Crustal Depths Research Articles

Metal remobilization from country rocks into the Jiaodong-type orogenic gold systems, Eastern China: New constraints from scheelite and galena isotope results at the Xiadian and Majiayao gold deposits

Orogenic gold deposits in the Jiaodong Peninsula contain >5000 t of known gold resources. Precambrian metamorphic rocks and Mesozoic granite represent the main host rocks for these deposits, but gold deposition (~120 Ma) is significantly younger and it remains unclear whether some of the metals are sourced from the local country rock to these mineral systems. This study uses in situ geochemistry and isotope signatures of scheelite (Sr isotopes) and galena (Pb isotopes) at the Xiadian and Majiayao gold deposits to determine the role, if any, of country rock compositions on the formation of gold ore systems in the Jiaodong Peninsula. Scheelite from the two gold deposits occurred as part of a syn-mineralization stage with quartz, siderite, ankerite, and pyrite in the main orebody and/or its footwall. New results show that scheelite yields high Sr (>200 ppm) and low Mo (<40 ppm) and As (<5 ppm) concentrations, bell to flat REE profiles, and variable Eu anomalies. These scheelite compositions are very similar to previously published results from orogenic-style gold deposits in greenstone belts, indicating fluids with similar geochemical properties. The Xiadian scheelite with negative Eu anomalies may be controlled by REE substitution mechanisms and the composition of parent fluids that may have interacted with graphitic formations of the Jingshan Group, while the Majiayao scheelite with positive Eu anomaly is most likely the result of crystallographic controls on REE partition. New scheelite Sr isotope results for the Xiadian (87Sr/86Sr: 0.71258–0.71540) and Majiayao (87Sr/86Sr: 0.71568–0.71930) deposits further support interaction between hydrothermal fluids and local to distal country rocks. The high radiogenic Sr isotope signature of scheelite from the Majiayao deposit is primarily derived from the regional Jingshan and/or Penglai Group meta-sedimentary rocks. Galena in the Xiadian deposit, which occurs with pyrite in the syn-mineralization stage, has Pb isotope signatures of 206Pb/204Pb: 17.185 ± 0.004 that may reflect mixing between the Linglong granite and/or metamorphic rocks. The Majiayao galena yields the least radiogenic Pb isotope results (206Pb/204Pb: 16.554 ± 0.040), which are consistent with sulfide in the Jiaodong Group meta-volcanic rocks (206Pb/204Pb: 16.270–16.718) and suggest remobilization of Pb from sulfide within the previously metamorphosed formation. New geochemistry and isotope results document a mixed metal contribution that likely occurred as a consequence of fluid-rock interaction between ore-forming fluids and wall rocks at shallow crustal depths (<10 km). Such processes may have played a critical role in generating similar metallogenic features of the Jiaodong gold deposits as well as syn-metamorphic orogenic gold deposits in other greenstone belts. Our study highlights that the remobilization and precipitation of metals (Pb, Sr, W, and possible Au) scavenged from country rocks are perhaps ubiquitous processes, whether in syn- and/or post-metamorphism orogenic gold systems.

Geochemical evolution of the Lebowa Granite Pluton in western Bushveld Igneous Complex, South Africa: More insight into the evolution of bimodal A‐type granitoid

The ~2.05 Ga Bushveld Igneous Complex (BIC) in the Kaapvaal Craton, South Africa, with an areal extent of ~65,000 km2, represents one of the largest bimodal A‐type granitoid suites in the world. The igneous activity of this complex begun with the Rooiberg Group of basalt–rhyolite bimodal volcanism, followed by the emplacement of the ultramafic–mafic Rustenburg Layered Suite (RLS), and finally the granitoid rocks of the Rashoop Granophyre (RG) and Lebowa Granite Suite (LGS). In this study, we present our new borehole description and whole‐rock chemical data on the LGS from the relatively unknown western BIC to constrain the origin of the LGS and try to evaluate their geochemical relationship to the Rooiberg Group felsic volcanics. In the western BIC, the LGS is locally represented by the hydrothermally altered Paalkraal Granite and the Kenkelbos Granite that together form a central dome‐like feature. The Veekraal Granite forms the peripheral part of the dome. These granitoids are, in general, coarse‐grained, porphyritic, or equigranular, showing granophyric intergrowth, and consist of quartz, perthitic microcline, and albitic plagioclase with minor opaque and interstitial hornblende/biotite. Our whole‐rock chemical data on western LGS, along with those from the Groblersdal area, eastern BIC (taken as reference), form a cluster in the ‘granite’ field in the normative Ab–An–Or diagram with mean Or/(Ab+Or) ~0.5 (0.11), and are peralkaline to weakly peraluminous in composition. These A‐type granitoids are strictly ‘ferroan’ and calc‐alkalic to alkali‐calcic in nature, and evolved through partial melting as evident mainly in process identification plots. In bulk chemical composition, the LGS and high‐SiO2 Rooiberg Group rhyolites overlap with the experimental melts generated during dehydration melting of tonalite–granodiorite, indicating the Archaean Kaapvaal TTG as their possible source. Constraints from experimental petrology further suggest the pressure and temperature of this relatively anhydrous (≤1 wt% H2O) melting were 2–1 kbar (i.e., ≤7.5 km crustal depth) and ~920°C, respectively. As suggested before, the parent magma of the voluminous mafic–ultramafic body (RLS) of the BIC, forming a voluminous magma chamber in shallow crustal depth, provided the heat for this partial melting event. The following igneous event as evident from field description in the literature, our new borehole description and whole‐rock chemical data, were a limited chemical mixing between the parent basaltic magma of the Rooiberg Group and the parent LGS magma under anorogenic condition to generate the metaluminous felsic magma represented by the Rooiberg Group dacite at present. This anorogenic mixing, perhaps guided by chemical diffusion and lasted for ≤950 ka, modified the major oxide compositions (except Na), and/or transitional elements (Sc, V, Co, and Ni), LILEs (Rb, Sr, and Ba), REEs, HFSE (Y, Zr, Nb, and Ta), actinide (Th and U), and possibly Sr‐isotopic compositions of both the parent LGS and Rooiberg Group basaltic magmas. The petrogenetic model observed for the BIC is perhaps unique in explaining geochemical and isotopic variations observed in bimodal A‐type granitoid/volcanic suites like the Karoo Igneous Province.

