Calc-alkaline Basalts Research Articles

Experimental evidence for decompression melting of metasomatized mantle beneath Colima Graben, Mexico

Primitive subduction zone magmas provide information about the composition and thermal structure of the underlying mantle wedge. In the Colima Graben, Mexico, primitive lavas erupted from cinder cones range from high magnesium calc-alkaline basalts to high-K trachybasalts. This chemical diversity suggests that the sub-arc mantle wedge from which they derive is heterogeneous. To explore the conditions of magma generation in the wedge beneath Colima we used an inverse experimental approach to constrain multiple saturation points on the liquidus surface of a primitive high-K basanite (COM-1). Equilibrium piston-cylinder experiments were carried out between 1.0 and 2.4 GPa under hydrous (1.8–3.8 wt% H2O) and oxidizing (ƒO2 = − 0.5 to 4.3 log units relative to NNO) conditions. COM-1 + 3.8 wt% H2O is shown to be multiply-saturated with a phlogopite-bearing spinel pyroxenite assemblage (cpx + opx + phl + sp) close to its liquidus at 1.9–2.4 GPa and 1300 ºC. Experimental mapping of the liquidus surface reveals a multiple saturation point (MSP) where a lherzolitic phase assemblage of ol + cpx + opx + sp + phl coexist. The topology of the MSP indicates a peritectic of the form cpx + opx + phl + sp = liquid + ol. Four bracketing experiments define the MSP of COM-1 as 1300 ± 10 °C, 1.7 ± 0.1 GPa, ∆NNO = 3.4 ± 0.5 log units, for melt containing 3.6 ± 0.4 wt% H2O. The MSP olivine is too forsterite-rich (Fo92-94) to be in equilibrium with mantle lherzolite, but matches phenocryst core compositions in the natural basanite. Thus, experimental results indicate that COM-1 was produced by incongruent melting of an olivine-free, phlogopite-pyroxenite source that itself is the result of metasomatism of mantle wedge by slab-derived fluids. These conditions provide a valuable constraint on the thermal structure and chemical composition of the mantle wedge beneath Colima.

Late Neoproterozoic Island-Arc Volcanic Associations in the Accretion Belt at the Southwestern Margin of the Siberian Craton (Predivinsk Terrane of the Yenisei Ridge)

Based on geological, geochemical, and isotope-geochronological studies, the geodynamic nature of the island-arc magmatic associations of the Predivinsk terrane of the Yenisei Ridge has been established. It is shown that the Predivinsk terrane consists of two blocks, which differ in age and composition of rock associations. The Eastern Block is composed of the following associations: (1) typical island-arc differentiated calc-alkaline basalt–andesite–dacite association dominated by andesites and dacites; (2) bimodal subalkaline metabasalt–trachyrhyolite association, which is enriched in incompatible elements, was formed under extensional conditions, and reflects the evolution of subduction magmatism, and (3) ophiolite fragments consisting of N-MORB metabasalts. The Western Block is composed of geochemically primitive metabasalt–plagiorhyolite and moderately differentiated calc-alkaline metabasalt–basaltic andesite–rhyodacite associations typical of oceanic island arcs. The magmatic associations of the Eastern Block are dated within 650–630 Ma, while volcanic and sedimentary associations of the Western block, within 620–600 Ma. The accretion of the island-arc complexes of the Predivinsk Terrane and their amalgamation to the Siberian margin occurred about 610–600 Ma.

Tectonic setting of the Bipindi greenstone belt, northwest Congo craton, Cameroon: Implications on BIF deposition

The banded iron formations (BIFs) in the Bipindi greenstone belt (GB) are hosted within metavolcano-sedimentary sequence of the Nyong Group at the northwestern edge of the Congo Craton (Ntem Complex) in southern Cameroon. New data on the BIFs and its associated rocks allow us to discuss the tectonic setting of this greenstone belt in a regional perspective. Metavolcanic rocks consist of epidosites, mafic granulites, and garnet-pyroxene gneisses, whereas the metasedimentary rocks comprise chlorite-epidote schists. These rocks have been metamorphosed under greenschist-to granulite-facies conditions. The geochemical features of the metavolcanic rocks such as the trace element ratios and rare earth elements (REE) patterns suggest that their protolith corresponded to tholeiitic to transitional and calc-alkaline basalts. The associated metasedimentary rocks are characterized by low La/Sc and Th/Sc discrimination ratios, and REE abundances, indicating that they were derived from mafic and intermediate igneous rocks. Similar protoliths were recently described for metamorphic rocks interbedded with BIFs at the Nyong Group, which were formed in a shallow basin near the continent. The Bipindi BIFs are further characterized by both weak (1.31–1.34) and slightly high (2.00–2.51) positive (Eu/Eu)SN ratio, that are typical of Superior- and Algoma-type BIFs respectively. However, the geochemical features of the host rocks (e.g., REE patterns, Th/Nb, La/Yb, and La/Sc ratios) suggest that the Bipindi BIFs were deposited at an extensional geodynamic setting likely related to a back-arc, similar to the depositional environment for many Algoma-type BIFs. Archean-Paleoproterozoic arc setting is widely distributed across the Congo Caton and the West African Craton, suggesting integration of the tectonic evolution of the Bipindi GB into a continental-scale paleogeography.

