Enriched MORB Research Articles

Halogen systematics (Cl, Br, I) in Mid-Ocean Ridge Basalts: A Macquarie Island case study

The abundance of halogens (Cl, Br and I) in the Earth’s mantle has the potential to provide information about mantle metasomatism and volatile recycling in subduction zones. Basalt melts sample different parts of the Earth’s mantle, but few data are available for Br or I in basalt melts, and the relative partitioning behaviour of these elements has not been investigated rigorously. To address these issues, we determined the abundances of Cl, Br and I in enriched Mid-Ocean Ridge Basalt (E-MORB) glasses from Macquarie Island in the southwest Pacific. The Macquarie Island glasses are fairly typical of Pacific MORB with MgO of ∼5.5. to 9wt%, 87Sr/86Sr of 0.70257–0.70276, 143Nd/144Nd of 0.51300–0.51306 and 3He/4He of ∼8Ra. These glasses provide a unique opportunity to investigate halogen partitioning behaviour, because their variable MgO contents and trace element signatures (e.g. La/Sm ∼1.4–7.9) result from different degrees of partial melting and fractional crystallisation.The combined measurement of Br/Cl, I/Cl and K/Cl, together with correlations between Cl and other trace elements, demonstrate that the halogens in the Macquarie Island glasses had a mantle source and were not influenced by seawater contamination. Log–log correlation diagrams indicate that Cl, Br and K were not statistically fractionated during partial melting, crystallisation or degassing of CO2 from these melts. The behaviour of I is less well constrained and minor fractionation of I/Cl cannot be precluded during multi-stage melting and enrichment processes. The data indicate the mantle source of the Macquarie Island glasses was characterised by K/Cl of 13±4, Br/Cl of (3.7±0.5)×10−3 and I/Cl of (130±100)×10−6 (2σ uncertainties; weight ratios). The K/Cl ratio of the Macquarie Island glasses is equivalent to the median of all published MORB data. This suggests the Br/Cl and I/Cl values may also be representative of average MORB mantle values.

Tholeiite–Boninite terrane in the North Qilian suture zone: Implications for subduction initiation and back-arc basin development

A tholeiite–boninite terrane occurs as a ~4.5-km-thick massif with lavas and intrusions in the Dachadaban (DCDB) area, the middle part of the North Qilian oceanic-type suture zone. It comprises two distinct lithological groups: the lower tholeiite unit and the upper boninite unit. The lower tholeiite unit consists of massive lava flows and subordinate gabbro intrusions with MORB-like characteristics that could represent 5–6% melting of an enriched MORB mantle. In contrast, the overlying boninite unit consists of pillow lavas, dolerite dykes and gabbro intrusions and shows high-Ca boninite features that may be formed by continuous melting of the extremely refractory mantle with the aid of a combination of the elevated mantle potential temperature of 1380–1460°C at depths of 42–66km and involvement of slab-derived hydrous fluids/melts. Zircon U–Pb SHRIMP dating shows that lower tholeiite magmatism lasted for at least 12M.y. from 517Ma to 505Ma and upper boninite volcanism occurred between 505 and 487Ma, which is consistent with the earliest age (486±7Ma) of the SSZ-type ophiolite belt immediately north of the Dachaidaban (DCDB) tholeiite–boninite terrane. The lower tholeiites are considered to represent the products of earliest infant arc magmatism by decompression-induced partial melting of the relatively “dry” and fertile upwelling mantle in response to the onset of subduction. The upper boninite unit with younger age of 487±9Ma is interpreted as earliest products of infant arc splitting and subsequent back-arc basin development. Therefore, the long-lived DCDB tholeiite–boninite sequence presents a key lithological record of early stages of supra-subduction zone magmatic activity evolved from subduction initiation at ~517Ma to back-arc extension at ~487Ma.

Geochemistry of mafic dykes from the Southeast Anatolian ophiolites, Turkey: Implications for an intra-oceanic arc–basin system

The Late Cretaceous–Tertiary accretionary prism in Eastern Turkey includes several ophiolitic megablocks and/or tectonic slivers (Mehmetalan, Mollatopuz and Alabayir) within a mélange complex, mainly comprising harzburgite, dunite and cumulate-textured gabbro. The diabases, which are the main focus of this study, cut across the ophiolites as parallel and variably thick dyke-swarms. Geochemistry of the diabases reveals three distinct groups, including a) supra-subduction zone (SSZ) type, which is characterized by marked Nb-anomaly and normal mid-ocean ridge basalt (N-MORB) like HFSE distribution, b) enriched MORB (E-MORB) type, showing some degree of enrichment relative to N-MORB, c) oceanic-island basalt (OIB) type with characteristic hump-backed trace element patterns, coupled with fractionated REE distribution. Among these groups, SSZ- and E-MORB-type signatures are acquired from the Mehmetalan and Mollatopuz suites, whereas OIB-type characteristics are found in the Alabayir suite. The melting models indicate involvement of both depleted and enriched sources for the genesis of the studied dykes. The close spatial relationship, similar ages (based on Ar–Ar dating) and the presence of variable subduction component displayed by Mehmetalan and Mollatopuz suites may indicate melt generation in an intra-oceanic SSZ within the southern branch of Neotethys. In spite of the solely OIB-like character of the Alabayir suite, the similar age obtained from these dykes may suggest their formation in a similar SSZ setting. Alternatively, the Alabayir suite may have represented an oceanic island or seamount formed in an intra-plate setting with or without plume influence. We suggest that decompression melting triggered by slab roll-back mechanism during the closure of the southern branch of the Neotethys during the Late Cretaceous may have been the main process that led to generation of magmas of both depleted and enriched characteristics.

