Gabbro Dikes Research Articles

A Proterozoic granulite-facies terrane on Roan Mountain, western Blue Ridge belt, North Carolina–Tennessee

Polymetamorphosed gneisses of Proterozoic age crop out on Roan Mountain, North Carolina–Tennessee, near the western edge of the Blue Ridge belt. These gneisses display Grenville-age, granulite-facies assemblages, including two-pyroxene metabasites and gar-net-sillimanite-orthoclase metapelites. All gneisses were intruded by diabase dikes of the late Precambrian Bakersville Gabbro. Paleozoic amphibolite-facies metamorphism locally formed garnet and amphibole in mafic rocks and kyanite in metapelitic rocks. Two pyroxene geothermometry and mineral stability curves define temperature-pressure ranges of 750–847 °C and 6.5–8.0 kbar for the granulite-facies metamorphism. This metamorphism probably occurred within the lower crust at depths of 23–28 km. Pressure-temperature conditions for the major Paleozoic metamorphism are not well constrained but are estimated to be 700–740 °C and 7.5–9.0 kbar.

Petrology of the Grenville swarm of gabbro dikes, Canadian Precambrian Shield

The Grenville tholeiitic dikes of Late Proterozoic or early Paleozoic age cut marbles and gneisses of the Grenville Province within and adjacent to the Ottawa rift structure. Where traversed the swarm consists of about 40 large dikes (up to 100 m wide) representing a crustal extension of 1 km. The principal minerals are olivine, augite, pigeonite, plagioclase, magnetite, ilmenite, K-feldspar, and quartz. Crystals of pigeonite around olivine and complex augite–pigeonite composite grains suggest that the reactions olivine + melt → pigeonite and pigeonite + melt → augite have taken place. Conspicuous zoning and grain to grain variation in the composition of augite are consistent with fractional crystallization, but crystal – melt equilibrium during crystal growth is indicated by a restricted range in pyroxene paleotemperatures of 1180–1060 °C. Rock textures (subophitic, combined ophitic–subophitic, and equigranular) in dike centres are related to dike width and are determined principally by the influence of cooling rate on the nucleation of augite. The K2O content of the gabbro (centres of 15 dikes, 3–100 mm wide) ranges from 0.2 to 1.2% and is closely correlated with other elements, positively with Ti, Na, P, Rb, Sr, Y, Zr, and Ba, and negatively with Mg, Ca, and Cr. Relatively minor within-dike variation exists in the form of local (centimetre-scale) inhomogeneity, slight enrichment in K and Na, and depletion in Ca and Mg in the centre of one large dike and complex rhythmic variation in K, Na, Ca, and Fe across one small dike. Centre–margin comparisons in several large dikes indicate minor residence con tamination (Ca, Mg) in the margins of some dikes cutting marble. Of the various possible causes for variation in composition, those favoured at present are Fractional crystallization at depth (separation of augite and plagioclase), the partial preservation of compositional heterogeneity during intrusion, and the gravitational sinking of early formed crystals (with the upward displacement of K-enriched melt) in the central regions of large dikes.

A Nd and Sr isotopic study of the Trinity peridotite; implications for mantle evolution

