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5. Multiple Paleozoic Metamorphic Histories, Fabrics, and Faulting in the Westminster and Potomac Terranes, Central Appalachian Piedmont, Northern Virginia and Southern Maryland

By Michael J. Kunk,1 Robert P. Wintsch,2 C. Scott Southworth,3 Bridget K. Mulvey,2 Charles W. Naeser,3 and Nancy D. Naeser3

1U.S. Geological Survey, Denver, CO 80225.
2Department of Geological Sciences, Indiana University, Bloomington, IN 47405.
3U.S. Geological Survey, Reston, VA 20192.

Introduction

This field trip is a progress report of research on the complex rocks of the Westminster and Potomac terranes (Horton and others, 1991) of Maryland, Virginia, and Washington, D.C. (fig. 1). The study of these rocks was begun more than 60 years ago with work by Jonas and Stose (1938), Cloos and Broedel (1940), Stose and Stose (1946), and Cloos and Cooke (1953). Research continued with Fisher (1963, 1970, 1971), Hopson (1964), and Drake (1986, 1989, 1994, 1998), and continues today. Geologic mapping at a scale of 1:24,000 by the U.S. Geological Survey (USGS) in this region was begun by Avery Drake in the 1970s and resulted in a number of novel concepts and interpretations, and the publication of a large number of geologic quadrangle maps (Drake, 1986, 1994, 1998; Drake and Froelich, 1986, 1997; Drake and Lee, 1989; Fleming and others, 1994; Drake and others, 1999; Southworth, 1999). Compilations of these data at a scale of 1:100,000 were recently published (Southworth and others, 2002; Davis and others, 2002) and constitute a summary of the latest understanding of the distribution of the rocks.

During the map compilation phase of the study, rocks were sampled for 40Ar/39Ar and fission-track dating to better understand the chronology of the tectonic assemblage in the region. Specifically, rocks were sampled across the major faults along an east to west transect along the Potomac River and its tributaries. Results of the recent argon and fission-track dating compel us to reconsider many earlier interpretations and are the motivation for this field trip. In particular, 40Ar/39Ar and fission-track data identify age and thermal discontinuities that give added significance to mapped faults and identify unmapped faults and shear zone boundaries.

Geologic Setting

The Westminster and Potomac terranes are exposed in southern Maryland, northern Virginia, and Washington, D.C. (fig. 1). Drake and others (1989) and Horton and others (1989) proposed that the Potomac terrane was thrust onto the Westminster terrane along the Pleasant Grove fault, and that the Westminster terrane was thrust westward along the Martic fault onto Cambrian and Ordovician continental margin strata, during the Ordovician Taconian orogeny. Horton and others (1989) also speculated that both thrust faults were reactivated with dextral strike-slip motion during the late Paleozoic Alleghanian orogeny.

Westminster terrane

The rocks of the Westminster terrane are dominated by phyllites and have been correlated with the Hamburg klippe in Pennsylvania and higher slices of the Taconic allochthon in New England and New York (Knopf, 1935; Lyttle, 1982; Drake, 1986; Drake and others, 1989; Horton and others, 1989). Both the Westminster and Hamburg terranes are considered to represent offshore, deepwater, post-rift deposits with no direct stratigraphic ties to Laurentia (Horton and others, 1989). The low-grade, polymetamorphic, and polydeformed rocks previously mapped by Jonas and Stose (1938), Cloos and Broedel (1940), Cloos and Cooke (1953), Hopson (1964), Froelich (1975), Fisher (1978), and Edwards (1986, 1988, 1994), have been mapped, compiled, and summarized by Southworth and others (2002). We will examine part of the terrane that is dominated by rocks assigned to the Marburg Formation.

