Mafic Intrusions Research Articles

Petrogenesis of the Cretaceous Intraplate Mafic Intrusions in the Eastern Tianshan Orogen, NW China

In this study, we conducted zircon U-Pb dating, and whole-rock geochemical and Sr-Nd isotope analyses on the Late Mesozoic dolerite dykes in the Bailingshan Fe deposit (Eastern Tianshan Orogen, NW China) to unravel their petrogenesis and regional tectonic significance. Zircon U-Pb dating on the dolerite yielded an Early Cretaceous age of 129.7 ± 1.4 Ma. The dolerite is calc-alkaline sodic (Na2O/K2O = 4.71 to 6.80), and enriched in LILEs (Rb, K, Sr, and Pb) but depleted in HFSEs (Nb, Ta, and Ti). The intermediate Nb/U (16.7 to 18.5) and Ce/Pb (6.33 to 6.90) values, and the presence of xenocrystic zircons in these dolerite dykes suggest crustal assimilation during the magma evolution. Petrological modeling suggests fractionation of olivine, pyroxene, garnet, and spinel. All the dolerite samples have low initial 87Sr/86Sr (0.7041 to 0.7043) and positive εNd(t) (+ 4.6 to + 5.1) values, indicative of a depleted asthenospheric mantle source. Partial melting modeling suggests that the melting has occurred in the spinel-garnet stability field. Integrating the data from ore deposit geology, geochronology, geochemistry and Sr-Nd isotopes, we proposed that the Late Cretaceous Eastern Tianshan mafic magmatism was developed in an intraplate extension setting.

The Mabja Dome Structure in Southern Tibet Revealed by Deep Seismic Reflection Data and Its Tectonic Implications

AbstractThe ongoing India‐Asia collision has led to the formation of the northern Himalayan gneiss domes belt in southern Tibet. The domes are the result of the ongoing convergence and were formed by geological processes that may include crustal thickening, metamorphism, partial melting, and exhumation of middle crustal rocks to the surface. A combination of compressional, extensional, and diapiric processes has been invoked to explain the formation and evolution of these domes. Differentiating among these competing hypotheses requires well‐defined geophysical images of the internal structure of the domes. The Mabja dome is the largest dome within the northern Himalayan gneiss domes belt. A 70‐km long deep seismic reflection profile across Mabja dome was acquired in 2016. The seismic data could provide new information about the structural elements beneath the domes to the depth of 25 Km. We address the structure of the Mabja dome by conducting an integrated analysis of shallow crustal velocity structure and a normal‐incidence seismic reflection image of deeper crust. Our work suggests that the Mabja dome is underlain by shear zones at depths of 10–15 km and two high‐velocity bodies at depths of 3 km possibly representing the eclogitic‐facies rocks or mafic intrusions. We propose that the dome formation may have been controlled by collision‐induced north‐south shortening expressed by thrust stacking of middle crustal rocks, which led to the doming of the upper‐crustal rocks. The proposed mechanism inferred for Mabja dome can be applied to interpret the widespread domes throughout the southern Tibet and other related structures in orogenic mountain belts.

Experimental settling, floatation and compaction of plagioclase in basaltic melt and a revision of melt density

Centrifuge-assisted piston cylinder experiments were conducted on plagioclase in basaltic melt at 1140–1250 °C, 0.42–0.84 GPa and mostly 1000 g. One set of experiments assesses the settling velocity of a dilute plagioclase suspension; a second sinks or floats plagioclase in a MORB-type melt exploring conditions of neutral buoyancy; and a third set examines floatation of plagioclase from an evolved lunar magma ocean composition. A compaction rate for plagioclase cumulates is established. The experiments demonstrate that neutral density of plagioclase An74 in a MOR-type tholeiitic basalt occurs at 0.59 ± 0.04 GPa (1200 °C), contrasting predictions by present models on melt density which yield a density inversion pressure at 0.10–0.15 GPa. In nature, the level of neutral buoyancy depends on melt composition; nevertheless, for the onset of plagioclase crystallization in dry tholeiitic basalts, our result is robust. As the molar volume of plagioclase is well known, the experimentally determined pressure of neutral buoyancy indicates a correction of -1.6% to previous density models for silicate melts. It follows that for (tholeiitic) layered mafic intrusions, plagioclase is negatively buoyant for early, relatively primitive, parent melts. In contrast, the extreme Fe enrichment of a fractionating lunar magma ocean leads to melt densities that let anorthite always float. Compaction φ/φ0 of experimental plagioclase cumulates is quantified to φ/φ0 = − 0.0582 log (Δρ·h·a·t) + 1.284, where φ0 is the porosity after settling (67 ± 2%), h the cumulate pile height, a acceleration and φ porosity as a function of time t. Gravitational-driven compaction in tens of m-thick plagioclase cumulate in basaltic magmas reaches down to ~ 40% porosity within hundreds of years, a timescales competing with characteristic cooling times of cumulate layers of mafic intrusions. To achieve plagioclase modes > 80% due to compaction, an additional overload of ~ 100 m (layers) of mafic minerals would be required. Compaction of a lunar anorthosite crust of 35 km to 20% porosity (i.e. ~ 90% plagioclase after crystallization of the interstitial melt) would require 30 kyrs.

