High Mantle Temperatures Research Articles

Geodynamic setting of Proterozoic massif-type anorthosites in the Eastern Canadian Shield

Massif-type anorthosites occur worldwide and were predominantly formed during the Paleo- to Mesoproterozoic Era, with an exceptionally large number emplaced in southeastern Laurentia, both within the Grenville Province and in the foreland of the Southeastern Churchill Province and North Atlantic Craton (Nain Province). Their secular nature infers that physical and chemical conditions necessary for their formation were optimized during that time period. A review of geochronological and ambient tectonic regimes that were operating during their emplacement suggests that they formed during four periods, ca. 1.65 Ga, ca. 1.45–1.30 Ga, ca. 1.16–1.14 Ga, and 1.08–1.01 Ga. The first two pulses overlap with continent–ocean convergence and subduction beneath the SE Laurentian margin. The last two pulses correlate with continent–continent collisional tectonics. We argue that higher mantle temperatures during the Proterozoic, as inferred in published models, were not sufficient, by themselves, to produce the necessary volume of basaltic underplate—a key factor in the generation of massif-type anorthosites—and that mantle re-fertilization through metasomatism and/or lateral accretion of oceanic lithosphere might have played a key role. High geothermal gradients acquired during the 2.0–1.2 Ga accretionary and continental arc phase may have helped sustain abnormally high temperatures during the 1.2–1.0 Ga collisional phase of the Grenville Province.

Phlogopite phenocrystals with inclusions of chrome spinelides from non-diamond-bearing kimberlites: the case of An-195 body of the West Ukukit field

The article presents the results of a microprobe study of the composition of phlogopite macrocrystals with inclusions of chrome spinelides from An-195 kimberlite body of the West Ukukit field; Yakut kimberlite province. The purpose of this study was to establish the genesis of such phlogopites: whether they are representatives of deep mantle matter or their formation is directly related to the formation of kimberlites composing An-195. It was found that the inclusions of chrome spinelides are confined only to chromic non–zonal phlogopites or to chromic sites of zonal macrocrystals (Cr2O3 0.75–1.55 wt.%; TiO2 2.67-3.84 wt.%; Mg# 83-88). Based on a comparative analysis of the studied phlogopites and chromespinelides with the composition of analogues from the parageneses of these minerals in various mantle xenoliths of ultrabasic rocks; eclogites and rocks of the MARID series; it was concluded that these phlogopites most likely represent a phenocrystal association of kimberlite minerals. They clearly differ from phlogopites of the MARID series rocks with equal Mg# and TiO2 in higher Al2O3 contents; from phlogopites of ultrabasic mantle rocks xenoliths – generally with lower magnesia content. Phlogopites from eclogites have a generally higher Cr2O3 content. Inclusions of high–chromium spinelides in phlogopite (Cr2O3 44-50 wt.%; TiO2 3-5.5 wt.%; Fe3+ < 0.35 f.e.) correspond in composition to spinelides from cataclysmic dunites and are products of their recrystallization under the influence of high-temperature mantle fluid. Inclusions of less chromic spinelides (Cr2O3 30-44 wt.%; TiO2 4-7.5 wt.%; Fe3+ > 0.35 fe) probably crystallized simultaneously with phlogopite macrocrystals and were captured by them during long-term co-growth.

Modern-style arc magmatism since ca. 2.2 billion years ago revealed by Nb/Ta-Dy/Yb systematics in igneous rocks

The subduction of oceanic lithosphere is a significant mechanism in plate tectonics, which regulates volcanic activity along convergent plate boundaries on the modern Earth. However, while there is evidence for at least episodic, hot subduction in the Archaean, modern-style subduction might have not occurred on the early Earth under a different tectonic regime, potentially due to the high mantle temperature. It remains uncertain when modern-style subduction and its corresponding arc magmatism started operating on a global scale. Here we show that the modern arc intermediate-felsic rocks exhibit a distinct Nb/Ta-Dy/Yb correlation compared to their Archean counterparts due to the contrasting petrogenesis. Our statistical analysis of a worldwide igneous rock database, spanning geological history, indicates the widespread occurrence of intermediate-felsic rocks with modern arc signatures since ca. 2.2 Ga, which coincides with the onset of the supercontinent cycle. Continuous subduction of the cold oceanic lithosphere not only gave rise to arc magmatism along active margins but also accelerated the cooling rate of the mantle, which largely terminated the widespread formations of intermediate-felsic rocks derived from crustal delamination and slab melting.

