The Petrogeneis of the Highlandcroft and Oliverian Plutonic Suites, New Hampshire: Implications for...

THE PETROGENESIS OF THE HIGHLANDCROFT AND OLIVERIANPLUTONIC SUITES, NEW HAMPSHIRE: IMPLICATIONS FOR THE

STRUCTURE OF THE BRONSON HILL TERRANE

MICHAEL J. DORAIS*, JENNY WORKMAN*, and JUGDEEP AGGARWAL**

ABSTRACT. Both Laurentian and peri-Gondwanan origins for the Oliverian andHighlandcroft Plutonic Suites of New Hampshire have been proposed. A peri-Gondwanan origin places the Red Indian Line, the principle Iapetus suture, to the westof the Bronson Hill Anticlinorium and requires the plutons to be allochthonous. Incontrast, a Laurentian margin emplacement setting places the Red Indian Line fartherto the east in New Hampshire. The combination of �Nd and 207Pb/204Pb values aremost compatible with the suites representing mixtures of arc-derived magmas andLaurentian crust, indicating that both suites were derived to the west of the Red IndianLine. The tectonic setting for the arc probably was a westerly dipping subduction zoneunder the Laurentian margin, resulting from a polarity flip after the collision of theShelburne Falls arc during the Taconic orogeny. Bulk-rock major and trace elements,as well as Nd, Sr, and Pb isotopic results, indicate that the Highlandcroft and OliverianPlutonic Suites of New Hampshire are both members of the same arc complex.

introductionThe traditional interpretation of the Taconic orogeny in New England is that the

orogen was the result of a collision between the Bronson Hill terrane, an island arcproduced by an easterly dipping subduction zone, and the continental margin ofLaurentia. This model explains the west-directed thrusts, the stratigraphic transitionfrom continental shelf to rift facies to oceanic accretionary prism to fore-arc and arcdeposits (Bradley, 1989; Kim and Jacobi, 1996), and the distribution of high-pressuremetamorphic rocks (Laird and others, 1984) across New York and Vermont. In thisinterpretation, island arc magmatism extended from � 480 to 440 Ma, the result ofprolonged easterly subduction (Stanley and Ratcliffe, 1985; Hollocher and others,2002).

Other geologists argue against extended easterly subduction, instead preferring atwo arc model (Karabinos and others, 1998, 2003). Farther to the west, the � 485 to470 Ma Shelburne Falls arc is separated from the Bronson Hill by the ConnecticutValley Trough and the Hartford Basin. Karabinos and others (1998, 2003) interpretthe Shelburne Falls arc as the colliding terrane that caused the Taconic orogeny at� 470 Ma which was followed by a subduction zone polarity flip to the west by � 450Ma. This two arc, polarity flip model is well cited to explain the Canadian andCaledonide portions of the orogeny (van Staal, 1994; Cawood and others, 1995;MacNiocaill and others, 1997; van Staal and others, 1998, 2007; Dewey and Mange,1999; Draut and Clift, 2001), but has met resistance in New England (Ratcliff andothers, 1998; Robinson and others, 1998; Hollocher and others, 2002).

The igneous rocks of the Bronson Hill terrane have bearing on this controversy.Here magmatism was diachronous with eruption of the � 475 to 465 Ma island arcmagmas, referred to as the Ammonoosuc Volcanics (Moench and Aleinikoff, 2003)and associated Joslin Turn, Chickwolnepy and Cambridge Black intrusions. Thesevolcanic and plutonic rocks constitute the Ammonoosuc sequence of Moench andAleinikoff (2003). Following a 10 to 15 Ma hiatus, the continental arc Quimbysequence magmas were erupted/emplaced. This younger magmatic event is expressed

*Department of Geological Sciences, Brigham Young University, Provo, Utah 84602, [email protected]** Keck Isotope Laboratory, Department of Earth Sciences, A232 Earth and Marine Sciences, University

of California, Santa Cruz, California 95064

[American Journal of Science, Vol. 308, January, 2008, P. 73–99, DOI 10.2475/01.2008.03]

73

as 1) the volcanic Quimby Formation and 2) as plutonic rocks in the cores of themantled gneiss domes, known as the Oliverian Plutonic Suite, that form the axis of theBronson Hill terrane. Temporally and spatially associated with the Oliverian Suite isthe Highlandcroft Plutonic Suite which is located just to the west of the Bronson Hillaxis. Both the Ammonoosuc Volcanics and younger volcanic rocks of the QuimbyFormation (443 � 4 Ma; Moench and Aleinikoff, 2003) mantle the Oliverian corerocks.

The magmatic sources and emplacement locations of the Oliverian and Highland-croft plutonic rocks are also controversial, the resolution of which has importantimplications for understanding the Taconic Orogeny in the Northern Appalachiansand the nature of the colliding tectonic elements and basement terranes. For example,Lyons and others (1986) suggested that the Oliverian magmas were generated inback-arc setting and the plutons intruded through coeval volcanics and volcanoclasticsediments. To the west of the Oliverian, the Highlandcroft suite plutons were inter-preted as forming in a fore-arc and trench environment where the Highlandcroftmagmas rose through and were contaminated by Grenvillian continental crust. Moenchand Aleinikoff (2003), agreeing with Lyons and co-workers on a Grenville marginalsetting, suggested that both suites were intruded west of the Red Indian Line, theprinciple Iapetan suture between Laurentia and peri-Gondwanan terranes, with theHighlandcroft suite being emplaced along the Laurentian continental margin. In thisview, the Red Indian Line lies to the east of the Bronson Hill, buried below themetasediments of the Central Maine trough [RIL (1) of fig. 1; Dorais and Paige, 2000;Moench and Aleinikoff, 2003]. An opposing view was presented by Robinson andothers (1998) who proposed that the Oliverian suite appears to have been built oneither oceanic or continental crust of Gondwanan affinity, east of the Red Indian Line,

Fig. 1. Generalized geologic map of New England showing the terranes of the northern Appalachiansand the location of the Bronson Hill terrane. Dashed line RIL (1) represents the Iapetus suture buriedbeneath the metasediments of the Central Maine trough (after Moench and Aleinikoff, 2003 and Dorais andPaige, 2000). Solid line RIL (2) along the western border of the Bronson Hill terrane represents the Iapetussuture of van Staal (2005).

74 M. J. Dorais and others—The petrogenesis of the Highlandcroft and Oliverian Plutonic

which, in this model, would lie along the western border of the Bronson Hill terrane[RIL (2) of fig. 1]. van Staal (2005) also places both intrusive suites to the east of theRed Indian Line on the Gondwanan side of the Iapetus suture. He suggested that theBronson Hill Arc may be continuous with the Popelogan-Victoria Arc of the Exploitssubzone, the peri-Gondwanan portion of the Dunnage zone of Maritime Canada.

In this paper, we present bulk-rock major, trace, and isotopic data to show 1) theOliverian and Highlandcroft Plutonic Suites are the products of a continental arc, incontrast to the island arc affinities of the older, mantling Ammonoosuc Volcanics; 2)the bulk-rock major and trace element abundances and the isotopic characteristics ofthe Oliverian and Highlandcroft rocks are similar to the volcanic rocks of the QuimbyFormation of northern New Hampshire; and 3) the isotopic characteristics of theOliverian and Highlandcroft rocks are indicative of a Laurentian crustal component.The geochemical characteristics of the Oliverian and Highlandcroft rocks are permis-sive of the subduction zone polarity flip model (Karabinos and others, 1998; van Staaland others, 1998) with the magmas having been generated over a westerly dippingsubduction zone along the Laurentian margin.

geological settingAlong the Bronson Hill terrane (fig. 1) lies a series of domes cored by the

Oliverian Plutonic Suite which consists of a near linear array of plutons extending fromthe shores of the Long Island Sound in Connecticut to the New Hampshire - Maineborder. The plutons show regional chemical variations with dominantly granitic rocksin the northern domes (Mascoma dome and northward, fig. 2) and dominantlytonalitic to granodioritic domes to the south (Hollocher and others, 2002). Hollocherand co-workers suggested that the northern plutons were derived from a moreK2O-rich intermediate or felsic crust, perhaps from the remains of older continentalcrusts upon which the arc was built. According to the Moench and Aleinikoff model(2003), this older crust would have been Laurentian whereas Robinson and others(1998) and van Staal (2005) think it was Gondwanan crust. South of the Mascomadome where more mafic rocks predominate, the magmas were probably derived froma more mafic source. Hollocher and others (2002) interpret this regional difference inmagma compositions as being comparable to the Aleutian arc that extends from anoceanic to continental setting (Kay and others, 1990).

