Ridge Basalt Type Research Articles

Mid-Ordovician stratigraphy and volcanism in the Hølonda area, Scandinavian Caledonides: complex tectonomagmatic development following arc–continent collision near the Laurentian margin of Iapetus

The Hølonda area of the central Scandinavian Caledonides is a key for models of the Caledonian orogen owing to its Ordovician fauna of Laurentian affinity, now stranded on the Baltic side during opening of the North Atlantic. Here, we present a revised stratigraphic and tectonomagmatic model based on remapping, sedimentology, igneous geochemistry, Nd and Sr isotopes and geochronology. The Hølonda Group ( c. 470–461 Ma) reflects a transition from subaerial and shallow-marine deposition on a continental shelf to deeper-water sedimentation along a subsiding slope. Adakitic and mid-ocean ridge basalt type magmatism at c. 468 Ma was succeeded by benmoreitic–rhyolitic, shoshonitic, calc-alkaline and ultra-alkaline volcanism at c. 467–465 Ma. The complex magmatism followed arc–continent collision along a microcontinent outboard of Laurentia, associated with subduction polarity flip and slab rollback. This led to rifting and opening of a wide basin and its adjoining shelf on thickened orogenic lithosphere. Associated mantle upwelling and partial melting of depleted and variably metasomatized mantle occurred in a tectonomagmatic setting comparable with that of the central Mediterranean. The Hølonda–Ilfjellet setting is unique along the Caledonian–Appalachian orogen, possibly reflecting interaction with Laurentia-derived continental terranes at the northeastern end of the Taconian–Grampian orogenic tract. Supplementary material: Analytical data and supplementary figures are available at https://doi.org/10.6084/m9.figshare.c.6368922 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

Contrasting mechanisms and timescales of subduction and exhumation as recorded by Paleoproterozoic and late Paleozoic high-pressure granulites in the North China Craton

Abstract The North China Craton records multiple metamorphic events related to supercontinent assembly during the Paleoproterozoic, forming Columbia, and again during the late Paleozoic, forming Pangea. Here we show that the Paleoproterozoic high-pressure granulites (HPGs) formed from enriched mid-ocean ridge basalt protoliths and record a clockwise pressure-temperature-time (P–T–t) path with prograde metamorphism at 7.8–10.0 kbar and 780–820 °C, peak granulite-facies metamorphism at 12–12.3 kbar and ∼860–880 °C, and retrograde metamorphism at 8.7–9.1 kbar and 850–855 °C. Subduction initiated prior to 1.90 Ga, with final collision and orogeny at 1.88 Ga, followed by post-collision/exhumation at 1.80 Ga, defining a prolonged exhumation period (∼90 m.y.) that occurred at a slow velocity of ∼0.16 ± 0.08 mm/y. Late Paleozoic HPGs are normal mid-ocean ridge basalt type and record a near clockwise P–T–t path, with peak/post-peak amphibolitefacies metamorphism at 11.0–12.5 kbar and 860–890 °C, isothermal decompression to 7.2–7.5 kbar and 810–820 °C, and retrogression to 5.5–7.2 kbar and 805–850 °C. Subduction initiated earlier than ca. 340 Ma, exhumation and uplift initiated at 335–309 Ma and continued to 297–287 Ma. The exhumation was short-lived (∼50 m.y.) and relatively fast (0.38 ± 0.14 mm/y). When compared to granulite-facies metamorphism documented in many Paleoproterozoic HPGs, late Paleozoic HPGs appear to commonly form with an initial period of steep subduction leading to eclogite-facies metamorphism, with subsequent exhumation to middle/lower levels of the crust. Our results further reveal that the exhumation velocity for supercontinent collision was facilitated and duration shortened through time, and that the exhumation mechanism might have been controlled by subduction angle, compression pressure, and temperature.

