Evolution of the Ballantrae Complex, SW Scotland: From the opening of the Iapetus to the Taconian-Grampian orogeny

The opening and closure of the Iapetus Ocean were key geodynamic events during the transition from Rodinia to Pangaea. Mantle-derived rocks formed during this period provide critical insights into the processes that shaped the mantle composition and structure during this tectonic transition. The Ballantrae Complex in Scotland is a well-preserved remnant of the Iapetus Ocean seafloor obducted during the Grampian Orogeny (ca. 475−465 Ma) and provides a window into the composition of the mantle at this period. The complex structure, and the overlying sedimentary rocks that cover the ophiolite, however, have made it difficult to parse the competing processes involved in the generation of the ophiolite. In this work we provide new whole-rock geochemical, Nd-Hf-Pb isotopic, and geochronological analyses on zircon, calcite, and titanite, guided by fieldwork observations, which help unravel the protracted evolution of the Ballantrae Complex. The oldest rock (571 ± 27 Ma) found in the region is a porphyritic dolerite at Bennane Head that contains zircon xenocrysts with a characteristic late Dalradian signature similar to that of the Southern Highlands Group. This dolerite is possibly an intrusion that occurred proximal to the Laurentian margin during the opening of the Iapetus Ocean. Our results reveal a two-stage evolution history for the development of the main body of the ophiolite. Rocks dated at ca. 500−480 Ma are light rare earth element (LREE)-depleted and have low Zr/Y ratios, reminiscent of depleted island-arc tholeiites and boninites, whereas rocks from ca. 475 Ma to 462 Ma are generally more LREE enriched and follow higher Zr/Y trends, akin to more evolved arc basalts and ocean-island basalt (OIB) compositions, including the Downan Point Lava Formation. We suggest that the late OIB-like basalts of the Ballantrae Complex are correlated with similar basalts in the Tyrone Volcanic Group of Northern Ireland and might be related to an intra-arc rifting event or the subduction of a slab window. The Ballantrae Complex, therefore, records a protracted history from the opening until the early-stage closure of the Iapetus Ocean.