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1.
对比分析了中国大陆科学钻探(CCSD)主孔100~2 000 m测井磁化率与岩心样本磁化率的测量结果,二者具有较好的一致性。测井磁化率统计结果表明CCSD主孔100~2 000 m超高压变质岩的磁化率从高至低依次为:蛇纹岩、石英榴辉岩、金红石榴辉岩、正片麻岩、退变质榴辉岩、多硅白云母榴辉岩、副片麻岩、绿泥石角闪岩、角闪岩。由CCSD主孔100~2 000 m磁三分量测量数据得到磁异常的△BZ、△BH和△BT值,结合前人所做的岩石学研究分析发现:主孔530~575 m的磁异常主要由岩石的金红石矿化、钛磁铁矿化和磁铁矿化引起,604~643 m和652~678 m的磁异常主要由岩石的蛇纹石化引起;钻孔中岩层的磁化率对磁三分量异常起着主要的决定作用,高磁化率岩层都表现出较大的磁三分量异常。  相似文献   

2.
Special features of linear marine magnetic anomalies and the magnetic measurements made on samples of the ocean crust require that the thickness of the marine magnetic source layer be thicker than the 0.5 km thick pillow lava layer (seismic layer 2A) alone. It is proposed here that the magnetic properties of the samples studied to date indicate a two-layered source to be the most likely: an upper 0.5 km thick pillow lava layer with a natural remanent magnetization of 5 A/m and a lower 3.5 km thick dike and upper gabbro layer with a magnetization of 0.5 A/m.  相似文献   

3.
From analysis of the geological and geophysical data (gravity, magnetic, seismic and petrophysics), we propose that geophysical anomalies are produced by a serpentinized mantle peridotite body (SMPB) situated in the middle to lower crust in the Sulu Belt. The SMPB was formed by crustal emplacement of mantle peridotites accompanied by ultrahigh-pressure (UHP) metamorphism. Our finding suggests an emplacement mechanism for the serpentinized mantle wedge (SMW), early in the subduction process. This is different from the classic view, which holds that the serpentinized forearc mantle is formed by in situ hydration processes (Blakely et al., 2005). The petrophysical properties of the SMPB are similar to those of the serpentinized forearc mantle or SMW in modern subduction-zones worldwide, but the formation mechanisms for SMPB and SMW are different. This observation is important for understanding the geodynamic processes that operated in the large UHP metamorphic belt in the Dabie-Sulu area, eastern China.  相似文献   

4.
柴北缘超高压带中锡铁山榴辉岩的变质时代   总被引：10，自引：8，他引：2  
宋述光  张聪  李献华  张立飞 《岩石学报》2011,27(4):1191-1197
锡铁山地体位于柴北缘超高压变质带的中部, 是柴北缘超高压变质带的重要组成部分。该地体由花岗质片麻岩、泥质片麻岩和相对较少的榴辉岩透镜体组成。大部分榴辉岩都经历了不同程度的后生合晶和角闪岩相退化变质改造。虽然近年来进行了大量的锆石U-Pb年代学研究,但榴辉岩相高压-超高压变质的时代一直存在争议,并且以前对锡铁山榴辉岩相变质时代的认识一直与相邻的绿梁山、鱼卡和其东部的都兰等地区的超高压变质年龄有明显的冲突。本文通过锡铁山榴辉岩锆石U-Pb年代学的研究,获得榴辉岩相变质锆石的206Pb/238U加权平均年龄为433±3Ma,与鱼卡地区榴辉岩的形成年龄一致,代表大陆俯冲时期的高压-超高压变质年龄。该研究对进一步了解锡铁山榴辉岩地体的变质演化和大陆地壳深俯冲有重要意义。  相似文献   

