LREE Depletion Research Articles

Age and geochemistry of granites in Gejiu area, Yunnan province, SW China: Constraints on their petrogenesis and tectonic setting

Gejiu is one of the largest polymetallic tin ore districts in the world. Located at the westernmost end of the South China tungsten–tin province (or Nanling tungsten–tin province), it is a granite-related (Gejiu granite) magmatic-hydrothermal system. Nine samples from two phases of Gejiu granitic intrusions have been analyzed by SHRIMP and/or LA-ICPMS zircon U–Pb techniques, yielding ages ranging from 77.4 ± 2.5 Ma to 85.0 ± 0.85 Ma. Whole rock analysis shows that both phases are high-K and alkali-rich granites and their ACNK values fall mainly into a small range of 1.0–1.1. Moreover, Harker diagrams indicate that granites experienced strong fractional crystallization during magmatic evolution. Most granites display relative enrichment in LREE and strong Eu depletion. The whole rock average ε Nd( t) values of the Gejiu granites vary from − 9.3 to − 6.9, whereas a range of − 8.12 < ε Hf( t) < 1.21 is defined by magmatic zircons. Sr–Nd–Hf isotope data indicate that the granites have been mainly derived from crustal melts with minor input of mantle component. Two stage Nd and Hf model ages, together with isotopic characteristics, indicate that the Gejiu granite magmas were possibly derived from partial melting of Mesoproterozoic continental crust, with minor input of mantle-derived melts, followed by extensive fractional crystallization.

Implications of subduction and subduction zone migration of the Paleo-Pacific Plate beneath eastern North China, based on distribution, geochronology, and geochemistry of Late Mesozoic volcanic rocks

Several major volcanic zones are distributed across the eastern North China Craton, from northwest to southeast: the Greater Xing’an Range, Jibei-Liaoxi, Xishan, and Songliao Basins, and the Yanji, Huanghua, and Ludong volcanic zones. The Huanghua depression within the Bohai Bay Basin was filled by middle Late Mesozoic volcanic rocks and abundant Cenozoic alkaline basalts. Zircon LA-ICP-MS and SHRIMP U–Pb dating show that basic–intermediate volcanic rocks were extruded in the Early Cretaceous of 118.8 ± 1.0 Ma (weighted mean 206Pb/238U age), before Late Cretaceous acid lavas at 71.5 ± 2.6 Ma. An inherited zircon from andesite has a Paleoprotoerozoic core crystallization age of 2,424 ± 22 Ma (206Pb/207Pb age) indicating that the basement of the Bohai Bay Basin is part of the North China Craton. Early Cretaceous basic and intermediate lavas are characterized by strong enrichments in LREE and LILE and depletions in HREE and HFSE, indicating a volcanic arc origin related to oceanic subduction. Depletion in Zr only occurs in basic and intermediate volcanic rocks, while depletions in Sr and Ti exist only in acid samples, indicating that the acid series is not genetically related to the basic–intermediate series. Formation ages and geochemical features indicate that the Late Cretaceous acid lavas are products of crustal remelting in an extensional regime. Combined information from all these volcanic zones shows that subduction-related volcanic rocks were generated in the Jibei-Liaoxi and Xishan volcanic zones during the Early Jurassic, about 60 Ma earlier than their analogues extruded in the Huanghua and Ludong volcanic zones during the Early Cretaceous. This younging trend also exists in the youngest extension-related volcanism in each of these zones: Early Cretaceous asthenosphere-derived alkaline basalts in the northwest and Late Cretaceous in the southeast. A tectonic model of northwestward subduction and continuous oceanward retreat of the Paleo-Pacific Plate is proposed to explain the migration pattern of both arc-related and post-subduction extension-related volcanic rocks. As the subduction zone continuously migrated, active continental margin and backarc regimes successively played their roles in different parts of North China during the Late Mesozoic (J1–K2).

Petrography and Geochemistry of St. Mary Islands, near Malpe, Dakshina Kannada District, Karnataka

The present paper documents the petrography, mineral chemistry and petrochemistry of the acid volcanics of St.Mary group of islands. These are essentially characterized by pyroxene bearing rhyolites with average 73.49 wt.% SiO 2 . All the rhyolites from these islands are acmite normative, hence peralkaline with phenocrysts of zoned plagioclase and sometimes hypersthene/augite. There is depletion of LREE with moderate negative Eu anomaly. The enhancement of HFSE elements indicates their compatibility with continental crust. These rhyolites have high equilibrium temperature (915°C) at relatively lower oxygen fugacity (10 -16 ). The acidic lava flows are anorogenic, within plate rhyolites, generated during northern migration of Indian plate over Marion hotspot.

REE and Sr-Nd isotope geochemistry for Yixian fluorite deposit, western Liaoning Province, China, and its geological implications

Up to now, there were no systematic studies of geochemistry and isotopic age for the Yixian (义县) fluorite deposit, western Liaoning (辽宁) Province, China. Based on the analysis of metallogenic geological setting, we studied the REE, Rb-Sr and Sm-Nd isotopes. The chondrite-normalized REE patterns of fluorite are characterized by moderate LREE depletion (LREE/HREE=0.95–3.57, (La/Yb)N=0.08–2.84) and enrichment of Sr (146×10−6-596×10−6) and moderately positive Eu anomalies (δEu=1.10–1.34), which are similar to those of the host Mesoproterozoic carbonate rocks. The fluorite display (87Sr/86Sr) t =−0.708 5, (143Nd/144Nd) t =−0.511 785, and e Nd(t)=−12.8, which are similar to those of the host Mesoproterozoic carbonate rocks and volcanic rocks of Middle Jurassic Lanqi (蓝旗) Formation. The REE and Sr-Nd isotope geochemistry suggest that the source of the ore-forming material may be the volcanic rocks of Lanqi Formation and host carbonate rocks. The Sm-Nd isochron age of 154±14 Ma (MSWD=0.23) indicates that the Yixian fluorite mineralization nearly corresponds to the period of Lanqi Formation. Based on the integrated geological and geochemical studies, coupled with previous studies, we suggest that Yixian fluorite deposit formed in the extension setting of postcollisional stage and may be attributed to the partial melting of ancient basaltic rocks in the lower crust induced by underplating of basic magma and to the reaction between the F-rich ore-forming fluids and the host carbonate rocks.

