Crystallization and uplift path of late Variscan granites evidenced by quartz chemistry and fluid inclusions: Example from the Land's End granite, SW England

Abstract

The megacrystic, coarse-grained granite of the Land’s End granitic complex, SW England, has been investigated by analyzing fluid inclusions, trace elements, and cathodoluminescence textures of quartz. By applying the TitaniQ geothermobarometer together with the cathodoluminescence textures, a two-stage emplacement process is proposed. K-feldspar and quartz phenocrysts crystallized in a deep magma chamber at ca. 18–20 km depth. The phenocrysts were transported together with the melt to a shallow emplacement depth at ca. 5–9 km in multiple intrusive events, causing the composite appearance of the granitic complex. This model of emplacement concurs with similar granites from the Erzgebirge. At the emplacement level, the magma exsolved an aqueous fluid with average salinity of 17.3% m/m NaCl and 9.7% m/m CaCl2. Fluids with higher salinities were exsolved deeper in the system, as the magma experienced stages of water saturation and water undersaturation during ascent from the deep magma chamber. The complex fluid inclusion textures are the results of multiple stages of entrapment of aqueous fluids in the host phases as multiple recharge events from the deeper magma chamber supplied fresh melts and aqueous volatiles. Titanium contents in quartz are closely related to the panchromatic cathodoluminescence intensity, and the Al/Ti ratio is reflected by the 3.26 eV/2.70 eV ratio of hyperspectral cathodoluminescence.