Significant Eu Anomalies Research Articles

Origin of rhythmic anorthositic–pyroxenitic layering in the Damiao anorthosite complex, China: Implications for late-stage fractional crystallization and genesis of Fe–Ti oxide ores

The ∼1.7Ga Damiao anorthosite complex (DAC) in the North China Craton contains abundant Ti–magnetite-dominated ore deposits. Both the Fe–Ti–P-rich silicate rocks and massive Fe–Ti–(P) ores occur as discordant late-stage dikes cross-cutting early-stage anorthosites with irregular but sharp boundaries. Field and petrographic observations indicate that some late-stage dikes are composed of unique oxide–apatite gabbronorites (OAGNs), whereas others comprise well-developed alternating late-stage anorthosites and Fe–Ti–P-rich pyroxenites defining rhythmic layers. Massive Fe–Ti–(P) ores are closely related to the Fe–Ti–P-rich pyroxenites. Plagioclase and whole-rock compositions of different rock types were analyzed to constrain the late-stage magma evolution and genesis of the Fe–Ti oxide ores. The similar mineralogical assemblages, REE and HFSE patterns suggest that the different rock types formed by differentiation from a common parental magma. Early-stage anorthosites are characterized by positive Eu anomalies and low REE contents, whereas the late-stage dike-like rocks display no significant Eu anomalies and high REE contents. Plagioclase compositions in the late-stage rocks show a decrease of An contents when compared to that of the early-stage rocks. Based on field relations, petrography and well-defined linear compositional trends, the sequence of crystallization is inferred as: early-stage anorthosites+leuconorites+norites, OAGNs, late-stage anorthosites+Fe–Ti–P-rich pyroxenites+massive Fe–Ti–(P) ores, and massive Fe–Ti–(P) ores. The OAGNs which underwent relatively rapid crystallization represent an early phase during the residual magma evolution after anorthosite separation, whereas the rhythmic layers formed by slow but extensive fractional crystallization of interstitial melt. High solubility of phosphorous played an important role in the formation of rhythmic layering. Massive Fe–Ti–(P) ores crystallized and segregated directly from the magma of Fe–Ti–P-rich pyroxenites, thus representing the most evolved products of the residual magma. Progressive and extensive fractional crystallization of residual ferrobasaltic magma is probably responsible for the formation of large-scale massive Fe–Ti–(P) ores.

Petrogenesis of mid-Carboniferous volcanics and granitic intrusions from western Junggar Basin boreholes: geodynamic implications for the Central Asian Orogenic Belt in Northwest China

The western Junggar Basin is located on the southeastern margin of the West Junggar terrane, Northwest China. Its sedimentary fill, magma petrogenesis, tectonic setting, and formation ages are important for understanding the Carboniferous tectonic evolution and continental growth of the Junggar terrane and the Central Asian Orogenic Belt. This paper documents a set of new zircon secondary ion mass spectrometry U–Pb geochronological and Hf isotopic data and whole-rock elemental and Sr–Nd isotopic analytical results for the Carboniferous strata and associated intrusions obtained from boreholes in the western Junggar Basin. The Carboniferous strata comprise basaltic andesite, andesite, and dacite with minor pyroclastic rocks, intruded by granitic intrusions with zircon secondary ion mass spectrometry U–Pb ages of 327–324 Ma. The volcanic rocks are calc-alkaline and show low high εNd(t) values (5.3–5.6) and initial 87Sr/86Sr (0.703561–0.703931), strong enrichment in LREEs, and some LILEs and depletion in Nb, Ta, and Ti. Furthermore, they also display high (La/Sm)N (1.36–1.63), Zr/Nb, and La/Yb, variable Ba/La and Ba/Th and constant Th/Yb ratios. These geochemical data, together with low Sm/Yb (1.18–1.38) and La/Sm (2.11–2.53) ratios, suggest that these volcanic rocks were derived from a 5–8% partial melting of a mainly spinel Iherzolite-depleted mantle metasomatized by slab-derived fluids and melts of some sediments in an island-arc setting. In contrast, the granitic intrusions represent typical adakite geochemical features of high Sr and low Y and Yb contents, with no significant Eu anomalies, high Mg#, and depleted εNd(t) (5.6–6.4) and εHf(t) (13.7–16.2) isotopic compositions, suggesting their derivation from partial melting of hot subducted oceanic crust. In combination with the previous work, the West Junggar terrane and adjacent western Junggar Basin are interpreted as a Mariana-type arc system driven by northwestward subduction of the Junggar Ocean, possibly with a tectonic transition from normal to ridge subduction commencing ca. at 331–327 Ma.

Changing elemental uptake of radish seedlings grown in Cd and Pb polluted smectite substrates

Radish seedlings were grown for one month in a smectite substrate under controlled laboratory conditions. The purpose was avoiding as much as possible side effects such as heterogeneity of natural soils, as well as fertilizer and atmospheric supplies. One set of seedlings was watered at the beginning of the survey with plain deionized water, whereas the others were with the same water containing Cd or Pb at 0.5 and 1mg/L concentrations, to quantify the impact of these metal pollutions on the plants. Significant changes were observed in the uptake of the seedlings watered with the contaminated solutions: Na and K uptakes decreased, whereas those of Al, Fe, Ca and Mn increased. Changes were also detected in the uptake of some rare earths and trace elements after watering with Cd or Pb contaminated solutions. The most concerning uptakes are for the pollutants Cd and Pb that increase by about 795 and 1240 times, respectively, in the contaminated seedlings, suggesting a high tolerance by the radishes for these two metals. The overall increasing elemental uptakes result from replacement of an ion exchange process during growth in the unpolluted substrate by an ion complexation process in the case of growth in the polluted substrate.Occurrence of a significant Eu anomaly and disappearance of the Gd anomaly in the rare-earth distribution patterns of the contaminated radishes were necessarily induced by organic activity; they also explain the suggested changing uptake process. Rare-earth element distributions represent therefore an efficient tracer to evaluate how much soil pollution by a given metal potentially affects the uptake process of plants, and to postulate that some types of metal pollution, Cd and Pb here, affect specific functions of enzymes and/or of bacterial micro-organisms.

