Mesozoic Mafic Dykes Research Articles

Zircon U-Pb geochronology, major, trace elemental, and Sr-Nd-Pb isotopic geochemistry: constraints on the age and magmatic origin of Mesozoic mafic dykes in the Guangxi Zhuang Autonomous Region, South China

ABSTRACT As a direct product of lithospheric extension, the mafic dyke group have become an ideal object for studying the formation, evolution and dynamics of both the crust and continental orogenic belts. Given the scientific significance of mafic dykes and their utility in addressing outstanding questions concerning the origin and development mafic dyke swarms Mesozoic age with-in the South China Plate, in this study, 54 representative mafic dykes within the Guangxi Zhuang Autonomous Region, South China, were examined. Research methods include laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb age-dating, whole rock geochemistry, and the Sr-Nd-Pb isotope geochemistry of the studied mafic dykes. The results indicate that these rocks formed between 139 Ma and 113 Ma and have typical doleritic and lamellar textures. They fall into the alkaline and shoshonitic series, are characterized by enrichment of light rare earth elements, some large ion lithophile elements (Rb, Ba, and Sr), Th, U, and Pb, and have depleted Nb, Ta, Hf, and Ti. Moreover, the mafic dykes studied have high initial87Sr/86Sr ratios (0.7055–0.7057), negative εNd(t) values (from −15.2 to −11.4), and relatively constant initial Pb isotopic ratios (that are Enriched Mantle-1 (EM I)-like; 206Pb/204Pb = 16.765–16.816, 207Pb/204Pb = 15.429–15.476, and208Pb/204Pb = 36.882–37.144). The above results indicate that the group of dolerites and lamprophyres studied herein were likely derived from magma generated via low-degree partial melting (1.0%–10%) of an EM1-like garnet-lherzolite mantle source. The parental magmas to these fractionated olivine-, clinopyroxene-, plagioclase, and Ti-bearing phases, with negligible crustal contamination during ascent and dyke emplacement. On the basis of existing research and evidence from this study, we propose a reasonable model for the origin of the mafic dykes: the ancient Pacific Plate subducting below the South China plate led to extension of the lithosphere, an upsurge of the asthenosphere, which triggered partial melting of the lower mantle lithosphere in the Early Cretaceous, and the formation of the parent magma to the Mesozoic mafic dykes in the Guangxi Zhuang Autonomous Region study area. The involvement of slab-derived fluids from the subduction of the ancient Pacific Plate was responsible for metasomatism of the mantle source to these mafic magmas.

Petrogenesis and tectonic setting of Early Cretaceous mafic dykes in the North Qinling Orogenic Belt, central China: constraints on the lithospheric lower crust delamination

Late Mesozoic mafic dykes, which are widely developed in the North Qinling Orogenic Belt (NQOB), include abundant geodynamic information. This paper describes the mafic dykes that intrude the Late Jurassic granite in the Dayu and Kuyu areas, and reports important petrological constraints for the late Mesozoic tectonic transition from compression to extension in the NQOB. Three zircon U–Pb results show that the minimum ages of the mafic dykes are 139.8 ± 1.4 Ma, 137.4 ± 1.7 Ma and 133.4 ± 0.9 Ma, indicating that the emplacement age of the Dayu and Kuyu mafic dykes is 140–133 Ma. Petrogeochemical analyses suggest that the mafic dykes belong to the high-K calc-alkaline shoshonite series with low SiO2 (46.93–56.73 wt%), MgO (1.88–9.10 wt%) and TiO2 (1.17–1.82 wt%), and high Al2O3 (13.98–17.46 wt%), TFe2O3 (7.81–10.92 wt%) and K2O (1.28–4.78 wt%). The mafic dykes are enriched in large ion lithophile elements (e.g. Rb, Ba, K, La, Sr) and depleted in high-field-strength elements (e.g. Nb, Ta, Zr, Ti). These samples have the right-sloping chondrite-normalised rare earth element patterns, which suggest light rare earth element enrichment and heavy rare earth elements depletion with no obvious Eu anomalies (δEu = 0.94–1.11). The I Sr, ε Nd(t), ε Hf (t) and T DM2(crust) values are 0.7056–0.7060, −10.60 to −5.98, −14.1 to −2.8, and 1382.4 ± 25.1 to 2081.9 ± 47.6 Ma, respectively. Both elemental and isotopic geochemistry show that the formation of Dayu and Kuyu mafic dykes is due to the partial decompression melting of previously enriched lithospheric mantle during a delamination process. The mafic dykes have undergone fractionation crystallisation of Mg–Fe phase minerals during magma ascent, accompanied by some crustal contamination. Combined with the regional tectonic setting, we suggested that the NQOB experienced intra-continental extension during the Early Cretaceous. KEY POINTS Early Cretaceous (140–133 Ma) mafic dykes have been discovered in the middle part of the North Qinling Orogenic Belt. The remote effect of the Paleo-Pacific Plate subduction has reached the middle of the North Qinling Orogenic Belt. The North Qinling Orogenic Belt entered the extensional stage in the Early Cretaceous (140–133 Ma).

