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1.
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.  相似文献   

2.
The CCSD‐PP1 drillhole penetrated a 110‐m‐thick sequence of the Zhimafang ultramafic body in the Sulu ultrahigh‐pressure (UHP) metamorphic belt, east China. The sequence consists of interlayered garnet‐bearing (Grt) and garnet‐free (GF) peridotite. Eleven layers of Grt‐peridotite, ranging from 1.2 to 9.5 m in thickness, have an aggregate thickness of 54.49 m, whereas eight layers of GF‐peridotite, ranging from 2.2 to 14.2 m in thickness, have a total thickness of 57.53 m. The boundaries between the two rock types are gradational. The Grt‐peridotites have slightly higher contents of Al2O3, CaO and SiO2, and lower Mg#s (0.90–0.92) than the GF‐peridotites (Mg#s 0.91–0.93). Both contain low TiO2 (<0.05 wt%) and have higher modal abundances of enstatite (average 10 vol.%) than diopside (1–5 vol.%), typical of depleted‐type upper mantle. The diopside in these rocks has high and relatively uniform Mg# members (0.93–0.95), but highly variable Al2O3 (0.2–2.3 wt%), Na2O (0.5–2.5 wt%) and Cr2O3 (0.38–2.09 wt%). Enstatite (En92?93) contains very low Al2O3 (0–0.3 wt%). Both porphyroblastic and equigranular garnet are present. The equigranular varieties are zoned, from core to rim in Cr2O3 (3.4–4.2 wt%), MgO (18.4–17.5 wt%) and Al2O3 (21.1–20.1 wt%). Titania is very low in all the garnet, mostly <0.05 wt%. Chromite or chromium (Cr)‐spinel occur both in the Grt‐ and GF‐peridotite, and are characterized by high contents of Cr2O3 (49–58 wt%) and FeO (24–43 wt%), similar to that in iron‐rich Alpine‐type peridotites. Based on the bulk‐rock MgO–FeO compositions, the Zhimafang Grt‐peridotite probably underwent 20–30% partial melting, whereas the GF‐peridotite may have undergone as much as 35–40% partial melting, suggesting that the two rock types owe their differences to different degrees of partial melting rather than to pressure differences during metamorphism.  相似文献   

3.
The Neoproterozoic Allaqi-Heiani suture （800-700 Ma） in the south Eastern Desert of Egypt is the northernmost linear ophiolitic belt that defines an arc-arc suture in the Arabian- Nubian shield （ANS）. The Neoproterozoic serpentinized peridotites represent a distinct lithology of dismembered ophiolites along the Allaqi-Heiani suture zone. The alteration of peridotites varies, some contain relicts of primary minerals （Cr-spinel and olivine） and others are extremely altered, especially along thrusts and shear zones, with development of talc, talc-carbonate and quartz-carbonate. The fresh cores of the chromian spinels are rimmed by ferritchromite and Cr- magnetite. The fresh chromian spinels have high Cr# （0.62 to 0.79）, while Mg# shows wider variation （0.35-0.59）. High Cr# in the relict chromian spinels and Fo content in the primary olivines indicate that they are residual peridotites after extensive partial melting. The studied ophiolitic upper mantle peridotites are highly depleted and most probably underwent high degrees of partial melting at a supra-subduction zone setting. They can be produced by up to -20%-22% dynamic melting of a primitive mantle source. The mineralogical and geochemical features of the studied rocks reflect that the mantle peridotites of the north part of the Wadi Allaqi district are similar to the fore-arc peridotites of a supra-subduction zone.  相似文献   

