Geology of the Hollins 1:24,000 Quadrangle, Alabama

GEOLOGY OF THE HOLLINS 1:24,000 QUADRANGLE, ALABAMA

David T. Allison, Jacob Grove, and Conner Antosz

LOCATION

The Hollins, Alabama, USGS 7½ minute quadrangle is located includes portions of southwest Clay County, northern Coosa, and southeast Talladega counties in the Appalachian Piedmont physiographic province. The topography consists of gently rolling hills, with sharp rugged ridges and valleys trending northeast-southwest. Major drainage in the quadrangle is dendritic, with most secondary streams feeding into Hatchet Creek, which drains southwestward through the southeast quadrant of the quadrangle. In the northwest quadrant of the quadrangle minor creeks feed the southwest trending Weogufka Creek. Elevations range from 546 feet (166 meters) on Hatchet Creek at the southeastern border, to 1265 feet (386 meters) at Locust Mountain in the central portion of the quadrangle. Numerous ridge crests throughout the area reach an elevation of 1000 feet (305 meters). The area is heavily wooded and rural. Cleared land is mostly pasture land. The quadrangle is traversed southeast to northwest by US Highway 280, and north to south by US Highway 231. Hollins (pop. 585) and Stewartville (pop. 1765) are the only incorporated towns in the quadrangle.

GEOLOGIC SETTING

The Hollins Quadrangle is within the northern Alabama Piedmont of the southern Appalachian orogenic belt and contains rocks of three separate fault blocks: a) the Talladega slate belt of the western Blue Ridge tectonic belt, b) the Coosa block, and c) the Tallapoosa block (Tull, 1978). The latter two fault blocks form part of the eastern Blue Ridge tectonic belt. The Talladega belt on the Hollins Quadrangle occurs as a roughly triangular polygon in the northwest quadrant of the quadrangle, with internal stratigraphy repeated by a thrust fault duplex trending along the Appalachian trend. The Coosa block extends southeast from the Hollins Thrust fault in the center of the quadrangle, to the Goodwater Fault in the southeast quadrant of the quadrangle. The Tallapoosa block occupies the southeastern third of the quadrangle southeast of the Goodwater Fault. Low grade metasedimentary and metavolcanic rocks of the Talladega belt within the quadrangle range in age between Lower Devonian and Lowermost Mississippian (?) (Butts, 1926; Tull and others, 1988; Gastaldo and others, 1993),

whereas medium and high grade metasedimentary and metavolcanic rocks of the Coosa and Tallapoosa blocks are considered to be Neoproterozoic in age (Tull, 1978). Granitic intrusive rocks in the Coosa and Tallapoosa fault blocks range in age from Cambrian to Devonian (Russell, 1978). The Talladega belt contains stratigraphy linked to the Laurentian cover rocks of ancient North America, whereas rocks of the Coosa and Tallapoosa blocks cannot be stratigraphically linked to Laurentia and constitute part of the eastern Blue Ridge composite suspect terrane (Jefferson Terrane of Horton and others, 1991). This composite terrane may include parts of the Laurentian outer margin cover sequence, as well as accreted components of accretionary prism, including terranes with ophiolitic and island arc affinity. Major structures in the quadrangle include: a) the Hollins Line thrust fault (Tull, 1978, 1994, 1995 a, b) separating the Talladega belt in the footwall from the Coosa block in the hanging-wall, b) the Goodwater fault (Tull,1978; Tull and others, 1985; Drummond, 1986), separating the Coosa and Tallapoosa blocks, and c) the southern limb of the F4 Millerville antiform, a regional cross fold that folds the following features: thrust sheets in the eastern Appalachian foreland, the Talladega belt, the Hollins Line fault, and the Coosa block (Plate 1). Mesoscopic fold phases and mesoscopic fabrics that correlate to deformation events D1 through D5 (Tull, 1978) are observed in all rock units in the quadrangle.

PREVIOUS INVESTIGATIONS

William F. Prouty laid the groundwork for future studies in this region with his pioneering work on the geology of Clay County (Prouty, 1923). Few Appalachian studies of this period rival Prouty’s work in terms of detail, accuracy, and insightfulness. The geology of the Hollins Quadrangle was described in field trip guidebooks and associated field stops published by the Alabama Geological Society (Neathery and Tull, 1975; Tull and Stow, 1979), and in an open file report to the Geological Survey of Alabama (Tull, 1976). Studies focusing on the Hillabee Greenstone in the quadrangle include work by Tull and others (1978), and Tull and Stow (1980), and thesis work by Long (1981) and Durham (1993). The southernmost part on the quadrangle borders on an area included in a study by Drummond and others (1994).

ACKNOWLEDGMENTS

The authors have worked closely over the years with several other geologists in researching the geology of the Hollins Quadrangle and the discussions and visits to the quadrangle with these individuals are gratefully acknowledged in adding to our understanding of this area. Thornton Neathery introduced the geology of the Alabama Piedmont to the geological community and his insight has been invaluable to multiple generations of geologists in identifying key problems for research in the region. James Tull, Lamar Long and Steve Stow conducted the first generation of detailed structural and geochemical studies of the northern Alabama Piedmont, particularly with respect to the Hillabee Greenstone and Paleozoic granitic rocks. The authors would also like to thank

the Jesse Edmondson of Charge Minerals, LLC, for logistical help, structural data from leased properties, aid in gaining access to properties, and for crystalline graphite analyses data.

STRATIGRAPHY

STRATIGRAPHIC NOMENCLATURE

The following outline is a brief summary of the stratigraphic nomenclature and known age relationships for units related to the geology of the Hollins Quadrangle:

Map Symbol (Plates 1 and 2) Unit Age rg Rockford-type Granitoids Devonian(?) hgs Hillabee Greenstone Devonian(?) hd Hillabee Metadacite Devonian(?) Dtjc/Dtes Jemison Chert/Erin Slate Devonian S-Dtbr Butting Ram Sandstone Siluro-Devonian S-Dtld Lay Dam undiff. Siluro-Devonian wec Wedowee Group Cragford Neoproterozoic (?) lithology weh Wedowee Group Neoproterozoic (?) Hackneyville lithology hc Hatchet Creek Group undiff. Neoproterozoic (?) hcg Hatchet Creek Group Neoproterozoic (?) metagreywacke hf Higgins Ferry Group undiff. Neoproterozoic (?) hfgr Higgins Ferry Group Neoproterozoic (?) graphitic schist hfmg Higgins Ferry Group Neoproterozoic (?) metagreywacke hfgq Higgins Ferry Group Neoproterozoic (?) graphitic quartz hfa Higgins Ferry Group Neoproterozoic (?) amphibolite

