甘肃景泰县老虎山地区蛇绿岩及其上覆岩系中枕状熔岩的地球化学特征

甘肃景泰县老虎山地区蛇绿岩之上覆盖着一套镁铁质的喷出岩和沉积岩组合，位于蛇绿岩的枕状熔岩之上，不是蛇绿岩的成员，为蛇绿岩的上覆岩系。蛇绿岩的枕状熔岩具Ｎ－ＭＯＲＢ的特征，而其上覆岩系中的枕状熔岩类似Ｅ－ＭＯＲＢ。前者来自扩张脊的下部岩浆房；后者推测源于扩张脊之外的岩浆房，也是软流圈地幔中等至高程度部分熔融的产物，不过，可能有少量来自下地幔的物质的加入     

Structural contribution from the Oman ophiolite to processes of crustal accretion at the East Pacific Rise

A comprehensive model for the activity of the elementary accretion segment at fast‐spreading ridges relies on integration of structural data from the Oman ophiolite and geophysical results from the East Pacific Rise (EPR) around 9°N, which are of comparable size and spreading rates. The axial melt lens at shallow crustal level provides a link between Deval segmentation at the seafloor and a lower melt sill at Moho level, imaged at the EPR as a crustal melt zone (CMZ) and mapped in Oman as the Moho transition zone (MTZ). Both are attached to a mantle upwelling at the EPR, and to a frozen diapir in Oman. The physical link between diapiric mantle uprising at the Moho and Devals segmentation at the seafloor is the melt being injected from the mantle into the lower MTZ, ponding there, and then being released by powerful injections into the upper melt lens. The magma chamber covers the diapir at a distance of 5 km from the ridge axis.     

Paragenesis of magma chamber internal wall discovered in Oman ophiolite gabbros

A detailed study in Wadi Farah reveals a singular point in Jebel Dihm (southern Oman ophiolite). Here, we investigated some primitive features of accretion at a fast‐spreading ridge that has exceptionally been exempt from hydrous alteration while crossing the TBL (thermal boundary layer). The conjunction of two factors has made this possible in the Farah area: its location at the point of sharp foliation rotation from the upper to the transitional gabbros and a locally twisted TBL. These factors may have favoured the intrusion within the TBL of hydrous wedges transverse to the ridge. Deeper into the TBL, near the active ridge, the wedges merge and are covered by veneers of totally fresh gabbros bearing glittering acicular clinopyroxene. These gabbros are interpreted as relics of the crystallizing internal wall of the magma chamber that have interacted with hydrous fluids. This internal TBL has anisotropic thermal properties and acts as a thermal blanket, maintaining high magma‐chamber temperatures.     

北祁连山中段早中奥陶世蛇绿岩中席状岩墙杂岩的发现及其地质意义

作为蛇绿岩套重要组成部分之一的席状岩墙杂岩(Sheeted dyke complex)近来在北祁连山中段肃南县大岔大坂北坡的早中奥陶世蛇绿岩中被发现。这一发现对于祁连山早古生代蛇绿岩来说尚属首次,而且对于研究蛇绿岩的发展演化及探讨奥陶纪时洋底扩张都有重要意义。席状岩墙杂岩由一系列具单向冷凝边的辉绿岩墙组成,以一墙挨一墙的形式产出,岩墙间无任何填充物。席状岩墙杂岩在矿物组合上,常量元素、稀土元素及痕量元素地球化学特征和配分模式,甚至金属硫化物矿化作用方面都有类似之处。这些证据表明席状岩墙杂岩是连通其下岩浆房与其上枕状熔岩的通道。席状岩墙的单向冷凝边为岩浆上升方式和扩张洋脊的存在提供了令人信服的证据。根据Zr/Y—Zr关系图式,得出该区洋脊的扩张速率大约为2cm/a。     

Geochemistry of hornblende gabbros from Sonidzuoqi,Inner Mongolia,North China: implications for magmatism during the final stage of suprasubduction‐zone ophiolite formation

