Dyke Swarm Research Articles

Geochemistry of Deccan Tholeiite Flows and Dykes of Elephanta Island: Insights into the Stratigraphy and Structure of the Panvel Flexure Zone, Western Indian Rifted Margin

Elephanta Island near Mumbai is an important area for understanding the stratigraphic and structural framework of the Deccan flood basalt province in the tectonically disturbed Panvel flexure zone on the western Indian rifted margin. Elephanta exposes a west-dipping, 66–65 Ma sequence of tholeiitic lava flows and dykes. Geochemical correlations with the thick, horizontal, 66–65 Ma Western Ghats sequence to the east show that lava flows of the Khandala and Ambenali formations are present at Elephanta, with two lava flows probably being locally derived. The Elephanta tholeiites have experienced crystal fractionation and accumulation, particularly of olivine. They have εNd(t) ranging from +5.4 to −7.9 and (87Sr/86Sr)t from 0.70391 to 0.70784, with most tholeiites little contaminated by continental lithosphere, probably lower crust. Field and geochemical data indicate a normal fault along the central part of Elephanta with a 220 m downthrow, consistent with a domino-type block-faulted structure of Elephanta, and the surrounding area as previously known. Seventeen of the 20 analyzed Elephanta intrusions, striking ~N–S, belong to the Coastal dyke swarm of the western Deccan province. Several of these are probable feeders to the Ambenali Formation in the Western Ghats sequence, requiring reconsideration of the current view that the voluminous Wai Subgroup lavas of the Western Ghats were erupted without organized crustal extension. East–west-directed extensional strain was already active at 66–65 Ma along this future (62.5 Ma) rifted continental margin. A young (~62 Ma) ankaramite dyke on Elephanta Island is a probable feeder to the Powai ankaramite flow in the 62.5 Ma Mumbai sequence 20 km to the northwest.

40Ar/39Ar and U-Pb SIMS zircon ages of Ediacaran dikes from the Arabian-Nubian Shield of south Jordan

A spectacular feature of the Arabian-Nubian Shield (ANS) is the abundance of well-exposed and extensive Neoproterozoic dike swarms of variable compositions. Most of these dikes are late to post-orogenic with respect to the East African Orogen (EAO) and are unmetamorphosed. We dated a composite dike with latite margins and a rhyolite core (607 ± 6 Ma, U-Pb), a biotite rhyolite dike (600 ± 4 Ma, 40Ar/39Ar age of biotite), an andesite dike (594 ± 3, 40Ar/39Ar age of amphibole) and a dolerite dike (~579 Ma, 40Ar/39Ar whole rock total gas age). We propose that the first three dikes represent one generation that was emplaced at different episodes extending between 607 and 590 Ma. Time and composition equivalent dikes are common in the northern ANS. The dikes crosscut late collisional granitoids and geochemically display a subduction-related character as evidenced by a negative Nb-Ta anomaly. These dikes are absent in the alkali feldspar A-type Humrat Syenogranite dated at 586 ± 5 Ma in Jordan and equivalent rocks in the northern ANS, which are crosscut only by the (~579 Ma) dolerite dikes. The within-plate character of the dolerite dikes is supported by the absence of the Nb-Ta anomaly and high field strength element geochemistry. We propose that the dolerite dikes are a generation, distinct from the ~607–590 Ma dikes, that reflects a change in tectonic regime and represents the last magmatic activity of the Neoproterozoic in the northern ANS. The ages of the dikes dated in this study agree with the published age range of the transitional stage from late orogenic calc-alkaline to extensional alkaline tectono-magmatic setting for the ANS. We propose that the magmatic activity was terminated ~50 m.y. before the age of the Cambrian unconformity at ~530 Ma. Correlation with ages of dikes and magmatic rocks in the northern ANS favors this supposition. The dike geochemistry and geochronology are compatible with a tectonic model that involves mantle lithosphere delamination from below the northern ANS after a significant crust-mantle thickening caused by the EAO, followed by thermal relaxation, subsidence and gradual denudation until the age of the unconformity at ~530 Ma.

Genesis of the Zaozigou gold deposit, West Qinling orogen, China: Constraints from sulfide trace element and stable isotope geochemistry

The West Qinling orogen, central China is endowed with dozens of world-class sediment-hosted gold deposits, most of which have formed from metamorphic fluids released from Paleozoic sedimentary rocks during the Triassic collisional orogeny. Here we present trace element and sulfur isotope data of gold-bearing pyrite and arsenopyrite, along with oxygen and hydrogen isotopes of ore-related sericite, to show that the Early Triassic Zaozigou gold deposit (142 t) in the Xiahe-Hezuo district may have contrasting origin. Gold mineralization at Zaozigou is dominated by fine-grained pyrite and arsenopyrite disseminations (Py1 and Asp1) in hydrothermally altered Triassic flysch and, less significantly, in Early Triassic granitoid dike swarm. The disseminated ores are overprinted by quartz-sulfide-ankerite-sulfide vein/veinlets, which contain less abundant, coarser pyrite and arsenopyrite (Py2 and Asp2) and are less economically significant. Gold occurs as invisible gold in fine-grained arsenian pyrite and arsenopyrite of the disseminated ores, with subordinate native gold grains locally observed in the quartz-sulfide-ankerite vein/veinlets. The two types of pyrite (Py1 vs. Py2) and arsenopyrite (Asp1 vs. Asp2) are consistently enriched in As, Au, Co, Ni, Mn, Cu, Zn, Ag, Sb, W, Pb, Te, and Bi and have well comparable δ34S values ranging from −12.0‰ to −5.5‰. Grain-scale variation has been observed for Au and other trace elements both in Py1 and Py2, likely reflecting changes in physio-chemical conditions such as fO2, fS2 and temperature due to fluid-rock interactions and/or fluid mixing. The δ34S values are broadly consistent with sulfur isotopic compositions of many Early Triassic ilmenite-series intrusions in the Xiahe-Hezuo district, and thus suggest a magmatic sulfur derivation from the reduced magmas that have assimilated carbonaceous sedimentary rocks. The δD and δ18O values of ore-fluids calculated from isotopic compositions of ore-related sericite range from −56 to −76‰ and 10.1 to 12.4‰, respectively, indicating a magmatic fluids exsolved from the above-mentioned ilmenite-series magma or a mixture of such as fluid with a metamorphic derived component. Synthesis of geochronological and geochemical data throughout the Xiahe-Hezuo district led us to conclude that the Zaozigou gold deposits can be considered as a shallow-seated, distal product of a reduced intrusion-related gold system.

