Petrology of mafic-ultramafic rocks along North Puruliya Shear zone,West Bengal

The North Puruliya Shear zone (NPSZ) is characterized by occurrence of mafic-ultramafic rocks aligned parallel to the shear zone, intruding the high grade Proterozoic rocks of Chhotanagpur Gneissic Complex. The ultramafic rocks occur as small lenses, pockets, veins, thin dykes and are intimately associated with mafic (gabbro, norite) rocks. Pyroxenites (viz. olivine websterite, websterite, plagioclase websterite) and hornblendite are the two important members of the ultramafic rocks containing clinopyroxene, orthopyroxene, olivine, plagioclase, amphibole, phlogopite and ilmenite. The mafic-ultramafic rocks show evidence of shearing and retrogressive metamorphism. Linear correlation of chemical attributes suggests fractionation-controlled magmatic differentiation. Enrichment of LILE and LREE in the mafic-ultramafic suite suggests an enriched mantle source and pronounced negative Eu-anomalies in all the rock types except hornblendite suggest fractionation of plagioclase under low f_O2 condition. Progressive iron enrichment trend in rocks of the mafic-ultramafic suite also indicate magmatic differentiation under low f_O2 condition. Early fractionation and accumulation of clinopyroxene and plagioclase from a basaltic magma may have given rise to the ultramafic rocks of the area. Little change in the Nb/Zr and Ce/Zr ratios of ultramafic and mafic rocks (except alkali norite) strongly support low crustal contamination. A few samples of norite and gabbro-norites appeared to be variably contaminated by a crustal component or affected by late granitic intrusion resulting in enrichment of alkali in the former.     

牛鼻子梁镁铁质-超镁铁质杂岩体岩石特征

牛鼻子梁岩体位于柴达木地块的北缘,出露面积约8 km²,平面形态呈长条状,主要由斜长二辉橄榄岩、斜长单辉橄榄岩、角闪二辉橄榄岩、角闪橄榄岩、角闪橄榄二辉岩、黑云母化二辉岩、角闪辉石岩、橄榄辉石角闪石岩、角闪橄榄辉长岩、细粒辉长岩、似斑状辉长岩、暗色辉长岩、辉长岩、淡色辉长岩、石英闪长岩和英云闪长岩组成。文章通过岩石学、矿物学、地球化学研究,得到锆石U-Pb年龄为（361.5±1.2） Ma,Sm-Nd等时线年龄为（347±26） Ma。研究认为,牛鼻子梁基性-超基性岩体含矿岩石产于大陆边缘环境。岩体形成于泥盆纪晚期。岩浆分异充分,岩石类型丰富,岩浆演化过程中主要发生了橄榄石和斜长石的分离结晶/堆晶作用。岩体的母岩浆应属于拉斑玄武岩质岩浆。从目前发现的矿化情况来看,牛鼻子梁基性-超基性杂岩体为含矿岩体,有很好的找矿前景。     

Geocheemical variations in a suite of granitoids and gabbros from Southland,New Zealand

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.     

Origin of ultramafic inclusions and megacrysts in a monchiquite dyke at Streap,inverness-shire,Scotland

Spinel lherzolite nodules, composed of olivine (Fo_88.7?89.2), clinepyroxene (6.5% Al₂O₃) and Al-rich spinel, and websterite nodules as well as megacrysts of clinopyroxene, orthopyroxene and magnetite occur in a monchiquite at Streap, Scotland. Petrographic data are given and microprobe analyses of coexisting phases in six spinel lherzolite nodules and one websterite nodule are reported, along with analyses of both types of pyroxene megacrysts. The spinel lherzolites show internal chemical homogeneity, and their mineral chemistries suggest equilibrium conditions of 1100–1200°C and 14–23 kb. The websterite nodules are, on the basis of mineral chemistry and petrography, considered to be crustal material. The megacrysts constitute a separate group, differing in composition from analogous phases in associated lherzolites and websterites as well as from monchiquite phenocryst phases, and show systematic chemical variations corresponding to low pressure crystal fractionation processes.     