4.
Dunite and serpentinized harzburgite in the Cheshmeh-Bid area, northwest of the Neyriz ophiolite in Iran, host podiform chromitite that occur as schlieren-type, tabular and aligned massive lenses of various sizes. The most important chromitite ore textures in the Cheshmeh-Bid deposit are massive, nodular and disseminated. Massive chromitite, dunite, and harzburgite host rocks were analyzed for trace and platinum-group elements geochemistry. Chromian spinel in chromitite is characterized by high Cr~#(0.72-0.78), high Mg~#(0.62–0.68) and low TiO_2(0.12 wt%-0.2 wt%) content. These data are similar to those of chromitites deposited from high degrees of mantle partial melting. The Cr~# of chromian spinel ranges from 0.73 to 0.8 in dunite, similar to the high-Cr chromitite, whereas it ranges from 0.56 to 0.65 in harzburgite. The calculated melt composition of the high-Cr chromitites of the Cheshmeh-Bid is 11.53 wt%–12.94 wt% Al_2O_3, 0.21 wt%–0.33 wt% TiO_2 with FeO/MgO ratios of 0.69-0.97, which are interpreted as more refractory melts akin to boninitic compositions. The total PGE content of the Cheshmeh-Bid chromitite, dunite and harzburgite are very low(average of 220.4, 34.5 and 47.3 ppb, respectively). The Pd/Ir ratio, which is an indicator of PGE fractionation, is very low(0.05–0.18) in the Cheshmeh-Bid chromitites and show that these rocks derived from a depleted mantle. The chromitites are characterized by high-Cr~#, low Pd + Pt(4–14 ppb) and high IPGE/PPGE ratios(8.2–22.25), resulting in a general negatively patterns, suggesting a high-degree of partial melting is responsible for the formation of the Cheshmeh-Bid chromitites. Therefore parent magma probably experiences a very low fractionation and was derived by an increasing partial melting. These geochemical characteristics show that the Cheshmeh-Bid chromitites have been probably derived from a boninitic melts in a supra-subduction setting that reacted with depleted peridotites. The high-Cr chromitite has relatively uniform mantle-normalized PGE patterns, with a steep slope, positive Ru and negative Pt, Pd anomalies, and enrichment of PGE relative to the chondrite. The dunite(total PGE = 47.25 ppb) and harzburgite(total PGE =3 4.5 ppb) are highly depleted in PGE and show slightly positive slopes PGE spidergrams, accompanied by a small positive Ru, Pt and Pd anomalies and their Pdn/Irn ratio ranges between 1.55–1.7 and 1.36-1.94, respectively. Trace element contents of the Cheshmeh-Bid chromitites, such as Ga, V, Zn, Co, Ni, and Mn, are low and vary between 13–26, 466–842, 22-84, 115–179, 826–-1210, and 697–1136 ppm, respectively. These contents are compatible with other boninitic chromitites worldwide. The chromian spinel and bulk PGE geochemistry for the Cheshmeh-Bid chromitites suggest that high-Cr chromitites were generated from Cr-rich and, Ti-and Al-poor boninitic melts, most probably in a fore-arc tectonic setting related with a supra-subduction zone, similarly to other ophiolites in the outer Zagros ophiolitic belt.  相似文献   

5.
西藏普兰地幔橄榄岩中尖晶石内的钙长石包裹体及其成因   总被引：6，自引：5，他引：1  
郭国林  徐向珍  李金阳 《岩石学报》2011,27(11):3197-3206
西藏普兰超镁铁岩体之东南缘与玄武岩接触界线附近的地幔橄榄岩中除有粒状半自形的钙长石产出外,还在尖晶石中发现有呈蠕虫状、浑圆状的钙长石包裹体存在.研究发现两种产状的钙长石An值都大于95且均无环带构造,说明钙长石从高Ca/Al比值的熔体中结晶时具有结晶时间短、结晶速度快的特点,可能形成于地壳较浅部位.从化学成分来看,包裹体形态的钙长石具有较高的Cr2O3含量,其寄主矿物尖晶石的Cr#值低且TiO2含量比深海橄榄岩中的尖晶石低得多,推断钙长石包裹体与寄主矿物尖晶石是在液相条件下几乎同时结晶的产物.综合研究表明钙长石包裹体的成因可能是玄武岩熔体在地壳较浅部位侵入方辉橄榄岩时,高温的玄武质熔体提供热源,使得方辉橄榄岩中尖晶石内的Cpx+ Opx细粒矿物包裹体在高温环境下发生熔融,发生Opx+ Cpx+ Sp→Ol+ Pl的反应,由于这种情况下尖晶石有剩余,故新生成的橄榄石和钙长石矿物仍然包裹于尖晶石内,从而形成尖晶石内部呈蠕虫状的钙长石包裹体.  相似文献   

6.
The Zedong ophiolites in the eastern Yarlung–Zangbo suture zone of Tibet represent a mantle slice of more than 45 km~2. This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite; a few dike-like bodies of dunite are also present. Mineral structures show that the peridotites experienced plastic deformation and partial melting. Olivine(Fo89.7–91.2), orthopyroxene(En_(88–92)), clinopyroxene(En_(45–49) Wo_(47–51) Fs_(2–4)) and spinel [Mg~#=100×Mg/(Mg+Fe)]=49.1–70.7; Cr~#=(100×Cr/(Cr+Al)=18.8–76.5] are the major minerals. The degree of partial melting of mantle peridotites is 10%–40%, indicating that the Zedong mantle peridotites may experience a multi–stage process. The peridotites are characterized by depleted major element compositions and low REE content(0.08–0.62 ppm). Their "spoon–shaped" primitive–mantle normalized REE patterns with(La/Sm)_N being 0.50–6.00 indicate that the Zedong ultramafic rocks belong to depleted residual mantle rocks. The PGE content of Zedong peridotites(18.19–50.74 ppb) is similar with primary mantle with Pd/Ir being 0.54–0.60 and Pt/Pd being 1.09–1.66. The Zedong peridotites have variable, unradiogenic Os isotopic compositions with ~(187)Os/~(188)Os=0.1228 to 0.1282. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes. All data of the Zedong peridotites suggest that they formed originally at a mid-ocean ridge(MOR) and were later modified in supra–subduction zone(SSZ) environment.  相似文献   

