Sm-Nd Isotopic Compositions Research Articles

Petrogenetic evolution of metabasalts and metakomatiites of the lower Onverwacht Group, Barberton Greenstone Belt (South Africa)

A well-preserved sequence, by Archean standards, of mantle-derived metabasalts and metakomatiites forms large parts of the lower Onverwacht Group of the Barberton Greenstone Belt (South Africa). To elucidate the origin of mafic and ultramafic rocks from this 3.55 to 3.45 Ga sequence, we present a comprehensive geochemical dataset including major and trace elements as well as LuHf and SmNd isotope compositions for a variety of metavolcanic rocks. These include metabasalts of the amphibolite-facies Sandspruit and Theespruit Formations as well as metabasalts and metakomatiites of the lower greenschist-facies Komati Formation.Based on their incompatible trace element patterns, the basalts of the Sandspruit and Theespruit Formations can be subdivided into a light rare earth element (LREE) depleted group, a LREE-undepleted group, and a LREE-enriched group. Positive εHf(t) and εNd(t) values of ca. +3 to +4 and 0 to +2, respectively, together with depletions in Th and LaCN/YbCN indicate derivation of the LREE-depleted basalts from a depleted mantle source. However, chondritic εHf(t) and εNd(t) values combined with positive Th and LaCN/YbCN of the LREE-enriched samples indicate a contribution from older granitoid crust in the petrogenesis of these samples.Trace element patterns of komatiites and basalts of the Komati Formation are generally flat relative to primitive mantle with slight depletions in heavy rare earth elements and Th and overall positive εHf(t) of +2.5 ± 3.5 (2 s.d.) and εNd(t) of +0.5 ± 2.2 (2 s.d.). The coherence in trace element characteristics suggests a common magmatic origin for basalts and komatiites. This study reveals that the two lavas were derived from the same mantle plume, i.e. komatiites were formed by high degrees of melting of a depleted mantle source containing residual garnet and the basalts were formed by moderate degrees of partial melting in shallower regions of the mantle.Based on the current dataset, combined with published data, we propose a geodynamic model for the oldest units of the Barberton Greenstone Belt that describes the development from a submerged continental setting (for the Sandspruit and Theespruit Formations) to a submarine plateau setting (for the Komati Formation) as a consequence of continental rifting.

Geochemistry and petrogenesis of sanukitoids and high-K anatectic granites from the Bundelkhand Craton, India: Implications for late-Archean crustal evolution

In this paper, we present in situ zircon U–Pb ages as well as whole-rock elemental and Sm-Nd isotopic compositions for late Archean sanukitoids of the Bundelkhand Craton, India. The LA–ICP–MS zircon U–Pb dating results of the sanukitoids (s.l.) indicate that these rocks were emplaced at 2577 ± 16 Ma, with inherited zircon at an age of 2644 ± 25 Ma, and are coeval with high-K anatectic granites (2544–2583 Ma). The sanukitoids (s.l.) are characterized by high-alumina (15–16 wt.%), Mg# (27–50) and TiO2 (0.31–0.87 wt.%). They have a strongly enriched signature in terms of incompatible trace elements (LREEs and LILE) pointing to a subduction-related or crustal source. The high-K anatectic granites are characterized by Al2O3 (12–15 wt.%), Mg# (13–39) and TiO2 (0.06–0.8 wt.%); they have enriched and variable concentrations of LILEs and REEs; and negative Eu anomalies (Eu/Eu* = 0.15–0.93). The whole-rock εNd(t) values (−3.64 to −1.66) indicate a crustal component. These isotopic and geochemical characteristics indicate that the sanukitoid plutons originated from the mixing of metasomatized mantle melts with anatectic melts and were followed by homogenization at shallow levels of emplacement, whereas the high-K anatectic granites indicate reworking of the pre-existing TTG crust during the cratonic stabilization ca. 2.5 Ga. This interpretation suggests a temporal evolution from TTG-domination through sanukitoids (s.l.) to anatectic leucogranites by an internal reworking of the continental crust, involving subduction-collision environments in the late Archean era. Such magmatic events and tectonics during the late-Archean in the Bundelkhand Craton resemble other Archean cratons, such as the Dharwar, North China, Karelian, Superior Province, Pilbara, Kaapvaal and Amazonian.

