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1.
Formation of rhythmical layering in intrusive basic and ultrabasic rock bodies is explained in different ways, in particular, by the movement of microparticles (mineral clusters) of plagioclase and pyroxene within the basalt melt under the influence of thermal and gravitational forces. A model of cluster movement is proposed as a consequence of the forces appearing when ultrasonic elastic fluctuations pass through the melt. The model is based on fundamental dynamic equations. Depending on cluster density, wave parameters, and magmatic chamber size, in the melt there can form different combinations of rhythmically alternating layers of different composition. 相似文献
2.
Mechanisms of fractional crystallization with simultaneous crustalassimilation (AFC) are examined for the Kutsugata and Tanetomilavas, an alkali basaltdacite suite erupted sequentiallyfrom Rishiri Volcano, northern Japan. The major element variationswithin the suite can be explained by boundary layer fractionation;that is, mixing of a magma in the main part of the magma bodywith a fractionated interstitial melt transported from the mushyboundary layer at the floor. Systematic variations in SiO2 correlatewith variations in the Pb, Sr and Nd isotopic compositions ofthe lavas. The geochemical variations of the lavas are explainedby a constant and relatively low ratio of assimilated mass tocrystallized mass (r value). In the magma chamberin which the Kutsugata and Tanetomi magmas evolved, a strongthermal gradient was present and it is suggested that the marginalpart of the reservoir was completely solidified. The assimilantwas transported by crack flow from the partially fused floorcrust to the partially crystallized floor mush zone throughfractures in the solidified margin, formed mainly by thermalstresses resulting from cooling of the solidified margin andheating of the crust. The crustal melt was then mixed with thefractionated interstitial melt in the mushy zone, and the mixedmelt was further transported by compositional convection tothe main magma, causing its geochemical evolution to be characteristicof AFC. The volume flux of the assimilant from the crust tothe magma chamber is suggested to have decreased progressivelywith time (proportional to t1/2), and was about 3 x 102m/year at t = 10 years and 1 x 102 m/year at t = 100years. It has been commonly considered that the heat balancebetween magmas and the surrounding crust controls the couplingof assimilation and fractional crystallization processes (i.e.absolute value of r). However, it is inferred from this studythat the ratio of assimilated mass to crystallized mass canbe controlled by the transport process of the assimilant fromthe crust to magma chambers. KEY WORDS: assimilation and fractional crystallization; mass balance model; magma chamber; melt transport; Pb isotope 相似文献
3.
利用探地雷达观测分析早春融雪前后沙丘表层土壤含水量的时空分布 总被引:1,自引:0,他引:1
积雪融水是古尔班通古特沙漠春季植物发育的重要水源, 快速获取早春沙丘的土壤水分变化具有十分重要的生态学意义. 2010年3、 4月分别使用探地雷达进行了多次测量实验, 结果显示: 1)融雪初期, 沙丘顶部土壤的自由水含量最大, 阳坡次之, 且融雪水在重力作用下沿坡面侧向缓慢流动, 在坡底汇集, 主导了融雪初期乃至整个春季沙丘表层土壤水分的分布格局; 2)融雪后期, 由于阴坡积雪和冻土消融相对滞后, 表层土壤含水量略高于阳坡, 而沙丘顶部由于融雪最早且融雪期间水分转移最多而表面最为干燥; 3)通过与时域反射仪的同步测量结果对比, 探地雷达的测量精度被有效控制在0.03范围内, 且探地雷达提供的连续数据更有利于从细节上把握土壤含水量的变化趋势, 为中小尺度土壤水分的动态研究提供了一种科学、 有效的技术手段. 相似文献
4.