Geology and petrology of the mafic dike swarms of the Araguaia Belt: Evidence for distinct events of intracontinental magmatism in central-northern Brazil and correlations with large igneous provinces

Extensive N–S and NNW-SEE-trending subparallel mafic dike swarms are intruded into metasedimentary rocks of the Tocantins Group, Araguaia Belt, Central-North of Brazil. They are under-examined, and there is little to no information about their origin and mantellic sources, as well as uncertainty about whether they are part of two different episodes of magmatism. Representative mafic dikes from the Santa Maria das Barreiras-Conceição do Araguaia region, at the boundary between the states of Pará and Tocantins, were studied in order to address these problems. It was possible to separate the dikes into two groups: one consisting of metadiabases affected by the Neoproterozoic regional metamorphism of the Araguaia Belt with varying degrees of transformations and mineral deformation; and the other consisting of unmetamorphosed and undeformed diabases and leucodiabases. The studied dikes are compositionally classified as subalkaline basalts with tholeiitic affinity and were interpreted to be originated in an intracontinental setting with the aid of Ti–V, Zr–Zr/Y, and Zr–Ti discrimination diagrams. However, metadiabases present an arc-like geochemical signature characterized by a pronounced Nb–Ta negative anomaly, whereas leucodiabases and diabases lack a negative Nb–Ta anomaly and show a LREE-enriched pattern, which resembles the signatures of plume-generated basaltic rocks. Also, there is evidence of an important contribution of enriched (EN) mantle components in the source of metadiabases and significant contribution of primitive mantle (PM) in the source of both leucodiabases and diabases. It was suggested, then, that the metadiabases represent the exposed plumbing system of arc-like intracontinental basalts which precede the regional Neoproterozoic metamorphism of the belt, and that the leucodiabases and diabases represent the exposed conduits of intracontinental basalts whose magmatism succeed the metamorphic event. Geochemical models were elaborated to demonstrate that fractional crystallization processes could account for the differentiation of the metadiabases and for the genesis of the leucodiabases via fractionation of the melt from which the diabases formed. The rocks from the earlier event share several similarities with Neoproterozoic mafic rocks from the eastern domain of the Araguaia Belt and nearby Tonian rocks of the 1100 Ma Rincón del Tigre-Huanchaca LIP event, while the rocks from the later event are remarkably similar to nearby CAMP basalts and diabase dikes.

Applicability of AMS technique as a flow fabric indicator in dykes: insight from Nandurbar–Dhule Deccan dyke swarm

Determination of the nature of magma flow (vertical or lateral) from rock fabrics in a dyke swarm holds significant importance in deciphering its emplacement mechanism, depth of magma chamber and its association with mantle plumes. Nandurbar–Dhule dyke swarm from Western India consists of ~ 210 mappable, tholeiitic, large, linear dykes intruding the country rocks of Deccan basalt. We discuss here the development of flow-controlled rock fabrics in these dykes as a function of their thicknesses. Magnetic fabric obtained from anisotropy of magnetic susceptibility (AMS) analysis of the samples collected from these dykes was used as a proxy for rock (flow) fabric. Samples from the central parts of the thicker dykes (strike perpendicular thickness > 10 m) show random and inconsistent development of magnetic fabrics than their marginal counterparts. Petrography suggests the development of relatively clear shape-preferred orientations of the elongated mineral grains only at the margin of a thick dyke. Thinner dykes (strike perpendicular thickness < 10 m) show both consistent magnetic fabrics and clear shape-preferred orientations. We propose that the center of a thick dyke fails to provide (due to high flow velocity, slower cooling rate, lack of friction-induced shearing, and draining back of magma) a favorable condition where (a) flow fabric can efficiently develop and (b) magnetic fabric can effectively mimic the flow fabric. Thinner dykes and the margins of the thicker dykes are, hence, better candidates for observing flow fabrics represented by magnetic fabrics. This work is also motivated by the reservations expressed by earlier researchers in using the AMS technique for magma flow characterization, especially for giant dyke swarms. We have critically evaluated their reservations in light of our results and commented on the applicability of the AMS technique on such studies.

