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1.
The material and isotope-geochemical peculiarities of zircons from dunites of the Kondyor massif (Aldan Province, Southeast Siberian Craton) have been described for the first time. This massif is associated with an economic platinum placer deposit. On the basis of detailed study of the morphology, internal structure, and geochemical characteristics, zircons have been subdivided into two types. The first type is represented by crystals of oval and rounded shapes, characterized by a bimodal distribution of ??ancient?? (2477 ± 18 and 1885 ± 52 Ma) U-Pb (SHRIMP-II) ages. The second type of zircons forms idiomorphic crystals and aggregates of prismatic habitus, forming two ??young?? age clusters (176 ± 1.2 and 143 ± 2.0 Ma). The obtained results allow us to make a conclusion about the long evolution of platinum-bearing dunites. The first type of zircons formed by metamictic cores and homogeneous peripheral rims indicates the time of metamorphism at the Archean-Proterozoic boundary (??2.5 Ga), implying an older age of porous zircon cores. Such zircons characterize the minimal age of the initial mantle substance or a close time when the platinum-bearing dunite was generated. Based on their formation time, the second type of zircons can be referred to the epoch of tectonic-magmatic activation of the Aldan Shield. The new geochronological data, along with the material identity of dunites from the Urals and the Aldan Province, argue for a common genetic origin of platinum-bearing dunites of the fold belts and ancient cratons.  相似文献   

2.
The paper presents new determinations of the U-Pb zircon age of high-Al chromitite from dunite of the mantle section of the Voikar-Synya massif at the Kershor site in the boundary zone with rocks of the dunite-wehrlite-clinopyroxenite complex. The high-Cr chromitite from dunite in the central part of the same massif contains zircon dated at ca. 0.6 Ga [10]. It is suggested that Paleoproterozoic (2.0?1.9 Ga) zircons from chromitites of the mantle section near the petrological Moho boundary were formed in the course of partial melting of peridotites and/or their interaction with migrating MORB-type melts. The occurrence of Vendian and Paleoproterozoic zircons in chromitites from different parts of the mantle section, as well as previously published petrological, geochemical, and geological data [2, 11, 22] allow us to suggest a complex multistage evolution of the mantle section in ophiolites. The arguments stated below show that chromitites and host dunites could have been formed at different times and were probably related to different processes. Thus, not only various complexes of the pre-Paleozoic oceanic crust reworked in the suprasubduction setting differ in age, but also the mantle rock of similar petrography, vary in the time of their formation.  相似文献   

3.
The Klyuveskoi gabbro-ultramafic massif is the most representative ophiolite complex on the eastern portion of the Uralian paleoisland arc part. The massif is composed of dunite-harzburgite (tectonized mantle peridotites) and dunite-wehrlite-clinopyroxenite-gabbro (layered part of the ophiolite section) rock associations. The U-Pb age was obtained for the accessory zircons from the latter association using a SHRIMP-II ion microprobe at the Center for Isotopic Research at the Karpinskii Russian Geological Research Institute. The euhedral zircon crystals with thin rhythmic zoning from dunites are 441.4 ± 5.0 Ma in age. Zircons from olivine clinopyroxenite show three age clusters with sharply prevalent grains 449.0 ± 6.8 Ma in age. Two points give 1.7 Ga, which is probably related to the age of the mantle generating the layered complex. One value corresponds to 280 Ma, which possibly reflects exhumation of ultramafic rocks in the upper crust during the collision of the Uralian foldbelt. Thus, dunites and olivine pyroxenites from the Klyuchevskoi massif are similar in age at 441–449 Ma. The bottom of the layered part of the ophiolite section corresponds to the M paleoboundary and, consequently, the age of the Mohorovicic discontinuity conforms with the Ordovician-Silurian boundary in this part of the Urals.  相似文献   

