Significant Negative Eu Anomalies Research Articles

Zircon U–Pb geochronological and Hf isotopic constraints on petrogenesis of Late Mesozoic intrusions in the southeast Hubei Province, Middle–Lower Yangtze River belt (MLYRB), East China

During the Late Mesozoic, extensive magmatism and associated ore deposits were developed in the southeast Hubei Province, Middle–Lower Yangtze River metallogenic belt (MLYRB), East China. To understand the petrogenesis of the intrusions in the area, we performed in situ zircon U–Pb and Hf isotopic analyses on 14 samples from the Fengshangdong stock, and from the Echeng, Tieshan, Wangbaoshan–Jinshandian and Yangxin batholiths. Three major suites of intrusive rocks are recognized in the southeast Hubei Province: (1) 141–146Ma granodiorite and granite porphyries (associated with 143–144Ma porphyry–skarn Cu–Mo deposits) have enriched radiogenic Hf isotopic compositions (εHf (t)=−9.5 to −1.6), comparable to those of Cretaceous pyroxene diorites, trachyandesites, and basalts (εHf (t)=−8.9 to −1.3) in the MLYRB, which were derived from an enriched lithospheric mantle. (2) Voluminous 136–143Ma gabbros, diorites, and quartz diorites (associated with 137–144Ma skarn Cu–Fe, Fe–Cu, and Au–Cu deposits) have variable Hf isotopic compositions (εHf (t)=−19.1 to −5.4), indicating an origin via the hybridization of enriched mantle-derived mafic magma and variable amounts of ancient lower-crustal components. (3) 127–133Ma diorites, quartz diorites, and granites (associated with 132–133Ma skarn Fe deposits) characterized by a wide range of εHf (t) values (−28.2 to −6.4), suggest the assimilation of various ancient lower-crustal materials by enriched mantle-derived basaltic magma. The intrusions of Suites 1 and 2 are characterized by high Sr contents (329–2087ppm), and high Sr/Yb (36–1710) and (La/Yb)N ratios(17.6–172), but low Yb contents (0.45–1.82ppm) and do not show negative Eu anomalies (δEu=0.74–1.4). The intrusions have radiogenic Sr–Nd isotopic compositions (εNd (t)=−10.0 to −3.4 and (87Sr/86Sr)i=0.7054–0.7084). Compared with the intrusions of Suites 1 and 2, rocks of Suite 3 have relatively low Sr contents (60.5–327ppm), and low Sr/Yb (33.0–315) and (La/Yb)N ratios (2.28–30.0), but relatively high Yb contents (1.04–5.17ppm) and significant negative Eu anomalies (δEu=0.22–0.73). The rocks of Suite 3 are characterized by a wide range of radiogenic Sr–Nd isotopes with εNd (t) values of −12.5 to −4.9 and (87Sr/86Sr)i=0.7057–0.7085. The proposed magma sources, combined with the geochemical and Sr–Nd isotopic differences between these three suites of intrusive rocks, indicate that Late Mesozoic magmatic activity in the southeast Hubei Province, MLYRB was associated with lithospheric thinning induced by asthenospheric upwelling at that time.

Silurian granulite-facies metamorphism, and coeval magmatism and crustal growth in the Tongbai orogen, central China

It is intriguing whether the formation of granulite in an orogenic belt is related to magmatism and crustal growth. The Tongbai orogen is one of the particular areas in the Qinling–Tongbai–Dabie–Sulu orogen that contains both granulites and associated arc magmatic rocks, and thus is an optimal place to address this issue. However, the formation age and genesis of the granulites and their relations to the magmatic rocks have not yet been well constrained. In situ LA (MC)-ICPMS U–Pb dating, and trace element and Lu–Hf isotope analyses were undertaken on zircon grains from gneiss, granulite, gabbro and granodiorite samples. Most zircons in three granulite samples and a gneiss sample have core–rim structures. The cores have oval or rounded shapes, oscillatory zones, high Th/U ratios and variable U–Pb ages, suggesting a detrital genesis. The youngest group of the detrital zircons yields a weighted mean 206Pb/238U age of 450±5Ma, which is interpreted as the maximum depositional age of their protoliths. A major age cluster is ca. 450–490Ma, corresponding to prolonged arc magmatism in the northern Tongbai and Qinling orogens. Another group of ages range from 660 to 950Ma, implying that the Qinling Group of the Tongbai orogen belongs to the South China Block (SCB). Metamorphic zircons in the granulites have weak or no zoning, low Th/U ratios and REE contents, negative Eu anomalies, and relatively flat HREE patterns, which indicate that they formed contemporaneously with garnet and plagioclase, and thus under granulite-facies metamorphic conditions. Their weighted mean age of 424±4Ma is taken as the best estimated age of the granulite-facies metamorphism. Zircons in a leucosome sample have significant negative Eu anomalies and relatively flat HREE patterns with a formation age of 428±4Ma, indicating partial melting coeval with the granulite-facies metamorphism. Metamorphic zircons in the gneiss show apparently negative Eu anomalies, and variable Th/U ratios and HREE contents. They yield a weighted mean 206Pb/238U age of 438±4Ma, which was suggested to record the timing of the prograde metamorphism. Zircon grains in a gabbro and a granodiorite yield U–Pb ages of 432±4 and 424±4Ma, respectively, which are interpreted as their formation ages and identical to the ages of the granulite-facies metamorphism. They have positive εHf (t) values of up to 5.1, showing clear evidence for crustal growth during their formation. The appearance of the Silurian granulite-facies metamorphism and magmatism in the Tongbai orogen provides a good paradigm of coeval granulite-facies metamorphism, magmatism and crustal growth in an arc-continent collision orogenic belt.

