Contributions to Mineralogy and Petrology

Contrib. Mineral. Petrol. 71, 151 155 (i979)

9 by Springer-Verlag 1979

Feldspar Geothermometry of the Hell Canyon Pluton, Boulder Batholith, Montana T h o m a s A. Vogel a n d R o n a l d E. W i d m a y e r Department of Geology, Michigan State University, East Lansing, Michigan 48824, USA

Abstract. T h e b u l k c o m p o s i t i o n s o f the g r o u n d m a s s alkali f e l d s p a r f r o m the Hell C a n y o n P l u t o n is 0.146 m o l e % albite. T h e c o m p o s i t i o n o f the o u t e r m o s t zone o f the o s c i l l a t o r y z o n e d p l a g i o c l a s e is 0.686 m o l e % albite, w h e r e a s the m o s t calcic cores have a c o m p o s i t i o n o f 0.43 m o l e % albite. T h e structural state o f the a l k a l i f e l d s p a r is n e a r orthoclase. B o t h c o m p o s i t i o n o f coexisting f e l d s p a r s a n d structural state o f the alkali f e l d s p a r are n e a r l y c o n s t a n t t h r o u g h o u t the pluton. Exsolved albite in the alkali f e l d s p a r have a c o m p o s i t i o n o f 0.965 m o l e % albite a n d the o r t h o c l a s e host has a c o m p o s i t i o n o f 0.032 m o l e % . Singe crystal X - r a y studies indicate t h a t the albite i n t e r g r o w t h s are c o h e r e n t with the host. E q u i l i b r i u m t e m p e r a t u r e s d e r i v e d f r o m the coexisting f e l d s p a r average 554 ~ C ; a b o u t 150 ~ C, t o o low for the m i n i m u m solidus t e m p e r a t u r e s for r e a s o n able e m p l a c e m e n t pressures (2 kb). If this m i n i m u m solidus t e m p e r a t u r e is a s s u m e d , t h e n the alkali felds p a r has lost a b o u t 0.15 m o l e % albite. This loss was m o s t likely caused b y h y d r o t h e r m a l s o l u t i o n s associated with the crystallizing m a g m a a n d e q u i l i b r a t e d at a b o u t 550 ~ C. H o w e v e r , b a s e d on the c o h e r e n t albite i n t e r g r o w t h s a n d the o r t h o c l a s e structure state it can be inferred t h a t the system was relatively free o f volatiles b e l o w 500 ~ C. F i n a l e q u i l i b i r i u m between o r t h o c l a s e host a n d albite i n t e r g r o w t h s o c c u r r e d at a b o u t 311 ~ C.

Introduction M u c h has been w r i t t e n a b o u t the p e t r o g e n e t i c applic a t i o n o f the s t r u c t u r a l state o f alkali feldspar, a n d the c o m p o s i t i o n s o f coexisting p l a g i o c l a s e a n d a l k a l i feldspars (see Smith, 1974; W h i t n e y a n d S t o r m e r , 1977b; a n d P a r s o n s , 1978 for recent reviews). But,

with few exceptions, (Tilling, 1968; W h i t n e y a n d Stormer, 1976, 1977a) there has been little d o n e with the a p p l i c a t i o n o f these variables to i n t e r p r e t i n g the d e t a i l e d t h e r m a l h i s t o r y o f p l u t o n i c bodies. It is the p u r p o s e o f this s t u d y to i n t e r p r e t the t h e r m a l h i s t o r y o f the Hell C a n y o n P l u t o n b a s e d on these variables. T h e Hell C a n y o n P l u t o n was selected for this s t u d y b e c a u s e it is believed to be one o f the latest p l u t o n s to be i n t r u d e d into the b a t h o l i t h a n d s h o u l d r e p r e s e n t a relatively simple c o o l i n g history w i t h o u t s e c o n d a r y r e h e a t i n g ( R o b i n s o n , et al., 1968; Tilling, et al., 1968).

