Biotite Flakes Research Articles

Structure and Mode of Emplacement of Caribou Mountain Pluton, Klamath Mountains, California

The Caribou Mountain trondhjemitic pluton is one of several Jurassic-Cretaceous granitic intrusions in the central metamorphic belt of the southern Klamath Mountains, California. This pluton, emplaced wholly within metacherts of the pre-Cretaceous Stuart Fork Formation, is elongated in a north-south direction and has an area of about 7 square miles. Both eastern and western contacts dip eastward at steep to moderate angles parallel to metamorphic foliation in the country rock. Foliation is markedly deflected around the intrusion except at the pluton9s southern end where foliation is crosscut. Contacts are sharp, and chilled zones are absent. Hornfelses (hornblende-hornfels facies) border the pluton in an aureole as wide as 1500 feet. The intrusion is characterized by a conspicuous primary igneous foliation defined principally by subparallel alignment of biotite flakes. Magmatic foliation and arcuate, fine-grained, trondhjemitic sheets parallel to foliation define a dome, the core of which is located asymmetrically in the northwestern corner of the intrusion. Mesozonal emplacement of this pluton and the adjacent Middle Fork and Canyon Creek plutons was primarily effected by pronounced forceful shouldering aside of country rock. Orientation of primary magmatic foliation in the Caribou Mountain intrusion indicates upward and outward expansion to the northwest of the dome during emplacement, resulting from addition of crystal-charged magma at depth and accompanied by attenuation of flow layers along the flanks of the intrusion. A well-developed system of primary cross joints and aplite-pegmatite dikes oriented radially about the core of the dome indicates that expansion of the pluton continued during late stages of consolidation. Rocks within the pluton are slightly more alkalic and silicic toward the core; this chemical variation is represented by a lithologic variation from calcic trondhjemite to trondhjemite. A trend toward trondhjemitic rocks is generally characteristic of Jurassic-Cretaceous plutons in the Klamath Mountains and Sierran Foothills and contrasts with the calc-alkaline variation of Late Cretaceous granitic rocks of the Sierra Nevada and Southern California batholiths. Petrological differences in Californian granitic rocks may be related to temporal controls rather than to the spatial controls suggested by Moore (1959).

Ludlovian Biotite-Bearing Bands

AbstractAllogenic biotite flakes, together with fish and conodont remains, are concentrated into bands at particular horizons within Upper Ludlovian rocks of Woolhope and other Welsh Borderland areas; and are present but more scattered within the Aymestry Limestone and Dayia Beds. Metamorphic rocks of Malvernian type appear to have provided the source of the biotite.

The Sandstone and Limestone of the Nakanosawa Formation

The sedimentary rocks of the upper Jurassic Nakanosawa formation were studied in this paper., They are classified into following ones., 1) A (dark gray) sandstone., This is abundant in matrix minerals and biotite flakes, and the diameters of the sand grains are small., : (Graywacke and subgraywacke after PETTIJ0HN'S classification)., 2) B (white) sandstone., : This is characterized by the large diameters of its sand grains, and contains a tew matrix mineral and biotite flake., (Orthoquartzite after PETTIJ0HN'S classification)., 3) C (calcareous) sandstone., : This yields abundant lime matrix, and its sand greins show large diameters., 4) Oolitic limestone., 5) Black, dense limestone., The B sandstone was the product under the condition of stronger water current than the A sandstone., The C sandstone and oolitic limestone were formed, when the depositional environments showed the relative increase of the deposition of lime, under rather strong water current., The relation between the volume percentage and the diameter of sand grains of the sandstones, and those of ooid of the oolitic limestone were examined, and it is concluded that the depositional condition of lime matrix of oolitic limestone was the same with that of calcareous sandstone, and sericite-like mineral as matrix of the A and B sandstone were washed away more easily than lime as matrix of the calcareous sandstone and oolitic limestone under a certain current., Composite ooids, composed of small simple ooids, fossil remains, and lime matrix, were formed when they rolled about on the sea bottom., The sandstones of the Tochikubo formation and the Tomisawa formation, which are overlain and underlain respectively by the Nakanosawa formation, have a good deal of feldspar grains, and the lower part of the Nakanosawa formation contains more feldspar grains than the upper part., The writer concluded that the backgrounds of the depositional basin rose before the deposition of the Tochikubo formation, and the Nakanosawa age was that of planation, and after the deposition of the Nakanosawa formation the backgrounds rose again., The limestone of the Nakanosawa formation was formed by the decrease of the deposition of terrigenous materials in the age of planation., The deposition of the lower part of the Koike limestone (the uppermost part of the Nakanosawa formation), mainly composed of oolitic limestone, was under the influence of rather strong water current, and the upper part of the lime stone, consisting of much black, dense limestone and oolitic limestone, was formed at deeper sea bottom than the lower part.,

Petrology of a Wealden Sandstone at Clock House, Capel, Surrey

The topmost band of sandstone in the Weald Clay at Capel (6) is now exposed for 100 yards, and varies in thickness from 8 inches at the southern end of the face to 1 foot at the northern end. The stone is glauconitic and highly micaceous, and carries so much fresh biotite on certain bedding-planes that they are heavily darkened. Flakes of biotite and muscovite commonly reach 2 mm. in diameter, and occasionally 3 mm. No petrological facies like it is known in the older Hastings Beds outcropping to the south and south-east. The petrography of the sandstone therefore merits description, and some discussion of the problems which it raises.

