Obvious Eu Anomalies Research Articles

The genesis of high Ba‐Sr adakitic rocks: Insights from an Early Cretaceous volcanic suite in the central North China Craton

The origin of high Ba‐Sr magmatic suites with adakitic features has remained controversial, although they provide important clues on the evolution of continental lithosphere. Here, we present petrological, geochemical, zircon U–Pb and Lu‐Hf data from an Early Cretaceous volcanic suite of andesitic‐dacitic composition in the Laiyuan complex of the central North China Craton, with a view to gain insights on the magma source, petrogenesis, and tectonic implications. The volcanic suite represented by rocks of andesitic‐dacitic composition shows enrichment in light rare earth elements (LREE) and large‐ion lithophile elements (LILE), and depletion in high‐field‐strength elements (HFSE) with no obvious Eu anomalies. They also show typical adakitic features such as high Sr/Y and La/Yb ratios, as well as high Ba‐Sr concentrations. Zircon U–Pb geochronology indicates that these rocks were formed in Early Cretaceous during 131 to 127 Ma, with εHf(t) values ranging from −23.5 to −19.4, suggesting enriched lithospheric mantle source. The geochemical and isotopic data show that the enriched lithospheric mantle source experienced fluid‐related subduction metasomatism. The distinct geochemical features of these identified in our study including high Ba‐Sr concentrations and adakitic affinities were not only inherited from their magma source, but are also a result of fractional crystallization during magma evolution including amphibole‐dominated fractional crystallization at depth and limited plagioclase removal. The volcanism in the Taihang Mountains occurred under extensional regime triggered by the subduction and rollback of the Paleo‐Pacific slab during Late Jurassic to Early Cretaceous and consequent thermal‐mechanical erosion of the lithospheric mantle.

Early Cretaceous mafic dikes in the northern Qinling Orogenic Belt, central China: Implications for lithosphere delamination

Mafic dikes preserve a considerable amount of geological information about crustal evolution. Yanshanian granitoids and granitites are widespread in the southern margin of the North China Craton (NCC) and northern Qinling Orogenic Belt (NQOB), respectively. However, further study is required to resolve the many contrasting hypotheses relating to the mechanisms of petrogenesis and origins. The present study therefore reports on geochemical, geochronological, and Sr-Nd-Pb isotopic data for a suite of mafic dikes in the Mokou area of the NQOB, central China. The Mokou dikes are composed of diabase and lamprophyre intruded into the Kuanping Group (Pt3). The zircon U-Pb ages of two diabases and one lamprophyre were 132 ± 3 Ma and 132 ± 1 Ma, and 132 ± 1 Ma, respectively. The ages of a total of 126 zircon grains from three rock samples were assigned to nine age groups: ~2.2 Ga, ~946 Ma, ~519 Ma, ~440 Ma, ~413 Ma, ~385 Ma, ~160 Ma, ~145 Ma, and ~132 Ma. This complex zircon age spectrum corresponds to regional magmatic or metamorphic events from the Paleoproterozoic to the Early Cretaceous. We determined that the Mokou dikes are dominantly alkaline rocks that belong to the shoshonite and high-K calc-alkaline series. The Mokou dike samples were enriched in large ion lithophile elements (e.g., Rb and Ba) and presented pronounced negative anomalies in the high-field-strength elements (e.g., Nb, Ta, Zr, and Hf). Both diabase and lamprophyre in the Mokou area were enriched in light rare earth elements (11.9 < (La/Yb)N < 30.8) with no obvious Eu anomalies (δEu = 0.92–1.03). They had high initial 87Sr/86Sr ratios (0.70625–0.71433), negative ε Nd (t) values (−12.9 to −8.3), and slightly variable initial ratios of 206Pb/204Pb (17.539–18.185), 207Pb/204Pb (15.518–15.693), and 208Pb/204Pb (38.090–38.996). Our results indicate that the Mokou dikes originated from an enriched lithospheric mantle and a contaminated asthenosphere. Continental crust materials in the magmas/fluid reservoirs were involved during the rapid intrusion of the Mokou mafic magma, as opposed to the assimilation of wall rocks in the ascent conduit. We conclude that the Mokou dikes are products of lithosphere delamination, which occurred ca. 132 Ma in the NQOB, thus suggesting that lithospheric thinning of the NCC also affected the NQOB.

Zircon U–Pb age and geochemical constraints on the origin and tectonic implication of the Tuotuohe Cenozoic alkaline magmatism in Qinghai–Tibet Plateau

