Download Fluorine and Chlorine in Granitoids from the Basin and Range

Economic Geology
Vol. 81, 1986, pp. 1484-1494
Fluorine and Chlorine in Granitoids from the Basin
and Range Province, Western United States
ERIC H.
CHRISTIANSEN~
Department of Geology, University of Iowa, Iowa City, Iowa 52242
AND DONALD E. LEE
U. S. Geological Survey, Box 25046, Federal Center, Denver, Colorado 80225-0046
Abstract
Analysis of fluorine and chlorine in 228 samples of granitoids' from the Basin and Range
province of the western United States suggests that at least three types of granitoids can be
distinguished: (1) fluorine-poor granitoids of the northwestern Great Basin (mean F = 0.041
wt %, maximum = 0.11 wt %) intrude a variety of allochthonous oceanic or island-arc terranes
that were probably accreted to North America during the Paleozoic and Mesozoic eras; (2)
fluorine~rich Precambrian granites generated during "anorogenic" magmatism of Proterozoic
-age in the southwestern United States (mean F = 0.118 wt %, maximum = Q.35 wt %); and
(3) a large group of granitoids with moderate fluorine contents (meanF = 0.053 wt %, maximum
= 0.13 wt %). Thislast group consists of granitoids of the eastern Great Basin and the southern
Basin and Range province that occur throughout the autochthonous continental terrane of the
western United States. These differences'in fluorine concentration do not appear to be the
result of regional differences in the degree of magma evolution. Instead, this variability is
attributed to magma contamination by, or generation from, a comparatively fluorine-rich reservoir in the continental crust underlying the southern and eastern portions of the -Basin and
Range province and to the absence of this reservoir in the northwestern Great Basin. This
interpretation is consistent with the geologically established boundaries of the" exotic" terranes
and with the Sr and Nd isotope compositions of rocks from these regions. Chlorine contents
(mean CI = 0.013 wt %, range = 0.005-0.050 wt %) show no regional differences and are
uniformly low in these granitoid rocks.
-
Introduction
INVESTIGATIONS of the' geochemistry of fluorine and
chlorine in igneous rocks are important in a variety
of studies including: the origin and distribution ofore
deposits and exploration for them (e.g., as pathfinder
elements for deposits of fluorite and fluorophile metals-Be, U, Ta, Sn, and perhaps Mo and W); the behavior of volatile elements in magmas (e.g., Burnham,
1979); and as ,indicators of the sources of magmas
(e.g., Bailey, 1977; Ishihara, 1981). In this paper we
report the results of fluorine and chlorine analyses of
228 specimens of granitoids from the Basin and Range
province of the western United States and speculate
about the origin and implications of the pronounced
regional differences in their fluorine content. Chlorine
shows considerably less variation (perhaps as a result
, of secondary processes which modified the original
magmatic CI content).
Regional differences in the distribution of fluorite
deposits across the western United States have been
recognized for decades. Lindgren (1933) pointed out
the concentration of these deposits in the eastern part
of the western cordillera. On the basis of the analysis
Present address: Department of Geology, Brigham Young
University, Provo, Utah 84602.
9
0361-0128/86/596/1484-11 $2.50
of fluorine ·in 170 specimens of unaltered glassy
rhyolitic rocks, Coats et al. (1963) showed that the
distribution of rocks with high contents of fluorine
(>0.10 wt %) correlated well with the distribution of
significant fluorite deposits. Fluorine-rich volcanic.
rocks are concentrated in the eastern part of the Basin
and Range province (as well as in Idaho, along the
Rio Grande rift, and in the Big Bend area of west
Texas). Peters (1958) suggested that these geochemical and metallogenic features were the result of a
persistent geochemical anomaly in the crust across
the region. Eaton (1984) has reiterated this concept.
Others, also noting the distribution of fluorite deposits
and their association with fluorine-rich rocks in the
western United States have suggested that the fluorine
enrichment occurred in the mantle-either during
generation of fluorine-rich "alkalic" magmas along a
subduction zone (Lamarre and Hodder, 1978) or
during magma generation accompanying extensional
tectonism (Shawe, 1976; Van Alstine, 1976). In addition, Bailey (1977) has questioned the existence of
fluorine proviI).ces over long geologic periods and
suggested instead that the regional differences in fluorine content of volcanic rocks noted by Coats et al.
(1963) are the result of differences in magmatic evolution or alkalinity. The results of our investigations
1484
148,5
F & CL IN GRANITOIDS, WEST U. S.
110°
support the early suggestions of Peters (1958) and
Coats et al. (1963) with regard to the importance of
the geographically variable composition of the crust
for establishing the concentrations of fluorine in silicic
igneous rocks from the Basin and Range province.
... .
;
/,'
,/,/,/'
!
40°
•
•
•
i,
87 Sr /86 Sr
.
=.706).
Sampling and Analytical Techniques
The specimens discussed here were collected as
part of a comprehensive regional study of the elemental and isotopic geochemistry of granitoid rocks
from the Basin and Range province. Other ·investigations of this same suite of specimens are reported
by Lee et al. (1980; Ba geochemistry), McNeal et al.
(1981; U and Th geochemistry), and Lee et al. (1981;
oxygen isotope geochemistry). Sr isotope ratios have
been determined by R. W. Kistler (1983 and in prep.).
Farmer and DePaolo (1983, 1984) present Nd and
Sr isotope analyses of several of these samples and
others from the same region. Quantitative major and
trace (F, CI, Rb, Ba, Li, Cs, As, Bi, Cd, and Zn) element
analyses of whole-rock samples are reported by Lee
(1984). To save space, we have not duplicated tables
of fluorine and chlorine contents for individual spec~mens. The specific results are available from the authors or by reference to Lee (1984).
Two rock specimens were collected and analyzed
from each of 114' plutons (Fig. 1) within the Basin
and Range province. These plutons were randomly
selected from the granitic (sensu lato) plutons in each
1 0 by 1 0 cell within the boundaries shown in Figure
1. All of the specimens were removed from fresh outcrops using a sledge hammer. Only areas deemed
typical of the intrusion were sampled; zones of aiteration, iron staining and friable rock were avoided.
Similarly, xenolithic material, dike rocks, and minor
intrusive variants were not sampled.
Bulk specimens were crushed in a steel jaw crusher
to about 10 mesh and then ground to - 100 mesh
using a plate grinder with ceramic plates. Subsequent
grinding in an agate shatterbox reduced grain sizes
to approximately -300 mesh. H. Neiman determined
the fluorine concentrations by means of an unpublished selective-ion electrode method devised by
D. ~. Norton and J. M. McDade. J. S. Wahlberg obtained the chlorine concentrations by X-ray fluorescence (Wahlberg, 1976). All analysts are affiliated
with the U. S. Geological Survey in Denver, Colorado.
