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1.
Nature and origin of eclogite xenoliths from kimberlites 总被引：16，自引：0，他引：16
Eclogites from the Earth's mantle found in kimberlites provide important information on craton formation and ancient geodynamic processes because such eclogites are mostly Archean in age. They have equilibrated over a range of temperatures and pressures throughout the subcratonic mantle and some are diamond-bearing. Most mantle eclogites are bimineralic (omphacite and garnet) rarely with accessory rutiles. Contrary to their overall mineralogical simplicity, their broadly basaltic-picritic bulk compositions cover a large range and overlap with (but are not identical to) much younger lower grade eclogites from orogenic massifs. The majority of mantle eclogites have trace element geochemical features that require an origin from plagioclase-bearing protoliths and oxygen isotopic characteristics consistent with seawater alteration of oceanic crust. Therefore, most suites of eclogite xenoliths from kimberlites can be satisfactorily explained as samples of subducted oceanic crust. In contrast, eclogite xenoliths from Kuruman, South Africa and Koidu, Sierra Leone stem from protoliths that were picritic cumulates from intermediate pressures (1–2 Ga) and were subsequently transposed to higher pressures within the subcratonic mantle, consistent with craton growth via island arc collisions. None of the eclogite suites can be satisfactorily explained by an origin as high pressure cumulates from primary melts from garnet peridotite. 相似文献
2.
Evidence for Archean ocean crust with low high field strength element signature from diamondiferous eclogite xenoliths 总被引：6，自引：0，他引：6
Late Archean (2.57 Ga) diamond-bearing eclogite xenoliths from Udachnaya, Siberia, exhibit geochemical characteristics including variation in oxygen isotope values, and correlations of δ18O with major elements and radiogenic isotopes which can be explained by an origin as subducted oceanic crust. Trace element analyses of constituent garnet and clinopyroxene by Laser-ICPMS are used to reconstruct whole-rock trace element compositions, which indicate that the eclogites have very low high field strength element (HFSE) concentrations and Zr/Hf and Nb/Ta ratios most similar to modern island arcs or ultradepleted mantle. Although hydrothermal alteration on the Archean sea floor had enough geochemical effect to allow the recognition of its effects in the eclogites and thus diagnose them as former oceanic crust, it was not severe enough to erase many other geochemical features of the original igneous rocks, particularly the relatively immobile HFSEs. Correlations of the trace element patterns with oxygen isotopes show that some, generally Mg-richer, eclogites originated as lavas, whereas others have lower δ18O and higher Sr and Eu contents indicating an origin as plagioclase-bearing intrusive rocks formed in magma chambers within the ocean crust. Major and trace element correlations demonstrate that the eclogites are residues after partial melting during the subduction process, and that their present compositions were enriched in MgO by this process. The original lava compositions were picritic, but not komatiitic, whereas the intrusives had lower, basaltic MgO contents. The HFSE signature of the eclogites may indicate that ocean floor basalts of the time were relatively close to island arcs and recycled material, which would be consistent with a larger number of smaller oceanic plates. Their composition appears to indicate that komatiitic ocean crust compositions were restricted to the early Archean which is not known to be represented among the eclogite xenolith population. 相似文献
3.
A. A. Shchipansky 《Geotectonics》2012,46(2):122-141
The diamond-bearing mantle keels underlying Archean cratons are a unique phenomenon of Early Precambrian geology. The common
stable assemblage of the Archean TTG early continental crust and underlying subcontinental lithospheric mantle clearly shows
their coupled tectogenesis, which was not repeated in younger geological epochs. One of the least studied aspects of this
phenomenon is concerned with the eclogitic xenoliths carried up by kimberlite pipes together with mantle-derived nodules.
