Geochemistry of the oldest MORB and OIB in the Isua Supracrustal Belt, southern West Greenland: Implications for the composition and temperature of early Archean upper mantle

Abstract Recent geological investigations of the Isua Supracrustal Belt (3.8 Ga), southern West Greenland, have suggested that it is the oldest accretionary complex on earth, defined by an oceanic plate‐type stratigraphy and a duplex structure. Plate history from mid‐oceanic ridge through plume magmatism to subduction zone has been postulated from analysis of the reconstructed oceanic plate stratigraphy in the accretionary complex. Comparison between field occurrence of greenstones in modern and ancient accretionary complexes reveals that two types of tholeiitic basalt from different tectonic settings, mid‐oceanic ridge basalt (MORB) and oceanic island basalt (OIB), occur. This work presents major, trace and rare earth element (REE) compositions of greenstones derived from Isua MORB and OIB, and of extremely rare relict igneous clinopyroxene in Isua MORB. The Isua clinopyroxenes (Cpx) have compositional variations equivalent to those of Cpx in modern MORB; in particular, low TiO₂ and Na₂O contents. The Isua Cpx show slightly light (L)REE‐depleted REE patterns, and the calculated REE pattern of the host magma is in agreement with that of Isua MORB. Analyses of 49 least‐altered greenstones carefully selected from approximately 1200 samples indicate that Isua MORB are enriched in Al₂O₃, and depleted in TiO₂, FeO*, Y and Zr at the given MgO content, compared with Isua OIB. In addition, Isua MORB show an LREE‐depleted pattern, whereas Isua OIB forms a flat REE pattern. Such differences suggest that the Early Archean mantle had already become heterogeneous, depending on the tectonic environment. Isua MORB are enriched in FeO compared with modern MORB. Comparison of Isua MORB with recent melting experiments shows that the source mantle had 85–87 in Mg? and was enriched in FeO. Potential mantle temperature is estimated to be approximately 1480°C, indicating that the Early Archean mantle was hotter by at most approximately 150°C than the modern mantle.     

Oceanic tholeiites on the Cape Verde Islands: petrochemical and geochemical evidence

The mineralogy, major element composition, REE patterns and Sc, Co, Hf, Cr, Sr, and Cu contents of nine pillow lavas from the pre-Cenozoic basement complex of Maio reveal the presence of oceanic tholeiites on the Cape Verde archipelago.These uplifted tholeiites constitute the oldest rocks exposed on the islands and are related to an Early Cretaceous (or still somewhat older?) oceanic ridge crest. It is suggested that the formation of east-west trending fracture zones in the Atlantic Ocean induced the alkalic character of the Cape Verde magmatism from the middle of the Cretaceous up to historical times.     

Rare earth elements in East Carpathian volcanic rocks

REE, Y, Rb, Sr, Cs, Ba, Pb, Th, U, Zr, Hf, and Sn are reported for a basalt, low-Si andesite, andesite, high-K andesite, dacite and rhyolite from the calc-alkaline volcanic belt of Calimani-Harghita Mountains (Rumenian Carpathians). The basalt, low-Si andesite and andesite show identical chondrite-normalized REE patterns with fractionated light REE (La-Sa) and unfractionated heavy REE (Gd-Yb). The dacite shows similar pattern but lower ΣREE. The high-K andesite and rhyolite have a distinctively different REE pattern strongly fractionated for both light and heavy REE. These differences point to different genetical mechanism for the high-K andesite-rhyolite and basalt-low-Si andesite-andesite-dacite magmas. The high-K andesite and rhyolite magmas are believed to represent primary melts of an undergoing oceanic slab; the basalt, low-Si andesite, andesite and dacite magmas are considered to be produced by partial melting of garnet pyroxenite bodies derived by reaction between the primary melts of the undergoing oceanic slab and the peridotitic mantle overlying the Benioff zone.     

