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Lithos
Elsevier Science B.V.
Lithos

Elsevier Science B.V.

0024-4937

Lithos/Journal LithosSCIISTP
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    The influence of tectonic and climatic factors on detrital zircon U—Pb age population of Paleozoic metasedimentary rocks in the Lhasa terrane

    Li-Ran ChenWang-Chun XuHong-Fei Zhang
    11页
    查看更多>>摘要：The origin of the Lhasa terrane in South Tibet remains enigmatic. In this study, we performed new U—Pb isotopic analyses on detrital zircons from Late Paleozoic metasedimentary rocks in the eastern end of the Lhasa terrane. The detrital zircon ages range from Archean to Paleozoic, with major peaks at ca. 2500 Ma, 1650-1500 Ma, ca. 1170 Ma, ca. 950 Ma, and ca. 580 Ma. Integrated with data from previous studies, the updated database shows that (1) the Early Paleozoic metasedimentary rocks in the Lhasa terrane contain abundant ca. 950 Ma detrital zircons, and (2) the Late Paleozoic metasedimentary rocks in the eastern end of die Lhasa terrane possess a higher proportion of ca. 950 Ma detrital zircons dian that in the rest of the Lhasa terrane. While the older (ca. 1170 Ma) Grenvillian detrital zircons could be supplied by multiple sources, the younger (ca. 950 Ma) Gren-villian detrital zircons have a single source from the Indian continent. Thus the above observations suggest that the Lhasa terrane has a strong affinity with Indian Gondwana during the Paleozoic. Abundance change in the distinctive ca. 950 Ma detrital zircons with stratigraphic age and position in the Lhasa terrane is interpreted to have been resulted from tectonic activity and climatic variation. We here emphasize that tectonic and climatic factors played an important role in the detrital zircon record of sedimentary rocks.
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    Early Cretaceous subduction initiation of the proto-Caribbean plate: geochronological and geochemical evidence from gabbros of the Moa-Baracoa ophiolitic massif, Eastern Cuba

    Hui-Chao RuiJing-Sui YangJian-Ping Zheng
    17页
    查看更多>>摘要：Ophiolites in Cuba are fragments of either die proto-Caribbean or Caribbean plate, however, neither die time of initial subduction nor the mechanism by which it was induced are well constrained. The Moa Baracoa ophiolitic massif, eastern Cuba, consists chiefly of harzburgite, layered gabbro, gabbro sills and dykes, and lavas. The gabbro sills and dykes yielded zircon U—Pb ages of 136.7 ± 1.8 Ma and 122.2 ± 0.5 Ma, respectively; in contrast, zircons from the layered gabbros yielded a wide range of ages (158 to 2556 Ma). The layered gabbros mostly show high MgO, AI2O3, and CaO, coupled with low TiO2, Na2O 4-K2O and incompatible elements, suggesting a cumulate origin. Their crystallization sequence (plagioclase before clinopyroxene), low plagioclase An values (69.8-73.5), and clinopyroxene Mg# values (80.4-81.6), are similar to those of mid-ocean ridge gabbros, however, modeled equilibrated melts based on clinopyroxene compositions have a forearc basalt geochemical affinity. In the gabbro sills, crystallization of clinopyroxene before plagioclase, high plagioclase An values (83.7-88.5), high clinopyroxene Mg# values (85.1-89.5), and the presence of magmatic amphibole require hydrous magmas from a supra-subduction zone. The gabbro dykes have moderate plagioclase An values (76.1-80.5) and clinopyroxene Mg# values (83.2-88.1), and modeled melts of the gabbro sills and dykes also show subduction-related geochemical features. High positive whole-rock eNd(t) (+8.48 to +10.81) and low (87Sr/86Sr); ratios (0.702500-0.703645), coupled with high positive eHf(t)(zircon) (+12.0 to +17.7) suggest that all the Moa-Baracoa gabbros were derived from a depleted source. However, strongly heterogenous δ~(18)O(zircon) values (0.41-6.32%) in the gabbro sills and dykes indicate addition of a subduction-related fluid. Our data suggest that die Moa-Baracoa ophiolitic massif originated in a forearc environment in die early Cretaceous during subduction initiation. Combined with die regional geological background, we propose that subduction of the proto-Caribbean plate was initiated before 137 Ma. Thus, the zircon in die layered gabbros (δz~(18)O = 6.28 to 8.39%o, eHf(r) =)-1.94 to-9.10) must be detrital grains derived from subducted crustal material. The youngest zircon in these rocks implies that some of the crustal material may have been recycled in as little as 20 Myr.
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    Polyphase growth history of peritectic garnet from a granite: Trace-element zonation, Lu-Hf ages and their significance for the duration of granite-forming processes

