TECTONIC IMPLICATIONS OF A NEW 450 MA MONAZITE DATE AND T/P DATA FROM THE MARTIC ZONE, PEQUEA "SILVER" MINE, SE PENNSYLVANIA

The complex Martic Zone of SE PA marks the shelf edge of the Cambro/Ordovician carbonate platform against deep-water sediments of the Octoraro seaway to the SE. The zone's history is: a) Ordovician transport across it of great allochthonous sheets from the old seaway, b) imbricate thrusting of edge and slope deposits, c) pervasive folding of these thrust sheets during regional nappe movements, and d) broad refolding of all these across the Tucquan/ Mine Ridge antiform in later Paleozoic time. The previously undated Pequea Silver Mine lies near the type Martic locality in S. Lancaster County. The deposit consists of a series of quartz / galena veins partly localized in hinge zones of the first set of folds; hence it is syn- or post-first folding. A 2 mm monazite crystal that grew contemporaneously in the cleavage of a galena crystal was collected from the Pequea district at 39 56' 27"N, 76 18' 29"W (shown on a color plate in “Mineralogy of Pennsylvania”). Dating by the University of Massachusetts Ultrachron electron microprobe resulted in a well-defined Th-U-total Pb date of 450 Ma +/- 4 (2 sigma count precision) with no sign of later overprinting. Primary aqueous H2O/CO2 fluid inclusions from an associated galena-bearing quartz vein yield Th = 215-265° C and a salinity of 0 wt. % NaCl. Secondary aqueous inclusions yield Th = 170-200° C with 10-15 wt. % NaCl. Using typical thermal gradients these values suggest depths ~ 10 km, i.e. by ~ 450 Ma the shelf-edge had become a mature mountain system. Fifty km to the NW (in today's distance but several 100 km then), the earliest signs of regional deformation are at ~ 459 Ma as dated by conodonts in downbowing of the local Myerstown basin. By Diplacanthograptus spiniferus time (450 Ma +/- 1 Ma), the final graptolites were being deposited in the youngest Martinsburg flysch that covers deep-water allochthons derived from S of the Martic Zone. Thus, at ~ 450 Ma even though alpine mountains near the shelf edge were near maturity, the distal foreland was still a sedimentary basin. In the following ~ 7 my the orogeny advanced from the SE and overran the foreland. The resulting mountains were eroded and by ~ 443 Ma their roots were being buried beneath the Silurian unconformity. Significant error bars exist on all these dates but it appears that the entire Taconian Orogeny in PA occupied only ~ 15 my.