Metamorphic convergence of the upper and lower plates of the Vincent thrust, San Gabriel Mountains, southern California, USA

The Vincent thrust of the San Gabriel Mountains, southern California, separates eugeoclinal Pelona Schist from overlying Precambrian to Mesozoic igneous and metamorphic rocks of North American continental affinity. The thrust is generally considered to be synmetamorphic because of similarity in structural orientations and mineral assemblages between the Pelona Schist and mylonites at the base of the upper plate. In this study, compositions of calcic amphibole and plagioclase in the upper plate and structurally high Pelona Schist were compared to further test this interpretation. Amphibole in the schist is mostly actinolite to actinolitic hornblende with high Na/Al ratio, indicating relatively high-P/low-T metamorphism. Individual grains are zoned, with concentrations of both Na and Al decreasing from cores to rims. Premylonitic amphibole in the upper plate is hornblende, tschermakite and pargasite with compositions indicative of low- or medium-P metamorphism. During mylonitization, this amphibole was replaced by actinolite to actinolitic hornblende with a similar range of Na and Al as amphibole rims in the Pelona Schist, but with slightly lower Na/Al ratio. This is consistent with the decrease of Na/Al up-section previously noted within the Pelona Schist of this area, and is considered to be the result of an inverted thermal gradient during thrusting. Convergence of composition between schist and upper plate also occurs for K and Ti contents of amphibole and An content of plagioclase. These features provide strong evidence that mylonitization of the upper plate is closely related in space and time to metamorphism of the Pelona Schist and therefore that the Vincent thrust is a remnant of the primary fault along which the Pelona Schist and correlative units were subducted beneath North America. Nonetheless, very fine-scale differences in amphibole composition between the schist and upper plate may indicate that metamorphic re-equilibration could not quite keep pace with movement on the fault.     

Mineralogy and pressure–temperature–time path of Cretaceous granulite gneisses, south-eastern San Gabriel Mountains, southern California

Mid-Cretaceous granulite gneisses crop out in a narrow belt in the Cucamonga region of the south-eastern foothills of the San Gabriel Mountains, southern California. Interlayered mafic granulites and pelitic, carbonate, calc-silicate and quartzofeldspathic metasediments record hornblende granulite subfacies metamorphism at approximately 8 kbar and 700–800°C. Regional deformation and formation of banded gneisses ceased by c. 108 Ma. although mafic-intermediate magmatism and high-grade metamorphism continued locally as late as c. 88 Ma. Garnet zoning in metapelitic gneisses suggests that peak metamorphism was followed locally by a period of near-isobaric cooling, but this interpretation requires diachronous cooling of the granulite belt which cannot be demonstrated without detailed thermo-chronological data. It is more likely that the entire terrane remained at granulite facies P–T conditions until 88 Ma, followed by rapid uplift associated with juxtaposition against adjacent middle and upper crustal arc terranes. Uplift occurred between c. 88 and 78 Ma at rates of approximately 1–2 km Ma^-1. The geotectonic evolution of the Cucamonga granulites is similar to mid-Cretaceous high- P granulites in the Sierra Nevada and Salinian block of central California. Late Cretaceous uplift common to these granulites may provide an important tectonic link between dismembered Mesozoic batholithic terranes in the California Cordillera.     

Oxygen isotopic study of Late Mesozoic cooling of the Mount Barcroft area,central White Mountains,eastern California