A large number of Eocene–Oligocene alkaline/alkali-rich igneous rocks were developed in the Tuotuohe region of the Qinghai-Tibetan Plateau. In this study, we present zircon U–Pb ages, Hf isotope data, and major and trace element compositions of the Cenozoic alkaline rocks from the Tuotuohe region in order to constraint the petrogenesis and tectonic evolution history of Qiangtang Block. Zircon U–Pb ages were measured via LA–ICP–MS to be 39.6, 37.6 and 32.0 Ma. The 39.6 Ma trachyte was characterized by low SiO2 and high K2O and MgO contents. The 37.6 and 32.0 Ma orthophyres show enrichment in SiO2 and K2O, but deficient in MgO. All of the samples belong to the alkaline rocks. These rocks display enrichment in REE, LREE, and LILE, depletion in HFSE, and no obvious Eu anomalies. Geological and geochemical features suggest that there were two possible mechanisms for the origin of the alkaline rocks in the Tuotuohe region: (1) the removed mafic lower crust dropped into the asthenosphere, forming the mix magma (Nariniya trachyte); (2) the upwelling asthenosphere triggered the crustal melting (Nariniya and Zamaqu orthophyre). The Eocene–Oligocene alkaline rocks in the study and adjacent areas are likely to be the result of the same tectonic–magmatic event of deep lithospheric evolution that is the crustal material melting triggered by lithospheric delamination. This conclusion extends the influence scope of lithospheric delamination eastward to the Tuotuohe region (~ 92°E) from Banda Co (~ 82°E).

Middle–Upper Ordovician radiolarians in Hunan and Jiangxi Provinces, South China: Implications for the sedimentary environment and nature of the Nanhua basin

The Yanxi and Duiershi formations of the Middle-Upper Ordovician in South China are composed of black, grayish black, and dark gray thin-bedded cherts and siliceous slates, rich in carbonaceous materials and pyrite. Horizontal beddings are developed. The strata contain randomly distributed fossils. Radiolarians are discovered in the Loudi, Yongzhou, Hengyang, Ji'an and Ganzhou sections. The Radiolaria from two sections are identified as Protoentactinia sp. Abundances of radiolarian in the central area of Hunan and Jiangxi Provinces are higher than in the southern area. The strata become thickened while the amounts of siliceous materials are decreased and argillaceous contents are increased from northwest to southwest for the Yanxi and Duiershi formations. The lithology gradually changes from carbonate rocks in the north to clastic rocks in the south. δ30Si values of chert and siliceous slate range from −0.6 to 1.18 with Al2O3/(Al2O3 + Fe2O3), MnO/TiO2, LaN/CeN, Ce/Ce*, and (La/Yb)N ratios of 0.65–0.97, 0.01–0.16, 0.99–1.27, 0.85–1.10, and 0.91–1.66, respectively. The SiO2 and TiO2 average contents are 89.08% and 0.27%. The SiO2 contents decrease while the TiO2 increase from northwest to southeast. They have similar the REE concentration with no obvious Eu anomalies. The Yanxi and Duiershi formations were deposited predominately in a bathyal environment on the continental slope–basin and the seawater was stagnant. In the Middle–early Late Ordovician, the Nanhua basin occupied a rather large area and sedimentary rocks in the region around Hengyang–Ji'an were deposited in the basin center with terrigenous source derived from the southeast.

Age and geochemistry of Early Ordovician A-type granites in the Northeastern Songnen Block, NE China

Early Ordovician A-type granites in the northeastern (NE) Songnen Block NE China were studied to better understand the geodynamic settings in this region. This research presents new zircon U–Pb ages and whole-rock geochemical data for the Early Ordovician granites in the NE Songnen Block. Zircon U–Pb dating indicates that the granite in the Cuibei, Hongxing, and Meixi areas in the NE Songnen Block formed in the Early Ordovician with ages of 471–479 Ma. The granites show geochemical characteristics of high SiO2 and K2O compositions and low FeOT, MgO, CaO, and P2O5 compositions. They belong to a high K calc-alkaline series and display a weak peraluminous feature with A/CNK values of 0.98–1.14. The rocks have a ∑REE composition of 249.98–423.94 ppm, and are enriched in LREE with (La/Yb)N values of 2.87–9.87, and display obvious Eu anomalies (δEu = 0.01–0.29). Trace elements of the studied granites are characterized by enrichment in Rb, Th, U, Pb, Hf, and Sm, and depletion of Ba, Nb, Ta, and Sr. They display geochemical features of high Zr + Y + Nb + Ce values (324–795 ppm) and Ga/Al ratios consistent with A-type granites. Based on particular geochemical features, such as high Rb/Nb (7.98–24.19) and Y/Nb (1.07–3.43), the studied A-type granites can be further classified as an A2-type subgroup. This research indicates that the Early Ordovician A-type granites were formed by the partial melting of ancient crust in an extensional setting. Lower Sr/Y and (Ho/Yb)N ratios indicate that plagioclase and amphibole are residual in the source, and garnet is absent, implying that the magma was generated at low levels of pressure. By contrast, the contemporaneous granites in the SE Xing’an Block suggest a subduction-related tectonic setting, and its adakitic property indicates a thickened continental crust. We suggest that the Paleo-Asian Ocean plate between the Xing’an and Songnen blocks subducted northward during the Early Ordovician. Meanwhile, the NE Songnen Block was exposed to a passive continental margin tectonic setting.