Petrogenesis of volcanic and intrusive rocks of the Zhuanqiao stage, Luzong Basin, Yangtze metallogenic belt, east China: implications for ore deposition

The Mesozoic Luzong volcanic basin is located in the Lower Yangtze River fault-depression, along the northern margin of the Yangtze Block. Volcanic and plutonic rocks are widely distributed in the basin and are spatially related to Cu, Fe, Au, Pb, and Zn ore deposits. The magmatic rocks are dominated by an alkali-rich shoshonitic suite of volcanics and intrusives, including both a trachybasalt-basaltic trachyandesite-trachyte series and a diorite-monzonite-syenite-alkali feldspar granite series. Volcanic sequences within the Luzong Basin are subdivided into four stages, namely the Longmenyuan, Zhuanqiao, Shuangmiao, and Fushan stages with magmatism occurring between 136 and 124 Ma. Most mineral deposits were formed during the Zhuanqiao stage from about 134 to 131 Ma, during a longer (140–130 Ma) period of transition from compression to extension within the Luzong Basin. Major element concentrations in the Zhuanqiao volcanic rocks range from 53.97 to 62.59 wt.% SiO2, 0.57–0.94 wt.% TiO2, 3.52–6.07 wt.% Na2O, and 3.60–8.66 wt.% K2O. Combined K2O + Na2O totals lie between 7.87 and 12.49 wt.%, and K2O + Na2O/Al2O3 ranges between 0.47 and 0.71. In comparison, the Zhuanqiao intrusions have SiO2 concentrations between 52.19 and 67.70 wt.%, TiO2 between 0.26 and 1.13 wt.%, Na2O between 1.33 and 6.55 wt.%, and K2O between 2.15 and 8.25 wt.%, with K2O + Na2O values ranging from 5.92 to 12.39 wt.% and K2O + Na2O/Al2O3 ratios between 0.37 and 0.75. The Zhuanqiao volcanic rocks have an average ∑REE (rare earth element) content of 223.17 ppm, whereas the average ∑REE of the Zhuanqiao intrusives is 250.06 ppm. Weak negative Eu anomalies (Eu/Eu* between 0.69 and 1.00) characterize the volcanics, whereas a larger range with more significant negative Eu anomalies (Eu/Eu* values of 0.35–0.94) is present in the plutonics. The Zhuanqiao volcanic rocks and plutons have (Nb/Th)PM ratios of 0.05–0.14 and 0.02–0.14 and (La/Sm)PM of 3.33–4.95 and 3.36–7.29, respectively. Initial 87Sr/86Sr and 143Nd/144Nd ratios and ϵSr(t) and ϵNd(t) of the Zhuanqiao volcanic rocks are 0.7059–0.7067, 0.51194–0.51218, 22.0–33.3, and −10.2 to −5.5, respectively; those of the intrusives are 0.7060–0.7082 and 0.51203–0.51214 and 23.3–54.7 and −8.6 to −6.4, respectively. The geochemistry presented here suggests that both volcanic and intrusive Zhuanqiao stage magmatic rocks were cogenetic and were probably derived from an enriched mantle (EM) source, most probably metasomatized mantle relating to the EM type-I (EM-I). The Zhuanqiao stage magmatism apparently was controlled by differentiation and assimilation of crustal material in a high-level magma chamber. Formation of the Zhuanqiao stage magmatic rocks coincided with the formation of much of the Cu and Fe mineralization within the Luzong Basin. This coeval magmatism and mineralization during the Zhuanqiao stage can be subdivided into three differing mineral deposit affinities: magmatism associated with magmato-hydrothermal vein Cu (e.g. the Jingbian, Shimenan, and Chuanshandong deposits), porphyry-hosted Fe (e.g. the Luohe and Nihe deposits), and skarn Fe (e.g. the Longqiao deposit). Combining geological characteristics with variations in SiO2, K2O + Na2O, and MgO enabled the division of the Zhuanqiao stage magmatism in the Luzong Basin into geochemically distinct units associated with these differing styles of mineralization.

Devonian A-type granitic magmatism on the northern margin of the North China Craton: SHRIMP U–Pb zircon dating and Hf-isotopes of the Hongshan granite at Chifeng, Inner Mongolia, China

We report first data for the Devonian Hongshan A-type granite along the northern margin of the Precambrian North China Craton. The granite is rich in silica (SiO 2 = 76.07–77.62 wt.%) and alkalis (Na 2O + K 2O = 8.45–8.78 wt.%), but poor in calcium (CaO = 0.28–0.70 wt.%) and magnesium (MgO = 0.08–0.20 wt.%). Chondrite-normalized REE patterns for the samples from the Hongshan granite show significant negative Eu anomalies (Eu/Eu* = 0.13–0.19). They contain high Rb, Ga, Y, Nb, and Zn, but low Ba and Sr contents, and in the multi-element spider diagrams normalized to primitive mantle, they show a similar pattern to that of A-type granites, such as negative Ba, Nb, Ta, Sr, P and Ti anomalies. Geochemical plots also show an affinity with A-type granites and suggest a post-orogenic setting (A 2 type granite). SHRIMP U–Pb zircon dating yielded a concordant weighted mean 206Pb/ 238U age of 387 ± 4 Ma, which defines a Devonian magmatic event. The zircons have negative ε Hf ( t) of − 8.4 to − 11.5 and give old Hf model ages of T DM2 = 2548–2820 Ma (mean 2667 Ma). Most 176Hf/ 177Hf and ε Hf values display Hf-isotope evolutional line of the lower continental crust. These results suggest that the Hongshan granite originated from melting of Archaean lower crust beneath the North China Craton in a post-orogenic extensional setting during the Devonian.