The Middle Jurassic Barcroft mafic granodiorite and Late Cretaceous, ternary-minimum McAfee Creek Granite are important components of the igneous arc sited along the SW North American margin. Bulk-rock analyses of 11 samples of the metaluminous, I-type Barcroft comagmatic suite have an average δ¹⁸O value of 7.4±0.6‰ (all values±1σ). Four Barcroft specimens average ε_Nd=?3.6±1.8, ⁸⁷Sr/⁸⁶Sr=0.707±0.001. The pluton consists of petrochemically gradational, Ca-amphibole-rich gabbro/diorite, granodiorite, metadiorite, and rare alaskite–aplite; for most of the pluton, oxygen isotope exchange of quartz, feldspar(s), biotite, and Ca-amphibole accompanied local deuteric alteration. Eight specimens of slightly peraluminous granitic rocks of the muscovite-bearing McAfee Creek series have an average δ¹⁸O of 8.6±0.5‰. Four McAfee-type samples average ε_Nd=?7.8±1.7, ⁸⁷Sr/⁸⁶Sr=0.711±0.004. For both plutons, bulk-rock evidence of exchange with near-surface water is lacking, suggesting ~5–10 km cooling depths. Barcroft minerals exhibit regular oxygen isotopic partitioning from high to low δ¹⁸O in the sequence quartz>plagioclase>K-feldspar>>amphibole≥biotite. Along the SE margin of the pluton, quartz and biotite in Lower Cambrian quartzites are higher in δ¹⁸O, and show slightly larger fractionations than igneous analogues. Exchange with fluids derived from these heated, contact-metamorphosed country rocks increased bulk ¹⁸O/¹⁶O ratios of Barcroft border rocks (and constituent plagioclase+subsolidus tremolite–actinolite), especially of granitic dikes transecting the wall rocks. Oxygen isotope thermometry for seven Barcroft pluton quartz–amphibole and six quartz–biotite pairs indicate apparent subsolidus temperatures averaging 519±49 °C. Quartz–plagioclase pairs from two Barcroft granodiorites yield values of 519 and 515 °C. A quartz–biotite pair from a quartzite adjacent to the Barcroft pluton yields an apparent temperature of 511 °C, in agreement with estimates based on contact metamorphic parageneses. Except for its SE margin, Barcroft pluton silicates evidently exchanged oxygen isotopes under local deuteric conditions. Compatible with Ca-amphibole thermobarometric analyses, areal distributions for quartz–plagioclase, quartz–amphibole, and quartz–biotite pairs reveal that putative annealing temperatures are lowest in NE-trending axial portions of the Barcroft body, so it simply cooled inwards. Intrusion ~70 million years later by the McAfee Creek Granite had no discernable effect on δ¹⁸O values of Barcroft minerals and bulk rocks.     

Small scale heterogeneity of Phanerozoic lower crust: evidence from isotopic and geochemical systematics of mid-Cretaceous granulite gneisses,San Gabriel Mountains,southern California

An elongate belt of mid-Cretaceous, compositionally banded gneisses and granulites is exposed in Cucamonga terrane, in the southeastern foothills of the San Gabriel Mountains of southern California. Banded gneisses include mafic granulites of two geochemical types: type 1 rocks are similar to high Al arc basalts and andesites but have higher HFSE (high-field-strength-element) abundances and extremely variable LILE (largeion-lithophile-element) abundances, while type 2 rocks are relatively low in Al and similar to alkali rich MOR (midocean-ridge) or intraplate basalts. Intercalated with mafic granulites are paragneisses which include felsic granulites, aluminous gneisses, marble, and calc-silicate gneisses. Type 1 mafic granulites and calcic trondhjemitic pegmatites also oceur as cross-cutting, synmetamorphic dikes or small plutons. Small-scale heterogeneity of deep continental crust is indicated by the lithologic and isotopic diversity of intercalated ortho-and paragneisses exposed in Cucamonga terrane. Geochemical and isotopic data indicate that K, Rb, and U depletion and Sm/Nd fractionation were associated with biotite +/- muscovite dehydration reactions in type 1 mafic granulites and aluminous gneisses during high-grade metamorphism. Field relations and model initial isotopic ratios imply a wide range of protolith ages, ranging from Early Proterozoic to Phanerozoic.     

