N-type Mid-ocean Ridge Basalts Research Articles

Age and petrology of Eocene-Oligocene calc-alkaline volcanism in Biga Peninsula (NW Turkey): Implications for magma origin and geodynamic evolution

In NW Anatolia, Eocene-Oligocene post-collisional volcanism is represented by K-rich volcanic rocks, including high K calc-alkaline products. Magmatic rocks in the Dededağ area (Biga Peninsula, NW Turkey) can be classified into 6 different groups with lithostratigraphic and chronostratigraphic data using 40Ar/39Ar methods (Eocene-early Oligocene, 42.08 ± 0.09 to 29.02 ± 0.29 Ma). From the Eocene period, there are calc-alkaline Laledağ granodiorite, Beyçayır volcanics with andesite and dacite composition, and also Kızıldam volcanics with alkaline and basic character (basaltic trachyandesite, trachybasalt). Furthermore, in the Oligocene period, calc-alkaline volcanism started with Dededağ volcanics with dacite and rhyolite intermediate-acidic composition, continued with Kırcalar volcanics consisting of basalt and basaltic andesite, and Harmancık volcanics ranging from basalt to rhyolite. The calc-alkaline Çamyayla pluton with quartz monzonite composition intruded into Oligocene Dededağ and Harmancık volcanics in the region. Alkaline lavas in the region are classified as high-K and shoshonitic, and calc-alkaline lavas are medium-high-K. On N-type mid-ocean ridge basalt and chondrite-normalized trace element diagrams, enrichment was observed for all volcanic units with regard to large ion lithophile elements and light rare earth elements. High field strength elements and heavy rare earth elements were depleted in all volcanic units. These compositions reveal the participation of a mantle source that was enriched by subduction components. Using geochemical data, all the volcanic rocks differentiated as a result of magma mixing/replenishment and contamination/assimilation processes. Consequently, the source of volcanism is thought to derive from the compressional regime associated with collision during the Eocene-Oligocene, and from a previously-enriched lithospheric mantle source with subsequent effects of the extensional regime.

Age and composition of dykes emplaced before and during the opening of the Tasman Sea—source implications

Dykes are common in the wave-cut platforms along the coast from Newcastle to Sydney. According to some authors, they may be related to the opening of the Tasman Sea that commenced ca 84 Ma ago. However, there are few detailed radiogenic dating and geochemical studies to evaluate this. We attempt to resolve this by K–Ar dating of plagioclase in and geochemical studies of, basaltic dykes intruding Permo-Triassic sequences on the wave-cut platforms and Carboniferous and Permo-Triassic sequences inland. The plagioclase separated from the dykes give K–Ar ages ranging from 266 to 53 Ma with the majority older than 84 Ma indicating that most dykes were emplaced before the Tasman Seafloor formation. The dykes are generally mildly alkaline, high-Ti basalts; fewer are tholeiitic and calc-alkaline, low-Ti basalts. Strongly light rare earth element (LREE)-enriched patterns typify the former and flat, LREE-depleted or slightly to moderately enriched LREE patterns, the latter. High-Ti basalts have ocean-island-basalt-like and low-Ti basalts, calc-alkaline or mid-ocean ridge basalt (MORB)-like patterns. Most high-Ti and some low-Ti basalts show plume-like characteristics, others N-type MORB and arc-like characteristics. Dykes intruding the Carboniferous sequences show a distinct contamination signature that could be crustal or due to subduction-related metasomatism of the subcontinental lithospheric mantle. The sources of the basaltic magmas vary substantially and in places changes with time. All alkali basalts are derived from enriched asthenospheric sources at varying depths (90–147 km) and most tholeiitic, low-Ti basalts have been extracted from asthenospheric and depleted asthenospheric–lithospheric sources indicating substantial compositional heterogeneity of the mantle. Further, Nd model ages varying from Neoproterozoic (940–580 Ma) to Paleozoic (460–370 Ma) suggest variation in the age of mantle sources for the basalts.

