Gabbroic Rocks Research Articles

Contrasting origins of A-type granites in the Late Triassic-Early Jurassic Pitou complex, southern Jiangxi province: Implications for Mesozoic tectonic evolution in South China

The origin of A-type granites is purely circumstantial. Granitic complexes are quite common and generally composed of different type granites. I- and A-type granite association in a granitic complex is often described in the literature. Here we present a granitic complex, i.e. the Pitou complex in southern Jiangxi province, South China that is composed of different varieties of A-type granites. Detailed zircon LA-ICP-MS U Pb chronology and in situ Hf isotope, mineral chemistry and whole-rock element and Sr Nd isotope data of this granitic complex are used to explore their origin and to further constrain the geodynamics of Mesozoic magmatism in South China. Our new data indicate that the Pitou complex was emplaced during the Late Triassic (237 Ma) (central to northern part) and Early Jurassic (188–185 Ma) (southern part), respectively and that both the Late Triassic and Early Jurassic granites belong to A-type but show contrasting origins. The Late Triassic granites show an association of alkali-feldspar granite-syenogranite-monzogranite and the rock type of the Early Jurassic granites is alkali-feldspar granite. All the granites are composed mainly of K-feldspar, quartz, plagioclase and biotite (ferri-biotite or annite) with biotite occurring as anhedral crystal, interstitial to quartz and feldspar. Both the Late Triassic and Early Jurassic granites have high SiO 2 (>70 wt%) and total alkalis (K 2 O + Na 2 O > 8 wt%) with high FeO T /(FeO T + MgO) and low Mg#. They are enriched in rare earth elements (except Eu) and depleted in Sr and Ba. They display Zr + Y + Ce + Nb >350 ppm, 10,000 × Ga/Al >2.6 and have high zircon saturation temperatures (>800 °C). Biotites in the granites are Fe-rich with very low Fe 3+ /Fe 2+ , indicating fairly reducing conditions. However, the Late Triassic and Early Jurassic granites also show some geochemical differences. The Late Triassic granites have lower FeO T /(FeO T + MgO) and higher Mg# than the Early Jurassic granites, with the Mg# of the former consistent with pure crustal melts and the Mg# of the latter much lower than pure crustal melts. In addition, the Late Triassic granites have lower zircon saturation temperatures (up to 843 °C, average 823 °C) than the Early Jurassic granites (up to 946 °C, average 875 °C). Furthermore, the Late Triassic granites have high 87 Sr/ 86 Sr (t) (0.7132–0.7226) and negative ε Nd (t) (−10.3 to −10.9) and ε Hf (t) (in situ zircon) (−11.5 to −11.6) that are similar to those of the early Paleozoic granitoids in the region whereas the Early Jurassic granites have low 87 Sr/ 86 Sr (t) (down to 0.7034) and positive ε Nd (t) (up to +3.0) and ε Hf (t) (in situ zircon) (up to +8.3) that are similar to those of the Early Jurassic gabbroic rocks in the region. Geochemical data and major element modeling suggest that the Late Triassic A-type granites were formed by shallow (ca. 15–20 km depth) dehydration melting of the early Paleozoic granitoids triggered by intraplating of basaltic magmas and the Early Jurassic A-type granites were produced by extensive fractionation (ca. 90% to 95%) from gabbroic magmas by removal of plagioclase, amphibole, K-feldspar, clinopyroxene and magnetite. We further suggest the origin of the Late Triassic A-type granites was related to the transtension of the regional NE–trending strike-slip faults caused by the early Indosinian continental collisions and the Early Jurassic A-type granites were emplaced in a back-arc extensional setting coupled with the slab rollback and subsequent slab break-off of the subducted Palaeo-Pacific plate. Our new data confirm that the early Mesozoic magmatism in South China was controlled by continental collisions during the Late Permian to Triassic amalgamation in Southeast Asia whereas the late Mesozoic magmatism was related to the northwestward subduction of the Palaeo-Pacific plate. A transition in the dominant tectonic regime from the Tethyan to the Pacific tectonic domain most likely took place during the Rhaetian. • The Pitou granitic complex emplaced during the Late Triassic and Early Jurassic. • Both Late Triassic and Early Jurassic granites are A-type but show contrasting origin. • Late Triassic granites formed by shallow dehydration melting of Paleozoic granitoids. • Early Jurassic granites formed by extensive fractionation from gabbroic magmas. • Origin of both A-type granites suggests different geodynamic mechanisms.

BELOMORIAN MOBILE BELT AS PART OF THE NUNA/COLUMBIA SUPERCONTINENT: NEW PALEOMAGNETIC AND GEOLOGICAL DATA

Analysis of the evolution of the Belomorian Mobile Belt (BMB) suggests that the metamorphism of gabbroic rocks was accompanied by their remagnetization and that lateral variations in magnetization constituents carry evidence for the time sequence of the extension of the western foreland of the Lapland-Kola orogen (LKO) to upper crustal levels. The latter formed while the Nuna/Columbia Supercontinent was being made up. The combined study of metamorphosed Paleoproterozoic igneous complexes has revealed a remagnetization trend in BMB rocks. The study has shown that the remagnetization front spread here from the north-west to the south-east and that it seems to be due to the collision pattern upon LKO formation. In addition, remagnetization in the southern BMB and in the southeastern Karelian Craton, dated at 1.65-1.60 Ga, was revealed. It seems to be due to the final episodes of formation of the Svecofennian orogen.

