Glastonbury Gneiss and Mantling Rocks

51 Pages·2010·7.02 MB·English

Glastonbury Gneiss and Mantling Rocks (a Modified
Oliverian Dome) in South-central Massachusetts and
North-central Connecticut; Geochemistry,
Petrogenesis, and Isotopic Age
GEOLOGICAL SURVEY PROFESSIONAL PAPER 1295

Glastonbury Gneiss and Mantling Rocks (a Modified
Oliverian Dome) in South-central Massachusetts and
North-central Connecticut: Geochemistry,
Petrogenesis, and Isotopic Age
By GERHARD W. LEO, ROBERT E. ZARTMAN, and DOUGLAS G. BROOKINS
GEOLOGICAL SURVEY PROFESSIONAL PAPER 1295
UNITED STATES GOVERNMENT PRINTING OFFICE, W AS H I N GTO N : 1 9 8 4
UNITED STATES DEPARTMENT OF THE INTERIOR
WILLIAM P. CLARK, Secretary
GEOLOGICAL SURVEY
Dallas L. Peck, Director
Library of Congress Cataloging in Publication Data
Leo, Gerhard W.
Glastonbury gneiss and mantling rocks (a modified
Oliverian dome) in south-central Massachusetts and north-
central Connecticut.
(Geological Survey professional paper ; P-1295)
Bibliography: p.
Supt. of Docs. no. : 119.16:1295
1. Domes (Geology) Connecticut. 2. Domes (Geology)
Massachusetts. 3. Gneiss Connecticut. 4. Gneiss-
Massachusetts. I. Zartmen, Robert E. II. Brookins,
Douglas G. III. Geological Survey (U.S.) IV. Title.
V. Series: Geological Survey professional paper ; 1295.
QE611.5.U6L46 1983 552'.4 83-600093
For sale by the Distribution Branch, U.S. Geological Survey,
604 South Pickett Street, Alexandria, VA 22304
CONTENTS
Page
- 1 Analytical results Continued
- 1 Trace elements Continued
Acknowledgments - 4 Rare-earth elements 24
Regional geology - 4 Monson Gneiss 24
Monson Gneiss - 4 Northern Glastonbury Gneiss 32
Ammonoosuc Volcanics - 6 Southern Glastonbury Gneiss 32
Partridge and Collins Hill Formations - 9 Isotopic ages 32
Glastonbury Gneiss - 9 "DiSCUSSiOn ________________________________________ 37
Structural character - 9 Northern Glastonbury Gneiss 38
Contact relationships - 9 Anatexis of Monson lithology 38
Lithologic character - 11 Middle Ordovician to Early Silurian heat flow
- 14 and metamorphism - 39
Major elements - 14 Southern Glastonbury Gneiss 40
Ammonoosuc Volanics and Monson Gneiss - 14 Plate-tectonic setting of the Glastonbury dome 40
Glastonbury Gneiss - 14 References cited 41
Trace elements-_ _~"~_-_ __ _ ._.__ - 24 Appendix: Sample field numbers, descriptions,
Trace elements other than rare earths 24 and locations 43
ILLUSTRATIONS
Page
FIGURE 1. Simplified geologic map of Bronson Hill anticlinorium - - 2
2. Geologic sketch map of Glastonbury dome - 5
3-5. Photographs of
3. Ammonoosuc Volcanics and Monson Gneiss
4. Monson Gneiss showing partial anatexis and plastic flow -
5. Structures and textures in northern Glastonbury Gneiss 10
6. Ternary diagram showing modal variations in Glastonbury Gneiss, Monson Gneiss, and felsic layers of
Ammonoosuc Volcanics 12
7. Photographs of structures and textures in southern Glastonbury Gneiss - 13
8. Normative An-Ab-Or diagram 20
9. Normative Q-Ab-Or diagram 21
10-12. Variation diagrams for
10. Major elements versus SiO2 - 22
11. Th, Co, and Sc versus SiO2 - --- - - 29
12. K versus Rb/Sr - 30
13. Plot of rare-earth-element patterns 31
14. Concordia diagram for zircon from Glastonbury and Monson Gneisses 35
15. Rb-Sr whole-rock isochron diagram for Glastonbury Gneiss 37
in
IV CONTENTS
TABLES
Page
TABLE 1. Major-element compositions, norms and modes of Glastonbury Gneiss, Monson Gneiss, and Ammonoosuc Volcanics 15
2. Trace-element and Sr-isotope data for Glastonbury and Monson Gneisses 25
3. Uranium-thorium-lead isotope ages of zircon from Glastonbury and Monson Gneisses 34
4. Whole-rock rubidium-strontium isotope age of the Glastonbury Gneiss 36
GLASTONBURY GNEISS AND MANTLING ROCKS
(A MODIFIED OLIVERIAN DOME) IN SOUTH-CENTRAL MASSACHUSETTS
AND NORTH-CENTRAL CONNECTICUT:
GEOCHEMISTRY, PETROGENESIS, AND ISOTOPIC AGE
By GERHARD W. LEO, ROBERT E. ZARTMAN, and DOUGLAS G. BROOKINS'
ABSTRACT plutonism in the Taconic than has generally been recognized. With
certain qualifications, the Glastonbury and associated volcanic rocks
The Glastonbury dome is a long, narrow structure trending approximately are compatible with recent plate-tectonic models involving the
70 km north-northeast through Connecticut and Massachusetts along the Bronson Hill anticlinorium.
