Gabbro Dikes Research Articles

Combined geochronological U-Pb, Hf isotopes and trace element zircon studies from Piedra Alta Terrane, Rio de la Plata Craton (West Uruguay, South America): A geodynamic model

This work delves into the intricate Paleoproterozoic geological setting of Uruguay, focusing on the Rio de la Plata Craton (RPC) and its prominent constituent Piedra Alta Terrane (PAT). The PAT constitutes the core of the RPC, characterized as a complex geological entity that encompasses a granite-migmatite basement, metamorphic belts, granite intrusions, and Proterozoic magmatism. To the west, it interfaces with the Transbrasiliano-Kandi tectonic corridor, bounded by the Late-Neoproterozoic Pampia Terrane. The PAT s southeastern boundary extends to Sierra de Tandil and perhaps to Sierra de La Ventana, demarcating its limits with Phanerozoic rocks of the North Patagonian Massif. The Sarandí del Yí shear zone defines the eastern limit of the PAT. Despite varied interpretations of the Piedra Alta Terrane’s significance, it is important to note that consensus prevails regarding its four major associations: a granite-gneiss-migmatitic basement complex, three volcanosedimentary belts of low to medium metamorphic grade, diverse late igneous intrusions, and extensional Statherian magmatism (1.79 Ga), represented by tholeiitic gabbroic dikes. In this study, a novel tectonic model for the PAT evolution emerges, supported by an extensive zircon dataset comprising new U-Pb ages, Hf isotopes, and trace element geochemistry. The analytical core involves U-Pb analyses on zircon for dating igneous rocks and establishing the provenance and maximum age of deposition of metavolcanosedimentary sequences. Lu-Hf isotopic analyses on the same zircon crystals provide petrogenetic insights, revealing episodes of crustal growth and minor recycling. Trace element geochemistry and zircon εHf(t) data further accentuate these findings, hinting at mantle source enrichment due to subduction within an intraoceanic magmatic-arc arc setting.

Dike Complexes in the Eastern Part of the Kaakhem Magmatic Area (Eastern Tuva): Composition, Age, Geological Position

This paper presents the results of isotope-geochronological and petrological studies of gabbroic and combined diorite-granite dikes located in the eastern part of the Kaakhem magmatic area. Both groups of dikes cut through diorite-tonalite-plagiogranite associations of different ages (489±5 and 476±4 Ma). Zircons from granitoid of mingling dikes have an age of 477±3 Ma (LA-ICP-MS). The age of the gabbroic dikes was determined by LA-ICP-MS (zircon) and Ar-Ar (amphibole) methods and is 454±10 and 450±6.3 Ma, respectively. Similar contents of major and trace elements in basic and intermediate rocks of dikes indicate their formation from a single source with subduction characteristics. The salic rocks of the combined dikes vary in composition and are close to the heterogeneous diorite-tonalite-plagiogranite-granite associations of the host rocks. The formation of dike complexes occurred at the collisional stage of development of the Kaakhem magmatic area and is associated with the development of local extension zones.

Geochemical and geochronological evidence of a Neoproterozoic island-arc on the easternmost Sierras Pampeanas. New U-Pb LA-ICP-MS data from amphibolites and metacarbonate depositional sequences

The first basement outcrops west of the Río de La Plata craton are metabasaltic and metasedimentary units of the Sierra Chica belt, part of the Eastern Sierras Pampeanas. These are orthoamphibolites and subordinate garnet-biotite gneisses and metacarbonate sequences that are intruded by Cambrian calc-alkaline and peraluminous rocks of the Pampean Orogeny. In accordance with one of the leading tectonic models for the region, new provenance data from this research suggest that the metabasaltic-metasedimentary association represents the proximal deposits of a Late Neoproterozoic island-arc. This contribution brings new geochemical and geochronological evidence from orthoamphibolites located on different sites along the Sierra Chica belt, which are interbedded with metacarbonate and gneisses outcrops, showing tholeiitic compositions, island-arc related tectonic settings, and Cryogenian to Ediacaran crystallization ages. Moreover, silicate-rich metacarbonates interbedded within the same sequences yielded a maximum depositional age of 569 Ma, reinforcing the assumed age for the sequence. All things considered, these rocks are interpreted as old basic lava flows and gabbro dikes linked to an island-arc activity, interspersed within the carbonatic and siliciclastic shelf sediments, together constituting the Neoproterozoic Córdoba Arc from the Sierra Chica belt. A depositional analogy with the present-day Caribbean island-arcs deposits suggests that these sequences could have been part of Oceanic Arc Depositional Systems (OADS) during the diachronous closure of the Goiás ocean and the final assembly of West Gondwana. Overlapping thermo-tectonic processes associated with the Pampean Orogeny are ubiquitous throughout all the studied outcrops, as evidenced by the petrography, the zircon morphology, and the U-Pb results, indicating a complex scenario towards a Late Cambrian slab break-off event.

