Geology and Gravity of the Lilesville Granite Batholith, North Carolina

JOHN D. WASKOM Department of Geology, Northwestern State University of Louisiana, Natchitocbes, Louisiana 71457

J. ROBERT BUTLER Department of Geology, Unit/ersity of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514

Geology and Gravity of the Lilesville Granite Batholith, North Carolina

named the Lilesville Granite, was exposed in ABSTRACT Anson and Richmond Counties, North The Lilesville Granite is the southernmost Carolina (Watson and Laney, 1906). Mann and granite intrusion of the Eastern Piedmont in Zablocki (1961, p. 198) reported a large posi- North Carolina. It is a sheet or tongue-shaped tive Bouguer gravity anomaly associated with concordant mass extending to a maximum the intrusion. Since granitic batholiths charac- depth of about 1.75 mi. It is surrounded by a teristically have a negative gravity anomaly contact aureole of metasedimentary mica schist (Bott, 1962), the situation seemed unusual. and mica gneiss and has an areal extent of 125 Furthermore, V. I. Mann (1963, oral com- sq mi. The granite is characterized by a porphy- mun.) found that the intrusion was the only ritic rapakivi texture with a medium- to coarse- large area of granite shown on the Geologic grained matrix of plagioclase, quartz, and Map of North Carolina (1958) that had a posi- biotite. The intrusion is compositionally zoned tive anomaly. Reconnaissance by H. D. Wa- and consists of adamellite, granodiorite, and gener (1962, oral commun.) disclosed that a tonalite. The granite is considered to have been small gabbro pluton was present in part of the emplaced as a crystal mush and to have crystal- area previously mapped as granite. lized in place. The modal variations and tex- The present report uses petrology and geo- tural features of the granite may be due to physics in an attempt to determine the three- crystal settling, contamination, and rapid late- dimensional configuration of the granite and stage crystallization (sudden escape of water gabbro intrusions, the nature and origin of the pressure). mineralogical variation in the granite, and the A large positive Bouguer anomaly associated geologic history of the region. with the granite is attributed to two major fea- The Lilesville Granite is one of a group of tures: (1) a gabbro body which intrudes the postmetamorphic granitic batholiths and stocks granite near its eastern margin, and (2) a mica located mainly in the Carolina Slate belt and gneiss unit which underlies the batholith. Charlotte belt (Butler and Ragland, 1969). The The proposed sequence of geologic events is: plutons are mostly adamellite and granodiorite, (1) deposition of felsic volcanic rocks and argil- and none studied so far has a significant propor- lites, (2) formation of the proposed anticline tion of closely related mafic rocks. and low-rank regional metamorphism, (3) in- The gabbro of the present study is one of trusion and crystallization of the Lilesville about 30 plutons in North and South Carolina Granite and the formation of the mica gneiss that comprise a gabbro-diorite-syenite differen- and mica schist units by thermal metamor- tiation suite (Butler and Ragland, 1969; Price, phism, (4) intrusion of gabbro body into the 1969). Lilesville Granite, (5) Triassic faulting and sedi- mentation, (6) intrusion of Triassic or Jurassic AREAL GEOLOGY dikes, and (7) deposition of Cretaceous and Tertiary sands and gravels. Field Procedure The previously existing geologic maps of the INTRODUCTION Lilesville Granite area did not indicate the gab- It was recognized more than sixty years ago bro unit or major dikes; therefore, an area of that a large granitic intrusion (Fig. 1), here about 325 sq mi was remapped during the sum-

Geological Society of America Bulletin, v. 82, p. 2827-2844, 12 figs., October 1971 2827

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-CRETACEOUS

kTRIASSIC

PRECAMBRIAN ? a PALEOZOIC SCOTLAND AFTER GEOLOGIC MAP OF NORTH CAROLINA (1958) \ Kbc V N

\

Feliic volcanic: \ Scalt of Miltt

Figure 1. Index map showing the geologic setting of the area studied.

