Apatite compositions from oceanic cumulates with implications for the evolution of mid-ocean ridge magmatic systems

Apatite distributions and compositions from cumulates from the EPR at Hess Deep, the MAR at the Kane Transform, and the SWIR at ODP Hole 735B were determined to assess the variability at each setting and to evaluate the potential utility of apatite in understanding the evolution of the lower ocean crust. Apatite in cumulates with low P₂O₅ contents are heterogeneously distributed along linear arrays or in tight clusters suggesting they crystallized from planar or pipe-like channels of evolved liquid. Most of the variation in the apatite composition is in the halogen site. The X_Ap^F and X_Ap^Cl are inversely correlated defining trends at near constant X_Ap^OH with EPR X_Ap^OH=0.25, MAR X_Ap^OH=0.45, and SWIR X_Ap^OH=0.65. These trends are defined both by sample averages and by the range of individual analyses in samples with large ranges in their F/Cl ratio. These trends are interpreted to reflect the reequilibration of apatite from variably evolved liquids/fluids that moved through the crystal mush. Comparison of the F–Cl–OH contents of apatite with the F–Cl–OH contents of glasses recovered from the same ridges show approximate correspondence: apatite and glasses from the EPR have the highest halogen contents while apatite and glasses from the MAR and SWIR have more OH. Some of the higher Cl contents in the apatite are interpreted to be produced by degassing of the cumulus pile while others reflect the assimilation of a seawater component. We suggest that systematic analysis of apatite from oceanic cumulates might allow cumulates that crystallized at shallow depths and assimilated seawater-derived components to be distinguished from those that crystallized below the level of seawater interaction.     

Thermal models for the magmatic accretion and subsequent metamorphism of continental crust

I present one-dimensional conductive relaxation models for the thermal evolution of continental crust which has undergone magmatic thickening in a predominantly recumbent tectonic regime. The sustained addition of magma, and associated heat, profoundly influences the conductive relaxation of the affected sialic crust. The mechanism of accretion, successively beneath (under-accretion), or successively above (over-accretion), previously added material, strongly affects the type of P-T-time history sustained by the rocks in the middle and deep crust. The pressure and temperature conditions which might be recorded by mineral assemblages equilibrating in such tectonothermal regimes may bear very little relation to conductive heat supplies from the upper mantle. P-T values obtained by geobarometry and geothermometry in such complexes must be used with caution to constrain steady state heat flows and geothermal gradients. In a contemporary plate tectonic context I consider the tectonothermal models developed here to apply particularly to island arcs and sites of continental collision associated with extensive plutonic igneous activity. Precambrian orthogneiss complexes, such as the Archaean of southern West Greenland, probably represent exhumed deep crustal elements involved in the intense magmatic thickening processes considered here.     

Regionalized models for the thermal evolution of the Earth

Traditional models for the heat loss in oceanic and continental regions are combined into a regionalized model for the thermal evolution of the Earth. The need for regionalization is obvious when one considers that the mantle loses 3 to 4 times as much heat per unit area in oceanic regions than in continental areas. The present-day rate of heat loss together with a geochemical estimate of the concentration of heat-producing elements in the Earth fixes the response time of the thermally convecting mantle. The response time in turn can be used to select the most reasonable representation for mantle convection in terms of the sensitivity of viscosity on temperature and layering versus mantle-wide circulation. Present geochemical estimates of the bulk composition of the Earth are most easily reconciled with the observed heat flow if the mantle is layered and its rheology is slightly less temperature dependent than generally assumed. The layered system can produce sufficiently high temperatures to explain the high-magnesian komatiites of the Archean. One difficulty with the models is that they predict widespread melting at shallow depth in the early stages of Earth history but do not address how such melting affects and alters the heat transfer mechanisms.     

On the structure and seismotectonics of the Kuril arc-trench system

Based on the new regional catalog of focal mechanisms for 396 strong (M ≥ 6.0) earthquakes in the Kuril-Okhotsk region and Japanese one in part for the period of 1964–2009 and with the data of seismic continuous profiling and multichannel common-depth point sounding by reflection method used, the peculiarities of the structure and seismotectonics of the Kuril arc-trench system have been analyzed. Additionally, the features of the Benioff and Tarakanov opposite seismofocal zones, associated with the Kuril arc-trench system, are studied. It is shown that the Benioff zone is a deep thrust through which the Kuril Arc (or Eurasian tectonic forefront) has thrust on the Pacific Plate by up to 50–70 km for the last 0.5–1.0 Ma (Pasadenian global phase of folding and orogeny). The thrusting process formed the middle and lower parts of the Pacific Ocean slope, the tectonic couple of Pegasus regional nappe and accretionary prism, the ramp structure of the Kuril Trench, and probably the opposite seismofocal zones.     

