Crustal Levels Research Articles

Formation and hydrothermal alteration of a volcanic center: Melt pooling and mass transfers at Langseth Ridge (Gakkel Ridge, Arctic Ocean)

Volcanic centers are characteristic features of ultraslow-spreading mid-ocean ridges, the least-explored parts of the global ridge system. Volcanic centers can provide insights into deep magmatic and metamorphic processes at these ridges. Here, we present data from the largest volcanic center on the Gakkel Ridge, the Langseth Ridge, situated at 60–62°E. Langseth is ∼10 km wide, consisting of three peaks that rise to 585 m water depth, some 3–4 km above the surrounding seafloor. It strikes perpendicular to Gakkel's spreading direction and can be traced for ∼40 km, which translates to an age of ∼8 Myr. Seafloor imaging revealed abundant (pillow) basalt but also fissures and geologic faults across the Langseth Ridge. Basaltic rocks were sampled at all summits and diabase at the slope of the northern summit that dips into the rift valley.Our samples are of normal to depleted mid-ocean ridge basalt composition and exhibit a wide range of major and trace element contents, due to magmatic processes, accumulation of macrocrysts, and hydrothermal alteration. Radiogenic isotope ratios, most notably 143Nd/144Nd and 208Pb/206Pb, trend from typical rift valley compositions to isotopically enriched values with increasing distance to the rift valley. This trend may imply melt pooling from different sources, potentially representing a shift from shallow melting beneath the rift valley to deeper melting of enriched sources and higher degrees of melting underneath Langseth. Mineral compositions and plagioclase sieve textures imply prolonged storage of magma at depth prior to eruption. Hydrothermal alteration occurred over a range of conditions. Basalt from the summits is weakly altered at temperatures ≪100 °C, which likely occurred in situ at the summit sites. Diabase samples experienced chloritization and albitization and display epidote and quartz veins, which formed at >300 °C. These assemblages and temperatures are typical for lower crustal levels and imply uplift of the samples of >1 km. Diabase samples from the Afanasenkov Seamount, another volcanic center on the Gakkel Ridge that we investigated for comparison, were altered under comparable conditions.Our findings suggest a combined volcanic–tectonic origin of the studied volcanic centers, potentially implying that such complexes may generally form due to the interplay of magmatism and tectonics. Researching volcanic centers has the potential to further our understanding of both deep and shallow crustal processes at ultraslow-spreading ridges, providing further insights into the role of these centers as linkages between lithosphere and hydrosphere and the (deep) biosphere they sustain.

CARACTERIZAÇÃO QUÍMICA E MINERALÓGICA DOS SUSPENSATOS DOS RIOS PURUS E JURUÁ NO ESTADO DO ACRE

The state of Acre, located in the southwest of the Brazilian Amazon, is drained by two large river basins, the Purus and Juruá rivers, which have their sources on the sub-Andean slopes of Peru and cross the state in a path almost parallel to each other and a southwest-northeast direction. They are meandering rivers, with a large amount of suspended sediment (suspended solids or load materials), which gives them their white or muddy color, in addition to transporting a lot of silty-sandy sediments by traction, forming significant beach bodies (pointed bars). The geological terrains drained by these rivers are made up of rocks from the Solimões Formation and sediments from the Alluvial Terraces. The present work sought to characterize the granulometry, mineralogy and chemical composition of suspensions from rivers in the State of Acre. These were subjected to granulometric analysis, mineral identification using X-ray diffraction (XRD) and scanning electron microscopy (SEM/SED) and total chemical analysis of trace elements. The highest concentration of suspended solids was found in the central part of the State both during the flood and dry periods, while the lowest are in the Moa and Paraná dos Mouras rivers. They are predominantly made up of silt with little fine sand and clay, averaging 12 microns. Mineralogically, they are made up of quartz, mica, smectite, illite, kaolinite, as well as albite and K-feldspar. These are sediments rich in SiO2 (55.69% +/- 3.89%), Al2O3 (18.68% +/- 1.76%), Fe2O3 (5.89% +/- 0.41%), with significant K2O values ​​(1, 93% +/- 0.07%), MgO (1.17% +/- 0.22%), Na2O (0.41% +/- 0.06%), in addition to TiO2 (0.85% +/- 0.10%). The values ​​of P2O5 (0.10% +/- 0.014%), CaO (0.14% +/- 0.17%) and MnO (0.10% +/- 0.021%) are at crustal levels. In terms of trace elements Rb, Sr, Cs, Ba, Th, U, Zr, Hf, Y, Nb, Cr, Co, Ni, Cu, Sc and V, are the elements that presented the highest concentrations. The average concentrations of these elements in the suspended materials when compared with beach sediments show enrichment in Al2O3, Fe2O3, MnO, MgO, Rb, Cs, Sc, and V. On the other hand, with the upper earth’s crust they show enrichment in Cs, Y, Cr, Co, Ni, Cu, Sc and V and with post-Archaean Australian shales only slightly enriched in Y. The presence of clay minerals (smectite, illite and kaolinite) and feldspars (albite and K-feldspars) are responsible for the high concentrations not only Al2O3 and Fe2O3, but also K2O, MgO and Na2O. The mineralogical and chemical composition shown by these suspensions, in addition to their granulometry, contribute to fertilizing both the floodplains (“varzeas”) as well as the beaches exposed by the rivers during the dry period, which are intensely cultivated with corn (Zea-mays) and beans (Vigna unguiculata (L) Walp) by riverside dwellers.

