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1.
P and S velocity and attenuation estimates in the lower crust are obtained from a set of wide angle reflection–refraction profiles in the region of active tectonics at the NE edge of the Tibetan Plateau and discussed together with respect to similar data at its Himalaya–south Tibet edge.The quality factor is estimated in the lower half of the crust by accounting for the differential effect on amplitude–frequency observed between waves of different penetrations, and both in P and S modes. Attenuation values allow to exclude a significant proportion of partial melt and to estimate the homologous temperature, ratio of in situ to solidus absolute temperatures. The latter depend on the physical conditions being of dry, wet or dehydration melting, which are found different among the regions of the northern Bayan Har and northern Qang Tang boundaries between blocks, as well as the Tethyan–Himalayas, south of the Indus–Tsangpo suture. Their in situ temperatures differ also as estimated from their different Vp for a similar felsic composition.Joint measurement of several parameters, Vp, Vs, Qp and Qs reveals the composition, mineralogy, temperature and hydration conditions of the lower half of the thickened crust of Tibet that may be discussed in terms of evolution. The material presently in the thickened crust, even its lower part, has a felsic composition, upper to middle crustal lithology, and the temperature conditions estimated suggest that basic material that could have underlain it could be eclogitized and not appear anymore above the seismic Moho.Under northern Qang Tang, the felsic material in the lower half of the crust appears as hot and dry. Its burial may have occurred earlier or may have been moderate in the postcollisional phase. This is consistent with a model of indentation of the Qang Tang crust by an originally thinner Bayan Har crust to bring part of its crust to greater depth, suggested from imaging the crustal architecture. Under northern Bayan Har, the material in the lower half of the crust appears as felsic, at low temperature and not dry conditions. This is evidence that it has been transported from a shallower depth, and this recently enough not to be yet dehydrated and temperature equilibrated in a conductive geotherm. It supports a model of recent overriding of the middle crust of the north Kun Lun block to the north independently suggested from the image of crustal architecture. The Tethyan Himalayas case appears bracketed by these two cases in northern Tibet for Vp and temperature conditions, but shows highest attenuation in the lower crust that is colder but less dry than under northern Qang Tang.  相似文献   

2.
Based on the results of a study of regional element abundance in eastern China and the 1:200 000 geochemical surveys in northern Xinjiang, the element geochemical characteristics of the exposed crust in 23 tectonic units of the continent of China are summarized. Compared with the global average abundance of the upper continental crust, the exposed crust of the continent of China is compositionally more evolved than the upper crust of the island arc, but less evolved than the mature Precambrian Canadian shield. The exposed crust of the North China and Yangtze platforms has a lower SiO2 content, but markedly higher CaO and MgO contents due to the presence of widespread carbonate strata, which suggests that we should not neglect the contribution of carbonate rocks in the study of the exposed crust and the element abundance of the upper crust. In comparison with two recently published average compositional models of the global upper continental crust, the exposed crust of the continent of China is depleted in Au, Hg, Mo, Sn, and W, which suggests that their abundance in the present global models is overestimated. The exposed crust of the North China platform and the Qinling-Dabieshan fold belt to its south has lower μ(238U/204Pb) values (<8), but other regions of the continent of China exhibit much higher μ values, which implies that the low μ feature of the North China platform and its adjacent regions does not have global significance. Considering the apparent lateral variation in composition of the exposed crust for the tectonic units of the continent of China, there is no adequate reason to take the average upper crust compositional model of the North China platform and its adjacent regions as a reliable composition representative for Chinese and global upper continental crust composition. Translated from Geological Bulletin of China, 2005, 24(10–11): 906–915 [译自: 地质通报]  相似文献   

3.
CO2 inclusions with density up to 1,197 kg m−3 occur in quartz–stibnite veins hosted in the low-grade Palaeozoic basement of the Gemericum tectonic unit in the Western Carpathians. Raman microanalysis corroborated CO2 as dominant gas species accompanied by small amounts of nitrogen (<7.3 mol%) and methane (<2.5 mol%). The superdense CO2 phase exsolved from an aqueous bulk fluid at temperatures of 183–237°C and pressures between 1.6 and 3.5 kbar, possibly up to 4.5 kbar. Low thermal gradients (∼12–13°C km−1) and the CO2–CH4–N2 fluid composition rule out a genetic link with the subjacent Permian granites and indicate an external, either metamorphogenic (oxidation of siderite, dedolomitization) or lower crustal/mantle, source of the ore-forming fluids.According to microprobe U–Pb–Th dating of monazite, the stibnite-bearing veins formed during early Cretaceous thrusting of the Gemeric basement over the adjacent Veporic unit. The 15- to 18-km depth of burial estimated from the fluid inclusion trapping PT parameters indicates a 8- to 11-km-thick Upper Palaeozoic–Jurassic accretionary complex overlying the Gemeric basement and its Permo-Triassic autochthonous cover.  相似文献   

