Degradation and Metamorphic Differentiation of the Keweenawan Tholeiitic Lavas of Northern Michigan, U.S.A.

The Portage Lake Lava Series of the Keweenaw Peninsula, northernMichigan, is composed of over 5000 m of tholeiitic lava flows.Chemical and petrologic study of thin undifferentiated flowsfrom this sequence shows that the bulk of the flows are olivinetholeiites; highly ironenriched tholeiites comprise up to 10per cent of the volume. In addition, a few small rhyolitic intrusives,exceedingly rich in alkalis, are exposed in the region. Duringburial the amygdular tops of each flow acted as channelwaysfor migrating fluids during a regional metamorphic event thatproduced a progressive sequence of secondary phases within theflow tops. In the upper part of the stratigraphic section, laumontite,analcime, albite, and chlorite dominate assemblages characteristicof the zeolite facies. Stratigraphically deeper. flow tops arecharacterized by metamorphic assemblages of the prehnite-pumpellyitefacies. Within the higher rank facies, rocks along fractureshave been transformed to calcium-rich monomineralic domains(metadomains) of epidote or pumpellyite. These metadomains gradeoutward through incompletely reconstituted rocks containingalbitized feldspars, and finally, in flow centers, to basaltdisplaying few signs of mineralogical readjustment. Extremechemical disparity exists within the altered flow tops, particularlywith respect to calcium and sodium content. Calculations revealthat bulk compositions of metamorphically adjusted flow topsare similar to unaltered basalts. Thus, the chemical variationdisplayed by the metamorphic rock types resulted from localizedmetamorphic differentiation. The Keweenawan sequence displays progressive dehydration fromthe top to the base of the stratigraphic section. At the topchlorite (H₂O = 12 per cent) and pumpellyite (H₂O = 6 per cent)metadomains formed by dehydration. The fact that most of theserocks were hydrated strongly suggests that fluid pressures wereless than total pressures during the metamorphic event. In rocksundergoing hydration in the uper part of the stratigraphic section,therefore, water pressures decreased away from channelways.As a result, the most hydrous rock types formed in these regions.Conversely, in rock undergoing dehydration water pressures increasedaway from channelwasys, so that the most dehydrated rocks formedin such regions. Water content of an individual rock is dependenton both position in the stratigraphic pile and position withrespect to fluid channelways.     

Primary curvature in the Mid-Continent Rift: Paleomagnetism of the Portage Lake Volcanics (northern Michigan, USA)

Rocks of the North American Mid-Continent Rift (MCR) in the Keweenaw Peninsula of northern Michigan display a change in structural trend from east to west, varying in strike from 100° in the east to 35° farther west, before returning to a more east–west trend of 80° near Wisconsin. In general, curvature can be described either as of primary origin, meaning that the arc developed in its present curved state, or as of secondary origin, when the arc formed from an initially straighter geometry. A powerful tool in evaluating the origin and degree of curvature in any deformed belt is paleomagnetism, where coincident change in declination and strike is evidence for secondary curvature. Several paleomagnetic studies have been completed on rift-related sedimentary rocks, as well as lava flows, from the MCR in the Lake Superior region. Paleomagnetic directions for the sedimentary rocks vary much more in declination than in inclination, possibly reflecting a vertical axis rotation. If secondary rotation has affected the sedimentary rocks, then underlying volcanic rocks should also demonstrate a similar rotation.Thirty-one sites were collected from the Portage Lake Volcanics in the most highly curved part of the Keweenaw Peninsula in the Upper Peninsula of Michigan. Sites were chosen to maximize the variation in structural trend. Thermal demagnetization results showed two components in samples from most sites, a lower temperature A component (< 580 °C) as well as a higher temperature B component (> 580 °C). Both components were tested for primary remanence using a conglomerate test. The A component, carried by magnetite, passes the conglomerate test at a 95% confidence level, and is therefore considered a primary remanence. The B component, carried by hematite, fails at the 95% level and is considered a secondary magnetization. Most importantly, declination of the primary (A) magnetization between sites shows no correlation with strike, demonstrating that vertical axis rotations cannot explain the curvature of the Mid-Continent Rift. We conclude that curvature in the Lake Superior Region is a primary feature, likely reflecting a pre-existing zone of weakness that was exploited during rifting.     