Magmatic evolution of the Schiel Alkaline Complex, Bushveld large igneous province, South Africa

The Paleoproterozoic Schiel Alkaline Complex (SAC) in the Limpopo Belt is a composite pluton that coevally formed with the layered intrusion of the Bushveld large igneous province. The SAC is mainly composed of alkaline felsic rocks, pyroxenite, gabbro, syenodiorite/−gabbro, and minor carbonatite and gabbroic dikes. Here we present a new geochemical, geochronological, and isotopic study of the felsic and (ultra)mafic rock suites of the western SAC. The whole rock SmNd isotope study of the felsic rocks revealed εNd2059 Ma values between −6.5 and − 3.8, while those of the syenodiorite/−gabbros and gabbros vary from −7.9 to −8.8, and − 4.3 to +1.1, respectively. The pyroxenites have εNd2059 Ma values of +7.4 to +2.7. The LuHf isotope study displays εHf2059 Ma values of −15.9 to −9.0 for the syenites, alkali granites and syenodiorite/−gabbros, and significantly higher εHf2059 Ma of −3.1 to +3.4 for the gabbros. LA-SF-ICPMS UPb zircon dating yields 2079 ± 10 Ma and 2088 ± 30 Ma for the syenodiorite/−gabbros, and 2086 ± 51 Ma and 2107 ± 30 Ma for the gabbros. In conjunction with published data from the eastern SAC, we propose a close petrogenetic link between the mafic and felsic rocks of the SAC. The positive εNd and εHf values of the pyroxenite and gabbro favor a depleted mantle source, while variable εNd and εHf values, and trace element ratios of the gabbros are ascribed to crustal contamination during magma ascent. The felsic rocks, however, mainly formed by fractional crystallization from the gabbros as revealed by major and trace element correlation trends, isotopic data and thermodynamic modelling.

Scandium Ore Occurrences in the Ancient Weathering Crust in the Nakyn Kimberlite Field of Yakutia

In the Malo-Botuobinsky, Sredne-Markhinsky, and Ygyattinsky diamond-bearing areas of the Western Yakutian kimberlite province, the prerequisites and signs of scandium deposits in ancient weathering crusts overlying Lower Paleozoic rocks and associated with zones of ancient reservoir and soil oxidation. According to X-ray fluorescence and ICP MS analyses, in the Nakyn kimberlite field of the Sredne-Markhinsky diamond-bearing region, promising ore occurrences and scandium concentration halos have been established in clay deposits of redeposited weathering crusts of the Dyakhtar Formation of Late Triassic–Early Jurassic age. They are confined to the erosional surface of Lower Paleozoic carbonate rocks and Middle Paleozoic traps, with monzonite porphyries and kimberlites cutting through them, covered on top by a sedimentary cover of Jurassic terrigenous deposits. Concentration anomalies of scandium in clays of the Dyakhtar Formation are localized mainly in deluvial clay deposits of paleowatersheds and gravitate to the intersections of tectonic disturbances. The depth of their occurrence does not exceed 100 m, which is quite favorable for the extraction of scandium by borehole in situ leaching. Concentrations of scandium were detected at one of the sites before 262 g/t in Late Triassic weathering crust, including material from a Middle Paleozoic gabbro dike, as well as overlying colluvial clays of the Dyakhtar formations with a maximum of 462 g/t.

Ore Genesis and the Magmatism of the Yuhaixi Mo(Cu) Deposit in Eastern Tianshan, NW China: Constraints from Geology, Geochemistry, Zircon U-Pb and Molybdenite Re-Os Dating

The Yuhaixi Mo(Cu) deposit is a new discovery in the eastern section of the Dananhu-Tousuquan island arc, Eastern Tianshan. However, the genesis of the Yuhaixi Mo(Cu) deposit is still not fully understood. The Yuhaixi intrusion is composed of monzonitic granites, diorites, granites, and gabbro dikes, among which disseminated or veinlet Mo and Cu mineralization is mainly hosted by the monzonitic granites. The LA-ICP-MS zircon U-Pb dating yields emplacement ages of 359.4 ± 1.6 Ma for the monzonitic granite, 298.8 ± 1.8 Ma for the diorite, and 307.0 ± 2.3 Ma for the granite. The Re-Os dating of molybdenite hosted by monzonitic granite yields a well-constrained 187Re-187Os isochron age of 354.1 ± 6.8 Ma (MSWD = 1.7) with a weighted average age of 344.5 ± 3.1 Ma. The Mo mineralization is closely associated with the Yuhaixi monzonitic granite. The Yuhaixi monzonitic granite rocks are characterized by high silica (SiO2 > 70 wt.%), low MgO (0.23–0.36), Ni, Cr contents, and they are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs: e.g., K, Ba, Pb and Sr), and depleted in heavy rare earth elements (HREEs) and high field-strength elements (HFSEs: e.g., Nb, Ta and Ti). They are weak peraluminous and have high εHf(t) (11.37–17.59) and εNd(t) (1.36–7.75) values, and varied initial 87Sr/86Sr (0.7037–0.7128) values. The Yuhaixi post-ore granites exhibit similar geochemical and isotopic signatures to the Yuhaixi monzonitic granite. These characteristics suggest that the Yuhaixi felsic rocks are likely sourced from the partial melting of the juvenile lower crust. The Yuhaixi diorite has low SiO2, and K2O contents, relatively high Na2O, MgO (Mg# = 45–53) contents, and depletions in HFSE (e.g., Nb, Ta, and Ti). These geochemical features, coupled with isotopic data such as low initial 87Sr/86Sr (≤0.7043), high εNd(t) (2.5 to 3.0) and εHf(t) (≥11.6) values, and young Hf model ages, suggest that their parental magmas possibly originated from the partial melting of the depleted lithospheric mantle that was metasomatized by hydrous melts or fluids from the subducting oceanic plate. Integrating our new results with previous works on the Dananhu-Tousuquan island arc belt, we suggest that the Yuhaixi Mo(Cu)deposit is likely sourced from the juvenile lower crust, which was formed in an arc setting, where the bipolar subduction of the North Tianshan oceanic slab forms the Dananhu Tousuquan belt to the north and the Aqishan-Yamansu belt to the south. The eastern section of the Dananhu-Tousuquan island arc is a promising target for late Paleozoic porphyry Mo(Cu) deposits.

