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1.
报道了徐淮地区早白垩世埃达克质岩中首次发现的含橄榄石单斜辉石岩捕虏体的岩相学与矿物化学资料, 该类捕虏体显示堆积结构、块状构造, 主要由单斜辉石(~80%)、斜方辉石(~5%)、橄榄石(~5%)和普通角闪石(~10%)组成.橄榄石外侧发育有斜方辉石反应边, 角闪石沿辉石粒间分布, 呈嵌晶结构.矿物化学分析结果表明: 橄榄石的镁橄榄石分子值(Fo)=77.7~79.3, Ni=623×10-6~773×10-6; 斜方辉石的Mg#=75.6~80.2, Cr=161×10-6~684×10-6, Ni=79×10-6~708×10-6; 单斜辉石的Mg#=84.5~86.4, CaO=21.59%~23.13%, Al2O3=1.72%~2.44%.上述矿物与中、新生代玄武岩中橄榄石、斜方辉石和单斜辉石斑晶以及堆积成因辉石岩中的斜方辉石和单斜辉石成分类似.此外, 单斜辉石的稀土配分型式以相对富含中稀土元素的上凸型为特征, 稀土元素含量较低(∑REE=10.14×10-6~12.71×10-6), 无明显的铕异常(δEu=0.90~1.16), 类似于新生代玄武岩中单斜辉石斑晶.捕虏体中的普通角闪石的Mg#=74.0~80.4、SiO2=43.2%~44.5%、Na2O=2.04%~2.29%, 稀土元素分馏不明显, 显示亏损高场强元素(HFSEs, 如Nb、Ta、Zr、Hf), 富集Sr、Rb、Ba的特征, 与新生代玄武岩中角闪石捕虏晶成分不同.结合其嵌晶结构, 普通角闪石应是寄主岩浆贯入结晶的产物.综合上述特征, 可以看出含橄榄石单斜辉石捕虏体为镁铁质岩浆高压堆晶成因.结合华北克拉通东部早白垩世双峰式火山岩组合的出现, 推断含橄榄石单斜辉石岩捕虏体可能是早白垩世基性岩浆底侵的产物. 相似文献
2.
Petrology and oxide mineral chemistry are presented on 5 kimberlite dikes that are classified into three groups: (1) one dike is highly carbonated and highly oxidized (> MH) and is characterised by chlorite+Mn-titanomagnetite+rutile+hematite (after chlorite)+maghemite (after titanomagnetite), with ilmenite and perovskite being absent; (2) three dikes are typified by atoll-textured spinels +phlogopite+euhedral Mn-picroilmenite, of intermediate oxidation state (WM-FMQ) with coexisting deuterically serpentinized olivine+Ni-Fe alloys and magnetite; (3) the remaining dike records an early crystallization event under very low ( WM) redox conditions that precipitated anionic-deficient spinels and a Mg-Ti-Cr-wüstite-type phase, followed by late stage more oxidizing (=FMQ) Mnpicroilmenite.Spinels are complexly zoned and crystallization trends among the dikes are diverse, underscoring the fact that no single compositional trend, or evolutionary sequence is typical of kimberlites. Ilmenites are euhedral, and criteria for groundmass crystallization are established. Extraordinarily high MnO (max 17 wt%) contents and high geikielite (62 mole%) concentrations expand the ilmenite field typically assigned to that of kimberlites. Zirconium, Nb and Cr are present in concentrations of 0.5–3 wt% (as oxides) in ilmenite. These highly incompatible elements, along with Mn, are concentrated in late stage melt fractions. The high pyrophanite contents, which are more typical of silicic alkali suites, are accompanied by phlogopite in the Koidu dikes.Objective evaluations of kimberlite-carbonatite relations, as outlined in the literature, cannot be made based on the oxide mineral group. Much of the compositional data for oxides in kimberlites are on mantle-derived xenolith suites and are not from oxides derived from the crystallization of kimberlitic melts.Assessments of the fO2's of kimberlites have considerable potential in evaluating diamond survival through redox reactions. Manganese-rich (+Nb, Zr, Cr) ilmenites are typical of many kimberlites and should be considered in the suite of index minerals employed in prospecting. 相似文献
3.
The Homestead kimberlite was emplaced in lower Cretaceous marine shale and siltstone in the Grassrange area of central Montana. The Grassrange area includes aillikite, alnoite, carbonatite, kimberlite, and monchiquite and is situated within the Archean Wyoming craton. The kimberlite contains 25–30 modal% olivine as xenocrysts and phenocrysts in a matrix of phlogopite, monticellite, diopside, serpentine, chlorite, hydrous Ca–Al–Na silicates, perovskite, and spinel. The rock is kimberlite based on mineralogy, the presence of atoll-textured groundmass spinels, and kimberlitic core-rim zoning of groundmass spinels and groundmass phlogopites.
