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VOL. 76, NO. 32
JOURNAL
OF GEOPHYSICAL
RESEARCH
NOVEMBER
10, 1971
OxygenIsotopeEvidence
forLarge-Scale
Interaction
between
MeteoricGroundWatersandTertiaryGranodiorite
Intrusions,
WesternCascade
Range,Oregon
HUGH P. TAYLOR,JR.
Division of Geologicaland Planetary Sciences
California Institute of Technology,Pasadena 91109
•80 data have been obtained on 7 of the 17 intrusive centersof diorite and granodiorite (Miocene?)that lie along a N-S belt about 50 km west of the High Cascades.Whole-rock $ values
(per mil relative to SMOW) in the intrusive stocksare: -[-2.3 to -t-4.9 (South Umpqua River),
-0.7 to -t-3.8 (Bohemia mining district), -t-4.3 to -t-4.6 (Vida stock), -0.2 to -t-5.5 (Nimrod
stock), --1.7 to -}-0.5 (Detroit stock), -2.1 to -[-0.7 (Laurel Hill stock, Mr. Hood area), and
-!-2.9 to -t-5.5 (Shellrock,Columbia River). The volcanic country rocks (mainly andesires)in the
vicinity of the intrusions are also depleted in •sO, with $ = -5.6 to -t-4.5. The rocks collected
more than 3 stock diameters from an intrusive contact have 'normal' $ values of -[-5.8 to -]-8.2.
Approximately 1200 km 2 (8% of the area of the Western Cascades)thus appears to be underlain
by propylitically altered igneous rocks that have suffered an average •sO depletion of about
5 to 7 per mil. These $•sOeffectsare similar to thosepreviously discoveredin the Tertiary intrusive
centersof westernScotlandand the San Juan Mountains, Colorado.They are typically associated
with (1)pervasive epidote and chlorite alteration; (2)'turbid' feldspars;(3)granophyric textures
and miarolitic cavities; and (4) young, jointed, flat-lying, volcanic country rocksthat are knowr•
to be highly permeable to ground-water flow. Convective circulation of heated ground waters
in the vicinity of the stocksmust have occurredthroughout a large part of their crystallization and
cooling history. The amounts of H20 involved are estimated to be about equal in volume to that
of the exchangedrock. Hence, much so-called 'deuteric' alteration of igneousrocks is probably
caused by such meteoric-hydrothermal waters rather than by H•O released during magmatic
crystallization.
INTRODUCTION
at hightemperatures
thuscanhavetheir •80/•0
Several recent oxygen isotope studies have ratios loweredby as much as 10 to 15 per rail.
In the Skaergaardintrusion, east Greenland,
demonstrated that certain epizonal igneous inand
at Skye, Mull, and Ardnamurchan in the
trusions(i.e., thoseemplacedat relatively shallow
Scottish
Hebrides,wherethe isotopicphenomena
depths in the earth's crust) have interacted
were
first
observed,the igneousrocks that are
strongly with meteoric ground waters during
their crystallizationand coolinghistory [Taylor, abnormally low in •sO characteristicallydisplay
1968; Taylor and Forester,1971]. The effects of the following geological,petrological,and isosuchinteractions are readily discernedby means topic features: (1) The intrusionsare emplaced
of •s0/•60 analyses,because'normal' igneous into young, highly jointed, flat-lying plateau
rocksthroughoutthe world have •s0/"0 values lavas that are very permeableto ground-water
that are uniformly 6 to l0 per mil heavier than movement.(2) In a givenrock, the feldsparsare
mean ocean water, whereas meteoric ground commonly isotopically exchangedto a greater
waters are typically 15to 10 per rail lighter than degree than the other coexistingminerals, and
oceanwater. Igneousrocks that have exchanged the feldsparsvery commonlyshowa 'clouding'or
with large quantities of heated ground waters turbidity (particularly the alkali feldspars).
(3) The primary igneouspyroxenesand olivines
are almost invariably partially altered to uralitic
• Contribution 1996 of the Division of Geological
and Planetary Sciences, California Institute of
Technology, Pasadena.
Copyright •
1971 by the American Geophysical Union.
amphibole,chlorite,Fe-Ti oxides,and/or epidote;
locally, this processhas gone to completionand
only pseudomorphsof the primary igneous
mineralsremain. (4) Granophyric(micrographic)
7855
7856
I-IuG• I ). TAYLOR,JR.
intergrowthsof turbid alkali feldsparand quartz
are ubiquitous.(5) Miarolitic cavitiesare locally
present
in the intrusives,
andveinsfilledwith 46øEXPLANATION
quartz, alkali feldspar, epidote, ohiorite, or
sulfidesare very commonin both the intrusives
and the surroundingcountry rocks.
A goodcasecan be made that these characteristics are wholly or in large part a result of
interactions
between
the
rocks
and
intrusives
altered
ASHINGTON
rocks
minXe
or Portland
•'•':'J
Mt.
•:•.•,....•,, Hood
heated
prospect
•,,.•:'? _v_.
meteoricgroundwaters set into convectivemovement by the heat emanating from the igneous
intrusions [Taylor, 1968; Taylor and Epstein,
1968; Taylor and Forester,1971]. Ground waters
•
.................
:z•??}•{•
6
:::::::::::::::::::::::::::::::::::::::::::::::::::
.....
areheatedin thevicinityof theintrusion,
and 45ø
becauseof lowered density they rise; they are
replaced by denser, cooler waters that migrate
radially inward toward the intrusive heat source.
These heated, low-•SO, meteoric-hydrothermal
waters exchangeoxygen with the minerals in
the intrusion and in the surrounding country
rocks. If enoughlow-•sO water moves through
•.:.: =====================
.:.:
.:.::.:.::.....::.::
..:.:
.:.::.:.:
.•.•........,.......:,....::.:.,.
:..:•.•
OREGON •::::::::::::::::::::::::::
"'--""•"••..:•"•
"• Mt.
';:::?"
'"'¾"•:{:•:•%::?::?'
":
.... Jefferson
::::::::::::::::::::::::::::::::::::::::::::
the rocks,the •s0/•60 ratios of the rocksare
:::::::::::::::::::::::::::::::::
:::::::::::::::::::::::::::::
......,...:...
.t..%...•
markedly lowered.
A very useful test of the above hypothesis
=========================
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
wouldbe (1) to pickan isotopically
unknown44ø
area in which the aforementioned geological
and petrologicalfeaturesare knownto be present;
(2) to predict that the intrusionsin suchan area
should show the low-•SOeffectstypically associated with hydrothermal ground-waterinteraction; and (3) then to isotopicallyanalyzesamples
from such an area to check this prediction.
Buddington and Callaghan [1936] have describedthe petrologyand geologyof a seriesof
dioritic to granodioriticintrusive bodies in the
:::::::::::::::::::::::::::::::::::
::::::::•.'•
::::::::::::::::::::::::::::::::::::::::::::::::::::::
i
:?:::t:::
::::::::::::::::::::::::::
• •
:::::::::::::::::::::
..•
;:.:::.
Western
Cascade
Range,Oregon,
andall the 43ø
123 ø
122 ø
features outlined above are present at least
locally in these intrusions, accordingto their
descriptions.These stocksall are intruded into
relatively fiat-lying Tertiary volcanic rocks,
principally basalts and andesires.Their exact
age of emplacementis unknown, but they are
thought to have been intruded in the Miocene
[Buddingtonand Callaghan, 1936; Peck et al.,
1964]. The intrusionsare now exposedas a result
of erosion by the westward-flowingrivers of
the Cascades.The stocks tend to be arranged
Fig. 1. Map of part of western Oregon. The
Tertiary volcanic rocks of the Western Cascade
Range are indicated by the dark stippled pattern;
also shown are the occurrencesof Tertiary diorites
and granodiorites,as well •s the areasof propylitic
alteration and mineralization that are commonly
associatedwith thesemedium-grainedigneousrocks
[modified after Peck et al., 1964]. The numbers
in clusters within
indicate
a narrow
N-S belt that
lies
10
I ....
0
10
I
I
20
I
I
MILES
I
I
I
0 10 20 30
krn
the various localities studied in this work'
(1) South Umpqua River (Figure 3); (2) Bohemia
about 50-60 km west of the Pleistocene and
Mining District (Figure 4); (3) Nimrod stock
Recent volcanoesof the High Cascades;this (Figure 5); (4) Vida stock; (5) Detroit Reservoir
belt extends across the entire width of Oregon (Figure 6); (6) Laurel Hill stock (Figure 7); (7)
(Figure 1). The intrusivebodiesrangein greatest Shell Rock intrusion, Columbia River.
INTERACTION BETWEEN GROUND WATERS AND INTRUSIONS
7857
dimension
from a few meterstoj,4 kin, and
Note that fresh, unaltered basaltsand andealthough most are augitc diorites or quartz sires throughout the world typically have very
diorite porphyries,someof the larger stocksare uniform 6180 values of -]-5.5 to -]-6.5. If they
principally granodiorite or quartz monzonite. have undergonesomeweatheringor low-temperaThe Western Cascades intrusive belt thus
ture (•100øC) hydrous alteration, they will
appearedto have all the characteristics
necessary usually have slightly higher 6 values; also,
to make it a worthwhile area on which to do an
plutohie granitic rocks generally have 61sOoxygenisotopestudy. It was hoped,first of all, -]-7.0 to -]-9.5 [Taylor, 1968]. Therefore, any
that low-x80rocks would be found, and second, igneousrock (intrusiveor volcanic)analyzedin
that the distributionof x80/q60ratios would the presentresearchthat has a 61sOless than
provide usefulinformationon the nature of the •-5.5 is immediatelysuspectedof having underpostulatedinteractionsbetweenintrusionsand gonehydrothermalexchangewith heatedlow-•SO
the waterspresentin the adjacentcountryrocks. meteoric waters. Such phenomenacannot be
In particular, it was hoped that the •sO data producedby hydrothermalexchangewith pricould be related to contact metamorphism, mary magmatic ground waters, becausesuch
regionalhydrothermalalteration,and the forma- waters have a relatively well-defined 6x80 of
tion of associated ore deposits. There are no about -]-6.0 to -]-8.5 [Taylor, 1967].