Petrogenesis of ilmenite-bearing mafic intrusions: A case study of Abu Ghalaga area, South Eastern Desert, Egypt

Neoproterozoic Abu Ghalaga ilmenite-bearing mafic rocks were subjected to geological, petrographical and geochemical studies integrated with mineral chemistry of ilmenite, pyrite and magnetite in order to deduce their petrogenesis. Abu Ghalaga mafic intrusions hosting the largest ilmenite reserve in the Egyptian part of the Arabian-Nubian Shield. Field observations revealed that the intrusions impound ilmenite ore, which occurs as interlayer of massive bands or disseminated within gabbroic rocks. Petrographically, Abu Ghalaga mafic intrusions include different kinds of gabbro; olivine pyroxene gabbro, pyroxene gabbro, pyroxene hornblende gabbro, altered gabbro and Fe-Ti-rich gabbro. They have calc-alkaline to tholeiitic magmas, which are depleted in high field strength elements (e.g., Ta, Nb and Th) relative to low field strength elements (e.g., Ba, Sr and Rb), and exhibit light rare earth elements enrichment relative to heavy rare earth elements with positive Eu anomaly (Eu/Eu*= 0.8-2.4). Variable mineralogical and wide variation of bulk rock geochemistry are ascribed to fractional crystallization and hydrothermal overprinting. Abu Ghalaga gabbroic rocks are derived from fractional crystallization of depleted mantle magmas that were modified by ascending fluids from subducted slab in volcanic arc setting. Fe-Ti oxides required high oxygen fugacity (ƒO2) and water contents by crystal settling of Ti-rich mantle which is applicable with subduction setting. It can be inferred that Abu Ghalaga mafic rocks are neither related to ophiolite nor to Alaskan type but to one of the layered intrusions hosting Fe-Ti oxides.

Brown Amphibole as Tracer of Tectono-Magmatic Evolution of the Atlantis Bank Oceanic Core Complex (IODP Hole U1473A)

Abstract Brown amphibole is a minor but common mineral component in lower oceanic crust. It is generally interpreted as products of migrating SiO2 and H2O-rich fluids or melts, which can be either residual melts from advanced magmatic differentiation of Mid-Ocean Ridge Basalt (MORB), or hydrothermal fluids including a seawater component. Within the lower oceanic crust exhumed at the Atlantis Bank Oceanic Core Complex, along the ultraslow Southwest Indian Ridge, brown amphibole is ubiquitous in all lithologies from olivine- to oxide-gabbros and diorites, including both undeformed and plastically deformed varieties. We here show the results of a systematic petrological study conceived to unravel the nature of the H2O-rich component recorded in brown amphiboles and document: (i) the evolution of migrating melts during the magmatic stage and (ii) different extents of melt-bearing deformation events recorded throughout the entire crustal transect. The low Cl contents and the light over heavy rare earth elements (LREE/HREE) ratios and high Ti contents in brown amphiboles indicate they crystallized from melts with a magmatic hydrous component. Consistently, their δ18O values are in equilibrium with Mid-Ocean Ridge Basalt (MORB) composition, except for diorite amphiboles that possibly record the local assimilation of altered minerals. In undeformed olivine gabbros, interstitial pargasite crystallized at hypersolidus conditions (~1000°C) from the melt residual after late stages of MORB differentiation. We speculate that before the olivine gabbro crystal mush reached fully solid state, some aliquots of residual melts were extracted and accumulated within discrete intervals. There, ferrobasaltic melts differentiated through the early crystallization of Fe-Ti oxides and clinopyroxene as liquidus phases, ultimately forming the oxide gabbros. This process promoted rapid Si enrichment and depletion in Fe, Ti, V in the residual melt, later extracted to form the crosscutting diorite veins. The mylonitic olivine gabbros record high-temperature plastic deformation (~900°C ± 50°C) under hypersolidus conditions, involving melts residual from previous crystallization of the gabbroic rock. Further solid-state plastic deformation led to substantial grain size reduction and, consequently, to an increase in porosity. This created pathways for subsequent melt focusing, which likely represent late-stage differentiated melts migrating throughout the lower crustal section. This study shows that brown amphibole in the Atlantis Bank lower oceanic crust is the crystallization product of melts residual from advanced magmatic differentiation, which are also locally involved in the plastic deformation events during crustal accretion.

