supra-subduction magmatism in the moretown terrane, a ... · mariana arc rift cram hill formation...

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I. ABSTRACT V. TECTONIC IMPLICATIONS VII. REFERENCES VI. CONCLUSIONS III. Field Relationships II. INTRODUCTION IV. GEOCHEMISTRY The western Vermont Appalachians expose rocks of the Proterozoic Laurentian basement, a Neoproterozoic rift and drift sequence, and Early Paleozoic passive margin sediments in the Champlain Valley and Taconic sequences. In central and eastern Vermont, terranes accreted to the Laurentian margin are exposed. The Moretown Terrane comprises the Moretown and Cram Hill formations. The Moretown Formation comprises mostly schist, granofels (quartzite), and greenstone (Walsh et al., 2010). The Cram HIll Formation consists mostly of phyllite and greenstone. Detrital zircons extracted from quartz-rich units of the Moretown by Ryan-Davis (2012) yielded age distributions that indicate their provenance is peri-Gondwanan (Ganderian) rather than Laurentian, as had previously been assumed. Furthermore, the maximum age, based on the youngest zircons, is 514 Ma (Macdonald et al., 2014). The Shelburne Falls arc was established on this peri-Gondwanan Moretown Terrane perhaps as early as 500 Ma but with a locus of magmatic activity at ~475 Ma ( Macdonald et al., 2014). By 460 - 470 Ma, the Shelburne Falls arc had collided with Laurentian and basins associated with it may have been receiving detritus from Laurentia as well as peri-Gondwanan terranes. Mafic rocks (greenstones) of uncertain age occur in both the Moretown and Cram Hill formations as dikes and possibly sills or flows. Here, we use geochemistry to show that the mafic rocks formed in supra-subduction regions, perhaps in response to two lithospheric delamination events in the tectonic history of the Vermont Appalachians. 0 500 1000 1500 2000 2500 3000 Age Probability Age (Ma) Ganderian Belts, NB & ME (Fyīe at al., 2009: n = 335) VT-MA Moretown (n = 764) Laurentian Margin, NL (Cawood and Nemchin., 2001: n = 342) VT-MA Rift-Drift (n = 578) The geochemistry of mafic rocks from the Moretown and Cram Hill formations indicates their origin from depleted mantle in a supra-subduction zone environment. Chemical differences between the two formations may be explained by derivation from slightly different mantle sources. The chemistry of the mafic rocks from the Moretown especially is similar to mafic rocks from the Ordovician Mount Norris Intrusive Suite and the Silurian Comerford Intrusive Suite. VIII. ACKNOWLEDGEMENTS Folded greenstone near Craftsbury, Vermont Concordant contact of greenstone with phyllite in Hubbard Park, Montpelier A few metamafic rocks are clearly folded by F 1 generation folds; others cut across S 2 foliations and exhibit chilled margins; many contacts simply parallel foliations with- out showing diagnostic relationships. So in the absence of radiometric age dates, we conclude that there are at least two generations of mafic rocks. Some are clearly dikes whereas others could have been sills, flows or dikes. All samples are metamorphosed to greenschist facies. Igneous texture is preserved in a few samples only. Mafic dike cutting the dominant foliation (S 2 ) at Putnamville, Vermont Thin section illustrating rarely preserved igneous texture in greenschist mineralogy: albite, chlorite, epidote, actinolite and titanite. greenstone phyllitic granofels F1fold F1fold greenstone phyllite with coticule metadiabase granofels 0 1 2 3 4 TiO 2 (wt. %) 10 15 20 Al 2 O 3 (wt. %) 2 4 6 8 10 12 70 60 50 40 30 20 CaO (wt. %) Mg # 0.0 0.2 0.4 0.6 0.8 1.0 P 2 O 5 (wt. %) 4 6 8 10 12 14 16 Fe 2 O 3 (wt. %) 0 2 4 6 8 70 60 50 40 30 20 MgO (wt. %) Mg # 1 10 100 200 Sample/Chondrite La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 1 10 100 Sample/Chondrite Mariana Trough Back-Arc Ocean Crust Mariana Arc Rift Cram Hill Formation Moretown Formation Th Nb La Ce Pr Nd Sm Ti Gd Tb Dy Y Er Tm Yb 0.1 1 10 100 Sample/MORB Cram Hill Formation Th Nb La Ce Pr Nd Sm Ti Gd Tb Dy Y Er Tm Yb 0.1 1 10 100 Sample/MORB Moretown Formation Th Nb La Ce Pr Nd Sm Ti Gd Tb Dy Y Er Tm Yb 0.1 1 10 100 Sample/MORB Mount Norris Intrusive Suite (MNIS) Th Nb La Ce Pr Nd Sm Ti Gd Tb Dy Y Er Tm Yb 0.1 1 10 100 Sample/MORB Comerford Intrusive Suite (CIS) 420 Ma 0.01 0.1 1 10 100 0.1 1 10 Nb 9.0 (ppm) Yb 9.0 (ppm) 5% 5% 15 25 40 FMM ƌĂŵ ,ŝůů &ŽƌŵĂƟŽŶ DŽƌĞƚŽǁŶ &ŽƌŵĂƟŽŶ /^ Ͳ ^ŝůƵƌŝĂŶ MNIS &ĞƌƟůĞ DKZ DĂŶƚůĞ Classification of mafic rocks as mostly basalts with the exception of three samples that plot as andesite or dacite. Rare earth element patterns are similar to those from basalts formed during arc and back-arc rifting in the western Pacific. Trace element contents in mafic rocks from Moretown and Cram Hill formations are consistent with formation in extensional environments near subduction zones. Calculated Nb and Yb values for primitive magmas (at 9% MgO) show that the Moretown mafic rocks formed by ~10% partial melting of fertile MORB mantle (Pearce & Par- kinson, 1993) whereas Cram Hill magmas may have formed from a slightly more depleted MORB mantle. Initial ε Nd values of +3 to +5 are consistent with this interpretation. Oxide variations with Mg number are consistent with fractionation of olivine, pyroxene & plagioclase. Extended element plots. All suites show negative Nb anomalies, characterisitc of subduction-influenced mantle. Mafic rocks of Moretown are similar to the Ordovician MNIS and Silurian CIS mafic rocks. Mafic rocks from the Cram Hill Formation have less enrichment in light rare earth elements. 0.001 0.01 0.1 1 5 0.01 0.1 1 10 Zr/TiO 2 Nb/Y Subalkaline Basalt Bas/And Alkali Basalt Andesite Dacite Rhyolite Basanite Phonolite Trachyte Trachy Andesite Comendite Pantellerite Cram Hill FormaƟon Moretown FormaƟon MF (SiO 2 > 53 %) Oceanic arc Alkaline arc Continental arc Major ocean ridge Ocean island 0.1 1 3 0.4 1 10 Nb'/La La/Yb 0.02 0.1 1 5 1 10 80 Th/Nb' La/Yb Major ocean ridge Ocean island Oceanic arc Continental arc Alkaline arc Y/15 La/10 Nb/8 1A calc-alkali basalts 1C arc tholeiites 2A continental basalts 2B back-arc basalts 3A rift alkali basalts 3B,3C E-type MORB 3D N-type MORB 3D 1C 1B 1A 2A 3A 3B 3C Back -Arc Basalts Diagrams after Hollocher et al. (2012) Cawood, P. A., and Nemchin, A. A., 2001, Paleogeographic development of the east Laurentian margin: Constraints from U-Pb dating of detrital zircons in the Newfoundland Appalachians: Geological Society of America Bulletin, v. 113, p. 1234-1246. Coish, R., Ryan-Davis, J., Amidon, W., Kim, J., and Dietsch, C., 2013, Origin of an Early Paleozoic arc-related sedimentary basin in the northern Vermont Appalachians: a detrital zircon study, AGU Fall Meeting Abstracts, p. 2404. Coish, R. A., 2010, Magmatism in the Vermont Appalachians, in Tollo, R., Bartholomew, M. J., Hibbard, J. P., and Karabinos, P., editors, From Rodinia to Pangea: The Lithotectonic Record of the Appalachian Region, Geological Society of America Memoir 206, p. 91-110. Fyffe, L. R., Barr, S. M., Johnson, S. C., McLeod, M. J., McNicoll, V. J., Valverde-Vaquero, P., van Staal, C. R., and White, C. E., 2009, Detrital zircon ages from Neoproterozoic and Early Paleozoic conglomerate and sandstone units of New Brunswick and coastal Maine: implications for the tectonic evolution of Ganderia: Atlantic Geology, v. 45, p. Pages 110-144. Hibbard, J. P., van Staal, C. R., Rankin, D. W., and Williams, H., 2006, Lithotectonic map of the Appalachian Orogen: Geological Survey of Canada Map 2096A, scale 1:1,500,000. Hollocher, K., Robinson, P., Walsh, E., and Roberts, D., 2012, Geochemistry of amphibolite-facies volcanics and gabbros of the Støren Nappe in extensions west and southwest of Trondheim, western gneiss region, Norway: A key to correlations and paleotectonic settings: American Journal of Science, v. 312, p. 357-416. Karabinos, P., Samson, S. D., Hepburn, J. C., and Stoll, H. M., 1998, Taconian orogeny in the New England Appalachians: Collision between Laurentia and the Shelburne Falls arc: Geology, v. 26, p. 215-218. Kim, J., Coish, R., Evans, M., and Dick, G., 2003, Supra-subduction zone extensional magmatism in Vermont and adjacent Quebec; implications for early Paleozoic Appalachian tectonics: Geological Society of America Bulletin, v. 115, p. 1552-1569. Kim, J., Gale, M., King, S. M., Orsi, C. M., and Pascale, L., 2003, Bedrock Geology of the Montpelier Quadrangle: Vermont Geological Survey Open File Map VG03-1, scale 1:24,000. Macdonald, F. A., Ryan-Davis, J., Coish, R. A., Crowley, J. L., and Karabinos, P., 2014, A newly identified Gondwanan terrane in the northern Appalachian Mountains: Implications for the Taconic orogeny and closure of the Iapetus Ocean: Geology, v. 42, p. 539-542. Moench, R. H., and Aleinikoff, J. N., 