Presence Of Orthopyroxene Research Articles

Petrogenesis of Late Cretaceous gabbronorites in southwestern Lhasa Terrane, Tibetan Plateau, China: Sediment melt-mantle interaction and magmatic flare-up in response to Neo-Tethys slab roll-back

The geodynamic processes responsible for the Late Cretaceous magmatic flare-up in the southern Lhasa Terrane, Tibetan Plateau, have been a matter of debate. In this study, we have reported on newly identified Late Cretaceous gabbronorites in the Dajiacuo Area of the southwestern Lhasa Terrane. These rocks show ophitic textures comprising plagioclase, orthopyroxene, and clinopyroxene, while hydrous minerals, such as amphibole and biotite, are not observed. They display geochemical affinities for high-Mg andesites (HMAs) given their SiO2 (53.5–59.7 wt. %), Mg# (56.8–59.9), and Cr (up to 178 ppm). Combined with their high Th/Yb and low Ba/La ratios and similar Sr–Nd–Hf isotopes to those of the Neo-Tethyan sediments, it is likely that the Dajiacuo gabbronorites were formed by the interaction between sediment-derived melts and the mantle. The Dajiacuo gabbronorites have similar Zr/Hf ratios to the Neo-Tethyan sediments and present no negative Zr–Hf anomalies. This indicates that the zircons in the subducted sediments may have been fully destroyed. This requires specific dynamic conditions to provide a hot subduction geothermal gradient. This is consistent with the high-temperature affinity of the parent magma inferred from Ti-in-zircon thermometry and the presence of orthopyroxene. Based on this, coupled with the synthesis of existing tectonomagmatic data for the southern Tibetan Plateau, we propose that the Late Cretaceous magmatic flare-up in the southern Lhasa Terrane was likely triggered by the roll-back of the northwards subducted Neo-Tethyan lithosphere.

Petrology and geochemistry of Charnockite patches and host Leptynites from Digapahandi area, EGB: Evidence for incipient growth of Charnockites

Occurrences of the small, elongated, irregular shape of charnockite patches along the foliation planes of the leptynites in Digapahandi area, Ganjam district, Odisha of Eastern Ghats Belt (EGB) are reported. Petrography and mineral chemistry of charnockite patches and host leptynites suggest that both the rocks have analogues mineral assemblages, except the presence of ortho-pyroxene in charnockite patches. The mineral chemistry studies revealed that mineral phases common in both rocks are chemically identical. Based on whole rocks geochemistry it is evident that both charnockite patches and host leptynites are iso-chemical. The adamellite composition, per-aluminous nature with positive Europium anomaly of host leptynite and charnockite patches suggests their co-genetic relation. Higher values of (La/Yb)N and (Gd/Lu)N of leptynites indicate the highly fractionated HREE pattern of leptynites. High concentrations of HREE in charnockite patches are due to the influence of fluid phase in granulite facies metamorphism. The charnockite patches are characterized by higher K2O content, higher HREE concentration, and less abundance of biotite indicating the in-situ growth of patchy charnockites.

Petrology and crustal evolution of the Tartarugal Grande Granulitic Complex - Northeastern Amazonian Craton

The Tartarugal Grande Complex (TGC) is an association of high-grade metamorphic rocks with Archean and Paleoproterozoic protoliths, located at the border of the Archean Amapá and Rhyacian Lourenço domains, in southeastern Guiana Shield, northeast of the Amazonian Craton. It consists of felsic granulites, and aluminous leucogneisses and rare mafic granulites. Both granulites and gneisses are affected by thrust and transcurrent shear zones along NW-SE trend. Migmatization with quartz-feldspathic neosomes, with or without orthopyroxene, and garnet-rich neosomes are present in felsic granulites and leucogneisses, respectively. The gneisses have high silica contents and are peraluminous due to the presence of biotite, garnet, and cordierite, indicating the derivation from metasedimentary sequences. The felsic granulites have medium to high silica content and geochemical signature of calc-alkaline magmatic arc series. The mafic granulites are small bodies embedded in the felsic granulites and leucogneisses. They display geochemical affinities with tholeiitic series and represent ancient diabase dykes. The metamorphic paragenesis in the granulite facies is represented by Pl-antiperhtite + Qtz + Mc-mesoperthite + Opx + Cpx + Bt ± Hbl (enderbitic granulite), Pl-antiperhtite + Qtz + Mc-mesoperthite + Opx ± Bt (charnoenderbitic granulite); Mc-mesoperthite + Qtz + Pl + Opx + Bt (charnockitic granulite), Pl (An60)+Opx + Cpx ± Hbl ± Grt (mafic granulite), and Qtz + Mc + Pl ± Bt ± Grt ± Crd (leucogneiss). The presence of orthopyroxene, clinopyroxene and hornblende in equilibrium in the granulites allows estimating temperatures at 800° ± 20 °C. In addition, non-extensive anatexis under anhydrous conditions produced charnockitic neosomes in the granulites and garnet-bearing leucogranite neossomes in the paragneisses. The presence of cordierite in leucogneisses indicates low-medium pressure conditions around 6–7 kbar. The partial or total replacement of pyroxenes by hornblende and/or biotite, garnet by biotite, clinopyroxene by titanite, and myrmekite intergrowths, indicate retrometamorphism and cooling during exhumation to higher crustal levels. The U–Pb LA-ICP-MS dating on zircon from a charnoenderbitic granulite yielded an age of 2082 ± 5 Ma for the crystallization of the igneous protolith of the granulites. Metamorphic zircons from an enderbitic granulite yielded a U–Pb LA-ICP-MS age of 2045 ± 14 Ma, which corresponds to the high-grade metamorphic episode that has affected the region at the end of the Rhyacian. The age of the protoliths is not constrained but 207Pb/206Pb dates at 2.57–2.70 Ga indicated inherited crystals and/or xenocrystals from Archean source. In agreement with previous works, we conclude that the TGC consists of calcium-alkaline magmatic rocks and pelitic sedimentary sequences formed during the development of a Rhyacian magmatic arc at the edge of the Archean Amapá Block. This was followed by a thermo-tectonic event at the end of Rhyacian, around 2.05 Ga, under high-grade conditions reaching the granulite facies, with restricted anatexis. Records of the tectonic exhumation of these terranes were defined by partial disequilibrium of some mineral phases in amphibolite facies. This Late-Rhyacian thermo-tectonic event is contemporaneous to the ultra-high temperature metamorphism (2.09–2.03 Ga) of the Bakhuis Granulite Belt in Suriname. The thermal differences between these two high-grade metamorphic terrenes may be related to the different crustal levels where they settled. Late-Rhyacian asthenospheric upwelling by crustal stretching during oblique continental collision seems to be the more plausible scenario as previously proposed, even if a slab break-off scenario cannot be ruled out.