Cl-rich amphiboles and micas in rocks of the Middle Riphean Kusa-Kopan complex of mafic layered intrusions (southern Urals, Russia)

High-Cl amphiboles and micas were found in rocks of the Kusa-Kopan complex of the Middle Riphean layered ore-bearing (Ti, Fe, V) mafic intrusions in the Southern Urals, Russia. Chloro-hastingsite (3.30–4.57 wt% Cl) was found in garnet-bearing orthoamphibolite in the Medvedevsky massif. Compositions of Cl-rich (3.26–4.84 wt% Cl) amphibole from borehole # 2 of the Kusinsky massif located to the north, correspond to “potassic-ferro-chloro-pargasite”, in some cases to potassic-chloro-hastingsite. Thus, in addition to the high-K trend in the evolution of Cl-rich amphiboles, a tendency to low-K content was revealed. Apart from amphibole, biotite with Cl content from 1.73 to 2.32 wt% corresponding to the annite-phlogopite series was observed. Calculations showed that Cl content reached 1.4 wt% in the melt/fluid during the crystallization of the high-Cl amphiboles of the Kusa part of the layered intrusion, and was about 6000 ppm during the crystallization of the “chloro-hastingsite” from the Medvedevsky massif. The same effect could be seen in lower-Cl varieties, where the differences reached a factor of three. The Cl content in the initial melt did not exceed 400 ppm, locally increasing up to 1000–4000 ppm. Chlorine-rich amphiboles are the result of a local excess concentration of chlorinated compounds of complex composition at the final stage of the magmatic system development (under subsolidus conditions). The emergence of an open system cannot be ruled out, in which Cl content in the fluid increases over time resulting from an external source, which is enriched in Cl due to the addition of mobilized brines from the host rocks additionally enriched with CO2.

Origin of Permian mafic intrusions in southern Chinese Altai, Central Asian Orogenic Belt: A post-collisional extension system triggered by slab break-off

Distinct types of mafic rocks, including gabbro-norite, gabbro-diorite, and monzogabbro-diorite, are exposed in the southern Chinese Altai. Zircon U-Pb dating of these intrusions reveals that they were emplaced at ca. 283–275 Ma. Geochemical data show that these intrusions are characterized by low Ce/Pb (3.1–10.3), low Nb/U (5.5–38.9), and high Th/Yb (0.4–2.4) ratios. They are slightly LREE enriched, with selective enrichments of LILE and LREE over HFSE and significant Nb-Ta and Zr-Hf depletions. These rocks have depleted Nd isotopic compositions (εNd(t) = 1.8–5.0). In addition, the monzogabbro-diorite samples contain abundant primary amphibole and biotite and show high Rb/Y ratios. The above geochemical characteristics and mineral associations indicate that their parental magmas were hydrous and likely resulted from partial melting of subduction-metasomatized sub-continental lithospheric mantle (SCLM). In contrast, the orthopyroxene-bearing gabbro-norite and gabbro-diorite intrusions show anhydrous and high-T mineral assemblages, implying that the hot asthenosphere must have been also involved in their magma genesis. Given the absence of oceanic island basalt (OIB)-like signals in our samples, the contribution of the coeval Tarim mantle plume can be precluded. A slab break-off scenario is proposed for the Chinese Altai after its collision with the East Junggar in Permian, with products of mafic magmas generated by the interaction between partial melts of SCLM and upwelling depleted asthenospheric mantle. Apart from the Altai Range, the coeval mafic-ultramafic magmatism in the Beishan and East Tianshan orogens share comparable petrogenesis, indicating a general tectonic regime of the western CAOB during this transitional period.

