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1.
《Precambrian Research》2001,105(2-4):183-203
Previous studies have shown that the 2.04 Ga Kangâmiut dyke swarm of SW Greenland was injected into an active tectonic environment associated with the formation of the Nagssugtoqidian orogenic belt. Major and trace element modelling of the swarm shows that its chemical evolution was controlled by simple clinopyroxene–plagioclase fractionation. However, such trends — although typical of continental flood basalts and mafic dyke swarms — are at variance with their mineralogy and petrography, which show that locally hornblende is the dominant primary ferromagnesian mineral. Modelling of intradyke fractionation alone shows that hornblende could locally have been an important crystallising phase within several dykes. Normal basaltic fractionation must have occurred before dyke injection at the exposed crustal levels, where the influx of water into the dykes is believed to be responsible for the transition from clinopyroxene–plagioclase (tholeiitic) to hornblende–plagioclase±oxides (calc–alkaline) crystallisation. Overall geochemical trends are dominated by tholeiitic fractionation because (1) hornblende fractionation tended to buffer chemical composition; (2) the presence of water in the surrounding country rocks may have resulted in the advection of heat away from the dyke and consequently resulted in rapid crystallisation, particularly in thin dykes. There is no evidence from trace element data, and particularly Pb isotopic ratios, of any significant assimilation of country rocks occurring during clinopyroxene–plagioclase fractionation, although this does not preclude contamination of the mantle source prior to magma generation. It is likely that the incompatible element enrichment within the dykes resulted from subduction-related mantle metasomatism. The Kangâmiut dyke swarm was both a syn-tectonic and thermal event, which triggered it may be linked to passage of a slab window underneath the metasomatised region, or a mantle plume ascending under a subduction zone.  相似文献   

2.
The Lower Permian complex from Bocca di Tenda (Corsica island, France) consists of a gabbroic sequence crosscut by chilled dykes ranging in composition from basalt to trachyandesite and peralkaline rhyolite. The gabbroic sequence is mostly composed of olivine gabbronorites, quartz gabbronorites/diorites locally displaying high ilmenite amounts, and hornblende-rich tonalites. The quartz gabbronorites/diorites and the hornblende-rich tonalites have similar initial εNd values (+0.9 to ?1.1) and record a fractional crystallization process driven by separation of plagioclase, pyroxene, and ilmenite. The olivine gabbronorites have slightly higher initial εNd than the quartz gabbronorites/diorites and the hornblende-rich tonalites, thereby documenting that the early evolution of the melts that gave rise to the gabbroic sequence was controlled by concomitant fractional crystallization and crustal assimilation. The trachyandesite dykes are rare and rich in dark mica. The selected trachyandesite has initial εNd of +0.4, which is slightly lower than the εNd of the basalt dykes. The basalt and the trachyandesite dykes are most likely genetically related through a process of fractional crystallization controlled by segregation of plagioclase, clinopyroxene and minor ilmenite, and assimilation of crustal material. The peralkaline rhyolites have initial εNd values ranging from +0.3 to ?0.3. Whole-rock chemical variations and trace element compositions of Na-amphibole (arfvedsonite) indicate that the peralkaline rhyolite dykes record a process of fractional crystallization mainly controlled by separation of alkali feldspar and minor ilmenite and arfvedsonite. A plausible petrogenetic hypothesis for the genesis of the peralkaline rhyolite melts implies a protracted process of fractional crystallization from the trachyandesitic melts. This fractionation process would be initially ruled by separation of plagioclase, dark mica, and minor ilmenite. An alternative hypothesis for the origin of the peralkaline rhyolite melts implies partial melting of nearly coeval amphibole-rich mafic intrusives, which formed by crustally contaminated mantle-derived melts. The genesis of the peralkaline rhyolites is in any case correlated with mantle-derived melts that experienced extensive crustal contamination.  相似文献   