Geochronology and geochemistry of early Paleozoic magmatism in the Qilian orogen: Constraints on closure of the Proto-Tethys Ocean

The Proto-Tethys Ocean was a Neoproterozoic to early Paleozoic ocean that opened upon the breakup of Rodinia and closed by Paleozoic oceanic and continental subduction. The Qilian orogenic belt is the northernmost orogen of the Proto-Tethys domain and consists of the North Qilian belt, the Central Qilian block and the South Qilian belt. The processes responsible for extensive early Paleozoic magmatism in the Qilian orogen are crucial to understanding the closure of the ocean. This paper presents a systematic investigation of early Paleozoic mafic and felsic rocks in the Qilian orogen. Gabbroic dikes from the Central Qilian block have LA-ICP-MS zircon U-Pb ages of 499–497 Ma with a wide range of εHf(t) values of −9.94 to + 4.28 and TDM1 model ages of 1557–929 Ma, and dioritic dikes have a LA-ICP-MS zircon U-Pb age of 489 ± 3 Ma, positive εHf(t) values of +7.61 to +10.53, TDM1 model ages of 684–790 Ma, and an amphibole 40Ar/39Ar age of 486.3 ± 4.3 Ma. Both sets of dikes were derived from partial melting of a heterogeneous spinel-facies lherzolite lithospheric mantle that was modified by fluids and melts from a subducted slab. The protolith of amphibolites from the Hualong block in the South Qilian belt was a mafic intrusion of 454 ± 2 Ma with negative εHf(t) values of −9.69 to −7.79 and TDM1 model ages of 1390–1478 Ma. Its trace element signature indicates that sediment-derived melts were involved in the magma formation. An amphibole 40Ar/39Ar age of 418.7 ± 3.8 Ma from an amphibolite records the timing of amphibolite facies metamorphism. Felsic volcanic tuffs and granitic intrusions from the Central Qilian block have zircon U-Pb ages of 507–440 Ma with a broad range of zircon εHf(t) values of −9.60 to + 11.87 and TDM2 model ages of 2037–584 Ma. They have geochemical characteristics similar to I-type and S-type granites that were sourced from partial melting of juvenile and ancient continental crust with minor input of mantle materials. The formation of the felsic rocks indicates the transition from an arc-related setting to a post-collisional setting. Combining our results with existing data, we suggest that the Central Qilian block was affected by both southward subduction of the North Qilian ocean and the northward subduction of the South Qilian ocean, resulting in the formation of an Andean-type continental arc along the northern margin of the Central Qilian block and a subduction-accretion system along the southern margin of the Hualong block at ca. 530–440 Ma. Collision of the Central Qilian, Hualong, Quanji and Qaidam blocks at ca. 440–418 Ma marks the final closure of the Proto-Tethys Ocean.

Precambrian tectono-magmatic evolution of the western margin of the Yangtze block, South China: evidence from zircon U-Pb-Lu-Hf isotopes, REE and trace elements of Yuanmou-Miyi complexes

ABSTRACT Here new evidence of the isotope geochronology and the geochemistry from the Yuanmou-Miyi complexes reveal the Precambrian tectono-magmatism geodynamic process in the study area. The Yangtze Block basement with 1.81- ~1.77 Ga and negative ε Hf(t) values of −7.8 to −0.7 corresponded to the assembly of the Columbia-supercontinent. However, the rifting magmatic event of ~ 1.70 Ga, which caused the continental crust extension-thinning in the study area, may exert influence on the subduction-magmatism in the Grenvillian period. The Yuanmou-Miyi complexes of 1.19- ~1.00 Ga are characterized by (La/Sm)N＞1, depletion in HFSEs, negative anomalies of Nb-Ta, and enrichment of LILEs, which show a genetic link with the subduction-magmatism. The temporal tendency that the magmatism from Yuanmou to Miyi gradually became younger, may be responsible for the subducting direction. The subduction caused the reactivation of the Anninghe-Lvzhijiag fault zone, and created the rupture windows of the subduction slab, leading to upwelling of the deep magma. These granitoids were derived from the partial melting of the mantle wedge, while the small-scale gabbro dikes intruded into the granitoids were likely spillover products from the asthenosphere mantle melts through the rupture windows.