Pyroxene control of H2 production and carbon storage during water-peridotite-CO2 hydrothermal reactions

Strategies of H2 production and CO2 mineralization were combined through olivine [(Mg,Fe)2SiO4] serpentinization and carbonation in a CO2-rich hydrothermal system. However, natural mantle peridotites commonly contain not only olivine but also orthopyroxene and/or clinopyroxene, which have effects that are not well understood. The present study investigated the reactions in H2O-olivine/orthopyroxene-CO2 systems by performing hydrothermal experiments in 0.5 M NaHCO3 solutions at 300 °C and 10 MPa.The yields of H2 and HCOOH initially were first suppressed in the presence of orthopyroxene; however, after orthopyroxene consumption, the rate of H2 production increased significantly. H2 yield increased to 348.3 mmol/kgmineral in 120 h with the presence of 20 wt% orthopyroxene at the beginning of the reaction. The initial suppression of H2 generation was due to incorporation of more Fe(II) into serpentine [(Mg,Fe)3Si2O5(OH)4] in the high SiO2(aq) concentration system. The presence of orthopyroxene also dramatically accelerated serpentine formation. In contrast, magnesite [(Mg,Fe)CO3] formation was inhibited upon addition of orthopyroxene, which also contributed to the release of Fe(II). Therefore, peridotite containing ≤20 wt% of pyroxenes is more suitable for long-term H2 production than pure olivine. When considering the reaction output of a water-peridotite-CO2 system, controlling the percentage of pyroxenes in the starting mineral may be more important than expected.

High-temperature S-type granitoids (charnockites) in the Jining complex, North China Craton: Restite entrainment and hybridization with mafic magma

The Liangcheng garnet granitoids are parautochthonous S-type granites that occur within granulite-facies metasediments in the ultrahigh-temperature (UHT) Jining Complex, eastern part of the Khondalite Belt, North China Craton. These garnet granitoids contain mafic microgranular enclaves and are intimately associated with numerous small intrusions of the Xuwujia gabbronorites, showing magma mingling and mixing textures. Typical mineralogy in the garnet granitoids is plagioclase + K-feldspar + quartz + garnet + biotite ± orthopyroxene. UPb ion microprobe dating of zircon suggests that garnet granitoids and associated gabbronorites were coeval with UHT metamorphism at ~1.93–1.92 Ga, and also recorded ~1.92 Ga metamorphism during slow cooling. Garnet granitoids are characterized by the presence of orthopyroxene, high modal proportion of garnet (up to 25%) and strongly peraluminous (ASI = 1.11–1.55) along with lower SiO2 (57.8–70.1 wt%) and higher FeOt + MgO (5.6–14.4 wt%) contents than typical S-type granites. Mineral chemistry (high Ti biotite and high Al orthopyroxene), high Al-in-orthopyroxene temperature (910–989 °C) and Zr saturation temperature (825–901 °C), and low whole-rock Al2O3/TiO2 ratios (11–28), indicate that garnet granitoids were derived from biotite dehydration melting of metasedimentary rocks under UHT granulite-facies conditions. Textural and compositional evidence suggests that a significant proportion of garnets in the Liangcheng garnet granitoids are potentially a peritectic phase entrained from the source. Garnet aggregates may also represent entrained garnet-rich restites. The large HREE variations of garnet granitoids also support the entrainment of garnet-rich restites. Garnet granitoids display nearly linear trends with the mafic enclaves and gabbronorite for most major elements that demonstrates the involvement of mafic magma in the granitoid petrogenesis. The εHf(t) values (−1.0 − +5.6) and δ18O values (9.2–11.6‰) of zircons in garnet granitoids partly overlap with those in gabbronorites (εHf(t) = −2.9 − +3.5, δ18O = 7.7–10.9‰), suggesting the strong interaction and mixing process between them. Major element modelling suggests that the addition of 20–40% restites (garnetites) and gabbronorites to the anatectic melts was responsible for the large chemical variations. Therefore, the Liangcheng garnet granitoids represent high-temperature (> 900 °C), H2O-undersaturated, strongly peraluminous granitoids, and are produced by large-scale melting of metasedimentary rocks and subsequent mixing between restite-rich anatectic melt and mafic magma. The voluminous high-temperature granitoids, coupled with UHT granulites in the Jining Complex, manifest the thermal response of a granulitic lower crust to the mafic underplating.

