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Lithos
Elsevier Science B.V.
Lithos

Elsevier Science B.V.

0024-4937

Lithos/Journal LithosSCIISTP
正式出版
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    The Calatrava paradox to decipher the origin of carbonatites: A petrological insight on Finca La Nava, Calatrava Province (central Spain)

    G. RosatelliE. Humphreys-WilliamsF. Wall
    22页
    查看更多>>摘要：The paper presents a detailed petrographic and geochemical study of the pyroclastites outcropping at La Nava maar (Calatrava Volcanic Province-CVP). Here, extrusive carbonatites mix with melilite nephelinite lapilli and bombs, crustal and mantle debris. Nephelinite and carbonatite are not characterised by a HFSE distribution typical of immiscible pairs. IREE and La/Lu ratios reflect primitive mantle carbonates and kimberlites. Igneous carbonates have 613C and 6180 isotopic ratios in the range of extrusive carbonatites and differ from local sedimentary carbonates. Carbonatites have 87Sr/86Sr and 144Nd/143Nd isotopic ratios similar to Calatrava leucitites but different to melilite nephelinite. The difference indicates that melilite nephelinites and carbonatites are co-eruptive but not comagmatic. The mingled magmatic convoy, underwent decarbonation processes, releasing Ca and Mg that reacted with parental silicate melt. Consequently, the nephelinitic melt evolved towards a melilititic composition producing Ca-rich overgrowths on mafic phenocrysts and reacted to produce high Mg#, low Cr + Ni forsterite. Mass balance calculations indicats that the nephelinite magma assimilated up to 30% of the carbonatite magma. Mixing modelling based on the Sr and Nd isotopic composition of mantle xenoliths and carbonatites indicate that La Nava carbonatite may represent the mantle metasomatic agent.
      原文链接:
    • NSTL

    HT overprint of HP granulites in the Oisans-Pelvoux massif: Implications for the dynamics of the Variscan collision in the external western Alps

    Emilie JanotsStephane GuillotJean-Baptiste Jacob
    23页
    查看更多>>摘要：The Oisans-Pelvoux massif belongs to the Paleozoic basement of the External western Alps and records high temperature (HT) metamorphism associated with intense migmatization during the syn to post-collisional stages of the Variscan orogeny. Metamorphic assemblages related to the early collision stages have been obliterated making it difficult to constrain the earlier evolution of the unit. In this study, we report the finding of new bodies of high pressure (HP) mafic granulites, which preserve relics of the prograde evolution. Relics of Mn-rich garnet cores containing inclusions of epidote, titanite, chlorite and rare white micas constrain an early prograde stage at 460-550 °C and 0.4-1.0 GPa. The HP assemblage consists in garnet + clinopyroxene+ quartz + rutile ±plagioclase ±amphibole ±biotite and yield peak-P conditions at 650-730 °C and 1.5-1.7 GPa. Decompression was associated with heating to HT granulite-facies conditions of 800-870 °C and 0.6-0.9 GPa, which led to the development of granoblastic polycrystalline mosaics over the resorbing HP assemblage. The U-Pb dating of magmatic zircon cores constrains the emplacement age of the mafic protoliths at 479 ±5 Ma. Timing of the prograde to HP evolution is estimated around 345-330 Ma based on rutile U-Pb dating, which is contemporaneous with HP recorded in the other crystalline massifs of the western Alps. Zircon metamorphic rims yields U-Pb dates scattering between 337 and 294 Ma, which are interpreted to record crystallization during decompression and heating to granulite facies. Metamorphic conditions during the early prograde stage precludes scenarios involving subduction of a cold passive margin and are better reconciled with thickening of an orogenic wedge during collision. In contrast with other Variscan HP relics of the external Alps, the HP rocks of the Oisans-Pelvoux massif record heating to HT granulitic conditions during decompression, which may result from longer residence time in the lower crust before exhumation.
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    • NSTL

    Unmixing of REE-Nb enriched carbonatites after incremental fractionation of alkaline magmas in the Shaxiongdong complex, Central China

