Gabbro Unit Research Articles

Constraints on the Tectonic Setting of the Andaman Ophiolites, Bay of Bengal, India, from SHRIMP U-Pb Zircon Geochronology of Plagiogranite

The Andaman ophiolites are well exposed in the Andaman group of islands, which is part of the Sunda-Burmese double-chain arc system in the Bay of Bengal, India. Plagiogranites occurring on the eastern margin of the southern part of South Andaman Island appear as interstitial vermicular and micrographic intergrowths of quartz and plagioclase. They are tonalitic to trondhjemitic in composition, and their Rb, Yb, Ta, and Y abundances are characteristic of a volcanic-arc affinity. Sensitive high-resolution ion microprobe U-Pb dating of zircons from a plagiogranite within the Andaman ophiolite has yielded a weighted mean 206Pb/238U age of Ma, interpreted as the age of its crystallization. The subduction-related plagiogranite has intruded a gabbro unit of the Andaman ophiolites as well as extrusives of the East Coast Volcanics at this time. Since the Andaman ophiolitic rocks predate the plagiogranite, they cannot have been generated in the currently active Late Miocene Andaman-Java subduction zone and were most likely obducted onto the leading edge of the Eurasian continent at an earlier phase of subduction activity during early Cretaceous time.

Compositional variations in the Mesoarchean chromites of the Nuggihalli schist belt, Western Dharwar Craton (India): potential parental melts and implications for tectonic setting

The chromite deposits in the Archean Nuggihalli schist belt are part of a layered ultramafic–mafic sequence within the Western Dharwar Craton of the Indian shield. The 3.1-Ga ultramafic–mafic units occur as sill-like intrusions within the volcano-sedimentary sequences of the Nuggihalli greenstone belt that are surrounded by the tonalite–trondhjemite–granodiorite (TTG) suite of rocks. The entire succession is exposed in the Tagdur mining district. The succession has been divided into the lower and the upper ultramafic units, separated by a middle gabbro unit. The ultramafic units comprise of deformed massive chromitite bodies that are hosted within chromite-bearing serpentinites. The chromitite bodies occur in the form of pods and elongated lenses (~60–500 m by ~15 m). Detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate-rich chromitite (~50% modal chromite) and serpentinite (~2% modal chromite) throughout the entire ultramafic sequence. However, the primary composition of chromite is preserved in the massive chromitites (~60–75% modal chromite) from the Byrapur and the Bhaktarhalli mining district of the Nuggihalli schist belt. These are characterized by high Cr-ratios (Cr/(Cr + Al) = 0.78–0.86) and moderate Mg-ratios (Mg/(Mg + Fe2+) = 0.38–0.58). The compositional variability occurs due to sub-solidus re-equilibration in the accessory chromite in the serpentinite (Mg-ratio = 0.01–0.38; Cr-ratio = 0.02–0.99) and in silicate-rich chromitite (Mg-ratio = 0.06–0.48; Cr-ratio = 0.60–0.99). In the massive chromitites, the sub-solidus re-equilibration for chromite is less or absent. However, the re-equilibration is prominent in the co-existing interstitial and included olivine (Fo96–98) and pyroxene grains (Mg-numbers = 97–99). Compositional variability on the scale of a single chromite grain occurs in the form of zoning, and it is common in the accessory chromite grains in serpentinite and in the altered grains in chromitite. In the zoned grains, the composition of the core is modified and the rim is ferritchromit. In general, ferritchromit occurs as irregular patches along the grain boundaries and fractures of the zoned grains. In this case, ferritchromit formation is not very extensive. This indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization. In some occurrences, the ferritchromit rim is very well developed, and only a small relict core appears to remain in the chromite grain. However, complete alteration of the chromite grains to ferritchromit without any remnant core is also present. The regular, well-developed and continuous occurrence of ferritchromit rims around the chromite grain boundaries, the complete alteration of the chromite grains and the modification of the core composition indicate the alteration in the Nuggihalli schist belt to be intense, pervasive and affected by later low-grade metamorphism. The primary composition of chromite has been used to compute the nature of the parental melt. The parental melt calculations indicate derivation from a high-Mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the Western Dharwar Craton. Tectonic discrimination diagrams using the primary composition of chromites indicate a supra-subduction zone setting (SSZ) for the Archean chromitites of Nuggihalli and derivation from a boninitic magma. The composition of the komatiitic basalts resembles those of boninites that occur in subduction zones and back-arc rift settings. Formation of the massive chromitites in Nuggihalli may be due to magma mixing process involving hydrous high-Mg magmas or may be related to intrusions of chromite crystal laden magma; however, there is little scope to test these models because the host rocks are highly altered, serpentinized and deformed. The present configurations of the chromitite bodies are related to the multistage deformation processes that are common in Archean greenstone belts.

PETROLOGY OF MANTLE PERIDOTITES AND INTRUSIVE MAFIC ROCKS FROM THE KERMANSHAH OPHIOLITIC COMPLEX (ZAGROS BELT, IRAN): IMPLICATIONS FOR THE GEODYNAMIC EVOLUTION OF THE NEO-TETHYAN OCEANIC BRANCH BETWEEN ARABIA AND IRAN