Origin and geotectonic evolution of Mir Tertiary basaltic andesite dykes, Western Desert, Egypt: Constraints from mineral and bulk‐rock chemistry

Compared with widespread Egyptian Tertiary basaltic rocks, Mir Tertiary basaltic dykes from the Western Desert of Egypt show distinct mineralogical and geochemical characteristics that provide insights into the nature of their mantle source and geotectonic evolution as well as melting and crystallization conditions. Their mineralogical features (i.e., presence of orthopyroxene and lack of olivine) distinguish them from the other Egyptian Tertiary basalts. They have basaltic andesite compositions (SiO2 = 53.55–55 wt.%, Na2O + K2O = 4.64–4.8 wt.%, MgO = 2.66–3.02 wt.%) with tholeiitic character. Temperature of crystallization ranges from 1,112°C to 1,140°C for plagioclase; from 1,085°C to 1,123°C for clinopyroxene; and from 1,034°C to 1,090°C for orthopyroxene. These results are in harmony with petrographic observations which suggest early crystallization of plagioclase, followed by simultaneous crystallization of plagioclase and clinopyroxene, and soon thereafter by orthopyroxene crystallization. Titanomagnetite–ilmenite intergrowths indicate oxygen fugacity (ƒO2) of −12.09 and −9.4 log units, suggesting crystallization at relatively oxidized conditions. Their temperature ranges from 959°C to 1,125°C, suggesting that the titanomagnetite joins the crystallized phases at early stages and continued until later stages. Mir basaltic andesite (MBA) has low Ni (15–27.9 ppm), Cr (19.8–48.5 ppm), and Sc (20–25 ppm) concentrations together with high FeOt/MgO ratios (3.18–3.58) relative to primitive basalts, arguing for their evolved nature. They have sub‐chondritic Nb/Ta ratios (15.5–17) similar to ocean–island basalts and higher than those of continental crust, ruling out a significant effect of crustal contamination. The high La/Nb (>1.4) with the negative Nb, Ta, and Ti anomalies on the primitive mantle‐normalized patterns most likely indicates that their primary magma was derived from partial melting of a mantle source previously metasomatized by influx of subduction‐related slab fluids/melts. Moreover, the super‐chondritic Zr/Hf ratios (39.55–55.21) and the negative K and Ti anomalies point to carbonate metasomatism of their mantle source. This inference is further supported by their high (K2O + Na2O)/TiO2 ratios (>1) which are comparable with experimentally produced carbonated peridotite melts. The high Zr/Y (average 9.75) and La/Yb ratios (8.6–10) of the MBA suggest that they were generated via low degrees of partial melting. Indeed, modelling calculations indicate generation by lower melting degrees (<10%) of a garnet‐bearing lherzolite mantle source. The melting region was at the garnet stability field suggesting a deeper origin (exceeds ~85 km) for MBA. Field and geochemical characteristics of the Egyptian Tertiary volcanism proved that it is spatially and temporally related to extension that has occurred during Red Sea rifting. Extension most likely due to the NE–SW tensional stresses which reactivated the old fractures and triggered mafic magma generation through passive upwelling of the asthenospheric mantle.

Pleistocene volcaniclastic units from North-Eastern Sicily (Italy): new evidence for calc-alkaline explosive volcanism in the Southern Tyrrhenian Sea

AbstractA well-preserved volcaniclastic sequence crops out in Pleistocene marine sediments along the Tyrrhenian coastline of the Calabrian-Peloritani arc (Sicily, Italy), testifying the occurrence of Lower-Middle Pleistocene volcanic activity in Southern Tyrrhenian Sea. The presence of dominant highly vesicular and minor blocky glassy particles indicates that the volcanic clasts were originated by explosive events related to the ascent and violent emission of volatile-rich magmas accompanied by and/or alternated with hydromagmatic fragmentation due to magma-sea water interaction. Field investigations and sedimentological features of the studied volcaniclastic units suggest a deposition from sediment-water density flows. The chemical classification of the pumice clasts indicates prevalent rhyolitic and dacitic compositions with calc-alkaline to high-K calc-alkaline affinity. The geochemical features of immobile trace elements together with the presence of orthopyroxene are indicative of a provenance from an arc-type environment. The age (from 980-910 to 589 ka), the chemical composition and the evidence of subaerial explosive volcanic activity constrain the origin nature and temporal evolution of the arc-type volcanism in the Southern Tyrrhenian domain. Finally, the new information here provided contribute to a better understanding of the temporal geodynamic evolution of this sector of the Mediterranean domain.

