Depletion and enrichment processes in the lithospheric mantle beneath the Kola Peninsula (Russia): Evidence from spinel lherzolite and wehrlite xenoliths

A suite of spinel lherzolite and wehrlite xenoliths from a Devonian kimberlite dyke near Kandalaksha, Kola Peninsula, Russia, has been studied to determine the nature of the lithospheric mantle beneath the northern Baltic Shield. Olivine modal estimates and Fo content in the spinel lherzolite xenoliths reveal that the lithosphere beneath the Archaean–Proterozoic crust has some similarities to Phanerozoic lithospheric mantle elsewhere. Modal metasomatism is indicated by the presence of Ti-rich and Ti-poor phlogopite, pargasite, apatite and picroilmenite in the xenoliths. Wehrlite xenoliths are considered to represent localised high-pressure cumulates from mafic–ultramafic melts trapped within the mantle as veins or lenses. Equilibration temperatures range from 775 to 969 °C for the spinel lherzolite xenoliths and from 817 to 904 °C for the wehrlites.

Laser ablation ICP-MS data for incompatible trace elements in primary clinopyroxenes and metasomatic amphiboles from the spinel lherzolites show moderate levels of LREE enrichment. Replacement clinopyroxenes in the wehrlites are less enriched in LREE but richer in TiO₂. Fractional melt modelling for Y and Yb concentrations in clinopyroxenes from the spinel lherzolites indicates 7–8% partial melting of a primitive source. Such a volume of partial melt could be related to the 2.4–2.5 Ga intrusion of basaltic magmas (now metamorphosed to garnet granulites) in the lower crust of the northern Baltic Shield. The lithosphere beneath the Kola Peninsula has undergone several episodes of metasomatism. Both the spinel lherzolites and wehrlites were subjected to an incomplete carbonatitic metasomatic event, probably related to an early carbonatitic phase associated with the 360–380 Ma Devonian alkaline magmatism. This resulted in crystallisation of secondary clinopyroxene rims at the expense of primary orthopyroxenes, with development of secondary forsteritic olivine and apatite. Two separate metasomatic events resulted in the crystallisation of the Ti–Fe-rich amphibole, phlogopite and ilmenite in the wehrlites and the low Ti–Fe amphibole and phlogopite in the spinel lherzolites. Alternatively, a single metasomatic event with a chemically evolving melt may have produced the significant compositional differences seen in the amphibole and phlogopite between the spinel lherzolites and wehrlites. The calculated REE pattern of a melt in equilibrium with clinopyroxenes from a cpx-rich pocket is identical to that of the kimberlite host, indicating a close petrological relationship.     

Genesis and evolution of the lithospheric mantle beneath the Buffalo Head Terrane, Alberta (Canada)

Mantle xenoliths and xenocrysts were retrieved from three of the 88–86 Ma Buffalo Hills kimberlites (K6, K11, K14) for a reconnaissance study of the subcontinental lithospheric mantle (SCLM) beneath the Buffalo Head Terrane (Alberta, Canada). The xenoliths include spinel lherzolites, one garnet spinel lherzolite, garnet harzburgites, one sheared garnet lherzolite and pyroxenites. Pyroxenitic and wehrlitic garnet xenocrysts are derived primarily from the shallow mantle and lherzolitic garnet xenocrysts from the deep mantle. Harzburgite with Ca-saturated garnets is concentrated in a layer between 135–165 km depth. Garnet xenocrysts define a model conductive paleogeotherm corresponding to a heat flow of 38–39 mW/m². The sheared garnet lherzolite lies on an inflection of this geotherm and may constrain the depth of the lithosphere–asthenosphere boundary (LAB) beneath this region to ca 180 km depth.