100 myr cycles of oceanic lithosphere generation in peri-Gondwana: Neoproterozoic–Devonian ophiolites from the NW African–Iberian margin of Gondwana and the Variscan Orogen

Abstract The Variscan Orogen in Iberia and the Anti-Atlas Mountains in Morocco contains a set of ophiolites formed between Neoproterozoic and Devonian times, during the complex evolution of the NW African–Iberian margin of Gondwana. During this time interval, the margin evolved from an active margin ( c. 750–500 Ma: the Reguibat–Avalonian–Cadomian arc) to the final collision with Laurussia in Devonian times to form Pangaea. In this context, one of the oldest recognized ophiolites is the Bou Azzer Ophiolite from the Anti-Atlas Mountains, dated at c. 697 Ma and containing two types of mafic rocks, the youngest of which has a boninitic composition. To the north, in the SW Iberian Massif, the Calzadilla Ophiolite contains mafic rocks also of boninitic composition dated at c. 598 Ma. Farther north, in the NW Iberian Massif, the Vila de Cruces Ophiolite is formed by a thick sequence of mafic rocks with an arc tholeiitic composition and minor alternations of tonalitic orthogneisses dated at c. 497 Ma. In the same region, the Bazar Ophiolite has a similar age of c. 495 Ma. Also in NW Iberia, there is a group of ophiolites with varied lithologies and dominant mafic rocks with arc tholeiitic composition (Careón, Purrido and Moeche ophiolites) dated at c. 395 Ma. The composition of all these peri-Gondwanan ophiolites is of supra-subduction zone type, showing no evidence of preserved mid-ocean ridge basalt type oceanic lithosphere. Consequently, these ophiolites were generated in the peri-Gondwanan realm during the opening of forearc or back-arc basins. Forearc oceanic lithosphere was promptly obducted or accreted to the volcanic arc, but the oceanic or transitional lithosphere generated in back-arc settings was preserved until the assembly of Pangaea. Based on the ages of the described ophiolites, the peri-Gondwanan realm has been a domain where the generation of oceanic or transitional lithosphere seems to have occurred at intervals of c. 100 myr. These regularly spaced time intervals may indicate cyclic events of mantle upwelling in the peri-Gondwanan mid-ocean ridges, with associated higher subduction rates at the peri-Gondwanan trenches and concomitant higher rates of partial melting in the mantle wedges involved. The origin of the apparent cyclicity for mantle upwelling in the peri-Gondwanan ocean ridges is unclear, but it could have possibly been related to episodic deep mantle convection. Cycles of more active deep mantle convection can explain episodic mantle upwelling, the transition from low- to fast-spreading type mid-ocean ridges and, finally, the dynamic context for the episodic generation of new supra-subduction zone type oceanic peri-Gondwanan lithosphere.

Rapid transition from continental breakup to igneous oceanic crust in the South China Sea

Continental breakup represents the successful process of rifting and thinning of the continental lithosphere, leading to plate rupture and initiation of oceanic crust formation. Magmatism during breakup seems to follow a path of either excessive, transient magmatism (magma-rich margins) or of igneous starvation (magma-poor margins). The latter type is characterized by extreme continental lithospheric extension and mantle exhumation prior to igneous oceanic crust formation. Discovery of magma-poor margins has raised fundamental questions about the onset of ocean-floor type magmatism, and has guided interpretation of seismic data across many rifted margins, including the highly extended northern South China Sea margin. Here we report International Ocean Discovery Program drilling data from the northern South China Sea margin, testing the magma-poor margin model outside the North Atlantic. Contrary to expectations, results show initiation of Mid-Ocean Ridge basalt type magmatism during breakup, with a narrow and rapid transition into igneous oceanic crust. Coring and seismic data suggest that fast lithospheric extension without mantle exhumation generated a margin structure between the two endmembers. Asthenospheric upwelling yielding Mid-Ocean Ridge basalt-type magmatism from normal-temperature mantle during final breakup is interpreted to reflect rapid rifting within thin pre-rift lithosphere.

