Zircon thermometry and U–Pb ion-microprobe dating of the gabbros and associated migmatites of the Variscan Toledo Anatectic Complex, Central Iberia

Abstract

In the Central Iberian Zone there are several large thermal domes in which small bodies of ultramafic, mafic and intermediate rocks appear intimately associated with crustal granites and migmatites. The closest spatial association between the ultramafic, mafic and intermediate rocks and migmatites is in the Toledo Anatectic Complex, where field relationships suggest that these rocks are coeval and have an age close to 340 Ma. This, and the recent discovery in the neighbouring Ossa Morena Zone of a large mid-crustal seismic reflector interpreted as a 335-350 Ma mafic sill, reinforce the hypothesis that heat for crustal melting was supplied from early Variscan mantle magmas emplaced in the middle crust. However, precise ionmicroprobe U-Pb zircon dating and Ti-in-zircon thermometry in Toledo do not support this idea. Whereas the mean age of four mafic bodies is 307 AE 2 Ma, the migmatites are c. 25 Ma older. The migmatites hosting ultramafic, mafic and intermediate bodies have the same age and Ti-in-zircon temperatures as migmatites far from any mafic intrusion. These data reveal that ultramafic, mafic and intermediate magmas are late Variscan; they were emplaced in already cooling anatectic zones once the extensional collapse was initiated, and their thermal impact on the mid-crustal Variscan anatexis of Central Iberia was negligible.

SHRIMP U–Pb zircon dating of anatexis in high-grade migmatite complexes of Central Spain: implications in the Hercynian evolution of Central Iberia

“…However, the age interval they obtained is similar to our data (350 to 320 Ma), suggesting that their older age could be biased due to a limited dataset (14 versus our 50 analyses) and poses the question of which would be the minimum amount of analyses necessary to properly characterize complex geologic processes like long-lived migma tiza tion events similar to the one described in Montero et al (2004). It is also noteworthy that our youngest migmatization ages overlap the age of the mafic magmatism from the Toledo area obtained by Bea et al (2006). Neverthe less, new additional data and future geochronological work are needed to clarify this discussion.…”

“…Recently, Bea et al (2006) present new results in two migmatite samples from the ACT. They propose an age for the migmatization surprisingly older (332 Ma).…”

“…At the starting point of this study, the geochronological data in the Toledo area was limited to a short communication on the anatectic granites from the southern ACT (Barbero and Rogers 1999), giving an age of 310 Ma (U-Pb ID-TIMS in monaz ites) for the anatectic event. However, recently Bea et al (2006) have published a SIMS U-Pb zircon study on the temporal relationships between the mafic magmatism and the migmatization in the Toledo area in which they obtained a mean migmatization age of 332 Ma.…”

Exhuming a cold case: The early granodiorites of the northwest Iberian Variscan belt—A Visean magmatic flare-up?

“…305-290 Ma), known classically as G3-G4 or late granites in the regional literature (Capdevila, 1969;Capdevila and Floor, 1970;Galán et al, 1996), intrude all the structural domains of the orogen, including the foreland fold and thrust belt, which makes the WEVB unique (see Gutiérrez-Alonso et al, 2011). The POS includes a large number of volumetrically minor intrusions of mafic and ultramafic rocks (e.g., Franco and García de Figuerola, 1986;Bea et al, 2006a;Villaseca et al, 2011;Orejana et al, 2012Orejana et al, , 2015. This magmatic event has been extensively studied and dated (e.g., Ugidos and Recio, 1993; 2 www.gsapubs.org | Volume 10 | Number 1 | LITHOSPHERE Moreno-Ventas et al, 1995;Bea et al, 1999Bea et al, , 2006aCastro et al, 1999;Villaseca et al, 1998Villaseca et al, , 1999Cuesta and Gallastegui, 2007;Fernández-Suárez et al, 2000Orejana et al, 2009;Gutiérrez-Alonso et al, 2011).…”

“…The POS includes a large number of volumetrically minor intrusions of mafic and ultramafic rocks (e.g., Franco and García de Figuerola, 1986;Bea et al, 2006a;Villaseca et al, 2011;Orejana et al, 2012Orejana et al, , 2015. This magmatic event has been extensively studied and dated (e.g., Ugidos and Recio, 1993; 2 www.gsapubs.org | Volume 10 | Number 1 | LITHOSPHERE Moreno-Ventas et al, 1995;Bea et al, 1999Bea et al, , 2006aCastro et al, 1999;Villaseca et al, 1998Villaseca et al, , 1999Cuesta and Gallastegui, 2007;Fernández-Suárez et al, 2000Orejana et al, 2009;Gutiérrez-Alonso et al, 2011). The POS has been interpreted as generated by lithospheric delamination triggered by the oroclinal bending of the mountain belt (Fernández-Suárez et al, 2000;Gutiérrez-Alonso et al, 2004, and references therein).…”