Contrasting provenance of Late Archean metasedimentary rocks from the Wutai Complex, North China Craton: detrital zircon U–Pb, whole-rock Sm–Nd isotopic, and geochemical data

Detrital zircon U–Pb ages, whole-rock Nd isotopic, and geochemical data of metasedimentary rocks from the Wutai Complex in the Central Zone, North China Craton, have been determined. Compositionally, these rocks are characterized by a narrow variation in SiO₂/Al₂O₃ (2.78–3.96, except sample 2007-1), variable Eu anomalies, spanning a range from significantly negative Eu anomalies to slightly positive anomalies (Eu/Eu* = 0.58–1.12), and positive ε _Nd (t) values (0.1–1.97). The 18 detrital zircons of one sample yielded age populations of 2.53 Ga, 2.60 Ga, and 2.70–2.85 Ga. Geochemical data reveal intermediate source weathering, varying degrees of K-metasomatism in the majority of these metasedimentary rocks, whereas other secondary disturbances seem to be negligible. Detailed analysis in detrital zircon U–Pb geochronology, whole-rock Nd isotope, and geochemistry shows that these metasedimentary rocks are derived from a mixed provenance. The predominant derivation is from the late Archean granitoids and metamorphic volcanics in the Wutai Complex, and there is also input of older continental remnants, except TTG gneisses, from the Hengshan and Fuping Complexes. The sediments were probably deposited in fore-arc or/and intra-arc basins within an arc system.     

Geochemistry and petrogenesis of granitoids of Lesser Himalayan crystallines,Western Arunachal Himalaya

Low to medium grade crystalline rocks locally known as Bomdila Group extensively covers the Lesser Himalayan region in Western Arunachal Himalaya. This Group consists dominantly of mylonitic gneisses of granitic composition of Palaeoproterozoic age, named as Bomdila mylonitic gneiss (BMG) and a small body of hornblende bearing granite of Mesoproterozoic age known as Salari granite (SG). The BMG is affinity to peraluminous (A/CNK > 1.1) with high content of SiO₂, K₂O/Na₂O ratio, normative corundum, high ratio of FeO^t/MgO in biotite (3.21–5.11) that shows characteristics of S-type granite whereas SG has granodiorite composition with high Na₂O, low K₂O, presence of hornblende, normative diopside, low A/CNK ratio (<1.1) and low FeO^t/MgO ratio in biotite (1.58–1.60) indicates metaluminous I-type granite affinity. The SG has more fractionated nature of REE [(Ce/Yb)_N = 9.06–18.53] and minor negative Eu anomalies [Eu_N/Eu* = 0.69–0.94] as compared to BMG which has less fractionation of REE [(Ce/Yb)_N = 5.95–9.16] and strong negative Eu anomalies [Eu_N/Eu* = 0.37–0.43]. Geochemical and petrological studies suggest that the SG and BMG are not genetically related; SG appears to have derived from igneous source whereas the BMG have been derived from sedimentary source, however these granitoids might have produced during the same thermal event.     

REE Geochemical Evolution and Its Significance of Early Precambrian Metamorphic Terrain,Wuyang,Henan

The supracrustal rocks of the Wuyang metamorphic terrain are divided into the Zhao anzhuang,Tieshanmiao and Yangshuwan Formations.These three Formations were dated at 3000-2550Ma,2550-2300Ma and 2300-2200Ma,respectively.∑REE and La/Yb)n of the Zhao anzhuang Formation volcanic rocks are obviously higher than those of the Tiesanmiao Formation equivalents,suggesting a sedimentary gap(2550 Ma boundary)between these two formations,The Zhao‘anzhuang Formation is older than the Tieshanmiao Formation.The sediments of these two Formations show no obvious differences in REE and are generally characterized by low ∑REE and positive Eu anomalies.On the contrary,the sediments of the Yangshuwan Formation are characterized by high ∑REE and negative Eu anomalies.Detailed discussions demonstrate that the Yangshuwan Formation was deposited in an oxidizing environment whereas the other two formations were formed in a reducing environment.At the end of the evolution of the Tieshanmiao Formation about 2300 Ma ago,the sedimentary environment was transformed from reducing to oxidizing .On the basis of the SHAB (soft/hard acid and base)theory,an oxidation-reduction model for sedimentary REE evolution has been established .It is proposed that the mantle tends to become gradually depleted in REE.especially in LREE,and the indices ∑REE and La/Yb) n of mantle-dervived volcanic rocks also tend to become lower and lower.     