In defense of magnetite-ilmenite thermometry in the Bishop Tuff and its implication for gradients in silicic magma reservoirs

Despite claims to the contrary, the compositions of magnetite and ilmenite in the Bishop Tuff correctly record the changing conditions of T and f O2 in the magma reservoir. In relatively reduced (ΔNNO 1, e.g., Fish Canyon Tuff, Pinatubo), and the d ( a TiO2)/ dT slope can be negative. Biotite, FeTi-oxides, liquid, and possibly plagioclase largely maintained equilibrium in the Bishop Tuff magma (unlike the pyroxenes, and cores of quartz, sanidine, and zircon) prior to and during a mixing event triggered by a deeper recharge, which, based on elemental diffusion profiles in minerals, took place at least several decades before eruption. Equilibrating phases and pumice compositions show evolving chemical variations that correlate well with mutually consistent temperatures based on the FeTi-oxides, sanidine-plagioclase, and Δ18O quartz-magnetite pairs. Early Bishop Tuff (EBT) temperatures are lower (700 to ~780 °C) than temperatures (780 to >820 °C) registered in Late Bishop Tuff (LBT), the latter defined here not strictly stratigraphically, but by the presence of orthopyroxene and reverse-zoned rims on quartz and sanidine. The claimed similarity in compositions, Zr-saturation temperatures and thermodynamically calculated temperatures (730–740 °C) between EBT and less evolved LBT reflect the use of glass inclusions in quartz cores in LBT that were inherited from the low-temperature rhyolitic part of the reservoir characteristic of the EBT. LBT temperatures as high as 820 °C, the preservation of orthopyroxene, and the presence of reverse-zoned minerals (quartz, sanidine, zircons) are consistent with magma recharge at the base of the zoned reservoir, heating the cooler rhyolitic melt, partly remelting cumulate mush, and introducing enough CO2 (0.4–1.4 wt%, mostly contained in the exsolved fluid phase) to significantly lower H2O-activity in the system.

UHP Metamorphism Documented in Ti-chondrodite- and Ti-clinohumite-bearing Serpentinized Ultramafic Rocks from Chinese Southwestern Tianshan

The southwestern Tianshan (China) metamorphic belt records high-pressure (HP) to ultrahigh-pressure (UHP) conditions corresponding to a cold oceanic subduction-zone setting. Serpentinites enclosing retrogressed eclogite and rodingite occur as lenses within metapelites in the UHP unit, which also hosts coesite-bearing eclogites. Based on the petrology and petrography of these serpentinites, five events are recognized: (1) formation of a wehrlite–harzburgite–dunite association in the mantle; (2) retrograde metamorphism and partial hydration during exhumation of the mantle rocks close to the seafloor; (3) oceanic metamorphism leading to the first serpentinization and rodingitization; (4) UHP metamorphism during subduction; (5) retrograde metamorphism during exhumation together with a second serpentinization. The peak metamorphic mineral assemblage of the serpentinized wehrlite comprises Ti-chondrodite + olivine + antigorite + chlorite + magnetite + brucite. A computed pseudosection for this serpentinized wehrlite shows that the Al content in antigorite is mostly sensititive to temperature but can also be used to constrain pressure. The average XAl = 0·204 ± 0·026 of antigorite (XAl = Al (a.p.f.u.)/8, where Al is in atoms per formula unit for a structural formula M48T34O85(OH)62, and M and T are octahedral and tetrahedral sites, respectively) included in Ti-chondrodite and average XAl = 0·203 ± 0·019 of antigorite in the matrix result in a well-constrained peak metamorphic temperature of 510–530°C. Peak pressures are less precisely constrained at 37 ± 7 kbar. The Tianshan serpentinites thus record UHP metamorphic conditions and represent the deepest subducted serpentinites discovered so far. The retrograde evolution occurs within the stability field of brucite + antigorite + olivine + chlorite and formation of Ti-clinohumite at the expense of Ti-chondrodite has been observed, suggesting isothermal decompression. The resulting P–T path is in excellent agreement with the metamorphic evolution of country rocks, indicating that the UHP unit in Tianshan was subducted and exhumed as a coherent block. To refine the metamorphic path of the ultramafic rocks, we have investigated the stability fields of Ti-chondrodite and Ti-clinohumite using piston-cylinder experiments. A total of 11 experiments were conducted at 25–55 kbar and 600–750°C in a F-free natural system. Combined with previous experiments and information from natural rocks we constructed a petrogenetic grid for the stability of Ti-chondrodite and Ti-clinohumite in F-free peridotite compositions. The formation of Ti-chondrodite in serpentinites requires a minimum pressure of about 26 kbar, whereas in Ti-rich systems it can form at considerably lower pressures. A key finding is that at UHP conditions, F-free Ti-chondrodite or Ti-clinohumite breaks down in the presence of orthopyroxene between 700 and 750°C, at temperatures that are significantly lower than those of the terminal breakdown reactions of these humite minerals. These breakdown reactions are an additional source of fluid during prograde subduction of serpentinites.

