Gabbroic Intrusions Research Articles

Geochemical, mineralogical and petrogenetic studies of the calc-alkaline younger gabbros, Sinai of Egypt with a special emphasis on the role of hydrous fluids

Neoproterozoic younger gabbroic intrusions cropping out in Wadi Watir, Gabal Minadir and Gabal Imleih, southern Sinai, Egypt were investigated. The Imleih gabbros are characterized by fresh mineral constituents while Watir and Minadir gabbros are slightly altered. They are layered and consist of pyroxenites, olivine gabbros, pyroxene gabbros, pyroxene-hornblende gabbros, hornblende gabbros and anorthosites. The clinopyroxenes are of subalkaline affinities and range in composition from augite to diopside, while the orthpyroxenes are clino-enstatite. The primary amphiboles are of mantle origin and represented mainly by pargasite and edenite whereas actinolite and magnesiohornblende represent the secondary one. The investigated gabbros were originated from high Al, hydrous basaltic magmas that were evolved by fractional crystallization at the base of magma chamber to produce calc-alkaline magmas that were emplaced in a transitional stage between volcanic arc and within plate settings. They were crystallized at temperature of ∼800–1200 °C; pressure ∼4.8–6 Kb (∼15–20 km) and high oxygen fugacity. The gabbroic magmas have undergone significant crustal contamination and differentiation which affect on the geochemical characteristics of them. The enrichment of LILE and LREE relative to HFSE and HREE, respectively reflects the effect of slab-derived fluids and metasomatism of their enriched mantle source.

Deep lithospheric structure beneath the Polish part of the East European Craton as a result of magnetotelluric surveys

A fundamental tectonic boundary between the Precambrian East European Craton (EEC) and the younger Phanerozoic mountain belts of Europe runs through Poland. Whereas the transition zone between both tectonical units was intensely investigated by means of electromagnetic methods in the past, the image of electric conductivity of the EEC itself has not been detailed so far. In the present study the lithospheric structure beneath the northeast of Poland is investigated by means of three parallel magnetotelluric profiles. As a result the Polish part of the craton does not appear as homogeneously highresistive as repeatedly reported. Beneath a shallow (from 2 to 4 km) sedimentary layer a less resistive structure (several hundred Ωm) under all three profiles in mid-crustal to upper mantle depth becomes apparent in the high-resistive background. It spatially coincides with a magmatic body of the Upper Proterozoic age called Śniardwy lake gabbro intrusion.

Multi-stage Reactive Formation of Troctolites in Slow-spreading Oceanic Lithosphere (Erro–Tobbio, Italy): a Combined Field and Petrochemical Study

Abstract Many recent studies have investigated the replacive formation of troctolites from mantle protoliths and the compositional evolution of the percolating melt during melt–rock interaction processes. However, strong structural and geochemical constraints for a replacive origin have not yet been established. The Erro–Tobbio impregnated mantle peridotites are primarily associated with a hectometre-size troctolitic body and crosscutting gabbroic dykes, providing a good field control on melt–rock interaction processes and subsequent magmatic intrusions. The troctolitic body exhibits high inner complexity, with a host troctolite (Troctolite A) crosscut by a second generation of troctolitic metre-size pseudo-tabular bodies (Troctolite B). The host Troctolite A is characterized by two different textural types of olivine, corroded deformed millimetre- to centimetre-size olivine and fine-grained rounded undeformed olivine, both embedded in interstitial to poikilitic plagioclase and clinopyroxene. Troctolite A shows melt–rock reaction microstructures indicative of replacive formation after percolation and impregnation of mantle dunites by a reactive melt. The evolution of the texture and crystallographic preferred orientation (CPO) of olivine are correlated and depend on the melt/rock ratio involved in the impregnation process. A low melt/rock ratio allows the preservation of the protolith structure, whereas a high melt/rock ratio leads to the disaggregation of the pre-existing matrix. The mineral compositions in Troctolite A define reactive trends, indicative of the buffering of the melt composition by assimilation of olivine during impregnation. The magmatic Troctolite B bodies are intruded within the pre-existing Troctolite A and are characterized by extreme textural variations of olivine, from decimetre-size dendritic to fine-grained euhedral crystals embedded in poikilitic plagioclase. This textural variability is the result of olivine assimilation during melt–rock reaction and the correlated increase in the degree of undercooling of the percolating melt. In the late gabbroic intrusions, mineral compositions are consistent with the fractional crystallization of melts modified after the reactive crystallization of Troctolites A and B. The Erro–Tobbio troctolitic body has a multi-stage origin, marked by the transition from reactive to fractional crystallization and diffuse to focused melt percolation and intrusion, related to progressive exhumation. During the formation of the troctolitic body, the melt composition was modified and controlled by assimilation and concomitant crystallization reactions occurring at low melt supply. Similar processes have been described in ultraslow-spreading oceanic settings characterized by scarce magmatic activity.