The Kermanshah ophiolitic complex consists of a melange formation, which includes dismembered ophiolitic sequences. These ophiolites are located along the Main Zagros Reverse Fault, which marks the ophiolitic suture zone between the Zagros belt and the Sanandaj-Sirjan zone. They represent the Neo- Tethyan oceanic lithosphere, which originally existed between the Arabian (to the south) and Eurasian (to the north) continental margins. The Kermanshah ophiolites were emplaced onto platform carbonate rocks, which represented the northeastern Arabian margin. The Kermanshah ophiolitic complex is composed of various partial sequences, which are represented by: (1) mantle tectonites consisting of depleted lherzolites and both clinopyroxene- (cpx-) rich and cpx-free harzburgites; (2) a troctolite-cumulate gabbro-isotropic gabbro sequence mainly showing pegmatoid texture; (3) a wehrlite-cumulate gabbro-isotropic gabbro sequence showing foliated texture; (4) a dyke complex; (5) very scarce pillow basalts. Mantle tectonites are volumetrically predominant and tectonically overlay the gabbroic sequences. A number of conclusions may be drawn based on petrographic observations, mineral chemistry, whole-rock chemistry, and rare earth elements (REE) modelling carried out on both mantle tectonites and intrusive rock associations. (1) The Foliated Gabbro Unit has an N-MORB chemical signature and represents a portion of oceanic crust generated in a mid-ocean ridge setting from an N-MORB-type sub-oceanic mantle. (2) The Pegmatoid Gabbro Unit displays an E-MORB signature and represents a portion of oceanic crust most likely generated from a sub-oceanic mantle source enriched in light REE (LREE). A comparison with the well-studied Oman ophiolites suggests that this sequence may have formed during the early stage of oceanic spreading. (3) The depleted lherzolites present mild depletions in heavy REE (HREE) and variable depletion in LREE. REE modelling shows that they may represent a residual mantle after 15-20% removal of N-MORB melts. Some lherzolites show a moderate enrichment in La and Ce with respect to Sm, suggesting that this residual MORB mantle was subsequently trapped in a supra-subduction zone (SSZ) mantle wedge and enriched in LREE by subduction-derived fluids. (4) The depleted harzburgites present a significant depletion in incompatible elements and REE, coupled with a marked LREE enrichment with respect to medium REE. REE modelling shows that they may represent a residual mantle after 25-30% removal of boninitic-type melts in an intra-oceanic arc setting.

Enrichment of PGE through interaction of evolved boninitic magmas with early formed cumulates in a gabbro–breccia zone of the Mesoarchean Nuasahi massif (eastern India)

The Mesoarchean Nuasahi chromite deposits of the Singhbhum Craton in eastern India consist of a lower chromite-bearing ultramafic unit and an upper magnetite-bearing gabbroic unit. The ultramafic unit is a ∼5 km long and ∼400 m wide linear belt trending NNW-SSE with a general north-easterly dip. The chromitite ore bodies are hosted in the dunite that is flanked by the orthopyroxenite. The rocks of the ultramafic unit including the chromitite crystallized from a primitive boninitic magma, whereas the gabbro unit formed from an evolved boninitic magma. A shear zone (10–75 m wide) is present at the upper contact of the ultramafic unit. This shear zone consists of a breccia comprising millimeter- to meter-sized fragments of chromitite and serpentinized rocks of the ultramafic unit enclosed in a pegmatitic and hybridized gabbroic matrix. The shear zone was formed late synkinematically with respect to the main gabbroic intrusion and intruded by a hydrous mafic magma comagmatic with the evolved boninitic magma that formed the gabbro unit. Both sulfide-free and sulfide-bearing zones with platinum group element (PGE) enrichment are present in the breccia zone. The PGE mineralogy in sulfide-rich assemblages is dominated by minerals containing Pd, Pt, Sb, Bi, Te, S, and/or As. Samples from the gabbro unit and the breccia zone have total PGE concentrations ranging from 3 to 116 ppb and 258 to 24,100 ppb, respectively. The sulfide-rich assemblages of the breccia zone are Pd-rich and have Pd/Ir ratios of 13–1,750 and Pd/Pt ratios of 1–73. The PGE-enriched sulfide-bearing assemblages of the breccia zone are characterized by (1) extensive development of secondary hydrous minerals in the altered parts of fragments and in the matrix of the breccia, (2) coarsening of grain size in the altered parts of the chromitite fragments, and (3) extensive alteration of primary chromite to more Fe-rich chromite with inclusions of chlorite, rutile, ilmenite, magnetite, chalcopyrite, and PGE-bearing chalcogenides. Unaltered parts of the massive chromitite fragments from the breccia zone show PGE ratios (Pd/Ir = 2.5) similar to massive chromitite (Pd/Ir = 0.4–6.6) of the ultramafic unit. The Ir-group PGE (IPGE: Ir, Os, Ru) of the sulfide-rich breccia assemblages were contributed from the ultramafic–chromitite breccia. Samples of the gabbro unit have fractionated primitive mantle-normalized patterns, IPGE depletion (Pd/Ir = 24–1,227) and Ni-depletion due to early removal of olivine and chromite from the primitive boninitic magma that formed the ultramafic unit. Samples of the gabbro and the breccia zone have negative Nb, Th, Zr, and Hf anomalies, indicating derivation from a depleted mantle source. The Cu/Pd ratios of the PGE-mineralized samples of the breccia zone (2.0 × 103–3.2 × 103) are lower than mantle (6.2 × 103) suggesting that the parental boninitic magma (Archean high-Mg lava: Cu/Pd ratio ∼1.3 × 103; komatiite: Cu/Pd ratio ∼8 × 103) was sulfur-undersaturated. Samples of the ultramafic unit, gabbro and the mineralized breccia zone, have a narrow range of incompatible trace element ratios indicating a cogenetic relationship. The ultramafic rocks and the gabbros have relatively constant subchondritic Nb/Ta ratios (ultramafic rocks: Nb/Ta = 4.1–8.8; gabbro unit: Nb/Ta = 11.5–13.2), whereas samples of the breccia zone are characterized by highly variable Nb/Ta ratios (Nb/Ta = 2.5–16.6) and show evidence of metasomatism. The enrichment of light rare earth element and mobile incompatible elements in the mineralized samples provides supporting evidence for metasomatism. The interaction of the ultramafic fragments with the evolved fluid-rich mafic magma was key to the formation of the PGE mineralization in the Nuasahi massif.