ASHLAND SUPERGROUP

Stratigraphic terminology in this region of complex geology has evolved over time as more structural and stratigraphic work and interpretations based upon this work have occurred. The “Ashland Series” was first described in Clay County by Prouty (1923) to include all of the rocks between the Hollins Line fault (then believed to be an extension of the Whitestone fault of north Georgia) and what is now mapped as the trace of the Goodwater fault, with the exception of a

narrow strip of rocks to the southeast, which Prouty referred to as “Altered Talladega”. Later, Butts (1926) referred to these two sequences in this area as the “Ashland mica schist” and the “Wedowee formation”. Neathery and Reynolds (1973) referred to both of these sequences as the Wedowee Group. Southeast of the trace of the Goodwater fault, in the Millerville Quadrangle, Prouty mapped the sequence as part of the “Ashland Series”, whereas Butts referred to it as “Ashland mica schist” and Neathery and Reynolds (1973) mapped it as Wedowee Group. Neathery (1975) and Tull (1978) recognized two lithologically distinct sequences in the northern third of the Millerville Quadrangle in rocks that Prouty (1923) had mapped as his “Ashland Series”, and Butts (1926) mapped as “Ashland mica schist”. Within these sequences formation level units can also be differentiated, so Tull (1978) elevated the Ashland to supergroup status (Ashland Supergroup), and subdivided it into a structurally lower (northwestern) sequence and a structurally upper (southeastern) sequence. At the latitude of the Millerville Quadrangle (northeast of Hollins quadrangle, 33° 11′ N), the Ashland Supergroup is folded into the regionally developed Millerville cross-antiform, which is decapitated down plunge to the east by the Goodwater fault. The south limb of the Millerville cross-antiform can be observed by tracing the curved contact of the Hollins Thrust in the northeast portion of the Hollins quadrangle (Plates 1 and 2). The Millerville cross-fold separates the Ashland Supergroup into a northeastern and a southwestern salient. In the southwestern salient, Neathery (1975) referred to the structurally lower sequence as the Higgins Ferry Formation and to the equivalent sequence in the northeastern salient as the Poe Bridge Mountain Formation. He referred to the structurally upper sequence in the southwestern salient as the Hatchet Creek Group, and that in the northeastern salient as the Mad Indian Group. Tull (1978) correlated the Higgins Ferry and Poe Bridge Mountain Formations and the Hatchet Creek and Mad Indian Groups across the Millerville antiform, upgraded the Higgins Ferry and Poe Bridge Mountain Formations to group status, and combined these two sets of equivalent groups into the Ashland Supergroup. In the Hollins Quadrangle the Ashland Supergroup is represented by the Higgins Ferry Group and the Hatchet Creek Group of the Coosa structural block, with the structurally overlying Wedowee Group of the Tallapoosa block exposed in the extreme southeastern portion of the quadrangle. The age of the Ashland Supergroup is not well constrained. No paleontologic materials are known to be preserved within it and the only constraints are isotopic age determinations, which generally date the approximate time of metamorphic cooling. The Ashland Supergroup is interpreted to be stratigraphically overlain by the Wedowee Group, which was intruded by the Cambro-Ordovician age Elkahatchee Quartz Diorite Gneiss. Therefore, this sequence is Neoproterozoic to early Paleozoic age. The similarity of this sequence to other major stratified sequences of the eastern Blue Ridge composite terrane (Ashe Metamorphic Suite, Lynchburg Group) also suggests that the Ashland is of probable Neoproterozoic in age.

HIGGINS FERRY GROUP (hf) This stratigraphic unit is exposed as the structural lowermost sequence of the Coosa structural block in the hanging wall of the Hollins thrust. The structural thickness of the Higgins Ferry Group approaches 4 km within the Hollins Quadrangle. Originally the sequence was thicker, but the base is cut off by the Hollins Line fault. Internal repetition may result from isoclinal folding, but major repetition of stratigraphy within this sequence has not been demonstrated and seems unlikely because major unit boundaries are not repeated. Because of its

tectonic base, the nature of the basement beneath this sequence is unknown. Topographically, this sequence forms sharp narrow ridges (e.g. Locust Mtn, Terrapin Hill) separated by narrow valleys. One of the characteristic features of these groups is the diverse array of distinctive, medium to coarse grained, repetitively interstratified lithologies that they contain, many of which constitute units mappable at the formation level. These include: feldspathic muscovite schist, roscoelite (vanadium mica) graphite quartz schist and quartzite, graphite quartz schist, biotite garnet and muscovite schist, garnet muscovite schist, biotite garnet feldspar gneiss, garnet quartzite, and orthoamphibolite. Among the most distinctive lithologic associations is the interlayered occurrence of roscoelite graphite quartz schist and quartzite, garnet quartzite (“garnetite”), and orthoamphibolite. The graphite quartz schists and quartzites hold up the sharp ridges mentioned above and elsewhere throughout the Coosa block. Among the distinctive lithologies found at several stratigraphic levels in the Higgins Ferry Groups is garnetiferous quartzite. Garnets are spessartine-rich, and weathering products of this rock display a characteristic black manganese encrustation surrounding fresher rock. Metamorphic grade is in the middle to upper amphibolite facies, with kyanite and staurolite locally present as index minerals. Sillimanite occurs locally in the eastern part of the Higgins Ferry Group in the Hollins quadrangle. Rare calc-silicate gneiss contains a garnet + clinopyroxene + hornblende + plagioclase assemblage that has a granular texture and borders on Granulite facies prograde T-P conditions based on geothermobarometry (Allison, 1992). Feldspar-rich schists are commonly migmatitic, with stringers and pods of simple pegmatite common. Graphitic quartz schist (hfgr): graphitic schist units containing flake graphite outcrop near the central portion of the Higgin Ferry Group belt. The main hfgr unit delineated on the geologic map and cross-section (Plates 1 and 2) is concordant to surrounding hf stratigraphy and is parallel to the trace of the Hollins Line thrust until the fault cuts up-section in the hanging wall in the northeast portion of the quadrangle. Crystalline graphite analyses of hfgr samples collected by Charge Minerals LLC averaged 2.76 wt. percent crystalline graphite on 79 samples from the southwestern portion of the quadrangle. The standard deviation was 1.04 wt. percent, the minimum value 0.14 wt. percent, and maximum value 5.36 wt. percent. Crystalline flake graphite content also seems to be correlated with roscoelite (Vanadium) mica content in schist. Graphitic quartzite (hfgq): metasedimentary units containing greater than 75% quartz are dispersed through the stratigraphic extent of the Higgins Ferry Group both stratigraphically and along strike. These units are erosionally resistant compared to other hf units and hold up ridges throughout the hf outcrop belt, for example Terrapin Hill in the southwest corner of the Hollins Quadrangle. Quartzite units do not display any primary sedimentary features such as relict grains, crossbeds, etc., therefore they are interpreted to have a bedded chert protolith. Some quartzite units are locally garnet-rich “garnetite”. Rarely garnet quartzite zones are intercalated with calc-silicate lenses containing garnet + plagioclase + sphene + hornblende + epidote + clinopyroxene + quartz mineral assemblages. Calc-silicate lenses may represent local carbonate- bearing protoliths intercalated with bedded cherts and pelites. Metagreywacke (hfmg): exposures of units with a recognizable feldspar + quartz content between 45-70% are designated as metagreywacke (hfmg). No mappable units of metarkose (feldspar >= 25%) were discovered in the hf outcrop belt. The single mapped hfmg unit is exposed as a discontinuous lens in the southwest corner of the quadrangle just northeast of