4.
The paper reports the results of mineralogical and isotope-geochronological study of zircons from the Uzyansky Kraka (UK) Massif, which represents the part of the large (more than 900 km2) lherzolite allochthon thrusted onto the Paleozoic sequences of the East European margin. The massif shows a distinct stratification (from the top downward): spinel lherzolites, garnet pyroxenites, and dunites. The formation of stratified section is considered to be related to the decompression uplift of mantle lherzolite block. Zircons from the massif rocks were dated using SHRIMP-II ion microprobe. The oldest relict datings characterizing endogenous transformations of protolith were established in the zircons from the lherzolites (2037 ± 20 and 1132 ± 6 Ma), garnet pyroxenites (953 ± 11 Ma), and dunites (632 ± 11 Ma). All rock associations contain zircons with ages within 590–550 and 445–390 Ma, which mark the stages of mantle stratification of lherzolite block into complementary series and their emplacement at the upper crustal level. Age values within 299–196 Ma were found only in the dunites and date the influence of the Paleozoic strike-slipping. Our studies led us to conclude that the modern structure of the Ural collision orogen contains the fragments of subcontinental lithosphere, which were previously described only for the massifs of the root zones of the Western and Central Europe. Some general petrogenetic questions of lherzolite massifs from orogenic regions are discussed.  相似文献   

5.
郑建平  赵伊  熊庆 《地球科学》2019,44(4):1067-1082
造山带橄榄岩记录了板块俯冲、碰撞、折返等复杂过程信息,可分为壳源和幔源两种类型.造山带橄榄岩（特别是幔源类型）中锆石极为罕见,锆石内部具有橄榄岩的特征矿物或组合包裹体说明这些锆石可以生长于地幔中.造山带橄榄岩在经历板块汇聚（例如超高压变质等）复杂作用过程中,经历了不同时期的熔/流体的交代作用,对橄榄岩的矿物组合和元素组成都能产生重要影响.橄榄岩中锆石作为典型的交代作用产物,它的形成受控于熔/流体的化学组成、来源属性以及形成物理化学环境等.幔源型造山带橄榄岩中锆石的形成过程可能包括:（1）锆石结晶能力强,在地幔环境变化中Zr优先与其他地幔硅酸盐矿物中的Si结合,形成锆石;（2）原始锆石的溶解和含Zr矿物相（如石榴石等）的破坏或晶间熔体析出,在亚固相线条件下形成锆石;（3）再循环地壳物质来源的熔/流体,交代地幔楔并结晶形成锆石.因此利用锆石可以揭示特定岩石圈域的演化历史,有助于深刻理解大陆克拉通及其边缘过程.   相似文献   

6.
The microstructure and magnetic properties of accessory Fe–Cr-spinels from the Kytlym massif of the Urals platinum-bearing belt were studied. Atypical Fe–Cr-spinels in the form of magnetic microareas in grains of primary nonmagnetic Fe–Cr-spinel have been revealed for the first time in the bed dunites of the Kytlym multiphase concentrically zoned massif, North Urals. These spinels are responsible for the magnetic properties of the dunites. It has been established that the microareas are separations in solid solution Fe2+(Cr2–xFex3+)O4, which are enriched in Fe3 + and are probably an intermediate product of the transformation of primary accessory Fe-Cr-spinel during the formation of the dunite massif. These are magnetic microphases with particular chemical composition, cation distribution, and corresponding reversed crystal lattice, which determine the main magnetic properties of the microarea: the magnitude and direction of magnetization vector and Curie temperature. The formation of this earlier unknown type of magnetic Fe–Cr-spinel is probably conjugate with the formation and concentration of PGE mineralization in the bed dunites of the Kytlym platinum-bearing massif. The presence of such magnetization carriers in rocks and ores must be taken into account in geophysical research at the Urals chromite and platinum–chromite deposits.  相似文献   

7.
The compositional and isotope–geochemical features of zircons from wehrlite of the Feklistov massif, which formed platinum coastal placers, are discussed in this paper for the first time. Zircons from wehrlite of the Feklistov massif, similarly to worldwide zoned clinopyroxenite–dunite massifs, are characterized by different morphology, composition and a wide spectrum of ages (from 2.717 to 0.373 Ga). The Late Devonian age (373.2 ± 7.5 Ma) of zircons allows us to characterize the timing of the formation of wehrlite from the Feklistov massif and to correlate its emplacement with a significant superplume event, which covered the Siberia and Laurussia continents. The geological meaning of this dating refers to limiting the lower age boundary for emplacement of the Feklistov clinopyroxenite–dunite massif into the Earth’s crust, which does not contradict geological observations.  相似文献   