Field evidence indicates that the Trinity peridotite was partially melted during its rise as a part of the upwelling convecting mantle at a spreading center. A Sm Nd mineral isochron for a plagioclase lherzolite yields an age, T = 427 ± 32 Ma and initial ε Nd = + 10.4 ∓ 0.4 which is distinctly higher than that expected for typical depleted mantle at this time. This age is interpreted as the time of crystallization of trapped melt in the plagioclase lherzolite P-T field. This time of crystallization probably represents the time when the massif was incorporated as a part of the oceanic lithosphere. The Sm Nd model age of the plagioclase lherzolite totalrock is T CHUR Nd = 3.4 AE . This suggests that the Trinity peridotite was derived from a mantle that was depleted rather early in earth history. The peridotite contains many generations of pyroxenite dikes and some microgabbro dikes. We report data for two dikes that clearly crosscut the main metamorphic fabric of the peridotite. A microgabbro dike yields a Sm Nd mineral isochron age of T = 435 ± 21 Ma and ε Nd = + 6.7 ∓ 0.3 . A pyroxenite dike yields an initial ε Nd = + 7.3 ± 0.4 . The initial ε Nd values for the pyroxenite and gabbro dikes are fairly similar to those for the depleted mantle at this time and are distinct from the lherzolite—demonstrating that they are not genetically related. Rb Sr data do not give any coherent pattern. However, some bounds can be put on initial Sr values of ε Sr ⩽ −21 for the plagioclase lherzolite and ε Sr ⩽ −8.7 for the microgabbro dike. It is plausible that the dikes represent cumulates left behind from island arc magmas that rose through the the oceanic lithosphere within the vicinity of a subduction zone. Major and trace elements and Sm Nd isotopic data indicate a multiple stage history for the Trinity peridotite; a small melt fraction was extracted from an undepleted source ∼ 3.4 AE or more ago to produce the proto-lherzolite; a large fraction of melt (∼ 12 to 23%) was extracted from the proto-lherzolite to produce the present rock; the lherzolite was then crosscut by dikes from average depleted mantle ∼ 0.44 AE ago. The data are compatible with the depleted mantle source being formed very early in earth history. Although most available data indicate that the depleted upper mantle has been relatively well stirred through time, the Trinity data suggest that very ancient Nd isotopic values are preserved and thus chemical and physical heteorgeneities are sometimes preserved in the depleted source of mid-ocean ridge basalts as well as the oceanic lithosphere which they intrude.

Mantle metasomatism: The REE story

Refractory rocks with light REE/heavy REE ratios greater than chondrite are common as xenoliths in basalts and kimberlites and are found in some oceanic peridotite massifs. This has led to the supposition that large parts of the upper mantle have been metasomatically altered by addition of light REE and other hyperfusible constituents. Structural and major-element geochemical evidence from xenoliths and alpine peridotites, however, suggest that the metasomatic effects are local and are related to emplacement of partial melts. The melts are represented by dikes of pyroxenite, hydrous minerals, and gabbro that occur in mantle peridotites of all origins and were emplaced in them in the same sequence as indicated by crosscutting relations. REE distributions in both the peridotite and the dikes may be explained as a result of metasomatic interaction between dikes and peridotite wall rock in which the peridotite is enriched in light REE and the dikes depleted in light REE relative to the original liquid. Differentiation of the intrusions and separation of residual liquids may further reduce the light REE/heavy REE ratio in pyroxenite dikes; these residual liquids (hydrous mineral veins) enriched in light REE extend the volume of metasomatized peridotite as they too interact with their wall rocks. Differences in the relative abundances of altered peridotite (reacted wall rock) in xenoliths and massifs are seen as a sampling problem rather than a difference in process.

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.

Peridotite and Gabbroic Structures in the Monte Maggiore Massif, Alpine Corsica

Penetratively deformed peridotites from the Monte Maggiore spinel/plagioclase lherzolite massif in Alpine Corsica are intruded by gabbroic dikes ranging in mineralogy from troctolites to Fe and Ti gabbros. The dikes cross-cut diffuse plagioclase segregations and veinlets which may represent trapped mafic melt within the residual peridotite. Elliptical shaped dunite pods with refractory compositions are often bounded by lace-like networks of plagioclase () which invade neighboring undepleted lherzolite. Lherzolite mineral compositions range from olivine , orthopyroxene , and clinopyroxene . Peridotite and gabbroic structures and textures suggest that the emplacement of the dikes was contemporaneous with the high temperature deformation of the peridotite, occurring under upper mantle or deep crustal conditions. Locally the intrusion of the dike magma followed preferential shear fracture planes related to flow structures in the enclosing peridotite. The similarity of differentiation patterns in the gabbroic dikes with gabbroic cumulates in the overlying Corsican ophiolitic section suggests that the Mt. Maggiore dikes may have fed these magma chambers.

Cross section through the peridotite in the Samail Ophiolite, southeastern Oman Mountains

