Hornblende-biotite Granite Research Articles

Geochronology of archean gneisses in the Lake Helen area, Southwestern Big Horn Mountains, Wyoming

Abstract The RbSr and UPb methods were used to study gneisses in the 7 1 2 - minute Lake Helen quadrangle of the Big Horn Mountains, Wyoming. Two episodes of magmatism, deformation and metamorphism occurred during the Archean. Trondhjemitic to tonalitic orthogneisses and amphibolite of the first episode (E-1) are cut by a trondhjemite pluton and a calc-alkaline intrusive series of the second episode (E-2). The E-2 series includes hornblende-biotite quartz diorite, biotite tonalite, biotite granodiorite and biotite granite. A RbSr whole-rock isochron for E-1 gneisses indicates an age of 3007 ± 34 Ma (1 sigma) and an initial 87Sr/86Sr of 0.7001 ± 0.0001. UPb determination on zircon from E-1 gneisses yield a concordia intercept age of 2947 ± 50 Ma. The low initial ratio suggests that the gneisses had no significant crustal history prior to metamorphism, and that the magmas from which they formed had originated from a mafic source. A RbSr whole-rock isochron for E-2 gneisses gives an age of 2801 ± 31 Ma. The 87Sr/86Sr initial ration is 0.7015 ± 0.0002 and precludes the existence of the rocks for more than 150 Ma prior to metamorphism. The E-2 magmas may have originated from melting of E-1 gneisses or from a more mafic source.

Rapakivi granite, anorthosite and charnockitic plutonism

THE close association of massive anorthosite and charnockitic granitic rocks is well documented1. Rare earth element (REE) investigations2–4 have indicated that the massive charnockite (mangerite) associated with anorthosite is not comagmatic with it but represents a distinct magma fraction. Bridgwater et al.5 describe a charnockitic mantle around a plutonic suite of norites, monzonites and rapakivi granites from the Kap Farvel district of south Greenland which they consider to represent a parallel situation to the anorthosite–mangerite association. They believe contact metamorphic processes accompanied by anatexis produced the charnockitic phases. The charnockites are, therefore, dependent on the associated igneous suites but are not comagmatic with them. Duchesne et al.7 have investigated the REE content of monzonoritic rocks considered consanguineous with the anorthosites of the South Rogaland district of southern Norway. The absence of any Eu anomaly in the monzonoritic rocks, as compared with the marked positive Eu anomaly found for the anorthosites, places severe constraints on any postulated common parent magma for this rock association. A magma produced by partial fusion of upper mantle Kaersutite has been suggested7. A link between anorthosite and rapakivi granite6 has been proposed, rapakivi granite being a high level crystallisation related to and contemporaneous with anorthosite emplacement at greater depth. From their study of the Pikes Peak Batholith, Colorado, Barker et al.8 proposed a composite model for the origin of the rock spectrum gabbro–anorthosite–syenite–granite (including rapakivi granite). Convecting, mantle-derived, alkaline olivine basalt magma is believed to produce a quartz–syenitic magma from K2O-poor lower crustal rocks which in turn reacts with the granodioritic to granitic rocks of higher crustal levels to produce biotite and biotite–hornblende granites. Anorthosite is a precipitate phase from intermediate liquids. Our results from an REE investigation of plutonic charnockitic rocks and associated granites from south-west Sweden suggest that rapakivi granite may be associated with the charnockites satellitic to anorthosite rather than with anorthosite itself.

Two ?S-type? granite suites with low initial 87Sr/86Sr ratios from the New England Batholith, Australia

Two “S-type” (pelitic) granite suites from the New England Batholith, N.S.W., have Upper Carboniferous ages, indicating that they predate by 40 m.y. the intrusion of hornblende biotite granites, and are the oldest plutons of the batholith. Mineralogically and geochemically both suites have “pelitic” characteristics, one suite containing an Al-rich biotite, muscovite and cordierite, the other an Al-rich biotite and rare pyrope-almandine garnet. Low initial 87Sr/86Sr ratios of 0.706 for both suites probably reflect the volcanoclastic nature and young age of the sedimentary source of these granites at the time of melting. The age of the suites coincides with the last stages of (Andean type?) volcanism along an andesite/dacite volcanic chain to the west, suggesting an origin for the “S-type” granitic magma by partial melting of deformed sediments marginal to a continental region.

