Abstract The geodynamic setting of the Xigaze ophiolite has long been debated. Structural and geochemical evidence suggest the Xigaze ophiolite was formed at a slow‐spreading ridge (Nicolas et al., 1981; Liu et al., 2016). Based on incompatible element concentrations, the Xigaze ophiolite volcanics are consistent with the ubiquitous subduction signature in suprasubduction zone (Bedard et al., 2009; Hebert et al., 2012; Dai et al., 2013). It is noteworthy that the Xigaze ophiolite is different from the Geotimes and Lasail and Velly units from Oman ophiolite, respectively. The mafic rocks of the Xigaze ophiolite generally resemble typical N‐MORB and Geotimes volcanics in composition except for slight depletions of Th and Nb (Fig.1a). Although the Xigaze rocks have similar Th and Nb concentrations to Lasail and Velly rocks, most incompatible elements in the Xigaze rocks are comparable to N‐MORB. Petrography in gabbro of Xigaze ophiolite shows that euhedral plagioclases are enclosed by clinopyroxenes suggesting that these minerals have crystallized from an anhydrous magma (Sisson and Grove, 1993). Although the Xigaze volcanic rocks are slightly depleted in Th and Nb, they have MORB‐like trace element characteristics implying that they are derived from an anhydrous MORB magma at spreading centre. Godard et al. (2006) suggested that the mantle source of the Oman ophiolite have element and isotopic characteristics similar to Indian Ocean MORB, where the mantle preserved some older slab materials. A negative Nb anomaly of Oman Geotimes volcanic rocks may be resulted from contamination of the slab materials via decompression melting of the convecting mantle. Moreover, the Xigaze rocks have 1.27–3.18 of (Th/Nb) N ratios similar with those of Geotimes volcanics ((Th/Nb) N =0.51–2.77) and lower than those of Lasail and Velly units ((Th/Nb) N =2.12–6.35). These features suggest that the Xigaze ophiolite may have formed at the spreading centre.
The Zhibo iron deposit is hosted in Carboniferous submarine volcanic rocks in Western Tianshan, NW China. A series of magnetite‐bearing intermediate‐mafic volcanic rocks are recognized in the periphery of the Zhibo ore district. Most of these volcanic rocks formed at 314 ± 2 Ma, possess tholeiitic–calc‐alkaline affinities, and display remarkable negative Nb, Ta, and Ti anomalies on primitive mantle‐normalized incompatible element diagrams. These features, together with those of their relatively complete rock assemblages and Th/Yb versus Nb/Yb diagrams, are indicative of their formation in an active continental margin arc setting. The wide compositional spectrum of SiO 2 values ranging from 47.11 to 62.75 wt.% and lower Mg # values (55–63) of basalts suggest that the Zhibo intermediate‐mafic volcanic rocks may have experienced magmatic differentiation. Their (Th/Ta) PM > 1, (La/Nb) PM > 1, Nb/Ta (11‐16), and Th/Ce (0.06‐0.23) values suggest that the source of these intermediate‐mafic volcanic rocks was significantly contaminated by crustal materials. The magnetites in the iron ore have lower contents of Al, Mn, Ti, and V, indicating that the mineralization of magnetite in the iron ore occurred under lower temperature and higher oxygen fugacity conditions than those in the intermediate‐mafic volcanic rocks. In addition, the magnetites in the Zhibo iron ores have lower contents of compatible elements (e.g., Ti, V, Mn, Co, Cr, and Zn) than those of the magnetite in the intermediate‐mafic volcanic rocks, suggesting that the Zhibo magnetites crystallized from late‐stage, residual iron‐rich magmatic melts/magmatic‐hydrothermal fluids. In addition, the textures of the volcanic rocks suggest that iron have ever enriched in the residual melt during the magmatic stage, and the iron‐rich fragments in andesitic volcaniclastic rocks indicate that the ore‐forming material was a high‐salinity fluid‐bearing iron‐rich melt. In combination of available information, including field observations and geochemical analyses, we interpret that the Zhibo iron deposit is magmatic‐hydrothermal in origin.
