The role of eclogites in the redistribution of water in the subcontinental mantle of the Siberian craton: results of determination of the water content in minerals from the Udachnaya pipe eclogites

A comprehensive study of 26 mafic mantle xenoliths from the Udachnaya kimberlite pipe was carried out. The contents of major and trace elements, equilibrium temperature parameters, and water content in the rock-forming minerals were determined. The temperatures of formation of the studied rocks are estimated at 800–1300 °C. According to IR spectroscopy data, the water content in clinopyroxenes from the studied eclogites varies from values below the detection limit to 99 ppm. The IR spectra of garnets lack bands of water. The water content in clinopyroxene and orthopyroxene from garnet websterite is 72 and 8 ppm, respectively. The water content in the average rock, calculated from the ratio of the rock-forming minerals, varies from a few to 55 ppm. No relationship among the water content, equilibrium temperatures, and rock composition is established. The low water contents in the eclogites are close to the earlier determined water contents in peridotites from the same pipe and are, most likely, due to the re-equilibration of the eclogites with the rocks of the peridotitic lithospheric mantle. The dehydration of the protolith during its subduction and the partial melting of eclogites before their removal by kimberlitic magma to the surface might be an additional cause of the low water contents in the mantle eclogite xenoliths.     

The evolution of refertilized lithospheric mantle beneath the northeastern Siberian craton: Links between mantle metasomatism,thermal state and diamond potential

Although the diamond potential of cratons is linked mainly to thick and depleted Archean lithospheric keels, there are examples of craton-edge locations and circum-cratonic Proterozoic terranes underlain by diamondiferous mantle. Here, we use the results of comprehensive major and trace-element studies of detrital garnets from diamond-rich Late Triassic (Carnian) sedimentary rocks in the northeastern Siberia to constrain the thermal and chemical state of the pre-Triassic mantle and its ability to sustain the diamond storage. The studied detrital mantle-derived garnets are dominated by low- to medium-Cr lherzolitic (～45%) and low-Cr megacrystic (～39%) chemistries, with a significant proportion of eclogitic garnets (～11%), and only subordinate contribution from harzburgitic garnets (～5%) with variable Cr₂O₃ contents (1.2–8.4 wt.%). Low-Cr megacrysts display uniform, “normal” rare-earth element (REE) patterns with no Eu/Eu* anomalies, systematic Zr and Ti enrichment (mainly within 2.5–5), which are evidence of their crystallization from deep metasomatic melts. Lherzolitic (G9) garnets exhibit normal or humped to MREE-depleted sinusoidal REE patterns and elevated Nd/Y (up to 0.33–0.41) and Zr/Y ratios (up to 7.62). Rare low- to high-Cr harzburgitic (G10) garnets have primarily “depleted”, sinusoidal REE-patterns, low Ti, Y and HREE, but vary significantly in Zr-Hf, Ti and MREE-HREE contents, Nd/Y (within 0.1–2.4) and Zr/Y (1.53–19.9) ratios. The observed trends of chemical enrichment from the most depleted, harzburgitic garnets towards lherzolitic (including high-Ti high-Cr G11-type) garnets and megacrysts result from either voluminous high-temperature metasomatism by plume-derived silicate melts or recurrent mobilization of less voluminous kimberlitic or related carbonated mantle melts, rather than the initially primitive, fertile nature of the Proterozoic SCLM. Calculated Ni-in-garnet temperatures (primarily within ～1150–1250 °C) indicate their derivation from at least ～220 km thick Cr-undersaturated lithosphere at the relevant Devonian to Triassic thermal flow of ～45 mW/m² or cooler. We suggest the existence of rare harzburgitic domains in the primarily lherzolitic diamond-facies SCLM beneath the northeastern Siberian craton at least by Triassic, whereas the abundance of eclogitic garnets, predominance of E-type inclusions in placer diamonds and specific morphologies argue for diamondiferous eclogites occurring within a ～50–65 kbar diamond window of the Olenek province by the same time.     

