Mafic Dike Swarms Research Articles

Early Cretaceous Metasomatized Lithospheric Mantle beneath the Central Jiangnan Orogen in South China: Geochemical and Sr‐Nd Isotope Evidence from the Tuanshanbei Dolerite

AbstractIt is well established that Cretaceous magmatism in the South China Block (SCB) is related to the Paleo‐Pacific subduction. However, the starting time and the associated deep crust‐mantle processes are still debatable. Mafic dike swarms carry important information on the deep earth (including mantle) geodynamics and geochemical evolution. In the Jiangnan Orogen (South China), there is no information on whether the Mesozoic magmatic activities in this region are also directly related to the Pacific subduction or not. In this study, we present detailed zircon U‐Pb geochronological, whole‐rock element and Sr‐Nd isotope data for Early Cretaceous Tuanshanbei dolerite dikes, and provide new constraints on the condition of the lithospheric mantle and mantle dynamics of the SCB during that time. LA‐ICP‐MS zircon U‐Pb dating suggests that this dolerite erupted in the Early Cretaceous (∼145 Ma). All samples have alkaline geochemical affinities with K2O + Na2O = 3.11–4.04 wt%, K2O/Na2O = 0.50–0.72, and Mg# = 62.24–65.13. They are enriched in LILE but depleted in HFSE with higher initial 87Sr/86Sr ratio (0.706896–0.714743) and lower εNd(t) (–2.61 to –1.67). They have high Nb/U, Nb/La, La/Sm and Rb/Sr, and low La/Nb, La/Ta, Ce/Pb, Ba/Rb, Tb/Yb and Gd/Yb ratios. Such geochemical signatures suggest that the fractional crystallization is obvious but crustal contamination play a negligible role during magmatic evolution. Tuanshanbei dolerite were most likely derived from low‐degree (2%–5%) partial melting of a phlogopite‐bearing mantle material consisted of ∼85% spinel peridotite and ∼15% garnet peridotite previously metasomatized by asthenosphere‐derived fluids/melts with minor subduction‐derived fluids/melts. Slab‐rollback generally lead to the upwelling of the hot asthenosphere. The upwelling of asthenosphere consuming the lithospheric mantle by thermo‐mechanical‐chemical erosion. The lithospheric mantle may have partially melted due to the heating by the upwelling asthenosphere and lithospheric extension. It is inferred that the Tuanshanbei dolerite might be associated with the initial slab rollback and corresponding lithospheric extension occurred potentially at ca. 145 Ma.

Correction to: ‘The Generation of Eocene Mafic dike Swarms During the Exhumation of a Core Complex, Biarjmand Area, NE Iran’ by Azizi et al. (2023), Journal of Petrology, 64, 1–118

Correction to: ‘The Generation of Eocene Mafic dike Swarms During the Exhumation of a Core Complex, Biarjmand Area, NE Iran’ by Azizi et al. (2023), Journal of Petrology, 64, 1–118

Paleomagnetism of the Rio Perdido mafic dike swarm (1110 Ma) and the paleogeography of Amazonia and its role for supercontinents Rodinia and Gondwana

The Rio Apa Terrane (RAT) is situated at the south end of the Amazonian Craton (AC) separated from it by the aulacogen Tucavaca belt. It encompasses basement rocks that evolved mainly during the Paleoproterozoic. Its relationship to the AC during Proterozoic is yet undefined, although geologic, geochronologic and isotopic evidence suggest it was close to the Ventuari-Tapajós and Rio Negro-Juruena Provinces at Paleoproterozoic and linked to southwestern AC at ca. 1320 Ma. Aiming to elucidate the paleogeographic relation of the RAT to the AC during the Proterozoic, we performed a paleomagnetic study of the 1110 Ma Rio Perdido mafic dike swarm that crosscut the RAT. These dikes strike preferentially WNW-ESE and are mostly undeformed and unmetamorphosed. Magnetic experiments (hysteresis curves, isothermal remanent magnetization (IRM) curves, thermomagnetic curves and first order reverse curves-FORCs) revealed pseudo-single domain (PSD) Ti-poor titanomagnetite as the main magnetic carrier in these dikes. Alternating field (AF) and thermal demagnetization were efficient in isolating southwestern and northeastern directions with low inclinations, whose mean direction (Dec = 51.8°, Inc = −0.8°, N = 14, α95 = 13.8°, k = 9.3) yielded the paleomagnetic pole (RP) at 34.8°S/197.3° (A95 = 10.9°). This pole passed a baked contact test and meets 6 out of 7 pole reliability criteria (R = 6). The Rio Perdido pole implies that RAT could be linked to the southwestern AC at 1110 Ma but rotated counterclockwise ∼ 110° related to its present position. The AC linked to RAT probably belonged to the 1100 Ma Umkondia megacontinent which later collided with Laurentia during the formation of Rodinia at ca. 1000–900 Ma. Also, paleomagnetic data suggest that the RAT and the southwestern AC (Paraguá Terrane) were linked to Ventuari-Tapajós and Rio Negro-Juruena provinces at ca. 1780 Ma and had a joint geological evolution during the Paleo- and Mesoproterozoic. Finally, during the amalgamation of West Gondwana, the RAT rotated clockwise to its present position.