Terrapin Hill. The hfmg units in the hf belt are similar in composition and texture to Hatchet Creek Group metasediments and are interpreted as impure sandstone protoliths deposited in turbidites sequences. Orthomphibolite (hfa): The presence of amphibolites within this sequence is important. These rocks form mappable units within the Higgins Ferry group, some of which can be traced for more than thirty kilometers in the Coosa Block. Within the Hollins Quadrangle orthoamphibolite is concentrated in the vicinity of Locust Mt. in the central portion of the quadrangle (“hfa” in Plates 1 and 2). A few of these units in other quadrangles (ex: Mitchell Dam Amphibolite, Ketchepedrakee Amphibolite) have been given formation status. Orthoamphibolite units are composed predominantly of pleochroic green to blue-green hornblende, and calcic plagioclase, with accessory quartz, garnet, sphene, epidote, biotite, clinopyroxene and opaques. Common mineral assemblages include: plagioclase + hornblende + biotite + epidote ± clinopyroxene ± zircon. Local occurrences of orthopyroxene relics, surrounded by coronas of hornblende suggest a volcanic or plutonic protolith for the amphibolite. Individual amphibolite bodies are concordant with surrounding stratigraphy, and are interlayered with and locally gradational into surrounding pelitic graphitic schists. The amphibolite layers range in thickness from a few centimeters to several hundred meters. A comprehensive study of the amphibolites of the Coosa block was undertaken by Stow and others (1984). Based upon geochemical, textural, and mineralogic criteria, and geologic setting, they interpreted these rocks to be derived from tholeiitic rift basalts. Thomas and others (1980) suggested a similar setting for the Coosa block amphibolites, interpreting them as ocean-floor basalts and associated sills intercalated with deep water metasediments. Locally, (at approximately 10 localities) outside the Hollins Quadrangle, several of the amphibolite units contain small pods and lenses of ultramafic rock, predominantly hornblendite, metapyroxenite, and metaperidotite up to 100 m thick, and extending for a few hundred meters in length (Neathery, 1975; Reynolds, 1973; Schafer and Coolen, 1983,1985; Zwaschka, 1986; Mies and Dean, 1994). Examination of lithologic contacts of these amphibolites indicates that they are not faulted, but in fact are interlayered with metasedimentary and other possible metavolcanic lithologies, similar to mafic and minor ultramafic rocks in several other parts of the Blue Ridge, such as those within the Lynchburg Group in Virginia (Glover and others, 1994). We therefore interpret these mafic and local ultramafic rocks to be a normal part of the tectono-lithofacies, and thus to be an integral member of the stratigraphic sequence, and see no reason to interpret them as tectonically interleaved and dismembered ophiolites. We interpret the ultramafic components to be minor cumulate parts of basaltic sills and flows which were intercalated with fine-grained marine sediments, possibly in the slope-rise portion of a rifted continental margin. As mentioned above, the Ashland Supergroup was detached at some level from its basement along the Hollins Line fault but the basement type is unknown. It could have been oceanic crust. An alternative hypothesis has been offered relative to the structural and tectonic setting of the amphibolites. Higgins and others (1988) mapped the amphibolites of the Coosa block as occurring within windows belonging to a separate thrust sheet from, and unrelated to, the Ashland Supergroup, being tectonically bounded above against the surrounding rocks of that sequence. They mapped the amphibolites as contiguous with and a part of the sequence referred to here as the Hillabee Greenstone (see below) and correlated these rocks with the Ropes Creek Amphibolite of the Alabama Inner Piedmont. This interpretation is not supported by any of the

observations of this study or any other published study of the amphibolites of the Coosa block. Other than the fact that the amphibolites are mafic rocks (metabasalts), we see no similarities with the Hillabee Greenstone, and find numerous difficulties with the structural hypothesis of Higgins and others (1988).

HATCHET CREK GROUP (hc) This group crops out in the southeastern third of the Hollins Quadrangle, and is much more internally homogeneous than the underlying Higgins Ferry Group, consisting dominantly of coarse-grained feldspathic quartz-muscovite-garnet schist, with lesser amounts of feldspathic garnet-biotite gneiss, biotite schist, micaceous quartzite, and calcsilicate gneiss. Quartz-rich muscovite schist interbedded with biotite gneiss holds up sharp ridges within this sequence, but regionally the topography within the Hatchet Creek Group is much more subdued than the sequences above and below (Plates 1 and 2; Cross-section A-A’). Units are locally carbonaceous, but graphitic units are not nearly as abundant as they are within the Higgins Ferry Group. Amphibolites are very rare in this group and are absent in the Hollins Quadrangle outcrop belt of the Hatchet Creek Group. Most schist and gneiss units are migmatitic, with ubiquitous feldspathic veins and pegmatites. Fibrolitic sillimanite is a common accessory mineral, and kyanite occurs locally. The basal contact of this unit with the Higgins Ferry Group is gradational and considered to be stratigraphic (Tull, 1978; Drummond 1986; Allison 1992). This contact is placed above and to the east of the uppermost zone of amphibolite and quartz-rich graphite schist and graphite quartzite in the Higgins Ferry Group, and can be traced consistently across both the northern and southern salients of the Coosa block. Interpretation of protoliths of rocks of the Ashland Supergroup is hampered by the lack of primary structures and features resulting from the high grade of metamorphic overprint, generally high degree of bulk strain, and migmatitic nature of the rocks. Volumetrically, the dominant lithologies are metapelitic, followed in order by metapsammites, feldspathic biotite gneiss, and amphibolite. No sedimentary protoliths were apparently coarse grained enough to have preserved primary detrital grains. The sequence is devoid of carbonate rocks, but rare occurrences of calcsilicate gneiss may represent calcareous siltstone protoliths. Cyclic packages of metapelite and feldspathic metagreywacke within both sets of groups of the Ashland suggest that these rocks may have originated as turbidites in a deep marine basin along a continental slope or rise (Thomas and others, 1980; Drummond and others, 1988; Allison, 1992). Those within the Higgins Ferry Group are commonly highly graphitic and thus must have contained significant amounts of organic material deposited as anoxic sands and muds. Units in the Poe Bridge Mountain Group were mined for many years in Clay County for flake graphite (Prouty, 1923). Currently mining interests have completed exploratory work near Locust Mt. and Terrapin Hill, and are awaiting investment capital to initiate commercial production of flake graphite. The amphibolites described above were intruded and extruded into this sequence as basaltic sills and flows with “ocean floor” geochemical signatures. Other lithologies may also be distinctive in terms of environmental interpretation. Garnet quartzites (garnetite) likely represent metamorphosed chemical sediments and silicate-facies exhalitives (Mn-rich chert?) and alteration products associated with submarine volcanism (Schafer and Coolen, 1983, 1985; Drummond, 1986).