The degree of metamorphism of both the Highlandcroft and Oliverian suitesvaries. Billings (1956) described the Oliverian Plutonic Suite as being characterized byfoliations and/or lineations and granoblastic textures, and since that early work, theOliverian plutons have traditionally been referred to as the cores of mantled gneissdomes. An exception to this interpretation is that of Kohn and Spear (1999) whoshowed that the mineral assemblages of some of the portions of the Oliverian rocks ofthe Keene and Alstead domes of southwestern New Hampshire preserve magmatictemperatures, confirming the igneous mineralogy of these rocks. This apparentcontradiction is resolved by considering that the degree of deformation across Oliver-ian plutons is variable with the most deformed rocks occurring at the contacts (Naylor,1987).

The Highlandcroft and Oliverian Plutonic Suites of western New Hampshire havelong been known to be compositionally similar (Billings and Wilson, 1965). Isotopicage determinations are also essentially the same for both suites (Oliverian, 435 – 456Ma, Highlandcroft, 441 – 452 Ma; Lyons and others, 1986, 1997; Moench andAleinikoff, 2003), leading some researchers to suggest that the two suites are consan-guineous. In spite of these similarities, the two suites occupy distinct structural settingsthat lend uncertainties as to the relationships between the two. The Oliverian PlutonicSuite occurs as mantled gneiss domes along the axis of the Bronson Hill Anticlinorium(fig. 1) whereas the Highlandcroft Plutonic Suite consists of sharply discordant plutonswest of the axis of the anticlinorium (fig. 2). The interpretation that the Highlandcroft

75Suites, New Hampshire: Implications for the structure of the Bronson Hill terrane

Fig. 2. Generalized geologic map of New Hampshire showing the locations of the Oliverian andHighlandcroft Plutonic Suites. Studied plutons include the Jefferson Batholith, Whitefield, Owls Head, andMascoma Domes of the Oliverian suite (white filled plutons) and the Lost Nation and Highlandcroft plutonsof the Highlandcroft suite (black filled plutons).


suite experienced greenschist facies metamorphism compared to the higher gradeOliverian suite led Naylor (1969) and Pogorzelski (ms, 1983) to suggest that theHighlandcroft was emplaced at higher crustal levels than the Oliverian suite, and theHighlandcroft may be decapitated Oliverian plutons downdropped to the west acrossthe Ammonoosuc fault, one of the major Triassic normal faults of central New England(Lyons and others, 1997; Moench and Aleinikoff, 2003).

Pertaining to this larger picture of tectonic emplacement and basement identifica-tion, the nature of the contact between the Oliverian plutons and the overlyingAmmonoosuc Volcanics has been highly debated. U-Pb zircon ages from quartz-phyrictuffs in the upper part of the Ammonoosuc and in the lower portion of the PartridgeFormation from the Bernardston area in Massachusetts gave ages of 453 � 2 and 449 �3 Ma respectively, ages that are indistinguishable from those of the Oliverian suite(Tucker and Robinson, 1990). Either the plutons were emplaced at shallow levels,intruding volcanic rocks of essentially the same age, or as preferred by Robinson (inTucker and Robinson, 1990 and Robinson and others, 1998), the contact between thevolcanic and intrusive rocks in Massachusetts could be a low angle extensionaldetachment formed during late arc history or early Silurian post-Taconian relaxation.He suggested that the contact could be a west-dipping detachment with major crustalextension that brought shallow back-arc basin deposits into direct contact with theintrusive core of the main arc as represented by the Oliverian suite (Robinson andothers, 1998).

This interpretation contrasts with observations in northern New Hampshire.Billings (1937), Leo (1991) and Moench and Aleinikoff (2003) concluded that theOliverian plutonic suite is intrusive into an older Ammonoosuc Volcanics and DeadRiver Formation. Resolution of this problem may be that the dated volcanic rocks inMassachusetts are actually part of what is called the younger Quimby sequence innorthern New Hampshire (Moench and Aleinikoff, 2003). Thus the Oliverian suitemay be intrusive into the older Ammonoosuc sequence in northern New Hampshirewhereas there may be a tectonic contact between the equivalent age, younger Quimbysequence of Moench and Aleinikoff (2003) in Massachusetts and southern NewHampshire (Hollocher and others, 2002).

The nature of the lower contact between the Oliverian plutons and their base-ment has also been the subject of considerable discussion. Researchers who considerthe Oliverian suite to have originated on peri-Gondwanan crust east of the Red IndianLine require allochthonous sheets to have carried the rootless Oliverian plutons overthe Laurentian margin (see for example, van Staal, 2005). Lyons and others (1996)attempted to resolve the issue by a geophysical study of several Oliverian plutons. TheMoody Ledge, Owls Head, Baker Pond, Smarts Mountain permit an interpretation thatthey may be intrusive sheets or floored by thrusts. In contrast, however, the MascomaDome is mushroom shaped, which the authors suggest does not support a thrusthypothesis for its floor.

Our studied plutons include the Lost Nation and Highlandcroft plutons of theHighlandcroft Plutonic Suite and the Jefferson batholith, Owls Head, and Mascomaplutons of the Oliverian Plutonic Suite (fig. 2). Difference of opinion exists as towhether the Whitefield pluton belongs to the Oliverian or the Highlandcroft suites. Ithas traditionally been included as an Oliverian pluton (Billings, 1956), but Moenchand Allienikoff (2003) grouped it with the Highlandcroft suite. Based on geochemicalsimilarities with the Jefferson Batholith, we follow Rankin (personal communication,2006) and include the Whitefield with the Oliverian suite.

instrumental methods

Bulk-rock major and trace element XRF analyses were conducted with a SiemensSRS 303 at BYU using fused disks for major elements and pressed powder pellets for


trace elements. Additional trace elements were determined by inductively coupledplasma mass spectrometry (ICP-MS) at ALS Chemex at Reno, Nevada.

Nd, Sr, and Pb isotopic compositions were measured at the University of Califor-nia Santa Cruz. Approximately 100mg of rock powder digested with 3ml concentratedhydrofluoric and 1ml concentrated nitric acid overnight on a hot plate. After diges-tion, samples were dried before being redissolved in 8M nitric acid and dried again.Samples were redissolved in 6M hydrochloric acid before a final drying. 0.5ml 0.1Mhydrobromic acid was added to each sample to dissolve the digest. After centrifuging,samples were loaded onto preconditioned lead ion exchange columns containingAG1X8 100-200 mesh cation exchange resin. Lead was eluted off the column with1.5ml of 6M hydrochloric acid and the sample dried prior to loading on a filament forthermal ionization mass spectrometry.

The non-Pb eluate was dried and redissolved in 2.5M hydrochloric acid beforebeing loaded onto a cation exchange column containing AG50X8 cation exchangeresin for separation of Sr from the matrix. Once collected, the Sr was dried for TIMSanalysis. Rare earth elements were eluted off the column using 6M hydrochloric acidand dried to a few microliters. This was then diluted to 100�l using 0.25M nitric acidbefore being loaded onto conditioned LnSpec ion exchange resin. Using 0.25M nitricacid, Nd was chromatographically separated from the other REE and the Nd cut wasdried for TIMS analysis.

