Geochemical and Isotopic Discrimination of Meta-Volcanics from the Rowe-Hawley Zone of Western New England: a Discussion Of

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Geochemical and isotopic discrimination of meta-volcanics from the Rowe-Hawley Zone of western New England: a discussion of

along-strike translation of tectonic formation

A thesis submitted to the

Graduate School

of the University of Cincinnati

in partial fulfillment of the

requirements for the degree of

Master of Science

in the Department of Geology

of the College of Arts and Sciences

Natashia Pierce

B.A. Cornell College

June 2013

Committee Chair: C. Dietsch, Ph.D.

ABSTRACT

A variety of tectonic models for the Lower Paleozoic evolution of the Rowe-

Hawley Zone (RHZ) in western New England describe the formation suprasubduction zone arc complexes and collision with Laurentia to produce the Taconic orogeny.

Models developed for parts of the RHZ (e.g., western Massachusetts and Connecticut

(Karabinos et al., 1998) and Newfoundland (van Staal et al., 2001)) have been translated along-strike. To test translation of various models along-strike across New England, geochemical data from major RHZ units in Vermont, Massachusetts, and Connecticut were compared for potential correlation. Element abundance data from previously analyzed meta-igneous rocks from the Mount Norris Intrusive Suite, North River Igneous

Suite, and Barnard Gneiss from Vermont; Hawley Formation and Charlemont Mafic

Intrusive Suite from Massachusetts; and Collinsville Formation of Connecticut were compiled; a set of meta-basalts from the Barnard Gneiss of Vermont were analyzed for their major and trace elements; and a set of 21 selected samples from these units were analyzed for Nd isotopes. These data were also compared to published data from units of the Bronson Hill Arc in central New England.

Geochemical analyses of meta-igneous rocks from the Hawley slices of eastern

RHZ in all three states show them to have suprasubduction zone affinity. Hawley subalkaline meta-basalts of Massachusetts can be differentiated from Collinsville subalkaline basalts and basaltic andesites of Connecticut using MORB-normalized multi- element plots. Hawley samples show large negative Nb, large positive P, and flat Zr to

Yb ~1, whereas Collinsville samples display small negative Nb, flat Zr to Yb 10 > x <

0.1, and flat to negatively sloping HFSE ~1. Mount Norris Intrusive Suite (MNIS)

subalkaline basalt from Vermont show backarc basin affinity (small negative Nb, Zr to

Yb ~1, and relatively flat HFSE ~1). These data, along with discrimination diagrams, indicate that Hawley and Collinsville rocks could have formed in forearc and backarc basins, respectively, of one complex similar to modern rifted suprasubduction arcs of the western Pacific (e.g., Izu-Bonin). The Taconic orogeny could have been the result of a sequence of accretionary events involving separate regions of one composite arc of the

RHZ.

Using TIMS thermal ionization mass spectrometry, the Nd isotopic ratios of a suite of twenty-one selected meta-basalt samples were analyzed to consider mantle source material, and continental and/or sedimentary magma components, and increase the validity of long-range correlation of major units of the RHZ. εNd values of twenty samples range between 1.4 and 7.4 with one outlier at -3.4. Their values evidence depleted mantle source proximal to the eastern (Gondwanan) side of Iapetus Ocean.

However, when added to published Nd data from central and eastern New England, an extensive range in εNd values for suprasubduction zone rocks in New England overlaps with Laurentian and Gondwanan crust. No one simple model for formation of arc terranes in the New England Appalachians can be applied along-strike. The location of rocks with

εNd values reflecting Gondwanan components along the RHZ and rocks reflecting

Laurentian components to the east may be from tectonic shuffling following the Taconic orogeny.

ACKNOWLEDGEMENTS

I would like to acknowledge the enormous support and guidance given by my advisor, Dr. Craig Dietsch of the University of Cincinnati, over the course of these two years. I would also like to express my gratitude for the other members of my committee,

Dr. Jonathon Kim of the Vermont Geological Survey and Dr. Elizabeth Widom of Miami

University, both of whom offered not only their insight and assistance throughout my project, but also some eye-opening discussions. Thanks is also due to Dr. Raymond Coish of Middlebury College who provided samples, information, direction, and essential advice that helped me from start to finish on this project. I would like to acknowledge Dr.

Barry Maynard of the University of Cincinnati for assistance in obtaining XRF trace element values for my samples as well as his advice in directions to follow for analysis.

My research was also made more efficient through the assistance of Nick Bose of the

University of Cincinnati and Fara Rosoazanamparany of Miami University during field work and lab work, respectively. I would also like to express appreciation for the assistance given by Dave Kuentz of Miami University who was a fountain of knowledge and advice during TIMS analysis. Finally, I would like to thank the Geological Society of

America, Vermont Geological Society, and University of Cincinnati for funding of the project as well as the University of Cincinnati and Miami University for use of the lab facilities.

TABLE OF CONTENTS

ABSTRACT ...... 2

ACKNOWLEDGEMENTS ...... 5

TABLE OF CONTENTS ...... 6

Chapter 1. INTRODUCTION ...... 7

Overview ...... 7 The evolution of tectonic models of pre-Silurian western New England ...... 11

2. UNIT DESCRIPTIONS ...... 15

Overview ...... 15 Units of northern Vermont ...... 16 Moretown Formation ...... 16 Cram Hill Formation ...... 17 Mount Norris Intrusive Suite ...... 18 Units of southern Vermont ...... 19 Barnard Gneiss ...... 19 North River Igneous Suite ...... 20 Units of western Massachusetts ...... 21 Hawley Formation ...... 21 Orthogneisses and amphibolites of the Shelburne Falls dome ...... 22 Units of central Massachusetts ...... 23 Ammonoosuc Volcanics ...... 23 Units of western Connecticut ...... 23 Collinsville Formation ...... 23 Units of eastern Connecticut ...... 24 Killingsworth dome ...... 24

3. RESULTS ...... 26

Overview ...... 26 Whole-rock Geochemistry ...... 27 Nd Isotopic Analysis ...... 31

4. DISCUSSION ...... 33

Overview ...... 33

Geochemical Discussion ...... 33 Isotopic Analysis Discussion ...... 38

5. CONCLUSION ...... 40

REFERENCES ...... 44

FIGURES ...... 54

TABLES ...... 78

APPENDICES ...... 101

Appendix A – Analytical Background...... 101 Geochemistry ...... 101 Isotopes and radioactivity ...... 104 Nd isotope ratios ...... 105 Continental crust extraction ...... 107 Mass Spectrometry ...... 109 Appendix B – Methods ...... 111 Appendix A – Sample preparation for analyses ...... 113 Preparation for XRF analysis ...... 113 Preparation for Nd isotopic analysis on TIMS ...... 113

INTRODUCTION

Overview

The bedrock geology of the New England Appalachians tells a complex story of how rock sequences formed, were deformed, metamorphosed, and intruded through the processes of the opening and closing of the Iapetus Ocean. Opening of Iapetus was part of the breakup of the Late Neoproterozoic supercontinent Rodinia and by the end of the

Cambrian, the continents Laurentia and Gondwana were separated by a wide ocean

(Scotese and McKerrow, 1991; Williams and Hiscott, 1987). Closing of Iapetus occurred through a complex array of subduction zones and at least four orogenic events that in some areas across the region partially overlapped: Taconian (Middle to Late Ordovician),

Salinic (Early Silurian), Acadian (Middle to Late Devonian), and Alleghanian (Middle

Carboniferous to Middle Permian) spanning the Paleozoic. Evidence of individual mountain-building events is, in places, difficult to pinpoint as a result of overprinting metamorphism and superimposed deformation. In conjunction with this, New England has been deeply eroded and bedrock exposure is limited.

There is an unresolved debate resulting from previous studies centered on three aspects of New England’s paleogeography and pre-Silurian tectonic evolution: 1) the number of arcs in the Iapetus Ocean, 2) the origin of these arcs; that is, whether they developed over east- or west-dipping subduction zones, and whether they were adjacent to Laurentia or Gondwana, and 3) the collision history of the arcs. An early, long- standing model was proposed by Stanley and Ratcliffe (1985) which describes a single arc formed above an east-dipping subduction zone off shore of Laurentia which collided with the Laurentian margin to produce the Middle Ordovician Taconic orogeny. A

second model (Karabinos et al., 1998) details the presence of two island arcs proximal to

Laurentia, which were created from two separate subduction zones and resulted in two separate collisions with the continent during the Middle and Late Ordovican. A third model (van Staal et al., 2001), based primarily on Newfoundland geology, calls for one or more peri-Laurentian arcs and one or more arcs on the opposite side of the ocean, adjacent to Gondwana (so-called peri-Gondwanan arcs) with arc accretion occurring from the Middle Ordovician through the Early to Middle Silurian.

Within western New England, a belt of mainly lower Paleozoic (pre-Silurian) suprasubduction zone volcanics, arc-related intrusive rocks, forearc sediments, and subduction complex rocks extends from Long Island Sound north to Québec, and continues northeastwards across Newfoundland (Fig. 1). This belt, the Rowe-Hawley

Zone (RHZ; Hibbard et al., 2007; Stanley and Hatch, 1988; Stanley and Ratcliffe, 1985), is a window into the evolution of the Iapetus Ocean and the Taconic orogeny during the

Ordovician. The RHZ includes the Collinsville Formation of western Connecticut, the

Hawley Formation of western Massachusetts, and the Barnard Gneiss, Mount Norris

Intrusive Suite, and North River Igneous Suite of Vermont (Fig. 2). Each of these meta- igneous units is associated with sedimentary units including the Sweetheart Mountain

Member of the Collinsville Formation in Connecticut, meta-sedimentary rocks mapped as

Hawley Formation in Massachusetts (mapped as Ohb by Hatch and Hartshorn (1968) on the USGS map of the Heath Quadrangle), and the Cram Hill and Moretown Formations in Vermont.

There has been significant study of the RHZ and additional models have been proposed to interpret the tectonic processes involved in formation of pre-Silurian western

New England (Chocyk-Jaminski and Dietsch, 2002; Kim and Jacobi, 2002, Aleinikoff et al., 2007; Dorais et al., 2008; Dorais et al., 2011; Coish et al., 2012), all still centered on the evolution of one or more suprasubduction zone volcanic arcs and the collision of this arc terrane (or terranes) with the Laurentian margin. Nevertheless, outstanding problems remain. Of major consideration is the lack of a clear-cut, widely accepted interpretation of the paleogeography, terrane affinity, and along-strike correlation of major map units of the RHZ on this western side of the orogen. For example, map units of the RHZ principally composed of meta-basalt have been interpreted to have formed in forearc

(Kim and Jacobi, 1996) and backarc (Chocky-Jaminski and Dietsch, 2002) settings: is it possible to correlate or differentiate suprasubduction zone settings along strike? Since the terrane affinity (that is, Laurentian, Gondwanan, or intra-Iapetan) of island arc-related orthogneisses cannot be made solely on the basis of igneous crystallization ages

(Zartman, 1988), can isotope data be used to identify crustal sources and so constrain paleogeography?

The focus of my study is to further understanding the evolution of the Taconian orogeny and to test tectonic models that describe the origin and evolution of a pre-

Silurian island arc (or arcs) in western New England (in Vermont, western

Massachusetts, and western Connecticut). Here, the suprasubduction zone rocks of the

Rowe-Hawley Zone (RHZ), a belt of meta-sedimentary and meta-igneous rocks formed as part of a forearc, island arc, and backarc terrane were accreted onto Laurentian continent during the Taconian orogeny. The geochemistry and Nd isotopes of mafic meta-volcanic rocks and dikes of the RHZ have been analyzed to test proposed along- strike correlations of major rock units, compare proposed paleo-tectonic settings, and

characterize magmatic source rocks. The use of geochemical analysis also provides a means to reconstruct paleogeography and test tectonic models of island arc formation as well as island arc-continental collision.

The evolution of tectonic models of pre-Silurian western New England

It is now widely accepted that during the Paleozoic, the New England

Appalachians developed through multiple accretionary events during the closure of

Iapetus. The late Precambrian (600-550 Ma) breakup of Rodinia that created Iapetus and left two large continents, Gondwana and Laurentia, on opposite sides of the ocean

(Scotese and McKerrow, 1991; Williams and Hiscott, 1987) has been further developed: the rifting created a passive margin on Laurentia which developed in two stages, initial rifting at ~570 Ma, opened the Iapetus Ocean and a second event at ~540-535 Ma rifted a continental block (a ribbon continent) from Laurentia to form the Taconian Seaway. The subsequent evolution of subduction zones, the development of arc complexes, and accretionary events that record the closing of the Iapetus have been debated as models of paleogeography and tectonics have been developed and countered throughout the literature. The models most applicable to western New England are described here.

