AN MINERA],OGIST,
VOL. 48, ]ULY-AUGUST,
GARNET-PYROXENE
BEAR MOUNTAIN,
1963
GNEISSES AT
NEW YORK
R. T. Dooo, Jn., Lunar-Planetary Erploration Bronch, Air
Force CambriilgeResearchLaboratories,L. G. Hanscom
F ield, B edford., M assachusetts.
ABsrRAcr
Iron-rich garnet-pyroxene gneisses occur as layers in hornblende granulite subfacies
metasediments at Bear Mountain, New York. Although these gneisses bear a superficial
resemblance to both eclogites and skarns, their chemical compositions suggest that they
are metamorphosed ferruginous sediments of the chert-carbonate type. Such rocks have
not been noted before in the Highlands metamorphic belt.
INrnooucrroN
The gneissesdescribedin this paper occur as concordantlayers in a sequenceof graphitic, iron sulfide-bearing,biotite-quartz-feldspargneisses
in the PrecambrianHighlands metamorphic complex at Bear Mountain,
New York (Figs. 1, 2). The Bear Mountain sequencealso contains small
amounts of marble, amphibolite,and skarn. It is thought to representthe
hornblendegranulite subfaciesof regionalmetamorphism(Dodd, 1962).
The garnet-pyroxenegneisses
at Bear Mountain werefirst describedby
Lowe, who called them eclogites(Lowe, 1950,p. 142)bft did not discuss
their mode of origin. Field and petrographicstudiesby the writer, supplementedby chemicalanalysesof the garnetsand pyroxenesof the gneisses,
suggestthat the rocks are metamorphosedferruginoussediments,probabiy similar to the chert-carbonate rocks of the Lake Superior iron formations (James,1954). Such metasedimentshave not been found elsewhere in the Hudson and New Jersey Highlands. fhey are of interest for
this reasonand becausethey permit an assessment
of the chemicalenvironment in which the Bear Mountain sedimentswere deposited.
Froro RBrerroNs
The principal layer of garnet-pyroxenegneissis sixteen feet thick. It
consistsof nine feet of a garnet-quartz-orthopyroxenerock (calledhereafter "garnet-orthopyroxenegneiss") overlain, with no apparent gradagneiss (called
tion, by seven feet of a garnet-magnetite-clinopyroxene
hereafter "garnet-clinopyroxenegneiss"). Several thinner layers of the
latter rock type occur in the overlying biotite-quartz-feldspargneisses,
through a stratigraphic interval of about eighty feet. The garnet-pyroxenegneissesare exposedfor about one-half mile along strike.
DnscnrprroNs ol rHE GenNnr-PvnoxENE GNBrssrs
Garnel-orl,hopyrownegneiss. Garnet-orthopyroxene gneiss is dark green
beneath strongly limonitized outcrop surfaces. It consists of an inter811
812
R T. DODD, JR,
N
I
I
\'- <Vr
-/,
P<l -'..
/F
l-
I
PAKrx
i
BEARMT.
VIT.
A R E\A
qt"oo'lrt,
%W:"j
(y(wi.
\.,vi_Y(rt
g
i \/-\:t\-/-Y
ot020
MILES
Frc. 1. Index map of the Hudson and New Jersey Highlands.
growth of coarseorthopyroxenecrystals (somemore than five cm across),
within which tiny grains (0.1-0.5mm in diameter) of dark red garnet and
gray qrartz are distributed in layers 6 to 12 mm thick. Coarsescalesof
graphite commonly occupy grain boundariesand cleavages.In addition
to the garnet- and qtartz-rich layers, the gneisscontainsscatteredlayers
of greenish-gray quartzite, which, like the other layers, parallel the foliation of the surroundinggneisses.
The microscopictexture of garnet-orthopyroxenegneissis shown in
Fig.3. The rock consists chiefly of eulite (Enrr), garnet (Alm56 Spza
GARN ET -P YROX EN E G/TEISSBS
GEOLOGY
PRECAMBRIAN
OF BEAR
M O U N T A I N , N EYW
ORK
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H
gt
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D
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o
/-:
\,\'\7\
/
/
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./\ \ \ /'-
l \ > \ \ / \ ' l H o R N B L E N 0GER A N t T E
fiiltmmlll
NE
NErss
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N
i;'ii:ji
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813
flt" lS:
F E L D S P A RG N E I S S
HYPERSTHENE-euARrzOLIGOCLASEGNEISS
A T T I T U D EO F F O L I A T I O N
74"OO
I MILE
Frc. 2. Precambrian geology of Bear Mountain, New York.
