High Heavy Rare Earth Elements Research Articles

Rare earth element cycling in the pore waters of the Bering Sea Slope (IODP Exp. 323)

We studied the diagenetic behavior of rare earth elements (REEs) in a highly productive passive margin setting of the Bering Sea Slope. Site U1345 was drilled during the Integrated Ocean Drilling Program Expedition 323 at a water depth of 1008m currently in the center of an oxygen minimum zone. Pore water concentrations of fourteen REEs were determined down to ~140meters below the seafloor (mbsf). The REE concentrations were higher in the pore water than the deep seawater, indicating that there was significant liberation from the sediments during diagenesis. There was a major peak at ~10mbsf that was more pronounced for the heavy REE (HREE); this peak occurred below the sulfate-methane transition zone (6.3mbsf) and coincided with high concentrations of dissolved iron and manganese. At ~2mbsf, there was a minor peak in REE and Mn contents. Below ~40mbsf, the REE concentration profiles remained constant. The Ce anomaly was insignificant and relatively constant (PAAS-normalized Ce/Ce*=1.1±0.2) throughout the depth profile, showing that the Ce depleted in seawater was restored in the pore water. HREE-enrichment was observed over the entire 140m except for the upper ~1m, where a middle REE (MREE)-bulge was apparent. REE release in shallow depths (2–4mbsf) is attributed to the release of light REEs (LREEs) and MREEs during the organoclastic reduction of Mn oxides in anoxic sediments. The high HREE concentrations observed at ~10mbsf can be attributed to the reduction of Fe and Mn minerals tied to anaerobic oxidation of methane or, less significantly, to ferromagnesian silicate mineral weathering. The upward diffusion flux across the sediment-water interface was between 3 (for Tm) and 290 (for Ce) pmolm−2y−1.

Late Carboniferous high εNd(t)–εHf(t) granitoids, enclaves and dikes in western Junggar, NW China: Ridge-subduction-related magmatism and crustal growth

We report results of petrologic, geochronological and geochemical investigation of the Late Carboniferous diorites, granodiorites, amphibole (Am)-bearing granites, and associated dioritic and monzonitic enclaves and mafic and granitic dikes in the Keramay area, of the western Junggar region of Central Asian Orogenic Belt (CAOB). Zircon U–Pb dating suggests that they were generated in the Late Carboniferous (316–304Ma). The diorite and granodiorite compositions extend over a wide range of SiO2 (53–70wt.%), Sr (240–602ppm), and Mg# (41–58) values, and are characterized by moderately fractionated rare earth element (REE) patterns, Nb–Ta depletion and relatively low Y and Yb contents. The mafic dikes consist of dolerites, diorite porphyries and minor granodiorite porphyries, and have variable SiO2 (51–59wt.%) and high Mg#, Cr and Ni values. With the exception of two samples with relatively high heavy REE (HREE) contents, the mafic dikes exhibit trace element characteristics similar to diorites and granodiorites. The Am-bearing granites and a granite porphyry dike sample have high levels of SiO2 (73–77wt.%), HREEs (e.g., Yb=3.46–15.7ppm) and low Mg#, Cr and Ni contents, along with clearly negative Eu, Ba and Sr anomalies, similar to typical A-type granites. All granitoids, enclaves and dikes in this region have high positive εNd(t) (+7.13 to +9.74) and zircon εHf(t) (+10 to +16) values and moderate initial 87Sr/87Sr ratios (0.7004–0.7049). Mineral composition data suggest that the parental magmas for mafic dikes are similar to Cenozoic sanukitoids in the Setouchi arc area (Japan) and were possibly generated under water-rich and high oxygen fugacity (NNO+1.5 to NNO+2.7) conditions. They most likely originated from partial melting of a mantle source variably modified by subducted oceanic crust-derived melts and minor fluids and subsequently underwent fractional crystallization. The diorites and granodiorites were possibly generated by magma mixing between enriched lithospheric mantle and juvenile lower crust-derived magmas coupled with minor crystal fractionation. The Am-bearing granites and granite porphyry dike were produced by partial melting of juvenile crustal materials at shallow crustal levels. Taking into account widespread contemporaneous magmatism including “MORB-type” basalts and slab-derived adakites in western Junggar, we suggest that the Keramay intrusive rocks were generated in a special arc setting related to ridge subduction and resultant slab window, which played an important role in the crustal growth of the CAOB.