The Pikes Peak batholith, Colorado front range, and a model for the origin of the gabbro—anorthosite—syenite—potassic granite suite

This study of the Pikes Peak batholith includes the mineralogy and petrology of quartz syenite at West Creek and of fayalite-bearing and fayalite-free biotite granite near Mount Rosa; major element chemistry of the batholith; comparisons with similar postorogenic, intracratonic, sodic to potassic intrusives; and genesis of the batholith. The batholith is elongate in plan, 50 by 100 km, composite, and generally subalkalic. It was emplaced at shallow depth 1,040 m. y. ago, sharply transects its walls and may have breached its roof. Biotite granite and biotite—hornblende granite are predominant; quartz syenite, fayalite granite and riebeckite granite are present in minor amounts. Fayalite-bearing and fayalite-free quartz syenite, fayalite-biotite granite and riebeckite granite show a well-defined sodic differentiation trend; the less sodic fayalite-free granites exhibit a broader compositional range and no sharp trends. Crystallization was largely at P H 2O < P total; P H 2O approached P total only at late stages. Aplite residual to fayalite-free biotite granite in the north formed at about 1,500 bars, or 5 km depth. Feldspar assemblages indicate late stages of crystallization at about 720°C. In the south ilmenite and manganian fayalite indicate f O 2 of 10 −17 or 10 −18 bars. Biotite and fayalite compositions and the ‘granite minimum’ imply completion of crystallization at about 700°C and 1,500 bars. Nearby fayalite-free biotite granite crystallized at higher water fugacity. All types of syenite and granite contain 5–6% K 2O through a range of SiO 2 of 63–76%. Average Na 2O percentages in quartz syenite are 6.2, fayalite granite 4.2, and fayalite-free granite 3.3 MgO contents are low, 0.03–0.4%; FeO averages 1.9–2.5%. FeO/Fe 2O 3 ratios are high. Fluorine ranges from 0.3 to 0.6%. The Pikes Peak intrusives are similar in mode of emplacement, composition, and probably genesis to rapakivi intrusives of Finland, the Younger Granites of Nigeria, Cape Ann Granite and Beverly Syenite, Mass., and syenite of Kungnat, Greenland, among others — allowing for different levels of erosion. A suite that includes gabbro or basalt, anorthosite, quartz syenite, fayalite granite, riebeckite granite, and biotite and/or hornblende granites is of worldwide occurrence. A model is proposed in which mantle-derived, convecting alkali olivine basaltic magma first reacts with K 2O-poor lower crust of granulite facies to produce magma of quartz syenitic composition. The syenitic liquid in turn reacts with granodioritic to granitic intermediate crust of amphibolite facies to produce the predominant fayalite-free biotite and biotite-hornblende granites of the batholith. This reaction of magma and roof involves both partial melting and the reconstitution and precipitation of refractory phases, as Bowen proposed. Intermediate liquids include MgO-depleted and Na 2O-enriched gabbro, which precipitated anorthosite, and alkali diorite. The heat source is the basaltic magma; the heat required for partial melting of the roof is supplied largely by heats of crystallization of phases that settle out of the liquid — mostly olivine, clinopyroxene and plagioclase.

The Effects of Pressure, Temperature, and XH2Oon Phase Assemblage in Four Synthetic Rock Compositions

The variation in stable phase assemblage has been determined over a wide range of temperature, pressure, and water content for four compositions within the system $$CaAl_{2}Si_{2}O_{8}-NaAlSi_{3}O_{8}-KAlSi_{3}O_{8}-SiO_{2}$$. The compositions used were based on the average analyses of (1) hornblende-biotite granite; (2) hornblende-biotite adamellite; (3) hornblende-biotite granodiorite; and (4) hornblende-biotite tonalite as compiled by Nockolds. The variable parameters studied were pressure (2-8 kb), temperature (625°-1,200°C), and $$H_{2}O$$ content (0-20 wt %). The results are compared with recent vapor-saturated rock-melting experiments on similar natural materials, and the approximate position of the bivariant liquid which coexists with plagioclase, alkali feldspar, quartz, and vapor in the system $$CaAl_{2}Si_{2}O_{8}-NaAlSi_{3}O_{8}-KAlSi_{3}O_{8}-SiO_{2}-H_{2}O$$ determined. The study suggests that quartz resorption should commonly occur during the intrusion of granitic magmas in the absence of...