Assimilation of peridotite in zoned calc-alkaline plutonic complexes: evidence from the Big Jim complex,Washington Cascades

The Big Jim complex is a concentrically zoned ultramafic to felsic plutonic complex which intruded the pelitic Chiwaukum schist. Most of the major plutonic rock types (from websterite through hornblendite, gabbronorite, hornblende gabbro and diorite, to granodiorite) enclose harzburgite and metaperidotite xenoliths similar to foliated metaperidotite lenses included in the Chiwaukum schist. The larger xenoliths preserve tectonite fabrics. All have Mg#'s (mole fraction MgO/(MgO+FeO^*)) from 0.90 to 0.89, the same as those of Chiwaukum metaperidotites, and distinctly different from undeformed Big Jim dunite (Mg#'s 0.84 to 0.82) and websterite (0.82 to 0.78). Contact relations indicate widespread, stepwise replacement of harzburgite by pyroxenite, hornblendite, gabbro and diorite. Thermodynamic modelling using an expanded regular solution model for silicate liquids (Ghiorso 1985; Ghiorso and Carmichael 1985) predicts that reaction between olivine (Fo90) and a liquid with the composition of Big Jim diorite +1.5 wt% H₂O, at 1,100° C and 3 kb, would produce websterite (Mg#'s 0.75 to 0.81) and dunite (0.79 to 0.82). This process is exothermic and results in a negative change in volume, since it increases total solid mass. Under conditions of decreasing temperature, modelled crystal fractionation with assimilation of olivine reproduces important features of the chemical variation observed in the Big Jim complex where crystal fractionation alone fails. The Big Jim complex has affinities with other ultramafic to felsic plutonic complexes such as the Bear Mountain complex (Snoke et al. 1981, 1982) and the Emigrant Gap complex (James 1971). The latter have wehrlite and clinopyroxenite, rather than websterite, but both have concentric zoning, with olivine-bearing rock types surrounded by successively more felsic pyroxenite, gabbro and diorite. In general, concentrically zoned complexes of this type may form where magma reacts with mantle-derived wall rock or ultramafic cumulates. Assimilation of peridotite in fractionating magma may be important in subduction-related magmatic arcs.     

Geochemistry of Proterozoic volcanic and granitic rocks from the Gold Hill-Wheeler Peak area,northern New Mexico

Early Proterozoic supracrustal and plutonic rocks from the Gold Hill-Wheeler Peak area in northern New Mexico define three populations: amphibolite—diorite—tonalite, hornblendite—cumulus amphibolite and felsic volcanics and porphyries. Also present are mid-Proterozoic granites. Amphibolites are similar in Ti, Zr, Cr, Ni and REE contents to young calc-alkaline and arc basalts and diorites and tonalites are similar in composition to young andesites and to high-Al₂O₃ tonalites, respectively. Felsic volcanics resemble young felsic volcanics from mature arc systems in their immobile-element contents. Geochemical model studies suggest that the amphibolites, hornblendites, diorites and tonalites are related by progressive fractional crystallization of a hydrous parent tholeiite magma produced from partial melting of undepleted lherzolite. Amphibolites represent parent tholeiites modified by olivine removal. Hornblendite is an early solid residue comprised chiefly of hornblende, clinopyroxene, and olivine; diorite and cumulus amphibolite represent respectively residual solid (clinopyroxene, plagioclase, hornblende) and liquid, after 50% crystallization. Tonalite represents a residual liquid after 80% crystallization. Felsic volcanic rocks are produced by partial melting of a tonalite or diorite source with granulite-facies mineralogy in the lower crust. Granites have a similar origin to felsic volcanics although requiring an inhomogeneous source with the presence of residual hornblende or garnet.The calc-alkaline igneous rocks in the Gold Hill-Wheeler Peak area suggest the presence of an arc system in northern New Mexico during the Early Proterozoic. The fact that these rocks interfinger with and are overlain by mature clastic sediments favors a model in which a continental arc system is uplited, eroded and buried by cratonic sediments from the north.     