High-precision measurement of B isotopes on low-boron oceanic volcanic rock samples via MC-ICPMS: Evaluating acid leaching effects on boron isotope compositions, and B isotopic variability in depleted oceanic basalts

Recent analytical advances have made the high precision (2SD, ±≤1‰) determination of boron (B) isotopic ratios in silicate rocks a possibility, but few such data are as yet available, especially for low B (<2 ppm) samples like oceanic basalts, where issues related to cryptic seawater alteration are a concern. This study carefully assesses the reproducibility of B isotope measurement via MC-ICPMS on silicate rocks. Based on repeated measurements of the standard reference materials B-5, B-6, JB-2, JR-2, AGV-2 and JB-3 and nature samples, the external reproducibility of MC-ICPMS measurements on the same solution is generally between 0.1‰ and 0.5‰ (2SD), a similar level of accuracy to that obtained based on long-term measurement of solution standard NIST SRM 951 of 0.4‰ (2SD). For different digestions, the average external reproducibility is 0.6‰ (2SD), which represents the whole procedure reproducibility, including chemical treatment. We have tested an acid leaching experimental procedure for its effects on the B isotope compositions of ~50 Ma boninites collected from a deep ocean setting. Fresh and moderately altered boninite samples were subjected to cold HCl leaching treatments for 1, 3, 5, 7 and 9 h, using 2 N and 6 N HCl, respectively. δ11B values for the fresh sample remained unchanged within error after leaching, suggesting that leaching did not lead to any B isotopic fractionation. δ11B values for different aliquots of an altered boninite sample show lower and uniform δ11B values after 1 h of 6 N HCl leaching, while aliquots leached in 2 N HCl show similar δ11B to the unleached altered sample. These results suggest that changes in B isotopes caused by at least moderate degrees of alteration, and/or adsorption effects in volcanic rocks preserved on the ocean floor can be effectively eliminated through short-duration 6 N HCl leaches before digestion and analysis. B isotope measurements on three documentably fresh, low-B N-type mid-ocean ridge basalt (MORB) glass samples from the Southwest Indian Ridge yield δ11B values from −6.62‰ to −7.39‰, within the range of the recommended global MORB value of ≈−7.1 ± 0.9‰, determined via SIMS methodologies (2SD at ±1.5‰; Marschall and Monteleone, 2015), and the range of four fresh glass samples from the Eastern Pacific Ridge of −7.3 ± 0.8‰, measured by in-situ laser-ablation ICP-MS (2SD at ±0.7–1.0‰; Le Roux et al., 2004).

Petrogenesis of the post-collisional Eocene volcanic rocks from the Central Sakarya Zone (Northwestern Anatolia, Turkey): Implications for source characteristics, magma evolution, and tectonic setting

Eocene volcanic rocks from the Central Sakarya Zone in the north of the Izmir–Ankara suture zone (IASZ) are predominantly intermediate-acidic lava flows and pyroclastics that crop out with a W–E orientation. The volcanic rocks include Bozanic lavas, agglomerates, Igdir lavas, and Kapikaya tuffs. The Bozanic lavas contain plagioclase, hornblende, clinopyroxene, and biotite, whereas the Igdir lavas consist of plagioclase, hornblende, biotite, and quartz. According to the total alkali–silica (TAS) diagram, the Bozanic lavas are mainly composed of andesite and dacite, with one sample of trachyandesite, whereas the Igdir lavas mainly contain dacite and minor andesite. Bozanic and Igdir lavas exhibit moderate- to high-K calc-alkaline character. Major oxide and trace element variations suggest the effects of fractional crystallization in the evolution of the volcanic rocks. N-type mid-ocean ridge basalt (MORB) normalized trace element patterns of the lavas exhibit enrichment in large ion lithophile elements (LILEs; K, Rb, Ba, Th) and depletion in high field strength elements (HFSEs; Zr, Ti, Y). In addition, the chondrite-normalized rare earth element (REE) plots of the rocks show moderately enriched and nearly concave-shaped patterns (LaN/YbN = 5.4–17.6 for the Bozanic lavas and 6.5–13.1 for the Igdir lavas), suggesting clinopyroxene (Cpx) and hornblende dominated fractionation. Negative Eu anomalies in the acidic lavas reveal plagioclase fractionation. Some trace element ratios of the lavas demonstrate a subduction signature and crustal contamination in the generation of the parent magma(s). Multi-dimensional tectonic discrimination diagrams suggest that the studied volcanic rocks have developed in a collisional setting.

