Low-Grade Metamorphism of Tuffaceous Rocks in the Karmutsen Group, Vancouver Island, British Columbia

RONALD C. SURDAM Department of Geology, The University of Wyoming, Laramie, Wyoming 82070

Low-Grade Metamorphism of Tuffaceous Rocks in the Karmutsen Group, Vancouver Island, British Columbia,

ABSTRACT ties had been explained by differences in the activity of H20, in the ratio nCO/MH2O, in The Triassic Karmutsen Group of central the rate of reaction, in the geothermal gradi- Vancouver Island consists of 18,000 ft of ents, and in the chemical composition of the pillow lava and breccia, aquagene tuff, massive parental rock. These factors can and probably amygdaloidal volcanic flows, and thin inter- do contribute to the observed mineralogical lava limestone beds. Low-grade metamorphism variations. However, the most significant vari- of the tuffaceous rocks has resulted in the de- able may be the composition of the aqueous velopment of laumontite, prehnite, pumpelly- solutions in contact with the solid phases ite, epidote, analcime, and albite. The glass during low-grade metamorphism. A study of alteration process in the tuffaceous rocks was the low-grade metamorphism of the volcanic one of hydration and solution. The large over- rocks of the Karmutsen Group illustrates the lap of critical minerals such as laumontite, importance of the cationic and anionic com- wairakite, prehnite, pumpellyite, and epidote position of the fluid phase, and suggests that is explained in terms of ionic equilibria. The even very subtle differences in the composition observed mineralogical differences are ex- of the fluid phase are significant. plained in terms of minor variations in the The two major purposes of this paper are to activities of ionic species in the aqueous phase, determine the mechanism of formation of rather than large changes in pressure and hydrous calcium aluminosilicate minerals in temperature. However, the gross regional pat- low-grade metavolcanic rocks, and to illustrate tern is explained in terms of the thermal the significance of aqueous solutions during stability of the hydrous calcium aluminosilicate low-grade metamorphism. minerals (that is, laumontite at the top of the The Buttle Lake area is near the center of section and prehnite at the base of the section). Vancouver Island, British Columbia, Canada (Fig. 1). The regional structural pattern of INTRODUCTION central Vancouver Island is a broad anticline The zeolite facies as defined by Turner (in that plunges to the north (Jeffery, 1963). In Fyfe and others, 1958) and Coombs (1970) the Buttle Lake area, this broad anticline is bridges the gap between sedimentary and cut by several regionally prominent vertical metamorphic environments. Some petrologists north-trending faults (Surdam, 1968). This suggest that the recrystallization is induced by uncomplicated structural pattern makes strati- a rise in temperature due to depth of burial, graphic reconstruction relatively simple and whereas others claim that recrystallization is facilitates a reasonable determination of depth a function of fluid pressure or, more accurately, of burial. the activity of water (aH2o). It is readily ap- The Karmutsen Group is underlain by the parent, after comparing zeolitic rocks from Permian Buttle Lake Formation (Fig. 2) and low-grade metamorphic terranes throughout is overlain successively by the upper Triassic the world, that there are significant differences Quatsino Formation, the Jurassic Bonanza in the stratigraphic position and in the se- Group, and the Cretaceous Nanaimo Group. quence of the critical mineral zones (Coombs From regional stratigraphy, the estimated and others, 1959; Packham and Crook, 1960; range in depth of burial for the top of the Surdam, 1967; Seki, 1969). These dissimilari- Karmutsen Group is 5,000 to 15,000 ft.

Geological Society of America Bulletin, v. 84, p. 1911-1922, 8 figs., June 1973 1911

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/6/1911/3428562/i0016-7606-84-6-1911.pdf by guest on 01 October 2021 1912 R. C. SURDAM

LOAD PRESSURE AND TEMPERATURE The stratigraphic reconstruction suggests that the top of the Karmutsen Group was subjected to lithostatic pressures of about 0.5 to 1 kb, whereas the bottom of the Karmutsen Group probably was subjected to lithostatic pressures of 2 to 3 kb. The lithostatic pressure at the time of metamorphism may have been lower because much of the reconstitution probably took place during the 20 to 30 m.y. that the Quatsino Formation and lower Bo- nanza Group were accumulating. The difference in the lithostatic pressure from the top to the bottom of the Karmutsen Group is approximately 2 kb. Reconstruction of the stratigraphic column also permits an estimate of the tempentures to which the rocks have been subjected, provided that a geothermal gradient can be in- Figure 1. Index map showing the location of the ferred. Depending on the geothermal gradient, Buttle Lake area. temperatures probably ranged from 50° to 150°C at the top of the Karmutsen Group and calcic plagioclase grains are commonly altered, from 230° to 330°C at the bottom. It is impor- the iron-titanium oxide minerals are less com- tant to note that these are estimates of temper- monly altered, and the pyroxene grains are ature during maximum overburden, and meta- rarely altered. Although primary textures are morphism may have started at lower tempera- preserved in most of the volcanic rocks, low- tures. grade metamorphism produced zeolitic and prehnite-pumpellyite-bearing assemblages. The PETROLOGY zonal mineral sequence in the Buttle Lake area The volcanic rocks of the Karmutsen Group is very similar to the classical zoned sequence of consist mainly of close-packed pillow lava, zeolites in the Triassic rocks of New Zealand pillow breccia, aquagene tuffs, and amygdaloi- (Fig. 3). dal flows (Surdam, 1970). Although the Kar- There are, however, significant mineralogical mutsen Group volcanic rocks have undergone differences between the two areas. In the classi- low-grade metamorphism, enough relict origi- cal New Zealand area, hydrous calcium alu- nal minerals remain to evaluate the original minosilicate minerals commonly replaced glass mineralogy. The original volcanic rocks were shards; whereas in the Buttle Lake area, the composed of four prominent phases: (1) calcic hydrous calcium aluininosilicate minerals gen- plagioclase laths and microlites, (2) pyroxene, erally surround the glass shards (Fig. 4). (3) irpn-titanium oxide minerals, and (4) glass. These primary phases are characterized by GLASS ALTERATION striking compositional variations. It has been An important aspect of the problem of low- suggested that these variations reflect inhomo- grade metamorphism is the alteration of glass geneities of the original bulk composition of because most of the rocks assigned to the the volcanic rocks (Surdam, 1970). zeolitic facies were originally tuffaceous or The Si02 and alkali contents of the relatively volcaniclastic. The formation of glass frag- unaltered feeder dikes and sills suggest that the ments in the aquagene tuff of the Karmutsen bulk composition of the Karmutsen Group Group has been well documented by Carlisle volcanic rocks was subalkaline, probably (1963). Until the alteration of the basaltic or tholeiitic (Surdam, 1970). In addition, the andesitic glass is understood, there is little dikes and sills do not contain enough sodium chance of reconstructing the chemical budget to be typical spilite. for the low-grade metamorphism of the Kar- The glass fragments in the Karmutsen mutsen Group volcanic rocks. Group metavolcanic rocks are altered, the The altered glass shards, globules, and frag-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/6/1911/3428562/i0016-7606-84-6-1911.pdf by guest on 01 October 2021 METAMORPHISM OF TUFFACEOUS ROCKS, BRITISH COLUMBIA 1913

