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1.
由于蕴含着潜在的油气勘探和资源储藏信息,岩床复合体在全球范围内被广泛研究。本文基于南海北部三维地震资料以及南极罗斯海地区二维多道地震资料,对岩床复合体的形态特征及分布进行分析,对两个地区岩床复合体的形成时间进行判断,阐述岩床复合体的侵位机制。研究发现,岩床复合体代表性产状除了菱形、碟形及复合型外,还存在沿着层位平行侵入的板状,推测它可能是夭折了的岩床侵入体,以及由几个小型岩床堆叠而成的透镜状岩床复合体。分布上,南海北部岩床主要发育于珠江口盆地白云凹陷三维工区西部,东部只零星可见;南极西罗斯海岩床主要发育于西北角的阿黛尔盆地内。根据岩床上覆的强制褶皱结合区域构造背景,推测南海北部岩床复合体的形成时间为早中新世(15.5 Ma),海底扩张结束之时。残余岩浆将盆地内裂谷时期形成的断层作为岩浆运移的通道,并最终在断层终点处即下中新统近海相砂泥岩地层中实现侵位。南极西罗斯海岩床复合体的形成时间则非常晚,不早于5.5 Ma。该区这期岩浆活动可能与构造作用无关,而是受到冰川消融的影响。研究发现岩床复合体内部的岩床个体互相供给,即深部的大岩床为上覆浅层的小岩床提供岩浆供给,并且互相组合,形成大的岩床复合体。然而,每个岩床复合体则可能由不同的岩浆供给发育而成。 相似文献
2.
冲绳海槽是一个处于弧后扩张作用早期的年轻的弧后盆地,是研究弧后扩张作用早期盆地演化和壳幔过程的天然实验室.随着调查研究工作的逐步展开和深入,也发现了一些新的、重要的、亟待解决的科学问题,而火山岩岩石系列归属的厘定又是其他研究工作的基础.在系统收集和整理迄今已有冲绳海槽火山岩资料的基础上,结合近期分析测试数据, 对冲绳海槽火山岩的岩石系列归属进行了重新厘定,探讨了火山岩的构造环境指示意义和浮岩与玄武岩之间的成因联系.研究结果表明:冲绳海槽火山岩分布具有以基性玄武岩和酸性(流纹)英安岩为主的双峰式特征,中性火山岩稀少,基性的玄武岩属于亚碱性系列的橄榄拉斑玄武岩,酸性浮岩可归属为亚碱性岩系的流纹英安岩或流纹岩;在构造环境判别上,冲绳海槽玄武岩表现出大洋中脊和岛弧构造环境的特点,既有别于大洋中脊扩张中心,也有别于成熟型弧后盆地,呈现出弧后早期扩张阶段盆地独特的构造环境特征;广泛分布于冲绳海槽的酸性浮岩表现出一定的岛弧环境的特点;酸性浮岩与玄武岩具有同源性,酸性岩是基性的玄武质岩浆经不同程度结晶分异和同化混染作用的产物. 相似文献
3.
Deformation of host rocks during growth of a laccolithic intrusion is analyzed using the theory of bending a stack of thin elastic plates. The theoretical model suggests that magma spreading laterally in the form of a sill will eventually gain sufficient leverage on the overlying strata to deflect them upward and form a laccolith. The amount of bending increases as the fourth power of the distance the magma spreads, whereas the overburden resists bending as the third power of its effective thickness. Effective thickness is the thickness of a single layer which has the same resistance to bending as a multilayer of similar length and elastic modulus. The effective thickness of overburden in the Henry Mountains is estimated as between
and
of the actual thickness. The form of bending is similar for Newtonian, pseudoplastic, and Bingham magmas. The magnitude of the bending depends upon the total upward force and its distribution and is not simply related to magma viscosity as has been suggested by several previous investigators.After elastic bending strata should fail over the periphery of an intrusion, the site of maximum bending strain and differential stress predicted by the theory. Field observations described in Part I correlate well with these predictions. Because bending strains are proportional to layer thickness, strata of comparable strength but different thicknesses fail at different stages of laccolith development. This leads to the different cross-sectional forms of laccoliths observed in the field.The effect of host rocks on sill form and growth is analyzed using the elastic solution for an elliptical hole under uniform pressure. The theory suggests that sill thickness increases in proportion to length. The concentration of high stresses near the sill termination should induce permanent deformation and account for the blunt terminations described in Part I. This blunting is most likely to occur in relatively ductile rocks whereas sills simply split brittle rocks and maintain sharp terminations. The driving pressure in sills can be calculated from measurements of length and termination radius of curvature, if the yield strength of the host rocks can be estimated. This driving pressure must be greater than the overburden pressure, but sills apparently do not form or propagate by lifting their overburdens. Instead they propagate by locally deforming the host rock. After spreading over a distance about three times the effective overburden thickness, the overlying layers begin to bend upward significantly. This stage marks the transition from a sill to a laccolithic intrusion. 相似文献
4.
