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:a hydrous suite characterized by the ubiquitous occurrence of (Ti-) pargasite and an anhydrous suite.The nineral chemistry reveals that the anhydrous suite and one associated phlogopite-bearing lherzolite are equilibrated under temperature conditions of 1000-1100℃,whereas amplhibole-bearing peridotites display distinct disequilibrum features,indicating partial reequilibration from 1050 to 850℃ and locally down to 750℃. The amplhbole-bearing peridotites were probably the uppermost part of the high temperature anhydrous suite which was modally modifed by fractionating H2O-rich metasomatic agent during regional upwelling.This relatively recent lithospheric uplift event followed an older uplift event 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 linked with the regional extension and widespread basaltic volcanism in Jiangsu-Anhui provinces since Early Tertiary and the formation of the nearby Subei(North Jiangsu) fault-depression basin during the Eocene,while the second event in association with the formation of amphiboles probably indicates the continuation but diminution of upwared mantle flux since Neogene in response to the change in tectonic regime for eastern china.     

Thermal state of the lithosphere beneath Central Mongolia: evidence from deep-seated xenoliths from the Shavaryn-Saram volcanic centre in the Tariat depression, Hangai, Mongolia

A suite of garnet-two pyroxene granulites, garnet pyroxenites and garnet peridotites from the pyroclastic facies of the Shavaryn-Saram volcanic centre in the Tariat depression in the northern part of the Hangai dome, Central Mongolia, yields pressure and temperature information for the lower crust and upper mantle in that region. Although a real geotherm cannot be constructed because of the common zoning of the minerals in some of the xenoliths, it can be inferred that the P-T locus from about 900 °C at 45 km to 1050 °C at 60 km defines a likely approximate geothermal gradient for the region around the time of entrainment of the xenoliths (about 1 Ma ago). This geothermal gradient is high relative to cratonic geotherms but is 50–100 °C lower than that for typical alkali basaltic provinces worldwide. The crust-mantle boundary inferred from the incoming of ultramafic rock types in this region is located at about 45 km and granulite rock types extend well into the mantle. This interpretation is consistent with the most recent seismic sections for the area.

Analytical data for major and trace elements (by electron- and proton-microprobe respectively) in clinopyroxenes indicate that the Cr-diopside series xenoliths are enriched in basaltic components (including Al₂O₃, Na₂O, TiO₂, Sr, Y and Zr).

The combination of elevated temperature and fertile composition of the uppermost mantle as revealed by the xenoliths could explain the observed anomalous seismic signatures seen beneath this region.     

华北克拉通东部中生代岩石圈减薄的过程与机制:中生代火成岩和深源捕虏体证据

基于最新的同位素年代学资料 ,华北克拉通东部中生代的岩浆作用可划分成四个阶段 ,即晚三叠世 ( 2 0 5～ 2 2 5Ma)碱性岩浆作用 ;中晚侏罗世 ( 1 5 5～ 1 6 0Ma)花岗质岩浆作用 ;早白垩世 ( 1 1 2～ 1 32Ma)双峰式岩浆作用和晚白垩世 ( 92～ 73Ma)碱性玄武质岩浆作用。徐淮地区中生代侵入岩中榴辉岩捕虏体的发现及其地质年代学资料 ( 2 1 9Ma)表明 ,华北克拉通东部中生代早期曾发生过一次重要的陆壳加厚过程。俯冲板片的断离以及高压—超高压变质岩的快速折返和晚三叠世 ( 2 0 5～ 2 2 5Ma)的碱性岩浆作用的存在均暗示 ,华北克拉通中生代岩石圈减薄已经开始。拆沉作用则是引起中生代早期岩石圈减薄的主要机制。中、晚侏罗世 ( 1 5 5～ 1 6 0Ma)花岗质岩浆作用形成于造山期后的伸展环境 ,代表了中生代岩石圈减薄的继续和发展。早白垩世 ( 1 1 2～ 1 32Ma)双峰式岩浆作用表明中生代岩石圈减薄达到了峰期。而幔源纯橄岩捕虏体中富硅质熔体的交代作用和玄武岩的高87Sr/ 86Sr值、低ε(Nd ,t)值特征表明 ,软流圈对岩石圈底部的化学侵蚀可能是导致该阶段岩石圈减薄的主导机制。晚白垩世 ( 92～ 73Ma)碱性玄武质岩浆作用和“海洋型”地幔捕虏体的存在代表了等温面的下降和岩石圈地幔的增生     

Lavas and their mantle xenoliths from intracratonic Eastern Paraguay (South America Platform) and Andean Domain, NW-Argentina: a comparative review