Sr-rich aragonite veins in Hyblean serpentinized peridotite xenoliths (Sicily, Italy): Evidence for abyssal-type carbonate metasomatism

Most of Hyblean serpentinized peridotite xenoliths (south-eastern Sicily, Italy) are characterized by the abundance of some incompatible trace elements, particularly Sr, whose origin is still debated. A cryptic metasomatic process is traditionally invoked, although there is also mineralogical evidence for ancient hydrothermal alteration. In this work, two selected peridotite xenoliths were investigated by combining scanning electron microscopy (SEM), electron probe microanalyses and vibrational (Infrared and Raman) spectroscopy. The results, based on both new and previously published data, confirm that the Sr contents in Hyblean partially serpentinized peridotite xenoliths correlate linearly with the loss on ignition (LOI, wt%), indicating that alteration strongly influences whole-rock compositions of these rocks. The Sr abundance is related mainly to the presence of aragonite veins (SrO = 0.65–5.10 wt%), substituting serpentine veins. A tentative timing of alteration of Hyblean deep-seated xenoliths, based on published data on hydrothermal zircons and the SmNd isotope compositions of some peridotite xenoliths, suggests a Permo-Triassic time range, in agreement with palaeo-geographical reconstructions suggesting that Sicily and much of the present Mediterranean are presently rooted in a preserved in-situ remnant of the (Early Devonian) Palaeo-Tethys realm.

Disturbance of the Sm-Nd isotopic system by metasomatic alteration: A case study of fluorapatite from the Sin Quyen Cu-LREE-Au deposit, Vietnam

The Neoproterozoic (840 Ma) Sin Quyen deposit in northwestern Vietnam contains replacement Cu-LREE-Au orebodies in Proterozoic metasedimentary rocks. In this deposit, LREE-bearing minerals include allanite-(Ce), monazite-(Ce), chevkinite-(Ce), and fluorapatite. Fluorapatite from orebodies has undergone variable degrees of metasomatic alteration. Samarium-neodymium isotopic analyses were conducted on altered fluorapatite, and also on allanite-(Ce) and monazite-(Ce), to investigate whether such metasomatism can affect the Sm-Nd isotope system. Allanite-(Ce) and monazite-(Ce) have Sm-147/Nd-144 ratios ranging from 0.0359 to 0.0549, and Nd-143/Nd-144 ratios from 0.51147 to 0.51172. Their initial Nd-143/Nd-144 values at the time of mineralization range from 0.51126 to 0.51148, but mostly cluster between 0.51135 and 0.51145. Thus, the primary ore-forming fluids were relatively homogeneous in their Sm-Nd isotopic compositions. In the Sm-147/Nd-144 vs. Nd-143/Nd-144 diagram, the compositions of allanite-(Ce) and monazite-(Ce) generally plot along a Sm-Nd isochron of 840 Ma, implying that the Sm-Nd isotopic systems of these minerals were either closed or only slightly modified. In contrast, altered fluorapatite crystals have Sm-147/Nd-144 ratios varying from 0.0667 to 0.1348, and Nd-143/Nd-144 ratios from 0.51160 to 0.51199. The calculated initial Nd-143/Nd-144 ratios range widely from 0.51114 to 0.51141, with most values lower than those of the allanite-(Ce) and monazite-(Ce). In the Sm-147/Nd-144 vs. Nd-143/Nd-144 diagram, their compositions mostly plot below the 840-Ma Sm-Nd isochron. Petrographic observations and trace elemental analyses show that metasomatic modification of fluorapatite grains led to increases of their Sm/Nd ratios. The unaltered domains in the grains have Sm/Nd ratios varying from 0.114 to 0.200, with an average value of 0.161; whereas the altered domains have Sm/Nd ratios varying from 0.111 to 0.254, with an average value of 0.183. The increased Sm/Nd ratios can cause the calculated initial Nd-143/Nd-144 ratios to be lower than actual initial isotopic ratios, and can also result in compositional deviations from the reference Sm-Nd isochron. This study demonstrates that the traditionally assumed inert Sm-Nd isotopic system can be metasomatically disturbed due to changes in the Sm/Nd ratio. Therefore, care must be taken when interpreting the Sm-Nd isotopic data from apatite/apatite-rich rocks that have undergone metasomatic alteration.