Fault controlled Carboniferous A-type magmatism in the proto-Andean foreland (Sierras Pampeanas, Argentina): Geochemical constraints and petrogenesis

The intrusion of granitoids into the Eastern Sierras Pampeanas in the Early Carboniferous took place after a long period of mainly compressional deformation that included the Famatinian (Ordovician) and Achalian (Devonian) orogenies. These granitoids occur as small scattered plutons emplaced in a dominant extensional setting, within older metamorphic and igneous rocks, and many of them are arranged along a reactivated large shear zone. A set of 46 samples from different granitic rocks: Huaco granitic complex, San Blas pluton, and the La Chinchilla stock from the Sierra de Velasco, Zapata granitic complex from Sierra de Zapata, and the Los Árboles pluton from Sierra de Fiambalá, display high and restricted SiO 2 contents between 69.2 and 76.4 wt.%. On both FeO/(FeO + MgO) vs. SiO 2 and [(Na 2O + K 2O) − CaO] vs. SiO 2 plots the samples plot in the ferroan and alkaline-calcic to calco-alkaline fields (FeO/(FeO + MgO) = 0.88–1.0%;[(Na 2O + K 2O) − CaO] = 6.3–8.3%), thus showing an A-type granitoid signature. The high concentrations for the High Field Strength Elements (HSFE), such as Y, Nb, Ga, Ta, U, Th, etc. and flat REE patterns showing significant negative Eu anomalies are also typical features of A-type granites. Our petrogenetic model supports progressive fractional crystallization with dominant fractionation of feldspar and a source mineral assemblage enriched in plagioclase. Biotites have distinctive compositions with high FeO/MgO ratios (7.8–61.5), F (360–5610 ppm), and Cl (120–1050 ppm). The FeO/MgO ratios together with the F and Cl content of igneous biotites seem to reflect the nature of their parental host magmas and may be useful in identifying A-type granitoids. The isotopic data (Rb–Sr and Sm–Nd) confirm that the A-type granites represent variable mixtures of asthenospheric mantle and continental crust and different mixtures lead to different subtypes of A-type granite (illustrating the lack of consensus about A-type magma origin). We conclude that prominent shear zones play an important role in providing suitable conduits for ascending asthenospheric material and heat influx in the crust, a hypothesis that is in accord with other recent work on A-type granites.

Geochemistry and provenance of stream sediments of the Ganga River and its major tributaries in the Himalayan region, India

Major, trace and REE compositions of sediments from the upper Ganga River and its tributaries in the Himalaya have been examined to study the weathering in the Himalayan catchment region and to determine the dominant source rocks to the sediments in the Plains. The Ganga River rises in the Higher Himalaya from the Higher Himalayan Crystalline Series (HHCS) bedrocks and traverses over the Lesser Himalayan Series (LHS) and the Himalayan foreland basin (Siwaliks) rocks before entering into the Gangetic Plains. The major element compositions of sediments, reflected in their low CIA values (45.0–54.7), indicate that silicate weathering has not been an important process in the Himalayan catchment region of the Ganga River. Along the entire traverse, from the HHCS through LHS and the Siwaliks, the sediments from the tributaries and the mainstream Ganga River show higher Na2O, K2O, CaO and silica. This, and the higher ratios of La/Sc, Th/Sc and lower ratios of Co/Th, suggest that the source rocks are felsic. The fractionated REE patterns and the significant negative Eu anomalies (Eu/Eu⁎ = 0.27–0.53) indicate highly differentiated source. Moreover, the comparison of the sediments with different source rock lithologies from the HHCS and the LHS for their major elements clearly suggests that the HHCS rocks were the dominant source. Further, comparison of their UCC (upper continental crust) normalized REE patterns suggests that, among the various HHCS rocks, the metasediments (para-gneiss and schist) and Cambro-Ordovician granites have formed the major source rocks. The Bhagirathi and Alaknanda River sediments are dominantly derived from metasediments and those in the Mandakini River from Cambro-Ordovician granites. The resulting composition of the sediments of the Ganga River is due to the mixing of sediments supplied by these tributaries after their confluence at Devprayag. No further change in major, trace and rare earth element compositions of the sediments of the Ganga River after Devprayag up to its exit point to the Plains at Haridwar, suggests little contribution of the Lesser Himalayan and Siwalik rocks to the Ganga River sediments.