As is shown in Figure 2, the mid-Tertiary
large minesin theseareas,but every one of the
intrusive localities has some associated mineraliintrusive igneous rocks and volcanic country
zation, notably the Bohemiamining district rocks from the Western Cascades show pro(Figure 1), from which about $1,000,000worth nounced differencesin 6xsOas compared with
of gold was removedfrom 1870 to 1940.
ANALYTICAL I•ESULTS
The •s0/•60 analysesobtainedin this study
the fresh volcanic rocks and a single intrusive
body (the Husband) from the younger High
Cascadesvolcanoes.The latter have perfectly
'normal' 6•s0 values, and arguments will be
are all plotted on generalizedgeologicmaps presented
belowthat, beforehydrothermalalterpresentedbelow, and the data are also shown
graphicallyin Figure 2. They are given as 6, co
derined as
R•_•mv•
_ 1)1000
6 ---(
xRstandard
L•
INCLUSION
r•
IN BRECCIA
co UNALTERED"
"•
VOLCANICS
Z
VOLCANIC
02
COUNTRY ROCKS
whereR = •s0/•60. The standardis SMOW,
•
INTRUSIVE
standard mean ocean water. Many of the
• STOCKS
O
•80//x60valuesweredeterminedin replicate,and
they shouldall be accurateto 4-0.2 per mil. • MEDICINE
LAKE
On the figures,the numbersin italicsareanalyses O NEWBERRY
of volcaniccountry rocks; the other numbers
THREE SISTERS
representanalysesof the intrusivebodies.Most
analysesare of whole-rock
samples,
whichrepreI,- MT. LASSEN
sent 25-mg splitsfrom a 10- to 15-gramsample
Z
of a hand specimenthat had been crushedand
groundto pass100 meshscreen.Mineral separateswerepreparedfor analysisby conventional
Ld MT. SHASTA
-5
0
+5
-4O
ao•8(%o)
techniques,exceptthat all quartz separateswere
givena final treatmentwith HF to removeany
Fig. 2. Comparison
of whole-rock•180valuesof
tracesof mineralimpurities(principallyfeldspar). Pleistocene and Recent volcanic rocks of the Eastern
Oxygenisotopicfractionations
amongcoexisting Cascades[Taylor, 1968; H. P. Taylor and I. Carminerals are conveniently reported as AAe,
definedas 1000in aAe, wherea•s is •sO/•60in
mineralA dividedby •sO/•O in mineralB. To a
very closeapproximation,A•s • •a -- •s.
michael,unpublisheddata] with data on Tertiary
igneousrocksof the WesternCascades.The in-
clusionin breccia (• - 4-0.3) is a diorite fragment
from •n igneousbrecci• dike in the SW 1/4 of sec.
32, T. 16S.,R. 5E., 15 km due E of Nimrod.
Hu•
P. TAYLOR,
ation, the igneousrocksof the WesternCascades
originally had 'normal' /PsOvalues as well. The
whole-rock alSO value of a sample of olivine
microdiorite,representativeof the north plug of
the Husband, a 300-meter-wide body in the
Three Sistersarea, is +6.2. This normal alSO
value is in keepingwith the fact that this small
Pleistocene stock was evidently emplaced at
very shallow depths and has apparently not
sufferedany hydrothermal alteration [Williams,
1944].
DISCUSSION OF INDIVIDUAL
LOCALITIES
South UmpquaRiver. The southernmostarea
studiedis on the SouthUmpqua River (Figure 1).
The generalgeologyof the area and the isotopic
results are shown in Figure 3. The section of
volcanic country rocks in this area is relatively
thin, probablylessthan 1000metersthick; these
are Eocene andesiresof the Colestin formation,
which unconformablyoverlie the pre-Tertiary
basementcomplex[Pecket al., 1964].
Two very small, elongateintrusive bodies of
diorite porphyry were sampled.The westernmost
body is apparently depleted in •sO by only a
small amount (/• = +4.9), but the easternmost
intrusive has $ -
+2.3
tuffs and andesires of the Little
Butte
volcanic
series[Peck et al., 1964]. Two small stocksless
than 3 km in diameter are present,the Champion
Creek stock and the Brice Creek stock, together
with a large number of much smaller dikes and
plugs. The smaller bodies are typically porphyritic augitc quartz dioritesor dacite porphyries,but the largerstocksare principallymediumgrained, equigranular augitc granodiorite. The
augires in all these rock types are generally
partly alteredto fibrousgreenamphibole,and to
a lesserextent to chlorite and epidote.
A well-defined
300 to 600 meter-wide
contact
metamorphicaureolesurroundsthe larger stocks
(see Figure 4). The most intensemetamorphic
effectsare representedby a tourmalinehornfels,
which consistsof black massesof tourmaline,
0.1 to 2 cm in diameter,in a dense,white groundmassof microcrystallinequartz. Other rock types
found in the aureole are epidote-tourmaline-
miles
5
4 km
Tc
and must have been
depletedin •s0 by about 4 per mil or more. Two
samplesof the volcaniccountryrocks,onewithin
a few meters of the intrusive contact ($ - +2.0)
and the other an inclusionin the diorite porphyry
(• = •L3.1), have also been depleted in lsO
relative to the country rocks well away from
these small intrusives; two samples collected
more than I km away have normal $1s0values
of +6.7. The low-isOigneousrocksall lie within
an area of propylitic alteration (i.e., an epidotechlorite-actinolite zone) outlined by Peck et al.
[1964] on their geologicreconnaissance
map of
the Western Cascades. This area of propylitic
alteration is roughly centered on the South
Umpqua River intrusives.
Bohemiamining district. A considerable
number of oxygenisotopedata have been obtained
on rocks and minerals in the vicinity of the
Bohemia mining district. The $•s0 values are
plotted on a generalizedgeologicmap in Figure 4.
The geologyof this area has been describedin
detail by Buddington and Callaghan [1936],
Callaghanand Buddington[1938], and by Lutton
[1962].The pile of volcaniccountry rocksin this
locality is about 3500 meters thick, principally
5
•*
Tc
Fig. 3. Generalizedgeologicmap of the South
Umpqua River area [after Peck et al., 1964], showing sample localities and whole-rock $1sOvalues
obtained in the present study. The italicized
numbers represent data on volcanic country rocks
or xenoliths, and the other numbersrepresentdata
on the intrusive igneousrocks.The diagonaldashed
pattern indicates the general area of propylitic
alteration. T c, Colestinformation (largelyandesite);
Tlt and Tlr, andesitic to rhyodacitic tuffs of the
Little
Butte volcanic series.
INTERACTION BETWEEN GROUND WATERS AND INTRUSIONS
*•'?
0• ,,,m,les
••
,,• ,km
•
+74
/I
.co
///
.• •o
BOHEMIA
MINING
DISTRICT,
Lane
County,Oregon
-•-•Diontic
Intrusive
rock
EXPLANATION
--
....-"-Boundary
ofcontact-
[---]Volcanic
country
rocks
I ß Sample
Iocahty
!
7859
1280•evalue
-
/•, • •
....
metamorphic
aureole
""•• 80•sContours
/-• -• -,•.•
Area
ofpropyhtlc
alteration
•
Fig. 4. Generalized
geologic
mapof theBohemiaminingdistrict[afterCallaghan
andBuddington,1938;andLutton,1962],showing
samplelocalitiesandwhole-rock$xsOdata.The largestock
at the top of the figureis theBriceCreekstock;the somewhat
smallerbodyin the centeris the
ChampionCreek stock.The italicizednumbersrepresentdata on volcaniccountryrocksor
xenoliths
(andesire
andbasaltflowsanddacitetuffsof the Little Buttevolcanic
series);
the other
numbers
represent
dataonthewhole-rock
samples
of the intrusives
exceptthat $ = 0.0F in the
ChampionCreekstockrefersto an analysisof an oligoclase
feldspar.More detailed•sO/•60
analyses
weremadeon 3 samples
nearthe centerof the Champion
Creekstock.A $•sOvalueof
-F1.8wasobtainedon a 0.5-cm-wide,
fine-grained,
quartz-feldspar-epidote
vein cuttingonesample
($ = -0.6a). A $•sOvalueof +2.8 wasobtainedon quartzfroman epidote-bearing
veinin the
granodiorite
that hasa whole-rock
$ = -0.4 b.The $xsO
valuesof coexisting
quartz,oligoclase,
andactinolitefromanothersample($ = -0.7 c) aregivenin Table1. Two samples
of volcanic
rockscollected1.5 and 2.0 milesdownstream(northwest)from the samplelocality ($ = +6.7)
in the upperleft cornerof the maphave$xsO
valuesof +7.5 and +7.1, respectively.
