首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
The isotopic compositions of Nd and Sr and concentrations of major and trace elements were measured in flows and tuffs of the Woods Mountains volcanic center of eastern California to assess the relative roles of mantle versus crustal magma sources and of fractional crystallization in the evolution of silicic magmatic systems. This site was chosen because the contrast in isotopic composition between Precambrian-to-Mesozoic country rocks and the underlying mantle make the isotope ratios sensitive indicators of the proportions of crustal- and mantle-derived magma. The major eruptive unit is the Wild Horse Mesa tuff (15.8 m.y. old), a compositionally zoned rhyolite ignimbrite. Trachyte pumice fragments in the ash-flow deposits provide information on intermediate composition magma types. Crustal xenoliths and younger flows of basalt and andesite (10 m.y. old) provide opportunities to confirm the isotopic compositions of potential mantle and crustal magma sources inferred from regional patterns. The trachyte and rhyolite have Nd values of -6.2 to -7.5 and initial 87Sr/86Sr ratios mostly between 0.7086 and 0.7113. These magmas cannot have been melted directly from the continental basement because the Nd values are too high. They also cannot have formed by closed system fractional crystallization of basalt because the 87Sr/86Sr ratios are higher than likely values for parental basalt. Both major and trace element variations indicate that crystal fractionation was an important process. These results require that the silicic magmas are end products of the evolution of mantle-derived basalt that underwent extensive fractional crystallization accompanied by assimilation of crustal rock. The mass fraction of crustal components in the trachyte and rhyolite is estimated to be between 10% and 40%, with the lower end of the range considered more likely. The generation of magmas with SiO2 contents greater than 60% appears to be dominated by crystal fractionation with minimal assimilation of upper crustal rocks.  相似文献   

2.
High-K mafic alkalic lavas (5.4 to 3.2 wt% K2O) from Deep Springs Valley, California define good correlations of increasing incompatible element (e.g., Sr, Zr, Ba, LREE) and compatible element contents (e.g., Ni, Cr) with increasing MgO. Strontium and Nd isotope compositions are also correlated with MgO; 87Sr/86Sr ratios decrease and ɛNd values increase with decreasing MgO. The Sr and Nd isotope compositions of these lavas are extreme compared to most other continental and oceanic rocks; 87Sr/86Sr ratios range from 0.7121 to 0.7105 and ɛNd values range from −16.9 to −15.4. Lead isotope ratios are relatively constant, 206Pb/204Pb ∼17.2, 207Pb/204Pb ∼15.5, and 208Pb/204Pb ∼38.6. Depleted mantle model ages calculated using Sr and Nd isotopes imply that the reservoir these lavas were derived from has been distinct from the depleted mantle reservoir since the early Proterozoic. The Sr-Nd-Pb isotope variations of the Deep Springs Valley lavas are unique because they do not plot along either the EM I or EM II arrays. For example, most basalts that have low ɛNd values and unradiogenic 206Pb/204Pb ratios have relatively low 87Sr/86Sr ratios (the EM I array), whereas basalts with low ɛNd values and high 87Sr/86Sr ratios have radiogenic 206Pb/204Pb ratios (the EM II array). High-K lavas from Deep Springs Valley have EM II-like Sr and Nd isotope compositions, but EM I-like Pb isotope compositions. A simple method for producing the range of isotopic and major- and trace-element variations in the Deep Springs Valley lavas is by two-component mixing between this unusual K-rich mantle source and a more typical depleted mantle basalt. We favor passage of MORB-like magmas that partially fused and were contaminated by potassic magmas derived from melting high-K mantle veins that were stored in the lithospheric mantle. The origin of the anomalously high 87Sr/86Sr and 208Pb/204Pb ratios and low ɛNd values and 206Pb/204Pb ratios requires addition of an old component with high Rb/Sr and Th/Pb ratios but low Sm/Nd and U/Pb ratios into the mantle source region from which these basalts were derived. This old component may be sediments that were introduced into the mantle, either during Proterozoic subduction, or by foundering of Proterozoic age crust into the mantle at some time prior to eruption of the lavas. Received: 28 February 1997 / Accepted: 9 July 1998  相似文献   

3.
The isotope-geochemical study of the Eocene-Oligocene magmatic rocks from the Western Kamchatka-Koryak volcanogenic belt revealed a lateral heterogeneity of mantle magma sources in its segments: Western Kamchatka, Central Koryak, and Northern Koryak ones. In the Western Kamchatka segment, magmatic melts were generated from isotopically heterogeneous (depleted and/or insignificantly enriched) mantle sources significantly contaminated by quartz-feldspathic sialic sediments; higher 87Sr/86Sr (0.70429–0.70564) and lower 143Nd/144Nd(ɛNd(T) = 0.06–2.9) ratios in the volcanic rocks from the Central Koryak segment presumably reflect the contribution of enriched mantle source; the high positive ɛNd(T) and low 87Sr/86Sr ratios in the magmatic rocks from the Northern Koryak segment area indicate their derivation from isotopically depleted mantle source without significant contamination by sialic or mantle material enriched in radiogenic Sr and Nd. Significantly different contamination histories of the Eocene-Oligocene mantle magmas in Kamchatka and Koryakia are related to their different thermal regimes: the higher heat flow beneath Kamchatka led to the crustal melting and contamination of mantle suprasubduction magmas by crustal melts. The cessation of suprasubduction volcanism in the Western Kamchatka segment of the continentalmargin belt was possibly related to the accretion of the Achaivayam-Valagin terrane 40 Ma ago, whereas suprasubduction activity in the Koryak segment stopped due to the closure of the Ukelayat basin in the Oligocene time.  相似文献   