Petrogenesis of Early Cretaceous volcanic rocks in Santos Basin, offshore SE Brazil: evidence for the Tristan da Cunha plume

ABSTRACT The Santos Basin comprises the most voluminous hydrocarbon accumulations in its ‘Pre-salt’ reservoirs. Magmatic rocks have been recorded in the basin mostly by the tholeiitic basalts (Camboriú Formation) within its rift megasequence at Hauterivian time. Recent studies have shown that magmatic rocks are also found interbedded with Aptian sedimentary rocks in the basin, related to its post-rift (mostly Aptian) megasequence. This paper presents petrological data obtained from sidewall core samples of magmatic rocks interbedded with Early Aptian coquina in a well drilled in the Santos Basin. The well comprises six magmatic sections of alkaline basalts being overlain and underlain by the Barra Velha limestones. The basalts are mostly plagioclase-phyric, amygdaloidal rocks but vary in texture and modal compositions. Magmatic compositions seem to have been slightly affected by alteration processes. Basalts comprise an alkaline, miaskitic series with relatively evolved compositions (MgO = 5.55 ± 1.50 wt%; Ni < 220 ppm; Cr < 400 ppm). The patterns in the Primitive Mantle normalized multielement diagram displays the typical Nb peak (0.3 < La/Nb(N)<0.7) of the OIB. Geochemical modelling showed that basalts within five magmatic sections are not cogenetic by differentiation, representing basaltic flows emitted from different vents. Modal batch partial melting model indicates that it is possible, although unlikely, to generate the differences in La/Yb(N) and Zr/Y ratios of the least evolved samples in the studied well by different partial melting degrees (0.5–11%) from the same mantle source. Binary mixing models using lithogeochemical datapoint to a role for the Tristan da Cunha plume tail in the petrogenesis of the alkaline basalts with no clear evidence for a possible role for the local subcontinental lithospheric mantle. The results of the petrogenetic models are consistent with a shallow MOHO and a high-temperature mantle below part of the Santos Basin in Early Aptian time.

Mo isotopes archive oceanic sediments in post-orogenic lithospheric mantle

The super-chondritic Mo isotopic composition of the continental crust has been attributed to contributions from subducted oceanic slabs and/or sediments, which introduce isotopically light Mo to the mantle. It has been hypothesized that dehydrated oceanic slabs subduct into the deep mantle and contribute to the genesis of ocean island basalts (OIBs). However, OIBs have similar average δ98/95Mo values (−0.22‰± 0.04‰; 2SE; n = 87) to the depleted mantle. Subducted oceanic sediments may create a light-δ98/95Mo reservoir in the subcontinental lithospheric mantle, but this has not been investigated in detail. Understanding the Mo isotopic systematics of the continental crust and mantle requires an understanding of the Mo isotopic composition of lithospheric mantle-derived volcanic rocks. The lithosphere in the Tibetan region has experienced multiple stages of oceanic subduction and is thus ideal for studying magmas derived from sediment-modified lithospheric mantle. Mafic ultrapotassic volcanic rocks (UPVRs) from the Sailipu area on the southern Tibetan Plateau have high Mg# values (67.1–72.5) and enriched Sr–Nd isotopic compositions and are primitive magmas derived from the lithospheric mantle. These rocks have higher Ce/Mo ratios (225–422) and lighter δ98/95Mo values (−0.45‰ to −0.13‰) than the depleted mantle, mid-ocean ridge basalts (MORBs), and OIBs, as well as high εHf(t) values at given εNd(t) values compared with the terrestrial array (ΔεHf = 4.59–5.73). The involvement of Ca-poor pelagic sediments rather than the lower continental crust and hydrated oceanic slab best accounts for the high Ce/Mo ratios and light δ98/95Mo values of the studied rocks. The Sailipu UPVRs also have higher Nb/Ta (18.9–20.8) and Th/La (2.6–3.4) ratios than arc-related igneous rocks and MORBs, suggesting that their source was phlogopite-bearing lithospheric mantle with a metasomatic vein component (rutile + monazite ± lawsonite). We propose a two-stage geodynamic process to explain the formation of the Sailipu UPVRs: (1) a metasomatic vein component with high Th/La and Ce/Mo ratios formed in the lithospheric mantle during multiple stages of oceanic subduction; (2) high mantle temperatures related to convective thinning of the lithosphere resulted in melting of the vein component and the ambient mantle, which produced hybrid melts of various mantle domains during the post-orogenic stage. The Sailipu UPVRs have enriched Sr–Nd isotopic compositions and high Th/La and Ce/Mo ratios, similar to lamproites that are widespread in the Tethyan orogenic realm from the Mediterranean to the southern Tibetan Plateau. Given that UPVRs with light δ98/95Mo values have also been identified in the Western Alps, we propose that the isotopically light Mo component is widespread in Tethyan lithospheric mantle. This isotopically light Mo in the lithospheric mantle may be a complementary end-member to the isotopically heavier continental crust. The recycling of subducted oceanic sediments has a key role in the deep recycling of Mo.