Detecting Peritectic Garnet in the Peraluminous Cardigan Pluton, New Hampshire

The peraluminous Cardigan Pluton of New Hampshire contains restitic garnetites with abundant peritectic garnet that serves as a basis for comparison with garnet in the host-rock. The peritectic garnet is inclusion-rich and contains high concentrations of heavy rare earth elements (HREE), Sc, and Zr and low concentrations of Zn, P, and U. The high HREE, Zr, and Sc result from biotite and zircon melting, buffering liquid compositions. Because the HREE and Sc are compatible in garnet, the peritectic garnet is rich in these elements. Some garnet crystals in the host-rocks have cores with identical inclusions and trace element compositions to the peritectic garnet in the garnetites. We infer that these are also peritectic garnet grains dispersed in the host magma. These cores are mantled by inclusion-poor garnet. These mantles, along with other garnet crystals that are inclusion-poor throughout, show rimward trends of decreasing HREE, Sc, and Zr, and increasing Zn, P, U, and Eu/Eu*. We interpret the trace element variations as records of fractionation as the melt decreased in HREE, Zr, and Sc and was enriched Zn, P, and U. The rimward increase in Eu/Eu* records the effect of feldspar fractionation. Thus the peritectic garnet preserves evidence of crystallization from buffered melts whereas the phenocrystic garnet shows variable trace element abundances indicative of crystallizing from fractionating melts. These differences provide criteria for the recognition of peritectic garnet in the host pluton.

Geochemical character and material source of sediments in the eastern Philippine Sea

Based upon analyses of grain-size, rare earth element (REE) compositions, elemental occurrence phases of REE, and U-series isotopic dating, the sediment characteristics and material sources of the study area were examined for the recently formed deep-sea clays in the eastern Philippine Sea. The analytical results are summarized as follows. (1) Low accumulation rate, poor sorting and roundness, and high contents of grains coarser than fine silt indicate relatively low sediment input, with localized material source without long distance transport. (2) The REE Contents are relatively high. Shale-normalized patterns of REE indicate weak enrichment in heavy REE (HREE), Ce-passive anomaly, and Eu-positive anomaly. (3) Elemental occurrence phases of REE between the sediments with and without crust are similar. REE mainly concentrate in residual phase and then in ferromanganese oxide phase. The light REE (LREE) enrichment, Ce-positive anomaly, and Eu-positive anomaly occur in residual phase. Ferromanganese oxide phase shows the characteristics of relatively high HREE content and Ce-passive anomaly. (4) There are differences in each above mentioned aspect between the sediments with and without ferromanganese crust. (5) Synthesizing the above characteristics and source discriminant analysis, the study sediments are deduced to mainly result from the alteration of local and nearby volcanic materials. Continental materials transported by wind and/or river (ocean) flows also have minor contributions.

Xenotime-(Y) veins in a Neoproterozoic metamudstone (Małopolska Block, S Poland)

Xenotime-(Y) veins in a Neoproterozoic metamudstone (Małopolska Block, S Poland)First occurrence of xenotime-(Y) veins in the Neoproterozoic metamudstone, NW margin of the Małopolska Block, is reported here. The veins are <5 μm thick, inhomogeneous and of varied morphology. Their textural characteristics, exhibiting very low U and Th contents (<1 wt%) and relatively high heavy rare earth elements (REE) contents (12-24 REE2O3 wt%), suggest hydrothermal genesis, though diagenetic precursors of the veins are not excluded. The veins provide good evidence of the mobility of Y, REE, U and Th in low-temperature conditions (anchimetamorphic to greenschist facies). The observed differences in the chemical composition of the veins may indicate multiple stages of formation, possibly during several hydrothermal events that affected the area.

