INTRODUCTIONExtraction of granite from lower crust, and its

emplacement at shallower levels, is the principalmechanism by which the continents have be-come differentiated. Thus, understanding howgranite moves through the crust is an importantstep toward understanding crustal evolution. Inmany convergent orogenic belts spatial and tem-poral relationships between granite and regionaltectonic structures suggest ascent and emplace-ment during contraction rather than during exten-sion (e.g., Hutton, 1997; Brown and Solar, 1998a).During orogenesis, melting occurs in a dynamicenvironment in which differential stresses actingupon anisotropic crust lead to heterogeneousdeformation at all scales, which enables graniteextraction, ascent, and emplacement (e.g., Brown,1994; Sawyer, 1994). Deformation leads to peri-odically connected melt flow networks (e.g.,Brown and Rushmer, 1997; Brown and Solar,1998b) and crustal-scale architectures, such asshear-zone systems (e.g., D’Lemos et al., 1992;Brown and Solar, 1998a), that allow melt extrac-tion and focus melt ascent through the crust.Regional tectonic structures are thought to playan important role during emplacement of somegranite plutons, either by creating space (e.g.,Hutton, 1988) or by arresting ascent (e.g.,Clemens and Mawer, 1992).

It is implicit in these relationships and inter-pretations that crustal anatexis and graniteextraction, ascent, and emplacement are syntec-tonic processes, and that deformation and melttransfer are synchronous. Synchroneity of meta-morphism and migmatization, and granite meltextraction, ascent, and emplacement can betested by precise determination of crystallizationages, which is the purpose of this paper.

GEOLOGY OF WEST-CENTRAL MAINEThe northern Appalachians of New Hamp-

shire and Maine include Ordovician metasedi-mentary and metavolcanic rocks, Neoproterozoicbasement rocks of the Bronson Hill belt, andmetasedimentary rocks of the Central Maine belt,which range in age from Llandovery to Emsian(e.g., Moench and Pankiwskyj, 1988; Fig. 1a).The Central Maine belt contains structuresbelieved to have formed primarily during Devo-nian dextral-transpressive deformation (Brownand Solar, 1998a).

In the Central Maine belt of west-centralMaine, zones of enhanced deformation are sug-gested by a high degree of parallelism betweencompositional layering and foliation, which main-tain a consistent northeast strike and steep south-east dip across the width of the zones, and awell-developed moderately to steeply northeast-plunging mineral elongation lineation (Brownand Solar, 1998a). These steep zones surroundzones (Figs.1b and 1c) in which foliation is nei-ther as strongly developed nor generally parallelto variably oriented, moderately dipping compo-sitional layering, although a well-developedmoderately to steeply northeast-plunging min-eral elongation lineation is pervasive. We inter-pret these intervening zones to record relativelylower strain (LSZs) within anastomosing zonesof higher strain (HSZs) (Solar and Brown,1998); these are part of the Central Maine beltshear-zone system. In HSZ rocks, a noncoaxialcomponent of deformation is suggested byasymmetric pressure shadow tails aroundporphyroblasts, by biotite “fish,” and by a con-sistent obliquity between boudinaged granite-pegmatite sheets and layering and/or foliation,and it is required to prevent space incompatibil-

ities between the structural zones. The kinemat-ics suggests the Central Maine belt shear-zonesystem accommodated dextral-reverse displace-ment during transpressive orogenesis (Brownand Solar, 1998a). Also, porphyroblasts of bio-tite, garnet, andalusite, and staurolite contain anincluded foliation inclined to surrounding matrixfoliation, consistent with coeval metamorphismand plastic deformation (Solar and Brown,1998). Smith and Barreiro (1990) determinedthe age of this syntectonic regional metamor-phism as 405–399 ± 2 Ma, using the U-Pbmethod on monazite crystallized at staurolite-grade conditions in schists unaffected by latercontact metamorphism.

At high metamorphic grade, partial meltingformed migmatites within the Tumbledown andWeld anatectic domains (Fig. 1b) (Brown andSolar, 1998a). Stromatic (layered) migmatitescontain a lower fraction of leucosome in com-parison to foliated inhomogeneous migmatites.Inhomogeneous migmatites vary from mica-richresidual varieties to leucosome-rich schliericgranite, which suggests progressive segregationof melt from residue by granular-flow–inducedcompaction. The boundaries between these twomigmatite types are approximately coincidentwith boundaries between structural zones(Fig. 1b); stromatic migmatites are in the HSZswhereas inhomogeneous migmatites are in theLSZs. Sheets of granite that are concordant orweakly discordant with respect to foliation inHSZs (Fig. 2) are interpreted to record chan-neled transfer of melt, and by analogy withveins at a lower metamorphic grade, may be intensile and dilational shear fractures (Brownand Solar, 1998b).