Comment on ‘The Generation of Eocene Mafic Dike Swarms During the Exhumation of a Core Complex, Biarjmand Area, NE Iran’ by Azizi et al. (2023), Journal of Petrology, 64, 1–18

Abstract Azizi, H., Daneshvar, N., Asahara, Y., Minami, M. & Anma, R. (2023). The generation of Eocene mafic dike swarms during the exhumation of a Core complex, Biarjmand area, NE Iran. Journal of Petrology64, 1–18 have attributed the E–W-oriented mafic dike swarm in the Biarjmand metamorphic core complex to an Eocene extensional event which is much younger than a previously suggested Late Jurassic–Early Cretaceous age. They proposed that the emplacement of these dikes occurred in a rapid extensional regime coeval with the exhumation of the core complex after gravitational instability in the Central Iran/Eurasia collision zone. I appreciate the opportunity this article provides to shed light on specific aspects of the Late Mesozoic–Early Cenozoic continental extension within the Eurasian sector of the Neotethys subduction system. However, I here bring to attention certain discrepancies within Azizi et al.'s publication. Specifically, the assignment of an Eocene age to the emplacement of the E–W-oriented dike swarm, even though purportedly supported by U–Pb zircon dating, appears to be at odds with field observations and previously published geochronological data. Furthermore, the article contains internal contradictions in its presentation of the core complex model for the study area. It is important to note that Malekpour-Alamdari, A., Axen, G. J., Heizler, M. T. & Hassanzadeh, J. (2017). Large-magnitude continental extension in the northeastern Iranian plateau: insight from K-feldspar 40Ar/39Ar thermochronology from the Shotor Kuh-Biarjmand metamorphic core complex. Geosphere13, 1207–1233 previously documented the geochronological-based metamorphic core complex model of the area. Regrettably, despite its direct relevance, these earlier works have not been acknowledged in Azizi et al.'s paper. In this comment, I outline the problems with the structural and regional geology, the zircon U–Pb age of the dike samples, the age of the dike swarm, and the geodynamic interpretations in Azizi et al.'s work.

Petrology of the Cretaceous Riacho do Cordeiro dike swarm: Unraveling the giant low-Ti tholeiitic plumbing system of the Equatorial Atlantic Magmatic Province

The formation of the Early Cretaceous Equatorial Atlantic Magmatic Province (EQUAMP), northeastern Brazil, is related to the fragmentation of the West Gondwana supercontinent that culminated in the separation of South America and Africa, during the Lower Cretaceous. The EQUAMP is characterized by its intrusive nature, where mafic dike swarms intrude the Proterozoic basement of the Borborema Province. The other components of this province are represented by sill complexes hosted between the Paleozoic sediments in the Parnaíba Basin, described as the Sardinha Formation. The magmatism related to the EQUAMP is tholeiitic, with high-Ti, and low-Ti magmas compositions that with similar features that are observed in other Mesozoic igneous provinces of Gondwana. The Riacho do Cordeiro dike swarm (RCo) represents the southernmost manifestation of the EQUAMP, consisting of a series of igneous bodies aligned along the N45E trend, arranged in an en echelon pattern. This structure extends for about 350 km, cutting the Precambrian basement of the southern segment of the Borborema Province. The dikes have a composition similar to tholeiitic basalts and basaltic andesites, but they are distinguished from the other components of the EQUAMP by their exclusively low-Ti content (TiO2 = 1.7–1.2 wt%). Magmatic differentiation is the main petrogenetic mechanism of the low-Ti magmas, typified by the systematic fractionation of major elements with respect to the variation of MgO (6.4–2.4% wt%). Trace (including rare earth) element patterns show enrichment in incompatible elements and negative anomalies in NbTa, a pattern recognized in continental basalts derived from (or contaminated with) lithospheric mantle. The Sr (87Sr/86Sr(i) 0.7072 to 0.7091) and Pb (206Pb/204Pb 18.360 to 18.785) isotopic signatures, combined with negative values for εNd(i) (−4.8 to −6.1) and TDM model ages >1.2 Ga, confirm the involvement of reservoir(s) in the upper mantle possibly modified by ancient subduction processes. The first reported 40Ar/39Ar ages for the low-Ti tholeiites of the RCo indicate ages from 134.53 ± 3.10 to 132.19 ± 1.28 Ma. Geochemically, the low-Ti tholeiites of the RCo are equivalent to low-Ti dikes that constitute part of the Rio Ceará Mirim dike swarm, the main component of the EQUAMP. Besides that, in a scenario external to this province, the same compositional correspondence is found within the Cretaceous dikes of the Vitória-Colatina swarm, southeastern Brazil, and the low-Ti type lavas of the Paraná-Etendeka Province. This compositional and geochronological connection between the tholeiitic magmatism located north and south of the São Francisco craton supports a possible genetic connection between these two Early Cretaceous LIPs and serve as base to future investigations that deal with what would have been the (global) mechanisms that led to the rifting of the Atlantic, from the southern segment to the equatorial margin.

