Enriched MORB Research Articles

Elemental and Sr–Nd isotopic geochemistry of the basalts and microgabbros in the Shuanggou ophiolite, SW China: implication for the evolution of the Palaeotethys Ocean

AbstractThe Early Palaeozoic Shuanggou ophiolite is the best-preserved part of the Ailaoshan ophiolite belt. The microgabbros (basaltic dykes) and basalts (basaltic lavas) show distinct characteristics in geochemistry, implying that their genetic mechanisms are different. With Al2O3 contents ranging from 14.7% to 17.0%, the microgabbros belong to low-alumina type. They exhibit normal mid-ocean-ridge basalt (N-MORB) -like trace elemental characteristics with positive εNd(t) values (9.7–11.6) and slightly variable (87Sr/86Sr)i ratios (0.7036–0.7046). In contrast, the basalts have high Al2O3 contents (19.5–23.2%), therefore belonging to high-alumina type. A plagioclase-accumulation model is used to account for the high alumina contents. Moreover, the basalts have enriched MORB (E-MORB) -like trace element characteristics with lower εNd(t) values (6.4–8.0) and (87Sr/86Sr)i ratios (0.7032–0.7036). Their incompatible element ratios exhibit linear correlation with the isotopic data, which is probably related to the contribution of a mixed lithosphere–asthenosphere source. In summary, a two-stage model is proposed to explain the formation of the Shuanggou ophiolite: (1) at the continent–ocean transition stage, the basalts were generated by low-degree partial melting of the mixed mantle near a slow-spreading embryonic centre; and (2) at the mature stage of the Ailaoshan Ocean, the microgabbros were produced by moderate-degree partial melting of the depleted asthenospheric mantle.

The Upper Cretaceous ophiolite of North Kozara – remnants of an anomalous mid-ocean ridge segment of the Neotethys?

Abstract This study sheds new light on the origin and evolution of the north Kozara ophiolite, a part of the Sava-Vardar Zone. The Sava-Vardar Zone is regarded as a relict of the youngest Tethyan realm in the present-day Balkan Peninsula. The north Kozara ophiolite consists of a bimodal igneous association comprising isotropic to layered gabbros, diabase dykes and basaltic pillow lavas (basic suite), as well as relicts of predominantly rhyodacite lava flows and analogous shallow intrusions (acid suite). The rocks of the basic suite show relatively flat to moderately light-REE enriched patterns with no or weak negative Eu-anomaly, whereas those of the acid suite exhibit steeper patterns and have distinctively more pronounced Eu- and Sr- negative anomalies. Compared to the known intra-ophiolitic granitoids from the Eastern Vardar Zone, the acid suite rocks are most similar to those considered to be oceanic plagiogranites. The new geochemical data suggest that the basic suite rocks are similar to enriched mid-ocean ridge basalts. The geochemical characteristics of the acid suite rocks indicate that their primary magmas most probably originated via partial melting of gabbros from the lower oceanic crust. Our study confirms the oceanic nature of the north Kozara Mts rock assemblage, and suggests that it may have formed within an anomalous ridge setting similar to present-day Iceland.

Protolith provenance and thermotectonic history of metamorphic rocks in eastern Jamaica: Evolution of a transform plate boundary

Two fault-bounded sequences of metamorphic rocks are exposed in the Blue Mountains of eastern Jamaica. Westphalia Schist is dominated by amphibolite facies hornblende schist and mica schist. Mt. Hibernia Schist is dominated by blueschist-greenschist facies metabasalts. New whole-rock geochemistry and 40 Ar/ 39 Ar ages clarify the tectonic setting of the protoliths, timing of post-metamorphic cooling, and evolution of the northern margin of the Caribbean plate. Westphalia Schist is geochemically variable, with mafi c igneous protoliths or vol caniclastic sedimentary protoliths. Regardless of the protolith, the trace-element geochemistry is consistent with an island-arc tectonic environment. These rocks most likely represent metamorphosed equivalents of the regionally extensive Early Cretaceous Greater Antilles arc that is preserved discontinuously along the present-day northern margin of the Carib bean plate. Mt. Hibernia Schist shows little geochemical variability, with an igneous protolith of subalkaline basaltic composition. Flat rare-earth-element patterns and fl at extended trace-element patterns are consistent with an enriched mid-ocean ridge basalt or oceanic plateau environment. However, in terms of immobile elements, Mt. Hibernia Schist is geochemically indistinguishable from nearby ca. 90 Ma basalt of the Bath-Dunrobin Formation, which is a product of Caribbean plate–forming ocean plateau magmatism; i.e., Caribbean large igneous province. Hence, an ocean plateau environment is inferred for the Mt. Hibernia protolith. The Westphalia and Mt. Hibernia Schists are currently juxtaposed along the Blue Mountain fault, yet were subjected to very different subduction-related metamorphic histories. Stratigraphic relationships require that the metamorphic rocks were uplifted, and exposed at the surface by the Early Paleo cene. 40 Ar/ 39 Ar ages indicate that the two units were affected differently by burial metamorphism related to Paleocene–Early Eocene transtensional tectonics. Final juxtaposition of Westphalia Schist and Mt. Hibernia Schist was accomplished through a combination of vertical and horizontal displacements during Neogene transpression along the Plantain Garden fault.