    Jian-Hui SuXin-Fu ZhaoXiao-Chun Li
    18页
    查看更多>>摘要：Carbonatites are often temporally and spatially associated with alkaline silicate rocks. However, petrogenetic models of formation of alkaline-carbonatite complex and the related rare metal element enrichment processes are sdll in a matter of debate. In this study, we present detailed petrographic observations and geochemical data for the Shaxiongdong complex, consisting of meta-pyroxenite, syenite, and carbonatite in the South Qinling Belt, Central China. Meta-pyroxenite crops out as a marginal face of the complex and has higher Mg, Fe, Ti, Ca and P contents relative to syenite, consistent with a cumulate of pyroxene, magnetite, ilmenite, titanite and apatite. Syenite is the main central phase of the complex and can cut the meta-pyroxenite as dykes or veins. Both meta-pyroxenite and syenite show various degrees of fenidzation, in which the residual syenite looks like breccia cemented by fenite. Carbonatite locally occurs as dykes of dozens of centimeters in width within syenite and meta-pyroxenite and hosts the most Nb and REE of the complex. Nb-rich minerals, e.g., pyrochlore, crystallized coeval with aegirine and apatite in carbonatite, implying a magmatic origin. In contrast, most REE minerals are spatially associated with barite and are interstitial to calcite, implying they were mainly crystallized from residual carbonatite magmas or magmatic-hydrothermal fluids. Carbonatite samples with REE mineralization have oxygen isotopes slightly higher than those of unmineralized and Nb mineralized samples, supporting the involvement of hydrothermal fluids in REE mineralization. U-Pb dating of titanite and zircon from meta-pyroxenite, syenite and the associated fenite yielded identical ages of ~440 Ma. Carbonatite, syenite, and meta-pyroxenite all have nearly identical Sr-Nd isotopes ((~(87)Sr/~(86)Sr)_i: 0.70298 to 0.70367; eNd(t):+3.6 to +4.6), suggesting that all of these rocks were derived from the same mantle source. Fractionation modeling suggests the syenites evolve from a parental alkali basalt magma by separation of cumulate pyroxenite in the complex and finally lead to the unmixing of Nb-REE enriched carbonatites. The subsequent fractionation of carbonatite melts led to Nb and REE mineralization in the Shaxiongdong carbonatite. Niobium minerals formed at early magmatic stage, while most REE minerals likely crystallized from the late magma residuals or precipitated from magmatic-hydrothermal fluids. This genetic model can be widely applicable to other alkaline-carbonatite complexes and provides a reasonable explanation for extreme enrichment of rare metals in such rocks.
      原文链接:
    • NSTL

    Pegmatites as geological expressions of spontaneous crustal flow localisation

    A. PlunderL. Le PourhietL. Rass
    13页
    查看更多>>摘要：Amongst the silicate-rich crystalline rocks that are produced in the continental crust, pegmatites are characterised by their large crystals which give them both an aesthetic and economic interest. Pegmatites crystallise either from fractionated magma derived from a parent granitic body or from the partial melting of meta-sediments or meta-igneous rocks (e.g. amphibolite). The mechanism of residual magma (or fluid) extraction from the parent granitic body has been thoroughly studied, but pegmatitic melt extraction after partial melting has received less attention. We present here a series of non-dimensional numerical experiments using a two-phase flow formulation that couples the Stokes problem to/with non-linear Darcy flow. This approach makes it possible to predict the movement of fluid inclusions (named porosity) in a deformable of a viscous rocks (named porous matrix). We find that the simulation produces either clusters or an isolated body of fluid inclusion depending on the compaction/decompaction ratio of the effectively viscous matrix in which they rise. Using a review of pegmatite natural properties, we propose a scaling of our numerical simulations that describes the ascent of a pegmatite-forming melt produced by partial melting. We then discuss possible travel distances and temperature effects. To discuss our results in light of field observations, we assume that the compaction-decompaction ratio is an accurate proxy for the influence of brittle processes at a scale smaller than die representative volume element, and therefore corresponds structural level variations at which pegmatites are emplaced. We find that our numerical simulation explain the statistical organisation, in terms of level of emplacement, of real fields of pegmatites possibly derived from partial melting of meta-sediments. Pegmatites in fact tend to organise as clusters around brittle faults in upper crustal levels, whereas they present a scattered distribution at mid to lower crustal levels. Our results therefore show that porosity waves are a possible mechanism for rapidly extracting and transporting pegmatite melts formed during low-degree (ca. 10%) partial melting at distances up to a few kilometres in the crust.
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    • NSTL

    The delamination of lower crust in continental back-arc basin: Evidence from Sr isotope and elemental compositions of plagioclase and clinopyroxene in andesites from Kueishantao, north of Taiwan, China

