Dating multiply overprinted granites: The effect of protracted magmatism and fluid flow on dating systems (zircon U-Pb: SHRIMP/SIMS, LA-ICP-MS, CA-ID-TIMS; and Rb–Sr, Ar–Ar) – Granites from the Western Erzgebirge (Bohemian Massif, Germany)

Abstract

The Variscan granites from the Western Erzgebirge were repeatedly dated by various methods, but no consensus has been reached about their exact intrusion ages. This study presents a multi-dating approach for the four largest intrusions from the Western Erzgebirge (Aue-Schwarzenberg, Bergen, Eibenstock, Kirchberg). We analysed several samples from each pluton/suite with zircon U–Pb CA-ID-TIMS (chemical abrasion-isotope dilution-thermal ionization mass spectrometry) to obtain robust temporal information on their age and tempo of intrusion. These data enable us for the first time to define three intrusive episodes of 1–2 Ma each, separated by quiet periods of several Ma. The Aue-Schwarzenberg suite represents the oldest granites that intruded at ~323–322 Ma followed by the granites from Bergen and Kirchberg 2–4 Ma later. The highly evolved ore-bearing granites from the Eibenstock pluton intruded after a time lag of ~5 Ma at ~315–314 Ma. The new data show that there is a resolvable age difference between the two known granite groups. Granite group 2 (also assigned as younger igneous complex, represented by the Eibenstock pluton) is ≥5 Ma younger than granite group 1 (assigned as older igneous complex, represented by granites from Aue-Schwarzenberg, Bergen and Kirchberg).Protracted magmatism and late-/post-magmatic fluid flow partly reset the U–Pb system of these granites to variable degrees, making a precise and accurate dating of their intrusion ages challenging. Pb loss in zircons is often combined with high common Pb (Pbc). SHRIMP/SIMS (sensitive high mass resolution ion microprobe/secondary ion mass spectrometry) and LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry) on non-CA zircons document that Pb loss and high Pbc is quite variable within zircon grains and may be located in micro-fractures. We demonstrate that chemical abrasion (CA) clearly minimizes or removes both Pb loss and Pbc. Results from prior LA-ICP-MS and SHRIMP dating on non-CA zircons from the same samples considerably helped the interpretation of the CA-ID-TIMS data when Pb loss was not completely erased by CA. In such cases we often had to choose the oldest analyses for mean age calculation in contrast to the common practice of the CA-ID-TIMS community to choose the youngest dates.Rb–Sr and Ar–Ar dating systems revealed age differences between the older group and the younger ore-bearing granites albeit with diverging absolute ages. Most Ar–Ar ages are identical with CA-ID-TIMS ages and would imply rapid cooling. However, samples from the older group have excess Ar that could have led to too old ages. In contrast, Rb–Sr ages for the older granites are 0–7 Ma younger than their intrusions. Fluid induced alteration led to the formation of Li-mica, fluorite and cassiterite (greisenization). For the youngest granite (Eibenstock), Li-mica was used to date the first greisenization. Samples without visible hydrothermal overprint yielded identical Ar–Ar and Rb–Sr ages as severely greisenized samples. This implies re-equilibration due to the hydrothermal overprint for all Ar–Ar and Rb–Sr ages from the Eibenstock pluton. According to Ar–Ar dating, the first ore formation (~315 Ma) is coeval with the CA-ID-TIMS intrusion age of the Eibenstock granite while it is delayed by ~6 (±3) Ma according to Rb–Sr dating (308 ± 3 Ma).