The Midcontinent Rift (MCR) of North America comprises a series of basaltic sheets, flows and intrusive rocks emplaced in the Lake Superior region during the Mesoproterozoic. The mafic rocks preserved on the northern flank of Lake Superior represent the older portions of the rift sequence and offer insights into the early development of the rift. New geochronological, geochemical and paleomagnetic data are presented for the dikes and sills located in and south of Thunder Bay, Ontario. Three sill suites are recognized within the study area; an earlier, spatially restricted ultramafic unit termed the Riverdale sill, the predominant Logan sills and Nipigon sills in the north of the study area. In addition three dike sets are recognized, the north-east trending Pigeon River swarm, the north-west trending Cloud River dikes and the Mt. Mollie dike. The geochemical data demonstrate that the majority of sills south of Thunder Bay are of Logan affinity and distinct from those of broadly similar age in the Nipigon Embayment to the north. The Pigeon River dikes that intrude the sills are geochemically coherent but distinct from the Logan sills and could not be feeders to the sills. The new age of 1109.2 ± 4.2 Ma for the Cloud River dike and its R polarity are consistent with published magnetostratigraphy. The Mt. Mollie dike age (1109.3 ± 6.3 Ma) indicates that it is not coeval with the spatially associated Crystal Lake gabbro as previously thought. The complexity of the dike and sill suites on the northern flank of suggests that the early phases of rifting occurred in distinct and changing stress fields prior to the main extensional rifting preserved in younger rocks to the south. The geochemistry and geochronology of the intrusions suggest a long-lived and complex magmatic history for the Midcontinent Rift. 相似文献
5.
Essentially isochemical thermal metamorphism of soda-rich Stockton, Lockatong and Brunswick formations of the Newark Group by diabase sills produced unusually varied and unique mineral assemblages, most of which are predominantly Na. feldspar and biotite. Within a meter of a sill Stockton arkose was altered to quartzo-feldspathic hornfels with common diopside and sphene. Within 50 m of a sill Lockatong calcitic and dolomitic mudstone formed calc-silicate hornfels with differing combinations of diopside-hedenbergite, andradite and grossular, prehnite, datolite, idocrase and wollastonite. Within a meter of a sill metamorphosed Lockatong calcareous feldspathic argillite contains sanidine-anorthoclase, aegirine, aegirine-augite, riebeckite and scapolite. Lockatong analcime-dolomite argillite was altered to unique feldspathoidal assemblages containing cancrinite, natrolite-thomsonite and rarely sodalite within 134 m, and nepheline within 30 m of the Byram Sill. Reddish-brown Brunswick mudstone produced spotted pelitic hornfels within a few 10's of meters of a sill.Response to thermal metamorphism varied directly with diminishing grain size. In both sandstone and mudstone Na. feldspar increases and K. feldspar decreases toward intrusions; quartz is rare or absent in highest-grade hornfels. Development of biotite was retarded by detrital clay minerals and hematite pigment, as well as by low temperature. Minor differences in composition among carbonate-rich and analcime-rich Lockatong deposits led to a diversity of closely associated assemblages. Aqueous solutions and relatively low temperature, probably in part during retrogressive metamorphism, produced hydrous minerals. Datolite, tourmaline, scapolite and fluorite suggest minor additions of volatiles, but the widespread feldspathoids were made from soda-rich sedimentary rocks without significant additions from an igneous source. 相似文献
6.
