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1.
In Maine, Siluro-Devonian turbidites were metamorphosed underhigh-T–low-P facies series conditions during deformationwithin a Devonian crustal-scale shear zone system, defined bykilometer-scale straight belts of apparent flattening strainthat anastomose around lozenges of apparent constrictional strain.At upper amphibolite facies grade, metapelites are partiallymelted, the onset of which is recorded by a migmatite front.The resulting migmatites are stromatic or heterogeneous, andsmaller-volume granites form sheets or cylinders according tothe structural zone in which they occur, suggesting that migmatitesand granites record syntectonic melt flow through the deformingcrust. Common leucogranite of the nearby coeval Phillips pluton,which was emplaced syntectonically, was sourced from crustalrocks with geochemical characteristics similar to those of thehost Siluro-Devonian succession. Migmatites have melt-depletedcompositions relative to metapelites. Leucosomes are peraluminousand represent the cumulate products of fractional crystallizationand variable loss of evolved fractionated liquid. Among theheterogeneous migmatites are schlieric granites, the geochemistryof which suggests melt accumulation before fractional crystallizationand loss of the evolved liquid. Smaller-volume granites areperaluminous with a range of chemistries that reflect variableentrainment of residual plagioclase and biotite, accumulationof products of fractional crystallization and loss of most ofthe evolved liquid. Common leucogranite of the Phillips plutonand larger granites in the migmatites have compositions thatsuggest crystallization of evolved liquids derived by fractionalcrystallization of primary muscovite dehydration melts. We inferthat the leucogranite represents the crystallized fugitive liquidfrom a migmatite source similar to that exposed nearby. Watertransported through the shear zone system dissolved in meltwas exsolved at the wet solidus to cause retrogression in sub-solidusrocks and retrograde muscovite growth in migmatites. KEY WORDS: anatexis of pelite; Maine; migmatite; peraluminous granite; plutons  相似文献   

2.
Anders Lindh 《Lithos》2005,80(1-4):249-266
A few tens of millions of years after the intrusion of the Early Svecofennian (1.87–1.85 Ma) granitoids in central Sweden, a renewed magmatic activity resulted in the emplacement of the Late Svecofennian granites, the tectonic setting of which remains obscure. S-type granites dominate this group, but both I-type and transitional granites are common. This study deals with one of these intrusions in east-central Sweden; a composite pluton that is insignificantly deformed and hosts both I- and S-type granites. One of the I-type granites shows a compositional trend from granodiorite to granite, which is uncommon among the Late Svecofennian granites. Major element and incompatible trace element compositions and Nd data show that two different sources, one igneous and one sedimentary, were involved. An important conclusion is that nearly coeval granites derived from different sources are found in close connection. The granites are suggested to have formed by partial melting in a thickened continental crust that was formed in an early stage of the Svecofennian event. Thermal models suggest that the slightly older, high-temperature I-type granite (granodiorite) was formed deeper in the crust than the S-type granite. The coexistence of essentially pure I- and S-type granites, rather than transitional mixtures, reflects the relative depths of the proposed sources and the varying thermal parameters of the lithologic units in the Svecofennian crust.  相似文献   

3.
Proterozoic granitoid rocks in Zhejiang Province were formed in the Shengongian period (1.8–1.9 Ga) and the Late Jinningian period (0.6–0.9 Ga), respectively. Petrogenetic problems are discussed based on chemical (major, trace elements and REEs) and Nd-Sr isotopic compositions. The Shengongian granites resulted from partial melting of the Badu Group and the Late Jinningian granites are of mantle derivation with or without contamination of crustal material. The crust in Zhejiang had undergone three major periods of growth during 2.6–2.7 Ga, 0.8–1.1 Ga and 0.10–0.12 Ga after it was generated in Archean time. Compositional fractionation in the process of crust evolution is not evident. The presence of Late Jinningian granites of mantle and mantle-crust-derivation along the Jiangshan-Shaoxing Fault is indicative of crust subduction at that time. This project was finantially supported by both the National Natural Science Foundation of China (No. 9490011) and the Zhongguancun Test Center.  相似文献   

