查看更多>>摘要：Early Paleozoic volcanic-intrusive rocks in the South Qinling Belt, central China, consist of bimodal alkaline suites ranging in composition from basalt to trachyte, with an apparent scarcity of intermediate silicic components (the so-called "Daly gap"). A series of trachytic rocks have been identified in the Quanxi area of the South Qinling Belt. In this work, we combine mineralogical evidence with whole-rock major-and trace-element data, Sr-Nd-Pb isotope data, and the results of thermodynamic modeling to gain a better understanding of the petrogenesis and evolutionary process of the Quanxi trachytic rocks, as well as their compositional discontinuity with coeval mafic rocks. Our samples can be clearly classified into two lithological groups based on their phenocryst content: Group 1, phenocryst-poor trachytes (<5 vol% phenocrysts; SiO2 = 63.91-66.13 wt%) and Group 2, trachytic tufflavas (30-45 vol% pyroclasts; SiO2 = 53.34-59.03 wt%). All of the samples exhibit typical oceanic island basalt-like enrichment in light rare earth elements and high field strength elements, as well as marked depletion of Ba, Sr, and Ti. A cogenetic metasomatized lithosphere source for the Quanxi trachytic rocks and contemporaneous mafic rocks is indicated by their homogeneous isotopic features (ε_(Nd)(t) = +2.3 to +3.4; initial ~(206)Pb/~(204)Pb = 17.92-18.69; initial ~(207)Pb/~(204)Pb = 15.53-15.57; initial 208Pb/204Pb = 37.96-38.99) and linear geochemical variations, with protracted fractional crystallization regarded as the key mechanism for the compositional variations among these cogenetic rocks. The results of Rhyolite-MELTS modeling predict 72%-80% crystallization involving plagioclase, pyroxene, biotite, apatite, and Fe—Ti oxide, to yield the magmatic compositions from an evolved mafic progenitor to the Quanxi trachytes under conditions of low pressure (1.5-2 kbar), high H2O content (2-3 wt%), and high fO2 (FMQ-0.5). Some incompatible elements in trachytic tufflavas deviate markedly from the liquid lines of descent from coeval mafic rocks to the Quanxi trachytes: this deviation is attributed to a combination of the involvement of large proportions of alkali-feldspar pyroclasts and post-magmatic hydrothermal alteration. The presence of the "Daly gap" in early Paleozoic alkaline suites can be explained by an accelerated differentiation rate (i.e., dSiCVdr) in the SiO2-intermediate field, which is driven by the simultaneous separation of a considerable scale of SiO2-poor phases.
查看更多>>摘要：The upper mantle is highly heterogenous and represents a mixture of depleted peridotites and recycled materials. Isotopic studies (Sm—Nd, Lu—Hf and Re—Os) on abyssal peridotites and oceanic basalts have revealed the recycling of ancient depleted peridotites that record melt extraction as old as 2 Ga. However, the nature, proportions and melting dynamics of these ancient domains are poorly constrained. Here, we present integrated petrological and geochemical data, particularly high-precision clinopyroxene Sm—Nd and Lu—Hf isotopes, on a suite of magnesium-rich and ultra-depleted cumulates in the plume-related Pengco ophiolite from central Tibet. Petrological and geochemical compositions of Pengco cumulates indicate they were derived from a water-poor, magnesium-rich, and ultra-depleted primary melt similar to the low-Ti basalts from Manihiki oceanic plateau. Clinopyroxene in Pengco cumulates exhibit modestiy radiogenic Nd isotopes [eNd (t) = +6.0 to +7.0] but highly radiogenic Hf isotopes [eHf (t) = +20.2 to +23.3], which deviate from the mantle array by as much as +11.4 epsilon units. Thus, Pengco cumulates represent the most Hf—Nd isotopic decoupling ever documented for oceanic crustal rocks. Based on geochemical models, we argue that the ultra-depleted melt compositions and extremely decoupled Nd—Hf isotopes of Pengco cumulates can be explained by re-melting of a source mainly composed of ancient depleted mantle that were recycled into the upper mantle. This agrees on the idea that ancient depleted peridotites might represent one of the dominant components in Earth's upper mantle. Melts from an ancient depleted mantle contribute prominently to the oceanic basalt compositions, although they have limited leverage on the Nd—Hf isotopic compositions of the fully aggregated melts due to its low incompatible element abundances. The unique Nd—Hf isotopic compositions preserved in tlie Pengco cumulates demonstrate that the melts preserved in the lowermost oceanic crust exhibit greater isotopic variability than the erupted magmas. Our results are consistent with the idea that full melt aggregation occurred at crustal level. Hf—Nd isotopes of the lowermost oceanic crust thus provide a critical re-evaluation of current models of the heterogeneity and melting dynamics in the upper mantle.