Gornet-Orf
hopyroxene
Gneiss
( Nol167;f ield= 12mm.)
Gneiss
Gornet-Clinopyroxene
( N o . 1 3 2 5f i;e l d = 9 m m . )
Fro. 3. Garnet-pyroxene gneisses from Bear Mountain, New York. Drawings were
traced from photographs. Literal symbols stand for garnet (gt), quartz (q), orthopyroxene
(opx), clinopyroxene (cpx), graphite (gr), and magnetite (mt). A single orthopyroxene
crystal fills the field in No. 1167: Note the contrasting habits of garnets in the two rock
types.
814
R. T. DODD, JR.
Andra Grs Pyra), and quartz. The proportions of these minerals vary
widely, but orthopyroxene (locally with minor clinopyroxene) always
makes up more than half of the specimen. Graphite and apatite are
present in amounts of about l/6. Minor accessoriesinclude biotite,
calcite, spheneand magnetite. The last of theseoccursboth as separate
grains and as inclusionsin garnet.
The eulite displays complex and puzzling exsolution textures. It contains lamellaeand irregular massesof augite, the f ormer parallel to ( 100).
The augitein turn containssparse,fine lamellaeparallel to its (001) plane.
These lamellae, of the order of 0.00x mm thick, cannot be identified
optically or by r-ray diffraction, but their orientation suggeststhat they
are either pigeonite or orthopyroxene after pigeonite (Poldervaart and
Hess, 1951,p. a83). The presenceof pigeonite suggeststemperaturesof
exsolutionin excessof 900oC. (Bowen and Schairer,1935).At such temperatures,the surroundinggneissesshould have beenextensivelymelted.
As field evidenceindicateslittle fusion of the gneisses,
there is an evident
and unexplaineddiscrepancybetweenfield data and theory.
Chemical and optical data for the eulite and garnet from a sample of
garnet-orthopyroxenegneissare presentedin Tables 1 and 3. A modal
analysisof the analyzedmaterial is given in Table 4.
Terlr; 1. Crpurcer, ANer,ysrs eNo Nonu ol Eulrtn lnou
GenNnr-OnrrropyRoxENE GNerss (Sample No. 1167/ 2)
Locot'ion: Perkins Memorial Drive, abandoned section, a few hundred feet rvest of
intersection with present Drive, Bear Mountain, Popolopen Lake Quadrangle, New York.
2 Fe3+
Fe3+
Z
Ti
Fe3+
I
Analysis
Fe3+
w
wt.7a
46 80
001
FeuOr
FeO
MgO
CaO
NarO
KrO
HzO+
HzOTiOr
PrOo
MnO
COr
si
Alo
780
793i
200
Fe3+
25
39.63
Fe2+
552
5.18
1.31
0.07
0.03
0 35
0 18
O- 12
0 24
4 15
Cations
to
Six 0
Mg
Ca
Na
K
ll
lvt n
o
2364i
Anatyst; M. Chiba
Car o Mgro a Feeo z
T o A lh z : o
Oplical date:
018
1-a:0
" t: 1 . 7 6 4
N"E:1 7s7
a:1 746
2 Yo : 9 1 2 c
Pale green; negligible
pleochroism
Renar hs:
1, The norm was prepared as outlined
by Hess (1949).
2. Most of the CaO
is thought to reside in ferroaugite
lanellae.
GARN ET-P VROX EN E GITEISSES
Tl'lln
815
2. Cnnurcnr, ANer,vsrs nNo Nonlr or, FERRoAucrrE FRoM
Genr.rrr-Cr.rNopyRoxENEGNnrss (Sample No. 1325)
Localion: southlvest side of Bear Mountain, Popolopen Lake Quadrangle, New York.