U–Pb geochronology and geochemistry of Bibi-Maryam pluton, eastern Iran: Implication for the late stage of the tectonic evolution of the Sistan Ocean

article i nfo Article history: Received 10 September 2013 Accepted 16 April 2014 Available online 2 May 2014 The Bibi-Maryam pluton crops out in the Sistan suture zone, eastern Iran. This pluton is a 1.5 × 2 km stock composed of leucocratic tonalite, granodiorite and granite. U-Pb zircon geochronology of a leucogranite indicates an emplacement age of 58.6 ± 2.1 Ma (95% confidence). The Bibi-Maryam rock suite is sodic with elevated Na2O/K2O (2.9 to 5.5), Sr/Y (15.6-62.2), La/Yb (13.3-22.2), and low MgO (0.86-1.81) abundances. It lacks significant Eu anomalies. Because of these geochemical characteristics, Bibi-Maryam rocks are similar to high-SiO2 adakites. Trace element modeling indicates that the Bibi-Maryam adakitic rocks could be produced by 5-8% non-modal batch partial melting from a source with composition of 95% N-MORB + 5% sediment in the presence of 35-40% amphibole + 5-10% garnet + 55-60% clinopyroxene + 1% apatite + 1% rutile. This source mineralogy is similar to hornblende eclogite or garnet amphibolites. Collectively, these data provide new constraints for the evolution of the Sistan suture zone and suggest that the Bibi-Maryam pluton formed via slab melting in an oceanic arc and pre-plate collision tectonic setting. This implies that the closure of the Sistan Ocean and Lut-Afghan continental blocks collision happened after the Bibi-Maryam emplacement

Geochemistry of the Desur Lavas, Deccan Traps: Case Study from the Vicinity of Belgaum, Karnataka and their Petrogenetic Inferences

Abstract: Geochemical characteristics of Desur-type basalt flows in the southern and southwestern part of Belgaum in Karnataka, India have been investigated to understand their petrogenesis. The basalts are compact, hard, massive, and show characteristic microporphyritic textures with abundant well-twinned and un-twinned plagioclase phenocrysts and minor clinopyroxene set in a fine-grained groundmass consisting of plagioclase, clinopyroxene, glass and Fe-Ti oxides. Thin sections show sub-ophitic, intergranular and intersertal textures. The basalts are Fe-rich tholeiites (13.4-13.8 wt %), characterized by high TiO2 (3.64 to 3.94 wt %); moderate MgO contents (4.79 to 5.41 wt %), low K2O contents (<0.58 wt %) and low Mg# (42.4-45.9). They are enriched in large ion lithophile elements, moderately enriched in the light rare earths (chondrite-normalized LaN/YbN 3.37-4.24), and exhibit nearly flat heavy rare-earth patterns that lack significant Eu anomalies (Eu/Eu* 0.86-1.10). Primitive-mantle-normalized element patterns for these rocks show characteristic troughs at K and Sr, absence of a Nb anomaly, and a low Zr/Nb ratio (<15), which suggest insignificant contamination by many types of continental crust, whereas, enrichments in the large ion lithophiles, La, P and Th could suggest enriched source characteristics. Based on the geochemical characteristics of the basalts, it is inferred that the Desur basalts representing the youngest flows of the Deccan Basalt Group are derived by partial melting of a peridotite source, and subsequent fractionation gave rise to the compositions of the basalts that are found in the Belgaum region.

Petrology and geochemistry of Permian mafic–ultramafic intrusions in the Emeishan large igneous province, SW China: Insight into the ore potential

Mafic–ultramafic layered intrusions constitute important components in the western part of the Emeishan large igneous province (LIP) in SW China. Here we present zircon U–Pb geochronology, geochemistry and Sr–Nd isotopic data from four representative barren mafic–ultramafic intrusions in Wupodan (WPD), Shangyi (SY), Louyi (LY) and Xiayi (XY). All these four intrusions were emplaced during the Late Permian, coeval with the formation of Emeishan flood basalts and associated mineralized intrusions. All rocks from the four intrusions exhibit mineralogical homogeneity, in the absence of any associated intermediate-felsic plutons. Chondrite-normalized REE patterns of LY, SY and WPD intrusions exhibit moderately fractionated patterns with (La/Yb)N of 3.55–5.12, whereas the XY intrusion shows pronounced LREE enrichment with (La/Yb)N of 11.61–12.94. Samples from the four intrusions lack any significant Eu anomalies, and have low εNd(t) values of −3.0 to −1.9 and variable initial 87Sr/86Sr ratios of 0.7057–0.7074, overlapping with the field defined by Permian Emeishan basalts and the related mineralized mafic–ultramafic intrusions. Clinopyroxenes from the SY, WPD and LY intrusions show relatively high equilibrium temperatures ranging from ~1300°C to ~1400°C, which can be regarded as a signature of plume-related origin. Although the clinopyroxenes from the XY intrusion exhibit a relatively lower equilibrium temperature (~1200°C), the geochemical signature resembling OIB suggests a plume-related origin. Thus, these intrusions appear to be part of the Emeishan LIP. These features, combined with moderate to pronounced depletion of HFSE (e.g. Nb, Ta and Ti) and enrichment of LILE (e.g. Rb, Ba, K), correlate the LY, SY and WPD intrusions to the interaction between mantle plume and subcontinental lithospheric mantle that had been previously modified by subduction-related materials. In contrast, the XY intrusion was derived from the plume source contaminated by subcontinental lithospheric mantle. We also attempt a comparison between the unmineralized and mineralized intrusions in terms of their parental magmas, crystallization history, nature of the mantle source, degrees of partial melting of source and the extent of crustal contamination. Some geochemical criteria that may be useful in discriminating between mineralized and unmineralized mafic–ultramafic intrusions are summarized, and insights into the ore potential of the four barren intrusions are addressed.