3.
The origin of ferroan A-type granites in anorogenic tectonic settings remains a long-standing petrological puzzle. The proposed models range from extreme fractional crystallization of mantle-derived magmas to partial melting of crustal rocks, or involve combination of both. In this study, we apply whole-rock chemical and Sm-Nd isotopic compositions and thermodynamically constrained modeling (Magma Chamber Simulator, MCS) to decipher the genesis of a suite of A1-type peralkaline to peraluminous granites and associated intermediate rocks (monzodiorite-monzonite, syenite) from the southwestern margin of the Archean Karelia craton, central Finland, Fennoscandian Shield. These plutonic rocks were emplaced at ca. 2.05 Ga during an early stage of the break-up of the Karelia craton along its western margin and show trace element affinities to ocean island basalt-type magmas. The intermediate rocks show positive εNd(2050 Ma) values (+1.3 to +2.6), which are only slightly lower than the estimated contemporaneous depleted mantle value (+3.4), but much higher than average εNd(2050 Ma) of Archean TTGs (–10) in the surrounding bedrock, indicating that these rocks were essentially derived from a mantle source. The εNd(2050 Ma) values of the peralkaline and peraluminous granite samples overlap (–0.9 to +0.6 and –3.2 to +0.9, respectively) and are somewhat lower than those in the intermediate rocks, suggesting that the mafic magmas parental to granite must have assimilated some amount of older Archean continental crust during their fractionation, which is consistent with the continental crust-like trace element signatures of the granite members. The MCS modeling indicates that fractional crystallization of mantle-derived magmas can explain the major element characteristics of the intermediate rocks. The generation of the granites requires further fractional crystallization of these magmas coupled with assimilation of Archean crust. These processes took place in the middle to upper crust (∼2–4 kbar, ∼7–15 km) and involved crystallization of large amounts of clinopyroxene, plagioclase and olivine. Our results highlight the importance of coupled FC-AFC processes in the petrogenesis of A-type magmas and support the general perception that magmas of A-type ferroan granites become more peraluminous by assimilation of crust. They further suggest that variable fractionation paths of the magmas upon the onset of assimilation may explain the broad variety of A-type felsic and intermediate igneous rocks that is often observed emplaced closely in time and space within the same igneous complex.  相似文献   

4.
The results of experimental studies and examination of variations in major elements, trace elements and Sr isotopes indicate that fractionation, assimilation and magma mixing combined to produce the lavas at Medicine Lake Highland. Some characteristics of the compositional differences among the members of the calc-alkalic association (basalt-andesite-dacite-rhyolite) can be produced by fractional crystallization, and a fractionation model reproduces the major element trends. Other variations are inconsistent with a fractionation origin. Elevated incompatible element abundances (K and Rb) observed in lavas intermediate between basalt and rhyolite can be produced through assimilation of a crustal component. An accompanying increase in 87Sr/86Sr from ∼ 0.07030 in basalt to ∼0.7040 in rhyolite is also consistent with crustal assimilation. The compatible trace element contents (Ni and Sr) of intermediate lavas can not be produced by fractional crystallization, and suggest a magma-mixing origin for some lavas. Unusual phenocryst assemblages and textural criteria in these lavas provide additional evidence for magma mixing. A phase diagram constructed from the low pressure melting experiments identifies a distributary reaction point, where olivine+augite react to pigeonite. Parental basalts reach this point at low pressures and undergo iron-enrichment at constant SiO2 content. The resulting liquid line of descent is characteristic of the tholeiitic trend. Calc-alkalic differentiation trends circumvent the distributary reaction point by three processes: fractionation at elevated pH2O, assimilation and magma mixing.  相似文献   

5.
The evolution of the late Archean Belingwe greenstone belt,Zimbabwe, is discussed in relation to the geochemistry of theultramafic to mafic volcanic rocks. Four volcanic types (komatiite,komatiitic basalt, D-basalt and E-basalt) are distinguishedin the 2·7 Ga Ngezi volcanic sequence using a combinationof petrography and geochemistry. The komatiites and D-basaltsare rocks in which isotopic systems and trace elements are depleted.Chemical variations in komatiites and D-basalts can be explainedby fractional crystallization from the parental komatiite. Incontrast, komatiitic basalts and E-basalts are siliceous anddisplay enriched isotopic and trace element compositions. Theirchemical trends are best explained by assimilation with fractionalcrystallization (AFC) from the primary komatiite. AFC calculationsindicate that the komatiitic basalts and E-basalts are derivedfrom komatiites contaminated with 20% and 30% crustal material,respectively. The volcanic stratigraphy of the Ngezi sequence,which is based on field relationships and the trace elementcompositions of relict clinopyroxenes, shows that the leastcontaminated komatiite lies between highly contaminated komatiiticbasalt flows, and has limited exposure near the base of thesuccession. Above these flows, D- and E-basalts alternate. Thekomatiite appears to have erupted on the surface only in theearly stages, when plume activity was high. As activity decreasedwith time, komatiite magmas may have stagnated to form magmachambers within the continental crust. Subsequent komatiiticmagmas underwent fractional crystallization and were contaminatedwith crust to form D-basalts or E-basalts. KEY WORDS: komatiite; crustal assimilation; Belingwe greenstone belt; continental flood basalt; plume magmatism  相似文献   