Enriched back-arc basin basalts from the northern Mariana Trough: implications for the magmatic evolution of back-arc basins

The composition of basalts erupted at the earliest stages in the evolution of a back-arc basin permit unique insights into the composition and structure of the sub-arc mantle. We report major and trace element chemical data and O-, Sr-, Nd-, and Pb- isotopic analyses for basalts recovered from four dredge hauls and one ALVIN dive in the northern Mariana Trough near 22°N. The petrography and major element chemistry of these basalts (MTB-22) are similar to tholeiites from the widest part of the Trough, near 18°N (MTB-18), except that MTB-22 have slightly more K₂O and slightly less TiO₂. The trace element data exhibit a very strong arc signature in MTB-22, including elevated K, Rb, Sr, Ba, and LREE contents; relatively lowK/Ba and highBa/La andSr/Nd. The Sr- and Nd- isotopic data plot in a field displaced from that of MTB-18 towards Mariana arc lavas, and the Pb-isotopic composition of MTB-22 is indistinguishable from Mariana arc lavas and much more homogeneous than MTB-18. Mixing of 50–90% Mariana arc component with a MORB component is hypothesized. We cannot determine whether this resulted from physical mixing of arc mantle and MORB mantle, or whether the arc component is introduced by metasomatism of MORB-like mantle by fluids released from the subducted lithosphere. The strong arc signature in back-arc melts from the Mariana Trough at 22°N, where the back-arc basin is narrow, supports general models for back-arc basin evolution whereby early back-arc basin basalts have a strong arc component which diminishes in importance relative to MORB as the back-arc basin widens.     

塔里木盆地石炭-二叠纪异常热演化及其对深部构造-岩浆活动的指示

本文分析了塔里木盆地大量实测镜质体反射率（Ro）数据,显示盆地西北部石炭系与二叠系之间Ro值演化不连续,记录了石炭-二叠纪间发生的构造-热事件,是地层抬升剥蚀和高温热事件共同作用的结果.热史模拟得出盆地在石炭纪末地温梯度开始升高,至～300 Ma达到峰值,分布在4.8～5.6℃/100 m之间,早二叠世迅速降低,随后进入缓慢稳定降低阶段.区域上高温热效应空间分布与塔里木大火成岩省的分布范围存在较好的相关性,但早于大规模玄武岩喷发的时间（290-288 Ma）,且后者热效应范围有限.因此推断这种高温异常是大规模的深部岩浆活动,即深部岩浆房相对长时间热烘烤的结果.该研究结果为塔里木盆地热演化机制及相关热效应提供了新的线索.     

Fractal approach of the temporal earthquake distribution in the Hellenic arc-trench system

The time clustering of earthquakes occurring in the Hellenic arc-trench system is quantitatively analyzed by means of the fractal dimension,D, of their time distribution in the time intervals of 1950–1985 (M _s >-4.5) and 1964–1985 (M _s 4.0). The results obtained imply that scale-invariant clustering holds over very large scale lengths of time,T, with 2²–2⁸T (in min) 2²⁰–2²², depending on the seismotectonic segment considered. In all segments a common feature is the relation between theD ₁,D ₂ andD ₃-values found for shallow, intermediate-depth and all-depth shocks, respectively:D ₃>D₁>D₂. TheD-values found for shallow shocks range between 0.137 and 0.191 with the exception of the Ionian Islands and Cretan segments where anomalously high values (D=0.221–0.251) have been determined. We discuss possible seismotectonic interpretations of the results.     

The magmatic evolution of the Seiland province,and Caledonian plate boundaries in northern Norway

The synorogenic intrusive activity of the eastern part of the Seiland province evolved from tholeiitic basalt low in K and Ti, through high-K calc-alkaline magmas and possible transitional basalt, to alkaline olivine basalt and picrite; finally, highly differentiated alkaline magmas and carbonatites were emplaced. For intrusive rocks with equivalent SiO₂ contents, K₂O, K₂O + Na₂O and K₂O/Na₂O increased with time, and the degree of iron enrichment in basaltic suites dimished. The western part of the province shows no equivalent evolution, tholeiitic magmas being emplaced at the same time as calc-alkaline magmas to the east.The magmatism is believed to have stemmed from a diapiric complex established in the mantle above a Benioff zone dipping to the east beneath the deforming Andean-type margin of the Baltic plate. Tectonic shortening of the continental edge and rearward movement of the underthrust plate relative to the asthenosphere resulted in migration of the plate junction, steepening of the seismic zone, and increasing depth to the magmagenetic region.     