LA‐ICP‐MS zircon U–Pb age, whole‐rock geochemistry, Sr–Nd–Pb isotope, and zircon Hf isotope constraints on the origin and dynamic setting of Mesozoic mafic dykes, Henan Province, southeastern North China Craton

Mafic dykes from the Henan Province within the southeastern North China Craton (NCC) were sampled. Herein, we present new zircon laser ablation‐inductively coupled plasma‐mass spectrometry U–Pb age, whole‐rock geochemical, Sr–Nd–Pb isotopic, and zircon Hf isotopic data for the five areas where these mafic dykes outcrop. The mafic dykes are all dolerite and were emplaced between 121.6 ± 1.1 Ma and 131.7 ± 1.2 Ma. These are characterized by enrichment in some of the large‐ion lithophile elements (e.g., Rb, Ba, Th, U, K, and Sr), light rare earth elements, and depletion in high‐field‐strength elements (Nb, Ta, Hf, and Ti) and Eu (Eu/Eu* is between 0.62 and 0.96) relative to chondrite‐ and primitive mantle‐values. The dykes have high initial Sr isotopic ratios ([87Sr/86Sr]i = 0.7056–0.7058), negative εNd (t) ratios (−12.3 to −12.1), relatively constant Pb isotopic ratios ([206Pb/204Pb]i = 16.771–16.778; [207Pb/204Pb]i = 15.461–15.467; [208Pb/204Pb]i = 36.850–36.857), negative εHf (t) (−14.9 to −3.5), as well as old Nd (TDM2; 2.22–2.54 Ga) and Hf (TDM2; 2.99–4.34 Ga) model ages. These geochronological, geochemical, and isotopic data indicate that the investigated Henan Province dykes were likely derived from magmas generated by low to moderate degree partial melting (1.0–10%) of an EM1‐like garnet lherzolite mantle source. Moreover, the mafic melts fractionated olivine, clinopyroxene, plagioclase, Ti‐bearing oxides (e.g., ilmenite, titanite, apatite, etc.) prior to emplacement, and assimilated only minimal amounts of crustal material. Several possible models have previously been proposed to explain the origin of Mesozoic magma activities in Henan Province. However, here we propose a foundering model for the mafic dykes studied, involving the foundering of eclogite from thickened lower crust due to the collision between the Yangtze Block, Qinling ocean crust, and southeastern NCC.

Geochronology and geochemistry of Late Mesozoic diabase in the Litun ore district, North China Craton

Mesozoic mafic dykes in the Litun ore district provide an opportunity to study the nature of the Mesozoic lithospheric mantle beneath the Luxi area of the North China Craton. U–Pb dating of zircon from the Litun diabase using laser ablation inductively coupled plasma mass spectrometry yields an age of 124.6 ± 1.7 Ma, indicating an Early Cretaceous emplacement age. Samples from the Litun diabase display a medium‐ to fine‐grained texture, consisting of plagioclase (60–65 vol%), clinopyroxene (30–35 vol%), and minor biotite (~2 vol%). They are characterized by enrichment of large‐ion lithophile elements (LILEs, such as Rb, Ba, Th, and U) and light rare‐earth elements (LREEs) without significant Eu anomalies (0.83–1.07), depletion of high‐field‐strength elements (HFSEs, such as Nb, Ta, and Ti). The Litun diabase has relatively high initial 87Sr/86Sr ratios (0.7049–0.7060) and negative εNd(t) values (−11.7 to −9.7). The Litun diabase also shows high MgO (10.52–13.84 wt%), Cr (956–1,162 ppm), Co (26.5–65.6 ppm), and Ni (240–514 ppm) contents and low Rb/Sr ratios (0.01–0.03). These geochemical features suggest that the Litun diabase may have originated from an enriched lithospheric mantle source with amphibole as an essential phase, and that the Litun diabase was not significantly affected by fractional crystallization and crustal contamination. In combination with the Sr–Nd isotopic compositions of mafic rocks in the Luxi area (such as the Mengyin mafic dykes and Jinan gabbro), our results suggest that the lithospheric mantle beneath the Luxi area may have been rapidly enriched between ~144 and ~124 Ma, which was induced by sequential hybridization of sinked lower crust.

