EVIDENCE FOR CARBONIFEROUS THERMOCHEMICAL OVERPRINTING IN THE CHEVIOTCOMPLEX

Previous studies of the Lower Devonian lavas of the Cheviot Hills have shown them to contain an unusually complex remanent magnetization, an d many lavas have given Carboniferous K-Ar ages which are not compatib le with the geological evidence. Furthermore, the Cheviot volcanics di splay pronounced low temperature alteration, significant jointing and faulting, and a phase of post-volcanic siliceous veining. Further pala eomagnetic and radiometric studies have been carried out in order to e stablish whether or not the anomalous magnetic and isotopic results ar e related to the tectono-thermochemical processes and mineral alterati on. K-Ar analyses of clay mineral concentrates of lava and vein materi al from eight sites all give Carboniferous ages; the lava-derived clay ages cluster around 325 Ma, while the clays from veins have ages clos e to 300 Ma. Palaeomagnetic studies of the intrusive rocks also show e xtensive low temperature oxidation of the Fe-Ti oxides, even in the co mpact and subglassy varieties. Magnetization is typically multicompone nt (dual-polarity) and is no less complex than that observed in the la vas. Inferred single-component directions are consistent with those re cognized in the extensive Permo-Carboniferous intrusive sheets of the region (the 'Whin Sill'). It is concluded that Hercynian tectonic reac tivation in the Cheviot region led to chemical and mineralogical modif ication of all of the rock units examined, finding explicit expression in low temperature oxidation, remagnetization, isotopic setting (or r esetting) of the K-Ar system in clay minerals from the lavas, and the development (or rejuvenation) of siliceous veining in the SE of the Ch eviot massif.