Gabbro Diorite Research Articles

Chemical composition of hydrothermal fluids buffered by veinand pegmatite-forming mineral assemblages: An experimental calibration at high temperatures and pressures

The Ning-Wu volcanic basin is an important part of the Middle-Lower Yangtze River Valley metallogenic belt in eastern China, where magnetite–apatite deposits are common. These magnetite–apatite deposits can be classified into two types. The first type includes the Washan, Taocun, Heshangqiao and Dongshan deposits, which are hosted in diorite porphyry and adjacent volcanic rocks. The second type includes Gushan, Baixiangshan and Hemushan deposits, developed at the contact between gabbro–diorite and sedimentary rocks. We used 40Ar–39Ar dating on phlogopite and U–Pb LA-ICP MS dating of zircon in ore-related diorite porphyry and gabbro–diorite to determine the timing of iron mineralization in the basin. The ages of the magnetite–apatite deposits such as Washan, Taocun, Heshangqiao Gushan Baixiangshan Hemushan deposits are from 130 Ma to 131 Ma. Thus the magnetite–apatite deposits in the Ning-Wu volcanic basin all formed at about 130 Ma, later than the skarn and porphyry Cu–Au deposits in the adjacent fault-uplift areas such as Edongnan, Jiu-Rui, Anqing-Guichi and Tongling clusters that are between 146 Ma and 135 Ma. Based on available petrological and geochemical data, the magnetite–apatite deposits in the Ning-Wu volcanic basin are interpreted to have formed under regional lithospheric extension and thinning, and ore-forming elements were derived from basaltic melts sourced from enriched lithospheric mantle and the crust.

Geology of the Annieopsquotch Complex, southwest Newfoundland

The Annieopsquotch Complex is an ophiolite that forms the Annieopsquotch Mountains of southwest Newfoundland. It contains rocks of the critical zone, gabbro zone (2.3 km thick), sheeted dyke zone (1.5 km thick), and pillow lava zone of a typical ophiolite. The zones trend northeast, face and dip southeast at approximately 50–70°, and are offset by faults.Cumulate rocks of the critical zone preserve graded layers, trough structures, and slump folds and locally are metamorphosed and deformed. The gabbro zone contains many textural varieties of gabbro, pegmatitic pods, layering, trondhjemite pods, and amphibolite near the base. It passes through a transition zone to a sheeted dyke zone that extends the full strike length of the ophiolite. Dykes trend northwest and are aphyric or plagioclase-phyric diabase. The pillow lava zone, besides pillow basalt, contains minor pillow breccia, hyaloclastite, and chert.The Annieopsquotch Complex is faulted against an Ordovician tonalite terrane to the northwest across the Lloyds River Fault and against the Victoria Lake Group to the southeast. It is cut by dykes and sills correlated with both these units. The complex is cut by two Late Ordovician gabbro–diorite intrusions and a granite of presumed Devonian age and is unconformably overlain by Early Silurian terrestrial sedimentary and volcanic rocks.Major-, trace-element, and clinopyroxene chemistry of the complex and other ophiolitic fragments between Buchans and King George IV Lake is typical of N-type MORB. These likely constituted one allochthon of Iapetus Ocean or marginal basin crust emplaced over the Ordovician continental margin of North America during the Taconic Orogeny.

Evaluation of and New Completion Recommendations for a Fractured, Crystalline, Igneous Reservoir, Texas Panhandle: ABSTRACT

In the Panhandle field of Texas, the lowest stratigraphic producing formation is a crystalline basement rock. The igneous rock is productive when it is naturally fractured sufficiently to hold hydrocarbons trapped in the same structure as the expansive Panhandle field. Research work to understand the crystalline reservoir better focused on the fracturing, mineralogy, and production potential of the basement interval. This trap is a true fractured reservoir; it has no more than 1% primary porosity in the nonfractured rock. The mineralogy of the host rock and fracture-filling minerals was established from drill and core samples. The host rock is a complex of granodiorite, dacite, and gabbroic diorite. Scanning electron microscopy, x-ray diffraction, and optical work confirmed the occurrence of at least 12 secondary fracture-fill minerals. Fluid inclusion analysis established formation temperatures for many of the minerals and allowed postulation of a detailed sequence of formation. The character of the fracture in differing host petrologies and the occurrence of clay minerals coating fracture surfaces were deemed to be critical factors in hydrocarbon production. New and specific enhancement treatments were designed for the subject reservoir, and these recommendations were used in a test well. Results from the test were favorable and appearmore » to confirm the ideas formed from this work. The new recommended treatments are less expensive than current practices and indicate potential for greater ultimate recoveries of oil.« less