Evidence that Laurentia and Gondwana began to move towards each other marking the end of growth of Iapetus is given by a passive margin on the Laurentian side becoming a subduction zone during the Ordovician (490-445 Ma). In early developed models, an oceanic island arc developed above an east-dipping subduction zone (Scotese and McKerrow, 1991) offshore of Laurentia and the westwards-drifting overriding plate brought the island arc to the Laurentian margin (McKerrow et al., 1991) where it

collided, initiating the Taconic orogeny, the first phase of collisional mountain building within the New England Appalachians. Subsequent models recognized older ophiolite obduction onto the Laurentian margin, ascribed to the closure of the Taconian Seaway as a west-facing subduction zone developed along the margin of the rifted continental block known as Dashwoods. The evidence for Dashwoods in New England is not yet compelling but Early Cambrian quartzo-feldspathic meta-sedimentary units could be eroded portion of Dashwoods (Karabinos, 2013).

A long-standing tectonic model for the Taconic orogeny in New England described the origin of the Bronson Hill arc above an east-dipping subduction zone off shore of Laurentia and a Middle Ordovician collision of this arc with the Laurentian margin. High precision zircon U-Pb ages of orthogneisses and meta-volcanics of the

Bronson Hill arc that ranged from 455 Ma to as young as 440 Ma (Tucker and

Robinson, 1996) cast doubt on this model since igneous rocks of the Bronson Hill arc were the same age or younger than the emplacement age of the Taconic allochthons, one of the classic constraints on the timing of the Taconic orogeny. Plutonic rocks with arc geochemistry exposed in the Shelburne Falls domes of western Massachusetts were subsequently dated (although principally with the Pb evaporation technique) and yielded older zircon ages (475 Ma) that would allow a Middle Ordovician arc-continent Taconic collision. Thus a second leading tectonic model for the pre-Silurian evolution of western

New England describes the pre-Middle Ordovician evolution of one arc, the Shelburne

Falls Arc (SFA), forming over an east-dipping subduction zone adjacent to Laurentia.

Collision of the SFA with the passive margin of Laurentia produced the Taconian orogeny. The Bronson Hill arc could be viewed as entirely separate from the SFA — with

even a peri-Gondwanan origin — or alternatively, as a younger component of a composite, peri-Laurentian arc whose collision with the Laurentian margin occurred in the Late, not Middle, Ordovician (Ratcliffe et al., 1998). The SFA has been identified as part of the RHZ within Massachusetts but it has not been directly correlated along-strike into western Connecticut. In Vermont, amphibolites and intermediate orthogneisses of the

North River Igneous Suite (NRIS) and the Barnard Gneiss (BG) have been considered to be part of the SFA by Karabinos et al. (1998).

Rocks of the RHZ that extend into the Newfoundland Appalachians have been extensively studied and a comprehensive model has been formulated to describe the tectonic evolution of this part of the orogeny where the grade of metamorphism is much lower than in New England and the orogeny is not as deeply eroded. This model, developed by van Staal et al. (2009), involves three Taconian phases (labeled Taconic 1,

2, and 3). Taconic 1 describes west-dipping subduction beneath the Lushs Bight oceanic tract (LBOT) culminating in obduction of Lushs Bight oceanic crust onto the peri-

Laurentian Dashwoods microcontinent at c. 495 Ma followed by a flip in subduction polarity eastwards beneath Dashwood. Taconic 2 consists of the first principle Middle

Ordovician arc-continent collisional event where the Laurentian continental margin is subducted eastwards beneath Dashwood. Taconic 2 is followed by west-dipping subduction outboard of accreted Dashwoods and finally, Taconic 3 records the collision of another island arc terrane to the now composite Laurentian margin. A similar multi- phase tectonic model for the Taconic orogeny has not been developed for New England.

In western New England, the origin and tectonic evolution of the RHZ – that is, the rock units of the pre-Silurian island arc complex – is a main determinant to resolve

the island arc debate. That is, does the RHZ represent only one island arc complex? Did arc and backarc igneous rocks of the RHZ form above an east- or west-dipping subduction zone? Was this arc (or arcs) peri-Laurentian or peri-Gondwanan? Processes that form island arc rocks leave distinct minor and trace element chemical signatures and so the tectonic setting of island arc rocks can be determined from their chemical analysis.

Moreover, volcanic and plutonic rocks formed adjacent to Laurentia versus those formed adjacent to Gondwana have different isotopic chemical signatures (Dorais et al., 2011).

Detailed analysis of island arc rocks of the RHZ can be used to distinguish between the different tectonic models of their formation.

There are three specific hypotheses that will be tested using geochemical and Nd isotopic data of this study: 1) the units of the RHZ in western New England constitute a single island arc and may, in fact, be a part of a composite arc that includes the Bronson

Hill Arc (data gathered from Hollocher et al., 2002) located east of the overlying Siluro-

Devonian sediments of central New England; 2) the multi-phase model created for the

Newfoundland Appalachians cannot, in fact, be stretched to the New England region, though some units may correlate; and 3) the best model for the formation of the island arc prior to accretion onto the Laurentian margin involves a peri-Laurentian setting.

UNIT DESCRIPTIONS

Overview

Analysis and interpretation of the Rowe-Hawley Zone (RHZ) units of western

New England have accelerated since the 1970s when the theory of plate tectonics first formed the framework for understanding the geologic evolution of mountain belts.

Studies of the RHZ including mapping the region at a variety of scales, analyzing their structural and metamorphic histories, dating of the meta-igneous rocks as well as analyses of their geochemistry, and, as described above, formulation of tectonic models.

Seminal papers on the rocks of the RHZ have established that the zone belongs to the

Iapetian realm separate from Laurentia and that the rocks of the RHZ were formed in a supra-subduction zone setting. These studies include those of Hatch and Stanley (1973;

Stanley and Hatch, 1988; Hatch and Stanley, 1988) who made the first comprehensive along-strike regional correlation of map units; Zen et al. (1983) who compiled a new bedrock geologic map of Massachusetts which delineated major regional faults bounding the RHZ; Stanley et al. (1984) who proposed that units of the RHZ containing slivers and small bodies of ultramafic rock formed as an accretionary prism; Laird et al. (1984) who discovered and dated high-pressure U-Pb zircon ages of Middle and Upper Ordovician for rocks of the Bronson Hill Arc east of the RHZ, effectively opening the door to consideration that rocks of the RHZ might include a separate, older arc; and Karabinos et al. (1995) who presented geochemical and age data to support the existence of such an arc in the RHZ (however, a comprehensive geologic argument against the existence of the Shelburne Falls Arc was developed by Ratcliffe et al. (1998). Summary descriptions of the rocks of the RHZ are given by Kim et al. (2003) for northern Vermont, by Kim and

Jacobi (1996) for western Massachusetts, and Dietsch et al. (2010) for western

Connecticut.

In this section, I present basic descriptions of the map units containing the meta- igneous rocks whose geochemistry I compiled and whose Nd isotopic ratios I analyzed.

The descriptions are organized geographically from north to south and the primary references for each unit are given at the beginning of each description. Units in Vermont are referenced to the recently published state geologic map of Vermont (Ratcliffe et al.,

2011).

Units of northern Vermont

Moretown Formation (Armstrong, 1994; Ratcliffe and Armstrong, 1998; Kim and Jacobi,

1996)

The Moretown Formation is composed of both meta-sedimentary and meta- igneous units. Structurally overlying the Rowe Formation, Moretown consists of various rock types including quartz-plagioclase granofels, laminated quartzite, schist, meta- diorite, and amphibolite. The lower to middle parts are made up of three specific units: 1) dark- and light-gray granofels, interbedded, well laminated quartzite (Omfp), 2) quartz- plagioclase-garnet schist (Omb and Omfs), and 3) dark green, well laminated mylonitic schist seen along the lower contact with the Rowe Formation (Oms). The upper portion of the Moretown is composed mostly of a plagioclase- and hornblende-rich granofels considered to be of volcanic origin (Omfg). The most prominent unit of the Moretown is a pinstriped granofels rock (Oml) which transitions into garnet schist and granofels

(Omgs). Interbedded with units Oml and Omgs is a biotite schist belt (Ombs) that is associated with masses of serpentinite and talc in varied sizes (OZu).

Generally speaking, amphibolites found near the contact of the Hawley Formation and Moretown are black to green, medium-grained, and composed of the assemblage hornblende-plagioclase-quartz-epidote ± garnet ± chlorite. These rocks are found boudinaged wihin meta-sediments of the Moretown on a local scale, with on another.

Fabrics found in the Moretown Formation are similar to those found in the underlying Rowe Formation. The dominant, regional schistosity, S2, found in the unit is considered to be a Late Ordovician, Taconian fabric, axial planar to F2 reclined, isoclinal folds and to open, west-verging to upright folds where the intensity of D2 is diminished.

The S2 schistosity is warped to strongly folded by up to four phases of superimposed

Acadian deformation

The age of the Moretown is constrained in southern Vermont by U-Pb zircon ages of 496 ± 8 Ma and 486 ± 3 Ma of a trondhjemitic and tonalitic orthogneiss, respectively, which intrude the Moretown (Ratcliffe et al., 1997).

Cram Hill Formation (Armstrong, 1994; Ratcliffe and Armstrong, 1998)

The Cram Hill Formation is composed of two rock types layered together in non- uniform sequences: 1) dark-gray to black, rusty-weathering sulfidic schist and 2) dark- gray quartzite up to 1 m thick in some layers. Throughout the type locality, the sulfidic schist of the Cram Hill is interlayered with various other rock types. For instance, in southern Vermont, the formation contains 1-10 cm thick layers of sandy schist and gray to black quartzite that are in sharp contact with the surrounding schist and commonly

contain graded beds and cross-bedding. Also, the lower contact of the Cram Hill within the northeastern Brattleboro quadrangle reveals the presence of a single horizon of quartz pebble conglomerate. The conglomerate consists of pebbles ranging from 1mm to 1.5 cm contained in a matrix of tan- to brown-weathering, fine-grained quartz-plagioclase- muscovite-chlorite schist with traces of opaques.

The Marlboro member of the Cram Hill Formation (Ocm) is composed of tan- to rusty-weathered, dark- to light-gray hornblende-garnet-chlorite-muscovite-plagioclase- quartz gneiss, muscovite-chlorite-plagioclase-quartz granofels, and hornblende- plagioclase gneiss well-bedded in layers throughout the unit.

Ratcliffe and Armstrong (1998) refer to the Cram Hill Formation as equivalent to

Hawley Formation meta-sediments and meta-igneous units in western Massachusetts, previousle mapped as both Ohb of Hatch and Hartschorn (1968; Stanley and Hatch,

1988) and Sanders Brook black slates of Kim and Jacobi (1996). Parts of the Cram Hill

Formation were also correlated along-strike with the St. Daniels Melange. The age of the

Cram Hill Formation in southern Vermont is constrained by a U-Pb zircon age of 484 ± 4

Ma from a felsic meta-volcanic layer (Ratcliffe et al., 1997).

Mount Norris Intrusive Suite (Kim et al., 2003)

Meta-diabase dikes within the Moretown and Stowe Formations in northern

Vermont make up the Mount Norris Intrusive Suite (MNIS). These dikes form boudins throughout these formation oriented parallel to the dominant regional S2 foliation but the rocks of the MNIS unit are recognized as intrusive dikes based on the presence of chilled margins, rare xenoliths of meta-sedimentary lithologies, and sharp contacts with the

surrounding units. These dikes are gray, massive to weakly foliated rocks, rounded, and granular. They contain distinct buff-colored weathering rings and some have plagioclase phenocrysts. An ophitic texture is prominent as intergrowths of albite and actinolite, potential pseudomorphs of plagioclase and clinopyroxene. The general mineral assemblage is albite-actinolite-epidote-chlorite-calcite-quartz which records greenschist facies metamorphism.

Kim et al., 2003 summarized the geologic evidence bearing on the age of the

MNIS, including correlation of the MNIS with the Bolton Igneous Group of southern

Québec and the Coburn Hill Volcanics of northernmost Vermont and concluded that it is pre-Late Ordovician.

Units of southern Vermont

Barnard Gneiss (Ratcliffe et al., 1997; Ratcliffe and Armstrong, 1998)

The Barnard Formation of Richardson (1924) in southern Vermont is composed of plagioclase-rich felsic and intermediate orthogneisses, amphibolites, hornblende- plagioclase gneisses, and mafic dikes. In its type locality near Ludlow, Vermont, much of the Barnard Gneiss is intrusive and cross-cuts various units of the Moretown Formation.

