Petrology of Spinel-Lherzolite Xenoliths from Mazéléand Others Northen Xenoliths Localities of Cameroon Volcanic Line: Exchange Reactions and Equilibrium State

The alkaline volcanism of the Cameroon Volcanic Line in its northern domain has raised many fresh enclaves of peridotites. The samples selected come from five (05) different localities (Liri, in the plateau of Kapsiki, Mazélé in the NE of Ngaoundéré, Tello and Ganguiré in the SE of Ngaoundéré and Likok, locality located in the west of Ngaoundé). The peridotite enclaves of the above localities show restricted mineralogical variation. Most are four-phase spinel-lherzolites, indicating that this is the main lithology that forms the lithospheric mantle below the shallow zone. No traces of garnet or primary plagioclase were detected, which strongly limits the depth range from which the rock fragments were sampled. The textures and the wide equilibrium temperatures (884˚C - 1115˚C) indicate also entrainment of lherzolite xenoliths from shallow depths within the lithosphere and the presence of mantle diapirism. The exchange reactions and equilibrium state established in this work make it possible to characterize the chemical composition of the upper mantle of each region and test the equilibrium state of the phases between them. Variations of major oxides and incompatible elemental concentrations in clinopyroxene indicate a primary control by partial melting. The absence of typical “metasomatic” minerals, low equilibration temperatures and enriched LREE patterns indicate that the upper mantle below septentrional crust of Cameroun underwent an event of cryptic metasomatic enrichment prior to partial melting. The distinctive chemical features, LREE enrichment, strong U, Ce and Pr, depletion relative to Ba, Nb, La, Pb, and T, fractionation of Zr and Hf and therefore ligh high Zr/Hf ratio, low La/Yb, Nb/La and Ti/Eu are all results of interaction of refractory peridotite residues with carbonatite melts.

Petrology of lherzolite xenoliths of HosséréSédévolcano(Adamawa plateau,Ngaoundéréarea,Cameroon)

Numerous mantle xenoliths 6–12 cm in size and sub-angular to rounded in shape occur within Mio-Pliocene basanite lavas of the monogenic volcano of HosséréSédé in the Adamawa plateau.Xenoliths of spinel lherzolite exhibit protogranular,equigranular or porphyroclastic texture.Microprobe chemical analyses show that olivine is highly magnesian(Fo_(88-90)),clinopyroxene crystals are diopside and augite(Wo_(41.6–49.6)En_(45.3–53.7)Fs_(4.2–6.2)),orthopyroxene crystals are enstatite(Wo 1.4–1.5 En_(88.6–89.0)Fs_(9.6–9.9))and spinel crystals are mainly Al-spinel associ-ated to minor Cr-spinel.Estimated temperatures and pressures through empirical formulas show that HosséréSédé noliths have equilibrated between 1085 and 1204℃and 1.08 to 1.57 GPa,corresponding to sampling depths of 36 and 52 km.Detailed petrographical and mineral chemistry of HosséréSédénoliths evidences the com-plex nature and composition of the subcontinental lithosphere under the Adamawa plateau.This may involve a probable uplift of the whole area after a limited exten-sional event,possible metasomatism through melt infiltration during shearing of the lithospheric mantle along the Pan African strike-slip fault system.