Geochemistry and petrogenesis of a peralkaline granite complex from the Midian Mountains, Saudi Arabia

A zoned intrusion with a biotite granodiorite core and arfvedsonite granite rim represents the source magma for an albitised granite plug near its eastern margin and radioactive siliceous veins along its western margin. A study of selected REE and trace elements of samples from this complex reveals that the albitised granite plug has at least a tenfold enrichment in Zr, Hf, Nb, Ta, Y, Th, U and Sr, and a greatly enhanced heavy/light REE ratio compared with the peralkaline granite. The siliceous veins have even stronger enrichment of these trace elements, but a heavy/light REE ratio and negative eu anomaly similar to the peralkaline granite. It is suggested that the veins were formed from acidic volatile activity and the plug from a combination of highly fractionated magma and co-existing alkaline volatile phase. The granodiorite core intrudes the peralkaline granite and has similar trace element geochemistry. The peralkaline granite is probably derived from the partial melting of the lower crust in the presence of halide-rich volatiles, and the granodiorite from further partial melting under volatile-free conditions.     

Geochemistry and petrogenesis of suprasubduction volcanic complexes of the Char shear zone,eastern Kazakhstan

The paper presents new petrographic, geochemical, and petrologic data from volcanic rocks of suprasubduction origin of the Char shear zone in eastern Kazakhstan. We discuss bulk rock composition (concentrations of major and trace elements), types of mantle sources and parameters of their melting, conditions of crystallization of mafic magma, and geodynamic settings of basalt eruption. According to the major element composition, the volcanic rocks are basalt, andesibasalt, and andesite of tholeiitic and transitional, from tholeiitic to calc-alkaline, series. They are characterized by low TiO₂ (0.85 wt.% on average) and crystallization trends in MgO–major elements plots. In terms of trace element composition, the volcanic rocks possess moderately LREE-enriched rare-earth element patterns and are characterized by negative Nb anomalies present on the multi-element spectra (Nb/La_pm = 0.14–0.47; Nb/Th_pm = 0.7–1.6). The distribution of rare-earth elements (La/Sm_n = 0.8–2.3, Gd/Yb_n = 0.7–1.9) and the results of geochemical modeling in the Nb–Yb system suggest high degrees of melting of a depleted mantle source at spinel facies depths. Fractional crystallization of clinopyroxene, plagioclase, and opaque minerals also affected the final composition of the volcanic rocks. Clinopyroxene monomineral thermometry calculations suggest that the melts crystallized within a range of 1020–1180 °C. We think that this volcanic complex formed at a western active margin of the Paleo-Asian Ocean.     

Geochemistry and Intrusive History of the Ashland Pluton, Klamath Mountains, California and Oregon

The Ashland pluton is a calc-alkaline plutonic complex thatintruded the western Paleozoic and Triassic belt of the KlamathMountains in late Middle Jurassic time. The pluton comprisesa series of compositionally distinct magma pulses. The oldestrocks are hornblende gabbro and two-pyroxene quartz gabbro withinitial ⁸⁷Sr/⁸⁶Sr = 0{dot}7044, ¹⁸O = 8{dot}7%, and REE patternswith chondrite normalized La/Lu = 7. These units were followedby a suite of tonalitic rocks (La_N/Lu_N = 7) and then by a suiteof K₂O- and P₂O₅ rocks of quartz monzodioritic affinity (La_N/Lu_N= 13–21; La_N/Sm_N = 2{dot}4–3{dot}) The quartz monzodioriticrocks were then intruded by biotite granodiorite and granitewith lower REE abundances but more fractionated LREE(La_N/Lu_N= 13–19; La_N/Sm_N = 4{dot}3–6 and they, in turn,were host to dikes and bosses of hornblende diorite. The latestintrusive activity consisted of aplitic and granitic dikes.Combined phase equilibria and mineral composition data, indicateemplacement conditions of approximately P_total = 2{dot}3kb,P_H2O between 1{dot}5 and 2{dot}2 kb, and f_O2 between the nickel-nickeloxide and hematite-magnetite buffers. Successive pulses of magma display increasing SiO₂ togetherwith increasing ¹⁸O and decreasing initial ⁸⁷Sr/⁸⁶Sr. The isotopicdata are consistent with either (1) combined fractional crystallizationof andesitic magma and concurrent assimilation of crustal materialcharacterized by low Sr₁ and high (¹⁸O or, more probably, (2)a series of partial melting events in which sources were successivelyless radiogenic but richer in ¹⁸O Each intrusive stage displaysevidence for some degree of crystal accumulation and/or fractionalcrystallization but neither process adequately accounts fortheir compositional differences. Consequently, each stage appearsto represent a distinct partial melting or assimilation event. The P₂O₅-rich nature of the quartz monzodiorite suite suggestsaccumulation of apatite. However, the suite contains abundantmafic microgranitoid enclaves and most apatite in the suiteis acicular. These observations suggest that magma mixing affectedthe compositional variation of the quartz monzodiorite suite.Mass balance calculations are consistent with a simple mixingprocess in which P₂O₅-rich alkalic basalt magma (representedby the mafic microgranitoid enclaves) was combined with a crystal-poorfelsic magma (represented by the tonalite suite), yielding aquartz monzodioritic magma that then underwent differentiationby crystal fractionation and accumulation.     

