A novel sample pre-screening methodology for accurate in situ U-Pb dating of zircon crystals

Internal inhomogeneity of the zircon crystals makes it difficult to date succedent geological processes. To overcome this limitation, a four-step pre-examination method is suggested before LA-ICP-MS based in situ U-Pb age determination of zircon crystals: 1. determination of morphology types of zircon, 2. description of internal textural patterns of zircon, 3. identification of mineral inclusions in zircon, 4. characterization of the structural state and chemical composition of zircon zones/areas. The novel four-step pre-examination protocol integrates established and partially established methods into comprehensive analytical techniques approach that enhances U-Pb dating. The paper demonstrates the effectiveness of this method using examples of zircons derived from granitoid rocks of the Variscan collision zone from the Mórágy Complex in southern Hungary and the Rastenberg Pluton in northern Austria. The study revealed that the areas of the primary textures within zircon crystals (growth zoning ± xenocrystic core, sector zoning) represent the effect of the primary magmatic event from both locations. The xenocrystic core suggests antecedent crustal origin. The secondary texture (convolute zoning) of zircon crystals reflects the post-magmatic event. It causes an overprinting of age data, highlighting the importance of a four-step pre-examination methodology in clarifying and separating nearly coeval geological processes. For age dating, 80 % of the core, 90 % of growth, sector zoning, and 100 % of convolute zoning of studied zircon crystals were appropriate. The suggested method can also be applied to U-Pb age dating of other mineral phases (allanite, apatite, titanite, rutile, xenotime, monazite) separated from igneous and sedimentary or metamorphic rocks.