武当地块西部席状基性侵入岩群地球化学特征：南秦岭古生代底侵作用的依据 总被引：5，自引：3，他引：5
武当地块西部顺层侵位了大量的基性岩席。本文对其进行的岩石学、地球化学和Sr、Nd同位素等特征研究表明，该基性岩席群的岩浆属于大陆拉斑玄武岩系列，来源于混合异常大陆岩石圈地幔型源区，为武当地块裂解和中古生代时南秦岭地区曾发生大规模的地幔岩浆底侵及壳幔相互作用重要标志之一。此外，该基性岩席群的岩石地球化学特征与勉略地区玄武岩及变辉绿岩总体的特点具有较大的一致性，结合该基性侵入岩席群的形成时代(401—407Ma)、武当地块伸展构造(425—260Ma)与勉略带火山岩(230Ma)同位素年代学的差异，武当地块西部基性岩席群很可能是由于勉略洋拉开前地幔柱活动使大陆岩石圈拉伸减薄发生拆离、深部岩浆上涌的结果。因此，它们应该是勉略洋打开的早期阶段在武当地块区的响应。 相似文献
7.
The Koktokay pegmatite-type rare-metal-bearing ore district in the Altai orogen is famous for both its large scale and its diversity of rare metals. However, the emplacement mechanisms of the ore-bearing pegmatite intrusions in the Koktokay ore district are still unclear. Based on field observations, the emplacement of the ore-bearing pegmatite intrusions falls into two types. The first type is typical of the formation of dykes and sills, whereby they intruded into fan shaped, moderate dipping, joints within plutonic rocks. The second type involves the formation of a punched laccolith that was fed by a pegmatite sill. Magmatic stoping is the main mechanism of the laccolith emplacement. The peripheral faults played an important role in helping the emplacement of the laccolith. The trend of dykes and sills indicate two potential prospecting areas, which are located in the western and northern regions of the Koktokay ore district. 相似文献
8.
Various types of igneous suites occurred between 930 Ma and 890 Ma in the North China craton are known to represent a large igneous province (LIP). However, the magma plumbing system of this LIP and its potential consequence to a breakup event are poorly understood. We report morphology, crystal size distribution of plagioclase, and whole-rock and mineral geochemistry profiles of this LIP represented by the two decoupled mafic sill complexes (Niutishan and Pingshan) in the Xuhuai basin. In terms of petrography and whole-rock geochemistry, unlike the lower-level doleritic Niutishan sill, the Pingshan sill is subdivided into an upper and a lower sequence that meets at a Sandwich Horizon but lacks stratification. The significant En reversals present an almost M-shaped profile that differs from a C-shaped profile as previously revealed in the Niutishan sill. Quantitative textural analyses show relatively strong plagioclase orientation in the lower sequence of the Pingshan sill, which is attributed to magma flow. The Pingshan sill is interpreted to have been constructed as multiple magma injections from crustal magma chambers based on the crystal size distribution and the varied crystallization pressure (5.1–7.5 kbar) of clinopyroxene crystals.Similar to the far-field radiating Dashigou dyke swarm (c. 925 Ma), the Niutishan sill is enriched in incompatible elements, which may represent the initiation of the Xuhuai rifting. However, the depleted features of the diamond-bearing Pingshan sill and the other coeval doleritic sills (916–890 Ma) were probably related to the subsequent process switched from continental rifting to lithospheric rupture in the divergent stage of the Wilson cycle. The occurrence of the intrusive rocks evolving from an early radiating dyke swarm to later sill complexes indicates a time-transgressive change from radial to planar patterns, implying that it was the upwelling asthenospheric plume material, instead of plate boundary processes, governed the initial and early evolution of this intracontinental rift. 相似文献
9.