Phenocryst He–Ar isotopic and whole-rock geochemical constraints on the origin of crustal components in the mantle source of Cenozoic continental basalt in eastern China

Continental basalts commonly exhibit similar geochemical compositions to oceanic island basalts, but their origin is still enigmatic in chemical geodynamics. This involves a resolution to the nature of crustal components in their mantle sources. For this purpose, we have studied the He and Ar isotopic compositions of mineral phenocrysts from Cenozoic continental basalts in eastern China. The results are combined with existing data for whole-rock geochemistry, yielding two trends between noble gas isotopic and other geochemical indicators. First is high 3He/4He ratios of 6.6 to 7.5RA and low 4He/40Ar* ratios of 0.07 to 1.39 in association with varying (La/Yb)N values from 10.4 to 31.8. Second is low 3He/4He ratios of 0.6 to 2.4RA and variable 4He/40Ar* ratios from 0.68 to 15.47 in association with low (La/Yb)N values of 11.3 to 12.0. Phenocryst 40Ar/36Ar ratios vary from 331.0 to 1677.4, in which the low ratios are associated with the low (La/Yb)N values. In combination with whole-rock Ba/Th and Sr/Y ratios as well as initial Sr and Nd isotopic ratios, we suggest three-component mixing between enriched MORB mantle, altered oceanic basalt and seafloor sediment to generate the mantle sources. The two kinds of crustal components would be incorporated into the mantle sources in the form of felsic melts. One is adakitic melt deriving from the altered oceanic basalt and thus low in Ba/Th ratios but high in Sr/Y, (La/Yb)N and εNd(t) values. The other is sialic melt originating from the seafloor sediment and thus high in Ba/Th ratios but low in Sr/Y, (La/Yb)N and εNd(t) values. The atmospheric Ar and crustal He noble gas components would be carried by the both seawater–hydrothermally altered basalt and seafloor sediment on the oceanic crust. These oceanic crust-derived melts would serve as a metasomatic agent to transfer the supracrustal He and Ar isotopic signatures to the mantle sources. The felsic melts would react with the mantle wedge peridotite during slab–mantle interaction in oceanic subduction channel. Therefore, the noble gas isotopic compositions of mantle sources for intraplate basalts are primarily determined by the budget between the mantle and crustal components during the melt–peridotite in oceanic subduction factory.

Diverse tectonic settings of formation of the metaigneous rocks in the Jurassic metamorphic accretionary complexes (Refahiye, NE Turkey) and their geodynamic implications

Two isolated metamorphic accretionary complexes of Jurassic age, the Refahiye and Kurtlutepe metamorphic rocks, crop out as tectonic slices within the coeval suprasubduction-zone ophiolite at the southern margin of the Eastern Pontides (NE Turkey), close to the İzmir-Ankara-Erzincan suture. The Refahiye metamorphic rocks are made up of greenschist, marble, serpentinite, phyllite and minor garnet amphibolite, garnet micaschist and metachert. The whole unit was metamorphosed under garnet-amphibolite-facies conditions and strongly retrogressed during exhumation. The Kurtlutepe metamorphic rocks consist of subgreenschist-facies metavolcanics, metavolcaniclastics, marble, calc-phyllite, and minor serpentinite and metachert. Metabasites in the Refahiye metamorphic rocks are represented by four distinct geochemical affinities: (i) cumulate “flavor,” (ii) alkaline oceanic island basalt (OIB), (iii) enriched mid-ocean ridge basalt (E-MORB) and (iv) tholeiitic island arc basalt (IAB). On the other hand, the Kurtlutepe metavolcanic rocks display only tholeiitic to calc-alkaline island arc geochemical affinities. The metabasic rocks with OIB affinities were interpreted as parts of the accreted oceanic islands, and those with E-MORB affinities as parts of accreted ridge segments close to oceanic islands and/or plume-distal mid-ocean ridges with a mantle previously metasomatized by plume components. The metabasic rocks with IAB affinities might have been derived from the overlying suprasubduction ophiolite and/or arc domain by a number of tectonic or sedimentary processes including tectonic slicing of accretionary complex and overlying fore-arc ophiolite, juxtaposition of the magmatic arc with subduction zone by strike slip faults, submarine gravity sliding and debris flows or subduction erosion. However, totally recrystallized nature of the metabasic rocks together with field relations does not allow any inference on the processes involved. The Kurtlutepe metavolcanic rocks might represent collided and accreted oceanic island arc with the subduction zone. Attempted subduction of an intraoceanic island arc may also explain the magmatic lull during Late Jurassic–Early Cretaceous in the Eastern Pontides.

