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

Breana Felix, Geology Department, Marshall University

AbstractThe 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 (X ab=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.

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

Clz

BtClz

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.

CcBt

QzBt Gt

Bt

QzGt

Bt

PlagBt

Plag

Petrography of Samples

E F G H I

Gt

Gt

Gt

Ilm

Bt

Chl

Ged

Ilm

Gt

Ksp

Bt Qz Clz

Bt

Ksp

BtQz

Plag

Qz Clz

Bt

Ged

Chl

BtIlm

Sill

Ap

Fig. 6: Selected textural relations. A-D are from the Garnet Mine outcrop, E-H are from the Savannah Church outcrop.

A B C

D

E F G H

Geologic Setting

Gt

Mn

Fe

Mg

Ca

Mn FeGt

BtSph

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

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 8000

10

20

30

40

50

60

70

80

GFT-24 Garnet #3 Zoning Profile

AlmandineGrossularPyropeSpessartineFe/(Fe+Mg)

Distance in Microns (μ)

%

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

50

60

70

80

90

GFT-24 Garnet #2 Zoning Profile

AlmandineGrossularPyropeSpessartineFe/(Fe+Mg)

Distance in Microns (μ)

%

Bt

Clz

Mineral ChemistryGarnets 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 μ10 μ10 μ10 μ

A

F M

Sylva

Bt

GtHb (Cats)

+Musc+Qz

Bt

Gt

A

F M

Savannah Church +Musc

+Qz

F

ChlGt

A

M

Ducktown

Bt

+Musc+Qz

A

F M

Beaucatcher Mountain

Sill

Bt

Gt

10 μ

C D

J K L

10 μ 10 μ 10 μ

10 μ10 μ

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

F M

Garnet Mine

Ath

Ged

Sill+Musc+Qz

PT ConditionsBased 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

Point P SD COR IR SigFit T SD P T

Murphy GFT7

GT1C1 6.2 1 0.59 5.00 1.61 528 14 6.7 562GT1C2 6.5 0.9 0.57 5.00 1.43 533 12 6.6 565GT1C3 6.4 0.9 0.57 5.00 1.51 533 12 6.8 582GT2C1 6.5 0.7 0.55 5.00 1.23 531 10 6.1 555GT2C2 6.3 1 0.58 5.00 1.61 534 13 6.7 564GT2C3 7.1 0.6 0.53 5.00 0.96 532 8 6.3 569GT2C4 6.6 1.1 0.57 5.00 1.71 533 14 6.5 552

AVERAGE 6.5 0.9 0.57 5.00 1.44 532 12 6.5 564

GT1R1 6.4 0.9 0.57 5.00 1.51 533 12 7.0 568GT1R2 6.6 1 0.59 5.00 1.66 536 14 6.3 551GT1R3 6.5 1.1 0.58 5.00 1.78 536 15 6.6 544GT1R4 6.6 0.8 0.56 5.00 1.35 535 11 6.4 529GT2R1 6.4 1 0.59 5.00 1.69 535 14 6.1 552GT2R2 6.5 0.8 0.56 5.00 1.36 532 11 7.4 557GT2R3 7.2 0.8 0.64 5.00 0.82 606 32 6.3 569GT2R4 6.2 1 0.59 5.00 1.61 530 13 5.5 523

AVERAGE 6.6 0.9 0.58 5.00 1.47 543 15 6.5 549

Sylva

SV4

GT1C2 14.1 1.7 0.94 8.00 2.42 757 52 18.0 557GT2C2 13.2 1.8 0.94 8.00 2.55 741 53 18.0 568GT4C1 13.2 1.8 0.93 8.00 2.63 742 54 18.0 551

AVERAGE 13.5 1.8 0.94 8.00 2.53 747 53 18.0 559

GT1R2 14.1 1.7 0.94 8.00 2.31 765 52 19.0 562GT1R5 13.9 1.8 0.94 8.00 2.55 765 55 18.0 550GT2R4 13.8 1.7 0.94 8.00 2.4 749 52 18.0 523GT2R5 12.0 1.7 0.93 8.00 2.66 704 54 18.0 552GT2R6 14.1 1.8 0.94 8.00 2.43 767 53 18.0 588GT4R2 14.0 1.7 0.94 8.00 2.29 761 50 17.5 540

