Definition of Penobscottian Orogeny

The northern Appalachians have

been divided into a number of different terranes or tectonostratigraphic

zones, reflecting different origins and accretionary

histories of different parts of the orogen. The

Laurentian craton and autochthonous sedimentary sequences

form the Humber zone, whereas fragments of peri-Gondwanan

continents are preserved in the Avalon zone. The Dunnage

terrane includes material accreted to the Laurentian and

Gondwanan continents during closure of the Paleozoic Iapetus

Ocean. Gander terrane rocks were initially deposited

adjacent to the Avalonian margin of Gondwana. A piece of

northwest Africa left behind during Atlantic rifting is preserved

in the Meguma terrane. For many years a pre-Middle

Ordovician (Taconic age) deformation event has been recognized

from parts of central Maine, New Brunswick, and a

few other scattered parts of the Appalachian orogen. This

Late Cambrian–Early Ordovician event probably occurred

between two exotic terranes in the Iapetus Ocean, before they

collided with North America. Understanding the nature of

this event, known as the Penobscottian orogeny, has become

one of the more enigmatic features of the northern

Appalachians. The overprinting of the original deformational

fabrics by the subsequent Taconic orogeny, despite the outboard

nature of the Penobscot, has led to numerous interpretations

of this particular event.

Because the Penobscot orogenic deformation is less

extensive than other orogenies in the Appalachians, interpretations

have been based on information in limited areas, and

thus a comprehensive model is difficult to propose. The

deformation is confined to the Gander terrane in the North-

ern Appalachians and the Piedmont in the Southern

Appalachians. The timing of this orogenic event can be best

constrained in western Maine where an exposure of sialic

basement, an ophiolite, and associated accretionary complexes

crop out. It has also been suggested that the Brunswick

subduction complex in New Brunswick, Canada, represents

the remains of the Penobscottian orogeny. In western Maine,

the major units include the Chain Lakes massif, interpreted to

be Grenville age; the Boil Mountain ophiolite, interpreted to

be a relict of oceanic crust obducted during accretion of the

Boundary Mountains terrane to the previously existing Gander

terrane; and the Hurricane Mountain mélange, expressing

the flysch deposits of the accretionary prism during amalgamation.

These rock units together record a portion of the

complex deformational history of the closing of the Iapetus

Ocean. The Boil Mountain ophiolite occurs in a structurally

complex belt of ophiolitic slivers, exotic microterranes (e.g.,

Chain Lakes massif), and mélange along the boundary

between the Dunnage and Gander terranes in central Maine.

Its geological evolution is critical to understanding the Late

Cambrian–Early Ordovician Penobscottian orogeny.

The Penobscot orogeny in the Northern Appalachians

was initiated sometime during the Late Cambrian. Isotopic

ages of the Boil Mountain ophiolite range from 500 million

to 477 million years old. Fossil evidence further supports this

Middle Cambrian to Early Ordovician age range. Sponges

of this age have been found in the black shales of the Hurricane

Mountain mélange constraining the maximum age for

the mélange.

The Boil Mountain ophiolite and the associated Chain

Lakes massif are instrumental in the interpretation of the

Penobscottian orogeny. In order to better comprehend their

relationship it is important to understand their lithologies

and regional context. The Chain Lakes massif is characterized

by (meta-) diamictite that is composed of mostly

metasandstone, minor amphibolite, granofels, and gneiss.

The structure has been interpreted to be an elongate dome,

and an exposed thickness of 9,840 feet (3,000 m) is estimated.

The unit is bounded by faults that strike northwest and

comes in contact with several intrusive bodies, including the

Attean batholith to the northeast and the Chain of Ponds pluton

to the southwest. Along its eastern and western margin,

Silurian and Devonian strata overlie the massif. The Boil

Mountain ophiolite is in fault-bounded contact with the

Chain Lakes massif on its southern and southeastern margin.

Seismic reflection profiling has shown that the Chain Lakes

massif is floored by a decollement dipping toward the southeast.

This was interpreted to represent a thrust that originated

to the southeast and occurred either during the Acadian or

the Taconic orogenies.

The massif may be divided into eight facies based on

structural aspects and lithology of the complex. The structurally

lowest sequence is first divided into three facies: (1)

the Twin Bridges semipelitic gneiss; (2) the Appleton epidiorite;

and (3) the Barrett Brook polycyclic epidiorite breccia.