5.
本文总结了榴辉岩的高温高压弹性波速测量结果,并将其应用于苏鲁超高压变质带地震资料的解释。由于榴辉岩具有高密度和高波速,它们和长英质片麻岩、大理岩、石英岩、角闪岩、麻粒岩、蛇纹石化橄榄岩的界面可以产生强反射。如果俯冲的陆壳物质以榴辉岩与围岩互层的形式在上地幔保留下来,就可能在造山带的上地幔产生地震反射。根据CCSD孔区地震剖面所建立的地壳成分模型表明:苏鲁超高压带地壳浅部的高速层可归因于夹在花岗质片麻岩、副片麻岩、角闪岩等岩石中的榴辉岩和超基性岩;中地壳主要由中酸性片麻岩、斜长角闪岩和副片麻岩组成;下地壳以中基性麻粒岩为主。在该超高压变质带现今的深部地壳,榴辉岩含量很少或几乎没有。因此,折返的超高压变质岩是以构造岩片的形式沿一系列剪切带逆冲并覆盖在正常的中下地壳之上,深部榴辉岩的缺乏可能与下地壳拆沉作用无关。  相似文献   

6.
A preliminary study of paleomagnetism and rock magnetism has been done on 55 eclogite samples collected from the Chinese Continental Scientific Drilling (CCSD) site at the Maobei (毛北) area, Donghai (东海) County, Jiangsu (江苏) Province. Also the isothermal remanence, hysteresis loop, magnetic fabric, thermal susceptibility were measured, and analyses were made by electron-probe and scanning electric microscope on some samples synchronously. The result indicates that there are two groups of stable remanence, the normal and reversed magnetization. The remanence orientations are: D=94.3o, I=-29.1o and D=273.7o, I=15.4o, respectively. The magnetization intensity and the density of the samples which carry the normal magnetization are very different from those bearing reversed magnetization. The magnetic anisotropy is weak, and the minimum axis is hardly determined. The isothermal remanence and the hysteresis loop show that the magnetic carriers of the eclogite are likely SD (single domain) and PSD (pseudo-single domain) magnetite. According to the magnetic property, the cause of formation of magnetic carriers, the mechanism of the remanence, and the significance for the tectonics are discussed.  相似文献   

7.
Various lines of evidence point to the lower crust as the source of the long-wavelength magnetic anomaly field measured by the POGO and Magsat satellites. Using seismically determined lower crust thicknesses and equivalent source inversion of the satellite anomaly data, magnetization for the lower crust for much of the United States has been calculated. The average magnetization for two hundred sixty-six 150 × 150 km areas is 3.5 A/m with a standard deviation of 1.1 A/m. These values are consistent with laboratory measurements of mafic-ultramafic rocks expected in the lower crust, and in agreement with previous estimates of lower crust magnetization based on long-wavelength aeromagnetic data. Average lower crust thickness for the same areas is 18.2 km (σ = 6.4 km). Thus, over large regions, it appears that variation in magnetization and variation in magnetic layer thickness contribute almost equally in causing the anomaly field variation at satellite altitude.  相似文献   

8.
Experiments on MORB?+?4 wt% H2O at 0.8–2.8 GPa and 700–950 °C (Liu in High pressure phase equilibria involving the amphibolite–eclogite transformation. PhD dissertation, Stanford University, Stanford, California, 1997; Liu et al. in Earth Planet Sci Lett 143:161–171, 1996) were reexamined for their major and trace element melt compositions and melting relations. Degree of melting diminishes at greater pressures, with corresponding evolution of melt from andesitic at the lowest pressures and hottest temperatures to high-silica rhyolitic at the greatest pressure and coolest temperature. Quartz contributes greatly to the production of near-solidus melts of basaltic eclogite, with the result that melt productivity falls markedly following quartz exhaustion. This limits the extent of melting attainable in the basaltic eclogite portions of sub-arc subducting plates to no more than ~?2?×?the modal wt% quartz in the mafic eclogite protolith. Synthesized residual mineral assemblages lack an epidote-series mineral at temperatures?>?750 °C, and as a result, melts from the rutile eclogite and rutile-amphibole eclogite facies have elevated concentrations of light rare earth elements, U, Th, have elevated Ba, K, and Sr, high Sr/Y, and are strongly depleted in Nb, Y, and the heavy rare earth elements. Models of eclogite partial melt reacting with peridotite of the mantle wedge reproduce major and trace element characteristics of parental arc magmas so long as the proportions of infiltrating melt to peridotite are relatively high, consistent with channelized ascent. Melt mass is estimated to increase roughly three- to ten-fold, consistent with H2O concentrations of 3–7 wt% in the magmas produced by reaction. Partial melts of subducting basaltic eclogite are predicted to have positive Sr concentration anomalies, relative to Ce and Nd, that persist through melt-peridotite reactions. Primitive arc magmas commonly have positive Sr anomalies, whereas such anomalies are smaller in estimates of the bulk continental crust. Overall, Sr anomalies diminish passing from primitive to more evolved arc volcanic rocks, consistent with extensive mineral-melt differentiation (crystallization, partial remelting) involving plagioclase. On the order of 50 wt% differentiation would be necessary to eliminate Sr positive anomalies, based on geochemical variations in the Cascade and western Aleutian magmatic arcs. Loss to the mantle of cumulates and restites with high Sr anomalies, in abundances broadly equal to the mass of the preserved crust, would be required to form the continents via processes similar to present-day subduction magmatism.  相似文献   