Fifty specimens were chosen at random for replicate
analysis and were analyzed along with the other specimens in a randomized sequence. The results of the
replicate analyses indicate a lack of bias and generally
high degree of precision in the analyses (±8% for F
and ±14% for CI). The stated detection limit for fluorine was 0.02 wt percent (200 ppm) and that for
chlorine was 0.01 wt percent (100 ppm). For computing summary statistics and constructing histograms, specimens with concentrations below these (8
•
J-Golconda Thrust
•
X
.,
•
.(
x
•
•
•
.\
.
.... ..... -......
::"""Limits of
Basin and
i
./flange Province
.~,)
..
/
)(
....
:
............/ .../
NV
r
XX
.....
;.(
(: ,
..
.....
.......
..
__ r_·
/.....
•
{
+
......
X
F>900 ppm
AZ
Precambrian granitoid
CI > 400 ppm
++.. \
-+t""~;:"'"''''''''+'''''
•
....................... "
CA.
UT
r-:::O------------J
............
·_
lo
.
.~
..: .........
•
.*\.
t
.
FIG. 1. Map of a portion of the western United States showing
the distribution of the granitoid plutons (e) which were sampled
in this study. Also shown are the locations of specimens which (1)
contain greater than 0.09 percent fluorine (e), (2) were collected
from plutons of Precambrian age (+), and/or (3) contain greater
than 0:04 percent chlorine (X). Each point represents two specimens. Also shown are the limits of the Basin and Range geomorphic province and the 87Sr/86Sr = 0.706 line as defined by
Kistler (1983) and Kistler and Peterman (1978) for Mesozoic plutonic rocks. Initial Sr isotope ratios exceed 0.706 to the east of
this line. The trace of the Golconda thrust is also shown in central
Nevada and is used in this paper to divide the western and eastern
Great Basin. These geologic features may be taken to represent
the western limits of sialic Precambrian craton.
contained less fluorine and 110 less chlorine) were
arbitrarily assigned values equal to one-half of the
respective detection limit. Means calculated in this
way vary by only small amounts from those calculated
by the methods of Miesch (1967). Using this latter
technique, the mean CI concentration is 0.010 wt
percent and the mean F concentration remains 0.060
wt percent.
Results
Chlorine
Chlorine concentrations range from below the detection limit to 0.05 wt percent. The distribution of
CI concentrations in our sample of Basin and Range
granitoids is positively skewed and similar to a lognormal distribution. The arithmetic mean of the anal-
1486
, E,' H; CHRISTIANSEN, AND D. E. LEE
yS.es~ "calculated as .described. above, is 0'.013 wt per
cent ± 0.011 (1 st~ndard deviation == 10-). The mean
of logarithinically .traqsfottneddata 0.010 wt per· gent (0.010-0.005 at llu).Onlyl?samples have,CI
concentrations greater thafi\O'.04'\¥~percent(Figs. 1
and 2) and seven of the~e come. froin.lar~l~tively small
,area in southeaste~nCalifo~r,iia."We"s,ee rio:other pattern to the distribution of CI concentrations in these
granitic rocks, and vatiolf§' stati~tical tests s,vgges~that
, ther~ is no detectable differenc~ among theimeans qr
va,xiances of ~he varibu~ geQgraIJ,hic subgrolJps in the
· l~egion (see below). These generally low.Cl concentrations are in accord with other studies of granitoid
.ro.oks (e.g'.·, Johns and Huang, 1967;~uge and Power,'
1969; Kesler et aI., 1975; Ishihara and Terashima,
1$77; Table I )whiehdemonsttatethat concentrations
of ohlorine below '0.05 wtpercent ate typical of granitoid rocks and reflect the sCflrcity of sites (principally
; flui:d inqlusions and the minerals apatite. and biotite)
that can be occupied by CI in fully crystal1ine,Si02~
satl,1ratedigneous rocks.. Whole--rockanalys,es, of other
than glassy materials,.are generally not considered to
convey .accurately the- magmatic. conc~ntrations of
· halogens (Noble'etaI., 19.67). Chlorine is probably'
de~leted inthe rocks relative to their parent magmas
by:'late-:- or postmagmatic. water--roc.k '.:interactipn..
NOhetheless,Ishihara (~981) and Nedachi(1980)
suggesf that high chlo.rine.concentratio~s,and cOn-sequently low F/Clratios (lessthan3)"ar~eharacter~
fsticof the· magnetite se~iesof,granitoids •. 'in' Japan,
~specially of .those. plutonsass'ociatedwith ,Pb-Zn
·thinerali~ation. We see no evidence' of a relationship
between F Ict ratibs and grallite type in' our data from
the western United Stat~s (Fig,. 3). Many ,of the Pre-'cambrian granitoids which have extremely 111ghF191
6 Precambrian 'Granitoid
• Phanerozoic Granitoid
is
E. Great Basl.n,
~=72
•../~
•.··;·8. Basin & Range
~.~. i·:.l
.'
"',
n=76
. i·····.J/ ". ,ONW Nevada
,.o.,<~'p:·/
. n=48
,,:;;... ,;•, .*.
0.02
•...
o1
,:.: : ; ~;~{;~; ?f;~·:' ' ' '_:.' 'AIi ~amPles
o.~... ~ ......... """"~
u.
2
5 10 20
40
60
80 90 98 99
Cumulative Per cent
EIC.~, ,Cumulative frequency Aiair~mforfluorin«alfdchlo""
rinein granitoids· from the Basin and ~ange ptovh1Qe',,\ lfhe, specitnenshave been grouped a.sdiscussedin the 'text A grbpp of
specimens collected from a normal' population .would define '. a
straight line with a positive slope on this dfagl'am.
.
'.".'
'. .......• ".
.
'.,.....
. '. ..•.
" ...••
'. '
','
"
·.. :
1 ';.'1"1
FIG.3~ . ,Fluorin~ atldchlorine contents'of granitoid's from tHe.'
BasIn and Range prb'vince (dots and circles) compared to magn~tite
(el1closed by dash-dot line) and ilmenite (enclosed by dashed,1ine)
series granitoids from Japan (Ishihara, 1981). A reference FjCJ
ratio, of is shown and has been used by Ishihara to distinguish
'magnetite series rocks (low F ICI) from ilmenite series granitoids~
THis distinction does not apply in the western United States. For'.
example, many of the F -i·ieh· Precambl'ian granitoids are of the
ma~netiteseries (Anderson, 1983).