4.
We determine the source parameters of three minor earthquakes in the Upper Rhine Graben (URG), a Cenozoic rift, using waveforms from permanent and temporary seismological stations. Two shallow thrust-faulting events (M L = 2.4 and 1.5) occurred on the rift shoulder just south of Heidelberg in March 2005. They indicate a possible movement along the sediment–crystalline interface due to tectonic loading from the near-by Odenwald. In February 2005, an earthquake with a normal-faulting mechanism occurred north of Speyer. This event (M L = 2.8) had an unusual depth of about 22 km and a similar deep normal-faulting event occurred there in 1972 (M L = 3.2). Other lower crustal events without fault plane solutions are known from 1981 and 1983. At such a depth, inside the lower crust, ductile behaviour instead of brittle faulting is commonly assumed and used for geodynamic modelling. Based on the newly available fault plane solutions we can confirm the brittle, extensional regime in the upper and lower crust in the central to northern URG indicated in earlier studies.  相似文献   

5.
In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine–gabbros to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those in erupted basaltic lavas from St. Vincent and in published high-pressure (4–10 kb) experimental run products of a St. Vincent high-Mg basalt in having higher An plagioclase coexisting with lower Fo olivine. The oxygen isotope compositions (δ18O) of cumulus olivine (4.89–5.18‰), plagioclase (5.84–6.28‰), clinopyroxene (5.17–5.47‰) and hornblende (5.48–5.61‰) and hydrogen isotope composition of hornblende (δD = −35.5 to −49.9‰) are all consistent with closed system magmatic differentiation of a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic compositions reported. δ18OOlivine is up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H2O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at depth within the crust, lowering MgO and Cr2O3 and raising Al2O3 and CaO of residual melt due to suppression of plagioclase. Low density, hydrous basaltic and basaltic andesite melts then ascend rapidly through the crust, stalling at shallow depth upon water saturation where crystallisation of the chemically distinct cumulus phases observed in this study can occur. Deposited crystals armour the shallow magma chamber where oxygen isotope equilibration between minerals is slowly approached, before remobilisation and entrainment by later injections of magma.  相似文献   

6.
Backstripping analysis has been carried out on nine outcrop sections of the Basque-Cantabrian Aptian and Albian of northern Spain which consist of shallow-water marine deposits up to 7000 m thick. It reveals four main pulses of common accelerated subsidence: I (Early Aptian), II (Early Albian), III (lower Late Albian), and IV (uppermost Late Albian) separated by tectonic quiescent intervals. Tectonic subsidence rates of 70 m Myr–1 for the Aptian and 101 m Myr–1 for the Albian were recorded (Sopuerta section). The calculated stretching factor referred to a pre-rift continental crust of 35 km is (βs=1.32) and represents approximately 8.5 km of crustal thinning. Points of change towards lesser subsidence rate in the curves are correlated with major unconformities in the sections, in the Basque-Cantabrian Basin and in Boreal Europe, suggesting geodynamic changes related with the opening of Bay of Biscay. Minor variations of spreading direction and intensity are proposed to explain the subsidence pulses.  相似文献   