Garnet xenocrysts are mainly Cr-pyropes, of which 2–12% are G10 compositions, crustal almandines are rare and eclogitic garnets are absent. Spinel xenocrysts have MgO and Cr2O3 contents ranging into the diamond inclusion field. Mg-ilmenite xenocrysts contain 7–11 wt.% MgO and 0.8–1.9 wt.% Cr2O3, with (Fe+3/Fetot) from 0.17–0.31. Olivine is the only obvious megacryst mineral present. One microdiamond was recovered from caustic fusion of a 45-kg sample.
Upper-mantle xenoliths up to 70 cm size are abundant and are some of the largest known garnet peridotite xenoliths in North America. The xenolith suite is dominated by dunites, and harzburgites containing garnet and/or spinel. Granulites are rare and eclogites are absent. Among 153 xenoliths, 7% are lherzolites, 61% are harzburgites, 31% are dunites, and 1% are orthopyroxenites. Three of 30 peridotite xenoliths that were analysed are low-Ca garnet–spinel harzburgites containing G10 garnets. Xenolith textures are mainly coarse granular, and only 5% are porphyroclastic.
Xenolith modal mineralogy and mineral compositions indicate ancient major-element depletion as observed in other Wyoming craton xenolith assemblages, followed by younger enrichment events evidenced by tectonized or undeformed veins of orthopyroxenite, clinopyroxenite, websterite, and the presence of phlogopite-bearing veins and disseminated phlogopite. Phlogopite-bearing veins may represent kimberlite-related addition and/or earlier K-metasomatism.
Xenolith thermobarometry using published two-pyroxene and Al-in-opx methods suggest that garnet–spinel peridotites are derived from 1180 to 1390 °C and 3.6 to 4.7 GPa, close to the diamond–graphite boundary and above a 38 mW/m2 shield geotherm. Low-Ca garnet–spinel harzburgites with G10 garnets fall in about the same T and P range. Most spinel peridotites with assumed 2.0 GPa pressure are in the same T range, possibly indicating heating of the shallow mantle. Four of 79 Cr diopside xenocrysts have P–T estimates in the diamond stability field using published single-pyroxene P–T calculation methods. 相似文献
4.
A xenolith from the kimberlite pipe of the Roberts Victor Mine, O.F.S. shows a marginal rim rich in garnet (Py50Alm35Gro15), presumably resulting from reaction between the grospyditic inclusion and kimberlitic host. Similarity between the reaction rim-garnets, and those of the common mafic inclusion of the Roberts Victor pipe, suggests that the rare grospydite inclusions are accidental xenoliths, not directly related in origin to the kimberlites in which they are found. 相似文献
5.
普兰蛇绿岩位于雅鲁藏布江缝合带西段,其中地幔橄榄岩由方辉橄榄岩、含单斜辉石方辉橄榄岩以及少量二辉橄榄岩及纯橄岩组成。尖晶石是地幔橄榄岩中常见的副矿物,可以作为重要的岩石学成因指示剂。在野外地质调查基础上,通过岩相观察、电子探针、尖晶石成分面分析、电子背反射衍射分析,可将普兰地幔橄榄岩铬尖晶石分为三类:第一类铬尖晶石呈自形,粒径较小(<100μm),或包裹于斜方辉石中,或杂乱分布于橄榄石和辉石之间,具有高Cr^#(>0.6)、低Mg^#(0.43~0.57)的特征,为部分熔融+玻安质熔体交代成因;第二类铬尖晶石呈半自形-他形,粒径较大(>100μm),常含有橄榄石、辉石包裹体,具有中Cr^#(0.17~0.42)、高Mg^#(0.63~0.77)的特点,主要受部分熔融作用影响;第三类铬尖晶石呈他形蠕虫状与辉石交生在一起构成后成合晶结构,粒径变化较大,具有低Cr#(0.17~0.28)、高Mg^#(0.67~0.77)的特点。EBSD分析结果显示尖晶石、辉石的结晶学优选方位(CPO)较为相似,表明为同一矿物分解而来,单斜辉石与大陆岩石圈地幔捕掳体中石榴子石的稀土元素对比表明构成后成合晶结构的辉石和铬尖晶石为具有大陆岩石圈地幔属性的高压石榴子石退变分解而成。综合分析表明:普兰蛇绿岩的地幔橄榄岩体在从石榴子石相深度上升过程中发生了石榴子石退变、岩石部分熔融及熔体渗透作用,岩体经历了威尔逊旋回初期的大陆裂谷阶段,主体经历了中-低程度的部分熔融,类似大洋中脊环境,局部受到了富硅、富镁玻安质熔体的影响。 相似文献
6.
