high-Ti Series Research Articles

Subduction and Oceanic Magmatism Records in Plutonic Rocks of the Kamchatsky Mys Ophiolite, Eastern Kamchatka

The paper presents petrographic, mineralogical, and geochemical data on dunites, pyroxenites, peridotites, and gabbroids of the Kamchatsky Mys ophiolite. These data were acquired to distinguish cogenetic assemblages of igneous rocks, gain an insight into their geodynamic settings, and test various criteria of genetic links between the different magmatic rocks of ophiolites. The ultramafic and mafic rocks are shown to belong to two series, which differ in the compositions of the primary minerals, bulk rocks, and estimated trapped melts. The rocks of these series are found out to have been produced by geochemically different melts in different geodynamic settings, and during different episodes of mantle magmatism. The rocks of the high-Ti series (gabbro of the Olenegorsk massif, dunite and melanogabbro xenoliths in them, and vein gabbro in these xenoliths) crystallized from N-MORB melts in an oceanic spreading center. The rocks of the low-Ti series (dunite, pyroxenite, and gabbro veins in the residual spinel peridotites of the Mount Soldatskaya massif, as well as pyroxenite, peridotite, and gabbro alluvium and diluvium in the central and western parts of the peninsula) crystallized from water-rich boninite melts in relation to initial subduction magmatism. Taken into account the absence of boninite lavas from the Kamchatsky Mys ophiolite, the plutonic ultramafic rocks (including the rocks of the veins) might be the only evidence of subduction boninitic magmatism in the ophiolites. It was demonstrated that conclusions about the geodynamic settings of plutonic ultramafic and mafic rocks and recognition of cogenetic relations of these rocks with spatially associated basalts are more reliable when derived from the compositions of the trapped melts, which are estimated from their bulk geochemistry and primary mineral compositions, than when they are based on the mineral compositions only.

Contrasting oxidation states of low-Ti and high-Ti magmas control Ni-Cu sulfide and Fe-Ti oxide mineralization in Emeishan Large Igneous Province

Magmatic Ni-Cu-(PGE) sulfide and Fe-Ti oxide deposits in plume-related large igneous provinces (LIPs) are commonly related to low-Ti and high-Ti series magmas, respectively, but the major factors that control such a relationship of metallogenic types and magma compositions are unclear. Magma fO2 controls sulfur status and relative timing of Fe-Ti oxide saturation in mafic magmas, which may help clarify this issue. Taking the Emeishan LIP as a case, we calculated the magma fO2 of the high-Ti and low-Ti picrites based on the olivine-spinel oxygen barometer, and the partitioning of V in olivine. The obtained fO2 of the high-Ti series magma (FMQ + 1.1 to FMQ + 2.6) is higher than that of the low-Ti series magma (FMQ − 0.5 to FMQ + 0.5). The magma fO2 of the high-Ti and low-Ti picrites containing Fo > 90 olivine reveals that the mantle source of the high-Ti series is likely more oxidized than that of the low-Ti series. The results using the ‘lambda REE’ approach show that the high-Ti series may have been derived from relatively oxidized mantle with garnet pyroxenite component. The S contents at sulfide saturation (SCSS) of the two series magmas were calculated based on liquid compositions obtained from the alphaMelts modeling, and the results show that the low-Ti series magma could easily attain the sulfide saturation as it has low fO2 with S being dominantly as S2−. In contrast, the oxidized high-Ti series magma is difficult to attain the sulfide saturation, but could crystallize Fe-Ti oxides at magma MgO content of ∼7.0 wt.%. Thus, contrasting magma fO2 of low-Ti and high-Ti series in plume-related LIPs may play an important role in producing two different styles of metallogeny.