O gS 5 o lOOOV g g Upper Massive Flows OC9 m Lower 750 Tuffaceous Argillite

Black Carbonaceous Limestone Quatsino Fm. 1650' Light Gray Limestone 300 yAmygdaloidol Flows 100'-400' yVPillow Lava a Pillow Breccia 0- 100' Black Carbonaceous Limestone 2000' \Gray Limestone Amygdaloidal Flows 0'- 100' = 1 'l'U Munii' 11 l'H Pillow Lavas, Pillow Breccias, m IHHlimili il "H HI 'HIHIWI1 ItWi W Minor Limestone n ' nm IH< iti u WJilluminiti 'HI) i)t nt>) i Hilpmil nII I I "|'l»1*1> 1IIN KIHH U'H NflT m|i < Hil«li>l i 1 5000" rtwMumiWIM W iinmmiii 11 ittflil I l'i Amygdaloidal Flows mimmiii "UHI • 'itiv " 'itili il 1ii'ii miniliii imi t '"MI ni tiiiiim

Karmutsen Group

Pillow Lava. Pillow Breccia a Aquagene Tuff

10.000'

Triossic 400' Argillite a Amygdaloidal Flows Permian Buttle Lake Fm. Limestone R.C Surdon, 1965 Figure 2. Composite section of the rocks in the Buttle Lake area.

ments in the aquagene tuff are typically very a 14A and 16A peak; whereas there is generally dark brown to light tan. Some are almost iso- no shift in the 14A peak after glyeolation for tropic, although they are commonly biréfrin- the "glassy" material from the lowermost part gent near their edge (Fig. 4). Good x-ray of the section. Furthermore, the altered diffraction patterns were obtained from most "glassy" material, particularly the cores of altered glass specimens, but some of the ma- larger fragments, commonly contain apprecia- terial may be amorphous to x-rays. The altered ble Ca (Table 1). The x-ray diffraction and glass fragments throughout the section give compositional information suggest that the a strong 14A x-ray peak. When the altered material in the upper part of the section is a "glassy" material from the upper half of the complex mixture of chlorite and interlayered section is glycolated, the 14A peak splits into chlorite and Ca-montmorillonite, whereas the

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/6/1911/3428562/i0016-7606-84-6-1911.pdf by guest on 01 October 2021 1912 R. C. SURDAM