Slab Subduction and Pull Link Magmatism at Active and Passive Continental Margins

AbstractSubduction leads to voluminous arc magmatism at active continental margins; however, the potential far‐field geodynamic connections with coeval magmatism along opposing passive margins, are rarely investigated. Here we report large‐scale and parallel mafic dike swarms on the South Qinling passive margin, whose geochemical features record the thick and cold subcontinental lithospheric mantle beneath passive margin was gradually replaced by thin and hot one during the early Paleozoic. The subduction of the Proto‐Tethys oceanic lithosphere led to arc magmatism along the active margin, shortly after the rift‐related mafic dike swarms occurred along the passive margin. Subsequent to the continuous oceanic subduction including the ridge‐margin collision, mantle upwelling and slab rollback eventually triggered large‐scale rift‐related magmatism and the opening of new ocean basin at passive margin, and also back‐arc magmatism on the active margin. We highlight slab subduction and pull link the geodynamic processes at active and passive continental margins, which improves our understanding of plate tectonics.

Mid-Neoproterozoic tectonic switch from orogeny to intraplate extension in NW Tarim Craton: New evidence from a composite dike of the Aksu Group

It is widely acknowledged that the Tarim Craton plays a crucial role in the supercontinent Rodinia. However, the timing and regime of the Neoproterozoic orogeny, as well as the subsequent tectonic evolution along the northern margin of the Tarim Craton, remain contentious. Recent research has revealed a composite dike in the Aksu blueschist terrane located in the NW Tarim, which sheds new light on the pivotal Neoproterozoic tectonic transition. This dike comprises deformed granites and undeformed diabases, resulting from two-episode igneous events shortly after the orogenic metamorphism. The granites exhibit S-type affinities, with high silica, calcic-alkalic, and peraluminous compositions. Enriched isotopic signatures and diagnostic inherited zircons suggest that they derived from the metasedimentary rocks of the accretionary complex. Conversely, the following diabase dike displays tholeiitic affinity, right-sloping REE patterns, enriched LILEs, and a pronounced depletion of high-field-strength elements. Moderately enriched Sr-Nd isotopes and chondrite-akin Hf isotopes of these diabases imply that they were likely derived from partial melting of the metasomatized subcontinental lithospheric mantle. Zircon U-Pb chronological results reveal that the crystallization of these granites occurred at around 780 Ma. When combined with the maximum depositional age of protolith of the Aksu Group metapsammite obtained by detrital zircon dating, the metamorphic age of the Aksu blueschist terrane is limited to a short duration of 800–780 Ma, which is similar to the syn-orogenic metamorphism and crustal melting in the NE Tarim margin constrained to 830–790 Ma. Structural and petrological analysis suggests that the granites underwent deformation and dynamic recrystallization, while the diabases did not. This likely reflects the post-orogenic processes followed by the extrusion of mafic dike swarm associated with lithospheric stretching. The composite dike highlights a rapid tectonic shift from orogeny to intraplate extension during the mid-Neoproterozoic (780–760 Ma). Widespread late Neoproterozoic intraplate mafic and A-type granitic magmatism occurred in the Tarim and its adjacent blocks (Central Tianshan, Yili, and Kazakhstan) along the southwestern Central Asian Orogenic Belt, indicating that these blocks shared a similar geodynamic evolution related to the breakup of Rodinia.

The Generation of Eocene Mafic Dike Swarms During the Exhumation of a Core Complex, Biarjmand Area, NE Iran