Combined oxygen isotope – compositional studies of some granitoids from the Grenville Province of Ontario, Canada: implications for source regions

Oxygen isotopic compositions of whole rocks and coexisting quartz–feldspar pairs have been determined for nine pre-, and syn- to late-kinematic granitoid plutons in the Grenville Province of Ontario. These new data demonstrate that granitoid rocks (Algonquin, Mulock) in migmatite terrain of the Ontario Gneiss Segment possess normal δ18O values (<9.0‰), whereas mesozonal to epizonal plutons (Elphin, Coe Hill, Deloro, Barber's Lake) in the Central Metasedimentary Belt (CMB) are characterized by significantly higher 18O contents (δ18O > 9.0‰), in accord with previous results.In the Algonquin sodic suite, a gross covariance of δ18O with compositional indices is present, from 6.4‰, SiO2 = 50.5 wt. % (gabbro) to 8.7‰, SiO2 = 72 wt. % (trondhjemite), resulting from combined assimilation–fractional crystallization. Mafic members of the sodic suite are 18O enriched overall (5.8–7.9‰) relative to fresh tholeiites (5.7 + 0.3‰), implicating some 18O contamination of the protolith. The dispersion of δ18O values in the Algonquin potassic suite, from 4.3 to 9.3‰, is independent of composition and attributed to isotopic exchange with low-18O thermal waters during emplacement. Biotite–hornblende granite of the Mulock batholith is characterized by a limited oxygen isotope compositional range, where the average δ18O = 8.1 ± 0.5‰; δ18O correlates with SiO2 but not with the zonal distribution of Ba, Rb, and Sr abundances.The Union Lake quartz diorite (δ18O = 8.5 ± 0.1‰) and White Lake trondhjemite (δ18O = 7.3 ± 0.6‰) have oxygen isotope compositions comparable to those of other trondhjemitic suites in the CMB. A systematic enrichment of ~1.2‰ in the Union Lake pluton, together with enhanced Ca, Mg, Fe, and Sr, can be accounted for by assimilation of ~5% marbles and 10% amphibolites from the country rock. Uniformly high δ18O values of 11.5 ± 0.8‰ characterize the Elphin granite–syenite complex. The largest values (11.7–12.7‰) and lowest SiO2 (54–56 wt. %) are in the partially assimilated host gabbro–diorite complex, endorsing the presence of 18O-enriched source regions. The Cheddar biotite–hornblende granite, one of a population of intrusions within the alkalic belt of the western CMB, has a restricted isotopic span, where δ18O = 8.8 ± 0.9‰. An unusual concave rare-earth-element (REE) distribution may result from interaction with a heavy rare-earth -element (HREE) enriched volatile phase. The Coe Hill biotite granite (δ18O = 10.4 ± 0.4‰) is isotopically in compliance with other granites and syenites of the CMB. Covariance of δ18O and SiO2, in conjunction with smooth and continuous geochemical trends, is interpreted in terms of assimilation–fractional crystallization.Peralkaline granite of the Deloro pluton includes a hypersolvus phase with high, scattered δ18O values (9.1–11.8‰) and a subsolvus counterpart attributed to late influx of water that induced isotopic reequilibration toward a more constrained range (δ18O = 9.2–10.2‰). REE distributions of a calcic syenite phase are compatible with its evolution by fractional crystallization of a low-K tholeiitic magma, and the high-18O character (δ18O = 11.1–12.6‰) requires 18O enrichment of the protolith and (or) 18O contamination of the magma. Peralkaline rhyolitic volcanics, compositionally coherent with the Deloro pluton and possibly representing extrusive equivalents, possess significantly higher and more variable δ18O values, from 11.7 to 14.2‰; this is attributed to 18O enrichment during low-temperature exchange with thermal waters, superimposed on a primary high-18O magma. The Barber's Lake two-mica granite contains enhanced abundances of U (15 ppm) and Th (36 ppm) in conjunction with systematically elevated δ18O values (10.4 ± 0.5‰). Geochemical constraints are compatible with its evolution from a trondhjemitic magma, but the isotopically enriched nature requires extensive 18O contamination of the protolith and (or) magma. These nine granites variously retain "memory" of primary and (or) secondary features, including δ18O of the source region, covariance of isotopic and compositional parameters, and sporadically superimposed disturbance by exchange with thermal waters. During metamorphism, quartz and feldspar were systematically reset to high-temperature fractionations, but the extent of open-system exchange with rock reservoirs was limited.Despite some probable disturbance by metamorphism and the limited data available, O–Sr isotope systematics of the Grenville granitoids indicate that (1) high-18O granites from the Frontenac Axis were derived from in situ anatexis of Grenville Supergroup metasediments, (2) synkinematic granites were derived by mixing of a primary magma generated at a lower crustal (granulite facies) or upper mantle level with the fusion products generated by partial melting of the Archean–Early Proterozoic type metasediments, and (3) the tonalite–trondhjemite suite in this part of the Grenville Province was derived from a similar lower crustal or upper mantle primary magma by direct fractional crystallization.