U–Pb baddeleyite ages, distribution and geochemistry of 925 Ma mafic dykes and 900 Ma sills in the North China craton: Evidence for a Neoproterozoic mantle plume

Numerous Neoproterozoic mafic dykes, referred to as the Dashigou swarm, are identified in the central and southeastern parts of the North China craton (NCC). They are 305–010° trending dykes, with widths of ~10–100m and exposed lengths of several to >10km. Precise U–Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS) measurements on baddeleyite grains separated from three dykes yield 207Pb/206Pb average ages of 924.0±3.7Ma (Dashigou dyke), 921.8±2.6Ma (Yangjiaogou dyke) and 925.8±1.7Ma (Taohuagou dyke). Baddeleyite grains from a late-stage pegmatite vein in the Dashigou dyke were analyzed by secondary ion mass spectrometry (SIMS) methods. These yield a 207Pb/206Pb average age of 920.4±5.7Ma. The Dashigou dyke swarm exhibits an overall radiating geometry (the overall fan angle is about 60°), with a focal point located along the southern margin of the eastern NCC, where a large ca. 900Ma sill swarm was previously recognized. The rift system hosting these slightly younger sills, named the Xu–Huai Rift System, could represent two breakup-parallel arms of a rift–rift–rift triple junction related to the initiation of the magma center that produced the Dashigou dykes. The sills have similar characteristics and could be cogenetic with the Dashigou dykes.The Dashigou dykes are coarse-grained, composed mainly of clinopyroxene and plagioclase feldspar, with or without olivine. One of the most primitive dykes has 47.79wt.% SiO2, 6.41wt.% MgO, 1.38wt.% TiO2, 17.77wt.% Al2O3, 10.47wt.% CaO and 0.62wt.% K2O. It shows slight enrichment in light rare earth elements and a slightly positiveEu/Eu* anomaly (1.1), and is slightly depleted in high field-strength elements compared to neighboring elements on a primitive mantle-normalized spidergram. The Dashigou dykes show some similarities with enriched-mid ocean ridge basalts (E-MORB) or ocean island basalts (OIB) and have εNdt values of +1.8 to +3.1 and 87Sr/86Srt values of 0.7019–0.7047 (t=920Ma). All these characteristics indicate that they are not likely derived from the ancient lithospheric mantle under the NCC, but rather from a mantle source below, in the asthenosphere. This 925–900Ma magmatism represents a second sub-lithospheric mantle upwelling event following the 1780–1730Ma event that occurred shortly (~70Ma) after the formation of the subcontinental lithospheric mantle (SCLM). Therefore the SCLM of the NCC was multiply metasomatized during asthenospheric upwellings (at least at 1780–1730 and 925–900Ma), before most of its eastern part was finally removed during the Mesozoic.Collectively, these ~925Ma dykes and ~900Ma sills constitute a large igneous province (LIP) with an areal extent of about 0.5Mkm2 and a diameter of about 1000km. This LIP probably resulted from a Neoproterozoic mantle plume centered along the present southern margin of the eastern NCC, and probably resulted in the break-off and rifting away of a separate crustal block. We speculate that this conjugate block could have been the combined São Francisco–Congo craton on the basis of precisely matched ages for the Bahia dykes (São Francisco craton) and Gangila–Mayumbian volcanic associations (western part of the Congo craton).

Geochemical evolution of Dalradian metavolcanic rocks: implications for the break-up of the Rodinia supercontinent

Abstract: Neoproterozoic basaltic magmatism in the Dalradian Supergroup of Scotland and Ireland was associated with the break-up of the Rodinia supercontinent. Magmas were erupted in rift-related basins along a strike length of at least 700 km and during a time period of c . 80 Ma. New major and trace element analyses of metabasalts from several formations are presented to trace the variations in magma compositions in time and space. The primary magmas resulted from variable degrees of mixing of melts derived from mantle sources similar to those of normal and enriched mid-ocean ridge basalts; some younger lavas also show evidence of contamination with continental crust. In contrast to speculations about magmatism elsewhere in Rodinia, the evidence here suggests that there was no involvement of a mantle plume in basalt generation. For example, the Scottish promontory of Laurentia drifted rapidly southwards through c . 25° over the duration of the magmatism, with no evidence of significant elevation above sea level, as might be expected from involvement of a plume. Generation of the primary magmas might have taken place predominantly through decompression melting in depleted upper mantle containing enriched streaks and blobs. Both the Dalradian lithostratigraphy and the metabasaltic compositions are consistent with extreme lithospheric stretching and possibly rupture during the earliest phase of magmatism, whereas generation of later magmatism appears to have been associated with major fault systems, possibly on a foundering continental margin. Supplementary material: Chemical analyses of Dalradian metavolcanic rocks (major elements recalculated to 100%, anhydrous) are available at www.geolsoc.org.uk/SUP18468 .

