spr avel Sregbyå ek sh SENOR RE a 20 SKR BIR Ck ++ ND JD i tive jake Vr i; Voeapg 34 4 såå lok ue på En peka AON 0 SepAAa MA ara so AN KeädgsA hag st Nha virtögara np 4 ra ER sk ÅN dv 4 . Å lad ER så yng NSSoN od nig prosa 1åna fd Li vd RE våg ej niotetan ” uu i ee sf Ae på lkdåd fet ' eta ka $ 1ä6d [ad jaf ke eri: SMA ls i PEPPRA IL 4 Hp I jkg t 4 ir St MEN SS åärd fokbyet te (bass gr ID) panda et sEar Tr beer + N FNL gNOR jord FER n UNDRA 2” LR snta vå ne ad 4 ne Hav fäder Nr . AR nd E RAR bes + ANA Il 14 NI 6 2004 oåA IRMA eldo RR SSÄLS nl T SAND ljuga der nä 4 arr gj ligg Ul i RERIREr FYI PVA EN HIA jan Vv Nad aren ELR Sä PAD RE 13307 SVEA åsell vå IA 4 vå rd pts ph Hata (nan Ky PRLNLETDEN sé pl MATS äh på ar SÄGA Fats FER EE Poe 020 made Rn rer ” MN sned g: Gipon brer br IR FRALJA Ma 1 ni bart NM sg 1 pers pil pd pd jag bl 08 ie AR He q UP Mel ål is bår 2 i Re SS lada a sa igt N iv ONA om TELE brå byn dd PER a RE ke bå Sd kl 3) IL Ed Hånkla . "i MN Sid an ENT EN Mod Er mad ud SS dr 4 UTE HEISpRe ot a eg ”n blid SE öjt vp N V 2 äräg fond a 7 SÅ ig! inb YT fe Rare HA broder »' else nh 6 Ah or Ao ör ” a ÖVERSIKT AV FINSKA VETENSKAPS-SOCIETETENS FÖRHANDLINGAR CXII. 1920—1921. A. MATEMATIK OCH NATURVETENSKAPER. HELSINGFORS 1921 =. HELSINGFORS CENTRALTRYCKERI OCH BOKBINDERI AKTIEBOLAG 9. INNEHÅLL: On the Igneous Rocks of Sviatoy Noss in Transbaikalia, by PENTTI ESKOLA. Redogörelse för fortgången av de astrofotografiska arbetena å observatoriet i Helsingfors under tiden juni 1919 till maj 1920, av ANDERS DONNER. Intensitätsverhältnisse der Trabanten einiger Spektrallinien, von HARALD LUNELUND. Lichtbrechung und Atombau. Die Refraktionsäquivalente der Ionen, von JARL A. WASASTJERNA. Experimentelle Beiträge zur Entwicklung des Bombinator- Herzens, von GUNNAR EKMAN. Mil 27 Textfiguren und öratel: Uber die konforme Abbildung von Sterngebieten, von ROLF NEVANLINNA. Me Sr p ' NN ; j ät Ra - vp La RE Re rd or a RS Sa fr in AGE Ko Översikt av Finska Vetenskaps-Societetens Förhandlingar. Bd. LXIII. 1920—1921. Avd. A. N:o 1. On the Igneous Rocks of Sviatoy Noss in Transbaikalia by PENTTI ESKOLA. Geological Synopsis of Sviatoy Noss. The vast mountainous area of crystalline rocks all around Lake Baikal represents an old peneplain, elevated and sub- jected to erosion at different geological times. Deep-seated igneous and metamorphic rocks and their mixtures were thus laid bare. There are, in many parts of this region, evidences of an unconformity, i. e. sediments and volcanic rocks have been deposited on the denuded roots of ancient mountain chains composed of injected ecrystalline schists and eruptives. The newly formed supercrustal rocks also have undergone metamorphism, but later denudation has not been great enough to reach such deep levels where pressed down supercrustal rocks may be injected with granitic masses and transformed into migmatites as the older rocks. Their metamorphic habit is therefore of another character, bearing evidence of folding at comparatively shallow depths. Russian geologists have designated this divi- sion of rocks as the metamorphic suite and the older deep- metamorphic complex of the Transbaikalian VE fölneld as the erystalline. suite It is probable, 2 Ke though not exactly proven, that the latter is of early paleo- i. — zoic age. The »crystalline suite» should be of pre-Cambrian — origin. [Ce] The latest elevation of the Baikalian Region took PR at late epochs of the Tertiary period, and the vast shield of rock-crust broke up, some parts forming deep depressions, as the basins of Lake Baical and the Bargousin River. The island of Olchon and the peninsula of Sviatoy Noss are smal- ler blocks of the broken up shield which have preserved a Fig. 1. The South Cape of Sviatoy Noss with elevated shore-terraces. Photo P. Eskola. horst-like position within the Baikalian depression and are bounded by faults. Sviatoy Noss, being joined to the conti- - nent only by an isthmus formed by the delta deposits of the Bargousin river, is therefore at the same time a part of the Baikalian shield and an isolated horst. The rock-crust of Sviatoy Noss (see the sketch-map) 5 belongs entirely to the »crystalline suite» in the sense named above. There occur crystalline schists largely injected with granite. No unconformity between the different rocks can be traced. And the peninsula being bounded by zones of — Pentti Eskola. KAL (EXET FÅ 2 displacement, there is no relation whatever between the 5: orography aid the geological structure, if not the fact is rr > v - + A N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 3 considered that the western and southwestern shores of Sviatoy Noss, like most of the other coast-lines of Baikal, are approximately parallel to the old strike of the foliated and granitized rocks. It was this character that suggested E. Suess to designate this kind of displacement as disjunc- tive ruptures. The faults bounding the peninsula generally are not exposed owing to the fact that there are Fig. 2. View of the Ridge of the Sviatoy Noss Mountain. C:a 1300 m above the lake level. Photo M. Sojusoff. rather extensive wave-cut terraces on the Baikal coasts. Travelling around the peninsula with a boat along the shore one sees almost constantly the lake floor through the translucent water of the Baikal, but going a few hundred meters from the shore, the ground dips abruptly to depths of several hundred, at places more than a thousand meters. It is there that the true fault planes are situated. Other conditions, however, are prevailing on the shores of the Tshivirkouy Bay. Here, around the narrow bay, the force of the waves is slighter than on the open shores of the Baikal, and therefore the terraces are less extensive, Just as the shore cliffs are lower, and traces of terraces from 4 Pentti Eskola. SM (LX ancient phases of denudation have been preserved above the actual one. Zones of displacement are here actually exposed, appearing in the form of intimate faulting an: crushing up of the rock. This phenomenon is especially striking in the limestones, e. g. at the Molodost Bay, on the isle of Kyltyghey, and on Mys Kourboulik. The limestone of Sviatoy Noss is generally coarsely crystalline and contains EA graphite in the form of scaly crystals. In all the localities named it may be seen from gradual transitions, how the bo limestone has been sheared, crushed and finally ground up 3 to such a fine grain that, if this ultimately mylonitized Er variety is examined alone, it may easily be mistaken for ET primarily fine-grained, i. e. slightly metamorphosed lime- stone. In this process the graphite at first is stretched out 2 into thin bands and then gradually ground to form a fine powder and mixed with the crushed up calcite. Thus the marble, formerly shining white in colour, has grown dull gray, sometimes almost black. The mylonitization appear also in siliceous rocks, though less striking. On Mys Kour boulik the faulting in the limestone is generally less intimate and the fault planes have been opened to form quartz-vein containing drusy cavities with. franca prismatic cryst of quartz. | The distribution of he occurrences of mylonite and fault breccias already indicates that the' faulting, at this side 0 Sviatoy Noss, is distributed among a series of parallel plane at short distances from one another. The amount of the displacement is probably not very considerable, as, on th bed of the Tshivirkouy Bay we meet, on many low island the same rocks that build up the neighbouring part of t peninsula. i Similar zones of displacement no doubt exist on be western side of the peninsula, though hidden beneath the water. , É RN J "MA NT ILE INU VY SVR JA Mn LO + d NV ( N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 5) quartz containing drusy cavities with crystals of quartz and fluorite. Å RT NI SN IVENTI Le SS Je i ” Evident as it is that the peninsula of Sviatoy Noss is at all sides bounded by zones of displacement, it is no less apparent that the peninsula is a broken up part of the vast - Baikalian shield, without any geological independence. It is neither anticline nor syncline. Its southeastern portion is built up of a granite and granodiorite mass which is also exposed on the low islands in the Tshivirkouy Bay and con- tinues in the Tshivirkouy Mountains and further in the Bargousin valley, having a very extensive occurrence. The main part of the peninsula consists of injected crystalline schists which are interbroken by displacement at the Tshi- -virkouy Bay, but on the eastern shore of the latter reappear. According to Kotoulskys map the same rocks continue far away towards the north along the