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1.
The evolution of Late Paleozoic granitoid magmatism in Transbaikalia shows a general tendency for an increase in the alkalinity of successively forming intrusive complexes: from high-K calc-alkaline granites of the Barguzin complex (Angara–Vitim batholith) at the early stage through transitional from calc-alkaline to alkaline granites and quartz syenites (Zaza complex) at the intermediate stage to peralkaline granitoids (Early Kunalei complex) at the last stage. This evolution trend is complicated by the synchronous development of granitoid complexes with different sets and geochemical compositions of rocks. The compositional changes were accompanied by the decrease in the scales of granitoid magmatism occurrence with time. Crustal metaterrigenous protoliths, possibly of different compositions and ages, were the source of granitoids of the Angara–Vitim batholith. The isotopic composition of all following granitoid complexes points to their mixed mantle–crustal genesis. The mechanisms of granitoid formation are different. Some granitoids formed through the mixing of mantle and crustal magmas; others resulted from the fractional crystallization of hybrid melts; and the rest originated from the fractional crystallization of mantle products or the melting of metabasic sources with the varying but subordinate contribution of crustal protoliths. Synplutonic basic intrusions, combined dikes, and mafic inclusions, specific for the post-Barguzin granitoids, are direct geologic evidence for the synchronous occurrence of crustal and mantle magmatism. The geodynamic setting of the Late Paleozoic magmatism in the Baikal folded area is still debatable. Three possible models are proposed: (1) mantle plume impact, (2) active continental margin, and (3) postcollisional rifting. The latter model agrees with the absence of mafic rocks from the Angara–Vitim batholith structure and with the post-Barguzin age of peralkaline rocks of the Vitim province.  相似文献   

2.
The Ulaan Tolgoi massif of rare-metal (Ta, Nb, and Zr) granites was formed at approximately 300Ma in the Eastern Sayan zone of rare-metal alkaline magmatism. The massif consists of alkaline salic rocks of various composition (listed in chronologic order of their emplacement): alkaline syenite → alkaline syenite pegmatite → pantellerite → alkaline granite, including ore-bearing alkaline granite, whose Ta and Nb concentrations reach significant values. The evolution of the massif ended with the emplacement of trachybasaltic andesite. The rocks of the massif show systematic enrichment in incompatible elements in the final differentiation products of the alkaline salic magmas. The differentiation processes during the early evolution of the massif occurred in an open system, with influx of melts that contained various proportions of incompatible elements. The magma system was closed during the origin of the ore-bearing granites. Rare-metal granitoids in the Eastern Sayan zone were produced by magmas formed by interaction between mantle melts (which formed the mafic dikes) with crustal material. The mantle melts likely affected the lower parts of the crust and either induced its melting, with later mixing the anatectic and mantle magmas, or assimilated crustal material and generated melts with crustal–mantle characteristics. The origin of the Eastern Sayan zone of rare-metal alkaline magmatism was related to rifting, which was triggered by interaction between the Tarim and Barguzin mantle plumes. The Eastern Sayan zone was formed in the marginal part of the Barguzin magmatic province, and rare-metal magmas in it were likely generated in relation with the activity of the Barguzin plume.  相似文献   