On the Pan-African transition of the Arabian–Nubian Shield from compression to extension: The post-collision Dokhan volcanic suite of Kid-Malhak region, Sinai, Egypt

The Pan-African Kid-Malhak Dokhan volcanic suite (609 ± 12 Ma) is exposed in the northernmost part of the Arabian–Nubian Shield. The suite consists of non-metamorphosed varicolored alternating succession of porphyritic lava flows of commonly felsic composition (rhyolite–dacite) interlayered with compositionally equivalent pyroclastic beds (dominantly ignimbrites). These Dokhan volcanics are quite evolved (SiO 2 ≈ 65–77 wt.%), with strong high-K calc-alkaline affinity and are characterized by relative enrichment in total alkalis, Ba, Y, Zr and total REEs, depletion in Sr, and a LREE-enriched REE patterns with significant negative Eu anomalies. The Kid-Malhak Dokhan lavas display geochemical characteristics of both orogenic arc-type and anorogenic within-plate environments, suggesting eruption in a transitional “post-collisional tectonic setting. The ages of emplacement of the Dokhan volcanics in Egypt including that of Kid-Malhak region (580–620 Ma) coincide with end of the documented collision between the juvenile Arabian–Nubian crust and Saharan Metacraton and the subsequent extensional collapse event. This post-collision transition from compression to extension is explained by the extensional collapse following continental collision, which was controlled mainly by lithospheric delamination and slab breakoff (passive rifting). Various trace element characteristics discussed herein have indicated that the studied Dokhan magma was highly likely generated from crustal sources and that assimilation–fractional crystallization (AFC) and crustal contamination have played a major role and are most probably superimposed on fractional crystallization during the magmatic evolution of Kid-Malhak Dokhan volcanic suite. The eruption of the high-K calc-alkaline post-collisional Dokhan volcanics in Egypt defines a tectono-magmatic transition between the older calc-alkaline arc-related and the subsequent alkaline magmatism in the northern part of the Arabian–Nubian Shield.

Geochemistry, age and tectonic significance of granitic rocks in north Altun, northwest China

Granitic rocks in the Altun terrane, NW China crop out in a 50-km-long, E–W-trending belt along the southern margin of the Bashikaogong Basin, north Altun. They intrude the north Altun subduction complex and are hosted in Precambrian sandstones, schists, mudstones and tuffs. Four lithologic types are recognized; (I) gray, very coarsely phyric granite, (II) red, fine- to medium-grained, aphyric granite, (III) gray, medium- to coarse-grained, moderately phyric granite and (IV) pink, medium- to coarse-grained, moderately phyric granite. All of the granites are peraluminous with normative corundum ranging from 0.8 to 3.9 wt.%. Silica contents vary between 65.1% and 75.7%, and total alkalis (Na2O+K2O) range from 7.5 to 9.0 wt.%, giving K2O/Na2O ratios of 1.1 to 2.7. Rittman indices vary from 1.8 to 2.8, with an average of 2.3. Total rare earth elements (∑REE) are between 89 and 335 ppm, and all samples show significant but variable negative Eu anomalies on chondrite-normalized REE patterns. On mantle-normalized trace element patterns all the granites have pronounced negative Ba, Nb, Sr, P and Ti anomalies. Zircons from the granites show slightly different SHRIMP U–Pb ages for the different lithologies; 474.3±6.8 Ma for type I granites, 446.6±5.2 Ma for type II, 434.5±3.8 Ma for type III, and 431.1±3.8 Ma for type IV. The geochemical and geological data indicate that these are S-type granites formed by partial melting of crustal materials during or shortly after collision of the Altun and Tarim continental blocks in the Paleozoic. The age and geochemistry of the north Altun granites suggest that they can be correlated with similar rocks in north Qilian on the opposite side of the Altun strike-slip fault.

Petrology, geochronology and tectonic implications of Mesozoic high Ba–Sr granites in the Haemi area, Hongseong Belt, South Korea

AbstractCollision between the North and South China continental blocks began in the Korean peninsula during the Permian (290–260 Ma). The Haemi area in the Hongseong collision belt (proposed as the eastern extension in South Korea of the Dabie–Sulu collision zone of China) within the Gyeonggi Massif comprises post‐collisional high Ba–Sr granite with intermediate enclaves that intruded into the Precambrian rocks. The intermediate enclaves have a shoshonitic affinity whereas the granite is a high‐K calc‐alkaline variety. The chondrite‐normalized rare earth element (REE) pattern with relative enrichment of LREE over HREE and absence of a significant negative Eu anomaly typifies both enclaves and granite. Geochemical similarities of enclaves and granite are attributed to the involvement of enriched mantle sources in their genesis. However, dominant crustal components were involved in the formation of high Ba–Sr granites. A granite crystallization age of 233 ± 2 Ma was obtained from SHRIMP U–Pb zircon dating. This age is slightly younger than the Triassic collision event in the Hongseong Belt. Geochemical data, U–Pb zircon age, and regional tectonics indicate that the Haemi high Ba–Sr granite formed in a post‐collisional tectonic environment. A Mesozoic post‐collisional lithospheric delamination model can account for the genesis of high Ba–Sr granite in the Haemi area.

Geochemistry of shales of the Upper Cretaceous Hayang Group, SE Korea: Implications for provenance and source weathering at an active continental margin