Analysis
Y
wt. ok
SiOr
w
4 7. 8 6
8.36
Alz0r
FqCr
I'eil
MgO
LaO
NarO
KrO
HzO+
HzO
Ticz
PzOr
MnO
COr
Cations
to
Six 0
|.94
3.31
18 24
4 99
21 92
o. 15
0 03
0. 30
O 20
0 07
0 19
0 76
Al
Fer+
Fez+
Mg
Ca
1\a
Til
Mn
o
38
42
254
121
391
.t
2.Ol
\\ XY
a17!
10+
2s00i
Analyst: M. Chiba
Carz oMgu :Feaz r
16 Al in Z:4.5
Opticel data:
-y-a:O O26
1:l 730
2Vt:59,6o
Z/\c=azo
Pleochroism:
X:very pale greeu
Y:Z :medium
green
Remarks:
1. The norm was prepared as outlined
by Hess (1949).
2. Contains lamellae
of 1?) pigeonite or
orthopyroxeDe.
Garnel-clinopyr ofienegneiss. Garnet-clinopyroxene gneissforms granular,
vuggy, badly weathered outcrops. Fresh rock is dark green, mediumgrained, and indistinctly layered. It consists principally of clinopyroxene, with subordinatebut variable amounts of dark red garnet and
magnetite and minor graphite. Calcite can be seenin some exposures.
The microscopictexture of the gneisscan be seenin Fig. 3. The principal minerals are ferroaugite (Fe3z1\tlg15
Caas),garnet (AimaeGrerAndrz
SpePyra),and magnetite (with very minor exsolvedilmenite). Accessory
mineralsinclude graphite,apatite, zircon,and hornblende,the Iast occurring as tiny euhedra in magnetite and, less commonly, intergrown with
ferroaugite.Calcite was not seenin thin section.
The ferroaugite contains exsolution lamellae, about 0.025 mm thick,
which, on the basisof their orientation, are thought to be either pigeonite
or orthopyroxeneafter pigeonite(seediscussionin the previoussection).
Chemicaland optical data for the pyroxeneand garnet from a sample
of garnet-clinopyroxenegneissare presentedin Tables 2 and 3. A modal
analysisof the analyzedsampleis given in Table 4.
AppnoxruerE RocK ColrposrrroNs
Analysesof garnets and pyroxenesfrom samples of garnet-orthopyroxenegneiss(No. 1167/2) and garnet-clinopyroxenegneiss (No. 1.325)
wereused,with modal analyses,to calculateapproximatetotal rock com-
816
R. T. DODD, JR.
Taer.a 3. ANar,vsns, Oprrcnr- exn X-nev Dera lon Ga.nxn:rs rnou G,tnNnrOnruopvnoxnNr GNrrss (1167/2) nNn GlRNrr-Cr-rNopyRoxENE Gnnrss (1325)
Analyst: M. Chiba
Sampie Number
Oxide
1325
1167/2
s6.87
19.12
3. 9 s
23.01
0.89
6.28
0 .1 3
003
0.16
o.20
tr.
o.42
947
SiOz
AhOa
FezOa
IieO
Mgo
CaO
NazO
KrO
HzO+
HzOTiOz
PzOs
MnO
COz
Total
Molecules:
3 7. 8 6
18.91
3.78
19.27
1. 0 1
14.61
0.08
0.05
0.26
o.20
0 .1 3
0.34
4.O9
100.
53
Andradite
Pyrope
Spessartite
Almandite
Grossularite
Cell edge (o)
Refractive index (+0.001)
100.59
12.06
4.02
8 .5 4
13.O2
3.64
22.92
55.21
5.21
30.65
1 1 . 6 1A4
| 799
1 1 . 6 eA1
r.799
1A
1a
positions. The modes and rock compositionsare given in Table 4. In
view of the layered characterof the gneisses,
especiallyNo. 1167/2,these
calculated analyses are very imprecise, but they will suffice for a comparisonof the presentrocks with other gneisses
and possibleparent rocks.