A Note on the Origin of Basic Xenoliths in Plutonic Rocks, with special reference to their Grain-size

SUMMARY AND CONCLUSIONSIt has been suggested that the grain-size of coarse basic plutonic rocks may be reduced in one or all of three ways:— (1) If the temperature of the invading magma be sufficient, the reduction may be effected by the formation of a granoblastic structure. It is pointed out that recrystallized inclusions are found more often in basic plutonic masses, where the earlier crystalloblastic structures are not obliterated by superimposed hybridization. In 1933 (p. 156) the writer suggested that relict felspars with granular inclusions (or clouding) may be useful criteria of a recrystallization antedating hybridization. (2) The grain-size may be reduced by the development of highly poikilitic crystals of hornblende and/or biotite, and by the final disruption of these. Such crystals may be formed as poikiloblasts in the initial recrystallization; but more often they may be attributed to hybridization, and in this process hornblende plays an important rôle. (3) The grain-size may be reduced by the formation of granular masses of pyroxene or by criss-cross flakes of biotite during hybridization. Thomas and Campbell Smith (1932) and Nockolds (1932) have discussed the development of large felspars within the fine grained xenolith and its final incorporation in the acid magma, but it is not the purpose of the present paper to deal with this stage. It is hoped that this discussion may have indicated a means by which coarse basic plutonic rocks may become fine grained xenoliths, and it is finally urged that it is not always necessary to postulate a fine grained origin for these inclusions.

The Petrology of the Pennant Series, East of the River Taff

SUMMARYThe occurrence of iron carbonate as a part of the cement in the Pennant Sandstone is not extraordinary, when the large amount of entombed organic matter is taken into consideration. The presence of organic material and the products of decay, the so-called humus acids, acting as reducing agents, would aid in the formation of iron carbonate and prevent its oxidation. Much of this carbonate probably owes its presence to a metasomatic replacement of calcium carbonate. The granules of siderite may have been derived from rocks similar to those of the Lower Coal Series of South "Wales. The alkaline carbonates which hold the humic substances in solution would readily dissolve silica, thus becoming effective agents in the solution and redeposition of silica in the rock. A solvent of this kind would be formed by the impregnation of alkaline salts by carbonic acid.1 The occurrence of so much secondary silica in the rock is probably the result of a redeposition of silica from such a solution. No oxides of iron have been found in any of the numerous separations, either by means of heavy liquids or the electro-magnet; their absence is easily explained when we consider the strongly reducing environment of the deposition and consolidation of the Pennant Sandstone. The perfectly rounded globules of sphserosiderite may have been precipitated whilst the sandstones were being deposited, but more probably derived from the denudation of fireclays similar to those of the Lower Coal Series of South Wales.2 The dominant mineral chlorite, existing as perfect cleavage flakes, is entirely distinct from the secondary aggregates of chlorite in the rock, and was probably deposited in its present form. I have examined a powdered specimen of the Delabole Slates of Cornwall (Upper Devonian), and have found in it a large proportion of chlorite flakes, exactly like the detrital chlorite of the Pennant Sandstone. The very few flakes of biotite found show little trace of alteration. The paucity of heavy minerals indicates that the Pennant sediments had not a granitic source; they were mainly derived from pre-existing sediments which were rich in chert, had abundant chloritic material present, and yet were capable of furnishing fairly fresh biotite. Metamorphosed shales would account for the presence of cordierite and corundum.

The Metamorphism of the Carrock Fell Gabbro; with a Note on the Origin of the Sulphide Veins of the Caldbeck Fells

WITHIN the last two years a cutting has been made at the south-east corner of Carrock Fell, a few yards north of the village of Mosedale, for the purpose of obtaining road-metal. This has opened an interesting section near the margin of the intrusion. The rocks exposed comprise two varieties of gabbro and enclosed Eycott lava. Of the two types of gabbro, that next the lava is of the quartz-bearing variety, exhibiting the prevailing dark-green tint in the hand specimen, and carries large flakes of biotite. This is succeeded by the same type of gabbro, but in a highly metamorphosed condition, at once evident in the hand specimen, which is lighter in colour, primarily owing to the decomposition of the felspars.

A Note on the Petrology and Physiography of Western Liberia, West Coast of Africa

I. Petrology . By the kind permission of Sir Harry Johnston, G.C.M.G., I am enabled to make the following brief contribution to the geology of part of Western Liberia. The district described lies to the north and north-east of Monrovia, in Montserrado county, and includes the lower part of the basin of the St. Paul or Ding River. These notes are the result of a seven months' journey in the Republic. Taking the southern part first, it should be noted that garnetschists and various gneisses, for the most part greatly decomposed, are exposed in the neighbourhood of Whiteplains and along the Caresburg Road. In typical specimens of the former collected near Whiteplains, garnets are abundant, much cracked and very irregular in outline. Flakes of biotite are common, quartz is occasionally plentiful; and the felspar is, at least partly, an acid plagioclase. Phenocrysts are rare. Kyanite-schists are probably associated with these rocks, for the distinguishing mineral is common in the gravels of the streams, but it has not yet been traced to the parent rock. These garnet-biotite-granulites or schists form a well-marked group, probably distinct from the commoner gneisses with which they are associated. In the latter the occurrence of a monoclinic, and possibly also of a rhombic, pleochroic pyroxene is worthy of note. Quartz is common, and occurs typically in elongated lenticles. The Whiteplains garnetiferous rocks may be correlated with others petrographically similar, found between Suen and Arthington on the western bank of the St. Paul, and with garnet-graphitegneisses and tremolite-schists

VIII.—Notes on the Probable Origin of Some Slates

In the deposits previously described what takes place appears to be about as follows. The larger flakes of biotite undergo an alteration which largely produces epidote, and not improbably granular rutile, though this is not at all proved. Crystals of rutile very rarely result during the process. Such of the iron of the biotite as is not combined in the epidote (which is strongly coloured) is removed in solution, some of it being deposited again as limonite diffused throughout the deposit, and some taking part in the formation of the thin bands of ironstone, being precipitated as carbonate.