Early Dykes Research Articles

Geochronological and Geochemical Constraints on the Petrogenesis of Permian Dolerite Dyke Swarms in the Beishan Orogenic Belt, NW China

Extensive Early Permian mafic-ultramafic intrusions, doleritic dykes, and basalts crop out within the Beishan area, southern Central Asian Orogenic Belt (CAOB). We present new geochronological and geochemical data for Gubaoquan dolerite dyke swarms in the Beishan orogenic belt. Zircon U-Pb Dating of the Gubaoquan dykes indicates that they were emplaced during the Early Permian (280.7 ± 4 Ma), that was coeval with Yinaoxia and Podong mafic dykes in Beishan area. The dykes are characterized by low Mg# (47–84) in the clinopyroxene crystals, and the content of whole-rock Fe2O3 (t), MgO, and alkali (Na2O + K2O) range from 12.5–17.4, 4.06–5.51, and 2.8–4.4 wt.%, respectively. The samples from the Gubaoquan dykes have high and variable Ba/La (5.93–14.2) and Ba/Nb (15.0–37.3) ratios but low Th/Yb (0.17–0.24) ratios. The rocks show slightly enrichments in LREE, HFSE, Th, and Hf, and depletion in Nb and Ta. The εNd (t = 280 Ma) values and initial 87Sr/86Sr ratios of the Gubaoquan dykes show variations ranging from 6.4 to 6.8 and 0.706240 to 0.707546, respectively. These data suggest that the parental magmas for the Gubaoquan dykes were probably derived from partially decompressed melting of upwelling depleted asthenosphere mantle that was metasomatized previously by subducted fluids.

Portrait of a giant deep-seated magmatic conduit system: The Seiland Igneous Province

The Seiland Igneous Province (SIP), Northern Norway, contains >5000km2 of mafic and ultramafic intrusions with minor alkaline, carbonatite and felsic rocks that were intruded into the lower continental crust at a depth of 25 to as much as 35km.The SIP can be geochemically and temporally correlated to numerous dyke swarms throughout Scandinavia at 560–610Ma, and is linked to magmatic provinces in W-Greenland and NE-America that are collectively known as the Central Iapetus Magmatic Province (CIMP).Revised mapping show that the SIP exposes 85–90% layered tholeiitic- alkaline- and syeno-gabbros, 8–10% peridotitic complexes, 2-5% carbonatite, syenite and diorite that formed within a narrow (<10Ma) time frame in the Ediacaran (560–570Ma). Large peridotite complexes were emplaced into the still hot and unconsolidated gabbro (no dating available) and are regarded as the main-conduit systems.Gravimetric data implies an average thickness of igneous rocks of 4–5km and also features six deep lithospheric roots of ultramafic rocks extending min 9km into the crust. Together, the root structures represent the main volcanic conduits conveying thousands of km3 of mafic-ultramafic melts from the asthenosphere to the lithosphere.The ultramafic complexes were predominantly emplaced into the layered gabbros at four major igneous centres, respectively, Nordre Brumandsfjord, Melkvann, Kvalfjord and Reinfjord. All complexes are situated in a right-way-up position and are steep sided forming large plugs. A marginal hybrid zone forms at the contact with country-rock and transitions gradually from olivine-mela-gabbro over pyroxenites that grades in to an olivine-clinopyroxenite zone, which is followed by a wehrlite zone and, finally, the centre of the complexes comprises pure dunite. From pyroxenite to dunite, olivine changes from Fo72 to Fo85 and clinopyroxene from Di80 to Di92 i.e. the complexes observe a reverse fractional crystallisation sequence with time.Parental melt compositions modelled from early dykes indicate komatiitic to picritic melts with 16–22wt% MgO, Cr of 1594ppm and Ni of 611ppm, which were emplaced at 1450–1500°C. Melt compositions calculated from clinopyroxene compositions from Reinfjord are OIB-like with LREE enriched over HREE.The high abundance of carbonatites and lamproites demonstrates the volatile-rich nature of the mantle source region and is further corroborated by the unusually high abundance of magmatic sulphides (0.5–1%) and carbonated and hydrous assemblages (c. 1%) throughout the region. In Reinfjord, they are also closely associated with PGE-Cu-Ni reef deposits.Essentially, the ultramafic complexes in the SIP comprises deep-seated transient magma chambers that facilitated mixing and homogenisation of a rich diversity of fertile asthenospheric melts en route to the upper parts of the continental crust.