Geochemical Characteristics of Cenozoic Basaltic High-K Volcanic Rocks from Maguan Area, Eastern Tibet

The major element, trace element and Nd-Sr isotopic composition of Cenozoic basaltic volcanic rocks from the Maguan area, eastern Tibet, indicates that the volcanic rocks are enriched in alkalis, especially K (K2O up to 3.81%) and depleted in Ti (TiO2 = 1.27%-2.00%). These rocks may be classified as two groups, based on their Mg# numbers: one may represent primary magma (Mg# numbers from 68 to 69), and the other, the evolved magma(Mg# numbers from 49 to 57). Their REE contents are very high (∑REE = 155.06-239.04μg/g). Their REE distribution patterns are of the right-inclined type, characterized by LREE enrichment [(La/Yb)N =12.0-19.2], no Ce anomaly (Ce/Ce*=1.0), and weak negative Eu anomaly (Eu/Eu*=0.9). The rocks are highly enriched in Rb, Sr and Ba (59.5-93.8μg/g, 732-999 μg/g, and 450-632 g/g, respectively), high in U and Th (1.59-2.31μg/g and 4.73-8.16 μg/g, respectively), and high in Nb, Ta, Zr and Hf (70-118 μg/g,3.72-5.93 μg/g, 215-381 μg/g, and 5.47-9.03 μg/g, respectively). In the primitive mantle-normalized incompatible element spidergram, Nb, Ta, Zr, Hf and P show positive anomalies, whereas Ba, Ti and Y show negative anomalies. The 87Sr/86Sr ratios range from 0. 704029 to 0.704761; 143Nd/144Nd from 0. 512769 to 0. 512949; and εNd from 2.6 to 6.1. These geochemical features might suggest that the potential source of the basaltic high-K volcanic rocks in the Maguan area is similar to the OIB-source mantle of Hawaii and Kergeulen volcanic rocks.     

Composition,sources, and genesis of granitoids in the Irtysh Complex,Eastern Kazakhstan

Intrusions of the Irtysh Complex are spatially restricted to the regional Irtysh Shear Zone (ISZ) and are hosted in blocks of high-grade metamorphic rocks (Kurchum, Predgornenskii, Sogra, and others) in the greenschist matrix of the ISZ. The massifs consist of contrasting rock series from gabbro to plagiogranite and granite at strongly subordinate amounts of diorite and the practical absence of rocks of intermediate composition (tonalite and granodiorite). The complex was produced in the Early Carboniferous, simultaneously with the onset of the origin of the ISZ itself. The granitoids composing the complex affiliate with diverse petrochemical series (from subaluminous plagiogranite of the andesite series to granite of the calc-alkaline series) and contain similar REE and HFSE concentrations [total REE = 103–163 ppm (La/Yb) _n = 3.59–5.44, Zr (200–273 ppm), Nb (7.6–10.6 ppm), Hf (6.1–7.6 ppm), and Ta (0.68–1.19 ppm)] but are different in concentrations in LILE [Rb (3–9 and 121–221 ppm), Sr (213–375 and 77–148 ppm), and Ba (67–140 and 240–369 ppm)] and isotopic composition of Nd (ɛ_Nd(T) from +5.3 in the plagiogranite to −1.2 in the granite) and O (δ¹⁸O from +9.4 in the plagiogranite to +14.5 in the granite). Data on the geochemistry and isotopic composition of metamorphic rocks of the Kurchum block and numerical geochemical simulations indicate that the granitoids were generated via the melting of a heterogeneous crustal source, which consisted of upper crustal metapelites and metabasites of the oceanic basement of the blocks of high-grade metamorphic rocks. The differences in the chemical and isotopic compositions of the granitoids were predetermined by the mixing of variable proportions of granitoid magmas derived from metapelite and metabasite sources.     

Geochemistry and petrogenesis of radioactive Palaeoproterozoic granitoids of Kinwat crystalline inlier,Nanded and Yeotmal districts,Maharashtra

Kinwat crystalline inlier exposes Palaeoproterozoic granitoids belonging to the northern extensions of younger phase of Peninsular gneissic complex (PGC) within Deccan Trap country in Eastern Dharwar Craton (EDC) and bounded in south by a major NW-SE trending lineament (Kaddam fault). Geochemically, the Kinwat granitoids are similar to high-K, calc-alkaline to shoshonite magnesian granitoids and subdivided into two major groups, i.e. felsic group (pink and grey granites) and intermediate to felsic group (hybrid granitoids). The felsic group (∼67–74% SiO₂) shares many features with Neoarchaean to Palaeoproterozoic high potassic granites of PGC such as higher LILE and LREE content and marked depletion in Eu, P and HFSE, especially Nb, Ti, relative to LILE and LREE. The hybrid granitoids (∼58–67% SiO₂) have comparatively higher Ca, Mg and Na contents and slightly lower REE content than the granitoids of felsic group. Both, felsic and hybrid granitoids are metaluminous to weakly peraluminous and belong to highly fractionated I-type suite as evidenced by negative correlation of SiO₂ with MgO, FeO^t, CaO, Na₂O, Al₂O₃, whereas K₂O, Rb and Ba show sympathetic relationship with SiO₂. Moderate to strong fractionated REE patterns (Ce/Yb_N: ∼54–387) and strong negative Eu anomalies (Eu/Eu*: 0.13–0.41) are quite apparent in these granitoids. The geochemical characteristics together with mineralogical features such as presence of biotite±hornblende as the dominant ferromagnesian mineral phases point towards intracrustal magma source, i.e. derivation of magma by partial melting of probably tonalitic igneous protolith at moderate crustal levels for felsic granites, whereas hybrid granitoids appear to be products of juvenile mantle-crust interaction, in an active continental margin setting.     