Petrological vestiges of the Late Jurassic-Early Cretaceous transition from rift to back-arc basin in southernmost Chile: New age and geochemical data from the Capitán Aracena, Carlos III, and Tortuga ophiolitic complexes

The ophiolitic remnants of the Upper Mesozoic Rocas Verdes basin in southernmost South America were studied from the perspectives of petrography, chemistry of minerals, bulk-rock geochemistry, and U-Pb geochronology. The study aimed to unravel the tectonic, magmatic, and metamorphic evolution of a suprasubduction rift zone that underwent a transition to a back-arc basin. The rifting phase and bimodal magmatism within the Rocas Verdes basin started prior to or during the Late Jurassic, as indicated by a gabbro in contact with pillow basalts that dated at 154 Ma. In the Late Jurassic Capitán Aracena and Carlos III complexes, tholeiitic basalts are geochemically comparable to enriched mid-oceanic ridge basalts. Back-arc basin development continued for 35 myr until the Early Cretaceous, as suggested by the ages of detrital zircons in cherty layers within pillow basalts and metamorphic titanite that crystallized during seafloor metamorphism near the spreading/magmatic axis. In the Early Cretaceous Tortuga Complex, tholeiitic basalts are comparable to normal mid-oceanic ridge basalts. Non-deformative metamorphism converted the primary mineralogy of the ophiolites to low- to intermediate-grade metamorphic assemblages formed during ocean-floor type alteration in a suprasubduction setting. Fossilized bacteria, preserved as rounded aggregates of titanite microcrystals, were identified in the pillow basalts up to the Early Cretaceous. The Rocas Verdes basin closed during the Andean orogeny, which started during the Late Cretaceous, and ophiolites were tectonically juxtaposed and thrust over the sedimentary infill of the quasi-oceanic basin in which they developed. The tectonic emplacement of the ophiolitic complexes was complete before the latest Cretaceous, as indicated by crystallization ages of granites intruded into the ophiolitic complexes.

The Eldivan ophiolite and volcanic rocks in the İzmir–Ankara–Erzincan suture zone, Northern Turkey: Geochronology, whole-rock geochemical and Nd–Sr–Pb isotope characteristics

Gabbros and dolerite dikes of the Eldivan ophiolite and basaltic volcanic rocks of the ophiolitic mélange in the central part of the İzmir–Ankara–Erzincan (IAE) suture zone were investigated for their 40Ar/39Ar age and whole-rock-major-trace element and Sr–Nd–Pb isotope compositions. Based on geological and geochemical characteristics basaltic volcanic rocks in the ophiolitic mélange are subdivided into two groups (Groups I and II) with ocean island basalts or enriched mid-ocean ridge basalt characteristics, respectively. Gabbros and dolerite dikes of the Eldivan ophiolite (Groups III and IV) have instead geochemical compositions indicative of a subduction-related environment. The volcanic rocks of Group I have 87Sr/86Sr(i) between 0.7037 and 0.7044, ƐNd(i)–DM of −4.5 to −5.6, and 206Pb/204Pb(i) ranging between 18.35 and 18.75. Group II volcanic rocks have higher 87Sr/86Sr(i) values (0.7049–0.7055), ƐNd(i)–DM ranging between −5.4 and −6.0, and 206Pb/204Pb(i) between 18.14 and 18.62. The Nd isotopic signatures and 207Pb/204Pb(i) values of the volcanic rocks of both groups point to a different source with respect to those of the Eldivan ophiolite. The low 206Pb/204Pb(i) values relative to the ophiolitic rocks seem to exclude a significant contribution from a HIMU reservoir, whereas the 207Pb/204Pb(i) values slightly above the NHRL might indicate some contribution from an EM2-type reservoir. Gabbros (Group III) of the Eldivan ophiolite and dolerite dikes (Group IV) cross-cutting the ultramafic part of the ophiolite show 87Sr/86Sr(i) between 0.7038 and 0.7053, ƐNd(i)–DM from −2 to −3.6 and 206Pb/204Pb(i) between 18.10 and 18.80. The gabbros yield ca. 150Ma 40Ar/39Ar amphibole-plateau ages, which, together with the geochemical data, indicate that they were produced above subducted oceanic lithosphere in the IAE ocean domain in Late Jurassic times. Therefore, the Eldivan ophiolite in the IAE suture zone constitutes a link between the Hellenide–Dinaride ophiolite belts to the west and the Armenian–Iranian ophiolites to the east.