Subsidence in magma chamber and the development of magmatic foliation in Oman ophiolite gabbros

In the Oman ophiolite, the horizon where the melt lens pinched during drifting away from the ocean ridge axis has been identified. Starting in the Root Zone of the Sheeted Dike Complex (RZSDC) located above this horizon, 18 sections down to the upper gabbros unit have been mapped in great detail, in selected areas of the southern massifs of this ophiolite. They are complemented by 133 sites, located throughout the entire ophiolite, where the transition from the RZSDC to the uppermost foliated gabbros is well exposed. Altogether, half the sites and 11 cross sections display, within a few tens of meters beneath the RZSDC, a magmatic foliation which is parallel to the overlying sheeted dikes. In the other sites and cross sections, the gabbro foliation is either flat-lying or steep but not parallel to the sheeted dikes. Compared to the RZSDC isotropic ophitic gabbros where clinopyroxene is interstitial between plagioclase laths, in the topmost steeply foliated gabbros, clinopyroxene is idiomorphic, becoming rapidly granular and tabular down section by recrystallization and peripheral alteration to hornblende. Moving down from these uppermost gabbros and over one hundred meters, the steep foliation becomes stronger and the poorly recovered top gabbros grade into the recrystallized, granular gabbros of the gabbro unit. These repeated observations indicate to ascribe these gabbros to subsidence of a compacting mush from the floor of the melt lens into the underlying, main magma chamber. The topmost gabbros beneath RZSDC, which were expelled from the melt lens by drifting very soon after settling on the melt lens floor, display in plagioclase a spectacular zoning pointing to a fast cooling. Moving downwards, stronger foliation, increased compaction and recovery–recrystallization are explained by the time spent by the subsiding mush inside the magma chamber increasing by one order of magnitude (~ 50 to 500 yr) over this vertical distance. This field-based study brings compelling field evidence supporting the former models of subsidence which were based on the assumption that the mush that settled onto the floor of the melt lens is sucked downwards during drifting of the crust away from the ridge axis.

The role of Fe–Ti oxide crystallization in the formation of A-type granitoids with implications for the Daly gap: An example from the Permian Baima igneous complex, SW China

The Baima igneous complex (BIC) is a member of the Permian Emeishan large igneous province, SW China, and is composed of a layered gabbroic unit and an isotropic syenitic unit. The gabbroic unit consists of four distinct zones, 1) lower cumulate zone, 2) massive Fe–Ti–V oxide ore zone, 3) olivine gabbro zone and 4) upper gabbro zone. SHRIMP zircon U–Pb dating results yield an age of 261 ± 2 Ma for the gabbroic unit, contemporaneous with the spatially associated syenitic unit. The gabbroic rocks show slightly LREE enriched patterns with (La/Yb) N values between 1.6 and 12.1 and positive Eu anomalies (Eu/Eu⁎ = 1.1–3.6). In contrast, the syenites display stronger enrichments of LREE (La/Yb N = 6.8–51.3) and negative Eu anomalies (Eu/Eu⁎ = 0.3–1.1). Rocks of the syenitic unit are chemically similar to the ferroan alkalic A-type granitoids (K 2O/Na 2O = 0.58–0.80, FeOt/(FeOt +MgO) = 0.80–0.91) and have a within-plate geochemical signature. Both units have similar trace element ratios (Zr/Nb, Th/U, Ni/Co, Cu/Ni, Hf/Ta and Zn/Ga) resembling an ocean–island basalt source. The εNd (T) values for the gabbroic unit (εNd (T) = + 1.6–+ 4.2) and syenitic unit (εNd (T) = + 2.5–+ 3.2) are within the range of the high-Ti Emeishan flood basalts. The geochemical and geochronological data indicate that the gabbroic and syenitic units are comagmatic and cogenetic. Geochemical modeling suggests that the two units originated by fractional crystallization of a common parental magma that resembles the high-Ti Emeishan flood basalts. The formation of the two units by fractional crystallization was likely responsible for the enrichment of Fe–Ti–V oxide minerals in the gabbroic unit and thus the development of the Daly gap. The results of this study indicate that silica saturated peralkaline A-type granitoids can be formed by fractionation of mantle derived mafic magmas.

Hydration due to high-T brittle failure within in situ oceanic crust, 30°N Mid-Atlantic Ridge

Analysis of an in situ fault zone within the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge) provides clues to the relevant deformation mechanisms and their temporal evolution within oceanic crust. IODP EXP304/305 drilled a succession of gabbroic lithologies to a final depth of 1415 m below the sea floor (mbsf), with very high recovery rates of up to 100% (generally ∼ 80%). We identified an intra-crustal fault zone between 720 and 780 mbsf in a section of massive gabbro, olivine gabbro, oxide gabbro units, and minor diabase intrusions. Of particular interest is the section between 744 and 750 mbsf, which unfortunately was marked by low recovery rates (17%). Electrical borehole-wall images show a 1-m-thick zone of east-dipping fractures within this interval, which is otherwise dominated by N–S dipping structures. Despite the high fracture density in this section, the hole walls are smooth, with rare breakouts, suggesting that the low recovery rate was due to a change in lithology rather than well conditions. The recovered rocks include ultracataclasite and possibly incohesive fault gouge that formed in the upper amphibolite regime, with mostly amphibole infill. Logging data suggest that the gabbroic rocks in this interval are rich in hydrous phases, consistent with increased amounts of amphibole found in the core. Equilibration temperature conditions of about 640 °C were obtained for plagioclase clasts and aluminous actinolite, assuming a pressure of 200 MPa. The permeability of the fault zone is in the range of 10 − 19 to 10 − 17 m 2. Although the permeability appears to be high within the fault zone relative to other parts of the section, it is no higher than that in typical lower crustal material. As a consequence, because brittle failure occurred at high temperatures, the fault zone was subsequently completely sealed by hydrous minerals, thereby preventing further fluid circulation and preserving water in the crust.

