The Petrology, Geochemistry, and Petrogenesis of E-MORB-type Mafic Rocks from the Guomangco Ophiolitic Mélange, Tibet

The Guomangco ophiolitic melange is situated in the middle part of the Shiquanhe- Yongzhu-Jiali ophiolitic melange belt （SYJMB） and possesses all the subunits of a typical Penrose- type ophiolite pseudostratigraphy. The study of the Guomangco ophiolitic melange is very important for investigating the tectonic evolution of the SYJMB. The mafic rocks of this ophiolitic melange mainly include diabases, sillite dikes, and basalts. Geochemical analysis shows that these dikes mostly have E-MORB major and trace element signatures; this is the first time that this has been observed in the SYJMB. The basalts have N-MORB and IAB affinities, and the mineral chemistry of harzburgites shows a composition similar to that of SSZ peridotites, indicating that the Guomangco ophiolitic melange probably originated in a back-arc basin. The Guomangco back-arc basin opened in the Middle Jurassic, which was caused by southward subduction of the Neo-Tethys Ocean in central Tibet. The main spreading of this back-arc basin occurred during the Late Jurassic, and the basalts were formed during this time. With the development of the back-arc basin, the subducted slab gradually retreated, and new mantle convection occurred in the mantle wedge. The recycling may have caused the metasomatized mantle to undergo a high degree of partial melting and to generate E- MORBs in the Early Cretaceous. E-MORB-type dikes probably crystallized from melts produced by about 20%-30% partial melting of a spinel mantle source, which was metasomatized by melts from low-degree partial melting of the subducted slab.     

Re-Os同位素体系在蛇绿岩应用研究中的进展

Re-Os不同于由亲石元素构成的同位素体系，在原始上地幔（PUN）部分熔融过程中，母体Re是中等不相容元素，优先进入熔体相，子体Os是强相容元素，富集在残留相中，是研究蛇绿岩的极好示踪剂。在蛇绿岩应用研究中已经取得了4个方面的进展：（1）明确了熔体相的Re／Os和^187Os／^188Os比值高，而残留相的低；（2）铬铁矿中铂族元素矿物（PGM）的Re亏损年龄（TRD）证实了蛇绿岩中复杂的超镁铁岩体是多阶段部分熔融的产物；（3）现代大洋橄榄岩和玄武岩的Re-Os同位素研究表明熔体相和残留相的^187Os／^188Os比值在高于亏损地幔值（DMM）的部分是一致的，而低于DMM的存在不一致性，为研究蛇绿岩中熔体相与残留相是否存在“耦合”关系提供了新的制约因素；（4）揭示了蛇绿岩地幔橄榄岩中含有古大陆岩石圈地幔，这是前所未知的。虽然取得了不少进展，但是由于Re-Os同位素体系用于蛇绿岩研究的时间较短，尚存在一些问题，如显生宙蛇绿岩地幔橄榄岩的定年问题，有待进一步深化研究。     

西藏雅鲁藏布江蛇绿岩带罗布莎地幔橄榄岩的成因

广泛分布在雅鲁藏布江蛇绿岩带的地幔橄榄岩是没有蛇纹石化的新鲜岩石,因而通过显微镜下观察和全岩化学分布探索其成因和地幔活动,是十分有利和简便的。最近几年来,利用岩石化学资料揭示远洋橄榄岩,认为它不是原始地幔岩经分或分离熔化作用萃取出玄武岩熔体后的直接残余物。罗布落蛇绿岩橄的镜下观察和岩石化学资料也证明,它不是原始地幔岩经局部熔化后的残余物,而且由来自更深层的地幔过渡带的硅酸盐超高压矿物底辟上升经熔取     

Mid-ocean ridge and supra-subduction affinities in the Pindos ophiolites (Greece): implications for magma genesis in a forearc setting

The Pindos ophiolitic massif is considered an important key area within the Albanide–Hellenide ophiolitic belt and is represented by two tectonically distinct ophiolitic units: (1) a lower unit, including an intrusive and a volcanic section; and (2) an Upper Ophiolitic Unit, mainly including mantle harzburgites. Both units share similar metamorphic soles and tectono-sedimentary mélanges at their bases.