Srlrrrln GNBrssBsErspwunnp
The garnet-pyroxenegneissesof Bear Mountain superficiallyresemble
eclogites(Lowe, 1950,p. I42) and skarns. However, they lack the omphacitic pyroxenesand pyrope-rich garnets which are characteristicof
eclogites,and theylack the andraditic garnet typical of skarns (Table 5).
In addition, garnet-orthopyroxenegneissis too poor in lime to have had a
calcareousparent.
fn mineralogy and chemistry the Bear Mountain gneissesresemble
GA RN ET -P YROX EN E GITZISSES
817
or GanNnrGNrrss
Tasrn 4. Moo.qr AuarvsEs AND C,trcur-.rrro Crnurcar. Anelvsns
OnrrropvnoxeNa GNrrss (1167/2) aNl GenNrr-ClntopvnoxeNr
In calculating analyses, oxides were assumed to be pure magnetite, hornblendes pure
hastingsite.
Calculated Analyses
Modes
t325
Mineral
Orthopyroxene
Clinopyroxene
Garnet
Quartz
Hornblende
Oxides
Apatite
Graphite
Total
Points Counted
52.7
t9
r 7. o
25.7
0.2
0.7
0.7
l t
100.0
1000
Oxide(Wt. %)
1167
/2
r325
SiOz
TiOz
AlzOa
Fezo:
FeO
MnO
55
tr
4
37
tr
72.4
1 3. 3
0.2
12.6
0.2
1.3
100.0
2200
27
A
Mso
CaO
NarO
KzO
HtO+
HzOPrOs
4
15
20
1
J
J
2
tr
tr
tr
tr
1
1
l7
u
tr
tr
tr
tr
1
ferruginousgneisses
describedby Tilley from Ross-shire,Scotland (1936)
and by Kranck from Labrador (1961). They are chemically similar to
some contact metamorphic rocks from the eastern Mesabi Range, but
represent a higher metamorphic grade. They differ from the eulysites
(seesummary by Tilley, 1936)in lacking manganiferousfayalite.
The calculatedanalysesof the Bear Mountain garnet-pyroxenegneisses
T.rsm 5. GnnNtrsrnou Gmrnr-OnrnopvnoxnmGxerss(1167
/2) eNnGenNr:rCr,rNopvnoxnNn
Gtnrss (1325),CourenEDwrrrr Avnnncrsor Ganrot
ANervsns lnou Srln,nar- Rocr Tvpns (Tndcnn, 1959)
Molecule
Pyrope
Almandine
Spessartite
Grossularite
Andradite
A-Average
B-Average
C-Average
1167/2
3.64
5 5. 2 1
22.92
5.21
13.02
r325
4.02
M.72
8 .5 4
30.65
12.06
of three garnets from eulysites
of fourteen garnets from eclogites
of fifty-five garnets from skarns
3+1
+ rr
64+
16++14
11+8
5+4
4s++7
1LL')
L2!L
38+7
III
14+6
3++4
1+1
t2+ t3
82+15
818
R. T. DODD, JR.
TAsr,n 6. CoupostloNs on Gennnr-pvRoxENE GNnrssrs Colrpanno wrrrt
Trrosr or Eur-vsrrns ,lNo Gnussps Rnr,ereo ro Eur,vsrrns
Garnet-Pyroxene Gneisses
Eulysites
Related Rocks
1325
SiOr
TiOr
AbOo
FezOr
FeO
MnO
Mgo
CaO
NarO
KrO
HrO+
HzOPrOr
C
etc.
Tota
Total Fe
Total Fef Mn
JJ
tr
44
315
27
4l
33
217
tr
tr
tr
tr
.'I
tr
20
tr
tr
tr
tr
Itr
l1
23
26
26
27
3 2. 8 5
020
302
988
30 45
584
394
tI
tr
nil
030
078
Dm,
99 81
30 c0
40 31
38 12
003
2.Ol
110
24 00
20 29
s.80
7.87
0 1l
016
0.18
37 06
016
0.80
3.01
46.36
5.01
4.37
247
037
030
o20
044
nm
010
038
n.m.
0.08
toO.21
1944
3 5 l. 4
100.s7
38 2l
42.O9
39 89
005
204
040
42.17
688
4 .50
335
000
000
000
000
025
n.m.
