THERMOCHRONOLOGY OF THE CORNUBIAN BATHOLITH IN SOUTHWEST ENGLAND - IMPLICATIONS FOR PLUTON EMPLACEMENT AND PROTRACTED HYDROTHERMAL MINERALIZATION

The metalliferous ore deposits of southwest England are associated wit h biotite-muscovite granites that intruded upper Paleozoic sediments a nd volcanic rocks at the end of the Hercynian Orogeny. The hydrotherma l mineralization can be subdivided into four stages: (1) exoskams; (2) high-temperature tin and tungsten oxide-bearing sheeted greisen borde red veins and Sn-bearing tourmaline veins and breccias; (3) polymetall ic uartz-tourmaline-chlorite-sulfide-fluorite-bearing fissure veins, w hich represent the main episode of economic mineralization; and (4) la te-stage, low-temperature polymetallic fluorite veins. U-Pb dating of monazite and xenotime and Ar-40/Ar-39 dating of muscovite were used to determine emplacement ages and cooling times for individual plutons w ithin the Cornubian batholith, as well as separate intrusive phases wi thin the plutons. In addition, Ar-40/Ar-39 ages from hornblende and se condary muscovite and Sm-Nd isochron ages from fluorite were employed to determine the relationship between pluton emplacement and different stages of mineralization. The U-Pb ages indicate that granite magmati sm was protracted from approximately 300 Ma down to approximately 275 Ma with no evidence of a major hiatus. There is no systematic relation between the age of a pluton and its location within the batholith. Th e U-Pb ages for separate granite phases within a single pluton are res olvable and indicate that magma emplacement within individual plutons occurred over periods of as much as 4.5 myrs. Felsic porphyry dike emp lacement was coeval with plutonism, but continued to approximately 270 Ma. The geochronologic data suggest that the Cornubian batholith orig inated from repeated melting events over 30 myrs and was formed by a s eries of small coalescing granitic bodies. Cooling rates of the main p lutons are unrelated to emplacement age, but decrease from the southwe st to the northeast from approximately 210-degrees-C myr-1 to approxim ately 60-degrees-C myr-1 with a mean of 100-degrees-C myr-1. These slo w cooling rates appear to reflect the addition of heat from multiple i ntrusive episodes. The mineralization history is distinct for each plu ton and ranges from coeval with, to up to 40 myrs younger than the coo ling age for the host pluton. Stage 2 mineralization is broadly synchr onous with the emplacement of granite magmas, is dominated by fluids e xpelled during crystallization, and may be repeated by the emplacement of younger magmas within the same pluton. Sm-Nd isochrons for fluorit e from stage 3 polymetallic mineralization give ages of 259 +/- 7, 266 +/- 3 and 267 +/- 12 Ma, postdating stage 2 mineralization by up to 2 5 myrs within the same deposit. The similarity in age of the main poly metallic mineralization hosted by the oldest and youngest plutons, sug gests that this stage of mineralization is unlikely to be related to h ydrothermal circulation driven by the emplacement and cooling of the h ost granite. The mineralization is more likely the product of regional hydrothermal circulation driven by heat from the emplacement and crys tallization of younger buried pulses of magma.