河南鹤壁地幔捕掳体内磷灰石的发现及地幔交代作用

目的地幔交代作用是岩石圈演化的重要方式之一。显性交代作用以出现交代成因的地幔矿物为特征,可为地幔交代作用提供直接证据,磷灰石是地幔中交代成因的典型矿物。中生代以来,太行山构造带东侧的华北板块岩石圈明显减薄,而西侧仍保留较厚的岩石圈,太行山构造带是华北克拉通东部被破坏区与西部稳定区的过渡带。因此,太行山构造带的地幔包体对研究华北克拉通破坏具有重要意义。鹤壁地区位于太行山构造带上,发育有金伯利岩等深源岩浆岩。方法本文以金伯利岩携带的地幔包体为研究对象,探讨鹤壁地区岩石圈地幔的性质。通过人工重砂的方法,从地幔包体中分选出数粒磷灰石和一粒被辉石包裹的磷灰石。结果体视显微镜下,磷灰石无色,透明,具玻璃光泽,呈短柱状;扫描电镜能谱分析表明,被包裹的矿物主要由Ca,P,O,F元素组成,寄主矿物主要由Si,Fe,Mg,Al,O元素组成;电子探针结果表明,磷灰石中w(CaO)为53.525%~56.514%,w(P_(2)O_(5))为37.372%~40.851%;实验岩石学研究认为,顽火辉石+方解石→橄榄石+透辉石+CO_(2),磷灰石等作为副矿物产出。因此,辉石包裹磷灰石的矿物组合属于地幔交代成因,指示了显性交代作用。结论磷灰石这种显性地幔交代成因矿物的发现,为华北克拉通岩石圈地幔经历显性交代作用提供了直接证据。

松辽盆地南部岩石圈地幔变形特征及各向异性分析——来自橄榄岩包体的约束

SKS测量结果显示松辽盆地快波方向分布较为复杂,由于缺乏深部岩石变形资料约束,制约了地震波各向异性成因解释。本文通过对松辽盆地南部双辽地区橄榄岩包体进行详细的岩石学、地球化学、显微结构、各向异性研究,结果显示双辽地区橄榄岩包体的平衡温度为893~1152℃,来源于岩石圈地幔。橄榄岩中橄榄石的晶格优选方位(CPO)类型主要为A型、D型和AG型,其中,AG型和D型CPO可能形成于西太平洋板块向欧亚大陆俯冲回撤导致的岩石圈变形,AG型橄榄石CPO也可能形成于熔体存在下的橄榄石变形。基于CPO计算获得的橄榄岩包体全岩VP各向异性(AVP)为4.79%~11.80%,最大剪切波各向异性(AVSmax)为3.13%~7.93%。结合地球物理测量结果,推断松辽盆地南部复杂的SKS各向异性的主要贡献可能来源于面理陡倾或直立的岩石圈地幔。

吉林保安屯地区侏罗世花岗闪长岩及其闪长质包体成因研究

本文对小兴安岭-张广才岭岩浆弧南段吉林保安屯出露的侏罗纪花岗闪长岩及其内部的闪长质包体开展了详细的地质调查及系统的岩石地球化学、锆石U-Pb年代学研究。研究表明寄主花岗闪长岩与闪长质包体的母岩浆不是同源的,寄主花岗闪长岩的源区壳源性质更明显,而闪长质包体的源区则更接近于幔源;此外,闪长质包体的形成温度和压力较寄主花岗闪长岩更高。二者均具有轻稀土富集、重稀土亏损、轻重稀土分馏明显的特征,在球粒陨石标准化分布型式图中显示,闪长质包体的Eu负异常更为明显。锆石U-Pb定年表明闪长质包体形成年龄为182.0±1.3Ma,为早侏罗世,而寄主岩体花岗闪长岩的U-Pb同位素年龄为168.1±1.6Ma~172.1±1.2Ma,属中侏罗世。上述地球化学特征及形成时代的差异表明闪长质包体可能代表晚期花岗闪长质岩浆上侵时捕获的围岩物质,其成因应属捕掳体,间接说明张广才岭南段侏罗世花岗岩类中广泛发育的暗色包体类型至少有两种,一是捕掳体成因,二是岩浆混合成因。

地幔榴辉岩包体的同位素定年方法研究进展

地幔榴辉岩包体是寄主岩浆从地球深部地幔携带到地表的俯冲洋壳碎片,作为一种高压变质岩,它可能记录着有关大洋地壳原岩、板块俯冲以及后期交代变质过程等重要信息,因此其年龄也显得格外重要。通过统计全球超过500个已发表的地幔榴辉岩包体的年龄结果,对比几种常用的地幔榴辉岩包体同位素定年方法,讨论了不同方法的优缺点及适用情况。结果表明:金红石U-Pb和全岩Rb-Sr、Sm-Nd、Lu-Hf全岩等时线法一般给出的是寄主岩浆的年龄或地幔交代的时间,而榴辉岩中的硫化物Re-Os等时线年龄,锆石U-Pb上交点年龄,Sm-Nd、Lu-Hf石榴子石-单斜辉石双矿物等时线年龄以及锆石Hf亏损地幔模式年龄对于其原岩年龄及板块俯冲时代有较为可观的应用前景;地幔榴辉岩包体最古老年龄的峰值分布在3.1~2.8 Ga,与Kaapvaal和西伯利亚克拉通构造拼合的时间吻合,可能代表着全球地幔榴辉岩包体的最早形成时代,也预示着板块构造可能于中太古代开启。

