Melt/peridotite interaction in the Southern Lanzo peridotite: Field, textural and geochemical evidence

This paper presents field, petrographic–structural and geochemical data on spinel and plagioclase peridotites from the southern domain of the Lanzo ophiolitic peridotite massif (Western Alps). Spinel lherzolites, harzburgites and dunites crop out at Mt. Arpone and Mt. Musinè. Field evidence indicates that pristine porphyroclastic spinel lherzolites are transformed to coarse granular spinel harzburgites, which are in turn overprinted by plagioclase peridotites, while strongly depleted spinel harzburgite and dunite bands and bodies replace the plagioclase peridotites. On the northern flank of Mt. Arpone, deformed, porphyroclastic (lithospheric) lherzolites, with diffuse pyroxenite banding, represent the oldest spinel-facies rocks. They show microstructures of a composite subsolidus evolution, suggesting provenance from deeper (asthenospheric) mantle levels and accretion to the lithosphere. These protoliths are locally transformed to coarse granular (reactive) spinel harzburgites and dunites, which show textures reminiscent of melt/rock reaction and geochemical characteristics suggesting that they are products of peridotite interaction with reactively percolating melts. Geochemical data and modelling suggest that <1–5% fractional melting of spinel-facies DMM produced the injected melts. Plagioclase peridotites are hybrid rocks resulting from pre-existing spinel peridotites and variable enrichment of plagioclase and micro-gabbroic material by percolating melts. The impregnating melts attained silica-saturation, as testified by widespread orthopyroxene replacement of olivine, during open system migration in the lithosphere. At Mt. Musinè, coarse granular spinel harzburgite and dunite bodies replace the plagioclase peridotites. Most of these replacive, refractory peridotites have interstitial magmatic clinopyroxene with trace element compositions in equilibrium with MORB, while some Cpx have REE-depleted patterns suggesting transient geochemical features of the migrating MORB-type melts, acquired by interaction with the ambient plagioclase peridotite. These replacive spinel harzburgite and dunite bodies are interpreted as channels exploited for focused and reactive migration of silica-undersaturated melts with aggregate MORB compositions. Such melts were unrelated to the silica-saturated melts that refertilized the pre-existing plagioclase peridotites. Finally, MORB melt migration occurred along open fractures, now recorded as gabbroic dikes.

Our data document the complexity of rock-types and mantle processes in the South Lanzo peridotite massif and describe a composite tectonic and magmatic scenario that is not consistent with the “asthenospheric scenario” proposed by previous authors. We envisage a “transitional scenario” in which extending subcontinental lithospheric mantle was strongly modified (both depleted and refertilized) by early melts with MORB-affinity formed by decompression partial melting of the upwelling asthenosphere, during pre-oceanic rifting and lithospheric thinning in the Ligurian Tethys realm.     

Gabbro-norite cumulates from strongly depleted MORB melts in the Alpine–Apennine ophiolites

Hundred-meter wide cumulate bodies and decimetric dykelets of gabbro-norites are widespread within the distal ophiolitic peridotites from the Jurassic Ligure-Piemontese oceanic basin, now emplaced in the Alpine–Apennine orogenic system. These peridotites derived from the sub-continental mantle of the pre-Triassic Europe–Adria lithosphere and underwent profound modifications of their structural and compositional characteristics via melt–rock interaction during diffuse percolation by porous flow of upwelling asthenospheric melts. Gabbro-norite cumulates show the peculiar association of high forsteritic olivine, high-Mg# clinopyroxenes and orthopyroxenes and high anorthitic plagioclase with respect to mineral compositions in common ophiolitic and oceanic MORB gabbros. Abundance and early crystallization of magnesian orthopyroxene suggests that parental magmas of the gabbro-noritic cumulates were relatively silica-rich basaltic liquids. Clinopyroxenes and plagioclase have anomalously low Sr and LREE, resulting in highly fractionated C1-normalized LREE patterns in clinopyroxenes and negatively fractionated C1-normalized LREE patterns in plagioclases.Modal mineralogy and mineral major and trace element compositions indicate that these gabbro-norites crystallized from MORB-type basaltic liquids that were strongly depleted in Na, Ti, Zr, Sr and other incompatible trace elements relative to any erupted liquids of MORB-type ophiolites and modern oceanic lithosphere. Computed melt compositions in equilibrium with gabbro-norite clinopyroxenes are closely similar to depleted MORB-type single melt increments after 5–7% of fractional melting of a DM asthenospheric mantle source under spinel-facies conditions.Present knowledge on the ophiolitic peridotites of Monte Maggiore indicate that they were formed by interaction of lithospheric mantle protoliths with depleted, MORB-type single melt increments produced by the ascending asthenosphere. Their composition was progressively modified from olivine-saturated to orthopyroxene-saturated by the early reactive melt–peridotite interaction (i.e., pyroxene dissolution and olivine precipitation).Gabbro-norite cumulates marked the change from diffuse porous flow percolation to intrusion and crystallization when cooling by conducive heat loss became dominant on heating by melt percolation. Progressive upwelling and cooling of the host peridotite during rifting caused transition to more brittle conditions and to hydration and serpentinization.The Monte Maggiore peridotite body was then intruded along fractures by variably evolved, Mg–Al- to Fe–Ti-rich gabbroic dykes. Computed melt compositions in equilibrium with clinopyroxenes from less evolved gabbro dykes are closely similar to aggregated MORBs. The event of gabbro intrusion indicates that aggregated MORB-type liquids: i) migrated through and stagnated in the mantle lithosphere and ii) underwent evolution into shallow ephemeral magma chambers to form the parental magmas of the gabbroic dykes and the basaltic lava flows of the Ligurian oceanic crust.     