Geochemistry and Geochronology of the Neoproterozoic Backarc Basin Khzama Ophiolite (Anti-Atlas Mountains, Morocco): Tectonomagmatic Implications

The Khzama ophiolite is a highly dismembered complex located in the Siroua inlier of the Moroccan Anti-Atlas Belt. It consists of ultramafic rocks, cumulate gabbros, sheeted dikes, pillow lavas, and an overlying volcano-sedimentary sequence. Three main tectonic slices of sheeted dike complexes are studied in detail along three rivers, exposing well preserved outcrops where individual dikes are clearly distinguishable from the intruded host rock (Assif n’Tinzla, Assif n’Tasriwine, and Assif n’Iriri). Sheeted dikes of the Khzama ophiolitic complex are basaltic to andesitic in composition, displaying a clear sub-alkaline nature. We identify two sets of dikes that originate from lower High-Ti series (HTS) lavas and overlying upper Low-Ti series (LTS) lava. The immobile trace-element signatures of these rocks point to a genesis on a backarc environment with magmas sourced in a supra-subduction zone (SSZ) at the spinel peridotite zone. The obtained SHRIMP U-Pb data of the gabbro represent the first radiometric age of zircon extracted from the mafic rocks that were intruded by the sheeted dike complex of the Khzama ophiolite. These grains yield a concordia age of 763 ± 5 Ma, which is consistent with the 761.1 + 1.9/−1.6 and 762 + 1/−2 Ma U-Pb zircon ages of plagiogranites of Siroua. Based on their mineralogy, modal proportions, and major element chemistry, the felsic dikes are classified as high silica–low alumina trondhjemites or plagiogranites. These plagiogranites were likely formed by the partial melting of mafic rocks rather than by extreme fractional crystallization. A plagiogranite dated at 777 ± 4.7 Ma (U-Pb on zircon) is significantly older than the ca. 762 Ma plagiogranites previously recorded for the Khzama locality, suggesting a long-lived supra-subduction zone (SSZ) with conditions for the hydrous melting of mafic rocks.

Elemental and Sr–Nd isotopic geochemistry of Permian Emeishan flood basalts in Zhaotong, Yunnan Province, SW China

This study presents new whole-rock elemental and isotopic data for the basalts from the Zhaotong area, located in the intermediate zone of the ~260 Ma Emeishan large igneous province (ELIP). The Zhaotong basalts belong to high-Ti series with TiO2 from 2.93 to 5.26 % and Ti/Y from 519 to 974. The parental magma was subjected to minor crustal contamination as indicated by slight Nb–Ta depletion (Nb/La: 0.72–1.10). Meanwhile, the relatively invariable Sr–Nd isotopes (eNd(t): −0.74 to +2.86, mostly +1.10 to +2.86; (87Sr/86Sr)i: 0.7050–0.7072) and the light rare earth elements (LREE) enrichment (La/Yb: 10.3–19.1) of the basalts prefer a mantle plume origin. A garnet-dominated peridotite mantle source was further suggested on the basis of the REE distribution patterns and high Sm/Yb and high La/Yb ratios. This study further confirms the geochemical zoning of the high-Ti basalts in the ELIP, which is in accordance with both the spatial distribution and the thickness of the basalts. The high-Ti basalts in the intermediate and outer zones of ELIP (e.g., Zhaotong and Guizhou) share similar Sr–Nd isotopic and elemental compositions, suggesting that they originated directly from the Emeishan mantle plume. By contrast, the high-Ti basalts in the inner zone (e.g., Longzhoushan and Binchuan) have variable compositions, indicating a rather heterogeneous mantle source possibly involved with subcontinental lithospheric mantle (SCLM) components.