Geochronology and geochemistry of a Neoproterozoic syn-tectonic granitic pluton in the Gari-Gombo area, East Cameroun: Implications for petrogenesis and tectonic evolution

Granites are widespread in many Precambrian orogenic belts worldwide; therefore, they can provide insights into orogenic processes and associated magmatism. Zircon UPb age, monazite Th-U-total Pb age and whole-rock geochemical data for a granite pluton from the Gari-Gombo area in the Adamawa-Yade domain of the Central African Fold Belt (CAFB) in East Cameroon are presented. The granite is composed dominantly of perthitic K-feldspars, quartz, plagioclase and minor biotite with accessory monazite, apatite and zircon. LA-ICP-MS zircon UPb dating yielded an age at ca 631–620 Ma, which is interpreted as age of emplacement that coincides with the onset of D2 Pan-African deformation. Monazite grains in Gari-Gombo granite follow strictly the huttonite substitution trend in Th + U vs Si coordinates. Monazites give consistent Neoproterozoic ages of 630 ± 4 Ma and 602 ± 4 Ma, indicating that growth history and crystallization age of monazites also correlate well with the Pan-African plutonism and granulite facies metamorphism (ca 614–600 Ma) in the Gari-Gombo area. The Gari-Gombo pluton samples show high-K calc-alkaline magnesian, slightly peraluminous signature, high SiO2 (70.16–78.80 wt%), K2O (4.39–5.38 wt%), and Rb (165–248 ppm), and low P2O5 ≤ 0.01 wt% and Sr (146–222 ppm) contents. They have highly-fractionated REE pattern ((La/Yb)N = 6.17–148.18), moderately Eu negative anomalies (Eu/Eu* = 0.53–0.93) and the obviously Nb and Ti negative anomalies. These geochemical features suggest that the Gari-Gombo pluton is a highly fractionated I-type granite generated by partial melting of older meta-igneous materials at middle to lower crustal levels. The 2.9 and 0.95 Ga inherited zircon grains identified within the studied granites further confirm the existence of ancient crust in this region.

Using U-Pb detrital zircon systematics to constrain the architecture and provenance of the Ronda peridotites’ crustal hanging wall, S Spain

Detrital zircon geochronology is frequently used for sediment provenance analysis in orogenic belts, but it can also be applied to locate the boundary between basement and sedimentary cover in successions and tectonometamorphic units where the paleontological record is missing, and primary stratigraphic relationships have been erased by intense deformation and/or metamorphism. This is critical to differentiate metamorphic mineral associations, P-T conditions and deformational events associated with superposed orogenic cycles. In this work, detrital zircon geochronology has been applied to the Ronda peridotites’ crustal hanging wall (hinterland of the Alpine Betic Cordillera, southern Spain), apparently made up of two major tectonic units – the Jubrique unit and the Maláguide complex – separated by a major Alpine tectonic contact camouflaged in between low-grade metamorphic rocks.Our results show that pre-Paleozoic peaks (c. 650, 1000, 2000, and 2600 Ma) are common along the entire studied crustal sequence, and Paleozoic ages are only recorded in the upper part of the Jubrique unit (c. 290, 350, and 475 Ma) and in the Maláguide complex (c. 350 and 475 Ma). Thus, the Paleozoic rocks of the Maláguide complex, constituting the highest crustal unit overlying the Ronda peridotites, represent syn-to-late orogenic Variscan sediments deformed and slightly metamorphosed prior to the Alpine cycle. In contrast, the underlying Jubrique unit is made of a post-Variscan sedimentary cover deposited on a Variscan basement, with the contact between both (hidden unconformity?) located within low- to medium-grade schists. These data support the presence of a major tectonic contact between the Maláguide complex (Late Carboniferous maximum depositional ages) and the Jubrique unit (Early Permian maximum depositional ages), i.e. the Maláguide complex would have overthrust the Jubrique unit during the Alpine orogeny. These results indicate a Permian or younger exhumation/emplacement up to crustal levels of the Ronda peridotites subcontinental mantle slab.Altogether, these detrital zircon age spectra support a relatively, but not entirely local derivation of the studied crustal sequences (fragments of the Iberian Massif) and suggest some Mesozoic drifting with respect to its present position. Nonetheless, the different age distribution of the Paleozoic populations between the Jubrique (Alpujárride complex) and Maláguide samples suggests that the former deposited relatively close to other samples from the eastern-central Alpujárride complex and Nevado-Filábride complex, which, in turn, show strong similarities with the Cantabrian zone and NE Iberia (Iberian Massif). In contrast, the Maláguide samples were probably deposited close to the Ossa-Morena Zone (Iberian Massif) . Thus, Alpujárride/Nevado-Filábride and Maláguide complexes would have been juxtaposed during the Triassic-Cretaceous Neotethys rifting/drifting and/or the subsequent Alpine collisional events.

Magmatic sulfide oxidation drives crustal PGE mobilization: Implications for hydrothermal PGE mineralization