shore of Lake Baikal. Turning to the geological structure of Sviatoy Noss we may at first discriminate those two great complexes: 1) the granite-granodiorite mass and 2) the complex of injected erystalline schists (see the sketch-map). The area of the first-named complex is, for the most part, underlain by coarse-grained porphyritic rocks whose composition varies between granite and granodiorite. It is occasionally intersected by dikes of lamprophyric rocks and "very frequently by such of pegmatite and aplite. Aplitic granite also forms larger masses within this complex. Going towards the boundary of the granite area, these -aplitic portions grow more and more frequent and nearest SA - 3 portion of the migmatites, have been invaded by intrusive rocks before the intrusion of the granite. These older in- oto the boundary towards the migmatitic area there is an : almost uninterrupted zone of light grey aplitic granite of medium grain. Still farther west one finds more and more numerous fragments of crystalline schists enclosed in this — granite, and thus the rock grades, by increase of the enclosed — materials, into the migmatites in which the granite occurs ers as veins and dikes. The crystalline schists, forming the older and larger 6 Pentti Eskola. . (exmn trusives have the composition of diorites and gabbros. They. i are foliated and metamorphosed to a considerable degree and seem to form sill-like bodies folded together with the =: other rocks of the metamorphic series. They are cut across by the dikes of aplite and pegmatite and also by the granitic | Veins. Among the crystalline schists of the migmatitic comin St the gneisses are represented in a great number of va- = rieties. Among these the various augengneisses call for special attention. They are biotite- or biotite-hornblende- = gneisses containing large »eyes» of microcline surrounded by scales of biotite. The eyes may pass over into small lenses of feldspar and quartz and further into granitic veinlets. In such cases the peculiar structure of the rock may either be referred to an ultra-metamorphism or to a beginning =: granitization. At other points the augengneiss has a ground- mass showing massive structure and medium grain. The - rock is then very like the coarse porphyritic granite: and - may have been derived from them. Some examples bear resemblance to quartz-porphyries, having a fine-crystalline ground mass. — Thus it seems to me probable that the augengneisses on Sviatoy Noss may be of various origin. Many examples of gneisses have a coarsely crystalline structure, so that their origin can not by any means be stated. There are grey mica-gneisses of medium grain, and plagioclase-amphibole-gneisses grading into amphibolites by decrease to the quartz. These varieties, as well as the augen- k gneisses, have commonly been injected with granitic veins to form adergneisses or arterites (fig. 3) representing the - most common kind of migmatite on Sviatoy Noss. The - primary nature of such rocks, of course, can not possibly 5 be interpreted. The amphibole-plagioclasegneisses and - the amphibolites (metamorphic plagioclase-hornblende- rocks) are very common in all parts of Sviatoy Noss, occurring also as fragments in the large masses of granite. The amphi- bole-bearing schists have all been very thoroughly meta- morphosed, and no traces of their primary structure have LÄ An IDE NORTE > A N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. been preserved. It is therefore very difficult to state anything with certainty regarding their origin. Probably they are various in this respect, just as the gneisses. On the south- eastern shore were observed many examples of amphibolite as dikes intersecting gneisses etc. Such varieties are almost certainly of intrusive origin. The same is probably the case with many larger occurrences whose form is not obvious. Thesé varieties have the composition of gabbros or diorites. Fig. 3. Adergneiss with cross-cutting dikes of aplite. SE-shore of Sviatoy Noss. Photo P. Eskola. Greater part of the amphibolites, however, is in all probability sedimentogenous, or, at least, tuffogeneous. This supposition is suggested by the bedded structure and interstratification with limmnestones, and by the occurrence of pyroxene-bearing layers which may be interpreted as metamorphic derivatives of marl or argillaceous limestones, and further by the very inconstant composition in their neighbouring layers, part of which may consist exclusively of plagioclase and hornblende, while others are pyroxene- or quartz-bearing and still others also contain almandite. 8 | ; Pentti Eskola. Some rocks related to the amphibolites may -posatkla VS have been formed by the pneumatolytic action of granitic magma on the limestone. A fact that may be suggestive of such a hypothesis is the occurrence of certain exceptional rocks found only as blocks on the shore-beach, and composed - chiefly of diopsidic pyroxene and very anorthitic plagioclase (bytownite), besides pale green hornblende and abundant titanite. For such a rock I can hardly find any other expla- nation than the well known theory of Adams concerning the formation of amphibolites by the action of granites. Besides this vague support for Adams” theory I have no other evi- dences and I believe it is, as far as the rocks of Sviatoy <: Noss are considered, applicable only on a very limited scale. The amphibolites are injected with the granite like - the gneissoid rocks. Those varieties which are quartz- bearing and at the same time strongly foliated, have often given rise to intimately mixed arterites, whilst more massive amphibolites, and especially those rich in hornblende, have been more resistant against the injection and have only - been invaded by the granite in the form of cross-cutting - dikes, the whole being what is generally called eruptive breccia. Very typical adergneiss derived from an amphibolite or amphibole-gneiss, is exposed in the promontory near the North Cape of Sviatoy Noss. These rocks may be studied in the blocks on the shore-beach (fig. 4). ONE Going southwards from the North Cape along the Tshivir- 8 kouy shore one meets with amphibolite-adergneiss in which the intrusive portion is largely made up of quartz. This - kind of rock which is apparently a transitional type between true injected migmatites and intensely metamorphosed 5 rocks with veinlets of quartz, is exposed on the isle of Kylser 3 hey and neighbouring shores of Sviatoy Noss. PÅ As a general remark on the migmatites of Sviatoy Noss, as compared e. g. with related rock types in Finland, we = may state that the former are represented by adergneisses and eruptive-breccias rather than by true remelted or anatec- = tic rocks. Such almost completely assimilated or »nebulitie» n SV ören Å N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 9 remnants of older rocks which are so characteristic of the anatectic granites in Finland, are only found on a limited scale on Sviatoy Noss. Crystalline limestone forms -a considerable part of the metamorphic rocks of Sviatoy Noss. As a rule, it occurs in the form of lenticular masses of all sizes, the largest being some hundred meters in thickness. The longest diameter of the masses usually is only a few times greater Fig. 4. Amphibolitic adergneiss. N. end of Sviatoy Noss. Photo M. Sojusoff. than the shortest one. In part the marble consists of very pure calcite having, however, almost always graphite in the form of thin scales as a minor constituent. Sometimes the -limestone contains considerable amounts of diopside or tremolite, the former occurring at places, e. g. on the west — shore near the North Cape, as euhedral though uralitized crystals of many centimeters in lenght. The limestone is — coarsely-crystalline (centimeter-grained). At places also fine- grained varieties occur, but these are probably always förvlonttic and have formerly been coarsely crystalline. Like all the other metamorphic rocks, the limestone is | frequently cut by dikes of pegmatite, aplite and lamprophyres. 