RECONNAISSANCE LITHOGEOCHEMICAL INVESTIGATION OF THE BULL ARM FORMATION AND SIGNIFICANCE OF DIAMICTITE IN THE OVERLYING BIG HEAD FORMATION IN THE ...
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Current Research (2021) Newfoundland and Labrador Department of Industry, Energy and Technology
Geological Survey, Report 211, pages 7396
RECONNAISSANCE LITHOGEOCHEMICAL INVESTIGATION OF THE
BULL ARM FORMATION AND SIGNIFICANCE OF DIAMICTITE IN THE
OVERLYING BIG HEAD FORMATION IN THE LONG HARBOUR–
PLACENTIA AREA, WESTERN AVALON PENINSULA, NEWFOUNDLAND
A.J. Mills and H.A.I. Sandeman1
Regional Geology Section
1
Mineral Deposits Section
ABSTRACT
A reconnaissance lithogeochemical investigation of volcanic rocks of the Bull Arm Formation in the Long Harbour–
Placentia area, western Avalon Peninsula, has identified three chemically distinct volcanic assemblages. Assemblage 1
includes green to brickred, locally scoriaceous, weakly calcalkaline basalt breccia, and mafic and intermediate tuffs chem
ically characterized by relatively steep multielement slopes (mean La/YbCN for mafic rocks = 4.5), and significant Nb troughs
(La/NbCN ~5; Th/NbCN ~9). Assemblage 2 includes mainly green, locally reddish (hematitized), amygdaloidal, plagioclase and
clinopyroxenephyric, tholeiitic basalt flows that exhibit flatter multielement patterns (mean La/YbCN = 3.3) and less pro
nounced Nb (and other HFSE) troughs (mean La/NbCN = 2.7; Th/NbCN = 4.7) than those of Assemblage 1. Assemblage 3
includes darkgrey to greengrey amygdaloidal basalt, agglomerate and volcanogenic conglomerate commonly interbedded
with red mudstone, and narrow (
CURRENT RESEARCH, REPORT 211
BAF
Bonavista Bay
0 50
km
Eastport Bonavista
Bonavista
Peninsula
ult
HB 600 Ma DP
a
r F uMG
x x
ve
591 Ma
Do
PCvb
x
589 Ma
Avalon LC-s Old Bonaventure
Terrane British Harbour
Connaigre Avalon MG
Peninsula Peninsula
ay
uMG
Bay de Verde
ty B
Burin CPG
Peninsula
i
Trin
Bay
uMG
tion
cep
BAF
Con
Isth
SJG
LHG 605 Ma
mu
MTG x St. John’s
Brigus
s
SJG
Long Harbour
Bay SHG
ne
tu
F or Study area
uMG
MTG Placentia
HMG
Placentia Bay BAF
CG
Gaskiers
Cape St. Mary’s St. Mary’s
Bay
LEGEND
AVALON ZONE
Bimodal, mainly subaerial volcanic rocks (Long Harbour Group)
DEVONIAN AND CARBONIFEROUS INTRUSIVE ROCKS Mainly subaerial volcanic rocks (unseparated Love Cove schist,
Mainly granites; posttectonic relative to latest deformation Bull Arm and Rocky Harbour formations, Musgravetown Group)
NEOPROTEROZOIC TO EARLY ORDOVICIAN STRATIFIED ROCKS Submarine to subaerial volcanic rocks (Harbour Main, Connaigre
Bay, Marystown and Broad Island groups)
Shallow-marine, mainly fine-grained, siliciclastic rocks, including
minor limestone and volcanic rocks Pillow basalt, mafic volcaniclastic rocks, minor siliciclastic rocks,
limestone and chert (Burin Group)
NEOPROTEROZOIC
NEOPROTEROZOIC TO CAMBRIAN INTRUSIVE ROCKS
Fluviatile and shallow-marine siliciclastic rocks (Signal Hill Group,
parts of Musgravetown Group) Mafic intrusions
Granitoid intrusions, including unseparated mafic phases
Marine deltaic siliciclastic rocks (St. John’s Group)
Marine turbidites (Conception and Connecting Point groups) SYMBOLS
Gaskiers Formation, Trinity facies (glacial diamictite; Geological contact, Fault
Conception and Musgravetown groups)
x Town, U–Pb (zircon) age
Figure 1. Simplified geology of the Avalon Terrane in Newfoundland showing the study area, and age constraints for Bull Arm
Formation rocks from the Isthmus, Eastport and Bonavista areas. Modified from ColmanSadd et al, (1990) and incorporat
ing changes suggested by Mills et al. (2020). BAF = Bull Arm Formation; (u)MG = (upper) Musgravetown Group; LCs =
Love Cove schist; LHG = Long Harbour Group; MTG = Marystown Group; CPG = Connecting Point Group; CG =
Conception Group; SJG = St. John’s Group; SHG = Signal Hill Group; HMG = Harbour Main Group; HB = Headland
basalt; DP = Dam Pond basalt; PCvb = Plate Cove volcanic belt (geographic name for Bull Arm Formation volcanic rocks
on Bonavista Peninsula).
74
A.J. MILLS AND H.A.I. SANDEMAN
NW SE
SJG
RF AG HG uMG WG SJG GHF
A PIS BIG
WG uSHG
BA HG
SCG F
G
AG
HMG
SH
Cg BHF
CPG CG HIS
CG
HMG
HMG? Unknown
basement rocks
Triassic dykes
NW SJG SE
B SJG GF GF
BHF
BAF SHG
uMG uMG CG
HIS CG
BAF CG
CG HMG HMG
HMG
CPG
CPG
Figure 2. Previously proposed schematic crosssections through the Avalon Terrane in Newfoundland. A) After King (1988);
B) After Brückner (1977). Glacial diamictite occurs within CG on ‘A’ and as thin blue line within CG and BHF on ‘B’. AG =
Adeyton Group; BIG = Bell Island Group; HG = Harcourt Group; RF = Random Formation; BAF = Bull Arm Formation;
BHF = Big Head Formation; uMG = upper Musgravetown Group; CPG = Connecting Point Group; CG = Conception
Group; Cg = Clarenville granite; GF = Gaskiers Formation; SJG = St. John’s Group; GHF = Gibbett Hill Formation (lower
Signal Hill Group); uSHG = upper Signal Hill Group; HMG = Harbour Main Group; HIS = Holyrood Intrusive Suite; WG
= Wabana Group; PIS = Powder Horn Intrusive Suite; SCG = Swift Current granite.