Episodic bimodal magmatism in Central Inner Mongolia, China: Insights from geochronological, geochemical, and Sr-Nd isotopic evidence

This study combines zircon U-Pb ages, whole-rock major and trace elements, and Sr-Nd isotope data to investigate the petrogenesis and tectonic implications of late Paleozoic magmatic rocks in central Inner Mongolia of the southeastern Central Asian Orogenic Belt. The results indicate the presence of two stages of bimodal magmatism from the late Carboniferous to early Permian. The late Carboniferous (ca. 323 Ma) bimodal associations consist mainly of gabbroic and granitic intrusive rocks, whereas the early Permian (ca. 273 Ma) bimodal suites are dominated by dolerite and granite porphyry dikes. The gabbroic intrusive rocks and dolerite dikes are calc-alkaline, slightly enriched in light rare earth elements and depleted in Nb, Ta, Ti, and have low initial 87 Sr/ 86 Sr ratios (0.70317–0.70510) and high ɛ Nd (t) values (+3.1 to +7.4). They are both interpreted to result from partial melting of a depleted spinel-lherzolite mantle source that was metasomatized by subduction-related components. The granitic intrusive rocks show I-type granite affinity and similar geochemical features to coeval gabbroic rocks, suggesting they were derived from a melt-induced anatexis of basaltic lower crust. The early Permian granite porphyry dikes show characteristics of LREE enrichment, negative Nb-Ta anomalies and positive Zr-Hf anomalies. They have higher La/Yb and Th/Zr ratios and ɛ Nd (t) values, and lower initial 87 Sr/ 86 Sr ratios than the coeval dolerite dikes. These features suggest that the early Permian felsic dikes were most likely generated by partial melting of a juvenile lower crust mixing with minor mantle-derived components. An integrated study with regional geological data suggests that the episodic bimodal suites in central Inner Mongolia formed in a post-collisional to within-plate rift setting after the closure of the Paleo-Asian Ocean. • Two stages of bimodal magmatism are identified in central Inner Mongolia. • The mafic rocks were derived from a depleted spinel-lherzolite mantle source. • The felsic rocks show a compositional variation from calc-alkaline to alkaline. • The bimodal suites formed in a post-collisional to within-plate rift setting.

Mineralogical-Petrographical Record of Melt-Rock Interaction and P–T Estimates from the Ozren Massif Ophiolites (Bosnia and Herzegovina)

The Dinaride Ophiolite Belt formed from the Jurassic part of the Neotethys. The investigated Ozren ophiolite complex in Bosnia and Herzegovina consists of peridotites, plagioclase peridotites, plagiogranites, troctolites and other gabbroic rocks, and fewer basalts. Lherzolites and harzburgites contain corroded ortho- and clinopyroxene1 porphyroclasts enclosed in the olivine matrix. The boundaries between olivine aggregates and pyroxene1 and spinel1 are infilled by medium-grained undeformed aggregates of clinopyroxene2, less orthopyroxene2, spinel2, and often clinopyroxene3-spinel3 symplectites. These textures indicate the final crystallization of peridotite in subsolidus conditions. Partial dissolution of deformed pyroxene1 porphyroclasts and coarse-grained spinel1 most likely occurred due to their reaction with the rest melt present in the grain boundaries. The Al decrease from pyroxene1 to pyroxene2 and 3, or the Cr decrease and Al increase from spinel1 to spinel2 and 3 is characteristic. Peridotites are associated with inferred remnants of a gabbro-dolerite layer, whereas basalts and radiolarites occur as rare dm-size fragments in an ophiolitic breccia. Troctolites display interstitial crystallization of plagioclase, clinopyroxene, less Na-Ti-rich amphiboles, and phlogopite in the olivine-spinel matrix, indicating the replacive character of impregnating melt within the dunite layers. Clinopyroxene-plagioclase-ilmenite-±amphibole gabbroic and fewer basaltic dykes in peridotites formed due to subridge extension, mantle thinning, and the deeper mantle melting. Iron-enriched olivines occur in the peridotite-dyke interfaces and troctolites. Hydrated ultramafics and mafics contain amphiboles, biotite, phlogopite, clinozoisite, epidote, and chlorite aggregates. Estimated magmatic to subsolidus T from peridotite two-pyroxene thermometry are 1000–850 °C, for the spinel facies. Ca-in-orthopyroxene1 thermometry provided T of 1028–1068 °C, and Ca-in-orthopyroxene2 thermometry gave 909–961 °C at estimated P of 1.1–0.9 GPa. However, the gabbroic dyke magmatic crystallization T was constrained to 1200–1100 °C at P of 0.45–0.15 GPa by single clinopyroxene thermobarometry. The obtained P–T conditions constrained the deeper mantle environment for the formation of peridotites than troctolites and crosscutting dykes. The ophiolitic thrust-sheet hanging wall conditions in an obduction-related accretionary wedge were estimated from amphibolites at 620 °C and 0.85 GPa by Ti-in-amphibole thermometry and amphibole-plagioclase thermobarometry. 300 °C and 0.5 GPa were determined from an exhumation shear zone using a combination of chlorite thermometry and Si-in-phengite barometry.