Geochemical characteristics and geological significance of Cretaceous phonotephrite from the Mid-Pacific Mountains

Systematic analyses were conducted including the petrographic features, major and trace elements, Sr and Nd isotopic compositions, and mineral structure and compositions of whole rocks. Mid-Pacific Mountain volcanic rocks are mainly phonotephrite with a porphyritic texture. Phenocrysts are mainly composed of Ca-rich plagioclase, clinopyroxene and nepheline. These volcanic rocks are significantly rich in large-ion lithophile and light rare earth elements, without obvious Eu anomalies ( δ Eu=0.99–1.03), and with relatively enriched 87Sr/86Sr (0.703829–0.704313) and 143Nd/144Nd isotopic ratios (0.512857–0.512871), suggesting that they have similar but more enriched features than the OIB magmatic source. These volcanic rocks may originate from relatively deep magma source with the existence of spinel-garnet Iherzolites, and have undergone partial melting at a low degree of 1–3%. In addition, The residual Nb-Ta minerals (such as sphene, rutile, perovskite) may remain in the mantle source, and the magma components have undergone metasomatism by carbonate melt/fluid or alkali-rich fluid, causing high contents of incompatible elements and significant loss of Nb, Ta and Ti in these volcanic rocks. There are many similarities between the phonotephrites in the Mid-Pacific Mountain and the volcanic rocks in the Line Islands based on the tectonic settings and the geochemical characteristics. We thus speculate that Site 313 volcanic rocks in the Mid-Pacific Mountain is most likely to be a continuation of the Line Islands.

Identifying Early Carboniferous bimodal volcanic rocks and geochemical characteristics in the Atengtao Mountain, Yili Block (Chinese western Tianshan)

The Early Carboniferous bimodal volcanic rocks are firstly recognized in the Atengtao Mountain, Yili Block (Chinese Western Tianshan). The bimodal volcanic rocks are composed of dominant rhyolite and subordinate basaltic andesite. The SiO2 contents of basaltic andesites span a range of 52.08 to 60.44 wt.%, whereas those of the rhyolites range from 72.50 to 76.78 wt.%, showing a sharp gap between 60.44 and 72.50 wt.% for SiO2 content. Zircon U–Pb dating of rhyolitic sample yielded crystallization age of 340 ± 6 Ma, which is interpreted as the extrusive age of the bimodal volcanic rocks. Basaltic andesite samples belong to the calc‐alkaline series, whereas rhyolites are peraluminous. The basaltic andesite samples display enrichment in LREE ((La/Yb)N = 3.16–6.27), and nearly no obvious Eu anomalies (δEu = 0.86–1.21), with relative enrichment in large ion lithophile elements (Rb, K, Sr, U) and depletion in high field strength elements (Nb, Ta). Compared to the basaltic andesites, the rhyolites show enriched LREE patterns ((La/Yb)N = 7.05–9.27) and significantly negative Eu anomalies (δEu = 0.36–0.62), with remarkably negative Nb, Ta, P, Ti, and Sr anomalies, which is consistent with A‐type granites. The rhyolites have positive εHf(t) values from 1.5 to 4.8 and TDM2(Hf) values from 1,036 to 1,247 Ma. The basaltic andesites are interpreted as resulting from partial melting of an enriched lithospheric mantle source that was metasomatized by subduction‐related components, whereas the rhyolites were derived from basaltic melt‐induced reworking of Proterozoic juvenile crustal material. Based on our data, and taking into account the regional geology, a model of back‐arc setting was proposed for the Early Carboniferous formation of the bimodal volcanic rocks in the Atengtao Mountain. Therefore, we suggest that the volcanic rocks were likely induced by the northward subduction of the South Tianshan Ocean. The subduction resulted in opening of a back‐arc basin in Atengtao Mountain, leading to upwelling of mantle that had produced the basaltic andesites. Moreover, this process provided heat to remelt the crustal materials and form the rhyolites.

Oceanic island basalts in ophiolitic mélanges of theCentralChinaOrogen: An overview

We present an overview of the internal structure of the 10 ophiolitic mélanges in the Central China Orogen (CCO) with a focus on the geochemical character and tectonic evolution of the ophiolitic mélange‐related ocean island basalt (OIB) and mafic rock assemblages. The ophiolitic mélanges in CCO are generally complicated and usually consist of metamorphic peridotites (serpentinite), cumulates, gabbros, basaltic lavas (pillows), and abyssal radiolarian cherts. The ages of ophiolitic mélanges range from Mesoproterozoic to Carboniferous. The OIB‐type basalts and mafic rocks in CCO occur as tectonic blocks within the mélanges that are composed of limestones, radiolarian cherts, and turbidites, possessing formation characteristics of seamounts (oceanic islands/plateau). The mafic rocks in ophiolitic mélanges of CCO display uniform chondrite‐normalized rare earth element (REE) patterns with light REE enrichment and heavy REE depletion, no obvious Eu anomalies or negative Nb, Ta, and Ti anomalies, and primitive mantle‐normalized trace element patterns with significant large‐ion lithophile element enrichment, similar to those of modern OIB and the Hawaiian alkaline basalts. The OIB‐type basalts and mafic rocks are considered as accreted seamount fragments in an accretionary complex of CCO and may represent plume‐related magmatism within the Proto‐Tethys Ocean and Paleo‐Tethys Ocean. Our study provides further insights into the processes of multiple subduction and long‐lasting accretionary histories with seamounts in the CCO.