A polygenetic origin for the Sikhoran ultramafic-mafic complex in South Iran

The nature and geodynamic setting of the Sikhoran ultramafic–mafic complex, southwest Iran, is debated, with multiple competing theories having been proposed in recent years. The Sikhoran ultramafic–mafic complex is located in the southern part of the Sanandaj-Sirjan metamorphic-magmatic zone (SSMMZ) and is comprised of porphyroclastic, transitional and layered ultramafic-mafic sequences of Carboniferous age. These units are cut by isotropic gabbros, diabasic and pegmatoid gabbroic dikes with ages ranging from Permian to the Cretaceous. These bodies also intrude into Upper Paleozoic metamorphic rocks, where they induced high-temperature contact metamorphism during the Late Carboniferous. Here we present new textural descriptions, mineral and whole-rock geochemical analyses, and the results of zircon UPb dating of magmatic mafic rocks and metamorphic host rocks from this region in order to reconstruct the petrogenesis of the Sikhoran ultramafic–mafic complex. Zircon UPb ages from layered gabbros indicate Late Carboniferous (320.8 ± 6.4 Ma) crystallization, which is similar to the age of the Upper Paleozoic host gneisses (334.6 ± 4.9 Ma). By contrast, isotropic gabbros that cross-cut the complex have a crystallization age of 178.3 ± 2.3 Ma, and anatectic quartz diorite/plagiogranite produced from partial melting of hosting amphibolite in contact with the gabbros yielded Late Triassic-Early Jurassic ages (187.2 ± 2.6 Ma). These data suggest a polygenetic origin and a range of tectonic settings for various parts of the ultramafic-mafic complex. Geochemical modeling shows that mantle plume-related melting (∼5–40%) of metasomatized mantle could have formed the ultramafic-mafic layered cumulate complex during the Late Carboniferous. A geochemical transition from island arc tholeiite (IAT) isotropic gabbros to cross-cutting enriched mid-ocean ridge basalts (E-MORB)–diabasic dikes occurred between the Late Triassic-Early Jurassic to the Late Cretaceous. The mantle-derived ultramafic parts of the Sikhoran ultramafic-mafic complex have similarities with other known ultramafic-mafic mantle plumes (e.g., Tinaqulllo, Lherz, and Ronda), and its layered ultramafic-mafic segment has similarity with layered intrusions (e.g., Pulur complex in Turkey). The highest observed crystallization temperature in porphyroclastic, transitional and layered ultramafic mantle sections of ∼1140 °C is considered as the mantle potential temperature, since the highest crystallization temperatures are observed in the most forsteritic olivine crystals. Based on amphibole chemistry, the crystallization conditions of transitional to layered ultramafic rocks at 900–1005 °C occurred at high H2O contents in a melt between nickel–nickel oxide (NNO) −8.5 and NNO −9.0, which lies along the NNO−2 buffer. These results are consistent with a model in which the opening of the Zagros Neotethys Oceanic basin in the Esfandagheh region initiated in the Late Carboniferous, alongside the formation of an extensional environment that allowed ascent of a subcontinental mantle plume. Subduction initiation of oceanic lithosphere began during the Late Triassic-Early Jurassic and ended in the Late Cretaceous.

Geochemical and geochronological constraints on the origin of the Sabzevar ophiolites (NE Iran): forced far-field subduction initiation in the upper-plate of the Neo-Tethys subduction zone

The Sabzevar ophiolites, located at the northern margin of the Central-East Iranian microcontinent (CEIM), are part of the Mesozoic-Paleogene Neotethyan suture zone developed along the Alpine-Himalayan convergence zone. These ophiolites consist mostly of oceanic lithospheric remnants, covered by early Campanian-late Maastrichtian volcano-sedimentary successions. A distinctive characteristic of the Sabzevar ophiolites is the occurrence of mafic dike swarms (gabbros, gabbronorites and diorites) with forearc-arc-tholeiitic geochemical signature, intruding the mantle section. Occurrence of orthopyroxene, development of pegmatitic texture, crystallization of clinopyroxene prior to plagioclase, and the presence of anorthite-rich plagioclase imply relatively high H2O content in the magmatic plumbing system. Rare plagiogranites (tonalite and trondhjemite compositions) show geochemical features compatible with a supra-subduction setting, whereas late (hornblende-bearing) gabbro dikes show a within-plate signature. The bimodal geochemical affinity (subduction vs. intraplate) is also attested by clinopyroxene compositions. The gabbroic, plagiogranitic and gabbronoritic samples yield Early Cretaceous SHRIMP zircon U-Pb ages of 96.7 ± 1, 98 ± 1 and 94 ± 1 Ma, respectively. A progression from tholeiitic MORB-like to more depleted high-Mg andesite and eventually alkaline affinities is here proposed, framing the magma evolution as generated in an evolving forearc setting that post-dated (of at least 9 Myr) the formation of the metamorphic sole during the infant stages of subduction of the Sabzevar Ocean.A scenario of far-field forced subduction initiation of the Sabzevar Ocean is proposed as consequence of propagation of the residual stresses transmitted from the Arabia-Eurasia convergence zone across the CEIM during Cretaceous times (Albian-Campanian).

