Neng Jiang

Ion microprobe zircon U–Pb age and geochemistry of the Myanmar jadeitite

Combined geochemistry and geochronology of the Myanmar jadeitite were determined. Bulk-rock trace element compositions display U-shaped REE patterns with pronounced positive Eu anomalies. The total REE abundances are very low, less than half chondritic, and the high field strength elements and some large ion lithophile elements are moderately enriched. These features indicate a metasomatic origin. There are three groups of zircons with different interior characteristics, cathodoluminescence, mineral inclusions, chemical compositions and sensitive high-resolution ion microprobe U–Pb ages. Group-I zircons, with a mean age of 163.2 ± 3.3 Ma, mostly have distinct oscillatory zoning, highest U and Th contents, and Na-free, Mg-rich mineral inclusions, and thus indicate an igneous (formation of oceanic crust) or hydrothermal (serpentinization and/or rodingitization) event in the Middle Jurassic. Group-II zircons, with a mean age of 146.5 ± 3.4 Ma, have bright luminescence without oscillatory zoning and include jadeite and jadeitic pyroxene inclusions, suggesting that formation of the Myanmar jadeitites, as well as subduction of the eastern Indian oceanic plate, occurred in the Late Jurassic. Group-III zircons have an age of 122.2 ± 4.8 Ma, which represents a later unknown thermal event. Discovery of the Middle Jurassic zircons provides geochronological constraint on the tectonic evolution of the eastern Indo-Burman Range.

Early Devonian alkaline intrusive complex from the northern North China craton: a petrological monitor of post-collisional tectonics

Abstract: Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating and geochemical data document the Early Devonian Sandaogou alkaline complex (409 Ma) from the northern margin of the North China craton. The rock suite includes pyroxene syenite, quartz syenite and monzonite. These rocks exhibit high contents of K2O (5–13 wt%), strong enrichments in large ion lithophile elements and light REE, slightly negative Eu anomalies, and pronounced depletions in high field strength elements. They are characterized by moderate 87Sr/86Sri ratios of 0.7052–0.7071, low εNd(t) values of −12.7 to −17.9 and zircon εHf(t) values from −27.8 to −32.3. These geochemical features and quantitative isotopic modelling suggest that they might have been formed through simultaneous fractional crystallization and lower crustal assimilation of a metasomatized mantle-derived alkali basaltic magma. These trachytic rocks, together with the Middle Devonian alkaline rocks and mafic–ultramafic complex from neighbouring regions, constitute a linear post-collisional magmatic belt along the northern North China craton, possibly formed under a tectonic regime of slab breakoff. They serve not only as a magmatic milestone for marking the termination of Early Palaeozoic orogenic cycle around the northern North China craton, but also as a spatial tracer for locating the position of the potential ancient slab breakoff at the surface. Supplementary material: Analytical methods, mineral composition data, field and zircon cathodoluminescence (CL) photographs are available at http://www.geolsoc.org.uk/SUP18403.

Petrology and geochemistry of the Shuiquangou syenitic complex, northern margin of the North China Craton

The Palaeozoic Shuiquangou complex is a major syenitic intrusion along the northern margin of the North China Craton that hosts a number of gold deposits. Despite the heterogeneity of the complex in terms of texture and modal mineralogy, mineral compositions, whole-rock chemistry as well as radiogenic isotope compositions show a well-constrained magmatic evolutionary trend. The whole-rock major element and REE trends are broadly consistent with fractional crystallization dominated by clinopyroxene, amphibole and accessory minerals. Initial Nd–Sr isotopic compositions suggest that the parent magmas to the complex were ultimately derived from an enriched mantle with relatively high initial 87Sr/86Sr ratios and negative ϵNd values. The enrichment of the mantle source was probably inherited from an older lithospheric mantle beneath the North China Craton. Sr isotopic data indicate that the Rb–Sr system was disturbed by later alteration processes, whereas Nd isotopes are essentially unaffected and exhibit a broad trend consistent with simultaneous fractional crystallization and country-rock assimilation (AFC). Quantitative modelling indicates that a very high bulk distribution coefficient for Nd and low ratios of assimilation to crystallization are needed to produce the observed Nd isotope evolutionary trend.