Platinum-group elements (PGEs) have a strong affinity for sulfur and tend to accumulate in the deep continental crust, either concentrated within magmatic Cu-Ni-PGE deposits or dispersed throughout disseminated sulfides. However, PGE enrichment in shallow magmatic-hydrothermal systems implies an obscure link to deep sulfide destabilization, which releases PGEs into ore-forming fluids. To bridge this gap, our study investigates the PGE composition of magmatic sulfides with oxidative textures in dacitic rocks from the southwestern Okinawa Trough. We identified three groups of magmatic sulfides, primarily precipitated as Cu-poor monosulfide solid solution (MSS), which formed at distinct stages of magma evolution from deep to shallow crustal levels. Group A sulfides manifest as small-sized inclusions (<30 μm) within most high-Mg olivines, whereas Group B and C sulfides are larger (50–500 μm) and occur within cognate xenoliths of mafic cumulate rocks and the groundmass of host dacites. Group B and C sulfides exhibit distinct oxidative textures and newly-formed mineral assemblages, including magnetite, hematite, goethite, and magnetite ± pyrite, alongside hydrothermal silicate minerals, respectively. We attribute the oxidation process to the infiltration of orthomagmatic fluids exsolved from mafic magma that had underplated the sulfide-bearing felsic magma reservoir, which was nearly solidified. By comparing the chemical compositions between pristine sulfides and their oxidative remnants, we observed extensive mobilization of Pd, Pt, Cu, Ag, Ni, and Co from the altered sulfides of Group B, while Au enrichment occurred as nanoparticles under high oxidation states. In contrast, Au was extracted along with other mobile metals from the altered sulfides of Group C, with Pt remaining in place under more reduced conditions. These distinct scenarios may lead to the formation of PGE-rich and Au-rich fluids, respectively. The formation of deep crustal MSS and subsequent hydrothermal oxidation under varying redox conditions thus provides a viable mechanism for trans-crustal PGE mobilization and inter-element fractionation, typical of PGE-rich magmatic-hydrothermal deposits.

Petrogenesis and Geodynamic Evolution of the Archean Shawmere Anorthosite Complex and Associated Gneisses, Kapuskasing Uplift, Superior Province, Canada

Abstract In this study, we integrated extensive field, petrographic, whole-rock major and trace element, and Nd–Pb–Sr–O isotope, and zircon U–Pb ages, trace element and Lu–Hf isotope data from the Neoarchean Shawmere Anorthosite Complex and surrounding gneisses to unravel their petrogenetic origin and tectonic history. The ~2765 Ma Shawmere Anorthosite Complex is interpreted to have been emplaced into a sequence of interlayered greywacke and basalt deposited in an intra-continental arc rift system above a north-dipping subduction zone. The complex consists mainly of anorthosite, leucogabbro, gabbro, and hornblendite that were emplaced as several batches of magmas and crystal mushes originating from sub-arc mantle sources. In contrast to the previous studies, our field and petrographic data suggest an igneous origin for the most hornblende in the complex, implying hydrous parental magmas. A hydrous magma origin is also consistent with the high-anorthite content (mostly 70–90%) of the plagioclase in the complex. Percolation of hydrous basaltic melts through gabbroic cumulates in crustal magma chambers led to extensive (&amp;gt;50%) replacement of igneous clinopyroxene by igneous hornblende. Continued subduction resulted in the closure of the intra-arc rift system and the intrusion of the complex by tonalite, granodiorite and diorite between 2765 and 2680 Ma in an Andean-type margin. The complex and surrounding gneisses underwent hornblende granulite-facies metamorphism mainly between 2680 and 2620 Ma, overlapping with mid-crustal east-west extension between 2660 and 2640 Ma. The granulite-facies metamorphism is recorded by the replacement of hornblende, plagioclase and clinopyroxene by garnet and the development of a garnet-orthopyroxene-plagioclase metamorphic assemblage with a granoblastic texture. Tectonic rebounding of mid-crustal rocks to upper crustal levels after 2620 Ma led to the formation of an extensive network of extensional fractures and retrograde metamorphism. Migration of CO2-rich hydrous fluids along the extensional fractures and grain boundaries resulted in the precipitation of many metasomatic minerals mainly at the expense of hornblende and plagioclase, including epidote, clinozoisite, tremolite, actinolite, paragonite, margarite, titanite, quartz, calcite, sillimanite, dolomite, and chlorite. Prevalent replacement of hornblende by garnet during prograde metamorphism and metasomatic replacement of hornblende and plagioclase by retrograde mineral assemblages disturbed the Sm–Nd, U–Th–Pb, and Rb–Sr isotope systems.

Petrogenesis and zircon U-Pb ages of the Late Paleozoic high-potassic Sonkul and Kokturpak plutons in Kyrgyz Tien Shan, with implications for related skarn-porphyry W-Au(-Cu-Mo) mineralization

ABSTRACT The Sonkul and adjacent Kokturpak multiphase plutons are situated along the ‘Major Structural Line of Tien Shan’ (known also as the ‘Nikolaev Line’) that extends for over 500 km and divides the terranes of the Middle and Northern Tien Shan. The plutons comprise a broad variety of high-potassic igneous rocks including the early olivine gabbro, intermediate monzonite to quartz monzonite, and later granodiorite, monzogranite, and leucogranite-alaskite, in turn followed by the late mafic dikes. U-Pb dating of zircon grains yields ages of 299 ± 2 Ma (olivine gabbro), 300 ± 3 Ma to 298 ± 3 Ma (monzonite to quartz monzonite), 299 ± 3 Ma to 298 ± 4 Ma to 297 ± 4 Ma (granodiorite), and 289 ± 4 Ma to 285 ± 2 Ma (monzogranite). These consecutive intrusive phases appear to have been produced by partial re-melting of a partially crystallized (mush) magma batch at deeper levels, which is evident by the presence of zircon antecrysts dated at some 306–311 Ma (up to 323 Ma?). Geochemical signatures of the igneous rocks correspond to high-K calc-alkaline to shoshonitic series intrusions emplaced in a transitional subduction-related to post-collisional setting. A low-degree partial melting of the metasomatically enriched upper mantle can be envisioned as the leading process of shoshonitic magma generation; this was followed by amphibole fractionation in a deep (lower crustal ?) magma chamber and possibly a mantle-induced generation of granitic magmas in the crustal protolith. Further mixing/mingling of the mafic magma and crustal granitic magma was followed by the magma fractionation and emplacement at shallow crustal levels. Assimilation of the old continental crust is consistent with the presence of ancient xenocrystic zircons (dated at ca. 1.5 to 2.5 Ga) in the studied rocks. The Late Carboniferous to Early Permian age and high-K calc-alkaline to shoshonitic composition are similar to those of highly mineralized intrusions of the other segments of the Middle Tien Shan. In this regard, several magmatic pulses of high-potassic intrusions and corresponding metallogenic pulses can be suggested in this region. Whereas the early (ca. 337 to 315 Ma) igneous suites accompanied by dominantly porphyry Cu-Au-Mo mineralization, the later (ca. 305 to 285 Ma) suites including the Sonkul and Kokturpak plutons are accompanied by W- and/or Au-dominant mineralization. This mineralization appears to pre-date more substantial intrusion-related to orogenic-type Au mineralization.