Structural architecture of the Western Alpine Ophiolites, and the Jurassic seafloor spreading tectonics of the Alpine Tethys

We present a regional synthesis of the structural architecture and tectonic evolution of the Western Alpine Ophiolites (WAO), exposed in NW Italy. The WAO represent the remnants of Alpine Tethys (Ligurian–Piedmont Ocean) that opened between Europe and Adria, and developed in four stages from the Middle Jurassic to Early Cretaceous. Emplacement of gabbroic intrusions into the extending lithospheric mantle of Europe–Adria marked the main magmatic event (Stage 1). Coalescent shear zones in the fossil upper mantle form lithospheric-scale detachment faults, which led to the exhumation of upper mantle peridotites and gabbros on the seafloor, and extensive serpentinization (Stage 2). Detachment faults, and serpentinized peridotites–gabbros in their footwalls, represent preserved fossil oceanic core complexes within the WAO. Emplacement of ophiolitic breccias and basaltic lava flows marked the syn-extensional phase (Stage 3). Radiolarian chert and limestone were deposited unconformably on this syn-extensional volcanic–sedimentary sequence, marking the post-extensional phase (Stage 4). Magmatic ages of gabbroic intrusions and mafic–felsic dykes, and depositional ages of post-extensional sequences in the WAO constrain the timing of the opening of the Ligurian–Piedmont Ocean to the Middle Jurassic ( c. 170–168 Ma), followed by a tectonic quiescence stage in the Late Jurassic–Early Cretaceous. Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection' available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

The mafic–ultramafic complex of Salem, southern India: An analogue for Neoproterozoic Alaskan‐type complex

The Salem Mafic–Ultramafic Complex occurs within the northern part of Cauvery Suture Zone (CSZ), Southern Granulite Terrane (SGT), India. The complex occurs as semicircular to elongated linear body with a thick mantle section and comparatively well‐developed crustal section comprising different lithological units, also referred to as the Chalk Hills of Salem. The major rock types in the complex comprise ultramafic cumulates of dunite, peridotite, wherlite, pyroxenite, and hornblendite, mafic and felsic intrusions of gabbros and amphibolites, and quartzo–feldspathic alkaline rocks together with several ultrapotassic dykes. Geochemical studies of amphibolites from this complex indicate tholeiitic parent magma with enrichment of LIL elements (Rb, Ba, Th, and Sr) and are relatively more enriched than N‐MORB as compared with arc‐related rocks. The enrichment of LIL elements over the HFS elements and with distinct Nb, Ta, and Ti depletion relative to other HFS elements suggest the involvement of subduction‐related components in the depleted mantle source and that the magmatism occurred in an oceanic convergent realm. The mineral chemistry of dunite and peridotite shows high content of forsterite (86.9–92.2) in olivine and high Cr# value (80.85–98.73) for spinel. The tectonic discrimination plots of spinel and olivine mineral chemistry together with clinopyroxene chemistry of amphibolites reveal arc signature typical of Alaskan type of complex. The U–Pb zircon analysis of quartz monzonite intrusion within the hornblendite from the complex yielded a weighted mean age of 819 ± 2.4 Ma, suggesting that the complex formed during the Neoproterozoic and can be correlated to the Alaskan‐type complexes described from the Arabian–Nubian Shield.

Subduction and rollback of the paleo-Pacific plate during the Early Cretaceous: Insights from the Jiangmiao gabbroic intrusion in the Lower Yangtze River Belt, central-eastern China