Petrogenesis and evolution of Arabian felsic plutonic rocks

Two overlapping stages can be distinguished in the formation of the Arabian Shield: (1) an ‘island-arc stage’, between about 900 and 660 Ma, during which the formation and nature of plutonic magmas was controlled by subduction-related processes; and (2) a ‘post-accretion stage’, between about 700 and 550 Ma, which was dominated by granitic magmas derived by massive crustal fusion. During the island-arc stage, the earliest plutons were calcic gabbro—diorite or gabbro—diorite—tonalite—trondhjemite complexes. These were superseded by calc-alkaline gabbro—diorite—tonalite—granodiorite complexes, some of which contain small volumes of monzogranite, and complexes composed predominantly of quartz diorite and tonalite. These plutonic complexes are analogous to intrusions emplaced in contemporary, immature to mature island arcs, and represent mantle (gabbro—diorite) and crustal (tonalite—trondhjemite—granodiorite) fusion products modified by fractionation processes. The post-accretion stage was characterized by a great diversity of rock-types reflecting complex interaction of mantle and crustal fusion products. Crustal fusion produced voluminous granitoid (predominantly granitic) magmas whose composition was mainly determined by the nature of the protolith and the degree of melting and differentiation. Fusion of primitive, island-arc protoliths produced low-K, tonalitic melts, whereas more evolved island-arc and continental-crust protoliths yielded moderately potassic granites and granodiorites. Re-fusion of refractory crust from which a granitic melt had already been extracted produced low-Ca, alkali-feldspar granite and alkali granite. Reaction-melting, involving mantle-derived mafic magma and granulitic crust, produced intrusions of gabbro—anorthosite—syenite and monzodiorite—monzonite.

Geology and petrology of Proterozoic mafic dikes, north-central Minnesota and western Ontario

Proterozoic diabase and gabbro dikes (~2120 Ma old) form a major northwest-oriented swarm extending about 300 km from the Mesabi iron range in Minnesota to the vicinity of Kenora, Ontario. The dikes were emplaced into Archean crust at about the time that the Early Proterozoic basin of the Lake Superior region was opening by rifting, and the swarm and the basin may therefore be tectonically related. The dikes are overlain unconformably by the Animikie Group, the upper sedimentary sequence in the Proterozoic basin in Minnesota, but may be approximately coeval with mafic volcanic rocks in the pre-Animikie Mille Lacs Group. A two-stage tectonic model involving (1) regional right-lateral crustal shear in the late Archean and (2) hot-spot rifting in the Early Proterozoic is proposed to account for the swarm.The dikes are iron-rich quartz tholeiites that are differentiated toward dioritic compositions. Late alteration to hydrous phases, including blue–green amphibole, chlorite, and sericite, together with lesser amounts of prehnite and epidote, is ubiquitous but variable in intensity, and is regarded as a deuteric phenomenon. The interior portions of some large dikes are compositionally layered parallel to contacts; the layers differ from each other in the proportions of primary hornblende, clinopyroxene, and plagioclase and thus range in composition across the gabbro–diorite boundary.Chilled margins of the dikes contain flow-aligned phenocrysts of plagioclase, clinopyroxene, titanomagnetite, and ilmenite. The clinopyroxenes occur as three morphotypes that have distinct compositions, indicating a complex intratelluric history. The dike magma was emplaced into cool Archean crust at an inferred temperature of about 1085 °C and was quenched in a matter of minutes at the dike walls. Complete solidification at the centers of dikes wider than 100 m appears to have taken more than 40 years.

