Geochemical characteristics of back-arc basin lower crust and upper mantle at final spreading stage of Shikoku Basin: an example of Mado Megamullion

This paper explores the evolutional process of back-arc basin (BAB) magma system at final spreading stage of extinct BAB, Shikoku Basin (Philippine Sea) and assesses its tectonic evolution using a newly discovered oceanic core complex, the Mado Megamullion. Bulk and in-situ chemical compositions together with in-situ Pb isotope composition of dolerite, oxide gabbro, gabbro, olivine gabbro, dunite, and peridotite are presented. Compositional ranges and trends of the igneous and peridotitic rocks from the Mado Megamullion are similar to those from the slow- to ultraslow-spreading mid-ocean ridges (MOR). Since the timing of the Mado Megamullion exhumation corresponds to the very end of the Shikoku Basin opening, the magma supply was subdued and highly episodic, leading to extreme magma differentiation to form ferrobasaltic, hydrous magmas. In-situ Pb isotope composition of magmatic brown amphibole in the oxide gabbro is identical to that of depleted source mantle for mid-ocean ridge basalt (MORB). In the context of hydrous BAB magma genesis, the magmatic water was derived solely from the MORB source mantle. The distance from the back-arc spreading center to the arc front increased away through maturing of the Shikoku Basin to cause MORB-like magmatism. After the exhumation of Mado Megamullion along detachment faults, dolerite dikes intruded as a post-spreading magmatism. The final magmatism along with post-spreading Kinan Seamount Chain volcanism were introduced around the extinct back-arc spreading center after the opening of Shikoku Basin by residual mantle upwelling.

Distribution of sulfides and PGE minerals in the picritic and taxitic gabbro-dolerites of the Norilsk 1 intrusion

ABSTRACT Disseminated ores in the Norilsk 1 intrusion were studied to elucidate the typomorphic features of sulfides and noble metal mineralizations in picritic and taxitic (or lower olivine) gabbro-dolerites. The former are characterized by the development of a low-sulfur sulfide association (troilite, Fe-rich pentlandite, talnakhite, chalcocite, native copper) while the latter exhibits a high-sulfur association (monoclinic pyrrhotite, Ni-rich pentlandite, pyrite, heazlewoodite). The contact between these types of rocks is geochemically and mineralogically contrasting. The mineralogical and geochemical zoning directed from the roof to the base of each layer is expressed by an increase in the Cu content (and chalcopyrite) in ores, an increase in the concentration of Ni in pentlandite and S in pyrrhotite in line with a decrease of the crystallization temperature, and an increase in sulfur fugacity in the same direction. Zoning of Pd(Pt) mineralization in picritic and taxitic (olivine) gabbro-dolerites is uniform and characterized by the distribution of Pd-Sn compounds in the upper parts (together with Pd-Pb minerals in picritic rocks) and Pd-As compounds in the lower parts of the sections according to a drop in temperature. Such reverse zoning contradicts the typical mechanism of differentiation by fractional crystallization, and possibly suggests a fluid-magmatic nature. Mineralogical and geochemical features in platinum group element-Cu-Ni-bearing rocks are consistent with the idea that different stages of multi-pulse intrusions of mafic-ultramafic magmas with different compositions formed the picritic and taxitic gabbro-dolerites of the Norilsk region.

Non-Dilatant Brittle Deformation and Strength Reduction of Olivine Gabbro due to Hydration

To investigate the influence of hydration on brittle deformation of oceanic crustal rocks, we conducted triaxial deformation experiments on gabbroic rocks with various degrees of hydration at a confining pressure of 20 MPa and room temperature, measuring elastic wave velocity. Hydrated olivine gabbros reached a maximum differential stress of 225–350 MPa, which was considerably less than those recorded for gabbros (~450 MPa), but comparable to those for serpentinized ultramafic rocks (250–300 MPa). Elastic wave velocities of hydrated olivine gabbros did not show a marked decrease even prior to failure. This indicated that the deformation of hydrated olivine gabbro is not associated with the opening of the stress-induced cracks that are responsible for dilatancy. Microstructural observations of the samples recovered after deformation showed crack damage to be highly localized to fault zones with no trace of stress-induced crack opening, consistent with the absence of dilatancy. These data suggest that strain localization of hydrated olivine gabbro can be caused by the development of shear cracks in hydrous minerals such as serpentine and chlorite, even when they are present in only small amounts. Our results suggest that the brittle behavior of the oceanic crust may considerably change due to limited hydration.

