Petrography of an unknown granitoid body in the Blue Ridge of northwest Georgia
Pearce, Darren, Columbus State University, Department of Earth and Space Science, Spring 2010
Abstract: Previous research in the Talladega belt and Eastern Blue Ridge of northwest Georgia has revealed the presence of two distinct metamorphosed granitoid bodies: the Mulberry Rock Gneiss and a group of unnamed metagranitoids with a distinctly different petrologic character. On the Yorkville 7.5’ quadrangle, an outcrop on U.S. Highway 278 contains a metagranitoid body identified by some researchers as being part of the Late Ordovician-Silurian Mulberry Rock Gneiss. Other researchers, however, have suggested that the rock belongs to the unnamed and undated metagranitoids of the Eastern Blue Ridge. My research suggests that this unnamed granitoid body may not be correlative with the Mulberry Rock Gneiss. First, shear fabrics along the margins of the granitoid suggest a fault may separate it from the adjacent rocks of the Talladega belt. Additionally, the presence of garnet in these rocks suggests that they may have been subjected to higher metamorphic conditions than the Mulberry Rock Gneiss. If the unnamed granitoid body is indeed in faulted contact with the surrounding rocks, with no distinct relationship to the Mulberry Rock Gneiss, this suggests that it provides no constraints on the age of the adjacent Talladega belt rocks. Using petrography to study the rocks in thin section, hand sample identification, and point-count analysis, I hope to determine if this granitoid body can be correlated with the Mulberry Rock Gneiss petrology, or if it resembles the numerous unnamed granitoid bodies within the region.
Stratigraphy Igneous Rock Classification Diagram, IUGS Talladega Group Faulted Relationship Talladega Group Intrusive Relationship
Jemison Chert/Erin Slate Mulberry Rock Gneiss Eastern Blue Ridge Alkali Feldspar Granite Butting Ram/Cheaha Quartzite
Lay Dam Formation
Mulberry Rock Gneiss Intrusion Possible Fault Contact
Unnamed Granitoid
Unnamed Granitoid
Petrography
Plagioclase Quartz Biotite K - Feldspar Biotite Muscovite Quartz Re-crystallized Quartz Quartz Potassium Feldspar Hornblende Quartz Muscovite Biotite K-Feldspar Quartz Biotite K -Feldspar Garnet
C-fabric
S-fabric
MULBERRY ROCK GNEISS GRANITE EASTERN BLUE RIDGE ALKALI FELDSPAR GRANITE A granodioritic gneiss of upper amphibolite facies consisting of 10% subhedral to euhedral This metagranitoid body consists of approximately 60 to 75% quartz some showing undulating extinction indicative of strain, and some This sample consists of approximately 25 - 30% subhedral to euhedral alkali feldspar, ranging from 0.25mm to hornblende ranging in size from 1mm to 5mm. Subhedral to euhedral quartz grains make up dynamic recrystallization and ranging in size from .25mm to 4mm. Subhedral to euhedral plagioclase and potassium feldspar grains range 3mm in size. Subhedral to euhedral quartz grains, some with sub-grain boundaries and undulating extinction, approximately ~ 50% and range in size from .01 to 4mm. Approximately 20% potassium in size from 0.1mm to 4mm in size and make up approximately 5% and 10% of the rock, respectively. Biotite is present in approximately 15 - range in size from 0.15mm to 3.5mm and make up approximately 35 - 40/% of the sample. Muscovite grains are feldspar in a subhedral to euhedral form are visible, possible perthite from exsolution lamellae 20% proportion ranging in size from .01mm to 1mm.. visible in approximately 10% concentration ranging in sizes from .10mm to 3mm. This granitoid also contains of plagioclase is present. Additionally, approximately 15% biotite is seen ranging in size from biotite in approximately 10% concentration and ranging in size from .01mm to 3mm. .01mm to 1mm and approximately 5% muscovite ranging in size from .01mm to 4mm.
