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1.
ABSTRACT

The Neo-Tethys-related Chaldoran ophiolite peridotites in NW Iran are remnants of mantle lithosphere, exhumed tectonically during the Late Cretaceous. Harzburgite is the predominant peridotite type, associated with oceanic lower crust cumulate gabbros occasionally. The ophiolite rocks are unconformably overlain by Late Cretaceous-Paleocene sediments. New whole-rock geochemistry of the variably serpentinized harzburgites shows a depleted nature, exemplified by low Al2O3, CaO, TiO2, V and Y and high Ni, Cr and Mg and also low rare earth element (REE) contents. The harzburgites present LREE enrichment. Positive correlations between some LREEs and high field strength elements (HFSE) suggest enrichment of LREEs by melt re-fertilization processes. Cr-spinels have Cr number of [Cr# = Cr/(Cr + Al) = 0.53–0.67], showing medium to high degree of partial melting (F = ~17-20%). Both whole-rock and mineral chemistry data show a supra-subduction zone setting and progressive depletion along with increase in spinel Cr# (MOR to fore arc). The cumulate gabbros have high MgO and SiO2, low TiO2 and Ti/V < 10 and also low chondrite normalized Dy (<8.5). The gabbro samples show enriched LREEs and LILEs and depleted HREEs and HFSEs with respect to MORBs.

Subduction initiation (SI) model in a fore-arc/proto-fore-arc environment is suggested for the upper mantle evolution of the Chaldoran ophiolite. The rocks have experienced depletion in a second melting process at the later stages of SI and compositions were probably modified by extraction of island arc tholeiitic (IAT) and possibly boninitic (BON) melts. The chemostratigraphic progression for ‘subduction initiation rule (SIR)’ is likely traceable in Chaldoran mafic-ultramafic sequence, which corresponds to the most Neo-Tethyan ophiolites and is similar to MOR to supra-subduction zone (SSZ) evolution of most Iranian ‘Inner’ and ‘Outer Zagros’ ophiolitic peridotites.  相似文献   

2.
错不扎蛇绿岩位于雅鲁藏布江缝合带西段北亚带,岩体呈北西-南东走向带状产出,主要由地幔橄榄岩和辉长岩脉组成。地幔橄榄岩主体为方辉橄榄岩,详细的矿物学及岩石地球化学研究表明,错不扎方辉橄榄岩中橄榄石为镁橄榄石,斜方辉石主要为顽火辉石,而单斜辉石主要为顽透辉石和透辉石,铬尖晶石具有高Al和高Mg(Mg#=60~70)特征。稀土配分图解显示其具有轻稀土亏损而重稀土富集的左倾型亏损地幔源区特征,(La/Yb)N=0.11~0.60,模拟结果显示其为经历了15%~20%部分熔融后的残余,与快速扩张大洋中脊环境下形成的深海橄榄岩的熔融程度(10%~22%)较为一致。此外,错不扎方辉橄榄岩轻稀土含量明显高于部分熔融模型中LREE的含量,而且,在微量元素原始地幔标准化图解中富集大离子亲石元素Rb、Sr和高场强元素Ta、Hf和Ti,这一特征指示错不扎方辉橄榄岩在大洋中脊环境形成后又受到后期俯冲带熔/流体的改造。结合南北两带不同蛇绿岩体构造环境的对比,笔者认为雅鲁藏布江西段南北两带蛇绿岩体具有相似的形成环境,两者在地理位置以及产状方面的差别可能是受到构造侵位的影响。  相似文献   

3.
青藏高原西部蛇绿岩类型:岩石学与地球化学证据   总被引：22，自引：0，他引：22  
对青藏高原西部地区的班公湖蛇绿岩、狮泉河蛇绿岩、雅鲁藏布江西段蛇绿岩和普兰—当穷蛇绿岩带中代表性岩体的地质学、岩石化学、稀土元素、微量元素、Pb、Sr同位素地球化学研究表明,青藏高原西部地区4条蛇绿岩中的地幔橄榄岩主要为方辉橄榄岩和少量纯橄岩,岩石化学成分具有富镁、贫铝、钙、碱的特点;论述了地幔橄榄岩轻稀土元素富集是由于先经历了较强的部分熔融,后经历了俯冲消减过程中的流体交代的二次过程;微量元素中大离子亲石元素Rb、不活动元素Nb、Zr、Hf和放射性生热元素Th等元素的丰度较高,以及Ti、Sm、Y、Yb等强不相容元素亏损的特点,与交代地幔岩特征类似;Pb、Sr同位素组成具有明显的壳源组分混入的特点,说明青藏高原西部的蛇绿岩曾受洋壳俯冲消减过程中的流体交代作用,蛇绿岩产于SSZ构造环境。对比青藏高原东部、三江、西昆仑地区以及形成于典型的SSZ环境的Troodos蛇绿岩中的地幔橄榄岩,就岩石化学富MgO、轻稀土元素富集而言,它们具有与青藏高原西部基本一致的地质地球化学特征,结合与俯冲岩浆作用有关的玻安岩和埃达克岩产出,说明可能包括三江、西昆仑库地在内的青藏高原不同时代蛇绿岩都主要形成于俯冲消减环境,属于SSZ型蛇绿岩。  相似文献   