    Jung S.J.A. PfanderK. Mezger
    13页
    查看更多>>摘要：Determining the ages and duration of high-grade metamorphic episodes during orogenesis and concomitant melt extraction events is pivotal in reconstructing and quantifying the chemical and petrological evolution of the Earth's continental crust over time. Major and trace elements combined with multichronometric garnet data from die Pan-African Damara belt of Namibia place absolute time constraints on high-grade regional meta-morphism and melting in the lower crust that led to the production of widespread granites emplaced in the upper crust. Some of these granites contain inherited peritectic garnet with inferred igneous rims that record their genesis and evolution from formation by partially melting in die deep crust to the emplacement of their host rock at shallower levels. Particular characteristics of these garnets are textural and chemical disequilibrium features such as resorption, local development of partly euhedral crystal faces and occurrence of quartz-feldspar stringers in the rim of the garnet and trace element abundances with sharp concentration gradients. These features document a previous melting event followed by garnet crystallization in an ascending melt. Widi die exception of the outermost rim, which records late-stage diffusional features, the garnet crystals have constant Mg and Mn concentrations. In contrast, Ca shows pronounced chemical zoning. Trace elements (HREE, LREE, Zr, Sr, Y) are depleted in the core and enriched in the rim region indicating disequilibrium conditions during garnet growth. A pseudosection calculated with the garnet rim composition and die composition of the host granite yields P-T conditions of 0.62 GPa and 760 °C for the formation of the rim. This result is consistent with a P-T estimate of 0.68 GPa and 760 °C derived from the conventional garnet-biotite thermobarometer. Regional metamorphic conditions were above the cordierite-andalusite stability field with estimated conditions of-550-600 °C and 0.4-0.5 GPa. This P-T regime indicates that the garnet originally crystallized at deep crustal levels. Diffusion modelling suggests tiiat the observed concentration gradient of Sm in garnet is consistent with a maximum duration of ca. 0.5-1.5 Ma for the inferred melting interval during peak metamorphism in the lower crust. High-precision multi-point Lu-Hf garnet-whole rock data yield dates of 522.0 ±1.0 and 522.3 ± 0.9 Ma for two samples. Because of presumed short intervals between melting in die lower crust and intrusion of the granite, these dates are interpreted as the age of the melting event during high-grade metamorphism and are temporally indistinguishable from the crystallization interval. Thus, dehydration melting in die deeper crust during peak metamorphism produced peritectic garnet tiiat was incorporated into the granitic melt followed by subsequent new growth of igneous garnet around peritectic cores during intrusion. Previously obtained Sm-Nd garnet whole rock ages are 20-25 Ma younger and likely represent cooling ages. This study shows mat granites enriched in mafic components such as garnet and biotite may intrude as mixtures of melt and the peritectic assemblage produced by incongruent melting involving dehydration-melting of biotite in the lower crust. Melting episodes represent short-lived processes relative to the absolute duration of orogenic cycles.
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    Feldspar crystallization and the behavior of fluorine in the Utagi trachyte, Oki-Dogo, Japan

    Satoshi NakanoKuniaki Makino
    15页
    查看更多>>摘要：Anti-rapakivi feldspar phenocrysts of ternary Or-Ab-An compositions in a sheet trachyte from Utagi Pass, Oki-I Dogo Island (Sea of Japan), show characteristic lamellar-wavy textures, commonly also with patchy texture. These textures have scales ranging from micrometers to decamicrometers, show narrow variation in composition, and preserve very fine cryptoperthites with high-temperature structural states. Several types of fluorite alignment are developed in the lamellar-wavy-patchy textures. Small fluorite grains (mostly <100 nm in size) are not distributed homogeneously or randomly but are heterogeneously or locally dispersed/clustered. Complex microtextures and alignment of fluorites in groundmass feldspars are also observed. In addition to these oc-I currences of fluorite, interstitial fluorite is found in the groundmass. Feldspars were examined using an electron microprobe analyzer, a scanning electron microscope, a transmission electron microscope, cathodoluminescence imager spectrometers and a Fourier transform-infrared spectrometer. It is inferred from the obtained data that the feldspar microtextures are products of complex, kinetically controlled magmatic crystallization with pre-I dominantly diffusion-controlled resorption reactions under H20-poor or near-anhydrous conditions, and they are developed in association with the crystallization of groundmass during magma ascent to final emplacement. In addition, fluorite alignments in the phenocrysts and groundmass feldspars are estimated to have formed by diffusion-controlled replacement through interaction with residual melt during magma ascent. The interstitial fluorite was the final crystallization product of the residual melt. These various occurrences of fluorite constitute a valuable record of the behavior of F in natural volcanic systems.
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    New U—Pb zircon and geochemical constraints on Late Devonian Back-arc basin origin of eclogite protoliths from northeastern Hainan Island, South China