    Kun GuoXiaoyuan WangShuai Chen
    19页
    查看更多>>摘要：Subduction zones (arcs and back-arcs) are major sites for elemental cycling via slab dehydration and subsequent mande metasomatism and melting; many models have been suggested by researchers to explain these processes. However, the influence of the overriding lithosphere, especially the lower crust, on the generation of back-arc magmas during the early spreading stage remains largely unknown. Kueishantao (KST), located at the tectonic junction of the southernmost part of the Okinawa Trough (SPOT) and the Ryukyu arc, is an ideal setting to investigate these issues because of the obvious differences between the subducted slab and the overriding continental lithosphere. To better address the contribution of the overriding lithosphere to a nascent back-arc basin, we examined the in situ major and trace element compositions of plagioclase (PI) and clinopyroxene (Cpx) phenocrysts and the in situ ~(87)Sr/~(86)Sr of PI phenocrysts in andesites from KST. The Pl and Cpx in KST andesites show obvious evolution (An contents of Pl, 39-91; Mg~# of Cpx, 73-92). The valid thermobarometers of Pl and Cpx imply crystallization temperatures and depths of 1059-1117 °C and 1-14 km, respectively, similar to those of the SPOT. The compositions of Cpx suggest that the KST andesitic magma belonged to the subalkaline, tholeiitic and calc-alkaline magma series. The dEu values of Pl are 9.27-25.73, and those of Cpx are 0.50-0.92, indicating that the magma experienced Pl crystallization and separation. The ~(87)Sr/~(86)Sr values of PI range from 0.705630 to 0.706985 and remain constant with varying An contents (52-85), indicating an enriched magma source not resulting from assimilation of continental crust. Taking into consideration all geochemical, trace elemental, ~(87)Sr/~(86)Sr isotope and geophysical constraints, the principal magma source of KST andesites was probably the lower continental crust, which foundered into the convecting mantle and subsequently melted and interacted with asthenospheric mantle materials. The over-thickened crust, mantle corner flow and addition of subducted fluids in the KST area led to delamination. The structure and geochemistry of Pl and Cpx imply a simple two- or multilayer magma chamber system in the continental crust and a mixing magma process between a relatively silicic magma (H2O-saturated and high fO2) and a primitive mafic magma at shallow depths.
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    • NSTL

    Monazite record for the Paleoproterozoic Svecofennian orogeny, SE Finland: An over 150-Ma spread of monazite dates

    Paula E. SalminenPentti HolttaRaimo Lahtinen
    23页
    查看更多>>摘要：The Svecofennian orogeny in Fennoscandia is an example of a long-lived orogeny characterised by low pressure, high temperature (LP-HT) metamorphism. In this paper, we present new LA-ICP-MS age data on monazite from fifteen paragneiss samples and two granitoid samples from the southeastern part of the Svecofennian orogen, including the Sulkava granulite complex. The high-grade rocks in the study area were metamorphosed at ca. 750-800 °C and 5-6 kbar, followed by near-isothermal decompression down to 3-4 kbar. The analysed monazites yielded ~(207)Pb/~(206)Pb dates from 1953 to 1737 Ma. The youngest dates are typically yielded by rims and overgrowths of monazites and by younger domains in patchy monazite grains. Psammitic and pelitic layers from the same outcrop display differences in monazite date distributions, with younger dates being more common in pelitic layers. Both detrital and metamorphic origins are possible for the =1.91 Ga monazite grains found in some samples. For the younger (<1.91 Ga) monazite grains, we found three main age peaks. The peak at 1.89-1.86 Ga fits well with the high-grade metamorphic event at 1.89-1.88 Ga observed elsewhere in southern and central Finland. The 1.87-1.86 Ga interval dates a near-isothermal decompression stage. The age peak at 1.83-1.82 Ga records the age of younger high-grade metamorphism in the study area. The age peak at 1.80-1.77 Ga is interpreted to represent isotopic resetting due to fluid-induced alteration during a shield-wide exhumation stage, when waning magmatism, leucosome crystallisation and a change towards brittle-ductile deformation led to localised fluid flow before cratonisation. The dates between the age peaks could be explained in part by mixed isotopic ages in patchy grains without clear growth zones.
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    • NSTL

    Ta-Nb mineralization in the shallow-level highly-evolved P-poor Shihuiyao granite, Northeast China