The GranitesThe dextral-reverse kinematics of the Central

Maine belt shear-zone system implies that suc-cessively shallower structural levels are exposedto the northwest across the study area in west-central Maine (Fig. 1b). Thus, within the block tothe southeast of the central HSZ, the Phillipspluton and the associated Weld anatectic domainrepresent the deepest structural level, while theRedington pluton and the northern lobe of theLexington pluton in the block to the northwest ofthe central HSZ represent shallower levels.

The three-dimensional shape of plutons can bededuced by combining geologic information

Geology;August 1998; v. 26; no. 8; p. 711–714; 3 figures; 1 table. 711

Granite ascent in convergent orogenic belts: Testing a modelGary S. SolarRachel A. Pressley Department of Geology, University of Maryland, College Park, Maryland 20742-4211Michael BrownRobert D. Tucker Department of Earth & Planetary Sciences, Washington University, St. Louis, Missouri 63130-4899

Data Repository item 9872 contains additional material related to this article.

ABSTRACTThe common spatial relationship in convergent orogenic belts between a crustal-scale shear-

zone system, high-grade metamorphic rocks, and granites suggests a feedback relation betweencrustal anatexis and contractional deformation that helps granite extraction and focuses graniteascent. Such a feedback relation has been proposed for ascent of Early Devonian granites in west-central Maine. This interpretation requires that deformation, metamorphism, and plutonismwere synchronous. We have determined precise U-Pb zircon and monazite ages that we interpretto record time of crystallization of syntectonic granite in metric to decametric sheets and kilo-metric plutons, and of schlieric granite within migmatites. Ages are in the range ca. 408–404 Ma,within 1 m.y. at 95% confidence limits. These ages are similar to extant U-Pb monazite ages ofca. 405–399 ± 2 Ma for syntectonic regional metamorphism in the same area. The coincidencebetween the age of peak metamorphism and crystallization ages of granite shows tectonics, meta-morphism, and magmatism were contemporaneous, in support of the feedback model.

with models based on geophysical data, gravitystudies in particular. Here, we summarize plutongeometries derived by Brown and Solar (1998b)using map information, thermal aureole widthand depth of pluton emplacement, cross sectionsof plutons modeled in a regional gravity study(Carnese, 1981), and a crustal model based on anintegrated geophysical study (Stewart, 1989).

The Phillips pluton is a hemiellipsoidal body inan LSZ (Fig. 1b). Where observed, principal con-tacts between granites and metasediments are

concordant with respect to regional structure.Steeply dipping magmatic fabrics (biotite-richschlieren, and modal and grain-size layering)occur locally and are oriented conformably withthe northeast-striking, subvertical foliation in sur-rounding metasedimentary units (Pressley andBrown, 1998). Although the Weld anatecticdomain to the south is poorly exposed, availableoutcrop data suggest it is composed of inhomoge-neous migmatite with lenses of schlieric granite(Brown and Solar, 1998b). Given the northeast-

plunging mineral elongation lineation in theCentral Maine belt rocks, it is implicit that rockssimilar to those exposed in the Weld anatecticdomain occur under the Phillips pluton to thenortheast (Fig. 1b). The Redington pluton has ir-regularly northeast-dipping contacts with wallrocks in the northeast and inward-dipping con-tacts in the southwest, where it is inferred to be incontact with wall rocks along a northeast-dippingsurface that represents the base of the pluton. Inthe southwest, aligned K-feldspar phenocrysts ingranite define a moderately northeast-dippingmagmatic foliation, subparallel to kilometer-scalescreens of weakly strained hornfelsic wall rock.The pluton is in an LSZ (Fig. 1b). Gravity model-ing suggests a horizontal wedge as much as2.5–3 km thick at the northeast margin thinning tothe southwest (Carnese, 1981). In contrast, theLexington pluton has a hybrid geometry in whicha hemiellipsoidal northern lobe is in an LSZ, butthe central-southern lobe has a tabular form thatthins to the south-southeast, cutting discordantlyacross the shear-zone system (Fig. 1b). Modelingby Unger et al. (1989) suggests the northern lobeis ≈12 km thick, with steep inward dipping con-tacts, in comparison with the central-southernlobe, which thins from ≈6 to ≈3 km across thestrike of the central HSZ. Sporadic outcrops in thecenter of the pluton exhibit northeast-striking,steeply southeast-dipping magmatic foliation.The eastern part of the Mooselookmegunticpluton is exposed in the west of the area (Fig. 1b).This is a large tabular pluton, shallowly northeastdipping and thinning to the southwest, that alsocuts the shear-zone system.

