Steep Rare Earth Element Patterns Research Articles

Relating steep REE patterns in modern volcanism and the development of an amphibole-rich middle to lower crust at the Colombian magmatic arc: A geochemical and receiver functions approach

In this study, I integrated geochemical modeling and receiver functions to test for magmatic fractionation above a garnet pyroxenite arc root structure beneath the Colombian magmatic arc (Northern Andes), as a feasible mechanism to explain the record of steep rare earth element (REE) patterns in modern volcanism and the development of an amphibole-rich middle to lower crust. Magmatic fractionation was evaluated through the geochemical modeling of volcanic rocks that show steep REE patterns with high Sr/Y and La/Yb ratios. A well-defined REE geochemical trend was accomplished for all samples at temperature and pressure estimates that are consistent with middle to lower crustal conditions (∼825-935 °C, ∼10.0–14.7 kbar). From the integration of receiver functions, after analyzing the middle to lower crustal signals at multiple stations on top of active volcanoes, the deep crust of the arc is interpreted as a seismically fast (Vs: ≥4.5 km/s) and dense arc root (≥3.3 g/cm3), above which, lies a low-velocity zone with a horizontal to tilted anisotropy axis. This anisotropic low-velocity region is proposed as a partially molten domain related to the evolution of the arc system. The proposed magmatic fractionating model explains satisfactorily the genesis of rocks with steep REE patterns and high Sr/Y (>40) and La/Yb (>20) ratios as a counterpart for a clinopyroxene amphibolite residuum composing deep levels of the arc crust. I suggest that these fractionating processes are playing an important role in the construction of the Colombian magmatic arc, enabling the formation of a dense garnet pyroxenite arc root, a genetically related amphibole-rich middle to lower crust, and the derivation of steep REE patterns with high Sr/Y and La/Yb ratios in the modern Colombian volcanism.

Petrogenesis of the Jiaoziding granitoids and associated basaltic porphyries: Implications for extensive early Neoproterozoic arc magmatism in western Yangtze Block

Middle Neoproterozoic (ca 860–750Ma) granitoids are widely distributed in the western margin of the Yangtze Block, China, yet their magma sources and tectonic settings are unclear. The geochronology and geochemistry of the granitoids and associated basaltic porphyries, which intruded the ~970Ma Tongmuliang arc volcanic rocks in the Jiaoziding area (east of Pingwu county), were investigated in this study. LA-ICP-MS zircon U-Pb dating indicates that the Jiaoziding granitoids and basaltic porphyries were formed at 795±6Ma and 790±20Ma, respectively. The granitoids have high SiO2 (69.2–76.9wt%), K2O (2.3–5.6wt%), and Na2O (3.2–5.1wt%) contents, and a low Al2O3 (12.4–14.5wt%) content. The basaltic porphyries contain high concentrations of TiO2 (~3wt%) and high field strength elements, have steep rare earth element patterns, and are depleted in Nd and Hf isotopes. Batch partial-melting modelling indicates that the Jiaoziding granitoids could have been derived by ~5% and 50-70% partial melting of Tongmuliang mafic rocks and quartz-keratophyres, respectively. Formation of the basaltic porphyries by melting of upwelling asthenospheric mantle would have been facilitated by extensive lithospheric delamination during the Neoproterozoic. This study established a link between mid-Neoproterozoic granitic magmatism and ~970Ma juvenile arc crust, indicating that extensive early Neoproterozoic juvenile arc crust, and partial melting of this crust in an extensional setting, favoured the formation of middle Neoproterozoic granitic rocks along the W–NW margin of the Yangtze Block.

Contrasting mantle-crust melting processes within orogenic belts: Implications from two episodes of mafic magmatism in the western segment of the Neoproterozoic Jiangnan Orogen in South China

Partial melting of crust and mantle is the dominant mechanism controlling Earth’s differentiation. The two melting processes are commonly associated because mantle-derived magmas could simultaneously provide heat and volatiles to promote partial melting of crust. To explore how the two melting regimes interact during the evolution of orogenic belts, we evaluated the petrogenesis and possible relationships with the Rodinia supercontinent of the two episodes of Neoproterozoic mafic magmatism in the western segment of the Jiangnan Orogen in South China. The first episode of mafic magmatism took place at ca. 830Ma in the Baotan area and was associated with widespread peraluminous granitoids. In contrast, the second formed at ca. 770Ma in the Longsheng area without significant granitic magmatism. The ca. 770Ma Longsheng mafic-ultramafic rocks show radiogenic and coupled Nd [εNd(t) values of −1.3 to +3.1)] and Hf isotopes [εHf(t) values of +2.6 to +6.7], whereas the ca. 830Ma Baotan mafic rocks show unradiogenic and decoupled Nd-Hf compositions [εNd(t)=−8.3 to −0.9, εHf(t)=−6.3 to −2.0]. Crustal contamination cannot result in the enriched isotopic features and the arc-like (such as strong deletions of Nb and Ta) geochemistry for the Baotan rocks because of the absence of correlations of Nd isotopes with incompatible elements and SiO2 contents, but crust-derived components may have been incorporated into the mantle source (<60km) by oceanic subduction. Geochemical modeling suggests that the Longsheng mafic-ultramafic rocks with steep rare earth element patterns formed from a deeper (deep to 120km) mantle source with little crustal contamination. We suggest that the Baotan mafic rocks formed in an active continental margin setting where a lot of volatiles can be released from the underplating mafic magmas to facilitate further crustal melting to generate the associated granitoid rocks. In contrast, the Longsheng rocks formed in a post-orogenic extensional setting where mafic magmas were apt to ascend so that extensive crustal melting was prohibited. The transition between the two episodes of mantle-derived magmas shed light on the secular evolution of lithospheric mantle in the western segment of the Jiangnan Orogen. The diverse mantle-crust melting processes in orogenic belts can serve as an indicator of tectonic transition of Precambrian orogenic belts in the history of the earth.