The Shangzhuang Fe-Ti oxide-bearing layered mafic intrusion, northeast of Beijing (North China): Implications for the mantle source of the giant Late Mesozoic magmatic event in the North China Craton

The Early Cretaceous Shangzhuang Fe-Ti oxide-bearing layered mafic intrusion in the Yanshan Belt northeast of Beijing is coeval with the giant Late Mesozoic igneous province in the eastern part of the North China Craton (NCC). This magmatic event was associated with lithospheric thinning and thus the igneous rocks have been used to characterize the nature of the Mesozoic mantle beneath the NCC. The Shangzhuang mafic pluton intruded a large granodioritic complex and crystallized at ~850–872°C at a depth of 13–14 km. It is composed, from the base upward, of troctolite, Fe-Ti oxide-bearing gabbronorite and gabbro. Rocks from this intrusion have low initial 87Sr/86Sr ratios (0.7053–0.7058), negative initial εNd values (−9.4 to −10.7), highly differentiated LREE and nearly flat HREE patterns indicative of an EMI-like mantle source unaffected by upper crustal contamination. The occurrence of Fe-Ti oxide ore layers, magnetite-ilmenite exsolution lamellae in hornblende and high TiO2 contents of the silicate rocks indicate that they formed from Fe- and Ti-rich ferrobasaltic magmas, which may have been generated by addition of magmas from a deeper mantle source. The presence of orthopyroxene, high-Mg ilmenite (up to 8.5 wt %), hornblende, biotite and high oxygen fugacities calculated from coexisting titanomagnetite-ilmenite pairs can be explained by derivation from an enriched EMI-type mantle modified by fluids from a subducted slab and mixed with asthenospheric or deeper-mantle materials in an extensional setting. Exposure of the complex occurred during large-scale uplift (at least 13 km) and exhumation of the Yanshan orogenic belt in the Early Cretaceous. Chemical metasomatism triggered by water and enhanced by heat from a deep magma source may have played an important role in removing the ancient cratonic root, generating partial melting of the lithospheric mantle and producing coeval magmatic activity in the Mesozoic eastern NCC.

The Kabul Block (Afghanistan), a segment of the Columbia Supercontinent, with a Neoproterozoic metamorphic overprint

We report field relationships, petrography and isotopic ages from two superposed basement units of the Kabul Block, the so called Lower Sherdarwaza and Upper Welayati formations. The Sherdarwaza Formation is represented mostly by migmatites and gneisses that are derived from pelitic and psammitic lithologies with lenses and layers of mafic and carbonate rocks. Several bodies of orthogneisses are also exposed in the Sherdarwaza Formation. The Upper Welayati Formation is characterized by micaschist, quartzite and amphibolites. SHRIMP U–Pb data on zircon from the orthogneiss in the Sherdarwaza Formation indicates a Neoarchean age of ca 2.5–2.8Ga for their magmatic crystallization. The rocks exhibit granulite facies conditions of 5–7kbar and 800°C that are documented by the presence of orthopyroxene and Ti-rich biotite in the orthogneiss and by olivine and phlogopite in some calc-silicate rocks at contact with marble. A Paleoproterozoic age of ca. 1.85–1.80Ga for this metamorphism was obtained using U-Pb SHRIMP dating on zircon and U-Th dating on monazite. Mineral textural relations also show a younger amphibolite facies metamorphism that is documented in both the Sherdarwaza and Welayati formations. This metamorphism occurred at relatively higher pressure conditions of up to 9kbar at ca. 650°C, compared to the granulite facies event. A Neoproterozoic age of ca 0.85–0.9Ga, for this metamorphism is confirmed by Ar-Ar data on biotite and white mica as well as by U-Th data on monazite. By combining the presented results on the metamorphic petrology, geochronology and geochemistry, we conclude that: (1) The Kabul basement is a fragment of an Archean block (craton); (2) the ca. 1.85–1.8 and 0.9–0.85Ga metamorphism marks an important orogenic events for the basement rocks of the Kabul Block which was stabilized during the early Precambrian; (3) the two metamorphic ages correlate well with global-scale orogenies related to the assembly of the Paleoproterozoic Columbia and Neoproterozoic Rodinia supercontinents; (4) based on metamorphic characteristics and ages, the Kabul basement rocks show an affinity to the Neoarchean rocks of the Tarim and/or South China cratons.

Refertilization process in the Patagonian subcontinental lithospheric mantle of Estancia Sol de Mayo (Argentina)

Anhydrous mantle xenoliths equilibrated at 1003–1040°C from Estancia Sol de Mayo (ESM, Central Patagonia, Argentina) and entrained in post-plateau alkaline lavas belonging to Meseta Lago Buenos Aires have been investigated aiming at reconstructing the depletion and enrichment processes that affected this portion of the Patagonia lithospheric mantle. Xenoliths are characterized by a coarse-grained protogranular texture and are devoid of evident modal metasomatism. They show two texturally different clinopyroxenes: protogranular (cpx1) and texturally related to spinel (cpx2). Three different types of orthopyroxenes are also recognized: large protogranular crystals with exsolution lamellae (opx1); small clean and undeformed grains without exsolution lamellae (opx2) and small grains arranged in a vein (opx3). Major element composition of clinopyroxenes and orthopyroxenes highlights two different trends characterized by i) a high Al2O3 content at almost constant mg# and ii) a slight increase in Al2O3 content with decreasing mg#. Clinopyroxenes are enriched in LREE and are characterized by prominent to slightly negative Nb, Zr and Ti anomalies. No geochemical differences are observed between cpx1 and cpx2, while a discrimination can be observed between opx1 and opx2 (LREE-depleted; prominent to slightly negative Ti and Zr anomalies) and opx3 (prominent positive Zr anomaly). Partial melting modeling using both major and trace elements indicates a melting degree between ~5% and ~13% (up to ~23% according to major element modeling) for lherzolites and between ~20% and ~30% for harzburgites (down to ~5% according to trace element modeling). La/Yb and Al2O3, as well as Sr and Al2O3 negative correlations in clinopyroxenes point to a refertilization event affecting this lithospheric mantle. The agent was most probably a transitional alkaline/subalkaline melt, as indicated by the presence of orthopyroxene in the vein and the similar geochemical features of ESM clinopyroxenes and those from Northern Patagonia pyroxenites which are derived from transitional alkaline/subalkaline lavas.