712 GEOLOGY, August 1998

L

M

10N

45°00'

70°15'

70°45'

0

10

15

km

70°15'50 km

central high

strain zone

70°00'

20

0

5

10

15

km

20

WAD

5

CMBrocks

BHBrocks

WAD

LSZ

LSZ

LSZ

LSZP

R

Avalon-likerocks

HSZ

TAD

M

M

SFL

FL FL L

L

Flagstaff Lake igneouscomplex, gabbro/biotite granite

Lexington pluton

Mooselookmeguntic pluton,leucogranite / granodiorite

Phillips pluton

Redington pluton

Sugarloaf pluton

Tumbledown and Weld anatectic domains(TAD and WAD); stromatic migmatite/inhomogeneous migmatite

S

R

P

M

L

FL

zone of fine-grained tectonite,boundary of BHB (NW)and CMB (SE)

foliation intersectionwith figure

Central Maine belt (CMB) rocks

boundary of structural zone;HSZ is higher strain zone,LSZ is lower strain zone

Bronson Hill belt (BHB) rocks

Grenvillian basement

Avalon-like rocks; stromatic migmatite/inhomogeneous migmatite

felsic plutonic rocks

mafic plutonic rocks

BHB rocks; stromatic migmatite/inhomogeneous migmatite

undifferentiated crustal rocks

A’A

0 10

0

10

c. Structure section

b. Block diagram

kmN

Canad

a

Bronson Hillbelt (BHB)

Central Mainebelt (CMB)

100 km

Atlantic

Ocean

70°W72°W

44°N

Boston

VT

ME

68°W

Nashobaterrane

BH

B

MA

Avaloncompositeterrane

NSZNorumbega

shear zonesystem

Sebagobatholith

Fig. 1b

CM

B

NSZ

Canada

46°N

NH

a. Location map

A’

A

HSZ

Sr Rangeley Formation

Sp Perry Mountain and Smalls Falls Formations

Dc Madrid and Carrabassett Formations

Sp

Sr

Sr

Sp

Sp

Sp

Dc Dc

DcDc

LSZLSZ

HSZ

Figure 1. a. Location of Central Maine belt (CMB) of New Hampshire and Maine in relation toBronson Hill belt (BHB) to northwest, and Norumbega shear-zone (NSZ) system and Avaloncomposite terrane to southeast. Outlined area north of Sebago batholith corresponds topart b. ME = Maine, VT = Vermont, NH = New Hampshire, and MA = Massachusetts. b. Blockdiagram constructed from simplified geological map and cross sections of the study area,west-central Maine, to show relationship between granite plutons and Central Maine belt(CMB) shear-zone system. Higher strain zones are indicated by lines representing orienta-tion of foliation and lower strain zones are unornamented; plutons are indicated by inclinedcrosses. c. Simplified structure section, northwest-southeast, with Redington and Phillipsplutons omitted.

Figure 2. Steeply east-dipping granite sheetsin stromatic migmatite within central highstrain zone along west side of Tumbledownanatectic domain, Swift River, Roxbury, Maine(view to north).