9.
Nature and origin of eclogite xenoliths from kimberlites   总被引：16，自引：0，他引：16  
D.E. Jacob   《Lithos》2004,77(1-4):295-316
Eclogites from the Earth's mantle found in kimberlites provide important information on craton formation and ancient geodynamic processes because such eclogites are mostly Archean in age. They have equilibrated over a range of temperatures and pressures throughout the subcratonic mantle and some are diamond-bearing. Most mantle eclogites are bimineralic (omphacite and garnet) rarely with accessory rutiles. Contrary to their overall mineralogical simplicity, their broadly basaltic-picritic bulk compositions cover a large range and overlap with (but are not identical to) much younger lower grade eclogites from orogenic massifs. The majority of mantle eclogites have trace element geochemical features that require an origin from plagioclase-bearing protoliths and oxygen isotopic characteristics consistent with seawater alteration of oceanic crust. Therefore, most suites of eclogite xenoliths from kimberlites can be satisfactorily explained as samples of subducted oceanic crust. In contrast, eclogite xenoliths from Kuruman, South Africa and Koidu, Sierra Leone stem from protoliths that were picritic cumulates from intermediate pressures (1–2 Ga) and were subsequently transposed to higher pressures within the subcratonic mantle, consistent with craton growth via island arc collisions. None of the eclogite suites can be satisfactorily explained by an origin as high pressure cumulates from primary melts from garnet peridotite.  相似文献   

10.
Along the Rio Muni transform margin, the transition from continental to oceanic crust occurs across a region of approximately 75-km width. The crust in this transition region, termed proto-oceanic crust (POC), is neither purely oceanic nor continental in composition and structure. Improved seismic reflection images from the PROBE deep-imaging dataset, combined with gravity modelling, have shed new light on the structural architecture of the margin and the composition of the POC. On these newly migrated seismic reflection sections, four fracture zones associated with large steps in the Moho are identified, splitting the POC into three segments. Models in which these segments are composed of oceanic or stretched continental crust do not provide satisfactory predictions of the gravity anomaly. A model of serpentinized peridotite for two segments of POC, with the third segment composed of oceanic crust in between, does satisfy the observed gravity anomaly. Three alternative geological scenarios are proposed to explain the segmentation and composition of the POC: (a) serpentinized upper mantle becoming unroofed and emplaced at basement surface level along detachment surfaces confined to discrete segments by the fracture zones, (b) oblique-slip on transform faults that allow the circulation of water into the mantle and emplacement of serpentinized upper mantle material; or (c) intense faulting of anomalous oceanic crust as a result of magma depletion allowing hydrothermal circulation and the emplacement of serpentinized peridotites.  相似文献   

11.
Daniel J. Schulze 《Lithos》2003,71(2-4):195-213
A new empirical method has been devised for classification of mantle-derived garnets in kimberlite. Simple chemical screens have been developed to distinguish between garnets from different parageneses, based on Mg, Fe, Ca, Cr, Ti and Na values of published analyses of garnets from >2000 ultramafic xenoliths in kimberlite. Although crustal garnets are typically uncommon as xenocrysts in kimberlite, the first step in the classification is to screen these from the mantle population, using data from >600 garnet-bearing crustal rocks. Such a screen may also prove useful in evaluating the source (crust vs. mantle) of garnet in kimberlite exploration samples. Subsequent steps divide mantle garnets into eclogite, peridotite and Cr-poor megacryst groupings, and sub-groups of the peridotite (lherzolite, harzburgite, wehrlite) and eclogite (Groups I and II and A, B, C and grospydite) populations. Important features of this classification include the fact that it is based on distinctions between groups of fundamental geological significance (e.g., peridotite vs. eclogite) and it is based on a large, well-documented and well-understood xenolith database. As it utilizes oxide values and molar ratios of major and minor elements, the rationale for the screens is readily understood and it is simple to use.  相似文献   