Marburg Formation

The Marburg Formation contains primarily phyllite, metasiltstone, and quartzite. The protolith of the metasiltstone with quartz ribbons within the Marburg Formation has been interpreted to be turbidites by Southworth (1999). A few lenses of quartzite and rare greenstone have been interpreted to reflect an influx of channel deposits and volcanic sediments. Because the Marburg Formation contains some rocks that are similar to those in the Ijamsville Phyllite to the west, Marburg Formation rocks are interpreted to be composed of deepwater-rise deposits beneath and eastward of the Ijamsville Phyllite. Rocks of the Marburg Formation have been thrust onto the rocks of the Sams Creek Formation to the west along the Hyattstown fault (fig. 2). Rocks of the Marburg Formation collectively constitute a wide fault zone with multiple foliations, retrogressive phyllonites, and polydeformed vein quartz between the Hyattstown fault and the Pleasant Grove fault.

Potomac terrane

The Potomac terrane is bounded on the west by the Pleasant Grove fault and covered by Cretaceous and Tertiary Coastal Plain deposits to the east (fig. 1). The mapped units of the Potomac terrane, from west to east, are the Mather Gorge, Sykesville, and Laurel Formations (Drake and Froelich, 1997; Drake, 1998) (fig. 2). The protoliths of these rocks are interpreted to be Neoproterozoic to Early Cambrian distal slope deposits and olistostromes (Drake, 1989). These formations include mélanges that contain ultramafic rocks, and they are intruded by Early to Middle Ordovician tonalitic to granodioritic rocks (Aleinikoff and others, 2002). The three formations are separated by major faults that trend northward. The Plummers Island fault separates rocks of the Mather Gorge Formation on the west from rocks of the Sykesville Formation on the east (Drake, 1989). The Rock Creek shear zone separates rocks of the Sykesville Formation intruded by Ordovician plutons on the west from diamictite of the Laurel Formation on the east (Fleming and others, 1994; Drake and Froelich, 1997; Fleming and Drake, 1998) (fig. 2). Multiple foliations in the rocks are common, and composite foliations are strongest in phyllonitic rocks in fault zones.

Regional aeromagnetic data reflect this complex geology (Southworth and others, 2002). Pronounced patterns of magnetic highs and lows define the Mather Gorge Formation (CZmg, fig. 2) and the presence of ultramafic and mafic rocks. A broad low magnetic anomaly defines the rocks of the Sykesville Formation, and magnetic lineaments mark the Rock Creek shear zone.

Mather Gorge Formation

Rocks defined as the Mather Gorge Formation by Drake and Froelich (1997) consist of the metamorphosed equivalents of well-bedded graywacke and mudstone, which are now granofelsic metagraywackes and quartz-mica schists and higher grade equivalents (CZmg, fig. 2). This north-northeast-striking belt of rock includes poorly exposed map-scale bodies of amphibolite, serpentinite, and talc schist that have been collectively mapped as ultramafic rocks (um, fig. 2). Both the metasedimentary rocks and amphibolite are intruded by the Ordovician Bear Island Granodiorite and associated pegmatites, and Devonian lamprophyre dikes intrude the schist. An apparent Barrovian metamorphic sequence of chlorite to sillimanite grade has been described that extends from the phyllitic rocks near the margin of the Culpeper basin eastward to the migmatites on Bear Island near Great Falls (Fisher, 1970; Drake, 1989). East of Great Falls (fig. 2, Stop 4), migmatitic rocks occur in a belt and grade eastward into a zone of retrograded chlorite-sericite phyllonites (Fisher, 1970) that are truncated on the east by the Plummers Island fault (Drake and Froelich, 1997) (fig. 2). The metagraywacke near Great Falls preserves bedding and soft-sedimentary slump features (Hopson, 1964; Fisher, 1970), and the package of rocks is interpreted to have been deposited as a sequence of turbidites (Drake and Froelich, 1997).

In this study, the rocks included in the Mather Gorge Formation are subdivided into three domains that are defined on the basis of lithology, metamorphic history, structure, and geochronology. From west to east, these are the Blockhouse Point, Bear Island, and Stubblefield Falls domains (fig. 2). The Blockhouse Point domain is characterized by chlorite-sericite phyllonites and some ultramafic rock bodies. The Bear Island domain is characterized by garnet-sillimanite-grade metagraywacke and schist that is migmatitic near the eastern boundary. Well-bedded metagraywacke (type locality of the Mather Gorge Formation), large ultramafic rock bodies, migmatite, granodiorite and pegmatite closely associated with amphibolite, and lamprophyre dikes characterize this domain. The Stubblefield Falls domain to the east is characterized by migmatitic schist that has been retrograded to chlorite-sericite phyllonitic schist, small bodies of amphibolite, very minor granodiorite, and a few areas mapped as diamictite.

