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1.
Gravity and magnetic anomalies have previously been interpreted to indicate strongly magnetic Permian or even Tertiary intrusive bodies beneath the Skagerrak waterway (such as the ‘Skagerrak volcano’) and beneath Silkeborg in Denmark. Our combined modelling of the magnetic and gravity anomalies over these rock bodies indicates that a steep upward magnetisation is required to explain the magnetic anomalies at the surface, reminiscent of the magnetic direction in the Sveconorwegian rocks of the Rogaland Igneous Province in southern Norway. The younger rocks of the Permian Oslo Rift region have intermediate and flat magnetisation that is inadequate to explain the observed magnetic field. The positive part of the Skagerrak aeromagnetic anomaly is continuous with the induced anomalies associated with the eastward extension of the Rogaland Igneous Province. This relation also suggests that rocks of the Rogaland Igneous Province and its offshore extension are responsible for the Skagerrak anomalies. Both the negative, remanence-dominated aeromagnetic anomaly and the positive gravity anomaly can be modelled using constraints from seismic reflection lines and available density data and rock-magnetic properties. A 7 km thick complex of ultramafic/mafic intrusions is located below a southward dipping 1–4 km thick section of Mesozoic sediments and 1–2 km of Palaeozoic sediments. The enormous body of dense, ultramafic/mafic rocks implied by the modelling could be the residue of the parental magma that produced the voluminous Rogaland anorthosites. The application of similar petrophysical properties in the forward modelling of the Silkeborg source body provides an improved explanation of the observed gravity and magnetic anomalies compared with earlier studies. The new model is constrained by magnetic depth estimates (from the Located Euler method) ranging between 6 and 8 km. Forward modelling shows that a model with a reverse magnetic body (anorthosite?) situated above a dense, mafic/ultramafic body may account for the Silkeborg anomalies. The anorthosites may have formed by differentiation of the underlying mafic intrusion, similar to the intrusive relations in the Rogaland Igneous Province. We conclude that there is strong evidence for a Sveconorwegian age for both the Skagerrak and the Silkeborg anomalous rock bodies.  相似文献   

2.
Two contrasting intrusions in Greenland are described which both have extreme compositions and mineralogy. Ilímaussaq is part of the Mezoproterozoic Gardar Province of southern West Greenland. It developed by extreme fractionation in the crust of non‐remarkable weakly alkaline basic magmas leading to extraordinarily high levels of many rare elements. Indeed, in this intrusion the zirconium mineral, eudialyte, attains rock‐forming status. Gardiner in the East Greenland sector of the Palaeogene North Atlantic Igneous Province, was formed from nephelinitic magmas formed at great depth in the mantle by very low degrees of partial melting. Here, the magmas were so silica‐poor that there are large amounts of rocks composed of the melilite group of minerals, generally with perovskite (CaTiO4) as a major phase. Thus, extreme compositions at Ilímaussaq were caused by fractionation to very small amounts of melt while at Gardiner it was caused by very small degrees of mantle melting. Both intrusions are known for fine specimens of rare minerals; in the case of Ilímaussaq over 200 have been described. Rocks such as those found here require their own names as they cannot be accommodated in the usual petrological nomenclature.  相似文献   

3.
Igneous rocks, silicate minerals and native sulphur have been analysed for their mercury content by a microwave-excited argon plasma. Basic and ultrabasic igneous rocks have abundances between 3 and 31 ppb Hg. The granitic rocks show much larger variations. The content in the silicate minerals is highly variable, ranging from less than 1 ppb Hg in an igneous biotite to 55 ppb Hg in a pegmatitic biotite. The analysed clay minerals give 110 and 200 ppb Hg. Volcanic elemental S contain from 11 to 9300 ppb Hg, the values on the lower side being from oceanic volcanoes and on the higher side for continental or marginal volcanoes.  相似文献   

4.
The contents of the platinum-group elements (PGEs: Os, Ir, Ru, Rh, Pt, Pd) in the Abulangdang ultramafic intrusion have been determined using ICP-MS after nickel sulfide fire assay preconcentration. Different samples show significant differences in absolute PGE abundance. They display a pronounced negative incline in mantle-normalized patterns which are characterized by strong enrichment in IPGEs (Os, Ir, Ru) and depleting to slight enrichment in PPGEs (Rh, Pt, Pd). The characteristics of PGE distribution in the Abulangdang rocks are due to the combined action of sulfide and non-sulfide (spinel/chromite or alloy or micro-granular aggregation of metals). In comparison with the mafic-ultramafic rocks which host Ni-Cu-PGE deposits in the Emeishan Large Igneous Province (ELIP), it is assumed that the Abulangdang ultramafic intrusion may be the product of early-stage magma activity in the ELIP.  相似文献   

