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243.
Ascertaining the emplacement mechanism of oceanic basaltic lavas is important in understanding how ocean floor topography is produced and oceanic plates evolve, particularly during the early stages of crustal development of a supra-subduction zone. A detailed study of the volcanic stratigraphy at International Ocean Discovery Program (IODP) Site U1438 in the Amami Sankaku Basin, west of the Kyushu–Palau Ridge, has revealed the development of lava accretion and ridge topography on the Philippine Sea plate at about 49 Ma. Igneous basement rocks penetrated at Site U1438 are the uppermost 150 m of ~6 km-thick oceanic crust, and comprise, in a downhole direction, sheet flows (12.6 m), lobate sheet flows (61.3 m), pillow lavas (50.7 m), and thin sheet flows (25.3 m). The lowermost sheet flows are intercalated with layers of limestone and epiclastic tuff. Lithofacies analysis reveals that the lowermost sheet flows, limestone, and tuff formed on an axial rise, the pillow lavas were emplaced on a ridge slope, and the lobate sheet flows formed off ridge on an abyssal plain. The lithofacies of the basement basalt corresponds to the upper portions of fast-spreading oceanic crust, suggesting that subduction initiation was associated with intermediate to fast rates of seafloor spreading. The surface sheet flows are olivine–clinopyroxene-phyric basalt and differ from the lower basalt flows that contain phenocrysts of olivine and plagioclase, with or without clinopyroxene. The depleted chrome-spinel composition and olivine–clinopyroxene phenocryst assemblage in the surface sheet flows suggests a slight contribution of water for magma generation not present for the lower basalt flows. Considering the lithological difference between the backarc and forearc oceanic crust in the Izu–Bonin–Mariana arc, with sheet flow dominant in the former, seafloor spreading occurred faster in the later stage of subduction initiation. 相似文献
244.
Recent seismic reflection studies of large-volume, anorogenic basaltic provinces at passive continental margins have challenged the traditional viewpoint that erupted magmas predominantly ascend through the lithosphere via dykes that exploit high-angle faults. However, such seismic-based methods are yet to be applied to identify the magmatic plumbing systems of low-volume basaltic provinces, such as the Cretaceous–Cenozoic Newer Volcanic Province (NVP) in southeastern Australia (total volume <20 000 km3). The South Australian Quaternary Basalt (SAQB) represents the most recent phase of activity both within the NVP and on the Australian continent. This province is located within the Otway Basin, and a large amount of seismic data from gas exploration is available for this region. Consequently, the SAQB represents a superb natural laboratory in which geochemical and seismic data can be combined in order to study the magmatic plumbing system of a low-volume continental basalt province and discriminate between the competing hypotheses for magma transport through the Earth's crust. Geochemical analyses and thermodynamic modelling suggests that the magma that fed the SAQB was generated by adiabatic decompressional melting of a secondary mantle plume. These models imply that melt segregation took place at successively lower pressures from about ~4000 to 3000 MPa during the ca 1 Ma SAQB eruptive history that culminated in the Northern Group, Mt Schank and Mt Gambier eruptions. During ascent, the magma underwent 34–41% fractional crystallisation and cooled ~200 °C, while residing in the crust for a time period on the order of days to weeks. Interpretation of a 3D seismic survey that overlaps with the northeastern part of the SAQB reveals a saucer-shaped sill with an unusual morphology, exhibiting a series of vertical concentric steps towards its outer rim. This sill appears to be fed by magma that intruded along steep normal faults from a feeder dyke that is hosted by the regional, NW–SE-trending Hungerford-Kalangadoo Fault. Our results suggest that the melt that fed the SAQB rose through the crust via dykes and high-angle normal faults, with less evidence for significant horizontal transport of magma than observed in large-volume basaltic provinces in sedimentary basins at rifted continental margins, possibly highlighting a correlation between the extent, volume and magma supply rate within a basaltic province and the nature of magma ascent. 相似文献
245.
