BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 39, PP. 555-578 JUNE 30, 1928 AN ORTHOSITE AND GRANITE AS DIFFERENTIATES OF A DIABASE SILL ON PIGEON POINT, MINNESOTA 1 BY FRANK F. GROUT (Presented by title before the Society December 29, 1927) CONTENTS Page Introduction and acknowledgments............................................................................. 555 General relations of the rocks of Pigeon Point.......................................................... 556 General features of the sill............................................................................................. 557 Components of the sill and their relations.................................................................. 558 Origin of the anorthosite................................................................................................ 562 Origin of the red rock (granite).................................................................................... 566 Points of agreement and disagreement................................................................ 566 Relation of granite to size of sill.......................................................................... 567 Granite magma and water..................................................................................... 567 Evidence of assimilation of Quartzite and of its consanguinity with granite 570 Shells of red rock around inclusions.............\ ..................................................... 572 Differentiation modified by inclusions................................................................ 573 Application of differentiation at Pigeon Point to the explanation of other occur­ rences of anorthosite................................................................................................... 573 Beaver Bay, Minnesota.......................................................................................... 573 The Adirondacks..................................................................................................... 574 Probable lack of anorthosite batholiths.............................................................. 575 Summary........................................................................................................................... 575 I ntroduction a n d A cknowledgments Anorthosite and granite or syenite are associated with masses of gabbro at so many places that many petrologists believe that they have a com­ mon origin, or that they are differentiates from a common magma. The great sill at Pigeon Point, Minnesota, shows some features that indicate rather clearly the genetic relations of such rocks, and that may throw' light on the general problem of rock differentiation. This sill seems to have been large enough to permit thorough differentiation, but not sc large that the intrusion of the later phases of the magma obscured the relations of the phases already solidified. The outcrops are numerous, 1 Manuscript received by the Secretary of the Society, February 27, 1928. Published by permission of the Director of the Minnesota Geological Survey. (555) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/39/2/555/3414852/BUL39_2-0555.pdf by guest on 29 September 2021 556 F. F. GROUT----ANORTHOSITE AND GRANITE ON PIGEON POINT and there may be no other single geologic unit in which the relations of these common rock types are so clearly and suggestively exposed. The field-work was done for the Minnesota Geological Survey. The writer has had the advantage of frank discussion with the Director of the Survey, Dr. W. H. Emmons, Dr. T. M. Broderick, Dr. G. M. Schwartz, and other members of the staff. He has also discussed the problems with Dr. W. S. Bayley and Dr. R. A. Daly, whose earlier studies and reports on Pigeon Point have made the locality a classic.2 The peculiar F i g u r e 1 .—Sketch Map showing Distribution of the pétrographie Phases of the Intrusive at Pigeon Point, Minnesota problem of the origin of the anorthosite here is considered in the light afforded by several recent papers, which are cited below at the proper places. The discussions have greatly clarified the writer’s ideas of the origin of the rocks, but the contributing geologists should not be held responsible for the theoretical conclusions here presented. G e n e r a l R e l a t io n s o f t h e R o ck s o f P ig e o n P o in t Pigeon Point is a peninsula that juts into Lake Superior at the ex­ treme east end of northern Minnesota. It is 4 miles long and about 2 W. S. Bayley : The eruptive and sedimentary rocks of Pigeon Point, Minnesota. U. S. Geol. Survey Bull. 109, 1893. R. A. Daly: The geology of Pigeon Point, Minnesota. Am. Jour. Sci., 4th ser., vol. 43, 1917, pp. 423-448. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/39/2/555/3414852/BUL39_2-0555.pdf by guest on 29 September 2021 GENERAL RELATION OF THE ROCKS 557 half a mile wide, and its point is about a mile and a half south of the Canadian shore of the lake. It is made up of Animikian sedimentary rocks and an intrusive sill about 500 feet thick. This sill is composed of a series of different, rocks, which have been so fulty exposed by glacia­ tion and wave erosion as to show clearly the relations of the members of the series (see figures 1 and 2). The sedimentary rocks above and below the sill are chiefly gray to black quartzite and slate, the overlying strata altered to a red contact rock. At places near the intrusive rock PIGEON POINT SUPER'0BI 5 Later? diabase dikes f7* -*«4 Chilled diabase, porphyry & anorthosite Granite , "red rock" I^ - nCV'I Intermediate rock I* x *1 Diabase gabbro, slightly porphyritic I-1- I Animikie sediments I I F i g u r e 2.—Sketch Map showing Distribution of pétrographie Phases of the Intrusive at Pigeon Point, Minnesota this contact rock contains some micrographie intergrowths of quartz and red feldspar that greatly resemble those in the red acid phase of the in­ trusive. Nearly all this alteration appears to have occurred in the over- lying strata ; the few exposures of the floor show relatively little contact metamophism, but even there some rock shows a little graphic inter­ growth. G e n e r a l F e a t u r e s o f t h e S il l The main intrusive body on Pigeon Point is a sill that in places cuts across heavy beds of the Animikie quartzite into which it is intruded (see figure 3). Daly has summarized very convincingly the evidence, both structural and pétrographie, that the intrusive mass is a sill,8 and * Opp. cit., pp. 426-430 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/39/2/555/3414852/BUL39_2-0555.pdf by guest on 29 September 2021 558 F. F. GEOUT---- ANORTHOSITE AND GRANITE ON PIGEON POINT several outcrops besides those mentioned in his paper show the con­ cordance of the upper part of the sill with the bedding of the quartzite. A particularly good exposure occurs south of the small area of gabbro shown on Bayley’s map, about a mile east of Marks Bay, and there are several others along the shore east of Little Portage Bay. The dis­ cordance of the sill contacts, however, is in places rather conspicuous. Aside from the blocky reentrants of granite in the roof, the variable width of the outcrop of the sill strongly suggests discordance. The sedimentary beds and the structural lines of the intrusive mass dip south at an angle of about 15 degrees, and the width of the exposure of the intrusive rock ranges from less than a quarter of a mile on the west side of Little Portage Bay to almost half a mile near the mouth of Pigeon River. The sill must vary greatly, perhaps, in some places not over 250 feet and in others as much as 700 feet thick. This great sill was first encountered at its west end, on Wauswaugon- ing Bay, and its outcrops are so numerous from there to Pigeon Point, over 6 miles east, that its continuity for that distance can hardly be questioned.4 The evidence that the several rocks of the sill form an intrusive unit rather than that they are the products of a succession of intrusions is well stated by Daly.5 C o m p o n e n t s o f t h e S il l a n d T h e ir R e l a t io n s From the roof of the sill down, the following rocks have been noted: 1. Chilled diabase containing in places abundant phenocrysts of labra- dorite and small masses of anorthosite. 2. “Red rock” (granite). 3. Intermediate rock. 4. A large volume of diabase gabbro. 5. Chilled diabase (near the floor). The petrography of most of these rocks has been described by Bayley6 and details that seem to have no important bearing on the problem of the origin of the rocks need not be considered here. 1. The chilled upper phase of the sill consists largely of diabase gab­ bro, of medium grain, in which the feldspar is labradorite. At some places there are abundant phenocrysts of labradorite, the largest several inches long, and the diabase gabbro grades into anorthositic gabbro. 4 Bayley’s map of the sill (his Plate XIV-XV) was made in part by interpolation, and he evidently missed some outcrops in a wooded swamp west of the area shown on figure 1 in the present paper. The area of numerous outcrops along the shore is confused by minor intrusions, but there is probably no such offset or fault as one might suppose from Bayley’s map. (See Daly’s paper, page 430, footnote.) 5 Op. cit., p. 434. 8 Op. cit. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/39/2/555/3414852/BUL39_2-0555.pdf by guest on 29 September 2021 COMPONENTS OP THE SILL AND THEIR RELATIONS 559 There are also small “bouldery” masses of coarse-grained true anorthosite. This upper phase is not continuous in outcrop. In the western part of the area mapped in figures 1 and 2, it forms a definite belt, but toward the east, where the exposures have been most thoroughly studied, the belt is broken by intrusives of later phases of the magma. 2. Beneath the diabase is a zone of “red rock” of variable thickness. This rock is a sodic granite that shows considerable variation in texture from place to place, and most of it contains abundant micrographie in­ tergrowths making the rock a granophyre. It intrudes the upper chilled gabbro and anorthosite with fairly sharp contacts and with some signs of assimilation.