.
3.
ratios belong to the, magnetite series (Anders'ort,
1983).
Fluorine
'~luorineconcentrationsrange from below th¢ de~,.
t~ct~on limit to: O'.3q wt percent. The overalldistrit:;.
bution' is positively skewed .andhas a prono.unceq
mode at 0.03 wt percent. The lognormal riatut:~.,of .
.the distribution appears to result from combiningun--'
Telated specimen's from several distinct but ovel1laIJ-':
ping populations, as. shown below. Cumulativy,..fre,7.'
. qu'ency Ctl!ves for these chronologie andg~9gr~phic:
subgroups ate· shown. in ,Figure· 2,andJt· is, app.arent:
thafuntransformed datain the separate groups a.ppear
~~J'oQrn'{~:from:ne~r--no;rmal popula.tipns.(N:ormal,p,op.-ji
111ations form straight lines, when· plott~d iu. thisfash~:i
fOll'.) Theryfofe, thecomput.edrnean&are, arithnl(:~ti~,;
and all, the',discl1ssibns employ thes,e ·untransform~d
fluorine concentrations.
,..
The mean' c()ncentrationQf fluorine (0.060·wt,.;%1
I '0.043 at 1 0") in these granitic rocks is compa~~bl~'
to the esthnate of Turekian .and., Wedepohl (1961),'
namely 0.052' wt' percent for highCa granites. The,
~imilarity. is. evencloser,'ifthePrecambrian~ granite~·
(n 7= 32) areexcludedftoJ,llthe computation; thty.IIl;eahi'
then becomes 0.050 wtperceIlt± 0.025 (n,,==-:196).
This 'mean is also ,similar to.thatofthedata,rep.otted
by,Kqnisawa (1979) and Kanisawa etal.(1979)for
granitic rocks, from Japan· (Table l},and slight~y high~r
thah that forplut()nicrocks.fromthe·Caribbeanisland~
and Central Ametica. (Kesle'f,et at" 1975). Fluorine..
rich rocks, such ;as peralkaHne granites' OF anorogenio,
biotite granites,'. are commonly much richer in fluorine
r
i
1487
F & CL IN GRANITOIDS, WEST U. S.
TABLE 1.
Fluorine and Chlorine Abundances in Selected Igneous Rocks
Fluorine (wt %)
Standard
deviation
Mean
Standard
deviation
0.060
0.041
0.057
0.049
0.118
0.082
0.043
0.019
0.026
0.026
0.075
0.013
0.012
0.014
0.012
0.011
0.011
0.008
0.011
0.011
0.009
0.093
0.46
0.140 2
0.042
0.034
0.300
0.046
0.064
0.054
0.050
0.019
0.034
0.017
0.019
0.025
0.042
0.059
0.003
0.012
0.015
Locality (n)
Mean
Basin and Range (228)
Western Nevada (48)
Eastern Great Basin (72)
Southern Basin and Range (76)
Precambrian (32)
Western United States (167)
Silicic volcanic rocks
Eolian Islands
Volcanic rocks (53)
Nigeria, Younger Granites (36)
Southwestern England granites (140)
Carribean, intrusive rocks (142)
Japan, granitoids
Kitakami Mountains (65)
San-in zone (6)
Abukama (28)
Ryoke zone (58)
Mid-Atlantic Ridge basalt
Normal ridge segments
Near Azores
Azores alkali basalt
1
- = information not available
2
Geometric mean
Chlorine (wt %)
1
This study
Coates et al., 1963
0.303
0.051
0.018
(up to 1.0 wt %; Table 1) than the Phanerozoic plutons
from the western United States which were sampled
, for this study. A cautionary note was sounded by
Bailey (1977), who summarized much of the earlier
work on fluorine geochemistry in granitic rocks (including that of Soviet workers), in his conclusion that
fluorine concentrations vary widely. Bailey suggests
. that values for the "average" fluorine content of
granitoid rocks are misleading.
One of the most important features of the fluorine
data is that there appear to be significant differences
in the fluorine content of some groups defined on the
basis of age or geographic distribution. We propose
that these differences are petrogenetically meaningful
and that they carry important implications regarding
the types of ore deposits that may be expected to be
associated with these granitoid groups. To examine
these differences we have separated the data into four
groups .. We have already alluded to the apparently
high concentrations of fluorine in the Precambrian
granites. Three other groups can be distinguished
based on geography, geologic history, and isotopic
differences of relevant igneous rocks (Fig. 1). These
groups consist of the Phanerozoic (Mesozoic and
Cenozoic) granitoids from: (1) northwestern Nevada,
(2) the eastern Great Basin consisting of eastern Nevada and western Utah, and (3) the southern Basin
and Range province consisting of parts of Arizona,
eastern California, and southern Nevada (exclusive of
Reference
0.101
2
Coradossi and Martini, 1981
Bowden, 1966
Fuge and Power, 1969
Kesler et al., 1975
Kanisawa, 1979
Kanisawa et al., 1979
0.003
0.034
0.035
0.001
0.023
0.021
Schilling et al., 1980
the Precambrian granitoids). Su.mmary statistics for
these groups are given in Table 1.
The three geographic regions show fundamentally
distinct geologic histories that appear to be reflected
in the nature and composition of the crust which underlies them.' Northwestern Nevada (and adjoining
areas in Oregon and eastern California) was accreted
to North America during the Paleozoic and Mesozoic
and its crust appears to be an amalgam of ocean floor
or island-arc terranes (e.g., Speed, 1979; Dickinson
et aI., 1983; Oldow, 1984). We have used the trace
of the Golconda thrust to mark the eastern boundary
of this terrane. Farmer and DePaolo (1983) note an
Nd isotopic discontinuity near this fault which they
correlate with the western extent of the Precambrian
craton. Silicic igneous rocks from this region have
relatively low 87Srj86Sr ratios (less than 0.706) and
relatively high 143Ndj 144 Nd isotope ratios (epsilon Nd
> -5.4) as reported by Farmer and DePaolo (1983),
Kistler (1983), and Wilson et aI. (1983). These results
suggest that the granitoids from this region are substantially of mantle derivation or have a component
from a young mafic crust: In contrast, the eastern
Great Basin was the site of thick sedimentation in a
Paleozoic miogeocline and appears to be underlain
by more sialic continental basement with Proterozoic
ages (Nelson et aI., 1983). Nd and Sr isotope ratios
(epsilon Nd = -9 to -20; 87Srj86Sr = 0.706 to greater
than 0.720) of silicic rocks from this region indicate
1488
E; H.CHRISTIANSEN: ANP
that they have,Qerived an,ii:rnportant componentfrom"
older crust with a 'morecohtinentalcharacter (high'"
lo'moderately high Rb/Sr ratios arid low Sm/No ratios)
as compared to crust in north,w7s~ernNevada.T,he
south~tn,Basin"and Range prox~pce '''fippears h),have
had an early.history similar to the 'eas'ternGreat'.:Basin
but' exposures ofPrec'ambrian r()',~ks are mnchmore
abundant here. An important component tbf these
Precambrian terranes are the .1. 4~)?j.Y. -old angrogenic
granitic plutons de~cribed by An,~erson (1983). These
plutons are, distinct in their geochemistry and mineralogy from older and younger orogenic gr~nitoids
and characteristically have high whole-rock Fe/(Fe
.+ ,Mg), ratios, fluorine..rich biotite) and occasionally
fluorite as well. Based on combined Nd and Srisotope .