Arc–Backarc Exchange Along the Tonga–Lau System: Constraints From Volatile Elements

Abstract Analyses of olivine-, pyroxene- and plagioclase-hosted melt (now preserved as glass) inclusions (MI) in submarine glasses and subaerial scoria from seven volcanoes along the volcanic front of the Tonga arc provide the first comprehensive dataset including volatile contents (H2O, CO2, S, Cl and F) for this arc. Maximum water contents at each volcano are typical (3–5 wt %) of other arc volcanoes worldwide, and within each volcano, water generally correlates with sulfur, consistent with degassing but not diffusive re-equilibration. The Tonga arc is notable for the eruption of magmas sourced from strongly depleted upper mantle, including boninites in the northern half of the active arc. A key question has been whether such boninites are derived from high degrees of melting actively occurring under the arc, driven by high mantle temperatures and water contents, or from high integrated degrees of melting including melting of depleted mantle in the backarc. This study supports the latter view, based on sub-arc mantle melt fractions of 17–23%, calculated from primary melt water contents and mantle-melt thermobarometry. Such fractions are not high enough to generate boninites from a typical, fertile mantle source in a single melting stage, and melt fractions do not increase north of 22°S, where the boninitic magmas appear. Instead, the northern Tonga arc boninites reflect high cumulative degrees of melting (&amp;gt;30%, with respect to fertile mantle) and occur along the volcanic front where adjacent backarc spreading is fast, proximal and mature. Slab tracers such as the Ba/La ratio peak at around 20–21°S, in the central part of the Tonga arc. A Ba/La peak also appears at about the same latitude in the Lau backarc basin. This peak is consistent with the coolest slab temperatures beneath the arc and backarc at this latitude, as the slab warms to the north due to an increase in mantle potential temperature, and to the south due to less heat extraction from melting. A new concept developed here is cooling of the mantle wedge by melting, due to the supply of the heat of fusion and melt removal. Such a process can cool the mantle by more than 100 degrees, and thus affect slab heating as well. We find a critical distance of slab influence in the Lau spreading centers; back-arc magmas erupted &amp;gt;70 km from the arc are &amp;gt;210 km above the slab, beyond the point of slab dehydration, and show no enrichment in Ba/La. The coupled dynamics of the Tonga arc-Lau basin are rich in their connections, and demonstrate how melting processes beneath the back-arc and arc can affect slab processes, which in turn can affect the composition of fluids that feed the arc and backarc.

Hydrous SiO2 in subducted oceanic crust and H2O transport to the core-mantle boundary

Subduction of oceanic lithosphere transports surface H2O into the mantle. Recent studies show that dense SiO2 in the form of stishovite, an abundant mineral in subducted oceanic crust at depths greater than ∼270 km, has the potential to host and transport a considerable amount of H2O into the lower mantle, but the H2O storage capacity of SiO2 phases at high pressure and temperature remains uncertain. We investigate the hydration of stishovite and its higher-pressure polymorphs, β-stishovite and seifertite, with in situ X-ray diffraction experiments at high pressures and temperatures. The H2O contents in SiO2 phases are quantified based on observed increases in unit cell volume relative to the anhydrous SiO2 system. Density functional theory (DFT) computations permit calibration of water content as a function of volume change based on interstitial substitution of H2O. Regression of our experimental data indicates an H2O storage capacity in stishovite of ∼3.5 wt% in the transition zone and shallow lower mantle, decreasing to about 0.8 wt% at the base of the mantle. We find that SiO2-bearing subducted oceanic crust can accommodate all the H2O in slab lithosphere that survives sub-arc dehydration. Hydration of silica phases in subducted oceanic crust and their unparalleled capacity to host significant amounts of H2O even at high mantle temperatures provides a unique mechanism for transport and storage of water in the deepest mantle.