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。     

Lithospheric mantle beneath the south-eastern Siberian craton: petrology of peridotite xenoliths in basalts from the Tokinsky Stanovik

We provide petrographic, major and trace element data for over 30 spinel peridotite xenoliths from the Tokinsky Stanovik (Tok) volcanic field on the Aldan shield to characterize the lithospheric mantle beneath the south-eastern margin of the Siberian craton, which formed in the Mesoproterozoic. High equilibration temperatures (870–1,010°C) of the xenoliths and the absence of garnet-bearing peridotites indicate a much thinner lithosphere than in the central craton. Most common among the xenoliths are clinopyroxene-poor lherzolites and harzburgites with Al₂O₃ and CaO contents nearly as low as in refractory xenoliths from kimberlite pipes (Mir, Udachnaya) in the central and northern Siberian craton. By contrast, the Tok peridotites have higher FeO, lower Mg-numbers and lower modal orthopyroxene and are apparently formed by shallow partial melting (3 GPa). Nearly all Tok xenoliths yield petrographic and chemical evidence for metasomatism: accessory phlogopite, amphibole, phosphates, feldspar and Ti-rich oxides, very high Na₂O (2–3.1%) in clinopyroxene, LREE enrichments in whole-rocks.Electronic Supplementary Material Supplementary material is available for this article at     

2D model of the crust and uppermost mantle along rift profile, Siberian craton

A two-dimensional model of the crust and uppermost mantle for the western Siberian craton and the adjoining areas of the Pur-Gedan basin to the north and Baikal Rift zone to the south is determined from travel time data from recordings of 30 chemical explosions and three nuclear explosions along the RIFT deep seismic sounding profile. This velocity model shows strong lateral variations in the crust and sub-Moho structure both within the craton and between the craton and the surrounding region. The Pur-Gedan basin has a 15-km thick, low-velocity sediment layer overlying a 25-km thick, high-velocity crystalline crustal layer. A paleo-rift zone with a graben-like structure in the basement and a high-velocity crustal intrusion or mantle upward exists beneath the southern part of the Pur-Gedan basin. The sedimentary layer is thin or non-existent and there is a velocity reversal in the upper crust beneath the Yenisey Zone. The Siberian craton has nearly uniform crustal thickness of 40–43 km but the average velocity in the lower crust in the north is higher (6.8–6.9 km/s) than in the south (6.6 km/s). The crust beneath the Baikal Rift zone is 35 km thick and has an average crustal velocity similar to that observed beneath the southern part of craton. The uppermost mantle velocity varies from 8.0 to 8.1 km/s beneath the young West Siberian platform and Baikal Rift zone to 8.1–8.5 km/s beneath the Siberian craton. Anomalous high Pn velocities (8.4–8.5 km/s) are observed beneath the western Tunguss basin in the northern part of the craton and beneath the southern part of the Siberian craton, but lower Pn velocities (8.1 km/s) are observed beneath the Low Angara basin in the central part of the craton. At about 100 km depth beneath the craton, there is a velocity inversion with a strong reflecting interface at its base. Some reflectors are also distinguished within the upper mantle at depth between 230 and 350 km.     

Prospects of search for diamondiferous kimberlites in the northeastern Siberian Platform