0.09
34 80
020
382
1860
19 70
7 50
4 34
1002
tr
tr
0.25
0.15
0 0q
n.m.
013
99 62
33 07
38 40
99 60
2 7. 4 4
34 24
37 00
018
282
2061
t9 44
s_93
8 79
255
tr
tr
1.20
0.30
1.27
n.m.
100 09
29.67
34-26
51 8q
721
236
18 68
7.08
9.18
093
031
081
(
I
l l
l'""
n.m.
054
100 32
16 27
21.75
A-Eulysite, DruiCeig Lodge, I och Duich, Ross-shire,Scot-and. The rock consistsof fayalite, hedenbergite,
iron-rich Iypersthene, magnetite,garnet, and apatite, in order of decreasingabundance.(Tilley,
r936,
pp. 333, 335)
B-Euiysite, Stora Utter*icks llage, Tunaberg, Sweden. (pa)mgren, 1916, p. 196)
C-Eulysite, MansjOMt., Loos, Sweden.(von Eckermann, 1922,p. 254)
D-Eulysite, Loberget Hill, Loos-Hamra region, sweden. (von Eckermann, 19J6, p, 166). The
rock consists
of fayalite, qtartz, garnet, apatite, and pyrite, in order of decreasingabundance,
E-Hedenbergite-garnet-magnetite rock, Druideig Lodge, Loch Duich, Ross-shire,Scotrand. (Tiiley,
1936,
p. 338)
F-Grunerite-garnet-magnetite rock, Druideig Lodge, Loch Duich, Ross-shire,
Scotland. (Tilley, 1936,
p. 33e)
C-Cummingtonite-garnet schist, 5/12 mile SW of outlet of Loch Bad-na-Sgalaig,
between Loch Maree
and Gairloch, Scotland. (Memoir Ceol, -Sura.Creal BriJoin, N.W. Hightands, l9O7)
are comparedwith analysesof eulysrtesand fayaiite-freerocks associated
with eulysitesin Table 6. The Bear nlountain rocks differ from eulysites
i' containing slightly more alumina and less of both iron and iron plus
manganese;they are very simiiar to the fayaiite-freeassociatesof eulysites.
that rock. Evidently manganesehas the effect of additional iron, as sugg e s t e db y K r a n c k ( 1 9 6 1 ) .
GARN ET -P YROX EN E GTE/SSES
819
SprruBNrenv P.q,nnNrsoF GARNET-PynoxBlvBGNBrssBs
A sedimentaryorigin has long beenacceptedfor the eulysitesand associated ferruginousgneisses(Tilley, 1936),on the basisof their compositions and observedgradationsbetweensuchrocks and unmetamorphosed
equivalents(James,1955;Gastil and Knowles, 1960). In the presentarea,
compositionalevidenceis supplementedby the associationof the garnetpyroxenegneisses
with graphitic and pyritic gneisses,
skarns,and marbles,
all of which can be safelyinterpreted as metasediments.
The high lime content of garnet-clinopyroxenegneisssuggestsa carbonate-rich parent, probably a chert-carbonatesediment such as those
described by James (1954, p. 585) and Goodwin (1956, p. 252). The
original assemblagewas probably chert- Fe, Ca, Mg, Mn carbonateorganicmatter. Someprimary magnetite may have beenpresent (Huber,
1958,p. 124),but it seemslikely that most of the abundant magnetite in
the gneissdeveiopedby breakdown of the original carbonatesin the absenceof enoughsilica to convert them entirely to silicates(James,1954,
p.247). The aiumina in the gneiss,which is concentratedin garnetiferous
layers,probably representssmall amounts of clasticmaterial intercalated
in the predominantly chemicalsediment.
The sedimentwhich produced garnet-orthopyroxenegneisswas probably aiso essentiallya chert-carbonaterock, but one in which the carbonate was a manganiferoussiderite and in which silica was plentiful
(witnessfree quartz in the gneissand the presenceof quartzite layers).In
this rock, too, clastic intercalationsare representedby the garnetiferous
Iayers.
