Bimodal Silurian and Lower Devonian 

Volcanic Rock Assemblages in the 

Machias-Eastport Area, Maine 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1184 

Bimodal Silurian and Lower Devonian 

Volcanic Rock Assemblages in the 

Machias-Eastport Area, Maine 

By OLCOTT GATES and ROBERT H. MOENCH 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1184 

Interpretation of chemical data for samples of 

weakly metamorphosed volcanic rocks of the 

coastal volcanic belt in southeastern Maine 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1981 
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary 

GEOLOGICAL SURVEY 

Doyle G. Frederick, Acting Director 

Library of Congress Cataloging in Publication Data 

Gates, Olcott, 1919 

Bimodal Silurian and Lower Devonian volcanic rock assemblages in the Machias-Eastport area, Maine. 

(Geological Survey Professional Paper 1184) 

Bibliography: p. 25 

Supt. of Docs. no.: I 19:16:1184 

1. Volcanic ash, tuff, etc.-Maine-Machias region. 2. Geology, Stratigraphic-Silurian. 3. Geology, Strl\tigraphicDevonian. 4. Geology-Maine-Machias region. 

I. Moench, Robert Hadley, 1926- joint author. II. Title. III. Series: United States Geological Survey Professional 

Paper 1184. 

QE46l.G277 552'.2'09741 80-607152 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 
CONTENTS 

Page 

Abstract.................................................................... 1 

Introduction................................................................. 1 

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Regional paleogeographic and tectonic relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Stratigraphy and structure of the Machias-Eastport area . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Definition and petrography of volcanic assemblages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

Silurian assemblage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

Older Devonian assemblage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

Younger Devonian assemblage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

Alteration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

Volcanic petrology............................................................ 13 

Silurian assemblage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

Older Devonian assemblage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 

Younger Devonian assemblage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 

Summary and origin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 

Tectonic implications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 

References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 

Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 

Appendix B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 

ILLUSTRATIONS 

Page 

FIGURE 1. Map of New England and Maritime Provinces showing Silurian and 

Lower Devonian volcanic tracts and principal paleotectonic features . . . 3 

2. Simplified map of the bedrock geology of the Machias-Eastport portion of 

the coastal volcanic belt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

3. Schematic cross section southeast from Oak Bay to the Bay of Fundy, 

showing relations of Silurian and Devonian formations to one another and 

to contemporaneous normal faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

4. Harker diagram showing major oxides, recalculated without H20 and C02 . . . 14 

5. Alkali-silica variation diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

6. AFM diagram..................................................... 16 

7. Variation of Si02, Ti02, and FeO* relative to solidification index. . . . . . . . . . . . 17 

8. Major-oxide and minor-element data for least altered basalts of the MachiasEastport area compared with data of McDougall for the Columbia River 

Basalt Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

TABLES 

Page 

TABLE 1. Volume of volcanic and hypabyssal intrusive rock types in the Eastport quadrangle, estimated from areas of each type in cross sections . . . . . . . . . . . . . . 8 

2. Major-oxide analyses of volcanic rocks of the Machias-Eastport area, Maine. . 12 

3. Selected and average minor-element data on volcanic rocks . . . . . . . . . . . . . . . . 19 

4. Silurian basalts from the Machias-Eastport area compared with high-alumina 

tholeiites and calc-alkaline basalts from other regions . . . . . . . . . . . . . . . . . . 20 

III 
IV CONTENTS 

METRIC-ENGLISH EQUIVALENTS 

[SI, International System of Units, a modernized metric system of measurement] 

SI unit 

millimeter (mm) meter (m) 

kilometer (km) 

centimeter2 (cm2) meter2 (m2) 

hectometer~ (hm2) 

kilometer2 (km2) 

centimeter:! (cma) 

decimeter3 ( dma) 

meter3 (m3) = 

= 

u.s. customary equivalent 

Length 

0.009 37 

3.281 

1.094 

0.621 4 

0.540 0 

Area 

0.155 0 

10.76 

1.196 

0.000 247 1 

2.471 

0.003 861 

0.3861 

Volume 

0.06102 

61.02 

2.113 

1.057 

0.264 2 

0.035 31 

35.31 

1.308 

264.2 

6.290 

inch (in) 

feet (ft) 

yards (yd) 

mile (mi) 

mile, nautical (nmi) 

inch2 (in2) 

feet2 (ft2) 

)'ards2 (yd2) acre 

acres 

section (640 acres or 1 mi2) 

mile2 (mi~) 

incha (ina) 

inches3 (ina) 

pints (pt) 

quarts (qt) gallon (gal) foot3 (ft3) 

feeta ( ft3) 

yardsa (yda) gallons (gal) 

barrels (bhl) (petroleum, 1 bbl=42 gal) 0.000 810 7 acre-foot (acre-ft) hectometer3 (hm3) = 810.7 acre-feet (acre-ft) kilometera (km3 ) = 0.239 9 muea (mia) 

Volume per unit time (includes flow) 

decimeter:! per second 

(dma/s) 0.035 31 

2.119 

foota per second (ftljs) 

feet3 per minute (ftl/ 

min) 

SI unit U.S. customary equivalent 

Volume per unit time (includes ftow)-Continued 

decimeter:! per second 

(dm3 /s) 

meter3 per second (mljs) 

gram (g) 

kilogram (kg) 

megagram (Mg) 

= 

= 

15.85 

543.4 

35.31 

15 850 

Mass 

0.035 27 

2.205 

1.102 

0.984 2 

gallons per minute 

(gal/min) barrels per day (bbl/d) (petroleum, 

1 bbl=42 gal) feet3 per second (ft3 /s) 

gallons per minute 

(gal/min) 

ounce avoirdupois (oz avdp) 

pounds avoirdupois (lb 

avdp) 

tons, short (2 000 lb) 

ton, long (2 240 lb) 

Mass per unit volume (includes density) 

kilogram per meter3 

(kg/rna) 

kilo pascal ( kPa) 

temp kelvin (K) 

temp deg Celsius (°C) 

= 

0.062 43 pound per foota (lb/fta) 

Pressure 

0.145 0 pound-force per inch2 

(lbf/in2) 

0.009 869 atmosphere, standard 

(atm) 

0.01 bar 

0.296 1 inch of mer<:ury at 60°F (in Hg) 

Temperature 

= [temp deg Fahrenheit (°F) +459.67] /1.8 

[temp deg Fahrenheit (°F)- 32] /1.8 
BIMODAL SILURIAN AND LOWER .DEVONIAN 

VOLCANIC ROCK ASSEMBLAGES IN THE 

MACHIAS-EASTPORT AREA., MAINE 

By OLCOTT GATES and RoBERT H. MoENCH 

ABSTRACT 

Exposed in the Machias-Eastport area of southeastern Maine is 

the thickest (at least 8,000 m), best exposed, best dated, and most 

nearly complete succession of Silurian and Lower Devonian volcanic 

strata in the coastal volcanic belt, remnants of which crop out along 

the coasts of southern New Brunswick, Canada, and southeastern 

New England in the United States. The volcanics were erupted 

through the 600-700-million-year-old Avalonian sialic basement. To 

test the possibility that this volcanic belt was a magmatic arc above 

a subduction zone prior to presumed Acadian continental collision, 

samples representing the entire section in the Machias-Eastport 

area of Maine were chemically analyzed. 

Three strongly bimodal assemblages of volcanic rocks and associated intrusives are recognized, herein called the Silurian, older Devonian, and younger Devonian assemblages. The Silurian assemblage contains typically nonporphyritic high-alumina tholeiitic 

basalts, basaltic andesites, and diabase of continental characterand calc-alkalic rhyolites, silicic dacites, and one known dike of 

andesite. These rocks are associated with fossiliferous, predominantly marine strata of the Quoddy, Dennys, and Edmunds Formations, and the Leighton Formation of the Pembroke Group (the 

stratigraphic rank of both is revised herein for the Machias-Eastport area), all of Silurian age. The shallow marine Hersey Formation 

(stratigraphic rank also revised herein) of the Pembroke Group, of 

latest Silurian age (and possibly earliest Devonian, as suggested by 

an ostracode fauna), contains no known volcanics; and it evidently 

was deposited during a volcanic hiatus that immediately preceded 

emergence of the coastal volcanic belt and the eruption of the older 

Devonian assemblage. The older Devonian assemblage, in the 

lagoonal to subaerial Lower Devonian Eastport Formation, contains tholeiitic basalts and basaltic andesites, typically with abundant plagioclase phenocrysts and typically richer in iron and 

titanium and poorer in magnesium and nickel than the Silurian 

basalts; and the Eastport Formation has rhyolites and silicic 

dacites that have higher average Si02 and K20 contents and higher 

ratios of FeO* to MgO than the Silurian ones. The younger Devonian assemblage is represented by one sa~ple of basalt from a 

flow in red beds of the post-Acadian Upper Devonian Perry Formation, and by three samples from pre-Acadian diabases that intrude 

the Leighton and Hersey Formations. These rocks are even richer in 

titanium and iron and poorer in magnesium and nickel than the 

older Devonian basalts. Post-Acadian granitic plutons exposed 

along the coastal belt for which analyses are available are tentatively included in the younger Devonian assemblage. The most conspicuous features of the coastal volcanics and associated intrusives 

are the preponderance of rocks of basaltic composition ( < 52 percent 

Si02 ) in the Silurian assemblage, and the near absence in all assemblages of intermediate rocks having 57-67 percent Si02 (calculated 

without volatiles). 

All the rocks are variably altered spilites and keratophyres. The 

basaltic types are adequately defined, however, by eight samples of 

least altered basalts having calcic plagioclase, clinopyroxene, and 

0.5 percent or less C02 • The more altered basalts are variably enriched or depleted in Na 20, K20, and CaO relative to the least altered ones. In the silicic rocks no primary ferromagnesian minerals 

are preserved. The Na 20 and K20 contents of the silicic rocks are 

erratic; they are approximately reciprocal, possibly owing to alkali 

exchange while the rocks were still glassy. 

We propose that the coastal volcanic belt extended along an axis 

of thermal swelling in the Earth's mantle and upward intrusion of 

partially melted mantle into the sialic Avalonian crust. These processes were accompanied by shoaling and emergence of the belt, and 

they produced the bimodal volcanism. Tholeiitic basaltic melts segregated from mantle material at rather shallow depths ( < 35 km). 

We are uncertain whether the Silurian basalts and the successively 

more fractionated older and younger Devonian basalts were produced by separate melting events, or by shallow fractionation of one 

to form the other. The silicic magmas segregated from sialic crust 

that was heated and partially melted by the mantle intrusion and its 

mafic segregations. At first, still-hydrous sialic crust yielded the 

calc-alkalic rhyolites and silicic dacites of the Silurian assemblage; 

sparse andesite may have formed by mixing of basaltic and rhyolitic 

magma. By earliest Devonian time the heated crust was significantly dehydrated and yielded the more potassic, silicic, and ironenriched older Devonian rhyolitic suite. Available data suggest that 

the post-Acadian coastal granites are even more enriched in the 

same components, and may have formed by partial melting of 

further-dehydrated crust. 

The strongly bimodal volcanics of the Machias-Eastport area are 

unlike known volcanic and plutonic suites that occur above documented subduction zones, where rocks of andesitic composition are 

typically abundant. Instead, bimodal volcanism implies extensional 

tectonism, a regime that is supported in the Machias-Eastport area 

by evidence of Silurian block faulting. The two authors favor somewhat different tectonic regimes, but both authors believe that the 

results of this study signal a major problem concerning the relations between magmatism and the Acadian orogeny along the 

coastal volcanic belt, and perhaps else~here in the northern Appalachians. 

INTRODUCTION 

Marine and subaerial metavolcanic rocks of Silurian 

and Early Devonian age crop out along the coasts of 

southern New Brunswick in Canada, and of southeastern Maine and eastern Massachusetts in the United 

States, forming a belt that was bordered by wide tracts 

1 
2 BIMODAL VOLCANIC ROCK ASSEMBLAGES, MACHIAS-EASTPORT AREA, MAINE 

of nonvolcanic, predominantly marine clastic deposits 

of the same age (fig. 1). This belt, called the coastal volcanic belt by Boucot (1968), has been assigned the role 

of a subduction-related volcanic arc that was active 

during closure of the presumed proto-Atlantic ocean in 

Silurian and Devonian time (Bird and Dewey, 1970; 

McKerrow and Ziegler, 1971; Dewey and Kidd, 1974; 

Wilson, 1966). However, this assignment was made 

without the benefit of petrologic data. Whether or not 

these volcanics are in fact akin to those that occur 

along any post-Triassic convergent plate boundary is 

the main question asked in this report. The MachiasEastport area is a critical one, because it contains the 

thickest, best exposed, best dated, and most nearly 

complete succession of Silurian and Lower Devonian 

volcanic strata in the coastal belt, and probably in the 

whole Appalachian-Caledonian orogen. 

For many years Gates has been mapping in the 

Machias-Eastport area, and a description and geologic 

map of the Eastport quadrangle is now available 

(Gates, 1975). In 1967, Moench began to study mineral 

deposits in the area. Spurred by the publication of the 

paper by Bird and Dewey (1970), in 1971 and 1972 we 

collected typical samples of all the known volcanic 

rock types that are exposed in the area. Our objectives 

were to test the Bird-Dewey hypothesis and to establish the nature of volcanic rocks that are associated 

with mineral deposits of the coastal area. This report 

presents major oxide data, preliminary minor element 

data, a summary of petrographic features, and interpretation,s that focus on tectonic setting. 

ACKNOWLEDGMENTS 

We wish to thank A. F. Shride and D. W. Rankin, 

U.S. Geological Survey, for their constructive criticisms of early versions of this report. The report also 

benefited from our discussions with Carl E. Hedge, 

Peter W. Lipman, and Harold J. Prostka, U.S. Geological Survey. The Maine Geological Survey supported 

mapping by Gates in the Machias-Eastport area. 

REGIONAL PALEOGEOGRAPHIC AND 

TECTONIC RELATIONSHIPS 

Magmas of the coastal volcanic belt probably 

erupted through the 600-700-m.y. (million years)-old 

Avalonian basement and its cover of lower Paleozoic 

platform metasedimentary rocks. Although the actual 

position of the northwestern margin of the Avalonian 

terrane has not been delineated precisely, it is at least 

as far northwest as the line shown in figure 1, drawn 

mainly on the basis of Naylor's (1975) work. Within the 

area of the coastal belt, the Silurian and Devonian volcanic suites surely lie above Avalonian rocks. 

Eruptions along the coastal volcanic belt that 

closely preceded those that produced the Silurian assemblage of this paper may be recorded in clasts found 

in two conglomerates. Volcanic and plutonic clasts are 

found in Silurian(?) slide conglomerates exposed immediately southwest of the Machias-Eastport area (Gilman, 1966), and in basal polymictic conglomerates of 

the Silurian Oak Bay Formation of Alcock (1946), exposed a few kilometers north of the Machias-Eastport 

area (fig. 1; Amos, 1963; Cumming, 1967; Ruitenberg, 

1967). These clasts deserve further study to see if they 

came from a volcanic tract that might have existed in 

earliest Silurian or before-prior to deposition of the 

Oak Bay or of the W aweig Formation of Ruitenberg 

(1967)-within or possibly immediately south of the 

Silurian and Devonian alinement of the coastal volcanic belt. A possible remnant of such a tract is the 

Bears Brook Volcanic Group of Late Ordovician(?) age, 

exposed along the northern coast of Nova Scotia some 

400 km northeast of the Machias-Eastport area (fig.1; 

Boucot and others, 197 4). 

According to published paleogeographic maps, the 

Silurian and Early Devonian coastal volcanic belt 

stands between tracts of approximately synchronous 

clastic deposits that are almost devoid of volcanic 

rocks (Boucot, 1968). On the southeast, in northern 

Nova Scotia (fig. 1), is the Arisaig belt composed of deposits that represent shallow marine and subaerial 

facies. The area of the Annapolis Valley paleogeographic belt farther south in Nova Scotia was emergent through much of Silurian time, but later in the 

Silurian and into Devonian time it subsided and received deep marine deposits. Northwest of the coastal 

volcanic belt are the Fredericton trough and the Merrimack synclinorium, underlain by enormous thicknesses of sparsely graptolitic, probably deep marine deposits of Silurian and Early Devonian age (Pankiwskyj 

and others, 1976; Osberg and others, 1968; McKerrow 

and Ziegler, 1971). 

The earlier ancestry of the Merrimack synclinorium 

is shown by the Aroostook-Matapedia belt of conformity between Ordovician and Silurian strata that 

extends at least from western Maine to Gaspe, Quebec 

(fig. 1; Pavlides and others, 1968). Elsewhere in the 

northern Appalachians, Silurian formations rest disconformably or unconformably on Middle Ordovician 

or older rocks. 

Farther northwest are scattered exposures of predominantly Lower Devonian and sparse Silurian vol-
REGIONAL PALEOGEOGRAPHIC AND TECTONIC RELATIONSHIPS 3 

68" 

46° 

FIGURE 1.- Map of New England and Maritime Provinces showing 

Silurian and Lower Devonian volcanic tracts and principal paleotectonic features. Names approximately coincide with axes of the features. Coastal volcanic, Arisaig, and Annapolis Valley belts and 

Bears Brook Volcanic Group modified from Boucot (1968) and 

Boucot and others (1974, figs. 27, 28). Fredericton trough from 

McKerrow and Ziegler (1971). Aroostook-Matapedia belt of Taconian 

conformity from Pavlides, Boucot, and Skidmore (1968, fig. 5-1). 

canic rocks, forming a poorly defined tract herein 

called the northern volcanic tract (fig. 1). Volcanics of 

Ordovician age or older are widely exposed along the 

same tract. As noted by Rodgers (1970, p. 126, 

135-136), however, during the Ordovician the deep 

marine nonvolcanic Aroostook-Matapedia trough appears to have separated the volcanic belt of northwestern Maine from that of northern New Brunswick. In 

Early Devonian time the northern tract was an area of 

100 

64° 62° 60° 

200 KILOMETERS 

Merrimack synclinorium from Osberg, Moench, and Warner (1968). 