General Description of the Hell Canyon Pluton, Boulder Batholith The Boulder Batholith is located in southwestern Montana. It is a composite igneous intrusion of over a dozen individual plutons exposed over an area of 6,000 to 7,000 square kilometers, with the main part of the batholith exposed as an elongate 50 x 100 km body (Klepper et al., 1971). Estimated thicknesses range from 5 km (Hamilton and Meyers, 1974) to over 15 km (Klepper et al., 1971). The age of emplacement is believed to have occurred over a time span from 78 to 68 m.y. ago during Late Cretaceous time (Robinson et al., 1968). The Hell Canyon Pluton is located at the southwestern tip of the Boulder Batholith and is in contact with Precambrian metamorphic rock to the south, east and west, and with prevolcanic sedimentary rock to the north. It is believed to be one of the last magmatic intrusions to occur in the Boulder Batholith based on K-Ar age dating (Robinson et al., 1968). Although there are no exposed contacts between this piuton and the rest of the batholith, it is considered to be part of the batholith from evidence derived from radiometric age dating and isotope ratios (Doe et al., 1968), composition and mineralogy (Tilling et al., 1968; Tilling, 1973, 1974), and proximity to the other plutons. The pluton is tear drop in shape, elongated in a northwest-southeast direction with the maximum width located at the northwest edge, approximately 7 km wide, and the smallest width located at the southeast edge. The maximum length of the pluton is approximately 17 kin. Elevation of the pluton generally increases in a northwest direction with the southeast region at an elevation of about 4,800 feet above sea level and the northwest region at an elevation of about 9,600 feet above sea level. A total of 70 samples were collected throughout the pluton.

0010-7999/79/0071/0151/$01.00

152 The Hell Canyon Pluton is a homogenous, porphyritic quartz monzonite of hypidiomorphic-granular texture (Mathews et al., 1977). The bulk of the groundmass is composed of oligoclase (40%), orthoclase (25%), quartz (20%), and biotite (nearly 7%). The remaining minerals include hornblende, sphene, magnetite, apatite and pyroxene. Secondary minerals include chlorite, muscovite and sericite. The plagioclase feldspar grains are most often euhedral with equidimensional to elongate shapes and sizes ranging from 0.1 to 10 mm in length. Most exhibit normal or oscillatory zoning with a composition, based on microprobe studies, that range from 30 to 35% anorthite at the grain boundaries to at least 57% anorthite within the cores. Albite rims are common and range in thickness from 0.001 to 0.2 mm. Composition of the albite rim edge was similar to that of the exsolved albite blebs in the alkali feldspar, averaging 95% albite. Although some grains are unaltered, most have sericitized zoned cores. A few grains are completely sericitized often only showing a thin unaltered albite rim. Inclusions consist of smaller plagioclase grains, sphene, biotite and chlorite. Orthoclase occurs as both phenocrysts and within the groundmass. The largest phenocryst observed was 33 x 37 x 25 mm although grains up to 30 x 60 mm are described by Mathews et al. (1977). These phenocrysts often have inclusions of zoned plagioclase. The groundmass orthoclase range in size from 0.2 to 15 mm i.d. and are generally interstitial, equidimensional and irregular in shape, although some smaller euhedral and subhedral grains were observed. Some orthoclase are quite clean; most are partially to highly altered to sericite. Inclusions of quartz, plagioclase, biotite, albite, amphibole, chlorite, magnetite, pyroxene, and sphene are common. Exsolved perthitic albite show up as films, irregular blebs, parallel linear blebs, small to large round, square or rectangular blebs, and combinations of two or three different types. The irregular blebs generally show no preferential orientation, whereas the linear blebs are aligned along crystallographic axes. Orthoclase grains containing little or no albite are also observed. Carlsbad twins are present but most grains appear untwinned. Microcline twins are exceptionally rare. The quartz grains .range in size from 0.1 to 8 mm diam. and are generally equidimensional and anhedral. They often contain inclusions of all other minerals present but otherwise are clean and unaltered. Sharp to slightly undulatory extinction is the general rule. In a few samples taken along the pluton contact the quartz grains show a mylonitic texture with quartz grains ranging in size from 0.01 to 0.2 mm i.d. Biotite is almost always altering to chlorite along fractures and generally occurs as irregular elongate grains. The remaining minerals make up about 8% of the total volume of the samples. Hornblende and sphene are more abundant than the other remaining minerals. Apatite and zircon are most often observed as inclusions within the biotite.