Petrology and geochemistry of early Carboniferous volcanic rocks in the Xinyuan region of western Tianshan: Implications for magma sources

Understanding the sources and processes that produce fossil continental arc magmas is still a challenging problem. To address this problem, here we perform detailed petrological investigations as well as whole-rock geochemical and SrNd isotopic analyses on the early Carboniferous volcanic rocks in the Xinyuan region. These volcanic rocks mainly consist of calc-alkaline basalt, trachy-andesite, and rhyolite, and show subduction-related geochemical and isotopic signatures. Petrographic observations and new geochemical data suggest that the rhyolite was generated by partial melting of juvenile crustal materials, while the basalt and trachy-andesite might undergo a series of open-system magmatic processes, such as fractional crystallization, crustal contamination, and magma replenishment and mixing. The primitive basaltic rocks with Mg# values of ~70 were least affected by crustal-level magmatic processes, and they could be used to constrain sub-arc mantle processes. The Nb/Yb, Zr/Yb, and Ta/Yb ratios of these basaltic rocks are similar to those of E-MORB, suggesting a relatively fertile mantle source prior to subduction, while their high Ba/Th, Sr/Nd, and Th/Nd ratios suggest a significant contribution of slab-derived components to the pre-subduction mantle wedge. Geochemical modelling based on trace elements and SrNd isotopic ratios indicates that the pre-subduction mantle wedge was modified by ~0.5% subducted sediment-derived fluids and ~2% bulk sediments. About 10% partial melting of such enriched mantle source could produce the early Carboniferous primitive magmas in the Xinyuan region.

Geochemistry and petrogenesis of sandstones and their basaltic interlayers of Shexing Formation from Linzhou basin, South Tibet

印度与亚洲大陆的碰撞是青藏高原演化的重要构造事件，碰撞过程被记录在拉萨地块南部的晚白垩世到古新世的沉积-岩浆作用中。林周盆地的晚白垩世设兴组及其之后不整合覆盖的林子宗火山岩，是解析碰撞过程的重要记录。本文对设兴组最高层位的砂岩和玄武岩夹层进行了岩石学、地球化学和年代学研究，探讨了岩石成因和构造意义。设兴组砂岩属于杂砂岩，碎屑物质主要来自中酸性岩浆岩源区；锆石Hf同位素指示设兴组大部分碎屑物质来源于盆地北面的中部拉萨地块，少部分来自盆地南部的冈底斯岩基；砂岩中最年轻的碎屑锆石年龄指示林周盆地设兴组是在98Ma之后接受沉积的。以夹层产出在设兴组顶部的玄武岩和玄武安山岩，富集轻稀土元素、亏损重稀土元素、弱负Eu异常，强烈富集Ba、Th、U、Pb等大离子亲石元素，显著亏损Nb、Ta等高场强元素，属于高钾钙碱性玄武岩系列，与典型安第斯型玄武岩特征吻合。玄武岩和玄武安山岩的锆石均为捕获锆石，其最年轻碎屑锆石年龄限定了设兴组玄武岩的喷发晚于110Ma。综合分析表明，林周盆地晚白垩世时期为夹持在冈底斯岩浆弧与中部拉萨地块之间的弧后盆地，新特提斯洋壳晚白垩世俯冲到冈底斯弧和弧后盆地之下，大约在98~110Ma之后喷发到林周盆地的很少量中基性岩浆构成了设兴组顶部的玄武岩和玄武安山岩夹层，是新特提斯洋俯冲相关的幔源岩浆作用。林周盆地设兴组（晚于98Ma）与上覆的林子宗火山岩（底部约为65Ma）之间呈大约33Myr的构造间断，可能代表了冈底斯弧的碰撞之前的隆升剥蚀过程。;The collision between India and Asia was the most important tectonic event in the evolution of the Tibetan Plateau, which was recorded in the Late Cretaceous to Paleogene sedimentary and magmatic rocks in the southern Lhasa terrane. The Late Cretaceous Shexing Formation strata and later eruption of the Linzizong volcanic succession (ca. 65~45Ma) is the best candidate to show the collisional processes. This study provides new petrological, geochemical and zircon U-Pb geochronological data on the sandstones and basaltic interlayers collected from the highest level of Shexing Formation in the Linzhou basin in southern Tibet. The materials of the Shexing greywacke were from the felsic magmatic source region. Hf isotope implies that the majority of the sediments came from the central Lhasa terrane to the north of the basin, and others from the Gangdese arc batholiths to the south of the basin. The youngest age of the detrital zircons in Shexing sandstone constrains the deposition were later than 98Ma in Linzhou basin. The interlayers close to the top of the Shexing Formation are basalts and basaltic andesites, with enriched light rare earth elements (REE), depleted heavy REE, weak negative Eu anomaly, strongly enriched large ionic lithophile elements (LILEs, such as Ba, Th, U, Pb), and depleted high-field strength elements (HFSEs, such as Nb, Ta). These basalts and basaltic andesites are high potassium calc-alkaline basalt series, with the affinity of the typical Andes-type basalts from continental arc. The detrital zircons from these interlayers define their eruption to be later than 110Ma. Our results suggest that the Linzhou basin is the back-arc tectonic setting, which located between the Gangdese arc and the central Lhasa terrane during the Late Cretaceous. The Neo-Tethys Ocean subducted beneath the Gangdese arc and the back-arc system and triggered the mantle partial melting to produce the eruption of the basalts and basaltic andesites after 98~110Ma. The interval of 33Myr between the upper most Shexing Formation (98Ma) and the Linzizong volcanics (65Ma), may represent the uplift and erosion processes of the Gangdese arc before the collision of India and Asia.