Earth's anomalous middle-age magmatism driven by plate slowdown

The mid-Proterozoic or "boring billion" exhibited extremely stable environmental conditions, with little change in atmospheric oxygen levels, and mildly oxygenated shallow oceans. A limited number of passive margins with extremely long lifespans are observed from this time, suggesting that subdued tectonic activity—a plate slowdown—was the underlying reason for the environmental stability. However, the Proterozoic also has a unique magmatic and metamorphic record; massif-type anorthosites and anorogenic Rapakivi granites are largely confined to this period and the temperature/pressure (thermobaric ratio) of granulite facies metamorphism peaked at over 1500 °C/GPa during the Mesoproterozoic. Here, we develop a method of calculating plate velocities from the passive margin record, benchmarked against Phanerozoic tectonic velocities. We then extend this approach to geological observations from the Proterozoic, and provide the first quantitative constraints on Proterozoic plate velocities that substantiate the postulated slowdown. Using mantle evolution models, we calculate the consequences of this slowdown for mantle temperatures, magmatic regimes and metamorphic conditions in the crust. We show that higher mantle temperatures in the Proterozoic would have resulted in a larger proportion of intrusive magmatism, with mantle-derived melts emplaced at the Moho or into the lower crust, enabling the production of anorthosites and Rapakivi granites, and giving rise to extreme thermobaric ratios of crustal metamorphism when plate velocities were slowest.

Formation of the Igneous Logi Ridge, NE Atlantic

AbstractThe igneous Logi Ridge is located in the NE Atlantic north of the West Jan Mayen Fracture Zone on oceanic crust. The origin of the ridge may relate to the increased influence of the Iceland plume in the Oligocene though the exact mechanism is uncertain. We estimate the igneous volume and magma production rate of the ridge from 10 transects, comprising a refraction profile, six reflection profiles, and three bathymetric profiles. The refraction profile shows up to 12 km crustal thickness of the ridge, and the forward gravity modeling and/or isostatic balancing shows that this is characteristic for most of the ridges. A 3D volume model was created by interpolating between profiles and subtracted the similarly made regional crustal thickness (5–7 km), which gives an excess magma volume of (1.7 ± 0.6) × 1013 m3. The admittance function between bathymetry and gravity indicates an elastic plate thickness of 3–5 km during the main ridge formation. This is consistent with the ridge being built on oceanic crust from Oligocene to earliest Miocene, indicating a main magmatic phase of ∼5 Ma duration, which gives a magma production rate of 0.07–0.15 m3/s, comparable to the Louisville Seamount Chain (LSC). However, the lower crustal velocities of the refraction profile differ from that of the Louisville Guyot in the LSC, suggesting that the melting event was not mainly caused by high mantle temperature, instead it may have been caused in part by the enriched mantle presently seen at the northern end of the Kolbeinsey Ridge to the south.

Effects of Hotspot‐Induced Long‐Wavelength Mantle Melting Variations on Magmatic Segmentation at the Reykjanes Ridge: Insights From 3D Geodynamic Modeling

AbstractSpatial variations in mantle melting induced by the Iceland hotspot have strong effects on meso‐scale mantle upwelling and crustal production along the slow‐spreading Reykjanes Ridge. The ridge‐hotspot interaction has been recorded by diachronous V‐shaped ridges and troughs extending away from Iceland, as well as by changes in ridge segmentation since 37 Ma. The origins of V‐shaped structures are widely debated, while the causes of the gradual erasion of ridge segments bounded by transform faults are rarely investigated. Through 3D time‐dependent geodynamic modeling, this study investigates how the hotspot‐induced regional mantle melting variations affect ridge segmentation. Periodic temperature perturbations were initially imposed beneath the melting zone to trigger buoyant upwelling cells, which corresponded to the offset ridge segments at the Reykjanes Ridge. Iceland hotspot‐induced long‐wavelength mantle melting variations were generated by applying a regional linear temperature gradient at the bottom of the model domain. Modeling reveals a two‐stage evolution of the buoyant upwelling cells that characterizes the segmentation transition at the Reykjanes Ridge. In Stage 1, the regional mantle melting variations trigger along‐axis pressure‐driven mantle flow, which alters the segment‐scale mantle upwelling and promotes the propagation of segment boundaries away from the region with relatively higher mantle temperature. In Stage 2, buoyant upwelling cells are destroyed progressively as along‐axis mantle flow dominants, leaving V‐shaped diachronous boundaries between the segmented and unsegmented crust. These results advance our understanding of the effects of long‐wavelength mantle melting variations induced by regional mantle heterogeneities on ridge segment evolution at slow‐spreading ridges.

