Diachronous and independent histories of plutonism and mineralization in the Cornubian Batholith, southwest England

Abstract

Concordant U-Pb dates of magmatic monazites in the dominant coarse-grained biotite (±muscovite) granites of the late Variscan Cornubian batholith demonstrate that the emplacement of the six main plutons was diachronous, ranging from 293.1 ±1.3 Ma (Carnmenellis) to 274.5 ± 1.4 Ma (Land's End, southern lobe). 40 Ar- 39 Ar age spectra for magmatic muscovites confirm that cooling to about 320 °C at the present level of exposure was accomplished within 4 to 5 Ma in each intrusion. Moreover, age spectra for hydrothermal muscovites show that development of the major Sn (-Cu) lodes of the South Crofty mine commenced before 286 Ma, only 2 to 3 Ma after cooling of the hosting Cam Brea granite, a satellite of the Carnmenellis pluton. The older plutons had therefore approached ambient temperatures, and had even experienced intense hydrothermal activity, before emplacement of the youngest.