studies (Farmer and DePaolo) 1984) metall1:mifious
granitoids of Mesozoic and Cenozoic age appear ,to
be deriv~d substantially from the crust, whereas peraluminous granitoids may be derived solely from continentalorust.
'
These three .regi9I1S also display different geo, physical charact~ristics)indicative'of their distinctive
FIG. ,4. Si0 2 vs. Duorinevarialion diagram. The granit6~a~ of
geologic histor'ies. For 'example, a gradient in' the Precambrian age
are in general enriched in fluorine relativ~: to
gravity fieldextengs across north-central Nevada with other granit,oids of comparable Si0 2 content, whereas ~hosefro~
approximately the same treQ-das the presumed edge nort,h\Vestern Nevada (enclosed by dashed ,line) are P?or i~. flu()fthe Precambrian craton (Eaton et aI., 19'78). Bou~ orine.''(he transition zone is! the area between the ,tra~e pfthe'
Golconda thrust on the east and the 878r/868r, =, 0.706 line (Ki;tler,
g'uer gravity values are, higher in western Nevadaand 1983;,Kistl.er and Peterman, 1978) on the west.
.
·adjqlning'.parts',of the province' than 'in the eastern
Great Basin. Likewise, western Nevada is marked by
numerous high amplitude magnetic anomalies (Mabey
et al., 1 978). Such anomalies are less common in east- ,'evo,lved character of the anorogenic group is":als()
ern Nevada' and Utah. These anomalies reflect the demonstrated by their higher Th contents (McN ~~J
greatet abundance of shallow igneous rocks in western et 801.) 1981) and high'er concentrations ofother:in~
'
Nevada than in magnetically ·quiet eastern Nevada compatible trace elements (Lee, 1984).
Another significant observation which can be rnad~
and western Utah. ~he southern Basin and Range
province is distinguished, by much higher ,Bouguer from Figure 4 Is that granitoids from 'northw~stern,
gravity values than either, of the, 'diyisionsof the Nevada (western Great BasiI1) have lower :fltI,9rip~
northern Basin and Rang~ (EatonetaL, 1 978).,
contents than' granitoids fromotherr~gions.rl;1~~\r~~
Some of the differences influorinecbn.tel1t can be lat~.()nship,is more ,cle'arly,: shQvyn inlfigure':J~wh"9h, "
seen' in' a piotofSi0 2 versus fluorin~. (fig.'4)..:.!17h~, '~,how~ ·the ,Huorine.confellt.Qfgranit~,ids,ft9D)d,~4H
Precambrian 'granitoids show a striking enr,ic4P1~nti~: ~Qr.th~rnBasi.nandRa~ge,.prQvipqe,.,vetsllstf1~i,r;.:lPPi
fluorine relative to the other granitoids, of cOIllparable gitude.. Clearly, granitoidsfromwesterhNe,yqq~;~r~
8i0 2 content. This is consistent with the an'orogeniq unifprmly poor'in fluorine. A continental pop,~I~~~~J}
.c~aracter (Anderson, 1983) of most of the Precam-,
ofg;rariitoJd's from eastern Nevada and westerntVt1}A
brianplutolls which were sampled for this study. An~ can be identified whose members have variably hfgbJt! .
orogenic granites· commonly show pronounced en- fluorine concentrations. Many granitoids with JqW;.
richments in Huorine relative to other types of gran- fluorine concentrations are' also found, in, this' latter.
itoids(e.g.) Collins et aI., 1982). The 'only group-it is the.upperbound which has ohanged'sig+
Precambrian pluton sampled thatdoe's,not'appearto nificantly. The transition zonepetw'ee,n;,,these tvv.?
be part oftheanorogenic group is.thePayson; Ari", , gr~ups( consis.ting of th,osepl~tonsexposed,betwe~ri
zona, plut~n studied by Ll1dwig and Silver(197.7)whp the: traqes of the 87Sr186Sr.= 0.7p6 line on, thewe~~
showed that this pluton is significantly older. (1 •. 7 b.y.) and the. Golconda thrust on ·theeast)contaj.ns, grani"7; .
tnan the generally accepted age {1.4 h.y.)fo:r;ano.. toids with intermediate. fluorine ~ontents. One anom~,
togenic magmatism in the American southwest (Lud~ alouslyhigh value (O.llwt%F) comeS fromon~
wig 'and Silver, 1977; Anderson,: 1983). Accordingly, specimen, (GR.-62). coUec~ed .from a., plutOl1,in, :the
thegranite:ofPayson has l(),W' concentrations of flu- llorthern S?n<>lila Range, (southwest of Golconda)i
orine (0.01 andO.02wt%). The,g~nerallymore This pluton'\also has a "high initial 87Sr/~6Sr ratio
1
I
Di E. LEE
1489
F & CL IN GRANITOIDS, WEST U. S.
0.20 . - - - - - - - - - - - - - - - - - - - - ,
• Northwestern Nevada
o Transition Zone
o Eastern Nevada & Utah
0.15
o
o
o
(1)
c:
0.10
o
f-
.~
o
® o~
0'2>
:::J
u::
8
0
0 0
0 00
0
0
00
0000
000
0.05
o
o
0
o
f-
. ".,...:
•
••
:
0
8
0
~
I
o
W
J
0
00
00
J
8
8
o
o
I
I
119 0
Longitude
FIG. 5. The fluorine-poor nature of the granitoids from the
western Great Basin is emphasiied in this plot of fluorine vs. longitude for granitoid specimens from the northern Basin and R~nge
province. Maximum observed fluorine concentrations at a given
latitude are much greater in the eastern Great Basin than in the
western Great Basin of northwestern Nevada.