Thermal ionization mass spectrometry was carried out on a VG Sector 54 instru-ment housed within the W.M. Keck Isotope Laboratory at the University of California,Santa Cruz. Lead samples were loaded with Si gel onto outgassed Re filaments.Strontium samples were run dynamically on outgassed Re filaments and Nd sampleswere run statically on triple filament assemblies on outgassed Re filaments. Strontiumisotope ratios were fractionation corrected using 86Sr/88Sr and Nd isotope ratios werecorrected using the 146Nd/144Nd. Lead isotope ratios were corrected using the averagefractionation correction per amu for the SRM NBS 981. Standard results (and longterm external precision with standard deviations) for NBS 987 gave a 87Sr/86Sr of0.710254 (0.710264 � 0.000031), La Jolla 143Nd/144Nd of 0.511859 (0.511838 �0.00017) and NBS 981 gave 206Pb/204Pb of 16.898 (16.9309 � 0.0026), 207Pb/204Pb15.443 (15.4840 � 0.0039), and 208Pb/204Pb 36.550 (36.6747 � 0.0070). All sampleswere run in an identical way to that of the standards.

bulk-rock geochemistry

Major ElementsRepresentative major and trace element analyses of the Lost Nation, Highland-

croft, Whitefield, Jefferson, Mascoma, and Owls Head plutons are given in table 1. TheNa2O � K2O versus SiO2 and the K2O versus SiO2 diagrams of figure 3 include ourOliverian and Highlandcroft analyses as well as those of the Ammonoosuc Volcanics(Aleinikoff, 1977; Schumaker, 1988) and, as defined by Moench and Aleinikoff (2003),the younger Quimby sequence volcanic rocks (Schumaker, 1988; Hingston, ms, 1992;Hollocher, 1993). These and all subsequent bulk-rock major and trace elementdiagrams also include Oliverian and Highlandcroft analyses from Pogorzelski (ms,1983), Leo (1991) and Hingston (ms, 1992).

The Oliverian rocks plot in two general fields, one at relatively low alkalis and theother, together with the Highlandcroft suite, at higher concentrations of alkalis (fig.3A). Samples from the Jefferson Batholith plot in both fields. In the K2O versus SiO2diagram (fig. 3B), the Jefferson Batholith samples show a range in K2O contents fromas low as 0.89 percent and as high as 6.9 percent. Leo (1991) noted the presence of atrondjemitic component in the Jefferson, an observation supported by our analyses.The Ammonoosuc Volcanics predominantly plot as low-K tholeiites with minor overlap


in the medium K, calc-alkaline field. The younger Quimby volcanic rocks mainly plotin the calc-alkaline field along with the K2O-poorer samples of the Jefferson Batholithand the Whitefield pluton. The Highlandcroft, together with the majority of theOliverian samples, plot in the high-K calc-alkaline and shoshonitic fields.

As noted by Hollocher and others (2002), the Oliverian Plutonic Suite innorthern New Hampshire consists primarily of gneisses of granodioritic and granitic

Table 1

Representative Analysis, Highlandcroft and Oliverian Plutonic Suites, New Hampshire

Suite HC* HC HC HC HC HC HC HC HC HC HC

Pluton Lost

Nation Lost

Nation Lost

Nation Lost

Nation Lost

NationLost

NationLost

NationLost

NationLost

Nation Lost

NationHighland-

croft Sample LN-1 LN-2 LN-3 LN-4 LN-5 LN-6 LN-7 LN-8 LN-9 LN-10 HC-1 SiO2 58.35 53.84 55.63 55.24 54.96 55.32 65.78 65.95 64.82 68.45 63.98 TiO2 0.73 0.85 0.89 0.82 0.74 0.78 0.45 0.42 0.39 0.34 0.38 Al2O3 16.11 17.62 16.15 16.92 16.65 17.12 15.29 15.67 14.94 15.33 14.06 Fe2O3 8.03 9.30 9.47 8.93 8.99 8.82 4.37 4.04 4.23 2.51 4.39 MnO 0.15 0.18 0.18 0.17 0.17 0.17 0.08 0.12 0.09 0.05 0.09 MgO 2.97 3.58 4.11 3.44 3.43 3.34 2.00 2.35 2.00 0.99 2.41 CaO 6.88 7.86 5.94 7.05 7.85 7.03 1.71 2.79 3.34 2.04 4.04 Na2O 3.51 3.54 3.21 3.43 4.08 2.94 4.15 2.94 3.27 4.81 1.10 K2O 2.00 2.09 1.83 2.21 1.87 2.07 3.76 3.87 3.94 4.07 6.00 P2O5 0.21 0.29 0.30 0.25 0.24 0.26 0.14 0.16 0.14 0.13 0.15 LOI 1.25 1.49 2.4 1.9 1.92 2.29 1.68 2.94 3.73 1.2 4.54 Total 100.177 100.639 100.1 100.347 100.9 100.136 99.4 101.26 100.87 99.91 101.15 Rb 54 59 60 65 65 60 82 109 100 86 166 Cs 1.3 1.5 1.2 0.9 0.6 2 Sr 500 553 429 498 548 513 469 440 449 902 241 Ba 1137 1288 1063 1300 1132 1080 1288 1259 1532 1085 1767 La 26.6 34.3 30.5 36.5 56.9 31.1 Ce 53.2 73.9 64.2 57.5 103.5 63.7 Pr 6.1 8.7 6.8 6.6 9 6.4 Nd 24.5 35.8 26.6 22.7 29.2 23.1 Sm 4.8 7 5.4 3.1 3.9 4 Eu 1.3 1.8 1.3 0.8 0.9 0.9 Gd 4.8 7.2 4.9 3.8 3.3 3.5 Tb 0.7 1.1 0.7 0.5 0.4 0.4 Dy 3.4 5.3 3.7 1.8 1.4 2 Ho 0.7 1.2 0.8 0.4 0.3 0.4 Er 2.1 3.3 2.3 1.1 0.8 1.2 Tm 0.3 0.5 0.3 0.2 0.1 0.2 Yb 1.8 2.9 2.0 1.0 0.7 1.2 Lu 0.3 0.5 0.3 0.2 0.1 0.2 Zr 163 205 101 159 164 154 149 142 132 219 121 Hf 4 7 5 4 6 5 Y 21 32 20 26 24 23 12 12 13 7 13 Nb 7 9 8 8 8 7 7 0 7 13 7 Th 3 7 3 2 3 5 14 12 11 26 12 U 0.7 1.9 2 2 2 1.1 2.9 2 3 2.6 1.8 Pb 12 13 13 13 13 11 25 33 11 15 22 Sc 25 29 22 24 28 28 12 10 10 0 16 V 201 234 239 242 215 224 98 99 84 44 142 Cr 19 22 26 30 23 24 46 39 34 12 69 Ni 11 14 15 14 14 15 20 15 13 9 21 Cu 22 31 30 22 28 39 27 13 8 4 41 Zn 72 88 100 92 85 87 59 54 47 34 40 Ga 17 18 17 17 17 18 15 17 15 19 17

Tzir 725 729 702 720 702 728 775 773 749 796 736

*HC � Highlandcroft


compositions, a statement supported by our analyses that show SiO2 contents between62 and 76 percent (fig. 4). In contrast, the Highlandcroft Plutonic Suite extends tomore mafic compositions with SiO2 contents ranging between 54 to 74 percent. Thesuites overlap in the Harker diagrams of figure 4, but differ in the aforementionedmedium K calc-alkaline group of Whitefield pluton and several Jefferson Batholithrocks (fig. 3).