Geochemistry and magmatic setting of Wadi El-Markh island-arc gabbro–diorite suite,central Eastern Desert,Egypt

Wadi El-Markh gabbro–diorite complex is composed of pyroxene hornblende gabbros, hornblende gabbros, diorites and quartz diorites. According to their bulk rock geochemistry and mineral chemistry, the gabbroic and dioritic rocks represent fractionates along a single line of descent and crystallized from a calc-alkaline mafic magma. When compared to the primitive mantle, all members of the gabbroic–dioritic rock suite are enriched in the large ion lithophile elements relative to the high field strength elements and display distinctive negative Nb and P₂O₅ anomalies. This signals an arc setting. Fractionation modeling involving the major elements reveals that the hornblende gabbros were generated from the parent pyroxene hornblende gabbros by 61.86% fractional crystallization. The diorites were produced from the hornblende gabbros by fractional crystallization with a 58.97% residual liquid, whereas the quartz diorites were formed from the diorites by 26.58% fractional crystallization. According to geothermobarometry based on amphibole mineral chemistry, the most primitive pyroxene hornblende gabbros crystallized at ～830 °C/～5 kbar. The crystallization conditions of the quartz diorites were estimated at ～570 °C/～2 kbar. In consequence the Wadi El-Markh gabbro–diorite complex represents a single magmatic suite of which fractionates crystallized in progressively shallower levels of an arc crust.     

The field and microstructural signatures of deformation‐assisted melt transfer: Insights from magmatic arc lower crust,New Zealand

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.     

Petrology of mafic inclusions from Itinome-gata,Japan

Various types of mafic inclusions up to 30 cm in size occur in lapilli tuff of alkali basalt at Itinome-gata crater, northeastern Japan. They are divided into the following four groups: amphibolite, hornblendite—hornblende gabbro, leucogabbro, and pyroxene gabbro. Also occurring with the mafic inclusions are lherzolite and websterite inclusions and megacrysts of Mg-rich olivine and chromian diopside. New analyses are presented for twenty five representative mafic inclusions, eight clinopyroxenes, six orthopyroxenes, and fifteen brown hornblendes. There are conspicuous chemical differences between the mafic inclusions and lherzolite and websterite inclusions: the former have higher TiO₂, Al₂O₃, total FeO, CaO, Na₂O, and K₂O, and lower MgO than the lherzolites and higher TiO₂, Al₂O₃, total FeO, and alkalis, and lower MgO than the websterites. The petrographic and chemical gradations among these three are not easily recognized. It is indicated that the Moho in this region is a boundary between mafic and ultramafic phases. The mineral assemblages of the mafic inclusions and the compositions of their essential minerals show that all of them recrystallized or crystallized under approximately the same temperature — pressure conditions, within the range of 600–1000° C and 6–9 kb. The following is hypothetically considered. The old and thick tholeiites or high-alumina basalts (may be pre-Silurian) making up the basement of the Japanese Islands had been subjected to the high T/P type metamorphism during Cretaceous time, and changed to amphibolites. In the cataclastic stage, complete melting of the lower part of the amphibolites occured locally and formed a gabbro magma. This gabbro magma moved upward slightly and produced hornblendite, hornblende gabbro and leucogabbro magmas by differentiation under wet conditions and a pyroxene gabbro magma under less wet conditions. Namely, the mafic inclusions are thought to be of fragments of the lower crust.     

Aluminous websterite and granulite xenoliths from the Chyulu Hills volcanic field, Kenya: gabbro–troctolitic cumulates subjected to lithospheric foundering

