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Archean Variolites: Quenched Immiscible Liquids (From the Abitibi Volcanic Belt) Exploration Géologique

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Archean Variolites: Quenched Immiscible Liquids (From the Abitibi Volcanic Belt) Exploration Géologique DPV 425 ARCHEAN VARIOLITES: QUENCHED IMMISCIBLE LIQUIDS (FROM THE ABITIBI VOLCANIC BELT) EXPLORATION GÉOLOGIQUE MINISTERE DES RICHESSES NATURELLES DIRECTION GENERALE DES MINES ~ ARCHEAN VARIOLITES- QUENCHED IMMISCIBLE LIQUIDS L . GELINAS C . BROOKS W . E TRZCIENSKI jr. DPV-,425 ARCHEAN VARIOLITES - QUENCHED IMMISCIBLE LIQUIDS L. GELINAS* C. BROOKS** W.E. TRZCIENSKI Jr.* * Département de Génie Minéral, Ecole Polytechnique. ** Département de Géologie, Université de Montréal. Document déposé au fichier ouvert, le 4 octobre 1976 Distribution sur demande seulement Document placed on Open File, October 4, 1976 Distribution on request only - I - ABSTRACT Variolitic lavas from Archean tholeiitic series north and south of Rouyn-Noranda (Abitibi Metavolcanic belt, Canada) con- tain large, sharply defined, spheroidal to subspheroidal felsic varioles (up to 5 cm in diameter) set in a ferruginous matrix of more mafic composition. Quench texture and flow differentia- tion studies indicate that the variolites were produced by ra- pid-cooling of a two-liquid magma, and that these liquids were in contact and chemically discrete prior to extrusion. Physical mixing models,do not adequately account for these contiguous mag- mas yet a liquid immiscible model demonstrably satisfies al- most all variolite field, microscopic, microprobe and chemical data. We conclude Archean variolites are formed by immiscible splitting of a magma of tholeiitic composition. — II — CONTENTS Page AB S TRACT I INTRODUCTION 1 GEOLOGICAL OUTLINE 4 Kinojevis Group Blake River Group FIELD OCCURRENCE 6 Pillowed variolites Massive-flow variolites Variolitic hyaloclastites MINERALOGY, TEXTURE AND PRE-ALTERATION NATURE 15 Varioles Matrix Pre-alteration nature Cooling histories CHEMISTRY AND SECONDARY ALTERATION 27 ORIGIN OF THE VARIOLITES 34 Physical mixing of contemporaneous magmas Liquid immiscibility — III — GEOCHEMICAL ASPECTS PERTAINING TO IMMISCIBILITY 43 CONCLUSIONS 47 ACKNOWLEDGEMENTS 50 REFERENCES 51 FIGURES 1 - Location map 3 2 - Geology, variolite occurrences and sampling locations 7 3 - Descriptive terms of the Archean variolites 15 4 - AFM plot of variolites 30 5 - Fe contents in 4 matrix-variole pairs from a variolitic lava from the Dufresnoy tholeiitic suite 40 6 - Pseudo-ternary diagram showing location of the proposed miscibility gap 46 7 - Archean variolites, varioles and matrix and the proposed miscibility gap 48 PLATES I - Modes of variole occurrence 12 II - Contact between the low-K rhyolitic varioles and the ferruginous matrix 13 III- Variole and matrix morphology in a variolitic lava 17 IV - Variole and matrix morphology in a variolitic lava 18 V - Variole and matrix morphology in variolites from Dufresnoy tholeiitic series 20 VI - Variole and matrix morphology in a variolitic lava showing the most advanced stage of crystallization 21 TABLES 1 - Crystal morphology as related to relative degree of supercooling 24 2 - Major element composition of Archean variolites, varioles and matrices 29 INTRODUCTION The concept of liquid immiscibility as a process of igneous differentiation is not new. Recently the application of the theory to silicate melts has been forcefully reviewed, and the combination of convincing experimental data (e.g. Roedder, 1951; Roedder and Weiblen, 1970; Massion and Koster van Groos, 1973; McBirney and Nakamura, 1974; Nakamura, 1974) and textural ob- servations (e.g. Carstens, 1963; Gélinas, 1974) suggest that immiscibility has indeed been operative in certain silicate sys- tems at temperatures that are of geological interest. In view of this renewed interest it is not surprising that other phenomena which texturally resemble globules produced by liquid immiscibility have been re-examined. For the most part these phenomena have been found in rocks of "unusual" composi- tion. Ferguson and Currie (1972) for instance suggest that the varioles in Archean basaltic komatiites of Transvaal are the pro- duct of immiscibility while evidences of the phenomenon are common in mafic alkaline intrusives (e.g. Dreyer, 1960; Philpotts and Hodgson, 1968; Philpotts, 1971; Ferguson and Currie, 1971; Car- stens, 1963; Strong and Harris, 1974; Carman, et. al., 1975). What is surprising however, is that there is much evidence sup- porting immiscibility in tholeiitic rocks. Roedder and Weiblen (1971) report the occurrence of two immiscible silicate liquids (one enclosed as droplets in another and quenched to form two - 2 - immiscible silicate glasses) form Hawaii (the 1965 lava lake in Makaopuhi; the 1959 lava lake in Rilauea Iki; the Prehistoric Makaopuhi lava lake), California (the Modoc lavas), and from