4.
在研究金溪—南城变质混合岩带中混合岩、花岗岩的矿物岩石学基础上,详细研究了它们的Nd、Sr、Pb、O同位素组成.混合岩、花岗岩初始钕同位素组成(143Nd/144Nd)i=0.511689～0.511853,在εNd-T图中位于该地区结晶基底变粒岩、片岩Nd同位素演化域上方,褶皱基底绢云千枚岩Nd同位素演化线下方;初始锶同位素组成(  相似文献   

5.
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6.
We report new occurrences of “two-phase” granitic textures from the Western Krušné hory/Erzgebirge pluton (central Europe) and use crystal-size distribution data and thermodynamic modeling to interpret their crystallization conditions. The two-phase texture consists of (1) early phenocrysts of quartz, plagioclase, K-feldspar and biotite, (2) medium-grained matrix of the same phases and (3) interstitial channels and patches of a late-stage, very fine-grained matrix. The porphyritic two-mica microgranites, which host two-phase textures, occur as minor intrusions in early low-F biotite granites or as marginal parts of evolved high-F Li-mica granites. Measurements of the crystal-size distribution of quartz revealed three grain populations: (1) early phenocrysts (0.5–3.0 mm) showing partial resorption by residual melt, (2) a medium-grained population of the equigranular rock matrix (0.05–0.50 mm) that experienced minor coarsening by subsolidus annealing and (3) a fine-grained population (<0.03 mm) in the interstitial channels and patches formed during rapid devolatilization; this quartz group shows no or poor grain coarsening. All samples exhibit similar fraction of the fine-grained population (44–52%) but proportions of phenocrysts to medium-grained matrix vary significantly. Thermodynamic modeling of liquidus equilibria and experimental data in the hydrous haplogranite system require: (1) ascent of a granitic suspension (15–25% phenocrysts) under H2O-undersaturated conditions at 25–45 bar/°C and a cooling rate of 40 J/(g kbar) in order to produce partial resorption of quartz phenocrysts and continued growth of feldspar phenocrysts, followed by (2) emplacement as discrete intrusions or bodies along pluton roof accompanied by sudden devolatilization. At the onset of matrix nucleation, disequilibrium undercooling of 70–85°C was inferred from the presence of micrographic intergrowths of quartz and K-feldspar. The two-phase granites in the Western Krušné hory/Erzgebirge pluton and in the Southeast Asian batholith form compositionally narrow groups with high-silica and moderate volatile enrichments but they differ in peraluminosity and phosphorus concentrations. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

7.
The Plechy pluton, southwestern Bohemian Massif, represents a late-Variscan, complexly zoned intrusive center emplaced near the crustal-scale Pfahl shear zone; the pluton thus provides an opportunity to examine the interplay among successive emplacement of large magma batches, magmatic fabric acquisition, and the late-Variscan stress field associated with strike-slip shearing. The magmatic history of the pluton started with the emplacement of the porphyritic Plechy and Haidmühler granites. Based on gravity and structural data, we interpret that the Plechy and Haidmühler granites were emplaced as a deeply rooted, ∼NE–SW elongated body; its gross shape and internal fabric (steep ∼NE–SW magmatic foliation) may have been controlled by the late-Variscan stress field. The steep magmatic foliation changes into flat-lying foliation (particularly recorded by AMS) presumably as a result of divergent flow. Magnetic lineations correspond to a sub-horizontal ∼NE–SW finite stretch associated with the divergent flow. Subsequently, the Třístoličník granite, characterized by steep margin-parallel magmatic foliation, was emplaced as a crescent-shaped body in the central part of the pluton. The otherwise inward-younging intrusive sequence was completed by the emplacement of the outermost and the most evolved garnet-bearing granite (the Marginal granite) along the southeastern margin of the pluton. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

