Isotopic and igneous record of the Avalonian-Cadomian arc in NW Iberia
U- Pb zircon age and tectonic meaning of the Cardenchosa pluton (Ossa-Morena Zone)
Edad U-Pb de zircones y significado tectónico del plutón de La Cardenchosa (Zona de Ossa- Morena)
A. Azor1, J.F. Simancas1, D.J. Martínez Poyatos1, P. Montero2, F. González Lodeiro1 and I. Pérez-Cáceres 1 Departamento de Geodinámica, Universidad de Granada, 18071 Granada. [email protected], [email protected], [email protected], [email protected], [email protected]
2 Departamento de Mineralogía y Petrología, Universidad de Granada, 18071 Granada. [email protected]
Abstract: The Cardenchosa pluton crops out in the northernmost Ossa-Morena Zone, immediately to the south of the boundary with the Central Iberian Zone. This pluton is made up of two granitic facies. Despite the lack of geochronological data, this intrusive body has been traditionally attributed to the Carboniferous magmatism of the so- called “Los Ojuelos Igneous Complex”. This work provides with the first radiometric age of the Cardenchosa pluton, obtained by Sensitive High Resolution Ion Microprobe U-Pb zircon dating from a sample of the central facies. This sample yielded an Ordovician age of 478-480 Ma, which is interpreted as corresponding to the intrusion of the pluton.
These new data indicate that the Cardenchosa pluton belongs to the widespread Cambrian-Ordovician igneous suite outcropping in the Ossa-Morena and Central Iberian Zones, attesting a tectonic setting of continental rifting at the onset of the Variscan cycle.
Key words:U-Pb zircon SHRIMP, deformed granite, Cambrian-Ordovician rifting, Variscan orogen.
Resumen: El plutón de la Cardenchosa aflora en la parte más septentrional de la Zona de Ossa-Morena, inmediatamente al sur del límite con la Zona Centroibérica. Está compuesto por dos facies de composición granítica.
Pese a la ausencia de dataciones radiométricas, este cuerpo ha sido atribuido tradicionalmente al magmatismo carbonífero del denominado “Complejo Ígneo de Los Ojuelos”. En este trabajo aportamos la primera datación radiométrica de este plutón. Para ello, hemos datado una muestra de la facies central con microsonda iónica mediante el método U-Pb en zircones. Hemos obtenido una edad ordovícica de 478-480 Ma, que interpretamos como la edad de intrusión del plutón. A la luz de estos nuevos datos, cabe concluir que este cuerpo granítico forma parte del numeroso cortejo ígneo de edad cambro-ordovícica aflorante en las Zonas de Ossa-Morena y Centroibérica, y que han sido ligados al contexto tectónico de rifting continental con el que se inició el ciclo varisco.
Palabras clave: U-Pb SHRIMP en zircones, granito deformado, rifting cambro-ordovícico, orógeno Varisco.
INTRODUCTION
The Ossa-Morena Zone (OMZ) is characterized by the abundance of igneous rocks, grouped into three time-spans for which different tectonic scenarios have been proposed (see Cambeses, 2015, for a recent review). The older ages are latemost Precambrian and correspond to a magmatic arc related to the Cadomian orogeny. The second igneous set clusters at Cambrian- Ordovician ages, witnessing the continental rifting that features the onset of the Variscan cycle. Finally, an important number of igneous bodies are Early Carboniferous in age and have been related to the Variscan orogeny, though their intrusion/extrusion probably occurred during an extensional/transtensional event that interrupted the ongoing collisional evolution (e.g. Pérez-Cáceres et al., 2015 and references therein).
The attribution of the different OMZ igneous rocks to each one of the abovementioned scenarios cannot be done drawing only on the geochemical signature and/or the intensity of the deformation affecting them.
Actually, the dating of a growing number of OMZ igneous intrusions, mostly by U-Pb on zircon grains (see Cambeses, 2015, for a compilation of ages), has yielded in some cases unexpected ages, since there is neither a straightforward relationship between deformation and age, nor between geochemical signature and age.