The geochronology and petrogenesis of Walash volcanic rocks, Mawat nappes: constraints on the evolution of the northwestern Zagros suture zone, Kurdistan Region, Iraq

Geological, geochemical, and geochronological investigations of the upper volcanics (UV) of Lower allochthon (LA) of Mawat nappes (MN) were used to elucidate the geologic relationships between the LA, the Qulqula Rise (i.e., accretionary complex terrane (ACT)) and peripheral Foreland Basin Assemblage (FBA), (Maastrichtian–Paleogene). The 40Ar-39Ar geochronological data suggest that the Middle to Late Paleogene magmatic history of Late Walash arc volcanic activity of UV can be divided into an early phase of andesitic volcanism around 43.1 ± 0.3 Ma and subsequent extensional arc phase magmatism of a time lapse between 40.1 ± 0.3and 32.3 ± 0.4 Ma. The geochemical data establish the existence of gradients in trace element ratios (i.e., Ba/La, Th/Yb, U/La, Ba/Th, Rb/Y, Ba/Y, and Nb/Y) throughout the studied samples and reflect differences in the degree of mantle wedge depletion influenced by addition of a considerable proportion of sedimentary derived input of the subducting Arabian slab. The trace, rare earth elements and isotopes of the studied samples have emphasized differences in the geochemical characteristics of andesites in comparison with other members of Walash. The andesites have subduction-related signature with the isotope (low Nd and high Sr isotope ratios) characteristic of a melt component input from cratonically-derived sediments. The diabases intruding in the basalts and andesites have a transitional composition from andesitic to N-type mid-ocean ridge basalts geochemistry with a noticeable subduction-related signature, and are interpreted to form in an extensional arc evolving into back arc basin setting. As evidenced from infant arc magmatism (Albian–Cenomenian), MN recorded protracted kinematic at convergent plate margins with a most notable was the development of the ACT (i.e., Qulqula Rise) adhered to the Arabian platform carbonates (Albian–Cenomenian) leading to the inception of peripheral FBA occurred by the erosion and unroofing of advancing ACT. Notably, the volcano flysch of Walash and Nummulite carbonate of Naopurdan were still accumulating on the remaining oceanic lithosphere further to the northeast and in part coeval with Paleogene molasses sedimentation on flexural FBA further to the southwest of the ACT.

Melt inclusions in olivine and plagioclase phenocrysts from Antarctic–Phoenix Ridge basalts: Implications for origins of N- and E-type MORB parent magmas

The Antarctic–Phoenix Ridge (APR) is a fossil spreading center in the Drake Passage, Antarctic Ocean. Spreading ceased in chron C2A (ca. 3.3Ma). Although the APR is a normal ridge that is not influenced by a hotspot, enriched (E-type) mid-ocean ridge basalt (MORB) coexists with normal (N-type) MORB in the ridge's axial region. The E-type APR basalt is relatively young (<3.1Ma) compared to the N-type basalt (>3.5Ma). The E-type basalt is characterized by elevated K2O/TiO2 (=0.4–0.8) and (La/Sm)N (=2.2–3.4) ratios, relative to the N-type basalt (K2O/TiO2=0.1–0.3; (La/Sm)N=0.7–0.8). To better understand the compositional variation in the APR basalts and their mantle source regions through time, silicate melt inclusions in primitive olivine (Fo87–89) and plagioclase (An85–89) phenocrysts from the N-type APR basalt were studied. Rehomogenized melt inclusions were analyzed by electron microprobe and LA-ICPMS for major and trace elements. The melt inclusions are more primitive than the host basalt, with Mg#s from 67.5 to 74.1. All inclusions exhibit patterns that are depleted in the light rare earth elements. The inclusions have K2O/TiO2 from 0.1 to 0.3 and (La/Sm)N ratios from 0.4 to 0.9; these values overlap with those of the N-type APR basalt. Furthermore, the melt inclusions have elevated (Lu/Hf)N and (Sm/Nd)N ratios compared to the E-type basalts. The N-type APR basalts do not contain any melt inclusions that are enriched in incompatible elements. The E-type basalt was generated by a low degree of partial melting of a relatively incompatible-element-enriched mantle source. In contrast, chemistries of melt inclusions and N-type basalts are compatible with high degrees of partial melting of an increasingly depleted mantle source. Assuming a veined or otherwise heterogeneous mantle, the absence of E-type inclusions from the N-type host has implications for cyclic magmatic activity beneath the APR. Multi-stage mantle melting and melt extraction from a composite source with sequential extraction of melt fluids might give rise to the primary melt diversity documented in the APR axis. The mantle source of the N-type melts may have been the residue from an earlier phase of melting that removed the easily melted, enriched components. The N-type APR basalt studied represents melt at the end of single cycle, whereas the E-type basalt may represent the early stage of a new pulse that was dominated by highly enriched components.