west side of the Bronson Hill anticlinorium. Structurally and stratigraphically
the dome is analogous to the Oliverian domes of New Hampshire. It is
INTRODUCTION
cored by Glastonbury Gneiss and is mantled by Ammonoosuc Volcanics
and Partridge Formation (or their equivalents) of Ordovician age. The The Glastonbury dome is a narrow, elongate structure
Glastonbury Gneiss intrudes the Ammonoosuc and, thereby, establishes the that extends from just south of the Belchertown Quartz
relative age of the two units. Monson Gneiss, which unconformably underlies
Monzodiorite pluton (Ashwal and others, 1979) in cen
the Ammonoosuc Volcanics in the Monson anticline to the east, is not in
tral Massachusetts to the vicinity of Middle Haddam,
contact with Glastonbury Gneiss except near Stafford Springs, Conn.,
where the contact may be gradational. In some places, Monson Gneiss Connecticut. The term "Glastonbury dome" as used
shows evidence of plastic flow and potential anatexis. The northern part of henceforth refers to the tectonic feature with consti
the Glastonbury Gneiss typically is leucocratic, granoblastic, relatively tuent lithologies, whereas "Glastonbury Gneiss" refers
potassium-poor gneiss that appears homogeneous in outcrop, but proves to
only to the core gneiss itself. A parallel distinction
be chemically and modally inhomogeneous over short distances, as shown
applies to the terms "Oliverian dome(s)" and "Oliverian
by variation diagrams and REE plots. The gneiss straddles the compositional
fields of trondhjemite, tonalite, and granodiorite, and partly overlaps that core gneiss(es)". The Glastonbury dome constitutes
of Monson Gneiss. The southern part of the Glastonbury Gneiss is consistently part of the Bronson Hill anticlinorium (Billings, 1956;
more potassic than the northern, having compositions ranging from granite Thompson and others, 1968), a complexly folded and
to granodiorite. All of the Glastonbury Gneiss show pervasive, strong folia
deformed sequence of lower Paleozoic metasedimentary,
tion, deformation, and local shearing related to the Acadian orogeny.