4.
 Nd, Sr and O isotope systematics were used to investigate the petrogenesis of two adjacent plutons of the Bethlehem Gneiss (BG) and the Kinsman Quartz Monzonite (KQM), exposed within the Central Maine Terrane (CMT) of New England. Both are Acadian-aged (≈413 Ma) synmetamorphic and syntectonic members of the New Hampshire Plutonic Series (NHPS). Potential source rocks analyzed for this study include Silurian and Devonian metasedimentary rocks of the CMT, and Ordovician metasedimentary rocks and granitic gneisses of the Bronson Hill Anticlinorium (BHA), which border the CMT to the west. The ɛSr(413),  ɛNd(413) and δ18O values for the KQM range from 56.3 to 120.0, 2.8 to −6.4, and 7.6‰ to 12.9‰, respectively; values for the BG range from 7.4 to 144.7, 0.6 to −9.3, and 8.3‰ to 11.3‰, respectively; and values for possible source rocks range from 38.1 to 654.2, −10.7 to 5.4, and 6.2‰ to 14.1‰, respectively. Both the BG and KQM have extremely heterogeneous initial isotopic compositions consistent with mixing of multiple crustal source rocks, and neither contains a volumetrically significant (i.e., ≥10%) mantlederived component. Overlapping values of ɛNd(413),  ɛSr(413) and δ18O values for both the BG and KQM samples resemble values for metasedimentary host rocks of the CMT and BHA. We observe no systematic correlations between ɛNd and ɛSr values for either the BG or the KQM. The ɛSr and δ18O values for the BG do not form any simple mixing trends, nor is there any direct correlation between the isotopic compositions of contact BG samples and their adjacent host rocks, in contrast to our observations for the KQM (Lathrop et al. 1994). We propose that the KQM and BG magmas were generated through anatexis of metasedimentary rocks from both the BHA and CMT in response to crystal thickening during the Acadian orogeny. Melting may have been initiated within CMT metasediments in response to high heat production in these mid-crustal rocks combined with crustal thickening, whereas melting of BHA rocks with normal crustal heat production, which were located at lower-crustal levels than CMT rocks, is likely to have been driven by crustal thickening alone. Following upward advection of mobile BHA magmas, BHA- and CMT-derived magmas may have mingled during complex Acadian deformation in the CMT, thus accounting for the isotopic similarities we observe between the BG and the KQM. Received: 13 September 1994/Accepted: 31 January 1996  相似文献   

5.
 Cerro Panizos, a large caldera in the central Andes Mountains, produced two large dacitic ignimbrites at 7.9 Ma and 6.7 Ma and many andesitic and dacitic lava flows and domes. The older rhyodacitic Cienago Ignimbrite represents the most silicic magma erupted by the system. The younger, much larger volume dacitic Cerro Panizos Ignimbrite is very crystal-rich, containing up to 50% biotite, plagioclase, and quartz crystals in the pumice. It is weakly zoned, with most of the zoning apparent between two main cooling units. Major and most trace elements show little variation through the Cerro Panizos Ignimbrite, but the small range of composition is consistent with typical fractionation trends. Sr, Nd, and Pb isotopic ratios are very “crustal”, with initial 87Sr/86Sr values of 0.711 to 0.715, ɛNd values of –7.5 to –10.2, and nearly invariant Pb isotopic ratios (206Pb/204Pb=18.85, 207Pb/204Pb=15.67, and 208Pb/204Pb=38.80). The limited zonation observed in the Cerro Panizos Ignimbrite is explained by impeded crystal settling due to high crystal content. The magma body was a crystal-liquid mush before ascent to the pre-eruption crustal levels. Crystals formed, but did not separate easily from the magma. Limited fractionation of plagioclase and biotite may have occurred, but the composition was largely controlled by lower crustal MASH processes. AFC modeling shows that the Cerro Panizos magmas resulted from a mixture of roughly equal proportions of late Miocene mantle-derived basalts and melts from ∼1.0 Ga (Grenville age) lower crust. This occurred in a MASH zone in the lower crust, and set the crustal isotopic ratios observed in the Cerro Panizos magmas. The great thickening of the crust beneath the central Andes Mountains sent upper and middle crustal rock types to lower crustal (and deeper) depths, and this explains the “upper crustal” isotopic signatures of the Cerro Panizos rocks. Minor upper crustal assimilation of early Miocene volcanic or subvolcanic rocks produced much of the isotopic variation seen in the system. The nearly invariant high Pb isotopic values and high Pb concentrations indicate that Pb came almost entirely from the crustal source, and was little altered by any subsequent upper crustal assimilation. This Pb signature is isotopically similar to that of the southern Bolivian Tin Belt, suggesting a widely distributed Pb source. The great difference between compositions of Miocene and Quaternary central Andean volcanic rocks is explained by crustal thickening in early Miocene time leading to abundant lower crustal water and associated fluxed melting during the time of the earlier eruptions. The lower crust dried out considerably by Quaternary time, so less crustal component is present. Received: 22 December 1994 / Accepted: 13 September 1995  相似文献   