8.
This paper presents results of isotopic (Cameca IMS1270 NORDSIM and SHRIMP-II ion microprobes) and geochemical (LA-ICP MS) study of zircons in three dunite samples of the Uralian-Alaskan-type massifs of the Urals: Kosva, Sakharin, and Eastern Khabarny. The zircons in the dunites share common features. Each sample contains the following genetic and age groups of zircons: (1) xenogenic zircons of the Archean and Proterozoic age; (2) zircons of magmatic appearance, which in age and geochemistry are close to the zircons from associated gabbroids; (3) postmagmatic zircons that presumably crystallized from hydrothermal solutions. The xenogenic zircons of the Archean age in each of three samples comprise transparent fragments, which are depleted in U and other trace elements and presumably have mantle origin. Xenogenic zircons of the Proterozoic age (1500–2000 Ma) occur as oval grains with surface abrasion, the traces of their redeposition. The geochemical features of the xenogenic zircons unequivocally demonstrate their affiliation to the continental crust—the basement of the Uralian orogen. The zircons of magmatic habit in all the dunite samples are close in age to the associated gabbroids: 435–432 Ma in the Kosva Massif, 378–374 in the Sakharin Massif, and 407–402 Ma in the Eastern Khabarny Massif, and mark the age of dunite formation. In addition, the magmatic zircons from dunites and associated gabbroids share similar geochemical features. These data could serve as additional argument in support of cumulate origin of dunites in the Uralian-Alaskan-type complexes. The postmagmatic zircons are most enriched in trace elements and were presumably formed from a fluid phase, which was responsible for the recrystallization of dunites and redistribution of Cr-spinel and PGE mineralization.  相似文献   

9.
The Qinling Orogenic Belt was formed by subduction and collision between the North and South China Blocks along the Shangdan suture. The Songshugou ultramafic massif located on the northern side of the Shangdan suture provides essential insights into the mantle origin and evolutionary processes during spreading and subduction of the Shangdan oceanic lithosphere. The ultramafic massif comprises harzburgite, coarse- and fine-grained dunites. The spinels from harzburgite exhibit low Cr# and high Mg# numbers, suggesting a mid-ocean ridge peridotite origin, whereas spinels from both coarse- and fine-grained dunites are indicated as resulted from melt-rock reaction due to their systematic higher Cr# and low Mg# numbers. This melt-rock reaction in the dunites is also indicated by the low TiO2 (mostly <0.4 wt%) in the spinel and high Fo (90–92) in olivines. Due to its relatively homogeneous nature in the mantle, oxygen isotopic composition is a sensitive indicator for the petrogenesis and tectonic setting of the Songshugou ultramafic rocks. Based on in-situ oxygen isotope analyses of olivines from twenty-six rock samples, most harzburgites from the Songshugou ultramafic massif show low δ18O values of 4.54–5.30‰, suggesting the olivines are equilibrium with N-MORB magmas and originally formed in a mid-ocean ridge setting. The coarse- and fine-grained dunites exhibit slightly higher olivine δ18O values of 4.69–6.00‰ and 5.00–6.11‰, respectively, suggesting they may have been modified by subduction-related boninitic melt-rock reaction. The δ18O values of olivines systematically increasing from the harzburgites, to coarse-grained dunites and fine-grained dunites may suggest enhancing of melt-rock reaction. The decreasing of Os concentration, 187Re/188Os and 187Os/188Os ratios from harzburgite to dunite suggest an 187Os-enriched, subduction zone melt was responsible for creating the melt channel for melt-rock reactions. Together with the high-temperature ductile deformation microstructures, these isotopic and mineral geochemical features suggest that the harzburgites represent mantle residues after partial melting at mid-ocean ridge or supra-subduction zone, while the dunites were probably resulted from reactions between boninitic melt and harzburgites in a supra-subduction zone. Re-Os geochronology yields a maximum Re depletion model age (TRD) of 805 Ma, constraining the minimum formation age of the harzburgites derived from oceanic mantle. Eight samples of whole rock and chromite yield a Re-Os isochron age of 500 ± 120 Ma, constraining the timing of melt-rock reactions. Combined with the regional geology and our previous investigations, the Songshugou ultramafic rocks favors a mantle origin at mid-ocean ridge before 805 Ma, and were modified by boninitic melt percolations in a SSZ setting at ca. 500 Ma. This long-term tectonic process from spreading to subduction might imply a huge Pan-Tethyan ocean between the Laurasia (e.g., North China Block) and Gondwana (e.g., South China Block) and/or a one-side subduction.  相似文献   