metavolcanic, and plutonic rocks trending south-
Field relations, textures, and chemistry of the northern part of the Glas
tonbury suggest an origin by anatexis of the premetamorphic Monson southwest from northern New Hampshire to Long
sequence at temperatures of about 690 °C to 750 °C and pressures of Island sound (fig. 1). The stratified rocks are intruded
<3kbars. The southern part of the Glastonbury appears to have been
by a variety of granitic plutons that constitute several
generated contemporaneously but not comagmatically from calc-
plutonic or plutonic-volcanic suites ("magma series" of
alkaline crust. U-Pb zircon ages for both the northern and southern
bodies are slightly discordant with 207Pb/208Pb ages of 445 to 467 m.y. Billings, 1937, 1956) and range in age from Middle
At first these results seem to contradict the known stratigraphic Ordovician to Cretaceous. The age of the Oliverian
position of the Glastonbury relative to the Monson, which yields dis Plutonic Suite in New Hampshire has been reliably
tinctly younger zircon 207Pb/ao8Pb ages of 428 to 440 m.y. However,
established as Middle Ordovician (Naylor, 1969; Poland
this apparent discrepancy in the radiometric ages younger Monson,
and Loiselle, 1980; Zartman and Leo, 1981). Oliverian
older Glastonbury could be resolved by postulating either (1) a
small component of old inherited zircon in the Glastonbury or (2) pref plutons form the cores of a number of gneiss domes
erential metamorphic overprinting of the zircon in the Monson. In mantled by the Ammonoosuc Volcanics of the Middle
any case, the isotopic age discrepancy is not so large as to render the Ordovician age (Naylor, 1968,1969; Leo, 1980a,b). The
proposed Monson anatectic model implausible. Rb-Sr whole-rock
Ammonoosuc Volcanics are a mostly bimodal sequence
data show a large amount of scatter on an isochron diagram and
of metamorphosed tholeiite and quartz keratophyre
hence do not permit a reliable estimate of age. This condition may
reflect inhomogeneities in the initial 87Sr/88Sr ratio or may have been tuff, locally associated with sills and stocks of trondhje
also induced by later Acadian or Alleghanian metamorphism. mite (Leo, 1980c; Leo and Gromet, 1981).
An early Silurian to Middle Ordovician age of the Glastonbury
Gneiss gives evidence of higher heat flow and more extensive "University of New Mexico
GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT
Southern part
43'
MAINE
0 10 20 30 40 50KM
73° 72°
FIGURE 1.-Simplified geologic map showing the central part of the domes (see explanation) as follows (generally north to south): J, Jeffer-
Bronson Hill anticlinorium, emphasizing the Oliverian domes and son; SH, Sugar Hill pluton; OH, Owls Head; M, Mascoma; L, Lebanon;
Ammonoosuc Volcanics. Geology east and west of the Ammonoosuc C, Croydon; U, Unity; A, Alstead; S, Swanzey; W, Warwick; P, Pelham;
Volcanics mostly omitted. Adapted from Robinson and Hall (1980, T, Tully; Mo, Monson Gneiss; G, Glastonbury; Wi, Willimantic; K, Kill-
fig. 1) and Billings (1955). Letter symbols for Oliverian and other ingworth.
INTRODUCTION
EXPLANATION
Layered rocks Intrusive rocks
including volcanic rocks but including orthogneisses
excluding granitic gneisses
Cretaceous
to
Portland Formation and related Triassic
Unconformity
Devonian
to
Silurian and Devonian rocks Felsic to intermediate plutonic Silurian
undivided rocks, mostly gneissic
Unconformity
Ammonoosuc Volcanics and Gneisses in cores of Oliverian Highlandcroft Plutonic Suite
Partridge Formation and related domes. (See (New Hampshire)
(Middletown Gneiss and figure captions for names
Collins Hill Formation, re corresponding to letter
spectively, in southern Con symbols)
necticut); also includes Or-
fordville Formation (southwest
New Hampshire) and Brim-
field Group (Middle Ordovi Ordovician
cian or older; Massachusetts
and Connecticut)
Albee Formation (New Hampshire)
Other Ordovician rocks, undivided
(southwest New Hampshire)
Unconformity
Proterozoic Z
Metasedimentary and Gneiss in cores of domes
metavolcanic rocks
Contact
Fault
FIGURE 1. Continued
GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT
The Glastonbury Gneiss resembles Oliverian core data). By contrast, field relationships as well as textur-
gneisses in that (1) it also is mantled by Ammonoosuc al and chemical features of the northern Glastonbury
Volcanics (and, at its southern end, by the Collins Hill Gneiss suggest that it originated in the upper crust by
Formation of Rodgers and others, 1959, hereby adopted anatexis of a premetamorphic Monson lithology. Never
for U.S. Geological Survey usage), which overlies the theless, as will be discussed below, the possibility that
Ammonoosuc Volcanics or its correlative, the Middle- the northern Glastonbury Gneiss represents a mag-
town Gneiss in Connecticut, and intrudes these strata matic Ammonoosuc phase, analogous to the trondhje
along much of the western side of the dome; and (2) the mites to the north, cannot be completely discounted.
