Constraining depositional ages of Potomac Terrane formations by zircon U/Pb isotope analysisEdward Kingman, Dr. Aaron Martin, Dr. Phillip Piccoli

University of Maryland, College Park Md.

II. Geologic description

The Potomac Terrane is a subdivision of the Piedmont province of the north American Appalachian

Mountains. The Potomac is interpreted by Kunk et al., (2005) to be distal slope deposits that where de-

posited in the Iapetus ocean during the Taconic Orogeny.

XII. Acknowledgements

The Problem:

What are the constraining ages of deposition for the

Potomac terrane units in question. How can the con-

straining ages be determined using detrital zircons.

Best age Ma

0 250 500 750 1000 1250 1500 1750 2000 2250 2500 27500

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207Pb/

235U

206P

b/2

38U

data-point error ellipses are 2σ

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Nu

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Upper depositional

constraint:

960 Ma

Blockhouse Point Domain, probability

distribution graph with histogram, and

Concordia plot (inset).

0 250 500

Best age Ma

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207Pb/

235U

206P

b/2

38U

data-point error ellipses are 2σ

0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750

Bear Island Domain, probability distri-

bution graph with histogram, and

Concordia plot (inset).

Best age Ma

0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750

Best age Ma

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207Pb/

235U

206P

b/2

38U

data-point error ellipses are 2σ

Upper depositional

constraint:

520 Ma

Laurel Formation, probability distribu-

tion graph with histogram, and Con-

cordia plot (inset).

VIII. U/Pb results (3 Metasedamintary

samples)

Upper depositional age constraints: Blockhouse Point Domain : 960 Ma

Bear Island Domain: 540 Ma

Laurel Formation : 520 Ma

Sample comparisons:1) All three sample age distributions contain three

age population between 800 Ma and 1600 Ma.

2) Bear Island and Laurel samples both contain a

similar, young population (~530 Ma) and old popula-

tion (~2700 Ma analysis in both)

3) The Blockhouse Point population contains no

analyses corresponding to the old and young

populations.

500

480

460

440

420

4000.064

0.068

0.072

0.076

0.080

0.48 0.52 0.56 0.60 0.64

207Pb/

235U

206P

b/2

38U

data-point error ellipses are 2σ

Bear Island Granodiorite magmatic growth ages

growth age.

Images taken by EPMA at University of Maryland Nanocenter

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Bear Island Granodiorite, probability distribution graph of zircon core

analyses demonstrate the Granodiorite zircons analyzed are inherited

Bear Island Domain zircons.

Best age Ma

Hypotheses: 1: The zircons from the Bear Island Granodio-

rite will yield a U/Pb crystallization age younger

than 570 Ma.

2: The zircons from the metasedimentary

units will yield a U/Pb core crystallization age

older than 700 Ma.

IX. Implications

X. Future work

III. Methods:A) Zircons were isolated from the pulverized

rock using magnetic and density separations

by a Frantz magnetic barrier separator and

dense liquid solution MethyleneIodide.

B) EPMA and sample mounts

C) LA-ICP-MS U-Th-Pb analyses

Measure:

- 232Th and its daughter 208Pb

- 238U and its daughter 206Pb

- 207Pb

- 204Pb

Concordia Age = 435.8 ±3.6 Ma

(1σ, decay-const. errs included)

MSWD (of concordance) = 0.91,

Probability (of concordance) = 0.34

Upper depositional

constraint:

540 Ma

V. Calculation of Error

Three data corrections:

1) Remove depth-dependent fractionation

2) Common Pb (from measurement of 204Pb)

3) Fractionation correction (from standard)

Laurel Fm. zircon epoxy mount

EPMA Backscatter electron

image of zircon mount. zir-

cons appear bright

ImFrants Magnetic separator collection trays, magnetic grains (left) non magnetic grains (right)

A very sincere thanks to the USGS for

providing the funding for this research.

IV. Method background

Isotope half lives -Decay of 238U to 206Pb (T1/2 = 4.468 Ga)

-Decay of 235U to 207Pb (T1/2 = 0.704 Ga)

-Decay of 232Th to 208Pb (T1/2 = 14.01 Ga)

Accounting for the blank

Sampling the zircon population:

Vermeesch (EPSL 2004) determined that in a sample

of 117 unknown grains, any population composing >5% of the

sample will be accounted for within a 95% confidence interval.

Sample Name orientations Collection site

coordinates

Lithology # of accepted

analyses

Blockhouse

Point domain

Foliation

198

39.05819

77.33440

Phyllonite 136

Bear Island

domain

Foliation

190

N38

W077

Metaquartzite 155

Laurel

Formation

Foliation

194

39.02996

77.00382

Melange 119

Bear Island

Granodiorite

Limb 1: 207

Limb 2: 222 43

38.98306

77.23048

Granodiorite 20 core

11 rim

correction 3 accounts for overall frac-

tionation of the analyses. This is

achieved by plotting the standard

analyses (blue dots) along a sliding

window average and uncertainty (pink)

with the unknown values (grey) with a

2% uncertainty.

Kunk, M. J., Wintsch, R.P., Southworth, S. C., 2004, Multiple Paleozoic Metamorphic Histories, Fabrics, and Faulting in the Westminster and Potomac Terranes, Central Piedmont, Northern Virginia and Southern Maryland. USGS Circular: 163-188.

- The Bear Island Domain and the Blockhouse point

domain may not be properly grouped together as part

of the Mather Gorge Formation

-The Bear Island Domain is more related to the Laurel

Formation than it is to the adjoining Blockhouse Point

domain.

-The sediments of the Bear Island Domain and of the

Laurel formation were indeed deposited in the Iapetus

ocean

Plot looks like this

1800

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0 1 2 3 4 5

207Pb/

235U

206P

b/2

38U

data-point error ellipses are 2σ

VI. Recognizing metamorphism

Metamorphic

analyses are

not concordant

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