10.
The data on the geochemistry and geochronology of zircons from wehrlites and clinopyroxenites of the dunite–wehrlite–clinopyroxenite banded complex that lies at the base of the crustal section of the ophiolite complex of the Nurali massif are presented. The obtained U–Pb age of the banded complex of 450 ± 4 Ma differs markedly from the previous age data. According to REE distribution patterns zircons from ultramafic rocks are attributed to the magmatic type and they indicate the age and supposed genetic similarity of the above rocks with lherzolites and dunites from the mantle section of the Nurali massif.  相似文献   

11.
Zircons in serpentinites from Nyashevo massif of the Ilmenogorskii complex were dated for the first time by means of the SHRIMP technique. The maximum date of 1892 ± 23 Ma for the zircons accounts for the minimum age of their mantle substrate probably constituting the restite residue. The date is comparable to those for metamorphic rocks of the Selyankino group, as well as of fenite–sand amphibolites of the Ilmenogorskii complex. The Upper Ordovician age limit of 443 ± 12 Ma is adequate for formation of the massif and conforms to the age of the Buldym massif and miaskites. The Early Permian dates of zircons (275.8 ± 2.1 Ma) represent late shear processes in the Ilmenogorskii complex.  相似文献   

12.
Zircons in dunites of the Sakharin and East Khararnin massifs, situated in the South Urals and part of the platinum-bearing Uralian belt, were investigated for the first time. Several types were identified in the polygene-polychronous zircon assemblage of both massifs. The first is represented by Proterozoic (from 1517 ± 12 to 2693 ± 9 Ma) crystals similar to those widely spread in the Riphean and more ancient Uralian deposits. The second type includes dunite varieties of typical magmatic habit and of ages from 377 ± 3.6 Ma to 402 ± 3 Ma. The third contains crystals and crystal fragments of a high degree of crystallinity. The concordant Archean dating (2808 ± 26 Ma) for zircons of this type determines the minimum age of the dunite substrate. Zircons of Type 4 are heterogeneous, consist of the relict (nuclear) part represented by crystals of Types I and III and the newly formed mantle, and are consistent with zircons of Type II.  相似文献   

13.
We present results of geochemical studies and isotope dating of eclogites and associated rocks from the Kuru-Vaara quarry, Belomorian Belt, Northeastern Baltic Shield. The southern and northern eclogites are similar in geochemical features. Their protoliths were primitive, mainly high-Mg basalts of oceanic affinity derived from a primitive mantle source rather than from a depleted mantle source characteristic of modern MORB. The post-eclogitic intrusive rocks show obvious evidence for crustal contamination. The eclogite-hosting tonalitetrondhjemite-granodiorite (TTG) gneisses form a coherent series including high-Al and low-Al varieties. The trace element data show that the TTG series formed through the hydrous partial melting of the southern eclogites in the presence of garnet and amphibole in the field of the rutile stability (>15 kbar). Zircons from the southern eclogites exhibit features of their strong re-equilibration by coupled dissolution–repre-cipitation processes but have locally preserved patches with a primary magmatic zoning. The geochemistry of the patches points to the oceanic provenance of protolithic zircons; their isotope dating (SHRIMP-II) yielded a concordant age of 2821 ± 21 Ma. Zircons from the trondhjemite gneiss with geochemical features of Archean adakite were dated at 2805 ± 11 Ma, which suggests the syneclogitic facies origin of the TTG melts. The concordant age of high-pressure zircons from the northern eclogites is 2722 ± 21 Ma, close to the age of the earlier described Gridino eclogites. The overview of the isotopically dated eclogite bodies show the presence of at least three temporally distinct groups of eclogites in the Belomorian Belt, ～2.86–2.87, ～2.82–2.80, and ～2.72 Ga, which is in a good accordance with the known isotopic ages for major crust-forming events in the belt. This, in turn, implies a close genetic relationship between the eclogites and the TTG origin, which might be consistent with the model of the short intermitted events of subduction of the thickened Archean oceanic crust. The presence of HP/UHP eclogites of different ages and the structural style of the Belomorian Belt permit it to be assigned to megamélange belts.  相似文献   

