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Ancient and Modern Volcanics and Proposed Miscibility in Silicate Systems Exploration Géologique

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Ancient and Modern Volcanics and Proposed Miscibility in Silicate Systems Exploration Géologique DPV 427 ANCIENT AND MODERN VOLCANICS AND PROPOSED MISCIBILITY IN SILICATE SYSTEMS EXPLORATION GÉOLOGIQUE MINISTERE DES RICHESSES s\ NATURELLES DIRECTION GÉNÉRALE DES MINES J l ç i ANCIENT AND MODERN VOLCANICS AND PROPOSED MISCIBILITY IN SILICATE SYSTEMS L . GELINAS C . BROOKS D PV-427 ANCIENT AND MODERN VOLCANICS AND PROPOSED MISCIBILITY IN SILICATE SYSTEMS L. GELINAS Département de Génie Minéral, Ecole Polytechnique C. BROOKS 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 ABSTRACT Evaluation of ancient and modern volcanic suites in terms of an experimentally based proposed miscibility gap (PRIG) reveals that of the main classes of volcanism it is only the tholeiites (ancient and modern) and ancient calc-alkaline volcanics which overlap or closely flank the PMG. This may explain the recently numerous observations of immisci- bility-produced-textures in tholeiitic rocks, and leads to a prediction that for modern tholeiites such observations will be most common in the more diversified low-K tholeiites of island-arcs. The restriction of the best and most common evidences of immiscibility to Archaean volcanic terrains (variolitic lavas) is related via partial melting and mantle- water considerations to a time-dependant, decreasing suscepti- bility of tholeiitic and low-K calc-alkaline magmas to undergo liquid immiscible-splitting. A time-independant tendency however is the apparent need for some degree of differentiation of a tholeiitic parent magma prior to immiscible splitting. — II — CONTENTS Page ABSTRACT I INTRODUCTION 1 PROPOSED MISCIBILITY GAPS 3 MODERN VOLCANIC SERIES AND THE PMG 6 Tholeiite suites Calc-alkaline suites Alkaline suites ANCIENT VOLCANICS AND THE PMG 13 DISCUSSION 15 ACKNOWLEDGEMENTS 24 REFERENCES 25 FIGURES Page 1 - Pseudo-ternary diagram 5 2 - Modern tholeiite suites and the PMG 7 3 - Modern cale-alkaline and alkaline suites and the PMG 10 4 - Ancient tholeiitic and low-K talc.--alkaline volcanics and the PMG 12 5 - South African Komatiites and the PMG 14 6 - Ar.hean variolites of Rouyn-Noranda region and the PMG 17 7 - Composite fields for modern clay volcanics in relationship to the PMG 18 8 - Composite modern and ancient tholeiite and low-K calc-alkaline fields and the PMG 19 INTRODUCTION Because of the renewed interest in immiscibility (e.g. Roedder and Weiblen, 1970; McBirney, 1975; Gélinas, 1974; Gélinas et al., 1975) many phenomena which texturally resemble globules produced by liquid segregation have been re-examined, and some found demonstrably immiscible with their enclosing fractions (e.g. Philpotts and Hodgson, 1968; Ferguson and Currie, 1971). Many of these examples come from mafic alkaline intrusives; however, surprisingly, some have been unearthed in tholeiitic associations. For instance Roedder and Weiblen (1971) report the occurrence of two immiscible silicate liquids (one enclosed as droplets in another and quenched to form two immiscible-glasses) in volcanics from Hawaii, California, Greenland and Canada. Similar observations have been made in the high-alumina olivine-tholeiite of Hat Creek, California (Anderson and Gottfried (1971) and in the tholeiitic Deccan Traps (De, 1974). De (1974) 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 immisci- bility, and this was subsequently experimentally verified by McBirney (1975). Further evidence linking immiscibility and tholeiitic magmatism has been advanced by Gélinas et al., (1975) who explain the origin of variolitic lavas in tholeiitic suites of Canadian Archaean volcanic piles by liquid immiscibility. 2 It would seem therefore that some form of association exists between liquid immiscibility and tholeiitic magmas. In many cases this association links immiscible splitting to only the late stages in magmatic history, however the tholeiitic Archaean variolites of Canada, the komatiitic Archaean variolites of South Africa (Ferguson and Currie, 1972) and the Skaergaard intrusion of Greenland all involve immisci- bility on a much grander scale. Are these examples merely artifacts of some peculiar geologic conditions, or is immisci- bility a ubiquitous phenomenon that continually shadows tholeii- tic magmatism, awaiting only the right physio-chemical conditions before manifesting itself? The purpose of this study is to examine this question by an appraisal of certain ancient and modern volcanic suites representative of the main classes of volcanism (tholeii- tic, calc-alkaline, alkaline) with the direct aim of a) evaluating the possible link between tholeiitic magmatism and immiscibility, and b) comparing the main classes of volcanism in the light of "susceptibility to immiscibility". We will demonstrate that not only is there a good case for tholeiitic magmas to be more commonly associated with immiscibility than either the calc- alkaline or alkaline volcanics, but also that there is a stronger likelihood of encountering evidences of immiscibility in the more ancient volcanics, as a result of a polarized, time- dependant "proneness" of certain magmas to undergo liquid segregation. 