Allochthonous hornblende‐rich gabbroic rocks at Sonidzuoqi constitute important components of the early to middle Palaeozoic orogen, which forms the southeastern part of the Central Asian orogenic belt in Inner Mongolia. Limited hornblende K–Ar and SHRIMP U–Pb zircon ages document the Late Silurian to Early Devonian gabbroic emplacement. The rocks are tholeiitic and are characterized by moderate large‐ion‐lithophile‐element (e.g. Th, U) abundances, high‐field‐strength‐element (e.g. Nb, Ta, Zr, Ti) depletions, high Ti/V ratios, and MORB‐like isotopic signatures [(⁸⁷Sr/⁸⁶Sr)_i≈0.7030 to 0.7042; ε_Nd(t)≈+4.35 to +7.80, (²⁰⁶Pb/²⁰⁴Pb)_i≈17.46 to 17.61]. These features argue for a hydrous basaltic parental magma. We postulate that the melt formed through the coupling of MORB‐type mantle upwelling with aqueous fluid influx derived from slab devolatilization. This petrogenetic scenario suggests that an active spreading centre entered the trench during ridge subduction, bringing to a close an episode of suprasubduction‐zone ophiolite formation. The Siluro‐Devonian hornblende gabbros, together with a pre‐490 Ma ophiolitic mélange of MORB‐OIB affinity, ～483–471 Ma arc intrusions, ～498–461 Ma trondhjemite‐tonalite‐granodiorite plutons, and ～427–423 Ma calc‐alkaline granites from the same area, provide documentation of multistage crustal generation processes during the life cycle of this suprasubduction‐zone ophiolite.     

Variations in basaltic geochemistry along a propagating rift of the late Ordovician marginal basin of the West Norwegian Caledonides

The late Ordovician Solund-Stavfjord ophiolite complex in the western Norwegian Caledonides records a multi stage seafloor spreading history of an Iapetus marginal basin and contains three structural domains with distinctive tectonomagmatic evolutionary paths. The NNE-trending Domain 1 consisting of high-level gabbro, sheeted dykes and extrusive rocks is interpreted to represent fossil oceanic crust developed along a spreading centre that propagated northwards into pre-existing oceanic crust in the marginal basin. Dyke swarms at the head of this inferred propagating rift range from primitive, high-MgO basalts to highly fractionated quartz-diorites. Southwards along-strike of Domain 1, the abundance of primitive basalts decreases and the proportion of FeTi-basalts increases to become predominant furthest behind the tip of the propagating rift. This geochemical evolution is comparable to that of the basalts of modern propagating rifts at the East Pacific Rise and the Galapagos spreading ridge. We suggest that the chemical variations of the metabasalts reflect changes in magma supply rates and in the increasing size of magma chamber(s) along-strike of the spreading centres.     

Trace and Minor Elements in Smithsonite from Huoshaoyun Nonsulfide Zinc Deposits in Southwest China: A LA–ICP–MS Study

The geodynamic setting of the Xigaze ophiolite has long been debated. Structural and geochemical evidence suggest the Xigaze ophiolite was formed at a slow‐spreading ridge (Nicolas et al., 1981; Liu et al., 2016). Based on incompatible element concentrations, the Xigaze ophiolite volcanics are consistent with the ubiquitous subduction signature in suprasubduction zone (Bedard et al., 2009; Hebert et al., 2012; Dai et al., 2013). It is noteworthy that the Xigaze ophiolite is different from the Geotimes and Lasail and Velly units from Oman ophiolite, respectively. The mafic rocks of the Xigaze ophiolite generally resemble typical N‐MORB and Geotimes volcanics in composition except for slight depletions of Th and Nb (Fig.1a). Although the Xigaze rocks have similar Th and Nb concentrations to Lasail and Velly rocks, most incompatible elements in the Xigaze rocks are comparable to N‐MORB. Petrography in gabbro of Xigaze ophiolite shows that euhedral plagioclases are enclosed by clinopyroxenes suggesting that these minerals have crystallized from an anhydrous magma (Sisson and Grove, 1993). Although the Xigaze volcanic rocks are slightly depleted in Th and Nb, they have MORB‐like trace element characteristics implying that they are derived from an anhydrous MORB magma at spreading centre. Godard et al. (2006) suggested that the mantle source of the Oman ophiolite have element and isotopic characteristics similar to Indian Ocean MORB, where the mantle preserved some older slab materials. A negative Nb anomaly of Oman Geotimes volcanic rocks may be resulted from contamination of the slab materials via decompression melting of the convecting mantle. Moreover, the Xigaze rocks have 1.27–3.18 of (Th/Nb)_N ratios similar with those of Geotimes volcanics ((Th/Nb)_N =0.51–2.77) and lower than those of Lasail and Velly units ((Th/Nb)_N =2.12–6.35). These features suggest that the Xigaze ophiolite may have formed at the spreading centre.     