Spatial variation of body wave attenuation in Garhwal-Kumaun Himalaya region, India

We have investigated the spatial variation of body wave attenuation in Garhwal-Kumaun Himalaya region from the datasets of 465 well located earthquakes, recorded at 52 broadband stations between 2017 and 2020. The body wave attenuation parameters QP and QS were estimated at each station by applying the extended coda normalization method for five different central frequencies ranging from 1.5 to 18 Hz. Strong frequency-dependent body wave attenuation was observed at each station over the whole region. Also, we found a significant variation of QP and QS from north to south which is consistent with geotectonic diversity beneath the study region. We have also made separate estimations of frequency-dependent relations for Garhwal and Kumaun Himalaya region in order to investigate the lateral variation in attenuation characteristics, and obtained the frequency-dependent relations as follows: QP=30±3f1.05±0.05, QS=143±20f0.88±0.07 for the Garhwal Himalaya region and QP=31±1f1.09±0.02, QS=121±11f1.00±0.04 for the Kuamun Himalaya region. Obtained Q values indicate strong body wave attenuation for both the region with no significant lateral variation. It may suggest the presence of crustal level folding, faulting, aqueous fluids, and metamorphic fluids below both the regions. Also, the ratios of QS/QP are high (>1) for the entire analyzed frequency range, suggesting a significant level of heterogeneity and tectonic complexities in the crust of the study region.

Early Triassic high-K granitoids and enclaves of the Daheba pluton, West Qinling (China): Implications for relative contributions of crust and mantle

Early Triassic granitoids are widespread in the northwestern West Qinling Orogen, China, but their petrogenesis and geodynamic implications remain unclear. In this study, we integrated new field and petrological observations, mineralogical compositions, zircon U-Pb dating, Hf isotopic and whole-rock geochemical analyses for the early Triassic granitic pluton in the Daheba area to determine its magma source and geodynamic scenario. The pluton is composed of granodiorite and syenogranite and carries microgranular enclaves (MEs) which formed at ca. 253–249 Ma. The granitoids show wide SiO2 contents of 63.73–77.49 wt% (av. 69.89), high K2O contents of 3.4–5.4 wt% (av. 4.1) and moderate Mg# of 16–51 (av. 36), belonging to high-K, calc-alkaline I-type granites. These rocks have high radiogenic but uniform Hf isotopic compositions with 176Hf/177Hf and ɛHf(t) of 0.282511–0.282658 and −3.85 – +1.25, respectively. The MEs hosed within the granite are characterized by high Mg# of 35–58 (av. 46) and variable εHf(t) of −4.64 – +9.35, which likely represent a hybridized melt derived from a slab-modified mantle and lower crust. In combination with coeval magmatic rocks in the western Gonghe-East Kunlun area, we propose a genetic model where mafic magmas derived from an enriched mantle and underplated beneath the overlying lower crust are considered to have produced the high-K felsic magma. Further, the hybridized melt ascended to shallower crustal levels to generate a series of rocks ranging from dioritic MEs to granodiorite to syenogranite. Mass balances modeling suggests that the generation of these rocks involved 36 % of the lower crustal-derived melt and 64 % of the SCLM (R2 = 0.9). Our new data, in tandem with published results suggest that the Daheba pluton formed during the subduction stage of the Paleo-Tethyan Ocean and that a local extensional episode occurred at 253–249 Ma in the western Gonghe area.

The Lac à l'Orignal phosphate deposit and constraints on high-quality phosphatic ore in massif-type anorthosite, Grenville Province, Canada

Phosphate is an indispensable component in the fertilizer industry. It has recently become a critical part of the green energy transition because of the demand for lithium ferro phosphate in rechargeable batteries. A significant amount of phosphate ore (∼90 %) is globally produced from sedimentary rocks. High-quality phosphate ore containing low amounts of toxic elements, however, is mostly produced from igneous carbonatite.The Lac à l'Orignal deposit (∼1069–993 Ma), Canada, in the central Grenville Province, contains a large amount of fluorapatite (≤ 20 %) hosted in oxide-bearing norite-gabbronorite-anorthosite layered intrusions. The deposit is hosted in the Vanel Anorthosite (1080 ± 2 Ma) near the northern border of the Mattawa Anorthosite (1016 ± 2 Ma). The major host rock oxide-apatite-gabbronorite (OAGN) is primarily composed of plagioclase, orthopyroxene, clinopyroxene, fluorapatite, ilmenite, and magnetite. The Al contents of the OAGN orthopyroxene indicate that the magmatic deposit was emplaced at mid-low crustal levels (∼2.0–3.6 kbar) and reverse zoning in the OAGN plagioclase suggests <2.5 kbar of pressure change during their crystallization. An overall increase of REE + Y concentrations in the OAGN fluorapatite with decreasing their fluorapatite contents indicates that the magma went through fractional crystallization. This study suggests that high abundances (ca. > 8 %) of fluorapatite in some OAGN were produced from crystal avalanching after initial crystal settling. Antithetic correlation between the Cl content of fluorapatite and the Mg# [Mg/(Mg + Fe2+)] of coexisting orthopyroxene suggests that phosphate ores with low Cl contents are expected for OAGN that formed at the initial stages of magma crystallization. Therefore, the Mg# of the OAGN orthopyroxene provides an exploration tool for targeting phosphate ore with low Cl content, which is preferred in the production of phosphoric acid, a major intermediate feedstock for the fertilizer and green energy industries. Apatite contents in carbonatite and average P2O5 content in carbonatite melt are generally similar to those of the Lac à l'Orignal OAGN. The average concentrations of REE, and toxic metals, such as Pb, Th, and U, in the OAGN fluorapatite are, however, lower than those in apatite from carbonatite worldwide. Therefore, the apatite of the OAGN appears to be of high quality and more environmentally-friendly as a source of phosphate ore.