The dynamics of Mesozoic tectonism recorded by the Lower Yangtze River Belt (LYRB), located in central-eastern China, are controversial. Between 148 and 123 Ma, three major and continuous stages of magmatism occurred in the LYRB: 1) high-K calc-alkaline intrusions, 2) andesitic shoshonitic volcanism, and 3) A-type granitic intrusions. The Jiangmiao gabbroic intrusion, which is located at the eastern end of the LYRB, is composed of gabbro and diorite and was emplaced at ~124 Ma (Dong et al., 2017; Sun et al., 2017a), simultaneous with the A-type granites of the third stage of Mesozoic magmatism in the LYRB. Petrography and mineral chemistry indicate that the Jiangmiao intrusion was formed by the intense crystal fractionation of an oxidized (ΔNNO ca. +1.0) and hydrous (3.5–4.5 wt% H2O) basaltic magma that produced the evolved dioritic magma and gabbroic cumulate. The accumulation of magnetite, olivine, clinopyroxene, and plagioclase resulted in abnormally high Fe2O3 (12.44–16.30 wt%) and low SiO2 (40.75–45.88 wt%) contents in the gabbro, as well as the enrichment of TiO2, Sr, Cr, Ni, Co, and V in the gabbro compared with the diorite. Intense crystal fractionation coupled with the fluid transport led to the enrichment of highly fluid-mobile elements (Rb, Th, U, and Pb) in the evolved diorite but depletion of these elements in the gabbro. The mineral chemistry of clinopyroxene and whole-rock Sr−Nd−Pb isotopic compositions indicate that the primitive basaltic magma originated from the partial melting of an enriched lithospheric mantle source in a subduction environment, but more asthenosphere mantle components were incorporated into the magma source compared with the earlier Mesozoic magmatic rocks in the LYRB. The coevality of the Jiangmiao gabbroic intrusion and the A-type granites supports the tectonic model of Mesozoic rollback of the westward-subducting paleo-Pacific plate. The Jiangmiao gabbroic intrusion likely represents the edge of the back-rolling slab at ~124 Ma, whereas the simultaneous A-type granites in the middle of the LYRB were formed as a result of intraplate extension generated by rollback.

Igneous Rocks, Fluidolites, and Rodingites of Paleocene Explosive Structures of the Taukha Terrane (Sikhote-Alin)

The Taukha terrane of the Sikhote-Alin hosts outcrops of rocks with unusual structural and textural features, the compositions of which vary from ultrabasic to intermediate and from subalkaline to alkaline. These are hornblendites, mica–feldspar, amphibole–feldspar, and carbonatite-like rocks of vent facies, a dike of phlogopite–olivine rocks, and a small intrusion of melanocratic amphibole gabbro. The magmatic rocks, together with fluidolites and rodingites, compose explosive structures of Paleocene age. Rodingites were formed via the development of explosive structures resulting from metasomatic replacement of basic vent-facies rocks and small intrusive bodies in sedimentary rocks. The Paleocene explosive structures of the Mokrushinskaya and Shirokopadninskaya areas of the Olginsky ore district and breaking of Triassic carbonaceous silicites are genetically related to Au–Pd–Pt mineralization, the specific features of which are “cupriferous gold” (including the Zn- and Ni-bearing varieties of cuproauride and auricupride), native elements, and intermetallic compounds.

Petrogenesis of Zeiatit gabbroic rocks in the Southern Eastern Desert of Egypt: Discrimination of arc-related Neoproterozoic gabbros

The area of Gabal Zeiatit, South Eastern Desert, Egypt represents one of a series of ophiolitic-island arc complexes found within the Arabian–Nubian Shield (ANS). The present study introduces new major, trace, and rare earth element analyses and mineral composition data of gabbroic rocks encountered in this area with the goal of constraining their geotectonic setting. The investigated area consists of two types of gabbroic rocks. The Zeiatit ophiolitic metagabbros (ZMG) are always associated with serpentinites of ophiolitic affinity and were subjected to metamorphism of greenschist to lower amphibolite facies and exhibit tholeiitic affinity and MORB-like characters. Serpentinites and metagabbros are tectonically thrusted over a thick pile of metavolcanic rocks. The Zeiatit intrusive gabbros (ZIG) are related to Egyptian Alaskan-type mafic-ultramafic complexes and intrude the studied ophiolitic rocks and island arc rocks. Geochemical data reveal that both studied gabbroic types originated in sub-arc setting. Mineral chemistry data for the two types of the studied gabbroic rocks indicate that they possess different characteristics. The NiO (<0.15 wt%) and Fo (<83 mol.%) contents of olivine from the investigated ZIG are lower than those of the Egyptian ophiolites. The clinopyroxene composition of the ZIG show arc related cumulate. Geochemically, the ZMG exhibit MORB-like and arc-like signatures as they display Ti/V ratios ranging from 14.52 to 21.09. Such phenomenon of hybrid mixture between island arc tholeiite (IAT) and MORB is generally acknowledged being unique to forearc tectonic regime. Moreover, depleted LREE and HFSE and relatively high CaO in the studied ZMG suggest derivation from Cpx-rich mantle at forearc tectonic setting. The ZIG have Nb/Yb (0.28–1.04) and Th/Yb (0.1–0.42) which are typical for oceanic island arcs. The ZIG are highly depleted in Nb, Ta, and Zr, but enriched in Ba, Pb, Sr, U and Th suggest their derivation from a metasomatized lithospheric mantle source that modified by hydrous fluids derived from the subducting slab in a subduction zone. ZIG have Th/Ta (0.69–2.05) and Ta/Hf (0.16–0.28) indicating that they were mainly derived from partial melting of a depleted mantle source, being mixed with crustal material. The present geochemical and mineralogical studies revealed that, the investigated ZMG are fragment of oceanic lithosphere probably developed in a supra-subduction zone environment and originated by 10–30% partial melting of a spinel lherzolite mantle in a sub-arc (forearc) setting at shallow depth which consistent with the most recent conclusions of the Egyptian ophiolitic metagabbros. However, the investigated ZIG are akin to the Alaskan-type rocks.