10 Pentti Eskola. (CXMÉ But the marble has never been injected with minute veins and lenses of granite. It seems, as though the limestone could not be migmatitized, a feature also very striking in the Archaean of Finland. ; At the boundaries between the limestones and the dikes of aplitic granites there are never any contact minerals. Dikes of pegmatite are sometimes coated with a contact- wall of diopside and amphibole and also scapolite. Where the limestone is, as larger masses, directly bounded by ordinary granites, either aplitic or porphyritic, there are no contact minerals whatever. But the granite often encloses fragments of limestone, and these have commonly been transformed into an dradite-skarn, a phenomenon of exceeding interest, as will be pointed out on later pages. Theintrusions within the migmatitic area of Sviatoy Noss are of two principal kinds: 1) those forming the younger injected portion in the migmatites, occurring in the form of thin veins or lenses, and 2) well-defined dik es. Most kinds of the latter also occur within the granite-grano- diorite area. Theinjected veins and lenses generally bear resemblance to pegmatite, being coarsely crystalline and, as a rule, poor in mafic minerals. Among the true dik es with sharp boundaries we may separate two groups of very different ages. The first group embraces all those dikes intruded simultaneously with or shortly after the intrusion of the granite batholith. They consist of phanerocrystalline rocks, showing true abyssal - characters. In the second group we include dikes of basalt and related rocks, being probably from the Tertiary period. The dike-rocks of this category show amygdaloid structure and other evidence of having consolidated at shallow depths:. They are, however, hypabyssal rather than volcanic, a fact easily intelligible, if the mode of their occurrence is considered along side with the development of the orography. The exposure of basalt observed is situated on the lake shore, i. e. on the floor of the wave-cut terrace and landside of the main displacement. In this point the amount of denudation ey) A N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 1 posterior to the displacement which probably took place at the same period as the eruption of the basalt, is certainly more than the actual height of the Sviatoy Noss mountain, i. e. considerably over 1,000 meters. The dike-rocks connected with the intrusion of he granite are of three principal types: 1) pegmatite, 2) aplite and 3) lam prophyric rocks. The dikes of peg matite are most closely connected with the granites, appearing within the masses of granite as primary dikes and occurring also in all parts of the mig- matite area. We may discriminate three different kinds of pegmatite 1) One kind is characterized by microcline and quartz as the main constituents. Dikes of this type of common pegmatite are equally distributed in the granite and in the migmatite area. 2) Another kind of pegmatite which will be called diopside-pegma tänk e; is characterized by oligoclase and diopside besides much titanite and a little quartz. This pegmatite occurs only in the limestone areas and,in their close neighbourhood. 3) A third kind of pegmatite will be termed as horn blend e- pegmatite. Its main constituents are a rather anorthitic plagioclase (andesine) and brown hornblende, while potash feldspar is absent and quartz is only sparingly present. A characteristic accessory of this pegmatite is orthite, a mineral which is, in small amounts and as minute crystals, also found in the granites and in the other kinds of pegmatite. The hornblende-pegmatite occurs only as intersecting the amphibolites in the northern portion:of Sviatoy Noss. The dikes of a p lit e are also found in evident geological connection with the granites, occurring within the granite area as well as in the migmatite. The aplitic granite forming a marginal zone of the granite batholith, is petrographically very like the aplite in the form of dikes, but having a little S larger quantity of micas. The aplite is the most common of all the dike-rocks on Sviatoy Noss. With the aplite are often associated dikes of I a m pr o- phyric rocks being either somewhat older or younger othan the former. Most of them are kersantites. Å 220 3 VTI 12 : Pentti Eskola. (LIITE Aplites and lamprophyric rocks often form composite dike YYTRE breccias. Within the migmatite area there are also larger masses of granites, porphyritic as well as aplitice. Some of these s intrusive masses, though evidently connected genetically AV 126; with the granites and granodiorites, have quite another composition. Part of them may be classed with syenites, : while others have a peculiar and uncommon composition, chemically characterized by the abundance of lime and - alkalies and mineralogically by the presence of the lime- ferri-garnet, andradite, the other constituents being those of a syenite. This rock will here be called sviatonossite. . The sviatonossite deserves exceeding interest from the petrological point of view, as it seems to exhibit a positive illustration of Daly's well-known theory concerning the for- mation of alkaline rocks by the action of magma on lime- stone. This rock-series will therefore be described in a detail that would be unnecessary in the case of more ordinary rocks. : All the rock groups occurring on the peninsula of Sviatoy Noss, mentioned in the order of their relative age, are the > following. The younger schistose formations (the metamor- phic suite) not occurring on Sviatoy Noss, but in other parts of the Baikalian Region are mentioned within parentheses. Basalt ( Tertiary). ( Crystalline schists of the metamorphic suite, Palaeozoic?). The Tshivirkouy batholith and other abyssal intrusives connected with the latter: Aplite, pegmatite, lam prophyric dike-rocks, granite, granodio- rite, andradite-syenite or sviatonossitenmn diorite, gabbro, hornblendite. Older intrusives in the crystalline schists: diorite and gabbro. Crystalline schists of the crystalline suite: Volcanogeneous (in part possibly plutonogeneous) rocks: amphibolites and augengneisses. Sedimentogenous rocks: Crystal line limestone, amphibolite, amphibole- gneiss and other gneisses, quartzite-gneisses. Ger ÖRE AA N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 13 Diorite. Among the rocks of Sviatoy Noss older than the igneous complex of the granites there are also such types which may be safely termed as primarily igneous or eruptivogeneous, and such of these that have some primary structural features preserved may be mentioned here. Those having a dioritic composition are all plutonogeneous. The amphibolites never have preserved any structural features pointing to a volcanic Origin. Diorite-gneiss. One of the most typical plutonogeneous rocks is a diorite- gneiss observed at several points in the southern part of — Sviatoy Noss, e. g. at points 7 and 35. At the former locality, near the South Cape, the diorite-gneiss occupies an elongated area, being, in the shore-cliff, exposed about 50 meters obliquely to the strike. The occurrence at point 35, a few kilometers further North, is somewhat less extensive. This rock is megascopically distinctly foliated and pre- sents a double granularity: The biotite and hornblende form flattened groups arranged along the strike and giving the rock its characteristic gneissoid appearance, and between them there are light spots of granular plagioclase. The following constituents were detected under the — microscope, named in the order of their relative abundance: Plagioclase, TIER quartz, hornblende, titanite, iron ore, — apatite. The plagioclase between crossed nicols appears spotted in consequence of unequal extinction, but no regular zonality can be observed. Its composition varies from about ÅAbss to Ab,,. | The hornblende is very dark-coloured and has : a strong pleochroism: « = yellowish green, p = dark olive green, nearly opaque in the section, y dark bluish green. FAbsorption: oy-c:y = about 22”, pg = b and the j negative acute axial angle 2 E is about 60” or 2 V =35", fupposing p to be 1,68. These characters point to a horn- 14 Pentti Eskola. (LXTIE blende rich in ferric as well as ferrous oxide. The bio- tite also is very dark (brown) in colour. Titanite is remarkably abundant, as well- formed . crystals. "Anatectic amphibolite. As already mentioned, there are, among the ampbibolites on the peninsula of Sviatoy Noss, many examples of rocks whose origin can not be stated with any certainty. Whatever they may have been originally, most of the amphibolites are, in their structure, very like igneous rocks, also in such cases, where their geological occurrence points to a sedi- mentogeneous origin. They may then have undergone some kind of anatectic recrystallization. Under the microscope such rocks are usually seen to be composed chiefly of plagio- clase and hornblende and quartz, the first-named ranging from oligoclase-andesine to labradorite. Thus they would. chiefly belong to the quartzdiorites, if treated as igneous rocks. A structural feature of interest is the common xeno- morphity of the hornblende towards the plagioclase, char- acteristic of many hornblende-gabbros and hornblende- diorites, while the average