Magmatic growth and batholithic root development in the northern Sierra Nevada, California

In contrast to the much-studied central and southern Sierra Nevada, relatively little is known about the growth and petrogenesis of the batholith in its northern reaches, making it difficult to evaluate range-wide, spatiotemporal trends in batholithic development and the regional extent of eclogite root production and/or loss. New U-Pb ages from northern Sierra plutons reveal a shift between the age of Cretaceous magmatism recorded in the northern Sierra and the timing of an apparent flare-up in the main batholith, indicating that: (1) the northern batholith was more spatially dispersed and emplaced into regions beyond the modern topographic range, and (2) the Cretaceous high-flux event may have occurred over a longer period of time than previously suggested. Relative to the southern Sierra, Nd and Sr isotopic signatures in northern plutons are more primitive, mimicking the predominantly juvenile nature of the terranes into which the plutons are built. Despite differences in isotopic character, however, major and trace element trends are remarkably similar between northern plutons and the rest of the batholith, suggesting that emplacement into juvenile and/or oceanic lithosphere does not inhibit the generation of evolved, arc-type magmatic products. Northern plutons have relatively high La/Yb and Sr/Y and steep rare-earth element patterns, with small to no Eu anomalies. Taken together, these trends are interpreted to indicate deep processing of magmas in equilibrium with a feldspar-poor, amphibolite-rich residue, containing modest amounts of garnet. It is therefore likely that the northern Sierra Nevada batholith was emplaced into relatively thick crust and developed a dense mafic to ultramafic root. Because it is not seismically imaged today, we posit that the root was subsequently lost, perhaps in response to encroachment of proto-Cascade arc volcanism.

The Somuncura Large Igneous Province in Patagonia: Interaction of a Transient Mantle Thermal Anomaly with a Subducting Slab

The Oligo-Miocene Somuncura province is the largest (∼55 000 km2) back-arc mafic volcanic field in Patagonia, and one of Earth's largest with no clear link to a hotspot or major extension. Major and trace element and Sr–Nd–Pb isotopic data suggest involvement of a plume-like component in the mantle magma source mixed with hydrous, but not high field strength element (HFSE)-depleted components, from a disintegrating subducting plate. Magmatism is attributed to mantle upwelling related to disturbances during plate reorganization, possibly at a time when the South America plate was nearly stationary over the underlying mantle. Melting was enhanced by hydration of the mantle during Paleogene subduction. Crustal contamination was minimal in a refractory crust that had been extensively melted in the Jurassic. Eruption began with low-volume intraplate alkaline mafic flows with depleted Nd–Sr isotopic signatures. These were followed by voluminous ∼29–25 Ma tholeiitic mafic flows with flat light and steep heavy rare earth element (REE) patterns, intraplate-like La/Ta ratios, arc-like Ba/La ratios and enriched Sr–Nd isotopic signatures. Their source can be explained by mixing EM1–Tristan da Cunha-like and depleted mantle components with subduction-related components. Post-plateau ∼24–17 Ma alkaline flows with steep REE patterns, high incompatible element abundances, and depleted Sr–Nd isotopic signatures mark the ebbing of the mantle upwelling.

Steep REE patterns and enriched Pb isotopes in southern Central American arc magmas: Evidence for forearc subduction erosion?

The appearance of adakitic magmas with steep rare earth element (REE) patterns in southern Costa Rica and Panama at ∼4 Ma coincides with the collision of the Cocos Ridge and the inception of slab shallowing along the margin. Distinctly higher 206Pb/204Pb and 208Pb/204Pb ratios in these adakitic lavas than in older Miocene lavas suggest that components enriched in radiogenic Pb also entered the mantle magma source at ∼4 Ma. Published Pb‐isotopic data for Central American arc lavas show that a similar radiogenic component is not present in lavas farther north and that maxima in post‐Miocene 206Pb/204Pb and 208Pb/204Pb ratios occur in central Costa Rica and western Panama. Cretaceous and early Tertiary ophiolites in the forearc, whose origins have been linked to the Galápagos hot spot, show a similar spatial pattern in Pb isotopic ratios. The incorporation of ophiolitic forearc crust into the mantle wedge by forearc subduction erosion can explain the along‐arc spatial and temporal pattern of Pb‐isotopic ratios in southern Central American arc lavas. Partial melting of crust removed from the base of the forearc and subjected to high‐pressure metamorphism in the subduction channel provides an explanation for the steep adakitic REE patterns in some Costa Rican and Panamanian arc lavas.