Sykesville Formation and Ordovician Plutonic Rocks

The Sykesville Formation of Hopson (1964) is a belt of metasedimentary rocks east of the Plummers Island fault and west of the Rock Creek shear zone (Cs, fig. 2). These rocks have been metamorphosed to upper amphibolite facies, and intruded by a suite of Ordovician tonalitic to granodioritic rocks (O, fig. 2). Aleinikoff and others (2002) used U-Pb zircon techniques to date intrusive rocks including the Norbeck, Falls Church, Dalecarlia, and Georgetown Intrusive Suites and the Kensington Tonalite. Rocks of the Sykesville Formation have been interpreted to structurally underlie rocks of the Mather Gorge Formation under the Plummers Island fault along the Potomac River (Drake, 1989) (fig. 2), but to overlie them southwest of Baltimore to the north (Muller and others, 1989).

The sedimentary protoliths of the Sykesville Formation were diamictites and sedimentary mélanges containing clasts of amphibolite, migmatite, schist, metagraywacke, gabbroic granofels, phyllonite, greenstone, and vein quartz supported in a matrix of quartz-feldspar-muscovite granofels and schist (Drake and Froelich, 1997). Clasts within the diamictite have been interpreted to be derived from already deformed and metamorphosed rocks of the Mather Gorge Formation (Drake, 1989; Muller and others, 1989), and map-scale bodies of migmatite, phyllonite, and ultramafic schists that appear to be surrounded by diamictite have been interpreted by Drake (1989) and Fleming and others (1994) to be large olistoliths.

Laurel Formation

Rocks of the Laurel Formation are considered by Drake (1989) to be part of a separate motif (Laurel-Loch Raven), east of the Mather Gorge-Sykesville motif. The Laurel Formation (Cl, fig. 2) (Hopson, 1964) forms a north-northeast-striking belt of diamictite that superficially resembles that of the Sykesville Formation. To the east it is covered by Cretaceous and younger Coastal Plain sediments, and to the west it is separated from the Sykesville Formation and Ordovician plutons by the Rock Creek shear zone (Fleming and others, 1994). These rocks were metamorphosed to upper amphibolite facies, then retrograded to phyllitic and mylonitic schists in the Rock Creek shear zone (Fleming and Drake, 1998).

In spite of the metamorphism, the protolith of the Laurel Formation is recognizably a diamictite, a sedimentary mélange with clasts of vein quartz, meta-arenite, biotite schist, actinolite schist, and local amphibolite supported by a quartzofeldspathic matrix (Fleming and others, 1994). The unit is similar to the Sykesville Formation but contains a greater number and variety of olistoliths (Drake, 1998).

Geochronology

Previous geochronology

U-Pb zircon ages reflect the time of igneous crystallization and later metamorphic overgrowths, and K-Ar, 40Ar/39Ar, and Rb-Sr mica ages have been interpreted to reflect the time of cooling and, in some cases, muscovite growth. Published data are not complete enough to constrain metamorphic histories in our field area, but they provide important additional ages in our study area.

Earlier attempts at U-Pb dating of zircons in this complex setting (Davis and others, 1958, 1960; Wetherill and others, 1966; Fisher, 1970; Sinha and others, 1989) are difficult to interpret in light of recent advances in our understanding of U-Pb systems, and they will not be discussed here. U-Pb SHRIMP and TIMS analyses of zircons from plutonic rocks that intruded the Sykesville Formation in our field area (Aleinikoff and others, 2002) reveal Early to Late Ordovician ages of emplacement.

