Geochemical and Nd isotope composition of detrital sediments on the north margin of the South China Sea: provenance and tectonic implications

A major re‐organization of regional drainages in eastern Tibet and south‐western China took place in the Cenozoic as deformation from the growing Himalayas and Tibetan Plateau affected an increasingly wider area. The effects of these changes on the regional sediment routing systems is not well constrained. This study examines the geochemical and Nd signatures of sedimentary rocks from the Ying‐Qiong and Nanxiong basins on the northern margin of the South China Sea to constrain and identify any significant changes in sediment source. Upper Cretaceous to Lower Eocene sedimentary rocks in the Nanxiong Basin show higher Th/Sc, La/Sc, Th/Cr and Th/Co ratios and lower Eu/Eu* ratios than PAAS (post‐Archaean Australian Shale), which indicates that Palaeozoic sedimentary rocks of the South China Block were the main basin sediment source. In contrast, Oligocene to Pleistocene sedimentary rocks of the Ying‐Qiong Basin show an abrupt change in these trace‐element ratios between 16·3 and 10·4 Ma, indicating a mid‐Miocene shift in provenance. ɛ_Nd values from the Ying‐Qiong Basin (range = −11·1 to −2·1) reinforce this, with pre‐13·8 Ma sedimentary rocks having average ɛ_Nd of −5·6 (range = −2·1 to −7·4), and post‐13·8 Ma sedimentary rocks having average ɛ_Nd of −9·3 (range = −8·7 to −11·1). During the Oligocene, the centre of rifting transferred south and basins on the north margin of the South China Sea experienced rapid subsidence. Further uplift and erosion then exposed Mesozoic and Cenozoic granites that supplied large amounts of granitic detritus, especially to the Ying‐Qiong Basin. Then a change occurred at ca 13 Ma resulting in less input from local sources (i.e. the fault blocks formed by Mesozoic‐Cenozoic tectonics and magmatism) to an increasing contribution of older continental material, mostly from Indochina to the west of the South China Sea.     

Geochemistry and Sm-Nd systematics of the 1.67 Ga Buanji Group of southwestern Tanzania: Paleo-weathering,provenance and paleo-tectonic setting implications

We present major and trace and Nd-isotopic data of the~1.67 Ga Buanji Group of southwestern Tanzania in order to constrain the nature of their protolith and the intensity of chemical weathering in the source terranes and make inferences on the nature of climatic conditions during the late Paleoproterozoic in the region. Major and trace element contents of the shales from three stratigraphic formations are comparable to those of the post-Archaean Australian Shale(PAAS) and the average Proterozoic Shale(PS). They are characterized by enrichments in LREE relative to HREE((La/Yb)_(CN)=9.07-13.2;(Gd/Yb)cN = 1.51-1.85) and negative Eu anomalies(Eu/Eu~* = 0.61-0.81); features which are comparable to those of PAAS and PS, indicating upper continental sources. Provenance proxy ratios,together with abundances of Cr, Ni, Co and V that increase up-stratigraphy suggest a decreasing input of felsic detritus up-stratigraphy. Chemical Indices of Alteration(CIA) for the lower, middle and upper Buanji formations are 81. 76 and 79, respectively. These indices largely indicate intermediate(ca. 60-80)to extreme(80) weathering intensities of the precursor rocks. These observations may suggest the prevalence of warm, humid climates during the late Paleoproterozoic in the region.The lower Buanji Formation yielded a depleted mantle Nd model age(T_(DM)) of 、2100 Ma which indicates an Eburnean parentage. T_(DM) ages of 2486-2155 Ma and 2535-2379 Ma obtained from middle and upper Buanji formations, respectively, suggest a progressive increase of sedimentary input from the Tanzania Craton up-stratigraphy. The Eburnean T_(DM) ages of the lower Buanji rocks are attributed to their derivation through denudation of a decaying topographic high composed predominantly of rocks that were generated during the Palaeoproterozoic Ubendian orogenesis, possibly in the realm of Columbian Supercontinent assembly. Overlapping T_(DM) ages between the middle and upper Buanji formations suggest multiple sources involving mixing of detritus from Archaean cratonic rocks and the Palaeoproterozoic Ubendian belt. However, the Archaean signal is relatively more pronounced in the upper Buanji Formation, suggesting sediments derivation from the craton, to the north of the basin. The middle Buanji Formation suggests more diverse protolith,given the relatively larger spread in the T_(DM) ages. The Nb/Ta, Zr/Sm and Ce/Pb ratios coupled with the negative Nb and Ta anomalies, relative to primitive mantle, suggest that the tectonic setting of the source rocks for the Buanji sediments was a subduction zone akin to that generating modern sland Arc Basalts. Thus, we suggest that the Buanji's palaeogeography is consistent with an extensional continental backarc basin during the late Paleoproterozoic.     