Permian-Triassic magmatic evolution of granitoids from the southeastern Central Asian Orogenic Belt: Implications for accretion leading to collision

Many orogenic belts in the world exhibit accretionary and collisional orogenic phases to varying extents. How accretion evolves into collision of the Central Asian Orogenic Belt(CAOB), the largest Phanerozoic accretionary orogenic belt,is an intriguing question. In this paper, we present new U-Pb age, geochemical and isotopic data for Permian-Triassic granitoids from middle Inner Mongolia, Northern China in the southeastern CAOB, and delineate the magmatic transition from subduction to(soft) collision. The magmatic record of soft collision is identified and characterized by thickened lower crust-derived high Sr/Y granitoids with a sub-linear distribution along the Solonker suture zone. Granitoids from Early Permian to Late Permian became more enriched in whole-rock Nd and zircon Hf isotopic compositions(ε_(Nd)(t) values from 2.4 to-19.5, ε_(Hf)(t) values from 11.6 to-33.7), indicating increasing incorporation of old crust. The change in peak timing of magmatism from west(ca. 264 Ma)to east(ca. 251 Ma) along the Solonker suture zone implies "scissor-like" closure of the Paleo-Asian Ocean. Integrated with previous studies, a three-stage tectonic model from the Permian to Triassic by accretion leading to collision on the south-eastern margin of CAOB is proposed.(1) Early Permian( ca. 285 Ma): Juvenile magmatism on an active continental margin with double-sided subduction of the Paleo-Asian Ocean;(2) Middle Permian to Middle Triassic(ca. 285–235 Ma): Magma source transition from juvenile to old crust induced by a tectonic switch from arc to "scissor-like" closure and subsequent intracontinental orogenic contraction;(3) Late Triassic( ca. 235 Ma): A-type and alkaline magmatism in response to post-collisional extension.     

Tectono-magmatic evolution of the 1.9-Ga great bear magmatic zone, Wopmay orogen, northwestern Canada

The 1875-1840-Ma Great Bear magmatic zone is a 100-km wide by at least 900-km-long belt of predominantly subgreenschist facies volcanic and plutonic rocks that unconformably overlie and intrude an older sialic basement complex. The basement complex comprises older arc and back-arc rocks metamorphosed and deformed during the Calderian orogeny, 5–15 Ma before the onset of Great Bear magmatism. The Great Bear magmatic zone contains the products of two magmatic episodes, separated temporally by an oblique folding event caused by dextral transpression of the zone: (1) a 1875-1860-Ma pre-folding suite of mainly calc-alkaline rocks ranging continuously in composition from basalt to rhyolite, cut by allied biotite-hornblende-bearing epizonal plutons; and (2) a 1.85-1.84-Ga post-folding suite of discordant, epizonal, biotite syenogranitic plutons, associated dikes, and hornblende-diorites, quartz diorites, and monzodiorites. The pre-folding suite of volcanic and plutonic rocks is interpreted as a continental magmatic arc generated by eastward subduction of oceanic lithosphere. Cessation of arc magmatism and subsequent dextral transpression may have resulted from ridge subduction and resultant change in relative plate motion. Increased heat flux due to ridge subduction coupled with crustal thickening during transpression may have caused crustal melting as evidenced by the late syenogranite suite. Final closure of the western ocean by collision with a substantial continental fragment, now forming the neoautochthonous basement of the northern Canadian Cordillera, is manifested by a major swarm of transcurrent faults found throughout the Great Bear zone and the Wopmay orogen.Although there is probably no single evolutionary template for magmatism at convergent plate margins, the main Andean phase of magmatism, exemplified by the pre-folding Great Bear magmatic suite, evolves as larger quantities of subduction-related mafic magma rise into and heat the crust. This results in magmas that are more homogeneous, siliceous, and explosive with time, ultimately leading to overturn and fractionation of the continental crust.     

Contribution to the knowledge of the magmatic evolution by the study of the variation of the coefficient of viscosity

The determination of the coefficient of viscosity of eruptive products gives useful elements to the knowledge of possible variations of composition and physical conditions of the magmas.     

Equivalent fixed-base models for soil-structure interaction systems

To simplify the consideration of the soil-structure interaction (SSI) effects, a single degree-of-freedom (SDOF) replacement oscillator has been successfully utilized to represent an SSI system with SDOF structural model. In the present paper, this approximation is first extended to an equivalent fixed-base model with modified system parameters. Based on this generalization, a methodology is then proposed to determine the equivalent fixed-base models of a general multi degree-of-freedom SSI system using simple system identification techniques in the frequency domain. Various fixed-base models are formulated and their accuracy is compared for a five-story shear building resting on soft soil. It is shown that the actual SSI system can be accurately represented with an appropriate fixed-base model.     