The eclogitic xenoliths reveal evidence for their subduction-related origin, but the Archean crustal counterparts of such
xenoliths remained unknown for a long time, and the question of their crustal source and relationships to the formation of
early continental crust remained open. The Archean crustal eclogites recently found in the Belomorian Belt of the Baltic Shield
are compared in this paper with eclogitic xenoliths from kimberlites in the context of the formation of both Archean subcontinental
lithospheric mantle (SCLM) and early continental crust. The crustal eclogites from the Belomorian Belt are identical in mineral
and chemical compositions to the eclogite nodules (group B), including their diamond-bearing varieties. The eclogite protoliths
are comparable in composition with the primary melts of the Meso- and Neoarchean oceanic crust, which was formed at a potential
temperature of the upper mantle which exceeded its present-day temperature by 150–250 K. The reconstructed pathways of the
Archean oceanic crust plunging in the upper mantle suggest that the Archean mantle was hotter than in the modern convergence
settings. The proposed geodynamic model assumes coupled formation of the Archean diamond-bearing SCLM and growth of early
continental crust as a phenomenon related to the specific geodynamics of that time controlled by a higher terrestrial heat
flow. 相似文献
4.
Snyder Gregory A.; Taylor Lawrence A.; Crozaz Ghislaine; Halliday Alex N.; Beard Brian L.; Sobolev Vladimir N.; Sobolev Nikolai V. 《Journal of Petrology》1997,38(1):85-113
Owing to the association with diamonds, eclogite xenoliths havereceived disproportionate attention given their low abundancein kimberlites. Several hypotheses have been advanced for theorigin of eclogite xenoliths, from the subduction and high-pressuremelting of oceanic crust, to cumulates and liquids derived fromthe upper mantle. We have amassed a comprehensive data set,including major- and trace-element mineral chemistry, carbonisotopes in diamonds, and Rb–Sr, Sm–Nd, Re–Os,and oxygen isotopes in ultrapure mineral and whole-rock splitsfrom eclogites of the Udachnaya kimberlite pipe, Yakutia, Russia.Furthermore, eclogites from two other Yakutian kimberlite pipes,Mir and Obnazhennaya, have been studied in detail and offercontrasting images of eclogite protoliths. Relative to eclogitesfrom southern Africa and other Yakutian localities, Udachnayaeclogites are notable in the absence of chemical zoning in mineralgrains, as well as the degree of light rare earth element (LREE)depletion and unradiogenic Sr; lack of significant oxygen, sulfur,and carbon isotopic variation relative to the mantle; and intermineralradiogenic isotopic equilibration. Several of these eclogitescould be derived from ancient, recycled, oceanic crust, butmany others exhibit no evidence for an oceanic crustal protolith.The apparent lack of stable-isotope variation in the Udachnayaeclogites could be due to the antiquity of the samples and consequentlack of deep oceanic and biogenically diverse environments atthat time. Those eclogites that are interpreted to be non-recycledhave compositions characteristic of Group A eclogites from otherlocalities that also have been interpreted as being directlyfrom the mantle. At least two separate and diverse isotopicreservoirs are suggested by Nd isotopic whole-rock reconstructions.Most samples were derived from typical depleted mantle. However,two groups of three samples each indicate both enriched mantleand possible ultra-depleted mantle present beneath Yakutia duringthe late Archean and early Proterozoic. The vast majority ofeclogites studied from the Obnazhennaya pipe also exhibit characteristicsof Group A eclogites and are probably derived directly fromthe mantle. However, the eclogites from the Mir kimberlite aremore typical of other eclogites world-wide and show convincingevidence of a recycled, oceanic crustal affinity. We concurwith the late Ted Ringwood that eclogites can be formed in avariety of ways, both within the mantle and from oceanic crustalresidues. KEY WORDS: diamonds; eclogite xenoliths; isotopic composition; REE; Yakutia 相似文献
5.
Summary Crust-derived xenoliths hosted by Miocene basaltic diatremes in the Hyblean Plateau (south-eastern Sicily, Italy) provide
new information regarding the nature of a portion of the central Mediterranean lower crust. These xenoliths can be divided
into three groups: gabbros (plagioclase + clinopyroxene + Fe–Ti oxides ± apatite ± amphibole ± Fe-rich green spinel), diorites
(An-poor plagioclase, clinopyroxene ± Fe–Ti oxides ± orthopyroxene) and mafic granulites (plagioclase + clinopyroxene + green
spinel ± orthopyroxene ± Fe–Ti oxides). Gabbros form the main subject of this paper. They represent cumulates whose igneous
texture has been locally obliterated by metamorphic recrystallization and shearing. They were permeated by Fe–Ti-rich melts
related to tholeiitic-type fractional crystallisation. Incompatible element ratios (Zr/Nb = 5–26; Y/Nb = 1.4–11) indicate
that these cumulate gabbros derived from MORB liquids. Late-stage and hydrothermal fluids caused diverse, sometimes important,
metasomatic trasformations. Petrographic and geochemical comparison with gabbroids from well-known geodynamic settings show
that the Hyblean lower crustal xenoliths were probably formed in an oceanic or oceanic-continent transition environment. 相似文献
6.