A continuous cross section through the peridotite of the Samail ophiolite south of Wadi Tayin offers a relatively undisturbed slice of oceanic mantle 9–12 km thick. The whole section consists of a regularly foliated harzburgite banded with orthopyroxenite except for a narrow zone at the base where the primary banding is dominantly dunitic. Websterite dikes, sometimes deformed and branching, are observed throughout the whole sequence, whereas undeformed gabbro dikes appear abruptly about 2–5 km above the base. The top of the foliated harzburgite is in contact with dunite cumulates that grade upward into wehrlite and gabbro cumulates. The websterite and gabbro dikes are formed by crystal accumulation from ascending, off‐axis olivine‐poor tholeiite magma that passes through the tectonized harzburgite. Discordant and deformed dunite within the harzburgite represent fractionated olivine accumulates derived from the deep‐seated primitive picritic tholeiite magmas as they moved through the harzburgite within the spreading axis. The homogeneous chemistry, extreme depletion, and absence of frozen partial melts demonstrate that the harzburgite tectonite is a highly refractory residue. It is suggested that the harzburgite tectonite does not represent the contemporaneous cogenetic residue from which the deep‐seated primitive picritic tholeiite was derived. The discordant dunite, gabbro, and websterite dikes record crystal fractionation products of ascending basaltic liquids derived from partial melting of fertile mantle at greater depths, whereas at least some of these dunites may represent a partial melt residue. A spinel lineation dipping slightly to the southwest, present all through the cross section, parallels isoclinal fold axes of the primary banding in the harzburgite. The tectonic fabric of the harzburgite exhibits clear maxima of olivine crystallographic axes related to the regional foliation and the spinel lineation. The magmatic reaction, the comparison of tectonic fabrics, size of neoblasts and subgrains in olivine with experimental data, suggest that the harzburgite sequence was deformed at or near solidus temperatures and under a deviatoric stress increasing from 300 to 400 bars in the upper part of the sequence and from 600 to 700 bars in the lower part. Disequilibrium between primary phases due to subsolidus transformation precludes establishment of temperatures and pressures of partial melting by using present KD ratios of coexisting phases in the harzburgite. Mylonitic textures in narrow bands at the base of the sequence grade upward into porphyroclastic textures with rapidly increasing neoblast size. These textures are the imprint of a superposed deformation at lower temperatures and under a deviatoric stress reaching 1 to 2 kbar that affected the narrow sole of the sequence. A narrow amphibolite‐facies zone at the base of the harzburgite tectonite nappe exhibits a similar fabric as the overlying mylonitic harzburgite. This deformation is thought to have occurred during thrusting and detachment of the Samail ophiolite nappes shortly after their formation as oceanic lithosphere.

Dolores Creek, Yukon; a disseminated copper mineralization in sodic metasomatites

The Dolores Creek mining area situated in the Proterozoic core of the Wernecke Mountains (northeastern Cordillera, Yukon) contains various types of copper, cobalt, and uranium mineralization in altered metasediments and diorite dikes.The 4,000 to 5,000-m-thick succession of Proterozoic rocks consists of two major sequences separated by an angular unconformity. These sequences crop out in a complex northeast-trending, northeast-plunging anticline.The lower part of the older, lower to middle Proterozoic sequence underlies most of the map area and consists of a thick, monotonous pile of marine siltites intergraded with argillaceous quartzites, silty argillites, and impure carbonates. These grade upward into argillites, impure arenites, and at the top into thick bedded, spectacularly orange-weathering dolomites. The metamorphic grade reaches the lower greenschist facies near the base and diminishes upward. Late Proterozoic (?) diorite or gabbro dikes are common in places.The younger, upper Proterozoic sequence occurs as erosional remnants in the immediate vicinity of the Dolores Creek area. Basal purple and green shales are its characteristic lithology.A conspicuous feature of the Dolores Creek area and of the Proterozoic terrane elsewhere in the Wernecke and Ogilvie Mountains is the widespread presence of altered breccias and pseudoconglomerates in Proterozoic metasediments. The breccias are both semiconformable and discordant, gradational to sharply outlined, and occur as elongate masses and lenses along faults or on strike continuations of diorite dikes.Most breccias are hydrothermally altered. In the Dolores Creek alteration-mineralization center, a roughly elliptical area 6 X 4 km has been altered into a central core of albite-(chlorite) metasomatites of syenitic appearance grading outward into albite-chlorite metasomatites with relict breccia fabrics; to fine albitized metasiltites and metacherts; to (albite)-paragonite + or - sericite, carbonate metasomatites; to quartz-sericite rocks; and finally spotted phyllites.Widespread, but so far uneconomic, mineralization within and around the Dolores Creek alteration-mineralization center is dominated by chalcopyrite which occurs with specularite as pervasive disseminations and fracture fillings in sodic metasomatites associated with breccias (Porphyry prospect), hair-thin bedded replacements in altered metasediments adjacent to diorite dikes, veins, and replacement masses in carbonate rocks. Cobaltite occurs in short discontinuous veins, and a pitchblende occurrence along fractures in altered breccias has been explored recently.The Porphyry occurrence resembles in some respects porphyry copper deposits, but the alteration zonality is reversed: the core of Na-feldspathization grades outward into Na-phyllic, K-phyllic, propylitic, and carbonatization zones. The geochemistry of mineralization at Dolores Creek shows many parallels with mineralized spilites, particularly with the homomineralic spilites described by Amstutz. The diorite dikes and their parent magmatic body are considered to have been the source of most metals and of the mineralizers.