新疆巴楚辉绿岩岩脉的岩石成因和大地构造意义

塔里木盆地西北部巴楚地区出露的二叠纪基性岩脉为研究塔里木盆地及其邻区晚古生代区域大地构造演化提供了重要信息。巴楚地区基性岩脉中辉绿岩脉富集大离子亲石元素、轻稀土元素以及高场强元素,La/Nb=0.84～1.23,Zr/Nb=5.80～7.81,Ce/Pb=2.6～15.4和 Nb/U=27.0～37.3,初始~(87)Sr/~(86)Sr=0.70495～0.70560,ε_(Nd)(t)=-1.7～ 2.0,明显不同于受俯冲交代地幔源区的产物,也区别于典型洋岛玄武岩(OIB),相反具有 OIB 型 富集岩石圈地幔的元素-同位素地球化学特征,能被理解为与二叠纪软流圈和岩石圈相互作用产物,其起源与软流圈地幔上涌有关,表明二叠纪时巴楚地区应处于软流圈上隆的岩石圈伸展构造背景。结合区域地质资料,可认为二叠纪时地幔源区属性自塔里木盆地内部向北至西南天山边缘呈规律性变化     

Olivine-normative dolerite dikes from western South Carolina: Mineralogy,chemical composition and petrogenesis

This investigation describes five Mesozoic dolerite dikes which intrude Paleozoic metamorphic and igneous rocks of the Inner Piedmont of western South Carolina. The dikes are vertical or nearly so and strike approximately N40° W. Three major northeast-trending faults also occur in the study area. Left lateral displacement of one dolerite is documented at a locality near Cleveland, South Carolina. Elsewhere, several of the dolerite dikes appear to terminate at or near the faults. — The dolerite dikes have subophitic to microporphyritic textures and consist principally of plagioclase (generally An_70–80), olivine (dominantly Fo_80–90) and augite with subordinate pigeonite, titanomagnetite, chromite and brown, partly glassy mesostasis. In one dike pyroxene compositions trend from augite to ferroaugite in contrast to an augitesubcalcic augitepigeonite trend characteristic of the other dolerites. The contrasting trends primarily result from differences in SiO₂ abundance in the dolerite magmas. — Major and trace element analyses indicate the presence of two different olivine-normative dolerite magma types. The two magma types are not related by near surface crystal fractionation. Models for genesis of the olivine-normative dolerite magmas by partial melting of a plagioclase peridotite upper mantle source region are presented. The models require that the source region be enriched in LREE and incompatible elements such as Rb, Ba, Hf and Th relative to Cl chondritic abundances. One magma type appears to represent a primary dolerite magma that ascended from the source region with little subsequent compositional change. The second magma type most likely experienced assimilation of clinopyroxene-garnet (eclogite) during ascent, thereby acquiring a REE pattern with a less steep negative slope for the LREE and a slight positive slope in the HREE.     

湖北铜绿山铜铁矿床中煌斑岩的矿物学特征及岩石成因初探

湖北铜绿山矽卡岩型铜铁矿床处于长江中下游铁铜成矿带西部的鄂东南矿集区。最近我们在该矿区工作中采集到两条侵入于含矿石英二长闪长玢岩中的煌斑岩脉。本文对这些煌斑岩脉进行了详细的镜下岩相学观察,并对其中造岩矿物黑云母、辉石和长石进行了电子探针成分分析。根据这些数据本文论述了煌斑岩的物源、形成环境以及岩浆体系的物理化学条件。研究表明,煌斑岩中云母为富镁黑云母,其形成的岩浆体系处于中低氧逸度(近Ni-Ni O);黑云母与全岩都具有高钛特征,表明煌斑岩的物源深度可能来自较深部地幔(如软流圈)。煌斑岩中辉石为单斜辉石,环带较为发育,有韵律环带和正环带,其韵律环带的形成可能是由于岩浆温压环境存在振荡性变化而形成的。铜绿山矿床中煌斑岩属于云煌岩;其岩浆来源具幔源性,受到地壳的混染作用不强。     

Anomalous isotopic compositions of Sr,Ar and O in the Mesozoic diabase dikes of Liberia,West Africa