The next four facies represent the principal diamictite

sequence and include (1) the McKenney Pond chaotic rheomorphic

granofels; (2) the Coburn Gore semipelitic gneissic

granofels; (3) the Kibby Mountain flecky gneiss; and (4) the

Sarampus Falls massive to layered granofels. The structurally

highest facies is the Bag Pond Mountain bimodal metavolcanic

section and feldspathic meta-arenite. The highest facies

are interpreted to represent the return to a passive margin

sequence after metamorphism and deformation. However, the

structural and metamorphic history of the area is complicated,

and it is difficult to be certain about stratigraphic relationships

across structural boundaries.

The metamorphic history of the Chain Lakes massif is

one of repeated deformation over a long span of time. The

metamorphism of this complex spans a period of 800 million

years and has been interpreted to record a pressure-temperature

time path of prograde and retrograde events that end

with the emplacement of the Late Ordovician batholith.

U/Pb, Rb/Sr, and Nd/Sm ages of the Chain Lakes massif range

from approximately 1,500 Ma to 684 Ma. The wide variance

in the ages may be caused by deposition of material in the

massif sometime between these ages as the deformational history

was proceeding, or the zircons that produced the Precambrian

dates must have been derived from a preexisting

Precambrian unit that was eroding into the basin. The

diamictite may have been deposited as a fanglomerate along

the rifted margin of the Iapetus and was subsequently overridden

by the Boil Mountain ophiolite complex.

The Boil Mountain ophiolite lies in fault-bounded contact

with the Chain Lakes massif along its southern boundary.

This complex extends about 20 miles (30 km) along

strike and has a maximum exposure across strike of about

four miles (6 km). Units typical of ophiolites such as serpentinite,

pyroxenite, metagabbro, mafic volcanics, and sediments

consisting of metaquartzwacke, metapelite, slate, and metaconglomerate

characterize the Boil Mountain ophiolite. The

stratigraphic units consist of, from bottom to top: an ultramafic

unit, a gabbroic unit, a tonalitic unit, mafic and felsic

volcanics, and metasedimentary units. Thus, the ophiolite has

all the components of an ophiolite sequence except the tectonite

ultramafic unit and the sheeted dike unit. The basal

contact is difficult to differentiate in the large scale, but any

contacts that are present suggest that ductile faulting accompanied

their emplacement. However, some of the units,

including the serpentinite, are in sharp structural contact with

the Chain Lakes massif along its southern boundary.

The complex may be divided based on chemistry into

several units, including ultramafics, gabbros, two mafic volcanic

units, and felsic volcanics. A felsic unit separates the two

mafic volcanic units, termed upper and lower. Trace element

and rare earth element (REE) patterns reveal two distinct crystallization

trends within the complex. The first magma to be

erupted was the lower mafic volcanic unit. The trace element

and REE trends suggest that the crystallization of olivine,

clinopyroxene, and plagioclase resulted in this magma composition.

This is accomplished by the formation of the ultramafics

and gabbroic unit. The upper mafic unit has

geochemical affinities to the Island Arc Tholeiites (IAT) zone

while the upper mafic unit is similar to Mid-Ocean Ridge

Basalts (MORB). Since there is such a large volume of felsic

material associated with the ophiolite, it is likely that there

were two phases of extrusion for the mafic volcanic unit.

Because the presence of tonalites and other felsic volcanic

rocks imply that there must be hydrous fluids, the first phase

must include a subduction zone. The lower mafic unit and the

felsics represent this phase. The upper mafic unit represents

the volcanism at a marginal basin because the mantle sources

for these units were not affected by the subducting slab.

The presence of tonalites in the Boil Mountain ophiolite,

with small amounts of trondhjemite, is an unusual feature

not found in “normal” ophiolite sequences. Other ophiolites

that include a unit of tonalite are the Semail ophiolite in

Oman and the Canyon Mountain ophiolite in California.

Tonalites suggest the presence of fluids during crystallization

and are instrumental in the interpretation of the ophiolites.

The placement of these ophiolites with respect to the rest of

the sequence is also of interest. The tonalite of the Boil

Mountain complex appears in the sequence above the gabbros

and below the volcanogenic units. It has the form of a

sill, with abundant intrusive contacts evident in float and

rare, poorly exposed outcrops in the northeast part of ophiolite.