T.A. Vogel and R.E. Widmayer: Feldspar Geothermometry grains are rare. Grains with straight sharp boundaries and thin albitic rims were used. Zoned plagioclase grains ~vere ideal for analysis because it is possible to obtain a complete temperature history of the grain. It is also possible to predict prehydrothermal compositions and temperatures based on a few assumptions discussed later. Grains of alkali feldspar and plagioclase that contained a high percentage of sericite and those showing evidence of myremekite were not analyzed. Plagioclase with corroded and embayed grain boundaries were avoided as well as those grains containing thick albitic rims. Temperature determinations are based on compositions of coexisting feldspars using the two-feldspar geothermometer first proposed by Barth (1934) and improved and refined by Stormer (1975) and Whitney and Stormer (1977b). Compositions of plagioclase, orthoclase and albite were determined using an ARL EMX microprobe with operating conditions of 15 kV and 0.1 gA beam current. LiF detector crystals were used to measure Ca K~ and K Kc~ and a RAP crystal was used to measure Na Ke. A point beam of approximately 0.5 g was used in the analysis of plagioclase, exsolved albite and inhomogeneous orthoclase while a 50 g defocused beam was used to measure homogeneous (orthoclase plus exsolved albite) compositions. Samples of 90.7% orthoclase, 100% albite and 95.75% anorthite were used as standards, and background counts were recorded for these standards and a few samples. Six 12-second counts were recorded for each area analyzed per grain. Between 3 and 6 areas per grain were analyzed and between 2 and 6 grains per sample were used. Plagioclase, orthoclase, and exsolved albite were analyzed in 25 different samples from the Hell Canyon Pluton. Analysis with the 50 g defocused beam on homogeneous orthoclase gave consistently reproducible values. When the point beam was used a greater range of values was obtained due to the inhomogeneity of the orthoclase on a microscopic scale. The X-ray diffraction method of Wright (1968) was done as a check on the defocused microprobe beam and also to determine the structural state of the alkali feldspar. Powdered alkali feldspar samples were analyzed by X-ray diffraction to obtain d-spacings of the (060), (204), and (201) crystallographic planes. The values were then graphically related to structural state and composition. This procedure was done twice for each orthoclase sample, first in its natural exsolved state, and then after the orthoclase had been homogenized by heating to 1,050~ C for 24 h. Homogenization was done to obtain the (201) peak which represents the homogeneous composition, before exsolution of the albite. The (060), (204), and (201) ~ values for each sample (Fig. 1) are plotted on a (060) vs (204) graph (Table 1) to obtain structural states of the samples (Wright, 1968).

Results Analytical Methods Grain Selection

A few requirements had to be met in choosing the alkali feldspar and plagioclase grains to be analyzed. Alkali feldspar with the smallest amount and size of exsolved albite was most desirable since this indicates an optically and probably compositionally homogeneous grain. Grains with a minimum of inclusions and alterations were also chosen to avoid possible related compositional variations. The only plagioclase grains used were those that were either completely enclosed within an alkali feldspar grain, or at least bordered on 3 sides by alkali feldspar. Clean unaltered plagioclase

Figure 1 is a plot of the Cu 20 (060) vs the Cu 20 (204) to determine the structural state of the alkali feldspar (Wright, 1968). Structural states were determined from two different outcrops (8 samples each) and from 9 samples across the pluton. There is nearly as much variation within an outcrop as there is across the entire pluton. All samples fall between Spencer B and SH 1070 of Wright (1968, Fig. 3). A comparison of the defocused microprobe beam and homogenized alkali feldspar X-ray diffraction method is shown in Table 1. There is a remarkably

T.A. Vogel and R.E. Widmayer: Feldspar Geothermometry

153 Table 2. Composition of the feldspars in mole percent albite and calculated temperatures

/,1.90 -

Max.

/

~,,0~

microcline X""

q)

~

~

o

High sdnadine

I 50.50

Xab ~

Xab '

No.