The Mesozoic diabase dikes of Liberia are tholeiites whose ⁸⁷Sr/⁸⁶Sr and ⁸⁷Rb/⁸⁶Sr ratios scatter widely on the Rb-Sr isochron diagram. The problem is attributed to differences in the initial ⁸⁷Sr/⁸⁶Sr ratios of these rocks which range from 0.70311 to 0.70792, assuming a uniform age of 186 Ma for the dikes and using (⁸⁷Rb)=1.42 × 10^–11y^–1. The range of values is similar to that observed in the Mesozoic basalt flows and dikes of other Gondwana continents.New whole-rock K-Ar dates confirm previous conclusions that the diabase dikes in the Liberian and Pan-African age provinces of Liberia absorbed extraneous ⁴⁰Ar after intrusion. Only the dikes in the Paynesville Sandstone have K-Ar dates that range from 117 Ma to 201 Ma and may not contain extraneous ⁴⁰Ar. However, dikes from all three age provinces of Liberia have elevated initial ⁸⁷Sr/⁸⁶Sr ratios. These results indicate that contamination with radiogenic ⁸⁷Sr occurred primarily before intrusion of the magma whereas the addition of extraneous ⁴⁰Ar occurred after emplacement and reflects the age and mineral composition of the country rock.The ¹⁸O values of the Liberian diabase range from +5.6/% to +9.10/% and correlate positively with initial ⁸⁷Sr/⁸⁶Sr ratios. The data can be modeled by fractional crystallization and simultaneous assimilation of crustal rocks by the magma. However, samples containing amphibole and biotite replacing pyroxene deviate from the Sr-O isotope trajectories of the model and appear to have been depleted in ¹⁸O and enriched in ⁸⁷Sr by interactions with groundwater at high temperature.Laboratory for Isotope Geology and Geochemistry Contribution No. 76     

浙闽晚中生代辉绿岩脉的岩石成因:年代学与地球化学制约

来自浙江文成和福建永泰两地的侵位于晚中生代中酸性火山岩地层中辉绿岩脉的形成于90～94Ma,为晚白垩世岩石圈伸展作用产物。两地辉绿岩的SiO2=48.61%～55.54%,Na2O=1.98%～3.89%,K2O=0.28%～2.26%,为钙碱性系列;岩浆在演化过程中经历了一定程度的镁铁质矿物、斜长石和副矿物的分离结晶作用。在微量元素特征上LILE(如Ba、Th)和LREE富集、HFSE亏损;具高放射成因Sr和低放射成因Nd同位素组成(文成辉绿岩:87Sr/86Sr(i)=0.7080～0.7097,εNd(t)=-7.7～-3.4;永泰辉绿岩:87Sr/86Sr(i)=0.7061～0.7062,εNd(t)=-1.2～+0.5),明显不同于亏损软流圈来源岩浆。地壳混染/AFC、岩浆混合等过程不足以解释这些辉绿岩的元素-同位素变化特征。结合主、微量元素和Sr-Nd同位素组成,我们认为区域晚中生代辉绿岩为伸展拉张背景下,软流圈上涌与富集岩石圈地幔相互作用,两种源区共同发生部分熔融的结果,其中文成辉绿岩有更大比例的富集岩石圈组分参与,而永泰辉绿岩的熔融源区中软流圈组分则更高。     

Strontium isotope composition and petrogenesis of the Kirkpatrick basalt,Queen Alexandra Range,Antarctica