New insights into the structural heterogeneity and geodynamics of the Indo-Burma subduction zone from ambient noise tomography

The absence of fine lithospheric-scale velocity models beneath Myanmar makes it difficult to understand the neotectonics and geodynamics along the Indo-Burma subduction zone. In this study, we present a high-resolution crustal and uppermost mantle 3-D shear-wave velocity (Vs) model of Myanmar to fill this knowledge gap, using ambient noise data from newly deployed seismic arrays. In the upper crust, our model reveals two thick (>10 km) N-S-elongated basins between the Indo-Burman Ranges and the Central Myanmar Basin. At middle to lower crustal depths, low velocities dominate the Indo-Burman Ranges, especially in its northern part where Vs is observed to be as low as ∼3.2 km/s in the lower crust. This feature is interpreted as sediments that were deposited west of the ranges and have since been subducted northeastward and accreted onto the overriding plate. Furthermore, our model reveals an N-S trending high-velocity anomaly beneath the Sagaing Fault, which could be explained by solidified basaltic magma that intruded upwards from the mantle where a low-velocity anomaly is imaged. In the upper mantle, the subducting Indian Plate is clearly imaged beneath Myanmar as an east-dipping high-velocity zone, overlain by a prominent wedge-shaped low-velocity body (Vs < 4.3 km/s). We interpret this low-velocity anomaly to represent partial serpentinization (19–38%) in the forearc mantle. The size and amplitude of this anomaly decrease towards the north, suggesting a northward reduction in serpentinization level within the forearc mantle, possibly related to a northward reduction of water in the subduction zone. This could be associated with lower water content in the subducting plate, as the thick sediments deposited in the north may have driven water out of the lowermost section, while the upper sedimentary section, which could still have been carrying water, would have been scraped off of the downgoing plate and accreted onto the overriding plate, forming part of the Indo-Burman Range.

The construction of the Donegal composite batholith, Irish Caledonides: Temporal constraints from U-Pb dating of zircon and titanite

Magmatic and tectonic processes can transport large volumes of magma generated in the deep crust as discrete pulses to shallower crustal depths, resulting in the incremental construction of large, composite batholiths over thousands to tens of millions of years. The Silurian to Early Devonian Donegal composite batholith in Ireland is a classic example of which regional geological syntheses and lithogeochemical data show that emplacement was syn- and post-kinematic with respect to the terminal phases (ca. 437−415 Ma) of the Caledonian orogeny. We used U-Pb dating of zircon and titanite to investigate the construction of the batholith over time. Imaging of these minerals reveals complex, zoned grains with distinct autocrystic (growth during pluton emplacement) and antecrystic (growth during lower crustal incubation) domains as well as xenocrysts (incorporated from wall rocks). To determine the ages of emplacement and of inherited domains, discrete growth zones were targeted for dating using laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS). Taken together, the zircon and titanite U-Pb isotopic data indicate that magmatism occurred over at least 30 m.y., between ca. 430 Ma and 400 Ma. Batholith emplacement is bracketed by the ca. 427−423 Ma Ardara pluton and the latest phases in the Main Donegal and Trawenagh Bay plutons (ca. 400 Ma). Although apparently volumetrically minor, U-Pb data from spatially associated mafic rocks (appinite suite, lamprophyre dikes, and mafic enclaves in granitoid plutons) yield ages ranging from ca. 431−416 Ma, which indicates ongoing mafic magmatism during emplacement of much of the Donegal composite batholith.