Messengers from the Magma Chambers: Petrostratigraphic Analysis of Plagioclase-Rich Flood Basalt Lavas in Turkana, Kenya

Abstract Continental large igneous provinces are among the largest magmatic events on Earth, characteristically manifesting at the surface as stacks of monotonous flood basalt lavas that form through a dynamic pattern of eruptive pulses separated by hiatuses. Such dynamic patterns may be difficult to detect from the composition of the lavas. Instead, lava crystal cargoes can be used to study how the eruption cycles inherent to continental large igneous provinces are moderated by episodic recharge and evacuation at multiple crustal levels. Here, we present a stratigraphically well-constrained series of 56 flood basalt flows from the Eocene Initial Phase of magmatic activity in East Africa. These flows and their crystal cargo permit temporal insight into the development of one of the youngest and best-preserved continental large igneous provinces. The stratiform mafic lavas exposed in northern Kenya consist of alternating aphyric and plagioclase-phyric lava packages consistent with periods of eruption punctuated by volcanic hiatus, where magmas stall and crystallize plagioclase at medium to shallow crustal levels. Plagioclase compositions (n = 541) exhibit little intra-crystal or intra-sample compositional diversity. Intra-crystal equilibrium calculations for Sr and Ti indicate internal chemical equilibrium, requiring storage at high temperatures over a prolonged time interval (10 000–100 000 years). Using a series of seven interlinked partial crystal fractionation models under equilibrium conditions, we replicate the observed stratigraphic patterns in plagioclase composition. We find that the balance between recharge, evacuation, and diffusive equilibration within a shallow magmatic system controls the composition of plagioclase in these flood basalts. We conclude that the shallow fractionation system modulates eruptive cycles and thus constitutes a critical component in studies of continental large igneous provinces.

The 3D Crustal Structure in the Epicentral Region of the 1980, Mw 6.9, Southern Apennines Earthquake (Southern Italy): New Constraints From the Integration of Seismic Exploration Data, Deep Wells, and Local Earthquake Tomography

AbstractWe present the first 3D crustal model of the epicentral region of the 1980, Mw 6.9, normal‐faulting Irpinia earthquake (southern Italy) determined by jointly interpreting the CROP‐04 deep seismic profile, a grid of commercial seismic lines, deep exploration wells, and a high‐resolution Local Earthquake Tomography. Despite numerous seismotectonic surveys and source studies of the background seismicity recorded by dense networks, a complete 3D geological model of the mid‐upper crust was still lacking in the region. The architecture of the Neogene fold‐and‐thrust belt is also debated, with competing thin‐ and thick‐skinned tectonic interpretations. We use the 3D geological model derived from subsurface exploration data to interpret the upper crustal tomographic velocities in terms of rock physical properties, while Vp and Vp/Vs anomalies provide inferences on the deep structural setting down to 12 km depth. We find that a thick‐skinned deformation style allows explaining the geometry of Pliocene fold‐and‐thrust systems deforming the Apulian carbonates but also deeper Permo‐Triassic metasediments and the Paleozoic crystalline femic basement. Inherited compressional structures and lithological heterogeneities control background seismicity occurring at two crustal levels. Fluid‐driven shallow seismicity (&lt;4–6 km) concentrates in a high‐Vp/Vs wedge of fractured, brine‐saturated Mesozoic stiff rocks delimited by the 1980 earthquake faults. Deep seismicity (9–14 km) clusters instead within the low‐Vp/Vs crystalline basement underneath the Apulian carbonate ramp‐anticlines. Commercial seismic data allow us to identify the Irpinia Fault, the main fault ruptured by the 1980 earthquake, reinforcing its previous interpretations as an immature structure with subtle geological and geophysical evidence.

Cu-Au-Platinum Group Element Mineralization in the Mbesa Prospect, Southern Tanzania: Unconventional Magmatic Sulfides