Areal geology and petrology of eclogites and associated metabasites of the Piemonte ophiolite nappe, breuil—st. Jacques area, Italian Western Alps

The Breuil—St. Jacques area is located in the upper Valtournanche and Ayas valleys on the north side of the middle Aosta Valley. The principal unit exposed here consists of calc-schists + greenstones of the Piemonte ophiolite nappe. This complex is interposed between the underlying Pennine Monte Rosa nappe and the overlying Austroalpine Dent Blanche + Sesia—Lanzo nappe. The juxtaposed representatives of two structural units occur within the Piemonte section; they differ in terms of lithologic associations, metamorphic assemblages and paleogeographic significance. 1. (1) The structurally lower Zermatt—Saas unit consists of an important basal sequence of largely serpentinized peridotite tectonites, an overlying group of discontinuous metagabbros locally containing magmatic clinopyroxene relics, and a capping series of various types of tholeiitic to slightly alkaline metabasalt. Syn- and post-volcanic sedimentary strata, now predominantly garnetiferous, ankeritic mica schists with associated marbles and minor calcareous, manganiferous metaradiolarites (spessartine-, piemontite- and braunite-bearing metacherts) form a superjacent cover series. The Zermatt—Saas unit exhibits a composite metamorphic character. Eclogites and early glaucophane schists of the eoalpine stage of recrystallization have been incipiently to pervasively replaced by a greenschistic assemblage produced during the Lepontine metamorphic event. This latter recrystallization involved the renewed growth or recrystallization of sodic amphibole. Serpentinite of the Zermatt—Saas unit contains numerous gabbroic dikes, some of which have been partly transformed to fine-grained rodingitic material; other rodingites represent metasomatic reaction rims between ultramafic material and lithologically diverse surrounding rocks. 2. (2) The structurally overlying Combin unit possesses a distinctive stratigraphy which contrasts with that of typical ophiolites and correlative oceanic crust. It consists of locally preserved Upper Permian (?), Triassic and Liassic strata of continental affinities, overlain by a section made up chiefly of regular intercalations of calc-schists, marbles and metavolcanic layers derived from submarine basaltic flows, hyaloclastites, tuffs and/or tuffites. The mafic rocks have been pervasively recrystallized to porphyroblastic albitebearing greenschists (prasinites). This volcanoclastic sequence contains interbeds of manganiferous metaradiolarite and quartz + albite-bearing chlorite schists, in part with associated stratiform Cu-Fe sulfides; it also includes some lenses — tectonic slivers and/or olistostromes — of metagabbro and serpentinite. The Combin unit does not exhibit the characteristic relict eclogitic association of the Zermatt—Saas unit; instead it displays only the effects of greenschist facies recrystallization (including very rare relics of sodic amphibole) attributable to the Lepontine metamorphic event, and corresponding to the greenschist overprinting of both the Zermatt—Saas unit and the lower tectonic member of the Austroalpine nappe. Bulk XRF analyses of seventeen mafic rock samples demonstrate that, although postigneous metasomatism has produced sodium enrichment, original lithologies possessed affinities with oceanic tholeiite. The Zermatt—Saas and Combin metabasites do not exhibit distinctive compositional differences. The relatively high-pressure metamorphic prograde path displayed by Zermatt—Saas mineral assemblages of eoalpine age is characteristic of subduction zone metamorphism, whereas the retrograde P—T trajectory represents nearly adiabatic decompression —hypothesized to have accompanied buoyant return of the imbricated subducted complex towards the surface after its detachment from the downgoing lithospheric slab. The greenschist facies overprinting, which according to isotopic age measurements is connected at least in part with the Lepontine metamorphic event, seems to be related to a post-collisional thermal reequilibration of the pile of nappes, including both Zermatt—Saas Combin units.