Djerfisherite, a Cl-bearing potassium sulfide (K6Na(Fe,Ni,Cu)24S26Cl), is a widespread accessory mineral in kimberlite-hosted mantle xenoliths. Nevertheless, the origin of this sulfide in nodules remains disputable. It is usually attributed to the replacement of primary Fe–Ni–Cu sulfides when xenoliths interact with a K-and Cl-enriched hypothetical melt/fluid. The paper is devoted to a detailed study of the composition and morphology of djerfisherite from a representative collection (22 samples) of the deepest mantle xenoliths—sheared garnet peridotite, taken from the Udachnaya-East kimberlite pipe (Yakutia). Four types of djerfisherite were distinguished in the mantle rocks on the basis of morphology, spatial distribution, and relationships with the rock-forming and accessory minerals in the nodules. Type 1 was found in the rims of polysulfide inclusions in the rock-forming minerals of the xenoliths; there, it was younger than the primary sulfide assemblage pyrrhotite + pentlandite ± chalcopyrite. Type 2 formed rims around large polysulfide segregations (pyrrhotite+ pentlandite) in the xenolith interstices. Type 3 formed individual grains in the xenolith interstices together with other sulfides, silicates, oxides, phosphates, and carbonates. Type 4 was present as a daughter phase in the secondary melt inclusions which occurred in healed cracks in the rock-forming minerals of the xenoliths. Along with djerfisherite, the inclusions contained silicates, oxides, phosphates, carbonates, alkaline sulfates, chlorides, and sulfides. The results indicate that djerfisherite from the xenoliths is consanguine with kimberlite. Djerfisherite both in the sheared-peridotite xenoliths from the Udachnaya-East pipe and in different xenoliths from other kimberlite pipes worldwide formed owing to the interaction between the nodules and kimberlitic melts. Djerfisherite forming individual grains in the melt inclusions and xenolith interstices crystallized directly from the infiltrating kimberlitic melt. Djerfisherite bounding the primary Fe–Ni ± Cu sulfides formed by their replacement as a result of a reaction with the kimberlitic melt. 相似文献
7.
Strontium isotopic studies of kimberlites reveal no significant differences between the respective whole-rock Sr87/Sr86 ratios of fissure and pipe kimberlites. Kimberlites from the Swartruggens fissure (calcareous micaceous kimberlite) have Sr87/Sr86 ratios of from 0.709 to 0.716, whilst those from the Wesselton pipe have Sr87/Sr86 ratios of from 0.708 to 0.715. Other kimberlites range from 0.706 to 0.715. Samples are considered to be late Cretaceous to early Tertiary and thus the ratios are approximately initial ratios. The Sr87/Sr86 ratios bear no relation to the Rb or Sr content of individual kimberlite bodies. The high initial ratios are not due to bulk assimilation of granitic material in either a kimberlite or carbonatitic magma. Rb-Sr data for garnet peridotites and eclogite xenoliths in kimberlite are not compatible with production of kimberlite by eclogite fractionation from a melt derived from garnet lherzolite. The Sr isotopic composition of kimberlite is compatible with partial melting of garnet mica peridotite. The isotopic composition of liquids formed by partial melting of this rock can be modified by (i) gross contamination with material of low Sr87/Sr86 ratio or (ii) selective diffusion of material of high Sr87/Sr86 ratio into kimberlitic fluids. 相似文献
8.