Tectonic evolution of syn- to late-orogenic sedimentary–volcanic basins in the central Norwegian Caledonides

We present new structural, geochemical and U–Pb zircon data from syn- to late-orogenic sedimentary–volcanic basins in the southwestern part of the Trondheim Nappe Complex, central Norwegian Caledonides. In this area, a succession of enriched mid-ocean ridge basalt type metabasalt, jasper, ribbon chert with associated sandstone and conglomerate, and green siltstone is interpreted to represent volcanism and sedimentation in a hitherto little-known spreading-dominated tectonic environment. This environment is different from the suprasubduction-zone ophiolite setting dominating the Iapetus rock record elsewhere in the Scandinavian Caledonides. This volcanic and sedimentary succession was overturned and isoclinally folded in a pre-427 Ma orogenic phase. Post-427 Ma cross-bedded sandstones were deposited on the eroded surface of the previously deformed rocks, representing a rare example of a late Silurian or younger sedimentary basin within the Scandinavian Caledonides. The cross-bedded sandstones are intercalated with and/or overlain by post-427 Ma intermediate volcanic or subvolcanic rocks of calc-alkaline composition, representing a hitherto unknown volcanic phase within the Trondheim Nappe Complex and elsewhere within the Scandinavian Caledonides. Their particular geochemical signature could be the result of late-stage subduction-zone volcanism just prior to the onset of continent–continent collision between Baltica and Laurentia, or much younger post-collisional extensional melting with inherited subduction signatures. Supplementary material: A description of analytical methods and a table of geochronological data from LA-ICP-MS analysis of zircons are available at https://doi.org/10.6084/m9.figshare.c.3994605 .

Enriched mid–ocean ridge basalt–type geochemistry of basalts and gabbros from the Nikoro Group, Tokoro Belt, Hokkaido, Japan

Enriched mid–ocean ridge basalt–type geochemistry of basalts and gabbros from the Nikoro Group, Tokoro Belt, Hokkaido, Japan

Möwe Bay Dykes, Northwestern Namibia: Geochemical and geochronological evidence for different mantle source regions during the Cretaceous opening of the South Atlantic

Dyke emplacement in the course of Paraná-Etendeka volcanism in northwestern Namibia has been considered as a short-lived event related to a specific magma source at approximately 135–130Ma. New geochemical, whole rock Sr-Nd-Pb isotope data and 40Ar/39Ar ages reveal that at least three geochemically and isotopically different tholeiitic dyke generations, with intrusion ages of 135.2±0.7, 124.1±0.8Ma and 113.0±0.5Ma, can be distinguished in the Möwe Bay area, Skeleton Coast of northwestern Namibia. Distinct mantle source components were identified in the petrogenesis of the various dyke generations. Magma composition of the two older dyke suites, both of which were emplaced in a tectonic setting dominated by E-W extension, evolved from a within-plate to an enriched mid-ocean ridge basalt type. Contamination by continental crust and/or lithospheric mantel is suggested by a high 238U/204Pb-value (=μ2) of 9.97, which is typical of the Kalahari Craton. The third dyke generation, which intruded in a tectonic regime dominated by SE-NW extension, corresponds to high-Ti ocean island-type basalt with unradiogenic isotopic compositions derived from a mantle source with a μ2-value of 9.35.The formation of the older dykes is attributed to the presence of a thermal anomaly in the upper sublithospheric mantle. This anomaly was most likely caused by the peripheral part of a mantle plume that impinged at the base of the lithosphere and caused erosion of the subcontinental mantle lithosphere and melting above the plume head. The initial break-up of SW Gondwana and the formation of early oceanic crust were most likely due to passive rather than active rifting. Continued plume upwelling facilitated by progressive thinning of SW-Gondwana crust led to the formation of the younger, c. 113Ma old dykes, which are chemically and isotopically identical to coeval rocks from the northeastern portion of the Walvis Ridge and thus are interpreted as onshore expressions of the Tristan-Gough plume head at that time. The difference in the dominant extension directions of the older and the younger dyke generations can be explained by rotation induced by the Aptian/Albian opening of the Equatorial Central Atlantic, accompanied by a substantial increase in the South Atlantic spreading rate.

