Geochemistry and petrogenesis of ultramafic and mafic plutonic rocks of the Dras ophiolitic melange, Indus suture (northwest Himalaya)

Geochemical data are presented for a suite of ultramafic and related rocks from the Dras ophiolitic melange of the Indus suture zone in the western Himalaya. Harzburgites from the suite have highly refractory chemistry. Lherzolites from the suite represent modified mantle material and are comparable to potential source rocks for MORB and to the lherzolites of the Bay of Islands ophiolites. Cumulus dunite, pyroxenite and gabbro units contain olivine, chromite, clinopyroxene and plagioclase as the major cumulus phases. Orthopyroxene is absent as a cumulus phase and in this respect the Dras ophiolite differs from the Marum, Betts Cove and Troodos ophiolites. However, the Dras cumulates are similar to the Vourinos and Bay of Islands cumulate sequence and are consistent with accumulation of low-pressure liquidus phases of mid-oceanic ridge-type magmas. Magmas parental to the Dras cumulate rocks contained high 100 Mg/(Mg + Fe²⁺) ratios of 77–79, high Ni, Cr and possibly Ca, low Ti and depleted LREE. Parent magmas were probably similar to those of normal MORB formed by two-stage (or dynamic) melting processes in the mantle. Peridotite fabrics suggest high-temperature plastic (mantle) deformation. Disruption, serpentinisation and melange formation were probably produced during emplacement in the Indus suture zone.     

Stored mafic/ultramafic crust and early Archean mantle depletion

Both early and late Archean rocks from greenstone belts and felsic gneiss complexes exhibit positive ε_Nd values of +1 to +5 by 3.5 Ga, demonstrating that a depleted mantle reservoir existed very early. The amount of preserved pre-3.0 Ga continental crust cannot explain such high ε values in the depleted residue unless the volume of residual mantle was very small: a layer less than 70 km thick by 3.0 Ga. Repeated and exclusive sampling of such a thin layer, especially in forming the felsic gneiss complexes, is implausible. Extraction of enough continental crust to deplete the early mantle and its destructive recycling before 3.0 Ga ago requires another implausibility, that the sites of crustal generation and of recycling were substantially distinct. In contrast, formation of mafic or ultramafic crust analogous to present-day oceanic crust was continuous from very early times. Recycled subducted oceanic lithosphere is a likely contributor to present-day hotspot magmas, and forms a reservoir at least comparable in volume to continental crust. Subduction of an early mafic/ultramafic “oceanic” crust and temporary storage rather than immediate mixing back into undifferentiated mantle may be responsible for the depletion and high ε_Nd values of the Archean upper mantle. Using oceanic crustal production proportional to heat productivity, we show that temporary storage in the mantle of that crust, whether basaltic as formed by 5–20% partial melting, or partly komatiitic and formed by higher extents of melting is sufficient to balance an early depleted mantle of significant volume with ε_Nd at least +3.0.     

Petrology of ultramafic and mafic xenoliths in picrite of Kahoolawe Island,Hawaii

A picrite lava (22 wt% MgO; 35 vol.% ol) along the western shore of the1.3–1.4 Ma Kahoolawe tholeiitic shield, Hawaii, contains small xenoliths of harzburgite, lherzolite, norite, and wehrlite. The various rock types have textures where either orthopyroxene, clinopyroxene, or plagioclase is in a poikilitic relationship with olivine. The Mg#s of the olivine, orthopyroxene, and clinopyroxene in this xenolith suite range between 86 and 82; spinel Mg#s range from 60 to 49, and plagioclase is An_75–80. A ⁸⁷Sr/⁸⁶Sr ratio for one ol-norite xenolith is 0.70444. In comparison, the host picrite has olivine phenocrysts with an average Mg# of 86.2 (range 87.5–84.5), and a whole-rock ⁸⁷Sr/⁸⁶Sr ratio of 0.70426. Textural and isotopic information together with mineral compositions indicate that the xenoliths are related to Kahoolawe tholeiitic magmatism, but are not crystallization products of the magma represented by their host picrite. Rather, the xenoliths are crystalline products of earlier primitive liquids (FeO/MgO ranging 1 to 1.3) at 5–9 kbar in the cumulate environment of a magma reservoir or conduit system. The presence of ultramafic xenoliths in picrite but not in typical Kahoolawe tholeiitic lava (6–9 wt% MgO) is consistent with replenishment of reservoirs by dense Mg-rich magma emplaced beneath resident, less dense tholeiitic magma. Mg-rich magmas have proximity to reservoir cumulate zones and are therefore more likely than fractionated residual liquids to entrain fragments of cumulate rock.     