5.
The Canadian Arctic Islands expose a complex network of dykes and sills that belong to the High Arctic Large Igneous Province (HALIP), which intruded volatile‐rich sedimentary rocks of the Sverdrup Basin (shale, limestone, sandstone and evaporite) some 130 to 120 million years ago. There is thus great potential in studying the HALIP to learn how volatile‐rich sedimentary rocks respond to magmatic heating events during LIP emplacement. The HALIP remains, however, one of the least well known LIPs on the planet due to its remote location, short field season, and harsh climate. A Canadian–Swedish team of geologists set out in summer 2015 to further explore HALIP sills and their sedimentary host rocks, including the sampling of igneous and meta‐sedimentary rocks for subsequent geochemical analysis, and high pressure‐temperature petrological experiments to help define the actual processes and time‐scales of magma–sediment interaction. The research results will advance our understanding of how climate‐active volatiles such as CO2, SO2 and CH4 are mobilised during the magma–sediment interaction related to LIP events, a process which is hypothesised to have drastically affected Earth's carbon and sulphur cycles. In addition, assimilation of sulphate evaporites, for example, is anticipated to trigger sulphide immiscibility in the magma bodies and in so doing could promote the formation of Ni‐PGE ore bodies. Here we document the joys and challenges of ‘frontier arctic fieldwork’ and discuss some of our initial observations from the High Arctic Large Igneous Province.  相似文献   

6.
《International Geology Review》2012,54(18):2249-2275
ABSTRACT

The Piqiang intrusion is one of the two important ma?c-ultrama?c layered intrusions that host giant Fe-Ti-V oxide deposits in the Permian Tarim Large Igneous Province, NW China. The intrusion mainly consists of gabbro, anorthosite and minor plagioclase-bearing clinopyroxenite in the marginal zone. Disseminated to massive Fe-Ti oxide ores occur as layers and lenses within the gabbro. SHRIMP zircon U-Pb results from both a gabbro from the Piqiang intrusion and a granite from the surrounding granitic dyke yield ages of ~270 Ma. Geochemically, the Piqiang silicate rocks are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), moderately depleted in high ?eld strength elements (HFSE), and have a limited range of Sr-Nd-Hf isotopic compositions. The similar mineralogy, mineral compositions, and trace element characteristics of the layered units suggest that all the rocks are co-magmatic. The parental magma is Fe-Ti-rich and is akin to the most primitive diabasic dyke which is associated with the Piqiang intrusion. Partial melting of the Tarim mantle plume with involvement of a subduction-metasomatized lithospheric mantle source best explains the geochemistry and petrogenesis of the parental magmas of the Piqiang intrusion. We propose that the lithospheric mantle source may have been metasomatized by subduction-related materials and the metasomatic enrichment of this source region which may be correlated with oceanic sediment recycling during southward subduction of the South Tianshan oceanic slab during the Early-Middle Paleozoic. Crystal settling and mechanical sorting is the predominant process responsible for the formation of the massive Fe-Ti oxide ores in the Piqiang intrusion. Central to ore formation is a combination of the protracted differentiation history of a Fe-Ti-enriched parental magma and the later addition of external H2O from the country rocks to the slowly cooling magma chamber.  相似文献   