Northern Silurian-Devonian volcanic tract from information on maps 

of Potter, Jackson, and Davies (1968), and Hussey (1967), and descriptions in Poole, Sanford, Williams, and Kelley (1970), Rankin 

(1968), Boudette, Hatch, and Harwood (1976, p. 12- 20), and other 

sources. Approximate limit of known Avalonian basement is based 

on Naylor (1975), Potter, Jackson, and Davies (1968), and Stewart 

(1974). 

shallow seas and volcanic islands. Although most postOrdovician volcanics of the northern tract are Devonian in age, at least widely scattered Silurian volcanic rocks are found in the Silurian Shaw Mountain 

Formation in Vermont, in unnamed units in the St. 

John Valley region of northwestern Maine (Boudette 

and others, 1976, p. 12-17), and in parts of the Pointe 

Aux Trembles Formation in the western Gaspe area 

(Lajoie and others, 1968, p. 627). 
4 BIMODAL VOLCANIC ROCK ASSEMBLAGES, MACHIAS-EASTPORT AREA, MAINE 

STRATIGRAPHY AND STRUCTURE OF 

THE MACHIAS-EASTPORT AREA 

A geologic sketch map (fig. 2) illustrates the main 

structural features and the general stratigraphic succession in the Machias-Eastport area. Bastin and 

Williams (1914) originally mapped the Eastport quadrangle, and their formation names are used in this report, with the following changes. Because volcanic 

rocks are locally abundant in the Quoddy Shale of 

Bastin and Williams (1914, p. 3), Gates (1975) preferred the name Quoddy Formation, and this usage is 

adopted herein. The volcanics are most abundant in 

the upper part of the Quoddy, where it crops out in the 

Cutler 15-minute quadrangle. The Quoddy of that area 

was originally called the Little River Formation by 

Gates (1961, p. 9-23), a name that he discarded in 

favor of the Quoddy as his mapping progressed into 

the Eastport quadrangle. Gates (1975) raised to formation status the former Leighton Gray Shale and 

Hersey Red Shale Members of the Pembroke Formation, and his stratigraphic rank assignment for the 

names Leighton and Hersey is adopted herein. The 

former Pembroke Formation is raised herein to group 

status. Except as noted in the following descriptions, 

age assignments follow those of Berry and Boucot 

(1970). 

The major structural features of post-Early Devonian age are (1) the large open Cobscook anticline in 

the Eastport and Gardner Lake quadrangles; (2) the 

companion Machias syncline, south of Machias; (3) the 

Lubec fault zone, a belt of sheared and tightly folded 

rocks of the Eastport and Quoddy Formations; and (4) 

the Quoddy block, a structural block about 8 km wide 

and at least 40 km long that lies between the Lubec 

fault zone and the Fundian fault (fig. 2). The Fundian 

fault may be the border fault for the Triassic rocks of 

the Bay of Fundy (Ballard and Uchupi, 1975, p. 

1046-1049; Gates, 1969, p. 500). Northwest of the 

Lubec fault zone, block faulting accompanied the 

Silurian volcanism, for several faults bring volcanic 

formations against older rocks and are overlapped by 

younger Silurian formations (fig. 3). Most of the sedimentary and volcanic rocks are at least weakly metamorphosed and regionally deformed by northeasttrending cleavage. This cleavage and the major folds 

originated prior to deposition of the Upper Devonian 

Perry Formation, but the Perry red beds and the folds 

and cleavage were locally faulted and folded during 

deposition of the Perry and probably during the Carboniferous. 

Rocks assigned to the Quoddy Formation are restricted to the Quoddy structural block (fig. 2), and no 

correlation of the formation has been firmly established as yet outside the block. The lower part of the 

Quoddy Formation is composed of dark graptolitebearing pyritic argillites, shale, siltstone, and thin 

graded beds of feldspar-rich tuff. The upper part of the 

Quoddy, exposed mainly in the southern part of the 

block, contains abundant marine basalts (some pillowed) and keratophyres, as well as local limestone and 

tuff breccia. Silurian graptolites found in shale and 

sparse brachiopods found in the coarse limestone and 

tuff breccia indicate a late Llandovery age for the 

Quoddy Formation. 

The Quoddy Formation occurs as many inclusions 

and septa engulfed in the Cutler Diabase, a name originally applied by Gates (1961) to complex gabbroic and 

diabasic intrusions exposed near the town of Cutler. In 

this report the name "Cutler Diabase" is applied for 

convenience to the mafic intrusions of the Quoddy 

structural block. Throughout the block the Cutler is a 

complex body of multiple emplaced massive and 

locally layered gabbro and diabase. Some of the diabase appears to have been intruded into the Quoddy 

Formation when the Quoddy sediments were still soft 

and water bearing. As a whole, the Cutler is interpreted to represent a subvolcanic complex emplaced 

mainly in Silurian time, penecontemporaneously with 

the accumulation of the Quoddy Formation, or only 

somewhat later. However, the Cutler may have been a 

source of basaltic lavas that were erupted during the 

accumulation of the Silurian Dennys, Edmunds, and 

Leighton Formations, and parts of the Cutler may be 

as young as Devonian. The Silurian and Lower Devonian formations exposed northwest of the Lubec 

fault zone are intruded by many dikes, sills, and plutons of diabase (too small to show in fig. 2) that mineralogically and texturally resemble the Cutler of the 

Quoddy block. As shown later, however, our chemical 

data suggest that the Cutler is petrologically akin to 

the Silurian basaltic flows, whereas the sampled diabase sills that intrude the Leighton and Hersey Formations are akin to basalt of the Upper Devonian Perry 

Formation. 

Silurian rocks, very poorly exposed and as yet unnamed, occupy the central part of the Columbia Falls 

quadrangle and the northern part of the Gardner Lake 

quadrangle. They include the slide conglomerates described by Gilman (1966), basaltic flows, silicic tuffs 

and tuff-breccias, and pyritic thinly bedded argillites 

and siltstones. Graptolites from black shales in the 

northwestern part of the Columbia Falls quadrangle 

suggest a late Llandovery to Wenlock age (W. B. N. 

Berry, written commun., 1962). The black shales are 

interbedded with silicic tuff-breccias that are in fault 
STRATIGRAPHY AND STRUCTURE OF THE MACHIAS-EASTPORT AREA 5 

EXPLANATION 

Contact 

Devonian or 

younger fault--

Dotted under 

water 

----Silurian fault 

~ Strike and dip 

of bedding 

Strike and dip 

of vertical bedding 

~___l_...L_j_ Indefinite metamorphic 

isograd- B, biotite; 

C, chlorite 

0£ 

.A .a Sites of samples--

Numbered or 

lettered samples 

listed in table 2 

0 10 15 20 

.----- ---'6::_JO,c-15,_' _ _____ ,66~ 

44" ~--- -- --- _J45' 

44" L_ _ _ _ _ __j[__ _ _ _ _ _J 30' 

INDEX TO QUADRANGLES 

25 KILOMETERS 

FIGURE 2.-Simplified map of the bedrock geology of the Machias-Eastport portion of the coastal volcanic belt. Simplified from Gates 

(1961,1975) and unpublished mapping by Gates and by Richard A. Gilman. 

contact with pre-Silurian schist. These unnamed 

Silurian rocks are labeled Quoddy(?) Formation in figure 3, on the assumption that they correlate with the 

Quoddy. 

The Silurian Dennys Formation, possibly of late 

Llandovery through Wenlock age, is the basal part of 

the volcanic sequence in the Cobscook anticline. It consists primarily of basaltic flows, agglomerate, and 

EXPLANATION FOR FIGURE 2 

Unit symbols (listed in order of increasing age): Dp, Perry Formation; Dgr, granite; Dg, gabbro and diorite; Dt, basaltic andesite of 

Mount Tom stock; De, Eastport Formation; all of Devonian age. Sh 

and Sl, respectively Hersey (which may also be Early Devonian age) 

and Leighton Formations of the Pembroke Group in the MachiasEastport area; Se, Edmunds Formation; Sd, Dennys Formation; 

Sq, Quoddy Formation; all of Silurian age. Su, undivided Silurian 

rocks. Sc, Cutler Diabase of Gates (1961) of Silurian(?) age in the 

Quoddy block. DC, Ordovician and Cambrian rocks. 
6 BIMODAL VOLCANIC ROCK ASSEMBLAGES, MACHIAS-EASTPORT AREA, MAINE 

NW 

::>. 

0 

t 

0 

SE 

CLASTIC ROCKS OF :e 

FREDERICTON TROUGH 8 VOLCANIC AND CLASTIC ROCKS 

OF MACHIAS-EASTPORT AREA 

B-

(f) 

LU "' 

4 

8 KILOMETERS 

0 10 20 KILOMETERS 

EXPLANATION 

Volcanic rocks 

Primarily shale 

Conglomerate 

FIGURE 3.-Schematic section across the coastal volcanic belt 

during late Eastport time, showing relations of Silurian and 

Devonian formations of the southernmost Fredericton trough 

and the Machias-Eastport area to one another and to contemporaneous normal faults. Heavy lines, faults; arrows show direction of movement. Solid thin lines, formation contacts, 

coarse .water-laid bedded tuffs deposited on the slopes 

of a basaltic volcano. The eruptive center of this volcano is an ellipsoidal mass of basalt and agglomerate 

in the east-central part of the Gardner Lake quadrangle. The Dennys Formation also contains a linear 

belt of keratophyric tuff-breccias, breccia pipes, and 

flow-banded and autobrecciated domes and shallow intrusions. This belt of eruptive centers may have been 

controlled by a contemporaneous fault zone. The fauna 

of the Dennys Formation consists of a diversified 

brachiopod suite together with trilobites, corals, and a 

few pelecypods. 

The Silurian Edmunds Formation, probably mainly 

of Ludlow age, is made up primarily of silicic green, 

purple, and maroon coarse tuff-breccia deposited as 

submarine pyroclastic debris flows or avalanche deposits (Gates, 1975, p. 4-5). As shown by drilling, one 

queried where uncertain; dashed lines, gradational boundaries 

between volcanic and sedimentary parts of formations. Digdeguash and Waweig Formations (both of Silurian age) and 

Flume Ridge Formation (of Silurian or Early Devonian age) 

are of Ruitenberg (1967). Correlation of Waweig with the 

Leighton Formation is from Pickerill (1976). 

graded avalanche deposit in the upper part of the 

Edmunds is at least 100m thick. A roughly circular 

area of very coarse breccia on the border between the 

Eastport and Gardner Lake quadrangles probably 

marks the vent that fed the avalanche deposits. The 

Edmunds Formation also contains a few thin basalt 

flows and basaltic agglomerates and several lensshaped domes and shallow intrusions of flow-banded 

and autobrecciated vitrophyre. The gray to black tuffaceous siltstones and shales of the Edmunds Formation carry a diversified fauna of brachiopods, trilobites, corals, pelecypods, gastropods, ostracodes, and 

orthoceroids. Watkins and Boucot (1975, p. 254) concluded that the brachiopod fauna indicates a nearshore 

environment. 

The Leighton Formation of latest Silurian (Pridoli) 

age is composed largely of gray to blue-gray well-
STRATIGRAPHY AND STRUCTURE OF THE MACHIAS-EASTPORT AREA 7 

bedded and somewhat calcareous siltstones and shales. 

A few thin dacitic to rhyolitic avalanche tuff-breccias, 

several thin basalt flows, lens-shaped bodies of basaltic 

agglomerate, and several domes of dacitic to rhyolitic 

vitrophyre comprise the volcanic components. Locally, 

as shown by inten'sive drilling in a small area, basaltic 

flows evidently poured in rapid succession into faultbounded depressions in the sea floor, where they accumulated to thicknesses of 200 m or more. The 

brachiopod fauna is a restricted shallow-water one 

(Watkins and Boucot, 1975, p. 254), accompanied by 

numerous gastropods, pelecypods, ostracodes (Berdan, 

1971), and a few trilobites. 

The Hersey Formation consists of maroon siltstones 

and shales and a few nodular limestone beds; it contains no known volcanic rocks. The Hersey pinches out 

to the southeast, where its stratigraphic position is 

occupied by shales and volcanic rocks assigned to the 

Eastport Formation. The Hersey carries a brackishwater fauna of pelecypods, gastropods, and ostracodes. The ostracode fauna suggests that the SilurianDevonian boundary lies within the Hersey (Berdan, 

1971). 

The Eastport Formation of Early Devonian age 

(Gedinne) is a diverse formation composed of basalt 

flows, coarse basaltic agglomerate, a few dacite and 

rhyolite tuff-breccia and ash-flow deposits, flowbanded and autobrecciated domes and shallow intrusions of rhyolitic vitrophyre, and gray to maroon siltstones, shales, and minor conglomerate. The fauna is a 

restricted one of lingulas, pelecypods, and ostracodes 

(Berdan, 1971). The volcanic rocks of the Eastport Formation erupted partly on land and partly on tidal flats 

and shallow lagoons, in contrast to the dominantly 

marine eruptions of the underlying Silurian formations. 

During the Acadian orogeny a bimodal suite of 

gabbro, granitic rocks, and subordinate quartz diorite 

or diorite, named the Bays-of-Maine Complex by Chapman (1962), intruded the volcanic section and neighboring pre-Silurian rocks along the coastal belt between Penobscot Bay and Passamaquoddy Bay. The 

complex is not shown in figure 1. The Bays-of-Maine 

Complex and older rocks were then intruded by 

granites of the Maine coastal plutons of Chapman 

(1968). Although field relationships clearly indicate 

that the coastal plutons are younger, no sharp distinction is seen in the isotopic ages. Granitic rocks assigned to the complex have yielded isotopic ages of 

about 400 m.y. (Spooner and Fairbairn, 1970; Rb-Sr 

whole rock and K-Ar biotite); whereas a spectrum of 

ages greater than 410-340 m.y. has been obtained by 

the same methods from the coastal granites (Brookins, 

1976, table 2). As shown by Brookins (1976), widely 

disparate ages have been obtained from a single pluton 

by different methods. Thus, it remains to be seen 

whether some or all of the coastal plutons are part of 

the Bays-of-Maine Complex or instead belong to a distinctly younger suite, as favored by Chapman (1968). 

The Perry Formation lies on a surface that truncates 

both the Silurian and Lower Devonian section and the 

400-m.y.-old Red Beach Granite of Amos (1963). The 

Red Beach is a large body that intrudes the Eastport 

Formation immediately north of the area of figure 2. 

This granite contributed coarse gravel to the Perry 

Formation (Amos, 1963; Spooner and Fairbairn, 1970). 

The Perry is mainly a post-Acadian red bed sequence 

among which Schluger (1973) has recognized scree, 

alluvial fan, overbank, and lacustrine facies. Block 

faulting formed the local basins in which the Perry conglomerate was deposited. One flow of altered basalt is 

exposed in the Eastport quadrangle. Plant fossils indicate a Late Devonian age (Smith and White, 1905, 

p. 35). 

Though cleaved, the Silurian and Lower Devonian 

formations are only weakly metamorphosed regionally. Detrital muscovite commonly is preserved in 

the shales as bent flakes subparallel to bedding. Metamorphic chlorite and muscovite are found most commonly where the rocks are strongly cleaved near faults. 

As described later, the volcanic rocks are partly to 

completely altered to mineral associations typical of 

spilites and keratophyres of greenschist facies or 

slightly below that metamorphic rank. Prehnite and 

pumpelleyite-rare on the southeast side of the Appalachians-were found in one specimen from a preAcadian(?) diabasic sill that intrudes the Leighton Formation. Hornfels is present along the borders of the 

principal plutons on the west; metamorphic biotite associated with the hornfels is present in a belt that extends a few kilometers east of the plutons, and in the 

Quoddy block (fig. 2). Actinolite is restricted to about 

the same areas. In small areas of sulfide mineralization, some of the rocks are hydrothermally altered. 

In summary, the volcanic and associated fossiliferous sedimentary rocks of the Machias-Eastport area 

record a long history of volcanism, often explosive. 

Volcanism lasted perhaps 20 m.y. in the Silurian, and a 

few more million years in Early Devonian time. Marine 

environments, becoming generally shallower through 

time, prevailed through the Silurian; in Early Devonian time the volcanics were erupted into shallow 

lagoons, onto tidal mudflats, and on land. The Eastport Formation represents the beginning of a general 

emergence of the coastal volcanic terrane that is shown 

on Boucot's paleogeographic maps (Boucot, 1968, figs. 

6-3 to6-6). 
8 BIMODAL VOLCANIC ROCK ASSEMBLAGES, MACHIAS-EASTPORT AREA, MAINE 

DEFINITION AND PETROGRAPHY OF 

VOLCANIC ASSEMBLAGES 

Volcanic rocks of the Machias-Eastport area divide 

into a strongly bimodal assemblage of basalts grading 

to basaltic andesites, for convenience called the basaltic suite, and rhyolites grading to silicic dacites, called 

the rhyolitic suite (table 1; fig. 4). The only known truly 

intermediate andesite is a dike that intrudes the 

Dennys Formation. If the Cutler Diabase were included in the measurements shown in table 1, the 

volume of the basaltic suite would greatly exceed that 

of the rhyolitic one. 