Geochemistry and petrogenesis of Proterozoic diabase in the southern Death Valley region of California

Diabase sills and dikes of Proterozoic age intrude crystalline basement and the overlying Crystal Spring Formation in the southern Death Valley region of California. Despite pervasive deuteric alteration, analyses of relict plagioclase (An_66-45), titaniferous augite, and ilmenite permit the calculation of initial crystallization temperatures of 1,165±25° C for plagioclase and 1,110±25° C for augite with an oxygen fugacity of 10^–11 atm. The early crystallization of plagioclase is consistent with the generally subophitic texture of the diabase.Geochemical arguments show that deuteric alteration has had little effect on the whole rock chemistry which has a mildly alkaline, transitional character. Mathematical models support the interpretation that chemical variations are principally the result of flow differentiation. Also, about 15 to 25% olivine or augite may have fractionated from the melt prior to intrusion. The mildly LREE enriched composition (normalized La/Yb=4.27 to 6.13) is best modelled by a 5% melt fraction from a moderately LREE-enriched source. Melt derived from a model garnet peridotite with accessory amphibole is compatible with the observed major and trace element chemistry. The diabase is compositionally akin to other middle Proterozoic basaltic rocks in North America which formed in extensional settings, and it is interpreted as a manifestation of widespread extensional tectonism.     

Geochemistry and petrogenesis of the tertiary alkaline volcanic suite of the Labe tectonovolcanic zone,Czech Republic

Summary The Tertiary alkaline, nepheline-normative olivine melanephelinite-basanite-phonolite suite in the Labe tectono-volcanic zone, Czech Republic, displays mineralogical and geochemical features indicative of the presence of products of more than one fractionation series and of more than one parental magma. The Mg- and Ni-rich olivine melanephelinites (ankaratrites), which contain disintegrated lherzolite xenoliths, represent low degree (c. 4%) partial melts of a highly enriched mande source. Many of the abundant basanitic rocks also represent primary or near-primary basaltic melts subject to a small but variable degree Fof olivine fractionation. Other basanites with lower mg-numbers and Mg and Ni contents had a crystallization history involving hornblende and phlogopite.Melaphonolites (tephriphonolites) provide a link between basaltic rocks and phonolites with plagioclase fractionation progressively playing a more significant role. The phonolites represent products of crystallization of highly evolved melts but poor correlation between SiO₂, MgO, CaO, Na₂O and degree of undersaturation as well as between major and trace element contents indicate that a single line of descent cannot have been in operation: the products of different parental magmas showing a wide compositional range that evolved under different conditions of fractionation are represented. A very advanced stage of magma evolution, characterized by the dominance of alkali feldspar fractionation is represented by phonolites that contain the highest Hf, Nb, Rb, Ta, Th, Y, Zr and REE in the whole Tertiary volcanic suite.