WEDOWEE GROUP (we) The extreme southeastern corner of the Hollins Quadrangle, southeast of the Goodwater fault, constitutes part of the Tallapoosa block of Tull (1978) and is occupied by the Wedowee Group which was originally named by Butts (1926). The Goodwater fault forms the northwest boundary of the Wedowee on this quadrangle and quadrangles to the north and south, but to the southwest of the Hollins Quadrangle, displacement apparently decreases on this fault and in the Richville AL Quadrangle the Wedowee Group is in stratigraphic contact with the underlying Hatchet Creek Group (Allison, 1992). This implies a stratigraphic succession of Wedowee Group above Hatchet Creek-Mad Indian Groups, that is in turn above Higgins Ferry- Poe Bridge Mountain Groups. No primary stratigraphic facing data have been obtained from any of these units, but it is highly unlikely that this > 10 km thick sequence is regionally overturned, and thus the Wedowee probably occurs at the stratigraphic top of the sequence. The Wedowee Group is a thick sequence of dominantly metapelitic rocks, commonly carbonaceous, with lesser amounts of quartzite. In the Hollins Quadrangle the Wedowee consists of the Cragford Phyllite that is generally the basal lithology of the Wedowee (Neathery, 1975). In proximity to intrusive granitic plutons the Wedowee may consist of a textural variant designated as the Hackneyville schist, and seems to be protected from phyllonitic transposition by the more mechanically competent intrusives. For this reason, the Hackneyville likely represents the original prograde Wedowee texture. Cragford Phyllite (wec): The Wedowee Group in the Hollins Quadrangle is almost exclusively made up of the Cragford Phyllite, a sequence of fine-grained graphite-garnet-biotite-muscovite- quartz schist, and minor feldspathic biotite quartzite (metagreywacke). Tourmaline, locally making up as much as 20% of the rock, is a common accessory mineral, and andalusite porphyroblasts occur rarely (Drummond, 1986). Much of this schist is phyllonitic, containing a transposed foliation caused by retrograde mylonitization and grain-size reduction of preexisting prograde schist (Tull, 1975,1978; Drummond, 1986). Because this textural variant of the Wedowee often lacks porphyroblasts, the lithology is easily mistaken for a low-grade phyllite. Hackneyville Schist (weh): On a megascopic scale the Hackneyville schist occurs structurally and probably stratigraphically above the basal Cragford lithology, however, this lithology is also preserved in the strain shadows of granitic intrusives on a mesoscopic scale within the Hollins Quadrangle Wedowee outcrop belt (Plates 1 and 2). The Hackneyville Schist, a sequence of coarse-grained, variably graphitic and garnetiferous biotite-muscovite schist, muscovite-biotite paragneiss, and quartzite (Drummond, 1986). The predominant mineral assemblage is quartz- muscovite-biotite-plagioclase, with accessory garnet, graphite, sillimanite, tourmaline, ilmenite, pyrrhotite, zircon, apatite, and staurolite (Drummond, 1986). Retrograde minerals in this sequence include epidote and chlorite. Units of the Wedowee Group are more internally homogeneous than those of the Ashland Supergroup. Most of the sequence within the Hollins Quadrangle consists of graphitic metapelite which exhibits little or no outcrop-scale compositional layering. Thus, most of this sequence originally consisted of carbonaceous shale, which was interlayered with minor amounts of fine- grained sandstones. Metabasaltic rocks (amphibolites) are only found in the upper (eastern) parts of the Wedowee in Randolph, Tallapoosa, and Chambers Counties, and are thus absent on the Hollins Quadrangle. The age constraints on the Wedowee Group are the same as those on the Ashland Supergroup (see above), and therefore it is also considered to be of Late Proterozoic to

early Paleozoic age.

INTRUSIVE IGNEOUS ROCKS

ROCKFORD-TYPE GRANITOIDS (rg) The Rockford-type granitoids in the Coosa and Tallapoosa blocks include the Rockford Granite, Bluff Springs Granite, and Almond Trondhjemite. Of these felsic intrusives, plutons of the Rockford-type granitoids are exposed within the Hollins Quadrangle in the extreme southeast corner of the quadrangle. The Rockford-type granitoids exhibit a range in composition. However, they are predominately muscovite-biotite granites with minor granodiorite, trondhjemite, and tonalite (Drummond, 1986; Allison, 1992; Drummond and others, 1987; Drummond and others, in press). Russell and others (1987) report a 366 Ma. Rb-Sr whole rock age estimate for the Bluff Springs granite, which is essentially equivalent to isotopic age estimates of dynamothermal activity. Textures within the granitoid intrusives agree with isotopic age estimates in that the granitoids are foliated strongly near contacts but retain igneous textures in the plutonic interior. Within the Hollins quadrangle, the Rockford-type is represented by three plutons, one of which spans the Goodwater Fault. Structurally the granitoids are concordant to their country rock and occupy a similar structural position near the Ashland Supergroup- Wedowee Group contact along strike. This may indicate that most, if not all, of the felsic intrusives in the eastern Blue Ridge of Alabama were intruded as sill-like bodies into a similar lithostratigraphic position. Geochemical characteristics of the Rockford Granites include a peraluminous bulk composition (S-type) indicative of a crustal protolith. Petrologic classifications based on normative mineralogy indicate granite to trondhjemite to granodiorite composition (Drummond, 1986). Drummond (1986), Drummond and others (1987), and Allison (1992) have proposed the migmatitic Hatchet Creek and Mad Indian feldspathic metasediments as appropriate protoliths for S-type granitoid formation via anatexis during an Acadian dynamothermal event. In this scenario, discrete granitoid diapirs intruded structurally higher metasediment stratigraphy near the Ashland Supergroup-Wedowee Group contact. Drummond (1986) and Drummond and others (1986) proposed a metasomatic origin for trondhjemite compositions that are present throughout the strike length of the Rockford-type granitoids. Numerous complex pegmatites and greissens are spatially related to granitoid bodies (Cook and others, 1987; Ford and Cook, 1989) (see below). Wesolowski and Drummond (1990) interpreted the genesis of Sn- and Ta-bearing greissen bodies to be related to Na-metasomatism of Rockford-type granite via biotite decomposition and subsequent liberation of Sn into a migrating fluid phase, leading to the close spatial relationship of trondhjemite granitoids to greisen and complex pegmatite.