Anne Nédélec Jean-Louis Paquette Jean-Luc Bouchez Philippe Olivier Bruno Ralison 《Geodinamica Acta》2013,26(1):48-56
AbstractStratoid granites constitute a major feature of the Precambrien basement of Madagascar. A detailed structural study was carried out NNW of Antananarivo. New zircon isotopic data on a typical alkaline granite ascertain their Panafrican age (585 Ma). The sheets of granites metric to kilometric of thickness, are interlayered with migmatitic gneisses and amphibolites. Their internal structures, determined by anisotropy of magnetic susceptibility measurements, everywhere yield foliations gently dipping to the west, and lineations striking WSW-ENE. These structures were mostly acquired at the magmatic stage in the granites, in the country-rocks they resulted from high-temperature plastic deformation.The very constant structural pattern, interpreted in terms of shear deformation of a section of the crust, as well as the low P (P = 4 - 5 kb) - high T (T # 750°C) conditions, suggest that the emplacement of the stratoid granites was coeval with a late-orogenic stage in the Panafrican Mozambique belt, and possibly linked to the thinning of the lithosphere. 相似文献
246.
Upper Devonian continental and subaqueous sedimentary rocks and bimodal volcanic rocks of the Boyd Volcanic Complex of the south coast of New South Wales were deposited in a rapidly subsiding, 330°‐trending, transtensional basin. Structural analysis of synvolcanic and synsedimentary deformational structures indicate that basin formation is related to a 330°‐orientated subhorizontal σ1 and a 060°‐orientated subhorizontal σ3, which account for the development of the observed intrusion and fracture orientations. Rhyolitic, basaltic and associated clastic dykes are preferentially intruded along extensional 330°‐trending fractures, subordinately along sinistral, transtensional 010°‐trending fractures and along 290°‐trending fractures. One of the implications of such a palaeotectonic reconstruction is that the so called north‐trending Eden‐Comerong‐Yalwal Late Devonian rift does not represent a simple, single palaeobasin entity, but is presently a north‐trending alignment of exposures of sedimentary and volcanic rocks probably emplaced in different basins or sub‐basins, mildly folded during the Carboniferous Kanimblan compression (which also formed the north‐trending Budawang synclinorium) and then extended to the east by the Tasman Sea opening during the Jurassic. The development of scattered, rapidly subsiding, basins characterised by bimodal volcanism during the Late Devonian throughout the Lachlan Fold Belt, can be interpreted in terms of extensional collapse of a forming mountain belt contemporaneous with a sharp decrease of compressional stress after the Middle Devonian Tabberabberan orogenic event. This would promote a reorientation of σ3 and transition from a compressional to a transtensional tectonic environment, which could also favour block rotation and formation of release basins. 相似文献
247.
The Renjiayingzi intermediate-acid pluton is located along a pre-existing ENE–WSW-trending dextral shear zone that forms part of the Xar Moron suture zone that marks the final closure of the Paleo-Asian Ocean. The pluton is composed of three small intrusions, which from northwest to southeast, are named the Shuangjianshan (SI), the Qianweiliansu (QI) and the Xingshuwabeishan (XI) intrusions. LA-ICPMS zircon U–Pb dating of a pyroxene diorite from the SI yields an age of 138 ± 1 Ma; the SHRIMP zircon U–Pb age of a tonalite from the QI records an age of 134 ± 2 Ma, whereas LA-ICPMS zircon U–Pb dating of a monzogranite from the XI has an age of 126 ± 1 Ma, suggesting the entire pluton was built up by three separate emplacement events that young to the ESE: this is further supported by the contact relations. Incremental growth of plutons by amalgamation of repeated small magma pulses is the most viable emplacement model. The pluton was probably emplaced by updoming of the roof along previous tensile fractures and by upward stacking of the three intrusions. The SI and QI have similar U–Pb ages and geochemical characteristics, and most likely had the same magma source and underwent similar petrogenetic processes. They have high MgO concentrations at low silica contents, are enriched in large ion lithophile elements, depleted in high field strength elements, have negative εNd(t) values of −1.8 to −3.7, with Nd model ages of 1.07–1.19 Ga. Pyroxene diorites of the SI also have variable zircon εHf(t) values (from −0.8 to +6.1), indicating that they were mainly derived from juvenile crust with minor crustal contamination and clinopyroxene-dominated fractional crystallization. The late monzogranites from the XI show weak negative εNd(t) values of −2.3 to −2.5, young Nd model ages of 0.99–1.00 Ga, positive zircon εHf(t) values (+1.3 to +4.6) and higher SiO2 and K2O contents, with strong depletion in Eu, P and Ti, indicating derivation from a distinct petrogenetic process from the two earlier intrusions. The monzogranites were the result of partial melting of juvenile crust in response to mantle-derived magma underplating, together with plagioclase-dominated fractional crystallization. 相似文献
248.