The initial ⁸⁷Sr/⁸⁶Sr ratios of twelve basalt flows of Jurassic age on Storm Peak in the Queen Alexandra Range are anomalously high and range from 0.7094–0.7133. The average value is 0.7112±0.0013 (1). The concentrations of rubidium and strontium have arithmetic means of 60.6±19.4 ppm and 128.8±11.9 ppm, respectively. The corresponding average Rb/Sr ratio is 0.47 which is also anomalously high for rocks of basaltic composition. In addition, these rocks have high concentrations of SiO₂ (56.50%) and K₂O (1.29%) and are depleted in Al₂O₃ (12.92%), MgO (3.44%) and CaO (7.91%) compared to average continental tholeiites. They are nevertheless classified as basalts on the basis of the composition of microphenocrysts.The initial ⁸⁷Sr/⁸⁶Sr ratios and all of the chemical parameters of the flows exhibit systematic stratigraphic variations. These are interpreted as indicating the occurrence of four eruptive cycles. In a typical cycle the initial ⁸⁷Sr/⁸⁶Sr ratios of successive flows and their concentrations of SiO₂, FeO (total iron), Na₂O, K₂O, P₂O₅, Rb and Sr decrease in ascending stratigraphic sequence while the concentrations of TiO₂, Al₂O₃, MgO, CaO and MnO increase upward. The initial ⁸⁷Sr/⁸⁶Sr ratios of the flows show a strong positive correlation with the strontium concentration. Similar correlations are observed between the initial ⁸⁷Sr/⁸⁶Sr ratios and all of the major oxide components. These relationships are incompatible with the hypothesis that these flows are the products of crystal fractionation of a-34 magma at depth under closed-system conditions. It is suggested that the flows resulted from the hybridization of a normal tholeiite basalt magma by assimilation of varying amounts of granitic rocks in the Precambrian basement which underlies the entire Transantarctic Mountain chain.Mixtures of two components having different ⁸⁷Sr/⁸⁶Sr ratios and differing strontium concentrations are related to each other by hyperbolic mixing equation. Such an equation was fitted by least squares regression of data points to a straight line in coordinates of initial ⁸⁷Sr/⁸⁶Sr and the reciprocals of the concentrations of strontium. This equation and plots of strontium versus other oxides were then used to estimate the chemical composition of the parent basalt magma and of the granitic contaminant by substituting reasonable estimates of their ⁸⁷Sr/⁸⁶Sr ratios. The chemical composition of the parent basalt (⁸⁷Sr/⁸⁶Sr=0.706) is generally compatible with that of average continental tholeiite, but is distinctive by having a low concentration of strontium (117 ppm). The chemical composition of the contaminant (⁸⁷Sr/⁸⁶Sr=0.720) is enriched in strontium (173 ppm), SiO₂, FeO (total iron) and the alkalies but is depleted in Al₂O₃, MgO and CaO. The data for strontium indicate that the lava flows on Storm Peak contain between 20 and 40% of this granitic contaminant. The contamination of basalt magma is not a local event but is characteristic of the Jurassic basalt flows and diabase sills throughout the Transantarctic Mountains and in Tasmania.Laboratory for Isotope Geology and Geochemistry, Contribution No. 33.     

Geology,geochemistry and petrogenesis of post-collisional adakitic intrusions and related dikes in the Khoynarood area,NW Iran

The Khoynarood area is located in the northwest of Iran, lying at the northwestern end of the Urumieh–Dokhtar volcano-plutonic belt and being part of the Qaradagh–South Armenia domain. The main intrusive rocks outcropped in the area have compositions ranging from monzonite–quartz monzonite, through granodiorite, to diorite–hornblende diorite, accompanied by several dikes of diorite–quartz diorite and hornblende diorite compositions, which were geochemically studied in order to provide further data and evidence for the geodynamic setting of the region. The SiO₂, Al₂O₃ and MgO contents of these rocks are about 58.32–68.12%, 14.13–18.65% and 0.68–4.27%, respectively. They are characterized by the K₂O/Na₂O ratio of 0.26–0.58, Fe₂O₃ + MnO + MgO + TiO₂ content about 4.27–13.13%, low Y (8–17 ppm) and HREE (e.g., 1–2 ppm Yb) and high Sr contents (750–1330 ppm), as well as high ratios of Ba/La (13.51–50.96), (La/Yb)_N (7–22), Sr/Y (57.56–166.25), Rb/La (1.13–2.96) and La/Yb (10–33.63), which may testify to the adakitic nature of these intrusions. Their chemical composition corresponds to high-silica adakites, displaying enrichments of LREEs and LILEs and preferential depletion of HFSEs, (e.g., Ti, Ta and Nb). The REE differentiation pattern and the low HREE and Y contents might be resulted from the presence of garnet and amphibole in the solid residue of the source rock, while the high Sr content and the negative anomalies of Ti, Ta and Nb may indicate the absence of plagioclase and presence of Fe and Ti oxides in it. As a general scenario, it may be concluded that the adakitic rocks in the Khoynarood were most likely resulted from detachment of the subducting Neo-Tethyan eclogitic slab after subduction cessation between Arabian and Central Iranian plates during the upper Cretaceous–early Cenozoic and partial melting of the detached slab, followed by interactions with metasomatized mantle wedge peridotite and contamination with continental crust.     