7.
The ophiolitic peridotites in the Wadi Arais area, south Eastern Desert of Egypt, represent a part of Neoproterozoic ophiolites of the Arabian-Nubian Shield (ANS). We found relics of fresh dunites enveloped by serpentinites that show abundances of bastite after orthopyroxene, reflecting harzburgite protoliths. The bulk-rock chemistry confirmed the harzburgites as the main protoliths. The primary mantle minerals such as orthopyroxene, olivine and chromian spinel in Arais serpentinites are still preserved. The orthopyroxene has high Mg# [=Mg/(Mg + Fe2+)], ~0.923 on average. It shows intra-grain chemical homogeneity and contains, on average, 2.28 wt.% A12O3, 0.88 wt.% Cr2O3 and 0.53 wt.% CaO, similar to primary orthopyroxenes in modern forearc peridotites. The olivine in harzburgites has lower Fo (93?94.5) than that in dunites (Fo94.3?Fo95.9). The Arais olivine is similar in NiO (0.47 wt.% on average) and MnO (0.08 wt.% on average) contents to the mantle olivine in primary peridotites. This olivine is high in Fo content, similar to Mg-rich olivines in ANS ophiolitic harzburgites, because of its residual origin. The chromian spinel, found in harzburgites, shows wide ranges of Cr#s [=Cr/(Cr + Al)], 0.46?0.81 and Mg#s, 0.34?0.67. The chromian spinel in dunites shows an intra-grain chemical homogeneity with high Cr#s (0.82?0.86). The chromian spinels in Arais peridotites are low in TiO2, 0.05 wt.% and YFe [= Fe3+/(Cr + Al + Fe3+)], ~0.06 on average. They are similar in chemistry to spinels in forearc peridotites. Their compositions associated with olivine’s Fo suggest that the harzburgites are refractory residues after high-degree partial melting (mainly ~25?30 % partial melting) and dunites are more depleted, similar to highly refractory peridotites recovered from forearcs. This is in accordance with the partial melting (>20 % melt) obtained by the whole-rock Al2O3 composition. The Arais peridotites have been possibly formed in a sub-arc setting (mantle wedge), where high degrees of partial melting were available during subduction and closing of the Mozambique Ocean, and emplaced in a forearc basin. Their equilibrium temperature based on olivine?spinel thermometry ranges from 650 to 780 °C, and their oxygen fugacity is high (Δlog ?O2?=?2.3 to 2.8), which is characteristic of mantle-wedge peridotites. The Arais peridotites are affected by secondary processes forming microinclusions inside the dunitic olivine, abundances of carbonates and talc flakes in serpentinites. These microinclusions have been formed by reaction between trapped fluids and host olivine in a closed system. Lizardite and chrysotile, based on Raman analyses, are the main serpentine minerals with lesser antigorite, indicating that serpentines were possibly formed under retrograde metamorphism during exhumation and near the surface at low T (<400 °C).  相似文献   

8.
The chromium number of spinel Cr#sp (atomic ratio of Cr/(Cr+Al)) is an important geochemical parameter for the estimation of the degree of partial melting, temperatures, and provenance in peridotites. In this study, a model has been developed in order to determine the effect of subsolidus reactions on the Cr#sp in ultramafic rocks. The final model includes temperature-dependent distribution coefficients of relevant reactions as well as solubility data and has been applied to lithologies common in mid-ocean ridge settings. Significant changes in the Cr#sp are predicted from the application of this model during cooling from 1300 to 800°C at mantle pressures. For spinel lherzolites and harzburgites, the Cr#sp is predicted to decrease proportional to the absolute values of the Cr#sp at (constantly) increasing spinel mass. Cpx-dunites show the same trend, although to a lower extent. Websterites show a different behavior with a slight increase in the Cr#sp due to their lack of olivine. Modal abundance of spinel correlates with the magnitude in Cr#sp change, too. Finally, these results were tested for possible effects on the calculated degree of partial melting as function of the Cr#sp. Application of the Cr#sp from a peridotite equilibrated down to 800°C would result in an underestimation of only 1.5 % in the degree of melting, justifying the use of Cr#sp for estimations of this parameter.  相似文献   