Two episodes of Late Paleozoic mafic magmatism in the western Tianshan Orogen: From Carboniferous subduction to Permian post-collisional extension

The evolution of the western Tianshan Orogen in the southern Central Asian Orogenic Belt involved prolonged accretion and collision processes in response to the closure of the Paleo-Asian Ocean during the Late Paleozoic. However, the timing for the tectonic transition from subduction to post-collision is still poorly constrained. Herein, we address this issue based on geochronological, geochemical and Sr-Nd-Hf isotopic investigations on representative mafic dikes from the Awulale Mountains in the heart of the western Tianshan. Zircon U-Pb dating suggest two episodes of mafic magmatism during the Late Carboniferous (ca. 324–310 Ma) and Early Permian (ca. 299–290 Ma). The Late Carboniferous gabbro dikes exhibit large variations in MgO (2.79–10.97 wt%) and Ni (7.58–244 ppm), and arc-like trace element patterns with high Th/Yb (0.55–2.26) ratios, relatively low εNd(t) (+0.2 to +2.5) and variable εHf(t) (+1.3 to +10.4) values, indicating that they were derived from fractional crystallization of a peridotite-bearing mantle source metasomatized by sediment-derived hydrous melts. In contrast, the Early Permian diabase dikes have slightly variable MgO (5.88–7.78 wt%) and Ni (59.90–104 ppm) with apparently low Th/Yb ratios (0.20–0.34, except for one sample: 0.86) and depleted εNd(t) (+5.0 to +7.8) and εHf(t) (+8.0 to +14.6) values, which indicates derivation from partial melting of a juvenile lithospheric mantle source with contributions from early subduction metasomatic components. In context with relevant published data, our study indicates that the Late Carboniferous magmatism of the Awulale Mountains was likely induced by breakoff of the North Tianshan oceanic slab during ongoing subduction; conversely, the Early Permian magmatism may have originated from delamination of the lower crust and lithospheric mantle in the subsequent post-collisional setting. Furthermore, the tectonic transition from Carboniferous subduction to Permian post-collisional extension of the western Tianshan suggests that terminal closure of the North Tianshan Ocean took place at Carboniferous-Permian boundary at ∼ 300 Ma.

Neoproterozoic tectonic shift from collisional orogenesis to intraplate extension in the Yili Block, southern Central Asian Orogenic Belt

Understanding the tectonic evolution of the many micro-continents in the Central Asian Orogenic Belt is a complex and controversial issue. One such micro-continent, the Yili Block is characterized by Neoproterozoic magmatic rocks, which can provide important constraints on the tectono-magmatic history of the region. Herein, we present a systematic investigation of the petrology, whole-rock geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes on newly identified leucogranite intrusions and gabbroic dikes in the Wenquan district of the northern Yili Block. Two leucogranite samples yielded zircon U-Pb ages of 983 ± 9 Ma and 905 ± 9 Ma, respectively. Both of muscovite-bearing leucogranites have S-type affinities and show distinctive geochemical compositions similar to peraluminous granites. Furthermore, the leucogranites exhibit low Rb/Sr ratios (0.10–0.49) and elevated zircon εHf(t) values (+3.04 - +13.59), indicating that they were derived from partial melting of supracrustal pelitic and mafic rocks. In contrast, one gabbroic dike sample yielded a zircon U-Pb age of 779 ± 5 Ma. The gabbroic samples have distinctly high Th/Yb (1.10–1.68) and low Nb/La (0.37–0.46) ratios, with enrichment in large ion lithophile elements and depletion in Nb, Ta and Ti, implying that they were sourced from a metasomatic subcontinental lithospheric mantle. Integrated with other available data, we propose that the Yili Block underwent continuously collisional orogenesis during the early Neoproterozoic (ca. 983–780 Ma), with a subsequent tectonic shift from collisional to an intraplate extensional setting at ca. 779–776 Ma. A comparable magmatic evolution also occurred in adjacent micro-continents of the Kazakhstan region and northern Tarim Craton, indicating that these micro-continents may have shared a common Neoproterozoic geodynamic evolution.

Petrogenesis of Miocene igneous rocks in the Tafresh area (central Urumieh‐Dokhtar magmatic arc, Iran): Insights into mantle sources and geodynamic processes

Cenozoic tectono‐magmatism in Iran is widely considered to be related to subduction of the Neo‐Tethys Ocean. We employed whole‐rock and mineral geochemistry and isotopic data of intrusive rocks from Tafresh, central Urumieh‐Dokhtar magmatic arc, to evaluate the role of the mantle in magmatism, to assess the timing of emplacement, and to interpret the tectonic setting. Rock compositions range from gabbro or gabbro‐diorite (plagioclase + pyroxene ± olivine), to diorite (plagioclase + amphibole ± pyroxene), to granodiorite (quartz + plagioclase + K‐feldspar + amphibole + biotite), exhibiting high‐alumina calc‐alkaline affinity. Major oxide and trace element variations vary systematically from less to more evolved rocks suggesting a major role for fractional crystallization processes. Zircon LA‐ICP‐MS U–Pb ages of major rock types are in the range of 24–19 Ma, whereas those of gabbroic dikes are ~17.5 Ma. ԐNd values range between −1.8 and 3.7, and (87Sr/86Sr)i is narrowly restricted to 0.705–0.706, suggesting a common mantle source. The enrichment in light rare earth element (REE) enrichment and flat heavy REE patterns couple depletion of Nb–Ta–Ti indicate that subducting oceanic crust had interacted with the overlying mantle wedge. High‐alumina, mid‐Mg# Tafresh plutonic rocks formed from hydrous melts from which Ca‐pyroxene and magnetite crystallized earlier than plagioclase, whereas late‐crystallizing zircon nucleated while magma traversed through lithospheric mantle and Cadomian crust. Modelling of isotope and incompatible‐element patterns suggests the contribution of no more than ~5% molten sediment or other crustal components in Tafresh magma, at a developmental stage before most plagioclase and amphibole had crystallized. The Miocene Tafresh plutons originated during the final stages of subduction, before the collision between the Arabian and Eurasian plates.