Southland, fragments), and it is evident that the alteration New Zealand (Coombs, 195M began from the outside and then proceeded Nag.BIBB toward the center of the fragments, probably Pumpellyite Epidoic along a solution interface. Prehnite Laumontite Taking the chemical composition of an Heulandlte average feeder dike trom the Karmutsen Group (Table 1, no. 3) as a reference, the cores of the Buttle Lake altered glass fragments show significant relative losses of Si and Na, and slight gains in Fe, AI, and Mg. With respect to the more altered bleached rims of the fragments, there are significant losses of Si, Ca, and Na, and relative gains of Fe and Mg. Microprobe scans have 0 1000 m also shown that the re is a complete loss of K. Thus, the progressive alteration of glass shows 0 500C ft relative losses of Si, Ca, Na, and K, and relative Figure 3. Comparison of the mineralogy of tujfa- gains of total Fe £.nd Mg. The altered glass ceous rocks from Southland, New Zealand (Coombs, also shows a large addition of H20. 1954) and Buttle Lake, British Columbia, Canada. The Textural and chemical evidence indicate that reference for the comparison is the analcime-albite the alteration of basaltic glass to the phyllo- transition. There is 5,000 to 15,000 ft of overburden silicate in the aquagene tuffs was a process of on top of Karmutsen Group. hydration and solution. This is in accord with altered material in the lowermost part of the the suggestion of Deffeyes (1959, p. 607) that section is mainly chlorite. For the sake of the zeolitic alteration of volcanic glass is character- discussion in the remainder of the paper, the ized by solution of material and not internal altered "glassy" material will be called phyllo- devitrification. Moreover, the suggestion that silicate. the glass in the Karmutsen Group altered by The average chemical compositions for sam- hydration and solution agrees with the recent ples of phyllosilicates are given in Table 1. observations concerning the nature and origin Analyses la and lb represent the typical of palagonite in the Honolulu Series on Oahu, composition of the cores and rims of the Hawaii (Hay and Iijima, 1968). Palagonite in phyllosilicate fragments. Analyses 2 and 3 the Pleistocene Honolulu Series formed by the represent, respectively, the average composi- reaction of ground water with sideromelane. tion of chilled "tachylytic" pillow lava rims Furthermore, about one-quarter of the Si02, and of feeder dikes. Compositionally these one-half of the AI2O3, and three-fourths or phyllosilicate fragments have significant simi- larities to the palagonitic material in the Honolulu Group described by Hay and Iijima (1968). It is apparent from the typical compositions (Table 1) that generally the chemical composition at the center of the phyllosilicate fragments approximates a basaltic composition more closely than the rims of the fragments. The composition of the bleached rims of the fragments more closely approximates the composition of chlorite. Some of the smaller phyllosilicate fragments show no chemical gradients and are characterized throughout by compositions similar to the rims of the larger fragments. Calculation of gain and loss of ma- Figure 4. Aquagene tuff from the uppermost terial in the transition of glass to phyllosilicate Karmutsen Group. Note the bleached rims of the is impossible without reference to some stan- "glass" (phyllosilicate) fragments. The plagioclase grains in this section are fresh. Laumontite or analcimc dard cell. Nevertheless, there are compositional commonly surrounds the "glass" fragments. Large differences (the greatest difference from a fragments in center of photo are approximately 2 mm basaltic composition being at the margins of wide.

TABLE 1. PARTIAL ELECTRON MICROPROBE Karmutsen Group the alteration of basaltic ANALYSES OF THE PHYLLOSILICATES glass to phyllosilicates was characterized by hydration and solution is compatible with la lb 2 3* observations on similar but much younger

Si02 35 .0 34 .0 48.3 49.5 rocks.

Al 203 16.. 0 14.. 0 13.1 13.7 Entrapped sea water was undoubtedly the main source of volatiles in the aquagene tuffs Fe203(total Fe) 15., 5 22. 5 15.8 12.8 of the Karmutsen Group. The physical nature MgO 8.. 0 12. 0 5.7 6.7 of the aquagene tuffs and their mode of forma- CaO 12.. 0 4.. 0 11.1 11.3 tion made them ideally suited for the entrap-

Na20 0 0 2.4 2.0 ment of sea water, particularly because most

Subtotal 86.. 5 86. 5 96.4 96.0 of the aquagene tuffs are overlain by relatively impermeable massive volcanic flows (Fig. 2). *X-ray fluorescence analysis. The conclusion that the alteration of basaltic la. Typical analysis of cores of phyllosilicate fragments, glass and subsequent formation of phyllosilicate lb. Typical analysis of bleached rims of phyllosilicate fragments. minerals is a hydration and solution process is 2. Typical analysis of chilled pillow lava rim, strengthened by examination of the "glassy" Karmutsen Group, ("tachylytic" rims) . 3. Typical x-ray fluorescence analysis of feeder dikes rims of pillow lavas from the Karmutsen Group. and sills in Karmutsen Croup (Surdam, 1970). The outermost selvage rimming the pillows commonly seems to be optically isotropic. The

more of the CaO, Na20, and K2O were lost in 1- to 2-cm-thick chilled rims are composed of converting sideromelane to palagonite. The material described by Carlisle (1963, p. 54) as components lost during the alteration of the devitrified sideromelane and tachylyte. These glass fragments were precipitated as zeolites, glassy selvages formed by abrupt quenching of clay minerals, opal, and calcite cement (Hay the lava at the time of pillow formation. The and Iijima, 1968). pillow lava rims characteristically have about It is possible to alter basaltic glass and form the same chemical composition as the unaltered zeolites in the laboratory. In some recent ex- feeder dikes and sills (Table 1, nos. 2 and 3). periments, the author subjected natural basaltic Thus, it is suggested that the "glassy" pillow glasses to alkaline solutions (NaOH) over a rims have not undergone the kind of alteration range in pH (9.1 to 11.5) at 80°C. These solu- typical of the "glassy" aquagene tuffs, even tions after 70 days are characterized by 20 to though presumably they had similar initial compositions. Textural differences probably 50 ppm Si02 and 1 to 5 ppm AI2O3. If the solutions are allowed to cool to room temperature, caused different types of alteration, because hydrous Na-Ca-K aluminosilicate gels form. the nearly impermeable pillow lava rims were Moreover, when these gels are heated to 80°C, not readily susceptible to hydration and solu- zeolites form from the gels. The net result of the tion, whereas the permeable aquagene tuffs experiment is basaltic glass fragments with were readily attacked by entrapped connate leached rims and zeolites (phillipsite, chabazite, water during burial. and erionite?). The experimental and analyti- Albite occurs in the Karmutsen Group in cal procedures for these experiments have been the Buttle Lake area as an alteration product described elsewhere (Mariner and Surdam, of calcic plagioclase and analcime. Most of the 1970). primary calcic plagioclase, particularly in the These experiments are particularly pertinent lower half of the section, has been altered to in light of the recent work of Alexandersson albite + hydrous calcium aluminum silicate. (1970), who studied the lithology and cementa- The hydrous calcium aluminum silicate is gen- tion of 6,000- to 7,000-yr-old hyaloclastics erally pumpellyite, but it is locally prehnite, (~ aquagene tuff) from the sea floor near laumontite, wairakite, or epidote. White mica Surtsey. Alexandersson described zeolites and and chlorite also occur in the alteration prod- gelatinous material surrounding palagonitic ucts of calcic plagioclase. The mixture of albite brown sideromelane and opaque tachylitic and pumpellyite or other alteration products, "glass" fragments. Compared to the sidero- in varying proportions, is pseudomorphous melane "glass," the cementing zeolites and after the calcic plagioclase. gels have a higher content of Si, Al, K, and Ca, Inasmuch as hydrous phases occur in the whereas Ti, Mg, and Fe are lost (Alexander- calcic plagioclase alteration products, H20 sson, 1970). Thus, the suggestion that for the must have been added during albitization.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/6/1911/3428562/i0016-7606-84-6-1911.pdf by guest on 01 October 2021 1912 R. C. SURDAM

