The Tectonomagmatic Evolution of the Land's End Granite

Abstract

The timing and kinematics of regional deformation play a key role in granite pluton construction, as well as controlling the distribution of evolved melts as precursors to magmatic-hydrothermal mineralisation. The Land’s End Granite pluton in the Permian Cornubian Batholith of South West England is an excellent area to assess the role of regional tectonics on pluton construction, magmatic-hydrothermal evolution, and distribution of mineralisation. The timing and association of regional tectonics and the batholith are well established, but the link between regional tectonics and the construction and evolution of individual plutons is poorly constrained. A comprehensive investigation in to the roof zone of the Land’s End Granite and associated aureole has been completed. Field-based collection of structural data and mapping was complemented by geometric and kinematic analysis, and petrographic, mineral-chemical, and geochronological studies. This study provides a clear case for the incremental emplacement of a laccolith-type pluton in a post-orogenic extensional tectonic setting. Results demonstrate a link between tectonic controls on magmatic evolution and the distribution of mineral deposits. Six deformation episodes associated with regional tectonic evolution are defined. D1 and D2 are associated with Variscan convergence and pre-date granite emplacement. Early ductile D3 pre-dates granite emplacement, and is associated with reactivation of Variscan thrusts and NW-SE-trending strike-slip faults during NNW-SSE extension. Continued crustal thinning brought about late-D3 moderately-steeply dipping ENE-WSW-striking extensional faults, coinciding with emplacement of the Land’s End Granite. The pluton was emplaced as a laccolith, constructed by the amalgamation of discrete intrusive episodes over a complex and protracted (c. 3 Ma) magmatic history. The Land’s End Granite formed as a result of the amalgamation of three smaller plutons. The older Zennor Lobe is composed of a coarse grained biotite granite carapace and is cross-cut by later medium- and fine-grained muscovite, Li-mica and tourmaline granites. The St Just Wedge intruded approximately 1.5 Ma after the Zennor Lobe, with the emplacement of Li-mica and tourmaline granites. ENE-WSW-striking faults controlled magma ascent, and a major sub-horizontal fault zone acted as a magma trap and controlled lateral migration. Granite emplacement was accommodated by roof uplift, and both tectonically- and magmatically-induced reactivation of ENE-WSW and NW-SE faults. Sn-Cu mineralisation has a close association with Li-mica and tourmaline granites, in particular the St Just Wedge. Devolatisation during crystallisation and upwards migration of volatiles were, in part, trapped in the apical parts of some granites causing the metasomatic alteration of crystal mushes and precipitation of hydrothermal minerals. Contemporary brittle deformation and pre-existing fracture networks controlled the migration of volatiles, mixing with meteoric fluids and the formation of mineralised veins. The change in orientation of mineralised veins from ENE-WSW in the east to NW-SE in the west is a function of the later emplacement age of the St Just Wedge and a change in tectonic regime, and reactivation of NW-SE faults.