stockhaveessentially
normal"$•sO
chlorite-magnetite-sericite-(pyrite)
hornfelsand Champion
tourmaline-specularite-sericite
hornfeb.The outer values greater than q-5.5. This includes two
part of the aureolegradesinto a zoneof propy- samplesthat lie to the NW of the map area shown
litic alteration (shownby the diagonaldashed in Figure 4. Thus in this locality an appreciable
patternon Figure4), termedan epidote-chlorite- outcrop area, including both intrusives and
(magnetite,pyrite) hornfelsby Buddington
and country rocks, exhibits a $xsOlower than
Callaghan[1936].The mineralogyof the propy- per mil; the rocksin this area all musthave been
litic zoneis described
by Pecket al. [1964,p. 41]. loweredin $•s0by at least 4 per rail. The average
Sufficient oxygen isotope data have been $•s0 value of 19 samplesof the altered volcanic
accumulated on this area to allow us to draw
countryrock is q-0.7. A central zone, approxiapproximate $xsOcontoursfor the rocks as mately 5 kin' in area, can be definedin which
shownin Figure 4. Note that the $xsOcontour essentiallyall rock types have a $xsOlessthan
pattern is very systematicand approximately zero. The axis of this central zone is displaced
coincides
with the patternof propyliticalteration somewhat asymmetrically to the east of the
outlined by Peck et al. [1964]. All the rocks axis of the two major stocks.This asymmetry is
collected more than 5 km distant from the most clear cut for the Brice Creek stock, but it
7860
Hua• 1•. TAYLOR,JR.
alsoseemsto be a feature of the Champion Creek
body.
The apparent asymmetry of the e]•aOpattern
conceivably is due to the greater abundance of
intrusive bodies to the east of the two large
stocksthan to the west; suchan outcrop pattern
might indicate the presenceof a larger intrusive
body at depth. Note that even though the
over-all e]•aOpattern is clearly geographically
related to the granodioriteand diorite intrusives,
except for the central parts of the two large
stocks,it seemsto make little differencewhether
a sample is from an intrusive body or from the
volcanic country rocks. This in itself suggests
that much or all of the •aO lowering occurred
after crystallizationof the intrusive bodies.
Of the intrusive bodies, only the Champion
Creek stock has been sampledin any detail. It is
relatively uniform in e]•sO,with e]- --0.7 to 0.0
throughout, except for a single sample near the
center of the body with •aO = q-1.4. A metamorphosed inclusion of country rock from an
agraafire zone near the southern contact is
slightly lower in •aO than the host granodiorite
(--0.5 versus--0.1). Two late-stage epidotebearing veins that cut the granodiorite are
slightly enriched in •0 relative to the granodiorite
itself.
This
is consistent
with
a lower-
temperature origin for the veins, becauseif we
assumethat the hydrothermal fluids had a constant e]•sO,a given mineral in equilibrium with
thesefluids would have a higher b•sOvalue, the
lower the temperature. If we assume that the
quartz in the granodiorite(e]qu•rt,.
= q-0.6 in the
sample with superscript C, Figure 4) was in
exchangeequilibrium with H20 at 600øC, the
•H20 would be --1.4. The quartz from the
epidotevein (• - q-2.8) couldhave crystallized
from an aqueousfluid with b•sO = --1.4 at a
temperature of 445øC (basedon the quartz-water
isotopic geothermometer of Clayton, O'Neil,
and Mayeda [O'Neil and Taylor, 1967]).
In the one granodiorite sample in which
coexistingminerals were analyzed (superscript
C), the quartz, feldspar, and uralite are clearly
not in exact isotopic equilibrium (Table 1,
comparewith data of Taylor and Epstein [1962]).
However, this mineral assemblage does not
exhibit the profound isotopic disequilibrium
typical of low-•aOgranites from Scotland where
quartz-feldspar A values of q-4 to q-8 are
common [Taylor, 1968; Taylor and Forester,
1971]. It is not certain why the quartz from the
Champion Creek granodiorite should be depleted in •sO to about the same extent as the
coexisting feldspar; it could be because this
quartz is much finer grained than in the Scottish
granites, or it could imply either that the granodiorite magma itself became •sO-depletedor
that the stock underwentcompleterecrystallization in the presenceof the low-•SOfluids. Grain
size clearly must play an important role during
interactions between rocks and hot aqueous
fluids. This may be the reason why the finegrained inclusion referred to above is lower
in •sOthan the host granodiorite.
Nimrod
stock.
The
Nimrod
stock
on
the
McKenzie River is the largest single intrusive
body in the Western Cascades(Figure 5). It is
also compositionallyunique in that it is principally a fine-grained, leucocratic,biotite quartz
monzonite [Buddingtonand Callaghan, 1936,
Figure 5, p. 433]. Subhedraloligoclaseis rimmed,
veined, and replaced by interstitial, highly
turbid K feldspar. Biotite is the most important
marie mineral, and is typically partially altered
to chlorite; locally, however, hornblendeis the
dominant mafic mineral. Epidote and sericite
are rare but ubiquitousalteration products,and
small miarolitic cavities lined with epidote are
also present.
The
•sO
data
obtained
on rocks from
the
Nimrod area are shownin Figure 5. The Nimrod
stock has not been mapped in detail, and the
outline shown in Figure 5 is only approximate.
The total thicknessand generallithology of the
flat-lying pile of volcanic country rocks are
similar to the situation at the Bohemia mining
district. The Nimrod pluton is isotopicallyquite
distinct from its surroundingvolcanic country
rocks. Except for one sample near the west,
contact with $•sO = q-0.1 (in which the marie
minerals are completely altered to actinolite,
chlorite, and epidote), the whole-rock $•sOvalues
in the stock are all greater than -]-2.0. The three
most centrally located samples have e]•sO =
q-2.8, q-3.6, and q-5.5. The two formersamples,
from the same locality, are relatively unaltered
hornblendegranite, whereasthe latter is a very
freshbiotite quartzmonzonite.There is, however,
only a rough correlation between the e]•sOand
the degreeof alteration of the mafic minerals,
becausethe sample with • = q-2.1 also contains
exceedinglyfresh,primary biotite (el = --0.8),
INTERACTION BETWEEN GROUND WATERS AND INTRUSIONS
7861
TABLE 1. Comparisonof •80/•60 Analysesof CoexistingMinerals*from Low-•80 IntrusiveBodiesin the
Western CascadeRange and the Inner Hebrides, Scotland, with Data on Some Isotopically 'Normal'
Plutonic
Sample
Granitic
Rocks from the United
•qtz
• feld
States
•amph
/kqt z-feld
/kfeld-amph
Oregon
Laurel Hill granodiorite
(•W r. -- +0.1)
+2.0
--0.1
--1.8
2.1
1.7
+0.5
--3.0
+1.9
+0.2
--2.2
1.7
2.4
+0.6
+6.8
+4.9
--0.5
+1.3i
+1.1
--1.4
1.1
5.5i
3.8
0.9
Ardnamurchangranophyre
Mull granophyre
Creag Strollamusgranite, Skye
Loch Ainort granite, Skye
-6.0
+3.3
+1.0
+2.9
-6.51
--0.8
- 3.1
--0.5
0.51
4.1
4.1
3.4
Red Hills granite, Skye
Maol na Gainmhich granite,
Skye
+4.5
--0.7
5.2
+5.6
--5.6
11.2
Alta granodiorite
Rubidouxleucogranite
+10.3
+9.7
+9.9
+8.9
+8.0
+8.8
+6.4
+5.5
+6.2
1.4
1.7
1.1
2.5
2.5
2.6
Bonsall tonalitc
San Jose tonalitc
+10.3
+9.7
+8.5
+8.0
+6.9
+6.6
1.8
1.7
1.6
1.4
Big Cliff Dam diorite porphyry
(• .... -- --0.3)
3.5
Detroit Reservoir granodiorite
(• ..... -- 0.0)
Champion Creek granodiorite
(&v.r. = --0.7)
Nimrod granite (• .... = +2.8)
Nimrod granite ($.... ----+2.1)
--0.8•:
Scotlandõ
'Normal' p!utonic granitic rocksl]
ShakeFlat quartz monzonite
* qtz, quartz; feld, feldspar; amph, amphibole; w.r., whole rock.
I Calculated from whole-rock •sO value by material balance.
$ Biotite •sO value.
õ Data from Taylor [1968] and Taylor and Forester[]971].
II Data from TaylorandEpstein[1962]and Taylor[1968].All are rocksfrom the SouthernCaliforniaand
Sierra Nevada batholiths, except the Alta stock, Wasatch Range, Utah.
whereas,in the sample with • = -]-2.3, the
biotite is completelydestroyedand replacedby
iddingsite, chicrite, magnetite, and epidote. A
fine-grainedxenolith of volcanic country rock
from the latter locality is even lower in •s0,
with • = --0.2. This isotopic difference is
somewhatlarger, but is in the same direction
as that described above for a xenolith from the
Champion Creek stock; again, the much finer
grain size and lower quartz content of the
xenolith may have renderedit more susceptible
to isotopicexchange.
All the analyzed volcanic country rocks surrounding the Nimrod stock are considerably
lower in •sOthan the stockitself, with • = -]-0.6
to --5.6. Thesesamplescomefrom the propylitically altered zone that envelopsthe stock and
extendsapproximately 15 km to the northeast.