Shales of the Upper Cretaceous Hayang Group in the Yeongyang Subbasin, southeastern Korea were deposited in fluvio-lacustrine environments as the Paleo-Pacific (Izanagi) Plate obliquely subducted beneath the Asian continent. Major and trace element compositions derived from shales analyzed from the Hayang Group reveal geochemical signatures that are different from Post-Archean average Australian Shale (PAAS), but are typical of igneous rocks found in the continental-margin island arc settings. For example, most major element concentrations are depleted relative to PAAS; among them MgO and MnO depletions are most pronounced in all shales. CaO enrichment is notable in some formations due to the presence of secondary calcite as dispersed micro-calcite nodules. Sc, Y, and Th concentrations are also slightly depleted relative to PAAS while chondrite-normalized REE data for shales from the Hayang Group show moderate to large uniform LREE enrichment but variable HREE depletions. In addition, they are more fractionated than PAAS, and have significant negative Eu anomalies. Moreover, the chemical index of alteration and A–CN–K relations comply that the source area was dominated by non-steady state weathering regimes indicative of active uplift along an active continental margin. These regions were punctuated by two periods of steady-state weathering or alternatively by enhanced recycling of sediments. The latter interpretation is in accord with the timing of exposure of a cherty sequence in the source area. Mixing calculations based on REE data suggest that the average shale in the Hayang Group was derived from sources composed of Precambrian granitic gneiss, arc basaltic rocks, Triassic Yeongdeok Granite in the subbasin, and the Mino accretionary complex of SW Japan with ratios around 43.7 (31.8–73.2):26.4 (15.8–36.7):15.4 (2.8–33.9):14.5 (0.0–33.8). However, these proportions change up sequence.

Geochronology and geochemistry of the Fangcheng Neoproterozoic alkali-syenites in East Qinling orogen and its geodynamic implications

Finding of Neoproterozoic syenites at Fangcheng in the northern Qinling region of the East Qinling orogen provides an important constraint on timing of tectonic transformation to extensional regime. The alkaline pluton consists mainly of nepheline syenite, aegirine syenite, and alkali-feldspar syenite. The syenites are of intermediate (SiO2 = 54%–62%), rich in alkali (K2O+Na2O = 12%–15%), aluminum (Al2O3 = 16.81%–23.26%) and large ion lithophile elements (LILE), without any obvious Nb, Ta, Zr, and Hf anomalies. The Fangcheng syenites are geochemically characterized by relative enrichment of LREE, minor differentiation of HREE, significant negative Eu anomalies (Eu = 0.13–0.23), less negative eNd(t) values of −1.37 to −3.90, young Nd model ages of 1364 to 1569 Ma, and high zircon saturation temperatures of 915 to 1044°C. The syenitic magmas probably originated from small proportion melting of upper mantle in an extensional regime of intraplate-anorogenic tectonic setting, and have been slightly contaminated by crustal materials during ascending and/or emplacement. LA-ICP-MS zircon U-Pb dating yields ages of 844.3±1.6 Ma (MSWD = 0.86), suggesting that the Fangcheng alkaline syenites formed in the early Neoproterozoic. They are the oldest Neoproterozoic alkaline rocks ever recognized in the Qinling orogen as well as in South China. This implies that the tectonic regime of the Qinling region would have transformed from post-collisional stretch to intraplate-anorogenic extension no later than 844 Ma.

Timing of UHP metamorphism in the Hong’an area, western Dabie Mountains, China: evidence from zircon U–Pb age, trace element and Hf isotope composition

The Hong’an area (western Dabie Mountains) is the westernmost terrane in the Qinling-Dabie-Sulu orogen that preserves UHP eclogites. The ages of the UHP metamorphism have not been well constrained, and thus hinder our understanding of the tectonic evolution of this area. LA-ICPMS U–Pb age, trace element and Hf isotope compositions of zircons of a granitic gneiss and an eclogite from the Xinxian UHP unit in the Hong’an area were analyzed to constrain the age of the UHP metamorphism. Most zircons are unzoned or show sector zoning. They have low trace element concentrations, without significant negative Eu anomalies. These metamorphic zircons can be further subdivided into two groups according to their U–Pb ages, and trace element and Lu–Hf isotope compositions. One group with an average age of 239 ± 2 Ma show relatively high and variable HREE contents (527 ≥ LuN ≥ 14) and 176Lu/177Hf ratios (0.00008–0.000931), indicating their growth prior to a great deal of garnet growth in the late stage of continental subduction. The other group yields an average age of 227 ± 2 Ma, and shows consistent low HREE contents and 176Lu/177Hf ratios, suggesting their growth with concurrent garnet crystallization and/or recrystallization. These two groups of age are taken as recording the time of prograde HP to UHP and retrograde UHP–HP stages, respectively. A few cores have high Th/U ratios, high trace element contents, and a clear negative Eu anomaly. These features support a magmatic origin of these zircon cores. The upper intercept ages of 771 ± 86 and 752 ± 70 Ma for the granitic gneiss and eclogite, respectively, indicate that their protoliths probably formed as a bimodal suite in rifting zones in the northern margin of the Yangtze Block. Young Hf model ages (TDM1) of magmatic cores indicate juvenile (mantle-derived) materials were involved in their protolith formation.

Geochemistry of black shales from the Lower Cretaceous Paja Formation, Eastern Cordillera, Colombia: Source weathering, provenance, and tectonic setting