3.
This paper summarizes the results of long-term geological, petrological, and geochemical investigations of the Mesozoic-Cenozoic complexes of the Stanovoy Range in order to determine the main reasons for their generation and evolution. The analysis of this material showed that the compositionally variable Late Mesozoic igneous complexes of the Stanovoy Range were formed in various depth facies, from abyssal to surficial. The majority of their salic complexes show minor compositional variations, whereas the mafic complexes are more variable, especially in the southeast of the region. The southeastern Stanovoy Range comprises comparable amounts of both subalkaline and low-alkali igneous rocks, whereas the central part is dominated by subalkaline rocks, and the northwestern part contains rocks only of the shoshonite-latite series. This zoning is fundamentally different from that of typical island arcs, which are characterized by the occurrence of volcanic rocks of similar alkalinity in each zone. Extrusive and intrusive rocks with similar alkali and silica contents (and schlieren-like inclusions in the granitoids of the region) were formed from common magmas of corresponding chemical compositions. In addition, the mafic and most of the salic magmas were formed as independent melting products, whereas the magmas of intermediate composition were formed mainly by mixing of chemically contrasting liquids (i.e., salic and basic). It was shown that the available information on the magmatism of the region is best interpreted in terms of the model of mantle diapirism. In particular, mantle diapirs ascended rather slowly during the Mesozoic and occurred over the whole territory of the Stanovoy Range during the Jurassic-Cretaceous stage (J3-K1), when alkaline and subalkaline basalts were formed. During the Early-Late Cretaceous stage, mantle diapirs produced alkali-poor basalts in the central and eastern parts. During the Cenozoic, the diapir ascended rather rapidly but only in a small area in the eastern part of the region forming alkali basalts. In contrast to the Cenozoic, the Earth’s crust was strongly affected by mantle diapirs and related mafic magmas in the Mesozoic. As a result, crustal sequences were reworked by fluids and subsequently yielded tremendous volumes of compositionally corresponding salic magmas, which interacted and mixed with mafic magmas producing the corresponding chemical zoning. The maximum generation of crustal magmas was confined to the axial zones of ascending diapirs, where the highest energy effects took place, whereas the role of autochthonous gneissic granites increased away from the axis at the expense of typical intrusive complexes.  相似文献   

4.
花岗岩研究的若干新进展与主要科学问题   总被引：5，自引：2，他引：3  
王孝磊 《岩石学报》2017,33(5):1445-1458
花岗岩研究进入到新的时期,需要寻找新的研究思路和研究方法。本文总结了近十年来国际上有关花岗岩前沿研究的新进展,归纳总结了七个方面的主要研究内容,包括:1)地球早期花岗岩与大陆演化;2)源区的不均一性与不平衡熔融;3)岩浆成分变化的同位素示踪;4)壳-幔岩浆混合与花岗岩的形成;5)地壳热带与中酸性岩的形成;6)花岗岩岩体的生长和结晶时间;7)金属稳定同位素在花岗岩研究中的应用。最后在此基础上对花岗岩的研究趋势,建议利用新视角、新方法等着重开展花岗岩源区和岩浆深部过程的精细研究,并揭示花岗岩与早期地壳形成之间的联系。  相似文献   

5.
Bernard Barbarin   《Lithos》2005,80(1-4):155-177
The calc-alkaline granitoids of the central Sierra Nevada batholith are associated with abundant mafic rocks. These include both country-rock xenoliths and mafic magmatic enclaves (MME) that commonly have fine-grained and, less commonly, cumulate textures. Scarce composite enclaves consist of either xenoliths enclosed in MME, or of MME enclosed in other MME with different grain size and texture. Enclaves are often enclosed in mafic aggregates and form meter-size polygenic swarms, mostly in the margins of normally zoned plutons. Enclaves may locally divert schlieren layering. Mafic dikes, which also occur in swarms, are undisturbed, composite, or largely hybridized. In central Sierra Nevada, with the exception of xenoliths that completely differ from the other rocks, host granitoids, mafic aggregates, MME, and some composite dikes exhibit a bulk compositional diversity and, at the same time, important mineralogical and geochemical (including isotopic) similarities. MME and host granitoids display distinct major and trace element compositions. However, strong correlations between MME–host granitoid pairs indicate interactions and parallel evolution of MME and enclosing granitoid in each pluton. Identical mafic mineral compositions and isotopic features are the result of these interactions and parallel evolution. Mafic dikes have broadly the same major and trace element compositions as the MME although variations are large between the different dikes that are at distinctly different stages of hybridization and digestion by the host granitoids. The composition of the granitoids and various mafic rocks reflects three distinct stages of hybridization that occurred, respectively, at depth, during ascent and emplacement, and after emplacement. The occurrence and succession of hybridization processes were tightly controlled by the physical properties of the magmas. The sequential thorough or partial mixing and mingling were commonly followed by differentiation and segregation processes. Unusual MME that contain abundant large crystals of hornblende resulted from disruption of early cumulates at depth, whereas those richer in large crystals of biotite were formed by disruption of late mafic aggregates or schlieren layerings at the level of emplacement. MME and host granitoids are considered cogenetic, because both are hybrid rocks that were produced by the mixing of the same two components in different proportions. The felsic component was produced by partial melting of preexisting crustal materials, whereas the dominant mafic component was probably derived from the upper mantle. However, in the lack of a clear mantle signature, the origin of the mafic component remains questionable.  相似文献   