Mobilization and enrichment of high-field strength elements during late- and post-magmatic processes in the Shuiquangou syenitic complex, Northern China

Abstract The Shuiquangou syenitic complex consists of melasyenite and leucosyenite. In the northeastern part, there is a small area where the leucosyenite was affected by alteration. The altered leucosyenite contains much higher concentrations of high-field strength elements (HFSE), especially Nb, Zr, REE and Y compared to fresh leucosyenite. REE patterns of the altered leucosyenite show moderate negative Eu anomaly, in contrast to the slightly positive to non-anomalous Eu signature of the fresh syenitic rocks. Allanite from the hydrothermal mineral assemblage in the altered leucosyenite also contains much higher concentrations of REE and Ti than the euhedral magmatic allanites in unaltered leucosyenite. The enrichment of HFSE in altered leucosyenite was interpreted to be caused by the hydrothermal fluids based on mineral chemistry and whole-rock data. Within the fresh leucosyenite, allanite occurs in two habits, euhedral ones indicating a primary magmatic origin and anhedral ones suggesting a late-magmatic origin. The latter has higher concentrations of REE and Ti than the former. The anhedral allanite was presumed to be formed coeval with the alteration of the euhedral allanite as a result of residual magmatic fluid derived directly from the syenitic magma. The alteration of magmatic euhedral allanite and the enrichment of REE and Ti in the anhedral late-magmatic allanite suggest that these elements were mobilized by the residual magmatic fluid. Both residual late-magmatic and hydrothermal fluids have an inferred composition characterized by high alkalic and F contents, suggesting that F may be the most ligand that complexed the high-valence cations.

Ba minerals in clinopyroxene rocks from the Myanmar jadeitite area: implications for Ba recycling in subduction zones

Barium (Ba) minerals identified in clinopyroxene rocks from the Myanmar jadeitite area include celsian, hyalophane, and hydrated barium aluminum silicate (an inferred barian zeolite). The hyalophane and hydrated barium aluminum silicate occur as interstitial phases and sometimes crosscut jadeite crystals, indicating they were formed at a later stage than jadeitite. The celsian has two modes of occurrence: (i) in association with jadeite in jadeitite; (ii) as a single-phase mineral forming part of multi-phase pseudomorphs in chromian omphacite rock (omphacitite). The latter rock type was formed predominantly during the same episode as the jadeitite under almost the same P-T conditions. Multi-phase pseudomorphs, mostly showing hexagonal form, contain celsian and kaolinite, with or without quartz, graphite and diaspore. The P-T condition constraints on celsian suggest that a precursor phase, probably cymrite, once existed under high-pressure conditions, and that celsian was formed by decomposition of the phase under decreasing pressure during uplift of the jadeitite. The frequent occurrence of Ba silicates in jadeitites worldwide reflects a Ba-enriched environment for the formation of jadeitite, which is inferred to be related to subducted barite-bearing slab sediments. Therefore, the jadeitite records metasomatism and metamorphism, as well as fluid interactions and phase changes in the BaO-Al 2 O 3 -SiO 2 -H 2 O system. In this way, jadeitite provides information on Ba phase transformations and Ba recycling within the subduction zone. Taken together with previous results, this study further suggests that jadeite-forming fluids are derived from the dehydration of the altered oceanic slab containing deep-sea sediments.

Geochemistry of the Ore-Forming Fluids in Gold Deposits from the Taihang Mountains, Northern China

Gold deposits in the Taihang Mountains, northern China, mainly consist of quartz sulfide veins in granitoid plutons. This paper describes the geological setting of the gold deposits, and presents the results of microthermometric, Fourier transform infrared spectra, and stable isotope analyses of ore—forming fluids for the purpose of examining the characteristics of these fluids. The ore—forming fluid was of high temperature (up to 380°C) and high salinity (33–41 wt% NaCl equiv.), represented by type I inclusions (with daughter minerals). This fluid evolved to low salinity at low temperatures recorded in type II (liquid-rich) and III inclusions (vapor—rich). Primary type II inclusions coexist with type III inclusions in quartz. Type III inclusions have almost the same homogenization temperatures as type II inclusions. This probably reflects boiling. The secondary fluid inclusions homogenized at lower temperatures, and have lower salinities than primary inclusions. Based on microthermometric data, we propose that the high—temperature fluid that separated from residual magma corresponded to the ore—forming fluid represented by type I inclusions. This fluid mixed with meteoric water in the upper part of the granitic pluton and was diluted. The diluted fluid boiled, probably due to abrupt pressure decrease, and formed liquid—rich type II inclusions and vapor—rich type III inclusions. The deposition of sulfide minerals and gold probably occurred during boiling.