Stress State in the Source Region of Mw2.2 Earthquake in a Deep Gold Mine in South Africa Determined from Borehole Cores

In December 2007, an Mw2.2 earthquake occurred in a gabbroic dike at 3.3 km depth in a deep gold mine in South Africa. The fore- and aftershock activity was analyzed in an effort to understand the preparation and generation processes of earthquakes; these findings have already been published. The present paper focuses on the stress state in the source region of the mainshock. A 90-m-long borehole across the mainshock fault was drilled ~ 1.5 years after the mainshock and logged by an optical televiewer. The fault can be identified by severe damage to the borehole wall at the point where the borehole intersected the aftershock cluster. Except for a 10-m section in the hanging wall of the fault, borehole cores were fully recovered. Borehole breakout (BO) and core disking (CD) were found to occur. Two stress measurement techniques [Deformation Rate Analysis (DRA) and Diametrical Core Deformation Analysis (DCDA)] were applied to the borehole cores. By combining their results with occurrence criteria for BO and CD, the principal stress state in the source region of the mainshock was determined. The principal directions in the hanging wall of the fault were nearly identical to the virgin stress state, while it was significantly disturbed in the footwall. The vertical stresses were 106 MPa and 40 MPa in the hanging wall and footwall, respectively. The significant difference in the vertical stress between the two sides of the fault can be explained by the stress redistribution associated with a nonuniform slip on a nonplanar fault.

Geology, U-Pb geochronology and stable isotope geochemistry of the Heihaibei gold deposit in the southern part of the Eastern Kunlun Orogenic Belt, China: A granitic intrusion-related gold deposit?

The Heihaibei gold deposit is a newly discovered gold deposit in the southern part of the Eastern Kunlun Orogenic Belt. Its most distinctive features are that the gold mineralization is hosted in monzogranite, and that the presence of pre-ore (possibly syn-ore) monzogranite and post-ore gabbro allows to constrain the mineralization’s formation age. Zircons from the monzogranites yield U-Pb ages of 454 ± 3 Ma, while zircons separated from the gabbro dikes cutting the monzogranites and gold mineralized body yield U-Pb ages of 439 ± 3 Ma, which is interpreted to be the minimum age of the Au mineralizing event. Combined with the regional geological background, we proposed that the Heihaibei Au mineralization occurred during the subduction stage of the Early Paleozoic Proto-Tethys ocean.The ore assemblage is dominated by pyrite, arsenopyrite and native gold. The hydrothermal alteration that has led to the peculiar enrichment of Au is not systematically distributed and displays no clear concentric zoning pattern. The main mineralization formed during three stages: the K-feldspar-quartz-pyrite (Py1)-arsenopyrite-sericite-epidote stage (I), the quartz-pyrite (Py2)-native gold-chlorite stage (Ⅱ), and the quartz-carbonate stage (III). The main gold mineralization occurred during stage Ⅱ. Fluid inclusion homogenization temperature and salinities decrease from stage I (Th., 268–412 °C; W., 6.87–16.63 wt% NaCl equiv.) to stage Ⅱ (Th., 183–288 °C; W., 3.69–14.84 wt% NaCl equiv.). The δ18O and δD values (δ18OH2O = 4.9 to 9.7‰; δDV-SMOW = -84.1‰ to −81.1‰) of quartz samples from stage I and stage Ⅱ are comparable to a magmatic-hydrothermal ore-forming fluid that possibly underwent fluid-rock interaction with the Nachitai Group metamorphic rocks during the early ore-forming stage. The relatively uniform δ34S values (δ34SV-CDT = 7.7 to 8.5‰) are slightly elevated compared to magmatic δ34S values, but could be derived from a magma if a significant crustal melt component is present. Moreover, the δ34S values are within the S isotopic composition range of a granitic reservoir, suggesting that they are probably inherited from the Heihaibei monzogranites. The Pb and Hf isotope compositions imply a close genetic association between the gold mineralization and granitic magmatism, which are both the products of the mixing of crustal and mantle sources. The trace element compositions of pyrite provide additional evidence that the gold mineralization in the Heihaibei deposit was related to the magmatism. Compared with the typical characteristics of orogenic gold and intrusion-related gold systems (IRGS) deposits, the Heihaibei gold deposit may instead be classified as a granitic intrusion-related gold deposit.