Triassic Subduction of the Paleo-Tethys in northern Tibet, China: Evidence from the geochemical and isotopic characteristics of eclogites and blueschists of the Qiangtang Block

A ∼500 km-long high-pressure metamorphic belt occurs in the Qiangtang Block of northern Tibet. The occurrence of high-pressure metamorphic rocks (eclogite and blueschist) has a significant bearing on the Paleo-Tethys subduction and plate suturing processes. A few studies have been conducted on petrology, mineralogy and geochronology of high-pressure rocks. In this paper we present the results of a geochemical and isotopic investigation on eclogite and blueschist from this belt, and discuss the probable geodynamic implications. Geochemical analyses show that the two rock types, eclogite and blueschist, have grossly similar bulk compositions, but they can be subdivided into two groups based on their TiO 2 contents. The low TiO 2 (<2.75 wt.%) rocks exhibit REE distribution patterns and trace element abundances similar to enriched mid-ocean ridge basalts (E-MORB), whereas the high TiO 2 (>2.75 wt.%) rocks are more akin to ocean island basalts (OIB). The positive ε Nd( T) values (ranging from 0.4 to 6.3) for all samples suggest that the basaltic protoliths of the eclogites and blueschists were derived from a long-term depleted mantle source. However, their geochemical characteristics require that the source be metasomatized and enriched in large-ion-lithophile and light rare-earth elements shortly before the formation of these basaltic protoliths, like for most OIB and E-MORB of the world. The recognition of oceanic basalts suggests that a Paleozoic ocean should be present in the central Qiangtang Block. This ocean was hypothesized to be the western extension of the Changning–Menglian Paleo-Tethys Ocean in the eastern margin of the Tibetan plateau. The eclogite and blueschist were formed by northward subduction of this Paleozoic ocean in the Triassic.

Magmatic degassing of ore-metals at the Menez Gwen: Input from the Azores plume into an active Mid-Atlantic Ridge seafloor hydrothermal system

Menez Gwen is an active seafloor hydrothermal system located on the Mid-Atlantic Ridge at 37°50′N under the Azores plume influence and within enriched MORB type rocks. Two distinct highly plagioclase phyric (HPP) lavas occur at Menez Gwen, type I and type II. Type I HPP lavas are vesicular and highly enriched in incompatible trace elements; type II HPP lavas are less vesicular and less enriched. Melt inclusions in plagioclase and olivine phenocrysts in these HPP basalt lavas were studied by laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). The LA-ICP-MS data support previous findings that mixing of melts occurred below Menez Gwen. Olivine crystals enclosing melt inclusions having compositions of both type I and type II magmas occur in type I HPP basalts. All plagioclase hosted melt inclusions have type II signatures regardless of their host rock character. The melt inclusions of the less incompatible element enriched type II lavas are more enriched in Cu than type I, an observation that is relevant to the origin of metals characteristic of these particular basalt-hosted seafloor hydrothermal systems. Furthermore, petrographic and analytical observations of melt inclusions indicate exsolution from the magma of an immiscible volatile phase carrying Cu and S. Subsequent injection into the magma chamber of the more enriched type I melt coupled with extensive pre-eruptive degassing of CO 2 and S likely triggered the mobilization of the exsolved ore-metals. High volatile contents and extensive magmatic degassing in a shallow magma chamber and mixing of enriched melts were contributors to the seafloor hydrothermal activity at Menez Gwen.

Geoquímica e petrogênese de metabasitos do grupo Votuverava (Terreno Apiaí, Cinturão Ribeira Meridional): evidências de uma bacia retroarco calimiana

The Votuverava Group (ca. 1500 - 1400 Ma), which is part of the Apiai Terrane, a segment of the Southern Ribeira Belt, is composed of distal turbidites with significant basic magmatism represented by concordant lenses of metabasite. Twenty metabasite samples were investigated using geochemical analyses (major and trace elements). Three groups (G1, G2, G3) were identified based on Ti/V ratios, rare earth element patterns and MORB-normalized multi-element diagrams. All samples have tholeiitic signatures. The G1 group has a volcanic arc signature, with Ti/V ratios of 15 - 20, enrichment of large ion lithophile elements (LILE) and depletion in high field strength elements (HFSE) relative to the MORB. The trace element pattern indicates an important contribution of the subduction component. The G2 group has a MORB-type composition, with Ti/V ratios of 22 - 28. Nevertheless, the trace element behavior indicates a small contribution of the subduction component. The G3 group has an enriched MORB signature, with Ti/V ratio of approximately 40 and enrichment of LILE and HFSE. The variability of the TiO2/Yb and Nb/Yb ratios suggests that the G1 and G2 rocks were generated by shallow melting (N-MORB and E-MORB mantle sources, respectively). The G3 rocks are enriched in Ti and Nb, suggesting interaction between MORB and plume mantle sources. The coexistence of volcanic arc basalt and mid-ocean ridge basalt compositions, the overall geochemical patterns, and the sedimentary environment of the Votuverava Group are consistent with a back-arc basin environment.

