A PETROGRAPHIC STUDY OF UNION ROCK CORRELATING IT TO THE PEABODY GRANITE AND DEDHAM GRANODIORITE

Union Rock is a locally famous glacial erratic located in Lynn Woods Reservation, Lynn, Massachusetts. There is a chilled margin down the center of the erratic with different rock on either side. On the east side of the chilled margin is an extremely coarse grained rock commonly identified as the Peabody Granite. The west side is a finer-grained rock known as the Dedham Granodiorite. A petrographic study comparing Union Rock with other felsic to intermediate igneous rocks from the surrounding area included analysis of eight thin sections, each stained for potassium feldspar. The thin sections were analyzed for texture and mineralogy then point counted to determine rock type. Microscopic study of the thin sections reveals that the three most abundant minerals seen in the samples, and used to collect point counting data, are plagioclase, alkali feldspars, mainly orthoclase, and quartz. Other common minerals include hornblende and biotite sometimes found in enclaves suggesting a possible mixing of magmas during crystallization. Perthitic texture of alkali feldspars dominates most of the thin sections. Of the eight samples, three are alkali feldspar granites, three are granites, one is a quartz-rich granitoid, and the other a quartz monzonite. The plotted data from the ternary diagram indicates that the rock on the east side of the contact is petrologically comparable to the Peabody Granite and that on the west side of the contact is petrologically comparable to the unit called the Dedham Granodiorite, although the latter plots as granite. Future research will include the use of a Scanning Electron Microscope and Energy Dispersive X-ray microanalysis to perform chemical comparisons on four polished thin sections.