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Geochemie und Petrogenese der tertiären alkalischen Vulkanit-Serie der Labe tektonovulkanischen Zone, Tschechische Republik

Zusammenfassung Die tertiäre alkalische Nephelin-normative Olivin Melanephelinit-Basanit-Phonolit Serie in der Labe tektono-vulkanischen Zone, Tschechische Republik, zeigt mineralogische und geochemische Eigenschaften, die auf die Anwesenheit von Produkten von mehr als einer Fraktionierungsserie und von mehr als einem Ausgangsmagma hinweisen. Die Mg- und Ni-reichen Olivin Melanephelinite (Ankaratrite), die umgewandelte LherzolitXenolite enthalten, stellen einen niedrigen Grad (ca. 4%) teilweiser Aufschmelzung einer stark angereicherten Mantelquelle dar. Viele der verbreiteten basanitischen Gesteine stellen auch primäre oder fast-primäre basaltische Schmelzen dar, die in kleinem, aber schwankendem Ausmaß von Olivinfraktionierung betroffen wurden. Andere Basanite mit niedrigen Mg-Zahlen und Mg- und Ni-Gehalten hatten eine Kristallisationsgeschichte in der Hornblende und Phlogopit eine Rolle spielte.Melaphonolite (Tephriphonolite) stellen ein Verbindungsglied zwischen basaltischen Gesteinen und Phonoliten dar, wobei Plagioklas-Fraktionierung eine zunehmend größere Rolle spielt. Phonolite sind Produkte der Kristallisation von Schmelzen, aber die schlechte Korrelation zwischen SiO₂, MgO, CaO, Na₂O und dem Grad der Untersättigung, wie auch zwischen Haupt- und Spurenelementen weist darauf hin, daß eine einzelne Entwicklungslinie nicht in Frage kommt. Die Produkte verschiedener Ausgangsmagmen zeigen ein weites Spektrum verschiedener Zusammensetzung, das sich unter verschiedenen Bedingungen der Fraktionierung entwickelt hat. Ein sehr fortgeschrittenes Stadium der Magmaentwicklung, charakterisiert durch die Vorherrschaft von Alkali-Feldspat-Fraktionierung wird durch Phonolite dargestellt, die die höchsten Gehalte an Hf, Nb, Rb, Ta, Th, Y, Zr und REE in der gesamten tertiären vulkanischen Abfolge führen.

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With 7 Figures     

新疆东天山白山钼矿深部岩体地球化学特征及成因意义

白山钼矿深部钻探表明矿体下部存在着矿化花岗斑岩体,所获岩芯显示其岩性主要为似斑状钾长花岗岩、黑云母斜长花岗岩和花岗斑岩.该岩体具有高Al、Na和Sr,低Mg、Y和Yb,以及高Sr/Y和La/Yb比值等特点,类似于埃达克质岩的地球化学特征,暗示来源于较深的含石榴子石的源区;岩石形成时代和地球化学构造判别图解表明,岩体形成于造山后的构造环境.因此,白山钼矿岩体可能为加厚下地壳熔融的产物.白山钼矿床的成矿作用可能与埃达克质岩浆活动有关,其成矿过程得益于高Mo丰度的地壳源区、成岩过程中提供流体和岩浆高氧逸度环境等.     