Garnet-bearing Granulite Facies Rock Xenoliths from Late Mesozoic Volcaniclastic Breccia, Xinyang, Henan Province

This paper presents the primary results of petrologic, mineralogical and petrochemical studies of garnet-bearing granulite facies rock xenoliths from Xinyang, Henan Province. These xenoliths, which are found in a pipe of late Mesozoic volcaniclastic breccia, are of high density (3.13–3.30 g/cm3) and high seismic velocity (Vp = 7.04–7.31 km/s), being products of underplating of basaltic magmas and had experienced granulite facies metamorphism. The underplating and metamorphism took place before the eruption of the host rock. Petrographical studies and equilibrium T-P calculations show that these xenoliths were captured at a 49 km depth and experienced at least a 16 km uplift before they were captured. The dynamics of the uplift could be related to the continent-continent collision between the North China plate and the Yangtze plate during the Triassic.

Underplating of Mesozoic Mantle-derived Magmas in Tongling, Anhui Province: Evidence from Megacrysts and Xenoliths

Lithological observations and mineralogical analyses on pyroxene and hornblende megacrysts and pyroxene and hornblende cumulates in xenoliths in the Mesozoic plutons of the Tongling region, Anhui Province, provide evidence for the magmatic underplating of mantle-derived alkali-olivine basalt at circa 140 Ma. The pyroxene and hornblende megacrysts and cumulates were formed through the AFC process at depths ranging from 27 to 35 km.

IMPLICATIONS OF THE DEEP-SEATED XENOLITHS IN CENOZOIC BASALT IN KANGXIWA, WEST KUNLUN,CHINA

Our understanding of solid earth is from the surface, but the depth we can reach is very limited. So, most of the interpretations of geological processes and mechanisms extrapolated from all kinds of the surface phenomena is greatly uncertain. Recently many researchers concentrate their efforts to the geological and geophysical studying of the deep processes of the solid earth. The International Lithosphere Project (ILP) started in 1981 now is also a frontier field. One of the concentrations of this project is the 3\|D structure, tectonic evolution and the dynamic models of lithosphere\|asthenosphere system (Li Xiaobo etc., 1997). The studying of the igneous rocks and bearing deep\|seated xenoliths, as one of the effective methods probing the structure and evolution of the lithosphere, plays a very important role in these aspects (Deng Jinfu et al., 1996; Lu Fengxiang, 1997).Commonly, magmas come to the surface in great speed and the covering lithosphere should be relatively thinner. For example, the thickness of the lithosphere in East China is about 60～80km in the era when the volcanoes erupted (Deng Jinfu et al. 1994). Recently, many deep\|seated xenoliths were found in several localities of southwest Tianshan (Han Baofu et al., 1998). But in Qinghai—Xizang plateau, “So far no any xenoliths of mantle rocks as peridotites, lherzolites and harzburgites and high pressure granulites were found (Deng Wanming et al., 1997)”.. But in the fieldwork of 1998 we find some deep\|seated xenoliths in Cenozoic basaltic rocks in Kangxiwa region, West Kunlun, China. This work is a part of the project “XinJiang DuShanZi—QuanShuiGou transect” managed by academician Xiao Xuchang.