Importance of vertical geochemical processes in controlling the oceanic profiles of dissolved rare earth elements in the northeastern Indian Ocean

Vertical profiles of dissolved rare earth elements (REEs) were obtained in the Bay of Bengal and the Andaman Sea. The REE concentrations at various depths in the Bay of Bengal are the highest in the Indian Ocean. This is attributable ultimately to the large outflow of the Ganges–Brahmaputra and Irrawaddy rivers, but the dissolved REE flux to surface waters alone cannot explain the large and near-constant REE enrichment throughout the entire water column. The underlying fan sediments serve as not a source but a sink for dissolved REE(III)s. Absence of excess ²²⁸Ra in the deep waters suggests that lateral input of dissolved REEs from slope sediments is also small in these regions. Partial (<0.3%) dissolution of detrital particles, which are carried by the rivers and lateral surface currents and subsequently settle through the water column, appears to be a predominant source for the dissolved REEs. Vertical profiles showing an almost linear increase with depth are common features for the light and middle REEs everywhere, but their concentration levels are variable from basin to basin and from element to element. This suggests that their oceanic distributions respond quickly to the variation of particle flux and its REE composition through reversible exchange equilibrium with suspended and sinking particles much like the case for Th. The relative importance of the vertical geochemical processes of reversible scavenging over the horizontal basin-scale ocean circulation with passive regeneration like nutrients decreases systematically from the light to the heavy REEs. Using a model, the mean oceanic residence times of REEs in the Bay of Bengal are estimated to range from 37 years for Ce to 140–1510 years for the strictly trivalent REEs. In the deep water of the Andaman Sea, isolated from the Bay of Bengal by the Andaman–Nicobar Ridge (maximum sill depth of ∼1800 m), the REE concentrations are almost uniform presumably due to rapid vertical mixing. The REE(III) concentrations are similar to that of ∼1250 m depth water in the Bay of Bengal, consistent with other oceanographic properties. However, the REE composition of the deep water appears to be altered slightly by preferential scavenging of the light REE(III) at the bottom boundary of the basin.     

Negative cerium anomalies in the rare earth element patterns of oceanic ferromanganese nodules

Ferromanganese nodules from the Bauer Basin of the south equatorial Pacific are unlike virtually all oceanic nodules so far analyzed in showing negative Ce anomalies in their REE abundance patterns. In comparison with similarly Cu-Ni-enriched nodules from the north equatorial Pacific they are depleted in REE by 50–80% and are heavy REE enriched relative to intermediate REE. The REE patterns can be accounted for by the input of hydrothermal iron oxyhydroxides and associated REE to the Bauer Basin and the transfer of the REE to the nodules because of diagenetic reactions in the sediment. The excess iron input also is reflected in lower Cu/Ni ratios in the nodules as compared with nodules from the north equatorial zone, apparently because of the larger proportions of a residual Fe phase in the nodules relative to todorokite. Cerium anomalies of the Bauer Basin nodules range from ?0.17 to ?0.29 as compared with +0.33 to +0.07 in the north equatorial Pacific but show a parallel sensitivity of Ce anomaly to Mn/Fe ratios of the nodules. Nodules with the more positive anomalies within each group have the smallest Mn/Fe ratios and have been subjected to the greatest seawater influence whereas nodules with the more negative anomalies have the largest Mn/Fe ratios and have been subjected to the greatest diagenetic influence.     

Gabbroic xenoliths from La Palma, Tenerife and Lanzarote, Canary Islands: evidence for reactions between mafic alkaline Canary Islands melts and old oceanic crust

Gabbroic and hornblendite xenoliths from La Palma, Tenerife and Lanzarote fall into three main groups based on petrography and chemistry. One group (comprising all xenoliths from Lanzarote and some from La Palma) consists of highly deformed orthopyroxene-bearing gabbroic rocks that show a strong affinity to N-MORB and oceanic gabbro cumulates in terms of mineral chemistry and REE relations. However, they show mild enrichment in the most incompatible elements (particularly Rb+Ba±K) relative to intermediate and heavy REE, and their Sr–Nd isotope ratios fall within or close to the N-MORB field. The second group (60% of the xenoliths from La Palma) are gabbroic cumulates with zoned clinopyroxenes (Ti–Al-poor cores, Ti–Al-rich rims) and reaction rims of hornblende, biotite and clinopyroxene on other phases. Their trace-element and Sr–Nd isotope relations are in general transitional between N-MORB cumulates and Canary Islands alkali basalts, but they show strong enrichment in Rb, Ba and K relative to other strongly incompatible elements. The third group (comprising some xenoliths from La Palma and all those from Tenerife) are undeformed gabbroic and hornblendite rocks in which hornblende and biotite appear to belong to the primary assemblage. These rocks show strong affinities to Canary Islands alkali basaltic magmas with respect to mineral, trace-element, and Sr–Nd isotope chemistry. The first two groups are interpreted as fragments of old oceanic crust which have been mildly to strongly metasomatized through reactions with Canary Islands alkaline magmas. The reaction process is a combination of enrichment in elements compatible with biotite (and hornblende), and simple mixing between N-MORB cumulates and trapped alkaline magmas. The third group represents intrusions/cumulates formed from mafic alkaline Canary Islands magmas. Modeling indicates that locally up to 50% new material has been added to the old oceanic crust through reactions with ocean island basalts. Reactions and formation of cumulates do not represent simple underplating at the mantle/crust boundary, but have taken place within the pre-existing oceanic crust, and are likely to have significantly thickened the old oceanic crust.     