Mesozoic mafic dykes in the North China Craton: magmatic evolution and implications for gold mineralization

ABSTRACT The North China Craton (NCC), which underwent multiple subduction and lithospheric destruction during the Mesozoic, is well known for hosting some of the world-class gold deposits. In many cases, Mesozoic mafic dykes are associated with the gold deposits, although there has been no systematic studies aimed to integrate the characteristics of these dykes in different regions of the NCC. Here we investigate diabase and lamprophyre dyke suites in the Xiaoqinling region along the southern margin of the NCC, and compile the published geochronological, geochemical and isotopic data for regional mafic dykes and gold deposits from the major gold belts including Jiaodong, Xiaoqinling and central NCC. Zircon U-Pb data show the mafic dykes in Xiaoqinling were emplaced in two episodes at 219–227 Ma and 121–137 Ma, and their εHf(t) values (ranging from −2.1 to 3.8 and −13.6 to 1.6 respectively) suggest increasing input of lithospheric materials in the Early Cretaceous dykes. The compiled geochemical data on the mafic dykes from different regions in the NCC show that the magmatic evolution involved olivine and clinopyroxene fractional crystallization with limited crustal contamination and subduction-related fluid metasomatism. The geochemical and isotopic signature suggests multiple sources for the parent magma of the mafic dykes ranging from convective asthenospheric mantle to ancient lithospheric mantle, with the magma source characterized by enriched mantle (EM2 or EM2+ EM1), and that signatures might be potential for gold mineralization. Further, most dykes are coeval and cospatial with gold deposits in the NCC. The geochronological data show that small amount of both Mesozoic dykes and gold deposits formed in Triassic whereas the peak formed in Cretaceous following slab roll back of the Pacific plate subducting from the east during 145–110 Ma. We envisage that the tectonic extension associated with Pacific slab subduction contributed to the magmatism and gold mineralization in the NCC.

Pyrite Rb-Sr, Sm-Nd and Fe isotopic constraints on the age and genesis of the Qingchengzi Pb-Zn deposits, northeastern China

Sr-Nd-Fe isotopes are reported for pyrites and whole rocks of ores and associated rocks for the Qingchengzi Pb-Zn deposits in the northeasern China, to investigate the age, sources and processes of the Pb-Zn mineralization. Rb-Sr leaching isotopic dating of pyrites that crystallized coeval with galena and sphalerite from five subsamples of ores yields isochron ages of 143 ± 12 Ma to 159 ± 12 Ma with a weighted mean age of 151.8 ± 5.2 Ma. This age can be interpreted to be the timing of Pb-Zn mineralization, coeval with the emplacement of Late Jurassic granites in the region. The pyrites have negative εNd (152 Ma) values of −17.6 to –22.0, among the Neoarchean TTG rocks, Paleoproterozoic meta-sedimetary rocks (i.e., wall rocks) and granites, and Mesozoic mafic dykes and granites. However, the initial 87Sr/86Sr ratio of 0.7188 ± 0.0004 is similar to those of Late Jurassic granites and Paleoproterozoic rocks (0.7082–0.7183), distinct from those of mafic dykes (0.7069–0.7110), indicating the ore-forming materials (i.e., Pb and Zn) were mainly derived from wall rocks and Late Jurassic granites. Combined with the previously-published H-O isotopes, the new age and Sr-Nd isotopic data indicate that the mineralizaing fluids were mainly derived from dehydration of granitic magmas, interacted with meteoric water that extracted Pb and Zn from the wall rocks. The δ56Fe values of pyrites in the Pb-Zn ores and Paleoproterozoic meta-sedimentary rocks range from +0.23‰ to +1.09‰ and −0.49‰ to +0.17‰, respectively, distinct from those of Paleoproterozoic basement rocks and Mesozoic rocks (−0.03‰ to +0.28‰). Combined with Nd isotopes, the large range of δ56Fe values for pyrites would be the result of rapid precipitation of pyrites in the wall rocks and contemperal deposition of pyrites in the residual fluids during interaction between wall rocks and ore-forming fluids. Collectively, our new Sr-Nd-Fe isotopes of pyrites not only constrain the age and sources of Pb and Zn mineralization, but also reveal the detailed mineralization processes, providing a new method to constrain the genesis of hydrothermal Pb-Zn deposits.