Genesis of the Yuanlingzhai porphyry molybdenum deposit, Jiangxi province, South China: Constraints from petrochemistry and geochronology

The newly discovered Yuanlingzhai porphyry molybdenum (Mo) deposit in southern Jiangxi province belongs to the group of Mo-only deposits in the Nanling region. The mineralization developed at contact zones between the Yuanlingzhai granite porphyry and Neoproterozoic metamorphic rocks of the Xunwu Formation. Precise LA–MC–ICPMS zircon U–Pb dating of the Yuanlingzhai porphyry, as well as the adjacent western Keshubei and eastern Keshubei granites, yielded ages of 165.49±0.59Ma, 159.68±0.43Ma, and 185.13±0.52–195.14±0.63Ma, respectively. Molybdenite Re–Os isochron ages of the ores are 160±1–162.7±1.1Ma, which is consistent with the age of large-scale W–Sn deposits in South China. The Yuanlingzhai porphyry is characterized by high K2O, P2O5, and A/CNK (1.33–1.59), and low CaO and Na2O. The rock shows relatively enriched LREE without significant Eu anomalies (Eu/Eu*=0.80–0.90). Geochemical and mineralogical characteristics indicate that the ore-hosting porphyry is a typical S-type granite generated from the partial melting of crustal material with only minor mantle contribution. Both Harker and evolutionary discrimination diagrams indicate that the Yuanlangzhai and western Keshubei granites are not products of co-magmatic evolution. The Keshubei granites and Xunwu Formation were not significant sources for the components in the porphyry mineralization, but the Yuanlangzhai granite may have supplied some ore-forming material. However, the main ore-forming material was carried by fluids from deep sources, as demonstrated by fluid inclusion and stable isotope data from the molybdenum deposit. The Mo porphyry deposit formed in an extensional setting, and was possibly associated with Jurassic subduction of the Izanagi Plate.

Geochemistry and zircon geochronology of the Archean granite suites of the Rio Maria granite-greenstone terrane, Carajás Province, Brazil

The Archean granites exposed in the Mesorchean Rio Maria granite-greenstone terrane (RMGGT), southeastern Amazonian craton can be divided into three groups on the basis of petrographic and geochemical data. (1) Potassic leucogranites (Xinguara and Mata Surrão granites), composed dominantly of biotite monzogranites that have high SiO2, K2O, and Rb contents and show fractionated REE patterns with moderate to pronounced negative Eu anomalies. These granites share many features with the low-Ca granite group of the Yilgarn craton and CA2-type of Archean calc-alkaline granites. These granites result from the partial melting of rocks similar to the older TTG of the RMGGT. (2) Leucogranodiorite-granite group (Guarantã suite, Grotão granodiorite, and similar rocks), which is composed of Ba- and Sr-rich rocks which display fractionated REE patterns without significant Eu anomalies and show geochemical affinity with the high-Ca granite group or Transitional TTG of the Yilgarn craton and the CA1-type of Archean calc-alkaline granites. These rocks appear to have been originated from mixing between a Ba- and Sr-enriched granite magma and trondhjemitic liquids or alternatively product of interaction between fluids enriched in K, Sr, and Ba, derived from a metasomatized mantle with older TTG rocks. (3) Amphibole-biotite monzogranites (Rancho de Deus granite) associated with sanukitoid suites. These granites were probably generated by fractional crystallization and differentiation of sanukitoid magmas enriched in Ba and Sr.The emplacement of the granites of the RMGGT occurred during the Mesoarchean (2.87–2.86 Ga). They are approximately coeval with the sanukitoid suites (∼2.87 Ga) and post-dated the main timing of TTG suites formation (2.98–2.92 Ga). The crust of Rio Maria was probably still quite warm at the time when the granite magmas were produced. In these conditions, the underplating in the lower crust of large volumes of sanukitoid magmas may have also contributed with heat inducing the partial melting of crustal protoliths and opening the possibility of complex interactions between different kinds of magmas.

