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Les diagrammes suivants illustrent la m6thode. 1 2 3 1 2 3 4 5 6 4f T :• ^Tm Wc' o 5>^*~^^^^^^^ -^^^^^^^^ Is v.- -»- j^ s ON SOME If)- i DIABASE DYKES OF THE RAINY LAKE REGION BY ANDREW C LAWSON, M.A., GF.OLOGIST TO GEOLOGICAL SURVEY OF CANADA. • .1 ti « * V so ' /I J ° • Op •, • >, ^ .1 , a •> '• » < a •• • • ) o » , » 1 ■; J J O0 *> i' ■•«•.,,• * I'D* ■■■ ' » , f, . . TOKONTC'f THE COPP, CLARK COMPANY, LIMIFED, PRINTERS, COLBORNE STREE:T l888. • •• •-••• /hi J, NOTES ON SOME DIABASE DYKES OF THE RAINY LAKE REGION BY ANDREW C. LAWSON, M.A., Geologist to Oeolojical Survey of Catrnda. The most recent of the crystalline rocks of the Rainy Lake region are comprised in a series of strong dykes of comparatively fresh diabase which are observed to cut, at different localities, the various members of the Archaean complex of formations. These dykes are not infrequent throughout the country lying between the eastern •confines of the first prairie st«ppe, which forms the basin of the Red River of the north, and the western border of the area of Animikie and later formations of the Lake Superior basin. Their occurranoe and some of their characters are briefly referred to in my report on the Lake of the Woods region.* As there observed, the occurence of these dykes cutting the older folded rocks, which in their eastward geographical continuation, pass under the flat-lying Animikie and Keweenawan formations, is suggestive of their possible connection with the bedded traps that form so large a part of the two latter geological series. With the question of the possible identity of character and age of these dykes with the traps of the Animikie or Keweenawan, or of both, is associated the equally interesting one of the extent of the earth's surface, over which, in €arly geological times, were in simultaneous operation, those parti- cular volcanic forces which appear to have bad their focus in the Lake Superior basin. The more notable field characters of these dykes are : their common strike throughout the region from N.W. and S.E. to N.N.W and S.S.E. ; the sharp, well defined nature of the gash or fissure which they fill, no matter what may be the character of the country rock ; the absence of inclusions of the country rock, or of apophyses of the dyke running into it, except in very occasional instances ; their generally uniform width under difierent conditions of occurrence in different localities, the limits being as a rule 60 and 150 feet; their continuity for one or several miles where exposures permit them to ♦ rmulated. A few notes regarding the microscopic features of these dykes, taken together with what has been said of their tield occurrence, may however, be of interest, and will serve as a report of progress of what is being done in this line of investigation in the tield west of Lake Superior.* One of tlie most characteristic of these dykes is one that traverses the coarse granitoid gneiss of the west arm of Jackfish Lake, which lies to the north-west of Rainy Lake. Its width is 1 35 feet and its contact with the country rock is well exposed as a sharp line. Fi'om a macroscopic examination the gneiss does not appear to have been altered perceptibly towards the contact. Specimens for microscopic examination were taken from different parts of the dyke, viz., at 60 feet, 20 feet, and 6 feet from the contact, and at the contact. At 60 feet from the contact, the rock is a coai-se-gmined mottled gray rock in which dirty white feldspar and black pyroxene are the prominent constituents. Under the microscope it presents the characters of a coarse-grained, comparatively fresh diabase. Augite of a pale mauve tinted gray colour is abundant and often occurs in masses that fill the field ctf the microscope when low powers are used. Sometimes these plates of augite are individual crystals. For the most part however, • Thes'? rcx^ks were studied microscopically in the laboratory of the Johns Hopkins University, Baltimore, under the guidance of Prof. G. H. Williams, for whose kind udvice and assistance the writer desiref to express his grateful acknowledgments. they are not single individuals. When examined between crossed nicols the plate of augite is seen at once to be resolved into an intimately interlocking mosaic of irregularly shaped gi-ains of diverse optical orientation. In ordinary light the boundaries between the different members of these " polysomatic "* masses of augit3 are traceable only with difl&culty and uncertainty. There is no interstitii. ' m&tter whatever, the different gi-ains being as intimately associated as in the case of interpenetration twins of feldspar. That they are not twins is shown by the fact that there are often as many as half- a-dozen grains il of different orientation thus combined in the same mass. The cleavage, by its lack of continuity over the field of courtie indicates a difference of orientation in different parts of it, but the <;leavage traces are not strongly marked, and attention is only directed to the discordance of the cleavage after the polysomatio character of the mass has been rendered prominent by the analyser of the microscope. This polysomatic structure of augite does not appear to be common. Rosenbusch does not mention it in his last compre- hensive summary of the present state of petrographical knowledge.! The nearest approac i to this structure that is at all well known is the polysomatic character of some chondri of olivine in certain meteorites such as are figured by Tschermakf and "Wadsworth.