Formation of plagioclase-bearing peridotite and plagioclase-bearing wehrlite and gabbro suite through reactive crystallization: an experimental study

Plagioclase-bearing peridotites are commonly associated with gabbroic rocks sampled around the Moho Transition Zone. Based on mineral chemistry, texture, and spatial relations, the formation of plagioclase-bearing peridotites has been attributed to impregnation of basalt into residual peridotites. We conducted reactive dissolution and crystallization experiments to test this hypothesis by reacting a primitive mid-ocean ridge basalt with a melt-impregnated lherzolite at 1,300 °C and 1 GPa and then cooling to 1,050 °C as pressure decreased to 0.7 GPa. Crystallization during cooling produced lithologic sequences of gabbro–wehrlite or gabbro–wehrlite–peridotite, depending on reaction time. Wehrlitic and peridotitic sections contain significant amounts of plagioclase interstitial to olivine and clinopyroxene and plagioclase compositions are spatially homogeneous. Clinopyroxene in the wehrlite–peridotite section is reprecipitated from the melt and exhibits poikilitic texture with small rounded olivine chadacrysts. Mineral composition in olivine and clinopyroxene varies spatially, both at the scale of the sample and within individual grains. Olivine grains that crystallized close to the melt–peridotite interface are enriched in iron due to their proximity to the basaltic melt reservoir. Consistent with many field studies, we observed gradual spatial variation in olivine and clinopyroxene composition across a lithologically sharp boundary between the gabbro and wehrlite–peridotite. Plagioclase compositions show no obvious dependence on distance from the melt–rock interface and were precipitated from late-stage trapped melts. Compositional trends of olivine, pyroxene, and plagioclase are consistent with previous experimental results and natural observations of the Moho Transition Zone. Different lithological sequences form based primarily on the melt–rock ratio, composition of the melt and host peridotite, and thermochemical conditions, but are expected to grade from gabbro to wehrlite or troctolite to peridotite. Plagioclase-bearing peridotite represents the low melt–rock ratio end member where pyroxene is only partially replaced by olivine and melt, whereas dunite is expected to form where melts overwhelm and consume all other phases. This study confirms that under nominally anhydrous conditions, the gabbro–wehrlite–plagioclase-peridotite sequence can be formed by reaction between basalt and lherzolite and subsequent crystallization at intermediate to low pressures. Melt–rock reaction is a fundamental process in the formation of new crust at the shallowest part of the melting column where pyroxene-undersaturated melts percolate through depleted peridotite.     

北秦岭松树沟橄榄岩与铬铁矿矿床的成因关系

松树沟橄榄岩体是秦岭造山带中规模最大的赋存铬铁矿床的超基性岩体。松树沟橄榄岩主要由细粒橄榄岩质糜棱岩和中粗粒橄榄岩组成。本文通过对松树沟橄榄岩的岩相学、主微量、稀土元素地球化学的系统研究,认为松树沟细粒方辉橄榄岩为洋脊扩张过程中地幔岩减压-近分离熔融产生的残留体,细粒纯橄岩主要由地幔橄榄岩熔融残留橄榄石、消耗辉石的减压熔融反应:aCpx+bOpx+cSpl=dOl+1Melt生成的橄榄石和少量的地幔方辉橄榄岩残留体组成,但均受到了后期渗滤熔体的再富集作用;中粗粒纯橄岩和方辉橄榄岩主要为上述反应产生的渗滤熔体被圈闭在迁移通道或减压扩容带内在热边界层(TBL)通过反应:MeltA=Ol+MeltB冷凝结晶而成,属堆晶橄榄岩。Pb-Sr-Nd同位素地球化学的证据显示,松树沟橄榄岩与基性岩具有共同的地幔源区,二者同为松树沟蛇绿岩的重要组成部分。通过矿床地质特征及铬铁矿电子探针测试研究,认为松树沟铬铁矿床是产于中粗粒堆晶纯橄岩中的层状铬铁矿床,形成于格林威尔期松树沟洋盆的扩张过程中,是中粗粒纯橄岩在热边界层(TBL)的冷凝结晶过程中岩浆分异作用的产物。     