As onemovesout of this altered zoneto the east,
the •80 valuesapparentlystart to go up (i.e., to
• = -]-3.0), but two samples1.5 to 3 km west of
the pluton, near the western edge of the zone
of propy]itic alteration mapped by Pect• et al.
[1964], have lower • values (-- 5.6, -- 4.6) than
any samples yet found in the entire Western
Cascades.Both of these samples come from
outcropswhere there has been a very massive
developmentof epidote (and chicrite) in segregations and veins. Thus, this is one examplewhere
the degree of •s0 depletion correlatesvery well
with the developmentof the mineralogical and
textural features connected with the hydrothermal alteration. The 'anomalously'lcw •s0
values might be a result of (1) the presenceof
another intrusive body at a shallow depth
directly underlying these localities (e.g., see
I-IuGI-I P. TAYLOR,JR.
7862
250 km*. [Peck et al., 1964]. Nine samples of
do basalts and andesires.If the Nimrod pluton
initially had •80 = -]-8.5 to-]-9.5 (typical
valuesfor a quartz monzonite[Taylor, 1968]), it
also must have undergonean over-all •0 depletion of about 6 to ? per mil, as its average
•0 is -]-2.7. The central zone, however,would
have been loweredby only about 4 per mil, and
so, by any model one chooses,
the degreeof •0
depletionappearsto progressivelyincreasewest-
volcanic
ward from the center of the stock.
Figure 9), or (2) simply to the vagaries of the
'plumbing system,' whereby these particular
rocks happen to be near a major conduit or
fracture systemand thus have suffereda greater
degreeof exchangewith a larger volume of the
meteoric-hydrothermM fluids.
The propylitic alteration zone that extends
NE of the Nimrod
stock covers an area of about
rocks collected from this area have an
average whole-rock 6•80 of --1.3. If we assume
that these effects originally extended over a
vertical distance of 1.5 km, this implies that
about 400 km• of rock mu•t have undergone
an over-M1 •80 depletion of at least 6 or 7 per
rail. The average 6•s0 of the Nimrod stock is
about 4 per mil higher than in these altered
volcanics, but this does not necessarilyimply
that the stockhas suffereda smaller•sOdepletion.
Calculation of the amount of 6•sO-loweringsuffered by a granitic pluton is difficult because
fresh, unaltered granitic rocks throughout the
world showa greater primary 6•s0 variation than
0
•
0
4
i
I
2 miles
i i
i i
2
5
¾ida intrusive body. Two samples were ana-
lyzed from a small diorite body at Vida, 11 km
west (downstream)from the Nimrod stock (see
Figure 1). These samplesare much lessdepleted
in •0 than the Nimrod samples (6 = -]-4.6,
-]-4.3), in keeping with the much smaller size
of the Vida intrusive. Thus, this locality is more
analogousto the SouthUmpqua River occurrence
in having a higher 6•80, smallerintrusivebodies,
and much smallerarea of propylitic alteration.
Detroit Reservoir. Compared with the locali-
ties discussed
above,the •s0/•60 data from the
Detroit Reservoir area (Figure 6) do not show
3
i
• km
ß
Tit
/
Tsa...........
Fig. 5. Generalizedgeologicmap of the areasurrounding
the Nimrod stockon the McKenzie
River [after Pecket al., 1964],showingsamplelocalitiesand whole-rock6tsOdata obtainedin the
presentstudy.Tsa, Sardineformation(largelyandesires);
Qtv, Plioceneto Quaternaryandesires
andbasalts;othernotationsimilarto that in Figures3 and4. tsO/t60ratiosof coexisting
minerals
from two samplesof the Nimrod granite (6 = +2.1 • and +2.8 b) are given in Table 1. 6tsO =
+0.3 was obtained on a diorite inclusionin a small breccia dike 15 km due east of the Nimrod
stock.
INTERACTIONBETWEENGROUNDWATERSAND INTRUSIONS
?S63
Tsa
.........
e¾$.2
North$ontiorn
•River
MILL
CITY
Tso
miles
•km
Fig. 6. Generalizedgeologicmap of the Mill City-Detroit Reservoirarea [after Pecket al.,
1964],showingsamplelocalitiesand wH,l•-r,eb •80 data obtainedin the presentstudy.The
notationis similarto that in Figures3, 4, and 5. •80/•O analysesof coexisting
mineralsfrom
samples
of the DetroitReservoirgranodiorite
(• = 0.0•) andthe Big Cliff Dam dioriteporphyry
(• = -0.3 •) are given in Table 1.
sucha goodcorrelationwith the area of propylitic alteration mapped by Pecl• et al. [1964].
In large part, this may be becausethe geology
is much more complicatedthan is shown on
Figure 6, but it shouldalso be remarked that,
on the basisof petrographicstudiesin this work,
the western boundary of propylitic alteration
should be extended at least 2 km farther down
the North Santiam River. In addition, in the
easternmostpart of Figure 6 the rocks just to
the north of the Detroit Reservoir are only
confinesof the pluton has 8•sO = --1.7. The
central granodioritesample containsoligoclase
phenocrysts(8 = DL-0.2)embeddedin a matrix
of interstitial fine-grainedturbid alkali feldspar
and quartz ((] = DL-1.9),much of it •n micrographicintergrowths.
Actinoliticamphibole(8 =
--2.2) is the only important mafic mineral
present. Chlorite and epidote are minor accessories.This area thus appearsto be somewhat
analogousto the Bohemiamining district in the
size,complexity,and nature of the granodiorite
intrusivespresent,and also in the fact that the
weakly altered. The area of interestis centered
on the Sardine syncline, and this area thus 8•sOvalues of the intrusives and the surrounding
representsby far the thickest pile of volcanic country rocks are almost indistinguishable.It
rocks of any area studiedin the presentwork. does differ, however, in that the isotopically
There are at least 5500 meters of volcanics, analyzed traverse shownin Figure 6 represents
cappedby andesiresof the Miocene Sardine only a very small part of a much larger area of
propylitic alteration that extends more than
formation [Pecl•et al., 1964].
Two samplesof porphyriticgranodioritewere 15 km to the NE and about 30 km to the SW of
analyzedfrom the stockat the west end of the the Detroit Reservoir [Peclcet al., 1964, and
Detroit Reservoir,one from near the center of Figure 1].
Another, much smaller intrusive body was
the pluton (• - 0.0) and one collected3 meters
inward from the east contact (• = +0.5); the sampledat Big Cliff Dam about 5 km NW of
latter contact is steepand very sharp.In addi- the Detroit stock. This diorite porphyry is
tion, a metavolcanicscreenwithin the mapped intenselyalteredto epidoteand chlorite,contains
7864
I-IuG• P. TAYLOR,
JR.
andesinephenocrysts(5 = -]-0.5) and actinolite
Pliocene,
approximately
7 m.y. ago[Wise,1969],
(• = -- 3.0), andhasa whole-rock•1sO= -- 0.3, and is thus probablyconsiderably
youngerthan
slightly lower than in the immediatelyadjacent the other intrusivcs of the Western Cascades.
volcanic country rocks. All the volcanic rocks The stockis principallymadeup of porphyritic
betweenthe Detroit granodioriteand the Big granodioriteand quartz monzonitc,but known
Cliff dioriteporphyryhavenegative•1sOvalues, variantsincludequartz diorite and granophyre.
ranging from --0.4 to --1.6. About 3 km west Actinolite and turbid K feldsparare present
of the Big Cliff body, however,the •1•O values throughout,and chloriteand epidoteare genof the volcanicsattain essentially'normal' values erally presentin minor amount.
of -]-5.8, and from there on west the • values
The oxygenisotopedata are shownin Figure7.
are all 'normal' (seeFigure 6).
Note that the two sampleslowestin lSO(both
The average whole-rock • value of all the
samplesshownin the easternhalf of Figure 6
is • =
-]-0.1. If this characterizes the entire
50-kin-long area of propylitic alteration, a
with $ = --2.1) werecollectednearthe margins
of the stock,whereasthe more centrallylocated
specimenis more than 2 per rail heavier. However, another samplenear the westernmargin
terrane at least 500 kin'- in extent must have
has $ = +0.7, so the 180 distribution is not a
undergonean over-all 1•O depletion of about simple function of position within the intrusion.
6 per rail.
Consideringthe relatively small amount of
Laurel Hill stock,Mr. Hood area. The Laurel alteration that the stock has undergone,the
Hill stock, studied by Wise [1969], may not $1sOvalues are remarkablylow. In fact, the
properly belong to the intrusive belt of the two sampleswith $1s0 = --2.1 representthe
Western Cascades, but it is included here most 1sO-depleted
intrusive samplesyet found
becauseit exhibitsthe texturalandmincralogical anywherein Oregon.We can only speculateas
characteristicsalready described.This stock, to why this shouldbe so, but two featuresare
about 2.5 km•-in area, lies just SW of Mr. Hood worth considering:
(1) The stockmay in fact be
and is in fact partially coveredby Mr. Hood appreciablylarger than is indicatedby its outvolcanics (Figure 7). It was eraplacedin the crop pattern on Figure 7. Wise [1969] believes
.:"•
I:::::::l
Iluvium
and
Mt. Hood Volcanics
Laurel
Hill
and
Pliocene
Basalts
Still
Creek
Intrusions
and Andesires
Fig. 7. Generalizedgeologic
map of the LaurelHill and Still Creekstocks[after Wise,1969],
showingsamplelocalitiesand whole-rock•1sOdata obtainedin the presentstudy. •sO/160data
on coexisting
mineralsin a samplefrom the centerof the LaurelHill stock(• = q-0.1) aregivenin
Table
1.