6.
The Los Encinos Volcanic Field (LEVF) consists of Miocene (10.6–13.6 Ma) hawaiitic volcanic necks and lava-capped mesas that crop out sparsely over an area of 11,500 km2 at the eastern margin of the Mexican Basin and Range Province (BRP). The LEVF rocks are similar to other early extensional hawaiites from the southern BRP, and provide numerous contrasts with younger basanites and alkali basalts that erupted during the Quaternary at the Ventura and Santo Domingo Volcanic Fields about 100 km to the south. A suite of 18 LEVF hawaiites was studied in thin section, and analyzed for mineral compositions, whole-rock major and trace element compositions, and Sr, Nd, and Pb isotopic ratios. All samples contain the stable minerals plagioclase (An53–64), olivine (Fo61–88), clinopyroxene, titanomagnetite, and minor biotite. Most samples also contain a complex assemblage of resorbed and reacted xenocrysts and megacrysts. Some of these minerals appear to have crystallized slowly from related, but more differentiated magmas, but other xenocrysts were clearly derived from lower-crustal, high-grade orthogneisses and paragneisses that are found as large xenoliths in the nearby Quaternary volcanic fields. Quartz xenocrysts are especially common in many hawaiites (up to 3.9 vol.%) and show a wide range of reaction styles. One sample contains microxenoliths of sillimanite- and quartz-bearing paragneiss with fine-grained domains that are interpreted as coronal structures related to original garnet xenocrysts. The geochemical effects of crustal contamination in the LEVF hawaiites vary widely. Five samples appear to be essentially uncontaminated (type U). Aside from being somewhat differentiated from a more primitive parent, the type-U samples can be used to infer the geochemistry of the mantle that was melting during the early stages of basin-and-range rifting. Type-U samples range up to εNd=+7.6 and down to 87Sr/86Sr = 0.70286 and 206Pb/204Pb = 18.74, compositions that are more extreme than any of the nearby Quaternary volcanic rocks. The other 13 samples are divided into two contamination types, A and B. Both types show trends toward higher 87Sr/86Sr (to 0.7040) and 206Pb/204Pb (to 18.98), lower εNd (to +3.1), and elevated Yb, which appear to reflect bulk or AFC-style contamination by granulites, particularly garnet-bearing paragneisses. Type-A hawaiites also show selective enrichments in Cs, Rb, Th, Sb, U, Pb, K, and Si. These elements were probably transferred into the type-A hawaiitic magmas through mixing with low-degree partial melts from deep-crustal granulites. The enrichments of these elements in type-A hawaiites complement the depletions of many of these same elements in high-grade granulites worldwide and provide insight into the origin of those depletions. Mixing models between type-U hawaiites and paragneiss xenoliths indicate that up to 45% of the Pb found in type-A hawaiites is crustally derived. In comparison with more mafic Quaternary basanitic rocks from the volcanic fields to the south, which carried large peridotite and granulite xenoliths to the surface, the LEVF hawaiites are relatively differentiated and megacryst rich, but free of large xenoliths, and show a wide variety of petrographic and geochemical evidence for crustal contamination. These differences probably reflect the slow and interrupted ascent of the LEVF hawaiites during early stages of basin-and-range extension in the Miocene, when the crust had a somewhat lower density and the entire lithosphere was relatively thick and cool. We argue that Quaternary basanites were able to ascend significantly faster through the thinner, hotter, and more fractured and extended lithosphere, whose crust was made denser by mafic intrusions during the preceding magmatic episode. Consequently the Quaternary basanites rose without stagnating and interacting with crustal lithologies, and without losing their entrained peridotite xenoliths. Received: 29 September 1993 / Accepted: 16 May 1994  相似文献   

7.
 Alkali basalts and nephelinites from the volcanic province of northern Tanzania contain pyroxene and nepheline that show evidence for chemical and/or isotopic disequilibria with their host magmas. Olivine, pyroxene, nepheline and plagioclase all appear to be partially xenocrystic in origin. Five whole rock/mineral separate pairs have been analyzed for Sr, Nd, and Pb isotopic compositions. The 206Pb/204Pb ratios are distinct by as much as 20.94 (whole rock) vs. 19.10 (clinopyroxene separate). The Sr and Nd isotopic disequilibria vary from insignificant in the case of nepheline, to Δ 87Sr/86Sr of 0.0002 and ΔɛNd of 0.7 in the case of clinopyroxene. The mineral chemistry of 25 samples indicates the ubiquitous presence of minerals that did not crystallize from a liquid represented by the host rock. The northern Tanzanian magmas are peralkaline and exhibit none of the xenocrystic phases expected from crustal assimilation. The disequilibria cannot be the result of mantle source variations. Rather the xenocrystic phases present appear to have been derived from earlier alkali basaltic rocks or magmas that were contaminated by the crust. Material from this earlier magma was then mixed with batches of magma that subsequently erupted on the surface. Disequilibrium in volcanic rocks has potentially serious consequences for the use of whole rock data to identify source reservoirs. However, mass balance calculations reveal that the 206Pb/204Pb isotopic compositions of the erupted lavas were changed by less than 0.25% as a result of this indirect crustal contamination. Received: 15 February 1995 / Accepted: 4 May 1996  相似文献   