    Zhenhua ZhouKarel BreiterSimon A. Wilde
    21页
    查看更多>>摘要：The Shihuiyao granite district in the southern Great Xing'an Range, Northeast China, is a newly discovered area of rare-metal-bearing granites (RMG). The latest Jurassic Shihuiyao granitic pluton, emplaced into the Lower Permian Linxi Formation, comprises porphyritic granite, leucogranite, and amazonite-bearing pegmatite, all enriched in rare metals (mainly Ta, Nb, Rb). The Shihuiyao granites are peraluminous (A/CNK = 1.02-1.27), characterized by high contents of SiO2 (69.4-78.0 wt%), Al2O3 (12.7-16.9 wt%) and total alkalis (6.8-12.8 wt %), enriched in Ta, Nb, Rb, Cs, and Sn, but depleted in Ba and Sr. They have strong negative Eu anomalies (Eu/Eu~* = 0.02-0.42) and show the M-type REE tetrad effect. Low Nb/Ta (<4) and Zr/Hf (<8) values indicate high degree of magmatic fractionation. Whole-rock and mineral compositions variations indicate that the Shihuiyao granites are a typical low pressure and P-poor RMG, which experienced intensive magmatic fractionation followed by intensive hydrothermal reworking. The narrow range of d~7Li values (-0.86±to 0.88± and Rayleigh distillation modeling suggest that the porphyritic granite and leucogranite represent products of the same magma source, but evolved independently at depth. LA-ICP-MS U-Pb dating of monazite from the porphyritic granite and leucogranite yield lower-intercept ages of 150.2 ±0.6 Ma and 147.0 ±.7 Ma, respectively. The ~(40)Ar-~(39)Ar plateau ages of mica from leucogranite (146.3 ±1.1 Ma) and an ore-bearing quartz vein (145.6 ±1.0 Ma), and the U-Pb age of columbite (146.7 ±4.7 Ma) further identify this as a latest Jurassic magmatic-hydrothermal mineralization event in NE China. Rapid crystallization and chemical quenching of magma resulted in neutralization of the acid fluoride solution, and a loss of the fluoride ligand during fluid evolution. This further triggered the subsequent crystallization of Nb-Ta oxides. As revealed from geochemical and textural characteristics of the columbite group minerals, the high-degree of fractional crystallization and Ta-rich hydrosilicate fluid metasomatism are the main factors leading to the enrichment of Ta-Nb within Shihuiyao RMG. In general, we conclude that rare metal-hosted magmatism took place in two episodes in NE China, namely, (i) Latest Jurassic to Early Cretaceous (150-130 Ma) peraluminous to metaluminous Ta-Nb-Sn-W RMG, and (ii) Early Cretaceous (130-120 Ma) alkaline Nb-Ta-Be-REE-Zr RMG, which highlight the prospecting fertility for Nb-Ta-REE (Sn-W) mineralization in NE China.
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    • NSTL

    Early Eocene high-Sr/Y magmas from the Urumieh-Dokhtar paleo-arc, Iran: Implications for the origin of high-flux events in magmatic arcs

    Mir Ali Asghar MokhtariHossein KouhestaniKwan-Nang Pang
    11页
    查看更多>>摘要：High-flux magmatic events, or flare-ups, are seen in many continental and transitional arcs but the underlying cause is not fully understood. Throughout Iran, voluminous calc-alkaline rocks emplaced between ～53 Ma and ～37 Ma are manifestations of a high-flux event. This high-flux event has been attributed to extension in the upper plate of one or more Neo-Tethyan subduction zones. Here, we present new findings in the Urumieh-Dokhtar paleo-arc, Iran where emplacement of Early Eocene granitic intrusions (ca. 55-52 Ma) immediately preceded the high-flux event. Geochemically, the studied rocks display high-K calc-alkaline compositions, continental crust-like trace element patterns, variable depletion of Y and heavy rare-earth elements, and Sr-Nd isotopes that are close to or slightly more enriched than the Bulk Silicate Earth. The Sr-Nd isotopic compositions indicate that the intrusions derived from upper plate materials. Relatively high Sr/Y and La/Yb of the studied rocks indicate that the residue of melt extraction, the crystalline assemblage that was left behind during magma differentiation, or both, to have existed in the deep arc crust. Following a ～20 Myr magmatic lull, the Early Eocene crustal thickening was most likely related to underthrusting of rear-arc crust towards the main arc, a process that not only promoted rapid root growth but also triggered the Eocene high-flux event. In the Iranian case example in particular, magmas formed during the high-flux event do not display elevated Sr/Y, implying that dense, locally thickened root might have foundered from the base of the arc lithosphere to the convecting mantle soon after their formation.
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    • NSTL