AF

PF (edge)

I 2 3 4 5 8 10 15 18 26 37 38 40 44 50 67 68 69

0.1471 0.1400 0.1416 0.1592 0.1382 0.1685 0.134I 0.1485 0.1422 0.1262 0.1495 0.1411 0.1519 0.1470 0.1478 0.1470 0.1483 0.1476

0.6781 0.6695 0.6703 0.6794 0.6702 0.6948 0.7116 0.6926 0.6950 0.7115 0.6879 0.6766 0.6808 0.7121 0.6842 0.6704 0.6746 0.6851 X= SD=

Edge T~ Microcline a (2 kb)

Edge T~ sanadine (2 kb)

JLab, Center PF T~ (center) sanadine b (2 kb)

585 577 579 602 575 610 555 582 572 543 585 577 591 573 584 588 588 585 581 X= 15 S D =

534 527 527 548 524 554 505 529 521 495 533 526 539 521 532 536 536 533 528 14

0.6263 0.4340 0.6046 -0.5663 0.5285 0.4447 0.5302 0.5214 0.5148 -0.4760 0.6287 ---

~ A

o,::;

c,q

41.50

Sample

50.90 (~04)~

Fig. 1. Plot of the (060) vs (204) 20 Cu Kc~ peaks (Wright, 1968). zx and 9 are samples from two separate outcrops. Filled Circles were collected along a SW-NE traverse across the entire Pluton

Table 1. Comparison of (201) X-ray diffraction method (Wright a n d Stewart, 1968) with defocused microprobe technique of determining the bulk composition of perthites

X-Ray Diffraction

Microprobe

Sample No.

Heated 1,050 ~ 24 b wt. % Or

50 g defocused beam wt. % Or

8 15 38 44 50 68 69 2 26 41

83.3 87.0 85.5 87.5 85.5 84.0 83.3 76.0 81.0 88.0

87.2 85.0 86.2 85.6 88.0 85.0 85.2 76.3 79.3 87.0

554 656 581

562 605 649 567 611 619 637 553

AF, is the total mole percent albite in the perthetic alkali feldspar Xab, PF (edge) is the mole percent albite for the outermost zone, except when pure albite is rimming the grain, then it is the zone in contact with the albite Jl~ab ,

Based on microcline solid-solution (Whitney and Stormer, 1977 b) b Based on sanadine solid-solution (Stormer, 1975) a

good correlation between the two independent methods and thus we consider the defocused microprobe technique is valid. Table 2 presents the microprobe data for both the coexisting alkali feldspar and plagioclase as well as the calculated temperatures based on Stormer, 1975 and Whitney and Stormer, 1977b. Two kilobars were considered a reasonable minimum pressure of crystallization for the pluton. The data from the perthites are shown in Table 3. In this case the compositions are based on focused

Table 3. Composition of exsolved albite (Xab, PF) and host alkali feldspar

Sample

Xab, A F

X,b, PF

T ~ (2kb)

1 3 4 8 15 37 38 40 43 44 67 68

0.0400 0.0322 0.0281 0.0357 0.0282 0.0342 0.0344 0.0303 0.0278 0.0388 0.0258 0.0323

0.9782 0.9671 0.9577 0.9706 0.9488 0.9645 0.9628 0.9674 0.9654 0.9658 0.9690 0.9669

353 335 325 344 326 341 341 330 323 35i 314 336 s SD=

a

b

Based on microcline solid-solution Based on sanadine solid-solution

12

Tb 305 285 276 295 277 292 293 281 274 303 265 287 )(=287 S D = 13

154

microprobe beam on exsolved albite and the alkali feldspar host.

T.A. Vogel and R.E. Widmayer: Feldspar Geothermometry Table 4. Summary of Hell Canyon Feldspar composition and calculated temperature Constant P = 2 k b