Mantle heterogeneity beneath the southern Mid-Atlantic Ridge: trace element evidence for contamination of ambient asthenospheric mantle

We report new trace element data for an extensive suite of quench basalt glasses dredged from the southern Mid-Atlantic Ridge (MAR) between 40°S and 52.5°S. Ratios between highly incompatible trace elements are strongly correlated and indicate a systematic distribution of incompatible element enriched mid-ocean ridge basalt (MORB) (E-type: Zr/Nb=5.9–19, Y/Nb=0.9–8.4, (La/Sm)n=1.0–2.9) and incompatible element depleted MORB (N-type: Zr/Nb=30–69, Y/Nb=11–29, (La/Sm)n=0.48–0.79) along this section of the southern MAR. A notable feature of N-type MORB from the region is the higher than usual Ba/Nb (4–9), La/Nb (1.2–2.4) and primitive mantle normalised K/Nb ratios (>1). Ba/Nb ratios in E-type MORB samples from 47.5 to 49°S are especially elevated (>10). The occurrence and geographic distribution of E-type MORB along this section of the southern MAR can be correlated with the ridge-centred Shona and off-axis Discovery mantle plumes. In conjunction with published isotope data for a subset of the same sample suite [Douglass et al., J. Geophys. Res. 104 (1999) 2941], a model is developed whereby prior to the breakup of Gondwana and the opening of the South Atlantic Ocean, the underlying asthenospheric mantle was locally contaminated by fluids/melts rising from the major Mesozoic subduction zone along the south–southwest boundary of Gondwana, leaving a subduction zone geochemical imprint (elevated (K/Nb)n and 87Sr/86Sr ratios, decreased 143Nd/144Nd ratios). Subsequent impingement of three major mantle plume heads (Tristan/Gough, Discovery, Shona) resulted in heating and thermal erosion of the lowermost subcontinental lithosphere and dispersal into the convecting asthenospheric mantle. With the opening of the ocean basin, continued plume upwelling led to plume–ridge interactions and mixing between geochemically enriched mantle derived from the Shona and Discovery mantle plumes, material derived from delamination of the subcontinental lithosphere, and mildly subduction zone contaminated depleted asthenospheric mantle.

The influence of mantle plume in the genesis of the Cache Creek oceanic igneous rocks: implications for the geodynamic evolution of the inner accreted terranes of the Canadian Cordillera

West of Prince George, British Columbia, the Cache Creek Terrane is composed of mafic lavas interlayered with both mid-Permian pelagic limestones and Upper Triassic siliceous shales and greywackes. Gabbro, basalt, dolerites, and foliated clinopyroxene-rich ultramafic rocks are exposed within the Pinchi Fault system. The mid-Permian lavas show affinities of oceanic island tholeiites. Among the Triassic lavas, three types of rocks have been distinguished. Type 1 is geochemically similar to the mid-Permian volcanic rocks. Type 2 differs from type 1 by higher TiO2 abundances and convex rare earth element patterns. Type 3 has the highest Zr, Nb, and Ta abundances and the greatest light rare earth element enrichment. The mafic rocks within the Pinchi Fault system are similar to N-type mid-ocean-ridge basalt (N-MORB), and the foliated ultramafic rocks are characterized by very low trace element contents, similar to extremely depleted harzburgites. Permian lavas and Triassic type 1 and igneous rocks from the Pinchi Fault system have the highest εNd(i) ratios (+7.4 to +9.6) and those of type 3 alkali have the lowest ratios (+2.0 to +5.3). The εNd(i) values of type 2 are intermediate between those of type 1 (~+7) and type 3 (~+4.9). This suggests that the Triassic rocks generated from a heterogeneous plume source or the mixing between depleted N-MORB and enriched oceanic island basalt sources. If the mafic igneous rocks sampled in central British Columbia are representative of the preserved parts of an oceanic crust, within the Cache Creek Terrane, then that crust was dominated by oceanic plateau components, perhaps due to the difficulty of subducting thick crust.