Table 1

(continued)

Suite HC HC HC HC OL** OL OL OL OL OL OL

Pluton Highland-

croft Highland-

croft Highland-

croft Highland-

croft Jefferson Jefferson Jefferson Jefferson Jefferson Jefferson Jefferson Sample HC-2 HC-3 HC-4 HC-5 JB-1 JB-2 JB-3 JB-4 JB-5 JB-6 JB-7 SiO2 65.40 59.40 62.00 58.84 71.48 71.82 74.80 74.60 70.70 66.10 66.80 TiO2 0.38 0.48 0.52 0.47 0.25 0.28 0.23 0.24 0.20 0.45 0.42 Al2O3 13.90 15.00 15.60 15.00 15.06 15.53 12.70 13.00 16.00 15.60 15.90 Fe2O3 4.71 6.34 5.88 5.91 1.65 1.70 2.51 2.03 1.26 3.49 3.36 MnO 0.10 0.12 0.09 0.11 0.04 0.04 0.04 0.05 0.02 0.07 0.07 MgO 2.41 3.63 3.19 3.29 0.32 0.33 0.65 0.72 0.19 1.41 1.27 CaO 3.24 4.70 2.41 4.73 1.68 1.18 1.31 1.35 1.33 2.88 2.60 Na2O 2.41 2.64 1.93 2.11 4.15 3.89 5.15 5.46 3.71 4.04 4.15 K2O 4.77 3.86 5.14 4.66 5.87 6.43 0.89 1.03 6.90 4.42 4.36 P2O5 0.14 0.22 0.23 0.23 0.09 0.06 0.09 0.14 0.04 0.25 0.22 LOI 3.92 4.36 4.14 4.92 0.24 0.34 1.41 0.93 0.24 0.83 0.72 Total 101.38 100.75 101.13 100.26 100.84 101.59 99.78 99.55 100.59 99.54 99.87 Rb 106 122 132 117 114 132 34 28 112 97 103 Cs 1.8 2 1.2 1.3 1.5 1.6 Sr 272 509 211 504 474 398 177 142 468 1154 1120 Ba 1341 1540 1810 1771 1020 932 236 330 1253 1123 1468 La 40.2 48 55.7 15.5 54 54.9 Ce 77.3 86.9 106 32.4 113.5 104 Pr 7.8 8.4 11.2 3.4 11.4 10.6 Nd 28 28.9 38.4 12.8 38.3 35.1 Sm 4.6 4.8 6.1 2.7 5.1 4.4 Eu 1.1 1.2 1.1 0.7 1.2 1.1 Gd 4.1 4.6 5.7 3 4.5 4.1 Tb 0.5 0.6 0.7 0.6 0.5 0.4 Dy 2.3 2.5 3.3 3.5 1.7 1.6 Ho 0.5 0.5 0.7 0.9 0.3 0.3 Er 1.4 1.5 2.1 2.7 1 0.9 Tm 0.2 0.2 0.3 0.4 0.1 0.1 Yb 1.3 1.4 1.9 3.0 0.9 0.8 Lu 0.2 0.2 0.3 0.5 0.1 0.1 Zr 141 163 145 169 180 186 130 145 146 235 231 Hf 5 6 4 4 5 5 Y 14 15 14 17 21 22 28 28 17 9 8 Nb 8 7 8 8 17 19 0 0 12 16 16 Th 17 11 21 17 23 43 12 13 21 44 20 U 3 3 3.7 3.5 5.1 5 0 1.6 3 6.2 4.5 Pb 14 20 22 17 27 24 6 5 23 20 23 Sc 13 22 20 19 0 0 10 8 0 5 0 V 112 162 151 148 29 29 24 24 18 57 57 Cr 75 136 106 110 1 1 1 0 0 10 10 Ni 21 40 30 32 0 0 0 0 0 8 8 Cu 13 45 61 34 0 0 0 1 0 7 3 Zn 35 49 45 50 7 10 7 12 7 30 30 Ga 13 16 17 16 13 13 12 14 13 18 17

Tzir 753 747 777 750 779 790 774 779 768 796 800

**OL � Oliverian


Trace ElementsRepresentative trace element abundances are also given in table 1. Rb and Ba

versus Sr concentrations are plotted in figure 5. Like the alkali versus SiO2 diagrams,these diagrams show two populations; one population has a loosely defined trend ofdecreasing Sr concentrations with a gradual increase in Rb and Ba consistent with

Table 1

(continued)

Suite OL OL OL OL OL OL OL OL OL OL OL

Pluton Whitefield Whitefield Whitefield Whitefield Whitefield Mascoma Mascoma Mascoma MascomaOwls Head

Owls Head

Sample WF-1 WF-2 WF-3 WF-4 WF-5 MD-1 MD-2 MD-3 MD-4 OH-2 OH-4 SiO2 73.73 64.19 74.10 63.33 64.53 73.87 73.53 74.10 74.40 71.70 75.10 TiO2 0.18 0.45 0.12 0.49 0.52 0.17 0.16 0.17 0.16 0.26 0.14 Al2O3 14.37 15.89 13.70 16.00 15.70 13.87 13.46 13.50 13.70 14.30 13.00 Fe2O3 2.86 6.19 2.13 6.88 6.80 1.60 1.50 1.59 1.73 2.59 1.06 MnO 0.07 0.13 0.09 0.13 0.11 0.05 0.04 0.05 0.06 0.08 0.05 MgO 0.74 1.98 0.48 2.16 2.17 0.45 0.39 0.36 0.42 0.62 0.26 CaO 2.40 5.46 2.76 5.41 5.22 1.50 1.31 1.47 1.56 1.98 0.79 Na2O 3.39 3.05 3.84 2.79 3.77 3.86 3.62 3.48 4.01 4.12 4.30 K2O 2.59 1.90 1.66 2.14 0.73 5.11 4.87 4.74 4.04 4.14 4.48 P2O5 0.05 0.09 0.03 0.12 0.10 0.06 0.07 0.14 0.07 0.11 0.07 LOI 0.72 0.85 0.63 0.93 0.30 0.73 0.28 0.22 0.27 0.33 0.23 Total 101.10 100.16 99.54 100.38 99.95 101.26 99.24 99.82 100.42 100.23 99.48 Rb 66 62 52 75 26 146 147 148 141 134 130 Cs 0.8 1.5 2 1.6 2.3 1.8 Sr 189 260 206 223 299 329 317 345 355 245 148 Ba 607 382 552 418 192 1796 1553 1703 1529 1492 1823 La 18.7 17.1 24.9 43.3 57.2 43.1 Ce 37 36.9 43.8 75.3 91 76.5 Pr 3.2 3.7 4.7 6.8 8.7 7.3 Nd 10.8 13.9 16.2 22 27 23.7 Sm 1.7 2.7 2.9 3.2 3.4 3.8 Eu 0.4 0.7 0.8 0.6 0.7 0.8 Gd 1.8 2.6 3.2 2.8 3.8 3.9 Tb 0.2 0.4 0.5 0.3 0.4 0.5 Dy 1.2 2.2 2.1 1.4 1.7 2.8 Ho 0.3 0.5 0.5 0.3 0.4 0.6 Er 0.8 1.4 1.4 0.9 1.2 2 Tm 0.1 0.2 0.2 0.1 0.2 0.3 Yb 0.9 1.4 1.3 0.9 1.2 1.8 Lu 0.2 0.2 0.2 0.1 0.2 0.4 Zr 89 102 93 99 111 136 128 135 142 174 101 Hf 2 3 3 4 4 5 Y 9 14 14 15 15 11 11 11 12 19 18 Nb 7 8 8 8 8 8 9 11 10 8 0 Th 7 6 16 6 11 23 29 34 28 23 24 U 1.4 1.9 0 1.5 0 5.1 5 6 5.4 4.2 0 Pb 23 24 11 17 12 33 31 31 38 18 28 Sc 6 18 5 17 21 0 0 0 0 0 0 V 28 110 19 123 120 17 15 14 17 28 4 Cr 2 8 1 10 9 0 0 0 0 2 0 Ni 0 6 1 5 5 0 0 0 0 1 0 Cu 58 23 3 15 24 0 0 0 0 2 0 Zn 43 70 22 67 43 12 9 7 9 17 6 Ga 12 17 11 16 17 10 11 11 12 12 10

Tzir 745 724 743 723 733 762 763 769 772 782 743


magmatic differentiation (Field 1). The second population plots at lower Ba and Rbvalues at low Sr concentrations (Field 2). This second population consists of the sameWhitefield and Jefferson samples that have medium K contents in figure 3. Note thatthe Quimby samples also plot in the low Rb and Ba fields.