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mers of 1963 and 1964. The collection of gran- eastward to a very dense bluish-gray phyllite, ite samples for the petrographic study on a grid which probably indicates an increase in the de- basis was not feasible for two reasons: (1) the gree of metamorphism from west to east. Lilesville Granite is deeply weathered over a The argillite along the eastern border of the large portion of the exposure, and (2) large Wadesboro basin has an average strike of N. patches of sand and gravel (Fig. 2) mask the 44° E. and dip 25° NW. (Randazzo, 1965, p. granite batholith. Consequently, 41 granite 12). Southeast of the Pee Dee River, the aver- samples were obtained to give a representative age strike of the argillite is N. 70° E. and dip coverage of the exposed batholith (Fig. 2), and 30° SE. A southwest-plunging anticline is in- additional samples were collected from other ferred on the basis of attitude of beds and loca- rock units. Waskom (1970) shows a location tion of older units (Fig. 2). The axial plane map for all samples collected in the present strikes about N. 58° E. This anticline is parallel study. to an anticline mapped by Floyd (1965) in the western part of Anson County and eastern Un- Field Descriptions and Relationships ion County, North Carolina, and is nearly par- The Lilesville Granite has an elliptical out- allel to the northeast trend of the Triassic basin. crop pattern with a total areal extent of approxi- The mica gneiss unit has an irregularly mately 125 sq mi (Fig. 2); an area of about 40 shaped outcrop pattern. It is gray, medium-to- sq mi of this total is covered by younger sedi- coarse textured, and consists of foliated biotite ments. gneiss and some sericite schist. The granite is prophyritic in texture and is The aureole is a narrow belt of rocks which medium-grained to very coarse-grained. The completely encircles the batholith except where dominant minerals are pink K-feldspar pheno- it is cut by Triassic faults (Fig. 2). This zone crysts in a matrix of light-gray plagioclase, col- consists of deeply weathered schist, phyllite, orless quartz, and biotite. In some outcrops, the and biotite gneiss. The outer portion of the long axes of the K-feldspar phenocrysts have a mica schist unit is gradational with the argillite preferred orientation in the horizontal plane. and felsic volcanic units. The contact between Muscovite and magnetite occur in minor the aureole and granite is generally sharp. Nu- amounts. Numerous mica gneiss xenoliths are merous quartz veins are present in the aureole found throughout the batholith. They are more but not in the granite. abundant near the outer margin of the batholith Where the aureole is exposed along the and near the contact adjacent to the mica gneiss northern, southern, and western edge of the unit. Granite appears to be chilled where it is batholith, thin to massive resistant bands of bio- in contact with large xenoliths near the mica tite gneiss strike parallel to the granite contact. gneiss unit. Xenoliths range from pea size to In hand specimen, the biotite gneiss is very small boulder size. similar in all respects to the mica gneiss unit. Three sets of vertical fractures occur in the The following interpretation is based on li- granite. One set has an average strike of N. thologic similarities between the mica gneiss 45 ° W., another set has an average strike of N. unit and the biotite gneiss zones within the au- 75° E., and a poorly developed set strikes reole, but few quantitative data are available. north-south. In most localities, the granite is The aureole represents both felsic volcanic and strongly fractured, particularly adjacent to the argillite rocks that were metamorphosed when gabbro intrusion and the margins of the bath- the granite batholith was emplaced. The mica olith. K-feldspar crystals in hand specimens col- gneiss unit and the zones of biotite gneiss in the lected at several of these localities are cut by aureole are therefore higher rank metamorphic numerous quartz veinlets. equivalents of the felsic volcanic unit. Also, the The metamorphic host rocks of the Lilesville mica schist and phyllite zones in the aureole are Granite are a felsic volcanic unit overlain by a the metamorphic equivalent of the argillite tuffaceous argillite unit. The volcanic unit is unit. mostly tuffs and flows(?) metamorphosed to In the eastern portion of the Lilesville Gran- phyllite and slate. Floyd (1965, p. 15) states ite, a gabbro intrusion is exposed over an area that the argillite is generally of low meta- of about 6 sq mi. This unit ranges in composi- morphic rank except in areas adjacent to the tion from a gabbro (central portion) to a diorite Gold Hill fault and intrusive bodies, where it is (outer portion). It is a massive, coarse-textured of medium metamorphic rank. In the northern rock composed of hornblende, olivine, plagio- portion of the study area, the argillite grades clase, pyroxene, and accessory minerals. Nu-

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merous rounded residual boulders were clays. Many large deposits of well-rounded observed. These boulders are relatively un- stream gravels (dominant) and some cobbles weathered, black to dark gray in color, and are present. pitted. The gabbro weathers to a relatively thick layer of red to brown residual soil. PETROLOGY The characteristics of the granite-gabbro con- tact were observed at several localities. The Analytical Procedure contact is generally very sharp and only a few Because of the coarse-grained texture of the feet wide. The dioritic rock at the contact may Lilesville Granite, stained slabs in addition to indicate assimilation of granite during the time thin sections were prepared for determination of emplacement. Xenoliths of granite are found of modal variation according to a procedure in the gabbro at the contact, indicating that the (Waskom, 1970) adapted from Bailey and Ste- gabbro intruded the granite. Along the western vens (I960). The slabs were mounted on a edge of the gabbro unit, just south of U.S. movable frame and covered with a sheet of Highway 74 (Fig. 2), rock is exposed which plastic on which a 2.5-mm grid had been in- grades into a chilled zone of more mafic fine- scribed. A binocular microscope was used to grained (marginal phase) gabbro. The granite count 1,000 to 1,500 points for each mode by marginal to the gabbro is veined with epidote identifying the grain that appeared at the inter- (probably deuteric) and quartz veinlets and is section of the grid lines. An appropriate area somewhat more mafic than that usually seen. At was used to assure an analytical error of less the southern border of the intrusion, gabbro than 2.45 percent (Chayes, 1956, p. 83). The dikes appear to have intruded the marginal reproducibility of modes from a single slab was zone of the granite; however, this is not conclu- calculated (coefficient of variation) to be 4 per- sive. cent. Numerous one 1- to 5-ft diabase dikes (not Modes determined from granite slabs are in- mapped) cut the granite batholith in its mar- cluded in Table 1 along with modes deter- ginal zone. Several larger dikes intrude the cen- mined from thin sections of the gabbro and tral portion (Fig. 2). These dikes range in width dike units. Standard point-count techniques from 10 ft to about 200 ft and vary greatly in were used to determine modes from these thin length. The largest of these dikes strikes ap- sections with an analytical error of less than 2 proximately N. 20° W. and extends for more percent (Chayes, 1956, p. 82). than 9 mi. It intrudes both the granite and mica gneiss unit. Modal and Textural Variations The Wadesboro basin, located in the north- The modal compositions shown on the K- western part of the area, has been studied by feldspar-plagioclase-quartz (Fig. 3) and the to- Randazzo (1965). The sedimentary sequence tal feldspar-mafics-quartz (Fig. 4) ternary exposed is Triassic in age and consists of con- diagrams are in good agreement with the mo- glomerates, fanglomerates, sandstones, silt- dal analyses of 260 thin sections of probable stones, and claystones. The Jonesboro fault magmatic granites from the eastern United which forms the southeastern border of the States (Chayes, 1951, p. 42). In the classifica- Wadesboro basin has an average strike of about tion of Peterson (1961, p. 35), the granitic N. 50° E. rocks range in composition from adamellite to The small Triassic basin just northeast of the granodiorite, with one specimen classified as batholith contains sedimentary rocks that are tonalite. similar in all respects to those described by Ran- Texture of the adamellite and granodiorite is dazzo in the Wadesboro basin. The western porphyritic hypautomorphic inequigranular. K- margin of this basin is faulted and is not be- feldspar occurs primarily as euhedral to anhe- lieved to be cut by any of the diabase dikes in dral phenocrysts ranging in size from 1 to 4 cm, the area. The strike of this fault is N. 45° E. and with the average size 2 cm. It also occurs as the average strike of the dikes is about N. small crystals (2 mm) in several specimens and 15° W. as interstitial grains between quartz grains. In Outliers of Coastal Plain deposits in the area many samples, the phenocrysts are highly frac- of study lie unconformably on the crystalline tured, and the fractures are filled with quartz. complex. The Coastal Plain sediments consist of Since no quartz veinlets were observed in Trias- cross-bedded, laminated, varicolored sands and sic sedimentary rocks or diabase, the veinlets