7.
Crustal xenoliths from basanitic dikes and necks that intruded into continental sediments of the Cretaceous Salta Rift at Quebrada de Las Conchas, Provincia Salta, Argentina were investigated to get information about the age and the chemical composition of the lower crust. Most of the xenoliths have a granitoid composition with quartz-plagioclase-garnet-rutile ± K-feldspar as major minerals. The exceedingly rare mafic xenoliths consist of plagioclase-clinopyroxene-garnet ± hornblende. All xenoliths show a well equilibrated granoblastic fabric and the minerals are compositionally unzoned. Thermobarometric calculations indicate equilibration of the mafic xenoliths in the granulite facies at temperatures of ca. 900 °C and pressures of ca. 10 kbar. The Sm-Nd mineral isochron ages are 95.1 ± 10.4 Ma, 91.5 ± 13.0 Ma, 89.0 ± 4.2 Ma (granitoid xenoliths), and 110.7 ± 23.6 Ma (mafic xenolith). These ages are in agreement with the age of basanitic volcanism (ca. 130–100 and 80–75 Ma) and are interpreted as minimum ages of metamorphism. Lower crustal temperature at the time given by the isochrons was above the closure temperature of the Sm-Nd system (>600–700 °C). The Sm-Nd and Rb-Sr isotopic signatures (147Sm/144Nd = 0.1225–0.1608; 143Nd/144Ndt 0 = 0.512000–0.512324; 87Rb/86Sr = 0.099–0.172; 87Sr/86Srt 0 = 0.708188–0.7143161) and common lead isotopic signatures (206Pb/204Pb = 18.43–18.48; 207Pb/204Pb = 15.62–15.70; 208Pb/204Pb = 38.22 –38.97) of the granitoid xenoliths are indistinguishable from the isotopic composition of the Early Paleozoic metamorphic basement from NW Argentina, apart from the lower 208Pb/204Pb ratio of the basement. The Sm-Nd depleted mantle model ages of ca. 1.8 Ga from granitoid xenoliths and Early Paleozoic basement point to a similar Proterozoic protolith. Time constraints, the well equilibrated granulite fabric, P-T conditions and lack of chemical zoning of minerals point to a high temperature in a crust of nearly normal thickness at ca. 90 Ma and to a prominent thermal anomaly in the lithosphere. The composition of the xenoliths is similar to the composition of the Early Paleozoic basement in the Andes of NW Argentina and northern Chile. A thick mafic lower crust seems unlikely considering low abundance of mafic xenoliths and the predominance of granitoid xenoliths. Received: 21 July 1998 / Accepted: 27 October 1998  相似文献   

8.
Mineral equilibria modeling involving solid solution calculations has been combined with mineral assemblage information from the alteration zones associated with gold mineralization to determine the T and X CO2 conditions for the formation of the Magdala gold deposit at Stawell, Victoria, Australia. Economic gold mineralization is primarily hosted within the stilpnomelane alteration zone of the Stawell Facies that is adjacent to the Magdala Basalt. Evolution of the Magdala gold deposit involved at least three fluid infiltration events: (1) a CO2-bearing fluid during the D2 deformation event produced carbonate spots throughout the chlorite zone; (2) a CO2–S–K-bearing fluid, accompanied the D3–4ab deformation and produced a muscovite zone and siderite rims on ankerite; and (3) a CO2–K–S–Au-bearing fluid during the D4c deformation event produced the stilpnomelane zone of the Stawell Facies, the proximal and distal alteration zones within the Magdala Basalt, and the main economic gold mineralization. Mineral equilibria modeling constrains the temperature of formation of the Magdala deposit to T = 345–390°C at 3kbar, substantially lower than indicated by other previous classical thermobarometry methods. Furthermore, this method has allowed the characterization of the mineralizing fluid and constrained its composition to X CO2 < 0.08 at 3kbar. The timing and composition of the mineralizing fluids are similar to that of metamorphic fluid generated from devolatilization of a greenstone pile with peak of metamorphism occurring earlier and at deeper levels in the crust.  相似文献   