On the 1961 bedrock geologic map of Vermont (Doll et al., 1961), these rocks are called the Barnard Volcanic Member of the Missisquoi Formation and are located east of the

Chester and Athens domes. On the 1961 map, the Barnard Volcanic Member can be traced southwards into western Massachusetts where it can be correlated with parts of the

Hawley Formation. Intrusive mafic, intermediate, and felsic orthogneisses of the old

Barnard Volcanic Member are now referred to as the North River Igneous Suite (see

below). The Barnard Gneiss now refers to orthogneisses and volcanic rocks west of the

Chester and Athens domes that can be traced northwards to their original type locality.

Ratcliffe et al. (1997) reported a U-Pb zircon age of 496 ± 8 Ma of a meta- trondjemite from the Barnard Gneiss.

The new state geologic map of Vermont (Ratcliffe et al., 2011) describes the

Barnard Gneiss as a light-gray to whitish-weathering, massive to gneissic tonalite of hornblende-biotite mineralogy and trondhjemite of biotite-muscovite-quartz-plagioclase mineralogy. Instances of rare hornblendite, metadiabase, and metapyroxenite occur as small stocks, inclusions, and dikes. U-Pb SHRIMP ages have been determined in two studies: 471.4 ± 3.7 Ma (Karabinos et al., 1998) and 496 ± 8 Ma (Aleinikoff et al., 2011).

North River Igneous Suite (Armstrong, 1994; Ratcliffe et al., 1997; Ratcliffe and

Armstrong, 1998)

The North River Igneous Suite (NRIS) includes meta-igneous rocks shown of the

1961 state geologic map of Vermont (Doll et al., 1961) as the Barnard Volcanic Member of the Missisquoi Formation east of the Chester and Athens domes. Included in the NRIS are dikes that cross-cut the Moretown and Cram Hill Formations. The lower and middle portions of the NRIS display well-layered chlorite-biotite-muscovite-garnet-quartz- plagioclase gneisses (SOnb) considered of volcanic origin. The NRIS cannot be physically traced into the Hawley Formation of western Massachusetts and so has not been correlated with the Hawley. Felsic orthogneisses of the NRIS crosscut layering within Moretown and Cram Hill Formations; two of these orthogneisses yielded U-Pb zircon crystallization ages of 496 ± 8 Ma and 486 ± 3 Ma (Ratcliffe et al., 1997).

Units of western Massachusetts

Hawley Formation (Kim & Jacobi, 1996)

In northwestern Massachusetts, the Hawley Formation is composed of amphibolites, black sulfidic schists, and minor plagioclase-quartz gneisses. Individually mapped members of the Hawley include the following units. The Warfield Mountain

Amphibolite (WMA), is a fine-grained, grayish-green hornblende-plagioclase-chlorite- epidote-quartz-Fe carbonate ± garnet schist. The WMA is fairly homogeneous and thinly foliated, displaying little or no compositional layering. Occurring locally within the

WMA are pillows, tuffs (some displaying cross-bedding), and tuff breccias.

Felsic meta-igneous units within the Hawley, include the Dell Metatrondhjemite, the Legate Hill Brooke Metadacite, and thin metatrondhjemite layers. The plutonic Dell

Metatrondhjemite is a white to light-gray, medium- to coarse-grained quartz-plagioclase- biotite-chlorite ± garnet schistose gneiss. The volcanic Legate Hill Brooke Metadacite

(LHB) is a fine-grained, schistose to massive, white to light gray quartz-plagioclase- hornblende-garnet-chlorite-apatite-metatrondhjemite that displays conspicuous quartz rims around garnet. Many thin (0.5-5m) discontinuous metatrondhjemite layers consisting of fine-grained schistose to massive quartz-plagioclase-hornblende-garnet- chlorite rocks with the quartz rims around garnets occur throughout the Hawley

Formation.

The Hawley Formation was given a minimum age of 462 ± 6 Ma (Ratcliffe et al.,

1997) based on a U-Pb zircon age of trondhjemitic intrusives that allegedly cross-cut the

Barnard Gneiss in southern Vermont. However, Aleinikoff et al. (2011) calculated the previous U-Pb zircon age and obtained an age of 502 ± 4 Ma. In addition, this rock has

been reinterpreted as a felsic volcanic layer within rocks correlative with the Hawley

Formation in western Massachusetts (Kim, pers. comm., 2013) and thus more accurately constrains the formation age of the Hawley.

Also included in the Hawley Formation is an intrusive mafic meta-igneous unit, the Charlemont Mafic Intrusive Suite (CMIS). Mapped as massive-weathering, granular, medium-grained, plagioclase-phenocryst bearing sills with chilled margins, the CMIS intrudes all units of the eastern portion of the Hawley Formation, including the Dell

Metatrondhjemite. Rocks of the CMIS are hornblende-plagioclase-epidote-quartz-Fe carbonate ± chlorite amphibolites with a foliation coplanar to the dominant regional foliation.

Orthogneisses and amphibolites of the Shelburne Falls dome (Kim & Jacobi, 1996)

Felsic gneisses and amphibolites exposed in the core of the Shelburne Falls dome located in northwestern Massachusetts are considered correlative with the rest of the

Hawley Formation (Hatch and Hartschorn, 1968; Hatch and Stanley, 1988).

Trondhjemitic orthogneisses of the Shelburne Falls dome are medium- to coarse-grained, quartz-plagioclase-biotite gneisses displaying sharp contacts with thin layers of amphibolite. Xenoliths of metatrondhjemite occur in amphibolite providing evidence that the amphibolites are intrusive. Karabinos et al. (1998) presented U-Pb zircon data of two metatonalites from the core of the Shelburne Falls dome. These data are slightly discordant but crystallization ages are best estimated as 475 ± 1.4 and 473 ± 1.5 Ma.

Units from central Massachusetts

Ammonoosuc Volcanics (Dorais et al., 2011; Hollocher et al., 2002)

The Ammonoosuc volcanics vary up to 1200 m in thickness. Containing its own stratigraphy, the Ammonoosuc is composed of a lower member of amphibolites, a middle member of garnet quartzite, and an upper, felsic gneiss member. Interbedded layers of calc-silicate rock, marble, and other sedimentary and volcaniclastic appear in minor amounts throughout the unit. Overlying the Ammonoosuc volcanics is the rusty weathering pelitic schist of the Partridge Formation. U-Pb zircon ages gives a crystallization age of 453 ± 2 Ma for the upper member of the Ammonoosuc and 449 +3/-

2 Ma for the overlying Partridge Formation (Tucker and Robinson, 1995; Schumacher,

1988).

Rocks of the Bronson Hill Arc (BHA) are now widely regarded as having a peri-

Gondwanan origin (van Staal et al., 1998; Hibbard et al., 2006) and the Ammonoosuc volcanics form the major meta-volcanic unit of the arc overlying the intermediate orthogneiss units exposed along its length.

Units of western Connecticut

Collinsville Formation (Hatch and Stanley, 1988; Stanley and Hatch, 1988; Chocyk-

Jaminski & Dietsch, 2002)

Amphibolites interlayered with lesser amounts of politic schist occurring beneath

Silurian-Devonian rocks across the RHZ in western Connecticut have been mapped as the

Collinsville Formation. The Collinsville Formation forms the core rocks of the

Collinsville, Granville, and Granby domes and also occurs as part of a nappe that overlies

the core rocks of the Waterbury dome. In the domes, Collinsville Formation amphibolites occur with felsic orthogneisses and are overlain by feldspathic garnet ± kyanite ± staurolite schist and plagioclase-quartz granofels which also contains boudins of amphibolites; these schists and granofels have been mapped as the Sweetheart Mountain

Member of the Collinsville Formation. In the Bristol dome, the Collinsville Formation overlies the Taine Mountain Formation which has been correlated with the Moretown

Formation of western Massachusetts. Beneath the Taine Mountain Formation in the

Bristol dome is the Bristol Gneiss, composed of plagioclase-quartz granofels and gneiss as well as interlayered garnet-biotite schist with amphibolite and hornblende gneiss. The

Bristol Gneiss may correlate with the NRIS of southern Vermont.

In the RHZ of western Connecticut west of the domes, the Collinsville Formation is cut by granodiorite of the Newtown Gneiss which has U-Pb zircon crystallization ages of 432 ± 2 and 438 ± 2 Ma (Sevigny and Hanson, 1995). Determining more accurate constraints of the crystallization age of the Collinsville Formation amphibolites remains a pressing problem.

Units from eastern Connecticut

Killingworth dome (Aleinikoff et al., 2007)

The Killingworth dome is located immediately east of the Hartford Basin in eastern Connecticut and is the southern-most dome of the Bronson Hill Arc (BHA), which exposes calc-alkaline orthogneisses in a belt extending throughout central New

England. Though previously considered as part of the suite of intermediate intrusive rocks of the BHA, more recent geochemical and isotopic analyses as well as a lack of

Acadian overprinting have both led to differentiation of rocks of the Killingworth dome from other BHA dome rocks. Petrologic and geochronologic evidence points towards an origin of the othrogneisses exposed in the Killingworth dome involving high grade metamorphism, partial melting, and ductile faulting during the Carboniferous. As a consequence, their tectonic affinity is now considered to be peri-Gondwanan.

Core rocks of the Killingworth dome consist of orthogneisses sandwiched between metasediments of the Central Maine and Merrimack terranes to the east and

Hartford Basin Mesozoic arkoses to the west, with a southern leg of Neoproterozoic orthogneisses and paragneisses. Killingworth orthogneisses are plagioclase- and quartz- rich with smaller abundances of biotite, hornblende, K-feldspar, magnetite, and traces of garnet. The gneisses have been divided into distinct units and only one, a unit in the eastern-most portion of the dome (the Turkey Hill belt of Monson Gneiss) is correlated with BHA rocks of north-central Connecticut and Massachusetts. More recent geochemical and isotopic data has resulted in the recognition of two separate intrusive complexes, both with crystallization ages between 455 and 465 Ma. In the core of the dome is a younger orthogneiss unit that has a Mississippian crystallization age of 339 ± 3

Ma. Structurally below and in fault contact with Ordovician orthogneiss in the core of the

Killingworth dome is the representative Gondwanan basement; the Clinton Granite

Gneiss which first crystallized at ~605 Ma.

RESULTS

Overview

The first portion of this study involves compiling and comparing geochemical data from previous studies as well as new data generated from samples of the Barnard gneiss collected and analyzed during this project. The second portion involves Nd isotopic analysis of 21 samples selected from the compilation. The following set of figures and their descriptions were created using data gathered from studies completed by

Kim et al. (2003), Chocyk-Jaminski and Dietsch (2002), Kim and Jacobi (2002), Ratcliffe and Armstrong (1995), Kim (unpublished data), and Dietsch (unpublished data). This chapter is split into two sections: whole-rock geochemistry and Nd isotopic analysis. Not all figures have the complete set of samples as not every sample was analyzed for all elements.

Samples were compiled from published studies of the area; Collinsville

Formation, Hawley Formation, and Mount Norris Intrusive Suites were collected from the above mentioned sources and can be found in Table 1. Barnard Gneiss samples were analyzed by XRF and their values are also included in Table 1. For other data (mentioned in the Discussion section later), see sources.

A description of trace element geochemistry and the Sm-Nd system and their use in tectonic discrimination of igneous rocks is presented in Appendix A. A detailed description of the data compilation, geochemical analysis of samples from the Barnard

Gneiss, and analysis of Nd isotopes is presented in Appendix B. Sample preparation for x-ray fluorescence (XRF) mass spectrometry and thermal ionization mass spectrometry

(TIMS) is presented in Appendix C.

Whole-rock Geochemistry

The meta-igneous rocks of the RHZ were classified using Winchester and Floyd’s

(1977) igneous rock classification plot based on Nb/Y versus Zr/TiO2 (Fig. 8). Samples within the RHZ plot in the fields of andesite, andesitic basalt, and subalkaline basalt with a smaller number of samples in the rhyodacite/dacite and alkaline basalt fields. There is significant overlap in the compositions of the five units that are plotted, but Hawley

Formation meta-basalts have the lowest Zr/TiO2 in the subalkaline and alkaline basalt fields and Hawley Formation meta-felsic rocks have the highest Zr/TiO2 in the rhyodacite/dacite field. There is no overlap between these Hawley samples and rocks of the MNIS. Most of the meta-basalts of the Collinsville Formation plot in the field of andesite/basalt.

Shervais (1982) formulated a Ti-V binary diagram to separate fields of arc tholeiite, MORB, and alkalic basalt, and meta-basalts with Ti/V ratios < 10, boninites and low-Ti arc tholeiites (Fig. 9). Meta-basalts within the Barnard Gneiss and Hawley

Formation are distinctly low in Ti, contrasting with meta-basalts found in the MNIS and

Collinsville Formation; however, a small cluster of low-Ti Collinsville meta-basalts does overlap the low-Ti Hawley samples. Meta-basalts of the MNIS follow a consistent trend within the MORB field, with a few samples plotting below Ti/V of 20 and two high-Ti samples located in the alkali basalt field. Collinsville Formation meta-basalts extend across the diagram, covering the full range of Ti values and maintaining V values from

150 to 550 ppm.