9.
The Neoproterozoic peridotite-chromitite complexes in the Central Eastern Desert of Egypt, being a part of the Arabian-Nubian Shield, are outcropped along the E–W trend from Wadi Sayfayn, Wadi Bardah, and Jabal Al-Faliq to Wadi Al-Barramiyah, from east to west. Their peridotites are completely serpentinized, and the abundance of bastite after orthopyroxene suggests harzburgite protoliths with subordinate dunites, confirmed by low contents of Al2O3, CaO and clinopyroxene (< 3 vol%) in bulk peridotites. The primary olivine is Fo89.3–Fo92.6, and the residual clinopyroxene (Cpx) in serpentinites contains, on average, 1.1 wt% Al2O3, 0.7 wt% Cr2O3, and 0.2 wt% Na2O, similar in chemistry to that in Izu-Bonin-Marian forearc peridotites. The wide range of spinel Cr-number [Cr/(Cr + Al)], 0.41–0.80, with low TiO2 (0.03 wt%), MnO (0. 3 wt%) and YFe [(Fe3 +/(Cr + Al + Fe3 +) = 0.03 on average)] for the investigated harzburgites-dunites is similar to spinel compositions for arc-related peridotites. The partial melting degrees of Bardah and Sayfayn harzburgites range mainly from 20 to 25% and 25 to 30% melting, respectively; this is confirmed by whole-rock chemistry and Cpx HREE modelling (~ 20% melting). The Barramiyah peridotite protoliths are refractory residues after a wide range of partial melting, 25–40%, where more hydrous fluids are available from the subducting slab. The Neoproterozoic mantle heterogeneity is possibly ascribed mainly to the wide variations of partial melting degrees in small-scale areas, slab-derived inputs and primordial mantle compositions. The Sayfayn chromitites were possibly crystallized from island-arc basaltic melts, followed by crystallization of Barramiyah chromitites from boninitic melt in the late stage of subduction. The residual Cpx with a spoon-shape REE pattern is rich in both LREE and fluid-mobile elements (e.g., Pb, B, Li, Ba, Sr), but poor in HFSE (e.g., Ta, Nb, Zr, Th), similar to Cpx in supra-subduction zone (SSZ) settings, where slab-fluid metasomatism is a prevalent agent. The studied chromitites and their host peridotites represent a fragment of sub-arc mantle, and originated in an arc-related setting. The systematic increase in the volume of chromitite pods with the increasing of their host-peridotite thickness from Northern to Southern Eastern Desert suggests that the thickness of wall rocks is one factor controlling the chromitite size. The factors controlling the size of Neoproterozoic chromitite pods are the thickness, beside the composition, of the host refractory peridotites, compositions and volumes of the supplied magmas, the amount of slab-derived fluids, and possibly the partial melting degree of the host peridotites.  相似文献   

10.
赣东北蛇绿岩作为华南少有的前寒武纪蛇绿岩长期都是华南大地构造研究中的热点。该蛇绿岩地幔橄榄岩以方辉橄榄岩为主,稀土总量为0.83×10-6~2.62×10-6,远低于原始地幔的含量,而Mg O含量高于原始地幔。稀土元素表现为LREE相对富集,微量元素表现为大离子亲石元素(LILE)富集,高场强元素Nb明显亏损的特征,这表明赣东北地幔橄榄岩具有亏损地幔源区特征,同时也有不同程度的俯冲带流体的交代特征。HREE模拟的部分熔融程度为10%~30%;同时,赣东北蛇绿岩地幔橄榄岩中同时包含Cr#60和Cr#60两种不同成因的铬尖晶石,说明其经历了MOR(mid-ocean ridge)和SSZ(supra-subduction zone)两种构造环境。结合前人资料,笔者推测赣东北蛇绿岩早先可能形成于洋中脊环境(约1060 Ma),随后在洋内俯冲作用下(约970 Ma),位于俯冲带上部的地幔橄榄岩受到了来自俯冲带的流体/熔体的交代,经历了SSZ环境的改造。  相似文献   

