274 D35. Petrology, Igneous (Posters)

Tuesday, 24 October 2017
9:00 AM - 6:30 PM
Halls 4EF

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

Paper number: 274-18
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-303897

© Copyright 2017 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.