Hundred-meter wide cumulate bodies and decimetric dykelets of gabbro-norites are widespread within the distal ophiolitic peridotites from the Jurassic Ligure-Piemontese oceanic basin, now emplaced in the Alpine–Apennine orogenic system. These peridotites derived from the sub-continental mantle of the pre-Triassic Europe–Adria lithosphere and underwent profound modifications of their structural and compositional characteristics via melt–rock interaction during diffuse percolation by porous flow of upwelling asthenospheric melts. Gabbro-norite cumulates show the peculiar association of high forsteritic olivine, high-Mg# clinopyroxenes and orthopyroxenes and high anorthitic plagioclase with respect to mineral compositions in common ophiolitic and oceanic MORB gabbros. Abundance and early crystallization of magnesian orthopyroxene suggests that parental magmas of the gabbro-noritic cumulates were relatively silica-rich basaltic liquids. Clinopyroxenes and plagioclase have anomalously low Sr and LREE, resulting in highly fractionated C1-normalized LREE patterns in clinopyroxenes and negatively fractionated C1-normalized LREE patterns in plagioclases.Modal mineralogy and mineral major and trace element compositions indicate that these gabbro-norites crystallized from MORB-type basaltic liquids that were strongly depleted in Na, Ti, Zr, Sr and other incompatible trace elements relative to any erupted liquids of MORB-type ophiolites and modern oceanic lithosphere. Computed melt compositions in equilibrium with gabbro-norite clinopyroxenes are closely similar to depleted MORB-type single melt increments after 5–7% of fractional melting of a DM asthenospheric mantle source under spinel-facies conditions.Present knowledge on the ophiolitic peridotites of Monte Maggiore indicate that they were formed by interaction of lithospheric mantle protoliths with depleted, MORB-type single melt increments produced by the ascending asthenosphere. Their composition was progressively modified from olivine-saturated to orthopyroxene-saturated by the early reactive melt–peridotite interaction (i.e., pyroxene dissolution and olivine precipitation).Gabbro-norite cumulates marked the change from diffuse porous flow percolation to intrusion and crystallization when cooling by conducive heat loss became dominant on heating by melt percolation. Progressive upwelling and cooling of the host peridotite during rifting caused transition to more brittle conditions and to hydration and serpentinization.The Monte Maggiore peridotite body was then intruded along fractures by variably evolved, Mg–Al- to Fe–Ti-rich gabbroic dykes. Computed melt compositions in equilibrium with clinopyroxenes from less evolved gabbro dykes are closely similar to aggregated MORBs. The event of gabbro intrusion indicates that aggregated MORB-type liquids: i) migrated through and stagnated in the mantle lithosphere and ii) underwent evolution into shallow ephemeral magma chambers to form the parental magmas of the gabbroic dykes and the basaltic lava flows of the Ligurian oceanic crust. 相似文献
7.