8.
The Eoarchaean (>3,600 Ma) Itsaq Gneiss Complex of southern West Greenland is dominated by polyphase orthogneisses with a complex Archaean tectonothermal history. Some of the orthogneisses have c. 3,850 Ma zircons, and they vary from rare single phase metatonalites to more common complexly banded migmatites. This is due to heterogeneous strain, in situ anatexis and granitic veining superimposed during younger tectonothermal events. In the single-phase tonalites with c. 3,850 Ma zircon, oscillatory-zoned prismatic zircon is all 3,850 Ma old, but shows patchy ancient loss of radiogenic Pb. SHRIMP spot analyses and laser ablation ICP-MS depth profiling show that thin (usually < 10 μm) younger (3,660–3,590 Ma and Neoarchaean) shells of lower Th/U metamorphic zircon are present on these 3,850 Ma zircons. Several samples with this simple zircon population occur on islands near Akilia. In contrast, migmatites usually contain more complex zircon populations, with often more than one generation of igneous zircon present. Additional zircon dating of banded gneisses across the Complex shows that samples with c. 3,850 Ma igneous zircon are not just a phenomenon restricted to Akilia and adjacent islands. For example, migmatites from Itilleq (c. 65 km from Akilia) contain variable amounts of oscillatory-zoned 3,850 Ma and 3,650 Ma zircon, interpreted, respectively, as the rock age and the time of crustal melting under Eoarchaean metamorphism. With only 110–140 ppm Zr in the tonalites and likely magmatic temperatures of >850°C, zircon solubility–melt composition relationships show that they were only one-third saturated in zircon. Any zircon entrained in the precursor magmas would thus have been highly soluble. Combined with the cathodoluminesence imaging, this demonstrates that the c. 3,850 Ma oscillatory zoned zircon crystallised out of the melt and hence gives a magmatic age. Thus the rare well-preserved tonalites and palaeosome in migmatites testify that c. 3,850 Ma quartzo–feldspathic rocks are a widespread (but probably minor) component in the Itsaq Gneiss Complex. C. 3,850 Ma zircon with negative Eu anomalies (showing growth in felsic systems) also occurs as detrital grains in rare c. 3,800 Ma metaquartzites and as inherited grains in some 3,660 Ma granites (sensu stricto). These demonstrate that still more c. 3,850 Ma rocks were present, but were recycled into Eoarchaean sediments and crustally derived granites. The major and trace element characteristics (e.g. LREE enrichment, HREE depletion, low MgO) of the best-preserved c. 3,850 Ma rocks are typical of Archaean TTG suites, and thus argue for crust formation processes involving important contributions from melting of hydrated mafic crust to the earliest Archaean. Five c. 3,850 Ma tonalites were selected as the best preserved on the basis of field criteria and zircon petrology. Four of these samples have overlapping initial ɛNd (3,850 Ma) values from +2.9 to +3.6± 0.5, with the fourth having a slightly lower value of +0.6. These data provide additional evidence for a markedly LREE-depleted early terrestrial mantle reservoir. The role of c. 3,850 Ma crust should be considered in interpreting isotope signatures of the younger (3,800–3,600 Ma) rocks of the Itsaq Gneiss Complex. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

9.
In the Variscan foreland of SW-Sardinia (Western Mediterranean sea), close to the leading edge of the nappe zone, nappe emplacement caused folding and repetition of stratigraphic successions, km-scale offset of stratigraphic boundaries and an extensive brittle-ductile shear zone. Thrusts assumed a significant role, accommodating a progressive change of shortening direction and forming complicated thrust triangle zones. During thrust emplacement of the nappes, strong penetrative deformation affected rocks beneath the basal thrust of the nappe stack and produced coeval structures with both foreland-directed and hinterland-directed (backthrusting) shear sense. Cross-cutting and overprinting relationships clearly show that the shortening direction changed progressively from N–S to E–W, producing in sequence: (1) E–W trending open folds contemporaneous with early nappe emplacement in the nearby nappe zone; (2) recumbent, quasi-isoclinal folds with axial plane foliation and widespread, “top-towards-the-SW”, penetrative shearing; (3) N–S trending folds with axial plane foliation, contemporaneous with late nappe emplacement; (4) backthrusts and related asymmetrical folds developed during the final stages of shortening, postdating foreland-verging structures. Structures at (3) and (4) occurred during the same tectonic transport “top-towards-the-E” of the nappe zone over the foreland. The several generations of folds, thrusts, and foliations with different orientations developed, result in a complex finite structural architecture, not completely explicable by the theoretical model proposed up to date.  相似文献   

