MOUNT-JOHNSON, QUEBEC - EXAMPLE OF SILICATE-LIQUID IMMISCIBILITY

Mount Johnson, a member of the Monteregian Hills petrographic province in Quebec, is a cylindrical, pluglike intrusion in which the silicic peripheral unit (pulaskite) appears to have been emplaced prior to the more mafic transition and core units (essexite). Liquid immiscibility has been invoked to explain this emplacement sequence. The partitioning of trace elements between ocelli-matrix pairs (which have been shown to be the result of liquid immiscibility) from two Monteregian sills is compared to the partitioning of the same elements between peripheral-unit rocks and transition-unit rocks at Mount Johnson. In general, the low-field strength elements are relatively enriched in the silicic material and the high-field strength elements are relatively enriched in the mafic material. The distribution of the trace elements is quite similar for the sills and Mount Johnson. It is concluded that two magmas were involved in the formation of Mount Johnson. The first magma split into two immiscible liquids to form the peripheral-unit and transition-unit rocks, and the second magma was subsequently intruded up the center of the conduit to form the core-unit rocks. © 1979 Geological Society of America.