Abstract Several swarms of parallel E–W dikes cuts late Neoproterozoic- to early Paleozoic (540 Ma) basement in the Biarjmand area, NE Iran. The microgabbroic to doleritic dike are several hundred meters long and 0.5 to 3 meters wide. U–Pb dating of their zircon grains yields a crystallization age of 46.0 ± 7.1 Ma for the dike swarms. The dikes have SiO2 contents of 46.0–49.3 wt%, and Fe2O3 contents of 9.80–14.8 wt% with variable MgO (4.92–9.16 wt%), TiO2 (1.47–2.65 wt%), and K2O contents (0.33–1.13 wt%). The dikes have low contents of high field strength elements (HFSEs), and have some similarities to transitional mid-ocean ridge basalts (T-MORB) based on their chemical composition. Positive ${\varepsilon}_{Nd}(t)$ values of +5.2 to +6.0 for the dike samples imply that the basaltic magma was generated from depleted lithospheric mantle. After the closure of the Sabzevar ocean and obduction of the large mantle peridotite body over the continental crust in the late Cretaceous-Paleocene, the gravitational instability in the central Iran/Eurasia plate collision zone triggered exhumation of old basement coeval with injection of the mafic magma. During exhumation, mylonitic deformation with brittle-ductile structures affected the basement metagranite and the host rocks. The metagranite and dike swarms cut the Cretaceous Sabzevar ophiolite, and the disparting of ophiolite members probably occurred during basement exhumation in the Cenozoic. The similar ages of the mafic dike swarms and other Eocene magmatic rocks of the Urumiah (Urmia) Dokhtar magmatic arc (UDMA), western Iran, is consistent with a rapid extensional regime over the Sabzevar suture zone on an earlier collision zone in NE Iran. This process provided a suitable setting for the exhumation of the old basement, the emplacement of the parallel dike swarm, and the development of shallow basins in this area.

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).

A petrological and structural assessment of mafic dike swarms in the Patagonian fold-thrust belt during seismic ridge collision

Between Viedma and San Martín lakes, in an area of 1200 km2 that partly covers the Southern Patagonian cordillera and the Miocene-Pliocene extra-Andean basaltic plateaus, mafic dike swarms crop out which have not been studied previously. As a result of mapping on satellite images and in the field more than 1500 dikes were identified. The dikes crop out in a zone between rear-arc calc-alkaline intrusions to the west and tholeiitic-alkaline plateau basalts to the east, within the internal and external fold-thrust belt. In order to understand the genetic relationship between the dikes, other igneous units in the region and the structures of the Andean orogeny, a petrological study (including petrography and geochemistry) and a structural analysis of the dike orientations were carried out. The dikes were emplaced close to the surface (<1 km). Three predominant compositions were recognized: basalts, which predominate, followed by tephrites and andesites, the latter restricted to a single sector. We propose preliminarily the following correlations: (i) the dikes of tephritic composition with the Río Carbón Essexite (Eocene), (ii) the andesites with the Chaltén Adakite (Miocene) and (iii) the basaltic dikes with the Pliocene basaltic plateaus. All these units are related in the literature to different stages of ridge subduction and slab window development. The dikes present variable, but not random, preferential orientations, with no relation with composition. Our analysis suggests that dike emplacement was controlled by previous anisotropies in a heterogeneous and very shallow crust. The anisotropy in the internal fold-thrust belt was achieved mainly during Eocene and/or Late Cretaceous contraction, whereas that in the external fold-thrust belt was developed during a less significant Miocene compressive event, thus resulting in a more homogeneous dike orientation. The vertical disposition of the dikes indicates horizontal stretching, as previously proposed for the last stages of Andean growth.

IN-SIMS zircon U-Pb geochronology from the Snowbird tectonic zone Large igneous Province (STZ LIP), western Churchill Province, Canada

IN-SIMS (in-situ secondary ionization mass spectrometry) micro-zircon geochronology resolves the age of the Chipman and Kazan mafic dike swarms exposed along the central segment of the Snowbird Tectonic Zone (STZ), the geophysically defined feature separating the Rae and Hearne subprovinces of the western Churchill Province. Long-considered syn-kinematic with respect to Paleoproterozoic transpressional deformation, the primitive composition of the STZ Large Igneous Province (LIP) and granulite grade metamorphic overprint has made establishing crystallization ages difficult. In-situ U-Pb isotopic analysis of micro-zircon yielded scattered and discordant results, except for four zircon crystals, which yielded a weighted mean of 2113 Ma +/- 22 (n = 8 analyses; MSWD: 0.46). This age is similar to other mafic complexes in the western Churchill Province including the Griffin Gabbro sills interpreted to have formed during opening of the Manikewan Ocean. Nd isotopic analyses of Kazan dikes paired with geochemical data suggest that the Chipman and Kazan dikes were asthenospherically-derived but contaminated by a metasomatized lithospheric root. At ca. 2.1 Ga, the STZ was the site of intracratonic extension within a coherent Rae-Hearne crustal block (Snowbird LIP Phase I). Existing data suggests that subsequent tectonism led to focused dextral transpression along the trace of the dike swarm and attests to a long-lived interplay of preexisting weaknesses and deformation.