8.
We report geochronological, geochemical and isotopic data for the Mesozoic Shangshuiquan granite from the northern margin of the North China craton. The granite is highly fractionated, with SiO2 > 74%. Occurrence of annitic biotite, high contents of alkalis (K2O + Na2O), Rb, Y, Nb and heavy rare earth elements, high FeOt/MgO, low contents of CaO, Al2O3, Ba, and Sr, and large negative Eu anomalies, makes it indistinguishable from typical A-type granites. A mantle-derived origin for the rocks of the granite is not favored because their high initial 87Sr/86Sr (≥0.706) and low εNd (t) (<−15) are completely different from either those of the lithospheric or asthenospheric mantle. In fact, their Sr–Nd isotopes fall within the range of Sr–Nd isotopic compositions of the Archean granulite terrains and are comparable to those of Mesozoic crustal-derived I-type granitoids in the region. Therefore, the Shangshuiquan granite is considered to be dominantly derived from partial melting of the ancient lower crust. Its parental magmas prove to be similar to I-type magmas and to have undergone extensive fractionation during its ascent. This is supported by the fact that some of the nearby Hannuoba feldspar-rich granulite xenoliths can be indeed regarded as the early cumulates in terms of their mineralogy, chemistry, Sr–Nd isotopes and zircon U–Pb ages and Hf isotopes. It is furthermore argued that some of highly fractionated granites worldwide, especially those with A-type characteristics and lacking close relationship with unfractionated rocks, may in fact be fractionated I-type granites. This suggestion can explain their close temporal and spatial associations as well as similar Sr–Nd isotopes with I-type granites. Our study also sheds new light on the petrogenesis of deep crustal xenoliths.  相似文献   

9.
Summary The Tyrrhenian border of the Italian peninsula has been the site of intense magmatism from Pliocene to recent times. Although calc-alkaline, potassic and ultrapotassic volcanism overlaps in space and time, a decrease of alkaline character in time and space (southward) is observed. Alkaline ultrapotassic and potassic volcanic rocks are characterised by variable enrichment in K and incompatible elements, coupled with consistently high LILE/HFSE values, similar to those of calc-alkaline volcanic rocks from the nearby Aeolian arc. On the basis of mineralogy and major and trace element chemistry two different arrays can be recognised among primitive rocks; a silica saturated trend, which resulted in formation of leucite-free mafic rocks, and a silica undersaturated trend, charactrerised by leucite-bearing rocks. Initial 87Sr/86Sr and 143Nd/144Nd values of Italian ultrapotassic and potassic mafic rocks range from 0.70506 to 0.71672 and from 0.51173 to 0.51273, respectively. 206Pb/204Pb values range between 18.50 and 19.15, 207Pb/204Pb values range between 15.63 and 15.70, and 208Pb/204Pb values range between 38.35 and 39.20. The general εSr vs. εNd array, along with crustal lead isotopic values, clearly indicates that a continental crustal component has played an important role in the genesis of these magmas. The main question is where this continental crustal component has been acquired by the magmas. Volcanological and petrologic data indicate continental crustal contamination to be a leading process along with fractional crystallisation and magma mixing. Considering, however, only the samples thought to represent primary magmas, which have been in equilibrium with their mantle source, a clearer picture emerges. A large variation of εSr vs. εNd is still observed, with εSr from −2 to +180 and εNd from + 2 to −12. A bifurcation of this array is observed in the samples that plot in the lower right quadrant, with mafic leucite-bearing Roman Province rocks buffered at εSr = + 100 whereas the mafic leucite-free potassic and ultrapotassic rocks point to strongly radiogenic Sr compositions. We may argue that mafic leucite-bearing Roman Province rocks point to εSr and εNd values similar to those of Miocene carbonate sediments whereas mafic leucite-free potassic and ultrapotassic rocks point to a silicate upper crust end-member. Lead isotopes plot well inside the field of island arcs, overlapping the values of pelagic sediments as well, but bifurcation between the samples north and south of Rome is observed. The main characteristic for the mantle source of Italian potassic and ultrapotassic magmas is the clear upper crustal signature acquired prior to partial melting through metasomatic agents released by the subducted slab. In addition, one lithospheric mantle source in the north and an asthenospheric mantle source, pointing to an HIMU reservoir, in the south were recognised. The chemical and isotopic differences observed between the northern and southern sectors of the magmatic region were possibly due to the presence of a carbonate-rich component in the crustal enriching agent in the south. One crustal component might have been generated by melting of silicate metasedimentary rocks or sediments from an ancient subducted slab. The second one might reflect the activity of mostly CO2-rich fluid released more recently by the incipient subduction of carbonate sedimentary rocks. Received February 16, 2000; revised version accepted September 6, 2001  相似文献   