(>0.708; R. W. Kistl~r, writ. commun., 1982) compared to surrounding plutons, possibly indicating that
the ~agma represented by this specimen was· contaminated by wall rocks or that the plu~on has a larger
component from a sedimentary s9urce. The 9ther
specimen (GR-61) fro~ this pluton contains only 0.06
wt percent F. This specimen also has a'·much lower
P 2 0 S content than GR-62 (0.16 vs. 0.'23 wt %). Con~
sequently, at least some the high fluorine content of
GR-62 might pe the result of elevated apatite cOD-tent,
anhnport~nt reservoir for fluorine in igneous rocks.
The not~on that the granitoids from the western Great
Basin are part of a distinct population in terms of their
low fluorine content is reinforced iq. Figure 1, which
shows the distribution of specimens which have fluorine concent~ationsequal to or greater than 0.09 wt
percent. A~l of these specimen~, with the one exception noted above, li~ on th~ continental side of the
Golconda thrust.
The fluorine contents of granitoiq.s from the soutpern Basin and Range province (excluding the Precambrian granitoids) are not notably different from those
of the eastern Great Bas.in (Fig. 4 and Table 1).
Statistical a!1'alysis
Application of the chi-squ;:tre statistic to these four,
groups suggests the existence 9f significant differences
(at the 99% confidence level) between the specimens
from western Nevada and those from the southern
Basin and J{.ange or the eastern Great Basin, with regard to their fluorine content. Moreover, thi~ test reveals no evidence that the latter two group's are different from one another at the same ~evel of significance (99%). In adqition, the Precambrian gran~t~s
are from an apparently distinct population. (Tukey's
multiple comparison procedure, a more robust or rigorous test, suggests that significant differences only
exist between the Precambrian and all of the Phanerozoic plutons from the region.)
Estimates of the variance in the mean of the fluorine
concentrations that is attributable to different sources
(among groups, among plutons within groups, and
between samples within plutons) ar~ shown in Table
2. The table includes an analysis of variance for all of
the data grouped together as described above. It
demonstrates that a large amount of variation arises
from differences among' the groups and that this variance is greater than the variation contributed by individual'pl~tons or contributed by ~pecimens to plutons. Th~se same conclusions are brought out in an
analysis of variance among the Phaner'ozoic plutons
alone (Table 2). The total variance is greatly reduced,
but a signi,ficarit (at the Q5% level) amount of variation
occurs at the intraregion level, as indicated by comparison of F ratios to the the critical values qf the F
distribution (F0,05),'
, Whereas there are significant differen~es among
the fluorine concentrations of specimens in several
groups, the group from the southern Basin and Range
is essentially indistinguishable from the granitoids ()f
the eastern Great Basin. However, this distinctiveness
of the gran~toids of northwestern Nevada does not
extend to their silica content. Only simple comparisons using a t test suggest a significant difference between the Si0 2 content of the grotip from wester~
Nevada (67.2% ±3.$8 at 1 (1) and that of the :precambrian group (70.6% ± 2.88 at 1 a)-the two
groups with th~ largest differences in average Si0 2
content. N e~ted analyses of variation models, such as
that outlined above for fluorine, show no significant
TABLE 2.
Nested An;11ysis of Variance for Fluorine in 13asin
and Range Granitoids
Source of
variation
Degrees of
freedom
Mean
squares
Variance
component
(%)
F
Fo.os
22.61
2.72
1.39
A. Basjn and Range granitoids
Among groupsl
Among plutons
Among sam:ples
3
110
114
4,477
198
61
Total
227
186
37.41
32.98
2Q.61
3.~5
100
B. Phanerozoic granitoids
Among groups2
Among plutons
Among samples
Total
2
95
98
375
95
22
195
62
6.92
57.54
35.54
3.95 '
4·32
3.07
1.38
100
1 Includes Precambrian, southern Basin and :&ange, western Great Basin,
and eastern Great Basin
2 sQuthern Basin and Range, western Great Basin, and eastern Great Basin
I
1490
intragroup variations with •reg~rd, to silida' cotit'ertb'~ cepttati()fi~'ofbiotiteihleucOGratichigh silicff granites
Likewise, Tukey's method of rrioltiple comparison te J : or t~yolit~s (Ewart, 1979) and to' thesigniftcaJ;itly
veals no sig'niHcant .qfH'ereuees in the rpeans of the Ibw'er partitioQc()e~ci~nt of 'Fb~twe~J?J1e-richbip,$i0 2 contents for the'yariousgroups. Thus it appears '~it~andrhy()liticrnelt.D~io~,~ayb~' as
as" 3 Pf 4
. that the differepces riJ;l.fltiOrinecontent. we. nota sec~
,Fe- ariqF~rich ,biQ~ites (Turley and Nash"l$'~O;
, 9nd-"order resultofregioua}dHferences in, sili~fl'pon- Kovalenko et aL, 1984;E. H. ,Christhinsen, unpub.
dat~) .'N onetheless,gfanit~s crystalliz'ed': from Such'
tent or t4~~egreeofm~g:rnatlc.d~fferehtiati6n."
'
I
low
for
m~(ts may"show d~clihingconcentr~ti~n~.ofF with
Trends with siliQ~. ponfent
It is important to fiote ftpmFigute ':4' th~t the'Jl~:~
l
'"
orine conte~t of th~ Preca.mbriangr~nitoid's:de'<?reases
with iQcteasing 8i0 2 • 'This trend ismirriicked.but:1ess
propo~nced· in .the other gro'ups aswe~L .Other· in..
ves~igators have noted similar negativ~· correlations
petween'fluorine and silicai'n granitip r<>,cks (e.g., .~ee
and Van' Loenen, 1971; ~hawe, 1976; Bailey, 1977;
Kanisawa, 1979). We attribute thisdeclin~rig trend
to at l~ast two different effects~ fluorine m~y behave
as a ~ompatible ~lement in oxidized,biotite~r~(Jh
granitoids, and in more silic~ous leucocratlc granite's
the' deolining H~or~n~ c~ptents' pf th~ ro'ckspr~babl¥
results from a decr.ea~e in their content of biotite-L.-,
,the principal host for fluorine .in ' granit~id· rocks
(Dodge et aI., 19q9; l3ailey, 1977; Speer, 1984)~
with differentiation.