Rapid fore-arc extension and detachment-mode spreading following subduction initiation

Most ophiolites have geochemical signatures that indicate formation by suprasubduction seafloor spreading above newly initiated subduction zones, and hence they record fore-arc processes operating following subduction initiation. They are frequently underlain by a metamorphic sole formed at the top of the downgoing plate and accreted below the overlying suprasubduction zone lithosphere immediately following ophiolite formation. Paleomagnetic analyses of ophiolites can provide important insights into the enigmatic geodynamic processes operating in this setting via identification of tectonic rotations related to upper plate extension. Here we present net tectonic rotation results from the Late Cretaceous Mersin ophiolite of southern Turkey that document rapid and progressive rotation of ophiolitic rocks and their associated metamorphic sole. Specifically, we demonstrate that lower crustal cumulate rocks and early dykes intruded into the underlying mantle section have undergone extreme rotation around ridge-parallel, shallowly-plunging axes, consistent with oceanic detachment faulting during spreading. Importantly, later dykes cutting the metamorphic sole experienced rotation around the same axis but with a lower magnitude. We show that these rotations occurred via a common mechanism in a pre-obduction, fore-arc setting, and are best explained by combining (hyper)extension resulting from detachment-mode, amagmatic suprasubduction zone spreading in a fore-arc environment with a recently proposed mechanism for exhumation of metamorphic soles driven by upper plate extension. Available age constraints demonstrate that extreme rotation of these units was accommodated rapidly by these processes over a time period of <∼3 Myr, comparable with rates of rotation seen in oceanic core complexes in the modern oceans.

Spatial and temporal evolution of kimberlite magma at A154N, Diavik, Northwest Territories, Canada

The A154N kimberlite pipe is part of the Diavik kimberlite cluster in the Northwest Territories, Canada. The pipe is filled by deposits that record a sequence of eruptive and intrusive events for which we have established relative times and modes of emplacement. As such, this sequence of kimberlite deposits provides a unique opportunity to explore the connections between magma properties, pipe or conduit geometry and eruption style. Deposits within the A154N pipe represent four discrete volcanic events including: (1) an array of pre-cursor dykes of coherent kimberlite that intrude the adjoining country rock (CK 1); (2) pyroclastic deposits recording the explosive eruption of A154N (PK 1); (3) late dykes of coherent kimberlite intruding the main infill of the pipe (CK 2); and (4) a sequence of water-laid pyroclastic deposits that sourced from another kimberlite volcano (PK 2). We use observations on polished slabs and thin sections to characterize the magma that erupted to produce each deposit. Image analysis techniques are used to quantify olivine content (abundance and size distributions) and abundances of vesicles within deposits and within individual juvenile pyroclasts. Our analysis shows that during the eruption cycle (CK 1 to PK 1 to CK 2), olivine content increased and volatile content decreased. Lastly, we estimate physical properties for the magmas that produced each deposit. The magma properties are combined with 3-D models for the geometry of the conduit at the time of eruption to constrain the mass flux and duration of each eruptive event. Steady-state assumptions for velocity and magma flux yield total eruption durations of minutes to hours. The diversity of textures and compositions recorded in pyroclastic kimberlite (PK 1) and dykes of coherent kimberlite (CK 1 and CK 2) are a possible manifestation of separated three-phase flow involving kimberlite melt and crystals, and an ever-increasing proportion of exsolved CO 2–H 2O fluid. Specifically, we suggest that the early dykes (CK 1), pyroclastic kimberlite (PK 1), and late-stage dykes (CK 2) are representative of three separate regimes of kimberlite magma created by the ascent of a vigorously degassing magma. We envisage this being composed of: (a) the gas-charged front of the magma created by decoupling the more buoyant gas phase from the silicate melt; (b) a transient, gas-rich body of magma in which later-stage exsolved fluids/gases are essentially coupled to the melt; and (c) a fluid-depleted tail of magma which remains buoyant within the lithosphere but ascends more slowly than the fluid and exsolving gas phases.

Generation of anorthositic magma by H2O-fluxed anatexis of silica-undersaturated gabbro: an example from the north Norwegian Caledonides

The Skattøra migmatite complex in the north Norwegian Caledonides consists of migmatized slightly nepheline-normative metagabbros that are net-veined by numerous (up to 90%) anorthositic and leucodioritic dykes. The average chemical composition of 17 anorthosite dykes is (wt %) 58.4% SiO2, 0.2% TiO2, 23% Al2O3, 1.8% FeOt, 0.7% MgO, 6.3% CaO, 7.8% Na2O, 0.2% K2O. A migmatite leucosome and a dyke have been dated by the U/Pb method on titanite to 456±4 Ma. In low melt fraction areas minor leucosomes are orientated parallel to the foliation. More intense anatexis formed stromatic to schlieric migmatites. The leucosomes are commonly connected to dykes, suggesting that melt segregated and left its source. Dyke thicknesses range from a few centimetres up to several metres. In general, early dykes are parallel to the foliation in the host rock, while the later dykes cut the foliation. Plagioclase (An20–50) is the dominant mineral (85–100%) in the dykes and the leucosome, but 0–15% amphibole is generally present. Field relations, geochemistry and preliminary melting-experiments strongly suggest that the anorthosites originated by H2O-fluxed anatexis of the gabbroic host rock.