Early Mesozoic Rare-Metal Granites and Metasomatites of Mongolia: Mineral and Geochemical Features and Hosted Ore Mineralization (Baga Gazriin Chuluu Pluton)

Abstract —We present results of petrographic, mineralogical, and geochemical study of all types of rocks of a multiphase pluton and consider the chemical evolution of igneous and metasomatic rocks of the Baga Gazriin Chuluu pluton, based on new precise analytical data. At the early stage of their formation, the pluton granites were already enriched in many trace elements (Li, Rb, Cs, Be, Nb, Ta, Th, and U), F, and HREE relative to the upper continental crust. They show strong negative Ba, Sr, La, and Eu anomalies, which is typical of rare-metal Li–F granites. The geochemical evolution of the Baga Gazriin Chuluu multiphase pluton at the postmagmatic stage was marked by the most intense enrichment of greisens and microclinites with lithophile and ore elements (Sn, W, and Zn) and the formation of ore mineralization. In the permeable rift zone where the Baga Gazriin Chuluu pluton is located, the fluid–magma interaction took place under the impact of a mantle plume. High-temperature mantle fluids caused melting of the crustal substratum, which determined the geochemical specifics of Li–F granite intrusions. Genesis of granitic magma enriched in Li, F, Rb, Sn, and Ta is possible at the low degrees of melting of the lower crustal substratum. The Baga Gazriin Chuluu pluton formed in the upper horizons of the Earth’s crust, where magma undergoes strong differentiation and the saturation of fluids with volatiles can lead to the postmagmatic formation of metasomatites of varying alkalinity (zwitters (greisens), microclinites, and albitites) producing rare-metal mineralization. By the example of the early Mesozoic magmatism area of Mongolia, it is shown that the formation of granites and associated rare-metal minerals is due to the interaction of mantle fluids with the crustal material and the subsequent evolution of granitic magmas.

Semibrittle seismic deformation in high-temperature mantle mylonite shear zone along the Romanche transform fault.

Oceanic transform faults, a key element of plate tectonics, represent the first-order discontinuities along mid-ocean ridges, host large earthquakes, and induce extreme thermal gradients in lithosphere. However, the thermal structure along transform faults and its effects on earthquake generation are poorly understood. Here we report the presence of a 10- to 15-kilometer-thick in-depth band of microseismicity in 10 to 34 kilometer depth range associated with a high-temperature (700° to 900°C) mantle below the brittle lithosphere along the Romanche mega transform fault in the equatorial Atlantic Ocean. The occurrence of the shallow 2016 moment magnitude 7.1 supershear rupture earthquake and these deep microearthquakes indicate that although large earthquakes occur in the upper brittle lithosphere, a substantial amount of deformation is accommodated in the semibrittle mylonitic mantle that resides at depths below the 600°C isotherm. We also observe a rapid westward deepening of this band of seismicity indicating a strong lateral heterogeneity.

Do Olivine Crystallization Temperatures Faithfully Record Mantle Temperature Variability?

AbstractCrystallization temperatures of primitive olivine crystals have been widely used as both a proxy for, or an intermediate step in calculating, mantle temperatures. The olivine‐spinel aluminum‐exchange thermometer has been applied to samples from mid‐ocean ridges and large igneous provinces, yielding considerable variability in olivine crystallization temperatures. We supplement the existing data with new crystallization temperature estimates for Hawaii, between 1282 ± 21 and 1375 ± 19°C. Magmatic temperatures may be linked to mantle temperatures if the thermal changes during melting can be quantified. The magnitude of this temperature change depends on melt fraction, itself controlled by mantle temperature, mantle composition and lithosphere thickness. Both mantle composition and lithosphere thickness vary spatially and temporally, with systematic differences between mid‐ocean ridges, ocean islands and large igneous provinces. For crystallization temperatures to provide robust evidence of mantle temperature variability, the controls of lithosphere thickness and mantle lithology on crystallization temperature must be isolated. We develop a multi‐lithology melting model for predicting crystallization temperatures of magmas in both intra‐plate volcanic provinces and mid‐ocean ridges. We find that the high crystallization temperatures seen at mantle plume localities do require high mantle temperatures. In the absence of further constraints on mantle lithology or melt productivity, we cannot robustly infer variable plume temperatures between ocean‐islands and large igneous provinces from crystallization temperatures alone; for example, the extremely high crystallization temperatures obtained for the Tortugal Phanerozoic komatiite could derive from mantle of comparable temperature to modern‐day Hawaii. This work demonstrates the limit of petrological thermometers when other geodynamic parameters are poorly known.

