U–Pb geochronology of zircon and monazite from Mesoproterozoic granitic gneisses of the northern Blue Ridge, Virginia and Maryland, USA

Precambrian Research 99 (2000) 113–146 www.elsevier.com/locate/precamres

U–Pb geochronology of zircon and monazite from Mesoproterozoic granitic gneisses of the northern Blue Ridge, Virginia and Maryland, USA John N. Aleinikoff a, *, William C. Burton b, Peter T. Lyttle c, Arthur E. Nelson b, C. Scott Southworth b a US Geological Survey, MS 963, Denver, CO 80225, USA b US Geological Survey, MS 926A, Reston, VA 22092, USA c US Geological Survey, MS 908, Reston, VA 22092, USA Received 29 March 1999; accepted 23 July 1999

Abstract Mesoproterozoic granitic gneisses comprise most of the basement of the northern Blue Ridge geologic province in Virginia and Maryland. Lithology, structure, and U–Pb geochronology have been used to subdivide the gneisses into three groups. The oldest rocks, Group 1, are layered granitic gneiss (1153±6 Ma), hornblende monzonite gneiss (1149±19 Ma), porphyroblastic granite gneiss (1144±2 Ma), coarse-grained metagranite (about 1140 Ma), and charnockite (>1145 Ma?). These gneisses contain three Proterozoic deformational fabrics. Because of complex U–Pb systematics due to extensive overgrowths on magmatic cores, zircons from hornblende monzonite gneiss were dated using the sensitive high-resolution ion microprobe (SHRIMP), whereas all other ages are based on conventional U–Pb geochronology. Group 2 rocks are leucocratic and biotitic varieties of Marshall Metagranite, dated at 1112±3 Ma and 1111±2 Ma respectively. Group 3 rocks are subdivided into two age groups: (1) garnetiferous metagranite (1077±4 Ma) and quartz-plagioclase gneiss (1077±4 Ma); (2) white leucocratic metagranite (1060±2 Ma), pink leucocratic metagranite (1059±2), biotite granite gneiss (1055±4 Ma), and megacrystic metagranite (1055±2 Ma). Groups 2 and 3 gneisses contain only the two younger Proterozoic deformational fabrics. Ages of monazite, separated from seven samples, indicate growth during both igneous and metamorphic (thermal ) events. However, ages obtained from individual grains may be mixtures of different age components, as suggested by backscatter electron (BSE) imaging of complexly zoned grains. Analyses of unzoned monazite (imaged by BSE and thought to contain only one age component) from porphyroblastic granite gneiss yield ages of 1070, 1060, and 1050 Ma. The range of ages of monazite (not reset to a uniform date) indicates that the Grenville granulite event at about 1035 Ma did not exceed about 750°C. Lack of evidence for 1110 Ma growth of monazite in porphyroblastic granite gneiss suggests that the Short Hill fault might be a Grenvillian structure that was reactivated in the Paleozoic. The timing of Proterozoic deformations is constrained by crystallization ages of the gneissic rocks. D1 occurred between about 1145 and 1075 Ma (or possibly between about 1145 and 1128 Ma). D2 and D3 must be younger than about 1050 Ma. Ages of Mesoproterozoic granitic rocks of the northern Blue Ridge are similar to rocks in other Grenville terranes of the eastern USA, including the Adirondacks and Hudson Highlands. However, comparisons with conventional U–Pb ages of granulite-grade rocks from the central and southern Appalachians may be specious because these ages may actually be mixtures of ages of cores and overgrowths. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Blue Ridge; Grenville; Mesoproterozoic; Monazite; U–Pb absolute age; Zircon

* Corresponding author. Fax: +1-303-236-4930. E-mail address: [email protected] (J.N. Aleinikoff ) 0301-9268/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 3 0 1- 9 2 68 ( 9 9 ) 0 00 5 6 -X

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1. Introduction The northern Blue Ridge Physiographic Province, about 150 km in length from central Virginia to south-central Maryland, is composed primarily of Mesoproterozoic granulite facies gneisses and mantling greenschist facies metasedimentary and metavolcanic cover sequence rocks of Neoproterozoic to Early Cambrian age. This package of rocks comprises a small part of the Blue Ridge Province, an upland belt that extends nearly 900 km from Georgia to Pennsylvania (Fig. 1). Recent detailed 1:24 000-scale maps ( Kline et al., 1991; Fauth and Brezinski, 1994; Southworth, 1994, 1995; Burton et al., 1995; Southworth and Brezinski, 1996; Nelson, 1997) have delineated more than 15 Mesoproterozoic basement units. Geologic maps at 1:100 000 scale (Burton et al., 1992; Lyttle et al., 1999; Southworth et al., unpublished data) synthesize the geology of this region (Fig. 2). The focus of this paper is the determination of ages of granitic gneisses found within 20 7.5∞

Fig. 1. Areas of exposed Mesoproterozoic rocks in the eastern North America.

quadrangles that have been partially or completely mapped as part of the Washington West and Frederick 30∞×60∞ quadrangle geologic mapping by the US Geological Survey in cooperation with the Maryland Geological Survey. Previous geochronologic work on Mesoproterozoic rocks to the south (cited in Bartholomew and Lewis, 1984; Clarke, 1984) indicated that these basement gneisses are about 1.0–1.2 Ga, with large uncertainty. During the course of the new mapping, Mesoproterozoic orthogneisses were subdivided into 13 varieties, on the basis of lithology and structure [presence of foliation(s)], 12 of which were collected for U–Pb zircon geochronology. In addition, monazite from seven samples was dated to either confirm the age of emplacement of the igneous protolith or determine the timing of metamorphism.

2. Geologic setting Mesoproterozoic gneisses and Neoproterozoic to Early Cambrian cover rocks of the northern Blue Ridge are bounded on the west by folded Paleozoic sedimentary rocks of the Great Valley of the Valley and Ridge Province and on the east by gently west-dipping Upper Triassic and Lower Jurassic clastic rocks and basalt of the Culpeper basin. Both basement gneisses and cover sequence rocks were deformed in the late(?) Paleozoic to form the Blue Ridge–South Mountain anticlinorium, a complex fold that is overturned to the west. The Mesoproterozoic gneisses of our study area are an extension of the Lovingston Massif of central Virginia (Sinha and Bartholomew, 1984), a relatively shallow terrane of amphibolite to granulite facies rocks, located to the east of the deeper level granulite facies Pedlar Massif. However, a charnockite (described below) in the northern Blue Ridge may be correlative with rocks of the Pedlar River Charnockite Suite (Sinha and Bartholomew, 1984). The term ‘Marshall Granite’ was originally used by Jonas (1928) for widespread Precambrian granite in the Blue Ridge basement of northern Virginia and Maryland. This granite was formally named the Marshall Metagranite by Espenshade (1986),

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who recognized both fine- and coarse-grained varieties ( Ym, Ymc, Fig. 2) in the Marshall Quadrangle. In the adjacent Orlean 7.5∞ quadrangle, Clarke (1984) mapped four varieties of granitic gneiss [including Flint Hill Gneiss ( Yfh, Fig. 2), porphyroblastic gneiss ( Ypg), augen gneiss ( Ypb), and Marshall Metagranite ( Ym)] and Howard (1991) recognized three varieties of granitic gneiss and a metanorite in his regional study of Mesoproterozoic rocks in northern Virginia. Mesoproterozoic rocks that make up the core of the northern Blue Ridge anticlinorium consist of weakly to strongly foliated gneisses exhibiting high-grade metamorphic mineral assemblages and textures. The orthogneisses can be divided into two groups on the basis of lithology: (1) moderately to strongly foliated granitic gneiss and weakly to moderately foliated granite gneiss that comprise over 90% by volume of the Mesoproterozoic basement (Fig. 2), and (2) volumetrically minor nongranitic rocks including quartz-plagioclase gneiss ( Yqp, Fig. 2) and garnet-graphite paragneiss, hornblende-pyroxene metanorite, and quartzite (undivided non-granitic rocks on Fig. 2) (Burton et al., 1992). Protoliths of the paragneiss, metanorite, and quartzite are considered to be older than the granitic protoliths (Burton and Southworth, 1993). A total of 11 of the 13 varieties of Mesoproterozoic granitic gneiss that have been mapped in the northern Blue Ridge anticlinorium (Fig. 2) contain biotite as the dominant mafic mineral, and some are locally very leucocratic. The terms ‘metagranite’ (a poorly foliated, generally mica-poor granite), ‘granite gneiss’ (a well-foliated, relatively mica-rich granite) and ‘granitic gneiss’ (a well-foliated, relatively mica-rich intermediate to felsic rock) are used to differentiate degree of foliation and possible protolith in these metaigneous rocks. All names for rock types are informal except Marshall Metagranite ( Espenshade, 1986). Samples are described below in order of decreasing age. This order is determined primarily by U–Pb geochronology, although local field relations, as noted below, have aided relative age assignments (also see Fig. 2). Layered granitic gneiss ( Ylg) is mostly mediumgrained and white, gray, or pink weathering, and

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is distinguished by an extremely variable migmatitic texture on an outcrop scale. Textures range from a well-foliated gneissic layering to totally massive, with diffuse or irregular boundaries between the textural domains. Complex fold patterns and cross-cutting relationships are typically seen in outcrop. Layered granitic gneiss may be a hybrid rock, such as layered felsic volcanic rock that was partially migmatized during injection of porphyroblastic granite gneiss ( Ypg), with which it is frequently associated. Hornblende monzonite gneiss ( Yhg) is largely restricted to an area west of the Short Hill fault in the northern part of the study area. It is typically a dark green, medium-grained, well-foliated rock in which hornblende is the dominant mafic mineral (about 10% by volume), quartz content is typically only 15–20%, and microcline is as much as 50%. Lighter-colored, more leucocratic phases also exist. It has a metaluminous composition, in contrast to all of the other granitic compositions, which are peraluminous (Burton and Southworth, 1996). Porphyroblastic granite gneiss ( Ypg) is widespread over the study area. It is yellowish-brown weathering and consists of ovoid porphyroblasts of microcline 1–3 cm long in a matrix of finergrained plagioclase and gray quartz. Biotite is the dominant mafic mineral and garnet is locally common. Despite its coarse grain size, this rock is typically well-foliated, as shown by the flattening and preferred elongation direction of the porphyroblasts. At a number of localities, this porphyroblastic granite gneiss is intruded by irregular dikelets of younger granitic material, including garnetiferous metagranite ( Ygt), pink leucocratic metagranite ( Yml ), and possibly Marshall Metagranite ( Ym). Coarse-grained metagranite ( Ymc) has a distinctive texture consisting of 1–2 cm long, densely packed white or pink, sub- to euhedral microcline porphyroblasts and lesser interstitial plagioclase and distinctive blue quartz. This rock typically has a coarse, indistinct Mesoproterozoic foliation consisting of crudely aligned porphyroblasts. It occurs largely as map-scale inclusions within Marshall Metagranite ( Ym), and was originally mapped as a coarser-grained facies of the Marshall ( Espenshade, 1986; Kline et al., 1991).

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Fig. 2. Generalized geologic map of the northern Blue Ridge geologic province, Virginia–Maryland: (a) northern portion; (b) southern portion.

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Fig. 2. (continued)

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One of the most abundant Mesoproterozoic granitic rocks in the Blue Ridge basement is the Marshall Metagranite ( Ym), first described by Jonas (1928) and later by Espenshade (1986). It occurs mostly to the east of the Short Hill fault (Fig. 2). It is an orange or pink weathering, weakly to moderately foliated, medium-grained biotite granite gneiss in which biotite content ranges from 10–15%. Leucocratic and more biotitic subvarieties also have been recognized. Thin pink or white finer-grained aplite and coarser-grained pegmatite veins commonly intrude this granite and locally define a layering parallel to foliation. Quartz-plagioclase gneiss ( Yqp) is restricted to the northern part of the basement as narrow lenses within biotite granite gneiss ( Ybg), leucocratic metagranite ( Yg), and garnetiferous metagranite ( Ygt). It is a white to gray-weathering felsic rock that ranges from massive to well-foliated, with little or no potassium feldspar and minor (0–10%) biotite. It has a trondhjemitic chemical composition (Burton and Southworth, 1996). Garnetiferous metagranite ( Ygt) is one of the predominant rock types at the north end of the exposed core of the Blue Ridge anticlinorium. It is white, light gray or cream weathering, mediumto fine-grained, and massive to weakly foliated. It is similar in appearance to leucocratic granite ( Yg, see below); these rocks are differentiated primarily on the presence or absence of scattered almandine garnets. Biotite content ranges from 0 to 10%. White leucocratic metagranite ( Yg) is similar in appearance to garnetiferous metagranite ( Ygt), except that it lacks garnets. White aplite and pegmatite sills also locally define layering in this rock. Pink leucocratic metagranite ( Yml ) borders the Marshall Metagranite ( Ym) (Fig. 2) and has the same orange to pink weathering aspect and grain size but a more variable biotite content (0 to 10– 15%). Its texture ranges from totally massive to locally well-foliated and/or lineated. The map pattern of Yml ( Fig. 2) suggests a thick sill-like body that intruded between the Marshall Metagranite and gneisses of the western Blue Ridge and enclosed a septum of porphyroblastic granite gneiss ( Ypg). Biotite granite gneiss ( Ybg) is orange to gray

weathering, medium- to fine-grained and generally well-foliated, with a biotite content of 10–15%. Garnetiferous metagranite ( Ygt) and leucocratic metagranite ( Yg) are interlayered with the biotite granite gneiss ( Ybg). Megacrystic metagranite ( Ypb) occurs over a large area in the southwestern part of the study area (Fig. 2). It is a coarse-grained, porphyroblastic rock that resembles porphyroblastic granite gneiss ( Ypg) and, locally, coarse-grained metagranite ( Ymc); however, Ypb is separated from the other two coarse-grained lithologies by a north–south-trending belt of the Neoproterozoic Robertson River Intrusive Suite ( Tollo and Aleinikoff, 1992). Compositional and textural differences that distinguish megacrystic metagranitic ( Ypb, an adamellite with single large microcline crystals) from porphyroblastic granite gneiss ( Ypg, a granite with porphyroblasts composed of microcline and quartz aggregates) are recognizable (Clarke, 1984). Medium- to coarse-grained, massive to well-foliated quartz–hornblende–orthopyroxene– potassium feldspar–plagioclase rock or charnockite ( Yc) occurs in linear and pod-like bodies that are mapped primarily on the basis of float. The charnockite is distinctive in having a dark green, fresh surface and a crusty, pitted, orange–yellow weathering rind 1–2 cm thick. Massive charnockite is found as a pod-like body within well-foliated hornblende monzonite gneiss ( Yhg) northwest of Hillsboro (Fig. 2), whereas farther south wellfoliated charnockite occurs as linear bodies within various granitic gneisses. Pink to white aplites and pegmatites commonly intrude the other granitic rocks on an outcrop scale (not shown in Fig. 2). They range in morphology from thin (1–2 cm) planar dikes and sills to thicker (several meters) irregular bodies. Because some of these bodies are highly deformed and others cross-cut all Mesoproterozoic structures, they probably have a range of intrusive ages. No dating of these rocks was attempted.