Petrogenesis and Tectonic Significance of the ~276 Ma Baixintan Ni-Cu Ore-Bearing Mafic-Ultramafic Intrusion in the Eastern Tianshan Orogenic Belt, NW China

The Baixintan mafic-ultramafic intrusion in the Dananhu-Tousuquan arc of the Eastern Tianshan orogenic belt is composed of lherzolite, olivine gabbro, and gabbro. Olivine gabbros contain zircon grains with a U-Pb age of 276.8 ± 1.1 Ma, similar to the ages of other Early Permian Ni-Cu ore-bearing intrusions in the region. The alkaline-silica diagrams, AFM diagram, together with the Ni/Cu-Pd/Ir diagram, indicate that the parental magmas for the Baixintan intrusion were likely high-Mg tholeiitic basaltic in composition. The Cu/Pd ratios, the relatively depleted PGEs and the correlations between them demonstrate that the parental magmas had already experienced sulfide segregation. The lower CaO content in pyroxenites compared with the Duke Island Alaskan-type intrusion and the composition of spinels imply that Baixintan is not an Alaskan-type intrusion. By comparing the Baixintan intrusion with other specific mafic-ultramafic intrusions, this paper considers that the mantle source of the Baixintan intrusion is metasomatized by subduction slab-derived fluids’ components, which gives rise to the negative anomalies of Nb, Ti, and Ta elements. Nb/Yb-Th/Yb, Nb/Yb-TiO2/Yb, and ThN-NbN plots show that the Baixintan intrusion was emplaced in a back-arc spreading environment and may be related to a mantle plume.

Oldest (ca. 518 Ma) Mariana type oceanic subduction initiation ophiolite: constraining initiation of modern oceanic plate tectonic regime

Initiation of stable Mariana type one-sided oceanic subduction zones requires rheologically strong oceanic lithosphere, which developed through secular cooling of Earth mantle. This enabled the development of focused high stress zones resulting in narrow weak zones of convergence with resultant oceanic subduction leading to mantle hydration and arc magmatism. Based on detailed study and identification of the oldest (518 Ma) Mariana type oceanic subduction initiation ophiolite (Munabulake ophiolite) on Earth from northern Tibet, along with compilation of oceanic subduction initiation ophiolites through Earth history, we argue for the initiation of modern plate tectonic regime by at least the early Cambrian. The mantle and crust members of the Munabulake ophiolite preserve a complete ophiolite stratigraphy. Blocks of layered marble and siliceous rocks interlayered with meta-basalt indicate a marine environment. Zircons from an olivine gabbro sample yield a concordant age of 518 Ma, along with mantle derived low δ18O (2.69‰ – 5.7‰) and high εHf(t) (11.1–13.6) values. The zircons also have varied H2O contents ranging from 109–1339 ppm with peaks at 260 and 520 ppm, indicative of hydration of mantle derived magma. The highly depleted peridotites display U–shaped REE patterns and varied Zr/Hf ratios, whereas spinel and olivine compositions within the peridotites indicate that they are residues of various degrees of melt extraction and evolved from abyssal to fore-arc peridotites. The crustal members of the ophiolite are mostly tholeiitic, display flat REE patterns and lower HFSEs, comparable to transitional lavas associated with Mariana subduction initiation ophiolite. Some rocks from the crustal section of the ophiolite display NMORB-like compositions but are also characterized by depletion in HSFEs. Therefore, the Munabulake ophiolite displays a chemical duality and progressively evolved from MORB (mid-ocean ridge basalt) to SSZ (supra-subduction zone) compositions, consistent with observations from zircon Hf-O isotopes and H2O contents. Furthermore, the ophiolite was formed during subduction initiation of the Proto-Tethys Ocean at the northern Gondwana margin, and coincided with an inferred slab roll back event in the southern Gondwana margin at ca. 530 − 520 Ma, indicative of a time of global tectonic re-organization. The early Cambrian Munabulake ophiolite indicates comparable slab strength and conditions to those that characterize modern plate tectonics. Such a tectonic regime coincided with final Gondwana assembly, and was associated with ca. 530 − 520 Ma global tectonic re-organization.