湘中印支期关帝庙岩体地球化学特征及成因

关帝庙岩体位于湘中盆地,据年龄资料可分为中三叠世末和晚三叠世中期两个形成时期。中三叠世花岗岩中发育闪长质暗色微粒包体,自早至晚依次为细中粒角闪石黑云母花岗闪长岩、细中粒斑状角闪石黑云母二长花岗岩、细-细中粒(斑状)黑云母二长花岗岩。晚三叠世花岗岩自早至晚依次为细中粒-粗中粒斑状二云母二长花岗岩、细粒二云母二长花岗岩。岩石高硅、富铝、高钾,(Na₂O+K₂O)含量为6.80%~8.87%,平均7.74%;K₂O/Na₂O比值在1.35~2.66之间,平均为1.58;ASI值为0.99~1.40。总体属镁质、高钾钙碱性系列过铝质花岗岩类。中三叠世花岗岩Ba、Nb、Sr、P、Ti表现为明显亏损,Rb、(Th+U+K)、(La+Ce)、Nd、(Zr+Hf+Sm)、(Y+Yb+Lu)等则相对富集。中、晚三叠世花岗岩ΣREE含量为121.60~197.56μg/g,平均为158.70μg/g;δEu值0.28~0.68,平均为0.53;(La/Yb)_N值为5.94~17.53,平均13.80。中三叠世花岗岩I_Sr值为0.71302~0.71758,εSr(t)值为121~186,εNd(t)值为-9.95~-8.74,t_2DM为1.72~1.82Ga。C/MF-A/MF图解显示源岩主要为碎屑岩、少量基性岩和泥质岩。地质地球化学特征表明,印支期关帝庙花岗岩属S型花岗岩,形成于碰撞-后碰撞构造环境。     

Geochemistry and petrogenesis of the Early Cretaceous continental rift-type volcanic rocks of the Mecsek Mountains, South Hungary

Early Cretaceous volcanic rocks (basanite to phonolite) from the Mecsek Mountains (South Hungary) represent the products of Late Mesozoic extension-related alkaline magmatism at the southern margin of the European plate. Two mafic groups have been distinguished: ankaramite-alkali basalt and Na-basanite-phonotephrite. Phonolites could have been formed from the Na-basanitic magma by low-pressure fractionation. The major and trace element characteristics of the Mecsek basalts are similar to those of alkaline basalts of other intraplate areas and have a St. Helena-type OIB affinity. The mantle source of the Mecsek volcanics could be similar to that proposed by Wilson and Downes (1991) as one of the mantle endmembers for extension-related Tertiary-Quaternary alkaline basalts in Europe. Geochemical modelling indicates that the primary magmas of the Na-basanite series were formed by about 4% partial melting, whereas ankaramites and alkali basalts originated by about 6% partial melting of a garnet-peridotite source.     

西藏中拉萨地块晚侏罗世许如错花岗岩地球化学与岩石成因

西藏中部拉萨地块晚侏罗世花岗岩的岩石成因与源区性质目前尚未得到很好约束。本文报道了中部拉萨地块西段许如错岩体的寄主花岗岩和闪长质包体的锆石U-Pb年龄、元素地球化学和锆石Hf同位素成分。许如错岩体寄主花岗岩年龄为155.1±0.7Ma,闪长质包体与寄主花岗岩同期(155.7±0.7Ma)形成。寄主花岗岩属I型偏铝质-弱过铝质高钾钙碱性系列岩石,富集Rb、K等大离子亲石元素和轻稀土元素,亏损Nb、Ta、P、Ti等高场强元素,锆石εHf(t)值(-16.6~-6.6)指示其可能来源于拉萨地块古老下地壳物质的重熔作用。闪长质包体为准铝质钙碱性系列,锆石εHf(t)值(-8.9~-3.8)具有总体为负但明显高于寄主花岗岩εHf(t)值的特征,表明这些闪长质包体代表了幔源物质组成的加入。许如错晚侏罗世花岗岩可能形成于班公湖-怒江特提斯洋洋壳在中晚侏罗世向南俯冲于拉萨地块之下,引起幔源物质与中部拉萨地块古老基底重熔所产生的酸性岩浆发生岩浆混合作用形成的。     