Mantle processes during ocean formation：Petrologic records in peridotites from the Alpine－Apennine ophiolites

Mantle peridotites were early exposed at the sea-floor of the Jurassic Tethys derived from the subcontinental mantle of the Europe-Adria system. During continental rifting and oceanic spreading, these lithospheric peri-dotites were percolated via diffuse reactive porous flowby melt fractions produced by near-fractional melting of the upwelling asthenosphere. Ascending melts inter-acted with the lower lithosphere, dissolving pyroxenes and precipitating olivine, and crystallized at shallower levels in the mantle column causing melt impregnation.Subsequent focused porous flow formed replacive dunitechannels, cutting the impregnated oeridotites, which were conduits for upward migration of MORB-type liq-uids. Melt migration produced depletionlrefertilization and significant heating of the percolatedlimpregnated mantle, i.e the thermochemical erosion of the litho-sphere. Impregnated and thermally modified lithos-pheric mantle was cooled by conductive heat loss dur-ing progressive lithosphere thinning and was intrudeaby MORB magmas, which formed Mg-rich and Fe-richgabbroic dykes and bodies. Alpine-Apennine ophiolitic peridotites record the deep-seated migration of melts which changed their compositions and dynamics during the rift evolution. The thermochemical erosion of the lithospheric mantle by the ascending asthenospheric melts, which induces significant compositional and rhe-ological changes in the lower lithosphere, is a major process in the evolution of the continent-ocean transi-tion towards a slow spreading oceanic system.     

Building of the deepest crust at a fossil slow-spreading centre (Pineto gabbroic sequence, Alpine Jurassic ophiolites)

This work presents new field and petrological data on a poorly known lower crustal section from the Alpine Jurassic ophiolites, the Pineto gabbroic sequence from Corsica (France). The Pineto gabbroic sequence is estimated to be ~1.5 km thick and mainly consists of clinopyroxene-rich gabbros to gabbronorites near its stratigraphic top and of troctolites and minor olivine gabbros in its deeper sector. The sequence also encloses olivine-rich troctolite and mantle peridotite bodies at different stratigraphic heights. The composition and the lithological variability of the Pineto gabbroic sequence recall those of the lower crustal sections at slow- and ultra-slow-spreading ridges. The gabbroic sequence considered in this study is distinct in the high proportion of troctolites and olivine gabbros, which approximately constitute 2/3 of the section. In particular, the lower sector of the Pineto gabbroic sequence shows the existence of large-scale fragments of the deepest oceanic crust displaying a highly primitive bulk composition. The mineral chemical variations document that the origin and the evolution of the Pineto gabbroic rocks were mostly constrained by a process of fractional crystallisation. The clinopyroxenes from the olivine gabbros and the olivine-rich troctolites also record the infiltration of olivine-dissolving, Cr₂O₃-rich melts that presumably formed within the mantle, into replacive dunite bodies. Cooling rates of the troctolites and the olivine gabbros were evaluated using the Ca in olivine geospeedometer. We obtained high and nearly constant values of ?2.2 to ?1.7 °C/year log units, which were correlated with the building of the Pineto gabbroic sequence through multiple gabbroic intrusions intruded into a cold lithospheric mantle.     

Multi-stage melt–rock interaction in the Mt. Maggiore (Corsica, France) ophiolitic peridotites: microstructural and geochemical evidence