Whole rock major and trace element abundances in aluminous garnet–spinel websterite, sapphirine-bearing Mg–Al granulite and hibonite-bearing Ca–Al granulite xenoliths from the Chyulu Hills volcanic field, Kenya, suggest that the samples represent a meta-igneous suite linked by fractionation. The incompatible major element contents increase from the websterites to the Mg–Al granulites and further to the Ca–Al granulites. High bulk rock Mg#s and very low concentrations of most incompatible trace elements indicate that the rocks are cumulates rather than crystallized melts. Elevated Ni abundances, impoverishment in Cr and HFSE and high contents of normative plagioclase and olivine in the granulites indicate that their protoliths were similar to troctolite. The textures and metamorphic reaction paths recorded in the granulites suggest igneous emplacement in the crust and cooling from igneous to ambient crustal temperatures accompanied or followed by compression. For the websterite xenoliths, there is an apparent contradiction between the results of P–T calculations that suggest high P and T of crystallization of early generation pyroxenes and elevated P–T conditions during final equilibration (1.4–2.2 GPa/740–980°C) on the one hand and the positive Eu anomaly that suggests shallow-level plagioclase accumulation on the other hand. This contradiction can be reconciled by a model of compression of a plagioclase-bearing (gabbroic) protolith to mantle depths where it recrystallized to an ultramafic assemblage, which requires foundering of dense lower crustal material into the mantle.     

Petrogenesis of Cretaceous Peraluminous Granite Suites with Low Initial Sr Isotopic Ratios, Kyushu Island, Southwest Japan Arc

Cretaceous granitic rocks are widely distributed in the northern to central parts of Kyushu Island, southwest Japan arc, and are divided into two types, granite and tonalite-granodiorite. These rocks are clearly defined by their mineral assemblage, that is, the granite contains mostly muscovite and/or garnet and does not have hornblende, whereas the tonalite-granodiorite is always observed with hornblende. Many workers suggest that most of the granite has low initial Sr isotopic ratios (SrI_{90 Ma}: 0.7052-0.7059) whose values are similar to those of tonalite-granodiorite (SrI_{90 Ma}: 0.7050-0.7056).

The SiO₂ contents of the granite (67.9 to 78.0 wt.%) are higher than that of tonalite-granodiorite (54.2 to 68.2 wt.%). The granite is characterized by peraluminous compositions, while the tonalite-granodiorite shows metaluminous to peraluminous compositions. The petrogenetic relation between granite and tonalite-granodiorite is examined by a modal batch melting model, which referred to results of already published experiments. The result of the model suggests that the petrogenesis of the peraluminous granitic magma can be explained by an anatexis of metaluminous tonalite in the tonalite-granodiorite under lower to middle crustal conditions and variable fH₂O.

Rocks of tonalite to granodiorite in a subduction system generally have low initial Sr isotopic ratios (<0.705), which imply that the magma derived from a highly metaluminous origin such as mafic magma, mafic lower crust and/or subducting slab. In this study, it is regarded that the peraluminous granitic magma can be directly produced by an anatexis of metaluminous tonalite. Therefore, peraluminous granite with low initial Sr isotopic ratios may imply to a first step of recycling of the granitic layer in an active plate margin.     

Geology and geochemistry of the Mendejin plutonicrocks, Mianeh, Iran

The Mendejin pluton is located in the Mianeh area, NW Iran, 550 km from Tehran. This pluton is probably of Oligo-Miocene age and is the result of extensive magmatism which occurred during and after the Alpine Orogeny. Similar plutons are common in the Alborz–Azarbaijan structural zone of Iran, and it is likely that there are concealed plutons related to this extensive Cenozoic magmatism, but due to their youth and low rates of erosion they have not yet been exposed. The Mendejin pluton is a composite body made up of four types of plutonic rocks: pink tonalite, grey tonalite, diorite and aplite. The pink tonalite is porphyritic and contains phenocrysts of plagioclase, K-feldspar and hornblende in a groundmass consisting of quartz, plagioclase, K-feldspar, hornblende, zircon, monazite, leucoxene, apatite and hematite. The grey porphyritic tonalite has more biotite, pyroxene and pyrite and less accessory phases compared with the pink tonalite. The diorite has a microporphyritic texture with phenocrysts of plagioclase, hornblende and augite. This rock also occurs as xenoliths in the Mendejin pluton. The aplitic dykes are the youngest magmatic products at Mendejin. The Mendejin tonalite contains more Cl, As, S, Cu, Ni and Zn than the global granite. These rocks are of I-type, peraluminous and calc-alkaline, with medium to high potassium, and were formed as part of a volcanic arc. The Mendejin pluton contains up to 8 ppb gold and could potentially have been the source of an economic gold deposit by leaching of Au from wall rocks and deposition in extensive hydrothermally altered marginal zones.     