The Neoproterozoic Kolet Um Kharit bimodal metavolcanic rocks, south Eastern Desert, Egypt: a case of enrichment from plume interaction?

Neoproterozoic metavolcanic rocks of Kolet Um Kharit (KUKh) in the southern Eastern Desert of Egypt have been traditionally regarded as a bimodal island-arc sequence. However, geological and geochemical arguments presented here make this interpretation doubtful. Geochemically, these rocks are classified into mafic (tholeiitic basalts) and felsic (high-K rhyodacites to rhyolites) groups. Both the KUKh mafic and felsic metavolcanic rocks show similar geochemical characteristics, implying a genetic link. They have comparable trace element ratios, such as Zr/Nb (27–30 vs. 20–36), Y/Nb (5.44–6.25 vs. 5.05–5.9), K/Rb (577–1164 vs. 573–937), Ba/La (4.29–25–9 vs. 11.4–16.2), Nb/Yb (1.82–2.03 vs. 1.76–1.99). Similarly both groups have parallel LREE-enriched patterns (La/Yb_CN=2.37–2.81 vs. 2.55–3.17); and negative Nb and Ta anomalies (Nb/La_pm=0.51–0.58 vs. 0.45–0.52 and Ta/La_pm=0.51–0.62 vs. 0.49–0.55). The observed negative Nb and Ta anomalies in the KUKh metavolcanic rocks cannot be attributed to crustal contamination or fractional crystallization. These rocks could represent either a remnant of break-up LIP or were derived from an enriched mantle source containing subduction components beneath an intraoceanic back-arc basin. The recognition of the KUKh rocks as derived from an enriched mantle source revives interest in models that involve enrichment from “plume” interaction during the evolution of the Arabian-Nubian Shield.     

Cenozoic volcanic rocks in the Belog Co area, Qiangtang, northern Tibet, China： Petrochemical evidence for partial melting of the mantle-crust transition zone

Neogene volcanic rocks in the Belog Co area, Qiangtang, northern Tibet, are represented by a typical intermediate-basic and intermediate alkaline rock association, with latite-trachyte as the main rock type. The results of chemical analysis are: SiO2=52%–62%, Al2O3>15%, Na2O/K2O>1 and MgO<3.30%. In addition, the volcanic rocks are LREE-enriched with LREE/HREE=10–13, (La/Yb)N=15–19, and show a weak negative Eu anomaly with δEu=0.71–0.89. The close relationship between Mg# and SiO2 and the co-variation of the magmatophile elements and ultra-magmatophile elements such as La/Sm-La and Cr-Tb indicate that this association of volcanic rocks is the product of comagmatic fractional crystallization. The rock association type and lower Sm/Yb values (Sm/Yb=3.23–3.97) imply that this association of volcanic rocks should have originated from partial melting of spinel lherzolite in the lithospheric mantle. On the other hand, the weak negative Eu anomaly and relative depletion in Nb, Ta and Ti reflect the features of terrigenous magma. So the Neogene Belog Co alkaline volcanic rocks should be the result of partial melting of the special crust-mantle transition zone on the Qinghai-Tibet Plateau.     

REE Geochemical Characteristics of Volcanic Rocks in Zhejiang and Jiangxi Provinces and Their Gological Significance

Based on the data of 64 samples ,the REE geochemical characteristics of volcanic rocks in northern Zhejiang and eastern Jiangxi provinces are discussed in this paper.The REE distribution patterns in acid and intermediate-acid volcanic rocks in these areas display some similarities,as indicated by rightward-inclined V-shaped curves with negative Eu anomalies,which are parallel to earch other.In addi-tion,their REE parameters(ΣREE,ΣLREE/ΣHREE,δEu,Ce/Yb,La/Sm,La/Yb,etc)also va-ry over a narrow range with small deviations.HREE are particularly concentrated in the volcanic rocks as-sociated with uranium mineralization.The initial ^87Sr/^86Sr ratio in the volcanic rocks is about 0.7056-0.7139.All these features in conjunction with strontium isotopic data indicate that the rock-forming materials come from the sialic crust.The REE distribution patterns and REE geochemical parameters of the volcanic rocks ,as well as La/Sm-La and Ce/Yb-Eu/Yb diagrams may be applied to the sources of rock-forming and ore-forming materials.     