Early–Middle Jurassic intra-oceanic subduction in the İzmir-Ankara-Erzincan Ocean, Northern Turkey

The İzmir-Ankara-Erzincan suture zone near Çankırı comprises an ophiolite-related rock suite of amphibolite, pyroxene-amphibolite and epidote-amphibolite. The geochemical data reveal that the basaltic protoliths of the Çankırı amphibolite suite are derived from different mantle sources. Amphibolites showing a LREE enrichment in chondrite-normalized REE pattern, relatively high La N/Yb N (8.5–13.1), Sm/Yb (2.8–3.7), Ce/Sm (8.8–9.8), Nb/Yb (14.4–20.5), Th/Nb (1.1–2.6) originate from an OIB-like (Type I amphibolites) mantle source. Those with flat chondrite-normalized REE pattern, low La N/Yb N (~ 0.6), Sm/Yb (~ 1.0), Ce/Sm (~ 2.7) Nb/Yb (~ 0.6), suggest derivation from a depleted MORB-like (Type II amphibolites) mantle source. A third type of amphibolite is characterized by La N/Yb N (1.54), Sm/Yb (1.1), Ce/Sm (4.5), Nb/Yb (1.1) and Th/Nb (0.3). It is derived from an enriched MORB source and shows characteristics similar to island arc basalts. Amphiboles from these rocks yielded 40Ar/ 39Ar plateau ages between 177.08 ± 0.96 Ma and 166.9 ± 1.1 Ma. These dates are interpreted as metamorphic ages of the amphibolitic rocks. They differ significantly from previous ages of amphibolitic rocks (the metamorphic sole rocks) related to Turkish ophiolites. However, similar metamorphic ages have been reported for metamorphic sole rocks of the Hellenic-Dinaric ophiolites further west. The metamorphic sole rocks have been generated in intra-oceanic environments via intra-oceanic subduction/thrusting events. The new age data of the Çankırı amphibolite suite suggest that the İzmir-Ankara-Erzincan ocean was subducted around Early–Middle Jurassic times, i.e. roughly coeval with the oceanic basin(s) generating the Hellenic-Dinaric ophiolites. In the Early–Middle Jurassic time, the tectonic regime in the İzmir-Ankara-Erzincan ocean was compressional and the ocean was closing at that time. The new age data presented here could imply that Jurassic granites which cross-cut basement rocks of the Pontides are related to the northward subduction of the İzmir-Ankara-Erzincan ocean during Jurassic time.

Occurrence and geochemistry of metamafic rocks from the Forquilha Eclogite Zone, central Ceará (NE Brazil): geodynamic implications

AbstractIn the northern region of the Borborema Province, a 30 km‐long mafic/ultramafic belt of high pressure (HP) rocks called the Forquilha Eclogite Zone has been recently discovered in the Central Domain of Ceará State. The belt comprises three groups of rocks: (1) garnet amphibolites, (2) retrograded eclogites and (3) clinopyroxene–garnet amphibolites, all of them being hosted in garnet ± kyanite + sillimanite schists and orthogneisses which are often migmatized. The geochemical analyses of 30 representative samples confirm differences among the three groups that were initially classified by petrography. The garnet amphibolites are the most depleted rocks, with relatively flat rare earth element (REE)PM pattern and (SPIDER)PM pattern with negative Rb, Th, Nb, Sr and Zr anomalies. It is the most fractionated group, ranging from picrobasalts to andesites. Fractional crystallization and mineral accumulation are indicated by Eu and Sr anomalies. The data show geochemical affinities with island‐arc basalts. The retrograded eclogites display flat (REE)PM, but without Eu anomalies. Nb/La ratios are low and variable, as for slightly enriched MORB from ocean floor or intra‐oceanic back‐arc environments. However, a genetic link with the Group 1 rocks cannot be completely discarded. The clinopyroxene–garnet amphibolites are the most enriched group. (REE)PM and (SPIDER)PM patterns show many features of alkaline basalts. In the Nb/Yb versus Th/Yb diagram the data cluster near the E‐MORB standard. Several geochemical aspects of these rocks fit well those from the HP/UHP Pan‐African/Brasiliano Suture Zone in the Dahomeyides/Hoggar regions of West Africa. However, differences in their extent and lack of alignment preclude a direct correlation among these zones. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Helium isotope variations between Réunion Island and the Central Indian Ridge (17°–21°S): New evidence for ridge–hot spot interaction