2002, Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England: Physics and Chemistry of the Earth, v. 27, p. 47-95. Rankin, D. W., Coish, R. A., Tucker, R. D., Peng, Z. X., Wilson, S. A., and Rouff, A. A., 2007, Silurian extension in the upper Connecticut Valley, United States and the origin of middle Paleozoic basins in the Quebec Embayment: American Journal of Science, v. 307, p. 216-264. Ratcliffe, N. M., Stanley, R. S., Gale, M. H., Thompson, P. J., and Walsh, G. J., 2011, Bedrock Geologic Map of Vermont: U.S. Geological Survey Scientific Investigations Map 3184, scale 1:100,000. Ryan-Davis, J., 2013, Origins of the Moretown Formation, northern Vermont: A detrital zircon study: B.A. thesis, Middlebury College, 74 p. Ryan-Davis, J., Coish, R., and Amidon, W. H., 2013, Origins of the Moretown Formation, Vermont: a detrital zircon study: Geological Society of America Abstracts with Programs, v. 45, no. 1, p. 95. Twelker, E., 2004, Geochemistry and field relations of greenstones in the Moretown and Cram Hill Formations, Montpelier Quadrangle, Vermont: B.A., Middlebury College, 66 p. van Staal, C. R., Whalen, J. B., McNicoll, V. J., Pehrsson, S., Lissenberg, C. J., Zagorevski, A., van Breemen, O., and Jenner, G. A., 2007, The Notre Dame arc and the Taconic orogeny in Newfoundland, in Hatcher Jr., R. D., Carlson, M. P., McBride, J. H., and Martínez Catalán, J. R., editors, 4-D Framework of Continental Crust, Geological Society of America Memoir 200, p. 511-552. Walsh, G. J., Kim, J., Gale, M. H., and King, S. M., 2010, Bedrock geologic map of the Montpelier and Barre West quadrangles, Washington and Orange Counties, Vermont: U.S. Geological Survey Scientific Investigations Map 3111, scale 1:24,000. We thank Evan Twelker and Scott Zolkos for their contributions to the early stages of this project. We thank Greg Walsh for discussion of aspects of the geology of the Montpelier- West Barre Quadrangle. We are grateful to Middlebury College for continuing support of student-faculty research and its commitment to maintaining first-class analytical facilities. B. SF arc SSZ ophiolite ~480 Ma upwelling asthenospheric C. Mafic Dikes Mafic Dikes ~470 Ma upwelling asthenosphere CMT CIS CVGT Ganderia ~420 Ma E. CMT Old SF arc Late BH arc Laurentia Ganderia ~460 Ma Old BH arc Slab retreat D. accretionary complex Laurentia amalgamated Laurentia/Ganderia Avalonia F. ~410 Ma Early Devonian granites Late Devonian granites ~375 Ma G. Avalonia Laurentia/Ganderia Laurentia Zone 2 Zone 1 Zone 3,4 A. ~550 Ma Moretown Fm (Gondwanan) A sequence of sketches showing the tectonic evolution of the New England Appalachians from Coish (2010), modified after van Staal et al. (2007), Karabinos et al. (1998), and Moench and Aleinikoff (2002). Mafic rocks of this study may have formed at two different times: 1) during the mid-Ordovician as a result of lithospheric delamination following collision of the Shelburne Falls arc with Laurentia, and 2) during the Silurian following collision of greater Ganderia with amalgamated Laurentia. Meta-sandstones of the Moretown terrane in western New England contain detrital zircons that reveal an affinity with the Gondwanan (eastern) rather than the Laurentian (western) side of the Cambrian-Ordovician Iapetus Ocean. In the Vermont Appalachians, metamorphosed mafic rocks, in the form of dikes and sills, cut this Moretown terrane. The meta-mafic rocks have geochemical characteristics similar to modern-day supra-subduction volcanics. Geochemistry suggests that the meta-mafic rocks geochemically were formed as mostly basalts or basaltic andesites. They have moderate TiO 2 contents (1- 2.5 wt %), are slightly enriched in the light-rare earth elements relative to the heavy rare earths, and have negative Nb-Ta anomalies in MORB-normalized extended rare earth element diagrams. ε Nd values for two samples are +3 and +5. The chemistry, taken together, suggests protoliths of the meta-mafic rocks may have formed in an extensional marginal basin(s), perhaps near a volcanic arc(s). The meta-mafic rocks of this study are similar in chemistry to the pre-Silurian Mount Norris Intrusive Suite (MNIS) of north-central Vermont, and also to the Silurian-aged Comerford Intrusive Suite (CIS) of northeastern Vermont. If the meta-mafic rocks of the Moretown are correlated with the MNIS, then the supra-subduction extensional environment could have formed in response to lithospheric