Because all of the Machias-Eastport rocks are at 

least mildly metamorphosed, we classify them mainly 

on the basis of their chemical composition, supplemented by relict primary petrographic features. The 

whole assemblage is subalkalic, as shown later. With 

modifications we follow the common Canadian practice 

of dividing subalkalic metavolcanic rocks into four 

broad categories: basalt, andesite, dacite, and rhyolite 

(Irvine and Baragar, 1971; Church, 1975). A plot of our 

data in Church's diagram (Church, 1975, fig. 8) shows 

that all samples from the basaltic suite are clearly 

basalts, despite the presence of as much as 56.5 percent Si02• In this report, the term basaltic andesite is 

used for individual basaltic rocks having more than 52 

percent Si02, as shown in figure 4. The term rhyolitic 

suite also is used for convenience, and the term silicic 

dacite applies to rocks of this suite having 67-70 percent Si02• The only sample of andesite, with 62.9 percent Si02, might be called dacite, but the analysis plots 

in the overlapping field for andesites and dacites in 

Church's diagram, and its cation norm plots in the 

field for calc-alkalic andesites in figure 7 of Irvine and 

Baragar (1971). Moreover, the rock has the "motheaten" porphyritic texture that is characteristic of 

andesites elsewhere. 

Norms were calculated for several of the basaltic 

analyses, but they are used only sparingly in this 

paper, because at least Fe20 3 to FeO ratios and alkali 

contents have been modified during alteration, even in 

some rocks that we call "least altered" (as defined 

TABLE I.-Volume by percent of volcanic and hypabyssal intrusive rock types in the Eastport quadrangle, estimated from areas of each type in cross sections; Cutler Diabase excluded 

Basaltic suite Rhyolitic suite 

Formation (basalt and basaltic Andesite (rhyolite and silicic 

andesite) dacite) 

East~ort 49 0 51 

Leig ton 58 0 42 

Edmunds 2 0 98 

Dennys 56 '5 39 

'Dike. 

later). The norm of one analysis-a sill that intrudes 

the Leighton Formation-shows a small amount of 

nepheline, but the other norms are saturated or quartz 

bearing. The analyses were prepared for norm calculations according to the recommendations of Irvine and 

Baragar (1971). 

The whole bimodal assemblage is further divisible 

into three assemblages on the basis of age, relict petrographic features, and chemical composition, herein 

called the Silurian, older Devonian, and younger Devonian assemblages. The Silurian assemblage comprises the basaltic and rhyolitic suites in the various 

Silurian formations. The Cutler Diabase of the Quoddy 

block and the andesite dike that intrudes the Dennys 

Formation are included in the Silurian assemblage, because their petrologic features and geologic settings 

suggest a genetic relation to the Silurian volcanics. 

The older Devonian assemblage contains the basaltic 

and rhyolitic suites of the Lower Devonian Eastport 

Formation. These rocks are chemically and petrographically distinct from those of the Silurian formations. Also assigned to the older Devonian assemblage 

is a plug of porphyritic basaltic andesite (the andesite 

at Mount Tom of Bastin and Williams (1914)), which intrudes the Leighton Formation and has some of the 

chemical characteristics of basalts in the Eastport. 

The basaltic tuff at the base of the Eastport Formation 

is chemically like the Silurian basalts, but no Eastporttype basalts are known at lower stratigraphic levels, 

and no Silurian-type basalts are known at higher 

levels. One of the silicic dacites in the Leighton Formation has chemical characteristics of both the Silurian 

and older Devonian basalts. With these exceptions, the 

Silurian and older Devonian volcanic rocks are chemically unlike one another. Most of the compositional 

shift seems to have taken place during a pause in volcanism represented by the Hersey Formation, and during the transition from marine to subaerial conditions. 

The younger Devonian assemblage is defined by the 

one sample we obtained from a basalt flow in the Perry 

Formation, but assigned to this assemblage are three 

sills of comparable chemistry but of probable prePerry age that intrude the Leighton and Hersey Formations. The age of the sills is uncertain, but available 

evidence, including the presence of Acadian cleavage, 

suggests that they were emplaced after Hersey deposition and prior to Acadian cleaving and metamorphism. 

No younger Devonian rhyolites are known. The 

youngest coastal granites might, however, be silicic 

companion rocks to the Perry basalt. Of seven chemical analyses of granites that are used in figures 5 and 7, 

four are from the Tunk Lake pluton (Karner, 1968), 

which has yielded a preliminary K-Ar age of 357±10 

m.y. (Karner, 1974, p. 190), and one is from the Vinal-
DEFINITION AND PETROGRAPHY OF VOLCANIC ASSEMBLAGES 9 

haven pluton, which has yielded a K-Ar age of 399 m.y. 

and an Rb-Sr whole rock age of 361±7 m.y. (Brookins, 

1976, table 2). If the younger age is the correct one for 

the Vinalhaven pluton, the approximate 360-m.y. date 

for the Vinalhaven and Tunk Lake bodies is not much 

older than the beginning of the Late Devonian. Moreover, the pre-cleavage diabase sills in the Leighton and 

Hersey Formations must be older than the Perry, and 

within the broad range of isotopic ages that have been 

reported for the coastal granites and the granitic rocks 

of the Bays-of-Maine Complex. 

In the following descriptions primary igneous textures and mineralogy are emphasized. Effects of alteration and low rank metamorphism are described 

later. 

SILURIAN ASSEMBLAGE 

Most of the basalts and basaltic andesites of the 

Silurian assemblage display intergranular, subophitic, 

and ophitic textures. The habit of chlorite, commonly 

filling intercrystalline pores, suggests formerly porous 

textures (diktytaxitic) that are common in some tholeiites. A few basalts have a subtrachytic texture of 

flow-alined laths of plagioclase and grains of augite. 

Most of the basalts are aphyric. Less common porphyritic basalts have sparsely scattered phenocrysts 

of plagioclase. Some basalts of the Edmunds and 

Leighton Formations have sparse phenocrysts of clinopyroxene as well. Olivine is uncommon, but sparse 

pseudomorphs of saponite(?) after small euhedral 

phenocrysts of olivine were found in two flows from 

the Edmunds Formation. Calcic plagioclase, where 

preserved, exhibits normal gradational zoning from 

calcic labradorite to sodic labradorite or calcic andesine. The augite is colorless or very pale brown, lacks 

exsolution lamelli, is only rarely zoned, and shows no 

evidence of marginal reaction. A few fine-grained intergranular basalts have grains of pigeonite along with 

augite. Hypersthene is absent. Some flows have small 

amounts of intergranular potassium feldspar. Accessory minerals are magnetite, ilmenite, sphene, and apatite. Amygdules composed variously of chlorite, 

quartz, calcite, and epidote are common, particularly 

at the base and tops of flows. Although many flows 

overlie or underlie fossiliferous marine siltstones and 

mudstones, pillows and columnar structures are rare. 

Most flows are a few to 20 m thick, but some are as 

much as 100 m thick. 

The diabase and gabbro of the Cutler Diabase have 

essentially the same textures and mineralogy as the 

Silurian basalts but are coarser grained. In addition, 

small amounts of ragged green, pale-blue, and colorless 

amphibole are present in the interstices of some 

gabbros. One specimen has small amounts of late magmatic deep-brown basaltic hornblende. Small amounts 

of metamorphic biotite are present in some rocks. In 

places complex compositional layering and cumulate 

textures indicate local gravitational fractionation of 

labradorite, augite, and titaniferous magnetite (Gates, 

1961, p. 39-41). 

Basaltic tuff-breccias of the Silurian assemblage are 

a mixture of angular to subrounded fragments of 

basalt as much as a meter across in a matrix of pumice 

lapilli, crystals of plagioclase and augite, disaggregated bedded basaltic tuffs, and clasts of siltstone and 

shale. 

Most of the Silurian rhyolites and silicic dacites contain sparse phenocrysts of albite widely scattered 

through a fine-grained matrix of intergrown albite, 

quartz, potassium feldspar, and alteration products. 

Phenocrysts of albite in the silicic domes and flows are 

euhedral; some are in glomeroporphyritic clots. The 

phenocrysts in pyroclastic rocks are commonly broken. 

The albite, having low temperature optics, is commonly dusty with clays and is partially replaced by epidote, or calcite, or both, suggesting that the original 

plagioclase was more calcic. Alkali feldspar occurs as 

phenocrysts in a few rhyolites, but more commonly it 

is intergrown with quartz in lithophysae or in the 

groundmass. It has disordered and intermediate structural states (Benson orthoclase, Spencer B), suggesting 

that some primary feldspar has survived metamorphism. Rounded and embayed phenocrysts of 

quartz occur along with albite in a few of the rhyolitic 

rocks. Original ferromagnesian minerals are missing, 

but they may be represented by ragged masses of 

chlorite and clusters of epidote grains. Accessory minerals are magnetite, leucoxene, and sparse zircons. 

Flow banding is common. Spherulites and perlitic 

cracks indicate that many of the silicic rocks were 

originally vitrophyres. Faint outlines of shards indicate that some of these rocks are tuffs; some may be 

ash-flow deposits. Evidence of welding, if any, has 

been destroyed by the metamorphism. Other rhyolitic 

rocks have a fine-grained granophyric texture or one of 

interlocking feldspar microlites. 

Coarse silicic mixed tuff breccias in the Edmunds 

and Leighton Formations are composed of bouldersized blocks and smaller fragments of rhyolites, porphyries, chert, tuffs, basalt, and sedimentary rocks; 

grains of quartz, albite, and sparse augite; and metamorphic epidote, chlorite, sericite, and carbonate. The 

sand-sized matrix of some of these deposits is feldspar 

crystal tuff, containing abundantly scattered broken 

and euhedral crystals of albite about a millimeter long. 

The andesite dike that intrudes the Dennys Formation has euhedral phenocrysts of saussuritized plagio-
10 BIMODAL VOLCANIC ROCK ASSEMBLAGES, MACHIAS-EASTPORT AREA, MAINE 

clase, and epidote pseudomorphous after phenocrysts 

of pyroxene(?). The phenocrysts are set in a felty 

matrix of ragged blue-green hornblende; laths of dusty 

albite; scattered grains and spots of chlorite, 

leucoxene, epidote, and magnetite; and sparse grains of 

quartz. 

OLDER DEVONIAN ASSEMBLAGE 

Most of the basalts and basaltic andesites of the 

Eastport Formation are porphyritic, a significant difference from most of the Silurian basalts. Unaltered 

phenocrysts of labradorite and augite, some in 

glomeroporphyritic clots, are set in an intergranular to 

subtrachytic matrix of the same minerals. Some of the 

more silica rich basalts and basaltic andesites have a 

percent or so of intergranular quartz. Small amounts of 

potassium feldspar are common in the interstices. 

Chlorite is ubiquitous; epidote or calcite or both may 

be present. The content of magnetite is at least twice 

that of the Silurian basalts. The actual iron contents of 

the older Devonian basalts are somewhat higher also, 

but the higher magnetite contents are probably more a 

function of the distinctly lower magnesium contents in 

the older Devonian basalts. This assumes that iron to 

magnesium ratios in the pyroxenes are about the same 

in basalts of both assemblages, but the pyroxenes have 

not been analyzed. Much of the iron in the older 

Devonian basalts has been oxidized to hematite, which 

gives the rocks a maroon color, in contrast to the darkgreen or black hues of the Silurian basalts. Many of the 

older Devonian basalts have red oxidized tops. 

The older Devonian silicic rocks contain scattered 

euhedral phenocrysts of albite, dusty with clays and 

other alteration products, in a fine-grained matrix. The 

matrix ranges from devitrified flow-banded glass to 

fine-grained granophyre. Potassium feldspars occur in 

small tabular crystals in the groundmass intergrown 

with quartz, in spherules, in lithophysae, and rarely as 

euhedral phenocrysts. Some of the potassium feldspars have intermediate structural states (Spencer B, 2 

samples)!, suggesting that some primary feldspar 

structure has survived metamorphism. One sample 

has ordered alkali feldspar (maximum microcline), suggesting a secondary origin, in accord with its habit as 

discrete layers and lenses along flow banding. A few 

samples contain rounded resorbed phenocrysts of 

quartz. Ragged masses and wisps of chlorite are the 

only ferromagnesian minerals. Magnetite, partly or 

1

X-ray studies done by U.S. Geological Survey, under supervision of C. G. Cunningham, 

Jr. Alkali feldspars (mainly groundmasst were separated from four Silurian and three older 

Devonian rhyolites; structural states determined by the method of Wright ( 19681. 

wholly altered to hematite, is more abundant than in 

the Silurian silicic rocks. A few devitrified vitrophyres 

of the Eastport Formation contain individual shards 

and collapsed pumice. 

The basaltic andesite at Mount Tom, like most 

basalt and the basaltic andesite in the Eastport Formation, is porphyritic. Saussuritized phenocrysts of 

labradorite are set in an intergranular matrix of small 

euhedral feldspar laths, epidote grains, chlorite, and 

abundant magnetite. Clots of chlorite and epidote 

rimmed by an unidentified opaque mineral have the 

outlines of original pyroxene phenocrysts. 

The coarse tuff-breccias, the finer grained tuffs, and 

the bedded water-laid tuffs consist wholly of rocks and 

minerals confined to the volcanic pile. Although clasts 

of pre-Silurian rocks are present in the basal and slide 

conglomerates in other parts of the coastal volcanic 

belt, no fragments of metamorphic or plutonic rocks 

from the presumed underlying Avalonian basement or 

from the pre-Silurian metamorphic rocks have been 

found either in the Silurian or Devonian volcanic rocks 

of the Machias-Eastport area. 

YOUNGER DEVONIAN ASSEMBLAGE 

The single sample of basalt from the Perry Formation contains sparse small euhedral phenocrysts of 

argillized labradorite in a fine-grained matrix of lathshaped argillized calcic andesine, grains of augite, interstitial chlorite, and a few grains of quartz. Some of 

the quartz grains are round and probably xenocrystic. 

Magnetite is abundant. The groundmass has finegrained reddish- to yellowish-brown platy to fibrous 

mineral (probably saponite), and a few small pseudomorphs of pyroxene containing bastite possibly after 

orthopyroxene. 

Diabase sills that intrude the Leighton and Hersey 

Formations have a relict ophitic or subophitic texture. 

The plagioclase is labradorite where unaltered. Magnetite and ilmenite are abundant, and small amounts of 

potassium feldspar are present in interstices. Apatite 

is rather abundant, as small rod-shaped crystals that 

penetrate plagioclase. Trace amounts of quartz were 

seen. One altered sample (table 2, sample No. 29) from 

the interior of a sill is coarse grained and composed of 

albite to sodic oligoclase, prehnite, sunbursts of 

vividly pleochroic blue-green to straw-yellow pumpelleyite (showing anomalous interference colors), epidote, deep-green chlorite, magnetite, augite, a few 

spots of calcite, and sparse saponite possibly after 

olivine. 
ALTERATION 11 

SAMPLING 

The sites of 44 samples that form the basis of this 

study are shown in figure 2. Of the 44 samples, 36 were 

collected from outcrops and roadcuts between Machias 

and Eastport, and 8 were collected from drill cores in 

the Pembroke prospect, a small area of epigenetic sulfide mineralization in the Leighton Formation in the 

northwest corner of the Eastport quadrangle. Sample 

A i~ hyW:othermally altered basalt from the prospect. 

MaJor oxide analyses of 32 samples are listed in table 

2, and all analyses, recalculated without volatiles are 

displayed in figure 4. Appendix A gives sites and descriptions of samples listed in table 2. Appendix B 

gives the same information and chemical analyses for 

all other samples. 

. Although our ~amples were not collected in proportion to the relative volumes shown in table 1 we believe that all the major types that define the Silurian 

and older Devonian assemblages are represented. Four 

of the seven silicic tuffs shown in figure 4 plot between 

67 and 71 percent Si02; they have the composition of 

silicic dacite and low-silica rhyolite. These samples represent mixed tuff breccias in the Edmunds and Leighton Formations, thought to be submarine pyroclastic 

flow deposits. As such they probably represent the 

average compositions of the source volcanoes. Our 

sampling of the younger Devonian assemblage is not 

adequate. We have one analysis of Perry basalt, and 

three of diabase sills in the Leighton. Seven analyses 

from the literature of the coastal granites (Karner, 

1968; Dale, 1907) are tentatively included in this assemblage. 

ALTERATION 

Thin sections show clear evidence of variable spilitic 

and keratophyric alteration, which produced saussuritized and albitized plagioclase, and also chlorite, epidote, carbonate, and leucoxene after the ferromagnesian and opaque minerals. At least minor changes in 

chemical composition are inevitable in such rocks, and 

a~e shown in fact by the vertical spread of data (especially CaO, Na20, and K20) in figure 4. Our problem is 

to find the rocks that have been modified the least. 

Furthermore, the vitrophyric silicic rocks must have 

been prone to alkali exchange during hydration (Lipman and others, 1969). In addition, studies of the Pembroke sulfide deposits have shown that hydrothermal 

alteration may be more intense and widespread than 

one might expect from casual inspection of outcrops 

and thin sections. There, a thick sequence of basalts in 

the Leighton Formation has been greatly enriched in 

potassium (in feldspar) and manganese (in carbonate 

and epidote). Data for one sample are shown in table 2 

and figure 4 (sample No. A). Without the spectacular 

manganese contents, these rocks could be mistaken for 

trachybasalts. Analyses that show more than 0.3 percent MnO and other obviously hydrothermally altered 

rocks were not included in this study. 