The Neoproterozoic to Triassic tectonic evolution of Jangbong Island in the northwestern Gyeonggi Massif on the Korean Peninsula

Jangbong Island is located in the coastal area of the northwestern Gyeonggi Massif, and its basement is composed of Paleoproterozoic gneiss. The basement is covered by Neoproterozoic metasedimentary rocks such as pelitic schist, calc-schist, and quartz schist. These rocks are intruded by mafic dikes metamorphosed under amphibolite facies conditions. In the study area, two different metamorphic conditions are confirmed: the first was intermediate-P/T peak metamorphism (680-630 °C/8.6-7.3 kbar), and the second was low-P/T metamorphism (600-560 °C/5.7-3.1 kbar) showing a clockwise P-T path. All rocks in the studied area are intruded by Triassic gabbro and granite. Age measurement using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields intrusion ages of 917 ± 33 Ma and 873 ± 32 Ma for metabasites. Two different metamorphic ages are also obtained from them: 255 ± 12 ~ 254.3 ± 1.3 Ma and 231.5 ± 1.9 ~ 229.8 ± 1.4 Ma for the first and second metamorphic episodes, respectively. The Triassic gabbro yields an intrusion age of 229.1 ± 0.57 Ma and formed in a post-collisional tectonic setting with Triassic granite. The geochemical analysis confirms that Jangbong Island metabasites formed in a within-plate tectonic setting and are all alkaline basalts and mostly tholeiitic. The 917-873 Ma rift-related mafic intrusion is well matched with the Neoproterozoic rift-related mafic intrusions in the northern Gyeonggi Massif and along the southeastern margin of the North China Craton (NCC). The 255-254 Ma intermediate-P/T peak metamorphism is well matched with the collisional event in the northern Gyeonggi Massif, and the 231-230 Ma low-P/T metamorphism and 229 Ma post-collisional igneous activity are well correlated with post-collisional events in the northern Gyeonggi Massif. Together with previous data, these data support the correlation between the northern Gyeonggi Massif and the NCC.

2.5D geophysical model of the Gällivare mining area: An integrated study to model the top 4 km of the subsurface and guide for future exploration activities

ABSTRACTPotential field and Slingram data alongside rock physical properties, drill‐core information and results from geological field mapping were used to investigate the geometry of the geological structures in the Gällivare area in regional scale. The main purpose of this study was to delineate the vertical and lateral extension of the Malmberget felsic volcanic rocks, the Dundret and Vassaravaara units as well as geological structures related to the crustal‐scale Nautanen deformation zone. Furthermore, we aimed at identifying new magnetite‐hematite and sulphide mineralizations and lithologies related to the mineralizations based on the results from regional‐scale potential field modelling which are delineated with rock physical properties and borehole data. The study result indicated that the dome‐shaped Dundret gabbro extends downwards to ∼4 km depth and has its maximum depth at its centre. Hematite and magnetite assemblages occur within the top 2 km of the Dundret complex. Felsic volcanic rocks in the Malmberget area extend vertically down to a maximum depth of ∼3 km and get considerably thinner towards the west. The model for the known magnetite‐rich mineralizations in Malmberget was inferred from earlier drilling activities and was integrated into the profile models, which indicates a reasonable fit to the measured data, in particular on the magnetic anomaly; whereas the small dimensions of the modelled structures make them invisible on the Bouguer anomaly data. Additional magnetite‐rich mineralizations are suggested within the Malmberget felsic rocks. High real component and low imaginary response of the Slingram data suggests conductive zones within the Nautanen deformation zone towards the NE, which given the geology of the area in the high strain zone can indicate disseminated sulphide mineralizations. Drill‐hole data at the eastern parts of W‐E profiles agreed well with the suggested model inferred from potential field data, suggesting presence of mafic intrusions within the ∼top 1 km of the subsurface. The integrated model based on geophysical, petrophysical and geological data improved earlier understanding about the regional geometry of the key structures in the Gällivare area and indicates new magnetite‐rich and sulphide mineralization prospects which can be used as a guide for future exploration activities in the area.

Seismic Structure, Gravity Anomalies and Flexure Along the Emperor Seamount Chain

AbstractThe Hawaiian‐Emperor seamount chain in the Pacific Ocean has provided fundamental insights into hotspot generated intraplate volcanism and the long‐term strength of oceanic lithosphere. However, only a few seismic experiments to determine crustal and upper mantle structure have been carried out on the Hawaiian Ridge, and no deep imaging has ever been carried out along the Emperor seamounts. Here, we present the results of an active source seismic experiment using 29 Ocean‐Bottom Seismometers (OBS) carried out along a strike profile of the seamounts in the region of Jimmu and Suiko guyots. Joint reflection and refraction tomographic inversion of the OBS data show the upper crust is highly heterogeneous withPwave velocities <4–5 km s−1, which are attributed to extrusive lavas and clastics. In contrast, the lower crust is remarkably homogeneous with velocities of 6.5–7.2 km s−1, which we attribute to oceanic crust and mafic intrusions. Moho is identified by a strongPmParrival at offsets of 20–80 km, yielding depths of 13–16 km. The underlying mantle is generally homogeneous with velocities in the range 7.9–8.0 km s−1. The crust and mantle velocity structure has been verified by gravity modeling. While top of oceanic crust prior to volcano loading is not recognized as a seismic or gravity discontinuity, flexural modeling reveals a ∼5.0–5.5 km thick preexisting oceanic crust that is overlain by a ∼8 km thick volcanic edifice. Unlike at the Hawaiian Ridge, we find no evidence of magmatic underplating.