Zircon and whole-rock Nd-Pb isotopic provenance of Middle and Upper Ordovician siliciclastic rocks, Argentine Precordillera

Graptolite‐bearing Middle and Upper Ordovician siliciclastic facies of the Argentine Precordillera fold‐thrust belt record the disintegration of a long‐lived Cambro‐Mid Ordovician carbonate platform into a series of tectonically partitioned basins. A combination of stratigraphic, petrographic, U‐Pb detrital zircon, and Nd‐Pb whole‐rock isotopic data provide evidence for a variety of clastic sediment sources. Four Upper Ordovician quartzo‐lithic sandstones collected in the eastern and central Precordillera yield complex U‐Pb zircon age spectra dominated by 1·05–1·10 Ga zircons, secondary populations of 1·22, 1·30, and 1·46 Ga, rare 2·2 and 1·8 Ga zircons, and a minor population (<2%) of concordant zircons in the 600–700 Ma range. Archaean‐age grains comprise <1% of all zircons analysed from these rocks. In contrast, a feldspathic arenite from the Middle Ordovician Estancia San Isidro Formation of the central Precordillera has two well‐defined peaks at 1·41 and 1·43 Ga, with no grains in the 600–1200 Ma range and none older than 1·70 Ga. The zircon age spectrum in this unit is similar to that of a Middle Cambrian quartz arenite from the La Laja Formation, suggesting that local basement rocks were a regional source of ca 1·4 Ga detrital zircons in the Precordillera Terrane from the Cambrian onwards. The lack of grains younger than 600 Ma in Upper Ordovician units reinforces petrographic data indicating that Ordovician volcanic arc sources did not supply significant material directly to these sedimentary basins. Nd isotopic data (n = 32) for Middle and Upper Ordovician graptolitic shales from six localities define a poorly mixed signal [ɛ_Nd(450 Ma) = −9·6 to −4·5] that becomes more regionally homogenized in Upper Ordovician rocks (−6·2 ± 1·0; T_DM = 1·51 ± 0·15 Ga; n = 17), a trend reinforced by the U‐Pb detrital zircon data. It is concluded that proximal, recycled orogenic sources dominated the siliciclastic sediment supply for these basins, consistent with rapid unroofing of the Precordillera Terrane platform succession and basement starting in Mid Ordovician time. Common Pb data for Middle and Upper Ordovician shales from the western and eastern Precordillera (n = 15) provide evidence for a minor (<30%) component that was likely derived from a high‐μ (U/Pb) terrane.     

Early Paleozoic Granite in the Talate Mining District,Chinese Altay,and its Geological Significance for the Altay Orogenic Belt

Zircon dating, geochemical and Nd-Sr isotopic analyses have been determined for samples from two granitic intrusions in the Talate mining district, Chinese Altay. Our data suggest that these intrusions were emplaced from 462.5 Ma to 457.8 Ma. These rocks have strong affinity to peralumious S-type granite and are characterized by prominent negative Eu anomalies(δEu=0.20–0.35), strong depletion in Ba, Sr, P, Ti, Nb, Ta and positive anomalies in Rb, Th, U, K, La, Nd, Zr, Hf. Nd-Sr isotopic compositions of the whole rock show negative εNd(t) values(-1.21 to-0.08) and Mesoproterozoic Nd model ages(T2 DM=1.20–1.30 Ga). Their precursor magmas were likely derived from the partial dehydration melting of Mesoproterozoic mica-rich pelitic sources and mixed with minor mantle-derived components, under relatively low P(≤1 kbar) and high T(746–796°C) conditions. A ridge subduction model may account for the early Paleozoic geodynamic process with mantle-derived magmas caused by Ordovician ridge subduction and the opening of a slab window underplated and/or intraplated in the middle–upper crust, which triggered extensive partial melting of the shallow crust to generate diverse igneous rocks, and provided the heat for the crustal melting and juvenile materials for crustal growth.     

Geochemistry, Nd Isotopic Characteristics of Metamorphic Complexes in Northern Hebei： Implications for Crustal Accretion

The middle segment of the northern margin of the North China Craton （NCC） consists mainly of metamorphosed Archean Dantazi Complex, Paleoproterozoic Hongqiyingzi Complex and unmetamorphosed gabbro-anorthosite-meta-alkaline granite, as well as metamorphosed Late Paleozoic mafic to granitoid rocks in the Damiao-Changshaoying area. The -2.49 Ga Dantazi Complex comprises dioritic-trondhjemitic-granodoritic-monzogranitic gneisses metamorphosed in amphibolite to granulite facies. Petrochemical characteristics reveal that most of the rocks belong to a medium- to high-potassium calc-alkaline series, and display Mg^# less than 40, right-declined REE patterns with no to obviously positive Eu anomalies, evidently negative Th, Nb, Ta and Ti anomalies in primitive mantlenormalized spider diagrams, εNd（t）=＋0.65 to -0.03, and depleted mantle model ages TDM=2.78-2.71 Ga. Study in petrogenesis indicates that the rocks were formed from magmatic mixing between mafic magma from the depleted mantle and granitoid magma from partial melting of recycled crustal mafic rocks in a continental margin setting. The 2.44-2.41 Ga Hongqiyingzi Complex is dominated by metamorphic mafic-granodioritic-monzogranitic gneisses, displaying similar petrochemical features to the Dantazi Complex, namely medium to high potassium calc-alkaline series, and the mafic rocks show evident change in LILEs, negative Th, Nb, Ta, Zr anomalies and positive P anomalies. And the other granitiod samples also exhibit negative Th, Nb, Ta, P and Ti anomalies. All rocks in the Hongqiyingzi Complex show right-declined REE patterns without Eu anomaly. The metamorphic mafic rocks with εNd（t） = -1.64 may not be an identical magmatic evolution series with granitoids that have εNd（t） values of ＋3.19 to ＋1.94 and TDM ages of 2.55-2.52 Ga. These granitic rocks originated from hybrid between mafic magma from the depleted mantle and magma from partial melting of juvenile crustal mafic rocks in an island arc setting. All the -311 Ma Late Paleozoic metamorphic mafic rocks and related granitic rocks show a medium-potassium calc-alkaline magmatic evolution series, characterized by high Mg^#, obviously negative Th, Nb, Ta anomalies and positive Sr anomalies, from no to strongly negative Ti anomalies and flat REE patterns with εNd（t） = ＋8.42, implying that the maflc magma was derived from the depleted mantle. However the other granitic rocks are characterized by right-declined REE patterns with no to evidently positive Eu anomalies, significantly low εNd（t） = -13.37 to -14.04, and TDM=1.97-1.96 Ga, revealing that the granitoid magma was derived from hybrid between maflc magma that came from -311 Ma depleted mantle and granitoid magma from Archean to Early Paleoproterozoic ancient crustal recycling. The geochemistry and Nd isotopic characteristics as well as the above geological and geochronological results indicate that the middle segment of the northern margin of the NCC mainly experienced four crustal growth episodes from Archean to Late Paleozoic, which were dominated by three continental marginal arc accretions （-2.49, -2.44 and 311 Ma）, except the 1.76-1.68 Ga episode related to post-collisional extension, revealing that the crustal accretion of this segment was chiefly generated from arc accretion and amalgamation to the NCC continental block.     

