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.
A model for trondhjemite genesis is proposed where granite is transformed to trondhjemite via infiltration by a Na-rich metamorphic fluid. The Rockford Granite of the Northern Alabama Piedmont serves as the case example for this process and is characterized as a synmetamorphic, peraluminous trondhjemite-granite suite. The major process operative in the conversion of granite to trondhjemite involves cation exchange of Na for K in the feldspar and mica phases through a volatile fluid medium. Whole-rock 18O values for the trondhjemites are negatively correlated with atomic proportion K/Na ratio indicating a partial reequilibration of the altered granitoids with a Na- and18O-rich metamorphically derived fluid. Biotite decomposition to an Al-epidote-paragonitic muscovite-secondary quartz assemblage is also associated with the sodium metasomatism, as are apatite replacement by Al-epidote and secondary zircon crystallization. Rare albitization of primary magmatic plagioclase and discontinuous grossularite reaction rim growth on magmatic garnet are present in the trondhjemites indicating the mobility of Ca during alkali metasomatism. The replacement of magmatic phases by me tasomatic phases exemplifies the chemical changes produced during infiltration metasomatism where the trondhjemites are depleted in P2O5, Th, Rb, U, K2O, V, Sn, F, MgO, Pb, TiO2, FeO* and Li and enriched in CaO, Na2O, Zr and Sr relative to the granites. Other elements, such as Cr, MnO, Cu, Zn, Co, Ba, SiO2, Ni, Al2O3, are shown to be relatively immobile during the metasomatism. The infiltration metasomatism probably occurred during prograde regional metamorphism, when a discrete fluid phase was produced in the surrounding amphibolite-grade metasediments. Foliation planes in the granite apparently served as conduits for fluid flow with reaction-enhanced permeability accompanying the 8% molar volume reduction during Na-for-K exchange in the feldspars. A source for the Na and Sr in the metamorphic fluid may have been paragonitic muscovite in the metasedimentary country rocks. Rubidium and K were probably retained in metasedimentary biotite. The Silent Lake pluton in southeastern Ontario is a possible analogue to the alkali metasomatic processes affecting the Rockford Granite.  相似文献   

2.
邹家山铀矿床碱交代蚀变广泛发育,与铀成矿作用关系极为密切。笔者通过野外宏观地质调查和室内岩相学、电子探针分析、常量及微量元素化学分析等方法,初步查明该矿床碱交代型矿石的矿物交代蚀变顺序为钠交代、钾交代、硅质交代。碱性成矿热液先是富Na,而后富K,且两者成分相似,但富K热液更利于铀成矿。与正常碎斑熔岩相比,碱交代型矿石SiO2含量减少,K2O或Na2O、Al2O3、Fe2O3、MgO、P2O5、CaO和U、Th、Zr、Hf、Sm、Ti及REE含量明显增高。与钾交代型矿石相比,钠交代型矿石Rb和REE含量较少,稀土分异强。碱交代作用有利于副矿物蚀变并释放铀,有利于对地层中铀的萃取,有利于铀的稳定迁移。  相似文献   