delamination, following collision of the Gondwanan Moretown terrane with either a Laurentian microcontinent or Laurentia itself in the Ordovician. If , on the other hand, they are correlated with the CIS, then the extensional environment may have formed by lithospheric delamination following collision of a Ganderia fragment with Laurentia in the Silurian. In either case, it is likely that the magmatism occurred after the peri-Gondwanan Moretown terrane became part of the Laurentian plate. Supra-subduction magmatism in the Moretown Terrane, a fragment of Gondwana in the western Appalachians COISH, Raymond 1 ; KIM, Jonathan 2 ; RYAN-DAVIS, Juliet 1 ; PIERCE, Natashia3; DIETSCH, Craig 3 1 Geology Department, Middlebury College, Middlebury, VT 05753, [email protected]; 2 Vermont Geological Survey, 1 National Drive, Davis 2, Montpelier, VT 05620-3902; 3 University of Cincinnati, Cincinnati, OH 45221 72°30'W 72°30'W 44°30'N 44°30'N 44°15'N 44°15'N 0 2 4 6 8 10 1 kilometers Putnamville Omq Dg Dgr DSu DSu Dg Ochu Omp Dg Ochu Ochp Ochuc Craftsbury Omc Ochu Omc Rift - Drift Sequence Moretown Terrane Silurian-Devonian Sequence Montpelier RMC RMC Dumpling Hill Belt Wrightsville Belt Shady Rill Fault Red Indian Line ! . # 0 Cram Hill Formation Moretown Formation Omc Dg Gile Mountain Formation Devonian granitoids Waits River, Northfield & Shaw Mt. formations undifferentiated Cram Hill Formation Moretown Formation phyllite member pin-striped granofels member quartzite/phyllite member carbonaceous schist member Umbrella Hill Conglomerate undivided member D Ochuc gr Ochp Ochu Omp Omq Explanation ( ( Thrust Fault Sample Locations Geology of the Montpelier - Craftsbury area northern Vermont Geology after Ratcliffe et al. (2011), Walsh et al. 2010, & Kim et al. (2003) Richardson Memorial Contact (RMC) DSu Burlington Craftsbury Middlebury Montpelier 73°W 73°W 72°W 72°W 45°N 44°N 44°N 43°N 43°N 45°N ² 0 10 20 30 40 50 5 kilometers Mesozoic Intrusives Devonian Intrusives Silurian Intrusives Connecticut Valley Trough Bronson Hill Belt Moretown Accreted Terrane The Taconic Allochthon Champlain Valley Belt Rift-Drift Terrane Laurentian Basement Laurentia Peri-Laurentian Units Peri-Gondwanan Arcs Ganderia Piedmont Accretionary Piedmont Avalonia Carolinia Meguma 0 1,000 500 Kilometers Lithotectonic Units in the Appalachians (after Hibbard et al., 2006) Generalized geologic map of Vermont (adapted from Ratcliffe et al., 2011). The Moretown Terrane includes the Moretown and Cram Hill Formations, which are the focus of this poster. Detailed map of the study area, showing sample locations of mafic rocks collected from the Montpelier Quadrangle and Craftsbury Township. Thick red line represents the postulated Red Indian Line in Vermont, as suggested by Ryan-Davis et al. (2013) and Macdonald et al., (2014). Detrital zircon data for the Moretown Formation metasedimentary rocks (Ryan-Davis, 2012; Ryan-Davis et al., 2012; Coish et al., 2013; Macdonald et al., 2014). Peaks at 600 - 800 Ma clearly indicate provenance of the Moretown is Gondwanan rather than Laurentian. 1. Metamorphosed mafic rocks in the Cram Hill and Moretown formations of north-central Vermont formed as dikes (and probably lava flows) in Early Ordovician to Silurian times. 2. Their geochemistry suggests they were mostly tholeiitic basalt or basaltic-andesite. 3. Rare earth element patterns and negative Nb anomalies (relative to Th and La) are consistent with derivation of primitive magmas in suprasubduction zone extensional environments. 4. The suites may have been derived by ~10 to 15% partial melting of fertile MORB mantle that had been variably enriched in selected elements mobilized from a subducting plate. The mantle source for the Cram Hill suite was less enriched than the source for the Moretown suite. 5. The magmas of the Cram Hill and some of the Moretown meta-mafic rocks may have formed after collision of the Shelburne Falls arc with Laurentia in the Middle Ordovician, perhaps as melts associated with slab break-off of an easterly-directed subducting plate, as postulated for the Mount Norris Intrusive Suite (Kim and others, 2003). Other meta-mafic rocks from the Moretown may have formed from delamination in the Late Silurian, by analogy with the Comerford Intrusive Complex (Rankin and others, 2007). Unfortunately, the chemistry of the mafic rocks cannot be used to distinguish different magmatic ages.