Two Types of Granites in the Western Yangtze Block and Their Implications for Regional Tectonic Evolution: Constraints from Geochemistry and Isotopic Data

In the western Yangtze Block, widespread Mesoproterozoic to Neoproterozoic rocks are the key to understanding the Precambrian tectonic-magmatic evolution of the region. However, their petrogenesis and tectonic setting are still controversial. In this paper, zircon U-Pb ages, Sm-Nd isotopic and whole-rock geochemical data are reported from selected fresh samples in the southern Dechang county, southwestern China, in order to constrain their emplacement age and magma source, as well as their petrogenesis and tectonic setting. They are mainly composed of biotite monzogranite, monzonitic granite, biotite granodiorites, and quartz diorite. Two ages of 1055 ± 43 Ma and 837.6 ± 3.8 Ma were obtained through zircon U-Pb dating by LA-ICP-MS and LA-MC-ICP-MS, respectively. According to their major element compositions, the Grenville-age granites are peraluminous calc-alkaline series calcic S-type granite. In contrast, the mid-Neoproterozoic granites are metaluminous calc-alkaline series alkalic I-type granite. Furthermore, the S-type granites are enriched in LREEs relative to HREEs with(La/Yb)_N ratios of 3.85–18.56 and underwent major fractionation with strongly negative Eu anomalies(Eu/Eu~* = 0.38–0.66). In the MORB-normalized trace element variation diagram, all the samples are enriched in Ce and large ion lithophile elements such as Rb, Th, and K, and depleted in high field strength elements such as Nb, and Ti, with negative Sr and Ti anomalies. The I-type granites are enriched in LREEs with slight negative Eu anomalies(Eu/Eu~* = 0.83–0.93). They are characterized by the enrichment of highly incompatible elements(such as K, Rb, Ba, Th) and LREEs, relative to MORB. Neodymium isotopic data show that the S-type granites display ~(143)Nd/(~(144) Nd) values of 0.51241–0.51256, and have ε_(Nd)(t = 1055 Ma) values of(-3.29) to(-3.81). Calculated t_(DM) ages yield values from 1.87 to 1.91 Ga with the t_(DM).2 stg ages of 1.86 to 1.9 Ga. The I-type granites have ~(143)Nd/(~(144) Nd) ratios between 0.51192 and 0.51195, corresponding to initial ε_(Nd)(t = 837 Ma) values of 1.22 to 5.63. Calculated t_(DM) ages yield values from 1.0 to 1.38 Ga and the t_(DM).2 stg ages yield values from 0.99 to 1.06 Ga. The S-type granites are distinguished as syn-collision granite, whereas the I-type granites were formed as arc magmas according to the Rb-(Yb+Ta) and R_1-R_2 tectonic discrimination diagrams. To conclude, there are two types of spatially associated granite, the Mesoproterozoic S-type granite which were derived from remelting of upper crustal mudstone and/or clastics and resulted from the convergence of two continental plates, and the mid-Neoproterozoic I-type granite which formed in continental arc and resulted from mantle-derived magma mixed crust material, in the western Yangtze Block. Furthermore, we suggest that collision between the Yangtze and Cathaysia blocks occurred at about 1055 Ma, and caused the Stype granite. The I-type granite related to the subduction of oceanic lithosphere eastward underneath the Yangtze Block in the mid-Neoproterozoic.     