Two magma series and associated ore deposit types in the Permian Emeishan large igneous province, SW China

The Late Middle Permian (∼ 260 Ma) Emeishan large igneous province in SW China contains two magmatic series, one comprising high-Ti basalts and Fe-rich gabbroic and syenitic intrusions, the other low-Ti basalts and mafic–ultramafic intrusions. The Fe-rich gabbros are spatially and temporally associated with syenites. Each series is associated with a distinctive type of mineralization, the first with giant Fe–Ti–V oxide ore deposits such as Panzhihua and Baima, the second with Ni–Cu–(PGE) sulfide deposits such as Jinbaoshan, Limahe and Zhubu. New SHRIMP zircon U–Pb isotopic data yielded 263 ± 3 Ma for the Limahe intrusion, 261 ± 2 Ma for the Zhubu intrusion and 262 ± 2 Ma for a syenitic intrusion. These new age dates, together with previously reported SHRIMP zircon U–Pb ages, suggest that all these intrusions are contemporaneous with the Emeishan flood basalts and formed during a major igneous event at ca. 260 Ma. The oxide-bearing intrusions have higher Al 2O 3, FeO (as total iron) and total alkalis (Na 2O + K 2O) but lower MgO than the sulfide-bearing intrusions. All intrusions are variably enriched in LREE relative to HREE. The oxide-bearing intrusions display positive Nb- and Ti-anomalies and in certain cases negative Zr–Hf anomalies, whereas the sulfide-bearing intrusions have obvious negative Nb- and Ti-anomalies, a feature of crustal contamination. Individual intrusions have relatively small ranges of ɛNd( t) values. All the intrusions, however, have ɛNd( t) values ranging from − 3.9 to + 4.6, and initial 87Sr/ 86Sr ratios from 0.7039 to 0.7105. The syenites have very low MgO (< 2 wt.%) but highly variable Fe 2O 3 (2.5 to 13 wt.%) with initial 87Sr/ 86Sr ratios ranging from 0.7039 to 0.7089. Magmas from both series could have derived by melting of a heterogeneous mantle plume: the high-Ti series from a Fe-rich, more fertile source and the low-Ti series from a Fe-poor, more refractory source. In addition, the low-Ti series underwent significant crustal contamination. The two magma series evolved along different paths that led to distinct mineralization styles.

Petrogenesis of greenstones from the Mino–Tamba belt, SW Japan: Evidence for an accreted Permian oceanic plateau

Permian greenstones in the Jurassic Mino–Tamba accretionary complex, southwest Japan, are divided into three distinct series on the basis of their geological occurrence, mineralogy, and geochemistry. A low-Ti series (LTS) is associated with Lower Permian chert and limestone, and is the most voluminous of the three series. The LTS shows slightly more enriched geochemical and isotopic characteristics than MORB. A transition series (TS) is mainly associated with Lower Permian chert, and has more enriched geochemical signatures than MORB. Its isotopic characteristics are divided into enriched and depleted types. A high-Ti series (HTS) occurs as sills and hyaloclastites within Middle Permian chert and as dikes intruding the TS. Some HTS rocks have high MgO contents. The HTS is characterized by enrichment in incompatible trace elements and an isotopic composition comparable to HIMU-type basalt. The geochemistry of the voluminous LTS is similar to that of the oceanic basalt series of the Kerguelen plateau, suggesting production by partial melting of a shallow mantle plume head below thick oceanic lithosphere in Early Permian time. We infer that the TS formed simultaneously at the margins of the mantle plume head. In contrast, the HTS may have resulted from partial melting of a deep mantle plume tail in Middle Permian time. Permian greenstones in the Mino–Tamba belt may have thus formed by superplume activity in an intra-oceanic setting. Given the presence of two known contemporary continental flood basalt provinces (Siberia and Emeishan) and some accreted oceanic plateau basalts, the vast magmatism of the Mino–Tamba oceanic plateau suggests a large-scale superplume pulse in Permian time. Accretion of oceanic plateaux may have played an important role in the growth of continental margins and island arcs in Japan and elsewhere in the circum-Pacific region.

