Livro de resumos 19 a 23 Outubro 2015, LNEG, Portugal

Editor: LNEG - Laboratório Nacional de Energia e Geologia Título: Livro de Resumos do X Congresso Ibérico de Geoquímica/XVIII Semana de Geoquímica Autor: Vários Suporte: Multimédia ISBN: 978-989-675-039-8

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Geochemical Evolution of Volcanism during the Upper CambrianOrdovician Extension of the North Gondwana Margin Evolución geoquímica del vulcanismo durante la extensión cambroordovícica del margen Norte de Gondwana Díez-Montes, A.1*, González Clavijo, E. J.1, Dias da Silva, I. F.2, Gomez Barreiro, J.3, Martínez 3 4 Catalán, J. R. , Castiñeiras, P. 1

Instituto Geológico y Minero de España. C/ Azafranal, 48, 1ºB, 37001 Salamanca, España; * [email protected] 2 Instituto Dom Luiz, FCUL, Campo Grande, Edfo. C8, Piso 3, 1749-016 Lisboa, Portugal 3 Departamento de Geología, Universidad de Salamanca. Plaza de la Merced, s/n, 37008 Salamanca, España 4 Departamento de Petrología y Geoquímica, Universidad Complutense, José Antonio Novais, 12, Ciudad Universitaria, 28040 – MADRID

Abstract Around the Morais Complex a continuous stratigrafic sequence, upper Cambrian to Silurian in age, displays four types of interlayered vulcanites. They have each distinctive lithogeochemical features interpreted as genetically related to the preVariscan break-up of the north margin of Gondwana. The spatial and temporal relationships between the stratigraphic package, mineralization, and deformation strongly support a distinct origin for these rocks: 1) the upper Cambrian Mora Vulcanites are bimodal-subalkaline; 2) the lowermost Ordovician Saldanha augen-gneisses present characteristics similar to the “Ollo de Sapo” formation. These two volcanic types are below the Armorican-type quartzite beds. Two other types of volcanism are located above this last formation: 3) the Middle-Upper Ordovician CVS-Peso volcanism, the most voluminous of the four groups, which is bimodal subalkaline bearing similar characteristics to the Mora vulcanism; and 4) the Carrapatas bimodal volcanism, occurring mainly as dykes, which is composed of alkali basalts, with OIB-type features, and peralkalines rhyolites.

Keywords: bimodal vulcanism, subalkaline-alkaline rocks, peralkaline rhyolites, North Portugal

Resumen Alrededor del Complejo de Morais una secuencia estratigráfica continua, de edad Cámbrico superior a Silúrico, presenta cuatro tipos diferentes de vulcanitas. Cada uno de ellos tiene unas características litogeoquímicas distintivas y se han interpretado como genéticamente relacionados con la extensión pre-varisca del margen Norte de Gondwana. Las distintas relaciones espaciales y temporales entre la secuencia estratigráfica, mineralización y deformación apoyan claramente distintos orígenes para estas rocas: 1) las vulcanitas cámbricas superiores de Mora son bimodales subalcalinas; 2) los gneises de Saldanha, del Ordovícico más inferior, presentan características semejantes a la Formación “Ollo de Sapo”. Estos dos tipos de vulcanismos se encuentran bajo el nivel guía definido por una Cuarcita de tipo Armoricano. Inmediatamente sobre estas capas se encuentran otros dos tipos de vulcanitas: 3) las vulcanitas del Ordovícico Medio-Superior de CVS-Peso, las más voluminosas de todas, las cuales son bimodales subalcalinas con características semejantes a las vulcanitas de Mora; y 4) el vulcanismo bimodal de Carrapatas, que aparece principalmente en diques, y está formado por basaltos alcalinos, con características de tipo OIB, y por riolitas peralcalinas.

Palabras clave: vulcanismo bimodal, rocas subalcalinas-alcalinas, riolitas peralcalinas, Norte Portugal

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Introduction and geological setting

rhyolites were only found in the Carrapatas Formation of the MCS (M.L. Ribeiro, 1986, 1987).

This contribution presents new geochemical data from 34 samples collected in vulcanites from two lithostratigraphic units: the Peso formation (Dias da Silva, 2014; equivalent to the Volcanic-Siliceous Complex –CVS- in Ribeiro et al., 2013); and the Carrapatas group (Ribeiro, 1986) at the Macedo de Cavaleiros Simplex (MCS). Both units can be correlated and crop out as a rim around the ophiolitic unit of the Morais Allochthonous Complex (MAC) following Ribeiro et al. (2013) and Dias & Ribeiro (2013).

