THE OCCURRENCE OF MICA IN THE CAMBRIAN GLENARM SERIES, BALTIMORE MARYLAND

Seven high grade metamorphic rocks were collected from the Cambrian Glenarm series north of Baltimore, MD. The samples include two carbonaceous gneisses, one amphibolite, two quartz schists and two pelitic schists. Common minerals observed in these samples include calcite, quartz, feldspars, biotite, muscovite, staurolite and sillimanite. Special attention is paid to the occurrence of micas. Except the amphibolite, mica existed in all other six samples.

Muscovite is only observed in associated with quartz. It is generally small (<0.5 mm) in size. But biotite is observed in other five samples and is varied in size from larger than 2 mm to smaller than 0.5 mm. According to the petrography, biotite could have three different occurrences: Type 1, biotite formed when the rock was in lower grade schist. Type 2: relict biotite that part of it was reacted into staurolite/sillimanite. And type 3: biotite which is a reaction product in the sillimanite zone.

Krol et al (1999) dated biotite and muscovite of a sample taken from Lock Raven Fm in the same area and found the biotite gave 40Ar/39Ar age significantly higher than that of coexisted muscovite and contributed it to excess Ar due to lower closing temperature. Observation in this study suggested an alternative interpretation that larger size biotite in some rocks of the Glenarm series may retain inherited Ar and would give older age than neocrystallized biotite in some other rocks.