Permian flood basalts and mafic intrusions in the Jinping (SW China)–Song Da (northern Vietnam) district: Mantle sources, crustal contamination and sulfide segregation

The ∼ 260-Ma Emeishan large igneous province (ELIP) in SW China and northern Vietnam includes high-Ti and low-Ti volcanic rocks and coeval mafic–ultramafic plutons. In the Jinping (SW China)–Song Da (northern Vietnam) district, the southernmost part of the ELIP, basalts and gabbros of the high-Ti series displays a narrow range of εNd( t) values (+ 0.5 to + 1.1) and low primitive mantle-normalized Th/Nb ratios (∼ 1.4), similar to those of an OIB-like enriched mantle source. The rocks have chondrite-normalized REE patterns enriched in LREE and depleted in HREE. This feature, together with low Al 2O 3/TiO 2 ratios (∼ 4), indicates that the primary melts were generated by small degrees of partial melting at garnet stable depths. The low-Ti series includes LREE-depleted and LREE-enriched picrites, basalts, olivine gabbros and gabbros. The LREE-depleted picrites have chondrite-normalized Ce/Yb ratios of 0.4–0.5, very positive εNd( t) values (+ 7.0), and very negative Th anomalies in primitive mantle-normalized trace element patterns. They were formed by high degrees of partial melting of a strongly depleted mantle source at relatively shallow mantle depths. The LREE-enriched picrites have very negative εNd( t) values (− 7.8) that indicate a mantle source locally modified by subducted oceanic crust. Highly variable chemical compositions and εNd( t) values (+ 6.4 to − 10.2) of the low-Ti basalts and gabbros are attributed to variable degrees of upper crustal contamination of the picritic parental magmas. Therefore, the extrusive and intrusive rocks in this region clearly formed from heterogeneous mantle sources. Crustal contamination triggered S-saturation of some low-Ti magmas and resulted in segregation of immiscible sulfide melts and generation of magmatic Ni–Cu–(PGE) sulfide deposits in this region. Eruption of magmas after sulfide segregation formed PGE-depleted low-Ti basalts.

Two melting regimes during Paleogene flood basalt generation in East Greenland: combined REE and PGE modelling

Previously published platinum group element (PGE) and rare earth element data (REE) from a sample suite of the Palaeogene flood basalts of the East Greenland rifted margin are used to approximate primary magma compositions by numerical models of mantle melting. Both high-Ti and low-Ti basalts are found intercalated in the coastal section “the Sortebre Profile” in central East Greenland, and the apparent lack of mixing between the two series indicates coexistence of two geographically separated melting regions and plumbing systems during continental breakup above the Palaeogene Iceland plume. The lavas show little or no sign of crustal contamination and the limited variation in La/Sm and Cu/Pd ratios can be interpreted to reflect mantle source composition and melting processes. Numerical modelling indicate that the low-Ti series formed by F~20% melting in a columnar melting regime from a slightly depleted upper mantle source with a relatively normal S-content (~180 ppm S). In contrast, the high-Ti series formed by much lower degrees of melting (F~6%) in a spreading-related, triangular melting regime from a relatively S-poor (~100 ppm S) source. The low-Ti suite was S-undersaturated at the stage of melt segregation from a shallow mantle source due to the high degree of melting. In contrast, the high-Ti suite probably formed from a S-poor source where some low degree melt batches were S-saturated at the stage of deep segregation in distal parts of the triangular melting regime. This suite shows a geochemical high pressure garnet-signature and adiabatic decompression could therefore have played a role in keeping the mantle-derived S in solution before Fe-enrichment related to fractional crystallisation also increased the S-capacity of these melts.