Ultrahigh-pressure metamorphism in the forbidden zone: the Xugou garnet peridotite, Sulu terrane, eastern China 总被引：21，自引：3，他引：21
The Xugou garnet peridotite body of the southern Sulu ultrahigh‐pressure (UHP) terrane is enclosed in felsic gneiss, bounded by faults, and consists of harzburgite and lenses of garnet clinopyroxenite and eclogite. The peridotite is composed of variable amounts of olivine (Fo91), enstatite (En92?93), garnet (Alm20?23Prp53?58Knr6?9Grs12?18), diopside and rare chromite. The ultramafic protolith has a depleted residual mantle composition, indicated by a high‐Mg number, very low CaO, Al2O3 and total REE contents compared to primary mantle and other Sulu peridotites. Most garnet (Prp44?58) clinopyroxenites are foliated. Except for rare kyanite‐bearing eclogitic bands, most eclogites contain a simple assemblage of garnet (Alm29?34Prp32?50Grs15?39) + omphacite (Jd24?36) + minor rutile. Clinopyroxenite and eclogite exhibit LREE‐depleted and LREE‐enriched patterns, respectively, but both have flat HREE patterns. Normalized La, Sm and Yb contents indicate that both eclogite and garnet clinopyroxenite formed by high‐pressure crystal accumulation (+ variable trapped melt) from melts resulting from two‐stage partial melting of a mantle source. Recrystallized textures and P–T estimates of 780–870 °C, 5–7 GPa and a metamorphic age of 231 ± 11 Ma indicate that both mafic and ultramafic protoliths experienced Triassic UHP metamorphism in the P–T forbidden zone with an extremely low thermal gradient (< 5 °C km?1), and multistage retrograde recrystallization during exhumation. Develop of prehnite veins in clinopyroxenite, eclogite, felsic blocks and country rock gneiss, and replacements of eclogitic minerals by prehnite, albite, white mica, and K‐feldspar indicate low‐temperature metasomatism. 相似文献
9.
We present high-precision iron and magnesium isotopic data for diverse mantle pyroxenite xenoliths collected from Hannuoba, North China Craton and provide the first combined iron and magnesium isotopic study of such rocks. Compositionally, these xenoliths range from Cr-diopside pyroxenites and Al-augite pyroxenites to garnet-bearing pyroxenites and are taken as physical evidence for different episodes of melt injection. Our results show that both Cr-diopside pyroxenites and Al-augite pyroxenites of cumulate origin display narrow ranges in iron and magnesium isotopic compositions (δ57Fe = ?0.01 to 0.09 with an average of 0.03 ± 0.08 (2SD, n = 6); δ26Mg = ? 0.28 to ?0.25 with an average of ?0.26 ± 0.03 (2SD, n = 3), respectively). These values are identical to those in the normal upper mantle and show equilibrium inter-mineral iron and magnesium isotope fractionation between coexisting mantle minerals. In contrast, the garnet-bearing pyroxenites, which are products of reactions between peridotites and silicate melts from an ancient subducted oceanic slab, exhibit larger iron isotopic variations, with δ57Fe ranging from 0.12 to 0.30. The δ57Fe values of minerals in these garnet-bearing pyroxenites also vary widely (?0.25 to 0.08 in olivines, ?0.04 to 0.25 in orthopyroxenes, ?0.07 to 0.31 in clinopyroxenes, 0.07 to 0.48 in spinels and 0.31–0.42 in garnets). In addition, the garnet-bearing pyroxenite shows light δ26Mg (?0.43) relative to the mantle. The δ26Mg of minerals in the garnet-bearing pyroxenite range from ?0.35 for olivine and orthopyroxene, to ?0.34 for clinopyroxene, 0.04 for spinel and ?0.68 for garnet. These measured values stand in marked contrast to calculated equilibrium iron and magnesium isotope fractionation between coexisting mantle minerals at mantle temperatures derived from theory, indicating disequilibrium isotope fractionation. Notably, one phlogopite clinopyroxenite with an apparent later metasomatic overprint has the heaviest δ57Fe (as high as 1.00) but the lightest δ26Mg (as low as ?1.50) values of all investigated samples. Overall, there appears to be a negative co-variation between δ57Fe and δ26Mg in the Hannuoba garnet-bearing pyroxenite and in the phlogopite clinopyroxenite xenoliths and minerals therein. These features may reflect kinetic isotopic fractionation due to iron and magnesium inter-diffusion during melt–rock interaction. Such processes play an important role in producing inter-mineral iron and magnesium isotopic disequilibrium and local iron and magnesium isotopic heterogeneity in the subcontinental mantle. 相似文献
10.