The major and trace element characteristics of black shales from the Lower Cretaceous Paja Formation of Colombia are broadly comparable with those of the average upper continental crust. Among the exceptions are marked enrichments in V, Cr, and Ni. These enrichments are associated with high organic carbon contents. CaO and Na 2O are strongly depleted, leading to high values for both the Chemical Index of Alteration (77–96) and the Plagioclase Index of Alteration (86–99), which indicates derivation from a stable, intensely weathered felsic source terrane. The REE abundances and patterns vary considerably but can be divided into three main groups according to their characteristics and stratigraphic position. Four samples from the lower part of the Paja Formation (Group 1) are characterized by LREE-enriched chondrite-normalized patterns (average La N/Yb N = 8.41) and significant negative Eu anomalies (average Eu/Eu ∗ = 0.63). A second group of five samples (Group 2), also from the lower part, have relatively flat REE patterns (average La N/Yb N = 1.84) and only slightly smaller Eu anomalies (average Eu/Eu ∗ = 0.69). Six samples from the middle and upper parts (Group 3) have highly fractionated patterns (average La N/Yb N = 15.35), resembling those of Group 1, and an identical average Eu/Eu ∗ of 0.63. The fractionated REE patterns and significant negative Eu anomalies in Groups 1 and 3 are consistent with derivation from an evolved felsic source. The flatter patterns of Group 2 shale and strongly concave MREE-depleted patterns in two additional shales likely were produced during diagenesis, rather than reflecting more mafic detrital inputs. An analysis of a single sandstone suggests diagenetic modification of the REE, because its REE pattern is identical to that of the upper continental crust except for the presence of a significant positive Eu anomaly (Eu/Eu ∗ = 1.15). Felsic provenance for all samples is suggested by the clustering on the Th/Sc–Zr/Sc and Gd N/Yb N–Eu/Eu ∗ diagrams. Averages of unmodified Groups 1 and 3 REE patterns compare well with cratonic sediments from the Roraima Formation in the Guyana Shield, suggesting derivation from a continental source of similar composition. In comparison with modern sediments, the geochemical parameters (K 2O/Na 2O, La N/Yb N, La N/Sm N, Eu/Eu ∗, La/Sc, La/Y, Ce/Sc) suggest the Paja Formation was deposited at a passive margin. The Paja shales thus represent highly mature sediments recycled from deeply weathered, older, sedimentary/metasedimentary rocks, possibly in the Guyana Shield, though Na-rich volcanic/granitic rocks may have contributed to some extent.

Nd‐Sr Isotopic Geochemistry of the Late Archean‐Paleoproterozoic Granitoids in the Lüliang‐Wutai Terrain, North China Craton, and Implications for Petrogenesis

Abstract: In this paper we report geochemical and Nd‐Sr isotopic data for a late Archean gneissic granitic pluton (Hengling pluton), an early Paleoproterozoic complex (Xipan complex) and a late Paleoproterozoic granitic pluton (Yunzhongshan granites) from the Lüliang‐Wutai terrain, North China, to trace the source of these late Archean‐Paleoproterozoic granitoids and, particularly, to understand the nature and mechanism of continental growth at that time. The Hengling granitic gneisses (ca. 2.51 Ga) are characterized by high Na2O and LILEs, TTG‐like REE patterns (highly depleted HREE and minor Eu anomalies) and moderately depleted Nd‐Sr isotopic compositions (∍Nd(t) =1.2‐2.7, ISr=0.7015‐0.7019), and were considered as being products of arc magmatism that was developed upon the North China craton. The Xipan complex (ca. 2.2 Ga) contain gabbroic diorite and monzonite, mostly being Na2O‐rich, highly fractionated REE patterns and isotopically enriched (∍Nd(t) =−1.5 to −4.1, ISr=0.7038‐0.706). The gabbroic diorites probably originated from melting of an enriched mantle source, but significantly contaminated by lower crustal material, and the monzonites probably represent a product of a mixture between the gabbroic dioritic magma and granitic melts of crustal origin. The Yunzhongshan post‐collisional granitoids (ca. 1.8 Ga) are characterized by high‐K affinity and highly‐enriched and homogeneous Nd isotopic compositions (∍Nd(t) = −4.9 to −5.7), although they split into two groups in terms of REE patterns: one group showing elevated HREE (and Sc, Y and Zr) with significant negative Eu anomalies and the other showing highly depleted HREE and, to a lesser extent, mid‐REE with negligible Eu anomalies. These granites are genetically related to a process of extensional collapse of a thickened orogen. They formed through magma mixing between mantle‐derived basaltic magmas and crust‐derived granitic melts, followed by significant fractionation of ferromagnesian phases (like hornblende and Cpx) and feldspar and accessory zircons. Some Yunzhongshan granites show very old Nd model ages (2.9‐3.0 Ga), suggesting the existence of continental crust older than 2.7 Ga, which is supported by our zircon Hf isotopic data for these granites.

Shrimp U–Pb zircon geochronology, geochemistry, and Nd–Sr isotopic study of contrasting granites in the Emeishan large igneous province, SW China