Abstract High-grade Cu mineralization was recently discovered in the Mbesa prospect in southern Tanzania, but its origin is poorly constrained. Herein, we present new major element, trace element, rare earth element (REE), and platinum group element (PGE) geochemistry and petrographic data to constrain the origin of the mineralization. The sulfide mineralization is dominated by chalcopyrite and bornite and characterized by high Cu/Ni over 100 in the massive and disseminated ores. The mineralization lacks pyrrhotite and pentlandite, which are found commonly in magmatic Ni-Cu-PGE sulfide deposits. Platinum group minerals present are michenerite (PdBiTe) and sudburyite (PdSb). PGE concentrations expressed as Pd + Pt + Au are up to 1.31 and 1.04 ppm in the massive and disseminated ores, respectively. The δ34S of chalcopyrite separates range from –3.9 to –0.6‰, bornite separates vary from –1.8 to 1.1‰, and bornite-chalcopyrite mixtures yield –1.3 and 0.6‰, mostly consistent with a magmatic sulfur origin. The sulfides are hosted by coarse-grained plagioclase amphibolite and hornblende gneiss with recrystallization textures suggestive of metamorphism. Both rocks are characterized by enrichment in large ion lithophile elements (LILEs), with negative anomalies of Zr and Nb, consistent with crustal contamination of magmas derived from partial melting of subcontinental lithospheric mantle (SCLM). An SCLM origin is further suggested by the close association of Cu sulfides with carbonates and apatite. It is suggested that the sulfides crystallized from the fractionated Cu-rich sulfide melt enriched in intermediate solid solution (iss) at shallow crustal levels. Sulfide were likely mobilized as droplets attached to CO2 vapor phase as expressed by close association between carbonates and apatite with the Cu-rich sulfides. Nickel-rich sulfides may remain undiscovered at deeper portions around the Mbesa prospect.

Contrasting Tectonomagmatic Conditions for Coexisting Iron Oxide-Apatite Deposits and Porphyry and Skarn Cu ± Au Deposits in the Middle-Lower Yangtze River Metallogenic Belt, China

Abstract Porphyry Cu ± Mo ± Au and iron oxide-apatite (IOA) deposits rarely occur in spatial and temporal proximity in Phanerozoic arc-related settings, and the formation of these mineral deposit types in an evolving arc setting remains poorly understood. Specifically, the roles of magma composition and the tectonic regime remain the subject of some debate. Here, we systematically estimated the P-T-fO2 conditions and H2O-S-Cl contents for dioritic to granodioritic source magmas for porphyry and skarn Cu ± Au (150–135 Ma) and IOA deposits (~130 Ma) that formed in transpressional and transtensional settings in the Middle-Lower Yangtze River metallogenic belt, China. Our estimates show that, compared to IOA deposits, the porphyry- and skarn-related magmas were relatively felsic, cooler, and more hydrous. These geochemical features are consistent with the tectonic transition from subduction to slab rollback of the paleo-Pacific plate in the East Asia continental margin at &amp;lt;135 Ma and concomitant crustal extension and steepening of the regional geothermal gradient. Apatite data reveal that the silicate melts associated with the porphyry and skarn Cu ± Au and IOA deposits had comparable predegassed S concentrations (~0.13 ± 0.06 wt % vs. ~0.16 ± 0.09 wt % on average), but that IOA-related melts contained higher predegassed Cl/H2O ratios (~0.11 ± 0.03 vs. ~0.04 ± 0.03 for porphyry- and skarn-related magmas) that decreased by one order of magnitude after magmatic degassing. Magmatic fO2 estimated using zircon and amphibole, reported in log units relative to the fayalite-magnetite-quartz (FMQ) redox buffer, gradually increased during cooling of the porphyry- and skarn-related magmas (ΔFMQ +0.7 to +2.5) at 950° to 800°C and decreased to ΔFMQ +1 at 700°C owing to fractionation of Fe2+-rich minerals and subsequent S degassing, respectively. In contrast, the magmatic fO2 values for the IOA-related source magmas varied significantly from ΔFMQ –1.5 to ΔFMQ +2.5 but generally show an increasing trend with cooling from 970° to 700°C that probably resulted from variable degrees of evaporite assimilation, fractionation of Fe2+-rich minerals, and Cl degassing. These results are consistent with the hypothesis that Cl enrichment of the IOA-related source magmas played a determinant role in their formation. We propose that the porphyry and skarn Cu ± Au deposits in the Middle-Lower Yangtze River metallogenic belt formed in a transpressional setting in response to paleo-Pacific flat-slab subduction that favored storage and evolution of S-rich hydrous ore-forming magmas at variable crustal levels. A subsequent extensional setting formed due to slab rollback, leading to rapid degassing of Cl-rich IOA-related magmas. For the latter scenario, assimilation of evaporite by mafic to intermediate magmas would lead to an enrichment of Cl in the predegassed magmas and subsequent exsolution of hypersaline magmatic-hydrothermal fluid enriched in Fe as FeCl2. This Fe-rich ore fluid efficiently transported Fe to the apical parts of the magma bodies and overlying extensional normal faults where IOA mineralization was localized. The concomitant loss of S, H2O, and Cu with Cl by volcanic outgassing may have inhibited sulfide mineralization at lower temperatures.

Petrology and Geochemistry of an Unusual Granulite Facies Xenolith of the Late Oligocene Post-Obduction Koum Granodiorite (New Caledonia, Southwest Pacific): Geodynamic Inferences

Pressure–temperature estimates of a xenolith found within a post-obduction granodiorite in southern New Caledonia provide evidence for subcrustal, granulite facies, peak crystallisation conditions (ca. 850 °C—8.5 ± 1.0 kbar), followed by isobaric cooling to 700 °C, and final decompression with partial rehydration at ca. 650 °C—3.5 kbar. The xenolith, dated at 24.7 Ma (U-Pb zircon), i.e., the same age as the granodiorite host rock, has low SiO2 (35.5 wt%) and high Al2O3 (33.2 wt%) contents, suggesting that it is the restite of a previous melting episode, while the elevated Ca (Ba and Sr) contents suggest mantle metasomatism. Although the concentrations of Rb, K, Ca, Ba, and Sr have been strongly modified, some geochemical (REE patterns and some “immobile” trace element ratios) and isotopic (Sr and Nd isotopic ratios, U-Pb zircon age) characteristics of the granulite facies xenolith are similar to those of the xenoliths found in other Late Oligocene intrusions in southern New Caledonia; therefore, this rock is interpreted to be related to an early magmatic episode. The rock protolith was emplaced and equilibrated at the base of the crust where it underwent ductile deformation. Younger ascending magma picked it up and they eventually crystallised together at a shallow crustal level, near the tectonic sole of the ophiolite. The recrystallisation and ductile deformation at ~8.5 kbar suggest that a rheological discontinuity existed at about 25–28 km, probably representing the Moho. It is concluded that a continental crust of normal thickness must have existed beneath New Caledonia at about 24 Ma, i.e., 10 Ma after obduction.