Zircon U‐Pb Dating and Hf Isotope of the Metavolcanics, Gabbro and Plagiogranite in the Mianxian‐Lueyang Areas of Mianlue Belt

AbstractThe Mianlue Belt is located in the central China and regarded as a tectonic mélange belt, representative of suture zone formed in the early Mesozoic between North and South China Blocks. However, its forming age has been controversial for a long time. This study provides the reliable LA‐ICPMS U‐Pb zircon dating and in‐situ Hf isotopes from the meta‐volcanics, gabbro and plagiogranite in this belt. Of which, two meta‐andesites yielded the ages of 934 Ma and 933 Ma, two meta‐dacites give the ages of 999 Ma and 873 Ma, and another plagiogranite define an age of 923 Ma, respectively, evidently older than those previously thoughted. Their Hf isotopes are quite coincident. The andesites have the εHf(t)= ‐0.03 ∼ +11.76 with the TDMC of 1032‐1629 Ma, εHf(t) = +9.81∼ +16.06 and the TDMC = 867‐1185 Ma for dacite and εHf(t) = +8.65 ∼ +16.66 and the TDMC = 805∼1123 Ma for plagiogranite, suggesting an important crustal growth in the early Neoproterozoic. In addition, the gabbro yielded younger ages of 224‐213 Ma and variable ages from 1225 Ma to 280 Ma in some inherited zircons, consistent with early Mesozoic mafic dykes in South Qinling belt. Their TDMC range from 1.2 to 2.7 Ga, indicating early‐middle Proterozoic crustal residence involved minor Neo‐Archean crustal materials. All new results lead us to propose that most of components in the Mianlue Belt were generated in the early Neoproterozoic and related to the subduction process in the northern margin of the Yangtze block, representative of an important juvenile crust growth in South Qinling during the Neoproterozoic.

Integrated elemental and Sr-Nd-Pb-Hf isotopic studies of Mesozoic mafic dykes from the eastern North China Craton: implications for the dramatic transformation of lithospheric mantle

Evolution of the lithospheric mantle beneath the North China Craton (NCC) from its Precambrian cratonic architecture until Paleozoic, and the transformation to an oceanic realm during Mesozoic, with implications on the destruction of cratonic root have attracted global attention. Here we present geochemical and isotopic data on a suite of newly identified Mesozoic mafic dyke swarms from the Longwangmiao, Weijiazhuang, Mengjiazhuang, Jiayou, Huangmi, and Xiahonghe areas (Qianhuai Block) along the eastern NCC with an attempt to gain further insights on the lithospheric evolution of the region. The Longwangmiao dykes are alkaline with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N >4.3) and EM1-like Sr-Nd-Pb-Hf isotopic signature ((87Sr/86Sr) i >0.706; ε Nd (t) < −6.3, (206Pb/204Pb) i > 16.6, (207Pb/204Pb) i >15.4, (208Pb/204Pb) i >36.8, ε Hf (t) < −22.4). The Weijiazhuang dykes are sub-alkaline with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N > 3.7), and display similar EM1-like isotopic features ((87Sr/86Sr) i >0.706; ε Nd (t) < −7.0, (206Pb/204Pb) i > 16.7, (207Pb/204Pb) i >15.4, (208Pb/204Pb) i >36.9, ε Hf (t) < −23.3). The Mengjiazhuang dykes are also sub-alkaline with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N > 2.4) and EM1-like isotopic features((87Sr/86Sr) i >0.706; ε Nd (t) < −18.4, (206Pb/204Pb) i > 16.7, (207Pb/204Pb) i >15.4, (208Pb/204Pb) i >36.9, ε Hf (t) < −8.6). The Jiayou dykes also display sub-alkaline affinity with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N > 3.7) and EM1-like Sr-Nd-Pb-Hf isotopic features ((87Sr/86Sr) i >0.706; ε Nd(t) < −15.3, (206Pb/204Pb) i > 16.7, (207Pb/204Pb) i >15.4, (208Pb/204Pb) i >36.9, ε Hf (t) < −18.4). The Huangmi dykes are alkaline (with Na2O+K2O ranging to more than 5.9wt.%)) with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N > 9.3) and EM1-like isotopic composition ((87Sr/86Sr) i >0.705; ε Nd (t) < −15.1, (206Pb/204Pb) i > 16.9, (207Pb/204Pb) i >15.5, (208Pb/204Pb) i >36.9, ε Hf (t) < −12.2). The Xiahonghe dykes are alkaline with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N=2.12-2.84) and similar EM1-like Sr-Nd-Pb-Hf isotopic signature ((87Sr/86Sr) i > 0.705; ε Nd (t)<-18.0, (206Pb/204Pb) i > 16.9, (207Pb/204Pb) i >15.5, (208Pb/204Pb) i >36.9, ε Hf (t)<−8.6). Our data from the various mafic dyke suites suggest that the magmas were derived from EM1-like lithospheric mantle, corresponding to lithospheric mantle modified by the previously foundered lower crust beneath the eastern NCC. Our results suggest contrasting lithospheric evolution from Triassic (212 Ma) to Cretaceous (123 Ma) beneath the NCC. These mafic dykes mark an important phase of lithospheric thinning in the eastern North China Craton.