Incomplete high P–T metamorphic transitions within the Kvamsøy pyroxenite complex, west Norway: a case study of disequilibrium

Abstract The Kvamsøy pyroxenite complex consists of olivine websterite, olivine gabbro and leucogabbro-norite which have been subjected to regional high P-T (HPT) metamorphism. The metamorphism has resulted in a range of disequilibrium textures with the development of coronas and pseudomorphism of the igneous phases. Reactions between felsic and mafic mineral domains have been controlled by variable and selective diffusion of elements, resulting in a variety of local plagioclase-breakdown reactions and in significant compositional variations for the product garnet. Restricted diffusion favours transient reaction stages with garnet ± spinel ± corundum ± zoisite after calcic plagioclase in olivine gabbro and olivine websterite and garnet ± spinel ± kyanite ± quartz + sodic plagioclase in leucogabbro-norite. Complete HPT reaction has produced garnet pyroxenite which consists of garnet + diopside + hornblende + zoisite in gabbroic rocks, while amphibolitization continued during the cooling and uplift history. Grt + Ky + Pl + Qtz geobarometry suggests pressures in the range 13-16 kbar for T = 750°C, comparable with the regional eclogite-forming metamorphism.     

Mineral chemistry constraints on the evolution of the 1.88–1.87 Ga post-kinematic granite plutons in the Central Finland Granitoid Complex

A suite of post-kinematic, 1.88–1.87 Ga, silicic plutons crosscut 1.89–1.88 Ga synkinematic granitoids in the Central Finland Granitoid Complex (CFGC) in south-central Finland. The plutons range from biotite±hornblende quartz monzonite to syenogranite and include pyroxene- and olivine-bearing varieties. Mineral chemical data on feldspars, biotite, amphibole, pyroxenes, olivine, and oxides of the post-kinematic plutons are presented. The data are interpreted to show that these plutons register (1) a considerable range in pressure from 2–4 kbar (amphibole barometry) to 5–7 kbar (olivine–pyroxene barometry), (2) temperatures mostly reflecting resetting during cooling (450–800°C; QUIlF thermometry), and (3) low fO₂ (log fO₂ ΔFMQ −0.3 to −1.5; QUIlF equilibria). In particular, plutons with olivine- and pyroxene-bearing margins and amphibole-dominated central parts record progressive oxidation and hydration upon cooling, shifting from the QUIlF equilibrium toward KUIlB. The post-kinematic granites can be considered post-collisional in regard to compressional events in the CFGC and display many of the characteristics of the anorogenic 1.6 Ga rapakivi granites further south. They were presumably derived from a deep and dry crustal source, like the rapakivi granites.     

Geology of the mineralized zone of the Wanapitei complex,Grenville Front,Ontario

The Wanapitei Complex (6 km×2.5 km), lying 0.4 km southeast of the Grenville Front, consists of a northwestern zone of gabbro and folded injection breccia and a southeastern layer of intensely folded hornblendeplagioclase gneiss. Disseminated Ni-Cu sulphides are unevenly distributed in a zone between the injection breccia and the folded gneiss.Rocks of the mineralized zone occur in southeastern and northeastern areas. The former area consists of hornblende norite, the major host rock of the sulphides, and olivine norite. Steeply-dipping cross-bedded primary layers and chemical trends indicate the top faces southeast. In the latter area olivine norite, hornblende norite, and hornblende gabbro grade eastward into recrystallized rocks and breccia. The olivine norites are characterized by corona reaction rims. Reactions are: olivine+plagioclase bronzite+diopside-spinel; olivine+pyroxene bronzite; and pyroxene+plagioclase diopside-spinel. Molecular proportion ratio variation diagrams suggest that rocks evolved from a common parent magma that underwent fractionation dominated by olivine and plagioclase. Sulphide mineralization (pyrrhotite, chalcopyrite, pentlandite, pyrite) is interstitial to the silicates and appears to be of primary magmatic origin.Northeasterly-trending shear zones, felsic dikes, and matic dikes are metamorphosed to the same degree as the rocks they cut (amphibolite facies). The sequence of events for the mineralized zone are: intrusion deep in the crust; tilting; brecciation; shearing; felsic and mafic dike emplacement; metamorphism; and injection of granite pegmatite dikes.Deceased (8-16-1986)     

