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Melting and reactions between minerals and melts are important processes in the evolution of the lithospheric mantle, and are usually inferred from their geochemical fingerprints in mantle samples. However, a suite of mantle-derived peridotite xenoliths from the Xilong area, South China, records the reaction of successive silicate melts of different compositions with mineral assemblages in the mantle, preserved by quenching during entrainment. These xenoliths form two groups and record a compositionally layered mantle. Group 1 has olivine Mg# ~ 91, (and is thus relatively refractory), is derived from depths of ~ 50–65 km, and shows the trace-element geochemical signature of “old” carbonatitic metasomatism. Group 2 is more fertile with olivine Mg# mainly ~ 89–90, is derived from ~ 40 to 55 km and has ubiquitous modal spinel. Xenoliths of both groups then show sequential infiltration by two compositionally distinct melts (Na-rich and K-rich) not long before eruption. The Na-rich melts are enclosed in spongy clinopyroxene and spinel rims and are inferred to have triggered the reactions that formed the spongy rims, which have lower Al 2 O 3 , Na 2 O and Mg#, but higher FeO, TiO 2 and Cr# than the primary phases. The undersaturated Na-rich mafic melts were probably formed in the asthenosphere by low-degree melting. The K-rich melts occur mainly in reaction zones around orthopyroxene and in reaction patches containing fine-grained secondary olivine, clinopyroxene and minor spinel. The melts have high contents of SiO 2 , K 2 O (mean 14.3 wt.%), Rb, Ba, and LREE but very low Na 2 O/K 2 O (0.01–0.29), positive anomalies in Eu and Sr, and variable HFSE anomalies. These compositional characteristics are consistent with an origin as low-degree partial melts of pre-existing phlogopite-bearing rocks. The K-rich melts also react with primary olivine, and the spongy-textured secondary clinopyroxene and spinel inferred to have formed by reaction with the Na-rich melts, yielding secondary olivine, vermicular clinopyroxene and spinel compositionally similar to that in the reaction patches. This observation suggests that the infiltration of K-rich melt occurred after the percolation of Na-rich melts. This study demonstrates the complexity of metasomatic processes in the lithospheric mantle, and emphasizes the caution required when interpreting the geochemical signatures of mantle-derived xenoliths in which multiple overprinting by compositionally diverse fluid and melt interactions with mantle minerals cannot be so clearly observed and characterized as in these samples. [ABSTRACT FROM AUTHOR]

Abstract: Lithospheric thinning beneath the North China Craton is widely recognized, but whether the Yangtze block has undergone the same process is a controversial issue. Based on a detailed petrographic study, a suite of xenoliths from the Lianshan Cenozoic basalts have been analyzed for the compositions of minerals and whole rocks, and their Sr–Nd isotopes to probe the nature and evolution of the subcontinental lithospheric mantle beneath the lower Yangtze block. The Lianshan xenoliths can be subdivided into two Types: the main Type 1 xenoliths (9–15% clinopyroxene and olivine-Mg#<90) and minor Type 2 peridotites (1.8–6.2% clinopyroxene and olivine-Mg#>90). Type 1 peridotites are characterized by low MgO, high levels of basaltic components (i.e., Al2O3, CaO and TiO2), LREE-depleted patterns in clinopyroxenes and whole rocks, and relatively high 143Nd/144Nd (0.513219–0.513331) and low 86Sr/87Sr (0.702279–0.702789). These features suggest that Type 1 peridotites represent fragments of the newly accreted fertile lithospheric mantle that have undergone ~1% of fractional partial melting and later weak silicate–melt metasomatism, similar to Phanerozoic lithospheric mantle beneath the eastern North China Craton. Type 2 peridotites may be shallow relics of the older lithospheric mantle depleted in basaltic components, with LREE-enriched and HREE-depleted patterns, relatively low 143Nd/144Nd (0.512499–0.512956) and high 86Sr/87Sr (0.703275–0.703997), which can be produced by 9–14% partial melting and subsequent carbonatite–melt metasomatism. Neither type shows a correlation between equilibration temperatures and Mg# in olivine, indicating that the lithospheric mantle is not compositionally stratified, but both types coexist at similar depths. This coexistence suggests that the residual refractory lithospheric mantle (i.e., Type 2 peridotites) may be irregularly eroded by upwelling asthenosphere materials along weak zones and eventually replaced to create a new and fertile lithosphere mantle (i.e., Type 1 xenoliths) as the asthenosphere cooled. Therefore, the subcontinental lithospheric mantle beneath the lower Yangtze block shared a common evolutional dynamic environment with that beneath the eastern North China Craton during late Mesozoic–Cenozoic time. [Copyright &y& Elsevier]