A loss of >20% partial melt is recorded by spinel lherzolites and up to 60% by the garnet harzburgites, which may be related to lithosphere formation. The mantle was subsequently modified during at least two metasomatic events. An older metasomatic event is evident in incompatible-element enrichments in homogeneous equilibrated garnet and clinopyroxene. Silicate melt metasomatism predominated in the deep lithosphere and led to enrichments in the HFSE with minor enrichments in LREE. Metasomatism by small-volume volatile-rich melts, such as carbonatite, appears to have been more important in the shallow lithosphere and led to enrichments in LREE with minor enrichments in HFSE. An intermediate metasomatic style, possibly a signature of volatile-rich silicate melts, is also recognised. These metasomatic styles may be related through modification of a single melt during progressive interaction with the mantle. This metasomatism is suggested to have occurred during Paleoproterozoic rifting of the Buffalo Head Terrane from the neighbouring Rae Province and may be responsible for the evolution of some samples toward unradiogenic Nd and Hf isotopic compositions.

Disturbed Re–Os isotope systematics, evident in implausible model ages, were obtained in situ for sulfides in several spinel lherzolites and suggest that many sulfides are secondary (metasomatic) or mixtures of primary and secondary sulfides. Sulfide in one peridotite has unradiogenic ¹⁸⁷Os/¹⁸⁸Os and gives a model age of 1.89±0.38 Ga. This age coincides with the inferred emplacement of mafic sheets in the crust and suggests that the melts parental to the intrusions interacted with the lithospheric mantle.

A younger metasomatic event is indicated by the occurrence of sulfide-rich melt patches, unequilibrated mineral compositions and overgrowths on spinel that are Ti-, Cr- and Fe-rich but Zn-poor. Subsequent cooling is recorded by fine exsolution lamellae in the pyroxenes and by arrested mineral reactions.

If the lithosphere beneath the Buffalo Head Terrane was formed in the Archaean, any unambiguous signatures of this ancient origin may have been obliterated during these multiple events.     

Geochemistry and petrogenesis of spinel lherzolite xenoliths from Boeun,Korea

Geochemical characteristics of spinel lherzolite xenoliths, enclosed in Miocene alkali basalt from Boeun, Korea, provide important clues for understanding the lithosphere composition, equilibrium temperature and pressure conditions, and depletion and enrichment processes of subcontinental lithospheric mantle beneath Boeun. The spinel lherzolite xenoliths with protogranular to porpyroclastic textures were accidentally trapped by the ascending alkali basalt magma. The spinel lherzolite xenoliths originated at depths between 50 and 63 km with equilibrium temperatures ranging from 847 to 1030 °C. These xenoliths may have undergone small degrees (1–2%) of partial melting and cryptic metasomatism by an alkali basaltic melt. Based on Sr and Nd isotope compositions, the subcontinental lithospheric mantle beneath Boeun was heterogeneous and similar to that beneath East China and Central Mongolia rather than the Japanese Island Arc.     

Genesis of Young Lithospheric Mantle in Southeastern China: an LAM-ICPMS Trace Element Study

Geological and geophysical evidence indicates that at least100 km of Archaean to Proterozoic lithospheric mantle has beenremoved from beneath large areas of eastern and southeasternChina during late Mesozoic to Cenozoic time. Mantle-derivedxenoliths in Tertiary basalts from several localities acrossthis region have been studied by X-ray fluorescence, electronmicroprobe and laser ablation microprobe–inductively coupledplasma-mass spectrometry to characterize this thinner lithosphere.Trace element patterns of clinopyroxenes in the peridotitesfrom southeastern China can be divided into four groups: fertilegarnet lherzolites, fertile spinel (± garnet) lherzolites,and depleted and enriched peridotites. The addition of Nb, Sr,light rare earth elements, but not of Ti and Zr, suggests ametasomatizing agent containing both H₂O and CO₂. This studyalso demonstrates that the negative Ti anomaly commonly observedin clinopyroxene from mantle peridotites cannot be balancedby the Ti in coexisting orthopyroxene, but can be explainedby small degrees of partial melting, using appropriate distributioncoefficients. Most of the peridotites from southeastern China,whether spinel or garnet facies, are highly fertile in termsof Al₂O₃ and CaO contents and mg-number; many resemble commonlyused primitive mantle compositions. Modelling of trace elementpatterns in clinopyroxene indicates that most spinel and garnetperidotites from the Nushan, Mingxi and Niutoushan localitiesexperienced less than 5%, and many less than 2%, partial melting.A few depleted spinel peridotites from Nushan, and all spinelperidotites from Mingxi, require 10–25% fractional partialmelting; almost all spinel peridotites from the Qilin localityshow evidence of higher degrees (6–25%) of fractionalpartial melting. At both Nushan and Mingxi, the more depletedcompositions occur in the upper part of the lithospheric mantle,which now is     