    Xiaochun LiuYi ChenMengmeng Xia
    14页
    查看更多>>摘要：Geochronological, geochemical, and Sr—Nd isotopic data are used to constrain the age, origin, and tectonic setting of the protoliths of newly discovered eclogites from Chaotanbi, northeastern Hainan Island, South China. New sensitive high-resolution ion microprobe (SHRIMP) U—Pb zircon dating reveals that the protoliths of the Chaotanbi eclogites formed at 364 ± 2 Ma and were metamorphosed at 297 ± 1 Ma. All samples collected from a coherent area of —1.8 km2 in the intertidal and subtidal zones on the island exhibit tholeiitic compositions, with initial ~(87)Sr/~(86)Sr and ε_(Nd) values of 0.705-0.707 and-1.1 to +7.4, respectively. The samples can be further categorized into three groups based on incompatible trace elements. Group #1 samples have a normal mid-ocean ridge basalt (N-MORB) affinity and were originated from relatively high-degree partial melting of a depleted spinel lherzolite mantle source slightly modified by slab-derived fluids. Group #2 samples have an enriched mid-ocean ridge basalt (E-MORB) affinity and were derived from an enriched spinel and spinel-garnet lherzolite transition mantle source at depths greater than that of Group #1. Group #3 samples have an island arc basalt (IAB) affinity and were generated by partial melting of an enriched spinel-garnet lherzolite mantle source modified by fluid-and sediment-related recycled components. The association of these diverse basalt types, with MORB-type samples accounting for —90% of all samples, suggest a Late Devonian mature back-arc basin existed on northeastern Hainan Island, as a part of the Jinshajiang-Ailaoshan-Song Ma-Hainan back-arc basin system. Therefore, the Chaotanbi eclogites are a fingerprint of the opening and closure of the eastern Paleo-Tethyan Ocean.
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    Carbonatite, aillikite and olivine melilitite from Zandkopsdrift, Namaqualand, South Africa: Constraints on the origin of an unusual lamprophyre-dominated carbonatite complex and the nature of its mantle source

    P.I. OgungbuyiP.E. JanneyC. Harris
    16页
    查看更多>>摘要：We present a chemical and stable and radiogenic isotope data set for carbonatite, aillikite (ultramafic lamp-rophyre) and olivine melilitite from the Zandkopsdrift carbonatite complex in Namaqualand, Soutii Africa. This carbonatite complex is associated with the 500-km-long, NE-SW oriented, age-progressive (40-80 Ma) Namaqualand-Bushmanland-Warmbad ultramafic alkaline magmatic lineation that consists mainly of olivine melilitite and kimberlite. Zandkopsdrift has a radiometric age of 55 ± 4 Ma and is unusual among southern African carbonatites because the dominant rock type exposed is aillikite (a variety of carbonated ultramafic lamprophyre), with carbonatite occurring mainly as a network of subvertical dykes in the southwest of the complex and olivine melilitite occurring as meter-sized blocks of float derived from the subsurface. Calcite and ferroan dolomite carbonatites are present but Mg-rich dolomitic carbonatite varieties are lacking. All rock types appear to have experienced at least moderate differentiation and all show elevated δ~(18)Osmow values for carbonate minerals, whereas phlogopite and clinopyroxene from these rock types fall near mantle values (with ranges of +13.3 to +21.8%o and + 4.9 to +5.4%o for carbonates and silicates, respectively), indicative of interaction with hydrous fluids at low-temperature. Sr-Nd-Pb isotope compositions indicate moderate to strong HIMU signatures (e.g., ~(206)Pb/~(204)Pbi = 19.4 to 21.0) with limited Sr—Nd isotope variation and subvertical Nd—Hf isotope arrays extending well below the ocean island basalt field and terrestrial array. Uncertainty regarding the differentiation history of the three rock types complicates estimation of primary magma compositions but strong chemical similarities between the carbonatites and aillikites, plus their close physical intermingling, strongly suggests tiiat they are related by fractional crystallization to a common parental magma, whereas their relationship to the melilitites is less straightforward. The aillikites and melilitites have compositions of the REE and other elements similar to near-primary versions of these rock types from other alkaline igneous provinces globally, which have been explained as the products of low degrees of partial melting at 4-7 GPa (for aillikites) and slightly higher degrees of melting at 3-3.5 GPa (for melilitites). Lower pressures of generation and distinct trace element characteristics of the Zandkopsdrift melilitites could be explained by these originating as melts of a garnet wehrlite halo around the magmatic conduit formed by metasomatic reaction between carbonated melts and lithospheric peridotite. Most Zandkopsdrift carbonatites have high Lu/Hf ratios which, combined with their relatively low 176Hf/177Hf values, suggests that their mantle sources could not have formed more than 30-50 Myr prior to emplacement of the complex. This implies tiiat a mantle plume had a role in their formation, either as a direct source of melts or as a source of heat with which to melt pre-existing metasomatic heterogeneities.
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    Multidisciplinary characterization of the "Ghiara" volcanic paleosoil (Mt. Etna volcano, Italy): Petrologic characters and genetic model