Paleoproterozoic Nb–enriched meta-gabbros in the Quanji Massif, NW China: Implications for assembly of the Columbia supercontinent

Diverse models have been proposed for the role of the Tarim Craton within the Paleoproterozoic Columbia supercontinent assembly. Here we report a suite of ∼1.71 Ga Nb-enriched meta–gabbro lenses in the eastern Quanji Massif, within the Tarim Craton in NW China. The meta–gabbroic rocks have Nb contents of 11.5–16.4 ppm with Nb/La ratios varying from 0.84 to 1.02 ((Nb/La)N = 0.81–0.98) and Nb/U ratios from 38.0 to 47.2. They show low SiO2 (45.1–48.5 wt.%) and MgO (5.96–6.81 wt.%) and Mg# (Mg# = Mg/(Mg + Fe) = 43.5–47.7), high FeOt (13.0–15.7 wt.%) and moderate TiO2 (1.70–2.51 wt.%), with tholeiitic affinities. These rocks possess low fractionated REE patterns without obvious Eu anomalies (Eu/Eu* = 0.87–1.02). Their primitive mantle-normalized elements patterns display significant Zr–Hf troughs, positive Nb anomalies, weak negative Ti and P anomalies, and high contents of Rb and Ba, resembling Nb-enriched basalts generated in arc-related tectonic settings. Their arc–like geochemical signatures together with whole rock ɛNd(t) values of 0.4–2.1 and corresponding old TDM (2.22–2.37 Ga) as well as (143Nd/144Nd)t and (87Sr/86Sr)t (t = 1712 Ma) values of 0.5104–0.5105 and 0.7030–0.7058, respectively, suggest that their precursor magma originated from mantle wedge peridotite metasomatised by subduction-derived melts. The results from our study reveal subduction along the eastern periphery of the Tarim Craton and marginal outgrowth continuing to ∼1.7 Ga within the Columbia supercontinent.

Geochronology, geochemistry, and tectonic implications of Jishou Cretaceous diabase, western Xuefengshan tectonic zone in South China

From the east of the Xuefengshan tectonic zone (XTZ) to the Pacific coast of the South China Block, there exist widespread Mesozoic magmatic rocks, which attract a great deal of attention for their forming mechanism and evolutional history. Among them, the Mesozoic Jishou diabase located at the west of the XTZ is reported in this study. Based on our geochronologic analysis, the diabase has a U–Pb age of 134 ± 2.3 Ma. The diabase belongs to calc‐alkaline series in a SiO2‐K2O diagram and illustrates significant enrichment in light rare earth elements and flat heavy rare earth elements without obvious Eu anomalies. Meanwhile, the diabase has negative εHf(t) and εNd(t) values and higher radiogenic 87Sr/86Sr(t) ratios, suggestive of EM2‐like Sr‐Nd isotopic compositions. The diabase shows high variations in 207Pb/204Pb(t; 15.609–15.671), 206Pb/204Pb(t; 17.943–18.742), and 208Pb/204Pb(t; 38.268–39.302). It is suggested that the Jishou diabase may be generated from the lithospheric mantle in response to the decompression melting accompanied by lithospheric extension during Pacific subduction process. During the Early Cretaceous (145–120 Ma), the upwelling and melting of the mantle occurred under the XTZ, causing intraplate Jishou diabase magma. Subsequently, the significantly descending of the subducted Paleo‐Pacific slab led progressively eastward generation and migration of subduction‐related magmas, resulting in a widespread distribution of igneous rocks in the Cathaysia Block.

Petrogenesis of ca. 1.95 Ga meta-leucogranites from the Jining Complex in the Khondalite Belt, North China Craton: Water-fluxed melting of metasedimentary rocks

Anatexis occurs in response to changing P-T-fluid conditions during orogenesis, is thus crucial for understanding the tectonic evolution of orogenic belts. Here we report an integrated petrological, geochemical, whole-rock oxygen isotopic and zircon U-Pb-Hf-O isotopic study of meta-leucogranites from the Jining Complex, North China Craton, to unravel its petrogenesis and constrain the tectonic evolution of the Paleoproterozoic Khondalite Belt. The meta-leucogranites are weakly peraluminous with homogeneous minimum-melt compositions, and are characterized by low Rb contents and Rb/Sr ratios, high Ba and Sr contents and Sr/Y ratios, and moderately fractionated REE patterns without obvious Eu anomalies. These trace-element characteristics differ from that of leucogranites derived from fluid-absent melting. Considering their low whole-rock Zr saturation temperatures (746–780°C) and comparable compositions with experimentally generated melts produced by water-fluxed melting, we infer that the meta-leucogranites are low-temperature granites, and were generated by water-fluxed melting of metasedimentary rocks at 6–8kbar. SIMS zircon U-Pb analyses suggest that the meta-leucogranites were emplaced at∼1.95Ga or slightly earlier, and were subsequently metamorphosed and cut by a pegmatite dike at∼1.92Ga. The abundant inherited zircons yield nearly concordant 207Pb/206Pb ages ranging from 2.24 to 1.98Ga, which are consistent with the detrital zircon age spectra of the metasedimentary rocks from the Khondalite Belt. The Hf isotopic compositions of inherited and magmatic zircons (εHf(t)=+0.2 to +7.7) closely overlap with the time evolved εHf range of detrital zircons from metasediments in the Khondalite Belt. Different types of zircons (inherited, magmatic and metamorphic zircons) have similar δ18O values of 7.2–9.2‰, which is lower than that of metamorphic zircons (10.0–13.6‰) from surrounding metasediments. Thus, we propose that the meta-leucogranites were formed by water-fluxed melting of metasedimentary rocks from the Khondalite belt at∼1.95Ga during the prograde upper amphibolite-facies metamorphism.