The objects of study are Triassic hypabyssal diamondiferous kimberlites with an age of 220-245 Ma, containing macrocrysts of unaltered olivine. The latter are close in the time of formation to the main stage of intrusion of the Siberian Trap Province (252 Ma), which lasted less than 1 Myr. A comparative high-precision analytical study of the Ti, Ca, Cr, and Al impurity patterns in about 1000 olivine macrocryst samples with a forsterite content Fo = (100Mg/(Mg + Fe)) of 78 to 93 has demonstrated the effect of traps on the lithospheric composition. A comprehensive comparative study of diamonds from northern placers and Triassic kimberlites, including determination of their carbon isotope composition, was performed. Chromatography-mass-spectroscopic analysis of submicron fluid inclusions in diamonds from northern placers and kimberlites has shown predominant hydrocarbons of a wide range of compositions and subordinate contents of N₂, H₂O, and CO₂. These findings, together with the results of previous studies of subcalcic Cr-pyropes and diamonds found in the Lower Carboniferous gritstones of the Kyutyungde graben, lead to the conclusion that the Toluopka kimberlite field is promising for Paleozoic kimberlites. The results of comprehensive studies of diamonds and indicator minerals and U/Pb isotope dating of numerous detrital zircon samples from the basal horizon of the Carnian Stage (Upper Triassic) of the Bulkur site in the lower reaches of the Lena River suggest the presence of diamondiferous kimberlites within the northeastern Siberian Platform. The age of the probable primary diamond sources in the study area can be evaluated by an integrated U/Pb isotope dating of zircons, perovskites, and rutiles from the developed diamond placers and the basal horizon of the Carnian Stage.     

Secular evolution of the lithospheric mantle beneath the eastern North China craton: evidence from peridotitic xenoliths from Late Cretaceous mafic rocks in the Jiaodong region,east-central China

The Mesozoic lithospheric mantle beneath the North China craton remains poorly constrained relative to its Palaeozoic and Cenozoic counterparts due to a lack of mantle xenoliths in volcanic rocks. Available data show that the Mesozoic lithospheric mantle was distinctive in terms of its major, trace element, and isotopic compositions. The recent discovery of mantle peridotitic xenoliths in Late Cretaceous mafic rocks in the Jiaodong region provides an opportunity to further quantify the nature and secular evolution of the Mesozoic lithospheric mantle beneath the region. These peridotitic xenoliths are all spinel-facies nodules and two groups, high-Mg# and low-Mg# types, can be distinguished based on textural and mineralogical features. High-Mg# peridotites have inequigranular textures, high Mg# (up to 92.2) in olivines, and high Cr# (up to 55) in spinels. Clinopyroxenes in the high-Mg# peridotites are generally LREE-enriched ((La/Yb)_N>1) with variable REE concentrations, and have enriched Sr–Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr = 0.7046–0.7087; ¹⁴³Nd/¹⁴⁴Nd = 0.5121–0.5126). We suggest that the high-Mg# peridotites are fragments of the Archaean and/or Proterozoic lithospheric mantle that underwent extensive interaction with both carbonatitic and silicate melts prior to or during Mesozoic time. The low-Mg# peridotites are equigranular, are typified by low Mg# ( < 90) in olivines, and by low Cr# ( < 12) in spinels. Clinopyroxenes from low-Mg# peridotites have low REE abundances (ΣREE = 12 ppm), LREE-depleted REE patterns ((La/Yb)_N < 1), and depleted Sr–Nd isotopic features, in contrast to the high-Mg# peridotites. These geochemical characteristics suggest that the low-Mg# peridotites represent samples from the newly accreted lithospheric mantle. Combined with the data of mantle xenoliths from the Junan and Daxizhuang areas, a highly heterogeneous, secular evolution of the lithosphere is inferred for the region in Late Cretaceous time.     