查看更多>>摘要：Indian-type mantle is extensively distributed in the marginal basins of the western Pacific; however, die origin of diis mantle remains unclear. The Celebes Sea is located between the Pacific and Indian oceans, and its igneous crustal rocks provide crucial constraints on if and on how this anomalous mantle developed in the region. However, limited knowledge has been obtained on the origin of mantle source of the Celebes Sea because only normal-type mid-ocean ridge basalts (N-MORBs) at ODP Site U770 in this basin have been examined. In this study, bodi N-MORBs and enriched (E)-MORBs were identified at Site U770, which were analyzed for whole-rock major and trace elements, Sr-Nd-Pb-Hf isotopes, and in situ olivine geochemistry. Site U770 basalt samples are Indian-type (i.e., plot above the Northern Hemisphere reference line [NHRL] on plots of 208Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pb) MORBs, based on their Sr-Nd-Pb-Hf isotopic compositions. An enriched mantle end-member exists in the MORB-type depleted mantle source, as evidenced by correlations between eNd and SmN/YbN, Sr/Sr*, and Eu/Eu*. Recycled igneous oceanic crust that experienced plagioclase fractionation can account for die geochemically enriched end-member in the mantle source, based on the positive correlation of eNd vs. Sr/Sr* and Eu/Eu*. The recycled igneous oceanic crust plus minor amounts of pelagic sediments can also explain the origins of Indian-type mantle in the Celebes Sea. The E-MORBs have higher Zn/Mn and Zn/Fe ratios, and similar olivine Ni, Ca, and Mn contents as compared to global MORBs, which is also consistent witii a recycled oceanic crust-derived, eclogite-rich, mantle source. This suggests recycled components contribute to die lidiological and geochemical heterogeneity in the mantle beneath the Celebes Sea.
查看更多>>摘要：Mafic igneous rocks are volumetrically limited in collisional orogens but are significant for revealing specific geological processes. The mafic intrusive rocks in the eastern Lhasa batholiths along the Parlung fault (the easternmost branch of the Jiali fault zone), southern Tibet, are commonly regarded as the easternmost mafic components of an inferred Shiquanhe-Nam Tso-Jiali-Parlung suture zone, which is assumed to mark a back-arc oceanic basin related to the southward subduction of the Bangong-Nujiang Meso-Tethyan Ocean. However, the petrogenesis and tectonic setting of die mafic intrusive rocks have been poorly constrained. In this paper, we present penological, geochemical and geochronological data for this mafic suite and evaluate the tectonic implications. The mafic intrusive rocks can be approximately divided into pyroxene-amphibole cumulate and medium-grained and pegmatitic hornblende gabbros that are characterized by the occurrence of very calcic plagioclase (An = 80-92) and abundant amphibole crystals, indicating an H20-enriched magmatic signature. Their whole-rock compositions, which include negative Nb and Ta anomalies, also suggest a subduction-modified setting. Zircon Lu—Hf isotope data show that their magmas were formed by magmatic mixing between melts derived from enriched subcontinental lithospheric mantle and liquids derived from upwelling asthenosphere. Zircon U—Pb dating results reveal crystallization ages of —114-110 Ma for this mafic suite, corresponding to the late stage of regional granitic magmatism (—135-110 Ma) that was related to the Lhasa-Qiangtang collision after the closure of the Bangong-Nujiang Meso-Tethyan Ocean. Given that coeval mafic and felsic rocks are distinctly controlled by the Parlung fault, we suggest that asthenospheric upwelling due to the break-off of the subducted Bangong-Nujiang oceanic slab not only induced partial melting of the enriched subcontinental lithospheric mantle but also resulted in regional rifting/extension and the initial development of the eastern Jiali (Parlung) fault.