12.
During the continental rifting the upper mantle was unroofed, and the mantle rocks were transformed into serpentinite at the ocean-continent transition of the west Galicia margin (Spain). The serpentinite layer, several km thick, extends probably eastwards, beneath the highly thinned continental crust of the margin.
The serpentinite layer was recently imaged by seismic reflection. It is discontinuously and deeply layered. As serpentinized peridotite can have densities and seismic velocities comparable to those of the lower continental crust, we suggest that undercrusting by serpentinite can play a part in building the lower seismic crust in highly stretched continental rifted areas.  相似文献   

13.
Mantle-derived peridotite bodies of Ariège are composed of spinel lherzolites and harzburgites ranging from remarkably fresh (less than 5% serpentinized) samples with protogranular texture to secondary foliated samples, which are generally 10%–20% serpentinized. The foliated samples have passed through two cycles of deformation and re-crystallization, the earlier ones occurring at temperatures above 950° C for 15 kbar pressure, the later ones at temperatures between 950° and 750° C for 8–15 kbar. Microscopic investigation of 140 samples reveals an accessoy sulfide component which is more abundant in lherzolie than in harzburgite. This component occurs in two differet textural locations, either as inclusions trapped within silicates during the first stage of re-crystallization or as interstitial grains among silicates. Mineralogy and chemistry of both sulfide occurrences are quite similar, at least in samples less than 5% serpentinized. In these fresh samples, sulfides are composed of complex intergrowths between nickel-rich pentlandite and pyrite, coexisting with minor primary pyrrhotite (Fe7S8) and chalcopyrite. Pentlandite and pyrite are interpreted as low-temperature breakdown products of upper mantle monosulfide solid solutions. The mineralogy and chemistry of interstitial sulfides in serpentinized rocks vary in parallel with the degree of serpentinization. In samples less than 10% serpentinized, primary pyrrhotite grades into FeS. In samples more than 10% serpentinized, pyrite is replaced by secondary pyrrhotite, and then disappears totally, whereas the coexisting pentlandite is Fe-enriched and replaced by mackinawite. This sequence of alteration indicates a decrease of sulfur fugacity, resulting from serpentinization of olivine at temperatures below 300° C. This is also the case for the inclusions which have been fractured during the tectonic emplacement of the host peridotites within the crust. The presence of non-equilibrium sulfide assemblages in both cases reflects the sluggishness of solid state reactions at near-surface temperatures. It is inferred from these results that sulfides disseminated within orogenic peridotite massifs are so sensitive to serpentinization that most sulfur fugacity estimates based on fractured inclusions and intergranular sulfides are unreliable.  相似文献   

14.
Three types of eclogite, together with a serpentinized harzburgite, coexist as blocks within granitic and pelitic gneisses along the Shaliuhe cross section, the eastern part of the North Qaidam continental-type ultrahigh-pressure (UHP) metamorphic belt, NW China. The olivine (Ol1) and orthopyroxene in the harzburgite are compositionally similar to present-day abyssal peridotites. The kyanite–eclogite is derived from a troctolitic protolith, whereas the epidote–eclogite from a gabbroic protolith, both having distinct positive Eu anomalies, low TiO2, and high Al2O3 and MgO. The kyanite–eclogite shows inherited cumulate layering. The phengite–eclogite has high TiO2, low Al2O3 and MgO with incompatible trace elements resembling enriched-type MORB. Sr–Nd isotope data indicate that the protoliths of both kyanite–eclogite and epidote–eclogite ([87Sr/86Sr]i ~ 0.703–0.704; εNd(T) ~ 5.9–8.0) are of mantle origin (e.g., ocean crust signatures). On the other hand, while the lower εNd(T) value (1.4–4.1) of phengite–eclogite is more or less consistent with an enriched MORB protolith, their high [87Sr/86Sr]i ratio (0.705–0.716) points to an additional enrichment in their history, probably in an subduction-zone environment. Field relations and geochemical analyses suggest that the serpentinized harzburgite and the three types of eclogite constitute the oceanic lithological section of an ophiolitic sequence from mantle peridotite, to cumulate, and to upper basaltic rocks. The presence of coesite pseudomorphs and quartz exsolution in omphacite plus thermobarometric calculations suggests that the eclogites have undergone ultrahigh pressure metamorphism (i.e., peak P ≥ 2.7 GPa). The harzburgite may also have experienced the same metamorphism, but the lack of garnet suggests that the pressure conditions of ≤ 3.0 GPa. Zircon U–Pb SHRIMP dating shows that the eclogites have a protolith age of 516 ± 8 Ma and a metamorphic age of 445 ± 7 Ma. These data indicate the presence of a Paleo-Qilian Ocean between Qaidam and Qilian blocks before the early Ordovician. The ophiolitic assemblage may be the relics of subducted oceanic crust prior to the subduction of continental materials during Ordovician–Silurian times and ultimate continent collision. These rocks, altogether, record a complete history of ocean crust subduction, to continental subduction, and to continental collision.  相似文献   