Evolution of spinel-bearing ultrahigh-temperature granulite in the Jining complex, North China Craton: constrained by phase equilibria and Monte Carlo methods

The peak temperature and timescale of ultrahigh-temperature (UHT) metamorphism are significant for understanding its thermal budget and geodynamic evolution. The spinel-bearing (sapphirine-absent) UHT granulite at the Xuwujia area in the Jining complex, North China Craton was revealed to have undergone a metamorphic evolution that involves the pre-Tmax (maximum temperature) heating decompression to the Tmax stage with an extreme temperature of ~1125 °C, the post-Tmax cooling to the fluid-absent solidus (~860 °C) at 0.8–0.9 GPa, and sub-solidus decompression. The Tmax condition was recorded by the inferred feldspar-absent peak assemblage of spinel ± garnet. The post-Tmax cooling evolution was indicated by the sequential appearances of plagioclase, K-feldspar, sillimanite and biotite, as well as the core-to-rim ascending grossular component in garnet. Moreover, some texturally zoned spinel has exsolved lamellae of magnetite in the core and shows rim-ward increase of Mg and Al, which suggests a temperature decrease from >1100 °C in local domains isolated from quartz. Spinel with exsolved magnetite lamellae or low Al/(Al + Fe3+) is therefore proposed as an indicator for UHT conditions in metapelites. Probability simulation on the collected zircon ages yields a timescale of ~40 Myr (95% confidence; during 1.90–1.94 Ga) for the supra-solidus cooling stage of the UHT metamorphism in the Jining complex. This extreme UHT metamorphism has reached the rock’s dry solidus (~1125 °C) and undergone slow cooling, which is interpreted to result from a post-orogenic plume activity with sufficient advective heating from hyperthermal mafic intrusions.

Baddeleyite Pb[sbnd]Pb geochronology and paleomagnetic poles for ~1.89–1.86 Ga mafic intrusions from the Dharwar craton, India, and their paleogeographic implications

We present new key paleomagnetic pole at 13°S, 152°E (k = 21, A95 = 7.8°) for recently identified 1864.4 ± 2.7 Ma (weighted mean age of four PbPb ages) mafic magmatic event, based on a detailed paleomagnetic study of dolerite dykes and sills intruding Archean basement rocks and Tadipatri formation of the Cuddapah basin, Dharwar craton respectively. The PbPb baddeleyite geochronology yields a crystallisation age of 1867.1 ± 1.0 Ma (MSWD = 1.02) for N77°E trending dyke in the southern region to Cuddapah basin. This new age obtained, confirms the presence of ~1864 Ma magmatic episode with a spatial extent of ~400 km in the Eastern Dharwar craton, within the brief period of ~5 Ma. The paleomagnetic results in these dykes revealed reverse polarity magnetisation direction with mean D = 107°, I = 24° (N = 13 sites, α95 = 10°). Here, we also update the normal polarity magnetic directions on ~1.89 Ga swarm, and the corresponding paleopole situated at 21°N, 336°E (N = 79 sites, A95 = 3.6°). The paleoposition of India is constrained around the equator during ~1.89–1.86 Ga time. The paleogeographic reconstructions were also been attempted at ~1.89 Ga and ~ 1.86 Ga with available key poles from other cratons, indicates the possibility of single plume acting as a source for two distinguishable radial emplacement of mafic dyke swarms across India (Dharwar and Bastar craton) and Western Australia (Yilgarn craton) within a time span of ~35 Ma. The individual movement of India, Baltica and Siberia with a drift rate of ~5.55 cm/yr towards the south, whereas Amazonia craton has moved rapidly to the north (~24.9 cm/yr), do not suggest the amalgamation of a supercontinent (Columbia/ Nuna) during ~1.88–1.86 Ga time.