Ordovician and Triassic mafic dykes in the Wudang terrane: Evidence for opening and closure of the South Qinling ocean basin, central China

We report zircon ages and geochemical composition for mafic dykes that intruded Neoproterozoic volcanic–sedimentary sequences in the southern part of Wudang area, South Qinling. The results indicate that the dykes were emplaced during the Early Paleozoic (c. 460Ma) and Early Mesozoic (c. 220Ma). The dykes share similar major element composition, but have distinctive trace element pattern and Sr–Nd–Pb isotope distribution. Early Paleozoic mafic dykes are characterized by enrichment in LREEs, LILEs and HFSEs and EM II-type isotopic features. These geochemical features suggest derivation from an OIB-type mantle source that had undergone metasomatism during earlier subduction events. The Early Mesozoic mafic dykes can be subdivided into two distinct geochemical groups. Dykes of Group 1 are depleted in LREEs, LILEs and HFSEs and show depleted isotope compositions, indicating an origin by partial melting of asthenospheric mantle material. Dykes of Group 2 have high Rb-, Ba-, and K-contents and EM I-type isotopic features, suggesting input of lower crustal material to the magma source during Mesozoic subduction. We propose that the Early Paleozoic dykes are related to the opening of an oceanic basin separating South Qinling from the Yangtze Block, while the Early Mesozoic dykes were derived from partial melting of up-welling asthenosphere during the final amalgamation of these two blocks in the Early Mesozoic. A slab break-off model could explain not only the petrogenesis of the Mesozoic mafic dykes, but also the distinct geological features between the Dabie–Sulu and South Qinling orogens. We propose that slab break-off occurred at great depth in the Dabie–Sulu orogen and hence rare magmatism occurred. Whereas in South Qinling the break-off occurred at a shallow depth, the asthenospheric mantle material could rise further up into the overlying mantle where it experienced decompression and melting. As a consequence, crustal sections were heated up to produce extensive granitic magmatism.

Petrogenesis of mafic dykes and high-Mg adakitic enclaves in the Late Mesozoic Fangshan low-Mg adakitic pluton, North China Craton

Late Mesozoic mafic dykes, high-Mg adakitic enclaves and the low-Mg adakitic host from the Fangshan pluton provide important insights into the process of crust-mantle interaction. The mafic dykes (134±2Ma) have a wide range of total alkali (Na2O+K2O: 3.41–7.11wt.%), MgO (2.6–6.5wt.%), Ni (60.1–541.7ppm), and Cr (138.2–749.5ppm) contents and high Mg# values (57.6–72.0). They are enriched in LREEs and LILEs (Rb, Ba, K) and depleted in HREEs and HFSEs (Nb, Ta, Zr, Hf), suggesting a garnet-bearing enriched mantle source. The host granodiorites (136±2Ma) have low-Mg adakitic geochemical compositions. They belong to the of high-K calc-alkaline series with low MgO contents (1.4–3.0wt.%) and Mg# values (41.9–48.1). They exhibit extremely high Sr and low Y and Yb contents, and are strongly enriched in LREEs resulting in high Sr/Y and (La/Yb)N ratios. The low-Mg adakitic granodoirites were probably generated by partial melting of thickened eclogitic lower continental crust (LCC). The enclaves (136±2Ma) with a strong affinity to the mafic dyke show high-Mg adakitic geochemical characteristics. They are alkaline series with relatively low SiO2 (52.2–59.6wt.%) and high MgO (Mg#=44.7–57.2) contents. They contain high Sr, Ni and Cr, low Y and Yb, and have very high Sr/Y and (La/Yb)N ratios, with depletion in HFSEs (Nb, Ta, Zr, Hf, Ti) but without Eu and Sr negative anomalies. Numerical simulation indicates that the high-Mg adakitic enclaves were likely produced by mixing of low-Mg adakitic melt with mantle- derived magma. We propose that the underplating of the mantle-derived magma might have triggered partial melting of the thickened LCC and resulted in the mixing between them in the Early Cretaceous.