Genesis and Geodynamic Significance of Chromitites from the Fuchuan Ophiolite, Southern China, as Evidenced by Trace Element Fingerprints of Chromite, Olivine and Pyroxene

The Fuchuan ophiolite is located in the northeasternmost segment of the Neoproterozoic Jiangnan orogen and consists mainly of harzburgites, with minor dunites, pyroxenite and gabbro veins and dykes. In order to investigate the genesis and tectonic setting of the Fuchuan ophiolite and chromitites, in situ analyses of unaltered chromites and silicates were carried out. Trace element analyses of unaltered chromites from the Fuchuan chromitites indicate the parental magma is of mid-ocean ridge basal...     

Chemical and Isotopic Characteristics of the Kuroko‐Forming Volcanism

The Miocene northeast Honshu magmatic arc, Japan, formed at a terrestrial continental margin via a stage of spreading in a back‐arc basin (23–17 Ma) followed by multiple stages of submarine rifting (19–13 Ma). The Kuroko deposits formed during this period, with most forming during the youngest rifting stage. The mode of magma eruption changed from submarine basalt lava flows during back‐arc basin spreading to submarine bimodal basalt lava flows and abundant rhyolitic effusive rocks during the rifting stage. The basalts produced during the stage of back‐arc basin spreading are geochemically similar to mid‐ocean ridge basalt, with a depleted Sr–Nd mantle source, whereas those produced during the rifting stage possess arc signatures with an enriched mantle source. The Nb/Zr ratios of the volcanic rocks show an increase over time, indicating a temporal increase in the fertility of the source. The Nb/Zr ratios are similar in basalts and rhyolites from a given rift zone, whereas the Nd isotopic compositions of the rhyolites are less radiogenic than those of the basalts. These data suggest that the rhyolites were derived from a basaltic magma via crystal fractionation and crustal assimilation. The rhyolites associated with the Kuroko deposits are aphyric and have higher concentrations of incompatible elements than do post‐Kuroko quartz‐phyric rhyolites. These observations suggest that the aphyric rhyolite magma was derived from a relatively deep magma chamber with strong fractional crystallization. Almost all of the Kuroko deposits formed in close temporal relation to the aphyric rhyolite indicating a genetic link between the Kuroko deposits and highly differentiated rhyolitic magma.     

北补连蛇绿岩的特征,形成环境及其构造意义

文中总结了北祁连蛇绿岩的特征，指出北祁连蛇绿岩大多具有ＭＯＲＢ的性质，有玻安岩产生，形成在弧后和岛弧环境，北祁连蛇绿岩大多侵位在岛弧增生楔或活动陆缘地体之上，蛇绿岩属于科迪勒拉型，早古生代的北祁连造山带属于科迪勒拉型造山带，部分蛇绿岩之上整合产出一套沉积一火山岩系，称为蛇绿岩的上覆岩系，指出蛇绿岩及其上覆岩系的枕状熔岩分别来自不同的源区，具有不同的构造意义，还讨论了北祁连早古生代板块构造格局，认为     

Architecture of the Oman–UAE ophiolite: evidence for a multi-phase magmatic history

The Oman–United Arab Emirates ophiolite is the world’s largest ophiolite. It is divided into 12 separate fault-bounded blocks, of which the northern three lie wholly or partly in the United Arab Emirates. Extensive mapping has shown that the United Arab Emirates blocks contain mantle and crustal sections which correspond to the classic ‘Penrose conference’ ophiolite definition but which are cut by a voluminous later magmatic sequence including ultramafic, mafic and felsic components. Samples from the later magmatic sequence are dated at 96.4?±?0.3, 95.74?±?0.3 and 95.2?±?0.3 Ma; the early crustal section, which has not been dated directly, is thus constrained to be older than c. 96.4 Ma. Petrological evidence shows that the early crustal section formed at a spreading ridge, but the later magmatic sequence was formed from hydrous magmas that produced different mineral crystallisation sequences to normal mid-ocean ridge basalt (MORB). Mineral and whole-rock geochemical analyses show that the early crustal rocks are chemically similar to MORB, but the later magmatic sequence has chemical features typically found in supra-subduction zone (SSZ) settings. The ophiolite in the United Arab Emirates thus preserves clear evidence for two stages of magmatism, an early episode formed at a spreading centre and a later episode associated with the onset of subduction. Similar two-stage magmatism has been recognised in the Oman sector, but the United Arab Emirates contains the most voluminous SSZ magmatism yet described from this ophiolite.     