Metamorphosed mafic rocks with N-type MORB geochemical features in Hainan Island—Remnants of the Paleo-Tethys Oceanic Crust?

A number of metamorphosed mafic rocks occurred within the Paleozoic strata in the Chenxing and Bangxi regions at the northern side of the Changjiang-Qionghai Fault in Central Hainan Island. These metamorphosed mafic rocks are tholeiites in chemistry. They are characterized by extreme depletion of Th, Nb, Ta and LREEs, resembling the depleted N-type mid-ocean ridge basalts (MORB). Field relations suggest that the protolith of the metamorphosed mafic rocks were likely formed in Paleozoic. These metamorphosed mafic rocks with N-type MORB geochemical features were probably the remnants of the Paleo-Tethys oceanic crust.

The geochemistry of olivine-hosted melt inclusions in a FAMOUS basalt ALV519-4-1

Some 22 melt inclusions hosted by high-Mg olivines (Fo 88–91) from a single basalt lava from the FAMOUS area of the Mid-Atlantic Ridge were analyzed for major and trace elements. The results demonstrate that large chemical variabilities are present in melts that existed in close temporal and spatial proximity at the site of host olivine growth. Major element compositions after corrections for growth of host olivine after entrapment show that they are very primitive (MgO ranging from 9 to 14 wt.%; Mg# around 0.73), and that variations of CaO, FeO and SiO 2 as a function of MgO are much greater than expected from fractionation of olivine from the most magnesian melt. Exactly 14 of the melt inclusions are consistent with equilibration with mantle minerals at 8.7±2 kbar and 1272±18°C. Abundance patterns of rare earth elements (REE) show that light REE (LREE)-enriched transitional mid-ocean ridge basalts (MORB) and LREE-depleted N-type MORB existed in close temporal and spatial proximity. It is argued that co-variations of trace element ratios, especially [La/Sm] N and Ti/Zr can be modeled by critical melting of a depleted MORB source at depths where garnet is stable to form LREE-enriched melts, and at depths of spinel lherzolite residue to form LREE-depleted melts. Abundance patterns of middle to heavy REE (M∼HREE) indicate, however, that important chemical modification has occurred by reactions between deep melts and shallow mantle in an AFC-type mechanism during upward migration of melts. This mechanism also appears to explain some of the major element characteristics, and suggests general importance of melt/mantle reaction during migration of MORB.

Geochemistry, petrogenesis and tectonic significance of the basic volcanic units of the Zildat ophiolitic mélange, Indus suture zone, eastern Ladakh (India)

AbstractBasic volcanic rocks within the Zildat ophiolitic melange of Indus suture zone in eastern Ladakh are medium to fine grained with partially preserved primary texture and mineralogy. These rocks are predominantly alkaline basalt with high Nb/Y and enriched incompatible trace element characteristics, similar to those of the oceanic island basalt (OIB). The minor sub-alkaline basaltic rocks resemble N-type mid ocean ridge basalt (N-MORB) but with much lower abundances of incompatible trace element including REE. The alkaline rocks probably generated through variable, but low degrees of partial melting of enriched mantle source and evolved through high pressure olivine and clinopyroxene fractionation. Low pressure plagioclase and Fe- Ti oxide do not appear to be major fractionating phases. Limited data on the sub-alkaline rocks suggest that their parental melts were derived from mantle sources some what similar to that of N- MORB. Significant role of added cumulates of olivine, clinopyroxene and Fe- Ti...

Origin of blueschist-facies clasts in the Mariana forearc, Western Pacific.