阿拉斯加型岩体的基本特征、成岩过程及成矿作用

阿拉斯加型岩体是一类具有独特的岩性环带状结构的镁铁-超镁铁质侵入体,常呈链状分布于汇聚板块边缘。其形成时代跨度较大,从元古代到新生代均有分布,以中生代最为发育。大部分阿拉斯加型岩体规模较小,出露面积约12～14 km²或更小,平面上呈近似同心环状结构,垂直剖面上呈管道状。岩体中心为纯橄岩,向外依次包括异剥橄榄岩、橄榄单斜辉石岩、单斜辉石岩、角闪单斜辉石岩、角闪石岩和辉长岩。造岩矿物为橄榄石、单斜辉石、角闪石等,副矿物为铬铁矿、磁铁矿、钛铁矿等,超镁铁质岩石中少或无斜方辉石,斜长石仅出现在边缘的辉长质岩石中。磁铁矿在单斜辉石岩和角闪石岩中为常见矿物,含量最高达15%～20%。阿拉斯加型岩体的主量元素成分揭示所有岩石均为与拉斑玄武质岩浆分异有关的亚碱性堆晶岩。微量元素成分上显示平坦的稀土元素配分型式和较低的微量元素含量,且富集大离子亲石元素,亏损高场强元素。矿物化学特征上,橄榄石富镁且Fo值变化较大;单斜辉石主要为富Ca的透辉石,其成分变化具有弧堆晶趋势;角闪石主要是镁角闪石和韭角闪石;铬铁矿富集Fe-Al,贫Cr。这些特征揭示,该类岩体成因明显不同于层状岩体和阿尔卑...     

Petrology and mineralogy of dredged gabbro from Gettysburg Bank,eastern Atlantic

Gabbros have been dredged from Gettysburg Bank, 110 km west of Portugal on the Azores/Gibraltar fracture zone. Primary minerals in olivine, pyroxene and brown hornblende gabbros are partially replaced by metamorphic minerals. Igneous textures are inhomogeneously overprinted by a granular polyhedral deformation and a cataclastic deformation. Amphiboles show characteristics which indicate a transition from crystallisation in a magma chamber to formation of amphibole in solid gabbro under metasomatic conditions. Of the amphiboles analysed, chlorine was present in the green amphiboles but below 0.05% in the brown suggesting the penetration of sea water after the formation of the brown amphibole but during the formation of the green.     

The role of amphibole in the evolution of arc magmas and crust: the case from the Jurassic Bonanza arc section, Vancouver Island, Canada

The Jurassic Bonanza arc, on Vancouver Island, British Columbia, represents an exhumed island arc crustal section of broadly diorite composition. We studied bodies of mafic and ultramafic cumulates within deeper levels of the arc to constrain the conditions and fractionation pathways leading from high-Mg basalt to andesite and dacite. Major element trends coupled with textural information show the intercumulus crystallization of amphibole, as large oikocrysts enclosing olivine in primitive cumulates controls the compositions of liquids until the onset of plagioclase crystallization. This process is cryptic, occurring only in the plutonic section, and explains the paucity of amphibole in mafic arc volcanics and the change in the Dy/Yb ratios in many arc suites with differentiation. The correlation of octahedral Al in hornblende with pressure in liquidus experiments on high-Mg basalts is applied as an empirical barometer to hornblendes from the Bonanza arc. It shows that crystallization took place at 470–880 MPa in H₂O-saturated primitive basaltic magmas. There are no magmatic equivalents to bulk continental crust in the Bonanza arc; no amount of delamination of ultramafic cumulates will shift the bulk arc composition to the high-Mg# andesite composition of bulk continental crust. Garnet removal from wet magmas appears to be the key factor in producing continental crust, requiring high pressures and thick crust. Because oceanic island arcs are built on thinner crust, the long-term process generating the bulk continental crust is the accretion of island arcs to continental margins with attendant tectonic thickening.     