Stress-induced redistribution of yttrium and heavy rare-earth elements (HREE) in garnet during high-grade polymetamorphism

Almandine garnet (Alm62.6Pyr11.4Sps8.4And6.5Grs4.0) exceptionally rich in Y2O3 (1.8–2.3 wt%), Sc2O3 (0.11–0.23 wt%), HREE2O3 (1.9–2.3 wt%), and Na2O (~0.3 wt%) occurs in Precambrian quartzofeld-spatic granulites ( T ~ 860 °C) on Hisaroya, Western Gneiss Region, Norway. The granulites were partially reequilibrated to eclogite-facies ( P = 14.9 ± 1.3 kbar, T = 649 ± 67 °C) and amphibolite-facies assemblages during the Caledonian Orogeny. The granulite-facies garnet is reequilibrated in an outer zone, typically ~4 μm thick, where both Y2O3 and HREE2O3 concentrations decrease to 1.2 wt%, and overgrown by Caledonian garnet with Y2O3 and HREE2O3 both below 0.1 wt%. The granulite-facies cores are also reequilibrated to lower Y2O3 and HREE2O3 (both ~1.8 wt%) along deformation structures including microfaults with horsetails, en-echelon bands, and splaying features around the tip of fractures. Locally, these reequilibrated zones, typically 10–20 μm across, have shoulders with higher Y and HREE (~2.9 wt%) than in the original garnet, suggesting closed-system behavior for these elements. In addition, the garnet locally displays a complex mesh-like pattern with high and low HREE and Y bands, possibly related to strain. Two charge-balancing mechanisms account for Y and HREE incorporation: (1) the yttrium aluminum garnet (Y2Al2O6, YAG) substitution involving incorporation of Al into the tetrahedral site is dominant in the granulite-facies garnet, and (2) coupled substitution with Na into the dodecahedral site is increasingly important during high- P reequilibration. The internal closed-system reequilibration of the granulite-facies garnet may be related to stress concentrations build up in garnet during the eclogite-facies event. Local stress concentrations during propagation of fractures may be the driving force behind the redistribution of elements seen within cores of granulite-facies garnet.

Accretionary orogenesis of the Chinese Altai: Insights from Paleozoic granitoids

Zircon U–Pb dating and whole-rock major oxide, trace element and Nd–Sr isotope compositions have been determined for four representative granitic intrusions in the SW Chinese Altai, in order to understand the orogenesis and history of crustal growth in the Central Asian Orogenic Belt (CAOB). The Ashile and Halatas bodies are relatively small intrusions formed in the Late Carboniferous (318 ± 6 Ma) and Permian (267 ± 4 Ma), respectively. The larger Tarlang (TB) and Keketuohai Batholiths (KB) have magmatic ages of 359 to 412 Ma. The granitoids consist chiefly of tonalite, granodiorite and granite, and are metaluminous to weakly peraluminous in composition (ASI = 0.8–1.1). The Ashile Pluton is characterized by relatively high MgO (2.73–3.54 wt.%) and Sr (395–456 ppm) contents, with low K 2O/Na 2O ratios (0.31–0.42) and mantle-like Nd–Sr isotopic compositions ( εNd T = + 2.85 to+ 3.26; initial 87Sr/ 86Sr = 0.7047–0.7051). It probably formed by fractional crystallization of a mantle-derived magma, with limited assimilation of old crustal material. The other intrusions have near-zero or negative εNd T values (− 0.67 to− 4.41) and higher Sr isotopic compositions (initial 87Sr/ 86Sr = 0.7067–0.7092), indicating a mixture of juvenile and old components in their genesis. These intrusions are characterized by relatively low K, Rb, Cs contents and low Rb/Sr ratios (mostly < 0.6), and generally exhibit moderate to high heavy rare earth element (HREE) concentrations (Yb N = 8–26) and weak HREE fractionation (Dy/Yb N < 1.5), with or without negative europium anomalies (Eu/Eu ⁎ = 0.5–1.0). These characteristics are consistent with an origin by dehydration melting of a hornblende-bearing, mid-crustal source (above the garnet stability field) dominated by mafic to intermediate rocks. This suggests that the deep crust of the Chinese Altai may contain a considerable proportion of juvenile material. Mantle-derived magmatism, probably in an active extensional continental margin setting, played an important role in the formation of the Chinese Altai, acting not only as a heat source for crustal melting but as a source of juvenile components. The relatively small volumes and distinct geochemical compositions of the Ashile and Halatas Plutons may imply a substantial change in the geodynamic setting in the late Paleozoic.