Geochemistry of ultramafic–mafic rocks of Mesoarchean Sargur Group, western Dharwar craton, India: Implications for their petrogenesis and tectonic setting

The Nuggihalli and Holenarsipur greenstone belts of the western Dharwar craton expose ultramafic–mafic rocks of the Mesoarchean. The rocks in these belts are geochemically considered as komatiites and komatiitic basalts with minor occurrences of tholeiitic and calc-alkaline basalts. The dominant ultramafic rocks of the Nuggihalli greenstone belt are layered and indicate fractionation processes at relatively shallower crustal levels. The Al-undepleted and Al-depleted signatures obtained could be attributed to magmatic differentiation processes and might be due to fractional crystallization of minerals such as hornblende and plagioclase, in addition to cumulus olivine and pyroxene. The chemical heterogeneity in the rocks of these greenstone belts might have therefore developed during the intrusion of the parental melts and their differentiation into a layered igneous complex. The differences in the lithological characteristics of the Holenarsipur and Nuggihalli greenstone belts can be explained by their different crustal levels of exposure. Presence of spinifex-textured komatiites need not necessarily imply that the sources have to be ultramafic and therefore of a deeper origin. This study indicates that the parental melts for unambiguous layered intrusive ultramafic–mafic complexes could be high-Mg basalts originating from relatively shallower levels. The probable geodynamic setting for the emplacement of the rocks of the two greenstone belts could be in a plume-modified mid-ocean ridge that was too thick and buoyant to be subducted, and the decompression-melted magma chamber developed igneous layering as the magma stalled in the lithosphere.

Late Neoarchean geodynamic evolution: Evidence from the metavolcanic rocks of the Western Shandong Terrane, North China Craton

Late Neoarchean metavolcanic rocks are widely distributed in the Western Shandong Terrane (WST). They are classified as ~2590–2580 Ma tholeiites (Group MB-1), ~2550–2530 Ma tholeiites (Group MB-2), calc-alkaline basalts (Group MB-3), high-Si adakites (Group MAD) and ~2520–2500 Ma tholeiites (Group MB-4) based on zircon U-Pb chronological and geochemical data. Their parental magmas have complex origins and were derived from a depleted mantle wedge enriched by slab-derived melts or fluids (Group MB-1); an unaltered depleted mantle (Group MB-2); the delaminated lower crustal materials (Group MAD); a strongly melt- and fluid-metasomatized depleted mantle (Group MB-3); and a fluid- and sediment-metasomatized asthenospheric mantle (Group MB-4). The late Neoarchean geodynamic evolution of the WST revealed by these multi-genetic volcanic rocks can be summarized as follows: (1) an ~2.62–2.53 Ga eastward subduction operated along the ancient continental margin, followed by delamination of unstable continental lithosphere in the back-arc region during ~2.60–2.53 Ga; and (2) delamination-derived mantle magmas ascended and caused the regional extension, further inducing the asthenosphere to passively rise and the back-arc basin to open during ~2.52–2.50 Ga. The above intense mantle magmatism and crust-mantle interactions have consumed abundant mantle energy and facilitated the continental stratification and final cratonization of the WST.

Origin and Age Determination of the Neotethys Meliata Basin Ophiolite Fragments in the Late Jurassic–Early Cretaceous Accretionary Wedge Mélange (Inner Western Carpathians, Slovakia)