10.
Orogeny, migmatites and leucogranites: A review   总被引：13，自引：0，他引：13  
The type ofP-T-t path and availability of fluid (H2O-rich metamorphic volatile phase or melt) are important variables in metamorphism. Collisional orogens are characterized by clockwiseP-T evolution, which means that in the core, where temperatures exceed the wet solidus for common crustal rocks, melt may be present throughout a significant portion of the evolution. Field observations of eroded orogens show that lower crust is migmatitic, and geophysical observations have been interpreted to suggest the presence of melt in active orogens. A consequence of these results is that orogenic collapse in mature orogens may be controlled by a partially-molten layer that decouples weak crust from subducting lithosphere, and such a weak layer may enable exhumation of deeply buried crust. Migmatites provide a record of melt segregation in partially molten crustal materials and syn-anatectic deformation under natural conditions. Grain boundary flow and intra-and inter-grain fracture flow are the principal grain scale melt flow mechanisms. Field observations of migmatites in ancient orogens show that leucosomes occur oriented in the metamorphic fabrics or are located in dilational sites. These observations are interpreted to suggest that melt segregation and extraction are syntectonic processes, and that melt migration pathways commonly relate to rock fabrics and structures. Thus, leucosomes in depleted migmatites record the remnant permeability network, but evolution of permeability networks and amplification of anomalies are poorly understood. Deformation of partially molten rocks is accommodated by melt-enhanced granular flow, and volumetric strain is accommodated by melt loss. Melt segregation and extraction may be cyclic or continuous, depending on the level of applied differential stress and rate of melt pressure buildup. During clockwiseP-T evolution, H2O is transferred from protolith to melt as rocks cross dehydration melting reactions, and H2O may be evolved above the solidus at lowP by crossing supra-solidus decompression-dehydration reactions if micas are still present in the depleted protolith. H2O dissolved in melt is transported through the crust to be exsolved on crystallization. This recycled H2O may promote wet melting at supra-solidus conditions and retrogression at subsolidus conditions. The common growth of ‘late’ muscovite over sillimanite in migmatite may be the result of this process, and influx of exogenous H2O may not be necessary. However, in general, metasomatism in the evolution of the crust remains a contentious issue. Processes in the lower-most crust may be inferred from studies of xenolith suites brought to the surface in lavas. Based on geochemical data, we can use statistical methods and modeling to evaluate whether migmatites are sources or feeder zones for granites, or simply segregated melt that was stagnant in residue, and to compare xenoliths of inferred lower crust with exposed deep crust. Upper-crustal granites are a necessary complement to melt-depleted granulites common in the lower crust, but the role of mafic magma in crustal melting remains uncertain. Plutons occur at various depths above and below the brittle-to-viscous transition in the crust and have a variety of 3-D shapes that may vary systematically with depth. The switch from ascent to emplacement may be caused by amplification of instabilities within (permeability, magma flow rate) or surrounding (strength or state of stress) the ascent column, or by the ascending magma intersecting some discontinuity in the crust that enables horizontal magma emplacement followed by thickening during pluton inflation. Feedback relations between rates of pluton filling, magma ascent and melt extraction maintain compatibility among these processes.  相似文献   