Rifting under steam—How rift magmatism triggers methane venting from sedimentary basins

Abstract During opening of a new ocean, magma intrudes into the surrounding sedimentary basins. Heat provided by the intrusions matures the host rock, creating metamorphic aureoles potentially releasing large amounts of hydrocarbons. These hydrocarbons may migrate to the seafloor in hydrothermal vent complexes in sufficient volumes to trigger global warming, e.g., during the Paleocene-Eocene Thermal Maximum (PETM). Mound structures at the top of buried hydrothermal vent complexes observed in seismic data off Norway were previously interpreted as sediment volcanoes, and the amount of released hydrocarbon was estimated based on this interpretation. Here, we present new geophysical and geochemical data from the Gulf of California suggesting that such mound structures could in fact be edifices constructed by the growth of black smoker–type chimneys rather than sediment volcanoes. We have evidence for two buried and one active hydrothermal vent systems outside the rift axis. The active vent releases fluids of several hundred degrees Celsius containing abundant methane, mid-ocean ridge basalt–type helium, and precipitating solids up to 300 m high into the water column. Our observations challenge the idea that methane is emitted slowly from rift-related vents. The association of large amounts of methane with hydrothermal fluids that enter the water column at high pressure and temperature provides an efficient mechanism to transport hydrocarbons into the water column and atmosphere, lending support to the hypothesis that rapid climate change such as during the PETM can be triggered by magmatic intrusions into organic-rich sedimentary basins.

Geology of metamorphic rocks deriving from paleohydrothermal systems in the Mesoproterozoic Serra do Itaberaba Group, São Paulo State, southeastern Brazil

ABSTRACTIn the central portion of the Ribeira fold belt, southeastern Brazil, the Mesoproterozoic volcanosedimentary Serra do Itaberaba Group was deposited in an ocean basin having normal mid-ocean ridge basalt type basalts that evolved to a back-arc environment. This succession was affected by two medium-grade regional metamorphic events and a third low-grade retrometamorphic event. This geological map covers an area of approximately 16 km2, between 23°16′41.824″ and 23°18′47.744″ latitudes S, and 46°20′57.056″ and 46°23′18.933″ longitudes W, as a scale of 1:5000. It encompasses the metamorphic products of tectonically deformed paleohydrothermal systems that developed in a back-arc environment, which are spatially and genetically linked to small metamorphosed andesitic-rhyolitic bodies. These systems were responsible for lixiviation of Ca and alkali in deeper parts, carbonatization in shallower parts, and a first large chloritic alteration zone (CZ1) that was crosscut by small chloritic (CZ2), silicification, and advanced argillic alteration zones. The metamorphic products of CZ1 are cummingtonite/anthophyllite rocks, whereas those related to CZ2 are Mg-rich chloritites. The metamorphic products of silicification and advanced argillic alteration zones are rocks composed of quartz ± specularite and of corundum, margarite, sericite, tourmaline, rutile, and Ca-plagioclase, respectively. Those associated with Ca and alkali depletions are garnet-hornblende amphibolites, whereas those related to carbonatization zones are composed of diopside, hornblende, tremolite/actinolite, carbonate, clinozoisite/epidote, plagioclase, and quartz. Cummingtonite/anthopyllite rocks and Mg-rich chloritites are similar to those associated with metamorphosed Kuroko-type volcanogenic massive sulfide deposits, whereas rocks composed of corundum, margarite, sericite, tourmaline, rutile, and Ca-plagioclase are genetically associated with oceanic high-sulfidation magmatic-hydrothermal gold mineralization.