This work provides with the first absolute age (U- Pb on zircons measured with ion microprobe) of the Cardenchosa pluton. This intrusive body has been attributed to the Early Carboniferous in previous
24 studies, though the data reported here do not support that assumption.
GEOLOGICAL SETTING
The Cardenchosa pluton is exposed just south of the boundary between the ZOM and the Central Iberian Zone (Fig. 1). To the SE, this boundary is cross-cut by the so-called “Los Ojuelos Igneous Complex”
(Delgado Quesada et al., 1985; Fig. 1b), which includes the Cardenchosa pluton and has been considered entirely as Early Carboniferous according to the presence of sediments of that age. This granitic body intruded the contact between the Sierra Albarrana
Unit and the remaining rocks of the ZOM (Azor, 1994;
Fig. 1b). Far from the granite, that contact consists of Carboniferous normal and left-lateral faults that place the Sierra Albarrana Unit in a lower structural position with respect to other OMZ rocks.
The Cardenchosa pluton is made up of two granitic facies (Garrote and Sánchez-Carretero, 1979), the most abundant being a coarse-grained slightly porphyritic granite with up to 4 cm feldspar phenocrysts. A second minor facies, mostly restricted to the western border of the pluton, is a chilled margin (Fig. 1b) composed of medium- to fine-grained leucocratic granite.
FIGURE 1: a) Geological sketch of southwest Iberia, showing the area in Fig. 1b (rectangle) and the three major zones of the Variscan orogen, as well as the Central Unit or Badajoz-Córdoba shear zone (dark blue). b) Simplified geological map of the Sierra Albarrana Unit and the Cardenchosa pluton. The dated sample (AA-513) is located with an asterisk.
The country rocks of the Cardenchosa pluton are as follows (Fig. 1b): (1) along the western border, medium-grade micaschists belonging to the Sierra Albarrana unit; (2) along the eastern and southern borders, slates with some quartzite intercalations belonging to the Lower-Middle Cambrian Azuaga Formation; and (3) along the northern border, basic igneous rocks belonging to Los Ojuelos Igneous Complex. The thermal aureole associated with the pluton is well developed along the western and southern contacts (1-2 km-wide band of hornfelses;
Garrote, 1976; Azor and Ballèvre, 1997). On the contrary, the eastern contact is affected by a high-angle reverse fault that hides the thermal aureole (Fig. 1b).
The highest grade assemblages in the hornfelses contain andalusite, biotite, fibrolitic sillimanite, and K-
feldspar. The microtextures shown by andalusite porphyroblasts indicate a static growth over a previous regional foliation. According to these assemblages, especially the coexistence of andalusite and K-feldspar in the innermost aureole, maximum pressures of no more than 2 kbar and temperatures between 500 and 600 ºC can be estimated. Consequently, the Cardenchosa pluton can be considered to have intruded in the upper crust at a maximum depth of 5-6 km.
The Cardenchosa pluton is strongly deformed, especially along its western border, where a penetrative solid-state foliation can be mapped (Simancas et al., 2000). Furthermore, to the NW it shows a cartographic tail that joins the left-lateral Azuaga Fault (Fig. 1b) and is affected by a planar-linear fabric with the same
kinematics as the fault. A gravimetric survey combined with surface geological data showed that the 3D geometry of the pluton corresponds to a lens-shaped body with flat and shallow floor located at 1-2 km depth (Simancas et al., 2000).
ZIRCON GEOCHRONOLOGY
We report the first geochronological data on the Cardenchosa granite. The geochronological work was done at the IBERSIMS laboratory of the University of Granada, equipped with a Sensitive High Resolution
Ion MicroProbe
(http://www.ugr.es/~ibersims/ibersims/Welcome.html).
The dated sample (AA-513) was collected from the coarse-grained facies in an outcrop located 2 km south of La Cardenchosa (Fig. 1b). This rock is slightly red- colored and contains K feldspar phenocrysts, plagioclase, quartz and biotite, with accessory zircon, apatite and ore minerals. Solid-state deformation is very scarce in this particular outcrop, as attested by a nearly isotropic fabric of the granite. Zircon crystals are euhedral, 50-200 m-long, transparent, pink to yellowish, clear grains. In cathodoluminescence, most crystals display a banded oscillatory zoning pattern.