FELSIC DIKES AND PEGMATITES (rg)

Felsic dikes and pegmatites pervasively intrude the Ashland Supergroup and Wedowee Group metasediments. Mineralogically simple pegmatites, in contrast to greisen and complex pegmatites, are formed from the intrusion of, and precipitation from, H2O-rich metamorphic fluids during peak dynamothermal activity rather than from a magmatic source. These

pegmatites are predominantly composed of biotite + muscovite + quartz + plagioclase ± K- feldspar. Felsic dikes commonly intrude both metasediments and larger intrusive bodies. The composition of felsic dikes within the eastern Blue Ridge terrane in Alabama is basically the same as the felsic intrusives, i.e. granite to trondhjemite to minor granodiorite (Moore and others, 1987). Exposures of felsic dikes, when fresh, typically exhibit an aplitic texture because of the lack of ferromagnesian components, especially in trondhjemite compositions. They may display incorporated xenoliths and schlieren of metasediment country rock (Moore and others, 1987). The timing of felsic dike emplacement is well constrained because of the genetic relationship with the synmetamorphic Rockford-type granitoids, and as demonstrated by fabric relationships. Early dikes are marked by a foliation development that is syn-metamorphic. These same early felsic dikes are geochemically identical to Rockford-type granitoids. Later less-foliated to non- foliated trondhjemite dikes have been altered by Na-metasomatism and therefore must have been intruded in the presence of Na-rich fluids during the waning stages of the metamorphic event, i.e. after the majority of the deformation fabric had been developed in older dike phases (Moore and others, 1987). Thus, felsic dike emplacement spanned the dynamothermal event and, therefore, records the progressive fabric and geochemical stages of felsic intrusive rocks within the eastern Blue Ridge terrane.

TALLADEGA GROUP

Rocks of the Talladega Group (Tull, 1982) are exposed in the northwest and north portions of the Hollins Quadrangle to the north and west of the Hollins Thrust Fault (Plates 1 and 2). These rocks include quartzite, micaceous quartzite, sericite-quartz phyllite, chlorite-sericite phyllite, and graphitic-quartz phyllite. Lithologies of the Talladega Group and the overlying Hillabee Greenstone rocks contain lower greenschist facies mineral assemblages. The Hillabee Greenstone is composed of metabasaltic lithologies associated with a mid-Paleozoic back-arc basin. Tull and Telle (1989) and Tull and Groszos (1991) have interpreted the Talladega Group as a middle Paleozoic successor basin sequence which developed unconformably above the Cambrian-Lower Ordovician carbonate bank facies along the Laurentian continental margin. This successor basin, dominated by turbidites and olistostromes in the lower part and shallow water sandstones, conglomerates, cherts, argillites, and carbonaceous shales in the upper part, formed in an extensional environment and was flanked initially by a Lower Paleozoic mantled Precambrian basement uplift on the southeast (Tull and Telle, 1989). This extensional environment was probably a response to partial back-arc rifting which culminated in formation of the Hillabee Greenstone. Rocks of the Talladega Group in the Hollins Quadrangle constitute the uppermost part of the successor basin sediments. Lay Dam Formation (S-Dtld): the Lay Dam formation is the lowermost exposed Talladega Group unit contained in the Hollins Quadrangle. It is composed of predominantly chlorite + sericite + quartz phyllite with minor interlayers of graphitic slate. Lenses of feldspathic blue- quartz diamictites occur in other quadrangles both northeast and southwest of the Hollins quadrangle where stratigraphy lower in the Lay Dam are exposed. Uppermost stratigraphy in the Lay Dam grades into metasandstone of the Butting Ram formation over an interval of

approximately 50m. The composition and geometry of metasediments in this formation indicate an initial rapidly filling depositional basin followed by more shallow marine depositional environments near the top of the formation. Graphitic units were probably organic-rich shale protoliths. Butting Ram Sandstone Formation (S-Dtbr): the Butting Ram formation consists of approximately 85m of medium bedded coarse-grained metasandstone and metaconglomerate. Some beds contain cross-beds, asymmetric ripple marks, and cut-and-fill structures that indicate that beds are not overturned. Rarely small lenses of phyllite and graphitic slate occur between beds of metasandstone. The size and geometry of sedimentary structures indicate a shallow marine depositional environment for this unit. Jemison Chert (Dtjc): the Jemison Chert formation is composed of “papery” thinly bedded metachert units that are approximately 830m thick in the vicinity of the Hollins Quadrangle. The metachert beds are intercalated with graphitic slate and phyllite, and it should be noted that along strike the Jemison Chert can undergo a facies change to the Erin Slate (Dtes), that is an very graphite-rich slate composition of equivalent age and stratigraphic position. Small pockets of Erin Slate were discovered in the Hollins Quadrangle, however, they did not prove mappable at 1:24,000 scale so they were consolidated into the Jemison Chert formation on the geologic map and cross-section (Plates 1 and 2)

HILLABEE GREENSTONE (hgs) The Hillabee Greenstone is the thickest and most laterally extensive metaigneous unit in the Appalachian western Blue Ridge, occurring at the structural top of the Talladega slate belt in east Alabama and west Georgia. In the Hollins Quadrangle the Hillabee obtains a maximum thickness of greater than 850m, but the original thickness is unknown because the unit is tectonically bounded above by the Hollins Line fault. The Hillabee is folded around the hinge of the Millerville antiform, the south limb of this antiform produces the change in attitude of the Hillabee belt and the trace of the Hollins thrust in the northeast corner of the Hollins quadrangle (Plates 1 and 2). The Hillabee is volumetrically dominated by metabasaltic rocks of tholeiitic character, but a significant portion (15-25%) of the exposed sequence also includes felsic rocks (metadacites; “hd”) of calc-alkaline character. A lens of metadacite is exposed within the Hillabee outcrop belt in the Hollins Quadrangle that measures 10,150 by 460 meters in outcrop area. Mafic rocks range from non-foliated massive greenstone to mafic phyllite. These rocks are chemically and mineralogically equivalent, fine-grained (0.1 to 1mm), and contain the mineral assemblage-actinolite (25-70%), epidote - clinozoisite - zoisite (10-60%), albite (20-30%), chlorite (1-10%) (Tull and others, 1978; Stow, 1982). Relict hypidiomorphic granular igneous textures are common in the greenstones. These include porphyritic, diabasic, and ophitic textures where relict shapes of pyroxene and plagioclase grains are preserved in a mosaic pattern. Plagioclase phenocrysts (up to 1 cm in diameter in the porphyritic rocks) are altered and saussuritized to albite and epidote/zoisite, and pyroxene phenocrysts (up to 3mm) are altered to actinolite/tremolite, with rarely preserved clino-(?) pyroxene cores. Greenstones are interpreted

