Mantle metasomatism by P- and F-rich melt/fluids: evidence from phosphate glass in spinel lherzolite xenolith in Keluo, Heilongjiang Province

Spinel lherzolite xenoliths were found in phonolitic alkaline basalt in the Keluo area, Heilongjiang Province. Detailed electron-microprobe study revealed abundant phosphates and associated metasomatic minerals between primary phases in xenolith. The phosphates are considered special residual phases (coagulation) of P- and F-rich mantle melt/fluid, most of which were identified as glass phases based on Raman spectroscopic analyses. Such melt/fluid also further metasomatized primary minerals, thus leading to formation of reaction rims successively composed of Cr-spinel symplectitic zone and olivine diopside zone. Therefore, the P- and F-rich melt/fluid played an important role in the upper-mantle metasomatism in the Keluo area. It is suggested that this kind of metasomstism may occur in some other places of eastern China. The present results may also have significance in study-ing types of metasomatic melt/fluid and its evolution in the lithospheric mantle beneath eastern China.     

Glass melt inclusion in clinopyroxene from Linqu Cenozoic basalt, Shandong Province, China

Melt inclusions or glasses usually occur in spinel- facies peridotitic xenoliths entrained in the global ba- salts[1―4], and olivine phenocrysts and/or xenocrysts carried in the intermediate-mafic volcanic rocks (such as basalts, basaltic andesites and h…     

Mantle olivine xenocrysts entrained in Mesozoic basalts from the North China craton：Implication for replacement process of lithospheric mantle

Mesozoic (125 Ma) Fangcheng basalts fromShandong Province contain clearly zoned olivines that arerare in terrestrial samples and provide first evidence for thereplacement of lithospheric mantle from high-Mg peridotitesto Iow-Mg peridotites through peridotite-melt reaction.Zoned olivines have compositions in the core (Mg# = 87.2--90.7) similar to those olivines from the mantle peridotiticxenoliths entrained in Cenozoic basalts from the NorthChina craton and in the rim (Mg# = 76.8--83.9) close to oli-vine phenocrysts of the host basalts (75.7--79.0). Thesecompositional features as well as rounded crystal shapes andsmaller grain sizes (300—800 μm) demonstrate that thesezoned olivines are mantle xenocrysts, i.e. disaggregates ofmantle peridotites. Their core compositions can representthose of olivines of mantle peridotites. The zoned texture ofolivines was formed through rapid reaction between the oli-vine xenocryst and the host basalt. This olivine-basaltic meltreaction could have been ubiquitous in the Mesozoic litho-spheric mantle beneath the North China craton, i.e. an im-portant type of the replacement of lithospheric mantle. Thereaction resulted in the transformation of the Paleozoic re-fractory (high-Mg) peridotites to the late Mesozoic fertile(Iow-Mg) and radiogenic isotope-enriched peridotites, lead-ing to the loss of old lithospheric mantle.     

Carboniferous adakites and Nb-enriched arc basaltic rocks association in the Alataw Mountains, north Xinjiang： interactions between slab melt and mantle peridotite and implications for crustal growth

Adakites and Nb-enriched arc basaltic rocks (NEABs) in arc setting, which are closely correlated in petrogenesis, have recently been widely followed with interest[1—9]. In general, adakite is derived from partial melting of subducting oceanic crust[1]. When adakitic magma (slab melt) passes through the mantle wedge, the interactions between slab melt and mantle peridotite will occur: slab melt is contaminated by peridotite, meanwhile peridotite is metasomated by slab melt. The NEABs are d…     

Mantle xenoliths from Late Cretaceous basalt in eastern Shandong Province： New constraint on the timing of lithospheric thinning in eastern China

Studies of mantle xenoliths hosted in both the Cenozoic alkali basalt and the Early Paleozoic kimberlite suggest that part of the subcontinental lithosphere as thick as more than 100 km has been lost from the Early Paleozoic to Cenozoic[1—8]. Neither the scale and mechanism nor the accurate timing of the lithospheric thinning has been precisely constrained[7-12]. One of the reasons for this is that there are only a few Mesozoic basalts cropped out, especially, few containing mantle-derived …     

Geochemical characteristics and petrogenesis of Mesozoic basalts from the North China Craton： A case study in Fuxin, Liaoning Province