Origin of Diverse Geochemical Signatures in Igneous Rocks from the West Philippine Basin: Implications for Tectonic Models

The West Philippine Basin (WPB), formed by seafloor spreading between 60 and 35 Ma, provides an excellent case study of relationships between basin tectonics and magma chemistry. At 48 Ma, the Izu-Bonin-Mariana (IBM) arc formed along the basin edge, orthogonal to the active spreading center; thus, WPB development is a key issue for this Margins Subduction Factory focus area. WPB basalts from the main spreading stage are normal to enriched mid-ocean ridge basalt (MORB) with an Indian Ocean MORB isotopic signature. Basalts from the Benham Rise and locations near the western Central Basin Spreading Center (CBSC) at 50-35 Ma are geochemically identical to oceanic island basalts. Late-stage CBSC basalts (35-26 Ma) are isotopically like main spreading-stage MORB, with widely varying and decoupled trace element enrichments. Based on basalt geochemistry, the WPB could be a trapped fragment of ancient Indian/Tethyan ocean ridge, as proposed in some models for the initiation of the IBM arc, or it could be a back-arc basin, provided plate configurations allowed replenishment of sub-Indian Ocean asthenosphere. Ocean island basalts were formed by decompression melting of an enriched source beneath the western CBSC, mixing with normal MORB sources to form enriched MORB. This was a transitory feature (15 Ma) related to spreading, rather than a deep-seated plume, and probably did not affect the early IBM arc. Magma formed in small, deep-seated batches as the extension waned. That CBSC activity continued for 22 Ma after the initiation of the IBM arc indicates that forces related to an additional subduction system influenced the WPB.

The Carboniferous ophiolite in the middle of the Qiangtang terrane, Northern Tibet: SHRIMP U–Pb dating, geochemical and Sr–Nd–Hf isotopic characteristics

Ophiolite plays a key role in identifying paleo-ocean and paleo-plate and rebuilding the evolutionary history of ancient orogen. Mafic–ultramafic rocks are distributed in a broadly E–W direction in the middle of the Qiangtang terrane, northern Tibetan plateau. However, interpretation of these rocks as ophiolite and subsequent tectonic implications have been much disputed, and they were regarded by some to have an origin in a continental rift setting. A detailed zircon dating and geochemical and Sr–Nd–Hf isotopic study of this suite of rocks has been undertaken in order to clarify this important issue. The mafic–ultramafic suite was collected from the Gangma Co and Guoganjianian localities and is composed of cumulate and isotropic gabbro, basalt, actinolitite and plagiogranite. All basaltic rocks are tholeiitic and have low rare earth element (REE) abundances with variable REE patterns and slight negative Nb and Ti anomalies. These features are comparable with those of normal mid-ocean ridge basalts (N-MORB) and/or enriched mid-ocean ridge basalts (E-MORB). The positive whole-rock εNd(t) and zircon εHf(t) values indicate that these rocks were derived from a long-term depleted mantle source. Zircon U–Pb dating using a sensitive high-resolution ion microprobe (SHRIMP) on two cumulate gabbros and two plagiogranites samples yielded Carboniferous ages of 357±2.5Ma, 356.1±3.0Ma, 354.7±4.7Ma and 345.4±4.6Ma. The ophiolite marks a Paleo-Tethys Ocean basin in middle of the Qiangtang terrane, and it is interpreted as the western extension of the Changning–Menglian Paleo-Tethys ophiolite in the eastern margin of the Tibetan plateau.

Early Middle Triassic mafic dikes from the Baoshan subterrane, western Yunnan: implications for the tectonic evolution of the Palaeo-Tethys in Southeast Asia

The time of final closure of the Palaeo-Tethys and the Sibumasu-Indochina collision in Southeast Asia represents a major unresolved geologic problem. Here, we present zircon chronology, whole-rock elemental, Sr–Nd, and zircon Hf isotopic geochemistry for newly discovered mafic dikes from the northern segment of the Sibumasu terrane, to provide constraints on this issue. Zircon U–Pb data indicate that the dikes were emplaced at 240 ± 3 Ma. These are the earliest Mesozoic magmatic rocks reported so far in the Sibumasu terrane, the late Palaeozoic passive margin of the Palaeo-Tethys. They are subalkaline tholeiites, showing geochemical characteristics similar to those of enriched mid-ocean ridge basalts (E-MORBs). They have 87Sr/86Sr(t) ratios of 0.703161–0.703826, ϵNd(t) of +4.8 to +7.5, and zircon ϵHf(t) of +9.2 to +13.3, implying strong mantle depletion. They were derived by partial melting of asthenospheric mantle and underwent subsequent fractional crystallization and lithospheric assimilation. The geologic–petrologic evidence suggests that the mafic dikes were generated in a collisional setting, when suturing of the Baoshan and Simao subterranes (the two subterranes are part of the Sibumasu and Indochina terranes, respectively) occurred. These early Middle Triassic mafic dikes provide an upper limit for Sibumasu–Indochina collision. In conjunction with previous work, we conclude that the final closure of the Palaeo-Tethys and collision of the Sibumasu and Indochina terranes took place during the late Permian to Early Triassic.