The intrusive section of the lower unit is composed by an alternation of troctolites with various ultramafic rock-types, including dunites, lherzolites, olivine-websterites, olivine-gabbros, anorthositic gabbros, gabbros and rare gabbronorites.

The volcanic and subvolcanic sequence of the lower unit can geochemically be subdivided into three groups of rocks: (1) basalts and basaltic andesites of the lower pillow section showing a clear high-Ti affinity; (2) basaltic andesites of the upper pillow section with high-Ti affinity, but showing many geochemical differences with respect to the first group; (3) very low-Ti (boninitic) basaltic and basaltic andesitic lava flows separating the lower and upper pillow sections, and dykes widespread throughout the Pindos ophiolites.

These different magmatic groups originated from fractional crystallization from different primary magmas, which were generated, in turn, from partial melting of mantle sources progressively depleted by previous melt extractions. Group 1 volcanics may have derived from partial melting (ca. 20%) of an undepleted lherzolitic source, while group 2 basaltic rocks may have derived from partial melting (ca. 10%) of a mantle that had previously experienced mid-ocean ridge basalt (MORB) extraction. Finally, the Group 3 boninites may have derived from partial melting (ca. 12–17%) of a mantle peridotite previously depleted by primary melt extraction of Groups 1 and 2 primary melts.

In order to explain the coexistence of these geochemically different magma groups, two petrogenetic models formerly proposed for the Albanian ophiolites are discussed.     

Origin of the Zedang and Luobusa Ophiolites,Tibet

The Zedang and Luobusa ophiolites are located in the eastern section of the Yalung Zangbo ophiolite belt,and they share similar geological tectonic setting and age.Thus,an understanding of their origins is very important for discussion of the evolution of the Eastern Tethys Ocean.There is no complete ophiolite assemblage in the Zedang ophiolite.The Zedang ophiolite is mainly composed of mantle peridotite and a suite of volcanic rocks as well as siliceous rocks,with some blocks of olivinepyroxenite.The mantle peridotite mainly consists of Cpx-harzburgite,harzburgite,some lherzolite,and some dunite.A suite of volcanic rocks is mainly composed of caic-aikaline pyroclastic rocks and secondly of tholeiitic pillow lavas,basaltic andesites,and some boninitic rocks with a lower TiO2 content （TiO2 ＜ 0.6％）.The pyroclastic rocks have a LREE-enriched REE pattern and a LILE-enriched （compared to HFSE） spider diagram,demonstrating an island-arc origin.The tholeiitic volcanic rock has a LREE-depleted REE pattern and a LILE-depleted （compared to HFSE） spider diagram,indicative of an origin from MORB.The boninitic rock was generated from fore-arc extension.The Luobusa ophiolite consists of mantle peridotite and mafic-ultramaflc cumulate units,without dike swarms and volcanic rocks.The mantle peridotite mainly consists of dunite,harzburgite with low-Opx （Opx ＜ 25％）,and harzburgite （Opx ＞ 25％）,which can be divided into two facies belts.The upper is a dunite-harzburgite （Opx ＜ 25％） belt,containing many dunite lenses and a large-scale chromite deposit with high Cr203; the lower is a harzburgite （Opx ＞25％） belt with small amounts of dunite and lherzolite.The Luobusa mantle peridotite exhibits a distinctive vertical zonation of partial melting with high melting in the upper unit and low melting in the lower.Many mantle peridotites are highly depleted,with a characteristic U-shaped REE pattern peculiar to fore-arc peridotite.The Luobusa cumulates are composed of wehrlite and olivine-pyroxenite,of the P-P-G ophiolite series.This study indicates that the Luobusa ophiolite was formed in a fore-arc basin environment on the basis of the occurrence of highly depleted mantle peridotite,a high-Cr2O3 chromite deposit,and cumulates of the P-P-G ophiolite series.We conclude that the evolution of the Eastern Tethys Ocean involved three stages：the initial ocean stage （formation of MORB volcanic rock and dikes）,the forearc extension stage （formation of high-Cr203 chromite deposits and P-P-G cumulates）,and the islandarc stage （formation of caic-alkaline pyroclastic rocks）.     