Mafic rocks The chemical differences between the two types of basalts are manifest in the Pearce (2008) diagrams. The mafic rocks of Mora and CVS-Peso, except the Carrapatas Fm alkali basalt, have a similar composition. The negative Nb anomaly is characteristic of the volcanic-rifted margin basalt, due to a magma-crust interaction process. In the Ti-Yb part of the spider-diagram both groups have a separate trend, a negative slope for the alkaline (OIB pattern) and flat, or slightly negative, for the other (characteristic of N- and E-MORB), according Pearce (2008). For alkali basalts, the existence of these gradient points to the presence of residual garnet, as Yb is much strongly partitioned in garnet than Ti. The gradient at the Nb-Ti part of the spider-diagram increases from tholeiitic to alkali composition, MORB to alkali basalts.

To complete the database, we have integrated the geochemical data of Ribeiro (1986), from samples gathered in vulcanites of the MCS, and from the samples of vulcanites collected by Dias da Silva (2014) at the eastern boundary of the MAC. The inclusion in our study of this last group is justified because Dias da Silva (2014) modified the position of the Main Trás-os-Montes Thrust (MTMT) in the sector to a lower position, under the uppermost lithostratigraphic unit of the Parautochthonous Nappe Complex on the Rodrigues et al. (2013) division. In this work, we considere the MCS and the Lower Alochthonous Complex (LAC) the same structural unit of the MAC, as it was proposed by Ribeiro et al. (2013). The lithostratigraphic sequence of this tectonic slice is continuous and presents several interlayered volcanic events. Following Dias da Silva et al. (2015a) all the here considered volcanic rocks extruded in an upper Cambrian to Upper Ordovician timespan. Lithogeochemistry Fig. 1 displays the compositional range of the entire group of samples. Two separate bimodal suites (basalts and rhyolites) can be identified: i) CVS-Peso-Mora-Saldanha, subalkaline with tholeiitic basalts and calcalkaline dacite-rhyolites; and ii) Carrapatas Group, alkaline with alkali basalts and peralkaline rhyolites. The per-alkaline

Fig. 1 - Modified Winchester and Floyd (1977) Zr/TiO2-Nb/Y discrimination diagram for rock classification (Pearce, 1996).

The alkali basalts display higher Nb/Yb Th/Yb ratios than tholeiitic basalts and plot

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in the OIB array. Some OIB samples are displaced to a higher Th/Yb place, which is distinctive of a recycled crustal component. Using the diagram with TiO2/Yb - Nb/Yb ratios the alkaline basalts plot in the OIB array, whereas the tholeiitics fit in the MORB array (Pearce, 2008).

arc field, close to the boundary with withinplate and anomalous granitoids (ORG, Pearce et al., 1984). The Middle-Upper Ordovician CarrapatasPeso felsic rocks (Dias da Silva et al., 2015a) are mainly of rhyolitic composition. They present high REE values with high LREE concentrations, (La/Sm)n=3.7-3.9, negative Eu anomaly (Eu/Eu*=0.15-0.19) and slight HREE fractionation, (Gd/Yb)n=1.4-1.8. The Primitive Mantle normalized spider diagram exhibits negative Nb and Ti anomalies. These felsic rocks plot inside the volcanic-arc field of Pearce et al. (1984).

On simple compositional criteria (Nb/Zr ratio, Fitton et al., 1997), at least two basic magma sources are represented in the Peso Formation. Low Nb/Zr ratio correspond to MORB source, while high Nb/Zr plot at OIB source field.

All the Carrapatas peralkaline rocks are of felsic composition, and they can be classified mainly as comenditic and pantelleritic rhyolites (Macdonald, 1974). There rocks are REE enriched with (La/Sm)n>1 and (Gd/Yb)n>1 and they show varying negative Eu anomalies reflecting significant plagioclase crystallization. In a primitive mantle normalized multi-elements diagrams (Sun and McDonough, 1989) the peralkaline rhyolites have low abundances of Ba, Sr, Eu, P and Ti, and high Nb and Ta contents significant greater than the Mora, Saldanha and CVS-Peso rhyolites. There peralkaline rhyolites have Zr>350 ppm and most have Zr>700 ppm and forming a good trend with Zr/Nb