INTERACTION
BETWEEN
GROUND
WATERS
ANDINTRUSIONS
the Laurel Hill stock to be connectedat depth
7865
all indicatethat theseassemblages
recrystallized
to the elongateStill Creek stockshownat the in near isotopicequilibrium.
The Nimrodquartzmonzonite
exhibitsvalues
bottomof Figure 7;if it is, this intrusivebody
(3.8,5.5) that aremuchlargerthan
wouldbe the largestof its kind in the Cascades, of Aqt•-fem
considerably
larger than the Nimrod stock.It those observed in the Oregon granodiorites
might thereforehave had a correspondingly(Table 1). These data clearly indicate nonlargereffecton the meteorichydrothermalcon- attainmentof isotopicequilibrium,and imply
with
vectionpatternin the surrounding
countryrocks. that the feldsparcontinuedto exchange
fluidsevenafter the
(2) The LaurelHill stockpresentlyoutcropsat the low-•80hydrothermal
a higherelevationand lies farther north than •80/•60 ratios of the coexistingquartz was
'frozenin.' Notethattheseabnormally
any of the intrusivesdescribedabove.Both essentially
situationswould favor ground waters of lower largeA valuesin the Nimrodgranitealsogo
lSOcontent,becausethe 81sOof rain and snow hand in hand with whole-rock 8•80 values that
tends to decreasewith both increasingelevation are largerthan in other intrusiverocksfrom
and latitude [Epsteinand Mayeda,1953; Fried- Oregon.Even thoughthere has over-allbeen
man et al., 1964; Craig,1961].Also,the climate markedlyless •80 depletionin the Nimrod
oftheNimrodfeldspars
in the Pliocenemay have been coolerthan in granite,the8•80lowering
the Miocene;this alsowouldtend to produce musthavebeensimilarto that of the granodiorite
surface waters of lower lS0 content.
Shellrock
intrusion,ColumbiaRiver. The intrusive body at ShellrockMountain on the
ColumbiaRiver (seeFigure1) cutsthe Miocene
feldsparsshownin Table 1.
CONCLUSIONS
Comparison
withotherareasof low-•80igneous
toisotopic
equilibrium
ColumbiaRiver basaltand alsomay not properly rocks. The closeapproach
by coexisting
minerals
in thegranodiobelongto the WesternCascadeintrusivebelt. exhibited
This porphyriticaugite-hypersthene
diorite is rites from Oregonis not commonlyfound in
much less altered than the plutons described other rocks that are known to have suffered
alteration. In
above,all the mineralsbeingfreshexceptthe intense meteoric-hydrothermal
datacanbe compared
with
hypersthene,
whichis partlyreplaced
by uralite. Table1, the Oregon
data from somegraniticintrusions
Two whole-rock samples of this body were analogous
[Taylor,
analyzedfor •sO, one of whichis essentially from Skye,Mull, and Ardnamurchan
1971].In the latter
normal(8 = -•5.5) andthe otheronlyslightly 1968;TaylorandForester,
wecommonly
observe
Aq•_fe•d
fractionadepleted(8 = •-2.9). Theserelativelyhigh samples
a
8•80 values are in keeping with the general tionsof 4 to 10 per mil. This clearlyrepresents
absenceof hydrothermalalterationeffectsin
this body.
nonequilibrium
situationproduced
because
the
feldspar
exchanged
muchmoreextensively
with
•80 Fractionationsamongcoexistingminerals. the low 180fluidsthan did the quartz. In addi-
to observe
8180values
Only a few sets of coexistingmineralswere tion,it is not uncommon
of
--4
to
--6
in
the
intrusive
igneous
rocksof
analyzedfor 180in the presentstudy,asis shown
the
Hebrides,
whereas
in
Oregon
the
only
in Table 1. The analysesof the granodiorites
intrusive whole-rock 8180 values less than --1
are remarkable,however,in their similarity.
are found in the Laurel Hill stock.
The quartz,feldspar,and actinoliticamphibole What are the reasonsfor the discrepancies
from a sampleof ChampionCreekgranodiorite, betweenthe Oregonrocks and the Scottish
LaurelHill granodiorite,
and Detroit Reservoir rocks?More work is needed,but the following
granodiorite
eachhavevery similar8180values. featuresare suggestedfor consideration
in
This is somewhat coincidental, since the 3
sampleschosenjust happento have similar
subsequent
detailedstudies:
1. The mostlikely explanationof the differ-
whole-rock8180values.Nonetheless,the isotopic encesin A valuesis that the finer-grainednature
similarities,together with the fact that the of the quartzin theOregon
rocks(muchof it as
made it more susceptible
to
measuredvalues of /•qtz-feldand /•feld-amr•h
are micropegmatite)
similar to values obtained on 'normal' igneous
exchange.
Quartz is much lessabundantin
than in
rocks[Taylorand Epstein,1962;Taylor,1968], thesequartzdioritesand granodiorites
7866
Huex• P. TAYLOR,JR.
the quartz-bearing Scottish intrusives, most of
which are true granites in which the quartz is
much coarsergrained. Also, in certain instances,
very fine-grained micropegmatite quartz from
Scotland has also been exchangeddown to very
low •sO values, as is shownby the data on an
Ardnamurchan granophyre given in Table 1.
The most conclusiveevidence on this point,
however,is given by the large valuesof
from the Nimrod granite. This body contains
quartz that is both coarser grained and more
abundant than in any of the granodioriteslisted
in Table 1, and the quartz-feldsparfractionations
of 3.8 to 5.5 are similar
to those observed in
several granitesfrom Skye and Mull.
2. The Oregon magmas conceivably might
have undergonea more pronounced•sOdepletion
before solidification than the Scottish ring
intrusions, and they almost certainly were involved in a much less complicated intrusive
history. In the Scottish Hebrides there was
long period of successiveigneous intrusion,
alternating with explosive volcanic activity,
that probably involved a complex overlapping
of meteoric-hydrothermM convection systems;
this would favor non-equilibriumphenomena.
3. The hydrothermal fluids in Scotland and
Oregon may have differed chemically. For
example, Taylor and Forester[1971] suggestthat
Pb and Sr isotopic compositionsof the Skye
rocks may have been drastically affectedby the
meteoric hydrothermal activity. This in turn
suggeststhat the Scottishhydrothermalsolutions
were carrying various materials in solution,
perhapsabundant NaC1, KC1, CaC12,etc. Such
alkali-chloride solutions have an enormously
greater potential of exchanging•sO and cartons
with feldsparsthan does relatively pure
O'Neil and Taylor [1967]have shownthat, during
cation exchangewith feldsparsin hydrothermal
solutions, the oxygen isotopes are always exchanged as well. In almost all cases, the Skye
and Mull feldsparsand whole rocks have undergonea greaterover-all depletionin •sOthan have
their counterpartsin Oregon. All of these considerationssuggestthat in Oregon we may be
dealing with meteoric-hydrothermal solutions
that have not picked up appreciable alkali
chloridesin solution,and thus that the effect of
differential exchangein coexistingfeldspar and
quartz is much less pronounced.This is reasonable in terms of the geologyalso, becausethe
ground waters at Skye and Mull certainly
penetratedthe Mesozoicand Precambriansandstone sections,as well as the overlyingplateau
lavas, whereas,at the presentlevels of exposure
in Oregon, they have only passed through
volcanics.It is probable that the former are a
much more likely source of saline solutions
(evaporites
and/or ancientbrine)thanthe latter.
The only chemicalconstituentsthat definitely
seem to have been carried by the meteorichydrothermal solutionsin the Oregonintrusive
belt are the boron and fluorine necessary to
explain the widespread tourmalinization, for
example,in the Bohemia mining district [Buddingtonand Callaghan,1936]. The volcanicsand
the intrusives are probably an adequate source
of B and F. It will be interesting to check these
ideas further by looking at the compositionsof
fluid inclusionsin minerals from the respective
areas.
4. There is evidence that in Oregon we are
dealingwith much broaderhydrothermalalteration systemsthan those that surroundedthe
Scottishintrusions.If the oxygenisotopiceffects
are spread out over a much larger volume of
country rock, we might a priori expect that,
other things being equal, the over-all •80 depletion would have to be correspondinglyweaker.
Although the 3-dimensional shapes of the
Oregon stocks are not very well known, if we
were to make the assumption that like the
Scottish intrusionsthey are roughly cylindrical
with steep or near-vertical contacts,the •80depletedzonessurrounding
the Oregonintrusives
are truly remarkable in their areal extent
(Figure 1). This is well shownin Figure 8, where
the whole-rock 5•sO results are plotted as a
function
of distance from the intrusive
contact.
In order to take into account the fact that the
larger intrusionsshouldin generalbe surrounded
by correspondingly
largermeteoric-hydrothermal
convection systems, the distances are plotted
in units of stock diameter. Note that the low-•sO
country-rockzonesin the Oregonlocalitiesextend
outward
at
least
2.5
to
3.0
stock
diameters
away from the intrusive contact. However, in
Scotland the meager data available suggestthat
these effects are confined
to within
about
0.5
'stock' diameter [Taylor and Forester,1971]. The
numbers for Scotland are perhaps somewhat
misleading,becausethe diametersof the Scottish
'stocks'
are assumed
to be those of an entire
II•TERACTION BETWEEl• GROUND WATERS Al•D INTRUSIONS
7867
+9
+8
+7
+6
--
ß
ß ß
ß............