3.
Late Precambrian granitoid rocks occurring within a 44,000 km2 area of the western Arabian Shield are subdivided on the basis of geology and petrology into older (820 to 715 Ma) and younger (686 to 517 Ma) assemblages. The older assemblage contains major complexes which can be assigned to either one of a granodioritic or trondhjemitic petrologic association. The earliest granitoid rocks are trondhjemitic tonalites (trondhjemite association), depleted in Ba, Ce, F, La, Li, Nb, Rb, Y and Zr compared to granitoids of the slightly younger granodiorite association, which are related to a calcic, calc-alkaline suite of rocks ranging in composition from gabbro through monzogranite. The plutonic rocks of the older assemblage were probably emplaced in the cores of contemporary island arcs.The younger plutonic assemblage is dominated by three, geochemically distinct, coeval granitic associations: the alkali granite, alkali-feldspar granite and monzogranite associations. The alkali granite association is composed of perthite granites (alkali granites and genetically related alkali-feldspar granites). Rocks of this association are marginally peralkaline or metaluminous and are characterized by low contents of Ba, Co, Li, Rb, Sc and Sr, and high contents of Be, Cu, F, REE, Nb, Sn, Y, Zn and Zr. The alkali-feldspar granite association is mainly composed of alkali-feldspar granites and syenogranites. Rocks of this association are marginally peraluminous or metaluminous and contain low Ba, Sr, and high F, Rb, Sn, Th and U. The monzogranite association consists mainly of monzogranites and granodiorites. Rocks of this association are peraluminous or marginally metaluminous and have the highest contents of Ba, Cu, Co, Li, Sc, Sr, Ta, and V, and the lowest contents of REE, Nb, Rb, Sn, Th, U, Y, Zn and Zr of the three granitic associations.These voluminous granitic magmas, together with the felsic component of a coeval sequence of bimodal volcanic rocks, are partial melts of the earlier island arc terrain produce during a prolonged fusion event. Subsolvus, highca granites of the monzogranite association have I-type features and represent partial melts of previously unfused crust, while low-Ca perthite granites of the alkali granite and alkali-feldspar granite associations have A-type features and represent partial melts of previously fused crust.This type of petrogenetic model can account for much of the petrologic diversity of the Pan-African granitic terrain of the Arabian Shield.  相似文献   

4.
  相似文献   

5.
Tourmaline bearing leucogranite occurs as a pluton with pegmatitic veins intruding the Archaean granodiorite in the Bastipadu area, Kurnool district of Andhra Pradesh. We present field and petrographic relations, mineral chemistry and geochemical data for the leucogranite. It is essentially a two-mica granite, composed of quartz, perthite, microcline, albite, tourmaline and muscovite along with minor biotite and titanite. The euhedral tourmalines are regularly distributed in the rock. The geochemical studies show that the leucogranite is calc-alkaline, peraluminous to metaluminous which formed in a syn-collisional to volcanic arc-related setting. It displays strong ‘S’ type signatures with high K/Na ratios, moderately fractionated light rare earth elements, relatively flat heavy rare earth elements with \(\hbox {[Ce/Yb]}_\mathrm{N} \le 27.8\) and a strong negative Eu anomaly. The geochemical characteristics indicate that the leucogranite melt might have been generated from partial melting of metasediments. Electron probe microanalyser data show the presence of alkali group tourmaline in leucogranite represented by schorl and dravite. Tourmaline compositions plot in the Li-poor granitoids and associated pegmatites and aplites and metapelites/metasammites fields. Partial melting of boron-enriched source rocks is linked with the development of tourmalines in the leucogranite.  相似文献   

6.
Rb/Sr data for seven basaltic provinces (K-Ar ages 50-0 Ma) in southeastern Australia imply isotopic heterogeneities in the mantle sources. The total range of 87Sr/ 86Sr is 0.7031–0.7054. Effects of crustal contamination are negligible, since the rocks analyzed represent primary or primitive magma compositions. The inferred scales of heterogeneity range from <1 km for small intraprovince variations, to in the order of 100 km for the larger differences between provinces.Correlation of regional high 87Sr/86Sr in basaltic rocks with the presence of amphibole-bearing upper mantle xenoliths suggests that the degree of metasomatic activity in the underlying mantle is a major control on the Rb/Sr and 87Sr/86Sr values of mantle source volumes and partial melts derived from these. Xenolith data also indicate that both pervasive metasomatism and the presence of crystallized melts or cumulates as veins and dykes in mantle wall rock are possible mechanisms for metasomatic additions.Mantle isochrons can be constructed both within some provinces and between provinces. However, episodic metasomatism in the mantle source regions, with correlated enrichment in Rb/Sr and 87Sr/86Sr, can produce artificial isochrons which may have no relevance to mantle differentiation events.  相似文献   