The lower oceanic crust at ultraslow-spreading Southwest Indian Ridge: The inside story

Owing to the lack of persistent magma chamber and the complex interplay between magmatism and tectonism at slow- to ultraslow-spreading ridges, the likeliness of fractional crystallization being the predominant process of evolution of crustal magmas is weak. Here, we report a detailed petro-geochemical investigation from the lower crustal gabbroic rocks recovered from Hole U1473A in Atlantis Bank oceanic core complex, Southwest Indian Ridge that may potentially provide new insights on the dynamic accretion process at ultraslow-spreading ridge. Sampling the entire lithological spectrum encompassing a single geochemical cycle in this study permit us to probe the underlying magmatic processes in ultraslow-spreading lower crust. Grain-size variation is ubiquitously recorded in the gabbroic lithologies of Hole U1473A, where fine-grained intervals represent relatively evolved chemistry than the coarser domains. This suggests that, they were crystallized from a common genetic melt during different degrees of evolution. The forward geochemical modelling approach have not only affirmed the long-established theory of assimilation (of early mushes’ crystals)-fractional crystallization to be the key mechanism in lower crust formation, but also provided explanation for the restricted whole rock geochemical cluster observed throughout all the rock types. We demonstrate for the first time the relationship between the progressive modal mineralogical evolution with respect to the different assimilation/fractionation ratios (r). The observed trend has been successfully replicated for the experiments with high r values = 0.8–0.9, previously predicted from this region. Integrating all results, our study supports the model of lower crustal formation via reactive porous flow of the squeezed-out melt percolating throughout the cumulate pile and segregated into different horizons.

The gabbro-diorite magmatism from the Narm area, western Kuh-e-Sarhangi (Central Iran): Evolution from Eocene magmatic flare up to Miocene asthenosphere upwelling

Geodynamically, the Kashmar-Kerman Tectonic Zone (KKTZ) is one of the most perplexing tectonomagmatic belts of the Central Iran Microcontinent (CIM), comprising two important districts, Bafq and Kuh-e-Sarhangi. The Late Neoproterozoic-Early Cambrian granitoids, metamorphic rocks, and mildly metamorphosed volcansedimentary sequences are the oldest geologic outcrops in the Narm area, which is located in the western part of Kuh-e-Sarhangi. Alkaline gabbro-diorites with relatively high contents of K2O (1.99–3.03 wt%) and Na2O (2.7–5.99 wt%) are among the youngest intrusive rocks in the area, representing a within-plate provenience. These rocks were emplaced into Paleozoic sedimentary units as mafic-intermediate stocks, sills and dykes. Geochemically, these rocks could have resulted directly from partial melting (e.g., FeOT/MgO>1, Nb/La>0.5) with no considerable indication of assimilation with crustal materials (e.g., Ti/Zr > 30, Ti/Y > 200). Assimilation and fractional crystallization cannot account for magma evolution of gabbro-diorite rocks in the Narm area, using rare earth element ratios and geochemical models. There are also some geochemical signatures of an asthenospheric origin for the Narm gabbro-diorite rocks, such as the low ratios of La/Nb (1.5) and La/Ta (22). U–Pb zircon ages show that the Narm gabbro-diorites formed during two major episodes of magmatism in Central Iran: 40.3 ± 0.1 Ma in the Late Eocene (Bartonian) for gabbroic units and 8.04 ± 0.05 and 7.86 ± 0.05 in the late Miocene (Tartonian) for diorite stocks and diorite sills, respectively. Despite a time difference of more than thirty million years, geochemical similarities between the Eocene gabbro rocks and the Miocene diorite from the Narm area are striking. It is proposed that the best scenarios for the west of Kuh-e-Sarhangi mafic-intermediate magmatic pulses along with the deep faults of the Central Iran, are an Eocene magmatic flare up and a Miocene asthenosphere upwelling. Temporally and spatially, these rocks are comparable to the Cenozoic alkaline intrusive rocks of the Bafq region.

Mg–Fe Isotopes Link the Geochemical Complexity of the Coldwell Complex, Midcontinent Rift to Metasomatic Processes in the Mantle

Abstract Mafic intrusions in the Coldwell Complex have previously been interpreted as forming from a metasomatized mantle source. To build upon our understanding of this metasomatism, the Mg–Fe isotope compositions of these rocks have been determined, and variations are assessed with respect to the magmatic processes that could have occurred at different stages of their formation. The mineralized Marathon Series (δ26Mg = −0.28‰ to −0.19‰), associated metabasalt (δ26Mg = −0.24‰ to −0.23‰), and the Geordie Lake gabbro (δ26Mg = −0.31‰ to −0.22‰) are characterized by δ26Mg values that are within the range of mantle values, whereas the unmineralized Layered Series (δ26Mg = −0.2‰ to −0.05‰) is heavier than mantle. In contrast, the δ56Fe values of all the Coldwell basaltic–gabbroic rocks (δ56Fe = 0.07 ± 0.08‰) are heavier than mantle but within the range of terrestrial basalts and mafic–ultramafic layered intrusions. We propose that the Mg–Fe isotope compositions of these rocks was not significantly modified by processes such as partial melting or garnet retention/fractionation in the mantle, fractional crystallization, or contamination during ascent through the crust, as the isotope values do not correlate with proxies for these processes (e.g. La/Sm and La/Yb, Gd/Yb, MgO–CaO–TiO2, and Th/Nb and Th/La, respectively). Their isotope compositions are, therefore, proposed to reflect the compositions of their metasomatized mantle sources. We conclude that metasomatism was not caused by a carbonate melt, subduction-altered oceanic crust and sediments, or an evolved silicate melt, as these processes generate light δ26Mg, variably fractionated δ56Fe, and heavy δ56Fe values, respectively, which are not observed in our dataset for the Coldwell Complex. The agent that metasomatized the mantle beneath the Coldwell Complex was likely slab-derived fluids characterized by isotopically heavy δ26Mg and basaltic δ56Fe values. This scenario can account for the lack of Fe isotope fractionation from basaltic values in all of the Coldwell rocks. The variably heavier δ26Mg of the Layered Series (−0.20 ± 0.01‰ to −0.05 ± 0.05‰) relative to the mantle (−0.25 ± 0.07‰) suggests that the magmas for the Coldwell rocks were derived by tapping of an isotopically heterogeneous mantle source that had undergone variable degrees of metasomatism. The distinctive geochemistry of mafic sequences in the Coldwell and numerous mafic dykes located in the northeast shoulder of the Midcontinent Rift suggests the presence of a variably metasomatized mantle source beneath a large area of the rift.