Basal lherzolites in the Bay of Islands Ophiolite: origin by detachment-related telescoping of a ridge-parallel melting gradient

Decompression melting under a spreading centre should generate a residual upper mantle section with a weak vertical gradient in the degree of chemical depletion. Chemical data from the 6 km-thick ophiolitic mantle section of the Table Mountain massif (Bay of Islands Ophiolite, Newfoundland) reveal, however, a depletion gradient of 1% melting/100 m in the basal mantle section of the massif. Within the framework of a segmented ridge we suggest that during ophiolite detachment intermittent accretion of progressively less depleted peridotites occurred to the (former) base of the ophiolite, amounting to a several hundred metre-thick lherzolitic sole. In this view, the basal mantle section in Table Mountain extends a structural concept previously established for the metamorphic sole: both metamorphic gradient in the sole and melting gradient in the basal mantle rocks are apparent and of tectonic nature. Like the sole, the basal mantle sequence may thus expose mantle material derived from a scale larger than the preserved ophiolite complex.     

Petrology, Geochemistry and Tectonic Significance of the Metamorphic Peridotites from Longmuco–Shuanghu Ophiolitic Melange belt, Tibet

Taoxinghu metamorphic peridotite is a firstly reported mantle sequence of ophiolite since Longmuco–Shuanghu–Lancangjiang suture zone (LSLSZ) was proposed, and it is also an important discovered for ophiolite studying in central Qiangtang. Based on detailed analyses of whole–rock geochemistry of Taoxinghu metamorphic peridotites and contrast to metamorphic peridotites in typical ophiolites worldwide, the paper investigates their petrogenesis and geological implication. The petrologic results show that the protolith of Taoxinghu metamorphic perdotites have the mineral assemblage and texture characteristic of mantle peridotite. Most metamorphic peridotites hav near global abyssal peridotites major elements contents, while the few is similar to SSZ–type peridotites. They exhibit typically U–shaped REE patterns, characterized by slight enrichment of LREE and HREE relative to MREE and a low fractionated LREE to HREE segment. Trace elements contents are low and all samples are strong enrichment in Cs, U, Pb, weak enrichment in Ba and depletion in Th, but negative Nb anomalies are only observed in few samples. That suggests Taoxinghu metamorphic peridotites have depletion mantle and suprasubduction affinities. A two–stage evolution history is considered: Taoxinghu metamorphic peridotites originated as the residue from melting at a ridge with 7%–20% degree of fraction melting and were subsequently modified by interaction with mafic melt and aqueous fluid within mantle wedge on subducted zone. Combined with previous studies, we preliminarily propose Taoxinghu metamorphic peridotites may be the Products of initial rifting of palo–Tethys, forming at middle Ordivician–upper Cambrian, and they may be the direct evidences for spreading of palo–Tethys.     