Tracking Water‐Rock Interaction at the Atlantis Massif (MAR, 30°N) Using Sulfur Geochemistry

AbstractHydrothermal alteration and serpentinization of ultramafic rocks at oceanic core complexes involve extensive element exchange between fluid and rock and a wide range of biogeochemical processes. These processes influence the global sulfur cycle due to both biogenic and abiogenic removal of seawater sulfate. Hence, ocean floor serpentinization connects the hydrosphere, biosphere, and lithosphere. This work presents a study of the sulfur geochemistry of highly altered mafic and ultramafic samples from the Atlantis Massif located at 30°N along the Mid‐Atlantic Ridge. The analyzed samples were drilled during International Ocean Discovery Program Expedition 357 and collected during Alvin dives in 2000, 2003, and 2005. Multiple sulfur isotope analyses of sulfide and sulfate phases indicate that several processes took place during progressive hydrothermal alteration: (1) incorporation of seawater sulfate, (2) thermochemical sulfate reduction during interaction with high‐temperature (high‐T; 350–400 °C), low‐pH fluids and input of H2S derived from leaching gabbroic intrusions, (3) microbial sulfate reduction, and (4) oxidation of sulfides at high water‐rock ratios. Petrological examinations show that high‐T fluids mostly postdated the bulk serpentinization and that these fluid pulses were relatively localized (&lt;1 dm scale) resulting in a highly heterogeneous mineralogy. Locally, high‐T fluid influx took place subsequent to microbial sulfate reduction and oxidation as indicated by geochemical modeling. Overall, this study documents the complex interplay of magmatic processes, fluid‐rock interaction, and microbial activity that take place during the formation of oceanic core complexes and where mantle rocks are exposed to seawater.

Variably evolved gabbroic intrusions within the Xigaze ophiolite (Tibet): new insights into the origin of ophiolite diversity

Ophiolites worldwide show striking diversities in their rock assemblage and structure (i.e., ophiolite diversity), raising a question whether ophiolites are originally similar before intense tectonic dismemberment. Comparison between ophiolites and oceanic lithospheres at modern mid-ocean ridges may provide key constraints on the origin of ophiolite diversity, because oceanic lithospheric structures are inherently controlled by spreading rates. Here, we present a case study of the Xigaze ophiolite in southern Tibet focusing on its gabbroic intrusions outcropping in three localities, i.e., Dazhuqu, Baigang and Jiding. Compared to the Jiding sequence, the Dazhuqu and Baigang gabbroic rocks are less evolved, characterized by higher Cr2O3 contents but lower contents of TiO2 and rare earth element in both clinopyroxene and bulk compositions. It is evident, hence, that the Xigaze ophiolite is characterized by variably evolved and discontinuously distributed gabbroic intrusions, rather by a continuous lower oceanic crust between the mantle and sheeted dike complex as the Penrose-type ophiolites. Our study, along with the identification in previous studies of oceanic detachment faults within the Xigaze ophiolite, demonstrates that the Xigaze ophiolite shows close similarities to oceanic lithospheres at modern slow- and ultraslow-spreading ridges. Hence, the significant structural distinctions between the Xigaze ophiolite and the Penrose-type ophiolites (e.g., the Oman ophiolite) may be inherently associated with different spreading rates of paleo-ridges. Considering the limited scale of the Xigaze gabbroic rocks, here we suggest the Xigaze ophiolite as a typical representative of fossil ultraslow-spreading ridges.