6.
 The southern Ivrea-Verbano Zone of the Italian Western Alps contains a huge mafic complex that intruded high-grade metamorphic rocks while they were resident in the lower crust. Geologic mapping and chemical variations of the igneous body were used to study the evolution of underplated crust. Slivers of crustal rocks (septa) interlayered with igneous mafic rocks are concentrated in a narrow zone deep in the complex (Paragneiss-bearing Belt) and show evidence of advanced degrees of partial melting. Variations of rare-earth-element patterns and Sr isotope composition of the igneous rocks across the sequence are consistent with increasing crustal contamination approaching the septa. Therefore, the Paragneiss-bearing Belt is considered representative of an “assimilation region” where in-situ interaction between mantle- and crust-derived magmas resulted in production of hybrid melts. Buoyancy caused upwards migration of the hybrid melts that incorporated the last septa and were stored at higher levels, feeding the Upper Mafic Complex. Synmagmatic stretching of the assimilation region facilitated mixing and homogenization of melts. Chemical variations of granitoids extracted from the septa show that deep septa are more depleted than shallow ones. This suggests that the first incorporated septa were denser than the later ones, as required by the high density of the first-injected mafic magmas. It is inferred that density contrasts between mafic melts and crustal rocks play a crucial role for the processes of contamination of continental magmas. In thick under plated crust, the extraction of early felsic/hybrid melts from the lower crust may be required to increase the density of the lower crust and to allow the later mafic magmas to penetrate higher crustal levels. Received: 2 May 1995 / Accepted: 1 November 1995  相似文献   

7.
The paper reports results of petrological-geochemical, isotope, and geochronological studies of the Preobrazhenka gabbro–granitoid massif located in the Altai collisional system of Hercynides, Eastern Kazakhstan. The massif shows evidence for the interaction of compositionally contrasting magmas during its emplacement. Mineralogical–petrological and geochemical studies indicate that the gabbroid rocks of the massif were formed through differentiation of primary trachybasaltic magma and its interaction with crustal anatectic melts. Origin of the granitoid rocks is related to melting of crustal protoliths under the thermal effect of mafic melts. The mantle–crust interaction occurred in several stages and at different depths. A model proposed here to explain the intrusion formation suggests subsequent emplacement of basite magmas in lithosphere and their cooling, melting of crustal protolith, emplacement at the upper crustal levels and cooling of the granitoid and basite magmas. It was concluded that the formation of gabbro-granitoid intrusive massifs serves as an indicator of active mantle–crust interaction at the late evolutionary stages of accretionary–collisional belts, when strike-slip pull-apart deformations causes the high permeability of lithosphere.  相似文献   

8.
The volcanics exposed in the northeast Niğde area are characterized by pumiceous pyroclastic rocks present as ash flows and fall deposits and by compositions ranging from dacite to rhyolite. Xenoliths found in the volcanics are basaltic andesite, andesite and dacite in composition. These rocks exhibit linear chemical variations between end‐member compositions and a continuity of trace element behaviour exists through the basaltic andesite–andesite–dacite–rhyolite compositional range. This is consistent with the fractionation of ferromagnesian minerals and plagioclase from a basaltic andesite or andesite parent. These rocks are peraluminous and show typical high‐K calc‐alkaline differentiation trends with total iron content decreasing progressively with increasing silica content. Bulk rock and mineral compositional trends and petrographic data suggest that crustal material was added to the magmas by subducted oceanic crust and is a likely contaminant of the source zone of the Niğde magmas. The chemical variations in these volcanics indicate that crystal liquid fractionation has been a dominant process in controlling the chemistry of the northeast Niğde volcanics. It is also clear, from the petrographic and chemical features, that magma mixing with disequilibrium played a significant role in the evolution of the Niğde volcanic rocks. This is shown by normal and reverse zoning in plagioclase and resorption of most of the observed minerals. The xenoliths found in the Niğde volcanics represent the deeper part of the magma reservoir which equilibrated at the higher pressures. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