5=5.5%ø
+5
+4
.-.
ß
o•' +2"
•o +4•
•
O,
ßß
I• ß
(•018values
of volcanic
country
ß •,
rocks,
Western
Cascade
Range,
;•e•, •
o
-3 --
• •
-4•--
•:•
•
0
Oregon
•)
I
•
2
3
4
5
6
7
8
9
•0
4•
•2
Distance from nearest intrusive contact(in units of stock diameter)
Fig. 8. Plot of whole-rock•180 values of volcanic country rocksfrom the Western Cascade
Range versusdistancefrom the closestintrusive contact (in units of stock diameter). At distances
greater than 3.0 stock diameters, the i•lsOvalues are essentially 'normal.'
plutoniccomplexrather than any singleintrusion
within the complex.However, even if we consider
only the diameter of a single igneous center
rather than the entire complex, the low-•SO
aureolesin Oregonare muchbroader,particularly
the ones surrounding the Nimrod and Detroit
localities(Figure 1).
We can only speculate as to the reason for
dippinguppercontacts.That this is actuallythe
casefor the ChampionCreekstock(seeFigure 4)
is suggestedby the myriad small intrusionsto
the east of the stock and the indication
that the
eastern contact dips gently eastward, roughly
concordant with the surrounding strata [Bud-
dingtonand Callaghan,1936, p. 426]. Also, the
Detroit Reservoir stock (Figure 6) has an essenthese differences in the relative sizes of the
tially fiat upper contact at about 600 meters
aureolesin the two regions.Obviously,a possible altitude [Pecket al., 1964, p. 39].
effect of importanceis the one pointed out above,
A possiblemodelof the meteoric-hydrothermal
namely, that the Scottish plutonic centers are alterationsystem. Several aspectsof meteoriclargely composed of a number of small ring
hydrothermalalteration systemsassociatedwith
intrusions emplaced in successionover a long epizonal igneousintrusionshave been discussed
interval of time. Thus at any one point in time
by Sheppardet al. [1969]and Taylor and Forester
the accompanyingmeteoric-hydrothermalsystem [1971] and will not be repeated here. However,
some of the unique features exhibited by the
might also be relatively small. Superpositionof
many such small systems over an extended Oregon bodies are worth considering.A hypoperiod of intrusive activity would explain two
features of the isotope data: (a) the relatively
thetical compositestock that incorporatesmost
smaller
sectionin Figure 9. The stock acts as a gigantic
'heat engine' that produces a complex pattern
of convective circulation in the adjacent local
ground waters.
The compositestock shownin Figure 9 was
areal extent
of the low-•SO aureoles in
Scotland,and (b) the fact that even though they
are relativelysmallerin size,the actual magnitude
of the •sO depletion (particularly in feldspars)
is much greater in Scotland.
Another feasible explanation of the data is
that the present outcrop areas of the Oregon
intrusionsonly representthe erodedtops of large,
broad intrusionswith relatively gentle,outward-
of these features
is shown
in vertical
cross
constructed as follows:
a. The stockis shownwith irregular, outwarddipping contactsin part for the reasonsgiven in
paragraph4 above.Almost all the intrusivesin
7868
ttu6•
the Western
P. TAYLOR,JR.
Cascades are associated with num-
diameterswide, even ignoringthe amountsnecessary to heat the large volumes of ground water.
This calculation is compatible with what is
actually observed in the Scottish Hebrides
intrusions, but it is totally at odds with the
Oregon data. Therefore, the Oregon intrusions
either broaden considerably with depth, as
indicatedin Figure 9, or the propylitic alteration
200øC(e.g., from about 50øCto 250øC),then zones are underlain by numerous igneous inwith a specificheat of 0.25 cal/g, approximately trusions that do not outcrop at the present
erous,small satellitic dikes and plugs, which are
probably offshootsof the larger pluton. The
clearestevidencefavoring this concept,however,
is given by a simple heat-balance calculation.
If the propylitically altered zone is considered
to have been producedby an averagerise in the
temperature of the country rocks of only about
erosion level.
50 cal/g of heat must be addedthroughoutthe
b. In light of the above considerations,it is
perhapsreasonablethat, the greater the outcrop
at a temperature
of 950øC,themaximumamount area of an intrusive body, the greater the depth
of heat that can be obtained during crystalliza- of emplacementof that part of the body. This
tion and coolingto 400øCis only 220 cal/g (this would mean that the Nimrod pluton would
includesabout80 cal/g to take into accountthe represent the most deeply eroded pluton and
latent heat of crystallization).Thus a cylindrical the South Umpqua River intrusions the least
stock only containsenoughheat to producean deeply eroded. It was on this basis that the
propylitically altered zones. If we assume a
cylindrical magma body initially entirely liquid
alteration
various
aureole at most about 0.6 to 0.7 stock
0
2
lOOO
\X
horizontal
HORIZONTAL DISTANCE (km)
6
8
10
12
4
So.
umFquo
R. /
\
sections
14
I•-
were
constructed
16
\ .... "x\
500
2000
5000
lOOO
400O
' /
Cho•mp•on
Cgek_/
,,
Derroll
'•
•
,
'
•
e
o•. •._ •.*'
1500
5000
.,xx
,,
•'•
6000
/
/ /
7000
8000
•'
"x...
.
'-',,'•'
•"
/ '-'
(
....
_x
.
,q)
'; •,.-g"',.>""•
_•
\
Br•ce Creek
'
2000
\ '•
--•
\
\
2500
X
9000
0
1
2
3
4
5
6
7
8
9
10
HORIZONTAL DISTANCE (miles)
Fig. 9. A vertical crosssection through a compositehypothetical granodiorite stock in the
Western CascadeRange. The depths below the land surface are only rough estimates, but the
horizontal distanceshave been scaled off the maps in Figures 3, 4, 5, and 6. The horizontal wavy
lines indicate approximate east-westsections(looking north) through the South Umpqua River,
Detroit, and Nimrod localities, and approximate SE-NW sections (looking SW) through the
Brice Creek and Champion Creek stocksin the Bohemia Mining District. The whole-rock •sO
data on Figures 3, 4, 5, and 6 have been transformed into a seriesof •sO contoursto indicate the
varying degreesof •sOdepletion sufferedby rocks in the vicinity of the stock. Arrows indicate the
radial inward flow of ground waters toward the intrusive stock, the rise of the heated, low-density
H•.O, and plausible convective circulations of these fluids through the margins of the stock and
in the adjacent volcanic country rocks.Certain stratigraphic units in the volcanicpile are probably
muchmorepermeablethan others.These'aquifers'mightlargelycontroltheconvectivecirculation.
INTERACTION BETWEEN GROUND WATERS AND INTRUSIONS
through the compositestock shownin Figure 9.
In supportof the arrangementshownin Figure9,
wealsonotethat: (1) The Nimrod stockpresently
crops out at the lowest elevation of any of the
intrusivebodies,only 275 metersabovesealevel,
in the middle of one of the largest and most
deeplydissectedcanyonsin the WesternCascade
Range. (2) The Brite Creek and Champion
Creek stocksare only 2 km apart horizontally,
but the samplesof the former were collectedat
an altitude of about 700 meters, whereasthose
of the latter are from an elevation of 900 meters
(see Figure 4). (3) The South Umpqua diorite
bodies intrude by far the thinnest section of
volcanic rocks of any of the localities studied.
(4) The Detroit Reservoir and Big Cliff Dam
bodies intrude by far the thickest volcanic
section,and these bodiespresentlycrop out at
7869
volcanic rocks are inherently much more susceptible to isotopic exchange than the stock
becauseof their finer grain size, this does not
necessarilymean that less H20 moved through
the periphery of the stock than through the
contact metamorphic aureole. In fact, in the
caseof the Champion Creek and Detroit stocks,
very low •sO values are characteristic of large
parts of the intrusive bodies.However, it is also
clear from Figure 9 that, as one moves inward
into the stock,at somepoint the relative amounts
of HaO circulatingthrough the rocksmust begin
to fall off sharply.The relatively high •sO values
in the central parts of the Champion Creek,
Brice Creek, and Nimrod stocks are clear
evidence of this effect.
The most probable explanation of the above
phenomenonis that the hydrothermalconvection
altitudes of 450 to 600 meters.
system begins operating in the country rocks
c. The relative
elevations
of the various
immediatelyuponintrusion(and perhapsreaches
localitiesshownon Figure9 are probablyreason- its maximum importanceat this time), whereas
ably well placed,but the actual depthsassigned as long as the stockis partly molten, no fractures
are obviously largely guesswork.Nonetheless, or joints can form and thus no convective
the various intrusiveshave textures and grain circulationof H20 can be set up insidethe body.
sizesthat are plausiblefor the assigneddepths. By the time the center of the stock has solidified
The relatively equigranular, medium-grained enough to fracture, the 'heat engine' is itself
granodioritesand granitesmust have formed at much smaller and the convection system is
depthsof at least 1 or 2 kin, particularly when beginningto turn itself off. Also, as a result of
it is rememberedthat thesebodiesundoubtedly mineral depositionalong the hydrothermal concrystallizedabnormally rapidly becauseof the duits,many of the routesof accessfor the aqueous
rapid cooling and heat loss brought about by fluids are partially sealedoff by this time.
large-scale
interactionswith the circulatingmeteIt goeswithout saying that heat-flow calculaoric ground waters.
tions that only involve solutionsof the simple
d. If the generalshapeand assigneddepthsof heat conductionequation [e.g., Jaeger,1961] are
the hypothetical stock shown in Figure 9 are totally useless in this type of environment.
acceptable, the rest of the diagram follows Essentially all the heat is removed through
directly from the •sOdata shownin Figures3, 4, heating and convectionof the circulatingground
5, and 6. The convective circulation of the
meteoric ground waters in the vicinity of the
stockis only schematic,however,and is probably
much more complicatedthan shown. Note that
the volumeof altered, •sO-depleted
countryrock
is about 30 times as large as the volumeof the
stockitself, demandingthat important quantities
of ground water circulate and exchangewith a
heat sourceat even greater depths than those
shown on Figure 9. The width of the alteration
zone obviously cannot continue to increase at
a constantrate with depth.