Mantle peridotites from the Western Pacific

We review petrographical and petrological characteristics of mantle peridotite xenoliths from the Western Pacific to construct a petrologic model of the lithospheric mantle beneath the convergent plate boundary. The peridotite varies from highly depleted spinel harzburgite of low-pressure origin at the volcanic front of active arcs (Avacha of Kamchatka arc and Iraya of Luzon–Taiwan arc) to fertile spinel lherzolite of high-pressure origin at the Eurasian continental margin (from Sikhote-Alin through Korea to eastern China) through intermediate lherzolite–harzburgite at backarc side of Japan island arcs. Oxygen fugacity recorded by the peridotite xenoliths decreases from the frontal side of arc to the continental margin. The sub-arc type peridotite is expected to exist beneath the continental margin if accretion of island arc is one of the important processes for continental growth. Its absence suggests replacement by the continental lherzolite at the region of backarc to continental margin. Asthenospheric upwelling beneath the continental region, which has frequently occurred at the Western Pacific, has replaced depleted sub-cratonic peridotite with the fertile spinel lherzolite. Some of these mantle diapirs had opened backarc basins and strongly modified the lithospheric upper mantle by metasomatism and formation of Group II pyroxenites.     

Ultramafic Xenoliths From a Kamafugite Lava in Cenozoic Volcanic Field of West Qinling, China and Its Geological Implication

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

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。     

Transformation of Archaean Lithospheric Mantle by Refertilization: Evidence from Exposed Peridotites in the Western Gneiss Region, Norway

Orogenic peridotites occur enclosed in Proterozoic gneissesat several localities in the Western Gneiss Region (WGR) ofwestern Norway; garnet peridotites typically occur as discretezones within larger bodies of garnet-free, chromite-bearingdunite and are commonly closely associated with pyroxenitesand eclogites. The dunites of the large Almklovdalen peridotitebody have extremely depleted compositions (Mg-number 92–93·6);the garnet peridotites have lower Mg-number (90·6–91·7)and higher whole-rock Ca and Al contents. Post-depletion metasomatismof both rock types is indicated by variable enrichment in thelight rare earth elements, Th, Ba and Sr. The dunites can bemodelled as residues after very high degrees (>60%) of meltextraction at high pressure (5–7 GPa), inconsistent withthe preservation of lower degrees of melting in the garnet peridotites.The garnet peridotites are, therefore, interpreted as zonesof melt percolation, which resulted in refertilization of thedunites by a silicate melt rich in Fe, Ca, Al and Na, but notTi. Previous Re–Os dating gives Archaean model ages forthe dunites, but mixed Archaean and Proterozoic ages for thegarnet peridotites, suggesting that refertilization occurredin Proterozoic time. At least some Proterozoic lithosphere mayrepresent reworked and transformed Archaean lithospheric mantle. KEY WORDS: Archaean mantle; Proterozoic mantle; Western Gneiss Region, Norway; mantle metasomatism; garnet peridotite     