Petrogenesis of the Mantle Sequence of the Jormua Ophiolite (Finland): Melt Migration in the Upper Mantle during Palaeoproterozoic Continental Break-up

Upper-mantle rocks are exposed as several fault-bounded blocks INTRODUCTION within the Palaeoproterozoic Jormua Ophiolite, northeastern FinOphiolites, in addition to mantle xenoliths and orogenic land. These blocks differ from each other on the basis of degree of lherzolite massifs, permit the direct study of the petrology, partial melting of the peridotites, type of intrusive mantle dykes geochemistry, and physical properties of the upper (dyke cumulates), and spatially associated ophiolite units. Detailed mantle. Exposed mantle fragments are invariably serfield mapping, and geochemical and Sm–Nd isotopic studies, together pentinized to some degree, but because of their larger with U–Pb zircon dating, indicate that these blocks represent size provide more representative mantle samples than an almost contiguous sequence across an ancient ocean–continent xenoliths. At present, only three early Proterozoic or transition, recording the accretion of a mantle diapir to the subolder rock complexes are known that satisfy the definition continental, most probably Archaean, lithospheric mantle during the of ophiolite (Anonymous, 1972), but only one of these— formation of an incipient oceanic basin ~1950 Ma ago. Rupture the ~1·95 Ga Jormua Ophiolite in northeastern Finof the subcontinental lithosphere was preceded by OIB-like magland—exposes a mantle section as well. For a more matism and coeval emplacement of clinopyroxene+amphibole detailed discussion of the ophiolite status of the Jormua ±garnet mantle dykes as well as pervasive metasomatism of mafic–ultramafic complex the reader is referred to Konsubcontinental peridotites (western block peridotites) at depths of tinen (1987) and Peltonen et al. (1996). Because of its 30–50 km, and emplacement of peralkaline granites in the crust. age, the Jormua Ophiolite has the potential to yield Soon after, parts of the recently accreted suboceanic mantle diapir information concerning the secular evolution of the upper (eastern mantle blocks) were uplifted and intruded by gabbroic mantle and ophiolite-forming processes during the latest mantle dykes, ‘upper-level’ ferrogabbros and plagiogranites, and the two billion years. In the accompanying paper (Peltonen sheeted dyke complex—all being related and progressively shallower et al., 1996) we examined the nature of early Proterozoic expressions of oceanic crust-forming magmatism. After a relatively mantle sources that influenced the chemical and isotopic quiescent period of ~50 Ma within the newly formed passive composition of sheeted dykes and lavas of the Jormua continental margin, fragments of oceanic crust and mantle and Ophiolite. One of our main findings was that two distinct continental mantle were obducted and transported over the Archaean suites of basalts are present in the Jormua Ophiolite: (1) craton to the present location of the Jormua Ophiolite. ‘early dykes’ with OIB-like trace element characteristics

Petrology and Geochemistry of Metabasalts from the 1.95 Ga Jormua Ophiolite, Northeastern Finland

The Jormua Ophiolite exposes a unique fragment of Red Seatype oceanic crust formed in a setting related to continental break-up 1950 Ma ago. Two distinct types of basalt are represented: the ‘early dykes’ and the ‘main basalt suite’. ‘Early dykes’ have fractionated (H) REE patterns, OIB-like trace element patterns, low Zr/Nb (≈6) and ENd(1.95 Ga)≈−0.6, indicative of their derivation from an enriched source. The remaining dykes and all lavas belong to the second, E-MORB-like ‘main basalt suite’, which is characterized by high mg-number and Cr contents, flat REE patterns, Zr/Nb = 6–17, chondritic Th/Ta and only moderately depleted isotopic signatures [END (1.95 Ga)≈+1.9]. Most ‘main suite’ samples cannot be related solely by fractional crystallization to a common parental magma. Rather, they represent distinct melt fractions that underwent variable amounts of chromite + olivine± plagioclase fractionation during ascent. A significant part of the compositional diversity of the ‘main basalt suite’ can be explained by mixing a depleted source with a relatively uniform proportion of an enriched component similar to that represented by the OIB-like ‘early dykes’. It is probable that during the latest stages of continental rifting the OIB-type melts metasomatized the upper part of the depleted asthenospheric mantle, which became the source of the ‘main basalt suite’ soon after the old continental lithosphere was ruptured. The complete absence of any evidence for a subduction-related component in the basalts implies that Jormua is not a back-are ophiolite.