Early Cretaceous Wulong intermediate-mafic dike swarms in the Liaodong Peninsula: Implications for rapid lithospheric delamination of the North China Craton

The North China Craton (NCC) underwent a dramatic change in the lithospheric architecture, and more than 100 km of the ancient and refractory lithospheric mantle was removed. However, the timing and mechanism of the lithospheric thinning remain controversial. New zircon UPb ages and whole-rock/in-situ geochemical and isotopic data of the Early Cretaceous intermediate-mafic dike swarms in the Liaodong Peninsula have been carried out to decipher the timing and mechanism of lithospheric thinning of the NCC. The dike swarms are chemically subdivided into the ca. 126 Ma diorite porphyry and coeval (ca. 119 Ma) low-Ti (TiO2 < 1.20 wt%, Ti/Y < 375) and high-Ti (TiO2 > 1.90 wt%, Ti/Y > 580) diabase dikes. The diorite porphyries exhibit a moderate silica (56.71–61.13 wt%) and high-K calc-alkalic signature, with low TiO2 and total Fe2O3 and high Cr and Ni compositions. They are enriched in large ion lithophile elements (LILEs) (e.g., Rb, K, Pb, and Sr) and depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, Ti, and P). They have high radiogenic Sr isotopes (whole-rock (87Sr/86Sr)i = 0.711553–0.714284, in-situ (87Sr/86Sr)i of plagioclase = 0.71392), low εNd(t) values (−19.9 to −14.5), and old Nd model ages (1.81–2.25 Ga). Meanwhile, some plagioclase phenocrysts have reverse zonings with Na-rich cores (An: 35–48) and Ca-rich rims (An: 48–57). These features imply that the diorite porphyries were generated by mixing between magmas from the Archean–Paleoproterozoic lower crust and the lithospheric mantle. The low-Ti diabase dikes are characterized by medium to high-K calc-alkaline series with high Al2O3 but low TiO2 contents. They show arc-like trace element signatures, such as enrichment in LILEs (Rb, K, Pb, and Sr) and depletion in HFSEs (Nb, Ta, Zr, Hf, Ti, and P). They also have relatively lower initial 87Sr/86Sr ratios (0.706331–0.708315 and 0.70837 for the whole-rock and in-situ Sr isotopes, respectively) compared to the porphyries and negative εNd(t) values (−13.6 to −5.0), indicating that they were derived from partial melting of an enriched lithospheric mantle. Their variable Th/Yb and Th/Nb ratios with constant Sr/Nd and U/Th values further suggest that the mantle source might have been previously modified by the subduction-related melt. In contrast, the high-Ti diabase dikes belong to the high-K calc-alkaline to shoshonitic series, with higher TiO2 and total Fe2O3 but lower Al2O3 contents. They have OIB-like trace elements (e.g., slight enrichments of Nb and Ta with no HFSE depletion, high Nb/U values (42 ± 5)) and SrNd isotopic compositions (e.g., positive whole-rock εNd(t) values (+3.1 to +4.3)), suggesting that they were generated from an asthenospheric mantle source. The coexisting Wulong low-Ti and high-Ti mafic dikes with similar formation ages (ca. 119 Ma) may indicate a transition from lithospheric to asthenospheric mantle source. In combination with previous studies on the Mesozoic–Cenozoic mafic rocks in the Eastern Block of the NCC, we propose that the lithospheric thinning beneath the Liaodong Peninsula initiated at ca. 119 Ma and this process beneath the NCC lasted for a narrow duration (ca. 13 Ma). The unsteady mantle flows due to the rapid subduction and rollback of the Paleo-Pacific plate might trigger the thickened, modified lithosphere to lose its gravitational stability, eventually leading to lithospheric delamination and thinning of the NCC.

Emplacement of dike swarms from the island of Ilhabela (SE Brazil) and its relationship with the South Atlantic Ocean opening revealed by magnetic fabrics

Magnetic fabric and rock-magnetism studies were performed on 153 dikes from diabase, lamprophyre and alkaline dike swarms that outcrop on the island of Ilhabela, NE São Paulo State. The dikes crosscut Archean and Proterozoic polymetamorphic rocks. Their thicknesses range from a few centimeters to 4 or 5 m. They trend predominantly N30-60E with a very steep (~60°) to vertical dip. Magnetic fabrics were determined using anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). Rock-magnetism measurements reveal that the magnetic mineral of the three swarms is magnetite mainly with grains in the range of 2–5 μm. For a few dikes, these grains are the carriers of bulk magnetic susceptibility but are not responsible for the AMS, which is due to Fe-bearing minerals, as shown by AARM. The main AMS fabric recognized in the swarms is due to magma flow, in which the Kmax–Kint plane is parallel to the dike's plane, and the magnetic foliation pole (Kmin) is perpendicular to it. The analysis of the Kmax inclination showed that the dikes were fed by horizontal to vertical flows. However, for the minority of the dikes, the AMS and AARM tensors are not coaxial. The AARMmax is oriented N30–60 W, approximately perpendicular to the Kmax. The AARMmax orientation is similar to the direction of a fault system located mainly in the Santos marginal basin which, was formed in the Cretaceous rifting during the South Atlantic opening. The AARM fabric is tectonic in origin, and is parallel with the extensional paleostress responsible for the Gondwana break-up. The comparison of AMS and AARM fabrics suggests that diabase and lamprophyre dikes were emplaced in three distinct events in the earliest stages of the South Atlantic opening. Later, the alkaline dikes were emplaced in the final stages of the Atlantic opening.