The Cida A-type granitic stock (∼ 4 km 2) and Ailanghe I-type granite batholith (∼ 100 km 2) in the Pan-Xi (Panzhihua-Xichang) area, SW China, are two important examples of granites formed during an episode of magmatism associated with the Permian Emeishan mantle plume activity. This is a classic setting of plume-related, anorogenic magmatism exhibiting the typical association of mantle-derived mafic and alkaline rocks along with silicic units. SHRIMP zircon U–Pb data reveal that the Cida granitic pluton (261 ± 4 Ma) was emplaced shortly before the Ailanghe granites (251 ± 6 Ma). The Cida granitoids display mineralogical and geochemical characteristics of A-type granites including high FeO ⁎/MgO ratios, elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios, which are much higher than those of the Ailanghe granites. All the granitic rocks show significant negative Eu anomalies and demonstrate the characteristic negative anomalies in Ba, Sr, and Ti in the spidergrams. It can be concluded that the Cida granitic rocks are highly fractionated A-type granitoids whereas the Ailanghe granitic rocks belong to highly evolved I-type granites. The Cida granitoids and enclaves have Nd and Sr isotopic initial ratios ( ε Nd( t) = − 0.25 to + 1.35 and ( 87Sr/ 86Sr) i = 0.7023 to 0.7053) close to those of the associated mafic intrusions and Emeishan basalts, indicating the involvement of a major mantle plume component. The Ailanghe granites exhibit prominent negative Nb and Ta anomalies and weakly positive Pb anomalies in the spidergram and have nonradiogenic ε Nd( t) ratios (− 6.34 to − 6.26) and high ( 87Sr/ 86Sr) i values (0.7102 to 0.7111), which indicate a significant contribution from crustal material. These observations combined with geochemical modeling suggest that the Cida A-type granitoids were produced by extensive fractional crystallization from basaltic parental magmas. In contrast, the Ailanghe I-type granites most probably originated by partial melting of the mid-upper crustal, metasedimentary–metavolcanic rocks from the Paleo-Mesoproterozoic Huili group and newly underplated basaltic rocks. In the present study, it is proposed that petrogenetic distinctions between A-type and I-type granites may not be as clear-cut as previously supposed, and that many compositional and genetically different granites of the A- and I-types can be produced in the plume-related setting. Their ultimate nature depends more importantly on the type and proportion of mantle and crustal material involved and melting conditions. Significant melt production and possible underplating and/or intrusion into the lower crust, may play an important role in generating the juvenile mafic lower crust (average 20 km) in the central part of the Emeishan mantle plume.

Pan-African Younger Granitoids of the Southern Eastern Desert, Egypt: Geology, Geochemical Constraints, and Mineralization

Younger granitoids of the Gabal El Farayid—Gabal El Marafawy district in the southern Eastern Desert, Egypt are compositionally syenogranites. Based on petrological and geochemical studies, they are classified as two distinct types: the biotite-hornblende subsolvus El-Marafaway I-type syenogranite; and the biotite-bearing hypersolvus El-Farayid A-type syenogranite. The subsolvus El-Marafawy syenogranite is characterized by low Rb/Sr ratios and high field strength element (HFSE) concentrations (Nb, Ta, Y), fractionated LREEs with flat HREEs, high LREE/HREE ratios, negative Nb-Ta anomalies, and lack of negative Ba and Eu anomalies. The hypersolvus El-Farayid syenogranite (HFS) is characterized by high Rb/Sr ratios and HFSE concentrations, slightly fractionated (HFSI) to unfractionated (HFSII) LREE with a positive slope for the HREE, absence of negative Nb-Ta anomalies and significant negative Ba, Ce, Zr, and Eu anomalies. Based on field and geochemical data, the late-tectonic subsolvus El-Marafawy syenogranite may have been produced by dehydration partial melting of an amphibolitic source, followed by high-pressure fractionation. The post-tectonic hypersolvus El-Farayid syenogranite probably formed from a residual magma by crystallization of biotite, plagioclase, K-feldspar and accessory monazite, allanite, and titanite in a shallow-level magma chamber emplaced along strike-slip faults of a convergent margin. Generally, the syenogranites were generated during a period of rapid tectonic transition from crustal thickening during subduction (subsolvus El-Marafawy syenogranite) to crustal thinning during extension (hypersolvus El-Farayid syenogranite), which represents the final magmatic stage of an extensive arc system. U and Th contents in the above rock types are comparable, indicating that these elements are mainly contained in accessory minerals that controlled their distributions.

Neoproterozoic Bimodal Intrusive Complex in the Southwestern Tarim Block, Northwest China: Age, Geochemistry, and Implications for the Rifting of Rodinia

We report here the first SHRIMP U-Pb zircon ages and bulk-rock geochemistry for a Neoproterozoic bimodal intrusive complex in the Tarim block and discuss the tectonic implications. The Kudi igneous complex (KIC) crystallized at 783 ± 10 Ma. Granite and gabbro comingled, indicating that the intrusions were coeval. The granite displays: high total alkalis (Na2O + K2O = 8-11 wt%); high abundances of Zr, Nb, Y, and total REE; and strong LREE enrichment [(La/Yb)N = 9-59] with a significant negative Eu anomaly (δdLEu = 0.17-0.44), characteristic of A-type granites. Furthermore, samples of the granite have homogeneous positive εNd(T) values of 1.4 to 2.1 and Mesoproterozoic Nd modal ages (∼1.3 Ga). We thus interpret the granite as probably generated by partial melting of the regional Mesoproterozoic arc and/or back-arc crust. The KIC gabbro is geochemically similar to intraplate alkaline basalts, with trace elements and isotope compositions indicating an OIB-type magma source, variably contaminated by continental crust. Field observations, petrology, and geochemistry indicate that the KIC was a bimodal alkaline complex formed by mixing of these two magma types of different origins in a continental rift, possibly related to a mantle superplume beneath the Rodinian supercontinent.

Geochemistry of the Willsher Group, southeast Otago, New Zealand, and comparison with the Murihiku and Dun Mountain‐Maitai Terranes

Triassic Willsher Group sandstones and siltstones in the Kaka Point Structural Belt of southeast Otago have geochemical provenance signatures and petrographic characteristics indicative of derivation from a moderately evolved, partially dissected, medium‐K volcanic arc, probably of continental island arc nature. Pre‐Etalian and Etalian sandstones are andesitic to dacitic in composition, whereas those in the Kaihikuan Stage are more felsic. These features are demonstrated by increased silica contents, immobile element ratios, and rare earth element parameters including LaN/YbNand LaN/SmN ratios. Significant negative Eu anomalies appear at the top of the Willsher succession. Comparison with the chemistry of coeval and older sandstones from the adjoining Dun Mountain‐Maitai Terrane shows the Willsher Group sediments are substantially more felsic, and close linkage between the two terranes can be ruled out. Comparison with coeval intervals in the adjacent Murihiku Terrane, including new rare earth element data presented and discussed here, shows that pre‐Etalian Willsher sandstones are more felsic, but that Etalian and Kaihikuan equivalents are similar in composition. In both the Willsher Group and the Murihiku Terrane, sandstones become more felsic stratigraphically upward, although the Willsher Group shows more variability within age groups, probably due to local variations in source proportions. These features and others previously described suggest the Kaka Point Structural Belt should be regarded as a separate subterrane of a greater Murihiku Terrane.

