gb 1978.book - Carolina Geological Society

PETROLOGY OF THE GREENWOOD PLUTON
pluton making the intrusion appear concordant. The forceful
injection of magma appears to be the major mechanism of
emplacement.
A gravity survey of the Greenwood area is nearly complete.
Gravity stations are on a one mile spacing dependent
upon access. A 20 milligal Bouger anomaly marks the
Greenwood gabbro. Further analysis of data should provide
a better indication of the shape of the intrusive body than the
poor exposure permits.
PETROGRAPHY
Microscopic examination of rock samples of the Greenwood
pluton reveals that plagioclase, olivine, augite, hypersthene,
and hornblende are the major primary minerals.
Biotite, magnetite, pyrite, apatite, and quartz are also
present. Alteration of varying intensity has produced chlorite,
serpentine, epidote, and sericite.
Point counting has been completed on selected samples from
within the pluton following the procedures outlined by
Chayes (1956). The major rock types identified are diorite,
gabbro, and olivine gabbro. (Table 1).
TABLE 1. Modal analysis data for Greenwood pluton rocks.
Values given are in volume percent.
Diorite Gabbro Olivine Gabbro
Plagioclase 51.0 50.9 62.4
Augite 5.9 20.3 13.3
Hypersthene 1.7 8.3 1.9
Olivine - 11.0 17.5
Magnetite 3.9 2.7 -
Quartz 1.6 - -
Biotite 2.9 4.0 0.5
Hornblende 25.7 2.7 1.2
Epidote 5.0 - -
Chlorite 2.2 Trace Trace
Apatite Trace - -
Serpentine - Trace -
99.9 99.9 99.8
The rocks of the Greenwood pluton are medium-grained
and display considerable textural variation including hypidiomorphic-granular,
ophitic, subophitic as well as porphyritic
textures. Reaction rims are obvious in many thin sections;
multiple rims are developed as predicted by Bowen’s reaction
series.
Plagioclase normally occurs as subhedral laths with
anorthite content ranges from 25 to 59 percent. Alteration of
plagioclase is a common occurrence. Sausuritization yielding
epidote and sericite is the most frequent alteration.
Subhedral grains of olivine reaching 4 mm in diameter
occur in the olivine gabbros. Alteration of olivine has produced
distinct grains and dendritic networks of magnetite
and serpentine. Hypersthene reaction rims often surround
olivine crystals.
The most abundant mafic mineral in the gabbroic rocks
is subhedral-anhedral augite. Alteration of augite generally
yields epidote, chlorite, and minor calcite or hornblende.
Hypersthene occurs as subhedral to anhedral crystals or
as coronas surrounding polygonal olivine grains. Strongly
pleochroic red coronas are found in samples of high modal
olivine content. Alteration of hypersthene produces a uralite
or chlorite.
Hornblende is the most abundant mafic mineral in the
dioritic rocks of the Greenwood pluton. It occurs as primary
subhedral grains with strong pleochroism (green-yellow
brown-brown) or as reaction rims enclosing grains of augite,
hypersthene, or magnetite.
REFERENCES CITED
Butler, J.R. and Ragland, P.C., 1969, A petrochemical survey of
plutonic intrusions in the Piedmont, southeastern Appalachians,
U.S.A.: Contr. Mineral. and Petrol., v. 24, p. 164-190.
Chalcraft, R.G., 1970, Petrography and geophysics of the Rock Hill
gabbro pluton, York County, South Carolina: Southeastern
Geol., v. 11, no. 3, p. 153-171.
Chalcraft, R.G., 1977, A petrographic study of the Ogden pluton,
York and Chester counties, South Carolina: Southeastern Section
Geol. Soc. Amer., Abstracts with Programs, v. 9. No. 2, p.
128.
Chayes, F., 1956, Petrographic Modal Analysis: New York, John
Wiley & Sons, 113 p.
Hermes, O.D., 1968, Petrology of the Mecklenburg gabbro-metagabbro
complex, North Carolina: Contr. Mineral. And Petrol.,
v. 18, p. 270-294.
McCauley, J.F., 1960, A preliminary report on the gabbros of Newberry
County, South Carolina: Div. of Geol., South Carolina
State Devel. Board, Geol. Notes, v. 5, p. 41-43.
McSween, H.Y., 1972, An investigation of the Dutchmans Creek
gabbro, Fairfield County, South Carolina: Div. of Geol., South
Carolina State Devel. Board, Geol. Notes, v. 16, no. 2, p. 19-42.
Medlin, J.H., 1969, Petrology of two mafic igneous complexes in
the South Carolina Piedmont: Southeastern Section, Geol. Soc.
Amer., Abstracts with Programs, v. 1, no. 4, p. 52.
Overstreet, W.C., and Bell, H., III, 1965, The crystalline rocks of
South Carolina: U.S. Geol. Surv. Bull. 1183, 126 p.
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