§ Olivine in a similiar condition in terrestrial rocks has recently been described and figured by Renard in specimens from Kerguelen Island in the Indian Ocean. || The polysomatic structure in augite is not so well known. Renard notes that the augites of the felds- pathic basalt of Heard Island, Indian Ocean, are grouped together at certain points,** and again in the same rocks in Marion Island that the augite is characterized by a tendency to form groups of individuals having their vertical axes parallel. ft Teall mentions " Granular Aggregates " of augite in the Hett and the High Green dykes in the north of England. J J Some of these appear from the figures given to ^ Adapted from Tsehermak's use of this word as applied t» a similar structure in the olivine •of certain meteorites.— V. Die Mikr. Ueschaff. der meteor. Stuttgart, 1885. t Mikr. Phys. der Mineralien und Gesteiue Stuttgart, 1886. ■ ,.. .. ♦ Die Mikr. Beschaft. der Meteor. Stuttgart, 1885, Taf. xv. Fig. 1 and 2. § Lithological Studies, Mem. Mus. Comp. Zool. Harvard, Vol. x., pi. 1. II Notice sur la geologle de 1' lie de Kerguelen, Bui. Mus. Roy. Hist. Nat. Belgique, Tome IV. No. 4., p. 233, fig. 1, pi. V. ** Notice sur les roche de 1' ile Heard. Bull. Mus. Roy. Hist. Nat. Belgique, 1886, 3 p. 260. tt Notice sur les roches de 1' lie Marion. Ibid. p. 250. tt Petrographical Notes on some north of England Dykes, Q. J. G. S., 1884, 158. p. 229 and 242. 6 be aggi-egates of grains of augite not in close juxtaposition with an interstitial base, although that figured in Plate xii, Fig. f), would seem to be a polysomatic augite, and if so is the only strictly parallel instance that I can find of this structure so common in this dyke and in others of the region. The augite is generally altered to hornblende at its periphery and occabionally the latter mineral entirely replaces the former. The process of alteration does not appear to proceed along the almost or quite imperceptible l'"ies of demarkation between the different individuals of the polysomatic augite, but extends fi*om the peri- phery of the mass as a whole in towards its centre. The plagioclase appears in two general forms a rather stout or tabular form which is the larger and usually the more cloudy with decomposition products, and a small long lath shaped feldspar which appears quite fresh and in which the polysynthetic lamellae are much more distinct than in the former. Magnetite occurs in irregularly bounded masses or is disseminated, often quite thickly, thi'ough the augite as inclusions of dusty or finely granular aspect. Pyrite also occurs and is discernable macro- scopically. Apatite is seen in occasionally colorless hexagonal sections and in slender prisms with rounded terminations. Water- clear quartz, with inclusions of apatite microlites and liquid inclusions with dancing bubbles, forms a considerable proportion of the mineral constituents of the rock and is characterined by having a common orientation for isolated sections over a wide area of the microscopic field, as in the micropegmatite structure. A few colorless garnets are also present. The rock, such being its characters, may be classed as a uralitic quartz diabase. At 20 feet from the contact the rock is very similiar to that at 60 feet but is much less coarse in texture. It differs from the latter in mineralogical composition in the fact that there is present an abundance of white or colorless garnets, all perfectly isotropic. They have a well defined border indicative of a high index of re- fraction and a perceptibly rough surface. Their shape is for the most part rounded, or, when rectilinear outlines are observable, they are hexagonal sections of the rhombic dodecahedron. The larger grains have a curved parting which may be demarkation lines between different individuals. The treatment of the slide with hy( ren are the Wi (at ale hydrochloric acid cold or hot, leaves them unaffected. The occur- rence of garnets in basic dykes is by no means unicjue. They are however regarded as a product of contact raetamorphism within the dyke. Speaking of the " Iron District of Lake Superior," Wadsworth says, "Most of the "diorites" (uralitic diabases) here (at Republic Mt.) contain garnets, this mineral being found principally along the edge of the intrusion while the centre was nearly if not entirely tree from it. The schist in like manner near the "diorite " frequently contains garnets both rocks appearing to have mutually reacted upon each other." * The garnets in the Jack Fish Lake dyke do not appear to be a product of contact