Melt must transfer through the lower crust, yet the field signatures and mechanisms involved in such transfer zones (excluding dykes) are still poorly understood. We report field and microstructural evidence of a deformation‐assisted melt transfer zone that developed in the lower crustal magmatic arc environment of Fiordland, New Zealand. A 30–40 m wide hornblende‐rich body comprising hornblende ± clinozoisite and/or garnet exhibits 'igneous‐like' features and is hosted within a metamorphic, two‐pyroxene–pargasite gabbroic gneiss (GG). Previous studies have interpreted the hornblende‐rich body as an igneous cumulate or a mass transfer zone. We present field and microstructural characteristics supporting the later and indicating the body has formed by deformation‐assisted, channelized, reactive porous melt flow. The host granulite facies GG contains distinctive rectilinear dykes and garnet reaction zones (GRZ) from earlier in the geological history; these form important reaction and strain markers. Field observations show that the mineral assemblages and microstructures of the GG and GRZ are progressively modified with proximity to the hornblende‐rich body. At the same time, GRZ bend systematically into the hornblende‐rich body on each side of the unit, showing apparent sinistral shearing. Within the hornblende‐rich body itself, microstructures and electron back‐scatter diffraction mapping show evidence of the former presence of melt including observations consistent with melt crystallization within pore spaces, elongate pseudomorphs of melt films along grain boundaries, minerals with low dihedral angles as small as <10° and up to <60°, and interconnected 3D melt pseudomorph networks. Reaction microstructures with highly irregular contact boundaries are observed at the field and thin‐section scale in remnant islands of original rock and replaced grains, respectively. We infer that the hornblende‐rich body was formed by modification of the host GG in situ due to reaction between an externally derived, reactive, hydrous gabbroic to intermediate melt percolating via porous melt flow through an actively deforming zone. Extensive melt–rock interaction and metasomatism occurred via coupled dissolution–precipitation, triggered by chemical disequilibrium between the host rock and the fluxing melt. As a result, the host plagioclase and pyroxene became unstable and were reacted and dissolved into the melt, while hornblende and to a lesser extent clinozoisite and garnet grew replacing the unstable phases. Our study shows that hornblendite rocks commonly observed within deep crustal sections, and attributed to cumulate fractionation processes, may instead delineate areas of deformation‐assisted, channelized reactive porous melt flow formed by melt‐mediated coupled dissolution–precipitation replacement reactions. 相似文献
11.
Ilmenite is one of the common kimberlitic indicator minerals recovered during diamond exploration, and its distinction from non-kimberlitic rock types is important. This is particularly true for regions where these minerals are present in relatively low abundance, and they are the dominant kimberlitic indicator mineral recovered. Difficulty in visually differentiating kimberlitic from non-kimberlitic ilmenite in exploration concentrates is also an issue, and distinguishing kimberlitic ilmenite from those derive from other similar rocks, such as ultramafic lamprophyres, is practically impossible. Ilmenite is also the indicator mineral whose compositional variety has the most potential to resolve provenance issues related to mineral dispersions with contributions from multiple kimberlite sources.
Various published data sets from selected kimberlitic (including kimberlites, lamproites, and various ultramafic lamprophyres) and non-kimberlitic rock types have been compiled and evaluated in terms of their major element compositions. Compositional fields and bounding reference lines for ilmenites derived from kimberlites (sensu stricto), ultramafic lamprophyres, and other non-kimberlitic rock types have been defined primarily on MgO–TiO2 graphs as well as MgO–Cr2O3 relationships. 相似文献
12.
David W. Mittlefehldt 《Contributions to Mineralogy and Petrology》1986,94(2):245-252
The Mount Carmel xenolith suite is composed of a series of garnet granulites of probable lower crustal origin, and a high pressure clinopyroxenite series. The clinopyroxenite series is petrologically diverse with the most common lithologies being garnet-clinopyroxenite, clinopyroxenite, garnet-amphibole-clinopyroxenite, amphibole-clinopyroxenite, amphibole-mica-clinopyroxenite plus megacrystalline nodules of clinopyroxene, garnet, amphibole and mica. Orthopyroxene is extremely rare (1 sample) and olivine is absent in the clinopyroxenite series xenoliths. The clinopyroxenite series is divided into three rock associations based on textures, mineralogy, mineral chemistry and equilibration temperatures: the metaclinopyroxenite, the magmatic garnet-clinopyroxenite and the amphibole-mica-clinopyroxenite associations. Many of the xenoliths contain late phases, largely amphibole, as microphenocrysts in glass and altered glass that was intruded into the xenoliths. Each of the three associations plus the late phases represents the crystallization products of one or more magma batchs. Garnet-clinopyroxene geothermometry and phase relations for alkaline basalts allow estimates of theP/T conditions of equilibration to be made for some of the xenoliths. The metaclinopyroxenites were equilibrated at 1.5–3 GPa and 990–1,115° and the magmatic garnet-clinopyroxenites were equilibrated at 2–3 GPa and 1,160–1,190°. The Mt. Carmel xenoliths are samples from the depth range 50–95 km and fall in the sampling gap between xenoliths typical of alkali basalts (d<60 km) and those typical of kimberlites (d> 90 km). 相似文献
13.