Geochemical Records of Mantle Processes in Mantle Xenoliths from Three Cenozoic Basaltic Volcanoes in Eastern China

Rare earth element (REE) contents, and Sr and Nd isotopic compositions were measured for three suites of mantle xenoliths from the Kuandian, Hannuoba and Huinan volcanoes in the north of the Sino-Korean Platform. From the correlations of Yb contents with Al/Si and Ca/Si ratios, the peridotites are considered to be the residues of partial melting of the primitive mantle. The chondrite-normalized REE compositions are diverse, varying from strongly LREE-depleted to LREE-enriched, with various types of REE patterns. Metasomatic alteration by small-volume silicate melts, of mantle peridotites previously variably depleted due to fractional melting in the spinel peridotite field, can account for the diversity of REE patterns. The Sr/ Ba versus La/Ba correlation indicates that the metasomatic agent was enriched in Ba over Sr and La, suggestive of its volatile-rich signature and an origin by fluid-triggered melting in an ancient subduction zone. The Sr and Nd isotopic compositions of these xenoliths, even from a single locality, vary widely, covering those of Cenozoic basalts in eastern China. The depleted end of the Sr-Nd isotope correlation is characterized by clearly higher 143Nd/144Nd and a broader range of 87Sr/86Sr compared to MORB. The low-143Nd/144Nd and high-87Sr/86Sr data distribution at the other end of data array suggests the existence of two enriched mantle components, like EM1 and EM2. The correlations between 143Nd/l44Nd and 147Sm/N4Nd ratios in these xenoliths suggest at least two mantle metasomatic events, i.e. events at 0.6-1.0 Ga and 280-400 Ma ago.

Special Xenoliths in an Aegirine-Augite Syenite Porphyry in Liuhe,Yunnan,China:Discovery and Implications

Three special types of xenoliths have recently been found in an aegirine-augite syenite porphyry in Liuhe, Yunnan, China. Petrographical, petrochemical, electron microprobe, and scanning electron microscopy studies indicate that pure calcite xenocrysts and quartz-bearing topaz pegmatite xenoliths result from the degassing of mantle fluids during their migration, and that black microcrystalline iron-rich silicate-melt xenoliths are the product of the extraction of mantle fluids accompanying degassing and are composed dominantly of quartz, chlorite, and iron-rich columnar and sheet silicate minerals with characteristic minerals, such as native iron, apatite, and zircon. According to the bulk-rock chemical and mineral compositions and crystallization states, the microcrystalline melt xenoliths are not the product of conventional magmatism, and especially the existence of native iron further proves that the xenoliths were mantle fluid materials under reduction or anoxic conditions. The study of the special xenoliths furnishes an important deep-process geochemical background of polymetallic mineralization in different rocks and strata in the study area.

ULTRAMAFIC XENOLITHS FROM A KAMAFUGITE LAVA IN CENOZOIC VOLCANIC FIELD OF WEST QINLING, CHINA AND ITS GEOLOGICAL IMPLICATION

Cenozoic potassic mafic volcanism (kamafugite magamtism) in West Qinling, Gansu province of China is a important section of Cenozoic volcanic belt of Qingzang (Tibet) plateau and adjacent area. The kamafugite magma show near\|primary magma characteristics. Strong incompatible elements concentrations in the volcanic rocks infer the kamafugite magmas may be origined from a enrichment mantle sources earlier.Four groups of deep\|seated xenoliths were collected from a kamafugite lava in West Qinling, Gansu province, China: (1)spinel Iherzolite and garnet Iherzolite; (2) harzburgite; (3) dunite and (4) clinopyroxenite. All suite of peridotite xenoliths are Cr\|diopside series. The main textures of peridotite xenoliths are granoblastic, porphyroclastic and granular, but the textures of clinopyroxenite are mainly poikilitic. The textural characteristics, mineral chemical variations and mineral geothermometric data show that the mantle source region of the Cenozoic kamafugite magam are very complex and undertook partial melting and enrichment associated with alkaline metasomatism by fluid and carbonatite melts. Geothermometry indicates the equilibration p and T conditions of the spinel lherzolite and garnet lherzolite are 970℃, 18 9GPa and 1219℃,27 61GPa corresponding to depth of 62km and 92km representatively. We suggest, combining with geophysical data, the spinel lherzolite formed at the top of upper mantle, the garnet lherzolite represent the base of lithosphere or the top of asthenosphere, but the clinopyroxenite is formed by cumulation in shallower magma reservoir. Kamafugite magmas formed within the stability field of garnet lherzolite by partial melting, the partial melting degree is only about 1%～2%. There is a superheating condition in Cenozoic mantle of West Qinling, which may be related to strong uplift of Qingzang (Tibet) plateau since cenozoic and extrusion east forward of asthenosphere beneath the Qingzang (Tibet) plateau.