Modeling the multi-level plumbing system of the Changbaishan caldera from geochemical, mineralogical, Sr-Nd isotopic and integrated geophysical data

Changbaishan, an intraplate volcano, is characterized by an approximately 6 km wide summit caldera and last erupted in 1903. Changbaishan experienced a period of unrest between 2002 and 2006. The activity developed in three main stages, including shield volcano (basalts), cone-construction (trachyandesites to trachytes with minor basalts), and caldera-forming stages (trachytes to comendites). This last stage is associated with one of the more energetic eruptions of the last millennium on Earth, the 946 CE, VEI 7 Millennium Eruption (ME), which emitted over 100 km3 of pyroclastics. Compared to other active calderas, the plumbing system of Changbaishan and its evolution mechanisms remain poorly constrained. Here, we merge new whole-rock, glass, mineral, isotopic, and geobarometry data with geophysical data and present a model of the plumbing system. The results show that the volcano is characterized by at least three main magma reservoirs at different depths: a basaltic reservoir at the Moho/lower crust depth, an intermediate reservoir at 10–15 km depth, and a shallower reservoir at 0.5–3 km depth. The shallower reservoir was involved in the ME eruption, which was triggered by a fresh trachytic melt entering a shallower reservoir where a comenditic magma was stored. The trachytes and comendites originate from fractional crystallization processes and minor assimilation of upper crust material, while the less evolved melts assimilate lower crust material. Syn-eruptive magma mingling occurred during the ME eruption phase. The magma reservoirs of the caldera-forming stage partly reactivate those of the cone-construction stage. The depth of the magma storage zones is controlled by the layering of the crust. The plumbing system of Changbaishan is vertically extensive, with crystal mush reservoirs renewed by the replenishment of new trachytic to trachyandesitic magma from depth. Unlike other volcanoes, evidence of a basaltic recharge is lacking. The interpretation of the signals preceding possible future eruptions should consider the multi-level nature of the Changbaishan plumbing system. A new arrival of magma may destabilize a part of or the entire system, thus triggering eruptions of different sizes and styles. The reference model proposed here for Changbaishan represents a prerequisite to properly understand periods of unrest to potentially anticipate future volcanic eruptions and to identify the mechanisms controlling the evolution of the crust below volcanoes.

Response of Fogo volcano (Cape Verde) to lunisolar gravitational forces during the 2014–2015 eruption

Volcanoes are complex systems that evolve in space and time as a result of their eruptive activity. Volcanic eruptions represent the ultimate expression of a complex interplay between internal and external processes that span across different time scales. Deciphering how internal and external processes interact at the time scale of eruptions may provide key insights on the temporal evolution of eruptions and also help to better evaluate associated volcanic hazards. Studies of the tidal influence on volcanic activity have fallen within this context, although the cause-effect relationship between tides and eruptions is still unclear. In this study, we used Singular Spectrum Analysis to analyze three time-series, namely the seismic tremor, SO2 emission and lava volume flow rate, which cover the first month of effusive activity at Fogo volcano, Cape Verde, in 2014–2015. We detect 9 tidal periodicities and up to 5 in each time-series ranging from semi-diurnal to fortnightly periods. We show that the movement of magma at crustal depths and at surface as well as gas emission during the effusive eruption are all modulated by lunisolar gravitational forces. We highlight the relevance of the volcano location on Earth, which together with the timing of the eruption, associated with a specific astronomical configuration, result in a specific combination of tides that directly influence the volcano eruptive activity. With this data set, we further investigate the response of Fogo volcano to this external forcing. We show that during the 2014–2015 eruption, Fogo volcano acted as a bandpass filter to quasi-permanent tidal oscillations.

Geochemical evidence for “cryptic amphibole fractionation” and lower crust melting for the generation of island arc tholeiitic rocks from northern Fossa Magna, central Japan

Amphibole plays an important role in arc-related magma differentiation. This relates to the fact that it contains a variety of elements and volatiles, and because it is one of the main minerals comprising the lower crust. Pliocene-Pleistocene Yoneyama volcanic rocks (northern Fossa Magna, central Japan) show a high-K sub-alkalic composition, and pargasite bearing ultramafic–mafic rocks and megacrysts are frequently contained within them. Volcanic activities are divided into five stages. Based on petrography and petrochemistry, “cryptic amphibole fractionation” was found to contribute to the magma differentiation process for the 1st stage rocks even though amphibole is rarely included as a phenocryst. Partial melting of amphibole bearing lower crust, on the other hand, was controlled the generation of the 3rd stage basaltic magma, with the lower crust being one of the important sources supplying H2O to basaltic magma. On the basis of mineral chemistry and isotopes, amphibole bearing ultramafic–mafic cumulates and megacrysts are considered to have crystallized from these basaltic magmas at a lower crustal depth. We conclude that these cumulates are the direct evidence for “cryptic amphibole fractionation” of Yoneyama volcanic rocks.