Also, the presence of pumpellyite, epidote. ogical elements seem unrelated to stratigraphie and chlorite in the alteration products requires position. Included in the former category are the addition of iron and magnesium. It is im- the analcime —> albite reaction, the phyllosili- portant to note that the albitized plagioclast: cate —» chlorite transition, and (to a lesser always contains appreciable amounts of hydrous extent) the albitization of calcic plagioclase. calcium alumino-silicate minerals. The altera- The analcime —> albite reaction seems to occur tion of glass probably provided the necessary over a relatively narrow stratigraphie interval, MgO, Fe2C>3, and FeO for the formation of whereas the phyllosilicate --» chlorite transition pumpellyite, epidote, and chlorite. Evidently, is very gradual and occurs over a very broad glass is the first phase to alter, because aquagene interval. The albitization pattern is much more tuffs in the upper part of the section contain complex; generally, the unaltered calcic plagio- fresh calcic plagioclase, but the glass is altered clase is at the top of the section, whereas the to phyllosilicates and is surrounded by zeolites. albitized plagioclase occurs in the lower part If hydration and solution of glass provided of the section. However, unaltered plagioclase the necessary material for the zeolites surround- is present locally throughout the stratigraphie ing the phyllosilicate fragments in the aquagene section. tuffs, the zeolitic reactions cannot be viewed only in terms of pressure, temperature, and IONIC EQUILIBRIA bulk composition of the solid phases. Moreover, As suggested by Eugster (1970) metamorphic the traditional view that the zonal distribution reactions involving a fluid phase should be con- of zeolites in low-grade metamorphic terranes sidered not only in terms of thermal stability, can best be explained in terms of a series of but also in terms of ionic stability. For ex- dehydration reactions is obviously too sim- ample, in the reaction laumontite —» prehnite, plistic. Instead, such parameters as salinity the thermal stability can be represented as and alkalinity of the solutions in contact with follows: the solid phases are as important as pressure and temperature. CaAl2Si40i2-4H20 + CaO -> Ca Al Si Oio(OH) An excellent illustration of the importance 2 2 3 2 + Si0 + 3H 0 (1) of solution chemistry in terms of diagenetic 2 2 reactions is the suite of authigenic minerals in and the ionic stability can be represented as the tuffaceous rocks of the Barstow Formation follows: (Sheppard and Gude, 1969). In the Mud Hills in a lateral direction in one tuff bed, the CaAl2SÍ40i2-4H20 authigenic minerals show the following reaction 4- Ca++ -> Ca2Al2Si3Oio(OH)2 relations: glass —> clinoptilolite —> analcime —> + Si02 + 2H20 4- 2H+ . (2) K-feldspar. This lateral variation of authigenic Reactions involving the hydrous calcium minerals in the Barstow Formation is explained aluminosilicates seem to be relatively unaf- in terms of salinity and alkalinity gradients in fected by stratigraphie position. For example, the solutions in contact with the glass or the distribution patterns of epidote, pumpelly- mineral precursors (Sheppard and Gude, 1969). ite, prehnite, laumontite, arid wairakite over- The Miocene Barstow Formation was never lap (Fig. 3). The geometry of the mineral buried to a depth of more than a few thousand patterns strongly suggests that pressure and feet. As is readily apparent, the horizontal temperature are not the primary controlling mineral sequences in the Barstow Formation factors of the mineral "stability" fields. are very similar to the vertical mineral se- Alternatively, the observed distribution pat- quences described in many low-grade meta- terns of the hydrous calcium aluminosilicate morphic terranes and assigned to the zeolite minerals can be explained readily in terms of mineral facies. The point is not to discredit ionic equilibria. The stability of these mineral previous work but simply to emphasize that phases, in terms of ionic equilibria, can be zeolitic reactions are particularly dependent examined most easily in tsrms of chemical on the composition of the solutions of the potential or activity diagrams prepared by individual low-grade metamorphic terranes. graphical techniques. Helgeson (1967) has Although several aspects of the low-grade shown the relation between traditional com- metamorphic mineral assemblages in the Kar- positional diagrams of solid phases and chemical mutsen Group apparently were controlled by potential diagrams in terms of ionic species "depth of burial," other important mineral- and has stressed the importance of the latter

TABLE 2. FORMULAS OF SELECTED HYDROUS CALCIUM ALUMIN0S1LICATE MINERALS

Moles O Mineral Formula CaO A1203 Si02 CVJ ^ 4 2 Wairakite* CaAl2Si^0i2'2H20 1 1 4 Uo Laumontite* CaAl2Si,012'4H20 1 1 4 c Pu-Ep