Seawater-like trace element signatures (REE + Y) of Eoarchaean chemical sedimentary rocks from southern West Greenland,and their corruption during high-grade metamorphism

Modern chemical sediments display a distinctive rare earth element + yttrium (REE + Y) pattern involving depleted LREE, positive La/La*_SN, Eu/Eu*_SN, and Y_SN anomalies (SN = shale normalised) that is related to precipitation from circumneutral to high pH waters with solution complexation of the REEs dominated by carbonate ions. This is often interpreted as reflecting precipitation from surface waters (usually marine). The oldest broadly accepted chemical sediments are c. 3,700 Ma amphibolite facies banded iron-formation (BIF) units in the Isua supracrustal belt, Greenland. Isua BIFs, including the BIF international reference material IF-G are generally considered to be seawater precipitates, and display these REE + Y patterns (Bolhar et al. in Earth Planet Sci Lett 222:43–60, 2004). Greenland Eoarchaean BIF metamorphosed up to granulite facies from several localities in the vicinity of Akilia (island), display REE + Y patterns identical to Isua BIF, consistent with an origin by chemical sedimentation from seawater and a paucity of clastic input. Furthermore, the much-debated magnetite-bearing siliceous unit of “earliest life” rocks (sample G91/26) from Akilia has the same REE + Y pattern. This suggests that sample G91/26 is also a chemical sediment, contrary to previous assertions (Bolhar et al. in Earth Planet Sci Lett 222:43–60, 2004), and including suggestions that the Akilia unit containing G91/26 consists entirely of silica-penetrated, metasomatised, mafic rock (Fedo and Whitehouse 2002a). Integration of our trace element data with those of Bolhar et al. (Earth Planet Sci Lett 222:43–60, 2004) demonstrates that Eoarchaean siliceous rocks in Greenland, with ages from 3.6 to 3.85 Ga, have diverse trace element signatures. There are now geographically-dispersed, widespread examples with Isua BIF-like REE + Y signatures, that are interpreted as chemically unaltered, albeit metamorphosed, chemical sediments. Other samples retain remnants of LREE depletion but are beginning to lose the distinct La, Eu and Y positive anomalies and are interpreted as metasomatised chemical sediments. Finally there are some siliceous samples with completely different trace element patterns that are interpreted as rocks of non-sedimentary origin, and include metasomatised mafic rocks. The positive La/La*_SN, Eu/Eu*_SN and Y_SN anomalies found in Isua BIFs and other Eoarchaean Greenland samples, such as G91/26 from Akilia, suggests that the processes of carbonate ion complexation controlling the REE − Y patterns were already established in the hydrosphere at the start of the sedimentary record 3,600–3,850 Ma ago. This is in accord with the presence of Eoarchaean siderite-bearing marbles of sedimentary origin, and suggests that CO₂ may have been a significant greenhouse gas at that time.     

Rare-earth elements as source indicators of Pan-African granites from Obudu Plateau,Southeastern Nigeria

The rare-earth element （REE） concentrations of representative granite samples from the southeast of the Obudu Plateau, Nigeria, were analyzed with an attempt to determine the signatures of their source, evolutionary history and tectonic setting. Results indicated that the granites have high absolute REE concentrations （190×10^-6-1191×10^-6; av.=549×10^-6） with the chondrite-normalized REE patterns characterized by steep negative slopes and prominent to slight or no negative Eu anomalies. All the samples are also characterized by high and variable concentrations of the LREE （151×10^-6-1169×10^-6; av.= 466×10^-6）, while the HREE show low abundance （4×10^-6-107×10^-6; av.=28×10^-6）. These are consistent with the variable levels of REE fractionation, and differentiation of the granites. This is further supported by the range of REE contents, the chondrite-normalized patterns and the ratios of LaN/YbN （2.30-343.37）, CeN/YbN （5.94-716.87）, LaN/SmN （3.14-11.68） and TbN/YbN （0.58-1.65）. The general parallelism of the REE patterns, suggest that all the granites were comagmatic in origin, while the high Eu/Eu＊ ratios （0.085-2.807; av.=0.9398） indicate high fo2 at the source. Similarly, irregular variations in LaN/YbN, CeN/YbN and Eu/Eu＊ ratios and REE abundances among the samples suggest behaviors that are related to mantle and crustal sources.     