Provenance of Ordovician clastic sequences of the San Rafael Block (Central Argentina), with emphasis on the Ponón Trehué Formation

The Ordovician Ponón Trehué Formation is the only early Palaeozoic sedimentary sequence known to record a primary contact with the Grenvillian-age basement of the Argentinean Cuyania terrane, in its southwards extension named the San Rafael block. Petrographic and geochemical data indicate contributions from a dominantly upper continental crustal component and a subordinated depleted component. Nd isotopes indicate ε_Nd of ? 4.6, ?_Sm/Nd ? 0.36 and T_DM 1.47 Ga in average. Pb-isotope ratios display average values for ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb of 19.15, 15.69 and 38.94 respectively. U–Pb detrital zircon ages from the Ponón Trehué Formation cluster around values of 1.2 Ga, indicating a main derivation from a local basement source (Cerro La Ventana Formation). The Upper Ordovician Pavón Formation records a younger episode of clastic sedimentation within the San Rafael block, and it shows a more complex detrital zircon age population (peaks at 1.1 and 1.4 Ga as well as Palaeoproterozoic and Neoproterozoic detrital grains). Detailed comparison between the two Ordovician clastic units indicates a shift with time in provenance from localized basement to more regional sources. Middle to early Upper Ordovician age is inferred for accretion of the Cuyania terrane to the proto-Andean margin of Gondwana.     

Geochemical characteristics of sandstones from Cretaceous Garudamangalam area of Ariyalur,Tamilnadu, India: Implications of provenance and tectonic setting

The Trichinopoly Group (later redesignated as Garudamangalam) has unconformable relationship with underlying Uttatur Group and is divided into lower Kulakanattam Formation and upper Anaipadi Formation. These calcareous sandstones are analysed major, trace and rare earth elements (REEs) to find out CIA, CIW, provenance and tectonic setting. The silica content of fossiliferous calcareous sandstone show wide variation ranging from 12.93 to 42.56%. Alumina content ranged from 3.49 to 8.47%. Higher values of Fe₂O₃ (2.29–22.02%) and low MgO content (0.75–2.44%) are observed in the Garudamangalam Formation. CaO (23.53–45.90) is high in these sandstones due to the presence of calcite as cementing material. Major element geochemistry of clastic rocks (Al₂O₃ vs. Na₂O) plot and trace elemental ratio (Th/U) reveal the moderate to intense weathering of the source rocks. The Cr/Zr ratio of clastic rocks reveal with an average of 1.74 suggesting of felsic provenance. In clastic rocks, high ratios of \(\sum \)LREE/\(\sum \)HREE, La/Sc, Th/Sc, Th/Co, La/Co and low ratios of Cr/Zr, and positive Eu anomaly ranges from (Eu/Eu* = 1.87–5.30) reveal felsic nature of the source rocks.     

Geochemistry,provenance and tectonic setting of the Late Cambrian–Early Ordovician Seydişehir Formation in the Çaltepe and Fele areas,SE Turkey

The major, trace and rare earth element (REE) contents of metapelite (MPL), metapsammite (MPS) and metamarl (MM) samples from the Cambro-Ordovician Seydi?ehir Formation were analyzed to investigate their provenance and tectonic setting. The MPS, MPL, and MM samples have variable SiO₂ concentrations, with average values of 72.36, 55.54, and 20.95 wt%, moderate SiO₂/Al₂O₃ ratios (means of 6.88, 3.23, and 3.80), moderate to high Fe₂O₃ + MgO contents (means of 5.14, 9.55, 3.56 wt%), and high K₂O/Na₂O ratios (means of 3.26, 3.64, 2.90), respectively. On average, the chemical index of alteration (CIA) values of the MPS and the MPL are 65.87 and 71.96, respectively, while the chemical index of weathering (CIW) values are 74.54 and 85.09, respectively. These data record an intermediate to high degree of alteration (weathering) of plagioclase to illite/kaolinite in the samples’ provenance. The chondrite-normalized REE patterns of all the sample groups are similar and are characterized by subparallel light rare earth elements (LREE)-enriched, relatively flat heavy rare earth elements (HREE) patterns with pronounced Eu anomalies (mean of 0.69) and moderate fractionation [average (La/Yb)_N = 8.7]. Plots of sediments in ternary diagrams of La, Th, Sc and elemental ratios (La/Sc, Th/Sc, Cr/Th, Eu/Eu*, La/Lu, Co/Th, La/Sc and Sc/Th), which are critical for determining provenance, and REE patterns indicate that the metaclastic units of the Seydi?ehir Formation were derived dominantly from felsic to intermediate magmatic rocks and not from a mafic source. The La–Sc–Th and Th–Sc–Zr/10 ternary diagrams of the Seydi?ehir Formation are typical of continental island arc/active continental margin tectonic settings. The geologic location and geochemistry of the Seydi?ehir Formation suggest that it was deposited in an Andean-type retroarc foreland basin during the Late Cambrian–Early Ordovician period. The Neoproterozoic intermediate to felsic magmatic rocks and metaclastic sediments with felsic origins of the Sand?kl?–Afyon Basement Complex (SBC) and their equivalent units, which are thought to be overlain by the younger units in the study area, may be the dominant source rocks for the Seydi?ehir Formation.     

