Gabriel Gutiérrez Alonso

Advances in U-Pb geochronology using a frequency quintupled Nd:YAG based laser ablation system (λ= 213 nm) and quadrupole based ICP-MS

The analytical potential of a 213 nm laser system coupled to a quadrupole based ICP-MS for U-Pb dating of zircon is investigated. The study was performed on zircons from a Neoproterozoic granitoid of the Cadomian belt of Brittany. Seven zircon fractions of the same sample including two single grains were dated by ID-TIMS to obtain a reference age and error to evaluate the accuracy and precision of LA-ICP-MS U-Pb results. The best estimate for the age of crystallisation obtained by ID-TIMS is 603 ± 2 Ma. LA-ICP-MS analyses were conducted in two sessions, one using only Ar as carrier gas and one using Ar + He. In both cases the age results are within analytical uncertainty of the ID-TIMS age (601 ± 9 Ma for the Ar analyses and 600 ± 2 Ma for the He analyses). It is demonstrated that the use of He carrier gas significantly improves reproducibility and precision of the elemental U-Pb ratios and allows the collection of data with a 2σ relative precision < ca. 1.5% for 206Pb/238U ratios and < ca. 2.5% for 207Pb/206Pb ratios at ca. 25 × 35 µm sample resolution. It is also demonstrated that the various sources of uncertainty associated with LA-ICP-MS U-Pb dating do not preclude the possibility of obtaining precise and accurate age results if an appropriate methodology is applied.

Assembly of the Armorica Microplate: A Strike‐Slip Terrane Delivery? Evidence from U‐Pb Ages of Detrital Zircons

Detrital zircons separated from the Ordovician Armorican quartzite in the foreland of the Variscan belt of Iberia were dated by laser ablation inductively coupled plasma mass spectrometry. The age groups found correspond to Proterozoic and Archean crust formation events. The Proterozoic events reflected by zircon ages are Avalonian‐Cadomian‐Pan‐African (550–800 Ma), Grenvillean‐Sunsas (900–1300 Ma), and Eburnian‐Transamazonian‐Icartian (1800–2200 Ma). Archean zircon ages are comprised between 2500 and 2800 Ma (Central‐Amazonian‐Liberian events). The presence of Mesoproterozoic (Grenvillean) zircons in the Armorican quartzite, which was deposited in a shallow platform bordering the NW African craton (north Gondwana margin), suggests that the NW Iberian terrane drifted from the South American margin, where it was located during the Neoproterozoic, toward Africa. The drift of the Iberian block, likely achieved via transform faults, was arrested by its docking and amalgamation with the peri–NW African Neoproterozoic terranes, resulting in the birth of the Paleozoic Armorica Terrane Assemblage.