Petrogenesis of Rabor-Lalehzar magmatic rocks (SE Iran): Constraints from whole rock chemistry and Sr-Nd isotopes

The Urumieh-Dokhtar magmatic arc (UDMA) of Central Iran has been formed during Neotethyan Ocean subduction underneath Eurasia. The Rabor-Lalehzar magmatic complex (RLMC), covers an area ～1000?km² in the Kerman magmatic belt (KMB), SE of UDMA. RLMC magmatic rocks include both granitoids and volcanic rocks with calc-alkaline and adakitic signatures but with different ages.Miocene adakitic rocks are characterd by relatively enrichmented in incompatible elements, high (Sr/Y)_(N) (>40), and (La/Yb)_(N) (>10) ratios with slightly negative Eu anomalies (Eu_N/Eu*≈ 0.9), depletion in HFSEs, and relatively non-radiogenic Sr isotope signatures (⁸⁷Sr/⁸⁶Sr?=?0.7048–0.7049). In contrast, the Oligocene granitoids exhibit low Sr/Y (<20) and La/Yb (<9) ratios, negative Eu anomalies (Eu_N/Eu*?≈?0.5), and enrichment in HFSEs and radiogenic Sr isotope signatures (⁸⁷Sr/⁸⁶Sr?=?0.7050–0.7052), showing affinity to the island arc rocks. Eocene volcanic rocks which crusscut the younger granitoid rocks comprise andesites and dacites. Geochemically, lavas show calc-alkaline character without any Eu anomaly (Eu_N/Eu*?≈?1.0). Based on the geochemical and isotopic data we propose that melt source for both calc-alkaline and adakitic rocks from the RLMC can be related to the melting of a sub-continental lithospheric mantle (SCLM). Basaltic melts derived from a metasomatized mantle wedge might be emplaced at the mantle-crust boundary and formed the juvenile mafic lower crust. However, some melts fractionated in the shallow magma chambers and continued to rise forming the volcanic intermediate-mafic rocks at the surface. On the other hand, the assimilation and fractional crystallization in the shallow magma chambers of may have been responsible for the development of Oligocene granitoids with calc-alkaline affinity. In the mid-Late Miocene, following the collision between Afro-Arabia and Iranian block the juvenile mafic crust of UDMA underwent thickening and metamorphosed into garnet-amphibolites. Subsequent upwelling of a hot asthenosphere during Miocene was responsible for partial melting of thickened juvenile crust of the SE UDMA (RLM complex). The adakitic melts ascended to the shallow crust to form the adakitic rocks in the KMB.     

Hydrothermal venting and basin evolution (Devonian,South China): Constraints from rare earth element geochemistry of chert

This paper presents rare earth element (REE) geochemistry of siliceous deposits from which hydrothermal activity and basin evolution are elucidated, in the Late Devonian, in the Yangshuo basin, South China, where siliceous deposits widely occurred as nodular chert in the deep-water limestones and bedded chert interbeded with tuffaceous chert in the early Late Devonian. Both nodular and bedded cherts are characterized by very low La abundances (avg. 2.07 and 2.49 ppm, respectively), intermediate negative Ce anomalies (Ce/Ce*: avg. 0. 69 and 0.61), slight to intermediate positive Eu anomalies (Eu/Eu*: avg. 1.33 and 1.57), and low to intermediate shale-normalized La_n/Yb_n values (avg. 0.86 and 0.52) and intermediate La_n/Ce_n values (avg. 1.61 and 1.72). These suggest both nodular and bedded cherts formed in the open marine basin of South China, rather than in the intracontinental rift basin as previously assumed, with involvement both with seawaters as indicated by intermediate negative Ce anomalies and generally LREE-depleted patterns, and hydrothermal vent fluids as indicated by convex, less LREE-depleted patterns with apparent positive Eu anomalies. In comparison with nodular and bedded chert, the tuffaceous chert has the highest La abundances (avg. 17.11 ppm), similar ranges of Ce anomalies (avg. 0.63) and La_n/Ce_n values (avg. 1.77), but lower La_n/Yb_n values (avg. 0.48) and no apparent positive Eu anomalies (avg. 0.97). This suggests that the tuffaceous fallouts were also significantly modified by the hydrothermal fluid and seawater. Rapid spatial variations of Eu/Eu* values and degree of LREE-depletions in the studied basin are recognized, characterizing a spatially differential hydothermal activity that is not well discriminated by major element features. Such a difference in hydrothermal activity is interpreted as having been related to the intensity and depth of syndepositional tectonic activity, reconciling the structural pattern unraveled by stratigraphic packages.     