Origin of Phenocrysts and Compositional Diversity in Pre-Mazama Rhyodacite Lavas, Crater Lake, Oregon

Phenocrysts in porphyritic volcanic rocks may originate in avariety of ways in addition to nucleation and growth in thematrix in which they are found. Porphyritic rhyodacite lavasthat underlie the eastern half of Mount Mazama, the High Cascadeandesite/dacite volcano that contains Crater Lake caldera, containevidence that bears on the general problem of phenocryst origin.Phenocrysts in these lavas apparently formed by crystallizationnear the margins of a magma chamber and were admixed into convectingmagma before eruption. About 20 km³ of pre-Mazama rhyodacite magma erupted during arelatively short period between400 and 500 ka; exposed pre-Mazamadacites are older and less voluminous. The rhyodacites formedas many as 40 lava domes and flows that can be assigned to threeeruptive groups on the basis of composition and phenocryst content.Phenocryst abundance decreases (from 32 to 8 vol.%) and SiO₂content increases (from 68 to 73 wt.%) in the apparent orderof eruption. Phenocrysts (plagioclase, orthopyroxene, augite,and Fe-Ti oxides) are commonly fragmental or form polycrystallineaggregates with interstitial glass. Discrete phenocrysts withcomplete euhedral outlines are rare except for small elongatedcrystals. The abundance of discrete phenocrysts increases withthat of aggregates. The grain-size of minerals in the aggregatescovers the range of discrete phenocrysts (0.2–4.2 mm).Rim compositions of phenocrysts and the range of chemical zoningare almost uniform among the three rhyodacite groups, regardlessof whether crystals are discrete or in aggregates. However,a small fraction of phenocrysts, especially small elongatedcrystals, have different compositions: plagioclase with Fe-richcores and augite with Wo-poor cores, both of which are characteristicof crystals in undercooled andesite enclaves in the rhyodacites.The majority of phenocrysts were derived by disintegration ofpolycrystalline aggregates; rare, small phenocrysts crystallizedin andesitic magma similar to that represented by the andesiteenclaves. The modal and chemical compositions of the rhyodacites can beexplained by different degrees of admixing of crystals, representedby the aggregates, into magma having 4 vol.% ‘true’phenocrysts, mainly plagioclase. The aggregates may be partsof the rind formed by in situ crystallization near the walland roof of the magma chamber. The rind was disrupted duringor just before eruption, and pieces were variably disaggregatedand incorporated into erupting magma. The amount of rind incorporateddeclined during the sequence of eruptions. Owing to vesiculationof interstitial liquid and shearing during flow, crystals inthe aggregates were separated and became phenocrysts. Pre-Mazamarhyodacite was erupted dominantly as lava, as opposed to thecompositionally similar rhyodacite pumice of the Holocene caldera-formingeruption of Mount Mazama, apparently because its source chamberwas crystallizing inward rather than actively growing.     

Geology and petrogenesis of the edgecumbe volcanic field,SE Alaska: The interaction of basalt and sialic crust

The Edgecumbe volcanic field is a Holocene volcanic province located on Kruzof Island, SE Alaska. Exposed within the 260 km² field are basalt, andesitic basalt, andesite, dacite and rhyodacite. The rhyodacites were erupted after the basalts and before the andesites. The volcanics, which are Al-rich (14–18 wt%) and lack an iron enrichment trend, range from tholeiites (47 wt% SiO₂) through rhyodacites (72%), but a compositional gap of approximately 9 wt% separates the dacites and rhyodacites. Initial ⁸⁷Sr/⁸⁶Sr ranges from 0.70297 in the basalts to 0.70440 in a pyroxene andesite. δ ¹⁸O increases across the suite: 5.8‰ to 7.9‰. Plagioclase (An_32–86) is the dominant phenocryst in all but one lava. Olivine (Fo_58–86) occurs in the basic lavas (<53 wt% SiO₂), but is replaced by orthopyroxene (En_43–73) and clinopyroxene (En₃₁Wo₄₁-En₄₈Wo₄₀) in the more siliceous volcanics. In the basalts and rhyodacites, plagioclase is weakly zoned, but extreme zoning (<30 mole% An) is characteristic of phenocrysts in the intermediate lavas. Fractionation of the observed phenocryst assemblages could not have produced the more silicous volcanics. Instead they were generated by partial melting of intrusive basement (⁸⁷Sr/⁸⁶Sr=0.70487; δ ¹⁸O: 8.7–9.3) by basaltic magma and subsequent assimilation. Mass balance calculations show the rhyodacites are almost pure partial melt (<5% basaltic component) whereas the intermediate lavas contain between 30 and 60% partial melt.     