4.
李平  王洪亮  徐学义  陈隽璐  过磊  奚仁刚 《岩石学报》2014,30(12):3553-3568
新疆西准噶尔北部地区的早泥盆世马拉苏组出露有少量富钠低钾的拉斑质中基性熔岩,这些分布于谢米斯台断裂北侧的玄武安山岩和玄武岩多呈夹层状断续产出于火山碎屑岩之中。马拉苏中基性熔岩的Mg#与主、微量元素协变关系及Th-Th/Nd图反映了其并非同源岩浆演化的结果。马拉苏火山岩中的玄武安山岩富集LILE、亏损HFSE,具有较高的Th含量及较低的Hf/Th和(Nb/Th)PM比值,显示出弧火山岩的地球化学特征。其中的玄武岩则具有略为平坦的稀土元素分配样式,较低的Th含量及较高的Hf/Th和(Nb/Th)PM比值,此同MORB地球化学特征极为相似;虽然其也显示有轻微的LILE富集、HFSE亏损,但是较高的La/Nb比值则暗示这同地壳或俯冲物质组分的卷入有关,且一系列构造环境判别图解也进一步印证了马拉苏组内的玄武岩应属似MORB基性熔岩。此外,两类岩石的高场强元素比值Zr/Nb、Hf/Ta同全球平均大洋中脊玄武岩的相应比值极为接近,反映了马拉苏组中基性火山岩的物质源区主体均为MORB地幔物质源区。La/Yb-Gd/Yb原始地幔标准化比值的模拟计算进一步显示了马拉苏组玄武安山岩与受改造(俯冲沉积物或地壳物质的混染)的石榴子石或尖晶石-石榴子石地幔橄榄岩物质源区的部分熔融作用有关,而似MORB型玄武岩则源自尖晶石地幔橄榄岩源区的部分熔融。结合区内同期的蛇绿岩、火山岩和碱性花岗岩的地球化学研究,我们可以进一步推断此类兼具有似MORB和弧火山岩地球化学特征的早泥盆世马拉苏火山岩应当是西准噶尔地块北部在早古生代受后期俯冲作用影响下经历弧后扩张形成的火山-岩浆地质记录。  相似文献   

5.
In situ zircon U–Pb ages and Hf isotope data, major and trace elements and Sr–Nd–Pb isotopic compositions are reported for coeval syenite–granodiorites–dacite association in South China. The shoshonitic syenites are characterized by high K2O contents (5.9–6.1 wt.%) and K2O/Na2O ratios (1.1–1.2), negative Eu anomalies (Eu/Eu* = 0.65 to 0.77), enrichments of Rb, K, Nb, Ta, Zr and Hf, but depletion of Sr, P and Ti. The adakitic granodiorite and granodiorite porphyry intrusions are characterized by high Al2O3 contents (15.0–16.8 wt.%), enrichment in light rare earth elements (LREEs), strongly fractionated LREEs (light rare earth elements) to HREEs (heavy rare earth elements), high Sr (438–629 ppm), Sr/Y (29.2–53.6), and low Y (11.7–16.8 ppm) and HREE contents (e.g., Yb = 1.29–1.64 ppm). The calc-alkaline dacites are characterized by LREE enrichment, absence of negative Eu anomalies, and enrichment of LILEs such as Rb, Ba, Th, U and Pb, and depletion of HFSEs such as Nb, Ta, P and Ti.Geochemical and Sr–Nd–Hf isotopic compositions of the syenites suggest that the shoshonitic magmas were differentiated from parental shoshonitic melts by fractional crystallization of olivine, clinopyroxene and feldspar. The parent magmas may have originated from partial melting of the lithospheric mantle with small amount contribution from crustal materials. The adakitic granodiorite and granodiorite porphyry have Sr–Nd–Pb isotopic compositions that are comparable to that of the mafic lower crust. They have low Mg# and MgO, Ni and Cr contents, abundant inherited zircons, low εNd(t) and εHf(t) values as well as old whole-rock Nd and zircon Hf model ages. These granodiorites were likely generated by partial melting of Triassic underplated mafic lower crust. The Hf isotopic compositions of the dacites are relatively more depleted than the Cathaysia enriched mantle, suggesting those magmas were derived from the partial melting of subduction-modified mantle sources. The coeval shoshonitic, high-K calc-alkaline and calc-alkaline rocks in Middle to Late Jurassic appear to be associated with an Andean-type subduction. This subduction could have resulted in the upwelling of the asthenosphere beneath the Cathaysia Block, which induced partial melting of the mantle as well as the mafic lower crust, and formed an arc regime in the coastal South China during Middle to Late Jurassic.  相似文献   