Ultra‐high temperature, mid‐crustal level, contact metamorphism imprinted on granulite facies paragneisses by a norite intrusion (São Gabriel da Baunilha, Araçuaí orogen, southeast Brazil)

AbstractLarge and hot intrusions may imprint relatively wide aureoles of ultra‐high temperature (UHT >900°C) contact metamorphism even on granulite facies rocks at mid‐crustal levels, especially on those still somewhat hydrated rocks like biotite‐rich paragneisses. After reaching UHT conditions, even followed by rapid cooling, such contact aureoles record reaction textures and newly formed mineral assemblages. Near intrusive contacts, high‐T to UHT contact aureoles present highly recrystallized hornfelsic rocks exhibiting conchoidal fracture that may also preserve inherited features. This paper describes a contact metamorphic aureole developed under high to ultra‐high temperatures at intermediate P conditions, imposed by a mafic intrusion on granulite facies paragneiss. In the Espírito Santo State (SE Brazil), many post‐collisional intrusions (the 525–480 Ma G5 supersuite) occur in the back‐arc region of the Araçuaí orogen. Among them, a greatly homogeneous norite forms a large intrusion hosted by Al‐rich migmatitic paragneisses previously metamorphosed to granulite facies during the collisional stage. Zircon U‐Pb (SHRIMP) data yield the age of 523 ± 7 Ma for the norite crystallization. This intrusion imprinted a contact metamorphism aureole on the migmatitic paragneiss, forming Al‐rich hornfelses free of biotite, near the intrusive contact, followed by recrystallized garnet‐rich rocks with relict gneissic fabrics progressively enriched in biotite towards the distal aureole. The Al‐rich hornfelses are very tenacious, massive, fine‐grained rocks with conchoidal fracture, resinous lustre, intergrowth of feldspars, simplectites of cordierite and hercynite, and show an increase calcium content in plagioclase towards the intrusion. The hornfelses comprise two mineral assemblages: (I) plagioclase–garnet–orthopyroxene–quartz and (II) plagioclase–garnet–quartz–cordierite–sillimanite–hercynite–K‐feldspar. Calculated temperatures for host rocks range from ~1,010°C, for a hornfels collected at 1 m from the intrusive contact, to ~732°C, for a paragneiss located at 262 m from the intrusion. Using Theriak‐Domino software, phase‐equilibrium modelling for the hornfels assemblage (II) provides temperatures between 970 and 1,010°C, at pressures from 5.6 to 6.0 kbar, characterizing UHT contact metamorphism at mid‐crustal level (21–22 km depth). Neocrystallized zircons with soccer ball morphology aged at 520 ± 19 Ma, and P–T conditions of 845°C at 6.35 kbar for a recrystallized paragneiss record the contact metamorphism at least up to 250 m distance from the intrusion. The presented data constrain a P–T–t peak ~5.8 kbar – 990°C – 520 Ma for the contact metamorphism while the P–T–t peak at 7.5 kbar – 785°C – 560 Ma was related to the collisional metamorphism of regional paragneisses in the literature. Therefore, the pressure–time relation between those metamorphic peaks gives a pressure difference of 1.7 kbar (~6.3 km thickness of continental crust) over 40 Ma, corresponding to low decompression (42.5 bar/Ma) and exhumation (157 m/Ma) rates from the collisional peak to the post‐collisional stage, in the studied region.

Petrogenesis and tectonic implications of the Neoproterozoic mafic intrusions in the Bikou Terrane along the northwestern margin of the Yangtze Block, South China

The Neoproterozoic mafic intrusions in the Bikou Terrane along the northwestern margin of the Yangtze Block record crucial information on evaluating the tectonic evolution of the Yangtze Block, South China. However, their petrogenesis and tectonic significance are still poorly constrained. In this paper, we present the results of a comprehensive study of zircon U–Pb geochronology and whole-rock geochemistry from the gabbro-diorites and gabbros of the Bikou Terrane. Zircon U–Pb dating results indicate that these gabbro-diorite and gabbro samples were formed coevally at ca. 880 Ma. The gabbro-diorite samples are calc-alkaline in composition and have intermediate contents of SiO2, Mg#, Ni, and Cr. They are enriched in LREEs, and depleted in HFSEs (i.e., Nb, Ta, Zr, and Hf). They have relatively high ratios of Th/Yb, and low ratios of Nb/Yb, Nb/La, Th/Nd, Th/Zr, and U/Th, indicating that they were derived from partial melting of a mantle wedge source metasomatized by slab-derived melts. The gabbro samples are commonly classified as sub-alkaline tholeiitic rocks, and have constantly low to medium values of MgO, Mg#, Cr, and Ni, with distinct spikes at Th, U, Nb and Ta concentrations. They show an E-MORB-like enriched LREEs, low ratios of Nb/La, Ta/Yb, Th/La, and Nb/Zr, and medium ratios of Nb/Yb and Th/Yb, implying that they were most probably originated from the interaction between the upwelling enriched asthenospheric mantle and the mantle wedge peridotite. This mechanism may be triggered by the slab rollback process in a subduction-related environment. Our new investigations, along with available observations from other scholars, support the persistent subduction-related system along the northwestern margin of the Yangtze Block during the early Neoproterozoic.