9.
The Koshrabad massif, referred to as the Hercynian postcollisional intrusions of the Tien Shan, is composed of two rock series: (1) mafic and quartz monzonites and (2) granites of the main phase. Porphyritic granitoids of the main phase contain ovoids of alkali feldspar, often rimmed with plagioclase. Mafic rocks developed locally in the massif core resulted from the injections of mafic magma into the still unconsolidated rocks of the main phase, which produced hybrid rocks and various dike series. All rocks of the massif are characterized by high f (Fe/(Fe + Mg)) values and contain fayalite, which points to the reducing conditions of their formation. Mafic rocks are the product of fractional crystallization of alkali-basaltic mantle melt, and granitoids of the main phase show signs of crustal-substance contamination. In high f values and HFSE contents the massif rocks are similar to A-type granites. Data on the geochemical evolution of the massif rocks confirm the genetic relationship of the massif gold deposits with magmatic processes and suggest the accumulation of gold in residual acid melts and the rapid formation of ore quartz veins in the same structures that controlled the intrusion of late dikes. The simultaneous intrusion of compositionally different postcollisional granitoids of the North Nuratau Ridge, including the Koshrabad granitoids, is due to the synchronous melting of different crustal protoliths in the zone of transcrustal shear, which was caused by the ascent of the hot asthenospheric matter in the dilatation setting. The resulting circulation of fluids led to the mobilization of ore elements from the crustal rocks and their accumulation in commercial concentrations.  相似文献   

10.
The Gredos massif is one the better exposed granitoid complexes of the Iberian massif. It is composed mainly of peraluminous granitoids with subordinate basic and ultrabasic complexes. The massif also contains mega-enclaves of migmatites with which the granitoids show transitional contacts. Two major magmatic associations have been distinguished in this study: (1) One comprises the granitoids with microgranular enclaves, the enclaves, and basic rocks; (2) the other is formed by leucogranites, intrusive into the former series and free of microgranular enclaves. Field relationships and microstructures indicate that the rocks of the first series are related by a dominant hybridization process. The Sr-Nd isotopic study reveals that this process is complex, relating different end-members of mantle and crustal affinities, and occurred around 295 Ma ago, late with respect to the main deformation phases of the Hercynian orogeny. The granitoids with microgranular enclaves (GME) are part of an overall mixing trend involving Palaeozoic mantle-derived magma and melts of older crustal material. Amphibole-bearing GME, in general, contain greater proportions of the mantle-derived component than the cordierite-bearing GME. The actual mixing processes took place on a variety of scales, sometimes between melts which were themselves hybrids. On a local scale this hybridization process can be modelled by simple binary mixing as documented in the case of a composite dyke. The isotopic signatures of the basic rocks are probably, to a large degree, the result of interaction with crustal melts, though additionally the presence of an enriched mantle source cannot be elmininated. Microgranular enclaves and their immediate hosts have differing initial Sr and Nd isotopic signatures, indicating that isotopic equilibrium was not attained. This suggests that the enclaves did not reside in their final granitic melt for long before cooling of the whole system. The enclaves are considered to have been derived from basaltic melts which had fractionated and hybridised to varying degrees. Late-stage peraluminous leucogranites have similar initial Nd isotopic compositions to the evolved GME; a crustal source with a radically different Nd isotopic composition or age does not need to be invoked in their petrogenesis.  相似文献   