9.
We present elemental and Sr–Nd–Pb isotopic data for the magmatic suite (~79 Ma) of the Harşit pluton, from the Eastern Pontides (NE Turkey), with the aim of determining its magma source and geodynamic evolution. The pluton comprises granite, granodiorite, tonalite and minor diorite (SiO2 = 59.43–76.95 wt%), with only minor gabbroic diorite mafic microgranular enclaves in composition (SiO2 = 54.95–56.32 wt%), and exhibits low Mg# (<46). All samples show a high-K calc-alkaline differentiation trend and I-type features. The chondrite-normalized REE patterns are fractionated [(La/Yb) n  = 2.40–12.44] and display weak Eu anomalies (Eu/Eu* = 0.30–0.76). The rocks are characterized by enrichment of LILE and depletion of HFSE. The Harşit host rocks have weak concave-upward REE patterns, suggesting that amphibole and garnet played a significant role in their generation during magma segregation. The host rocks and their enclaves are isotopically indistinguishable. Sr–Nd isotopic data for all of the samples display I Sr = 0.70676–0.70708, ε Nd(79 Ma) = −4.4 to −3.3, with T DM = 1.09–1.36 Ga. The lead isotopic ratios are (206Pb/204Pb) = 18.79–18.87, (207Pb/204Pb) = 15.59–15.61 and (208Pb/204Pb) = 38.71–38.83. These geochemical data rule out pure crustal-derived magma genesis in a post-collision extensional stage and suggest mixed-origin magma generation in a subduction setting. The melting that generated these high-K granitoidic rocks may have resulted from the upper Cretaceous subduction of the Izmir–Ankara–Erzincan oceanic slab beneath the Eurasian block in the region. The back-arc extensional events would have caused melting of the enriched subcontinental lithospheric mantle and formed mafic magma. The underplating of the lower crust by mafic magmas would have played a significant role in the generation of high-K magma. Thus, a thermal anomaly induced by underplated basic magma into a hot crust would have caused partial melting in the lower part of the crust. In this scenario, the lithospheric mantle-derived basaltic melt first mixed with granitic magma of crustal origin at depth. Then, the melts, which subsequently underwent a fractional crystallization and crustal assimilation processes, could ascend to shallower crustal levels to generate a variety of rock types ranging from diorite to granite. Sr–Nd isotope modeling shows that the generation of these magmas involved ~65–75% of the lower crustal-derived melt and ~25–35% of subcontinental lithospheric mantle. Further, geochemical data and the Ar–Ar plateau age on hornblende, combined with regional studies, imply that the Harşit pluton formed in a subduction setting and that the back-arc extensional period started by least ~79 Ma in the Eastern Pontides.  相似文献   

10.
J. Ebbing   《Tectonophysics》2004,380(1-2):89-104
The transition between European and Adriatic crust is an important feature related to the plate collision that formed the European Alps. The diversity of seismic and geological information allows the construction of two alternative 3D density models, which both match the observed gravity field. Different seismic experiments suggest a thickness for the Adriatic crust between 30 and 40 km. The thick crust model requires an unusually dense lower crust (>3050 kg/m3) to reproduce the observed Bouguer anomaly. To evaluate the two alternative models, the isostatic implications of the geometry and density distribution within both 3D models are investigated, using local (Airy) and regional (Vening Meinesz) isostasy.Airy isostatic investigations show that the Eastern Alps are not isostatically compensated and the residuals correlate strongly with exposed geological formations. Subsequently, subsurface loading is an important factor controlling isostatic processes. The different geometry and densities in the two 3D models imply different loading at the crust–mantle boundary. The subsurface loads calculated from the 3D density models were used to estimate regional isostasy by a convolution method. In general, small rigidity values (D<10×1021 Nm) are determined for the Eastern Alpine lithosphere. In the model with a 40-km-thick Adriatic crust, high flexural rigidities are inferred for the Adriatic plate (>100×1021 Nm), but these values are unusual for an active orogenic region. The results point to the interfingering of European and Adriatic crust that results in the squeezing of European crust between Adriatic crust and mantle with additional contamination by mantle material.  相似文献   