We report new helium abundance and isotope results for submarine basaltic glasses from the Central Indian Ridge (CIR) between the Marie Celeste (16.7°S) and Egeria fracture zones (FZ) (20.6°S); the adjacent Gasitao, Three Magi, and Rodrigues ridges; and for olivine separates from lavas and cumulate xenoliths from the Mascarene Islands (Reunion, Mauritius, and Rodrigues). Helium isotope ratios in basaltic glasses range from 7.1 to 12.2 R A (where R A = air 3He/4He) and lie between values of Mid‐Ocean Ridge Basalt (MORB) (8 ± 1 R A) and samples from Reunion Island (11.5 to 14.1 R A). The highest 3He/4He values (up to 12.2 R A) are found in glasses recovered off axis from the Three Magi and Gasitao ridges. Along the CIR axis, MORB‐like 3He/4He ratios are found near the Egeria FZ, and there is a marked increase to values of ∼11 R A between ∼19° and 20°S. The lowest 3He/4He values (<8 R A) are found immediately south of the Marie Celeste FZ, where incompatible trace element ratios (e.g., La/Sm) are highest. These low 3He/4He ratios can be explained by closed system radiogenic 4 He ingrowth in either (1) a fossil Reunion hot spot mantle component, embedded into the subridge mantle when the CIR migrated over the hot spot at ∼34 Ma or (2) trace element enriched MORB mantle. In contrast, the high 3He/4He ratios observed on the CIR axis adjacent to the Gasitao Ridge, and along the off‐axis volcanic ridges, are consistent with flow of hot spot mantle material from Reunion (∼1100 km to the west) toward the CIR.

Age and geochemistry of the oceanic Manihiki Plateau, SW Pacific: New evidence for a plume origin

We present 40Ar/ 39Ar age and geochemical (major and trace element and Sr–Nd–Hf–Pb isotope) data from submarine samples recovered from the basement of the Manihiki Plateau during the R/V Sonne research expedition SO193. The samples, predominately tholeiites, with minor occurrences of basaltic andesites and hawaiites, give a mean age of 124.6 ± 1.6 Ma from four different localities on the plateau. Based on TiO 2 content, we define two groups of volcanic rocks that differ in trace element and isotopic compositions. Partial melting modeling suggests that the low-Ti group lavas were derived through large degrees of melting (c. 30%) of a peridotitic source at mantle potential melting temperatures of c. T p = 1510 °C, more than 100 °C above the ambient mantle potential melting temperature. Since the primary water contents of both groups of lavas are low (0.1–0.3g wt.%) and the source is peridotitic, excess temperature is most likely the reason for the large degrees of melting producing the large volume of plateau basalts, consistent with the involvement of a mantle plume. The incompatible element contents of the low-Ti group lavas show a multistage history with enrichment in the most incompatible elements of a previously highly depleted source. They have isotopic compositions similar to enriched mid-ocean-ridge basalt (EMORB) and similar to the common focal zone (FOZO) component. The high-Ti group lavas have more enriched incompatible element compositions overall. Their isotopic compositions tend towards an enriched mantle (EMI)-type endmember, similar, although less extreme, than lavas from the Pitcairn Islands. The geochemistry of the Manihiki Plateau can best be explained by a plume containing three components: 1) a dominant peridotitic FOZO-type component, 2) delaminated EMI-type subcontinental lithospheric mantle (SCLM), and 3) a HIMU (recycled oceanic crustal)-type component possibly in the form of eclogite/pyroxenite. The similarity in age and geochemical composition of Manihiki, Hikurangi and Ontong Java basement lavas, including volcanism in some adjacent basins, suggests that the Greater Ontong Java Volcanic Event covered c. 1% of the Earth's surface with volcanism.

Crustal contamination and mantle source characteristics in continental intra-plate volcanic rocks: Pb, Hf and Os isotopes from central European volcanic province basalts