Flat rare earth element patterns as an indicator of cumulate processes in the Lesser Qinling carbonatites, China

The Lesser Qinling carbonatite dykes are mainly composed of calcites. They are characterized by unusually high heavy rare earth element concentrations (HREE; e.g. Yb > 30 ppm) and flat to weakly light rare earth element (LREE) enriched chondrite-normalized patterns (La/Yb n = 1.0–5.5), which is in marked contrast with all other published carbonatite data. The trace element contents of calcite crystals were measured in situ by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Some crystals show reduced LREE from core to rim, whereas their HREE compositions are relatively constant. The total REE contents and chondrite-normalized REE patterns from the cores of carbonate crystals are similar to those of the whole rock. The carbon and oxygen isotopic compositions of calcites fall within the range of primary, mantle-derived carbonatites. The initial Sr isotopic compositions (0.70480–0.70557) of calcites are consistent with an EM1 source or mixing between HIMU and EM1 mantle sources. However these sources cannot produce carbonatite parental magmas with a flat or slightly LREE enrichment pattern by low degrees of partial melting. Analyses of carbonates from other carbonatites show that carbonates have nearly flat REE pattern if they crystallize from a LREE enriched carbonatite melt. This implies that when carbonates crystallize from a carbonatite melt the calcite/melt partition coefficients ( D) for HREE are much greater than the D for the LREE. The nearly flat REE patterns of the Lesser Qinling carbonatites can be explained if they are carbonate cumulates that contain little trapped carbonatite melt. Strong enrichment of HREE in the carbonatites may require their derivation by small degrees of melting from a garnet-poor source.

Contrasting Group I and Group II Eclogite Xenolith Petrogenesis: Petrological, Trace Element and Isotopic Evidence from Eclogite, Garnet-Websterite and Alkremite Xenoliths in the Kaalvallei Kimberlite, South Africa

Eclogite xenolith varieties at Kaalvallei include accessory-free bimineralic xenoliths, diamond-bearing eclogite, corundum-bearing eclogite, garnet-websterite, alkremite and spinel-bearing eclogite. The xenoliths can be accurately classified into previously defined Group I and Group II varieties on both petrographic and geochemical principles. Kaalvallei Group I eclogites (including diamond-bearing eclogite) are considered to derive from a heterogeneous protolith. Eclogite genesis might have been by residue formation associated with the dehydration and partial melting of a protolith consisting of variably mixed, subducted ocean floor basalt and sediment. Kaalvallei Group II eclogite xenoliths are likely to have formed through crystallization of small-volume melts within conduits in old, enriched subcontinental lithosphere. Kaalvallei websterite xenoliths might be petrogenetically related to Group II xenoliths. Isotopic data for Kaalvallei corundum-bearing eclogite and alkremite xenoliths do not provide constraints on petrogenesis. Spinel-bearing eclogite xenoliths are ultradepleted in virtually all trace elements, with very low light rare earth element contents, relatively high heavy rare earth element concentrations, extreme 87Sr/86Sr (∼0·915) and extreme 143Nd/144Nd (0·517) isotopic compositions. These xenoliths are considered to be the residues of a partial melting event.