Reed and others (1970) dated two biotite separates from a lamprophyre dike at Great Falls (Stop 4) at 360±13 Ma and 363±13 Ma (2?) using conventional K-Ar techniques. Muth and others (1979) dated muscovite cooling from the Bear Island Granodiorite within the Mather Gorge Formation, at 469±20 Ma and 469±12 Ma (2σ) using Rb-Sr techniques. Becker and others (1993) report 40Ar/39Ar ages of amphibole and muscovite from migmatitic rocks along Difficult Run (Stop 5, fig. 2) of 490 Ma and 422 Ma, respectively. They interpreted the amphibole age to represent cooling from the Cambrian-Ordovician, Penobscottian orogeny. We reinterpret the spectrum age as being the result of extraneous argon. Their muscovite age spectrum is sigmoidal because of the presence of more than one generation of muscovite, so the minimum apparent cooling age probably reflects a mixture of muscovite populations. Krol and others (1999) used 40Ar/39Ar dating of muscovite and biotite from rocks within the Pleasant Grove fault “zone.” Their study did not provide any ages from the Mather Gorge, Sykesville, or Laurel Formations. They interpreted their 40Ar/39Ar data from muscovite to indicate a possible thermotectonic event between 368 and 348 Ma (Acadian), and dextral shearing of the central and northern parts of the Pleasant Grove fault “zone” at 311 Ma (Alleghanian).

New geochronology

New argon and fission-track data from the Westminster and Potomac terranes (Mulvey, 2003; Kunk and others, in press; and M.J. Kunk, unpub. data) together with published data require modifications of the previously interpreted regional framework. A summary of these data is presented in figure 3. Our sampling strategy was designed to take advantage of detailed geologic mapping summarized by Southworth and others (2002), along a traverse that extends (west to east) from near the Hyattstown fault in Maryland (Stop 1) across the Rock Creek shear zone in the District of Columbia.

Amphibole 40Ar/39Ar ages

All amphibole samples were collected from rocks that experienced upper amphibolite facies metamorphism (Drake, 1989). They contain more than 50 percent coarse-grained amphibole coexisting with, and locally including, plagioclase, magnetite, biotite, and epidote. Many samples also contain retrograde epidote and chlorite that may or may not define a late fabric. In most rocks the amphiboles define at least one foliation. In some samples, a coarser grained gneissosity is overprinted by a more pervasive schistosity, and one sample appears to retain an igneous texture. Amphibole in metagabbro intrusive to the Laurel Formation is typical, containing equant grains of amphibole up to 0.5 mm in diameter that are overgrown by acicular needles that define a second foliation (S2, fig. 4A). The high metamorphic grade of the samples is supported by the amphibole textures which indicates temperatures >600°C (Poirier, 1985).

The time of cooling of each of the three high-grade domains of figure 2 is estimated by at least two amphibole analyses. Three samples from the Bear Island domain contain excess argon, with two samples yielding minimum and isochron ages of 475 Ma (Stops 4 and 5). The third sample produces an overlapping isochron age of 455±23 Ma. These ages are consistent with the Rb-Sr ages of muscovite of 469±12 Ma and 469±20 Ma (Muth and others, 1979), which are also considered to have a closure temperature of ~500°C (Jäeger, 1979). The ~455- to 475-Ma age range of these samples is accepted as the best estimate for the time of cooling of the Bear Island domain through ~500°C.

Two samples of amphibole from rocks that intrude the Sykesville Formation also agree with each other within analytical uncertainty. A sample from the Falls Church pluton produced a near-plateau age spectrum with a minimum age of 401 Ma. This amphibole probably had a relatively simple igneous crystallization history, followed by slow cooling with relatively little deformation. The 405-Ma isochron age of a sample from the Georgetown Intrusive Suite confirms the time of cooling. Hence the Early Devonian age of ~401 Ma is a reasonable estimate of the time of regional cooling of the intrusive rocks and the country rocks of the Sykesville Formation through 500°C.

Two amphibole samples from rocks that intrude the Laurel Formation have minimum ages of 404 Ma and 398 Ma. This indicates Early Devonian cooling through 500°C for the intrusive rocks and rocks of the Laurel Formation. These ages are indistinguishable from the amphibole cooling ages of the Sykesville Formation.