REE and HFSE mobility due to protracted flow of basinal brines in the mesoproterozoic Belt-Purcell Supergroup, Laurentia

Argillites from the Belt-Purcell Supergroup record three types of REE patterns. Type 1 (T1), near-primary compositional pattern is characterized by: (1) a PA-UCC-like REE pattern, and Th/Sc ratios reflecting a dominantly Paleoproterozoic catchment; (2) depletion of Ca, Sr and Na due to weathering in the provenance of the sediments; (3) variable depletion of Nb–Ta and Zr–Hf from winnowing of titanite and zircon, respectively, with Nb/Ta and Zr/Hf ratios similar to PA-UCC; and (4) K-enrichment, with variable Th/U and Ce/Ce* ratios from diagenesis. Sandstones with T1 patterns are compositionally equivalent to argillites diluted with quartz, and show the same four T1 compositional features, albeit at lower absolute abundances. Relative to T1 argillites, Type 2 (T2) and Type 3 (T3) patterns both feature enriched and fractionated HREE, but T2 exhibits LREE depletion. Relative to T1 these constitute T2 low − ΣLREE/ΣHREE (15 ± 3) and T3 high − ΣLREE/ΣHREE (24 ± 3) patterns showing progressive enrichment of HREE, erratic ratios of Zr/Hf, Ti/Sm, Y/Ho, Y/Yb and Al/Yb. Collectively, T2 and T3 patterns record diagenetic transfer of REE as well as REE/HFSE and HFSE/HFSE fractionations. Such fractionations are more pronounced in sandstones due to greater hydraulic conductivity for diagenetic fluids. Hence REE and HFSE patterns in the argillites and sandstones are interpreted as diagenetic processes superimposed on a PA-UCC provenance signature, specifically to pervasive and episodic migration of oxidizing-alkaline diagenetic basinal brines in the Belt-Purcell Supergroup. During these diagenetic processes Al, Th and Ga behaved isochemically. Dominantly positive Ce anomalies coupled with mobility of U (as recorded by variable Th/U ratios) is consistent with oxidizing conditions, which may have been internally generated within the basin by reactions between clays and carbonate. Alkalinity of the fluids could have been promoted by some combination of: (1) enrichment of K as indicated by mass balance calculations and the presence of illite; (2) hydrothermal and/or diagenetic alteration of mafic igneous units lower in the stratigraphic sequence; and (3) dissolution of evaporitic units, evident in relict halite casts, in the Belt-Purcell sequence. Supporting this interpretation, previous studies revealed two populations of monazite, differentiated in terms of texture, composition and age. Detrital monazites are >1400 Ma, whereas diagenetic monazites are euhedral and chemically distinctive, spanning ∼1400–∼300 Ma. The diagenetic monazites may record episodic brine activity for ∼1000 Ma that induced T2 and T3 compositional features, and redistributed HREE and Zr–Hf at a basin scale.

On the origin of silicate-bearing diamondites

Garnets and clinopyroxenes, intergrown with diamonds in 37 diamondites (“bort”, “polycrystalline diamond aggregates”, “polycrystalline diamond”, “framesite”), presumably from southern Africa, were analyzed for trace element contents by LA-ICP-MS. The intimate diamond-silicate intergrowths suggest that both precipitated from the same fluids during the same crystallization events. In this study we distinguish 5 chemical garnet groups: “peridotitic” (P), intermediate (I) and 3 “eclogitic” groups (E1, E2 and E3). Chondrite-normalized trace element patterns for the garnet groups roughly correlate with major element abundances. Most of P garnets show complex, mildly sinusoidal REEN patterns with relatively flat HREEN-MREEN, a small hump at Sm-Nd and depleted LREEN, and have relatively high contents of Nb, Ta, U, and Th. The REEN abundance patterns of E garnets differ by showing a continuous increase from LREE to HREE and depletion in LREE and highly incompatible elements relative to the P garnets. Of all garnet groups, E3 garnets are the poorest in highly incompatible trace elements and in Mg. Model equilibrium fluids for P garnets suggest crystallization from magnesian carbonate-bearing fluids/melts, which were very rich in incompatible trace elements — similar to kimberlites. Hypothetical equilibrium melts for E1 and E2 garnets are also magnesian and poorer in LREE and highly incompatible elements relative to typical kimberlitic or carbonatitic melts. Fluids that crystallized the P and most of the E garnets have similar mg numbers indicating a peridotitic source for both. The differences in Cr and highly incompatible element contents can be the result of differences in fluid formation and/or evolution rather than different source rock. The positive correlation of Cr2O3 and mg with the abundances of highly incompatible elements in garnets indicate fluid-rock fractionation processes rather than igneous fractional crystallization processes being responsible for the evolution of the diamondite-forming fluids.