Formation also comprises rocks of different ages and petro REGIONAL GEOLOGY
chemical characteristics (Mills and Sandeman, 2015; Mills
et al., 2017, 2020). As the upper components of both the The Long Harbour–Placentia area is situated south of
Harbour Main Group and Bull Arm Formation are overlain the Isthmus of Avalon on the west side of the Avalon
by ca. 580 Ma glaciogenic diamictite (Pu et al., 2016), pre Peninsula, or the northern part of the Cape St. Mary’s sub
vious stratigraphic nomenclature and regional correlations peninsula (black box, Figure 1). The area is underlain by
across the Avalon Terrane in Newfoundland (e.g., mainly volcanic and clastic sedimentary rocks of the Bull
McCartney, 1967; King, 1988) must be reevaluated and Arm and Big Head formations, respectively (McCartney,
revised (see Figure 2). 1967; King, 1988). The study area was previously mapped
at ~1:50 000 scale (Argentia map sheet; McCartney, 1956),
This contribution focuses on the lithogeochemistry of compiled with adjacent maps at 1 inch to four miles
the Bull Arm Formation in the Long Harbour–Placentia area (McCartney, 1967), and recompiled at 1:250 000 (King,
(McCartney, 1967; King, 1988) and aims to: 1) establish the 1988). The mainly volcanic Bull Arm Formation is the
chemical character of the volcanic rocks in the study area; 2) stratigraphically lowest unit in the area and includes
discern whether the formation comprises rocks of one or andesite to basalt flows and breccias, intercalated tuff, red
more lithogeochemical signature(s); and 3) to establish the and green arkose, siltstone, slate, conglomerate and minor
relative timing of emplacement of its constituent assem rhyolitic lava flows and breccias (see McCartney, 1956,
blage(s). In addition, field descriptions are included for 1967). It occupies the core of a northeasttrending anticline
diamictites from two localities in the Long Harbour area south of Long Harbour, interpreted by King (1988) to be
(see also Brückner, 1977) and the regional significance of doublyplunging (Figure 3). Only small slivers of the Bull
these rocks is highlighted. This investigation is based upon Arm Formation outcrop north of the Long Harbour inlet.
field and petrographic observations and lithogeochemistry The volcanic rocks are overlain by predominantly fine
for 21 samples from 13 sites (Figure 3). grained clastic sedimentary rocks of the Big Head
Formation, Musgravetown Group (McCartney, 1967; King,
75
CURRENT RESEARCH, REPORT 211
F
54°0'0"W AЄ MHC
F
ЄR ЄR
MHT
F
SYMBOLS MHT MHHD
MHBld
+
MHHD ЄS
Contact (defined, approximate, MHC
assumed).................................................... MHBl HЄO
MHHC
+
Fault (defined, approximate,
MHM
assumed).................................................... MHM
M
MHBld
MHHD
MHBld AЄ
F
Fold axis (anticline, syncline,
+
with direction of plunge)............................... MHBl
M F
r ЄR
rb ou LH
Diamictite..................................................... lDg MHM Ha Mnt A.
ng
+
lDm Lo MHBAb
MHBl ЄR
SAMPLE LOCATIONS/GEOCHEMISTRY KEY
MHM
SHF ЄS
+
Dolerite Assemblage 2
MHM
Assemblage 3 MHBAp
Basalt
Basalt clast Assemblage 1
MHT
Diabase dyke Intermediate rocks MHBu ЄR
ЄS
Basalt Mafic rocks
)
" MHBAb ЄS
ЄR
lDg
ur AЄ
a rbo
i pH MHBl
AЄ
47°20'0"N
Sh MHHD
ЄR
47°20'0"N (
!
F
M
Fox Harbour MHBld AЄ
MHBl MHBAt ЄS
MHHD
F
Argentia
³
p
BA
MHHC
MH
+
$ * ЄR
#
AЄ
MHBAb
MHM
Placentia
0 2 4 6 8 MHBAb
km
MHBl MHBAa 53°45'0"W
54°0'0"W
LEGEND
SPREAD EAGLE GABBRO (and equivalents) HEART’S CONTENT FORMATION
MHHC Grey to black shale containing wispy sandstone laminae
Є Diabase and gabbro (may feed Cambrian volcanic rocks)
S
HARCOURT GROUP MATURIN PONDS FORMATION
Red sandstone and mudstone exhibiting distinctive wavy bedding;
HЄO Mainly black shale MHM
minor conglomerate
ADEYTON GROUP
BIG HEAD FORMATION
AЄ Mainly green and red shale, minor limestone MHBu Grey to red arkose and granule conglomerate
RANDOM FORMATION
Wavy bedded, grey to green tuffaceous siltstone and arkose; locally includes
White orthoquartzite interbedded with green, grey and red arkose and MHBl
ЄR Whiteway Member consisting of red sandstone and siltstone
siltstone; local basal conglomerate
MUSGRAVETOWN GROUP MHBld Green-grey to maroon volcanogenic mixtite or diamictite
CROWN HILL FORMATION
Red pebble conglomerate and sandstone; locally, red siltstone at base; BULL ARM FORMATION
MHC MHBAa Felsic flows and tuffs, and clastic sedimentary rocks
minor green conglomerate
TRINNY COVE FORMATION MHBAb Mafic flows; includes minor felsic flows and clastic sedimentary rocks
Olive-green and red sandstone, siltstone and conglomerate; minor grey
MHT
shale containing wispy sandstone laminae MHBAt Predominantly crystal and lithic tuffs, commonly reworked
HEART’S DESIRE FORMATION
MHBAp Mafic to felsic variegated flows, and pyroclastic and clastic sedimentary rocks
MHHD Olive-green sandstone
Figure 3. Geology of the Long Harbour–Placentia area showing the sample locations. Modified from King (1988). Key: Mnt
A. = Mount Arlington Heights; LH = community of Long Harbour; SHF = Ship Harbour fault.
76
A.J. MILLS AND H.A.I. SANDEMAN
1988). A distinctive diamictite unit occurs roughly in the In the Eastport area, earliest magmatism is calcalkaline
middle of the Big Head Formation and is exposed on the and includes felsic to intermediate tuff of the ca. 620 Ma
east and west limbs of a syncline, at the communities of Broad Island Group and younger mafic flows, dykes and
Long Harbour and Mount Arlington Heights, respectively schist of the Cannings Cove Formation (Mills et al., 2020).