Muscovite 40Ar/39Ar ages

The rocks of the Westminster terrane (Stops 1 and 2) never exceeded lower greenschist facies conditions, and thus were always below the closure temperature for diffusion of argon in muscovite (~350°C). Many of the muscovite separates (and one whole-rock sample) produced either sigmoidal or climbing age spectra (Mulvey, 2003). The shapes of these age spectra can be caused by (1) the presence of a (usually) very small population of much older detrital muscovite in the sample, (2) the presence of multiple generations of metamorphic muscovite in the sample, and (3) the presence of inseparable, intimately intergrown chlorite. Because of these complications, the muscovite age spectra provide only approximations of the maximum ages of the most recent muscovite growth and minimum ages of earlier muscovite foliations or detrital components in the Westminster terrane (Mulvey, 2003). These results are nonetheless useful at the orogenic level.

Three samples collected from the western part of the Marburg Formation in the Westminster terrane gave ages of ~435 to 430 Ma, whereas two samples from the eastern part of the Marburg Formation gave ages of ~382 to 375 Ma (Stops 1 and 2) (fig. 2) (Mulvey, 2003; M.J. Kunk, unpub. data). All of these ages are interpreted to approximate the time of growth of cleavage-forming muscovite in these samples.

All of the rocks in the Potomac terrane have been metamorphosed to at least biotite grade (Kunk and others, in press), thus any detrital muscovite in sedimentary protoliths has been either recrystallized or thermally reset. In addition, we were able to separate muscovite that was free of chlorite intergrowths in all but one of the samples from the Potomac terrane (Kunk and others, in press). Nonetheless, most of the muscovite samples from the Potomac terrane also have sigmoidal or climbing age spectra. In these rocks, older (S1) high-grade mineral assemblages have been partly overprinted by younger (S2), lower grade foliation(s) (crystallized below the muscovite closure temperature for argon diffusion). For these samples we have interpreted the minimum age in the spectrum as the maximum age of muscovite growth below closure, and the maximum age in the spectrum as a minimum age for cooling of the higher grade muscovite through 350°C. These age pairs for each sample are plotted in figure 3, and rocks of the Mather Gorge Formation are summarized as follows:

  1. The westernmost muscovite sample from the Blockhouse Point domain (figs. 2 and 3) produced a plateau age of 362±2 Ma (1σ) that we interpret as the time of white mica growth (S2) in the sample. Two samples farther to the east gave ~371 Ma cooling ages (S1) and ~362 Ma growth ages (S2).
  2. In the Bear Island domain, cooling-age estimates from the muscovite age spectra range from 422 to 411 Ma (S1), and below-closure growth-age estimates range from 385 to 373 Ma (S2).
  3. In the Stubblefield Falls domain, cooling-age estimates from the muscovite age spectra range from 375 to 354 Ma (S1), and growth-age estimates range from 350 to 328 Ma (S2).

East of the Mather Gorge Formation and Plummers Island fault, cooling-age estimates from the muscovite age spectra from the Sykesville Formation range from 354 to 340 Ma (S1), and growth-age estimates range from 332 to 304 Ma (S2).

Biotite 40Ar/39Ar ages

Only one biotite sample was dated because of the dearth of unaltered biotites in the field area. The biotite (Stop 5, fig. 2) is from the Bear Island Granodiorite in the Bear Island domain and produced a 40Ar/39Ar total fusion age of 364±2 Ma. This age is the same, within the limits of analytical precision, as K-Ar ages reported by Reed and others (1970) for two biotite samples (360±13 Ma and 363±13 Ma) collected nearby from a lamprophyre dike (Stop 4, fig. 2). Because the biotite from our sample grew in a metamorphic environment well above its argon closure temperature, we interpret both this age and the ages from the dike to represent the timing of cooling of the biotite through ~300°C in a regional thermal gradient.

