The implications of Cl-rich minerals in granulite facies rocks are discussed. Results from ironstones of the Beartooth Mountains, Montana are discussed. It is suggested that CO2-brine immiscibility might be applicable to granulite facies conditions, and if so, then aqueous brines might be preferentially adsorbed onto mineral surfaces relative to CO2

Henry, D. J.

English

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67 - 
CL-RICH MINERALS IN ARCHEAN GRANULITE FACIES IRONSTONES 
FROM THE BEARTOOTH MOUNTAINS, MONTANA, USA: IMPLICATIONS FOR 
FLUIDS INVOLVED IN GRANULITE METAMORPHISM. 
D. J. Henry, Department of Geology and Geophysics, Louisiana 
State University, Baton Rouge, Louisiana 70803 USA 
INTRODUCTION. Although C1-rich minerals have been recognized 
to develop in a number of petrologic environments ranging from 
submarine hydrothermal vents C 1 1  to late mafic pegmatitic dikes 
and Pt-rich horizons in layered magmatic intrusives C2,31, they 
are most commonly found in granulite facies rocks. I t  is probably 
the earlier work on the high grade terrains of South India that 
most clearly demonstrated this Ce.g. 4,5,6,7,81. Nonetheless, 
most of the investigations on C1-rich minerals from granulites 
dealt primarily with their chemical characterization but have not 
considered their petrologic implications. Clearly, before fully 
understanding granulite facies metamorphism the role of C1 in 
granulites must be explored. 
There are three factors that influence the incorporation of 
C1 into hydrous silicates and phosphates: fluid composition, 
temperature and crystallochemical constraints. I t  has been 
experimentally confirmed that as chlorinity and acidity in an 
aqueous fluid increase and/or as temperature increases, the C1 
concentrations in the coexisting micas also increase C9,lOI. 
Experimental studies have also established that the compositional 
parameters (especially increasing ferrous Fe contents) that 
enable the hydroxyl site to approach ideal hexagonal symmetry in 
hydrous silicates strongly favor C1 incorporation Ell]. 
With these factors in mind, the hydrous minerals of the 
granulite facies ironstones from the Beartooth Mountains, Montana 
are considered in this investigation. 
BACKGROUND. The eastern portion of the Beartooth Mountains, 
composed predominantly of 2800 Ma granitic to tonalitic 
granitoids, gneisses and migmatites, contain enclaves of various 
supracrustal lithologies that range in size f r o m  a f e w  cm to a 
few km C121. The supracrustal lithologies, including pelitic 
schists, felsic gneisses, amphibolites, mafic gneisses, 
quartzites and ironstones, typically display mineral assemblages 
that are indicative of granulite facies metamorphism C131. 
Application of a series of geothermobarometers indicate that the 
peak metamorphism took place at pressures of 5-6 kbar and 
temperatures of 750-800 C. Some of the lithologies are partially- 
to-completely reset by a subsequent amphibolite facies 
metamorphism that is attributed to the major magmatic event at 
2800 Ma. The Rb-Sr isotopic systematics on the supracrustal 
approximately 3400 Ma that has been interpreted as the time of 
the granulite facies metamorphism C121. 
lithologies produce a poorly constrained isochron of 
MINERALOGY AND PETROLOGY OF THE IRONSTONES. The iron-rich 
metasedimentary rock of the eastern Beartooth Mountains is a 
typical quartz-magnetite banded iron formation with bulk 
compositions ranging from 47 to 61 weight % SiO,, 30 to 40 % F e O  
https://ntrs.nasa.gov/search.jsp?R=19890012840 2020-03-20T02:32:12+00:00Z
CL-RICH MINERALS IN GRANULITES 