6.
The study of geology, geochemistry, rare earth elements, trace elements, Pb and Sr isotopes of representative ophiolite bodies from four ophiolitic belts in the western Qinghai-Tibetan Plateau, shows that the mantle peridotites of these ophiolites are mainly harzburgite in composition, with minor dunite. They are characterized by high magnesium (MgO) and low aluminum, calcium and alkali oxide contents. Enrichment of light rare earth elements in mantle peridotites may be due to two geological processes: relatively strong partial melting; and later metasomatism by the liquids released during the subduction of oceanic crust. Mantle peridotites are characterized by low contents of the trace elements Sr, Ti and Y and relatively high contents of Rb, Nb, Zr, Hf and Th, similar to metasomatic pyrolite. The isotopic compositions of Sr and Pb show evidence of contamination by a crustal component. All the evidence indicates that the four ophiolite belts in the western Qinghai-Tibetean Plateau have undergone metasomatism by liquids released during the subduction of oceanic crust, suggesting that they were formed in a supra-subduction zone (SSZ) tectonic setting. The mantle peridotites in ophiolite belts located in eastern Qinghai-Tibetan Plateau, e.g. Sanjiang and West Kunlun, may be compared with the Troodos, which is regarded as a typical SSZ complex, having the same geochemical characteristics, i.e. high MgO and LREE-rich. The geochemistry, combined with the occurrence of boninite and adakite rocks, which are associated with subduction magmatism, suggest that ophiolites formed at different times in Qinghai-Tibetan Plateau, including Sanjiang and West Kunlun, are all SSZ-type ophiolites formed in a supra-subduction zone tectonic setting.  相似文献   

7.
ABSTRACT

The Ordovician plutons in the Erguna Block, NE China, can be classified into two groups: Early Ordovician diorites with zircon U–Pb ages ranging from 486 to 485 Ma and Middle Ordovician gabbros and granites with zircon U–Pb ages ranging from 466 to 463 Ma. The diorites are calc-alkaline in nature and are characterized by weak to moderate enrichments of large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE) and heavy rare earth elements (HREE). The gabbros and granites have high total alkali contents, and all samples are enriched in LREE and LILE and depleted in HFSE such as Nb, Ta, and Ti. Isotopically, Early Ordovician diorites display values that are less radiogenic [εHf(t) = + 9.9–+16.8] compared to those of Middle Ordovician gabbros [εHf(t) = ? 3.0–+5.0]. Middle Ordovician granites have positive εHf(t) values of +1.4 to +4.3 and two-stage Hf model ages (TDM2) of 1167 to 1356 Ma. These data indicate that the diorites may have been generated by the partial melting of a recently metasomatized mantle source, whereas the gabbros and granites may have been formed by the partial melting of enriched lithospheric mantle and Mesoproterozoic crust, respectively. Our results, combined with other regional results, suggest that Early Ordovician magmatism was likely associated with the northward subduction of the Heihe–Xilinhot oceanic plate beneath the Erguna–Xing’an Block, whereas the Middle Ordovician gabbros and granites were most likely formed in an extensional setting controlled by the rollback of this subducted oceanic plate.  相似文献   