北祁连蛇绿岩的特征、形成环境及其构造意义

文中总结了北祁连蛇绿岩的特征，指出北祁连蛇绿岩大多具有ＭＯＲＢ的性质，有玻安岩产出，形成在弧后和岛弧环境。北祁连蛇绿岩大多侵位在岛弧增生楔或活动陆缘地体之上，蛇绿岩属于科迪勒拉型，早古生代的北祁连造山带属于科迪勒拉型造山带。部分蛇绿岩之上整合产出一套沉积－火山岩系，称为蛇绿岩的上覆岩系。指出蛇绿岩及其上覆岩系的枕状熔岩分别来自不同的源区，具有不同的构造意义。还讨论了北祁连早古生代板块构造格局，认为北祁连洋盆属于古亚洲洋的一部分，可能曾经是一个较大规模的洋盆。献中通常把它当成增生或俯冲杂岩带的一部分来看待〔１３，１６－１７〕；大岔大坂蛇绿岩带，其向两侧的延伸情况不清楚；九个泉（或塔墩沟）蛇绿岩带，向东可连到景泰县老虎山蛇绿岩，有人认为，向西可与榆树沟蛇绿岩相连〔２０〕。早先认为，北祁连存在新元古代、中寒武和早－中奥陶世三个时代的蛇绿岩〔２，１１〕，经过多年研究，目前大多数同意蛇绿岩主要是晚寒武－奥陶纪的〔１３，１６〕。图１北祁连早古生代蛇绿岩分布图１．前寒武纪基底；２．俯冲杂岩带；３．蛇绿岩。图中数字：１．九个泉；２．大岔大坂；３．边马沟；４．玉石沟；５．小八宝；６．百经寺；７．老虎山；８．榆树沟山２北祁连几     

Petrogenesis and Tectonic Significance of the Cuobuzha Peridotite,Yarlung Zangbo Suture Zone,China

The Yarlung Zangbo suture zone (YZSZ) in southern Tibet includes the remnants of Neo‐Tethyan oceanic lithosphere and marks a major suture between the Indian plate to the south and the Lhasa terrane of Tibet to the north. The upper mantle section of the Cuobuzha ophiolite in the northern subbelt of the western YZSZ comprises mainly clinopyroxene (cpx)‐rich and depleted harzburgites. Spinels in the cpx‐harzburgites show lower Cr^# values (12.6–15.1) than the spinels in the harzburgites (26.1–34.5), and the cpx‐harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites. The harzburgites have subchondritic Os isotopic compositions (0.11624–0.11699), yielding Re‐depletion model ages (TRD) ages from 1.8 to 1.7 Ga, indicating that the Cubuzha mantle underwent at least one ancient melt extraction event ca. 1.8‐1.7Ga; whereas the cpx‐harzburgites have suprachondritic ¹⁸⁷Os/¹⁸⁸Os ratios (0.12831–0.13125) with higher Re concentrations (0.380–0.575 ppb), indicating subsequent addition of Re following the last partial melting event that occurred during mid‐ocean ridge melt evolution processes. Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid‐oceanic ridge–type upper mantle units, their melt evolution trends reflect different mantle processes. The cpx‐harzburgites formed from low‐degree partial melting of a primitive mantle source, and they were subsequently modified by melt‐rock interactions in a mid‐oceanic ridge environment. The depleted harzburgites, however, were produced by remelting of the cpx‐harzburgites, which later interacted with mid‐oceanic ridge basalt– or island‐arc tholeiite–like melts, possibly in a trench–distal backarc spreading center. Our new isotopic and geochemical data from the Cuobuzha peridotites confirm that the Neo‐Tethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its seafloor spreading evolution.     