Crustal Footprint of Mantle Upwelling and Plate Amalgamation Revealed by Ambient Noise Tomography in Northern Vietnam and the Northern South China Sea

AbstractA high‐resolution crustal and uppermost mantle velocity model of northern Vietnam and the northern South China Sea (SCS) is essential for understanding the evolution of the SCS and its relationship to the Hainan plume, which is still controversial. Based on ambient seismic noise analysis, we processed ambient noise data from 65 broadband stations in southwestern China and northern Vietnam to obtain the crustal and uppermost mantle, S‐wave velocity model, through the direct surface‐wave tomography method, with a lateral resolution of 100–150 km. Our results show distinct lateral velocity variation at shallow crustal depths, such as significant low velocity in the Beibu Gulf Basin and the Yinggehai Basin and relatively higher velocity in Hainan Island. We suggest that the low velocity of the lower crust and uppermost mantle beneath the Yinggehai Basin indicates the present upwelling of upper mantle material. However, the observed high‐velocity upheaval feature beneath the Leizhou Peninsula may represent the ancient magmatic underplating, which has a relation to the Hainan plume. Moreover, our results demonstrate different crustal structures of the two flanks of the Red River Fault, that is, in the middle and lower crust, the S‐wave velocity in the region north of the Red River Fault is higher than that in the south. This may be associated with the collision and amalgamation of the South China and Indochina Blocks. In addition, our results also indicate a crustal footprint of the amalgamation of the Yangtze and Cathaysia Blocks.

Upper crustal shear wave velocity and radial anisotropy beneath Jeju Island volcanoes from ambient noise tomography

SUMMARYJeju Island offshore of the southern Korean Peninsula is an isolated intraplate volcano formed by multiple basaltic eruptions from the Pleistocene (∼1.8 Ma) to the Holocene (∼3.7 ka). Due to the lack of available seismic data, magma structures at upper crustal depths of the island have not been clearly revealed. In this study, we imaged upper crustal isotropic and radial anisotropic structures beneath the island using ambient noise data from a temporary seismic network. A series of transdimensional hierarchical Bayesian inversions were performed to construct upper crustal (1–10 km) isotropic and anisotropic structures. Surface wave (Rayleigh and Love wave) group and phase velocity dispersion data were jointly inverted for 2–15 s. The results show that layers of negative anisotropy (VSH &amp;lt; VSV) are predominant at shallower (&amp;lt;2 km) and deeper (&amp;gt;5 km) depths, which was interpreted as reflecting dyke swarms responsible for the more than 400 cinder cones at the surface and the vertical plumbing systems supplying magma from deeper sources, respectively. Additionally, a layer with significantly positive radial anisotropy (VSH &amp;gt; VSV, up to 5 per cent) was found at middle depths (2–5 km), and was interpreted as horizontally aligned magma plumbing systems (e.g. sills) through comparisons with several other volcanoes worldwide. In comparison with the isotropic structure, the positive anisotropic layer was separated into upper and lower layers with locally neutral to slightly fast and slower shear wave velocities, respectively, beneath the largest central crater (Mt Halla). Such a structure indicates that the cooled upper part of the magma plumbing systems formed within the horizontally developed sill complex, and is underlain by still-warm sill structures, potentially with a small fraction of melting. With dykes predominant above and below, the island-wide sill layer and locally high-temperature body at the centre explain the evolution of the Jeju Island volcanoes by island-forming surface lava flows and central volcanic eruptions before and after the eruptions of cinder cones.

Fault Pattern and Seismotectonic Style of the Campania – Lucania 1980 Earthquake (Mw 6.9, Southern Italy): New Multidisciplinary Constraints

New fault trace mapping and structural survey of the active faults outcropping within the epicentral area of the Campania-Lucania 1980 normal fault earthquake (Mw6.9) are integrated with a revision of pre-existing earthquake data and with an updated interpretation of the CROP-04 near-vertical seismic profile to reconstruct the surface and depth geometry, the kinematics and stress tensor of the seismogenic fault pattern. Three main fault alignments, organized in high-angle en-echelon segments of several kilometers in length, are identified and characterized. The inner and intermediate ones, i.e. Inner Irpinia (InIF) and Irpinia Faults (IF), dip eastward; the outer Antithetic Fault (AFA) dips westward. Both the InIF and the IF strike NW-SE along the northern and central segments and rotate to W-E along the southern segments for at least 16 km. We provide evidence of surface coseismic faulting (up to 1 m) not recognized before along the E-W segments and document coseismic ruptures with maximum vertical displacement up to ∼1 m where already surveyed from other investigators 40 years ago. Fault/slip data from surface data and a new compilation of focal mechanisms (1980 – 2018) were used for strain and stress analyses to show a coherent NNE-directed least principal stress over time and at different crustal depths, with a crustal-scale deviation from the classic SW-NE tensional direction across the Apennines of Italy. The continuation at depth of the outcropping faults is analyzed along the trace of the CROP-04 profile and with available hypocentral distributions. Integrating all information, a 3D seismotectonic model, extrapolated to the base of the seismogenic layer, is built. It outlines a graben-like structure with a southern E-W bend developed at depth shallower than 10–12 km, at the hanging wall of an extensional NE- to E-dipping extensional basal detachment. In our interpretation, such a configuration implies a control in the stress transfer during the 1980 earthquake ruptures and provides a new interpretation of the second sub-event, occurred at 20 s. Our reconstruction suggests that the latter ruptured a hanging wall NNE-dipping splay of the E-W striking main fault segment and possibly also an antithetic SSW-dipping splay, in two in-sequence episodes.