Phlogopite in mantle xenoliths and kimberlite from the Grib pipe,Arkhangelsk province,Russia:Evidence for multi-stage mantle metasomatism and origin of phlogopite in kimberlite

We present petrography and mineral chemistry for both phlogopite,from mantle-derived xenoliths (garnet peridotite,eclogite and clinopyroxene-phlogopite rocks) and for megacryst,macrocryst and groundmass flakes from the Grib kimberlite in the Arkhangelsk diamond province of Russia to provide new insights into multi-stage metasomatism in the subcratonic lithospheric mantle (SCLM) and the origin of phlogopite in kimberlite.Based on the analysed xenoliths,phlogopite is characterized by several generations.The first generation (Phl1) occurs as coarse,discrete grains within garnet peridotite and eclogite xenoliths and as a rock-forming mineral within clinopyroxene-phlogopite xenoliths.The second phlogopite generation (Phl2) occurs as rims and outer zones that surround the Phl1 grains and as fine flakes within kimberlite-related veinlets filled with carbonate,serpentine,chlorite and spinel.In garnet peridotite xenoliths,phlogopite occurs as overgrowths surrounding garnet porphyroblasts,within which phlogopite is associated with Cr-spinel and minor carbonate.In eclogite xenoliths,phlogopite occasionally associates with carbonate bearing veinlet networks.Phlogopite,from the kimberlite,occurs as megacrysts,macrocrysts,microcrysts and fine flakes in the groundmass and matrix of kimberlitic pyroclasts.Most phlogopite grains within the kimberlite are characterised by signs of deformation and form partly fragmented grains,which indicates that they are the disintegrated fragments of previously larger grains.Phl1,within the garnet peridotite and clinopyroxeneephlogopitexenoliths,is characterised bylow Ti and Cr contents (TiO2<1 wt.%,Cr2O3<1 wt.% andMg#=100×Mg/(MgtFe)>92) typical of primary peridotite phlogopite in mantle peridotite xenoliths from global kimberlite occurrences.They formed during SCLM metasomatism that led to a transformation from garnet peridotite to clinopyroxene-phlogopite rocks and the crystallisation of phlogopite and high-Cr clinopyroxene megacrysts before the generation of host-kimberlite magmas.One of the possible processes to generate low-Ti-Cr phlogopite is via the replacement of garnet during its interaction with a metasomatic agent enriched in K and H2O.Rb-Sr isotopic data indicates that the metasomatic agent had a contribution of more radiogenic source than the host-kimberlite magma.Compared with peridotite xenoliths,eclogite xenoliths feature low-Ti phlogopites that are depleted in Cr2O3 despite a wider range of TiO2 concentrations.The presence of phlogopite in eclogite xenoliths indicates that metasomatic processes affected peridotite as well as eclogite within the SCLM beneath the Grib kimberlite.Phl2 has high Ti and Cr concentrations (TiO2 > 2 wt.%,Cr2O3 > 1 wt.% and Mg#=100 × Mg/(Mg + Fe)< 92) and compositionally overlaps with phlogopite from polymict breccia xenoliths that occur in global kimberlite formations.These phlogopites are the product of kimberlitic magma and mantle rock interaction at mantle depths where Phl2 overgrew Phl1 grains or crystallized directly from stalled batches of kimberlitic magmas.Megacrysts,most macrocrysts and microcrysts are disintegrated phlogopite fragments from metasomatised peridotite and eclogite xenoliths.Fine phlogopite flakes within kimberlite groundmass represent mixing of high-Ti-Cr phlogopite antecrysts and high-Ti and low-Cr kimberlitic phlogopite with high Al and Ba contents that may have formed individual grains or overgrown antecrysts.Based on the results of this study,we propose a schematic model of SCLM metasomatism involving phlogopite crystallization,megacryst formation,and genesis of kimberlite magmas as recorded by the Grib pipe.

