Presentation on theme: "P-T Conditions of Selected Samples across the Blue Ridge Province Breana Felix, Geology Department, Marshall University Abstract The Blue Ridge province."— Presentation transcript:
P-T Conditions of Selected Samples across the Blue Ridge Province Breana Felix, Geology Department, Marshall University 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 (X alm = 0.65 – 0.75, X prp = 0.07 – 0.2, X grs < 0.1), with the exception of samples from Sylva which have garnets with X prp = 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 (X ab =0.50-0.70, X an = 0.21-0.50, X or =0.0-.02), with the exception of samples from Sylva which are more calcic with X an =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. 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 by Hatcher 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. 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). Introduction 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. 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. Fig.2a: Map of the different over and underthrusting terranes with outcrop sample locations (Hatcher, 2010). Fig.2b: Cross section of the different terranes and the approximate ages of each (Hatcher, 2010). 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. Clz Qz Gt Cc Clz Bt Clz Amph 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. Cc Bt Qz Bt Gt Bt Qz Gt Bt Plag Bt Plag Petrography of Samples E F G H I Gt Ilm Bt Chl Ged Ilm Gt Ksp BtQz Clz Bt Ksp Bt Qz Plag Qz Clz Bt Ged Chl Bt Ilm Sill Ap Fig. 6: Selected textural relations. A-D are from the Garnet Mine outcrop, E-H are from the Savannah Church outcrop. ABC D EFGH Geologic Setting Gt Mn Fe Mg Ca Mn Fe Gt Bt Sph Qz Clz Rt Fig. 5: Zoning Patterns in Garnet from Sylva. Note high Spss occurs where Bt is in contact with Gt. Mn Fe Mg A B 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-evaluated due to the discovery of mineral assemblages higher than what they were mapped. Conclusions Fig.3 Bt Clz 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). 10 μ A FM Sylva Bt Gt Hb (Cats) +Musc +Qz Bt Gt A FM Savannah Church +Musc +Qz F Chl Gt A M Ducktown Bt +Musc +Qz A FM Beaucatcher Mountain Sill Bt Gt 10 μ C D JKL 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. Fig. 5: AFM diagrams for each of the locations. A 10 μ Gt Bt Qz Op 10 μ Rt Bt Op B St Bt Gt A FM Garnet Mine Ath Ged Sill +Musc +Qz 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. Table 1 Name ThermoCalc Reche/Martinez PT Calc PointPSD COR IRSigFitTSDPT Murphy GFT7 GT1C16.21 0.59 5.001.61528146.7562 GT1C26.50.9 0.57 5.001.43533126.6565 GT1C36.40.9 0.57 5.001.51533126.8582 GT2C16.50.7 0.55 5.001.23531106.1555 GT2C26.31 0.58 5.001.61534136.7564 GT2C37.10.6 0.53 5.000.9653286.3569 GT2C46.61.1 0.57 5.001.71533146.5552 AVERAGE6.50.9 0.57 5.001.44532126.5564 GT1R16.40.9 0.57 5.001.51533127.0568 GT1R26.61 0.59 5.001.66536146.3551 GT1R36.51.1 0.58 5.001.78536156.6544 GT1R46.60.8 0.56 5.001.35535116.4529 GT2R16.41 0.59 5.001.69535146.1552 GT2R26.50.8 0.56 5.001.36532117.4557 GT2R37.20.8 0.64 5.000.82606326.3569 GT2R46.21 0.59 5.001.61530135.5523 AVERAGE6.60.9 0.58 5.001.47543156.5549 Sylva SV4 GT1C214.11.7 0.94 8.002.427575218.0557 GT2C213.21.8 0.94 8.002.557415318.0568 GT4C113.21.8 0.93 8.002.637425418.0551 AVERAGE13.51.8 0.94 8.002.537475318.0559 GT1R214.11.7 0.94 8.002.317655219.0562 GT1R513.91.8 0.94 8.002.557655518.0550 GT2R413.81.7 0.94 8.002.47495218.0523 GT2R512.01.7 