Elemental geochemistry and Nd isotopic characteristics of the metasedimentary rocks from the metamorphic belt in central Jiangxi： Provenance and tectonically environmental constraints

The metamorphic belt in central Jiangxi, located in the compound terrain within the Cathaysia, Yangtze Block and Caledonian fold zone of South China, is composed dominantly of meta-argillo-arenaceous rocks, with minor amphibolite. These rocks underwent amphibolite-facies metamorphism. The meta-argillo-arenaceous rocks show large variations in major element composition, but have similar REE patterns and trace element composition, incompatible element and LIE enrichments [ high Th/Sc (0.57-3.59) , La/Sc ( 1.46 - 12.4), La/Yb (5.84 - 19.0) ] and variable Th/U ratios, with ∑REE = 129-296μg/g, δEu =0.51 -0.86, and (La/Yb)N = 3.95 -12.9. The Nd isotopic model ages tDM of these rocks vary from 1597 to 2124 Ma. Their 143 Nd/144 Nd values are low [εNd (0) = - 11.4 to -- 15.8]. Some conclusions have been drawn as follows: (1) The metamorphic rocks in central Jiangxi Province are likely formed in a tectonic environment at the passive continental margin of the Cathaysia massif. (2) The metamorphosed argillo-arenaceous rocks are composed dominantly of upper crustal-source rocks (Al- and Krich granitic or/and sedimentary rocks of Early Proterozoic), which experienced good sorting, slow deposition and more intense chemical weathering. (3) According to the whole-rock Sm-Nd isochron ages (1113±49 to 1199 ± 26 Ma) of plagioclase-amphibole (schist) and Nd isotopic model age tDM ( 1597 - 2124Ma) of meta-argillo-arenaceous rocks, the metamorphic belt in central Jiangxi Province was formed during the Middle Proterozoic ( 1100 - 1600 Ma).     

Petrogenesis of the early Cretaceous Valle Chico igneous complex (SE Uruguay): Relationships with Paraná–Etendeka magmatism

The early Cretaceous (130 Ma) igneous complex of Valle Chico (SE Uruguay) is made up of felsic plutonic and subordinate volcanic rocks and dykes cropping out over an area of about 250 km². This complex is strictly linked with the formation of the Paraná–Etendeka Igneous Province and the first stages of the South Atlantic Ocean rifting. The plutonic rocks range from quartz-monzonite to syenite, quartz-syenite and granite. The volcanic rocks and the dykes range from quartz-latite to trachyte and rhyolite; no substantial differences in term of chemical composition have been found between plutonic and volcanic rocks. Only a sample of basaltic composition (with tholeiitic affinity) has been sampled associated with the felsic rocks. The Agpaitic Index of the Valle Chico felsic rocks range from 0.72 to 1.34, with the peralkaline terms confined in the most evolved samples (SiO₂>65 wt.%). Initial ⁸⁷Sr/⁸⁶Sr₍₁₃₀₎ of the felsic rocks range from 0.7046 to 0.7201, but the range of ⁸⁷Sr/⁸⁶Sr of low-Rb/Sr samples cluster at 0.7083; ¹⁴³Nd/¹⁴⁴Nd₍₁₃₀₎ ratios range from 0.5121 (syenite) to 0.5117 (granite). The tholeiitic basalt show more depleted isotopic compositions (⁸⁷Sr/⁸⁶Sr₍₁₃₀₎=0.7061; ¹⁴³Nd/¹⁴⁴Nd₍₁₃₀₎=0.5122), and plots in the field of other early Cretaceous low-Ti basaltic rocks of SE Uruguay. The radiogenic Sr and unradiogenic Nd of the Valle Chico felsic rocks require involvement of lower crustal material in their genesis either as melt contaminant or as protolith (crustal anatexis). In particular, most of the Valle Chico (VC) felsic rocks define a near-vertical array in Sr–Nd isotopic spaces, pointing toward classical EMI-type composition; this feature is considered to reflect a lower crust involvement as observed for other mafic and felsic rocks of the Paraná–Etendeka Igneous Province. Decompression melting of the lower crust related to Gondwana continental rifting before the opening of the South Atlantic Ocean or the presence of thermal anomalies related to the Tristan plume may have induced the lower crust to partially melt. Alternative hypothesis considers contamination of upper mantle by a mafic/ultramafic keel composed of lower crust and uppermost mantle after delamination and detachment processes. This interaction may have occurred after the continent–continent collision during the last stages of the Panafrican Orogeny. This “lower crust” model does not exclude active involvement of upper crust as contaminant, necessary to explain the strongly radiogenic ⁸⁷Sr/⁸⁶Sr₍₁₃₀₎ isotopic composition of some VC SiO₂-rich rocks. Mineralogical (sporadic presence of pigeonite, Ca–Na and Na clinopyroxene, calcic- and calco-sodic amphibole) and geochemical evidences (major and trace element as well as Sr–Nd isotopic similarities with the felsic early Cretaceous volcanic rocks of the Arequita Formation in SE Uruguay) allow us to propose for the VC rocks a transitional rock series (the most abundant rock types are of syenitic/trachytic composition) preferentially evolving towards SiO₂-oversaturated compositions (granite/rhyolite) also with a strong upper crustal contribution as melt contaminant. This conclusion is in contrast with previous studies according which the VC complex had clear alkaline affinity. Many similarities between VC and the coeval Paresis granitoids (Etendeka, Namibia) are evidenced in this paper. The genetic similarities between VC and the rhyolites (s.l.) of SE Uruguay may find counterparts with the genetic link existing between the early Cretaceous tholeiitic-alkaline Messum complex and the quartz latites (s.l.) of the Awahab Formation (Etendeka region, Namibia).     