Meso- and Neoarchean Banded Iron Formations and Genesis of High-Grade Magnetite Ores in the Anshan-Benxi Area, North China Craton

AbstractThe Anshan-Benxi area in the North China craton has numerous occurrences of Algoma-type banded iron formations (BIFs) with subordinate high-grade magnetite ores. These ores provide insight into iron metallogenesis and early evolution of the North China craton. In this paper, we present Sm-Nd-Fe-O isotope, mineralogical, and structural data for four BIF-type iron deposits to place constraints on their depositional ages and formation mechanism.Previous SIMS and LA-ICP-MS zircon U-Pb dating results indicated a Mesoarchean age (ca. 3.10 Ga) for the Dagushan BIF and a Neoarchean age (ca. 2.55 Ga) for other regional BIFs (Dai et al., 2012, 2013, 2014).This is confirmed by Sm-Nd isochron ages of these BIFs, high-grade magnetite ores, and host metavolcanics, which yield two regression lines and match apparent ages of 3149 ± 85 Ma (MSWD = 1.2) for Dagushan, and 2671 ± 120 Ma (MSWD = 3.0) for the other three deposits. Our new chronological data thus suggest Meso- and Neoarchean BIF deposition and potentially significant BIF-type iron deposits at depth.The regional high-grade magnetite ores are all hosted in the BIFs that occur in the same orientation and have transitional boundaries between them. They also show similar Sm-Nd isotope compositions and magnetite rare earth elements + yttrium (REY) profiles, indicating that the Anshan-Benxi BIFs were most likely the source beds. The high-grade magnetite ores contain abundant pyrite and actinolite, with systematically lower δ56Fe values (0.67–0.40‰) when compared to the BIFs (1.88–0.64‰), suggesting a hydrothermal origin. In the field, some high-grade orebodies with schistose textures are adjacent to undeformed granitic plutons. This geologic relationship implies that the high-grade magnetite ores were formed earlier and probably did not result from magmatic hydrothermal fluids. Therefore we suggest that the Anshan-Benxi high-grade magnetite ores were most likely produced by infiltration of metamorphic fluids into primary BIFs, based on the following: (1) magnetite δ18O values within the high-grade magnetite ores (+2.5 to −0.6‰) are significantly lower than those in the BIFs (9.2–2.6‰); (2) magnetite (avg 0.39 ppm) and pyrite (avg 0.098 ppm) in the high-grade magnetite ores have much lower REY abundances than magnetite in the BIFs (avg 14.6 ppm); (3) skeletal quartz in the high-grade magnetite ores shows systematically higher FeOtolal contents (1.36–0.56 wt %) than those in laminated chert bands (0.06–0.00 wt %); and (4) hydrothermal zircons within the Nanfen BIF yield a U-Pb age of 2480 Ma, which is comparable to ca. 2.48 Ga regional metamorphism (Zhu et al., 2015).Furthermore, microstructural textures indicate a maximum regional deformation temperature of up to 500°C, which is lower than the plastic flow temperature (>600°C) of magnetite. Finite strain measurements and electron backscatter diffraction analyses suggest a general flattening deformation and similar crystallographic preferred orientation for all magnetite crystals. These structural features reveal that magnetite in the high-grade magnetite ores never experienced a separate tectonic event. Our microscopic studies also show that microfractures at the interfaces of BIF bands contain fragmented quartz crystals and are filled with abundant metamorphic minerals (e.g., actinolite and chlorite). Considering that the Anshan-Benxi high-grade magnetite ores are commonly adjacent to weak structural planes (e.g., faults), we propose that macro- and microscopic fractures probably provided channels for metamorphic fluids.Recent zircon U-Pb geochronology has indicated widespread BIF formation at ca. 2.55 Ga in the North China craton, corresponding to a pronounced peak in BIF deposition of other Precambrian cratons. It is thus implied that a global geologic event triggered the extensive occurrence of BIFs. We correlate the Neoarchean tectonic evolution of the North China craton with the 2.7 to 2.5 Ga Kenorland supercontinent. Significantly, planar distribution signatures of the North China craton BIFs indicate ca. 2.5 Ga cratonization through the amalgamation of at least seven microblocks that were welded by several Neoarchean greenstone belts. Hf-Nd isotope studies have highlighted the Archean episodic crustal evolution of the North China craton, and the Meso- and Neoarchean BIF deposition could have benefitted from these geologic processes. The Anshan-Benxi high-grade magnetite ores that formed at ca. 2.48 Ga were closely related to important metamorphic events during the North China cratonization process.