Petrogenesis and Metallogenesis of the Niumaoquan Gabbroic Intrusion Associated with Fe‐Ti Oxide Ores in the Eastern Tianshan, NW China

AbstractThe Niumaoquan layered gabbroic intrusion is in the southern margin of the Central Asian Orogenic Belt in North Xinjiang, China, and hosts a Fe‐Ti oxide deposit in its evolved gabbroic phases. In this paper, we report zircon U‐Pb age, Sr‐Nd‐Hf isotopes, plagioclase chemistry, and whole‐rock geochemistry of the Niumaoquan layered gabbroic intrusion. Zircon grains separated from an anorthosite sample analyzed by laser ablation inductively coupled plasma mass spectrometry yielded a concordia age of 314.7±0.74 Ma, indicating that the Niumaoquan ore‐bearing gabbroic intrusion was emplaced during the Late Carboniferous. The olivine gabbro texture and plagioclase chemistry suggest that plagioclase was an early crystallized silicate phase that crystallized prior to olivine. Fractional crystallization and accumulation of plagioclase significantly controlled the evolution of the Niumaoquan gabbroic intrusion and contributed to the formation of anorthosite layers, causing metallogenic elements to become enriched in the residual melt. The Niumaoquan gabbroic intrusion is characterized by the enrichment of large ion lithophile elements and depletion of high field strength elements, positive zircon εHf(t) values (+2.1 to +12.2), positive εNd(t) values (+3.3 to +5.2), and low initial 87Sr/86Sr ratios (0.7039 to 0.7047), suggesting that the parental magma was produced by interactions between metasomatized lithospheric mantle and depleted asthenospheric melts at an early post‐collision stage. The Fe‐Ti oxide mineralization of the Niumaoquan intrusion benefited from interactions between depleted asthenospheric melts and lithospheric mantle, and fractional crystallization of abundant plagioclase and magnesian minerals.

Structural style and kinematic evolution of Al Ji'lani area, Ad Dawadimi terrane, Saudi Arabia

This article discusses the structural control of the Precambrian Al Ji'lani basic intrusion, which is one of numerous oval igneous intrusions scattered within different terranes of the Arabian-Nubian Shield. The area in the Ad Dawadimi vicinity exposes 75 km long northwest-trending sinistral en echelon fault systems. During East African Orogen, the master fault segments propagate as sinistral system creating a rhomboidal shape releasing stepover area, defined in this work as Al Ji'lani pull-apart structure. The fault kinematic analysis shows significant variation in stress tensors between data measured outside and inside Al Ji'lani pull-apart structure. The stress tensor, computed from fault population inside the pull-apart, typify a NW-SE to NNW-SSE pure tectonic extension. Outside the pull-apart structure, the stress tensor illustrates a pure strike slip stress regime with a minimum stress axis that varies from NW-SE to NNW-SSE, and maximum stress axis that varies from ENE-WSW to NE-SW. The pull-apart northwestern corner develops, at the end stage, a circular to elliptic basic intrusion known as the Al Ji'lani layered gabbro intrusion. The intrusion is located in the releasing stepover's corner and characterized by inward dipping layers, suggesting an evolution genetically related to the pull-apart growth. Field data and faults kinematic analysis reveal that the Al Ji'lani structure is an ancient Ediacaran–Cryogenian gabbro intrusion developed in pull-apart corner, widely recognized as preferential crustal weakness zone in recent systems of fault-related-volcanism. The results support a predominant relationship between brittle tectonic and volcanism rather than shearing as it was previously accepted.

Petrogenesis of gabbroic intrusions in the Valerianov-Beltau-Kurama magmatic arc, Uzbekistan: The role of arc maturity controlling the generation of giant porphyry Cu—Au deposits

