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Howell Bosbyshell , Leeann Srogi and Gale Blackmer Advances in constraining the timing of deformation and metamorphism: Howell Bosbyshell1, LeeAnn Srogi1 and Gale Blackmer2 1 2 Salinic through Acadian transpressional tectonics in the Central Appalachians West Chester University Pennsylvania Geological Survey Simplified Geologic Map of the Central Appalachian Piedmont 44069 (monazite) Ge-06-33 A Mt. Cuba Gneiss 39 40 40 Coastal Plain 0.0688 Wissahickon Formation 429 o o o W o Wissahickon Formation Pa. P-T conditions in Mt. Cuba Gneiss in the Mill Creek nappe, This poster focuses on 30'N N 30'N TIMS data P-05-02 Md. 76 Mesozoic Basin 300 Yorklyn, De (loc. 44069) are estimated to have exceeded 427 the West Grove Meta- 0.0684 James Run arc/BMC 426 morphic Suite 750°C, based on the presence of perthitic feldspars, at pres- U 320 Wissahickon Fm 238 MAP AREA sures in the sillimanite stability field, but greater than 0.6 0.0680 424 (WGMS)- the Mt. Cuba Pb/ 340 Gneiss and Doe Run Wilmington Complex GPa, based on a lack of cordierite (Bosbyshell et al., 2011). 206 o W 422 0.0676 75 Mt. Cuba Gneiss SHRIMP and TIMS monazite ages, 426 ± 3 Ma and 424.9 ± 0.4 360 Schist and the arc- Concordia Age = 424.9 ± 0.4 Ma N probability of concordance = 0.42 Laurels & Doe Run Schist Ma, respectively (Aleinikoff, et al., 2006), constrain the timing 420 related Wissahickon 0.0672 380 Metasedimentary rocks of high-T metamorphism and deformation. 0.512 0.514 0.516 0.518 0.520 0.522 0.524 0.526 0.528 Formation. 207 235 0 km 25 Pb/ U 400 Clastics and carbonates Modified from Faill (1997), Hanan and Sinha (1989). Grenville gneiss (Ma) Age 420 Mill Creek Nappe Geologic Map of Southeastern Pennsylvania and Northern Delaware Mill Creek Nappe 440 460 2 mm 10000 WG-43b 480 340 While complexly deformed, the eastern Wissahickon Formation Ge-06-33 is characterized by a classic Barrovian, st-ky-sil metamorhpic field U 500 U 360 9000 D kyanite Z - biotite gradient and exhibits little evidence of the Silurian high-T meta- D S Wiss-1N Honey Brook Upland sillimanite y e l l a n V do 380 morphism. Monazite ages indicate that this metamorphism is B-22 ing t n Hu - ilmenite 8000 Devonian, approximately 384 Ma(Bosbyshell, 2008). 380 + rutile 400 -staurolite e n i L P-05-02 +kyanite Mine Ridge c 390 i pl ilm t t Ca Mn s r 420 +ms U B22 a u 7000 grt bt M r (Ma) Age h T D sil qtz e l l i liq qfm v 400 e 440 e -chl r Pressure [Bar] b +liquid pl ilm Z -muscovite m 6000 Ca Mn S E grt bt Rim domains N West Chester Massif 460 410 -staurolite sil qtz U +cordierite D t B-22 H2O qfm l W E GW-05-54 u a -H2O t 480 F 420 5000 -biotite +sillimanite C-207 d n S a e o CC-7205 t r e o S R t - sillimanite +cordierite m - D o k m a i i n e r o e s h T o Avondale Massif 10 Km Woodville R 430 23.01.2012 - 10:53:11 CPU time: 5h 04m 32.46 Monazite is complexly zoned and may contain 80º ? Nappe 79º 78º 77º 76º ? 42º 42º 4000 ? 75º (Ma) Age two age populations: 419 ± 9.7 Ma and 437 ± 11.0 ? ? 550 650 750 850 440 Pa. WG-216 41º 41º ? Ma. Both inclusions and matrix grains contain Del. Cores 75º Temperature [C] ? U ? compositional domains which yield both ages. 450 40º 40º ? 75º The dominant foliation in the Mount Cuba Gniess above the Street Road Fault (sample WG-43) 44069 e ? 80º 79º 78º 77º 76º While there are instances where textures are con- n i WG-43 l ? is defined by aligned sillimanite and biotite. Staurolite contains inclusions of foliation-parallel c i Wilmington Complex t sistent with this age difference, such examples are 460 ? n DEL. Bc21-b sillimanite and cross-cuts sillimanite fabrics. Kyanite occurs as small crystals which overprint A ? Legend WG-101 k e not ubiquitous. Thus it is not clear that these re- re ? C l Triassic rocks sillimanite and high Ca-rims are present on garnet. This suggests a second period of mineral il ? 470 M sults indicate two distinct periods of monazite ? Stockton Fm. growth during isobaric cooling from peak temperatures. ? growth. The average of all analyses is 427 ± 14 Ma, ? 