Mafic and ultramafic xenoliths in a basaltic cone at The Anakies in south-eastern Australia are geochemically equivalent to continental basaltic magmas and cumulates. The xenolith microstructures range from recognizably meta-igneous for intrusive rocks to granoblastic for garnet pyroxenites. Contact relationships between different rock types within some xenoliths suggest a complex petrogenesis of multiple intrusive, metamorphic and metasomatic events at the crust/mantle boundary during the evolution of south-eastern Australia. Unaltered spinel lher-zolite, typical of the uppermost eastern Australian mantle, is interleaved with or veined by the metamorphosed intrusive rocks of basaltic composition. Geothermobarometry calculations by a variety of methods show a concordance of equilibration temperatures ranging from 880°C to 980°C and pressures of 12 to 18 kbar (1200-1800 mPa). These physical conditions span the gabbro to granulite to eclogite transition boundaries. The water-vapour pressure during equilibration is estimated to be about 0.5% of the load pressure, using amphibole breakdown data. Large fluid inclusions of pure CO2 are abundant in the mineral phases in the xenoliths, and it is suggested that flux of CO2 from the mantle has been an important heat source and fluid medium during metamorphism of the mafic and ultramafic protoliths at the lower crust/upper mantle boundary. The calculated pressures and temperatures suggest that the south-eastern Australian crust has sustained a high geothermal gradient. In addition, the nature of the mineral assemblages and the contact relationships of granulitic rock with spinel lherzolite, characteristic of mantle material, suggest that the Moho is not a discrete feature in this region, but is represented by a transition zone approximately 20 km thick. These inferences are in agreement with geophysical data (including seismic, heat-flow and electrical resistivity data) determined for south-eastern Australia. Underplating at the crust/mantle boundary by continental basaltic magmas may be an important alternative or additional mechanism to the conventional andesite model for crustal accretion. 相似文献
8.
中国西部柴北缘—阿尔金的超高压变质榴辉岩及其原岩性质探讨 总被引：45，自引：5，他引：45
中国西部祁连山柴北缘地区和南阿尔金地区存在一条被阿尔金断裂错开 4 0 0km ,但构造上相连的早古生代超高压变质带。通过对柴北缘地区大柴旦、锡铁山、都兰和南阿尔金地区且末一带榴辉岩的岩石地球化学研究 ,发现榴辉岩原岩主要由玄武岩和苦橄岩两类岩石组成 ,进一步分为高Ti型 (w(TiO2 ) =2 %～ 5 % ) ,中Ti型 (1%～ 2 % )和低Ti型 (<1% ) 3种类型 ,识别出榴辉岩的原岩类型有洋脊玄武岩、岛弧拉斑玄武岩和洋岛玄武岩类等产在不同环境的岩石类型。榴辉岩的Nd同位素组成与现代洋脊玄武岩类相似 ,ε(Nd ,0 )主要为正值 ,少量为轻微负值 ,表明榴辉岩的原岩曾是海底玄武岩 ,并且经过了消减俯冲作用 ,混入了部分的地壳物质。榴辉岩的超高压变质年龄为 5 0 0～ 4 4 0Ma,原岩年龄分别为 80 0～ 75 0Ma和～ 10 0 0Ma。研究表明 ,柴北缘滩涧山群中存在两套时代不同的基性超基性岩 ,一套为产在绿梁山的新元古代时期形成的蛇绿岩组合 ,新获得的年龄值为 (76 8±39)Ma(Rb Sr)和 (780± 2 2 )Ma(Sm Nd) ,另一套主要为产在赛什腾山的晚寒武世岛弧火山岩 ,形成时代约在 5 15～ 4 86Ma。榴辉岩的岩石化学成分和Nd同位素组成 ,以及 80 0～ 75 0Ma的原岩时代与其中的新元古代基性岩类可以对比。初步认为它们是同一套岩石? 相似文献
9.
华北地台麻粒岩包体（汉诺坝）与麻粒岩地体（西望山、蔓菁沟）存在明显的岩石地球化学特征差异，麻粒岩包体总体成分偏基性，麻粒岩地体总体成分偏长英质。形成时间、空间及构造背景的不同是造成成分差异的重要因素。汉诺坝麻粒岩包体是在拉线环境下玄武质岩浆底侵下地壳后结晶分异过程中堆晶作用的结果，其主、微量元素成分受控于辉石和斜长石堆晶的相对比例；下地壳物质的混染作用不是造成麻粒岩包体Nb、Ta、Th、U亏损的惟一因素，底侵的玄武质岩浆可能发生了含钛矿物相的结晶分离，造成了汉诺坝麻粒岩包体Nb、Ta亏损，而下地壳的麻粒岩相变质作用则使Th、U发生亏损。麻粒岩地体存在两种不同性质的微量元素组成，反映了两种不同构造环境的并置，是构造挤压抬升的结晶。 相似文献
10.