to have originated as basalt flows. Mafic phyllites are interpreted to represent both more highly deformed greenstone protoliths and basaltic tuff. The latter preserve delicate compositional layering on a millimeter and centimeter scale defined by alterations of plagioclase-rich and mafic mineral-rich laminae. Relict primary volcanic features however are rare. Paris and Cook (1989) and Paris (1990) have interpreted scattered elliptical epidote-quartz masses as lapilli fragments and rounded radiating epidote crystals as filled amygdules. Pillow structures have not yet been conclusively identified. Felsic rocks of the Hillabee Greenstone average 67% SiO2 and are compositionally equivalent to dacite and rhyodacite. They occur throughout the Hillabee outcrop belt as thick tabular sheets of relatively uniform thickness, interlayered with mafic rocks. One of these tabular units occur in the northeast portion of the Hollins Quadrangle (Plates 1 and 2). Contacts of metadacite with mafic rocks (metabasalts) are sharp and non-gradational. There is no evidence of localized high shear strain at these contacts and they are interpreted as non-faulted pre- metamorphic primary volcanic contacts. A distinctive feature of these rocks is the presence of large porphyroclasts of amphibole (0.5 - 10 mm long; up to 12 volume %) and plagioclase (1 mm in diameter; up to 9 volume %) which are interpreted as original volcanic phenocrysts or pyroclastic crystal fragments (Tull and others, 1978; Long, 1981; Durham, 1993). Based upon petrography, geochemistry, dimensions, and field relationships, these rocks are interpreted as ash flow crystal tuffs which represent major eruptive events (ignimbrites). The age of the Hillabee is controversial. U/Pb studies by Russell (1978) and Russell and others (1984) yielded slightly discordant ages of 460 Ma., whereas Rb-Sr whole-rock studies of the metadacite yield an age of 395 ± 20 Ma. (Durham, 1993). The zircons may contain an inherited xenocrystic component. Structural and stratigraphic studies on the other hand argue that the Hillabee occurs stratigraphically above the Middle Paleozoic rocks of the Talladega Group (Tull and others, 1978; Tull and Stow 1980; Paris and Cook, 1989; Paris 1990; German 1990). Geochemical Relationships Hillabee rocks are distinctly bimodal, with modes of 48-50% (basalt) and 66-68% (dacite) SiO2. All rocks are subalkaline; mafic rocks are tholeiitic, but are more alkaline than low-K tholeiites typical of spreading centers, and felsic rocks are calc-alkaline. No geochemical data suggest that the two modal suites are petrogenetically related (Stow, 1982; Durham, 1993). Each of these suites displays igneous trends on various major, trace, and REE element diagrams, and each contains relic igneous minerals and textures (Tull and others, 1978; Tull and Stow, 1980; Stow, 1982; Durham,1993). On a “total alkali-silica” diagram, mafic rocks plot as basalt and basaltic andesite (Stow,1982) and felsic rocks plot as dacite and rhyodacite (Durham, 1993). No ultramafic rocks are known from the Hillabee. On tectonic discrimination diagrams, Hillabee mafic rocks plot in a mixture of settings, including ocean floor, island arc, and continental settings. REE patterns show a strong MORB-like pattern, which is most similar to “plum- related” MORBs. Other characteristics also suggest enriched (“plume-related”) MORBs. Durham (1993) determined that Hillabee metadacite rocks plot in either the syn- collisional or volcanic arc fields using the tectonic discrimination trace element techniques of Pearce and others (1984) for granitic rocks. Using the major element tectonic discrimination plots of Maniar and Peccoli (1989) Durham placed the Hillabee metadacites into the island arc- continental arc-continental collisional granite field. As stated above, the two coeval modal suites (basalts and dacites) are intimately

interstratified and boundaries are sharp, non-faulted, metamorphosed igneous contacts. Thus, although the felsic rocks are not from the same magma series as the metabasalts, their spatial and temporal relationships indicate that they must have formed in essentially the same tectonic setting. The most likely setting which is compatible with all of the available data is an extensional back-arc setting as originally proposed by Stow (1979) and Tull and Stow (1980). The “MORB-ocean floor” settings identified in some tectonic discrimination diagrams of the mafic rocks are also inclusive of back-arc environments and the felsic rocks could have been emplaced into the back-arc as crustal melts. This model is also consistent with the interpretation of the stratigraphically underlying Talladega Group as an extensional successor basin formed above thinned Laurentian crust (Tull and Telle, 1989).

STRUCTURE The Hollins Quadrangle contains a complex array of large and small-scale structures which formed during various stages of deformation associated with the evolution of the Appalachian orogenic belt. These features are described below in chronological order. Deformation phase D1: All rocks of the quadrangle were subjected to an early phase of deformation that approximately coincided with peak metamorphic conditions (see METAMORPHISM below). It was during this phase that maximum strain was imposed on most rocks of the quadrangle. Associated with this strain and peak metamorphism was the growth of the major microfabric in most rocks. In rocks of the Talladega belt this fabric is a pronounced slaty cleavage common to all rocks, with the exception of the massive greenstones of the Hillabee, which were not well cleaved by this event. In the Hollins Quadrangle slaty cleavage (S1)strikes northeast and dips at moderate angles to the southeast, except in the extreme northeast portion of the quadrangle where S1 is deflected to a more east-west strike orientation because of the effect of the south limb of the Millerville Antiform. In the plane of this cleavage there is an assortment of related coaxial lineations. These include bedding-cleavage intersection lineations (I1), mineral growth lineations, and primary grain (sand, pebble, phenocryst) stretching lineations (L1). These L1 lineations trend predominantly southeast at shallow plunge angles (Plates 1 and 2). Locally, outcrop-scale isoclinal folds have been observed in the Talladega belt which are coaxial with the above described lineations and which have the slaty cleavage parallel to their axial planes. A similar, if not the same, deformational event is manifested in rocks of the Coosa and Tallapoosa blocks, except that the metamorphic grades, and thus the temperatures and corresponding bulk strains were greater. Rocks of the Ashland Supergroup and Wedowee Group and their included intrusive rocks contain a pervasive foliation (schistosity or gneissosity) (S1) defined by the constituent metamorphic minerals or metamorphically derived compositional layering. An assortment of lineations exists within the plane of the foliation. These include dimensionally preferred mineral lineations (biotite, muscovite, hornblende, quartz, and flake graphite), mineral clot lineations, and compositional layering (bedding in many cases)-schistosity intersection lineations. These lineations are coaxial with first-phase (F1) mesoscopic isoclinal flow folds in these sequences and the schisosity (S1) is parallel to the axial planes of these folds. A geometric analysis of these lineations shows that in the Hollins Quadrangle they plot on a lower hemisphere equal area net in a northeast striking, moderately southeast dipping girdle, with maxima to the northeast and southeast, with a