Orthopyroxene–omphacite- and garnet–omphacite-bearing magmatic assemblages, Breaksea Orthogneiss, New Zealand: Oxidation state controlled by high-P oxide fractionation

Abstract The Breaksea Orthogneiss comprises a monzodioritic host partially recrystallised to omphacite–garnet–plagioclase–rutile granulite at 850 °C and 1.8 GPa, with metre to decametre-scale, cognate inclusions ranging from ultramafic through gabbroic to monzodioritic composition. Coarsely layered garnetite and diopsidic clinopyroxenite cumulate preserves igneous textures, whereas garnet–omphacite cumulate shows a partial metamorphic overprint to eclogite. Garnet and omphacite in undeformed to weakly deformed rocks have similar major and rare earth element characteristics reflecting their common igneous origin, pointing to a lack of metamorphic recrystallisation. Inclusions of omphacite–orthopyroxene–plagioclase–ulvospinel orthogneiss have whole-rock compositions almost identical to the host monzodiorite. Reaction zones developed along contacts between the orthopyroxene-bearing inclusions and host contain metamorphic garnet that is microstructurally and chemically distinct from igneous garnet. The presence of orthopyroxene is interpreted to reflect redox distinctions: early, oxidised magma crystallised orthopyroxene and ulvospinel at high- P (~ 1.8 GPa), garnet crystallisation having been suppressed. Progressive fractionation of oxygen into early formed phases (ulvospinel, magnetite, orthopyroxene, ferric iron-rich omphacite and rare garnet) drove the magma to less oxidising conditions, resulting in the more common igneous assemblage of garnet, omphacite and rutile in the main host.

Petrographic and structural analysis of the Precambrian rocks in the Zalbi Sector, northwest Léré, Chad

The northwestern part of southwest Chad is one of the best regions to find exposed Precambrian terrains and their Phanerozoic cover. In this area, it is easy to establish the lithostratigraphic and structural relationships between sedimentary and granitic rocks, greenstone belts and dolerite dykes. Based on petrographic and structural studies, we distinguished the greenstone belts that belong to the Neoproterozoic Zalbi Series. The Neoproterozoic greenstone belts that are intruded by three generations of Pan-African granitoids (gabbro-diorite, biotite monzogranite and leucogranite) host the mineral resources in the region. The Precambrian rocks are covered by Cretaceous shale, sandstone and microconglomerate in the Kebbi Basin, and intruded by dolerite dykes. These late dolerite dykes, have a continental tholeiitic composition, which is supported by the presence of orthopyroxene in the mineralogical assemblage. Structurally, two major deformation events are present in the Zalbi Series. The earliest D1 event is the most intensive and is associated with vertical to subvertical north-trending S1 foliation, a vertical L1 lineation and P1 folds; the metamorphism associated with this phase is equilibrated in greenschist facies conditions. The second D2 event is discrete and is characterised by large-amplitude folds and fractures.

Composition of the Rheasilvia basin, a window into Vesta's interior

The estimated excavation depth of the huge Rheasilvia impact basin is nearly twice the likely thickness of the Vestan basaltic crust, so the mantle should be exposed. Spectral mapping by the Dawn spacecraft reveals orthopyroxene‐rich materials, similar to diogenite meteorites, in the deepest parts of the basin and within its walls. Significant amounts of olivine are predicted for the mantles of bulk‐chondritic bodies like Vesta, and its occurrence is demonstrated by some diogenites that are harzburgite and dunite. However, olivine has so far escaped detection by Dawn's instruments. Spectral detection of olivine in the presence of orthopyroxene is difficult in samples with <25% olivine, and olivine in Rheasilvia might have been diluted during impact mixing or covered by the collapse of basin walls. The distribution of diogenite inferred from its exposures in and around Rheasilvia provides a geologic context for the formation of these meteorites, but does not clearly distinguish between a magmatic cumulate versus partial melting restite origin for diogenites. The former is favored by geochemical arguments, and crystallization in either a magma ocean or multiple plutons emplaced near the crust‐mantle boundary is permitted by Dawn observations.