11.
山东的无棣大山和栖霞方山是两个以霞石岩为主的新生代火山,含有大量的地幔橄榄岩捕掳体,橄榄岩捕掳体中的尖晶石常见黑色反应边。在BSE图像上,这些反应边根据结构的不同又可以进一步分为两种:筛状边和均一边。有时两种反应边在同一颗尖晶石中共存并相互过渡。尖晶石捕掳晶和捕掳体中与寄主岩直接接触的尖晶石常发育均一边;发育筛状边的尖晶石则一般不直接与寄主岩接触,但位于捕掳体的边缘。尖晶石的筛状边呈多孔状,为富Cr尖晶石或铬铁矿。筛状边成分上与核部(Cr_2O_3=7.52%~36.75%,Cr#=7.80~44.20,Mg#=44.70~74.48)区别明显,具有高的Cr_2O_3含量(23.42%~65.96%)、Cr#值(78.97~92.49)以及低的Mg#值(17.22~43.02)。另外,筛状边相对其核部(TiO 2=0.00%~0.53%,MnO=0.04%~0.35%)还显示偏高的TiO 2(0.20%~3.60%)和MnO(0.29%~1.93%)。在筛状边附近存在富Al_2O_3(8.00%~17.57%)和MgO(17.89%~26.02%)的玻璃。尖晶石的均一边无孔洞但多发育裂理,成分上与核部突变,并以富TiO 2(20.90%~6.64%)和FeO T(70.79%~53.92%)为特征,最外部为钛磁铁矿。均一边具有明显的成分分带,表现为由内至外TiO 2、FeO T含量逐渐增高,Al_2O_3(0.04%~16.34%)、Cr_2O_3(0.77%~25.69%)和MgO(0.50%~7.16%)含量逐渐降低。尖晶石的两种反应边与寄主岩密切的空间关系说明其成因与寄主岩浆有关。虽然尖晶石筛状边的Cr#值(79.0~92.5)明显高于核部(7.8~44.2),但是其相对核部偏高的TiO 2和MnO含量,以及显著降低的Mg#值均与单纯的部分熔融趋势不吻合。根据尖晶石两种反应边的结构特征、成分特征和相互的空间关系,我们认为筛状边与均一边是岩浆与尖晶石反应不同阶段的产物。当橄榄岩捕掳体被岩浆捕获后,由于成分上的不均衡两者之间将发生溶解反应。反应过程中,尖晶石中不同元素向熔体迁移的速率区别明显,其中Cr是最难熔的元素也是迁移最慢的元素。在反应的初始阶段,由于易溶组份Al、Mg等元素随熔体迁出,尖晶石发育筛状边,并同时伴随Cr#值的显著升高和Mg#值的显著降低。此时,寄主岩浆中的Ti、Mn等元素也部分扩散进入筛状边中的残留尖晶石。如果熔体足够多,反应得以持续进行,筛状边中的Cr最终也将被熔体带走。在反应产生的混合熔体中金属氧化物最早达到饱和并结晶。随着反应的持续进行,结晶出的矿物将继续生长、变粗,并相互连接,直至孔洞消失形成均一边。同时伴随着矿物的不断结晶,溶解界面上的熔体成分也不断变化,从而使均一边具有明显的成分分带。在橄榄岩捕掳体内部,由于渗进捕掳体中的岩浆有限,渗透岩浆与尖晶石之间的反应多限于早期阶段,反应产物为筛状边。在捕掳体边缘,与寄主岩浆直接接触的尖晶石由于参与反应的岩浆量足够多,因此一般产生反应的终端产物———均一边。  相似文献   

12.
Serpentinites (massive and schistose) and listvenite occur as tectonic sheets and lenses within a calcareous metasedimentary mélange of the Tulu Dimtu, western Ethiopia. The massive serpentinite contains high-magnesian metamorphic olivine (forsterite [fo] ~96 mol%) and rare relict primary mantle olivine (Fo90–93). Both massive and schistose serpentinites contain zoned chromian spinel; the cores with the ferritchromite rims preserve a pristine Cr/(Cr+Al) atomic ratio (Cr# = 0.79–0.87), suggesting a highly depleted residual mantle peridotite, likely formed in a suprasubduction zone setting. Listvenite associated with serpentinites of smaller ultramafic lenses also contain relict chromian spinel having identical Cr# to those observed in serpentinites. However, the relict chromian spinel in listvenite has significantly higher Mg/(Mg+Fe2+) atomic ratios. This suggests that a nearly complete metasomatic replacement of ultramafic rocks by magnesite, talc, and quartz to prevent Mg–Fe2+ redistribution between relict chromian spinel and the host, that is, listvenite formation, took place prior to re-equilibration between chromian spinel and the surrounding mafic minerals in serpentinites. Considering together with the regional geological context, low-temperature CO2-rich hydrothermal fluids would have infiltrated into ultramafic rocks from host calcareous sedimentary rocks at a shallow level of accretionary prism before a continental collision to form the East African Orogen (EAO).  相似文献   

13.
张利  杨经绥  刘飞  连东洋  黄健  赵慧  杨艳 《岩石学报》2016,32(12):3649-3672
西藏阿里地区的南公珠错蛇绿岩产在公珠错的南侧,空间上属于雅鲁藏布江缝合带西段之南亚带蛇绿岩。该蛇绿岩主要由地幔橄榄岩和辉长岩等基性岩类组成。地幔橄榄岩中约80%为方辉橄榄岩,20%为二辉橄榄岩,纯橄岩较少。南公珠错地幔橄榄岩矿物化学特征表现为橄榄石具有较低的Fo(89.3~91.4)值、辉石具有较高的Al_2O_3含量(1.89%~6.06%)、尖晶石具有较低的Cr~#(12.7~28.3)值。与原始地幔相比南公珠错地幔橄榄岩的全岩地球化学特征具有较高的MgO含量和较低的Al_2O_3、CaO和TiO_2等易熔元素含量;方辉橄榄岩和二辉橄榄岩的稀土元素总含量分别介于0.66×10-6~1.10×10-6和0.90×10~(-6)~3.78×10~(-6)之间,明显低于原始地幔值,其稀土元素配分模式为轻稀土元素轻微富集型;在原始地幔标准化微量元素蜘蛛图中,南公珠错地幔橄榄岩显示出强烈的U正异常、Nd轻微正异常和强不相容元素Zr的负异常;方辉橄榄岩和二辉橄榄岩的铂族元素总量分别介于15.26×10~(-9)~25.23×10~(-9)和18.74×10~(-9)~26.86×10~(-9)之间,二者含量的变化较小,南公珠错地幔橄榄岩PGEs球粒陨石标准化图解显示其为接近于原始地幔的"平坦型"。南公珠错地幔橄榄岩的矿物化学和全岩地球化学特征与深海橄榄岩相似,指示它们可能形成于大洋扩张脊环境。定量模拟估算表明,南公珠错地幔橄榄岩可能来源于地幔中的尖晶石相二辉橄榄岩源区,系经历了至多16%部分熔融的残余。LREE的微富集和较高的Pd/Ir、Rh/Ir比值指示它们还经历了岩石-熔体反应作用。初步结论认为南公珠错地幔橄榄岩形成于大洋脊环境,为尖晶石相二辉橄榄岩地幔源区较低程度部分熔融的残余,但经历了后期岩石-熔体反应作用。  相似文献   

