The alteration petrology of the Cheviot granite

Abstract

The Cheviot Granite complex is a high-level pluton made up of six intrusive phases intruded into a pile of andesites of very similar composition to the bulk of the plutonics. The whole represents the eroded remnant of a lower Devonian volcano, erupted immediately after the continental collision that destroyed the Iapetus Ocean. The igneous history comprises two igneous cycles, both starting with basic granodiorite and including a late porphyritic ring dyke. The two cycles can be distinguished geochemically using Zr and K contents. Fractionation during each cycle involved precipitation of a biotite diorite cumulate. The second cycle ends with a highly evolved leucocratic microgranite. The geochemistry shows a rather shoshonitic - chemistry to the series, and trace elements are consistent with an immediately post-collisional origin. Hydrothermal alteration occurs in two phases, one associated in space and time with the porphyritic granodiorite of the first igneous cycle, and the other with that of the s~cond cycle. In both systems both potassic and sericitic alteration assemblages are found, and there is a wide development of propylitic alteration around these higher temperature zones. The two hydrothermal phases can be distinguished by the abundance of tourmaline in sericitic and propylitic rocks of the second cycle, and the abundance of calcium (mostly as calcite) in the sericitic rocks of the first phase. Geochemical flux calculations show that silica has been widely introduced to the granite during the alteration (in amounts up to 10\) and Ca and Sr removed. Other element fluxes are more complex, and may be coupled together. Comparison with other granites of N England and S Scotland shows that the Loch Doon complex and the Shap granite are very similar to Cheviot, that the Criffell Dalbeattie granite has a very similar early phase but diverges later, and that the Skiddaw, Weardale and Cairnsmore of Fleet granites are very different from Cheviot, being essentially granitic rather than granodioritic.