3. Mesoproterozoic structures and metamorphism Mesoproterozoic structures and mineral assemblages, produced during the 1.1–1.0 Ga Grenville

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orogeny, are discernible in granitic and nongranitic rocks despite the fact that rocks of the northern Blue Ridge anticlinorium developed a regional cleavage and experienced metamorphic retrogression during late (?) Paleozoic deformation. All of the granites sampled in this study have a Grenvillian metamorphic foliation, indicating that deformation occurred or continued after intrusion of the youngest granitic protoliths. Grenvillian foliation is defined by platy or tabular mafic minerals such as biotite or hornblende, flattened, unstrained quartz and feldspar in a granoblasticelongate fabric and, locally, a migmatitic layering defined by thin, lit–par–lit sills of aplite or pegmatite. Gneissic banding composed of alternating thin mafic and felsic layers is rare but is seen in layered granitic gneiss ( Ylg). Open to isoclinal folds in foliation and ptygmatic folds in cross-cutting aplite and pegmatite veins indicate ductile conditions during deformation. A stretching lineation consisting of biotite streaks and/or rodded quartz and feldspar occurs locally on foliation surfaces. This lineation is best developed in the biotite-bearing granitic gneisses such as Marshall Metagranite ( Ym) and biotite granite gneiss ( Ybg). Where isoclinal folds and mineral lineations occur together, the fold hinges and stretching lineations are collinear, suggesting a common origin (Burton et al., 1994; Burton and Southworth, 1996). Orientations of Grenvillian structures and crosscutting relationships suggest multiple episodes of deformation. An older episode of foliation development (D1) is recorded by one exposure of porphyroblastic granite gneiss ( Ypg, Fig. 2), where a vein of garnetiferous metagranite ( Ygt), elsewhere well-foliated, truncates well-developed, northwest-trending D1 foliation in the host rock (Burton et al., 1994). If unrotated this foliation records NE–SW compression. Northwest-trending foliation in hornblende monzonite gneiss ( Yhg) and layered granitic gneiss ( Ylg) is also inferred to be of D1 age (Fig. 2). Grenvillian D2 foliation has two main trends, northwest and northeast, with vertical to northeast or southeast dips respectively. The collinear folds and mineral stretching lineations are developed in this foliation and plunge southeast, perhaps indicating SE–NW transport after initial development of D2 foliation.

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Outcrop-scale, open to tight, upright folds in foliation and migmatitic layering (D3) are locally developed and have gently north- or south-plunging hinges. These folds arch D2 foliation and are partly responsible for its NE to NW range in strike (Burton and Southworth, 1996). Mineral assemblages, in textural equilibrium that indicates granulite-facies and upper amphibolite-facies metamorphism, are found in some rocks in the map area. These include hornblende–orthopyroxene–microcline-bearing charnockite ( Yc, Fig. 2), and brown hornblende–orthopyroxenebearing metanorite (included in non-granitic rocks, Fig. 2). Kline et al. (1994) studied relict peridotitic (olivine–pyroxene-bearing) and gabbroic (hornblende–pyroxene) mineral assemblages in a small composite ultramafic body that they interpreted as igneous. Hornblende from this body yielded a late Grenvillian 40Ar/39Ar age of 995 Ma, which Kline et al. (1994) interpret as a cooling age following intrusion of the ultramafic body; 40Ar/39Ar ages of 1.0 to 0.92 Ga from hornblende in hornblende monzonite ( Yhg) are interpreted as a cooling ages following regional metamorphism ( Kunk et al., 1993). No P–T–t studies have been conducted on these rocks, primarily because their mineralogic compositions are not amenable to such an analysis.

4. U–Pb geochronology 4.1. Procedures Approximately 25 kg of each sample were processed through the standard mineral separations procedure, including crushing, pulverizing, and concentration of heavy minerals using a Wilfley Table, methylene iodide, and magnetic separator. All samples were obtained from outcrops except sample Bl-1-94 which was collected from a large pile of rock that had been removed from a pasture as in situ float. The most important criteria for selection of zircons for analysis was clarity (as clear as possible with a minimum of cracks and inclusions) and color (in general, light tan grains contain less uranium than darker grains). Although ideally we would prefer to select euhedral zircons, in many instances fragments of grains

0.092 0.065 0.062 0.070 0.114 0.075

( Ypg) (−150+200)C1 (−150+200)C2 (−150+200)C3 (−150+200)C4 monazite(dark)e monazite( light)e ( Ypg) (−150+200)EC1 (−150+200)EC2 (−150+200)EC3

J329-90 2a 2b 2c 2d 2e 2f BR1700 3a 3b 3c 0.025 0.020 0.018

0.051 0.053 0.025 0.042 0.006 0.006 0.008 0.006 0.006 0.007 0.003 0.002 0.002 0.012 0.001 0.013 0.048 0.030 0.031 0.014 0.026 0.019

U

513.5 617.4 460.1

1022 279.3 396.1 379.0 1807 2408

552.3 559.0 673.9 674.8 604.2 633.9 464.8 907.2 704.1 631.9 910.7 1050 1619 446.1 2041 1289 4690 4208 4585 2850 4384 6882

PB

91.52 110.6 87.76

200.1 52.91 74.58 72.47 947.5 2763

98.37 100.4 118.4 115.5 124.3 108.7 80.44 158.4 126.6 124.3 200.0 214.6 290.4 89.68 413.5 217.6 3811 3636 5282 2272 8144 4397

5186.0 4870.7 1389.5

4455.8 3130.4 9896.3 6650.9 2638.8 5283.4

2359.4 2387.5 2796.1 2086.6 318.38 1538.5 1955.3 1400.7 1726.9 1279.7 1051.0 1420.3 902.10 4278.0 1339.2 1662.2 13 294 25 550 14 354 25 763 29 301 3091.1

206Pb/ 204Pb

7948.5 7354.7 1613.1

5499.6 10 149 54 322 13 526 2733.7 5735.2

2765.5 2778.2 3955.6 2141.5 341.58 2390.9 3609.5 1806.2 2630.9 1661.1 1497.6 2870.4 1186.6 11 423 2714.2 1835.9 14 817 39 349 17 727 141 800 44 636 3194.1

206Pb/ 204Pb

13.394 18.203 17.708 13.966 0.461 73 0.159 31

15.309 14.422 14.301 12.940 5.9903 19.177 21.634 17.708 20.672 11.428 4.9175 16.329 10.618 12.020 10.167 10.408 0.250 60 0.227 63 0.146 92 0.258 82 0.091 07 0.349 48

206Pb/ 208Pb

12.831 19.472 12.770 19.984 11.629 11.748

12.405 12.613 12.807 12.689 12.255 12.962

12.076 12.102 12.419 11.903 8.5115 12.154 12.298 11.861 12.117 11.543 11.386 10.911 11.409 12.571 11.971 11.991 12.987 13.134 13.456 13.032 13.361 12.410

206Pb/ 207Pb

Weight Concentrations Measured Pb compositionb (mg) (ppm)

U250-92 ( Ylg) 1a (−100+150)EC1 1b (−100+150)EC2 1c (−100+150)EC3 1d (−100+150)EC4 1e (−100+150)EC5e 1f (−100+150)EC6e 1g (−100+150)EC8e 1h (−100+150)EC9e 1i (−100+150)EC11e 1j (−100+150)EC12e 1k (−100+150)EC13e 1l (−100+150)EC14e 1m (−100+150)EC15e 1n (−100+150)EC16e 1o (−100+150)EC17e 1p (−100+150)EC18e 1q monazite1e 1r monazite2e 1s monazite3e 1t monazite4e 1u monazitese 1v monazite6e

Graina

(0.25) (0.32) (0.22) (0.27) (0.39) (0.19)

(0.20) (0.14) (0.17) (0.82) (0.34) (0.37) (0.29) (0.20) (0.21) (0.24) (0.39) (0.51) (0.31) (0.46) (0.45) (0.28) (0.21) (0.23) (0.70) (0.46) (0.40) (0.78) 2.114 2.082 2.070 2.070 1.955 1.849

1.920 1.927 1.876 1.834 1.847 1.837 1.895 1.858 1.953 2.075 2.111 2.452 1.813 2.167 2.138 1.719 1.940 1.905 1.709 1.969 1.826 1.956 (0.26) (0.33) (0.24) (0.28) (0.39) (0.19)

(0.20) (0.15) (0.19) (1.2) (0.39) (0.39) (0.32) (0.22) (0.23) (0.28) (0.53) (0.52) (0.37) (0.47) (0.48) (0.29) (0.21) (0.23) (0.70) (0.47) (0.40) (0.78)

207Pb/ 235U

0.1831 (0.22) 1.923 (0.24) 0.1841 (0.44) 1.939 (0.44) 0.1873 (1.2) 1.994 (1.2)

0.1964 0.1939 0.1930 0.1930 0.1856 0.1796

0.1793 0.1802 0.1769 0.1702 0.1765 0.1745 0.1776 0.1762 0.1837 0.1927 0.1954 0.2050 0.1738 0.2007 0.1980 0.1648 0.1850 0.1824 0.1686 0.1863 0.1777 0.1863

206Pb/ 238U

Ratios (% error)c

(0.06) (0.09) (0.07) (0.07) (0.04) (0.04)

1156 1143 1137 1138 1098 1064

1063 1068 1050 1013 1048 1037 1054 1046 1087 1136 1150 1202 1033 1179 1165 983 1094 1080 1000 1102 1055 1101

1089 1095 1114

1153 1143 1139 1139 1100 1063

1088 1090 1073 1058 1062 1059 1080 1066 1100 1141 1152 1258 1050 1171 1161 1016 1095 1083 1012 1105 1055 1100

1099 1105 1126

1148 1144 1142 1141 1106 1060

1139 1135 1120 1151 1092 1104 1131 1107 1125 1149 1156 1356 1086 1155 1155 1086 1096 1089 1028 1112 1056 1099

206Pb/ 207Pb/ 207Pb/ 238U 235U 206Pb

0.076 15 (0.08) 1084 0.076 38 (0.05) 1089 0.077 20 (0.07) 1107

0.078 04 0.077 88 0.077 82 0.077 76 0.076 40 0.074 67

0.077 68 (0.05) 0.077 53 (0.05) 0.076 94 (0.10) 0.0781 (0.84) 0.0759 (0.18) 0.076 34 (0.10) 0.077 38 (0.11) 0.076 45 (0.10) 0.077 13 (0.10) 0.0781 (0.14) 0.0784 (0.34) 0.086 77 (0.12) 0.0757 (0.20) 0.078 31 (0.09) 0.0783 (0.14) 0.075 66 (0.06) 0.076 04 (0.03) 0.075 78 (0.04) 0.073 51 (0.04) 0.076 63 (0.11) 0.074 52 (0.04) 0.076 14 (0.04)

207Pb/ 206Pb

Ages (Ma)d

Table 1 U–Pb isotopic data for zircon and monazite from granitic gneisses, northern Blue Ridge, Virginia–Maryland [constants: 235l=9.8485×10−10/yr; 238l=1.551 25×10−10/yr; 238U/235U=137.88 (Steiger and Ja¨ger, 1977)]

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HF1-90 5a 5b 5c 5d 5e 5f

( Ygt) (−100+200)C1f (−100+200)C2f (−100+200)C3f (−100+200)C4f (−150+200)EC5 (−150+200)EqC 0.048 0.046 0.058 0.061 0.031 0.021

0.037 0.023 0.025 0.026 0.017 0.004 0.003 0.003 0.001 0.007 0.008 0.017 0.020

LN398 4a 4b 4c 4d 4c 4f 4g 4h 4i 4j 4k 4l 4m

( Ymc) (−100+150)EC1 (−100+150)EC2 (−100+150)EC4 (−100+150)EC5 (−100+150)EC7 (−100+150)EC8e (−100+150)EC9e (−100+150)EC10e (−100+150)EC11e (−200)EC1 (−200)EC2 (−200)EC3 (−200)EC4

0.028 0.031 0.016 0.013 0.010 0.010 0.014 0.013 0.115 0.062 0.006 0.009 0.009 0.010 0.005 0.025 0.011 0.020 0.016 0.030 0.022 0.005

3d (−150+200)EC4 3c (−150+200)EC5 3f (+100)EqC1e 3g (+100)EqC2e 3h (+100)EqC3e 3I (+100)EqC4e 3j (+100)EqC5e 3k (+100)EqC6e 3l monazite(dark)e 3m monazite( light)e 3-LT3 monazite Uz 3-LT6 monazite Uz 3-LT7 monazite Uz 3-LT21 monazite Uz 3-LT29 monazite Sp 3-LT41 monazite Sp 3-LT43 monazite Sp 3-LT51 monazite FOZ 3-DK29 monazite Uz 3-DK31 monazite Uz 3-DK36 monazite FOZ 3-DK43 monazite FOZ

551.4 601.8 615.4 624.0 735.3 830.0

457.6 483.2 497.8 577.3 566.2 491.5 691.7 1044 1333 424.2 768.5 616.6 447.7

810.1 763.5 637.3 1182.4 480.5 1021 665.3 626.2 3306 2828 1557 3337 3190 1717 3922 2570 2334 2397 3077 2912 2200 3799

105.1 116.0 110.4 112.6 132.0 155.2

87.64 92.48 94.96 109.4 107.7 94.42 142.5 226.7 255.8 85.57 142.9 115.4 83.56

149.5 139.4 143.4 212.9 105.3 179.8 128.4 116.4 3308 2796 2124 3251 3980 2433 3801 3088 2775 2840 2891 2822 3301 4681

772.94 994.17 14 744 9648.4 1931.8 443.30

5239.8 3309.6 3407.0 5384.2 6793.6 1284.0 497.30 2363.7 1428.0 620.10 893.16 1434.0 3051.7

2842.2 8351.7 240.0 842.80 347.48 6570.0 432.90 1314.3 5349.9 8560.2 124.32 251.81 502.35 822.82 1173.1 545.6 157.83 532.56 427.50 790.86 287.53 1606.8

793.91 1026.5 22 913 12 331 2590.6 486.26

10 765 6551.6 6286.9 13 428 16 722 2766.3 603.88 6924.6 10 430 924.90 1189.4 1626.0 4315.2

3182.8 11957 244.37 886.59 366.40 15 278 450.20 1531.7 5501.4 9332.0 126.64 255.70 518.81 896.62 1309.7 554.76 159.52 543.60 434.74 805.28 291.01 1888.1

10.725 11.026 13.055 12.888 12.281 9.6529

12.762 12.603 12.610 12.770 12.856 12.259 9.9572 13.036 12.638 10.845 11.396 11.760 12.594

12.266 12.829 7.4362 11.092 8.5937 13.266 9.4459 11.817 12.940 12.697 5.3365 7.6916 9.7886 10.997 11.627 9.9247 6.086 9.8224 9.3055 10.817 8.0772 12.176

9.7591 11.063 17.066 18.029 15.818 7.9388

10.233 9.9729 9.5526 10.696 9.8489 9.0041 7.4207 2.9876 11.275 7.3309 8.3085 9.1285 9.4719

15.448 15.289 5.6333 10.248 7.4873 13.675 8.9100 7.4884 0.185 17 0.205 49 0.153 84 0.203 33 0.14395 0.12711 0.195 18 0.154 90 0.176 25 0.161 64 0.207 40 0.196 44 0.120 08 0.144 92

0.1809 0.1864 0.1836 0.1849 0.1812 0.1703

0.1890 0.1882 0.1868 0.1878 0.1874 0.1861 0.1882 0.1781 0.1907 0.1872 0.1762 0.1802 0.1824

0.1862 0.1854 0.1850 0.1727 0.1940 0.1781 0.1769 0.1752 0.1778 0.1917 0.1772 0.1735 0.1730 0.1790 0.1778 0.1779 0.1792 0.1818 0.1758 0.1771 0.1725 0.1765

(0.50) (0.23) (0.20) (0.15) (0.18) (0.93)

(0.16) (0.26) (0.23) (0.22) (0.13) (0.43) (0.45) (0.43) (0.63) (0.55) (0.61) (0.36) (0.26)

(0.13) (0.21) (0.21) (0.22) (0.27) (0.28) (0.20) (0.15) (0.38) (0.23) (0.22) (0.14) (0.28) (0.23) (0.24) (0.24) (0.24) (0.30) (0.20) (0.25) (0.21) (0.23)

1.878 1.975 1.923 1.949 1.897 1.745

2.008 2.002 1.984 2.000 1.988 1.961 1.996 1.833 2.045 1.984 1.842 1.896 1.914

(0.54) (0.25) (0.21) (0.16) (0.19) (1.0)

