Pegmatitic Gabbro Research Articles

The origin and emplacement of the Serpentine Hill Complex, Western Tasmania

Abstract The Serpentine Hill Complex is an ophiolite dismembered by tectonic emplacement and subsequent faulting. It consists of ultramafics, gabbros, dolerites, and volcanics (mainly basaltic). The layered ultramafics (dominantly orthopyroxenite and harzburgite) and layered hypersthene gabbro are thought to be cumulates, and have been intruded by pegmatitic gabbro and microgabbros. Three types of serpentinite can be recognized in the western part of the complex, black serpentinites, green serpentinites, and sheared contact serpentinites. The black serpentinites are generally unsheared, consist essentially of lizardite, and have preferentially replaced olivine‐bearing ultramafics. The sheared contact serpentinites consist mainly of chrysotile and antigorite, and may have formed during tectonic emplacement. The formation of the green serpentinites (lizardite‐chrysotile mixtures) appears to have accompanied localized deformation which occurred after tectonic emplacement of the complex. It is postulated that...

ON THE TABITO COMPOSITE MASS IN THE ABUKUMA PLATEAU

The Tabito composite mass in the southern Abukuma plateau, consisting of a granitic body and small gabbroic masses, is studied petrographically and petrochemically by the present author. The granitic body is mainly composed of granodiorite and quartz diorite and is divided into three rock types, namely Myojin-ishi, Komuro and Iritabyuto. The Myojin-ishi type has prismatic hornblendes characteristically and often contains a small amount of cummingtonite and pyroxenes. The Komuro type does not contain cummingtonite and pyroxenes, and is intruded by many leucocratic veins derived from the Iritabyuto type. The Iritabyuto type is the most leucocratic of the three and contains dark inclusions ubiquitously. Four gabbroic masses consist mainly of hornblende gabbro with a small amount of cortlandtite and gabbro pegmatite. Modal analyses on 53 rocks and new chemical analyses on 41 rocks are given. As to modal compositions and variation trends on the SiO2-Al2O3, SiO2-MgO diagrams, there is a discontinuity between the Iritabyuto type rocks and other rocks. Dark inclusions are always more mafic than the host rocks.

Iron—titanium oxide minerals in the Series, Kap Edvard Holm, East Upper Layered Greenland

Summary Electron-probe data are presented for coexisting magnetite and ilmenite from seven gabbro cumulates and a gabbro pegmatite, and bulk FeO-Fe2O3-TiO2 analyses for ten magnetite-ilmenite assemblages. The oxides equilibrated at 580 to 720 °C and oxygen fugacities of 10−15·0 to 10−20·5 bar. Late-stage alteration, associated with growth of hydrothermal silicates, resulted in the breakdown of ilmenite to rutile and the dissolution of magnetite. Higher levels in the intrusion equilibrated at lower temperatures than deeper levels. All features described are consistent with the retention of the initial water content of the magma within the walls of the intrusion.

ON THE GABBROIC IN THE NORTHERN ABUKUMA MOUNTAINS

There are many gabbroic masses intruded prior to the emplacement of the older group granite in the Abukuma plateau. These intrusives are mainly hornblende gabbros with a small amount of cortlandite. Two representative intrusions, that is, Hayama and Utsushigatake‾bodies occupying about 3×2km. respectively, were described petrographically in some detail. Chemical and modal analyses were done on fifteen rocks including a hornblende gabbro pegmatite and two cortlandites. They are characterized by high Al2O3 and CaO, and relatively low SiO2 contents. Measured age of K-Ar dating on biotite from a gabbro is 86×106 years. This obtained age is apparently corresponding to that of the younger group granite.

PETROLOGY OF THE MUROTOMISAKI GABBROIC COMPLEX

The Murotomisaki differentiated gabbroic complex locates at the extreme point of the Cape Murotomisaki, Shikoku Island. This complex intruded into the Murotohanto Group of Eocene in age. The zonal arrangement of which complex shows the indication of the fractional crystallization with gravitational sinking. Principal rock types of the complex are as follows: chilled marginal dolerite, picrite gabbro, fine-grained hyperathene bearing olivine augite gabbro, medium-grained hyperathene bearing olivine augite gabbro, augite gabbro pegmatite and ferrogabbro. The pegmatitic rocks of the latest stage intruded as small veins. Anygrain of pigeonite or inverted pigeonite is not found, whereas a small amount of hypersthene is observable as internal precipitates. Iron concentration in olivine and in augite is conspicuous at the upper part of the complex. Ilmenite is the predominant opaque minerals through the course of fractionation and magnetite appears at the latest stage. Fractional crystallization of OLIVINE THOLEIITE to intermediate iron concentration under low vapour pressure and low oxygen partial pressure is studied on these bases.