10.
Middle to Late Jurassic plutonic rocks in the central Mojave Desert represent the continuation of the Sierran arc south of the Garlock fault. Rock types range from calc-alkaline gabbro to quartz monzonite. Chemical and isotopic data indicate that petrologic diversity is attributable to mixing of crustal components with mantle melts. Evidence for magma mixing is scarce in most plutons, but emplacement and injection of plutons into preexisting wallrocks (e.g. pendants of metasedimentary rocks) suggests that assimilation may be locally important. Field and petrographic evidence and major and trace element data indicate that the gabbros do not represent pure liquids but are, at least partly, cumulates. The cumulate nature of the gabbros coupled with field evidence for open-system contamination means that trace element contents of gabbros cannot be used to fingerprint the Jurassic mantle source, nor can isotopic data be unequivocally interpreted to reflect the isotopic composition of the mantle. Correlation of Sr and Nd isotropic composition with bulk composition allows some constraints to be placed on the mantle isotopic signature. Gabbros and mafic inclusions from localities north of Barstow, CA have the most depleted mantle-like isotopic signatures (Sr ( i )≈0.705 and ɛNd (t)=≈0 to +1). However, these rocks have likely seen some contamination as well, so the mantle source probably has an even more depleted character. Gabbros with the lowest Sr( i ) and highest ɛNd (t) are also characterized by the highest 207Pb/204Pb and 206Pb/204Pb in the entire data set. This may be a feature of the mantle component in the Jurassic arc indicative of minor source contamination with subducted sediment as has been observed in modern continental arcs. Locally exposed Precambrian basement and metasedimentary rocks have appropriate Sr, Nd and Pb isotopic signatures for the crustal end members and are possible contaminants. Incorporation of these components through combined anatexis and assimilation can explain the observed spread in isotopic composition. Evidence for a depleted mantle component in these gabbros contrasts with the enriched subcontinental mantle component in Jurassic arc plutons further to the east and suggests there may have been a major mantle lithosphere boundary between the two areas as far back as the Late Jurassic. Crustal boundaries and isotopic provinces defined on the basis of initial isotopic composition (Sr( i )=0.706 isopleth) are difficult to delineate because of the correlation of bulk composition with Sr and Nd isotopic composition and because values may differ depending on the age of the rocks sampled within a given area. Data from plutons intruded into rocks known or inferred to be Precambrian are, however, shifted dramatically (highest Sr( i ) and lowest ɛNd(t)) toward Precambrian values. The least isotopically evolved rocks (lowest Sr( i ) and highest ɛNd(t)) occur within the eugeoclinal belt of the Mojave Desert. This zone has been previously identified as a Precambrian rift zone but more likely represents a zone where mantle magmas have been intruded into isotopically similar crustal rocks of the eugeocline with minor input from old Precambrian crust. Received: 12 August 1993/Accepted: 8 July 1994  相似文献   

11.
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   

12.
Mafic and intermediate granulite xenoliths, collected from Cenozoic alkali basalts, provide samples of the lower crust in western Saudi Arabia. The xenoliths are metaigneous two-pyroxene and garnet granulites. Mineral and whole rock compositions are inconsistent with origin from Red Sea rift-related basalts, and are compatible with origin from island arc calc-alkaline and low-potassium tholeiitic basalts. Most of the samples are either cumulates from mafic magmas or are restites remaining after partial melting of intermediate rocks and extraction of a felsic liquid. Initial87Sr/86Sr ratios are less than 0.7032, except for two samples at 0.7049. The Sm-Nd data yield TDM model ages of 0.64 to 1.02 Ga, similar to typical Arabian-Nubian Shield upper continental crust. The isotopic data indicate that the granulites formed from mantle-derived magmas with little or no contamination by older continent crust. Calculated temperatures and pressures of last reequilibration of the xenoliths show that they are derived from the lower crust. Calculated depths of origin and calculated seismic velocities for the xenoliths are in excellent agreement with the crustal structure model of Gettings et al. (1986) based on geophysical data from western Saudi Arabia. Estimation of mean lower crustal composition, using the granulite xenoliths and the Gettings et al. (1986) crustal model, suggests a remarkably homogeneous mafic lower crust, and an andesite or basaltic andesite bulk composition for Pan-African juvenile continental crust.  相似文献   

13.
The paper presents data on the Nd-Sr systematics of magmatic rocks of the Khaidaiskii Series of the Anginskaya Formation in the Ol’khon region, western Baikal area, and rocks of the Talanchanskaya Formation on the eastern shore of Lake Baikal. Geochemical characteristics of these rocks are identical and testify to their arc provenance. At the same time, the ɛNdtof rocks of the Khaidaiskii Series in the Ol’khon area has positive values, and the data points of these rocks plot near the mantle succession line in the ɛNdt-87Sr/86Sr diagram, whereas the ɛNdt values of rocks of the Talanchanskaya Formation are negative, and the data points of these rocks fall into the fourth quadrant in the ɛNdt-87Sr/86Sr diagram. This testifies to a mantle genesis of the parental magmas of the Khaidaiskii Series and to the significant involvement of older crustal material in the generation of the melts that produced the orthorocks on the eastern shore of the lake. These conclusions are corroborated by model ages of magmatic rocks in the Ol’khon area (close to 1 Ga) and of rocks of the Talanchanskaya Formation (approximately 2 Ga). The comparison of our data with those obtained by other researchers on the Nd-Sr isotopic age of granulites of the Ol’khon Group and metavolcanics in various structural zones in the northern Baikal area suggests, with regard for the geochemistry of these rocks, the accretion of tectonic nappes that had different isotopic histories: some of them were derived from the mantle wedge and localized in the island arc itself (magmatic rocks of the Anginskaya Formation) or backarc spreading zone (mafic metamagmatic rocks of the Ol’khon Group), while others were partial melts derived, with the participation of crustal material, from sources of various age (metagraywackes in the backarc basin in the Ol’khon Group and the ensialic basement of the island arc in the Talanchanskaya Formation).  相似文献   