Compositional Variation in the Tunk Lake Granite Pluton, Southeastern Maine

The Tunk Lake granite pluton is exposed over a circular area of 70 square miles in southeastern Maine. It intrudes the Ordovician (?) Ellsworth Schist, the middle Paleozoic Bays-of-Maine gabbroic-granitic complex, and Middle Devonian biotite granite and quartz monzonite. From the margin of the pluton to the core, six concentric gradational zones are recognized: 1 magnetite-aegirine augite granite chill zone; II hornblende-aegirine augite granite; III hornblende granite; IV hornblende-biotite granite; V biotite granite; VI biotite quartz monzonite. Modal and chemical data for the pluton show regular compositional variation from margin to core. Quartz and oligoclase increase inward, whereas total feldspar, perthite, percentage of albite in perthite, and total mafics and accessories decrease. The dominant mafic mineral varies inward according to the series magnetite-aegirine augite-hornblende-biotite. SiO2 increases toward the core of the pluton paralleling the quartz variation, whereas Al2O3, Na2O + K2O, FeO + Fe2O3, and CaO decrease corresponding to the variation in total feldspar and total mafics and accessories. Emplacement of the pluton at an assumed depth from one to three miles occurred by piecemeal stoping along marginal zones of ring fracturing and by cauldron subsidence or stoping of large blocks in the central zones. The magma at the time of intrusion is assumed to have had a composition, indicated by a calculated average for the pluton, approximately equal to the biotite granite of zone V, a volatile content from 2 to 3 percent, and a temperature of 800° C. Crystallization began in the outer portions of the pluton with precipitation of magnetite and aegirine augite followed by sodium-rich alkali feldspar and quartz. Exsolution of feldspar was followed by the partial replacement of primary mafic minerals by hornblende and biotite. During crystallization, silica-rich aqueous fluids migrated from the margins to the center of the pluton. The marginal rocks were left quartz-poor, and feldspar- and mafic-rich. The core rocks became quartz-rich. In the central zones additional water lowered crystallization temperatures allowing oligoclase, hornblende, and biotite to form as primary minerals and also facilitated late-stage recrystallization. Recrystallization fluxed by hydrous fluids accounts for fine-grained intergranular material in the central granites and also for sheets, lenses, and irregular bodies of fine-grained rock throughout the pluton. The pluton is related to several circum-Atlantic groups of intrusions which are similar in size, shape, petrography, and emplacement. Mineralogical and compositional variations in the granitic rocks of the White Mountain plutonic-volcanic series of New Hampshire and the “Younger Granites” of Northern Nigeria are similar to trends observed in the zones of the Tunk Lake pluton. In the Tunk Lake body, the variations occur within a single, gradational intrusion with a relatively simple crystallization history which may closely parallel those of the White Mountain and Nigerian intrusive complexes.

Virginia Dale Precambrian Ring-Dike Complex, Colorado-Wyoming

The Virginia Dale ring-dike complex is a roughly circular granitic feature bout nine miles in diameter in the northern Front Range, straddling the Colorado-Wyoming boundary. The deeply eroded complex is a part of a large batholith of Sherman Granite, of Precambrian age, and discordantly intrudes metamorphic rocks. It consists of four main units: an outer granite ring dike, which merges to the north with the main Sherman batholith; a composite zone of metamorphic rocks and mafic igneous rocks; and two inner zones of quartz monzonite, comprising the core. The outer ring dike, about a half mile wide, consists of coarse equigranular hornblende-biotite granite, and the two core zones consist of porphyritic biotite quartz monzonite. Both the granite and quartz monzonite are facies of Sherman Granite. Between the outer ring and the core in the southern portion of the complex is a composite zone consisting of pre-Sherman metamorphic rocks, and of diorite, andesite, quartz monzonite, and hybrid rocks. These rocks show much evidence of brecciation, mechanical mixing accompanied by flow, and metasomatism. Field relations and petrographic analysis suggest that the diorites are of hybrid origin. Chemical analyses, petrographic data, and trend analysis suggest that granite in the outer ring dike of the complex is of composition slightly different from that of granite outside the complex, and that it crystallized at higher water pressure. The quartz monzonite may have crystallized at even higher water pressure than that of granite and in addition was contaminated by mafic material. Relatively high magma volatile contents, as well as hybridization, seem to be characteristic of central complexes, even where erosion has been as deep as in the Virginia Dale complex. Gravity data indicate that the core did not form by cauldron subsidence. Rather, it appears to be a dome emplaced along old ring fractures. The outer ring dike may be a collapse structure, although the batholith to which it is connected to the north may have been emplaced as large sheets.

A preliminary note on the syenitic rocks form the Osawa-gawa area near Matsumae-mati, Oshima Province, southwestern Hokkaido

The Matsumae district is situated in the southern parts of the Oshima Peninsula in Hokkaido. Acidic and basic igneous rocks occur in the present area as a small mass at some places, that is, Osawa-gawa, Araya-gawa and Matsukura-gawa. The rocks consist mainly of pyroxene-biotite-hornblende diorite, hornblende-biotite granite, hornblende gabbro and pyroxene peridotite. The related minor rocks such as pegmatite, aplite and fine-grained granite are found in the dioritic complex. The writer describes the petrographical characters of the syenodiorite and melanocratic hornblende syenite found in the dioritic mass of the Osawa-gawa, and gives some considerations on their genesis.