Rare lower crustal xenoliths found in Cenozoic alkali basalts from the Tariat region in central Mongolia and the Dariganga Plateau in south-eastern Mongolia are the only direct samples of lower crustal material known so far from central and eastern Asia. They are two-pyroxene granulites, including some garnet granulites, as well as scarce amphibolite-facies rocks. The xenoliths are broadly basaltic to andesitic in bulk chemical composition. Their igneous protoliths appear to represent underplated fractionated liquids and cumulates from such liquids. The xenoliths yield equilibration temperatures of 840 ± 30 °C (Wells, 1977) and, for Tariat garnet granulites only, pressures of 14 ± 1.5 kbar. For central Mongolia, these estimates indicate unusually great depths of origin which, however, are in line with some geophysical models for that area.
Extensive to complete kelyphitisation has affected the garnets where originally present in the Tariat suite; nevertheless, the kelyphite has largerly preserved the major element and REE compositions of the original garnet. Mineral and whole-rock Sm-Nd data obtained for three samples from Tariat and Dariganga indicate, within large errors, low or zero ages. These may either indicate that the rocks are young (Cenozoic) or that ambient temperatures in the lower crust were high enough to permit continuous isotopic equilibration on a mineral-to-mineral scale. 相似文献
11.
Brian Mason 《Contributions to Mineralogy and Petrology》1968,19(4):316-327
Eclogitic xenoliths consisting of tschermakitic augite and pyrope garnet, together with variable amounts of kaersutitic hornblende, are common in a volcanic breccia of Lower Oligocene age at Kakanui, New Zealand. The breccia also contains xenocrysts of these minerals, and xenoliths of peridotite. Modal analyses are given of a number of the eclogitic xenoliths, and chemical analyses of two of them and their component minerals. They are compared with similar xenoliths from Hoggar (Algeria), Salt Lake Crater (Hawaii), and Delegate (Australia), with eclogite xenoliths from kimberlites, and with garnet peridotites. These three types of igneous eclogites can be characterized by the nature of their clinopyroxene: tschermakitic in the xenoliths from basaltic rocks, jadeitic in the xenoliths from kimberlites, and chrome diopside in the garnet peridotites. The eclogitic xenoliths in basaltic rocks probably crystallized in the mantle at depths of about 60 km, but their rarity in contrast to the numerous occurrences of peridotite xenoliths poses some significant problems. 相似文献
12.
Jean H. Bédard 《Contributions to Mineralogy and Petrology》2001,141(6):747-771
Equilibrium melt trace element contents are calculated from Proterozoic Nain Plutonic Suite (NPS) mafic and anorthositic cumulates, and from plagioclase and orthopyroxene megacrysts. Assumed trapped melt fractions (TMF) <20% generally eliminate all minor phases in most mafic cumulate rocks, reducing them to mixtures of feldspar, pyroxene and olivine, which would represent the high-temperature cumulus assemblage. In anorthosites, TMF <15% generally reduce the mode to a feldspar-only assemblage. All model melts have trace element profiles enriched in highly incompatible elements relative to normal mid-ocean ridge basalt (NMORB); commonly with negative Nb and Th anomalies. Most mafic cumulates yield similar profiles with constant incompatible element ratios, and can be linked through fractional crystallization. High K-La subtypes probably represent crust-contaminated facies. Mafic cumulates are inferred to belong to a tholeiitic differentiation series, variably contaminated by upper and lower crustal components, and probably related to coeval tholeiitic basaltic dyke swarms and lavas in Labrador. Model melts from anorthosites and megacrysts have normalized trace element profiles with steeper slopes than those calculated from mafic cumulates, indicating that mafic cumulates and anorthosites did not crystallize from the same melts. Orthopyroxene megacrysts yield model melts that are more enriched than typical anorthositic model melts, precluding an origin from parental melts. Jotunites have lower K-Rb-Ba-Y-Yb and higher La-Ce than model residues from fractionation of anorthositic model melts, suggesting they are not cosanguineous with them, but provide reasonable fits to evolved mafic cumulate model melts. Incompatible element profiles of anorthositic model melts closely resemble those of crustal melts such as tonalites, with steep Y-Yb-Lu segments that suggest residual garnet in the source. Inversion models yield protoliths similar to depleted lower crustal granulite xenoliths with aluminous compositions, suggesting that the incompatible trace element budget of the anorthosites are derived from remobilization of the lower crust. The similarity of the highly incompatible trace elements and LILE between anorthositic and mafic cumulate model melts suggests that the basalts parental to the mafic cumulates locally assimilated considerable quantities of the same crust that yielded the anorthosites. The reaction between underplating basalt and aluminous lower crust would have forced crystallization of abundant plagioclase, and remobilization of these hybrid plagioclase-rich mushes then produced the anorthosite massifs. 相似文献
13.