Evolution of Sverdrup Basin, Arctic Canada

Sverdrup basin, a structural depression near the northern margin of the North American craton, contains a succession up to 13,000 m thick of Lower Carboniferous to Tertiary, marine and nonmarine sedimentary rocks, basalt flows, and gabbro dikes and sills. The basin evolved in five phases: (1) late Paleozoic, when evaporites and marine muds were deposited in the axial region and carbonates and mature sands on the margins; (2) early Mesozoic, when great thicknesses of siltstone and shale accumulated in axial parts of the basin; (3) middle Mesozoic, when terrigenous clastic deposits accumulated slowly; (4) late Mesozoic, when clastic deposits widely overstepped former basin margins; and (5) late Mesozoic-Cenozoic, when the basin underwent three stages of tectonism (Eurekan orogeny). Some evaporite diapirs in the basin developed halokinetically, beginning probably in the early Mesozoic; others were generated during Tertiary folding and faulting. Mafic volcanism and intrusion occurred episodically at times of basin foundering. Depending on the availability of imported terrigenous detritus, phases of mafic activity (and foundering) were accompanied by widespread marine transgression or by vigorous basin filling. The thick sedimentary fill masks the fundamental character of the basin. If acutely starved depositional conditions had prevailed through its Paleozoic and Mesozoic history, Sverdrup basin in the latest Cretaceous would have been a small ocean basin with steep sides, a basaltic floor about 3,500 m deep, and a thin succession of deep marine sediments--fundamental attributes of some modern small ocean basins.

Rare earth geochemistry of fused ophiolitic and alpine lherzolites—I. Othris, Lanzo and Troodos

Lherzolites from two Mediterranean peridotite masses have major and trace element data compatible with an origin as a fragment of relatively undepleted mantle. Field observations indicate a close association with in situ basaltic melt (gabbroic dikes and segregations) and a barren refractory residue (harzburgite) produced by the removal of the melt fraction. Two lherzolites Othris (ophiolite) and Lanzo (alpine ∗ ∗ Alpine peridotites will be referred to as ‘mantle peridotite massifs’ throughout this text. Such peridotites may be cumulate or tectonite in nature and exist in either the plagioclase. spinel or garnet Iherzolite fields. Those stable in the plagioclase Iherzolite field (<9kbar) also occur as mantle sections in ophiolites. periodotite) have approximately chondritic rare earth abundances with a slight depletion in light rare earths. The refractory material is moderately to heavily depleted in light REE dependent on the efficiency of removal of basaltic melt. Lherzolite xenoliths from the Massif Central probably contain an interstitial light REE enriched fraction as the recalculated lherzolite is depleted and not light REE enriched like the actual whole rock. These basaltic xenoliths are similar in major, trace and REE profile to the Lanzo and Othris mantle lherzolites, giving some indication of source homogeneity in the Mediterranean area. Partial fusion calculations on the Othris and Lanzo peridotites reveal that tholeiitic liquids could be generated by 10–30% partial melting. Such tholeiitic liquids separated from the Othris mantle section and probably formed early sea floor in a small ocean basin. Alkalic basalts are also associated with the Othris ophiolite as an early rifting sequence, and such liquids could have been generated from the source Iherzolite but difficulties would occur in removing such a liquid from the refractory residue.

Tangihua volcanics in the Opouteke—Pakotai area, Northland, New Zealand

Abstract The area discussed is a 65 km2 block situated in the southern part of the Mangakahia Ranges (N19), near Pakotai, mid-Northland. It consists largely of Tangihua Volcanics comprising a variety of submarine basaltic lavas and volcaniclastics intruded by numerous dikes of dolerite, gabbro, diorite, and granophyre; a large multiply-intruded gabbro-diorite-granophyre dike complex, the Onetai Intrusion, occurs in the southern part of the area. Twelve new chemical analyses are presented.

Dating of Igneous Dike, Eastern Uinta Mountains

Age of a gabbroic dike which intrudes the Precambrian metasediments of the Uinta Mountains in T. 1 S., Rs. 18 and 19 E. , Uintah County, Utah, has been determined by the K-Ar method. Analyses of the whole rock and a feldspar concentrate from it were run. The whole-rock determination gave an age of 412 ± 17 my and the feldspar concentrate 453 ± 17 my. The whole-rock determination would place the age in the uppermost Silurian; the feldspar concentrate determination would place the age as medial to uppermost Ordovician. Geochron Laboratories Division of Krueger Enterprises, Inc., Cambridge, Massachusetts, who performed the analyses, commented that this rock is at least lower Paleozoic in age, and almost certainly it is pre-Devonian.