Episodic Nature of Magmatic Ascent in a Dynamic Conduit System: Evidence From a Late Gabbroic Intrusion Associated with the Eagle Ni-Cu Sulfide Deposit in Northern Michigan, USA

Abstract New exploration studies have revealed a near-vertical, plug-like gabbroic intrusive body emplaced along the feeder conduit of the funnel-shaped Eagle East ultramafic intrusion in the Upper Peninsula of Michigan. U-Pb baddeleyite analysis on this gabbro yielded an age of 1103.4 ± 1.2 Ma. Although a sulfide-deficient gabbro is the principal rock unit, peripheral zones of the intrusion contain feldspathic pyroxenite and melagabbro with minor blebs of chalcopyrite and pyrrhotite. A small pocket-like body of peridotite with significant quantities of sulfide minerals occurs along the edge of the main gabbroic unit. The modes of occurrence of sulfide minerals in the pocket peridotite are similar to the larger Eagle East and Eagle intrusions nearby. The composition of olivine in the main gabbroic rock is Fo24–30, but olivine in pocket peridotite shows a composition of ~Fo74. The Ni-concentration in the olivine of the main gabbro unit is below 100 ppm, but in the Mg-rich olivine of the pocket peridotite, it is between 2,100 and 2,800 ppm. Although δ34S values of sulfide minerals in the pyroxenite rim and in the pocket peridotite range from –0.6 to 7.0‰, the average value is higher than those reported from sulfides in the Eagle East and Eagle intrusions. The origin of the gabbro unit could be explained by a closed-system crystallization of a fractionated late derivative of an original high-MgO magma, while the peridotite pocket is a xenolithic representative of the old peridotitic intrusion dislodged and entrained in a late magmatic pulse.

Whole-rock geochemistry and mineral compositions of gabbroic rocks and the associated Fe–Ti (–V) oxide deposit in the Gonamsan intrusion, South Korea

The NS trending sill-like Gonamsan gabbroic intrusion in the central region of the Korean Peninsula intruded Neoproterozoic metasedimentary rocks composed of schists intercalated with gneisses, marbles, and quartzites. The intrusion has an east zone (EZ) composed of diorite–monzodiorite, diorite–gabbro, and oxide gabbro–monzodiorite, and a west zone (WZ) composed of quartz monzodiorite. These silicate rocks have similar primitive-mantle normalized trace elements and chondrite-normalized REE patterns, suggesting that they are co-magmatic. Plagioclase anorthite (An) contents (1.9–54.0), pyroxene MgO/ (MgO + FeO) (Mg#) (35.4–73.7), and olivine forsterite (Fo) values (14.5–20.9) show a wide range through the intrusive silicate suite, but each unit has narrow ranges, and reverse fractionation is absent. The Fe–Ti (–V) oxide mineralization only occurs in the EZ and is characterized by variations in morphology, size, mineral assemblages, and chemical compositions of the Fe–Ti oxide minerals. Stratiform Fe–Ti (–V) oxide mineralization interlayered with diorite–monzodiorite is restricted to metasedimentary host rocks in the central region of Gonamsan and comprises magnetite and ilmenite with lesser amount of olivine, tschermakite, apatite, spinel, pyroxene, and plagioclase. Lenticular mineralization occurs along the western margin of the diorite–gabbro and consists of magnetite and ilmenite with minor hornblende, apatite, chlorite, and spinel. Oxide gabbro interlayered with relatively thin monzodiorite bands forms cyclical units irregularly distributed throughout the EZ and consists of olivine and pyroxene with minor interstitial magnetite, ilmenite, and apatite. Magnetite is the principal Fe–Ti oxide mineral, and varies in chemical composition and microtextures in different ore zones. Overall, the compositions of magnetite from all mineralized zones fall in the compositional fields for magmatic Fe–Ti, V deposits defined by Dupuis and Beaudoin (2011). Magnetite from the stratiform Fe–Ti oxide mineralization is compositionally consistent with those from high-temperature silicate melts and displays complex ilmenite and spinel exsolution textures that resulted from re-equilibration of ulvöspinel–magnetite solid solution during cooling. In contrast, magnetite from the lenticular ore and oxide gabbro are compositionally consistent with being derived from evolved low-temperature melt. Based on the whole-rock geochemistry, mineral composition, and mineralization distribution, the Fe–Ti (–V) oxide mineralization in the Gonamsan intrusion seems to be related to Fe–Ti-rich melt formed from magmatic fractionation and migration along a relatively permeable zone, rather than other magmatic and post-magmatic processes.