8.
Ophiolites exposed across the western Tauride Belt in SW Turkey represent tectonically emplaced fragments of oceanic lithosphere incorporated into continental margin following the closure of the Neotethys Ocean during the Late Cretaceous. The mantle sections of the ophiolites contain peridotites with diverse suites of geochemical signatures indicative of residual origin by melt depletion in both mid‐ocean ridge (MOR) and supra‐subduction zone (SSZ) settings. This study uses a laser‐ablation inductively‐coupled plasma‐mass spectrometry (LA‐ICP‐MS) for in situ measurements of trace elements in primary mantle phases in order to identify the upper mantle petrogenetic processes effective during variable stage of melt extraction in these discrete tectonic settings and to discriminate between the effects of reaction with chemically distinct mantle melts migrating through the solid residues. Trace element signatures in pyroxenes suggest small‐length scales of compositional variations which may be interpreted to be a result of post‐melting petrogenetic processes. Relative distribution of rare earth elements and Li between coexisting orthopyroxene‐clinopyroxene pairs in the peridotites suggests compositional disequilibrium in sub‐solidus conditions, which possibly reflects differential effects of diffusive exchange during melting and melt transport or interaction with subduction melts/fluids. On the basis of Ga abundances and Ga–Ti–Fe+3# [Fe+3/(Fe+3 + Cr + Al)] relationships of chrome‐spinels it is documented that the peridotites have experienced the combined effects of partial melting and variable extent of melt‐solid interaction. The MOR peridotites have spinels with geochemical signatures indicative of melt‐depleted residual origin with subsequent incompatible element enrichment through melt impregnation, while the Ga–Ti–Fe+3# relationships of chrome‐spinels in SSZ peridotites indicate that these highly depleted peridotites are not simple melt residues, but have been subject to significant compositional modification by interaction with subduction related melts/fluids. The observed compositional variations, which are related to long‐term tectonic reorganisation of oceanic lithosphere, provide evidence for a time integrated evolution from a mid‐ocean ridge to a supra‐subduction zone setting and may be a possible analogue to explain the coexistence of geochemically diverse MOR–SSZ suites in other Tethyan ophiolites. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

9.
Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountainsvolcanic field (Montana, USA), derived from the lower lithosphereof the Wyoming craton, can be divided based on textural criteriainto tectonite and cumulate groups. The tectonites consist ofstrongly depleted spinel lherzolites, harzburgites and dunites.Although their mineralogical compositions are generally similarto those of spinel peridotites in off-craton settings, somecontain pyroxenes and spinels that have unusually low Al2O3contents more akin to those found in cratonic spinel peridotites.Furthermore, the tectonite peridotites have whole-rock majorelement compositions that tend to be significantly more depletedthan non-cratonic mantle spinel peridotites (high MgO, low CaO,Al2O3 and TiO2) and resemble those of cratonic mantle. Thesecompositions could have been generated by up to 30% partialmelting of an undepleted mantle source. Petrographic evidencesuggests that the mantle beneath the Wyoming craton was re-enrichedin three ways: (1) by silicate melts that formed mica websteriteand clinopyroxenite veins; (2) by growth of phlogopite fromK-rich hydrous fluids; (3) by interaction with aqueous fluidsto form orthopyroxene porphyroblasts and orthopyroxenite veins.In contrast to their depleted major element compositions, thetectonite peridotites are mostly light rare earth element (LREE)-enrichedand show enrichment in fluid-mobile elements such as Cs, Rb,U and Pb on mantle-normalized diagrams. Lack of enrichment inhigh field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf)suggests that the tectonite peridotites have been metasomatizedby a subduction-related fluid. Clinopyroxenes from the tectoniteperidotites have distinct U-shaped REE patterns with strongLREE enrichment. They have 143Nd/144Nd values that range from0·5121 (close to the host minette values) to 0·5107,similar to those of xenoliths from the nearby Highwood Mountains.Foliated mica websterites also have low 143Nd/144Nd values (0·5113)and extremely high 87Sr/86Sr ratios in their constituent phlogopite,indicating an ancient (probably mid-Proterozoic) enrichment.This enriched mantle lithosphere later contributed to the formationof the high-K Eocene host magmas. The cumulate group rangesfrom clinopyroxene-rich mica peridotites (including abundantmica wehrlites) to mica clinopyroxenites. Most contain >30%phlogopite. Their mineral compositions are similar to thoseof phenocrysts in the host minettes. Their whole-rock compositionsare generally poorer in MgO but richer in incompatible traceelements than those of the tectonite peridotites. Whole-rocktrace element patterns are enriched in large ion lithophileelements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb,Ta Zr and Hf) as in the host minettes, and their Sr–Ndisotopic compositions are also identical to those of the minettes.Their clinopyroxenes are LREE-enriched and formed in equilibriumwith a LREE-enriched melt closely resembling the minettes. Thecumulates therefore represent a much younger magmatic event,related to crystallization at mantle depths of minette magmasin Eocene times, that caused further metasomatic enrichmentof the lithosphere. KEY WORDS: ultramafic xenoliths; Montana; Wyoming craton; metasomatism; cumulates; minette  相似文献   