Geochemical make-up of oceanic peridotites from NW Turkey and the multi-stage melting history of the Tethyan upper mantle

We present the whole-rock and the mineral chemical data for upper mantle peridotites from the Harmanc?k region in NW Turkey and discuss their petrogenetic–tectonic origin. These peridotites are part of a Tethyan ophiolite belt occurring along the ?zmir-Ankara-Ercincan suture zone in northern Turkey, and include depleted lherzolites and refractory harzburgites. The Al₂O₃ contents in orthopyroxene and clinopyroxene from the depleted lherzolite are high, and the Cr-number in the coexisting spinel is low falling within the abyssal field. However, the orthopyroxene and clinopyroxene in the harzburgites have lower Al₂O₃ contents for a given Cr-number of spinel, and plot within the lower end of the abyssal field. The whole-rock geochemical and the mineral chemistry data imply that the Harmanc?k peridotites formed by different degrees of partial melting (~%10–27) of the mantle. The depleted lherzolite samples have higher MREE and HREE abundances than the harzburgitic peridotites, showing convex-downward patterns. These peridotites represent up to ~16 % melting residue that formed during the initial seafloor spreading stage of the Northern Neotethys. On the other hand, the more refractory harzburgites represent residues after ~4–11 % hydrous partial melting of the previously depleted MOR mantle, which was metasomatized by slab-derived fluids during the early stages of subduction. The Harmanc?k peridotites, hence, represent the fragments of upper mantle rocks that formed during different stages of the tectonic evolution of the Tethyan oceanic lithosphere in Northern Neotethys. We infer that the multi-stage melting history of the Harmanc?k peridotites reflect the geochemically heterogeneous character of the Tethyan oceanic lithosphere currently exposed along the ?zmir-Ankara-Erzincan suture zone.     

The Ronda high temperature peridotite: Geochemistry and petrogenesis

The Ronda peridotite in southern Spain is a large (~300 km²) exposure of upper mantle which provides direct information about mantle processes on a scale much larger than that provided by mantle xenoliths in basalt. Ronda peridotites range from harzburgite to lherzolite, and vary considerably in major element content, e.g., Al₂O₃ from 0.9 to 4.8%, and trace element abundances, e.g., Sr, Zr and La abundances vary by factors of 20 to 40. These compositional variations are systematic and correlate with (pyroxene + garnet)/olivine ratios and olivine compositions. The data are consistent with formation of residual peridotites by variable degrees of melting (~0 to 30%) of a compositionally homogeneous peridotite. None of the peridotites have geochemical characteristics of residues formed by extensive (?5%) fractional melting and the data can be explained by equilibrium (batch) melting, possibly with incomplete melt segregation in some samples. Based on compositional differences between Ronda peridotites, the segregated melts were picritic (12–22% MgO) with relative rare earth element abundances similar to mid-ocean ridge basalt (MORB). Prior to the melting event the Ronda peridotite body was a suitable source for MORB. The compositional characteristics of Ronda peridotites are consistent with diapiric rise of a fertile mantle peridotite with relatively small degrees of melting near the diapir-wall rock interface yielding residues of garnet iherzolite, and larger degrees of melting in the diapir interior yielding residues of garnet-free peridotite. Subsequently these residual rocks were recrystallized at sub-solidus conditions (Obata, 1980), and emplaced in the crust by thrusting (Lundeen, 1978).     

西藏西南部达巴-休古嘎布蛇绿岩带中地幔橄榄岩的成因

本文报道了西藏西南部达巴-休古嘎布蛇绿岩带中橄榄岩的矿物化学资料.橄榄岩中主要造岩矿物化学成分的分析研究表明,该区橄榄岩为残余地幔成因,但它们不是地幔简单熔化的残余物.尖晶石中Cr#及辉石中Ti的广泛变化表明它们具有复杂的熔融历史及地幔交代作用的印记,其形成过程可能经历了两种构造环境的转变.早期在MORB环境下形成低Cr#(尖晶石)橄榄岩;其后由于洋内俯冲作用,早先形成的低熔橄榄岩被消减到岛弧之下再度发生熔融形成高Cr#(尖晶石)橄榄岩.从而,在古大洋消失之后形成的碰撞带上同时保存了MORB型和SSZ型两类蛇绿岩.     