14.
Arc magmas ranging in composition from basaltic andesites to rhyolites and intrusive equivalents were emplaced into the western margin of the North American craton starting in Late Triassic time giving way to rift0related sedimentation in the Late Jurassic. The region of this study cuts across Proterozoic basements of contrasting Nd model ages, 1.7–1.8 Ga (average ɛNd∼−11) in eastern Arizona and 2.0 to 2.3 GA (average ɛNd∼−18) in western Arizona and eastern California (Bennett and DePaolo 1987). The Mesozoic rocks have initial ɛNd of -3.4 to-6.4 in the eastern part of the study area and -7.1 to -9.2 in the western part. All of the rocks have elevated 87Sr/87Sr initial ratios (>0.706). Trends in initial ɛNd values of Mesozoic arc rocks are directly correlated with the Nd model ages of the basement through which they passed. Simple two-component mixing calculations indicate that recycled continental crust in the arc magmas represents on average about 65%. A minimum of 35% mantle input into continental arc magmas, as recent as the Mesozoic, represents a significant contribution to the growth of the continental crust, in the absence of a return flow of continental material into the mantle of similar magnitude. In a detailed study in the Santa Rita Mountains. Arizona, there is a pattern of increase of ɛNd with time: early basaltic andesites have more negative ɛNd than later felsic rocks. A correlated pattern of depletion with time is also observed with trace element and major element data. We attribute this either to progressive hybridization of the lower crust by repeated injection of mantle magmas, or the progressive thinning of the continental crust during prolonged arc magmatism. The present data do not allow distinction between the two models. Progressive decrease in crustal contribution to arc magmas with time may be an important feature of continental arc evolution. Hybridization of the lower crust due to repeated injection of mantle melts during arc magmatism may help contribute to small-scale heterogeneities in lower crust inferred from seismic and xenolith data. Similarly, whether there is a well defined MOHO or sharp crust-mantle boundary in any given segment of the continental crust may in part depend on the extent of crust modification as a result of continental arc magmatism.  相似文献   

15.
Subduction-related Mesozoic to Cainozoic granites s.l. in western Palmer Land, Antarctic Peninsula, have similar chemical compositions to Archean tonalite-trondhjemite-granodiorite (TTG) suites, Phanerozoic slab-melts (adakites), and to experimental partial melts of basaltic material in equilibrium with amphibole ± pyroxene ± garnet. They are predominantly sodic, metaluminous and most have Al2O3 > 15 wt% and Y < 18 ppm. All are light rare earth element (LREE)-enriched (2 < La/Ybn <30) and most have small Eu anomalies. They have a wide range of initial ɛNd(t) (−6.8 to +4.5) and ɛSr(t) (+293.4 to −3.7), but most Pb isotope compositions deviate by < 0.3% from their mean. The Pb isotope data indicate a crustal component to all the granites, which Sr and Nd isotope variations suggest is pre-Triassic–Triassic. The 207Pb/204Pb(t) range from 15.602 to 15.666 and appear to preclude a significant Proterozoic, or older, crustal component. The granites have chemical and isotopic compositions that suggest they are not partial melts of subducted oceanic lithosphere, as has been suggested for some Archean and Phanerozoic TTG magmas. We conclude that they were produced by mixing between basaltic-andesitic arc magmas, partial melts of juvenile basaltic lower crust and pre-Triassic crust. The low H(heavy)REE+Y content of some of the granites requires that garnet was a residual phase in the crust during partial melting, indicating a crustal thickness of >36 km. Between Triassic and Tertiary times the initial ɛNd(t) of the magmatism increased and ɛSr(t) decreased, suggesting that new continental crust was produced during this period. Underplating by mafic magma was an important crustal growth mechanism in the arc: the generation of abnormally thick crust, and its subse quent fusion, is considered to be a consequence of ca. ≥ 180 Ma of subduction and associated magmatism in the region. An implication of the model is that dense garnet-amphibolite and eclogite residues from partial melting of the lower crust will accumulate. In theory, the setting was appropriate for such residues to detach from the base of the crust and to sink into the convecting mantle. Such a process would leave the rest of the crust enriched in large ion lithophile elements/LREE, but depleted in HREE+Y. Received: 2 October 1995 / Accepted: 5 January 1997  相似文献   