AVERAGE 13.7 1.7 0.94 8.00 2.44 752 53 18.1 553

SV7

GT1C1 11.9 1.6 0.90 9.00 2.07 724 60 18.2 968GT1C2 10.6 1.5 0.91 9.00 2.02 696 59 15.9 908GT1C3 12.0 1.6 0.90 9.00 2.1 736 62 19.9 1044GT1C4 12.0 1.6 0.90 9.00 2.08 736 61 18.4 982

AVERAGE 11.6 1.575 0.90 9.00 2.0675 723 61 18.1 975

GT1R1 12.6 1.4 0.96 9.00 1.18 766 53 12.8 760GT1R2 11.5 1.7 0.91 9.00 2.22 709 65 17.2 925GT1R3 12.1 1.9 0.90 9.00 2.46 748 72 21.2 1099GT1R4 11.1 1.6 0.90 9.00 2.15 718 63 17.2 970GT1R5 11.7 1.7 0.91 9.00 2.31 720 67 18.5 983GT1R6 12.3 1.9 0.90 9.00 2.43 739 71 20.0 1040GT1R7 10.9 1.7 0.91 9.00 2.23 703 66 15.3 876

AVERAGE 11.7 1.7 0.91 9.00 2.14 729 65 17.5 950

Savannah

Church

SVC5

GT1C1 8.1 1 0.82 4.00 1.84 486 23 9.6 667GT1C2 8.1 0.9 0.82 4.00 1.82 486 22 9.2 670GT1C3 8.0 1.0 0.82 4.00 1.86 485 23 9.2 645

AVERAGE 8.1 1.0 0.82 4.00 1.84 486 23 9.3 661

GT1R1 8.8 1.3 0.80 4.00 2.37 494 29 8.9 615GT1R2 8.1 1.2 0.79 4.00 2.28 447 27 11.0 701GT1R3 7.7 0.9 0.83 4.00 1.71 480 21 9.0 656GT1R4 7.6 0.9 0.82 4.00 1.83 477 22 9.1 680

AVERAGE 8.1 1.1 0.81 4.00 2.05 475 25 9.5 663

Little Pine Gt-

Mine

LP7

GT1C1 11.3 2 0.83 11.00 2.54 896 137 6.9 545GT1C2 11.1 1.4 0.83 11.00 1.83 887 98 5.5 495

GT1C3 11.6 2 0.83 11.00 2.52 915 139 7.4 582

GT1C4 11.3 1.8 0.83 11.00 2.39 887 126 9.3 574GT1C5 11.5 2.1 0.83 11.00 2.59 907 142 7.2 549GT1C6 10.9 1.9 0.83 11.00 2.51 877 133 5.9 503GT1C7 11.5 1.9 0.84 11.00 2.43 906 134 9.0 569GT1C8 11.1 2.2 0.84 11.00 2.75 885 151 5.3 480GT1C9 11.2 2.2 0.84 11.00 2.65 885 146 7.1 476

AVERAGE 11.3 1.9 0.83 11.00 2.47 894 134 7.1 530

GT1R1 11.1 2 0.83 11.00 2.5 897 137 5.6 504GT1R2 11.1 2 0.83 11.00 2.49 884 134 6.7 475GT1R3 11.3 2.1 0.83 11.00 2.63 896 143 7.1 543GT1R4 11.2 2.1 0.83 11.00 2.63 890 147 5.0 481GT1R5 11.7 1.5 0.84 11.00 1.86 917 103 6.9 567GT1R6 11.2 2.2 0.84 11.00 2.69 893 148 5.8 497GT1R7 11.6 1.7 0.85 11.00 2.08 912 119 6.0 509