We investigate the evolution of the salt field in a bar-built estuary after the tidal inlet is closed by sediment, isolating the estuary from the ocean. We show that seawater trapped by inlet closure in the Russian River Estuary, CA, undergoes a two-stage landward intrusion process that leads to widespread salt stratification throughout the estuary. This salinity intrusion extends to distances of several kilometers from the beach—into the “inner estuary” that is separated from the “outer estuary” by shallow sills and typically devoid of saline waters during tidal conditions when the mouth is open. We describe landward movement of saline waters during six closure events in 2009 and 2010, based on repeat boat-based conductivity-temperature-depth (CTD) surveys and bottom-mounted acoustic Doppler current profilers (ADCPs). While sills block the initial landward motion of dense saline waters due to gravitational adjustment (first stage of intrusion), these same sills facilitate a wind-induced, one-direction valve mechanism through which saline waters are pumped into the inner estuary. Saline waters that crest the shallow sill can drain into deeper pools in the inner estuary as a pulsed gravity current (second stage of intrusion). We use empirical orthogonal function (EOF) analysis to identify an internal seiche in the outer estuary that results in uplift of pycnocline waters during the night at the boundary to the inner estuary. EOF analysis of inner estuary currents and a horizontal Richardson number are used to suggest that nocturnal gravity current events in the inner estuary (beyond the blocking sill) occur as pulses initiated by the internal seiche in the outer estuary. 相似文献
10.
Hornblende gabbro sill complex at Onion Valley,California, and a mixing origin for the Sierra Nevada batholith 总被引：9，自引：0，他引：9
T. W. Sisson T. L. Grove D. S. Coleman 《Contributions to Mineralogy and Petrology》1996,126(1-2):81-108
The steep crest of the Sierra Nevada, California, near Onion Valley, exposes natural cross sections through a mafic intrusive
complex that formed as part of the Mesozoic Sierra Nevada batholith. Sheeted sills of hornblende gabbro to hornblende diorite,
individually as thick as 1.5 m, form the upper 200 to 300 m of the complex. Thicker, multiply-injected sills, as well as mafic
stocks, lie underneath at elevations below 3600 m. Lens-shaped cumulate bodies, as thick as 200 m and more than 700 m broad,
lie near the base of the sheeted sill suite. Cumulates are flat-lying, modally layered hornblende gabbro with subsidiary ultramafic
olivine hornblendite, plagioclase hornblendite, and late-mobile hornblende-plagioclase pegmatite. Fine grain size, scarce
phenocrysts and xenocrysts, and quench mineral textures are evidence that hornblende gabbro sills injected in a largely liquid
state and preserve basaltic melt compositions. Most sills reached volatile saturation, as shown by tiny miarolitic cavities
that are also widespread in cumulates. Although some sills chilled directly against others, most chilled against septa, millimeters
to a few centimeters thick, of medium-grained diorite to granodiorite. Mutually crosscutting relations, as well as chilling,
show that the septa were partly molten at the time the sills injected and likely formed the lower portions of an overlying
more silicic magma chamber that has since been removed by erosion. Sill compositions range from evolved high-alumina basalt
to aluminous andesite with major and trace element abundances similar to those of modern arc magmas. Experimental phase equilibria
indicate dissolved water contents near 6 wt% (Sisson and Grove 1993a). The sills show unequivocally that hydrous arc basaltic
magmas reached shallow levels in the crust during formation of the largely granodioritic Sierra Nevada batholith. The basaltic
magmas appear to have been produced from an enriched mantle source with 87Sr/86Sr ∼0.7065, ɛNd ∼−4.3, 206Pb/204Pb ∼18.6, 207Pb/204Pb ∼15.6, 208Pb/204Pb ∼38.6. Although crystal fractionation contributed to forming the sill suite and the associated cumulates, nearly constant
concentrations of Na2O, P2O5, Nb, Zr, and light rare earth elements in the sills indicate that mixing between sill basaltic and more evolved septa magmas
was important for producing sills with andesitic compositions. Average Sierran granodiorite major and trace element concentrations
are readily reproduced by a simple mixture of average basaltic sill from Onion Valley and average Sierran low-silica granite.
This result supports the inference that Sierran granitoids formed chiefly by mixing between crustal and mantle-derived magmas,
although in some cases these crustal melts may have been derived by refusion of earlier mafic intrusions near the base of
the crust. The common mafic inclusions (enclaves) in Sierran granodiorites bear a superficial resemblance to Onion Valley
mafic sills; however, high concentrations of lithophile elements in the inclusions point to extensive chemical exchange between
inclusions and their host magmas. The prevalence of hornblende-rich mafic intrusive rocks at Onion Valley, elsewhere in the
Sierra Nevada, and in other shallow subduction batholiths stems from two effects of high melt water concentrations (∼4–6 wt%
H2O). The hydrous parent basaltic and basaltic andesite magmas had low liquidus temperatures, compared to nearly dry basaltic
melts, and thus were chilled less during ascent through the crust and were more capable of ascent as liquids. More importantly,
their high water concentrations led to low melt densities, higher than granitoid liquids, but comparable to or less dense
than partly solidified granitoid magmas. Thus, the hydrous basaltic and basaltic andesite magmas were neutrally or positively
buoyant and were capable of penetrating and rising through partly crystallized granitoids and their partly molten source regions
to reach upper crustal emplacement levels. Drier basaltic magmas were probably abundant at depth and contributed heat and
mass to granite generation, but were insufficiently buoyant to ascend to shallow levels.