(0.17) (0.27) (0.24) (0.23) (0.16) (0.48) (0.49) (0.46) (0.65) (0.58) (0.63) (0.38) (0.28)

1.978 (0.15) 1.963 (0.23) 1.948 (0.49) 1.765 (0.30) 2.078 (0.33) 1.828 (0.30) 1.810 (0.34) 1.820 (0.18) 1.831 (0.38) 2.042 (0.23) 1.834 (0.37) 1.778 (0.19) 1.7826 (0.31) 1.853 (0.25) 1.8427 (0.26) 1.843 (0.30) 1.857 (0.34) 1.897 (0.31) 1.811 (0.22) 1.826 (0.26) 1.782 (0.28) 1.816 (0.25)

0.0753 (0.19) 0.076 87 (0.08) 0.076 98 (0.07) 0.076 44 (0.05) 0.075 94 (0.06) 0.0743 (0.44)

0.077 04 (0.07) 0.077 18 (0.08) 0.077 04 (0.08) 0.077 25 (0.08) 0.076 94 (0.09) 0.0764 (0.21) 0.0769 (0.19) 0.0747 (0.16) 0.0778 (0.17) 0.0769 (0.19) 0.0758 (0.17) 0.076 30 (0.11) 0.076 11 (0.08)

0.077 07 (0.07) 0.076 76 (0.08) 0.0764 (0.41) 0.0741 (0.19) 0.0777 (0.18) 0.074 45 (0.08) 0.0742 (0.25) 0.075 34 (0.10) 0.074 69 (0.04) 0.077 24 (0.04) 0.0751 (0.29) 0.074 32 (0.12) 0.0747 (0.14) 0.075 06 (0.07) 0.075 15 (0.10) 0.0751 (0.17) 0.0751 (0.23) 0.075 66 (0.09) 0.074 73 (0.09) 0.074 77 (0.08) 0.0749 (0.18) 0.074 59 (0.09)

1072 1102 1087 1094 1073 1014

1116 1111 1104 1109 1107 1100 1112 1057 1125 1106 1046 1068 1080

1101 1097 1094 1027 1143 1057 1050 1041 1055 1131 1052 1032 1029 1062 1055 1056 1063 1077 1044 1051 1026 1048

1074 1107 1089 1098 1080 1025

1118 1116 1110 1116 1112 1102 1114 1057 1131 1110 1061 1088 1086

1108 1103 1098 1033 1142 1056 1049 1053 1057 1130 1058 1038 1039 1065 1061 1061 1066 1080 1050 1055 1039 1051

1077 1118 1095 1107 1094 1050

1122 1126 1122 1128 1120 1107 1119 1059 1141 1118 1090 1103 1098

1123 1115 1105 1044 1139 1054 1048 1078 1060 1127 1070 1050 1061 1070 1072 1072 1072 1086 1061 1062 1066 1057

J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146 121

0.040 0.047 0.027 0.023 0.053 0.020 0.028

WW22-90 ( Ym) 7a (−100+150)EC1 7b (−100+150)EC2 7c (−100+150)EC3 7d (−100+150)EC4 7e (−100+150)EC5 7f (−150+200)E1 7g (−150+200)E2

0.021 0.022 0.008 0.004 0.006 0.008 0.006 0.007 0.012 0.013 0.026 0.023 0.123 0.076 0.013 0.045 0.089 0.103 0.013 0.024 0.043 0.014 0.008 0.014 0.011 0.018

(−150+200)EqC (−150+200)EqC (−100+150)EqCe (−100+150)EqCe (−100+150)ECe (−100+150)ECe (−100+150)ECe (−100+150)ECe monaziteC1e monaziteC2e monaziteC3e monaziteC4e monaziteC5e monaziteP1e monaziteP2e monaziteP3e monaziteP4e monaziteP5e monaziteP6e

U

454.8 469.7 448.1 419.9 456.1 529.0 831.3

377.4 501.9 367.8 400.6 523.7 14 310 11 026

1095 734.8 615.5 972.4 1034.5 1392 982.4 578.0 2610 3737 2404 2180 3867 3683 3886 3179 2917 3068 4636

PB

85.96 87.42 77.11 81.12 86.95 100.9 162.0

67.75 87.25 70.43 73.68 97.52 5826 5464

188.2 127.9 107.6 156.2 187.0 243.6 167.1 99.32 2135 3325 1784 1744 3224 3037 3295 2269 2318 2195 3302

12 186 13 118 2391.0 1466.6 4636.1 1167.2 1063.3

2589.8 6397.6 1127.3 1309.8 1866.3 9055.8 8650.6

3926.6 1435.1 3159.0 2698.0 2918.0 475.10 645.20 942.10 3582.1 1504.0 5497.4 2027.6 42 137 18 474 2296.3 1804.0 6560.9 705.01 11 364.6

206Pb/ 204Pb

78 293 52 983 3756.4 1956.8 8904.1 1432.9 1153.2

5140.5 18 284 2048.9 9273.2 3979.9 10 715 9827.5

8424.3 2088.3 7285.7 7572.7 4832.6 494.32 716.09 1187.8 5091.5 1640.2 7090.2 2268.4 54 894 22 335 2637.9 1870.7 7042.3 711.35 16 890

206Pb/ 204Pb

12.973 13.015 12.433 11.929 12.812 11.584 11.265

12.674 13.092 11.986 12.849 12.489 13.215 13.225

13.162 12.285 13.133 13.141 12.543 9.6762 10.578 11.304 12.860 11.950 12.988 12.295 13.270 13.222 12.417 12.071 13.015 10.566 13.152

206Pb/ 207Pb

Weight Concentrations Measured Pb compositionb (mg) (ppm)

WW29-90 ( Ym) 6a (−100+200)C1 6b (−150+200)C2 6c (−150+200)C3 6d (−150+200)C4 6e (−150+200)C5 6f monazite1e 6g monazite2e

5g 5h 5i 5j 5k 5l 5m 5n 5o 5p 5q 5r 5s 5t 5u 5v 5w 5x 5y

Graina

Table 1 (continued )

10.404 12.030 10.385 9.3508 10.074 8.8254 7.5897

12.319 18.226 9.6522 11.193 12.055 0.631 93 0.452 93

115.388 15.097 14.551 30.089 20.271 10.788 11.999 13.832 0.243 65 0.213 01 0.282 14 0.249 48 0.237 13 0.241 11 0.208 77 0.280 75 0.249 46 0.252 42 0.291 98

206Pb/ 208Pb

0.1873 0.1865 0.1692 0.1869 0.1879 0.1826 0.1827

0.1793 0.1786 0.1859 0.1829 0.1853 0.1770 0.1742

0.1748 0.1749 0.1771 0.1680 0.1856 0.1640 0.1638 0.1696 0.1819 0.1760 0.1851 0.1802 0.1819 0.1822 0.1657 0.1759 0.1801 0.1595 0.1826

206Pb/ 238U

(0.21) (0.17) (0.18) (0.24) (0.55) (0.22) (0.16)

(0.21) (0.17) (0.43) (0.66) (0.30) (0.37) (0.28)

(0.47) (0.39) (0.48) (0.57) (0.27) (0.12) (0.19) (0.34) (0.15) (0.42) (0.14) (0.29) (0.23) (0.29) (0.32) (0.13) (0.39) (0.29) (0.20)

1.986 1.969 1.789 1.974 1.981 1.924 1.927

1.882 1.861 1.961 1.924 1.955 1.814 1.782

1.790 1.799 1.812 1.719 1.965 1.686 1.686 1.789 1.880 1.820 1.914 1.865 1.884 1.884 1.717 1.825 1.858 1.642 1.893

(0.22) (0.18) (0.21) (0.27) (0.75) (0.26) (0.17)

(0.22) (0.18) (0.45) (0.68) (0.31) (0.37) (0.28)

(0.48) (0.43) (0.50) (0.58) (0.29) (0.19) (0.23) (0.36) (0.16) (0.42) (0.15) (0.30) (0.23) (0.29) (0.37) (0.17) (0.40) (0.31) (0.22)

207Pb/ 235U

Ratios (% error)c

0.075 90 (0.05) 0.076 57 (0.05) 0.076 65 (0.10) 0.076 58 (0.11) 0.0765 (0.51) 0.076 42 (0.12) 0.076 46 (0.06)

0.076 14 (0.07) 0.075 60 (0.05) 0.076 50 (0.11) 0.0763 (0.17) 0.076 50 (0.08) 0.074 34 (0.04) 0.074 16 (0.03)

0.074 29 (0.06) 0.0746 (0.18) 0.0742 (0.14) 0.074 22 (0.11) 0.076 79 (0.10) 0.0746 (0.14) 0.0747 (0.13) 0.076 51 (0.12) 0.074 97 (0.05) 0.075 01 (0.05) 0.074 99 (0.07) 0.075 07 (0.09) 0.075 10 (0.03) 0.074 99 (0.03) 0.0751 (0.16) 0.075 25 (0.10) 0.074 81 (0.09) 0.074 63 (0.l1) 0.075 19 (0.09)

207Pb/ 206Pb

1107 1103 1008 1105 1110 1081 1082

1063 1059 1099 1083 1096 1050 1035

1039 1039 1051 1001 1097 979 978 1010 1077 1045 1095 1068 1077 1079 989 1045 1067 954 1081

1111 1105 1041 1107 1109 1089 1090

1075 1068 1102 1089 1100 1051 1039

1042 1045 1050 1016 1104 1003 1003 1041 1074 1053 1086 1069 1075 1076 1015 1055 1066 986 1079

1119 1110 1112 1110 1107 1106 1107

1099 1085 1108 1103 1108 1051 1046

1049 1057 1047 1047 1116 1057 1060 1108 1068 1069 1068 1070 1071 1068 1073 1075 1064 1059 1074

206Pb/ 207Pb/ 207Pb/ 238U 235U 206Pb

Ages (Ma)d

122 J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146

0.101 0.062 0.126 0.047 0.031 0.016 0.027 0.037 0.042 0.035 0.059 0.040 0.050 0.059 0.040 0.109 0.048 0.035 0.029

BR11-90 ( Yg) 9a (−100+150)C1 9b (−100+150)C2 9c (−100+150)C3 9d (−100+150)C4 BR1313-92 ( Yml ) 10a (−100+150)EC1 10b (−100+150)EC2 10c (−100+150)EC3 10d (−100+150)EC4 10e monazite1e 10f monazite2e BR554-90 ( Ybg) 11a (−100+150)C1 11b (−100+150)C2 11c (−100+150)C3 11d (−100+150)C4 11e (−100+150)C6 11f monazitee ( Ypb) (−100+150)EC1 (−100+150)EC2 (−100+150)EC3 ( Yc) (−100+150)EqCA1e (−100+150)EqCA2e (−100+150)ElCA3e (−100+150)ElCA4e (−100+150)ElCA5e (−100+150)ElCA6e (−100+150)ElCA7e (−100+150)ElCA8e

U39-90 12a 12b 12c Bl-1-94 13a 13b 13c 13d 13e 13f 13g 13h 0.011 0.007 0.007 0.008 0.004 0.005 0.003 0.007

0.039 0.027 0.026 0.044 0.022 0.029 0.041

( Yqp) (−100+150)E1 (−100+150)E2 (−100+150)E3 (−100+150)E4 (−100+150)E5 (−100+150)E6 (−100+150)E7

BR1674 8a 8b 8c 8d 8c 8f 8g

84.18 205.9 320.8 192.8 278.7 457.0 535.0 241.9

115.8 230.6 211.1

748.5 770.5 740.4 805.3 768.7 4109

223.2 219.4 255.9 234.2 246.2 1783

186.2 612.2 334.1 563.4

775.9 755.9 1140 835.5 738.5 756.0 793.6

17.24 45.26 66.11 43.71 70.32 115.2 120.6 67.84

20.95 41.72 38.58

130.7 133.6 128.1 139.6 140.0 3839

43.31 44.45 49.54 47.38 1875 3544

39.23 124.4 69.11 116.6

145.9 133.1 196.7 148.1 140.3 136.2 138.1

467.20 434.2 641.20 303.50 178.51 229.58 320.20 147.52

2312.2 3820.0 2487.0

4119.4 13 240 17 211 15 117 1299.0 40 078

2662.0 880.10 4903.0 8743.7 1671.0 1383.7

9240.5 8890.0 12 146 4886.6

826.18 5387.9 12 537 13 680 638.49 5819.1 8279.4

766.04 554.16 813.29 351.88 197.34 245.23 373.39 159.54

24 947 7997.2 8514.5

4943.7 55 009 98 569 34 557 1404.1 53 001

8186.4 1514.8 13 408 89 693 1956.4 1441.3

67 750 15 224 24 886 9495.8

857.25 8254.9 28 623 26 900 684.34 10 819 14 980

15.212 16.961 16.396 18.296 10.777 0.195 53

5.2305 4.9132 4.9739 4.0809 0.020 26 0.087 43

3.3827 4.1181 3.5626 3.7174

10.941 15.974 19.604 13.805 9.2126 11.994 18.630

9.5939 9.159 10.302 8.189 6.5081 7.2211 8.3482 5.9108

4.8564 5.4736 7.1280 4.4549 3.4549 3.8139 4.5009 2.7626

13.312 9.5401 13.114 9.5300 13.121 8.9308

12.948 13.363 13.402 13.328 11.813 13.559

13.093 11.767 13.220 13.378 12.343 11.825

13.356 13.223 13.307 13.134

10.889 13.012 13.245 13.224 10.448 13.074 13.145

(0.21) (0.26) (0.17) (0.18) (0.38) (0.32)

(0.37) (0.51) (0.19) (0.17) (0.16) (0.42)

(0.16) (0.10) (0.18) (0.15)

(0.36) (0.18) (0.62) (0.49) (0.41) (0.18) (0.13)

1.814 1.829 1.819 1.836 1.824 1.766

1.827 1.882 1.809 1.831 1.754 1.864

1.838 1.841 1.821 1.836

(0.22) (0.27) (0.19) (0.20) (0.40) (0.32)

(0.39) (0.52) (0.20) (0.21) (0.18) (0.51)

(0.19) (0.15) (0.22) (0.16)

1.877 (0.38) 1.858 (0.19) 1.841 (0.63) 1.858 (0.49) 1.8449 (0.42) 1.869 (0.21) 1.854 (0.15)

0.1825 0.1948 0.1914 0.1886 0.1880 0.1956 0.1887 0.1913

(0.92) (0.53) (0.60) (0.60) (0.68) (0.35) (0.54) (0.86)

1.905 2.089 2.019 1.991 1.964 2.097 1.990 2.014

(0.96) (0.59) (0.81) (0.63) (0.78) (0.39) (0.57) (0.95)

0.1781 (0.41) 1.831 (0.42) 0.1777 (0.18) 1.824 (0.20) 0.1785 (0.34) 1.835 (0.35)

0.1770 0.1779 0.1771 0.1784 0.1775 0.1743

0.1775 0.1806 0.1760 0.1781 0.1725 0.1810

0.1785 0.1788 0.1770 0.1784

0.1809 0.1794 0.1781 0.1794 0.1786 0.1804 0.1790

(0.12) (0.06) (0.06) (0.04) (0.10) (0.10) (0.07)

1051 1055 1051 1058 1053 1036

1053 1070 1045 1057 1026 1072

1059 1060 1051 1058

1072 1064 1056 1064 1059 1069 1061

0.0757 0.0778 0.0765 0.0766 0.0758 0.0777 0.0765 0.0764

(0.26) (0.24) (0.50) (0.20) (0.35) (0.17) (0.18) (0.37)

1080 1147 1129 1114 1111 1152 1114 1128

0.074 55 (0.12) 1057 0.074 42 (0.09) 1054 0.074 54 (0.09) 1059

0.074 35 (0.04) 0.074 57 (0.05) 0.074 47 (0.08) 0.074 62 (0.08) 0.0745 (0.13) 0.073 48 (0.03)