Metallogenesis on a Mesozoic passive continental margin,Antalya Complex,southwest Turkey

A variety of Fe, Mn and trace-metal-enriched Mesozoic pelagic sediments are associated with the tectonically emplaced Antalya Complex in southwestern Turkey. Palaeotectonic settings represented within the complex comprise a continental platform, passing laterally through a Mesozoic passive margin into a zone of marginal oceanic crust, formed during the early stages of continental separation. The origins of the metalliferous sediments are elucidated using mineralogical, major, trace element and REE data, and comparisons with oceanic and ophiolite-related sediments.Late Triassic deposition during the initial continental separation was mostly terrigenous, including detrital carbonate derived from adjacent reef complexes. During the Jurassic and Early Cretaceous the passive margin underwent accumulation of fine-grained terrigenous matter and biogenic silica in deep water below the carbonate compensation depth. Argillaceous mudstones deposited during a regional hiatus at the end of the Upper Triassic show unusual Fe and trace metal enrichment, together with a marked positive Ce anomaly, indicative of slow hydrogenous accumulation.The marginal oceanic crustal zone also shows dominantly terrigenous and siliceous biogenic deposition but with the addition of an important hydrothermal component represented by Fe-Mn deposits. These occur within and immediately above the Upper Triassic lavas of the oceanic crust and as intercalations in the overlying Lower Cretaceous radiolarian chert sequence. Most of these sediments show strong Fe-Mn fractionation; several show a negative Ce anomaly implying rapid incorporation of the REEs from seawater.The Upper Triassic Fe-Mn deposits associated with the lavas are relatively trace-element-depleted and record rapid localised precipitation from relatively high-temperature hydrothermal solutions. By contrast, the more manganiferous and trace-element-enriched metalliferous horizons in the Jurassic to Lower Cretaceous chert sequences represent more dilute low-temperature hydrothermal discharge. Regional comparisons suggest that dominantly manganiferous deposits free of sulphides are characteristic of the early formed Mesozoic ocean crust compared with well established spreading axes like the Troodos Massif, Cyprus.     

Large scale isotopic Sr, Nd and O isotopic anatomy of altered oceanic crust: DSDP/ODP sites417/418

Large-scale compositional domains at DSDP/ODP drill sites 417A, 417D and 418A were analyzed for O, Sr and Nd isotope ratios, and REE, U, K, Rb and Sr abundances, to constrain the bulk chemical composition of the oceanic crust that is recycled at subduction zones. The combination of the three sites gives the composition of the upper oceanic crust in this region over a distance of about 8 km. The δ¹⁸O_SMOW and⁸⁷Sr/⁸⁶Sr_meas of compositional domains 10–100 m in size correlate well, with a range of 7.7–19.2 and 0.70364–0.70744, and mean of 9.96 and 0.70475, respectively. The Rb inventory of the upper crust increases by about an order of magnitude, while Sr contents remain constant. U abundances increase moderately under oxidizing alteration conditions and nearly triple in the commonly reducing alteration environments of the upper oceanic crust. REEs are influenced by alteration only to a small extent, and recycled oceanic crust is similar to MORB with respect to¹⁴³Nd/¹⁴⁴Nd. Even though the average composition of the upper oceanic crust is well defined, the large scale composition varies widely. Highly altered compositional domains may not have a large impact on the average composition of the oceanic crust, but they may preferentially contribute to fluids or partial melts derived from the crust by prograde metamorphic reactions.     