Protogranular spinel-peridotite mantle xenoliths and their host sodic alkaline lavas of Cretaceous to Paleogene age occur at the same latitude ≈26°S in central eastern Paraguay and Andes. Na- alkaline lavas from both regions display similar geochemical features, differing mainly by higher Rb content of the Paraguayan samples. Sr, Nd, and Pb isotope ratios are also similar with predominant trends from depleted to enriched mantle components. The mantle xenoliths are divided into two main suites, i.e. relatively low in potassium and incompatible elements, and high in potassium and incompatible elements. The suite high in potassium occurs only in Paraguay. Compositions of both suites range from lherzolite to dunite indicating variable “melt extraction”. Clinopyroxenes from the xenoliths display variable trace element enrichment/depletion patterns compared with the pattern of average primitive mantle. Enrichment in LREE and Sr coupled with depletion of Nb, Ti and Zr in xenoliths from both areas are attributed to asthenospheric metasomatic fluids affecting the lithospheric mantle. Metasomatism is apparent in the sieve textures and glassy drops in clinopyroxenes, by glassy patches with associated primary carbonates in Paraguayan xenoliths. Trace element geochemistry and thermobarometric data indicate lack of interaction between xenoliths and host lavas, due to their rapid ascent. Sr and Nd isotope signatures of the Andean and Paraguayan xenoliths and host volcanic rocks plot mainly into the field of depleted mantle and show some compositional overlap. The Andean samples indicate a generally slightly more depleted mantle lithosphere. Pb isotope signatures in xenoliths and host volcanic rocks indicate the existence of a radiogenic Pb source (high U/Pb component in the source) in both areas. In spite of the distinct tectonic settings, generally compressive in the Central Andes (but extensional in a back-arc environment), and extensional in Eastern Paraguay (rifting environment in an intercratonic area), lavas and host xenoliths from both regions are similar in terms of geochemical and isotopic characteristics.     

Petrology and geochemistry of peridotite xenoliths from the Letlhakane kimberlites, Botswana

The diamondiferous Letlhakane kimberlites are intruded into the Proterozoic Magondi Belt of Botswana. Given the general correlation of diamondiferous kimberlites with Archaean cratons, the apparent tectonic setting of these kimberlites is somewhat anomalous. Xenoliths in kimberlite diatremes provide a window into the underlying crust and upper mantle and, with the aid of detailed petrological and geochemical study, can help unravel problems of tectonic setting. To provide relevant data on the deep mantle under eastern Botswana we have studied peridotite xenoliths from the Letlhakane kimberlites. The mantle-derived xenolith suite at Letlhakane includes peridotites, pyroxenites, eclogites, megacrysts, MARID and glimmerite xenoliths. Peridotite xenoliths are represented by garnet-bearing harzburgites and lherzolites as well as spinel-bearing lherzolite xenoliths. Most peridotites are coarse, but some are intensely deformed. Both garnet harzburgites and garnet lherzolites are in many cases variably metasomatised and show the introduction of metasomatic phlogopite, clinopyroxene and ilmenite. The petrography and mineral chemistry of these xenoliths are comparable to that of peridotite xenoliths from the Kaapvaal craton. Calculated temperature-depth relations show a well-developed correlation between the textures of xenoliths and P-T conditions, with the highest temperatures and pressures calculated for the deformed xenoliths. This is comparable to xenoliths from the Kaapvaal craton. However, the P-T gap evident between low-T coarse peridotites and high-T deformed peridotites from the Kaapvaal craton is not seen in the Letlhakane xenoliths. The P-T data indicate the presence of lithospheric mantle beneath Letlhakane, which is at least 150 km thick and which had a 40mW/m² continental geotherm at the time of pipe emplacement. The peridotite xenoliths were in internal Nd isotopic equilibrium at the time of pipe emplacement but a lherzolite xenolith with a relatively low calculated temperature of equilibration shows evidence for remnant isotopic disequilibrium. Both harzburgite and lherzolite xenoliths bear trace element and isotopic signatures of variously enriched mantle (low Sm/Nd, high Rb/Sr), stabilised in subcontinental lithosphere since the Archaean. It is therefore apparent that the Letlhakane kimberlites are underlain by old, cold and very thick lithosphere, probably related to the Zimbabwe craton. The eastern extremity of the Proterozoic Magondi Belt into which the kimberlites intrude is interpreted as a superficial feature not rooted in the mantle. Received: 19 March 1996 / Accepted: 16 October 1996     

Mineralogy of the upper mantle: A review of the minerals in mantle xenoliths from kimberlite