Theseinferred sedimentaryassemblages
imply a specificchemicalenvironment. The combination of silica, iron-manganesecarbonates,and
organic matter is stable in a slightly alkaline (pH:7.0-7.8), slightly reducing environment (Krumbein and Garrels, 1952, p. 26). Primary
magnetite is also theoretically possiblein this setting (Huber, 1958, p.
r24).
Thesechemicalconditionswould be satisfiedby a basin with restricted
circulation. That the Bear Mountain sedimentswere depositedin such a
basin is also suggestedby the presenceof graphite and iron sulfidesin the
bio tite-quartz-feldspargneisses.
Suulreny ol CoNcLUSToNS
The garnet-pyroxenegneissesat Bear Mountain are thought to represent ferruginouschert-carbonatesedimentswhich were part of a seriesof
dominantly argillaceoussedimentsdepositedin a restricted basin under
reducing conditions.
Such metasedimentshave not been reported elsewherein the High-
820
R. T. DODD, JR.
lands. The Bear Mountain rocks may be unique, or, perhaps,someof the
garnet-pyroxenerocks mapped elsewhereas skarns are of the same type
and derivation.
AcrlrowlrlGMENTS
The writer expresses
his gratitude to ProfessorJ. O. K. Kalliokoski of
Princeton University, under whosedirection the thesisof which this paper
is a part was prepared. ProfessorKurt E. Lowe of The City Collegeof
New York introduced the writer to the problem and followed the work
with interest.The manuscript was reviewedby ProfessorsH. H. Hessand
A. F. Buddington of Princeton University.
Honoraria from the New York State Museum and ScienceServicedefrayed field expenses.The four chemicalanalyses,preparedby NI. Chiba
of the Japan Analytical Chemistry Researchfnstitute of Tokyo, were
purchasedby Princeton University.
RrlnnrNcrs
Bowrx, N. L. ,q.No
(1935)
F.
Scuarnrn
ThesystemMgO-FeO-SiOz.
Am.Jour. Sci.29,
J.
t5t-2r7.
y.
Doon, R. T., Jn. (1962) Precambrian Geology of the Popolopen Lake
euadrangle, N.
Ph.D. Diss., Pr'i,ncetronUnia.
Gesrrr-, G. aNo D. M. KNowr.os (1960) Geology of the Wabush Lake area, southwestern
Labrador and eastern Quebec, Canada. Bult. Geol. Soc.Am.71, 7243-1254.
Gooowrx, A. M. (1956) Facies relations in the Gunflint iron formation. Econ. Geol. Sl,
505-590.
Huron, N. K. (1958) The environmental control of sedimentary iron minerals. Econ. GeoI.
53, 123-140.
Jeuos, H. L. (1954) Sedimentary facies of iron formation. L,con.Geol,.49,235-285.
--(1955) Zones of regional metamorphism in the Precambrian of northern Michigan.
Bull. Geol.Soc. Am.66, 1455-1487.
Knaucr, s. H. (1961) A study of phase equilibria in a metamorphic iron formation. Jour.
Petrol.2, 137-184.
KnuurrrN, w. c. elro R. M. Gannns (1952) origin and classification of chemical sediments in terms of pH and Eh. Jour. Geol.6O, l-33.
Lown, K. E. (1950) Storm King granite at Bear Mountain, New york. Bull. Geol. Soc.
Am. 6r, 137-190.
Par.ucnrx, J. (1916) Die Eulysite von Sodermanland. Bul,l. Geol. Insl. Unit. Upsala, 14,
108-225.
Pol-onnva.Anr, A. eNn H. H. Hess (1951) Pyroxenes in the crystallization of basaltic
magma. Jour. GeoL 59, 47 2-489.
Trr.r.nv, C. B. (1936) Eulysites from Rossshire. Mineral. Mag.24, 33L-342.
Tndcun, E. (1959) Die Granatgruppe: Beziehungen zwischen Mineralchemismus und
Gesteinsart. Neues fahrb. Mineral., Abh. 93, l-M.
voN EcrrnueNN, H. (1922) The rocks and contact minerals of the Mansjii Mountain.
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-(1936) The Loos-Hamra region region. Geol. Fiiren. Fijrh.58, 129-343.
Manuscripl recehted.,
January 16, 1963; accepteilJor publication, February 16, 1963.