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.

Seismic features of the Permian Basin Region from receiver function analysis

We used receiver function (RF) analysis to derive new seismic models of Moho depth, crustal Vp/Vs ratio, depth of sedimentary velocity discontinuity, and corresponding sedimentary Vp/Vs ratio in the Permian Basin region from the seismic waveform data of 260 moment magnitude (Mw) ≥ 6 teleseismic events. We obtained the optimal models by searching one- and two-layer models and comparing similarity between the synthetic and observed RFs. We find the first-order velocity discontinuity in sedimentary basins is shallower than the basin basement. The Delaware Basin (DB) exhibits a bowl-shaped sedimentary velocity discontinuity, with depths ranging from 1.7 km to 5.8 km, whereas the Central Basin Platform (CBP) has a shallow sedimentary velocity discontinuity, with 0.2 km to 2.7 km depths. Moho depth ranges from 36 km to 46 km in the Permian Basin region. The median Vp/Vs ratios of the crust are 1.75, 1.79, and 1.78 in the DB, CBP, and the Diablo Platform (DP), respectively. Our interpretation of the different Vp/Vs ratio in the three subregions is that the crust in the DB includes more low-Vp/Vs-ratio felsic rocks, and the crust in the CBP and DP contains more high-Vp/Vs-ratio gabbroic and basaltic rocks from the Proterozoic and Phanerozoic mafic intrusion.

High Ti-bearing Gabbros from Chalk Hills of Salem, Southern India: A Co-genetic Origin during Neoproterozoic Alaskan-type Evolution

ABSTRACT The Chalk hills of Salem complex also called Salem mafic and ultramafic complex (SMUC), occurs in northern part of Cauvery suture zone (CSZ), southern granulite terrane (SGT), India with the lithological sequence of ultramafic cumulates, mafic intrusions of gabbros and amphibolites with quartzo-feldspathic veins and ultrapotassic dykes. Recently the complex has been described as Neoproterozoic Alaskan-type. The Ti-rich gabbros are in association with serpentinized dunites and pyroxenites in the form of several intrusions at various locations of the complex. The petrography of these gabbros shows sub-alkaline to alkaline in nature with mineralogy of two pyroxenes and plagioclases of calcic-sodic assemblages along with magnetite-titanomagnetite-ilmenite (Ti-spinels). The whole rock geochemistry indicates that they are tholeiitic in nature with enrichment of LILE (Rb, Ba, Th, Sr) and depletion of HFSE (Nb, Ta, Ti) with reference to N-MORB. The mineral chemistry of clinopyroxenes from these gabbros show slightly higher Mg# (0.68-0.80), TiO2 (0.40-0.73 wt%) with lower Al2O3 (4.4-5.5 wt%), Na2O (0.53-0.94 wt%) and Cr2O3 (0.1-0.17 wt%) contents. The orthopyroxenes also show low Al2O3 (1.4-2.3 wt%), Cr2O3 (up to 0.13 wt%), and relatively higher MgO (17.11-21.79 wt%). The Ti-bearing oxides (ilmenites) of these gabbros are characterized by higher contents of TiO2 (38-51 wt%) and FeO (46-56 wt%), similar to that of Ti-Fe rich Alaskan-type of mafic rocks. On various tectonic discrimination diagrams of whole rock and clinipyroxene mineral chemistry of these gabbros reveal that they are derived from subduction related components of arc magmas of titanium rich, evolved through the process of high degree of crystal fractionation. From the two-pyroxene thermometry, the temperature of formation of these gabbros are determined as 893-1014°C and the clinopyroxene barometry results variable pressures of 8-13 kbars. The tectonic setting with the available age relationships from the complex suggest these intrusions are syntectonic and co-genetic of late emplacements along with the ultramafics during Neoproterozoic subduction events. Such occurrences of high Ti-bearing gabbros are common in many Alaskan-type of complexes like Neoproterozoic Arabian Nubian Shield and other parts of the world.

Nano-Micron Exsolved Spinels in Titanomagnetite and Their Implications for the Formation of the Panzhihua Fe-Ti-V Oxide Deposit, Southwest China.