Sedimentary geochemistry of the Cambrian-Ordovician cherts: Implication on archipelagic ocean of North Qilian orogenic belt

The Caledonian North Qilian orogenic belt lies between the North China plate and the Qaidam mi-croplates, and resulted from the collision among the Qaidam microplate, mid-Qilian block and the North China plate. The orogen initiated from the rifting of the Late Proterozoic Rodinia, and then it experi-enced stages of Cambrian rift basin and Ordovician archipelagic oceanic basin, and foreland basin during Silurian to Early-Middle Devonian. The average ratios of Al/(Al Fe Mn), Al/(Al Fe), δ Ce, Lan/Ybn and Lan/Cen from cherts of Cambrian Heicigou Formation are 0.797, 0.627, 1.114, 0.994 and 1.034 re-spectively. In the NAS standardized REE distribution pattern, the cherts from Xiangqianshan is slightly HREE enriched, and the cherts from Ganluci and Shiqingdong are plane. All of these features indicated that Cambrian cherts of the Heicigou Formation originated from a continental margin rift background. On the contrary, the average ratios of Al/(Al Fe Mn), Al/(Al Fe), δ Ce, Lan/Ybn, Lan/Cen of the Ordovician chert from Dakecha, Cuijiadun, Shihuigou, Laohushan, Heicigou, Maomaoshan, Bianmagou, Da-chadaban, Baiquanmen, Jiugequan and Angzanggou, are respectively 0.72, 0.58, 0.99, 1.09 and 0.96 respectively. Their NAS standardized REE distribution patterns of most Ordovician cherts are plane mode or slightly HREE enriched. The REE distribution pattern of few samples of cherts are slightly LREE enriched. Characteristics of sedimentary geochemistry and tectonic evolution demonstrated that the Cambrian-Ordovician cherts, associated with rift, oceanic, island arc and back-arc volcanic rocks, was not formed in a typical abyssal oceanic basin or mid-oceanic ridge. On the contrary, they formed in a deepwater basin of continental margin or a archipelagic ocean tectonic setting. Several Early Paleo-zoic ophiolite belts in North Qilian and adjacent periphery Qaidam microplate imply that an archipelagic ocean during Ordovician existed in the east of Pro-Tethys.     

Bay of Islands ophiolite suite,Newfoundland: Petrologic and geochemical characteristics with emphasis on rare earth element geochemistry

Characteristic geochemical features of the ophiolite suite from the Bay of Islands Complex have been determined by major and trace element analyses of 13 rocks. Based on elements, such as rare earth elements (REE), whose abundances are relatively immobile during alteration and metamorphism, we find that (1) the pillow lavas and diabases are relatively depleted in light REE similar to most tholeiites occurring along spreading oceanic ridges, in back-arc basins and comprising the early phases of volcanism in island arcs; (2) the gabbros, composed of cumulate plagioclase and olivine with poikilitic clinopyroxene, have REE contents consistent with formation as cumulates precipitated from magmas represented by the overlying pillow lavas and diabases; (3) as in most harzburgites from ophiolites, the Bay of Islands harzburgite and dunite have relative REE abundances inconsistent with a genetic relationship to the overlying basic rocks — this inconsistency may be primary or it may result from late-stage alteration, contamination and/or metamorphism; (4) some Bay of Islands lherzolites have major and trace element abundances expected in the mantle source of the overlying basic rocks. Overall, the geochemical features of this Bay of Islands ophiolite suite are similar to those from Troodos and Vourinos, but these data are not sufficient to distinguish between different tectonic environments such as deep ocean ridge, small ocean basin or young island arc.     

The assessment of REE patterns and Nd/Nd ratios in fish remains

The REE content and isotopic composition of Sr and Nd have been determined in fish teeth ranging in age from the Trias to the present and from various localities mostly around the Atlantic. These measurements have been carried out on Selachian and Teleost remains from the same locality in Togo and show no appreciable difference, which suggests, with the help of a mass balance calculation of the Ce anomaly, that diagenetic effects are not responsible for the REE enrichment of biogenic phosphates.One group of fossil teeth has about 3 times the REE abundances of shale and a shale-normalized pattern with a minimum at Sm: it is thought to reflect deposition in the open-sea environment. A second group has REE concentration about 10 times higher than the first group with either a regular light REE enrichment or, more frequently, a maximum in the middle REE, both being probably indicative of deposition in estuarine or near-shore conditions. The shape of the REE spectra and the size of the Ce anomaly can be used semi-quantitatively to determine the depth of deposition. The results presented here on Late Cretaceous/Eocene fish teeth samples from Morocco reflect an increasing influx of deep waters with a lowLa/Yb ratio and strong negative Ce anomaly, which agrees well with the evolution of sediment chemistry and microfauna associations.In contrast, _Nd is typical of the water mass in which the fish debris decayed. Examples of nearly isolated basins identified with Nd isotopes include the South Atlantic prior to the Lutetian (_Nd ≈ −13.5), the Miocene Persian Gulf (_Nd = −3.1), and Bolivia during the Late Cretaceous (_Nd = −12.8). Togo and Guinea-Bissau results suggest that, in the South Atlantic, the meridional oceanic circulation had not started before 45 Ma ago.Combination of REE and_Nd data suggests that the assignment of Jurassic-Cretaceous samples measured so far to open-sea water masses is still ambiguous.     