Open-system magmatic evolution and crystallization conditions of the Ediacaran shoshonitic rocks from the São João do Sabugi Pluton, Borborema Province, NE Brazil

Abstract The Sao Joao do Sabugi Pluton (SJSP) (∼579 Ma) is one of the most expressive occurrences of Ediacaran, syn- to post-collisional shoshonitic rocks in the Rio Piranhas-Serido Domain, Borborema Province, NE Brazil. The petrography, whole rock and mineral composition of this pluton were investigated to characterize its open-system magmatic differentiation processes. The pluton is composed of three rock groups: gabbro-diorite, with clinopyroxene ± orthopyroxene ± amphibole ± biotite; monzodiorite, with amphibole and biotite (± pyroxenes); and granodiorite, with biotite as the main mafic mineral. Evidences of open-system processes, including partially resorbed metasediment xenoliths, are widespread. These rocks are metaluminous, alkali-calcic and magnesian, and have shoshonitic affinity. They show relative LILE and LREE enrichment and pronounced negative Nb-Ta anomalies. Major and trace element geochemical modeling favors magma differentiation by fractional crystallization (56–62%) after crustal assimilation of ∼30% of local paragneisses and schists from the Serido Group in a relatively stationary magma chamber at crustal depth, under pressures of 5–6 kbar, temperatures of ∼800–1,000°C and relatively oxidizing conditions. The less evolved gabbro-diorite was generated by ∼18% modal dynamic partial melting of a metasomatized mantle source.

Fault-controlled upwelling of low-T hydrothermal fluids tracked by travertines in a fold-and-thrust belt, Monte Alpi, southern apennines, Italy

Active faults within the southern Apennines fold-and-thrust belt modulate the present-day upwelling of hydrothermal fluids at surface. In the Monte Alpi area, we investigated the structural and geo-morphological settings of a ~10 km2-wide area, and texture and stable isotope composition of Quaternary travertine deposits to identify the present day fault-controlled pathways of low-T hydrothermal fluids at shallow crustal depths. Results of 1:10,000 geological mapping and microstructural analyses show that the Monte Alpi travertines were deposited in a paleo-fluvial environment consisting of cascades and pool/barrage systems, including shallow caves. Travertines precipitated from CO2-rich meteoric-derived paleo-fluids that yielded δ18O compositions from −7.7‰ to −10.1‰. Values in this range overlap the δ18O values of water from active hydrothermal springs in the study area. Accordingly, we assess that travertines precipitated from fluids similar to those currently emerging from low-T (~22 °C) hydrothermal springs active in the study area. In particular, the SO4 chemical signature of hydrothermal springs indicates a complex circulation implying: (1) the infiltration at depth of original meteoric fluid into the Monte Alpi carbonate massif; (2) fluid warming by the geothermal gradient, CO2 enrichment, and interaction with the local stratigraphy (Messinian sulphate-rich deposits); and (3) fluid upwelling through highly permeable fault zone compartments that correspond to fault intersections. Structural analysis and aerial photo interpretation show that sub-parallel fault scarps and landslide escarpments controlled the depositional setting of the travertines. Moreover, travertines only occur above carbonates rocks, whereas travertines are virtually absent along fault scarps and landslide escarpments in shales and metapelites. We conclude that, at fault intersections, high-permability fluid conduits localize along fault zones that sub-parallel the current σhmax of the southern Apennines. In contrast, we infer that clay smearing along fault segment that cut shales and metapelites resulted in low-permeability fault zones, which hampered fluid upwelling.

Geology and crystallization conditions of the Särkiniemiintrusion and related nickel-copper ore, central Finland – implications for depth of emplacement of 1.88 Ga nickel-bearing intrusions