We report new Os–Pb–Hf isotope data for a suite of alkaline to basaltic (nephelinites, basanites, olivine tholeiites to quartz-tholeiites) lavas from the Miocene Vogelsberg (Germany), the largest of the rift-related continental volcanic complexes of the Central European Volcanic Province (CEVP). 187Os/ 188Os in primitive (high-MgO) alkaline lavas show a much wider range than has been observed in alkaline basalts and peridotite xenoliths from elsewhere in the CEVP, from ratios similar to those in modern MORB and OIB (0.1260–0.1451; 58.9–168 ppt Os) to more radiogenic ratios (0.1908 and 0.2197; 27.6–15.1 ppt Os). Radiogenic Os is associated with high ε Hf and ε Nd, low 87Sr/ 86Sr and does not correlate with Mg ∗ or incompatible trace elements (e.g. Ce/Pb), suggesting the presence of a radiogenic endmember in the mantle rather than crustal contamination as the source of radiogenic Os. This contrasts with another high-Mg alkaline lava characterized by highly radiogenic 187Os/ 188Os (0.4344, 10.3 ppt Os), lower ε Hf and ε Nd, higher 87Sr/ 86Sr, and Pb isotope signatures than the other alkaline lavas with similar trace element composition suggestive of contamination with crustal material. Hafnium (ε Hf: +8.9 to +5.0) and Pb isotope compositions ( 206Pb/ 204Pb: 19.10–19.61; 207Pb/ 204Pb: 15.56–15.60) of the alkaline rocks fall within the range of enriched MORB and some OIB. The Vogelsberg tholeiites show even more diverse 187Os/ 188Os, ranging from 0.1487 in Os-rich olivine tholeiite (31.7 ppt) to ratios as high as 0.7526 in other olivine-tholeiites and in quartz-tholeiites with lower Os concentrations (10.3–2.0 ppt). Low- 187Os/ 188Os tholeiites show Pb–Hf isotope ratios ( 206Pb/ 204Pb:18.81; 207Pb/ 204Pb: 15.61; ε Hf: +2.7) that are distinct from those in alkaline lavas with similar 187Os/ 188Os and originate from a different mantle source. By contrast, the combination of radiogenic Os and low 206Pb/ 204Pb and ε Hf in the other tholeiites probably reflects crustal contamination. The association at Vogelsberg of primitive alkaline and tholeiitic lavas with a range of MORB- to OIB-like Os–Pb–Hf–Nd–Sr isotopic characteristics requires at least two asthenospheric magma sources. This is consistent with trace element modelling which suggests that the alkaline and tholeiitic parent magmas represent mixtures of melts from garnet and spinel peridotite sources (both with amphibole), implying an origin of the magmas in the garnet peridotite-spinel peridotite transition zone, probably at the asthenosphere–lithosphere interface. We propose that uncontaminated Vogelsberg lavas originated in ‘metasomatized’ mantle, involving a 3-stage model: (1) early carbonatite metasomatism several 10–100 Ma before the melting event (2) deposition of low-degree asthenospheric melts from carbonated peridotite at the lithosphere–asthenosphere thermal boundary produces hydrous amphibole-bearing veins or patches, and (3) remobilization of this modified lithospheric mantle into other asthenospheric melts passing through the same area later. In keeping with ‘metasomatized’ mantle models for other continental basalt provinces, we envisage that stage (2) is short-lived (few Ma), thus producing a prominent lithospheric trace element signature without changing the asthenospheric isotopic signatures. Models of this type can explain the peculiar mix of lithospheric (prominent depletions of Rb and K) and asthenospheric (OIB-like high 187Os/ 188Os, 143Nd/ 144Nd and 176Hf/ 177Hf) signatures observed in the Vogelsberg and many other continental basalt suites.

Perspective on the Genesis of E-MORB from Chemical and Isotopic Heterogeneity at 9–10°N East Pacific Rise

The discovery of chemically and isotopically enriched mid-ocean ridge basalts (E-MORB) has offered substantial insight into the origin, time scales, and length scales of mantle heterogeneity. However, the exact processes involved in producing this E-MORB enrichment are vigorously debated. Additionally, because the ages of E-MORB are not well constrained, the petrogenetic, temporal, and geological relationships between E-MORB and normal (N)-MORB are not known. To investigate these relationships and to explore how melting and melt transport processes contribute to or modify enriched mantle source compositions and generate E-MORB melts beneath mid-ocean ridges, we measured major and trace elements, and Sr, Nd, Hf, Pb, and U–Th–Ra isotopes for a suite of lavas that were collected off-axis, including several E-MORB, at 9–10°N along the East Pacific Rise (EPR). These data show coherent mixing trends among long-lived radiogenic isotopes, U-series nuclides, and incompatible trace elements, implying that mixing of melts from different sources occurs at different depths. Our results are consistent with previous studies that show that melting occurs in a two-porosity melting regime, with high-porosity channels forming deeply in the presence of garnet and transporting enriched melts with large 230Th excesses to the crust, whereas low-porosity channels transport melts more slowly, allowing them to equilibrate at shallow depths and develop large 226Ra excesses at the expense of diminished 230Th excesses. Forward modeling of the trace element data also is consistent with mixing of melts in a two-porosity melting regime. U-series age constraints suggest that E-MORB neither erupt at systematically different times from N-MORB, nor necessarily through different pathways. Previous studies of E-MORB at 9–10°N have suggested that E-MORB compositions could be explained by off-axis eruption. However, when considered in light of previously published magnetic paleointensity and U-series age constraints, recent geological studies, and the most widely accepted contemporary understanding of volcanic construction at 9–10°N EPR, the asymmetric, off-axis distribution of E-MORB at 9–10°N EPR is consistent with, and more simply explained by, a model in which E-MORB erupted within the axial summit trough (AST) and flowed down the ridge flanks (∼0–3 km). These E-MORB subsequently spread away from the AST, and, finally, were preserved on the seafloor through asymmetric construction of the extrusive layer. Taken together, the range of ages of E-MORB at 9–10°N EPR and the geochemical and isotopic mixing trends suggest that enriched melts are continuously supplied to the ridge axis, but because of their small proportions relative to the volumetrically and volcanically dominant N-MORB, E-MORB preservation and exposure is comparatively scarce.