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Page 1: Supra-subduction magmatism in the Moretown Terrane, a ... · Mariana Arc Rift Cram Hill Formation ... Kim, J., Gale, M., King, S. M., Orsi, C. M., and Pascale, L., 2003, Bedrock Geology

I. ABSTRACT V. TECTONIC IMPLICATIONS

VII. REFERENCES

VI. CONCLUSIONS

III. Field Relationships

II. INTRODUCTION

IV. GEOCHEMISTRY

The western Vermont Appalachians expose rocks of the Proterozoic Laurentian basement, a Neoproterozoic rift and drift sequence, and Early Paleozoic passive margin sediments in the Champlain Valley and Taconic sequences. In central and eastern Vermont, terranes accreted to the Laurentian margin are exposed. The Moretown Terrane comprises the Moretown and Cram Hill formations. The Moretown Formation comprises mostly schist, granofels (quartzite), and greenstone (Walsh et al., 2010). The Cram HIll Formation consists mostly of phyllite and greenstone. Detrital zircons extracted from quartz-rich units of the Moretown by Ryan-Davis (2012) yielded age distributions that indicate their provenance is peri-Gondwanan (Ganderian) rather than Laurentian, as had previously been assumed. Furthermore, the maximum age, based on the youngest zircons, is 514 Ma (Macdonald et al., 2014). The Shelburne Falls arc was established on this peri-Gondwanan Moretown Terrane perhaps as early as 500 Ma but with a locus of magmatic activity at ~475 Ma ( Macdonald et al., 2014). By 460 - 470 Ma, the Shelburne Falls arc had collided with Laurentian and basins associated with it may have been receiving detritus from Laurentia as well as peri-Gondwanan terranes. Ma�c rocks (greenstones) of uncertain age occur in both the Moretown and Cram Hill formations as dikes and possibly sills or �ows. Here, we use geochemistry to show that the ma�c rocks formed in supra-subduction regions, perhaps in response to two lithospheric delamination events in the tectonic history of the Vermont Appalachians.

0 500 1000 1500 2000 2500 3000

Age

Pro

babi

lity

Age (Ma)

Ganderian Belts, NB & ME(Fy e at al., 2009: n = 335)

VT-MA Moretown(n = 764)

Laurentian Margin, NL(Cawood and Nemchin., 2001: n = 342)

VT-MA Rift-Drift(n = 578)

The geochemistry of ma�c rocks from the Moretown and Cram Hill formations indicates their origin from depleted mantle in a supra-subduction zone environment. Chemical di�erences between the two formations may be explained by derivation from slightly di�erent mantle sources. The chemistry of the ma�c rocks from the Moretown especially is similar to ma�c rocks from the Ordovician Mount Norris Intrusive Suite and the Silurian Comerford Intrusive Suite.

VIII. ACKNOWLEDGEMENTS

Folded greenstone near Craftsbury, VermontConcordant contact of greenstone with phyllite in Hubbard Park, Montpelier

A few metama�c rocks are clearly folded by F1 generation folds; others cut across S2 foliations and exhibit chilled margins; many contacts simply parallel foliations with-out showing diagnostic relationships. So in the absence of radiometric age dates, we conclude that there are at least two generations of ma�c rocks. Some are clearly dikes whereas others could have been sills, �ows or dikes. All samples are metamorphosed to greenschist facies. Igneous texture is preserved in a few samples only.

Ma�c dike cutting the dominant foliation (S2) at Putnamville, Vermont

Thin section illustrating rarely preserved igneous texture in greenschist mineralogy: albite, chlorite, epidote, actinolite and titanite.

greenstone

phyllitic granofels

F1fold

F1fold

greenstone

phyllitewith coticule

metadiabase

granofels

0

1

2

3

4

TiO

2 (w

t. %

)

10

15

20

Al2O

3 (w

t. %

)

2

4

6

8

10

12

70 60 50 40 30 20

CaO

(wt.

%)

Mg #

0.0

0.2

0.4

0.6

0.8

1.0

P 2O

5 (w

t. %

)

4

6

8

10

12

14

16

Fe2O

3 (w

t. %

)

0

2

4

6

8

70 60 50 40 30 20

MgO

(wt.

%)

Mg #

1

10

100

200

Sam

ple/

Cho

ndrit

e

La Ce Pr Nd

Sm Eu Gd Tb Dy

Ho Er Tm Yb Lu

1

10

100

Sam

ple/

Cho

ndrit

e

Mariana TroughBack-Arc Ocean Crust

Mariana Arc Rift

Cram Hill Formation

Moretown Formation

Th Nb La Ce Pr

Nd

Sm Ti Gd

Tb Dy Y Er

Tm Yb

0.1

1

10

100

Sam

ple/

MO

RB

Cram Hill Formation

Th Nb La Ce Pr

Nd

Sm Ti Gd

Tb Dy Y Er

Tm Yb

0.1

1

10

100

Sam

ple/

MO

RB

Moretown Formation

Th Nb La Ce Pr

Nd

Sm Ti Gd

Tb Dy Y Er

Tm Yb

0.1

1

10

100

Sam

ple/

MO

RB

Mount Norris Intrusive Suite(MNIS)

Th Nb La Ce Pr

Nd

Sm Ti Gd

Tb Dy Y Er

Tm Yb

0.1

1

10

100

Sam

ple/

MO

RB

Comerford Intrusive Suite (CIS)420 Ma

0.01

0.1

1

10

100

0.1 1 10

Nb 9

.0 (p

pm)

Yb9.0 (ppm)

5%

5%

152540

FMM

MNIS

Classi�cation of ma�c rocks as mostly basalts with the exception of three samples that plot as andesite or dacite.