Petrographic and geochemical characteristics of Mesoproterozoic Kumbalgarh clastic rocks,NW Indian shield: implications for provenance,tectonic setting,and crustal evolution

The Kumbalgarh Group of the south Delhi fold belt are the main bedrock series exposed in the axial region of the Aravalli craton. Quartzites and greywackes, the chief clastic constituents of this group, are well exposed. Petrographic and bulk-rock analyses of these rocks permit determination of their provenance, tectonic setting of the basin, and the Archaean to Proterozoic crustal evolution. Greywackes comprise quartz, plagioclase, amphiboles, K-feldspar, and rock fragments. Based on mineralogy, we divided the quartzites into three categories: QTZ1 is chiefly composed of quartz with a silty matrix and a minor quantity of feldspars and QTZ2 contains significant mafic minerals as well as quartz and feldspars, whereas QTZ3 is more feldspathic than the other groups. All the lithounits have SiO₂/Al₂O₃ ratios <～10 suggesting textural immaturity consistent with their sedimentary petrography. Greywackes display the least fractionated rare earth elements (REEs) (La/Yb _N : avg. 2.55) with positive Eu anomalies (avg. Eu/Eu* = 1.34). QTZ1 contains strongly fractionated REE patterns (avg. La/Yb _N : 13.56, avg. Eu/Eu* = 0.60), QTZ2 shows moderate REE fractionation (avg. La/Yb _N : 4.97, avg. Eu/Eu* = 0.61), and QTZ3 possesses the least fractionated V-shaped REE patterns (avg. La/Yb _N : 1.97, avg. Eu/Eu* = 0.51). Weathering attributes including chemical index of alteration (CIA), plagioclase index of alteration (PIA), chemical index of weathering (CIW), and A–CN–K plots assign a low to moderate degree of weathering to the Kumbalgarh sediments under a subtropical climate. Based on our synthesis of the petrographic and geochemical data, we suggest a provenance comprising basalts, tonalite–trondhjemite–granodiorite (TTG), and granite. Geochemical attributes indicate deposition of the detritus in an extensional backarc basin receiving sedimentary input from opposite directions. The opening and then closure of the South Delhi Basin was the last phase of the break-up of the supercontinent, columbia, which began by abortive rifting of the Udaipur belt and culminated in separation of the Aravalli–Bundelkhand–Dharwar block in the east and the East African orogen in the west.     

Geochemistry and Petrogenesis of Neoarchean Metamorphic Mafic Rocks in the Wutai Complex

Neoarchean metamorphic mafic rocks in the lower and the middle Wutai Complex mainly comprise metamorphic gabbros, amphibolites and chlorite schists. They can be subdivided into three groups according to chondrite normalized REE patterns. Rocks in Group #1 are characterized by nearly flat REE patterns （Lan/Ybn=0.86-1.3）, the lowest total REEs （29-52 ppm）, and weak negative to positive Eu anomalies （Eun/Eun=0.84-1.02）, nearly flat primitive mantle normalized patterns and strong negative Zr（Hf） anomalies. Their geochemical characteristics in REEs and trace elements are similar to those of ocean plateau tholeiite, which imply that this group of rocks can represent remnants of Archean oceanic crust derived from a mantle plume. Rocks in Group #2 are characterized by moderate total REEs （34-116 ppm）, LREE-enriched （Lan/Ybn=1.76-4.34） chondrite normalized REE patterns with weak Eu anomalies （Eun/Eun=0.76-1.16）, and negative Nb, Ta, Zr（Hf）, Ti anomalies in the primitive mantle normalized spider diagram. The REE and trace element characteristics indicate that they represent arc magmas originating from a sub-arc mantle wedge metasomatized by slab-derived fluids. Rocks in Group #3 are characterized by the highest total REEs （61-192 ppm）, the strongest LREEs enrichment （Lan/Ybn=7.12-16） with slightly negative Eu anomalies （Eun/Eun=0.81-0.95） in the chondrite normalized diagram. In the primitive mantle normalized diagram, these rocks are characterized by large negative anomalies in Nb, Ta, Ti, negative to no Zr anomalies. They represent arc magmas originating from a sub-arc mantle wedge enriched in slab-derived melts. The three groups of rocks imply that the formation of the Neoarchean Wutai Complex is related to mantle plumes and island-arc interaction.     

Geochemistry and petrogenesis of Proterozoic granitic rocks from northern margin of the Chotanagpur Gneissic Complex (CGC)

This study presents the geochemical characteristics of granitic rocks located on the northern margin of Chotanagpur Gneissic Complex (CGC), exposed in parts of Gaya district, Bihar and discusses the possible petrogenetic process and source characteristics. These granites are associated with Barabar Anorthosite Complex and Neo-proterozoic Munger–Rajgir group of rocks. The granitic litho-units identified in the field are grey, pink and porphyritic granites. On the basis of geochemical and petrographic characteristics, the grey and pink granites were grouped together as GPG while the porphyritic granites were named as PG. Both GPG and PG are enriched in SiO₂, K₂O, Na₂O, REE (except Eu), Rb, Ba, HFSE (Nb, Y, Zr), depleted in MgO, CaO, Sr and are characterised by high Fe* values, Ga/Al ratios and high Zr saturation temperatures (GPG_avg～ 861 ^°C and PG_avg～ 835 ^°C). The REE patterns for GPG are moderately fractionated with an average (La/Yb)_N～ 4.55 and Eu/Eu* ～ 0.58, than PG which are strongly fractionated with an average (La/Yb)_N～ 31.86 and Eu/Eu* ～ 0.75. These features indicate that the granites have an A-type character. On the basis of geochemical data, we conclude that the granites are probably derived from a predominant crustal source with variable mantle involvement in a post-collisional setting.     

