The complex and prolonged fracturing and chemical alteration history of the Rolvsnes granodiorite on the Bømlo archipelago in southwestern Norway

Reconstructing the tectonic and geomorphological history of geological terranes poses significant challenges, particularly when interpreting deeply buried and exhumed settings. The Rolvsnes granodiorite terrane exposed in the Bømlo archipelago on Norway’s southwestern coast, provide invaluable insights into these processes. This granodioritic to granitic intrusive body serves as an onshore counterpart to offshore basement reservoirs, such as those on the Utsira basement high, where altered basement rocks are overlain by Permian to Cretaceous sediments. These analogs enable researchers to link surface observations to subsurface conditions, offering a rare opportunity to understand complex fracture and alteration histories.

We provide new evidence from multidisciplinary investigations constraining the fracturing and alteration history of the Rolvsnes granodiorite. Multiple chemically altered bedrock outcrops associated with fracture zones were identified across Bømlo, with samples collected for geochemical, mineralogical, and isotopic analyses. We characterize secondary clay assemblages and constrain the timing of alteration processes. Additionally, three bedrock cores drilled through prominent fracture zones were logged and sampled to enhance the dataset with subsurface information.

K-Ar geochronology dates range from the Carboniferous to the Paleogene, suggesting multiple alteration events over extended periods. Geochemical and mineralogical data indicate significant leaching of alkali and alkaline-earth elements, with the formation of kaolinite, smectite, interstratified illite-smectite, illite, and lepidocrocite in the altered material. Scanning electron microscopy reveals small but significant differences between alteration zones. In some zones, K-feldspar is altered into a mixture of kaolinite, smectite, and illite, while plagioclase (particularly Na-rich laminae) and biotite remain relatively unaffected. In other zones, biotite transforms into vermiculite, illite, and iron (hydro-)oxides, while plagioclase alters into smectite and kaolinite, leaving K-feldspar relatively intact. These findings suggest alteration predominantly by low-temperature meteoric water. Many samples did not yield Kübler Index determinations due to poorly defined 10 Å peaks, but acquired illite-crystallinity data indicate fluid temperatures ranging from 120 to 295 °C. Thus, K-Ar ages should be interpreted cautiously, as multiple alteration events may occur along the same fracture zone at different times.

The data suggests that faulting and hydrothermal alteration initiated as early as the Carboniferous, continuing through the Permian. Late Triassic brittle faulting may have coincided with supergene weathering under humid, tropical conditions, resulting in saprolitic weathering of the crystalline basement along pre-existing fractures. During subsequent marine transgressions, most saprolitic material was eroded, leaving remnants buried beneath sedimentary cover, which was in turn largely removed during the Plio-Pleistocene. The Rolvsnes granodiorite appears to have experienced additional fracturing and alteration events beneath this sedimentary cover, as indicated by K-Ar dates extending into the Early Cretaceous and Paleogene. 

This study highlights the inherent difficulties of reconstructing complex tectonic and geomorphological histories in such terranes. The Bømlo archipelago offers a compelling case study for linking onshore observations to offshore settings, but challenges remain in disentangling overlapping alteration processes and correlating them to specific tectonic events.