High-Mg adakitic rocks and their complementary cumulates formed by crystal fractionation of hydrous mafic magmas in a continental crustal magma chamber

Understanding how adakitic magmas form is important for understanding the formation of the continental crust. Generating such high-Sr/Y rocks by crystal fractionation of basalts/basaltic andesites in magma chambers has been proposed in a wide range of tectonic settings. However, the complementary cumulates predicted by this scenario have rarely been observed. The late Triassic (~227Ma) Ningcheng complex from the North China Craton is composed of a websterite - (Ol−/Hbl-) pyroxenite - gabbro unit and a quartz-diorite unit. They are interpreted as the products (cumulates and derivative melts, respectively) of fractionation from hydrous mafic magmas at mid- to lower-crustal pressures (4.9~8.3kbar). The quartz diorites are high-Mg intermediate rocks with moderate SiO2 (57.0~62.9wt%), high Mg# (>49) and adakitic trace element signatures, such as high Sr (≥636ppm) and light rare earth elements (REEs), low Y (≤17ppm) and heavy REEs (Yb≤1.8ppm), lack of obvious Eu anomalies, and high Sr/Y (≥31) and La/Yb (≥24)). These adakitic signatures reflect differentiation of hydrous mantle-derived magmas in the deep crust, leaving behind a plagioclase-free residual solid assemblage in the early stages, which is represented by the coeval websterite-pyroxenite complex. This study therefore not only demonstrates that hydrous crystal fractionation is an important mechanism to form adakitic rocks, but also presents an example of a preserved fractionating system, i.e. high-Sr/Y rocks and their complementary cumulates. A geochemical comparison is made between representative adakitic rocks formed by fractionation of hydrous magmas and Archean TTGs. It is suggested that crystal fractionation is an efficient process for making Phanerozoic high Sr/Y rocks but was not responsible for the formation of Archean granitoids.

OIB- and P-type ophiolites along the Yarlung- Zangbo Suture Zone (YZSZ), Southern Tibet: Poly- Phase melt history and mantle sources of the Neotethyan oceanic lithosphere

We present an overview of the internal structure of the ophiolite massifs along the Yarlung Zangbo suture zone (YZSZ) in southern Tibet with a focus on the geochemical character and tectonic evolution of the Ocean Island Basalt (OIB) and mafic alkaline rock assemblages associated with these ophiolites. The Jurassic - early Cretaceous lavas, massive diabase and gabbroic rocks are either tectonically intercalated with the early Cretaceous, subduction-influenced ophiolitic units, or occur as thrust sheets or blocks with an early Cretaceous melange and in a Jurassic-Cretaceous flysch unit structurally beneath these ophiolites. They display uniform chondrite-normalized REE patterns with light rare earth element (LREE) enrichment and heavy rare earth element (HREE) depletion, no obvious Eu anomalies or negative Nb, Ta and Ti anomalies, and primitive mantle normalized trace element patterns with significant large-ion lithophile element (LILE) enrichment, similar to those of modern OIB and the Hawaiian alkaline basalts. These mafic alkaline rock assemblages represent OIB- and Plume-type (P-type) oceanic crustal rocks (with no subduction influence) that formed from magmas produced by partial melting of plume-metasomatized asthenospheric mantle source during the early stages of the opening of a Neotethyan seaway between Proto-India and Eurasia. Subsequent consumption of this OIB-P-type mid-ocean ridge (MOR) oceanic lithosphere at an intraoceanic subduction zone produced the ~130-120 Ma forearc to backarc, SSZ oceanic crust within the same Neotethys. The evolutionary history of the YZSZ ophiolites thus reflects a poly-phase melt history and different mantle melt sources. The final tectonic juxtaposition of the older OIBand P-type oceanic crustal rocks with SSZ-type oceanic lithosphere fragments took place as the northern passive continental margin of Proto-India collided with and underplated the intraoceanic subduction-accretion system in the late Cretaceous. The YZSZ displays a complete Wilson cycle record of the rift-drift, seafloor spreading and subduction zone tectonic evolution of the Mesozoic Neotethys.