0.074 64 (0.11) 0.075 60 (0.12) 0.074 58 (0.06) 0.074 59 (0.12) 0.073 74 (0.09) 0.0747 (0.27)

0.074 66 (0.09) 0.074 69 (0.12) 0.0746 (0.13) 0.074 64 (0.07)

0.075 25 0.075 13 0.075 00 0.075 09 0.074 92 0.075 17 0.075 12

1083 1145 1122 1113 1103 1148 1112 1120

1057 1054 1058

1051 1056 1052 1058 1054 1033

1055 1075 1048 1057 1029 1068

1059 1060 1053 1058

1073 1066 1060 1066 1062 1070 1065

1088 1141 1108 1110 1089 1140 1108 1105

1056 1054 1056

1051 1057 1054 1058 1055 1027

1059 1084 1057 1058 1034 1060

1060 1060 1057 1059

1075 1072 1069 1071 1066 1073 1072

J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146 123

(−100+150)ElCA10e 0.005 (−100+150)ElCA11e 0.005 0.037 0.033 0.041 0.015 0.017 0.014 0.011 0.007 0.007 0.005 0.016 0.015 0.013 0.016 0.047 0.052

( Yhm) (−100+150)EC1 (−100+150)EC2 (−100+150)EC3 (−100+150)EC4 (−100+150)EC6 (+100)EqC1e (+100)EqC2e (+100)EqC3e (+100)EqC4e (+100)EqC5e (+100)EC1 (+100)EC2 (+100)EC3 (+100)EC4

BR1529 14a 14b 14c 14d 14e 14f 14g 14h 14i 14j 14k 14l 14m 14n HF2-90 ( Yhm) 15a (−100+150)EC1 15b (−100+150)EC2

U

416.6 454.9

308.3 326.3 264.2 224.3 356.6 302.9 288.7 462.8 637.6 1916 285.0 310.8 297.5 333.1

447.4 426.7

PB

78.66 87.29

58.46 60.47 51.73 44.53 69.49 70.19 57.03 85.78 142.9 316.5 58.51 71.19 58.30 47.34

85.76 85.71

8466.5 6470.3

6354.9 6420.1 1651.2 1644.6 1701.7 2196.8 3040.0 2042.0 5719.0 1666.4 650.23 342.32 1538.3 1139.5

701.50 495.70

206Pb/ 204Pb

19 558 22 312

12 110 13 399 1898.6 2756.4 3210.7 3994.7 13 254 4450.1 13 420 1989.5 990.98 409.61 39 958 4016.6

1423.8 779.42

206Pb/ 204Pb

206Pb/ 208Pb

8.7357 8.6632 6.5731 6.8663 7.0162 2.3841 9.7141 9.0337 2.5488 16.251 5.7585 4.2719 9.1018 8.3940 13.130 7.0939 13.142 6.4584

12.856 12.899 11.984 12.202 12.412 12.823 12.683 12.788 13.243 12.107 11.056 8.9153 12.854 12.395

10.488 7.5688 9.6401 6.1186

206Pb/ 207Pb

Weight Concentrations Measured Pb compositionb (mg) (ppm)

13i 13j

Graina

Table 1 (continued )

207Pb/ 235U

(0.46) (0.28) (0.38) (0.38) (0.38) (0.33) (0.31) (0.32) (0.22) (0.22) (0.36) (0.38) (0.38) (0.33)

1.950 1.902 1.911 1.978 1.934 1.835 2.082 1.867 1.817 1.732 1.946 2.023 2.047 1.458

(0.48) (0.37) (0.39) (0.43) (0.40) (0.34) (0.33) (0.36) (0.23) (0.24) (0.38) (0.43) (0.41) (0.37) 0.1802 (0.29) 1.874 (0.29) 0.1810 (0.19) 1.883 (0.20)

0.1846 0.1804 0.1825 0.1868 0.1842 0.1788 0.1941 0.1805 0.1770 0.1665 0.1854 0.1892 0.1917 0.1371

0.1846 (0.73) 1.932 (0.75) 0.1852 (0.78) 1.942 (0.81)

206Pb/ 238U

Ratios (% error)c

1092 1069 1080 1104 1090 1061 1144 1070 1050 993 1096 1117 1131 828

1092 1095

1072 1075

1099 1082 1085 1108 1093 1058 1143 1069 1052 1020 1097 1123 1131 913

1092 1096

1080 1081

1111 1107 1094 1116 1099 1053 1141 1069 1054 1081 1098 1136 1133 1125

1093 1096

206Pb/ 207Pb/ 207Pb/ 238U 235U 206Pb

0.075 43 (0.06) 1068 0.075 46 (0.05) 1073

0.076 61 (0.11) 0.0765 (0.24) 0.075 96 (0.08) 0.0768 (0.20) 0.076 15 (0.10) 0.074 42 (0.09) 0.077 78 (0.10) 0.0750 (0.16) 0.074 45 (0.07) 0.075 45 (0.08) 0.076 11 (0.12) 0.0776 (0.18) 0.0774 (0.14) 0.0771 (0.15)

0.0759 (0.16) 0.0760 (0.19)

207Pb/ 206Pb

Ages (Ma)d

124 J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146

(−100+150)EC4 (−100+150)EC5 (−100+150)EC7 (−150+200)EC8 (−150+200)EC9 (+100)EC1e (+100)EC2e (+100)EC3e (+100)EC4e (+100)EC5e (+100)EC6e (+100)EC7e (+100)EC8e (+1000EC9e (+100)EC10e (+100)EC11e (+100)EC12e (+100)EC13e (+100)EC14e

0.024 0.029 0.032 0.017 0.006 0.007 0.015 0.012 0.012 0.018 0.011 0.012 0.009 0.017 0.010 0.024 0.010 0.018 0.025

369.1 423.2 439.1 513.5 462.1 448.2 568.2 673.7 553.0 724.1 567.5 859.6 494.6 452.5 389.1 732.4 237.4 315.3 374.8

72.32 79.45 82.99 102.0 90.93 82.02 107.1 130.7 106.6 142.3 108.6 171.2 95.70 93.69 79.00 136.6 85.15 67.42 77.12

2283.0 3028.6 4321.6 1184.3 1323.0 1053.6 2233.4 2081.5 1729.5 6360.3 1941.7 1654.7 692.90 870.83 476.02 2481.7 82.358 524.60 893.78

4015.4 5197.3 8745.8 1338.6 2168.8 8617.2 6913.7 6458.2 5265.6 17 882 10 216 2821.9 861.30 1004.8 560.14 3051.7 84.496 588.11 1014.9

12.502 12.633 13.047 11.636 12.032 12.787 12.956 12.997 12.795 13.293 13.12 12.566 10.967 10.850 9.9539 12.623 4.0994 9.8548 11.166

7.0672 10.271 6.6158 5.8951 7.6387 7.6661 7.3955 5.1221 6.1605 4.8338 5.9639 4.9992 7.1007 7.8309 5.5048 7.4476 1.7250 5.9230 5.2252

0.1857 0.1849 0.1786 0.1814 0.1869 0.1757 0.1802 0.1767 0.1799 0.1779 0.1785 0.1797 0.1788 0.1939 0.1782 0.1776 0.1813 0.1898 0.1834

(0.21) (0.23) (0.41) (0.20) (0.44) (0.99) (0.32) (0.18) (0.26) (0.28) (0.27) (0.19) (0.32) (0.21) (0.34) (0.31) (0.27) (0.22) (0.21)

1.957 1.948 1.847 1.884 1.973 1.854 1.867 1.821 1.872 1.826 1.841 1.846 1.840 2.087 1.845 1.825 1.893 2.024 1.911

(0.23) (0.25) (0.42) (0.25) (0.50) (1.0) (0.33) (0.22) (0.28) (0.29) (0.28) (0.23) (0.36) (0.22) (0.36) (0.33) (0.54) (0.26) (0.24)

0.076 45 (0.09) 0.076 42 (0.11) 0.075 02 (0.05) 0.0753 (0.14) 0.0766 (0.22) 0.0766 (0.19) 0.075 13 (0.08) 0.074 74 (0.11) 0.075 46 (0.11) 0.074 43 (0.08) 0.074 82 (0.10) 0.074 54 (0.12) 0.0747 (0.15) 0.078 06 (0.08) 0.075 07 (0.11) 0.074 56 (0.11) 0.0757 (0.44) 0.0774 (0.13) 0.075 55 (0.12)

1098 1094 1059 1075 1105 1043 1068 1049 1067 1056 1059 1065 1060 1142 1057 1054 1074 1120 1086

1101 1098 1062 1075 1106 1065 1069 1053 1071 1055 1060 1062 1060 1144 1062 1055 1079 1124 1085

1107 1106 1069 1077 1110 1110 1072 1061 1081 1053 1064 1056 1059 1149 1070 1057 1088 1131 1083

a All zircons were abraded [Aleinikoff et al., 1990b; modified from Krogh (1982)]. Abbreviations: E (elongate), C (clear), Eq (equant), P (pitted/frosted ), Uz (unzoned), Sp (spotted), FOZ (faint oscillatory zoning). b Blank (which decreased from about 50 to 10 pg, 50%, during the course of this study) and fractionation (0.140.03%) corrected. Assumed blank composition is 204:206:207:208=1:18.8:15.65:38.65. c 2s uncertainties. d Common lead corrections from Stacey and Kramers (1975) model. e Single grain. f Mixed fraction of elongate and equant grains.

15c 15d 15e 15f 15g 15h 15i 15j 15k 15l 15m 15n 15o 15p 15q 15r 15s 15t 15u

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were chosen because they had the fewest imperfections. All grains thought to be igneous in origin have deformed, pitted faces, indicative of the highgrade metamorphic conditions these rocks have undergone during the Grenville orogeny, but they preserve euhedral oscillatory zoning. In addition, equant, multifaceted clear to light tan grains (thought to have formed during metamorphism) were obtained from two samples and one sample yielded spherical, abraded grains interpreted as detrital. All analyzed zircons were hand-picked under alcohol, abraded moderately (to remove crystal faces, resulting in lozenge-shaped grains) or severely (resulting in spheres) (Aleinikoff et al., 1990b, modified from Krogh, 1982), and leached in 7 N HNO and 6.5 N HCl prior to dissolution. 3 Zircon fractions weighed between 1 and 126 mg. Most fractions were composed of about five to ten grains, although some samples with complex isotopic systematics necessitated analysis of individual grains ( Table 1). Individual monazite grains were hand-picked for clarity, color ( light yellow to dark orange), and lack of cracks or inclusions. Monazite was leached in 3N HNO and 3N HCl prior to 3 dissolution. All zircons were loaded into PFA microcapsules in a standard Parr bomb (modified from Parrish, 1987), to which was added a mixed 205Pb–233U– 236U spike, concentrated HF, and concentrated HNO . Dissolution required about 3 days in an 3 oven at about 210°C. Monazite was dissolved in 12 N HCl in a screw-top container on a hot plate at 150°C for several days. Extraction of Pb and U from zircon and monazite followed procedures of Krogh (1973) and Parrish (1987), with minor modifications. Isotopic ratios of U, loaded on a single Re filament with suspended graphite (aquadaug), and Pb, loaded on a single Re filament with silica gel and 0.5N H PO , were measured on 3 4 a VG Micromass 54E mass spectrometer with a single Faraday cup collector and Daly multiplier, using the ANALYST program of Ludwig (1992). Measured Pb isotopic ratios were corrected for the presence of common Pb using the Stacey and Kramers (1975) model. Data reduction and plotting were done using the PBDAT and ISOPLOT programs, respectively (Ludwig, 1991a,b). Most

ages for zircon samples cited below are concordia intercept ages calculated using a best-fit regression through a linear data array. In rare cases of overlapping data with little or no spread in Pb/U ages, the age is calculated using the weighted average of the 207Pb/206Pb ages. U–Pb data from monazite are treated similarly; however, when only one grain was analyzed from a sample and the results are reversely discordant (i.e. plot above the concordia curve), the age of the monazite is taken to be the 207Pb/235U age [assuming that the reverse discordance is caused by excess 206Pb (Parrish, 1990)]. Zircons from sample HF2-90 were also analyzed using SHRIMP II ( Table 2) at the Research School of Earth Sciences, Australian National University, Canberra. Selected grains were mounted in epoxy, ground to half-thickness and polished with 6 mm and 3 mm diamond paste. Zircon standard AS3 (1099 Ma zircon from gabbroic anorthosite from the Duluth Complex, Paces and Miller, 1993) was used to calibrate 206Pb/238U ages. All grains were imaged in cathodoluminescence (CL) and photographed in both transmitted and reflected light prior to SHRIMP analysis to identify cores and overgrowths and crack- and inclusion-free areas. Analytical procedures followed the methods described in Compston et al. (1984) and Williams and Claesson (1987). SHRIMP results are displayed as: (1) a Tera–Wasserburg plot (238U/206Pb versus 207Pb/206Pb; Tera and Wasserburg, 1972) using data that has not been corrected for common Pb content — this plot merely provides information for an assessment of which data points should be used for the age calculation; (2) weighted averages plot of 207Pb/206Pb ages corrected for common Pb. 4.2. Results: zircon U–Pb geochronology U–Pb ages obtained by conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS ) for 15 samples of felsic metaigneous gneisses span a range of about 100 m.y. (from about 1150–1050 Ma), and are divisible into three groups, based on age and degree of deformation. Group 1 includes five samples that are mostly strongly foliated and range in age from 1153±6

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J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146 Table 2 SHRIMP U–Th–Pb data for sample HF2-90 Sample

Spot loc., grain colora

Ub (ppm)

Thb (ppm)

Th/U

206Pb/ 204Pb

238U/ 206Pbc

Errord

207Pb/ 206Pbc

Errord

% concordant

207Pb/ 206Pbe (Ma)

Errord (Ma)

1.1 2.1 3.1 3.2 4.1 4.2 5.1 6.1 7.1 8.1 9.1 9.2 10.1 11.1 12.1 13.1 14.1 15.1 16.1 17.1 18.1 19.1 20.1 20.2 21.1 21.2 22.1 23.1

c, p-rd r, rd r, cl c, cl r, cl c, cl r, cl r, rd c, cl c, rd c, cl r, cl c, rd c, cl r, rd c, rd c, rd c, cl c, cl c, cl c, p-rd c, cl c, rd r, rd c, cl r, cl c, cl c, rd

681 801 906 1580 3564 508 1243 963 382 463 835 506 1121 266 892 424 532 512 463 324 321 572 895 1469 397 2319 374 846

215 274 810 824 56 452 797 836 125 189 463 196 428 99 664 181 223 250 162 169 125 279 325 827 70 328 234 482

0.32 0.34 0.89 0.52 0.01 0.89 0.64 0.87 0.33 0.41 0.55 0.39 0.38 0.37 0.74 0.43 0.42 0.49 0.35 0.52 0.39 0.49 0.36 0.56 0.18 0.14 0.63 0.57

7231 5554 5435 642 45 249 1414 6957 12 191 4004 5307 7503 2538 13 077 1980 7675 2577 3178 3277 4084 3704 2086 5110 6868 12 618 16 633 23 725 2898 7543

5.1010 5.1190 5.6647 5.1582 5.7026 5.1741 5.4567 5.5358 5.2572 5.1182 5.6268 5.1766 4.9905 4.9444 5.5752 5.0342 5.0529 4.8513 4.8386 4.9759 5.2739 5.0639 4.9370 5.3270 5.4894 5.5645 5.1237 5.5385

0.0954 0.1464 0.0863 0.1173 0.1104 0.1552 0.1326 0.0906 0.1034 0.1230 0.0772 0.0531 0.1019 0.1036 0.1176 0.1975 0.1394 0.0659 0.1496 0.0943 0.1169 0.0847 0.1242 0.1617 0.1078 0.0781 0.1046 0.1067

0.081 65 0.079 80 0.075 27 0.098 80 0.074 35 0.090 02 0.074 80 0.075 42 0.081 15 0.082 33 0.076 41 0.080 63 0.080 40 0.081 45 0.076 27 0.084 96 0.081 24 0.080 36 0.079 81 0.081 14 0.082 97 0.079 71 0.079 24 0.075 46 0.0765 0.075 11 0.078 37 0.075 57

0.000 76 0.001 57 0.000 75 0.000 42 0.000 40 0.001 05 0.000 45 0.000 62 0.001 30 0.000 83 0.000 51 0.000 82 0.000 37 0.001 21 0.000 46 0.000 93 0.000 88 0.002 11 0.000 85 0.001 56 0.001 22 0.000 75 0.000 69 0.000 42 0.001 29 0.0005 0.002 34 0.0007

97 102 104 100 100 94 108 102 98 97 100 106 100 112 101 98 104 110 109 104 101 104 105 106 99 101 111 103

1189 1127 1004 1112 1042 1197 1007 1048 1137 1189 1055 1069 1180 1048 1053 1184 1115 1096 1104 1129 1099 1119 1126 1050 1086 1055 1026 1033

32 46 28 40 11 65 19 22 52 31 20 33 16 56 24 49 35 64 31 55 47 41 21 17 43 20 81 26

a Abbreviations: c (core), r (rim), p-rd (pale red ), rd (red ), cl (colorless). b Concentrations are probably 20%. c Uncorrected for common Pb content. d 1s, absolute uncertainty. e Corrected for common Pb content using 204Pb content.

to about 1140 Ma. One of these samples (HF2-90; hornblende monzonite gneiss) was also dated by in situ analysis using SHRIMP. Group 2 includes two samples that are moderately to strongly deformed and have identical ages of 1111±2 and 1112±3 Ma. Group 3 is composed of six samples that are weakly to moderately deformed and range in age from 1077±4 to 1055±2 Ma (subjectively subdivided into subgroups 3A and 3B). In addition, zircon from charnockite (sample Bl-1-94) did not yield a precise age due to very complicated isotopic systematics, although we suspect (on the basis of 207Pb/206Pb ages) that this rock belongs

in Group 1. The geochronologic results are discussed in order of decreasing age.