14.
The Ulaan Tolgoi massif of rare-metal (Ta, Nb, and Zr) granites was formed at approximately 300Ma in the Eastern Sayan zone of rare-metal alkaline magmatism. The massif consists of alkaline salic rocks of various composition (listed in chronologic order of their emplacement): alkaline syenite → alkaline syenite pegmatite → pantellerite → alkaline granite, including ore-bearing alkaline granite, whose Ta and Nb concentrations reach significant values. The evolution of the massif ended with the emplacement of trachybasaltic andesite. The rocks of the massif show systematic enrichment in incompatible elements in the final differentiation products of the alkaline salic magmas. The differentiation processes during the early evolution of the massif occurred in an open system, with influx of melts that contained various proportions of incompatible elements. The magma system was closed during the origin of the ore-bearing granites. Rare-metal granitoids in the Eastern Sayan zone were produced by magmas formed by interaction between mantle melts (which formed the mafic dikes) with crustal material. The mantle melts likely affected the lower parts of the crust and either induced its melting, with later mixing the anatectic and mantle magmas, or assimilated crustal material and generated melts with crustal–mantle characteristics. The origin of the Eastern Sayan zone of rare-metal alkaline magmatism was related to rifting, which was triggered by interaction between the Tarim and Barguzin mantle plumes. The Eastern Sayan zone was formed in the marginal part of the Barguzin magmatic province, and rare-metal magmas in it were likely generated in relation with the activity of the Barguzin plume.  相似文献   

15.
The investigation of melt inclusions in Cr spinels yielded direct information on the physicochemical parameters of the magmatic processes responsible for the formation of the Konder platiniferous alkaline-ultrabasic massif (southeastern Aldan Shield). The comparative analysis of the composition of the chromites containing the inclusions revealed that the Cr spinels from the Konder dunites differ significantly from their counterparts in the ultrabasic complexes of ophiolites and the modern oceanic crust. In terms of their composition and REE distribution, the clinopyroxene microcrystals from the Konder chromites are significantly different from the pyroxenes in the basic-ultrabasic ophiolite complexes of associations and identical to the minerals from the Kytlym platiniferous massif (the Urals). With respect to the distribution of the major components, the high-magnesian inclusions are identical to alkaline biotite-pyroxene picrites, testifying to the active participation of ultrabasic (picritic) alkaline magmatic systems in the formation of the dunites in the Konder Massif. The results of the ion probe investigation of the inclusions indicate a high water content (up to 0.54 wt %) in the melts. The data on the distribution of the rare and rare-earth elements in the inclusions suggest that the dunites of the Konder Massif crystallized with the involvement of water-saturated magmas at minimal temperatures of about 1230°C. Such temperatures are consistent with the earlier estimates for the melt inclusions in the olivine of the Konder Massif.  相似文献   

16.
The U-Pb age was obtained for the coexisting baddeleytte-zircon system from dunites of the Konder massif, Aldan shield. Four groups of zircons are heterogeneous by morphology, habit, age, and geochemistry in contrast to homogeneous baddeleytte. The studied zircon groups are characterized by several U-Pb age clusters in the range of 1895 ± 50 to 125.8 ± 3.8 Ma, which indicates their long evolution in Pt-bearing dunites. The young assemblage of baddeleytte and zircon (124.9 ± 1.9 and 125.8 ± 3.8 Ma, respectively) also differs from ancient zircons in the morphology and geochemistry and probably dates to a much later event of diapir evolution, which may be referred to the formation of apatite-phlogopite ore pyroxenites inside the dunite core and tectono-magmatic activation of the Aldan shield.  相似文献   