Prehnite* Ca2Al2SijOio(OH)2 2 1 3 1

Pumpellyite' Ca2Al3Si30„(0H)3 2 1.5 3

Epidote* Ca2Al3Si 3012(OH) 2 1.5 3

*Formula from Deer and others, 1962a 1962b 196S. 2 nAI fFormula from Gottardi , 1965. / 2°3 Schematic composition diagram for the system Ca0-Al203-Si02-H20 plotted on orthogonal diagrams in petrological problems involving coordinates for an unspecified temperature and pressure solutions. in the presence of water. Pr = prehnite, Pu = pum- The important relation that allows graphi- pellyite, Ep = epidote, Lm = laumontite, Wr = cal conversion from compositional diagrams wairakite. plotted on orthogonal coordinates to ionic activity diagrams is the fact that the slopes of It is obvious that within an appropriate the tie lines on the orthogonal composition range of temperature and pressure and in the diagram are equal to the negative reciprocals presence of H20, any of the hydrous calcium of the corresponding field boundaries on alumino-silicates being discussed could be a chemical potential diagram (Korzhinskii, stable, depending on the activities of Ca++, + 1957). The details of moving from one diagram Si02, and H . The gross overlap of critical to the other are thoroughly described in mineral zones in low-grade metamorphic ter- Korzhinskii (1957) and Helgeson (1967). ranes can be explained in terms of the activi- Assuming the mineral phases of interest are ties of ionic species in the aqueous phase. stoichiometric (Table 2), they can be plotted This point is well illustrated in the amygda- on orthogonal coordinates (Fig. 5). Moreover, loidal volcanic flows in the Karmutsen Group. by assuming the conservation of aluminum, a On the scale of a thin section, wairakite and set of equations describing the ionic reactions laumontite occur in adjacent amygdales. between the phases can be written: Stoichiometrically, wairakite and laumontite are the same, except that laumontite contains CaAl2Si40i2-4H20 + Ca++ two more water molecules. However, electron laumontite microprobe studies of the amygdaloidal min- = Ca2Al2Si3Oio(OH)2 + Si02 + 2H20 + 2H+ prehnite (3)

3CaAl2Si40i2-4H20 + Ca++ laumontite

= 2Ca2Al3Si3On(OH)3 + 6Si02 + 8H20 + 2H+ pumpellyite (4)

3Ca2Al2Si3Oio(OH)2 + 4H+ = Pu-Ep prehnite Lm-Wr

2Ca2Al3Si3Ou(OH)3 + 2Ca++ + 3Si02 + 2H20 . pumpellyite (5) In the above equations, wairakite can be substituted for laumontite, or epidote for pumpellyite, necessitating only a change in the coefficient of H20. These equations represent I o g the reactions at the boundaries in an activity Figure 6. Activity diagram depicting phase rela- diagram such as Figure 6. Due to a lack of tions of hydrous calcium aluminosilicate minerals in thermochemical data, this diagram cannot be the Karmutsen Group metavolcanic rock system at an quantified, but the geometry of the stability unspecified temperature and pressure in the presence fields is probably correct. of water.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/6/1911/3428562/i0016-7606-84-6-1911.pdf by guest on 01 October 2021 1912 R. C. SURDAM

erals have suggested almost complete solid Buttle Lake area, laumontite may be unstable solution in the mineral series analcime-wairak- at temperatures as low as 100°C; whereas in ite, whereas Na substitution is virtually ab- New Zealand, laumon;:ite may be stable at sent in laumontite (Surdam, 1965). Therefore, temperatures as high as ,500°C. a plausible explanation is that the laumontite/ The observation thai analcime + quartz wairakite relation is controlled by small dif- persists to higher grades than laumontite in ferences in the activities of Na+ and Ca++. some terranes (for example, Puerto Rico, The presence or absence of pumpellyite or Karmutsen Group) and not in others (Southern epidote in the tuffaceous rocks is probably a New Zealand, New Sot th Wales) can be ex- function of jxC02. The alteration products in plained if the curve for t rie reaction laumontite the tuffaceous rocks show a close association + CaO = prehnite + quartz -f water inter- between CaCOa and epidote, and a correlation sects the analcime + quf.rtz = albite + water between the presence of CaCC>3 and the ab- curve. Campbell and Fyfe (1965) proposed an sence of pumpellyite. Pumpellyite and epidote equilibrium curve for the reaction analcime + can be related as follows: quartz = albite -f water. In addition, the AV term for the formation of prehnite from Ca2Al3Si30u(0H)3 — Ca,Al3Si3012(0H) + H20 laumontite is negative whether or not PH„O < pumpellyite epidote (6) Ptotai• If tne AS for the reaction is assumed to be positive, the reaction curve should have a A high MCO/jiH 0 would favor epidote, 2 negative slope. whereas a low /iC0/VII20 would favor pumpellyite. This example illustrates the effect of If the diagram of Camp bell and Fyfe (1965) fiC02 on low-grade metamorphic reactions, r. is valid and if the laumontite-prehnite rela- point that was first made by Zen (1961). tions have been interpreted correctly, a hypo- Electron microprobe analyses have shown thetical diagram can be constructed depicting that generally the mole percent CaO, Si02, the spatial relations of the reaction curves of and total Fe are nearly identical in epidote interest (Fig. 7). As can be seen from Figure 7, and pumpellyite from the Karmutsen Group with a relatively high gíiothermal gradient, (Surdam, 1967). However, the pumpellyite analcime would persist to higher temperatures may contain 1 to 2 wt percent MgO, whereas than does laumontite; whereas with a lower the epidote usually contains less than 1 wt geothermal gradient, laumontite would persist percent (Surdam, 1967); also, the Fe+2/Fe+3 to higher temperatures than does analcime. may be different. Thus, subtle differences in On the other hand, if the ionic stability of ++ 1-1 the aMe or aFe" * in the aqueous phase also laumontite is emphasized, another explanation may be a factor in stabilizing pumpellyite or must be considered. Figure 6 demonstrates epidote. that only minor fluctuations in the activities ++ + To further illustrate the importance of ionic of Ca , and (or) H , and (or) Si02 in the stability, consider the problem of laumontite aqueous solution can stabilize either laumontite distribution in low-grade metamorphic ter- or prehnite. Thus, instead of calling on large ranes. Field and petrographic observations in differences in geothermal gradients or thermal the Buttle Lake area suggest that the upper histories, the observed mineralogical differences stability limit of laumontite is stratigraphically can be explained in terms of chemical varia- higher, and thus at lower temperatures, than tions in the aqueous phase. Moreover, in addi- the analcime-albite transition. Campbell and tion to the traditional pararr.eters of bulk com- Fyfe (1965) suggested that in the presence of position, temperature, and pressure, the activi- quartz the upper stability of analcime is 200°C. ties of the ions in the aqueous phase will be An estimated temperature for the upper determined by such parameters as solu- stability of laumontite based on stratigraphic tion:solid, glass:mineral, plagioclase:total-min- position in the Buttle Lake area is 150° ± 50°C. eralogy ratios, grain size, salinity, permeability, Coombs and others (1959, p. 91), suggested and alkalinity. These parameters will determine that a gradual transition from laumontite to the path the solution takes in an activity prehnite assemblages at temperatures of about diagram such as Figure 6 during low-grade 300°C is compatible with field occurrences in metamorphism and will also determine the the New Zealand geosyncline. Obviously there order of appearance of the critical mineral is a disparity in the proposed thermal stability zones. Once the glass has been converted to relations of laumontite in the two areas. In the chlorite and the plagioclase albitized (for ex-