metamorphism since they are found in the middle of the dyke and very much more abundantly at 20 feet from the contact than at 6 feet from it, or immediately at the contact, where their presence has not been detected. Beyond the abundance of garnets, the dyke at 20 feet has the same charactei's as at 60 feet. The polysomatic structure of the augite is pronounced. At 6 feet from the contact the rock is tine grained and the ophitic structure of typical diabase is much more characteristically developed than in the coarser grained parts of the dyke. In this part of the dyke there is first observed a differentiation of the rock into con- stituents of different periods of crystalization, the order being first plagioclase in more or less idiomorphicf lath-shaped individuals lying in all positions, then augite genei-ally allotriomorphic,t sometimes hypidiomorphicf and finally a base or matrix of both these minerals in a very much more finely crystalline state together with magnetite. The structure of the base is rather obrfciiro, the cliloritic substance usually present in diabase rocks being more prominent here than in the coarser grained part of the dyke when it is almost or perhaps entirely wanting. Quartz is present but in smaller quantities than in the coarser grained portions of the dyke. ' The augite occurs both in simple individxials and in polysomatic masses. The uralitization of the augite, which is generally observable, is much more pronounced in the irregularly bounded polysomatic masses than in the simple allotriomorphic development of the same mineral. A few garnets are present as inclusions in the feldspar but were not identified with * Notes on the Geol. of the Iron and Copper Districts of Lake Superior. Zoffl. Harvard, 1880, pp. 46, 46, 47. t Terms introduced by Roienbusch. Cf. op. cit. p. 11. ' Bull. Mas. Comp. 6 certainty. In this respect this portion of the dyke differs markedly from the more contra! portions examined. The most interesting constituent of this portion of the dyke remains, liowever, to be mentioned. It is tlie non-pleochroic colorless rhombic pyroxene, enstatite; it occurs in idiomorphic development showing the char- acteristic obtuse demos in some of the sections. It shows regular cleavage parallel to co P (HO), upon which the angle of extinction is zero, and characteristic cross parting along which partial alteration of the mineral to bastite or serpentine is apparent. This enstatite is not abundant and j)lay8 the role of an accessory mineral. Its occurrence in a rock of well marked diabase structure is interesting. Rosenbusch remarks that it is present in only a few diabases which have a gabbro-like structure,* and Teall has recorded the occurrence of the allied rhombic pyroxene bronzite in the Whin-Sill of the north of England as an accessory. t Enstatite also occurs in a variety of the allied rock diabase porphyrito from Schaumberge, which has been described by Laspeyres and Strong under the name Palatinite. This enstatite was not observed in the coarser parts of the dyke but occurs, as will be noted, in the still liner grained diabase at the contact. At the immediate contact the dyke assumes microscoj^ically the characters of a very compact grayisli black aphani*^.ic rock in which can be occasionally detected minute glistening facets of porphyritic crystals. With low powers of the microscope the matrix is not resolvable but appears as an uniformly yellowish to greenish gray ground thickly dotted with grains of magnetite. Under the higher powers this is seen to be made up, in addition to magnetite, of a fine felt-work of minute lath-shnped crystals of pla- gioclase imbedded in hazy, somewhat yellowish green flocculent chlorite substance derived presumably from the alteration of the augite, since that mineral cannot with certainty be identified in the base. The porphyritic character of this part of the dyke is well marked, though the imbedded crystals are small. These are augite in small irregular polysomatic masses, with a hazy margin or fringe of greenish decomposition product, and long lath-shaped pla- gioclase and occasionally stouter broken fragments. Besides these there are porphyritic crystals of enstatite much more altered and * Mik. Phys. derMossigen Gestine, 2na Ed., 1806, p. 188. + Q. J. G. S.,1884,p. 652. ' ■ -1- -. IV lesH plentiful than at 6 feet from the contact. Neither ouartz nor garnets are ooHervable in the contact rock. Considering then the dyke with reference to its variation in structure and mineral composition the points of interest to be noted are : The passage of the coarse grained central portions of the dyke to the compact aphanitic rock at the contact ; the absence of porphyritic structure in the middle of the dyke as contrasted with the well marked development of the same as the rock becomes finer grained towards the dyke walls ; the absence of the characteristic chloritic substance of diabase in the centre of the dyke and its abundance towards the contact , the presence of quartz in greater quantity in the coarse grained middle portions than at the sides ; the presence of garnets in the coarsest parts of the dyke, their abundance in the medium grained parts and their rarity or total absence in the neighborhood of tlie contact ; the presence of the rhombic pyroxene enstatite in typical idiomorphic porphyritic crystals in the fine grained parts near the contact and its absence in the coarser central parts ; the diminution in size of the porphyritic crystals near the contact in co- extension with the increasing fineness of the ground mass ; and finally the " polysomatic " structure common to the augite througliout the dyke. 