Chondrite-normalized REE patterns for the Highlandcroft and Oliverian PlutonicSuites and the Quimby Formation are plotted in figure 6. Both plutonic suites are verysimilar with LaN ranging from � 100 to 250 for the Highlandcroft Plutonic Suite (fig.6A) and 160 to 230 times chondrites for the majority of the Oliverian suite (Group 1 infig. 6B). Small negative Eu anomalies and relatively flat HREE profiles characterize

Fig. 3. Total alkali verus SiO2 and K2O versus SiO2 diagrams. Ammonoosuc volcanics plot as low-Ktholeiites whereas Oliverian, Highlandcroft, and Quimby rocks are richer in alkalis, specifically K2O. Quimbysamples and several samples of the Whitefield pluton and the Jefferson batholith have medium K contents;other Highlandcroft and Oliverian samples are high K.


both suites. The medium K Jefferson Batholith sample JB-4 and the Whitefield samples(Group 2 in fig. 6B) are not only alkali and Ba-poor compared to other Jefferson andOliverian rocks, they also have flatter REE patterns with a (La/Yb)N values of �3.5

Fig. 4. Harker diagrams for Oliverian and Highlandcroft Plutonic Suites. Figure includes analyses fromPogorzelski (ms, 1983), Leo (1991), and Hingston (ms, 1992). Many of the Highlandcroft suite rocks plot atlower SiO2 values than the Oliverian rocks but the two suites generally form continuous trends. Quimbysamples are similar in composition to the Oliverian and Highlandcroft samples.


versus �20 of Group 1 Oliverian rocks with similar SiO2 contents. Quimby sampleshave the same patterns as the LREE-poor Group 2 Oliverian rocks (fig. 6C).

Multi-element normalized diagrams (fig. 7) also show similarities between theHighlandcroft and Oliverian suites. Negative Nb and Ti anomalies are present for bothand the overall range in trace element concentrations is similar. Some Highlandcroftsuite samples show positive Th anomalies, others from the same suite show negativeanomalies whereas with the exception of two Whitefield pluton samples, the Group 1Oliverian rocks have positive Th anomalies. The medium K and LREE-poor Oliveriansamples (Group 2 in fig. 7B) have the lowest highly incompatible element abundancesand the highest Tb-Yb values of the Oliverian suite. As with previously discussed

Fig. 5. Rb and Ba versus Sr diagrams. Oliverian and Highlandcroft samples define two populations;most samples form a general trend (field 1) of decreasing Sr with increasing Rb and Ba. However Jefferson,Whitefield, and Quimby samples with low Na2O�K2O values of figure 3 are also poor in Rb, Ba and Sr(field 2).


Fig. 6. Chondrite-normalized REE patterns. Both suites show the same general range in REE abun-dances with minor negative Eu anomalies and flat HREE (6A and group 1 of 6B). They differ however, inthat the Highlandcroft suite rocks show an increase in REE with increasing SiO2 whereas the Oliverian suiterocks decrease. Oliverian Group 2 rocks are LREE-poor compared to Group 1, as well as having lower alkaliand alkaline earth contents show in figures 3 and 5. Quimby samples are similar to the low LREE Group 2Oliverian rocks (6C).


diagrams, the Quimby rocks show strong compositional similarities with OliverianGroup 2 rocks (fig. 7C).

Highlandcroft and Oliverian suites and Quimby rocks plot in the volcanic arc andsyncollisional fields in the Y versus Nb diagram (fig. 8A). Although both Nb and Y arerelatively immobile during metamorphism, this diagram does not distinguish between

Fig. 7. Multi-element, spider diagrams for Highlandcroft and Oliverian suite rocks (Normalizationconstants after Thompson and others, 1984). Patterns for Highlandcroft and Oliverian Group 1 rocks aresimilar, showing negative Nb, Sr, P, and Ti anomalies. Oliverian rocks have positive Th anomalies whereasHighlandcroft rocks, with 3 exceptions, show negative Th anomalies. As in previous variation diagrams,Oliverian Group 2 rocks are similar to Quimby samples (10C).


Fig. 8. Y versus Nd (11A), Y�Nb versus Rb (11B), Yb�Ta versus Rb (11C), and Yb versus Ta (11D)diagrams (after Pearce and others, 1984; WPG � within plate granites, VAG � volcanic arc granites,Syn-COLG � syn-collisional granites; ORG � ocean ridge granites). Highlandcroft and Oliverian PlutonicSuites and the Quimby volcanics plot in the volcanic arc field.


syncollisional and volcanic arc settings. Both suites plot in the volcanic arc field in theY�Nb versus Rb diagram (fig. 8B) and in the Yb � Ta versus Rb diagram (fig. 8C). Thesuitability of tectonic discrimination diagrams based on Rb contents is uncertain forplutonic rocks that have experienced variable degrees of metamorphism, however, themajority of the Highlandcroft, Oliverian, and Quimby rocks also plot in the volcanicarc field in the Ta versus Yb diagram (fig. 8D).

The Ti versus Zr diagram (fig. 9) shows two compositional trends. The majority ofthe � 470 Ma Ammonoosuc volcanics define a high Ti trend whereas the youngerOliverian, Highlandcroft, and Quimby samples have lower Ti contents. Again, thereare strong similarities between the Quimby volcanic rocks and the Oliverian andHighlandcroft suites.

Nd, Sr, and Pb Isotopic CompositionsNd, Sr, and Pb isotopic compositions for selected Oliverian and Highlandcroft

Plutonic Suite rocks are given in table 2. Initial 87Sr/86Sr versus εNd (450 Ma) values areplotted in figure 10 which includes four additional Oliverian analyses from the BakerPond and Owls Head plutons, two Ammonoosuc and five Quimby volcanics samplesfrom Hingston (ms, 1992). The majority of Oliverian, Highlandcroft and Quimbysamples plot between 0 and -4 εNd (450 Ma) and between initial 87Sr/86Sr values of0.70377 and 0.7078. Three Whitefield pluton samples and one Quimby sample show amore significant crustal component than other Highlandcroft or Oliverian sampleswith εNd (450 Ma) values between -3.7 and -7.72 and initial 87Sr/86Sr values of 0.7078 and0.7102 respectively. Other than these samples, there is good overlap between both theHighlandcroft and Oliverian samples and the Quimby rocks with no obvious petroge-netic differences between the three suites in terms of Nd and Sr isotopes.

Bulk-rock Pb isotopic values are listed in table 2 with age corrected 207Pb/204Pbversus εNd (450 Ma) values plotted in figure 11. Tomascak and others (2005) compiledNd and Pb isotopic data for rocks of several terranes of the Northern Appalachians.They combined the two isotopic systems in one diagram, permitting a better assess-ment of possible sources for plutonic rocks of New England and Maritime Canada. Onthis diagram, Laurentian crust plots at low 207Pb/204Pb values (� 15.57) and negativeε

Nd (450 Ma)values (� -4). In contrast peri-Gondwanan, non-North American crust plots at

Fig. 9. Zr versus Ti (ppm) diagram. Ammonoosuc volcanics plot at high Ti contents whereas theHighlandcroft, Oliverian and Quimby rocks are poorer in Ti.


high 207Pb/204Pb values between 15.75 and 15.60 and at εNd (450 Ma) values � -5. Alsoplotted are initial isotopic values for Mesozoic tholeiites from northeastern NorthAmerica to represent the lithospheric mantle and values for basaltic rocks from the

Table 2

Nd, Sr and Pb Isotopic Data for Oliverian and Highlandcroft Plutonic Suites

Sample 143 /144Nd % Standard Error

Nd Sm εNdo εNdt

Highlandcroft Plutonic Suite LN-2 0.512325 0.0004 35.8 7 -6.11 -1.68 LN-6 0.512286 0.0004 26.6 5.4 -6.87 -2.70 LN-7 0.512160 0.0004 22.7 3.1 -9.32 -2.88 LN-10 0.512282 0.0004 29.2 3.9 -6.94 -0.39 HC-1 0.512264 0.0004 23.1 4 -7.29 -2.10 HC-4 0.512246 0.0004 28 4.6 -7.65 -2.15 HC-5 0.512244 0.0005 28.9 4.8 -7.68 -2.24 Oliverian Plutonic Suite JB-1 0.512305 0.0003 38.4 6.1 -6.50 -0.82 JB-6 0.512278 0.0004 38.3 5.1 -7.03 -0.47 WF-1 0.512147 0.0016 10.8 1.7 -9.60 -3.70 WF-2 0.512073 0.0016 13.9 2.7 -11.60 -6.50 WF-4 0.511987 0.0010 16.2 2.9 -12.71 -7.72