Geochemistry of Late Proterozoic amphibolites and ultramafic rocks, southeastern Canadian Cordillera

Late Proterozoic amphibolites and ultramafic rocks from the southeastern Canadian Cordillera have been analysed for major and trace elements in order to determine the nature and origin of the protoliths. Geologic relations indicate that these rocks were produced during an episode of continental rifting in the Precambrian. Based on rare-earth-element (REE) patterns, immobile-incompatible-element ratios, and characteristic elemental abundances, amphibolites are subdivided into alkaline and tholeiitic metabasalts. Alkaline basalts are recognized by their steep REE patterns, high Zr/Y, high TiO2 and P2O5 abundances, and low Y/Nb and Ti/Zr. Tholeiitic basalts are subdivided into three groups: (I) high-Mg#, high-field-strength-element (HFSE)-depleted, light-REE (LREE)-enriched tholeiites with flat heavy REE (HREE) patterns; (II) LREE-enriched tholeiites depleted in HREE; and (III) low-Mg# tholeiites with flat REE patterns. Ultramafic rocks occur as boudins of partially recrystallized Cr-spinel-bearing harzburgite or therzolite, enriched in LREE (Ce/Sm = 1.7–1.9), HFSE, CaO, Al2O3, and TiO2 relative to depleted mantle.Geochemical data suggest that the basalts were derived from a heterogeneous mantle source that underwent different degrees of partial melting with variable amounts of subsequent crystal fractionation of the melts. High Mg#, high Cr and Ni abundances, low HFSE abundances, and high olivine saturation temperatures suggest that group I tholeiites are primary mantle melts produced by a relatively high degree of partial melting of a LREE-enriched, HFSE-depleted source. Group II and III basalts have undergone moderate olivine and pryoxene and limited plagioclase fractionation. Mass-balance calculations suggest that the ultramafic rocks represent a crustally contaminated primary-mantle-derived melt.Les éléments majeurs et traces des amphibolites et des ultramafites, d'âge protérozoïque tardif, du sud-est de la Cordillère canadienne ont été analysés dans le but de déterminer la nature et l'origine des protolithes. Les relations géologiques indiquent que ces roches se sont formées durant un épisode de rifting continental dans le Précambrien. Les diagrammes des terres rares, les rapports des éléments immobiles et incompatibles et les compositions chimiques caractéristiques ont permis de subdiviser les amphibolites en métabasaltes tholéiitiques et alcalins. Les basaltes alcalins sont reconnus par les courbes abruptes dans les diagrammes des terres rares, les rapports Zr/Y élevés et les fortes teneurs en TiO2 et P2O5 et les rapports Y/Nb et Ti/Zr faibles. Les basaltes tholéiitiques sont subdivisés en trois groupes : (I) avec Mg# élevé, appauvrissement en éléments de force de champ élevée, tholéiites enrichies en terres rares légères avec courbe horizontale des terres rares lourdes; (II) tholéiites enrichies en terres rares légères et appauvries en terres rares lourdes; et (III) tholéiites avec Mg# faible et avec courbe horizontale des variations des terres rares. Les ultramafites se présentent en boudins formés d'harzburgite incluant un spinelle chromifère partiellement recristallisé ou de therzolite qui sont enrichies en terres rares légères (Ce/Sm = 1,7–1,9), en éléments à force de champ élevée, en CaO, Al2O3 et TiO2, comparativement à un manteau appauvri.

Geochemistry of Archean metasedimentary rocks from West Greenland

Archean metasedimentary rocks occur as components of the Isua supracrustals, Akilia association and Malene supracrustals of southern West Greenland. Primary structures in these rocks have been destroyed by metamorphism and deformation. Their chemistry and mineralogy is consistent with a sedimentary origin, but other possible parents ( e.g. acid volcanics, altered pyroclastic rocks) cannot be excluded for some of them. There is little difference in the composition of metasedimentary rocks from the early Archean Isua supracrustals and probable correlative Akilia association. Both have a wide range in rare earth element (REE) patterns with La N Yb N ranging from 0.61−5.8. The REE pattern of one Akilia sample, with low La N Yb N , compares favourably with that of associated tholeiites and it is likely that such samples were derived almost exclusively from basaltic sources. Other samples with very steep REE patterns are similar to felsic volcanic boulders found in a conglomeratic unit in the Isua supracrustals. Samples with intermediate REE patterns are best explained by mixing of basaltic and felsic end members. Metasedimentary rocks from the Malene supracrustals can be divided into low silica (≤55% SiO 2) and high silica (>77% SiO 2) varieties. These rocks also show much variation in La N Yb N (0.46−14.0) and their origin is explained by derivation from a mixture of mafic volcanics and felsic igneous rocks. The wide range in trace element characteristics of these metasedimentary rocks argues for inefficient mixing of the various source lithologies during sedimentation. Accordingly, these data do not rigorously test models of early Archean crustal composition and evolution. The systematic variability in trace element geochemistry provides evidence for the bimodal nature of the early Archean crust.