Zircon U–Pb–Hf isotopes and geochemistry of two contrasting Neoarchean charnockitic rock series in Eastern Hebei, North China Craton: Implications for petrogenesis and tectonic setting

The Zunhua–Qinglong microblock in Eastern Hebei, North China Craton, exposes two contrasting charnockitic rock series in the Yuhuzhai–Taipingzhai and Cuizhangzi areas: charnockitic plagioclase gneiss series with chemical compositions of dioritic and tonalitic rocks, and charnockite series showing tonalitic and granodioritic components. Petrographic observation reveals that two types of orthopyroxenes are preserved in these rocks. Medium-fine grained anhedral orthopyroxenes are preserved in both series, and coarse grained subhedral orthopyroxenes are preserved only in the charnockite series. In situ trace element analyses reveal that the coarse grained subhedral orthopyroxenes exhibit typical chemical features of magmatic orthopyroxene, whereas the medium-fine grained anhedral orthopyroxenes display lower medium to heavy rare earth elements (REE) and no obvious Eu anomalies, resulting in left-inclined REE patterns. Combined with petrographic features, these new data indicate that the medium-fine grained anhedral orthopyroxenes were formed by granulite–facies metamorphism caused by dehydration reactions, whereas the coarse grained subhedral orthopyroxenes were directly crystallized from their magmatic precursors. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) zircon U–Pb isotopic dating reveals that the magmatic precursors of the charnockitic plagioclase gneiss series were emplaced between 2530±17 and 2523±12Ma, and those of the charnockite series were emplaced between 2527±28 and 2515±22Ma. We conclude that these magmatic precursors are contemporaneous emplacements of two Neoarchean granitoid magmatic events, which were followed by regional granulite–facies metamorphism at ∼2.45Ga.The charnockitic plagioclase gneiss series and charnockite series exhibit similar ɛHf(t) values of −2.4 to +5.3 and −3.2 to +5.5, indicating either depleted mantle or juvenile crust affinity, and involvement of ancient crustal components. The charnockitic plagioclase gneiss series exhibit relatively low SiO2 contents (54.1–65.2%), with various MgO contents (1.93–5.36%), Mg# (0.36–0.55), and (La/Yb)N values (3.49–56.00); the charnockite series exhibit higher SiO2 contents (60.1–69.2%), with similar (La/Yb)N values (14.61–42.69), MgO contents (2.10–3.99%), and Mg# (0.29–0.52). These geochemical and zircon Lu–Hf isotopic features, together with chemical modeling, suggest that the magmatic precursors of the charnockitic plagioclase gneiss series may have been primarily derived from the partial melting of a depleted mantle that was metasomatized by slab-derived melts/fluids, and experienced fractional crystallization processes with hornblende and plagioclase as primary fractionated phases. The aforementioned features further suggest that the charnockite series originated from a partial melting of the subducted slab, and it was strongly contaminated by mantle peridotites. These new data, combined with the previous studies of Neoarchean basement rocks in the Zunhua–Qinglong microblock in Eastern Hebei, suggest that the magmatic precursors of these two contrasting charnockitic rock series were formed in a back-arc-related tectonic environment at a convergent plate margin.

Early Permian–Late Triassic Magmatism in the Tuotuohe Region of the Qinghai–Tibet Plateau: Constraints on the Tectonic evolution of the Western Segment of the Jinshajiang Suture

Abstract:In this paper we present new zircon U–Pb ages, whole‐rock major and trace element analyses, and zircon Hf isotopic data for magmatic rocks in the Tuotuohe region of the western segment of the Jinshajiang suture. Our aim is to constrain the Early Permian–Late Triassic tectonic evolution of the region. Zircons from the magmatic rocks of the Tuotuohe region are euhedral–subhedral in shape and display fine‐scale oscillatory zoning as well as high Th/U ratios (0.4–4.6), indicating a magmatic origin. The zircon U–Pb ages obtained using LA–ICP–MS are 281 ± 1 Ma, 258 ± 1 Ma, 244 ± 1 Ma, and 216 ± 1 Ma, which indicate magmatism in the Early Permian–Late Triassic. A diorite from Bashihubei (BSHN) has SiO2= 57.18–59.97 wt%, A12O3= 15.70–16.53 wt%, and total alkalis (Na2O + K2O) = 4.46–6.34 wt%, typical of calc‐alkaline and metaluminous series. A gabbro from Bashibadaoban (BSBDB) belongs to the alkaline series, and is poor in SiO2 (45.46–54.03 wt%) but rich in A12O3 (16.19–17.39 wt%) and total alkalis (Na2O + K2O = 5.48–6.26 wt%). The BSHN diorite and the BSBDB gabbro both display an enrichment of LREEs and LILEs and depletion of HFSEs, and they have no obvious Eu anomaly; they have relatively low MgO contents (2.54–4.93 wt%), Mgs# values of 43 to 52, and low Cr and Ni contents (8.07–33.6 ppm and 4.41–14.2 ppm, respectively), indicating they differentiated from primitive mantle magmas. They have low Nb/U, Ta/U, and Ce/Pb ratios (1.3–9.6, 0.2–0.8, and 0.1–18.1, respectively), and their initial Hf isotopic ratios range from +9.6 to +16.9 (BSHN diorite) and +6.5 to +12.6 (BSBDB gabbro), suggesting their primary magmas were derived mainly from the partial melting of a mantle wedge that had been metasomatized by subduction fluids. Taking all the new data together, we conclude that the western and eastern segment of the Jinshajiang suture regions underwent identical processes of evolution in the Early Permian–Late Triassic: oceanic crust subduction before the Early Permian, continental collision during the Early–Middle Triassic, and post‐collisional extension from the Late Triassic.