Geological history and supercontinent cycles of the Arctic

AbstractThe geological history of the Arctic is constrained within the framework of the assembly and breakup of three supercontinents. The first of these was preceded by the crystallization of the oldest dated rocks on Earth and consolidation of the Arctic region’s Archean cratons between 2.82 and 2.54 Ga. Following the emplacement of regional mafic dike swarms between 2.51 and 2.03 Ga, the cratons were amalgamated into the Nuna (Columbia) supercontinent between 2.0 and 1.6 Ga, and the distribution of low-thermalgradient eclogite (indicative of continental subduction) and ophiolite (indicative of obduction of oceanic crust onto a continental margin) suggests that diagnostic plate-tectonic processes were well in place by the early Paleoproterozoic. Basin formation, flood basalts, and dike swarms are features of the partial(?) breakup of Nuna (Columbia) by 1.5–1.27 Ga. The extent to which specific dike swarms led to continental breakup and a rift-to-drift transition remains unclear. Assembly of the second supercontinent (Rodinia, 1.4–0.9 Ga) is recorded by a network of Grenvillian and Sveconorwegian collisional orogenic belts. Prominent features of Rodinia breakup (780–615 Ma) in the Arctic are extensive dike swarms and regional-scale glacial-periglacial deposits associated with the Sturtian (717–661 Ma) and Marinoan (ca. 645 ± 6 to ca. 635 Ma) snowball Earth glaciations. Assembly of the third supercontinent, Pangea, between 600 Ma and ca. 250 Ma, was accomplished through stitching of four orogens in the Arctic (Timan-Varanger, Caledonian, Ellesmerian, and Urals-Taymyr). Pangea breakup (rifting since 250 Ma and oceanic spreading since the Cretaceous) led to the emplacement of Cretaceous and Paleogene flood basalts, new oceanic crust in the Labrador Sea, North Atlantic Ocean, and Arctic Ocean, and orogens characterized by relatively small but far-traveled accreted terranes with provenance in Laurentia, Baltica, and Siberia. Paleogeographic similarities and geological correlations among Laurentia, Baltica, Siberia, and the North China craton suggest that Rodinia formed following incomplete breakup of Nuna (Columbia) and/or by introversion, whereas unique paleogeographic traits for Pangea within the Arctic region point to supercontinent formation by extroversion.

Mesoscopic and Microscopic Magmatic Structures in the Quxu Batholith of the Gangdese Belt, Southern Tibet: Implications for Multiple Hybridization Processes

The Quxu batholith of the Gangdese magmatic belt, southern Tibet, comprises predominantly Early Eocene calc-alkaline granitoids that feature a variety of types of magmatic microgranular enclaves and dikes. Previous studies have demonstrated that magma mixing played a crucial role in the formation of the Quxu batholith. However, the specific processes responsible for this mixing/hybridization have not been identified. The magmatic microgranular enclaves and dikes preserve a record of this magma mixing, and are therefore an excellent source of information about the processes involved. In this study, mesoscopic and microscopic magmatic structures have been investigated, in combination with analyses of mineral textures and chemical compositions. Texturally, most of the enclaves are microporphyritic, with large crystals such as clinopyroxene, hornblende, and plagioclase in a groundmass of hornblende, plagioclase, and biotite. Two types of enclave swarms can be distinguished: polygenic and monogenic swarms. Composite dikes are observed, and represent an intermediate stage between undisturbed mafic dike and dike-like monogenic enclave swarms. Our results reveal three distinct stages of magma mixing in the Quxu batholith, occurring at depth, during ascent and emplacement, and after emplacement, respectively. At depth, thorough and/or partial mixing occurred between mantle-derived mafic and crust-derived felsic magmas to produce hybrid magma. The mafic magma was generated from the primitive mantle, whereas the felsic end-member was produced by partial melting of the preexisting juvenile crust. Many types of enclaves and host granitoids are thus cogenetic, because all are hybrid products produced by the mixing of the two contrasting magmas in different proportions. In the second stage, segregation and differentiation of the hybrid magma led to the formation of the host granitoids as well as various types of magmatic microgranular enclaves. At this stage, mingling and/or local mixing happened during ascent and emplacement. In the final stage, mafic or hybrid magma was injected into early fractures in the crystallizing and cooling pluton to form dikes. Some dikes remained undisturbed, whereas others experienced local mingling and mixing to form composite dikes and eventually disturbed dike-like monogenic enclave swarms. In summary, our study demonstrates the coupling between magmatic texture and composition in an open-system batholith and highlights the potential of magmatic structures for understanding the magma mixing process.