赣南中侏罗世流纹岩地球化学及成因研究：上地壳成因的的微量元素和Pb-Nd-Sr同位素地球化学制约

江西南部龙南-寻邬地区余田群菖蒲组的流纹岩形成于中侏罗世(164．8．1Ma～174．9Ma)，为双峰式火山岩组合的酸性端元，属弱过铝质(ACNK平均为1．04)钙碱性系列(σ平均为1．90)。稀土元素含量高(平均为278．9μg/g)，轻稀土富集(IMH平均为7．94)，Eu亏损中等(δEu平均为0．41)。微量元素以富集Rb，Ba，Th，U，Zr，Y，Pb，而亏损Sr，Nb为特征。Pb，Nd，Sr同位素组成为：(^206Pb/^204Pb)t=17．89～18．58，(^207Pb/^204Pb)t=15．58～15．70，(^208Pb/^204Pb)t=37．94～38．82；εNd(t)=-7．44～-11．9；ISr=0．71126～0．71228。对赣南流纹岩微量元素采用蛛网图和模糊聚类分析方法进行对比研究的结果及Pb-Nd-Sr同位素相关特征为其上地壳成因提供了地球化学佐证。赣南流纹岩是由进入上地壳内高位岩浆房的石英拉斑玄武岩浆的底侵作用直接使上地壳部分熔融形成的产物。     

Origin,age and petrogenesis of Neoproterozoic composite dikes from the Arabian‐Nubian Shield,SW Jordan

The evolution of a Pan‐African (c. 900–550 Ma) suite of composite dikes, with latite margins and rhyolite interiors, from southwest Jordan is discussed. The dikes cut the Neoproterozoic calc‐alkaline granitoids and high‐grade metamorphic rocks (c. 800–600 Ma) of the northern Arabian‐Nubian Shield in Jordan and have been dated by the Rb‐Sr isochron method at 566±7 Ma. The symmetrically distributed latite margins constitute less than one‐quarter of the whole dike thickness. The rhyolite intruded a median fracture within the latite, while the latter was still hot but completely solidified. The dikes are alkaline and bimodal in composition with a gap in SiO₂ between 61 and 74 wt%. Both end members display similar chondrite‐normalized rare earth element patterns. The rhyolites display the compositional signature of A‐type granites. The (La/Lu)_N values are 6.02 and 4.91 for latites and rhyolites, respectively, and the rhyolites show a pronounced negative Eu anomaly, in contrast to the slight negative Eu anomaly of the latites. The chemical variability (e.g. Zr/Y, Zr/Nb, K/Rb) within and between latites and rhyolites does not support a fractional crystallization relationship between the felsic and mafic members of the dikes. We interpret the magma genesis of the composite dikes as the result of intrusion of mantle‐derived mafic magma into the lower crust in an extensional tectonic regime. The mafic magma underwent extensive fractional crystallization, which supplied the necessary heat for melting of the lower crust. The products of the initial stages of partial melting (5–10%) mixed with the fractionating mafic magma and gave rise to the latite melts. Further partial melting of the lower crust (up to 30%) produced a felsic melt, which upon 50% fractional crystallization (hornblende 15%, biotite 5%, feldspars 60%, and quartz 20%) gave rise to the rhyolitic magma. Copyright © 2004 John Wiley & Sons, Ltd.     

Oxygen fugacity and geochemical variations in the martian basalts: implications for martian basalt petrogenesis and the oxidation state of the upper mantle of Mars

The oxygen fugacity of the Dar al Gani 476 martian basalt is determined to be quartz-fayalite-magnetite (QFM) −2.3 ± 0.4 through analysis of olivine, low-Ca pyroxene, and Cr-spinel and is in good agreement with revised results from Fe-Ti oxides that yield QFM −2.5 ± 0.7. This estimate falls within the range of oxygen fugacity for the other martian basalts, QFM −3 to QFM −1. Oxygen fugacity in martian basalts correlates with ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and La/Yb ratios, indicating that the mantle source of the basalts is reduced and that assimilation of crust-like material controls the oxygen fugacity. This allows constraints to be placed on the oxidation state of the martian mantle and on the nature of assimilated crustal material. The assimilated material may be the product of early and extensive hydrothermal alteration of the martian crust, or it may be amphibole- or phlogopite-bearing basaltic rock within the crust. In either case, water may play a significant role in the oxidation of basaltic magmas on Mars, although it may be secondary to assimilation of ferric iron-rich material.     

