P-T Conditions of Selected Samples across the Blue Ridge Province

1. P-T Conditions of Selected Samples across the Blue Ridge Province Breana Felix, Geology Department, Marshall University A Sylva +Musc +Qz Abstract The Blue Ridge province of the Appalachian Mountains has a complex geologic history characterized by more than one metamorphic and deformational event. Outcrops from Ducktown in the western Blue Ridge (WBR), and Savannah Church and Sylva in the central Blue Ridge (CBR), and Beaucatcher Mountain in the eastern Blue Ridge (EBR) expose pelitic and psammitic rocks metamorphosed under amphibolite to granulite facies conditions. The Ducktown metapelites contain the assemblage garnet, biotite, staurolite, and plagioclase, whereas Savannah Church and Beaucatcher Mountain contain the assemblage garnet, biotite, plagioclase, and hornblende. The Sylva outcrop in the middle of the CBR contains the assemblage garnet, biotite, plagioclase, sillimanite, gedrite, and anthophyllite. All samples from the CBR and EBR are characterized by almandine rich garnets (Xalm = 0.65 – 0.75, Xprp = 0.07 – 0.2, Xgrs < 0.1), with the exception of samples from Sylva which have garnets with Xprp = 0.17 – 0.25. All garnets are unzoned with the exception of some retrograde rim re-equilibration. Feldspars from these samples are albite rich plagioclases (Xab=0.50-0.70, Xan= 0.21-0.50, Xor=0.0-.02), with the exception of samples from Sylva which are more calcic with Xan=0.40-0.50). Using the average P-T routine of Thermocalc 3.26 and conventional Gt-Bt and GASP thermobarometry, peak metamorphic conditions for the Ducktown samples (WBR) were calculated at 530°C, 6.5 kbar. Conditions for Savannah Church were calculated at 650-700°C, 9-11 kbar with Beaucatcher Mountain having conditions at 700°C, 7 kbar. On the other hand, P-T conditions for Sylva are estimated at 700°C, 18 kbar. These results show that the P-T conditions across the Blue Ridge are only broadly consistent with the isograds mapped for the terrane. The most striking anomaly is the unusually high pressures calculated for the Sylva and Savannah Church outcrops, which are inconsistent with the overall trend of decreasing temperatures and pressures from Winding Stair Gap in the south to outcrops in the north. Petrography of Samples Hb (Cats) D C Savannah Church: A) Bt showing crenulation pattern around a Gt crystal. Garnet Mine: B) Plane polarized light photomicrograph of oriented opaques and large Rt crystals included in Bt. C) strongly pleochroic St crystal with oriented inclusions of Ap and Ilm. D) Gt with Sph inclusions. Bt Bt Bt A B Gt Seventeen samples were selected for petrographic analysis (two from BCM, five from Little Pine, three from Savannah Church, two from Sylva, and one from each of the other localities mentioned above). Petrographic analysis of each of these samples was carried out, analyzing textures, mineral assemblages, and modal percentages. Bt Op St Gt Op Rt Gt Qz F M Fig.3 G Amph F E H J K L I Bt Plag Gt Plag Gt Qz Clz Bt Bt Hb A Bt Garnet Mine St Clz Bt Sill The rocks of the Blue Ridge record a tectonometamorphic history that ranges from the Grenville orogeny to the Late Paleozoic Alleghanian orogeny (Stewart et al., 1997). Like the Appalachians, the Blue Ridge consists of different terranes accreted at different times. The nature of many of the boundaries between these terranes is not well understood or agreed upon. The metamorphic facies are found in complex combinations that make it difficult to interpret their origins and with which orogeny they formed. A detailed petrological study of the pelitic rocks from the Blue Ridge had not been carried out, nor had there been any systematic study of thermobarometric conditions or P-T time paths. The metamorphic facies map (Figure 1) compiled byHatcher et al. (2005) is based on extensive fieldwork from numerous outcrops, but limited petrological studies. However, careful petrographic studies of samples from several