15.
L. G. Medaris  Jr. 《Lithos》1980,13(4):339-353
Core and rim compositions of minerals in garnet-bearing assemblages in the Lien peridotite define a retrograde metamorphic trend from 820° C, 28.1 kbar, to 645° C, 17.6 kbar. Eclogites in Basal Gneiss near the peridotite contain a record of prograde metamorphism which converges with the retrograde trend of the ultramafic rocks. The Lien peridotite appears to have been derived from the upper mantle under eclogite facies conditions and emplaced into unusually thick continental crust during a Caledonian eclogite facies metamorphic event.  相似文献   

16.
Granulitic and eclogitic inclusions from basic pipes at Delegate,Australia   总被引：4，自引：0，他引：4  
Basic breccia-nephelinite pipes at Delegate (N.S.W., Australia) contain abundant two-pyroxene granulite, garnet granulite and fassaite eclogite inclusions and rare spinel pyroxenite, peridotite and charnockite inclusions.Petrographic, mineralogical and chemical data on the inclusions and their co-existing phases are consistent with the hypothesis that the fassaite eclogite, garnet granulite and spinel pyroxenite inclusions all crystallized or recrystallized in about the same temperature-pressure region, within the range 7–15 kb and 700–1200° C. This means that these particular inclusions were formed within the uppermost part of the mantle and/or the lowermost part of the crust.The two-pyroxene granulites may also have crystallized in the same region but there are some data which are indicative of crystallization at lower pressures within the crust. The charnockite inclusion is also considered to be of crustal origin.  相似文献   

17.
2010年笔者完成了西藏朗县秀沟工区1∶5 000高精度重磁勘探,重力测量总精度达0.069 4 mGal。根据岩矿石物性特征建立了铬铁矿地球物理模型与找矿标志。局部重力高异常0.1～0.6 mGal、宽度几十米至一二百米,并且磁异常中等强度、反向磁化的重磁异常组合特征是识别铬铁矿的标志;局部重力高异常带与环状镶边的正磁异常带的组合特征是识别超基性岩带的标志。运用小波分析提取铬铁矿与超基性岩体局部重磁异常,倾斜角(tilt-angle)法识别岩体边界,Paker法密度填图及2.5D交互反演推断了蛇纹石化橄榄岩的范围及14个铬铁矿与矿化体的重磁远景异常,其中6个远景异常已经得到证实。指出在西藏进行铬铁矿勘探,不仅要求野外施工精度高,而且要求室内处理解释工作精细,应充分运用各种数据处理的新方法技术,才能够获得良好的地质效果。  相似文献   

18.
A few long-range airborne magnetic profiles flown at an altitude of 7.5 km a.s.l. across the Indian shield are analysed and interpreted in terms of magnetization in the lower crust. The wavelengths of the crustal anomalies are in the range of 51–255 km and this is used to separate them from signals originating at shallow depths. Spectral analysis of these profiles provided a maximum depth of 34–41 km for the long-wavelength anomalies and 9–10 km for the shallow sources identified as Mohorovic̆ić discontinuity and the basement respectively. The magnetic “high” recorded in satellite observations over the Indian shield is interpreted as due to a bulge of 3–4 km in the Moho under the Godovari graben, with a magnetization of 200 nT in the direction of the Earth's present-day magnetic field. Similarly the magnetic lows observed over the Himalaya are interpreted in terms of thickening of the granitic part of the crust from 18 to 23.5 km with a magnetization contrast of 200 nT in the direction of the Earth's present-day magnetic field.  相似文献   