Trace elements record depositional history of an Early Archean stromatolitic carbonate platform

Rare earth elements and selected trace elements were measured in 48 samples of carbonate and chert from stromatolites and associated facies in the 3.45 billion year old Strelley Pool Formation, Pilbara Craton, Western Australia. The samples show coherent REE+Y patterns that vary systematically with sedimentary facies. Chert samples from bedded cherts beneath the Strelley Pool Formation and from the upper bedded chert members in the formation show REE+Y patterns consistent with originating by precipitation from hydrothermal and mixed marine-hydrothermal fluids. In contrast, carbonates and cherts from the stromatolitic reef member share the essential shale-normalized characteristics of other Archean marine precipitates (LREE depletion, positive La and Gd anomalies, absence of a negative Ce anomaly and a strongly superchondritic Y/Ho ratio). The close correspondence between REE+Y signatures and independent sedimentary facies interpretations is viewed as strong evidence for the primary nature of REE+Y patterns. They can thus be used as a proxy for the fluids from which sediments precipitated. Mixing hyperbolae can be constructed that reproduce the chemistry of cherts and carbonates by mixing of hydrothermal and marine fluid endmembers throughout the entire vertical succession from beneath the Strelley Pool Formation to the uppermost cherts. The mixing hyperbolae provide semi-quantitative confirmation that the trace element compositions across the suite of cherts represent different mixtures of ambient seawater and hydrothermal fluids. Our results indicate that the Earth's oldest supracrustal carbonates and associated cherts record important aspects of the REE geochemistry of the waters in which they precipitated, and provide valuable information on possible habitats of some of Earth's earliest biota.

Geochemical systematics of basalts of the Lower Basalt Unit, 2.7 Ga Kambalda Sequence, Yilgarn craton, Australia: Plume impingement at a rifted craton margin

The 2.7 Ga Kambalda Sequence is a mafic and ultramafic volcanic sequence in the Kalgoorlie Terrane of the Yilgarn craton, Western Australia. The Sequence is divided into the Lower Basalt and Upper Basalt Units, separated by the komatiite lava flow units, and was erupted onto an autochthonous volcanic sequence, the 2.8 Ga Penneshaw Formation. The Lower Basalt Unit includes six basalt suites or formations with well preserved pillows and vesicles, at mid-greenschist facies: these are the Woolyeenyer and Burbanks Formations, and Lunnon, Wongi, Scotia, and Missouri basalts. Basaltic flows of the Woolyeenyer Formation, and Lunnon and Wongi Basalts, are compositionally restricted tholeiites with Mg# and Ni contents of 56–52, 110 ppm, 57–51, 190–160 ppm, and 63, 300–210 ppm, respectively. Heavy-REE (HREE) are flat at 4–6 times chondrite, with mild LREE depletion at (La/Sm) N = 0.77–0.94, and Nb/Th ratios of 8–17 signifying that these basalts were erupted through the autochthon without crustal contamination as there is no correlation of Nb/Th with (La/Sm) N . The Scotia Basalt is distinctive in having a range of HREE fractionation where (Gd/Yb) N = 1.1–1.4 indicative of melting over a range of depths from above to below the garnet stability field at ∼ 90 km. Basalts of the Burbanks and Penneshaw Formations have major element compositions akin to the Woolyeenyer Formation, but near-flat REE at 9–11 times chondrite. Both formations have two populations: the most primitive have greater Th contents but Nb/Th < 8, whereas the more evolved show lower Th in conjunction with Nb/Th > 8, in keeping with the hottest liquids having assimilated crust but the evolved flows being uncontaminated. Missouri basalts are all crustally contaminated. Collectively, the uncontaminated Woolyeenyer, Lunnon, Wongi, Scotia basalts and Burbanks Formation have ε Nd values spanning from 4.5 to 1.5, whereas ε Nd values for the contaminated Missouri Basalt and Penneshaw Formation span between 2 and 1; there is a weak trend of ε Nd with Nb/Th. Collectively, the results are interpreted in terms of a mantle plume impinging at the edge of rifted cratonic lithosphere mantle, melting over a range of depths, where hotter liquids assimilated crust but cooler liquids did not.

Geochemistry of coexisting depleted and enriched Paringa Basalts, in the 2.7 Ga Kalgoorlie Terrane, Yilgarn Craton, Western Australia: Evidence for a heterogeneous mantle plume event