Seismic Imaging of Dante's Domes Oceanic Core Complex From Streamer Waveform Inversion and Reverse Time Migration

AbstractEarly arrival traveltime tomography and full waveform inversion were conducted on downward continued streamer seismic data at Dante's Domes oceanic core complex (OCC), providing unprecedented details of shallow P wave velocity structure. Together with reverse time migration images, seafloor morphology, in situ geological samples, magnetic and gravity data, the seismic constraints are used to infer the lithological distribution along the seismic profiles. Based on the striking similarity in velocity structure beneath the corrugated domes with other OCCs and drilling results from Atlantis Massif, we confidently reconfirmed the Southern Dome as dominantly gabbroic rocks, and the Northern Dome as serpentinized peridotites. A series of isolated gabbroic bodies embedded in the diabase and basaltic layers is observed in the breakaway zone, suggesting that the initiation of Dante's Domes OCC occurred over a long period during which there were several failed attempts to form a long‐lived detachment fault. This early development of the OCC probably occurred under a regime of alternating magma starvation and magma replenishment. The predominantly gabbroic section, beneath the Southern Dome and extending to termination, indicates the OCC has been created with relatively high magma flux. We also imaged distinct shallow subseafloor reflections which are also termed as D reflectors underneath the corrugated domes. The location of the D reflectors is similar to those in the Atlantis Massif, with depths well correlated with the top of exhumed gabbroic bodies and the discontinuities in the D reflectors between gabbroic bodies. Our findings contribute to the understanding of processes controlling the OCCs initiation and evolution at slow spreading ridges.

Recording the largest gabbro-anorthositic complex worldwide: The Kunene Complex (KC), SW Angola

The Kunene Complex (KC) represents a large Mesoproterozoic igneous body, mainly composed of anorthosites and gabbroic rocks that extends from SW Angola to NW Namibia (18000 km2, N-S trend, and ca. 350 km long and 25–50 km wide). Although the KC has been studied from a cartographic and geochemical point of view, little is known about its structure at depth below the sedimentary deposits of the Kalahari basin. Hence, we use available satellite gravity data to estimate its extent and to unravel its morphology at depth. The Bouguer anomaly map depicts a gravity gradient from the coast (+200 mGal) towards eastern Angola (-150 mGal), which is explained by the transition from a young, dense and thin basaltic oceanic crust, formed during the Mesozoic Atlantic rifting, to an old, light and thick Archaean to Proterozoic continental crust (Congo Craton), to the east. The outcropping KC interrupts the gravity trend, showing at the western, southwestern and northeastern sides, several positive and isolated gravity anomalies linked to gabbroic intrusions associated to KC (ca. 50 km wavelength and −90 mGal). In contrast, the anomalies found at the central part of the massif (50 km wavelength and < -110 mGal) correspond to the dominant anorthositic members, according to the spatial correlation of the mapping. Five 2.5D gravity profiles have been modelled to investigate the unexposed eastern boundary, reconstructing the surface crustal structure (between 0 and 15 km depth) overlaid by the thin sedimentary cover of the Kalahari basin. The gravity modelling helps us to show that the KC was emplaced in the Upper Crust and extends in depth up to ca. 6 km, showing a lobular geometry and following a large NE-SW to NNE-SSW linear trend, presumably inherited from older Palaeoproterozoic structures. The lateral continuation of the KC to the east (between 50 and 125 km) beneath the Kalahari sediments suggests an overall size of at least twice the outcropping dimension (about 42500 km2). This statement clearly influences in the economic potential of this massif, related to the prospecting of raw materials and certain types of economic mineralization (Fe-Ti oxides, metallic sulphides or platinum group minerals).

Petrology and geochemistry of the Batchingou anorthositic suite rocks (Bana volcano‐plutonic complex, Cameroon Volcanic Line): Inference on their origin and relation with host granites