Geochemical and Sr–Nd isotopic characteristics of granitic rocks from northern Vietnam

Five major felsic igneous suites from northern Vietnam, with ages from mid-Proterozoic to early Cenozoic, were studied. Representative granitic rocks from the Posen Complex (mid-Proterozoic) and the Dienbien Complex (late Permian to early Triassic) show geochemical characteristics similar to those of calc-alkaline to high-K calc-alkaline I-type granites. However, the former, located in the South China block, has significantly higher initial Nd isotopic ratios [ε_Nd(T)=+0.7 to +1.5] and older Nd isotopic model ages (TDM∼1.7 Ga) than the latter [ε_Nd(T)=−4.7 to −9.7; T_DM∼1.3–1.5 Ga] which were emplaced south of the Song Ma Suture and thus in the Indochina block. The generation of both complexes may be attributed to subduction-related processes that occurred in two distinct crustal provenances with different degrees of mantle inputs. On the other hand, Jurassic to Cretaceous granitic rocks from the Phusaphin Complex, contemporaneous rhyolites from the Tule Basin, and late Paleogene granitic rocks from the Yeyensun Complex, all exposed in the South China block between the Ailao Shan–Red River shear zone and the Song Ma Suture, display geochemical features similar to those of A-type granites with intermediate ε_Nd(T) values (+0.6 to −2.8) and younger T_DM ages (0.6–1.1 Ga). These magmas are suggested to have been generated as a consequence of intraplate extension in the western part of the South China block (Yunnan), and to have been transported to their present position by mid-Tertiary continental extrusion along the Ailao Shan–Red River shear zone related to the India–Asia collision. Overall, the isotopic and model age data, reported in this study indicate that in northern Vietnam, the most important crust formation episode took place in the Proterozoic. Likewise, repeated mantle inputs have played a role in the petrogenesis of Phanerozoic granitic rocks.     

Shoshonite-latite series of the Eastern Transbaikalia: 40Ar/39Ar age,geochemistry, and Sr-Nd isotope composition of rocks from the Akatui volcano-plutonic association of the Aleksandrovskii Zavod depression

The paper presents new data on age, geochemistry, and Sr and Nd isotope composition of rocks from the Akatui massif and comagmatic rocks from the lower unit of the Kailas Formation (Akatui volcano-plutonic association), localized within the Aleksandrovskii Zavod depression. The amphibole ⁴⁰Ar/³⁹Ar age date the monzogabbro of the early phase of the Akatui massif at 154.8 ± 4.4 Ma; the monzonite of the main phase yields a ⁴⁰Ar/³⁹Ar age of 160.7 ± 3.9 Ma, and the shoshonite basalt of the lower unit of the Kailas Formation yields a ⁴⁰Ar/³⁹Ar age of 161.5 ± 1.7 Ma. The leading petrogenetic mechanism for the Akatui volcano-plutonic association is crystal fractional differentiation of melts with minor crustal contamination, which can be suggested from the mineralogical and petrographic features and geochemical and isotope characteristics of rocks. The geochemical data for the Akatui volcano-plutonic association show LILE, LREE, U, Th, and Pb enrichment with a characteristic depletion in high-field strength elements (HFSE), such as Nb and Ti. They are also depleted in P. Sr-Nd isotope data (⁸⁷Sr/⁸⁶Sr(₁6_{0 Ma}) = 0.70642-0.70688 and £Nd(160 _Ma) = − 0.6 to − 2.2) suggest an EMII-type mantle source and could also indicate a negligible degree of crustal contamination in the evolved melts.     

The Neoproterozoic-early Paleozoic metamorphic and magmatic evolution of the Eastern Sierras Pampeanas: an overview

The Eastern Sierras Pampeanas were structured by three main events: the Ediacaran to early Cambrian (580?C510?Ma) Pampean, the late Cambrian?COrdovician (500?C440?Ma) Famatinian and the Devonian-Carboniferous (400?C350?Ma) Achalian orogenies. Geochronological and Sm?CNd isotopic evidence combined with petrological and structural features allow to speculate for a major rift event (Ediacaran) dividing into two Mesoproterozoic major crustal blocks (source of the Grenvillian age peaks in the metaclastic rocks).This event would be coeval with the development of arc magmatism along the eastern margin of the eastern block. Closure of this eastern margin led to a Cambrian active margin (Sierra Norte arc) along the western margin of the eastern block in which magmatism reworked the same crustal block. Consumption of a ridge segment (input of OIB signature mafic magmas) which controlled granulite-facies metamorphism led to a final collision (Pampean orogeny) with the western Mesoprotrozoic block. Sm?CNd results for the metamorphic basement suggest that the T _DM age interval of 1.8?C1.7?Ga, which is associated with the less radiogenic values of ??Nd₍₅₄₀₎ (?6 to ?8), can be considered as the mean average crustal composition for the Eastern Sierras Pampeanas. Increasing metamorphic grade in rocks with similar detrital sources and metamorphic ages like in the Sierras de Córdoba is associated with a younger T _DM age and a more positive ??Nd₍₅₄₀₎ value. Pampean pre-540?Ma granitoids form two clusters, one with T _DM ages between 2.0 and 1.75?Ga and another between 1.6 and 1.5?Ga. Pampean post-540?Ma granitoids exhibit more homogenous T _DM ages ranging from 2.0 to 1.75?Ga. Ordovician re-activation of active margin along the western part of the block that collided in the Cambrian led to arc magmatism (Famatinian orogeny) and related ensialic back-arc basin in which high-grade metamorphism is related to mid-crustal felsic plutonism and mafic magmatism with significant contamination of continental crust. T _DM values for the Ordovician Famatinian granitoids define a main interval of 1.8?C1.6, except for the Ordovician TTG suites of the Sierras de Córdoba, which show younger T _DM ages ranging from 1.3 to 1.0?Ga. In Devonian times (Achalian orogeny), a new subduction regime installed west of the Eastern Sierras Pampeanas. Devonian magmatism in the Sierras exhibit process of mixing/assimilation of depleted mantle signature melts and continental crust. Achalian magmatism exhibits more radiogenic ??Nd₍₅₄₀₎ values that range between 0.5 and ?4 and T _DM ages younger than 1.3?Ga. In pre-Devonian times, crustal reworking is dominant, whereas processes during Devonian times involved different geochemical and isotopic signatures that reflect a major input of juvenile magmatism.     

