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 differ- ence 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, pro- vided 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.