Geology, mineralization, and fluid inclusion characteristics of the Agylki reduced tungsten (W-Cu-Au-Bi) skarn deposit, Verkhoyansk fold-and-thrust belt, Eastern Siberia: Tungsten deposit in a gold-dominant metallogenic province

The Agylki deposit is a large (>100 Kt WO3, possibly underexplored) and high-grade (average 1.46% WO3, 2.79% Cu, 0.5 g/t Au, and 0.03–0.04% Bi) reduced W skarn deposit situated in the Verkhoyansk fold-and-thrust belt, the latter formed after a passive continental margin, part of the Verkhoyansk-Kolyma gold-dominant metallogenic province. The belt comprises intrusion-related Au mineralization and roughly parallels the other main metallogenic belts of the region hosting numerous gold (Au, Au-Bi, Au-Sb, etc.) and W-Sn deposits. The deposit could be part of a broader regional-scale magmatic-hydrothermal system comprising nearly coeval intrusion-related reduced W skarn deposits and vein/stockwork Au(±W) deposits. The deposit is related to a swarm of Late Jurassic-Early Cretaceous monzodiorite-porphyry, granodiorite- and granite-porphyry dikes that intrude a Triassic terrigenous sequence. The dikes comprise weakly reduced-weakly oxidized (ilmenite-titanite-bearing), high- to medium-K calc-alkaline, metaluminous to weakly peraluminous, I-type granitoid rocks.The deposit is represented by a single lenticular flat-lying to shallow-dipping mineralized zone that was formed by the replacement of a limestone horizon. This may correspond to a distal level in an integrated structural model of W skarn deposits that occurs well above causative pluton. The deposit bears signatures of reduced W skarn deposits, with dominant pyroxene skarn containing pyrrhotite and scheelite. Propylitic alteration assemblages (amphibole-epidote-chlorite-calcite-quartz with abundant pyrrhotite, scheelite and minor chalcopyrite) overprint skarn and are in turn overprinted by zones of phyllic alteration (quartz-sericite-Fe-carbonate) comprising abundant apatite and arsenopyrite followed by scheelite, chalcopyrite and other sulfides as well as Bi, Te and Au minerals.Fluid inclusions data indicate formation of retrograde (mainly pyroxene-quartz-amphibole) skarn from aqueous, dominantly magnesian-sodic-chloride, moderate-salinity (14.0–17.3 wt% NaCl-eq.), high pressure (1.4 ± 0.2 kbars), moderately-hot (~400–500 °C) fluid. Propylitic alteration assemblages were formed from homogenous, low-carbonic, lower salinity (8.8–13.7 wt% NaCl-eq.), lower pressure (1.1–0.8 kbars) and lower temperature (~300–400 °C) Ca-enriched fluid. Early phyllic alteration assemblages were formed from a boiling fluid, with separation of carbonic-rich (particularly methane-enriched) gaseous fluid and coexisting low-carbonic aqueous fluid at ~320–280 °C and ~0.9 ± 0.1 kbar. Late phyllic alteration assemblages were formed from homogenous, low-carbonic, moderate-salinity (14.0–17.8 wt% NaCl-eq.) low-temperature (250–200 °C) fluid.

Geometry and field relations disclose the emplacement dynamics of the SW Sinai Dyke Swarms (Egypt)

The first results on the structural attributes and emplacement mechanics of mafic and felsic dyke swarms exposed in the Precambrian basement in SW Sinai (Egypt) are presented. These results are based on data analysis of remote sensing, dyke geometries and relationships, and trace element chemistry available from literature. A total of 2376 dyke segments (2056 mafic and 320 felsic) formed parallel (N25°E-oriented mafic and N35°E-oriented felsic) and N35°W-oriented radial dyke swarm (eastern and western) sets. The dykes have a variety of attributes (length, thickness, and aspect ratio), inferred magma chamber overpressures, and depths, indicating different sources of origin. The average magma overpressures are estimated at 14, 29, 14, and 13 MPa, at depths of 20, 23, 17, and 5 km, for the parallel mafic swarm, parallel felsic swarm, the eastern, and western occurrences of the radial dyke swarms, respectively. The mafic dykes were fed by deep-seated basaltic magma reservoirs or chambers located at the crust-mantle boundary or in the lower crust, close to the Moho (20–25 km) and the Conrad (15–18 km) discontinuity in the area. The felsic dykes propagated from shallow-level, 5 km deep, rhyolitic chamber(s) in the upper crust.Three extensional episodes of dyke emplacement were recognized. The first episode is marked by a regional maximum tensile stress field (σ3) which is responsible for the formation of the parallel mafic swarm that strikes N25°E. Felsic dykes oriented N35°E were affected by the same stress field oriented N55°W flipped 10° clockwise from the preceding field emplaced coevally or shortly after the dykes of the first episode. Later, N35°W-trending mafic dykes were radially emplaced affected by local stress field evolved from volcanic edifices developed from two successive phases of local minimal stress fields σ3.The parallel set of dykes show calc-alkaline affinities with negative NbTa anomalies; typical of basaltic and rhyolitic magma for the mafic and felsic dykes. These were emplaced at active continental margins in a subduction-related setting mostly parallel to the plate boundaries at the time of their emplacement. The radial swarms have a transitional to mildly alkaline chemistry, without negative NbTa anomalies consistent with a continental tectonic setting of the source region. Insights concluded enabled comparisons to worldwide dyke swarms.