Contrasting Middle Jurassic and Early Cretaceous mafic intrusive rocks from western Liaoning, North China craton: petrogenesis and tectonic implications

Zircon U–Pb dating, whole-rock major oxide, trace element and Sr–Nd isotopic data are presented for the Late Mesozoic mafic intrusive rocks from Yiwulushan of western Liaoning along the eastern segment of the Yanshan belt, North China craton, with two episodes of magmatism documented. Middle Jurassic hornblende-rich gabbros show enrichment of large ion lithophile elements and light REE, and prominent depletion in high field strength elements, and possess moderately enriched isotopic compositions with ( 87 Sr/ 86 Sr) i ranging from 0.7056 to 0.7065 and ɛ Nd (t) from −5.0 to −7.1. These features suggest that the gabbros were derived from an amphibole-bearing harzburgitic lithospheric mantle source metasomatized recently by slab-derived fluids. By contrast, Early Cretaceous mafic dykes are gabbroic dioritic to dioritic in composition, with comparable trace element characteristics to continental crust and depleted isotopic signatures (( 87 Sr/ 86 Sr) i = 0.7048–0.7055, ɛ Nd (t) = 0 to −3.0). They probably originated from partial melting of a relatively fertile asthenospheric mantle in the spinel stability field, with subsequent lower crustal assimilation and fractional crystallization. These two contrasting mafic intrusive suites, together with multiple Mesozoic mafic volcanic rocks from western Liaoning, documented a localized lithospheric thinning process, mainly through prolonged hydro-weakening or melt–rock interaction and triggered by gravitational collapse, possibly within an evolved post-collisional to within-plate extensional regime.

Links between Mesozoic intermediate–felsic igneous rocks and lower crustal granulite xenoliths, North China Craton: O and Pb isotopic evidence

We present and compare whole-rock and zircon O and Pb isotopic compositions for the Hannuoba granulite xenoliths and Mesozoic intermediate-to-felsic igneous rocks from the Zhangjiakou region, northern margin of the North China Craton, northeast China. The xenoliths have an overall Pb isotopic range similar to rocks from the regionally exposed Neoarchaean granulite terrain. Mesozoic zircons from different types of granulite xenoliths have a narrow range of δ18O values (6.0–7.7‰) higher than normal mantle δ18O values (∼5.7‰). Mesozoic intermediate–felsic igneous rocks have O and Pb isotopic compositions indistinguishable from the Hannuoba intermediate–mafic granulite xenoliths. Our new data suggest that the Mesozoic igneous rocks and granulite xenoliths are genetically linked and that both were derived from the late Neoarchaean lower crust. This argues against previous proposals that the granulite xenoliths are either products of Mesozoic basaltic underplating or formed by mixing between mantle-derived and pre-existing crustal magmas.

Geochemistry and Mineralogy of Two Contrasting Cretaceous Lavas: Implications for Lithospheric Mantle Evolution beneath the Northeastern North China Craton

A combined geochemical and mineralogical study has been conducted on a volcanic sequence at Jianguo, northeastern North China craton (NCC). This sequence contains two contrasting rock types, the lower andesitic lava and the upper basaltic lava. The lower lava is comprised of Cr-spinel (chromite mostly)-bearing andesites, whereas the upper lava consists of titanomagnetite (up to 59-65 mol% Usp)-bearing basalts. 40Ar/39Ar dating shows that the Jianguo andesites and basalts are 116.8 ± 3.0 Ma and 105.5 ± 0.5 Ma, respectively. The basalts have initial Sr-Nd isotopic ratios characteristic of mid-ocean ridge basalt (MORB) and have been interpreted as derived from MORB-type asthenosphere. The andesites, with evolved Sr-Nd isotope features and negative Ta and Nb anomalies, are interpreted to have been ultimately derived from MORB-type asthenosphere accompanied by combined assimilation and fractional crystallization. This suggestion is applicable to the genesis of two other coeval andesites nearby. These geochemical contrasts imply that the lithospheric mantle beneath the eastern part of NCC was considerably thinned by ca. 117 Ma.