4.2.1. Group 1 A sample of layered granitic gneiss ( Ylg; sample number U250-92) was collected from a wellexposed, low outcrop in a pasture (sample locality 1, Fig. 2). The outcrop is composed of finely layered felsic rock that appears to be of volcanic origin, with other layers and dikelets of porphyroblastic granite gneiss ( Ypg) and garnetiferous metagranite ( Ygt). It is impossible to determine the

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exact nature of the cross-cutting relationships in this outcrop. Zircons from sample U250-92 have a wide range of shapes [Fig. 3(a)] and colors. Approximately two-thirds of the grains are medium to dark brown, euhedral, and prismatic, with length-to-width ratios l/w=2–7. Many of the more elongate grains are doubly terminated with somewhat rounded tips, and nearly all are moderately deformed with

pitted and slightly bent faces. We tentatively interpret this morphology to indicate that these zircons are igneous in origin. The remaining one-third of the grains are spherical to lozenge shaped, with frosted surfaces. These grains have a wide range in color, including pale orange, deep red, and very dark brown. The morphology and color variability of these zircons are quite suggestive of a detrital origin. They lack the typical characteristics of

Fig. 3. Scanning electron microscope images of representative zircons from granitic gneisses of the northern Blue Ridge. Grains in (a) and (b) are from Group 1, (c) is from Group 2, (d ) and (e) from Group 3, (f ) is unknown age but thought to be in Group 1. (a) Elongate and spherical grains (detrital origin?) in sample U250 ( layered granitic gneiss). (b) Elongate, partially resorbed and overgrown grains from sample J329-90 (porphyroblastic granite gneiss). (c) Elongate grains from sample WW22-90 (Marshall Metagranite). (d) Elongate and equant grains from sample HF1-90 (garnetiferous metagranite). Equant grains have faces, suggestive of metamorphic origin. (e) Elongate grains from sample BR11-90 (white leucocratic metagranite). (f ) Elongate grains from sample Bl-1-94 (charnockite).

J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146

metamorphic zircons ( low U, multiple facets, clear and colorless) and thus it is unlikely that they formed during metamorphism. It is possible that the elongate zircons are also detrital. A volcanic origin is suggested for this rock because of its fine grain size, layering, and the presence of spherical zircons interpreted as detrital. U–Pb data from four multigrain fractions of elongate zircon from sample U250-92 (Table 1) are discordant, with 207Pb/206Pb ages between 1120 and 1151 Ma and Pb/U ages ranging from 1013 to 1090 Ma [ Table 1, Fig. 4(a)]. The data are not

Fig. 4. U–Pb data for sample U250-92 ( layered granitic gneiss): (a) zircon multigrain fractions; (b) zircon single grains; (c) monazites.

129

collinear, and thus no definitive age information was obtained from the multigrain fractions beyond the suggestion that the rock is of ‘Grenville’ age. A total of 12 zircons were analyzed individually, in an attempt to eliminate the possibility of mixing grains of different ages [Fig. 4(b)]. Five single grains (1g, 1j, 1k, 1o, 1n) form a collinear array (two are slightly discordant, two are slightly reversely discordant, and one is very discordant). A linear regression through these five data points yields intercept ages of 1152.7±5.6 and 287±131 Ma. Grain 1l (not shown on Fig. 4) has a 207Pb/206Pb age of 1356 Ma ( Table 1) and probably is detrital (although it may be a xenocryst, i.e. inherited by melting or assimilation processes). Six other grains (1e, 1f, 1h, 1i, 1m, 1p) have younger 207Pb/206Pb ages (1086–1131 Ma, Table 1); the data are discordant and scattered [Fig. 4(b)]. We suggest that the isotopic systematics in these zircons were modified by Pb-loss and possibly the formation of metamorphic overgrowths during the Grenville orogeny at about 1.05 Ga (see Section 5.2 on ages of metamorphic monazite). The collinear array of five apparently relatively undisturbed grains leads to the conclusion that the protolith of the layered gneiss is about 1150 Ma. This interpretation is in agreement with field relations observed at other locations that suggest that the layered gneiss is intruded by the porphyroblastic granite gneiss and the garnetiferous metagranite. However, if all of the zircons are detrital, then the age of the layered granitic gneiss is unknown, but thought to be older than the porphyroblastic granite gneiss and garnetiferous metagranite (see below). Six grains of monazite from sample U250-92 were analyzed individually, yielding 207Pb/206Pb ages of 1028 to 1112 Ma ( Table 1). Isotopic data from three monazites (1r, 1q, and 1v) are collinear with intercept ages of 1098±3 and 600±118 Ma [Fig. 4(c)]. Isotopic data from two monazites (1s and 1u) form a line with intercept ages of 1057±5 and 624±81 Ma. A regression calculated through monazite 1t and anchored at 600±50 Ma results in an upper intercept age of 1121±6 Ma. It is possible that some of these grains contain mixtures of ages (see discussion of monazite ages in Section 5.2; cf. Hawkins and Bowring, 1997;

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Fanning and Aleinikoff, 1998), but three grains yielded concordant ages of about 1099 Ma, 1096 Ma, and 1056 Ma [1v, 1q and 1u respectively, Fig. 4(c)]. The age of 1121±6 Ma is regarded as a minimum age for the protolith of the layered granitic gneiss. Two samples of hornblende monzonite gneiss ( Yhg, sample localities 14 and 15, Fig. 2) were analyzed for U–Pb zircon geochronology by conventional isotope dilution methods. Outcrop patterns and foliation in this unit strike NW, parallel to regional D1 foliation (Fig. 2). Garnetiferous metagranite ( Ygt) truncates the hornblende monzonite gneiss in map pattern at the north end of the study area in Maryland. Although no crosscutting relationships have been observed in outcrops, the truncation may be an intrusive contact. The significant D1 foliation and the contact with garnetiferous metagranite suggest that the hornblende monzonite gneiss is older. The zircon populations in both samples of hornblende monzonite gneiss are heterogeneous. Morphologic varieties exist in shape (equant to elongate, l/w=1–6), color (pink, deep red, light to dark brown), crystallinity (anhedral to euhedral ), and crystal shape (simple elongate prisms with or without pyramidal terminations to multifaceted equant grains). CL imaging ( Fig. 5) reveals that many of the grains in sample HF2-90 are overgrown by one or more zones of younger zircon. 207Pb/206Pb ages of multigrain fractions and single zircons range from 1149±2 to 1053±2 Ma [ Table 1, Fig. 6(a)]. Isotopic data from zircon from a second sample [BR1529, Fig. 6(b)] collected about 20 km north of HF2-90 are also complicated. 207Pb/206Pb ages of multigrain fractions and single zircons range from 1141±2 to 1053±2 Ma ( Table 1). It is likely that most grains in both samples of hornblende monzonite gneiss contain mixtures of zircon that may differ in age by as much as 100 m.y. Standard concordia plots [Fig. 6(a) and (b)] of HF2-90 and BR1529 do not yield easily interpretable ages. The scatter of conventional U–Pb data for zircons from the hornblende monzonite gneiss indicated that in situ SHRIMP analysis would be necessary to decipher the isotopic systematics. Prior to SHRIMP analysis, zircons from HF2-90

were subdivided into two populations on the basis of color (as seen in the epoxy ion probe mount): (1) deep to pale red, and (2) very pale red to colorless. U–Pb data from 28 spots on a total of 23 grains result in two groupings of ages [ Table 2, Fig. 6(c)]. However, a priori discrimination of zones based on color, morphology, or CL zoning did not result in distinct age groupings. Data from red and colorless grains that show oscillatory zoning yield an age of 1149±19 Ma (MSWD= 1.12), whereas overgrowths on red grains, plus several wholly colorless grains and a few cores of red grains, are dated at 1042±11 Ma (MSWD= 0.71) [Fig. 6(c)]. Thus, cores and overgrowths were found to be both old and young, suggesting complex igneous and metamorphic growth processes. We interpret the older age as the time of emplacement of the protolith of the gneiss and the younger age as the time of regional prograde metamorphism of the rock. Surprisingly, these ages are in excellent agreement with the oldest and youngest ages as determined by isotope dilution U–Pb geochronology (1149±2 Ma and 1053±2 Ma, Table 1), implying that some of the grains contain only one age component. Although in situ spot analysis by SHRIMP allows the nearly unequivocal interpretation of the age of a particular zone within a grain, the uncertainties are large due to the very small amount of material being analyzed. Conventional U–Pb analysis yields precise ages but interpretation of the data is severely limited by a lack of understanding of exactly what was analyzed (i.e. how many age domains are contained within the zircon). The most robust interpretations are those based on results from both techniques. Two samples of porphyroblastic granite gneiss ( Ypg) were collected and dated. Sample J329-90, from the southern end of the study area ( locality 2, Jeffersonton 7.5∞ quadrangle), and BR1700, from the central part of the study area ( locality 3, Bluemont 7.5∞ quadrangle), are identical in appearance. This rock also is similar in appearance to a megacrystic metagranite ( Ypb) in the southwestern part of the study area ( Fig. 2), although subtle textural and chemical differences permitted Nelson (1997) to map them as different units, substantiated by U–Pb geochronology (see below). At local-

J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146

131

Fig. 5. CL images of zircon from hornblende monzonite gneiss sample HF2-90. Note oscillatory zoned cores overgrown by unzoned rims. White ellipses indicate locations of SHRIMP analyses.

ity 2, the porphyroblastic granite gneiss is cut by numerous dikes of leucocratic granitic rock of unknown age. At locality 3, the porphyroblastic granite gneiss crops out within about 100 m of charnockite. However, because the charnockite occurs only as float at this locality, the contact relations between the charnockite and the porpyroblastic granite gneiss are unknown. Zircons from both samples of porphyroblastic granite gneiss are medium brown and stubby to elongate with l/w=3–6 [Fig. 3(b)]. The zircon population ranges in shape from subhedral to euhedral; most grains are deformed, with rounded terminations. Many of the grains are fragments of zircon crystals. Four fractions from J329-90 form a collinear array with one concordant point and

three slightly discordant points (one of which is less than 1% reversely discordant). A best-fit regression through the data yields intercept ages of 1143.8±1.5 and 491±159 Ma [Fig. 7(a)]. We interpret the upper intercept as the crystallization age of the protolith of the porphyroblastic granite gneiss; the lower intercept suggests loss of Pb in the Paleozoic but due to the large uncertainty we are unable to evaluate when exactly that occurred. Monazite in this rock occurs in two color varieties: light yellow and dark yellow. One grain of each was dated, yielding slightly discordant U–Pb data with 207Pb/206Pb ages of 1106 Ma and 1060 Ma respectively. However, as discussed below in detail, these ages may be mixtures of different ages of growth domains.

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Fig. 6. U–Pb data for samples of hornblende monzonite gneiss: (a) zircon from sample HF2-90; (b) zircon from sample BR1529; (c) Tera–Wasserburg and weighted averages plots of SHRIMP data from sample HF2-90.

J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146

Fig. 7. U–Pb data for samples of porphyroblastic granite gneiss: (a) zircon and monazite from sample J329-90; (b) zircon and monazite from sample BR1700.

U–Pb data from sample BR1700 are much more scattered than the isotopic data from sample J329-90 [Fig. 7(b)]. Five multigrain fractions are all somewhat discordant (1.4–2%), with 207Pb/ 206Pb ages ranging from 1099 to 1126 Ma. The non-linear data array suggests that these fractions are composed of zircons that formed at two or more times. These different ages may occur either as discrete grains or as metamorphic overgrowths. Because of the possibility of mixing of grains of different ages, we analyzed six zircons individually. U–Pb data from three of these grains plot on a reference chord between 350 and 1050 Ma, one grain is quite discordant with a 207Pb/206Pb age of 1078 Ma, one grain is slightly discordant with a 207Pb/206Pb age of 1105 Ma and one grain is concordant at about 1139±4 Ma [ Table 1, Fig. 7(b)]. This oldest zircon is essentially the same age as the upper intercept age (1144±2 Ma) of the other sample of porphyroblastic granite gneiss (J329-90). Although we are unable to determine an age for sample BR1700, we suggest that it is

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about 1140 Ma, on the basis of identical mineralogic and textural appearance and the age of one zircon. This interpretation implies that the younger zircons are metamorphic in origin and that the multigrain fractions are composed of mixtures of grains of different ages. Two grains of monazite, light yellow and dark yellow, yield ages of 1127 Ma and 1060 Ma respectively [ Table 1, Fig. 7(b)]. The significance of these ages, possibly the result of mixing of different age components, is discussed in detail in Section 5.2. Zircons from a sample of coarse-grained granite gneiss ( Ymc, sample LN398, locality 4, Fig. 2) are medium–dark brown. They are prismatic (l/w=2– 5), with rounded tips and edges. U–Pb isotopic data from five multigrain fractions from the (−100+150) size fraction are slightly (0.5–1%) discordant and are not collinear [Fig. 8(a)]. They have 207Pb/206Pb ages ranging from 1120–1128 Ma and 206Pb/238U ages ranging from 1104–1116 Ma. A regression through the data, anchored at 400±100 Ma, yields an upper intercept age of 1129±8 Ma, with a large MSWD of 20 [Fig. 8(a)].

Fig. 8. U–Pb data for sample LN398 (coarse-grained metagranite): (a) (−100+150) multigrain fractions; (b) (−100+150) single grains and (−200) multigrain fractions.