《International Geology Review》2012,54(3):263-286
The ENE-trending, ca. 306–287 Ma, Totoltepec pluton is part of a Carboniferous–Permian continental magmatic arc on the western Pangaean margin. The 15 km?×?5 km pluton is bounded by two N–S Permian dextral faults, an E–W thrust to the south, and an E–W normal fault to the north. Thermobarometric data indicate that the main, ca. 289–287 Ma, part of the pluton was emplaced at ≤20 km depth and ≥700°C and was exhumed to 11 km and 400°C in 4 ± 2 million years. We have documented the following intrusive sequence: (1) the 306 Ma northern marginal mafic phase; (2) the 287 Ma main trondhjemitic phase; and (3) ca. 289–283 Ma sub-vertical dikes that vary from (a) N39E, undeformed with crystal growth perpendicular to the margins, through (b) ca. N50–73E, foliated and folded with sinistral shear indicators, to (c) N73–140E and boudinaged. The obliquity of the boundary between the folded and stretched dikes relative to the N–S dextral faults suggests sequential emplacement in a transtensional regime (with 20% E–W extension), followed by different degrees of clockwise rotation passing through a shortening field accompanied by sinistral shear into an extensional field. The ca. 289–287 Ma intrusion also contains a steep ENE-striking foliation and hornblende lineations varying from sub-horizontal to steeply plunging, probably the result of emplacement in a triclinic strain regime. We infer that magmatism ceased when some of the dextral motion was transferred from the western to the eastern bounding fault, causing thrusting to take place along the southern boundary of the pluton. This mechanism is also invoked for the rapid uplift and exhumation of the pluton between ca. 287 Ma and 283 Ma. The distinctive characteristics of the Totoltepec pluton should prove useful in identifying similar tectonic settings within continental arcs. 相似文献
249.
“花岗岩锆石U—Pb年龄能代表花岗岩侵位年龄”质疑——花岗岩锆石U-Pb年龄与全岩Rb-Sr等时线年龄对比证据 总被引:10,自引:0,他引:10
通过花岗岩体的64对锆石U-Pb年龄(tZr)与全岩Rb-Sr等时线年龄(tRb)之间差值Δt(Δt=tZr-tRb)进行的频数统计分析表明:Δt直方图呈对称正态分布(偏度系数CSK=0.150;峰度系数CKU=4.274);Δt既有正值又有负值;Δt的众数值为2.0Ma,但远低于花岗岩基冷却—结晶所需的时间(>16Ma);采用最小二乘法计算,花岗岩体64对锆石U-Pb年龄(tZr)和全岩Rb-Sr等时线年龄(tRb)拟合出相关系数很高(r=0.9967),回归系数接近l(斜率为0.99354)的线性回归方程(tRb=0.99354tZr 2.15163)。该回归方程的常数项(2.15163Ma)与Δt众数(2.0Ma)基本一致。这些特征表明,从总体来看,花岗岩体的全岩Rb-Sr等时线定年测定结果与锆石U-Pb定年测定结果是一致的,从而为花岗岩锆石U-Pb年龄不能代表花岗岩侵位年龄提供了证据。 相似文献
250.
通过花岗岩体的64对锆石U-Pb年龄(tZr)与全岩Rb-Sr等时线年龄(tRb)之间差值Δt(Δt=tZr-tRb)进行的频数统计分析表明: Δt直方图呈对称正态分布(偏度系数CSK=0.150; 峰度系数CKU=4.274); Δt既有正值又有负值; Δt的众数值为2.0Ma, 但远低于花岗岩基冷却—结晶所需的时间(>16Ma); 采用最小二乘法计算,花岗岩体64对锆石UPb年龄(tZr) 和全岩Rb-Sr等时线年龄(tRb) 拟合出相关系数很高(r=0.9967), 回归系数接近l(斜率为0.99354)的线性回归方程(tRb=0.99354tZr +2.15163) 。该回归方程的常数项(2.15163 Ma)与Δt众数(2.0Ma)基本一致。 这些特征表明, 从总体来看,花岗岩体的全岩Rb-Sr等时线定年测定结果与锆石U-Pb定年测定结果是一致的, 从而为花岗岩锆石U-Pb年龄不能代表花岗岩侵位年龄提供了证据。 相似文献