11.
A. B. Vrevsky 《Petrology》2011,19(5):521-547
This paper reports new geochemical and isotope data on the volcanogenic complexes of the Arvarench sequence of the Imandra-Varzuga paleorift structure of the Fennoscandian shield. It was established that these complexes are made up of komatiites, basalts, high-Mg andesites, and dacites and occupy a Sumian chronostratigraphic position with U-Pb (SHRIMP) age of 2429 ± 6.6 Ma in the regional Early Precambrian stratigraphic scale of the Kola-Norwegian province of the Fennoscandian shield, thus constraining the Sumian Subhorizon of the Lower Karelian Complex of the Northeastern Fennoscandian shield within 2450–2430 Ma. The high negative εNd, LREE enrichment, and the presence of different-age Archean zircons with REE patterns indicative of disequilibrium crystallization suggest that the parental dacitic melts were derived by anatectic melting of polychronous (3.2, 2.9, 2.8, 2.7 Ga) lithological complexes of the Archean continental crust of the Kola-Norwegian province of the Fennoscandian shield. Numerical petrological-geochemical modeling of generation and evolution of primary melts of the metavolcanic rocks made it possible to establish that the isotope-geochemical peculiarities of the komatiites, basalts, and basaltic andesites can be best described by fractional crystallization of primary komatiite melt contaminated by ∼ 2% of the Archean crustal material of tonalitic composition. The mantle protolith of primary melt in terms of its isotope-geochemical parameters was similar to the “enriched” mantle source of the Paleoproterozoic (2430–2450 Ma) mafic-ultramafic layered intrusions of the Kola-Norwegian province and Sumian metavolcanic rocks of the Fennoscandian shield. The high-Mg andesites of the Arvarench sequence were derived by fractionation of crustally contaminated (∼ 2%) magnesian basalts with elevated Al content (Al2O3 ∼ 15.6 wt %) in equilibrium with 40–50% Cpx 40-Ol 20-Opx 10-Pl 10-Mag 20 assemblage at P < 1 GPa. Obtained isotope-geochemical data and modeling results could be interpreted by off-subduction geodynamic model of the evolution of mantle plume and its interaction with the Archean continental lithosphere at the early stage of intracratonic rifting.  相似文献   

12.
The Afar Depression offers a rare opportunity to study the geodynamic evolution of a rift system from continental rifting to sea floor spreading. This study presents geochemical data for crustal and mantle xenoliths and their alkaline host basalts from the region. The basalts have enriched REE patterns, OIB-like trace element characteristics, and a limited range in isotopic composition (87Sr/86Sr = 0.70336–0.70356, ε Nd = +6.6 to +7.0, and ε Hf = +10.0 to +10.7). In terms of trace elements and Sr–Nd isotopes, they are similar to basalts from the Hanish and Zubair islands in the southern Red Sea and are thus interpreted to be melts from the Afar mantle. The gabbroic crustal xenoliths vary widely in isotope composition (87Sr/86Sr = 0.70437–0.70791, ε Nd = −8.1 to +2.5, and ε Hf = −10.5 to +4.9), and their trace element characteristics match those of Neoproterozoic rocks from the Arabian–Nubian Shield and modern arc rocks, suggesting that the lower crust beneath the Afar Depression contains Neoproterozoic mafic igneous rocks. Ultramafic mantle xenoliths from Assab contain primary assemblages of fresh ol + opx + cpx + sp ± pl, with no alteration or hydrous minerals. They equilibrated at 870–1,040°C and follow a steep geothermal gradient consistent with the tectonic environment of the Afar Depression. The systematic variations in major and trace elements among the Assab mantle xenoliths together with their isotopic compositions suggest that these rocks are not mantle residues but rather series of layered cumulate sills that crystallized from a relatively enriched picritic melt related to the Afar plume that was emplaced before the eruption of the host basalts.  相似文献   

13.
 The densification and structural changes in SiO2 glass compressed up to 43.4 GPa by shock experiments are investigated quantitatively by the X-ray diffraction technique. Direct structural data (average Si–O and Si–Si distances and Si–O–Si angles, coordination number of the Si atom) of these shock-densified SiO2 glasses have been obtained by analyzing the radial distribution function curves, RDF(r), calculated with X-ray diffraction data. The coordination number of all densified glasses is about 4 and shows almost no pressure variation. The SiO2 glass has shown density increase of 11% at a shock compression of 26.3 GPa. This density evolution could not be explained by the coordination change. The reduction of the average Si–O–Si angle (144° at 0 GPa to 136° at 26.3 GPa) obtained from RDF(r) data may account for this density increase. This Si–O–Si angle change may be caused by shrinkage of the network structure and the increase of small rings of SiO4 tetrahedra. For higher shock pressure, a decrease in the Si–O–Si angle to 140° was observed. This is consistent with the decrease in density at 32.0 and 43.2 GPa. This decrease in the Si–O–Si angle and density could be attributed to an annealing effect due to high after-shock residual temperature. This pressure dependence of average Si–O–Si angles in shock-densified SiO2 glass agrees with the results of our previous Raman spectroscopic study. On the other hand, the pressure variation for the first sharp diffraction peak (FSDP) was analyzed to estimate the evolution of intermediate range structures. It is suggested that the mean d value (d m ) obtained from the position of FSDP strongly depends on the shock and residual temperature, as well as shock pressure. Received: 29 June 2001 / Accepted: 14 November 2001  相似文献   