Oxidation of titanomagnetites in mafic and felsic intrusive rocks

We report opaque mineralogical observations and magnetic properties of primary titanomagnetites in Tertiary submarine gabbros from DSDP, Legs 30 and 37 and in a late Archean, continental granitic pluton, the Shelley Lake granite. The titanomagnetites and silicates in all the submarine gabbros have been deuterically oxidized. There is no indication of subsequent low-temperature oxidation, although serpentization of olivines is pervasive in the deeper Leg 37 units. The Leg 30 samples, from a single thick sill, contain abundant coarse (≈100 μm) titanomagnetites with fully developed ilmenite exsolution lamellae. Curie temperatures are 515–550°C; there are no low Curie temperatures that would indicate surviving unoxidized titanomagnetite. The unserpentinized Leg 37 gabbros contain scarce opaques with pure magnetite Curie points that are barely resolvable microscopically; most occur as inclusions in pyroxene. In the Shelley Lake granite, on the other hand, many samples exhibit bimodal blocking-temperature spectra, with blocking temperature peaks at 250–300°C and 550–575°C. The low-blocking-temperature phase is unidentified. No pyrrhotite was seen in thin section. Optically homogeneous grains coexist with fully exsolved neighbours, but the electron microprobe indicates no titanium. The lamellae appear to be haematite, not ilmenite, and the primary composition of the opaques is pure magnetite. The oxidation state of the opaques is very inhomogeneous, even on a fine scale.     

Uranium and boron distributions in some oceanic ultramafic rocks

From mica fission-track maps the serpentinized and weathered portions of four ultramafic rocks from oceanic ridge systems contained 0.5 to 2.4 ppm U compared to only 2.5 ppb in clinopyroxene, 0.6 ppb in chromite and less than 7.0 ppb in olivine. Orthopyroxene grains contained 0.4 ppb U which is three orders of magnitude lower than had previously been reported.Long thin tracks from (n, α) reactions with boron were recorded in cellulose nitrate plastic and were counted like fission tracks. The track density from boron was 2×10⁴ times higher than that from uranium fission alone. Boron in serpentine was variable on a 50-μm scale attaining 155 ppm concentrations. Orthopyroxene grains, in contrast, had maximum concentrations of 0.8 ppm.Most of the uranium and boron in the rocks is believed to have been introduced during serpentinization. From known crystal-melt partitioning ratios the uranium and boron distributions are consistent with the ultramafic rocks being cumulates or residues from partial melting events.     

Electrical conductivity of ultramafic rocks to 1820 kelvin

Electrical conductivity, σ, of six ultramafic rocks (garnet-bearing peridotites and an eclogite) has been investigated in the temperature range 670–1820 K under known environment. Between 670 and ～1320 K σ increases with 1/T monotonically, first slowly, by 1–1.5 orders of magnitude. Above 1320 K σ increases sharply; an increase of 3–4 orders of magnitude is observed between 1320 and 1670 K. In the same temperature range the σ values for all the six rocks fall within 1.5 orders of magnitude, the lowest conductivity being for a spinel lherzolite. The range of σ values is narrowed at higher temperatures. The differences in σ values above 1470 K may be explained on the basis of varying degrees of partial melting in the rocks. Over the entire range of temperature, the σ values for the ultramafic rocks are lower than those reported for basalts but higher (by 1–2 orders of magnitude) than those for single-crystal olivines.     

Platinum-group element geochemistry of mafic rocks from the Dongchuan area,southwestern China