Timing and genesis of the adakitic and shoshonitic intrusions in the Laoniushan complex, southern margin of the North China Craton: Implications for post-collisional magmatism associated with the Qinling Orogen

The NWW-striking Qinling Orogen formed in the Triassic by collision between the North China and Yangtze Cratons. Triassic granitoid intrusions, mostly middle- to high-K, calc-alkaline, are widespread in this orogen, but contemporaneous intrusions are rare in the southern margin of the North China Craton, an area commonly considered as the hinterland belt of the orogen. In this paper, we report zircon U–Pb ages, elemental geochemistry, and Sr–Nd–Hf isotope data for the Laoniushan granitoid complex that was emplaced in the southern margin of the North China Craton. Zircon U–Pb dating shows that the complex was emplaced in the late Triassic (228 ± 1 to 215 ± 4 Ma), indicating that it is part of the post-collisional magmatism in the Qinling Orogen. The complex consists of, from early to late, biotite monzogranite, quartz diorite, quartz monzonite, and hornblende monzonite, which span a wide compositional range, e.g., SiO 2 = 55.9–70.6 wt.%, K 2O + Na 2O = 6.6–10.2 wt.%, and Mg # of 24 to 54. The biotite monzogranite has high Al 2O 3 (15.5–17.4 wt.%), Sr (396–1398 ppm) and Ba (1284–3993 ppm) contents and relatively high La/Yb (mostly 14–30) and Sr/Y (mostly 40–97) ratios, but low Yb (mostly 1.3–1.6 ppm) and Y (mostly14–19 ppm) contents, features typical of adakitic rocks. The quartz monzonite, hornblende monzonite and quartz diorite have a shoshonitic affinity, with K 2O up to 5.58 wt.% and K 2O/Na 2O ratios averaging 1.4. The rocks are characterized by strong LREE/HREE fractionation in chondrite-normalized REE pattern, without obvious Eu anomalies, and show enrichment in large ion lithophile elements but depletion in high field strength elements (Nb, Ta, Ti). The biotite monzogranite (228 Ma) has initial 87Sr/ 86Sr ratios of 0.7061 to 0.7067, ε Nd(t) values of − 9.2 to − 12.6, and ε Hf(t) values of − 9.0 to − 15.1; whereas the shoshonitic granitoids (mainly 217–215 Ma) have similar initial 87Sr/ 86Sr ratios (0.7065 to 0.7075) but more radiogenic ε Nd(t) (− 12.4 to − 17.0) and ε Hf(t) (− 14.1 to − 17.0). The Sr–Nd–Hf isotope data indicate that the rocks were likely generated by partial melting of an ancient lower continental crust with heterogeneous compositions, as partly confirmed by the widespread presence of early Paleoproterozoic inherited zircons. Mafic microgranular enclaves (MMEs), characterized by fine-grained igneous textures and an abundance of acicular apatites, are common in the Laoniushan complex. Compared with the host rocks, they have lower SiO 2 (48.6–53.7 wt.%) and higher Mg # (51–56), Cr (122–393 ppm), and Ni (24–79 ppm), but equivalent Sr–Nd isotope compositions, indicating that the MMEs likely originated from an ancient enriched lithospheric mantle. The abundance of MMEs in the granitoid intrusions suggest that magma mixing plays an important role in the generation of the Laoniushan complex. Collectively, it is suggested that the Laoniushan complex was a product of post-collisional magmatism related to lithospheric extension following slab break-off. Formation of the adakitic and shoshonitic intrusions in the Laoniushan complex indicates that the Qinling Orogen had evolved into a post-collisional setting by about 230–210 Ma.

Geology, geochemistry, and geochronology of the Miaowan ophiolite, Yangtze craton: Implications for South China's amalgamation history with the Rodinian supercontinent

We report the presence of a Grenvillian ophiolite on the northern margin of the Yangtze craton, drastically changing current ideas about South China's role in plate reconstructions of the Rodinia supercontinent. Strongly deformed amphibolites that locally show relict pillow lavas, isotropic and layered metagabbro, diabase dikes, serpentinized dunite and harzburgite with podiform chromite are dated at circa 1100–985Ma (U–Pb zircon). The ophiolite is structurally dismembered and thrust over the Proterozoic shelf sequence that covers the north margin of the Yangtze craton, and overrode a flysch to conglomerate-wildflysch unit shed from the ophiolite and a magmatic arc terrane and deposited on the older Yangtze carbonate platform. The youngest clasts in the conglomerate are circa 861–813Ma (U–Pb zircon), giving a maximum age for ophiolite emplacement. Fine-grained layered amphibolites exhibit slightly depleted-flat type REE curves with no obvious Eu anomalies, and are N-MORB type tholeiites. Metagabbro has typical cumulate textures, flat REE distributions and obvious positive Eu anomalies. The REE characteristics of serpentinized dunites show a U-shape of slight loss of middle REE, representing cumulates metasomatized by LREE slightly enriched mantle. All these features indicate that the metamafic–ultramafic rocks from the Proterozoic Miaowan Formation form a structurally dismembered ophiolite resting above an ophiolitic wildflysch, sitting on top of the Proterozoic shelf sequence on the Yangtze craton. The ophiolite is contemporaneous with an arc sequence preserved to the north on the edge of the Yangtze craton, suggesting that the entire ophiolitic forearc–arc was accreted to the Yangtze craton between 1000 and 850Ma. Xenocrystic zircons in granite clasts in the basal wildflysch unit have ages consistent with Australian affinity, and detrital zircons in the arc sequence also show derivation from Australia, suggesting that the arc formed on the Australian segment of Rodinia before collision with the Yangtze craton. The discovery of the Proterozoic Miaowan ophiolite supplies important evidence for the existence of a Neoproterozoic oceanic basin on the north margin of the Yangtze craton, and demonstrates that the Yangtze craton first collided with Rodinia on its northern margin, with subsequent accretion of the Cathaysian block on the southern margin of the craton.