Stacked gabbro units and intervening mantle: A detailed look at a section of IODP Leg 305, Hole U1309D

Hole U1309D (Integrated Ocean Drilling Program (IODP) Legs 304/305) penetrated 1415 m into the seafloor of the Atlantis Massif, an oceanic core complex at 30°N, Mid‐Atlantic Ridge. More than 96% of all recovered rocks are gabbroic. On the basis of a mineral chemical overview, we suggest that between ≤800 and 1100 m below sea floor (mbsf), a magmatic unit occurs, ranging from olivine gabbro and troctolite in the lower part to gabbronorite and oxide gabbro in the upper part. Below 1235 mbsf, massive gabbronorites/oxide gabbros were drilled and they may represent the roof of an underlying magmatic unit. The focus here is on the zone where both units interact and screens, totaling 50 m, of a microstructurally distinct, olivine‐rich troctolite occur. We argue that the olivine‐rich troctolite is a former mantle rock which was converted to a crust‐mantle transition zone dunite at the base of the upper magmatic unit. Later, as melts derived from the lower magmatic unit percolated through it, it was equilibrated to a more evolved chemistry and transformed to a fine‐grained, olivine‐rich troctolite. Our main arguments against a possible cumulate nature of the olivine‐rich troctolite are the lack of a systematic downhole trend in compatible elements within the olivine‐rich troctolite, its distinctly fine‐grained microstructure, the high Cr content of cpx, and its Ni‐rich olivine composition. The high NiO for a given Mg/(Mg + Fe) in the olivine‐rich troctolite can be modeled by simple equilibration of relict mantle olivine with a mildly evolved melt. Evidence for the percolation of evolved melts through the olivine‐rich troctolites are Ti‐rich, interstitial pyroxenes and, as inclusions in Cr‐spinel, highly evolved amphiboles and orthopyroxenes plus the occurrence of millimeter‐scale noritic veins. The percolation by evolved melts would also be the major difference to otherwise conceptually similar rocks from the ophiolitic crust‐mantle transition zone.

Geochemistry and geothermobarometry of the Um Eleiga Neoproterozoic island arc intrusive complex, SE Egypt: genesis of a potential gold-hosting intrusion

Gabbro-diorite-tonalite±granodiorite complexes, derived from subduction-related, calc alkaline magmas represent a distinct unit in the Precambrian basement of Egypt. These intrusive complexes are commonly intruded into ophiolitic mélange, schistose or gneissic rock assemblages. Well-constrained geological mapping of the Um Eleiga complex, combined with geochemical studies, should enable a better understanding of its genesis, especially its potential as a host for gold mineralisation. This complex represents a composite pluton intruded into the allochthonous, ophiolitic Abu Dahr massif and its associated undifferentiated metavolcano-sedimentary mélange matrix in the southern Eastern Desert of Egypt. Field and petrographic investigations have revealed a compositional continuum from gabbro to granodiorite through diorite and subordinate tonalite. Petrography and bulk rock geochemical data have been used to determine the magma type and tectonic setting of the Um Eleiga complex, whereas the electron microprobe data were valuable for the purpose of determining the pressure–temperature and oxygen fugacity conditions. The compositional continuum, pattern of zonation, shallow magma origin and marked common negative Nb anomalies suggest that a close petrogenetic relationship between the gabbro, diorite, tonalite and granodiorite units of the Um Eleiga complex. Subducted upper mantle wedge is proposed as being the source region for the gabbroic magma, whereas zoning of the complex is attributed to interplay of fractional crystallisation and assimilation. Pressure estimates based on the Al(iv)-in-hornblende geobarometry indicate that the gabbroic and dioritic rocks crystallised under 4–5 kbar, whereas tonalite and granodiorite were formed at 3–4 kbar. The hornblende-plagioclase thermometer indicates a formation temperature range of about 720 to 880°C for the complex. Varied oxidation states are inferred from the mineral assemblages, including magnetite, titanite and ilmenite and checked by Fe/(Fe+Mg) ratios of biotite in the granitoid rocks. The tectonic setting, mineralogical, geochemical characteristics and varied redox state of the Um Eleiga intrusive complex are typical for arc-related intrusives potential for hosting intrusion-related gold in a collisional environment.

A Bushveld-related high-Ti igneous suite (HITIS) derived from an alkali to transitional basaltic magma, South Africa

A suite of high-Ti mafic to felsic igneous rock (acronym: HITIS) was emplaced at ~2055 Ma as km-sized bodies to the south of the Bushveld Complex, South Africa, stretching from Parys and Potchefstroom in the south-west to Marble Hall in the north and Badplaas in the east. Currently known members of the suite include the Marble Hall sills and related intrusives and breccia, the Basal Gabbro Unit of the Uitkomst Complex, the Lindeques Drift and Heidelberg intrusions, the volcanic Roodekraal Complex, the Rietfontein Complex, the Koedoesfontein Complex, the Schoongezicht (previously Kaffirskraal) Complex, the Vredefort alkali granite, as well as the high-Ti basalt and FeTiP-enriched basaltic lava of the Rooiberg Group. The volcanic rocks extruded on mildly folded and denuded metasedimentary rocks and lava of the Pretoria Group, whereas the plutonic rocks intruded at a level between the upper Witwatersrand Supergroup and the lower Pretoria Group, but largely within the dolomite of the Chuniespoort Group. The mafic rocks in this suite are dominated by clinopyroxene (salite to augite), FeTi-oxide (magnetite-ilmenite), amphibole (largely edinitic and hastingsitic), olivine (Fo 44 to Fo 80 ), orthopyroxene and plagioclase (An ), forming an array of dioritic and subordinate gabbroic rocks with associated clinopyroxene±magnetite(-ilmenite)±olivine±plagioclase cumulates. Locally, hybrid calc-silicate rocks derived from reaction of the magma with dolomite of the Chuniespoort Group are developed. The felsic rocks are syenodiorite and alkali granite with K-feldspar, albite, alkali pyriboles and biotite. Trace element systematics suggests that the HITIS rocks are derived from an alkali to transitional basaltic parent. The most primitive rocks representing liquid fractions are developed in the chill zone of the Basal Gabbro Unit of the Uitkomst Complex (Mg# 0.55 to 0.68) and in the Marble Hall diorite sills (Mg# = 0.53 to 0.65). These are followed by the more evolved high-Ti basalt (Mg# = 0.45 to 0.50) of the Rooiberg Group, the mugearite lavas (Mg# = 0.29 to 0.47) of the volcanic Roodekraal Complex, the FeTiP lava (Mg# = 0.17 to 0.30) of the Rooiberg Group, and finally the Vredefort alkali granite (Mg# = 0.26 to 0.72). This rock series resulted from deep-seated (lower crustal?) amphibole fractionation, followed by shallow level clinopyroxene±magnetite(-ilmenite)±olivine±plagioclase fractionation with the FeTi-oxide phases becoming more dominant in the later stages. The Vredefort alkali granite represents the final liquid in the liquid line of descent. The HITIS magma marks an early stage in the Bushveld magmatic event and has a close temporal relationship with the boninitic B1-type magma, regarded as parental to the Lower Zone of the Bushveld Complex. It also appears to have been derived from the same mantle source that yielded the tholeiitic B3-type magma, which is in part responsible for the Main Zone of the Bushveld Complex. Minor copper and PGE concentrations are associated with cumulate rocks of the HITIS.