7.
北祁连中段北缘冷龙岭铀矿带处于祁连-龙首山NW向海西碱交代热液型铀成矿带,是典型的钠交代型铀矿带。铀成矿与钠交代有着十分密切的关系,矿体几乎都产在钠交代岩内。矿床内钠交代岩矿物组成上的最大特点是原岩矿物大多消失,代之以红色的钠长石。钠交代使岩石有效孔隙度增大,并且降低了抗压强度,从而成为构造活动的薄弱环节。钠交代促使原岩中铀的赋存状态发生改变,使铀发生活化转移。在钠质交代过程中,许多铀元素的载体矿物被钠长石交代,也是使铀发生活化转移的重要因素。以上事实充分说明,钠交代不仅为铀沉淀提供了良好的空间,而且为铀成矿奠定了物质基础。  相似文献   

8.
A series of striking migmatitic structures occur in rectilinear networks through western Fiordland, New Zealand, involving, for the most part, narrow anorthositic dykes that cut hornblende‐bearing orthogneiss. Adjacent to the dykes, host rocks show patchy, spatially restricted recrystallization and dehydration on a decimetre‐scale to garnet granulite. Although there is general agreement that the migration of silicate melt has formed at least parts of the structures, there is disagreement on the role of silicate melt in dehydrating the host rock. A variety of causal processes have been inferred, including metasomatism due to the ingress of a carbonic, mantle‐derived fluid; hornblende‐breakdown leading to water release and limited partial melting of host rocks; and dehydration induced by volatile scavenging by a migrating silicate melt. Variability in dyke assemblage, together with the correlation between dehydration structures and host rock silica content, are inconsistent with macroscopic metasomatism, and best match open system behaviour involving volatile scavenging by a migrating trondhjemitic liquid.  相似文献   

9.
In the ultra-high pressure Metamorphic Kimi Complex widespread tonalitic–trondhjemitic dykes, with an intrusion age ca. 65–63 Ma, cross-cut boudins and layers of amphibolitized eclogites. Geochemical investigation proclaims the tied genetic relationship of the amphibolitized eclogites and the associated tonalitic–trondhjemitic dykes. The major and trace element contents and rare earth element patterns of the amphibolitized eclogites indicate formation of their protoliths by fractional crystallization of tholeiitic magmas in a back-arc environment. The tonalites and trondhjemites are characterized by moderate to high Sr contents (>130 ppm), and low Y (<8.2 ppm) and heavy rare earth element contents (Yb content of 0.19–0.88 ppm). The chemical composition of the tonalitic and trondhjemitic dykes are best explained by partial melting of a tholeiitic source like the amphibolitized eclogites with residual garnet and amphibole, at the base of a thickened crust during Early Tertiary subduction/accretion at the southern margins of the European continent.  相似文献   