480 Pa. similar to previously reported TIMS and SHRIMP Md. Del. Rocks of known and probable Laurentian affinity results (above, Aleinikoff, 2006). Coastal PlainLower Paleozoic Cambrian Tectonic Speculation Octoraro Formation West Grove Metamorphic Suite Street Road Fault Wilmington Complex Avondale Massif Ordovician - ~480 Ma Rocks of arc affinity Chickies, Harpers, Antietam Fms. Mt. Cuba Gneiss Wissahickon Fm. Sample B-22 demonstrates the metamorphic and deformational Ganderia ? undifferentiated meta-clastic rocks Laurels and Doe Run Schist Doe Run Schist Iapetus Avalonia Silurian plutonic/metaplutonic rocks Undifferentiated Ca history of the western, probably lower portion of the Wissa- Granite/ WG-216 Gabbro granodiorite meta-carbonate rocks Kennett Square Amphibolite C-207 hickon Formation. Garnet maps show a low-Ca core, which Ordovician metaigneous rocks Peters Creek Schist White Clay Creek Amphibolite staurolite staurolite formed during M1, high-T low-P metamorphism (andalusite- Wilmington Complex is intra-Iapetan arc. Volcanic Plutonic Glenarm Group sillimanite grade), with a high-Ca overgrowth the formed during Silurian ~430 Ma Salinic Orogeny Precambrian M2, higher-P, kyanite-grade metamorphism. The fabric in this Ordovician meta-clastic Avondale massif Wiss Fm. rock, S3 wraps the M1 garnet core, but is included within M2 Laurentia Doe Run Mt.Cuba Avalonia and meta-volcanic rocks Anorthosite Sycamore Mills Fm. Wissahickon Fm overgrowth. Monazite cores (433 ± 7 Ma) likely record the time Quartz monzonite gneiss Undifferentiated gneiss sillimanite Rosemont Shear Zone of M1. Asymmetric high-U rims on monazite constrain the timing Felsic and intermediate gneiss West Chester Massif Undifferentiated of S3 (average, 415 ± 14 Ma) and are included in high-Ca garnet Granulite facies garnet High-T low-P metamorphism in Wissahickon, then Mt. Cuba ultramafic rocks Graphitic felsic gneiss sillimanite rims. Amphibolite facies Granulite metamorphism of Wilmington Complex Modified from Blackmer, 2005; Pyle et al., 2006. Ca x-ray map of garnet from WG-216. Low Ca rim is in- Width of field ≈ 6mm for both photos Rosemont Shear Zone Early Devonian ~410 Ma CC-7205 Early stages of Acadian orogeny West Grove Metamorphic Suite Wissahickon Formation terpreted to correspond to garnet growth during high m1 WG-216 320 Rosemont Shear Zone T staurolite breakdown (Srogi et al., 2007). Late garnet Street Road Fault 1035 Ma 1050 Ma 1195 Ma Wiss-1N GW-05-154 960 Ma Bc21-b C-207 and sil after st (photomicrographs) is syn- to post- Laurentia Avalonia Wissahickon Fm. 434 C-207 Mt. Cuba Gneiss Doe Run Schist tectonic. Isopleths of this garnet rim compositon inter- 430 418 340 340 Ashland, De Chatham, Pa Wissahickon Creek sect at approximately 700C and 5 kb. n = 90 360 Dextral transpression n = 95 895 Ma Philadelphia, Pa Rims n = 139 10000 471 360 Burial and metamorphism of Doe Run Schist WG-216 380 Deformation onset of crustal thickening in Wissahickon Fm. 574 Ma 491 546 Ma -chlorite 427 kyanite Outer core Middle Devonian ~385 Ma 9000 400 380 sillimanite 388 ± 12 Ma End of Acadian orogeny 1050 Ma 44069 960 Ma 1470 Ma P-05-02 GW-05-154 - ilm 420 -chloritoid (n = 24) Rosemont Shear Zone Mt. Cuba Gneiss + rt 400 Street Road Fault Wissahickon Fm. +chlorite 945 Ma Doe Run Schist 8000 Embreeville Thrust Yorklyn, De Fairmont Park Monazite with young, low-Y, high- 440 0.5 mm Laurentia Avalonia n = 49 Unionville, Pa n = 124 Philadelphia, Pa Th rims is present in the matrix and 420 Aleinikoff et al. 2006 +liquid -H2O pl ilm 460 n = 143 7000 grt bt as inclusions in the high-Ca garnet (Ma) Age Above, syn-tectonic 554 Ma sil qtz Barrovian metamorphism of Wissahickon Fm. liq qfm rims in WG-216. High-T metamor- garnet in ultramylonite 480 440 Cores References Pressure [Bar] phism can be no older than 410 ± from the RSZ (loc. CC- Aleinikoff, J.L, Schenck, W. S., Plank, M. O., Srogi, L. A., Fanning, C. M., Kamo, S. L., and Bosbyshell, H., 2006, Deciphering igneous and 6000 st-bearing metamorphic events in high-grade rocks of the Wilmington Complex, Delaware: Morphology, cathodoluminescence and backscattered assemblages 11 Ma. Textural considerations on 520 500 1000 1500 2000 2500 7205). P-T estimates of electron zoning, and SHRIMP U-Pb geochronology of zircon and monazite: Geological Society of America Bulletin, v. 118, p. 39-64. 1020 Ma Wg-101 1200 Ma CC46-2 460 Blackmer, G. C., 2005, Preliminary bedrock geologic map of a portion of the Wilmington 30- by 60-minute quadrangle, southeastern 610 Ma sps the timing of monazite growth Ma 600C at 8 kb were ob- Pennsylvania: Pennsylvania Geological Survey, 4th ser., Open-File Report OFBM 05–01.0, 16 p., Portable Document Format (PDF) file. Wissahickon Fm. grs 540 Mt. Cuba Gneiss 5000 similarly constrain the age of high-T Bosbyshell, H., Blackmer, G.C., Srogi, L., Mathur, R., Crawford, M., Valencia, V., and Schenck, W.S., 2012, Detrital zircon ages from the Wissa- +cordierite tained using the composi- hickon Formation, southeast Pennsylvania and northern Delaware: Regional tectonic implications: Geological Society of America 965 Ma alm The West Grove Metamorphic Glen Riddle, Pa Abstracts with Programs, v.
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