Depleted and enriched Paringa Basalts form the uppermost part of the Upper Basalt Unit of the Kambalda Sequence in the 2720–2680 Ma Kalgoorlie Terrane, and provide a window into compositionally and isotopically heterogeneous asthenosphere. Depleted examples of the basalts are compositionally restricted with Mg# between 61 and 57, and Ni between 166 and 73 ppm. On REE and primitive-mantle normalised diagrams they are characterised by: (1) near-flat REE patterns with a small range of LREE depletion ((La/Sm) N 1.01–0.76); (2) neither Ce nor Eu anomalies; (3) Nb/Th ratios of 8.7–12, variably greater than the primitive mantle value of 8; and (4) no negative primitive-mantle normalised P or Ti anomalies relative to neighbouring REE. They have compositions similar to Neoarchean tholeiitic basalts associated with komatiites, and ɛ Nd 2.7 Ga values of +1.3 to + 2.2, in keeping with the majority of crustally uncontaminated Neoarchean basalts and komatiites. Enriched counterparts of the Paringa Basalt are compositionally varied, with Mg# 76–53 and Ni 391–73 ppm. On REE and primitive-mantle normalised diagrams, they feature: (1) systematically fractionated LREE [(La/Sm) N = 2.1–3.1)]; (2) HREE mostly ∼1, with outliers to 1.6; (3) neither Ce nor Eu anomalies; (4) low Nb/Th ratios of 0.7–1.6; and (5) strong negative primitive-mantle normalised P and Ti anomalies. Epsilon Nd values are −1.7 to −4.4; and there is no correlation of ɛ Nd with indices of crustal contamination such as (La/Sm) N or Nb/Th ratios. This basaltic suite has previously been interpreted as either boninitic or crustally contaminated komatiite. Assimilation-fractional crystallisation (AFC) modeling was conducted using a komatiite as parental liquid. Contaminants used in this modeling were local trondhjemite-tonalite-dacite (TTD) Black Flag Group (BFG), average high-Ca granites of the Yilgarn Craton, and average upper-, middle-, or lower continental crust of Taylor, S.R., McLennan, S.M. (1985). The Continental Crust: Its Composition and Evolution. Blackwell Scientific Publications, Oxford, 312 pp.]. Neither any of the average Archean crustal compositions match the AFC trends, nor does the BFG. High-Ca granites do match the trends, as does average modern upper crustal composition but the former have depleted mantle Nd-isotope compositions. There are numerous problems with a model of assimilation of continental crust in the genesis of the enriched Paringa Basalts, including coherent REE patterns over a range of Mg#, and Nb-, P-, and Ti-anomalies that do not increase with Th or (La/Sm) N. Enriched Paringa Basalts are compositionally distinct from Mg-rich continental flood basalts that feature greater contents of Ti, and other incompatible elements, with fractionated HREE, or boninites, picrites, or medium-K basalts from intraoceanic arcs. Rather, these basalts are interpreted to result from recycling of older continental crust into the mantle source of the plume from which the Kambalda Sequence komatiites and basalts erupted. The two distinct compositional and isotopic types record a heterogeneous mantle plume, possibly erupted at a rifted craton margin.

Remelting of subducted continental lithosphere: Petrogenesis of Mesozoic magmatic rocks in the Dabie-Sulu orogenic belt

The Dabie-Sulu orogenic belt was formed by the Triassic continental collision between the South China Block and the North China Block. There is a large area of Mesozoic magmatic rocks along this orogenic belt, with emplacement ages mainly at Late Triassic, Late Jurassic and Early Cretaceous. The Late Triassic alkaline rocks and the Late Jurassic granitoids only crop out in the eastern part of the Sulu orogen, whereas the Early Cretaceous magmatic rocks occur as massive granitoids, sporadic intermediate-mafic intrusive and volcanic rocks throughout the Dabie-Sulu orogenic belt. Despite the different ages for their emplacement, the Mesozoic magmatic rocks are all characterized not only by enrichment of LREE and LILE but depletion of HFSE, but also by high initial Sr isotope ratios, low ɛNd(t) values and low radiogeneic Pb isotope compositions. Some zircons from the Jurassic and Cretaceous granitoids contain inherited magmatic cores with Neoprotozoic and Triassic U-Pb ages. Most of the Cretaceous mafic rocks have zircon δ18O values and whole-rock δ13C values lower than those for the normal mantle. A systematic comparison with adjacent UHP metaigneous rocks shows that the Mesozoic granitoids and mafic rocks have elemental and isotopic features similar to the UHP metagranite and metabasite, respectively. This indicates that these magmatic and metamorphic rocks share the diagnostic features of lithospheric source that has tectonic affinity to the northern edge of the South China Block. Their precursors underwent the UHP metamorphism and the post-collisional anatexis, respectively at different times and depths. Therefore, the Mesozoic magmatic rocks were derived from anatexis of the subducted continental lithosphere itself beneath the collision-thickened orogen; the geodynamic mechanism of the post-collisional magmatisms is tectonic collapse of orogenic roots in response to lithospheric extension.

Pt–Re–Os and Sm–Nd isotope and HSE and REE systematics of the 2.7 Ga Belingwe and Abitibi komatiites

High-precision Pt–Re–Os and Sm–Nd isotope and highly siderophile element (HSE) and rare earth element (REE) abundance data are reported for two 2.7 b.y. old komatiite lava flows, Tony’s flow (TN) from the Belingwe greenstone belt, Zimbabwe, and the PH-II flow (PH) from Munro Township in the Abitibi greenstone belt, Canada. The emplaced lavas are calculated to have contained ∼25% (TN) and ∼28% (PH) MgO. These lavas were derived from mantle sources characterized by strong depletions in highly incompatible lithophile trace elements, such as light REE (Ce/Sm N = 0.64 ± 0.02 (TN) and 0.52 ± 0.01 (PH), ε 143Nd(T) = +2.9 ± 0.2 in both sources). 190Pt– 186Os and 187Re– 187Os isochrons generated for each flow yield ages consistent with respective emplacement ages obtained using other chronometers. The calculated precise initial 186Os/ 188Os = 0.1198318 ± 3 (TN) and 0.1198316 ± 5 (PH) and 187Os/ 188Os = 0.10875 ± 17 (TN) and 0.10873 ± 15 (PH) require time-integrated 190Pt/ 188Os and 187Re/ 188Os of 0.00178 ± 11 and 0.407 ± 8 (TN) and 0.00174 ± 18 and 0.415 ± 5 (PH). These parameters, which by far represent the most precise and accurate estimates of time-integrated Pt/Os and Re/Os of the Archean mantle, are best matched by those of enstatite chondrites. The data also provide evidence for a remarkable similarity in the composition of the sources of these komatiites with respect to both REE and HSE. The calculated absolute HSE abundances in the TN and PH komatiite sources are within or slightly below the range of estimates for the terrestrial Primitive Upper Mantle (PUM). Assuming a chondritic composition of the bulk silicate Earth, the strong depletions in LREE, yet chondritic Re/Os in the komatiite sources are apparently problematic because early Earth processes capable of fractionating the LREE might also be expected to fractionate Re/Os. This apparent discrepancy could be reconciled via a two-stage model, whereby the moderate LREE depletion in the sources of the komatiites initially occurred within the first 100 Ma of Earth’s history as a result of either global magma ocean differentiation or extraction and subsequent long-term isolation of early crust, whereas HSE were largely added subsequently via late accretion. The komatiite formation, preceded by derivation of basaltic magmas, was a result of second-stage, large-degree dynamic melting in mantle plumes.