Tracing the fluid evolution of the Kiruna iron oxide apatite deposits using zircon, monazite, and whole rock trace elements and isotopic studies

The ore genesis of the Paleoproterozoic iron oxide apatite deposits in the vicinity of Kiruna in northern Sweden is poorly understood, despite a century-long mining history and 2500Mt of iron ore with grades of 30 to 70wt% Fe produced in the region to date. Zircon grains from the ore, recently dated at ca. 1874Ma, show very different appearances compared to zircon from surrounding host rocks (ca. 1880Ma) and related intrusions (ca. 1880 and ca. 1874Ma), particularly an inclusion-rich rim. In contrast, zircon from the host rocks, and a proximal granite intrusion, exhibit typical igneous growth zoning. Electron microprobe results show near stoichiometric composition for Zr, Si, and Hf in the host rock zircon grains. The ore zircon crystals have low analytical totals with significant concentrations of Ca, Fe, Y, and P and infrared spectroscopy showed several weight percent of water. These ore zircon grains further show Fe-rich inclusions, zones and/or veins in elemental X-ray maps, and light rare earth elements (LREE) enrichment. Transmission electron microscopy (TEM) shows that the LREE are not due to micro- or nano-inclusions in the zircon, but are likely hosted as LREE oxides in amorphous regions of the grains. Based on these characteristics, the rims on ore zircon grains are interpreted to be of hydrothermal origin. Uranium-Pb in monazite from the ore, measured by SIMS, suggests a secondary event influencing the area at ca. 1624Ma, a period of known geologic activity in Fennoscandia. Electron microprobe X-ray mapping of these monazite grains shows no zoning and relatively low U and Th concentrations.Stark contrasts are visible between the ore (depleted mantle influence) and host rocks (crustal influence) in the whole rock Lu-Hf and Sm-Nd data. The depleted mantle signature of the ore could be related to the Kiruna greenstone group as a potential source region for the iron. The Sm-Nd isotopic composition of monazite from the ore shows a crustal influence, and indicates that the younger event has not disturbed the whole rock Sm-Nd signature of the ore. The hydrothermal nature of the ore zircon grains and the isotopic signatures point to a hydrothermal influence on the ore formation, with a high temperature magmatic fluid related to the intrusions as most likely heat and fluid source.

Initiation of modern-style plate tectonics recorded in Mesoarchean marine chemical sediments

The chemistry of the oceans in part reflects a balance between inputs from the continents and mantle. Traditionally, it has been thought that Archean ocean chemistry was dominated by mantle sources, but recent work has suggested that continental weathering during the Archean provided a much higher flux to the oceans than previously recognized. Here, we present new Rb-Sr and Sm-Nd isotope compositions on carbonate (dolomite and limestone) from the 2.94Ga Red Lake and 2.80Ga Steep Rock groups in the Superior Province, Canada to assess the potential impact continental weathering had on ocean chemistry during the Mesoarchean, a time when initiation of modern-style plate tectonics has been proposed to have occurred. The low Rb contents of all carbonate samples suggest that clastic contamination does not affect the Sr isotope compositions. Using O and Sr isotope modeling, we identified unaltered samples and estimate a 87Sr/86Sr ratio of 0.70173 for seawater at 2.94Ga and 0.70182 at 2.80Ga. Strontium isotope compositions from both Red Lake and Steep Rock indicate that seawater was significantly more radiogenic than contemporaneous mantle, and suggests that weathering of evolved continental crust was an important input to seawater. Continental weathering likely affected seawater chemistry through uplift of continental lithosphere during the initiation of modern-style plate tectonics at 3.2Ga, a model that is contrary to those that suggest the Archean continents were small in extent and largely submerged. Initiation of modern-style plate tectonics and associated continental weathering had an important effect on the biosphere, including increased nutrient delivery, as well as creation of ecological niches that allowed development of the first biologically produced shallow marine redox gradients.

Combining Nd isotopes in monazite and Hf isotopes in zircon to understand complex open-system processes in granitic magmas

Abstract Mapping the age and trace element and Sm-Nd isotope compositions of monazite grains from a peraluminous Cretaceous granite using laser ablation–split stream analysis reveals a wide range in Nd isotope and rare earth element (REE) compositions within and between single grains. These data corroborate isotopic variability indicated by Hf isotope analysis of zircon in the same granite sample. The REE variations indicate that monazite grew during fractional crystallization. Hf and Nd isotopes indicate that the granitic magma was generated from at least two distinct Proterozoic sources of approximately the same age: one component that had highly radiogenic initial 176Hf/177Hf and 143Nd/144Nd and a second component that was notably less radiogenic. This study highlights the utility of in situ REE and Sm-Nd isotope data in monazite in magmatic systems. Further, it refines the zircon-based constraints on magmatic processes because of sensitivity of light REEs to fractional crystallization, lower probability of complications owing to inheritance, and smaller analytical volumes required.