In the Mariana forearc region, blueschist-facies metabasite clasts have been recovered from a serpentinite seamount during Ocean Drilling Program Leg 125. To clarify the origin of these metamorphic clasts, major and rare earth elements (REE), Ba, and Sr were measured in seven blueschist-facies metabasite clasts. Two out of seven clasts have zig-zag REE patterns reflecting the lanthanide tetrad effect, suggesting intense interaction with sea water. The other five clasts, however, show REE patterns with light REE depletion similar to N-type mid-ocean ridge basalts. Barium and Sr enrichments, relative to light REE abundances, are not conspicuous and most clasts show Sr and/or Ba depletions, indicating that they have chemical affinity with mid-ocean ridge basalt. This result is supported by the similarity of the TiO2-Al2O3 systematics of the clasts to those of mid-ocean ridge basalts. These MORB-like clasts metamorphosed under blueschist-facies conditions must have been taken into uprising serpentinite diapir at a depth where materials from oceanic crust extensively accreted. Two possible sources for the MORB-like protolith of blueschist-facies clasts are considered: (1) the oceanic crust of the subducted Pacific plate, (2) oceanic crust trapped within the area between trench axis and volcanic front when the subduction of the Pacific plate started. The second possibility is likely because the subducting Pacific plate crust is overlain by 600-800 m of oceanic sediment which is not represented in the clast population. It is proposed that the clasts provide evidence of tectonic erosion of oceanic crust which was trapped in the Mariana forearc at the initiation of subduction.

Geochemical and Nd isotopic study of Palaeozoic bimodal volcanics in Hainan Island, South China—Implications for rifting tectonics and mantle reservoirs

Geochemical and SmNd isotopic results are presented for Carboniferous bimodal metavolcanics from the central part of the Hainan Island, south China. The volcanics are composed of both tholeiites and quartz rhyolites, which are interbedded with clastic metasediments. This volcanic sequence hosts the Shilu FeCoCu Deposit, the richest iron deposit in China. Study of the volcanics is thus important for understanding of both the geochemistry of the subcontinental mantle and the genesis of the Shilu Deposit. Tholeiite samples display large geochemical variations but remarkably uniform Nd isotopic signatures. Overall, at comparable MgO level, tholeiites from the western portion of the volcanic belt have lower TiO 2 and P 2O 5, and higher Al 2O 3 TiO 2 and CaO TiO 2 ratios than those from the eastern portion. A tholeiite sample taken from the Junying area, western portion of the volcanic belt has characteristics of N-type mid-ocean ridge basalts (MORB) in having a LREE-depleted pattern, positive ϵ Nd( T) value (+6.8), as well as relatively high Zr Nb (25), Y Nb (10) and low Zr Y (2.4), La Nb (0.81) ratios. On the other hand, tholeiites occurring in the eastern portion of the volcanic belt are relatively enriched in LIL, HFS and LRE elements. Geochemical features, such as positive Nb anomalies on spiderdiagram and generally high Zr Y ratios, indicate that these rocks are analogous to oceanic island tholeiites (OIT) or initial rifting tholeiites (IRT). However, least contaminated tholeiites in the eastern portion have limited ranges of relatively low La Nb ratios (0.82–1.03) and high ϵ Nd( T) values (+4.2∼+7.6), which are similar to the MORB-type tholeiites in the western portion. The above geochemical features can be best explained in terms of different degrees of partial melting of an isotopically similar depleted source(s). The OIT-like tholeiites from the eastern portion were derived by lower degree partial melting, whilst the N-MORB-like ones from the western portion, by higher degree of partial melting. Some tholeiite samples with lower ϵ Nd( T) values show evidence for crustal contamination during their petrogenesis. Quartz rhyolites from the volcanic belt are enriched in SiO 2 (72–80%), K 2O (4.64–6.42%), Rb, depleted in CaO, Na 2O, Sr, and display strong negative Eu anomalies. Their negative ϵ Nd( T) values (−6.9∼−7.6) are similar to that of the bulk continental crust of south China and suggest a derivation by partial melting of a continental crustal source. We propose that Palaeozoic passive rifting tectonics was responsible for the magmatic activity in this region. The early stage of extensional rifting caused increasing degrees of decompressional partial melting of an upwelling asthenospheric mantle and resulted in the production of tholeiitic magmas. The ascent of these magmas into the crust provided heat which caused partial melting at lower/middle crustal levels and yielded rhyolitic magma. The N-type MORB-like affinities exhibited by tholeiites in the western portion of the belt indicate that the rift in this area may have developed into a small oceanic basin.