Occurrence of Cretaceous basalts in Fuxin County, Liaoning Province provides us an opportunity to understand Mesozoic mantle processes beneath the northern margin of the North China Craton (NNCC). Fuxin Jianguo basalts occur as volcanic channel phases with well-developed columnar jointings and contain few spinel lherzolite and pyroxenite xenoliths. They are poor in silica and rich in alkalis, Ti and Al, belonging to alkaline basalts. In trace element compositions, Jianguo basalts are moderately enriched in LREE and LILE, but not depleted in HFSE. They have low Sr and high Nd and Pb isotopic ratios. These geochemical characteristics suggest that Jianguo basalts originated from the depleted asthenosphere, representing an undifferentiated and uncontaminated primitive magma. Presence of these basalts indicates that the lithosphere beneath the region had thickness less than 65 km at the time of basalt eruption and was mainly composed of fertile pargasite-bearing spinel Iherzolite and plagioclase pyroxenite. The voluminous basaltic-andesitic magmatism during the early Jurassic-late Cretaceous time indicates that the commencement and accomplishment of lithosphere thinning in the NNCC was much earlier than that in the southern margin, since the mafic-intermediate volcanism only occurred at the Cretaceous time in the southern margin and the basalts with an asthenosphere isotopic signature at the Tertiary. This shows that highly spatial and temporal heterogeneity existed in the Mesozoic lithosphere evolution.     

An in situ zircon Hf isotopic,U-Pb age and trace element study of banded granulite xenolith from Hannuoba basalt： Tracking the early evolution of the lower crust in the North China craton

Backscattered electron images, in situ Hf isotopes, U-Pb ages and trace elements of zircons in a banded granulite xenolith from Hannuoba basalt have been studied. The results show that the banded granulite is a sample derived from the early lower crust of the North China craton. It is difficult to explain the petrogenesis of the xenolith with a single process. Abundant information on several processes, however, is contained in the granulite. These processes in-clude the addition of mantle material, crustal remelting, metamorphic differentiation and the delamination of early lower crust. About 80% of zircons studied yield ages of 1842 ±40 Ma, except few ages of 3097-2824 Ma and 2489-2447 Ma. The zircons with ages older than 2447 Ma have high εHf (up to +18.3) and high Hf model age (2.5-2.6 Ga), indicating that the primitive materials of the granulite were derived mainly from a depleted mantle source in late Archean. Most εhf of the zircons with early Proterozoic U-Pb age vary around zero, but two have     

The oldest known dinoflagellates： Morphological and molecular evidence from Mesoproterozoic rocks at Yongji, Shanxi Province

Abundant and well-preserved organic-walled microfossils including acanthomorphic acritarchs have been found in Mesoproterozoic Beidajian Formation in the Yongji area of Shanxi Province, North China. The morphological and ultrastructural features of these acanthomorphic acritarchs resemble living dinofiagellates (e.g. double-walled and polygonal structures), which leads to the interpretation of these fossils as probably the oldest dinofiagellates. The detection of dinosterane, a dinofiagellate biomarker, from pyrolytic product of these fossils further supports the morphological inference. This finding is consistent with molecular clock estimate that dinofiagellates may have diverged 700 to 900 million years (Ma) before previously known fossil record.     

The age of the Gaojiacun mafic-ultramafic intrusive complex in the Yanbian area,Sichuan Province： Geochronological constraints by U-Pb dating of single zircon grains and ^40Ar／^39Ar dating of hornblende

The Gaojiacun mafic-ultramafic intrusivecomplex in the Yanbian area, Sichuan Province, is a strati-form intrusive body that has undergone intensive magmaticdifferentiation. This intrusive body involves two magmaticaccumulating cycles. Systematic U-Pb dating of single zircongrains and ~(40)Ar/~(39)Ar dating of hornblende were conducted,and the results showed that the age of hornblende gabbro,which was formed at the main phase of intrusion of the Gao-jiacun intrusive complex, is 840±5 Ma, casting doubt on theconcept of "Yanbian Ophiolite". It is believed that the for-mation of the Gaojiacun intrusive complex seems to be re-lated to a super-mantl3 plume underneath the su-per-continent Rodinia. The above research results are helpfulfor us to get a better understanding of the characteristics ofNeoproterozoic tectonic evolution of the Yanbian area inSichuan Province.     

Fluid/melt inclusions in Cenozoic mantle xenoliths from Linqu,Shandong Province,eastern China: Implications for asthenosphere-lithosphere interactions

Fluid and melt inclusions in mantle xenoliths are thought as direct samples to study mantle liquids. Here we apply Raman mi- crospectroscopy and microthermometry to fluid/melt inclusions in lherzolite xenoliths in Qiaoshan basalts, a Miocene volcano in Linqu, Shandong Province, eastern China. These inclusions include (1) early CO2 fluid inclusions, (2) early carbonate melt inclu-sions, (3) late CO2 fluid inclusions, and (4) late silicate melt inclusions. Among the early CO2 fluid inclusions, most consist of...