Halogen and trace-element chemistry in the Gardar Province, South Greenland: Subduction-related mantle metasomatism and fluid exsolution from alkalic melts

XRF analyses of 152 magmatic dyke samples from a broad area (150×60km) of the Gardar Province in Southern Greenland span the time 1280 to 1163Ma and represent a wide compositional range (transitional olivine basalts to trachytes, alkalinity index of 0.3 to 1.5). Among those, 16 dyke samples were additionally analysed for Cl and Br.Generally, the dykes represent a continuous fractionation trend from relatively unfractionated basalts to more highly fractionated trachytes. Dykes from different areas exhibit a diverse geochemistry suggesting a heterogeneous and metasomatised mantle source. Enrichment in LILE, LREE and Sr and depletion in HFSE, Nb and Ti suggest that some of the metasomatism may have been associated with subduction processes pre-dating Gardar activity by some 600Ma (Ketilidian orogeny).The dykes are characterised by high F contents up to 1.2wt.%, particularly in the vicinity of the Ivigtut fluoride deposit. F was probably derived from partial melting of lithospheric mantle enriched in F-apatite and F-phlogopite. High Cl/Br (>500) and low Cl/F (<1) ratios of the dykes point to a fluid degassing/separation process which is supported by mineral/rock and fluid inclusion data from the Ilímaussaq and Ivigtut intrusions. There, the analysed rocks and minerals generally show high Cl/Br (>300) and low Cl/F (<1) weight ratios whereas the fluid inclusions have complementary low ratios (Cl/Br ±100; Cl/F >10). Our investigations are in accordance with experimental data which show that F is preferentially enriched in the melt whereas Cl and especially Br are lost with the fluid phase. Accordingly, the halogens show an increase in incompatible behaviour in the magma in the order of F<Cl<Br.

Island arc tholeiite to boninitic melt evolution of the Cretaceous Kizildag (Turkey) ophiolite: Model for multi-stage early arc–forearc magmatism in Tethyan subduction factories

The Kizildag ophiolite in Turkey is a remnant of the late Cretaceous suprasubduction zone (SSZ) oceanic lithosphere that was developed in Southern Tethys, and is part of a peri-Arabian ophiolite belt including the Troodos (Cyprus), Baër-Bassit (Syria) and Semail (Oman) ophiolites in the eastern Mediterranean region. The internal structure, petrology and geochemistry of the Kizildag intrusive and extrusive rocks show a magmatic progression from island arc tholeiite (IAT) to boninitic compositions. Similar structural architecture and geochemical trends in the coeval Troodos and Baër-Bassit ophiolites suggest that this multi-stage SSZ magmatism was common in most Tethyan ophiolites. The sheeted dikes and the majority of extrusive rocks in Kizildag are composed of basalt, basaltic andesite and andesite with relatively low La/Sm N (0.5–0.7), low MgO (7–9 wt.%) and high TiO 2 (0.7–1.0 wt.%) and exhibit high Dy/Yb N ratios. These IAT suites also include rare, younger dacitic dikes showing strong enrichment in the LREE. The late-stage basaltic dikes and lavas (sakalavites) have high La/Sm N (0.8–1.0), high MgO (9–11 wt.%) and low-TiO 2 (0.3–0.4 wt.%) contents and display strong depletion in both LREE and HREE. This suite of rocks represents the boninitic component of the Kizildag ophiolite that developed from shallow and greater degrees of melting of hydrous and highly refractory peridotites beneath the extended forearc environment. The progressive evolution of Kizildag magmas from IAT to boninitic compositions in time and space was a result of variable mixing, aggregation, and differentiation of geochemically different melt batches, which formed in different levels (polybaric) within the melting column. Rapid slab rollback-induced corner flow, mantle return flow, and asthenospheric diapirism (providing more fertile melt) directly affected the evolution of this melting column and may have triggered advanced degrees of shallow melting of the hydrous and highly depleted forearc mantle, producing refractory liquids of the boninitic stage. These geochemical patterns of the Kizildag ophiolite reflect a multi-stage evolution of SSZ magmatism in Tethyan subduction factories that is analogous to the earlier arc volcanism in the Eocene–Oligocene Izu–Bonin–Mariana system.

Characterization of enriched lithospheric mantle components in ∼2.7 Ga Banded Iron Formations: An example from the Tati Greenstone Belt, Northeastern Botswana