14.
《International Geology Review》2012,54(17):2184-2210
ABSTRACT

The Purang ultramafic massif, located in the Yarlung-Zangbo Suture Zone (YZSZ) of the Tibetan Plateau, consists mainly of harzburgites and minor lherzolites. The spinel-bearing lherzolites of the NW part of the massif display a granular texture, consisting of large olivine and pyroxene crystals with curvilinear grain boundaries. These lherzolites contain chromian spinel (Cr-spinel) of low Cr# [100 × Cr/(Cr +Al) = 24.7–30.2], enstatite with high Mg# [100 × Mg/(Mg + Fe2+) = 90.0–91.2] and relatively high Al2O3 content (3.3–4.1 wt%), and diopside with high Mg# (90.2–93.3) and Al2O3 content (4.6–5.0 wt%). These compositions are analogous to those of spinel and pyroxenes from residual peridotites. However, the Purang lherzolites show U-shaped primitive mantle (PM)-normalized rare earth element (REE)-profiles, which are not consistent with a potential origin as melting residues. The high LREE contents and positive Ti anomalies shown by the investigated lherzolites coupled with the low TiO2 content of their mineral constituents imply that these rocks possibly stored LREE- and Ti-bearing arc-related melts/fluids in their groundmass.

A mineral assemblage composed of diamond, super-reduced [(SuR) moissanite, native Cr] and crustal-derived minerals (zircon, corundum, rutile), has been separated from the Purang lherzolites. Uranium-Pb geochronological dating of zircons yielded an age range between 1718 and 465 Ma, indicating that they represent ancient crustal material delivered into the upper mantle via previous subduction events. Diamonds and old zircons (± crustal minerals) were carried to shallow mantle levels by asthenospheric magmas produced during a slab rollback-induced decompression melting process. The recovery of SuR minerals is consistent with fluid percolation and crystallization of alteration-related minerals in the lithospheric parts of a (hydrated) mantle wedge, resulting in the formation of highly reduced micro-environments.  相似文献   

15.
The Jurassic Bangong Lake ophiolite, NW Tibet, is a key element within the western part of the Bangong–Nujiang suture zone, which marks the boundary between the Lhasa and Qiangtang blocks. It is a tectonic mélange consisting of numerous blocks of peridotite, mafic lavas and dikes. The mantle peridotites include both clinopyroxene-bearing and clinopyroxene-free harzburgites. The Cpx-bearing harzburgite contains Al-rich spinel with low Cr#s (20–25), resembling peridotites formed in mid-ocean ridge settings. On the other hand, the Cpx-free harzburgite is highly depleted with Cr-rich spinel (Cr# = 69–73), typical of peridotites formed in subduction zone environments. Mafic rocks include lavas of N-MORB and E-MORB affinity and boninites. The N-MORB rocks consist of pillow lavas and mafic dikes, whereas the E-MORB rocks are brecciated basalts. The boninites have high SiO2 (53.2–57.9 wt%), MgO (6.5–12.5 wt%), Cr (166–752 ppm) and Ni (63–213 ppm) and low TiO2 (0.22–0.37 wt%) and Y (5.34–8.10 ppm), and are characterized by having U-shaped, chondrite-normalized REE patterns. The N-MORB and E-MORB lavas probably formed by different degrees of partial melting of primitive mantle, whereas the boninites reflect partial melting of depleted peridotite in a suprasubduction zone environment. The geochemistry of the ophiolite suggests that it is a fragment of oceanic lithosphere formed originally at a mid-ocean ridge (MOR) and then trapped above an intraoceanic subduction zone (SSZ), where the mantle peridotites were modified by boninitic melts. The Bangong–Nujiang suture zone is believed to mark the boundary between two blocks within Gondwanaland rather than to separate Gondwanaland from Eurasia.  相似文献   