Petrogenesis of anomalous K-enriched MORB from the Southwest Indian Ridge: 11°53′E to 14°38′E

Glassy pillow basalts with unusual geochemical characteristics for mid-ocean ridge basalt (MORB) have been dredge sampled from the Southwest Indian Ridge between 12 and 15°E during Leg ANT IV/4 of the F.S. POLARSTERN. Lavas from 4 of 6 dredges are moderately nepheline normative, highly K-enriched (0.5–1.77 wt% K2O) alkali basalts and hawaiites. Mg-numbers indicate that many of the lavas are fairly primitive (Mg No.=63–67), yet show extreme enrichment in incompatible elements; e.g. Nb (24–60 ppm), Ba (170–470 ppm) and Sr (258–460 ppm). Incompatible-element ratios such as Zr/Nb (3–5) and Y/Nb (0.46–1.1) are extremely low even for E-type (enriched) MORB, whereas (La/Yb)n ratios are particularly high (3.4–7.8). 87Sr/86Sr (0.70290–0.70368), 143Nd/144Nd (0.51302–0.51284) and 206Pb/204Pb (18.708–19.564) isotopic ratios further indicate the geochemically ‘enriched’ nature of these lavas, which range from the compositional field for depleted N-type (normal) MORB towards the composition of Bouvet Island lavas. Mutually correlated incompatible-element and Sr-, Nd- and Pb-isotopic ratios allow a fairly well constrained model to be developed for the petrogenesis of these unusually alkalic mid-ocean ridge lavas. The alkalic nature and degree of enrichment in incompatible elements is ascribed to particularly low degrees of partial melting (3–5 wt%), at greater than usual depth, of a source region that has experienced prior geochemical enrichment (by veining) related to the upwelling Bouvet mantle plume. To account for the observed compositional variations, a model is proposed whereby mixing between partial melts derived from these geochemically enriched silicate veins, and an incipient to low percentage (±2%) melt from the surrounding geochemically depleted suboceanic asthenosphere occurs as a consequence of increasing degree of melting with adiabatic upwelling. Eruption of these alkalic lavas in this spreading ridge environment is attributed to a temporary hiatus in tholeiitic volcanism and associated spreading along this section of the Southwest Indian Ridge, related to readjustment of spreading direction to a more stable plate geometry.

Modified Gulf of California model for South Georgia,north Scotia Ridge, and implications for the Rocas Verdes back-arc basin, southern Andes

New field mapping and an integrated trace element and Nd isotopic study have been undertaken on the Jurassic-Cretaceous Larsen Harbour Complex on the island of South Georgia, recognized as a displaced part of the southern Andes ensialic, autochthonous Rocas Verdes marginal basin. Five basalt groups (Groups I-V) are identified from trace element data; initial Nd isotopic compositions suggest that they are derived from only two sources. Magmas produced during early stages of continental lithospheric attenuation (Groups I-III) were derived by varying degrees of partial melting and fractional crystallization from a large ion lithophile element (LILE)-enriched, low-{epsilon}{sub Nd} mantle source relative to normal (N)-type mid-ocean ridge basalt (MORB). Magmas produced during later stages of rifting (Groups IV and V) were, however, derived from a high-{epsilon}{sub Nd} asthenospheric mantle source similar to N-type MORB, unaffected by earlier LILE enrichment. Our data do not occur with previous suggestions that the Larsen Harbour Complex and, by implication, the Rocas Verdes formed in a supra-subduction-zone setting. On the basis of geologic and geochemical evidence, we favor basin formation along an oblique-slip margin akin to that of the Gulf of California.

Geochemistry of basic dikes in the Lanzo massif (Western Alps): Petrogenetic and geodynamic implications

The Alpine peridotite massif of Lanzo (Italy) contains three generations of basic dikes (gabbros and basalts). The older gabbros are plagioclase-rich mantle segregates while the younger gabbro dikes are cumulates very similar in chemical composition to recent oceanic gabbros and gabbros from ophiolitic complexes. They both were derived from the N-type mid-ocean ridge basalt (MORB) magmas which were progressively more depleted in incompatible elements and were probably generated during a dynamic melting of a rising mantle diapir. The basaltic dikes are the N-type MORB and closely resemble the Alpine-Apennine ophiolitic basalts. They were derived from a different upper mantle source than the parental magmas of the gabbros. The source of the basalts was less depleted in light REE. The presence of basic magmas with N-type MORB affinities in the Lanzo massif is consistent with the close genetic relationship between the Alpine peridotite body and the ophiolites of the Liguro-Piemontese basin.