Mulberry Rock Gneiss from Barineau and Pearce, 2010 “ The Mulberry Rock Gneiss, exposed in an ‘eyelid window’ southwest of Cartersville, Ga., has traditional been interpreted as Grenville basement (McConnell and Abrams, 1984; Higgins et al, 1988; Osberg et al, 1989; Higgins et al, 2003), and more recently as intruding the Lay Dam Formation (Crawford and Kath, 2008; Higgins and Crawford, 2008). The 420 to 460Ma Mulberry rock (Das, 2006; Holm-Denoma, 2006; Mueller, personal communication) is a “medium grained, slightly metaluminous, two-mica granite with sub equal amounts of quartz, Conclusions: plagioclase, and k-feldspar” with uncommon garnet ( Holm-Denoma, 2006) and lies at the boundary between the Unidentified • The unnamed granitoid studied in this research is petrographically and lithologically distinct from the Mulberry Rock Gneiss showing eastern and western Blue Ridge, in the footwall of the Altoona fault. Multiple workers ( Holm-Denoma, 2006; granitoid body both mineralogical and grain size differences. These differences indicate that it is not an intrusion of the Mulberry rock into the Lay Dam Higgins et al, 2003, Crawford and Kath, 2008) have mapped the type section of the Mulberry as being completely as suggested by previous researchers and therefore has no bearing on the age of the Lay Dam Formation. Also, the Eastern Blue Ridge fault-bounded in the vicinity of Yorkville, Ga. Crawford and Kath (2008) however, map a granitoid exposure on the consists of a variety of igneous and high-grade metamorphic rocks including many granite bodies of different lithology. This research recently constructed Rockmart highway ( Hwy 278 ) on the Yorkville 7.5’ quadrangle as intruding the adjacent indicates that this particular granitoid may be one such body. interlayered schists and metawackes of the Lay Dam Formation - and interpretation echoed by Higgins and Crawford (2008). This interpretation is complicated by the following: •The presence of garnet in this unnamed granitoid, if metamorphic in origin, would suggest it is at a higher metamorphic grade, probably amphibolite grade, than the adjacent Talladega belt. The Talladega belt is of lower greenschist facies falling below the garnet isograd • There are no exposures of the Mulberry Rock on the Yorkville 7.5’ quadrangle which would allow definitive supporting the idea that it is in faulted contact with the Lay Dam Formation and not an intrusion of the Mulberry Rock Gneiss. analysis of a fault vs. intrusive relationship between it and the surrounding metasedimentary rocks of the • S-C fabrics in this unnamed granitoid indicate high shear strain and further supports the idea that this granitoid is in faulted contact Talladega belt due to the scarcity of outcrop and deep weathering of exposed outcrops. To conclude that the Mulberry Rock Gneiss relationship between the Mulberry Rock Gneiss and the adjacent Talladega belt would necessitate an exposure with the adjacent Talladega belt and Lay Dam Formation possibly by the nearby Altoona fault. which clearly eliminates the possibility of faulted contact. • The granitoid mapped by Crawford and Kath (2008) as intruding the Lay Dam lies more than 2 miles west and north of the Mulberry Rock exposures which yielded the Late Ordovician to Silurian ages (Holm-Denoma, 2006)
and is found in a separate thrust panel than the dated Mulberry Rock Gneiss. References: • The granitoid interpreted as Mulberry Rock by Crawford and Kath (2008) is petrographically distinct from the Barineau, C.I., and Pearce, D., 2010, Age of the Lay Dam Formation, Talladega Belt, Southernmost Appalachians: Stratigraphic and Structural Constraints; Northeastern large exposure of Mulberry Rock to the east and south. Section (45th Annual) and Southeastern Section (59th Annual) Joint Meeting: Abstracts with programs - Geological Society of America, v. 42, no. 1: p. 93. Crawford, T.J., and Kath, R.L. , 2009, Geologic Map Illustrating the Stratigraphy and Structural Geology of the Yorkville, Georgia, 7.5 Minute Quadrangle, Geological Society • The granitoid interpreted as Mulberry Rock by Crawford and Kath (2008) contains garnet, which may suggest it of America, Abstracts with Programs, vol.41, pp. 12 is at a higher metamorphic grade than the adjacent lower greenschist facies rocks of the Talladega belt. Holm-Denoma, C. S. (2006). Characterization of Paleozoic terranes and terrane accretion at the southeastern margin of Laurentia Georgia and Alabama Appalachians. Additionally, the presence of shear fabrics on its western contact with the Lay Dam Formation suggest the Thesis (Ph. D.)--Florida State University, 2006. http://etd.lib.fsu.edu/theses/available/etd-11132006-173338. possibility that the two are separated by a fault. Holm-Denoma (2006) mapped the Altoona fault in the vicinity of Streckeisen, A. & Le Maitre, R. W., 1979. A chemical approximation to the modal QAPF classification of the igneous rocks. Neues Jahrbuch für Mineralogie, Abhandlungen 136, 169–206 the contact between the unnamed granitoid ( interpreted as Mulberry Rock Gneiss By Crawford and Kath, 2008), and it is possible that it separates the unnamed granitoid from the underlying Talladega belt metasedimentary rocks. Further structural and petrologic study will be essential for establishing a more definitive interpretation of the nature of the contact between the Lay Dam and this unnamed granitoid body. “ This research was funded in part by an Undergraduate Research Grant from the Office of the Provost, Columbus State University