长江中下游地区蝌蚪山晚中生代玄武岩的地球化学研究: 岩石圈地幔性质与演化的制约

长江中下游地区繁昌火山盆地蝌蚪山晚白垩世玄武岩为硅饱和岩石, SiO2含量在 47.63%～ 50.02%之间,在 TAS图上位于碱性和亚碱性的分界线上,多数属于玄武岩,少数为粗面玄武岩. MgO含量较低( 3.72%～ 5.58%),但 Mg#值较高( 61～ 71).岩石富集大离子亲石元素 Ba、 Th、 U、 LREE和 Pb,亏损高场强元素 Nb、 Zr和 Ti,具富集的 Sr、 Nd和 Pb同位素组成.初始 87Sr/86Sr(t)比值介于 0.706 5～ 0.706 6之间,ε Nd(t)值介于-5.5～-7.3之间.在 87Sr/86Sr(t)-ε Nd(t)相关图上投在富集的第Ⅳ象限,并趋向于 EMⅡ地幔端员. 206Pb/204Pb(t)、 207Pb/204Pb(t)和 208Pb/204Pb(t)比值分别为 17.928～ 18.311、 15.426～ 15.621和 37.785～ 38.525,在 Pb同位素相关图上处在 DMM和 EMⅡ地幔端员之间.蝌蚪山玄武岩的地球化学特征表明其原始岩浆来源于岩石圈地幔,底侵到壳幔边界后曾有一段时间的滞留,并经历了一定程度的以橄榄石和斜方辉石为主的结晶分异,随后在上升过程中没有受到明显的地壳物质的混染.本地区晚中生代时期陆下岩石圈地幔具有同位素富集的性质,曾受到过古老俯冲事件中析出流体 /熔体的交代.与本地区新生代玄武岩地球化学性质的对比表明,从晚中生代至新生代,由于岩石圈拉张和软流圈的上涌,长江中下游地区发生了岩石圈的减薄,其过程和华北地块东部的岩石圈减薄事件基本一致.造成华北地块东部和华南地块东部的岩石圈减薄具有一致的动力学机制和背景,很可能是晚中生代时期古太平洋板块向亚洲大陆之下俯冲造成的弧后盆地的拉张减薄.     

Historical seismicity on the southern margin of the Siberian craton: new data

We discuss historical evidence for seismicity on the southern margin of the Siberian craton collected from old local newspapers. The reported earthquakes vary in magnitude from M = 2.5 to 4.5, and their macroseismic locations agree well with the regional tectonic framework. The new data prove seismic activity in the area and can be used in seismic risk assessment.     

Paleoproterozoic accretion in the Northeast Siberian craton: Isotopic dating of the Anabar collision system

Geochronological database considered in the work and characterizing the Anabar collision system in the Northeast Siberian craton includes coordinated results of Sm-Nd and Rb-Sr dating of samples from crustal xenoliths in kimberlites, deep drill holes, and bedrock outcrops. As is inferred, collision developed in three stages dated at 2200–2100, 1940–1760, and 1710–1630 Ma. The age of 2000–1960 Ma is established for substratum of mafic rocks, which probably originated during the lower crust interaction with asthenosphere due to the local collapse of the collision prism. Comparison of Sm-Nd and Rb-Sr isochron dates shows that the system cooling from ≈700 to ≈300°C lasted approximately 300 m.y. with a substantial lag relative to collision metamorphism and granite formation. It is assumed that accretion of the Siberian craton resulted in formation of a giant collision mountainous structure of the Himalayan type that was eroded by 1.65 Ga ago, when accumulation of gently dipping Meso-to Neoproterozoic (Riphean) platform cover commenced.     

橄榄岩-熔体的相互作用:岩石圈地幔组成转变的重要方式

橄榄岩-熔体/岩浆的相互作用常被用来解释蛇绿岩套橄榄岩、造山带橄榄岩、超镁铁质侵入杂岩体、地幔橄榄岩捕虏体中某些具有不平衡结构和矿物组成的岩石的形成过程。橄榄岩-熔体的反应主要有两种方式,即消耗橄榄石(和单斜辉石)生成斜方辉石或消耗斜方辉石生成橄榄石(和单斜辉石)。反应的结果不仅造成矿物百分含量的变化,而且造成矿物组成的变化;后者更重要但未引起足够的重视。华北东部中生代玄武质岩石中具有环带状结构的橄榄石和辉石捕虏晶,特别是具有环带状结构的地幔橄榄岩捕虏体的发现,暗示这种橄榄岩-熔体的相互作用在华北东南部中生代岩石圈地幔中很可能普遍存在,为岩石圈地幔组成转变和快速富集的重要方式。这是全球首例由橄榄岩-熔体相互反应造成的岩石圈地幔大规模的组成变化。反应熔体来源途径主要有地壳来源和软流圈地幔来源。来源不同的熔体与橄榄岩的反应造成的组成变化完全不同。     