This study reports the Neotethyan Meliata Basin ophiolite fragments in the Late Jurassic–Early Cretaceous accretionary wedge mélange in the southern part of the Inner Western Carpathians (IWC). Here we present new lithostratigraphical, petrographical, geochemical, and geochronological data obtained from the mélange blocks used to reconstruct the Meliaticum paleotectonic zones in a tentative evolutionary model of this accretionary wedge. The Dobšiná mélange block continental margin carbonatic and siliciclastic sediments have calc-alkaline basalt intercalations. The basalt Concordia age dated to 245.5 ± 3.3 Ma by U–Pb SIMS on zircon most likely indicates the pre-oceanic advanced early Middle Triassic continental rifting stage. The evolving marginal oceanic crust is composed of Middle to Upper Triassic cherty shales to radiolarites. The detrital zircon U–Pb SIMS Concordia ages of 247 ± 4 Ma and 243 ± 4 Ma from a cherty shale, and the xenocryst zircon population Concordia age of 266 ± 3 Ma from a 0.5 m thick “normal” mid-ocean ridge (N-MOR) basalt layer in this cherty shale reveal the connection of the oceanic basin to the adjacent rifting continental margin. The chertified reddish limestone transition to radiolarite indicates syn-rift basin deepening. Upwards, regular alternating N-MOR basalts and radiolarites are often disturbed by peperite breccia horizons. The Nd isotope values of these basalts (εNd240 = 7–8) are consistent with their chondrite normalized rare earth element (REE) patterns and indicate a depleted mantle source. The Triassic ophiolitic suite also comprises rare ocean island (OI) basalts (εNd240 = 5) and serpentinized subduction unrelated peridotites. The Middle to Late Jurassic shortening and southward intra-oceanic and continental margin subduction at approximately 170–150 Ma enhanced the formation of the trench-like Jurassic flysch succession which preceded the closure of the Meliata Basin. The flysch sediments form a mélange matrix of olistolithic unsubducted, obducted, and MP–HP/LT metamorphosed exhumed blocks of the Triassic to Lower Jurassic successions. Blocks of peridotites, rodingites, blueschists, greenschists, rare amphibolites, deep-water shaly sediments and shallow- to deep-water carbonates are typical members of the mélange. The Meliatic accretionary wedge mélange nappe outliers were incorporated in the IWC orogenic wedge in the late Early Cretaceous according to metamorphic rutile U–Pb SIMS ages of 100 ± 10 Ma determined from a Jaklovce metabasalt.

Magmatic plumbing system beneath a fossil continental arc volcano in western Tianshan (NW China): Constraints from clinopyroxene and thermodynamic modelling

Determining the processes of magma ascent from magma source to the surface is paramount to understand the evolution of magmatic plumbing systems. Here, we perform systematic petrological investigations and thermodynamic modelling on the early Carboniferous calc-alkaline basalt in the Xinyuan region of western Tianshan. As the most common phenocryst in basalt, clinopyroxene with diverse textures and zoning patterns can be classified into 6 types. Type 1 clinopyroxene, occurring throughout the whole basaltic sequence is unzoned or normally zoned. Type 2 to type 6 clinopyroxenes, only observed in the late erupted basalt, show disequilibrium textures including resorbed cores, sieve textures, oscillatory zoning, and patchy zoning. These textural and compositional features of clinopyroxene suggest that fractional crystallization controlled the early stage of basaltic magma evolution, and the late emitted basaltic magmas might be formed by combined processes of fractional crystallization, recycling of early-formed crystals, repeated recharge of hot, primitive magmas with subsequent mixing, and convection of residing magmas at depth. Thermobarometric calculations and thermodynamic modelling with the rhyolite-MELTS software suggest at least two magma storage zones: a deep magma chamber at ∼22–25 km depth and a shallow magma chamber at ∼ 2–3 km depth. Chromite and clinopyroxene with Mg# values of 77–84 crystallized from melts with ∼1 wt % H2O at QFM-1 conditions in the deep magma chamber. Plagioclase (An = 59-70) started to co-crystallize with Fe-rich clinopyroxene (Mg# = 74–78) in the shallow magma chamber in more oxidized conditions (close to the QFM buffer).

Geology and Petrogenesis of Igneous Rocks from Batur Paleovolcano, Gunungkidul, Yogyakarta: Evidence from their Textures, Mineralogy, and Major Elements Geochemistry

Batur paleovolcano is located in Wediombo Beach area, Gunungkidul Regency, Yogyakarta and is being part of Wuni Formation. Several volcanic products including lava flow, autoclastic breccia and volcanic breccia can be found associated with diorite intrusions. This research is aimed to characterize geological, mineralogical andgeochemical variations of igneous rocks from Batur paleovolcano to understand its petrogenesis. Detailed geological mapping with scale of 1:12,500 is conducted to identify geological aspects and delineate igneous rocks distributions. Igneous rocks and selected wall rocks samples were prepared for laboratory analysis including 8 samples for petrography and 5 samples for ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) analysis. Several geochemical data from previous study are also added to investigate the geochemical variations. Geological condition of the research area consists of four rock units including colluvial deposit, limestone, andesite lava and diorite intrusion. Geological structures found are normal fault and shear joint where the main stress direction is north–south. Petrography analysis showed that igneous rocks in this research area consist of diorite intrusion and andesite lava with phorphyritic texture. Plagioclase become the most abundant minerals found both as phenocryst phase and groundmass. Hornblende only occur as phenocryst phase in minor amounts as accesory mineral. Major elementsgeochemistry analysis showed the rocks are characterized by intermediate silica with low alkali content. They are can be categorized as calc-alkaline series. However, some samples are fall into tholeiitic series. Major elements variation and textural study also indicate the magma is experienced differentiation process by fractional crystallization mechanism. This study suggests that igneous rocks from Batur paleovolcano is formed by two phases of formation. Earlier phase is the formation of andesite lava in island arc tholeiitic tectonic setting then at the later phase is formation of diorite intrusion in the calc-alkaline basalts tectonic setting.