Were the Deccan Flood Basalts Derived in Part from Ancient Oceanic Crust Within the Indian Continental Lithosphere? 总被引：5，自引：1，他引：5
Deep mantle plumes supposedly incorporate deeply subducted eclogitized oceanic crust, and continental flood basalts (CFBs) are now thought by some to be derived from such eclogite-bearing peridotite plumes. Eclogite-peridotite mixtures have much lower solidi (and produce much greater melt fractions for a given temperature) than peridotite. Fe-rich (eclogite- or pyroxenite-bearing) sources have been inferred for many CFBs. However, plumes with considerable amounts of eclogite should have difficulty in upwelling owing to the high density of eclogite. Besides, CFBs are always located along pre-existing lithospheric structures (suture zones, edges of thick cratons) and commonly associated with lithospheric rifting and continental breakup. India's major late Mesozoic CFB, the Deccan Traps, erupted through rift zones and a new continental margin that had developed along ancient suture zones traversing the subcontinent. Many Deccan basalts are too Fe-rich to have been in equilibrium with a peridotite mantle source, and have commonly been considered to be significantly fractionated derivatives of picritic liquids. However, it is possible to view them as relatively less evolved liquids derived from a source with extra fertility (i.e., an Fe-rich source). A new non-plume, plate tectonic model for Icelandic hotspot volcanism involves melting of a shallowly recycled slab of eclogitized Iapetus oceanic crust formerly trapped along the Caledonian suture. The model explains the geochemical-petrological characteristics of Icelandic basalts, and is consistent with passive upper mantle upwelling under Iceland inferred from recent seismic tomography. Based on the petrological and geochemical features of the Deccan flood basalts of the type section, in the Western Ghats, I propose that old, eclogitized oceanic crust trapped in the ancient Indian suture zones could have produced voluminous basaltic melts during the Deccan event. 相似文献
14.
徐淮地区中生代深部地壳的物质组成及热状态——深源捕虏体证据 总被引：4，自引：0，他引：4
徐淮地区中生代侵入杂岩中含有丰富的深源岩石捕虏体,包括榴辉岩类、单斜辉石岩、角闪岩类和片麻岩类。深源捕虏体的岩相学研究表明,它们经历了早期的榴辉岩相变质和晚期的角闪岩相退化变质,然后被寄主岩浆所捕获。捕虏体中矿物平衡温压估算结果显示,榴辉岩相变质的温压条件为:T=657～992℃,P=1.33～2.78GPa,所反映的地温梯度介于克拉通地温(40mW/m2)和大洋地温(60mW/m2)之间;晚期角闪岩相退变的温压条件为:T=523～878℃,P=0.5～1.02GPa,这与中国东部新生代地温梯度相一致。本区中生代深部地壳主要由一套混杂堆积岩所构成,包括退变榴辉岩、退变石榴辉石岩、角闪岩类和片麻岩类。该区深源捕虏体的P-T演化暗示中生代早期华北地块东部深部地壳和/或岩石圈地幔曾存在一次重要的加厚过程;苏鲁造山带的形成可能是扬子地块沿北西方向俯冲于华北地块之下的产物;中生代郯庐断裂带并不存在巨大左行平移。 相似文献
15.
下地壳拆沉作用及大陆地壳演化 总被引：13，自引：0，他引：13
讨论了下地壳拆沉作用的地球化学示踪方法,并以近来对秦岭－大别造山带和美国西部内华达岩基地区的研究成果为例,说明了下地壳拆沉在两地区壳－幔演化方面可能起的重要作用。 相似文献
16.
The role of amphibole in the evolution of arc magmas and crust: the case from the Jurassic Bonanza arc section, Vancouver Island, Canada 总被引：6，自引：4，他引：2
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. 相似文献
17.