The nano-micron exsolved spinels with various mineralogical characteristics in titanomagnetite from Fe-Ti oxide gabbros in the Panzhihua Fe-Ti-V oxide deposit, SW China, have been studied by field emission scanning electron microscopy (FE-SEM) and electron probe microanalysis (EPMA) based on comparisons of physical and chemical conditions at different stratigraphic heights to investigate the compositional inheritance between titanomagnetite and exsolved spinel and further explore the relationship between the morphology and growth of exsolved spinels. Restored chemical data for titanomagnetite combined with evidence from petrography and whole-rock geochemistry imply fractional crystallization of the Panzhihua Fe-Ti-V oxide deposit, where the titanomagnetite of thick massive oxides at the bottom of the No. VIII orebody represents the early crystallizing phase characterized by high temperature and oxygen fugacity. The chemical variation in the exsolved spinel, which has the same trend as the restored composition of titanomagnetite, represents inheritance from the parent rock within the Panzhihua deposit. Exsolved spinel continuously adjusts morphology and grain size to decrease the total energy of the manganate-spinel system from fine-grained spinels parallel to the {100} plane of titanomagnetite to spinels with complex stellate morphology to bulky granular spinels with high degrees of idiomorphism. The unusual multiple magma replenishment during the mineralizing process and at different stratigraphic heights in the Panzhihua intrusion had an important influence on the thermal evolution history of the orebody, resulting in the identifiable spatial distribution patterns of spinel morphology and grain size. Using spinel exsolution as a discriminator for the provenance of magmatic ore deposits may provide intuitive and easy mineralogical evidence to qualitatively discuss the evolution of the metallogenetic environment and the ore-forming conditions for similar large mafic intrusions.

PGE and isotopic characteristics of Shergol and Suru Valley Ophiolites, Western Ladakh: Implications for supra-subduction tectonics along Indus Suture Zone

Present study reports the PGE-geochemistry of mantle peridotites and Nd-isotope geochemistry of arc related mafic rocks from the Indus Suture Zone (ISZ), western Ladakh. The total PGE concentration of the Shergol and Suru Valley peridotites (∑PGE = 96–180 ppb) is much higher than that of the primitive mantle and global ophiolitic mantle peridotites. The studied peridotites show concave upward PGE-distribution patterns with higher palladium-group PGE/Iridium-group PGE ratios (i.e., 0.8–2.9) suggesting that the partial melting is not the sole factor responsible for the evolution of these peridotites. The observed PGE-distribution patterns are distinct from residual/refractory mantle peridotites, which have concave downward or flat PGE-distribution patterns. Relative enrichment of palladium-group PGE as well as other whole-rock incompatible elements (e.g., LILE and LREE) and higher Pd/Ir ratio (1.1–5.9) reflects that these peridotites have experienced fluid/melt interaction in a supra-subduction zone (SSZ) tectonic setting. Also, the Shergol mafic intrusives and Dras mafic volcanics, associated with the studied peridotites, have high 143Nd/144Nd ratios (i.e., 0.512908–0.513078 and 0.512901–0.512977, respectively) and positive εNd(t) (calculated for t = 140 Ma) values (i.e., +5.3 to +8.6 and + 5.1 to +6.6, respectively), indicating derivation from depleted mantle sources within an intra-oceanic arc setting, similar to Spongtang and Nidar ophiolites from other parts of Ladakh Himalaya. The transition from SSZ-type Shergol and Suru Valley peridotites to Early Cretaceous tholeiitic Shergol mafic intrusives followed by tholeiitic to calc-alkaline Dras mafic volcanics within the Neo-Tethys Ocean exhibit characteristics of subduction initiation mechanism analogous to the Izu-Bonin-Mariana arc system within western Pacific.

Ti–V magnetite stratigraphy of the Upper Zone of the Windimurra Igneous Complex, Western Australia

Layered mafic intrusions (LMI) are stratified igneous bodies, with many examples that are economically significant for a number of metals including chromium, platinum group elements, vanadium, titanium, and iron. The Windimurra Igneous Complex (WIC) of the central Murchison Domain, Western Australia, the largest layered mafic intrusion in Australia, comprises a giant (2500 km2) LMI, with an interpreted thickness of approximately 11 km. The Upper Zone is free of primary hydrous minerals and contains ore-grade Fe-enrichment discoveries up to 66.6 wt% FeOt. Elevated Ti, V and Cr in magnetite at the base of the Upper Zone, and high Cr in the Middle Zone, combined with structural observations in the field, are consistent with injection of two separate magmatic pulses. In this study, four deep diamond drill cores and 92 reverse circulation drill holes from the Upper Zone and uppermost Middle Zone of the WIC sample a near continuous section of 820 m in the Upper Zone, including numerous magnetite-rich horizons. An increase in the whole-rock TiO2/V2O5 upward in the Upper Zone, a decrease in the frequency of magnetite-rich horizons, and an increase upwards in P content, have been used in combination with airborne magnetic surveys, to develop a magnetic stratigraphy for the Upper Zone. The stratigraphy reflects the prospectivity for magnetite, vanadium and chromium, all of which are more abundant at or near the base of the Upper Zone. Such exploration vectors can be used to better target local iron, vanadium as well as chromium mineralization, much of which appear spatially associated with the Upper Zone’s basal contact.