Rare Earth Element and Trace Element Features of Gold‐bearing Pyrite in the Jinshan Gold Deposit, Jiangxi Province

Abstract:Jinshan gold deposit is located in northeastern Jiangxi, South China, which is related to the ductile shear zone. It has a gold reserve of more than 200 tons, with 80% of gold occurring in pyrite. The σREE of gold‐bearing pyrite is as higher as 171.664 ppm on average, with relatively higher light rare earth elements (LREE; 159.556 ppm) and lower HREE (12.108 ppm). The σLREE/σHREE ratio is 12.612 and (La/Yb)N is 11.765. These indicate that pyrite is rich in LREE. The (La/Sm)N ratio is 3.758 and that of (Gd/Yb)N is 1.695. These are obvious LREE fractionations. The rare earth element (REE) distribution patterns show obvious Eu anomaly with average δEu values of 0.664, and δCe anomalies of 1.044. REE characteristics are similar to those of wall rocks (regional metamorphic rocks), but different from those of the Dexing granodiorite porphyry and Damaoshan biotite granite. These features indicate that the ore‐forming materials in the Jinshan gold deposit derived from the wall rocks, and the ore‐forming fluids derived from metamorphic water. The Co/Ni ratio (average value 0.38) of pyrite suggests that the Jinshan gold deposit formed under a medium–low temperature. It is inferred from the values of high‐field strength elements, LREE, Hf/Sm, Nb/La, and Th/La of the pyrite that the ore‐forming fluids of the Jinshan gold deposit derived from metamorphic water with Cl&gt;F.

Late Mesozoic magmatism from the Daye region, eastern China: U–Pb ages, petrogenesis, and geodynamic implications

Late Mesozoic dioritic and quartz dioritic plutons are widespread in the Daye region, eastern Yangtze craton, eastern China. Detailed geochronological, geochemical, and Sr–Nd isotopic studies have been undertaken for most of these plutons, in an attempt to provide a comprehensive understanding in the age, genesis and geodynamical control of the extensive magmatism. SHRIMP and LA-ICP-MS zircon U–Pb dating indicate that the plutons were emplaced in the range of latest Jurassic (ca. 152 Ma) to early Cretaceous (ca. 132 Ma), which was followed by dyke emplacement between 127 and 121 Ma and volcanism during the 130–113 Ma interval. Both diorites and quartz diorites are sodic, metaluminous, high-K calc-alkaline, and characterized by strongly fractionated, sub-parallel REE patterns without obvious Eu anomalies. The rocks are enriched in highly incompatible elements and large ion lithophile elements, but depleted in high field strength elements. Samples of diorite and quartz diorite have similar Sr–Nd isotopic compositions that are consistent with the early Cretaceous basalts and mafic intrusions throughout the eastern Yangtze craton. The geochemical and isotopic data, together with results of geochemical modeling, indicate an enriched mantle source for the plutonic rocks. The quartz diorites have geochemical signatures resembling adakites, such as high Al2O3 (15–19 wt.%), Sr (630–2,080 ppm), Na2O (>3.5 wt.%), negative Nb–Ta anomalies, low Y (7–19 ppm), Yb (0.5–1.8 ppm), Sc (5–15 ppm), and resultant high Sr/Y (45–200) and La/Yb (31–63) ratios. Genesis of the adakitic quartz diorites is best explained in terms of low-pressure intracrustal fractional crystallization of cumulates consisting of hornblende, plagioclase, K-feldspar, magnetite, and apatite from mantle-derived dioritic magmas. Mantle-derived magmatism broadly coeval with that of the Daye region also is widespread in other regions of the eastern Yangtze craton, reflecting large-scale melting of the lithospheric mantle during the Late Mesozoic. The large-scale magmatism was most likely driven by lithospheric extension associated with thinning of lithospheric mantle beneath the eastern China continent.

REE geochemical characteristics of apatite, sphene and zircon from alkaline rocks

The accessory minerals apatite and sphene are the main carriers of REE in alkaline rocks. Their chondrite-normalized REE patterns decline sharply to the right as those of the host rocks. In the patterns an obvious negative Eu anomaly and a positive Ce anomaly can be seen in apatite and sphene, respectively. Zircon from alkaline rocks is different in REE pattern, i. e., a nearly symmetric “V” -shaped pattern with a maximum negative Eu anomaly. Compared with the equivalents from granites, apatite, sphene and zircon from alkaline rocks are all characterized by higher (La/Yb)n ratio and less Eu depletion. As to the relative contents of REE in minerals, apatite, sphene and zircon are enriched in LREE, MREE and HREE respectively, depending on their crystallochemical properties.