11.
The Ordovician Sierras Pampeanas, located in a continental back-arc position at the Proto-Andean margin of southwest Gondwana, experienced substantial mantle heat transfer during the Ordovician Famatina orogeny, converting Neoproterozoic and Early Cambrian metasediments to migmatites and granites. The high-grade metamorphic basement underwent intense extensional shearing during the Early and Middle Ordovician. Contemporaneously, up to 7000 m marine sediments were deposited in extensional back-arc basins covering the pre-Ordovician basement. Extensional Ordovician tectonics were more effective in mid- and lower crustal migmatites than in higher levels of the crust. At a depth of about 13 km the separating boundary between low-strain solid upper and high-strain lower migmatitic crust evolved to an intra-crustal detachment. The detachment zone varies in thickness but does not exceed about 500 m. The formation of anatectic melt at the metamorphic peak, and the resulting drop in shear strength, initiated extensional tectonics which continued along localized ductile shear zones until the migmatitic crust cooled to amphibolite facies P–T conditions. P–T–d–t data in combination with field evidence suggest significant (ca. 52%) crustal thinning below the detachment corresponding to a thinning factor of 2.1. Ductile thinning of the upper crust is estimated to be less than that of the lower crust and might range between 25% and 44%, constituting total crustal thinning factors of 1.7–2.0. While the migmatites experienced retrograde decompression during the Ordovician, rocks along and above the detachment show isobaric cooling. This suggests that the magnitude of upper crustal extension controls the amount of space created for sediments deposited at the surface. Upper crustal extension and thinning is compensated by newly deposited sediments, maintaining constant pressure at detachment level. Thinning of the migmatitic lower crust is compensated by elevation of the crust–mantle boundary. The degree of mechanical coupling between migmatitic lower and solid upper crust across the detachment zone is the main factor controlling upper crustal extension, basin formation, and sediment thickness in the back-arc basin. The initiation of crustal extension in the back-arc, however, crucially depends on the presence of anatectic melt in the middle and lower crust. Consumption of melt and cooling of the lower crust correlate with decreasing deposition rates in the sedimentary basins and decreasing rates of crustal extension.  相似文献   

12.
The isotopic–geochemical features of late and postorogenic granites of the S type and ambient migmatites are studied within the Russian part of the Svecofennian orogen of the Fennoscandinavian Shield. The spatial association of leucosomes of migmatites and granites of the S type and their similar petro- and geochemistry and distribution of Pb isotopes are evidence of the genetic similarity of their parental melts. The Borodinskoe pluton has a more primitive 206Pb/207Pb ratio, which indicates the presence of upper and U-poor lower crustal material in the source of granitic magmas. This conclusion is supported by the ?Nt(t) lower value of granites of this pluton relative to those of other plutons of the region.  相似文献   

13.
The Gede Volcanic Complex (GVC) of the Sunda island arc (West Java, Indonesia) consists of multiple volcanic centres and eruptive groups with complex magmatic histories. We present new petrological, mineralogical, whole-rock major and trace element and Sr–O isotopic data to provide constraints on the relative importance of fractional crystallisation and magma mixing in petrogenesis, as well as on the role and nature of the arc crust. Banded juvenile scoria from Young and Old Gede provide unequivocal evidence for the (late-stage) interaction of distinct magmas at Gede volcano. However, the relatively small-degree compositional zoning observed in plagioclase phenocrysts of all eruptive groups (up to ~20 mol% An) may be attributed to physical changes in magma properties (e.g. P, T, and PH2O) rather than changes in melt composition. Major element and trace element variations within each eruptive series are inconsistent with magmatic evolution through simple mixing processes. Instead, mixing of variably fractionated magma batches is suggested to account for the significant scatter in some element variation diagrams. No correlation is observed between textural complexity and/or mineral disequilibrium and whole-rock geochemistry. REE data and geochemical modelling indicate that fractional crystallisation involving amphibole in the mid- to lower crust, and fractionation of plagioclase, clinopyroxene, Fe–Ti oxide ± olivine ± orthopyroxene provide strong control on the geochemical evolution of GVC rocks. Two-pyroxene geothermobarometry provides pre-eruption crystallisation temperatures of 891–1,046°C and pressures of 3.4–6.5 kbar, equivalent to ~13–24 km depth beneath the volcanoes (mid- to lower crust). Low, mantle-like clinopyroxene δ18O values of GVC lavas and poor correlation of Sr isotope ratios with indices of differentiation precludes significant assimilation of isotopically distinct crust during magmatic differentiation. Therefore, we suggest that the geochemical character of the moderately thick West Javan arc crust is relatively immature compared to typical continental crust. Trace element ratios and strontium isotopes show that the magmatic source composition of the older geographical units, Gegerbentang and Older Quaternary, is distinct from the other GVC groups.  相似文献   