Evidence of melt– and fluid–rock interactions in the refractory forearc peridotites and associated mafic intrusives from the Tuting–Tidding ophiolites, eastern Himalaya, India: Petrogenetic and tectonic implications

We present a comprehensive geochemical data set of whole‐rock geochemistry and mineral phases from the mantle peridotites and mafic intrusives of the Tuting–Tidding Suture Zone (TTSZ) ophiolites, eastern Himalaya, north‐east India. Modal mineralogy, low Al2O3 and CaO, high Cr# of Cr‐spinels, and forsterite number (Fo90–92) in primary olivine indicate the highly refractory nature of the TTSZ peridotites. LREE‐enriched patterns, cumulative olivines in dunites, and Cr‐spinel and olivine compositions further suggest that the peridotites were subjected to high‐temperature melt–rock interaction from a percolating boninitic melt in the nascent forearc of an intra‐oceanic subduction zone. The associated mafic intrusives are tholeiitic in nature and their geochemical characteristics [∑REE = 23.34–59.12; nearly flat rare earth elements (REE) pattern (LaN/YbN = 1.49–2.58); negative anomalies of Nb and Ti] show mid‐oceanic ridge basalt affinity. Trace elemental modelling of the mafic intrusives along with their mineralogy and geochemistry suggests that they formed due to different degrees of partial melting with the involvement of a subduction component in the spreading regime of the forearc region. Based on the presence of hydrous minerals like Cr‐chlorite and tremolite in the peridotites and P–T modelling (525–575°C, 1.05–1.09 GPa) of the mafic intrusives, it is considered that TTSZ ophiolites underwent low‐temperature metamorphism by fluid–rock interaction either during the later phase of subduction (in a cool mature subduction zone) or during Himalayan Orogeny. We also infer that the TTSZ ophiolites resemble other Neo‐Tethyan ophiolites of the Indus‐Tsangpo Suture Zone in terms of their geochemical and petrogenetic aspects.

Palaeoproterozoic turbidite deposition in the Liaodong Penisula, northeastern North China craton – Constraints from the Gaojiayu formation of the Liaohe Group

The Palaeoproterozoic sedimentary environment is difficult to reconstruct because rocks often experience multiple stages of deformation and metamorphism; however, it is critical to understand the Precambrian evolution of sedimentary basin. In this study, we report two un-metamorphosed sections in the Gaojiayu Formation of the North Liaohe Group, central part of the Jiao-Liao-Ji orogenic belt, eastern North China Craton. Rocks consist of shales, carbonaceous slates, calcareous mudstones and sandstones, marlstones, calcarenites, and brecciated limestones in these sections. Detailed field observations indicate that all kinds of original bedding and syn-depositional structure developed in these rocks. Microscope observations suggest that detritus in rocks predominantly contain feldspars, quartz, and calcites, occasionally with volcanic fragments and pyrites. Palaeocurrent study indicates that the provenance of detritus in rock of these two sections was from NW to SE. Detrital heavy minerals mainly consist of pyrite and hematite, occasionally with barite, which are all from the Li'eryu Formation that contains these minerals in sedimentary layers. The increase of percentages of pyrite in a cross-section indicate that the sedimentary environments transformed from an oxidizing to a reducing depositional environment because the depositional setting became deeper. According to the mafic intrusion dated in this study and regional geochronological data such as the age of underlying volcanic rocks, the depositional age of the Gaojiayu Formation is constrained between 2.17 and 2.16 Ga and 2.11 Ga. Archaean basement (2.5 Ga) is recognized as the major provenance providing clastic materials for the Gaojiayu Formation; volcaniclastic rocks in the Li'eryu Formation were also a significant source. Accordingly, in this paper we interpret the Gaojiayu Formation as turbiditic sedimentary rocks that were deposited in an arc-related abyssal and semi-abyssal environment, probably in an extensional back-arc basin setting. Palaeoproterozoic turbidite deposition combined with a 2.17 Ga volcanic arc developed in the study area, we conclude that the modern trench-arc-basin system may had already existed in the early Palaeoproterozoic and deeply affected the tectonic evolution of the Jiao-Liao-Ji orogenic belt, northeastern North China Craton.