10.
《地学前缘(英文版)》2018,9(6):1755-1775
The ophiolite suite from south Andaman Islands forms part of the Tethyan Ophiolite Belt and preserves the remnants of an ideal ophiolite sequence comprising a basal serpentinized and tectonised mantle peridotite followed by ultramafic and mafic cumulate units, basaltic dykes and spilitic pillow basalts interlayered with arkosic wacke. Here, we present new major, trace, rare earth(REE) and platinum group(PGE) element data for serpentinized and metasomatized peridotites(dunites) exposed in south Andaman representing the tectonized mantle section of the ophiolite suite. Geochemical features of the studied rocks, marked by Al_2 O_3/TiO_2 23, LILE-LREE enrichment, HFSE depletion, and U-shaped chondrite-normalized REE patterns with(La/Sm)N 1 and(Gd/Yb)N 1, suggest contributions from boninitic mantle melts. These observations substantiate a subduction initiation process ensued by rapid slab roll-back with extension and seafloor spreading in an intraoceanic fore-arc regime. The boninitic composition of the serpentinized peridotites corroborate fluid and melt interaction with mantle manifested in terms of(i) hydration, metasomatism and serpentinization of depleted, MORB-type, sub-arc wedge mantle residual after repeated melt extraction; and(ii) refertilization of refractory mantle peridotite by boninitic melts derived at the initial stage of intraoceanic subduction. Serpentinized and metasomatized mantle dunites in this study record both MOR and intraoceanic arc signatures collectively suggesting suprasubduction zone affinity. The elevated abundances of Pd(4.4-12.2 ppb) with highΣPPGE/∑IPGE(2-3) and Pd/Ir(2-5.5) ratios are in accordance with extensive melt-rock interaction through percolation of boninitic melts enriched in fluid-fluxed LILE-LREE into the depleted mantle after multiple episodes of melt extraction. The high Pd contents with relatively lower Ir concentrations of the samples are analogous to characteristic PGE signatures of boninitic magmas and might have resulted by the infiltration of boninitic melts into the depleted and residual mantle wedge peridotite during fore-arc extension at the initial stage of intraoceanic subduction. The PGE patterns with high Os + Ir(2-8.6 ppb)and Ru(2.8-8.4 ppb) also suggest mantle rejuvenation by infiltration of melts derived by high degree of mantle melting. The trace, REE and PGE data presented in our study collectively reflect heterogeneous mantle compositions and provide insights into ocean-crust-mantle interaction and associated geochemical cycling within a suprasubduction zone regime.  相似文献   

11.
康穷蛇纹岩出露于班公湖-怒江缝合带的中西段,是康穷蛇绿混杂岩的超基性岩石端元。依据全岩地球化学元素含量,可将康穷蛇纹岩划分为A组和B组2种。两者均富集大离子亲石元素(Rb、Cs)和高场强元素(U、Nb)。经估算,A组蛇纹岩原岩是原始地幔经过10%~17%部分熔融的地幔残留,B组蛇纹岩原岩经历过更高程度的部分熔融(17%~23%),表明康穷蛇纹岩原岩至少经历过两阶段的部分熔融。初步研究认为,A组蛇纹岩原岩是洋内俯冲初始阶段上涌软流圈地幔部分熔融的地幔残留,受类MOR型熔体交代作用而仍然亏损轻稀土元素(LREE),却富集U和Nb,属类MOR型地幔橄榄岩;B组蛇纹岩原岩是A组蛇纹岩原岩在洋内俯冲带环境再熔融的产物,受玻安质熔体交代作用而相对富集轻稀土元素,属弧前地幔橄榄岩。两者均发生蛇纹石化作用而转变成蛇纹岩,最终因大洋碰撞闭合而残存于缝合带内,是班公湖-怒江新特提斯洋洋内俯冲演化阶段的岩石记录。  相似文献   

12.
造山带幔源（M型）橄榄岩虽然在高压/超高压变质带分布不多,但由于其来自俯冲板块上覆的岩石圈地幔,因此是研究俯冲隧道内俯冲板片与地幔楔之间相互作用的重要对象,对于还原超高压变质带的演化有重要意义.柴北缘鱼卡榴辉岩-片麻岩区边部附近的开屏沟存在一套橄榄岩,其岩石类型、成因、时代等都缺乏研究.对开屏沟橄榄岩全岩的主量和微量元素及铂族元素、橄榄石主量元素、锆石U-Pb年龄和Hf同位素进行了研究.结果显示,其全岩具有高的Mg#、Mg/Si和Ni值,同时表现出亏损难溶的HFSE和HREE,轻微富集LILE和LREE中与流体活动性相关的元素;橄榄石具有较高的Fo值（90.11~92.77）与NiO含量（0.32%~0.45%）、低的CaO含量（< 0.02%）;PGEs的球粒陨石标准化配分模式与交代橄榄岩和残留橄榄岩近似;两组变质锆石年龄为459.5±3.6 Ma和417.5±2.7 Ma,对应的εHf（t）值为-0.71~9.45和-11.96~-1.20,分别反映了洋壳流体（或早期大陆俯冲板片流体）和陆壳流体交代的性质和时限.开屏沟橄榄岩来源于俯冲带上覆地幔楔,遭受了不同来源流体不同程度的交代作用而获得地壳特征,同时为柴北缘大洋俯冲到陆陆碰撞的构造演化提供了新证据.   相似文献   