Figure 10 is a Ti-Zr binary plot developed by Pearce and Cann (1973) to separate the fields of ocean floor basalt (OFB), low-potassium tholeiite (LKT), and calc-alkaline

basalt (CAB) that allows rocks of the North River Intrusive Suite (NRIS) to be compared to the other units. MNIS and NRIS meta-basalts overlap, except for one low-Ti sample, in the low-K tholeiite and ocean floor basalt fields as they follow the same positive trend as meta-basalts from the Collinsville Formation. The lowest Ti and Zr samples from this trend are set apart from the low-Ti samples of the Hawley Formation, Barnard Gneiss, and Collinsville Formation; these specific low-Ti samples have Zr values less than 50 ppm. Another set of low-Ti samples of the same three units (including one sample of

NRIS) maintain Zr values between 50 and 100 ppm. Throughout all units, there is an absence of samples plotting within the calc-alkaline basalt field with the exception of two samples (one from MNIS and one from the Collinsville Formation), both plotting adjacent to the upper left boundary.

Pearce and Norry (1979) compared Zr and Zr/Y to differentiate among the fields of within-plate basalt (WPB), oceanic island arc basalt (IAB), and MORB (Fig. 11). This diagram also effectively discriminates between basalts produced in oceanic arcs (lower

Zr/Y) from those produced at continental margins (higher Zr/Y); continental arcs plot towards the field of within plate basalt as both are produced from enriched sources

(Pearce, 1983). Here, MNIS and NRIS samples cluster together within the MORB field with slight extension into the overlapping IAB field as well as the extent of the WPB region. HF and BG samples plot on the left lateral exterior of the IAB field, though a few samples plot within the MORB-IAB overlapping region. Once more, CF samples cover all fields and envelop all other sample groups. As in Figure 9, samples with the lowest Zr include meta-basalts of the Hawley, Barnard, and a subset of Collinsville. Among these samples are the lowest Zr/Y ratios, which are found in Hawley and Collinsville meta-

basalts. Samples with the highest Zr/Y ratios, and mostly plotting in the within-plate basalt field, include all but one of the NRIS samples, two samples of the MNIS, and a subset of Collinsville samples. Most samples of the MNIS and Collinsville plot within the MORB field.

Meta-basalts of the MNIS, NRIS, Hawley Formation, and Collinsville Formation include samples that were analyzed for rare earth elements as well as trace elements allowing these samples to be plotted on MORB-normalized (“spider”) diagrams (Figs. 12 and 18), which were described in detail by Pearce (1982) and are described above. All sample groups have the overall characteristic pattern of a negative slope but there are important differences among the units even though there is considerable overlap. The spider diagrams of each unit (Fig. 12) show that the MNIS samples form a tight pattern characterized by enrichment in Th up to ten times MORB, negative Nb values, and a mostly flat HFSE pattern with the majority of values at slightly less than one. A subset of four NRIS samples closely follow this pattern, but three others are significantly different: one highly enriched in LILE as well as Zr, Hf, and Sm; one enriched in Th and Ta, and depleted in P and Ti; and one strongly depleted in HFSE. Meta-basalt samples from the

Hawley Formation can be characterized by distinctive features: they are depleted in both

Ce and, to a lesser extent, Ti (with one sample more strongly depleted in Ti). Two

Hawley samples are enriched in LILE but all of them are strongly depleted in Zr, Hf, Sm,

Ti, Y, and Yb. Meta-basalts of the Collinsville Formation show a wide range of patterns, though all are enriched in LILE. Several samples are depleted in Th, and most are enriched in Ta and slightly depleted in Nb. Nearly two-thirds are strongly depleted in Ce through Yb, similar to the Hawley Formation meta-basalts, but without enrichment in P.

Pearce (1983; Kim et al., 2003; Chocyk-Jaminski and Dietsch, 2002) developed a bivariate diagram comparing Ta/Yb to Th/Yb in order to show basalt that has subduction zone enrichment specifically related to a sediment melt component and/or has been contaminated by continental crust — both of which increase Th — comparing MORB and within plate basalt unaffected by subduction-related processes that plot along a well- defined trend with a positive slope of one (Fig. 13). In this plot, MNIS samples plot above the MORB-within plate basalt trend range as do a subset of Hawley and

Collinsville Formation samples. Almost half of the Collinsville samples do plot with the

MORB/within plate basalt trend as do two Hawley samples with another two Hawley samples plotting along the trend’s upper boundary.

Another diagram was developed to illustrate the influence of subduction zone enrichment and fractional crystallization on basalts of N- and E-MORB chemistry through comparison of Nb/Yb and Th/Yb (Fig. 14; Pearce, 2008). Collinsville Formation has the lowest Th/Yb ratios (located within the MORB field), but ranges up to overlap

Hawley Formation and MNIS meta-basalts above the upper boundary of the MORB field.

Hawley Formation meta-basalts are spread out with all but four samples above the

MORB field, but no higher than a Th/Yb ratio of 2. MNIS meta-basalts cluster together at just under Th/Yb of 1 and just above Nb/Yb of 1 with two outliers (one at low Nb/Yb and

Th/Yb as well as one opposite at high Nb/Yb and Th/Yb). All MNIS samples are above the MORB field, though five of them border the upper boundary of the MORB field.

Nd Isotopic Analysis

As detailed above in the Methods sections, from the database of samples with whole-rock geochemical data (Table 1) a subset of twenty-one samples was chosen for

Nd isotopic analysis (locations illustrated on Figure 15). These samples, their formations, locations, and ages are given in Table 2. The ages of these 21 samples are best estimates gathered from literature and are based on regional correlation to dated rocks, the ages of cross-cuttings intrusions, and/or the ages of interlayered felsic meta-volcanic rocks that have been determined by U-Pb dating of zircon described in previous studies. Of these twenty-one samples, the Nd isotopic composition of sixteen of them was successfully analyzed and these samples and their Sm and Nd values are given in Table 3. Rocks of the Hawley Formation have been identified as Legate Hill volcanics and Charlemont intrusives in Tables 2 and 3 following Kim and Jacobi’s (1996) subdivision of the

Hawley into informal mappable members and their identification of intrusive bodies. The

Legate Hill volcanics are meta-dacites within the Hawley Formation and the Charlemont intrusives are sills of amphibolites with chilled margins that intrude the eastern part of the

Hawley Formation. Three other analyzed samples of the Hawley were collected from the

Shelburne Falls dome in western Massachusetts. Samples of amphibolite of the

Collinsville Formation have been subdivided geographically in Tables 2 and 3 into amphibolites from the Granville dome in southern Massachusetts and the Collinsville dome in central-western Connecticut.

Measured 143Nd/144Nd values determined by TIMS analysis at Miami University are reported in Table 3 (2σ error is ± 7 x 10-6 based on the long-term external reproducibility of the La Jolla Nd standard). These values were used in conjunction with

the respective Sm/Nd ratios and geologically constrained ages to create a range of initial

143Nd/144Nd values for the analyzed samples (Table 4). As described above in the

Analytical Background section, calculations were used to find the intial εNd values. All

εNd values range from 1.4 to 7.4 with one negative outlier at -3.4.

Comparison of the initial εNd values of the sixteen analyzed samples and εNd values of a depleted mantle source, the chondrite uniform reservoir (CHUR), and 1.0-1.2

Ga crust values over time are shown in Figure 16 (Schoonmaker et al., 2011). All samples plot at positive values relative to CHUR except one sample of the Hawley

Formation from the Shelburne Falls dome which has a value of -3.4 and plots on the upper boundary of billion year old crust. Thirteen of the samples form a vertical pattern of similar εNd values extending from the lower boundary of the depleted mantle array to a value of 1.4 for a Collinsville amphibolite from the Collinsville dome. The amphibolites of the Charlemont Hill intrusives have the most positive values (7.1 – 7.4) and overlap the lower boundary of the depleted mantle field.

Meta-basalt samples with initial εNd values between 3.6 and 1.4 — one from the

MNIS, one from the Legate Hill volcanics, and two from the Collinsville — plot within the field for Neoproterozoic Avalonian basement and the Hawley sample from the

Shelburne Falls dome with the negative initial εNd values (-3.4) plots within the field of

Neoproterozoic Ganderian basement (Fig. 17; Dorais et al., 2012). The initial εNd values of these five samples are comparable to values of the Ammonoosuc Volcanics of the

Bronson Hill arc in northern New Hampshire (Dorais, personal communication).

DISCUSSION

Overview

As part of this study, the analyzed samples from the RHZ units have been compared to previously analyzed samples from units located elsewhere in New England and from the Newfoundland Appalachians. Units from previous studies that have comparable geochemical data include the Fourmile Gneiss from the Pelham dome and

Monson Gneiss from the Monson dome of the BHA (Hollocher et al., 2002), the

Ammonoosuc Volcanics of the BHA in New Hampshire (Dorais et al., 2011; 2005), the

Highlandcroft and Oliverian Plutonic Suites of the New Hampshire BHA terrane (Dorais et al., 2008), rocks from the core of the Killingsworth dome from south-central

Connecticut (Aleinikoff et al., 2007), and the Taconian Notre Dame arc of Newfoundland

(van Staal et al., 2007). Diagrams in this section show the sixteen RHZ samples selected and analyzed for Nd ratios compared to data from these previous studies. Again, some charts may not display all samples as not all samples have published data for all elements.

Geochemical Discussion

When comparing RHZ samples to one another alone, there is a definite overlap between meta-basalts of the MNIS and NRIS, though BG samples from Vermont are chemically distinct from these other two units. The HF is significantly different from other units and displays a forearc signature based on the boninite pattern seen throughout

(Kim and Jacobi, 1996). CF samples cover a range of various different signatures and patterns, which is potentially characteristic of a backarc region of an island arc based on comparison of known backarc geochemistry (Dietsch, 2013).

These general interpretations can be further explored by comparing the multi- element spider diagrams of the four RHZ units to characteristic patterns for enriched

MORB (E-MORB), ocean island basalt (OIB), boninites, enriched back-arc basin basalt

(E-BABB), and depleted back-arc basin basalt (D-BABB) gathered from Falloon and

Crawford (1991), Fretzdorff et al. (2002), and Pearce and Stern (2006) (Fig. 18). The

MNIS and NRIS sample ranges most closely follow the patterns of MORB and BABB, consistent with the interpretation of Kim et al. (2003) for a suprasubduction zone, backarc setting for the MNIS. Samples of the HF follow the general pattern of boninites, first noted by Kim and Jacobi (1996). Samples of the CF rocks display a wide range of values as they do on other geochemical diagrams, overlapping the trends of MORB and BABB most closely. Meta-basalts analyzed by Chocyk-Jaminski and Dietsch (2002) contain predominantly MORB compositions, indicating a back-arc setting for the Collinsville.

Expanding comparisons to surrounding units of the region reveals overlap in the igneous rock types between RHZ units and mafic rocks of the Monson Gneiss and the

Ammonoosuc Volcanics on the igneous classification diagram (Fig. 19; Winchester and

Floyd, 1977). The Fourmile Gneiss, Monson Gneiss, and Highlandcroft-Oliverian plutonic suites of the BHA as well as the Killingsworth dome rocks include samples with higher ratios of Zr to Ti, including rhyodacite/dacite , rhyolite, and for the Monson

Gneiss in particular, trachyandesite. Considering rock types alone, the Bronson Hill arc exposes more felsic and alkalic orthogneisses, potentially representative of plutonic roots from the once active front of a volcanic island arc.

Considering Shervais’ (1982) Ti vs. V plot (Fig. 20), Monson Gneiss,

Highlandcroft-Oliverian plutonic suites, and the Ammonoosuc Volcanics overlap the

MORB and Arc regions in conjunction with samples of the Collinsville Formation. The

Monson Gneiss follows two positive trends: 1) low-Ti felsic and trachyandesite compositions are located within the arc field with a few samples falling on the arc-

MORB boundary, and 2) andesite and basalt compositions plot in the MORB field.