(Brückner, 1977; MHBId unit of King, 1988). The Big Head The Love Cove schist (Widmer, 1949; see also Dec et al.,
Formation is overlain by redbeds of the Maturin Ponds 1992) is 589 ± 2 Ma at its type locality (see Figure 1 for
Formation (McCartney, 1967; King, 1988). location), and these chemically transitional pyroclastic rocks
are interpreted to be strongly tectonized correlatives of the
AGE AND CHEMICAL CONSTRAINTS Bull Arm Formation (Mills et al., 2020). Alkaline rocks in
ON THE BULL ARM FORMATION the Eastport area, including the 568.7 ± 1.4 Ma Wolf Island
rhyolite, are part of the younger Rocky Harbour Formation
No age constraints are, as of yet, available for rocks of (O’Brien and King, 2004; Mills et al., 2020).
the Bull Arm Formation on the western Avalon Peninsula. In
In summary, Musgravetown Group magmatism on
the Isthmus of Avalon, and Bonavista and Eastport peninsu
northwestern Avalon Terrane in Newfoundland includes ca.
las, U–Pb (zircon) geochronological and lithogeochemical
600 Ma and older calcalkaline magmatism, ca. 589 Ma and
studies have shown a timeprogressive change in the chem
older tholeiitic and transitional magmatism (Bull Arm
istry of magmatic Musgravetown Group rocks, formerly all
Formation), and ca. 580–569 Ma alkaline magmatism
included in the Bull Arm Formation. On the Isthmus of
(Rocky Harbour Formation).
Avalon, 20 km south of the type locality of Bull Arm (Figure
1), the banded rhyolite having transitional (between calcalka
line and alkaline) chemistry has been dated at ca. 605 Ma, is
PREVIOUS FIELD DESCRIPTIONS OF
considered basal Musgravetown Group, and is overlain by DIAMICTITE WITHIN THE BIG HEAD
tholeiitic and alkaline basalts (Mills et al., 2017, 2020). FORMATION, LONG HARBOUR–
MOUNT ARLINGTON HEIGHTS AREA
On the Bonavista Peninsula, ca. 600 Ma basal
Musgravetown Group cobble to boulderconglomerate Tillite was reported in the Big Head Formation at Long
(Cannings Cove Formation; Jenness, 1963) and interbedded Harbour and Mount Arlington Heights by Brückner (1977).
calcalkaline basalt (Headland basalt of Mills and Whereas ‘tillite’ is today defined as “unsorted sediment
Sandeman, 2015) unconformably overlie ca. 605 Ma shal deposited directly from glacier ice” (Hambrey and Glasser,
lowmarine rocks of the upper Connecting Point Group 2012), its connotation in 1977 more loosely included “rocks
(Mills et al., 2016). Bull Arm Formation rocks outcrop far of glacial origin” (Brückner, 1977). Big Head, on the north
ther east (Figure 1), in the internally deformed and steeply shore of the inlet of Long Harbour, is the type area for the
eastdipping Plate Cove volcanic belt, which broadly sepa Big Head Formation and is divided by McCartney (1967)
rates the Connecting Point Group (620–605 Ma: Mills et al., into three subdivisions: lower “red conglomerate and arkose
2020) to the west from Musgravetown Group rocks to the beds” (interpreted by Brückner (1977) as pyroclastic
east (e.g., Jenness, 1963; Mills, 2014). The chemically tran deposits ranging from breccias to graded tuffs); medial grey
sitional (tholeiitic to weakly calcalkaline) basalts of the belt green siliceous siltstone, chert and minor arkose; and upper,
are interpreted as continental tholeiites that erupted in a ten green to locally pinkish arkose. The distinct diamictite beds
sional or transtensional setting (Mills and Sandeman, 2015). occur within McCartney’s (1967) middle division. Brückner
The transitional chemistry of Bull Arm Formation magma (1977) estimates the diamictite (his “tillite bed”) thickness
tism marks a change from early arclike magmatism to later to be 15–20 m at Long Harbour and ~10 m at Mount
extensionrelated magmatism (Mills et al., 2020). Although Arlington Heights. The lower contact is not exposed at
the basalts from the Plate Cove volcanic belt have not been either location. The upper contact is exposed only at the
dated, felsic tuffaceous rocks from the west and east side of Mount Arlington site, where it grades into bedded chert
the belt yielded ages of 592 ± 2.2 Ma and 591.3 ± 1.6 Ma, without clasts (Brückner, 1977). Clasts in the diamictite are
respectively (Mills et al., 2017). The basalts are interpreted unsorted, range from angular to rounded, 5–12 cm in diam
to be similar in age to the dated tuffs. Alkaline basalts in the eter, and mainly consist of mafic or felsic volcanic litholo
northeastern (Dam Pond) and southwestern (British gies with lesser fragments of vein quartz, arkose and silt
Harbour) parts of the Bonavista area (Figure 1) occur below stone. Brückner (1977) suggested that the volcanic clasts in
and above, respectively, the Trinity facies (Normore, 2011), the diamictite were all locally derived from rocks of the
a glacial diamictite unit dated at 580 Ma near Old underlying Bull Arm Formation. Although he did not dis
Bonaventure and correlative to the 580Ma Gaskiers cern glacial striae on the clast surfaces, he asserts that the
Formation (Williams and King, 1979; Pu et al., 2016). poor sorting of both clasts and matrix material, the range of
77
CURRENT RESEARCH, REPORT 211
clast lithology, shape and size, and the general appearance of cm or are truncated by overlying layers (Plate 1A). The
the Long Harbour diamictite, which “so closely resemble nature, thickness and lateral continuity of these beds/lami
those of the Gaskiers tillite”, are sufficient criteria upon nae vary along strike; planar, wavy, lenticular and slumped
which to interpret the former as “genuine tillite”. beds or laminae all occur within 10s of metres along strike.
A disconformity (Plate 1B) marked by pebble to boulder,
NEW FIELD OBSERVATIONS OF porphyritic and/or amygdaloidal basalt clasts (pebble lag or
lapilli/bombs?) occurs close to the transition between under
DIAMICTITE WITHIN THE BIG HEAD
lying maroon mudstone–grey sandstone facies and overly
FORMATION, LONG HARBOUR– ing greengrey laminated mudstone ± sandstone facies
MOUNT ARLINGTON HEIGHTS AREA (Plate 1C). The overlying greengrey mudstone ± sandstone
facies (Plate 2) is highly siliceous and is characterized by
Whereas most of Brückner’s (1977) observations come submmscale rhythmic laminations in the mudstone (Plate
from the hilltop at Mount Arlington (immediately north and 3A). Compared to the underlying maroon facies, sandstone
east of the Mount Arlington diamictite location on Figure 3), beds in the upper facies are thicker (dmscale), have more
new observations documented here come from the rockcut pebbles and contain more outsized clasts. Outsized clasts
along the north side of the road at the base of the same hill. comprise 1–3% of the greengrey laminated mudstone and
The lowermost strata there comprises sandy mudstone with up to 15% of the sandstone. Compositionally, the clasts
A.J. MILLS AND H.A.I. SANDEMAN
A B
Plate 2. A) Greengrey laminated mudstone–sandstone facies at Mount Arlington Heights having subangular to subrounded
clasts of mudstone, siltstone, sandstone, basalt and rhyolite/felsic tuff, and showing B) sheath fold in laminated greengrey
mudstone (red dashed arrow shows antiformal geometry of the fold).