Atsushi Utsunomiya Bor-ming Jahn Kazuaki Okamoto Tsutomu Ota Hironao Shinjoe 《Chemical Geology》2011,280(1-2):97-114
New geochemical and Sr–Nd isotopic data for the Iratsu eclogite and surrounding metamorphic rocks of the Sanbagawa belt, Japan, show that, while the protoliths of the metamorphic rocks formed in a variety of tectonic settings, the Iratsu body represents a deeply subducted and accreted island arc. The igneous protoliths of eclogites and garnet amphibolites were probably generated from a mantle source that had components of both a depleted mantle modified by slab-released fluid (as seen in a negative Nb anomaly) and an enriched mantle, similar to that of ocean island basalts (OIB). Fractional crystallization modeling indicates that the protoliths of some garnet clinopyroxenites from the Iratsu body are cumulates from a basaltic magma that crystallized under high O2 and H2O fugacities in the middle to lower crust. The source characteristics and crystallization conditions suggest that the protoliths of the Iratsu rocks formed in an oceanic island arc. Quartz eclogites from the marginal zone of the Iratsu body have geochemical signatures similar to turbidites from the Izu–Bonin island arc (as seen in a negative Nb anomaly and a concave REE pattern). The protoliths might be volcaniclastic turbidites that formed in a setting proximal to the oceanic island arc. Geochemical and isotopic signatures of the surrounding mafic schists are similar to normal (N-) and enriched (E-) mid-ocean-ridge basalt (MORB), and distinct from the rocks from the Iratsu body. The protoliths of the mafic schists likely formed in a plume-influenced mid-ocean ridge or back-arc basin. Pelitic schists from the surrounding rocks and pelitic gneisses from the marginal zone of the Iratsu body have evolved, continental geochemical signatures (as seen in a negative εNd(t) value (~?5)), consistent with their origin as continent-derived trench-fill turbidites. 相似文献
18.
Sonja Aulbach Suzanne Y. O’Reilly Norman J. Pearson 《Contributions to Mineralogy and Petrology》2011,162(5):1047-1062
New trace-element data of rutile in kimberlite-borne ~1.85 Ga eclogite and pyroxenite xenoliths from the central Slave craton,
as well as ~110 Ma MARID xenoliths from the Kaapvaal craton, provide constraints on the origins of lithospheric and sublithospheric
mantle variability in high field strength element ratios. Rutiles in eclogites and pyroxenites have Zr/Hf ranging from 20
to 62 and Nb/Ta ranging from 10 to 40. Rutiles in MARID xenoliths have Zr/Hf from 24 to 33 and Nb/Ta from 10 to 41. Calculated
whole-rock Zr/Hf is suprachondritic for eclogites with suggested gabbroic protoliths and subchondritic for boninite-like eclogites;
the latter is consistent with cpx-controlled depletion in the protolith source. Within each eclogite type, positive correlations
of Zr/Hf with La/Lu and negative correlations with Lu/Hf likely reflect fractionation of cpx and/or plagioclase during crystallisation
of the protoliths. Zr/Hf–Nb/Ta relationships of some MARID-type rocks, which are products of lithospheric mantle metasomatism,
and eclogite xenoliths plot on a silicate differentiation trend, whereas other samples have higher Nb/Ta at a given Zr/Hf.
Fractionation of a few percent rutile from an HFSE-rich mafic melt can generate a trend towards strongly increased Nb/Ta at
minimally changed Zr/Hf in the residual melt. Superposition of rutile fractionation on the effects of silicate differentiation,
which fractionates Zr/Hf more strongly than Nb/Ta, can explain the Zr/Hf–Nb/Ta relationships of most eclogites from the central
Slave craton as well as those of MARID rocks, metasomatised peridotites and group II kimberlites. By contrast, Zr/Hf–Nb/Ta
relationships suggest that Group I kimberlites are mixtures between depleted peridotite and carbonatite. Thus, high Nb/Ta
is a signature of lithospheric processes and may not be important in deeply subducted eclogites that bypass extended residence
in the lithosphere. Conversely, considerable primary Zr/Hf variability was inherited by the eclogites, which is indicative
of the compositional diversity of ancient subducted oceanic crust, which is expected to have generated substantial heterogeneity
in sublithospheric basalt sources. 相似文献
19.