Root zone of the sheeted dike complex in the Oman ophiolite

In the Oman ophiolite crustal section, a contact zone between the gabbro unit and the volcanics and diabase sheeted dikes, called the root zone of the sheeted dike complex, has been recently mapped at a fine scale in a selected area. The Oman ophiolite is derived from a fast spreading ridge which had a melt lens located between the main gabbro unit and the root zone of the sheeted dike complex. With a few exceptions accounted for, this horizon has a fairly constant thickness, ∼100 m, and a crude internal pseudo‐stratigraphy. At the base of the root zone are isotropic ophitic gabbros interpreted as a thermal boundary layer. This layer is transitional between the magmatic system of the melt lens, convecting at 1200°C, and a high‐temperature (<1100°C) hydrothermal system, convecting within the root zone. Above this level, the isotropic gabbros have been, locally, largely molten due to an influx of seawater, at ∼1100°C, thus generating varitextured ophitic and pegmatitic gabbros. These latter gabbros constitute the upper part of the root zone and are associated with trondjhemitic intrusions as screens in the lower sheeted dikes. Diorites and trondjhemites were also generated by hydrous melting, at temperatures below 1000°C. The whole root zone is a domain of very sharp average thermal gradient (∼7°C/m). At the top of the root zone, a new thermal boundary layer, with diabase dikes hydrated in amphibolite facies conditions, separates the preceding high‐temperature convective system from the well‐known greenschist facies (<450°C) hydrothermal system operating throughout the sheeted dike complex, up to the seafloor. The isotropic gabbros near the base of the root zone are intruded by protodikes with distinctive microgranular margins and an ophitic center. Protodike swarms are exceptional because, intruding a medium at ∼1100°C, they are largely destroyed by dike‐in‐dike intrusions and by hydrous melting. However, they demonstrate that this zone was generated by melt conduits issued from the underlying melt lens. Each dike of the sheeted dike complex is thus fed by one protodike. As this zone has been recently drilled by IODP in the eastern Pacific Ocean, a brief comparison is proposed.

Geology and Lithostratigraphy of the Ultramafic-Mafic Rocks and Associated Mineralizations, Wadi Khamal Area, West-Central Arabian Shield

Field, petrography and major elements geochemistry of the ultramafic -mafic rocks of Wadi Khamal area led to the recognition of the following rock groups, namely: 1) ophiolitic suite, melagabbro- norite and cumulate gabbronorite to the south of Wadi Mahalah (southern ultramafic-mafic rocks) and 2) Wadi Khamal anorthosite- gabbro complex to the north. The ophiolitic rocks in the southern area include serpentinites, peridotites and pyroxenites. The serpentinites contain large blocks of amphibolites, schists, gneisses, and metagabbros. These rocks were in- truded by melagabbronorite and cumulate gabbronorite. On the AFM diagram, these rocks plot along the MgO-FeO* side in the tholeiitic field except those of the melagabbronorite which plot within the calc- alkaline field. The Wadi Khamal complex consists of four major units; mar- ginal gabbro unit, anorthosite unit, central gabbroic unit and northern gabbronorite unit. The marginal gabbro unit and anorthosite unit have a calc alkaline differentiation trend while those of the central gabbro unit (olivine gabbro, melagabbronorite and Fe-Ti oxides-apatite gab- bronorite) and the northern gabbronorite show a distinct different trend parallel to the FeO*-MgO side line and belong to the tholeiitic differentiation trend. Based on major element geochemistry, the southern ultramafic - mafic rocks were derived from a more mafic liquid source rather different from magma source(s) which formed the Khamal anorthosite-gabbro complex.