中天山白石泉镁铁-超镁铁质岩体岩石学与矿物学研究

白石泉地区镁铁一超镁铁质岩体处于塔里木板块前缘活动带与中天山地块接合部位，是中天山地块华力西中期岩浆活动的产物。主要岩石类型有辉石橄榄岩（斜方辉石橄榄岩、斜长二辉橄榄岩）、橄榄辉石岩、橄长岩、辉长岩及角闪辉长岩等，主要造岩矿物为橄榄石、斜方辉石、单斜辉石、角闪石、斜长石及黑云母。橄榄石均为贵橄榄石，其Fo值（78-85）位于含铜镍硫化物矿橄榄石的Fo值范围之内；辉石主要有顽火辉石、古铜辉石、紫苏辉石、透辉石等；斜长石的环带构造较为发育；角闪石的FeO含量随着岩浆的演化逐渐增加。它们与造山带环境中的东疆型镁铁一超镁铁杂岩中的造岩矿物具有相同的特征。这些特征表明了白石泉地区的镁铁一超镁铁质岩体的原始岩浆为高镁的拉斑玄武质岩浆。     

Geochemistry of an Alaskan-type mafic-ultramafic complex in Eastern Desert,Egypt:New insights and constraints on the Neoproterozoic island arc magmatism

Mikbi intrusion(MI) is a part of the Neoproterozoic Nubian Shield located along the NE-SW trending major fracture zones prevailing southern Eastern Desert of Egypt. In this study, we present for the first time detailed mineralogical and bulk-rock geochemical data to infer some constraints on the parental magma genesis and to understand the tectonic processes contributed to MI formation. Lithologically, it is composed of fresh peridotite, clinopyroxenite, hornblendite, anorthosite, gabbronorite, pyroxene amphibole gabbro, amphibole gabbro and diorite. All rocks have low Th/La ratios(mostly <0.2) and lack positive Zr and Th anomalies excluding significant crustal contamination. They show very low concentrations of Nb, Ta, Zr and Hf together with sub-chondritic ratios of Nb/Ta(2-15) and Zr/Hf(19-35),suggesting that their mantle source was depleted by earlier melting extraction event. The oxygen fugacity(logfO_2) estimated from diorite biotite is around the nickel-nickel oxide buffer(NNO) indicating crystallization from a relatively oxidized magma. Amphiboles in the studied mafic-ultramafic rocks indicate relative oxygen fugacity(i.e. ΔNNO; nickel-nickel oxide) of 0.28-3 and were in equilibrium mostly with 3.77-8.24 wt.% H_2 O_melt(i.e. water content in the melt), consistent with the typical values of subduction-related magmas. Moreover, pressure estimates(0.53-6.79 kbar) indicate polybaric crystallization and suggest that the magma chamber(s) was located at relatively shallow crustal levels. The enrichment in LILE(e.g., Cs, Ba, K and Sr) and the depletion in HFSE(e.g., Th and Nb) relative to primitive mantle are consistent with island arc signature. The olivine, pyroxene and amphibole compositions also reflect arc affinity. These inferences suggest that their primary magma was derived from partial melting of a mantle source that formerly metasomatized in a subduction zone setting. Clinopyroxene and bulkrock data are consistent with orogenic tholeiitic affinity. Consequently, the mineral and bulk-rock chemistry strongly indicate crystallization from hydrous tholeiitic magma. Moreover, their trace element patterns are subparallel indicating that the various rock types possibly result from differentiation of the same primary magma. These petrological, mineralogical and geochemical characteristics show that the MI is a typical Alaskan-type complex.