The eight large symbols shown in figure 4 represent 

our least altered basalts and basaltic andesites, and are 

our "best" analyses. These rocks have recognizable 

calcic plagioclase, augite, very little calcite, and less 

than about 0.5 percent C02 (table 2, sample Nos. 1, 4, 5, 

7, 8, 18, 20, 30). The plagioclase in these rocks may be 

dusty with alteration products, and the augite is embayed by chlorite. Furthermore, the least altered rocks 

are variably oxidized, having FeO to Fe20 3 ratios of 3.3 

to 6.8 in the five marine Silurian basalts, about 1.5 in 

the two lagoonal or subaerial Eastport basalts, and 0.3 

in the single subaerial Perry basalt (table 2). Nonetheless, these rocks are the best guides we have to the 

original nature of the basalts and the chemical changes 

that accompanied alteration of the other samples. Because the least altered silicic rocks cannot be defined 

as rigorously, we have not attempted to identify 

"best" analyses of dacites and rhyolites. 

In figure 4, 2Fe0*, MgO, Ti02, and P20 5 in the 

Silurian basalts plot within narrowly defined fields 

that are almost completely separate from the fields for 

these oxides in the Devonian basalts. This relationship 

holds regardless of degree of alteration, suggesting 

that these four oxides were little affected by alteration. 

This conclusion finds support in Val1ance's (1974) 

study of chemical changes that accompanied the 

spi~tic alteration of Deccan basalt near Bombay, 

India. There, Ti02 appears to have been most stable 

followed by MgO and total iron (FeO*). Although th; 

oxidation states of iron are extremely variable in our 

basalts and in the Deccan basalt, the total iron contents evidently are little affected by alteration. 

In contrast, the CaO and N a20 contents evidently 

have been changed significantly. On the assumption 

that the CaO and Na20 contents of the least altered 

Silurian basalts are primary or nearly so, CaO has been 

strongly depleted and N a20 has been enriched in some 

samples of more altered basalts. Depletion of calcium 

may have occurred where C02 pressure was too low to 

fix the calcium as calcite in the rocks, for two samples 

'FeO* = FeO + 0.9 Fe,O, (in weight percent I. 
12 BIMODAL VOLCANIC ROCK ASSEMBLAGES, MACHIAS-EASTPORT AREA, MAINE 

TABLE 2.-Major oxide analyses in weight percent of volcanic rocks of the Machias-Eastport area, Maine 

[Starred site no.,least altered basalt, basaltic andesite, and diabase; has at least clinopyroxene and calcic plagioclase; < 0.5 percent CO,. Site Nos. 14 and A, analyses by standard methods 

by Vertie C. Smith; all other analyses by rapid methods described by Shapiro and Brannock (1962) supplemented by atomic absorption; analyzed by P. Elmore, J. Glenn, R. Moore, H. 

Smith, and S. Botts, all U.S. Geological Survey. N, no phenocrysts; P, plagioclase in basalts, alkali feldspar in silicic rocks; C, clinopyroxene; 0, olivine. Leaders(-·), not determined; <, 

less than] 

Assembla~ ............... ________________________________________ Sil_'u_na_·_n __________________________________________________________ __ 

Formation ............... . Dennys Edmunds Leighton Cutler Dennys Edmunds 

Sample No................ *1 Laboratory No.. . . . . . . . . . . . D160 

372W Rock class .. . .. .. .. .. .. .. . Flow Phenocrysts . . . . . . . . . . . . . . N 

SiO, ..................... 49.7 Al,O, .................... 16.6 Fe,O,.................... 1.6 FeO..................... 7.0 MgO.................... 7.7 

CaO .................... . Na,O ................... . K,O .................... . H,o· .................... . H,o-.................... . 

TiO, .................... . P,O, .................... . MnO ................... . CO, .................... . Cl,F,S .................. . 

Subtotal ........... . LessO ............ . 

9.2 

2.1 

1.0 

2.8 

.13 

1.3 

.25 

.16 

.05 

2 W176 

692 

Flow 

N 

47.7 

16.7 

3.4 

7.2 

8.1 

6.4 

3.1 

1.2 

3.3 

.27 

1.6 

.22 

.21 

<.05 

.oos 

Total . . . . . . . . . . . . . . . 99.59 99.50 

SiO,..................... 51.4 Al,O, .................... 17.2 Fe0*(Fe0+0.9 Fe,O,) . . . . . . 8.8 MgO.................... 8.0 CaO..................... 9.5 

Na,O.................... 2.2 K,O..................... 1.0 TiO,..................... 1.3 P,O,..................... .26 MnO.................... .17 

Assemblage ................... . 

49.7 

17.4 

10.7 

8.5 

6.7 

3.2 

1.3 

1.7 

.23 

.22 

3 *4 W176 W176 

691 689 Flow Flow P C,O 

50.8 

15.6 

4.1 

5.3 

6.3 

7.4 

4.4 

.83 

2.8 

.16 

1.2 

.15 

.15 

.36 

.oos 

48.9 

16.6 

2.0 

7.5 

6.6 

9.1 

3.1 

.56 

2.8 

.21 

1.2 

.19 

.22 

.15 

.oos 

99.55 99.13 

52.8 

16.2 

9.4 

6.5 

7.7 

4.6 

.86 

1.3 

.16 

.16 

51.0 

17.3 

9.7 

6.9 

9.5 

3.2 

.58 

1.3 

.20 

.23 

*5 6 W176 W178 686 301 Flow Flow N P,C 

46.4 

16.8 

2.4 

8.0 

8.0 

10.5 

2.1 

.15 

3.3 

.24 

1.4 

.15 

.15 

<.05 

.00 

50.6 

15.9 

2.8 

5.2 

5.9 

9.3 

2.8 

.45 

2.5 

.32 

1.3 

.37 

.09 

1.7 

*7 *8 

W176 W176 682 680 Intrusives 

N N 

46.8 

16.3 

1.4 

9.5 

7.4 

10.0 

2.5 

1.1 

2.8 

.11 

1.5 

.25 

.22 

.15 

.08S 

48.8 

15.1 

1.6 

8.2 

8.2 

10.7 

2.5 

.72 

2.3 

.11 

1.3 

.22 

.19 

<.05 

.00 

9 10 W176 W176 698 696 Flow Flow p p 

73.0 

15.0 .68 

1.2 

.53 

1.1 

6.4 

1.3 

.62 

.02 

.20 

.02 

.00 

<.05 

.028 

74.2 

13.2 .54 

1.6 

.48 

.92 

4.1 

3.7 

.69 

.03 

.28 

.03 

.10 

.06 

.15S 

11 

Dl60 

371W 

Flow p 

73.7 

13.3 

2.0 

.84 

.37 

.35 

4.2 

3.9 

.73 

.21 

.26 

.03 

.10 

.01 

99.59 99.23 100.11 99.94 100.09 100.08 100.00 

48.3 

17.5 

10.6 

8.3 

10.9 

2.2 

.16 

1.5 

.16 

.16 

Recalculated without volatiles 

53.4 

16.8 

8.1 

6.2 

9.8 

3.0 

.48 

1.4 

.39 

.10 

48.3 

16.8 

11.1 

7.6 

10.3 

2.6 

1.1 

1.5 

.26 

.23 

Older Devonian 

50.0 

15.5 

9.8 

8.4 

11.0 

2.6 

.74 

1.3 

.23 

.19 

73.4 

15.1 

1.8 

.53 

1.1 

6.4 

1.3 

.20 

.02 

.00 

74.8 

13.3 

2.1 

.48 

.93 

4.1 

3.7 

.28 

.03 

.10 

74.4 

13.4 

2.3 

.37 

.35 

4.2 

3.9 

.26 

.03 

.10 

12 

D160 

373W 

Flow 

p 

71.6 

14.4 

2.5 

.80 

.37 

.85 

4.0 

3.1 

1.2 

.24 

.44 

.09 

.04 

.30 

99.93 

72.9 

14.7 

3.2 

.38 

.87 

4.1 

3.2 

.45 

.09 

.04 

------------------------------------------------------------------------

Formation .................... . Eastport Mt. 

Tom 

Sample No... . . . . . . . . . . . . . . . . . . . 16 17 

Dl60 

375W 

Flow 

*18 19 *20 21 22 

Wl76 

684 

Stock 

P,C 

Laboratory No. . . . . . . . . . . . . . . . . . Wl76 

671 Rock class . . . . . . . . . . . . . . . . . . . . . Tuff 

Wl76 W176 Wl76 W176 674 673 672 679 Flow Flow Flow Flow Phenocrysts. . . . . . . . . . . . . . . . . . . . N p P P,C P,C P 

SiO, ......................... . 53.3 45.4 50.4 50.7 53.6 53.5 52.2 Al,O, ......................... . 15.1 18.0 17.2 14.5 16.4 14.8 17.8 Fe,O, ........................ . 3.0 5.4 4.0 6.2 3.8 .85 6.1 FeO .......................... . 6.0 5.3 6.1 5.5 5.9 10.5 4.2 MgO ......................... . 6.6 2.8 5.1 4.1 3.7 2.7 4.3 

CaO ......................... . 4.4 8.1 7.2 6.0 7.8 4.9 7.3 Na,O ......................... . 2.9 3.4 3.3 4.7 3.1 4.0 3.7 K,O .......................... . 1.1 .61 1.5 .32 1.5 .41 .85 H,o· ......................... . 3.8 2.5 2.6 2.2 1. 7 3. 7 1.4 H,o- ......................... . .75 1.4 .28 .24 .25 .20 .12 

TiO, .. .. .. .. .. .. .. .. .. .. .. .. .. 1.4 1.7 1.7 2.7 1.4 1.9 1.3 P,O, . . . . . . . . . . . . . . . . . . . . . . . . . . .23 .27 .28 .46 .35 .73 .21 MnO... . . . . . . . . . . . . . . . . . . . . . . . .16 .24 .20 .23 .15 .24 .24 CO,.. .. .. .. .. .. .. .. .. .. .. .. .. . 1.3 4.2 < .05 2.0 .35 1.4 < .05 s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .00 .00 .00 .00 .00 .00 

23 

Wl76 

675 

Flow 

p 

67.1 

13.0 

4.3 

1.8 

.43 

2.1· 

4.7 

3.0 

1.1 

.12 

.50 

.08 

.18 

1.5 

.01 

24 

D160 

379W 

Tuff 

p 

72.4 

13.9 

2.7 

.48 

.23 

.27 

4.2 

4.5 

.70 

.11 

.30 

.06 

.06 

.04 

Eastport 

25 

W176 

678 

Flow 

N 

69.5 

12.8 

2.8 

2.1 

.37 

1.6 

2.9 

5.5 

.80 

.10 

.39 

.04 

.18 

.89 

.14 

26 27 

Dl60 D160 378W 376W Flow Flow p p 

74.1 

12.9 

1.3 

1.7 

.15 

.09 

2.8 

5.3 

1.0 

.09 

.27 

.05 

.07 

.02 

74.9 

13.2 

1.7 

.28 

.04 

1.20 

4.9 

3.5 

.69 

.07 

.19 

.04 

.01 

.02 

Leighton 

13 

Dl60 

380W 

Flow p 

66.3 

15.9 .64 

3.6 

.31 

14 

D102 

345 

Flow p 

65.72 

14.46 

1.00 

3.02 

1.56 

1.8 2.67 2.0 4.48 5.7 1.70 1.8 1.68 .09 .12 

.54 .60 .08 .18 .07 .21 1.2 2.13 

(.02Cl, 

.04F, 

.lOS) 

99.69 .07 100.03 99.62 

68.4 68.7 16.4 15.1 4.3 4.1 

.32 1.6 1.9 2.8 

2.1 4.7 5.9 1.8 .56 .63 .08 .19 .07 .22 

Unnamed Leighton 

15 

Dl60 

370W 

Dike 

P,C? 

62.1 

16.2 

2.1 

3.8 

2.8 

5.2 

4.0 

1.5 

1.1 

.12 

.83 

.21 

.12 

.01 

100.09 

62.8 

16.4 

5.8 

2.8 

5.3 

4.1 

1.5 

.84 

.21 

.12 

A 

D102 

348 Alt. flow 

N 

46.61 

16.25 

1.40 

7.29 

3.56 

6.77 

1.94 

4.33 

3.63 

.16 

1.55 

.20 

1.51 

4.55 

(.01 Cl, 

.07F, 

.068) 

99.89 .06 99.83 

51.0 

17.8 

9.4 

3.9 

7.4 

2.2 

4.7 

1.7 

.22 

1.7 

Younger Devonian 

Unnamed 

28 29 Wl78 W176 302 683 Sills N N 

48.7 

14.6 

4.0 

8.7 

5.0 

8.0 

3.6 

.77 

2.4 

.20 

3.1 

.56 

.16 

.17 

47.0 

13.2 

3.7 

10.9 

5.2 

9.2 

4.2 

.12 

2.0 

.12 

3.4 

.45 

.24 

.26 

.00 

Perry Unnamed 

*30 

W176 

677 

Flow 

N 

48.9 

15.8 

10.8 

3.4 

4.8 

6.7 

3.9 

.70 

1.6 

.75 

2.1 

.49 

.15 

<.05 

.00 

31 

W178 

303 

Sill 

p 

47.5 

14.3 

2.9 

8.4 

4.6 

8.3 

2.0 

.77 

3.1 

.21 

2.6 

.57 

.11 

4.6 

----------------------------------------------------------------------------------------------- Total. . . . . . . . . . . . . . . . . . . . . 100.04 99.32 99.86 99.85 100.00 99.83 99.72 99.92 99.95 100.11 99.84 99.74 

Recalculated without volatiles 

SiO, ......................... . Al,O, ......................... . Fe0*(Fe0+0.9 Fe,O,) ........... . MgO ......................... . CaO ......................... . 

Na,O ......................... . K,O .......................... . TiO, ......................... . P,O, ......................... . MnO ......................... . 

56.6 

16.0 

9.2 

7.0 

4.7 

3.1 

1.2 

1.5 

.24 

.17 

49.8 

19.7 

11.1 

3.1 

8.9 

3.7 

.67 

1.9 

.30 

.26 

52.0 

17.7 

10.0 

5.3 

7.4 

3.4 

1.6 

1.8 

.29 

.21 

53.1 

15.2 

11.6 

4.3 

6.3 

4.9 

.34 

2.8 

.48 

.24 

54.9 

16.8 

9.5 

3.8 

8.0 

3.2 

1.5 

1.4 

.36 

.15 

56.6 

15.7 

11.9 

2.9 

5.2 

4.2 

.43 

2.0 

.77 

.25 

53.2 

18.1 

9.9 

4.4 

7.4 

3.8 

.86 

1.3 

.21 

.24 

69.1 

13.4 

5.8 

.44 

2.2 

4.8 

3.1 

.51 

.08 

.19 

73.0 

14.0 

2.9 

.23 

.27 

4.2 

4.5 

.30 

.06 

.06 

70.8 

13.0 

4.7 

.38 

1.6 

3.0 

5.6 

.40 

.04 

.18 

75.1 

13.1 

2.9 

.15 

.09 

2.8 

5.4 

.27 

.05 

.07 

75.7 

13.3 

1.8 

.04 

.20 

5.0 

3.5 

.19 

.04 

.01 

99.96 99.99 

50.1 

15.0 

12.7 

5.2 

8.2 

3.7 

.79 

3.2 

.58 

.16 

48.1 

13.5 

14.6 

5.3 

9.4 

4.3 

.12 

3.5 

.46 

.25 

100.09 

50.0 

16.2 

13.4 

4.9 

6.9 

4.0 

.72 

2.2 

.50 

.15 

99.96 

51.6 

15.5 

12.0 

5.0 

9.0 

2.2 

.84 

2.8 

.62 

.12 
VOLCANIC PETROLOGY 13 

of Silurian basalts that have the lowest CaO contents 

(fig. 4) also have little or no C02. 

Although the K20 contents of most of the basalts are 

too small and variable to tell much about possible enrichment or depletion, potassium may have been 

strongly depleted in two samples (table 2): sample 5, a 

least altered Silurian basalt with 0.16 percent K20, and 

sample 29, a more altered younger Devonian diabase 

with 0.12 percent K20. These K20 contents are onethird to one-eighth that of the other basalts or diabases 

of their respective assemblages. Both samples also 

have exceptionally small amounts of barium and 

rubidium, as shown for sample 5 in table 3. Although 

these low abundances might be primary, it seems more 

likely that potassium, rubidium, and barium were 

selectively removed. 

In the rhyolitic rocks of both assemblages, N a20 and 

K20 contents are extremely erratic (fig. 4). High Na20 

contents are typically accompanied by low K20 contents, however, so that the total alkali contents are 

much less variable. This approximately reciprocal relationship may express alkali exchanges that took place 

while the rocks were still glassy. 

Although none of the sampled rocks were truly 

closed systems during alteration, there is little evidence of massive chemical changes that would significantly affect the identification bf a rock from its chemical analysis. Andesites were not changed to rocks that 

we now call basalts or rhyolites, or vice versa. Our confidence in the validity of the major oxide analyses is 

strengthened by the minor-element data, which as 

shown later have remarkably consistent minor-element 

signatures within each assemblage and with pristine 

basalts of other parts of the world. 

VOLCANIC PETROLOGY 

The bimodal distribution of the volcanic rock types 

and their major chemical features are shown in figures 

4-7. The Silurian and older Devonian assemblages are 

distinguished by almost consistently different 

FeO*/MgO, Fe0*/Si02, and Mg0/Si02, and contents of 

MgO and total alkalis in both the basaltic and rhyolitic 

suites. Basalt in the Perry Formation and the diabase 

sills in the Leighton and Hersey Formations, and the 

tentatively included coastal granites (figs. 5, 7) comprise a third compositionally distinctive bimodal suite. 