Sanukite（赞岐岩）的地球化学特征、成因及其地球动力学意义

赞岐岩 (サヌカイト ,sanukite)是指发现于日本四国北部的一种富Mg的火山岩 ,主要产于日本中新世 (11～14Ma)Setouchi火山岩带 ,是一种黑色玻璃质的火山岩。其化学成分以富Si质 (安山英安质 )、具很高的Mg# 值 (>0 .6 )、高的Cr、Ni丰度和K/Na值 (0 .33～ 0 .5 2 )为特征。赞岐岩的形成与菲律宾海板块年轻的热的岩石圈俯冲和四国盆地的张开有关 ,产于岛弧的弧前或弧后盆地环境。赞岐岩不只代表火山岩 ,也包括侵入岩。Shirey和Hanson(1984 )将该术语引入太古宙 ,将太古宙具上述赞岐岩特征 (Si过饱和、Mg# 高和Ni、Cr、LILE含量高 )的深成岩和火山岩称为sanukite岩套。赞岐岩与埃达克岩具有大体类似的地球化学特征 ,但前者更富Mg、Cr和Ni,表明赞岐岩可以直接由地幔岩部分熔融形成 ,而埃达克岩只能由玄武岩部分熔融形成。现代的赞岐岩很少见 ,而太古宙的赞岐岩比较常见 ,暗示太古宙导致板片熔融的消减的岩石圈本身或上地幔可能具有更高的温度。赞岐岩集中出现在晚太古代 (<3.0Ga) ,表明板块消减作用可能在 3.0Ga之后才成为一个重要的过程 ,晚太古代赞岐岩的出现可能标志着现代类型板块构造的开始     

特提斯喜马拉雅带中段桑秀组玄武岩的地球化学和岩石成因

桑秀组玄武岩仅仅分布在特提斯喜马拉雅带中段东部,古地理属大印度北缘.分别采用 XRF、 ICP MS和全岩同位素稀释法对这些玄武岩的主元素、微量元素和 Sr Nd同位素进行了系统分析,并用来推论其成因.结果显示,桑秀组玄武岩除了 MgO含量和相容元素含量较低表明其为演化岩浆以外,其高 TiO2、 TFeO和 P2O5含量 (分别平均为 3.46%、 10.90%和 0.51% )、轻重稀土明显分馏 [(La/Yb)N=8.4～ 10.2]、 Ti/V、 Ti/Y和 Zr/Y比值高以及富集 Ba和 Th等不相容元素和高场强元素等特征与洋岛玄武岩 (OIB)相似;桑秀组玄武岩高 Sr [(87Sr/86Sr)t=0.707 370～ 0.709 904]和较低ε Nd(t)(=- 1.71～ 2.00)同位素组成以及微量元素地球化学指标等又显示了岩石圈地幔物质的印记;微量元素特征显示桑秀组玄武岩为大陆边缘裂谷背景下的碱性玄武岩,在源区物质低度部分熔融过程中不断有橄榄石等铁镁矿物的分离结晶,没有遭受地壳混染;桑秀组玄武岩的地球化学特征和同位素组成可与印度东缘的 Rajmahal玄武岩、印度洋 90° E海岭玄武岩和 Kerguelen OIB对比,提出桑秀组玄武岩可能是岩石圈地幔与地幔热柱或热点物质相互作用的产物,这种地幔热柱或热点可能与 Kerguelen热点的早期活动有联系.     

Geochemistry and petrogenesis of the Laramie anorthosite complex,Wyoming

A geochemical investigation of the Laramie anorthosite complex determined that monsonite associated with the complex are characterized by positive Eu anomalies and display a regular variation in composition with distance from the monzonite/county rock contact. Anorthositic rocks have major and trace element abundance typical of similar complexes. The internal variations in the monzonite were produced by in situ fractionation and contamination. The data indicate that anorthosite and monzonite cannot be comagmatic. It is proposed that the anorthosite and monzonite of the complex evolved from two distinct magmas, and that two stages of anatectic melting contributed to the evolution of the monzonite. An initial stage of partial melting was induced by intrusion of a gabbroic anorthosite magma into the lower crust; a second partial melting event occurred after emplacement where heat from the intrusions melted country rocks resulting in extensive contamination ofthe monzonite.     