Boron recycling in the mantle: Evidence from a global comparison of ocean island basalts

Radiogenic and noble gas isotopes have been integral for demonstrating the existence, source, and age of (geo) chemical reservoirs in the mantle, yet, the volatile element composition of the Earth’s interior remains poorly characterized. Boron isotopes have the potential to constrain the processes that generate distinct mantle reservoirs, as they fractionate strongly at the surface of the Earth and during subduction but are little perturbed during high-temperature mantle processes, and so can inform our understanding of mantle volatile cycling. Here, we present the largest, internally consistent, high-precision B isotope dataset from ocean island basalt (OIB) glasses and olivine-hosted melt inclusions measured by Secondary Ion Mass Spectrometry (SIMS) to date, including new data derived from the Pitcairn Islands, Tristan da Cunha, St. Helena, Ascension Island, the MacDonald (Ra) Seamount, and Fogo (Cape Verde Islands) in addition to previously published data from La Réunion, La Palma (Canary Islands), Iceland, and Hawai’i. This dataset allows a comparison of ocean island basalts that contain heterogeneous recycled components (e.g., Pitcairn Islands) to those with primordial components (e.g., La Réunion) in their sources. We focus on basaltic glass and melt inclusions (>6 wt% MgO) as they are least affected by shallow differentiation and assimilation processes. We find that our new OIB data show a limited spread in average δ11B (−5.9 ± 3.0‰ to −10.8 ± 0.7‰), which is smaller compared to previous OIB studies. These data generally overlap with mid-ocean ridge basalts (MORB; −7.1 ± 0.9‰) and display lighter values when compared to mafic arc magmas (∼−9 to +20‰). Importantly, some trace element enriched OIB endmembers display lighter δ11B values and lower B/P and B/Zr, indicative of a source with lower B concentrations relative to the primordial mantle. This suggests that the deeper mantle is becoming increasingly B-depleted over time because boron is effectively stripped from recycled lithologies during subduction and slab dehydration. In addition, the results highlight the decoupling of B isotopes from radiogenic (Sr, Pb) isotopes providing a new perspective on volatile recycling.

Petrogenesis of Granitic Rocks in the Hisakajima Island, Goto Archipelago, Southwestern Japan: A Geochemical Study

Whole-rock chemical compositions including rare earth elements for the granitic rocks from the Hisakajima Island, Goto Archipelago, southwestern Japan were measured in order to constrain their origin and petrogenesis. The granites were divided into two types—a granodioritic group (GD) and a high Fe/Mg ratio granitic group (HFG). The granitic magma was formed by the upwelling of high-temperature mantle material, which might be related to the extension of the Japan Sea around the Middle Miocene. The origin of the GD magma was attributed to the mantle material, while the origin of the HFG magma was attributed to partial melting of the crust by upwelling of the high-temperature mantle. The amount of rare earth elements revealed the secondary addition of light rare earth elements through hydrothermal processes for the granites. Chondrite normalized rare earth element patterns revealed that the HFG rocks were not well differentiated.

High-temperature melting of different crustal levels in the inner zone of the Emeishan large igneous province: Constraints from the Permian ferrosyenite and granite from the Panxi region

The Late-Permian Emeishan mantle plume event caused large-scale flood basalts and related mafic-ultramafic layered mafic intrusions in the inner zone of the Emeishan large igneous province (ELIP). However, the related crustal magma response of this mantle plume event in the inner zone of the ELIP remains poorly understood. In this contribution, we report contemporaneous (~259 Ma) ferrosyenite and A-type granite in the inner zone of the ELIP, and their chronological and geochemical data suggest simultaneous crustal melting at different depths. The ferrosyenites display moderate SiO2 (60.14 to 63.58 wt%), high Fe2O3T (4.28 to 8.52 wt%) and alkaline (Na2O + K2O = 10.91 to 12.75 wt%) contents. In combination with their positive zircon εHf(t) values (+1.5 to +12.9), we propose that the ferrosyenites were formed by high temperature melting of Fe-rich refractory juvenile lower crust at H2O-poor condition, which was induced by underplating of the high-temperature mantle plume. Rhyolite–MELTS modeling results show that ~7% melting of ferrodiorite-like source at ~1120 °C and 8 kbar can produce similar compositions with least evolved ferrosyenite under a relatively dry condition. The Baima granites have high SiO2 (70.58 to 74.26 wt%) contents and 10000*Ga/Al ratios (3.00 to 3.28), indicating the affinity to A-type granites. Their evolved zircon εHf(t) values (−8.1 to −0.6) indicate an evolved crustal source region. Rcrust modeling indicates that ~9% partial melting of ancient granodiorite-like crustal material under low pressure condition (4 kbar) can produce the observed granitic compositions. Considering the close spatial and temporal relationship between ferrosyenites and A-type granites, we propose that the emplacement of ferrosyenitic magma resulted in low-degree partial melting at a relatively shallow crustal level, responsible for the formation of the A-type granites. In summary, this study highlights the significant impact on crustal magma response to the high-temperature mantle plume event in the inner zone of ELIP.