7.
Alkali-bearing Ti oxides were identified in mantle xenoliths enclosed in kimberlite-like rocks from Limeira 1 alkaline intrusion from the Alto Paranaíba Igneous Province, southeastern Brazil. The metasomatic mineral assemblages include mathiasite-loveringite and priderite associated with clinopyroxene, phlogopite, ilmenite and rutile. Mathiasite-loveringite (55–60 wt.% TiO2; 5.2–6.7 wt.% ZrO2) occurs in peridotite xenoliths rimming chromite (～50 wt.% Cr2O3) and subordinate ilmenite (12–13.4 wt.% MgO) in double reaction rim coronas. Priderite (Ba/(K+Ba)< 0.05) occurs in phlogopite-rich xenoliths as lamellae within Mg-ilmenite (8.4–9.8 wt.% MgO) or as intergrowths in rutile crystals that may be included in sagenitic phlogopite. Mathiasite-loveringite was formed by reaction of peridotite primary minerals with alkaline melts. The priderite was formed by reaction of peridotite minerals with ultrapotassic melts. Disequilibrium textures and chemical zoning of associated minerals suggest that the metasomatic reactions responsible for the formation of the alkali-bearing Ti oxides took place shortly prior the entrainment of the xenoliths in the host magma, and is not connected to old (Proterozoic) mantle enrichment events.  相似文献   

8.
The Qingkuangshan Ni-Cu-PGE deposit, located in the Xiaoguanhe region of Huili County, Sichuan Province, is one of several Ni-Cu-PGE deposits in the Emeishan Large Igneous Province (ELIP). The ore-bearing intrusion is a mafic-ultramafic body. This paper reports major elements, trace elements and platinum-group elements in different types of rocks and sulfide-mineralized samples in the intrusion. These data are used to evaluate the source mantle characteristics, the degree of mantle partial melting, the composition of parental magma and the ore-forming processes. The results show that Qingkuangshan intrusion is part of the ELIP. The rocks have trace element ratios similar to the coeval Emeishan basalts. The primitive mantle-normalized patterns of Ni-Cu-PGE have positive slopes, and the ratios of Pd/Ir are lower than 22. The PGE compositions of sulfide ores and associated rocks are characterized by Ru depletion. The PGE contents in bulk sulfides are slightly depleted relative to Ni and Cu, which is similar to the Yangliuping Ni-Cu-PGE deposit. The composition of the parental magma for the intrusion is estimated to contain about 14.65 wt% MgO, 48.66 wt% SiO2 and 15.48 wt% FeOt, and the degree of mantle partial melting is estimated to be about 20%. In comparison with other typical Ni-Cu-PGE deposits in the ELIP, the Qingkuangshan Ni-Cu-PGE deposit has lower PGE contents than the Jinbaoshan PGE deposit, but has higher PGE contents than the Limahe and Baimazhai Ni-Cu deposit, and has similar PGE contents to the Yangliuping Ni-Cu-PGE deposit. The moderate PGE depletions in the bulk sulfide of the Qingkuanghan deposit suggest that the parental magma of the host intrusion may have undergone minor sulfide segregation at depth. The mixing calculations suggests that an average of 10% crustal contamination in the magma, which may have been the main cause of sulfide saturation in the magma. We propose that sulfide segregation from a moderately PGE depleted magma took place prior to magma emplacement at Qingkuangshan, that small amounts of immiscible sulfide droplets and olivine and chromite crystals were suspended in the ascending magma, and that the suspended materials settled down when the magma passed trough the Qingkuangshan conduit. The Qingkuangshan sulfide-bearing intrusion is interpreted to a feeder of Emeishan flood basalts in the region.  相似文献   

9.
耿马一带古近纪花岗岩类岩浆成因及演化   总被引：1，自引：0，他引：1  
从地质学、岩石学、岩石地球化学等方面，探讨耿马一带古近纪岩浆岩类成因类型和演化、形成的构造背景及其地球动力学。侵入于红色建造K1n及河湖相Em中，比三叠纪花岗岩更偏基性，地球化学特征显示同时具同碰撞及火山弧花岗岩性质。下地壳的拆沉作用，地壳加厚及陆内造山驱动力的影响，富铁超镁铁质堆晶体拆沉于地幔中。堆晶体发生部分熔融，拆沉物熔融岩浆与地壳熔融岩浆于地壳岩浆房产生完全混合作用，形成古近纪具同碰撞及弧火山属性的岩浆。这种混合岩浆上升运移，产生较强的分离结晶作用，多次脉动定位形成古近纪不同岩性单元花岗岩。结合同位素测年结果及其它地质事件，确定其于古近纪侵入于陆内，是陆内造山晚期导致拆沉作用的产物。  相似文献   