Ammonoosuc Volcanics samples split in two groups: a cluster of arc-affinity samples as well as three samples plotting within the MORB field. Though samples are not plotting in a distinct trend, samples from the Highlandcroft-Oliverian plutonic suites cover a range from very low Ti and V to 8000 ppm Ti and 300 ppm V, with one outlier outside the upper boundary of the alkalic field. Low-Ti Hawley Formation meta-basalts and Barnard

Gneiss meta-volcanics form a near-vertical trend in proximity to the arc field and these samples have equivalent samples from the Fourmile Gneiss, Monson Gneiss, and

Highlandcroft-Oliverian plutonic suites of the BHA as well as Notre Dame arc rocks from Newfoundland. MNIS samples are among samples with the highest Ti and V and plot in fields of MORB and alkalic basalt. The rocks of arc-affinity are dominated by samples of the Monson Gneiss, Ammonoosuc Volcanics, and Fourmile Gneiss. Aside from meta-basalts of the Collinsville Formation (Fig. 9), rocks from RHZ units have minimal representation in the arc field. As each grouping is distinct from the others, a potential hypothesis is that each unit, and maybe even subsets of the Collinsville, maintains different affinities. More detailed differentiation between units along-strike in the RHZ is needed in future studies.

Further comparison using trace element discrimination in the form of a Ti-Zr bivariate plot (Fig. 21; Pearce and Cann, 1973) highlights the presence of low-Ti felsic rocks from the Fourmile Gneiss, Monson Gneiss, Notre Dame Arc, and Killingsworth

dome with no counterparts in the RHZ units compiled for this study. These low-Ti felsic rocks cluster on the lower end and exterior of the LKT and CAB regions. Ammonoosuc

Volcanics and mafic rocks of the Monson Gneiss extend into the OFB region and beyond into very high abundances of Zr and Ti and there are also distinct positive trends from

LKT to CAB regions within samples of the MNIS and Collinsville Formation. The CAB field remains mostly empty, populated by samples of Monson Gneiss, Highlandcroft-

Oliverian plutons, and Ammonoosuc Volcanics only reaching the exterior borders of the field. As an explanation for this, it is possible that much arc volcanic material has been eroded from the region.

The compiled Zr v Zr/Y diagram (Figure 22; Pearce and Norry, 1979) is a bit more difficult to read, though it appears to illustrate the presence of two separate clusters of samples. The first is of Fourmile Gneiss, Monson Gneiss, Ammonoosuc Volcanics,

Notre Dame Arc, Highlandcroft-Oliverian plutonic suites, and Killingsworth dome rocks in the top middle portion of the plot, only partly within the WPB field. The high Zr/Y ratios of these samples indicate a source contribution from a continental margin or continental crust. In contrast, other samples of Fourmile Gneiss and Monson Gneiss plot within and on the lower exterior of the IAB field at low Zr/Y ratios and relatively low Zr; these are mafic rocks with a more primitive source. Connecting these two groups are

RHZ units as well as Fourmile Gneiss, Monson Gneiss, one Ammonoosuc Volcanics sample, Notre Dame Arc rocks, and Killingsworth Dome Rocks located mostly within the

MORB and MORB/IAB overlapping fields (with Zr around 100 ppm and a range of Zr/Y values from 2 to 5). Overall distribution of the samples may signify a mixing between

both a mantle source and continental contaminants, which would imply the presence of suprasubduction zone magmatic processes near the continental margin.

A comparison of incompatible trace element ratios, seen in the Ta/Yb vs. Th/Yb diagram (Fig. 23; Pearce, 1983; Kim et al., 2003; Chocyk-Jaminski and Dietsch, 2002) shows that there is very little overlap between RHZ samples and units of the Bronson Hill arc and others east of the RHZ. Fourmile Gneiss, Monson Gneiss, Ammonoosuc

Volcanics, Notre Dame arc rocks, Highlandcroft-Oliverian plutonic suite, and the

Killingsworth Dome rocks all plot in the island arc and continental margins fields at higher values of Th/Yb for a given value of Ta/Yb. MNIS samples also display this subduction zone signature, but less strongly than those units previously mentioned.

Hawley and Collinsville samples, in particular, appear to have more primitive sources lacking an obvious continental component. Collinsville maintains the most primitive source with Hawley having elevated Ta/Yb. These ratio comparisons illustrate that

Collinsville and Hawley samples are geochemically distinctive from units east of the

RHZ.

The influence of subduction zone enrichment and fractional crystallization can be further illustrated with the incompatible trace elements ratios Th/Yb and Nb/Yb (Fig. 24;

Pearce, 2008). Aside from samples of Hawley, Collinsville, and three from the

Killingsworth dome which plot within the MORB trend, both RHZ units and units to the east show a significant influence of subduction zone enrichment. Fractional crystallization trends reflect the presence of more felsic compositions in rocks of the

Bronson Hill arc. Units of the Bronson Hill arc have higher ratios of Th/Yb than any of

the RHZ units, providing evidence for more significant enrichment from a subduction zone component compared to meta-basalts of the RHZ.

Isotopic Analysis Discussion

It appears that all sixteen RHZ samples analyzed for Nd isotope ratios have a depleted mantle source component. What remains unclear is whether and to what extent these rocks have either Grenvillian or Ganderian crustal material as a secondary source component. The general locality (peri-Laurentian, intra-Iapetian, or peri-Gondwanan) of the suprasubduction zone setting for the RHZ units during their formation could potentially be pinpointed through interpretation of their crustal component. In addition, on the basis of Nd isotopes alone, it can be said that the RHZ samples are only minimally similar to those of the Bronson Hill arc units (discussed below).

Initial Nd values are plotted against various ratios of incompatible elements (Fig.

25) in an attempt to characterize different source components. No clear trends are apparent, although there are weak negative trends in Yb/Y, Th/Yb, and Th/Hf, with much more scatter shown in other ratios (Zr/Y and Zr/Hf). Correlations between incompatible ratios will need to be further studied in future projects.

Samples from units east of the RHZ display a wide range of εNd values (Fig. 26;

Schoonmaker et al., 2011), extending to much more negative values than units of the

RHZ. Highlandcroft-Oliverian plutons, Ammonoosuc Volcanics, Notre Dame arc, and

Killingsworth dome samples cluster within the field of 1.0-1.2 Ga crust. Other samples of these same units overlap with the RHZ samples with positive values. Notre Dame arc rocks and Highlandcroft-Oliverian plutonic suites have values in two distinct groups: one

that plots near the depleted mantle region and another that plots with the billion year old crust. In general, units of the RHZ maintain a more narrow range of positive values compared to units to the east.

When comparing to continental trends over time (Fig. 27; Dorais et al., 2012), the majority of units east of the RHZ fall within Ganderian or Avalonian values. Some RHZ units also overlap with values of Avalonian crust. One distinction to point out is that

Highlandcroft-Oliverian plutonic suite samples may display three grouping of values: 1) a large cluster within the Ganderian affinity region, 2) two samples in the Avalonian affinity range, and 3) three samples within the transition zone. Not as distinct are the

Killingsworth Dome rocks, which also appear to be split between the three regions. The wide range of values of units east of the RHZ is evidence of mixing between depleted mantle and a peri-Gondwana crustal component; mixing models are needed to quantify this potential relationship. Nevertheless, an important geochemical difference exists between units of the RHZ and units of the Bronson Hill and others to the east.

CONCLUSION

As discussed in the previous chapter, there is significant overlap of the Mount

Norris Intrusive Suite and North River Igneous Suite of Vermont; however, the Barnard

Gneiss samples do not correlate through any discrimination attempts. These differences are evident in the geochemistry, however Nd isotopic analysis reveals similarities in regards to their source material.

Hawley Formation samples reveal distinct differences from all other units as well as a forearc signature through each discrimination diagram. The distinct differences are displayed in the bivariate discrimination plots of incompatible elements. The forearc pattern of the samples comes out in the multi-element spider diagrams as the unit has an enrichment of LIL elements and a depletion of HFS elements compared to the more flat- lying HFS element pattern of an intra-arc environment. Despite one outlier within

Hawley samples, isotopic values are consistent with other RHZ samples, pointing towards similar source material.

Collinsville Formation samples are characteristic of a backarc region of an island arc, which tend to have a range of values from forearc to intra-arc signatures. All across the discrimination diagrams, Collinsville Formation rocks cover multiple fields and overlap the majority of other unit samples. The large range of values seen in the

Collinsville samples brings up the potential for further discrimination within the single unit. Isotopic analysis reveals a slightly larger range in values than MNIS and the majority of Hawley samples. Initial Nd values of the Collinsville Formation form three small groups (Figs. 16 & 17), a fact which provides further support for discrimination within the single unit.

The similarity in recorded ages leads to the potential correlation along-strike, however, the differences in geochemistry and isotopic evidence allow some speculation.

There is enough similarity within the trace element analysis to perpetuate a correlation across units. This correlation is not strict, but rather allows for variation within the single island arc. In other words, there appears to be meta-basalts generated in forearc, backarc, and intra-arc settings along-strike within the RHZ. As for correlation east to west across strike, there is minimal potential for correlation to the Bronson Hill Arc based on geochemistry. More age data is needed to make more stringent correlation of units along strike as well as east to west across the New England Appalachians.

Despite the lack of overlap within error of initial Nd isotopic values within the

RHZ (Fig. 28), general similarity of the units leads to an interpretation of the mantle source of the units. Most samples plot in the positive range proximal to the depleted mantle evolution over time, therefore it is assumed that the samples came from a depleted mantle source with contribution from a secondary contamination source. Also, comparison of Nb/Yb to Th/Yb provides definitive evidence of a supra-subduction zone setting for all units with potentially minimal influence of fractional crystallization.

When compared to rocks of the Bronson Hill arc and other units east of the RHZ, and to the Newfoundland Appalachians, there is minimal geochemical overlap and correlation to some extent, but the high variation of values for units outside the RHZ is too extensive to narrow tectonic settings for comparison. An interesting turn of events is the chemical fingerprint of different tectonic settings within samples of the same unit.

The range of geochemical and isotopic data appear to be split into distinct trends and clusters that cause speculation as to the correlation of such samples together. It is evident

that the New England Appalachians needs more extensive discrimination of individual units — beginning with new mapping — not only within the RHZ, but potentially in the surrounding region as well.

Comparison of Nd values to basement regions of distinctive crustal evolution over time may reveal contamination by Grenville or Ganderia, providing evidence of a suprasubduction locality within the Iapetus Ocean. Specifically, the RHZ units appear to be peri-Ganderian due to its slightly Avalonian signature, however, the data are not completely conclusive. Having said this, it is important to consider the information needed for future studies. A priority for studies within the RHZ should include determination of an age for the Collinsville Formation as ages used are based on minimum and maximum estimates. Also, as trace elements of the RHZ are not entirely conclusive, it is important to look at the isotopic evidence of the area. Therefore, further analysis of isotopes within the area should be completed. The difficulty lies in finding material to analyze as metamorphism has reworked and overprinted all rock units of the region. There may be potential to find accessory minerals to analyze for Sr and Pb isotopes, but whether they might preserve the initial ratios of their source material is unclear. What is clear is the need for further and more detailed chemical characterization within the RHZ itself. The high range of values along strike points to the need to discriminate between smaller geologic units, specifically within the Collinsville

Formation.

Returning to questions mentioned in the beginning, we can make some conclusions, though others remain unanswered. It is possible to recognize a suprasubduction zone setting along-strike, however, the correlation of a specific

suprasubduction zone throughout the units is non-conclusive without further discrimination among rocks of the RHZ. There is definite potential for interpreting paleogeography of the region based on isotopic analysis and geochemical discrimination, though more analysis is needed to make definitive conclusions regarding the tectonic past of the region. For instance, extensive analysis of Grenville and Gander basement material is needed for comparison to units within the RHZ as well as further isotopic study of the

RHZ itself.

Though there is geochemical and isotopic correlation with the BHA east of the

RHZ, it is minimal, leading to the conclusion that, while the RHZ itself may represent a single arc model, it is not part of a composite arc with the BHA. Also, while it was previously assumed that a peri-Laurentian setting was the best model for island arc formation of the RHZ region, a peri-Ganderian setting seems to be more likely based on

Nd isotopic interpretation. A more conclusive model may be developed after more extensive isotopic analysis.

In the end, the driving reason behind this study was to provide evidence for or against a previously developed tectonic model of formation for the RHZ. Despite this, it has been determined that more isotopic and age analyses are needed to make more detailed interpretation regarding a specific model. Due to the high level of complexity in this region, it is most reasonable that multiple models of individual sections of the region are required to interpret along-strike variation rather than the previous assumption of defining a single model to translate along-strike throughout the RHZ.

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Figures

Figure 1 – Generalized geologic map of the Laurentian, Peri-Gondwanan, and transitional realms of the Appalachian Orogen through Newfoundland, New England, and the southern Appalachians. From Hibbard et al., 2006.