吉林汪清—辉南地幔透辉石微量元素及其记录的地幔熔融与交代作用

根据吉林汪清(WQ)中新世和辉南(HN)上新世玄武岩捕虏体橄榄岩透辉石的激光探针(LAM-ICPMS)微量元素分析结果,分析了华北北缘与兴蒙造山带结合部位陆下岩石圈地幔所经历的部分熔融作用和地幔交代作用.新生代时无论是靠近华北地块一侧的辉南,还是靠近兴蒙造山带一侧的汪清陆下岩石圈除少量难熔地幔残留外,主体上均表现为显生宙‘大洋型’的地幔特征,即经历了8％以下的部分熔融作用.这些地幔又记录着复杂的地幔交代作用过程,交代介质类似于含水的、含碳酸岩质的硅酸岩熔体.这样的地幔特征与中国东部其他地区新生代时的主体岩石圈相似,也有少量古老地幔残余,由此进一步说明中国东部中、新生代岩石圈减薄作用经历了软流圈地幔对古老岩石圈的侵蚀和最终的置换过程.     

Chemical Variations in Peridotite Xenoliths from Vitim, Siberia: Inferences for REE and Hf Behaviour in the Garnet-Facies Upper Mantle

Peridotite xenoliths in a Miocene picrite tuff from the Vitimvolcanic province east of Lake Baikal, Siberia, are samplesof the off-craton lithospheric mantle that span a depth rangefrom the spinel to garnet facies in a mainly fertile domain.Their major and trace element compositions show some scatter(unrelated to sampling or analytical problems), which is notconsistent with different degrees of partial melting or metasomatism.Some spinel peridotites and, to a lesser degree, garnet-bearingperidotites are depleted in heavy rare earth elements (HREE)relative to middle REE (MREE), whereas some garnet peridotitesare enriched in HREE relative to MREE, with Lu abundances muchhigher than in primitive mantle estimates. Clinopyroxenes fromseveral spinel peridotites have HREE-depleted patterns, whichare normally seen only in clinopyroxenes coexisting with garnet.Garnets in peridotites with similar modal and major elementcompositions have a broad range of Lu and Yb abundances. Overall,HREE are decoupled from MREE and Hf and are poorly correlatedwith partial melting indices. It appears that elements withhigh affinity to garnet were partially redistributed in theVitim peridotite series following partial melting, with feweffects for other elements. The Lu–Hf decoupling may disturbHf-isotope depletion ages and their correlations with meltingindices. KEY WORDS: garnet peridotite; lithospheric mantle; Lu–Hf isotope system; Siberia; trace elements     

Evidence from mantle xenoliths for relatively thin (<100 km) continental lithosphere below the Phanerozoic crust of southernmost South America

Garnet peridotite xenoliths in the Quaternary Pali-Aike alkali olivine basalts of southernmost South America are samples of the deeper portion of continental lithosphere formed by accretion along the western margin of Gondwanaland during the Phanerozoic. Core compositions of minerals in garnet peridotites indicate temperatures of 970 to 1160°C between 1.9 and 2.4 GPa, constraining a geothermal gradient which suggests a lithospheric thickness of approximately 100 km below this region. Previously, this lithosphere may have been heated and thinned to ≤80 km during the Jurassic break-up of Gondwanaland, when widespread mafic and silicic volcanism occurred in association with extension in southern South America. Subsequent cooling, by up to >175°C, and thickening, by about 20 km, of the lithosphere is reflected in low-temperature (<970°C) spinel peridotites by chemical zonation of pyroxenes involving a rimward decrease in Ca, and in moderate- and high-temperature (>970°C) peridotites by textural evidence for the transformation of spinel to garnet. A recent heating event, which probably occurred in conjunction with modal metasomatism related to the genesis of the Pali-Aike alkali olivine basalts, has again thinned the lithosphere to <100 km. Evidence for this heating is preserved in moderate- and high-temperature (>970°C) peridotites as chemical zonation of pyroxenes involving a rimward increase in Ca, and by kelyphitic rims around garnet. The majority of moderate- and high-temperature (>970°C) xenoliths are petrochemically similar to the asthenospheric source of mid-oceanic ridge basalts: fertile (>20% modal clinopyroxene and garnet), Fe-rich garnet lherzolite with major element composition similar to estimates of primitive mantle, but large-ion-lithophile and light-rare-earth element depletion relative to heavy-rare-earth elements, and with Sr, Nd, Pb, Os, and O isotopic compositions similar to MORB. In contrast, infertile, Mg-rich spinel harzburgite is predominant among low-temperature (<970°C) xenoliths. This implies a significant chemical gradient and increasing density with depth in the mantle section represented by the xenoliths, and the absence of a deep, low density, olivine-rich root below the southernmost South American crust such as has been inferred below Archean cratons. With respect to both temperature/rheology and chemistry/density, the subcontinental mantle lithosphere below southernmost South America is similar to that below oceanic crust. It is interpreted to have formed by tectonic capture, during the Paleozoic, of a segment of what had previously been oceanic lithosphere generated at a late Proterozoic mid-oceanic spreading ridge.     