Late Paleozoic mafic–ultramafic intrusions in southern Central Asian Orogenic Belt (NW China): Insight into magmatic Ni–Cu sulfide mineralization in orogenic setting

The mafic–ultramafic intrusions hosting magmatic Ni–Cu sulfide ore deposits, usually present within the Precambrian cratons and on their margin, have been extensively studied, whereas those occurring within the Phanerozoic orogenic belts have received little attention. The southern margin of the Central Asian Orogenic Belt is a region of late Paleozoic mafic–ultramafic intrusions emplaced during post-orogenic stage. In order to recognize the mineralization features, this study compares barren or weakly mineralized mafic–ultramafic intrusions in the Beishan with those hosting Ni–Cu sulfide ore deposit in the Altay and Eastern Tianshan in petrology, mineralogy and geochemistry aspects. Several mineralization features can be recognized with respects to petrology, mineralogy and geochemistry from comprehensive comparisons. Among the mineralization indicators, we highlight strong alteration accompanying sulfide emplacement, post-erosion exposure of magma conduit, abundant primary hydrous minerals and potential transportation mechanism of ore-forming elements. Other indicators are the presence of orthopyroxene and pyroxenite for potential sulfide formation through crystal fractionation, restricted Fo and Ni ranges in olivines for sulfide segregation, and some other special geochemical characteristics related to crustal contamination. The variation in mineralization could be ascribed to different degrees of partial melting, crustal assimilation and subduction-related modification of a mantle source. We suggest that the Ni–Cu sulfide mineralization of mafic–ultramafic intrusions within orogenic belts is remarkably different from that of cratonic and continental marginal settings in respects of petrogenesis and mineralizing mechanism. Magmatic conduit-type Ni–Cu sulfide ore deposit is a target for future prospecting and exploration for mafic–ultramafic intrusions in the orogenic belts.

Constraints from melt inclusions and their host olivines on the petrogenesis of Oligocene-Early Miocene Xindian basalts, Chifeng area, North China Craton

The geochemical characteristics of melt inclusions and their host olivines provide important information on the processes that create magmas and the nature of their mantle and crustal source regions. We report chemical compositions of melt inclusions, their host olivines and bulk rocks of Xindian basalts in Chifeng area, North China Craton. Compositions of both bulk rocks and melt inclusions are tholeiitic. Based on petrographic observations and compositional variation of melt inclusions, the crystallizing sequence of Xindian basalts is as follows: olivine (at MgO > ~5.5 wt%), plagioclase (beginning at MgO = ~5.5 wt%), clinopyroxene and ilmenite (at MgO 6 wt%), indicate that Xindian basalts are possibly derived from a pyroxenite source rather than a peridotite source. In the CS-MS-A diagram, all the high MgO melt inclusions (MgO > 6.0 wt%) project in the field between garnet + clinopyroxene + liquid and garnet + clinopyroxene + orthopyroxene + liquid near 3.0 GPa, further suggesting that residual minerals are mainly garnet and clinopyroxene, with possible presence of orthopyroxene, but without olivine. Modeling calculations using MELTS show that the water content of Xindian basalts is 0.3–0.7 wt% at MgO = 8.13 wt%. Using 20–25 % of partial melting estimated by moderately incompatible element ratios, the water content in the source of Xindian basalts is inferred to be ≥450 ppm, much higher than 6–85 ppm in dry lithospheric mantle. The melting depth is inferred to be ~3.0 GPa, much deeper than that of tholeiitic lavas (<2.0 GPa), assuming a peridotite source with a normal mantle potential temperature. Such melting depth is virtually equal to the thickness of lithosphere beneath Chifeng area (~100 km), suggesting that Xindian basalts are derived from the asthenospheric mantle, if the lithospheric lid effect model is assumed.

Interaction of adakitic melt-peridotite: Implications for the high-Mg# signature of Mesozoic adakitic rocks in the eastern North China Craton

Abundant dunite and harzbugite xenoliths are preserved in Early Cretaceous high-Mg# [63–67, where Mg# = molar 100 × Mg/(Mg + Fe tot)] diorite intrusions from western Shandong in the North China Craton (NCC). Dunite and some harzburgite xenoliths typically preserve areas of orthopyroxenite (sometimes accompanied by phlogopite) either as veins or as zones surrounding chromite grains. Harzburgite is chiefly composed of olivine, orthopyroxene, minor clinopyroxene and chromian-spinel. High Mg#'s (averaging 91.4) and depletions in Al 2O 3 and CaO (averaging 0.52 wt.% and 0.29 wt.%, respectively) in harzburgite and dunite xenoliths suggest that they are residues formed by large degrees of polybaric melting. However, olivines and orthopyroxenes from dunite xenoliths spatially associated with orthopyroxenite display lower Mg#'s (i.e., 82–87 and 83–89, respectively), suggesting that an adakitic melt–peridotite reaction has taken place. This is consistent with the production of veined orthopyroxene or orthopyroxene + phlogopite in dunite and some harzburgite xenoliths in response to the introduction of adakitic melt into the previously depleted lithospheric mantle (i.e., harzburgite and dunite xenoliths). The presence of orthopyroxene in veins or as a zones surrounding chromite in peridotite xenoliths is thought to be representative of adakitic melt metasomatism. The dunite and harzbugite xenoliths are relatively rich in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), poor in heavy rare earth elements (HREEs) and high field strength elements (HFSEs), and lack Eu anomalies on chondrite normalized trace element diagrams. The initial 87Sr/ 86Sr ratios and εNd( t) values for the xenoliths range from 0.7058 to 0.7212 and + 0.18 to − 19.59, respectively. Taken together, these features, combined with the strong depletion in HFSE and the existence of Archean inherited zircons in the host rocks, suggest that the adakitic melt was derived from the partial melting of early Mesozoic delaminated lower continental crust. The interaction of the adakitic melt with peridotite is responsible for the high-Mg# character of the early Cretaceous diorites in western Shandong.