Evolution of recycled crust within the mantle: Constraints from the garnet pyroxenites of the External Ligurian ophiolites (northern Apennines, Italy)

Research Article| October 01, 2015 Evolution of recycled crust within the mantle: Constraints from the garnet pyroxenites of the External Ligurian ophiolites (northern Apennines, Italy) Alessandra Montanini; Alessandra Montanini 1DIFEST, Università di Parma, Parco Area delle Scienze 157a, 43100 Parma, Italy Search for other works by this author on: GSW Google Scholar Riccardo Tribuzio Riccardo Tribuzio 2Dipartimento di Scienze della Terra e dell’Ambiente, Università di Pavia, Via Ferrata 9, 27100 Pavia, Italy3CNR–Istituto di Geoscienze e Georisorse–U.O.S. (Unità Operativa di Supporto) Pavia, via Ferrata 1, 27100 Pavia, Italy Search for other works by this author on: GSW Google Scholar Author and Article Information Alessandra Montanini 1DIFEST, Università di Parma, Parco Area delle Scienze 157a, 43100 Parma, Italy Riccardo Tribuzio 2Dipartimento di Scienze della Terra e dell’Ambiente, Università di Pavia, Via Ferrata 9, 27100 Pavia, Italy3CNR–Istituto di Geoscienze e Georisorse–U.O.S. (Unità Operativa di Supporto) Pavia, via Ferrata 1, 27100 Pavia, Italy Publisher: Geological Society of America Received: 12 Apr 2015 Revision Received: 07 Aug 2015 Accepted: 09 Aug 2015 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2015 Geological Society of America Geology (2015) 43 (10): 911–914. https://doi.org/10.1130/G36877.1 Article history Received: 12 Apr 2015 Revision Received: 07 Aug 2015 Accepted: 09 Aug 2015 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Alessandra Montanini, Riccardo Tribuzio; Evolution of recycled crust within the mantle: Constraints from the garnet pyroxenites of the External Ligurian ophiolites (northern Apennines, Italy). Geology 2015;; 43 (10): 911–914. doi: https://doi.org/10.1130/G36877.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The pyroxenite-peridotite sequence from the External Ligurian (northern Apennines, Italy) ophiolites is evidence of the evolution of recycled crust within the mantle. We present new major and trace element and Nd-Hf isotopic compositions of garnet clinopyroxenites and websterites from this mantle section. The garnet clinopyroxenites display clinopyroxene and bulk-rock rare earth element patterns with distinct positive Eu anomalies, which argue for the involvement of plagioclase-rich precursors in their origin. We propose that the garnet clinopyroxenites formed by crystallization of eclogite-derived melts that underwent negligible interaction with the host peridotites. The garnet websterites are interpreted to have been produced by reactions between the eclogite-derived melts and peridotites, thereby giving rise to hybrid, second-stage pyroxenites with a crustal geochemical fingerprint. In our petrogenetic scenario, a rifting-related event at ca. 220 Ma caused melting of eclogites originating from a mid-oceanic ridge basalt–type gabbroic sequence. These mafic protoliths underwent a long-lived evolution of recycling in the mantle (1.5–1.0 Ga). We show that heterogeneity of crustal protoliths, age of recycling, and interaction with the host peridotites may lead to a significant compositional and isotopic diversity of crust-derived mantle pyroxenites. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

High-Pressure Tourmaline Formation and Fluid Activity in Fe–Ti-rich Eclogites from the Kreuzeck Mountains, Eastern Alps, Austria

A tourmaline-bearing eclogite from the Kreuzeck Mountains, Eastern Austria, is described in this study as part of an eclogite suite situated SE of theTauern Window.These rocks formed during Eoalpine high-P metamorphism of the Austroalpine basement. They have high and variable Fe, Ti, P, Zr and Sr contents and unlike other Eoalpine eclogites their Ti^V^Zr^P^Nb^Y systematics indicates a within-plate rather than mid-ocean ridge basalt type environment for the formation of their protoliths. Major and trace element variations are consistent with progressive differentiation of a mafic magma with Fe^Ti oxide and apatite as important igneous phases and with metasomatic addition of K and Sr. The eclogites