Spinel and plagioclase peridotites from the Mt.Maggiore (Corsica, France) ophiolitic massif record a composite asthenosphere–lithosphere history of partial melting and subsequent multi-stage melt–rock interaction. Cpx-poor spinel lherzolites are consistent with mantle residues after low-degree fractional melting (F = 5–10%). Opx + spinel symplectites at the rims of orthopyroxene porphyroclasts indicate post-melting lithospheric cooling (T = 970–1,100°C); this was followed by formation of olivine embayments within pyroxene porphyroclasts by melt–rock interaction. Enrichment in modal olivine (up to 85 wt%) at constant bulk Mg values, and variable absolute REE contents (at constant LREE/HREE) indicate olivine precipitation and pyroxene dissolution during reactive porous melt flow. This stage occurred at spinel-facies depths, after incorporation of the peridotites in the thermal lithosphere. Plagioclase-enriched peridotites show melt impregnation microtextures, like opx + plag intergrowths replacing exsolved cpx porphyroclasts and interstitial gabbronoritic veinlets. This second melt–rock interaction stage caused systematic chemical changes in clinopyroxene (e.g. Ti, REE, Zr, Y increase), related to the concomitant effects of local melt–rock interaction at decreasing melt mass, and crystallization of small (<3%) trapped melt fractions. LREE depletion in minerals of the gabbronoritic veinlets indicates that the impregnating melts were more depleted than normal MORB. Preserved microtextural evidence of previous melt–rock interaction in the impregnated peridotites suggests that they were progressively uplifted in response to lithosphere extension and thinning. Migrating melts were likely produced by mantle upwelling and melting related to extension; they were modified from olivine-saturated to opx-saturated compositions, and caused different styles of melt–rock interaction (reactive spinel harzburgites, vs. impregnated plagioclase peridotites) depending on the lithospheric depths at which interaction occurred. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Reaction between slab-derived melts and peridotite in the mantle wedge: experimental constraints at 3.8 GPa

Laboratory experiments on natural, hydrous basalts at 1–4 GPa constrain the composition of “unadulterated” partial melts of eclogitized oceanic crust within downgoing lithospheric slabs in subduction zones. We complement the “slab melting” experiments with another set of experiments in which these same “adakite” melts are allowed to infiltrate and react with an overlying layer of peridotite, simulating melt:rock reaction at the slab–mantle wedge interface. In subduction zones, the effects of reaction between slab-derived, adakite melts and peridotitic mantle conceivably range from hybridization of the melt, to modal or cryptic metasomatism of the sub-arc mantle, depending upon the “effective” melt:rock ratio. In experiments at 3.8 GPa, assimilation of either fertile or depleted peridotite by slab melts at a melt:rock ratio 2:1 produces Mg-rich, high-silica liquids in reactions which form pyrope-rich garnet and low-Mg# orthopyroxene, and fully consume olivine. Analysis of both the pristine and hybridized slab melts for a range of trace elements indicates that, although abundances of most trace elements in the melt increase during assimilation (because melt is consumed), trace element ratios remain relatively constant. In their compositional range, the experimental liquids closely resemble adakite lavas in island-arc and continental margin settings, and adakite veins and melt inclusions in metasomatized peridotite xenoliths from the sub-arc mantle. At slightly lower melt:rock ratios (1:1), slab melts are fully consumed, along with peridotitic olivine, in modal metasomatic reactions that form sodic amphibole and high-Mg# orthopyroxene.     

地幔橄榄岩中橄榄石的指示意义

橄榄石是地幔橄榄岩和辉石岩的主要组成矿物,但也经常以斑晶和捕虏晶的形式出现在玄武质岩石中。结合近年来在地幔橄榄岩的主要元素（如Mg和Fe）组成特征以及Li、Mg和Fe稳定同位素地球化学方面的研究成果,重点对橄榄石的地球化学特征与华北克拉通岩石圈地幔演化过程之间的联系进行了讨论,旨在加深对华北克拉通岩石圈地幔演化过程的理解。现有研究表明：地幔橄榄岩中橄榄石的矿物学特征、元素和同位素地球化学组成能够很好地指示岩石圈地幔的特征及其演化过程,因而具有重要的意义。对于克拉通地区的地幔橄榄岩来说,橄榄石的Mg#通常可以指示岩石圈地幔的属性,古老、难熔的地幔橄榄岩中的橄榄石一般具有较高的Mg#（〉92）,而新生的岩石圈地幔橄榄岩中的橄榄石则具有较低的Mg#（〈91）。因此,地幔橄榄岩中橄榄石的Mg#在一定程度上具有年龄意义。橄榄岩中橄榄石的Li、Mg和Fe同位素组成也可以明确指示岩石圈地幔的属性及其所经历的演化过程,正常地幔的δ7Li、δ26Mg和δ57Fe组成相对均一,如果上述同位素组成偏离正常地幔值,则说明岩石圈地幔经历了熔体/流体的交代作用。华北克拉通地区地幔橄榄岩捕虏体中橄榄石的Li、Mg和Fe同位素组成研究表明：该区的岩石圈地幔经历了多个阶段、不同来源的熔体/流体的改造过程。     

