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Origin of the Ultramafic Complex at Duke Island, Southeastern Alaska

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Origin of the Ultramafic Complex at Duke Island, Southeastern Alaska MINERALOGICAL SOCIETY OF AMERICA, SPECIAL PAPER 1, 1963 INTERNATIONALMINERALOGICALASSOCIATION,PAPERS, THIRD GENERAL MEETING ORIGIN OF THE ULTRAMAFIC COMPLEX AT DUKE ISLAND, SOUTHEASTERN ALASKA T. N. IRVINEl M cM aster University, Hamilton, Ontario ABSTRACT The Duke Island ultramafic complex crops out in two areas that total 9 square miles. Rocks within the complex are comprised of olivine, clinopyroxene, hornblende and ferriferous oxides and are classified as dunite, peridotite, olivine pyrox- enite and hornblende pyroxenite; all are devoid of plagioclase and orthopyroxene. A pegmatite of hornblende and anor- thite is closely associated and probably comagmatic, but abundant gabbro surrounding the complex has been altered near its margin and is apparently older. Remarkable gravitational layering, showing grain-size grading, occurs intermittently in the olivine-bearing ultramafic rocks through a possible thickness of 2 miles. Although the Fe/Mg ratios of olivine and clinopyroxene increase slightly in the ultramafic units in the order listed above, cryptic layering is slight or absent. Counter- parts in the layering of cross-bedding, slide conglomerates and slump structures suggest accumulation under the influence of magmatic currents, and a marked structural break, locally analogous to an angular unconformity with basal conglomer- ate, marks the contact of olivine pyroxenite and the dunite-pcridotite zone and indicates at least two major intrusions of magma into the complex. A variety of structural, mineralogic and petrologic evidence suggests that the ultramafic rocks crystallized from a magma of ultrabasic composition. Hornblende pyroxenite generally forms a border zone around the olivine-bearing rocks, and together with the alteration aureole, seems to represent peripheral reactions that tended to pro- duce a succession of zones in local equilibrium along thermal and chemical gradients between the ultrabasic magma and its country rock. The possible composition and temperature of the ultrabasic magma are discussed. INTRODUCTION AND ACKNOWLEDGMENTS morphosed volcanic and clastic sedimentary forma- Duke Island is at the southern end of southeastern tions. These generally fringe the igneous core of the Alaska (55°55'N, 131°20'W) and has an area of 59 island, which is comprised of gabbroic, ultramafic square miles. The ultramafic complex exposed on the and granitic plutons that apparently were emplaced island is a member of a belt of 35 or more ultramafic in that order. The metamorphic rocks are not in bodies that occur along the 350-mile length of the contact with the ultramafic complex, but where they Alaskan panhandle. A brief description of the belt is are closer than i-mile to the main gabbro mass, they available in Taylor and Noble (1960). The present have mineral assemblages indicative of the horn- paper is a summary of the petrology of the Duke blende hornfels facies and, locally, the pyroxene Island complex; a more comprehensive report is in hornfels facies (Fyfe et al., 1958). Elsewhere, they preparation. belong to the albite-epidote hornfels or green schist The information presented here was largely col- facies. lected while the author was attending the California Primary (pyroxene) gabbro. The primary gabbro is Institute of Technology, and the contributions of comprised of 40-50 per cent plagioclase and 45-55 many people there by way of discussion, assistance, per cent pyroxene and hornblende combined. and criticism are gratefully acknowledged. Particu- Olivine and biotite are present locally; ilmeno- lar thanks go to Dr. James A. Noble for sponsoring magnetite and apatite are common accessories. the field work and supervising the study. During the Plagioclase ranges from An40 to An95, An50_75pre- academic year 1958-1959, support was provided by dominating (Fig. 2). Orthopyroxene and clinopyrox- the Foster Hewitt Fellowship in Field Geology. ene are generally both present, but rocks having only Grants from the Geological Survey of Canada have one or the other do exist. By optical properties (Figs. covered reproduction of some of the diagrams. G. V. 3, 4), orthopyroxene ranges from En47 to En76, and Middleton and D. M. Shaw have made several help- clinopyroxene from about Ca37Mg53Felo to Ca3SMg?9- ful suggestions with regard to exposition. Fe33. As shown in Fig. 3, the Fe/Mg ratio of each pyroxene increases as plagioclase becomes more GEOLOGICAL DESCRIPTION OF DUKE ISLAND sodic. Olivine changes in the same way, its range of Metamorphic rocks. The geology of part of Duke varia tion being F027 to FOn Island is shown in Fig. 1. The oldest rocks are meta- The pyroxene gabbro underlies most of the east- ern half of Duke Island and thus, is abundant. Un- 1 Present address: Geological Survey of Canada, Ottawa, fortunately, little is known about its structure: the Canada. rock is largely massive; topographic relief is low; 36 DUKE ISLAND ULTRAMAFIC COMPLEX 37 ~~/- contact AN INTERPRETATION ...