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LILESVILLE GRANITE B-6A 22.9 33.8 28.5 14.8 C-5A 22.7 34.8 29.9 12.6 C-6A 21.2 37.4 25.0 16.4 C-7A 29.4 34.6 21.9 14.1 C-7B 27.5 30.5 27.4 14.6 D-4B 33.2 32.8 20.6 13.4 D-4C 26.2 34.4 21.3 18.1 D-5A 24.9 40.4 21.1 13.6 D-6A* 25.9 35.5 21.2 17.4 D-6B 13.6 42.0 19.2 25.2 D-7A 19.2 41.3 23.7 15.8 E-3F 27.5 35.3 25.6 11.6 E-4C 25.6 33.8 27.7 12.9 E-5B 26.8 36.8 24.3 12.1 E-6A. 30.2 29.1 34.8 5.9 E-7B 11.2 43.1 24.6 21.1 F-3A 30.7 33.1 21.1 15.1 F-3C 27.4 36.4 21.7 14.5 F-3D 21.9 40.9 22.0 15.2 F-5A+ 2.3 45.7 21.4 30.6 F-6D 21.7 35.0 27.7 15.6 F-7A 16.7 32.9 30.5 19.9 G-4A 21.3 41.1 24.9 12.7 G-4B 23.4 32.7 29.6 14.3 G-4Ct 21.7 40.0 23.0 15.3 G-5A 16.1 40.5 23.4 20.0 G-6A* 26.9 36.7 18.6 17.8 G-7B 6.5 41.1 32.2 20.2 G-7C 29.2 28.2 28.4 14.2 G-7D 20.9 39.3 22.9 16.9 G-8A 25.2 38.7 20.7 15.4 6-8E 24.5 36.4 25.0 14.1 H-3A 31.0 31.6 25.8 11.6 H-4A, 30.3 29.3 22.3 18.1 H-6A 14.4 41.9 24.8 18.9 H-7A 42.4 26.4 24.7 6.5 I-4A 26.6 30.1 28.7 14.6 I-4D -3* 17.6 38.6 23.3 20.5 I-6D 19.5 38.0 25.2 17.3 I-6E 35.5 18.3 40.7 5.5 I-7B 15.8 35.9 30.1 18.2 Aver. 2T1 3578 25.2 T577 GABBRO UNIT H-5A 64.9 2.4 11.5 6.0 1.6 9.2 4.2 0.2 H-5B 57.4 1.1 12.9 6.2 5.8 8.8 7.3 0.5 H-5D 51.2 4.2 7.2 3.8 17.4 13.8 2.2 0.2 I-4C 53.4 2.4 13.4 4.4 7.0 12.4 7.0 0 I-5B 55.2 1.6 14.8 3.4 6.8 11.4 6.6 0.2 I-6B 58.8 1.8 16.4 5.1 5.5 5.8 6.1 0.5 I-6C 54.8 0.2 13.2 3.8 7.4 13.4 6.2 1.0 Aver. 5674 ~270 12.8 4.7 7.5 10.7 5.6 0.4 DOLERITE DIKES E-3A 52.7 19.0 24.9 2.1 1.3 E -4 61.1 14.4 20.4 2.9 1.2 F-3B 48.6 20.4 28.2 2.8 0 F-8A 60.8 9.8 28.5 0.9 0 F-8B 51.1 24.8 21.0 3.1 0 G-3A 48.6 27.2 22.5 1.7 0 G-7A 55.8 20.4 19.6 2.4 1.8 H-4B 53.6 19.0 24.0 1.9 1.5 H-5C 41.2 24.0 27.6 5.2 2.0 I-2B 56.0 19.2 21.4 1.9 1.5 I-2C 53.8 21.4 20.5 2.9 1.4 I-3B 45.1 30.7 15.3 3.7 5.2 J-3B 52.9 1.0 26.4 4.4 15.3 K-2A 59.1 3.9 22.7 3.6 10.7 Aver. BT3~ 18.5 23.1 "279 TT