Progressive metamorphism of the Taitao ophiolite; evidence for axial and off-axis hydrothermal alterations

We estimated metamorphic conditions for the ∼ 6 Ma Taitao ophiolite, associated with the Chile triple junction. The metamorphic grade of the ophiolite, estimated from secondary matrix minerals, changes stratigraphically downwards from the zeolite facies, through the prehnite–actinolite facies, greenschist facies and the greenschist–amphibolite transition, to the amphibolite facies. The metamorphic facies series corresponds to the low-pressure type. The metamorphic zone boundaries are subparallel to the internal lithological boundaries of the ophiolite, indicating that the metamorphism was due to axial hydrothermal alteration at a mid-ocean ridge. Mineral assemblages and the compositions of veins systematically change from quartz-dominated, through epidote-dominated, to prehnite-dominated with increasing depth. Temperatures estimated from the vein assemblages range from ∼ 230 °C in the volcanic unit to ∼ 380 °C at the bottom of the gabbro unit, systematically ∼ 200 °C lower than estimates from the adjoining matrix minerals. The late development of veins and the systematically lower temperatures suggest that the vein-forming alteration was due to off-axis hydrothermal alteration. Comparison between the Taitao ophiolite with its mid-ocean ridge (MOR) affinity, and other ophiolites and MOR crusts, suggests that the Taitao ophiolite has many hydrothermal alteration features similar to those of MOR crusts. This is consistent with the tectonic history that the Taitao ophiolite was formed at the South Chile ridge system near the South American continent (Anma, R., Armstrong, R., Danhara, T., Orihashi, Y. and Iwano, H., 2006. Zircon sensitive high mass-resolution ion microprobe U–Pb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications. The Island Arc, 15(1): 130–142).

Magmatic Stratigraphy of the Tilted Tottabetsu Plutonic Complex, Hokkaido, North Japan: Magma Chamber Dynamics and Pluton Construction

The steeply tilted Tottabetsu plutonic complex (TPC) provides an exceptional cross‐sectional view of a frozen felsic magma chamber. The entire pluton is apparently cylindrical in shape, but the active magma chamber at any one time was much thinner, tabular in shape. The pluton is stratified, with a topmost granitic unit (zone III), an underlying dioritic unit (zone II), and a lowermost gabbroic unit (zone I). Field, petrographic, and geochemical data suggest that zones I and II developed at the aggrading floor of a felsic magma chamber. Only part of zone III appears to have crystallized from the roof. Successive stoping events, recorded as included country‐rock blocks in zones I and II, apparently prohibited growth of the roof solidification zone. The complex record of hotter replenishments and fractional crystallization is preserved in the stratigraphy of zones I and II as either alternating comagmatic sheets and cumulate layers or an upward fractionation trend with a major compositional reversal exhibited by the cumulates. The magmas injected during the development of zone I were basalts to basaltic andesites, while those injected during the development of zone II were andesites to dacites. The overall compositional difference between zones I and II is essentially attributed to the compositional difference of the injected magmas. By contrast, zone III comprises monotonous granites whose geochemical variation suggests that they represent mixtures of fractionates complementary to the cumulates formed from both the resident felsic magma and the recharged intermediate magma. Buoyant fractionates released from the crystallizing recharge magma presumably rose as plumes and mixed with the resident magma. Thus, the TPC reveals the protracted history of pluton construction from a long‐lasting, melt‐rich magma chamber.

40Ar– 39Ar age, geochemistry and Sr–Nd–Pb isotopes of the Neoproterozoic Lengshuiqing Cu–Ni sulfide-bearing mafic–ultramafic complex, SW China

The Lengshuiqing Cu–Ni sulfide deposit of the Yanbian area in Sichuan Province, SW China, is hosted by relatively small mafic–ultramafic bodies that intruded Mesoproterozoic metavolcanic rocks and schists. The mafic–ultramafic complex is mainly composed of the olivine-hornblende pyroxenite and hornblende gabbro units. 40Ar/ 39Ar age, elemental and Sr–Nd–Pb isotopic results of the complex are reported in this paper. 40Ar/ 39Ar dating of hornblende yielded an age of 821 ± 1 Ma for the mafic–ultramafic complex. This age is similar to the SHRIMP U–Pb zircon age of 825 ± 12 Ma for the adjacent Gaojiacun mafic–ultramafic complex. This suggests that the Lengshuiqing mafic–ultramafic complex originated from Neoproterozoic mafic magma. The olivine-hornblende pyroxenite unit is characterized by high MgO contents (24.25–28.08%), Mg # values (74.5–78.0) and compatible elements (Cr = 1179–2336 ppm, Ni = 732–1303 ppm), low ΣREE (19.5–44.6 ppm) and other incompatible element contents. The hornblende gabbro unit has MgO = 7.32–12.51%, Mg # = 55.0–69.3, Cr = 37.7–1131 ppm, Ni = 50.0–243 ppm and ΣREE = 37.2–75.4 ppm. Most of the rocks are moderately enriched in LREE ((La/Yb) N = 2.28–7.67), and significantly depleted in Nb–Ta ((Nb/La) PM = 0.17–0.47; (Ta/La) PM = 0.19–0.58), with slightly moderately positive Eu anomalies ( δ Eu = 0.95–1.65). The trace element feature of the ore-bearing samples is similar to that of the rocks in the complex. The variations in major and trace elements can be well explained by magma crystal fractionation. Isotopically, the rocks in Lengshuiqing mafic–ultramafic complex are characterized by low 87Sr/ 86Sr(i) (0.7039–0.7044), positive ɛ Nd(i) (+2.2 − +4.3) and low radiogenic Pb values ( 206Pb/ 204Pb(i) = 17.382–17.913, 207Pb/ 204Pb(i) = 15.518–15.578, 208Pb/ 204Pb(i) = 37.227–38.019). The ores also have positive ɛ Nd(i) (+2.4 − +5.4), and low radiogenic Pb values ( 206Pb/ 204Pb(i) = 17.256–17.733, 207Pb/ 204Pb(i) = 15.484–15.555, 208Pb/ 204Pb(i) = 36.924–37.665). All the samples exhibit positive ɛ Nd(i) values, suggesting that their parental magmas were derived from a long-term depleted asthenospheric mantle. The relatively large variations in 87Sr/ 86Sr(i) of the ores (0.7012–0.7046) could be ascribed to the interaction between magma and country rocks. A good positive correlation between Nb/La and Nb/Th reveals that crustal assimilation was involved in their genesis. This contamination resulted in the observed depletion of Nb and Ta in rocks and ores. The crustal contamination and fractional crystallization, similar to the process of magmatic assimilation and fractional crystallization (AFC), are thus critical for the formation of the Lengshuiqing Cu–Ni ore deposit. The Neoproterozoic Lengshuiqing Cu–Ni sulfide-bearing mafic–ultramafic intrusions formed in an intraplate continental rift, which was most likely related to a mantle plume underneath the supercontinent Rodinia.