A B
fc
1 cm
1 cm
Plate 3. Closeup photographs of greengrey laminated mudstone–sandstone facies from Mount Arlington Heights. A) Finely
laminated, greengrey siliceous mudstone–sandstone showing truncated layers (truncating layers shown as dashed yellow
lines), folded and slumped laminae (dashed red lines), sandstone dykes and sills (dotted yellow lines), weakly developed shear
bands (orange dashed lines), and an outsized (>1 cm) faceted clast (fc); B) Finely laminated greengrey mudstone scoured
and truncated by overlying pebbly sandstone; note ~9 cm long sandstone dyke.
(Plate 4A), faceted (Plate 3A) and bulletshaped (Plate 4A) to clastrich, intermediate diamictite (poorly sorted, matrix
clasts (Boulton, 1978) are common. Beddingconfined supported conglomerate having a matrix with a sandtomud
slumps, convolutions, sheath folds and possible glaciodislo ratio between 33–66%; Hambrey and Glasser, 2012).
cations (Chumakov, 2015) are also common in the laminat Diffuse, palegreen bands were noted (Plate 4B). The pale
ed mudstone (Plates 2B and 3A). bands could result from a compositional difference and, if
so, may indicate that the rock is crudely stratified. However,
A few small (
CURRENT RESEARCH, REPORT 211
analysis, including four samples collected in 2017 by the
A second author. Brief petrographic descriptions are presented
in Table 1. All samples, with one exception, were collected
from rocks mapped as Bull Arm Formation (McCartney,
1956, 1967). The single exception is an amydaloidal basalt
boulder sampled from the diamictite unit within the overly
ing Big Head Formation (Plate 1C) at Mount Arlington
Heights. The Bull Arm Formation is generally poorly
exposed over an elongate, northnortheasttrending, 5–6 km
wide by 16 km long, antiformal structure cored by mainly
basaltic rocks (Figure 3). The samples include massive
basalt flows, pyroclastic rocks including scoriaceous basalt
and a range of tuffaceous rocks, the amygdaloidal basalt
boulder from the base of the diamictite at Mount Arlington
B Heights, two basaltic dykes cutting massive basalt, and a
dolerite that lacks exposed contacts.
The subdivision of the rocks exposed in the Bull Arm
Formation antiform between the communities of Long
Harbour and Placentia (Figure 3) based on lithofacies alone
is problematic owing to the variability and heterogeneity of
the formation’s constituent lithologies. Therefore, an infor
mal subdivision of Bull Arm Formation strata into three
assemblages is proposed based on petrology and lithogeo
chemistry.
Assemblage 1 includes green to brickred fragmental
basaltic rocks or breccias (Plate 5A), and crystal lithic, crys
tal ash (Plate 5B), and lapilli tuffs. Assemblage 2 includes
Plate 4. Further examples of possible periglacial deposits mainly green, locally hematitized to reddish, amygdaloidal,
from the study area. A) Finely laminated greengrey plagioclase and clinopyroxenephyric basalt flows (Plate
siliceous mudstone–sandstone facies from Mount Arlington 5C). Assemblage 3 includes darkgrey to greengrey amyg
showing outsized clasts with rounded (red arrow), flatiron daloidal basalt, greengrey to red fragmental basalt interbed
(yellow arrow, dotted line), and bulletshaped (orange ded with basaltclastbearing red mudstone (Plate 5D) and
arrow, dotted line) morphologies; B) Clastpoor intermedi narrow (30–200 cm wide), northwesttrending, subvertical
ate diamictite with cobblesized clasts of basalt; yellow basalt dykes that cut massive basalt of Assemblage 2 (Plate
arrow indicates the diffuse palegreen banding common in 5E). Igneous rocks of all three assemblages are interbedded
outcrops in the vicinity of the community of Long Harbour. with sandstone, siltstone, mudstone and pebble conglomer
ate with no clear lithological association distinctive to any
greengrey sandstone, siltstone, and rare quartzite clasts particular volcanic assemblage. Variably hematitized frag
were also noted. The thickness of the diamictite at Long mental basalt occurs within assemblages 1 and 3. Rocks of
Harbour was not ascertained, but Brückner’s (1977) esti Assemblage 2 are massive, not fragmental, but reddening/
mate of 15–20 m is consistent with field observations. A hematitization was noted locally.
variably developed, subvertical cleavage is locally accentu
ated by flattened black lithic fragments. In thin section, polycrystalline chlorite appears to have
completely replaced possible mafic volcanic fragments in
intermediate lithic ash tuff of Assemblage 1 (Plate 6A).
FIELD AND PETROGRAPHIC
Fragmental basaltic rocks of Assemblage 1 contain highly
OBSERVATIONS OF THE BULL ARM vesicular to scoriaceous fragments in a glassy groundmass.
FORMATION, LONG HARBOUR– These rocks contain abundant plagioclase; no primary mafic
PLACENTIA AREA minerals were identified in thin section. Basalts of
Assemblage 2 are commonly clinopyroxenebearing, exhib
Twentyone samples were collected from the Long it variably developed trachytic textures, and contain rare
Harbour–Placentia area (Figure 3) for lithogeochemical sievetextured plagioclase phenocrysts (Plate 6B). The
80
Table 1. Location data and field and petrographic descriptions of samples from the Long Harbour–Placentia area
Lab# Sample# RockType UTMEast UTMNorth Zone Datum Field Description Petrographic Description
10740338 19AM012A01 basalt 273858 5239741 22 NAD 27 Dolerite from Argentia; hypabyssal texture, possible Gabbro – Pl + Cpx; ophitic texture – Pl laths completely embedded in
dyke or sill; margins not exposed Cpx; well preserved/fresh; minor interstitial Chl
Assemblage 3
10740328 19AM006B01 basalt 276705 5238564 22 NAD 27 30–50 cm thick diabase dyke crosscuts basalt flows; Tiny, equant Cpx in groundmass + Pl + opaque; 2 mm Chl blebs =
trends 320/70 possible Cpx pseudomorphs
10740329 19AM006B02 basalt 276705 5238564 22 NAD 27 1.5–2 m thick dyke cuts locally epidotized basalt; Similar to 19AM006B01 but more strongly altered; possible prehnite
trends 310/80 prehnite associated with Chl+Cal
10740324 19AM001A01 clast 284067 5256514 22 NAD 27 Amygdaloidal basalt boulder in diamictite; ~20 cm Altered Pl, fine matrix; trace Ti; amygdales filled mainly with Qtz, Cal
diameter
10740337 19AM011A01 basalt 279603 5247991 22 NAD 27 Amygdaloidal basalt flow at Ship Harbour; contacts Rare 1–2 mm Cpx phenocrysts; common 2+ mm amygdales filled with
not exposed; massive/structureless Chl+ Cal; possible Ep after Cpx pseudomorphs (1–2 mm)
10740346 19AM018A01 mafic 278931 5238962 22 NAD 27 Subrounded to angular volcanic fragments in fine Mafic volcanic fragments in glassy groundmass; minor carbonate blebs
agglomerate grained, brickred matrix and veinlets
10740347 19AM018A02 mafic 278931 5238962 22 NAD 27 Greygreen basalt bomb in red mudstone; west Altered, subanhedral Pl phenocrysts; black, veryfinegrained needles
agglomerate outcrop (rutile?); carbonate and black (glass?) infilling veinlets
10740348 19AM018A03 mafic 278931 5238962 22 NAD 27 Greygreen basalt bomb in red mudstone; east Trachytic textured basalt; foamy/bubbly textured fragments and
outcrop spherical opaques = volcanic glass (?)