土耳其中部阿哲格尔火山湖新生代玄武岩的地球化学与岩石成因

阿哲格尔火山湖玄武岩是位于卡帕多利亚火成岩省(CVP)东部的第四纪火山岩之一。与CVP中其他火山岩相比,玄武岩具有演化程度较低,成分上更接近原始岩浆的特征。玄武岩记录了两期深部岩浆活动:橄榄石捕掳晶Fo值高达89,尖晶石包裹体具有低Cr~#、高Al的特征,表明了早期岩浆源区可能为受俯冲板片流体交代的岩石圈地幔;除样品TCA02外,玄武岩ΣCe/ΣY比值为1.39~2.11,~(143)Nd/~(144)Nd比值为0.51287~0.51288,Th/Nb比值为0.15~0.16,具有大陆裂谷玄武岩的特征,反应了晚期的碱性岩浆起源于软流圈地幔。与CVP早-中更新世钙碱性岩浆相对比,晚期碱性岩浆的高场强元素(如Nb、Ta)具有弱的负异常,K、Ba等大离子亲石元素富集程度较低,揭示了岩浆在上升演化过程中存在富集交代的岩石圈地幔物质的加入。     

Boninitic and tholeiitic basaltic lavas and dikes from dispersed Jurassic East Othris ophiolitic units,Greece: petrogenesis and geodynamic implications

ABSTRACT

Pillow lavas, massive lava flows, and sub-volcanic dikes of tholeiitic basaltic composition are found to be members of the Vrinena, Aerino, Eretria, and Velestino dispersed Middle–Upper Jurassic ophiolitic units in East Othris. The Vrinena and Eretria ophiolitic units appear to have been emplaced onto the Pelagonian continental margin during the Upper Jurassic–Lower Cretaceous, whereas the Aerino and Velestino units seem to have been finally emplaced during post-Palaeocene times. Geochemically these are divided into two groups: Group I includes subduction-related boninites and low-Ti basalts from the Vrinena and Aerino units, and Group II high-Ti basalts show spreading-type characteristics occurring in the Eretria and Velestino units. Primary magma of the Group I volcanics appears to have been formed after high partial melting degrees (~18%) of a highly depleted harzburgitic mantle source, under relatively high temperatures (mantle potential temperature ~1372°C). Petrogenetic modelling also suggests that the primary magma of the Group II volcanics were formed after lower partial melting degrees (~7%) of a moderately depleted mantle source. The petrological and geochemical data from the East Othris dispersed and diversely emplaced ophiolitic units provide evidence of a common intra-oceanic supra-subduction zone (SSZ) origin within the Pindos oceanic strand of the Western Tethys. Specifically, Group I lavas and dikes from Vrinena seem to represent the extrusive part of an almost complete fore- to island-arc ophiolitic sequence. Dikes of Aerino most likely correspond to fore-arc magmatic material that intruded within exhumed serpentinized ultramafic rocks through a subduction channel that developed close to the slab and towards the fore-arc and the accretionary prism. The Group II volcanics either corresponded to a fore-arc magmatic expression, which extruded earlier than Group I volcanics and prior to the establishment of a mature subduction zone, or represent back-arc to island-arc magmatism that was contemporaneous to the fore-arc magmatic activity during rollback subduction.     

峨眉山大火成岩省东区普安玄武岩系年代学、地球化学及成因研究

广泛分布于中国西南川、滇、黔三省的峨眉山玄武岩是我国最早被国际认可的大火成岩省,受到了国内外学者的广泛关注.前人对大火成岩省西区玄武岩已达成多项共识,而对东区玄武岩的岩石组合、火山活动时限、岩石成因等方面还存在诸多争议.本文以峨眉山大火成岩省东区贵州普安玄武岩系为研究对象,通过解析典型剖面,明确该区玄武岩系岩石类型从底...     

西藏拉萨地体南缘汤白地区始新世辉绿岩脉——新特提斯洋壳断离的证据

拉萨地体南缘汤白地区广泛分布新生代的辉绿岩脉。为探讨该辉绿岩脉形成时代、岩石成因和地质意义,对其开展了详细的岩相学、地球化学、锆石U-Pb年代学及Hf同位素研究。LA-ICP-MS锆石U-Pb定年结果显示辉绿岩脉的结晶年龄为(54±1)Ma,表明其形成于早始新世。微量元素地球化学特征显示富集大离子亲石元素(LILEs:如Rb、Sr和Ba),亏损高场强元素(HFSEs:如Nb、Ta和Ti)。与典型的弧岩浆岩、区域上叶巴组和桑日岩群中的玄武岩相比具有较高的Nb、TiO_2和Zr含量,在微量元素构造环境判别图解中显示出板内玄武岩地球化学属性。微量元素地球化学特征结合锆石Hf同位素表明岩浆源区除被俯冲板片释放的流体交代的岩石圈富集地幔外,还有软流圈亏损地幔物质加入。汤白辉绿岩脉侵入年龄与区域上林子宗群火山活动峰期接近(52 Ma)。同时结合岩石成因及构造背景,作者认为汤白辉绿岩脉是54～52 Ma新特提斯洋壳断离诱发岩浆作用的产物。根据最新大陆碰撞及板片断离的三维数值模型,暗示了印度板块与欧亚大陆碰撞的起始时间为65 Ma或者更早。     