Received: 2 August 1995 / Accepted: 26 June 1996 相似文献
11.
Varioles and matrix have been studied in the typical globular rocks??variolites of the Yalguba Range and Suisari Island in the Onega structure, Central Karelia. It was determined that the cores of the varioles are significantly enriched in silica, Na, K, Rb, Cl, and Ba, and have lower K/Na ratio as compared to matrix. In addition, varioles strongly differ from matrix in oxygen isotope composition (??18Ov-??18Om varies from 1.6 to 2.6??). The consideration of possible mechanisms of the formation of the Onega variolites with allowance for available isotopic and geochemical data (major element composition, REE and trace element distribution) demonstrated that the observed geochemical characteristics of matrix and globules of the Yalguba Range and Suisari Island variolites cannot be formed by melt mixing. Such processes as greenstone alteration and crystallization of spherulites during melt overcooling also did not define the isotopic and geochemical peculiarities of the Onega variolites, but played only subordinate role. Except for significant oxygen isotope shift in the variole-matrix system, the obtained data are best consistent with liquation model. However, few available experimental data on the distribution of trace elements and oxygen isotopes during liquid immiscibility make it impossible to reach decisive conclusion concerning liquation genesis of the Onega variolites. 相似文献
12.
《Chemie der Erde / Geochemistry》2021,81(1):125689
The widespread records of mafic intrusives (both sills and dykes) are reported from the Proterozoic sedimentary basins of the Indian Shield. Amongst them, the Bijawar basin is also intruded by Paleoproterozoic (ca. 1.98−1.97 Ga) mafic sills. We provide first hand information on petrological and geochemical characteristics of these mafic sills together with a few NW-trending mafic dykes belong to the Jhansi swarm emplaced within the Bundelkhand craton, adjacent to the Bijawar basin. These Paleoproterzoic mafic intrusive rocks, i.e. sills and dykes, are believed to be integral parts of the Jhansi LIP, identified in the Bundelkhand craton. The studied mafic magmatic samples are medium- to coarse-grained and contain doleritic mineral compositions and textures. Geochemically, the mafic sill samples of the Bijawar basin, which belong to the Darguwan-Surjapura mafic sills (DSMS), are sub-alkaline basaltic-andesite to andesite in character. They are co-genetic in nature and show close geochemical similarities with a set of NW-trending mafic dykes (low-Ti) emplaced in the Bundelkhand craton. On the other hand, another set of NW-trending mafic dykes (high-Ti) of the Bundelkhand craton have distinct geochemical nature; likely to have different genetic history. The rare-earth element contents and trace-element modeling suggest that the DSMS and low-Ti dyke samples are likely to be derived from a melt generated ≥20 % melting of a shallower mantle source (spinel stability field), whereas the high-Ti dyke samples show their derivation from a melt generated through ≤15 % melting of the similar mantle source but at deeper level (garnet or garnet-spinel transition stability field); with a substantiate percentage of olivine fractionation of melts before crystallization. Their emplacement in an intracratonic tectonic regime and role of plume in the genesis of these rocks are suggested. The geochemical signature also indicates the role of an ancient (Archean) subduction event that has metasomatized the mantle before the cratonization. Their spatiotemporal correlation with other similar magmatic events of the globe indicate that the Bundelkhand craton was closer to the Karelia-Kola craton (Baltica Shield), North China craton and northern Superior craton, which could be part of the Columbia supercontinent, during its assembly. 相似文献
13.