The importance of ultramafic and eclogitic xenoliths in kimberlite as representing the rocks and minerals of the upper mantle has been widely perceived during the last decade. Studies of the petrology and mineral chemistry of these mantle fragments as well as of inclusions in diamond, have led to significant progress in our understanding of the mineralogy and chemistry of the upper mantle. For example, it is now known that textural differences in the ultramafic xenoliths (lherzolite, harzburgite, pyroxenite and websterite) are partially reflected in chemical differences. Thus xenoliths that display a ‘fluidal’ texture, indicative of intense deformation are less depleted in Ca, Al, Na, Fe and Ti than those xenoliths in which granular textures are predominant. It is believed this relative depletion may indicate the sheared (fluidal texture) xenoliths are representative of primary, undifferentiated mantle. This material on partial melting would produce ‘basaltic-type’ material, and leave a residuum whose chemistry and mineralogy is reflected by the granular xenoliths.Also present in kimberlite are large single phase xenoliths that may be either one single crystal (xenocryst, megacryst) or an aggregate of several crystals of the same mineral (discrete xenolith, or discrete nodule). These large single phase samples consist of similar minerals to those occurring in the ultramafic xenoliths but chemically they are distinct in being generally more Fe-rich. The relation between these xenocrysts to their counterparts in the ultramafic xenoliths is unknown. Also unknown, at the present time, is the exact relation between diamond and kimberlite. Evidence obtained from study of the mineral inclusions in diamond suggests that diamond forms in at least two chemically distinct environments in the mantle; one eclogitic, the other, ultramafic. Interestingly, this suggestion is true for diamonds from worldwide localities.The mineral-chemical results of studies on xenoliths and inclusions in diamond have been convincingly interpreted in the light of experimental studies. It is now possible based on several different geothermometers and barometers to determine relatively reasonable physical conditions for the final genesis of many of these mantle rocks. For the most part the final equilibration temperatures range between 1000 and 1400°C and pressure in the region 100–200 km. These conditions are consistent with an upper mantle origin. Future studies will undoubtedly attempt to more concisely, and accurately, define these conditions, as well as understand better the chemical and spatial relationship of the rock-types in the mantle.     

Nature of the mantle roots beneath the North American craton: mantle xenolith evidence from Somerset Island kimberlites

The recently discovered Nikos kimberlite on Somerset Island, in the Canadian Arctic, hosts an unusually well preserved suite of mantle xenoliths dominated by garnet–peridotite (lherzolite, harzburgite, dunite) showing coarse and porphyroclastic textures, with minor garnet–pyroxenite. The whole rock and mineral data for 54 Nikos xenoliths indicate a highly refractory underlying mantle with high olivine forsterite contents (ave. Fo=92.3) and moderate to high olivine abundances (ave. 80 wt.%). These characteristics are similar to those reported for peridotites from the Archean Kaapvaal and Siberian cratons (ave. olivine Fo=92.5), but are clearly distinct from the trend defined by oceanic peridotites and mantle xenoliths in alkaline basalts and kimberlites from post-Archean continental terranes (ave. olivine Fo=91.0). The Nikos xenoliths yield pressures and temperatures of last equilibration between 20 and 55 kb and 650 and 1300°C, and a number of the peridotite nodules appear to have equilibrated in the diamond stability field. The pressure and temperature data define a conductive paleogeotherm corresponding to a surface heat flow of 44 mW/m². Paleogeotherms based on xenolith data from the central Slave province of the Canadian craton require a lower surface heat flow (40 mW/m²) indicating a cooler geothermal regime than that beneath the Canadian Arctic. A large number of kimberlite-hosted peridotites from the Kaapvaal craton in South Africa and parts of the Siberian craton are characterized by high orthopyroxene contents (ave. Kaapvaal 32 wt.%, Siberia 20 wt.%). The calculated modal mineral assemblages for the Nikos peridotites show moderate to low contents of orthopyroxene (ave. 12 wt.%), indicating that the orthopyroxene-rich mineralogy characteristic of the Kaapvaal and Siberian cratons is not a feature of the cratonic upper mantle beneath Somerset Island.     

Ophicalcites from the northern Pyrenean belt: a field,petrographic and stable isotope study