Late Paleozoic adakites and Nb‐enriched basalts from northern Xinjiang, northwest China: Evidence for the southward subduction of the Paleo‐Asian Oceanic Plate

Abstract Volcanic rocks consisting of adakite and Nb‐enriched basalt are found in the early Devonian Tuoranggekuduke Group in the northern margin of the Kazakhstan‐Junggar Plate, northern Xinjiang, northwest China. The geochemical characteristics of the andesitic and dacitic rocks in this area resemble that of adakites. The relatively high Al2O3, Na2O and MgO content and Mg♯ values indicate that the adakites were generated in relation to oceanic slab subduction rather than the partial melting of basaltic crust. A slightly higher SrI and a lower ɛNd(t = 375 Ma) compared to adakites of mid‐oceanic ridge basalt (MORB) imply that slab sediments were incorporated into these adakites during slab melting. The Nb‐enriched basalt lavas, which are intercalated in adakite lava suite, are silica saturated and are distinguished from the typical arc basalts by their higher Nb and Ti content (high field strength element enrichment). They are derived from the partial melting of the slab melt‐metasomatized mantle wedge peridotite. Apparently, positive Sr anomalies and a slightly higher heavy rare earth element content in these adakites compared to their Cenozoic counterparts indicate that the geothermal gradient in the Paleo‐Asian Oceanic subduction zone and the depth of the Paleo‐Asian Oceanic slab melting are between those of their Archean and Cenozoic counterparts. The distribution of the adakites and Nb‐enriched basalts in the northern margin of the Kazakhstan‐Junggar Plate, northern Xinjiang, indicates that the Paleo‐Asian Oceanic Plate subducted southward beneath the Kazakhstan‐Junggar Plate in the early Devonian period.

The case for Archaean boninites

Rare Archaean light rare earth element (LREE)-enriched mafic rocks derived from a strongly refractory mantle source show a range of features in common with modern boninites. These Archaean second-stage melts are divided into at least two distinct groups—Whundo-type and Whitney-type. Whundo-type rocks are most like modern boninites in terms of their composition and association with tholeiitic to calc-alkaline mafic to intermediate volcanics. Small compositional differences compared to modern boninites, including higher Al2O3 and heavy REE (HREE), probably reflect secular changes in mantle temperatures and a more garnet-rich residual source. Whundo-type rocks are known from 3.12 and 2.8 Ga assemblages and are true Archaean analogues of modern boninites. Whitney-type rocks occur throughout the Archaean, as far back as ca. 3.8 Ga, and are closely associated with ultramafic magmatism including komatiites, in an affiliation unlike that of modern subduction zones. They are characterised by very high Al2O3 and HREE concentrations, and their extremely depleted compositions require a source which at some stage was more garnet-rich than the source for either modern boninites or Whundo-type second-stage melts. Low La/Yb and La/Gd ratios compared to Whundo-type rocks and modern boninites either reflect very weak subduction-related metasomatism of the mantle source or very limited crustal assimilation by a refractory-mantle derived melt. Regardless, the petrogenesis of the Whitney-type rocks appears either directly or indirectly related to plume magmatism. If Whitney-type rocks have a boninitic petrogenesis then a plume related model similar to that proposed for the modern Tongan high-Ca boninites might apply, but with uniquely Archaean source compositions and source enrichment processes. Second-stage melts from Barberton (S. Africa –3.5 Ga) and ca. 3.0 Ga rocks from the central Pilbara (Australia) have features in common with both Whundo- and Whitney-types, but appear more closely related to the Whitney-type. Subduction zone processes essentially the same as those that produce modern boninites have operated since at least ~3.12 Ga, while a uniquely Archaean boninite-forming process, involving more buoyant oceanic plates and very inefficient mantle-source enrichment, may have occurred before then.