The tonalite was probably derived from partial melting

of the lower mafic volcanics and gabbros, which then intruded

as a sill.

The Boil Mountain ophiolite tonalite has yielded a U/Pb

zircon age of 477±1 million years. The age of 477 million

years places a minimum plutonic age for the tonalites of the

Boil Mountain ophiolite, which is significantly less than any

previously determined age associated with the ophiolite. A

Late Cambrian to Early Ordovician age for the Boil Mountain

ophiolite has been previously suggested based on several

pieces of information. Felsic volcanics in the upper part of the

ophiolite give a U-Pb zircon age of 500 Ma±10 million years.

The age of 477 million years for the Boil Mountain tonalites

is interpreted as a late-stage intrusive event, possibly related

to partial melting of hydrated oceanic crust and the intrusion

of a tonalitic extract.

A comparison of the age of the Boil Mountain ophiolite

with nearby ophiolitic sequences in the Taconic allochthons

of Quebec shows that ophiolite obduction was occurring on

the Humber zone of the Appalachian margin of Laurentia, at

similar times as the Boil Mountain ophiolite was being

emplaced over the Chain Lakes massif, interpreted as a piece

of the Gander margin of Gondwana. Hornblendes from the

metamorphic sole to the Thetford Mines ophiolite have yielded

40Ar/39Ar ages of 477±5 million years, with an initial

detachment age of 479±3 million years for the ophiolitic

crust. Detachment of the circa 479-million-year-old sheet

began at a ridge segment in a fore-arc environment, in contrast

to an older, circa 491±11-million-year-old (40Ar/39Ar

from amphibole) oceanic slab preserved as the Pennington

sheet in the Flintkote mine that is interpreted as a piece of

oceanic crust originally attached to Laurentia. Thus, there

may be a protracted history of Taconian ophiolite obduction

in the Quebec Appalachians. In Gaspe a 40Ar/39Ar analysis on

basal amphibolite tectonite gave an emplacement age of

456±3 million years for the Mount Albert ophiolite.

The age and origin of the tonalites of the Boil Mountain

ophiolite, and their relationships to the Chain Lakes massif,

have considerable bearing on the Penobscottian orogeny.

First, since tonalitic intrusives are confined to the

allochthonous Boil Mountain ophiolite, it can be inferred

that the ophiolite was not structurally emplaced in its final

position over the Chain Lakes massif until after 477 million

years ago (Arenigian). This does not, however, preclude earlier

emplacement (previous to tonalite intrusion) of the ophiolite

in a different structural position. Dates for the Taconic

orogeny range from 491 million to 456 million years ago.

Early ideas for the Penobscottian orogeny suggested that it

took place prior to the Taconic orogeny. However, it seems

more likely that these two orogenies were taking place at the

same time, although not necessarily on the same margin of

Iapetus. Geochemical data suggest a two-stage evolution for

the Boil Mountain ophiolite, including an early phase of arc

or forearc volcanism, followed by a tholeiitic phase of

spreading in an intra-arc or back arc basin. The first phase

includes felsic volcanics dated at 500±10 million years, and

the second phase, associated with partial melting of the older

oceanic crust and the formation of the upper volcanic

sequence, occurred at 477±1 million years ago. This tectonic

setting is compatible with the IAT characteristics of the lower

volcanic unit and the MORB characteristics of the upper

mafic volcanic unit of the ophiolite. Sedimentary rocks intercalated

with the upper volcanic unit include iron formation,

graywacke, phyllite, and chert, consistent with deposition in

a back arc basin setting. The tonalites formed by hydrous

melting of mafic crust of the lower volcanic unit, during initial

stages of back arc basin evolution.

The Boil Mountain ophiolite and Chain Lakes massif

thus reveal critical insights about the Penobscottian orogeny.

The lower volcanic group of the Boil Mountain ophiolite are

island arc tholeiites formed within an immature arc setting

500 million years ago. This phase of volcanism is similar to

other Exploits subzone (Dunnage zone) ophiolites, such as

the 493-million-year-old Pipestone Pond complex, the 489-

million-year-old Coy Pond complex, the South Lake ophiolite

of Newfoundland, and 493-million-year-old rhyolites associated

with serpentinites of the Annidale area of southern New

Brunswick. The Pipestone Pond and Coy Pond complexes

both occur as allochthons overlying domal metamorphic

cores of Gander zone rocks (Mount Cormack and Meelpaeg

Inliers), in a structural arrangement reminiscent of the Boil

Mountain complex resting on the margin of the Chain Lakes

massif. These ophiolites, including the Boil Mountain ophiolite,

probably represent part of the Penobscot arc, which

developed in the forearc over a west-dipping subduction

zone, near the Gander margin of Gondwana.