Platinum group elements zoning and mineralogy of chromitites from the cumulate sequence of the Nurali massif (Southern Urals, Russia)

The Nurali lherzolite massif is one of the dismembered ophiolite bodies associated with the Main Uralian Fault (Southern Urals, Russia). It comprises a mainly lherzolitic mantle section, an ultramafic clinopyroxene-rich cumulate sequence (Transition Zone), and an amphibole gabbro unit. The cumulate section hosts small chromitite bodies at different stratigraphic heights within the sequence. Chromitite bodies from three different levels along a full section of the cumulate sequence and two from other localities were investigated. They differ in the host lithology, chromitite texture and composition, and PGE content and mineralogy. Chromitites at the lowest level, which are hosted by clinopyroxenite, form cm-scale flattened lenses. They have high Cr# and low Mg# chromites and are enriched in Pt and Pd relative to Os and Ir. At a higher, intermediate level, the chromitites are hosted by dunite. They form meter thick lenses, contain low Cr# and high Mg# chromites, have high PGE contents (up to 26,700 ppb), and are enriched in Os, Ir and Ru relative to Pt and Pd, reflecting a mineralogy dominated by laurite–erlichmanite and PGE–Fe alloys. At the highest level are chromitites hosted by olivine–enstatite rocks. These chromitites have high Cr# and relatively low Mg# chromites and very low PGE content, with laurite as the dominant PGE mineral. The platinum group minerals (PGMs) show extreme zoning, with compositions ranging from erlichmanite to almost pure laurite and from Os-rich to Ru-rich alloys, with variable and irregular zoning patterns. Two chromitite bodies up to 6 km from the main sequence can be correlated with the latter based on geochemistry and mineralogy, implying that the variations in chromitite geochemistry are due to processes that operated on the scale of the massif rather than those that operated on the scale of the outcrop. Pertsev et al. [Pertsev, A.N., Spadea, P., Savelieva, G.N., Gaggero, L., 1997. Nature of the transition zone in the Nurali ophiolite, Southern Urals. Tectonophysics 276, 163–180.] propose that the Transition Zone formed by solidification of a series of small magma bodies that partially overlapped in time and space. The magmas formed by successive partial melting of the underlying mantle. We suggest that this process determined the changing PGE geochemistry of the successive batches of magma. The PGE distribution fits a model of selected extraction from the mantle, where monosulphide solid solution–sulphide liquid equilibrium was attained until complete melting of the monosulphide solid solution. Later and localized variations in fS 2 resulted in the formation of different PGMs with complex zoning patterns.

SEGMENTATION IN OMAN OPHIOLITE, IMPLICATIONS FOR FAST SPREADING RIDGES TECTONICS AND HIGH TEMPERATURE HYDROTHERMAL ACTIVITY

New, fine scale mapping in the large NW-SE ridge segment formerly identified in the Oman ophiolite (Nicolas and Boudier, 1995) has revealed that this structure is composed of smaller nestled segments, each being centered on the small mantle diapirs already mapped. The contacts with adjacent lithosphere and the tips of these segments have been mapped in detail. Their nature and structure depend on the difference in age between the two lithospheres. When the difference in age is in the range of 1 Myr, strike slip shear zones, 1-2 km wide, are developed in the mantle of the new segment. When this difference drops to ~0.5 Myr, the shear zones are small and diffuse but, in the mantle wedge at the tip of the segment which penetrates the older lithosphere, spectacular deformations are observed. The mantle and lower crust of the older lithosphere near the Moho are shovelled vertically and kilometer-sized folds develop in the gabbro unit. In contrast, the lid is not affected, suggesting that, at present day fast spreading ridges, similar major tectonic structures, as seen in Oman thanks to deep sections, may also be present. Contacts and tips of new segments are invaded by mafic dikes and sills issued from the segment magmatic activity and trapped against these colder boundaries. Melt reaction in the uppermost mantle at the tip of propagators generates large volumes of dunites and wehrlites. An important contribution to this magmatism results from massive seawater penetration down to the Moho, possibly favored by the segment tectonic activity. This water is heated at very high temperatures (VHT), 1000C° and possibly above. Inside crystallizing magma chamber, the hydrous reaction (Koepke et al.,2004) rejuvenates magmatic activity and generates orthopyroxene gabbros, injected and stretched parallel to the olivine gabbros layering. Outside the magma chamber, VHT seawater produces, by hydrous anatexis, copious melts which mix and react with the indigenous segment melts and crystallize as orthopyroxene- or pargasitebearing, clinopyroxene gabbros, diorites and trondjhemites. It is suggested that the geochemical signature of such hybrid melts should be looked for in present day ridges.

Assimilation and crystal accumulation in a mid-crustal magma chamber: the Sausfjellet pluton, north-central Norway