Co-existing low-Ti and high-Ti dolerites in two large dykes in the Gap Dyke swarm, southeastern Karoo Basin (South Africa)

AbstractThis paper presents major, trace element and Sr-Nd isotope data on two large, east-west trending dolerite dykes in the southeastern part of the Karoo Basin, the South Gap (155 km long, ≤275 m wide) and the North Gap (150 km long, ≤190 m wide) dykes. The Gap dykes represent a rare case in the Karoo Large Igneous Province (LIP) where low-Ti (Gap1: &amp;lt;1.7 wt% TiO2, &amp;lt;130 ppm Zr, 200 to 330 ppm Sr, 6 to 17 ppm La, 87Sr/86Sr183: 0.7045 to 0.7075, ɛNd183: +0.31 to -7.5, ΔNb: +0.4 to -0.2) and high-Ti rocks (Gap2: 2.4 to 3.1 wt% TiO2, 110 to 240 ppm Zr, 260 to 390 ppm Sr, 12 to 24 ppm La, 87Sr/86Sr183: 0.7066 to 0.7074, ɛNd183: -3.2 to -4.1, ΔNb: +0.2 to -0.4) alternate along the same dykes. The aim of the study is to unravel the origin and petrogenesis of the two rock groups. The sample groups show the chemical traces of fractional crystallization in increasing concentrations of incompatible trace elements with decreasing MgO, and pivot points where elements such as Ti, Ca, P, and Sc change from trends of increasing to decreasing concentrations with decreasing MgO. Trends of increasing 87Sr/86Sr183 with decreasing ɛNd183 ratios indicate crustal contamination. However, three Gap1 samples from the South Gap dyke with the least enriched Sr-Nd isotope ratios and highest ΔNb may be derived from uncontaminated plume-type mantle melts. Modelling of Sr-Nd isotopes indicate that the Gap1 samples were subjected to assimilation of crustal melts (≤8%) and fractional crystallization in deep crustal magma chambers. There are no restrictions on the mantle source for the Gap2 rocks. However, because both geochemical signatures occur at different localities along the two Gap dykes, we favor a SA (South African) type mantle source for the two groups. The Gap2 rocks appear to have a two-stage assimilation history with one stage in the SCLM (subcontinental lithospheric mantle) or deep crust (decreasing the ɛNd183, increasing the 87Sr/86Sr183 ratios, and inducing the high-Ti character), followed by a second stage of contamination and fractional crystallization in the upper crust (increasing Sr isotope and (Sm/Yb)N ratios, with only minor decrease in ɛNd183).

Rifting of the Kaapvaal Craton during the early Paleoproterozoic: Evidence from magmatism in the western Transvaal subbasin (South Africa)

The Neoarchean to Paleoproterozoic (ca. 2.65–2.06 Ga) Transvaal Supergroup of the Kaapvaal Craton is dominated by clastic and chemical sedimentary rocks, with few strata-bound igneous units. Two ca. 2.2 Ga igneous units in its upper part (Pretoria Group), the Machadodorp Member of the Silverton Formation and the Mashishing dyke swarm, have so far only been recognized in the eastern part of the Transvaal subbasin. As these units are near-contemporaneous with an early Paleoproterozoic geochemical excursion to higher Th/Nb ratios compared to both older and younger igneous units, they probably mark a change in the geodynamic evolution of the craton. In the present contribution, we provide new data on the igneous units within the Silverton Formation (one lava and 5 sills) from a borehole (BB11) in the south-western part of the Transvaal subbasin. Two of these igneous units are particularly interesting because they show strong geochemical similarities to the Machadodorp Member and Mashishing dyke swarm, which are thus more spatially extensive than previously thought. In combination with existing data, our results further show that the Machadodorp Member corresponds to two separate, possibly not co-magmatic, igneous events, as documented by low- and high-Th basalts. The low-Th lavas show flat mantle-normalized REE and multi-element patterns, along with depleted Nd and Hf isotopic signatures, which are unusual among Precambrian magmatic rocks in the craton. This signature can be explained by partial melting of a depleted spinel peridotite incorporated into the subcratonic lithospheric mantle (SCLM) by a prior subduction event. A failed rifting event can have triggered melting of such a shallow source in a cratonic context. Even though comparable in age, the high-Th ca. 2.2 Ga Mashishing dyke swarm comes from a very different source, likely a part of the SCLM that had experienced subduction-related fluid metasomatism around the Archean – Proterozoic transition. About 150 million years later, the Bushveld Complex possibly exploited the same rifting structures during its intrusion.

Aeromagnetic patterns in Southern Uruguay: Precambrian-Mesozoic dyke swarms and Mesozoic rifting structural and tectonic evolution

New high-resolution airborne magnetic data of Uruguay allowed constructing new maps concerning the spatial distribution of dyke swarms, main faults and other magnetic bodies, which compose the Uruguayan Shield. We combined geophysical analyses (vertical derivatives, upward continuation, Euler deconvolution), structural analyses of the magnetic maps and previous geological data in order to discriminate the main structural features of the Uruguayan Shield and contribute to a better understanding of its tectonic evolution.The magnetic maps revealed several outstanding features in the Uruguayan Shield. The Paleoproterozoic dyke swarm is larger, denser, more widespread and complex than originally thought, suggesting a possible plume origin. In addition, a new Mesozoic dyke swarm, as complex as the previous one, was identified crosscutting the Paleoproterozoic dyke swarm and the Neoproterozoic orogenic structures. Moreover, this swarm is connected to volcanic calderas in the Merín basin, and shows displacements along Neoproterozoic shear zones, in the magnetic maps, revealing its brittle reactivation during Mesozoic times.The new observations clarify how Proterozoic basement structures controlled the development of the Mesozoic rift. Paleoproterozoic dyke swarms were reactivated as normal faults and Neoproterozoic structures hindered the rift growth, deflecting the deformation in transcurrent movements. Meanwhile, the Mesozoic dyke swarm was developed in a perpendicular direction to the Neoproterozoic structures. Moreover, these findings contradict the current rift model for Uruguay and rise a new model in which the Mesozoic rift developed as two rift basins connected by a central transfer zone, generated by the reactivation of Dom Feliciano Belt structures, between the Sierra Ballena and Sarandí del Yí Shear Zones.