New insights on the origin of troctolites from the breakaway area of the Godzilla Megamullion (Parece Vela back‐arc basin): The role of melt‐mantle interaction on the composition of the lower crust

The troctolites and olivine‐gabbros from the Dive 6 K‐1147 represent the most primitive gabbroic rocks collected at the Godzilla Megamullion, a giant oceanic core complex formed at an extinct spreading segment of the Parece Vela back‐arc basin (Philippine Sea). Previous investigations have shown that these rocks have textural and major elements mineral compositions consistent with a formation through multistage interaction between mantle‐derived melts and a pre‐existing ultramafic matrix. New investigations on trace element mineral compositions basically agree with this hypothesis. Clinopyroxenes and plagioclase have incompatible element signatures similar to that of typical‐MORB. However, the clinopyroxenes show very high Cr contents (similar to those of mantle clinopyroxene) and rim having sharply higher Zr/REE ratios with respect to the core. These features are in contrast with an evolution constrained by fractional crystallization processes, and suggest that the clinopyroxene compositions are controlled by melt‐rock interaction processes. The plagioclase anorthite versus clinopyroxene Mg#[Mg/(Mg + Fe^Tot)] correlation of the Dive 6 K‐1147 rocks shows a trend much steeper than those depicted by other oceanic gabbroic sections. Using a thermodynamic model, we show that this trend is reproducible by fractionation of melts assimilating 1 g of mantle peridotite per 1 °C of cooling. This model predicts the early crystallization of high Mg# clinopyroxene, consistent with our petrological observation. The melt‐peridotite interaction process produces Na‐rich melts causing the crystallization of plagioclase with low anorthite component, typically characterizing the evolved gabbros from Godzilla Megamullion.     

Petrochemical evidence for off-ridge magmatism in a back-arc setting from the Yakuno ophiolite, Japan

Abstract The Permian ophiolite emplaced in the Yakuno area, Kyoto Prefecture, consists of metavolcanic sequences, metagabbro and a troctolitic intrusion. The metavolcanics are associated with thick mudstone through a contact that shows the flowage of lava over unconsolidated mud layers on the sea floor. The metavolcanics and metagabbro have rare earth element (REE) patterns that are similar to enriched (E)‐ and transitional (T)‐types ([La/Yb]_N = 0.77–11.2) of mid‐oceanic ridge basalts (MORB), whereas their Nb/La ratios (0.40–1.20) are as low as those of back‐arc basin basalts (BABB). Cr‐spinels in the metavolcanic rocks have Cr? of 40–73 and an Fe³⁺? of 9–24, numbers which are comparable to the values of BABB. These lines of evidence suggest that the Yakuno ophiolite originated more likely from an early stage back‐arc basin rather than from an oceanic plateau, as has been suggested by some researchers. The troctolitic body that intrudes as a 0.5‐km long lens in the metagabbro is composed of troctolite, olivine gabbro and microgabbro. The troctolite is marked by an olivine–plagioclase crystallization sequence, different from the commonly observed olivine–clinopyroxene sequence in other mafic/ultramafic cumulates of the Yakuno ophiolite. The microgabbro, with a composition close to that of the parental magma of the troctolite, is depleted in light REE ([La/Yb]_N = 0.18–0.55) so that it has an REE pattern that mimics normal (N)‐type MORB. The interstitial clinopyroxene of the troctolite has highly variable TiO₂ contents (0.2–1.4 wt%), which is interpreted to result from postcumulus crystallization of heterogeneous intercumulus melts. The troctolitic intrusion may represent a late stage intrusion that formed in an off‐ridge environment during sea floor spreading of the back‐arc basin. The geochemical variation observed in the Yakuno ophiolite, ranging from N‐ to E‐MORB affinities, reflects the changes in both mantle source compositions and processes involved in magma generation during the evolution of the back‐arc basin.     