Petrology and Nd-Sr isotopic composition of alkaline lamprophyres from the Early to Late Cretaceous Mundwara alkaline complex, NW India: Evidence of crystal fractionation, accumulation and corrosion in a complex magma chamber plumbing system

The Early to Late Cretaceous Mundwara alkaline complex (comprising the Musala, Mer and Toa plugs) displays a broad spectrum of alkaline rocks closely associated in space and time with the Deccan Large Igneous Province (DLIP) in NW India. Petrology and Nd-Sr isotopic data on two youngest and altogether compositionally different lamprophyre dykes of the Mundwara alkaline complex are presented in this paper to understand their petrogenesis and also to constrain the magmatic processes responsible for generation of the rock spectrum in the complex (pyroxenite, picrite ankaramite, carbonatite, shonkinite, olivine gabbro, feldspathoidal and foid-free syenite). The two lamprophyre dykes occurring in the Mer and the Musala hills are referred to as basaltic camptonite I and camptonite II, respectively. The basaltic camptonite-I is highly porphyritic and contains olivine, clinopyroxene and magnetite macrocrysts embedded within the groundmass of microphenocyrsts composed of clinopyroxene, phlogopite, magnetite and feldspar. Whereas camptonite-II, with more or less similar texture, contains amphibole, biotite, magnetite and clinopyroxene within the microphenocrystic groundmass of amphibole, biotite, apatite and feldspar. Pyroxenes are chemically zoned and display corrosion of the cores revealing that they are antecrysts developed during early stages of magma evolution and later on inherited by more evolved magmas. Mineral chemistry and trace element composition of the lamprophyres reveal that fractional crystallisation was a dominant process. Early segregation of olivine + Cr-rich clinopyroxene + Cr-spinel from a primary hydrous alkali basalt within a magmatic plumbing system is inferred which led to the generation of basaltic camptonitic magma (M1) forming the Mer hill lamprophyre. Subsequently, progressive fractionation of pyroxene and Fe-Ti oxides from the basaltic camptonitic (M1) magma generated camptonitic (M2) magma forming the Musala hill lamprophyre. Both lamprophyre dykes on the Sr-Nd isotopic array reflect plume type asthenospheric derivation which largely corresponds to the Réunion plume and other alkaline rocks of the Deccan LIP. Our study brings out a complex sequence of processes such as crystal fractionation, accumulation and corrosion in the magmatic plumbing system involved in the generation of the Mundwara alkaline complex.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5277073

New Geochemical and Mineralogical Data on Rocks and Ores of the NE Flank of the Oktyabr’skoe Deposit (Norilsk Area) and a View on Their Origin

The Oktyabr’skoe deposit in the Norilsk ore district is the largest platinum-copper-nickel deposit in the world. It contains a huge main orebody (2.4 km3) of massive sulfide ores and some smaller sulfide bodies. Almost all publications on this deposit are devoted to the main orebody. However, to solve the problems of the deposit genesis, it is necessary to take into account the geological structure of the entire area and the composition of all orebodies. For the first time we present data on the inner structure, geochemical and mineralogical characteristics of the intrusive body, and related the disseminated and massive sulfide ores (orebody number C-5) in the northeastern flank of the deposit. The intrusion studied in the core of the borehole RG-2 consists of several horizons including the following rock varieties (from bottom to top): olivine gabbro-dolerites, taxitic gabbro-dolerites, picritic gabbro-dolerites, troctolites, olivine-free gabbro-dolerites, ferrogabbro, and leucogabbro. The intrusion shows a strong differentiated inner structure where high-Mg rocks (up to 25 wt.% MgO troctolites and picritic gabbro-dolerites) in the bottom are associated with low-Mg rocks (6–7 wt.%, gabbro-dolerites, leucogabbro, ferrogabbro) without intermediate differentiated members (8–12 wt.% MgO olivine gabbro-dolerites). Rocks are characterized by low TiO2 content (≤1 wt.%). Taxitic gabbro-dolerites, picritic gabbro-dolerites, and troctolites contain disseminated sulfide chalcopyrite-pyrrhotite mineralization (32 m thick). Cu and Ni concentrations reach up 0.74 and 0.77 wt.%, respectively. Massive ores (27 m) occur in the bottom part of the intrusion. The ores consist of pentlandite, chalcopyrite and pyrrhotite, the latter mineral dominates. Their chemical composition is stable: Cu/Ni ~1, Pd/Pt varies from 5 to 6. The C-5 orebody is similar to the C-3 orebody in terms of mineral and chemical compositions, and differ from the nearby the C-4 orebody which is characterized by a Cu/Ni ratio changing from 5 to 8. On the basis of geochemical and mineralogical data, it is assumed that orebodies C-3 and C-5 are associated with one intrusion, while the orebody number C-4 is related to another intrusive body. Thus, the deposit has a more complex structure and includes several more intrusions than is usually considered.