The Longwoods Complex of Southland, New Zealand is part of an extensive terrane consisting of intrusives, volcanics, and sediments,
which outcrops in the southern and north-western portions of the South Island. This terrane represents a volcanic arc which
was active from Permian to Jurassic times (Grindley, 1958; Challis, 1968, 1969; Coombs et al., 1976). Between Pahia Point
and Oraka Point on the southern coast of the South Island a section across the Longwoods Complex is well exposed and intrusives
ranging in composition from ultrabasic cumulate rock, high-Al gabbro and gabbroic diorite to quartz diorite and granite outcrop.
Two models have been considered for the origin of the rocks of the Pahia Point-Oraka Point section: (a) the rocks constitute
one suite, the members of which are related by a crystal fractionation process; (b) the rocks constitute two suites which
are not directly related.
The ultrabasic rocks, and quartz diorites are complementary and are derived from a high-Al gabbro parent by crystal fractionation
involving pyroxene, olivine, plagioclase and hornblende, but considerations of viscosity and the geochemistry of the granite
preclude derivation of the high-Si rocks by continuation of the crystal fractionation model. Furthermore, the quartz-diorites
are of two types: xenolith bearing foliated quartz-diorites and xenolith deficient unfoliated types. The latter rock type
appears to group with the gabbros on variation diagrams and partitioning of Ti between mica and amphibole supports the view
that two distinct suites of rocks are involved: (a) a suite derived by fractional crystallization from a high-Al gabbro parent
and consisting of cumulate ultramafic rocks, high-Al gabbro, gabbroic diorite and quartz-diorite; (b) a suite of foliated
quartz diorites, formed by partial melting of lower crustal igneous rocks. The xenoliths in the foliated quartz-diorites represent
modified residue left after partial melting. Melt and residue have unmixed to varying degrees during diapiric rise and a range
of compositions has resulted.
The association of the two suites is tectonic. Gabbroic melts are generated in the lithosphere during plate subduction beneath
a continental margin and rise of these melts into the lower continental crust results in partial melting and generation of
quartz-diorite magmas. 相似文献
14.
Srinanda Chaudhuri Jyotisankar Ray Christian Koeberl Martin Thni Riya Dutta Abhishek Saha Mousumi Banerjee 《Geological Journal》2014,49(2):111-128
The present article describes, for the first time, petrological and geochemical details of the Mawpyut differentiated complex which is related to the Sylhet trap located at Jaintia Hills district, Meghalaya, northeastern India. The Mawpyut complex occurs as an arcuate body that intrudes into the surrounding Shillong Group rocks. The complex in general contains ‘ultramafic’ and ‘mafic’ rocks, as well as minor syenitic veins that postdate the main units. The lithotypes correspond to cumulate and noncumulate units. The cumulate unit is represented by olivine clinopyroxenite, clinopyroxenite, plagioclase‐bearing ultramafic, olivine gabbronorite, mela‐gabbronorite, melagabbro, orthopyroxene gabbro, and gabbro, all with a pronounced cumulus texture. The noncumulate unit is marked by gabbro, monzonite, monzodiorite, and quartzsyenite. The use of several major and trace element variation diagrams suggests that magmatic differentiation led to the formation of cumulate and noncumulate units. In chondrite‐normalized REE diagrams the cumulate rocks show flat LREE and MREE patterns and a moderate positive Eu anomaly (in plagioclase‐bearing ultramafics) due to plagioclase cumulation. The rocks of the noncumulate unit show a strongly fractionated REE pattern and no Eu anomaly. The noncumulate mafic rocks are geochemically comparable to high‐phosphorous/high‐titanium basalts (HPT) indicative of low pressure fractional crystallization. In a primitive mantle‐normalized multielement diagram some of the cumulate rocks show pronounced negative anomalies for K and P, indicating anorogenic mafic magmatism in a within‐plate setting. The rocks of the noncumulate unit show a slight negative anomaly for Yb and a Nb–Ta trough, indicating a subduction‐related signature that perhaps is inherited from subducted sedimentary rocks incorporated during crustal contamination of the derived magma (left after crystal cumulation) with country rocks. Various trace element ratios for the cumulate mafic rocks indicate parent EMI/EMII/HIMU sources with a very limited crustal signature. The noncumulate mafic rocks (corresponding to the derived evolved magma) indicate EMI/EMII/HIMU sources with a pronounced crustal contamination. The Sr–Nd isotopic compositions of the Mawpyut samples typically plot in the continental flood basalt field, with an affinity to the EMII source. The isotopic compositions of the noncumulate rocks also clearly indicate crustal contamination. We suggest that partial melting (involving garnet in the residue) of the enriched mantle source EMI/EMII/HIMU could have derived the parental melt; this melt, in turn, underwent assimilation and fractional crystallization to produce the variety of cumulate‐noncumulate lithologies of the Mawpyut complex. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
15.
