TEXTURAL, MINERAL AND BULK CHEMICAL EVIDENCE FOR MAGMA MIXING AT ANDREWS POINT, CAPE ANN MASSACHUSETTS

At Andrews Point, Cape Ann Granite engulfs a large (decimeter) xenolith of Beverly Syenite (?) that contains angular to sub rounded blocks of basalt. The basalt blocks have been interpreted to represent an intrusive dike that pillowed as it intruded the still partially molten syenite. This interpretation, however has been challenged due to the angular shape and sharp contacts between the two members. This research looks at textural, mineral and bulk chemistries across these contacts to determine the nature of the relationship between these units. In outcrop, the contacts between the basalt and syenite are sharp but the basalt is injected by many small (mm) scale veins of felsic material. In thin section, the contacts show evidence for rafting of mm scale clusters of syenitic material into the adjacent basalt. These clusters or rafts contain rounded spongy orthoclase crystals with plagioclase overgrowths. The host basalt away from these syenitic rafts typically contains clinopyroxene, hornblende, plagioclase and opaque oxide minerals. Within mm of the syenitic rafts with the resorbed orthoclase, the basalt contains little clinopyroxene but abundant biotite and hornblende. The potassium released during the resorption of orthoclase feldspar may have driven the contaminated basalt to crystallize biotite in favor of clinopyroxene at the interface. The mm scale of these chemical interactions suggests that both the syenitic rafts and the host basalt were at their respective liquidus temperatues. The outcrop scale observation of veins of syenitic material injecting into the basalt also supports a liquid/crystal mush interaction. The thermally shocked basalt contracted and pillowed as it intruded into the syenite crystal mush causing extraction of syenite melt along with rafts of clusters of crystals into the basalt. These data strongly argue for a comingling of an injected basaltic liquid into a felsic crystal mush.