amphibolites having typical granoblastic structure have idiomorphic, or idioblastic, hornblende, this mineral having a more advanced position in the crystalloblastic series than the plagioclase. Another feature common in these amphibolites is the zonal structure in plagioclase with cores more anorthitic than the margins. Granites and Granodiorites. The inner part of the Tshivirkouy Bay and the eastern Sviatoy Noss are apparently underlain by coarse-grained granitic and granodioritic rocks. This vast massif continues, in an easterly direction, over the Tshivirkouy Mountains into the Bargousin valley. In the migmatitic area of Sviatoy Noss were rocks of characteristically similar structure and composition occasion- ally met with as small lenticular intrusions. A N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 15 The composition of the large massif is fairly uniform, its varieties ranging from subalkalic. granites to granodiorites. The porphyritic rocks are frequently intersected by dikes of lamprophyric and aplitic rocks, and the massif seems to be bordered towards the migmatite area by a zone of aplitic granite. As the latter is often seen to cut the porphyr- itic coarse-grained rocks, while the opposite case was never observed, the latter have been solidified earlier, though great difference in the age is not by any means probable. Porphyritic coarse-grained gran o- / diorite. ; As representative of the chemical composition of these coarse-grained porphyritic rocks a specimen taken about one kilometer northwest of Mys Besimyanniy on the east shore of the Tshivirkouy Bay, was analyzed by the writer. The result was as follows. | Mol. |i Mol. DER MÖR 07, Nörm SiO, | 65.06 | 1084 | 71.27 | Quartz....... "14.102, Q=14.10 Al,O, |: 15.84 | 155 | 10.19 | Orthoclase ...... I) | Fe,0, | 1.74] — 110] ALDIber ba Rd aa 35.11) - F=73.76 | FeO 2.51 i 3.81 | Anorthite ........ 0 öd) | "MnO | 0.08 | Z sal 87.86 | MgO Sal 33 | 2.50 CaSiO, 1.97 CaO 3.28 60 3.94 | Diopside . .Y MgSiO, lg Na,O 4.16 67 4.41 l FeSiO, 0.79) | P == 7.51 F-K;O 4.82] 51:| 3.35 MgSiO, 2.20 f HO, |. 0,63 261,00.53 Hypersthene | FeSiO, äs)?05 P30; 0.26 21] — Magnetifet öste sve va 2.55 Fl Ho 0.57 Ilinenite «..... . . io 2 STR APAELLC TNG mat site AS ehe e 0.67 A = 0,67 ; Z fem 11.95 99.81 100.26 | 100.00 | | AN dd -k ba Nana AR og FSD SONEN 16 | Pentti Eskola. ; (LXI Toscanose (1, 4, 2, 3). The Osann figures: S A & F (ARG JES k T 71.80” 7.76, 2430 17:82. 18:51 3.080 SS The constituent minerals are, named in the order [01 ENE their abundance: Plagioclase, microcline with some albite, quartz, biotite (with secondary chlorite), hornblende (dark green), titanite (euhedral crystals and leucoxene), magnetite, apatite, orthite and epidote. The actual feldspars are plagioclase (oligoclase) and microline-perthite. The plagioclase forms, together with quartz and some microcline, the medium- granular (millimeter-grained) ground-mass of the porphyritiec rock. It shows, in sections | PM, an extinction angle of + 4”. Its refractive indices are all higher than that of the Canada balsam. A comparison with the quartz gave the result: «' w. Thus the composition of the plagio- clase 1s approximatively Ab,s;. Towards the microcline the plagioclase grains are bounded by albitic borders. Orthite, with e pi d ot e, is remarkable because being a very constant accessory in all the igneous rocks of Sviatoy Noss. The mode of occurrence is always the same: minute euhedral and strongly pleochroic brown crystals of orthite are surrounded by a much larger zone of faintly greenish epidote. They will be described in more detail under the sviatonossite in which they are still more prominent consti- tuents. The other minerals do not call for any comments. In the qualitative system of igneous rocks, as modified - by Iddings, this rock classes itself with the granodiori- t es 1), having alkali-feldspar and lime-soda-feldspar in nearly equal amounts, within the ratios 5 : 3 and 3: 5 and the latter exceeding. As, in the present paper, I will generally follow Idding's classification, this term will be used here. It may, however, be remarked that in this particular case it would perhaps be more rational to call this rock quartz-monzonite, which term, according to Iddings, is applied to rocks closely 2) J. Iddings, »Igneous Rocks>, II, p. 69. New York, 1913. | A N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. ky related to granodiorites, but having alkali-feldspar in excess over the lime-soda-feldspar. The name quartz-monzonite is in this case in a better accord with the high percentage of alkalies, equalling that of some true syenites, as well as with a percentage of lime, lower than what is customary in granodiorites, and, moreover, a large part of it being con- tained in the mafic minerals. In Washington's tables, many analyses Ae a similar composition, refer to granodioritic rocks from Western North-America, that well-known calci-alkalic petrographic province. Other similar analyses may be found among those of rocks called adamellites and banatites. We may safely state that the analysis of the granodiorite from Mys Besimyanniy represents the composition of a rock-type of very wide-spread occurrence in the Tshivirkouy batholith as well as in its continuation in the Bargousin valley and neighbouring regions of Transbaikalia. Among the analyses of specimens collected by the Radium expedition in 1914, that of a granodiorite from »Camp Mountain» in the Namama Region near the water-shed between the Bargousin and Angara rivers, has a striking similarity to that in question. The analysis, made by the writer, gave the following figures: Mol. Mol. | numb. |, Rp SiO, | 66.88f| 1115 | 73.36 | Quartz ..... RES fb 0 OO Al,O, | 15.24 | 149 | 9.81 | Orthoclase. ...... 22.24] Fe,O, | 1.10 TIN (re SÄND 1te fed ana EL äN 38.77 F = 70.74 Fe0 2.20 31 | 3.03 | Anorthite . ...... 19.73) MnO | 0.11 1 = IE dal 8944 MgO 1.32 33 2.17 CaSiO, 1.74 CaO 3.14 56 | 3.68 | Diopside . .y MgSiO, oo ) Na,O | 4.60 74 | 4.86 l FeSiO, 0.66: P= 742 SO | ao | 201 25 | yperstrene MeSIO, 230) PaOg. |: 0.25 21 — | Magnetite ....... AGNE ND = Molég H,O 0.43 5 ILmenite:r. oa och revs 1.06 $ ADALILe: 3115 oxe ed re 0.67 A = 0.67 Zz fem 10.77 99.21 99.66 I 100.00 | 18 | Pentti Eskola. ER (LXII - Lassenose (1, 4, 2, 4), near FOR ORONE The Osann fonres S Å C FAONKe Ve ji n VR 73.82. 7:49. 112,32 — 6:50,5-09.0 73:08:05 EEE The actual minerals of this rock are plagioclase (about Abg0), orthoclase, quartz, green hornblende, titanite, biotite, diopside, apatite, magnetite, chlorite and epidote. There are many features in the structure and mineral development of exceeding interest, and they will (RET TeeR be deseRned in some detail. At the first sight this rock shows the most bybibalt primary consolidation-structure : possible, and every one of the con- stituent minerals has its given place in the series of idio- morphism. Among the four main constituents the common green hornblende is idiomorphic towards all the others showing in fact quite an ideal crystalline form, combined of the simple forms (110), (001) and (101). Thereafter follows the plagioclase, which has a marked zonal structure. Its maxi- mum extinction angles in symmetrical sections are about 5”, but with variable sign in different zones, the percentage of albite thus varying from about 75 to 85. Myrmekitiec intergrowth of quartz and plagioclase is frequent. The largest crystals of plagioclase measure about 2 cm in dia- meter, the hornblende prisms averaging 1.5 x 1 x 0.5 cm. The interstices between the hornblende and the plagioclase are filled up with a mass of orthoclase and quartz. . The former shows an idiomorphic development towards the latter and forms the very largest crystals in the rock, being, more- over, striking by their beautiful salmon-red colour which contrasts against the white plagioclase. At places, quartz and orthoclase form a micropegmatitic intergrowth around crystals of: orthoclase. No microcline is present, neither simple nor twinned. | Thus the hornblende apparently was the earliest main mineral to crystallize out. Strange to tell it is, however, of a secondary origin in such a sense that it had originated at the expense of other minerals that had crystallized out still Kg , q , A N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 19 earlier and then been resorbed. This appears from the follow- ing facts: Enclosed in the hornblende are frequent ragged remnants of diopside, and all around the crystals there occur grains of magnetite, titanite and epidote. The diopside apparently. represents the original material and the others are second-hand