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.     

Geochemistry and provenance of the Middle Ordovician Austin Glen Member (Normanskill Formation) and the Taconian Orogeny in New England

The Austin Glen Member of the upper Middle Ordovician Normanskill Formation is a sandstone-shale flysch succession deposited in the foreland of the Taconian Orogen. Petrographic, major and trace element, and Nd–Pb isotopic data provide substantial constraints on its provenance. Lack of K-feldspar and paucity of plagioclase, in addition to the dominance of sedimentary rock fragments, indicate that the source was dominated by recycled, sedimentary components. Major and trace element data support this conclusion and indicate that the provenance of both shales and sandstones was the same. No evidence of an ophiolitic or volcanic component was observed. Interpretation of Nd isotopic characteristics are complicated by a partial resetting of the Nd isotope system at about the time of sedimentation but indicate that the provenance of the Austin Glen Member had a long-term history of light rare earth element (LREE) enrichment (average T_DM = 1·8 Ga). Furthermore, Nd isotopic compositions are extremely homogeneous (?_Nd = –8·1 ± 0·6; 1 s.d.; n = 23) at 450 Ma, the approximate depositional age, indicating either a single source or very well-mixed sources. ²⁰⁷Pb/²⁰⁴Pb ratios are variable but within the range of Pb isotopic compositions typically described as Grenvillian. The range of ²⁰⁷Pb/²⁰⁴Pb is greater than expected for the range of ²⁰⁶Pb/²⁰⁴Pb and suggests an additional component of Pb, possibly introduced during diagenesis. The immediate source of the Austin Glen Member may have been the accretionary prism that developed as older sediments of the Laurentian margin were scraped off the basin floor, incorporated within the accretionary prism and shed into the basin. No evidence indicating the arrival of an undifferentiated island arc or continental fragment during the Taconian Orogeny has been found. The data acquired during this study can be explained almost exclusively by Grenville Province source components but with possible additional contributions from older Laurentian terranes and Late Proterozoic rift volcanics that are not readily quantified but likely to have been minor. Accordingly, we conclude that the Taconian Orogeny in New England involved either: (1) a continental arc that involved exclusively Laurentia; (2) collision of a continental block with identical geochemical characteristics as Laurentia; or (3) essentially no detritus from any exotic colliding block (island arc or continental fragment) reached the foreland basin at the time of Austin Glen deposition.     

Nd Isotopic Composition and Material Source of Pre—and Post—Sinian Sedimentary Rocks in Xiushui Area,Jiangxi Province

Reported in this paper are the Nd isotopic compositions of the pre-Sinian and Sinian-Cambrian sedimentary rocks in the Xiushui area,Jiangxi Province.Significant differences are noticed between them in their Nd isotopic dompositions.As for the pre-Sinian lightly metmorphozed sedimentary rocks,^143Nd/^144Nd=0.512000-0.512214,CNd(T)=-8.04-9.99,and TDM=18332426Ma are suggested for the Sinian-Cambrian sedimentary rocks .These differences would reflect the diversity of material source for the sedimentary rocks deposited before and after the Sinian period.Mantle material ap-pears to have been involved in the formation of the pre-Sinian sedimentary rocks while the post-Sinian sedimentary rocks are composed mainly of recycled detritus from the continental crust.     

Mississippian volcanism in the south-central Andes: New U–Pb SHRIMP zircon geochronology and whole-rock geochemistry

In the northern extension of the Famatina and the southern Puna (NW Argentina) prominent rhyolitic volcanic rocks traditionally referred to as Ordovician are exposed, resting on metamorphic basement and covered by thick Late Paleozoic siliciclastic successions. We report new U–Pb SHRIMP ages from these rhyolites that show them to be of Mississippian (348–342 Ma) age, thus identifying a previously unknown volcanic event in this portion of western Gondwana. Whole-rock geochemistry and Sr–Nd isotopic analyses suggest a crustal source for these rocks but with a juvenile input (ε_Nd(t) between ? 2.91 and ? 0.3, and T_DM values between 1.09 and 1.1 Ga). This is different from the Early Paleozoic magmatism of western Argentina where crustal recycling took place without any involvement of mantle material. The Carboniferous magmatism is compatible with an extensional environment developed along the Terra Australis accretionary orogen as a result of tectonic switching processes. These rhyolites may be related to the coeval Mississippian A-type granites exposed to the east, in the Sierras Pampeanas, confirming the regional character of this magmatism.     