Spatial and temporal distribution patterns of Precambrian mafic dyke swarms in northern Mauritania (West African craton): analysis and results from remote-sensing interpretation, geographical information systems (GIS), Google Earth ™ images, and regional geology

We used remote sensing, geographical information systems, Google Earth™ images, and regional geology in order to (i) improve the mapping of linear structures and understand the chronology of different mafic dyke swarms in the Ahmeyim area that belongs to the Archean Tasiast-Tijirit Terrane of the Reguibat Shield, West African craton, NW Mauritania. The spatial and temporal distributions with the trends of the dyke swarms provide important information about geodynamics. The analysis of the mafic dyke swarms map and statistical data allow us to distinguish four mafic dyke swarm sets: a major swarm trending NE-SW to NNE-SSW (80%) and three minor swarms trending EW to ENE-WSW (9.33%), NW-SE to WNW-ESE (9.06%), and NS (1.3%). The major swarms extend over 35 km while the minor swarms do not exceed 13 km. The Google Earth™ images reveal relative ages through crossover relationships. The major NE-SW to NNE-SSW and the minor NS swarms are the oldest generations emplaced in the Ahemyim area. The NW-SE-oriented swarm dykes which are cutting the two former swarms are emplaced later. The minor E-W to WSW-ENE swarms are probably the youngest. A precise U-Pb baddeleyite age of 2733 ± 2 Ma has been obtained for the NNE-SSW Ahmeyim Great Dyke. This dyke is approximately 1500 m wide in some zone and extends for more than 150 km. The distinct mafic dyke swarms being identified in this study can potentially be linked with coeval magmatic events on other cratons around the globe to identify reconstructed LIPs and constrain continental reconstructions. (Less)

Reliable Euler deconvolution estimates throughout the vertical derivatives of the total-field anomaly

We propose a novel methodology to select reliable Euler deconvolution estimates throughout the vertical derivatives of the total-field anomaly, grounded on the capability of this quantity to locate anomalies due to its higher signal decay with distance. In applying Euler deconvolution to a small moving-data window, we compute the standard deviation of the vertical derivatives of the total-field anomaly for each data window. Then, we define the reliable source-location estimates as those estimates that are obtained by using the data windows with the largest standard deviations of the vertical derivatives of the total-field anomaly. For all tentative values of the structural index (SI), the reliable estimates with tight clustering define the correct SI and the mean of these estimates define the source position. We compared our methodology to select reliable Euler source-position estimates with two available methodologies in the literature based on the rejection criteria of data amplitude and of depth uncertainty. We conducted tests on synthetic noise-corrupted data to investigate the sensitivity of our method to deal with the presence of: i) an additive nonlinear background that simulates a regional field; ii) interfering anomalies with distinct amplitudes; iii) weak-, mid-, and strong-interfering anomalies; and iv) distinct noise levels. Most of tests in the sensitivity analysis shows that our methodology to select reliable Euler source-position estimates yielded better interpretation of the simulated magnetic sources than the methodology based on the rejection criteria of data amplitude and of depth uncertainty. The only exception was the tests with high noise levels in which the reliable Euler estimates selected either by our method or by the rejection criteria yielded poor interpretations. Applications to a real aeromagnetic survey from southern Brazil interpreted an isolated plug intrusion over the Anitápolis anomaly and a swarm of shallow-seated dikes with northwest-southeast trend over the Paranaguá Terrane.

Multi-stage evolution of the Xuhuai rift: Insights from the occurrence and compositional profiles of doleritic sills in the southeastern margin of the North China Craton

Various types of igneous suites occurred between 930 Ma and 890 Ma in the North China craton are known to represent a large igneous province (LIP). However, the magma plumbing system of this LIP and its potential consequence to a breakup event are poorly understood. We report morphology, crystal size distribution of plagioclase, and whole-rock and mineral geochemistry profiles of this LIP represented by the two decoupled mafic sill complexes (Niutishan and Pingshan) in the Xuhuai basin. In terms of petrography and whole-rock geochemistry, unlike the lower-level doleritic Niutishan sill, the Pingshan sill is subdivided into an upper and a lower sequence that meets at a Sandwich Horizon but lacks stratification. The significant En reversals present an almost M-shaped profile that differs from a C-shaped profile as previously revealed in the Niutishan sill. Quantitative textural analyses show relatively strong plagioclase orientation in the lower sequence of the Pingshan sill, which is attributed to magma flow. The Pingshan sill is interpreted to have been constructed as multiple magma injections from crustal magma chambers based on the crystal size distribution and the varied crystallization pressure (5.1–7.5 kbar) of clinopyroxene crystals.Similar to the far-field radiating Dashigou dyke swarm (c. 925 Ma), the Niutishan sill is enriched in incompatible elements, which may represent the initiation of the Xuhuai rifting. However, the depleted features of the diamond-bearing Pingshan sill and the other coeval doleritic sills (916–890 Ma) were probably related to the subsequent process switched from continental rifting to lithospheric rupture in the divergent stage of the Wilson cycle. The occurrence of the intrusive rocks evolving from an early radiating dyke swarm to later sill complexes indicates a time-transgressive change from radial to planar patterns, implying that it was the upwelling asthenospheric plume material, instead of plate boundary processes, governed the initial and early evolution of this intracontinental rift.