11.
The study provides new petrologic and isotope geochemical data for rocks of the 465 ± 5 Ma Bulka massif (Borodina et al., 2011). The primary amphibole from granitoid stocks cutting across the layered series of the massif yielded an Ar–Ar age of 415.9 ± 3.7 Ma. The rocks of the Bulka massif have 143Nd/144Nd ratio of 0.513243 and εNd (Т) values of +12.00. The granitoids have 143Nd/144Nd ratios between 0.512919 and 0.512961 and εNd (Т) values between +8.03 and +9.25. The Nd isotope composition indicates that the rocks of the Bulka massif and granitoids were derived from a depleted mantle source. Depletion of the rocks of the massif in LILE, LREE, and HFSE over LILE is inherited from the mantle source, which has geochemical signatures of N-MORB and subduction-related components. Granitoids are metaluminous I-type granites, which were probably generated either by differentiation of intermediate to mafic mantle-derived magmas or by melting of metabasites. The rocks of the granitoid stocks are characterized by enrichment in LILE and LREE and depletion in HFSE over LILE, which suggests derivation from arc-related parental magmas.  相似文献   

12.
The Archean Eon was a time of geodynamic changes. Direct evidence of these transitions come from igneous/metaigneous rocks, which dominate cratonic segments worldwide. New data for granitoids from an Archean basement inlier related to the Southern São Francisco Craton (SSFC), are integrated with geochronological, isotopic and geochemical data on Archean granitoids from the SSFC. The rocks are divided into three main geochemical groups with different ages: (1) TTG (3.02–2.77 Ga); (2) medium- to high-K granitoids (2.85–2.72 Ga); and (3) A-type granites (2.7–2.6 Ga). The juvenile to chondritic (Hf-Nd isotopes) TTG were divided into two sub-groups, TTG 1 (low-HREE) and 2 (high-HREE), derived from partial melting of metamafic rocks similar to those from adjacent greenstone belts. The compositional diversity within the TTG is attributed to different pressures during partial melting, supported by a positive correlation of Dy/Yb and Sr/Zr, and batch melting calculations. The proposed TTG sources are geochemically similar to basaltic rocks from modern island-arcs, indicating the presence of subduction processes concomitant with TTG emplacement. From ～2.85 Ga to 2.70 Ga, the dominant rocks were K-rich granitoids. These are modeled as crustal melts of TTG, during regional metamorphism indicative of crustal thickening. Their compositional diversity is linked to: (i) differences in source composition; (ii) distinct melt fractions during partial melting; and (iii) different residual mineralogies reflecting varying P–T conditions. Post-collisional (～2.7–2.6 Ga) A-type granites reflect rifting in that they were closely followed by extension-related dyke swarms, and they are interpreted as differentiation or partial melting products of magmas derived from subduction-modified mantle. The sequence of granitoid emplacement indicates subduction-related magmatism was followed by crustal thickening, regional metamorphism and crustal melting, and post-collisional extension, similar to that seen in younger Wilson Cycles. It is compelling evidence that plate tectonics was active in this segment of Brazil from ～3 Ga.  相似文献   

13.
Abstract. Granitoids and related rocks of eastern Australia can be classified according to their metallogenic potential using a scheme based on compositional character, degree of compositional evolution, degree of fractionation, and oxidation state. The scheme is based on empirical and theoretical considerations and satisfactorily describes the known distribution of granite‐related mineralisation. The granitoids range from unevolved, mantle compatible compositions to highly evolved and fractionated. They exhibit age‐ and region‐specific variations in silica content, compositional evolution and oxidation state. The most unevolved intrusive igneous rocks comprise those of the Ordovician of the Lachlan Orogen, and the Devonian of the New England Orogen. Strongly fractionated and evolved I‐type granites occur in western Tasmania, the southern New England Orogen, and far north Queensland. Other fractionated suites tend to occur relatively rarely in the Lachlan Orogen and elsewhere. Oxidation states vary markedly. The most consistently oxidised rocks occur in the Ordovician of the central Lachlan Orogen, and the northernmost New England Orogen. The Carboniferous I‐types of the northeastern Lachlan Orogen are consistently more oxidised than other Lachlan Orogen I‐types. Gold‐rich, Cu‐poor systems associated with felsic I‐types in eastern Australia are associated with W‐Mo mineralised suites with gold occurring within a predictable metallogenic zonation. Gold mineralised I‐types comprise weakly to moderately oxidised, high‐K granitoid suites that, at least in the east Australian context, have low K/Rb ratios and show strong fractionation trends. Gold is readily removed from granitic magmas through the early precipitation of sulfides, or to a lesser extent by magnetite. Crystallisation of Fe‐poor, silica‐rich granitic magmas in a relatively narrow oxidation window between the FMQ and NNO buffers may provide conditions where retention of Au in magmas in felsic granitic magmas is optimised.  相似文献   