U-Pb VEIN CALCITE DATING REVEALS THE AGE OF CARLIN-TYPE GOLD DEPOSITS OF CENTRAL YUKON AND A CONTEMPORANEITY WITH A REGIONAL INTRUSION-RELATED METALLOGENIC EVENT

AbstractLaser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of hydrothermal calcite associated with the waning stage of mineralization is used to constrain the age of Neoproterozoic-hosted Carlin-type gold deposits in central Yukon, Canada. Samples exhibiting minor textural complexity show low dispersion of individual spot analyses and define Tera-Wasserburg lower intercept ages between 75.1 ± 2.5 and 71.2 ± 4.3 Ma. Samples with U-rich microfractures and/or a second generation of calcite associated with vein reopening record the primary stage of calcite growth related to hydrothermal activity at ca. 74 Ma and a younger ca. 48 Ma event related to fluid-assisted U mobility. The minimum age for mineralization at ca. 74 Ma overlaps with a maximum age based on a zircon U-Pb date of 74.1 ± 1.0 Ma for a gabbro dike that is locally altered and mineralized. These time constraints indicate that mineralization is ca. 74 Ma and penecontemporaneous with dike emplacement. Furthermore, mineralization is contemporaneous with a regional gold-bearing, intrusion-related, metallogenic event in eastern Alaska and central Yukon. This temporal link between Carlin-type mineralization and other mineralization types bears similarities with world-class sedimentary rock-hosted gold deposits of Nevada. This study demonstrates the potential of using LA-ICP-MS U-Pb dating on calcite to constrain the age of mineral deposits lacking synmineralization minerals amenable to classic dating techniques.

Juxtaposition of Cenozoic and Mesozoic ophiolites in Palawan island, Philippines: New insights on the evolution of the Proto-South China Sea

The proto-South China Sea (proto-SCS) is a fully consumed Mesozoic plate that once existed between the South China-Eurasian margin and Borneo. Remnants of this oceanic domain are believed to be emplaced as ophiolitic lithologies in Palawan and Zambales, Philippines and Borneo. Cretaceous and Eocene ophiolitic lithologies are exposed in the central and southern portions of Palawan island. The Eocene central Palawan Ophiolite (CPO) is composed of mantle peridotites, isotropic and layered gabbros, and massive and pillow lavas. In lieu of a sheeted dike complex, mafic dikes with distinct bake-and-chill margins cut the CPO, from the metamorphic sole to the crustal section. The Cretaceous southern Palawan Ophiolite (SPO) comprises of peridotite and pillow basalt exposures. Isotropic olivine gabbros are mainly preserved as floats. Troctolite and olivine gabbro dikes also cut the peridotites in the SPO.The CPO crustal rocks and mafic dikes have dominantly back-arc basin basalt affinities. The SPO lithologies exhibit ocean island basalt-like signatures with a mafic dike showing island arc basalt-like compositions. A plagiogranite sample from Puerto Princesa indicates an Eocene age (40.01 ± 0.54 Ma) for the CPO. U-Pb dating of olivine gabbro and syenite floats yielded a minimum age of Early Cretaceous (100.73 ± 1.07 and 102.97 ± 1.07 Ma, respectively) for the SPO. This is the first reported radiometric age for the now completely subducted Mesozoic proto-SCS exposed in southern Palawan, which complements existing paleontological dates. Geochemical and geochronological characteristics of the SPO and CPO are similar to those of the Darvel Bay and Telupid Ophiolites in Borneo, which are interpreted to originate from the proto-SCS. The CPO and SPO are thus construed to represent fragments of the proto-SCS emplaced in Palawan. Complex tectonic processes in the region during the Cretaceous to Eocene led to the inception of multiple subduction and rifting events involving the proto-SCS.

Late Carboniferous – Early Permian mafic dikes and granitoids in the heart of the Western Tianshan Orogen, NW China: Implications for a tectonic transition from a syn- to post-collisional setting

Syn- to post-collisional magmatism in the Chinese western Tianshan Orogen is recorded in the Late Carboniferous to Early Permian magmatic rocks of the Awulale Mountains. However, the orogen's tectono-magmatic evolution remains controversial. In particular, two contrasting genetic models have been proposed, i.e., post-collisional extension vs. mantle plume. Here, we report zircon U-Pb ages, whole-rock geochemical data and Nd-Hf isotopic data from the mafic dikes and granitoids of the western Awulale Mountains. These data better constrain the rock's petrogenesis and geodynamic processes following the amalgamation of the Tianshan Orogen. In combination with previous data, we have identified two magmatic episodes in the Awulale Mountains. The Late Carboniferous (ca. 319–307 Ma) episode emplaced the mafic gabbro dikes and intermediate-acidic granitoids with I-type and adakitic affinity. The latest Carboniferous-Permian (ca. 302–286 Ma) episode emplaced bimodal intrusions containing mafic diabase dikes, acidic adakite and I- and A-type granites. Mafic dikes and granitoids of both episodes show depleted whole-rock Nd and zircon Hf isotopic compositions (εNd(t): +2.21 ~ +7.54, εHf(t): +6.48 ~ +15.97), indicating a juvenile magmatic source. However, the mafic dikes have variable geochemical features that clearly suggest their derivation from two distinct mantle sources during the two magmatic episodes. The earlier gabbro dikes have lower Th/Yb (0.08–0.12) and Zr/Yb ratios (15.32–25.45) that resemble a normal mid-ocean ridge basalt (N-MORB) from a depleted mantle source. In contrast, the later diabase dikes show enrichment in the light rare earth elements (LREE) with relatively higher Th/Yb (0.22–0.86) and Zr/Yb ratios (38.05–68.97) that are similar to an enriched mid-ocean ridge basalt (E-MORB) mantle source. Also, the Nb/Ta ratios from the earlier adakitic granitoids (21.26–23.28) differ markedly from the later acidic adakites (0.41–19.50), which implies the transition from a thicker to a thinner crust. Integrating our results with the previous geological records, leads us to propose a two-stage tectonic model for the Chinese western Tianshan Orogen's transition from syn- to post-collision. The Late Carboniferous stage of magmatism was triggered by the break-off of the North Tianshan Oceanic slab as the Yili Block and Junggar Terrane collided. The subsequent latest Carboniferous-Permian stage followed the switch to a post-collisional setting with delamination of the thickened lower crust and the intrusion of voluminous lithospheric mantle-induced bimodal magmatism.

