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

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 ion-microprobe 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 ± 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.伊比利亚中部带（Central Iberian Zone）发育多处大型热穹窿，其中超镁铁质、镁铁质与中性岩小岩体与地壳花岗岩及混合岩（migmatites）紧密伴生。超镁铁质、镁铁质、中性岩与混合岩最紧密的空间伴生关系见于托利多深熔杂岩（Toledo Anatectic Complex），野外地质关系显示这些岩石为同期形成，年龄接近340 Ma。这一发现，以及邻近奥萨-莫雷纳带（Ossa Morena Zone）新近发现的、被解释为335~350 Ma镁铁质岩床的大型中地壳地震反射体，进一步强化了“地壳熔融所需热量来自侵位至中地壳的早华力西期（Variscan）地幔岩浆”这一假说。然而，托利多地区开展的高精度离子探针锆石U-Pb定年与锆石钛含量测温结果，并不支持该假说。4个镁铁质岩体的平均年龄为307±2 Ma，而混合岩的形成年龄则比其早约25 Ma。赋存超镁铁质、镁铁质与中性岩体的混合岩，其形成年龄与锆石钛含量测温所得温度，均与远离任何镁铁质侵入体的混合岩一致。上述数据表明，超镁铁质、镁铁质与中性岩浆形成于华力西晚期；它们是在伸展塌陷启动后，侵位于已经开始冷却的深熔带中，且对伊比利亚中部中地壳华力子期深熔作用的热影响可忽略不计。