Rare earth element patterns are similar to those from basalts formed during arc and back-arc rifting in the western Paci�c.

Trace element contents in ma�c rocks from Moretown and Cram Hill formations are consistent with formation in extensional environments near subduction zones.

Calculated Nb and Yb values for primitive magmas (at 9% MgO) show that the Moretown ma�c rocks formed by ~10% partial melting of fertile MORB mantle (Pearce & Par-kinson, 1993) whereas Cram Hill magmas may have formed from a slightly more depleted MORB mantle. Initial εNd values of +3 to +5 are consistent with this interpretation.

Oxide variations with Mg number are consistent with fractionation of olivine, pyroxene & plagioclase.

Extended element plots. All suites show negative Nb anomalies, characterisitc of subduction-in�uenced mantle. Ma�c rocks of Moretown are similar to the Ordovician MNIS and Silurian CIS ma�c rocks. Ma�c rocks from the Cram Hill Formation have less enrichment in light rare earth elements.

0.001

0.01

0.1

1

5

0.01 0.1 1 10

Zr/T

iO2

Nb/Y

SubalkalineBasalt

Bas/AndAlkali Basalt

Andesite

Dacite

Rhyolite

Basa

nite

Phonolite

Trachyte

Trachy Andesite

Comendite Pantellerite

Cram Hill Forma onMoretown Forma onMF (SiO 2 > 53 %)

Oceanic arc

Alkaline arcContinental

arc

Major ocean ridge Ocean island

0.1

1

3

0.4 1 10

Nb'

/La

La/Yb0.02

0.1

1

5

1 10 80

Th/N

b'

La/Yb

Major ocean ridge

Ocean island

Oceanic arc Continental arc Alkaline arc

Y/15

La/10 Nb/8

1A calc-alkali basalts1C arc tholeiites2A continental basalts2B back-arc basalts3A rift alkali basalts3B,3C E-type MORB3D N-type MORB

3D

1C

1B

1A2A 3A

3B

3C

Back -Arc Basalts

Diagrams afterHollocher et al. (2012)

Cawood, P. A., and Nemchin, A. A., 2001, Paleogeographic development of the east Laurentian margin: Constraints from U-Pb dating of detrital zircons in the Newfoundland Appalachians: Geological Society of America Bulletin, v. 113, p. 1234-1246.

Coish, R., Ryan-Davis, J., Amidon, W., Kim, J., and Dietsch, C., 2013, Origin of an Early Paleozoic arc-related sedimentary basin in the northern Vermont Appalachians: a detrital zircon study, AGU Fall Meeting Abstracts, p. 2404.

Coish, R. A., 2010, Magmatism in the Vermont Appalachians, in Tollo, R., Bartholomew, M. J., Hibbard, J. P., and Karabinos, P., editors, From Rodinia to Pangea: The Lithotectonic Record of the Appalachian Region, Geological Society of America Memoir 206, p. 91-110.

Fy�e, L. R., Barr, S. M., Johnson, S. C., McLeod, M. J., McNicoll, V. J., Valverde-Vaquero, P., van Staal, C. R., and White, C. E., 2009, Detrital zircon ages from Neoproterozoic and Early Paleozoic conglomerate and sandstone units of New Brunswick and coastal Maine: implications for the tectonic evolution of Ganderia: Atlantic Geology, v. 45, p. Pages 110-144.

Hibbard, J. P., van Staal, C. R., Rankin, D. W., and Williams, H., 2006, Lithotectonic map of the Appalachian Orogen: Geological Survey of Canada Map 2096A, scale 1:1,500,000.

Hollocher, K., Robinson, P., Walsh, E., and Roberts, D., 2012, Geochemistry of amphibolite-facies volcanics and gabbros of the Støren Nappe in extensions west and southwest of Trondheim, western gneiss region, Norway: A key to correlations and paleotectonic settings: American Journal of Science, v. 312, p. 357-416.

Karabinos, P., Samson, S. D., Hepburn, J. C., and Stoll, H. M., 1998, Taconian orogeny in the New England Appalachians: Collision between Laurentia and the Shelburne Falls arc: Geology, v. 26, p. 215-218.

Kim, J., Coish, R., Evans, M., and Dick, G., 2003, Supra-subduction zone extensional magmatism in Vermont and adjacent Quebec; implications for early Paleozoic Appalachian tectonics: Geological Society of America Bulletin, v. 115, p. 1552-1569.

Kim, J., Gale, M., King, S. M., Orsi, C. M., and Pascale, L., 2003, Bedrock Geology of the Montpelier Quadrangle: Vermont Geological Survey Open File Map VG03-1, scale 1:24,000.

Macdonald, F. A., Ryan-Davis, J., Coish, R. A., Crowley, J. L., and Karabinos, P., 2014, A newly identi�ed Gondwanan terrane in the northern Appalachian Mountains: Implications for the Taconic orogeny and closure of the Iapetus Ocean: Geology, v. 42, p. 539-542.