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Four multigrain fractions from the (−200) size fraction were analyzed because the smaller grains tend to be less deformed and cracked, and presumably less susceptible to Pb-loss. In addition, four elongate, clear grains from the (−100+150) fraction were analyzed individually ( Table 1). Of these eight analyses, three of the (−200) fractions (4j– 4l ) plus one coarser grain (4g) plot collinearly, with intercept ages of 1127.7±9.9 and 550±99 Ma (MSWD=1.5) [Fig. 8(b)]. The other single grains are concordant or slightly discordant with 207Pb/206Pb ages of 1141, 1107, and 1059 Ma. The fourth (−200) fraction has a 207Pb/206Pb age of 1098 Ma. By analogy with the isotope dilution U– Pb systematics of zircon from the hornblende monzonite gneiss, the oldest grain (4i; 207Pb/206Pb age of 1141±3 Ma) could be the minimum age for the time of emplacement of the coarse-grained granite gneiss. However, the paucity of data at about 1140 Ma makes this conclusion suspect. The single grain with the concordant age of 1059 Ma probably represents the time of overgrowth. All other grains contain mixtures of ages between about 1140 and 1060 Ma. However, because numerous single grains and multigrain fractions have 207Pb/206Pb ages between about 1118 and 1128 Ma, it is possible that the time of emplacement was 15–20 m.y. younger than the main plutonic event dated (see above) at about 1145 Ma. In this scenario, the grain with 1141 Ma age would be interpreted as xenocrystic. We suggest an age of about 1140 Ma for the coarse-grained granite gneiss. 4.2.2. Group 2 Two samples of Marshall Metagranite ( Ym) were collected from the Rectortown 7.5∞ quadrangle ( Fig. 2). Sample WW22-90 is a biotitic granitic gneiss whereas sample WW29-90 is a more leucocratic granitic gneiss (sample localities 6 and 7, Fig. 2). Field relations suggest that the leucocratic granitic gneiss cuts the biotitic granitic gneiss. D2 foliation with northwest to northeast strikes and southeast-plunging folds and mineral lineations occurs in the rocks mapped as Marshall Metagranite, suggesting that the emplacement of the granite was post-D1 and pre-D2. Zircons from both rocks are medium to dark brown, subhedral

to anhedral, subequant to elongate (l/w=1–5) and highly fractured [Fig. 3(c)]. Four of five fractions from the (−100+150) size fraction from sample WW29-90 yield U–Pb data that are collinear with intercept ages of 1112±3 and 336±72 Ma [Fig. 9(a)]; the fifth fraction plots to the left of the discordia, with a 207Pb/206Pb age of 1085 Ma. Isotopic data from two grains of monazite (one of which is concordant) form a line with intercepts at 1051±2 and 406±129 Ma. Zircons from sample WW22-90 have isotopic systematics that are similar to the data from sample WW29-90 [Fig. 9(b)]. A best-fit discordia through five of seven fractions has intercept ages of 1111±2 and 201±89 Ma. The two fractions plotting off the discordia have 207Pb/206Pb ages of 1112 Ma and 1119 Ma respectively. No monazite was recovered from sample WW22-90. 4.2.3. Group 3 Quartz-plagioclase gneiss ( Yqp) occurs as thin lenses in the northern part of the study area

Fig. 9. U–Pb data for samples of Marshall Metagranite: (a) zircon and monazite from sample WW29-90; (b) zircon from sample WW22-90.

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(Fig. 2). It is a relatively minor rock type but was considered important in understanding the evolution of the terrane because of its chemical similarity to ~1.3 Ga metatonalite and metatrondhjemite of the Green Mountain massif, Vermont (Burton and Southworth, 1996). Zircons from sample BR1674-93 ( locality 8, Fig. 2) are medium to dark brown, subequant to elongate (l/w=1–4), anhedral to euhedral, and moderately fractured. Many grains are irregularly shaped, a morphology suggestive of a multistage (xenocrystic) history. Isotopic data from six of seven fractions form a collinear array with intercept ages of 1077±4 and 462±211 Ma (Fig. 10). One fraction, with a 207Pb/206Pb age of 1066 Ma, plots slightly to the left of the discordia. This fraction has uncharacteristically low 206Pb/204Pb ( Table 1). The relatively young Grenvillian age for the quartz-plagioclase gneiss ( Yqp) rules out any definitive correlation with Green Mountain metatonalite and metatrondhjemite. Garnetiferous metagranite ( Ygt) crops out over a large area, particularly in the northern half of the northern Blue Ridge (Fig. 2). In many localities, this rock is found cross-cutting porphyroblastic granite gneiss ( Ypg), one of the few unambiguous field relations in the study area. The ubiquitous occurrence of coarse garnet in this rock, perhaps indicative of a metasedimentary source, is unique amongst granitic gneisses of the northern Blue Ridge. The zircon population from the garnetiferous metagranite is very heterogeneous [Fig. 3(d)]. Grains vary in color from

Fig. 10. U–Pb data for sample BR1674-93 (quartz-plagioclase gneiss).

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light brown to dark brown to deep red, and vary in shape from equant (with some faces) to spherical (with no planar faces) to prismatic elongate (euhedral to subhedral; l/w=1–5) to irregular (anhedral ). Spherical grains tend to have pitted, frosted surfaces whereas more elongate grains have pitted faces and rounded edges. Equant grains with multifaceted, ‘soccer ball’ morphology rather than detrital spherical shape are similar in appearance to metamorphic zircon. The wide range of zircon morphologies and the aluminous character of the rock suggest that many of these zircons may have cycled through a sedimentary source prior to incorporation in the felsic magma. In order to attempt to date the time of crystallization, only elongate euhedral or equant euhedral grains were analyzed (i.e. spherical, frosted zircons were not selected ). Monazite occurs as medium to dark yellow subhedral to anhedral grains. The color variation is gradational, with no distinct color populations. However, some grains have pitted and frosted surfaces, whereas others are adamantine. A total of 11 grains comprising the two populations of monazite were analyzed for U–Pb geochronology. U–Pb isotopic systematics of zircon from garnetiferous metagranite sample HF1-90 ( locality 5, Fig. 2) are very complicated ( Fig. 11). The first four fractions (5a–5d ) analyzed consisted of mixtures of equant and elongate grains, chosen primarily because they were relatively clear and crack-free. The data are scattered and range from marginally concordant (fraction 5a) to slightly discordant [Fig. 11(a)], having 207Pb/206Pb ages of 1077 to 1118 Ma ( Table 1). Four additional multigrain fractions, composed only of either elongate or equant grains {5e: elongate [Fig. 11(a)]; 5f–5h: equant [Fig. 11(b)]} were analyzed to separate the different age components of the previous four fractions. Fraction 5e has a 207Pb/206Pb age of 1094 Ma, whereas fractions 5f–5h have 207Pb/206Pb ages of 1050 Ma, 1049 Ma and 1057 Ma respectively. Because single grain 5e has an older 207Pb/206Pb age than fraction 5a, we realized that even within a carefully hand-picked fraction of grains of similar morphology it is likely that grains of different ages were combined. Thus, four individual elongate grains (fractions 5k–5n), plus two individual equant, multifaceted grains

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Fig. 11. U–Pb data for sample HF1-90 (garnetiferous metagranite): (a) multigrain fractions (mixed elongate and equant) and single elongate grains; (b) multigrain fractions (equant only) and single equant grains; (c) monazites (pitted and clear).

(fractions 5i and 5j) were analyzed. 207Pb/206Pb ages for the elongate grains range from 1057 to 1116 Ma. The two equant grains both have 207Pb/206Pb ages of 1047 Ma, one of which is concordant. A best-fit line calculated through four of five fractions (both single grain and multigrain) of equant zircon has intercept ages of 1048±6 and −23±287 Ma [Fig. 11(b)].

A total of 11 individual grains of monazite from garnetiferous metagranite, representative of the pitted/frosted and clear/adamantine populations, was analyzed for U and Pb isotopes [Fig. 11(c)]. The presumption was made that pitted grains might be detrital in origin, whereas the clear grains might have grown during metamorphism. Grains 5r (clear) and 5w (pitted ) yielded concordant data, with 207Pb/206Pb ages of 1070 Ma and 1064 Ma respectively. Data from five grains (5o, 5s, 5t, 5q, 5y) are reversely discordant and data from four grains (5p, 5u, 5v, 5x) are normally discordant (total range of 207Pb/235U ages is 986 to 1086 Ma). The data from all 11 monazite grains form a linear array, with intercept ages of 1070±3 and 56±100 Ma. Two important conclusions from the monazite data are: (1) no history prior to 1070 Ma is preserved, despite morphologic differences; (2) 1070 Ma is the minimum age of the garnetiferous metagranite, which implies that the equant zircons (upper intercept age of 1048±6 Ma) are metamorphic in origin. We are unable to determine the emplacement age of the garnetiferous metagranite because the isotopic data from elongate zircons are scattered. Two elongate fractions (5b and 5k) have 207Pb/206Pb ages of 1118 Ma and 1116 Ma respectively, similar to the age of the Marshall Metagranite, suggesting that these grains are inherited. Fraction 5a is concordant at 1077±4 Ma, and fractions 5c, 5d, and 5e have 207Pb/206Pb ages between 1077 and 1116 Ma. Thus, the crystallization age of the garnetiferous metagranite is interpreted as being somewhere between about 1077 Ma (age of youngest elongate grains) and about 1070 Ma (age of monazite). If this interpretation is correct, then the garnetiferous metagranite ( Ygt) and quartz-plagioclase gneiss ( Yqp) are very similar in age. Alternatively, the garnetiferous metagranite might be as old as about 1118 Ma. Samples of four different granitic rock types ( Yg, Yml, Ybg, and Ypb; Fig. 2) have the youngest ages of granitic gneisses in the northern Blue Ridge, spanning a range of about 1055–1060 Ma. Although the ages of these rocks are nearly identical, the morphologies of zircons they contain are quite variable. With the exception of Ybg, the

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rocks are poorly foliated and contain only the D2 fabric. Thus, even prior to U–Pb geochronology, these units were considered to be amongst the youngest Mesoproterozoic gneisses of the northern Blue Ridge on the basis of their fabrics. Sample BR11-90 ( locality 9, Fig. 2) is a white leucocratic metagranite ( Yg) that in the field appears to be similar to the garnetiferous metagranite except that it lacks garnets. However, the morphologies of zircon from the two rocks are quite different. Zircons from the white leucocratic granite gneiss are light to medium brown, euhedral, doubly terminated prisms (l/w=3–6), with very few cracks or inclusions (Fig. 3e). U–Pb data from four fractions yielded three concordant points and one slightly discordant point [Fig. 12(a)]. Intercept ages calculated from a best-fit regression are 1059.6±2.2 and 381±384 Ma. Sample BR1313-92 ( locality 10, Fig. 2), a pink leucocratic metagranite ( Yml ), contains medium to dark brown prismatic (l/w=2–4) zircons, many

Fig. 12. U–Pb data for samples of leucocratic metagranite: (a) zircon from white leucocratic metagranite (sample BR11-90); (b) zircon and monazite from pink leucocratic metagranite (sample BR1313-92).

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of which have deformed and pitted faces. A small proportion of grains was nearly colorless and clear, contained few inclusions or cracks, and was considered appropriate for U–Pb geochronology. Three of four fractions yielded isotopic data that are collinear, with intercept ages of 1058.2±2 and 128±204 Ma [Fig. 12(b)]. The fourth fraction has an older 207Pb/206Pb age of 1084 Ma ( Table 1). Ages from two monazite grains differ by about 25 m.y. Monazite 10f is reversely discordant and plots on the discordia derived from the zircon results, suggesting that the cause of reverse discordance is due to loss of U or gain of radiogenic Pb, not excess 206Pb. Monazite 10e is slightly normally discordant with a 207Pb/206Pb age of 1034±2 Ma. We interpret these data as indicating two episodes of monazite growth, one during crystallization of the granite magma, and a second period during reheating about 25 m.y. later. Sample BR554-90 ( locality 11, Fig. 2), a biotite granite gneiss ( Ybg), contains dark reddish-brown zircons in a variety of shapes including prisms (l/w=3–5), spheres, and lozenge-shaped grains. Most of the grains are anhedral, irregular shapes, although a small proportion was subhedral to euhedral crystals with pitted faces, some of which were selected for analysis. All five analyzed fractions yielded concordant isotopic data, with 207Pb/206Pb ages ranging from 1058 to 1051 Ma [Fig. 13(a)]. Because the data plot on concordia with very little spread in the Pb/U ages, we calculate an age of 1055.2±3.5 Ma using the weighted average of the 207Pb/206Pb ages ( Table 1), rather than determining concordia intercepts of a best-fit regression. Because there is some spread in the data along concordia, suggesting the possibility of very minor Pb loss in the past, this date should be regarded as a minimum age for the biotite granite gneiss. Monazite from this sample is reversely discordant with a 207Pb/235U of 1033±3 Ma. Sample U39-90 ( locality 12, Fig. 2), a megacrystic metagranite ( Ypb) similar in appearance to J329-90 ( Ypg), has zircons that are notable because they are light brown, transparent, and lack cracks or inclusions. Most of the grains are prismatic (l/w=4–7) and have only slightly bent

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Fig. 13. U–Pb data for samples of granitic gneiss: (a) zircon and monazite from biotite granite gneiss (sample BR554-90); (b) zircon from megacrystic metagranite (sample U39-90).

gies, only single grains were analyzed, including two equant grains (13a, 13b) and eight elongate grains (13c–13j) ( Table 1). 207Pb/206Pb ages range from 1088 to 1141 Ma and the data are scattered ( Table 1, Fig. 14). There is no obvious relationship of morphology to age in zircon from the charnockite. For example, equant grains are 1140 and 1090 Ma, and elongate grains of very similar shape and color occur in all three age groups. Because of the lack of any independent evidence for the age of this rock, we are unable to determine its emplacement age. The two older age groups, ~1140 Ma and ~1110 Ma, correspond closely to the ages of the porphyroblastic granite gneiss and Marshall Metagranite respectively, both of which are widespread throughout the northern Blue Ridge. It is likely that this rock is older than 1140 Ma (assuming that this age is a mixture of ages of core and overgrowth, analogous to zircon from the hornblende monzonite gneiss); however, the older zircons could be xenocrysts. CL imaging and SHRIMP dating are required to unravel the complex growth characteristics.

5. Summary of geochronologic results faces. Three fractions yielded concordant data with a weighted average of the 207Pb/206Pb ages of 1055±2 Ma [Fig. 12(b)]. 4.2.4. Age uncertain Zircon from charnockite ( Yc) was analyzed for U and Pb isotopes, but because of very complicated isotopic systematics, no unambiguous age was determined. Owing to very poor exposure, the mapped distribution of charnockite was based primarily on diagnostic soil and float. Although not observed in outcrop, a foliation is evident in sawn and stained sections of charnockite. The charnockite was not found in cross-cutting relationship with any other rock in the study. Zircons from charnockite sample Bl-1-94 ( locality 13, Fig. 2) are pale to dark pink. Many of these grains are equant and multifaceted, whereas others are elongate (l/w=3–6) with slightly bent faces and rounded tips. Many of the grains show complex cores and overgrowths when imaged in CL. Because of the wide variety of zircon morpholo-

5.1. Results: zircon U–Pb geochronology In summary, U–Pb ages from zircon indicate that the protoliths of 12 samples of granitic gneiss from the northern Blue Ridge were emplaced over a period of about 100 m.y., from about 1150 to

Fig. 14. U–Pb data for zircons from charnockite (sample Bl1-94).

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1050 Ma ( Table 3). During this time span, there appears to have been three pulses of magmatism, at about 1150–1140 Ma, 1110 Ma, and 1075– 1055 Ma (perhaps divisible into two subgroups at about 1075 and 1060–1055 Ma). By comparison with samples from other exposures of Grenvillian crust (i.e. Felchville tonalite, Ratcliffe et al., 1996; Corbin Metagranite, Heatherington, et al., 1996; Baltimore Gneiss, Aleinikoff et al., 1997), the U– Pb isotopic systematics of zircon from most of the granitic gneisses in the northern Blue Ridge are unusually simple. Only some of the Group 1

samples are complicated by inheritance and/or overgrowths. Of nomenclatural significance is the decision here to limit the term ‘Marshall Metagranite’ to the foliated biotitic granite gneisses dated at 1112±3 and 1111±2 Ma; these rocks occur mostly east of the Short Hill fault (Fig. 2). 5.2. Results: monazite U–Pb geochronology Six granite gneiss samples contain monazite; in five of these samples (BR554-90, BR1313-92, BR1700, J329-90, and U250-92), some of the

Table 3 Summary of U–Pb zircon and monazite geochronology of Mesoproterozoic rocks of the northern Blue Ridge, Virginia–Maryland Fig.