南秦岭铧厂沟碧口群玄武岩LA-ICP-MS锆石U-Pb年龄及岩石成因研究

铧厂沟地区碧口群玄武岩呈带状和透镜状分布于凝灰岩和凝灰质千枚岩中。在详细野外观察基础上,利用显微镜观察、主微量元素及放射性同位素分析等综合分析技术对玄武岩的岩石学、地球化学以及年代学特征进行了研究。结果表明:碧口群玄武岩普遍经历了绿片岩相的变质作用,主要矿物为斜长石、绿泥石和绿帘石;SiO_2(44.67%~49.76%)、TiO_2(1.14%~1.34%)含量较低,TFe_2O_3(12.03%~15.47%)、MgO(7.57%~9.3%)、CaO(7.29%~10.54%)含量较高。稀土总量较低,轻稀土亏损,重稀土富集。岩石微量元素和(?)石Hf同位素特征显示玄武岩形成环境可能类似N-MORB,起源于软流圈,并在上升过程中混入了古老的地壳物质。LA-ICP-MS锆石测年结果显示玄武岩的形成年龄为(316.3:±6.0)Ma(MSWD=0.78;n=7),表明碧口群火山岩系至少是两期(新元古代时期和晚古生代时期)火山作用的产物。其大规模形成时期为新元古代,后经晚古生代火山作用的叠加改造而成。     

华南长城岭晚白垩世斜斑玄武岩的岩浆作用过程与岩石成因制约

斜斑玄武岩能较好地记录岩浆作用过程、动力学机制和构造环境。本文对华南南岭中段晚白垩世长城岭斜斑玄武岩的斜长石环带结构进行了详细研究,以制约其岩浆作用过程和岩石成因机制。长城岭斜斑玄武岩富集大离子亲石元素,亏损Nb-Ta,具强烈Pb正异常和轻微Ti正异常;其全岩~(87)Sr/~(86)Sr较高且较均匀(0.7088~0.7089),较低(-0.95~-0.94),而斜长石斑晶原位~(87)Sr/~(86)Sr变化相对较大(0.706~0.710)。斜长石斑晶普遍显示反环带结构,从核部到边部的An组分以及MgO、FeO、TiO_2含量明显升高而K_2O含量降低;另外,反环带结构斜长石斑晶的核部An含量不均匀,但边部成分一致,指示岩浆补给作用。总体上,斜长石斑晶的边部相对较窄,指示补给岩浆在岩浆房中的驻留时间较短,补给岩浆的加入造成了玄武岩浆的快速喷发。斜长石斑晶的~(87)Sr/~(86)Sr比值与Sr含量呈负相关关系,表明岩浆房演化过程中存在地壳物质的同化混染作用。~(39)Ar-~(40)Ar定年结果显示,由基质斜长石获得的坪年龄(83.45±0.44Ma)略老于由斜长石斑晶获得的坪年龄(79.02±0.43Ma),为驻留时间较短的补给岩浆受地壳混染作用影响而具有偏低的初始~(39)Ar/~(40)Ar的结果。长城岭斜斑玄武岩在2Nb-Zr/4-Y图解中落在板内拉斑玄武岩区域内,并具有与板内拉斑玄武岩相似的斜长石An(60~70),应形成于板内拉张环境。华南地区晚白垩世基性岩浆活动,包括沿海地区的永泰玄武岩(~85Ma)和内陆地区的长城岭斜斑玄武岩(~79Ma)、衡阳玄武岩(~70Ma)及禾埠玄武岩(~63Ma)等,显示逐渐从岛弧玄武岩转变为板内碱性玄武岩的趋势,其Sr-Nd同位素也显示逐渐亏损的特征;它们的时空分布特征表明,华南内陆在晚白垩世期间经历了持续的拉张作用,可能与古太平洋板块后撤有关。