Geochronology of the Uasilau-Yau Yau porphyry copper prospect, New Britain, Papua New Guinea

K-Ar ages for the three major intrusive episodes of the Uasilau-Yau Yau intrusive complex have been determined. Group 1 rocks (gabbro and quartz diorite) are at least 30 m.y. old. Group 2 intrusions (quartz diorite, tonalite, and granodiorite), which form the main volume of the complex, were emplaced as a number of comagmatic phases at about 28.4 m.y. Intrusion of group 3 tonalite at about 23.5 m.y. was followed closely by hydrothermal alteration and copper mineralization. Emplacement of quartz-feldspar porphyry took place after mineralization. Igneous activity in the Uasilau-Yau Yau intrusive complex spans a period of at least 6.6 m.y. It is suggested that all porphyry copper mineralization in New Britain may be the result of a late Oligocene igneous event.

Geocheemical variations in a suite of granitoids and gabbros from Southland, New Zealand

The Longwoods Complex of Southland, New Zealand is part of an extensive terrane consisting of intrusives, volcanics, and sediments, which outcrops in the southern and north-western portions of the South Island. This terrane represents a volcanic arc which was active from Permian to Jurassic times (Grindley, 1958; Challis, 1968, 1969; Coombs et al., 1976). Between Pahia Point and Oraka Point on the southern coast of the South Island a section across the Longwoods Complex is well exposed and intrusives ranging in composition from ultrabasic cumulate rock, high-Al gabbro and gabbroic diorite to quartz diorite and granite outcrop. Two models have been considered for the origin of the rocks of the Pahia Point-Oraka Point section: (a) the rocks constitute one suite, the members of which are related by a crystal fractionation process; (b) the rocks constitute two suites which are not directly related.

The Moruya Batholith and geochemical contrasts between the Moruya and Jindabyne suites

Abstract Rocks of the Moruya Batholith range from gabbros and gabbroic diorites through quartz diorites and tonalites to granodiorites and rare adamellites. The gabbros and gabbroic diorites appear as small, early bodies intruded and enclosed by quartz diorites and tonalites. These early gabbroids are petrographically and chemically distinct from the granitoids. The latter occur as a meridionally‐oriented sequence of nine separate plutons. Mafic xenoliths are most abundant in the quartz diorites and tonalites; they are petrographically similar to their host granitoids and are chemically a more mafic extension of the variation in granitoid compositions. The various granitoid bodies are considered to be derived from similar source rocks, with the xenoliths representing modified material relict from partial melting of that source. Comparison of chemical data from the Moruya granitoids with those of the I‐types of the Jindabyne Suite in the Kosciusko Batholith, shows that the potassium content is indistinguis...

Petrochemistry of the Sakainokami plutonic body, Kitakami mountains, northeastern Japan.

Sakainokami plutonic body is one of the many Cretaceous intrusive bodies in the northern Kitakami mountains. The present study revealed that the plutonic body is composed of three rock types, namely: type A, type B and type C, which have been emplaced successively in that order. Types A and C are further subdivided into three and two subtypes, respectively, as follows: Type A Subtype A-1: Biotite-bearing olivine augite hornblende gabbro Subtype A-2: Biotite, hypersthene-bearing augite hornblende gabbro Subtype A-3: Augite-bearing biotite hornblende gabbro Type B: Augite-bearing biotite hornblende gabbro—diorite Type C Subtype C-1: Biotite hornblende monzodiorite—granodiorite Subtype C-2: Biotite hornblende granodiorite. Within subtypes A-2 and A-3, two groups differing in the crystallization sequence of plagioclase and hornblende are identified. In one group, hornblende has crystallized earlier than plagioclase and there is an accumulation of more hornblende than normally. In the other group, plagioclase has crystallized earlier than hornblende and there is an accumulation of opaque minerals in certain portions. The former group is designated as group h and the latter is designated as group p. Without considering the presence of these groups the plots of type A rocks are segregated in accordance with these groupings three different trends emerge. These trends are designated as trend I, trend II and trend III. Trend I represents the case where there is accumulation of hornblende; trend II, where there is accumulation of opaque mineral; and trend III, representing the calc-alkaline trend.

Time of Folding of the Yahgan Formation and Age of the Tekenika Beds, Southern Chile, South America

The marine Lower Cretaceous to Upper Jurassic(?) Yahgan Formation of southern Chile was orogenically folded between late Early Cretaceous and middle Late Cretaceous time. Biotite and hornblende from diorite bodies intrusive into the folded Yahgan Formation have been dated at 79 ± 5 m.y. by the Rb-Sr and K-Ar methods, thus setting the younger limit to the age of folding of the formation. Hornblende from greenstone at Bertrand Island, which is considered contemporaneous with deposition of the Yahgan Formation, but recrystallized during the folding of the sedimentary sequence (Katz and Walters, 1966), has a K-Ar date of 92 ± 4 m.y. At Burleigh Peninsula, the Tekenika conglomeratic beds, generally accepted as Jurassic in age, are considered as Late Cretaceous to early Tertiary. Paleontologic evidence and a K-Ar total rock date of 116 ± 5 m.y. for a boulder of dioritic gabbro from the Tekenika beds support this stratigraphic revision.