Elemental responses to subduction-zone metamorphism: Constraints from the North Qilian Mountain, NW China

Subduction zone metamorphism (SZM) and behaviors of chemical elements in response to this process are important for both arc magmatism and mantle compositional heterogeneity. In this paper, we report the results of our petrographic and geochemical studies on blueschist and eclogite facies rocks of sedimentary and basaltic protoliths from two metamorphic sub-belts with different metamorphic histories in the North Qilian Mountain, Northwest China. The protolith of low-grade blueschists is basaltic in composition and is most likely produced in a back-arc setting, while the protoliths of high-grade blueschists/eclogites geochemically resemble the present-day normal and enriched mid-oceanic ridge basalts plus some volcanic arc rocks. The meta-sedimentary rocks, including meta-graywacke, meta-pelite, meta-chert and marble, show geochemical similarity to global oceanic (subducted) sediments. Assuming that high field strength elements (HFSEs) are relatively immobile, the correlated variations of rare earth elements (REEs) and Th with HFSEs suggest that all these elements are probably also immobile, whereas Pb and Sr are mobile in rocks of both basaltic and sedimentary protoliths during SZM. Ba, Cs and Rb are immobile in rocks of sedimentary protoliths and mobile in rocks of basaltic protolith. The apparent mobility of U in rocks of basaltic protolith may be inherited from seafloor alterations rather than caused by SZM.On the basis of in situ mineral compositional analysis (both major and trace elements), the most significant trace element storage minerals in these subduction-zone metamorphic rocks are: lawsonite, pumpellyite, apatite, garnet and epidote group minerals for REEs, white micas (both phengite and paragonite) for large ion lithophile elements, rutile and titanite for HFSEs. The presence and stability of these minerals exert the primary controls on the geochemical behaviors of most of these elements during SZM. The immobility of REEs, Th and U owing to their redistribution into newly formed minerals suggests that subduction-zone dehydration metamorphism will not contribute to the enrichment of these elements in arc magmatism. These observations require the formation and contribution of supercritical fluids or hydrous melts (these can effectively transport the aforementioned incompatible elements) from greater depths to arc magmatism. In addition, the overall sub-chondritic Nb/Ta ratio retained in rutile-bearing eclogites indicates that the subducting/subducted residual ocean crust passing through SZM cannot be responsible for the missing Nb (relative to Ta) in the bulk silicate earth.

Late Paleozoic subduction system in the southern Central Asian Orogenic Belt: Evidences from geochronology and geochemistry of the Xiaohuangshan ophiolite in the Beishan orogenic belt

The Xiaohuangshan ophiolite of the Beishan (Inner Mongolia) is located in the southern margin of the Central Asian Orogenic Belt. It consists of several blocks composed dominantly of serpentinized ultramafic rocks, cumulative gabbros and basalts. The geochemical data of gabbros and basalts obtained from the Xiaohuangshan ophiolite are similar to tholeiitic rocks. They all have low TiO2 and high Al2O3 contents. Their light rare earth elements are slightly enriched, (La/Yb)N=3.62–6.80, similar to the typical enriched mid-ocean ridge basalts. The mafic rocks display enrichments in large ion lithophile elements and depletions in high field strength elements, as well as significantNb–Ta–Ti negative anomalies, similar to subduction-derived rocks. All these geochemical characteritics indicate that the Xiaohuangshan ophiolite would form in a subduction zone from a slightly enriched mantle source. Ion microprobes (SHRIMP) U–Pb dating were conducted on zircons from the basalt and gabbro. The weighted mean ages are 336.4±4.1Ma and 345±14Ma, which are considered as the crystallization ages of the basalt and gabbro, respectively. Together with other two units, the Dongqiyishan arc belt and the Yueyashan–Xichangjing ophiolite, the Xiaohuangshan ophiolite forms a Late Paleozoic arc-basin system, indicating that the Paleo-Asian Ocean did not close in the early Carboniferous. Based on the geochemical characteristics of adjacent geological bodies and their settings, the Xiaohuangshan ophiolite is considered as an indicator of a suture zone between the different epicontinental belts in the Beishan region.