14.
A large portion of the lower continental crust may be amphibolitic in composition and without a free fluid phase. As a consequence, H2O-undersaturated or fluid-absent melting of amphibolites may be responsible for the formation of some granites and migmatites produced during major orogenic events. In an attempt to determine the systematics of melting under fluid-absent conditions, a series of piston-cylinder experiments was conducted on two natural amphibolites; one, a meta-alkali basalt (ABA) with a total water content of ∼ wt% contained in hornblende, and the other, a meta-island-arc tholeiite (IAT) which has ∼1–1.3 wt% water contained in hornblende, cummingtonite and biotite. The experimentally determined melting ranges of the two amphibolites showed that the solidus temperatures, and sta temperature interval over which amphibole was stable, were controlled by the amphibolites' different bulk compositions and their resulting metamorphic assemblages. The volume % of melt produced by melting of the two amphibolites were compared with estimated amounts, based on Burnham's (1979) water-melt solubility model and the fluid-absent melting model presented by Clemens and Vielzeuf (1987). The observed melt volumes were greater than estimated. As the water content of melt largely detemines the volume % of melt produced, independent measurements of the water-content of the glass formed during partial melting in the ABA were made by thermogravimetric analyses. The water content of the ABA glass is ∼2 wt%, which is less than the assumed “melt-water” content (water content of the melt) used in previous modeling of fluidabsent anatexis in mafic lithologies. As a consequence, more melt can be expected during fluid-absent partial melting of mafic lower crust, as is observed in the experiments. A modification of the Clemens and Vielzeuf (1987) fluid-absent melting model for mafic compositions has been made using the experimental data available on melting in basaltic systems and is presented here for pressures of 5, 8 and 10 kbar. Tectonic scenarios in which the crust is thickened (i.e. by collision) then undergoes extension or where a previously thinned crust is later rethickened, provide enough heat so that amphibolite melting under fluid-absent conditions can become importan and hence responsible for some melts produced during post-collisional magmatism. The results may also have applications to melting in hydrated oceanic crust in subduction zones and in island arc terains.  相似文献   

15.
《Precambrian Research》2003,120(1-2):37-53
The Puutsaari intrusion is a potassium-rich magmatic complex in the eastern part of the Svecofennian domain close to the Archaean border. The intrusion is generally undeformed in contrast to 1880–1875 Ma-old country rock tonalitic migmatites and diatectites. The main rock types are: (1) mafic rocks of a gabbro–norite–diorite–quartz monzodiorite series; (2) quartz diorite–tonalite–granodiorite; and (3) coarse-grained microcline granite. The three rock-types intruded coevally forming a peculiar three-component mingling system. The mafic rocks, enriched in K, P, Ba, Sr and LREE, have marked shoshonitic affinities (K2O=1.97–5.40, K2O/Na2O=0.6–2.37). On a regional scale they demonstrate transitional geochemistry between less enriched syn-orogenic 1880 Ma-old gabbro–tonalite complexes and strongly enriched 1800 Ma post-collisional shoshonitic intrusions. The microcline granite as well as the tonalite–granodiorite rocks are geochemically similar to crustal anatectic granitoids of the NW Ladoga Lake area. The three rock groups do not form a single trend on Harker-type diagrams and are unlikely to be related by fractional crystallisation or mixing. Zircons from the Puutsaari microcline granite and from the mafic rock series have been dated by ion-microprobe (NORDSIM) at 1868.2±5.9 and 1869±7.7 Ma, respectively. Most zircons recovered from a granite sample had zoned or homogeneous cores and unzoned fractured rims. No statistically significant variation of zircon core and rim ages from the granite was established in the course of this study. Zircons from the mafic rock are unzoned. It is suggested that the mafic rocks at Puutsaari were derived from an enriched mantle shortly after the main Svecofennian collisional event and the roughly 1.88 Ga regional metamorphic culmination. The emplacement of the mafic melt caused anatectic melting of various crustal protoliths and produced coeval granitic and tonalitic compositions.  相似文献   