Crustal thickening along the West Antarctic Gondwana margin during mid-Cretaceous deformation of the Triassic intra-oceanic Dyer Arc

Subduction-related Mesozoic mafic dykes in eastern Palmer Land, Antarctic Peninsula, record the development of an intra-oceanic arc terrane, the Dyer Arc, probably of late Permian–Triassic age, represented by a tholeiitic dyke group. Arc rocks were deformed in late Triassic–early Jurassic and mid-Cretaceous times on the Gondwana margin. Eruption of syn- to post-mid-Cretaceous orogenesis magmatism is represented by a calc–alkaline group of dykes. The tholeiitic dykes intruded immature granitic crust of the Eastern Zone sub-terrane of the Central Domain, one of the magmatic terranes of the Pacific margin of Gondwana. The calc–alkaline dykes straddle the major tectonic boundary between the Central Domain and the continental margin Eastern Domain. 132 dykes were studied, 82 of which are calc–alkaline, 48 tholeiitic and two shoshonitic, across an area of approximately 4000km2. Tholeiitic dykes strike broadly NNW–SSE, transposed into parallelism with the strike of the tectonic boundary during Cretaceous orogenesis whereas calc–alkaline dykes strike tightly ESE–WNW at a high angle to the boundary. The tholeiites pre-date late Triassic deformation and metamorphism, and are interpreted as dominantly Triassic in age, whereas Ar–Ar dated calc–alkaline dykes are younger (~100Ma and ~97Ma) and field relations indicate that they overlap with the waning phase of the mid-Cretaceous Palmer Land Event. The tholeiites have trace element abundances similar to, but more depleted than, those of modern intra-oceanic arcs, as recorded by Zr/Hf ratios. Nb/Yb versus TiO2*/Yb and Sm/Yb versus La/Sm plots are used to model depths of partial melting. The tholeiites were mostly generated at shallow depths corresponding to 2.5 and certainly less than 3.0GPa in largely garnet-free mantle. The calc–alkaline magmas were generated at pressures greater than 3.0 to possibly >3.5GPa, in the garnet zone. The deepening of the mantle source, and onset of calc–alkaline magmatism are interpreted to have been caused by deformation of the intra-oceanic arc terrane on the Gondwana margin, and lithospheric thickening during the Palmer Land Event.

Petrogenesis of Late Mesozoic mafic dykes in the Jiaodong Peninsula, eastern North China Craton and implications for the foundering of lower crust

Late Mesozoic (122.5–127 Ma) mafic dykes (dolerite-porphyries) in the Beibo gold district, Shandong Province, eastern North China Craton, can be subdivided into two groups (high-K and low-K) based upon K2O content. They are characterised by enrichment in light rare earth element (LREE) and some large ion lithophile elements (LILEs) (e.g., Rb, Ba and Sr), similar concentrations of heavy rare earth elements (HREEs), high (87Sr/86Sr)i ranging from 0.7094 to 0.7101, low εNd(t) values from −15.3 to −13.9, 206Pb/204Pb=16.98–17.70, 207Pb/204Pb=15.47–15.55 and 208Pb/204Pb=37.61–37.89. In addition, the high-K dolerite-porphyries are distinguished by negative Eu, Nb, Ta, P anomalies in chondrite-normalised REE and primitive-mantle-normalised trace element diagrams. These features suggest that they were derived from a common, enriched garnet-lherzolite mantle beneath the North China Craton, and that this was metasomatised by multiple enrichment events induced by hybridism of foundering lower crust at mantle depths. The low-K dolerite-porphyries were produced by 10–15% partial melting of the garnet-lherzolite source that had undergone metasomatism by carbonate-rich fluids producing secondary phlogopite and amphibole, whereas the high-K group was derived by relatively low-degree (3–5%) partial melting of a source that had experienced metasomatism by a carbonate-rich fluid with crystallisation of only secondary amphibole. The Pd-subgroup platinum-group element (PPGE) (Rh, Pt and Pd) enrichment and Ir-subgroup platinum-group element (IPGE) (Ir and Ru) depletion of these mafic dykes suggest derivation from an S-unsaturated magma. Geochemical and PGE features indicate that fractionation of olivine, clinopyroxene, hornblende, plagioclase, ilmenite, titanite, apatite, laurite and Os–Ir–Ru alloys was important in the formation of the high-K mafic magma. Additionally, separation of all the above phases with the exception of plagioclase and apatite played an important role in the origin of low-K dolerite-porphyries. Minor crustal contamination also occurred during ascent of these mafic magmas.