Plagioclase lherzolite-residual mantle relationships within two eastern mediterranean ophiolites

The ophiolites of Othris, northern Greece, and Troodos, Cyprus, are of mixed lherzolite-harzburgite and harzburgite sub-type respectively. Within both ophiolites an entire spectrum of harzburgite, plagioclase harzburgite, lherzolite and plagioclase lherzolite interpreted respectively as residual and highly, moderately and slightly modified upper mantle has been recognised. Plagioclase lherzolite frequently contains gabbroic segregations generated by partial melting and incomplete extraction. Othris shows the full range of mantle peridotites while Troodos is dominated by harzburgite with very minor occurrences of plagioclase harzburgite and lherzolite. Chemically, the plagioclase lherzolites have low contents of the basaltophillic minor elements, and a composition slightly more residual than postulated upper mantle compositions, suggestive of a preceding phase of minor depletion. The Othris and Troodos ophiolites seem to have formed under fundamentally different environments — Othris as a marginal ophiolite at the inception on rifting of continental crust, and Troodos later in such an event when spreading was well established.     

Peridotites from the Kamchatsky Mys: evidence of oceanic mantle melting near a hotspot

A suite of mantle peridotites sampled in the Kamchatsky Mys includes spinel lherzolite, clinopyroxene-bearing harzburgite, and harzburgite. Mineral chemistry of olivine, chromian spinel, and clinopyroxene show strongly correlated element patterns typical of peridotite formed by 8% to more than 22% partial melting. Clinopyroxene in the Kamchatka peridotites is compositionally different from that of both abyssal and suprasubduction varieties: Clinopyroxene in lherzolite is depleted in LREE relative to abyssal peridotite and that in harzburgite has very low LREE and Sr unlike the subduction-related counterpart. These composition features indicate that the rocks ultra-depleted in basaltic components originated in the vicinity of a hotspot, possibly, proto-Hawaiian plume, which provided high temperature and melting degree of the MORB source mantle at mid-ocean ridge.     

阿尔金红柳沟蛇绿岩研究进展

实测了2条阿尔金地区红柳沟蛇绿混杂带的地质剖面.地球化学的研究发现混杂带中含有2种基性火山岩:MORB型和OIB型,其稀土配分型式分别为平坦型和轻稀土富集型,结合同位素ε_Nd值,推断混杂带中变基性火山岩来自于不同的岩浆源.变质橄榄岩稀土总量低,具有亏损型和平坦型2种不同的稀土配分型式.这种MORB和OIB的组合代表了地幔柱物质与洋壳物质的混合作用.     

特提斯洋对流上地幔Os同位素组成估算的新方法及初步运用

对流上地幔Os同位素组成的准确估算是运用Re-Os同位素体系探讨地幔演化的基础。前人研究主要是以地幔橄榄岩为研究对象,由于地幔橄榄岩Os同位素存在明显的不均一性,因而直接影响估算值的准确性。对流上地幔中包含的不同亏损程度的难熔组分在部分熔融过程中难以熔融,对形成的熔体相的Os同位素组成贡献很少或者没有。因此,与对流上地幔具有相同的Os同位素组成初始值的早期分离结晶岩石(如堆晶岩),结合堆晶岩中锆石的准确定年,可以用来估算对流上地幔Os同位素组成。本文根据这一方法测试了那曲地区弧后盆地堆晶岩的Os同位素组成和锆石U-Pb年龄,推测那曲地区新特提斯洋对流上地幔Os同位素组成为碳质球粒陨石型的。根据这一模型,对比了罗布莎和东巧铬铁矿岩、含矿围岩以及不含矿围岩的Os同位素特征,揭示出矿石及围岩均具有古老大陆岩石圈地幔信息,而不含矿围岩(泽当岩体)的Os同位素组成为碳质球粒陨石型的,无古老大陆岩石圈地幔信息。     

Extensional to compressive Mesozoic magmatism at the SE Eurasia margin as recorded from the Meratus ophiolite (SE Borneo,Indonesia)