Granodion'te Tonal He

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are thought to be pre-Triassic. Inclusions of plagioclase, quartz, and biotite are common in K-feldspar crystals. Approximately 60 percent of all stained slabs studied contain small xenoliths. Plagioclase phenocrysts are common throughout the batholith; however, most of the euhedral to anhedral crystals of plagioclase are considered part of the groundmass, which has an average grain size of about 6 mm. Rapakivi texture was observed in all but two specimens and has the following characteristics: 1. K-feldspar is 1.5 to 2.5 cm in size and oval in shape, with mantles of plagioclase. 2. Mantles range in thickness from 1 to 4 mm and are commonly anhedral toward the central K-feldspar core and have a euhedral exterior Figure 3. Point diagram of volume percent of K-feld- surface. Xenoliths with mantles of plagioclase spar (Kf), plagioclase (PI) and quartz (Qtz) in 41 stained slabs of the Lilesville Granite, North Carolina. are also common. 3. There is an occurrence of two groups (mantled and unmantled) of K-feldspar crys- tals. 4. Plagioclase is commonly arranged within K-feldspar phenocrysts as inclusions. Stewart (1956) suggests that this is also rapakivi tex- / \ ture. Quartz occurs as anhedral grains with an av- erage size of about 8 mm and is highly frac- tured. Quartz also occurs as inclusions in K-feldspar and plagioclase. The adamellite-granodiorite consists of mi- crocline, plagioclase, quartz, biotite, apatite, epidote, magnetite, and muscovite. Hematite, chlorite, pyrite, and sericite are common sec- ondary minerals. Microcline has well- developed grid twinning and is generally Figure 4. Point diagram of volume percent of quartz perthitic. Plagioclase (sodic oligoclase) is com- (Qtz), total feldspar (Kf+ P1) and color index (C.I.) in 41 monly zoned with centers slightly more calcic stained slabs of the Lilesville Granite, North Carolina. than the rims and shows some alteration to seri- cite. Curved cleavage planes in biotite are ap- parent in most thin sections. The pleochroic formula for biotite is: X = grayish yellow (5 Y Crystallization of the Lilesville Granite 8/4), Y or Z = olive gray (5 Y 3/2) or moder- The pattern of modal and textural variations ate olive brown (5 Y 4/4) (Goddard, 1963). can be explained by several different processes. Chlorite and epidote appear to be associated The present data are not sufficient for unique with biotite. Minute zircon crystals occur as in- solutions. The occurrence of large K-feldspar clusions in biotite surrounded by pleochroic phenocrysts (microcline) suggests that K-feld- haloes. Quartz has undulatory extinction and spar was the fastest growing of several minerals sutured contacts. Small veinlets of quartz are to crystallize during the cooling of the bath- common. A few grains of magnetite intergrown olith. The batholith may have been intruded as with ilmenite are present. a granodiorite magma consisting of a mush of Areal modal variation maps (Figs. 5 and 6) K-feldspar, quartz, and some plagioclase crys- indicate a distinctive series of northeast-south- tals. Uneven contamination by assimilation of west-trending highs and lows with a well- stoped felsic material is considered a possible developed inverse variation between K-feld- factor because of numerous xenoliths present spar and color index. throughout the batholith.

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According to Shaw (1965), crystal settling ence of fine-grained feldspar and quartz in the may be a major factor in the crystallization of matrix of the Lilesville Granite is possibly the granitic magmas. The "layer" of granodiorite result of rapid crystallization due to escape of in what are now the thinner parts of the bath- water (Tuttle and Bowen, 1958, p. 98) in the olith may have resulted from crystal settling of last stages of crystallization. mafic constituents. In the absence of convection An alternate possibility is that at least two in the magma chamber, crystallization would periods of intrusion occurred, one immediately probably produce a pattern of differentiation by after the other, resulting in a mixing of the crystal settling (Shaw, 1965, p. 150). This diffe- magma to give the indicated gradations. The rentiation would result in the more mafic com- first was an adamellite magma which resulted in positions near the margin and at depth. This is adamellite rock in the thicker and outer por- generally the case as indicated by plagioclase tions of the batholith. The second was a and color index in the central and southeastern granodiorite magma intruding the central and portions. southeastern portion of the batholith. Both in- Contamination of the magma might create a trusions would have to crystallize in such a way disequilibrium condition, making possible the as to result in the rapakivi texture. formation of rapakivi texture and zoned plagio- Regardless of the theory chosen for the crys- clase crystals. Stewart (1956, p. 313) suggests tallization of the Lilesville batholith, it is appar- that under disequilibrium conditions, the K- ent that the batholith was strongly fractured feldspar crystals will react with the liquid to after crystallization. This created favorable con- form plagioclase. This plagioclase will precipi- ditions for the passage of chemically laden tate as rapakivi mantles (resulting in observed fluids. It is possible that heat and fluids from the characteristics) on existing K-feldspar crystals gabbro intrusion resulted in partial metasoma- or as new centers of crystallization. The exist- tism of the granite batholith. The irregular mo-

POTASSIUM FELDSPAR

Variation Map

35° 00

EXPLANATION Contour Interval = 4 percent

4-^ Granite; \folume percent SCALE 3MILES 34° 50

"8 IP Figure 5. Potassium feldspar variation map.

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COLOR INDEX

Variation Map

3^00

EXPLANATION Granite, Volume percent

Contour Interval = 4 percent SCALE 0 _ I 2_ 3 MILES -34° 50

Figure 6. Color index variation map.