Discussion Table 4 is a summary of the feldspar compositions and calculated temperature for the Hell Canyon Pluton. Two temperatures were calculated, one using the sanadine equation of Stormer, 1975 and the other, assuming an intermediate structural state, using an average of the microcline equation (Whitney and Stormer, 1977 b) and the sanadine equation. The 528 ~C temperature would be the lowest equilibrium temperature possible if sanadine were the structural state of the alkali feldspar in equilibrium with the outermost zone of the plagioclase. If an intermediate structural state is assumed, the temperature is 555~ (the numbers in brackets are one standard deviation). The most anorthite rich core observed contained a composition of Xab =0.43. This would represent an equilibrium temperature of 656 ~ C if the alkali feldspar (sanadine) were in equilibrium with this composition or 678 ~ C for intermediate alkali feldspar. The 555 ~ C temperature clearly cannot represent solidus temperature and most likely represents a hydrothermal equilibrium event. If the alkali feldspar and the plagioclase compositions (Xab, A F = 0 . 1 4 6 ; Xab, PF = 0.686) both represent primary solidus compositions, they would represent a minimum equilibrium pressure of 7.7 kb and temperature of about 625 ~ C based on the water saturated granite solidus (Fig. 2). This level of emplacement is not supported by the estimated depths of emplacement (Hamilton and Meyers, 1974; Klepper, 1971). Based on the water saturated granite solidus (Fig. 2), a realistic minimum equilibrium temperature for a shallowly emplaced pluton would be about 700 ~ C at 2 kb. If an alkali feldspar (sanadine) were in equilibrium with the edge of the plagioclase at this temperature, its composition would be Xab, AF = 0.297. Thus the composition of the alkali feldspar presently observed (Xab, AF=0.146) represents a maximum loss of about 0.15 mole% albite in alkali feldspar. This represents a maximum possible loss, if 2 kb is the minimum crystallization pressure; at higher pressures the loss would be nearly proportionally less. It is likely that alkali feldspars have changed composition rather than the plagioclase for the following reasons. First, delicate oscillatory zones are preserved in most plagioclase grains and is difficult to imagine that these zones would be maintained if subsolidus recrystallization had occurred. Second, it is much ea-

Xau, A F

Matrix (18) Edge (Av.)

JL~b, PF

0.146 (_+0.018) 0.686

Core (Min.)

(_+0.030)

528 ( + 2 8 ) u

554 ( _+ 28)a 656b 678a

0.43

Exsolved (12) Perthites a b

Temperature

oC

0.032 (_+0.009) 0.965 (_+0.014) 311 (_+ 13)

Average of microcline and sanadine temperatures Sanadine temperature

8.00

A m

m e • +

7,00 A H

6.00

+

Edge(O) Edge solidus Edge(S) Solidus

O

5.00

~4,oo

~xEl

+

q~

o ~2 3.00

2.00

1.00

Arn

4D

4-

+ I I [ I I 0.00 /,80.00 560.00 640.00 720.00 800.00 880.00 Degrees celsius Fig. 2. Possible pressure-temperature conditions as indicated by the coexisting feldspars. Pluses are estimated P-T distribution for granite saturated solidus. Triangles are the P-T equilibrium for s a n a d i n e - plagioclase of the Hell Canyon pluton for the outermost zone on the plagioclase assuming a sanadine structure. Squares assume an intermediate structure for the alkali feldspar. Octagons show the P - T equilibrium assuming that the sanadine - plagioclase intersects the solidus at 2 kb. This equilibrium T-P intersection represents an increase in alkali feldspar albite content 0.15 mole%. En: Edge (o); 9 Edge solidus; • Edge(S);+ :Solidus

sier to substitute Na for K in alkali feldspar than the simultaneous substitution of Na for Ca and Si for A1, because of the high bond energy of Si-O and A1-O in the plagioclase. If the alkali feldspar at the solidus were also in equilibrium with the plagioclase cores, they would

T.A. Vogel and R.E. Widmayer: Feldspar Geothermometry

have a minimum temperature of 1,060 ~ C. This equilibrium is very unlikely because the textures indicate that the alkali feldspars crystallized late. Nevertheless, 1,060 ~ C would represent a minimum temperature for crystallization of these plagioclase cores because the equilibrium temperature would be higher for higher pressures and higher in the alkali feldspar absent field. Compositions of exsolved albite in the perthites are Xab, P F = 0 . 9 6 5 and the alkali feldspar ~host is Xau, AF=0.032. This results in an equilibrium temperature of 311 ~ C. Most of the perthitic feldspar appear as film type perthites. Two of these were X-rayed by single crystal precession techniques to determine if the lattice of the exsolved albite was coherent with the host. In both cases the diffuse lattice reflections indicate coherent intergrowths (X-ray precession photographs were done by Professor S.W. Bailey at the University of Wisconsin-Madison). Parsons (1978) suggested that if hydrous solutions were present at temperatures below 400 ~ C, incoherent intergrowths would be facilitated. Coherent intergrowths would indicate the absence of fluids. Parsons (1978) also interprets the presence of intermediate structural states as indicating the lack of fluids.