The Cuddapah Basin is one of many Proterozoic, intracontinental sedimentary basins across Peninsular India. The basin comprises several unconformity-bounded successions, the lowermost of which (the Papaghni Group and overlying Chitravati Group) are intruded by dolerite sills that contact metamorphosed their host rocks. A mafic-ultramafic sill from the base of the Tadpatri Formation in the Chitravati Group was previously dated at c. 1885 Ma, and interpreted to be part of a large igneous province (LIP). We have dated two samples of a felsic tuff from the upper part of the Tadpatri Formation at 1864 ± 13 Ma and 1858 ± 16 Ma; combining data from the two samples yields a weighted mean date of 1862 ± 9 Ma. Mafic sills intrude rocks stratigraphically above the tuffaceous beds, indicating that mafic magmatism continued until after c. 1860 Ma. Given that the sills intruded lithified rocks, some of the sills may be considerably younger than 1860 Ma. Mafic volcanic rocks are also known from below the unconformity at the base of the Chitravati Group, within the basal Papaghni Group (> c. 1890 Ma). Collectively, these data indicate that mafic sill emplacement spanned more than 30 myr so that it is likely to have been a protracted event or a series of events, and, therefore unlikely to represent a LIP. The time span for mafic magmatism is more compatible with episodic, lithospheric extension (passive rifting) during basin evolution than it is with a mantle plume (active rifting). 相似文献
14.
《International Geology Review》2012,54(8):1020-1038
ABSTRACTThe La Tinta mélange is a small but singular ultramafic mélange sheet that crops out in eastern Cuba. It is composed of dolerite-derived amphibolite blocks embedded in a serpentinite matrix. The amphibolite blocks have mid-ocean ridge basalt (MORB)-like composition showing little if any imprint of subduction zone component, similar to most forearc and MOR basalts worldwide. Relict Cr-spinel and olivine mineral chemistry of the serpentinized ultramafic matrix suggest a forearc position for these rocks. These characteristics, together with a hornblende 40Ar/39Ar age of 123.2 ± 2.2 Ma from one of the amphibolite blocks, suggest that the protoliths of the amphibolite blocks correspond to forearc basalt (FAB)-related rocks that formed during the earlier stage of subduction initiation of the Early Cretaceous Caribbean arc. We propose that the La Tinta amphibolites correspond to fragments of sills and dikes of hypoabyssal rocks formed in the earlier stages of a subduction initiation scenario in the Pacific realm (ca. 136 Ma). The forearc dolerite-derived amphibolites formed by partial melting of upwelling fertile asthenosphere at the beginning of subduction of the Proto-Caribbean (Atlantic) slab, with no interaction with slab-derived fluids/melts. This magmatic episode probably correlates with Early Cretaceous basic rocks described in Hispaniola (Gaspar Hernandez serpentinized peridotite-tectonite). The dikes and sills cooled and metamorphosed due to hydration at low pressure (ca. 3.8 kbar) and medium to high temperature (up to 720ºC) and reached ca. 500ºC at ca. 123 Ma. At this cooling stage, serpentinite formed after hydration of the ultramafic upper mantle. This process might have been favoured by faulting during extension of the forearc, indicating an early stage of dike and sill fragmentation and serpentinite mélanges formation; however, full development of the mélange likely took place during tectonic emplacement (obduction) onto the thrust belt of eastern Cuba during the latest Cretaceous. 相似文献
15.
Chao Xu Yuanping Cheng Ting Ren Liang Wang Shengli Kong Shouqing Lu 《Natural Hazards》2014,71(1):109-134
In the Yangliu Mine in Huaibei Coalfield, violent magma intrusion produced two layers of the igneous cover sills and the annular dike and caused the ejection of gas and water in the bed splitting from the surface well that destroyed the gas extraction system on the surface. The source, flow characteristics and hazard of the gas in bed splitting under igneous sills were researched via laboratory experiments, theoretical analyses and numerical simulations. The experimental results indicate that the shorter the distance from the igneous sill and the larger the thickness of the igneous sill, the greater will be the amount of the thermal evolution, the pyrolysis stoma, the coal adsorption and desorption ability and the risk of outburst, six coal samples collected at different distances from the igneous sill. The first breaking length of the igneous sill, the volume of the bed splitting, the gas pressure and flow rate in the bed splitting and the accumulative volume of the gas ejected from the surface well #2 were theoretically calculated. The laws of the bed splitting development under the igneous sills were simulated, consistent with the theoretical calculation results. The regional control and prevention technologies for coal seam gas under igneous sills were proposed and applied. Then the safety of the first-mining seam production and the gas extraction in the adjacent seam and bed splitting via the surface well was ensured, and no accidental gas and water ejection occurred. 相似文献
16.