Assemblage 2
8941358 HS17091C massive 281410 5446712 22 NAD 27 Dark green, mediumgrained, locally pebbly sand Weak flow alignment of subhedral Pl; Chl interstitial to Pl, possibly
basalt stone with
CURRENT RESEARCH, REPORT 211 Plate 5. Field photographs of igneous rocks sampled from the study area. A) Amygdaloidal basalt clast in basaltic fragmen tal rocks (possible flowtop breccia; HS17092A; Assemblage 1 mafic volcanic rock); B) Potassium feldsparbearing lithic tuff (HS17093; Assemblage 1 intermediate tuff); C) Weakly epidotized and hematitized massive basalt (HS17094; Assemblage 2); D) Basalt breccia interbedded with red mudstone (19AM018A; Assemblage 3); E) 30–50 cm thick alkaline dyke (trend: 320/70) with distinct chill margins cuts patchily epidotized massive tholeiitic basalt (19AM006B01 of Assemblage 3, and 19AM006A01 of Assemblage 2, respectively), view to the north; F) Cut slab of dolerite from Argentia showing radial plagio clase glomerocrysts (19AM012A). 82
A.J. MILLS AND H.A.I. SANDEMAN
A B
Ep
Chl
Pl
19AM005 19AM007A
C D
Pl
Ep Cpx
Chl
Cbt
Ti
19AM011 19AM006B
E F
Cpx Cpx
Chl Pl Chl Pl
Cpx Cpx
19AM012 19AM012
Plate 6. Photomicrographs of rocks sampled from the study area. A) Polycrystalline chlorite (Chl) bleb interpreted to have replaced a mafic
volcanic fragment in lithic ash tuff; the red dotted lines highlight primary layering (19AM005; Assemblage 1 intermediate tuff); B) Sieve
textured, subhedral plagioclase phenocryst (Pl) and epidote (Ep) crystals (late with respect to crystallization of the rock; possibly filling
amygdales) in finegrained, weakly trachytic, plagioclaserich groundmass (19AM007A; Assemblage 2); C) Amygdaloidal basalt from Ship
Cove showing probable primary clinopyroxene crystals entirely pseudomorphed by epidote and carbonate (Cbt) (19AM011A01; Assemblage
3); D) Titanite (Ti) and clinopyroxene microphenocryst (Cpx) which is partially replaced by chlorite in alkaline dyke (19AM006B01;
Assemblage 3); E, F) Dolerite from Argentia showing ophitic texture, best viewed under crosspolars where optical continuity of the clinopy
roxene oikocryst highlights the size of the crystal; viewed in crosspolar and planepolar light, respectively.
83CURRENT RESEARCH, REPORT 211
Assemblage 3 basalt from Ship Harbour (Figure 3) contains ceptible to most fluid–rock alterations (e.g., Pearce and
subhedral, 2–3 mm clinopyroxene phenocrysts, pseudomor Cann, 1973; Wood et al., 1980; Middelburg et al., 1988) are
phed by polycrystalline epidote and plagioclase, and 2–5 emphasized and more heavily relied upon for petrologic and
mm amygdales filled with mainly chlorite and/or carbonate tectonic interpretations.
(Plate 6C), set in a trachytic textured finegrained ground
mass. Assemblage 3 basalt dykes contain clinopyroxene ASSEMBLAGE 1: MAFIC ROCKS
crystals partially replaced by chlorite, strongly altered (saus
suritized) plagioclase, minor magnetite and titanite (Plate Assemblage 1 includes two subsets. The mafic subset
6D). Large (up to 1 cm) amygdales are most common in includes three basalt breccias. These are basalt, basaltic
assemblages 2 and 3, although fragments in Assemblage 1 andesite and trachyandesite (Le Maitre et al., 1989; Figure
rocks are highly vesicular or scoriaceous. Basalts of assem 4B) based on their majorelement compositions, and basalt
blages 1 and 3 both have variably glassy groundmasses and based on their traceelement abundances (Pearce, 1996;
plagioclase microlites that indicate quenching. Trachytic Figure 4C). These rocks exhibit a wide range in majorele
textures were observed locally in all three assemblages. ment contents: e.g., SiO2 (42–53%), Mg# (40–63), CaO
(3–8.9%). No iron enrichment trend is discernible within
A dolerite sampled at Argentia has unexposed contacts; this small sample population (n=3). Traceelement contents
a cut slab shows glomerocrystic plagioclase crystals radiat also vary widely: e.g., Zr (73–133 ppm), Cr (77–126 ppm),
ing from a central black mass (Plate 5F). In thin section, the Ba (230–3069 ppm). The multielement patterns of this sub
rock exhibits well developed ophitic to subophitic texture, set are characterized by moderately steep slopes (mean La/
with elongate prismatic to acicular plagioclase chadacrysts YbCN = 4.5; Table 2), prominent Nb troughs (mean La/NbCN
partially to completely embedded within 2–8 mm clinopy 5.2; Th/NbCN 8.9), and slightly negatively anomalous
roxene oikocrysts (Plate 6E, F). Glomerocrystic plagioclase HSFEs (Figure 5). Relative to basaltic rocks of Assemblages
crystals are larger than the chadacrysts (2–5 mm vs.A.J. MILLS AND H.A.I. SANDEMAN
Figure 4. Rock classification diagrams showing the compositions of rocks from the Long Harbour–Placentia area. A)
Alteration boxplot (Large et al., 2001); B) Based on major element content – silica vs. total alkalis silica diagram normal
ized to 100% water free (after Le Maitre et al., 1989); C) Based on trace element content (after Pearce, 1996). Fields corre
spond to mafic rock assemblages from the Bonavista Peninsula (Mills and Sandeman, 2015). HB = calcalkaline Headland
basalt; PCvb2 = continental tholeiite series 2 of the Plate Cove volcanic belt; PCvb1 = continental tholeiite series 1 of the
Plate Cove volcanic belt; DP = Dam Pond alkaline basalt; Br Hr = British Harbour alkaline basalt.