products of the formation of the hornblende. Still there are ragged crystals of biotite, enclosed in the feldspar as well as in the hornblende and apparently of an earlier date than the latter. Biotite and diopside were the mafic minerals earlier than the hornblende and for greater part now resorbed. But it is quite probable that there had been still earlier crystalliza- tions, perhaps olivine. Some aggregates of hornblende met with in dark segregations in the granodiorite may in fact be interpreted as pseudomorphs after olivine, their contours agreeing fairly well with the most common crystalline forms of that mineral. We have here an example of neon and destruction of minerals at the magmatic stage, occurring in a rock which never was disturbed by movements, neither during nor after its consolidation. Similar phenomena have often been met Wwith in less well preserved rocks and have there usually been attributed to later agencies. It seems to me, that the results arrived at in this case may be generalized widely. This is also in fair agreement with many results won in the later years by experimental physicochemical work. The granodiorite of »Camp Mountain» forms a large mass. 3 A quaternary glacier has worked out a glacial cirque in this remarkable rock, and boulders of it, transported by the ice, are widely spread around the springs of the Bargousin river. In the marginal zone the granodiorite grades into quartz- diorite showing similar magmatic resorption of the earlier diopside as the granodiorite. - A microscopic investigation of the specimens collected from the surroundings of the Tshivirkouy Bay, in numerous — cases, established an almost the same mineral composition as the analyzed rocks. The composition of the plagioclase here varies between narrow limits, from about 74 to 78 percent 20 Pentti Eskola. = (LX OSA albite. The quartz is not abundant, while hornblende and = biotite are present in conspicuous amounts. Such hornblende- SS biotite-granodiorites were observed by the writer e.g. at SA Mys Pokoyniky and at Irkana; in the neighbourhood of Mys — - Besimyanniy it is widely distributed. Mr. Viscont collected = from point 163B a specimen of hornblende-granodiorite having small amounts of quartz and a plagioclase that Sd contains only about 70 percent albite. Here and there the granodiorite passes by gradual Rn tion into more femic types, which, however, do not occupy any large areas, but are rather like inclusions. Similar rocks : are seen near the contacts with the limestones and gneisses. Apparently some of the dioritic or gabbroid intrusive rocks met with in the migmatitic area also are in a comagmatie = - relation to the prophyritic coarse-grained granodiorites. The gradual transition of the normal granodiorite into EE more femic types takes place by means of increase in the amount of hornblende, the composition of the plagioclase being little affected. Thus, a specimen of a hornblenditic rock from Mys Monachoff consists chiefly of green horn- blende and contains a little plagioclase, microcline and Sd quartz. The refractive indices of the plagioclase are all = lower than w of the quartz and the feldspar cannot, there- fore, contain more than about 15 percent anorthite. Porphyritic coarse-grained granite. The varieties more femic than the main type being quantitatively of little importance, more salic types of a the coarse-grained porphyritic rocks occupy large portions of the great batholith. Greater part of them are true grani- = tes, the alkali-feldspar exceeding largely the lime-soda-feld- = - spar. At the same time the Körner often though not = always disappears. Specimens of such granites were collected from point. 156 A, from Mys Pokoyniky and from the island of Baklani. The distribution of the granitic and granodioritic portions of the batholith does not show any regularity. Both types 2 "i M LJ S oo A N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 21 are found in the central part of the mass as well as near the boundaries and in the apophysal intrusions in neighbour- ing migmatitic areas. As representative of the mineralogical characters and structure of the porphyritic granites may be taken the speci- men from point 156 A. The rock consists of phenocrysts of micro-perthite, 10—20 mm in diameter, embedded in a millimeter-grained mass of plagioclase, microcline, quartz, biotite, hornblende, titanite, magnetite and apatite. oPlagioclase is found as euhedral crystals enclosed in the perthitic PASS and as anhedral grains in the ground-mass. A section ! PM showed extinction parallel to 3 the trace of M, SR to the composition Abgp. Towards the microcline the grains have albitic border-zones. The potash feldsparin the ground-mass is cross- hatched microcline destitute of perthitic albite. The pheno- crysts of potash feldspar contain abundant albite in perthitic intergrowth showing distinct zonal development, the albite laths being most abundant in zones arranged along certain crystalline faces. The potash feldspar in this zonal perthite seems to be orthoclase; at places very fine microcline twinning may be observed, but on the whole the quadrille-structure if existing has submicroscopic dimensions. There are, how- ever, large amounts of coarsely cross-hatched microcline also, —— the microcline structure being apparently of secondary origin. Often such microcline traverses the whole individual along lines that seem to represent deformation zones. Cracks are often seen in these. The microcline portions are, in most striking manner, very poor if not quite devoid of the perthi- tic albite laths. ; The other minerals do not need any special description. Many other specimens contain only cross-hatched micro- . cline as potash feldspar. Myrmekitic intergrowth of -plagioclase and quartz is common in such varieties. To these microcline-granites also belong the porphyritic granites found as intrusives in the migmatite area, e. g. at > points 7, 31 and 33. The specimens collected from these Points consist of the following minerals: microcline, plagio- 92 NE Pentti" Bskola ; FER clase (Abg,), biotite, quartz, dark green hornblende, magne- = tite, titanite and apatite. The large phenocrysts of cross- = hatched microcline contain narrow perthitic albite-laths in 4 small amounts. Grains of plagioclase surrounding the micro- cline or included in it have developed abundant myrmekite, ä which is often restricted to the albitic border zones of the plagioclase, the late ORO of the implication being thus: 2 apparent. ÅR The small quantities of an EA present in most of the porphyritic granites, in the large batholith as well as in the small intrusions within the migmatite area, is very striking. The composition must in many cases be that of a quartz- = = syenite. As the rocks are, at the same time, poor in mafic SE constituents, the alkali content must be high. Such a sub- = - alkaline character is really a feature common to the Trans- i baikalian igneous areas on the whole, so far as appears from Er. the investigation T the Radium Fapenieen RA bä The Aplitic Zone round the Tshivirkouy Batholith. As mentioned above the vast massif of mainly porphyritie Er granite and granodiorite is, towards the West, bordered by light-coloured medium- and even-grained granites of rather = - aplitic character. "Near Makarova, on the southeastern half of the peninsula, this granite separates the porphyritic variety from the migmatite area. The field-observations are, briefly summarized, as follows. e On the southeastern coast of Sviatoy Noss northeast of AN Makarova most part of the shore-drift consists of light grey even-grained granite, poor in biotite. The whole southern slope of the mountain ridge is made up of this granite, often cut by' primary dikes of still lighter aplite. The main rock is homogeneous and could very well be used for technical purpo- = ses. An ill developed jointing in the direction of the general strike of the schistose formations in the Northwest, being = about N 55” E, vertical, is always visible. Blocks of amphi- = bolite are sparingly enclosed in the granite and towards the : S migmatite area they grow gradually more and more numerous. RE — AN:o1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 23 > Southwest of Makarova the migmatite dominates. In the form of dikes the aplitic granite, however, is common in all the exposures of the southern part of the peninsula. On the coast of the Tshivirkouy Bay, aplitic granite was observed at several localities, which seem to be separate occurrences, some of them dike-formed, other forming smaller massifs in the mixed formation. A few of the occur- rences may