Adakitic rocks derived from the partial melting of subducted continental crust: Evidence from the Eocene volcanic rocks in the northern Qiangtang block

Eocene is a critical time for the elevation of Tibetan Plateau and global climate change, and previous studies suggested that the Eocene elevation was caused by intra-continental subduction of the Songpan–Garze block beneath the Qiangtang block. This paper reports zircon U–Pb age and geochemistry of the Eocene volcanic rocks from the Zuerkenwula mountain area in the northern part of Qiangtang block, and proposes that both slab break-off of the Neo-Tethys oceanic slab along the Bangong–Nujiang suture and intra-continental subduction of the Songpan–Garze block beneath the Qiangtang block caused the extensive partial melting of lithospheric mantle and subducted Songpan–Garze continental crust, which resulted in the significant elevation of the Tibetan Plateau. The volcanic rocks have LA-ICP MS U–Pb zircon age of 40.25±0.15Ma (MSWD=2.1, 2σ), which is contemporaneous with the Eocene eclogites in the Great Himalayan and K-rich lavas in the southeastern Tibet. They display some adakitic characteristics with SiO2=57.44 to 68.72%, TiO2=0.38 to 0.81%, Na2O=2.89 to 4.35%, K2O=2.77 to 4.48%, Al2O3=13.92 to 18.22%, A/CNK=0.69 to 1.03, MgO=0.27 to 5.86% with Mg# ranging from 13.2 to 72.0, strongly depleted in heavy rare earth elements (HREEs) (Yb=0.92 to 1.51ppm and Y=10.1 to 24.1ppm), in combination with their positive Sr anomalies, high Sr/Y ratios and no significant Eu anomalies, which suggest a garnet-in and plagioclase-free source residue. These volcanic rocks can be divided into high-Mg# (>45) and low-Mg# (<45) groups. Both of the two groups share evolved Sr–Nd–Pb isotopic compositions with 87Sr/86Sr=0.707412–0.708284; εNd(t)=−4.0 to −5.7; 206Pb/204Pb=18.7499–18.8189, 207Pb/204Pb=15.7189–15.7384; 208Pb/204Pb=39.166–39.262. The geophysical data and regional geological setting suggest that the low-Mg# adakitic rocks were derived from the decompression melting of a subducted lower continental crust, when low-Mg# adakitic melts in the overlying peridotite mantle wedge captured some olivine crystals, resulting in their elevated Mg# and MgO values.

Petrology and geochemistry of calc-alkaline volcanic and subvolcanic rocks, Dalli porphyry copper–gold deposit, Markazi Province, Iran

Early Miocene igneous rocks associated with the Dalli porphyry ore body are exposed within the Urumieh-Dokhtar Magmatic Arc (UDMA). The Dalli porphyry Cu–Au deposit is hosted by subduction-related subvolcanic plutons with chemical composition from diorite to granodiorite, which intruded andesitic and dacitic volcanic rocks and a variety of sedimentary sequences. 40Ar/39Ar age data indicate a minimum emplacement age of ∼21 million years for a potasically altered porphyritic diorite that hosts the porphyry system. The deposit has a proven reserve of 8 million tonnes of rock containing 0.75 g/t Au and 0.5% Cu. Chondrite-normalized rare earth element (REE) patterns for the subvolcanic rocks are characterized by light REE enrichments [(La/Sm) n = 2.57–6.40] and flat to gently upward-sloping profiles from middle to heavy REEs [(Dy/Yb) n = 0.99–2.78; (Gd/Yb) n = 1.37–3.54], with no significant Eu anomalies. These characteristics are generated by the fractionation of amphibole and the suppression of plagioclase crystallization from hydrous calc-alkaline magmas. In normalized multi-element diagrams, all analysed rocks are characterized by enrichments in large ion lithophile elements and depletions in high field strength elements, and display typical features of subduction-related calc-alkaline magmas. We used igneous mineral compositions to constrain the conditions of crystallization and emplacement. Biotite compositions plot above the nickel–nickel oxide (NNO) buffer and close to oxygen fugacity values defined by the hematite–magnetite (HM) buffer, indicating oxidizing conditions during crystallization. Assuming a minimum crystallization temperature of 775°C, the oxygen (fO2) and water (fH2O) fugacities are estimated to be 10−10.3 bars (∼ΔNNO+4) and ≤748 bars, respectively, during the crystallization of biotite phenocrysts. The temperature and pressure conditions, estimated from temperature–corrected Al-in-hornblende barometry and amphibole-plagioclase thermometry, suggest that the hornblende phenocrysts in Dalli rocks crystallized at around 780 ± 20°C and 3.8 ± 0.4 kbar. An alternative method using the calcic amphibole thermobarometer indicates that the Dalli magmas were, on average, characterized by an H2O content of 4.3 wt.%, a relatively high oxygen fugacity of 10−11.0 bars (ΔNNO+1.3), and a hornblende phenocryst crystallization temperature of 880 ± 68°C and pressure of 2.6 ± 1.7 kbar.

REE Characteristic of Hydrothermal Calcite in Langdu Skarn Copper Deposit

Langdu is one of the highest grade, skarn copper deposits in Zhongdian area, Yunnan province. The chondrite-normalized REE patterns for syn-sulfide calcite from Langdu deposit are unique, being HREE-enriched, LREE-depleted and have significant positive Eu anomalies. REEs could derive from magmatic fluid exsolved from near melt. This distribution behavior is predominately constricted by REE patterns of hydrothermal fluid and the stability of complex in solutions than crystallographic factors. The DREE are increase with the decrease of ion radius. The optimum ion radius of substitutional site for REEs in the hydrothermal calcites is 0.1032nm. Eu occurs as Eu 2+ and complex species can explain the positive Eu anomalies

Geochronology/geochemistry of the Washan dioritic porphyry associated with Kiruna-type iron ores, Middle-Lower Yangtze River Valley, eastern China: implications for petrogenesis/mineralization