Figure 2 – Generalized geologic map of the New England Appalachians showing major lithotectonic unit packages of the western New England region. The Bronson Hill Arc (BHA) seen in the east, the Rowe-Hawley Zone (RHZ) in the west is the region under consideration in this study, and the Connecticut Valley Trough (CVT) and Hartford Basin (HB) cover the transition between the two arc regions. Modified from Dietsch et al., 2010 through compilation with geologic maps from the USGS database.

Figure 3 – Diagram by Allègre (2008) displaying the standard model of the mantle in layers, each of which produces either MORB or OIB material. (p.243)

Figure 4 – Isotope growth curve of the continental crust and mantle created by Allègre (2008). (p.260)

Figure 5 – Graphical representation of the method of model age calculation (p. 270; Allègre, 2008).

Figure 6 – Depiction of the general mechanics of the TIMS thermal ionization mass spectrometer by Allègre (2008). (p. 5)

Figure 7 – Generalized geologic map of the RHZ region within the western New England Appalachians. Locations of sample collection are outlined for each major unit analyzed. Each sample site region is labeled with unit name, reference study, and the figure from that reference where detailed sample site information can be found. Modified from Dietsch et al., 2010 through compilation with geologic maps from the USGS database.

Figure 8 – Winchester and Floyd (1977) igneous rock classification plot based on Nb/Y versus Zr/Ti, demonstrating chemical differences and similarities throughout the RHZ. MNIS = Mount Norris Intrusive Suite, BG = Barnard Gneiss, NRIS = North River Igneous Suite, HF = Hawley Formation, and CF = Collinsville Formation. See text for discussion.

Figure 9 – Diagram demonstrating chemical differences among RHZ samples using Ti/1000 versus V based on Shervais (1982). MNIS = Mount Norris Intrusive Suite, BG = Barnard Gneiss, HF = Hawley Formation, and CF = Collinsville Formation; no published V data for NRIS. See text for discussion.

Figure 10 – Differentiation diagram of Zr versus Ti (Pearce and Cann, 1973) comparing RHZ units: MNIS = Mount Norris Intrusive Suite, BG = Barnard Gneiss, NRIS = North River Igneous Suite, HF = Hawley Formation, and CF = Collinsville Formation. Tectonic environments include: LKT = low-K tholeiite, OFB = ocean floor basalt, and CAB = calc-alkaline basalt. See text for discussion.

Figure 11 – Discrimination diagram of Zr versus Zr/Y (Pearce and Norry, 1979) comparing potential tectonic settings (WPB = within-plate basalt, MORB = mid-ocean ridge basalt, and IAB = island arc basalt) of the RHZ: MNIS = Mount Norris Intrusive Suite, BG = Barnard Gneiss, NRIS = North River Igneous Suite, HF = Hawley Formation, and CF = Collinsville Formation. See text for discussion.

Figure 12 – Individual N-MORB normalized spider diagrams for the RHZ units. Normalizing values are from Pearce (1982).

Figure 13 – Discrimination diagram of Ta/Yb vs. Th/Yb (Pearce, 1983) showing the influence of subduction zone enrichment and continental crust contamination on meta- basalts of the RHZ. Units in this study: MNIS = Mount Norris Intrusive Suite, NRIS = North River Igneous Suite, HF = Hawley Formation, CF = Collinsville Formation; tectonic settings: WPB = within-plate basalt, MORB = mid-ocean ridge basalt; Contamination components: s = subduction, c = continental crust, w = within-plate source, f = fractional crystallization.

Figure 14 – Diagram illustrating the influence of subduction zone enrichment and fractional crystallization on the chemistry of RHZ units through comparison of Nb/Yb to Th/Yb (Pearce, 2008). Units include: MNIS = Mount Norris Intrusive Suite, BG = Barnard Gneiss, NRIS = North River Igneous Suite, HF = Hawley Formation, and CF = Collinsville Formation. See text for discussion.

Figure 15 – Sample locations for those rocks selected for TIMS analysis. Samples are overlain on a geologic map generated from USGS database maps.

Figure 16 – Diagram illustrating εNd values over time with comparative fields for depleted mantle and 1.0-1.2 Ga crust (Schoonmaker et al., 2011). MNIS = Mount Norris Intrusive Suite, HF = Hawley Formation, and CF = Collinsville Formation. Samples are listed based on detailed locality (found in Table 2). See text for discussion.

Figure 17 – Diagram illustrating εNd values over time with a comparative field of depleted mantle as well as Ganderia, Avalonia, and the transition zone between them (Dorais et al., 2012). See Figure 17 for sample key. See text for discussion.

Figure 18 – A) Shaded ranges of N-MORB normalized values for the RHZ units. Normalizing values are from Pearce (1982). Overlain on the ranges are characteristic tectonic settings including: E-MORB = enriched MORB, OIB = ocean island basalt, Boninites, E-BABB = enriched back-arc basin basalt, and D-BABB = depleted back-arc basin basalt. B) Compilation of all shaded ranges and characteristic settings.

Figure 19 – Winchester and Floyd (1977) igneous rock classification plot based on Nb/Y versus Zr/Ti, demonstrating chemical differences and similarities between the RHZ and surrounding units within the New England and Newfoundland Appalachians. MNIS = Mount Norris Intrusive Suite, HF = Hawley Formation, CF = Collinsville Formation, FG = Fourmile Gneiss, MG = Monson Gneiss, AV = Ammonoosuc Volcanics, NDA = Notre Dame arc, HOP = Highlandcroft-Oliverian plutons, KD = Killingsworth Dome. See text for discussion.

Figure 20 – Diagram demonstrating chemical differences between RHZ samples and surrounding New England and Newfoundland Appalachian samples using Ti/1000 versus V based on Shervais (1982). Mount Norris Intrusive Suite, HF = Hawley Formation, CF = Collinsville Formation, FG = Fourmile Gneiss, MG = Monson Gneiss, AV = Ammonoosuc Volcanics, NDA = Notre Dame arc, HOP = Highlandcroft-Oliverian plutons, KD = Killingsworth Dome. See text for discussion.

Figure 21 – Differentiation diagram of Zr versus Ti (Pearce and Cann, 1973) comparing RHZ units and units in the surrounding New England and Newfoundland Appalachians: Mount Norris Intrusive Suite, HF = Hawley Formation, CF = Collinsville Formation, FG = Fourmile Gneiss, MG = Monson Gneiss, AV = Ammonoosuc Volcanics, NDA = Notre Dame arc, HOP = Highlandcroft-Oliverian plutons, KD = Killingsworth Dome. Tectonic environments include: LKT = low-K tholeiite, OFB = ocean floor basalt, and CAB = calc-alkaline basalt. See text for discussion.

Figure 22 – Discrimination diagram of Zr versus Zr/Y (Pearce and Norry, 1979) comparing potential tectonic settings (WPB = within-plate basalt, MORB = mid-ocean ridge basalt, and IAB = island arc basalt) of the RHZ to those of surrounding New England and Newfoundland Appalachian units: Mount Norris Intrusive Suite, HF = Hawley Formation, CF = Collinsville Formation, FG = Fourmile Gneiss, MG = Monson Gneiss, AV = Ammonoosuc Volcanics, NDA = Notre Dame arc, HOP = Highlandcroft- Oliverian plutons, KD = Killingsworth Dome. See text for discussion.

Figure 23 – Discrimination diagram of Ta/Yb vs. Th/Yb (Pearce, 1983) showing the influence of subduction zone enrichment and continental crust contamination on meta- basalts of the RHZ as well as surrounding units. Units: MNIS = Mount Norris Intrusive Suite, NRIS = North River Igneous Suite, HF = Hawley Formation, CF = Collinsville Formation, AV = Ammonoosuc Volcanics, NDA = Notre Dame Arc, HOP = Highalandcroft-Oliverian plutonic suite, KD = Killinsworth Dome; tectonic settings: WPB = within-plate basalt, MORB = mid-ocean ridge basalt; Contamination components: s = subduction, c = continental crust, w = within-plate source, f = fractional crystallization. RHZ samples are not limited to those analyzed for Nd isotopes as those samples were lacking in available element data.

Figure 24 – Diagram illustrating the influence of subduction zone enrichment and fractional crystallization on the chemistry of RHZ and surrounding New England and Newfoundland Appalachian units through comparison of Nb/Yb to Th/Yb (Pearce, 2008). Units include: Mount Norris Intrusive Suite, HF = Hawley Formation, CF = Collinsville Formation, FG = Fourmile Gneiss, MG = Monson Gneiss, AV = Ammonoosuc Volcanics, NDA = Notre Dame arc, HOP = Highlandcroft-Oliverian plutons, KD = Killingsworth Dome. See text for discussion.

Figure 25 – Diagrams displaying potential mixing trends through comparison of initial Nd values to various incompatible trace element ratios.

Figure 26 – Diagram illustrating εNd values over time with comparative fields for depleted mantle and 1.0-1.2 Ga crust (Schoonmaker et al., 2011). Samples are listed based on detailed locality (found in Table 2). MNIS = Mount Norris Intrusive Suite, HF = Hawley Formation, and CF = Collinsville Formation. Added samples include those from previous studies and are listed by locality. See text for discussion.

Figure 27 – Diagram illustrating εNd values over time with a comparative field of depleted mantle as well as Ganderia, Avalonia, and the transition zone between them (Dorais et al., 2012). Samples are listed based on detailed locality (found in Table 2). Added samples include those from previous studies and are listed by locality. See Figure 25 for sample key. See text for discussion.

Figure 28 – Comparison of calculated initial Nd values of samples arranged geographically from north to south. Circles are samples from Vermont, squares are samples from Massachusetts, and diamonds are samples from Connecticut. Error bars on samples are based on calculating initial Nd with ± 7 ppm error. The diagram illustrates the lack of overlap among initial Nd values.

TABLES

Table 1: A compilation of data gathered from previous studies on the RHZ. See text for references

Table 2 – RHZ samples selected for TIMS analysis. Samples are listed based on geographic location.

Sample General Formation Locality/Unit Latitude Longitude Age (Ma) GD8816 Moretown Mount Norris intrusives 44.972494 -72.382345 470 ME-94-22 Moretown Mount Norris intrusives 44.821756 -72.438594 470 6-25-96-1 Moretown Mount Norris intrusives 470 SZ72110-4 Moretown VT greenstone 44.618879 -72.405467 470 JK8902-4 Moretown VT greenstone 44.324684 -72.570910 504 ET110803-9 Moretown Cram Hill intrusives 44.270468 -72.578336 488 SFD1 Hawley Shelburne Falls Dome 42.605278 -72.738333 475 SFD6 Hawley Shelburne Falls Dome 42.605278 -72.738333 475 SFD9 Hawley Shelburne Falls Dome 42.605278 -72.738333 475 SFD10 Hawley Shelburne Falls Dome 42.605278 -72.738333 475 CH2 Hawley Charlemonte Hill intrusives 42.632500 -72.872778 434 CH14 Hawley Charlemonte Hill intrusives 42.632500 -72.872778 434 CH17 Hawley Charlemonte Hill intrusives 42.632500 -72.872778 434 LHB12 Hawley Legate Hill volcanics 42.645556 -72.897222 484 8-13-92-3 Hawley Legate Hill volcanics 42.645556 -72.897222 484 961201 Collinsville Granville Dome 42.080833 -72.842222 475 961202 Collinsville Granville Dome 42.080833 -72.842222 475 961206 Collinsville Granville Dome 42.080833 -72.842222 475 961601 Collinsville Collinsville Dome 41.811389 -72.888056 475 961604 Collinsville Collinsville Dome 41.811111 -72.888056 475 942503 Collinsville Collinsville Dome 41.807222 -72.925556 475

Table 3 – Final calculations of Nd values for RHZ samples selected for TIMS analysis.

General 143 144 143 144 εNd Sample Locality/Unit Nd (ppm) Sm (ppm) Nd/ Ndm Nd/ Ndi Formation (rounded) Mount Norris 6-25-96-1 Moretown 10.9 3.31 0.512778 0.512216 3.6 intrusives JK8902-4 Moretown VT greenstone 26.7 6.82 0.512768 0.512261 5.3 ET110803-9 Moretown Cram Hill intrusives 15.3 4.93 0.512981 6.9 0.512361 Shelburne Falls SFD1 Hawley 9.86 2.23 0.512274 0.511851 -3.4 Dome Shelburne Falls SFD6 Hawley 10.58 3.13 0.512883 0.512330 5.9 Dome Shelburne Falls SFD10 Hawley 7.11 2.38 0.512874 0.512248 4.3 Dome Charlemonte Hill CH2 Hawley 26.53 7.4 0.512921 0.512445 7.1 intrusives Charlemonte Hill CH14 Hawley 20.99 5.74 0.512911 0.512444 7.1 intrusives Charlemonte Hill CH17 Hawley 17.64 5.19 0.512963 7.4 intrusives 0.512460 Legate Hill 8-13-92-3 Hawley 15.8 4.87 0.512773 volcanics 0.512164 3.4 961201 Collinsville Granville Dome 2.5 0.64 0.512769 0.512290 5.1 961202 Collinsville Granville Dome 6 1.65 0.512824 0.512310 5.5 961206 Collinsville Granville Dome 15 4.69 0.512946 0.512361 6.5 961601 Collinsville Collinsville Dome 9 3.39 0.513085 0.512381 6.9 961604 Collinsville Collinsville Dome 1.7 0.38 0.512795 0.512376 6.8 942503 Collinsville Collinsville Dome 11 3.29 0.512657 0.512098 1.4

Table 4 – Randomly selected samples and their Nd values calculated multiple times to display the minimal variation experienced through age and collection error.