16.
The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ±orthopyroxene phenocrysts. The compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from ∼ Fo81 to Fo91. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning profiles. However, there are significant differences between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 × Mg/(Mg+Fe+2)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts with similar reverse zonation. The compositional variations of these phenocrysts can be explained by mixing of a multiphase (plagioclase, augite and orthopyroxene) saturated, evolved magma with more mafic magma saturated only with olivine. The differences in the compositional ranges of plagioclase, augite and orthopyroxene crystals between samples indicate that these samples were derived from isolated magma chambers which had undergone distinct fractionation and mixing histories. The samples containing plagioclase and pyroxene with small compositional variations reflect magmas that were buffered near the olivine + melt ⇒Low-Ca pyroxene + augite + plagioclase reaction point by frequent intrusions of mafic olivine-bearing magmas. Samples containing plagioclase and pyroxene phenocrysts with large compositional ranges reflect magmas that evolved beyond this reaction point when there was no replenishment with olivine-saturated magma. Two of these samples contain augite cores with Mg# of ∼71, corresponding to Mg# of 36–40 in equilibrium melts, and augite in another sample has Mg# of 63–65 which is in equilibrium with a very evolved melt with a Mg# of ∼30. Such highly evolved magmas also exist beneath the Puna Ridge of Kilauea volcano. They are rarely erupted during the shield building stage, but may commonly form in ephemeral magma pockets in the rift zones. The compositions of clinopyroxene phenocryst rims and associated glass rinds indicate that most of the samples were last equilibrated at 2–3 kbar and 1130–1160 °C. However, in one sample, augite and glass rind compositions reflect crystallization at higher pressures (4–5 kbar). This sample provides evidence for magma mixing at relatively high pressures and perhaps transport of magma from the summit conduits to the rift zone along the oceanic crust-mantle boundary. Received: 8 July 1998 / Accepted: 2 January 1999  相似文献   

17.
华南中生代大地构造研究新进展   总被引：33，自引：0，他引：33  
华南地区中生代构造动力体制经历了从特提斯构造域向滨太平洋构造域的转换,由此产生了强烈的陆内造山作用和岩浆活动,形成了复杂构造组合的晚中生代陆内造山带和火成岩省。本项研究在下列几个方面取得了新的进展:(1)通过对雪峰山地区沅麻盆地的野外调查和构造测量,确定了该盆地晚中生代-早新生代5期构造应力场及其演替序列:中晚侏罗世近W—E向挤压、早白垩世NW—SE向伸展、早白垩世中晚期NW—SE向挤压、晚白垩世近N—S向伸展、古近纪晚期NE—SW向挤压。构造应力场方向的变化记录了不同板缘的动力作用对该区的影响。(2)识别了湖南地区晚古生代-早中生代海相地层中发育的横跨叠加褶皱构造,并基于地层接触关系和已有火成岩同位素年代学数据分析,认为该地区横跨叠加褶皱构造记录了中生代两期构造挤压和地壳增厚事件:早期近东西向褶皱构造是对三叠纪华南地块南北边缘大陆碰撞和增生作用的远程响应,晚期NE—NNE向褶皱构造则是对中晚侏罗世古太平洋板块向华南大陆之下低角度俯冲作用的变形响应。(3)对湖南衡山西缘拆离断裂带的变形结构和运动学特征进行了详细的调查和构造测量,确定了衡山变质核杂岩构造,并对拆离带中韧性剪切变形的钠长岩脉的锆石进行了SHRIMP U-Pb测年,从而确定了华南地区伸展构造的起始时代约137 Ma,即早白垩世早中期。(4)通过锆石U-Pb年代学测试分析,揭示了东南沿海长乐—南澳构造带早白垩世2期构造-岩浆事件:早期(147~135 Ma)表现为强烈的混合岩化作用和深熔作用形成的片麻状花岗岩、花岗片麻岩等;晚期(135~117 Ma)岩浆岩以含石榴子石花岗岩为主。这个结果表明东南沿海构造带是晚中生代陆缘造山带,造山作用可能起始于晚侏罗世,于早白垩世早中期(135 Ma)以来发生伸展垮塌。在上述研究结果的基础上,探讨了华南地区三叠纪"印支运动"和中、晚侏罗世"燕山运动"的表现及其产生的板块构造动力体制及其转换时代、早白垩世从挤压构造应力体制向伸展构造应力体制转变的时间节点。  相似文献   