19.
High-pressure (HP) metamorphic rocks, including garnet peridotite, eclogite, HP granulite, and HP amphibolite, are important constituents of several tectonostratigraphic units in the pre-Alpine nappe stack of the Getic–Supragetic (GS) basement in the South Carpathians. A Variscan age for HP metamorphism is firmly established by Sm–Nd mineral–whole-rock isochrons for garnet amphibolite, 358±10 Ma, two samples of eclogite, 341±8 and 344±7 Ma, and garnet peridotite, 316±4 Ma.

A prograde history for many HP metamorphic rocks is documented by the presence of lower pressure mineral inclusions and compositional zoning in garnet. Application of commonly accepted thermobarometers to eclogite (grt+cpx±ky±phn±pg±zo) yields a range in “peak” pressures and temperatures of 10.8–22.3 kbar and 545–745 °C, depending on tectonostratigraphic unit and locality. Zoisite equilibria indicate that activity of H2O in some samples was substantially reduced, ca. 0.1–0.4. HP granulite (grt+cpx+hb+pl) and HP amphibolite (grt+hbl+pl) may have formed by retrogression of eclogites during high-temperature decompression. Two types of garnet peridotite have been recognized, one forming from spinel peridotite at ca. 1150–1300 °C, 25.8–29.0 kbar, and another from plagioclase peridotite at 560 °C, 16.1 kbar.

The Variscan evolution of the pre-Mesozoic basement in the South Carpathians is similar to that in other segments of the European Variscides, including widespread HP metamorphism, in which P–T–t characteristics are specific to individual tectonostratigraphic units, the presence of diverse types of garnet peridotite, diachronous subduction and accretion, nappe assembly in pre-Westphalian time due to collision of Laurussia, Gondwana, and amalgamated terranes, and finally, rapid exhumation, cooling, and deposition of eroded debris in Westphalian to Permian sedimentary basins.  相似文献   


20.
Caledonian eclogite facies shear zones developed from Grenvillian garnet granulite facies anorthosites and gabbros in the Bergen Arcs of western Norway allow direct investigation of the relations between macroscopic structures and crystallographic preferred orientation (CPO) in lower continental crust. Field relations on the island of Holsnøy show that the eclogites formed locally from granulite facies rocks by progressive development of: (1) eclogite adjacent to fractures; (2) eclogite in discrete shear zones (> 2 m thick); (3) eclogite breccia consisting of >80% well-foliated eclogite that wraps around rotated granulite blocks; and (4) anastomosing, subparallel, eclogite facies shear zones 30–100 m thick continuous over distances > 1 km within the granulite terrane. These shear zones deformed under eclogite facies conditions at an estimated temperature of 670 ± 50°C and a minimum pressure of 1460 MPa, which corresponds to depths of >55 km in the continental crust. Detailed investigation of the major shear zones shows the development of a strong foliation defined by the shape preferred orientation of omphacite and by alternating segregations of omphacite/garnet-rich and kyanite/zoisite-rich layers. A consistent lineation throughout the shear zones is defined by elongate aggregates of garnet and omphacite. The CPO of omphacite, determined from five-axis universal stage measurements, shows a strong b-axis maximum normal to foliation, and a c-axis girdle within the foliation plane with weak maxima parallel to the lineation direction. These patterns are consistent with deformation of omphacite by slip parallel to [001] and suggest glide along (010). The lineation and CPO data reveal a consistent sense of shear zone movement, although the displacement was small. Localized faulting of high-grade rocks accompanied by fluid infiltration can be an important mode of failure in the lower continental crust. Field relations show that granulite facies rocks can exist in a metastable state under eclogite facies conditions and imply that the lower crust can host differing metamorphic facies at the same depth. Deformation of granulite and partial conversion to eclogite, such as is exposed on Holsnøy Island, may be an orogenic-scale process in the lowermost crust of collisional orogens.  相似文献   

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