3 PROPOSED MISCIBILITY GAPS Grieg (1927) showed that of the major rock forming oxides, Na2O, K2O and Al2O3 are miscible with S102 whereas CaO, FeO, MgO and Fe2O3 are immiscible with SiO2 at high SiO2 concentrations. Grieg presented a pseudo-ternary diagram of (CaO + MgO + EFeO(+TiO2)) versus (Na2O + K2O + Al 2O3) versus (SiO2) to show the limited extent of the primitive immiscibility field in the polycomponent system. In this system, Grieg's primitive field is restricted to a narrow zone high on the silica portion of the cafemic oxide-SiO2 edge (figure 1). Although experimentally verified, Grieg considered immiscibility unlikely as a process in the origin of igneous rocks, on the bases that: (1) the experimentally established minimum tempera- tures were too high (nearly 1700° C); (2) no igneous rocks approached the compositions found to show immiscibility in the laboratory; and (3) the textural evidence for immiscibility in natural rocks could be readily assigned to other processes. It was not until Roedder (1951) described silicate immiscibility in the system leucite-fayalite-silica, and in the parent quater- nary system K2O - FeO - Al2O3 - SiO2 (1953), at much lower temperatures (minimum < 1100° C), and in compositions much closer to igneous rocks, that the geological possibility of immiscibility became reasonable. In the development of knowledge concerning this second field of immiscibility (denoted PTIG in figure 1), both Ilolgate (1954) and Roedder (1956) noted that two systems relevant to silicate melts display flat liquidus surfaces which according to Grieg (1927) are indicative of possible metastable fields of liquid immiscibility. These systems are NaA1SiO4 FeO - SiO2 (Bowen and Schairer, 1938), where the fayalite field displays a flat: liquidus surface, and the CaNgSi2O6 - SiO2 (Schairer and Bowen, 1938), where the plateau-shaped diopside field shows a peculiar inverse curvature. With the aid of Greig's (1927) pseudo-ternary diagram, Ilolgate (1954) presented Roedder's (1951) stability field of immiscibility within the system leucite-fayalite-silica, together with the locations of the axes of the metastable miscibility gaps for the systems NaA1SiO4 FeO - SiO2 and KA1Si2O6 - CaMgSi2O6 - SiO2. The coincidence of these axes with Roedder's field of immiscibility was striking. The final validity of this field was obtained in the laboratory. Roedder and Weiblen (1970), Ferguson and Currie (1972), Massion and Van Groos (1973) and McBirney (1975) have obtained immiscible, liquid-pairs during experiment which plot within the PMG shown in figure 1. Hence the PMG can be confidantly considered as a verified, geologically-applicable, two-liquid field. Presumably, magmas with compositions plotting in or on the flank of the PMG would be prone to split into two-liquids and we use this property throughout our evaluation of the volcanic rock series. Because we use the PMG in our appraisal of volcanic rock suites, it is pertinent to here consider some of Si0 2 PRIMITIVE T.WO LIQUID FIELD EN f" M, DI /" PMG B+' 50, / 50 FO ~ 2 Ca0+Mg0+EFe0+Ti0 Na20 + K20 + A1203 WEIGHT FIGURE 1 - Pseudo-ternary diagram showing location of the primitive and proposed miscibility gaps (latter denoted as PMG). Tie-lines within the PMG join experimentally established liquid immis- cible fractions (data from Roedder and Weiblen (R) 1970; Massion and Van Gross (M) 1973; McBirney (B) 1975). The open fields are for lunar glass inclusions (N 50) of Roedder and Weiblen). 6 the more important properties of this field as revealed on the pseudo-ternary diagram of figure 1. The PMG is an extended ellipsoid that roughly parallels the silica-cafemic oxides boundary. This means that a magma composition plotting on the alkaluminous side of the PMG would be driven into the PMG by any processes leading to a relative decrease in the alkali and alumina contents of that magma. If this process also leads to a lower alumina to alkalis ratio, then immiscible splitting is even further favored, although as pointed out by Watson and Dickey (1975) the ratio, more appriopriately, should be alumina to alkalis plus water. These features have importance in the comparison of volcanic suites to follow. MODERN VOLCANIC SERIES AND THE PMG Phenocryst-free,
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