Zircon U–Pb geochronology and major, trace elemental and Sr–Nd–Pb isotopic geochemistry of mafic dykes in western Shandong Province, east China: Constrains on their petrogenesis and geodynamic significance

Mesozoic mafic dykes in western Shandong Province (Luxi), SE, the North China Craton (NCC) provide an opportunity to examine the nature of their mantle source and the secular evolution of the Mesozoic lithospheric mantle beneath SE NCC. Chronological, geochemical and Sr–Nd–Pb isotopic analyses were carried out on two selected Mesozoic mafic dykes from Mengyin and Zichuan in western Shandong, respectively. Detailed SHRIMP zircon U–Pb dating yields emplacement ages of 144 ± 2 Ma for the Mengyin mafic dykes and of 143 ± 2 Ma for the Zichuan mafic dykes. Both are enriched in LILE (Rb, Ba, Sr, Pb) and LREE without Eu anomalies, but are depleted in HFSE (Nb, Ta and Ti). The dykes have relatively radiogenic Sr ( 87Sr/ 86Sr) i (0.706 ~ 0.707) and negative ε Nd(t) (− 6.5 to − 4.4), but remarkably unradiogenic Pb ( 206Pb/ 204Pb = 16.99–18.36). Both dykes may have experienced crystal fractionation of olivine and clinopyroxene. The Mengyin mafic dykes are characterised by high MgO (9.66–17.97 wt.%), Mg # (66–74), Cr (809–1208 ppm) and Ni (171–390 ppm), indicative of derivation from mantle-derived melts with minor fractionation. In contrast, the Zichuan mafic dykes have relatively low MgO (3.38–4.14 wt.%), Cr (24–56 ppm) and Ni (14–24 ppm), suggesting that they may have originated from an extremely evolved magma. There is evidence in support of multiple enrichment events induced by hybridism of foundering lower crust at mantle depths. For example, phlogopite, amphibole and rutile metasomatism may have affected the lithospheric mantle beneath Mengyin, whereas phlogopite and carbonatite metasomatism modified the lithospheric mantle below Zichuan. This study sheds light also on the geodynamic significance of the 144–143 Ma mafic dykes, for example, we propose that lithospheric thinning underneath southeastern NCC of eastern China was caused by the removal of the lower lithosphere (mantle and lower crust).

Geochemistry of Late Mesozoic mafic dykes in western Fujian Province of China: Sr-Nd isotope and trace element constraints

The Bancun diabase dyke and the Bali hornblende gabbro dyke in western Fujian Province were emplaced in the Early and Late Cretaceous periods, respectively; the former is designated to calc-alkaline series and the latter to K-high-calc-alkaline rock series. Both the dykes are characterized by such geochemical characteristics as high Al and Na2O>K2O. As for the Bancun dyke, Al2O3=16.32%−17.54% and K2O/Na2O=0.65−0.77; as for the Bali dyke, Al2O3=16.89%−17.81% and K2O/Na2O=0.93−0.99. Both the Bancun and Bali mafic dykes are relatively enriched in LILE and LREE, but depleted in HSFE, displaying the geochemical characteristics of continental marginal arc, with high initial Sr isotopic ratios and low ɛNd(t) values. The (87Sr/86Sr)i ratios of the Bancun diabase dyke are within the range of 0.708556–0.70903 and their ɛNd(t) values vary between −6.8 and −6.3; those of the Bali hornblende dyke are within the range of 0.710726–0.710746 and their ɛNd(t) values are −4.7–−4.9, showing the characteristics of enriched mantle EM II. The isotope and trace element data showed that the mafic dykes have not experienced obvious crustal contamination, and metasomatism caused by subduction fluids is the main factor leading to LILE and LREE enrichments. The enriched mantle is the source region for the mafic dykes, and mixing of subduction fluid metasomatized enriched mantle and EM II-type mantle constituted the mantle source region of both the Bancun and Bali mafic dykes. Upwelling of the asthenosphere mantle provided sufficient heat energy for the generation of magmas. In accordance with the discrimination diagram of their tectonic settings as well as their trace element geochemical characteristics, it is considered that the dykes both at Bancun and Bali possess the characteristics of continental marginal arc, revealing the tectonic environment of formation of the mafic dykes, the continental dynamic background as an intraplate tensional belt in which the mafic dykes were emplaced. Meanwhile, it is also indicated that the tensional tectonic stress mechanism is responsible for the formation of the mafic dykes in western Fujian Province.