10.
《Gondwana Research》2002,5(2):287-305
Large volumes of granitoids were emplaced in the Hercynian Central Iberian Zone during the last ductile deformation phase (D3, 300-320 Ma). The biotite-rich granitoids are the most abundant: (1) syn-D3 granodiorites-monzogranites (313-319 Ma) with calc-alkaline and aluminopotassic affinities; (2) late-D3 granodiorites-monzogranites (306-311 Ma), related to subalkaline and aluminopotassic series. These granitoids are associated with coeval gabbro-norite to granodiorite bodies and/or mafic microgranular enclaves. Both granitoids and basic-intermediate rocks show petrological, geochemical and isotopic evidence of interaction between felsic and mafic magmas.The mantle-derived melts, represented by shoshonitic gabbro-norites, were probably derived from an enriched and isotopically homogeneous source (Sri = 0.7049 to 0.7053, eNd = -2.1 to -2.5). In some syn- and late-D3 plutons there are evidences of essentially crustal granites, represented by moderately peraluminous monzogranites of aluminopotassic affinity. They have similar Nd model ages (1.4 Ga) but different isotopic compositions (Sri = 0.7089 to 0.7106, eNd = -5.6 to -6.8), revealing a heterogeneous crust. Potential protoliths are metasedimentary (immature sediments) and/or felsic meta-igneous lower crust materials. Large amounts of hybrid magmas were generated by the interaction of these coeval mantle- and crust-derived liquids, giving rise to slightly peraluminous monzogranites/granodiorites of calc-alkaline and subalkaline affinities, which display more depleted isotopic compositions than the crustal end-members (Sri = 0.7064 to 0.7085, eNd = -4.4 to -6.2). Petrogenetic processes involving mingling and/or mixing and fractional crystallization (at variable degrees) in multiple reservoirs are suggested.A major crustal growth event occurred in late-Hercynian times (∼305-320 Ma) related to the input of juvenile mantle magmas and leading to the genesis of composite calc-alkaline and subalkaline plutons, largely represented in the Central Iberian Zone.  相似文献   

11.
江西庐山─星子地区早前寒武纪片麻岩基底的初步研究   总被引：3，自引：1，他引：2  
庐山-星子地区原称的混合花岗岩和混合岩实为以奥长花岗质片麻岩为主体的灰色片麻岩（TTG岩套）和花岗质片麻岩．后者是前者经钾质成分改造交代的结果。其常量元素和微量元素特征与早前寒武纪低铝型灰色片麻岩一致,构成扬子克拉通的晚太古代基底。其上的星子群属表壳岩,与赣北片麻岩一起构成花岗-绿岩地体,因燕山运动和新构造而出露地表。  相似文献   

12.
The Ayopaya province in the eastern Andes of Bolivia, 100 km NW of Cochabamba, hosts a Cretaceous alkaline rock series within a Palaeozoic sedimentary sequence. The alkaline rock association comprises nepheline-syenitic/foyaitic to ijolitic intrusions, carbonatite, kimberlite, melilititic, nephelinitic to basanitic dykes and diatremes, and a variety of alkaline dykes. The carbonatites display a wide petrographic and geochemical spectrum. The Cerro Sapo area hosts a small calciocarbonatite intrusion and a multitude of ferrocarbonatitic dykes and lenses in association with a nepheline-syenitic stock. The stock is crosscut by a spectacular REE-Sr-Th-rich sodalite-ankerite-baryte dyke system. The nearby Chiaracke complex represents a magnesiocarbonatite intrusion with no evidence for a relationship to igneous silicate rocks. The magnesiocarbonatite ( REE up to 1.3 wt%) shows strong HREE depletion, i.e. unusually high La/Yb ratios (520–1,500). Calciocarbonatites ( REE up to 0.5 wt%) have a flatter REE distribution pattern (La/Yb 95–160) and higher Nb and Zr contents. The sodalite-ankerite-baryte dyke system shows geochemical enrichment features, particularly in Na, Ba, Cl, Sr, REE, which are similar to the unusual natrocarbonatitic lavas of the recent volcano of Oldoinyo Lengai, Tanzania. The Cerro Sapo complex may be regarded as an intrusive equivalent of natrocarbonatitic volcanism, and provides an example for carbonatite genesis by late-stage crystal fractionation and liquid immiscibility. The magnesiocarbonatite intrusion of Chiaracke, on the other hand, appears to result from a primary carbonatitic mantle melt. Deep seated mantle magmatism/metasomatism is also expressed by the occurrence of a kimberlite dyke. Neodymium and strontium isotope data (Nd 1.4–5.4, 87Sr/86 Sr<Bulk Earth) indicate a depleted mantle source for the alkaline magmatism. The magmatism of the Ayopaya region is attributed to failed rifting of western South America during the Mesozoic and represents the only occurrence of carbonatite and kimberlite rocks in the Andes.  相似文献   