10.
马言胜  陶琰  朱丹  郝义 《地球化学》2012,(4):359-370
云南朱布镁铁-超镁铁岩体赋存中型铜镍铂族元素矿床,侵位于元谋群片岩和花岗片麻岩中,岩体垂直分异明显,自下而上为橄榄岩、橄辉岩、辉石岩、辉长岩等相带.矿体以底部“边缘矿”为主,上部有呈透镜状产出的少量“上悬矿”.本文报道了朱布岩体主元素、微量元素、铂族元素(PGE)和 Sr-Nd 同位素组成新的测试结果.分析表明朱布岩体具有拉斑玄武质岩浆分异演化趋势,富集 LREE 的分布模式,弱的Nb 异常和较明显的 Sr 负异常,并与峨眉山大火成岩省(ELIP)苦橄岩相类似,暗示两者可能存在成因上的联系.朱布岩体的铂族元素相对分布模式为“Pt-Pd”富集型,原始地幔标准化曲线向左陡倾.较高的(87Sr/86Sr)i (0.7096~0.7107)和较低的εNd(t)(-3.1~-2.3),表明朱布岩浆受到了地壳物质不同程度的混染.通过岩浆演化过程反演,得出其母岩浆性质为苦橄质,并估算地壳混染程度在3%~20%之间,发现在 R (岩浆与熔离硫化物的比例)值为1000~5000时比较吻合朱布样品中硫化物的实际测定值,证实了朱布岩体可能为开放系统的岩浆房,经过多级富集过程,先熔出的硫化物从后续多期次岩浆中吸收了大量 PGE,岩浆房中同时存在堆晶和岩浆演化,分别形成了底层橄榄岩和上部的辉长岩,中间过渡为橄辉岩和辉石岩  相似文献   

11.
 Plagioclase recrystallization microstructures and petrofabrics in the unmetamorphosed, 1.43 Ga Poe Mountain anorthosite, Wyoming, are indicative of very high-temperature deformation and recrystallization during the emplacement of the anorthosite body. The Poe Mountain anorthosite consists of a core of recrystallized, massive anorthosite transitional with a series of layered anorthositic cumulates at the margin of the intrusion. Irregular grain boundaries and dissected grain microstructures in the massive core and transitional anorthosites suggest that the anorthositic rocks recrystallized by “fast” grain boundary migration and possibly subgrain rotation recrystallization, at very high temperatures (≈1050°C) during emplacement of the intrusion in the mid-crust (3 kbar). The deformation and recrystallization of the Poe Mountain anorthosite was continuous from subliquidus to subsolidus temperature conditions during the emplacement of the intrusion. Anorthosites with the lowest modal percentages of ferromagnesian minerals and Fe-Ti oxides are always the most recrystallized. This suggests that melt interstitial to the plagioclase-crystal framework was removed during deformation and recrystallization of the intrusion. Bulging of plagioclase grain boundaries around Fe-Ti oxides together with deformed oikocrystic ferromagnesian minerals and plagioclase chadacrysts indicate that the deformation and recrystallization of the intrusion continued after the crystallization of the interstitial melt minerals. Received: 28 February 1995/Accepted: 20 July 1995  相似文献   

12.
The Astrophyllite Bay Complex in East Greenland (part of the Palaeogene North Atlantic Igneous Province) consists of an alkaline diorite plug, with detached trachyandesitic pillows, surrounded by co-magmatic syenite that was emplaced into Archaean basement. The diorite intrusion has yielded a 47.11 ± 0.68 Ma Rb-Sr isochron age. Saw-cut profiles through pillow-syenite-gneiss sections have been taken to resolve close spatial elemental and isotopic (Sr-Nd-Hf-Pb-O) variations. The diorite and syenite formed from alkaline basaltic, mantle-derived, melts with complex histories of prolonged assimilation and fractional crystallisation. Each evolved to different extents in separate magma chambers during the establishment of new plumbing systems in the Kangerlussuaq area. The diorite is dominated by lower crustal, granulite facies contamination, whereas the syenite shows evidence for greater degrees of upper crustal amphibolite facies contamination, indicating stalling and fractionation of magmas at different levels within the crust. The syenite and diorite magmas were subsequently emplaced as separate pulses into the basement gneisses at Astrophyllite Bay giving rise to superimposed local contamination trends between pillow/syenite and syenite/gneiss, respectively.  相似文献   