localities that were mapped as belonging to a certain metamorphic zone revealed that they contained mineral assemblages incompatible with these metamorphic zones. Based on these new relationships, it is difficult to identify which orogenic event is related to the peak mineral assemblage in each area. It is also difficult to determine whether metamorphism preceded or post-dated movement along the terrane boundaries. Petrological studies may also lead to a better understanding of the tectonic history of the various terranes. • Sylva: E) Gt rimmed by Clz in crossed-polarized light.F) Bt showing symplectic texture. Taken under crossed-polarized light. G) Clz showing symplectic texture. Taken under crossed-polarized light. H) crossed-polarized light photomicrograph of Plag showing a myrmikitic texture. I) crossed-polarized photomicrograph showing large Plag crystal with inclusions of Gt and Bt. J) Hb crystals near small grained Gt. K) Staurolite showing poikiloblastic and blastoporphyritic textures with quartz, biotite and ilmenite inclusions. L) Gt crystal with inclusions of Qtz and rimmed in part by Bt under crossed-polarized light. Qz Introduction St 10 μ 10 μ 10 μ 10 μ 10 μ 10 μ 10 μ 10 μ 10 μ 10 μ 10 μ 10 μ Clz Gt Clz Gt Cc Gt Ged F M Plag Ath Bt Bt Fe A Gt Cc Bt Mn B Qz +Musc +Qz Mn Fe Bt A A Savannah Church +Musc +Qz Ducktown +Musc +Qz Bt Mg Clz Qz Fig. 1: Isograd map of the different metamorphic facies of the Blue Ridge area showing the different sample areas and the Winding Stair Gap along with the major fault zones (HF: Hayesville Fault, CF: Chattahooche Fault) (Hatcher, 2010). • Objectives of this study: • Identify peak metamorphic mineral assemblages in several areas throughout the Blue Ridge Province. • Constrain peak P-T conditions of metamorphism for areas throughout the Blue Ridge. • Compare P-T conditions to identify any patterns. • Compare P-T conditions within and between the different terranes. • Identify relationships between metamorphism and major structural elements. Fig. 4: AFM diagrams for each location showing peak mineral assemblages. Chl Gt Ca Gt PT Conditions Based on both ThermoCalc and the Reche and Martinez PT Calculation, the WBR showed pressures and temperatures lower than the rest of the areas shown averaging about 6.5 kbars and 530˚C. Towards Sylva, pressures and temperatures reach an anomalous 14 kbars and 750˚C. Farther east, crossing into the DGB, Savannah Church pressures decrease to about 8 kbars and decrease in temperature to500 ˚C. Up to the north, the Little Pine Garnet Mine, according to the ThermoCalc program, pressures reached 11 kbars, however, after using the AFM diagram above for the Garnet Mine, the Reche and Martinez calculation seems to be more realistic, since not only was Sill present, but St also. Therefore, with the presence of such large staurolite crystals, temperatures seem more reliable at the 6.5-7 kbars and 500 ˚C. The Beaucatcher Mountains of the EBR have similar conditions to the garnet mine. Pressures at this location were about 6.5 kbars with temperatures at about 750 ˚C. Fig. 5: AFM diagrams for each of the locations. Mn Bt F M F M Fe Bt Geologic Setting Mg A Rt Beaucatcher Mountain Sill The main focus for collecting rock samples was a 120 mile stretch along highway 64, State Rt. 23, and highway 74 starting in Ducktown Tennessee, and ending north west of Asheville, North Carolina. Rocks were collected from 5 outcrops along this transect; Ducktown and Murphy in the western Blue Ridge (WBR), Savannah Church (SVC), Sylva (SV), and Little Pine Garnet Mine in the central Blue Ridge (CBR), and finally Beaucatcher Mountain (BCM) in the eastern Blue Ridge (EBR). According to Hatcher et al.’s (2005) map showing structural boundaries, when my sample locations are placed on this map, the Ducktown outcrop is located in the Laurentian terrane, Murphy outcrop is situated in the Murphy Syncline, the Sylva outcrops are located in the Cartoogechaye terrane (Sylva being right