熔体再富集作用对大陆岩石圈地幔的影响

         二辉橄榄岩通常被认为是低程度部分熔融的残留,但熔体再富集作用为其成因提供了一种新的解释。熔体再富集作 用通常是指软流圈来源的玄武质熔体加入到难熔的方辉橄榄岩或纯橄岩形成更为饱满的二辉橄榄岩的过程。除了使主量元 素富集之外,熔体再富集作用还可以使微量元素与Sr-Nd 同位素从方辉橄榄岩中的富集特征转变为二辉橄榄岩所呈现的亏 损特征。对于熔体再富集作用是否改变橄榄岩的Os 同位素组成还存在较大的争议,它主要取决于加入熔体的比例,熔体中 硫的饱和程度以及熔体再富集作用发生的时间等因素。对于以低熔/岩比例为主的大陆岩石圈地幔来说,熔体再富集作用对 橄榄岩的Os 同位素组成的影响可能较为有限。除了化学成分上的影响之外,熔体的加入也会改变大陆岩石圈地幔的物理特 征。这一过程使得岩石圈地幔的渗透率增大和黏滞度降低,从而会破坏大陆岩石圈地幔的稳定性。虽然熔体再富集作用可 以影响和改变岩石圈地幔的性质,但它是否导致克拉通地幔的减薄以及克拉通破坏尚有疑问。     

Petrogenesis of basalts and gabbros from an ancient continent–ocean transition (External Liguride ophiolites, Northern Italy)

Remnants of a fossil continent–ocean transition similar to that of the modern non-volcanic continental margins are preserved in the Jurassic External Liguride units. They consist of fertile lherzolites of subcontinental origin, MOR-type basalts and rare gabbroic intrusives, together with continental crust bodies exhumed during the rifting phases preceding the oceanization. The gabbroic rocks include troctolites, (olivine) gabbros, Fe–Ti oxide-bearing gabbros and diorites. Trace element and Nd isotope compositions indicate that these rocks were derived from N-MORB melts variably evolved through fractional crystallisation. In the gabbroic rocks, high-temperature ( 900 °C) shearing along ductile shear zones is locally overprinted by amphibolite-facies recrystallization (T  650 °C), which was most likely assisted by seawater-derived fluids. Basalts crop out as lava flows and as dykes crosscutting mantle lherzolites and gabbroic rocks. They display nearly flat REE patterns and high Y/Nb values (5–14), similar to modern N-MORB. Basalts are also characterised by weak Zr enrichment relative to neighbouring REE (Zr/Zr = 1.1–1.7) and high (Sm/Yb)_DM ratios (1.5–1.8). Their Nd isotope compositions are close to typical depleted mantle (initial _Nd = +7.6 to + 9.4). The geochemical features of parental melts of basaltic and gabbroic rocks may be attributed to melting of a MORB-type asthenospheric source. Trace element modelling shows that low-degree (≤ 6%) fractional melting of a depleted spinel peridotite cannot account for the elevated Sm/Yb ratios of basalts. Low-degree melting of a mixed source of spinel peridotite with small amounts of garnet pyroxenite has been proposed to explain the trace element signature of basalts.     

Lithium isotopic systematics of peridotite xenoliths from Hannuoba, North China Craton: Implications for melt-rock interaction in the considerably thinned lithospheric mantle

Li concentrations and isotopic compositions of coexisting minerals (ol, opx, and cpx) from peridotite xenoliths entrained in the Hannuoba Tertiary basalts, North China Craton, provide insight into Li isotopic fractionation between mantle minerals during melt-rock interaction in the considerably thinned lithospheric mantle. Bulk analyses of mineral separates show significant enrichment of Li in cpx (2.4-3.6 ppm) relative to olivine (1.2-1.8 ppm), indicating that these peridotites have been affected by mantle metasomatism with mafic silicate melts. Bulk olivine separates (δ⁷Li ∼ +3.3‰ to +6.4‰) are isotopically heavier than coexisting pyroxenes (δ⁷Li ∼ −3.3‰ to −8.2‰ in cpx, and −4.0‰ to −6.7‰ in opx). Such large variation suggests Li elemental and isotopic disequilibrium. This conclusion is supported by results from in situ SIMS analyses of mineral grains where significant Li elemental and isotopic zonations exist. The olivine and opx have lower Li concentrations and heavier Li isotopes in the rims than in the cores. This reverse correlation of δ⁷Li with Li concentrations indicates diffusive fractionation of Li isotopes. However, the zoning patterns in coexisting cpx show isotopically heavier rims with higher Li abundances. This positive correlation between δ⁷Li and Li concentrations suggests a melt mixing trend. We attribute Li concentration and isotope zonation in minerals to the effects of two-stage diffusive fractionation coupled with melt-rock interaction. The earliest melts may have been derived from the subducted oceanic slab with low δ⁷Li values produced by isotopic fractionation during the dehydration of the seawater-altered slab. Melts at later stages were derived from the asthenosphere and interacted with the peridotites, producing the Li elemental and isotopic zoning in mineral grains. These data thus provide evidence for multiple-stage peridotite-melt interaction in the lithospheric mantle beneath the northern North China Craton.     

