogenetic ocean island basalt-type and shoshonitic volcanism.

The geochemical variations of the upper cenozoic volcanism along the Calama–Olacapato–El Toro transversal fault system in central Andes (~24°S): petrogenetic and geodynamic implications

In this paper, we present new geochemical and Sr–Nd isotopic data for several Upper Miocene volcanic centres aligned along one of the most extensive transcurrent lineament in the Central Andes, the Calama–Olacapato–El Toro (COT). The transversal volcanic belt along COT is constituted by large composite volcanoes and a caldera structure; they are, from NW to SE, Puntas Negras, Rincon, Tul Tul, Del Medio and Pocitos (TUMEPO), Quevar Aguas Calientes and Tastil. In order to compare chemical data from the different centres along the COT transect, differentiation effects were minimised by using data extrapolated at 60% SiO2 with least-square regression method. In the western sector of the COT, the volcanic products of Puntas Negras and Rincon show relatively high K2O and 87Sr/86Sr and low Rb/Cs, Ta/Th, La/Yb, 143Nd/144Nd. To the east, the TUMEPO products have high Sr and 143Nd/144Nd, La/Yb and Ba/Rb and low Y, 87Sr/86Sr. In the easternmost COT sector, Quevar, Aguas Calientes and Tastil volcanic complexes exhibit low La/Yb, high87Sr/Sr86 and low 143Nd/144Nd. On the basis of these data, we propose a petrogenetic and geodynamical model for Central Andes at 24°S. In correspondence of Miocene–Quaternary volcanic arc (Puntas Negras and Rincon), the magmas inherited a calcalkaline signature partly modified by upper crustal and/or sediment assimilation. In the central eastern sector, melting, assimilation, storage and homogenisation (MASH) processes occurred at the base of a thickened crust. In this COT sector, TUMEPO products show an evident lower crust signature and could be considered representative for MASH derived magmas. In the easternmost sector, Quevar, Aguas Calientes and Tastil products could represent magmas generated by partial melting of underthrusted Brasilian shield and mixed with magmas derived by MASH processes.

Miocene magmatism and tectonics of the easternmost sector of the Calama–Olacapato–El Toro fault system in Central Andes at ~24°S: Insights into the evolution of the Eastern Cordillera

The Miocene Las Burras–Almagro–El Toro magmatic complex lies ~300 km to the east of the Central Andes volcanic arc, in the easternmost sector of the transverse Calama–Olacapato–El Toro fault zone. The magmatic rocks of the Las Burras–Almagro–El Toro complex comprise a monzogabbro to monzogranite laccolith like intrusion and basaltic andesite to dacite volcanic rocks that include seven lithostratigraphic members. New Rb-Sr dates indicate that the intrusive rocks are ca. 14 Ma, and K-Ar dates suggest emplacement ages of ca. 12.8–6.4 Ma for the volcanic rocks. The emplacement of the intrusion was controlled by N-S–striking strike-slip faults in a context of oblique convergence; the volcanism, which occurred along WNW-ESE– and N-S–striking extensional faults, relates to the Calama–Olacapato–El Toro fault zone. Two magmatic phases were recognized. Intrusive and volcanic rocks of the older magmatic phase (ca. 14–13 Ma) are characterized by Ba/Nb (7–14), La/Ta (11–18), and isotopic ratios ( 87 Sr/ 86 Sr: 0.704339–0.705281, 143 Nd/ 144 Nd: 0.512713–0.512598), which are intraplate characteristics. The source of the older magmas was isotopically depleted lithospheric mantle rich in K, Rb, and Th. Energy constrained–assimilation and fractional crystallization (EC-AFC) modeling indicates that fractional crystallization and crustal assimilation moderately modified magma composition during its residence in the crust. The products of the younger magmatic phase (ca. 11–6 Ma) have higher Ba/Nb (24–42) and La/Ta (24–30) and 87 Sr/ 86 Sr (0.706738–0.708729) and lower 143 Nd/ 144 Nd (0.512433–0.512360). The results of EC-AFC modeling exclude a significant role for the upper crust in the generation of the most primitive magmas of this phase. Their compositions can be explained by (1) contamination of the primary magmas having originated in a depleted mantle with a mafic crust, or (2) the contribution of isotopically enriched mantle zones. Shallow differentiation and moderate contamination by continental crust can explain the composition of the intermediate and evolved products of the younger phase. The variation of the magma source characteristics at 11 Ma is discussed in the frame of the complex geo-dynamical setting in this region.

Combined U-Pb and Lu-Hf isotope analyses by laser ablation MC-ICP-MS: methodology and applications

The Lutetium-Hafnium isotopic system represents one of the most innovative and powerful tools for geochronology and isotopic studies. Combined U-Pb and Lu-Hf in situ analyses on zircon by LA-MC-ICP-MS permit to characterize isotopically the host magma from which it crystallized furnishing significant information for sediment provenance and crustal evolution studies. In this paper we describe the Lu-Hf systematic by LA-MC-ICP-MS developed in the laboratory of Geochronology of the University of Brasilia and report the results obtained by repeated analyses of 176 Hf/ 177 Hf isotopic ratio of three zircon standards: GJ-1 = 0.282022 ± 11 (n=56), Temora 2 = 0.282693 ± 14 (n=25) and UQZ = 0.282127 ± 33 (n=11). The 176 Hf/ 177 Hf ratio (0.282352 ± 22, n=14) of gem quality zircon used as in-house standard have been also characterized. As a geological application, we analyzed two complex zircons selected from a migmatitic rocks from the Borborema Province, NE Brazil. On the basis of U-Pb and Lu-Hf data, two main crystallization events have been identified in both studied zircons. An older event at ca. 2.05 Ga recognized in the inherited cores represents a well-characterized paleoproterozoic magmatic event that affected the whole Borborema Province. A second crystallization event at � 575 Ma, recognized at the rims, represents a Neoproterozoic (Brazilian) high grade metamorphic-magmatic event.