16.
The distribution of platinum-group elements (PGEs), together with spinel composition, of podiform chromitites and serpentinized peridotites were examined to elucidate the nature of the upper mantle of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco. The mantle section is dominated by harzburgite with less abundant dunite. Chromitite pods are also found as small lenses not exceeding a few meters in size. Almost all primary silicates have been altered, and chromian spinel is the only primary mineral that survived alteration. Chromian spinel of chromitites is less affected by hydrothermal alteration than that of mantle peridotites. All chromitite samples of the Bou Azzer ophiolite display a steep negative slope of PGE spidergrams, being enriched in Os, Ir and Ru, and extremely depleted in Pt and Pd. Harzburgites and dunites usually have intermediate to low PGE contents showing more or less unfractionated PGE patterns with conspicuous positive anomalies of Ru and Rh. Two types of magnetite veins in serpentinized peridotite, type I (fibrous) and type II (octahedral), have relatively low PGE contents, displaying a generally positive slope from Os to Pd in the former type, and positive slope from Os to Rh then negative from Rh to Pd in the latter type. These magnetite patterns demonstrate their early and late hydrothermal origin, respectively. Chromian spinel composition of chromitites, dunites and harzburgites reflects their highly depleted nature with little variations; the Cr# is, on average, 0.71, 0.68 and 0.71, respectively. The TiO2 content is extremely low in chromian spinels, <0.10, of all rock types. The strong PGE fractionation of podiform chromitites and the high-Cr, low-Ti character of spinel of all rock types imply that the chromitites of the Bou Azzer ophiolite were formed either from a high-degree partial melting of primitive mantle, or from melting of already depleted mantle peridotites. This kind of melting is most easily accomplished in the supra-subduction zone environment, indicating a genetic link with supra-subduction zone magma, such as high-Mg andesite or arc tholeiite. This is a general feature in the Neoproterozoic upper mantle.  相似文献   

17.
丁青蛇绿岩位于班公湖-怒江缝合带东段,是该缝合带出露面积最大的蛇绿岩。为查明岩体成因,在丁青东岩体中实施了一口165.19m的钻孔。除最顶部有约0.5m厚的第四系残坡积物外,其余均为地幔橄榄岩。结合显微镜鉴定将岩心划分出17个岩性单元层,岩性主要以方辉橄榄岩为主,夹少量纯橄岩和含铬铁矿纯橄岩。地幔橄榄岩中橄榄石的Fo变化于88.79~93.73,铬尖晶石的Cr#变化于44.33~81.66,揭示丁青地幔橄榄岩可能经历过约20%~40%的中高度部分熔融作用;全岩地球化学分析表明其具有富镁（MgO=45.98%~49.45%）、贫铝（Al2O3=0.19%~1.37%）和贫钙（CaO=0.28%~0.70%）的特点,属于熔融程度较高的地幔残余物质。岩石具有明显不同于阿尔卑斯蛇绿岩的轻稀土元素富集特征,指示区内地幔橄榄岩先经历了较强程度的部分熔融,后经历了俯冲消减过程中的流体交代。利用地幔橄榄岩中的铬尖晶石成分计算母熔体Al2O3含量对应的FeO/MgO值,与不同构造环境原始岩浆成分相比较,发现丁青地幔橄榄岩母熔体大多处于玻安岩中。纯橄岩氧逸度估算FMQ=-3.05~-0.71,方辉橄榄岩氧逸度FMQ=-3.89~+1.47,显示丁青地幔橄榄岩有俯冲作用的参与。通过丁青钻孔岩心的研究,提出丁青东岩体可能形成于俯冲带之上的弧前环境这一观点。  相似文献   

18.
The Ospino-Kitoi and Kharanur ultrabasic massifs represent the northern and southern ophiolite branches respectively of the Upper Onot ophiolitic nappe and they are located in the southeastern part of the Eastern Sayan(SEPES ophiolites).Podiform chromitites with PGE mineralization occur as lensoid pods within dunites and rarely in harzburgites or serpentinized peridotites.The chromitites are classified into type I and type Ⅱ based on their Cr~#.Type I(Cr~# = 59-85) occurs in both northern and southern branches,whereas type Ⅱ(Cr~# = 76-90) occurs only in the northern branch.PGE contents range from ∑PGE 88-1189 ppb,Pt/Ir0.04-0.42 to ∑PGE 250-1700 ppb,Pt/Ir 0.03-0.25 for type I chromitites of the northern and southern branches respectively.The type Ⅱ chromitites of the northern branch have ∑PGE contents higher than that of type Ⅰ(468-8617 ppb,Pt/Ir 0.1-0.33).Parental melt compositions,in equilibrium with podiform chromitites,are in the range of boninitic melts and vary in Al_2O_3,TiO_2 and FeO/MgO contents from those of type I and type Ⅱ chromitites.Calculated melt compositions for type Ⅰ chromitites are(Al_2O_3)_(melt) = 10.6—13.5 wt.%,(TiO_2)_(melt) = 0.01-0.44 wt.%,(Fe/Mg)_(melt) = 0.42-1.81;those for type Ⅱ chromitites are:(Al_2O_3)_(melt) = 7.8-10.5 wt.%,(TiO_2)_(melt) = 0.01-0.25 wt.%,(Fe/Mg)_(melt) = 0.5-2.4.Chromitites are further divided into Os-Ir-Ru(Ⅰ) and Pt-Pd(Ⅱ) based on their PGE patterns.The type Ⅰ chromitites show only the Os-Ir-Ru pattern whereas type Ⅱ shows both Os-Ir-Ru and Pt-Pd patterns.PGE mineralization in type Ⅰ chromitites is represented by the Os-Ir-Ru system,whereas in type Ⅱ it is represented by the Os-Ir-Ru-Rh-Pt system.These results indicate that chromitites and PGE mineralization in the northern branch formed in a suprasubduction setting from a fluid-rich boninitic melt during active subduction.However,the chromitites and PGE mineralization of the southern branch could have formed in a spreading zone environment.Mantle peridotites have been exposed in the area with remnants of mantle-derived reduced fluids,as indicated by the occurrence of widespread highly carbonaceous graphitized ultrabasic rocks and serpentinites with up to 9.75 wt.%.Fluid inclusions in highly carbonaceous graphitized ultrabasic rocks contain CO,CO_2,CH4,N_2 and the δ~(13)C isotopic composition(-7.4 to-14.5‰) broadly corresponds to mantle carbon.  相似文献   