dome is pervasively metamorphosed at middle am- If the mode of origin here proposed for the northern
phibolite facies grade and exhibits Acadian deforma Glastonbury Gneiss is correct, it implies a higher heat
tion. flow related to the Taconic orogeny than has heretofore
The Glastonbury Gneiss (fig. 2) can be divided into been proposed or documented.
two parts, northern and southern, on the basis of min-
erology and chemistry. The northern part of the gneiss
(henceforth referred to informally as "northern Glas ACKNOWLEDGMENTS
tonbury Gneiss" or "northern gneiss") is typically low
Thanks are due Richard S. Naylor for providing
in potassium (compositionally trondhjemite to tonalite),
GWL with insights to the Oliverian domes and for re
although the composition is variable on a local scale.
viewing an early draft of the manuscript. We also wish
The southern part of the gneiss (henceforth referred to
to thank Rosalind T. Helz, Michael W. Higgins, Joseph G.
informally as "southern Glastonbury Gneiss" or "south
Arth, and especially Richard Goldsmith for reviews of
ern gneiss") is much more potassic and is mostly gran
later versions of the paper. The authors, however, take
ite. Field relationships and chemical data suggest that
sole responsibility for the interpretations and conclu
the northern gneiss was produced by anatexis of Middle
sions herein.
Ordovician volcanic rocks (the protolith of the Monson
Gneiss).
In several of the northern domes, notably the Mas-
REGIONAL GEOLOGY
coma dome (Naylor, 1969; fig 1), K-poor, felsic, strati
The layered rock units described below are arranged
fied rocks of volcaniclastic aspect mantle the intrusive
in order from oldest to youngest based on field relation
core gneiss. Naylor applied the term "stratified core
ships and not on the somewhat anomalous radiometric
gneiss" to these rocks in the Mascoma dome and re
dates for the Monson and Glastonbury Gneisses (to be
garded them as an integral part of the Oliverian lithol-
described below). The Glastonbury Gneiss is listed last
ogy, although he left open the possibility that they
(youngest) because it cuts Ammonoosuc Volcanics;
might represent a downward continuation of the Ammo
however, the Glastonbury has not been shown to cut
noosuc Volcanics. Further study (Leo, unpub. data) has
the Partridge and (or) the equivalent Collins Hill For
shown that somewhat similar rocks underlie the Ammo-
mations that conformably overlie Ammonoosuc Vol
sue in several, but by no means all, of the other Oliver
canics. It is therefore conceivable that the Glastonbury
ian domes. It seems appropriate, therefore, to put such
is actually older than the Partridge/Collins Hill. This
rocks in a special category, for example, an informally
relationship is not basic to the subject of this paper.
designated "pre-Ammonoosuc volcanic section," and to
discontinue the use of the term "Oliverian" as part of
their name. MONSON GNEISS
In New Hampshire and Vermont, intrusive trondhje The Monson Gneiss (Monson granodiorite of Emerson,
mite is associated with the Ammonoosuc Volcanics in 1917, p. 241-243) underlies the Ammonoosuc Volcanics
several of the Oliverian domes, commonly as sills and from southern New Hampshire to Long Island Sound.
other minor intrusions. In three small domes, trondhje The Monson is exposed in broad domes in northern
mite constitutes the entire core gneiss (Leo and Gromet, Massachusetts ("main body" and "Tully dome" of Robin
1981). These trondhjemites differ somewhat from the son and Hall, 1980); and in southern Connecticut (Kill-
northern Glastonbury Gneiss in that they contain rela ingworth dome) (fig. 1). Between these two areas it con
tively lower K2O and CaO and higher Na2O. Thus they stitutes a stratigraphic layer, locally much faulted and
define a different compositional field on normative dia displaced (fig. 2). Quartz-plagioclase gneisses in the
grams (table 1; figs. 8 and 9). Trace element abundances, Willimantic dome (fig. 1) and the Selden Neck dome to
including rare earths, are also somewhat different and the south, traditionally correlated with Monson Gneiss
suggest a mantle origin for at least some of the New to the west across the Chester syncline (not shown in
Hampshire and Vermont trondhjemites (Leo, unpub. fig. 1), are reported to show chemical and