Geodynamic significance of ophiolites within the Calabrian Arc

Abstract Slices of oceanic lithosphere belonging to the neo‐Tethys realm crop out discontinuously in the northern Calabrian Arc, Southern Apennines. They consist of high‐pressure–low‐temperature metamorphic ophiolitic sequences formed from metaultramafics, metabasites and alternating metapelites, metarenites, marbles and calcschist. Ophiolites occupy an intermediate position in the northern Calabrian Arc nappe pile, situated between overlying Hercynian continental crust and the underlying Apenninic limestone units. In the literature, these ophiolitic sequences are subdivided into several tectonometamorphic units. Geochemical characteristics indicate that metabasites were derived from subalkaline basalts with tholeiitic affinity (transitional mid‐oceanic ridge basalt type), and a harzburgitic‐lherzolitic protolith is suggested for the serpentinites. The pressure–temperature‐deformation paths of the metabasites from different outcrops display similar features: (i) the prograde segment follows a typical Alpine geothermal gradient up to a metamorphic climax at 350°C and 0.9 GPa and crystallization of the high‐pressure mineral assemblage occurs along a pervasive foliation developed during a compressive tectonic event; and (ii) the retrogression path can be subdivided in two segments, the first is characterized by nearly isothermal decompression to approximately 400°C and 0.3 GPa and the second follows a cooling trajectory. During low‐pressure conditions, a second deformation event produces millimetric to decametric scale asymmetric folds that describe west‐verging major structures. The third deformation event is characterized by brittle extensional structures. The tectonometamorphic evolution of the ophiolitic sequences from the different outcrops is similar. Both thermobarometric modeling and tectonic history indicate that the studied rocks underwent Alpine subduction and exhumation processes as tectonic slices inside a west‐verging accretionary wedge. The subduction of oceanic lithosphere was towards the present east; therefore, the Hercynian continental crust, overthrusted on the ophiolitic accretionary wedge after the neo‐Tethys closure, was part of the African paleomargin or a continental microplate between Africa and Europe.

Reconstructing early Himalayan tectonic evolution and paleogeography from Tertiary foreland basin sedimentary rocks, northern India

The latest Paleocene–middle Eocene Subathu Formation and the Oligocene–Miocene Dagshai and Kasauli Formations of the Indian foreland basin record the early evolution of the Himalayan fold-thrust belt. Sandstone petrography of the Subathu Formation shows a predominantly recycled sedimentary source, with a distinct ophiolitic and volcanic influence that was drastically reduced by the time of deposition of the Dagshai Formation. Sandstones in the Dagshai and Kasauli Formations consist predominantly of metapelitic detritus. The metamorphic grade of metapelitic lithic grains increases with time, from dominantly very low grade at the base of the Dagshai Formation to dominantly low grade in the Kasauli sandstones. Mudstone geochemistry documents the presence of a mafic-ultramafic source during the time of deposition of the Subathu Formation that becomes significantly less important by the time of deposition of the Dagshai Formation. Compositions of Subathu Formation detrital spinels show they were either derived from both mid-ocean ridge basalt–type and arc-type ophiolites or from an ophiolite of composite origin, and that southerly derivation from the Deccan Trap continental flood basalts is unlikely. Kasauli Formation garnet compositions suggest derivation from medium-grade metamorphic rocks to amphibolite facies. Subathu Formation composition reflects provenance influence from the Indus suture zone during latest Paleocene–middle Eocene time, indicating initiation of continent-continent collision and development of the foreland basin by this time. Suture-zone influence was drastically reduced by the time of deposition of the Dagshai Formation, when the embryonic thrust belt provided a barrier sufficient to partially separate the suture zone from the Himalayan foreland basin. The first appearance of Himalayan metamorphic detritus occurs in the Dagshai Formation at the close of Oligocene time, whereas the Kasauli Formation records erosion to deeper metamorphic levels during earliest Miocene time. The occurrence of garnet and higher grade metamorphic lithic grains during early Neogene time is coincident with the timing of displacement along the Main Central thrust and South Tibetan detachment zone. Composition of early foreland basin sediments from Pakistan (Balakot, Murree and Kamlial Formations) to Nepal (Bhainskati and Dumri Formations) and Bangladesh (Kopili, Barail Formations; Surma Group) indicates diachronous arrival of ophiolitic to low-grade metamorphic detritus derived respectively from the Indus Suture and early Himalayan thrust sheets in the north. This is consistent with progressively later closure of Neotethys along the suture, from latest Paleocene time in the west to Eocene time or even later in the east.

