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.