Zircon fission-track ages

Zircon fission-track ages determined for five samples from the Potomac terrane (one sample from the Blockhouse Point domain of the Mather Gorge Formation, three samples from the Sykesville Formation, and one sample from the Laurel Formation), are statistically indistinguishable, suggesting that the rocks cooled through the zircon fission-track closure temperature (~235°C; Brandon and others, 1998) from 298±46 Ma to 262±20 Ma (±2 standard errors of the mean; fig. 3). The mean zircon age calculated for the five samples is 282±13 Ma, suggesting that the rocks cooled through ~235°C in earliest Permian time.

Apatite fission-track ages

Seven samples yielded sufficient apatite for fission-track analysis. The samples span most of the Potomac composite terrane, from the shear zone separating the Blockhouse Point and Bear Island domains in the Mather Gorge complex in the west to Rock Creek Park (Laurel Formation) in the east. Apatite fission-track ages range from 198±21 Ma to 131±16 Ma (±2σ), with ages generally becoming younger to the west. The apatite fission-track age and track-length data indicate that rocks presently exposed at the surface cooled through the apatite fission-track closure temperature (~90°C–100°C in these rocks) from the Early Jurassic to the Early Cretaceous, with the time of cooling becoming younger to the west.

Summary of recent studies

The rocks of the Potomac terrane have been metamorphosed to at least biotite grade, and contain one or more higher grade schistosities (collectively called “S1”), which formed above the ~350°C closure temperature of muscovite. The 40Ar/39Ar ages recorded by S1 muscovites should represent the time of their passage through ~350°C. In addition, most of these rocks and the rocks of the Marburg Formation in the Westminster terrane also contain one or more younger schistosities (collectively called “S2”), that grew below ~350°C in the greenschist facies; their 40Ar/39Ar muscovite ages should record the time of these muscovites’ crystallization. S1 and S2 as referred to here are clearly not the same in the various tectonic blocks that are discussed. While S1 ages represent only the last passage of the rocks through the ~350°C isotherm, S2 ages can represent composite schistosities and multiple episodes of mineral growth.

Westminster terrane

The muscovite growth-age estimates in the western part of the Marburg Formation (Stop 1) are ~435 to 430 Ma, in contrast to those in the eastern part of the Marburg Formation (Stop 2) where they range from 382 to 375 Ma (fig. 3). The ~50 m.y. discontinuity in age between these two groups of samples is most easily explained by a fault within the Marburg Formation between Stops 1 and 2.

Potomac terrane

Mather Gorge Formation

Blockhouse Point domain

Maximum estimates of muscovite growth ages of two samples are 362 Ma, and a third sample in the westernmost part of the Blockhouse Point domain has a plateau age of 362±2 Ma. We interpret 362 Ma as the age of the lower greenschist facies muscovite growth (Stop 3). Because the metamorphic grade of these rocks reached biotite-grade conditions in the Middle Devonian, any argon isotopic evidence of an earlier metamorphic event in these muscovites has been reset.

Bear Island domain

Higher metamorphic temperatures here resulted in formation of migmatite (Stops 5 and 6). Our amphibole data indicate that cooling through 500°C occurred between 475 and 455 Ma (fig. 3). The estimate from muscovite data for the timing of cooling through 350°C (S1) is >422 Ma. Maximum age estimates for below-closure-temperature growth of muscovite (S2) range from 385 to 373 Ma. A comparison with the muscovite samples of the Blockhouse Point domain shows a striking difference in the estimated time of cooling through 350°C (371 Ma versus 422 Ma), suggesting the presence of a fault zone between the two domains.

Stubblefield Falls domain

The rocks west of the Plummers Island fault were migmatitic (Stop 6), but they were later retrograded to a chlorite-sericite phyllonite (Fisher, 1970). We extend the Ordovician cooling history of the Bear Island domain here based on that earlier high-temperature history. Muscovite age spectra indicate minimum estimates for cooling through 350°C that range from 375 Ma to 354 Ma, younging to the east, suggesting reheating in the Devonian to reset the muscovite ages. Maximum estimates for the time of subsequent growth below closure of S2 muscovite also show a general pattern of younging to the east, and range from 350 Ma in the western part of the domain to 328 Ma near the Plummers Island fault.