The Sausfjellet pluton is made up of two intrusive units emplaced into high-grade metamorphic rocks of the Helgeland Nappe Complex of the Uppermost Allochthon in the Norwegian Caledonides. The eastern part of the pluton intruded marble and less voluminous calc-silicate and pelitic rocks. The western half is hosted predominantly by semi-pelitic migmatite with intercalated marble. Remelting of the migmatite during pluton emplacement occurred in a thermal aureole as much as 1000 m wide. The early gabbroic unit forms the southeastern part of the body; it consists of hornblende-bearing to hornblende-rich gabbro and diorite which is thought to have crystallized from an H 2O-rich andesitic parental magma. The younger dioritic unit underlies the central and western parts of the pluton, as well as a zone as much as 200 m wide that separates the rest of the pluton from its host rocks (herein the “annular zone”). The interior or central zone of the dioritic unit is pyroxene diorite that is locally interlayered with anorthosite. The western and annular zones are, by comparison, mineralogically heterogeneous. They range from diorite to quartz monzonite and from biotite-bearing two- and three-pyroxene assemblages to biotite–hornblende assemblages. Neither rock type nor mafic assemblage is correlated with position in the pluton or proximity to a contact. Stoped blocks of a distinctive coarse-grained diorite, as well as pyroxene-rich calc-silicates, are present in the gabbroic unit and the central zone of the dioritic unit. The few stoped blocks observed in the western zone of the dioritic unit are predominantly quartz-rich gneiss. Chemical variation in the central zone of the dioritic unit is interpreted to result from accumulation of pyroxenes+plagioclase from an H 2O-poor andesitic parent. These rocks have approximately constant δ 18O of +6.6±0.2‰ and lack evidence of in situ assimilation. Heterogeneities in the western and annular zones of the dioritic unit are reflected in variable, anomalously enriched incompatible element contents and in δ 18O, which ranges from +6.7‰ to +8.6‰. Petrologic models indicate that the magma parental to the central zone could also be parental to the western and annular zones. If so, evolution of the western and annular zone magma was by crystal accumulation and assimilation of metapelitic host rocks. As much as 20% of the mass of the western and annular zones can be ascribed to assimilated material, which apparently entered the magma by stoping. Therefore, the asymmetrical zoning of the pluton is due to differences in host rock compositions and the relative ability of the magma to assimilate its host rocks.

Petrology, geochemistry and paleogeographic reconstruction of the East Sulawesi Ophiolite, Indonesia

The East Sulawesi Ophiolite (ESO) is tectonically dismembered and widely distributed in Central and East Sulawesi. It comprises, from base to top, residual mantle peridotite and mafic–ultramafic cumulate through layered to isotropic gabbro, to sheeted dolerites and basaltic volcanic rocks. Residual peridotite is dominantly spinel lherzolite intercalated with harzburgite and dunite. Ultramafic rocks from different locations display significant differences in rock composition and mineral. However, the clinopyroxene of peridotite displays REE pattern similarities with those of mid-ocean ridge (MOR) origin, rather than those of suprasubduction zone (SSZ) origin. The gabbroic unit consists of massive gabbro, layered gabbro, mafic and ultramafic cumulate and anorthosite. The observed crystallization sequence of gabbroic unit, which is olivine→(spinel)→plagioclase→clinopyroxene→(orthopyroxene)→(hornblende), and the mineral chemistry data indicate that the ESO gabbro has similarities with MOR setting. Major and trace element geochemistry of basalt and dolerite suggests MOR, oceanic plateau and minor SSZ origins. A possible oceanic plateau origin is supported by the following: (i) the 15-km thickness is comparable with the thickness of oceanic plateau rather than normal oceanic lithosphere; (ii) there are no or only minor olivine phenocrysts in the basalt; and (iii) predominance of aphyric texture in the basalts. The REE pattern of ESO basalt exhibits N-MORB-like signatures. However, a negative Nb anomaly in the trace element spider diagram may be attributed to mantle heterogeneity of an OPB source. The geochemical variations and disparities for both peridotite and basalt and the noncogenetic relationship between crust and mantle sections in several locations suggest that the ESO may have been formed at one tectonic setting and was later overprinted by magmatism in different environments through its birth to emplacement. A possible Cretaceous origin of an oceanic plateau component of the ESO is indicated on the basis of calculated paleopositions using plate trajectory analyses together with previously published paleolatitude data. The ESO can be traced back to the proximity of the presently active region of the SW Pacific Superplume.

Geology and petrology of the plutonic complexes in the Wadi Fizh area: Multiple magmatic events and segment structure in the northern Oman ophiolite

Multiple magmatic events are recorded in the gabbroic unit in the Fizh area of the northern Oman ophiolite. Gabbroic blocks intruded by sheeted dike complex and upper gabbros of the main crustal sequence show the oldest event. Gabbronorite sills in the gabbroic blocks are nearly coeval with the host gabbro. Wehrlitic intrusions (wehrlite I) mark the third event of magmatism. These three magmatic events occurred at the retreating (dying) ridge axis because all these rocks are intruded by dolerite dike swarm, which is generally regarded as a precursor of advancing ridge axis. The next stage of magmatism is a main phase of oceanic crust generation in this area. Wehrlite II and then gabbronorite dikes intrude the still hot main gabbro unit. All of these above rocks have similar signatures with respect to clinopyroxene compositions and covariations between plagioclase and mafic minerals, though slight differences are present in the compositional ranges and clinopyroxene compositions of each unit. After considerable cooling of the main gabbro unit, primitive basalt dikes intrude the main gabbro unit, which may correspond to the Lasail unit. Finally, the Fizh‐South complex intrudes into considerably cooled crustal sequence, being below the brittle‐plastic transition temperatures. The Fizh‐South complex, which was regarded as a common wehrlitic intrusion, is significantly different from all of the above mentioned rocks, with respect to the covariation between plagioclase and associating mafic minerals, crystallization order, and clinopyroxene compositions. The clinopyroxenes are characterized by extremely low Ti and Na contents, comparable with those of the V2 unit (Alley volcanics), suggesting that the Fizh‐South complex correlates with the plutonic facies of the V2 unit during arc stage. Layered gabbros in the Wadi Zabin area, about 10 km north of the Fizh area, may be a northern extension of the gabbro blocks of the Fizh area, because they are intruded by numerous dolerite dikes. On the other hand, basin‐like structure of the main gabbroic unit in the northern end of the Fizh area may demonstrate a fossilized magma chamber beneath the advancing ridge axis due to the ceasing of magmatic crustal accretion. On the basis of these lines of evidence, we propose that the Fizh area indicates the northward propagation tip of the ridge axis.