Li-Sr-Nd isotope signatures of the plume and cratonic lithospheric mantle beneath the margin of the rifted Tanzanian craton (Labait)

Lithium concentrations and isotopic compositions of olivine and ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd of coexisting clinopyroxene from peridotite xenoliths from the Quaternary Labait volcano, Tanzania, document the influence of rift-related metasomatism on the ancient cratonic mantle. Olivines show negative correlations between Fo content and both δ⁷Li and Li concentrations. Olivines in iron-rich peridotites (Fo_85–87) have high Li concentrations (3.2–4.8 ppm) and heavy δ⁷Li (+5.2 to +6.6). In contrast, olivines in ancient, refractory peridotites have lower Li concentrations (∼2 ppm) and relatively light δ⁷Li (+2.6 to +3.5). This reflects mixing between ancient, refractory cratonic lithosphere and asthenosphere-derived rift magmas. A uniquely fertile, deformed, high-temperature garnet lherzolite, interpreted to be from the base of the lithosphere, has a ⁸⁷Sr/⁸⁶Sr of 0.7029 and ¹⁴³Nd/¹⁴⁴Nd of 0.51286, similar to HIMU oceanic basalts. It provides the best estimate of the Sr–Nd isotope composition of the upwelling mantle (i.e., plume, sensu lato) underlying this portion of the East African Rift, and is slightly less radiogenic compared to previous estimates of the plume that were based on rift basalts. Although elevated δ⁷Li are not exclusive to HIMU source regions, the data collectively indicate that the plume beneath Labait has HIMU characteristics in Sr, Nd and Li isotope composition. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Neoproterozoic zircon inheritance in eastern North China craton (China) Mesozoic igneous rocks: derivation from the Yangtze craton and tectonic implications

We propose that inherited Neoproterozoic zircons in Mesozoic igneous rocks from the eastern portion of the North China craton (NCC) were initially derived from the Yangtze/South China block, rather than from the NCC itself. The mechanism that introduced these zircons into the NCC was likely tectonic underplating during Triassic continental subduction/collision of the Yangtze block beneath the NCC. The addition of abundant crustal materials represented by the exotic zircons, probably along the Moho or weak interfaces within the NCC crust, led to the crustal thickening of the NCC. These sialic materials contributed significantly to the Mesozoic igneous rocks, either as source rocks or as contaminants of magmas generated during an extensional environment following crustal thickening. Crustal thickening was spatially linked to lithospheric thinning, with both occurring mainly in the eastern segment of the NCC, suggestive of an intrinsic relationship between thickening and thinning events during Mesozoic evolution of the NCC.     

Oxidation of the Kaapvaal lithospheric mantle driven by metasomatism

The oxidation state, reflected in the oxygen fugacity (fO₂), of the subcratonic lithospheric mantle is laterally and vertically heterogeneous. In the garnet stability field, the Kaapvaal lithospheric mantle becomes progressively more reducing with increasing depth from Δlog fO₂ FMQ-2 at 110 km to FMQ-4 at 210 km. Oxidation accompanying metasomatism has obscured this crystal-chemical controlled depth-fO₂ trend in the mantle beneath Kimberley, South Africa. Chondrite normalized REE patterns for garnets, preserve evidence of a range in metasomatic enrichment from mild metasomatism in harzburgites to extensive metasomatism by LREE-enriched fluids and melts with fairly unfractionated LREE/HREE ratios in phlogopite-bearing lherzolites. The metasomatized xenoliths record redox conditions extending up to Δlog fO₂ = FMQ, sufficiently oxidized that magnesite would be the stable host of carbon in the most metasomatized samples. The most oxidized lherzolites, those in or near the carbonate stability field, have the greatest modal abundance of phlogopite and clinopyroxene. Clinopyroxene is modally less abundant or absent in the most reduced peridotite samples. The infiltration of metasomatic fluids/melts into diamondiferous lithospheric mantle beneath the Kaapvaal craton converted reduced, anhydrous harzburgite into variably oxidized phlogopite-bearing lherzolite. Locally, portions of the lithospheric mantle were metasomatized and oxidized to an extent that conversion of diamond into carbonate should have occurred. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