Petrogenesis of the Natkusiak continental basalts, Victoria Island, Northwest Territories, Canada

The Natkusiak flows (~1100 m thick), sills, and dykes of Victoria Island, Northwest Territories, are part of an extensive, late Hadrynian magmatic event in the northern Canadian Shield. The flows are subaerially erupted plateau basalts and together with associated doleritic sills and dykes display many geochemical features typical of continental tholeiites. In addition to fractional crystallization, the rocks were affected by crustal contamination but to a smaller degree than usually encountered in continental tholeiites elsewhere. The basalts were derived from a depleted upper mantle source very similar to that of N-type mid-ocean ridge basalts, although it is suggested that the melting residue contained small amounts of garnet (~2%).

Petrological and geochemical variations along the Mid-Atlantic Ridge between 46°S and 32°S: Influence of the Tristan da Cunha mantle plume

Basalts from a section of the Mid-Atlantic Ridge close to the active volcanic island of Tristan da Cunha in the South Atlantic have been analysed to investigate the influence of the mantle plume on the geochemistry of basalts being erupted at the spreading center. Although petrographically the rocks show only limited variation, two basaltic types were determined to be erupting in this region based on their major, trace and REE compositions. One group shows depletion in the incompatible and LRE elements, and can be characterised as N-type mid-ocean ridge basalts. The second group shows “enriched” geochemical characteristics and is similar to T-type MORBs. Mixing hyperbolae for the incompatible element and REE ratios suggest that extensive mixing of an end-member, characteristic of a plume region with an end-member of normal depleted MORB, canaccount for the occurrence of the T-type MORBs in this region.Based on the nature and development of the Tristan da Cunha mantle plume over the past 100 Ma, a composite model of evolution is suggested,in which a ridge-centered hotspot progressed to a near ridge hotspot, and finally to a totally intraplate situation. The fact that Tristan da Cunha is highly alkalic now, but that an irregular geochemical anomalyis also present on the Mid-Atlantic Ridge at this latitude would suggest an intermediate stage between the near-ridge and totally intraplate situation. This model leads to the conclusion that, as the Mid-Atlantic Ridge migrated away from the Tristan hotspot, a preferential sublithospheric flow towards the Ridge was established. This discontinuous feature can explain the geochemical variations seen along the Mid-Atlantic Ridge by providing a mechanism for mixing of a depleted N-type MORB component with an enriched component originating through processes active at the Tristan da Cunha mantle plume.

A re-appraisal of the use of trace elements to classify and discriminate between magma series erupted in different tectonic settings

The hygromagmatophile element composition of basic lavas from several tectonic environments are compared with the estimated composition of the primordial mantle. The observed variations are used to subdivide mid-ocean ridge basalts (MORB) into two main types according to the tectonic character of the ridge segment from which they were erupted. Ridge segments with positive residual gravity, depth and heat flow anomalies erupt E-type MORB which are predominantly enriched in the more hygromagmatophile elements, but also include magma types which are depleted in most of these elements. Both enriched and depleted E-type MORB can be distinguished from the basalts erupted at normal ridge segments (N-type MORB) by their La/Ta ratios (in E-type MORB La/Ta ∼10, in N-type MORB La/Ta is ∼15) and by Hf/Ta ratios (in E-type MORB Hf/Ta> 7, in N-type MORB Hf/Ta> 7). E-type MORB can be distinguished from the basalts erupted at ocean islands by their higher Hf/Ta ratios (>2). A Th-Hf-Ta triangular diagram is used to discriminate between the different ocean floor basalts as well as those erupted at destructive plate margins, which are depleted in Ta and Nb. This diagram can also distinguish between silicic lavas from the different tectonic environments as well as identifying lavas that have been contaminated with continental crust.