    Early Jurassic intra-oceanic subduction initiation along the Bangong Meso-Tethys: Geochemical and geochronological evidence from the Shiquanhe ophiolitic complex, western Tibet

    Xiao-Wen ZengMing WangHang Li
    15页
    查看更多>>摘要：Due to the unclear tectonic setting of the Jurassic ophiolites within the Bangong-Nujiang suture, how and when intra-oceanic subduction initiated within the Bangong Meso-Tethys remain controversial. Based on the new penological, geochemical, and geochronological data reported in this paper, ～190-180 Ma MORB-like ophiolites and OIB-like mafic rocks were identified from the Shiquanhe ophiolitic complex in the western Bangong-Nujiang suture. The MORB-like gabbros exhibit strong light rare earth elements (LREEs) and heavy rare earth elements (HFSEs) depletion and were derived from partial melting of a highly refractory depleted mantle source with limited slab-derived fluids. The OIB-like mafic rocks are enriched in LREEs and HFSEs and were originated from the melting of upwelling asthenospheric mantie without subduction additions. The data for the MORB-like ophiolites reported in this paper and previously published data demonstrate the formation of Early Jurassic fore-arc ophiolites in response to the Early Jurassic intra-oceanic subduction initiation within the Bangong-Nujiang suture. The Early Jurassic OIB-like rocks formed in the extensional tectonic setting, together with regional geological evidence, suggest that the Early Jurassic initial subduction event within the Bangong Meso-Tethys resulted in broad upper plate extension. Our study not only reveals the intra-oceanic subduction initiation age of the Bangong Meso-Tethys but also implies that subduction initiation can cause strong extension in a wider region of the upper plate in the Tethys oceanic system, not just in the fore-arc region.
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    • NSTL

    S-type like granites and felsic volcanic rocks in the Mahabad area, NW Iran: Late Neoproterozoic extensional tectonics follow collision on the northern boundary of Gondwana

    Somayeh GholipourFariborz MasoudiHossein Azizi
    16页
    查看更多>>摘要：The Late Neoproterozoic basement is exposed in the Mahabad area of northwest Iran, and it is classified into three main groups. The first is metamorphic rocks that include gneiss, schist, phyllite, and slate. The second is sedimentary rocks, which comprise shale, sandstone, and dolomite and the third is igneous rocks including granite, rhyolite, felsic tuff, and ignimbrite, which are the subject of this research. Zircon U-Pb ages for three igneous samples show that crystallization occurred at 558-552 Ma in the late Neoproterozoic (Ediacaran). The felsic magmatic rocks have SiO2 contents of 64.3 to 78.9 wt%, Al2O3 contents of 12.0 to 16.8 wt%, low TiO2 contents (0.08 to 0.69 wt%), and large variations in K2O (0.05 to 5.46 wt%) and Na20 (1.17 to 6.70 wt%). The felsic magmatic rocks are enriched in light rare earth elements (REEs) such as La and Sm, have flat heavy REE patterns, and strong negative Eu anomalies. They are mainly weakly to strongly peraluminous and calc-alkalic to alkali-calcic rocks. A wide range of initial ~(87)Sr/~(86)Sr_((556Ma)) ratios (0.7056 to 0.7203) and negative eNd(t) values (-6.0 to -2.9) do not show clear trends of mixing/contamination among mantle, slab fluids, and/or altered oceanic crust. The Mahabad magmatic rocks were produced mainly by partial melting of graywacke, psammite, and shale at high temperatures (～800 °C), based on the zircon saturation index. S-type like granites usually form in syn- to post-collisional systems, but the Mahabad felsic rocks, which have almost the same ages as the associated sedimentary rocks, were generated in an extensional regime according to geochemical discrimination diagrams. This can be explained as follows: sediments were deeply buried during the late Neoproterozoic amalgamation of microcontinents at a subduction zone along the northern boundary of Gondwana, and then erosion of the roof and/or thinning owing to extension led to upwelling of the asthenosphere and intrusion of mafic magma, which was responsible for partially melting the buried metasedimentary rocks to generate S-type like granites and felsic volcanic rocks.
      原文链接:
    • NSTL