Sample number

Rock type

Zircon agea (Ma)

Group 3B Fig. 13(b) Fig. 13(a) Fig. 12(b)

U39-90 BR554-90 BR1313-92

megacrystic metagranite ( Ypb) biotite granite gneiss ( Ybg) pink leucocratic metagranite ( Yml )

1055±2 1055±4 1059±2

Fig. 12(a)

BR11-90

white leucocratic metagranite ( Yg)

1060±2

Group 3A Fig. 11(a)–(c)

HF1-90

garnetiferous metagranite ( Ygt)

Fig. 10

BR1674-93

quartz-plagioclase gneiss ( Yqp)

1077±4 1048±6 (mz) 1077±4

Group 2 Fig. 9(a) Fig. 9(b)

WW29-90 WW22-90

leucocratic Marshall Metagranite ( Yin) 1112±3 biotitic Marshall Metagranite ( Yin) 1111±2

Group 1 Fig. 8(a) and (b) LN398 Figs. 7(b) and 15 BR1700

coarse-grained metagranite ( Ymc) porphyroblastic granite gneiss ( Ypg)

~1140 ≥1140±4 ~1050 (mz)

Fig. 7(a)

J329-90

porphyroblastic granite gneiss ( Ypg)

1144±2

Fig. 6(a)–(c)

HF2-90+BR1529

hornblende monzonite gneiss ( Yhm)

Fig. 4(a)–(c)

U250-92

layered granitic gneiss ( Ylg)

1149±19 (S) 1042±11 (S, mz) 1153±6

Age uncertain Fig. 14

Bl-1-94

charnockite ( Yc)

>1145(?)

Monazite age (Ma) Reliabilityb

1033±2 1059±2c 1034±2

1 1 1 1

1070±3

2 1

1051±2

1060±1 (dark)d 1130±1 ( light)d 1071±1 (uz) 1062±1 (uz) 1050±2(uz) 1060±1 ( light)d 1106±1 (dark)d

1 1 3 3

1 1

1057±5 1098±3 1121±6

2

3

a S: SHRIMP age; mz: metamorphic zircon; uz: unzoned. b 1: excellent (concordant data or very good linearity of data; no complications); 2: moderate (some linear data, but with some complications); 3: complicated (several splits of different ages). c Upper intercept age; data are about 1% reversely discordant. d Possibly average of multi-age components.

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monazite is younger than coexisting zircon and, therefore, is considered to be metamorphic in origin. In contrast, the ages of some monazites in samples HF1-90, BR1313-92, and BR1700 are within uncertainty of the inferred age of coexisting zircon. The ages of individual monazites, even within a sample, do not define a single high-grade, ‘Grenville’ event. For example, monazite in sample U250-92 (Group 1; about 1145 Ma) has ages of about 1110, 1100, 1055, and 1028 Ma. The broad spectrum of monazite ages can be explained as being due to: (1) numerous periods of monazite growth, implying thermal events at about 1130, 1110, 1100, 1080, 1060, 1050, and 1030 Ma; (2) igneous crystallization during emplacement of the granitic magma and metamorphic growth (partial resorption and overgrowth) at about 1030 Ma; or (3) a combination of (1) and (2). During the early stages of the study, the prevailing interpretation was that monazite is a mineral that contains relatively simple U–Pb isotopic systematics, lacking multiple age components within a single grain. However, Hawkins and Bowring (1997), Miller et al. (1997), and Fanning and Aleinikoff (1998) have shown, using isotope dilution and ion microprobe U–Pb geochronology and BSE imaging, that some multiply deformed rocks contain monazite with multiple age domains. In some cases, oscillatory-zoned cores (interpreted as igneous in origin) occur within one or more unzoned overgrowths. Other grains from the same sample are completely unzoned, suggesting the presence of only one age domain. Thus, some U– Pb ages of whole, single monazite grains from Blue Ridge granite gneisses may be mixtures of multiple age components, analogous to U–Pb data from zircon from sample HF2-90. Preliminary U–Pb geochronology of monazite from sample BR1700 yielded two ages: a lightcolored grain was about 1130 Ma, whereas a dark grain was about 1057 Ma ( Table 1). These grains were chosen for analysis on the basis of examination in transmitted and reflected light only; they were relatively free of inclusions and cracks at the limit of detection of a standard laboratory binocular microscope. To determine if mixing of age components is a factor in Blue Ridge monazites,

about 100 grains from sample BR1700 were mounted in epoxy and polished for BSE imaging on the USGS Jeol 5800LV scanning electron microscope (SEM ). The monazites were subdivided into two groups of 50 grains each on the basis of color: medium brown or pale yellow (referred to as ‘dark’ and ‘light’ respectively in subsequent text and in Table 1). BSE imaging ( Fig. 15) of both dark and light monazite grains from sample BR1700 reveals that some grains have irregular, oscillatory-zoned cores that are overgrown by one or more unzoned rims or overgrowths. Other grains have an irregular blotchy or spotted zoning pattern (herein referred to as ‘spotted’) and some are completely unzoned. A total of 12 half-grains of monazite were removed from the mount, including four light unzoned grains, two dark unzoned grains, three light grains with spotted zoning, one light grain with a faint oscillatory-zoned core and spotted-zoned overgrowth, and two dark grains with faint oscillatory zoning within unzoned overgrowths. Although analysis of monazite usually yields concordant age data, results from all 12 grains are discordant (0.8–3.8%). Another unusual feature of the isotopic data is that the common Pb contents are quite high (0.2–2 ng) as expressed by low 206Pb/204Pb ratios ( Table 1). Both of these atypical characteristics were probably caused by processing in the laboratory. Half-grains (resulting from grinding and polishing for SEM-BSE imaging) picked out of the mount may have lost radiogenic Pb due to sample preparation and probably gained common Pb from either adhering epoxy or embedded grinding compounds, despite leaching in very weak HCl and HNO prior to dissolution. In 3 essence, modern Pb-loss probably was induced in the laboratory and thus, regressions of linear data arrays are forced through 0±50 Ma. Neither of the laboratory problems caused insurmountable difficulties in the geochronologic aspects of the study because the monazite grains contain very high concentrations of U and radiogenic Pb. Isotopic data from monazite of sample BR1700 can be subdivided into at least three groups ( Table 1, Fig. 16): (1) five grains (all light; two unzoned, three spotted) form a linear array with an upper intercept age of 1071±1 Ma; (2) three

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Fig. 15. SEM-BSE images of monazite from sample BR1700 (porphyroblastic granite gneiss), showing representative zoning patterns: (a) grains with spotted zoning; (b) grain with oscillatory-zoned core invaded and overgrown by unzoned material; (c) broad unzoned rim on spotted core; (d) mostly unzoned grain with faint remnant of resorbed core; (e) completely unzoned grain.

grains (two dark, one light; all unzoned ) with an upper intercept age of 1062±1 Ma; (3) one light unzoned grain with a 207Pb/206Pb age of 1050±2 Ma. Three other grains, containing mixtures of oscillatory-zoned and unzoned material, have 207Pb/206Pb ages of 1057, 1066, and 1086 Ma and do not plot on any linear arrays. We conclude that metamorphic monazite grew in sample BR1700 at about 1070, 1060 and 1050 Ma. These ages correspond fairly closely to thermal events associated with emplacement of plutons (Table 3). Because the monazites represent several different ages (i.e. were not reset to some common age), it is likely that Grenvillian high-grade metamorphism

of rocks of the northern Blue Ridge did not exceed about 750°C (closure temperature for the U–Pb system in monazite, Parrish and Whitehouse, 1999). The youngest growth age of monazite in BR1700 (~1050 Ma) is very similar to the age of three single grains of zircon from the same sample ( Tables 1 and 3). No monazite grains in sample BR1700 were found to be ~1033 Ma, the youngest dated monazites in this study from samples BR1313-92 and BR554-90. Both of these rocks occur to the east of the Short Hill fault, whereas the porphyroblastic granite gneiss primarily crops out to the west of the fault. The potential significance of this fault is discussed below.

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Fig. 16. U–Pb data for monazite from sample BR1700 (porphyroblastic granite gneiss). Filled black ellipses represent data from grains that contained mixtures of faint remnant zoning and unzoned overgrowths. The ellipse with horizontal stripes has a 207Pb/206Pb of 1050±2 Ma.

6. Timing of Grenvillian deformation and metamorphism Northwest-trending D1 foliation is well-developed in porphyroblastic granite gneiss ( Ypg), hornblende monzonite gneiss ( Yhm), and layered granitic gneiss ( Ylg). These rocks have Group 1 U–Pb zircon ages, suggesting that D1 occurred after about 1145 Ma. D1 foliation is poorly developed in Group 1 coarse-grained metagranite ( Ymc), suggesting either that this rock is somewhat younger than the porphyroblastic granite [1128±10 Ma, as suggested by some of the zircon data, Fig. 8)] or that this rock was somehow resistant to foliation development. D1 foliation in Ypg is discordantly cut by Group 3A garnetiferous metagranite, indicating that the minimum age for D1 is about 1077±4 Ma. Cooling and strain hardening of Group 1 gneisses may have promoted preservation of D1 foliation. Northeast- to northwest-striking D2 foliation and southeast-plunging lineation are well-developed in Group 2 [Marshall Metagranite ( Ym)] and Group 3 [biotite granite gneiss ( Ybg)] rocks. D2 deformation must have occurred, therefore, after the latest intrusion, or about 1055 Ma. Upright D3 folds arch D2 folia-

tion and, therefore, must be even younger in age (Burton et al., 1994). Comparison of monazite growth ages of 1070, 1060, and 1050 Ma with inferred timing of formation of foliations suggests that thermal perturbations in the study area are not necessarily correlative with the formation of fabrics. The youngest monazite age (about 1035 Ma) may coincide with either D2 or D3. Hornblende 40Ar/39Ar ages of about 900–1000 Ma in rocks of the Blue Ridge basement ( Kunk et al., 1993) record the last time that this Mesoproterozoic terrane cooled below about 500°C. Two ages notably absent from the isotopic data preserved in monazite from sample BR1700 are 1110 Ma (time of emplacement of the Marshall Metagranite) and 1033 Ma (age of monazite in samples BR1313-92 and BR554-90). The main body of Marshall Metagranite occurs less than 5 km to the east of porphyroblastic granite gneiss ( Ypg) sample BR1700 and the two units are within 0.5 to 1 km locally. However, separating these two granitic gneisses is the Short Hill fault, a postMiddle Cambrian normal fault that places Lower and Middle Cambrian carbonate shelf limestone and dolomite of the Tomstown, Waynesboro, and Elbrook Formations against metabasalt and metarhyolite of the Neoproterozoic Catoctin Formation (570 Ma; Aleinikoff et al., 1995) and Lower Cambrian Weverton and Harpers Formations. The displacement in the Paleozoic cover rocks is limited to a few kilometers (Southworth and Brezinski, 1996). This displacement may be sufficient to account for the absence of Marshall-age monazite in sample BR1700. Alternatively, the absence of 1110 and 1033 Ma monazite in the porphyroblastic granite gneiss suggests that the monazite in the porphyroblastic granite gneiss was not affected by the intrusion of Marshall Metagranite. Thus, the Short Hill fault may previously have had significant displacement, juxtaposing Marshall Metagranite and younger granitic gneisses against porphyroblastic granite gneiss long after emplacement of the protoliths of the granitic gneisses. If so, the small area of rock classified as Marshall Metagranite north of sample BR1700 and west of the Short Hill fault (Fig. 2) may be reinterpreted as 1055 Ma biotite granite gneiss ( Ybg).

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7. Comparison with recent U–Pb crystallization ages from other Grenvillian terranes in the eastern USA The three major Mesoproterozoic magmatic pulses indicated by our geochronologic data from rocks of the northern Blue Ridge have possible correlatives in other parts of the USA portion of the Grenville Province. Three clusters of U–Pb ages of plutonic rocks in the Adirondack Highlands of northern New York appear to match fairly well those determined for the northern Blue Ridge: 1134–1156 Ma mangeritic and charnockitic gneisses and hornblende granitic gneisses, equivalent to Blue Ridge Group 1; 1095–1100 Ma hornblende granitic gneiss (equivalent to Group 2); and 1050–1075 Ma alaskitic gneiss, anorthosite, and metagabbro (Group 3) (McLelland et al., 1988). Syn- to post-intrusive deformational fabrics in the two younger groups of rocks in both areas are evidence for the late Grenvillian Ottawan Orogeny of Moore and Thompson (1980). Two episodes of magmatism in the Adirondacks and Green Mountain Massif of Vermont have no known equivalents in the northern Blue Ridge, including 1.30–1.35 Ga tonalitic and trondhjemitic gneisses (McLelland and Chiarenzelli, 1990; Ratcliffe et al., 1991), and 1.25 Ga granites (McLelland et al., 1988; Aleinikoff et al., 1990a), of the Elzevirian Orogeny of Moore and Thompson (1980). Also, a suite of 0.96 Ga megacrystic granites occurs in Vermont ( Karabinos and Aleinikoff, 1990), whereas no thermal or metamorphic activity younger than 1.0 Ga has been found in the northern Blue Ridge. In the Hudson Highlands of southern New York, the oldest well-dated rock is the hornblendebearing Storm King Granite, whose U–Pb crystallization age of about 1130 Ma (Ratcliffe and Aleinikoff, 1990; 1134±2 Ma, Aleinikoff, unpublished data) falls between Group 1 and Group ages of the northern Blue Ridge, whereas the Canada Hill Granite (1010 Ma; Aleinikoff et al., 1982) is younger than any dated rocks in the northern Blue Ridge. Metavolcanic rocks of the Baltimore Gneiss are distinctly older (about 1.25 Ga), but the youngest event affecting zircons in these rocks (about

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1.17 Ga) approaches the age of northern Blue Ridge Group 1 plutonism (Aleinikoff et al., 1997). Specific correlation of rock units in the northern Blue Ridge basement with possible equivalents in the central Blue Ridge is hampered by a lack of recent, high-precision U–Pb dates for the latter region. The central Blue Ridge rocks are similar in that they are largely plutonic and have U–Pb crystallization ages in the range of 1.15–1.05 Ga (Sinha and Bartholomew, 1984). However, the very complicated growth characteristics and isotopic systematics in zircon from the hornblende monzonite gneiss suggest that rocks of similar lithology in the central Blue Ridge probably are equally complex. Thus, conventional U–Pb age data on the central Blue Ridge high-grade rocks may represent mixtures of age components and should be viewed with some skepticism. Zircons from the Corbin Gneiss of northern Georgia have a crystallization age of about 1106±13 Ma, very similar to the emplacement age of northern Blue Ridge Group 2 rocks. Overgrowths formed at about 1040 Ma, slightly post-dating the northern Blue Ridge Group 3, and much later at about 950–980 Ma (Heatherington et al., 1996; Aleinikoff, unpublished data). In the broader tectonic context, rocks of the northern Blue Ridge probably belong to the collisional Shawinigan (~1.19–1.14 Ga) and Ottawan (1.08–1.02 Ga) phases of the Grenville Orogeny as defined in the Grenville Province of eastern Canada (Rivers, 1997). However, the emplacement of the Marshall Metagranite at about 1110 Ma does not fit this regional scheme. Close correspondence among ages of Blue Ridge and Adirondack rocks suggests that discrete magmatic pulses within this phase of the Grenville Orogeny in the USA may have been widespread. Evidence for plutonism associated with or predating the ~1.25 Ga Elzevirian Orogeny is lacking in the northern Blue Ridge, either because the orogeny never occurred in this region or the rocks were subsequently destroyed during later plutonism.