14.
Early Paleozoic granitoids of autochthonous and allochthonous facies in the Baikal area (Ol’khon Island, Khamar-Daban Ridge) are in close spatial association with gneisses, migmatites, and plagiogranites and are usually confined to granite–gneiss domes. They are virtually not subjected to magmatic differentiation. Formation of granitoids of the Solzan massif and Sharanur complex lasted 26–28 Myr, which might be considered an indicator of collisional granitoid magmatism. Collisional granitoids of different provinces have a series of indicative features: They are peraluminous and highly potassic and are enriched in crustal elements (Rb, Pb, and Th) but sometimes have low contents of volatiles. In contrast to collisional magmatism, petrogenesis of intraplate granitoids does not depend on the composition and age of the enclosing rocks. The geochemical evolution of intraplate granitoid magmatism in the Baikal area is expressed as an increase in contents of F, Li, Rb, Cs, Sn, Be, Ta, Zr, and Pb and a decrease in contents of Ba, Sr, Zn, Th, and U during the differentiation of multiphase intrusions. The geochemical diversity of these granitoids formed both from crustal and from mantle sources and as a result of the mantle–crust interaction, might be due to the effect of plume on the geologic evolution of intraplate magmatism. The wide range of compositions and geochemical types of igneous rocks (from alkali and subalkalic to rare-metal granitoids) within the Late Paleozoic Baikal magmatism area suggests its high ore potential.  相似文献   

15.
Geological, mineralogical, petrographic, geochemical, and geochronological data are reported for granitoids of the Aturkol Massif (Gorny Altai). It is shown that it was formed in within-plate setting in the Early Triassic, nearly simultaneously with flood basalts of the Kuznetsk Basin and alkalic basite and lampropyre dike swarms in the western Altai-Sayan Fold Region. At the same time, the mineralogical-petrographic, geochemical, and isotope characteristics of the considered granitoids are close to those of I-type granites. Intraplate signatures (elevated HFSE and REE) are recognized only in the least silicic rocks (granosyenites). Obtained data suggest mantle–crustal nature of the granitoids. They were formed by mixing of lamprophyre magmas with high pressure (>10 kbar) crustal melts derived from a mixed source consisting mainly of N-MORB-type metabasites with insignificant admixture of high-Ti basalts and metasedimentary rocks. The contribution of mantle component in the granitoids was insignificant (<20%). Proposed petrogenetic mechanism can provide the formation of large volumes of granitoid magmas with “crustal” geochemical and isotope signatures in an intraplate setting.  相似文献   

16.
Garnet, an uncommon accessory mineral in igneous rocks, occurs in seven small peraluminous granitoid plutons in the southeastern Arabian Shield; textural equilibrium between garnet and other host granitoid minerals indicates that the garnets crystallized from their host magmas. Compositions of the garnets form three groups that reflect host-granitoid compositions, which in turn reflect source compositions and tectonic regimes in which the host magmas were generated. Garnets from the seven plutons have almandine-rich cores and spessartine-rich rims. This reverse zoning depicts host magma compositional evolution; i.e. rimward spessartine enrichment resulted from progressive, host-magma manganese enrichment. The garnets are heavy rare-earth element enriched; (Lu/La) N ranges from 13 to 355 and one of the garnets contains spectacularly elevated abundances of Y, Ta, Th, U, Zn, Zr, Hf, Sn, and Nb. Involvement of garnets with these trace element characteristics in magma genesis or evolution can have dramatic effects on trace element signatures of the resulting magmas. Other researchers suggest that Mn-enriched magmas are most conducive to garnet nucleation. Although the garnetiferous granitoids discussed here are slightly Mn enriched, other genetically similar peraluminous Arabian granitoids lack garnet; Mn enrichment alone does not guarantee garnet nucleation. The presence of excess alumina in the magma may be a prerequisite for garnet nucleation.  相似文献   