Back-arc and post-collisional volcanism in the Palaeoproterozoic Granites-Tanami Orogen, Australia

Pillow basalt, dolerite sills, and doleritic dykes are the major products of volcanism in the Granites-Tanami Orogen, and provide a window for understanding the magmatism and tectonic evolution of the orogen. The pillow basalt and dolerite sills from the Stubbins Formation have similar geochemical features, suggesting that they are genetically related. The pillow basalt and dolerite sills (ca. 1864Ma) have tholeiitic affinity, with high TiO2 contents (0.9–1.88wt.%), low Mg# (40–55), and positive ɛNd(t) values (+0.6 to +2.8). Enriched mid-ocean ridge basalt (E-MORB)-normalized REE and primitive mantle-normalized trace element patterns for these rocks show relatively flat REE distribution, slight enrichment of LILE (Rb, Th, U, and K) and weak negative anomalies of HFSE (Nb and P), which are the characteristics similar to those of the enriched back-arc basin basalt (E-BABB), but different from those of modern normal-MORB (N-MORB), E-MORB, oceanic island basalt (OIB), depleted back-arc basin basalt (D-BABB), and island arc basalt (IAB). The pillow basalt and dolerite sills are interpreted to be the results of a high-degree decompressional melting (5–15%) of the asthenosphere source in the spinel stability field. The mantle source of these rocks have primitive mantle signature with input of 3–4% subduction material. Convective flow operating during subduction, circulating the enriched material from wedge corner into the asthenospheric mantle beneath the Granites-Tanami back-arc basin and generated the mild enriched mafic magma that was emplaced as dolerite sills or erupted as basalt.“Late” (ca. 1800Ma) doleritic dykes in the region have high contents of MgO (7.15–14.0wt.%), Cr (262–757ppm), K2O (2.25–4.85, wt.%), with high Mg# (57–72), K2O/Na2O>2, and negative ɛNd(t) values range from −4.5 to 0.3. Their REE and trace element characteristics, such as enrichment of Rb, Ba, Th, U, K, LREE, and negative Nb–Ta anomalies, are similar to those of the upper (UCC) and middle continental crust (MCC), but more enriched than those of the lower continental crust (LCC). The interpreted continental–continental collision between the Granites-Tanami and Arunta orogens during the Tanami Orogeny (DGTO2; ca. 1800Ma) resulted in the thickening of the lower crust and lithospheric mantle root along the Willowra Lineament. The thickened lithospheric mantle root has higher density than the upper asthenospheric mantle leading to gravitational instability and resulting in convective removal. The detached denser lithospheric mantle root sank into asthenosphere triggered upwelling of the hot asthenospheric mantle material and extension. It is proposed here that melts derived from partial melting of the previously subduction-metasomatized lithospheric mantle with crustal assimilation generated the doleritic dykes, and modelling of geophysical data supports this interpretation. This post-collisional magmatism is approximately synchronously with the ca. 1800Ma gold mineralization and extensive granitic magmatism in the Granites-Tanami Orogen.

Petrogenesis of Early Paleozoic basalts and gabbros in the western Cuyania terrane: Constraints on the tectonic setting of the southwestern Gondwana margin (Sierra del Tigre, Andean Argentine Precordillera)

Early Paleozoic mafic igneous rocks from the Central Andean basement are located in N–S belts interpreted as suture zones among different terranes. In the western Cuyania terrane (Andean Argentine Precordillera), Sierra del Tigre mafic rocks are part of one of these belts and consist of clinopyroxene+plagioclase basalts and gabbros of tholeiitic–transitional geochemistry. These rocks were emplaced as volcanic–subvolcanic bodies during the Mid/Late Ordovician/Silurian. Distinctive geochemical features include moderate to slightly elevated light rare earth element (LREE) concentrations; low to moderate large ion lithophile element (LILE)/high field strength element (HFSE) ratios; absence of negative Nb-Ta anomalies; low Rb concentrations; negative anomalies of K (strong) and positive Ba anomalies. This geochemistry differs from that of normal-type mid-oceanic ridge basalt (N-MORB) or oceanic island basalt (OIB) and excludes the existence of subduction components. REE patterns and LILE/HFSE ratios are similar to those of enriched MORB (E-MORB) and some continental flood basalts. Geochemical modeling suggests that the parental magmas of these basalts and gabbros derived from mantle sources with compositions similar to that of the primordial mantle in the garnet-spinel transition zone. Further magmatic evolution consisted of olivine crystal fractionation and olivine+clinopyroxene+plagioclase equilibrium crystallization under low pressure, H2O, and oxidation conditions. Continental crust contamination may have occurred, but only in low proportions. Post-magmatic hydrothermal alteration consisted of slight to moderate metasomatism that added Ba, removed K±Rb, and generated the observed secondary mineral assemblage (mainly chlorite, epidote, and carbonates). The most probable tectonic setting for the generation and emplacement of this mafic suite is an evolved rifted margin (tholeiitic–transitional nature, E-MORB) within a thin continental crust that was unrelated to subduction-type margins. This model suggests that a western terrane (Chilenia) rifted partially from Cuyania during the Late Ordovician.