Several Ni-Cu deposits occur within the Kotalahti area, central Finland, in proximity to an Archaean gneiss dome surrounded by a Palaeoproterozoic craton-margin supracrustal sequence comprising quartzites, limestones, calc-silicate rocks, black schists and banded diopside amphibolites. The geology of the area and age of the Ni-bearing intrusions (1.88 Ga) are similar to the Thompson Ni belt in the Canadian Trans-Hudson Orogen. The small mafic-ultramafic and Ni-Cu -bearing Särkiniemi intrusion, closely associated with the Archaean basement core of the Kotalahti Dome, is composed of a western peridotite and eastern gabbro body, both of which are mineralized. The eastern gabbro has a contact aureole several meters thick, consisting of orthopyroxene +/- cordierite bearing hornfels between the intrusion and the migmatites. Geochemically, the Särkiniemi intrusion shares many features in common with other Svecofennian mafic-ultramafic intrusions, including crustal contamination and nickel depletion. The related Ni-Cu deposit has a low Ni/Co value (15) and low nickel content in the sulphide fraction (2.8 wt.%), together with a low estimated magma/sulphide ratio of around 170. Svecofennian 1.88 Ga mafic-ultramafic intrusions occur in terrains of variable metamorphic grade (from low-amphibolite to granulite facies) and are likely to represent emplacement at different crustal depths. Multi-equilibrium thermobarometry indicates that the contact aureole at Särkiniemi reached equilibrium at pressures of 4.5–6 kbar (15–20 km depth) and temperatures of 600–670 °C. Combined with the results of earlier research on the Svecofennian intrusions, this study indicates that a depth of 15–20 km crustal level was favourable, along with other critical factors, for nickel sulfide deposition at 1.88 Ga.

The closure of the Rocas Verdes Basin and early tectono-metamorphic evolution of the Magallanes Fold-and-Thrust Belt, southern Patagonian Andes (52–54°S)

The Western Domain of the Magallanes Fold-and-Thrust Belt (MFTB) between 52°-54°S is part of a poorly studied hinterland region of the southernmost Andean Cordillera. This domain consists of NNW-SSE trending tectonic slices of pre-Jurassic basement units and Upper Jurassic-Lower Cretaceous ophiolitic complexes and volcano-sedimentary successions of the Rocas Verdes Basin (RVB). New detrital zircon U–Pb ages of metatuffs and metapsammopelites constrain episodes of Late Jurassic rift-related volcanism (ca. 160 Ma) followed by Early Cretaceous sedimentation (ca. 125 Ma) during the opening of the RVB. Shear bands developed in the RVB units further record the initial phases of the Andean Orogeny. The 30-km wide thrust stack located on top of the Eastern Tobífera Thrust consists of mylonitic metatuffs, metapelites and metabasalts with a NE-verging brittle-ductile S1* foliation. Phengite-bearing metatuffs commonly record pressure-temperature (P-T) conditions between ~3–6 kbar and ~210–460 °C, consistent with underthrusting of the RVB beneath the parautochthonous magmatic arc in the west. Peak metamorphic conditions of ~6 kbar and 460 °C are derived from a metapsammopelite with textures of contact metamorphism overprinting early mylonitic structures (at least S1*). A back-arc quartz-diorite, intruded at ca. 83 Na, is in contact with the metapsammopelite and constrain the minimum age of deformation at deep crustal depths. Campanian-Maastrichtian (ca. 70–73 Ma) 40Ar/39Ar phengite dates from a mylonitic metapelite indicate the timing of thrusting and backthrusting during the initial uplift of the underthrusted crustal stack. These findings reveal a ~400 km along-strike connection of mylonite belts in a continent-verging thrust structure that became active at the onset of the Andean orogeny during the closure of the Rocas Verdes back-arc marginal basin.

The missing craton edge: Crustal structure of the East European Craton beneath the Carpathian Orogen revealed by double-difference tomography

The Trans-European Suture Zone (TESZ) is the most important and extensive continental suture in Europe, marking the edge of the East European Craton (EEC), from the North Sea to the Black Sea. It corresponds to significant changes in surface geology and deep crustal structure, evident in seismic, gravitational and magnetic studies. However, the TESZ disappears beneath the Eastern Carpathians accretionary nappes and Neotethys ophiolites, thrusted over the subducted EEC passive margin in Romania that may have experienced progressive southward break-off generating post-collisional volcanism and anomalous seismicity in the Vrancea region of the South-East Carpathians. To illuminate the missing TESZ section and investigate the change in crustal properties from the Precambrian EEC across the collisional orogen and the impact of related volcanism, we determined a 3D seismic model of P and S wave velocities of the Eastern Carpathians in Romania. With the advent of new permanent broadband and short-period seismic stations of the Romanian Seismic Network, we were able to lower the earthquake magnitude detection threshold to 0.5 ML, largely expanding the earthquake database and create seismic images of the crust across the Carpathians. Using double-difference tomography and waveform cross-correlation differential times, we relocated local crustal earthquakes between 2010 and 2017 and jointly inverted for the 3D P and S -wave velocity structure down to the Moho discontinuity. Our study provides the highest resolution 3D crustal seismic model of this area to date and emphasizes the manifestation of surface tectonic boundaries at lower crustal depths. The TESZ is highlighted at mid-to lower-crustal depths beneath the Carpathian nappes, east of the post-collisional volcanic belt, as a gently-dipping transition from high Vp in the eastern footwall, the dipping Precambrian basement, to low Vp beneath the Carpathian Orogen to the west and a downward decrease in Vp/Vs. In parts of the volcanic region, Vp/Vs ratios increase suggesting the presence of fluids, mafic material and partial melts that may have altered the seismic structure of the TESZ. Relocated hypocenters are vertically distributed in the velocity transition zone from low to high especially beneath the youngest volcanoes in the south, consistent with a hypothetically active magmatic plumbing system. In the northern segment, where volcanoes are older, relatively high velocities are estimated and seismicity tends to cluster in the top 10 km, likely connected to a recently reactivated fault system. Seismicity in the foreland of the East Carpathians aligns along the NW-SE trending major crustal faults and continues beneath the thrust belt, suggesting the East European Craton basement extends beneath the orogen as far as the volcanic belt.