The Early Cretaceous Washan dioritic porphyry is spatially and temporally associated with Kiruna-type iron oxide deposits in the Ningwu basin, Middle-Lower Yangtze River Valley (MLYRV). We present new LA-ICP-MS U–Pb dating + zircon Lu–Hf isotopic studies, as well as bulk-rock major + trace element and Sr + Nd isotopic compositions of the porphyry. LA-ICP-MS U–Pb zircon analyses suggest that the pluton formed at 130.8 ± 0.9 Ma. Analysed zircon ϵHf(t) values range from –7.0 to –4.1. The dioritic rocks are significantly enriched in Pb and light rare earth elements, relative to high-field strength elements (Nb + Ti), coupled in the absence of significant Eu anomalies. They exhibit age-corrected ϵNd(t) (t = 130 million years) values of −3.5 to −3.9 and initial 87Sr/86Sr ratios of 0.70553–0.70653. The ore-bearing dioritic porphyry was derived from a parental basaltic liquid that was produced by partial melting of an enriched spinel-facies lherzolite in the Yangtze lithospheric mantle. This basaltic melt underwent a fractionation of plagioclase and clinopyroxene during ascent towards the surface, which led to the relative enrichment of iron in the residual melt. This type of magma was widespread in the MLYRV area but did not generate widespread Fe mineralization. In the Ningwu area, the dioritic magma was modified by minor assimilation of phosphorus-bearing rocks in the Yangtze upper crust. The special crustal characteristics of the Ningwu basin, i.e. phosphorus-rich strata, were likely a crucial factor controlling the formation of Kiruna-type iron oxide deposits.

The Cretaceous Duimiangou adakite-like intrusion from the Chifeng region, northern North China Craton: Crustal contamination of basaltic magma in an intracontinental extensional environment

Zircon U–Pb ages, major and trace element and Sr, Nd and Pb isotope compositions of the Duimiangou (DMG) quartz monzonite from the Chifeng region on the northern North China Craton (NCC) were studied to investigate its derivation, evolution and geodynamic significance. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating yields an emplacement age of 128±1Ma for this intrusion, with numerous Mesozoic inherited zircons clustering at 219±12Ma and 161±3Ma, along with some ancient zircons with ages of 2.5Ga, 1.77Ga and 324Ma. Bulk-rock analyses show that this intrusion is characterized by variable SiO2 (63.4–69.4wt.%), Al2O3 (14.5–16.3wt.%), Na2O+K2O (8.01–8.95wt.%), and Mg# (41.3–48.0). They are enriched in large ion lithophile elements and light rare earth elements without significant Eu anomalies (mostly between 0.89–1.10), and depleted in heavy rare earth elements and high field strength elements, with high Sr/Y (63.7–101.7) and (La/Yb)N (20.5–31.0) ratios. The DMG intrusion formed in an intracontinental extensional setting contemporaneous with the formation of pull-apart basins, metamorphic core complexes and intense magmatism, rather than in a convergent margin. It has homogeneous Sr ((87Sr/86Sr)i=0.7059–0.7066), Nd (εNd(t)=−6.2 to −7.2) and Pb ((206Pb/204Pb)i=17.289–17.375, (207Pb/204Pb)i=15.359–15.463, (208Pb/204Pb)i=37.130–37.472) isotope compositions. Sr–Nd isotope modeling results, plus relatively young Nd model ages (1522–1618Ma) and the presence of relict zircons, suggest that this intrusion could have originated from crustal contamination of newly formed basaltic melts derived from asthenospheric mantle, accompanied by fractional crystallization of K-feldspar, biotite, apatite, Fe–Ti oxides and minor hornblende and plagioclase. Thus, the DMG adakite-like intrusion may record the magmatic event associated with underplating of asthenospheric magma in an intracontinental extensional environment, and both asthenospheric magma underplating and lithospheric delamination may have played important roles in the lithospheric thinning of the northern NCC.

Mineral chemistry of Plio-Quaternary subvolcanic rocks, southwest Yazd Province, Iran