13.
Summary This paper describes corundum formation in a metasomatic reaction zonation around an ultramafic body within a metapelitic sequence. The investigated body is about 100 m in diameter and is located in the Saualpe of the Austroalpine nappe complex in the Eastern Alps. The body is surrounded by a 10 m wide reaction zone (here called zone 3) containing the paragenesis garnet–staurolite–biotite–margarite–chlorite–corundum. Beyond a further metasomatic transition zone (here called zone 2), there are undisturbed metapelitic host rocks (zone 1) that have the metamorphic peak paragenesis garnet–biotite–plagioclase–staurolite–muscovite–quartz. It is shown that reaction zonation formed around 7.2kbar and 615°C during regional metamorphism, just above the serpentine breakdown reactions in the system MgO–SiO2–H2O. Detailed analysis of the whole rock compositions shows that the reaction zonation formed by infiltration metasomatism that caused significant mass loss in the two alteration zones. These zones are particularly depleted in SiO2, Na2O and possibly K2O. An XNa2O–XSiO2 thermodynamic pseudosection is presented that includes the parageneses of both the unaltered metapelitic host rock and the corundum-bearing parageneses. This suggests that the metasomatic process can be explained by the transfer of SiO2 and Na2O alone. We interpret that the process is driven by water liberated from the previously serpentinised ultramafic body during prograde dehydration during regional Eo-Alpine metamorphism. This fluid flowed outwards from the ultramafic body depleting the surrounding pelites in silica and causing margarite formation from plagioclase and muscovite. This interpretation of the driving mechanism is consistent with our knowledge of the low water activities of the Saualpe during the Eo-Alpine orogenic cycle.  相似文献   

14.
The Acadian-age Waldoboro Pluton Complex (WPC), mid-coastalMaine, consists of seven granitoid units surrounded by migmatitic,peraluminous gneisses and schists (predominantly Bucksport orSebascodegan Formation). The complex (area >340 km2) cross-cutsthe westward-vergent St. George thrust fault, which may markthe boundary between the Avalon and Gander composite terranes.Field and petrologic data indicate in situ formation of theperaluminous, syntectonic granitoids: contacts with Bucksportparagneisses are transitional and concordant; abundant country-rockenclaves show evidence for melting; restitic garnet, biotite,and plagioclase in the granitoids are identical in compositionto garnet, biotite, and plagioclase in the country rock. Chemicalvariations among the main granitoid phases (gneissic granite,granite, and leucogranite) reflect varying degrees of melt-restiteunmixing. Major and trace elements define mixing trends betweenrefractory Bucksport lithologies and leucogranites which approximatemelt compositions. Petrographic and whole-rock chemical dataare consistent with restitic plagioclase, garnet, biotite, tourmaline,zircon, apatite, sphene, and an accessory phase such as monazite.Quantitative major-oxide mass-balance models indicate that gneissicgranite represents a mixture of 55% melt–45% restite whereasgranite represents a mixture of 76% melt–24% restite.Melt-restite proportions calculated from trace element dataagree with those calculated from major oxide data for the gneissicgranite, but are different (85% melt–15% restite) fromthose calculated from major oxide data for the granite. Thisis attributed to inhomogeneous distribution of minor phasesand the effects of metasomatism. High K2O, Rb, Ba, Cs, Li, B,K/Rb, K/Ba, Rb/Sr, and Th/U along the eastern mylonitic marginand elsewhere within the WPC reflect post-solidification metasomaticprocesses. Intrusion of mafic magmas during uplift after crustal thickeningappears to have caused high-temperature metamorphism and anatexisof Bucksport country rocks at relatively low pressure (0.4 GPa).Dehydration melting of muscovite to produce magmas saturatedor nearly saturated with H2O explains the formation of migmatitesin the vicinity of the WPC. Formation of granites by 50–60%fluid-absent melting of Bucksport source rocks containing 20%biotite requires that fusion occurred at T860C and consumedall of the biotite in the source rock. Phase equilibrium dataand estimated temperatures of formation provide evidence thatthe granitoids formed at T<860C, whereas petrographic dataindicate that not all biotite in the source rock was consumedduring anatexis. Therefore, the WPC granitoids could have formedby fluid-absent melting if the source rocks contained >20%biotite (the maximum amount observed). However, it is also possiblethat influx of aqueous fluid before or during anatexis allowedproduction of relatively large volumes of melt at T<860C.Available data do not allow these possibilities to be rigorouslytested. The WPC granitoids have many characteristics of S-type granitesand preserve a chemical and mineralogical record of their sourcerocks, indicating that granites can image their sources evenin tectonically complex regions.  相似文献   