Chemistry of the Shiant Isles Main Sill, NW Scotland, and Wider Implications for the Petrogenesis of Mafic Sills 总被引：3，自引：0，他引：3
Major and trace element data for the Tertiary, Shiant IslesMain Sill, NW Scotland, are used to discuss its complex internaldifferentiation. Vertical sections through the sill exhibitsharp breaks in chemistry that coincide with changes in texture,grain size and mineralogy. These breaks are paired, top andbottom, and correspond to the boundaries of intrusive units,confirming a four-phase multiple-intrusion model based on fieldrelations, petrography, mineralogy and isotopes. Whole-rockchemistry is consistent with this model and necessitates onlyminor revisions to the intrusive and differentiation mechanismspreviously proposed. The rocks contain strongly zoned minerals(e.g. olivine Fo70–5, clinopyroxene Mg# = 75–5,plagioclase An75–5) indicating almost perfect fractionalcrystallization, but whole-rock compositions do not show suchextreme variations. Thus, while residual liquids became highlyevolved in situ, they mainly became trapped within the crystalnetwork and did not undergo wholesale inward migration. Someinward (mainly upward) concentration of residual liquids didoccur to form a ‘sandwich horizon’, but the morevolatile-rich, late-stage liquids that did not crystallize insitu appear to have migrated to higher levels in the sill toform pegmatitic horizons. Parental liquid compositions are modelledfor the intrusive units and it is concluded that the originalparent magma formed by partial melting of upper mantle thatwas more depleted in LREE than the sources of most ScottishTertiary basaltic rocks. Incompatible trace elements in thepicrodolerite–crinanite intrusive unit support isotopeevidence that its parent magma was contaminated by crustal material.Attempts to reconcile the chemical characteristics of the sillwith a recently proposed petrogenetic model based on a singleintrusion of magma differentiated by novel, but controversial,processes fail comprehensively. It is predicted that the complexpetrogenetic history of the Shiant Isles sill is not unusualand could become the model for other large (>50 m thick)sills. KEY WORDS: alkali basalt; differentiation; geochemistry; multiple intrusion; Shiant Isles; sill 相似文献
17.
GUO Rui ZHANG Gongcheng ZHANG Jinwei ZHAO Xingbin LIU Junbang YUAN Dawei SONG Shuang 《《地质学报》英文版》2013,87(5):1306-1318
An extensive suite of igneous sills was intruded into the Tertiary sedimentary section of the Jiaojiang sag,East China Sea.This suite has been well imaged offshore through 2D and 3D seismic surveys,showing a close relationship with CO2 content in nearby gas discoveries.A new observational model,which incorporates simple upward propagation,simple horizontal propagation,and transgressive propagation,was proposed to interpret these sill intrusions.In this model,the source magma of the saucer-shaped sills was injected from their lowest points near the center.The transgressive propagation can be interpreted as a combination of the vertical and horizontal propagation.Most shallow sub-volcanic intrusions in the Jiaojiang sag exhibit fingered characteristics,both vertically and horizontally.The vertical fingered propagation produced saucer-shaped sills arranged in the flower style.Along the brims of lower sills could usually be found the upper sills,which are interpreted to have formed simultaneously with or later than the lower feeder sills.In the second type,the chilled paths of the older sills were reutilized by subsequent intrusions.The horizontal fingered propagation formed sheet intrusions that exhibit groove,lobate,tubular,and crevasse splaylike geometry in plan view.In 3D view,the sheet intrusions have still preserved the lower center and higher rim that similar with the sub-rounded saucer-shaped sills.Although fracture propagation may be important,flow inflation was the key mechanism of magma intrusion.Further consideration of the mechanisms underlying sill formation may help explain the fingered characteristics of sill propagation. 相似文献
18.