Initial Nd Calculations Age Sample -7ppm value +7ppm Minimum 465Ma 6-25-96-1 0.512215 0.512222 0.512229 Maximum 470Ma 6-25-96-1 0.512209 0.512216 0.512223 Minimum 483Ma 8902-4 0.512276 0.512283 0.51229 Maximum 504Ma 8902-4 0.512255 0.512262 0.512269 Minimum 484Ma 8-13-92-3 0.512179 0.512186 0.512193 Maximum 502Ma 8-13-92-3 0.512158 0.512165 0.512172

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Appendix A –Analytical Background

Geochemistry

Elements used in the study of igneous rocks include major, minor, and trace elements. Major elements are those that generally make up more than 1.0 weight percent of the rock; minor elements are those that are between 0.1 and 1.0 weight percent of the rock; and trace elements are those that compose less than 0.1 weight percent of the rock.

Major and minor elements tend to have a lower atomic number than the trace elements and the former are expressed in weight percent oxides while the latter are expressed in parts per million (ppm) of the element. Totals of major and minor elements will not always sum to 100% as there are errors with individual analyses and some oxides are left out of analysis.

Major elements are used for preliminary methods of classifying rocks and interpreting the magmatic processes of formation. Trace elements, on the other hand, tend to be more sensitive to fractionation and partial melting processes as these elements are incorporated or excluded from various mineral phases much more selectively than major elements. Trace elements can be divided into two groups based on their behavior during melting: 1) incompatible trace elements are more highly concentrated in the melt phase compared to unmelted solid phases (their bulk distribution coefficient, D, < 1 where D = concentration in the bulk solid divided by concentration in the liquid) whereas 2) compatible trace elements maintain higher concentration in the solid than the melt and have D > 1. Incompatible elements can be further subdivided into two groups based on valence-to-ionic radius ratios. Smaller, more highly charged elements are known as high field strength elements (HFSE) and larger elements with low field strengths are called

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large ion lithophile elements (LILE). LILEs are considered more mobile in the presence of a fluid phase.

Rare Earth Elements (REE) include the 15 elements from atomic number 57 (La) to 71 (Lu) and are split between lower atomic number (the light rare earth elements;

LREE) and higher atomic number (the heavy rare earth elements; HREE) with some intermediate values referred to as middle REE. They are geochemically very similar making them useful for geochemical discrimination as depletions and enrichments are more readily detected (Hanson, 1980) compared to other HFSE. With REE, there is a tendency for elements of even atomic number to have higher concentrations and odd to have lower concentrations due to the atomic odd-even effect. To counter this effect, REE are normalized to the concentrations of REE in chondrites. Chrondrite values are chosen as they are considered to represent the composition of Earth’s primitive mantle. However, the variety of analyzed chondrite values leads to uncertainty with regard to interpreting

REE patterns and their application to determining tectonic settings, so chondrite normalization is used in conjunction with other discrimination diagrams (Rollinson,

1993). In addition, the REE behave in similar ways to one another, increasing the need for alternate methods of discrimination through analysis of other chemically diverse element suites in conjunction with chondrite normalized diagrams.

Normalized multi-element (“spider”) diagrams are used to characterize basalt compositions as they have a larger range of trace elements compared to diagrams that only use the REE. In spider diagrams, the abundances of trace elements in samples are normalized to values of a uniform primitive mantle or another reservoir — typically

MORB — to compare their estimated abundances in those reservoirs. I follow the

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element ordering scheme of Pearce (1982; 1983) based on ionic potential of the elements and their bulk distribution coefficient for the elements between a garnet lherzolite and melt. The distribution coefficient is a measure of the incompatibility of an element in small amounts of partial melt and all the elements plotted are incompatible: the most incompatible elements plot on the left-hand side of the diagram, and the least incompatible elements plot on the right-hand side. The LILE Sr, K, Rb, and Ba on the left-hand side of the diagram are regarded as mobile in fluids. From extensive analysis of rock types on this diagram, certain patterns emerge. The characteristic pattern of island arc basalt, for example, on a MORB-normalized spider diagram shows an enrichment of

LILEs (and sometimes Th and Ta) as well as a depleted to flat-lying HFSE range from

Nb through Cr. High Ba and Rb may suggest contamination by a crustal component due to the ease of LILE extraction from the mantle and subsequent concentration in the continental crust.

There are also a set of commonly used trace element discrimination diagrams including Nb/Y versus Zr/TiO2; Zr versus Zr/Y; and Ti versus V. In these diagrams, trace elements and trace element ratios display patterns or trends and fields of various tectonic settings have been established based on the analysis of volcanics from known settings.

These diagrams typically use the following settings (and abbreviations) in order to infer settings of the ancient (meta-) volcanics:

WPB - within plate basalt

IAT - island arc tholeiite

CAB - calc-alkaline basalt

MORB - mid-ocean ridge basalt

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OIT - ocean island tholeiite

OA - ocean island alkaline basalt

IAB - island arc basalt

Isotopes and Radioactivity

Isotopes are atoms of an element that contain different numbers of neutrons within their nucleus. Some isotopes are stable and their nuclei will remain unchanged indefinitely. Other isotopes are known as radioactive based on their tendency to decay into stable isotopes of another element. Radiogenic “parent” isotopes lose energy through release of an ionizing particle (see below) which changes the number of protons and neutrons in their nucleus to that of the stable “daughter” isotope.

Radioactivity and the consequent decay of parent isotopes occur in a variety ways: β- decay, β+ decay, electron capture, α decay, or spontaneous fission. β- decay occurs when a neutron spontaneously decays into a proton and an electron, causing the nucleus to emit an antineutrino (ῡ) and an electron. Due to the neutron to proton conversion, the daughter is a new element of a higher atomic number on the Periodic

Table (eg: 87Rb  87Sr + β- + ῡ). For β+ radioactivity, the nucleus emits a positron (e+; anti-electron) and a neutrino (υ) at the same time, causing a proton to alter into a neutron

(eg: ; where A and B are the parent and daughter isotopes,

respectively of atomic mass A and atomic number Z). Atomic number decreases.

Electron capture involves the incorporation of a peripheral electron into the nucleus and a proton is converted into a neutron and the atomic number decreases by one (eg: 40K + e-

40 r + υ). α decay occurs when a helium nucleus is emitted by a parent nucleus (eg:

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147 143 Sm  Nd + He), with associated recoil energy release. Finally, spontaneous fission

can occur, especially in high atomic number elements such as uranium, in which the nucleus splits into 2 unequal parts and many neutrons.

Isotope ratios of some elements vary in nature as a result of radioactive decay and can be used as tracers for major geological and geodynamic processes. Isotope ratios can reflect the process(es) of how rocks form and can record the movement of material among major geochemical reservoirs of the Earth (crust, mantle, core, atmosphere, etc.).

Nd isotope ratios

143Nd is formed through α decay of 147Sm, which has a half-life of 1.5576 x 1011 years and a decay constant of 6.42 x 10-12. Rocks with high Sm/Nd ratios have greater

143Nd/144Nd as time passes compared to rocks with low Sm/Nd ratios. Both Sm and Nd are incompatible elements and their concentrations both increase in igneous rocks with increasing degrees of differentiation. Because Nd is slightly more incompatible than Sm, during partial melting and fractional crystallization, Nd is more concentrated in the liquid relative to Sm. Rocks that are depleted in light REEs (Nd with atomic number 60 is slightly more depleted than Sm with atomic number 62) have relatively high Sm/Nd ratios and relatively high 143Nd/144Nd. However, variations are on the order of 1 part per

10,000 making the necessity of a precise mass spectrometer critical. Variations in

143Nd/144Nd values in diverse mantle and crustal reservoirs have been determined through extensive analyses:

MORB ratios range from 0.513000 to 0.513300;

OIBs ratios range from 0.512312 to 0.513095;

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Continental flood basalt ratios range from 0.512210 to 0.512925;

Subduction-related volcanics ratios range from 0.512120 to 0.513032;

Upper crust ratios range from 0.510236 to 0.512790;

Lower crust granulite ratios range from 0.509818 to 0.512951, providing a basis on which to compare unknown samples (Rollinson, 1993). In fact, there is even differentiation in ratios measured from different ocean basins: Atlantic Ocean is from 0.5119 to 0.5122; the Pacific Ocean is from 0.5120 to 0.5122; and the Indian Ocean is from 0.5124 to 0.5126 (Rollinson, 1993).

As the ratios tend to vary only slightly from one another, it is more efficient to express the Nd ratios in the form of εNd notation, which displays Nd ratios relative to a reference chondrite standard (0.512638). The equation is as follows:

( ) ( )

ε = x [1], Nd

( )

[ ] and it expresses the relative variation per 104, simplifying reporting and manipulation, but most importantly, allowing samples to be compared at a given time in the past.

Continental crust is enriched compared to the mantle in incompatible elements

(e.g. K, Rb, Cs, REE, Th, and U). As elements are extracted from the mantle during continental crust formation, the source mantle becomes depleted in those elements.

Continental crust is a quasi-permanent part of the crust that isn’t dense enough to be completely recycled into the mantle during subduction. However, sediments derived from the erosion of continental crust can be subducted and ultimately contribute to the composition of suprasubduction zone melts. Continental crust is more enriched in Nd

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than Sm as extraction of these elements into the crust occurs, giving it a lower Sm/Nd ratio than the underlying mantle. As the mantle is continually mixing by convection, material that makes its way to MORB settings will be mostly homogeneous, recording a narrow range of isotopic ratios. It was once thought that more primitive (or less depleted) mantle likely characterizes the entire lower mantle, allowing for limited mixing to give a distinctly depleted geochemical signature within the upper mantle. Today there is no strong evidence for completely primitive mantle anywhere evidence does exist for limited mixing between deep lower mantle and shallow depleted mantle. OIB material originates in deeper mantle regions and becomes contaminated as it travels through upper mantle plumes that have been constantly mixing. Thus there are a variety of sources that contribute to suprasubduction zone magmas, including subducted continental crust, subducted sediments derived from the erosion of continental crust, depleted mantle, enriched mantle, and mixtures between these two mantle sources, so the isotopic range of subduction related magmas can be quite varied (Fig. 3).

In igneous rocks, there is a negative correlation between 87Sr/86Sr and 143Nd/144Nd because Rb (87Rb being the isotopic parent of 87Sr) is more incompatible than Sr, so partial melting and fractional crystallization result in melts with elevated Rb/Sr (but low

Sm/Nd.

Continental crust extraction

The main process behind chemical differentiation of the uppermost mantle involves the extraction of the continental crust and there are geochemical consequences in the underlying mantle. Models attempting to explain continental crust extraction

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include continual extraction and single episode extraction. The first model involves the continual extraction of continental crust over time from the mantle whereas the second focuses on the single extraction of the continental crust early in Earth formation and the continuous recycling of old material through erosion, sedimentation, and tectonic activity. Quantitative modeling reveals an age of 2.5-2.7 Ga for the average age of crust- mantle differentiation (Dickin, 2005; Rollinson, 1993), which provides evidence against a model of single extraction at the time of Earth formation and has also been interpreted as evidence against continuous extraction. The average age of crust-mantle differentiation has been interpreted in terms of a two-stage model of original extraction as well as continual extraction since then (Allègre, 2008). Continental extraction includes processes from both ridges and subduction zones.

As the average age for continental crust has been formulated, questions have arisen as to the isotopic development through time of both the continental crust and the underlying mantle. In order to recreate this progression of isotope ratios through time, ratios in ophiolite massifs and komatiites representing the evolving mantle and in shale representing differentiated continental crust through time have been analyzed. Age and isotopic ratio data have given an average change in isotope abundances over time. Sm and Nd are less soluble and less mobile than Rb and Sr, making them the method of analysis of choice over time. The εNd notation is used once more, but primitive mantle values used are those from the time period under study. This alters the equation 1 as follows:

Nd Nd ( ) ( ) p [ Nd ] x [2] ( ) p

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which ultimately, when plotted against time, illustrates the primitive mantle evolution as a horizontal line and the upper mantle evidence relative to it along a line from the following equation:

Nd Nd ( ) (present day) [3] p p p with the following variables:

T is the age of the sample

Nd (present day) = 0.512638 and is the 143Nd/144Nd isotope ratio of the primitive p

mantle of present day

µp = 0.196 and is the 147Sm/143Nd parent-daughter isotope ratio of the primitive

mantle

λ is the decay constant of the Sm-Nd system.