Combined bulk-rock Hf- and Nd-isotope compositions of Mesoarchaean metavolcanic rocks from the Ivisaartoq Supracrustal Belt, SW Greenland: Deviations from the mantle array caused by crustal recycling

Bulk-rock Lu-Hf and Sm-Nd isotope compositions, as well as major and trace element data are presented for metavolcanic rocks from the Mesoarchaean (ca. 3075 Ma) Ivisaartoq Supracrustal Belt in the Nuuk region of southern West Greenland. The εHft calculated at 3075 Ma range from +0.8 to +3.1 and the corresponding εNdt values range from +0.7 to +3.6, which forms an array that is displaced off the mantle array for these two isotopic systems. Primitive mantle normalized trace element plots of the metabasalts display negative Nb- and Ti-anomalies in combination with the elevated Th abundances, which is consistent with a subduction zone affinity as proposed by previous studies of this metavolcanic belt. No significant correlations are observed between the isotope compositions and proxies of shallow crustal contamination in the Ivisaartoq rocks, despite clear evidence for inherited Eoarchaean zircon [Polat et al. (2009a) Chemical Geology 268, 248-271], which would have dominated the bulk-rock Hf-isotope budget. Furthermore, the measured samples are less radiogenic than the estimate for the depleted mantle composition at 3075 Ma. The lack of isotope and trace element correlation suggests incomplete equilibration between the crustal contaminant and the parental Ivisaartoq melts. We prefer a petrogenetic model with some combination of slab-derived metasomatism of the mantle source region for the Ivisaartoq magmas, which homogenized their trace element contents, in combination with the incorporation of granitoid residue with unradiogenic Hf-isotope composition at higher degrees of partial melting and finally the eruption of mechanically entrained Eoarchaean crust without significant chemical equilibration. The geochemical arc-affinity and lower than depleted mantle (DM) isotope compositions of these metavolcanic rocks support the notion that crustal recycling and plate tectonics has been operating on Earth since at least the Mesoarchaean Eon.

Magma source feature and eruption age of volcanic rocks in the Tram Tau district, Tu Le Basin

Magma source feature and eruption age of volcanic rocks in the Tram Tau district, Tu Le Basin

Assimilation, differentiation, and thickening during formation of arc crust in space and time: The Jurassic Bonanza arc, Vancouver Island, Canada

Research Article| March 01, 2016 Assimilation, differentiation, and thickening during formation of arc crust in space and time: The Jurassic Bonanza arc, Vancouver Island, Canada Rameses J. D’Souza; Rameses J. D’Souza † 1School of Earth and Ocean Science, University of Victoria, Victoria, British Colombia V8W 3P6, Canada †rdsouza@uvic.ca Search for other works by this author on: GSW Google Scholar Dante Canil; Dante Canil 1School of Earth and Ocean Science, University of Victoria, Victoria, British Colombia V8W 3P6, Canada Search for other works by this author on: GSW Google Scholar Robert A. Creaser Robert A. Creaser 2Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar GSA Bulletin (2016) 128 (3-4): 543–557. https://doi.org/10.1130/B31289.1 Article history received: 30 Jan 2015 rev-recd: 21 Aug 2015 accepted: 06 Oct 2015 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Rameses J. D’Souza, Dante Canil, Robert A. Creaser; Assimilation, differentiation, and thickening during formation of arc crust in space and time: The Jurassic Bonanza arc, Vancouver Island, Canada. GSA Bulletin 2016;; 128 (3-4): 543–557. doi: https://doi.org/10.1130/B31289.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Continental arcs and island arcs, eventually accreted to continental margins, are thought to have been the locus of continental growth since at least the Proterozoic Eon. The Jurassic Bonanza arc, part of the Wrangellia terrane on Vancouver Island, British Columbia, exposes the stratigraphy of an island arc emplaced between 203 and 164 Ma on a thick preexisting substrate of noncontinental origin. We measured the bulk major- and trace-element geochemistry, and Rb-Sr and Sm-Nd isotope compositions of 18 plutonic samples to establish if differentiation involved contamination of the Bonanza arc magmas by the pre-Jurassic basement rocks. The 87Sr/86Sr and 143Nd/144Nd isotope ratios of the plutonic rocks at 180 Ma vary from 0.70253 to 0.7066 and 0.512594 to 0.512717, respectively. Assimilation–fractional crystallization modeling using trace-element concentrations and Nd and Sr isotope ratios indicates that contamination by a Devonian island arc in the Wrangellia basement is less than 10%. Rare earth element modeling indicates that the observed geochemistry of Bonanza arc rocks represents two lineages, each defined by two stages of fractionation that implicate removal of garnet, varying in modal proportion up to 15%. Garnet-bearing cumulate rocks have not been reported from the Bonanza arc, but their inference is consistent with our crustal thickness estimates from geological mapping and geobarometry, indicating that the arc grew to at least 23 km total thickness. The inference of garnet-bearing cumulate rocks in the Bonanza arc is a previously unsuspected similarity with the coeval Talkeetna arc (Alaska), where garnet-bearing cumulate rocks have been described. Geochronological data from the Bonanza arc show a continuum in plutonic ages from 203 to 164 Ma, whereas the volcanic rocks show a bimodal age distribution over the same span of time with modes at 198 and 171 Ma. We argue that the bimodal volcanic age distribution is likely due to sampling or preservation bias. East-west separation of regions of young and old volcanism could be produced by rollback of a west-dipping slab, forearc erosion by an east-dipping slab, or juxtaposition of two arcs along arc-parallel strike-slip faults. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Neodymium isotopic compositions of the standard monazites used in U[sbnd]Th[sbnd]Pb geochronology