High pressure basic inclusions from the Kayrunnera kimberlitic diatreme in New South Wales,Australia
A. C. Edwards J. F. Lovering John Ferguson 《Contributions to Mineralogy and Petrology》1979,69(2):185-192
Basic inclusions of two types occur in a kimberlitic diatreme at Kayrunnera in northwestern New South Wales. Type I inclusions comprise assemblages of clinopyroxene+garnet+rutile±plagioclase ±quartz±K-feldspar±scapolite±sphene±apaite. Type II inclusions have assemblages of clinopyroxene +garnet+kyanite+quartz±plagioclase and are lower in Ti, total Fe, and higher in Al and have a higher Mg/Mg+gSFe ratio than the Type I inclusions. Experimental and theoretical data indicate that both inclusion types equilibrated at between 850–900 ° C and 18–23 kb. Due to their low concentrations of incompatible elements, the Type I inclusions are considered to represent a basaltic melt derived from an Fe-rich mantle source rock, and not to be the product of fractionation. The Type II inclusions are believed to represent cumulates which formed from a basaltic magma. The presence of sulphur rich scapolite in the Type I inclusions extends the range of P-T conditions from which this mineral has been reported thus adding further credence to the hypothesis that it may act as a stable repository for S and CO2 in the crust and upper mantle. 相似文献
16.
One mantle xenolith from a basanite host of the Mt. Melbourne Volcanic Field (Ross Sea Rift) is extraordinary in containing
veins filled with leucite, plagioclase, clinopyroxene, nepheline, Mg-ilmenite, apatite, titaniferous mica, and the rare mineral
zirconolite. These veins show extensive reaction with the dunitic or lherzolitic host (olivine+spinel+orthopyroxene+clinopyroxene).
The reaction areas contain skeletal olivine and diopside crystals, plagioclase, phlogopite, aluminous spinel and ilmenite
in a fine grained groundmass of aluminous spinel, clinopyroxene, olivine, plagioclase and interstitial leucite. The vein composition
estimated from modal abundances and microprobe analyses is a mafic leucite-phonolite with high amounts of K, Al, Ti, Zr and
Nb but low volatile contents. The melt is unrelated to the host basanite and was probably derived by smallscale melting of
incompatible element-enriched phlogopite-bearing mantle material and must have lost most of its volatile content during migration,
crystallization and reaction with the host dunite. While the veins are completely undeformed the dunitic host shows slight
deformation. Vein minerals crystallized at high temperatures above 1000°C and pressures below 5 kbar according to the phase
assemblage including leucite, nepheline and K-feldspar. Spinel/olivine geothermometry yielded 800–920°C for the re-equilibration
of the host peridotite. Thus the xenolith must have been at shallow depth prior to and during the late veining event. Mantle
material at shallow depths is consistent with rifting and the regional extreme displacement at the transition from the rifted
Victoria Land Basin in the Ross Sea to the uplifted Trans-Antarctic Mountains. 相似文献
17.
E. Bali G. Falus C. Szabó D. W. Peate K. Hidas K. Török T. Ntaflos 《Mineralogy and Petrology》2007,90(1-2):51-72
Summary We present a detailed textural and compositional study of two orthopyroxene-rich olivine websterites. One occurs as a vein
in a harzburgite xenolith and the other is an individual xenolith, both found at Szentbékkálla in the Bakony–Balaton Highland
Volcanic Field (central Pannonian Basin, western Hungary). The textural features of these orthopyroxene-rich rocks suggest
that they crystallized from silicate melts to form veins in peridotite mantle rock. Their geochemical features, such as the
presence of Al2O3-poor orthopyroxenes, Cr-rich spinels, and clinopyroxenes with U-shaped chondrite-normalized REE-patterns, indicate that the
vein material formed from Mg-rich silicic (boninitic) melts at mantle depths. The olivine fabric investigation of both the
veins and the wall-rock suggest that the development of the veins was followed by subsequent recrystallization during the
Cenozoic evolution of the Carpathian–Pannonian region. 相似文献
18.