Geochronometry of the Bridge River Camp, southwestern British Columbia

Mineralization at the Bralorne mesothermal gold vein deposit is closely related to a suite of early Late Cretaceous to early Tertiary dykes. Premineral albitite dykes (91.4 ± 1.4 Ma by U–Pb on zircons) and postmineral lamprophyre dykes (43.5 ± 1.5 Ma by K–Ar on biotite) set definite age limits on the mineralizing event. A late intra- to post-mineral green hornblende dyke set (85.7 ± 3.0 Ma by K–Ar on hornblende) that forms a transitional series to the albitites may further restrict the age. Thus, mineralization occurred long after emplacement of the host Bralorne intrusions, dated as Early Permian (minimum age of approximately 270 ± 5 Ma by U–Pb on zircons, 284 ± 20 Ma by K–Ar on hornblende, and 40Ar/39Ar plateau at 276 ± 31 Ma). Lithologically similar intrusions 20 km to the north near Gold Bridge are also Early Permian (287 ± 20 Ma by K–Ar on hornblende and 320 ± 80 Ma by a Rb–Sr whole-rock isochron). Geochronology, radiogenic and stable isotopes, and fluid-inclusion studies suggest that there were several pulses of mineralizing activity adjacent to and east of the Coast Plutonic Complex (CPC). Decreasing temperatures and younger age of mineralization with increasing distance from the CPC imply that plutons of the CPC were the main heat source responsible for mineralization. The main pulses were about 90 Ma for mesothermal Au–Ag–As ± W,Mo mineralization at Bralorne near the CPC, ranging outwards to 65 Ma for Ag–Au–Sb–As ± Hg mineralization at the Minto and Congress deposits, to 45 Ma for Ag–Au epithermal mineralization at Blackdome, 100 km east of the CPC.The Bralorne intrusions may have been emplaced below the sea floor in a spreading-ridge oceanic environment, as suggested by the petrology of the intrusive suite, which includes serpentinized ultramafite, hornblende diorite, and soda granite (trondhjemite), typical of an ophiolite association. The chemistry of volcanic rocks mapped as Cadwallader Group, which host these intrusive bodies, is transitional from mid-ocean-ridge basalts to island-arc tholeiite, suggesting a back-arc-basin setting. Gradational contact relations between the hornblende diorite and the volcanic rocks suggest that the diorite intruded its own volcanic products. Intrusive contacts of the diorite with adjacent elongate ultramafic bodies imply that the ultramafic rocks are of Permian or older age and had been structurally emplaced into crustal levels by the time of diorite intrusion. In the Bralorne fault block the Bralorne intrusions appear to cut the adjacent Cadwallader and Bridge River groups, implying an Early Permian or older age for at least parts of these groups. Thus, rocks mapped as Cadwallader Group in the Bralorne area could be distinct from and older than lithologic equivalents exposed elsewhere, although they are similar in terms of their petrology and major- and trace-element chemistry.

Late Caledonian dyke‐swarms in Southern Scotland: New field, petrological and geochemical data for the Wigtown Peninsula, Galloway

AbstractSeveral hundred, dominantly NE‐trending dykes invade Silurian flysch sediments of the Wigtown Peninsula. The most basic dykes are K‐rich biotite‐lamprophyres, hornblende‐lamprophyres and appinites ('diorites' of previous maps). More acidic (qz and cm normative) dykes include restricted andesitic compositions ('porphyritcs', c. 57 per cent SiO2) and a continuum from dacitic to rhyolitic compositions ('acid porphyrites' and 'porphyries', c. 63–71 per cent SiO2). The relative abundance of basic to intermediate + acidic dykes, based on examination of 180 thin sections and 70 new whole‐rock analyses, is approximately 7:1. The biotite‐ and hornblende‐lamprophyres are distinct in chemistry and distribution, biotite‐lamprophyres being confined to the south of the Peninsula. Neither type has any compositional equivalent among the plutonic rocks of the area. Porphyrite and porphyry dykes do not represent simple lamprophyre fractionates, but may be derived from at least two (intermediate and acidic) parent magmas, which may represent crustally‐contaminated lamprophyre or entirely separate crustal melts. Emplacement of all dyke compositions was apparently episodic through Wenlock to early Devonian times (c. 425‐395 Ma). Early dykes post‐date the main deformation (D1) but predate minor (D2) folding, both D1 and D2 being related to thrusting. On the current accretionary model for the Southern Uplands, this implies that dyke emplacement commenced before the end of accretion in a compressional tectonic regime—scarcely consistent with the moderate (up to 6 per cent) crustal extension represented by the dykes. The geochemistry of the lamprophyres requires a deep mantle origin several hundred kilometres behind any subduction zone. This suggests that the 'lapetus suture', at the time of dyke emplacement, was much farther south than its currently inferred position and places the Southern Uplands in a back‐arc situation.