Opening of the Proto-Tethys Ocean: Implications from the late Neoproterozoic mafic dike swarms in the South Qinling Belt, South China

Magmatism in the passive margin records continuum rift of continental lithosphere and onset of seafloor spreading. The ca. 650 Ma ultramafic-mafic dike swarms are the latest magmatic episode in the South Qinling Belt during the Neoproterozoic, and therefore provide a window to address their mantle sources, and the continental rift to drift transition. These mafic dikes have distinct geochemical signatures, and can be grouped as N-MORB-, E-MORB-, and Arc-affinities. The Shiziyan amphibole gabbros have low Rb/Y (<0.75), Nb/Y (<0.15), Nb/Zr (<0.05), Th/Zr (<0.01) ratios, positive εNd (+4.3 to +5.4), low Sri (0.70650–0.70759) and moderate 207Pb/204Pb (15.48) and 206Pb/204Pb (17.57), similar to N-MORB, suggesting that they were derived from an asthenosphere mantle. The Yindongshan pyroxenites display enrichment of LILEs and HFSEs, similar to the E-MORB characters. They also have low Sri (0.70384–0.70410), high εNd (+4.2 to +5.4), and 207Pb/204Pb (15.53–15.54) and 206Pb/204Pb (18.18–18.37). They have high zircon εHf (+13.1 to +15.0), but low δ18O values (+4.43 to +5.17‰). These chemical features indicate an enriched mantle source with involvement of recycled component. The Duchongshan olivine gabbros show arc-like geochemical features, with high Nb/Zr (0.07–0.16), Nb/Yb (3.52–20.1) ratios, and high Sri (0.70545–0.70586), negative εNd (−6.1 to −5.2), large variable zircon εHf (−0.5 to +4.2) and high δ18O (+5.41 to +6.41‰). These features favor that they were derived from a subduction-modified lithosphere mantle followed by significant crustal contamination. The northern Yangtze Block experienced secular crustal extension and developed into a volcanic rift margin during the middle to late Neoproterozoic. Mafic dike swarms emplaced into the volcanic successions and unconformably overlayed by late Ediacaran neritic to abysmal deposition. The magmatic termination and sedimentary facies transition indicate that the northern Yangtze Block broke up during the late Neoproterozoic and the Proto-Tethys Ocean opened at ca. 650 Ma.

Eocene thickening without extra heat in a collisional orogenic belt: A record from Eocene metamorphism in mafic dike swarms within the Tethyan Himalaya, southern Tibet

Abstract Knowledge of the nature of the earliest metamorphism experienced by collisional orogenic belts is essential for reconstruction of tectonic processes that build high mountain chains and their environmental consequences. Understanding the metamorphic nature of Eohimalayan-phase orogeny of the Himalayan orogen, one of the typical examples of orogenic belts worldwide, could provide some important constraints to test different tectonic models (shallow continental subduction vs. slab breakoff) for the early phases of the development of large-scale orogenic belts. As exhumed middle- to lower-crustal rocks in the Kangmar gneiss dome, the garnet amphibolites with a protolith age of 176.4 ± 3.6 Ma experienced a phase of metamorphism at 47.2 ± 1.8 Ma with an increase in pressure as well as temperature from 3–5 kbar and 550–600 °C to over ~11 kbar and 650 °C. This suggests that the middle- to lower-crustal rocks experienced heating at least by ~50 °C while they underwent compression and thickening. Heat-flow estimation further demonstrates that the self-produced heat was high enough to achieve the observed pressure-temperature conditions recorded by the garnet amphibolite. Therefore, an additional heat supply is not required during early Eocene metamorphism. A breakoff of the leading part of the subducting Indian continental slab, if it occurred, should be younger than ca. 47 Ma.

Geochemistry and paleogeographic implications of Permo-Triassic metasedimentary cover from the Tauern Window (Eastern Alps)