Combined Sm–Nd and Lu–Hf age and isotope data indicate that Mg- and Cr-rich ultramafic rocks at Sandvik, Western Gneiss Region
(WGR), Norway, originated from depleted Archean lithospheric mantle that was chemically and physically modified in Middle
Proterozoic time. The Sandvik outcrop consists of garnet peridotite and garnet-olivine pyroxenite and thin garnet pyroxenite
layers. These contain two principal mineral assemblages: an earlier porphyroclastic assemblage of grt + opx + cpx ± ol (1,200–1,000°C,
40–50 kbar) and a later kelyphitic assemblage of grt + spl + am ± opx ± ol (700–750°C; 12–18 kbar). A CHUR Hf model age indicates
a period of melt extraction at ca. 3.3 Ga for garnet peridotite, reflecting extremely high Lu/Hf ratios and very radiogenic
present-day 176Hf/177Hf (εHf=+2,165). Lu–Hf garnet-cpx-whole rock ages of two olivine-bearing samples (garnet peridotite and garnet-olivine pyroxenite)
from the outcrop are ca. 1,255 Ma, whereas two olivine-free garnet pyroxenites yield Lu–Hf ages of ca. 1,185 Ma. All Sm–Nd
garnet-cpx-whole rock ages of these samples are significantly younger (ca. 1,150 Ma for garnet peridotite and ca. 1,120 Ma
for garnet pyroxenite). The isotope systematics indicate that the Lu–Hf ages represent cooling from an earlier period of formation/recrystallization
for garnet peridotite whereas they probably reflect formation/recrystallization ages of the garnet pyroxenite. The Sm–Nd ages
and isotope systematics of the garnet peridotite samples are consistent with an episode of LREE metasomatism, perhaps facilitated
by a fluid of carbonatitic composition that strongly decoupled Sm–Nd and Lu–Hf. The Sm–Nd ages of the garnet pyroxenite may
represent either LREE metasomatism or cooling, and, like the peridotites, Lu–Hf ages are older than Sm–Nd ages. The age data,
as well as the inferred Nd isotope composition of the fluid that affected the olivine-bearing samples, suggest that these
rocks were not in contact during the LREE metasomatic event. Moreover, the pyroxenite layers cannot have been emplaced as
magmas into the host peridotite. The pyroxenite layers are interpreted to be tectonically juxtaposed with the host olivine-bearing
samples sometime after 1,150 Ma but before development of kelyphite. 相似文献
19.
Ian S. Sanders 《Lithos》1978,11(1):15-22
Aluminous clinopyroxenite is described from an exposure in the Eastern Lewisian metamorphic complex at Glenelg in NW Scotland. One specimen (S527) has exsolution lamellae of Ca-rich garnet in an omphacitic host, while another specimen (S526) contains Ca-Tschermakitic augite and green hercynitic spinel with reaction rims of garnet. Garnet exsolution is attributed to cooling to about 600°C at eclogite-stabilizing pressure, and is shown to depend on the Ca + Na content of the pyroxene. Pyroxene stoichiometry is questioned. Originally the pyroxenite was probably a high-pressure cumulate which crystallized and cooled within an eclogite-facies environment. Alternatively, it may have formed during an early phase of very high-grade metamorphism. 相似文献
20.
Petrogenesis of Dunite Xenoliths from Koolau Volcano, Oahu, Hawaii: Implications for Hawaiian Volcanism 总被引：2，自引：0，他引：2
Ultramafic xenoliths from Koolau Volcano on the island of Oahu,Hawaii, are divided into spinel lherzolite, pyroxenite, anddunite suites. On the basis of a study of the petrography andmineral compositions of 43 spinel lherzolites, 12 pyroxenites,and 20 dunites, the following characteristics of the dunitesin relation to the other nodule types and to Hawaiian lavasemerge. (1) The forstente content of olivines in the Koolaudunites (Fo82.6-Fo89 7 ) overlap those of Hawaiian tholeiiticand alkalic lavas and are generally lower than those in abyssallherzolites and dunites and in Koolau spinel lherzolites. (2)Most of the dunites contain no orthopyroxene, all except twocontain chrome spinel, and a few contain interstitial plagioclaseand clinopyroxene. (3) Chrome spinels from the Koolau dunitesare distinctly higher in Cr/(Cr+Al), lower in Mg/(Mg+ Fe2+)and higher in TiO2 than those from abyssal basalts and peridotites.Chrome spinels in the dunites correspond closely in compositionto chrome spinels in Hawaiian tholeiitic and alkalic lavas.(4) The abundance of dunite relative to other nodule types decreasesoutward from the central part of the volcano. The dunites areinterpreted not as residues of partial fusion of the mantlebut as crystal accumulations stored at shallow depths beneaththe central part of Koolau Volcano and derived from picriticmagmas parental to the shield-building tholeiitic lavas. 相似文献