As an important carrier of thorium, uranium and trace elements, monazite is widely used in UThPb geochronology and SmNd isotopic studies. The combination of UThPb age and SmNd isotopic composition makes monazite one of the most versatile minerals in the study of crustal evolution. Nevertheless, most monazites are small and have complex internal textures that require isotopic measurements with high spatial resolution. The widely utilized in situ analyses, such as secondary ion mass spectrometry (SIMS) and inductively coupled plasma mass spectrometry (ICP-MS), have emphasized the importance for uniform and homogeneous monazite standards for external calibration. Here we performed in situ simultaneous measurement of UThPb and SmNd isotopes, as well as trace element determinations, of several monazite standards using a 193nm excimer laser ablation system coupled with an Agilent 7500a quadruple (Q)-ICP-MS and a Neptune multi-collector (MC)-ICP-MS. For UThPb isotopic analysis, instrumental mass bias and elemental fractionation were corrected by calibrating against a homogeneous monazite standard (44069), which makes it possible to precisely and accurately date other monazite grains. In terms of the SmNd isotopes, the interference of 144Sm on 144Nd was corrected by using the interference-free 147Sm/149Sm for the Sm mass fractionation, and the fractionation between 144Sm and 144Nd was corrected by calibrating against an external monazite standard Manangoutry, which is homogeneous in SmNd isotopic composition. The obtained SmNd isotopic compositions for synthetic LREE glass and natural monazite samples are all consistent with the values obtained by isotope dilution thermal ionization mass spectrometry (ID-TIMS), which indicates the reliability and robustness of our method. Our results demonstrate that the Manangoutry monazite from Madagascar and the Namaqualand monazite from South Africa are relatively uniform and homogeneous in their SmNd isotopic compositions, and hence can serve as standards for external calibration during in situ SmNd isotopic analysis.

Preserved Sm-Nd Isotopic Composition as Useful Provenance Indicators in Neoproterozoic Sandstones in the Voltaian Basin, Ghana

The provenance of sandstones derived from theLower Voltaian Kwahu-Morago Group and the Middle Voltaian Oti-Pendjari Group of the Neoproterozoic Voltaian basin are discriminated by their Sm-Nd Isotopic compositions. Plots from the Sm-Nd data suggested provenance of the Kwahu-Morago Group to be from the Birimian metasediments and associated “basin type”granitoids. The Sm-Nd studies have further revealed an average TDM model age of whole rock samples in the Kwahu-Morago Group to be 2.2 Ga which shows that this portion of the Voltaian Supergroup represents eroded remnants of “basin type” granitoids. Sm-Nd data from the Oti-Pendjari Group suggested provenance from the Birimian volcanic rocks and probably with contribution from the Pan African rocks. Its average TDM model age of whole rock samples was 2.0 Ga, which generally falls in the range of the model ages for the basement Birimian volcanic rocks as well as the model ages for the granitoid rocks and thus suggests the major source rock of the Oti-Pendjari Group as coming from the volcanic belts. The model ages for both groups seem to indicate clastic supply from an early Proterozoic crustal provenance. This study shows that whole rock isotopic analyses can also be complementary in providing an insight into the origin and development of sedimentary successions.