along the edge of the Dahlonega Gold Belt), Savannah Church is located in the Dahlonega Gold Belt, and Little Pine Garnet Mine is located in the grenvillian gneisses margin, and Beaucatcher mountain is in the Tugaloo terrane. Gt Qz Fig.2a: Map of the different over and underthrusting terranes with outcrop sample locations (Hatcher, 2010). Fig. 5: Zoning Patterns in Garnet from Sylva. Note high Spss occurs where Bt is in contact with Gt. Clz Bt Gt Sph F M Bt Fig.2b: Cross section of the different terranes and the approximate ages of each (Hatcher, 2010). A B C Ilm Conclusions Ilm Ilm Bt • After determining pressures and temperatures for each of the localities, it appears there is a trend of increasing pressure from southwest to northeast and then a decrease farther northeast past the Sylva locality and into the Beacatcher Mountain locality. As for the temperature, there doesn’t seem to be a particular trend. The most astounding results were from the Sylva outcrop, where pressures reached an anomalous high of 18 kbars with a temperature of about 950°C, 13 kbars and 800°C respectively. This data allows for future research to be done to determine whether the isograds precede or post-date movement. According to El-Shazly et. al., the CBR (Cartoogechaye T.) was thrust under the WBR (Laurentia), followed by the Dahlonega gold belt being thrust on top of the CBR. These events were concluded by the collision of the Tugaloo terrane, which was thrust on top of all three earlier terranes. When the DGB and Tugaloo T. collided with the CBR, it caused the Cartoogechaye T. to be first thrust under Laurentia and then pushed out with the final collision. This would explain the higher temperatures and pressures in the CBR. • WBR has the lowest PT assemblages in Ducktown. • Temperatures and pressures in the CBR are higher than the WBR and EBR. • Thermal axis seems to be the highest temperature and pressure in Sylva, not Winding Stair Gap. It was possibly a thrusted slice that was pushed back up from greater depths, not fitting in with the rest of the CBR, which would explain higher temperatures. • Savannah Church is part of the Dahlonega Gold Belt and not the Cartoogechaye terrane, which would explain its lower results. • Sill was found in the Garnet Mine location (Gt zone on Fig. 1), meaning the metamorphic facies for that area should be re-evaluateddue to the discovery of mineral assemblages higher than what they were mapped. D Chl Sill Gt Ged Mineral Chemistry Garnets from the Murphy location were almandine rich and significantly high in spessartine and pyrope (Xalm=0.55-0.56, Xprp=0.15-0.17, Xgrs=0.2-0.5, Xspss= 0.21-0.22). Feldspars here were albite rich (Xan=0.21-0.22, Xab-0.78-0.80). Garnets from the Sylva outcrop were characterized by almandine rich garnets but with substantial amounts of grossular and pyrope (Xalm= 0.48-0.59, Xprp=0.22-0.24, Xgrs= 0.21-0.23, Xspss= 0.02). Feldspars from Sylva exhibited albite rich plagioclases (Xab= 0.37-0.60, Xan= 0..50-0.62). Savannah Church garnets were characterized by almandine rich garnets with small to moderate amounts of pyrope and grossular, and substantial amounts of spessartine (Xalm= 0.66-0.70, Xprp=0.12-0.14, Xgrs= 0.06-0.07, Xspss= 0.08-0.16). Feldspars from Savannah Church were characterized by albite rich plagioclases (Xab= 0.69-0.71, Xan= 0.06-0.22). Garnets from the garnet mine outcrop were almandine rich (Xalm= 0.61-0.62, Xprp=0.25-0.30, Xgrs= 0.05-0.07, Xspss= 0.02). Feldspars were characterized by albite rich plagioclases (Xab= 0.54-0.63, Xan= 0.19-0.25). Beaucatcher mountain garnets were almandine rich (Xalm= 0.73-0.75, Xprp=0.09-0.12, Xgrs= 0.03-0.05, Xspss= 0.07-0.14). Feldspars from Beaucatcher mountain were characterized by albite rich plagioclases (Xab=0.63-0.69, Xan= 0.29-0.33). Bt Ged Ap Chl Bt Bt Gt Gt E F G H Bt Qz Ksp Clz Plag Ksp Fig. 6: Selected textural relations. A-D are from the Garnet Mine outcrop, E-H are from the Savannah Church outcrop. Gt Qz Bt Qz Clz