line (Figure 6A, B) and, as expected for silicic rocks, plot multielement patterns [mean (La/Yb)CN = 3.3] and less pro
farther off the mantle array relative to the mafic rocks of nounced Nb troughs [average (La/Nb)CN = 2.7; average
Assemblage 1 (Figure 6C, D). (Th/Nb)CN = 4.7] than those of Assemblage 1 (Figure 5;
Table 2). With one exception, these rocks lack Ti troughs, in
ASSEMBLAGE 2: THOLEIITES contrast to the mafic rocks of Assemblage 1. The sample
with a Ti trough also has considerably higher Th and lower
Rocks of Assemblage 2 are basaltic trachyandesite P relative to the other five samples. This sample is more
(n=3), basalt (n=1), trachybasalt (n=1) and trachyandesite highly fractured, with epidote and chlorite veinlets, and
(n=1) in terms of their majorelement compositions (Figure patchy carbonate and chlorite dispersed throughout its
4B). They are all basaltic based on their traceelement com groundmass, and is therefore more highly altered. Despite
positions, with the exception of one andesite to basaltic their moderate Nb troughs, consistent with an apparent calc
andesite (Figure 4C). Assemblage 2 basalts have flatter alkaline affinity (Figures 5 and 6A), these rocks are transi
85CURRENT RESEARCH, REPORT 211
tional (Figure 6B), as indicated by their flatter multielement ARGENTIA DOLERITE
patterns (Figure 5) and typically subdued Nb troughs [lower
(La/Nb)CN and (Th/Nb)CN relative to Assemblage 1 rocks As the margins of the Argentia dolerite are not exposed,
(Table 2)]. In TiO2/Yb vs. Nb/Yb space (Pearce, 2008), they its host rock could not be established in the field, and there
plot on the EMORB side of the mantle array (Figure 6C) fore its relative timing of emplacement cannot be ascer
and, like the Assemblage 1 basalts, Assemblage 2 basalts tained. Chemically, it is classified as trachybasalt based on
plot above, albeit closer to the mantle array in Th/Yb vs. its majorelemental composition, and as basalt based on its
Nb/Yb space (Figure 6D). traceelement content (Figure 4B, C). Its multielement pat
tern is flatter than that of any other rock from the study area
ASSEMBLAGE 3: ALKALINE BASALTS (La/Yb)CN = 2.2, showing minor LREE enrichment and no
notable negative anomalies in the HFSEs (Figure 5). The
Assemblage 3 basalts are basalt, trachybasalt, basaltic dolerite is EMORBlike, and plots close to EMORB on most
trachyandesite and trachyandesite based on their major discrimination diagrams (Figure 6A, C, D).
element compositions (Figure 4B), and alkali basalt based
on their trace elements (Figure 4C). These basalts have the DISCUSSION
steepest multielement slopes (mean (La/Yb)CN = 10.9) of
the three basaltic assemblages (Figure 5). They also have the INTERPRETATION AND IMPLICATIONS OF
highest Mg# and V, Cr and Ni contents of all three assem DIAMICTITES IN THE LONG HARBOUR AREA
blages (Table 2). Their steep multielement patterns lack Nb
and Ti troughs, and exhibit very minor dips in Hf and Zr Brückner’s (1977) tillite reportedly outcrops farther
(Figure 5). In La–Y–Nb space (Figure 6A), these rocks upsection (at the top of the hill) relative to the diamictites
straddle the continental and intercontinental rift (alkaline) examined in the current investigation. The diamictites of
fields. In both TiO2/Yb vs. Nb/Yb and Th/Yb vs. Nb/Yb the Big Head Formation, described here, may have origi
space, Assemblage 3 rocks plot close to OIB along the man nated in a periglacial environment (see below). However,
tle array (Figure 6C, D). nonglacial depositional processes can also produce unsort
ed sediments that resemble glacial products
Table 2. Summary of salient lithogeochemical features of volcanic assem (Carto and Eyles, 2012a, b). For example,
blages from the Long Harbour–Placentia area King (1990) interpreted a mixtite unit with
in the Conception Group on northeastern
Assemblage 1 Assemblage 2 Assemblage 3 Avalon Peninsula to be volcanogenic in ori
Mafics Intermediates gin, despite similarities in lithology and
stratigraphic position that led Hsu (1978) to
SiO2 4253 4865 4758 4349 interpret the same mixtite unit as a “tilloid”
(La/Yb)CN 4.5 6.9 3.3 10.9 correlative to the Gaskiers Formation on
(La/Sm)CN 1.9 2.5 1.7 2.7 southern Avalon Peninsula. Volcanogenic
(Gd/Yb)CN 1.7 1.7 1.6 2.6 deposits, however, should compositionally
(Th/La)CN 1.8 2.8 1.6 0.9 reflect the volcanic source, resulting in a
(La/Nb)CN 5.2 4.0 2.7 0.8 largely monolithic clast composition. At
(Th/Nb)CN 8.9 10.4 4.7 0.7 Mount Arlington, the basalt clastrich hori
Eu/Eu* Range .941.03 .79.86 0.781.89 .881.11 zon lying between maroon mudstone–grey
Th 2.33.7 7.412.3 1.07 2.43.9 sandstone facies and overlying grey lami
Sr 287381 371406 161363 247652 nated mudstone ± sandstone facies is large
Zr 73133 162224 72102 134301 ly monolithic and possibly volcanogenic.
ΣREE 71126 132265 6190 117234 The majority of all Mount Arlington
Mg# 4063 1043 4858 5163 Heights and Long Harbour diamictites,
TiO2 1.11.5 0.50.9 0.71.8 2.13.0 however, are polylithic, reflecting mixed
V 106282 48137 173351 227361 provenance, which is more consistent with a
Cr 77126 622 82155 2405 glacial origin (Arnaud and Etienne, 2011).
Ni 2490 828 3283 15199 As is true for the Trinity facies on Bonavista
Sc 3245 1023 2841 1328 Peninsula, the Long Harbour diamictites are
commonly volcaniclastic, as a high propor
Note: Mg# = [molecular MgO/(MgO + FeOT)]*100; subscript CN = chon tion of the contained clasts are volcanic, but
drite normalized; ƩREE = sum of rare earth elements. All values cited are the siliciclastic clasts ranging from chert to
mean for each assemblage sandstone are also represented indicating a
86A.J. MILLS AND H.A.I. SANDEMAN
100 Assemblage 3 100
Assemblage 1 basalts basalt
diabase dyke
tillite boulder
10 10
British Harbour basalt field
Headland basalt field
Dam Pond basalt field
1 1
100 Assemblage 1 tuffs Argentia dolerite 100
10 10
1 1
Nb Ce P Zr Sm Gd Tb Ho Yb
Th La Pr Nd Hf Eu Ti Dy Y Lu
100 Assemblage 2 basalts
Figure 5. Multielement diagrams (normalized to primitive
mantle; normalizing values from Sun and McDonough,
1989) for samples from the study area. Also shown, for com
10 parison, are fields (as per Figure 4) for the multielement
patterns for mafic rocks from the Bonavista area, discussed
in the text.
