Early Devonian rift-related felsic igneous rocks in the western Looe Basin, SW England
Geoscience in South-West England
The Ussher Society
© The Author
The Looe Basin records the extension of South-West England continental lithosphere during the Early Devonian. Its syn-rift infill, comprising the essentially non-marine Dartmouth Group (?Lochkovian-Pragian) and largely marine Meadfoot Group (Pragian- Emsian), includes low volume bimodal intrusive and extrusive igneous rocks that are predominantly mafic. Felsic igneous rocks are rare and have been described from several locations in the eastern Looe Basin, most notably the area around Whympston, south of Modbury, where they have also been investigated for their mineralisation potential. In the western Looe Basin a single occurrence, the poorly exposed Hendra Felsite, was recognised by Ussher during re-mapping of the Bodmin sheet at the beginning of the 20th Century. Previously undescribed Early Devonian felsic igneous rocks, defined here as the Hoblyn’s Cove Felsite, crop out in the structurally complex coastal section through the Start-Perranporth Zone between Ligger Point and Holywell Bay. The felsites exhibit bedding-parallel contacts with the host Trendrean Mudstone Formation (Meadfoot Group), share its Variscan high strain composite S1/S2 fabric, and are interpreted to have originated as horizontal quartz- and feldspar-phyric rhyolite sills. Thinlybedded crystal-lithic volcaniclastic sandstones, sandy mudstones and possible felsic pyroclastic rocks occur in the host rock succession in the vicinity of the intrusive felsite sheets and suggest that felsic magmatism was both contemporaneous with sedimentation and had a volcanic expression. Contemporaneous felsic magmatism is compatible with previous suggestions of a sedimentary or volcanic exhalative origin for the earliest sulphide-rich paragenesis of the nearby Perran Iron Lode. The felsite whole-rock trace element geochemistry is similar to that reported for Early Devonian felsites from the eastern Looe Basin. The Hoblyn’s Cove Felsite and associated volcaniclastic and pyroclastic rocks were re-orientated, during latest Carboniferous-early Permian post-Variscan extension, to their sub-vertical attitude within the steep limb of the large-scale S-verging monoformal F3 fold that defines the Start-Perranporth Zone. Early Permian elvans are readily distinguished from the Early Devonian felsites by their lack of Variscan foliation and distinctive trace element whole-rock geochemistry (low Zr, high P2O5 at low TiO2).
Vol. 13, No. 4, pp. 471-482