Mill Creek Nappe

Avondale Massif

West Chester Massif

Mill Creek Nappe

Street Road Fault

Embreeville thrust

Advances in constraining the timing of deformation and metamorphism: Salinic through Acadian transpressional tectonics in the Central Appalachians

Howell Bosbyshell1, LeeAnn Srogi1 and Gale Blackmer2

1West Chester University 2Pennsylvania Geological Survey

ReferencesAleinikoff, J.L, Schenck, W. S., Plank, M. O., Srogi, L. A., Fanning, C. M., Kamo, S. L., and Bosbyshell, H., 2006, Deciphering igneous and metamorphic events in high-grade rocks of the Wilmington Complex, Delaware: Morphology, cathodoluminescence and backscattered electron zoning, and SHRIMP U-Pb geochronology of zircon and monazite: Geological Society of America Bulletin, v. 118, p. 39-64.

Blackmer, G. C., 2005, Preliminary bedrock geologic map of a portion of the Wilmington 30- by 60-minute quadrangle, southeastern Pennsylvania: Pennsylvania Geological Survey, 4th ser., Open-File Report OFBM 05–01.0, 16 p., Portable Document Format (PDF) file.

Bosbyshell, H., Blackmer, G.C., Srogi, L., Mathur, R., Crawford, M., Valencia, V., and Schenck, W.S., 2012, Detrital zircon ages from the Wissa-hickon Formation, southeast Pennsylvania and northern Delaware: Regional tectonic implications: Geological Society of America Abstracts with Programs, v. 44, no. 2, p. 87.

Bosbyshell, H., 2008, Bedrock geologic map of a portion of the Philadelphia quadrangle, Montgomery and Philadelphia Counties, Penn-sylvania: Pennsylvania Geological Survey, 4th ser., Open-File Report OFBM 08–05.0, 21 p., Portable Document Format (PDF).

Faill, R. T., 1997a, A Geologic history of the North-Central Appalachians. Part 1. Orogensis from the Mesoproterozoic through the Taconic orogeny: American Journal of Science, v. 297, pp. 551-619.

Hanan, B. B., and Sinha, A. K., 1989, Petrology and tectonic affinity of the Baltimore mafic complex, Maryland: in Mittwede, S. K., and Stoddard, E. F., eds., Ultramafic rocks of the Appalachian Piedmont: Geological Society of America Special Paper 231, p. 1-18.

Pyle, J.M., Bosbyshell, H., and Blackmer, G.C., 2006, Refining the metamorphic and tectonic history of the southeastern Pennsylvania Piedmont: Recent results from monazite and zircon geochronology and accessory-phase thermometry, in Pazzaglia, F.J., ed., Excursions in Geology and History: Field Trips in the Middle Atlantic States: Geological Society of America Field Guide 8, p. 83-112.

Srogi, L., Blackmer, G.C. and Bosbyshell H., 2007, Contrasting metamorphic histories within the Wissahickon Formation: Evidence for subdivision and tectonic implications: Geological Society of America Abstracts with Programs, v. 39, no. 1, p. 86.

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Geologic Map of Southeastern Pennsylvania and Northern Delaware

Pa.

Del.Md.

S t r e e t R o a d F a ul t

E mb

r e e v i l l e T h r u s t

H u n t i n g d o n V a l l e y S Z

UD

U

D

U

DU

D

Pa.Del.

Wilmington Complex

Coastal Plain

Avondale MassifWest C

hester Massif

Honey Brook Upland

Mine Ridge

Mill Creek

Anticli

ne

M a r t i c L i n e

Woodville

Nappe

Legend

Stockton Fm.

Triassic rocks

10 Km

N

S

EW

40º

41º41º75º

78º80º

77º42º

79º

40º

76º

80º

75º

76º

77º

78º42º

75º

79º

DEL.

Octoraro FormationChickies, Harpers, Antietam Fms.undifferentiated meta-clastic rocksUndifferentiatedmeta-carbonate rocks

Lower PaleozoicRocks of known and probable Laurentian affinity

Cambrian West Grove Metamorphic Suite

Peters Creek Schist

Mt. Cuba Gneiss

Laurels and Doe Run Schist

Glenarm GroupPrecambrian

Anorthosite

Quartz monzonite gneiss

Felsic and intermediate gneiss

Graphitic felsic gneiss

West Chester MassifGranulite faciesAmphibolite facies

Avondale massif

Undifferentiated gneissSycamore Mills Fm.

Kennett Square Amphibolite

White Clay Creek Amphibolite

Silurian plutonic/metaplutonic rocks

Ordovician metaigneous rocksGabbro Granite/

granodiorite

Volcanic Plutonic

Undifferentiatedultramafic rocks

Wissahickon Fm

Ordovician meta-clastic and meta-volcanic rocks

Rocks of arc affinity

This poster focuses on the West Grove Meta-morphic Suite (WGMS)- the Mt. Cuba Gneiss and Doe Run Schist and the arc-related Wissahickon Formation.