Enriched and depleted characters of the Amnay Ophiolite upper crustal section and the regionally heterogeneous nature of the South China Sea mantle

The volcanic section of the Middle Oligocene Amnay Ophiolite in Mindoro, Philippines has previously been shown to be of normalmid-oceanic ridge basalt (NMORB) composition. Here we report for the first time an enriched mantle component that is additionally recorded in this crustal section. New whole rock major and trace element data are presented for nine mafic volcanic rocks from a section of the ophiolite that has not been previously examined. These moderately evolved tholeiitic basalts were found to have resulted from the bulk mixing of ∼10% ocean island basalt components with depleted mantle. Drawing together various geochemical characteristics reported for different rock suites taken as representatives of the South China Sea crust, including the enriched MORB (EMORB) and NMORB of the East Taiwan Ophiolite, the NMORB from previous studies of the Amnay Ophiolite and the younger ocean floor eruptives of the Scarborough Seamount–Reed Bank region, a veined mantle model is proposed for the South China Sea mantle. The NMORB magmatic products are suggested to have been derived from the more depleted portions of the mantle whereas the ocean island basalt (OIB) and EMORB-type materials from the mixing of depleted and veined/enriched mantle regions.

Trace element and isotopic fingerprints in HP–LT metamorphic rocks as a result of fluid–rock interactions (Ile de Groix, France)

Whole rock trace element and isotopic compositions of different HP–LT metamorphic rocks of the Ile de Groix were analysed to characterise geochemical fingerprints during subduction and exhumation in a late Palaeozoic HP metamorphic terrain. Massive metabasites of hydrothermally altered enriched mid-ocean ridge basalt (E-MORB) origin are in association with banded metabasic rocks of volcano-sedimentary origin and metapelites. Fluid-rock interactions that likely occurred during seafloor hydrothermal alteration and early subduction metasomatism increased δ18O values, as well as K2O, Na2O, MgO, and LILE contents and decreased CaO contents of metabasites. Most metabasites have retained their early-subduction and pre-HP trace element and isotopic composition, even for rocks metamorphosed to lower eclogite-facies P–T conditions. Micaschists also preserved apparent pelitic protolith trace element values and oxygen isotopic compositions. During retrograde metamorphism related to the exhumation, metabasites were rehydrated by fluids in equilibrium with the host rock compositions, which were likely derived from the basic rocks. This style of fluid–rock interaction formed a greenschist facies mineral assemblage. Metabasites that underwent pervasive alteration by seafloor hydrothermal and metasomatism processes prior to peak metamorphism, show greater effects of retrogression and albitisation, probably because they were richer in H2O and Na2O. The variety of metamorphic assemblages on the Ile de Groix is thus directly related to the pre-HP rock composition. The extent of retrogression in the western part of the Ile de Groix primarily reflects stronger metasomatic intensities than in the eastern part.

Paleocene MORB and OIB from the Resolution Ridge, Tasman Sea

Altered lavas have been dredged from three locations on the Resolution Ridge, west of New Zealand's South Island. On the basis of whole-rock geochemistry, Sr, Nd and Pb isotope data and Ar–Ar ages, they can be divided into two suites: 62–60 Ma enriched mid-ocean ridge basalt (E-MORB), and 57 Ma trachybasalt and trachyandesite of ocean island basalt (OIB) affinity. The E-MORBs from the Resolution Ridge are only the second place from which Tasman Sea abyssal oceanic crust has ever been sampled, they have Indian MORB-like isotope compositions, and their ages support a recent interpretation of a 100 km sinistral offset of the southern part of the Tasman Sea spreading ridge. The slightly younger OIB suite erupted shortly after oceanic crust formation and has FOZO to HIMU source characteristics similar to the well-known SW Pacific Diffuse Alkaline Magmatic Province (DAMP). The close occurrence and isotopic mixing relationships of both Paleocene volcanic suites on the Resolution Ridge may be explained by a heterogeneous upper mantle in which the more fertile OIB component was extracted during a later melting event away from the spreading ridge. The dredged lavas predate formation of Southeast Tasman oceanic crust that borders the Resolution Ridge to the south.