The behaviour of platinum-group elements in basalts from the East Greenland rifted margin

The continental flood basalts of the East Greenland volcanic rifted margin were extruded during continental breakup above the ancestral Iceland mantle plume at 55 Ma. Three distinct magma types, the low-Ti, high-Ti and very high-Ti series (LTS, HTS and VHTS respectively), are found intercalated in the ~6-km-thick Plateau Lava sequence. Incompatible trace elements indicate that the LTS are derived from a more depleted mantle source compared to HTS and VHTS. The LTS is characterised by increasing Cu (105 to 248 ppm) and Pd (7 to 24 ppb), constant Cu/Pd ratio (~10,000), and decreasing Ir (1.1 to <0.05 ppb) and Ru (1.8 to <0.3 ppb) concentrations during magmatic differentiation (16 to 7 wt% MgO). The constant Cu/Pd ratio reflects silicate- and chromite-dominated fractionation without concurrent segregation of sulphide. S-undersaturated differentiation is also indicated in the HTS, which also displays increasing Pd (6–16 ppb) and decreasing Ir concentrations (1 to <0.05 ppb) during differentiation, and the Cu/Pd ratios for the entire series average 21,000. However, some HTS samples have elevated Cu/Pd ratios (up to 33,000). Cu/Pd ratios in the HTS do not correlate with MgO, and this is interpreted to reflect varying Cu/Pd ratios of HTS parental magmas rather than S-saturated differentiation. During S-undersaturated differentiation of the LTS and HTS, Pt/Pd ratios decrease from 1.3 to 0.11 and 1.1 to 0.2 respectively, which indicates that Pd is much more incompatible than Pt during S-undersaturated differentiation. The VHTS consists exclusively of highly evolved samples with low MgO (6.6–6.1 wt%) and Pd/Ir ratios 98–228. Here, Cu/Pd ratios increase from 17,500 to 35,000 with decreasing Cr concentrations which indicate that these magmas experienced silicate fractionation with concurrent segregation of sulphide. The LTS represent melting of a depleted source and show high PGE concentrations and constant Cu/Pd ratios during S-undersaturated differentiation. Melting of a normal depleted upper mantle source generates S-saturated melts (MORB), and the depleted LTS source is therefore considered an extraordinary S-poor component within the ancestral Iceland plume. Of the three series, the VHTS contain the largest contribution from enriched mantle portions. The VHTS have similar PGE but much higher Nb concentrations for instance compared to the most evolved LTS and HTS samples, indicating that the enriched source contributes Nb but not PGE.

The Kraemer Island macrodyke, East Greenland: solidification of a flood basalt conduit

The Kraemer Island macrodyke that is exclusively exposed on Kraemer Island about 7 km west of the Skaergaard Intrusion belongs to a regional dyke swarm termed the ‘Skaergaard-like dykes’ (or FG-1 dykes). Weakly modally layered olivine gabbros dominate the exposed parts of the intrusion that has a width of 650 m to 1000 m. Plagioclase (core An68±2) and Ca-rich pyroxene (core Mg no. 79±1) grains are normally zoned, whereas olivine grains (Fo50–65) are homogeneous. Calculated mineral–magma equilibria, based on experimentally determined Mg–Fe magma–olivine and magma–clinopyroxene partition coefficients, suggest that the observed olivine and clinopyroxene compositions in the gabbros cannot have formed from a common parental magma. The unzoned nature of olivine grains and their iron-rich com-positions relative to clinopyroxene suggest post-cumulus Mg–Fe exchange between olivine and interstitial melt. A gabbroic pegmatite is developed in the centre of the intrusion along its entire exposed 5 km strike length. Here, mineral zonation is limited and compositions are similar to rims of cumulus minerals in the enveloping olivine gabbros. The pegmatite could therefore represent interstitial melt mobilized from gabbroic cumulates that later accumulated and crystallized at its present stratigraphic location. Cumulus olivines in the gabbros are close to equilibrium with Ca-rich pyroxene in the pegmatite. This is interpreted as reflecting interstitial melt mobility during the late stages of solidification of the macrodyke. Chilled margins are well preserved at the intrusion margins and are rich in Fe (14–15.3% FeOTOT), Ti (3–3.3% TiO2) and light rare-earth elements ([La/Sm]N = 1.2–1.3), similar to magmas well represented in the overlying sequence of contemporaneous High-Ti Series flood basalts. It is therefore likely that extensive wall-rock melting adjacent to the macrodyke reflects continuous feeding of the overlying flood basalts through the Kraemer Island macrodyke.