The Val Biandino Intrusive Suite (central Southern Alps, N Italy): new geochronological and geochemical data on the Early Permian magmatic activity in the Southalpine Domain

The Early Permian in the present-day Europe area was a time when a major tectonic shift occurred, leading from the tectonic collapse of the Variscan orogeny to the crustal extension and thinning that characterized the Early Permian times. Crustal extension was associated with extensive magmatism at different crustal levels: from gabbro in the lower crust or at the mantle/crust transition to subaerial high-SiO2 volcanic activity. In the whole Southalpine Domain, the Early Permian intrusive bodies occur from the west (e.g. Ivrea-Verbano Complex and “Graniti dei Laghi”) to the east (Ifinger, Brixen and Cima d’Asta intrusive complexes). Among these, in the central Southern Alps (comprised between the Giudicarie Belt and the Lake Como), minor intrusive complexes also occur. The Val Biandino Intrusive Suite consists of two magmatic units: the Val Biandino Quartz-Diorite (VBQD) and the Valle di San Biagio Granite (VSBG). The first of them consists of gabbro-diorite to granodiorite bodies associated with leucocratic cordierite-bearing granitic dikes that intruded the pre-Permian basement. To the west, a W-dipping normal fault of Permian age represents the boundary between this unit and the Valle di San Biagio porphyric granite. All rock varieties of the Val Biandino Intrusive Suite display a high-K calc-alkaline affinity with metaluminous to peraluminous character. Field crosscutting relationships point to a late generation of the cordierite granites of the Val Biandino Quartz-Diorite unit with respect to the more mafic types. SHRIMP U–Pb zircon dating provided an age of 285.2 ± 1.9 Ma for a cordierite granite of the Val Biandino Quartz-Diorite unit and 283.2 ± 1.9 Ma for the porphyric Valle di San Biagio Granite. Geochemical data suggest that gabbro-diorite, quartz-diorite, granodiorite and leucogranite are not co-magmatic. The existing gaps in term of SiO2 wt% and the higher HREE contents in mafic and intermediate rocks with respect to granite coupled with similar LREE in all rocks support this hypothesis. The high Rb/Sr ratios (> 1) in leucogranite, together with the occurrence of white mica and cordierite point to a significant contribution of crustal partial melting in their genesis. The Val Biandino Intrusive Suite was likely formed through the interaction of magma genesis at the mantle/crust transition and partial melting of the heterogeneous pre-Permian basement of the Southalpine Domain. This scenario explains the strong heterogeneity displayed by the relatively small intrusive complex and the strong crustal signature exhibited by all the magmatic types of the Val Biandino Intrusive Suite.

Tectono-metamorphic evolution of paragneiss in the Chengde metamorphic Complex, northern Paleoproterozoic Trans-North China Orogen

Metasedimentary rocks can provide important information for reconstructing tectonic evolution of an orogen. In this study, we report the newly identified metapelite and metagraywacke samples from the Chengde Metamorphic Complex, the Trans-North China Orogen in the North China Craton. The garnet-bearing paragneiss generally occurs as country rocks of mafic granulite and amphibolite, and displays compositional layering with distinct gneissosity. The metapelite mainly consists of garnet, sillimanite, biotite, muscovite, plagioclase, and quartz, while the metagraywacke commonly comprises garnet, plagioclase, K-feldspar, quartz, and biotite/chlorite. Application of conventional geothermobarometers and phase equilibrium modelling constrains diverse peak metamorphic pressure–temperature (P-T) conditions (CD35, ∼800 ℃/10.5 kbar determined by phase equilibrium modelling; CD37, 677−716 ℃/9.6−9.8 kbar determined by geothermobarometers; CD45, 665−691℃/9.1−9.6 kbar and ∼ 745 ℃/11.2 kbar determined by geothermobarometers and phase equilibrium modelling respectively). The metapelite sample CD35 also records two retrograde stages, defining a clockwise P-T path with isothermal decompression (ITD) process. U-Pb dating of zircon and monazite yields metamorphic ages of 1910−1796 Ma, implying the timing of subduction to the lower crust. Combining previous studies in the Chengde Metamorphic Complex, we conclude that this complex is composed of imbricate rocks with different peak metamorphic P-T conditions, metamorphic grades, metamorphic ages, protolith origins, or protolith ages, indicating that these rocks were subducted to different depths in the subduction channel and then exhumed together to shallower crustal levels.

Inside the Ryoke Magmatic Arc: Crustal Deformation, High-T Metamorphism, and Magmatic Pulses

The Ryoke belt represents the root of a volcanic arc exposed across SW Japan. It records successive deformation phases, high-temperature metamorphism, and several magmatic pulses that occurred during the Late Cretaceous. Successive magma intrusions at different crustal levels raised the overall geothermal gradient of the arc crust, and their thermal influence was contrastingly recorded in metamorphic zircon and monazite. Despite a broadly similar duration of magmatic activity (20–30 My) along the belt, the timing and periodicity of magma pulses varied. An along-arc variation in lower crustal magma generation together with a fluctuating crustal stress regime likely controlled the formation and evolution of this magmatic arc section.