查看更多>>摘要：Constraining the growth and evolution of die continental crust provides insight into the secular change of tectono-magmatic processes dirough time. Radiogenic isotope pairs such as ~(176)Lu/~(176)Hf are commonly used to track the growth rates of the continental crust. Previous studies utilizing global compilations of Lu—Hf isotopes of zircon to query continental growth through time. However, we show that sample clustering and spatial bias lead to erroneous interpretations and casts doubt as to the representativeness of global zircon compilations. To I ameliorate this issue, we apply a geographic declustering algorithm to remove the spatial bias in the nonrep-resentative dataset. A more representative declustered dataset indicates that the continental crust grew much faster tiian previous assessments have proposed. We further speculate that the transition to modern-style tec-I tonics (as heralded by the appearance of low temperature-high pressure metamorphism) was accompanied by little to no net continental growth and propose tiiat continental growtii may be a feature of pre-plate tectonic magmatic processes.
Mohammad Reza GhorbaniHossein MahmoudiFatemeh Sepidbar
19页
查看更多>>摘要：The Sawlava ophiolite is situated on the Iran-Iraq border within the Walash-Kermanshah ophiolitic belt and represents remnants of the Cenozoic southern Neo-Tethys Ocean, which was located between the Arabian plate and the Sanandaj-Sirjan zone. It contains a complete ophiolitic sequence started from harzburgites with foliated/ discordant dunites, gabbroic bodies, a dyke complex, and pillow as well as massive basaltic lavas. The whole-rock and mineral chemistry of the Sawlava peridotites suggest that they are a mantle residuum after ca. 20-30% partial melting and melt extraction in the suprasubduction zone (SSZ) setting. The volcanic rocks generally range from island arc tholeiite to calc-alkaline affinities. Whole-rock chemistry indicates that volcanic units have compositions akin to depleted-and enriched-types mid-ocean ridge basalts (E-MORB). Layered-and isotropic gabbros show geochemical affinity similar to the normal mid-ocean ridge basalt (N-MORB) and ￡-MORB compositions, respectively. Their llight rare earth elements (LREE) observed in both N-MORB-like and E-MORB-like rocks, define two main basic geochemical types (LREE-enriched and LREE-depleted), suggesting several magmatic pulses from different mantle compositions that reveal mantle evolution and heterogeneity across the study area. Basaltic lavas weakly depleted in LREE show relatively flat REE patterns and (La/Yb)N values of 0.77 to 1.24, whereas LREE-enriched rocks have a (La/Yb)N mean values of 1.7, and lack an Eu anomaly. These subgroups of basaltic rocks show different extrusion ages-Paleocene-Eocene (59-50 Ma) and Oligocene (29 Ma), respectively. Petrogenetic modeling shows that mafic N-and E-MORB type rocks of Sawlava, which have (La/Yb)w = 0.97 and 1.7 and (Dy/Yb)^ = 1.13 and 1.1, respectively, formed due to partial melting of a mantle source less enriched in LREE than an garnet lherzolite ((La/Yb)w = 0.88 and (Dy/Yb)jv = 0.94) source or, alternatively, from very low degree (<2%) partial melting of a spinel-bearing DMM. The La/Yb-Dy/Yb sys-tematics of these rocks are interpreted to be compatible with ~2 to 10% and ~ 20% of partial melting of a garnet bearing spinel lherzolite source for the N-and E-MORB type rocks, respectively. The geochemical data and results reveal that magmatic-originated rocks from both Paleocene-Eocene and Oligocene lavas seem to be less heterogeneous compared with the mantle sources of the Walash-Kermanshah ophiolites, likely due to different slab-fluid components and variations of partial melting degrees. The studied volcanic rocks therefore possibly record chemical heterogeneity due to magmatic activity at an oceanic spreading center alongside slab rollback and back-arc extension at the Eurasian continental margin.
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