Simplified Geologic Map of the Central Appalachian Piedmont

Mesozoic Basin

Wissahickon FmWilmington Complex

James Run arc/BMC

Coastal Plain

Mt. Cuba GneissLaurels & Doe Run SchistMetasedimentary rocksClastics and carbonatesGrenville gneiss

0 km 25

A

N

39 o30'N

40 o30'N

40 oN

75o W

76o WPa.Md.

MAP AREA

Modified from Faill (1997), Hanan and Sinha (1989).

R o s e m

o n

t

S

Z

CC-7205

B-22

Ge-06-33

Wiss-1N

P-05-02

WG

WG-43

C-207GW-05-54

44069

-216

Bc21-bWG-101

GW-05-154Doe Run SchistUnionville, Pa

n = 124

C-207Doe Run Schist

Chatham, Pan = 90

500 1000 1500 2000 2500Ma

Bc21-bMt. Cuba Gneiss

Ashland, Den = 95

44069Mt. Cuba Gneiss

Yorklyn, Den = 49

Aleinikoff et al. 2006

Wg-101Mt. Cuba GneissWest Grove, Pa

n = 95

0 500 1000 1500 2000 2500Ma

525Ma

0 500 1000 1500 2000 2500 3000Ma

The West Grove Metamorphic Suite is no longer considered part of the Wissahickon Formation, based on differences in tectono-thermal history and detrital zircon provenance. (Bosbyshell et al, 2012).

West Grove Metamorphic Suite Wissahickon Formation

CC46-2Wissahickon Fm.Glen Riddle, Pa

n = 116

Wiss-1NWissahickon Fm.

Wissahickon CreekPhiladelphia, Pa

n = 139

P-05-02Wissahickon Fm.

Fairmont ParkPhiladelphia, Pa

n = 143

610 Ma 1200 Ma

1470 Ma960 Ma

554 Ma

574 Ma

1050 Ma960 Ma

1035 Ma

546 Ma

1050 Ma

945 Ma

1020 Ma

965 Ma

547 Ma

895 Ma

1195 Ma

Doe Run Schist

WG-216GW-05-154C-207

340

360

380

400

420

440

460

480

520

540

560

Age

(Ma)

staurolite

sillimanite

WG-216

Width of field ≈ 6mm for both photos

Ca

434

430

471

491

427

418

m1

The Embreeville Thrust places the West Grove Metamorphic Suite and all struc-turally higher rocks above the green-schist grade Peters Creek Formation.

staurolite

sillimanitegarnet

C-207

Ca x-ray map of garnet from WG-216. Low Ca rim is in-terpreted to correspond to garnet growth during high T staurolite breakdown (Srogi et al., 2007). Late garnet and sil after st (photomicrographs) is syn- to post-tectonic. Isopleths of this garnet rim compositon inter-sect at approximately 700C and 5 kb.

400 500 600 700 800Temperature [C]

4000

5000

6000

7000

8000

9000

10000

Pres

sure

[Bar

]

pl ilm grt bt sil qtz liq qfm

st-bearingassemblages

WG-216

kyan

itesil

liman

ite

-sta

urol

ite

-bio

tite

+liquid-H

2O

+cordierite

+sill

iman

ite

-chloritoid

+ rt- ilm

+chlorite

-chlorite

T h

e r i

a k - D o m i n o

Monazite with young, low-Y, high-Th rims is present in the matrix and as inclusions in the high-Ca garnet rims in WG-216. High-T metamor-phism can be no older than 410 ± 11 Ma. Textural considerations on the timing of monazite growth similarly constrain the age of high-T metamorphism and deformation in C-207 to 411 ± 11 Ma.

alm

grs

pyr

sps

Mt. Cuba Gneiss

2 mm

Rose

mon

t She

ar Z

one

340

360

380

400

420

440

460

480

Age

(Ma)

Monazite is complexly zoned and may contain two age populations: 419 ± 9.7 Ma and 437 ± 11.0 Ma. Both inclusions and matrix grains contain compositional domains which yield both ages. While there are instances where textures are con-sistent with this age difference, such examples are not ubiquitous. Thus it is not clear that these re-sults indicate two distinct periods of monazite growth. The average of all analyses is 427 ± 14 Ma, similar to previously reported TIMS and SHRIMP results (above, Aleinikoff, 2006).