Dehydration-melting experiments were conducted on granulite and Al-gneiss,the two most representative rock-types of the khondalite series in the northern segment of the Helan Mountain.The critical melting fraction was determined to be 30% in volume,which is of great significance with respect to the P-T-t path of metamorphism and granite generation in the region. 相似文献
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《International Geology Review》2012,54(13):1641-1659
Eocene mafic volcanic rocks occurring in an E–W-trending, curvilinear belt along and north of the Izmir–Ankara–Erzincan suture zone (IAESZ) in northern Anatolia, Turkey, represent a discrete episode of magmatism following a series of early Cenozoic collisions between Eurasia and the Gondwana-derived microcontinents. Based on our new geochronological, geochemical, and isotope data from the Kartepe volcanic units in northwest Anatolia and the extant data in the literature, we evaluate the petrogenetic evolution, mantle melt sources, and possible causes of this Eocene volcanism. The Kartepe volcanic rocks and spatially associated dikes range from basalt and basaltic andesite to trachybasalt and basaltic trachyandesite in composition, and display calc-alkaline and transitional calc-alkaline to tholeiitic geochemical affinities. They are slightly to moderately enriched in large ion lithophile (LILE) and light rare earth elements (LREE) with respect to high-field strength elements (HFSE) and show negative Nb, Ta, and Ti anomalies reminiscent of subduction-influenced magmatic rocks. The analysed rocks have 87Sr/86Sr(i) values between 0.70570 and 0.70399, positive ?Nd values between 2.7 and 6.6, and Pb isotope ratios of 206Pb/204Pb(i) = 18.6–18.7, 207Pb/204Pb(i) = 15.6–15.7, and 208Pb/204Pb(i) = 38.7–39.1. The 40Ar/39Ar cooling ages of 52.7 ± 0.5 and 41.7 ± 0.3 Ma obtained from basaltic andesite and basalt samples indicate middle to late Eocene timing of this volcanic episode in northwest Anatolia. Calculated two-stage Nd depleted mantle model (TDM) ages of the Eocene mafic lavas range from 0.6 to 0.3 Ga, falling between the TDM ages of the K-enriched subcontinental lithospheric mantle of the Sakarya Continent (1.0–0.9 Ga) to the north, and the young depleted mantle beneath central Western Anatolia (0.4–0.25 Ga) to the south. These geochemical and isotopic features collectively point to the interaction of melts derived from a sublithospheric, MORB-like mantle and a subduction-metasomatized, subcontinental lithospheric mantle during the evolution of the Eocene mafic volcanism. We infer triggering of partial melting by asthenospheric upwelling beneath the suture zone in the absence of active subduction in the Northern Neotethys. The geochemical signature of the volcanic rocks changed from subduction- and collision-related to intra-plate affinities through time, indicating an increased asthenospheric melt input in the later stages of Eocene volcanism, accompanied by extensional deformation and rifting. 相似文献
8.
《International Geology Review》2012,54(4):405-423
ABSTRACTMetatexite and diatexite migmatites are widely distributed within the upper amphibolite and granulite-facies zones of the Higo low-P/high-T metamorphic terrane. Here we report Nd–Sr isotopic and whole rock composition data from an outcrop in the highest-grade part of the granulite-facies zone, in which diatexite occurs as a 3 m-thick layer between 2 m-thick layers of stromatic-structured metatexite within pelitic gneiss. The metatexite has Nd–Sr isotopes and whole rock compositions similar to those of the gneiss, but the diatexite shows the reverse. The diatexite has a higher εNd(t) and 147Sm/144Nd ratio (εNd(t) = ?0.5; 147Sm/144Nd = 0.1636) than the gneiss (εNd(t) = ?2.1; 147Sm/144Nd = 0.1287) and metatexite (εNd(t) = ?3.1; 147Sm/144Nd = 0.1188). The (87Sr/86Sr)initial and 87Rb/86Sr of the diatexite ((87Sr/86Sr)initial = 0.70568; 87Rb/86Sr = 0.416) are lower than those of the gneiss ((87Sr/86Sr)initial = 0.70857; 87Rb/86Sr = 1.13) and metatexite ((87Sr/86Sr)initial = 0.70792; 87Rb/86Sr = 1.11). The metatexite and gneiss show enrichment of Th and depletion of P and Eu and have a similar chondrite-normalized REE pattern, which shows steep LREE–MREE-enriched and gently declining HREE patterns and negative Eu anomalies, whereas the diatexite shows enrichment of Sr and depletion of Th and Y, and exhibits gently declining LREE and steeply declining HREE pattern and weak Eu depletion. The metatexite migmatite is interpreted to have formed by in situ partial melting in which the melt did not migrate from the source, whereas the diatexite migmatite included an externally derived melt with a juvenile component. The Cretaceous high-temperature metamorphism of the Higo metamorphic terrane is interpreted to reflect emplacement of mantle-derived basalts under a volcanic arc along the eastern margin of the Eurasian continent, and mass transfer and advection of heat via hybrid silicic melts from the lower crust. 相似文献
9.