8. Conclusions (1) Large volumes of granitic magma were emplaced into the crust of what is now the core

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of the Blue Ridge–South Mountain anticlinorium in northern Virginia and Maryland during a period of about 100 million years, from about 1150 to 1050 Ma. Within this time span, three pulses of plutonism occurred: at about 1150–1140 (or 1125 Ma) (Group 1); 1110 Ma (Marshall Metagranite, Group 2); and 1075–1055 Ma (Group 3, perhaps divisible into two subgroups at about 1075 and 1060–1055 Ma). The name, Marshall Metagranite, formerly applied to most of the granitic gneisses of the northern Blue Ridge, is now restricted to weakly to moderately foliated, medium-grained biotite granitic gneisses that are about 1110 Ma. (2) Although most of the granitic gneisses in this study are amenable to dating by conventional zircon U–Pb geochronology, zircons in hornblende monzonite gneiss are very complex mixtures of cores and overgrowths, necessitating use of the ion microprobe to decipher the age components. On the basis of this experience, similar strategies should be considered when determining the ages of comparable high-grade metamorphic rocks in the central and southern Blue Ridge. (3) On the basis of presence or absence of foliations in the dated samples, D1 is constrained to have occurred between about 1145 and 1075 Ma (or possibly between about 1145 and 1128 Ma). Even the youngest dated samples contain a foliation, which means that the younger foliationproducing deformation (D2) must be younger than about 1050 Ma. 1035 Ma monazite may have grown in response to D2 or the subsequent deformation (D3) that folded D2 foliation. (4) Complex zoning in monazite may be caused by compositional variation during magmatic or metamorphic growth or by inclusion of xenocrystic monazite (comparable to inheritance in zircon). Because the closure temperature of the U–Pb system in monazite is greater than temperatures at which metamorphic monazite can form, care must be taken (e.g. by using SEM-BSE imaging) to analyze monazite grains from high-grade rocks that appear to be composed of only one age component. (5) Monazite ages (1070, 1060, and 1050 Ma) from porphyroblastic granite gneiss (1145 Ma) indicate that several thermal pulses occurred

between deformations D1 and D2 in the northern Blue Ridge. The final high-grade event, probably at about 1035 Ma, did not reset older monazite ages, indicating that the temperature of this event (and all previous events except the first one) was less than about 750°C. (6) Lack of evidence for 1110 Ma growth of monazite in porphyroblastic granite gneiss suggests that the Short Hill fault might be a Grenvillian structure that was reactivated in the Paleozoic. (7) Mesoproterozoic rocks of the northern Blue Ridge are broadly age-correlative with rocks in other Grenville terranes of the USA, particularly the Adirondacks. The 100 m.y. span of ages of Blue Ridge granitic gneisses corresponds to the Shawinigan and Ottawan pulses of the Grenville Orogeny in eastern Canada. However, correlation of ages with other high-grade rocks in the central Appalachians may be specious because of the very complex growth characteristics of the zircons that may have resulted in mixed ages. Conventional U–Pb geochronology may not be able to decipher multi-age isotopic systematics; in situ analysis by ion microprobe probably is the only method capable of analyzing individual age components within complexly zoned grains.

Acknowledgements Marianne Walter and Rebecca Sauer ably completed mineral separations and chemical extractions. We thank Mark Fanning and Ian Williams for help using the ion microprobe in Canberra. We thank Wright Horton, Robert Ayuso, Fernando Corfu, and Calvin Miller for detailed critical reviews of earlier versions of the manuscript. Their comments resulted in significant improvements to the text, figures, and tables; however, the authors assume sole responsibility for all conclusions and interpretations presented herein.

References Aleinikoff, J.N., Grauch, R.I., Simmons, K.R., Nutt, C.J., 1982. Chronology of metamorphic rocks associated with uranium

J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146 occurrences, Hudson Highlands, New York–New Jersey. Geol. Soc. Am. Abstr. Prog. 14, 1–2, 1. Aleinikoff, J.N., Ratcliffe, N.M., Burton, W.C., Karabinos, P., 1990a. U–Pb ages of Middle Proterozoic igneous and metamorphic events, Green Mountains, Vermont. Geol. Soc. Am. Abstr. Prog. 22 (2), 1. Aleinikoff, J.N., Winegarden, D.L., Walter, M., 1990b. U–Pb ages of rims of zircon: a new analytical method using the air-abrasion technique. Chem. Geol. (Iso. Geo. Sect.) 80, 351–363. Aleinikoff, J.N., Zartman, R.E., Walter, M., Rankin, D.W., Lyttle, P.T., Burton, W.C., 1995. U–Pb ages of metarhyolites of the Catoctin and Mount Rogers Formations, central and southern Appalachians: evidence for two pulses of Iapetan rifting. Am. J. Sci. 295, 428–454. Aleinikoff, J.N., Fanning, C.M., Horton Jr, J.W., Drake Jr, A.A., Sauer, R.R., 1997. The Baltimore Gneiss re-revisited: new SHRIMP (zircon) and conventional (titanite) U–Pb ages. GAC/MAC Annual Meeting Abstr. Vol. 22: A-2. Bartholomew, M.J., Lewis, S.E., 1984. Evolution of Grenville massifs in the Blue Ridge geologic province, southern and central Appalachians. In: Bartholomew, M.J., Force, E.R., Sinha, A.K., Herz, N. ( Eds.), Geol. Soc. Am. Spec. Paper 194, 229–254. Burton, W.C., Southworth, C.S., 1993. Garnet–graphite paragneiss and other country rocks in granitic Grenvillian basement, Blue Ridge anticlinorium, Northern Virginia and Maryland. Geol. Soc. Am. Abstr. Prog. 25 (4), 6. Burton, W.C., Southworth, C.S., 1996. Basement rocks of the Blue Ridge province in Maryland. In: Brezinski, D.K., Reger, J.P. ( Eds.), Studies in Maryland Geology, Maryland Geol. Surv. Spec. Publ. 3, 187–203. Burton, W.C., Froelich, A.J., Schindler, J.S., Southworth, C.S., 1992. Geologic Map of Loudoun County, Virginia. US Geol. Surv. Open-File Map 92-716. scale 1:100 000. Burton, W.C., Aleinikoff, J.N., Southworth, C.S., Lyttle, P.T., 1994. Grenvillian history of intrusion and deformation, northern Blue Ridge, Virginia. Geol. Soc. Am. Abstr. Prog. 26 (4), 5. Burton, W.C., Froelich, A.J., Pomeroy, J.S., Lee, K.Y., 1995. Geologic Map of the Waterford Quadrangle, Virginia and Maryland, and the Virginia Portion of the Point of Rocks Quadrangle, US Geol. Surv. Bull. 2095, 31, scale 1:24 000. Clarke, J.W., 1984. The Core of the Blue Ridge Anticlinorium in Northern Virginia, Geol. Soc. Am. Spec. Paper 194, 155–160. Compston, W., Williams, I.S., Meyer, C., 1984. U–Pb geochronology of zircons from lunar breccia 73217 using a sensitive high-resolution ion-microprobe Proc. 14th Lunar Sci. Conf., J. Geophys. Res. B 98, 525–534. Espenshade, G.H., 1986. Geology of the Marshall Quadrangle, Fauquier County, Virginia, US Geol. Surv. Bull. 1560, 60. Fanning, C.M., Aleinikoff, J.N., 1998. Complex zoning in monazite as revealed by backscatter imaging and SHRIMP analysis. Geol. Soc. Am. Abstr. Prog. 30, A214. Fauth, J.L., Brezinski, D.K., 1994. Geologic Map of the Middletown Quadrangle, Frederick and Washington Counties,

145

Maryland. Maryland Geological Survey, Baltimore. scale 1:24 000. Hawkins, D.P., Bowring, S.A., 1997. U–Pb systematics of monazite and xenotime: case studies from the Paleoproterozoic of the Grand Canyon, Arizona. Contr. Mineral. Petrol. 127, 87–103. Heatherington, A.L., Mueller, P.A., Smith, M.S., Nutman, A.P., 1996. The Corbin Gneiss: Evidence for Grenvillian magmatism and older continental basement in the southernmost Blue Ridge. Southeast. Geol. 36, 15–25. Howard, J.L., 1991. Lithofacies of the Precambrian basement complex in the northernmost Blue Ridge province of Virginia. Southeast. Geol. 31 (4), 191–202. Jonas, A.I., 1928. Geologic map of Virginia. Charlottesville, Vir. Div. Min. Res., scale 1:500 000. Karabinos, P., Aleinikoff, J.N., 1990. Two distinct ages of augen gneisses from the Green Mountain massif and Chester dome, Vermont. Am. J. Sci. 290, 959–974. Kline, S.W., Lyttle, P.T., Froelich, A.J., 1991. Geologic map of the Loudoun County portion of the Middleburg quadrangle, Virginia. US Geol. Surv. Open-File Rep. 90-641, scale 1:24 000. Kline, S.W., Hertzog, M.G., Lupo, V.E., Swinsky, E.M., Kunk, M., Fleisher, C.J., 1994. Grenville-age ultramafic plutons in the northern Virginia Blue Ridge. Vir. Min. 40, 3–4, 17–28. Krogh, T.E., 1973. A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determination. Geochim. Cosmo. Acta 37, 485–494. Krogh, T.E., 1982. Improved accuracy of U–Pb zircon ages by the creation of more concordant systems using an air abrasion technique. Geochim. Cosmo. Acta 46, 637–649. Kunk, M.J., Lyttle, P.T., Schindler, J.S., Burton, W.C., 1993. Constraints on the thermal history of the Blue Ridge in northernmost Virginia: 40Ar/39Ar age dating results. Geol. Soc. Am. Abstr. Prog. 25 (2), 31. Ludwig, K.R., 1991a. PBDAT — A computer program for processing Pb–U–Th isotope data, Version 1.20. US Geol. Surv. Open-File Rep. 88-542, Rev. March 19, 1991. Ludwig, K.R., 1991b. ISOPLOT — A plotting and regression program for radiogenic-isotope data. US Geol. Surv. OpenFile Rep. 91-445. Ludwig, K.R., 1992. ANALYST — A computer program for control of a thermal-ionization, single collector massspectrometer, version 2.20. US Geol. Surv. Open-File Rep. 92-543. Lyttle, P.T., Aleinikoff, J.N., Drake Jr, A.A., Horton Jr, J.W., Kasselas, G., McCartan, L., Mixon, R.M., Nelson, A.E., Pavlides, L., Weems, R.E., 1999. Geologic Map of the Washington West 30∞×60∞ Quadrangle Virginia and Maryland. US Geol. Surv. Misc. Invest. Ser. Map I-xxxx. scale 1:100 000. McLelland, J., Chiarenzelli, J., 1990. Geochronological studies in the Adirondack Mountains and implications of a Middle Proterozoic tonalite suite. In: Gower, C., Ryan, B. ( Eds.), Mid Proterozoic Laurentia–Baltica, Geol. Assoc. Can. Spec. Paper 38, 175–194.

146

J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146

McLelland, J., Chiarenzelli, J., Whitney, P., Isachsen, Y., 1988. U–Pb zircon geochronology of the Adirondack Mountains and implications for the their geologic evolution. Geology 16, 920–924. Miller, C.F., D’Andrea, J.L., Ayres, J.C., Coath, C.D., Harrison, T.M., 1997. BSE imaging and ion probe geochronology of zircon and monazite from plutons of the Eldorado and Newberry Mountains: age, inheritance and sub-solidus modification. Am. Geophys. Un. Fall Meet. EOS Trans. 78 (46), F783. Moore Jr, J.M., Thompson, P.H., 1980. The Flinton Group: a late Precambrian metasedimentary succession in the Grenville Province of eastern Ontario. Can. J. Earth Sci. 17, 1685–1707. Nelson, A.E., 1997. Geology of the Upperville 7.5-minute quadrangle, Fauquier and Loudoun Counties, Virginia. US Geol. Surv. Open-File Rep. 97-708, scale 1:24 000. Paces, J.B., Miller Jr., J.D., 1993. Precise U–Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota; geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagnetic processes associated with the 1.1 Ga Midcontinental Rift System. J. Geophys. Res. B, Solid Eath and Planets 98, 13 997–14 013. Parrish, R.R., 1987. An improved microcapsule for zircon dissolution in U–Pb geochronology. Chem. Geol. 66, 99–102. Parrish, R.R., 1990. U–Pb dating of monazite and its application to geological problems. Can. J. Earth Sci. 27, 1435–1450. Parrish, R., Whitehouse, M., 1999. Constraints on the diffusivity of Pb in monazite, its closure temperature, and its U–Th–Pb systematics in metamorphic terrains, from a TIMS and SIMS study. J. Conf. Abstr. 4, 711. Ratcliffe, N.M., Aleinikoff, J.N., 1990. Speculations on structural chronology and tectonic setting of middle Proterozoic terranes of the northern U.S. Appalachians based on U–Pb dating. Geol. Soc. Am. Abstr. Prog. 22 (2), 64. Ratcliffe, N.M., Aleinikoff, J.N., Burton, W.C., Karabinos, P., 1991. Trondhjemitic, 1.35–1.31 Ga gneisses of the Mount Holly Complex of Vermont: evidence for an Elzevirian event

in the Grenville basement of the U.S. Appalachians. Can. J. Earth Sci. 28, 77–93. Ratcliffe, N.M., Aleinikoff, J.N., Hames, W.E., 1996. 1.4 Ga U–Pb zircon ages of metatrondhjemite of the Chester dome, VT and probable Middle Proterozoic age of the Cavendish Formation. Geol. Soc. Am. Abstr. Prog. 28 (3), 93. Rivers, T., 1997. Lithotectonic elements of the Grenville Province: review and tectonic implications. Precam. Res. 86, 117–154. Sinha, A.K., Bartholomew, M.J., 1984. Evolution of the Grenville terrane in the central Virginia Appalachians. In: Bartholomew, M.J., Force, E.R., Sinha, A.K., Herz, N. ( Eds.), The Grenville Event in the Appalachians and Related Topics, Geol. Soc. Am. Spec. Paper 194, 175–186. Southworth, C.S., 1994. Geologic Map of the Bluemont Quadrangle, Loudoun and Clarke Counties, Virginia. US Geol. Surv. Geol. Quad. Map GQ-1739. scale 1:24 000. Southworth, C.S., 1995. Geologic Map of the Purcellville Quadrangle, Loudoun County, Virginia. US Geol. Surv. Geol. Quad. Map GQ-1755. scale 1:24 000. Southworth, C.S., Brezinski, D.K., 1996. Geology of the Harpers Ferry Quadrangle, Virginia, Maryland, and West Virginia. US Geol. Surv. Bull. 2123. Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet. Sci. Lett. 26, 207–226. Steiger, R.H., Ja¨ger, E., 1977. Subcommission on geochronology, convention on the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359–362. Tera, F., Wasserburg, G.J., 1972. U–Th–Pb systematics in three Apollo 14 basalts and the problem of initial Pb in lunar rocks. Earth Planet. Sci. Lett. 14, 281–304. Tollo, R.P., Aleinikoff, J.N., 1992. The Robertson River Igneous Suite, Virginia, Blue Ridge: a case study in multiplestage magmatism associated with the early stages of Iapetan rifting. Geol. Soc. Am. Abstr. Prog. 24 (2), 70. Williams, I.S., Claesson, S., 1987. Isotopic evidence for the Precambrian provenance and Caledonian metamorphism of high grade paragneisses from the Seve Nappes, Scandinavian Caledonides, II. Ion microprobe zircon U–Th–Pb. Contr. Mineral. Petrol. 97, 205–217.