14.
By combining the carbon cycle model with the records of carbonate and organic (kerogen) carbon isotope, this paper presents the calculation of the fraction of organic carbon burial (f org) of beds 23–40 at the global boundary stratotype section and point (GSSP) of the Permian-Triassic boundary at Meishan, Zhejiang Province. The resulting calculation produces two episodes of f org maxima observed to occur at beds 23–24 and 27–29, which respectively corresponds to the two episodic anoxic events indicated by the flourish of green sulfur bacteria. Two episodic f org minima occurred at beds 25–26 and 32–34, generally coincident with the flourish of cyanobacteria (bed 26 and upper part of beds 29 to 34) as shown by the high value of 2-melthyhopnoanes. It appears that the f org is related to the redox conditions, with greater f org values observed under the reductive condition. The relationship between f org and the total organic carbon (TOC) content was complex. The f org value was low at some beds with a high TOC content (such as bed 26), while high observed at some beds with a low TOC content (e.g. bed 27). This association infers the important contribution of primary productivity to the TOC content. The original organic burial could be thus calculated through the configuration of the function of the primary productivity and f org, which can be used to correct the residual TOC measured today. This investigation indicates that compiling the organic-inorganic carbon isotopes with the carbon cycle model favors to understand the fraction of organic carbon burial, providing information for the reconstruction of the coupling among biota, environments and organic burial. Journal of China University of Geosciences, 2007, 32(6): 767–773 [译自: 地球科学—中国地质大学学报]  相似文献   

15.
1ＩｎｔｒｏｄｕｃｔｉｏｎＴｈｅＡｉｌａｏｓｈａｎ ＪｉｎｓｈａｊｉａｎｇＣｅｎｏｚｏｉｃａｌｋａｌｉ ｒｉｃｈｐｏｔａｓｓｉｃｍａｇｍａｔｉｃｂｅｌｔｅｘｔｅｎｄｓｏｖｅｒａｄｉｓｔａｎｃｅｏｆ370 0ｋｍｉｎｔｈｅＮＷ ＮＷＷｄｉｒｅｃｔｉｏｎ (ＺｈａｎｇＹｕｑｕａｎａｎｄＸｉｅＹｉｎｇｗｅｎ ,1 997) .Ｓｔｕｄｉｅｓｏｆｐｅｔｒｏｌｏｇｙａｎｄｇｅｏｃｈｅｍｉｓｔｒｙｉｎｄｉｃａｔｅｔｈａｔｔｈｅｉｎｔｅｒｍｅｄｉａｔｅｔｏｉｎｔｅｒｍｅｄｉａｔｅ ａｃｉｄａｌｋａｌｉ ｒｉｃｈｐｏｒｐｈｙｒｙｇｒ…  相似文献   

16.
Quantitative thermobarometry of inclusions in zoned garnet from a Franciscan eclogite block record a counter-clockwise P–T path from blueschist to eclogite and back. Garnet retains prograde zoning from inclusion-rich Alm52Grs30Pyp6Sps12 cores to inclusion-poor Alm62Grs25Pyp12Sps1 mantles, with overgrowths of highly variable composition. Barometry using the Waters–Martin version of the garnet–phengite–omphacite thermobarometer yields conditions of 7–15 kbar, 400–500°C (garnet cores), 18–22 kbar, ∼550°C (mantles), and 10–14 kbar, 350–450°C (overgrowths), in agreement with clinozoisite–sphene–rutile–garnet–quartz barometry. These pressures are ∼10–15 kbar less than those obtained using more recent, fully thermodynamic calibrations of the phengite–omphacite–garnet thermobarometer. Low early temperatures suggest that the block was subducted in a thermally mature subduction zone and not at the inception of subduction when prograde temperature is expected to be higher. Franciscan high-grade blocks likely represent crust subducted throughout the history of this convergent margin, rather than only at the inception of the subduction zone.  相似文献   

