Gabbroic Dykes Research Articles

Structural mapping in the Oman ophiolites: Mantle diapirism along an oceanic ridge

Structures induced in peridotites by high-temperature plastic flow at the oceanic spreading centre source have been mapped in the Oman ophiolite. They can be reset to the geometry of the spreading system by rotating the boundary between these peridotites and the layered gabbros (the Moho) to the horizontal and by equating the trend of the sheeted dyke complex with that of the ridge. A number of typical asthenospheric flow patterns thus emerge which are more specifically described in a companion paper ∗ ∗ Ceuleneer et al. (this issue). . The most spectacular is a structure characterized by divergence of flow planes and flow lines from the vertical to the horizontal above and around mantle diapirs 10 × 15 km in diameter. Such diapirs are also recognized by traces of an intense magmatic activity (magmatic impregnation, and an abundance of gabbro dykes and intrusions and chromite pods). The general structure of the 500 km long Oman palaeo-ridge is dominated by these diapirs which are spaced approximately every 50 km along strike. In the preliminary picture which is revealed by this study, no other mantle structure typical of a ridge segmentation is documented with the possible exception of a transform structure within the Wadi Tayin massif. Evidence converges to indicate that Oman ophiolites correspond to a fast-spreading ridge.

Un exemple de métasomatisme alcalin albite–uranium dans le bassin des Monts Otish, Québec

Metallographic studies of uraniferous occurrences in the Otish basin (Quebec) have shown that these mineralizations are characterized by the association albite–uranium (successively, brannerite–uraninite). This paragenesis corresponds to a sodium metasomatic event related to Hudsonian (1730 ± 30 Ma) gabbroic sills and dykes that intrude both Archean granitic basement and the overlying Aphebian detritic cover. The mineralization (1723 ± 3 Ma) is interpreted as a late Hudsonian event and is in many ways comparable to other occurrences in the Precambrian shields of Brazil, Scandinavia, and the Ukraine. A comparison of these different occurrences with the Otish mineralization has resulted in the recognition of several characteristics common to late-orogenic metallogenic events from mid-Proterozoic time in Canada and elsewhere up to late Hercynien in Europe. To explain specific assemblages of tellurides, selenides, and sulphides within a dolomitic gangue, one suggests that deep basinal Na-rich brines were mobilized to form mineralizing fluids during a regional tectono-magmatic event. The pitchblende–calcite association in the Otish basin corresponds to the latest mineralized event (306 Ma) caused by a remobilization of the primary Na–U metallogeny.

Geochemistry of the felsic metavolcanic rocks of the Wakeham Group: a metamorphosed peralkaline suite from the eastern Grenville Province, Quebec, Canada

The Wakeham Group is a suite of low-grade (greenschist facies) rocks located in the eastern Grenville Province. In order of volumetric importance, it consists of sandstones, acid volcanics, and gabbro dykes and sills. This report deals with the acid volcanic member of the group. Primary textural features have in large part been preserved. The minerals present include stilpnomelane, white mica, albite, titanite, and zircon. Calculations using 33 chemical analyses show that 29 of the compositions have normative corundum. The rocks would therefore appear to be predominantly peraluminous; however, incompatible trace-element data show affinities with other anorogenic, peralkaline suites. It is proposed that alkali loss, probably during the subsequent Grenvillian orogeny, imposed a peraluminous composition on originally peralkaline rocks. The Wakeham Group cannot be definitively correlated with other metavolcanic rocks found in the region.

Geochemistry of eclogites and associated rocks of the southeastern area of the French Massif Central: Origin of the protoliths

Within the Barrovian metamorphic series of the French Massif Central, most of the eclogites are closely associated with metagabbros and meta-ultrabasites. The protoliths of the eclogites may represent basaltic or gabbroic dykes or sills of oceanic affinities erupted in either a continental domain, or during early oceanization or opening up of an ensialic back-arc basin. Both the nature and type of magma fractionation and of post-magmatic evolution are consistent with a shallow crustal origin for the protoliths. Eclogitization postdates the low-pressure magmatic differentiation and the volcano-sedimentary supra-crustal evolution.

Metasomatic control of K/Rb ratios in amphibolites

Amphibolite facies metamorphism of gabbroic dykes, together with their quartzo-feldspathic, granulite facies country rock (Archean basement, west Greenland), transformed the gabbroic rocks into amphibolites with remarkably high K/Rb values (up to 2300, average 727), reflecting correspondingly high K/Rb values in the country rock. A similar dyke transformation in an amphibolite facies complex (Precambrian basement, southwest Sweden) produced amphibolites with more common K/Rb values (average 375) similar to those found in the country rock. As the protolith chemistry and the P- T conditions of origin for the two amphibolite occurrences are very similar, it is concluded that the difference in K/Rb distribution is caused by the difference in country rocks. The K and Rb values in the two amphibolite occurrences lie between protolith and country-rock values, suggesting an exchange of K and Rb between basic rock and quartzo-feldspathic country rock, probably through the hydrothermal fluids which assisted in the amphibolite formation.

The role of possible transverse faults in the development of the Sabzevar ophiolites, North-East Iran, with special reference to magma chamber tectonics

In the Sabzevar ophiolites, North-East Iran, structural studies have shown that accretion have rather occurred along an oblique spreading axis. A discontinuous ridge may have developed either at a ridge-transform intersection with a small offset or in a pull-apart basin linked to a wrench fault subparallel to the mountain belt. Special attention was paid to the origin of the setting of the layered cumulates which probably were formed in several disrupted magma chambers. The layering, of variable dip ahd strike, results from crystallization taking place either along inclined planes linked to fault activities or near temporary subhorizontal floor regulated by gravitational forces. Tectonics in accretion zones also control the shape and size of the chamber, the evolution of the more or less asymmetric channel, the setting of gabbroic dykes and flow-layered secant cumulates.