Moench, R. H., and Aleiniko�, J. N., 2002, Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England: Physics and Chemistry of the Earth, v. 27, p. 47-95.

Rankin, D. W., Coish, R. A., Tucker, R. D., Peng, Z. X., Wilson, S. A., and Rou�, A. A., 2007, Silurian extension in the upper Connecticut Valley, United States and the origin of middle Paleozoic basins in the Quebec Embayment: American Journal of Science, v. 307, p. 216-264.

Ratcli�e, N. M., Stanley, R. S., Gale, M. H., Thompson, P. J., and Walsh, G. J., 2011, Bedrock Geologic Map of Vermont: U.S. Geological Survey Scienti�c Investigations Map 3184, scale 1:100,000.

Ryan-Davis, J., 2013, Origins of the Moretown Formation, northern Vermont: A detrital zircon study: B.A. thesis, Middlebury College, 74 p.

Ryan-Davis, J., Coish, R., and Amidon, W. H., 2013, Origins of the Moretown Formation, Vermont: a detrital zircon study: Geological Society of America Abstracts with Programs, v. 45, no. 1, p. 95.

Twelker, E., 2004, Geochemistry and �eld relations of greenstones in the Moretown and Cram Hill Formations, Montpelier Quadrangle, Vermont: B.A., Middlebury College, 66 p.

van Staal, C. R., Whalen, J. B., McNicoll, V. J., Pehrsson, S., Lissenberg, C. J., Zagorevski, A., van Breemen, O., and Jenner, G. A., 2007, The Notre Dame arc and the Taconic orogeny in Newfoundland, in Hatcher Jr., R. D., Carlson, M. P., McBride, J. H., and Martínez Catalán, J. R., editors, 4-D Framework of Continental Crust, Geological Society of America Memoir 200, p. 511-552.

Walsh, G. J., Kim, J., Gale, M. H., and King, S. M., 2010, Bedrock geologic map of the Montpelier and Barre West quadrangles, Washington and Orange Counties, Vermont: U.S. Geological Survey Scienti�c Investigations Map 3111, scale 1:24,000.

We thank Evan Twelker and Scott Zolkos for their contributions to the early stages of this project. We thank Greg Walsh for discussion of aspects of the geology of the Montpelier- West Barre Quadrangle. We are grateful to Middlebury College for continuing support of student-faculty research and its commitment to maintaining �rst-class analytical facilities.

B. SF arc

SSZ ophio

lite

~480 Ma

upwellingasthenospheric

C. Ma�c Dikes

Ma�c Dikes

~470 Ma

upwelling asthenosphere

CMT

CIS

CVGT

Ganderia

~420 Ma

E.

CMTOldSF arc

LateBH arc

Laurentia Ganderia

~460 Ma

OldBH arc

Slab retreat

D. accretionarycomplex

Laurentia

amalgamatedLaurentia/Ganderia AvaloniaF.

~410 MaEarly Devoniangranites

Late Devoniangranites

~375 Ma

G. AvaloniaLaurentia/Ganderia

LaurentiaZone 2Zone 1 Zone 3,4A.

~550 Ma

Moretown Fm(Gondwanan) A sequence of sketches

showing the tectonic evolution of the New England Appalachians from Coish (2010), modi�ed after van Staal et al. (2007), Karabinos et al. (1998), and Moench and Aleiniko� (2002).

Ma�c rocks of this study may have formed at two di�erent times: 1) during the mid-Ordovician as a result of lithospheric delamination following collision of the Shelburne Falls arc with Laurentia, and 2) during the Silurian following collision of greater Ganderia with amalgamated Laurentia.

Meta-sandstones of the Moretown terrane in western New England contain detrital zircons that reveal an a�nity with the Gondwanan (eastern) rather than the Laurentian (western) side of the Cambrian-Ordovician Iapetus Ocean. In the Vermont Appalachians, metamorphosed ma�c rocks, in the form of dikes and sills, cut this Moretown terrane. The meta-ma�c rocks have geochemical characteristics similar to modern-day supra-subduction volcanics. Geochemistry suggests that the meta-ma�c rocks geochemically were formed as mostly basalts or basaltic andesites. They have moderate TiO2 contents (1- 2.5 wt %), are slightly enriched in the light-rare earth elements relative to the heavy rare earths, and have negative Nb-Ta anomalies in MORB-normalized extended rare earth element diagrams. εNd values for two samples are +3 and +5. The chemistry, taken together, suggests protoliths of the meta-ma�c rocks may have formed in an extensional marginal basin(s), perhaps near a volcanic arc(s). The meta-ma�c rocks of this study are similar in chemistry to the pre-Silurian Mount Norris Intrusive Suite (MNIS) of north-central Vermont, and also to the Silurian-aged Comerford Intrusive Suite (CIS) of northeastern Vermont. If the meta-ma�c rocks of the Moretown are correlated with the MNIS, then the supra-subduction extensional environment could have formed in response to lithospheric delamination, following collision of the Gondwanan Moretown terrane with either a Laurentian microcontinent or Laurentia itself in the Ordovician. If , on the other hand, they are correlated with the CIS, then the extensional environment may have formed by lithospheric delamination following collision of a Ganderia fragment with Laurentia in the Silurian. In either case, it is likely that the magmatism occurred after the peri-Gondwanan Moretown terrane became part of the Laurentian plate.