dal variation pattern may therefore partly ville Granite area (Table 1) indicate that the reflect metasomatic processes; however, the ob- mineralogy is similar to dikes in the Deep River served variation shows no consistent relation to basin (Fig. 7) described by Hermes (1964). the gabbro body. The mica gneiss unit located in the south- central portion of the Lilesville Granite (Fig. 2) Gabbro, Dike, and Mica Gneiss Units consists of plagioclase, quartz, biotite, musco- A detailed study of the gabbro unit was not vite, apatite, and chlorite. The average grain undertaken; however, modal data (Table 1) size in the groundmass is about 0.08 mm and is were obtained and the general characteristics of generally granoblastic in texture. A few grains the rock unit determined from seven thin sec- of mica porphyroblasts are as large as 4 mm. tions. Mica occurs as xenoblastic crystals, and a few In a typical thin section, the average grain discontinuous bands of quartz were observed. size is 1.5 mm, and the texture is hypauto- morphic seriate inequigranular. The gabbro GRAVITY consists of plagioclase (labradorite?), olivine, clinopyroxene, orthopyroxene, hornblende, General Statement apatite, biotite, antigorite, opaque minerals (il- A Worden gravimeter no. 121 having a sen- menite, magnetite), and a trace of muscovite. sitivity of 0.320 mgal per dial division was used Plagioclase is partly altered to sericite and is in conducting the gravity survey. Gravity sta- commonly zoned. Clinopyroxene is partly re- tions were located at the most accessible and placed by hornblende and hornblende by bio- accurately determined points along roads. tite. Olivine occurs as small rounded crystals These stations were located on U.S. Geological and is commonly rimmed by other minerals. Survey topographic maps north of an east-west Modes of the major diabase dikes in the Liles- line through the city of Morven, North

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Carolina, and on county base maps south of this TABLE 2. MEAN DENSITY OF MAJOR LITHOLOGIC UNITS line. Waskom (1970) shows the 260 established Lithologic Number Mean „ Range of Mean group unit of density density density gravity stations, the 100 previous gravity sta- samples gm/cc gm/cc gm/cc tions established by Zablocki (1959), and the type of elevation control. Granite 36 2.64 2 40-2.72 Intrusive Gabbro 7 2.98 2 91-3.03 igneous Dolerite 13 2.91 2 86-2.98 2.71 Gravity Survey Volcanic and Argil lite 5 2.61 2 40-2.81 metamorphic Field work was carried out using the loop Mica Gneiss 3 2.73 2 67-2.80 2.65 method. All base stations in Anson County were tied to a gravity station with a value of No attempt was made to correct for slight effects of 979.7554 gals (Mann, 1963) in the town of weathering. Pee Dee, North Carolina. Those in Richmond County were tied to a gravity station with a the sedimentary rocks of Triassic and Creta- value of 979.7539 gals, also established by ceous age were not determined. The effect of Mann, in the town of Rockingham, North the densities of these rocks is considered small Carolina. For the purpose of instrument drift with the exception of the Wadesboro Triassic control, the station at Pee Dee or that at Rock- basin in the northwestern portion of the study ingham, North Carolina, was occupied at the area and the small Triassic basin in the beginning and end of each day. northeastern portion (Fig. 2). Elevations for stations north of Morven, Standard data reductions were made for the North Carolina, are believed to be accurate effects of instrument drift, latitude, and total within 10 ft for all stations. The maximum error elevation (combined free-air and Bouguer in simple Bouguer reduction caused by error in effect). Due to moderate relief (Fig. 2, topo- elevation as much as 10 ft is 0.6 mgal. Eleva- graphic profile A-A') in the area, no terrain tions south of Morven were obtained exclu- corrections were made. It is estimated that a sively with a Paulin Altimeter. For the purpose maximum error of 0.5 mgal due to terrain may of altimeter drift control, a base station was be expected. A combined elevation correction reoccupied at least once every hour. The max- of 0.06 mgal per foot was used. This factor imum error in elevation due to altimeter drift includes a Bouguer correction for an infinite is 20 ft. Thus, the maximum error in gravity at slab of material of density 2.67 gm/cc. Latitude any one station due to error in elevation is 1.2 correction data were taken from the interna- mgal and is believed to be less than 0.6 mgal for tional formula (Nettleton, 1940, p. 137-143). most of these stations. Waskom (1970) lists the Bouguer anomaly Rock densities were determined on most of values to the nearest 0.10 mgal. A maximum the major rock types (Table 2). The densities of error of 0.3 mgal is estimated for instrument reading and drift combined, and a maximum error of 0.2 mgal is estimated for latitude cor- rection due to possible error in locating stations on the base map. These errors, combined with those previously discussed, give a maximum er- ror of 1.60 mgal in the Bouguer gravity values at any one station north of Morven, and 2.20 mgal at any one station south of Morven, North Carolina. Most stations are estimated to have a maximum error of less than 1.2 mgal. Any er- ror introduced by assuming an average density of 2.67 gm/cc is neglected. All gravity data are contoured on a 2 mgal interval. Because of the regular contour pattern and good agreement with the geology, such an interval is considered justified. The theoretical gravity was computed using Figure 7. Diagram showing variation in percentage of major minerals of dolerite dikes as compared with the two-dimensional graticule developed by Hermes (1964). Area outlined represents variation in Hubbert (1948). End effects were not applied percentage of major minerals indicated by Hermes. in computing the gravity profiles.