GT1R10 11.2 2.3 0.85 11.00 2.77 895 158 6.2 456GT1R11 11.7 1.8 0.85 11.00 2.11 924 123 5.5 488GT1R12 11.3 2.6 0.85 11.00 3.04 907 176 5.0 455GT1R15 11.4 20 0.85 11.00 2.34 900 132 4.5 443GT1R16 11.0 2.1 0.84 11.00 2.92 880 162 5.0 441GT1R17 11.3 2.3 0.84 11.00 2.77 907 157 5.5 502GT1R19 11.4 2.3 0.85 11.00 2.71 906 155 6.2 484GT1R20 11.6 1.9 0.85 11.00 2.26 919 133 4.8 466

AVERAGE 11.3 3.3 0.84 11.00 2.52 902 142 5.7 487

Beaucatcher

Mountain

BCM2

GT3C1 6.9 3.2 0.80 3.00 2.15 843 289 6.2 723

GT4C1 6.3 2 0.83 3.00 1.5 725 168 5.8 645GT5C1 8.2 3.5 0.85 3.00 2.11 926 323 7.7 765GT6C1 7.5 1.4 0.83 3.00 0.72 673 100 7.5 605

AVERAGE 6.9 2.6 0.84 3.00 1.60 838 222 7.2 711

GT2R1 7.2 2.1 0.85 3.00 1.55 758 177 7.4 715GT2R2 8.1 2.4 0.86 3.00 1.63 840 210 8.5 767GT3R1 6.7 2.4 0.86 3.00 1.77 778 215 5.1 620GT3R2 6.5 3 0.82 3.00 2.19 798 275 3.9 579GT4R1 6.8 1.4 0.82 3.00 0.99 698 109 5.4 561GT4R2 6.9 1.5 0.82 3.00 1.06 702 116 4.3 531GT6R1 6.5 1.8 0.83 3.00 1.46 676 146 5.9 612

AVERAGE 7.0 2.1 0.84 3.00 1.52 750 178 5.8 626

Table 2. Modal contents of samples.

  GFT-24 GFT-7 GFT-5 GFT-23 LP-1 LP-10 LP-7 LP-12 LP-13 SVC-7 SVC-3 SVC-5 BCM-2 SV-4 SV-7

Quartz 2 50 36 – 1 30 1 1 – 21 30 38 33 8 39

Biotite 33 5 10 20 15 10 30 10 54 15 35 25 15 15 30

Muscovite – 10 5 1 – 1 – 2 – – 1 3 – –

Gedrite – – – – 49 – 26 43 – – – – – 20 –

Anthophyllite– – – – – – 10 18 – – – – – 15 –

Hornblende 8 1 5 20 5 – 5 – 1 2 1

Garnet 10 5 25 8 14 1 1 7 15 10 5 5 15 1

Plagioclase 15 1 65 5 1 35 10 10 36 15 11

Orthoclase – – – 1 – – – – – – – 10 – 1 1

Rutile– – – – – – 1 – – – – – – 1 –

Ilmenite 1 20 3 – 15 10 10 8 11 10 9 3 1 3 3

Chloritoid – – – – 2 10 – – – 1 – – – – –

Clinozoisite 7 – – 2 – – – 10 – – – – – 1 3

Epidote 5 – – 1 – – – – – – – 1 – – –

Sill – – – – – – 1 1 – – – – – – –

Staurolite – 8 – – 7 2 1 – – – – – 0 –

Clinopyroxene6 – – 1? – – – – – – – – – – –

Calcite 5 – – – – – – – – – 1 – 1 – 10

Chlorite – 1 20 – 5 – 10 5 20 – – 3 – – –

Zircon – – 1 1 1 – – – – 1 1 1 1 1 1

Apatite 1 – – – – 1 1 – – 1 1 1 – 1 –

Sphene 6 – – – – – 1 – – – – – 1 2 –

Monazite 1 – – – – – – – – 1 1 1 1 1 –

Xenotime – – – – – – – 1 1 – 1 1 1 1 –

St

Bt

Gt

Bt

Hb

Plag

Bt

Gt

Qz

St

Fig. 4: AFM diagrams for each location showing peak mineral assemblages.