Figure 5 shows alternative boundaries between 

alkaline and subalkaline or tholeiitic basaltic rocks. We 

favor Irvine and Baragar's line (1971) for the MachiasEastport rocks, because it yields the simpler classification (almost all subalkaline), and because the line of 

Macdonald and Katsura (1964) does not divide the 

Machias-Eastport rocks in a compositionally meaningful way. This preference is supported by data on the 

oxides and minor elements that Floyd and Winchester 

(1975) and Winchester and Floyd (1976) considered to 

be immobile during alteration, namely: P 205, Ti02, 

niobium, yttrium, and zirconium. According to criteria 

established by these authors, almost all the Eastport.: 

Machias basalts are clearly tholeiitic. The younger 

Devonian basalts are partial exceptions, for they may 

be classed as alkalic on the basis of their high contents 

of Ti02 and P 20 5, and high P 20 5 relative to zirconium; 

or they may be classed as tholeiitic on the basis of low 

contents of niobium relative to yttrium. These relationships may be seen when our data from table 3 are 

plotted in various diagrams of Winchester and Floyd 

(1976) and Floyd and Winchester (1975) that utilize all 

the listed immobile elements. Although the older and 

younger Devonian basalts have higher niobium contents than the Silurian basalts (table 3), alkalic basalts 

elsewhere typically have at least twice as much 

niobium as the 14 ppm (parts per million) niobium in 

sample 30 (table 3), a younger Devonian basalt. (See 

Engel and others, 1965, table 2; Lipman and Moench, 

1972, table 1.) 

Figure 6 is the commonly used AFM diagram, and 

figure 7 is the type of diagram used by Kuno (1968, fig. 

4, 16-18) to illustrate the variation of major oxides 

relative to solidification index, or the amount of MgO 

that shows in an AFM diagram. In figure 7 the variations of FeO*, Ti02, and Si02 relative to solidification 

index effectively illustrate the compositional evolution 

of both the basaltic and rhyolitic suites of all three age 

assemblages. 

Table 3 summarizes minor-element data. In this 

report these data are used only for comparative purposes, and further interpretations are deferred until 

other analyses are completed. 

Pearce and Cann (1973) prepared three diagrams 

that are intended to permit one to identify the tectonic 

setting of a suite of basalts on the basis of their minorelement contents; specifically, titanium, zirconium, 

yttrium, and strontium. These diagrams do not appear 

to be appropriate for identifying the tectonic setting of 

the basalts of the Machias-Eastport area. The main 

problem, also recognized by Gottfried, Annell, and 

Schwarz (1977, p. 106, fig. 6), is the lack of adequate 

data on a wide range of continental tholeiites. For 

example, data for the Columbia River Basalt Group 

(McDougall, 1976, tables 1-3) and for basalts of the 
14 BIMODAL VOLCANIC ROCK ASSEMBLAGES, MACHIAS-EASTPORT AREA, MAINE 

BASAL TIC SUITE RHYOLITIC SUITE 

BASALT BASAL TIC ANDESITE ANDESITE SILICIC DACITE RHYOLITE 

~ 

48 

M 

0 i6 e15 e13 N 16 <( 0 •• 0 

0 0 

.& .& 

12 X .& 

"';; 

0 • N A Q) 9 * LL. 00? 

(1)0 LL+ 6 0 • .& 

Q) 0 

LL. ... o• 

3 0 0 eA oO 

• 

9 0 

A 

0 6 

C> 

~ A 

I- 3 .& • z 

w 0 •• Cb u 

a: 13~. 

w • a.. 

I-

:J: 9 

(!) 

w • ~ 0 

CQ 6 u ~ • 0 0 

3 0 0 

•• 0 • .& o.& 0 

•• A 

8 

0 • N 

CQ 5 z .& ~· 'b • • A 0 .& 

2 • 

13. .& 

5 6 

0 

N 

~ 0 o• • 

2 •• io 6 • Cb .& 

N 3 

0 

t= ~ 

• 0 -· ~ 

ll) 0.7 • 0 

N 

a.. 0.3 

Si02, IN WEIGHT PERCENT 
VOLCANIC PETROLOGY 15 

10.----.------------.-----------~------------~--~--------.-----------~--~ 

1-

z 

w 

u 

a: 

w 

Q.. 

1-

:I: 

(!) 

Irvine and Baragar (1971) 

13• • 

• :\...~AMacDonald and Katsura (1964) 

• ~~0~ •15 

jjj 

~ 

~ 

• 

,.~ 

/4/• • 

•• x / •• . / 

0 5 N 

::.:: 

+ 

0 

N 0 

/ ./ . t'CI 

z 

• •• 

• 2~--~------------~----------~------------~------------~----------~--~ 

50 55 60 65 70 75 

Si02, IN WEIGHT PERCENT 

FIGURE 5.-Alkali-silica variation diagram. Field of coastal granites encloses four analyses from the Tunk Lake 

pluton (Karner, 1968), and one each from the Jonesboro pluton at Jonesboro, the Vinalhaven pluton on Hurricane 

Island, and an unnamed pluton on High Island in southwestern Penobscot Bay (Dale, 1907). Large symbol, least 

altered basalts. Open symbol, volcaniclastic rocks; all others, lava rocks. Circle, Silurian assemblage; triangle, 

older Devonian assemblage; X, younger Devonian assemblage. Samples 13, 15, and 16 (table 2) are labeled. 

Rio Grande depression (Lipman, 1969; Lipman and 

Mehnert, 1975)-all clearly continental tholeiites-plot 

variously in the fields for ocean-floor basalts, lowpotassium tholeiites of island arcs, and calc-alkalic 

basalts in Pearce and Cann's (1973) diagrams. The diagrams provide, however, a convenient way for comparing minor-element associations in different groups of 

basalts. For example, in their diagrams the available 

data for basalts of the Rio Grande depression plot 

closely with our data for the Silurian basalts of the 

Machias-Eastport area. The remarkable similarity of 

these basaltic rocks of widely different age and setting 

is shown also by the major oxide data listed in table 4. 

• FIGURE 4 (facing page).-Harker diagram showing major oxides, 

recalculated without H20 and C02. Large symbol, best analyses 

of least altered basalts. Open symbol, volcaniclastic rocks, including four analyses of mixed tuff breccias between 67.5 and 71 percent Si02. All others, lava rocks. Circle, Silurian assemblage; triangle, older Devonian assemblage; X, younger Devonian assemblage. Samples 13, 15, and 16 (table 2) are labeled. Sample A, 

hydrothermally altered Silurian basalt from the Pembroke prospect. Envelopes enclose samples of Silurian basalts. 

Figure 8 shows average major-oxide and minor-element data for least altered basalts of the MachiasEastport area on the Harker-type diagrams of McDougall (1976, figs. 5, 6) for basalts of the Columbia 

River Group. These diagrams provide a convenient 

way of showing how minor-element signatures change 

sympathetically with changes in major-oxide composition in the Silurian to older and younger Devonian progression. The diagrams also show a striking similarity 

that exists (in direction, not amount) between these 

changes and those that accompany the Columbia River 

progression, despite obvious differences in tectonic 

setting. All basalts of the Machias-Eastport area are 

much more aluminous than those from the Columbia 

River area, but the differences in FeO*, MgO, CaO, 

Ti02, P 20 5, and the minor-element contents of the 

somewhat magnesian Silurian basalts, of the silicic 

older Devonian basalts, and of the younger Devonian 

basalts (exceptionally rich in iron and titanium), are 

remarkably similar to the differences between the Picture Gorge, lower, and middle basalts of the Yakima 

Subgroup. 
16 BIMODAL VOLCANIC ROCK ASSEMBLAGES, MACHIAS-EASTPORT AREA, MAINE 

F 

SILURIAN ASSEMBLAGE 

SILURIAN 

ASSEMBLAGE 

FIGURE 6.-AFM diagram (weight percent basis). Large symbol, least altered basalts. Open symbol, volcaniclastic rocks; all others, lava 

rocks. Circle, Silurian assemblage; triangle, older Devonian assemblage; X, younger Devonian assemblage. Samples 13, 15, and 16 

(table 2) are labeled. 

FIGURE 7 (facing page).-Variation of Si02, Ti02, and FeO* (total .... 

iron as FeO) relative to solidification index (weight percent basis). 

Approximate boundaries of hypersthenic field in Japan from 

Kuno (1968, figs. 16, 17). Skaergaard trend from Wager and 

Brown (1968, tables 4, 9, 10). Field of coastal granites encloses 

four analyses from the Tunk Lake pluton (Karner, 1968), and one 

each from the Jonesboro pluton at Jonesboro, the Vinalhaven 

pluton on Hurricane Island, and an unnamed pluton on High 

Island in southwestern Penobscot Bay (Dale, 1907). Large symbol, least altered basalts. Open symbol, volcaniclastic rocks; all 

others, lava rocks. Circle, Silurian assemblage; triangle, older Devonian assemblage; X, younger Devonian assemblage. Samples 

13, 15, and 16 (table 2) are labeled. 
1-

z 

w 

(.) 

tt: 

w 

a.. 

1-

::I: 

(.!) 

w 

:: 

75 

70 

65 

N 

0 

u; 60 

N 

0 

i= 

* 0 

Q) 