Multi‐Stage Hydrothermal Veins in Layered Gabbro of the Oman Ophiolite: Implications for Focused Fluid Circulation in the Lower Oceanic Crust

AbstractAn outcrop in the layered gabbro section from the Wadi Wariyah of the Oman ophiolite has been investigated to provide new insights into the hydrothermal activities that cover a large temperature range in the lower crust at fast‐spreading mid‐ocean ridges. Our observations reveal a complex veining system composed of gabbroic dikes, hydrous mineral (amphibole, epidote, and prehnite) veins and associated alteration, which strongly overprinted the layered gabbros shortly after the solidification of the gabbroic cumulates. The general strike of the veins is roughly perpendicular to the magmatic foliation of the layered gabbro. Mineral‐based geothermometry indicates a wide temperature range (ca. 1,000−250°C) of hydrothermal activities, spanning from hydrous suprasolidus stage (VHT) through amphibolite facies (HT) and epidote‐amphibolite facies (MT) down to the prehnite‐pumpellyite facies (LT), which induced significant differences in selective element depletion and enrichment as well as in exchange of Sr and O isotopes. A key observation is the omnipresence of relics from a previous temperature stage in a given hydrothermal paragenesis formed at lower temperatures, implying that the same pathways were used for hydrothermal veining in a broad temperature interval spanning about 750°C. Our observations support a model that focused fluid circulations, in the form of combined microcracking (grain boundary spacing) and macrocracking (veining), developed shortly subsequent to the complete solidification of the layered gabbros, which may have facilitated effective cooling for magma crystallization in the lower crust.

Serial interaction of primitive magmas with felsic and mafic crust recorded by gabbroic dikes from the Antarctic extension of the Karoo large igneous province

Two subvertical gabbroic dikes with widths of ~ 350 m (East-Muren) and ≥ 500 m (West-Muren) crosscut continental flood basalts in the Antarctic extension of the ~ 180 Ma Karoo large igneous province (LIP) in Vestfjella, western Dronning Maud Land. The dikes exhibit unusual geochemical profiles; most significantly, initial (at 180 Ma) εNd values increase from the dike interiors towards the hornfelsed wallrock basalts (from − 15.3 to − 7.8 in East-Muren and more gradually from − 9.0 to − 5.5 in West-Muren). In this study, we utilize models of partial melting and energy-constrained assimilation‒fractional crystallization in deciphering the magmatic evolution of the dikes and their contact aureoles. The modeling indicates that both gabbroic dikes acquired the distinctly negative εNd values recorded by their central parts by varying degrees of assimilation of Archean crust at depth. This first phase of deep contamination was followed by a second event at or close to the emplacement level and is related to the interaction of the magmas with the wallrock basalts. These basalts belong to a distinct Karoo LIP magma type having initial εNd from − 2.1 to + 2.5, which provides a stark contrast to the εNd composition of the dike parental magmas (− 15.3 for East-Muren, − 9.0 for West-Muren) previously contaminated by Archean crust. For East-Muren, the distal hornfelses represent partially melted wallrock basalts and the proximal contact zones represent hybrids of such residues with differentiated melts from the intrusion; the magmas that were contaminated by the partial melts of the wallrock basalts were likely transported away from the currently exposed parts of the conduit before the magma–wallrock contact was sealed and further assimilation prevented. In contrast, for West-Muren, the assimilation of the wallrock basalt partial melts is recorded by the gradually increasing εNd of the presently exposed gabbroic rocks towards the roof contact with the basalts. Our study shows that primitive LIP magmas release enough sensible and latent heat to partially melt and potentially assimilate wallrocks in multiple stages. This type of multi-stage assimilation is difficult to detect in general, especially if the associated wallrocks show broad compositional similarity with the intruding magmas. Notably, trace element and isotopic heterogeneity in LIP magmas can be homogenized by such processes (basaltic cannibalism). If similar processes work at larger scales, they may affect the geochemical evolution of the crust and influence the generation of, for example, massif-type anorthosites and “ghost plagioclase” geochemical signature.