Mineral chemistry and thermo-barometry of intrusives from northern side of Shahroud river around Zardkuh in Guilan province (northern Iran)

The study area is located in northern Iran and is part of the Alborz mountain range. The exposed rock units in the study area are mostly Eocene volcanic rocks with some Oligomiocene intrusive masses included and locally Paleocene, Jurassic, Permian and Carboniferous sedimentary outcrops. The intrusive rocks of the study area are mostly gabbro and olivine gabbro in terms of mineralogical composition, but intermediate types such as porphyry monzonite are also rarely seen. Based on the results of microprobe analysis, pyroxenes have augite and diopside compositions, orthopyroxenes are hypersthenes, biotites have magnesium biotite and phlogopite compositions, olivines have hyalosiderite compositions and amphiboles are pargasite and ferropargasite compositionally. The results of temperature and pressure evaluation indicate a temperature range of 650 to 1200 ° C and pressure range >2-5 KB. Based on the chemistry of clinopyroxene mineral of the mentioned intrusions, magma that made these intrusions is of the sub-alkaline type and is formed in a volcanic arc tectonic setting.

Petrography and geochemistry of intrusive magmas in Varmaqan – Sardare ghobadi in the west of Iran

The study area is a quadrilateral of 155 km2 between eastern longitude 47˚ and 40 ′ to 47˚ and 52 ′ and northern latitudes 35˚ and 00 ′ to 35˚ and 04 ′ that is located in west of Iran, north of Sonqor city and between Varmaqan and Sardare Ghobadi villages of Kermanshah province. In this range, the intrusive rocks are alkaline granite, granite, granodiorite, tonalite, quartz alkaline syenite, quartz monzonite, quartz monzodiorite, quartz diorite, alkaline syenite, monzonite, diorite, gabbro diorite, gabbro, and olivine gabbro as they were injected in the iron ores of cretaceous which has resulted in contact metamorphism and created hornfels at the site of contact. After comprehensive sampling of all required igneous rocks and according to the thesis objectives, thin sections were prepared and after petrography and some samples were selected for geochemical experiments. XRF analysis, ICP and alkaline fusion were performed on some samples. According to geochemical and petrological studies, the magmas forming these intrusive igneous rocks are from one region and because of magmatic differentiation or fractional crystallization, they from basaltic to acidic terms. Samples of this quadrilateral have a meta-alumina nature and granitoids are in the range of arc islands granites, continental arc granitoids and continental collision granitoids. The mineralogical and chemical composition of the acidic rocks in the area show that the granites in this study are type I.

Mineral chemistry and geothermobarometry of gabbroic rocks from the Gysel area, Alborz mountains, north Iran

The gabbroic rocks in the Gysel area of the Central Alborz Mountains in north Iran are intruded into the Eocene Volcano-sedimentary units. The main gabbroic rocks varieties include gabbro porphyry, olivine gabbro, olivine dolerite and olivine monzo-gabbro. The main minerals phases in the rocks are plagioclase and pyroxene and the chief textures are sub-hedral granular, trachytoidic, porphyritic, intergranular and poikilitic. Electron microprobe analyses on minerals in the rock samples shows that plagioclase composition ranges from labradorite to bytonite, with oscillatory and normal chemical zonings. Clinopyroxene is augite and orthopyroxene is hypersthene to ferro-hypersthene. Thermometry calculations indicate temperatures of 650˚C to 750˚C for plagioclase crystallization and 950˚C to 1130˚C for pyroxene crystallization. Clinopyroxene chemistry reveals sub-alkaline and calc-alkaline nature for the parental magma emplaced in a volcanic arc setting.