Geochemistry of Mid-Ocean Ridge Mafic intrusives from the Manipur Ophiolitic Complex, Indo-Myanmar Orogenic Belt, NE India

Abstract Mafic intrusives emplaced within the mélange zone of the Manipur Ophiolitic Complex are subalkaline-tholeiitic affinity with Fe-enrichment. Based on the field occurrences, textures-mineralogy and whole-rock compositions, these mafic intrusives can be identified as type-I (gabbro intrusives) and type-II (basalt-dolerite dykes). The type-I resembling enriched-type mid-ocean ridge basalt (E-MORB) shows moderate LREE enrichment (LaN/SmN = 2.5-2.6), slightly enriched MORB normalized HFSE patterns possibly represent melts derived from enriched MORB sub-oceanic mantle sources by small degree of partial melting. The other type-II has normal-type mid-ocean ridge basalt (N-MORB) geochemical features, as it exhibits nearly flat to depleted LREE (LaN/SmN = 1.0-0.6), flat MORB normalized HFSE patterns with slight LREE/HREE depletion (CeN/YbN = 1.37-0.46). It might have been derived from depleted MORB type sub-oceanic mantle source. The MORB signature displayed by these mafic intrusives indicates that they are dismembered fragments of oceanic crust generated at mid-ocean spreading ridge system and support the hypothesis that the Manipur ophiolites was initially formed in the divergent plate margin.

OIB/seamount recycling as a possible process for E‐MORB genesis

This paper deals with the origin of enriched MORB independent from any hot spot activity. Indeed, MORB enrichment was readily attributed to a ridge/hot spot interaction and in absence of identified neighboring hot spot, to more questionable processes (e.g., incipient plume or plume activity residue). More recently, the existence of enriched MORB away from any identifiable hot spot was attributed to different origins (i.e., recycled oceanic crust and/or enriched mantle after subduction metasomatism). Within this frame, we present here a new set of geochemical analyses of major and trace elements and Sr, Nd and Pb isotopes on samples collected by submersible on both intersections of the 15°20′N fracture zone with the spreading axis of the Mid‐Atlantic Ridge. This area is characterized by an increasing enrichment of the lava compositions from north to south through the fracture zone. Results show that the geochemical enrichment observed with a different intensity on both sides of the fracture zone is linked to the 14°N topographic and geochemical anomaly. Our modeling shows that both trace element and isotopic compositions are consistent with a binary mixing between the regional depleted MORB mantle source and a recycled OIB/seamount, as previously proposed to explain the observed enrichment at 14°N. This model can also account for other enriched MORB i.e., the 18°–20°S region of the Central Indian Ridge, illustrating that it does not represent an isolated and local process. On the basis of our results and on the DMM isotopic evolution, the age of the recycled OIB/seamount is estimated to be ∼250 Ma, suggesting a recycling within the upper mantle. Considering the huge number of ocean islands and seamounts upon the ocean floor, their recycling into the upper mantle is a plausible process to produce enriched MORB.

Melting of a Two-component Source beneath Iceland

New trace element and Hf^Nd isotope data on post-glacial basalts from Iceland’s main rift zones are used in conjunction with literature data to evaluate the relative importance of source heterogeneity and the melting process for the final melt composition. Correlations between Hf and Nd isotope compositions and trace element ratios indicate that at least two source components are sampled systematically as a function of the degree and pressure of melting beneath Iceland. Strong depletion in Rb, Ba, U and Th and enrichment in Nb and Ta compared with La in the most enriched samples from the Reykjanes Peninsula and Western Rift Zone suggests that the enriched source component is similar to ancient recycled enriched mid-ocean ridge basalt (E-MORB) crust. Highly incompatible trace element ratios such as Nb/La and Nb/U and Pb isotope ratios are variable across Iceland. This observation suggests that either the enriched component is intrinsically heterogeneous, or that there is a larger proportion of the enriched source component beneath the Southwestern Rift Zone compared with the Northern Rift Zone. The relative effect of source heterogeneity and melting on the final melt composition was evaluated with a one-dimensional polybaric melt mixing model in which accumulated melts from a depleted MORB mantle and a recycled E-MORB crust are mixed in different ways. Two styles of melt mixing were simulated: (1) complete mixing of melts with variable proportions of the depleted mantle and recycled E-MORB components; (2) incomplete mixing with a fixed initial proportion of the two source components. Calculated pressure-dependent compositional changes using these simple two-component melting models can explain the observed trends in trace element ratio and isotope ratio diagrams for Icelandic basalts, even in cases where conventional binary mixing models would require more than two source components. The example of Iceland demonstrates that melt mixing during extraction from the mantle is a key process for controlling the trace element and isotope variability observed in basaltic lavas and must be evaluated before inferring the presence of multiple source components.