Geochemical approach to magmatic evolution of Mt. Erciyes stratovolcano Central Anatolia, Turkey

Erciyes stratovolcano, culminating at 3917 m, is located in the Cappadocian region of central Anatolia. During its evolution, this Quaternary volcano produced pyroclastic deposits and lava flows. The great majority of these products are calc-alkaline in character and they constitute Kocdag and Erciyes sequences by repeated activities. Alkaline activity is mainly observed in the first stages of Kocdag and approximately first-middle stages of Erciyes sequences. Generally, Kocdag and Erciyes stages terminate by pyroclastic activities. The composition of lavas ranges from basalt to rhyolite (48.4–70.5 wt.% SiO₂). Calc-alkaline rocks are represented mostly by andesites and dacites. Some compositional differences between alkaline basaltic, basaltic and andesitic rocks were found; while the composition of dacites remain unchanged. All these volcanics are generally enriched in LIL and HFS elements relative to the orogenic values except Rb, Ba, Nb depleted alkaline basalt. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotopic composition of the volcanics range between 0.703344–0.703964, 0.512920–0.512780 for alkaline basalts and change between 0.704322–0.705088, 0.512731–0.512630 for alkaline basaltic rocks whereas calc-alkaline rocks have relatively high Sr and Nd isotopic ratios (0.703434–0.705468, 0.512942–0.512600). Low Rb, Ba, Nb content with high Zr/Nb, low Ba/Nb, La/Yb ratio and low Sr isotopic composition suggest an depleted source component, while high Ba, Rb, Nb content with high La/Yb, Ba/Nb, low Zr/Nb and low ⁸⁷Sr/⁸⁶Sr ratios indicate an OIB-like mantle source for the generation of Erciyes alkaline magma. These elemental and ratio variations also indicate that the different mantle sources have undergone different degree of partial melting episodes. The depletion in Ba, Rb, Nb content may be explained by the removal of these elements from the source by slab-derived fluids which were released from pre-collisional subduction, modified the asthenospheric mantle. The chemically different mantle sources interacted with crustal materials to produce calc-alkaline magma. The Ba/Nb increase of calc-alkaline samples indicates the increasing input of crustal components to Erciyes volcanics. Sr and Nd isotopic compositions and elevated LIL and HFS element content imply that calc-alkaline magma may be derived from mixing of an OIB-like mantle melts with a subduction-modified asthenospheric mantle and involvement of crustal materials in intraplate environments.     

Pressure and volume changes in magmatic systems due to the vertical displacement of compressible materials

The problem of a pressure or volume increase due to the vertical redistribution of two substances with differing compressibilities is considered and compared with a previous treatment. Modified expressions for the pressure and volume changes are obtained using the general equation of state with constant compressibility. The effects of gas and melt compressibilities, and also of the solubility of gas in floating bubbles, on pressure and volume changes in magmatic systems are discussed. Very different conclusions from those in the previous treatment are found.     

The influence of rheologic crustal properties of the crust on the location of ore-forming hydrothermal magmatic systems

Based on a comprehensive study of hydrothermal magmatic systems at island arcs and a review of available mechanisms that cause elasto-plastic deformation in rocks, we considered the conditions for interaction between a convective magmatic cell and a convective hydrothermal cell in different rheologic zones of the crust. Three models have been developed to describe the generation of hydrothermal circulation systems: (1) the magma chamber is localized in a plastic zone, (2) partial and (3) complete penetration of the chamber into a brittle crust. It is shown that the last of these models is highly consistent with the structure of presentday high-temperature hydrothermal magmatic systems at depths greater than 1.0?C1.5 km and with the structure of Miocene to Pliocene ore-bearing volcano-plutonic complexes that are eroded to different depths in different geologic blocks within these complexes.     

A critical discussion of numerical models for multiaquifer systems

Leaky aquifers constitute complicated hydrological structures, whose inclusion in numerical models of hydrological systems is difficult, because of their three-dimensional nature. Methods for treating such systems can be classified as fully three-dimensional and quasi-three-dimensional. The latter have clear numerical advantages when applicable. In this paper a critical discussion of existing quasi-three-dimensional models is presented.     

Relations of cinder cones to the magmatic evolution of Mount Mazama,Crater Lake National Park,Oregon

Cinder cones at Crater Lake are composed of high-alumina basaltic to andesitic scoria and lavas. The Williams Crater Complex, a basaltic cinder cone with andesitic to dacitic lava flows, stands on the western edge of the caldera, against an andesite flow from Mount Mazama. Bombs erupted from Williams Crater contain cores of banded andesite and dacite, similar to those erupted during the climatic eruption of Mount Mazama.Major- and trace-element variations exhibit an increase in incompatible elements and a decrease in compatible elements, consistent with crystal fractionation of olivine, plagioclase, clinopyroxene, orthopyroxene, and magnetite. LREE patterns in the rocks are irregular; each successive basalt is enriched in LREE relative to the preceding andesite.Compositional variations in the magmas of the cinder cones suggest that three magmatic processes were involved, partial melting, fractional crystallization, and magma mixing. Partial melting of more than one source produced primary basaltic magma(s). Subsequent mixing and fractional crystallization produced the more differentiated basaltic to andesitic magmas.