Type-II xenoliths and related metasomatism from the Nógrád-Gömör Volcanic Field, Carpathian-Pannonian region (northern Hungary–southern Slovakia)

Xenoliths from the upper mantle and lower crust are abundant in Plio–Pleistocene alkali basalts of the Nógrád-Gömör Volcanic Field (NGVF; northern Pannonian Basin, northern Hungary/southern Slovakia), representing a valuable ‘probe’ of lithospheric structures and processes. Ultramafic xenoliths have been divided into two groups: (1) Type-I, composed mostly of olivine with subsidiary orthopyroxene, clinopyroxene and spinel, and (2) Type-II, containing mostly Al- and Ti-rich clinopyroxene with subordinate olivine, spinel and plagioclase. Both types often contain amphibole and, to a lesser extent, mica. The refractory character of Type-I xenoliths suggests they represent mantle depleted by prior episodes of partial melting. In contrast, Type-II series (wehrlites, olivine clinopyroxenites, clinopyroxenites and plagioclase-bearing ultramafic lithologies), on the basis of their textural features, thermobarometric histories and major and trace element variation, appear to have formed as magmatic cumulates. Petrologic and geochemical studies of Type-II xenoliths from Nógrád-Gömör suggest they crystallized from basaltic melts emplaced within the lithospheric mantle and lower crust, prior to the onset of Plio–Pleistocene volcanic activity. After their consolidation, metasomatic agents reacted with the anhydrous cumulate phases producing amphiboles and micas at the expense of olivine and clinopyroxene. The metasomatic agents appear to have been adakitic rather than basaltic in composition, possibly linked to a retreating arc–forearc system. Large-scale contamination of the lithospheric mantle can therefore be attributed to fluid and melt fractions related to subduction beneath the outer Carpathian arc.     

造山带橄榄岩起源和大陆俯冲带壳幔相互作用

俯冲地壳衍生流体交代地幔楔,是产生俯冲带岩浆作用的重要机制.但是,目前人们对俯冲大陆物质改造地幔楔的岩石学过程和机理仍缺乏深入认识,造山带橄榄岩是解析这一问题的直接样品.通过对大别-苏鲁造山带橄榄岩进行系统的矿物学、岩石学和地球化学研究,发现橄榄石Ni/Co比值可有效区分幔源和壳源造山带橄榄岩,揭示幔源造山带橄榄岩起源于华北岩石圈地幔.苏鲁李家屯纯橄岩在进入俯冲带之前就已在地幔内部经历了碳酸盐熔体交代.大别毛屋和苏鲁蒋庄橄榄岩及其交代脉体记录了约170~200 km深度的俯冲带壳幔相互作用过程.深俯冲陆壳释放的富Si-Al质熔体可不同程度地改造地幔楔底部,形成富石榴石和富辉石的交代岩,并引发强烈的Os同位素分馏效应.该过程不仅改变地幔楔岩性和化学组成,还能够改变交代介质成分,为俯冲带各类深部地幔岩浆提供源区物质.因此,大陆深俯冲是导致上地幔不均一的重要途径.      

Lithium enrichment and isotopic variation in minerals from peridotite xenoliths from northwestern Ethiopian plateau