Early Cambrian multiple-sourced plutonism in the Eastern Sierras Pampeanas, Córdoba, Argentina: Implications for the evolution of the early Paleozoic Gondwana margin

Four major igneous suites with different geochemical characteristics are identified in the accretionary complex of the Pampean orogeny from the southern Sierras de Córdoba, Argentina. Crystallization ages of gabbroic and trondhjemitic plutonic rocks are Early to Middle Cambrian. The plutonic suites include mafic to intermediate alkaline OIB-like rocks, mafic tholeiitic N-MORB rocks, transitional mafic rocks, and minor trondhjemitic rocks with adakitic geochemical signature. The multiple igneous suites are spatially and temporally related to what reveals simultaneous tapping of distinct source reservoirs with their respective melts remaining distinctive from extraction to emplacement at middle crustal depths. The alkaline mafic-intermediate rocks with OIB-like geochemistry were generated by the partial melting of an enriched or metasomatized mantle source. By contrast, tholeiitic mafic rocks with N-MORB geochemical signature came out from a depleted mantle source. Trace element abundances of the transitional mafic magmas reflect variable mixtures between OIB-like and MORB-type magmas. Trondhjemitic rocks exhibit geochemical features highlighting their parent melts formed by low degree melting of a metasomatized subducting oceanic crust. The most reliable setting to generate all these diverse magmatic suites at the same time is a convergent margin where the opening of a slab window followed ridge-trench interaction. A corollary is that widespread mafic magmatism enhanced heat advection and gave rise to extensive intra-crustal anatexis in an accretionary complex of crustal extension.

Multi-stage Jurassic magmatism in the Liaodong Peninsula: Constraints on crustal evolution beneath the eastern North China Craton

Whole-rock geochemical and in situ zircon UPb and HfO isotope compositions have been measured for the Jurassic granitoids in the Liaodong Peninsula, in order to investigate their genesis and reveal the Jurassic crustal evolution of the eastern North China Craton. Our new data, combined with field and petrological observations, indicate multi-stage Jurassic magmatism, now represented by Early to Middle Jurassic biotite granites and monzogranites, and Late Jurassic two-mica monzogranites. The Early to Middle Jurassic granitoids have relatively low SiO2 contents (64.4–69.6 wt%) and K2O/Na2O ratios (0.4–1.0), adakitic characteristics with enriched LREE and depleted HREE contents, high Sr/Y ratios (52–189), positive Eu and Ba anomalies, and negative Nb and Ta anomalies. Together with their highly negative zircon ɛHf(t) (−26.6 to −16.1) and mantle-like δ18O (4.6‰ to 6.3‰) values, partial melting of a basaltic lower crust, which formed at ca. 2.5 Ga, can be proposed for the magma source of the Early to Middle Jurassic granitoids. These source rocks have not experienced significant water–rock alteration after formation. In contrary, the Late Jurassic two-mica granitoids exhibit relatively differentiated geochemical characteristics, with high SiO2 contents (>72 wt%) and K2O/Na2O ratios and variable Eu anomalies. Their parental magma was also enriched in LREEs and depleted in HREEs, with high Sr/Y (49.8–134) and (La/Yb)N (22.6–38.0) ratios. The magmatic zircons have extremely negative ɛHf(t) (−37.5 to −19.6) and relatively high δ18O (6.9‰ to 9.6‰) values, indicating the involvement of an ancient crustal component with high δ18O values, such as metasedimentary rocks. We propose, therefore, that the Late Jurassic two-mica monzogranites originated from partial melting of reworked ancient crustal materials at the depth of lower crust, and with the magma subsequently evolving as a result of plagioclase-dominated fractional crystallization. During Early to Middle Jurassic, westward subduction of the Paleo-Pacific plate, resulted in the partial melting of the mafic lower crust and the burial of ancient crustal materials in to deep crust, while crustal stretching in Late Jurassic might account for the decompression melting of the ancient crustal source. Thus, multi-stage Jurassic magmatism in the Liaodong Peninsula recorded a complex crustal evolution history, which closely related to the westward subduction of the Paleo-Pacific plate.