Oceanic plateau accretion inferred from Late Paleozoic greenstones in the Jurassic Tamba accretionary complex, southwest Japan

Abstract The Jurassic Tamba accretionary complex is divided into two tectono‐stratigraphic suites (Type I and II nappe groups), which are further divided into six complexes (nappes) each of which is characterized by a rock sequence of Late Paleozoic greenstone/limestone, Permian to Jurassic chert and Jurassic terrigenous clastic rocks. The mode of occurrence of the greenstone is divided into two types. The major basal type occurs as a large coherent slab associated with Permian chert and limestone, constituting the basal part of each complex, and the minor mixed type occurs as fragmented allochthonous greenstone blocks and lenses mixed with chert, limestone and sandstone in the Jurassic mudstone matrix. Most of the basal greenstones have uniform geochemical characteristics, which indicate enriched‐mid‐oceanic ridge basalt (MORB) affinity. Their geochemical compositions are akin to the reported Permo‐Carboniferous and Triassic oceanic plateau basalts. Mixed greenstones are divided into two petrochemical types: (i) tholeiitic basalt with normal‐MORB affinity, which is predominant in the uppermost complex of the Type II suite (upper nappe group); and (ii) tholeiitic and alkalic basalts of oceanic island or seamount origin, which are common in all complexes of the Tamba Belt. Geochemical characteristics of the greenstones thus vary in accordance with their occurrences and the structural units to which they belong. This relationship reflects the difference in topographic relief and crustal thickness of the accreted oceanic edifices – the remnants of thick oceanic plateau crust tended to accrete to the continental margin as a large basal greenstone body, whereas thin normal oceanic crust with small seamounts or oceanic islands accreted as mixed greenstones because of their mechanical weakness. The Type II suite (upper nappe group) contains the basal and mixed greenstones, whereas the Type I suite (lower nappe group) includes only mixed greenstones. This distinction may reflect the temporal change of subducting edifices from a thick oceanic plateau to a thin normal oceanic crust, and suggests that the accretion of a large oceanic plateau may be responsible for building accretionary complexes with thick basal greenstones slabs.     

An Archean oceanic felsic dyke swarm in a nascent arc: the Hunter Mine Group, Abitibi Greenstone Belt, Canada

The 2730-Ma-old Hunter Mine Group (HMG), a dominantly felsic subaqueous volcanic sequence, was formed during early arc construction in the Abitibi greenstone belt (Quebec, Canada). The western part of the HMG contains a felsic dyke swarm up to 1.5 km wide and traceable up-section for 2.5 km. Five distinct generations were identified: (1) aphanitic to feldspar-phyric dykes; (2) quartz-feldspar-phyric dykes with < 5% quartz phenocrysts; (3) quartz-feldspar-phyric dykes with 10–25% quartz phenocrysts; (4) dacitic feldspar-phyric dykes; and (5) mafic dykes. The felsic dykes collectively constitute more than 90% of the dyke swarm. Geochemically, they resemble modern calc-alkaline dacites and rhyolites. Their mantle-normalized incompatible trace-element patterns display a moderate enrichment of Th and light REE relative to HFSE and heavy REE as well as negative Nb, Ta, Eu and Ti anomalies. Most of the major- and trace-element abundance variations in these rocks can be explained by crystallization of feldspars. Geochemical data including depleted mantle-like _Nd values suggest that an older sialic substrate was not involved in their genesis. We infer that the felsic rocks were generated by melting of mafic oceanic crust. The swarm was emplaced during nascent oceanic island-arc development and was related to rifting of the arc. The conformably overlying MORB-like basalts and basaltic komatiites of the Stoughton-Roquemaure Group used the same conduits and further indicate splitting of the arc. HMG and associated parts of the Abitibi greenstone belts bear a strong resemblance to modern rifted intraoceanic arcs of the western Pacific.     

Trace element geochemistry of some continental tholeiites

Continental tholeiites from four regions (Late Cretaceous to Early Eocene Deccan Trap lavas of central India, Early Mesozoic tholeiites from the Atlantic margins of Northwest Africa-Morocco, and northeastern North America-Nova Scotia, Canada and Precambrian Coppermine River basalts from Northwest Territories, Canada) differ from MORB by higher concentrations of K, Rb, Ba and Th and to a lesser degree light REE. Their chondrite-normalized trace element patterns show negative Nb anomalies. The distribution and variation of trace elements indicate that the rocks from all the areas studied were affected by interaction with the continental crust. It is suggested that continental tholeiites have been generated from a similar source as oceanic tholeiites and many of their geochemical differences are related to crustal contamination.     

