THE GEOCHEMISTRY AND SIGNIFICANCE OF PLUGS INTRUDING THE TERTIARY MULL-MORVERN LAVA SUCCESSION, WESTERN SCOTLAND

The Tertiary plugs of north Mull and Morvern, can be divided into thre e broad groups; (1) trachytes and mugearites; (2) diorites and tholeii tic dolerites with < 6 wt. % MgO and; (3) predominantly tholeiitic dol erites with < 6 wt. % MgO. Two trachytic trends are discernible, a low -iron and a high-iron trend. However, only the high-iron trend is foun d within the Tertiary Mull lava sequence. The origin of these two diff erentiation trends may be related to the composition of the primary ma gma, with more tholeiitic magmas fractionating to produce the lower-ir on trachytes. The more MgO-rich plugs are invariably tholeiitic and ar e chemically unlike any of the preserved lavas within the Mull success ion. The absence of this composition and of the low-iron trachyte comp ositions within the lava succession suggests that these magmas were po ssibly erupted near the top of the lava pile and have since been erode d away. The plugs may have acted as substantial feeders of the lava su ccession during its later stages of formation, while in the earlier st ages the lavas were probably erupted from fissures. The absence of plu gs associated with the Skye lavas, contrasts with the Mull and Antrim lava fields, and may be due to the more-intermittent nature of the Ter tiary volcanic activity on Skye.