Petrography and Geochemistry of Peridotite Xenoliths from Hannuoba and Significance for Lithospheric Mantle Evolution

The compositions of the whole rocks and trace elements of minerals in peridotites can reflect the characteristics of the lithospheric mantle. The nature and evolution of the Cenozoic lithospheric mantle beneath Hannuoba （汉诺坝）, located on the north edge of the intra-North China orogenic belt, are discussed based on the in-situ LAM-ICPMS detected trace element compositions of clinopyroxenes in the Hannuoba peridotitic xenoliths combined with detailed petrography and geochemistry studies. The Hannuoba lithospheric mantle was formed by different partial meltings of the primitive mantle. Most of the samples reflect the partial melting degree of lower than 5% with a few samples of 15%-20%. Major element compositions of the whole rocks and geochemical compositions of clinopyroxenes reveal the coexistence of both fertile and depleted mantle underneath the Hannuoba region during the Cenozoic. This was probably caused by the asthenospheric mantle replacing the aged craton mantle through erosion, intermingling and modification. Our conclusion is further supported by the existence of both carbonatitic magmatic material and silicate melt/ fluid metasomatism as magnified by the trace elements of the clinopyroxencs from the Hannuoba lithospherJc mantle.

Mantle Partial Melting and Melt-peridotite Interaction: A Case Study of Peridotite Xenoliths in Cenozoic Basalt of Nanjing, SE China

The peridotite xenoliths are widely distributed in the Cenozoic basalts, Eastern China. However, their petrogenesis is hotly controversial. The peridotite xenoliths of Nanjing are found embedded in Cenozoic alkali basalt. Most of the xenoliths are rounded and small to moderate in size(typically 5~10 cm in diameter), though larger ones have been found. Nearly all small xenoliths are harzburgite and dunite. However, the big ones have zoned structure: lherzolite core and harzburgite or dunite rim with new growth clinopyroxene(Cpx) as eyeliner along their margins. Petrology, mineralogy, and Major and trace element compositions of the Nanjing peridotite xenoliths in the Cenozoic basalts are measured to provide an insight into the nature of their mantle sources and processes. Our works suggest that they were suffered from a partial melting process and subsequently underwent a process of melt-peridotite interaction. The evidences of partial melting are as follows. Firstly, the lherzolite core is mostly composed of olivine(Ol) + orthopyroxene(Opx) + Cpx with minor spinel(Spl), however, the harzburgite or dunite rim is mostly complosed of Ol + Opx with minor Cpx. Secondly, from the lherzolite core to the harzburgite or dunite rim, Ol and Opx contents are gradually increased, inversely, Cpx contents are decreased, and the Spl disappears. Thirdly, Mg# values of Ol are increased from the core(~89.5) to the rim(>92) of the peridotite xenolith, but FeO contents(from 11.0 to 8.1) in Ol are opposite.Forthly, Mg#(from 90 to 93) and Cr#(from 4 to 17) values of the Opx are increased, but its Al2O3 contents(from 5.0 to 2.0) are decreased from the core to the rim. The evidences of meltperidotite interaction are mostly from clinopyroxenes. The clinopyroxenes can roughly divided into two groups: original clinopyroxenes of the peridotite and new growth clinopyroxenes generated by melt-peridotite interaction. The original clinopyroxenes are generally in the inner of peridotite xenoliths such as lherzolite. They are mostly coarse-grained, euhedral and bottle-green. However, the new growth clinopyroxenes generally occur as eyeliner along the margin of the peridotite xenoliths. They are generally fine-grained, irregular and light green. Compared with the original clinopyroxenes, the new growth ones have low Na2O and Al2O3 and high CaO contents. The Nanjing peridotite xenoliths in the Cenozoic alkali basalts suggest that the SCLM beneath eastern China might be a fertile mantle which has had a complex history, and is now a mixture of refractory and fertile mantle domains modified by a number of events.

Variation in mantle lithology and composition beneath the Ngao Bilta volcano,Adamawa Massif,Cameroon volcanic line,West-central Africa

Mantle peridotites entrained as xenoliths in the lavas of Ngao Bilta in the eastern branch of the continental Cameroon Line were examined to constrain mantle processes and the origin and nature of melts that have modified the upper mantle beneath the Cameroon Line.The xenoliths consist mainly of lherzolite with subordinate harzburgite and dunite.They commonly contain olivine,orthopyroxene,clinopyroxene and spinel although the dunite is spinel-free.Amphibole is an essential constituent in the lherzolites.Mineral chemistry differs between the three types of peridotite:olivines have usual mantle-like Mg#of around 90 in lherzolites,but follow a trend of decreasing Mg#(to 82)and NiO(to 0.06 wt.%)that is continuous in the dunites.Lherzolites also contain orthopyroxenes and/or clinopyroxenes with low-Mg#,indicating a reaction that removes Opx and introduces Cpx,olivine,amphibole and spinel.This is attributed to reaction with a silica-undersaturated silicate melt such as nephelinite or basanite,which originated as a low-degree melt from a depleted source as indicated by low Al2O3 and Na2O in Cpx and high Na2O/K2O in amphibole.Thermobarometric estimates place the xenoliths at pressures of 11–15 kbar(35–50 km)and temperatures of 863–957
C,along a dynamic rift geotherm and shallower than the region where carbonate melts may occur.The melt/rock reactions exhibited by the Ngao Bilta xenoliths are consistent with their peripheral position in the eastern branch of the Cameroon Volcanic Line in an area of thinned crust and lithosphere beneath the Adamawa Uplift.