Sykesville Formation and Ordovician intrusive rocks

The rocks of the Sykesville Formation were intruded by Middle to Late Ordovician plutonic rock (Aleinikoff and others, 2002) prior to or during peak amphibolite facies metamorphic conditions. Amphibole samples from the Falls Church pluton and the Dalecarlia Intrusive Suite have disturbed 40Ar/39Ar ages of 401 Ma and 405 Ma, respectively, indicating cooling through 500°C in the Early Devonian. The muscovite age spectra from the Sykesville Formation give minimum estimates for cooling through 350°C that range from 357 to 340 Ma (S1). Maximum age estimates for subsequent muscovite below-closure growth (S2) range from 304 Ma in the Plummers Island fault to 332 Ma near the Rock Creek shear zone, and show a dramatic decrease in age to the west (fig. 3). We interpret the 304-Ma age to represent a maximum age for the time of final Alleghanian movement on the Plummers Island fault.

Laurel Formation and intrusive rocks

Two amphiboles were dated from rocks that intrude the Laurel Formation. Both had relatively flat age spectra and suggest an age of ~400 Ma for their time of cooling through 500°C.

Thermal Histories Across the Plummers Island Fault

Cooling curves provide comparison of the thermal history across the Plummers Island fault (fig. 5). The best estimate for the time of cooling of the Bear Island domain through ~500°C is 475 to 455 Ma, obtained from the three amphibole 40Ar/39Ar ages and two Rb-Sr muscovite ages. The minimum age estimate for cooling of muscovite through its closure temperature, 422 Ma, provides a data point at ~350°C. Our biotite 40Ar/39Ar age of 364±2 Ma adds a point on the curve at ~300°C. Regional zircon fission-track ages of ~280 Ma add a point at ~235°C. Together these points generate the concave-up cooling trend (fig. 5) typical of many metamorphic terranes. Extrapolating back from this curve, migmatitic conditions of 650 to 700°C appear to have occurred in these rocks in the Early Ordovician (migmatite patterned polygon, fig. 5). This cooling curve also shows that metamorphic conditions did not exceed greenschist facies after mid-Paleozoic times. The planar lamprophyre dikes in fractures suggests no significant contractional deformation occurred after about 365 Ma.

Migmatitic textures in the diamictite of the Sykesville Formation indicate upper amphibolite facies metamorphism. The Ordovician ages of the five dated plutonic rocks (Aleinikoff and others, 2002) provide data points at the near-solidus temperatures of ~700°C. Our ~400-Ma cooling ages of amphiboles provide a point at ~500°C. The minimum cooling-age estimate from the muscovite data provides a data point at 357 Ma and ~350°C, and a fourth point is established by the zircon fission-track data at 280 Ma and 235°C.

The cooling curves show independent paths throughout the Paleozoic that do not converge until ~300 to 280 Ma (fig. 5). In particular, migmatitic rocks of the Bear Island domain had cooled to greenschist-facies conditions by the time that the Sykesville Formation was still at upper amphibolite facies in the Late Silurian. These temperature contrasts reflect 3 to 6 km of net vertical displacement, depending on the geothermal gradients in the two blocks of rock. Vertical displacement may have been accompanied by an unknown, but possibly considerable, component of strike-slip movement. This difference in cooling history is not consistent with the interpretation of a Cambrian-age premetamorphic Plummers Island fault that shed clasts of metamorphosed Mather Gorge Formation into sediments that became the Sykesville Formation (Drake, 1989). Muller and others’ (1989) interpretation of the Sykesville Formation being deposited in Late Cambrian to Early Ordovician time from erosion of an uplifted Mather Gorge Formation (their Morgan Run Formation) is also unlikely, based on the difference in cooling histories during much of the Paleozoic.

Rocks of the Bear Island domain cooled continuously from Middle Ordovician to Late Carboniferous time, and were cooler at any given time than the rocks of the Sykesville Formation (fig. 5). Nonetheless, the cooling rate of the Sykesville Formation exceeded that of the Bear Island domain from the middle Silurian through the Pennsylvanian (fig. 5). The net crustal displacement across the fault from the Ordovician through Pennsylvanian is east side up.

