The longevity of subcontinental lithospheric mantle beneath Jiangsu-Anhui Region——The Os isotope model age of mantle-derived peridotite xenoliths

The basalt-borne peridotite xenoliths from Jiangsu-Anhui provinces were analyzed for whole rock Os isotopic compositions in two laboratories of USTC, China and CRPG, France, respectively. The ^(187)Os/^(188)Os ratio of the sample set ranges from 0.119 to 0.129 (25 samples, USTC) and from 0.117 to 0.131 (17 samples, CRPG). The Os isotopic compositions of most samples are less than 0.129 and depleted relatively to the primitive mantle, showing a good correlation with the major element compositions. With the ^(187)Os/^(188)Os-AI2O3 alumichron, the samples yield a model age of 2.5±0.1 Ga (data of USTC) and 1.9±0.1 Ga (data of CRPG), late Archean to early Pro-terozoic. The two samples with the lowest ^(187)Os/^(188)Os ratio (0.119 and 0.117) have the T_(RD) (Re depleted age) of 1.1 Ga (USTC) and 1.4 Ga (CRPG), mid-Proterozoic. The Os isotope model age shows that the peridotite xenoliths from Cenozoic alkali basalt in Jiangsu-Anhui provinces have an old formation age (early- to mid- Proterozoic). They are not newly produced mantle after the Phanerozoic replacement of the lithosphere mantle, but residual fractions of Proterozoic mantle.

Petrological and Os Isotopic Characteristics of Zedong Peridotites in the Eastern Yarlung–Zangbo Suture in Tibet

The Zedong ophiolites in the eastern Yarlung-Zangbo suture zone of Tibet represent a mantle slice of more than 45 km2. This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite; a few dike-like bodies of dunite are also present. Mineral structures show that the peridotites experienced plastic deformation and partial melting. Olivine （Fos9.7-91.2）, orthopyroxene （Enss-92）, clinopyroxene （En4-49Wo47-slFS2-4） and spinel [Mg^#=lOOxMg/（Mg＋Fe）]=49.1-70.7; Cr^#=（100xCr/（Cr＋Al）=18.8-76.5] are the major minerals. The degree of partial melting of mantle peridotites is 10%-40%, indicating that the Zedong mantle peridotites may experience a multi-stage process. The peridotites are characterized by depleted major element compositions and low REE content （0.08-0.62 ppm）. Their ＂spoon-shaped＂ primitivemantle normalized REE patterns with （La/Sm）N being 0.50-6.00 indicate that the Zedong ultramafic rocks belong to depleted residual mantle rocks. The PGE content of Zedong peridotites （18.19-50.74 ppb） is similar with primary mantle with Pd/Ir being 0.54-0.60 and Pt/Pd being 1.09-1.66. The Zedong peridotites have variable, unradiogenic Os isotopic compositions with 187Os/18Os=0.1228 to 0.1282. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes. All data of the Zedong peridotites suggest that they formed originally at a mid-ocean ridge （MOR） and were later modified in supra-subduction zone （SSZ） environment.

Tracing crustal contamination of the Cenozoic basalts with OIB-affinity in northern marginal region of North China Craton:An Os perspective

The Cenozoic basalts with OIB-affinity in northern marginal region of the North China Craton are thought to experience minor even no crustal contamination during the magma evolution.The whole-rock Sr-Nd-Pb-Hf isotopes are attributed to a two-component mixing between depleted and enriched mantle sources,while the major element variations are controlled by the fractional crystallization of olivine and clinopyroxene.However,in this study,the new Os isotopic data proposes an opposite model for the Cenozoic basalts in northern marginal region of the North China Craton.In this model,the Jining basalts were contaminated by the Archean mafic rocks during the magma storage and ascent.The crustal contamination process is supported by(1)the highly radiogenic Os isotopic compositions,and(2)the positive correlation between 187Os/188Os and 1/Os of the Jining basalts.By modeling the Os isotopic composition of the basalts,an incorporation of<10%mafic granulites/amphibolites to the parental magma can successfully explain the initial values of highly radiogenic Os.In contrast,the unradiogenic and uniform Os isotopic compositions of the Chifeng basalts suggest negligible crustal contamination.Os isotopic data acts as an indicator of crustal contamination during magma evolution,providing us a novel insight into the evolution of the intra-continental OIB-like basalts worldwide.