Magma flow inferred from preferred orientations of plagioclase of the Rio Ceará-Mirim dyke swarm (NE Brazil) and its AMS significance

Abstract Low-field magnetic and plagioclase fabrics were compared in Mesozoic mafic dykes of the Rio Ceará-Mirim swarm. Coarse titanomagnetites with pervasive ilmenite lamellae constitute the main carrier of the magnetic anisotropy. The hysteresis parameters of the mafic dykes fall in the pseudo-single domain field. The resulting AMS ellipsoid is usually oblate and has a very low anisotropy (&lt;3%). Textures indicate that the oxi-exolution processes and size reduction of the ferrimagnetic domains occurred at subsolidus temperatures on cooling of the dykes. The magmatic fabric was determined by the shape preferred orientation of plagioclase laths. It rarely matches the magnetic fabric. Besides their contrasting shape ellipsoids, prolate and oblate respectively, their corresponding principal directions diverge from each other or exchange their positions depending on the symmetry of the ellipsoids. These discrepancies are attributed principally to small differences in the net shape of Ti-poor magnetite after exsolution of ilmenite and in the inherently oblique fabric of grains with different shapes. These results draw attention to the need to use independent methods to confirm the conclusions about flow fabrics of weakly anisotropic mafic dykes based only in AMS.

The Late-Variscan fault network in central–northern Portugal (NW Iberia): a re-evaluation

The fault network in central–northern Portugal, especially the fault system with mean strike N25°, has been used to help deduce the late Palaeozoic dynamics of Western Europe. On the other hand, the N80° strike–slip fault system was recognized in previous works as Late-Variscan and left lateral, but was scarcely mapped and its importance neglected. This study shows that the N25° faults were dextral in the late stages of the Variscan Orogeny and sinistral during the Alpine Cycle due to pervasive reactivation. Fault rocks and intrusions are clearly different, according to age: mostly high temperature quartz infillings, but locally muscovite, tourmaline, and aplite dykes in Late-Variscan times, and low temperature cataclasites, fault gouges and Mesozoic mafic dykes in younger Alpine times. We dated dextral N45° segments of the N25° fault system because they are less prone to reactivation by the NNW–SSE Alpine compression, and can thus preserve the Variscan record. We used K–Ar in muscovite concentrates and obtained a minimum age of the analysed faults of ca. 312 Ma, which sets a lower limit to the so-called Late-Variscan wrench-faulting period. The present study shows that the N80° fault set is pervasive and sinistral in central–northern Portugal, and therefore, that it only admits one brittle dextral conjugate, the N25° fault system. Both were generated by a maximum compressive stress bearing between N50° and N55° in azimuth. We did not find evidence of a Variscan sinistral strike–slip movement in the N25° fault system, and therefore, this kinematics is believed to represent only the displacements accommodated during the Alpine Cycle.

Geochemistry of mafic dykes from Perhentian and Redang islands: an example of petrogenesis of the younger (dolerite) dykes from the Eastern Belt of Peninsular Malaysia

Mesozoic mafic dykes in the Perhentian islands and their surrounding area can be divided into two types, based on their field occurrence, i.e. the older and younger dykes. The older dykes are synplutonic to their felsic host and the younger dykes post date their felsic host. The younger dykes, which are more abundant, are mainly doleritic in composition and are similar to those found throughout the Eastern Belt of Peninsular Malaysia. They are made up of plagioclase, clinopyroxene, amphibole, iron ore and chlorite. The silica content of the dykes is between 47.17 to 53.7% and can be classify as basalt, trachybasalt and basaltic trachyandesite. Geochemical study shows that the younger dykes formed in a continental within plate tectonic setting.

Basement geology and Palaeozoic–Mesozoic mafic dykes from the Cape Meredith Complex, Falkland Islands: a record of repeated intracontinental extension

Mafic dykes (Groups A–D) intruded into Mesoproterozoic basement amphibolites, gneisses, and granitoids of the Cape Meredith Complex on the southern tip of West Falkland, provide an important record of at least three periods of lithospheric extension during Palaeozoic and Jurassic times. Group A dykes are calc-alkaline lamprophyres that were generated by partial melting of an enriched subcontinental lithospheric mantle in Cambro-Ordovician times. Group B dykes are Ordovician dolerites derived from an asthenospheric mantle source, perhaps during the same extensional episode as Group A dykes. Group C dykes were also derived from an asthenospheric source and are possibly of Silurian age. The youngest, Group D, dykes are part of the widespread Jurassic Gondwana province. This group contains an oceanic island basalt-like sample and an enriched sample similar to both Group A lamprophyres and to the Jurassic Ferrar province in Antarctica. These correlations have interesting implications for the composition and evolution of mantle sources through time; the co-existence of Cambrian lamprophyres and Jurassic Ferrar-type magmas in the Cape Meredith Complex demonstrate for the first time that the enriched lithospheric mantle source postulated for the Ferrar magmas existed as far back as Cambrian times.

Mesozoic mafic dykes of northeastern South America and correlations with similar dyke swarms in West África and eastern North America

Mesozoic mafic dykes of northeastern South America and correlations with similar dyke swarms in West África and eastern North America