Petrography and geochemistry of epidote alteration patches in gabbro dykes at Matagami, Quebec

Light colored patches, ranging to 0.5 m across, are common in some gabbro dykes and volcanic rocks at Matagami. Skeletal ilmenite and outlines of plagioclase grains, traced from patches to fresh gabbro, indicate an origin by alteration and metasomatism. Gabbro near patches is composed of actinolite, hornblende, labradorite, and composite ilmenite–magnetite grains, with minor epidote, albite, quartz, and chlorite. Patches are mainly epidote, quartz, hornblende, with minor albite, chlorite, apatite, and sphene. Compared to gabbro, patches are enriched in CaO, SiO2, and P2O5, and depleted in FeO*, MgO, and alkalis.Present features of the gabbro developed in three stages: (1) magmatic, (2) autometamorphic (or deuteric), and (3) regional metamorphic. The patches formed during stage 2 by channeling of fluids through permeable zones in cooling dykes and overlying volcanic rocks.

The geology of Tugtutôq and neighbouring islands, South Greenland. Part III. Olivine gabbros, syeno-gabbros and anorthosites. Part IV. The nepheline syenites of the Hviddal composite dyke

Large Precambrian (Gardar) ENE-WSW dykes traverse the Tugtutôq region and can be traced for some 50 km. Thicknesses of up to 800 m in the ENE diminish rapidly towards the WSW. The earliest of the large dykes is composite with intrusion of syeno-gabbro followed by median intrusion of nepheline syenite. The syenogabbro has feldspar zoned from ca. An67 to Or21∙5Ab65An13∙5 with olivines ranging from Fa54-64, augitic pyroxene, biotite, ore and apatite. Subsequent intrusion of high aluminous alkali basalt magma produced massive dykes of troetolitic gabbro in the granitic basement complex, a large sill-like body along the unconformity separating the granitic rocks from the Gardar series of sediments and volcanics, and smaller sills within the Gardar series. The gabbro dykes have synformal internal structures defined by mineral banding and feldspar lamination developed at irregular intervals along their length as does also the large overlying sill. The gabbro intrusion as a whole is regarded as a layered body displaying limited cryptic and phase layering. The gabbroic rocks are orthocumulate sequences formed by bottom accumulation of crystals in a period of tectonic instability.As a result of tilting towards the ENE, lower level rocks are exposed towards the WSW of the intrusion where the cumulus phases were feldspar ca. An 72 and olivine Fa35-40. Interstitial pyroxene is a titanaugite, Ca45∙5Mg37∙5Fe17. In the higher rocks towards the ENE the cumulus feldspar (An65) and olivine are joined by cumulus ilmenomagnetite and fluorapatite. In one sector a gabbro dyke is composite with median intrusion of syeno-gabbro followed by quartz syenite.This syeno-gabbro has feldspar zoned from Or4Ab62An34 to Or8Ab73An19, with early clinopyroxene (Ca44∙5Mg36Fe19∙5 → Ca44∙5Mg32∙5Fe23), olivine, (Fa72), biotite, ore and fluorapatite.Four rock analyses and three pyroxene analyses are presented for the gabbros and syeno-gabbros. Inclusions of labradoritite and gabbroic anorthosite, considered to be samples from a large anorthositic body at depth, are brought up by the large dykes and also by a later swarm of hybridised dolerite dykes. The inclusions range from layered labradorite-orthocumulates ((with An57) and interstitial olivine (Fa33) and augite) to cataclastic varieties with no fresh ferromagnesian minerals. A late Gardar swarm of ENE dykes and associated sills of camptonitic dolerite is believed to be genetically related to the earlier syeno-gabbros and gabbros.

The geology of Tugtutôq and neighbouring islands, South Greenland. Part II. Nordmarkitic syenites and related alkaline rocks

Among the Precambrian (Gardar) intrusions in the Tugtutôq area, syenitic rocks are abundantly represented. Quartz-bearing alkalic rocks were first intruded after the emplacement of a number of large gabbro dykes. Although early Gardar nepheline syenites occur on Tugtutôq, no post-gabbro undersaturated syenites occur within the area under discussion. Quartz syenites and their faster cooled equivalents occur either as ENE directed dykes or as ring-dykes and stocks within a late Gardar central complex. The earliest quartz syenite occurs as the central component in a composite sector of one of the large gabbro dykes. This was succeeded by multiple swarms of dykes with alkalic types ranging from 'rhomb porphyry' microsyenite to comendite. Dyke intrusion was virtually at an end by the time of formation of the central ring-complex. The total petrographic range of the oversaturated alkalic rocks extends from ferroaugite-fayalite syenites with relatively calcic feldspar (ca. Or25Ab66An9) to highly acid riebeckite-bearing rock types. The various intrusions are considered to represent magma batches supplied, at irregular intervals, from a deep-seated syenite complex which was crystallising throughout the latter part of the Gardar period. The inferred sub-surface complex is taken to be closely comparable to the Gardar complexes exposed at Nunarssuit and Kungnât, and to have differentiated by process of crystal fractionation. By analogy with the Kungnât complex the parental magma of the underlying magma chamber(s) is believed to have passed through a larvikitic stage. The perthosite stock which forms the latest major intrusion on Tugtutoq is thought to indicate a slight divergence from the general larvikite-nordmarkite-alkali granite differentiation sequence. The magmas of this main differentiation sequence changed in composition along a "thermal valley" dose to that of the synthetic system Or-Ab-SiO2-H2O.