This study focusses on field mapping, petrographic description, and geochemical analyses of the Batchingou anorthositic suite rocks (Bana Plutono‐volcanic Anorogenic Complex, Cameroon Volcanic Line). This suite consists of gabbroic (gabbro, gabbronorite, norite, and anorthosite) and charnockitic (mangerite, jotunite, enderbite, and monzogranite) rocks and intrude Ypresian granites (51 ± 1 Ma), within an elliptically shaped corridor of 1200 × 600 m2. Gabbroic rocks evolved by crystal fractionation of olivine, hypersthene, augite, plagioclase, and ilmenite following the path: gabbro → gabbronorite → norite → anorthosite. There is little or no crustal assimilation in this suite, contrarily to the charnockitic suite which likely evolved from a jotunite melt by assimilation of the Pan‐African granito‐gneissic basement rocks. Anorthosite has high Eu/Eu* (2.04–2.66), typical of anorthosites formed as cumulates. The La/Ta ratios (9.75–20.1 in gabbro, 21–45 in anorthosite and 23–76 charnockitic rocks), La/YbN (4.2–19.5), Nb/U (22–90), and Nb/Th (5–23) suggest that the parental magma of the Batchingou anorthositic suite rocks is an alkaline basaltic melt deriving from a garnet‐bearing asthenospheric mantle, but was variably modified during ascent by lithospheric spinel lherzolites and crustal rocks. The Batchingou gabbro has characteristics of evolved tholeiites and match the composition of the Laramie high‐Al gabbro, whereas jotunite shows compositional affinity to ferrodiorites from Proterozoic anorthosite complexes. The gabbro Ni (19–39 ppm) and Cu (44–118 ppm) contents are low, indicating that their parental melt is not primitive. The Batchingou anorthositic suite differentiated from a different magma batch postdating the host granitoids and is similar to the anorthosite dykes of the Mboutou ring complex (Cameroon) and most of the ring complexes of the Aïr Province (Niger).

Zircon from Intraplate Gabbroic Rocks of Western Chukotka (Anyui Plutonic Complex) and Interpretation of Its Age

Zircon from Intraplate Gabbroic Rocks of Western Chukotka (Anyui Plutonic Complex) and Interpretation of Its Age

Multi-stage evolution of the Lost City hydrothermal vent fluids

Serpentinization-influenced hydrothermal systems, such as the Lost City Hydrothermal Field (LCHF), are considered as potential sites for the origin of life. Despite an abundance of reducing power in this system (H2 and CH4), microbial habitability may be limited by high pH, elevated temperatures, and/or low concentrations of bioavailable carbon. At the LCHF, the relative contribution of biotic and abiotic processes to the vent fluid composition, especially in the lower temperature vents, remain poorly constrained. We present fluid chemistry and isotope data that suggest that all LCHF fluids are derived from a single endmember produced in the hotter, deeper subsurface essentially in the absence of microbial activity. The strontium isotope composition (87Sr/86Sr) of this fluid records the influence of underlying mantle and/or gabbroic rocks, whereas sulfur isotope composition indicates closed-system thermochemical sulfate reduction. Conductive cooling and transport is accompanied by continued sulfate reduction, likely microbial, and mixing with unaltered seawater, which produce second-order vents characterized by higher δ34Ssulfide and lower δ34Ssulfate values. Third-order vent fluids are produced by varying degrees of subsurface mixing between the first- and second-order fluids and a seawater-dominated fluid. Additional biotic and abiotic processes along different flow paths are needed to explain the spatial variability among the vents. Relationships between sulfur geochemistry and hydrogen concentrations dominantly reflect variations in temperature and/or distance from the primary outflow path. Methane concentrations are constant across the field which point to an origin independent of flow path and venting temperature. At Lost City, not all vent fluids appear to have zero Mg concentrations. Thus, we propose an extrapolation to a Sr isotope-endmember composition as an alternative method to estimate endmember fluid compositions at least in similar systems where a two-component mixing with respect to Sr isotopes between seawater and endmember fluids can be established.

Effects of Water Saturation on the Dynamic Compression and Fragmentation Response of Gabbroic Rock

Effects of Water Saturation on the Dynamic Compression and Fragmentation Response of Gabbroic Rock

The effect of metamorphism on the aggregate properties of gabbroic rocks

Granitic rocks are durable materials sought after for the production of road and railroad aggregates. Granitic bedrock commonly, however, includes gabbroic components, which may enhance or decrease the aggregate performance. This study evaluates the variation in resistance to fragmentation (Los Angeles value, LA) and wear/abrasion (micro-Deval value, MDE) for the fraction 10/14 mm of gabbro in different metamorphic states. Samples were collected along a 150-km profile where metamorphic conditions grade from epidote–amphibolite to high-pressure granulite-facies, and the degree of metamorphic recrystallization varies with the amount of hydrous fluid. Rocks with no or incipient metamorphic recrystallization preserving their primary igneous fabric and interlocking texture meet the criteria for both asphalt base course and track ballast in railroad, with LA and MDE values below 25% and 14%, respectively. Mafic granulite and fine-grained amphibolite have LA values below 25% and can be used in unbound layers. Mafic granulites crystallize at high temperatures but commonly preserve a relict igneous texture due to limited hydration. Coarse-grained amphibolite and migmatitic amphibolite have the poorest performance. They recrystallized at hydrous conditions, leading to complete recrystallization, grain coarsening, and loss of interlocking igneous texture. This study shows that both temperature and infiltration of hydrous fluids significantly affect the technical properties. Even at high metamorphic temperatures, gabbroic rocks may yield aggregates of high technical performance. At hydrous conditions, however, recrystallization results in rock aggregates suitable for unbound layers only. The variation in metamorphic grade and hydration is easily assessed by the geologist in the field and by using standard petrographic microscopy.