Plate Cove volcanic belt basalt field
1
Nb Ce P Zr Sm Gd Tb Ho Yb
Th La Pr Nd Hf Eu Ti Dy Y Lu
broad provenance that is not restricted to local volcanic phologies are consistent with dropstone rainout from a raft
sources. Bent and ruptured laminae (e.g., Plate 4A) are con edice source (e.g., Benn and Evans, 1998; Arnaud and
sistent with a dropstone interpretation. Although projectile Etienne, 2011).
volcanic detritus can result in the deflection and penetration
of laminae (Bennett et al., 1996), none of the observed Mass transport processes, such as deep marine gravity
clasts resemble the fusiform, spindle, or almond shape flows (particularly at extensional margins), have been inter
characteristic of pyroclastic materials. Instead, clast shapes preted as the depositional mechanism for diamictite deposits
range from rounded to angular, including faceted, bullet that were previously interpreted to be glacial (e.g., King,
shaped and flatiron clasts (Plate 4A). These clast mor 1990; Carto and Eyles, 2012a, b; Arnaud and Eyles, 2002).
87CURRENT RESEARCH, REPORT 211
Y/15
A
B
NMORB 10
* Back arc basin CA
Dolerite
Fields
VAT Assemblage 3
* Tillite boulder Br Hr
Diabase dyke
DP
Trans. 1
Th/Yb
Basalt
Assemblage 2 PCvb1
EMORB Basalt
PCvb2 TH
Assemblage 1 .1
Tuff HB
Alkaline
Basalt
Calc-alkali Cont. Intercontinental
rifts .01
La/10 Nb/8 1 Zr/Y 10
10 10
C D deep crustal
recycling
rray
rca OIB
ni ca
a
volc
Th Alk 1
deep melting
OIB
Th/Yb
TiO 2 /Yb
1 EMORB
shallow melting magma-crust
EMORB interaction .1
NMORB
NMORB
.1 .01
.1 1 10 100 .1 1 10 100
Nb/Yb Nb/Yb
Figure 6. Tectonic discrimination diagrams for samples from the study area. A) Ternary plot of La–Nb–Y (after Cabanis and
Lécolle, 1989); B) Logarithmic plot of Th/Yb vs. Zr/Y (after Ross and Bédard, 2009); C) Melting depth proxy, TiO2/Yb vs.
Nb/Yb (after Pearce, 2008); D) Crustal input proxy, Th/Yb vs. Nb/Yb (after Pearce, 2008).
Matrixsupported, unsorted conglomerates, clastic dykes, tions, which are interpreted to form as fragments of semi
and slump folds ranging from recumbent to sheathlike, are consolidated (or frozen) underlying strata were rotated or
not unique to glacial deposits, and are common in debrites plucked by the deforming overlying sediment (Chumakov,
or olistostromes (e.g., Tohver et al., 2018). It is therefore 2015). Similar features can also form in a slope environ
possible that some of the facies described from the study ment, and are therefore not diagnostic of a glacial setting.
area may have been redeposited by masstransport process The faceted, bulletshaped and flatiron clasts, the deforma
es, but the faceted, bulletshaped and flatiron clast mor tion and local penetration of lamellae by outsized clasts,
phologies are nevertheless consistent with a glacial origin interpreted as dropstones, and lithological and stratigraphic
that could have been affected by nominal subsequent trans similarities between diamictites in the Big Head Formation
port, permitting preservation of these distinctive clast at Long Harbour, and those of the Trinity facies rocks on
shapes. The truncation of mudstone laminae by overlying Bonavista (see Plate 7 of Normore, 2011), lead the authors
planar laminated mudstone results in common dislocation to concur with Brückner’s (1977) interpretation of a glacio
surfaces (e.g., Plate 3A) that closely resemble glaciodisloca genic origin for these rocks. Correlation with the ca. 580 Ma
88A.J. MILLS AND H.A.I. SANDEMAN
Trinity and Gaskiers diamictites is proposed but remains to Harbour, on the north side of the southwest projection of the
be unequivocably established. Ship Harbour fault (Figure 3). One Assemblage 1 tuff from
northeast of Placentia overlies tholeiitic flows of
REGIONAL STRATIGRAPHIC IMPLICATIONS OF Assemblage 2. However, this sample is a crystal lithic tuff
DIAMICTITES IN THE LONG HARBOUR AREA that contains siltstone and other lithic fragments (Table 1),
so that its wholerock chemistry is a mix of pyroclastic and
Whether the diamictites in the study area were deposit epiclastic material and not representative of the magma
ed directly by glacial/deglacial processes or were (re)de source. This is also likely the case for the other tuffs of
posited by massflow processes, their lithology and facies Assemblage 1. Based on Pearce’s (2008) use of Ti–Yb ratio
associations bear striking resemblance to diamictites of the as a proxy for melting depth, the calcalkaline Assemblage
glacial Trinity facies on Bonavista Peninsula (Normore, 1 basalts were likely derived from an undepleted, shallow
2011; Pu et al., 2016). If these glaciogenic units are correla asthenospheric source (Figure 6C). Their relatively high
tive, then King’s (1988) designation as Big Head Formation, Th/Yb ratios, a proxy for subduction zone influence or
for siliciclastic rocks stratigraphically above the Gibbett Hill crustal input (Pearce, 2008), indicates their mantle source
Formation (Signal Hill Group) on the Bay de Verde sub was modified by suprasubduction zonederived fluids/
peninsula, becomes problematic (see Figure 2). Although magma (Figure 6D). Assemblage 1 basalts are similar to the
there are no age constraints on the Gibbett Hill Formation, calcalkaline basalts from the Bonavista area, albeit less
the Signal Hill Group is interpreted to conformably and gra LREE and Thenriched (Figure 6A, B, D).
dationally overlie the St. John’s Group (King, 1990), which
in turn conformably overlies the Conception Group. U–Pb Assemblage 2 basalts are chemically similar to one of
(zircon) age constraints from southern Avalon Peninsula the two tholeiitic basalt series (PCvb2 of Mills and
include 565 ± 3 Ma (Benus, 1988), 566.25 ± 0.35 Ma Sandeman, 2015) identified within the Plate Cove volcanic
(Canfield et al., 2020), and 565.00 ± 0.16 Ma (Matthews et belt on the Bonavista Peninsula (see Figures 5 and 6).