Mafic intrusions and dykes are well preserved in the Yinmin and Lanniping districts,located within the western margin of the Yangtze Block,SW China.Although these mafic rocks from the two areas formed during different periods,they share similar ranges of PGE concentration.Most of the Yinmin gabbroic dykes contain relatively high PGE concentrations(PGEs = 13.9-87.0 ppb) and low S contents(0.003%-0.020%),higher than the maximum PGE concentrations of mafic magmas melting from the mantle.Two exceptional Yinmin samples are characterized by relatively low PGE(PGEs = 0.31-0.37 ppb) and high S(0.114%-0.257%) contents.In contrast,most samples from the Lanniping gabbroic intrusion have low PGE concentrations(PGEs — 0.12-1.02 ppb) and high S contents(0.130%-0.360%),except that the three samples exhibit relatively high PGE(PGEs = 16.3-34.8 ppb) and low S concentrations(0.014%-0.070%).All the Yinmin and Lanniping samples are characterized by the enrichment of PPGE relative to IPGE in the primitive-mantle normalized diagrams,and the high-PGE samples exhibit obvious Ru anomalies.This study suggests that during the ascent of the parental magma,removal of Os-Ir-Ru alloys and/or chromite/spinel leads to high Pd/Ir ratios and Ru anomalies for the Yinmin high-PGE samples and relatively lower Pd/Ir ratios and Ru anomalies for the Lanniping low-PGE samples.We propose that the magmas parental to the Yinmin gabbroic dykes are initially S-unsaturated,and subsequently,minor evolved magma reached sulfur saturation and led to sulfide segregation.Although the Lanniping parental magmas are originally not saturated in S,the high Cu/Pd ratios(3.8 × 10~4 to 3.2 ×10~6) for most of the Lanniping samples indicate the S-saturated state and sulfide segregation.A calculation shows that the PGE-poor magmas might have experienced 0.01%-0.1% sulfide segregation in the magma chamber.Therefore,our study provides a possible opportunity to discover PGE-enriched sulfide mineralization somewhere near or within the Lanniping mafic intrusion.     

Duplex of metamorphic units including ultramafic rocks in the central region of Tokunoshima,Southwest Japan

Geological observations in the central part of Tokunoshima in the Amami Islands, Southwest Japan, reveal that discrete layers of serpentinite, dioritic gneiss, and amphibolite are intercalated into pelitic schist and these rock bodies form a northwest‐dipping tectonic stack. A subhorizontal psammitic schist layer overlies them. These rocks underwent ductile deformation that is denoted by penetrative foliation and mineral lineation. Microstructures of the sheared metamorphic rocks and serpentinite indicate top‐to‐the‐east, ‐southeast or ‐south (hanging‐wall up) displacements. The en echelon array of rock bodies is interpreted as a duplex with the psammitic schist layer on its top and the pelitic schist layer on its bottom. It is inferred that the serpentinite‐bearing duplex was formed due to the tectonic erosion and the subsequent accretionary growth operated in a Cretaceous or older subduction zone. Tokunoshima has been considered to belong to the Shimanto Belt. However, regional low‐pressure and high‐temperature type amphibolite‐facies metamorphism and related ductile deformation have not been recognized in the other areas of the Shimanto Belt. There is no metamorphic rock occurrence comparable to that of Tokunoshima in the neighboring islands. The metamorphic rocks in Tokunoshima can be correlated to any of low‐pressure/temperature type metamorphic regions in Kyushu.     

Magma mixing in mafic alkaline volcanic rocks: the evidence from relict phenocryst phases and other inclusions

Green clinopyroxenes, commonly rounded and anhedral and richer in Fe, Na and Mn than the pyroxenes of the surrounding groundmass are a common feature of mafic alkaline volcanic rocks (e.g. basanites, monchiquites, leucitites). Some are accompanied by one or more of the following phases: Fe-rich kaersutite and biotite, anorthoclase, sodic plagioclase, apatite, magnetite, sphene, which are believed to be cognate with the green pyroxenes. We review evidence that these minerals have crystallized from mugearite, trachyte or phonolite magmas, and their presence in mafic alkaline rocks is due to magma mixing. The intermediate and salic magmas may sometimes be generated at mantle depths, possibly by melting of mantle material enriched in Fe, Na and volatiles.     

Geochemistry of mafic cainozoic volcanic rocks from Sardinia (western mediterranean)

The Cainozoic volcanism of Sardinia (Italy) can be divided into two main cycles with different magmatic and geodynamic significance. The early cycle — Oligo-Miocenic in age (29-13 My ago) — shows the calc-alkaline character typical of converging plate areas. The later activity, ranging from Lower Pliocene (about 5 My ago) to recent Pleistocene, produced mostly basic lavas extruded onto a continental plate («within plate basalts»). It was related to a period of tensional tectonics which had affected the western Mediterranean area prior to, and during, the volcanic activity. Intermediate and acid volcanic products were associated with the mafic rocks of the latest magmatic episodes. The main groups of rocks — the basic ones, already classified from their petrographic features and geological setting — can be characterized very well when a statistical elaboration of their chemical analyses is used. In fact, from chemical data it is possible to distinguish the Oligo-Miocene volcanic products from those of Plio-Pleistocene age. Moreover, within this latter group basanites, alkalic and subalkalic basaltic rocks can clearly be distinguished. Samples that had not clearly been defined on the basis of their petrographic characteristics (anonymous samples) have been attributed to one or the other of the main groups by means of discriminant functions. Chemical variations in the Plio-Pleistocenic rocks are due to fractionation episodes at shallow depths superimposed on primary magmatic variations. A model of partial fusion of the mantle accounts for many but not all the observed original chemical variations. Different physical melting conditions, the effects of minor mineral phases in the mantle and, probably, crustal contamination were also effective in creating the observed chemical variations.     