Trace elements and Hf isotopic compositions of four ancient zircon megacrysts from Yakutian kimberlite, Siberia: Implications for identifying mantle and old zircon megacrysts from Kimberlites

金伯利岩中的锆石按照颗粒大小可以分为细粒锆石（一般小于200μm）和巨晶锆石（一般大于500μm）。前人的研究结果显示在金伯利岩中粒径较大的巨晶锆石的U-Pb体系在高温的地幔中一直保持着开放状态，直到寄主金伯利岩浆的喷发才使地幔锆石的U-Pb体系封闭，因此这些巨晶锆石是确定金伯利岩年龄的重要矿物之一。然而，近年来的研究表明，金伯利岩中还存在一些时代远老于金伯利岩年龄的锆石，也具有较大的粒径（以下称古老锆石巨晶）。它们的存在无疑影响了利用锆石U-Pb方法确定金伯利岩年龄的准确性。本文以西伯利亚雅库特（Yakutia）金伯利岩省中的四颗古老锆石巨晶为研究对象，通过形态学、年代学、微量元素和Hf同位素组成，讨论古老锆石巨晶的来源。同时，我们统计和对比了全球多个金伯利岩中能够确定金伯利岩年龄的锆石和古老锆石巨晶的形态学、U、Th含量和微量元素组成、Hf-O同位素等特征。研究结果显示，金伯利岩中的古老锆石巨晶的t_DM年龄和O同位素组成与可以用来确定金伯利岩年龄的锆石巨晶具有明显的差别。这些手段在未来的研究中可以用来区分可确定金伯利岩年龄的锆石巨晶和古老的锆石巨晶。;Zircon from kimberlite can be divided into fine-grained zircon (<200μm) and megacrystal zircon (>500μm) according to its grain size. Previous studies have shown that the U-Pb system of mantle zircon is an open state in the high temperature mantle until the eruption of host kimberlite. Therefore, mantle zircon is one of the important minerals for dating the age of kimberlite. However, recent studies have shown that some zircon megacrysts from kimberlite with ages much older than kimberlite also have large grain sizes. Their presence undoubtedly influences the quasi-dating of kimberlite ages. In this study, four ancient zircon megacrysts from the Siberian kimberlite were studied,and the origin of the ancient zircon megacrysts was discussed through morphology, chronology, trace elements and Hf isotopic compositions. Moreover, the morphology, U and Th contents, trace element composition, and Hf-O isotopes of mantle zircon and old zircon from cratonic kimberlite rocks in the world are collected and compared. The results show obvious differences in t_DM ages and O isotopic compositions between the large old zircon and mantle zircons. These methods can be used to distinguish old zircon from mantle zircon megacrysts in kimberlite in future studies.

Investigation Of Petrological Characteristics Of The Upper Mantle In Hadji-Abad Ophiolitic Complex (South Of Iran): Based On Mineral Chemistry

The mantle peridotites of the Hadji-Abad ultramafic complex in Hormozgan province, show some petrological evolutions of the upper mantle of southern Iran. The complex includes harzburgite, lherzolite, dunite and chromitite. Evidences such as different generations of minerals, lobate boundaries between grains, elongation of cr-spinels and pyroxenes, incongruent melting related textures and exsolution lamellae of clinopyroxene in orthopyroxene show that the rocks in this complex first underwent high temperature-pressure in the upper mantle, and then emplaced in the crust. The chemical composition of chromites in the Hadji Abad dunites and chromitites is similar to that of boninite melts, while, the mineral chemistry of the harzburgitic and lherzolitic show that the host peridotites belong to the upper mantle and have been depleted from incompatible elements due to 15 to 20 percent partial melting. Geo-thermometric calculations reveal that the studied peridotites equilibriated in upper mantle and spinel-peridotite stability field. Using tectonomagmatic discrimination diagrams shows that the Hadji-Abad ultramafic complex is part of an oceanic lithosphere above suprasubduction zone which has undegone partial melting, high temperature deformations and mantle metasomatism processes associated with this environment. These evidences along with the geological and tectonic setting of the Hadji Abad complex adjacent to the Zagros thrust indicate that the complex probably was created in the mantle section under a back-arc basin and then tectonically emplaced as part of Esfandagheh-Hadji-Abad melange in the current situation during the upper Cretaceous. These informations confirm the dependence of the Esfandagheh-Hadji-Abad ophiolite melange on the Neotethyan oceanic lithosphere in southern Iran.