    G. LanzafameM.C. CaggianiC. Finocchiaro
    13页
    查看更多>>摘要：The large expanses of lava field and soils spread out around Mt. Etna volcano (Italy), make unique the landscape of the whole area. In this work, we investigated a sample set of a peculiar aggregate locally named with the term "Ghiara" collected in different outcrops of Mt. Etna volcano southern flank. A distinctive character of this material is that it is found exclusively beneath solidified lava flows. In past centuries, Ghiara was widely employed in construction field for its reddish hue and the high pozzolanic activity; for the same reasons it has now raised die interest of the scientific community involved in the formulation and production of eco-friendly materials as alternative to the traditional ones. The aim of this paper is to define its chemical and mineralogical characters and to provide a model for its genesis. We conducted a multidisciplinary analytical approach, complementary to the few investigations reported in literature, in order to define the chemical compositions by means of X-ray fluorescence (XRF), the mineralogy through synchrotron radiation X-ray diffraction (XRD) analyses and spectroscopic features by Raman and Fourier-transform infrared (FT-IR) methods. The analytical results show that the original Etnean paleosoils, mainly given by pyroclastic fall and accumulation, underwent marked modifications that produced the unique mineral assemblage of the Ghiara, characterized by the presence of hematite, a mineral responsible for its reddish appearance. The heterogeneity of the samples, however, does not allow us to define a univocal genetic model. On the basis of the outcrops geometry and the environmental conditions we propose a range of models that fit the characters of each sample. In detail, the genesis of this material can be influenced by one or all die following phenomena: i) chemical oxidation, ii) biological oxidation and iii) hydrothermal transformation by interaction of meteoric waters with volcanic volatiles from lavas. This work represents a first contribute in the knowledge of Ghiara characters and genesis, which on one side will help to a better use in its employment for construction purposes, on the other will contribute to constrain the hematite formation processes in different environments.
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    Peritectic and phenocrystic garnet accumulation and the origin of strongly peraluminous granitic rocks: The Flagstaff Lake Igneous Complex, Maine

    Michael J. DoraisStephen Campbell
    20页
    查看更多>>摘要：The Flagstaff Lake Igneous Complex of Maine contains a metaluminous phase that ranges from diorite to granite, and three phases of strongly peraluminous, garnet-bearing rocks with silica contents that range between 43 and 59 wt%. These peraluminous rocks have increasing amounts of normative corundum with increasing maficity, indicating that they do not represent liquid compositions, rather garnet accumulation accounts for their bulk-rock A/CNK compositions. Garnet in these rocks displays a variety of textures, inclusion patterns, and trace element compositions. The Loon Lake phase is characterized by garnet with abundant, randomly distributed apatite inclusions. Some grains have spiral inclusion patterns that are peritictic grains formed during partial melting of source rocks while they were undergoing differential stress. In contrast, the Quarry Phases A and B contain inclusion-free garnet with euhedral grain boundaries. These grains have trace element profiles indicative of crystal fractionation processes and are interpreted to be phenocrysts. Some of these phenocrysts have inclusion poor, resorbed cores that are mantled by inclusion-free overgrowths. The cores have lower concentrations of compatible trace elements and higher concentrations of incompatible elements than the mantles. These cores are resorbed phenocrysts from an evolved magma that were mingled in a more primitive, garnet crystallizing magma. Also present in Quarry Phase B rocks are a minor population of inclusion-rich garnets with random inclusion distributions. These are also interpreted as peritectic garnet, entrained from the source, or from dehydration melting of country rock biotite. They are mantled by inclusion-free garnet with the same composition as the phenociysts and are phenocrystic overgrowths. Thus, the original Flagstaff Lake anatectic melts were mildly peraluminous with the current bulk-rock compositions of the garnet tonalites dominated by the accumulation of variable portions of phenocrystic garnet or the entrainment of peritictic garnet, both of which generated the increasing peraluminosity with increasing maficity of the complex. Magmatic, mafic enclaves present in the peraluminous rocks are similar to the metaluminous, main phase rocks of the complex, suggesting that the coeval basaltic magmas provided the heat for anataxis to generate the peraluminous magmas.
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    Zoisite in cratonic eclogite xenoliths-Implications for water in the upper mantle