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.

Granulite-facies Middle-Lower Crustal Xenoliths from Nushan Alkali Basalt in Northeastern Anhui Province,China

Metamorphic xenoliths within the Nushan alkali basalt of northeastern Anhui (NEA),China ,are from the middle-lower crust.They could be divided into two end-members:basic and acid.Interme-diate xenoliths are scarcely found.Basic two-pyroxene granulites(pyriclasites) were formed at 720-810℃ and 7-8kb.Petrological and geochemical studies indicate that the primary magma of the protoliths of basic granulites was derived from the metasomatized upper mantle, while the pa-rental magma of the acid end-member was probably produced by partial melting of the basic rocks. The protoliths of charnockites and grey gneisses represent respectively the early and late crystallization products of the granitic magma.The Nushan granulites are much different in many aspects from the granulites exposed in the northern part of North China ,which implies the inhomogeneity regarding to the early evolution of the North China terranc.

Amphibole-Bearing Peridotite Xenoliths from Nushan,Anhui Province:Evidence for Melt Percolation Process in the Upper Mantle and Lithospheric Uplift

The spinel peridotite xenoliths of Group I in Quaternary basanites from Nushan, Anhui Province, can be classified as two suites: 8 hydrous suite characterized by the ubiquitous oc-currence of (Ti-) pargasite and an anhydrous suite. The mineral chemistry reveals that the an-hydrous suite and one associated phlogopite-bearing lherzolite are equilibrated under temperatureconditi0ns of 1000 - 1110℃, whereas amphibole-bearing peridotites display distinct disequilibrium features, indicating partial reequilibration from 1050 to 850℃ and locally down to 750℃.This amphibole-bearing peridotites were probably the uppermost part of the high temperatureanhydrous suite which was modally modified by fractionating H2O-rich metasomatic agent during regional upwelling. This relatively recent lithospheric uplift event foll0wed an older upliftevent recognized from pyroxene unmixing of domains in local equilibrium, as well as the dominant deformation texture in the anhydrous suite. The first thermal disturbance can be linkedwith the regional extension and widespread basaltic volcanism in Jiangsu-Anhui provinces sinceEarly Tertiary and the formation of the nearby Subei (North Jiangsu) fault-depression basinduring the Eocene, while the second event in association with the formation of amphibolesprobably indicates the continuation but diminution of upward mantle flux since Neogene in response to the change in tectonic regime for eastern China.

Chemical and Isotopic Characteristics of Gabbroic Xenoliths from Hannuoba,China

Nine pieces of gabbroic xenoliths from Hannuoba were examined for their major and trace elements and Nd,Sr and Pb isotopes.The results show that the gab-broic xenoliths are of more mafic basaltic composition .Their abundances show narrow variations in major elements.The trace element contents are highly variable in contrast with those of host basalts and lherzolite xenoliths.The gabbroic xenoliths are rich in Nd(0.51159-0.51249),Sr(0.70491-0.70768) and low in radiogenic Pb(16.283-17.046, 15.191-15.381 and 36.999-37.476),significantly different from basalts and lherzolites in isotopic space.The calculated Nd and Pb model ages are about 3.0-3.5 Ga.The rocks have relatively low equilibrium T(-850℃) and P(0.8-0.9 Gpa).They could be inter-preted to be the product of upper mantle melting at the boundary between the lower crust and the upper mantle.Their chemical and isotopic variations can be ascribed to different degrees of melting,segregation and long-term evolution.