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Trend Surface Analysis physical data. The quintic residual is used for the purpose of gravity interpretation. The purpose of the trend surface analysis was Gravity Analysis to separate the "regional effect" or "trend" from local variations of the geophysical data in The Bouguer anomaly gravity map (Fig. 8) an objective manner. Krumbein and Graybill has a pronounced maximum "high" of +40 (1965, p. 321) define trend-surface analysis as mgal (12 to 14 mgal residual) over the Liles- follows: "A procedure by which each map ob- ville batholith. This large east-west-trending servation is divided into two or more parts: anomaly is not centered directly over the bath- some associated with the 'large-scale' systematic olith but is slightly offset to the south. The part changes that extend from one map edge to the of the anomaly greater than 30 mgal can be other, and the others associated with 'small- directly associated with the gabbro intrusion. scale' apparently non-systematic fluctuations The gabbro intrusion and the mica gneiss unit that are superimposed on the large-scale pat- that underlie the batholith (Fig. 2) are consid- terns." This definition and the term "trend" as ered responsible for such a large gravity max- discussed by Grant (1957, p. 310) are accepted imum over an otherwise felsic igneous for this paper. intrusion. A nonorthogonal (irregularly spaced points) Another large positive anomaly of +24 trend-surface analysis of all gravity data was mgal is located in the northern portion of the performed by H. G. Goodell. To separate the study area. This anomaly is associated with the relatively large-scale systematic changes in argillites which generally had higher density mapped data (the trend) from essentially non- than those south of the batholith. systematic small-scale variations due to local For the purpose of discussing the quintic effects (the residuals on the trend), a computer residual map (Fig. 9), it is assumed that the zero program for a least-square regression fit was isogal outlines the various residual anomalies. used (H. G. Goodell, 1964, personal com- The very close correlation between the residual mun.). This procedure consisted of fitting a anomalies and the geology of the area (Fig. 2) polynomial to the observed data up through the should be noted. Figure 10 shows the gravity fifth degree (quintic). A surface was computed quintic trend surface fitted to the observed which has the elements of the regional trend. data. Any deviations from this surface (residuals) are The largest anomaly (approximately +15 considered to be local effects. mgal) is associated with the gabbro unit. The The equation of the polynomial of fifth de- theoretical mass necessary to produce this gree used in the trend surface analysis is as anomaly (Fig. 11) along profile No. 7 (Fig. 9) follows: is calculated, using a method developed by Skeels (1963). According to this calculation, the maximum total depth to which the gabbro unit can extend is approximately 2.66 mi. The computed geologic interpretation (Fig. 11) of + A n this gabbro body, using a maximum density A X5 + A X4Y + A 3P + . . . etc. le 17 1 contrast (^p) of 0.34 gm/cc between gabbro and granite, is a thin slab which thins to the where values for A are coefficients; X and Y are north. A possible feeder tube with a maximum the geographic co-ordinates of the point; total depth of 2.66 mi is centered directly under Z' is the computed value of Z at that point. the gravity maximum. This gabbro body is in- Now R = (Z — Z') where Z is the data at the terpreted to be a tongue-shaped structure. The point (X,Y) and R is the residual of difference gravity effect of the mica gneiss with a density between the actual data Z and the computed contrast of 0.09 gm/cc (located south of the Z'. exposed gabbro unit) must be considered in The X,Y co-ordinates of all established order to satisfy the quintic residual anomaly. gravity stations used in the computer program The Jonesboro fault along the northwestern were determined with the origin at the south- margin of the batholith has little or no apparent western area shown on the map (Fig. 2). effect on the gravity anomaly present. Six Waskom (1970) lists the quintic computed gravity profiles (Fig. 9) were plotted (Fig. 11) surface values (Z') and the quintic residual val- at right angles to the contact aureole. The ues (observed Z — computed Z') for the geo- slope-angle calculations (Bott, 1962) for the

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BOUGUER ANOMALY GRAVITY MAP

- 34° 5!)

INDEX MAP

34° 50 SCALE I 2 3 MILES

CONTOUR INTERVAL Zmgal.

Figure 8. Bouguer anomaly gravity map.

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GRAVITY,

QUINTIC TREND RESIDUAL 35 00

] 34 55

34° 50 SCALE 0 I 2 3 MILES

CONTOUR INTERVAL 2mgol.

Figure 9. Gravity, quintic trend residual map.

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GRAVITY MAP

QUINTIC TREND SURFACE

35°00 FITTED TO OBSERVED DATA

34° 55

INDEX MAP

34° 50' SCALE I 2 3 MILES

CONTOUR INTERVAL 2mgal.

Figure 10. Gravity, quintic trend surface fitted to observed data.

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margin of the granite indicate steep outward margins of the batholith are believed due to the dips, with one exception. Profile No. 2 (Fig. less dense granitic rocks and represent the 11) suggests that the granite margin in this area thickest portions. Two theoretical gravity pro- slopes steeply inward (Fig. 2, profile B-B'). files (Fig. 12) were calculated assuming differ- The negative anomalies located within the ent density contrasts (Table 3). These gravity

SSE

F-l. 05/1.52-0.69 ASSUME A/" 0.34 THEN M-15.65/1.52 X 1.609 X 0.34 «18.8 FROM CHART FOR CIRCULAR ANOMALIES (SKEELS, 1963) SSW NNE N-0.28 PROFILE* 3 D, = 1.52 X 0.28 = 0.426 MILES (KNOWN DEPTH «0) Oil -3-4 "5.8 Di/Di = O.I6, R/Di«0.50.'. Di-0.426/O.I6=266MILES .'.R-2.66X0.50=1.33MILES (KNOWN R-I.85MILES) -4 Ag MAX.=6.65' COMPUTED GRAVITY NE sw.4 in PROFILE*4 2 QUINTIC ^RESIDUAL GRAVITY -j —2.8 -4

W =2-4 /K PROFILE*5 is THEORETICAL Df2.66MILES GEOLOGIC _4 INTERPRETATION 2>r NNW SSE PROFILE* 6

S«o Lavel

J-6 COMPUTED SLOPE-ANGLE CALCULATIONS GEOLOGIC FOR THE MARGIN OF THE GRANITE INTERPRETATION SLOPE PROFILE -*« 6"-* Z' Z2 ANGLE * 1 0.571 0.091 66° # 2 1.222 0.091 130° 3 2- 1012 HORIZONTAL SCALE IN MILES * 3 0.584 0.091 68° GRAVITY PROFILE*? AND INTERPRETATION * 4 LO 0.091 go- GEOLOGIC CROSS-SECTION * 5 1.0 O.O9I go" *6 0.414 0.091 56° (AFTER BOTT, 1962)

Figure 11. Gravity and interpretative geologic cross section and slope-angle calculations for the margin of the granite.

