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Allen J. Dennis and Others
graphic examination of the undeformed diorite shows igneous
textures and mineralogy is preserved (Fig. 14). Every
indication is that this rock is a post-metamorphic pluton.
Dennis and Wright (1993, 1996) dated three size fractions
of zircon from the diorite at this site by the U-Pb
method and interpreted the resulting upper intercept at
535±4 Ma as a crystallization age. Comparing this age with
that reported by Dennis and Wright (1993, 1996) for Mean
Crossroads metadiorite-diorite gneiss, one concludes that the
metamorphic fabric observed in these rocks of the Charlotte
belt must be older than 535±4 Ma. The Acado-Baltic fossils
from the Carolina slate belt reported by Samson and others
(1990) are middle Cambrian in age. Thus, we conclude that
the metamorphic fabric observed here did not form as a consequence
of collision of the Carolina arc with Laurentia, but
must record events on the fringes of Gondwana (Dennis and
Wright, 1995). Jim Hibbard and Scott Samson (1995) have
correlated this deformation with the Virgilina orogeny of
Glover and Sinha (1973).
STOP 7: MEAN CROSSROADS METADIORITE/
DIORITE GNEISS
LOCATION
: The outcrops are located on dirt tracks leading north
and west around the prominent hill located 0.2 mile west of
the S-42-12 bridge over Fairforest Creek. (Fig. 9, Jonesville
quadrangle)
DESCRIPTION
Loose boulders and cobbles of metadiorite and diorite
gneiss of the Mean Crossroads complex (Dennis and Shervais,
1991) litter the slopes of this prominent hill. Some samples
show little fabric at all with randomly oriented coarse
hornblende needles, others have a mylonitic foliation. What
is common to all samples is the ubiquitous development of
epidote at the expense of plagioclase (Fig. 15). This is interpreted
to indicate intrusion of the Mean Crossroads diorite
relatively late in the late Precambrian-early Cambrian orogenic
event, when rather than a zone of broad, distributed
strain, deformation was restricted to narrow bands irregularly
distributed through a pluton that was intruded into a
semicontinuously deforming zone.
Dennis and Wright (1993, 1996) dated four size fractions
of zircon from this site by the U-Pb method. The data
plot in a highly linear array with an upper intercept at 538±5
Ma, interpreted to be the crystallization age of the diorite and
the age of the Mean Crossroads complex.
The Mean Crossroads complex was identified by Dennis
(1988) and described by Dennis and Shervais (1991,
1995). The Mean Crossroads complex is zoned intrusive
complex that has a hornblendite-clinopyroxenite core, surrounded
by varieties of gabbro, and mantled by a thick rind
of diorite. This entire package was metamorphosed at greenschist
to lower amphibolite facies. It is believed that at least
some of the mafic metavolcanic rocks in this area were
derived from the Mean Crossroads complex because
ankaramite and ultramafic dikes crosscut the plutonic rocks.
Dennis (1988) and Dennis and Shervais (1991, 1992, 1995)
argue that the Mean Crossroads complex, Wildcat Branch
complex, the Hammett Grove Meta-Igneous Suite, York-
Chester mafic-ultramafic complex, Davie County Complex
(NC), Latimer (SC)-Nancy Hart (GA) complex, and Berner
mafic complex represent loci of arc-rifting along the western
edge of the Carolina arc while the arc was in a periGondwanide
position. Probably the Caswell County complex
described by Wilkins and others (1995) also belongs in this
group. This arc-rifting occurred in the western Carolina terrane
at the same time or slightly younger than eruption of
Persimmon Fork Formation, Lincolnton metadacite, Uwharrie
Formation, ca. 550 Ma (Dennis, 1995; Dennis and Shervais,
1995).
STOP 8: BUFFALO GABBRO/SYENITE CON-
TACT
LOCATION
The outcrops are located on S-42-210, 0.7 mile south of
the intersection with S-42-33. (Fig. 16, Cross Anchor quadrangle)
DESCRIPTION
These road cuts and boulders in the adjacent woods
show the Buffalo gabbro and the associated felsic rocks
mapped on its south margin, and the contact between these
two rock types (Medlin, 1966; Medlin and others, 1972).
The gabbro here is medium to coarse-grained and black in
color. In fact, the mafic rocks of the Buffalo gabbro grade in
no apparently systematic way from gabbro to norite (Medlin
and others, 1972). Medlin and others (1972) report modal
percentages of olivine from 0-20.9% (norites) and 0-23%
(gabbros); orthopyroxene from 12-42.2% (norites) and 0.6-
21.9% (gabbros); and clinopyroxene from 7.2-36.3%
(norites) and 7.8-58% (gabbros). Medlin and others (1972)
report 8 olivine, 3 orthopyroxene, and 6 clinopyroxene mineral
analyses: olivines range from Fo68-Fo80.6; orthopyroxenes
from En75-En78; and, clinopyroxenes from
Wo41.5-En44.5 to Wo44En41.
The felsic rocks are bluish-grey with bluish potassium
feldspar phenocrysts. Medlin (1966) terms these rocks
“biotite quartz monzonite,” and Willis (1984) calls them
“syenodiorite.” Two modal analyses (Medlin and others,
1972) plotted on the 1976 IUGS classification scheme
(Streckeisen, 1976) yield points in the quartz monzonite
(QAP -> 14-43-43) and quartz syenite (9-62-29) fields Medlin
and others (1972) did not think that there was a necessarily
a petrogenetic or magmatic relation between the felsic
and mafic rocks, but without explicitly establishing the rela-
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