the usual rate of accumulation of pelagic sediments is enough to impede the alteration of basaltic glass and the formation of zeolites at or near the sediment-water interface in a marine environment. Certainly the rate of accumulation of the aquagene tuffs in the Karmutsen Group was at least several times greater than 1 cm per 1,000 yrs. Observations made by Alexandersson (1972) studying volcaniclastic rocks near Surtsey bear directly on this problem. The 10,000-yr- old marine volcaniclastic rocks near Surtsey, strikingly similar in many respects to the aquagene tuffs and pillow breccias of the Karmutsen Group, contain vitric material Figure 7. Pn3o-temperature equilibrium diagram for the reaction analcime + quartz —> albite + H2O characterized by only incipient alteration. The (Campbell and Fyfe, 1965) and a hypothetical dashed incipient alteration consists of gelatinous coat- curve for the laumontite to prehnite reaction. ings and cement surrounding the volcaniclastic fragments. In some of the scanning electron micrographs taken by Alexandersson (1972, ample, greenschist assemblages), these param- Fig. 5), a few small (1 X 3 n) crystals are eters become less critical; and temperature, recognizable in the gel phase. pressure, and bulk composition become more After the Karmutsen Group volcanic rocks critical. accumulated, the rate of burial became negli- gible. The next 20 to 30 m.y. (Karnian through Timing of Alteration at least Sinemurian) were characterized by rela- One of the most interesting facets of the tive volcanic quiescence, while 2,400 ft of glass-alteration process in the Karmutsen marine sediments accumulated. During this Group is the timing of the alteration. The lack interval the uppermost tuffaceous rocks were of alteration of basaltic glass has been noted in at temperatures of 10° to 40°C and under less the cores from the preliminary drilling phase than 1 kb total pressure, whereas the tuffaceous of the Mohole project (Riedel and others, rocks in the lower half of the section were at a 1961; Murata and Erd, 1964). Relatively un- temperature of 140° to 240° C and under 1 to altered vitric pyroclastic material, particularly 2 kb pressure. If the comparison with modern of basaltic composition, occurs to a depth of submarine volcanic rocks is appropriate, altera- 170 m below the sea floor in the preliminary tion of glass in the Karmutsen Group would Mohole core. These findings do not rule out the have begun only after burial. possibility of incipient palagonitization, be- The absence of heulandite in any part of the cause it would indeed be surprising if volcanic section at Buttle Lake supports the hypothesis glass of any composition remained totally unal- that alteration of glass started only after burial. tered after being subjected to a hydrous envi- In most sections characterized by a vertical ronment. Nonetheless, they do suggest that for zonation of zeolites, heulandite is found at the some reason the glass alteration reactions in top (Hay, 1966, p. 70). Coombs and others these sediments were retarded. (1959, p. 91) have suggested that heulandite is Riedel and others (1961, p. 1798) concluded stable at lower temperatures and pressure than that the unaltered vitroclastic material along is laumontite. This suggestion is supported by with the unoxidized character of the Mohole field observations, because in many sections, sediments reflected a rate of accumulation more heulandite is typically replaced by laumontite rapid than usual for pelagic clays. A rate of 1 4- quartz at depth. Observations at Wairakei cm per 1,000 yrs is suggested for the Mohole suggest that heulandite reacts to form laumont- sediments, whereas the average rate of accumu- ite at approximately 200°C (Coombs, 1961, lation for pelagic clays in the Pacific Ocean is p. 206). However, Hay (1966, p. 90) indicated thought to be 1 mm per 1,000 yrs (Riedel and that there are exceptions to this general rela- others, 1961). If their hypothesis is accepted, a tion by citing several examples where laumont- rate about an order of magnitude greater than ite 4- quartz coexists in deposits whose maxi-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/6/1911/3428562/i0016-7606-84-6-1911.pdf by guest on 01 October 2021 1912 R. C. SURDAM