"
Case 1: The, partition coefficient for .fluQrine between biotite and 'the flllid from which it crystallizes
is a strong fup.ctioq of the Fe/Mg ratio of-the biotite
: (e~g., MUhOZ, 1984; 8peer,J984), and hence, oxygeii'
f~gacityat:td'melt cgmpositipn. Ac~ording to the',ex~
periments. of.A~Hl~gov~~t>~L. (lQ77), the partitio~
coefficient b~tween magut1 s jahbiotite 'and granitic
melt'is approxirnaf~ly5'to'10(average 7.4 at 1 kb
and 780?C iiI' uQbuffer~d runs in gold capsules). Natural ghiss-biotite pairs sllggestranges of between 10
a:rtq' 20 fot thep~titiollco~ffi.cieo~ of fltiorinebetw~~n
magne~ia~ ~iotite anq melt (~~ldreth, 1977;. Kovalen'ko' et :al.; '1984; Luhr et aI., i'984). As, a consequep.ce, Ql1 ly. 5 to. '15 percenf,biotittf' fil'11st ·be'Jractionating to produce
bulk·· par,tition coefficient
gr~ater than ,l.0for,flQor~ne. (The amount of biotite
,would be ev~n lower . if the· relatively 'highcryst~ll
, liquid part~tion for f in fe~dspars and'qu~rtz as' given
by' Kovalenkq et at., (1984) are even approxirnately
correct.)- U rider these conditions, fll}.orine concentra·tions in aresiqual nqui<;l,'wouldde~Hrieas a tesulfof
fractio'nal erystal~ization, 'Sliqp: 'proportions of 1?iotite '
are I1pt unusu~li~ gtariitoi9S(Dodg'e etal., 1 Q~9; ~ee
and Van Loenen, 1971) and removal of1Ji~titei~ these
proportionsc9uld pro~tice~sotneof the negative cor':' .
relations between fluorine and'sili9a.
',Case 2: In fi1any c~ses, It can be sQown that'Huo:rine
, beha;ves as'an incompat'ible element in diffel~entiating
silicic melts. Fluqrine' cp~monlyaecu~ulatesdurIng
the differenti~tionof gJassy. volcanic rock~. (llacon at
al.;J 981;- ~ildl'eth; 1981; Christia~senet at, 1984).
The incompatibil~typfF :cah be-attributed to 10""" co~I
I
a
~
.
increasing p'onceritr.ations of Si0 2 '. The!comm.6nly
,II
obs¢rved 'decline/in the, abon:dallce of ~iotite!, horn.
'.
.',. •..
,"
.J.,'
... '
, ' ,
1>le~de, titanite, aIi(l ~p~titedil1jihg dJfferenliati()n may
help to: ~xplain. ~hi~ . fflct .•''Yith, advaricing~tli~ere'ntia-'
tiqn, this process' c6uld,' tesult~iIi· increasingly' fluorin~riQhhiotites in inCreasingly fluorine-depletedw4ole
rocks (e.g., :Kahis~wa; lQ79;'K\inisaWQ'et at, 1~79)'.
This process fs 'depicted scherrHltically iriF,igure '6,
which shows the evolut~on of 'a granitic ~el~ and 'a
1.0
'"'
f'i'
~ O.~
d>
~b
0.6
r1l
.5,
d>
c
0.4
'-
o
~ 0.2
D~!O~7.4
G().F=0~04
FluQr'in~ !r:M~I! 'or Rock :(wt~).'
{;FIG. ·6~ plot Of. fluori~e:pontent..of melt. apd .crysta,lliZ~(l:ro,dk
vs .•••. the. fluorin~,contentofsimuHaneously.formed hibttt~~"The
it#t~al concentration. o'f fl uorine( CO,F, in w~, ~)iti the me1J an·~.th~
par~hion 'coefHci~Ilt (D~i~~)' for' f1 ubtine' betw'eenh{otit~,aHa li~uid
ar'esho\Vn. in the diagra~. The ·in~lt'ahdio~k evohltionA)vere;;cal~'
cul~ted, ~sslimingmultisequenceR~yleighJracti6natib,11;iJ\$ ::de~
scrib~d by'Allegte ahd ~Minster (197~n:.1'geJraction·~f residual
liqti~d remaini~g (f) at'the end of each of a series 9fcryst~l1ization'
steps is'labeled. Th,e ''Veight fJ;action pf~iotite (Xblot)wasii\pr~;'
m:enfallychanged in orde:r to estimatethe eftepts 'Of decreasin~
biotite concent~ation with crystalHzati~n of leucocratic granites,
~n<:ltheir'par~ntalmelts. In the~odel shown~Biot = 0.10, (f}for
f~ 1.0, 0.'8, O.6~ 0.4, 0.2,' 'and O;l.'VVith inereasingdegre,~sJ~f
cry~tanization, (declining f), th~rri~ltbecomes e~riched in Huor~ne
as, (Ides th~ bi~tite fqrmed inequilibriu~withit.However;b~c~use'
of the paucity of mineralogic sites, the fluorine content t>r~ftiny
c~~sfanized rock formed fr9m this Q1~I~,ma~bemg<?hlb~~r,~r'Iq
fact~th~'di~erentiationof~. gr~hitoidmagma:hl~k~d~yad~bt'efuJ~
In' the amount Qfbiotite, may res'vJt in'. the cleeline of bulk4 rock
~ll()rine content with increasing evolution of the rocks· J~S '~H9~rl'
by, thehypotheti9al, crystallization path., In thissimPJemodel, biq..
tite':was used as, the qIily F~bearing·,phase. In ,naturalexamples
the buiI~ D F ~o~ld :probab~y b~soInewhat highera,nd th~ fluorine
elldchment of the residual liquid and the fluorine Clepleti'bn' of
the'Qryst~lline~oc~would notpe~s extreme.
I
' .
",
,
F & CL IN GRANITOIDS, WEST U. S.
solidified granite whose fluorine content is solely the
result of fluorine retention in a changing proportion
of biotite; the rest of the fluorine is presumably lost
during solidification duriIig loss of fluids. A variety of
F versus Si0 2 curves can thus be envisioned-the
trend deperiding on the, proportion of minerals in the
rock which can tie up fluorine. Trends showing fluorine increases or fluorine decreases with advancing
Si0 2 or which show variable trends can all be imagined. Each of these trends have been identified from
specific granitoid complexes (summarized by Bailey,
1977). The model in Figure 6 is for biotite granite,
but similar relationships would hold if hornblende,
titanite, or apatite were included because they also
decline in abundance during granitoid differentiation.
