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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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    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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    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.
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    Compositional signatures of ophiolitic rocks from the Dongco massif: Novel insights into the evolution of the central Tibetan Meso-Tethyan oceanic plateau

    Peng YangQiang-tai HuangKai-Jun Zhang
    18页
    查看更多>>摘要：The role of the Meso-Tethyan oceanic plateau in the growth of the Tibetan Plateau is debated. In this paper, we report coexistence of large-scale mid-ocean ridge basalt (MORB)-like and enormous ocean island basalt (OIB)-type mafic rocks from the Dongco ophiolitic rocks of the Bangong-Nujiang Suture Zone. The MORB-like rocks host tholeiitic geochemical characterizes, and have supra-subduction zone affinity such as enrichments in large ion lithophile elements (LILEs), depletions in high field strength elements, high e_(Nd)(t) values (+6.5 to +9.5) and low initial ~(87)Sr/~(86)Sr ratios (0.7033-0.7050). The OIB-type rocks have an alkaline affinity and are characterized by enrichments in LILEs and light rare earth elements, strongly negative Sr anomalies, variable e_(Nd)(t) values of -3.7 to +7.3, and low initial ~(87)Sr/~(86)Sr ratios ranging from 0.7034 to 0.7050. Metamorphic zircons from amphibolites in the sole of the ophiolitic rocks yielded U-Pb ages of 104-100 Ma. Detrital zircons from the upper Cretaceous sandstone of the Abushan Formation (K2ab) yielded young U-Pb ages of 100-86 Ma. Considering the regional geology, we suggest that the Dongco MORB-like rocks were derived from a depleted mantle source and formed from an intra-oceanic subduction-related setting, representing remnants of normal oceanic crust, whereas the OIB-type rocks were derived from an enriched mantle source representing remnants of a Meso-Tethyan oceanic plateau. Our preferred hypothesis is that the Meso-Tethyan oceanic plateau experienced a phase of accretion at 104-100 Ma, which probably increased local subduction erosion of the upper plate and providing provenance for the deposition of the K2ab sandstone. The maximum depositional ages of K2ab sandstone constraint the minimum ages for closure of the Bangong-Nujiang Tethys Ocean most likely at 100-86 Ma.
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    Zircon mineral chemistry and implications for magmatic-hydrothermal evolution of the granite-hosted Zaaiplaats Sn deposit, Bushveld Large Igneous Province, South Africa

    Teimoor Nazari-DehkordiLaurence Robb
    16页
    查看更多>>摘要：This study presents trace element compositions for zircons from a series of co-genetic granitic rocks of the Province (LIP). In this area, the LGS comprises the coarse-grained Nebo and Bobbejaankop granites, and the finegrained Lease granite, of which the two latter are variably altered and host Sn mineralisation. Under cathodoluminescence, zircons of the Nebo granite display long prismatic crystals and well-developed internal oscillatory zoning, whereas those of the Bobbejaankop and Lease granites are porous with faint zoning and vermicular to spongy internal textures. Trace element contents of zircons in the Bobbejaankop and Lease granites show significant enrichments in all the trace elements (e.g., Hf, Nb, Ta, U, Th, Ti, and REE) suggesting a hydrothermal origin for the zircons of the Bobbejaankop and Lease granites, that are different to those for the magmatic zircons of the Nebo granite. Moreover, the inclusion-free hydrothermal zircons of the Bobbejaankop and Lease granites point to lower Ti-in-zircon derived crystallisation temperatures (450-600 °C) and oxygen fugacities [as log (fO2) values in the range - 20 to -26] than those of the Nebo granite [where T = 570-625 °C and log (fO2) values = -19 to -21]. Collectively, the data record an apparent magmatic differentiation from a single magma, showing derivation of the hydrothermal zircons of the Bobbejaankop and Lease granites from a highly-fractionated late-magmatic-hydrothermal fluid enriched in F, CI and B, as indicated by non-chondritic Y/ Ho and Zr/Hf ratios under reducing conditions, as required typically for the formation of the granite-hosted Sn deposits.
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    • NSTL