LL 

55 

50 

45 

I 

31-

2r11-•• 

I 

20 

15 

5 

40 

0 

0 

VOLCANIC PETROLOGY 

e15 

~~ 

~~~ 

~" 616 ¥:-~ 

f(/q:; ~ 

~<l. ~ 

... • 

• 

X X 

X 

SKAERGAARD TREND 

I I I X I 

X 

X • 

X 16 

• .Ja • • • •• • • 

• 

I I I I 

• 

• 

• 

I 

• 

•• • J • o o e e\Al 

COASTAL GRANITES~·. ( ., 1 

\ .... 

• 

I 

-

-

-

• • ..13 

tt ..... /:W&. I o I o 

COASTAL•13 

GRANITESAk, 

. ...~:. /.._, .. ~ .. · 

35 30 25 20 15 1 0 5 0 

SOLIDIFICATION INDEX, MgQx 1 OO/Fe0+0.9Fe203+Mg0+ Na20+K20 

17 
18 

..... 

z 

w 

u 

a: 

w 

a.. 

..... 

:I: 

(!) 

jjj 

~ 

~ 

en 

w 

c 

x 

0 

17.0 

16.0 

M 

0 15.0 N 

<( 

14.0 

13.0 

14.0 

* 

~ 12.0 LL 

0 

C) 

~ 

0 

co 

u 

0 

N 

co 

z 

0 

N 

~ 

N 

0 

i= 

ll'l 

0 

N 

a.. 

10.0 

8.0 

6.0 

4.0 

10.0 

8.0 

BIMODAL VOLCANIC ROCK ASSEMBLAGES, MACHIAS-EASTPORT AREA, MAINE 

500 

> 

300 

> 

60 

40 

80 

~ 2~ 

20 • 

600 

co 

m 

400 

200 

50 

.J:l 

a: 30 @ cSS/ 

::::i 

...J 

0 

z 

250 

10 ~ 

~ N 

a: 150 w 

a.. 

(/) 

..... 60 a: 

<( (,) 

a.. (/) 

~ 40 

cn' • ..... 

z 30 w co 

~ (!) 

w I 

...J 20 .,. w 

• 

100 

z 60 

20 2 

45 

0 

u 

35 

300 

50 52 54 

Si02, IN WEIGHT PERCENT u 200 

100 

50 52 54 

Si02, IN WEIGHT PERCENT 

FIGURE 8.-Major-oxide and minor-element data for least altered basalts of the Machias-Eastport area compared with data 

of McDougall (1976, figs. 5, 6) for the Columbia River Basalt Group. 
VOLCANIC PETROLOGY 19 

TABLE 3.-Selected and average minor-element data on volcanic rocks 

[Starred sample Nos., average of several values. N,looked for but not detected; leaders~-), 

not looked for; < , less than. Numbers in parentheses, number of samples for which data 

are available. Data in parts per million except SiO, and K,O, in percent ~from table 2). In 

all samples, Sr and Rb detected by X·ray fluorescence analyses. In all samples, U 

detected by delayed neutron determinations. All samples, except samples 13 and 15, analyzed forTh, La, Yb, Ba, Co, Cr, Cs, Hf, Sb, Sc, Ta, and Zr by instrumental neutron activation. For samples 13 and 15, Th detected by delayed neutron determinations. For 

samples 13 and 15, La, Yb, Ba, Co, Cr, Cs, Hf, Sb, Sc, Ta, and Zr detected by semiquanRock type .................... ______ B_a_sal_t ____ _ Basaltic 

andesite 

Age assemblage ............... ___ S_il_unan_· ___ _ 

Sample No .................... * 1,4,8 

Si02................... 50.8 1

K20................... .77 

1

Sr ..................... 516 1Rb .................... 15 ~u ..................... .4o 1

Th.................... 1.77 1

La .................... 15.0 

Yb.................... 2.12 1

Ba .................... 227 

Co .................. ·· 46.1 

Cr .................... 380 

Cs · · ....... · · · · · · · ·... 1.24 

Hf.................... 2.84 

Sb. .. . . . . .. . . . . . . . . . .. .23 

Sc.................... 33.5 

Ta. .. . ... .. . . ..... .. . . .26 1

Zr ..................... 115 

Be .................... <1 

Ga .................... 22 1

Nb.................... 4.3(2) 

Ni .................... 120 

v ..................... 233 

Y ..................... 28 

Ag................... .34(2) 

Mo................... 4.3 

Pb ................... 6.2 

Sn . . . . . . . . . . . . . . . . . . . 9.5(2) 

Zn .................... 127(2) 

5 

48.3 

.16 

374 

2 

.23 

.75 

7.77 

2.17 

84.8 

55.8 

345 

.83 

2.19 

1.34 

34.2 

.11 

85.7 

<1 

23 

4 

172 

300 

32 

.29 

6.2 

7 

<3.2 

129 

18 

52.0 

1.6 

472 

35 

.76 

2.24 

15.2 

2.66 

290 

43.4 

162 

2.7 

4.47 

.63 

30.8 

.45 

194 

1.5 

28 

9.6 

41 

200 

31 

.31 

4.0 

6.1 

7.9 

117 

Older 

Devonian 

20 

54.9 

1.5 

375 

29 

1.25 

4.01 

24.9 

2.95 

330 

32.5 

47.3 

1.41 

4.72 

.40 

28.2 

.52 

222 

1.9 

24 

6.0 

11 

300 

36 

.21 

< 1.5 

5.3 

7.9 

116 

titative emission spectrographic analyses by computerized techniques. In all samples, 

Be, Ga, Nb, Ni, V, Y, Ag, Mo, Pb, Sn, and Zn detected by semiquantitative emission spectrographic analyses by computerized techniques. Analysts: X-ray fluorescence analyses 

by W. P. Doering; delayed neutron determinations by H. T. Millard, Jr., and D. A. Bickford; instrumental neutron activation analyses by R. J. Knight and H. T. Millard, Jr.; 

semiquantitative emission spectrographic analyses by computerized techniques by A. F. 

Dorzapf, supplemented by visual method by J. L. Harris, J.D. Fletcher, B. W. Lanthorn. 

All analysts, U.S. Geological Survey] 

Basalt 

Younger 

Devonian 

30 

50.0 

.72 

550 

16 

1.12 

3.18 

27.4 

2.83 

307 

39.9 

58.0 

1.77 

4.39 

2.31 

32.3 

.83 

231 

1.8 

30 

14 

55 

300 

45 

.47 

5.4 

14 

15 

169 

Andesite Rhyolite Dacite 

Silurian 

15 

62.9 

1.5 

412 

47 

1.92 

9.17 

50 

3 

300 

20 

50 

N 

N 

30 

150 

1 

20 

7 

50 

150 

30 

N 

3 

15 

N 

N 

*9,10,11,12 

73.9 

3.0 

140 

74 

3.17 

11.7 

39.7 

4.87 

554 

2.42 

20.4 

1.25 

8.60 

.99 

9.53 

1.04 

325 

2.4 

19 

8.5 

6.1 

12.(3) 

13 

68.4 

5.9 

85 

158 

5.26 

22.3 

100 

15 

1,000 

N 

N 

20 

5 

30 

1,000 

2 

30 

30 

70 

3 

57 150 

<0.1 N 

2.7(1) 5 

15 20 

5.9(2) N 

36.(2) N 

Dacite 

Older 

Devonian 

*23,25 

69.5 

4.4 

62 

128 

3.05 

13.5 

72.7 

10.8 

676 

1.27 

16.6 

.65 

18.7 

.89 

6.57 

2.49 

802 

6.9 

46 

22. 

2.8 

12. 

177 

.35 

2.8 

16 

10 

141 

Rhyolite 

75.4 

4.5 

38 

144 

4.64 

15.6 

133 

8.59 

636 

1.01 

17.6 

.98 

10.7 

1.25 

12.8 

1.79 

443 

3 

25 

25 

13 

15 

100 

N 

N 

10 

N 

N 

'Elements or oxides of elements that are strongly excluded from rock-forming minerals of the Earth's mantle; called excluded or incompatible. Others are included in mantle minerals; 

called included or compatible elements ~Green and Ringwood, 1967, p. 17 4, 175; Jamieson and Clarke, 1970). 

EXPLANATION FOR FIGURE 8 

~ From oldest to youngest: 1, Picture Gorge Basalt; 2, lower basalt, 

and 3, middle basalt of the Yakima Basalt Subgroup; all of the 

Columbia River Basalt Group. Machias-Eastport data, from oldest 

to youngest: dot, average of three Silurian basalts (sample Nos. 1, 4, 

8); triangle, average of two older Devonian basalts (sample Nos. 18, 

20); X, younger Devonian basalt (sample No. 30); all data in tables 2 

and3. 
20 BIMODAL VOLCANIC ROCK ASSEMBLAGES, MACHIAS-EASTPORT AREA, MAINE 

SILURIAN ASSEMBLAGE 

In major-oxide and minor-element composition and 

in the absence of a transition to abundant andesites, 

the Silurian basalts may be called high-alumina 

tholeiites of continental character. Olivine tholeiites 

and quartz tholeiites are represented. The basalts are 

not strongly fractionated, for many are nonporphyritic, and FeO* to MgO ratios are rather uniform. 

This characterization is based mainly on the paucity of 

andesites, for many similarities can be seen in the composition of calc-alkalic orogenic basalts, high-alumina 

continental tholeiites, and the Silurian basalts. All the 

examples shown in table 4 plot within Kuno's field for 

high-alumina basalts (Kuno, 1960; 1968, fig. 1). On the 

basis of its major-oxide composition alone, the Silurian 

basalt of sample 5 (table 2, recalculated without volatiles) is indistinguishable from the average ocean floor 

tholeiite basalt of Engel, Engel, and Havens (1965, 

table 3). The only conspicuous differences between 

these two and all the others are the low K20 contents 

and the high Ti02 contents, particularly in comparison 

with the calc-alkalic basalts. As shown by Pearce and 

Cann (1973, fig. 2), most calc-alkalic basalts have less 

than about 7,500 ppm titanium, whereas the Silurian 

basalts contain 7,500 to nearly 10,000 ppm titanium. 

The Ti02 content alone, however, is not a sure indica· 

tion of tectonic association of basalts. 

The data on minor elements indicate that the 

Silurian basalts are not akin to oceanic tholeiites or 

tholeiites of island arc or back-arc settings, but no 

sharp distinction can be drawn from these data between the Silurian basalts, calc-alkalic basalts, and 

some continental basalts. The Silurian basalts contain 

more potassium and far greater abundances of the 

whole suite of excluded minor elements (listed and 

defined in table 3) in comparison with oceanic tholeiites. (Compare table 3 with Jakes and Gill, 1970, 

table 1; Jamieson and Clark, 1970, table 2; Engel and 

others, 1965, tables 1, 2; Kay and others, 1970, table 4.) 

Data on rare-earth elements are particularly diagnostic. The Silurian basalts have 7.8-18.5 ppm lanthanum and lanthanum to ytterbium ratios that range 

at least from 3.5 to 8, indicating significant enrichment 

in the light rare-earth elements (table 3, average La 

and Yb given for sample Nos. 1, 4, 8). These abundances are not out of line with rare-earth element data 

for calc-alkalic basalts, but they stand in contrast with 

the small lanthanum abundances and low lanthanum 

to ytterbium ratios that are characteristic of ocean 

floor, island arc, and back-arc tholeiites (Jakes and 

Gill, 1970; Jakes and White, 1972, table 2B; Hart and 

others, 1972; Kay and others, 1970). Abundances of 

the included minor elements, on the other hand, are not 

conspicuously different. In diagrams of Miyashiro and 

Shido (1975), which utilize titanium, vanadium, 

chromium, and nickel, the Silurian basalts plot consistently with the ocean floor tholeiites-but so do the 

most basaltic calc-alkalic rocks. 

Petrologists commonly distinguish calc-alkalic from 

tholeiitic rocks on the basis of alkali-enrichment versus 

iron-enrichment respectively (Miyashiro, 197 4), but as 

noted by Irvine and Baragar (1971, p. 529), this distinction is not always sharply defined. The standard 

AFM diagram may be misleading if used uncritically. 

In figure 6, for example, the area for the Silurian 

basalts bulges slightly toward A, suggesting an incipient calc-alkalic trend. This bulge probably is an 

expression of metasomatic sodium-enrichment, howTABLE 4.-Silurian basalts from the Machias-Eastport area compared with high-alumina tholeiites and calc-alkaline basalts 

from other regions 

[Data in weight percent] 

Silurian assemblage Tholeiitic basalts Calc-alkalic basalts 

Northern New Ocean Greenland Northern 

Machias-Eastport area Mexico Rio Grande 

depression floor Skaergaard 

chill border California Warner basalt Japan Papua 

Reference No .............. 1A 1B 2 3 4 5 6 7 No. of analyses ............ 4 1 19 10 1 1 1 1 

Si02 ............... ~ 50.2 48.3 50.9 49.94 48.71 49.20 48.10 50.79 

Al20 3 •••••••••••••• 16.7 17.5 16.3 17.25 17.44 17.16 16.68 16.45 

FeO* .............. 9.9 10.6 11.2 8.71 9.48 10.28 11.24 8.45 

MgO .............. 7.7 8.3 7.4 7.28 8.73 7.88 8.89 9.05 

CaO ............... 10.1 10.9 8.9 11.86 11.53 11.45 10.48 9.60 

Na20 ............... 2.65 2.2 3.0 2.76 2.40 2.58 2.51 2.92 

K20 ............... .83 .16 .64 .16. .25 .23 .46 1.08 

Ti02 ............... 1.35 1.50 1.2 1.51 1.19 .89 .73 1.06 

P205 ............... .24 .16 .16 .16 .10 .09 .15 .21 

MnO .............. .21 .16 .16 .17 .16 .17 .54 .12 

SOURCES OF DATA 

1. A, average four least altered basalts; B, sample No. 5 (table 21. 5. Kuno (1968, table 2, col. 6; recalculated without volatilesl. 

2. Lipman and Mehnert (1975, table 1, col. 121. 6. Kuno (1968, table 2, col. 51. 

3. Engel, Engel, and Havens (1965, table 3, col. 11. 7. Jakes and Smith (1970, table 2, col. 1; recalculated without volatilesl. 

4. Kuno (1968, table 2, col. 7; recalculated without volatilesl. 
VOLCANIC PETROLOGY 21 

ever, for it is composed entirely of basalts that do not 

qualify as least altered. An alkali-enrichment trend is 

shown also in Aoki's AFM diagram for basalts of the 

Rio Grande depression (Aoki, 1967, fig. 5), but this 

trend is formed by one alkali andesite from a cone that 

is younger than the basalts of the Servilleta Formation 

(the most voluminous upper Cenozoic basalts in the 

northern part of the depression) and by three alkalic 

olivine basalts that may have their source outside the 

depression (Lipman, 1969, p. 1349). In Aoki's AFM 

diagram, all the tholeiites plot within a small area that 

shows no obvious trend one way or the other. Thus, in 

view of these and the foregoing considerations, the 

only valid distinction between high-alumina tholeiites 

and calc-alkalic basalts is the absence or presence respectively of a transition to abundant andesites. In the 

absence of such a transition, we conclude that the 

Silurian basaltic suite is tholeiitic. Its large abundance 

of excluded minor elements identifies the suite as continental. 

Most analyses of the Silurian rhyolitic suite and the 

andesite dike plot along a typical calc-alkaline trend in 

the AFM diagram (fig. 6), and within the hypersthenic 

series of Kuno (1968) in figure 7. With the exception of 

sample 13, K20 contents in rocks of the rhyolitic suite 

are rather low (table 2; fig. 4). Contents of K20 and 

Na20 tend to be roughly reciprocal, suggesting that 

alkali exchange took place while the rocks were still 

glassy (Lipman and others, 1969). As shown in figure 

4, most of the Silurian rhyolites are mor~ aluminous 

than those of the older Devonian rhyolitic suite. 

The analysis of one sample of silicic dacite in the 

Leighton Formation (table 2, sample No. 13) plots with 

the older Devonian silicic dacites in figures 4-7, expressing the high content of total alkalis, and the high 

FeO* to MgO ratio in this sample. This rock appears to 

have characteristics of both the Silurian and older 

Devonian assemblages. Its alumina content is high, 

actually higher than any of the other Silurian rhyolites; it also has the highest K20 and lowest Na20 contents of both rhyolitic suites. Minor-element abundances in sample 13 are exceptional, but they are most 

comparable to those of the older Devonian rhyolitic 

suite (table 3). The cobalt and nickel contents of sample 

13 are comparable to those of the andesite. 

In summary, the Silurian assemblage divides into a 

suite of tholeiitic high-alumina basalts of continental 

character, and a suite of calc-alkalic rhyolites and 

silicic dacites and one known andesite. Although this 

association may seem incongruous, Irvine and Baragar 

(1971, p. 529) pointed out that tholeiitic basalts and 

calc-alkalic silicic rocks are in fact closely associated in 

some areas, as in the Yellowknife volcanic belt of 

Canada. If calc-alkalic andesites of Silurian age were 

much more abundant than they are in the MachiasEastport area, the whole Silurian assemblage would 

not be unlike the calc-alkalic volcanics of the Cascade 

Range. The same reasoning applies, however, to the 

basalts of the Rio Grande depression. 

OLDER DEVONIAN ASSEMBLAGE 

The older Devonian basaltic suite is clearly tholeiitic, according to Irvine and Baragar's criteria (fig. 

5), and it has predominantly tholeiitic abundances of 

immobile minor elements, as discussed previously. As 

shown in figure 6, these basalts clearly define an ironenrichment trend. The older Devonian basalts and 

basaltic andesites have a wider range and higher average Si02 content than the Silurian ones, and distinctly 

more FeO*, Ti02, P20 5, and Na20, but less CaO and 

MgO. Thus, FeO* to MgO ratios are generally higher 

than those of the Silurian suite. Among the included 

minor elements, chromium and nickel contents range 

widely but are distinctly less abundant in the older 

Devonian basaltic suite (table 3). Other differences in 

minor-element signatures are shown in figure 8. Petrographically, the older Devonian basalts and basaltic 

andesites, typically having abundant magnetite and 

phenocrysts of plagioclase, appear to represent derivative magmas. The low-magnesium and low-nickel contents indicate that olivine fractionated from the 

magmas. 

In contrast, the basaltic tuff at the base of the Eastport Formation has chemical characteristics of the 

Silurian basalts (table 2, sample No. 16; figs. 4-7), 

except for its high Si02 and low CaO contents, which 

(since it is a tuff) might be a result of contamination by 

quartz-bearing sediments, and its low CaO content, 

probably a result of calcium-depletion during alteration. This basaltic tuff is directly overlain by a basaltic 

flow (table 2, sample No. 20) having a high FeO* to 

MgO ratio, a high P 20 5 content, and petrographic features and the minor-element signature of the older Devonian basalts (table 3). No basalts like those of the 

Silurian assemblage are known higher in the section, 

and no Eastport-type basalts are known lower in the 

section. Among the basalts, therefore, a shift toward 

iron-enriched basaltic magmas evidently took place 

abruptly between eruptions of the basal tuff and the 

next overlying flow of the Eastport. This shift closely 

followed the volcanic hiatus represented by the Hersey 

Formation and accompanied the general trend toward 

shoaling and emergence of the area of the coastal volcanic belt. 

Our six samples of older Devonian rhyolites and 

silicic dacites have 69-75.7 percent Si02. The average 

Si02 content of these rocks is distinctly higher than 
22 BIMODAL VOLCANIC ROCK ASSEMBLAGES, MACHIAS-EASTPORT AREA, MAINE 

that of the Silurian rhyolitic suite, even if the Silurian 

andesite and the mixed tuff breccias are excluded from 

the average. The older Devonian rhyolitic suite also 

has distinctly higher FeO* to MgO ratios, higher average K20 and total alkali contents, and distinctly lower 

alumina contents (figs. 4-7). 

Figure 7 illustrates the contrast between the Silurian 

and older Devonian rhyolitic suites to best advantage. 

Whereas all of the Silurian suite, except sample 13, 

conforms to the calc-alkalic hypersthenic series of 

Kuno (1968), the older Devonian rhyolitic suite plots 

along a trend that diverges only slightly from that of 

the strongly tholeiitic Skaergaard trend. Significant 

differences between the Silurian and older Devonian 

rhyolitic suites are seen also in the minor-element signatures (table 3). The older Devonian rocks are poorest 

in strontium, but richest in the other excluded minor 

elements. 

YOUNGER DEVONIAN ASSEMBLAGE 