Nature and Evolution of Cenozoic Lithospheric Mantle beneath Shandong Peninsula,Sino-Korean Craton,Eastern China

The Shanwang and Qixia basalts lie within the North China block and were erupted in Miocene to Pliocene time (18.1 to 4.3 Ma) and Pliocene time (6.4 to 5.9 Ma), respectively. The Shanwang area lies astride the Tancheng-Lujiang (Tanlu) fault zone, a major lithospheric fault, whereas the Qixia area lies east of the fault zone. The basaltic rocks (alkali olivine basalts, basanites, nephelinites) carry abundant deep-seated xenoliths including spinel lherzolite (dominant), dunite, and pyroxenite, and a megacryst suite including augite, anorthoclase, phlogopite, ilmenite, and garnet. Xenoliths with coarse-grained microstructures are common in the Qixia xenolith suite, but are absent in Shanwang. Reconstructed bulk compositions of the lherzolites range from relatively depleted (<3% modal diopside) to fertile (>12% modal diopside). Equilibration temperatures of 850° to 1020°C indicate entrainment of these lherzolites from depths ≤45 km, within the lithosphere; the geotherm may have been higher beneath Shanwang. The Shanwang suite contains less-depleted lherzolites, and more pyroxenites, than the Qixia suite. The chondrite-normalized REE patterns in clinopyroxenes of the Shandong xenoliths vary from LREE depleted, through concave shaped, to LREE enriched; spidergrams for the clinopyroxenes can be divided into depleted, fertile, and metasomatic types. Progressive depletion in Na and Al is accompanied by depletion in moderately incompatible elements such as Y, Yb, and Zr, and an increase in Mg#. Ti and Zr in clinopyroxenes have not been affected by the metasomatic process, and MREE have been little disturbed, whereas the light rare-earth elements, Nb, and Sr have been strongly enriched during metasomatism; this suggests that carbonate-rich fluids/melts were the metasomatic agent. The mantle beneath the Shandong Peninsula sampled by these basalts is dominantly Phanerozoic in character rather than Archean or Proterozoic lithospheric mantle. This mantle probably represents a mixture of older lithospheric mantle and newly accreted material that replaced the Archean lithospheric keel through extension, thermal erosion, and fluid/melt metasomatism. The differences in micro-structures, chemistry, temperature, and fluid/melt activity between Shanwang and Qixia are ascribed to their spatial relationships to the Tanlu fault, which is a major translithospheric suture that hasplayed an important role in the Cenozoic replacement of the pre-existing Archean lithospheric mantle.     

