40AR/39AR DATING CONSTRAINTS ON THE METAMORPHIC AND COOLING HISTORIES OF THE BALTIMORE COMPLEX SOUTHWEST, WEST AND NORTH OF BALTIMORE, MARYLAND

White mica (WM), phlogopite, biotite, and K-feldspar samples from rocks collected in three 6-23 km long transects across the Baltimore Complex were analyzed using ⁴⁰Ar/³⁹Ar age spectrum dating techniques. Three samples from the southern transect about 30 km SW of Baltimore appear to contain only one generation of WM in rocks that are in the garnet zone. The age spectra of these samples are interpreted to contain excess argon. Nonetheless, inverse isotope correlation analysis of the age spectrum data gives cooling ages (~350°C) that range from 325 to 299 Ma and become younger to the east.

WM cooling ages from the Prettyboy Schist and the Baltimore Gneiss in the middle transect, 15-38 km west of Baltimore, range from 330 to 302 Ma, and are again younger in the east. Biotites from these rocks appear to contain variable amounts of excess argon. K-feldspar age data are interpreted to represent cooling through argon closure (~250°C and ~150°C) and range from 316-287 Ma and 250-231 Ma respectively. These data suggest a moderate cooling rate for these rocks between ~350°C and ~250°C.

A single WM sample collected between the middle and northern transects in the Prettyboy Schist NW of Baltimore contains two generations of WM one of which grew in the lower greenschist facies below the closure temperature of WM. The age spectrum from this sample is sigmoidal in shape and is interpreted to represent both cooling of the sample through ~350°C at > 341 Ma and retrograde growth of WM at < 330 Ma.

All samples collected for WM from the Prettyboy schist in the northern transect 30 km north of Baltimore contain more than one generation of white mica, at least one of which grew below its closure temperature. All of these samples have complex age spectra that are consistent with cooling through ~350°C at > 420 Ma and below closure growth of WM at < 382 Ma. A single phlogopite sample from the Cockeysville Marble N of Baltimore yielded an inverse isotope correlation age of 364 Ma.

The results demonstrate diachronous cooling and subsequent growth of WM in the Baltimore Complex. White mica in the southern and middle transects cooled through its argon closure temperature in the Pennsylvanian, while samples from the northern traverse cooled through closure temperature at > 420 Ma with subsequent below closure temperature growth of WM at < 382 Ma likely during the Acadian orogeny.