The Lakeview Mountains Pluton, Southern California Batholith Part I: Petrology and Structure

The Lakeview Mountains pluton, a part of the southern California batholith, is a steep-walled body with a tear-shaped ground plan, exposed discontinuously over an area of 100 to 130 km 2 . It is located at an abrupt local deflection of the regional structural grain and is concordant with planar structures in the enclosing rocks. These are gabbro, granitic rocks, mixed rock composed of granitic and metamorphic rocks, and regionally metamorphosed rocks of the cordierite-amphibolite facies. Most of the adjacent granitic rocks are older than the Lakeview Mountains pluton, although some may be of the same age. The pluton is almost entirely coarse-grained hornblende-biotite quartz diorite that lacks potassium feldspar. Schlieren are ubiquitous, and small lenticular inclusions, common throughout the pluton, parallel the schlieren. The schlieren, which make the rock extremely inhomogeneous on a small scale, geometrically fall into three orientation groups. One group, the most pronounced, is concordant to the outline of the pluton and planar structures in the adjacent rocks; the other two, discordant with respect to the shape of the pluton, strike northeast and northwest. Some outcrops exhibit schlieren intersecting at a large angle, and mesoscopic fabric analysis indicates that mineral orientation of much of the massive-appearing rocks is complex on the scale of an outcrop. Granitic pegmatite dikes, masses of hypersthene gabbro, and both mafic and leucocratic quartz diorite are concentrated in the geometrically and structurally deduced center of the pluton. It is concluded that the concordant schlieren resulted from outward growth of the pluton and the discordant sets in response to regional forces. The concordance between the outline of the pluton and its wall rocks and the abrupt deflection of the regional structural grain resulted from the outward growth of the pluton. The pegmatite represents the fugitive constituents of the pluton-forming magma, and the sites of pegmatite concentration in the wider part of the tear-shaped body mark the last portions of the pluton to crystallize.

V.—The Petrology of Byne Hill, Ayrshire

SynopsisByne Hill, two miles south of Girvan, Ayrshire, is composed of Lower Ordovician igneous rocks which form partof the Girvan-Ballantrae igneous series. Byne Hill consists of serpentinized harzburgite, gabbro, diorite and trondhjemite which probably form a N.E.–S.W. elongated dome with trondhjemite occupying the core. There are transitional passages from gabbro into dioritic gabbro, diorite, quartz diorite and trondhjemite with no sharp contacts, and gabbroidal minerals and textures persist into the marginal hornblendic phases of the trondhjemite. The mineralogical changes are mainly progressive amphibolization and albitization of the gabbro accompanied by increases in SiO2 and Na2O. Geochemical culminations of FeO, MnO and P2O5 occur in the intermediate dioritic gabbro and diorites. Comparing the intermediate rocks with similar rock series from other areas suggests that mineralogical and chemical criteria do not always distinguish intermediate rocks of metasomatic origin from those produced by fractional crystallization. On Byne Hill the textural evidence suggests that the intermediate rocks are hybrids produced by reaction between crystalline gabbro and silicic sodium-rich solutions related to the trondhjemite, although there is evidence that the trondhjemite is itself metasomatic in origin. Serpentinization and rodingitization of the Byne Hill gabbro at contacts with serpentinite are described, and it is concluded that the gabbro was emplaced into the ultrabasic rocks prior to their serpentinization.

Geological and petrological studies on the Tanzawa mountainland (4)