DISTRIBUTION OF RADIOACTIVITY OF GRANITIC ROCKS IN THE TAKANAWA PENINSULA ,EHIME PREFECTURE BY CAR-BORNE RADIOMETRIC SURVEY1

A car-borne radiometric survey was carried out to study the distribution of radioactivity of granitic rocks in the Takanawa peninsula, Ehime prefecture. The maintenance of equipment and the dynamic character of special ratemeter system used in the car-borne survey are discussed. The effect of weathering to the radioactivity of rocks and the geometric effect in the measurement of gamma rays are examined. The results of the car-borne survey revealed that the radioactivity is stronger in biotite granite than in hornblende-biotite granite, but the latter has strong radioactivity near the contact with the Ryoke metamorphic rocks. Auth.

DISTRIBUTION OF URANIUM AND THORIUM IN THE GRANITES OF CENTRAL KAZAKHSTAN

A total of 65 radiochemical uranium and thorium analyses of igneous rocks from the Hercynian Kylcha and Kuu granitic intrusions of Central Kazakhstan is presented. Data on leaching studies and on the uranium contents of mineral separates of Kylcha rocks are given. For comparative purposes, 14 uranium and thorium analyses of Zhirov's for Hercynian Shet granitic rocks are included. Petrographical data include general descriptions of the rocks, modal analyses, and six whole rock chemical analyses. Geologically the Kylcha biotite- and biotite-hornblende granites are determined to be early Hercynian in age whereas the leucogranites of Kuu and Shet intrusions are late Hercynian. Average thorium and uranium contents, and Th/U ratios for the three intrusions are respectively: 31 ppm, 4.2 ppm, and 7.4 for Kylcha granites; 42 ppm, 11 ppm, and 3.8 for Kuu granites; and 46 ppm, 10 ppm, and 4.6 for Shet granites. The uranium and thorium data are treated statistically by the authors and they conclude that there is a real difference between the distributions of radioactive elements in these early and late Hercynian granitic intrusions.--Z. Peterman.

The Petrology of the District around Bauchi, Northern Nigeria

The area studied lies around Bauchi in the northeast of Nigeria and within the basement complex of Nigeria. The rocks here consist of metasedimentary rocks, migmatites, granites, quartz monzonite, and diorite. These are disposed in an aureole with the diorite and monzonite forming a core which is surrounded successively by zones of a biotite-hornblende granite, a finer grained biotite granite, and migmatites. The rocks are described with some petrographic detail. An account of the study made of the myrmekite structure commonly associated with the feldspars of the granites and the monzonite is given; and the structure is suggested as evidence of microclinization by metasomatism. Some features suggested as evidence of probable origin by plutonic process are: the spatial distribution of the rocks, the gradational nature of the boundaries between the principal rocks, the presence of a northeast-southwest lamination common to all the rocks, the crystalloblastic outline of the microcline crystals in the different rocks and the presence of schlieren and such petrographic structure as the myrmekite. This plutonic process probably involves an early introduction of metasomatic microcline and quartz, and a later soaking of the granites by a ferrous-iron-rich migma to produce diorite and monzonite. The migma probably originated partly from the unstable material displaced from the zone of granitization above and partly from the zone of transformation to eclogite below. A similarity of the quartz diorite and monzonite to the Indian charnockite is suggested, and the problem of charnockite genesis is discussed in the light of the present investigation.