'm ■■'/■'^'i^'''y'r '-"."■ ' •' v ' ■ ' : Three quarters of a mile from the exposure where the specimens whose characters have just been given were collected, there occurs, on the o2)posite side of the bay in the line of the strike of the dyke, another exposure of the sair^e dyke. On the islands of the bay which lie intermediate between these two localities the outcrop of the dyke is observable, so that there is no doubt of cheir both being exposures of the same dyke. The rock here was not studied in so great detail as at the last exposure. The specimens taken were of the same grade of coarseness as those taken at 20 feet from the contact on the north side of the bay. The feldspars are more decomposed and the twinning lamellae often obscure, and the small quantity of quartz which is associated with them appears to be of secondary origin ; whereas the origin of the quartz noted in the same dyke on the north side of the bay seemed much more problematic. In the latter case the common micropegmatitic character of the quartz and the occurrence in it of needles of apatite, which in no way differ from those in the feldspar, together with the not m. • >i m in^'vequent occurrence of one individual of apatite partially included in quartz and partially in an adjacent feldspar, would argue for the orimar^ character of the quartz. The augite in the dyke on the so'ith side of the bay resembles that already described occur- ring both in simple individuals and in polysomatic masses. It is larpely altered to uralite. Titanic iron with its alteration product leucoxene shows characteristic barred structure of the cleavage ti-aces parallel to the planes of the rhombchedron. The leucoxene is frequently accompanied by a margin more or less extensive, of secondary brown mica. Apatite ra present in comparative abundance. Chlorite occurs in vaguely defined masses and the garnets which, as before, are present, are associated with i*^^. On the south-east shore of Pipestone Lake about a mile west of Stone-dam Portage occurs smother of these dykes cutting transversely schists which have a strike of N.E. to E.N.E. The specimen taken from the middle of the dyke has the characters of an uralitic quartz diabase. The feldspar as a rule is remarkably fresh and occurs in the usual lath-shaped twinned crystals of plagioclase. The crystals are commonly observed to be cracked transversely and the cracks filled with a brownish yellow material which shows aggregate polarization. The augite occurs more commonly in poTysomatic masses than in simple indi- viduals. The magnetite is often suri*cunded by rims of secondary brown mica. The quartz is apparently original and has numerous inclusions of an opaque granular char- acter together with fluid inclu- sions with dancing bubbles, gas pores with black borders and glass inclusions oval and circular. On the south shore of the PiQ- i- North-west Bay of Rainy Section of diabase, from Pipestone liuke dyke, I^ke, a similar dyke CUtS showing large polysoniatic grain of augite in three _ ^ • < i_ granules of diverse orientation a 6 c ; d uralitic both the biotito gnClSS OI the hornblende ; e magnetite. X 28. , . i •. region and the red granite which is intrusive through it. Tt is an uralitic quartz diabase. II — 1 The feldspar is in rather stout crystals in the coarser grained part of the dyke, though usually lath-shaped. It is much decom- posed and is partially replaced by quartz and chlorite. The poly- somatic character of the augite is not prominent but this may be due to the fact that it is about half altered to Hornblende and to chlorite. The augite individuals are often twinned and the cleavage traces are unusually well defined. The magnetite shows a tendency to peripheral airangement around the altered augite indicative of its secondary origin. Quartz is present which is probably origina besides that which is clearly secondary. Apatite in long slender needles and leucox^ne in irregular masses, are the accessory con stituents. In the same dyke, nearer the contact where the texture is fine grained, the rock is much more uralitized, traces of augite being observable only in cores of the compact green hornblende, which has almost entirely replaced it. Apatite appears more abun- dant, as do also the secondary quartz and chlorite. Garnet of a pale yellowish color occurs sparingly. At the contact the dyke rock is a compact aphanitic base in which can be detected minute porphyritic crystals. Under the microscope