Supra-subduction magmatism in the Moretown Terrane, a fragment of Gondwana in the western AppalachiansCOISH, Raymond1; KIM, Jonathan2; RYAN-DAVIS, Juliet1; PIERCE, Natashia3; DIETSCH, Craig3

1Geology Department, Middlebury College, Middlebury, VT 05753, [email protected]; 2Vermont Geological Survey, 1 National Drive, Davis 2, Montpelier, VT 05620-3902; 3University of Cincinnati, Cincinnati, OH 45221

72°30'W

72°30'W44

°30'

N

44°3

0'N

44°1

5'N

44°1

5'N

0 2 4 6 8 101

kilometers

Putnamville

Omq Dg

Dgr

DSu

DSu

Dg

Ochu

Omp

Dg

OchuOchp

Ochuc

Craftsbury

Omc

Ochu

Omc

Rift - Drift Sequence Moretown

Terrane

Silurian-DevonianSequence

Montpelier

RMC

RMC

Dumplin

g Hill

Belt

WrightsvilleBelt

Shad

y Ri

ll Fa

ult

Red

Indi

an L

ine

!.

#0Cram Hill FormationMoretown Formation

Omc

Dg

Gile Mountain Formation

Devonian granitoids

Waits River, Northfield &Shaw Mt. formationsundifferentiated

Cram Hill Formation

Moretown Formation

phyllite member

pin-striped granofels member

quartzite/phyllite membercarbonaceous schist member

Umbrella Hill Conglomerate

undivided member

D

Ochuc

gr

OchpOchu

Omp

Omq

Explanation

(( Thrust Fault

Sample Locations

Geology of theMontpelier - Craftsbury area

northern VermontGeology after Ratcli�e et al. (2011),Walsh et al. 2010, & Kim et al. (2003)

Richardson Memorial Contact (RMC)

DSu

Burlington

Craftsbury

Middlebury

Montpelier

73°W

73°W

72°W

72°W

45°N

44°N 44°N

43°N 43°N

45°N

²

0 10 20 30 40 505

kilometers

Mesozoic Intrusives

Devonian Intrusives

Silurian Intrusives

Connecticut Valley Trough

Bronson Hill Belt

Moretown Accreted Terrane

The Taconic Allochthon

Champlain Valley Belt

Rift-Drift Terrane

Laurentian Basement

Laurentia

Peri-Laurentian Units

Peri-Gondwanan Arcs

Ganderia

Piedmont Accretionary

Piedmont

Avalonia

Carolinia

Meguma

0 1,000500

Kilometers

Lithotectonic Units in theAppalachians

(after Hibbard et al., 2006)

Generalized geologic map of Vermont (adapted from Ratcli�e et al., 2011). The Moretown Terrane includes the Moretown and Cram Hill Formations, which are the focus of this poster.

Detailed map of the study area, showing sample locations of ma�c rocks collected from the Montpelier Quadrangle and Craftsbury Township. Thick red line represents the postulated Red Indian Line in Vermont, as suggested by Ryan-Davis et al. (2013) and Macdonald et al., (2014).

Detrital zircon data for the Moretown Formation metasedimentary rocks (Ryan-Davis, 2012; Ryan-Davis et al., 2012; Coish et al., 2013; Macdonald et al., 2014). Peaks at 600 - 800 Ma clearly indicate provenance of the Moretown is Gondwanan rather than Laurentian.

1. Metamorphosed ma�c rocks in the Cram Hill and Moretown formations of north-central Vermont formed as dikes (and probably lava �ows) in Early Ordovician to Silurian times.

2. Their geochemistry suggests they were mostly tholeiitic basalt or basaltic-andesite.

3. Rare earth element patterns and negative Nb anomalies (relative to Th and La) are consistent with derivation of primitive magmas in suprasubduction zone extensional environments.

4. The suites may have been derived by ~10 to 15% partial melting of fertile MORB mantle that had been variably enriched in selected elements mobilized from a subducting plate. The mantle source for the Cram Hill suite was less enriched than the source for the Moretown suite.

5. The magmas of the Cram Hill and some of the Moretown meta-ma�c rocks may have formed after collision of the Shelburne Falls arc with Laurentia in the Middle Ordovician, perhaps as melts associated with slab break-o� of an easterly-directed subducting plate, as postulated for the Mount Norris Intrusive Suite (Kim and others, 2003). Other meta-ma�c rocks from the Moretown may have formed from delamination in the Late Silurian, by analogy with the Comerford Intrusive Complex (Rankin and others, 2007). Unfortunately, the chemistry of the ma�c rocks cannot be used to distinguish di�erent magmatic ages.