In the Tanzawa mountainland, there are various types of plutonic rocks such as norite, tonalite and schistose gabbro in the plutonic mass which have been known as the Tanzawa Quartz diorite mass. The present paper described on the petrography and discussed on the petrogeny of the plutonic rocks based on the field and microscopic observations. They are summarized as follows. 1. The Tanzawa plutonic mass consists of gabbroic rocks and dioritic rocks which were formed in successive order. 2. Gabbroic rocks are composed of mylonitic hornblende gabbro, plagioclase porphyritic hornblende gabbro, schistose hornblende gabbro, metasomatic gabbro, dioritic gabbro, tonalitic gabbro, and diabasic gabbro, which are grouped into two series. Gabbro amphibolite series and hornblende gabbro series. Both of the series are originated from diabasic gabbro, but one metamorphosed to the amphibolite by mylonitization and recrystallization and other to the hornblende gabbro by the acidification, though the two courses were displayed simultaneously. 3. The dioritic rocks are composed of gabbroic diorite, normal hornblende diorite, gneissose biotite diorite, plagioclase porphyritic diorite, quartz porphyritic diorite, and tonalitic diorite, which were formed in successive order. The first two are related to the initial stage, the middle one to the upheaval stage, and the last three to the intrusion stage of the complex. These dioritics are derived from gabbroics resulting from the strong enrichment of SiO2 and Na2O. 4. Gabbro amphibolite series often gradually changes into or intercalate with amphibolites while the dioritic rocks of the intrusion stage has hornfelses around them. The other series or types of gabbroic and dioritic rocks has least relation with metamorphic rocks. They are in close strict relation to each other as main constituents of the dioritization process of gabbro in the plutonic mass.

Geological and petrological studies on the tanzawa mountainland

In the Tanzawa mountainland, there are various types of plutonic rocks such as norite, tonalite and schistose gabbro in the plutonic mass which have been known as the Tanzawa Quartz diorite mass. The present paper described on the petrography and discussed on the petrogeny of the plutonic rocks based on the field and microscopic observations. They are summarized as follows. 1. The Tanzawa plutonic mass consists of gabbroic rocks and dioritic rocks which were formed in successive order. 2. Gabbroic rocks are composed of mylonitic hornblende gabbro, plagioclase porphyritic hornblende gabbro, schistose hornblende gabbro, metasomatic gabbro, dioritic gabbro, tonalitic gabbro, and diabasic gabbro, which are grouped into two series. Gabbro amphibolite series and hornblende gabbro series. Both of the series are originated from diabasic gabbro, but one metamorphosed to the amphibolite by mylonitization and recrystallization and other to the hornblende gabbro by the acidification, though the two courses were displayed simultaneously. 3. The dioritic rocks are composed of gabbroic diorite, normal hornblende diorite, gneissose biotite diorite, plagioclase porphyritic diorite, quartz porphyritic diorite, and tonalitic diorite, which were formed in successive order. The first two are related to the initial stage, the middle one to the upheaval stage, and the last three to the intrusion stage of the complex. These dioritics are derived from gabbroics resulting from the strong enrichment of SiO2 and Na2O. 4. Gabbro amphibolite series often gradually changes into or intercalate with amphibolites while the dioritic rocks of the intrusion stage has hornfelses around them. The other series or types of gabbroic and dioritic rocks has least relation with metamorphic rocks. They are in close strict relation to each other as main constituents of the dioritization process of gabbro in the plutonic mass.

Minor elements in a metasomatic zone related to a copper-bearing pyrite deposit

Metasomatic alteration of sheared hornblende gabbro and quartz diorite which accompanied mineralization in the Hitachi mine district, northeast Japan, resulted in development of anthophyllite-cordierite and biotite-cordierite rocks in the ore zone. Both types of metasomatic rocks exhibit considerable enrichment in Mg and Fe and a decrease in Ca. Among changes in the minor element composition, an increase in Ba, a decrease in Sr and V, and little variation in Ni, Cr, Co, and Ga are noted.

On the diabase at the Meguro district, Hidaka Province, Hokkaido

There is a distinct difference in the distribution of the igneous rocks between the western and the eastern part of the Hidaka metamorphic zone. The peridotites and gabbroic rocks are widely distributed in the western part. The granites mainly occur in the eastern part. Meguro district is situated in the south-eastern part of the Hidaka Mountains. From petrological features, metamorphic zone of this district can be classified into the following six zones. 1. hornfels, 2. schistose hornfels, 3. banded gneiss, 4. cordierite migmatite, 5. gneissose migmatite, 6. granitic migmatite. These metamorphic rocks and migmatites are intruded by dikes of basic igneous rocks, such as diabase, dioritic gabbro, hornblende gabbro and amphibolite. It seems probable that the above-mentioned basic plutonic complexes and the basic dikes are geneticaly related. The diabases which occur as minor dikes (2-3m in average width) are more abundant than the other dike rocks. The diabase may furnish clues that will suggest further data necessary to solve a problem of the mutural relation between the migmatites and the basic igneous complexes. In the present paper, the writer have described the mode of occurrences and the microscopical features on the diabases. Some considerations on the distribution, alteration and intrusion of the diabases are given.