U-Pb zircon and titanite age of the Christiansø granite, Ertholmene, Denmark, and correlation with other Bornholm granitoids

Abstract

A granitic sample from the Danish island of Christiansø in the Ertholmene island group north of Bornholm is described petrographically and geochemically, and dated using U-Pb in zircon and titanite. Zircon systematics in the sample are complicated by abundant Pb loss and a large population of zircons interpreted as being inherited. Removal of highly disturbed zircons, imprecise analyses, and assumed inherited zircons yield an upper intercept date of 1500 ± 18 Ma (MSWD = 13, n = 58). Removal of zircons with high common Pb from this population yields an identical result of 1500 ± 22 Ma (MSWD = 8, n = 34). Zircons that are ≤3% discordant give a weighted average 206Pb/238U age of 1458 ± 12 Ma (MSWD = 3.0, n = 18), and a weighted average 207Pb/206Pb age of 1495 ± 14 Ma (MSWD = 4.7, n = 19). Titanites from the sample yield a lower intercept age of 1448 ± 15 Ma (MSWD = 6.8, n = 45). The sample contains a significant number of inherited grains indicative of ages around 1.7–1.8 Ga. The relatively large MSWDs for these age determinations indicate geological complexity, likely reflecting Pb loss, and the possible presence of inherited zircons which suffered major Pb loss during incorporation in the granitic magma. The zircon and titanite dates agree reasonably well with previous age determinations on felsic lithologies from the Bornholm mainland, as well as from the Blekinge Province of southern Sweden. Petrographically and geochemically, the Christiansø granite is indistinguishable from, and can be correlated with, the A-type granites and gneisses which occur on Bornholm. The high abundance of disturbed and inherited zircons (c. 1.7–1.8 Ga) may indicate that the granite was intruded into and assimilated a nearby region of unexposed Transscandinavian Igneous Belt rocks. The somewhat altered nature of the rock, and overall disturbance of U-Pb zircon systematics, suggest alteration associated with fluid-flow along nearby faults defining the northern margin of the Sorgenfrei–Tornquist Zone.