Major and trace element, samarium (Sm)–neodymium (Nd) and lead (Pb) isotopic analyses of individual mesobands of five Banded Iron Formations (BIFs) and associated volcanic and sedimentary rocks from the Neoarchean Tati Greenstone Belt (TGB, Northeastern Botswana) were conducted in order to characterize the source(s) and depositional environment(s). Rare earth element (REE)–yttrium (Y) patterns of individual BIF mesobands show features characteristic of other Archean BIFs with LREE depletion relative to MREE and HREE, positive La/LaPAAS*, Eu/EuPAAS*, Y/Ho ratios and no Ce/CePAAS* anomalies. The REY patterns are comparable to modern seawater and together with low concentrations of high-field strength elements these features are indicative of an essentially detritus-free precipitation. Elevated Eu anomalies in the TGB BIFs are a general feature observed in ∼2.7Ga BIFs worldwide and possibly result from widespread magmatic activity and associated high-temperature fluid fluxes to the oceans at around this time.Uranogenic Pb isotope data for the BIFs define correlation lines with slopes corresponding to apparent ages of ∼2.7Ga which brackets the depositional timeframe. Pb isotope data on sulfides and Pb-stepwise leaching (PbSL) data on garnets define a correlation line with an apparent age of 1976±88Ma. This age is similar to tectono-metamorphic events within the adjacent Limpopo belt. Elevated 207Pb/204Pb relative to 206Pb/204Pb ratios of BIFs are indicative of a high-μ (238U/204Pb) prehistory of their source materials which can best be modeled by a 3.0–3.2Ga extraction of these sources from an older Archean mantle reservoir.The TGB BIFs show evidence of two periodically interacting water masses during the deposition. The first is characterized by elevated Sm/Nd ratios and a negative inferred ɛNd(2.7Ga) value of −2.5 and is associated with high Fe fluxes. The second source, associated with high Si fluxes, is characterized by lower Sm/Nd ratios and a less negative inferred ɛNd(2.7Ga) value of −0.4. While the association of high Fe concentrations and elevated Sm–Nd in BIF mesobands is characteristic of hydrothermal seawater input, the Sm–Nd isotopic characterization of this source, unlike other Archean BIFs, points to a significantly LREE enriched mantle source. This finding is compatible with the potential existence of a sub-continental lithospheric mantle reservoir beneath the Zimbabwe and Kaapvaal craton. The old (up to ∼3.5Ga) Nd (TDM) model ages, particularly of iron-rich mesobands of the TGB BIFs, support such a scenario. In contrast, Si-rich solutes were likely derived from weathering of mafic continental crust.

Petrogenesis of keratophyes in the Pingshui Group, Zhejiang: Constraints from zircon U-Pb ages and Hf isotopes

Zircon LA-ICP-MS U-Pb ages and Hf isotopic as well as whole-rock geochemical data are reported for keratophyes in the Pingshui Group, Zhejiang. The results are used to discuss their petrogenesis and geological significance. The keratophyes were dated at 904±8 to 906±10 Ma. These intermediate-felsic rocks are characterized by high LREE contents and depletion of HREE and HFSE (e.g., Nb, Ta, Ti, P), resembling arc-derived rocks. The keratophyes exhibit positive eHf(t) values of 8.6 to 15.4, consistent with their eNd(t) values of 6.4 to 7.9 but far away from those of crust-derived rocks. Such features indicate that they were likely originated from prompt reworking of juvenile crust by arc-continent collision during the early-Neoproterozoic assembly between the Cathaysia and Yangtze Blocks. Combining with their Hf model ages, we suggest that there may exist not only remarkable growth of juvenile crust at ca.1.3—1.1 Ga but also production of juvenile arc-derived crust along the southeastern margin of the Yangtze Block (e.g., the Pingshui area) at ca.1.0–0.9 Ga.

Rare Earth Element Patterns in the Karst Terrains of Guizhou Province, China: Implication for Water/Particle Interaction

The authors determine the concentrations of dissolved (<0.22 μm) rare earth elements (REE) and suspended particulate matter (SPM) of typical karst rivers in Guizhou Province, China during the high-flow period. The concentrations of acid-soluble REE extracted from SPM using diluted hydrochloric acid are also obtained to investigate water/particle interaction in the river water. The dissolved REE contents in the river water are extremely low in the rivers of the study. The dissolved REE distribution patterns normalized by the Post Archean Australia Shale (PAAS) in the karst rivers are not flat, show slight enrichment of heavy REE to light REE, and also have significant negative Ce and Eu anomalies. The acid-soluble REE appears to have similar distribution patterns as characterized by MREE enrichment and slight LREE depletion, with unremarkable Ce and Eu anomalies. The PAAS-normalized REE distribution patterns of SPM are flat with negative Eu anomalies. The contents and distribution patterns of REE in the SPM are closely related to the lithological character of the source rocks. The SPM contains almost all the REE produced in the process of surficial weathering. This demonstrates that particle-hosted REE are the most important form of REE occurrence. REE fractionation, which takes place during weathering and transport, leads to an obvious HREE enrichment in the dissolved loads relative to the SPM. Y/Ho ratio can be used to shed light on REE behaviors during water/particle interaction.

Enriched and depleted arc basalts, with Mg-andesites and adakites: A potential paired arc–back-arc of the 2.6 Ga Hutti greenstone terrane, India

The ∼2.6 Ga Hutti greenstone belt is one of several Neoarchean greenstone terranes of the eastern Dharwar Craton. There are prevalent mafic volcanic flows with subordinate felsic volcanic units and siliciclastic sedimentary rocks. All lithologies show variable intensities of submarine hydrothermal alteration, polyphase deformation and greenschist to amphibolite grade metamorphism, yet pillow, cumulus, and other primary volcanic features are locally preserved. Well exposed interlayered metabasalts, Mg-andesites (MA), and felsic flows outcrop along an 11 km sector in the SE of the terrane. Based on combined petrographic and geochemical characteristics, two tholeiitic basalt populations have been identified within the metabasalts: (1) those with enriched LREE at 20–50 times chondrite, and (2) an depleted LREE population at 12–20 times chondrite. The former has fractionated LREE, where (La/Sm) N = 1.2–1.7, but flat HREE, and negative anomalies at Nb, P, and Ti relative to neighbouring REE. The latter has lower absolute abundances of compatible and incompatible elements, mildly fractionated LREE, smaller anomalies at Nb, P, and Ti, with (Gd/Yb) N = 1.1–1.6. Several samples have the “N-MORB” signature of LREE depletion coupled with positive Nb anomalies. On the Th/Yb vs. Nb/Yb discrimination diagram depleted basalts plot near the MORB field whereas enriched basalts overlap the backarc and arc fields, consistent with a paired arc–back-arc. Mg-andesites feature SiO 2 57–61 wt.%, multielement pattens similar to enriched basalts, coupled with Cr, Co, Ni contents greater than “normal” andesites. Felsic volcanic rocks are characterized by low Y, high (La/Yb) N , and Zr/Sm, but low Nb/Ta, with zero to positive Eu anomalies, thus conforming to most of the compositional criteria of Archean and Phanerozoic adakites. Similar associations of enriched and depleted arc basalts, with adakites, are known from Neoarchean greenstone terranes of the Superior Province. During intraoceanic subduction, slab dehydration-wedge melting generated arc basalts whereas slab melting-wedge hybridization, generated adakites and Mg-andesites.

