CONCOMITANT MIXING AND DIFFUSION OF GRANITE WITH AN EVOLVING BASALT IN CONTROLLING THE COMPOSITION OF HYBRID MAGMAS

The Paleozoic Isle au Haut Igneous Complex, Maine (424 ± 1 Ma.), consists of an enclave-bearing granite, overlying a composite layered sequence of alternating gabbroic and dioritic units, which in turn overlies a thick and massive gabbroic unit, beneath which is an enclave-void granite. Field and textural relationships indicate that complex hybridization has taken place between the lower, massive gabbroic unit and the underlying granite while both were liquid or largely liquid. The base of the gabbro has chilled against the underlying granite, forming a composite zone of chilled basaltic pillows in a thin (<1 meter) hybrid matrix along the gabbro-granite contact. Petrography of the individual units shows varying degrees of disequilibrium. These include crystals with heterogeneous dissolution textures and mineral assemblages not expected in those units. The composition of the hybrid matrix does not conform to a process consistent with linear bulk mixing between the massive gabbro and the underlying granite. Instead, the hybrid compositions are usually skewed to one side or the other of an hypothetical linear bulk mixing line between the two proposed end-members. The skewed compositional trends observed in the Isle au Haut hybrids are due to two main processes. Fractional crystallization of a dry basalt produced evolved "phantom" end-member compositions. These end-member compositions mixed with the granite and produced hybrid compositions along a linear bulk mixing line between the granite and "phantom" end-members. Actual end-member compositions of evolved basalt remain either hidden interstitially in the, now, gabbro, or in the mixing with the granite to produce the granites.