Henry, D. J. 
68 
and 1 to 3 % Ale?O=. The bulk compositions and REE abundance5 
indicate an origin of the ferruginous sediment on a continental 
shelf rather than a eugeoclinal depositional environment Cl 2 3 .  
The mineral assemblages of the ironstones are, in general, 
similar to other granulite facies ironstones C 1 4 3  with the most 
common assemblage being quartz + magnetite + ferrohypersthene + 
almandine + clinopyroxene. In addition, there are trace or minor 
amounts of a blue-green amphibole and dark brown biotite that are 
found as both inclusions and matrix phases. A s  such, they are 
interpreted as having been involved in the prograde metamorphism 
of the ironstones. Very minor amounts of cummingtonite after 
ferrohypersthene and actinolite after clinopyroxene localized 
along later fractures indicate the subsequent amphibolite facies 
metamorphism did not significantly affect the ironstones. 
Based on calculations of coexisting mineral compositions i t  
is estimated that the fluid phase has an X(HEO)<0.3 and is 
relatively oxidizing (near the NNO buffer). "Primary" fluid 
inclusions are apparently CO,-rich suggesting CO, is the dominant 
component in the fluid phase. 
Both the biotites and amphiboles found in these ironstones 
have some of the highest C1 levels that have been documented to 
date (with the biotites and amphiboles in the matrix containing 
more C1 than the biotite and amphibole inclusions in garnet and 
ferrohypersthene). The biotites not only contain up to 2.9 wt % 
C1 (22 % of the OH site), but also have substantial amounts of Ba 
(up to 10.5 wt%) and Ti (up to 6.9 wt % )  (see Table 1 ) .  The 
amphiboles contain up to 2.8 wt X C1 (40 % of the total OH site) 
and range in composition from a ferroan pargasite to a Cl-rich 
potassium hastingsite (see Table 1 ) .  In these minerals there is a 
general positive correlation among C1 levels and Fe, Ba and K (ia 
amphiboles). These trends are in accordance with the expected 
crytallochemical controls of C1 incorporation. 
Not all o f  the C1 variations, however, are purely controlled 
by the crystallochemical constraints. Based on the C1 contents of 
the biotites, the relative f(H,O)/f(HCl) ratios C 1 0 3  range from 
0.02 to 1 suggesting that some o f  the biotite variability must 
result from changes in the local fluid compositions. These 
calculations indicate the that the amount of HC1 in the fluid 
must be substantial and are probably only attained in an aqueous 
fluid. Furthermore, the increasing C1-enrichment of the matrix 
biotite and amphibole relative to the included biotite and 
amphibole suggest an increasing chlorinity with grade. 
Nonetheless, based on fluid calculations and fluid inclusion work 
it is known that the dominant fluid is a C0,-rich fluid. Such a 
fluid will not support the high levels of C1 necessary to produce 
the elevated C1 contents in the silicates Ccf. 151. Consequently, 
we must entertain the possibilty of an immiscible highly saline 
and acidic aqueous fluid coexisting with the dominant CO, fluid 
typically found in granulites. 
IMPLICATIONS. Evidence for COet-brine immiscibility have 
been found in some medium grade metamorphic rocks Ce.9. 15,161. 
However, this current study indicates that this immiscible 
behavior of fluids may extend into granulite facies conditions. 
CL-RICH MINERALS IN GRANULITES 
69 
Henry, D. J. 
The presence of such a high grade immiscible fluid m a y  have 
significant affects on the behavior of granulite fluids. The 
aqueous brines may be preferentially absorbed on the surfaces o f  