15.
Major and 31 minor elements have been determined in 39 large samples of Variscan granitoids from 6 plutons or intrusions from the South Bohemian Batholith (Rastenberg, Weinsberg, Mauthausen, Schrems, Eisgarn and Gebharts). The granitoids are mainly granites but also diorites, tonalites, trondhjemites, granodiorites. Average concentrations of Ba, Th, U, La, Ce, Pb, Nd, Sr and K in the Weinsberg, Mauthausen and Schrems granites exceed those in average felsic I- and S-type granites by factors ranging between 2.1 and 1.3. The granites melts formed at waterundersaturated conditions and intruded at 10 to 15 km depth during late-tectonic and post-tectonic phases of the Variscan orogeny (about 330 to 300 Ma ago). Hydrothermal or low temperature alteration is excluded for the majority of samples from a study of oxygen isotopes. The thickness of the plutons is estimated at about 6 km from heat balance constraints. By analogy with experimental partial melting, three different sources of the granitoids can be identified and chemically characterized: (1) The trondhjemites, tonalites and diorites in the early Rastenberg pluton are products of 15 to 40% melting respectively of a mafic (partly amphibolitic) lower crust. Redwitzites from the West Bohemian Massif which are comparable in age partly resemble the Rastenberg rocks. The mafic sources of the Rastenberg granitoids and redwitzites are crustally contaminated as reflected in their Sr-Nd isotopes. (2) The very large syn-tectonic Weinsberg pluton was formed from about 30% partial melting of a tonalitic lower crust at 800 to 850°C. Its low proportion of ca. 10% restite has a ferrodioritic composition. The post-tectonic fine-grained Mauthausen and Schrems granites which tend to a granodioritic mode, are very low in restite and are also products of melting of a tonalitic source. (3) The youngest (leuco-)granite, the Eisgarn pluton (high in Si, P, Li, Rb, Cs, U,87Sr/86Sr and low in Ca, Sr, Ba) reflects a pelitic source. The change from mafic to tonalitic to pelitic source composition for the granitoid sequence may indicate that the depth of melt formation decreased with time. The concentration of heavy rare earth elements decreased from Weinsberg to Eisgarn granites which indicates an increasing proportion of garnet in the source. The orogenic heat conformable with a heat flow of about 100 mWm-2 was provided by mafic intrusions. An alternative would be a drastic increase of the crustal thickness which cannot be recognized by barometry of the associated metamorphic rocks. Exposed metamorphic country rocks occur in higher amphibolite facies indicating about 5 kbar pressure. Mafic intrusions contain gabbros (Kleinzwettl) or have formed (quartz-)diorites (Gebharts), the latter being contaminated by granitic melts from partial melting of the wall rocks (MASH process). Largescale contamination by crustal materials can be observed in 18O and in Sr-Nd isotopes. The major mafic activity was probably caused by depression of solidus temperatures in the mantle wedge above a subduction zone where water was available from dehydration of subducted ocean crust. This water initiated partial melting of ultramafic rocks and metasomatism in the uppermost mantle above the level of melting. The water also mobilized highly incompatible elements (Ba, Th, U, La, Ce, Pb, Nd, Sr and K) from the uppermost mantle and transported them into the lower crust. Indicators of a nearby subduction or collision zone of Late Variscan age in addition to the specific association of granitoidal rocks are abundant upper mantle tectonites. An alternate or additional source of metasomatic fluids may have been dehydration of lower crustal rocks during Variscan high-grade metamorphism.Dedicated to Prof. Dr J. Zemann on the occasion of his 70th birthday  相似文献   