Brecciated and fractured peridotites with a carbonate matrix, referred to as ophicalcites, are common features of mantle rocks exhumed in passive margins and mid-oceanic ridges. Ophicalcites have been found in close association with massive peridotites, which form the numerous ultramafic bodies scattered along the North Pyrenean Zone (NPZ), on the northern flank of the Pyrenean belt. We present the first field, textural and stable isotopic characterization of these rocks. Our observations show that Pyrenean ophicalcites belong to three main types: (1) a wide variety of breccias composed of sorted or unsorted millimeter- to meter-sized clasts of fresh or oxidized ultramafic material, in a fine-grained calcitic matrix; (2) calcitic veins penetrating into fractured serpentine and fresh peridotite; and (3) pervasive substitution of serpentine minerals by calcite. Stable isotopic analyses (O, C) have been conducted on the carbonate matrix, veins and clasts of samples from 12 Pyrenean ultramafic bodies. We show that the Pyrenean ophicalcites are the product of three distinct genetic processes: (1) pervasive ophicalcite resulting from relatively deep and hot hydrothermal activity; (2) ophicalcites in veins resulting from tectonic fracturing and cooler hydrothermal activity; and (3) polymictic breccias resulting from sedimentary processes occurring after the exposure of subcontinental mantle as portions of the floor of basins which opened during the mid-Cretaceous. We highlight a major difference between the eastern and western Pyrenean ophicalcites belonging, respectively, to the sedimentary and to the hydrothermal types. Our data set points to a possible origin of the sedimentary ophicalcites in continental endorheic basins, but a post-depositional evolution by circulation of metamorphic fluids or an origin from relatively warm marine waters cannot be ruled out. Finally, we discuss the significance of such discrepancy in the characteristics of the NPZ ophicalcites in the frame of the variable exhumation history of the peridotites all along the Pyrenean realm.     

Geochemistry and petrology of Tertiary volcanic rocks and related ultramafic xenoliths from the central and eastern Oman Mountains

The Tertiary volcanic rocks of the central and the eastern parts of the Oman Mountains consist mainly of basanites with abundant upper mantle ultramafic xenoliths. The lavas are alkaline (42–43 wt.% SiO₂; 3.5–5.5 wt.% Na₂O + K₂O). They include primitive (11–14 wt.% MgO) features with strong OIB-like geochemical signatures. Trace element and Sr–Nd isotope data for the basanites suggest mixing of melts derived from variable degrees of melting of both garnet- and spinel lherzolite-facies mantle source. The associated xenolith suite consists mainly of spinel and Cr-bearing diopside wehrlite, lherzolite and dunite with predominantly granuloblastic textures. No significant difference in chemistry was found between the basanites and xenoliths from the central and eastern Oman Mountains, which indicate a similar mantle source. Calculated oxygen fugacity indicates equilibration of the xenoliths at − 0.43 to − 2.2 log units above the fayalite–magnetite–quartz (FMQ) buffer. Mantle xenolith equilibration temperatures range from 910–1045 + 50 °C at weakly constrained pressures between 13 and 21 kbar. Xenolith data and geophysical studies indicate that the Moho is located at a depth of  40 km. A geotherm substantially hotter (90 mW m^− 2) than the crust–mantle boundary (45 mW m^− 2) is indicated and probably relates to tectonothermal events associated with the local and regional Tertiary magmatism. The petrogenesis of the Omani Tertiary basanites is explained by partial melting of an asthenospheric mantle protolith during an extension phase predating opening of the Gulf of Aden and plume-related alkaline volcanic rocks.     

中国东部及西秦岭地区新生代岩石圈地幔中的相转变带及其温压条件

五相(橄榄石 斜方辉石 单斜辉石 石榴石 尖晶石)共存的地幔橄榄岩捕虏体是来自岩石圈地幔相转变带的直接样品。中国东部及西秦岭地区晚第三至第四纪碱性火山岩携带的少量五相共存的地幔橄榄岩捕虏体为探讨这些地区新生代岩石圈地幔中相转变带提供了宝贵的样品。本文根据地幔橄榄岩捕虏体中石榴石和尖晶石的产出状况,将这些橄榄岩捕虏体分为三类:第一类橄榄岩中尖晶石为粒状残核,尖晶石外缘被石榴石的反应边包围。这种橄榄岩捕虏体代表尖晶石一石榴石相转变带的上限,故称为尖晶石带橄榄岩;第二类橄榄岩中尖晶石和石榴石以单颗粒零散分布为特征,二者共存但未见明显的相转变关系。这类橄榄岩多位于相转变带中部,拟称为尖晶石-石榴石过渡带橄榄岩;第三类橄榄岩中以石榴石为主,尖晶石和辉石等微晶构成石榴石反应边。这类橄榄岩代表尖晶石-石榴石相转变带的下限,故称为石榴石带橄榄岩。因此,根据不同类型橄榄岩捕虏体中矿物的组成,结合温度压力估算即可确定岩石圈地幔中相转变带的深度和厚度。本文通过对中国东部及西秦岭地区晚第三至第四纪碱性火山岩携带的尖晶石-石榴石二辉橄榄岩捕虏体的温度压力估算来进一步厘定中国东部新生代岩石圈地幔中的相转变带深度和厚度。     