The NW–SE-trendingLate Cretaceous–Cenozoic Urumieh-Dokhtar Magmatic Arc (UDMA) in southwest Iran hosts numerous Plio-Quaternary subvolcanic porphyritic andesitic to rhyodacitic domes intruded into a variety of rock sequences. Bulk-rock geochemical data show that the calc-alkaline dacitic to rhyodacitic subvolcanic rocks share compositional affinities with high-silica adakites, including high ratios of Na2O/K2O >1, Sr/Y (most >70), and La/Yb (>35), high Al2O3 (>15 wt.%), low Yb (<1.8 ppm) and Y (<18 ppm) contents, no significant Eu anomalies, and flat to gently upward-sloping chondrite-normalized heavy rare-earth element (HREE) patterns. All analysed rocks are characterized by enrichment in large-ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs). They also display typical features of subduction-related calc-alkaline magmas. In chondrite-normalized rare-earth element patterns, the light rare-earth elements (LREEs) are enriched ((La/Sm) N = 3.49–7.89) in comparison to those of the HREE ((Gd/Yb) N = 1.52–2.38). Except for the G-Aliabad Dome, plagioclase crystals in the Shamsabad, Ostaj, Abdollah, and Bouragh Domes are mostly oligoclase to andesine (An19–49). Amphibole and biotite are abundant ferromagnesian minerals in the subvolcanic rocks. Calcic amphiboles are dominantly magnesiohornblende, magnesiohastingsite, and tschermakite with Mg/(Mg + Fetot) ratios ranging from 0.58 to 0.78. In all the studied domes, amphiboles are typically ferric iron-rich, but that those the Shamsabad Dome have the highest Fe3+/(Fe3+ + Fe2+) ratios, between 0.69 and 0.98. Amphiboles from the Ostaj and Shamsabad Domes are relatively rich in F (0.39–1.01 wt.%) in comparison to the other studied domes. This phase commonly shows pargasitic and hastingsitic substitutions with a combination of tschermakitic and edenitic types. Temperature-corrected Al-in-hornblende data show that amphibole phenocrysts from the Ostaj, Abdollah, and G-Aliabad Domes crystallized at pressures ranging from 2.14 to 3.42 kbar, 3.49 to 3.96 kbar, and 2.02 to 3.47 kbar, respectively. Temperatures of crystallization calculated with the amphibole–plagioclase thermometer for the Ostaj, Abdollah, and G-Aliabad subvolcanic domes range from 735°C to 826°C (mean = 786 ± 29), 778°C to 808°C (mean = 791 ± 13), and 866°C to 908°C (mean = 885 ± 12), respectively. In the annite–siderophyllite–phlogopite–eastonite quadrilateral, biotite from the G-Aliabad, Bouragh, and Ostaj Domes are characterized by relatively low total Al contents with variable Fetot/(Fetot + Mg) values from 0.26 to 0.43. All biotite analyses define a nearly straight line in the X Mg versus Fetot plot, with r = –0.96 correlation coefficient. In comparison to other domes, the F content of biotite from the G-Aliabad Dome shows high concentrations in the range of 1.80–2.57 wt.% (mean = 2.20). Inferred pre-eruptive conditions based on the calcic amphibole thermobarometric calculations for the Shamsabad, Abdollah, and Ostaj Domes show that the calc-alkaline subvolcanic magma chamber, on average, was characterized by a water content of 6.10 wt.%, a relatively high oxygen fugacity of 10–10.66 (ΔNNO + 1.28), a temperature of 896°C, and a pressure of 2.75 kbar.

U–Pb zircon geochronology, geochemical, and Sr–Nd isotopic constraints on the age and origin of basaltic porphyries from western Liaoning Province, China

Basaltic porphyries from the northeast North China craton (NCC) provide an excellent opportunity to examine the nature of their mantle source and the secular evolution of the underlying mantle lithosphere. In addition, the study helps to constrain the age and the mechanism of NCC lithospheric destruction. In this paper, we report geochronological, geochemical, and Sr–Nd isotopic analyses of a suite of mafic lavas. Detailed laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb dating yielded an age of 223.3 ± 1.1 million years, which we regard as representing the crystallization age of the basaltic porphyries. The bulk-rock analysed samples are enriched in both large ion lithophile elements (LILEs) (i.e. Ba, Sr, and Pb) and light rare earth elements (LREEs), but depleted in high field strong elements (HFSEs) (i.e. Nb, Ta, Zr, Hf, and Ti) and heavy rare earth elements (HREEs), without significant Eu anomalies (Eu/Eu* = 089–0.98). The basaltic porphyries have undergone low degrees (∼5%) of partial melting of a garnet-bearing lherzolite mantle. The rocks display very uniform (87Sr/86Sr) i (0.70557–0.70583) and negative ϵNd (t) values (–11.9 to –10.1). These features indicate that the western Liaoning basaltic porphyries were derived from a common enriched lithosphere mantle that had previously been metasomatized by fluids related to subduction of Palaeo-Asian sedimentary units. However, the mafic melts were not affected to a significant degree by crustal contamination. Based on earlier studies, these findings provide new evidence that the northeast margin of the NCC had undergone a phase of post-orogenic extensional tectonics during the Middle Triassic. Furthermore, lithospheric thinning occurring across the northern NCC might have been initiated during Early Triassic times and was likely controlled by the final closure of the Palaeo-Asian Ocean, as well as the collision of Mongolian arc terrenes with the NCC.

Adakite-like granitoid porphyries in the Eastern Pontides, NE Turkey: Potential parental melts and geodynamic implications

The tectonic setting of the Eastern Pontides during the late Mesozoic to early Cenozoic remains a subject of debate. Petrogenesis of adakite-like granitoid porphyries plays a critical role in determining the nature of the lower continental crust and mantle dynamics during orogenic processes in the region. Here we describe, for the first time, the late Paleocene to early Eocene adakite-like granitoid porphyries from the northern part of the Eastern Pontides although their counterparts in the southern part have recently been found. The adakitic porphyries, which emplaced into the subduction-related Turonian–Santonian volcanics from northern part of the region, consist of I-type calc-alkaline quartz monzonite–tonalite (SiO2=62.89–65.07wt.%) and high-K calc-alkaline granodiorite–granite associations (SiO2=69.06–70.43wt.%). The former displays peraluminous to metaluminous signatures, whereas the latter shows peraluminous geochemical character. The granite–granodiorite porphyries have high K2O (3.32–3.84wt.%), and low Na2O (3.48–4.61wt.%) and MgO (0.91–1.04wt.%) relative to the quartz monzonite–tonalite association (K2O=1.50–1.92 wt.%; Na2O=4.08–6.45 wt.%; MgO=1.44–2.07 wt.%). Ar–Ar geochronology studies on the amphibole separates reveal that the adakite-like porphyries have a crystallization ages of 51.34±0.27 to 53.55±0.34Ma. Here, we contend that these rocks were formed by partial fusion of a mafic lower continental crust in a collisional phase but not in a subduction setting. All the samples exhibit the typical geochemical characteristics of adakite, that is, high Sr (250–1141ppm), high Sr/Y ratios (16–147), low Y (6.8–14.8ppm) and low HREE concentrations; they are similar to adakites formed by slab melting associated with the subduction zone. However, the rocks exhibit heterogeneity in isotopic composition, with ISr ranging from 0.70554 to 0.70986, εNd (51Ma) from –8.5 to –0.9 and Nd model ages from 0.72 to 1.26 Ga. The samples are characterized by relatively high Th, Th/U and no significant Eu anomalies, implying that garnet was stable in their source during partial melting. The compositional diversity between rock groups is probably related to partial melting of heterogeneous lower crustal source. All of the features are inconsistent with a slab-melting origin and slab-related petrogenetic model, but instead, they favor an origin by melting of heterogeneous lower continental crust due to a thermal anomaly that was induced by the upwelling of the asthenosphere through a slab break-off in a collisional setting. These interpretations argue against the evidence for the presence of an early Cenozoic arc setting in the Eastern Pontides. Instead, the early Cenozoic time in the region may be attributed to a geodynamic response to a post-collisional uplift phase that occurred along the continent-continent collision between the Pontide and the Anatolide-Tauride blocks.