영남육괴 북동부 덕구온천지구 우백질 화강암의 희토류원소 분포도, Sm-Nd, Rb-Sr 연대 및 지구화학적 의의

Here we report major element composition, trace and rare earth element abundance, Sm-Nd and Rb-Sr isotopic composition from Deokgu leucogranite. Chondrite-normalized REE pattern and its Eu anomaly are divided into 3 types systematically, and have close relationship with contents. Such geochemical characteristic indicates that the leucogranite was derived by feldspar fractionation from a common source magma. Sm-Nd and Rb-Sr whole rock ages are (initial $^{143}Nd/^{144}Nd\;ratio

Whole-rock Pb and Sm-Nd isotopic constraints on the growth of southeastern Laurentia during Grenvillian orogenesis

The conventional view that the basement of the southern and central Appalachians represents juvenile Mesoproterozoic crust, the final stage of growth of Laurentia prior to Grenville collision, has recently been challenged. New whole-rock Pb and Sm‑Nd isotopic data are presented from Mesoproterozoic basement in the southern and central Appalachians and the Granite-Rhyolite province, as well as one new U-Pb zircon age from the Granite-Rhyolite province. These data, combined with existing data from Mesoproterozoic terranes throughout southeastern Laurentia, further substantiate recent suggestions that the southern and central Appalachian basement is exotic with respect to Laurentia. Sm-Nd isotopic compositions of most rocks from the southern and central Appalachian basement are consistent with progressive growth through reworking of the adjacent Granite-Rhyolite province. However, Pb isotopic data, including new analyses from important regions not sampled in previous studies, do not correspond with Pb isotopic compositions of any adjacent crust. The most distinct ages and isotopic compositions in the southern and central Appalachian basement come from the Roan Mountain area, eastern Tennessee–western North Carolina. The data set indicates U-Pb zircon ages up to 1.8 Ga for igneous rocks, inherited and detrital zircon ages >2.0 Ga, Sm-Nd depleted mantle model (T DM ) ages >2.0 Ga, and the most elevated 207 Pb/ 204 Pb observed in southeastern Laurentia. The combined U-Pb geochronologic and Sm-Nd and Pb isotopic data preclude derivation of southern and central Appalachian basement from any nearby crustal material and demonstrate that Grenville age crust in southeastern Laurentia is exotic and probably was transferred during collision and assembly of Rodinia. These new data better define the boundary between the exotic southern and central Appalachian basement and adjacent Laurentian Granite-Rhyolite province.

Geochemical and isotopic signatures of nonsubduction mechanisms of formation of the Neoarchean continental lithosphere of the Fennoscandian shield

This study presents new data on the geochemical and Sm-Nd isotope compositions, as well as the U-Pb age and geodynamic nature, of the Neoarchean basalt-andesite-dacite (BAD) association from the Kolmozero-Voron’ya greenstone belt. As it was first demonstrated by the example of the Neoarchean greenstone belt, the formation of BAD associations within a single Neoarchean greenstone structure may be explained by the long-lasting evolution of separate mantle or crustal sources not related to subduction processes.

Sm-Nd isotopic compositions of Cambrian-Ordovician strata at the Jinggangshan area in Jiangxi Province: Tectonic implications

Whole-rock Sm-Nd isotopes were investigated for Cambrian-Ordovician strata from the Jinggangshan area between the Yangtze Block and Cathysia Block in South China. These strata were deposited as a greatly thick unit of muddy-sandy laminas with intercalated carbonate and organic carbon-bearing layers. They have low ɛNd(t) values of −13.9 to −7.9 and old Nd model ages of 1842 to 2375 Ma. In tDM-tStr diagram, they are far away from the concordant line but fall within the evolution zone of the Proterozoic crust of South China. This indicates that the Cambrian-Ordovician strata are mainly composed of matters eroded from ancient Paleoproterozoic crust that may mainly consist of continental-derived detrital sediments with high maturity in the Cathysia Block. However, the Ordovician Jueshangou Formation and Dui’ershi Formation have ɛNd(t) values of −10.5 and −7.9 at the higher end of the above range and Nd model ages of 1842 to 2059 Ma at the lower end of the above range. This suggests involvement of more detritus that were eroded from the relatively juvenile crust from Late Paleoproterozoic to the Early Neoproterozoic. All the Nd model ages for the Cambrian-Ordovician sedimentary rocks in the Cathysia Block and the southeastern margin of the Yangtze Block are older than 1800 Ma, suggesting that no material from the Early Paleozoic depleted mantle-derived magmas was involved in these regions.