13.
In this paper, we present whole-rock and mineral geochemistry of serpentinized peridotites from the Suru Valley ophiolite slice Ladakh Himalaya, in an attempt to put constraints on their petrogenesis and tectonic evolution in the context of Mesozoic Neo-Tethys Ocean. On the basis of petrographic study, Suru Valley serpentinized peridotites can be identified as serpentinized harzburgites. Relative to primitive mantle these rocks have depleted major and rare earth element (REE) geochemical characteristics comparable to ocean floor mantle rocks reflecting their mantle residual nature. However, higher abundance of highly incompatible large ion lithophile elements (e.g., Rb, Ba, Th, U, Pb and Sr), reflect metasomatism in a subduction zone environment. The presence of silicate assemblage includes Mg-rich olivine (Fo90-92) and orthopyroxene (En91-93 Fs6.4-8.7) of supra-subduction zone affinity. Evaluation of mineral and whole-rock geochemistry suggests that the Suru Valley ophiolitic peridotites represent residues left after moderate degrees of partial melting thereby underwent metasomatism in a supra-subduction zone environment related to north dipping intra-oceanic island arc during Cretaceous in the context of Mesozoic Neo-Tethys ocean.  相似文献   

14.
The Jiangzhuang ultrahigh‐pressure (UHP) metamorphic peridotite from south Sulu, eastern China occurs as a layer within gneiss with eclogite blocks, and consists of coarse‐grained garnet porphyroblasts and a fine‐grained matrix assemblage of garnet + forsterite + enstatite + diopside ± phlogopite ± Ti‐clinohumite ± magnesite. Both types of garnet are characterized by high MgO content and depletion of light rare earth element (LREE) and enrichment of heavy rare earth element, but the matrix garnet has lower MgO, TiO2 and higher Cr2O3 and REE contents. Diopside displays LREE enrichment, and has low but variable large‐ion lithophile element (LILE) contents. Phlogopite is a major carrier of LILE. Ti‐clinohumite contains high Nb, Ta, Cr, Ni, V and Co contents. The P–T conditions of 4.5–6.0 GPa and 850–950 °C were estimated for matrix mineral assemblages. Most peridotites are depleted in Al2O3, CaO and TiO2, and enriched in SiO2, K2O, REE and LILE. In contrast to phlogopite‐free peridotites, the phlogopite‐bearing peridotites have higher K2O, Zr, REE and LILE contents. Zircon occurs only in the phlogopite‐bearing peridotites, shows no zoning, with low REE contents and Th/U ratios, and yields tight U–Pb ages of 225–220 Ma, indicating the peridotites experienced consistent Triassic UHP metamorphism with subducted supercrustal rocks. These data demonstrate that the Jiangzhuang peridotites were derived from the depleted mantle wedge of the North China Craton, and experienced various degrees of metasomatism. The phlogopite‐free peridotites may have been subjected to an early cryptic metasomatism at UHP conditions of the mantle wedge, whereas the phlogopite‐bearing peridotites were subjected to a subsequent strong metasomatism, characterized by distinctly enrichment in LILE, LREE, Zr and K as well as the growth of zircon and volatile‐bearing minerals at UHP subduction conditions. The related metasomatism may have resulted from the filtration of fluids sourced mainly from deeply subducted supracrustal rocks.  相似文献   

15.
Clinopyroxene/melt and garnet/melt partition coefficients have been determined for Ti, Sr, Y, Zr, Nb, Hf, and rare earth elements from 19 doped experiments on 1921 Kilauea basalt. The experiments were carried out from 2.0 to 3.0 GPa and 1310° to 1470 °C. The purpose was to derive a set of partition coefficients for high-field-strength elements (HFSE) and rare earth elements (REE) in a systematic, linked set of experiments at P and T conditions relevant to basalt petrogenesis. These data are used in melting models to understand the development of negative HFSE anomalies observed in many abyssal peridotite clinopyroxenes. It is shown that melting can account for the observed trace element patterns in some residual peridotites, but that other processes may also be needed to account for most residual mantle compositions in mid-ocean ridge systems. It is also shown that REE are more strongly fractionated by garnet at these P-T conditions than previously thought. Received: 1 July 1997 / Accepted: 11 May 1998  相似文献   