Conclusion

The composition of the alkali feldspar and the outermost edge of the zoned plagioclase would indicate an equilibrium temperature of about 555 ~ C for reasonable estimated depths of emplacement (Fig. 2). This temperature is about 145 ~ C below the minimum granitic solidus. This temperature probably represents a late hydrothermal reequilibration of the alkali feldspar and would indicate a loss of albite from the alkali feldspar of about 15 mole% (Fig. 2). Minimum emplacement temperatures of 1,060 ~ C are indicated by the plagioclase cores. The general sequence of crystallization indicated by the feldspars are shown as follows: (1) a minimum emplacement temperature of 1,060 ~ C with final solidus equilibrium crystallization of alkali feldspar and plagioclase at 700 ~ C with an assumed 2 kb pressure (about 6.6 km), (2) a hydrothermal event, probably related to magmatic fluids, equilibrating at about 555 ~ C, (3) exsolution of albite occurring after the hydrothermal event; the system is interpreted to be relatively dry during exsolution because of the coherent perthitic intergrowths and the intermediate structural state of the alkali feldspar, and (4) final equili-

155

brium of the albite intergrowths with the alkali feldspar host occurring at about 311 ~ C.

References Barth, T.F.W. : Polymorphic phenomena and crystal structure. Am. J. Sci. 5, 273 286 (1934) Doe, B.R., Tilling, R.I., Hedge, C.E., Klepper, M.R.: Lead and strontium isotope studies of the Boulder Batholith, southwestern Montana, Econ. Geol. 63, 884406 (1968) Hamilton, W., Meyers, W.B.: Nature of the Boulder Batholith of Montana. Geol. Soc. Am. Bull. 85, 365 378 (1974) Klepper, M.R., Robinson, G.D., Smedes, H.W.: On the nature of the Boulder Batholith. Geol. Soc. of Am. Bull. 82, 1563-1580 (1971) Mathews, G.W., McClain, L.K., Johanns, W.M. : Petrogenetic aspects of the Hell Canyon Pluton and its relation to the Boulder Batholith, southwestern Montana. Northwest Geol. 6-2, 77 84 (1977) Parsons, I. : Feldspars and fluids in cooling plutons. Mineral. Mag. 42, 321, 1 17 (1978) Robinson, G.D., Klepper, M.R., Obradovich, J.D.: Overlapping plutonism, volcanism, and tectonism in the Boulder Batholith region, western Montana. Studies of Volcanology: Geol. Soc. Am. Mem. 116, 557-576 (1968) Smith, J.V.: Feldspar Minerals. II. Chemical and Textural Properties. 690 p. Berlin, Heidelberg, New York: Springer 1974 Stormer, J.C. : A practical two-feldspar geothermometer. Am. Mineral. 60, 667 674 (1975) Tilling, R.I.: Zonal distribution of variations in structural state of alkali feldspars within the Radar Creek Pluton, Boulder Batholith, Montana. J. Petrol. 9, 331-357 (1968) Tilling, R.I. : Boulder Batholith, Montana: A product of two contemporaneous but chemically distinct magma series. Geol. Soc. Am. Bull. 84, 3879 3900 (1973) Tilling, R.I. : Composition and time relations of plutonic and associated volcanic rocks, Boulder Batholith region, Montana. Geol. Soc. Am. Bull. 85, 1925-1930 (1974) Tilling, R.I., Klepper, M.R., Obradovich, J.D.: K-Ar ages and time span of emplacement of the Boulder Batholith, Montana. Am. J. Sci. 266, 671 689 (1968) Whitney, J.A., Stormer, J.C. Jr. : Geothermometry and geobarometry in epizonal granite intrusions : a comparison of iron-titanium oxides and coexisting feldspars. Am. Mineral. 61, 751 761 (1976) Whitney, J.A., Stormer, J.C. : Two feldspar geothermometry, geobarometry in Mesozonal granitic intrusions: three examples from the Piedmont of Georgia. Contrib. Mineral. Petrol. 63, 51 64 (1977a) Whitney, J.A., Stormer, J.C. Jr.: The distribution of NaA1Si30 a between coexisting microcline and plagioclase and its effect on geothermometric calculations. Am. Mineral. 62, 687 691 (1977b) Wright, T.L. : X-ray and optical study of alkali feldspars: II. An X-ray method for determining the composition and structural state from measurements of 20 values for three reflections. Am. Mineral. 53, 88-104 (1968)

Received August 1, 1979; Accepted September 12, 1979