TRANSVERSE ISOTROPIC CRUST STRUCTURE BENEATH THE NORTHWEST AND CENTRAL NORTH ANATOLIA REVEALED BY SEISMIC SURFACE WAVES PROPAGATION

The Anatolian crust, which is abnormally hot, is widely deformed by subduction related volcanism. Suture zones, transform faults, thrusts and folds and metamorphic core complexes add to the geological complexity. Volcanic provinces such as Western, Central and Eastern Anatolia and Galatea are recognized as distinct features in the region. The middle-to-lower crust depths appear to be intruded by horizontal sills and the upper crust by vertical dykes. Both horizontal sills and vertical dykes leave anisotropic signs detected as Vertical Transverse Isotropy (VTI) that is explored by Love and Rayleigh surface wave inversions, i.e., Love-Rayleigh wave discrepancy which arises because the dykes and sills act differently against the Love and Rayleigh surface waves. The current study gives emphasis to the Northwest and Central North Anatolia utilizing both single-station and two-station tomography techniques to recover the two-dimensional group and phase speed charts from which one-dimensional dispersion inversions are implemented. The one-dimensional inversions are joined to construct the three-dimensional crust of the studied region. The shear-wave anisotropy is used to locate the anisotropy in the crust. The vertical dykes in the upper crust fit into negative VTI around -10% while the horizontal sills in the middle-to-lower crust yield positive VTI around 12%. The vertical magma flows within the vertical dykes and the horizontal magma flows within the horizontal sills contribute constructively to the anisotropy created by the special shape orientations of sills and dykes. The earthquakes hypocenter distribution and high and low speeds alongside the VTI provide significant clues to differentiate between diverse geological districts.

Geochemical, fluid inclusion and O–H–S isotope constraints on the origin of the Rangraz copper deposit, Central Iran

The Rangraz copper deposit (5 Mt @ 0.83% Cu) is located 20 km north of Saveh city, Markazi Province, Iran, in the central Uromieh-Dokhtar volcano-plutonic belt. Host rocks are mainly composed of Eocene volcanic and volcaniclastic rocks that have been intruded by late Eocene quartz monzodiorites and basaltic-andesitic dikes. The dominant copper-related hydrothermal alteration types include silicification, sericitization, and chloritization-kaolinitization. The ore body takes the form of fault-hosted, NW-SE-striking lodes comprising pyrite, chalcopyrite, bornite, specular hematite and magnetite as the principal hypogene ore minerals. Gold (maximum spot assays of up to ca. 0.37 ppm Au) and silver (up to 20.6 ppm Ag) have also been recorded. The ore minerals are mainly confined to veins or veinlets showing crustiform, colloform, open space-filling, dissemination and replacements characteristics in texture. The principal gangue minerals are quartz, barite and calcite. Most of the fluid inclusions identified in quartz-chalcopyrite ± specular hematite veins contain two liquid and vapour phases at room temperature. The primary and pseudo-secondary fluid inclusions have low to intermediate homogenization temperatures between 134 and 338 °C and salinities between 3.55 and 16.89 wt% NaCl equivalents. The fluid inclusion data are consistent with a two-stage evolution of the mineralizing system with initial metal precipitation at crustal depths of <2 km followed by a later pulse of metal deposition at shallower crustal levels of <1 km. The calculated 34SH2S values of sulphide mineral separates from the Rangraz copper deposit range from −9.1‰ to −7.3‰, whereas the calculated δD and δ18O values of the ore fluids range from −3.2 to 0.4‰ and −85 to −75‰, respectively. The δ34S values of the Rangraz samples are consistent with a magmatic sulphur source, implying a possible link between copper mineralization and the Rangraz quartz-monzodiorite. The H, S and O isotope data are consistent with extensive mixing of primary magmatic fluids with cooler, oxidizing meteoric waters occurred in the Rangraz deposit and was responsible for triggering the precipitation of ore minerals. Collectively, the fluid inclusion and isotope data point towards the copper-bearing fluids having been subjected to dilution and cooling, probably as a consequence of their mixing with meteoric waters and boiling. Taken as a whole, the geological setting, ore and gangue mineralogy, vein textures, hydrothermal alteration types, and geochemical, stable isotope, and fluid inclusion evidence may be taken to imply that the Rangraz copper deposit belongs to a class of vein-hosted low sulfidation epithermal mineral systems, several of which have been recorded in the Uromieh-Dokhtar magmatic arc.

Extensive Sills in the Continental Basement from Deep Seismic Reflection Profiling

Crustal seismic reflection profiling has revealed the presence of extensive, coherent reflections with anomalously high amplitudes in the crystalline crust at a number of locations around the world. In areas of active tectonic activity, these seismic “bright spots” have often been interpreted as fluid magma at depth. The focus in this report is high-amplitude reflections that have been identified or inferred to mark interfaces between solid mafic intrusions and felsic to intermediate country rock. These “frozen sills” most commonly appear as thin, subhorizontal sheets at middle to upper crustal depths, several of which can be traced for tens to hundreds of kilometers. Their frequency among seismic profiles suggest that they may be more common than widely realized. These intrusions constrain crustal rheology at the time of their emplacement, represent a significant mode of transfer of mantle material and heat into the crust, and some may constitute fingerprints of distant mantle plumes. These sills may have played important roles in overlying basin evolution and ore deposition.