16.
Nd, Sr and O isotopic data were obtained from silicic ash-flow tuffs and lavas at the Tertiary age (16–9 Ma) Timber (Mountain/Oasis Valley volcanic center (TMOV) in southern Nevada, to assess models for the origin and evolution of the large-volume silicic magma bodies generated in this region. The large-volume (>900 km3), chemically-zoned, Topopah Spring (TS) and Tiva Canyon (TC) members of the Paintbrush Tuff, and the Rainier Mesa (RM) and Ammonia Tanks (AT) members of the younger Timber Mountain Tuff all have internal Nd and Sr isotopic zonations. In each tuff, high-silica rhyolites have lower initial Nd values (1 Nd unit), higher87Sr/86Sr, and lower Nd and Sr contents, than cocrupted trachytes. The TS, TC, and RM members have similar Nd values for high-silica rhyolites (-11.7 to -11.2) and trachytes (-10.5 to -10.7), but the younger AT member has a higher Nd for both compositional types (-10.3 and -9.4). Oxygen isotope data confirm that the TC and AT members were derived from low Nd magmas. The internal Sr and Nd isotopic variations in each tuff are interpreted to be the result of the incorporation of 20–40% (by mass) wall-rock into magmas that were injected into the upper crust. The low Nd magmas most likely formed via the incorporation of low 18O, hydrothermally-altered, wall-rock. Small-volume rhyolite lavas and ash-flow tuffs have similar isotopic characteristics to the large-volume ash-flow tuffs, but lavas erupted from extracaldera vents may have interacted with higher 18O crustal rocks peripheral to the main magma chamber(s). Andesitic lavas from the 13–14 Ma Wahmonie/Salyer volcanic center southeast of the TMOV have low Nd (-13.2 to -13.8) and are considered on the basis of textural evidence to be mixtures of basaltic composition magmas and large proportions (70–80%) of anatectic crustal melts. A similar process may have occurred early in the magmatic history of the TMOV. The large-volume rhyolites may represent a mature stage of magmatism after repeated injection of basaltic magmas, crustal melting, and volcanism cleared sufficient space in the upper crust for large magma bodies to accumulate and differentiate. The TMOV rhyolites and 0–10 Ma old basalts that erupted in southern Nevada all have similar Nd and Sr isotopic compositions, which suggests that silicic and mafic magmatism at the TMOV were genetically related. The distinctive isotopic compositions of the AT member may reflect temporal changes in the isotopic compositions of basaltic magmas entering the upper crust, possibly as a result of increasing basification of a lower crustal magma source by repeated injection of mantle-derived mafic magmas.  相似文献   

17.
The Closepet batholith in South India is generally considered as a typical crustal granite emplaced 2.5 Ga ago and derived through partial melting of the surrounding Peninsular Gneisses (3.3 to 3.0 Ga). In the field, it appears as a composite batholith made up of at least two groups of intrusions. (a) An early SiO2-poor group (clinopyroxene quartz-monzonite and porphyritic phyritic monzogranite) is located in the central part of the batholith. These rocks display a narrow range in both initial 87Sr/86Sr (0.7017–0.7035) and Nd(–0.9to –4.1). (b) A later SiO2-rich group (equigranular grey and pink granites) is located along the interface between the SiO2-poor group and the Peninsular Gneisses. They progressively grade into migmatised Peninsular Gneisses, thus indicating their anatectic derivation. Their isotopic characteristics vary over a wide range (87Sr/86Sr ratios=0.7028–0.7336 and Nd values from-2.7 to-8.3, at 2.52 Ga). Field and geochronological evidence shows that the two groups are broadly contemporaneous (2.518–2.513 Ga) and mechanically mixed. This observation is supported by the chemical data that display well defined mixing trends in the Sr vs Nd and elemental variation diagrams. The continuous chemical variation of the two magmatic bodies is interpreted in terms of interaction and mixing of two unrelated end-members derived from different source regions (enriched peridotitic mantle and Peninsular Gneisses). It is proposed that the intrusion of mantle-derived magmas into mid-crustal levels occurred along a transcurrent shear zone; these magmas supplied additional heat and fluids that initiated anatexis of the surrounding crust. During this event, large-scale mixing occurred between mantle and crustal melts, thus generating the composite Closepet batholith. The mantle-derived magmatism is clearly associated with granulite facies metamorphism 2.51±0.01 Ga ago. Both are interpreted as resulting from a major crustal accretion event, possibly related to mantle plume activity.  相似文献   

18.
Granitoids within the Precambrian basement of north-eastern and southern Somalia are subdivided on the basis of geology, geochronology and petrology into three different assemblages. The post-kinematic assemblage in north-eastern Somalia ( 630 Ma) comprises granodiorites and granites which belong to a medium-K calc-alkaline suite. Average initial Sr, Nd and Pb isotopic ratios [Sri = 0.7048, Nd = –1.8,206Pb/204Pb(i) = 17.704 and207Pb/204Pb(i) = 15.611] indicate that these melts were derived from a mantle or juvenile crustal source with only slight involvement of pre-existing crust as a contaminant. Two different assemblages are found in southern Somalia. The older assemblage is composed of crustal anatectic, synkinematic, parautochthonous granites ( 600 Ma) related to amphibolite facies retrogression of an intensively reworked pre-Pan-African crust [Sri = 0.7100, Nd = –8.4,206Pb/204Pb(i) = 15.403 and207Pb/204Pb(i) = 15.259]. These monzo- and syenogranites are moderately potassic and peraluminous. The younger assemblage ( 470 Ma) consists of post-kinematic monzonites to syenogranites with A-type affinities. Initial Sr, Nd and Pb isotopic data for this metaluminous assemblage [Sri = 0.7114, Nd = –13.1,207Pb/204Pb(i) = 16.913 and207Pb/204Pb(i) = 15.512] indicate a significant lower crustal component but, however, also a mantle signature. The late Proterozoic to early Palaeozoic granitoids in Somalia thus express contrasting regimes, characterized by strong juvenile input in the north, close to the Arabian-Nubian Shield, whereas intense crustal reworking with little addition of juvenile material prevailed in the south. Somalia was definitively not a cratonic area during the Pan-African, but a zone of high crustal mobility.  相似文献   