Late Neoproterozoic-to-recent long-term t–T-evolution of the Kaoko and Damara belts in NW Namibia

This research aims to reconstruct the Late Neoproterozoic-to-recent long-term time–temperature-evolution of the NW Namibian Kaoko and Damara belts combining numerical modeling of new thermochronological data with previously published geochronological data, i.e., U–Pb, Sm–Nd, and Rb–Sr analyses, and K/Ar, 40Ar/39Ar low-temperature thermochronology. Consequently, we retrieve a coherent long-term time–temperature-evolution of the NW Namibian Neoproterozoic basement rocks including rates of exhumation and subsidence periods over the last ~ 500 Myr. Neoproterozoic basement rocks indicate fast post-Pan African/Brasiliano cooling and exhumation, reheating, or rather subsidence during the development of the Paleozoic-to-Mesozoic SW Gondwana intraplate environment and a significant thermal overprint of the rocks during South Atlantic syn- to post-rift processes, and therefore, resemble the opponent SE Brazilian time–temperature-evolution. We provide an overview of thermochronological data including new apatite and zircon fission-track data derived from Neoproterozoic, Late Paleozoic, and Lower Cretaceous rocks. Apatite fission-track ages range from 390.9 ± 17.9 Ma to 80.8 ± 6.0 Ma in the NW Kaoko Belt with youngest ages confined to the coastal area and significant age increase towards the inland. New zircon apatite fission-track data reveal ages between 429.5 ± 47.8 and 313.9 ± 53.4 Ma for the rocks of the Kaoko Belt. In the central Damara Belt, new apatite fission-track ages range between 138.5 ± 25.3 Ma to 63.8 ± 4.8 Ma. Combined apatite fission-track age distributions from Angola to Namibia and SE Brazil correlate for both sides of the South Atlantic passive continental margin and the reset AFT ages overlap with the lateral Parana–Etendeka dike swarm distribution.

Phoscorites of the Salitre I complex: Origin and petrogenetic implications

The Salitre I complex is a kidney-shaped alkaline-carbonatite-phoscorite complex part of the Late Cretaceous Alto Paranaíba Igneous Province, central Brazil. This complex is a key example where the phoscorite-carbonatite association was preserved from metasomatism by late-stage fluids and, thus, provides an opportunity to address the genesis of phoscorites. Salitre I is composed of two main coalescing intrusions: a) a northern one dominantly containing perovskite bebedourites (B1), with swarms of ring dykes of carbonatites and phoscorites and, b) a southern one containing melanite bebedourites (B2). Salitre I phoscorites can be subdivided into P1 and P2 based on modal abundance, mineral texture, chemistry and mode of emplacement. P1 phoscorites contain essential olivine, phlogopite and apatite, with accessory magnetite and traces of perovskite whereas P2 phoscorites are composed of the same essential phases but also contain carbonate-rich pockets, accessory pyrochlore and rare phlogopite. Carbonate C-O isotopes and whole-rock Sr-Nd show that Salitre I phoscorites crystallized from mantle-derived parental magmas, similar to other APIP carbonatite complexes. However, chemical and textural discrepancies between P1 and P2 suggest derivation from different magma sources and through distinct processes. For instance, P1 Al- and Ti-rich phlogopite, accessory perovskite and general trace element geochemical behaviour can be associated with the evolution of B1 perovskite bebedourites, i.e. genesis through fractional crystallization from a bebedourite magma. P2 phoscorites, on the other hand, have their evolution linked to a magmatic phase that underwent melt immiscibility, as suggested by trace element fractionation. Additionally, P2 contains pyrochlore, which is generally lacking in bebedourites. Therefore, whereas P1 phoscorites might have derived from a bebedourite magma, P2 phoscorites were generated from a carbonated magma enriched in phosphate and iron oxides.

Late Silurian-Early Devonian slab break-off beneath the Canadian Appalachians: Insights from the Nashwaak Granite, west-central New Brunswick, Canada

Late Silurian-Early Devonian granites are widespread in the Miramichi Highlands of New Brunswick, but their petrogenesis is controversial and hinders an understanding of the geological evolution of the Canadian Appalachians. Here we present a detailed geochemical and geochronological study of the Nashwaak Granite, and compare it with coeval granites in this region to indicate a Late Silurian-Early Devonian slab break-off regime in the Canadian Appalachians. The Nashwaak Granite consists of two-mica granite, biotite granite, and biotite granite dyke swarms. These rocks intruded at ca. 420 Ma and their compositions show a highly siliceous, calc-alkaline, and peraluminous characteristic. They show similar REE distribution patterns, similar depletion of Ba, Sr, Nb, P, and Ti, and similar Nd isotopes as contemporaneous granites inboard of the Salinic suture zone (Bamford Brook Fault). Together, these data suggest that Late Silurian-Early Devonian granites in the Miramichi Highlands may be derived from partial melting of the Neoproterozoic to lower Cambrian Ganderian basement resembling the Brookville terrane. Enriched subcontinental lithospheric mantle may have also been involved in the magma system (where coeval gabbro occurs within the Granite), but a contribution from asthenospheric mantle is absent. These linearly-distributed and post-collisional granites are most likely formed by Salinic slab break-off, with corresponding uplift and extension recorded by 422-419 Ma bimodal volcanic rocks, unconformities in sedimentary rocks, and extensional deformation (D3) in the Brunswick subduction complex of northern New Brunswick.