The flow and sills of the younger Devonian assemblage are iron- and titanium-rich tholeiitic basalts of 

strongly continental character. The four samples have 

48-51.5 percent Si02, distinctly lower than most of the 

older Devonian basalts. The higher FeO*, Ti02, and 

P 205 contents of these rocks are higher than those of 

most basalts of the Eastport, but FeO* to MgO ratios 

are about the same (table 2; fig. 4). In figure 5 one 

analysis plots barely in the alkaline field of Irvine and 

Baragar (1971). This analysis (table 2, sample No. 29) 

has a trace of nepheline in the norm, possibly because 

its Fe20 3 content is too low. According to criteria established by Floyd and Winchester (1975), the abundances of immobile elements have characteristics of 

both alkalic and tholeiitic basalts, owing to the high 

Ti02 and P 20 5 contents (common in alkalic basalts) and 

low niobium contents (common in tholeiitic basalts). In 

the AFM diagram the four samples plot within a small 

area closer to the FM side than do the analyses of older 

Devonian basalts, and in figure 7 they plot closer to 

the Skaergaard trend. 

The younger Devonian basalts and the two least 

altered older Devonian basalts have rather comparable 

minor-element signatures that are quite different from 

the basaltic Silurian signature (table 3; fig. 8). In the 

younger, magnesium and nickel contents are low, as in 

the older Devonian basalts, indicating that olivine 

fractionated from the magma. 

Available analyses of granitic plutons of the coastal 

belt plot within the small areas that are shown in figures 5 and 7. As shown in figure 5, these granites have 

generally higher total alkali contents-owing mainly 

to higher K20-than the older Devonian and Silurian 

rhyolitic suites. In figure 7 the area of the granites is 

closest to the Skaergaard trend. Thus, in all the silicic 

rocks of all three assemblages, the most conspicuous 

changes from the older to younger suites are toward 

higher K20 contents, toward higher average and more 

restricted Si02 contents, and toward increasingly tholeiitic crystallization trends. Unfortunately, absence of 

published analyses of the Bays-of-Maine Complex prevents determination of whether or not the rocks of this 

complex, intermediate in age between the older Devonian assemblage and the younger coastal granites, 

share this trend. 

SUMMARY AND ORIGIN 

The most conspicuous feature of the volcanic suite in 

the Machias-Eastport area is the andesite gap. Within 

the fossiliferous Silurian and Lower Devonian sequence, sedimentary beds and volcanic rocks of widely 

contrasting mafic and silicic composition are complexly interstratified. Plutonic rocks further emphasize the bimodal distribution of igneous rock compositions in the Bays-of-Maine Complex (Chapman, 1962; 

Pajari, 1973) and evidently throughout New England 

as well (Wones, 1976). Major intrusives exposed in and 

near the Machias-Eastport area are the Cutler Diabase 

(considered a subvolcanic mafic complex related to the 

Silurian assemblage of volcanics), and the Bays-ofMaine Complex, intruded in turn ·by the coastal 

granites (tentatively considered the silicic end member 

of the younger Devonian assemblage). Bimodal magmatism thus seems to have been time-persistent in the 

Silurian and Devonian before and possibly during and 

after the Acadian orogeny. Whether it was characteristic of the whole northern Appalachians for this time 

remains to be ascertained. Although andesites of this 

age are in fact abundant locally (Shride, 1976; Williams 

and Gregory, 1900; Howard, 1926), descriptions in 

most available reports seem to emphasize the predominance of basaltic and rhyolitic rocks. Andesites are described in all the principal reports on other parts of the 

coastal volcanic belt, but no chemical analyses have 

been published. It remains to be determined whether 

these andesites are akin to those of typical subductionrelated magmatic arcs, or instead are like the porphyritic iron- and titanium-rich silicic basalts and 

basaltic andesites of the Eastport Formation; and, for 

example, the lower basalts of the Yakima Subgroup of 

the Columbia River Group (fig. 8; Waters, 1961; 

Wright and others, 1973; McDougall, 1976). 

Chemically and petrographically, basalts of the 

Machias-Eastport area have characteristics of continental tholeiites, in accord with the likelihood that 
TECTONIC IMPLICATIONS 23 

they were erupted through the Avalonian sialic crust. 

Moreover, as time passed the basaltic lavas became 

more continental in character, in a manner that seems 

analogous to the Columbia River progression (fig. 8), 

changing from the rather magnesian Silurian lavas to 

the silicic older Devonian basalts and basaltic andesites, variably enriched in iron and titanium and depleted in magnesium and nickel, and then to the 

younger Devonian basalts, which also are most enriched in iron and titanium and in several of the excluded minor elements. The silicic magmas also 

changed, starting with calc-alkalic types and ending 

with more potassic and more silicic magmas having 

the greatest abundances of excluded minor elements 

(table 3), and a closer alinement to the Skaergaard 

trend (figs. 6, 7). These changes appear to have continued across the time of the Acadian orogeny, for the 

available analyses of the coastal granites show even 

closer alinement to the Skaergaard trend. The most 

conspicuous shift toward iron-enrichment was abrupt; 

it closely followed a volcanic hiatus represented by the 

Hersey Formation, and it accompanied the general 

emergence of the coastal volcanic belt in earliest Devonian time. The younger Devonian basalts evidently 

erupted both before and after the formation of the 

Acadian cleavage. Thus, the compositional shift was 

fully developed in both the basaltic and rhyolitic suites 

before the Acadian. It was in reverse, moreover, to the 

changes that mark the transition from immature to 

mature island arcs, according to Miyashiro (1974). 

Nicholls and Ringwood (1973) have shown that partial melting of ultramafic upper mantle under watersaturated conditions can yield quartz tholeiites and olivine tholeiites at depths as great as 70 km and 100 km, 

respectively. Under dry conditions the origin of such 

magmas and the fractionation of olivine from them are 

restricted to much shallower depths: no more than 

about 15 km for quartz tholeiites, and 35 km for highalumina basalts (Green and Ringwood, 1967; Green 

and others, 1967) or perhaps high-alumina tholeiites 

like those of the Silurian assemblage. The most favorable tectonic setting for deep origins under hydrous 

conditions is in the wedge-shaped mantle that lies 

below an island arc. By definition the resulting tholeiitic basalts are arc tholeiites-poor in titanium, magnesium, nickel, and chromium relative to abyssal tholeiites (Jakes and Gill, 1970), and quite unlike any 

basalts of the Machias-Eastport area, which are of continental character. Shallow origins thus seem to be required for basalts of the Machias-Eastport area. 

A scheme that we currently favor postulates thermal 

expansion of the mantle, and diapiric intrusion into the 

sialic crust along the trend of the coastal volcanic belt. 

These processes were accompanied by shoaling and 

emergence of the belt, and they produced the bimodal 

volcanism. At depths that may have been significantly 

less than 35 km, high-alumina tholeiitic magma segregated from the partially melted mantle diapir. If the 

crust was in fact much thinner than 35 km, it was 

probably thinned or fragmented by extension. The fact 

that the older and younger Devonian basalts are 

greatly depleted in magnesium and nickel relative to 

the Silurian ones indicates that large amounts of olivine fractionated from the parent. We are not certain, 

however, that the Silurian basaltic magma was in fact 

the parent of the Devonian magmas. All three types 

may have had another parent; or, alternatively, each 

type might represent separate melting events. These 

questions remain for future consideration. 

According to our scheme, heat furnished to the crust 

by the mantle diapir caused partial melting, resulting 

in the silicic magmas. At first, in Silurian time, melting yielded calc-alkalic rhyolites and silicic dacites 

from deformed and metamorphosed but still hydrous, 

largely pelitic sedimentary material. Sparse andesite 

magma might have formed by mixing of basalt and 

rhyolite, known to occur at least locally elsewhere 

(Eichelberger, 197 4). By earliest Devonian time the 

heated crust was partly dehydrated; partial melting 

produced more silicic and more potassic rhyolitic 

magmas that solidified along iron-enrichment trends, 

in accord with experimental evidence. (See Wyllie, 

1973, fig. 8.) The shift toward iron-enrichment is probably an expression of lower oxygen fugacities that inhibited the early crystallization of magnetite (Kennedy, 1955; Osborn, 1962) or amphibole (Boettcher, 

1973; Cawthorn and O'Hara, 1976). We are unable to 

explain, as yet, the close correspondence between the 

emergence of the coastal volcanic belt and the shift 

toward iron-enrichment that is shown by both the 

basaltic and rhyolitic suites; but an important factor 

undoubtedly was the progressive dehydration. 

TECTONIC IMPLICATIONS 

In documented post-Triassic plate tectonic settings, 

it is well known that fundamentally basaltic or 

strongly bimodal basalt-rhyolite igneous suites are 

characteristic of continental regions undergoing plate 

separation and extension; whereas, most suites along 

convergent plate boundaries are more or less unimodal, 

having a wide spectrum of igneous rock compositions 

(Martin and Piwinskii, 1972; Miyashiro, 1974, 1975; 

Lipman and others, 1972; Christiansen and Lipman, 

1972; and many others). In extensional settings only 

very locally is andesite the predominant rock type; 

typically, it is greatly subordinate to more mafic and 
24 BIMODAL VOLCANIC ROCK ASSEMBLAGES, MACHIAS-EASTPORT AREA, MAINE 

silicic rocks, and commonly andesite is absent. In convergent settings andesite is rarely absent; commonly, 

it is the predominant rock type. Exceptions to these 

rules should be noted. Bimodal or even trimodal volcanic suites occur locally in convergent settings, as in 

some individual volcanoes of the Cascades (McBirney, 

1968, 1969) and on individual islands of the Lesser Antilles (Brown and others, 1977, fig. 2); but andesite is 

abundant in these suites. The Newberry volcano of the 

east side of the Cascade Range is strongly bimodal, 

composed largely of basalt, but having about 30 percent rhyolite and dacite and only 5 percent andesite in 

the immediate vicinity of the caldera (Higgins, 1973). 

Interestingly, the Newberry and the rather similar 

Medicine Lake Highland volcanoes (Condie and Hayslip, 197 5) are on the western margin of a region of 

great extent that is known to be undergoing extension, 

possibly in a relationship analogous to extensional 

basins behind island arcs (Karig, 1971; Christiansen 

and Lipman, 1972; McDougall, 1976). The primitive 

Kermadec arc seems to have a bimodal suite composed 

of tholeiitic basalt and sparse silicic dacite (Brothers 

and Martin, 1970; Brothers and Searle, 1970). Unlike 

the coastal volcanic belt, the Kermadec arc is underlain 

by a thin oceanic crust; and its tholeiitic basalts have 

only small abundances of excluded minor elements 

(Miyashiro, 1974; Jakes and Gill, 1970). While these 

exceptions must be taken into account, the association 

of regionally extensive, time-persistent, strongly bimodal volcanism with extensional tectonism in postTriassic settings seems to be a general rule that should 

apply to ancient orogenic belts. Pending further 

studies elsewhere along the coastal volcanic belt, we 

believe that the bimodal suites of the Machias-Eastport area express extensional tectonism in Silurian and 

Devonian time. The coastal belt does not seem to be 

the remains of a volcanic arc above a subduction zone, 

as proposed by McKerrow and Ziegler (1971) and 

Dewey and Kidd (197 4). 

The presence of calc-alkalic silicic rocks in the 

Silurian assemblage requires explanation, for such 

rocks are most common in volcanic arcs above subduction zones (Miyashiro, 1975). Though uncommon in extensional settings, calc-alkalic rocks are in fact known 

to occur there: in parts of the Karroo dolerites, in the 

British Tertiary (Miyashiro and Shido, 1975, p. 267), 

and in the Bushveld Complex (Walker and Poldervaart, 1949, fig. 35). According to our scheme for the 

origin of the volcanics in the Machias-Eastport area, 

the calc-alkalic Silurian rhyolitic suite was produced 

by partial melting of still-hydrous sialic crust; whereas, the tholeiitic older Devonian rhyolitic suite and the 

younger Devonian granites were produced by partial 

melting of dehydrated crust. If this model is correct, 

the presence of calc-alkalic silicic rocks does not necessarily identify a subduction-related magmatic suite. 

If the coastal volcanic belt is not the remains of a 

subduction-related volcanic arc, then what is it? Conceivably, the bimodal Machias-Eastport volcanism 

took place in an extensional area behind an andesitic 

arc, as suggested by McDougall (1976) and others for 

the Columbia River Basalt Group, but if so, where is 

the arc? From presently available information, no major belt of andesitic volcanics of Silurian or Early Devonian age can be identified in the northern Appalachians. We have no definite answers that we can 

agree upon. 

Gates (1978) proposed that during the Late Ordovician or earliest Silurian time the spreading center of 

the Cambrian and Ordovician proto-Atlantic was subducted beneath the northwest margin of the Avalonian 

continental plate. This brought to a halt spreading and 

subduction in the region of the present Merrimack synclinorium, and left a remnant linear basin, the 

Fredericton trough, which subsequently filled with 

Silurian and Lower Devonian clastics. The subducted 

spreading center powered the extensional block faulting and bimodal volcanism of the Machias-Eastport 

area and perhaps, but yet to be demonstrated, the entire coastal volcanic belt. Renewed compression during 

the Acadian orogeny squeezed the magmas that had 

fed the volcanoes upwards into the overlying volcanics 

to form the Bays-of-Maine Complex, a bimodal plutonic complex. The normal faults that formed the 

Perry terrestrial alluvial basin tapped subcrustal 

magmas for the Perry basalts. 

Moench agrees that the proto-Atlantic ocean did not 

close completely during the Taconian orogeny, as 

shown by the Aroostook-Matapedia belt of Taconian 

conformity (fig. 1; Pavlides and others, 1968). He believes, however, that the thick prism of marine sediments of the Merrimack portion of the belt of conformity accumulated in a deep extensional fault trough 

that was tectonically active through Late Ordovician, 

Silurian, and Early Devonian time (Moench, 1970; 

Moench and Pankiwskyj, 1978). During the Silurian 

the axis of the trough evidently shifted from the Aroostook-Matapedia belt to the present alinement of the 

Merrimack synclinorium and Fredericton trough (fig. 

1). Subsidence continued along this new alinement 

while contemporaneous block faulting, bimodal volcanism, shoaling, and emergence took place along the 

coastal volcanic belt. According to Moench, the coastal 

volcanic belt was a tract of thermal inflation and arching that lay directly above an axis of mantle upwelling, 

whereas the deep Merrimack-Fredericton seaway coincided with an active extensional fault trough that lay 

along the northwestern flank of the tract of arching. 
REFERENCES CITED 25 

Both the trough and the arch were expressions of tectonic extension that prevailed through the Silurian and 

Early Devonian. 

But did mantle upwelling end with the onset of the 

Acadian orogeny? Although Acadian deformation in 

New England was compressional, little evidence has 

been found so far-provided our conclusion that the 

coastal volcanic belt is not the remains of an andesitic 

arc is correct-that the orogeny was preceded by subduction during the Silurian and Early Devonian, leading to continental collision. Moench sees no major 

chemical differences in the available data-admittedly 

too meager for definite conclusions-between the 

coastal volcanics and the New Hampshire Plutonic 

Series. Both are strongly bimodal; the silicic member 

of New Hampshire plutonics is calc-alkalic tonalites 

to granites having about the same compositional range 

as the rhyolitic suite of the Silurian assemblage. 

Moench proposes, therefore, that the coastal volcanism, and perhaps the more sporadic volcanism of 

the northern belt, were forerunners of the far more extensive and voluminous magmatism that resulted in 

the syn-Acadian and post-Acadian New Hampshire 

plutons and batholiths. The process of mantle upwelling, segregation of basaltic magma, and partial melting of sialic crust to produce the silicic magmas was 

the same before, during, and shortly after the Acadian. 

Because the process began earliest along the coastal 

volcanic belt, the crust was dehydrated earliest there, 

resulting in the shift from calc-alkalic to tholeiitic 

magma types well before the Acadian. The calc-alkalic 

New Hampshire Plutonic Series northwest of the 

coastal volcanic belt, on the other hand, represents 

partial melting of predominantly pelitic material that 

was still hydrous in the Devonian. 

Both authors agree on the essential conclusions that 

the Merrimack synclinorium and the Fredericton 

trough do not represent a former trench along a 

Silurian and Early Devonian subduction zone and that 

the coastal volcanic belt is not the remains of an andesitic arc above a subduction zone. 

Firm conclusions regarding the tectonic setting of 

the entire southeastern margin of the New England 

and Maritime Appalachians during the Silurian and 

Devonian obviously cannot be made from this petrologic study of a single small area. We have found few 

published investigations of the chemistry of Silurian 