Arc maturity is considered to play an important role in the mineralization of porphyry Cu—Au deposits. However, the early stage of arc is rarely preserved leading to controversies over arc evolution. Here we investigate gabbroic intrusions from the Akcha, Beleuti and Kalmakyr arc in relation to the Almalyk porphyry Cu—Au orefield in Uzbekistan. The Akcha biotite gabbro shows medium to fine grained texture, and is composed of plagioclase and biotite with minor clinopyroxene, orthopyroxene, quartz and magnetite. The Beleuti and Kalmakyr gabbros display coarse-grained texture, and are dominated by plagioclase, amphibole and clinopyroxene. Zircon LA-ICP-MS U—Pb dating yields ages of 339.9 ± 2.0 Ma for Akcha biotite gabbro, 339.0 ± 3.3 Ma for Beleuti gabbro and 335.0 ± 2.4 Ma for Kalmakyr gabbro. The emplacement ages of these intrusions are older than those of ore-bearing porphyries in the Almalyk orefield, indicating that the gabbros were emplaced during early stage of the arc evolution. The gabbroic rocks are characterized by high Mg# [100 × molar Mg2+/(Mg2++Fe2+), 41–61], low SiO2 contents (41.6–50.9 wt%), enrichment in large-ion lithophile elements and negative anomalies for high field strength elements, together with relatively depleted Sr-Nd-Hf isotopic compositions (0.7044–0.7057, −1.59-1.96, 3.38–9.96, respectively). The geochemical features suggest that these rocks were mostly derived from the partial melting of mantle wedge modified by subduction-related fluids. Estimates on water contents based on the composition of amphibole show 4.9–5.3 wt% for the Akcha biotite gabbro, and 5.6–6.8 wt% for the Beleuti gabbro. The oxygen fugacity estimated by zircon Ce/Nd, Ce4+/Ce3+ ratios are lower than those of porphyries from the giant porphyry deposits in the Central Asian Orogenic Belt. Our results, together with previously reported data on porphyries in the Almalyk orefield suggest that the gabbro was formed in an immature arc setting associated with the northward subduction of the Paleo-Turkestan oceanic plate, whereas the ore-bearing porphyries were formed in a mature arc.

Петролого-геохимические особенности рудовмещающих эффузивных и интрузивных пород Николаевского месторождения золото-порфирового типа (Южный Урал)

Object. The article considers the results of geochemical studies of ore-bearing effusive and intrusive rocks of the Nikolaevka gold deposit, located in the zone of the Main Uralian fault in the Southern Urals. The deposit belongs to the gold-porphyry type which unconventional for the Urals and poorly studied here. Methods. The rock composition was determined by chemical analysis (IG UFRC RAS), mass-spectrometry with inductively coupled plasma on the ELAH 9000 quadrupole mass-spectrometer (IGG UB RAS) and X-ray fluorescence analysis on the VRA 30 spectrometer (IG UFRC RAS). Results. It is established that gold-porphyry mineralization associated with an island-arc volcanic-intrusive complex that unites plagiophyric and pyroxene-plagiophyric basalts, their tuffs, gabbro intrusions and ore-bearing series of dikes plagiophyric dolerite-porphyrites and gabbro-diorite-porphyrites. The rocks of the complex, including ore-bearing dikes, have normal alkalinity, tholeitic and transition from tholeitic to calc-alkaline composition. Among the volcanogenic formations, faunistically dated in the zone of the Main Ural fault in the Southern Urals, by a number of geochemical parameters (for example, the ratios of Zr/Nb and Nb/Th) the effusive and intrusive rocks of the deposit are the closest to the volcanics of the pyrite-bearing Baymak-Buribay Formation (D 1 e 2 ), lying at the base of the section of the Magnitogorsk island arc, and, in all probability, are their age analog. At the same time, the gold-bearing complex differs from the Baymak-Buribay Formation by a general increased iron and titanium content, with a reduced magnesia of all rock types, as well as the absence of boninites and acid volcanics, the predominance of porphyry rock types, which can be due to the geodynamic conditions of its formation. Conclusion. Geodynamic analysis of geochemical data using diagrams (La/Sm)N-TiO 2 and V-Ti/1000 suggests that the Nikolaevka deposit was formed in the articulation zone of the island arc and the back-arc basin. Gold-porphyry mineralization associated with the Late Emsian volcanic processes was isolated in the Southern Urals for the first time.