Silurian A-type metaquartz-syenite to -granite in the Eastern Anatolia: Implications for Late Ordovician-Silurian rifting at the northern margin of Gondwana

The Anatolide-Tauride block in the Tethyan belt represented passive northern continental margin of Gondwana throughout Paleozoic time. In this paper, we report on a relatively large amphibolite-facies metaquartz-syenite to -granite complex (~ 60 km2) in the Eastern Anatolia, the easternmost part of the Anatolide-Tauride block. Igneous textures in the metaintrusive complex are variably preserved. The preserved igneous minerals include mesoperthite + quartz ± aegirine-augite ± sodic amphibole ± calcic amphibole ± biotite and ± allanite. Mesoperthite is the sole feldspar in well-preserved igneous textures, suggesting hypersolvus nature of the original quartz syenite to granite. In the well-recrystallized samples, mesoperthite is partially or totally replaced by discrete grains of plagioclase and K-feldspar. U–Pb dating on igneous zircons from three samples yielded Silurian igneous crystallization ages (430–440 Ma). All rock types have typical geochemical characteristics of A-type syenite to granite with relatively high abundances of FeO*, Na2O, K2O, Nb, Ga, Zr, Ce, and Zn, and low abundances of MgO, CaO and Sr, resulting in high ratios of Fe/Mg and Ga/Al. Geochemical features such as (i) the absence of negative Nb–Ta anomalies on multielement variation diagrams, (ii) the enrichment of Nb and Ce relative to Y, and (iii) Y/Nb, Yb/Ta and Ce/Nb ratios similar to those in ocean island basalts point to A1-type syenite and granite which are thought to be differentiates of ocean island basaltic magmas. Estimated magma temperatures based on zirconium saturation range from 850 to 990 °C, indicating derivation of the original quartz syenite and granite from high-temperature juvenile magmas. Initial εHf values of the igneous zircons from two samples are 2.59 ± 1.14 and 2.30 ± 1.29, while δ18O values of the zircons range from 5.41 to 7.32‰ with a median of 6.50 ± 0.48‰. All these isotopic characteristics suggest that the quartz syenite to granite were derived from high-temperature mantle magmas with insignificant crustal assimilation. The Late Ordovician-Silurian A-type igneous rocks in the northern part of the Anatolide-Tauride block were probably related to a rifting event at the northern margin of Gondwana, leading to the opening of the Paleo-Tethys. This interpretation is also supported from (i) regional geological features of the Early Paleozoic sedimentary successions suggestive of extensional setting during the Late Ordovician-Silurian, (ii) the presence of Silurian anorogenic magmatism in continental blocks detached from Gondwana during Early Paleozoic time, and (iii) the presence of Silurian deep sea sedimentary blocks in Paleo-Tethyan accretionary complexes.

Three-dimensional high-resolution velocity structure imaging and seismicity study of the Tangshan fault

Based on the 4415 earthquakes body wave arrival times recorded by the 47 fixed and 33 temporary stations in Tangshan and its surrounding areas, the Tangshan fault’s area focal parameters and three-dimensional fine velocity structure model were inversed by using the consistency-constrained double-difference tomography method. The results revealed that the relocated earthquakes were roughly vertical to the fault’s strike, with the maximum depth exceeding 25 km. The Tangshan fault’s area Vp, Vs and Vp/Vs values demonstrate obvious inhomogeneity, and high speed bodies prone to the brittle fault are dominant on both sides of the fault in the upper crust. Small low velocity bodies appeared initially at the Ms7.8 Tangshan earthquake source in the 14–16 km range of the middle crust, followed by a gradual expansion of the low velocity area with an increase in depth. Variation in the velocity structure below the Tangshan earthquake source shows that the high temperature mantle source invading the crust has weakened the upper crust’s high-speed area, leading to stress accumulation in the local area, which is an important factor for the occurrence of earthquakes in the Tangshan region. The relationship between the regional seismic structure and Tangshan seismic sequence, causing migration of surrounding blocks and seismic activity in recent years was analyzed based on the Tangshan fault’s 3D highresolution velocity structure imaging. It is considered that the Tangshan area unique velocity structure not only plays a pivotal role in promoting horizontal tectonic stress during seismic activity, but is also the root cause of the aftershock evolution process and the fault’s morphological characteristics.