and Lower Devonian volcanic rocks elsewhere in either 

the coastal volcanic belt or the northern belt (fig. 1). 

Yet, one of the best clues we have to ancient plate tectonic regimes lies in comparisons of the chemistry of 

their magmatic products with the chemistry of magmatic products of current plate tectonic regimes. Our 

conclusions disagree with published models for the 

Appalachians. We hope that this disagreement will 

spur others to make more thorough studies in the New 

England and the Maritime Appalachians to further 

test the Wilson cycle model as applied to Silurian and 

Early Devonian time and the Acadian orogeny. 

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28 BIMODAL VOLCANIC ROCK ASSEMBLAGES, MACHIAS-EASTPORT AREA, MAINE 

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APPENDIX 
30 BIMODAL VOLCANIC ROCK ASSEMBLAGES, MACHIAS-EASTPORT AREA, MAINE 

APPENDIX A 

(Samples listed in table 2: Sites on 7'/z-minute quadrangles and Gardner Lake 15-minute quadrangle, and sample descriptions. Mineral abbreviations: Q, quartz; Ksp, K-feldspar; Alb, 

albite; Oli, oligoclase; Pc, calcic plagioclase; Lab, labradorite; Byt, bytownite; Cpx, clinopyroxene; Aug, augite; Pig, pigeonite; 01, olivine; Amp, amphibole; Hb, hornblende; Ac, actinolite; Cht, chlorite; Bi, biotite; Mu, muscovite; Set, sericite; Sap, saponite; Op, opaques; Mt, magnetite; II, ilmenite; Leu, leucoxene; Hem, hematite; Py, pyrite; Sph, sphene; Ru, rutile; 

Pr, prehnite; Pu, pumpellyite; Ep, epidote; Ca, calcite; Zr, zircon; Ap, apatite; Sp, sphalerite] . 

Sample No. Description 

1 Machias Bay quadrangle; roadside outcrop U.S. highway 

1, approx. 0.16 km (0.1 mi) east of Indian Lake. Dennys 

Formation. Basalt flow, massive, aphyric, relict dictytaxitic texture. Minerals: Aug, Byt, Cht, sparse Ep, 

Ac, Mt, Leu, Ksp. 

2 Machias Bay quadrangle; roadside outcrop U.S. highway 1, approx. 0.3 km (0.2 mi) west of Indian Lake. 

Dennys Formation. Basalt flow, massive, aphyric with 

sparse Ca-Ep amygdales. Minerals: Aug, Alb, Cht, 

sparse Ksp, Ep, Set, Ac, Op, trace Ca. 

3 Whiting quadrangle; roadside outcrop U.S. highway 1 

about 2.4 km (1.5 mi) east of Indian Lake. Edmunds 

Formation. Massive aphyric basalt flow with relict 

dictytaxitic texture. Minerals: Alb, Aug, Cht; scattered Ksp, Mt, Ep, and trace of Ca. 

4 Whiting quadrangle; outcrop on west shore of Whiting 

Bay, 1 km (0.6 mi) azimuth N. 78° E. of summit of 

Littles Mountain, Cobscook Bay State Park. Edmunds 

Formation. Basalt flow with small phenocrysts of 

Aug and saponitized 01, and sparse Cht-Ca amygdales. Minerals: Pc (dusty with clay and saussuritic 

alteration), 01, Aug, Cht, minor Sap, Ep, Ru, Ca, Ksp. 

5 Pembroke quadrangle; roadside outcrop U.S. highway 

1; 0.8 km (0.5 mi) azimuth N. 75° W. from summit of 

Oak Hill. Leighton Formation. Massive aphyric basalt 

flow with inconspicuous secondary foliation; relict 

ophitic texture. Minerals: Pc, Aug, Cht, minor Ru, 

Leu, Ep, Sap. 

6 Pembroke quadrangle; outcrop in woods 0.5 km (0.3 mi) 

N. 70° E. of Big Hill summit. Leighton Formation. 

Massive aphyric basalt flow with inconspicuous secondary foliation; small phenocrysts of Pc and Aug. 

Minerals: Pc, Aug, Cht, Ca, minor Ep, Mt, Leu, and 

Set. 

7 Cutler quadrangle; quarry 1.6 km (1 mi) west of Cutler 

village. Cutler Diabase. Coarse relict ophitic texture 

with scattered Ep pods, avoided in sampling. Minerals: 

Pc, Pig, brown Hb, blue-green and colorless Amp, 

sparse brown Bi, minor Il, Cht, Ep, and sparse Ca. 

8 West Lubec quadrangle; quarry at NE. end of Ellis 

Hill. Cutler Diabase. Coarse ophitic. Minerals: Pc, 

Aug, pale green to colorless Amp, Bi, Cht, minor Il, 

Leu, Sph, Ep, and unidentified white mica. 

9 Machias Bay quadrangle; outcrop on State route 191, 

1.6 km (1 mi) south of East Machias. Dennys Formation. Gray flow-banded felsite with inconspicuous 

secondary foliation; 5 percent Alb phenocrysts in 

dense xenomorphic granular matrix. Minerals: Alb, 

Q, white mica, minor Cht, Mt, Bi, and Ep. 

10 Machias quadrangle; roadside outcrop on U.S. highway 1 about 1.5 km (0.9 mi) SW. of East Machias. 

Dennys Formation. Felsite with about 5 percent 

phenocrysts of Alb in a dense xenomorphic granular 

matrix. Minerals: Q, Alb, Ksp, minor Bi, Cht, sparse 

Set, and traces of Ca and Py. 

11 Gardner Lake 15' quadrangle; SE. cor.; roadside outcrop on U.S. highway 1, 0.8 km (0.5 mi) east of Indian 

Lake. Dennys Formation. Pale-red felsite with sparse 

blocky phenocrysts of alkali feldspar (partly replaced 

by Ksp) in interlocking matrix. Minerals: Q, Alb, Ksp, 

and traces of Mt, Ap, and metamorphic Bi. 

Sample No. Description 

12 Pembroke quadrangle; roadside outcrop on U.S. highway 1, 1.2 km (0.75 mi) south of Hobart Stream. Edmunds Formation. Grayish-red felsite with 5-10 percent albite phenocrysts in a matrix of interlocking 

feldspars and Q, in part trachytic. Minerals: Q, Alb, 

Ksp, minor Cht, Mt, trace of Ca. 

13 Eastport quadrangle; outcrop on hill east of road, 1.6 

km (1 mi) south of Denbow Point on Denbow Neck. 

Leighton Formation. Gray felsite with amygdales 

filled with Cht, Set, and Ca, and a few grains of Py 

and Sp. Major minerals are Q, Ksp, and Alb. 

14 Pembroke quadrangle; drill core from Pembroke prospect; hole S-67, 1.2 km (0.75 mi), azimuth N. 55° W. 

of summit of Big Hill; approx. 2.6X2.6-cm (1X1-in.) 

fragments taken at 1.5-m (5-ft) intervals between 98 

and 134 m (320 and 440 ft) below collar. Represents 

lower part of lenticular flow in Leighton Formation. 

Pale-gray mottled to banded felsite with 10-20 percent of small Alb phenocrysts. Minerals: Q, Alb, 

minor Cht, and Ca, and sparse Set. 

15 Machias Bay quadrangle; roadside outcrop about 0.8 

km (0.5 mi) west of Gardner Lake and 1 km (0.6 mi) 

east of Whiting town line. Andesite dike that cuts 

Dennys Formation. Dark gray, dense, nonfoliated. 

Minerals: Alb (containing Ep), Q, ragged Hb, minor 

Ksp,andOp. 

A Pembroke quadrangle; drill core from Pembroke prospect; hole A-8, 0.3_km (950 ft), azimuth N. 80° E. of 

summit of Big Hill; approx. 2.6 x 2.6 em (1 x 1 in.) fragments taken at 3-m (10-ft) intervals between 61 and 

91 m (200 and 300 ft) below collar. Represents upper 

part of thick, hydrothermally altered flow sequence 

in Leighton Formation. Dark-gray, dense, massive 

altered basalt; has scattered amygdales mainly filled 

with green Cht, some with Q, Ca. Minerals: Alb, Set, 

Ca (probably Mn-bearing), Il, Leu, and sparse Cpx, 

Ep. 

16 Pembroke quadrangle; roadside outcrop on U.S. highway 1 about 0.3 km (0.2 mi) NW. along road from east 

border of quadrangle. Basaltic tuff at base of Eastport Formation; angular clasts as much as 1 em across 

of porphyritic, dense, and scoriaceous basalts and 

sparse clasts of silty mudstone; matrix is quartz-bearing feldspathic tuff. Minerals: Alb, Cht, Ca, Set, Sph, 

Ksp. 

17 Eastport quadrangle; roadside outcrop on U.S. highway 1 

about 1.6 km (1 mi) east of west quadrangle border. 

Eastport Formation. Basalt flow with abundant phenocrysts of Alb. Minerals: Alb, Cht, Ca, Il, Hem; pseudomorphs of Ca and Cht after Cpx. 

18 Eastport quadrangle; roadside outcrop U.S. highway 1 

about 2.3 km (1.4 mi) east of west quadrangle border. 

Eastport Formation. Basalt flow with abundant phenocrysts of Lab as much as 6 mm across. Minerals: argillized Lab, Aug, Cht, Mt, Sph, Ksp. 

19 Eastport quadrangle; roadside outcrop U.S. highway 1 

about 2.4 km (1.5 mi) east of west border of quadrangle. 

Eastport Formation. Flow above that of sample 18; 

dense basalt with small phenocrysts of feldspar and 

Aug in a trachytic matrix. Minerals: Alb, Cht, Aug, 

Mt, Ep, Sph, Ca. 
32 BIMODAL VOLCANIC ROCK ASSEMBLAGES, MACHIAS-EASTPORT AREA, MAINE " 

Lab. No. Description 

W176693 Machias Bay quadrangle; roadside outcrop U.S. highway 1, 1.6 km (1 mi) SW. of northern quadrangle 

boundary. Dennys Formation. Large sample of gray 

mixed tuff breccia; felsites and scattered dark-gray 

clasts in feldspar-rich clastic matrix. Minerals: Alb, 

Ksp, Q, Bi, Set, Cht, Ep. Analyses: 71.7 Si, 13.3 A, 

0.69 Fe2, 2.1 Fe, 1.1 M, 2.7 C, 3.7 N, 1.8 K, 0.92 H, 

0.06 h, 0.39 T, 0.08 P, 0.10 Mn, 0.40 CO. 

W176690 Whiting quadrangle; roadside outcrop U.S. highway 

1, 2.6 km (1.6 mi) north of Whiting. Edmunds Formation. Mixed tuff breccia; angular clasts as long 

as 2 em of felsites in matrix of feldspar crystal tuff. 

Minerals: Alb, Q, minor Ep, Cht, Set, Op, Ca. Analyses: 69.5 Si, 14.3 A, 2.6 Fe2, 1.3 Fe, 1.2 M, 3.5 C, 

4.0 N, 1.1 K, 1.4 H, 0.11 h, 0.51 T, 0.10 P, 0.10 Mn, 

0.22 CO, 0.00 S. 

Wl 76688 Whiting quadrangle; outcrop on west shore of Whiting 

Bay approx. 61 m (200 ft) south of site of sample 

No. 4 (App. A). Edmunds Formation. Mixed tuff 

breccia; angular to rounded clasts of felsites in 

matrix of feldspar crystal tuff. Minerals: Alb, Q, 

minor Cht, Set, Ep, Op, Ksp. Analyses: 65.4 Si, 

14.8 A, 2.8 Fe2, 2.4 Fe, 1.6 M, 2.9 C, 3.3 N, 2.7 K, 

1.7 H, 0.16 h, 0.76 T, 0.16 P, 0.13 Mn, 1.2 CO, 0.00 S. 

W176687 Pembroke quadrangle; roadside outcrop U.S. highway 

1 at corner near Dennysville-Pembroke town line. 

Edmunds Formation. Mixed tuff breccia; felsites 

and darker volcanics in matrix of crystal tuff. Minerals: Alb, minor Ksp, Q, Cht, Set, Ca, Op. Analyses: 64.9 Si, 13.3 A, 2.4 Fe2, 1.2 Fe, 0.57 M, 4.6 C, 

4.2 N, 2.5 K, 1.4 H, 0.13 h, 0.62 T, 0.13 P, 0.20 Mn, 

3.5 CO, 0.00 S. 

Lab. No. Description 

W176685 Pembroke quadrangle; roadcut U.S. highway 1, 0.65 

km (0.4 mi) west of West Pembroke junction. Edmunds Formation. Mixed tuff breccia; unsorted 

angular clasts of felsites, and a few silty mugstones 

in a matrix of feldspar crystal tuff. Mi~als: Alb, 

Q, Cht, Set. Analyses: 68.9 Si, 15.0 A, 0.88 Fe2, 3.2 

Fe, 1.5 M, 1.5 C, 4.7 N, 1.2 K, 2.1 H, 0.13 h, 0.5 T, 

0.08 P, 0.08 Mn, 0.25 CO, 0.06 S. 

D102344 Pembroke quadrangle; same site as sample No. 14 

(App. A); approx. 2.6 x 2.6-cm (1 x l-in.) fragments 

taken at 1.52-m (5-ft) intervals between 61 and 98 m 

(200 and 320 ft) below collar. Represents middle of 

thick lenticular flow in Leighton Formation. Analyses: 65.04 Si, 14.65 A, 0.74 Fe2, 3.47 "~ 1.70 M, 

2.85 C, 4.13 N, 1.67 K, 2.02 H, 0.17 h, O.bZ T, 0.16 P, 

0.23 Mn, 2.14 CO, 0.01 Cl, 0.04 F, 0.22 S. 

W178306 Pembroke quadrangle; same site as sample No. 14 

(App. A); single 2.6 x 5-cm (1 x 2-in.) specimen taken 

114m (375ft) below collar. Analyses: 64.7 Si, 14.4 A, 

0.50 Fe2, 2.5 Fe, 1.7 M, 4.8 C, 3.7 N, 1.5 K, 1.9 H, 

0.15 h, 0.52 T, 0.22 P, 0.24 Mn, 3.5 CO. 

D160374W Pembroke quadrangle; same site and basalt flow as 

sample No. 20 (App. A). Analyses: 52.4 Si, 16.8 A, 

3.4 Fe2, 5.8 Fe, 4.0 M, 7.8 C, 2.7 N, 1.4 K, 2.0 H, 

0.26 h, 1.6 T, 0.35 P, 0.17 Mn, 0.75 CO. 

D160377W Eastport quadrangle; roadside outcrop State route 

190, 0.65 km (0.4 mi) south of Perry. Eastport Formation. Reddish-brown felsite; spherulitic and intergrown blades of alkali feldspar, intergranular 

Q, and scattered phenocrysts of Alb as long as 3 mm. 

Minerals: Alb, Ksp, Q, spots Hem-dusted Cht, trace 

Zr. Analyses: 74.5 Si, 13.2 A, 2.1 Fe2, 0.84 Fe, 

0.16 M, 0.43 C, 5.0 N, 1.7 K, 0.74 H, 0.08 h, 0.20 T, 

0.03 P, 0.06 Mn, 0.22 CO. 

• 

~U.S. GOVERNMENT PRINTING OFFICE' 1980-777.034/34 
• 

APPENDIX 31 

Sample No. Description 

20 Pembroke quadrangle; roadside outcrop U.S. highway 1 at 

east border of quadrangle. Eastport Formation. Basaltic flow above basal Eastport tuff (sample 16); euhedral 

____ phenocrysts of Pc and Aug. Minerals: Pc, Set, Aug, Cht, 

Mt, sparse Ca. 

21 Eastport quadrangle; outcrop on south shore of Buckman 

Head, Moose Island, Eastport, Eastport Formation. 

Bulbous body that intrudes shale, evidently penecontemporaneously: finely porphyritic basalt. Minerals: 

Alb, Cht, Ca, Sph, Leu. 

22 Pembroke quadrangle; outcrop at south end of Mount 

Tom, about 1.6 km (1 mi) NW. of Ayers Junction. Stock 

of porphyritic basaltic andesite that intrudes the Leigh-

_ton ,Formation; blocky argillized Lab phenocrysts 3-5 

imn· across in a trachytic matrix. Minerals: Lab, Cht, 

and Ep (partly pseudomorphous after Cpx), abundant 

M t, and sparse Ksp. 

23 Eastport quadrangle; roadside outcrop on U.S. highway 1 

about 2.9 km (1.8 mi) east of west quadrangle border. 

Eastport Formation. Flow-banded felsite with alternating layers of spherulitic and interlocking bladed feldspar 

and Q; sparse blocky alkali feldspar phenocrysts. Minerals: Ksp, Alb, Q, minor Mt, Cht, and Ca. 

24 Eastport quadrangle; outcrop on west shore of Moose Island, 1.2 km (0. 75 mi) north of Shackford Head. Eastport 

Formation. Rhyolitic tuff with clasts of pale-red felsite 

as much as 1 em long in a dense gray matrix; conspicuously foliated; matrix composed of smaller felsites and 

Alb crystals. Minerals: Q, Alb, Ksp, Cht, Hem. 

25 Eastport quadrangle; roadside outcrop on State route 190 

at Redoubt Hill, Moose Island. Eastport Formation. 

Grayish-red flow-banded felsite; interlocking blades of 

alkali feldspar closely intergrown with Q. Minerals: Q, 

allrs.li feldspar, minor Ca, Mt, sparse Cht, Hem, Py. 

Sample No. Description 

26 Eastport quadrangle; outcrop on State route 190, Moose 

Island in Eastport, 1.6 km (1 mi) SE. of Redoubt Hill. 

Eastport Formation. Flow-banded felsite with sparse 

1-mm phenocrysts of Alb in a dense matrix of Q, Ksp, 

·sparse Cht, and Op. 

27 Eastport quadrangle; roadside outcrop on U.S. highway 

1 about 2.4 km (1.5 mi) south of town of Perry. Eastport 

Formation. Grayish-red flow-banded felsite with spherulitic and interlocking textures. Minerals: Q, Alb, Ksp, 

minor Op, and accessory Zr. 

28 Pembroke quadrangle; drill core from hole S-51, Pembroke prospect; hole is 0.7 km (0.42 mi) azimuth N. 58° 

W. from Big Hill summit. Diabase sill that cuts the 

Leighton Formation: approx. 2.6 x 5-cm (1 x 2-in.) fragment taken 105 m (344 ft) below collar; middle of 24-m 

(80-ft) thick sill. Relict diabasic texture. Minerals: Alb, 

Cht, Aug, Ep, Ksp, Q (1.6 percent), 11, Mt, Py, Ap, Ac, Ca. 

29 Pembroke quadrangle; roadcut on U.S. highway 1 near 

SE. end of Pennamaquan Lake. Diabase sill that cuts 

the Hersey Formation. Coarsely ophitic diabase from 

center of sill. Minerals: Aug, Alb-Oli, Cht, trace of brown 

Hb, minor Ep, Pr, Pu, and Sap. 

30 Eastport quadrangle; outcrop on shore of Passamaquoddy 

Bay, 0.8 km (0.5 mi) north of Gleason Point. Perry Formation. Brownish-gray dense basalt with sparse 3-5-mm 

phenocrysts of plagioclase in a trachytic matrix. Minerals: argillized Pc, Aug, Mt, Hem, sparse amygdales 

with Q and Cht. 

31 Pembroke quadrangle; drill core from hole S-51, 0.7 km 

(0.42 mi), azimuth N. 58° W. of Big Hill summit. Diabase sill that cuts the Leighton Formation; approx. 

2.6 x 5-cm (1 x 2-in.) fragment taken from a thin sill at 

17 m (55 ft) below the collar. Reli"ct diabasic texture; 

clear Lab corroded by Ca and Set. Minerals: Lab, Cht, 

Ca, Set, Mt, 11, sparse Py. 

APPENDIXB 

[Samples not listed in table 2 or numbered in figure 2: sites on 71/•·minute quadrangles, 

sample descriptions, chemical analyses, Samples Dl02344, Dl02347 analyzed by 

standard methods by Vertie C. Smith; all others by rapid methods by P. Elmore, J. 

Glenn, R. Mom·e, H. Smith, S. Botts, U.S. Geological Survey. Mineral abbreviations: Q, 

quartz;- Ksp, }{-feldspar; Alb, albite; Oli, oligoclase; Pc, calcic plagioclase; Lab, labradorite; BYr.'Qytownite; Cpx, clinopyroxene; Aug, augite; Pig, pigeonite; 01, olivine; Amp, 

Lab. No. Description 

W176695 Machias Bay quadrangle; roadside outcrop U.S. highway 1, 0.8 km (0.5 mi) east of Gardner Lake. Dennys 

Formation. A 10-in block basaltic tuff in basaltic 

agglomerate; clasts of altered scoriaceous and mas-

• sive basalts in fine-grained clastic matrix of basaltic 

composition. Minerals: Alb, Ep, Cht, Bi, and Amp. 

Analyses: 51.8 SL 15.9 A, 2.5 Fe2, 7.7 Fe, 8.0 M, 4.2 C 

2.4 N, 1.6 K, 3.9 H, 0.09 h, 1.2 T, 0.34 P, 0.15 Mn, 

< 0.05 CO, 0.01 S. 

D102347 Pembroke quadrangle. Drill core, Pembroke prospect; 

hole E-4, 0.76 km (0.47 mi), azimuth S. 55° W. from 

Big Hill summit. Sample is 2.6 x 2.6-cm (1 x l-in.) 

fragment taken every 1.5 m (5 ft) from 62.5 to 78 m 

(205 to 255 ft) below collar; represents several basaltic flows. Leighton Formation. Massive basalt with 

evenly scattered amygdales (Q, Ca, Cht) and inconspicuous pervasive secondary foliation. Minerals: 

Cht, Ca, Alb, minor Set, Ep, Sph. Analyses: 48.39 

Si, 15.13 A, 0.91 Fe2, 7.85 Fe, 5.91 M, 6.50 C, 3.14 

N, 0.95 K, 4.18 H, 0.24 h, 1.38 T, 0.17 P, 0.25 Mn, 

4.63 CO, 0.01 Cl, 0.03 F, 0.03 S. 

amphibole; Hb, hornblende; Ac, actinolite; Cht, chlorite; Bi, biotite; Mu, muscovite; Set, 

sericite; Sap, saponite; Op, opaques; Mt, magnetite; 11, ilmenite; Leu, leucoxene; Hem, 

hematite; Py, pyrite; Sph, sphene; Ru, rutile; Pr, prehnite; Pu, pumpellyite; Ep, epidote; 

Ca, calcite; Zr, zircon; Ap, apatite; Sp, sphalerite. Oxide abbreviations: Si, SiO,; A, Al,O,; 

Fe2, Fe,O,; Fe, FeO; M, MgO; C, CaO; N, Na,O; K, K,O; H, H,o+; h, H,o-; T, TiO,; P, 

P,O,; Mn, MnO; CO, CO,. Oxides measured in weight percent] 

Lab. No. Description 

W178304 Same site and rock type as D102347. Single 2.6 x 5-

cm (1 x 2-in.) fragment at about 62.5 m (205 ft)~ least 

altered available. Leighton Formation. Analyses: 

47.8 Si, 15.2 A, 0.00 Fe2, 7.8 Fe, 5.4 M, 8.0 C, 3.2 N, 

0.70 K, 3.9 H, 0.30 h, 1.2 T, 0.29 P, 0.18 Mn, 5.4 CO. 

W178307 Same site as sample No. A (App. A). Single 2.6 x 5-

cm (1 x 2-in.) fragment at about 229 m (750 ft). Least 

altered available. Leighton Formation. Dense aphyric 

basalt; inconspicuous secondary foliation. Minerals: 

Pc, Alb, Aug, Cht, Amp, Ep, Ca, minor Leu, Ru, 

Set. Analyses: 50.0 Si, 15.9 A, 1.4 Fe2, 7.4 Fe, 6.3 

M, 8.4 C, 2.4 N, 0.96 K, 3.9 H, 0.23 h, 1.3 T, 0.26 P, 

0.23 Mn, 1. 7 CO. 

W176697 Machias quadrangle; roadcut U.S. highway 1, 1.5 km 

(0.9 mi) south of bridge at East· Machias. Dennys 

Formation. Mixed tuff breccia; clasts of many felsites in a clastic matrix of alkali feldspars, Q, and 

small felsites. Minerals: Q, Alb, Ksp, Bi, and Mu 

(metamorphic), minor Ca, Cht, Ep. Analyses: 73.2 

Si, 13.0 A, 0. 77 Fe2, 2.3 Fe, 0.92 M, 1.8 C, 4.3 N, 

2.1 K, 0.90 H, 0.03 h, 0.39 T, 0.08 P, 0.11 Mn, 0.16 

C0,0.20S.