19.
Alkalic and tholeiitic basalts were erupted in the central Arizona Transition Zone during Miocene-Pliocene time before and after regional faulting. The alkalic lava types differ from the subalkaline lavas in Sr, Nd and Pb isotopic ratios and trace element ratios and, despite close temporal and spatial relationships, the two types appear to be from discrete mantle sources. Pre-faulting lava types include: potassic trachybasalts (87Sr/86Sr = 0.7052 to 0.7055, Nd= –9.2 to –10.7); alkali olivine basalts (87Sr/ 86Sr = 0.7049 to 0.7054, Nd= –2 to 0.2); basanite and hawaiites (87Sr/86Sr = 0.7049 to 0.7053, Nd= –3.5 to –7.8); and quartz tholeiites (87Sr/86Sr = 0.7047, Nd= –1.4 to –2.6). Post-faulting lavas have lower 87Sr/86Sr (<0.7045) and Nd from –3.2 to 2.3. Pb isotopic data for both preand post-faulting lavas form coherent clusters by magma type with values higher than those associated with MORB but within the range of values found for crustal rocks and sulfide ores in Arizona and New Mexico. Pb isotopic systematics appear to be dominated by crustal contamination. Effects of assimilation and fractional crystallization are inadequate to produce the Sr isotopic variations unless very large amounts of assimilation occurred relative to fractionation. It is impossible to produce the Nd isotopic variations unless ancient very unradiogenic material exists beneath the region. Moreover the assumption that the alkalic lavas are cogenetic requires high degrees of fractionation inconsistent with major- and trace-element data. Metasomatism of the subcontinental lithosphere above a subduction zone by a slab-derived fluid enriched in Sr, Ba, P and K could have produced the isotopic and elemental patterns. The degree of metasomatism apparently decreased upward, with the alkalic lavas sampling more modified regions of the mantle than the tholeiitic lavas. Such metasomatism may have been a regional event associated with crustal formation at about 1.6 Ga. Disruption and weakening of the subcontinental lithosphere in the Transition Zone of the Colorado Plateau by volcanism probably made deformation possible.  相似文献   

20.
Trace element and isotopic compositions of mid-Tertiary siliceous magma sequences from two localities of the Sierra Madre Occidental, northern Mexico, display differences that reflect the composition and age of the basement through which they erupted. The crust beneath the section at San Buenaventura is thicker and more evolved and forms part of the North American basement, while that under El Divisadero consists of allochthonous terranes of island arc/oceanic? crust accreted during the Mesozoic.The volcanics are highly differentiated and range in composition from basalt to rhyolite (SiO2=50–76%). Those erupted through the accreted terranes display a small range of isotope ratios and have lowest initial (age-corrected) Sr isotope ratios (>0.7044) and the highest Nd (<0.5126) and Pb isotope ratios (206Pb/204Pb ∼18.9). Isotope ratios of the continental suite are more variable and form an array which trends away from that of the accreted terrane suite toward compositions more typical of old crust (to 87Sr/86Sr ∼0.710 and 143Nd/144Nd ∼0.5123). The volcanics in the continental zone are relatively more enriched in moderately incompatible elements compared with those within the accreted terranes (Ce/Yb=25–45 vs. 13–33, respectively), but are depleted in some highly incompatible elements such as U and Rb (e.g., Th/U=3.8–7.5 vs. 2.5–4.0, respectively). Those higher in the stratigraphic sections have higher 87Sr/86Sr, 208Pb/204Pb, and Th/U ratios, and lower 143Nd/144Nd ratios than those lower in the sections.The data have implications for the nature of the sources and the petrogenesis of these volcanics. The isotope ratios of both suites fall between those of mafic magma compositions from the Sierra Madre Occidental, and intermediate and felsic lower crustal xenoliths in northern Mexico and the southwestern USA. The relationship between the isotope ratios of the sequences and the age of the basement, combined with the fact that the overall data set forms well-defined isotopic arrays, demonstrates the strong effects of the crust on the chemistry of the silicic magmas. In the continental suite, isotope ratios covary with Th/Pb and U/Pb ratios, approaching the compositions found in the intermediate and felsic granulite facies xenoliths, strongly indicating that they are not anatectic melts of the lower crust but rather reflect interaction between mantle-derived basaltic parental magmas and the crust. Crustal contributions appear to be large, on the order of 20–70%. The small range of isotope ratios in the accreted terrane suite appears to reflect interaction of the basaltic parent with relatively juvenile crust whose isotopic composition is similar to the mantle-derived magmas. High Th/U and Th/Rb ratios indicate that the crustal contamination occurs in the lower crust. Moreover, the less radiogenic 206Pb/204Pb and 207Pb/204Pb ratios in the continental suite indicate that the depletion in highly incompatible elements in the continental lower crust is an old feature. The secular changes in the isotope ratios within the stratigraphic sections indicate increasingly shallow crustal contributions with time, initially by predominantly mafic deep lower crust and later by more felsic middle crust. Using lavas from outside of the two heavily sampled stratigraphic sections, the differences in the isotopic compositions between volcanics erupted through the accreted terranes and the continental basement help to delineate the location of the boundary.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号