16.
This study presents new geochemical data on rocks from the Vespor suite, an important mafic unit from the Juruena arc, Roosevelt-Juruena terrain, SW Amazonian craton, northwest Mato Grosso, Brazil, attempting to define their tectonic setting and type of mantle source. The Juruena arc may be part of a magmatic belt (Jamari and Juruena arcs) at the southwestern Amazonian craton during assembly of the Columbia supercontinent. The investigated rocks represent a Paleoproterozoic subduction-related mafic suite of sigmoidal bodies, composed mainly of gabbro, norite, gabbronorite and diorite, that underwent amphibolite facies metamorphism. Here we present also preliminary petrology aspects and zircon U–Pb geochronology. Geochemical character and variation trends of major and trace elements as well as selected trace element ratios suggest that Vespor suite rocks have a tholeiitic lineage of arc affinity controlled by fractional crystallization with a prominent iron enrichment trend. Gabbros, norites and gabbronorites are characterized by enrichment of LILE and weakly to moderately differentiated HFSE patterns, suggesting their deviation from an enriched heterogeneous lithospheric mantle source. Vespor suite rocks are characterized by depletion of Nb–Ta, P and Ti, with flat distribution of HFSE, markedly large variations in most of the LILE, positive anomalies displayed by Ba, K, Th, Sr, Pb and weak negative anomalies of Hf–Zr. These features reflect limited degrees of crustal contamination associated with a subduction-related magma process where the mantle wedge was chemically modified. In addition, the enrichment in LILE and Pb, low values of the ratios (Lan/Smn – 0.83 to 4.58) and (Nbn/Lan – 0.04 to 0.45), but high Th/Yb ratios, gently to moderately sloping REE profiles (La/Ybn = 2.53–7.37), negative anomalies in HFSE (Ta, Zr, Hf, and Ti), and positive anomalies in LILE (Th, Ba, Sr), suggest derivation from a metasomatized lithospheric mantle source above a subduction zone with weak crustal contamination. Both the composition of the mantle source and the degree of partial melting that produced the parental magmas of these rocks, determined by using REE abundance and ratios, indicate that gabbroic/dioritic melts were generated at different degrees of melting of the source: about 5–20% partial melting of a garnet-spinel lherzolite, around 1–10% partial melting of spinel lherzolite source, and approximately 1–5% partial melting of intermediate source composition, and crystallizing between 1.773 and 1.764 Ma.  相似文献   

17.
中国大陆科学钻探工程预先导孔(CCSD-PP1) 打在苏鲁超高压变质带芝麻坊超镁铁岩体上,钻孔穿透超镁铁岩体115m.超镁铁岩体由二辉橄榄岩、方辉橄榄岩和少量单辉橄榄岩和纯橄岩组成,与上下围岩接触的橄榄岩被强烈蛇纹石化.多数橄榄岩含有石榴石或其假象,普遍含有金云母和菱镁矿,少量样品中有钛斜硅镁石.在化学成分上,橄榄岩的Mg#指数变化于90.392.6之间,MgO含量(36.61%49.15%,平均45.17%) 与Na2O (0.01%0.25%)、Al2O3 (0.07%3.71%,多数 < 2.0%,平均1.46%)和CaO (0.12%2.53%,一个高达3.30%,平均1.00%) 呈负相关关系.与主量易熔元素相对亏损的特点相反,橄榄岩中显示了稀土元素富集、分馏和配分曲线显示近于平行和相似的特点,(La/Lu) N比值为3.1833.05;此外,多数样品具有高Ba (最高比原始地幔高100多倍) 含量,在蛛网图上显示Rb、Nb、Ta、Zr、Hf和Sr的负异常,Ti/Eu比值均低于1300.岩相学特征和难熔主量元素与不相容元素之间的无相关性表明橄榄岩至少受到了形成金云母和菱镁矿的2次交代作用.富含金云母的橄榄岩(例如C25-143-61,C32-149-71) 具有富钾趋势,并且显示K2O与Rb、Ba和Th等大离子亲石元素的正相关关系,未见K2O和稀土元素、Sr和Ca之间的相关关系.这些特征表明橄榄岩被含水、硅铝质碱性熔体交代,之后又被高Ba低Rb和高场强元素的镁质(菱镁矿) 碳酸岩熔体交代,并且强烈地改变了Ba的丰度和显示了特定地幔碳酸盐的稀土元素配分型式.全岩具有不均一的高放射性Sr (87Sr/86Sr=0.70840.7201)和低放射性Nd (εNd (t) =-1.14-8.55) 组成,结合已有的氧同位素研究资料,表明预先导孔PP1中的橄榄岩所代表的地幔可能在早期就遭受了来自深部的介质的交代作用.   相似文献   