Geochemistry and petrogenesis of the mandidarla volcanics,northwestern Himalayas

The Proterozoic Mandi-Darla Volcanics (MDV) occur as flows intercalated with the low-grade metasediments of the Sundarnagar Group of the Lesser Himalayas. These volcanics are aphyric and have quenched textures with plagioclase-clinopyroxene skeletal microphenocrysts. They are of tholeiitic composition, and are enriched in FeO^t, TiO₂, incompatible trace and rare earth elements. They have relatively high SiO₂ abundances for their MgO levels, which are attributed to high PH₂O pertaining during melt generation. Fractional crystallisation does not appear to have played any major role in the evolution of their bulk chemistry. Instead, they appear to have been generated by different extents of isobaric partial melting of a non-pyrolitic and heterogeneous source with enriched and variable Fe/Mg ratios. REE modelling suggests that the REE enriched source consisted of variable proportions of at least two end members differing in their MREE and HREE contents, but with similar LREE abundances. The enrichment of the source is attributed to mantle metasomatism by a melt-dominated phase penecontemporaneous with magma generation.     

Geochemistry,mineralogy and petrogenesis of the northeast Niğde volcanics,central Anatolia,Turkey

The volcanics exposed in the northeast Niğde area are characterized by pumiceous pyroclastic rocks present as ash flows and fall deposits and by compositions ranging from dacite to rhyolite. Xenoliths found in the volcanics are basaltic andesite, andesite and dacite in composition. These rocks exhibit linear chemical variations between end‐member compositions and a continuity of trace element behaviour exists through the basaltic andesite–andesite–dacite–rhyolite compositional range. This is consistent with the fractionation of ferromagnesian minerals and plagioclase from a basaltic andesite or andesite parent. These rocks are peraluminous and show typical high‐K calc‐alkaline differentiation trends with total iron content decreasing progressively with increasing silica content. Bulk rock and mineral compositional trends and petrographic data suggest that crustal material was added to the magmas by subducted oceanic crust and is a likely contaminant of the source zone of the Niğde magmas. The chemical variations in these volcanics indicate that crystal liquid fractionation has been a dominant process in controlling the chemistry of the northeast Niğde volcanics. It is also clear, from the petrographic and chemical features, that magma mixing with disequilibrium played a significant role in the evolution of the Niğde volcanic rocks. This is shown by normal and reverse zoning in plagioclase and resorption of most of the observed minerals. The xenoliths found in the Niğde volcanics represent the deeper part of the magma reservoir which equilibrated at the higher pressures. Copyright © 2002 John Wiley & Sons, Ltd.     

Eolian deposits in the Neoproterozoic Big Bear Group,San Bernardino Mountains,California, USA

Strata interpreted to be eolian are recognized in the Neoproterozoic Big Bear Group in the San Bernardino Mountains of southern California, USA. The strata consist of medium- to large-scale (30 cm to > 6 m) cross-stratified quartzite considered to be eolian dune deposits and interstratified thinly laminated quartzite that are problematically interpreted as either eolian translatent climbing ripple laminae, or as tidal-flat deposits. High index ripples and adhesion structures considered to be eolian are associated with the thinly laminated and cross-stratified strata. The eolian strata are in a succession that is characterized by flaser bedding, aqueous ripple marks, mudcracks, and interstratified small-scale cross-strata that are suggestive of a tidal environment containing local fluvial deposits. The eolian strata may have formed in a near-shore environment inland of a tidal flat.The Neoproterozoic Big Bear Group is unusual in the western United States and may represent a remnant of strata that were originally more widespread and part of the hypothetical Neoproterozoic supercontinent of Rodinia. The Big Bear Group perhaps is preserved only in blocks that were downdropped along Neoproterozoic extensional faults. The eolian deposits of the Big Bear Group may have been deposited during arid conditions that preceded worldwide glacial events in the late Neoproterozoic. Possibly similar pre-glacial arid events are recognized in northern Mexico, northeast Washington, Australia, and northwest Canada.