U-Pb zircon dating of the plutonic Hisingen suite in SW Sweden: geochemical classification and evaluation of field methods for differentiation

Abstract

The aim of this study was to investigate the c. 1.5 Ga Hisingen suite and the c. 1.3 Ga Kungsbacka bimodal suite by LA–ICP–MS U–Pb zircon dating, geochemical classification, magnetic susceptibility, gamma ray spectrometry and handheld X–ray fluorescence analyses. Four rock samples were collected across a previously mapped contact between the two suites, and mineral separation was performed to isolate zircons for analysis. Zircon dating of two of the samples gave ages of 1556±8 Ma and 1557±6 Ma for this thesis, while the two others were dated by Winblad (2022), which gave ages between 1548–1538 Ma. These ages ascribe the rocks to the main part of the Hisingen suite continental arc magmatism at 1588–1522 Ma. None of the analyses showed ages coeval with the intraorogenic period when the Kungsbacka bimodal suite intruded. Previously collected and analyzed rocks were used to compare with and classify the newly collected rocks. The newly collected rocks were classified as magnesian, calc-alkalic granites and granodiorites, which are described by the literature as common in the plutonic parts of magmatic arcs. Trace element analysis revealed a possibly diagnostic signature of the Heavy Rare Earth Elements (HREE) between the Hisingen and some of the Kungsbacka suite rocks. No other diagnostic trends were identified by magnetic susceptibility, gamma ray radiation or XRF analysis. A comparison between handheld XRF analysis and whole-rock chemical analysis showed that the XRF performs well for certain elements that can be useful when characterizing granitic rocks.