megascopic F3 hinge statistically calculated at 046, 07 attitude. Maxima also indicate F3 folding as tight, with asymmetric long limbs dipping coplanar with the Appalachian trend, and short limbs dipping northwest at steep angles. A study by Moore and others (1987) has shown that felsic (granitic and trondhjemitic) dikes intruded the Elkahatchee Quartz Diorite Gneiss at various stages during this deformation phase along NW-SE extension fractures parallel to the field of active shortening during progressive deformation. Deformation phase D2: This deformation phase is best recognized in rocks of the Wedowee Group, but also affected rocks of the Ashland Supergroup to a lesser degree (Tull, 1978). It is responsible for modifying the earlier prograde tectonite fabrics of these rocks and is responsible for the major retrogression of mineral assemblages of rocks within the Coosa and Tallapoosa structural blocks. Similar transposition structures have been recognized locally in rocks of the Talladega belt but these features may be associated with events later than the deformation phase described here. Best recognized in pelitic rocks of the Wedowee Group’s Cragford Phyllite, this event produced variably developed phyllonite schists and phyllitic mylonites, resulting in significant grain-size reduction (Tull, 1978; Drummond, 1986). The D2 microstructure involves tight F2 kink folding of the S1 fabric into microfolds which transpose the S1 schistosity into an F2 axial plane cleavage (S2). Thus, the S2 slaty cleavage becomes the dominant planar fabric in much of the Cragford Phyllite. With the exception of S2 slaty cleavage, mesoscopic and megascopic D2 structures are not recognized within the northern Alabama Piedmont. Deformation Phase D3: A third fold phase (F3), ranging in scale from fine hair-like crenulation folds to map-scale folds with wavelengths of several kilometers, affected rocks of the Ashland Supergroup and Wedowee Group, and refolds earlier planar fabrics (compositional layering, S1, and S2). These structures have not been definitively recognized in the Talladega belt. Regionally, these structures consist of tight, overturned, shallowly and doubly plunging, moderately inclined folds (Tull, 1978; 1987; Allison, 1992). Axial planes strike northeast and dip moderately southeast, and hinge lines plunge gently to the northeast and southwest (Plates 1 and 2). Hinge lines are coaxial in this area with linear features formed during the D1 deformation phase. In the Holins Quadrangle F3 hinge lines are doubly plunging. Digitations that deflect contacts away from the Appalachian trend indicate F3 map-scale fold hinges, for example, the “hfa” amphibolite contacts northeast of Locust Mt. (Plates 1 and 2; sec. 16, T24N, R10E) Deformation Phase D4- Hollins Line fault- The Hollins Line thrust fault is a fundamental boundary structure separating the eastern and western Blue Ridge in Alabama and west Georgia, and as such, represents a terrane boundary and one of the most tectonically significant faults in the southern Appalachians. Its trace can be mapped for 250 km, and it forms the roof thrust of a family of associated faults collectively forming a footwall duplex known as the Hollins Line fault system (Moore and others, 1983; Moore and Tull, 1989; Tull, 1984, 1994, 1995a,b;). Regionally, the fault dips at a low angle of about 15-20° and planes through the Ashland Supergroup stratigraphy at a high angle, cutting stratigraphically downward through >6 km of this sequence in the northwest direction of displacement (Tull and others, 1978; Tull, 1995a). On the Hollins Quadrangle the fault strikes along a consistent northeast trend for most of the outcrop extent until it becomes more east-west near the northeast corner of the quadrangle. This deflection is the onset of folding by the Millerville F4 cross-fold located northeast of the Hollins Quadrangle. At this point the Hollins thrust begins to cut up-section in the hanging wall stratigraphy (Ashland Supergroup) and up-section in the footwall (Talladega Belt), suggesting

the presence of a lateral ramp structure. The repetition of stratigraphy in the footwall is produced by a fault horse – along strike to the north and south the footwall of the Hollins thrust contains an extensive duplex system. The Hollins fault is remains concordant to stratigraphy in the footwall for most of the strike length in the Hollins Quadrangle except when encountering the Millerville cross-fold south limb where the fault begins to cut down-section in the footwall (Plates 1 and 2; northeast corner of quadrangle). The Hollins Line fault always marks a sharp contact between the medium to high grade metamorphic rocks of the hanging-wall Ashland Supergroup, and low- grade rocks (usually the Hillabee Greenstone) of the footwall, Thus the Hollins thrust fault constitutes a significant metamorphic “break” thrust boundary. Rocks of the Ashland Supergroup never occur below (northwest of) this boundary, and along much of its trace, the fault marks a topographic escarpment, with rocks of the more resistant hangingwall holding up ridges adjacent to the footwall valley. Fault zone fabrics are well developed in the immediate hangingwall over a thickness of tens of meters and include composite planar (S-C) fabrics (ex: button schists) indicative of a top to the northwest displacement. Displacement on this fault may be greater than on any fault in the state of Alabama. Measurements in the Millerville, AL, region indicate a minimum horizontal component of net slip of 18 km (Tull and others, 1978). Removing this displacement still results in juxtaposition of significantly disparate metamorphic grades and stratigraphic ages (Late Proterozoic against Middle Paleozoic), so it is clear that the actual net slip was probably on the order of many tens of kilometers or more. The stratigraphic offset indicates a vertical component of net slip of greater than twenty kilometers. Recent work by Tull (1994, 1995a,b) has shown that displacement on this fault also involved a significant component of oblique dextral slip in addition to the thrust slip during emplacement of the eastern Blue Ridge onto the Talladega belt. Deformation Phase D4: Following emplacement of the eastern Blue Ridge onto the Talladega belt along the Hollins Line fault, rocks of the Coosa block, the Talladega belt, the Talladega- Cartersville fault below it, and rocks in the footwall of this latter fault were deformed together by a regional cross-fold phase referred to as the “Millerville-Childersburg generation” by Tull (1984). On the Hollins Quadrangle, this phase is represented by the south limb of the Millerville antiform where Talladega Belt, Ashland Supergroup, and the Hollins thrust are all deflected to a more east-west strike in the northeast corner of the quadrangle. The axial trace of F4 fold phases is oriented approximately east-west, and refold earlier structures such as S1 and L1. Mesoscopic F4 folds are detectable at some outcrops as open upright folds with east trending hinges with shallow plunge, and vertical axial planes. Deformation Phase D5- Goodwater- Enitachopco fault: The latest major structural feature to form on the Hollins Quadrangle was the Goodwater fault (Tull, 1978; Plates 1 and 2). This fault decapitates folds of phases three and four above (Plates 1 and 2), and has a linear N45°E trace, dipping steeply (65°) to the southeast. An exposure of this fault in the Millerville Quadrangle (NE¼, SW¼, sec.22, T. 21 S., R. 7 E.) was described in Neathery and Tull (1975, Stop A-2) as a 10-15 m wide schuppen zone containing breccia blocks and slices of several different lithologies separated by an anastomosing cataclastic foliation. Tull and others (1985) argued on the basis of geometry and stratigraphic separation that this structure is a normal fault. To the southwest near Goodwater, Drummond (1986) found kinematic evidence for normal displacement on this fault. Based upon the geometric relationships of fold decapitation in the Millerville and Goodwater areas (Drummond, 1986), displacement on this fault in this area must be on the order of several

kilometers. However, in southwestern Coosa County, along the projected trace of the fault, Allison (1992) mapped a stratigraphic contact between the Wedowee and Hatchet Creek Groups and found no evidence for the existence of a major fault in this position. Thus, displacement on the Goodwater- Enitachopco fault must die to the southwest and the fault must be hinged in Coosa County between the areas mapped by Allison (1992) and Drummond (1986).