The 513-million-year-old Tally Pond volcanics of the Lake

Ambrose volcanic belt of south-central Newfoundland may

also be time-correlative with the lower volcanics of the Boil

Mountain ophiolite; both preserve a mixed mafic-felsic volcanic

section with arc related geochemical affinities, and both

are interpreted as parts of the Penobscot-Exploit’s arc accreted

to Gondwana in the Early Ordovician. Thus, the Boil Mountain,

Coy Pond, and Pipestone Pond ophiolites formed the

fore-arc to the Penobscot-Exploit’s arc, and the Lake Ambrose

volcanic belt represents volcanism in more central parts of the

arc. Obduction of Exploit’s subzone forearc ophiolites over

Gander zone rocks in Newfoundland and New Brunswick

occurred in Tremadocian-Arenigian (490–475 million years

ago), was followed by arc reversal in the Arenigian (475–465

million years ago), which formed a new arc (Popelogan arc in

northern New Brunswick) and a back arc basin (Fournier

Group ophiolites at 464 Ma) that widened rapidly to accommodate

the collision of the Popelogan arc with the Notre

Dame (Taconic) arc by 445 million years ago. This rifting

event may have detached a fragment of Avalonian basement

and Gander Zone sediments, with overlying allochthonous

ophiolitic slabs, now preserved as the Chain Lakes massif and

the lower sections of the Boil Mountain ophiolite.

A second episode of magma generation in the Boil

Mountain ophiolite is represented by the upper tholeiitic volcanic

unit and by the 477-million-year-old tonalite sill, generated

by partial melting of the lower volcanic unit. This phase

of magma generation is associated with development of the

Tetagouche back arc basin behind the Popelogan arc during

Late Arenigian-Llanvirnian. The timing of this event appears

to be similar in southern Newfoundland, where several

Ordovician granites, including the 477.6±1.8-million-yearold

Baggs Hill granite, and the 474±3-million-year-old Partridgeberry

Hills granite, intrude ophiolites obducted onto

the Gondwanan margin during the early Ordovician.

Since the Boil Mountain tonalites are allochthonous and

are not known to occur in the rocks beneath the ophiolite,

final obduction of the Boil Mountain complex probably

occurred after collision of the Popelogan and Notre Dame

(Taconic) arcs, during post-450-million-year-old collision of

the Gander margin of Avalon with the active margin of Laurentia.

This event could be represented by the 445-millionyear-

old Attean pluton.

As an alternative model to that presented above, the entire

Boil Mountain ophiolite complex may have formed in the forearc

of the Popelogan arc, which formed after the Penobscot arc

collided with the Avalonian margin of Gondwana. In this

model, the Boil Mountain ophiolite would be correlative with

other 480–475-million-year-old ophiolites of the Robert’s

Arm–Annieopsquotsch belt, that occur along the main Iapetus

suture between the Notre Dame (Taconic) arc accreted to Laurentia,

and the Penobscot-Exploit’s arcs accreted to Gondwana.

This model would help explain why no evidence of pre-477-

million-year-old obduction-related fabrics have been documented

from the Boil Mountain complex–Chain Lakes massif

contact, but it does not adequately account for the Cambrian

ages of the lower volcanic unit of the Boil Mountain ophiolite.

The Penobscottian orogeny has presented difficulties to

geologists for quite some time. However, recent studies of the

exposure in Maine, including the Chain Lakes unit and the

Boil Mountain ophiolite, have led to new models for the tectonic

evolution of this complex terrane. New isotopic dates

and geochemical evidence show that the Taconian and Penobscottian

orogenies were most likely simultaneous events. The

models presented in this entry take into account several factors,

including the formation of the Chain Lakes unit, the timing

of emplacement of the ophiolite, and the timing of the

intrusion of tonalites that are thought to represent the suture

of the Gander to the Boundary Mountains terrane. However,

further work needs to be done to truly constrain the timing

and sequence of events that created the rocks of the Penobscottian

orogeny.

See also APPALACHIANS; CALEDONIDES.

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