Sample 87Sr/86Sr % Standard

Error Rb Sr Initial Sr

Highlandcroft Plutonic Suite LN-2 0.706910 0.0008 59 553 0.704967 LN-6 0.707091 0.0009 60 513 0.704961 LN-7 0.708738 0.0031 82 469 0.705553 LN-10 0.706490 0.0012 86 902 0.704753 HC-1 0.716558 0.0008 166 241 0.703773 HC-4 0.716383 0.0013 132 211 0.704772 HC-5 0.710912 0.0009 117 504 0.706605 Oliverian Plutonic Suite JB-1 0.709435 0.0007 114 474 0.704924 JB-6 0.706168 0.0007 97 1154 0.704592 WF-1 0.714766 0.0009 66 189 0.708290 WF-2 0.714600 0.0009 62 260 0.710170 WF-4 0.715251 0.0007 75 223 0.708897

Sample 208Pb/204Pb % Standard

Error

207Pb/204Pb % Standard Error

206Pb/204Pb % Standard Error

Highlandcroft Plutonic Suite LN-2 38.653 0.002 15.600 0.002 18.635 0.001 LN-6 38.421 0.002 15.606 0.002 18.547 0.002 LN-10 39.453 0.002 15.624 0.002 19.109 0.002 HC-4 38.794 0.002 15.618 0.002 18.896 0.002 HC-5 38.960 0.002 15.616 0.002 18.878 0.002 Oliverian Plutonic Suite JB-1 38.809 0.002 15.584 0.000 18.868 0.001 JB-6 40.474 0.002 15.632 0.000 19.581 0.002 WF-1 38.328 0.001 15.573 0.001 18.383 0.001 WF-2 38.067 0.001 15.563 0.001 18.293 0.001 WF-4 38.442 0.003 15.590 0.002 18.395 0.002 OH-2 40.018 0.003 15.638 0.000 19.564 0.002 OH-4 38.501 0.002 15.608 0.000 18.984 0.002


Antilles as an additional estimation of mantle signatures. We lack Nd isotopic values topair with our Pb data for our two Owls Head samples and have plotted the average oftwo Owls Head εNd (450 Ma) values from Hingston (ms, 1992) for these samples.

With one Owls Head sample exception that plots at slightly higher 207Pb/204Pbvalues, our age corrected, bulk-rock analyses plot between the two gray lines thatrepresent the high and low 207Pb/204Pb values of Oliverian and Highlandcroft alkalifeldspars from Moench and Alienikoff (2003). The majority of our samples plotbetween the non-North American/lithospheric mantle and Laurentian fields andshow no differences in isotopic compositions between the Highlandcroft and Oliveriansamples. The three Whitefield pluton samples differ in that they plot well within theLaurentian crust field.

discussion

Comparison of Highlandcroft and Oliverian Plutonic Suites and Quimby VolcanicsThe chemical compositions of the Highlandcroft and Oliverian Plutonic Suites

have long been known to be similar, leading to the inference that the two suites arecomagmatic. Our data support this conclusion. Chondrite-normalized REE patterns(fig. 6), multi-element diagrams (fig. 7), and tectonic discrimination diagrams (fig. 8)show no differences between the two suites, and the differences on Harker diagrams(fig. 4) can be explained as the result of a greater amount of differentiation of theOliverian magmas. Oliverian rocks tend to be richer in U and Th, which is consistentwith the overall character of greater SiO2 enrichment, suggesting that the Oliverianmay be the differentiated equivalent of the Highlandcroft. Isotopic analyses for Nd, Sr,and Pb show no differences as well. εNd (450 Ma), initial 87Sr/86Sr, and age-corrected207Pb/204Pb values are identical for both suites. The sum of all the bulk-rock data andisotopic age determinations indicate that the two suites are consanguineous and inspite of the different contact relations, are magmas from the same arc with theOliverian rocks representing more differentiated portions of the magmatic system.

Fig. 10. Initial 87Sr/86Sr versus εNd(450 Ma) values for Oliverian and Highlandcroft Plutonic Suite rocks.Most samples of both suites have the same range in εNd and initial 87Sr/86Sr ratios, but the Whitefield plutonand one Quimby sample plot at lower εNd and higher initial 87Sr/86Sr ratios, indicating that they contain ahigher crustal component than their cousins. Error bars are smaller than symbol size.


Group 2 of the Oliverian suite is distinguished from the majority of their Oliveriancousins by their lower Na2O � K2O, Rb, Ba, Sr and LREE contents. Although the alkalielements may be mobile during metamorphism, the lower abundances in all incompat-ible elements in this group, including the REE, suggests that the distinction betweenthis group and their more alkali and REE-rich Oliverian cousins is not because ofmetasomatic effects or variable degrees of metamorphism. This incompatible element-poorer Oliverian group is identical to the Quimby volcanics in all measured elements.These geochemical similarities, combined with the same age for the Oliverian andQuimby rocks, permits a consanguineous relationship between the two. In contrast,the Ammonoosuc volcanics are not only older than the Oliverian/Highlandcroft suitesby 10 to 15 m.y., they are also distinct on all geochemical variation diagrams. TheAmmonoosuc rocks are dominated by island arc tholeiites whereas the Oliverian andHighlandcroft Suites, as well as the Quimby volcanics, have continental arc signatures.

A continental arc setting for the Oliverian, Highlandcroft, and Quimby is indi-cated by 1) the tectonic discrimination diagrams (figs. 8 and 9); 2) initial 87Sr/86Srversus εNd (450 Ma) values that plot in the lower right quadrant of figure 10. Thecompilation of Tatsumi and Eggins (1995) shows that island arc rocks are character-ized by depleted isotopic values, plotting in the upper left quadrant of figure 10,whereas continental arcs have isotopic values that extend from the upper left to thelower right quadrants with increasing crustal contents in the magmas; and 3) asdiscussed below, the 207Pb/204 Pb versus εNd (450 Ma) diagram that suggests the rocksrepresent mixtures of Laurentian crust and mantle-derived magmas if not dominantlyLaurentian-derived magmas.

Zircon Geothermometry and Inherited ZirconsTIMS analyses of zircons from six Highlandcroft plutons were presented by Lyons

and others (1986). Four of these plutons showed an inherited zircon component with

Fig. 11. εNd(450 Ma) versus 207Pb/204Pb diagram (after Tomascak and others, 2005) showing fields forLaurentian, non-North American, Gander and Miramichi, and Mesozoic basalts rocks (See Tomascak andothers, 2005 for references). Recent basalts from the Antilles are also shown (Thirlwall and Graham, 1984;Sen and others, 1988; Lebron and Perfit, 1994; Turner and others, 1996; Thirlwall and others, 1996; Heathand others, 1998; Frost and others, 1998; Jolly and others, 1998). Age corrected Oliverian and Highlandcroftrocks plot between Laurentian crust and Mesozoic and Antilles basalts, thought to represent lithosphericmantle values. Horizontal gray lines bracket feldspar 207Pb/204Pb values of Moench and Aleinikoff (2003).See text for discussion.


an upper intercept age of 1.5 Ga. From these results, Lyons and coworkers concludedthat the Highlandcroft plutons ascended through or were derived from LaurentianProterozoic crust. Since that publication, it has been commonly accepted that theHighlandcroft and Oliverian suites differ in that inherited zircons are present in theHighlandcroft whereas they are absent in the Oliverian (Robinson and others, 1998;Hollocher and others, 2002; Moench and Aleinikoff, 2003). This inferred distinctionbetween the suites has been the foundation of models that indicate different tectonicsettings for the two. For example, the presence of inherited zircons in the Highland-croft suite has been cited as indicating emplacement along the margin of Laurentia, aforearc setting according to Lyons and others (1986) and the absence of inheritance inthe Oliverian suite as indicating emplacement offshore in a backarc setting.