Paleomagnetism of Igneous Rock Units from the Coast of Labrador

Paleomagnetic results are reported for the first time from the coast of Labrador. The rocks are from (1) 20 sites of gently folded basalts of the Mugford Series, dated 948 ± 90 m.y., near Cape Mugford, and (2) 12 gabbroic dikes in a swarm dated 2080 ± 42 m.y., near Indian Harbour. Both dates are from preliminary, published K–Ar determinations. After partial alternating-field (AF) or thermal demagnetization, nearly all samples from both rock units revealed a stable thermoremanence that is most probably of primary origin. For the Mugford basalts, a mean remanence direction relative to bedding planes was calculated, after AF treatment, from 16 stably magnetized sites ([Formula: see text] flows). The corresponding pole is 49 °N, 143 °W, with dp = 9°, dm = 11°, which is northeastward of most North American poles from rocks dated 1200-1000 m.y. The pole calculated from the 12 Indian Harbour dikes, after AF treatment, is 6 °S, 117 °W, with dp = 6°, dm = 12°.

The Aston Formation (? Proterozoic) on Prince of Wales Island, Arctic Canada

Substantial exposures of a thick basal sedimentary sequence, the Aston Formation of Somerset Island, occur on eastern Prince of Wales Island and incorporate at least 2600 ft (793 m) of arkose, subarkose, and orthoquartzite, with minor dolostone, chert, and mudstone. The detrital sediments, derived from contemporary uplands on the site of the present Boothia Arch, accumulated in shallow marine to lagoonal environments. A thin, extensive, and distinctive stromatolitic dolostone in the two major exposures of the formation has an environmental and possibly a regional time–stratigraphic significance. Gabbro dikes and sills, some of them exceeding 1000 ft (305 m) in thickness, intrude the Aston Formation.

Contributions to mineral chemistry of Hawaiian rocks

Small gabbroic dikes of high TiO2 content transect massive hawaiite in the Kaena Quarry, Waianae Range, Oahu. One dike studied consists of two rock types: (a) border zone alkali gabbro of high titanomagnetite and titanaugite content and, (b) interior mugearite that contains iron-rich pyroxenes and K-feldspar. The dike probably formed as an in situ latestage segregation enriched in TiO2, SiO2, and alkalis.

The coast-parallel dike swarm of southwest Greenland in relation to the opening of the Labrador Sea

Coast-parallel gabbro dikes in southwest Greenland have been dated as Jurassic; their distribution and age suggest that initial rifting between Greenland and Labrador took place at about the middle of the Mesozoic Era.

Geology and genesis of ultrabasic nickel-copper-pyrrhotite deposits at the Pacific Nickel property, southwestern British Columbia

This paper deals with geology and genesis of some pipelike nickel-copper-pyrrhotite deposits on the property of Pacific Nickel Mines in southwestern British Columbia. These deposits occur almost exclusively within a stocklike ultrabasic mass one and a half miles across. The ultra-basic body consists essentially of pyroxenite with cores of peridotite, with hornblendic replacement phases, and with a remarkable reaction margin of pegmatitic hornblendite. The mineralization consists of disseminated and massive pyrrhotite with subordinate pentlandite and chalcopyrite among fresh olivine, bronzite, augite, and hornblende. The ultrabasics intermingle with, and partly cut, a larger batholithic mass of genetically related diorites of Late Mesozoic age. Both rocks and ore are cut by small ultrabasic and gabbroic dikes, and by veins and alteration zones. Some of the ore occurs as sulfidic olivine-rich cores or shells with bronzitic borders, forming remarkably zoned, steeply plunging pipe- or parsnip-shaped magnesian ultrabasic structures 100 feet or more in diameter. These features suggest replacement origin. The remainder of the ore forms massive sulfide-silicate bodies similar in size, attitude, and mineralogy to the zoned ones but more irregular in cross section and showing flow lines, banding, drag folds, sharp contacts, inclusions, and hornblendic reaction rims--all suggestive of injection origin.Genetic characteristics of other similar nickel deposits and rocks of the world are summarized and compared, then two theories are presented for genesis of the ore at Pacific Nickel. Magmatic segregation is suggested with injection of sulfides in an incipiently molten state (perhaps 700-800 degrees C) followed by minor replacement and reconstitution. Alternatively, a new twist is added to the hydrothermal replacement theory by suggesting that ascending water vapor above 650 degrees C could deposit the sulfides and, by removal or addition of silica, convert bronzite to olivine or vice versa to produce the zoned structures and other features.