We report Lithium (Li) concentrations and isotopic compositions for co-existing olivine, orthopyroxene (opx), and clinopyroxene (cpx) mineral separates from depleted and metasomatised peridotite xenoliths hosted by basaltic lavas from northwestern Ethiopian plateau (Gundeweyn area). The peridotites contain five lherzolites and one harzburgite and are variably depleted and enriched in LREE relative to HREE. In both depleted and enriched lherzolites, Li is preferentially incorporated into olivine (2.4-3.3 ppm) compared to opx (1.4-2.1 ppm) and cpx (1.4-2.0 ppm) whereas the Li contents of olivines (5.4 ppm) from an enriched harzburgiteare higher than those of lherzolites. Olivines from the samples show higher Li abundances than normal mantle olivines (1.6-1.9 ppm) indicating the occurrence of Li enrichments through melt-preroditite interaction. The average δ⁷ Li values range from +2.2 to +6.0‰ in olivine, from -0.1 to +2.0‰ in opx and from -4.4 to -0.9‰ in cpx from the lherzolites. The Li isotopic composition (3.5‰) of olivines from harzburgite fall within the range of olivine from lherzolites but the opxs show low in δ⁷Li (-2.0‰). Overall Li isotopic compositions of olivines from the peridotites fall within the range of normal mantle olivine, δ⁷Li values of ~+4±2‰ within uncertainty, reflecting metasomatism (enrichment) of the peridotites by isotopically heavy Li-rich asthenospheric melt. Li isotope zonation is also observed in most peridotite minerals. Majority of olivine grains display isotopically heavy cores and light rims and the reverse case is observed for some olivine grains. Orthopyroxene and clinopyroxene grains show irregular distribution in δ⁷Li. These features of Li isotopic compositions within and between grains in the samples reflect the effect of diffusion-driven isotopic fractionation during meltperidotite interaction and cooling processes.     

Refertilization of ancient lithospheric mantle beneath the central North China Craton: Evidence from petrology and geochemistry of peridotite xenoliths

The petrology and geochemistry of peridotite xenoliths in the Cenozoic basalts from Fanshi, the central North China Craton (NCC), provide constraints on the evolution of sub-continental lithospheric mantle. These peridotite xenoliths are mainly spinel-facies lherzolites with minor harzburgites. The lherzolites are characterized by low forsterite contents in olivines (Fo < 91) and light rare earth element (LREE) enrichments in clinopyroxenes. In contrast, the harzburgites are typified by high-Fo olivines (> 91), high-Cr# spinels and clinopyroxenes with low abundances of heavy REE (HREE). These features are similar to those from old refractory lithospheric mantle around the world, and thus interpreted to be relics of old lithospheric mantle. The old lithospheric mantle has been chemically modified by the influx of melts, as evidenced by the Sr–Nd isotopic compositions of clinopyroxenes and relatively lower Fo contents than typical Archean lithospheric mantle (Fo > 92.5). The Sr–Nd isotopic compositions of harzburgites are close to EM1-type mantle, and of the lherzolites are similar to bulk silicate earth. The latter could be the result of recent modification of old harzburgites by asthenospheric melt, which is strengthened by fertile compositions of minerals in the lherzolites. Therefore, the isotopic and chemical heterogeneities of the Fanshi peridotite xenoliths reflect the refertilization of ancient refractory lithospheric mantle by massive addition of asthenospheric melts. This may be an important mechanism for the lithospheric evolution beneath the Central NCC.     

深海橄榄岩研究进展

很久以来,深海橄榄岩一直被认为是洋中脊下地幔熔融残留体,但最近深海橄榄岩实际矿物组分研究和全岩化学成分重建的结果表明,深海橄榄岩是洋脊下地幔熔融残留体和过量橄榄石的混合物。深海橄榄岩的成因是洋脊下上隆地幔降压熔融作用和熔体上升通过地幔最上部的热边界层时发生冷却、结晶出橄榄石的联合作用的结果。地幔熔融程度越高,深海橄榄岩越亏损,则含有越多的过量橄榄石。为此,只要把深海橄榄岩中过量的橄榄石剔除后,其实际矿物组分就能有效地反映洋脊下地幔熔融作用和熔融萃取过程。     

Petrogenesis of the ~740 Korab Kansi mafic-ultramafic intrusion,South Eastern Desert of Egypt: Evidence of Ti-rich ferropicritic magmatism