Petrology of the ultramafic and basic rocks of betancuria massif,fuerteventura Island (Canarian Archipelago)

The petrological and geochemical aspects of the ultramafic and basic plutonic rocks of Betancuria Massif are examined. The rocks consist of gradational varieties of wehrlite, pyroxenite, olivine-gabbro and gabbro formed mainly by magnesium-rich olivine, clinopyroxene and plagioclase. The Complex exhibits structures and textures characteristic of layered igneous rocks;i.e., banding, layering, lamination, etc...; rocks are therefore considered as cumulates or magmatic sediments. The Complex has undergone a deep process of alkalinisation produced by the syenite and trachyte ring-dykes intrusion and by the existence of carbonatite veins. Rocks close to the syenite-trachyte intrusions show an increase in alkali feldspars. The presence of amphibole (kaersutite) is the first steep in the alkalinisation process and it is related to the disappearence of pyroxene. Extreme transformations of the gabbroidic rocks are found when they come in contact with the carbonatites. The age of the Complex is unknown but field evidence lead us to conclude that it is older than any other visible formation of the Island.     

The water content and hydrogen isotope composition of continental lithospheric mantle and mantle-derived mafic igneous rocks in eastern China

The water contents of minerals and whole-rock in mantle-derived xenoliths from eastern China exhibit large variations and are generally lower than those from other on- and off-craton lithotectonic units. Nevertheless, the water contents of mineral and whole-rock in Junan peridotite xenoliths, which sourced from the juvenile lithospheric mantle, are generally higher than those elsewhere in eastern China. This suggests that the initial water content of juvenile lithospheric mantle is not low. There is no obvious correlation between the water contents and Mg^# values of minerals in the mantle xenoliths and no occurrence of diffusion profile in pyroxene, suggesting no relationship between the low water content of mantle xenolith and the diffusion loss of water during xenolith ascent with host basaltic magmas. If the subcontinental lithospheric mantle (SCLM) base is heated by the asthenospheric mantle, the diffusion loss of water is expected to occur. On the other hand, extraction of basaltic melts from the SCLM is a more efficient mechanism to reduce the water content of xenoliths. The primary melts of Mesozoic and Cenozoic basalts in eastern China have water contents, as calculated from the water contents of phenocrysts, higher than those of normal mid-ocean ridge basalts (MORB). The Mesozoic basalts exhibit similar water contents to those of island arc basalts, whereas the Cenozoic basalts exhibit comparable water contents to oceanic island basalts and backarc basin basalts with some of them resembling island arc basalts. These observations suggest the water enrichment in the mantle source of continental basalts due to metasomatism by aqueous fluids and hydrous melts derived from dehydration and melting of deeply subducted crust. Mantle-derived megacrysts, minerals in xenoliths and phenocrysts in basalts from eastern China also exhibit largely variable hydrogen isotope compositions, indicating a large isotopic heterogeneity for the Cenozoic SCLM in eastern China. The water content that is higher than that of depleted MORB mantle and the hydrogen isotope composition that is deviated from that of depleted MORB mantle suggest that the Cenozoic continental lithospheric mantle suffered the metasomatism by hydrous melts derived from partial melting of the subducted Pacific slab below eastern China continent. The metasomatism would lead to the increase of water content in the SCLM base and then to the decrease of its viscosity. As a consequence, the SCLM base would be weakened and thus susceptible to tectonic erosion and delamination. As such, the crust-mantle interaction in oceanic subduction channel is the major cause for thinning of the craton lithosphere in North China.     