Abstract. The chemical composition of metasediments is a valuable source of paleogeographic information about the protolith's sedimentary environment. Here, we compile major- and trace-element whole-rock data, including B contents, and 10/11B-isotope ratios from the Permo-Triassic metasedimentary cover of the Pfitsch–Mörchner basin, overlying the Variscan basement in the western Tauern Window, Eastern Alps (Austria and Italy). The basement consists of orthogneiss (“Zentralgneis”, metamorphosed Variscan granitoids with intrusion ages between 305 and 280 Ma), and the roof pendant consists of granites (amphibolites, paragneiss, and minor serpentinites). The Zentralgneis is partly hydrothermally altered into pyrite quartzite with high Al–S contents, low Na–Sr–Ca–Mg contents, and very strong depletion of the light rare earth elements. Comparison with published detailed mapping of this and other time-equivalent basins in the western Tauern Window, with radiometric age data in the literature, and with unmetamorphosed basins in the South Alpine realm yields a late Permian to Early Triassic age of sedimentation. Although during Alpine metamorphism all rocks were strongly deformed, the whole-rock chemical compositions of the metasediments were not pervasively changed during deformation. We show that the sediments were deposited in a small, probably lacustrine–fluviatile, intramontane basin, under arid to semi-arid climatic conditions. The sequence starts with metaconglomerates, which can be interpreted as a mixture of the different basement rocks, based on a combination of major-element ratios Na2O / (Na2O + K2O) and MgO / (MgO + Fe2O3) with concentrations of trace elements Cr, V, and Ni. The sequence is overlain by a fining-upwards sequence of clastic sediments, in which the behavior of K, Rb, and Sr allows the reconstruction of intense diagenetic K–B metasomatism, which raised the K2O contents up to ∼ 10 wt %. The average B content of 218 µg g−1 is well above the B content of common sediments, and the B-isotope composition reaches extremely low values of down to −33 ‰ δ11B. The top of the sequence is a lazulite quartzite, interpreted as a former conglomeratic phosphatic sandstone, which marks the transition from a closed Permian basin to an open Triassic basin. Within the clastic sequence, the presence of hydrothermal tourmalinite veins documents a hydrothermal event after deposition but before the onset of Alpine metamorphism. A metamorphosed mafic dike swarm in the orthogneiss indicates a post-Variscan event of basaltic magmatism, and this event is tentatively correlated with increased heat flow in the Triassic basin and hydrothermal activity. A consistent conceptual model of this basin and its diagenetic modifications, based on a combination of geochemical data with petrographical and field information, provides the geodynamic context of the European margin at the onset of the Alpine orogeny.

Middle Paleozoic archipelago amalgamation and tectonic transform in the northern West Junggar, NW China: Constraints from magmatism and deformation

The West Junggar (WJ) is generally accepted as a Paleozoic archipelago-type orogenic system and records a multiple arcs amalgamation history; however, some of the associated processes require further examination. In this paper, we conducted detailed field investigations and large-scale mapping in the Saier Mountains, northern WJ, and identified several episodes of magmatism and deformation events. The felsic rocks (dioritic porphyrite and granitic porphyry), characterized by a dense cleavage, were emplaced at the Early–Middle Silurian (433–423 Ma). These rocks exhibit geochemical characteristics comparable to those of the coeval adakite-like rocks and A2-type granites in the Xiemisitai Mountains. The undeformed mafic dike swarms yielded SHRIMP zircon age of 382 ± 6 Ma and intruded the Hebukesaier ophiolitic mélange (HOM) as well as the surrounding rocks. The HOM, Silurian felsic intrusions and pyroclastic rocks (422 ± 9 Ma) in the Saier Mountains commonly display cleavage attributed north–south shortening during 422–382 Ma. Deep-sea bedded cherts and turbidites in the area also experienced deformation and exhibited cleavage, with subduction-related fold deformation partially preserved in the HOM. The fold deformation was greatly related to a subduction-accretion process of the Hongguleleng–Balkybey Ocean during the Ordovician. Considering the geochronology and geochemistry data, the extensive Silurian–Early Devonian magmatism in the northern WJ indicates a prolonged (~40 Ma) and episodic post-collisional extension in this orogen and records an obvious trend of crustal temperature increasing and thickness thinning from 435 to 410 Ma. The regional extrusion deformation was attributed to the southward subduction of the Irtysh–Zaysan Ocean during the Middle Devonian, and then the slab roll-back offered an extensional setting which favored the emplacement of mantle-derived mafic dike swarms. We propose that the northern WJ probably experienced a complicated evolution involving archipelago amalgamation and tectonic transform in the middle Paleozoic, providing a new sight to reveal the orogenic process of the Central Asian Orogenic Belt (CAOB).

Geophysical insights into Paleoproterozoic tectonics along the southern margin of the Superior Province, central Upper Peninsula, Michigan, USA

The southern margin of the Archean Superior Province in the central Upper Peninsula of Michigan was a nexus for key Paleoproterozoic tectonic events involved in the ~2.1 Ga rifting of proposed Archean supercraton Superia and subsequent assembly of Laurentia. Interpretations of the region’s tectonic history have historically been hampered by extensive Pleistocene glacial and Paleozoic sedimentary cover and a lack of appropriate geophysical data. These rifting and orogenic events formed geologic effects that are readily mappable with modern geophysical methods. New aeromagnetic and gravity data provide a critical means of mapping and interpreting the complex geological framework through cover, allowing development of significantly richer geographical and process-based perspectives on all these tectonic events. Interpretations of Precambrian contacts and structure are here, for the first time, carried >30 km eastward under Paleozoic cover. Effects of ~2.1 Ga rifting are strongly expressed geophysically, including the Dickinson Group, perhaps a unique record of the progression of rift-related sedimentation and magmatism, shown here to be a geographically extensive and largely concealed tectonic feature of the southern Superior Province. The geophysical evidence for plausible ~2.1 Ga rift-related intrusive magmatism includes a previously unrecognized swarm of northeast-striking mafic dikes cutting Archean rocks and gravity lows produced by granites. Effects of the ~1.87–1.83 Ga Penokean orogeny include gravity and magnetic gradients and pattern breaks along the Niagara fault zone suture, abundant evidence for thin-skinned thrusting and folding in the Menominee iron district, and speculative emplacement of an allochthonous sedimentary sequence in the Calumet trough. Numerous east–west trending structures imaged geophysically likely originated, or were significantly reactivated by, post-Penokean deformation. Metamorphic events at ~1.76 Ga and ~1.65 Ga may correspond to orogenies involving younger, outboard Paleoproterozoic crustal provinces recognized in southern Laurentia. For example, the previously unrecognized West Branch fault, separating the Dickinson Group from Archean rocks, is shown to be a major structure in the region, and is a proposed expression of ~1.76 Ga thick-skinned deformation. Oblique disruptions of crudely east–west striking structures have robust geophysical expressions and are speculatively connected to transpressive deformation at ~1.65 Ga. These new geophysical observations and interpretations collectively help illuminate a critical period in the tectonic evolution of Laurentia, as it transitioned from a disparate array of Archean cratons to a more coherent, growing continent.