be mentioned: At point 141 A, is exposed aplitic granite, forming stretched out masses along the main strike and interstratified with limestone. At point 142 A occurs a larger body of grey granite showing distinct parallel arrangement along the strike, N 70? E. The rock is poor in coloured constituents and rich in quartz. Going further — upwards in a northwesterly direction the granite grades, at point 143 A, into migmatite, enclosed fragments of ampbhi- bolite becoming more and more numerous. | Further south, at points 156 A and 157 A the main part of the rock is a porphyritic gray granite which, however, frequently grades into the even-grained aplitic variety. This is especially the case around fragments of amphibolite and in apophyses from the granite into the amphibolite. The — even-grained granite is here commonly hornblende-bearing. At point 158 A was found only even-grained hornblende- — granite intruding limestone. Fragments of andradite-skarn are included in the granite. Such is also the case at point 160 A. Going farther south one finds the porphyritic variety oto become 'dominating, but there are transitions into the — aplitic variety, e. g. at points 170 A and between 172 and 175 A, where numerous schlieric portions richer in biotite than the main rock are apparently remnants of assimilated —. amphibolite fragments. Still at Irkana, in the inner part of — the Tshivirkouy Bay, within the granodiorite area, a few larger masses of even-grained granite occur. — From the observations quoted it appears that the even- grained aplite-like granite chiefly appears at the margins of the batholith whose main parts are built up by the por- — Phyritic variety. The field-observation was not quite suffi- v. cient to show, how regular the aplitic marginal zone may be. 24 2 Pentti Eskola. c (LXIH KE On the coast of the Tshivirkouy Bay, at least, it is frequently | interrupted by the porphyritic variety. For chemical study of the even-grained granite was selected a specimen collected from point 5, at the summit of the Sviatoy Noss ridge near Makarova. The analysis, made by the writer, gave the follow- | ing result. | | WG Re Då Norm SiO, | 70.88 | 11.81 | 78.01 | Quartz ..... AI a 93:34 GE SSR Al,O, | 14.80 | 145 | 9.57 | Orthoclasé. . .'.... 26.13 Fe,OZz | 1.00 | 6 l Albite. - . sms är a 40:35:09 5 El sARAS FeO 1.08 15 | 185 | Anorthite ....... 5.00 MnO 0.10 1 Corundum....... 0.31 C= 03 MgO | 0.28 7 | 0.46 2 sal 95.13 ' | | CaO 1.33 24 | 1.58 ÖR a : Hypersthene P=0K30 Na,O | 4.80 77 |... 5.08 MgSiO, 0.70 K,O 4.42 47 | 3.10 | Magnetite ....... så MSE TiO, 0:39] 5 0.33 | Ilmenite ........ 0.76 Bl P,O; | 0.30 NES JA Pabite lyser ks 0.67... A= 0,67 SR HÖNS ne ER BR z fem 4.18 : ; | . | RN | 99.77 | + | 99.98 |. Toscanose, near liparose (1, 4, 2, 3). The Osann figures: S VANN G F a (FUN Ear) FÖRR kos 78.34" 8,18.1:1.39 7 2.50-13.5 2.0 714 621 14477 The normative plagioclase is Aby,. The actual plagioclase — - is nonhomogeneous, the somewhat turbid cores having in sections | PM extinction angles of — 8”, corresponding to the composition Absg, while the marginal. portions show extinction angles up to — 12” which indicates the compo- sition Aby,. The other minerals are cross-hatched microcline, quartz, rather pale brown biotite, muscovite, magnetite and KEGKACN:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 25 apatite. Assuming the albite, magnetite and apatite to be present in the same amounts as they appear in the norm and rating the amount of muscovite approximately, the actual mineral composition or mode of this rock may be calculated as follows: (0 TATE Moss SS SS SIE SM efaae YT Se 24.66 NIICROCIING 2 stra star NA ge NS 220 AN TIST GC SS Sätea tafsare LS 40.35 SAST ORM GCK sar ba Ia NAR 3.89 | ATOM ESR se SAGA SIR AR SR AD SMS NIA 5.44 VILSE OMIEE! «eo RER sas LG 0.50 ECO TATT G Sd sg obs styrs tel Ten ere bre 0.78 MaOnhel ber ss Esra sees 1530 TUTTAR SNR EA RIA A ap SA SEE 0.67 99.92 About the structure of this rock, little of interest can be said. All the minerals are devoid of crystalline forms and > their boundaries are ragged or serrated. - No perthite is present. The quartz shows very little strain shadows. K Specimens of the aplitic granites from other localities examined microscopically' do not exhibit anything of parti- cular interest. The structure is always the same, aplitic. The mineral composition is invariably characterized by the pre- — valence of alkali feldspars. Orthite with surrounding epidote was observed in some specimens. Magnetite is always rather — abundant as an accessory. The Dike-rocks. Alpi iten Dk es: Dikes of granite-aplite are met with on Sviatoy Noss Wherever rock is exposed. In the even-grained aplitic granite = they present the aspect of primary dikes having ill defined . : — boundaries and the dike-rock being often nearly identical With its country-rock, but containing a little smaller amounts ' 26 | Pentti Eskola. = 6 NTE of biotite. In the porphyritic granite of the great batholith these dikes are sharply defined, and in the migmatite area they appear in the manner of true fissure-dikes independent of the strike. The dikes also cut the intrusive granitic por- tions of the migmatites, a fact indicating them to be the = — youngest intrusions. At places, however, there are still younger dikes of melanocratic rocks. . E For illustrating the chemical composition of the granite- SO aplite, I executed a chemical analysis on a specimen collected from one kilometer northwest of Mys Besimyanniy. The SS dike cuts the porphyritic granodiorite which in this tract SN is typical and homogeneous. It is a dike-breccia, blocks of SM G a melanocratic rock being enclosed in the aplite. The analysis = gave the following results: i | Sd | | JA Mol. Mol. NIO numb Yo | SiO, 73.89 TS CEO: SANS ÖT LTA Ts vios tele Ta ES så 28.62 Al,O, | 13.37 | 131| 8.51 | Orthoclase....... 37.25 Fe,O, | 0.57 4 ATDIEC Na csr es SSR JA 26.20 FeO 1.22 IN 1.69 | Anorthite ...... - 3.89) MnO |: 0.05 if | Z sal 95.96 : "| MgO. | 0.41 10 | 0.65 CaSiO, 0.46 SR | ICaO NE kas 21 |. 1.36 | Diopside. . .2 MgSiO, 0.20 cl TR Na,O | 3.14 50 | 3.25 Så FeSiO, ja P=278) K,0 6.261 GH ASO LA RR SN ; Hypersthene , 1.86 TION SANN ONS i FeSiO, 1.06 i P,O, 0.12 TG Magnetite ....... a RN i H,0 0.33 TImenitej. see 0.46 + Ål paliter. ovalt AR 0.34 A — 0.34 2 fem 4.51 6 . 100.47 | 100.87 | | 100-00 | Liparose (1, 4, 1, 3). The Osann figures: ; | NH S A C F a c FA NR 80.18.) 7:61 ,0.907,:1:2.807 13.5 15N SO em 1.0 LA N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 27 - The actual minerals of this rock are, named in the order — of their abundance: Microcline, plagioclase, quartz, biotite, hornblende, titanite, apatite, orthite with epidote, magnetite. — No calculation of the mode was carried out, because it is very uncertain as to the måfic constituents in such cases when both hornblende and biotite are present. The norma- tive plagioclase is Abgg. In a thin section the plagioclase showed, in symmetric sections, a maximum extinction angle I of — 7” which should give a value of about Abg. The occurrence of hornblende is most probably a conse- Hd quence of assimilation of the fragment rock which is a horn- blende-kersantite. That resorption has actually taken place, appears from the rounded forms of the fragments. Most of the other specimens from aplite dikes examined under the microscope were also found to contain microcline in excess over the plagioclase, like the analyzed example. I have not examined specimens from a sufficient number of —dikes to state how general is really this potassic character of the aplite, but certainly it is not invariable. Thus a speci- men from topographical point 18, collected by Mr. Viscont, showed quite other proportions: The rock consists chiefly of feldspars and quartz. Among the former, plagioclase is far more abundant than the potash feldspar and forms some- what larger grains which are idiomorphic. They contain — potash feldspar in antiperthitic intergrowth. The plagioclase shows only very small extinction angles, thus being probably near AÅbg, in composition. As a mafic mineral there are very E small quantities of pale green hornblende. Dikes of Melanocratic Rocks and Composite Dikes. 