the base is seen to V>e made up of minute lath-shaped crystals of fresh plagioclase augite grains, magnetite and chloritic sv.bstance. The porphyritic crystals are lath- shaped feldspars occasionally brok- en and showing the lamellae in some instance bent, as the result of pressure of one individual against an angular part of another, and augite gener- ^'^ ^• ally surrounded with an irregular border of dytr^^ortCe't'lay! secondary hornblende, which, in turn, has an SnaV''ai^rS"tS outer girdle or wreath of granules of magnetite EertH*. e'n""." that have separated out in the process of urali- ar7m:'Sl;r '""'"'" tization as in fig. 3. In the south part of the Rainy Lnke and on the Rainy River a number of these dykes have been observed. One cuts the coarse granitoid gneisp of the river between Couchiching and Fort g 3. Plag-ioclose f r ni diab'vsfc dyke, Northwest Bay, Rainy Lake, sh'wingefifectof pressure of one crystal against another. Frances on tlie sotitli side of the river, and another crosses the river at the Manitou rapids. Neither of these have yet been examined microscopically. On the lake near the extremity of Gash Point one of these dykes cuts the schists with a strike of N. W. and S. E. across the whole breadth of the point and traverses the islands on both sides of it. Here it is traceable on the point and on the islands for a distance of a mile. Three miles to the south east in the line of the strike of the dyke, a dyke occurs cutting the schists on the islands off the south shore of the lake which is px'obably a continuation of that of Gash Point. From this point it is traceable for two miles across the islands to the main shore on the south side of Grassy Nari'ows. Thus, this dyke has a length of at least six miles and has an extension to the north-webt and south-east of the points observed, for a distance that is probably very much greater. A sjtecimen from the central part of this dyke, proved on examination to have the chHractei-s of a ui'alitic quartz diabase. The j)lagiocla8e occurs in long, i-ather stout, lath-shaped crystals, which are generally so cloudy as to obliterate the twinning in most cases. The augite occurs both in simple individuals and in polysomatic masses. It exhibits the usual marginal alteration to hornblende and there is besides a certain amount of chlorite. Original magnetite is frequently surround- ed by a margin of secondary biotite, Micro- pegmatitic quartz is abundant. It is often intimately intergrown with the feldspar, and as the latter is much decomposed, would seem to replace it as a partial pseudomorph, but apatite needles of the same aspect as those which occur as inclusions in feldspar, aug- ite, and quartz, are often seen to be inclosed partly in a feldspar «ind partly in quartz grain. The primary origin of the quartz in spite of its micropegmatitic character, is however, not beyond doubt. It is to be noted that were the quartz original we would hardly expect to find it in such close association with the feldspar. The plagioclase of these rocks affords unmistakable evidence in its idiomorphic character of its having first crystallized from the magma. The augite crystallized next, enclosing the lath- Fig. h. Polysomatic jj r a i ii ci f augite— Grassy Narrows dyke Kainy Lake, a and h are twins —the othe' sfanules are of diverse optical orientation. X2S. ■ ■ t f 13 ■•'flSfi shaped plagioclase ; and the quartz, which would be the last to crystallise, we would expect to find sepai'ate from the plagioclase by the augite, i.e., to fill in the interstices between the augite. Again although single apatites are often found extending from a quartz grain to a feldspar grain, a condition of things favoiing the notion of a common primary origin of both the latter minerals, yet such a phenomenon is not incompatable with a secondarj' origin for the quartz, sin 36 the replacement of feldspar by quartz must necessarily be a slow operation and proceed particle by particle. Further, if the quartz were original we should hardly expect to find in it inclusions of crystals of the first generation like apatite, which would be liable to be enclosed for the most part in the earlier secretions like feldspar and au^^ite, rather than in the residual silica of the magma. The non-existence, however, of quartz in some diabases which are very much decomposed and its presence in fresh ones, militates against the theory of the secondary origin of the quartz in these rocks, so that the question of how much of the quartz is primary and how much secondary in an old diabase is a question that as yet does not appear susceptible of definite settlement. , , • About a mile to the west of this dyke where it crosses Gi-assy Narrows Island is another nearly parallel dyke converging on the former at a small angle towards the south. The rock is an uralitic quartz diabase and in its courser portions, near the mitldle, the textxire is more granular than tliat of ty[iical diabase. The pla- gioclase is cloudy with decomposition products and quartz is abundant. The augite is entirely rci)laoc