19.
东波超镁铁岩体产在雅鲁藏布江缝合带的西段,与周边白垩纪沉积岩地层和火山岩以断层接触.航磁资料显示该岩体约400km2规模,地表出露连续,地下有一定延深.超镁铁岩体由亏损的地幔橄榄岩组成,主要有高镁的方辉橄榄岩、纯橄岩和少量二辉橄榄岩.方辉橄榄岩和二辉橄榄岩中橄榄石和斜方辉石属高镁型,分别为Fo=89.5～91.5和Mg#=90～91.5.但二辉橄榄岩中的Al2O3和CaO含量明显高于方辉橄榄岩.方辉橄榄岩中单斜辉石Mg#=92～95,二辉橄榄岩的Mg#=92～93,两者的值也重叠.二辉橄榄岩中的Al2O3和CaO含量要明显高于方辉橄榄岩.这些均为阿尔卑斯型地幔橄榄岩的典型特征.纯橄岩中的橄榄石Fo=92～93.2,其斜方辉石和单斜辉石的Mg#=～93,但Al2O3和CaO的含量比方辉橄榄岩和二辉橄榄岩的低.三种岩石的成分变化规律,反映了地幔部分熔融程度的差异.二辉橄榄岩铬尖晶石的Cr#值20～30,反映为典型深海橄榄岩特征,指示MOR环境.与其不同的是,方辉橄榄岩的铬尖晶石的Cr#=20～75,指示MOR和SSZ两者兼有环境.岩石的原始地幔标准化的REE和微量元素蛛网图模式支持了上述的认识.东波地幔橄榄岩中的岩石学特征与产有大型铬铁矿床的罗布莎地幔橄榄岩可对比,岩体中已多处发现块状铬铁矿石,其铬铁矿的Cr2O3含量56％～59％,表明东波是寻找铬铁矿大矿和富矿甚具前景的一个超镁铁岩体.  相似文献   

20.

Serpentinized rocks closely associated with Paleoproterozoic eclogitic metabasites were recently discovered at Eseka area in the northwestern edge of the Congo craton in southern Cameroon. Here, we present new field data, petrography, and first comprehensible whole-rock geochemistry data and discuss the protolith and tectonic significance of these serpentinites in the region. The studied rock samples are characterized by pseudomorphic textures, including mesh microstructure formed by serpentine intergrowths with cores of olivine, bastites after pyroxene. Antigorite constitutes almost the whole bulk of the rocks and is associated (to the less amount) with tremolite, talc, spinel, and magnetite. Whole-rock chemistry of the Eseka serpentinites led to the distinction of two types. Type 1 has high MgO (> 40 wt%) content and high Mg# values (88.80) whereas Type 2 serpentinite samples display relatively low MgO concentration and Mg# values (< 40 and 82.88 wt%, respectively). Both types have low Al/Si and high Mg/Si ratios than the primitive mantle, reflecting a refractory abyssal mantle peridotite protolith. Partial melting modeling indicates that these rocks were derived from melting of spinel peridotite before serpentinization. Bulk rock high-Ti content is similar to the values of subducted serpentinites (> 50 ppm). This similarity, associated with the high Cr contents, spinel-peridotite protolith compositions and Mg/Si and Al/Si ratios imply that the studied serpentinites were formed in a subduction-related environment. The U-shaped chondrite normalized-REE patterns of serpentinized peridotites, coupled with similar enrichments in LREE and HFSE, suggest the refertilized nature due to melt/rock interaction prior to serpentinization. Based on the results, we suggest that the Eseka serpentinized peridotites are mantle residues that suffered a high degree of partial melting in a subduction-related environment, especially in Supra Subduction Zone setting. These new findings suggest that the Nyong series in Cameroon represents an uncontested Paleoproterozoic suture zone between the Congo craton and the São Francisco craton in Brazil.

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