2 At places dikes of dark hornblende-bearing rocks are fairly frequent on Sviatoy Noss. They accompany the aplite —— dikes, and sometimes have formed composite dikes with the latter. Most occurrences of the dark dikes were met with On the southeast shore of Sviatoy Noss. Another tract where 28 "Pentti Eskola. | (LX they are numerous is the east shore of the Tshivirkouy Bay. | In the following I shall quote some AN my notes Ponge their occurence. At point 8 is exposed a nearly massive amphibolite cut 2 by several intrusions (fig. 5). The oldest are schlieric granite intrusions tending to give the whole an aspect of ader-gneiss, and are no doubt to be parallelized to those intrusions which Fig. 5. Many intrusions in amphibolite. SE-shore of Sviatoy Noss. Photo P. Eskola. in more distinctly schistose rocks have given rise to ader- gneiss. This migmatite has been intersected by a dark dike rich in amphibole, seen at the left hand side in the figure. Thereafter followed intrusions of aplite cutting all the above- named rocks and forming a composite dike with the older dark dike-rock. Then another intrusion of dark hornblende- biotite-rock followed cutting one of the large aplite dikes (in the middle part of the figure). At last still narrow dikes and veins of aplite were intruded. Thus no less than five generations of dikes and veins can be traced at this place. f A N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 29 | At point 19 was observed a fine example of composite dike (fig. 6), intruding an adergneiss. The middle part of the dike consists of a dark kersantitic rock composed of — hornblende, biotite and feldspar. This rock has been broken | SE EV ' IT NET Ah Fig. 6. A Composite dike-breccia in adergneiss, SE-shore of Sviatoy Noss. Photo P. Eskola. (The hammer-shaft is 42 cm in length.) a sd VTLy up into fragments which, somewhat rounded by resorption, are enclosed in the aplite. 3 At point 26 an amphibolite-migmatite is invaded by a grey minette dike. This cuts across the aplite dikes and the granite veins of the migmatite, but it is itself inter- ; sected by other narrow (2cm in bredth) dikes of aplite. The minette, examined under the microscope was found to consist of plagioclase (Abg,), biotite, quartz and a little microcline. Nå inr a Se NE IG Jes rar NGT AA JL abe LAS SYN RR ESS ÅN Å AA ' LA Ä 30 Pentti Eskola. (LXIIE a migmatite with ek and such mixtures are - > intersected by the aplite. The mig- matite is also invaded by dark dikes, Så some of which are older and other younger than the Pr dikes (fig. DR S At point 38 Å occurs a differan - tiated dike whose breadth is åppr. 20 Sd meters. This dike invades amphibol- - ite-adergneiss in a north-southerly SS : direction obliquely to the strike of 6. å the latter and in a vertical position. RV The western border is made up of =: a dark-coloured hornblende-gabbro which passes gradually into a hornblende-granite, on the 3 eastern side. Close to this dike in W there is a parallel 23 simple dike of the same kind as the most femic portion of d Nå the differentiated dike. | ; 9 At point 42 A a dark dike intersects coarse-grained — 2 graphite-limestone. This dike-rock consists of biotite, pale brown hornblende, colourless pyroxene and smaller amounts — d of plagioclase (labradorite). : 3 Near Mys Besimyanniy composite dike-breccias of kerså Es santite and aplite invaded the porphyritic granodiorite, AS a 4 already mentioned. Both the aplite and the kersantite were subjected to chemical analysis. That of the aplite was quoted - SR above (p. 26). The analysis of the FREE made by the writer, gave the IeToRna result. ; KE Ne Fig. 7. Dikes in migmatite SE-shore of SE Noss. va bust, AE 08 RSS SR RAN Mn é , wj 1 SE CRT A N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 31 | ov Mol. Mol. NEO ROTADE —--& " sr -==—=————>— jpb py. wnwnnwnnninhlhNhhnxnmnhllbmlnnlxKeevv;=,=""U SiO, 51.03 | 851 | 57.93 | Orthoclase .....« (EN A1,O; | 20.15 | 198 13:48 | Albite ; us. Ca PE 11:49 Fe,0, 2.02 | . 13 Anorthite..... » « 27.80 FeO 7.48 | 104 9.06 | Nephelite . ... : ... 1:42 LE NA2 MnO 0.21 3 Z sal 78.91 MgO LST 44 2.99 rg 1.63 CAO =729 130 |-8.85 | Diopside. RR SR :Na,O 4,24 68 4,63 FeSiO, 1.19 P = 13.19 K,O 2.80 | 30 2.04 Mg,SiO, 2.731. : Olivine .. do. a. j9.87 TiO, 1.20 | 15 1.02 Fe,SiO, 7.14 PIO5 0:76 Hl te ARN 020 20; Magnetite RR WENBRB0 H,0O 0.42 SEARLIM EINES alise sket ös 2.28 APablle idol Rre 1.68 A = "1.68 | id Zz fem 20.17 | 98.08 [99.37 | 4 [100-00 ] Andose (II, 5, 3, 4). The Osann figures: S RA C F a C i n N 35.95 >-6.067 6.81" 14:09 5.0 —- 5.0 10:0: 76:94" 0-82 The mineral constituents, named in the order of their abundance, are plagioclase, biotite, hornblende, titanite and apatite. The plagioclase builds up a little more than a half of the rock-mass; the biotite is only a little more abundant than the hornblende. The plagioclase occurs in the form of isometric - polygonal grains and shows rarely a few twinning lamellae. The extinction angle in sections | PM is about + 6”, the refringence being considerably high as compared with the Canada balsam. The composition should therefore conform to Ab;,. No potash feldspar is actually present, the potash content entering into the biotite in which the ratio K.O: 32 Pentti Eskola. (LXTII Si0, being less than 1:6 causes the appearance of norma- - ; tive nephelite, as is often the case in rocks free from quartz but with abundant biotite. The hornblende is of the common green variety having rather dark colours. The biotite also has dark colours (mark the high ratio FeO : MgO in the rock). The mica is idiomorphic towards the hornblende and both Sd are idiomorphic towards the plagioclase. The structure of this dike-rock is really, so far as I can judge, quite identical with that of many crystalloblastic amphibolites. The pave- ment-like fabric of the plagioclase mass is the same, so is also the idiomorphism or, as one would call it in a meta- morphic rock, the idioblasticity, of the mafic constituents, and the similarity is still accentuated by a marked parallel arrangement of most of the hornblende and biotite crystals. The average grain is about 0.3 millimeters in diameter. = The crystals of hornblende and biotite are somewhat larger than the feldspar grains. All the constituents are perfectly : unaltered. This rock has here been called kersantite to which it belongs according to the definition of Rosenbusch, being a melanocratic dike-rock of the granite-dioritic series characterized by the mineral combination plagioclase-biotite. The additional hornblende defines it as a horn blen de- 2 kersantite. It may be remarked that in its composition this horn- blende-kersantite differs from all the typical kersantites whose analyses are quoted in Rosenbusch” text-book ?) in being considerably richer in alumina. Mineralogically this deviation means a higher proportion of feldspar in the pre- sently discussed rock. Chemically it is more closely related to many gabbroid rocks and especially to the essexites, but in its geological occurrence it is no doubt a typical dike-rock. There is still another chemical feature distinctive of the rock under consideration: its high proportion of ferrous oxide as compared with the amount of magnesia present. Generally such a feature is characteristic of granites and other 1) H. Rosenbusch, »Elemente der Gesteinslehre>», p. 235 (1901). 22 A N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 33 highly siliceous rocks, but in this case the FeO: MgO ratio is higher in the basic dike-rock than in its granodioritic country- rock, from whose magma it has obviously been derived. It was suggested from the field observation concerning this and other similar dikes that the aplite and the kersantite should represent a diaschistic suite. We may therefore com- pare the analyses of both dike-rocks with that of the surround- ing granodiorite. I is the analysis of the aplite, II that of the kersantite and III that of the granodiorite, all from Mys KR IV is the average of 5 parts of I and 3 parts of II. IE II. III. IV. Sl035 738015, 103 05,065 05.32 KSO ESS OLSEN dOBÅ LG KesOs- 057 2.02 1.74 1.11 FeO 1.22 7.48 2.51 3.57 MnO = 0.05 0.21 0.08 0.11 MORA ST 1.31 0.92 CaO 1.18 7.29 3.28 3.47 Na0 = 3.14 4.24 4.16 3.55 KROAT 0:20 2.80 4.82 4.96 TiO. 20:33 1.20 0.63 0.66 12 AR NS AS ANN Os 0.26 0.36 30: 0:33 0.42 0.57 0.36 100.387 99.37 —100.26 100.24 The ratio 5: 3 was chosen, because a simple calculation showed the aplite and kersantite, if mixed in this proportion, to give approximately the same percentage of silica as that of the granodiorite. It is now quite astonishing to find a very close agreement in all the other oxides also. Greatest disagreement is found in the iron oxides, but the deviation in the ferric oxide is opposite to that in the ferrous oxide. The diaschistic nature of these and other dike series thus seems obvious. But the relations must not be under- stood as if the average composition of all the existing dikes should be identical with that of the granodiorite-granite- ; 3 FE NE Sk 2 a 1 NOEN LEN GER SN SNES SN 34 