EMPLACEMENT OF THREE GRANITIC PLUTONS IN SOUTHEASTERN ONTARIO, CANADA

The manner and history of emplacement of three post-tectonic granitic plutons in Hastings and Peterborough counties, Ontario, were studied using lithological and structural mapping, petrographic studies (including modal analyses, chemical analyses, radio-metric and decrepitometric studies), and petrofabric studies. The country rocks consist of Grenville metasedimentary rocks which were folded in two stages along northeast-and east-southeast-trending axes respectively, before emplacement of the plutons. The elliptical (5 by 3 miles) Chandos Lake pluton is made up of a central body of foliated syenite-monzonite, surrounded by later foliated biotite granite grading to a granodiorite. Both components are steep-dipping, crosscutting, funnel-shaped bodies emplaced by forceful intrusion at the site of a pre-existing structural basin. The elongated body of the foliated Wollaston biotite-hornblende granite (6 by 1.5 miles) was forcefully intruded along the eastern flank of a regional anticline as several steep, east-dipping bodies of granite, separated by narrow screens of paragneissic country rocks, parts of which were granitized to a granite gneiss. The composite Deloro pluton (20 square miles) consists of an earlier crescent-shaped (on plan), forceful intrusion of perthite granite grading to syenite along the western border, followed closely by the intrusion of a smaller body of granophyric granite in the east. The biotite granite-granodiorite of Chandos Lake pluton and the Wollaston biotite-hornblende granite show unsystematic variations of the modal quartz (Q)-feldspars (F)-mafic minerals (M) and the quartz (Q)-potassium feldspar (Kf)-plagioclase (PI) proportions and of the α activity; assimilation of varied rock types by the granite magmas accounts for such variations. In the Deloro pluton assimilation of gabbro-diabase by the granite magma accounts for the syenitic western border, but magmatic differentiation by fractional crystallization caused a systematic trend of acidification toward the central and eastern parts of the pluton and the location of the Q-Pl-Kf plots of the perthite granites within the thermal valley of Bowen. The cooling of the plutonic rocks was accompanied by extensive recrystallization of quartz in all the plutons, formation of myrmekite and of albite rims on plagioclase in the Chandos Lake and Wollaston plutons, and segregation of albite from the perthites in the Deloro pluton.

The Geology of South-west Jersey, Channel Islands

South-west Jersey is formed of a granite complex emplaced in sediments of presumed Pre-Cambrian age. The granites, considered to be Armorican in age, are of four main types: (i) The marginal facies of an older medium-grained biotite-horn-blende granite in contact with the sediments of the St. Aubin area; (ii) the older biotite-hornblende granite which extends from Corbière to Noirmont; (iii) a younger micro-granite occupying the approximate centre of the area and (iv) a porphyritic granite surrounding the younger granite mass. Basic dykes of various ages traverse Pre-Cambrian sediments and granites alike in north-south and east-west directions. Mica lamprophyres are distributed sporadically. In the St. Aubin area the fine-grained marginal granite, characterised by its variable porphyritic texture and its contamination by the sediments, forms three lobes in slightly indurated Pre-Cambrian sediments. At Belcroute Bay, the type locality, the marginal granite is over a hundred and forty yards in width, but at La Carrière, in the north-west, it is only a few feet in width. The degree of contamination in the marginal granite decreases away from its contact with the Pre-Cambrian. The former is characterised by crystalloblastic textures; porphyroblastic oligoclase, perthite and quartz are commonly developed, and there is considerable micro-brecciation. Sedimentary xenoliths are abundant in the marginal granite and common in the main biotite-hornblende granite. Large xenoliths of altered Pre-Cambrian sediments, orientated approximately east-west, occur in a few localities. The origin of such xenoliths, and of other xenoliths within the granites, is discussed. The porphyritic granite, which is extremely variable in composition and texture, is considered to be altered older granite surrounding the younger microgranite masses. Micrometric and textural analysis of un-orientated specimens collected along traverses across the porphyritic granite (from the microgranite to the older granite) have revealed zoned variations in composition and replacement textures. The extent of alteration in the older granite round cupolas and domes of microgranite has been found to depend mainly upon the size of such microgranite masses. These zonal variations, which have been found both vertically and horizontally round a small microgranite mass at Portelet, are considered to be the result of cyclic metasomatism, both during and immediately after the emplacement of the microgranites.

BIOTITE CRYSTALS FROM THE MYOGO-HO, MIYAGI PREFECTURE, JAPAN

The biotite crystals described in this paper are found as an ingredient mineral of hornblende biotite granite exposed on Myogo-ho, Kawasaki-machi, Shibata-gun, Miyagi Prefecture. They are also contained abundantly in the sand derived from the weathered rock. These crystals occur as six-sided short prisms or plates. Hardness: 2.5-3. Colour: brownish black with brownish streak. Indices of refraction: α=1.620-1.621, β=1.658, γ=1.658-1.659. Further, they are characterized by the small optic angle ?? (-)2V ?? 0° ?? and weak dispersion (υ>ρ). A chemical analysis gave the following result for this mineral: SiO2 33.92, TiO2 2.91, Al2O3 13.88, Fe2O3 3.39, FeO 26.66, MnO 1.35, MgO 4.86, CaO 0.75, K2O 7.46, Na2O 1.70, H2O (+) 3.53, H2O (-) 0.16, total 100.57%. Moreover, the X-ray powder diagram of this specimen agrees with that of biotite rich in ferrous iron on its octahedral layer.