华北东部中、新生代岩石圈地幔的不均一性：来自橄榄石的组成填图结果

华北中、新生代玄武质火山岩和基性脉岩携带的地慢橄榄岩捕虏体中橄榄石和／或橄榄石捕虏晶系统的组成填图显示华北东部中、新生代岩石圈地幔存在明显的时空分布规律和不均一性。这与通过岩石圈地幔源基性岩石的地球化学反演获得的华北中生代岩石圈地幔的时空不均一性及其块体特征完全一致。太行山和鲁皖地区新生代岩石圈地慢的差异演化主要反映古老地幔橄榄岩与熔体相互作用时熔体性质和来源的不同。同时,橄榄石Fo填图还揭示了郯庐断裂对华北东部中、新生代基性岩浆活动及其岩石圈地幔演化的重要制约作用。而且,华北东部中生代岩石圈减薄后尚存古老岩石圈地幔残留。因此,华北东部岩石圈减薄的整体拆沉模式很难成立。     

北部湾涠洲岛橄榄岩中熔体包裹体——大陆岩石圈地幔交代作用的新证据

根据北部湾涠洲岛晚更新世火山岩中尖晶石二辉橄榄岩的矿物化学和形成条件推测,它们是来自地下约40km-50km的大陆岩石圈地幔样品。涠洲岛地幔橄榄岩中同时出现高硅（64％～68％）和低硅（49％～57％）两种熔体成分,它们普遍富碱、Al2O3和H2O、CO2等挥发组分（2％-5％）。研究认为,两种熔体存在不同的成因机制。高硅熔体可能与软流圈上升的玄武质岩浆和斜方辉石反应或下地壳物质循环进入地幔后的部分熔融有关。低硅熔体直接提供了富硅熔体与橄榄石发生交代作用的证据。这可以用富SiO2熔体＋Ol→贫SiO2熔体＋Opx反应解释,其结果是消耗橄榄石生成斜方辉石。涠洲岛橄榄岩中两种熔体的发现,进一步提供了发生在大陆岩石圈地幔流（熔）体与橄榄岩交代作用的普遍性、复杂性,以及地幔熔体多样性的新证据。     

Early Proterozoic granitoids of the Olenek complex (northern Siberian craton): petrogenesis and geodynamic setting

The paper deals with geological and geochemical studies of granitoids of the Olenek complex in the Olenek uplift of the basement of the northern Siberian craton. The age of these granitoids was earlier estimated at 2036 ± 11 Ma. The granitoids of the Olenek complex correspond in composition to high-alumina quartz diorites, granites, and leucogranites of the normal petrochemical series. According to geochemical and mineralogical characteristics, the quartz diorites can be assigned to granites of the transitional I-S type, and the granites and leucogranites, to S-type granites. The 8_Nd(T values in the granites of the Olenek complex vary from -0.2 to + 1.4, and the Nd model age is 2.4-2.5 Ga. The quartz diorite is characterized by 8_Nd(T) = + 3.0 and a Nd model age T(DM) = 2.2 Ga. The geochemical characteristics of the granites and leucogranites indicate their formation through the melting of a source of graywacke composition, whereas the quartz diorites resulted, most likely, from the mixing of granitic and basaltic melts. The fact that the granitoids of the Olenek complex intruded the folded rocks of the Eekit Formation but stay virtually undeformed massive bodies suggests that they formed at the postdeformation stage of the regional evolution after the completion of the Paleoproterozoic orogenic events. The intrusion of granitoids marks the completion of the formation of the Early Proterozoic Eekit fold belt on the western (in the recent coordinates) margin of the Birekta terrane of the Olenek superterraine and the final formation of the superterrane structure. At the next stage of magmatism (1.98-1.96 Ga), best pronounced in the uplifts of the basement of the northern Siberian craton, all terranes forming the Anabar and Olenek superterranes assembled into a single structure.     