Pentti Eskola. (LÄTT mass: the number and quantity of the melanocratic dikes is quite trifling as compared with the aplites. No less difficult = to explain is also the frequent recurrence of the lampro- phyric and -aplitic intrusions observed in so many places. The composition of the lamprophyres appears to be variable = and not conformable to the typical dike-rock families. = The dike-rocks on Sviatoy Noss thus offer many inet teresting features, but the observation does not give any — positive evidence, as to the mode of origin of the diaschistie - rocks. Further discussion therefore seems unnecessary, until more knowledge about the physico-chemical Progesren At such differentiation. is arrived at. SS The pegmatite. Common pegmatite and its contact- products. Common pegmatites are called all such pegmatites which have microcline-perthite, quartz and mica as their main constituents. Such pegmatites are found in every part of Sviatoy Noss and penetrating all the other kinds of rocks. In the fine-grained aplite-like granite forming a zone around the great granite mass the pegmatites often EN å a zonal structure, elongated crystals of feldspar protruding - from the salbands in a parallel Hg towards the : a central parts. är Pegmatitic Scertod td or NV veins s similar to those and larger masses of aplitic granites invading the schistone oc rock and also in such masses which are themselves dike- formed. The pegmatites intruded in the coarse-grained = granitic rocks are more independent in their occurrence than Ey those in the aplitic granites and generally have sharp boun- = daries. The central zone of the dikes often is made up of quartz. 'Orthite in the form of minute crystals of the typical shape is fairly common, though never : Tönne in any Targa 5 quantities. i | "Ry RER ; SÅ SP pM 22 FTSE SR ft + TV SUP TI . A N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 32 Within the area of mixed rocks on the southwestern shore of the peninsula pegmatites also occur, though not in any great frequency. The rock is composed almost exclusively of potash feldspar and quartz, often in graphic intergrowth, and with irregular lumps of magnetite, which seems to be a regular constituent of the pegmatites in this part of blie peninsula. Near topogr. point 7 a limestone mass is penetrated by numerous dikes of aplitic pegmatite at whose contacts a wall of contact minerals has been developed. Nearest to the pegmatite occurs a zone of dark hornblende and thereafter another zone of light green or nearly colourless diopside in contact with the limestone. Around this compact wall of the lime-silicate minerals a zone of limestone is still impreg- nated with numerous rounded grains of the pyroxene. At places there is, between the pegmatite and the hornblende zone, a third compact zone of scapolite. In part the pegma- tite dikes are very narrow, being nevertheless surrounded by exomorphous contact zones of a considerable thickness, often thicker than the pegmatites themselves. And where the dikes taper out, their continuation is formed by fissures which are still surrounded by the same contact zones, the scapolite wall forming their central part. The pegmatite dikes have thus graded over into scapolite dikes. This phenomenon gives clear evidence of iron- and magnesium- silicates being transferred from a magma to its country-rock, as is postulated e. g. by the theory of the skarn formation in limestones by the influence of the granite magma. Such contact walls are, however, of quite an exceptional occur- rence. By far the greater part of the pegmatite dikes inter- secting limestones have sharp boundary lines without any: contact minerals whatever. At point 7, near the above-named occurrence of scapolite, the limestone encloses pyroxene in the form of large lumps, the structure of the pyroxene mass as well as the character of the mineral, which is a nearly colourless diopside, being quite the same as in that occurring at the contact wall of the pegmatite dikes. More irregularly formed inclusions occur 36 Pentti Eskola. (LXIT in the mylonitic limestone on the shores of the Tshivirkouy Bay, e. g. at the Molodost Bay and on the isle of Kyltyghey. Here were found inclusions of ordinary pegmatite as well as pyroxene- and amphibole-rocks, and also fragments of a scapolite-rock that closely resembles the rock of the »scapo- lite-dike» forming the continuation of the pegmatite dike near point 7. The last-named inclusions of pegmatite or its contact products in the mylonitic limestone could be easily under- stood as simple fragments of dike masses which have been broken up at the same time that the mylonitic structure of the limestone originated. It is remarkable, however, that at places there are contact walls or reaction rims around the ends of such broken up fragments. ; Those inclusions occurring in the coarsely crystalline limestone cannot be interpreted in the above-named way as there is no mechanical deformation bearing evidence of movements in late geological times, posterior to the final recrystallization of the rock masses. These inclusions are no more simple results from the influence of the contact action of the neighbouring granite masses, as at the imme- diate contacts of the latter towards the limestones there are no contact minerals whatever, and it does not therefore seem probable that they might have given rise to pneumato- lytical products at a distance. It was observed, moreover, that the occurrence of the inclusions is not in any way de- pendent on the presence of granite masses: they are not at all commoner in the vicinity of the granites than else- where. It remains to explain these fragments to be formed at an earlier stage, before the perfect chemical metamorphism and the granitization of these formations took place. Crustal movements might at that time have caused mechanical de- formations and breaking up of rock masses. Movements, of course, continued still at the same time as the intrusion of granites' and pegmatites had begun. We may therefore assume that those pegmatites occurring in the form of frag- ments in the limestone have originated at the earlier stages få FER PYroFCES 2 SR ) A N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. JA of the great ancient orogenetic period, before the main part of the chemical metamorphism had been performed. Diopside-pegmatite. On the western shore of Sviatoy Noss, pegmatites as well as other rocks were observed mainly as boulders of the shore- drift. Here was observed a special type of pegmatite which was later found to be largely distributed and characteristic of the Transbaikalian crystalline region, being found in all the tracts where limestones have an extensive occurrence. This pegmatite is characterized by the occurrence of diopside as the main mafic mineral. In the following it will therefore be designated as diopside-pegmatite. Another very invariable and often abundant constituent is titanite. When first observed as blocks among those of the sviatonossite, this pegmatite was considered as belonging to the latter rock, but later observation proved this supposition erroneous: the diopside-pegmatite is really confined to the limestone and to the vicinity of the limestone masses. It occurs in all parts of the peninsula, but it does not form very large masses. Nor are all pegmatites intruded in limestone areas diopside- bearing: the common pegmatites are much commoner also in the limestones. The mineralogical composition of these pegmatites was found to be remarkably constant, though there is much variation in the quantitative proportions between the differ- ent minerals. Often plagioclase is the most abundant constituent. Studied microscopically, the plagioclase was found to have the composition of oligoclase, about Abgp. This mineral then forms the largest individuals, and m i- crocline is present as a fine-grained mortar between the plagioclase crystals. The latter also contain spot-like inclu- sions of microcline as a kind of antiperthitic intergrowth. In other cases the microcline is the dominant feldspar, form- ing the largest individuals. It is of a gray colour. Quartz is usually only present in small amounts. The diopside is greenish gray or nearly colourless, often having the pro- 38 . Pentti Eskola. = = (CSA perties of the baikalite from the south shore of Lake Baikal, which, in fact, occurs in quite the same kind of pegmatite. 2 This mineral often shows fine euhedral development. The = end planes usually are ill developed or wanting. All around = the pyroxene there are smaller prisms of a pale green 2 amphibole in which the angle c:y = 20 and o=B = yellowish