华北东部橄榄岩与岩石圈减薄中的地幔伸展和侵蚀置换作用

对比分析了华北不同时代捕虏体橄榄岩及其南部超高压地质体橄榄岩的矿物化学。具古老难熔岩石圈地幔特征的橄榄岩是古生代金伯利岩捕虏体和早中生代苏鲁变质带地质体的主要岩石类型。具这一性质的橄榄岩也构成了河南鹤壁上新世玄武岩捕虏体的主体部分,并可以在辽宁阜新晚中生代玄武岩中被发现。具饱满岩石圈地幔性质的橄榄岩则是阜新晚中生代火山岩、特别是郯庐断裂带(山旺)及其附近地区(栖霞)中新世玄武岩捕虏体的主要类型。从华北东部已有的捕虏体橄榄岩及地质体橄榄岩所表现出的新生饱满与古老难熔地幔的时、空分布特点,即有些地区捕虏体橄榄岩表现出不同性质地幔共存现象(如阜新、鹤壁)或橄榄石Mg#呈渐变关系看:克拉通岩石圈地幔因扬子板块俯冲所引起的早期(如早中生代)地幔伸展、和晚中生代—渐新世因太平洋俯冲所引起的热扰动的软流圈物质上涌对古老地幔产生强烈的侵蚀作用(引起岩石圈的巨大减薄);中新世以来的软流圈热沉降作用出现新生岩石圈地幔并表现为岩石圈的小幅增厚,从而实现地幔置换过程和华北东部岩石圈的整体减薄过程。岩石圈幔内薄弱带及岩石圈深断裂(如郯庐断裂带)起了软流圈物质侵蚀古老岩石圈地幔的通道作用并导引着深部物质运移和不规则减薄作用等。个别地区(如阜新)强烈的软流圈上涌于晚中生代就已经开始,显示地幔置换作用的强烈不均一性。     

Geochemistry of subalkaline and alkaline extrusives from the Kermanshah ophiolite,Zagros Suture Zone,Western Iran: implications for Tethyan plate tectonics

The Kermanshah ophiolite is a highly dismembered ophiolite complex that is located in western Iran and belongs to the Zagros orogenic system. The igneous rocks of this complex consist of both mantle and crustal suites and include peridotites (dunite and harzburgite), cumulate gabbros, diorites, and a volcanic sequence that exhibits a wide range in composition from subalkaline basalts to alkaline basalts to trachytes. The associated sedimentary rocks include a variety of Upper Triassic to Lower Cretaceous deep- and shallow-water sedimentary rocks (e.g., dolomite, limestone, and pelagic sediments, including umber). Also present are extensive units of radiolarian chert. The geochemical data clearly identifies some of the volcanic rocks to have formed from two distinct types of basaltic melts: (i) those of the subalkaline suite, which formed from an initial melt with a light rare earth elements (LREE) enriched signature and incompatible trace element patterns that suggest an island arc affinity; and (ii) those of the alkaline suite with LREE-enriched signature and incompatible trace element patterns that are virtually identical to typical oceanic island basalt (OIB) pattern. The data also suggests that the trachytes were derived from the alkaline source, with fractionation controlled by extensive removal of plagioclase and to a lesser extent clinopyroxene. The presence of compositionally diverse volcanics together with the occurrence of a variety of Triassic–Cretaceous sedimentary rocks and radiolarian chert indicate that the studied volcanic rocks from the Kermanshah ophiolite represent off-axis volcanic units that were formed in intraplate oceanic island and island arc environments in an oceanic basin. They were located on the eastern and northern flanks of one of the spreading centers of a ridge-transform fault system that connected Troodos to Oman prior to its subduction under the Eurasian plate.     

Mapping lithospheric boundaries using Os isotopes of mantle xenoliths: An example from the North China Craton

The petrology, mineral compositions, whole rock major/trace element concentrations, including highly siderophile elements, and Re-Os isotopes of 99 peridotite xenoliths from the central North China Craton were determined in order to constrain the structure and evolution of the deep lithosphere. Samples from seven Early Cretaceous-Tertiary volcanic centers display distinct geochemical characteristics from north to south. Peridotites from the northern section are generally more fertile (e.g., Al₂O₃ = 0.9-4.0%) than those from the south (e.g., Al₂O₃ = 0.2-2.2%), and have maximum whole-rock Re-depletion Os model ages (T_RD) of ∼1.8 Ga suggesting their coeval formation in the latest Paleoproterozoic. By contrast, peridotites from the south have maximum T_RD model ages that span the Archean-Proterozoic boundary (2.1-2.5 Ga). Peridotites with model ages from both groups are found at Fansi, the southernmost locality in the northern group, which likely marks a lithospheric boundary. The Neoarchean age of the lithospheric mantle in the southern section matches that of the overlying crust and likely reflects the time of amalgamation of the North China Craton via collision between the Eastern and Western blocks. The Late Paleoproterozoic (∼1.8 Ga) lithospheric mantle beneath the northern section is significantly younger than the overlying Archean crust, indicating that the original lithospheric mantle was replaced in this region, either during a major north-south continent-continent collision that occurred during assembly of the Columbia supercontinent at ∼1.8-1.9 Ga, or from extrusion of ∼1.9 Ga lithosphere from the Khondalite Belt beneath the northern Trans-North China Orogen, during the ∼1.85 Ga continental collision between Eastern and Western blocks. Post-Cretaceous heating of the southern section is indicated by high temperatures (>1000 °C) recorded in peridotites from the 4 Ma Hebi suite, which are significantly higher than the temperatures recorded in peridotites from the nearby Early Cretaceous Fushan suite (<720 °C), and likely reflects significant lithospheric thinning after the Early Cretaceous. Combining previous Os isotope results on mantle xenoliths from the eastern North China Craton with our new data, it appears that lithospheric thinning and replacement may have evolved from east to west with time, commencing before the Triassic on the eastern edge of the craton, occurring during the Jurassic-Cretaceous within the interior, and post-dating 125 Ma on the westernmost boundary.     