GEOLOGY OF THE LOS TEQUES–CUA REGION, VENEZUELA

The Los Teques-Cua portion of the Coast Range of north-central Venezuela consists essentially of a granitic basement overlain by two metasedimentary and metaigneous complexes of probable Cretaceous age, locally covered by unmetamorphosed volcanic and Tertiary sedimentary rocks. The Sebastopol orthogneiss forms the basement, overlain unconformably by the Caracas metasedimentary series. Although now gneissic, the Sebastopol was probably unmetamorphosed, coarse-grained granite when the sediments of the Caracas series were deposited. The Older Metamorphic Complex, the Caracas series, is chiefly metamorphosed sediments seemingly deposited on a stable platform in a transgressing early Cretaceous sea. This sequence of arkoses, conglomerates, orthoquartzites, dolomitic and reef limestones, and calcareous and noncalcareous shales was metamorphosed to microcline dolomite rocks, and schists and marbles of the Las Brisas formation, the Antimano limestone, and the Las Mercedes formation. An amphibolite layer, possibly representing a metamorphosed basaltic flow, occurs interbedded in the sequence. Well-defined schistosity and lineation were developed during the folding and metamorphism. The sediments of the Younger Metamorphic Complex, the Paracotos formation, lie unconformably above the Older Complex and represent rapid deposition under eugeosynclinal conditions. They consist of a mixture of graywackes, conglomerates, and cherts, interbedded with tuffaceous and limy layers. Most of the clastic material for the graywackes was derived from the already exposed Caracas series schists and the Sebastopol basement. The Paracotos sediments were intruded by peridotitic igneous rocks and a hornblende diorite with related dikes of soda granite. Metamorphism of this complex to the muscovite-chlorite subfacies of the green-schist facies shortly after the emplacement of the igneous rocks converted the sediments to phyllites and metagraywackes and imparted a slight schistosity and microfold lineation to all the rocks affected. The ultramafic intrusives were also partly altered to antigorite in the northern outcrops. In neither this nor the earlier metamorphism of the Caracas series was a thick cover of sediments present. The Tiara volcanics consist of gabbroic dikes and sills and basaltic flows emplaced immediately after the metamorphism of the Younger Complex. These, together with the earlier metamorphics, were being eroded by Coniacian time, supplying material to the Late Cretaceous, Tertiary, and Quaternary sediments. Structurally the region lies on the southern limb of the major anticline which trends N.75°E. across the northern part of the area. All magnitudes of folding occur in these rocks, from the large isoclinal folds to the microscopic fold lineation. Faulting is also prominent chiefly as N.80°E. transcurrent faults and their accompanying N.60°W. normal faults. Thrust faults, probably older than the transcurrent faults, occur locally. The region is rugged. Youthful streams are cutting rapidly into an uplifted erosion surface.

Petrology and structure of the Moa Chromite district, Oriente Province, Cuba

Chromite deposits In the Moa district, on the north coast of Oriente Province, Cuba, form one facies of the ultramafic complex in which they occur. Two planar structures, one a compositional layering due to variations in the relative proportions of olivine, pyroxene, plagioclase feldspar, and chromite, and the other a foliation caused by the orientation of pyroxene crystals in peridotite, are in general parallel, but diverge in a few places. Dips are fairly constant, being moderate to the northwest over most of the area, but sharp reversals occur immediately adjacent to irregularities in the chromite deposits. Two prominent joint sets are present: (1) Cross joints striking parallel to the strike of the foliation and dipping approximately at right angles to it; and (2) longitudinal joints striking parallel to the direction of dip of the foliation and nearly vertical. Gabbro dikes follow the first set, chrysotlle veins the second. Within the ore bodies dikes are commonly coarser‐grained, larger, and far more abundant than in peridotite; they enclose coarse, angular fragments of chromite.