More recent work on the Highlandcroft suite has determined that two of theplutons examined by Lyons and others (1986) are actually not members of theHighlandcroft suite. The Fairlee pluton belongs to the Devonian Bethlehem Gneiss ofthe New Hampshire Plutonic Suite (Moench and Aleinikoff, 1987; Leo, 1991) and theEast Inlet pluton is too young to be a Highlandcroft pluton (Lyons and others, 1997).That leaves the Highlandcroft and Lost Nation plutons as the only Highlandcroft suiteplutons with reported inherited zircon fractions.

Lyons and others (1986) analyzed three zircon fractions from the Highlandcroftpluton, two of which show identical 207Pb/206Pb ages of � 453 Ma. The third fractionhas a slightly older 207Pb/206Pb ages of � 458 Ma that lies within analytical uncertain-ties of the other two fractions. More recent work by Moench and Aleinikoff (2003)includes SHRIMP analyses of the Highlandcroft pluton, no inheritance was noted.Thus the evidence of inherited zircon in the Highlandcroft pluton is negligible.

Better evidence of inheritance was found for the Lost Nation pluton. Lyons andothers (1986) found that four zircon fractions did not form a linear array in 207Pb/235Uversus 206Pb/238U diagram (their Fig. 4). Ages ranged from 480 to 639 Ma with anupper intercept of � 1525 Ma. However, the lack of SHRIMP confirmation ofinheritance in the Lost Nation pluton and the absence of inheritance for otherHighlandcroft plutons (Moench and Aleinikoff, 2003), weakens the argument thatzircon inheritance is a characteristic of the Highlandcroft Plutonic Suite that distin-guishes it from the Oliverian Plutonic Suite.

Zircon geothermometry has a bearing on the inheritance problem. If inheritanceis characteristic of the Highlandcroft Suite, one would expect relatively high Zrconcentration in the rocks, values that would lead to high zircon saturation tempera-tures. Hollocher and others (2002) cite one of the Highlandcroft pluton analyses ofLeo (1991) that contains 1598 ppm Zr as a prime candidate for inherited zircon basedon the zircon saturation experiments of Watson and Harrison (1983). However, noneof our analyses of the suite, including five from the Highlandcroft pluton and ten fromthe Lost Nation pluton, show anomalously high Zr concentrations. Zircon saturationtemperatures (table 1) range between 702 and 796oC for the Highlandcroft suite witha mean of 742oC, slightly lower than the 743 to 800o C range of the Oliverian suite(mean of 773oC). We therefore find the evidence tenuous at best that the Highland-croft Plutonic Suite can be distinguished from the Oliverian Suite based on thepresence of inherited zircons, and suggest that distinct petrogenetic and tectonicmodels for the two suites based on the inferred presence of inherited zircons may needto be re-evaluated, especially because there appears to be no geochemical and isotopicdifferences between the two suites. Hollocher and others’ (2002) suggestion that theHighlandcroft had relatively pristine continental sources less diluted by large volumesof mafic arc magmas compared to the Oliverian magma sources may not be validbecause 1) the Highlandcroft extends to more mafic compositions than the Oliverian,and 2) the identical Nd, Sr, and Pb isotopic values for both suites in northern NewHampshire does not support a difference in sources.


Isotopic Evidence for the Origin of the Highlandcroft and Oliverian Plutonic SuitesThe most comprehensive assessment of the isotopic characteristics of the various

terranes of the northern Appalachians is that of Tomascak and others (2005). Asmentioned above, these authors defined fields for Laurentian, peri-Gondwanan non-North American, and Gander/Miramichi crust in an εNd versus 207Pb/204Pb diagram.In our diagram, recalculated at 450 Ma, the Highlandcroft and Oliverian samples plotbetween the Laurentian and non-North American/mantle fields (fig. 11). The Pbisotopic signature is Laurentian, whereas, with the exception of the Whitefield plutonsamples, the Nd isotopic signature is non-North American. At first approach, these twoisotopic systems seem contradictory, but two possibilities can account for these isotopiccompositions. First, Oliverian/Highlandcroft magmas may represent mixtures ofLaurentian and non-North American or peri-Gondwanan crust. Ayuso (1986) andAyuso and Bevier (1991) interpret Pb isotopic data for the Acadian plutons of theCentral Maine province as mixtures of Grenville-type and Avalon-type basements.Moench and Aleinikoff (2003) take a slightly different approach, suggesting that theintermediate isotopic signature is the result of mixing of lead in sedimentary apronsshed oceanward from the Laurentian margin and the approaching peri-Gondwananterranes rather than the basement complexes themselves. While either model explainsthe origin of the syncollisional Acadian and Alleghanian plutons of central Maine, themajority of which are peraluminous as expected from partial melting of graywackesand pelites (Patino Douce, 1999), we think they are inadequate to explain thepetrogenesis of the metaluminous continental arc Oliverian and Highlandcroft Plu-tonic Suites. First, the peri-Gondwanan terranes that comprise the non-North Ameri-can field of figure 11 were not adjacent to Laurentia at this time, and secondly,although Gander/Miramichi may have been proximal, the Oliverian and Highland-croft samples do not plot between Gander/Miramichi crust and Laurentia, suggestingthat the plutons are not mixtures of these two sources.

Tomascak and others (2005) also plotted tholeiitic basalts of the Mesozoic EasternNorth American province as representative of the lithospheric mantle (fig. 11).Although these magmas were emplaced � 250 Ma after the Oliverian and Highland-croft magmas (Marzoli and others, 1999; Hames and others, 2000), the analyses mayserve as the best approximation of the εNd and 207Pb/204Pb values of the mantle. Wealso plot the initial isotopic values of basalts from the Antilles arc as an additionalestimation of mantle values. With these approximations, we note that the Oliverianand Highlandcroft rocks plot between the field for Mesozoic/Antilles basalts and theLaurentian crust in figure 11, suggesting that the suites may represent mixtures of amantle-derived arc component and the Laurentian crust. Note that plutons that definethe Gander, Miramachi and non-North American crust plot at higher 207Pb/204Pbvalues than the Oliverian and Highlandcroft rock; mixing of an arc component withthese peri-Gondwanan sources should have produced magmas with similarly elevated207Pb/204Pb ratios, values not seen in the Oliverian and Highlandcroft plutons. Wetherefore suggest that the Pb and Nd isotopic data best fit a model of the Oliverian andHighlandcroft magmas as having originated along the Laurentian continental marginas espoused by Lyons and others (1986) and Moench and Aleinikoff (2003) instead ofeast of the Red Indian Line on Gondwanan oceanic or crustal blocks.

The Devonian plutons of the Northeast Kingdom batholith of northern Vermontplot in the same εNd and 207Pb/204Pb space as the Oliverian and Highlandcroft plutons(Tomascak and others, 2005; their figure 6). These plutons were emplaced overLaurentian basement and the primitive rocks of the batholith have also been inter-preted as mixtures of mantle-derived and Laurentian-derived crustal magmas (Arthand Ayuso, 1997).

Additional evidence for a Laurentian crustal component in the Oliverian PlutonicSuite is found in the isotopic compositions of the three Whitefield pluton samples;


these plot squarely within the Laurentian crust field in figure 11, signifying that theymay be representative of the Laurentian crustal endmember inferred to be present inthe other Highlandcroft and Oliverian samples.

Implications for the Structure of the Bronson Hill Terrane and the Polarity of SubductionThe Laurentian isotopic signature of the Oliverian and Highlandcroft Plutonic

Suites has bearing on the structure of the Bronson Hill terrane and the tectonics of theTaconic orogeny. Traditionally, the Taconic orogeny was thought to be the result ofthe collision of the Bronson Hill terrane, an island arc assumed to have developed overan easterly dipping subduction zone, with the Laurentian margin (Rowley and Kidd,1981; Stanley and Ratcliffe, 1985).