The Siberian lithosphere traverse: mantle terranes and the assembly of the Siberian Craton

The kimberlite fields scattered across the NE part of the Siberian Craton have been used to map the subcontinental lithospheric mantle (SCLM), as it existed during Devonian to Late Jurassic time, along a 1000-km traverse NE–SW across the Archean Magan and Anabar provinces and into the Proterozoic Olenek Province. 4100 garnets and 260 chromites from 65 kimberlites have been analysed by electron probe (major elements) and proton microprobe (trace elements). These data, and radiometric ages on the kimberlites, have been used to estimate the position of the local (paleo)geotherm and the thickness of the lithosphere, and to map the detailed distribution of specific rock types and mantle processes in space and time. A low geotherm, corresponding approximately to the 35 mW/m² conductive model of Pollack and Chapman [Tectonophysics 38, 279–296, 1977], characterised the Devonian lithosphere beneath the Magan and Anabar crustal provinces. The Devonian geotherm beneath the northern part of the area was higher, rising to near a 40 mW/m² conductive model. Areas intruded by Mesozoic kimberlites are generally characterised by this higher, but still ‘cratonic' geotherm. Lithosphere thickness at the time of kimberlite intrusion varied from ca. 190 to ca. 240 km beneath the Archean Magan and Anabar provinces, but was less (150–180 km) beneath the Proterozoic Olenek Province already in Devonian time. Thinner Devonian lithosphere (140 km) in parts of this area may be related to Riphean rifting. Near the northern end of the traverse, differences in geotherm, lithosphere thickness and composition between the Devonian Toluopka area and the nearby Mesozoic kimberlite fields suggest thinning of the lithosphere by ca. 50–60 km, related to Devonian rifting and Triassic magmatism. A major conclusion of this study is that the crustal terrane boundaries defined by geological mapping and geophysical data (extended from outcrops in the Anabar Shield) represent major lithospheric sutures, which continue through the upper mantle and juxtapose lithospheric domains that differ significantly in composition and rock-type distribution between 100 and 250 km depth. The presence of significant proportions of harzburgitic and depleted lherzolitic garnets beneath the Magan and Anabar provinces is concordant with their Archean surface geology. The lack of harzburgitic garnets, and the chemistry of the lherzolitic garnets, beneath most of the other fields are consistent with the Proterozoic surface rocks. Mantle sections for different terranes within the Archean portion of the craton show pronounced differences in bulk composition, rock-type distribution, metasomatic overprint and lithospheric thickness. These observations suggest that individual crustal terranes, of both Archean and Proterozoic age, had developed their own lithospheric roots, and that these differences were preserved during the Proterozoic assembly of the craton. Data from kimberlite fields near the main Archean–Proterozoic suture (the Billyakh Shear Zone) suggest that reworking and mixing of Archean and Proterozoic mantle was limited to a zone less than 100 km wide.     

中国大陆科学钻探CCSD-PP3钻孔地幔岩的尖晶石相部分熔融证据

CCSD PP3超镁铁岩表现为高 Mg 和 Cr,低 Ca 和 Al 特征,是中国东部和超高压变质带上极其罕见的亏损地幔岩,通过地球化学和矿物学和 Re-Os 同位素研究发现岩石来自古元古代岩石圈地幔,并在地幔浅部尖晶石相条件下发生部分熔融,最近一次部分熔融作用发生在中-晚元古代,后经岩石圈厚度加大、拆沉或深俯冲过程,岩石转变为石榴石橄榄岩。     

Petrology and geochemistry of spinel peridotite xenoliths from Hannuoba and Qixia, North China craton

We report mineralogical and chemical compositions of spinel peridotite xenoliths from two Tertiary alkali basalt localities on the Archean North China craton (Hannuoba, located in the central orogenic block, and Qixia, in the eastern block). The two peridotite suites have major element compositions that are indistinguishable from each other and reflect variable degrees (0–25%) of melt extraction from a primitive mantle source. Their compositions are markedly different from typical cratonic lithosphere, consistent with previous suggestions for removal of the Archean mantle lithosphere beneath this craton. Our previously published Os isotopic results for these samples [Earth Planet. Sci. Lett. 198 (2002) 307] show that lithosphere replacement occurred in the Paleoproterozoic beneath Hannuoba, but in the Phanerozoic beneath Qixia. Thus, we see no evidence for a compositional distinction between Proterozoic and Phanerozoic continental lithospheric mantle. The Hannuoba xenoliths equilibrated over a more extensive temperature (hence depth) interval than the Qixia xenoliths. Neither suite shows a correlation between equilibration temperature and major element composition, indicating that the lithosphere is not chemically stratified in either area. Trace element and Sr and Nd isotopic compositions of the Hannuoba xenoliths reflect recent metasomatic overprinting that is not related to the Tertiary magmatism in this area.     