17.
The Bundelkhand massif of Archean-Palaeoproterozoic age is primarily a granite-gneiss complex. Three distinct granitoid suites have been identified within the massif hornblende granitoids, biotite granitoids and leucogranitoids, in order of decreasing age. These granitoids were emplaced in previously deformed basement consisting of gneisses, banded iron formations and other metasediments, mafic to felsic volcanics.
The granitoids exhibit a large compositional range from quartz diorite to syenogranite and show a calc-alkaline trend. They are metaluminous to peraluminous and have I-type characteristics. The SiO2 content ranges from 49 to 77 wt%. Low K2O/Na2O characterizes the granitoids. The oldest hornblende granitoids have low Rb and Yb contents compared to the younger biotite granitoids and leucogranitoids. Rb/Sr values for most of the granitoids are low (< 1). K/Rb ratios range from 95 to 373 which is, in general, comparable with other calc-alkaline suites. Y/Nb ratios of the granitoids are > 1.2 which is a characteristic feature of magmas derived from sources chemically similar to island arc or continental margin basalts.
The features mentioned above coupled with concentrations of Rb, Y, Nb, Yb, Ta and Th indicate a volcanic-arc tectonic setting for the granitoids. It is proposed that the massif represents subduction-related magmatism of an ocean in the southern part of the massif (an Andean plate margin).  相似文献   

18.
The paper reports data on the Nd isotopic composition and the evaluated composition of the sources of magmatism that produced massifs of alkali and basic rocks of the Khaldzan-Buregtei group. The massifs were emplaced in the terminal Devonian at 392–395 Ma in the Ozernaya zone of western Mongolia. The host rocks of the massifs are ophiolites of the early Caledonian Ozernaya zone, which were dated at 545–522 Ma. The massifs were emplaced in the following succession (listed in order from older to younger): (1) nordmarkites and dolerites syngenetic with them; (2) alkali granites and syngenetic dolerites; (3) dike ekerites; (4) dike pantellerites; (5) rare-metal granitoids; (6) alkali and intermediate basites and quartz syenites; and (7) miarolitic rare-metal alkali granites. Our data on the Nd isotopic composition [?Nd(T)] and conventionally used (canonical) ratios of incompatible elements (Nb/U, Zr/Nb, and La/Yb) in rocks from the alkaline massifs and their host ophiolites indicate that all of these rocks were derived mostly from mantle and mantle-crustal enriched sources like OIB, E-MORB, and IAB with a subordinate contribution of N-MORB (DM) and upper continental crustal material. The variations in the ?Nd(T) values in rocks of these massifs suggest multiple mixing of the sources or magmas derived from them when the massifs composing the Khaldzan-Buregtei group were produced. The OIB and E-MORB sources were mixed when the rocks with mantle signatures were formed. The occurrence of nordmarkites, alkali granites, and other rocks whose isotopic and geochemical signatures are intermediate between the values for mantle and crustal sources testifies to the mixing of mantle and crustal magmas. The crustal source itself, which consisted of rocks of the ophiolite complex, was obviously isotopically and geochemically heterogeneous, as also were the magmas derived from it. The model proposed for the genesis of alkali rocks of the Khaldzan-Buregtei massifs implies that the magmas were derived at two major depth levels: (1) mantle, at which the plume source mixed with an E-MORB source, and (2) crustal, at which the ophiolites were melted, and this gave rise to the parental magmas of the nordmarkites and alkali granites. The basites were derived immediately from the mantle. The mantle syenites, pantellerites, and rare-metal granitoids were produced either by the deep crystallization differentiation of basite magma or by the partial melting of the parental basites and the subsequent crystallization differentiation of the generated magmas. Differentiation likely took place in an intermediate chamber at depth levels close to the crustal (ophiolite) level of magma generation. Only such conditions could ensure the intense mixing of mantle and crustal magmas. The principal factor initiating magma generation in the region was the mantle plume that controlled within-plate magmatism in the Altai-Sayan area and the basite magmas related to this plume, which gave rise to small dikes and magmatic bodies in the group of intrusive massifs.  相似文献   