Geochemistry of metamorphic rocks and mineralization in the Golgohar iron ore deposit (No. 1), Sirjan, SE Iran: Implications for paleotectonic setting and ore genesis

The Golgohar iron mine is located southwest of Sirjan in the Sanandaj-Sirjan structural zone. The Golgohar ore deposit occurs mainly in the forms of massive, dispersed in altered rocks and rarely stratabound mineralizations, which is related to a fault zone. The mineralization is hosted by metapelites and metabasites. To clarify the paleotectonic, P-T condition of metamorphism and the genesis of the Golgohar ore deposit we report ICP-MS data of metamorphic rocks and EPMA data of feldspar, biotite, amphibole, and magnetite of host rocks and the mineralization. The protolith of metapelites are classified as shale and Fe-bearing shale. The chondrite-normalized REE patterns show enrichment in LREE, fairly flat HREE and negative Eu anomaly. The REE and other geochemical features of the shales support an undifferentiated arc precipitation environment. Based on the relative age of the protoliths, the shales formed during the Late Precambrian-Early Cambrian Pan-African orogeny. The protoliths of amphibolites were calc-alkaline basalt to andesite, which formed in a continental island arc setting. Geochemical data of the amphibolites suggest that the magma was derived from a depleted mantle that was contaminated by melts of subducted sediments. The igneous rocks were also formed during the Pan African orogeny. At the orogenic episode, the deep faulting generated a hydrothermal conduit to form the mineralization in this area. The compositional variations of the magnetite in the mineralization zones and the altered rocks were controlled by the compositions of fluid and the host rocks, the fugacity of oxygen and sulfur, the temperature, and the solubility of elements in the fluid. Large volume of the mineralization crosscutting the host rock is clearly controlled by structure. The presence of dispersed alteration zones, geochemical features such as high values of transitional elements (Co, V) in the magnetite mineral, the presence of xenoliths with high V and Ti contents in the mineralization zones and the stable isotope data indicate that the Golgohar ore deposit formed from high-density magmatic fluids in a subduction zone. The amphibolite host rock contains repidolite, magnesio-hornblende, pargasite, tschermakite, actinolite, phlogopite, andesine-oligoclase-albite, K-feldspar, and quartz. The sedimentary, igneous, and mineralized rocks had undergone metamorphism in the upper greenschist and amphibolite facies (450–700 °C and 6–10 kbar) at the Middle Jurassic, during the Middle Cimmerian tectonic phase. After these events, the deposit has been exposed, oxidized, weathered, and then buried by the Quaternary sediments. Thus, sedimentation and magmatism events occurred during Proto-Tethys subduction in the Cadomian arc, which is related to the Pan-African orogeny. Subsequently, in the late stage of subduction, magmatic fluids were responsible for the multiphase mineralization in the Golgohar area, while at the lateral stage of mineralization, they were accompanied by the mixing of sulfur-rich fluids (evaporative sulfate).

Magma Genesis at the South Aegean Volcanic Arc

The South Aegean volcanic arc consists of five volcanic fields, with products that range from medium- and high-K calc-alkaline basalts to rhyolites. Parental magmas are generated by variable proportions of decompression and flux melting of a mantle source metasomatized by sediment melts and aqueous fluids released from the subducted slab. Fluid/sediment ratios are lowest in Santorini (Greece) where high lithospheric extension results in a predominance of decompression melting, shallower magma storage, and more mafic volcanism than elsewhere in the arc. Contributions from slab sediment melt decrease from west to east. With the lowest convergence rate and surface heat flux of any continental arc worldwide, the South Aegean is an ideal natural laboratory for studying arc magmatism at low magma production rates.

Geochemical constraints on the origin of Neoarchean magmatic rocks in the Lüliang Complex, North China Craton: Tectonic implications