13.
The appearance in 1997 of the British Geological Survey's memoir on Rum was followed by a period of intense research, leading to upwards of 35 papers, books and other articles. The scope of these publications, and the research progress over the last 15 years since publication of the memoir, is reviewed here. Igneous activity on Rum was short lived, possibly only ca. 500 ka, and, at about 60.5 Ma. The Rum central complex thus pre‐dates the nearby Skye central complex. The earliest, acidic and mixed acidic/basic magmatism on Rum involved both shallow intrusions and ignimbrite eruptions into a collapsing caldera bound by the Main Ring Fault, a structure which probably also exercised a structural influence on subsequent mafic and ultrabasic magmatism. Subsequent emplacement of gabbros and ultrabasic rocks caused only limited thermal metamorphism of the surrounding Torridonian sandstones, contrasting markedly with the intense alteration of uplifted masses of Lewisian gneiss within the ring fault. Detailed textural studies on the gabbroic and ultrabasic rocks allow distinction between intrusive peridotites and peridotite that formed as part of the classic layered units of Rum and, furthermore, this work and that on the chromite seams and veins in these rocks shows that movements of trapped magma and magma derived from later intrusions, may produce textures and structures hitherto regarded as primary features of cumulate rocks. Rare picritic dykes provide an indication of likely parent magma for the mafic and ultrabasic rocks, but these and other magmatic rocks on Rum have all undergone varying degrees of crustal contamination, involving both Lewisian granulite and amphibolite crust but, notably, not Moine rocks as at Ardnamurchan. Sulphides in the chromite seams and ultrabasic rocks show possible influences from assimilated Jurassic sediments. From recent apatite fission track studies it seems likely that Rum, in common with other Palaeogene centres, underwent a brief, but significantly younger (Mesozoic) heating event.  相似文献   

14.
The British Tertiary Volcanic Province (BTVP) comprises within-platecentral igneous complexes associated with plateau lavas andregional dyke swarms. Lundy is the southernmost complex of theBTVP and comprises granite ({small tilde}90%) emplaced intodeformed Devonian sedimentary rocks within the Hercynian Cornubiangranite province of southwest England. The complex is intrudedby a northwest-southeast trending dyke swarm. In common withother BTVP igneous complexes, Lundy is associated with positivegravity and magnetic anomalies which are interpreted in termsof the presena of an underlying basic intrusion at shallow depth,with a volume exceeding that of the overlying granite. The Lundy intrusion is a coarse-grained megacrystic granitecontaining up to 20% alkali feldspar megacrysts in a coarse-grainedgroundmass composed of alkali feldspar, quartz, lithium-bearingmuscovite, and ‘biotite’ (lithian siderophyllite),with a range of aaxssory minerals. The main granite has a coarse-grained(locally miarolitic) pegmatitic facies and is intruded by thinsheets and veins of fine-grained aplite and microgranite. Themineralogy indicates crystallization of the Lundy granite froma highly fractionated H2O- and halogen-rich magma at a relativelyshallow crustal level. The main Lundy granite is a peraluminous leucogranite with Na2O=3–4%,K2O{small tilde}5%, low TiO2, MeO, CaO, Zr, and Sr, and highRb and Rb/Sr in comparison with many other peralurninous granites,including those from the Cornubian batholith and the BTVP. Anew Rb-Sr whole-rock isochron for the granite yields an ageof 58?7?1?6 Ma with an initial 87Sr/86Sr of 0?715?0?006. Ndvalues for the granite (–0?9 to –1?9) plot betweencontemporaneous mantle (positive Nd and Cornubian granites (Nd=ca.–11). The trace element data (Rb, Y, Nb) show affinities with syn-collisionand within-plate granites. As the Sr isotope data indicate amajor crustal component, and the Nd isotope data suggest bothmantle and crustal components, we propose that the Lundy graniteis derived from a parental magma comprising crustal components(derived from a similar source to that of the Cornubian granitebatholith) and a mantle-derived component (derived from a differentiateof contemporaneous basaltic magma This magma experienced fractionalcrystallization of plagioclase, alkai feldspar, Fe-Mg minerals,and REE-bearing accessory minerals before emplacement, and theLundy granite experienced further in situ fractional crystallization,associateded with crustal contamination by the Devonian shaleafter emplacement.  相似文献   