Updated map of the mafic dike swarms of Brazil based on airborne geophysical data

Identification of mafic dike swarms and LIPs (Large Igneous Provinces) are of vital importance in geologic history because they provide information on geodynamics, mantle geochemistry, and paleomagnetism. These data provide key information for paleogeographic reconstructions with the aid of barcode matches and precise radiometric ages. Considering such issues, the Brazilian Precambrian shield can be used as a case for refining the cartography of the relevant intraplate activity (e.g., dikes, sills, flood basalts) in space and time.This work presents an updated map of Brazilian mafic dike swarms produced from airborne geophysical maps (Series 1000 – Geological Survey of Brazil). Linear and strong anomalies found on aeromagnetic maps using First Vertical Derivative of the Magnetic Field and Amplitude of the Analytic Signal were mapped on a GIS platform. The obtained data were compared to ternary radiometric maps and geological maps in order to exclude those that do not correspond to mafic dikes. The remaining structures - those believed to represent mafic dikes - were classified based on data compiled from the literature.The updated map exhibits more than 5000 elements, including dikes and magmatic suites, in which about 75% were geologically identified and divided into 60 dike swarms and 10 igneous suites and/or units. The dikes were grouped into sixteen extensional episodes from the Archean to the Cenozoic, although some are related to extension/transtension domains within regional compressive zones akin to orogenic settings. The most frequent records refer to the Proterozoic, representing intraplate episodes, some of them consistent with LIPs. The dataset also includes a large record of the Mesozoic age, which corresponds to major LIP events related to the opening of the Atlantic Ocean and the fragmentation of Gondwana.

Multiple mantle melting events for two overlapping ca. 2.21-2.18 Ga mafic dyke swarms in the Dharwar craton, India

ABSTRACT Two distinct NW- to WNW-trending mafic dike swarms of the previously dated ca. 2.21 Ga Anantapur-Kunigal and ca. 2.18 Ga Mahbubnagar-Dandeli swarms, emplaced in the Dharwar craton, are studied in greater petrological detail. Petrography classifies these as dolerite, olivine-dolerite, and meta-dolerite. The bulk rock geochemistry of 39 new and 11 published samples of both dike swarms are compared to those of three ca. 2.21 Ga and two ca. 2.18 Ga samples previously dated through baddeleyite U-Pb geochronology. All 50 samples are sub-alkaline tholeiite basalt to basaltic andesite in compositions and both swarms have similar differentiation/crystallization trends. Nonetheless, based on the slightly enriched REE patterns of ca. 2.21 Ga samples, as opposed to practically flat ca. 2.18 Ga REE patterns; we propose that 41 samples constitute the ca. 2.21 Ga Anantapur-Kunigal dike swarm and 9 samples belong to the ca. 2.18 Ga Mahbubnagar-Dandeli swarm. Petrogenetic modelling suggests that each swarm was derived through different degrees and proportions of partial melting of both an ambient asthenospheric and metasomatized sub-continental lithospheric mantle (SCLM) and further differentiated through similar fractional crystallization processes. Both mantle melts segregated within the garnet to garnet-spinel transition zone. Geodynamic conditions suggest that dikes of both swarms were emplaced in a similar tectonic system within a single LIP, which was likely triggered through the heating by the same (pulsating?) mantle plume. Such an indirect influence from a mantle plume, not recorded in geochemical signatures, is supported by ~1576°C potential mantle temperature (Tp) estimates for more magnesian parents within both swarms. Available geochronological data on mafic dikes and Palaeopositions of the Dharwar and North American cratons during ca. 2.21–2.18 Ga are well supportive of their link to the Superia supercraton reconstruction.