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profiles with their respective geologic interpre- maximum and minimum thickness for the gran- tations are self-explanatory. The mica gneiss is ite batholith and different shapes for the gabbro interpreted to be the metamorphic equivalent intrusion (Fig. 12). of the felsic volcanic rocks. The mica schist is From the gravity data, the following conclu- considered to be the metamorphic equivalent sions are made concerning the nature of the of the argillite. Lilesville batholith and its associated rock units: The assumed density contrast of —0.12 1. The margins of the batholith dip steeply gm/cc for the granite (profile 1) is somewhat outward except for the southeastern margin, large, and a density contrast of —0.06 gm/cc which dips steeply inward. (profile 11) is believed small. A density con- 2. The quintic residual indicates that the trast of —0.09 gm/cc would probably give a granite is thinnest in the central and southeast- more realistic interpretation. However, the ern portions. density contrasts assumed were selected to give 3. The batholith is sheet- or tongue-like, hav-

Computed Gravity for Profile H

Profile I

Profilell Figure 12. Gravity profile B-B' and interpretative geologic cross section; profiles I and II.

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ing an irregular floor and a maximum thickness TABLE 3. MEAN DENSITIES AND DENSITY CONTRASTS ASSUMED FOR THE CALCULATIONS OF GRAVITV PROFILES I AND II of about 1.75 mi in its northern portion. The mica gneiss unit is probably part of the floor of Li thologic Mean Density the intrusion rather than a roof pendant. unit density contrast 4. The gabbro unit is asymmetrical to the gm/cc gm/cc south. It is a tongue-shaped body and thins to the north and west. It has a probable thickness PROFILE I Gabbro (gb) range of 0.62 to 1.50 mi. 2.98 +0.22 Country Rock 2.76* 5. A southwest-plunging anticline is inter- (mgn and msh) -0.12 preted in the central portion of the batholith. Granite (gr) 2.64

SEQUENCE OF GEOLOGIC EVENTS PROFILE II Gabbro (gb) Based on field relationships, structural data, 2.98 +0.34 Granite (gr) and petrography, the following sequence of 2.64 -0.06 Country Rock 2.70 events is indicated: (mgn and msh} 1. Accumulation of a thick sequence of felsic volcanic and argillaceous rocks (late Precam- Mica schist (Daly and others, 1966, p. 26), brian - Cambrian). The rocks into which the Lilesville Granite intruded are probably strati- criteria of Buddington (1959, p. 677-680), em- graphically lower than metarhyolites 30 mi to placed in the epizone (less than about 4 to 6 mi the north, which yielded a Rb-Sr whole-rock below the surface) along a major fold axis, isotopic age of 494 ^ 14 m.y. (Hills and But- forming a sheet or tongue-shaped partly con- ler, 1969). cordant structure about 2.4 mi thick. The intru- 2. Formation of the proposed anticline by sion is a porphyritic rapakivi adamellite- regional compression and low-rank regional granodiorite batholith. metamorphism (early to middle Paleozoic). The granite probably was emplaced as a crys- 3. Intrusion and crystallization of the Liles- tal mush. With few exceptions, granodiorite ville Granite batholith and the formation of dominates the thinner portions of the batholith. mica gneiss and mica schist units by thermal This places the granodiorite and the one sample metamorphism (late Mississippian to early of tonalite near the axial surface of the anticline. Pennsylvanian). The Lilesville intrusion is simi- Adamellite occurs downlimb where the bath- lar in general field relationships, petrography, olith is much thicker, and the K-feldspar con- and geochemistry to a batholith near Liberty tent tends to increase toward the thicker Hill, South Carolina, 60 mi southwest of Liles- portion of the batholith. This suggests that the ville (Butler and Ragland, 1969). The Rb-Sr batholith is compositionally zoned. whole-rock age for the Liberty Hill intrusion is The positive Bouguer anomaly associated 306 ± 18 m.y. (Fullagar, 1969), and the initial with the batholith can be explained by the pres- Sr87/Sr86 ratio is 0.7039 ± 0.0014, which ence of the gabbro unit and the mica gneiss unit suggests that the granitic magma was derived which underlies the central portion of the gran- from the lower crust or upper mantle (Faure ite along the axis of the anticline. and Hurley, 1963). The term batholith, as generally used, im- 4. Intrusion of the gabbro body and possible plies extension of the intrusion to considerable metasomatism (Pennsylvanian or Permian). depth. None of the field evidence indicated 5. Formation of Triassic faults (including that the Lilesville Granite was a thin body. How Jonesboro fault), resulting in the Wadesboro many other batholiths are shallow features? Triassic basin. We, of course, have no information on the 6. Intrusion of Late Triassic (or Jurassic) original configuration of the intrusion above dikes. the present erosion surface. Perhaps other simi- 7. Extensive erosion. lar intrusions in this region are cut at different 8. Deposition of Cretaceous and Tertiary levels by the present erosion surface and addi- sands and gravels. tional geophysical studies would help to define 9. Extensive erosion, resulting in the present the original shapes. surface. ACKNOWLEDGMENTS CONCLUSIONS The writers are grateful to Paul C. Ragland, The Lilesville Granite was, according to the University of North Carolina at Chapel Hill,