If muscovite, topaz, or fluorite occur, .as in two-mica
and A-type (anorgenic; Collins et aI., 1982) granites,
less fluorine is lost after crystallization. Usually, however, these minerals contribute' little to the total fluorine budget of a rock. A further indicatio~ of the
importance of the accumulation of fluorine-bearing
minerals to ~he fluorine content of granitoid rocks
comes from the comnion observation that biotite-poor
but chemically evolved late aplite dikes in granitic
plutons are commonly fluorine poor (Bailey, 1977;
Christianseri et aI., 1983a). In contrast, fluorine commonly accumulates during the differentiation of glassy
volcariic rocks geochemically analogous to aplites in
terms of less fugitive bomponents (Bacon et aI., 1981;
Hildreth, 1981; Christiansen et al., 1984).
Origin of the Fluorine Contrasts
We suggest that the ultimate origin of the fluorineenriched granitoids from~ the eastern and. southern
Basin and Range province lies in their int~raction with
or derivation from the Precambrian continental crust
which underlies much of the. western United States.
The isotopic and elemental geochemistry of the anorogenic group ofPrecambri~ngranites has led other
investigators to suggest that they were derived by direct partial melting of the crust (Anderson, 1983).
Partial melting of crustal rocks is pJ;obably controlled
not by the water-saturated solidus of granite or
granodiorite but instead by the' fluid-absent decomposition of hydrous rriinerals such as biotite and hornblende (e.g., 13urnham, 1979). Hydrous minerals may
be relatively fluorine rich in high~grade metamorphic
roc~s, such a~ those which give rise to anatectic melts
in the l<;>we'r crust, because of the higher thermal stability of the fluor end members of the common amphibol~s and biotites compared to their hydrous
equivalents (Holloway 'and Ford, 1975; Holloway,
1977; Manning and,Pichavant, 1983). White (1966)
has shown that F/(F + OH) ratios in biotites from
migmatites are higher than in biotites from their parent schists (0.06 vs 0.18% F). In addition, Fillippov
et aI. (1974) have shown that biotites fr0Il?- granulite
1491
facies metamorphic rocks contain greater amounts of
fluorine than comparable rocks in amphibolite facies
(0.65 vs 0.24 to 0.38% f). Pursuing this notion,
Christiansen et aI. (1983a, b) and Collins et at. (1982)
have proposed that 3.J)orogenic magmas derived from
the crust should be fluorine rich as the result of the
thermal decomposition of refractory fluorine-rich
biotite. In like manner, the degree ofpart~al melting
is also controlled by the amount of hydrous minerals
present in the protolith (Burnham, 197~). Thus, upon
the decorriposition of the small quantities of biotite
and amphibole found in some granulitic rocks, sma~l
quantities of fluorine-rich melt could be produced. It
may be this low degree of partial melting which pro~
duces the enrichment of incompatible trace elements
found iIi many anorogenic magma suites and not any
abnormal endowment of these elemerits in their
sources (Christiansen et aI., 1983b).
The origin of the less pronounced fluorine enrichment of the Phanerozoic plutons of the eastern Great
Basin and the southern Basin and Range province may,
in some cases, be the result of similar anatectic processes. Such an origin was pr()posed by Christiansen
et aI. (1983b) for the T~rtiary Sheeprock, granite of
~estern Utah, which is fluorine rich (>0.30 wt % F)
but which was not included in the random design of
this study. However, it is likely that many of the plutons from these regions contain a substaptial component fro~ the mantle (e.g., Farrrier and DePaolo,
1983, 1984), and we suggest that their relatively
slight enrichment in fluorine is tpe result of the interaction. ofmantle-clerived magmas with a fluorinerich continental crust,. Likewise; the granitoids of the
northwestern Great Basin may .have the saine proportions of mantle and c~ustal components but ~ppear
to have interacted with a fluorine-poor crust of arc or
ocean-floor origin. This latter possibility is suggested
by the relatively high oxygen isotope .ratios (commonly greater than 9%0 a18 0-values typical of altered ocean crust) displayed by the granitoids from
western Nevada (Lee et aI., 1981) at relatively low
87Sr/86Sr ratios «0.706) and relatively high epsilon
Nd (-6- t9 + 1) (Farmer and DePaolo, 1983; Kistler,
1983). These isotopic compositions probably result
from the exchange of c9mponents between mantlederived magma and hydrothermally, altered or
weathered material of sea floor-arc origin (kistler,
1983). Two simple models sho~ing how fluorine contents, are affected by assimilation of these distinct
crustal components are shown in Figures 7 and 8.
Figure 7 shows the re~ult of the combined assimilation
and fractional crystallization (DePaolo, 1981) of a
mantle-derived magma digesting fluorine-poor arctype crust. Only moderate fluorine enrichments are
produced over the range of fractionation expected in
a typical intrusive complex. The preserved fluorine
content of the rocks resulting from the crystallization
1492
E.11. (JHRISTIANSENAND'D.E.\LEE
fi'mtl their ohservedisotopic and elemeritalgeQchetri~
u
ls~rY9:nddistributibn,'BtirtetaL.(1982)ahd,Chri~~
'S
C"
:J
tiallsenetaL; (1983a,b) suggest that fhe fluorin'eAridh'
.8
melts were ',derived from the cot1tine~talcrust,Trhey'
~
0.08
point" 0tlt·, t~e 'spatial and., temporal coincid~nee/of
l
,§ 0.06
Mb,.W ,'(wo1framite);ariq B,emineraliiatio.o'Wifh:dg4
o
neQus rocks ,of ,high fluorine' content., BrimhalLetaLf
:::J
0.04
u::
(1983)" hilvealsoreiterated the. assoGiation.6f Sri",W;
and Mo'deposits with igneous rocks qfhigh F/CLrati@~
o
a~.indicat~d by bio,tite,compositions and.sugg~stthat
such·,ma.gmas' are derived from thecrtist by anatexis
FlO. 1. the changing concerttration of fluorine· in a magma
in~uc~d by the decoinp~sitionof phyllosilicateswith:
evolving by combined assimilation and. fractional cl'ystalHzation
,
high
F/C!. Sato (1980) has also pointed to the impor~
(DePaolo, 1981). The bulk partition coefficient (DF) and the con',qentratton 9f fluorine in the original lfiagma. (Co)-ass~med to tanceof qrustal.c6rnposition 'to the generation ,d(Shhave been derived from the mantle (e.g., Schillinget aI:, 1980)~ .W and fluorite deposits arid their associatedf1uQrine~
a~ld in theassimilant (CA)-'-:"assurned to be an arc-type crust '(e.g.
richigneousropks.·
.'