    I.B. RaduB.N. MoineN. Bolfan-Casanova
    17页
    查看更多>>摘要：A significant portion of the Earth's water is found at trace levels in nominally anhydrous rock forming minerals in the crust and mantie. Its distribution has far-reaching implications in melt-generating processes, mantle rheology and the stability of the lithosphere. We report petrographic, major and trace element, Fourier transform infrared spectroscopy and secondary ion mass spectrometry data on four eclogite xenoliths from the Obnazhennaya kimberlite, Siberia that contain hydrous minerals at upper mantle conditions. Al-rich (>9.9 wt% AI2O3) omphacitic clinopyroxene, a major mineral of eclogites, contains crystallographically controlled exsolutions of CaO-rich (up to 23.6 wt%) garnet (coronae, lenses), garnet-zoisite (lamellae) or amphiboles (needle/blade-like exsolutions). The omphacite hosting the exsolved lamellae shows enrichment or depletion in major (Al, Si, Mg) and trace (e.g., HREE) elements compatible with garnet and zoisite, proportional to the abundance of exsolutions. We argue that zoisite likely formed concomitantly with garnet exsolution from a water-and Al-rich precursory omphacite, without any fluid addition. The compositional gradients are consistent with diffusion-controlled partitioning in a closed system, and their preservation indicates the exsolution likely took place shortly prior to eclogite entrapment by the kimberlite. Large, non-exsolved, omphacite grains contain minimum —870-1500 ppm wt. H2O (determined as OH). We infer that intertwined lamellae of hydrous garnet (600-960 ppm wt. H20) and minor (<1%) zoisite (2 wt% H2O) formed at mantle conditions due to progressive exsolution, without external fluids. Recalculated whole rock water contents range from-320 to 970 ppm wt. H20. These estimates exceed by far those for the surrounding peridotitic mantle and suggest that, although a minor component, eclogites may locally be an important water reservoir in the cratonic mantle.
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    Petrogenesis, tectonic setting and geodynamic implications of Ouaden, Doumba Bello, and Ngoura granitic plutons (Eastern Cameroon): Constraints from elemental and Sr-Nd—Hf isotopic data and zircon U-Pb ages

    Yanick Brice LemdjouHuan LiScott A. Whattam
    19页
    查看更多>>摘要：The Ouaden, Doumba Bello, and Ngoura granitic plutons, eastern Cameroon, comprise segments of the Central African Fold Belt (CAFB) situated along the northern edge of the Congo craton. A geochemical, isotopic and geochronological study was carried out on these three plutons to determine their petrogenesis and tectonic environment of formation. Granodiorites and granites (SiO2 of 65-73 wt%, Mg# of 14-46) of these plutons are potassic and weakly peraluminous with A/CNK ratios of 0.99-1.10. On primitive-mantle normalized trace element diagrams, all intrusives are depleted in Nb, Ta, and Ti. LA-ICP-MS zircon U-Pb dating indicate that the Ouaden, Doumba Bello and Ngoura granitic plutons were emplaced coevally at 640 ± 3 Ma, 641 ± 3 Ma, and 638 ± 4 Ma, respectively. The granodiorites and granites have variable 87Sr/86Sr(Q ratios of 0.71117-0.71831, relatively unradiogenic whole-rock eNd(t) values of-9.6 to-6.3, and zircon eHf(t) values ranging from-10.1 to-2.1; the occurrence of mafic microgranular enclaves suggests mixing with mantle-derived magmas, and such mixing possibly resulted in the scattering of zircon Hf data. Two-stage Nd-Hf model ages of 2115-1856 Ma and 2218-1708 Ma, and AFMQ of +5.3-+2.1 indicate that the studied granitoids were derived from partial melting of Paleoproterozoic crust under variable oxidizing conditions. All granitoids yield inherited zircon ages (736-685 Ma), indicating contamination of Early Neoproterozoic crust during ascent. A combined examination of the geological record, bulk rock chemistry and isotope signatures suggests that the granodiorites and granites formed by mixing of mantle-and crust-derived melts during a late-subduction early-collision setting between the Metacraton and the Congo craton.
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