Structure of the crust and uppermost mantle of the Yanshan Belt and adjacent regions at the northeastern boundary of the North China Craton from Rayleigh Wave Dispersion Analysis

We have used Rayleigh wave dispersion analysis and inversion to produce a high resolution S-wave velocity imaging profile of the crust and uppermost mantle structure beneath the northeastern boundary regions of the North China Craton (NCC). Using waveform data from 45 broadband NCISP stations, Rayleigh wave phase velocities were measured at periods from 10 to 48 s and utilized in subsequent inversions to solve for the S-wave velocity structure from 15 km down to 120 km depth. The inverted lower crust and uppermost mantle velocities, about 3.75 km/s and 4.3 km/s on average, are low compared with the global average. The Moho was constrained in the depth range of 30–40 km, indicating a typical crustal thickness along the profile. However, a thin lithosphere of no more than 100 km was imaged under a large part of the profile, decreasing to only ~ 60 km under the Inner Mongolian Axis (IMA) where an abnormally slow anomaly was observed below 60 km depth. The overall structural features of the study region resemble those of typical continental rift zones and are probably associated with the lithospheric reactivation and tectonic extension widespread in the eastern NCC during Mesozoic–Cenozoic time. Distinctly high velocities, up to ~ 4.6 km/s, were found immediately to the south of the IMA beneath the northern Yanshan Belt (YSB), extending down to > 100-km depth. The anomalous velocities are interpreted as the cratonic lithospheric lid of the region, which may have not been affected by the Mesozoic–Cenozoic deformation process as strongly as other regions in the eastern NCC. Based on our S-wave velocity structural image and other geophysical observations, we propose a possible lithosphere–asthenosphere interaction scenario at the northeastern boundary of the NCC. We speculate that significant undulations of the base of the lithosphere, which might have resulted from the uneven Mesozoic–Cenozoic lithospheric thinning, may induce mantle flows concentrating beneath the weak IMA zone. The relatively thick lithospheric lid in the northern YSB may serve as a tectonic barrier separating the on-craton and off-craton regions into different upper mantle convection systems at the present time.     

安徽女山地幔橄榄岩捕虏体的同位素组成:中国东部新生代岩石圈地幔时代制约

位于安徽省境内的女山新生代碱性玄武岩中含有大量而且类型丰富的地幔橄榄岩包体,主要类型有尖晶石相、石榴石相、尖晶石-石榴子石过渡相二辉橄榄岩以及少量的方辉橄榄岩,其中部分尖晶石二辉橄榄岩样品中出现富含挥发分的角闪石、金云母和磷灰石。本文选择该区的尖晶石二辉橄榄岩和方辉橄榄岩包体进行了较为详细的岩石学、矿物学、地球化学研究工作。结果显示,除2个方辉橄榄岩表现难熔特征外,其它25件尖晶石相二辉橄榄岩均具有饱满的主量元素组成。二辉橄榄岩样品的Sr-Nd-Hf同位素均表现为亏损地幔的性质,不同于古老克拉通型难熔、富集的岩石圈地幔。富含挥发份交代矿物的出现以及轻稀土元素不同程度的富集,表明女山岩石圈地幔经历了较为强烈的交代作用,然而Re-Os同位素及PGE分析结果表明交代作用并没有显著改变Os同位素组成。二辉橄榄岩样品均具有较高的Os同位素组成,结合其饱满的主量元素组成,亏损的同位素特征,表明女山地区岩石圈地幔整体为新生岩石圈地幔。但1个方辉橄榄岩样品给出了较低的Os同位素比值0.1184,其Re亏损年龄为1.5Ga,它可能来自于软流圈中残留的古老难熔地幔。     