The Meratus ophiolitic series (SE Borneo) present a specific assemblage that have recorded (1) a continental extensional episode mostly seen within the peridotites and 2) later subduction-related magmatic events marked by the emplacement of calc-alkaline magmas. These events relate the magmatic activity and geodynamic evolution of the SE Eurasia margin in Mesozoic times. The ophiolitic series comprise ultramafic rocks with minor metavolcanic rocks. The ultramafic rocks include dominant lherzolites and pyroxenites with rather scarce harzburgites and dunites. Spinel peridotite, mineral chemistry data and bulk rock Rare Earth Element (REE) abundances show that most rocks underwent a low degree of partial melting. However, a few samples display significant depletions in Light REE (LREE), which are interpreted as the result of fractional melting under shallow conditions. Plagioclase-bearing peridotites are characterized by high REE abundances which also point to a very low degree of melting followed by reequilibration in the plagioclase facies, as seen from phase chemistry data. These peridotites are locally crosscut by dikelets containing high-temperature K-and Cr-rich amphiboles. Lavas closely associated with the Meratus peridotites have REE compositions ranging from the ones typical of enriched MORB (E-MORB) to normal MORB (N-MORB) types. We believe that the Meratus peridotites represent a fragment of subcontinental lithospheric mantle that locally suffered a low degree of fractional melting during the last stages of a continental rifting phase, in agreement with the presence of metamorphic K- and Cr-rich amphiboles in the peridotites. The E-MORB basalts might result from the melting of an enriched subcontinental lithosphere thermally eroded during the rifting phase by rising asthenosphere which might have produced N-MORB volcanic rocks. Back-arc basin basalts (BABB) now associated with E-MORB and N-MORB have also been found in the metamorphic soles of the peridotites. These rocks would have formed in a back-arc basin now accreted to the eastern margin of Eurasia. The latter are partly covered by calc-alkaline magmatism (Alino Formation). The ophiolitic series was later crosscut by calc-alkaline melts (Manunggul Formation). The Meratus ophiolitic series hence displays a dual origin. They witness 1) a continental episode mostly seen within the peridotites. and 2) later subductionrelated events marked by the emplacement of calc-alkaline magmas.     

Petrology and Geochemistry of the Dangqiong Ophiolite,Western Yarlung-Zangbo Suture Zone,Tibet,China

The Dangqiong ophiolite, the largest in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)ophiolite belt in southern Tibet, consists of discontinuous mantle peridotite and intrusive mafic rocks. The former is composed dominantly of harzburgite, with minor dunite, locally lherzolite and some dunite containing lenses and veins of chromitite. The latter, mafic dykes(gabbro and diabase dykes), occur mainly in the southern part. This study carried out geochemical analysis on both rocks. The results show that the mantle peridotite has Fo values in olivine from 89.92 to 91.63 and is characterized by low aluminum contents(1.5–4.66 wt%) and high Mg# values(91.06–94.53) of clinopyroxene. Most spinels in the Dangqiong peridotites have typical Mg# values ranging from 61.07 to 72.52, with corresponding Cr# values ranging from 17.67 to 31.66, and have TiO2 contents from 0 to 0.09%, indicating only a low degree of partial melting(10–15%). The olivine-spinel equilibrium and spinel chemistry of the Dangqiong peridotites suggest that they originated deeper mantle(20 kbar). The gabbro dykes show N-MORB-type patterns of REE and trace elements. The presence of amphibole in the Dangqiong gabbro suggests the late-stage alteration of subduction-derived fluids. All the lherzolites and harzburgites in Dangqiong have similar distribution patterns of REE and trace elements, the mineral chemistry in the harzburgites and lherzolites indicates compositions similar to those of abyssal and forearc peridotites, suggesting that the ophiolite in Dangqiong formed in a MOR environment and then was modified by late-stage melts and fluids in a suprasubduction zone(SSZ) setting. This formation process is consistent with that of the Luobusa ophiolite in the eastern Yarlung-Zangbo Suture Zone and Purang ophiolite in the western Yarlung-Zangbo Suture Zone.     