Late Paleozoic potassic igneous rocks of the Adyrtor intrusions in the eastern Kyrgyz Tien Shan: geochemistry, isotope U-Pb zircon geochronology, and related W-Mo(-Cu-Au) mineralization

ABSTRACT The Adyrtor multiphase pluton is situated in the easternmost part of the ‘Main Structural Line of Tien Shan’, or the ‘Nikolaev Line’, and intrudes the Paleoproterozoic metamorphic sequence representing the oldest (1.8–2.5 Ga) continental basement fragment in the Tien Shan orogenic belt. The intrusions comprise mafic to intermediate (monzodiorite, monzonite, quartz syenite) and silicic (quartz monzonite, monzogranite, and leucogranite-alaskite) rocks. Geochemical signatures of the igneous rocks correspond to high-K calc-alkaline to shoshonitic series intrusions, with a strong A-type granite affinity, emplaced in post-collisional setting. Based on the isotopic U-Pb data, quartz syenite from the Adyrtor intrusions crystallized at 330.7 ± 4.3 Ma, and quartz monzonite – at 329.5 ± 5.8 Ma. Although the ages identified correspond to those of the subduction-related magmatism in the western part of the Middle Tien Shan, geochemical characteristics of the rocks support rather post-collisional setting of the plutons, principally due to a ‘scissor-like’ closure of the Turkestan paleoocean, starting from the east. Magmatic evolution included a generation of shoshonitic magma by low-degree partial melting of the metasomatically-enriched upper mantle, followed by amphibole fractionation in a deep (lower crustal ?) magma chamber. Then, under the influence of mantle-supplied fluids and heat, granitic magmas in the crustal protolith could have been generated, with further mixing/mingling of the mantle-derived mafic (shoshonitic) magma and mantle-induced crustal granitic magma, followed by the magma fractionation and emplacement at shallower crustal levels. Assimilation of an old continental crust is also envisioned, consistent with the presence of the xenocrystic zircons dated at ca. 1.6 Ga to 2.5 Ga and the A-type granitoid affinity of the igneous rocks. The associated skarn-porphyry W-Mo(-Cu-Au) mineralization is consistent with post-collisional setting of the productive intrusions. This mineralization complements the group of similar deposits and occurrences associated with high-K calc-alkaline to shoshonitic series intrusions in the Middle Tien Shan and globally. The high endowment in W and Mo can be explained by the fertilization of subduction-modified subcontinental lithospheric mantle in these metals, with its further involvement in the magma generation in post-collisional setting. Also, a stronger W-Mo mineralization can reflect a greater role of a crustal protolith in the magma generation, as revealed by the distinct A-type affinity of the igneous rocks.

Trace elements in zircon record changing magmatic processes and the multi-stage build-up of Archean proto-continental crust

Zircon trace element geochemistry has become an increasingly popular tool to track crustal evolution through time. This has been especially important in early-Earth settings where most of the crust has been lost, but in some fortuitous instances detrital zircons derived from that lost crust have been preserved in younger sediments. To study the formation and geochemical evolution of continental crust from the Hadean to the Paleoarchean, the 3.6 to 3.2 Ga Barberton Greenstone Belt in southern Africa is an excellent target due to its outstanding preservation and presence of detrital zircons that span almost a billion years. Here, we use trace elements, in combination with hafnium and oxygen isotopes, of 3.65 to 3.22 Ga detrital and tuffaceous zircons of the Moodies and Fig Tree groups and compare their geochemistry to previously studied 4.2 to 3.3 Ga detrital zircons from the Green Sandstone Bed of the Onverwacht Group. The major detrital zircon age clusters in the former at 3.55 Ga, 3.46 Ga, and 3.26–3.23 Ga overlap with episodes of TTG emplacement and felsic volcanism in the Barberton area, suggesting a local provenance. In contrast, age clusters at 3.65 Ga and 3.29 Ga of the Moodies and Fig Tree groups as well as 4.2 to 3.3 Ga detrital zircons from the Green Sandstone Bed do not have known intrusive sources and were likely derived from outside the present-day Barberton belt. This indicates that more than half of the felsic igneous events in the detrital zircon record do not have a whole-rock representation that can be directly studied. The similar compositions and inferred crustal evolution histories recorded in zircons from the Fig Tree and Moodies groups, as well as from the Green Sandstone Bed, suggest that they were derived from connected terranes experiencing similar crustal processes diachronously. Together, they show three phases of felsic continent formation, reflecting different crustal processes: (1) long-lived protocrust formed in the Hadean from undepleted mantle sources. These zircons are vastly different from younger zircons and, hence, Barberton TTGs are not good analogues of Hadean crust formation. (2) At 3.8 Ga, onset of significant crustal growth though cyclic juvenile additions and hydrous melting, possibly within a volcanic plateau setting but an arc-like setting cannot be excluded based on this data. (3) Between 3.4 and 3.3 Ga, felsic crust is generated through a previously unrecognized episode of crustal growth by shallow melting of mafic, mantle-derived sources. This is immediately followed by the onset of crustal thickening through the transport of surface-altered, hydrated materials to deep crustal levels. Since there is geological evidence for extension and shortening at that time this may reflect the onset of horizontal movement. Whether this last geodynamic setting reflects modern-style plate tectonics or not, continent formation and the onset of plate tectonics in the Barberton area occurred through complex multi-stage processes spanning almost a billion years, most of which is only accessible through the detrital zircon record.