Nature of the Late Cretaceous mantle source beneath the western Lhasa terrane, southern Tibet: Insights from the newly discovered mafic intrusion

Recent studies indicate that the mantle source of eastern Lhasa terrane was still depleted during the late stage of oceanic subduction (100–50 Ma). However, the mantle source beneath the western Lhasa terrane during this period was poorly constrained mainly due to the rare exposure of mafic rocks in this region. Here, we report the detailed petrological, geochronological, geochemical, and Sr–Nd–Hf–Pb isotopic data for recently identified gabbroic rocks in the Taruocuo area, western Lhasa terrane, southern Tibet. The zircon U-Pb dating results indicate that the Taruocuo gabbroic rocks were emplaced during the Late Cretaceous (~90 Ma). These gabbroic rocks have low SiO2 (50.9–55.0 wt%), and moderately high MgO (4.10–5.60 wt%) contents with arc-like trace-element patterns, such as enrichment in large ion lithophile elements, light rare earth elements, and depletion in high field strength elements. Meanwhile, they have high Th/La, low Ba/La values, and exhibit relatively enriched and uniform Sr–Nd–Hf–Pb isotopic compositions (87Sr/86Sr(i) = 0.7048–0.7058, εNd(t) = −1.8 to −1.2, εHf(t) = +3.1 to +3.8, and 206Pb/204Pb = 18.60 to 18.66). Such geochemical features of the Taruocuo gabbroic rocks suggest that they would originate from an enriched and fertile mantle source that was generated by the metasomatic reaction between the mantle wedge peridotite and the subducted Neo-Tethyan oceanic sediment-derived melts. Taking the regional tectonic evolution into consideration, we favor the model of Neo-Tethyan oceanic-ridge subduction having taken place to account for the Late Cretaceous (~90 Ma) magmatic event in the Lhasa terrane. Under such geological background, partial melting of this enriched mantle would have produced the Taruocuo gabbroic rocks in the western Lhasa terrane. Given that the Late Cretaceous Taruocuo gabbroic rocks have the relatively enriched Sr–Nd–Hf isotopic compositions, distinctive from the contemporaneous mafic rocks in the eastern Lhasa terrane that have the depleted Sr–Nd–Hf isotopic compositions, we propose that the mantle source beneath the Lhasa terrane was probably isotopically segmented during the Late Cretaceous (~90 Ma).

Multiple crustal and mantle inputs in post-collisional magmatism: Evidence from late-Variscan Sàrrabus pluton (SE Sardinia, Italy)

The Sàrrabus pluton is formed by multiple short-lived intrusions emplaced at about 286 Ma at shallow crustal levels within the external part of the South Variscan Orogenic Belt. A chemical and Sr and Nd isotopic study on the Variscan post-collisional magmatism from the Sàrrabus pluton reveals the repeated bimodal character of the intrusions, in which heterogeneous crustal sources and mantle-derived calcalkaline magmas are involved. Products of this magmatic activity occur as intrusive units and mafic/felsic dykes intruded in post-collisional regime along extensional faults during tectonic exhumation. Pluton growth started with an early stage of emplacement of broadly granodioritic magma with subordinate mafic magma batches (stage 1) followed by large intrusions of metaluminous to subaluminous and subordinately peraluminous granites (stage 2). In stage 1, the occurrence of remnants of stratified olivine-bearing gabbroic rocks indicates the intrusion of mafic magmas which experienced low-pressure crystal/liquid fractionation. Mafic magmas may represent an external heat supply for melting of different crustal materials belonging to an inferred Precambrian crystalline basement underlying the Paleozoic rocks of the Variscan nappe pile. Strong evidence for heterogeneous crustal sources is constrained by isotope data. Peraluminous granites and felsic dikes display initial 87Sr/86Sr in the range of 0.7140÷0.7215 and a roughly constant εNd286 (−7.4 to −7.5). Conversely, a peculiar less radiogenic character, in the range of 0.7030÷0.7067/−5.5÷ − 6.2, is observed for metaluminous to subaluminous varieties. Calculated Neodymium Crustal Index (NCI) confirmed a progressive increase in crustal magmas generation during the pluton growth, from stage 1, recording minor mixing processes between mantle- and crustal-derived peraluminous melts, to stage 2, where extensive crustal melting occurred, originating metaluminous to subaluminous granites. Possible crustal sources for metaluminous/subaluminous and peraluminous granites are (Pan-African) amphibolites and metasedimentary rocks, respectively. Two-stage depleted-mantle Nd model ages cluster at 1.4 and 1.6 Ga for metaluminous/subaluminous and peraluminous granites, respectively, well comparable with other segments of the European Variscan belt. Remarkably, last magmatic pulses resulted in widespread subalkaline bimodal mafic/felsic dykes that overlapped the SrNd signature recorded by major intrusions. This similar geochemical affinity between late dykes and the first intrusives may be related to decreasing temperature in the crust, which inhibited extensive mixing processes with the upwelling melts. At the same time, the presence of felsic intrusions in the shallow crust would have prevented the rise of more primitive basic magmas, which would have consolidated at depth. Finally, the high radiogenic character of Sàrrabus mafic products, compared to coeval Variscan mafic rocks of Corsica and northern Sardinia, may be indicative of previous fractionation and mixing processes, possibly related to magmatic underplating of the lower crust.