西北印度洋脊的厘定及其地质构造特征

西北印度洋的洋脊系统目前以"中印度洋脊"和"卡尔斯伯格脊"分别指示南北两段,两者的分界点被认为是澳大利亚板块与印度板块的板块边界与洋脊的交点,但具体分布位置不明确.基于已有的地质、地球物理和地球化学等多方面特征,认为卡尔斯伯格脊和中印度洋脊可以统一称为"西北印度洋脊",从罗德里格斯三联点一直延伸到欧文断裂带.新的洋脊厘定将有助于更全面地了解整个西北印度洋的洋脊演化和地球动力学过程.西北印度洋脊地形上南北两端断裂较少,中间断层密集,形似吸管的弯折部位,调节洋脊的转向.重力异常显示沿脊轴方向两端高中间低的特征,表明两端岩浆供给相对充足,而中间断层密集区岩浆量少.磁异常特征显示清晰的分带性,指示多阶段的洋脊扩张历史.岩石地球化学特征显示南北两个同位素相对富集洋脊段,可能与热点作用相关,或与残留岩石圈或地壳物质对亏损软流圈地幔的富集改造有关.     

Petrology of the mafic rocks of the Xigaze ophiolite,Tibet

The Xigaze ophiolite (Yarlung-Zangbo suture zone, Southern Tibet, China) shows an unusual crustal sequence characterized by a lack of large masses of cumulate gabbros, by dolerites intrusive throughout the whole ophiolite sequence, and by the injection of dolerites in already serpentinized peridotites. The abyssal tholeiitic nature of all the mafic rocks indicates that they have been generated at an oceanic ridge. All the geological arguments and petrological and textural data on the mafic rocks point to very low heat production and large heat losses through widespread intensive sea-water circulation, for the spreading centre in which they have been formed, in good agreement with a slow-spreading ridge origin.     

Petrologic Evolution of Large Failed Rifts in the Eastern Pacific: Petrology of Volcanic and Plutonic Rocks from the Mathematician Ridge Area and the Guadalupe Trough

The Mathematician failed rift was a fast spreading oceanic ridgeprior to abandonment about 6.5 Ma. but presently has the morphologyof a slow spreading ridge crest. Dredges of its inner rift valleywalls indicate that it also has the petrologic characteristicsof a slow-spreading ridge: the dredges recovered serpentinizedperidotite, cumulate and isotropic gabbro, diabase and mid-oceanridge basalt (MORB) lava. The gabbros include fresh troctolites,cpx gabbros and two pyroxene gabbros, all of which have relativelyiron-rich mafic minerals and sodic plagioclase. Other gabbrosamples are sheared and metamorphosed and presently have lowergreenschist to lower amphibolite mineral assemblages. Peak metamorphictemperatures calculated from coexisting amphibole and plagioclaseare about 600 ?C at 1-2 kb pressure but many of the assemblagesreflect much lower temperatures. The MORB lavas from the riftvalley walls are relatively fresh and very primitive (100 ?Mg/(Mg?Fe^2?) = 72–74). This suggests that they were eruptedin the last stages of rift abandonment after the disappearanceof the shallow crustal magma chamber that may have been presentwhen the rift was a fast-spreading one. One dredge of the Guadalupefailed rift also recovered MORB lava, but it is more evolvedthan that at the Mathematicians. Dredges of failed transformfaults in the Mathematicians recovered fresh alkali basalt.This post-abandonment alkali basalt may be analogous to alkaliclavas erupted off-ridge near active spreading ridges like theEast Pacific Rise. Dredges of an E-W bathymetric linear featurenear the Mathematician ridge south of the Clarion fracture zonealso recovered very fresh alkalic lavas. These are distinctfrom the post-abandonment alkalic rocks in the Mathematiciansand include hawaiites and trachytes, some of which were ‘popping’when first brought to the surface. We conclude that in somecases of fast-spreading rift abandonment, the rift first slowsdown and ceases to have a steady state magma chamber in thecrust. This is followed by the eruption of alkali basalts atfailed transforms as the lithosphere thickens in place.     