Elemental and Sr?Nd isotopic systematics of the early Mesozoic volcanic sequence in southern Jiangxi Province, South China: petrogenesis and tectonic implications

Elemental and Sr–Nd isotopic results are presented for the early Mesozoic volcanic sequence (~172 Ma) in southern Jiangxi Province, South China. The sequence is voluminously composed of ~45% subalkaline basaltic rocks (group 1), <5% high-mg andesite–dacites (group 2) and ~50% rhyolites (group 3). The group 1 rocks are characterized by (La/Yb)cn = 3.8–7.2, Eu/Eu* = 0.65–1.15, Nb/La = 0.64–0.99, 87Sr/86Sr(t) = 0.70602~0.70822 and eNd(t) = −1.63 to +0.11, similar to those of an EMII-like source. The group 2 rocks have mg=0.42~0.60, SiO2=60.24~66.71%, MgO=2.65~ 5.54%, Ni=24~102 ppm and Cr=84~266 ppm, classified as high-mg andesitic rocks. These rocks are more enriched in LILEs and LREE with more significant negative Eu anomaly (0.63~0.79), are more depleted in HFSEs with Nb/La ratios of 0.40–0.56 and have lower eNd(t) (−9.44 to −7.78) and higher 87Sr/86Sr(t) (0.70985~0.71016), in comparison with the group 1 rocks. They most likely originated from metasomatised veins in the lithospheric mantle. The origination of the group 1 and group 2 magma suggests the development of a peridotite-plus-vein lithospheric mantle during early Mesozoic era beneath the interior of the Cathaysian block. The group 3 rhyolites are characterized by high SiO2 (72.75~77.97%), Zr (99~290 ppm), Hf (3.9~9.7 ppm) and Ga/Al (2.76~3.87) and significant Nb–Ta, Ba–Sr and P–Ti depletions. These rhyolites exhibit Sr–Nd isotopic compositions (87Sr/86Sr(t) = 0.70962~0.71104, eNd(t) = −4.63 to −5.80) similar to the contemporaneous Zhaibei and Pitou A-type granites in the area. Such characteristics suggest that they might be derived from the underplating basaltic magma contaminated by crustal materials. Therefore, an early Mesozoic rifting model in response to intracontinental lithospheric extension is proposed to account for the early Mesozoic volcanism in southern Jiangxi Province, South China.

A Jurassic garnet-bearing granitic pluton from NE China showing tetrad REE patterns

NE China is characterized by the massive distribution of Phanerozoic granitoids. Most of them are of I- and A-type granites, whereas S-type granites are rarely documented. The present work deals with the Dongqing pluton, a small granitic body emplaced in the southern Zhangguangcai Range. The pluton comprises a two-mica (±garnet) granite and a garnet-bearing muscovite granite; the latter occurs as veins in the former. The pluton shows a gradational contact with the surrounding host granites. Rb–Sr and Sm–Nd isotope analyses on whole-rocks and minerals reveal that the two types of granites were emplaced synchronously at about 160 Ma. The pluton was emplaced coeval with the surrounding I-type granitic pluton, and had a rapid cooling history. It is characterized by an initial Sr isotopic ratio of ∼0.706, slightly negative ε Nd( T) values (−0.5 to −1.9) and young depleted-mantle model ages (970–1090 Ma). This suggests that the parent magma originated from partial melting of relatively juvenile crust, which is largely compatible with the general scenario for much of the Phanerozoic granitoids emplaced in the Central Asian Orogenic Belt. Geochemically, the granites of the Dongqing pluton are peraluminous, with a Shand Index (molar ratio A/CNK) of 1.0–1.1 for the two-mica granites and 1.2–1.3 for the garnet-bearing granites. All the garnet-bearing granites and some of the two-mica granites show tetrad REE patterns (=tetrad group), whereas most two-mica granites show normal granitic REE patterns (=normal group). The normal group granites exhibit depletion in Nb, Ta, P and Ti in spidergrams, and generally weak positive Eu anomalies in REE patterns. By contrast, the tetrad group granites manifest depletion in Ba, Nb, Ta, Sr, P, and Ti and significant negative Eu anomalies. The trace element data constrain the parental magmas to having undergone extensive magmatic differentiation. During their late stage magmatic evolution, intense interaction of residual melts with aqueous hydrothermal fluids resulted in the non-CHARAC (charge and radius controlled) trace element behavior and the tetrad effect in REE distribution patterns. This, in turn, leads to the invalidation of the commonly used tectonic discrimination criteria derived from trace element abundances of normal granites. In view of this and previous studies, we conclude that there were probably no S-type granites produced in NE China during the Phanerozoic. Consequently, weathered sedimentary material did not play an important role in the genesis of the strongly peraluminous granites in the Zhangguangcai Range.