Triassic shoshonitic dykes from the northern North China craton: petrogenesis and geodynamic significance

AbstractZircon U–Pb ages, major and trace element geochemistry and Sr, Nd and Pb isotope compositions of diorite and diorite porphyry dykes from the Jinchanggouliang (JCGL) gold ore field on the northern margin of the North China craton (NCC) were studied to investigate their sources, petrogenesis and geodynamic significance. LA-ICP-MS zircon U–Pb dating reveals three major age groups of 2500 Ma (n = 2), 253 ± 7 Ma (n = 5) and 227 ± 1 Ma (n = 9). The inherited ages of 2500 Ma, contemporary with the Archaean NCC continental growth, imply that crustal material was involved in the magma source. The igneous zircons with a concordia age of 227 ± 1 Ma may record the emplacement age of the JCGL dykes. Both diorite and diorite porphyry exhibit a wide range of SiO2 and MgO contents and are characterized by high concentrations of Na2O+K2O and Al2O3, and low abundances of P2O5 and TiO2. They are enriched in large ion lithophile elements and light rare earth elements without significant Eu anomalies, and depleted in high-field-strength elements; all are categorized as shoshonitic rocks. All samples show a narrow range of Sr isotope compositions with initial 87Sr/86Sr ratios from 0.70394 to 0.70592, variable εNd(t) values (1.1 to −12.0) and TDM2 ages of 913–1972 Ma. Their Pb isotope compositions form continuous variation trends and plot in the fields between enriched mantle 1 (EM1) and lower continental crust (LCC). The above results suggest that the JCGL dykes studied could have been derived from mixing of lower crust, lithospheric mantle of the NCC and ascending asthenospheric melt in a post-orogenic extensional geodynamic setting. These shoshonitic dykes, together with the geochronological data of regional ENE-trending retrograded eclogites, ophiolites, continental arc magmatic belt, A-type granite, alkaline intrusions and metamorphic core complex from the northern NCC and Central Asian Orogenic Belt (CAOB) suggest that closure of the Palaeo-Asian Ocean (i.e. stage of pre-collision to collision) had completed during latest Permian to earliest Triassic time, and that the CAOB was subsequently tectonically dominated by post-orogenic extensional regimes. The involvement of asthenospheric melt in the magma source implies that the sub-continental lithospheric mantle (SCLM) of the NCC had been modified, and the onset of lithospheric destruction and thinning beneath the northern NCC may have occurred in Middle–Late Triassic time as a result of post-orogenic subducting slab detachment and lithospheric delamination.

Origin of LateTriassic high-Mg adakitic granitoid rocks from the Dongjiangkou area, Qinling orogen, central China: Implications for subduction of continental crust

The origin of high-Mg adakitic granitoids in collisional orogens can provide important information about the nature of the lower crust and upper mantle during the orogenic process. Late-Triassic high-Mg adakitic granite and its mafic enclaves from the Dongjiangkou area, the Qinling orogenic belt, central China, were derived by partial melting of subducted continental crust and underwent interaction with the overlying mantle wedge peridotite. Adakitic affinity of the different facies of the Dongjiangkou granite body are: high Sr, Ba, high La/Yb and Sr/Y, low Y,Yb, Yb/Lu and Dy/Yb, and no significant Eu anomalies, suggesting amphibole + garnet and plagioclase-free restite in their source region. Evolved Sr-Nd-Pb isotopic compositions [( 87Sr/ 86Sr) i = 0.7050 to 0.7055,ε Nd(t) = –6.6 to –3.3; ( 206Pb/ 204Pb) i = 17.599 to 17.799, ( 207Pb/ 204Pb) i = 15.507 to 15.526, ( 208Pb/ 204Pb) i = 37.775 to 37.795] and high K 2O, Rb, together with a large variation in zircon Hf isotopic composition (ε Hf(t) = −9.8 to + 5.0), suggest that the granite was derived from reworking of the ancient lower continental crust. CaO, P 2O 5, K 2O/Na 2O, Cr, Ni, Nb/Ta, Rb/Sr and Y increase, and SiO 2, Sr/Y and Eu/Eu* decrease with increasing MgO, consistent with interaction of primitive adakitic melt and overlying mantle peridotite. Zircons separated from the host granites have U-Pb concordia ages of 214 ± 2 Ma to 222 ± 2 Ma, compatible with exhumation ages of Triassic UHP metamorphic rocks in the Dabie orogenic belt. Mafic microgranular enclaves and mafic dykes associated with the granite have identical zircon U-Pb ages of 220 Ma, and are characterized by lower SiO 2, high TiO 2, Mg# and similar evolved Sr-Nd-Pb isotopic composition. Zircons from mafic microgranular enclaves (MMEs) and mafic dykes also show a large variation in Hf isotopic composition with ε Hf(t) between −11.3 and + 11.3. It is inferred that they were formed by partial melting of enriched mantle lithosphere and contaminated by the host adakitic granite magma. In combination with the regional geology, high-Mg# adakitic granitoid rocks in the Dongjiangkou area are considered to have resulted from interaction between subducted Yangtze continental crust and the overlying mantle wedge. Triassic continental collision caused detachment of the Yangtze continental lithosphere subducted beneath the North China Craton, at ca. 220 Ma causing asthenosphere upwelling and exhumation of the continental crust. Triassic clockwise rotation of the Yangtze Craton caused extension in the Dabie area which led to rapid exhumation of the subducted continental lithosphere, while compression in the Qinling area and high-P partial melting (amphibole ± garnet stability field) of the subducted continental crust produced adakitic granitic magma that reacted with peridotite to form Mg-rich hybrid magma.