Xenolithic inclusions in calc-alkaline andesite from Mt. Moffettvolcano, Adak Island, Aleutian arc, reveal a nearly continuousrecord of crystallization of basaltic magmas in the crust, andpossibly upper mantle, of the arc. The record is more detailedand continuous than that obtained from study of calc-alkalinevolcanic rocks in the arc. Cumulate xenoliths form a progressiveseries in modal mineralogy from ultramafic, hornblende-bearingolivine clinopyroxenite to both hornblende-bearing and hornblende-freegabbros. The cumulate hornblende gabbro xenoliths are typicalof those found in island arc andesites worldwide. Xenolithicinclusions without cumulate textures, here termed compositexenoliths, are characterized by forsteritic olivine, zoned Cr-diopsideand hornblende, and are interpreted to have resulted from reactionand chilling upon magma mixing at depth. The olivine and clinopyroxene in both cumulate and compositexenoliths show the largest and the most complete variation trendsfor Ni, Cr, and FeO/MgO ratio yet reported in igneous xenolithsfrom island arc volcanic rocks. Variation of Ni in olivine indicatesthat the parent magmas for the xenoliths had minimum MgO contentsof 9 wt. per cent. Variation of Cr in clinopyroxene indicatesthat the magmas were basaltic rather than picritic, probablyin equilibrium with spinel lherzolite at near Moho depths. Successiveinjections of batches of primary melt into a magma chamber fractionatingolivine and clinopyroxene can reproduce observed compatibleelement depletion trends. A steady-state process of cotecticcrystallization in a magma chamber continually replenished withbasaltic magma is a possible mechanism for producing large accumulationsof olivine and clinopyroxene, suggesting that Alaskan-type ultramaficcomplexes are related to hydrous basaltic magmas in island arcs.This steady-state open-system crystallization process can alsoyield the abundant high-alumina basalt type in the Aleutianarc. Continued crystallization of high-alumina basalt in lowercrustal magma chambers, recorded in a mineralogically coherentseries of pyroxenite to hornblende gabbro xenoliths, can yieldbasaltic to andesitic magmas of the calc-alkaline series. No xenoliths with a sedimentary protolith have been found atMt Moffett, evidence that the arc crust is igneous in origin,with the lower crust formed of gabbro crystallized from mantle-derivedmelts. Ultramafic cumulates may reside in both the lower crustor upper mantle beneath the arc. A model is proposed wherebythe cumulate crystallization products of hydrous, mantle beneaththe arc. A model is define the upper mantle and lower crustof the arc over time.The net composition added to the crustof the arc is that of high-alumina basalt. 相似文献
20.
P. D. Kempton H. Downes E. V. Sharkov V. R. Vetrin D. A. Ionov D. A. Carswell A. Beard 《Lithos》1995,36(3-4):157-184
Lower crustal xenoliths entrained in a Paleozoic ultramafic lamprophyre breccia pipe on Elovy island, Kola peninsula, Russia, represent some of the oldest lower crustal material yet investigated from Europe. The xenoliths vary from feldspar-poor, garnetrich rocks which resemble eclogites, to feldspar-rich garnet granulites. Quartz-rich felsic granulites, as well as pyroxenites and amphibole-rich rocks are also present.
The mafic granulites/eclogites represent a suite of gabbros and norites that is related by olivine fractionation. The igneous protoliths may have formed in a manner analogous to lower crustal rocks from most other European xenolith localities, i.e. by basaltic underplating, but magmatic cumulates are not in evidence.
The Kola lower crust was subjected to one or more metasomatic events which introduced up to 45% phlogopite and/or amphibole into both eclogites/granulites and pyroxenites. The resulting rocks have strong enrichments in Rb, Ba, and K, indicating that the lower crust is not uniformly depleted in LIL and heat-producing elements. Siliceous (65% SiO2) and mafic (< 50% SiO2) lithologies coexist in migmatitic xenoliths, which provide evidence for partial melting processes and restite formation in mafic metaigneous lower crust. The relationship, if any, between partial melting and metasomatism is unclear. 相似文献