17.
北秦岭地区的早古生代超高压变质作用是整个秦岭-桐柏-红安-大别-苏鲁造山带内最古老的超高压变质记录, 代表华南与华北板块之间最早的一期增生碰撞事件, 然而目前对于该地区早古生代的构造演化过程仍存在较大争议.对北秦岭官坡超高压榴辉岩中的一个长英质脉体开展了详细的锆石形态学、微量元素和U-Pb年代学研究.结果表明, 脉体中的锆石呈自形的棱柱状晶形, 发育弱的振荡环带、面状分带或无明显分带特征, 具有高的HREE、Y、U含量, 低的Th含量和Th/U比值, 说明锆石生长自含水熔体.另外, 这些变质锆石具有LREE亏损、HREE相对富集的配分模式以及明显的Eu负异常, 表明含水熔体形成于角闪岩相退变质过程.锆石的206Pb/238U加权平均年龄为494±10 Ma (MSWD=2.2), 与北秦岭超高压变质作用的峰期年龄(490.4±5.8 Ma)在误差范围内基本一致.北秦岭地体经历深俯冲作用之后发生快速折返, 并在折返过程中发生角闪岩相退变质作用诱发俯冲板片部分熔融产生含水熔体.   相似文献   

18.
《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   

19.
The paper presents data on the geochemical and geochronological characteristics of zircons from mafic rocks of part of the Monchegorsk layered complex represented by the Vurechuaivench massif. Ages of zircons (SHRIMP-II) from samples V-l-09 (anorthosite) and V-2-09 (gabbronorite) are dated back to 2508 ± 7 and 2504 ± 8 Ma, respectively. The chondrite-normalized REE patterns confirm the magmatic nature of zircons. The data unequivocally indicate that the U–Pb age of zircon from both gabbronorite and anorthosite corresponds to the age of melt crystallization in a magmatic chamber. The mantle origin of gabbroic rocks of the Vurechuaivench massif is confirmed by the REE patterns of three zircon generations with different crystallization sequences. The wide range of the Ce/Ce* ratio (9.96–105.24) established for zircons from gabbroic rocks of the Vurechuaivench massif indicates sharply oxidative conditions of zircon crystallization. For deepseated mantle rocks, these data can only be explained by significant contamination of the melt with country rock material.  相似文献   

20.
The Bulqiza ultramafic massif, which is part of the eastern Mirdita ophiolite of northern Albania, is world renowned for its high-Cr chromitite deposits. High-Cr chromitites hosted in the mantle section are the crystallized products of boninitic melts in a supra-subduction zone (SSZ). However, economically important high-Al chromitites are also present in massive dunite of the mantle-crust transition zone (MTZ). Chromian-spinel in the high-Al chromitites and dunites of the MTZ have much lower Cr# values (100Cr/(Cr+Al)) (47.7–55.1 and 46.5–51.7, respectively) than those in the high-Cr chromitites (78.2–80.4), harzburgites (72.6–77.9) and mantle dunites (79.4–84.3). The chemical differences in these two types of chromitites are reflected in the behaviors of their platinum-group elements (PGE). The high-Cr chromitites are rich in IPGE relative to PPGE with 0.10–0.45 PPGE/IPGE ratios, whereas the high-Al chromitites have relatively higher PPGE/IPGE ratios between 1.20 and 7.80. The calculated melts in equilibrium with the high-Cr chromitites are boninitic-like, and those associated with the high-Al chromitites are MORB-like but with hydrous, oxidized and TiO2-poor features. We propose that the coexistence of both types of chromitites in the Bulqiza ultramafic massif may indicates a change in magma composition from MORB-like to boninitic-like in a proto-forearc setting during subduction initiation.  相似文献   

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