Two curves are formulated based on the Nd evolution, one for depleted mantle and one for continental crust (Fig. 4).

As significant Sm/Nd fractionation does not occur readily through surface processes, a model age can be associated with the extraction time from the mantle. The slope of primitive mantle evolution is -0.196 and the slope of continental crust evolution is -0.11. Therefore, a sample’s model age can be estimated graphically using a figure similar to Figure 5.

Mass Spectrometry

There are three main parts to a mass spectrometer: source, magnet, and collectors.

The source generates ions from atoms and accelerates them in a beam through calibrated slits in a set of high-voltage plates. From here, the magnet deflects the ions in circular 109

paths with the radius dependent on the mass to charge ratio, thus producing individual ion beams of each mass, which can be measured simultaneously in multiple collectors. The collector, aside from collecting the charged ions, converts ion beams into an electrical current, which is conducted along a wire to an electrical resistor. The potential difference across resistors is calculated using Ohm’s law and the difference is amplified electronically to increase sensitivity. A good vacuum is needed to minimize collisions of ions with air molecules during travel of the ions from source to collector.

The first task of a mass spectrometer is the ionization of the elements. In early methods, the atoms in gaseous state were bombarded with electrons, causing them to lose their own electrons and converting the atoms to ions. With the thermal ionization mass spectrometer (Fig. 6), which is a solid-state mass spectrometer, a small portion of the sample is placed on a filament which is then heated by the Joule effect (an increasing electric current). Elements are volatilized and ionized at different temperatures depending on the element, and can form either positive or negative ions that are accelerated through the magnetic field.

Prior to mass spectrometry, samples must be prepared using chemical separation to purify the element under study. Chemical separation removes major elements from the sample in order for the trace elements to become concentrated so that they can be effectively ionized, and also serves to eliminate isobars (ions of a different element but the same mass, e.g. 144Sm and 144Nd), which cannot be distinguished from the mass of interest in the mass spectrometer. A common technique for chemical separations in liquid solutions is ion-exchange column chemistry (Aldrich et al., 1953).

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Appendix B - Methods

Major, minor, and trace element abundances were compiled from previous studies for geochemically analyzed whole-rock samples of the Collinsville Formation (Chocyk-

Jaminski and Dietsch, 2002; Dietsch, unpublished data), Hawley Formation (Kim and

Jacobi, 1996; Kim, unpublished data), Mount Norris Intrusive Suite (Kim et al., 2003), and North River Intrusive Suite (Ratcliffe and Armstrong, 1998) within the RHZ, as well as from the Bronson Hill Arc (Hollocher et al.,2002; Dorais, et al., 2012; Dorais et al.,

2005). Figure 7 illustrates the generalized location of samples used for comparison within this study. Sample data were plotted and compared using MORB-normalized multi- element diagrams (“spider diagrams”), Pearce Element Ratio diagrams, and other bivariate discrimination diagrams in order to display similarities and differences of the units. A sample set of 5 specimens from the Barnard Gneiss from western Vermont (VB-

1, VB-2, VB-3, VB-4, and VB-5) was collected and analyzed for trace elements using a

Rigaku 3070 X-Ray Spectrometer at the University of Cincinnati. A set of 21 samples selected from the larger set of rocks with whole-rock geochemical data were analyzed for their Nd isotopic compositions. Details of the sample preparation methods are given in

Appendix A.

Diagrams used for analysis of major, minor, and trace elements include: the igneous rock classification diagram from Winchester and Floyd (1977) based on Nb/Y versus ZrTiO2; a Ti (ppm/1000) versus V (ppm) diagram formulated by Shervais (1982) to separate the fields of mid-ocean ridge basalt (MORB), island arc basalt (IAB), and alkalic basalt; a plot of Zr (ppm) versus Ti (ppm) created by Pearce and Cann (1973) to differentiate low-potassium tholeiite (LKT), ocean floor basalt (OFB), and continental arc

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basalt (CAB); a diagram of Zr versus Zr/Y (Pearce and Norry, 1979) meant to differentiate IAB, MORB, and within-plate basalt (WPB) rock types; MORB-normalized spider diagrams (Pearce, 1982) used to reveal patterns of large ion lithophile elements

(LILE) and high field strength elements (HFSE); and, finally, a diagram comparing

Nb/Yb to Th/Yb developed by Pearce (2008) to display the impact of subduction zone

LILE and volatile element enrichment and fractional crystallization on primary magma compositions. In addition to the primary sources for each diagram used, discussion of the utility of these diagrams and their application to interpreting the origin of suprasubduction zone magmas can be found in Winter (2009), Wilson (1989), and

Rollinson (1993).

The set of 21 samples analyzed for their Nd isotopic compositions were selected to represent the major meta-volcanic and intrusive units along-strike within the RHZ.

Within Vermont, 3 samples were selected from the Mount Norris Intrusive Suite (6-25-

96-1, GD8816, ME-94-22), 2 from the Moretown Formation (8902-4, 72110-4), and 1 from the Cram Hill Formation (ET110803-9). Within Massachusetts, samples of the

Hawley Formation include 4 from the Shelburne Falls area (SFD1, SFD6, SFD9,

SFD10), 3 from Charlemont Hill (CH2, CH14, CH17), and 2 from Legate Hill Brooke

(LHB12, 8-13-92-3). Within Connecticut, samples of the Collinsville Formation include

3 from the Granville Dome (961201, 961202, 961206) and 3 from the Collinsville Dome

(961601, 961604, 942503).

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Appendix C – Sample preparation methods for X-ray fluorescence (XRF) analysis and Nd isotopic analysis using thermal ionization mass spectrometry (TIMS)

Preparation for XRF analysis –

Samples were first ground to a powder using a disk mill and pressed into pellets through applying mechanical force of 20 tons of pressure using an X-Press hydraulic press; sample powders were placed in aluminum containers to stabilize the pressed pellet.

Samples were then loaded into a Rigaku 3070 X-Ray Spectrometer. After collection of radiation measurements, corrections are made using a multiple regression matrix with coefficients established by measuring a standard set of samples from the United States

Geological Survey (USGS) and the Japan Geological Survey. The standard SDO-1 was used to correct for machine drift.

Preparation for Nd isotopic analysis on TIMS –

Powdered samples were taken to Miami University in Oxford, OH for chemical separation and preparation for thermal ionization mass spectrometry (TIMS) analysis.

Samples were dissolved and purified in sets of eight including either a blank or replicate in each set. First, samples were weighed out based on their Nd concentrations to acquire approximately 500 ng of Nd. Those with less than 7 ppm of Nd were measured into approximately 100 mg aliquots whereas those with greater Nd concentrations were measured into approximately 50 mg aliquots. Once placed in 15mL beakers, the samples were then dissolved using a concentrated HF-HNO3 solution of a 2:1 ratio with 1 mL per

50 mg of sample being dissolved. This process breaks down the silicates within the

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sample. After leaving these in the dissolution step overnight, the samples were evaporated until dry, then ~1 mL of HNO3 was added to the dried sample. They were evaporated once more and another ~1 mL of HNO3 was added. These two steps aid in prevention of fluoride formation during the dissolution process. Another evaporation step takes place before the samples are covered in 1 mL concentrated HCl and heated to break down any insoluble fluorides that formed. Beakers were capped and samples were left for an hour before adding E-pure H2O to about half the height of the beaker. This allows for the entire sample to go into solution. Sample solutions were left on a hot plate overnight and the samples were once more evaporated down to dryness.

While prepping the resin in the cation columns for rare earth element (REE) separation with a cleaning of 2 sets of 10 mL of 6N HCl and charging of 5 mL of 4N

HCl, the samples were put through their own preparation of the addition of 1 mL HCl, heating on a hot plate, and centrifuging for 10 minutes. From here, the samples were loaded into the columns and eluted with 4 mL of 4N HCl (1 set of 1 mL and a second set of 3 mL) before being collected along with the next 10 mL of 4N HCl (2 sets of 5 mL).

After collection, samples were set on the hot plate under the heat lamp for another round of evaporation while the columns were cleaned with 6 sets of 10 mL of 6N HCl followed by 5 mL E-Pure H2O and backwash.

The samples were then prepared for Ln-Spec columns for Sm-Nd separation by adding 0.25 mL of 0.25N HCl. The resin within the columns was cleaned using a full reservoir (~20 mL) of 6N HCl followed by a full reservoir of E-pure H2O. Next, the resin was charged with 5 mL of 0.25N HCl. The samples were loaded and eluted in 4 mL of

0.25N HCl through four steps: (1) adding the 0.25 mL of sample from the beaker; (2)

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adding 0.25 mL to the beaker, scrubbing the beaker with transfer pipette to get all residues of the sample, and adding this to the columns; (3) adding 0.5 mL to the columns;

(4) adding 3 mL to the columns. After elution, the Nd fraction was collected in 6 mL of

0.25N HCl and set on the hot plate under the heat lamp to evaporate. The columns were cleaned using two full reservoirs of 6N HCl. They were equilibrated with 5 mL of 1N

HCl. After the samples were dry, one drop of concentrated HCl and one drop of HNO3 were added, the samples were dried again, one drop of HNO3 was added, and they were dried a final time.

The next step was to load the samples onto the evaporation filament of a double

Re filament assembly in preparation for TIMS analysis. After loading the wheel of samples, the mass spectrometer must sit overnight to return to the proper pressure for vacuum analysis. As each sample ran, the evaporation filament was heated enough to cause the sample to evaporate and the ionization filament, which is kept at a much higher temperature, ionized the sample atoms. After acceleration of the ions and magnetic separation of the masses, the ion beams were measured simultaneously using multi- collector Faraday cups.

Data collection included all isotopes of Nd as well as 147Sm to correct for the isobaric 144Sm interference. Mass fractionation corrections were based on 144Nd/146Nd =

0.7219. Initial Nd isotope ratios were calculated using respective sample 147Sm/144Nd ratios (based on measured Sm and Nd concentrations) and the sample ages, from which initial εNd values were further calculated. The following equations were used:

( ) , (1)

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where i signifies initial and m refers to the measured value. C is a constant defined by the following equation:

, (2)

where Nd refers to the atomic weight of Nd, Sm refers to the atomic weight of Sm, 147Sm refers to the abundance of 147Sm in the Earth, and 144Nd refers to abundance of 144Nd in the Earth. Finally, λ is the decay constant of Sm (λ=6.54E-12) and t is the time of formation.

From here, εNd initial or present day can be calculated. For our study, the initial value was used in order to compare to values of potential crustal and mantle sources at the time of formation. The equation for εNd initial or present day requires finding the Nd isotope ratios of the chondrite uniform reservoir (CHUR) at the time of formation.

CHUR assumes that the Nd in the Earth’s mantle evolved in a uniform reservoir, which has a Sm/Nd ratio equal to that of chondrite meteorites. The equation for time-based

CHUR is as follows:

( ) ( ) ( )

. (3)

In this case, t=0 refers to present day and the 143Nd/144Nd CHUR of the present day has been calculated as 0.512638 (Rollinson, 1993). The 147Sm/144Nd CHUR of the present day has been determined as 0.1967 (Rollinson, 1993). The decay constant and time values remain the same as in equation (1).

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The present day CHUR value and the initial 143Nd/144Nd ratio are put into the following equation to give the εNd initial value:

( )

( ) . (4)

All previous definitions of values remain the same for equation (4). εNd values can be plotted in multiple ways on bivariate plots. One way is to plot εNd against age in order to compare and contrast the sample values to those of specific crustal signatures or known mantle or igneous rock types (E-MORB, N-MORB, etc.). Another possibility is to plot

εNd against other measured isotopic ratios, particularly Pb and Sr. Due to the metamorphism undergone by the rocks in this study as well as the mobile nature of Pb and Sr during metamorphism, Nd was the only isotope analyzed.

Ages used in calculations are best estimates as the crystallization ages of mafic igneous rocks within the RHZ, which relies on dating intermediate or felsic meta-igneous rocks associated with them. Accurate crystallization ages of intermediate and felsic orthogneisses and meta-volcanic rocks, typically done by high precision U-Pb dating of zircon, are relatively scarce throughout the RHZ (for a typical example, see Kim et al.,

2003). Therefore, εNd values were calculated using large age ranges to reflect the associated errors.

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