Reassessing the intrusive tempo and magma genesis of the late Variscan Aar batholith: U–Pb geochronology, trace element and initial Hf isotope composition of zircon

Abstract

The Variscan orogeny was responsible for the formation of a significant volume of igneous basement throughout present-day Europe. Detailed understanding of these rocks has, however, been obfuscated by significant overprinting during younger geologic events. In order to better understand the formation of this basement, we present U–Pb dates, trace element concentrations and Hf isotope compositions of zircon from 17 intrusions of the Variscan Aar batholith, located in the Aar Massif, Central Alps, Switzerland. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) was used to generate a large set of U–Pb dates, trace element and Hf isotope compositions on untreated zircon, as well as zircon pretreated by chemical abrasion. Furthermore, a subset of samples was also analyzed for high-precision U–Pb geochronology using chemical abrasion, isotope dilution, thermal ionization mass spectrometry (CA-ID-TIMS). The U–Pb dates of both dating techniques are significantly dispersed, indicating that they are influenced by multiple forms of complexity, including inheritance, domains of secondary alteration likely related to Alpine overprint or growth, decay damage related Pb-loss, and potentially protracted magmatic growth. Decay-damage related Pb-loss is likely a subordinate source of age scatter within the data, therefore chemical abrasion pretreatment is not capable of completely mitigating the observed analytical scatter. After rejection of outliers, the remaining data still exhibit excess scatter of several percent among 206Pb/238U dates in individual samples, however it is possible to interpret reasonable geologic ages from these data. These new U–Pb zircon age interpretations indicate the Aar batholith grew incrementally through four major magmatic pulses, which occurred at approximately 348, 333, 309 and 298 Ma. Based on the trace element and Hf isotope geochemistry, the melt source(s) of the Aar batholith evolved throughout the duration of batholith formation and growth. The transitioning from (i) melting of depleted mantle at 348 Ma during a stage of active continental arc magmatism (εHf = + 12 to + 10), (ii) melting of metasomatically enriched lithospheric mantle, possibly contaminated by crust during the 333 Ma pulse (εHf = − 10 to − 3), followed by (iii) an increasing incorporation of a juvenile mantle components during the 309 and 298 Ma pulses (εHf = − 3 to + 6). Finally, these new U–Pb ages yield a more detailed understanding of the Variscan Aar batholith by integrating the new detailed mapping of Aar Massif for the Geological Atlas of Switzerland, allowing for more accurate characterization and categorization of variably deformed heterogeneous intrusive bodies.

Epidote dissolution–precipitation during viscous granular flow: a micro-chemical and isotope study

“…The Aar Massif consists of a Paleozoic polymetamorphic basement and Permian intrusives, including the post-Variscan Central Aar Granite. This granitoid was emplaced 299 ± 2 Ma (Schaltegger and Corfu, 1992;Ruiz et al, 2022) and bears clear evidence of Alpine deformation (e.g., Choukroune and Gapais, 1983;Bambauer et al, 2009;and references therein). This Alpine deformation is expressed by a large number of ductile shear zones, and in the southern Aar Massif it can be subdivided into two major phases: (1) a reverse faulting phase with green biotite stable in the shear zones at >400 • C (ca.…”

“…Thus, the differences in Pb and Sr isotopic data of Epidote-A grain and micro-separates are interpreted to document mixing between different contributions: (1) one reflecting the isotopic composition of Epidote-A incorporated upon vein formation and (2) one reflecting the mixing of dissolved Epidote-A material with extraneous Pb and Sr brought to the site of Epidote-B crystallization. The existence of open-system conditions bears the potential for fluid-mediated transfer of chemical 299 ± 2 Ma old host granitoid (Schaltegger and Corfu, 1992;Ruiz et al, 2022) contains minerals with high concentrations of U, Th, and Rb (e.g., allanite, biotite), even small and variable contributions of radiogenic Pb and Sr leached from these minerals may produce a measurable shift in Pb-Sr isotope ratios without a resolvable effect on Sr and Pb fluid concentrations. Also, crystallization of Epidote-A and Epidote-B most likely occurred under comparable T conditions (ca.…”

Identifying crustal contributions in the Patagonian Chon Aike Silicic Large Igneous Province

“…Given the excess scatter in all dated rocks, even after removal of anomalously old or young analyses, the reported weighted mean ages of the selected population of analyses may not be accurate within uncertainty determined purely from data scatter. In addition to the error calculated in IsoplotR that we take as a minimum error, we follow the method of Ruiz et al ( 2022 ) in calculating an extended error. This extended error, even though probably overestimated, allows for a more accurate age interpretation from scattering U–Pb dates, which is more important than apparent precision.…”

Discovery of Variscan orogenic peridotites in the Pelvoux Massif (Western Alps, France)

“…First, the opening of Devonian-Tournaisian basins, presumably in a back-arc setting, was associated with intrusion of trondhjemite dykes and sills around 350 Ma (Fréville et al, 2018). Tonalite plutons of the same age have also been dated in the Aar massif (Ruiz et al, 2022). This low-K magmatism was followed by emplacement of K-rich sub-alkaline granitoids from the Visean to the early Permian (ca.…”