Deep melting and sodic metasomatism underneath the highly oblique-spreading Lena Trough (Arctic Ocean)

Abyssal peridotites collected along the highly oblique-spreading Lena Trough north of Greenland and Spitsbergen have mineral compositions that are similar to residual abyssal peridotites, except for high sodium concentrations in clinopyroxene (cpx). Most samples are lherzolites with light rare earth element (REE)-depleted cpx trace element patterns, but significantly fractionated middle to heavy REE ratios at relatively high heavy REE concentrations. Such characteristics can only be explained by initial melting of a garnet peridotite followed by low degrees of melting in the stability field of spinel peridotite. The residual garnet signature requires either a high potential temperature of the upwelling mantle, or elevated solidus-lowering water contents. The limited spinel field melting suggests a deep cessation of melt extraction, possibly because of the presence of a thick lithospheric cap. This is consistent with the extremely low effective spreading rate and the vicinity to a passive continental margin, which allow conductive cooling to reach deeper levels than commonly estimated for faster mid-ocean ridges. High sodium concentrations in cpx are neither explainable by melt refertilization, nor by a simple diffusion mechanism. The efficient fractionation of sodium from the light REE requires post-melting metasomatism, which is typically restricted to the subcontinental lithosphere. This might imply that the Lena Trough peridotites represent unroofed subcontinental mantle, from which no melt was extracted during the opening of the Lena Trough. It is more likely that sodic metasomatism occurred after partial melting underneath the Lena Trough, and that such an enrichment process is responsible for elevated sodium concentrations in abyssal peridotites elsewhere. Sodium in cpx of residual peridotites can therefore not serve as an indicator of partial melting or melt refertilization.

A post-collisional magmatic plumbing system: Mesozoic granitoid plutons from the Dabieshan high-pressure and ultrahigh-pressure metamorphic zone, east-central China

Three groups of Mesozoic shoshonitic or high-K calc-alkaline intrusive rocks are identified in Dabieshan high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic zone, east-central China and they are related to: (I) slab breakoff; (II) magmatic underplating; and (III) doming. Group-I, the slab breakoff-type, consists of late Triassic (∼210 Ma) mafic monzodiorites. It has moderate to high Sr, and low Rb and Ba abundances, and moderate light rare earth element (LREE)/heavy rare earth element (HREE) and K/Rb ratios. Group-II, the underplating-type, consists mainly of middle Jurassic–early Cretaceous (160–120 Ma) hornblende quartz monzonitic, biotite monzogranitic, and syenogranitic plutons, characterized by relatively high LREE/HREE and K/Rb ratios, and by a large range in concentration of Sr and Ba, coupled with much smaller range in Rb. Group-III, the doming-type, is represented by Cretaceous (125–95 Ma) granitic stocks and granitic porphyry. Compared with group-II, it has high Rb, Y and HREE abundances, low Sr and Ba abundances and low LREE/HREE and K/Rb ratios. All groups have similar Nd and Sr isotopic compositions. Among the three groups, post-collisional granitoid magmatism (group-II) with ages of 160 to 120 Ma, post-dating the HP and UHP metamorphism at 245 to 220 Ma, is the most abundant in the Dabieshan area. The post-collisional granitoid plutons were initially emplaced at different levels ranging from mid-crust to near-surface. This study shows that the whole-rock chemistry of the granitoids vary systematically with crystallization pressures. For example, K 2O, normative Or, Rb and Zr show the strongest increase with decreasing pressure, whereas Ba, Nb, Nd, Yb, MnO, and normative An decrease upward in the Dabie Block. It is suggested that ascent of differentiated, buoyant liquids, combined with fractionation paired with assimilation (AFC), is responsible for the vertical variation. Geological, geochemical and petrological data indicate that group-I could have been generated by partial melting of enriched subcontinental lithosphere mantle due to slab breakoff. Group-II rocks could have been produced mainly from crustal assimilation/melting and fractional crystallization of mantle-derived magmas, whereas group-III magma could have derived from anatexis of the Dabie complex and was highly evolved in a hot doming setting. The late Triassic-early Jurassic slab breakoff may be responsible for the exhumation of UHP rocks through the mantle. The voluminous granitic emplacement together with an episode of rapid denudation suggests that magmatic underplating and inflation could have played a role in the Middle Jurassic–Early Cretaceous rapid exhumation of Dabieshan.

Distinctive REE patterns for tholiitic and calc-alkaline magma series co-occurring at Adatara volcano, Northeast Japan.