Karabinos and others (1998) presented a problem with this paradigm, that is thecooling ages of Laurentian margin rocks involved with Taconic metamorphism (�470Ma, Laird and others, 1984) are older than the plutons of the Bronson Hill arc. Theysuggested that the orogeny was not caused by the collision of the Bronson Hill arc, butfrom the older, more westerly Shelburne Falls arc which has an age of � 485 to 470 Ma.Karabinos and others (1998) proposed that after the obduction of the Shelburne Fallsarc and the Taconic orogeny, subduction zone polarity flipped to the west with theBronson Hill continental arc forming over the Taconic modified Laurentian margin(fig. 12). Our isotopic data suggests that the Oliverian and Highlandcroft PlutonicSuites consist of a mantle-derived arc component that was contaminated by Laurentiancrust, evidence that is permissive of the reversal in subduction polarity model ofKarabinos and others (1998) with the arc forming along the modified Laurentianmargin. To some degree, the Laurentian Pb isotopic signatures could have beenderived from subducted Laurentian-derived sediments in a continuously easterlydipping subduction zone as proposed by Hollocher and others (2002), but we thinkthat the overwhelming Laurentian crustal signature of the Whitefield pluton is moreconsistent with crustal anatexis above a westerly dipping subducting slab. A polarity flipwould also account for the 10 to 15 Ma gap between the Ammonoosuc volcanics andthe Oliverian/Highlandcroft plutons.

Metamorphism of the Chain Lakes massif of northwestern Maine can be inter-preted in the context of the polarity flip model. Gerbi and others (2006) demonstratedthat the complex underwent metamorphism and anatexis at anomalously shallowpressures at 469 Ma, suggesting a significant increase in heat flux from the mantle.They envision the heat source as the asthenosphere having risen as a result of slabbreakoff or lithospheric delamination. The age of anatexis and metamorphism of theChain Lakes massif lies between the ages of the Ammonoosuc/Shelburne Falls rocksand the younger Oliverian/Highlandcroft Plutonic Suites and inferred associatedQuimby Formation Volcanics, suggesting that asthenospheric upwelling could haveoccurred after slab break-off from the easterly dipping subduction zone and before theinitiation of westerly polarity.

Recently, van Staal and others (2007) proposed that the Taconic orogeny inNewfoundland is the result of three accretionary events (Taconic 1, 2, and 3), eachwith its own distinct magmatic event. The three magmatic pulses were generatedbetween 489 to 477 Ma, 469 to 459 Ma, and 445 to 435 Ma with distinct gaps betweenpulses. Taconic 1 resulted from a west-directed obduction of the Lushs Bight oceanictract onto the peri-Laurentian Dashwoods microcontinent. Associated igneous rocks(489 – 477 Ma) are from an island arc setting. Clogging of the easterly dippingsubduction zone by the Dashwoods microcontinent caused stepping back of subduc-tion to the Humber Seaway between Laurentia and its offshore Dashwoods block. Thisyounger, easterly dipping subduction zone produced the Baie Verte oceanic tract withassociated arc magmas. Closure of the Humber Seaway caused the collision ofDashwoods and its accreted arcs with the Laurentian margin, and after a gap of 7 to10 m.y., a magmatic flare-up of both arc and non-arc composition tonalites occurred


Fig. 12. Schematic depiction of the tectonic setting spanning before, during, and after the Taconicorogeny in New England (after Karabinos and others, 2003). Between 485 to 470 Ma, an easterly dippingsubduction zone produced the Shelburne Falls island arc (15A), the collision of which caused the Taconicorogeny (15B). Following obduction of the Shelburne Falls arc, subduction polarity flipped to the west with ayounger �450 Ma continental arc, the Bronson Hill arc, forming along the Taconic modified Laurentianmargin (15C).


between 464 to 459 Ma. This waning stage Taconic 2 magmatism is thought to haveresulted from slab break-off of the Humber Seaway subducting slab. Taconic 3magmatism was produced by the initiation of westward-dipping subduction afteraccretion of the Dashwoods microcontinent to Laurentia. Whalen and others (2006)documented that the pulse initially consisted of arc magmas (440 – 435 Ma) followedby the appearance of non-arc magmas (Nb/Th values � 7) at 435 Ma. The change isthought to represent slab break-off in the westerly dipping subduction zone.

The Oliverian and Highlandcroft plutons range in age between 435 to 456 Ma(Moench and Aleinikoff, 2003), younger than the Newfoundland Taconic 2 magmas,but overlapping the oldest of the � 435 Ma Taconic 3 magmas. Both the Oliverian/Highlandcroft and the oldest Newfoundland Taconic 3 rocks have arc compositions,and both are dominated by medium to high K rocks, but also include low K andshoshonitic compositions (Whalen and others, 2006). Thus it appears that the NewHampshire Oliverian and Highlandcroft suites best correlate with NewfoundlandTaconic 3 arc magmas. The significance of this inferred correlation is that 1) theOliverian and Highlandcroft magmas were derived from a westerly-dipping subductionzone that originated west of the Red Indian Line on the Laurentian margin (van Staaland others, 2007); and 2) the Oliverian and Highlandcroft magmas therefore do notcorrelate with the Popelogan-Victoria Arc of the peri-Gondwanan Exploits subzone ofthe Dunnage zone of Maritime Canada (van Staal, 2005). Not only do the Oliverianand Highlandcroft Plutonic Suites have isotopic signatures more consistent with aLaurentian component, their ages are considerably younger than the � 470 MaPopelogan-Victoria Arc.

Finally, while emplacement along the Laurentian margin does not require theOliverian and Highlandcroft plutons to be authocthonous, it removes a requiredallochthonous setting if the suites were emplaced in peri-Gondwanan crust that wassubsequently thrust over the Laurentian continental margin. Based on geophysicaldata, Lyons and others (1986) minimized the possibility that the Mascoma dome of theOliverian suite is floored by a thrust fault, instead suggesting that the Mascoma is insitu, a setting that is permissible in the westerly dipping subducition zone model. Ourresults do not resolve the long-standing controversy over the contact relationshipsbetween the Ammonoosuc Volcanics and the Oliverian plutons. We note however, thatif the contact is tectonic in nature as suggested by Robinson and others (1998) andKohn and Spear (1999), then the island arc signature of the Ammonoosuc Volcanicsdoes not necessitate an island arc setting for the Oliverian magmas. Indeed, if thejuxtaposition of Ammonoosuc Volcanics and Oliverian plutons occurred during theAcadian orogeny, � 40 million years after the emplacement of the Oliverian magmas,there need be no coupling of tectonic settings for the volcanic and plutonic rocks.Hence, the concept of the Oliverian plutons representing in situ emplacement inLaurentian crust is permissible.

conclusions

Other than the Oliverian Plutonic Suite extending to more differentiated compo-sitions than the Highlandcroft, major and trace element compositions, as well as Nd,Sr, and Pb isotopic data, indicate that there are no differences between the Oliverianand Highlandcroft Plutonic Suites. Nd and Pb isotopic data are most compatible withthe two suites being mixtures of mantle-derived arc magmas with Laurentian crust,indicating derivation west of the Red Indian line on the Laurentian continentalmargin. A Laurentian margin origin alleviates the need for the two suites to beallochthonous; they may represent plutons of an in place, continental arc as suggestedby the gravity data of Lyons and others (1996). We suggest that the westerly dippingsubduction model espoused by Karabinos and others (1998, 2003) best explains the


presence of a Laurentian crustal component in the Oliverian and Highlandcroft rocks,and especially the overwhelming Laurentian crustal signature of the Whitefield pluton.

acknowledgmentsWe are grateful for helpful journal reviews by Ray Coish and Paul Karabinos and to

Paul Tomascak and Kurt Hollocher for comments on an earlier version of themanuscript. This research was funded by EAR-0510857 to Wintsch and Dorais, by theCollege of Physical and Mathematical Sciences at Brigham Young University and byBYU in the form of an undergraduate mentoring grant for J. Workman.

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