Geochemistry and geodynamic origin of the Mesoarchean Ujarassuit and Ivisaartoq greenstone belts, SW Greenland

The Ivisaartoq (ca. 3075 Ma) and Ujarassuit (ca. 3070 Ma) greenstone belts are the largest Mesoarchean supracrustal lithotectonic assemblages in the Nuuk region, SW Greenland. Both greenstone belts underwent polyphase deformation and amphibolite facies metamorphism, and were in due course variously dismembered. Pillow lavas, pillow breccia, magmatic layering, and relic sedimentary structures are well preserved in the Ivisaartoq belt. Volcanic rocks include basalts, and minor andesites and picrites. Boninite-like rocks occur in the Ujarassuit belt. Metasedimentary rocks in the Ujarassuit belt occur as thin layers (0.5–1.0 m) of biotite schists, and in the Ivisaartoq belt as ~ 500 m-thick biotite schists intercalated with minor quartzitic gneisses. There is no field evidence indicating that the Ivisaartoq and Ujarassuit supracrustal rocks were deposited on older continental basement, and their volcanic rocks do not exhibit any geochemical trends indicating contamination by Archean upper continental crust. Four groups of amphibolites (metavolcanic rocks) were recognized in the Ujarassuit greenstone belt on the basis of their REE and HFSE characteristics: (1) Group 1 is characterized by near-flat REE patterns (La/Sm cn = 0.77–1.14; La/Yb cn = 0.84–1.24) and moderate negative Nb anomalies (Nb/Nb⁎ = 0.60–0.79); (2) Group 2 displays LREE-depleted patterns (La/Sm cn = 0.53–1.02; La/Yb cn = 0.32–0.61) and pronounced negative Nb anomalies (Nb/Nb ⁎ = 0.32–0.67); (3) Group 3 consists of LREE depleted patterns (La/Sm cn = 0.69–0.84; La/Yb cn = 0.55–0.91) and absence of significant Nb anomalies (Nb/Nb⁎ = 0.92–1.15); and (4) Group 4 has concave-upward REE patterns (La/Sm cn = 1.64–2.42, Gd/Yb cn = 0.57–1.01) and large negative Nb anomalies (Nb/Nb ⁎ = 0.28–0.42). The depletion of LREE in Group 2 amphibolites appears to have resulted from mobility of these elements during metamorphism. Group 1, 3, and 4 amphibolites retain their near-primary geochemical signatures and their trace element patterns are comparable to those of Phanerozoic oceanic island arc tholeiites (IAT), normal-mid-ocean ridge basalts (N-MORB), and boninites, respectively. The trace element patterns of the least altered meta-ultramafic rocks (La/Yb cn = 2.48–1.35; Nb/Nb⁎ = 0.31–0.60) are comparable to those of modern subduction-related picrites. Amphibolites with an andesitic composition show large negative Nb anomalies (Nb/Nb⁎ = 0.21–0.55), enriched LREE patterns (La/Yb cn = 6.29–15.64), and fractionated HREE (Gd/Yb cn = 2.61–3.12) implying deep melting in equilibrium with residual garnet in the source. Biotite schists and quartzitic gneisses have low chemical indexes of alteration values (CIA = 46 to 62) and trace element characteristics indicating volcaniclastic sedimentary protoliths derived from poorly weathered felsic to mafic source rocks. Collectively, the geochemical features of metavolcanic and metasedimentary rocks suggest that the Ivisaartoq and Ujarassuit greenstone belts represent dismembered fragments of Mesoarchean supra-subduction zone oceanic crust formed in an arc-forearc-backarc tectonic setting.

The world-class Natalka gold deposit, northeast Russia: REE patterns, fluid inclusions, stable oxygen isotopes, and formation conditions of ore