Geochemistry of the E-MORB type ophiolite and related volcanic rocks from the Wushan area,West Qinling

The mafic-ultramafic assemblages, which thrustthrust into the Wushan-Tangzang boundary fault as some blocks and outcropped in the Yuanyangzhen, Lijiahe, Lubangou and Gaojiahe area, consist mainly of meta-peridotites, gabbros and basalts. The meta-peridotites are characterized by high SiO₂ and MgO contents, low ΣREE, as well as their chondrite-normalized rare earth element patterns show some similarities to that of middle oceanic meta-peridotite. The basalts from the Yuanyangzhen, Lijiahe and Lubangou area are characterized by relatively high TiO₂ content, low Al₂O₃ content and Na₂O>>K₂O. Above all, it is the slight enrichment or flat REE distribution patterns and the unfractionated in HFS elements in the primitive-normalized trace elements distribution patterns that indicate these basalts are similar to that of the typical E-MORB. In comparison, the basalts from the Gaojiahe section are featured by depletion in Nb and Ta contents and enrichment in Th content which show that these were derived from an island-arc setting. From studies of the regional geology, petrology, geochemistry, geo-chronology and all above evidence, it can be suggested that the mafic-ultramafic rocks from the Wushan area are mainly dismembered E-MORB type ophiolite, which represent the fragments of the lithosphere of the Early-Paleozoic Qinling ocean. It is preferred that these rocks were formed in an initial mid-ocean ridge setting during the beginning stage of the oceanic basin spreading. This ophiolite together with the Gaojiahe island-arc basalts shows that there exists an ophiolitic mélange along the Wushan-Tangzang boundary fault, and marks the suture zone after the closure of the Qinling ocean in early Paleozoic.

     

Geochemistry of early Paleozoic alkali dyke swarms in south Qinling and its geological significance

The abundant research achievements about Qinling orogeny going through central China were obtained in the last decade[1—3]. However, there is a debate about the early Paleozoic tectonic evolution of south Qinling. Thus, it is undoubted that the systemati…     

Geochemistry and petrogenesis of ophiolites from Northern Pindos (Greece)

The ophiolitic complex of Northern Pindos (Greece) contains ocean-floor basalts and low-Ti mafic rocks. The former rocks are similar to recent mid-ocean ridge basalts with a light REE depletion and a La/Yb ratio < 2. The low-Ti rocks resemble boninites in their high Mg and very low Ti and Zr contents and in their REE patterns which have convex-downwards shape with a slight light REE enrichment. However, their Zr/Ti, Ti/V and Zr/Y ratios are lower than in boninites. Both rock-types could be generated by dynamic partial melting of a rising upper mantle diapir. Slight enrichment in light REE, Sr, Rb and Ba in low-Ti rocks could be the result of either metasomatic or alteration processes. Although a subduction zone origin of the sequence is possible, the geochemical data do not necessarily imply such a setting.     

The role of sphene as an accessory phase in the high-pressure partial melting of hydrous mafic compositions

In a high-pressure experimental study of reactions and possible melt products occurring in the deep continental crust or in subducted oceanic crust, sphene has been identified over a pressure range of 10–18 kbar and to temperatures of 1020°C. Sphene may be a refractory phase with up to 60% partial melting for hydrous mafic compositions. Sphene breaks down at lower pressure than the maximum pressure stability of amphibole in hydrous mafic compositions, and rutile rather than sphene is the important Ti-bearing accessory phase at pressures greater than 16–18 kbar. Sphene and rutile coexist to pressures as low as 14 kbar. This implies that amphibole eclogites containing primary sphene and no rutile have most likely formed at depths less than 45 km.The presence of minor sphene as a residual phase in equilibrium with low-Ti silicic liquids may have a marked effect on the REE distribution in derivative liquids. Thus melts in equilibrium with a garnet and sphene-bearing residuum may have less light-REE-enriched patterns than those predicted when garnet is a residual phase without coexisting sphene. This effect is modelled using REE patterns for sphenes from high-grade metamorphic terrains of western Norway.Both the new REE data and the experimental study have important implications for the genesis of low-Ti magmas formed in continental margins and island arcs.