Geochemistry of Late Mesozoic dioritic porphyries associated with Kiruna-style and stratabound carbonate-hosted Zhonggu iron ores, Middle–Lower Yangtze Valley, Eastern China: Constraints on petrogenesis and iron sources

Several small Mesozoic sodium-rich dioritic porphyries associated with huge iron oxide deposits occur in the Zhonggu iron ore field, in the eastern part of the Middle–Lower Yangtze River Belt (MLYRB) in the Yangtze craton. We present bulk-rock major and trace element and Sr and Nd isotopic compositions of the three representative dioritic porphyries: the Gushan, Hemushan and Baixiangshan dioritic intrusions. These porphyries are significantly enriched in Pb and light rare earth elements (LREE), relative to high field strength elements (HFSE; Nb, Ta and Ti), coupled with the absence of significant Eu anomalies. They exhibit negative ε Nd( t) (− 5.2 to − 6.8), and highly radiogenic Pb ( 206Pb/ 204Pb = 18.27–18.45). The similar geochemical characteristics of these iron ore-bearing dioritic stocks suggest that they were derived from a common parent basaltic melt, which was produced by partial melting of the enriched lithospheric mantle. However, the parental melt experienced assimilation of Yangtze Block upper crust, accompanying the fractionation of plagioclase and clinopyroxene during ascent to the surface, which led to the extreme enrichment of iron in a highly evolved magma. The contribution of phosphorus and potential CO 2 from the country rocks might have been the crucial factor that led to liquid immiscibility and formation of the unique ore-magma type mineralization.

Magma mixing origin for the post-collisional adakitic monzogranite of the Triassic Yangba pluton, Northwestern margin of the South China block: geochemistry, Sr–Nd isotopic, zircon U–Pb dating and Hf isotopic evidences

Petrogenesis of high Mg# adakitic rocks in intracontinental settings is still a matter of debate. This paper reports major and trace element, whole-rock Sr–Nd isotope, zircon U–Pb and Hf isotope data for a suite of adakitic monzogranite and its mafic microgranular enclaves (MMEs) at Yangba in the northwestern margin of the South China Block. These geochemical data suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived from subcontinental lithospheric mantle (SCLM) may account for the origin of high Mg# adakitic rocks in the intracontinental setting. The host monzogranite and MMEs from the Yangba pluton have zircon U–Pb ages of 207 ± 2 and 208 ± 2 Ma, respectively. The MMEs show igneous textures and contain abundant acicular apatite that suggests quenching process. Their trace element and evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.707069–0.707138, and eNd(t) = −6.5] indicate an origin from SCLM. Some zircon grains from the MMEs have positive eHf(t) values of 2.3–8.2 with single-stage Hf model ages of 531–764 Ma. Thus, the MMEs would be derived from partial melts of the Neoproterozoic SCLM that formed during rift magmatism in response to breakup of supercontinent Rodinia, and experience subsequent fractional crystallization and magma mixing process. The host monzogranite exhibits typical geochemical characteristics of adakite, i.e., high La/Yb and Sr/Y ratios, low contents of Y (9.5–14.5 ppm) and Yb, no significant Eu anomalies (Eu/Eu* = 0.81–0.90), suggesting that garnet was stable in their source during partial melting. Its evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.7041–0.7061, and eNd(t) = −3.1 to −4.3] and high contents of K2O (3.22–3.84%) and Th (13.7–19.0 ppm) clearly indicate an origin from the continental crust. In addition, its high Mg# (51–55), Cr and Ni contents may result from mixing with the SCLM-derived mafic magma. Most of the zircon grains from the adakitic monzogranite show negative eHf(t) values of −9.4 to −0.1 with two-stage Hf model ages of 1,043–1,517 Ma; some zircon grains display positive eHf(t) of 0.1–3.9 with single-stage Hf ages of 704–856 Ma. These indicate that the source region of adakitic monzogranite contains the Neoproterozoic juvenile crust that has the positive eHf(t) values in the Triassic. Thus, the high-Mg adakitic granites in the intracontinental setting would form by mixing between the crustal-derived adakitic magma and the SCLM-derived mafic magma. The mafic and adakitic magmas were generated coevally at Late Triassic, temporally consistent with the exhumation of deeply subducted continental crust in the northern margin of the South China Block. This bimodal magmatism postdates slab breakoff at mantle depths and therefore is suggested as a geodynamic response to lithospheric extension subsequent to the continental collision between the South China and North China Blocks.