华北克拉通东部中生代高Mg闪长岩——对岩石圈减薄机制的制约

在华北克拉通东部鲁西—徐淮地区,存在一套辉石闪长岩-二长闪长岩-花岗闪长岩组成的adakitic岩石。锆石SHRI MP和LA-ICP-MS U-Pb定年结果表明它们形成于早白垩世(130~132Ma)。该类岩石具有较高的MgO含量(质量分数为1·46%~9·76%)、高的Mg#值(0·46~0·68)和高的Sr/Y值(主体介于30~52之间,个别高达410)。这些特征类似于由俯冲大洋板片部分熔融形成的adakitic岩石。然而,它们所表现出来的相对较高的87Sr/86Sr初始比值(0·7051~0·7077)和较低的εNd(t)值(-4·43~-15·92)则反映岩浆形成或演化过程中应有陆壳物质的参与。徐淮地区该类岩石中榴辉岩类捕虏体和石榴石捕虏晶的存在和鲁西辉石闪长岩中众多地幔橄榄岩捕虏体的发现,以及这些捕虏体中普遍发育富硅质交代作用,由此可以判定该类岩浆应起源于拆沉下部大陆地壳的部分熔融及其在上升过程中与地幔橄榄岩的反应,石榴石作为残留相。华北克拉通东部早白垩世adakitic岩石的存在以及榴辉岩类捕虏体的年代学表明,中生代早期曾存在一次重要的陆壳加厚过程,之后相继出现的加厚岩石圈的拆沉应是中生代岩石圈减薄的主导机制。     

Metasomatic Mantle Xenoliths from the Bismarck Microplate (Papua New Guinea)--Thermal Evolution, Geochemistry and Extent of Slab-induced Metasomatism

A suite of ultramafic mantle xenoliths from the TUBAF and EDISONseamounts in the Bismarck Archipelago NE of Papua New Guineawas sampled by video-guided grab. The xenoliths, which weretransported to the sea floor by rift-related, Quaternary trachybasalts,mainly represent part of the oceanic mantle. Mineral zoningin peridotite xenoliths testifies to slow cooling after mantleformation at a mid-ocean ridge. Cooling rates in the range of1°C/Ma were calculated from zoning of Ca in olivine usingthe Lasaga algorithm. Subsequent to this cooling, a strong metasomatismaffected the mantle peridotites when metasomatic agents emergedfrom the underlying slab of a subduction zone, which was stalledabout 15 my ago. This resulted in the formation of orthopyroxene-,clinopyroxene-, phlogopite- and hornblende-bearing veins crosscuttingspinel peridotites and olivine clinopyroxenites, as well aspervasively metasomatized plagioclase lherzolites. The metasomaticxenoliths reveal strong chemical disequilibria between the metasomaticminerals and the adjacent, unaltered host rock minerals, whichare especially prominent in the veined samples. Temperaturesduring the metasomatic overprint, estimated using spinel–olivinethermometry, range between 660 and 950°C. Oxygen barometryreveals an elevated oxygen fugacity, with     

Petrological significance of high-pressure ultramafic xenoliths from ultrapotassic rocks of Central Italy