15.
The Taihe intrusion is one of the layered intrusions situated in the central zone of the Emeishan Large Igneous Province (ELIP), SW China. The cyclic units in the Middle Zone of the intrusion are composed of apatite-magnetite clinopyroxenite at the base and gabbro at the top. The apatite-rich oxide ores contain 6–12 modal% apatite and 20–50 modal% Fe-Ti oxides evidently distinguished from the coeval intrusions in which apatite-rich rocks are poor in Fe-Ti oxides. Most of apatites of the Taihe Middle and Upper Zones are fluorapatite, although four samples show slightly high Cl content in apatite suggesting that they crystallize from a hydrous parental magma. Compared to the apatite from the gabbro of the Panzhihua intrusion, situated 100 km to the south of the Taihe intrusion, the apatite of the Taihe rocks is richer in Sr and depleted in HREE relative to LREE. The calculated magma in equilibrium with apatite of the Taihe Middle and Upper Zones also shows weakly negative Sr anomalies in primitive mantle normalized trace element diagrams. These features indicate that the apatite of the Taihe Middle and Upper Zones crystallizes after clinopyroxene and before plagioclase. The apatite of the Taihe Middle and Upper Zones shows weakly negative Eu anomalies suggesting a high oxygen fugacity condition. The high iron and titanium contents in the oxidizing magma result in crystallization of Fe-Ti oxides. Crystallization of abundant Fe-Ti oxides and clinopyroxenes lowers the solubility of phosphorus and elevates SiO2 concentration in the magma triggering the saturation of apatite. The positive correlations of Sr, V, total REE contents and Ce/Yb ratio in apatite with cumulus clinopyroxene demonstrate approximately compositional equilibrium between these phases suggesting they crystallized from the same ferrobasaltic magma. Early crystallization and accumulation of Fe-Ti oxide together with apatite produced the apatite-rich oxide ores at the base of the cyclic units of the Taihe Middle Zone.  相似文献   

16.
The Kangerdlugssuaq intrusion, East Greenland, consists of quartzsyenites, syenites, pulaskites and foyaites. The age and petrogenesis of the intrusion has been investigated by strontium and oxygen isotope analyses of the major rock types (and some separated minerals) and the surrounding country rocks. Crystallization and rapid cooling of the intrusion close to 50 m.y. ago is indicated by concordance of an Rb-Sr mineral isochron (49.9±1.0 m.y.) and an Rb-Sr whole-rock isochron (50.0±1.9 m.y.) with previously published mineral dates. The feldspathoid-bearing rocks of the intrusion, which were the last to crystallize, have uniformly depleted oxygen (18O = +3.9, SMOW) and homogeneous initial 87Sr/86Sr ratios (0.70450±7). This is ascribed to equilibration of the magma prior to the crystallization of these rocks with about 10% by weight of meteoric ground water. The concommittant increase of to about 1 Kb (the lithostatic load pressure) would depress the liquidus surfaces in the system Ne-Ks-Qz by about 200 ° C, allowing the magma to evolve continuously down temperature from oversatuated to undersaturated compositions. The chemical mechanism responsible for this trend has not been uniquely identified, but probably involved reduction of SiO2 content in an open system. The outer, quartz-normative, rocks of the intrusion have 18O values ranging up to +5.5 and initial 87Sr/86Sr ratios ranging up to 0.7095. This is due to interaction of the solid rocks, down to temperatures approaching 500 ° C, with ground water which had been enriched in 18O and 87Sr by previous exchange with the Precambrian country rocks. Minimum water/rock ratios are lower than in certain other known cases of interaction in the North Atlantic Tertiary Igneous Province.  相似文献   

17.
塔什库尔干新生代碱性杂岩造岩矿物化学成分及成因意义   总被引：1，自引：0，他引：1  
新疆塔什库尔干碱性杂岩体主要由苦子干碱性正长岩体和卡日巴生碱性花岗岩体组成，是帕米尔地区最大的新生代碱性杂岩体。本文在岩相学和矿物化学的基础上，着重研究了苦子干岩体主要造岩矿物的种属、共生关系和结晶顺序。研究表明，苦子干岩体中的不同岩石类型系同源岩浆演化的产物；岩浆在整个演化过程中平衡结晶作用占主导，分离结晶作用的影响极小。据岩浆房中矿物结晶时的温度和压力条件、矿物的结晶特征及演化趋势，推测岩浆上升速度较快，侵位较浅。  相似文献   