Contrasting geochemical and Sm-Nd isotopic compositions of Archean metasediments from the Kongling high-grade terrain of the Yangtze craton: evidence for cratonic evolution and redistribution of REE during crustal anatexis

Twenty-three clastic metasediments from the Kongling high-grade terrain of the Yangtze craton, South China were analyzed for major, trace and rare earth elements and Sm-Nd isotopic ratios. Associated dioritic-tonalitic-trondhjemitic (DTT) and granitic gneisses as well as amphibolites were also analyzed in order to constrain provenance. The results show that the clastic metasediments can be classified into 3 distinct groups in terms of mineralogical, geochemical and Sm-Nd isotopic compositions. Group A is characterized by having no to slight negative Eu anomalies (Eu/Eu1 = 0.82–1.07), being high in Cr (191–396 ppm) and Ni (68–137 ppm), and low in Th (3.3–7.8 ppm) and REE (ΣREE = 99–156 ppm). These characteristics are similar to those of metasediments from Archean greenstone belts. In addition, the Group A metasediments have the value of the Chemical Index of Alteration (CIW) close to felsic gneisses. Their Sm-Nd isotopic, REE and trace element compositions can be interpreted by mixtures of the DTT gneisses and amphibolites. Dating of detrital zircons from 2 Group A samples by SHRIMP reveals a major concordant age group of 2.87–3.0 Ga, which is identical to the age of the trondhjemitic gneiss. These results strongly suggest that Group A was principally the first-cycle erosion product of the local Kongling DTT gneiss and amphibolite. Moreover, the higher than amphibolite Cr content and slight Eu depletion exhibited by some samples from this group infer that ultramafic rocks like komatiite and granite of probably 3.0–3.3 Ga in age also played a role.Group B is characterized by the presence of graphite and shows a more evolved composition similar to post-Archean shales with a prominent negative Eu anomaly (Eu/Eu1 = 0.48–0.77) and high CIW. On paired Cr/Th vs La/Co and Co/Th plots, Group B samples conform to a two-end member mixing line of the Kongling granitic gneiss and amphibolite. However, data on Nd model age and CIW suggest that the granite component should be younger than the sampled granitic gneiss and derived from a distal source.Both Groups A and B exhibit a clear positive correlation between CIW and T_DM and a negative one between CIW and Eu/Eu1. These correlations point to the crustal evolution of the Yangtze craton towards coupled increasing CIW and Eu depletion with decreasing age. This in turn reflects the change of granitoid magmatism from local Na-rich dioritic-tonalitic-trondhjemitic rocks to widespread K-feldspar granite. The change led to the intracrustal differentiation, stabilization and growth of the craton.Group C is restite and contains abundant sillimanite and garnet and unusually high ilmenite (7–11vol%), which can be seen to be dehydration melting products of biotite under the microscope. This group shows extremely varied REE distributions from LREE enriched to depleted and from negative to strong positive (Eu/Eu1 = 1.63) Eu anomalies. Compared to Groups A and B, Group C is severely depleted in Na₂O, K₂O, LREE, Rb and Ba, whereas TiO₂, Co, V, Sc and HREE and Y are considerably enriched. This is accompanied by anomalous high Sm/Nd (0.21–0.28), ¹⁴⁷Sm/¹⁴⁴Nd (0.1361–0.1738) and ¹⁴³Nd/¹⁴⁴Nd (0.511589–0.511958) ratios. T_DM correlates clearly with Sm/Nd ratio and 2 out of 3 samples give significantly older to unrealistic T_DM (3.9–4.9 Ga). The results document redistribution of REE and an open behavior of the Sm-Nd isotope system during the biotite dehydration melting of metasediments.     

Constraints on metasomatized mantle under Central South America: evidence from Jurassic alkaline lamprophyre dykes from the Eastern Cordillera, NM Argentina

In the Río Grande Valley, NW Argentina, several porphyritic panidiomorphic, ocelli-bearing dykes and sills intrude the Neoproterozoic to lower Paleozoic basement of the Eastern Cordillera. New petrographical and geochemical data permit us to classify these rocks as ocellar-analcime monchiquites, a feldspar-free variety of alkaline lamprophyre composed of Ti-rich-diopside/augite, Ti-rich biotite/phlogopite, forsteritic olivine, titanian-pargasite and analcime, with abundant ocelli filled with analcime/carbonate. In terms of geochemical compositions they are characterized by LILE and LREE enrichment and lack of Nb-Ta and Eu anomalies. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd initial ratios range between 0.70377 to 0.70781 and 0.512506 and 0.512716 respectively, and T_DM model ages vary between 0.25–0.64 Ga. A K-Ar age of 163?±?9 Ma suggests that these rocks are related to the pre-rifting stage of the Mesozoic-Cenozoic continental Salta Rift in NW Argentina. Partial melting of a heterogeneous enriched metasomatized lithospheric mantle, magma mixing and fractionation are envisaged to explain the petrographic, geochemical and isotope characteristics of these magmas.