Petrochemical study of the Kagura ultramafic body, Kitakami mountainland

Abstract: A small ultramafic body which is a member of the Hayachine ultramafic complex crops out at Kagura, Kawai district, central Kitakami mountainland. This Kagura body consists of serpentinite, serpentinized pyroxene peridotite, pyroxenite, hornblende hornblende pyroxenite or pyroxene hornblendite, hornblendite, pyroxene hornblende gabbro, hornblende gabbro and hornblende gabbro pegmatite. And a little rodingitic rocks occur as wein like form within gabbroic rocks. The petrochemical features of the Kagura body and discussed by means of the chemical analyses of thirteen representative rocks. It is seemed that all the rock types are derived from and ultramafic magma in the order listed above and are formed by the succession of noted below: peridotite dunite pyroxene peridotite hornblendite (largely liquid) hornblende gabbro pyroxene hornblendite (liq.+cryst.) pyroxene hornblende gabbro hornblende gabbro pyroxenite. (largely crystalline) hornblende gabbro pegmatite Some opinion on the chemical character of the rodingitic rocks which are formed by hydrothermal reaction is give.

ALKALIC PEGMATITES OF THE AFRIKANDA MASSIF

Paragenetic sequences observed in ore-bearing and barren pegmatites associated with alkalic ultramafic rocks are described. The Afrikanda massif is located in the western part of the Kola peninsula and is represented by nepheline pyroxenite. In its central part are ore-bearing pegmatites containing concentrations of titanomagnetite, knopite, schorlomite garnet, nepheline, pyroxene, and phlogopite. Processes of replacement are described and a comparison is made of geochemical features of these pegmatites as compared to granitic pegmatites, nepheline syenite pegmatites, and gabbroic pegmatites. --M. Russell.

PETROLOGY OF BASALTIC ROCKS IN PART OF THE LABRADOR TROUGH

Basalts and sedimentary rocks in the eastern half of the Labrador trough are profusely intruded by gabbro sills. In the Ahr Lake area, Quebec, where the igneous rocks were studied in detail, the gabbros are divided into normal gabbros, leopard rocks, and metagabbros. Metagabbros, which are confined to the eastern part of the area, are equivalent to normal gabbros except for two unusual varieties, leuco- and melanometagabbros. Normal gabbro sills have differentiated into olivine-bearing lower, and pegmatitic upper, portions. The pegmatite of one sill (albite gabbro pegmatite) has sodic plagioclase in contrast with andesine in other pegmatites and a mesostasis interpreted as devitrified glass. Glass formation is attributed to volatile deficiency in the magma, but development of sodic plagioclase may have been due to later metamorphism facilitated by the presence of glass. Leopard rock is a coarse-grained feldspathic gabbro spotted with 6- to 15-cm aggregates of altered plagioclase. Sills of leopard rock have narrow zones of medium-grained, sparsely prophyritic gabbro along their margins, and some are composite with intrusions of normal gabbro along their centers. Leopard rock is probably equivalent to normal gabbro with a high concentration of plagioclase, and it may have formed by partial melting of the mantle at high pressures or under conditions of moderately high water pressure. Magma with a load of suspended plagioclase may assume a flow structure upon intrusion such that plagioclase is concentrated in a central sheet and the margins are free of plagioclase clusters. Thus the marginal zones with few feldspathic clots are explained. Melano- and leuco metagabbros occupy the lower and upper parts respectively of two sills and are separated by a narrow gradational zone. Each also occurs separately. Melanometagabbro is predominantly amphibole but shows relic olivine structures; leucometagabbro is predominantly altered plagioclase. Sills of melano- and leucometagabbros cannot be due to normal differentiation, and their origin is uncertain. Petrographic, mineralogical, spectrographic, and chemical data for gabbros and basalts are given. Normal gabbros are tholeiites which show an iron-rich trend similar to the Skaergaard trend but with negligible alkali enrichment. Basalts of the area are equivalent to the parent magma of the normal gabbros, and differentiation was in situ . Low potassium and strontium are characteristic of the province. Trace-element behavior is similar to that in other tholeiitic provinces. Metamorphism decreases westerly across the Ahr Lake area from quartz-albite-epidote-biotite subfacies of the greenschist facies through quartz-albite-muscovite-chlorite subfacies to subgreenschist facies rocks. Local development of pumpellyite and prehnite in the latter may indicate the zeolite facies.

Ultramafic and mafic rocks of Shimo-Ina district, Nagano Prefecture

The Sanbagawa, Mikabu metamorphics and the Chichibu Paleozoics are diatributed in this district. Green rocks occuring in the crystalline schist, are composed of massive green rock and the green intrusive rock (the Mikabu intrusive), which include meta-diabase, meta-gabbro and amphibolite. Most of the ultramafic rocks concordantly intrude along the boundary between the massive green rock and the schist. They are mainly composed of dunite, banded diallage peridotite, diallage peridotite, fine-grained and coarse-grained gabbro, and are companied by gabbro pegmatite and albitite. Most of the ultramafic rocks are affected more or less by serpentinization and other metasomatic actions. Although these alterations are varied in different rocks, they are always characterized by the transformation of olivine to antigorite and of plagioclase to grossularite. Some gabbroic rocks are also altered into rodingite. Compared with the ordinary grossularite, the grossularite in these rocks has lower refraction indices (1.720-1.733) and smaller density (3.50) and lattice constant (11.88 A). From the properties, it is considered that the grossularite probably inculdes the hydrogrossular molecules.