minerals relative to the nonpolar CO, fluid phase resulting in 
differential movement o f  the unmixed brine and CO, fluids C173. 
Due to its capacity to form complexes, C1-rich aqueous brines 
have been implicated in the movement o f  elements such as Pt group 
elements, Pb and rare earth elements C2,183. 
References, C 1 1  Vanko, D. A. (1986) &n, PJinerall, 21, p. 51- 
59. C23 Boudreau, A. E . ,  Mathez, E.  A .  and McCallum, I. S. (1986) 
----- Jour. -- Petrol., 32, 967-986. C33 Stumpfl, E.  F. and Ballhaus, C. 
G. (1986) Eo_ct_s_c_h_r_, M_inera_l,, 6_4, 205-214. C43 Leelanadam, C. 
(1969) Pl~~~l~l, fi~q,,3_2, 362-365. C51 Leelanadam, C. (1969) fl, 
3_b_, Mineyell, Monat., Was_, 461-468. C63 Leelanadam, C. (1970) 
@nL Mineral., -__----- 25, 1353-1358. C77 Blattner, P. (1980) EL J_b_, 
Plin_e_la_LLL M_o_n_lt_,, WS_O_, 283-288. C 8 3  Kamineni, D. C., Bonardi, M. 
and R a o ,  A. T. (1982) em_, Pljn_eral., a?, 1001-1004. C93 Munoz, J. 
L. and Swenson, A .  (1981) E _ ~ o _ n _ ~  G_e_o_L,, 26, p. 2212-2221. C 1 0 3  
Volfinger, M . ,  Robert, J. L . ,  Vielzeu?; D. and Neiva, A. M. 
(1985) g_e_o_c_hi_m, _C_o_s_mochiml e_c_t_a, 52 ,  p. 37-48. El27 Mueller, P.  
A . ,  Wooden, J. L., Henry, D. J. and Bowes, D. R .  (1985) _M_o_n_t_a_n_a 
B_uIl _Mi_n!e_s _ G _ e _ o l l  SEFL _ P _ u _ b . L 9  -- 92, p. 9-20. C133 Henry, D. J., 
Mueller, P. A . ,  Wooden, J. L., Warner, J. L. and Lee-Berman, H. 
(1982) Monta_n_a_ B_u_J& yjn_e_s_ G _ e _ o _ ~ ,  S_ee_c_, P_u_b_L,, f33, p.147-156. C143 
Klein, C. (1983) in Trendall, A .  F. and Morris, R .  C. gyo_n_ 
-_-----_-- Formation: ---_- F a c t s  --- and -------- Problems, p. 417-469. C 1 5 3  Trommsdorff, V.  
and Skippen, G. (1986) Eontrib. _Mi_n_ey_all _P_e&'_o12, 25 ,  p. 317-322. 
C161 Sisson, V .  B., Crawford, M. L. and Thompson, P. H. (1981) 
C_o_n_t_rib. M_gn_e,ya_L, P_e_t_yo_L,, ZS,, p. 371-378. C173 Thompson, A. E. 
(1Y87) J _ o _ u _ ~ -  G_e_o_L, S _ O _ C _ , ~  L_o_n_d_~_n_, 144, p. 309-312. C 1 8 3  Allen, P .  
Condie, K .  C. and Narayana, B. L. (1985) GeochFfn, C_o_s_fno_&h_in, 
Munoz, J. L. (1984) k v _ ,  M_jn_e_la_L,, 13, p. 469-493. C 1 1 1  
---- Acta, 5 2 ,  p. 323-336. 
TAPLE I .  Representative C1-rich biotite and amphibole analyses 
Sample name QC82-45 QC82-45 QCBI-113 QCE2-45 QC82-45 
knalysrs p t  BIOTITE I BIOTITE 2 BIOTITE 3 NIPHIB 1 AHPHIB 2 
hemar k t l A T A I X  OPX INCL OPX INCL MATRIX GAR lNCL 
si02 
A I  203 
T i 0 2  
Cr203 
FeO 
fin0 
flqO 
C*O 
Na20 
K20 
BaO 
CI 
F 
SO3 
Total 
28.79 30.58 33.28 37. I I 37.97 
14.12 13.88 15.05 11.38 10.86 
6.29 5.84 1.02 1.40 1.84 
0.00 0.00 0.02 0.04 0.04 
29.08 29.37 27. I 4  27.28 25.75 
0.11 0.10 0.13 0.17 0.17 
2.29 3.96 6.66 3.83 4.96 
0.02 0.01 0.03 11.50 1 I .62 
0.00 0.05 0.OE 0.65 1.06 
5 . 4 7  6.93 8.18 3.20 2.39 
9.65 7.25 2.90 0.55 0.53 
2.74 2.26 2.41 2.61 1 .e2 
0.10 0.07 0.07 0.08 0.25 
0.00 0.17 0.00 0.03 0.04 
98.66 100.47 96.97 99.81 99.30 
------ ------ ------ -----_ ------ 
O=F,CI 0.66 0.54 0.57 0.62 0.52 ------ ------ ----_- ------ ------ 
TOTAL 98.00 99.93 96.40 99.19 98.79 
QC-IS 
AHPHIB 3 
HATRl X 
39.70 
11.78 
0. I2 
0.01 
24.96 
0.26 
5.70 
1 I .52 
0.61 
2.97 
0.19 
1.34  
0.21 
0.00 
99.37 
0.39 
98. 98 
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Cl-rich minerals in Archean granulite facies ironstones from the Beartooth Mountains, Montana, USA: Implications for fluids involved in granulite metamorphism

Cl-rich minerals in Archean granulite facies ironstones from the Beartooth Mountains, Montana, USA: Implications for fluids involved in granulite metamorphism

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