Sulfide-melt inclusions in mantle xenoliths from the Changbaishan district,Jilin province,China

Mantle xenoliths are common in the Cenozoic basalts of the Changbaishan District,Jilin Province,China.Sulfide assemblages in mantle minerals can be divided into three types:isolated sulfide grains,sulfide-meh inclusions and filling sulfides in fractures.Sulfide-meh inclusions occur as single-phase sulfides,sulfide-silicate melt,and CO_2-sulfide-silicate melt inclusions. Isolated sulfide grains are mainly composed of pyrrhotite,but cubanite was found occasionally.Sulfide-meh inclusions are mainly composed of pontlandite and MSS,with small amounts of chalcopyrite and talnakhite.The calculated distribution coefficient K_(D3)for lherzolite are similar to that of mean experimental value.The bulk sulfides in lherzolite were in equilibrium with the enclosing minerals, indicating immiscible sulfide melts captured in partial melting of upper mantle.Sulfide in fractures has higher Ni/Fe and(Fe+Ni)/S than those of sulfide melt inclusions.They might represent later metasomatizing fluids in the mantle.Ni/Fe and(Fe+Ni)/S increase from isolated grains,sulfide inclusions to sulfides in fractures.These changes were not only affected by temperature and pressure,hut by geochemistry of Ni,Fe and Cu,and sulfur fugacity as well.

Characteristics of Lower Crustal Granulite Xenoliths from the East Qinling Orogenic Belt and Their Tectonic Significance

The Tongbai granulites are present mainly as xenoliths in granodioritic gneisses. The xenoliths with a zircon age of 470Ma are older than the host rocks of granodioritic gneisses which yield a zircon age of 435Ma. It is suggested that the granulites were transported from the lower crust to the upper level along with granodioritic magma. Geothermometrical and geobarometrical studies based on the coexisting minerals (Opx-Cpx and Opx-Gar) show that the granulites were crystallized at 818-840℃ and 9.5-9.8 ×108 Pa corresponding to the lower crust. Tectonically, the Shangdan suture zone conshtutes the boundary between the North China and Yangtze plates. The zone is characterized by the occurrence of ophiolites in the western part and by that of granulites in the eastern part. So the western part marks the upper crustal level of the Qinling belt, while the eastern part represents the exposure of a deeper level. The results of isotopic dating and the geochemical characteristics of the xenoliths are consistent with those of meta-tholeiltes of the ophiolites in the western part.Therefore, it is assumed that both ophiolites found in the west and granulites found in the east all represent the renmants of the ancient Qinling ocean plate. The difference is that the ophiolites are pieces of obductal fragments from the ocean floor during the subduction in the Early Palaeozoic.However, in the Tongbai area, when the ocean floor was subducting towards the lower crust, it underwent a granulite facies metamorphism. Subsequently, granodioritic magma formed by partial melting trapped some fragments of granulite upwards.

Helium and Argon Isotopic Composition of Cenozoic Alkali Basalts and Mantle-Derived Xenoliths from Kuandian, Liaoning Province, China

The noble gas isotopic composition and content data of 2 alkali basalts, 3 lherzolite xenoliths and one clinopyroxene megacryst from the Kuandian region have confirmed the occurrence of a fractionation of noble gases during magmatism. Light noble gases such as He and Ne are high in mobility and appear to be incompatible as compared with heavy ones (such as Kr and Xe). Therefore, light noble gases are abundant in volcanics, especially in the volcanics with bubbles; lherzolite xenoliths have relatively high heavy noble gases. The clinopyroxene megacryst has the lowest abundance of noble gases, probably due to its high P-T origin. Noble gas isotopic composition of the clinopyroxene megacryst reveals that the mantle source beneath the Kuandian area has an MORB-like reservoir with 3He/4He ratio of ～10 Ra (Ra: atmospheric 3He/4He ratio) and 40Ar/36Ar ratio of 345.6. The lherzolite xenoliths possess moderate 3He/4He ratios of 2.59-4.53 Ra, reflecting the loss of primary helium during rock deformation or metasomatism caused by enriched mantle fluids during the up-lifting. The alkali volcanics have very low 3He/4He ratios (0.47-0.61 Ra), indicating a contribution of radiogenic 4He, probably having resulted from crust contamination. Most of the samples have excess 21Ne and 22Ne as compared with atmospheric neon, but Kr and Xe isotopic compositions are indistinguishable from atmospheric values within uncertainties with only individual samples having excess 129Xe, 134Xe and 136Xe.