44069 (monazite)Wissahickon Formation

TIMS data

206

Pb/23

8U

207 Pb/235 U

429

427

426

424

422

4200.0672

0.0676

0.0680

0.0684

0.0688

0.512 0.514 0.516 0.518 0.520 0.522 0.524 0.526 0.528

Concordia Age = 424.9 ± 0.4 Maprobability of concordance = 0.42

P-T conditions in Mt. Cuba Gneiss in the Mill Creek nappe, Yorklyn, De (loc. 44069) are estimated to have exceeded 750°C, based on the presence of perthitic feldspars, at pres-sures in the sillimanite stability field, but greater than 0.6 GPa, based on a lack of cordierite (Bosbyshell et al., 2011). SHRIMP and TIMS monazite ages, 426 ± 3 Ma and 424.9 ± 0.4 Ma, respectively (Aleinikoff, et al., 2006), constrain the timing of high-T metamorphism and deformation.

T h

e r i

a k - D o m i n o

550 650 750 850Temperature [C]

4000

5000

6000

7000

8000

9000

10000

Pres

sure

[Bar

]

23.01.2012 - 10:53:11 CPU time: 5h 04m 32.46

- sillimanite

kyan

itesil

liman

ite

- bio

tite

-sta

urol

ite

-staurolite

-bio

tite

+liquid-H

2O

-mus

covi

te

+cordierite

+cordierite

+kyanite

+sill

iman

ite

-chl

+ms

+ rutile- ilmenite

pl ilm grt bt sil qtz

H2O qfm

pl ilm grt bt sil qtz liq qfm

WG-43b

Ca Mn

The dominant foliation in the Mount Cuba Gniess above the Street Road Fault (sample WG-43) is defined by aligned sillimanite and biotite. Staurolite contains inclusions of foliation-parallel sillimanite and cross-cuts sillimanite fabrics. Kyanite occurs as small crystals which overprint sillimanite and high Ca-rims are present on garnet. This suggests a second period of mineral growth during isobaric cooling from peak temperatures.

B22Ca Mn

U

Wissahickon Formation

Age

(Ma)

Age

(Ma)

300

320

340

360

380

400

420

440

460

480

500

380

390

400

410

430

450

470

420

440

460

480

B-22

Rim domains

320

340

360

380

400

420

440

460

480

Outer core

388 ± 12 Ma

(n = 24)

Cores

Rims

CC-7205

Above, syn-tectonic garnet in ultramylonite from the RSZ (loc. CC-7205). P-T estimates of 600C at 8 kb were ob-tained using the composi-tion of this garnet with recrystallized matrix pla-gioclase and biotite. Syn-tectonic monazite is Devonian, 388 ± 12

Ma. The shear zone cuts high-T, low-P pelitic migmatite. Monazite cores record this earlier metamorphism and are similar in age to zircon and monazite in the adjacent Wilmington Complex (Aleinikoff et al., 2006).

0.5 mm

Rosemont Shear Zone

Sample B-22 demonstrates the metamorphic and deformational history of the western, probably lower portion of the Wissa-hickon Formation. Garnet maps show a low-Ca core, which formed during M1, high-T low-P metamorphism (andalusite-sillimanite grade), with a high-Ca overgrowth the formed during M2, higher-P, kyanite-grade metamorphism. The fabric in this rock, S3 wraps the M1 garnet core, but is included within M2 overgrowth. Monazite cores (433 ± 7 Ma) likely record the time of M1. Asymmetric high-U rims on monazite constrain the timing of S3 (average, 415 ± 14 Ma) and are included in high-Ca garnet rims.

While complexly deformed, the eastern Wissahickon Formation is characterized by a classic Barrovian, st-ky-sil metamorhpic field gradient and exhibits little evidence of the Silurian high-T meta-morphism. Monazite ages indicate that this metamorphism is Devonian, approximately 384 Ma(Bosbyshell, 2008).

Ge-06-33P-05-02

Cores

Tectonic Speculation

Modified from Blackmer, 2005; Pyle et al., 2006.

Wilmington ComplexWissahickon Fm.

Iapetus Ganderia ? Avalonia

Ordovician - ~480 Ma

Silurian ~430 Ma

AvaloniaDoe Run Mt.Cuba Wiss Fm.Laurentia

AvaloniaLaurentia

Wilmington Complex is intra-Iapetan arc.

High-T low-P metamorphism in Wissahickon, then Mt. Cuba Granulite metamorphism of Wilmington Complex

Early Devonian ~410 Ma

Salinic Orogeny

Street Road Fault

Dextral transpressionBurial and metamorphism of Doe Run SchistDeformation onset of crustal thickening in Wissahickon Fm.

Rosemont Shear Zone

AvaloniaLaurentia

Street Road FaultEmbreeville Thrust

Rosemont Shear Zone

Early stages of Acadian orogeny

Middle Devonian ~385 MaEnd of Acadian orogeny

Barrovian metamorphism of Wissahickon Fm.