《International Geology Review》2012,54(14):1768-1785
ABSTRACTWe analysed whole-rock major and trace elements and Sr-Nd-Pb-Hf isotopes of the late Cenozoic volcanic rocks in the Leizhou Peninsula, South China to investigate their mantle source characteristics. These volcanic rocks, collected from Jiujiang, Tianyang and Huoju areas of the Leizhou Peninsula, are characterized by incompatible element enrichment but variable isotopic depletion. The volcanic rocks from Jiujiang and Tianyang show prominent primitive-mantle-normalized positive Nb, Ta and Sr anomalies and depleted Sr-Nd-Pb-Hf isotope compositions, whereas those from Huoju show slight positive to negative Nb and Ta anomalies, a prominent positive Pb anomaly, and more enriched Sr-Nd-Pb-Hf isotope compositions. Two types of mantle metasomatism are required to explain the geochemical characteristics of these rocks. The Jiujiang and Tianyang samples were largely derived from a mantle source metasomatized recently by a low-F melt. Such low-F melt is generated within the asthenospheric mantle, which is enriched in volatiles and incompatible elements with positive Sr anomaly and depleted Sr-Nd-Pb-Hf isotope compositions. The Huoju samples were largely derived from a mantle source metasomatized by recycled upper continental crust material. These two types of mantle metasomatism beneath the Leizhou Peninsula are consistent with trace element characteristics of mantle mineralogy (e.g. clinopyroxene vs. amphibole), which reflects source evolution in space and time (e.g. tectonic setting change). 相似文献
10.
http://www.sciencedirect.com/science/article/pii/S1674987113000595 总被引:11,自引:0,他引:11
Mei-Fu Zhou Wei Terry Chen Christina Yan Wang Stephen A.Prevec Patricia Pingping Liu Geoffrey H.Howarth 《地学前缘(英文版)》2013,4(5):481-502
Magmatic oxide deposits in the~260 Ma Emeishan Large Igneous Province(ELIP),SW China and northern Vietnam,are important sources of Fe,Ti and V.Some giant magmatic Fe-Ti-V oxide deposits, such as the Panzhihua,Hongge,and Baima deposits,are well described in the literature and are hosted in layered mafic-ultramafic intrusions in the Panxi region,the central ELIP.The same type of ELIP- related deposits also occur far to the south and include the Anyi deposit,about 130 km south of Panzhihua,and the Mianhuadi deposit in the Red River fault zone.The Anyi deposit is relatively small but is similarly hosted in a layered mafic intrusion.The Mianhuadi deposit has a zircon U-Pb age of~260 Ma and is thus contemporaneous with the ELIP.This deposit was variably metamorphosed during the Indosinian orogeny and Red River faulting.Compositionally,magnetite of the Mianhuadi deposit contains smaller amounts of Ti and V than that of the other deposits,possibly attributable to the later metamorphism.The distribution of the oxide ore deposits is not related to the domal structure of the ELIP.One major feature of all the oxide deposits in the ELIP is the spatial association of oxide-bearing gabbroic intrusions,syenitic plutons and high-Ti flood basalts.Thus,we propose that magmas from a mantle plume were emplaced into a shallow magma chamber where they were evolved into a field of liquid immiscibility to form two silicate liquids,one with an extremely Fe-Ti-rich gabbroic composition and the other syenitic.An immiscible Fe-Ti-(P) oxide melt may then separate from the mafic magmas to form oxide deposits.The parental magmas from which these deposits formed were likely Fe-Ti-rich picritic in composition and were derived from enriched asthenospheric mantle at a greater depth than the magmas that produced sulfide-bearing intrusions of the ELIP. 相似文献