Two suites of ultramafic xenoliths have been found in ultrapotassic lavas from the 0.9 Ma old Torre Alfina volcano sited at the northern border of the Vulsinian district (Central Italy). One group of Xenoliths consists of spinel-bearing lherzolites, harzburgites, minor wherlites and dunites with a maximum size of 3–4 cm. Some samples contain discrete laths of phlogopite. A second class consists of phlogopite-rich, glass-bearing peridotites. The first suite displays textural characteristics such as triple points, deformed olivine with well developed kink banding and porphyroclastic textures indicating equilibration at high pressure. Pressure estimates give values in the range 1.3–2.5 GPa, corresponding to mantle depths in the area, where the present-day Moho is about 25 km deep. Equilibration temperatures have been estimated in the range between 950–1000°C. The chemical composition of some phases, such as the very high Fo contents of olivines (up to Fo₉₄ in harzburgites), Mg content of orthopyroxenes and Cr/Cr+Al ratios of clinopyroxenes and spinels, suggest that these xenoliths represent peridotites which suffered different degrees of partial melting before being incorporated into the Torre Alfina magma. On the other hand, the occurrence of phlogopite speaks for metasomatic events. The phlogopite-rich, glass-bearing xenoliths consist of phlogopite, olivine, clinopyroxene, rare orthopyroxene and glass. Apatite is the most common accessory. Olivine is present in both euhedral and strained crystals. A few relics of protogranular textures are also observed. Textural and chemical evidence suggests that these xenoliths represent mica-rich peridotites which have undergone phlogopite breakdown during rapid rise to the surface with the development of a K-rich liquid which reacted with mafic phases producing a rapid growth of olivine and, to a lower extent, pyroxene. Originally, these xenoliths may have represented intensively metasomatized upper mantle. However, a cumulitic origin from previous potassic magmatic events cannot be excluded. The host lavas have compositions intermediate between high-silica lamproite and Roman-type ultrapotassic rock. They have high abundances of incompatible elements and radiogenic Sr, coupled with high Mg content, MgO/CaO, Ni and Cr. These features support a genesis in a residual upper mantle which has suffered partial melting with the extraction of basaltic liquids, followed by metasomatic events which caused an enrichment in incompatible elements and radiogenic Sr. The presence of mantle-derived ultramafic xenoliths in the torre Alfina lavas testifies for a rapid uprise of the magma which reached the surface without suffering fractional crystallization and significant interaction with the upper crust. Accordingly, the Torre Alfina lavas represent an unique example of primitive potassic liquid in Central Italy.     

The Hyblean xenolith suite (Sicily): an unexpected legacy of the Ionian–Tethys realm

The extensive study of a great number of deep-seated xenoliths from Tortonian tuff-breccia pipes in the Hyblean area (Sicily) revealed the following fundamental evidence: (1) typical continental crust rocks are completely absent in the entire xenolith suite; (2) mantle ultramafics are more abundant than gabbroids; (3) sheared oxide–gabbros, closely resembling those from oceanic fracture zones, are relatively common; (4) secondary mineral assemblages, compatible with alteration processes in serpentinite-hosted hydrothermal systems, occur both in peridotites and gabbros. Among the products of this hydrothermal activity, organic compounds, having abiotic origin via Fischer–Tropsch synthesis, occur in some hydrothermally altered gabbro and ultramafic xenoliths, as well as in hydrothermal clays. Moreover, the U–Pb dating of hydrothermal zircon grains, hosted in a xenolith of metasomatized tectonic breccia, indicated an Early–Middle Triassic age of the fossil hydrothermal system. Another line of evidence for the oceanic nature of the Hyblean–Pelagian basement is the complete absence of continental crust lithologies (granites, felsic metaigneous, and metasedimentary rocks) in outcrops and in boreholes, and the oceanic affinity of the Tertiary volcanic rocks from the Hyblean Plateau and the Sicily Channel (Pantelleria and Linosa Islands), which lack of any geochemical signature for continental crust contamination. A reappraisal of existing geophysical data pointed out that serpentinites form the dominant lithologies in the lithospheric basement of the Hyblean–Pelagian area down to a mean depth of 19 km, which represents the regional Moho considered as the serpentinization front, marking the transition from serpentinites to unaltered peridotites. On these grounds, we confirm that Hyblean xenoliths contain mineralogical, compositional, and textural evidence for tectonic, magmatic, and hydrothermal processes indicating the existence of fossil oceanic core complexes, in the geotectonic framework of the Paleo–Mesozoic, ultra-slow spreading, Ionian–Tethys Ocean forming the present Ionian–Hyblean–Pelagian domain.     

Ultramafic complexes and associated mineral deposits in the Precambrian of eastern Minas Gerais,Brazil

Based on field work, prospection and petrographic studies ultramafic complexes and their mineral deposits from eastern Minas Gerais in southeastern Brazil are generally outlined to form a basis for future investigations in this region. The bodies dealt with occur at Ipanema, Córrego Novo, Bela Vista de Minas, Rio Pomba and Liberdade. These ultramafic bodies are generally enclosed in high-grade gneisses and consist of serpentinized peridotites and harzburgites which were metamorphosed together with their country rocks in upper amphibolite to granulite facies. Weathering of these rocks gives rise to nickeliferous laterite, while metamorphism has resulted in anthophyllite asbestos and talc deposits.     