al., 2020) for the Mistaken Point Formation (upper Basalts of the Plate Cove volcanic belt are intercalated with
Conception Group) and 562.5 ± 1.1 Ma for lower St. John’s silicic tuffaceous rocks locally dated at ca. 592 and 591 Ma
Formation rocks (Canfield et al., 2020). The Gibbett Hill (Mills et al., 2016), providing strong evidence that tholeiitic
Formation is therefore expected to be younger than ca. 562 volcanism followed calcalkaline volcanism in the
Ma. If the Big Head Formation diamictites at Long Harbour Bonavista area. According to Pearce’s (2008) Ti–Yb proxy
are correlative to the Trinity facies and Gaskiers Formation, for melting depth, Assemblage 2 basalts were derived from
then either the rocks on Bay de Verde subpeninsula a weakly enriched, slightly deeper source than that of
assigned to Gibbett Hill Formation are not correlative to Assemblage 1 rocks (Figure 6C). Based on their lower
those on eastern Avalon Peninsula, or rocks of the Big Head Th/Yb ratios relative to Assemblage 1 mafic rocks, the
Formation unit on Bay de Verde are not correlative to the source of Assemblage 2 rocks had either a reduced supra
unit defined at its type locality at Long Harbour (see Figure subduction zone metasomatic modification, and/or involved
2). Possibly, significant thrust emplacement, hitherto unrec less lithospheric recycling in the petrogenesis of its parental
ognized, could be invoked to place the older Big Head magma (Figure 6D). Based on our limited sampling,
Formation rocks above the younger Gibbett Hill Formation Assemblage 2 basalts are apparently spatially restricted to
rocks at Bay de Verde. As such structural imbrication is the Bull Arm Formation anticline south of the Ship Harbour
unlikely to be restricted to the interface between these two fault (Figure 3).
units, this interpretation is considered unlikely. It is more
probable that the lithostratigraphic designation of either Assemblage 3 basalts are alkaline (Figures 4B, 5 and 6)
Gibbett Hill Formation or Big Head Formation on the Cape and occur near the top of the Bull Arm Formation in the
de Verde peninsula (or both) is incorrect. study area (Figure 3). North of Placentia, two dykes having
alkaline chemistry (Assemblage 3) crosscut tholeiitic basalts
INTERPRETATION AND IMPLICATIONS OF BULL of Assemblage 2 (Figure 6E), demonstrating that
ARM FORMATION VOLCANIC ROCKS Assemblage 3 rocks are younger. The alkaline flow at Ship
Harbour is at the top of the Bull Arm Formation and overlies
Mafic and intermediate rocks of Assemblage 1 are calcalkaline Assemblage 1 rocks, indicating that
weakly calcalkaline, and are less LREE and LILE Assemblage 3 is younger than Assemblage 1. In addition, the
enriched than the Headland basalt from the Bonavista basalt boulder sampled from the tillite within the Big Head
Peninsula (see Figures 5 and 6A, B). Assemblage 1 mafic Formation (Figure 3) is also alkaline, indicating that alkaline
rocks have lower (La/Yb)CN ratios and Th contents than the magmatism preceded the deposition of the diamictite. It is
Headland basalt (Figures 5 and 6A, B, D). The mafic rocks possible that some alkaline mafic magmatism was concomi
of Assemblage 1 are spatially restricted to north of Ship tant with deglaciation in the Long Harbour area, but further
89CURRENT RESEARCH, REPORT 211
studies of the basalt clastrich horizon between maroon mud Recommended future work should include age con
stone facies and overlying grey laminated mudstone facies straints for the diamictite in the Long Harbour area and tar
are required to definitively confirm this proposal. geted studies to better understand the lithostratigraphy of the
Musgravetown Group, particularly on the Bay de Verde sub
Crosscutting relationships and superposition clearly peninsula. Age and petrochemical constraints are also
show that emplacement of Assemblage 3 postdates that of required for other parts of the Bull Arm Formation including
Assemblages 1 and 2. The relative timing of Assemblages 1 the felsic dome south of the current study area, mafic and
and 2 cannot be firmly established based on this reconnais felsic units north and south of Bull Arm, and the two north
sance investigation. Based on age and petrochemistry con trending Love Cove schist units south of the Eastport area
straints on Bull Arm Formation rocks at the Isthmus of (Figure 1). Detailed sedimentological and stratigraphic stud
Avalon, Eastport and Bonavista areas (see ‘Age and ies of McCartney’s (1967) Snows Pond/Whiteway and Big
Chemical Constraints on the Bull Arm Formation’), the Head formations are recommended to resolve possible erro
same trend is expected for Bull Arm Formation rocks in the neous lithocorrelations that were retained in King’s (1988)
Long Harbour–Placentia area – i.e., calcalkaline magma compilation for Avalon Peninsula. Additionally, northeast
tism preceded tholeiitic (and/or transitional magmatism), trending faults, such as the Ship Harbour fault (Figure 3),
which was, in turn, followed by alkaline magmatism. should be investigated to determine the nature and magni
Further mapping, geochronological, and lithogeochemical tude of offset.
studies are recommended for the southwestern Avalon
Peninsula, targeting the felsic core of the Cape St. Mary’s ACKNOWLEDGMENTS
subpeninsula (see Figures 1 and 3), in particular.
The first author acknowledges the sound field assis
On the Bonavista Peninsula, the timing of alkaline mag tance of Ofure Onodenalore and discussions in the field with
matism relative to the ca. 580 Ma deglaciation event (Pu et visiting scientists Phil McCausland (Western University),
al., 2016) is apparently diachronous. On the northeast part and Boris Robert, Mathew Domeier and Hans Jørgen Kjøll
of the Peninsula, the alkaline Dam Pond basalts (Mills and (University of Oslo) during the 2019 field season. The sec
Sandeman, 2015) occupy a structural culmination or dome ond author thanks Cody Spurrell for solid field assistance in
flanked by rocks that include the Trinity diamictite. On the 2017. We thank Chris Finch and the Geological Survey
southwestern part of the Peninsula, however, alkaline frag Branch’s laboratory team for timely delivery of quality lith
mental basalt locally overlies the Trinity diamictite, with ogeochemical data. Kim Morgan of the Department of
approximately 50 m of intervening finegrained siliciclastic Industry, Energy and Technology drafted the map figures.
strata (see also Pu et al., 2016). Farther south, at British Critical reviews by Dr. David Lowe and Dr. Alana Hinchey
Harbour, alkaline basalts occur within an upwardcoarsen helped to improve the manuscript. The manuscript was
ing sequence (Rocky Harbour Formation) near the transition typeset by Joanne Rooney.
to coarsegrained, terrestrial red beds of the Crown Hill
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