Permian dyke swarm with bimodal affinity from the Hegenshan ophiolite-arc-accretionary belt, Central Inner Mongolia: Implications on lithospheric extension in a Carboniferous continental arc

The Hegenshan ophiolite-arc-accretionary belt adjacent to the Solonker suture in central Inner Mongolia marks the closure of a back-arc ocean basin and an Early Permian extensional setting is a critical region to understand the final closure of the Paleo-Asian Ocean in relation to the construction of the Central Asian Orogenic Belt (CAOB). Since the Paleo-Asian Ocean still existed in Solonker during Permian, the Early Permian extensional regime as indicated by the widely distributed Permian A-type granites in this accretionary belt remains enigmatic. In order to address this issue, we investigate the large-scale Early Permian EW-trending dyke swarm in the Hegenshan belt through an integrated study on petrology, zircon U-Pb isotopic ages, whole rock major-trace elements, and Sr-Nd-Pb isotopes. These dykes were emplaced in the volcanic-plutonic complex of the Carboniferous continental arc. Our zircon U-Pb results show an age range of 289.9 ± 3.6 Ma to 274.6 ± 4.6 Ma for the dyke emplacement, which is slightly younger than the post-subduction bimodal volcanic-plutonic suite (305–299 Ma) which is closely associated with the dyke swarm. Based on field characteristics and geochemical data on the dykes, we recognize two magma types: calc-alkaline dykes and A-type granitic dykes. The calc-alkaline dykes include diorite and granodiorite, whereas the A-type granitic dykes are composed of granite porphyry and rhyolite, corresponding to a major phase of Early Permian bimodal magmatism in the Hegenshan belt. The diorite-granodiorite dykes exhibit a continuous evolutionary trend of calc-alkaline magma from gabbro through diorite to granodiorite. They also display enrichment in LILEs and LREEs, and depletion of HFSEs. These characteristics are consistent with the Late Carboniferous calc-alkaline plutons which are spatially and temporally associated with the diorite-granodiorite dykes. We therefore infer a common magma system, with the parent magma sourced from enriched lithosphere mantle. The granite porphyry-rhyolite dykes show marked consistency in geochemical and Sr-Nd-Pb isotopic compositions with the Late Carboniferous A-type granite and rhyolite in this area. Compared with the calc-alkaline diorite-granodiorite dykes, the granite porphyry-rhyolite dykes have significantly higher Rb, Th, HFSEs (Nb, Ta), HREEs (Yb) and Y, and lower Sr and Ba. These rocks exhibit the features of A2-type granites, possibly inherited from pre-existing subduction-related juvenile lower crust. The large-scale dyke swarm with bimodal signature suggests an extensional setting in the Carboniferous continental arc. Combined with the age data (305–299 Ma) of the Late Carboniferous bimodal magmatic rocks in the study area which denote the maximum age of the slab break-off, the emplacement time of the dyke swarm (289–274 Ma) provides robust constraints on the Early Permian extensional tectonic regime (305–274 Ma) in the Hegenshan accretionary belt. The Early Permian extension in this belt occurred prior to the closure of the Paleo-Asian Ocean and formation of the Central Asian Orogenic Belt, and the Hegenshan belt likely represents a back-arc formed through the northward subduction of the Paleo-Asian Ocean during Permian.

Origin of late Neoarchean granitoid diversity in the Western Shandong province, North China Craton

Diverse late Neoarchean granitoids are widely exposed in the Western Shandong province (WSP). According to geological relationships, lithological features, magmatic zircon U–Pb–Lu–Hf isotopes and whole-rock chemical characteristics, eight main lithological assemblages are identified, and their petrogenesis is determined. (1) The ~2.54 Ga and ~2.51 Ga potassic granites show the highest SiO2 contents (67.06–75.84 wt%) and were sourced from partial melting of metamorphic greywackes. (2) The ~2.53 Ga quartz syenites exhibit the highest alkali contents (10.32–11.23 wt%) and were sourced from mantle-derived basaltic rocks and recycled sediments. (3) The ~2.53–2.52 Ga quartz diorites (less-differentiated sanukitoids) formed from partial melting of metasomatized mantle. (4) The ~2.53 Ga hornblendites have crystallization ages and Lu–Hf isotopes similar to those of the quartz diorites, implying that they are cogenetic cumulates from the quartz dioritic magmas. (5) The ~2.56–2.52 Ga tonalite-trondhjemite-granodiorite (TTG) gneisses were formed by melting of thickened crust. (6) The ~2.53 Ga high-Mg granodiorites (differentiated sanukitoids) were formed by mixing of crust-mantle melts. (7) The ~2.51–2.49 Ga gabbros originated from partial melting of fluid-metasomatized mantle. (8) The ~2.54–2.53 Ga high-Mg andesites (boninites) originated from partial melting of refractory mantle that experienced the extraction of basaltic magma at earlier stage and was then enriched in LILEs and LREEs by slab-derived melts.Integrating the petrogenesis of these late Neoarchean diversified granitoids and associated volcanic rocks, we consider that the late Neoarchean was a crucial period of crust-mantle interaction in the WSP, and partial melting of the metasomatized mantle produced intermediate-mafic melts, then underplating of the mantle-derived magmas caused the partial melting of crustal materials, further led to the mixing of crust-mantle magmas. The above complex crust-mantle interactions triggered the transfer of energy and materials from the Earth’s interior to the surface, forming modern-like continental crust thickness and geothermal gradient. Moreover, combining late Neoarchean intense granitic magmatism and ~2.49 Ga mafic dike swarms implies that the North China Craton experienced cratonization processes at the end of the Neoarchean.