The Neoproterozoic Korab Kansi mafic-ultramafic intrusion is one of the largest (100 km²) intrusions in the Southern Eastern Desert of Egypt. The intrusion consists of Fe-Ti-bearing dunite layers, amphibole peridotites, pyroxenites, troctolites, olivine gabbros, gabbronorites, pyroxene gabbros and pyroxene-hornblende gabbros, and also hosts significant Fe-Ti deposits, mainly as titanomagnetite-ilmenite. These lithologies show rhythmic layers and intrusive contacts against the surrounding granites and ophiolitic-island arc assemblages. The wide ranges of olivine forsterite contents (Fo_67.9-85.7), clinopyroxene Mg# (0.57–0.95), amphibole Mg# (0.47–0.88), and plagioclase compositions (An_85.8-40.9) indicate the role of fractional crystallization in the evolution from ultramafic to mafic rock types. Clinopyroxene (Cpx) has high REE contents (2–30 times chondrite) with depleted LREE relative to HREE, like those crystallized from ferropicritic melts generated in an island-arc setting. Melts in equilibrium with Cpx also resemble ferropicrites crystallized from olivine-rich mantle melts. Cpx chemistry and its host rock compositions have affinities to tholeiitic and calc-alkaline magma types. Compositions of mafic-ultramafic rocks are depleted in HFSE (e.g. Nb, Ta, Zr, Th and U) relative to LILE (e.g. Li, Rb, Ba, Pb and Sr) due to the addition of subduction-related hydrous fluids (rich in LILE) to the mantle source, suggesting an island-arc setting. Fine-grained olivine gabbros may represent quenched melts approximating the primary magma compositions because they are typically similar in assemblage and chemistry as well as in whole-rock chemistry to ferropicrites. We suggest that the Korab Kansi intrusion crystallized at temperatures ranging from ~700 to 1100 °C from ferropicritic magma derived from melting of metasomatized mantle at <5 Kbar. These hydrous ferropicritic melts were generated in the deep mantle and evolved by fractional crystallization under high ƒO₂ at relatively shallow depth. Fractionation formed calc-alkaline magmas during the maturation of an island arc system, reflecting the role of subduction-related fluids. The interaction of metasomatized lithosphere with upwelling asthenospheric melts produced the Fe and Ti-rich ferropicritic parental melts that are responsible for precipitating large quantities of Fe-Ti oxide layers in the Korab Kansi mafic-ultramafic intrusion. The other factors controlling these economic Fe-Ti deposits beside parental melts are high oxygen fugacity, water content and increasing degrees of mantle partial melting. The generation of Ti-rich melts and formation of Fe-Ti deposits in few layered intrusions in Egypt possibly reflect the Neoproterozoic mantle heterogeneity in the Nubian Shield. We suggest that Cryogenian-Tonian mafic intrusions in SE Egypt can be subdivided into Alaskan-type intrusions that are enriched in PGEs whereas Korab Kansi-type layered intrusions are enriched in Fe-Ti-V deposits.     

Geochronology, geochemistry and petrogenesis of mafic and ultramafic rocks from Southern Beishan area, NW China: Implications for crust–mantle interaction

The Liuyuan mafic and ultramafic rocks are exposed in Southern Beishan, which is along the southern branch of the Central Asian Orogenic Belt (CAOB). Zircon SHRIMP U–Pb dating showed that Liuyuan gabbros intruded during the early Permian (~ 270–295 Ma) coeval with the basalts and the ultramafic rocks were emplaced at about 250 Ma. The basalts are within–plate tholeiites with slight enrichment in light rare earth elements (LREE) relative to heavy rare earths (HREE) and small negative anomalies of Nb and Ta. Gabbros including olivine gabbros, olivine gabbronorites and troctolites are grouped into two: the cumulate gabbros are depleted in LREE and show small negative Nb and Ta anomalies but distinct positive Sr and Eu anomalies; non–cumulate gabbros resemble tholeiitic basalts. Lamprophyres and cumulate ultramafic rocks are characterized by large enrichment of LREE relative to HREE with depletion in Nb and Ta. The enriched Sr–Nd isotopic trend from DM towards the EM II end member component implies that the lithospheric mantle was progressively enriched with depth by the involvement of subducted crustal material due to the delamination of thickened mantle lithosphere after collision. The digestion of subducted crustal material into the mantle resulting in the metasomatized and enriched mantle is inferred to be an important process during crust–mantle interaction.     

大陆碰撞造山带的两类橄榄岩——以柴北缘超高压变质带为例

论述了大陆俯冲碰撞带中地幔橄榄岩的基本特征和成岩类型,并重点讨论柴北缘超高压变质带中不同性质的橄榄岩及其成因。根据岩石学特征,我们确定柴北缘超高压带中发育有两种类型的橄榄岩:(1)石榴橄榄岩,岩石类型包括石榴二辉橄榄岩、石榴方辉橄榄岩、纯橄岩和石榴辉石岩,是大陆型俯冲带的标志性岩石。金刚石包裹体、石榴石和橄榄石的出溶结构、温压计算等均反映其来源深度大于200km。地球化学特征表明该橄榄岩的原岩是岛弧环境下高镁岩浆在地幔环境下堆晶的产物。(2)大洋蛇绿岩型地幔橄榄岩,与变质的堆晶杂岩(包括石榴辉石岩、蓝晶石榴辉岩)和具有大洋玄武岩特征的榴辉岩构成典型的蛇绿岩剖面,代表大洋岩石圈残片。这两类橄榄岩的确定对了解柴北缘超高压变质带的性质和构造演化过程有重要意义。