Diversity of late Neoarchean K-rich granitoid rocks derived from subduction-related crust/mantle interactions in the Jiaobei terrane,North China Craton

The appearance of voluminous K-rich granitoids within nearly all ancient cratons represents one major characteristic of late Archean Earth, which hold the key to understand the transitional geodynamic regimes globally during this period. The genetic regimes and links among different K-rich granitoids and their implications for crustal growth and evolution remain controversial. A series of late Neoarchean K-rich granitoids, including quartz dioritic, granodioritic, and monzogranitic gneisses, occur in the Jiaobei terrane of North China Craton. Zircon U-Pb isotopic data reveal that they emplaced during ~2544–2494 Ma, coeval with regional ~2530–2470 Ma high-grade metamorphism.The quartz dioritic-granodioritic gneisses are magnesian rocks, and they show low Si and Ti, but high K and Mg, similar to Archean low-Ti sanukitoids. The Sr/Y and (La/Yb)_N are high (mostly 59.99–119.32 and 8.56–61.42), with moderate to high Nb (up to 11.79 ppm). Geochemical modeling, combined with depleted zircon εHf(t₂) (+0.5 − +7.2) and the presence of minor xenocrystic zircons, indicate that these low silica samples were derived from a metasomatized depleted mantle source with inputs of slab-derived fluids and melts, and minor contamination by ancient crustal materials. The monzogranitic rocks are ferroan rocks showing high Si, K, and Fe, but low Mg. They are divided into two subgroups: the first displays low TREE of 44.00–127.00 ppm and positive Eu anomalies (Eu_N/Eu*_N = 1.06–1.60), whereas the second shows high TREE of 85.76–819.02 ppm but negative Eu anomalies (Eu_N/Eu*_N = 0.51–0.62). Geochemical modeling and depleted zircon εHf(t₂) of +2.6 − +8.4 suggest their formation by partial melting of juvenile crustal sources involving tonalitic and some metasedimentary rocks at diverse crustal levels.Combined with regional geological data, these late Neoarchean K-rich granitoids were generated by coupled melting of metasomatized depleted mantle and dominantly juvenile crustal materials during crustal stabilization. Furthermore, the Jiaobei terrane experienced ~2.6–2.5 Ga crustal growth under a subduction-accretion setting.     

Nature of the crust in Maine,USA: evidence from the Sebago batholith

 Neodymium and lead isotope and elemental data are presented for the Sebago batholith (293±2 Ma), the largest exposed granite in New England. The batholith is lithologically homogeneous, yet internally heterogeneous with respect to rare earth elements (REE) and Nd isotopic composition. Two-mica granites in the southern/central portion of the batholith (group 1) are characterized by REE patterns with uniform shapes [Ce_N/Yb_N (chondrite normalized) = 9.4–19 and Eu/Eu* (Eu anomaly) = 0.27–0.42] and ɛ _Nd(t) = −3.1 to −2.1. Peripheral two-mica granites (group 2), spatially associated with stromatic and schlieric migmatites, have a wider range of total REE contents and patterns with variable shapes (Ce_N/Yb_N = 6.1–67, Eu/Eu* = 0.20–0.46) and ɛ _Nd(t) = −5.6 to −2.8. The heterogeneous REE character of the group 2 granites records the effects of magmatic differentiation that involved monazite. Coarse-grained leucogranites and aplites have kinked REE patterns and low total REE, but have Nd isotope systematics similar to group 2 granites with ɛ _Nd(t) = −5.5 to −4.7. Rare biotite granites have steep REE patterns (Ce_N/Yb_N = 51–61, Eu/Eu* = 0.32–0.84) and ɛ _Nd(t) = −4.6 to −3.8. The two-mica granites have a restricted range in initial Pb isotopic composition (²⁰⁶Pb/²⁰⁴Pb = 18.41–18.75; ²⁰⁷Pb/²⁰⁴Pb = 15.60–15.68; ²⁰⁸Pb/²⁰⁴Pb = 38.21–38.55), requiring and old, high U/Pb (but not Th/U) source component. The Nd isotope data are consistent with magma derivation from two sources: Avalon-like crust (ɛ _Nd>−3), and Central Maine Belt metasedimentary rocks (ɛ _Nd<−4), without material input from the mantle. The variations in isotope systematics and REE patterns are inconsistent with models of disequilibrium melting which involved monazite. Received: 8 December 1995 / Accepted: 29 April 1996     