The North China Craton suffered a strong tectonothermal event in the late Neoarchean, with its crystalline basement dominantly composed of ca. 2.6–2.5 Ga granitoid gneisses and metamorphosed volcano-sedimentary sequences. However, controversial issues have long existed on the geodynamic setting of this event. We present new in-situ LA-(MC-) ICP-MS zircon U-Pb and Lu-Hf isotope data, and whole-rock geochemical data for Neoarchean magmatic rocks from the Lüliang Complex. Zircon U-Pb dating reveal that the basalts, andesites and felsic volcanics yield crystallization ages of 2535–2486 Ma, followed by crustal anatexis and metamorphism at ∼2489 Ma. The basalts and andesites yield positive εHf(t) values of 5.1–11.6 and 2.2–6.5 for zircons, corresponding to average Hf model ages of 2547 Ma and 2660 Ma, respectively. The felsic volcanic rock yields positive zircon εHf(t) values of 1.4–6.3, with Hf model ages averaging 2775 Ma. Such ages are consistent with 207Pb/206Pb ages of 2.8–2.7 Ga for relict zircons, indicating reworking of crustal rocks. The basalts and andesites have geochemical compositions of island arc tholeiite (IAT) and calc-alkaline basalt (CAB), suggesting that they were both derived from partial melting of subarc mantle sources that were generated by metasomatic reaction of the mantle wedge peridotite with subducting crust-derived fluids. Compared with the basalts, the andesites were also originated from a subarc mantle source with incorporation of more subducting sediment-derived melts. Thus, both the basalts and andesites would most likely form in a continental arc setting. In this regard, plate subduction and arc magmatism in the North China Craton can be traced back to at least 2.65 Ga in the Lüliang area. The 2.6–2.5 Ga tectonothermal event manifested as widespread accretionary orogeny in the North China Craton, resulting in significant growth of the juvenile mafic crust as well as extensive reworking of the old basement dominated by 2.8–2.7 Ga crustal rocks.

Haida Gwaii (British Columbia, Canada): a Phanerozoic analogue of a subduction-unrelated Archean greenstone belt

Understanding the formation and evolution of Precambrian greenstone belts is hampered by gaps in the rock record and the uncertainty of the tectonic regime that was operating at the time. Thus identifying a modern analogue of a Precambrian greenstone belt can be problematic. In this paper we present geological, geochemical and petrological evidence outlining the case for Haida Gwaii (British Columbia, Canada) as a modern example of a greenstone belt. Haida Gwaii is comprised of two rift-related volcano-sedimentary sequences. The older (Early Triassic) Karmutsen volcanic sequence consists of subaqueous ultramafic-mafic volcanic rocks that are capped by marine carbonate and siliciclastic rocks. The younger (Paleogene) Masset bimodal volcanic sequence consists of tholeiitic and calc-alkaline basalt along with calc-alkaline silicic volcanic and intrusive rocks that are capped by epiclastic sandstones. The Karmutsen and Masset volcanic rocks have indistinguishable Sr-Nd-Pb isotopes demonstrating they were derived from a similar mantle source. Some of the Masset calc-alkaline rocks are compositionally similar to magnesian andesites (SiO2 = 56–64 wt%; Mg# = 0.50–0.64) that are typical of subduction-related Archean greenstone belts. We show that the calc-alkaline signature observed in the bimodal sequence of the Masset Formation is likely due to fractional crystallization of a tholeiitic parental magma under relatively oxidizing (ΔFMQ + 0.7) conditions indicating that a calc-alkaline signature is not prima facie evidence of a subduction setting. Given the geological and geochemical evidence, Haida Gwaii represents one of the best analogues of a modern subduction-unrelated Archean greenstone belt.

The Barreiro suite in the central Ribeira Belt (SE-Brazil): a late Tonian tholeiitic intraplate magmatic event in the distal passive margin of the São Francisco Paleocontinent

New geochemical, U-Pb, Lu-Hf and Sr-Nd data from the Barreiro Suite metabasites in comparison with enclaves within the distal Andrelândia Group and the orthogranulites of the Juiz de Fora Complex are presented. Geochemical data suggest intraplate setting, with high and low- TiO 2 , TDM Nd ages between 1.80 and 1.41 Ga, negative ENd t and ( 87 Sr/ 86 Sr) i between 0.714 and 0.747. Results contrast with part of the Juiz de Fora Complex enclaves, with island arc tholeiites- calcalkaline basalts (IAT-CAB) geochemical signatures, TDM Nd ages between 2.58 and 2.16 Ga, positive ENd t values and ( 87 Sr/ 86 Sr) i between 0.700 and 0.712. U-Pb data for the Barreiro Suite yielded a crystallization age of 766 ± 13 Ma and a metamorphic overprint of 619 ± 6 Ma. The results indicate three episodes of mafic magmatism in the Occidental terrane of the Ribeira Belt. The two older episodes are related to Rhyacian arc evolution (ca. 2.2 to 2.1 Ga) and to the Statherian (ca. 1.7 Ga) tectonics, and occur only within the Juiz de Fora Complex, while the younger ca. 766 Ma episode constrains the timing of distal passive margin evolution. An important implication is that these late Tonian metabasic rocks could have been a source of detrital zircons for the sedimentation along the distal Andrelândia basin.