Comparative Study of Magmatism in East Pacific Rise Versus Nearby Seamounts: Constraints on Magma Supply and Thermal Structure Beneath Mid‐ocean Ridge

Major elements of 2202 basalts from the East Pacific Rise (EPR) and 888 basalts from near-EPR seamounts are used to investigate their differences in magma crystallization pressures and mantle melting conditions. Crystallization pressure calculation from basalts with 5.0wt%相似文献   

Anomalous phenomena preceding earthquakes (one more approach to discussion on SES nature and VAN-prediction reliability)

Seismic electric signals (SES) preceding strong earthquakes (EQ) are reliable recorded and separated by VAN group on special station network in Greece. Traditional commonly accepted idea that original cause of EQ and all its precursors is the stress accumulation in focal zone meets difficulties in explanation of SES features: long distance appearance, selectivity, lack of reversibility, alternation. Recently proposed (Guterman & Khazan, 1994) magmatic mechanism of shallow crustal EQ preparation allows to construct the model for explanation of most enigmatic SES features. Basic model consists of mantle chamber, the central crustal magma chamber and magma channel connecting mantle and crustal chambers. Basic model can be supplemented by set of secondary crustal chambers with their magma channels spreading quasiradially from centre. Data of geology on giant radiating dike swarms confirm plausibility of the model.     

Crustal accretion and dynamic feedback on mantle melting of a ridge centred plume: The Iceland case

A major consequence of the interaction of a plume with an oceanic ridge is the enhanced melt production and associated crust generation. In the case of Iceland crustal thickness as large as 20 to 40 km has been reported. Crustal seismic velocities are high, and have to be explained by thermal or chemical effects. In the first part of the paper we address the question whether extraction of melt out of the plume beneath a slowly spreading ridge and deposition of extracted basalt volumes at the surface produces a dynamic feedback mechanism on mantle melting. To study this question we solve the convection equations for a ridge centred plume with non-Newtonian rheology including melting, melt extraction associated with deposition of cold crust at the surface of the model, and using a simplified approach for compaction. The assumption of cold crust is justified if the thickness of each deposited basaltic layer is less than roughly 1 km. Depending on the buoyancy flux of the plume, crustal thicknesses between 10 and 40 km are modelled, showing characteristic dipping structures resembling the rift-ward dipping basaltic layers of East- and Western Iceland. Comparing the resulting crustal thickness and magma generation rate with models in which the dynamic effect of crust deposition has been suppressed indicates, that melt generation beneath a slowly spreading ridge is considerably damped by the dynamic feedback mechanism if the plume buoyancy flux exceeds 400 to 600 kg/s. Based on the observed crustal thickness of Iceland our models predict a plume buoyancy flux of 1140 kg/s.In the second part we study the accretion of the Icelandic crust by a thermo-mechanical model in more detail based on the Navier–Stokes-, the heat transport and the mass conservation equations including volumetric sources. Hot (1200 °C) molten crustal material is injected into the newly forming crust with a constant rate at different crustal source regions: a) deep, widespread emplacement of dykes and sills including crustal underplating, b) magma chambers at shallow to mid-crustal level, and c) surface extrusions and intrusions in fissure swarms at shallow depth connected to volcanic centres. We identify the material from the different source regions by a marker approach. Varying the relative dominance of these source regions, characteristic crustal structures evolve, showing shallow dipping upper crustal layers with dip angles between 10 and 15°. The thermal structure of the crust varies between cold crust (shallow-source region dominating) and hot crust (deep-source region dominating). We use observations of maximum depth of seismicity to constrain the depth of the 650 °C isotherm and seismological inferences on the lower crust to constrain temperatures in that region. The best agreement with our models is achieved for crust formation dominated by deep dykes and underplating with a considerable influence of magma chamber accretion.     

Supra-subduction and abyssal mantle peridotites of the Coast Range ophiolite,California

The Coast Range ophiolite (CRO) of California is one of the most extensive tracts of oceanic crust preserved in the North American Cordillera, but its origin has long remained controversial. We present here new data on mineral compositions in mantle peridotites that underlie crustal sections of the ophiolite, and show that these are dominantly refractory harzburgites related to high apparent melting in a supra-subduction zone (SSZ) setting. Abyssal peridotite (characterized by high-Al spinels and relatively high Ti, Na, Nd, Sm, Lu, and Hf in pyroxene) occurs at one location where it is associated with SSZ mantle peridotite and volcanic rocks with both oceanic and arc-like geochemistry. SSZ mantle peridotites (characterized by intermediate-Cr/Al or high-Cr spinels, and by extremely low Ti, Na, Nd, Sm, Lu, and Hf in pyroxenes) are associated with crustal sections containing arc-related volcanic rocks, including boninites. This convergence between conclusions based on crustal lithologies and their underlying mantle sections confirms previous proposals that link the CRO to SSZ processes, and seriously undermines hypotheses that invoke formation of the ophiolite at a mid-ocean ridge spreading center.