TIMING OF DEFORMATION Earliest deformation in the Northern Alabama Piedmont was synchronous with metamorphism. In the Tallapoosa block, the Cambro-Ordovician Elkahatchee Quartz Diorite Gneiss predated this deformational event but this event is believed to be syntectonic with later granites and trondjhemites (Tull, 1975, 1987; Drummond, 1986, 1987; Moore and others, 1987; Allison, 1992). Most studies interpret the “younger” granites to be Devonian in age (Russell, 1978; Tull, 1978; Russell and others, 1987; Drummond, 1987, Moore and others, 1987) and this is supported by isotopic determinations of age of metamorphism in both the Talladega belt and Coosa and Tallapoosa blocks. K-Ar whole rock slate ages from the Talladega belt average 390 Ma (Kish, 1990), K-Ar hornblende ages from the Talladega belt are 382 ± 14 Ma (Russell, 1978) and 348 Ma. from the Coosa block (Wampler and others, 1970). Rb/Sr whole-rock ages from the Talladega belt are 379 Ma. (Weimer, unpb. data) and 395 ± 20 Ma. (Durham, 1993), and 366 ± 25 Ma. from the Coosa block (Russell, 1978). All of these data are compatible with interpreting the dynamothermal events associated with the first phase of deformation (D1) as being manifestations of the Devonian Acadian orogeny. Recently however, Gastaldo and others (1993) reported the discovery of a rare earliest Mississippian plant species from the uppermost Talladega Group which thus far has only been found in three other world localities. This discovery implies that the D1 event may be late Paleozoic and associated with the Alleghanian orogeny. As yet however, no other Carboniferous fossils have been documented to have come from the Talladega belt and this single report awaits other supporting paleontologic and isotopic evidence. Age constraints on deformation phases D2 and D3 are poor. They may be associated with either the Acadian or Alleghanian orogenies. Tull (1984) has presented evidence that deformation phase D4 is Alleghanian in age. Deformation phase D5 is either Alleghanian or younger. If Alleghanian, it may be related to late extensional collapse of the orogen. Alternatively, the Goodwater fault may be a Mesozoic structure, and related to early extensional events associated with the Mesozoic rifting.

METAMORPHISM

INTRODUCTION Metamorphic grade within the areal extent of the Hollins Quadrangle varies from lower greenschist facies in the Talladega belt northwest of the Hollins Line fault system, to middle and upper amphibolite facies in the Ashland and Wedowee belts southeast of the fault (Tull, 1978).

Rare occurrences of calc-silicate gneiss in the Higgins Ferry group may preserve pockets Granulite facies metamorphic assemblages. The Hollins Line fault system represents a significant tectonic break in the west-to-east trend of increasing metamorphic grade (Tull, 1978).

METAMORPHIC ASSEMBLAGES IN THE TALLADEGA BELT ROCKS The Talladega belt is composed of a diverse package of metasedimentary and metavolcanic compositions. Within the Hollins quadrangle only the uppermost units of the belt are exposed. These compositions include graphite + chlorite + quartz + sericite phyllite, metachert, chlorite + epidote + actinolite + plagioclase greenstone, and hornblende-bearing metadacite. Pyroxene present in the greenstones and hornblende in metadacites is interpreted by Tull and others (1978) to represent relic magmatic phases rather than metamorphically-derived equilibrium phases. The Talladega belt was metamorphosed to lower-greenschist facies during Acadian dynamothermal activity. However, the Acadian metamorphic isograds were tectonically truncated by a later (D4) development of the Hollins Line fault.

METAMORPHIC ASSEMBLAGES IN THE ASHLAND SUPERGROUP AND WEDOWEE GROUP Metamorphic grade within the exposed rocks of the Ashland Supergroup and Wedowee Group within the areal extent of the Hollins Quadrangle ranges from middle to upper- amphibolite facies. Because this metamorphic grade affects a variety of metasedimentary and metavolcanic compositions, the mineralogy and texture of metamorphic rocks in the Ashland Supergroup and Wedowee Group vary. Pelitic compositions at middle amphibolite facies are typically garnet-bearing mica schists with kyanite as the aluminosilicate phase. In upper- amphibolite facies pelitic rocks, garnet mica schist commonly contains fibrolitic sillimanite as the aluminosilicate phase. Units within the Mad Indian Group are richer in feldspar component which is exclusively andesine plagioclase in most samples. While middle amphibolite facies feldspathic schists develop as plagioclase + garnet mica schist in the Poe Bridge Mountain and Wedowee Groups, the equivalent compositions in the upper-amphibolite facies Mad Indian Group were selectively partially melted to form abundant migmatite. Metavolcanic units internal to the Higgins Ferry and Poe Bridge Mountain stratigraphy were metamorphosed to garnet + hornblende + zoisite orthoamphibolite. Calcium-silicate zones intercalated between orthoamphibolite units consist of clinopyroxene + grossular garnet + zoisite + sphene + An80-90 plagioclase gneiss. The protoliths of the calc-silicate layers are interpreted to represent calcareous marl originally intercalated with volcanic flows and/or ash deposits (Tull, 1978; Drummond, 1986).

PRESSURE-TEMPERATURE ESTIMATES Drummond (1986) and Allison (1992) have estimated the pressure, temperature, and fluid composition (P-T-X) of metamorphism within the Ashland Supergroup and Wedowee Group metasediments. Drummond estimated temperature averages of 660°, 586°, and 665̊ C for the Hatchet Creek Group, Wedowee Group, and Higgins Ferry Group respectively. Average pressures were 6.84, 5.53, and 4.54 kb respectively. These values suggest a lower metamorphic grade in the Wedowee Group as compared to the other units. The P-T conditions prevalent in the Hatchet Creek/Mad Indian Group are proximal to the wet granite solidus in P-T space, a fact

which agrees well with the occurrence of abundant migmatite in appropriate compositions. Drummond calculated fluid compositions in equilibrium with graphitic pelitic units to be dominantly H2O with mole fraction ranges of 0.77 to 0.88. Other significant metamorphic fluid components included CO2 (0.052-0.100) and CH4 (0.0.052-0.100). Allison (1992) estimated essentially equivalent metamorphic fluid compositions and lithostatic P ranges but somewhat larger T ranges (502-690°C) with a mean value of 613°C. Drummond and others (1987) interpret the array of P-T estimates for Ashland Supergroup and Wedowee Group rocks analyzed as representing a P-T-time path indicative of a thermal doming of Hatchet Creek rocks. The axis of the thermal upwarp would roughly coincide with the strike length of the Hatchet Creek and Mad Indian Groups, although this hypothesis is currently untested by P-T-X data within the Mad Indian Group.

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