Figure 9 showsthat the maximum •O depletion of the rocks occurs at the stock contact or
in the adjacent countryrocks.Inasmuchas the
waters.
OnthebasisofmanyD/H and•sO/•øO
analyses
of modern geothermalwaters from a variety of
areas, Harmon Craig has shown that all hotspring waters are at least 95% of surfacederivation [e.g., White, 1968]. Thus the model shown
in Figure 9 may apply equally as well to many
present meteoric-hydrothermalsystems.One of
the major difficultiesin interpretingsuchsystems
has been the problem of heat transfer between
the heat source(i.e., the intrusion) and the H20.
This is generally shown as occurringby simple
heat conduction[White, 1968, Figure 3]. On the
basisof the data in the presentstudy, the heattransfer problem can probably be adequately
7870
HUGH P. TAYLOR,JR.
explainedby migration of the H20 directly into
the heat sourceitself. The heat exchangethereby
occursthrough intimate physicalcontactbetween
the t:[20 and the igneousintrusion.
As well as direct heat and nO exchangewith
the solidified intrusive body, some diffusion of
meteoricH20 into the magma undoubtedlytakes
place. The augRe diorite and granodiorite magmas in the Western Cascadeswere undoubtedly
undersaturated in H20 at the time of intrusion
and would thus act as a 'sink' for any mobile
H20
available in the local environment.
This
would producesomenO depletion,but we know
that most of the 8nO lowering in the Nimrod
stock, for example,occurredafter crystallization,
because the feldspar exchanged much more
extensively with the aqueous fluids than did
coexisting quartz in the same hand specimen
(seeTable 1).
Amounts of H•O involved in the meteorichydrothermalsystems. We know the initial 8nO
values of the volcanic country rocks before
hydrothermal alteration with a high degree of
accuracy:+6.5 4- 1.0 per mil. However, the
initial 8nO of the ground waters before heating
by the Western Cascades intrusions is more
difficult to fix. They almost certainly must have
had 8nO values between about --5 and --15,
becausethese are typical for present-day fresh
surface waters throughout the world (excluding
the arctic regions,oceanicislands,and extremely
high elevations). A closer estimate can perhaps
be made on the basis of D/H analysesof OHbearing minerals in the Oregon intrusive bodies.
H. P. Taylor, Jr., and S. Epstein (unpublished
manuscript, 1971) have shown that 4 actinolites
and chlorites from these bodies have 8D =
--85
to --100 per mil (relative to SMOW). If we
assume a 8D fractionation of about 30 per mil
between H•O and these minerals, which is a
reasonablevalue at •500øC (T. Suzuoki and
S. Epstein, unpublishedmanuscript, 1971), the
8D valuesof thesemeteoric-hydrothermalwaters
must
have been about
--55
to --70.
Inasmuch
as enormousamountsof H20 are involved, these
8D values must also be identical
to those of the
original, cool ground waters. Using the meteoric
water equation
•iD = 88•s0 --[- 10
[Craig, 1961] we obtain estimated initial
valuesof --8 to --10 for theseground waters.
A reasonablechoicefor the original •inOvalue
of the meteoric ground water is therefore --9.
However, several processescan continuously
operate to modify this •i value before the water
enters the hydrothermal exchangesystem (isotopic exchange with the surrounding rocks,
mixing with magmatic or connatewaters, etc.).
Therefore it is important to considerhow much
such processesmight modify the 8nO values of
the ground waters.
Marine connatewaters initially have •inO• 0
and primary magmatic waters have •inO•--_+6
to -4-8;thus additionof either type of H20 would
enrich the ground water in nO. Effects of exchange with the surrounding rocks are more
difficult to evaluate becausesuch processesare
very sensitive to both temperature and time at
temperatures
of 50ø to 200øC.In all subsequent
discussions,
we shallmake the reasonable
approxi-
mationthat •irock
and l•plagioclase(Anao
) are essentially identical at equilibrium so that we can
utilize the feldspar-water isotope geothermometer of O'Neil and Taylor [1967]. The applicable
equation is then
•rock-- •H,O• 2.68(10øT
-2) -- 3.53
(T in øK)
This means that, at temperatures less than
about100øC,smallamounts
of waterin exchange
equilibrium with isotopically 'normal' volcanic
rocks can attain
8nO values less than --9.
How-
ever, rates of exchangeare so low at such tem-
peraturesthat this is probably not an important
process,particularly whenlarge amountsof water
are involved.
In light of the above considerations,
it is not
necessaryto consider initial •iw=te,values lower
than --12 or higher than --3, and it is felt that
a •iw=ter
• --9 is by far the most likely. These
resultsare incorporatedin Figure 10, where for
variousvaluesof 8w=te
r we plot 8•o• as a function
of the water/rock ratio calculatedon the basis
of the simplest possible closed system model
(e.g.,Table 2, and Sheppardet al. [1060,p. 770]).
The data in Figure 10 indicate that as a
practical matter it is impossibleto explain the
lowest-nO
volcanic
rock
at
Nimrod
with
a
8w•te•= --6 much lessthan with a 8w•t•, = --3.
Absurdlyhighwater/rockratios(>>10)wouldbe
required,assumingany reasonabletemperature
lower than 600øC.Even for •iw=t• = --9, a
INTERACTION BETWEEN GROUNd) WATERS aN•) INTRUSIONS
z¸ +a
"%..?¾.:..
3 -6•
0.1
7871
-
rock
(Nimrod)"
.......................
•
J
0.2
0.3
0.4
WATER/ROCK
0.6
0.8 q.0
2.0
3.0
4.0
6.0 80 •0.0
RATIO (ATOM % OXYGEN)
Fig. 10. Plot of whole-rock•so of the volcaniccountryrocksof the WesternCascadeRange
versus the calculated ratio of the amount of meteoric-hydrothermalwater to the amount of
exchanged,•sO-depletedvolcanic rock (log scale). It shouldbe rememberedthat this is an integrated water/rock ratio, in that it refersto the total amount of water that has migrated through
and exchangedwith the rocks.The 4 diagonalbandsrepresent4 different •so valuesof the initial
groundwater enteringthe exchangesystem (•wa,•r = --12, --9, --6, and --3). The upper and
lower limits on eachband are calculatedon the basisof isotopicequilibriumbetweenrock and
water at 400øC and 600øC, respectively.Most of the country rocksin the vicinity of the igneous
intrusionswereprobablyheatedto suchtemperatures.However,if a lowertemperatureof 300øC
were chosenfor a particular band, the upper limit of that band would lie approximatelyin the
middle of the next overlyingband, and correspondingly
greater amountsof water would thus
be necessaryto producea given •so depletionin a rock. The horizontal lines indicate the average
•sO valuesof the volcaniccountryrocksin 4 differentlocalitiesstudiedin the presentwork, as
well as the •so value of the most •sO-depletedrock found in Oregon (from the Nimrod area).
The band for •wat•r= --9 is thought to closelyapproximateconditionsin the Western Cascade
meteoric-hydrothermalexchangesystems,and under thesecircumstancesthe averagewater/rock
ratios for the variouslocalitieswould be given by the crosses
in the •w•t• = -9 band. If initial
/iw•,• values of -6 or greater are chosen,it is impossibleto explain the low-•so rock from Nimrod
with any reasonablewater/rock ratio (i.e., <10).
relativelyhigh water/rock ratio of about 6.0 is
terns in the Western
required (Figure 10). Using this /•w•e• and the
average/•'sOvaluesof the volcaniccountryrocks
at variouslocalitiesin the Western Cascades,we
areas, we must have had interaction between
roughly equal amounts of water and rock
(expressedas atomic per cent oxygen). Note
that about 8% of the area of the Western
Cascadesis probably underlain by rocks that
have beendepletedin •sOby about 5 to 6 per rail;
obtainthe averagewater/rockratios shownin
Figure 10 and Table 2. It is of interest that the
smallestintrusions(South Umpqua River) are
associatedwith the smallestwater/rock ratio,
Cascades. In most of the
therefore, at least in this region, such low-•SO
whereas the reverse effect is observed for the
igneousrocks cannot be consideredto be uncom-
large Nimrod stock. Figure 10 also clearly
demonstratesthat the lower the temperature of
mon. The developmentof such large-scale•sO
depletion requires a total volume of water of at
least 2000 km 3. If this water was suppliedfrom a
drainagebasin« the area of the presentWestern
Cascades,and if we assumean annual rainfall of
75 cm (30 inches), of which only 5% ultimately
is added to the deep ground-water circulation
hydrothermalalteration,the larger the water/
rock ratio necessaryto producea given/•ro•k.
Table 3 presents some approximate calculations of the total amounts of H20 involved in
various meteoric-hydrothermalalteration sys-
7872
HuG• P. TAYLOR,JR.