Single-grain detrital muscovite Rb-Sr isotopic composition as an indicator of provenance for the Carboniferous sedimentary rocks in northern Dabie, China

Low-grade metamorphic Paleozoic sedimentary rocks are exposed along the northern margin of the Qinling-Dabie orogenic belt that formed by the collision of the North and South China blocks during the Early Mesozoic. Geological provenance and tectonic setting of these low-grade sedimentary rocks are strongly debated. Recent studies have shown that they contain Paleozoic, Late Proterozoic and Late Archean detrital zircons, which points to an origin from different sources including rocks from the South and North China blocks. This study presents single-grain mineral Rb-Sr results for detrital muscovites and biotites from Carboniferous sedimentary sequences of the Beihuaiyang low-grade metamorphic zone exposed along the northern margin of the Dabie Mountains. About 15% of detrital muscovite grains are phengitic in composition with high Si/Al ratios of >3.3. Single grain Rb-Sr isotopic analyses show that both muscovites and phengites give a pooled age of about 420 Ma, implying Middle Paleozoic thermal and high-pressure metamorphic events in the source area(s). Rb-Sr isotopic evidence from detrital biotites suggests at least two different sedimentary sources of old crustal rocks and juvenile material. The latter probably comprised magmatic rocks related to the Paleozoic convergence along the southern margin of the North China block. Whole-rock Sm-Nd isotopic composition of the sedimentary rocks also demonstrates a mixture of sedimentary sources of different ages that confirm conclusions proposed previously from evidence of detrital zircon ages. We conclude that the Paleozoic detrital phengites originate from Paleozoic subduction and subsequent collision proposed for the North Qinling terrane. Our study shows that, single-grain detrital mica Rb-Sr isotopic characteristics can be a potential indicator for the provenance of geological complex terranes.

Paleoarchean tonalite-trondhjemite complex in the northwestern part of the Sharyzhalgai uplift ( southwestern Siberian craton): results of U-Pb and Sm-Nd study

In the northwestern part of the Sharyzhalgai uplift of the Siberian craton (Bulun block), the earliest sialic crust (grey-gneiss complex) is composed of plagiogneisses, their migmatized varieties, and subordinate plagiogranitoids. The petrochemical, trace-element, and Sm-Nd isotope compositions of rocks were studied, and U-Pb dating of zircons (SHRIMP II) was performed. Plagiogneisses and plagiogranitoids of trondhjemite and, more seldom, tonalite compositions are predominant; their compositions are typical of rocks of Archean tonalite-trondhjemitegranodiorite (TTG) complexes (Al 2O 3 ≥ 15%, Mg# = 28–38, (La/Yb) n = 23–66, Sr/Y = 27–135, Eu/Eu ⁎ = 0.7–1.1). Plagiogneisses of meta-andesite-rhyodacite association are subordinate (SiO 2 = 59–69%, (La/Yb) n = 7–32, Sr/Y = 11–24, Eu/Eu ⁎ = 0.5–0.7). Cathodoluminescent study of zircons revealed “magmatic cores” and metamorphic rims; most of the rims differ from the cores in U and Th contents and low or greatly varying Th/U ratios. In migmatized plagiogneisses of trondhjemite composition, two zircon generations of different morphologies have been recognized. The protoliths of the grey-gneiss complex rocks formed in the Paleoarchean as a result of two discrete magmatic events, at ∼3.3 and 3.25 Ga, and their metamorphism and migmatization took place at ∼3.2 Ga. The isotopic and geochemical features of rocks evidence that the primary melts were produced mainly through the melting of metabasic sources at different depths of the thickened crust. Plagiogneisses of trondhjemite composition apparently resulted from magma generation involving ancient sialic material.

Calcite Sm-Nd isochron age of the Shuiyindong Carlin-type gold deposit, Guizhou, China

Shuiyindong is one of the largest and highest grade, stratabound Carlin-type gold deposits in China. It is hosted in Permian bioclastic limestone near the axis of an anticline. Hydrothermal calcite is common on the periphery of central zones of ferroan carbonate dissolution that provided the main source of iron that was sulfidized during gold deposition. These calcite veins contain relatively high concentrations of rare earth elements with MREE-enriched patterns, and yield Sm-Nd isochron ages of 134 ± 3 Ma to 136 ± 3 Ma, with significant different initial ε Nd values of − 12.9 and − 2.0, respectively. These ages are interpreted to record age of decarbonation and gold deposition during the late stages of Yanshanian Orogeny. The initial Nd isotopic compositions of the calcites suggest that Nd was primarily derived from the Permian Longtang Formation that hosts all ore bodies at the deposit. This study shows that Sm-Nd isotopic compositions of hydrothermal calcites can be used to constrain the age and source of Nd in Carlin-type gold deposits.