55 million years of continuous anatexis in Central Iberia: single-zircon dating of the Peña Negra Complex

Abstract

Single-zircon evaporation and ion-microprobe dating of migmatites and anatectic granites in the Peña Negra Complex of Central Iberia reveal that the Variscan anatexis occurred continuously from 352 to 297 Ma, with a maximum at 335–305 Ma. Anatexis began coeval with the main collision of continental masses. A limited melting event, probably related to syncollision crustal-scale shear zones, produced a population of zircons with ages of c . 350 Ma. The production of new zircons decreased to a minimum at c . 343 Ma but then increased swiftly as the internal thermal evolution of the thickened Central Iberian crust led to widespread anatexis in the Peña Negra region at 332 Ma. Shortly after this, the melt resident in the migmatites was locally segregated into small bodies that crystallized as cordierite leucogranites at 321 Ma. Simultaneously, extensional subhorizontal shear zones were preferentially developed over layers of the migmatite series that, owing to their elevated heat production and fertility, had the highest melt fraction. Shearing provoked further anatexis and contributed significantly to the in situ production of high melt-fraction granodiorites and adamellites from the migmatites. This process occurred from 325 to 305 Ma, with a maximum at 309 Ma marking the peak of the Variscan extensional collapse in Central Iberia. After c . 305 Ma the melt fraction decreased quickly, so that the production of new zircons was insignificant at 300 Ma and had stopped completely by 297 Ma.