SYN-MAGMATIC INTRUSIVES, TRANSITIONAL PECULIARITIES AND POST-MAGMATIC REPLACEMENTS: KEEPING TRACK OF THE MAGMATIC-HYDROTHERMAL TRANSITION IN THE LAND’S END GRANITIC COMPLEX, SW ENGLAND

The Land’s End granite of the Cornubian Batholith, SW England displays a range of syn- and post-magmatic textures and structures on the outcrop to micro scale. A megacrystic coarse-grained granite (CGG), comprising most exposed outcrops, are intruded by pulses of lesser porphyritic granites and more evolved fine-grained granites (FGG). Flow textures of K-feldspar megacrysts and distinct chemical differences between matrix and porphyry quartz indicate a two-stage crystallization, where the pheno- and megacrysts crystallized at a deeper setting than the matrix. The arrangement of K-feldspar megacrysts vary from wavy and dense to a gradual decrease in both size and frequency representing different styles of magma recharge of coarse-grained granites. The FGG postdates the CGG but K-feldspar phenocryst protruding into the FGG, and enclaves of partially dissolved CGG in the FGG, indicate that the CGG was not fully crystallized when the fine-grained granite intruded. Late- and post-magmatic textures are represented by various quartz-tourmaline aggregations. In the CGG, quartz-tourmaline orbicules, interpreted to have formed from immiscible borosilicate liquids, are unevenly distributed. In the fine-grained granite, sheets and dykes of quartz and tourmaline are formed both by diffusion limited growth and localized hydrothermal replacement. In the examples formed by diffusion limited growth, tourmaline crystallized as rosettes evenly distributed in the center of the fine-grained granite sheets, and towards the contact to the coarse-grained granite the granite is near tourmaline free where 5-20 cm wide quartz-tourmaline aggregations crystallized close to the contact. These can be followed on and off for 100s of meters. Occasionally, these structures grade into miarolitic cavities, representing the full transition into an aqueous-driven system. Quartz-tourmaline (+/- K-feldspar) formed by hydrothermal replacement can be distinguished from the igneous varieties by the mineral chemistry. The Land’s End granite provides a field laboratory that excellently exposes the magmatic-hydrothermal transition, and careful assessment of the field relations in combination with mineral chemistry and textures has led us to a better understanding of this enigmatic transition