16.
The geological position, composition of mineral assemblages, and typomorphism of major minerals from garnet-bearing rocks at the Berezitovoe gold-base-metal deposit in the Upper Amur Region have been studied in detail. These are ore-bearing metasomatic rocks and metamorphosed porphyritic dikes. The garnet-bearing metasomatic rocks reveal zoning, which is caused by various degrees of metasomatic transformation of the Paleozoic porphyritic granodiorite that hosts the ore zone. The metasomatic replacement of granodiorite was accompanied by loss of Na, Ca, Ba, Sr and gain of K, Mn, and Rb. Garnet-bearing metamorphosed intermediate dikes occur within the metasomatic zone. The PT conditions of metamorphism and metasomatism are similar and estimated at 3.9 kbar and 500°C from various mineral equilibria. The results of physicochemical simulation of garnet-bearing mineral assemblages carried out by minimizing the Gibbs free energy and the geological data show that garnet-bearing mineral assemblages arose at the Berezitovoe deposit as a result of local high-temperature thermal metamorphism of previously formed low-temperature metasomatic rocks close in composition to classic beresite. In this connection, we propose considering garnet-bearing metasomatic rocks as high-temperature metamorphosed beresites.  相似文献   

17.
We investigated the contact zone between peridotite lenses and host gneisses located on the northern side of the Hochwart peak, also known as Vedetta Alta (Ulten Zone, Alto Adige -Südtirol) where metasomatic contact bands occur. The country rocks are gneisses consisting mainly of quartz, K-feldspar, garnet, kyanite, biotite and muscovite. The ultramafic body consists of a hectometre-sized garnet peridotite and harzburgite lens. The reaction zone shows mineralogic zoning from phlogopite-rich to tremolite-anthophyllite-talc-rich rocks from the host gneiss towards the peridotite. In some cases, lenses of serpentine and talc in association with chlorite, and trondhjemitic pods develop at the ultramafic rocks border to the gneisses. Trondhjemite dikes with pegmatoid texture also crosscut the peridotite body. Phlogopite aggregates with accessory zircon, Cl-apatite and tourmaline and phlogopite-hornblende aggregates also occur. The combination of petrography, mineral chemistry and mass balance calculations constrains the gains and losses of elements during metasomatism. Reaction zone formation involved extensive addition of H2O, K2O and LILE from the fluid, whereas MgO, CaO and Al2O3 were removed from the peridotite. Thus, the formation of the reaction zones between the mantle rocks and the gneisses was triggered by considerable fluid/melt circulation, causing crystallisation of mainly phlogopite, anthophyllite and talc, and the release of a trondhjemitic residual melt. Field mapping provides evidence that the internal structures of the host migmatites (folds) and those of the peridotites (foliation, fluid texture) are discordant. Pseudosection calculations give insights into the P-T conditions (T 660–700°C; P 0.5–0.7 GPa) of metasomatism responsible for the formation of reaction zones, which is related to the retrograde path of the Ulten Zone peridotites. Our results suggest that the redistribution of major and trace elements in subduction zones is strongly influenced by metasomatic reactions occurring at the slab-mantle interface.  相似文献   

