PETROGENESIS OF THE CONCORD SYENITE: PARTIAL MELTING OF AN ALKALIC GABBRO

The Concord Syenite Ring Dike and the adjacent Concord Gabbro (CG) have been the focus of extensive geological study for more than sixty years. The studies to date have suggested that the quartz- bearing syenite was produced from the gabbro by extensive (~95%) fractional crystallization (FC). Close examination of these previous FC models reveals major inconsistencies with both the modal mineralogy and the major and trace element abundances of the proposed parent/daughter melts. Our analysis uses new and existing major and trace element data and a newly developed dynamic fractionation calculation scheme, which mimics the incremental chemical and mineralogical changes that take place during fractionation. The results clearly demonstrate that FC of a melt similar to that of the gabbro cannot produce a silica oversaturated syenitic composition.

Our petrogenic modeling instead indicates that the Concord Syenite was produced by the partial melting of an alkalic gabbro, similar to that of the CG. Approximately 10% partial melting of an enriched gabbro, similar to that of the CG will reproduce the major and trace element abundances of the syenite. A partial melting model for the petrogenesis of the syenite also explains the lack of intermediate rocks observed at this complex.

In order to better understand the crystallization history of the ring dike, petrographic observations are also combined with cumulate elemental modeling to evaluate the amount and chemical composition of the intercumulus (IC) liquid within the syenitic intrusion. The results of the modeling demonstrate the IC liquid is a syenitic melt that varies in volume from 5 to 85 %. The textures observed in the Concord Syenite range from porphyritic to adcumulate. The high abundance of antiperthite and perthite cumulates lower the bulk silica content of the syenite, which explains its nepheline-normative nature. The IC liquid, however, is silica saturated, which led to post-cumulus production of myrmekitic quartz in the IC plagioclase.

Furthermore, a partial melting model also explains the formation of the ring dike. This melt, with its variable cumulate/IC mush, was intruded around a subsiding gabbroic pluton at a depth of 12-15 km’s. The intrusive process led to the formation of the observed ring dike structure.