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for his helpful criticisms and suggestions. Spe- Abstracts with Programs for 1969, pt. 4, p. 26. cial acknowledgment goes to H. D. Wagener, Geologic Map of North Carolina, 1958, North The Citadel, for his field assistance and to G. R. Carolina Dept. Conserv. and Devel. Div. Min- eral Resources Map. MacCarthy, University of North Carolina at Goddard, E. N., 1963, Rock-color chart: New York, Chapel Hill, for his encouragement and assist- Geol. Soc. America. ance with the geophysical portion of this inves- Grant, F., 1957, A problem in the analysis of geo- tigation. The writers wish to express physical data: Geophysics, v. 22, p. 309-344. appreciation to H. G. Goodell, University of Hermes, O. D., 1964, A quantitative petrographic Virginia, and to Florida State University for study of dolerite in the Deep River basin, North furnishing the computer program and com- Carolina: Am. Mineralogist, v. 49, p. 1718- puter time which made the gravity trend-sur- 1729. face analysis possible. The writers are grateful Hills, F. A., and Butler, J. R., 1969, Rubidium- strontium dates for some rhyolites from the to C. Mugnier for the drafting of all maps and Carolina slate belt of the North Carolina Pied- figures. mont: Geol. Soc. America, Abs. for 1968, Spec. Funds for field work and the preparation of Paper 121, p. 445- petrographic thin sections were provided by Hubbert, M. K., 1948, A line-integral method of the Smith Fund of the University of North computing the gravimetric effects of two-dimen- Carolina and the Division of Mineral Resources sional masses: Geophysics, v. 13, p. 215-225. of the North Carolina Department of Conser- Krumbein, W. C., and Graybill, F. A., 1965, An vation and Development. introduction to statistical models in geology: New York, McGraw-Hill Book Co., Inc., 475 P. Mann, V. I., 1963, The force of gravity at selected localities in North Carolina: North Carolina REFERENCES CITED Dept. Conserv. and Devel. Div. Mineral Re- sources, Inf. Circ. 18, 19 P- Bailey, E. H., and Stevens, R. E., I960, Selected Mann, V. I., and Zablocki, F. S., 1961, Gravity staining of potassium-feldspar and plagioclase features of the Deep River-Wadesboro Triassic on rock slabs and thin sections: Am. Mineralo- basin in North Carolina: Southeastern Geology, gist, v. 45, p. 1020-1025. v. 2, p. 191-215. Bott, M.H.P., 1962, A simple criterion for interpret- Nettleton, L. L, 1940, Geophysical prospecting for ing negative gravity anomalies: Geophysics, v. oil: New York, McGraw-Hill Book Co., Inc., 27, p. 376-381. 444 p. Buddington, A. F., 1959, Granite emplacement: Peterson, D. W., 1961, Descriptive modal classifica- Geol. Soc. America Bull., v. 70, p. 671-748. tion of igneous rocks, Table 1 and 2: Geotimes, Butler, J. R., and Ragland, P. C., 1969, A petro- v. 5, p. 35. chemical survey of plutonic intrusions in the Price, Vaneaton, Jr., 1969, Distribution of trace ele- Piedmont, southeastern Appalachians, U.S.A.: ments in plutonic rocks of the southeastern Contr. Mineralogy and Petrology, v. 24, p. 164- Piedmont [Ph.D. dissert.]: Univ. North 190. Carolina at Chapel Hill, 87 p. Chayes, F., 1951, Modal composition of granites: Randazzo, A. F., 1965, The stratigraphy of the Carnegie Inst. Washington Year Book, v. 50, p. Wadesboro Triassic basin in North and South 41. Carolina [M.S. thesis]: Univ. North Carolina at 1956, Petrographic modal analysis: New York, Chapel Hill, 52 p. John Wiley & Sons, Inc., 113 p. Shaw, H. R., 1965, Comments on viscosity, crystal Daly, R. A., Manger, G. E., and Clark, S. P., 1966, settling, and convection in granitic magmas: Density of rocks, p. 20-26 in Clark, S. P., ed., Am. Jour. Sci., v. 263, p. 120-152. Handbook of physical constants: Geol. Soc. Skeels, D. C., 1963, An approximate solution of the America Mem. 97, 587 p. problem of maximum depth in gravity interpre- Faure, G., and Hurley, P. M., 1963, The isotopic tation: Geophysics, v. 28, p. 724-735. composition of strontium in oceanic and conti- Stewart, D. B., 1956, Rapakivi granite from eastern nental basalts: Application to the origin of igne- Penobscot Bay, Maine: Internal. Geol. Cong., ous rocks: Jour. Petrology, v. 4, p. 31-50. 20th, Mexico City, sec. 1 IA, p. 293-320. Floyd, E. O., 1965, Geology of ground-water re- Tuttle, O. F., and Bowen, N. L., 1958, Origin of sources of the Monroe area, North Carolina: granite in the light of experimental studies in North Carolina Dept. Water Resources the system NaAlSi3O8-KASi3O8-SiO2-H2O: Ground-Water Bull. no. 5, 109 p. Geol. Soc. America Mem. 74, 153p. Fullagar, P. D., 1969, Whole-rock rubidium-stron- Waskom.J. D., 1970, Geology and geophysics of the tium ages of the Liberty Hill pluton, S.C., and Lilesville Granite batholith, North Carolina the Salisbury pluton, N.C.: Geol. Soc. America, [Ph.D. dissert.]: Univ. North Carolina at

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Chapel Hill, 78 p. River-Wadesboro Triassic basin of North Watson, T. L, and Laney, F. B., 1906, The building Carolina [M.S. thesis]: Univ. North Carolina at and ornamental stones of North Carolina: Chapel Hill, 44 p. North Carolina Geol. Survey Bull., no. 2, p. 15-21. MANUSCRIPT RECEIVED BY THE SOCIETY MAY 11, 1970 Zablocki, F. S., 1959, A gravity study in the Deep REVISED MANUSCRIPT RECEIVED FEBRUARY 19, 1971

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