The behaviour of tungsten during mantle melting revisited with implications for planetary differentiation time scales

Tungsten is a moderately siderophile high-field-strength element that is hydrophile and widely regarded as highly incompatible during mantle melting. In an effort to extend empirical knowledge regarding the behaviour of W during the latter process, we report new high-precision trace element data (W, Th, U, Ba, La, Sm) that represent both terrestrial and planetary reservoirs: MORB (11), abyssal peridotites (8), eucrite basalts (3), and carbonaceous chondrites (8). A full trace element suite is also reported for Cordilleran Permian ophiolite peridotites (12) to better constrain the behaviour of W in the upper mantle. In addition, we report our long-term averages for a number of USGS (BIR-1, BHVO-1, BHVO-2, PCC-1, DTS-1) and GSJ (JA-3, JP-1) standard reference materials, some of which we conclude to be heterogeneous and contaminated with respect to W. The most significant finding of this study is that many of the highly depleted upper mantle peridotites contain far higher W concentrations than expected. In the absence of convincing indications for alteration, re-enrichment or contamination, we propose that the W excess was caused by retention in an Os-Ir alloy phase, whose stability is dependent on fO₂ of the mantle source region. This explanation could help to account for the particularly low W content of N-MORB and implies that the lithophile behaviour of W in basaltic rocks is not an accurate representation of the behaviour in the melt source. These findings then become relevant to the interpretation of W-isotopic data for achondrites, where the fractionation of Hf from W during melting is used to infer the Hf/W of the parent body mantle. This is exemplified by the differentiation chronology of the eucrite parent body (EPB), which has been modeled with a melt source with high Hf/W. By contrast, we explore the alternative scenario with a low mantle Hf/W on the EPB. Using available eucrite literature data, a maximum core segregation age of 1.2 ± 1.2 Myr after the closure of CAIs is calculated with a more prolonged time between core formation and mantle fractionation of ca. 2 Myr. This timeline is consistent with most recent published chronologies of the EPB differentiation based on the ⁵³Mn-⁵³Cr and ²⁶Al-²⁶Mg systems.     

西藏班公湖—怒江缝合带中段蓬湖蛇绿岩中的洋脊型二辉橄榄岩

蓬湖蛇绿岩产于西藏藏北湖区的蓬湖西侧,属班公湖-怒江缝合带中段白拉拉弄-依拉山亚带。该蛇绿岩主要由地幔橄榄岩、堆晶岩和辉绿岩等组成。其中地幔橄榄岩由方辉橄榄岩和二辉橄榄岩组成。蓬湖二辉橄榄岩的橄榄石Fo值介于88.85～90.33之间、斜方辉石的Al2O3含量范围在4.26%～6.60%。与原始地幔相比,蓬湖二辉橄榄岩岩石有较高的MgO含量和较低的Al2O3、CaO和TiO2等易熔组分含量;稀土元素总量介于1.11×10-6～1.53×10-6之间,明显低于原始地幔值,配分模式为轻稀土轻微亏损。在原始地幔微量元素蛛网图中,蓬湖二辉橄榄岩显示Rb、Zr亏损,U、Ta、Sr强烈富集特征。蓬湖二辉橄榄岩的铂族元素总量介于22.9×10-9～27×10-9之间,PGEs球粒陨石标准化图解显示其为接近原始地幔的"平坦型"。以上特征与深海橄榄岩相似,指示它们可能形成于大洋中脊环境。定量模拟估算表明,蓬湖二辉橄榄岩可能来源于地幔中尖晶石相二辉橄榄岩源区,系经历了约5%～10%的部分熔融残余。蓬湖堆晶岩矿物结晶顺序为橄榄石-单斜辉石-斜长石,其中异剥橄榄岩中的单斜辉石Mg#值介于86.92～89.93之间、橄榄石Fo平均值为84.45,明显不同于MOR型蛇绿岩堆晶岩。蓬湖堆晶岩的矿物组成、岩浆结晶顺序和矿物成分均与俯冲带上SSZ型蛇绿岩形成的堆晶岩类似。以上结果表明,蓬湖二辉橄榄岩形成于大洋脊环境,为尖晶石二辉橄榄岩源区经历了不超过10%部分熔融的残余,后期由于洋内俯冲作用经历了岩石-熔体反应,形成了SSZ型堆晶岩和含较高Cr#值尖晶石的方辉橄榄岩。     