青岛灵山岛南背来石剖面发育了两层大型不规则砂脉。其主要特征如下：上层大型不规则砂脉呈分散团块状顺黄褐色凝灰质泥岩分布,断续延伸约60 m,宽数米;有17个侵入砂团块。砂团块多呈浅灰色,主要岩性为砂岩、砂砾岩;砂团块直径在1~2 m。砂团块的形态非常复杂,侧向极易尖灭或突然中止。边界与围岩多突变,某些地方与节理发育方向相同。下层大型不规则砂脉浅灰色,主要底部为砂岩、顶部为砂砾岩,呈透镜状顺层产于灰黑色薄层砂泥岩中,均一团块状和边缘含大量泥砾;厚18 m,宽度和高度不详;上层砂脉发育在浅湖中;下层大型不规则砂脉发育在三角洲前缘。与此同时还探讨了上层砂脉的形成的动力学机制,建立了顺层砂脉的形成深度与液化层的深度之间的关系式（h=0.29H）。该公式可以用来预测顺层的侵位深度和寻找砂脉,为含油气盆地砂脉储层预测提供理论依据和新方法。通过流化公式计算获得了形成上层砂脉的流体上侵速度为1.26 m/s。通过本文的论述,进一步为理解灵山岛地区下白垩统的沉积环境和沉积动力学特征提供了新的岩石学证据。 相似文献
19.
Jing-Yu Jiang Yuan-Ping Cheng Lei WangWei Li Liang Wang 《International Journal of Coal Geology》2011,88(1):55-66
Since the early 1980s, fifteen outbursts have occurred in the Huaibei Coalfield of China. These outbursts were reported to be associated with sills. To study the effect of sill intrusions on coal seam and their relationship to methane outbursts, eleven samples from the No. 10 coal seam were taken from the Wolonghu Mine at various distances from a diorite sill. Comparisons were made between unaltered and heat-affected coals using petrographic and chemical data, micropore characteristics, adsorption properties of coal, and gas outburst indexes from field. Approaching the intrusion, vitrinite reflectance levels increased from 2.74% to 5.03%, and the thermal aureole of the sill ~ 60 m (from the sill boundary to sample 9). Three zones along this gradient were identified as corresponding to (1) thermal evolution zone No. 1 (0-5 m from sill), (2) thermal evolution zone No. 2 (5-60 m from sill), and (3) unaltered zone. The methane adsorption capacity of coal samples in the thermal evolution zone No. 2 was generally higher than in the two other zones, and the unaltered zone higher than the thermal evolution zone No. 1. It is concluded that the contact-metamorphism decreased the adsorption capacity of coal and the thermal evolution of sill increased it. The trap effect of sill, combined with the mudstone and siltstone roof and floor of the No. 10 coal seam, provided a seal for the formation of a gas pocket. Abnormally high formation pressures at the No. 10 coal seam led to two outbursts. 相似文献
20.
塔里木大火成岩省是我国境内发现的两个重要的二叠纪大火成省之一,不仅有大规模的溢流相玄武岩喷发,还有复杂多样的侵入岩及其组合。以往的研究主要集中在二叠系等显生宙地层发育区,对于前寒武系等古老层系中是否存在二叠纪岩浆侵入未引起重视。本文通过野外地质调查,在阿克苏地区前寒武系中识别出数条侵入南华系的基性岩墙和较大规模侵入震旦系的基性岩床。岩墙、岩床与围岩之间均显示出典型的侵入接触关系,包括岩体内部结晶粗、边部结晶细的淬冷边结构、岩床顶底面显著的切层现象以及包裹围岩团块等;显微镜下观察表明,辉绿岩具有典型的辉绿结构和嵌晶含长结构。锆石U-Pb年代学结果表明,侵入下震旦统的辉绿岩岩床和侵入南华系的辉绿岩岩墙的侵位时代在误差范围内基本一致,约为290Ma。岩石地球化学特征表明,辉绿岩岩床具有与洋岛玄武岩(OIB)相似的稀土及微量元素配分模式,结合Sr-Nb-Pb同位素测试结果,认为它们来源于碳酸盐化的富集型岩石圈地幔部分熔融,而辉绿岩岩墙具有Nb、Ta、Zr、Hf等负异常的地球化学特征,可能来自曾被俯冲相关流体/熔体交代的岩石圈地幔源区。本文的早二叠世辉绿岩岩床和岩墙的形成时代、岩浆源区和岩浆演化... 相似文献