Petrology, geochemistry and U–Pb geochronology of the Betic Ophiolites: Inferences for Pangaea break-up and birth of the westernmost Tethys Ocean

The Betic Ophiolitic Unit, part of the Mulhacén Complex in the Betic Cordillera (SE Spain), is composed of several tectonic slices of eclogitized mafic and ultramafic rocks, together with oceanic metasediments. A comprehensive review of the petrological characteristics of the Betic Ophiolites shows notable analogies with other ophiolites from the Alps–Apennines system, indicating that they represent relics of oceanic lithosphere created at the westernmost end of the Western Tethys. U–Pb dating by SHRIMP (Sensitive High Resolution Ion MicroProbe) on zircon grains carefully investigated by cathodoluminescence images has been carried out on an eclogitized metagabbro (Lugros outcrop) and a metadolerite dyke intruded in ultramafic rocks (Almirez outcrop), giving consistent magmatic ages of around 185 ± 3 Ma. This age marks the beginning of the Betic oceanic magmatism (transitional to enriched MORB) that followed Pangaea break-up and the earliest oceanic accretion of the Western Tethys Ocean. Subsequently, ultraslow mid-ocean ridge spreading developed from the Betic oceanic basin northeastward to the Ligurian and Alpine Tethys domains, mainly around 165 to 140 Ma.

Petrografia e Geoquímica de Rochas Metamáficas e Metaultramáficas da região de Cambuci, Rio de Janeiro

In the northern Rio de Janeiro State, mafic and ultramafic rocks occur locally showing evidences of metamorphism and deformation. They are (meta)clinopiroxenites, (meta)piroxenites, (meta)diorites to quartzdiorites and mainly (meta)gabbros, (meta)norites and (meta)gabbronorites. The rocks present isotropic to foliated fabric marked by the orientation of piroxene/amphibole and feldspar or even mafic clots. Contrasting with samples that exhibit clear recristalized fabric, primary textures as granular, cumulatic and flow are found in many samples of the different lithotypes. Geochemical studies indicate tholeiitic basaltic composition and ocean floor signature, particularly transitional to enriched MORB. The obtained data allow interpreting these rocks as paleo-ocean remnants tectonically positioned in the regional paragneisses.

Subducted Precambrian oceanic crust: geochemical and Sr–Nd isotopic evidence from metabasalts of the Aksu blueschist, NW China

Abstract: The Aksu Proterozoic blueschist terrane in northwestern China is regarded as one of the oldest well-substantiated Precambrian blueschist terranes in the world. Previous work has focused on the high-pressure metamorphism: both the age and P – T conditions of the metamorphic event have been well discussed. However, little attention has been paid to protolith identification of the blueschist terrane. In this paper, we present geochemical and Sr–Nd analyses of metabasalt samples from the blueschist terrane. The results show that the protoliths of these mafic schists were enriched mid-ocean ridge basalt (E-MORB). Crystal fractionation has led to differentiation of trace element concentrations. This provides the opportunity to obtain a protolith Sm–Nd isochron age of 890 ± 23 Ma (MSWD = 0.68). We conclude that the Aksu blueschist terrane represents part of an ocean crust at the NW edge of Rodinia around Tarim at c . 890 Ma. It experienced subduction and exhumation followed by accretion to the Tarim craton. If the north margin of present Tarim faced outboard of Rodinia during subduction, it might form one section of the arcuate subduction zone around the supercontinent. Alternatively, the subduction zone may have been located between Tarim and Australia and would later be closed.

Vestige of an Early Cambrian incipient oceanic crust incorporated in the Variscan orogen: Letovice Complex, Bohemian Massif

Abstract: The Letovice Complex, composed of metamorphosed enriched mid-ocean ridge basalts with trondhjemitic sheets, as well as gabbroic and ultrabasic bodies, is exposed within an important Variscan suture separating the Moldanubian Domain (to the west) from the Brunovistulicum (to the east) in the eastern part of the Bohemian Massif, Czech Republic. A polyphase tectonic evolution of the study area was connected with underthrusting and subsequent exhumation of the former oceanic realm, which resulted in three deformation phases (D 1 –D 3 ). New U–Pb laser ablation inductively coupled plasma mass spectrometry dating of magmatic zircons from amphibolite and trondhjemite yielded statistically identical concordia ages of 530 ± 6 Ma and 529 ± 7 Ma (2σ), respectively. These are interpreted as dating intrusions of the Early Cambrian protoliths of the studied rocks. Based on this age as well as whole-rock geochemical and Nd isotopic data, the Letovice Complex is interpreted as a vestige of an incipient oceanic basin developing on attenuated crust. The most likely geotectonic setting was a post-Cadomian extensional regime at the northern margin of Gondwana that marked the onset of its break-up. Supplementary material: Photomicrographs of microstructures, sample locations, descriptions of analytical techniques, and selected electron microprobe analyses of the main rock-forming minerals are available at http://www.geolsoc.org.uk/SUP18424 .