18.
Two-phase quartz intergrowths with garnet, cordierite and tourmaline occur commonly in prograde high-temperature migmatites, granulites, as well as in the last crystallization stages of biotite granites. Structural, microtextural and mineralogical data show that they result from the breakdown of biotite in the presence of a melt phase associated with incongruent dissolution of feldspars into the melt and silica release (giving quartz in silica saturated rocks). Biotite breakdown and growth of Al-rich ferromagnesian minerals, occurring at the solid–liquid transition in the crust (early melting or final crystallization), is kinetically controlled by Fe and Mg mass transport, the network-forming cations Si and Al being locally compensated for by feldspar dissolution/crystallization. This process leads to significant changes with respect to equilibrium dehydration-melting reactions wherein quartz is a reactant and K-feldspar a reaction product. Therefore, quartz inclusions commonly occurring in garnets from granulite-facies metapelites and metagraywackes are not simply grains passively included during garnet growth. They may also correspond to newly crystallized phases. Resorption of feldspar may lead to more alkaline melt and to crystalline residue richer in Al than expected under equilibrium conditions. Hence, excess alumina in granulite-facies rocks is not necessarily related to initial alumina-rich whole-rock compositions (as currently considered), but may be due, at least partly, to kinetics of melting.  相似文献   

19.
Geological studies on saturated to oversaturated and subsolvus aegirine-riebeckite syenite bodies of the Pulikonda alkaline complex and Dancherla alkaline complex were carried out. The REE distribution of the Dancherla syenite shows a high fractionation between LREE and HREE. The absence of Eu anomaly suggests source from garnet peridotite. The Pulikonda syenite shows moderate fractionation between LREE and HREE as reflected by enrichment of HREE and moderate enrichment of LREE. The negative Eu anomaly indicates role of plagioclase fractionation.Three distinct co-eval primary magmas i.e. mafic syenite-, felsic syenite- and alkali basalt magmas — all derived from low-degrees of partial melting of mantle differentiates and enriched metasomatised lower crust played a major role in the genesis and emplacement of the syenites into overlying crust along deep seated regional scale trans-lithospheric strike-slip faults and shear zones following immediately after late-Archaean calc-alkaline arc magmatism at different time-space episodes i.e. initially at craton margin and later on into the thickened interior of the Eastern Dharwar craton. The ductile sheared and folded Pulikonda alkaline complex was evolved dominantly from the magmas derived from partial melting of lower crust and minor juvenile magmas from mantle. Differentiation and fractionation by liquid immiscibility of mafic magma and commingling-mixing of intermediate and felsic magmas followed by fractionational crystallisation under extensional tectonics during waning stages of calc-alkaline arc magmatism nearer to the craton margin were attributed as the main processes for the genesis of Pulikonda syenite complex. Commingling and limited mixing of independent mantle derived mafic and felsic syenitic magmas and accompanying fractionation resulting into soda rich and potash rich syenite variants was tentatively deduced mechanism for the origin of Dancherla, Danduvaripalle, Reddypalle syenites and other bodies belonging to Dancherla alkaline complex at the craton interior. The Peddavaduguru syenite was formed by differentiation of alkali mafic magma (gabbro to diorite) and it’s simultaneous mingling with fractionated felsic syenitic magma under incipient rift. Vannedoddi and Yeguvapalli syenites were derived due to desilicification and accompanying alkali feldspar mestasomatism of younger potash rich granites along Guntakal-Gooty fault and along Singanamala shear zone respectively.  相似文献   

20.
Integration of geochemical, mineralogical, isotopic, and geochronological data with geodynamic considerations suggests that the Variscan granites in the Erzgebirge-Slavkovsky les domain originated from repeated melting events and were emplaced over a period of about 40 Ma (330–290 Ma). Several lines of evidence exist supporting the idea that Erzgebirge granites assigned to different types (biotite granites, two-mica granites, strongly peraluminous P-rich Li-mica granites, and slightly peraluminous P-poor granites) are in most cases not genetically related via continuous fractional crystallization from a common magmatic reservoir. The genesis of the Slavkovsky les granites, however, might be discussed in terms of an uninterrupted fractionation series. Geological models of Sn-W deposits based upon geochemical and structural results imply that the main ore depositional events followed immediately the emplacement and solidification processes of melt via fluid-melt immiscibility, breccia-pipe formation and/or pervasive rock-fluid interactions. Received: 4 August 1998 / Accepted: 5 January 1999  相似文献   

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