Late Cretaceous subduction initiation and Palaeocene–Eocene slab breakoff magmatism in South-Central Anatolia,Turkey

The Late Cretaceous Alihoca ophiolite in the Inner Tauride suture zone (ITSZ) of South-Central Turkey represents part of a single ophiolitic thrust sheet that originated from the Inner Tauride ocean. The ophiolite contains upper mantle peridotites, cumulate wehrlites, layered-to-isotropic gabbros, and microgabbroic-to-doleritic dikes. An ophiolitic mélange beneath the Alihoca ophiolite includes blocks of limestone, peridotite, dolerite, basalt, and deep-sea sedimentary rocks (radiolarite, chert) in a matrix comprising sheared serpentinite and mudstone. Isotropic gabbro and dolerite dike rocks show enrichment in Sr, K, Rb, Ba, and Th (LILE) and depletion of Ta, Nb, Zr, Ti, and Y (HFSE), indicating an island arc tholeiite (IAT) affinity. Relatively younger dolerite rocks display low TiO₂ (<0.5 wt.%) contents, concave REE profiles with low HREE concentrations, and high LREE values, typical of boninitic affinities. The Alihoca ophiolite, hence, displays an IAT to boninitic geochemical progression in its magmatic evolution, reminiscent of many other Tethyan ophiolites in the region. It represents the remnant of a forearc oceanic crust, which developed during the early stages of subduction within the Inner Tauride ocean. Volcanic, volcano-sedimentary, and sedimentary rocks of the Uluk??la–Çamard? basin north of the ITSZ disconformably overlie the mafic-ultramafic rocks of the Alihoca ophiolite. Pillowed and massive lavas of the latest Cretaceous–Palaeocene Uluk??la Formation have alkaline basalt-to-basaltic andesite compositions, displaying relatively enriched LILE and LREE patterns with negative Nb and Ta anomalies. These geochemical features suggest that magmas of the Uluk??la–Çamard? volcanic rocks formed from partial melting of a metasomatized lithospheric mantle. This melting event was triggered by the influx of asthenospheric heat through a slab breakoff-induced window in the downgoing Tethyan oceanic lithosphere.     

Geochemistry of basic dikes in the Lanzo massif (Western Alps): Petrogenetic and geodynamic implications

The Alpine peridotite massif of Lanzo (Italy) contains three generations of basic dikes (gabbros and basalts). The older gabbros are plagioclase-rich mantle segregates while the younger gabbro dikes are cumulates very similar in chemical composition to recent oceanic gabbros and gabbros from ophiolitic complexes. They both were derived from the N-type mid-ocean ridge basalt (MORB) magmas which were progressively more depleted in incompatible elements and were probably generated during a dynamic melting of a rising mantle diapir. The basaltic dikes are the N-type MORB and closely resemble the Alpine-Apennine ophiolitic basalts. They were derived from a different upper mantle source than the parental magmas of the gabbros. The source of the basalts was less depleted in light REE. The presence of basic magmas with N-type MORB affinities in the Lanzo massif is consistent with the close genetic relationship between the Alpine peridotite body and the ophiolites of the Liguro-Piemontese basin.