The 920–900 Ma Bahia-Gangila LIP of the São Francisco and Congo cratons and link with Dashigou-Chulan LIP of North China craton: New insights from U-Pb geochronology and geochemistry

A widespread mafic sill and dyke unit (the Pedro Lessa suite) adjacent to the southeastern São Francisco (SF) craton (eastern South America) is dated as 940 ± 42 Ma, 912 ± 17 Ma, and 986 ± 64 Ma (based on combined ID-TIMS and LA-ICPMS analyses on baddeleyite grains) on the Santa Maria de Itabira gabbroic intrusion. These ages suggest membership in the previously defined ca. 920–900 Ma Large Igneous Province (LIP) (herein referred to as the Bahia-Gangila LIP) of the reconstructed São Francisco and formerly adjacent Congo craton (Africa). Additional mafic and felsic units throughout the SF craton are also linked (based on a literature review), thus expanding the size of the reconstructed LIP to ca. 1,500,000 km2. Geochemistry reveals high Ti OIB signature for most units and low Ti E-MORB composition for the Gangila flood basalts, marking the initiation of attempted ocean opening. Geochemistry of the Bahia-Gangila LIP is also compared with the ca. 920–900 Ma Dashigou-Chulan LIP of the North China craton, and implications for three possible reconstructions of SF/Congo craton with the North China craton are considered.

Breaking up continents at magma-poor rifted margins: a seismic v. outcrop perspective

The break-up of continents at magma-poor rifted margins is a complex yet little understood process accounting for intricate interactions between tectonic and magmatic processes. Whereas high-resolution seismic data may resolve the first-order architecture of ocean–continent transitions (OCTs), direct access to rocks remains limited to deep-sea drilling. In this study, we combine seismic observations from the East Antarctica margin (Geoscience Australia Survey 228) with field observations from the Bracco–Levanto area (Northern Apennines, Italy), representing modern and fossil examples of ultra-distal magma-poor rifted margins respectively. The combination of detailed structural mapping and petrological studies from fossil examples with architectural features observed in seismic sections from present-day OCTs enables to bridge the different observation scales. Field evidence indicates that the magmatic budget is not zero, as magma is present throughout the exhumation process. Extensional detachment faults that exhumed serpentinized mantle and deep-seated gabbro intrusions are truncated by later high-angle normal faults, which most probably acted as feeders for the emplacement of massive syn-extensional basalts. These observations suggest a polyphase tectonic and magmatic evolution of the ultra-distal margin prior to the formation of the first true oceanic crust, which can be studied in detail only by combining seismic and outcrop observations.

Geochemistry and Petrogenesis of metabasites from the Gol-e-Gohar Complex in southern Sanandaj-Sirjan metamorphic zone, South of Iran; Evidences for crustal extension and magmatism at early Palaeozoic

In Gol-e-Gohar metamorphic Complex from south-eastern Sanandaj-Sirjan metamorphic zone (Kerman province, Iran), there are two types of metabasites contain layered metamorphosed lava flows and the younger meta-gabbros. The protoliths formed in the Paleozoic era and were metamorphosed during the early Cimmerian orogenic phase in the late Triassic, under temperatures of 640–680oC and pressures of ~7–10.5kbar (amphibolite facies). These rocks are garnet-bearing amphibolites, garnet free amphibolites and metamorphosed gabbros. Many mineralogical and chemical aspects of these metabasites are similar, although the layered metabasites show tholeiitic and the meta-gabbros depict alkaline affinities. Evidences such as whole rock geochemical characteristics, Sr and Nd isotopic data, ( 143 Nd/ 144 Nd initial =0.511913–0.512067; eNd 550Ma =-0.31–2.68), relatively flat patterns of chondritenormalized Rare Earth Elements and multi-elemental diagrams, the enrichment in TiO 2 (average content ~2.16) and high Zr/Y ratios (3–8), indicate that all of Gol-e-Gohar metabasites are formed in an extentional intracontinental rift zone from tholeiitic to alkaline magmas. The data suggest that the paren magmas could derived by low degrees of partial melting of spinel-lherzolite sources in subcontinental lithospheric mantle. These evidences confirm the existence of extentional environments in southern part of the Sanandaj-Sirjan metamorphic zone in the Paleozoic era, when large extensional depressions developed in the Sanandaj-Sirjan metamorphic zone and underlying asthenosphere ascent and partially melted during this time. Gradually, thick sequences of continental detritic sediments and tholeiitic lava flows accumulated in these troughs. Subsequent magmatic event in the area characterizes by emplacement of alkaline gabbro intrusions. At the early Cimmerian orogeny, these sedimentaryigneous rocks associations metamorphosed and the Gol-e Gohar metabasites formed.