The zoned zircons have very variable contents of U (74-7920 ppm), Th (13-1508 ppm), and Th/U ratios (0.08-3.49), yielding concordia ages of 478 ± 7 (206Pb/238Pb) and 480 ± 5 Ma (207Pb/235U) (Fig. 2). We interpret these Early Ordovician ages as corresponding to the crystallization of the granitic magma in upper crustal levels. Two xenocrystic zircons have yielded Cadomian (≈ 570 Ma) and Paleaeoproterozoic (≈ 1900 Ma) concordant ages.
FIGURE 2: Wetherill concordia plot (common-lead uncorrected and less than 5 % of discordance) with 23 zircon grains of the sample AA-513 used to calculate the intrusion age of the Cardenchosa granite.
DISCUSSION AND CONCLUSION
The U-Pb zircon age of the Cadenchosa pluton supports that it intruded at Ordovician time (478-480
Ma) and not at Carboniferous times as supposed until now. This fact calls attention to the need of radiometric dating of the different intrusive bodies grouped in the so-called Los Ojuelos Igneous Complex. Actually, the only igneous rocks of this area whose age can be considered as certain are the acidic volcanic rocks intercalated within Lower Carboniferous sedimentary rocks of the Benajarafe basin.
The Ordovician age of the Cardenchosa granite leads us to include it in the Cambrian-Ordovician rifting episode that marks the onset of the Variscan cycle. This rift-related magmatic episode was widespread all across the Central Iberian Zone and the OMZ (e.g. Simancas et al., 2004 and references therein), being characterized by plutonic and volcanic rocks of very diverse geochemical signature. By composition and age, the Cardenchosa granite is quite similar to two ortghoneissic bodies located inside the Central Unit (Minillas [474 Ma] and Ribera del Fresno [475 Ma]). These two bodies appear strongly elongated on map, showing a fully penetrative and prominent planar-linear fabric associated with ductile left-lateral shearing at the boundary between the OMZ and the Central Iberian Zone. On the contrary, the Cardenchosa pluton is as a whole less intensely deformed, generally depicting a rough subvertical planar fabric marked by elongated quartz grains. However, in the western margin the planar fabric is very prominent and the external granite contact is affected by hectometer-scale folds (Simancas et al., 2000); in the same way, the NW cartographic tail of the pluton depicts a planar-linear fabric with left-lateral kinematics, i.e. it is affected by the same shearing as the Central Unit.
In SW Iberia, the Cambrian-Ordovician rifting episode could have eventually given way to the development of oceanic-like crust at the northern (narrow oceanic realm?) and southern (Rheic ocean?) boundaries of the OMZ. Another particular feature of the Cambrian-Ordovician rifting in SW Iberia was the formation of several migmatitic domes with preserved early Palaeozoic extensional planar-linear fabrics (Valuengo and Monesterio; Expósito et al., 2003). In the northernmost OMZ, the Sierra Albarrana Unit might have recorded a similar evolution, as attested by Late Cambrian to Early Ordovician SHRIMP U-Pb ages on zircons from migmatitic leucosomes (497 Ma) and 40Ar-39Ar on amphibole concentrates (481 Ma) (Azor et al., 2012). However, the preservation of tectonic fabrics and major structures associated with this migmatitic low-pressure event still need to be unraveled. In this regard, the fact that the contact aureole metamorphic assemblages at the western contact of the Cardenchosa pluton are superposed onto the main regional foliation in the host rocks of the Sierra Albarrana Unit suggests that this planar fabric is pre-Variscan and formed during the
26 Late Cambrian extensional tectonics associated with the rifting episode.
ACKNOWLEDGMENTS
Financial support was provided by grants numbers CGL2011-24101 and CGL2015-71692-P of the Spanish Ministry of Economy and Competitiveness.
We thank A. Castro for the critical reading of the manuscript.
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