Characterization of Mineralogy, Petrography, Geochemistry and Petrogensis of Basaltic Outcrops in Jurf Ed Darawish Area, Central Jordan

This research was conducted to investigate the mineralogy, petrography, geochemistry and petrogensis of the basaltic flows in Jurf Ed Darawish (JDB) area of central Jordan. Sexton representative basalt rock samples were selected from the studied JDB outcrops. Modally, JDB consists of plagioclase, olivine, pyroxene (diopside), opaque’s, calcite and iddingsite minerals. Petrographically, basalt is holocrystalline, hypidiomorphic fine to medium grained and exhibited aphanitic to porphyritic texture. The common textures of the JDB rock samples were aphanitic, porphyritic, trachytic, glomeroporphyritic, sub ophitic, vesicular, and amygdaloidal. Geochemically, all of the inspected samples of JDB are located within Trachy basalt and plate alkaline basalt. The tectonic setting of JDB was plotted within the calcalkaline basalt and continental basaltic field. The rare-earth elements showed enrichment of the Ba and K, depletion of Ce relative to K, and enrichment of Nb and Pb with depletion of Y and positive Nb, Zr and Ti anomalies. Negative anomalies of Ba, Sr, Ti and P may be attributed to the fractionation of feldspar for Ba and Sr depletion apatite for P depletion. The positive Nb peak conforms to the tertiary as well as to recent continental alkali basalt provinces and acts as an indicator to the JDB product for the lithosphere from upwelling of the asthenosphere mantle.

Petrography, Geochemistry and Petrogensis of Pleistocene Basaltic Flow from Northwest Atarous Area, Central Jordan

Fifteen basaltic rock samples were collected from central Jordan at the Atarous volcanism basaltic flow area. The samples cover about 8 km2 from the Atarous Basalt flow (AB). The AB flow was introduced in the Miocene to Pleistocene periods. The samples analyze major and trace elements by using XRF. Petrography, Geochemistry and Petrogensis have investigation to carried out for the AB. The petrography analyses of the AB rocks show they are composed of plagioclase (labradorite and bytownite), pyroxene (augite), and olivine (forsterite); accessory minerals include apatite and secondary minerals magnetite, ilmenite, spinel and iddingsite. The AB is classified within alkaline to sub-alkaline and tholeiitic to Calc-Alkaline basalt. The Mg# range between 0.39 and 0.49 of basalt samples exhibits different degrees of fractionation with a low degree of melting < 15% as indicated from the varying concentration of incompatible trace elements Ba, Rb, Sr. Trace elements of primary magna show low variable abundances of compatible and incompatible elements, which reflecs a homogenous source. Geochemical parameters such as Mg# and high Ti contents indicate that the corresponding magmas are of primary origin. The tectonic setting of AB is explained by using discrimination diagrams, Ti-Zr-Sr and Nb-Zr-Y and Ti-Zr-Y, the AB plotted within the plate basalt, alkali basalt and Calk alkaline basalt field, respectively. The spider diagram shows the samples AB enrichment of the Ba, K, Nb and Ce, depletion of Nb and Y. The AB exhibited positive Nb, Ce and Ti anomalies, and negative anomalies of Ba, Sr, and P. It is attributed to the fractionation of feldspar for Ba and Sr and apatite for P depletion. The spider diagram showed a positive Nb peak, which conforms to the tertiary and to recent continental alkali basalt provinces and indicates that the AB is the product of lithosphere from upwelling asthenosphere mantle.

Magma storage below Cascades shield volcanoes as inferred from melt inclusion data: A comparison of long-lived and short-lived magma plumbing systems

In the central Oregon Cascades, mafic volcanic edifices include both short-lived cinder cones and a spectrum of much larger, longer-lived volcanoes. We analyzed olivine-hosted melt inclusions from explosive eruptions from three of the larger edifices to compare crystallization, volatile contents, and storage depths to those of the shorter-lived cinder cones. The melt inclusions were sampled from calc-alkaline basalt to basaltic andesite tephra collected from Belknap, Mount Washington, and North Sister volcanoes. Olivine host compositions are Fo80–83, Fo79–82, and Fo74–81, respectively. These Fo values are lower than the published values for nearby cinder cones (mostly Fo83–85), indicating greater extents of differentiation beneath the longer-lived edifices. The H2O contents of melt inclusions from each of the three volcanoes form a continuous range of values from ≤0.5 to ~2.5 wt% H2O. Many of the inclusions contain a vapor bubble, and the bubbles are mostly in the range of 1.0–6 vol% of the inclusion. After correcting bubble-bearing inclusions for the amounts of CO2 lost to the bubbles, our data show that variable degassing and olivine crystallization occurred beneath the larger edifices over a wide range of depths from > 7 km (> 2–5 kbar) to as shallow as ~0.1 km (~10–30 bars). In contrast, olivine-hosted melt inclusions from nearby cinder cones record entrapment at > 4–5 km depth, with no evidence for shallow crystallization and storage (e.g. Ruscitto et al., 2010). Our results demonstrate that the longer-lived edifices develop a vertically extensive zone of storage and crystallization that extends from the middle to the upper crust and continues into the core of the edifice itself. Modeled primary magma compositions for Belknap, Mount Washington, and North Sister are similar to those from nearby cinder cones, indicating that shield volcanoes and nearby cinder cones are supplied by the same magma sources. The results demonstrate that the shield volcanoes have more complex, shallow magmatic plumbing systems than that of the cinder cones.