New U-Pb geochronology evidence for 2.3 Ga detrital zircon grains in the youngest Huronian Supergroup formations, Canada

Detrital zircon U-Pb ages from one sandstone and one claystone unit in the upper Huronian Supergroup, near Flack Lake, Ontario, Canada, provide new evidence for the maximum depositional age of the two youngest formations and reinterpretation of the depositional history of the upper Huronian Supergroup. The average age of the youngest zircon grains constrains the age of deposition to sometime after 2315 ± 5 Ma, but prior to intrusion of gabbro (Nipissing) dykes approximately 95 m.y. later. Clusters of late Archean age zircon are prominent in both samples, but a considerable addition from post-Archean sources is also present. The sample from the Bar River Formation contains a greater proportion of post-Archean material, which may be a result of grain-size bias. Our results are similar to U-Pb zircon ages determined from stratigraphically lower purported tuff beds in the Gordon Lake Formation, however our field and Scanning Electron Microscope (SEM) analyses demonstrate that the samples in the present study were not part of tuff units.

Polycyclic Palaeozoic evolution of accretionary orogenic wedge in the southern Chinese Altai: Evidence from structural relationships and U–Pb geochronology

Structural analysis and U–Pb geochronological study on zircons from the southern Chinese Altai (the Kalasu area, SE of the Altai city) show that the Cambro-Ordovician accretionary wedge (ca. 520–492 Ma) underwent four major geological events: 1) emplacement of Early Devonian magmas (ca. 410–400) associated with formation of a volcano-sedimentary cover, 2) major Middle Devonian (ca. 390–374 Ma) tectono-metamorphic event, 3) Late Devonian-Early Carboniferous folding without apparent metamorphism, and 4) a regional folding with localized Early Permian high- to ultrahigh-temperature reworking (ca. 300–280 Ma). The Early Devonian magmatism is characterized by emplacement of mafic rocks and granitoids in the centre of the NE-SW profile, coevally with granitoid magmatism and rhyolite volcanism in the southwest and northeast, respectively. The whole volcano-sedimentary and magmatic edifice was transposed by sub-horizontal metamorphic fabric associated with variable metamorphic degrees in different areas ranging from greenschist facies in the northeast (mu + bi±g) to amphibolite facies in the southwest (st + g ± sill) and granulite facies in the centre (g + sill+kfs). This metamorphic architecture, distribution of magmatism and character of metamorphic zircon populations allow to correlate these areas with upper, middle and lower orogenic crust that developed during important vertical shortening and horizontal flow in Middle Devonian. Subsequently, the whole edifice was affected by regional NE-SW trending upright (possibly Late Devonian-Early Carboniferous) folding. Finally, Early Permian shortening produced NW-SE trending regional upright folds in the southwest and northeast and a crustal-scale vertical, tabular deformation zone in the centre. The Permian deformation is accompanied by granulite facies (kfs + cd + sill+g) metamorphism and anatexis reworking the Devonian lower orogenic crust, with extensive resetting and growth of new zircons and with intrusions of Permian granites and gabbros. This study suggests that the Early Permian event was related to massive perturbation of thermal structure of the mantle lithosphere due to the collision of the Junggar arc with the Chinese Altai terrane.

Geochemistry and SHRIMP U‐Pb Zircon Dating of Mafic Rocks North of Zunhua City, Eastern Hebei, North China Craton: Paleoproterozoic Gabbro rather than Neoarchean Ophiolite

Abundant mafic‐ultramafic blocks and dikes occur in the area north of Zunhua City, eastern Hebei Province, and were previously suggested to be part of a late Archean ophiolitic assemblage. We employed SHRIMP zircon dating and a geochemical study on these mafic and surrounding rocks to test the ophiolite hypothesis. The SHRIMP data suggest that three metagabbro samples were metamorphosed at ∼1.8 Ga. Numerous ∼2.5 Ga zircons display strong oscillatory zoning, characteristic of zircons from granitoid rocks but not from gabbro, so we suggest that these are xenocrystic grains. The age of these xenocrystic zircons and their metamorphic rims suggests that these mafic blocks formed in Paleoproterozoic. The surrounding gneiss of intermediate composition also contains 2.5 Ga zircons with oscillatory zoning and 1.8 Ga metamorphic rims. Fractionated REE patterns and Nb, Ta, Zr, Hf negative anomalies to variable extent were observed in the mafic blocks and surrounding rocks, also supporting a significant difference in the chemistry of ophiolitic rocks. Our data suggest that many mafic blocks in northern Zunhua are not part of a late Archean ophiolite complex but part of a tectonically dismembered Paleoproterozoic intrusive gabbro complex. This study shows that late Paleoproterozoic metamorphism occurred in the western part of eastern Hebei Province.