Magmagenesis at Soufriere Volcano, St Vincent, Lesser Antilles Arc

Soufriere volcano of St Vincent ( 3 wt %, whereas various projections onto phase diagrams are more consistent with relatively anhydrous magmas. Primary magmas at Soufriere were generated by around 15% melting of mid-ocean ridge basalt type mantle sources which had been modified by addition of fluids released from the slab containing contributions from subducted sediments and mafic crust.

Differentiation and source of the Nipissing Diabase intrusions, Ontario, Canada

Major and trace element data are presented for 2.2 Ga Proterozoic diabase sills from across the Nipissing magmatic province of Ontario. In situ differentiation of the magma coupled with assimilation of Huronian Supergroup roof sediments is responsible for the variation in composition between quartz diabase and granophyric diabase seen within many of the differentiated intrusions. Uniform trace element and isotope ratio signatures, such as La/Sm (2.8 – 3.7) and εNdCHUR (−2.7 to −5.9) characterize chilled margins and undifferentiated quartz diabases. These chemical signatures suggest the existence of a single magma source that was parental to intrusions throughout the magmatic province; this magma has higher La/Sm and lower Ti/Y than primitive mantle and is displaced towards the composition of shales. Most chilled diabases and quartz diabases have a similar Mg# (0.64 and 0.60) and Ni content (98 and 127 ppm), and it is argued that the magma differentiated at depth and was emplaced as a uniform low-Mg magma. The Wanapitei intrusion and Kukagami Lake sill are an exception in that although the quartz diabase has La/Sm similar to the Nipissing magma type, which suggests that they came from the same source, the Mg# (0.68–0.71) and Ni content (130–141 ppm) are higher, which may suggest that they are either slightly more primitive examples of the normal Nipissing magma or that cumulus hypersthene has been resorbed. The light rare earth element enriched signature of the Nipissing magmas was perhaps introduced from the continental crust as the magma migrated from the mantle to the surface, but a remarkably constant and large amount (>20%) of crustal contamination would be required. An addition of 1 –3% shale to the source of a transitional mid-ocean ridge basalt type magma can broadly reproduce the compositional features of the Nipissing magma type. The source characteristics were perhaps imparted during subduction accompanying the terminal Kenoran orogeny.

Alkaline rocks and their inclusions: a window on the Earth’s interior

Summary The majority of inclusion-bearing alkaline rocks have a source region identical with that of ocean island basalts regardless of whether they were erupted through oceanic or continental crust. Models have been proposed where oceanic alkaline rocks are derived from the asthenosphere and during passage to the surface there have occurred variable amounts of interaction with lithosphere of the mid-ocean ridge basalt type. A similar scenario would account for most inclusion-bearing continental alkaline rocks. Genesis of micaceous kimberlites and lamproites, however, may require that asthenospheric plumes have impinged on ‘nuggets’ of aged heterogeneous enriched sub-continental lithosphere. This would lead to mixing of isotopically distinct asthenospheric and lithospheric melts.

A new Tertiary sill complex of mid-ocean ridge basalt type NNE of the Shetland Isles: a preliminary report

ABSTRACTBasic intrusive rocks recently encountered in wells N and NNE of the Shetland Isles are probably parts of a single large sill complex which extends for over 130 km along the edge of the Faeroe-Shetland Trough. The sills intrude thick Mesozoic sediments which almost certainly overlie continental crust but the complex also appears to underlie, and extend beyond the SE edge of, the Faeroes basaltic lava plateau. Petrographic and geochemical analyses of drill core samples recovered from some of these sills reveal that they are of mid-ocean ridge basalt (MORB) type; an observation which provides evidence regarding the plate tectonic history of this area of the North Atlantic and has major implications for the nature of the continental/oceanic crust transition.