mum burial depths range from less than 3,0C0 to 8,000 ft, suggesting temperatures in the range 50° to 100°C. Furthermore, Hay (1966) suggested that the formation of laumontite rather than heulandite at low temperatures in the presence of quartz, may be a kinetic phenomenon. The laumontite in the aquager.e tuffs in the Buttle Lake section does not appear to represent the reaction product of heulandite, because much of the laumontite does not occur with quartz. Prehnite in tuffaceous rocks of the uppermost Karmutsen Group is iron poor (Table 3) and occurs within laumontite as needles and veins intimately associated with quartz. Tex- Figure 8. Relict glass fragment consisting of turally, the prehnite and quartz seem to prehnite and chlorite from aquagene tuff in the lower- represent the decomposition of laumontite. most Karmutsen Group. The plagioclase grains from These prehnite needles contain less than 1.3 wt this section have been albitii:ed. Large fragment at percent Fe2C>3 (total Fe). center of phcto is approximately 2 mm wide. The prehnite in the lowermost part of the Karmutsen Group is iron rich (Table 3) anc associated with laumontite, whereas the iron- occurs locally in tuffaceous rocks replacing glass rich prehnite at the bottom of the section is fragments but occurs mainly as patches and the direct result of glass alteration, is best veinlets, generally without quartz. Texturally, explained if the glass altered after burial. Be- this prehnite seems to be the direct alteration cause of rapid burial, the glassy material in the of glass (Fig. 8). The average Fe2C>3 (total Fe) lowermost part of the Karmutsen Group passed content of prehnite from the lower half of the through the stability fields of heulandite and Karmutsen Group is 8.0 wt percent. laumontite. When the alteration began, the The absence of heulandite and the fact that rocks were in the thermal stability field of iron-poor prehnite at the top of the section is prehnite. This prehnite accepted appreciable amounts of iron into its crystal structure be-

TABLE 3. PARTIAL ELECTRON MICROPROBE ANALYSES OF PREHNITE* cause it formed from glassy material rich in iron. The minute amount of prehnite at the A. Prehnite from the uppermost aquagene top of the section formed from iron-poor tuff (Karmutsen Group) 12 3 4 laumontite.

SiO, 44.8 44.7 44.2 44.0 ROLE OF VOLATILES CaO 25.1 25.0 25.3 25.8 From mineralogical evidence, the activity

A120, 25.3 25.2 25.2 25.8 of water was a very significant physio-chemical

FejO,(total Fe) 0.3 0.6 0.4 tr. variable during the low-grade metamorphism of the Karmutsen Group. The relatively unal- Range of Fe 0 0.1-0.6 0.2-1.3 0.1-0.9 0.0-0.2 2 3 tered pillow rinds have suffered only minor Subtotal 95.5 95.3 95.2 95.6 hydration because ignition loss determinations of pillow rinds average about 0.80 wt percent. B . Prehnite from the lowermost aquagene tuff (Karrcutsen Group) If water passed through, or otherwise gained 5 6 7 8 access to the glassy rinds, the glass would have been hydrated. In the pillow breccias and Si02 43.8 44.6 43.0 44.6 aquagene tu.is where connate waters had access CaO 24.6 25.6 26.3 25.2 to the smaller glass fragments, there has been AI2O3 18.7 17.4 17.6 20.0 appreciable hydration. Ignition loss determina-

Fe20,(total Fe) 8.2 8.0 7.6 5.0 tions for the tuffaceous matrix of pillow breccias and aquagene tuffs average 5.3 wt percent. Range of Fe203 7.5-8.8 4.4-10.3 0.4-9.2 3.3-6.9 The solution interfaces displayed by some of Subtotal 95.3 95.6 94.5 94.8 the incompletely hydrated "glassy" fragments * For all prehnite analyses, the ngo, NaiO, and K2O indicate that volatile activity gradients oc- contents are less than 0.5S. curred across very small domains. Therefore,