17.
The melting behaviour of three carbonated pelites containing 0–1 wt% water was studied at 8 and 13 GPa, 900–1,850°C to define conditions of melting, melt compositions and melting reactions. At 8 GPa, the fluid-absent and dry carbonated pelite solidi locate at 950 and 1,075°C, respectively; >100°C lower than in carbonated basalts and 150–300°C lower than the mantle adiabat. From 8 to 13 GPa, the fluid-present and dry solidi temperatures then increase to 1,150 and 1,325°C for the 1.1 wt% H2O and the dry composition, respectively. The melting behaviour in the 1.1 wt% H2O composition changes from fluid-absent at 8 GPa to fluid-present at 13 GPa with the pressure breakdown of phengite and the absence of other hydrous minerals. Melting reactions are controlled by carbonates, and the potassium and hydrous phases present in the subsolidus. The first melts, which composition has been determined by reverse sandwich experiments, are potassium-rich Ca–Fe–Mg-carbonatites, with extreme K2O/Na2O wt ratios of up to 42 at 8 GPa. Na is compatible in clinopyroxene with D\textNa\textcpx/\textcarbonatite = 10-18 D_{\text{Na}}^{{{\text{cpx}}/{\text{carbonatite}}}} = 10{-}18 at the solidus at 8 GPa. The melt K2O/Na2O slightly decreases with increasing temperature and degree of melting but strongly decreases from 8 to 13 GPa when K-hollandite extends its stability field to 200°C above the solidus. The compositional array of the sediment-derived carbonatites is congruent with alkali- and CO2-rich melt or fluid inclusions found in diamonds. The fluid-absent melting of carbonated pelites at 8 GPa contrasts that at ≤5 GPa where silicate melts form at lower temperatures than carbonatites. Comparison of our melting temperatures with typical subduction and mantle geotherms shows that melting of carbonated pelites to 400-km depth is only feasible for extremely hot subduction. Nevertheless, melting may occur when subduction slows down or stops and thermal relaxation sets in. Our experiments show that CO2-metasomatism originating from subducted crust is intimately linked with K-metasomatism at depth of >200 km. As long as the mantle remains adiabatic, low-viscosity carbonatites will rise into the mantle and percolate upwards. In cold subcontinental lithospheric mantle keels, the potassic Ca–Fe–Mg-carbonatites may freeze when reacting with the surrounding mantle leading to potassium-, carbonate/diamond- and incompatible element enriched metasomatized zones, which are most likely at the origin of ultrapotassic magmas such as group II kimberlites.  相似文献   

18.
We compiled a database containing more than 480000 determinations for 73 elements in melt inclusions in minerals and quenched glasses of volcanic rocks. These data were used to estimate the mean contents of major, volatile, and trace elements in igneous melts from main geodynamic settings. The following settings were distinguished: (I) oceanic spreading zones (mid-ocean ridges); (II) zones of mantle plume activity on oceanic plates (oceanic islands and plateaus); (III) and (IV) settings related to subduction processes, including (III) zones of island-arc magmatism generated on the oceanic crust and (IV) magmatic zones of active continental margins involving the continental crust into magma generation processes; (V) intracontinental rifts and continental hot spots; and (VI) back-arc spreading centers. The histogram of SiO2 contents in the natural igneous melts of all geodynamic settings exhibits a bimodal distribution with two maxima at SiO2 contents of 50–52 wt % and 72–74 wt %. The range 62–64 wt % SiO2 comprises the minimum number of determinations. Primitive mantle-normalized spidergrams were constructed for average contents of elements in the igneous melts of basic, intermediate, and acidic compositions from settings I–V. The diagrams reflect the characteristic features of melt compositions for each geodynamic setting. On the basis of the analysis of data on the composition of melt inclusions and glasses of rocks, average ratios of incompatible trace and volatile components (H2O/Ce, K2O/Cl, Nb/U, Ba/Rb, Ce/Pb, etc.) were estimated for the igneous melts of all of the settings. Variations of these ratios were determined, and it was shown that, in most cases, the ratios of incompatible elements are significantly different between settings. The difference is especially pronounced for the ratios of elements with different degrees of incompatibility (e.g., Nb/Yb) and for some ratios with volatile components (e.g., K2O/H2O).  相似文献   