Geochemistry and intrusive directions in sheeted dikes in the Troodos ophiolite: Implications for mid‐ocean ridge spreading centers

Sheeted dikes at mid‐ocean ridge volcanoes represent the link between deep magma production and storage processes and shallow processes such as volcanism and hydrothermal activity. As such, they are crucial for the interpretation of many observations at mid‐ocean ridges or other volcanoes with pronounced rift zones, including topography, hydrothermal systems, petrology, and geochemistry. We carried out a structural, magnetic, and chemical investigation of a 4 × 10 km sheeted dike section in the Troodos ophiolite, Cyprus. On the basis of major and trace element geochemistry, we distinguish dikes that may be correlated with the basal high‐Ti series (HTS) lavas from those of the overlying low‐Ti series (LTS) lavas. All dikes studied are nearly parallel to each other, with vertical or steeply dipping planes whose strike likely indicates the orientation of the spreading center. Anisotropy of magnetic susceptibility measurements suggests that the HTS and LTS dikes intrude in fundamentally different ways. HTS dikes reflect the intrusive behavior of dikes in the vicinity of a magma supply system and define ridge parallel intrusive sheets that radiate out from the magma chamber. LTS dikes show a bimodal, orthogonal set of intrusive directions, one shallow and one near vertical. Near‐lateral propagating dikes provide a means for delivery of magma into distant portions of a rift system, and near‐vertical dike propagation directions are probably associated with feeder dikes to down‐rift surface flows. Our study suggests that the types of dike intrusive behavior in the Troodos ophiolite may also be typical for “normal” mid‐ocean ridges or other major shield volcanoes with well‐developed rift zones.

Petrological and Chemical Evolution of the Northeastern Troodos Extrusive Series, Cyprus

Four magma series are distinguished in the northeastern Troodos Extrusive Series: (A) a Lower Low-Ti Series (Lo-LTS) of basaltic andesites, (B) a High-Ti Series (HTS) of basaltic andesites to rhyodacites, (C) a Low-Ti Series(DLTS), the last two being of basaltic andesite. Trace-element characteristics vary systematically from Series A to D and are interpreted in terms of a variable contribution of three major source components (SCs). LILE-enriched water-rich fluids (SCI) derived from dehydration of a subducted lithosphere slab were continuously added to the overlying mantle wedge. Increasing LILE/HFSE and LILE/REE ratios and decreasing absolute HFSE and REE concentrations from Series A to Dindicate progressive depletion of the actual mantle source (SCII). A negative Ta anomaly in the lavas decreases from Series A to D and is interpreted to have resulted from partial melting of the lower crust (SCII) where Ta-Nb-Ti may be fractionated by Ti-rich accessory phases. The contribution of SCIII decreases when the eruptive sites successively move away from the central axial zone and the temperature of the lower crust decreases, preventing partial melting of the lower crust. Chemical compositions of fresh glass separates and phenocrysts indicate a change of major petrogenetic processes from series A to D. Lo-LTS and IITS lavas are intrepreted to be directly related by open-system fractional crystallization in crustal magma chambers. Removal of observed phenocryst phases clinopyroxene, orthopyroxene, plagioclase, and magnetite, and repeated subsequent mixing of dacitc to rhyodacitic magmas with batches of replenshing basaltic andesites are the major processes, possibly induced by vesicle formation in the mafic layer after a period of some crystallization. LTS and DLTS magmas were directly fed to the surface without stagnating at crustal levels, with feeder dykes positioned marginal to the central rift zone and thus by-passing the central magma chambers. These magmas apparently experienced only limited fractional crystallization of 10–15 wt.% olivine+clinopyroxene+chromite, probably at the mantle-crust boundary.