GEOLOGIC HISTORY OF CHESTER COUNTY, CAROLINA TERRANE, SOUTH CAROLINA

The area is a composite infrastructural complex with a long history of sedimentation, volcanic arc collision, fragmentation and accretation, metamorphism, igneous intrusion, ductile shearing and faulting and at least three tectonic - intrusive episodes.

The county is 95 % Charlotte Belt (the west) and about 5% Carolina Slate Belt (CSB) at the eastern boundary.

The oldest and most widespread rocks are late Protorozoic or Cambrian amphibolites and granitic gneiss cut by metagranites and metadiorites. The rocks are complexly folded metamorphosed sedimentary and volcanics, now mica and biotite gneiss, schistose amphibolites, hornblende gneiss and quartzites. A metamorphosed granite dike complex, aplites, quartz veinlets and irregular pegmatites also cut these rocks. Low dipping faults offset nearly vertical aplite dikes and an older set of mafic dikes; right lateral movements of up to 15 meters are present.

Those rocks are host for the Great Falls metagranite, 543 +/- 63 Ma, a 100 km. sq. discordant stock, which is cut by aplite dikes, irregular pegmatites and vertical, faulted metamorphosed mafic dikes (12 cm. to 1 m. wide).

Near Chester and Fort Lawn four irregular stocks of younger (400 Ma) gabbro, diorite and metagabbro are outlined by magnetic highs and boulder float.

The Lowry's pluton, a leucocratic post-metamorphic adamellite (399 +/-4 Ma) to granite crops out in western Chester and southern York counties; it is porphyritic to equigranular, composed of 45% microcline, 5% plagioclase, 30 % quartz with accessory biotite, muscovite and epidote.

One northeast-striking, steeply dipping 4-5 m wide orbicular quartz diorite dike (5-15 % oval one cm orbicules) intrudes granite gneiss and metagranites, single-shell, dissimilar-cores are surrounded by radiating hornblende; the matrix is plagioclase, hornblende, biotite and epidote.

Northwest to N5E striking vertical Triassic/Jurassic diabase dikes cut all older rocks.

Narrow near vertical fracture filling and replacement laumontite veinlets cut most granitic rocks.

The CSB occurs in the NE part of the county, The Gold-Hill - Silver-Hill shear zone (400-385 Ma), phyllonites and unsheared greenschist facies metasedimentary and metavolcanics are exposed in the northeast area extending south from York county.