----- fault OF THE STRUCTURE OF ~ -layering :';::::::---.<approximate limit of THE DUKE ISLAND ULTRAMAFIC COMPLEX _/ mafic pegmatite and - hornblende gabbro FIG. 1 contacts are poorly exposed; and the pattern of same relation is indicated for this locality and others mineral variation is not obviously systematic. by the graded rhythmic layering that occurs in the Rhythmic layering does, however, occur· at two ultramafic complex as described below. localities, and one, denoted "A" in Fig. 1, is notable. The layering there is continuous and regular for Description of the ultramafic complex several tens of feet, and individual layers have (a) Petrography and mineralogy thicknesses up to one foot. Each layer has its base Most of the ultramafic rocks crop out in two areas clearly defined by a marked concentration of pyrox- tha t to tal 9 square miles, but the areas are believed ene: proceeding upward, the proportion of mafic to be the exposure of one large body at depth (Fig. minerals decreases in a sharp gradation, and the rock 1). This in terpreta tion is based partly on an aero- becomes normal or slightly feldspathic and shows magnetic map and partly on geological inference, marked igneous lamination. The layering is approxi- some of which is mentioned below. If it is correct, mately parallel to the nearby contact of the gabbro then it is imprecise to refer to each exposure as a against the ultramafic complex, and the direction of "body." The word "outcrop" is therefore used in its tops of layers, as indicated by grading, suggests broader sense, and the exposures are called the Hall that the gabbro underlies the ultramafic rocks. The Cove and Judd Harbor ultramafic outcrops. 38 T. N. IRVINE 6 4 2 C/) ...~ 0 c( FIG. 2. Histograms of the composition of plagioclase in the ~ 8 ::IE different types of mafic rocks occurring at Duke Island, Alaska. In Q: the hornblende gabbro histogram, the dark part represents plagio- ~ 6 clase from hornblende gabbro near the granitic intrusions; the rest '"o 4 IL represents plagioclase in hornblende gabbro near the ultramafic 0 Q: 2 rocks. ~'" 0 ::> z 8 6 4 2 0 MOLE PER CENT ANORTHITE 90 80 70 60 50 40 30 100 _J 1.735 55 ORTHOPYROXENE 1.730 • VS • 50 PLAGIOCLASE 1.725 • 45 1.120 APPROXIMATE 40 MOLE RATIO nz 1.715 Fe X 100 M9+ Fe 35 1.710 1.105 30 1.700 25 1.695 1.705 30 1.700 25 APPROXIMATE 1.695 20 MOLE RATIO Q ny I Fe X 100 1.690 e 1-15 Co+ M9 + Fe 1.685 CLINOPYROXENE VS ~~ 1.680 • PLAGIOCLASE 1.675:t-r-~:--r-:::c---y--'--....--'r-""T'-.--.--__,.-~_J 100 90 80 70 60 50 40 30 MOLE PER CENT ANORTHITE FIG. 3. Relation of the chemical variations in the pyroxenes and plagioclase of the primary gabbro at Duke Islands as determined by optical properties. The plots of coexisting pyroxenes are joined by "tie lines." In the clinopyroxene, the proportion of Ca is roughly constant near 40 per cent. DUKE ISLAND ULTRAMAFIC COMPLEX 39 The ultramafic rocks are essentially comprised of 2VII~-'-'--'-~'--'-'~-'-' --' o dunile and peridotite three minerals: olivine; clinopyroxene (diopsidic 57 o olivine pyroxenite o hornblende pyroxenite augite); and hornblende. The results of modal 56 • pyroxene gabbro tr B 0 analyses are given in Figs. SA and B, and the rock 55 o ~ 0 0 8 o classification scheme is apparent in Fig. SA. The 54 o 00 0 o principal units are dunite, peridotite, olivine pyroxe- 53 o o 00 8 nite, and hornblende pyroxenite. Much of the horn- 52 blende pyroxenite contains 13-17 per cent magnetite, 51 2-3 per cent ilmenite, and minor hercynitic spinel. 50 Chromite or chromiferous magnetite is a sparse 49 accessory in dunite and peridotite. Olivine is gener- 48 ally partly altered to serpentine and secondary 47 magnetite, but otherwise, the rocks are fresh. Orthopyroxene and plagioclase are notable by their absence. Figure 5C shows the approximate areal abun- dances of the different rocks. The histograms were FIG. 4. Plot of 2V against ny for clinopyroxene from Duke Island rocks. This is roughly equivalent to a plot of Ca content against prepared by weighting each modal analysis by the Fe/ mg ratio. map area the specimen is believed to represent. Al- though perhaps not statistically elegant, this pro- has many of the features of the rhythmic layering in cedure does give proper weighting to specimens that the Skaergaard, Stillwater, and other intrusions, but come from small occurrences of a particular rock which is distinct in that it is graded by grain size type, and the result graphically portrays distribu- rather than mineral density (Fig. 8). The distribu- tion features that are very obvious in the field. tion and attitude of the layering is shown in Fig. 1 Olivine pyroxenite containing 15-30 per cent olivine by trend lines. It is almost exclusive to olivine-bear- (plus serpentine alteration) is an extremely abundant ing rocks and involves only two primary-precipitate rock type and has a histogram mode distinct from minerals, clinopyroxene and olivine.
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