REE patterns of hydrothermally altered rocks, fluid inclusions, and stable oxygen isotopes of quartz were studied at the Natalka gold deposit. Metasomatic rocks formed under decompression reveal gradual depletion in LREE and HREE relative to siltstone of the protolith. The HREE patterns of metasomatic rocks formed under decompression are uniform; an insignificant removal of LREE can be noted. The progressive extraction of REE with increasing alteration of rocks could have been due to the effect of magmatogenic or meteoric fluid. Because a Ce anomaly is absent, the participation of oxidized meteoric water was limited. The inverse correlation between the total REE content and the Eu anomaly value in altered rocks indicates a substantial role of magmatogenic fluid. The REE patterns of altered rocks formed under compression show that the role of metamorphic fluid was not great. All metasomatic rocks are enriched in LREE, so that the enrichment of fluid in LREE as well may be suggested. Three fluid compositions were captured as fluid inclusions: (1) H2O-CO2-NaCl-MgCl2 with a salinity of 1.0–4.9 wt % NaCl equiv, (2) CO2-CH4, and (3) H2O-NaCl-MgCl2 with a salinity of 7.0–5.6 wt % NaCl equiv. Compositions (1) and (2) coexisted in the mineral-forming system at 250–350°C and 1.1–2.4 kbar as products of phase separation under conditions of decreasing P and T. The interaction of this fluid with host rocks resulted in the formation of extensive halos of beresitized rocks with sulfide disseminations. The precipitation of arsenopyrite and pyrite led to the substantial depletion of mineral-forming fluid in H2S and destabilization of the Au(HS)2− complex. The fluid with the third composition arose due to the boiling of the H2O-CO2-CH4-NaCl-MgCl2 liquid and was responsible for metasomatic alteration of host rocks. The late mineral assemblages were deposited from this fluid at the initial stage of ore formation. The high methane concentrations in the ore-forming fluid were likely caused by interaction of hydrothermal ore-bearing solutions with carbonaceous host rocks. The δ18O values of quartz from quartz-scheelite-pyrite-arsenopyrite and sulfide-sulfosalt mineral assemblages vary from +11.6 to +14.1‰ and +11.2 to +13.5‰, respectively. The parental fluids of the early and late mineral assemblages probably were derived from a magmatic source and were characterized by \( \delta ^{18} O_{H_2 O} \) = +6.3 to +8.8‰ at 350°C and +3.6 to +5.9‰ at 280°C, respectively. The narrow interval of oxygen isotopic compositions shows that this source was homogeneous. The data obtained allow us to suggest that the deposit formation was related to magmatic activity, including the direct supply of ore components from a magma chamber and mobilization of these components in the processes of dehydration and decarbonation during contact and regional metamorphism.

Chemical geodynamics of continental subduction-zone metamorphism: Insights from studies of the Chinese Continental Scientific Drilling (CCSD) core samples

The Dabie–Sulu orogenic belt of east-central China has long been a type location for the study of geodynamic processes associated with ultrahigh-pressure (UHP) tectonics. Much of our understanding of the world's most enigmatic processes in continental deep-subduction zones has been deduced from various records in this belt. By taking advantage of having depth profiles from core samples of the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen, a series of combined studies were carried out for UHP metamorphic rocks from the main hole (MH) at continuous depths of 100 to 5000 m. The results provide new insights into the chemical geodynamics of continental subduction-zone metamorphism, especially on the issues that are not able to be resolved from the surface outcrops. Available results from our geochemical studies of CCSD-MH core samples can be outlined as follows. (1) An O isotope profile of 100 to 5000 m is established for the UHP metamorphic minerals, with finding of 18O depletion as deep as 3300 m. Along with areal 18O depletion of over 30,000 km 2 along the Dabie–Sulu orogenic belt, three-dimensional 18O depletion of over 100,000 km 3 occurs along the northern margin of the South China Block. (2) Changes in mineral O isotope, H isotope and water content occur in eclogite-gneiss transitions, concordant with petrographic changes. The contact between different lithologies is thus the most favorable place for fluid action; fluid for retrogression of the eclogites away from the eclogite-gneiss boundary was derived from the decompression exsolution. For the eclogites adjacent to gneiss, in contrast, the retrograde metamorphism was principally caused by aqueous fluid from the gneiss that is relatively rich in water. Inspection of the relationship between the distance, petrography and δ 18O values of adjacent samples shows O isotope heterogeneities between the different and same lithologies on scales of 20 to 50 cm, corresponding to the maximum scales of fluid mobility during the continental collision. (3) Studies of major and trace elements in the two continuous core segments indicate high mobility of LILE and LREE but immobility of HFSE and HREE. Some eclogites have andesitic compositions with high SiO 2, alkalis, LREE and LILE but low CaO, MgO and FeO contents. These features likely result from chemical exchange with gneisses, possibly due to the metasomatism of felsic melt produced by partial melting of the associated gneisses during the exhumation. On the other hand, some eclogites appear to have geochemical affinity to refractory rocks formed by melt extraction as evidence by strong LREE and LILE depletion and the absence of hydrous minerals. These results provide evidence for melt-induced element mobility in the UHP metamorphic rocks, and thus the possible presence of supercritical fluid during exhumation. In particular, large variations in the abundance of such elements as SiO 2, LREE and LILE occur at the contact between eclogite and gneiss. This indicates their mobility between different slab components, although it only occurs on small scales and is thus limited in local open-systems. (4) Despite the widespread retrogression, retrograde fluid was internally buffered in stable isotope compositions, and the retrograde fluid was of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl and molecular water in nominally anhydrous minerals. (5) A combined study of petrography and geochronology reveals the episode of HP eclogite-facies recrystallization at 216 ± 3 Ma, with timescale of 1.9 to 9.3 Myr or less. Collectively, the Dabie–Sulu UHP terrenes underwent the protracted exhumation (2–3 mm/yr) in the HP-UHP regime. (6) Zircon U–Pb ages and Hf isotopes indicate that mid-Neoproterozoic protoliths of bimodal UHP metaigneous rocks formed during supercontinental rifting along preexisting arc-continent collision orogen, corresponding to dual bimodal magmatism in response to the attempted breakup of the supercontinent Rodinia at about 780 Ma. The first type of bimodal magmatism was formed by reworking of juvenile Late Mesoproterozoic crust, whereas the second type of bimodal magmatism was principally generated by rifting anatexis of ancient Middle Paleoproterozoic crust. In conclusion, the geochemical studies of CCSD-MH core samples have placed important constraints on the nature and scale of fluid action and element mobility during the continental subduction and UHP metamorphism.