TABLE 2. Calculated 'Temperatures'* of Iso- undergone appreciable •s0 enrichment through
topic Exchange between H20 and the Volcanic exchangewith the heated rocks through which
Country Rocks, Assuminga Simple Closed-System
it has passed.It is probablyat presentimpossible
Model (see text) and Constant Water/Rock Ratios
of 0.8 and 1.0
to separate these two effects.
(Under these conditions, the f•sO contours in the
The variations in r]•s0in the volcanic country
country rocks in Figures 4 and 9 also represent rocks are perhaps more amenable to analysis.
isotherms)
Temperature, øC
f.-ook
The outwardincreasein r]•s0in the propylitically
altered zones cannot, of course,be due to the
water having been abnormally enrichedin xs0
due to exchange. Just the reverse would be
expected.There is, however,probablya tendency
w/r = 0.8
w/r = 1.0
+6
+5
+4
+3
+2
115 ø
140 ø
175 ø
215 ø
275 ø
110 ø
135 ø
165 ø
200 ø
245 ø
for somewhatlarger water/rock ratios to be
+1
360 ø
305 ø
0
--1
--2
505 ø
845 ø
•
395 ø
540 ø
875 ø
exchange rates are so much slower. Second,
because of radial inward migration of water
* Calculated assumingthat
frock• •plagioclase(Anao),
utilizing the following equation'
frock -- 6.5
--9 -- (frock -- •)
= w/r = 0.8 or 1.0
where 6.5 is the original fx80 of the unaltered
volcanic rocks, --9 is assumed to be the original
fx80 of the meteoric ground waters, and A is the
x80 fractionation between rock and water; A =
2.68(106T-•-) -- 3.53, where T is in øK [O'Neil and
Taylor, 1967].
found in the vicinity of the stock margins.First,
at temperaturesbelow 100øC the effective
water/rockratiois very small,because
isotopic
toward the igneousintrusion, the water/rock
ratio would be expectedto progressivelyincrease
toward the stock because,assumingcylindrical
geometry, the rock volume decreasesinward as
the squareof the distance.However, throughout
the zones of upward streaming or convective
circulation, the water/rock ratio should be
roughly constant.
If the water/rock ratio is in fact roughly
constant throughout an appreciablevolume of
rock, the r]•s0contoursin the rocks should also
representapproximateisotherms,as is indicated
in Table 2 for two differentwater/rock ratios,
0.8 and 1.0. Thus the increase in r]•s0 outward
system, it would require only about 7000 years
to produce the observed effects. In particular,
note that in young volcanic terranes most of
the local rainfall quickly migrates down into
the jointed rocks, very little moving off immediately as surfacerunoff.
Obviously the period of intrusion, crystallization, and cooling of the Oregon plutonic bodies
extended over a much longer time period than
7000 years. In fact, even today many hot springs
are found throughout the Western Cascades,
though they generally tend to occur somewhat
to the east of the axis of mid-Tertiary intrusions
shown in Figure 1.
Temperature gradients in the hydrothermal
alteration
aureole.
It
is clear that
the inward
increasein b•sOin the hypothetical stock shown
in Figure9 is dueto progressively
smallerwater/
rock ratios as one moves into the stock. Also, it
is in part a result of the fact that by the time the
H•.O gets well into the stock it has already
from the stock would be principally due to
falling temperatures as one moves away from
the heat source. In the region between the
contoursr]rook
- -[-6 and-[-4, wherethe water/
rock ratio is likely to be relatively smaller, it
can be shown that the calculated temperatures
are not stronglydependenton the water/rock
ratio. For example,at •rock= +4:, the calculated
'temperatures'are 235øC and 195øC, respectively, for water/rock ratios of 0.4 and 0.6
(comparewith data in Table 2). For (•rock> +4:,
the temperature differences are practically
negligible.
It is probably going to be difficult to directly
apply oxygen isotope geothermometry to coexisting minerals in the propylitic alteration
zones becauseof the general lack of attainment
of isotopicequilibrium (see Table 1). Therefore,
we must have recourse to indirect
methods like
those shownin Table 2. In fact, however,for a
water/rock ratio of 0.8 or 1.0 (which from
INTERACTION BETWEEN GROUND WATERS AND INTRUSIONS
7873
Figure 9 and Table 3 seemto be typical values
in the Western Cascades),theseindirect methods
give very reasonable temperatures. The horizontal temperature gradient outward from the
Champion stock (Figure 4) would, on the basis
All the above occurrencesare, however, of
that low-•sOigneousrocks are nowhere near so
rare as was once thought. Areas of at least 1000
km 2 in Oregon and 1000 km 2 in the Scottish
Hebrides are apparently underlain by rockswith
•sO - --6 to -]-4. Very extensive areas of
igneousbodies that have interacted with super-
Cretaceousor Tertiary age; very little work has
been done to try to identify low-•SOigneous
rocksof pre-Cretaceousage. The only definitely
known occurrencesare singlesamplesof the late
of sucha calculation,fall off from about 550øC Precambrian SeychellesIsland granite [Taylor,
in the peripheryof the stock to about 110øC 1968] and the Mesozoicring-dike complexin the
at the b = •-6 contour. This correspondsto an Ossipee Mountains, New Hampshire (H. P.
averagedropof about100ø/kmto the westand Taylor, unpublisheddata, 1971). Inasmuchas we
70ø/km to the east of the stock,the gradient are dealing with phenomenathat take place in
becoming less steep away from the intrusive shallow parts of the earth's crust, it would be
body.
expectedthat becauseof erosionand burial by
Occurrences
of low-•SOigneousrocks. The data younger rocks the older occurrenceswould be
in the present study and that given by Taylor much less common. It is neverthelessvery im[1968] and Taylor and Forester [1971] indicate portant to searchfor and identify Precambrian
low-•sO rocks also occur in the San Juan Moun-
tains, Colorado(H. P. Taylor and R. W. Forester,
unpublisheddata, 1971). They also have been
found in parts of the Boulder batholith (S. M. F.
Sheppard and H. P. Taylor, Jr., unpublished
manuscript, 1971), the Skaergaard intrusion in
Greenland [Taylor and Epstein, 1962], and in the
Corona micropegmatitegranite of the Southern
California batholith (H. P. Taylor, unpublished
data, 1971).
ficial waters,becausethe b•sOand bD valuesof
such waters are determined in large part by
the bD and b•sO values of the oceans that existed
at the time of intrusion. Thus, this may be a
way to trace the variation of b•sOand bD in
ocean waters and rain waters through geologic
time.
This method could have some great
advantages compared with the utilization of
sedimentarymaterialslike chertsand carbonates
to obtain data on the isotopic evolution of the
ancient oceans[e.g., Perry, 1967]. The principal
advantage is that one would be dealing with
much more coarsely crystalline rocks which
have an enormouslybetter chanceof preserving
TABLE 3. Data on the Propylitically Altered Areas of the Western Cascade Range, and Calculated
Amounts of H20 Involved in the Various Meteoric-Hydrothermal Alteration Systems
Vertical
Area
Avg. •so c.r.*
Area, km2
Extent,
meterst
Volume
of
Altered Rock, Water/Rock
km 3
Ratio :•
Volume of
H20, km3õ
South Umpqua
River
Bohemia district
•- 3.0
•-0.7
25
75
400-1000
1000-3000
10-25
75-225
0.35
0.7
5-13
75-225
Nimrod
- 1.3
250
1500-3500
375-875
1.2
650-1500
Detroit Reservoir
•-0.1
500
1500-5000
750-2500
0.8
900-3000
=0.0
= 1200
--0.8
-- 2000-4500
Entire Western
Cascades
(15,000 km •)
-- 1500-3000
-- 1800-3600
* Average •180 of samples of the propylitically altered volcanic country rocks (for the South Umpqua
River area, the average includesthe •sO analysesof the small intrusive bodies; see Figure 3).
I Assumedvertical distance (in meters) over which the low-xsOalteration effectsprobably extend; this is
estimated from the areal extent of the alteration zone and from the thicknessof the volcanic pile in the given
locality [after Peck at el., 1964].
$ The calculated water/rock ratio (in terms of atomic per cent oxygen) as given in Figure 10, assuming
that the H•O has an initial •O
•
--9.
õ Approximate volume of meteoric ground water involved in each hydrothermal alteration syslem,
assumingthe density of the tt•O is 1.0 g/cm s. Note that these are minimum values becauseit was probable
that isotopicequilibrium was not invariably attained between the rocks and the It•O.
7874
HUGH •P. TAYLOR,JR.
their 180/160(andD/H?) ratiosthroughgeologic
Oregon, U.S. Geol. SurveyProf. Pap. JJ9, 56 pp.,
1964.
time. Also, the time coordinatecan in theorybe
Perry, E. C., The oxygen isotope chemistry of
exactlyfixed, becausethe originalisotopicinterancient cherts, Earth Planet. Sci. Lett., 3, 62-66,
action with ground waters had to occur during
1967.
Sheppard,S. M. F., R. L. Nielsen, and H. P. Taylor,
crystallizationand coolingof the pluton.
A clcnowledgments.
This research was supported
by the National Science Foundation, grant GA12945.
Discussionswith A. F. Buddington, R. W. Forester, and D. E. White have been helpful. E. M.
Taylor kindly providedtwo of the samplesanalyzed
in this study, and P. Yanagisawa did much of the
laboratory work.
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