although volatile activities in the aquagene on the sea-floor: Surtsey Research Prog. Rept. tuffs may have been high relative to those in V., p. 83-89. pillow lavas, volatile gradients did exist across 1972, The sedimentary xenoliths from Surt- very small domains in the aquagene tuffs. sey: Turbidites indicating shelf growth: Surtsey Prog. Rept. VI, p. 1-16. Remnants of calcic plagioclase and incomplete- Campbell, A. S„ and Fyfe, W. S., 1965, Analcite- ly hydrated glass low in the stratigraphic albite equilibia: Am. Jour. Sci., v. 263, p. section can be explained by such volatile 807-816. gradients. Carlisle, D., 1963, Pillow breccias and their aquagene tuffs, Quadra Island, British Columbia: CONCLUSIONS Jour. Geology, v. 71, no. 71, p. 48-71. Some vertically zoned Ca-zeolite sequences Coombs, D. S., 1954, The nature and alteration of are considered to result from a series of de- some Triassic sediments from Southland, New Zealand: Royal Soc. New Zealand hydration reactions caused by elevated tem- Trans., v. 82, p. 65-109. peratures (Coombs and others, 1959). This is 1961, Some recent work on the lower grades not the case for the Karmutsen Group. As was of metamorphism: Australian Jour. Sci., v. 24, suggested, the glass at the top of the strati- p. 203-215. graphic section altered to laumontite and 1970, Present status of the zeolite facies: phyllosilicate, whereas the glass in the lower- Second Internat. Conf. on molecular sieve most part of the sequence altered to prehnite zeolites: Am. Chem. Soc., p. 317-327. and phyllosilicate directly. Therefore, the Coombs, D. S., Ellis, A. J., Fyfe, W. S., and Taylor, gross vertical zonation with respect to laumont- A. M., 1959, The zeolite facies, with comments ite and prehnite is not the result of a dehydra- on the interpretation of hydrothermal syn- tion reaction. The formation of both laumont- theses: Geochim. et Cosmochim. Acta, v. 17, p. 53-107. ite in the upper Karmutsen Group and prehnite Deer, W. A., Howie, R. A., and Zussman, J., in the lower Karmutsen Group actually is the 1962a, Rock forming minerals. Vol. 1, Ortho- result of hydration reactions. In each case, and ring silicates: New York, John Wiley water is added to nearby anhydrous basaltic & Sons, Inc., p. 333. glass, and laumontite + phyllosilicates or 1962b, Rock forming minerals. Vol. 3, Sheet prehnite + phyllosilicates result. The glass- silicates: New York, John Wiley & Sons, alteration process is characterized by hydration Inc., p. 270. and solution, not by dehydration. 1963, Rock forming minerals. Vol. 4, Frame- work silicates: New York, John Wiley & In the Karmutsen Group, as well as in many Sons, Inc., p. 435. other low-grade metamorphic terranes, the Deffeyes, K. S., 1959, Zeolites in sedimentary large overlap of critical minerals such as rocks: Jour. Sed. Petrology, v. 29, p. 602-609. laumontite, wairakite, prehnite, pumpellyite, Eugster, H. P., 1970, Thermal and ionic equilibria and epidote can best be explained in terms of among muscovite, K-feldspar and alumino- ionic equilibria. Laumontite and prehnite, in silicate assemblages: Fortschr. Mineralogie, v. reaction relation, overlap more than 10,000 47, p. 106-123. stratigraphic feet 100°C) and cannot be Fyfe, W. S., Turner, F. J., and Verhoogen, J., explained in terms of thermal stability; how- 1958, Metamorphic reactions and metamorphic facies: Geol. Soc. America Mem. 73, ever, they can be explained in terms of ionic p. 260. stability. Gottardi, G., 1965, Die Kristallstruktur von pumpellyit: Tschin. Mineralog. u. Petrolog. ACKNOWLEDGMENTS Mitt. 10, p. 115-119. Hay, R. L., 1966, Zeolites and zeolitic reactions The Geological Society of America, Na- in sedimentary rocks: Geol. Soc. America tional Science Foundation, and UCLA Geology Spec. Paper 85, p. 130. Department helped support this research. Field Hay, R. L., and Iijima, A., 1968, Petrology of excursions and discussions were held with D. palagonite tuffs of Koko Craters, Oahu, Carlisle and W. G. Jeffery. An earlier draft Hawaii: Contr. Mineralogy and Petrology 17, of this manuscript was reviewed by fames R. p. 141-154. Boles and Richard Sheppard. Helgeson, H. C., 1967, Solution chemistry and metamorphism, in Abelson, P. H., ed., Re- searches in geochemistry, Vol. 2: New York, REFERENCES CITED John Wiley & Sons, Inc., p. 362-404. Alexandersson, T., 1970, The sedimentary xeno- Jeffery, W. G., 1963, Preliminary map of Buttle liths from Surtsey: Marine sediments lithifield Lake: Victoria, British Columbia Dept. Mines.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/6/1911/3428562/i0016-7606-84-6-1911.pdf by guest on 01 October 2021 1912 R. C. SURDAM

Korzhinskii, D. S., 1957, Physicochemical basis San Bernardino County, California: U.S. Geol. of the analysis of the paragenesis of minerals: Survey Prof. Paper 634, p. 35. London, Chapman and Hall, 142 p. Surdam, R. C., 1965, Analcime-wairakite mineral Mariner, R. H., and Surdam, R. C., 1970, Alka- series: Geol. Soc. America Spec. Paper 87, linity and formation of zeolites in saline p. 169-170. alkaline lakes: Science, v. 170, p. 977-980. 1967, Low-grade metamorphism of the Murata, K. J., and Erd, R. C., 1964, Composition Karmutsen Group, Buttle Lake area, Van- of sediments from the experimental Mohoie couver Island, British Columbia [Ph.D. project (Guadalupe site): Jour. Sed. Petrology, thesis]: Univ. California, Los Angeles, 313 p. v. 34, p. 633-655. 1968, The stratigraphy and volcanic history Packham, G. H., and Crook, K.A.W., 1960, The of the Karmutser. Group, Vancouver Island, principles of diagenetic facies and some of its B. C.: Contr. Geology, v. 7, p. 15-26. implications: Jour. Geology, v. 68, p. 3S.Î-407. • 1970, The petrology and chemistry of the Riedel, W. R., Ladd, H. S„ Tracy, J. I., and Karmutsen Group volcanic rocks: Contr. Bramlette, M. N., 1961, Preliminary drilling Geology, v. 9, no. 1, p. 9-12. phase of Mohoie project; II. Summary of Zen, E-An, 1961, The zeolite facies: An interpreta- coring operations: Am. Assoc. Petroleum tion: Am. Jour. Sci., v. 259, p. 401-409. Geologists Bull., v. 45, no. 11, p. 1793-1798. Seki, Y., 1969, Facies series in low-grade metamorphism: Geol. Soc. Japan Jour., v. 75, p. 255-266. MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 20, Sheppard, R. A., and Gude, A. J., 1969, Diagenesis 1972 of tuffs in the Barstow Formation, Mud Hills, REVISED MANUSCRIPT RECEIVED OCTOBER 6, 1972

PRINTED IN U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/6/1911/3428562/i0016-7606-84-6-1911.pdf by guest on 01 October 2021