Formation and Modification of the Shallow Sub-continental Lithospheric Mantle: a Review of Geochemical Evidence from Ultramafic Xenolith Suites and Tectonically Emplaced Ultramafic Massifs of Western and Central Europe

The petrology and geochemistry of shallow continental lithosphericmantle (SCLM) can be studied via (1) tectonically emplaced ultramaficmassifs and (2) mantle xenoliths entrained in alkaline magmas.Data from these two separate sources are used to identify processesthat have formed and modified the SCLM. In western and centralEurope where the continental crust consolidated in Phanerozoictimes, both sources of information are available for study.Rock types found in ultramafic massifs in Europe are generallysimilar to those found in ultramafic xenolith suites. The mostfrequent lithology is anhydrous spinel lherzolite, grading towardsharzburgite. Massifs reveal pyroxenite layering, harzburgitebands and cross-cutting mafic and ultramafic dykes. The PhanerozoicEuropean SCLM xenoliths and massifs show broad mineralogicaland chemical similarities to Phanerozoic continental spinelperidotites world-wide. The main process that controls the geochemistryof the SCLM is depletion by removal of basaltic melt. Differencesfrom this norm reflect significantly different processes inthe SCLM, such as interaction with melts and fluids. Such processesprobably gave rise to hornblendite veins and pyroxenite layers,although the latter have also been interpreted as recycled oceaniccrust. Rare earth element data for whole-rock peridotites andtheir constituent clinopyroxenes show a variety of patterns,including light rare earth element (LREE) depletion as a resultof removal of basaltic melt, LREE enrichment caused by metasomatism,and U-shaped REE patterns that are probably due to interactionwith carbonatite melts. Extended mantle-normalized incompatibletrace element patterns for whole rocks show enrichment in Rband Ba in peridotites considered to have been subduction-metasomatized,whereas those considered to be carbonate-metasomatized havestrong negative anomalies in Zr, Nb and Hf. Mantle amphibolesare strongly enriched in LREE when found in veins, but can beLREE depleted if they are interstitial. Radiogenic isotope ratiosfor xenoliths and massifs largely overlap, although the xenolithsshow a significant clustering around a ‘plume-component’identical to the Neogene alkaline magmatism of Europe. Thiscomponent is lacking in the massifs, most of which were emplacedinto the crust before the onset of Neogene plume activity. Infiltrationof carbonatite melts is observed petrographically in some xenolithsand evidenced by low Ti/Eu ratios in bulk rocks, but is veryrare. The effect of passage of hydrous fluids from subductingslabs is also seen in some suites and massifs, being exhibitedmainly as unusual Sr and Pb isotope ratios, although enrichmentin K, Rb and Ba, and the presence of modal phlogopite, may alsopoint to subduction-metasomatism. KEY WORDS: peridotites; xenoliths; orogenic massifs; Europe     

Trace element compositions of minerals in garnet and spinel peridotite xenoliths from the Vitim volcanic field, Transbaikalia, eastern Siberia

Peridotite xenoliths from the Bereya alkali picrite tuff in the Vitim volcanic province of Transbaikalia consist of garnet lherzolite, garnet–spinel lherzolite and spinel lherzolite varieties. The volcanism is related to the Cenozoic Baikal Rift. All peridotites come from pressures of 20–23 kbar close to the garnet to spinel peridotite transition depth, and the presence of garnet can be attributed to cooling of spinel peridotites, probably during formation of the lithosphere. The peridotites show petrographic and mineral chemical evidence for infiltration by an alkaline silicate melt shortly before their transport to the Earth's surface. The melt infiltration event is indicated petrographically by clinopyroxenes which mimic melt morphologies, and post-dates outer kelyphitic rims on garnets which are attributed to an isochemical heating event within the mantle before transport to the Earth's surface. Single-mineral thermometry gives reasonable temperature estimates of 1050±50°C, whereas two-mineral methods involving clinopyroxene are falsified by secondary components in clinopyroxene introduced during the melt infiltration event. Excimer Laser–ICP-MS analysis has been performed for an extensive palette of both incompatible and compatible trace elements, and manifests the most thorough dataset available for this rock type. Orthopyroxene and garnet show only partial equilibration of trace elements with the infiltrating melt, whereas clinopyroxene and amphibole are close to equilibration with the melt and with each other. The incompatible element composition of the infiltrating melt calculated from the clinopyroxene and amphibole analyses via experimental mineral/melt partition coefficients is similar to the host alkali picrite, and probably represents a low melt fraction from a similar source during rift propagation. The chemistry and chronology of the events recorded in the xenoliths delineates the series of events expected during the influence of an expanding rift region in the upper mantle, namely the progressive erosion of the lithosphere and the episodic upward and outward propagation of melts, resulting in the evolution of the Vitim volcanic field.