Entrained Macrocryst Minerals as a Key to the Source Region of Olivine Nephelinites: Humberg, Kaiserstuhl, Germany

Olivine nephelinites commonly contain macrocrysts of olivine and clinopyroxene. Some of these macrocrysts might represent fragments of the source region of the host magma transported to the Earth’s surface. If this hypothesis is correct these fragments can be used to characterize the composition of the source region and to put constraints on the magma generation process. In this study, we investigate the origin of macrocrysts and mineral aggregates from an olivine nephelinite from the Kaiserstuhl, Germany. We focus on clinopyroxenes (Cpx), which can be divided into three groups. Cpx I is relict Cpx from aggregates with deformed olivine that is depleted in Ca and characterized by strong light rare earth element (LREE) fractionation, low Ti/Eu and negative high field strength element (HFSE) anomalies. Its geochemical signature is consistent with formation by carbonatite metasomatism and with equilibration in the presence of orthopyroxene. Cpx II is Ca-rich Cpx, forming both aggregates with deformed olivine and individual macrocrysts. The LREE, as for Cpx I, are strongly fractionated. Convex REE patterns may be present.The depletion in HFSE is less pronounced. Cpx III is oscillatory zoned Cpx phenocrysts showing enrichment in Ca, convex REE patterns and no HFSE anomalies.The transition in the trace element abundances between the Cpx of the three groups is gradual. However, Cpx I and II did not crystallize from the host magma, as demonstrated by the presence of kink-bands and undulose extinction in the associated olivine and by the composition of alkali aluminosilicate glass inclusions in Cpx II. Based on the Cpx relationships, we interpret the studied suite of macrocrysts and mineral aggregates as a mixture of disintegrated fragments of the source region of the host olivine nephelinite.The process of melt generation was multi-stage. A primary carbonatite melt ascending from deeper levels in the mantle, probably from the dolomite^garnet peridotite stability field, reacted with mantle peridotite along the solidus ledge in the system lherzolite^CO2 (520^22 kbar) and started to crystallize carbonate minerals. Because of its low solidus temperature, the resulting carbonate-wehrlite assemblage melted incongruently with the formation of additional clinopyroxene. The carbonatite melt evolved during crystallization of carbonate minerals and concomitant incongruent melting of the carbonate-wehrlite, accompanied by the segregation of incipient alkali aluminosilicate melts. As a consequence of fast reaction rates in the presence of a carbonatite melt, this process probably took place under disequilibrium conditions. Further melting of the assemblage wehrliteþ alkali aluminosilicate melt led to the generation of the olivine nephelinite magma. It entrained fragments of the wehrlite and brought them to the surface.

Microstructures of olivine-plagioclase corona in meta-ultramafic rocks from Sefuri Mountains, NW Kyushu, Japan

Two types of reaction rims occur between olivine and plagioclase in ultramafic rocks from the Sefuri Mountains, NW Kyushu, Japan, which were metamorphosed under granulite-facies conditions. One occurs as a thin film of orthopyroxene along the boundary between olivine and plagioclase (orthopyroxene zone). The other is composed of two zones: symplectite of calcic amphibole and spinel on the plagioclase-side (symplectite zone) and calcic amphibole with sporadic orthopyroxene on the olivine-side (tremolite zone). In the tremolite zone, calcic amphibole shows a systematic decrease in Al content and increase in Mg/(Fe +Mg) with decreasing distance from olivine. Local equilibria maintained during the diffusion-controlled corona-forming reaction enable us to apply equilibrium thermodynamics to calcic amphibole and adjacent orthopyroxene. An integrated formulation of the Gibbs method for an Fe–Mg exchange reaction constrains the equilibrium temperature recorded in the tremolite zone to be 600–710 °C. It is significantly lower than the temperature of the granulite-facies metamorphism (800–900 °C) estimated using conventional geothermobarometry. Except for H 2O, the association of calcic amphibole and spinel in the symplectite zone is chemically equivalent to the association of olivine, plagioclase and orthopyroxene that was stable before the corona formation. This suggests that the following orthopyroxene-consuming reaction describes the paragenetic change taking place between 800–900 °C and 600–710 °C, olivine + plagioclase + orthopyroxene + aqueous fluid = calcic amphibole + spinel. In contrast, the overall reaction inferred from microstructures produces orthopyroxene as well as calcic amphibole and spinel at the expense of olivine and plagioclase. This reaction requires removal mainly of MgO that is also responsible for destabilizing the local association of olivine and plagioclase. These features suggest that the presence of orthopyroxene as a product in the corona is not always indicative of an orthopyroxene-producing reaction being responsible for the change of paragenetic relation. Microstructural features should be carefully applied to infer the reaction describing paragenetic change by which we argue the P– T path of the rocks.