The ModelOn the basis of field mapping and micro-

structural interpretations in west-central Maine(Fig. 1), Brown and Solar (1998a, 1998b) haveproposed a model in which metamorphism,migmatization, and granite melt transfer weresynchronous with deformation in a crustal-scaleshear-zone system. In nonmigmatitic rocks,porphyroblast-matrix relations show metamor-phic crystallization was syntectonic (Solar andBrown, 1998). For anatectic rocks, at melt frac-tions greater than threshold permeability duringactive contractional deformation, end-memberrheological models are as follows: (1) percolativemelt flow parallel to the principal finite elongationdirection in the plane of flattening, recorded bythe mineral elongation lineation and the foliation,and (2) en masse flow of melt with residue (e.g.,by granular flow), in which differential flow ratesmay enable melt and residue to segregate. Flowmay be channeled, as illustrated by centimeter-scale stromatic (layered) migmatite structures andmeter-scale sheets of internally layered granitearrested during ascent (Fig. 2). Embrittlement dueto a buildup of melt pressure may have enabledtensile and dilatant shear fractures to form in stro-matic migmatite, and granular flow may becomedilatant, leading to localization of deformationthat enables melt exfiltration (Brown and Solar,1998a, 1998b). Cyclic fluctuations of melt pres-sure result in pulsed flow of melt consistent withinternal layering in granite sheets. Granite ex-hibits magmatic foliation but does not recordsolid-state fabrics. This suggests late syntectonictransfer of melt through the shear-zone system asdeformation waned, followed by crystallizationthat proceeded without penetrative strain, al-though smaller sheets exhibit crenulate margins,and some sheets are deformed into pinch andswell structures (Fig. 2).

Existing Age DataPrevious age determinations on granites in this

area include Rb/Sr whole-rock isochron ages of371 ± 6 Ma (regressed from nine samples of theMooselookmeguntic pluton and satellite bodies,and recalculated from data in Moench andZartman, 1976) and 399 ± 6 Ma (based onsamples from both the northern and central-southern lobes of the Lexington pluton, H. E.Gaudette, personal commun., cited in Dickersonand Holdaway, 1989, p. 499). A U-Pb monaziteage of 363± 2 Ma from the southern part of theMooselookmeguntic pluton was reported bySmith and Barreiro (1990). DeYoreo et al.(1989) reported Late Devonian through Car-boniferous 40Ar/ 39Ar ages from hornblende,muscovite, and biotite mineral separates from theMooselookmeguntic and Phillips plutons.

RESULTSZircon selected for analysis was needle or

prism shaped (<75 µm size fraction), of high

optical quality, and free of optically visibleinclusions. We interpret these zircons to beigneous (Pupin, 1980). On this basis, we expectthat the zircons may incorporate the least inher-ited component, thus permitting the interpreta-tion of ages as recording the time of crystalliza-tion. Honey-yellow monazite selected foranalysis was round and ~50 µm in grain size.Preparation, chemical purification, and analyti-cal techniques follow the procedures developedby Krogh (1973, 1982), with slight modification(Tucker et al., 1990). In the conventional con-cordia diagrams of Figure 3, individual zirconfractions plot concordantly or with slight discor-

dance, suggesting modern-day Pb loss. Allmonazite fractions plot concordantly. The dataare given in Table A,1 and ages, quoted at 95%confidence limits, are summarized in Table 1.The general consistency between zircon andmonazite ages and reproducibility of multiplefractions from the same granite samples (Fig. 3)suggest that the ages may be interpreted torecord the time of crystallization of the granites.