Interpreted Tectonic Assemblage

By the Early to Middle Ordovician, the rocks of the Bear Island domain and, by inference, also the Stubblefield Falls domain in the Mather Gorge Formation, had been metamorphosed and were cooling through ~500°C (fig. 5), while the Sykesville Formation was still at upper amphibolite-facies prograde metamorphic conditions and was being intruded by plutons (Aleinikoff and others, 2002). The metamorphic and necessarily structural divergence precludes a geological connection between the formations in the Ordovician. Rocks of the Blockhouse Point domain had not been detectably metamorphosed, so they were also physically separated from the Bear Island domain.

By the Early Silurian the rocks of the westernmost Marburg Formation had been metamorphosed to lower greenschist facies when an S1 cleavage was produced, while the eastern Marburg Formation apparently remained unmetamorphosed. During the Early Devonian (figs. 5, 6), hot rocks of the Sykesville Formation and associated plutons were being thrust over the Stubblefield Falls domain, heating the muscovite to temperatures well above argon closure, along an early Plummers Island shear zone (Schoenborn, 2002). At ~400 Ma, the Laurel and Sykesville Formations simultaneously cooled through 500°C, the closure temperature for argon diffusion in amphibole, suggesting that they were at the same structural level at that time.

By the Late Devonian, the Blockhouse Point domain had cooled from biotite-grade metamorphism (>400°C). A shear zone with strain distributed over a zone as much as 12 km wide, from the Blockhouse Point domain west into the Westminster terrane, produced S2 cleavage defined by muscovite that crystallized below its closure temperature. At about the same time, rocks of the Stubblefield Falls domain, and later of the Sykesville Formation, cooled through ~350°C while the rocks of the Bear Island domain cooled through 300°C, demonstrating that these blocks were at different structural levels at the time (figs. 5, 6).

In the Late Pennsylvanian, the rocks of the Stubblefield Falls domain of the Mather Gorge Formation moved up relative to the Sykesville Formation on the steep, west-dipping Plummers Island fault and mylonite zones (Schoenborn, 2001) within an existing Plummers Island shear zone (figs. 5, 6). Shearing formed S2 cleavage with below-closure muscovite growth and more pervasive S2 cleavage in the Sykesville Formation. By the earliest Permian, all of the rocks in the Potomac terrane had cooled through 235°C (figs. 3, 5). Apatite fission-track data indicate cooling through ~90°C to 100°C in Early Jurassic to Early Cretaceous time, with increasing ages to the east, suggesting kilometer-scale rotation of the Potomac terrane in the Cretaceous and (or) Tertiary, with the west side up.

Faults and Shear Zones: Plummers Island, Pleasant Grove, and Rock Creek

In summary, the assemblage of the rocks of the Potomac and Westminster terranes occurred during the Acadian orogeny in the Devonian, with fault reactivation during the Alleghanian orogeny in Pennsylvanian time. The apparent Barrovian metamorphic sequence of chlorite to sillimanite grade (west to east) (Fisher, 1970), and the Potomac River fan structure (fig. 7) (Drake, 1989), both interpreted to be of Ordovician age (Drake, 1989), are obviously a composite of different events throughout the Paleozoic.

If our analysis is correct, the 5-km-wide Early Devonian (Acadian) Plummers Island shear zone was reactivated in the Carboniferous (Alleghanian) under lower greenschist-facies conditions as the Plummers Island fault (fig. 6). The muscovite cooling (S1) and growth ages (S2) in the Blockhouse Point domain are similar to growth ages in the eastern part of the Westminster terrane. Therefore, Late Devonian (Acadian) deformation in a 12-km-wide Pleasant Grove shear zone may have extended from the Blockhouse Point domain west across the Pleasant Grove fault into the Marburg Formation of the Westminster terrane. There is no isotopic evidence of pre-Devonian or significant post-Devonian movement on this part of the Pleasant Grove fault. However, shear-band cleavage with dextral strike-slip kinematics supports Alleghanian movement of unknown significance within the Pleasant Grove shear zone.

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