Major and trace elements and water contents were analyzed in 16 peridotite xenoliths embedded by the Cenozoic basalts in Pingnan (southeastern Guangxi Province), to constrain the chemical composition and evolution of the lithospheric mantle located in the central part of the South China Block (SCB). The peridotites are mainly moderately refractory harzburgites and lherzolites (Mg#-Ol = 90.3-91.7) and minor fertile lherzolites (Mg#-Ol = 88.9-89.9). Clinopyroxenes in the peridotites show LREE-depleted pattern, and commonly exhibit negative anomalies in Nb and Ti, suggesting the peridotites probably represent residues after 1-10% of partial melting without significant mantle metasomatism. Water contents range from 146 to 237 ppm wt. H2O in clinopyroxene, and from 65 to 112 ppm wt. H2O, in orthopyroxene but are below detection limit (2 ppm wt. H2O) in olivine. Calculated bulk water contents, based on the mineral modes and partition coefficient, range from 14 to 83 ppm wt. H2O (average 59 ppm wt. H2O). There is a correlation between melting indices (such as Mg#-Ol, Ybn in clinopyroxene) and water contents in clinopyroxene and orthopyroxene, but no correlation is observed between the whole-rock water contents and the redox state (Fe3+/∑Fe ratios in spinel), suggesting that water contents in the peridotites are mainly controlled by the degree of partial melting rather than by oxygen fugacity. The lithospheric mantle beneath the interior of the SCB may not be compositionally stratified; fertile and moderately refractory mantle coexist at the similar depths. Geochemical data and water contents of the studied peridotites are similar to the proposed MORB source and indicate that the ancient refractory lithospheric mantle was irregularly eroded or reacted by the upwelling asthenosphere, and eventually replaced by juvenile fertile accreted mantle through the cooling of the asthenosphere. [ABSTRACT FROM AUTHOR]

The distribution of the water content in the subcontinental lithospheric mantle plays a crucial role in the geodynamic processes. To better comprehend the nature and evolution of the mantle, we carried out detailed petrological and geochemical studies and analyzed the water content of the peridotite xenoliths from Xinchang and Xilong (Zhejiang Province), South China. The peridotites are the (spinel) lherzolites with minor harzburgites exhibiting porphyroclastic and protogranular textures. The Mg# values of the olivines from Xinchang and Xilong xenoliths were in the range 89.0–90.7 and 89.3–91.2, respectively. The lithospheric mantle is moderately refractory and fertile, it had undergone low to moderate degree of melt extraction. Peridotite xenoliths from both the localities display various rare earth element (REE) patterns of pyroxenes. They are enriched in incompatible elements with relatively low clinopyroxene 87Sr/86Sr ratios (0.7013–0.7037) and are metasomatized by silicate melts derived from the asthenospheric mantle. Based on the mineral modes and partition coefficient, the calculated whole-rock water content of the peridotites is 39–94 ppm and 48–89 ppm, respectively, lower than the previously published data from Jiande which is located in the northern part of the studied areas. The variations in the water contents of the studied lithospheric mantle might have been caused not only by partial melting, but also by geodynamic processes. Combined all the observations, we suggest that the ancient refractory lithospheric mantle beneath Zhejiang Province along with the entire South China Block, might have been removed before the early Cenozoic era, followed by accretion together with the juvenile fertile mantle during asthenospheric upwelling driven by lithospheric extension during the Eocene period. • Water content of Xinchang and Xilong lithospheric mantle is reported. • Water content of studied peridotites were affected by complex geological processes. • The lithospheric mantle beneath the SCB recorded accretion and replacement process [ABSTRACT FROM AUTHOR]