MAGMA MIXING IN THE OAK POINT FACIES OF THE DEER ISLE PLUTON, MAINE

The Devonian Deer Isle pluton is subdivided into two distinct facies: Oak Point (Opg) and Stonington (Sg) granites. Both granites are coarse-grained seriate, titanite and magnetite bearing, biotite granites that contain widespread plagioclase-mantled potassium feldspar and plagioclase with complex zoning. The Opg is recognized by its orange-red colored alkali feldspar, minor hornblende, abundant mafic enclaves, higher color index, and local occurrence of mafic schlieren. Feldspar and disc-shaped enclaves define a prominent NW strike-SW dip foliation. Fine-grained leucocratic and rare composite (mafic/felsic) dikes have random orientations. Rounded “pillow-shaped” fragments of mafic rocks are included within a felsic matrix. In contrast the Sg has salmon pink alkali feldspars, scant mafic enclaves, and rare hornblende. Fine-grained leucocratic granite dikes are common. Domains comprised of tightly packed feldspar phenocrysts occur locally. Major element variation diagrams (excluding the mafic component of the composite dikes) typically exhibit highly correlated (R > 0.9) linear arrays. Trace element abundances are more variable. The local occurrence of mafic and felsic cumulates (e.g., mafic schlieren and tightly packed feldspars) suggest a minor role for crystal liquid fractionation which may be reflected in trace element content. However, the exceptional linear arrays defined by major element data are best modeled by two component mixing. The composition of leucocratic dikes and felsic enclaves represent the high silica component whereas the mafic component is best approximated by the composition of the most primitive mafic enclaves. Episodic inputs of mafic magma, into a felsic magma chamber via dikes, ponded upon crystal cumulates (defined by the feldspar foliation) on the floor of the chamber. The mafic magma evolved by crystal fractionation until density contrasts between it and the overlying granitic magma were minimized and mixing could occur. Locally, large feldspar crystals were incorporated into the enclave producing magmas. Enclaves were transported to higher levels in the chamber by convective currents driven by the thermal input from the invading mafic magma.