Source characteristics and tectonic setting of mafic–ultramafic intrusions in North Xinjiang, NW China: Insights from the petrology and geochemistry of the Lubei mafic–ultramafic intrusion

The newly discovered Lubei sulfide-bearing mafic–ultramafic intrusion forms the western extension of the Huangshan–Jin'erquan mafic–ultramafic intrusion belt in East Tianshan, NW China. The Lubei intrusion comprises hornblende peridotite, lherzolite, and harzburgite in its southern portion, gabbro in its middle portion, and hornblende gabbro in its northern portion. Intrusive relationships indicate that three magma pulses were involved in the formation of the intrusion, and that they were likely evolved from a common primitive magma. Estimated compositions of the Lubei primitive magma are similar to those of island arc calc-alkaline basalt except for the low Na2O and CaO contents of the Lubei primitive magma. This paper reports on the mineral compositions, whole-rock major and trace element contents, and Rb–Sr and Sm–Nd isotopic compositions of the Lubei intrusion, and a zircon LA–MC–ICP–MS U–Pb age for hornblende gabbro. The Lubei intrusion is characterized by enrichment in large-ion lithophile elements, depletion in high-field-strength elements, and marked negative Nb and Ta anomalies, with enrichment in chondrite-normalized light rare earth elements. It exhibits low (87Sr/86Sr)i ratios of 0.70333–0.70636 and low (143Nd/144Nd)i ratios of 0.51214–0.51260, with positive εNd values of +4.01 to +6.33. LA–ICP–MS U–Pb zircon ages yielded a weighted-mean age of 287.9 ± 1.6 Ma for the Lubei intrusion. Contemporaneous mafic–ultramafic intrusions in different tectonic domains in North Xinjiang show similar geological and geochemical signatures to the Lubei intrusion, suggesting a source region of metasomatized mantle previously modified by hydrous fluids from the slab subducted beneath the North Xinjiang region in the early Permian. Metasomatism of the mantle was dominated by hydrous fluids and was related to subduction of the Paleo-Asian oceanic lithosphere during the Paleozoic. Sr–Nd–Pb isotopic compositions suggest that the mantle source was a mixture of depleted mid-ocean-ridge-basalt mantle and enriched-mantle I components. The Permian mafic–ultramafic intrusions in North Xinjiang were formed from tholeiitic basaltic magmas derived from decompression partial melting of the metasomatized mantle in a post-collision extensional tectonic setting. The tholeiitic basaltic magmas are equivalent to the voluminous underplated basaltic magmas that formed during vertical crustal growth of the Central Asian Orogenic Belt in the later Paleozoic.

The southwestern extension of the Jiao-Liao-Ji belt in the North China Craton: Geochronological and geochemical evidence from the Wuhe Group in the Bengbu area

The Wuhe complex is located at the southeastern margin of the North China Craton. The complex consists of metamorphosed Paleoproterozoic potassic granitoids and supracrustal rocks, of which the latter include the Fengyang and Wuhe groups. Meta-mafic rocks from the lower Wuhe Group have igneous zircon U-Pb ages of 2126 ± 37 Ma with εHf(t) values of −6.22 to +8.38, and xenocrystic zircons of 2.39–2.36 Ga, 2.55–2.54 Ga and 2.77–2.69 Ga. Geochemically, the meta-mafic rocks can be classified into two groups. Group 1 island arc tholeiites display flat to slightly right declined REE patterns and moderately negative Nb, Ta, Zr, and Ti anomalies. Group 2 mature arc calcalkaline basalts display strongly fractionated chondrite-normalized REE patterns and evidently negative Nb, Ta and Ti anomalies. These meta-mafic rocks formed by partial melting of sub-arc depleted mantle wedge which had been modified by slab-derived melts at an active continental margin. Depositional age of the group can be constrained in the period of 2.16–2.10 Ga based on ages of the youngest detrital zircons and latter intrusions. U-Pb ages of detrital zircons yield major age peaks of 2.69 Ga and 2.52 Ga, with minor peaks at 2.88 Ga, 2.78 Ga, 2.35 Ga and 2.17 Ga, most of which are derived from the late Mesoarchean to early Paleoproterozoic granitoids in the Wuhe complex and the Jiaodong Terrane. Metamorphic zircons in the marbles coexisting with garnet amphibolites or granulites occur as either single grains or overgrowth (or recrystallization) rims surrounding magmatic zircon cores and yield ages of 1882 ± 19 Ma to 1844 ± 15 Ma. The comparable ca. 2.1 Ga potassic granites with A-type granite affinity, the ca. 2.1 Ga meta-mafic rocks with arc-like geochemical features, the 2.1–1.9 Ga meta-sedimentary units and the 1.9–1.8 Ga subduction- and collision-related granulite-facies metamorphism suggest that the Wuhe complex and the Jiao-Liao-Ji Belt share the same late Paleoproterozoic tectonic evolution process and the former is the southwestern extension of the latter.