19.
Petrogenesis of high Mg# adakitic rocks in intracontinental settings is still a matter of debate. This paper reports major and trace element, whole-rock Sr–Nd isotope, zircon U–Pb and Hf isotope data for a suite of adakitic monzogranite and its mafic microgranular enclaves (MMEs) at Yangba in the northwestern margin of the South China Block. These geochemical data suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived from subcontinental lithospheric mantle (SCLM) may account for the origin of high Mg# adakitic rocks in the intracontinental setting. The host monzogranite and MMEs from the Yangba pluton have zircon U–Pb ages of 207 ± 2 and 208 ± 2 Ma, respectively. The MMEs show igneous textures and contain abundant acicular apatite that suggests quenching process. Their trace element and evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.707069–0.707138, and εNd(t) = −6.5] indicate an origin from SCLM. Some zircon grains from the MMEs have positive εHf(t) values of 2.3–8.2 with single-stage Hf model ages of 531–764 Ma. Thus, the MMEs would be derived from partial melts of the Neoproterozoic SCLM that formed during rift magmatism in response to breakup of supercontinent Rodinia, and experience subsequent fractional crystallization and magma mixing process. The host monzogranite exhibits typical geochemical characteristics of adakite, i.e., high La/Yb and Sr/Y ratios, low contents of Y (9.5–14.5 ppm) and Yb, no significant Eu anomalies (Eu/Eu* = 0.81–0.90), suggesting that garnet was stable in their source during partial melting. Its evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.7041–0.7061, and εNd(t) = −3.1 to −4.3] and high contents of K2O (3.22–3.84%) and Th (13.7–19.0 ppm) clearly indicate an origin from the continental crust. In addition, its high Mg# (51–55), Cr and Ni contents may result from mixing with the SCLM-derived mafic magma. Most of the zircon grains from the adakitic monzogranite show negative εHf(t) values of −9.4 to −0.1 with two-stage Hf model ages of 1,043–1,517 Ma; some zircon grains display positive εHf(t) of 0.1–3.9 with single-stage Hf ages of 704–856 Ma. These indicate that the source region of adakitic monzogranite contains the Neoproterozoic juvenile crust that has the positive εHf(t) values in the Triassic. Thus, the high-Mg adakitic granites in the intracontinental setting would form by mixing between the crustal-derived adakitic magma and the SCLM-derived mafic magma. The mafic and adakitic magmas were generated coevally at Late Triassic, temporally consistent with the exhumation of deeply subducted continental crust in the northern margin of the South China Block. This bimodal magmatism postdates slab breakoff at mantle depths and therefore is suggested as a geodynamic response to lithospheric extension subsequent to the continental collision between the South China and North China Blocks.  相似文献   

20.
In this paper we consider an anisotropic scaling approach to understanding rock density and surface gravity which naturally accounts for wide range variability and anomalies at all scales. This approach is empirically justified by the growing body of evidence that geophysical fields including topography and density are scaling over wide range ranges. Theoretically it is justified, since scale invariance is a (geo)dynamical symmetry principle which is expected to hold in the absence of symmetry breaking mechanisms. Unfortunately, to date most scaling approaches have been self-similar, i.e., they have assumed not only scale invariant but also isotropic dynamics. In contrast, most nonscaling approaches recognize the anisotropy (e.g., the strata), but implicitly assume that the latter is independent of scale. In this paper, we argue that the dynamics are scaling but highly anisotropic, i.e., with scale dependent differential anisotropy. By using empirical density statistics in the crust and a statistical theory of high Prandtl number convection in the mantle, we argue that is a reasonable model for the 3-D spectrum (K is the horizontal wavevector and K is its modulus, k z is a vertical wavenumber), (s,H z ) are fundamental exponents which we estimate as (5.3,3), (3,3) in the crust and mantle, respectively. We theoretically derive expressions for the corresponding surface gravity spectrum. For scales smaller than ≈100 km, the anisotropic crust model of the density (with flat top and bottom) using empirically determined vertical and horizontal density spectra is sufficient to explain the (Bouguer) g z spectra. However, the crust thickness is highly variable and the crust-mantle density contrast is very large. By considering isostatic equilibrium, and using global gravity and topography data, we show that this thickness variability is the dominant contribution to the surface g z spectrum over the range ≈100–1000 km. Using estimates of mantle properties (viscosity, thermal conductivity, thermal expansion coefficient, etc.), we show that the mantle contribution to the mean spectrum is strongest at ≈1000 km and is comparable to the variable crust thickness contribution. Overall, we produce a model which is compatible with both the observed (horizontal and vertical) density heterogeneity and surface gravity anomaly statistics over a range of meters to several thousand kilometers.  相似文献   

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