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  1. 1. THE GEOLOGY OF ROWAN COUNTY, CENTRAL CAROLINA TERRANE , N.C PRIVETT, Donald R., President, S.T.A.R. Environmental, 1 Circle Street, Great Falls, S.C. 29055, Rowan County in central North Carolina Piedmont has had a long history of sedimentation, volcanism, terrane accretation and at least three intrusive episodes. Most of the county is Carolina Terrane; about 15% at the eastern boundary is Carolina Slate Belt. The western county boundary is very close to the IP boundary and Cats Square terrane. Phyllite, metasandstones and metavolcanics are folded into a plunging syncline east of the Gold Hill - Silver Hill faults. The oldest and most wide spread rocks in the Charlotte Belt are early Paleozoic amphibolites and granitic gneiss cut by metagranites and metadiorites, later gabbros, meta-adamellite, granites (Woodleaf and porphyritic Churchland and China Grove) and Jurassic diabase dikes and widespread laumonatization. The proportion of granite to diorite varies from a few granite dikes cutting the diorite; in other exposures the granite is so abundant that it produces a complex of diorite xenoliths. Areas of predominately diorite are shown separately on the geologic map. The complex is deeply weathered; exposures of unweathered diorite are found only in streams, unjointed residual boulders and in deep roadcuts & quarries. The diorites are medium- grained hypidiomorphic-granular and are composed of approximately 45-60% subhedral plagioclase (0.3-3.0 mm), 4O-45% hornblende (0.5-2.5 mm) and 3- 15% quartz with accessory biotite (1.0.-2.0 mm), magnetite, pyrite, and sphene and secondary epidote, calcite and chlorite. These older metadiorites and metagranites cut by younger post- metamorphic gabbro stocks probably 400 m.y.o. A large positive gravity anomaly over the Cleveland gabbro suggests a deep seated intrusion as interpreted from the gravity map (Mann, 1964).The rocks of the intrusion vary from olivine gabbro to hornblende gabbro. The rock is dark gray to black and is fine to medium grained; subophitic to hypidiomorphic granular textures and are composed of plagloclase (An55 average) (0.4 -5.0 mm), hypersthene (0.5 - .0 mm), augite (1.0 -3.0 mm), hornblende, (up to 6 -7 mm~ mostly 0.2-3.0 mm), olivine (1.0 mm) and accessory biotite (2.0 mm), quartz, apatite, magnetite and spinel. The Woodleaf quarry exposes an excellent contact reaction zone between younger 293-+ 7 m.y.o. unmetamorphosed granite to quartz monzonite stock and host diorite. The fluid and intrusive nature of the granite is shown by abundant dikes and sills; diorite xenoliths and roof pendants. Flow around the larger diorite xenoliths is very common. Granite dikes cut
  2. 2. the xenoliths and flow aligned biotite schileren are common. Both the granite and diorite are cut by late stage vuggy quartz microcline veins containing abundant euhedral crystal of pyrite, quartz, epidote, chalcopyrite, molybdenite, calcite and zeolites The Woodleaf granite is medium-grained (0.5-3.0 mm) hypidiomorphic granular, containing microcline, (1.0-3 mm), quartz (3 mm) muscovite and plagioclase (1-4 mm), plus accessory biotite (0.7-1.4 mm), sphene, and secondary epidote (1.5-1.0 mm), chlorite, and calcite Two porphyritic alkali granite plutons characterized by large euhedral microcline phenocrysts are exposed in parts of Iredell, Rowan, Davie and Cabarrus Counties. The rock is holocrystalline porphyritic containing 10-50 percent euhedral microcline phenocrysts set in a groundmass of 5-20 percent biotite, 15-30 percent plagioclase, and 20-25 percent quartz. Accessory minerals include hornblende, magnetite, sphene, ilmenite, zoned zircon and secondary epidote, leucoxene, and chlorite. Small metamorphosed ultramafic rocks crop out poorly in the western part of the county on the Rowan Mills Quadrangle. The rock outcrops appear to be lenses and are composed of talc, chlorite, and tremolite The Salisbury shear zone is the name given to a 45 mile long, 2-5 mile wide zone of nearly striking N20-60 E., vertically dipping (80-85 NW), mylonite gneiss and ultramylonite schists and unsheared intrusive igneous and large blocks of-interdispersed metasedimentary relics. The shear zone appears to be a major internal tectonic feature in the Charlotte Terrane. The preservation of small unsheared relic blocks of granite and diorite in with ultramylonite schist, mylonite gneiss, protomylonites, and cataclasites provides the only evidence for the nature of the original rocks. Sheared rocks have undergone considerable recrystallization and neomineralization and fluxion structure. The shear zone is 2-5 miles wide and is 45 miles long extending from Cabarrus across Rowan and most of Davidson County into Guilford. Salisbury and Yadkin alkali granite plutons have been emplaced on the southeast and northwest sides of the shear zone. Smaller lens like bodies are also present. The Salisbury pluton most common texture is cataclastic. Full agar, (1971) described the age the pluton to be 410-+10 m.y Several 1 km. long ridges of quartzite crop out across Rowan County. These rocks are mostly quartz and muscovite but sometimes contain kyanite and sillimanite, they have lepidoblastic to porphyroblastic textures; quartz is broken and recrystallized with sutured contacts. Mica layers delineate chevron folds. The Carolina slate belt, is poorly exposed in the southeastern part of the county as a thick sequence of metamorphosed mafic to intermediate to felsic volcanic (predominantly calc-alkaline) and volcanoclastic rocks interbedded
  3. 3. with locally derived mudstone shales, siltstones, and argillites of late Precambrian to Cambrian and possibly Ordovician age, metamorphosed to the lower greenschist facies. They have been folded into a plunging anticline cut by a shear zone. The Gold-Hill Silver-Hill shear zone is a northeast trending 120 km. long, 3-7 km wide zone of phyllonites and unsheared rocks.