Petrology and geochemistry of the early Mesozoic Caraquet dyke, New Brunswick, Canada

The Caraquet dyke of New Brunswick is one of three early Mesozoic dykes recognized in Atlantic Canada. It has a quartz-normative tholeiitic composition intermediate in character between "low-Ti" and "high-Ti" Appalachian diabase. Major-element (e.g., MgO, SiO2) and trace-element concentrations (e.g., Ba, Zr) suggest that the dyke is less evolved than the Shelburne dyke but slightly more evolved than the Avalon dyke. Although related to continental rifting and opening of the Atlantic Ocean, the dykes of the Atlantic Provinces show little in common with continental rift volcanic rocks. They resemble MORB except that their K, Rb, and Ba concentrations are much higher. Semi-quantitative modelling suggests that the elevated large-ion lithophile-element concentrations reflect assimilation of continental crust. Differences in composition among the three dykes may be due to variations in the quantity and composition of assimilated material as well as the degree of differentiation. As with other early Mesozoic dykes of eastern North America, the emplacement of these rocks was structurally controlled: (1) their orientations reflect a rift-induced stress field and (2) their location reflects control by Paleozoic lithotectonic terranes of the Applachian Orogen.

REE distributions in a high-grade Archaean gneiss complex in Scotland: Implications for the genesis of ancient sialic crust

Seventeen rocks from the Lewisian Gneiss of the Inner Hebrides of Scotland, which represent three distinct lithological types at granulite to greenschist facies of metamorphism show rare-earth element patterns which seem not to have been disturbed by their complex metamorphic history. Some indication of their origin can be obtained by simple geochemical models. Three tonalitic pyroxene gneisses are characterized by: (1) light REE enrichment and heavy REE depletion; (2) low total REE contents; (3) moderate Eu enrichment. Their REE chemistry can be approximated by a model involving 10% partial melting of various garnet-bearing basaltic source materials. Alternatively, they may be some form of crystal cumulate, preserving their original anhydrous mineralogy, representing 30% crystallization of a parent tonalitic magma. Three tonalitic to granodioritic hornblende-biotite gneisses are characterized by: (1) light REE enrichment and heavy REE depletion; (2) significantly higher total REE contents than the pyroxene gneisses; (3) moderate Eu depletion. Their REE patterns can be approximated by a residual silicic melt in a model involving 30% fractional crystallization of solids with the modal mineralogy of the pyroxene gneisses or 40% removal of pure anorthosite from a parent dacitic magma. Two strongly metasomatised diopside-actinolite gneisses and one highly sheared epidote-chlorite gneiss have REE patterns which are not significantly different from the hornblende-biotite gneisses which were their precursors before metasomatism and late greenschist-facies shearing. This suggests that strong alteration has not enciphered the REE systematics of the gneisses. Basic gneisses of quartz tholeiite composition occurring as early dykes, which shared the same metamorphic history as the tonalitic to granodioritic gneisses, are characterised by: (1) slight enrichment in light REE relative to heavy REE; (2) variable total REE contents; (3) little difference between granulite and amphibolite facies types. Their REE patterns can be matched by models involving 5–15% partial melting of ultrabasic mantle with 3 times chondritic REE abundances, leaving a residue of olivine and orthopyroxene.

Early Transverse Dykes in the Main Donegal Granite

AbstractCertain microtonalite dykes cut across the flow banding of the Main Donegal Granite in the direction of an important cross-joint, yet they and their granite host are shown to have crystallized together and in the same manner as both the darker flow bands in the granite and the pelitic xenoliths within it. It appears that an embryonic joint system existed in the Donegal Granite before consolidation and crystallization were complete. It is suggested that the early dykes represent metasomatized pelitic enclave material and are not differentation products of granitic magma. Comparison is made with the basic analogue provided by the beerbachite enclaves and dykes of the hypersthene-gabbro of Ardnamurchan.