GEOLOGY, August 1998 713

0.480 0.485 0.490 0.495 0.500

0.0630

0.0635

0.0640

0.0645

0.0650

0.0655

0.0660

398

6

406

7

8

6 - pale brown zircon7, 8 - colorless zircon

SWIFT RIVER, ROXBURY, MAINE

410

402

0.480 0.485 0.490 0.495 0.500

14

16

15

407.9 1.9 Ma

404.3 1.9 Ma

NORTH ROXBURY, MAINE

13, 14 - colorless zircon15, 16 - monazite

0.0630

0.0635

0.0640

0.0645

0.0650

0.0655

0.0660

two-mica leucogranite

two-mica granite

13

0.480 0.485 0.490 0.495 0.500

398

406

9

11

10

12

9, 10, 11 - colorless zircon12 - monazite

SOUTH ROXBURY, MAINE

410

402

0.0630

0.0635

0.0640

0.0645

0.0650

0.0655

0.0660

Pb/

U23

820

6

408.2 2.5 Matwo-mica granite

Pb / U235207

0.476 0.480 0.484 0.488 0.4920.0630

0.0635

0.0640

PHILLIPS PLUTON

396

398

406

18

23

21400

0.435 0.440 0.445 0.450

0.0580

0.0590

0.0600375

40

0.0610 40, 41 - monazite

0.455 0.460 0.465

380

38535

38

34, 36, 37, 38 - colorless needles35 - colorless short prisms

MOOSELOOKMEGUNTIC PLUTON

0.0620

404

19

3022

29

370

388.9 1.6 Mabiotite granite

370.3 1.1 Matwo-mica leucogranite

41

40217

27

28

17, 18 - pale brown zircon19 - monazite

403.5 1.6 Maleucogranite

21 - pale brown zircon22, 23 - monazite

403.6 2.2 Magray granite

27, 28 - colorless zircon

405.3 1.8 Maschlieric granite

398

406

410

402

Pb / U235207

Pb / U235207

Pb / U235207

Pb / U235207

Pb/

U23

820

6Pb

/U

238

206

K

K K

K K

K

K

K

31

32

33

31 - pale brown zircon32 - colorless zircon33 - very pale brown zircon

LEXINGTON AND REDINGTON PLUTONS0.0660

0.0650

0.0640

0.0630

0.0620

0.0610

404.2 1.8 MaLexington biotite granite

0.460 0.470 0.480 0.490

385

395

390

400

405

410

1, 2, 3, 4 - pale brown zircon

407.6 4.7 MaRedington biotite granite

1

2

Pb / U235207

K

K

4

3

34

36, 37

Figure 3. 206Pb/ 238U vs. 207Pb/ 235U concordia plots of analytical data. Three samples fromRoxbury area are sheets of granite within the central high strain zone. Ages are given at 95%confidence level.

1GSA Data Repository item 9872, Table A, Zirconand Monazite Analyses, is available on request fromDocuments Secretary, GSA, P.O. Box 9140, Boulder,CO 80301. E-mail: editing@geosociety.org.

DISCUSSION AND CONCLUSIONSIn west-central Maine, granite sheets in the

central HSZ, schlieric granite in migmatites, andgranite plutons yield precise crystallization agesin the range ca. 408–404 Ma, consistent withinerror with the age of 405–399 ± 2 Ma for thesynkinematic metamorphism (Smith and Barreiro,1990) and with the range for plutons farthernortheast along strike, 410–400 Ma (Hubacherand Lux, 1987; Bradley et al., 1996). The appar-ent contradiction between crystallization ages ofthe granites and fossil ages of the youngestmetasedimentary rocks they intrude (Emsian) isresolved by the new Devonian timescale ofTucker et al. (1998), in which the base of theDevonian is ca. 418 Ma and the base of theEmsian is ca. 409.5 Ma. Thus, the new datareported in this paper support a model of contem-poraneous deformation, metamorphism, andgranite ascent through the crust (Brown andSolar, 1998a, 1998b); viewed at the crustal scale(Fig. 1b), granite extraction, ascent, and emplace-ment were syntectonic.

Our data for the Mooselookmeguntic plutonshow that it is composite, having been con-structed by at least two separate plutonic events.The younger age of ca. 370 Ma for leucograniteis consistent with U-Pb monazite ages reportedby Smith and Barreiro (1990) of 369–363 ±2 Ma from metasedimentary rocks within thecontact aureole of this pluton and close to themonazite age of 363 ± 2 Ma for a satellite bodyof leucogranite. Southwest along strike, in NewHampshire, the range of monazite ages reportedby Eusden and Barreiro (1988) from meta-morphic rocks is ca. 402–376 Ma and fromsmall plutons and sheets of granite and pegma-tite is ca. 401–359 Ma. These ages from theMooselookmeguntic pluton and farther south-west suggest orogen-parallel diachroneity inthe age of tectonic events in the northernAppalachians.

ACKNOWLEDGMENTSU-Pb zircon and monazite analyses were supported

by National Science Foundation grants EAR-9304142and EAR-9506693 to R. D. Tucker, and manuscriptpreparation was supported by National Science Founda-tion grant EAR-9705858 to M. Brown. We ack-nowledge discussions with P. B. Tomascak, technical

assistance from Z. X. Peng at Washington University,and constructive reviews by K. Benn, C. F. Miller,S. Seaman, P. B. Tomascak, and C. R. van Staal.

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Hutton, D. H. W., 1997, Syntectonic granites and theprinciple of effective stress: A general solution tothe space problem?, in Bouchez, J. L., Hutton,D. H. W., and Stephens, W. E., eds., Granite:From segregation of melt to emplacementfabrics: Dordrecht, Netherlands, Kluwer Aca-demic Publishers, p. 189–197.

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Manuscript received November 6, 1997Revised manuscript received April 29, 1998Manuscript accepted May 19, 1998

714 Printed in U.S.A. GEOLOGY, August 1998