Rare earth element (REE) concentrations were precisely determined by inductively coupled argon plasma/atomic emission spectrometry (ICP) for 9 tholeiitic and 13 associated calc-alkaline rocks from Adatara volcano, located at the volcanic front of Northeast Japan arc. The tholeiitic samples commonly display nearly flat REE patterns, whereas the calc-alkaline samples characteristically show concave light REE-enriched patterns. In both suites, the REEs, with the exception of Eu, are positively correlated to varying extents with K2O and other incompatible elements. Eu concentrations in the calc-alkaline suite do not vary significantly, resulting in significant negative Eu anomalies. In the tholeiitic suite, chondrite-normalized (Ce/Yb)CN ratios are low and show little variation (1.54–1.80) over a significant range in SiO2 content (52–62 wt%). In contrast, (Ce/Yb)CN in the calc-alkaline suite show much greater variation (2.17 to 2.94) over a smaller range in SiO2 content (56 to 62 wt%) and increase systematically with SiO2. REE data for the tholeiitic suite are consistent with a crystal fractionation model that successfully reproduces the observed major and trace-element variations. A similar fractionation model is compatible with major-element and some trace-element variations for the calc-alkaline suite, but results in excessively high heavy REE contents, and fails to reproduce the elevated (Ce/Yb)CN ratios for the most acidic samples. The preferential enrichment of light REEs may reflect additional processes such as: (1) assimilation, (2) incorporation of a differentiated liquid (or evolved magma) that has undergone fractionation of heavy REE-rich minerals, and (3) mixing of primary magma derived by a small degree of partial melting from a common (amphibolitic or hornblende gabbroic) source. Although the tholeiitic and calc-alkaline suites possess distinct 87Sr/86Sr, Rb/Ba and Zr/Nb ratios, different magma sources are not required to produce the distinct REE patterns of the coexisting suites. An equilibrium batch partial melting model can reproduce both the flat (tholeiitic) and concave (calc-alkaline) REE patterns from a common peridotitic source by larger (10–15%) and smaller (5%) degrees of partial melting, respectively. Therefore, both tholeiitic and calc-alkaline suites could have been derived from a common lherzolitic magma source, provided that the calc-alkaline suite underwent incorporation (assimilation) of isotopically distinct (lower crust-derived) materials prior to evolution.

Early Proterozoic bimodal volcanic rocks in central Colorado, U.S.A., Part II: Geochemistry, petrogenesis and tectonic setting

The early Proterozoic supracrustal rocks of the Salida area of central Colorado consist of strongly bimodal sequences of volcanogenic rocks. The mafic rocks — basalts, basaltic volcaniclastics, and related gabbro sheets — are distinctly tholeiitic, display a strong iron-enrichment trend, and typically contain less than 50% SiO 2. The felsites are rhyolites to dacites and contain more than 70% SiO 2. Major and trace element modeling show that the mafic rocks underwent two stages of crystal fractionation, the first involving olivine and plagioclase, the second involving plagioclase and clinopyroxene. Fractionation occurred within individual injections as they rose toward the surface rather than in a single magma chamber at depth. Field relations and major element data support the derivation of the felsic rocks from a magma generated by anatexis of sialic crust. However, the low Sr and high heavy REE concentrations in these rocks are not compatible with a partial melting model and suggest that the felsic volcanic rocks could have been derived by extensive fractional crystallization of the mafic magma. Normalized trace element abundances and trace element ratios of the mafic rocks are most like continental flood basalts such as the Columbia River basalts. They also display some similarity to immature back-arc basin tholeiites developed on continental crust, such as those of the Sarmiento complex. The felsic rocks have strong chemical affinities to within-plate rhyolites rather than calc-alkaline rhyolites from orogenic areas. The chemical data, as well as the petrographic, stratigraphic, and regional field data all indicate that the early Proterozoic supracrustal rocks of the Salida area developed along a continental margin, probably within an immature back-arc basin underlain by sialic crust. Remnants of the arc system of similar age may lie to the south in northern New Mexico and southwestern Colorado.