中国东部部分地区新生代岩石圈地幔的成因:主量和微量元素制约

为厘定中国东部新生代岩石圈地幔的成因,进而探讨岩石圈减薄的方式,本文选择汉诺坝、女山、盘石山和方山尖晶石二辉橄榄岩作为研究对象,系统分析了其主、微量元素。结果表明,这些地区新生代岩石圈地幔具有"新生"的性质,为上涌的软流圈物质经过小于10%的部分熔融程度后的残留相形成,后又经历了类似新生代玄武岩性质的富硅熔体的交代,交代介质的物质贡献小于3%。新生代岩石圈地幔与古老克拉通岩石圈地幔在主、微量元素上的显著差异以及新生代岩石圈地幔微量元素特征的拟合,支持拆沉为华北克拉通东部减薄的主要方式。     

东北和华北地区上地幔成分和热状态对比及对中国东部岩石圈减薄的启示

地幔成分与其上覆地壳年龄存在相关关系,年龄越老,地幔越亏损玄武质组分。本文对产于东北和华北地区的尖晶石相橄榄岩包体的成分进行了统计分析,结果显示东北地区橄榄岩包体比华北地区包体更亏损玄武质组分。这说明东北岩石圈地幔比华北上地幔更难熔,但其上覆地壳年龄却远小于华北地区地壳的年龄。这种地壳年龄和地幔组分之间的解耦暗示东北和华北地区的岩石圈地幔形成之后发生了大规模的改造。华北地区的壳幔解耦与中生代岩石圈减薄和增生有关,而东北地区的壳幔解耦则是该区地壳的多期改造和中生代岩石圈减薄和增生等过程综合作用的结果。两地区地幔成分的差异显然与部分熔融程度的不同有关,但影响部分熔融程度的因素很多,目前尚不能确定。包体的平衡温度统计和地温线对比显示东北岩石圈的地温梯度低于华北的地温梯度,可能是东北地区岩石圈减薄的时间要早于华北地区,或者华北岩石圈减薄程度可能大于东北地区的结果,因此中国东部岩石圈减薄存在时空不均一性。     

Petrogenesis of the Early Jurassic Nandaling flood basalts in the Yanshan belt, North China Craton: A correlation between magmatic underplating and lithospheric thinning

Integrated geochemical and Sr–Nd–Pb isotopic studies of the Early Jurassic Nandaling flood basalts (NFB) in the Yanshan belt, northern margin of the North China Craton (NCC), are presented in this paper. These sub-alkaline basalts evolved from a more magnesium-rich parental magma through fractional crystallization of olivine and clinopyroxene. The primitive magma of the NFB originated from 2–5% partial melting of spinel to garnet transitional peridotite at about 70–80 km depth in the Mesozoic lithosphere mantle. The NFB contain a distinctive lithospheric component, characterized by Nb (Ta), Th, U and Ti depletions, LREE enrichments, moderate Sr, and low Nd and Pb initial isotopic ratios, as a result of an interaction between lower crust (15–25%) and primitive magma evoked by magmatic underplating at crust–mantle boundary. The Early Jurassic NFB extruded in an intraplate extensional setting related to post-orogenic collapse in the northern margin of the NCC, indicating an event of lithospheric modification earlier than that in the southern margin (Early Cretaceous). The temporal similarity of the Jurassic–Cretaceous mantle-derived mafic rocks to lower crust replacement, and the decoupling of surface shortening with lithospheric thinning during the Late Jurassic–Early Cretaceous, suggest the important role of magmatic underplating and subsequent crust–mantle interaction accompanied by asthenosphere upwelling on the evolution of the Mesozoic lithosphere of the NCC. The correlation between lithospheric thinning and magmatic underplating may be an important process in continental rifting.