',.". ';),")I:~l",j
Gatcia et aI., 1979).:..-are shown. Altho~ghthe' ainQuritof fluorille
'
:
'B,ased
,on·
thisstttdy.ofthe
'flu()rine~nd.,'<?hlbririe
increases with' larger values for ,assimilation rate/crystallization'
content's of granitoids from the BasinahdRahgeprov~:
rate, (1'), the fluorine content of fheresultarifmagrilais stiUrela~l1ce,tht3, F-rich .anQrogenic.grariitpid~ .of thePtot~ro­
tively low~The fluorine content ofa rock crystallized frotn this
magrna may be lower still.
zoiC (ca. 1~4h.y.),aswell as the,geocherliically.similat
,•. late. Ge'nqzoic anorogenid rhyolites ofbimopal·assq~'
cl~t.i6ns; would be ,the most likely to be assocHited
.
of these model magmas will probably be lo\¥er as' with such lithophile element rri.ineral~zation.Thetwo
noted above. Figtire.·g shows that moderate fluorine models for magmatic fluorine enrichment outlined
~nr~chments will b~ produced by reasonable amounts above~n~tnely,magma derivation from the. c.J;ustor
of assimilation of fluorine-rich continental crust even contamination of mantle-derived melts bY90ntinental
for. a parental fluorine-poor, mantle-derived melt crust-may be used to explain the associfitionof,F
Again, .loss bf fluorine during magma 'c~ystallizatidn with Mo, W> Be, and Sn as a reflection of the associ~
, is ex;pected, but taking this into acc9unt, reasonable atlon of these elements in the continental soutce;rna~
predictions of the fluorine coritent of granitoids from terials of these granitoids. It appears that the ,Precaln~
the cOJ.?tinental portion of the western United States brian C(a.tori of the western United States is relatively
enrich,ed in these elements, indicating a more felsic
result from these simple' ~ssimilation'models. .
. The data gathered in this study do, not preclude ~omposition, compared to the Phanerozoic arc-ocean
the pos,sibility that the Fenr,ichment stems from con- , floor crust which underlies the northwestern Great
trasting mantle ~ourceregions (Le., F . .rich granites'
may arise from parent m'agmas deri"edin oldF-rich
'sub90ritin~ntalmantlewh,ereas the F-poorgval1itoids
"0
,'S
of the northwestern Great Basin were derived from
'0o
yotlng~r F-poormaritle" tecentlyaccteted to the con~
,5
tinental' margin.) However, .we feel· that· th~·occur. .
(6'
rence of F-rich granitoiqs in those regions which have
:i
,.~ .0.06
granitoids with elevated initial 87 Sr/ 86 Sr ah~ low ep'i:
o
silonNd(Farmer and DePaolo, 1983, 1984; Kistler,
.rl:,0.04
1983) ~rgues strongly for th~presenceof a light rare"
0.02 ~...a-,......I-..-"--L_-'-'----L------'"--'--'---..J.--L---I
earth element- and Rb-enriched.granite protblif4 to
1.0
0.6
0.8
0.4
0.0
0~2
tl:te east. Higher potassiumc()ncentrations in ,eastern
Fraction of Liquid' Remaining
granitoids also correlat~"wlth the crustal discontinuity
marked by isotopic studies. These, correlations
FIG; '8. The (1oncentration of nuorihei~a magrit~ evolviIig by
strongly poirit toward their ,co~rhon 'origin, in. one' , fractfoQal. crystallization combined with assimilation of corithiept~l
m.agmatic reservoir which can Teasonably he corre~ crusb The ~ssumptions, fo.r this ~odel are thesameasfort~~onE1
l.ated· ,W'ith ·the ·c·.ontin·en't·.a·l C'. r ;ls't,.',
pr~sented iriJr.ig~re 7 with the exception of the valu~ USe?: '(or
-
•
.•..•
',',
1
j
,
'
0""
I
1
'.
u
.
Metallogenic.lm,'p,1licatiorls,
.
. Burt et al. (1982) ha~el'ecently,sutnmarizedthe
,p.<?ssible types of ore dep(jsits~ss9ciatedwith flu.orine~
.
rich ~gnedus rockS withsp'echd ,reference to topaz
I
,rhyolites from. the· western·United States. Reasoning
the concentration of fluorine'irithe.assimilant (CA== O.06%cori},.
qentr~tion of fluorine), taken t,oreptesentsialiccohHrtellhiLctust
(Taylor, 1964). Moderate co'ncentrations of fluorine in an eyplving
melt can be producedwith~uch lesscrystallizathmthah~n.the
previo\ls~xalnple. This modelcan beus~~ to explain the apparent
.t1hl-ichmentof.Huorinein gl'ariitoids emplaced thtQtig~, co~tin~ntal
crust in the western United States as compared to those emplaced
through accreted arc', or ocean. floor ,terranes.
'
'1
F & CL IN GRANITOIDS, WEST U. S.
Basin and adjoining regions. The crust in this region
must be relatively mafic and poor in these elements,
but its clear association with Cenozoic mercury deposits suggests that it is Hg enriched (Eaton, 1984).
Truly anomalous concentrations of the lithophile
metals or of fluorine in the Precambrian crust are not
required. The compositional difference between "average" continental crust and "average" oceanic crust
is sufficient to account for the differences in fluorine
content in granitic rocks from the two terranes. Simultaneously, we acknowledge the fact that many
factors other than the enrichment of elements in
magmas control the ultimate development of ore deposits associated with igneous rocks. N onetheles.s, the
relationships outlined here are at least gross indicators
of the geochemical character of a variety of granitoids
and of the crust across the region. As such they may
yield invaluable clues to possible ore associations.
Acknowledgments
This work was initiated while ERC was a National
Research Council-V. S. Geological Survey Postdoctoral Research Associate. This study received institutional support from the University of Iowa and the
U. S. Geological Survey.
August 17,1984; July 24,1985
REFE~NCES
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trace element behavior in magmatic processes: Earth Planet.
Sci. Letters, v. 38, p. 1-25.
Anfilogov, V. N., Bushlyakov, I. N., Vilisov, V. A., and Bragina,
G. 1.,1977, Distribution of fluorine between coexisting biotite
and amphibole and granitic melt at 780°C and 1000 atm pressure: Geochemistry Internat., v. 14, no. 2, p. 95-98.
Anderson, J. L., 1983, Proteroz;oic anorogenic granite plutonism
of North America: GeoI. Soc. 'America Mem. 161, p. 133-154.
Bacon, C. R., Macdonald, R., Smith, R. L., and Baedecker,P. A.,
1981, Pleistocene high-silica rhyolites of the Coso volcanic field,
Inyo County, California: Jour. Geophys. Research, v. 86, p.
10223-10241.
Bailey, J. C., 1977, Fluorine in granitic rocks and melts: A review:
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