A detrital record of lower oceanic crust exhumation within a Miocene slow-spreading ridge: Macquarie Island, Southern Ocean

Research Article| January 01, 2011 A detrital record of lower oceanic crust exhumation within a Miocene slow-spreading ridge: Macquarie Island, Southern Ocean Ryan A. Portner; Ryan A. Portner † GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney NSW 2109, Australia †E-mail: ryan.portner@mq.edu.au Search for other works by this author on: GSW Google Scholar Melissa J. Murphy; Melissa J. Murphy GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney NSW 2109, Australia Search for other works by this author on: GSW Google Scholar Nathan R. Daczko Nathan R. Daczko GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney NSW 2109, Australia Search for other works by this author on: GSW Google Scholar GSA Bulletin (2011) 123 (1-2): 255–273. https://doi.org/10.1130/B30082.1 Article history received: 30 Apr 2009 rev-recd: 25 Jan 2010 accepted: 27 Jan 2010 first online: 08 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 Ryan A. Portner, Melissa J. Murphy, Nathan R. Daczko; A detrital record of lower oceanic crust exhumation within a Miocene slow-spreading ridge: Macquarie Island, Southern Ocean. GSA Bulletin 2011;; 123 (1-2): 255–273. doi: https://doi.org/10.1130/B30082.1 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 SocietyGSA Bulletin Search Advanced Search Abstract Uplift, exhumation, and denudation of the lower oceanic crust are recorded by sedimentary rocks of Macquarie Island (54°30′S, 158°54′E), which were deposited within the slow-spreading proto–Macquarie spreading ridge between ca. 9 and 12 Ma. Measured stratigraphic sections typically contain basal basaltic breccia lithofacies that are overlain by a thick sequence of enriched mid-ocean-ridge basalt (E-MORB) with thin intercalations of gabbroic sedimentary lithofacies. Basaltic detritus has zeolite to lower-greenschist metamorphic grades typical of the upper oceanic crust, and gabbroic detritus has upper-greenschist to amphibolite metamorphic grades typical of the lower oceanic crust. Breccia clast counts and sedimentary structures indicate that basaltic lithofacies were locally derived from the footwalls of adjacent spreading-related faults. Sedimentary structures, detrital clinopyroxene major- and trace-element geochemistry, and 206Pb/238U zircon geochronology indicate that the gabbroic lithofacies were more distally derived from a Paleogene-aged tholeiitic MORB source. Detrital zircon populations of ca. 27 and ca. 33 Ma correspond to oceanic magnetic anomalies 8o and 13o, respectively, and exclude ca. 8.5 Ma gabbroic rocks of Macquarie Island as a potential source. Geodynamic reconstructions show that anomaly 8o crust from the Southeast Indian Ridge was juxtaposed against the active proto–Macquarie spreading ridge when sedimentary rocks of Macquarie Island were deposited and was a likely source for the gabbroic lithofacies. The proto–Macquarie spreading ridge and Southeast Indian Ridge were connected by the Jurru long-offset transform, which has undergone significant transpression since 27 Ma. This transpression formed a bathymetric transverse ridge that was composed of structurally isolated blocks of heterogeneously aged Paleogene source crust, which provided the source for Macquarie Island's gabbroic sedimentary lithofacies. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geochemistry and petrology of late Eocene Cascade Head and Yachats Basalt and alkalic intrusions of the central Oregon Coast Range, U.S.A.

The Oregon Coast Range underwent extension and basaltic magmatism accompanying tectonic rotation during the Tertiary. Basaltic magmatism was expressed initially in submarine eruptions, but mostly in younger subaerial flows. A tholeiitic basalt series, represented by Yachats units, evolved from a depleted, low K, Rb source through a process of fractional crystallization and magma mixing. An alkalic basalt series, represented by Cascade Head units, evolved primarily by fractional crystallization from parental magmas created by lower degrees of partial melting at greater depth than the Yachats series. Camptonite, found in sills and dikes, was formed by even smaller degrees of melting of sources similar to those of Cascade Head. Small nepheline syenite intrusions, emplaced as sills into middle Eocene Tyee Formation, appear to have formed through extensive fractional crystallization of parental basanite, which is sparsely represented in the Yachats and Cascade Head basaltic lava units. Individual syenite intrusions are not directly related to each other, as each intrusion represents a relatively small batch of magma separated at different evolutionary stages from underlying magma chambers. P–T segregation conditions were estimated for primitive Coast Range basalts. These include depths of ~ 120–160 km and temperatures of 1600–1700 °C for the alkalic series represented primarily by Cascade Head units and depths of ~ 60–120 km with temperatures of ~ 1500–1600 °C for tholeiitic units. A few analyses from other Coast Range igneous units (Tillamook and Goble volcanics) suggest T–P segregations at shallower depths (~ 55–33 km) and lower temperatures (~ 1400 °C). Paired Nd and Sr isotopic analyses of four Coast Range samples fall in the enriched MORB field. The depths and high temperatures indicated for the Central Oregon Coast ancestral basalts require magma generation in the garnet peridotite stability field, with the alkalic series probably originating below the subducting Juan de Fuca plate. This would suggest an origin for the magmas in a slab window or mantle plume environment. We further suggest that major extension documented for the Coast Range may have triggered upwelling of mantle asthenosphere giving birth to a mantle plume.