NATURE AND TIMING OF MINERALISING FLUIDS DURING THE APPALACHIAN-CALEDONIAN OROGENY: EVIDENCE FROM THE SHAP AND WEARDALE GRANITES, ENGLAND

We present new fluid inclusion and Re-Os molybdenite data to constrain the magmatic-hydrothermal fluid and the timing of mineralization and by genetic association the minimum age of the Shap and Weardale intrusions of northern England. Fluids inclusions associated with the quartz-molybdenite bearing veins of the post Acadian (Emsian) cleavage Shap Granite show fluids were the result of boiling processes and possess salinities of 1-8 eq. wt. % NaCl and TH values of 275-448°C (to the liquid and vapour phases). A late-stage low TH (100-200°C) and low salinity (1-7 eq. wt. % NaCl) hydrothermal fluid was also recorded. A mean Re-Os molybdenite age of 405.4 ± 0.4 Ma (0.1% 2sigma, n = 2) records the timing of mineralization and also provides a minimum emplacement age for the Shap granite due to the robust nature of the Re-Os isotopic system in molybdenite. The Re-Os age is in excellent agreement with a Rb-Sr single plagioclase age (405 ± 2 Ma), however both are significantly older than previous U-Pb (390 ± 6 Ma), Rb-Sr (394 ± 3 Ma) and K-Ar (397 ± 7 Ma) ages, which likely represent disturbance to these isotopic systems. Fluid inclusions from a quartz-molybdenite bearing vein of the Weardale granodiorite indicate fluids of low salinity (0-8 eq wt % NaCl), with a wide range of TH values (140-425°C), with some evidence of boiling. A single Re-Os molybdenite analysis yields an age of 398 ± 1.6 Ma, which also provides a minimum determination of granite emplacement. Previous Rb-Sr isotopic ages are broadly similar (390 ± 8 Ma; 410 ± 10 Ma). The Re-Os ages of late phase Early Devonian magmatism of the Caledonian Orogeny in the English Lake District are similar to those of the Galway Granite, which forms a key component of the late phase of the Caledonian Orogeny in western Ireland. However, unlike the Galway Granite no evidence for carbonic fluids are recorded suggesting significant variations in the geochemical characteristics of the magmatic-hydrothermal fluid associated with molybdenite mineralization across the Caledonian Orogeny. The Re-Os ages propose a ~ 37 million year apparent quiescience of magmatism during the Caledonian Orogeny, with the Shap granite Re-Os molybdenite age suggesting that the country rock cleavage developed before ~ 405 Ma.