Petrography and geochemistry of the Ngaoundéré Pan-African granitoids in Central North Cameroon: Implications for their sources and geological setting

The Nagoundéré Pan-African granitoids in Central North Cameroon belong to a regional-scale massif, which is referred to as the Adamawa-Yade batholith. The granites were emplaced into a ca. 2.1 Ga remobilised basement composed of metasedimentary and meta-igneous rocks that later underwent medium- to high-grade Pan-African metamorphism. The granitoids comprise three groups: the hornblende–biotite granitoids (HBGs), the biotite ± muscovite granitoids (BMGs), and the biotite granitoids (BGs). New Th–U–Pb monazite data on the BMGs and BGs confirm their late Neoproterozoic emplacement age (ca. 615 ± 27 Ma for the BMGs and ca. 575 Ma for the BGs) during the time interval of the regional tectono-metamorphic event in North Cameroon. The BMGs also show the presence of ca. 926 Ma inheritances, suggesting an early Neoproterozoic component in their protolith.The HBGs are characterized by high Ba–Sr, and low K₂O/Na₂O ratios. They show fairly fractionated REE patterns (La_N/Yb_N 6–22) with no Eu anomalies. The BMGs are characterized by higher K₂O/Na₂O and Rb/Sr ratios. They are more REE-fractionated (La_N/Yb_N = 17–168) with strong negative Eu anomalies (Eu/Eu^* = 0.2–0.5). The BGs are characterized by high SiO₂ with K₂O/Na₂O > 1. They show moderated fractionated REE patterns (La_N/Yb_N = 11–37) with strong Eu negative anomalies (Eu/Eu^* = 0.2–0.8) and flat HREE features (Gd_N/Yb_N = 1.5–2.2). In Primitive Mantle-normalized multi-element diagrams, the patterns of all rocks show enrichment in LILE relative to HFSE and display negative Nb–Ta and Ti anomalies. All the granitoids belong to high-K calc-alkaline suites and have an I-type signature.Major and trace element data of the HBGs are consistent with differentiation of a mafic magma from an enriched subcontinental lithospheric mantle, with possible crustal assimilation. In contrast, the high Th content, the LREE-enrichment, and the presence of inherited monazite suggest that the BGs and BMGs were derived from melting of the middle continental crust. Structural and petrochemical data indicate that these granitoids were emplaced in both syn- to post-collision tectonic settings.     

Archean metaterrigenous rocks: Major geochemical constraints

The paper summarizes data on the geochemistry of metaterrigenous rocks from 26 reference Archean territories: the Pilbara and Yilgarn blocks; Isua and Akilia complexes; Wittwatersrand, Swaziland, Pongola, and Yellowknife supergroups; Khapchanskaya and Gimol’skaya groups; Kan, Sharyzhalgai, Chupa, Slyudyanka, and Onot complexes; etc. The general sets of data points and the calculated median values of the concentrations of trace elements and their ratios are compared to those of Archean and post-Archean shales. In Ce/Cr-Co/Hf, Eu/Eu*-Gd_N/Yb_N, Ce/Cr-Th/Sc, Th/Sc-Sc, Th-La, La/Sm-Sc/Th, Yb-Gd_N/Yb_N, Th/Sc-Cr, Ni-Cr, and some other diagrams, the fields in which the most data points of Archean metaterrigenous rocks group are outlined. The results of this research indicate that there are no values of geochemical parameters that are inherent only in Archean or only in post-Archean fine-grained terrigenous rocks. Within 80–85% confidence levels, most individual compositions of Archean metaterrigenous rocks are characterized by the following geochemical parameters: (1) Th/Sc < 0.6–0.7, (2) Ce/Cr < 0.6, and (3) Eu/Eu* > 0.70–0.75. If the median values are used, these ranges can be further constrained to (i) Th/Sc < 0.55, (ii) Ce/Cr < 0.4, (iii) Cr/Th > 25, and (iv) Th < 12 ppm. Compared to PAAS, Archean metaterrigenous rocks are characterized by higher median concentrations of Cr and Ni and the Eu/Eu*, Sc/Th, Cr/Th, and Co/Hf ratios, whereas the Nb, La, Ce, Yb, Hf, Th, and U concentrations and the La/Sm and Ce/Cr ratios of PAAS are, conversely, lower. The median values of the La_N/Yb_N ratios of reference Archean terranes can be either higher or lower than in PAAS, likely depending on the proportions of various rock types in the sources of the terrigenous material. The medians of the Gd_N/Yb_N ratios of ～60% of the reference Archean metaterrigenous terranes in our databank are slightly higher than the Gd_N/Yb_N ratios of PAAS. The median values of the La_N/Sm_N ratios of Archean terrigenous rocks are mostly slightly lower than the typical PAAS ratios.