Subhorizontal tectonic framework of the Horoman peridotite complex and enveloping crustal rocks,south‐central Hokkaido,Japan

Geological observations in the Horoman area, south‐central Hokkaido, show that the Horoman peridotite complex of the Hidaka metamorphic belt is a tectonic slice about 1200 m thick. The peridotite slab is intercalated into a gently east‐dipping structure. The underlying unit is a Cretaceous–Paleogene accretionary complex. Riedel shear planes in the sedimentary layers of the accretionary complex near the structural bottom of the peridotite slab indicate top‐to‐the‐west (thrust) displacement. The overlying unit is composed of felsic–pelitic gneisses and mafic–felsic intrusive rocks (the Hidaka metamorphic rocks). The boundary surface between the peridotite complex and metamorphic rocks forms a domal structure. Microstructures of sheared metamorphic rocks near the structural top of the peridotite slab indicate top‐to‐the‐east (normal) displacement. The results combined with previous studies suggest that the Horoman peridotite complex was emplaced onto the Asian margin (Northeast Japan) during the collision between the Asian margin and the Hidaka crustal block.     

Petrochemistry and tectonic setting of mafic volcanic rocks in the Chon Daen–Wang Pong area,Phetchabun, Thailand

The mafic volcanic rocks and hypabyssal rocks in the Chon Dean‐Wang Pong area are possibly the southern extension of the western Loei Volcanic Sub‐belt, Northeast Thailand. They are least‐altered, and might have been formed in Permian–Triassic times. The rocks are commonly porphyritic, with different amounts of plagioclase, clinopyroxene, orthopyroxene, amphibole, Fe–Ti oxide, unknown mafic mineral, and apatite phenocrysts or microphenocrysts, and are uncommonly seriate textured. The groundmass mainly shows an intergranular texture, with occasionally hyalophitic, intersertal and ophitic–subophitic textures. The groundmass constituents have the same minerals as the phenocrysts or microphenocrysts and may contain altered glass. The groundmass plagioclase laths may show a preferred orientation. Chemically, the studied rock samples can be separated into three magmatic groups: Group I, Group II, and Group III. These magmatic groups are different in values for Ti/Zr ratios. The averaged Ti/Zr values for Group I, Group II, and Group III rocks are 83 ± 6, 46 ± 12, and 29 ± 5, respectively. In addition, the Group I rocks have higher P/Zr, but lower Zr/Nb relative to Group II and Group III rocks. The Group I and Group II rocks comprise tholeiitic andesite–basalt and microdiorite–microgabbro, while the Group III rocks are calc‐alkalic andesite and microdiorite. According to the magmatic affinities and the negative Nb anomalies on normal mid‐oceanic ridge basalt (N‐MORB) normalized multi‐element plot, arc‐related lavas are persuasive. The similarity between the studied lavas and the Quaternary lavas from the northern Kyukyu Arc, in terms of chondrite‐normalized rare earth element (REE) patterns and N‐MORB normalized multi‐element patterns, leads to a conclusion that the mafic volcanic rocks and hypabyssal rocks in the Chon Daen–Wang Pong area have been formed in a volcanic arc environment.     

The ReOs systematics of the Ronda Ultramafic Complex of southern Spain

The Os isotopic compositions of twelve ultramafic and six mafic layer samples from the Ronda Ultramafic Complex of southern Spain have been determined. Among the ultramafic rocks, ¹⁸⁷Os/¹⁸⁶Os varies from 0.98 to 1.12. A weak correlation is observed between ¹⁸⁷Os/¹⁸⁶Os and Re/Os. A much stronger correlation exists between Os isotopic ratio and Mg#, suggesting that the Re/Os ratios have been perturbed to some extent. Two alternatives are proposed to explain the relationship between Os composition and Mg#: (1) Continuous processes in the convecting mantle; (2) Radiogenic ingrowth since an ancient melt depletion event. No relationship is observed between ¹⁸⁷Os/¹⁸⁶Os and ¹⁴³Nd/¹⁴⁴Nd. This is probably because the Nd systematics were strongly affected by a recent metasomatic event, which apparently had little effect on the Os isotopic compositions.The Os isotopic ratios of the mafic layers range from 1.7 to 47.9. Within a single thick layer, the ratios vary from 16.5 to 47.9. These high ratios demonstrate that the layers are ancient features. Among the mafic samples, Os isotopic ratio is found to decrease strongly with increasing Os concentration, which ranges from 0.009 ppb to 1.16 ppb. One layer, which had a SmNd model age of less than 200 Ma, yielded a ReOs model age of about 2 Ga. This implies that neither system can be trusted to give accurate information about the time of mafic layer formation.