华北东南部中生代岩石圈地幔性质、组成、富集过程及其形成机理

本文通过对我国华北东南部中生代幔源岩浆活动的时空分布规律及其地球化学特征的系统总结来进一步厘定该地区中生代岩石圈地幔的性质和组成,并通过与华北内部如鲁中地区中生代岩石圈地幔的对比研究探讨华北东部岩石圈的时空演化规律、富集过程及其形成机理。幔源岩石的 Sr-Nd-Ph 同位素特征表明华北东部中生代岩石圈地幔存在明显的时空不均匀性,其中心部位如鲁中地区以弱富集地幔为主体;而东南部如鲁西南和胶东地区则为类似 EM2型地幔(~(87)Sr/~(86)Sr_i 可高达0.7114)。华北东南部中生代岩石圈地幔随时间的演化特征也很明显。这些幔源岩石的地球化学特征和玄武岩中地幔岩捕虏体(橄榄岩和辉石岩)和捕虏晶(橄榄石和辉石)的组成和结构特征皆证明华北东南部中生代岩石圈地幔曾受到过富硅熔体的强烈改造。橄榄岩-熔体的相互反应是该区岩石圈改造和组成转变的重要方式,从而造成古生代高镁橄榄岩转变为晚中生代低镁橄榄岩和辉石岩。进入岩石圈地幔的熔体具下/中地壳物质重熔的特征,从而导致该区晚中生代岩石圈地幔的快速富集。有关华北东部中生代岩石圈减薄和改造的时限、过程和机制等问题也进行较详细的讨论。     

Age and tectonic setting of the Kengurak-Sergachi gabbro-anorthosite massif (the Selenga-Stanovoi superterrane,southern frame of the Siberian craton)

The results of geochemical and geochronological study of the Kengurak-Sergachi gabbroanorthosite massif in the Selenga-Stanovoi superterrane, southern frame of the Siberian craton, are presented. According to geochemical peculiarities, the massif rocks are close to the autonomous “massif-type anorthosite.” The massif age corresponds to 1866 ± 6 Ma based on the results of U-Pb zircon dating. The Kengurak-Sergachi massif was intruded most likely in post-collision epoch concurrently to formation of the South Siberian giant post-collision magmatic belt (1.87–1.84 Ga) extending along the southwestern flank of the Siberian craton.     

华南显生宙时期岩石圈地幔的性质与形成时代

火山岩携带的橄榄岩捕虏体是研究深部岩石圈地幔成分特征与热结构状态的最直接样品。湖北大洪山早奥陶世钾镁煌斑岩携带的石榴二辉橄榄岩具有富集的地球化学特征,指示当时的岩石圈厚度可达110 km。华南内陆地区宁远和道县早侏罗世玄武岩所携带的地幔包体具有饱满的成分特征,代表遭受了较低程度部分熔融的地幔残留。宁远地幔包体的全岩Re-Os同位素特征显示该地区中生代大陆岩石圈地幔为从软流圈新增生而形成的新生地幔。这表明内陆地区古生代存在的富集地幔被完全拆沉,并被新生地幔所取代;中生代内陆地区的岩石圈拆沉作用可能与该地区自225 Ma以来大规模的岩石圈伸展作用有关。华南新生代地幔包体主要分布在沿海地区。通过地幔包体矿物成分估算获得的温度与压力资料揭示新生代沿海地区岩石圈厚度约为80～90 km,并具有热的地温梯度。无论是全岩还是硫化物的Re-Os同位素特征都表明沿海地区在新生代仍残留有古元古代岩石圈地幔,表明新生代沿海地区的拉张作用仅导致了岩石圈地幔的部分拆沉和减薄。