18.
The Parnell Quartz Monzonite in the Pilbara Block of Western Australia is a Proterozoic (1731 ± 14 Ma) pluton characterized by high modal K‐feldspar and a greater abundance of hornblende relative to biotite, as is typical of Phanerozoic monzonitic rocks in eastern Australia. The only geochemical features reflecting its setting in an Archaean terrain are high Na2O, Ni and Cr. The pluton is zoned, with an increase in K‐feldspar, quartz and biotite and a decrease in plagioclase and hornblende from margin to core. Chemically, this zoning is reflected by systematic variation of CaO, K2O, Na2O, Sr and Rb, but ferromagnesian elements have irregular trends, implying preferential extraction of feldspars relative to mafic minerals during differentiation of the magma. The unusual geochemical trends are explained by a model involving ‘in situ’ feldspar fractionation of a K‐rich residual liquid from a mafic crystalline mush.

A parent magma similar to the average rock composition of the pluton is deduced because high ferromagnesian trace element abundances preclude extensive fractionation of mafic minerals. Geochemical and isotopic constraints suggest that the ultimate source was chemically similar to a shoshonitic basaltic andesite, that must have been emplaced beneath the eastern margin of the Pilbara Block in the Early Proterozoic. Subsequent partial melting of this postulated underplated source at ～ 1700 Ma to produce the Parnell Quartz Monzonite was probably associated with tectonism in the Gregory Range Complex.  相似文献   

19.
The igneous rocks of the British Tertiary Volcanic Province(BTVP) comprise intrusive central complexes and associated lavafields in northwest Scotland and northern Ireland. These centresare associated with linear dyke swarms which are radial aroundthe major central complexes. The most extensive dyke swarm isrelated to the Mull intrusive complex and includes the Clevelanddyke, which appears to extend some 430 km from Mull throughthe Scottish Midland Valley (SMV) to the coast of northeastEngland. The dyke may have been emplaced by lateral magma migrationfrom Mull, by vertical magma migration, or by a combinationof these processes associated with the emplacement of the Mullcentre and the presence of a regional stress field in northernBritain. Petrographic, mineralogical, and geochemical data for samplescollected across and along the Cleveland dyke have been usedto evaluate its petrogenesis and emplacement mechanism. Thesegment of the dyke north of, and along, the Southern UplandsFault, the southern boundary of the SMV, is not comagmatic withthat to the south, which is now defined as the Cleveland dykesensu stricto. The Cleveland dyke is an olivine-free, plagioclase-and pyroxene-phyric basaltic andesite. Plagioclase mineralogyand bulk composition indicate that it experienced a complexmagmatic history involving polybaric fractional crystallizationand minor crustal contamination. Despite this complex evolution,the dyke magma is relatively homogeneous and shows chemicalcharacteristics closely similar to tholeiitic rocks from Mull.The data substantiate lateral emplacement from this BVTP centre,rather than by vertical emplacement through heterogeneous lithosphere. Numerical modelling of dyke dynamics is consistent with emplacementof the Cleveland dyke as a single pulse of magma from the Mullcentre, flowing in a manner transitional between laminar andturbulent conditions. According to this model, the dyke (volumec. 85 km3 was initiated in a large magma chamber below Mullsubject to a small excess magmatic pressure. Lateral migrationat relatively high velocity (1–5 ms–1) caused emplacementof the dyke in 1–5 days. Following emplacement, minorvertical ascent of magma may have contributed to the local enechelon distribution of dyke segments.  相似文献   

20.
The Ben Nevis igneous complex consists of a central mass ofrhyodacites surrounded by four plutonic intrusions. In orderof intrusion, these are: the Outer and Inner Quartz Diorites,which are complex rock types with varying chemical and mineralogicalcompositions; the Porphyritic Outer Granite; and the Inner Granite.The plutonic rocks crystallized under comparatively low pressureand thus contain no primary aluminou shornblende, its placebeing taken in all but the most acid rocks by primary pyroxene,which was commonly replaced by an alumina-poor hornblende atlower temperatures. The ferromagnesian minerals in the acidrocks are more magnesian than those in the more basic rocks.This is considered to be due to the partial pressure of oxygenremaining constant at an unusually high level as the magmasconsolidated, causing the minerals to be increasingly enrichedin magnesium as crystallization proceeded.  相似文献   

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