Greenland (the Disko Tertiary high-iron basalts) . Anderson and Gottfried (1971) report similar observations from the high-alumina olivine tholeiite of Hat Creek, California, while De (1974) noted the occurrence of quenched immiscible silicate glasses in the tholeiitic Deccan Traps. De, as well. as Holgate (1954) and Philpotts (1967), also suggested that the trend of differentiation in the Upper Zone of the Layered Series of the Skaergaard Intrusion was in part controlled by liquid immiscibility, and experimental work by McBirney and Nakamura (1974), and McBirney (1975) confirm that in its later stages, the magma se- parated into two immiscible liquids. The purpose of this paper is to document field, textural, and chemical evidence in favor of the origin of certain tholeiitic va- riolitic lavas by liquid immiscibility. These variolites come from Archean sections in the Rouyn-Noranda region of the Abitibi vol- canic belt (Figure 1) however they are a common phenomenon in many Archean metavolcanic.belts (e.g. Yellowknife belt, Henderson and Brown, 1952). In this paper, we will present a detailed description of the field occurrence of these variolitic lavas, as well as pertinent microscopic, microprobe and chemical. data. We will show that when taken together these data furnish • a 44v A • 4 e'rrr”ntn...t a • Aliir fr 4" A ,A 4 ....... fr 4,4AP4 . 4 4 e. a Av vV . 46W a • A • •••••••- A 4 A4 v 4.4414,,,, 4 :A•l a • A ‘AIDUriCAKUUt. I • n r. °. 4.,444'• 4`1•4.• vv: 4 :A444 . 4"a 4a• V• a 4 •r - dome 404 /V 110•4' Om° 4"... „jowl/ '.46....60 "••••••/..." S 6' ..... V LAVA•• ••• Aiv 0 fi) I L DUPARQUET °N • MA NN UJ 2 ' CY I IL DA SSERA 0 ...—•:"-.*` 15• • .• • • • • • • • • • eZ,P.:-1/.4.46-41jj.0•6 •,• • • P. •••• •• • • ill • • • •••• ••• • • •• • •••• • ' 41', ip.,•_r Yiltf O 5M 064.• • ?...rf". r`t, • 40 4 • • • • • • ! • 1.• • •ar't I 1 t • LEGEND ••..•• • • •• SEDIMENTARY ROCKS • + + • GRANITE ARCHEAN r" MOSTLY MAFIC VOLCANIC ROCKS MOSTLY MAFIC AND INTERMEDIATE VOLCANIC ROCKS • FELSIC VOLCANIC ROCKS • T1'71, AGE UNCERTAIN GNEISSES AND SCHISTS FIGURE 1 — Location map, showing the regional geology of that part of the Abitibi Belt investigated. - 4 - strong evidence for the existence of immiscible processes associa- ted with Archean tholeiitic volcanics, which in turn further strengthens the link between tholeiitic volcanism in general and the process of immiscibility. An excellent literature summary of the origin and nature of variolites is presented by Carstens (1963) ; for our purposes we wish merely to clarify the terminology followed. The term var- iolite is used by us to denote a volcanic rock in which varioles occur, set within a matrix. We do not further subdivide this terminology (e.g. based on the composition of the variolite), but do modify the term to describe field occurrences (e.g. var- iolitic pillow, variolitic flow, etc..). GEOLOGICAL OUTLINE The southern part of the Superior Province contains east- trending metavolcanic belts, alternating with meta-sedimentary belts. The metavolcanic belts are typically arcuate and com- plexely deformed, with lenticular granitic rocks intruded along antiforms. The Abitibi volcanic belt•(Goodwin and Ridler, 1970) in the eastern part of the Superior Province is one of the more extensive of these Archean belts, and from size consideration alone it is unique even when compared to South African and. Australian examples. The belt is complexely folded about generally east-west trending axes. These folds may or may not superpose original stratigraphic trends, however the present-day result is a somewhat contorted belt in which the diagnostic and trace- able ultramafic, mafic and volcano-sedimentary horizons are 5 - oriented mostly east-west. Our traverse within the Abitibi belt along which the variolitic rocks were encountered was oriented north-south, in order to intersect at a high angle the dominant structural trends (Figure 2). Structurally the Abitibi belt is characterized by large- scale isoclinal folds generally displaying two superposed schis- tosities and subvertical minorfolds (Dimroth, et. al., 1973). The intensity of deformation within the belt varies considerably, and certain strongly-deformed, linear-zones have been used to segment the belt. One such zone of deformation termed the "Du- parquet-Destor-Manneville break" (Dimroth, et. al., 1973 here- after the 'DDM break') cuts our line of traverse. The large expanse and thickness of the Abitibi metavolcanic belt has undoubtedly contributed to the preservation of pri- mary textures and mineralogies. The overall metamorphic grade of this belt is prehnite-pumpellyite to low-greenschist facies, a feature we attribute to the thermal buffering effects of con- siderable thickness of volcanic rocks during post-formational orogenesis. Delicate and ornamental quench textures are pre- served in many of the basalts and andesites (Gélinas
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