The origin of the Bayan Obo ore deposit, the largest REE deposit in the world, has long been debated and various hypotheses have been proposed. Among them is that the Bayan Obo ore deposit is correlated with and has the same origin as the Sailinhudong micrite mound in the southern limb of the Bayan Obo synclinorium. To test this model, the Bayan Obo ore deposit and the Sailinhudong micrite mound are systematically compared for their geological features, elemental geochemistry, and C, O, and Mg isotopic geochemistry. We show that the Bayan Obo ore deposit and the Sailinhudong micrite mound are both calcareous, lens-like in shape, lack bedding features, and are both hosted in a sedimentary formation that consists of clastic sediments and carbonates, unconformably overlying the Archaean–Palaeoproterozoic crystalline basement. However, their geochemical characteristics differ markedly. Compared with the Sailinhudong micrite carbonates, the Bayan Obo ore-hosting dolomite marbles are strongly enriched in LREEs, Ba, Th, Nb, Pb, and Sr, and have very different (PAAS)-normalized REE patterns. Sailinhudong micrite carbonates have higher δ13CPDB and δ18OSMOW values, falling into the typical sedimentary field, but the Bayan Obo ore-hosting dolomites are isotopically intermediate between primary igneous carbonatite and typical sedimentary limestone. The δ26 Mg values of the Sailinhudong micrite carbonates are lighter than those of normal Mesoproterozoic sedimentary dolostone, while those of the Bayan Obo ore-hosting dolomite marble are isotopically heavier, similar to δ26 Mg of mantle xenoliths and Bayan Obo intrusive carbonatite. We conclude that the Bayan Obo ore deposit is not correlated with the Sailinhudong micrite mound; it is neither a micrite mound nor an altered micrite mound.
Carbonatite, an unusual carbonate-rich igneous rock, is known to be sourced from the mantle which provides insights into mantle-to-crust carbon transfer.To constrain further the Ca isotopic composition of carbonatites, investigate the behaviour of Ca isotopes during their evolution, and constrain whether recycled carbonates are involved in their source regions, we report δ 44/42 Ca for 47 worldwide carbonatite and associated silicate rocks using a refined analytical protocol.Our results show that primary carbonatite and associated silicate rocks are rather homogeneous in Ca isotope compositions that are comparable to δ 44/42 Ca values of basalts, while nonprimary carbonatites show detectable δ 44/42 Ca variations that are correlated to δ 13 C values.Our finding suggests that Ca isotopes fractionate during late stages of carbonatite evolution, making it a useful tool in the study of carbonatite evolution.The finding also implies that carbonatite is sourced from a mantle source without requiring the involvement of recycled carbonates.
A combination of major and trace element, whole-rock Sr, Nd and Hf isotope, and zircon U–Pb isotopic data are reported for a suite of dolerite dikes from the Liaodong Peninsula in the northeastern North China Craton. The study aimed to investigate the source, petrogenesis and tectonic setting of the dikes. Sensitive high-resolution ion microprobe U–Pb zircon analyses yield a Late Triassic emplacement age of ∼213 Ma for these dikes, post-dating the collision between the North China and Yangtze cratons and consequent ultrahigh-pressure metamorphism. Three geochemical groups of dikes have been identified in the Liaodong Peninsula based on their geochemical and Sr–Nd–Hf isotope characteristics. Group 1 dikes are tholeiitic, with high TiO2 and total Fe2O3 and low MgO contents, absent to weak negative Nb and Ta anomalies, variable (87Sr/86Sr)i (0·7060–0·7153), εNd(t) (− 0·8 to −6·5) and εHf(t) (−2·7 to −7·8) values, and negative ΔεHf(t) (−1·1 to −7·8). They are inferred to be derived from partial melting of a relatively fertile asthenospheric mantle in the spinel stability field, with some upper crustal assimilation and fractional crystallization. Group 2 dikes have geochemical features of high-Mg andesites with (87Sr/86Sr)i values of 0·7063–0·7072, and negative εNd(t) (−3·0 to −9·5) and εHf(t) (−3·2 to −10·1) values, and may have originated as melts of foundered lower crust, with subsequent interaction with mantle peridotite. Group 3 dikes are shoshonitic in composition with relatively low (87Sr/86Sr)i values (0·7061–0·7063), and negative εNd(t) (−13·2 to −13·4) and εHf(t) (−11·0 to −11·5) values, and were derived by partial melting of an ancient, re-enriched, refractory lithospheric mantle in the garnet stability field. The geochemical and geochronological data presented here indicate that Late Triassic magmatism occurred in an extensional setting, most probably related to post-orogenic lithospheric delamination.
Sm–Nd isotopic measurements were undertaken to constrain the chronology of REE mineralization events (REE = rare-earth elements) at the Bayan Obo REE–Nb–Fe deposit in northern China. The earliest REE mineralization event was dated precisely using samples of coarse-grained dolomite from the Bayan Obo Orebodies and the carbonatite dikes in their vicinity, which yielded a Sm–Nd isochron age of 1286 ± 27 Ma. The Sm–Nd data indicate that during this early event, the rare-earth elements were sourced from the mantle. A significant thermal event at ca. 0.4 Ga resulted in the formation of late-stage veins with coarse crystals of REE minerals. REE mineralization developed during this event resulted from REE remobilization within the ore-bodies with minimal contribution from external sources. A series of ages between 1.3 and 0.4 Ga reported for Bayan Obo ores in some previous studies resulted from thermal disturbance and do not imply the existence of multiple events.
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