18.
《International Geology Review》2012,54(13):1715-1734
This study examines the geochemistry of major and trace elements of abyssal peridotites from the Southwest Indian Ridge (SWIR) (53° E amagmatic segment), to determine the influence of mafic melts on mantle peridotites during melt extraction. The results show a great geochemical variability in the ~90 km-long ridge segment, with a degree of mantle melting ranging from 4% to 24%. An ancient melting event may explain the presence of highly depleted peridotites at the ultraslow-spreading ridge. The 53° E segment peridotites show enrichment of light rare earth elements (LREEs) (average LaN/SmN = 1.87) and significant positive anomaly of U and Pb normalized to primitive mantle (PM). The positive correlations between LREEs (La, Ce, Pr, Nd) and high field strength elements (HFSEs; e.g. Nb and Zr) suggest that the enrichment of LREEs is caused by melt refertilization, which is also supported by prevalent magmatic microstructures in the peridotites. The melt refertilization model shows that the addition of 0.02–2.7% basaltic melts to peridotites can be responsible for the LREE enrichment. We suggest that the positive anomaly of U is probably attributed to fluid alteration whereas the enrichment of Pb is probably attributed to both melt refertilization and fluid alteration. Melt refertilization in the 53° E segment peridotites may be a result of melt–rock reaction and crystallization of melts trapped in peridotites. These processes may be enhanced by increased melt permeability in the mantle owing to the refractory peridotites produced by ancient melting and the decreasing efficiency of melt extraction in the cold and thick lithosphere at the 53° E ridge segment. The presence of melt refertilization implies that melt extraction is incomplete in the ridge mantle, which may be one of the reasons for the extremely thin and irregular variation of the crustal thickness at ultraslow-spreading ridges.  相似文献   

19.
休古嘎布蛇绿岩块位于雅鲁藏布缝合带(YZSZ)西段,主要由地幔橄榄岩和侵入其中的基性岩墙所组成。基性岩墙具有弧后盆地地球化学亲缘性,其LREE亏损,(La/Yb)N为0.39~0.55;具有明显的Nb、Ti负异常及Sr、Ba正异常。Sr、Nd同位素特征表明它们起源于亏损地幔,并且受到了板片析出流体的影响。4个基性岩墙的Sm-Nd同位素样品获得内部等时线年龄为126.2±9.1Ma(MSWD=0.44)。地幔橄榄岩具有汤勺形和U形两组REE分布型式,显示出不同程度部分熔融和地幔交代作用的特征,具有弧-盆体系地球化学亲缘性。第一组橄榄岩的LREE弱富集或近于平坦,尖晶石的Cr#值低而且变化不大,部分熔融程度较低(15%~20%),可能形成于弧后扩张盆地;第二组橄榄岩的LREE明显富集,尖晶石的Cr#值高而且变化较大(0.4~0.77),部分熔融程度较高(25%~30%),并经历了强烈的交代富集作用,可能与洋内岛弧有关。  相似文献   

20.
Tectonically emplaced peridotites from North Hebei Province, North China Craton, have retained an original harzburgite mineral assemblage of olivine(54%–58%) + orthopyroxene(40%–46%)+minor clinopyroxene(1%)+spinel. Samples with boninite-like chemical compositions also coexist with these peridotites. The spinels within the peridotites have high-Al end-members with Al_2O_3 content of 30 wt % –50 wt %, typical of mantle spinels. When compared with experimentally determined melt extraction trajectories, the harzburgites display a high degree of melting and enrichment of SiO_2, which is typical of cratonic mantle peridotites. The peridotites display variably enriched light rare earth elements(REEs), relatively depleted middle REEs and weakly fractionated heavy REEs, which suggest a melt extraction of over 25% in the spinel stability field. The occurrence of arc-and SSZ-type chromian spinels in the peridotites suggests that melt extraction and metasomatism occurred mostly in a subduction-related setting. This is also supported by the geochemical data of the coexisting boninite-like samples. The peridotites have ~(187)Os/~(188)Os ratios ranging from 0.113–0.122, which is typical of cratonic lithospheric mantle. These ~(187)Os/~(188)Os ratios yield model melt extraction ages(TRD) ranging from 981 Ma to 2054 Ma, which may represent the minimum estimation of the melt extraction age. The Al_2O_3-~(187)Os/~(188)Os-proxy isochron ages of 2.4 Ga–2.7 Ga suggest a mantle melt depletion age between the Late Achaean and Early Paleoproterozoic. Both the peridotites and boninite-like rocks are therefore interpreted as tectonically exhumed continental lithospheric mantle of the North China Craton, which has experienced mantle melt depletion and subduction-related mantle metasomatism during the Neoarchean-Paleoproterozoic.  相似文献   

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