18.
Major and trace element geochemistry of Proterozoic granitoids from the Dirang and Galensiniak Formations, of Lesser and Higher Himalayas, respectively, emplaced in and around Dirang and Tawang regions of the western Arunachal Himalaya, is discussed. In general, these granitoids are massive as well as foliated in nature and are characterized by granitic mineralogical compositions. Porphyritic and hypidiomorphic textures are common in massive type, whereas others show porphyroblastic and foliated textures. Augen structure is also observed in a number of samples. Geochemical and normative compositions together with petrographic features classify them as peraluminous granitoids. Major and trace element geochemistry of most of these granitoids shows granitic nature, while few samples also show monzonitic characteristics. Observed geochemical characters, such as their peraluminous and alkali-calcic/calcic-alkalic nature, crudely defined geochemical patterns, different multi-element and rare-earth element patterns, together with low Mg# (Mg number) of these granitoids suggest their derivation from lower crustal material rather than a mantle source. Multi-element and rare-earth element patterns corroborate their genesis from different crustal melts. It is difficult to explain variations observed in granitoid rocks by partial melting alone; definitely different other processes like migration of melts, magma mixing, assimilation and fractional crystallization also played important role in the genesis of these granitoids. These melts were likely generated at low temperature (730–760 °C) and low pressure (2–5 GPa). The chemical compositions suggest that most of these Paleoproterozoic granitoids are emplaced within the syn-collisional tectonic setting, while few granitoid samples also indicate their volcanic-arc nature. Probably, later group of granitoids are slightly younger to the syn-collisional type.  相似文献   

19.
Detailed geochemical, isotope, and geochronological studies were carried out for the granitoids of the Chuya and Kutima complexes in the Baikal marginal salient of the Siberian craton basement. The obtained results indicate that the granitoids of both complexes are confined to the same tectonic structure (Akitkan fold belt) and are of similar absolute age. U–Pb zircon dating of the Kutima granites yielded an age of 2019±16 Ma, which nearly coincides with the age of 2020±12 Ma obtained earlier for the granitoids of the Chuya complex. Despite the close ages, the granitoids of these complexes differ considerably in geochemical characteristics. The granitoids of the Chuya complex correspond in composition to calcic and calc-alkalic peraluminous trondhjemites, and the granites of the Kutima complex, to calc-alkalic and alkali-calcic peraluminous granites. The granites of the Chuya complex are similar to rocks of the tonalite–trondhjemite–granodiorite (TTG) series and are close in CaO, Sr, and Ba contents to I-type granites. The granites of the Kutima complex are similar in contents of major oxides to oxidized A-type granites. Study of the Nd isotope composition of the Chuya and Kutima granitoids showed their close positive values of εNd(T) (+ 1.9 to + 3.5), which indicates that both rocks formed from sources with a short crustal history. Based on petrogeochemical data, it has been established that the Chuya granitoids might have been formed through the melting of a metabasitic source, whereas the Kutima granites, through the melting of a crustal source of quartz–feldspathic composition. Estimation of the PT-conditions of granitoid melt crystallization shows that the Chuya granitoids formed at 735–776 °C (zircon saturation temperature) and > 10 kbar and the Kutima granites, at 819–920 °C and > 10 kbar. It is assumed that the granitoids of both complexes formed in thickened continental crust within an accretionary orogen.  相似文献   

20.
The mineralogical-petrographic, petrochemical, and geochemical characteristics of the trondhjemite-granodiorite intrusions widespread among the volcanoterrigenous rocks of the eastern part of the Voronezh crystalline massif are studied. The rocks are characterized by cotectic quartz-plagioclase differentiation and differentiated (La/Yb = 31) REE pattern and are enriched in LILE with a decrease in the K/Rb and Zr/Hf ratios and an increase in the Th/U ratio by 1.5–2 times relative to subduction trondhjemites and their volcanic analogs from the modern and young island arcs. These data allow us to conclude that generation of the trondhjemite magma is a result of melting of the rocks of the continental crust during collision. The concordant (SHRIMP) age of trondhjemites is 2047 ± 11 Ma, and the age of zircon cores probably entrapped from the host rocks by the trondhjemitic melt is 2172 ± 17 Ma.  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