19.
The problems of tectonic control of composition, size, and morphology of synkinematic crustal granitoids are discussed by the example of the Western Sangilen granites (South-East Tuva). Comparative analysis was performed for felsic bodies and massifs spatially confined to tectonic zone (Erzin shear zone): Erzin migmatite–granite complex (510–490 Ma), Matut granitoid massif (510–490 Ma), Bayankol polyphase gabbro-monzodiorite–granodiorite–granite massif (490–480 Ma), and the Nizhneulor Massif (480–470 Ma). It is shown that synkinematic felsic melts during the transition from collisional compression to transpression were formed at different crustal levels. An increase of shear component provided favorable conditions for the migration of felsic melts, increase of size and morphology of intrusive bodies from vein type to harploith (likely, loppoliths and laccoliths) and further to stocks. All kinematic granitoids of the Erzin tectonic zone are ascribed to the crustal S-type granites. Dispersion and average chemical composition of the synkinematic granites strongly depend on the degree of their “isolation” from protolith. From auto- and paraautochthonous granitoids to allochthonous granites, the compositional dispersion decreases and the chemical composition is displaced toward I-type magmatic rocks.  相似文献   

20.
The article describes the petrochemical evolution of oceanicrocks in terms of plate tectonics with special reference toIceland. The compositional variation along the rift zone isrelated to different production rates of mantle-derived olivinetholeiite of invariant composition which is added to the crustfrom below and modified by mixing with anatectic melts in thecrust and concomitant crystal fractionation. The kinematic processes of crustal accretion cause rocks depositedin the rift zone to subside towards higher temperatures wherethey suffer hydration and progressive metamorphism before becominga part of the stable crustal plate. Rocks deposited near therift-centre assume the highest metamorphic grade (greatest depth)while rocks deposited at the rift-margins follow a shallow pathbefore being carried towards lower temperatures in the stableplate. The material transport through stationary metamorphiczones produces the layering of the oceanic crust. As the hydrated rocks cross their solidus isotherm, silicicmagma is formed by incongruent partial melting. The meltingcontinues until rocks crossing the boundary between the amphiboliteand granulite fades are finally dehydrated by the break-downof amphibole. This reaction boundary defines the surface ofthe upper mantle. The segregation and retention of crust-derived magmas withinthe rift zone results in chemical fractionation in the oceaniccrust, for its lower sections are depleted in elements enteringthe early melt fractions, which are silicic and enriched inthe dispersed elements. The last melt-increments from the samesubsiding pile are ne-normative basalts. The rift-zone rocks are shown to be mantle-derived olivine tholeiitemodified by minor amounts of crustal rhyolite and nephelinebasalt, while volcanism outside the rift zone is dominated bythe crust-derived magmas themselves. All mixtures undergo furthermineralogical evolution towards invariant compositions in thebasalt system, resembling the olivine tholeiite, quartz tholeiite,and nepheline basalt of synthetic systems. The dispersed-elementgeochemistry of the oceanic rocks is but slightly modified bycrystal fractionation, and reflects the mixing ratios of theolivine tholeiite and the different crustal magmas. The geochemistry of radiogenic isotopes is controlled by continuousprocesses of crustal fractionation separating mother and daughterelements. The oxygen-isotope geochemistry can be referred tomagma mixing, for rhyolites formed by anatexis in the hydratedcrust are enriched in light oxygen relative to the mantle-derivedmelts.  相似文献   

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