0.93 8.002.667045418.0552 GT2R614.11.8 0.94 8.002.437675318.0588 GT4R214.01.7 0.94 8.002.297615017.5540 AVERAGE13.71.7 0.94 8.002.447525318.1553 SV7 GT1C111.91.6 0.90 9.002.077246018.2968 GT1C210.61.5 0.91 9.002.026965915.9908 GT1C312.01.6 0.90 9.002.17366219.91044 GT1C412.01.6 0.90 9.002.087366118.4982 AVERAGE11.61.575 0.90 9.002.06757236118.1975 GT1R112.61.4 0.96 9.001.187665312.8760 GT1R211.51.7 0.91 9.002.227096517.2925 GT1R312.11.9 0.90 9.002.467487221.21099 GT1R411.11.6 0.90 9.002.157186317.2970 GT1R511.71.7 0.91 9.002.317206718.5983 GT1R612.31.9 0.90 9.002.437397120.01040 GT1R710.91.7 0.91 9.002.237036615.3876 AVERAGE11.71.7 0.91 9.002.147296517.5950 Savannah Church SVC5 GT1C18.11 0.82 4.001.84486239.6667 GT1C28.10.9 0.82 4.001.82486229.2670 GT1C38.01.0 0.82 4.001.86485239.2645 AVERAGE8.11.0 0.82 4.001.84486239.3661 GT1R18.81.3 0.80 4.002.37494298.9615 GT1R28.11.2 0.79 4.002.284472711.0701 GT1R37.70.9 0.83 4.001.71480219.0656 GT1R47.60.9 0.82 4.001.83477229.1680 AVERAGE8.11.1 0.81 4.002.05475259.5663 Little Pine Gt-Mine LP7 GT1C111.32 0.83 11.002.548961376.9545 GT1C211.11.4 0.83 11.001.83887985.5495 GT1C311.62 0.83 11.002.529151397.4582 GT1C411.31.8 0.83 11.002.398871269.3574 GT1C511.52.1 0.83 11.002.599071427.2549 GT1C610.91.9 0.83 11.002.518771335.9503 GT1C711.51.9 0.84 11.002.439061349.0569 GT1C811.12.2 0.84 11.002.758851515.3480 GT1C911.22.2 0.84 11.002.658851467.1476 AVERAGE11.31.9 0.83 11.002.478941347.1530 GT1R111.12 0.83 11.002.58971375.6504 GT1R211.12 0.83 11.002.498841346.7475 GT1R311.32.1 0.83 11.002.638961437.1543 GT1R411.22.1 0.83 11.002.638901475.0481 GT1R511.71.5 0.84 11.001.869171036.9567 GT1R611.22.2 0.84 11.002.698931485.8497 GT1R711.61.7 0.85 11.002.089121196.0509 GT1R1011.22.3 0.85 11.002.778951586.2456 GT1R1111.71.8 0.85 11.002.119241235.5488 GT1R1211.32.6 0.85 11.003.049071765.0455 GT1R1511.420 0.85 11.002.349001324.5443 GT1R1611.02.1 0.84 11.002.928801625.0441 GT1R1711.32.3 0.84 11.002.779071575.5502 GT1R1911.42.3 0.85 11.002.719061556.2484 GT1R2011.61.9 0.85 11.002.269191334.8466 AVERAGE11.33.3 0.84 11.002.529021425.7487 Beaucatcher Mountain BCM2 GT3C16.93.2 0.80 3.002.158432896.2723 GT4C16.32 0.83 3.001.57251685.8645 GT5C18.23.5 0.85 3.002.119263237.7765 GT6C17.51.4 0.83 3.000.726731007.5605 AVERAGE6.92.6 0.84 3.001.608382227.2711 GT2R17.22.1 0.85 3.001.557581777.4715 GT2R28.12.4 0.86 3.001.638402108.5767 GT3R16.72.4 0.86 3.001.777782155.1620 GT3R26.53 0.82 3.002.197982753.9579 GT4R16.81.4 0.82 3.000.996981095.4561 GT4R26.91.5 0.82 3.001.067021164.3531 GT6R16.51.8 0.83 3.001.466761465.9612 AVERAGE7.02.1 0.84 3.001.527501785.8626 Table 2. Modal contents of samples. GFT-24GFT-7GFT-5GFT-23LP-1LP-10LP-7LP-12LP-13SVC-7SVC-3SVC-5BCM-2SV-4SV-7 Quartz 25036–13011–21303833839 Biotite 3351020151030105415352515 30 Muscovite –1051–1–2––13–– Gedrite ––––49–2643–––––20– Anthophyllite ––––––1018–––––15– Hornblende 815205–5–121 Garnet 10525814117151055151 Plagioclase 15165513510 361511 Orthoclase –––1–––––––10–11 Rutile ––––––1––––––1– Ilmenite 1203–1510 8111093133 Chloritoid ––––210–––1––––– Clinozoisite 7––2–––10–––––13 Epidote 5––1–––––––1––– Sill ––––––11––––––– Staurolite –8––721–––––0– Clinopyroxene 6––1?––––––––––– Calcite 5–––––––––1–1–10 Chlorite –120–5–10520––3––– Zircon ––111––––111111 Apatite 1––––11––111–1– Sphene 6–––––1–––––12– Monazite 1––––––––11111– Xenotime –––––––11–1111– St Bt Gt Bt Hb Plag Bt Gt Qz St Fig. 4: AFM diagrams for each location showing peak mineral assemblages.