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.

Water in peridotite xenoliths from South China

The water contents of nominally anhydrous clinopyroxene （cpx）, orthopyroxene （opx） and olivine （ol） in peridotite xenoliths hosted by Cenozoic basalts from Yingfengling, Zhangchouchun, Fujitian and Lindi, South China, were measured by Micro- FTIR. All cpx and opx grains contained a certain amount of water, which was indicated by the presence of hydroxyls in the crystal structure. The water contents （H20, ppm） of cpx and opx from peridotite xenoliths of the study areas were 293-981, 183-752, 73-586 and 51-423 ppm, and 82-471, 74-571, 53-170 and 9-135 ppm, respectively. No prominent OH absorption bands were detected for any ol grains, indicating that the water contents were below the detection limit （approximately 2 ppm）. The entire rock contents recalculated according to mineral volume proportions were 49-163, 48-168, 21-111 and 8-40 ppm, respectively. Combined with previously reported data describing the water contents of peridotite xenoliths worldwide, the results presented here suggest that water distribution in the continental lithospheric mantle is spatially heterogeneous at a global scale. The lithospheric mantle of South China is much richer in water than that of the North China Craton, and is close to that typical of off-craton localities, such as the Basin and Range Province, and Massif Central.

Lithospheric structure and evolution of the North China Craton:An integrated study of geophysical and xenolith data

Determination of the physical and chemical structures of the inaccessible continental lithosphere by comprehensive geophysical and geochemical studies can provide valuable information on its formation and evolution.Extensive studies from various disciplines have revealed complex lithospheric modification of the North China Craton(NCC),but less attention has been paid to an integrated study from different fields.Here we provide an integrated constraint on the lithospheric mantle structure of the NCC by comprehensive semiology,gravity and thermal studies with xenolith data involving depth(levels in the lithosphere),property(chemical and physical),and timing(formation and reworking ages).Our results suggest that the NCC has a relatively heterogeneous lithospheric mantle.Its margins and internal weak zones,especially in the eastern NCC,are generally underlain by the fertile,weakly metasomatized mantle with generally young formation ages.In contrast,its core tends to preserve the refractory,strongly metasomatized mantle with ages roughly coupled to the overlying Archean crust.Such a lithospheric structure shows the preferential modification of the lithospheric mantle in the eastern NCC and in the peripheral regions of the western NCC.The interior of the craton,especially most of the western NCC,remains stable and has been weakly modified.

Formation of Melt Pocket in Mantle Peridotite Xenolith from Western Qinling, Central China: Partial Melting and Metasomatism

Two types of melt pockets, closed melt pocket （CMP） and open melt pocket （OMP）, are recognized from the peridotite xenoliths entrained in the Cenozoic kamafugites in western Qinling （秦岭）, Central China. The Haoti （好梯） CMPs have a mineral assemblage of olivine＋ clinopyroxene＋amphibole＋K- feldspar, whereas the Baiguan （白关） CMPs are composed of olivine＋clinopyroxene＋Umenite＋carbonate. The components of the OMPs are more complicated. In the Haoti OMPs, there are olivine, ciinopyroxene, glass, low modal abundances of amphibole, K-feldspar （Kfs）, Umenite, sulfide, chlorite, perovskite, chromite and phlogopite. The Baiguan OMPs contain olivine, clinopyroxene, glass, chlorite and chromite. Compositionally, ofivines in the CMPs and OMPs are both apparently depleted in Ni, and those in the OMPs are also depleted in Fe and Mg, and enriched in Ca compared to the primary ones. Ciinopyroxenes display large and systematical compositional variations between the CMPs and OMPs, particularly in Al, Cr, Na, Ca and Ti. Glasses are generally depleted in Si compared to the worldwide glasses in melt pockets, although they still have large variations. Amphiboles and K-feldspars have relatively restricted compositional variations. The petrographical observations and mineral chemistry suggest that the Haoti and Baiguan CMPs were generated by the in-sitn decompression melting of orthopyroxenes, olivines and clinopyroxenes, and by the addition of minor external K-rich and Ca-rich melt/fluids. The OMPs formed during the latest metasomatic event in the lithospheric mantle beneath the western Qinling.