6.
At Medicine Lake volcano, California, andesite of the Holocene Burnt Lava flow has been produced by fractional crystallization of parental high alumina basalt (HAB) accompanied by assimilation of granitic crustal material. Burnt Lava contains inclusions of quenched HAB liquid, a potential parent magma of the andesite, highly melted granitic crustal xenoliths, and xenocryst assemblages which provide a record of the fractional crystallization and crustal assimilation process. Samples of granitic crustal material occur as xenoliths in other Holocene and Pleistocene lavas, and these xenoliths are used to constrain geochemical models of the assimilation process.A large amount of assimilation accompanied fractional crystallization to produce the contaminated Burnt lava andesites. Models which assume that assimilation and fractionation occurred simultaneously estimate the ratio of assimilation to fractional crystallization (R) to be >1 and best fits to all geochemical data are at an R value of 1.35 at F=0.68. Petrologic evidence, however, indicates that the assimilation process did not involve continuous addition of granitic crust as fractionation occurred. Instead, heat and mass transfer were separated in space and time. During the assimilation process, HAB magma underwent large amounts of fractional crystallization which was not accompanied by significant amounts of assimilation. This fractionation process supplied heat to melt granitic crust. The models proposed to explain the contamination process involve fractionation, replenishment by parental HAB, and mixing of evolved and parental magmas with melted granitic crust.  相似文献   

7.
The study area in the northwest Sinai represents one of the most significant regions in the Egyptian basement intensely invaded by post-orogenic calc-alkaline dyke swarms. Two post-orogenic dyke swarms have been recognized in NW Sinai namely: (1) mafic dykes of basalt, basaltic andesite and andesite composition and (2) felsic dykes of dacite, rhyodacite and rhyolite composition. These basaltic to rhyolitic dykes intruded contemporaneously and shortly after the intrusion of the post-orogenic leucogranite. The mafic and felsic dykes are enriched in incompatible elements, especially in the large ion lithophile elements (e.g. K, Rb, Ba) and depleted in high field strength elements with negative P, Ti and Nb anomalies. Major and trace element geochemistry indicates that investigated mafic and felsic magma types are not related via fractional crystallization. The protoliths of the mafic and felsic dykes appear to have evolved from different parental magmas. The incompatible trace element patterns favour a derivation of the mafic dykes from melting of a garnet peridotite source followed by fractional crystallization of olivine, clinopyroxene amphibole and zircon. The felsic dykes, on the other hand, could be generated by melting of garnet-free source modified subsequently by fractional crystallization of plagioclase, apatite and titanomagnetite. This implies variable source characteristics at the end of the Pan-African in the NW Sinai.The mafic and felsic dykes can be related to an intracontinental setting and that this was accompanied by a chemical evolution of the subcontinental lithosphere. Magma generation and ascent in the area was favoured by extensional movements, which is already known from other areas in NE Africa.  相似文献   

8.
Despite the fact that some greenstone belts preserve the record of contemporaneous komatiitic and tholeiitic volcanism, a genetic link between the two is not widely accepted. The significance of a compositional gap seperating these magma types and differences in their respective degree of light rare earth element (LREE) enrichment, cited as evidence against a derivative relationship, are complicated by the possibility of crustal assimilation by magmas of komatiitic affinity. In the Archean La Grande Greenstone belt of northern Quebec a succession of metamorphosed tholeiitic basalts and younger, high-Mg, LREE-enriched andesites are preserved. The tholeiites are differentiated basaltic rocks whose chemical compositions appear to have been controlled by low pressure, gabbroic fractional crystallization and are similar to Type 1 MORB. Parental magmas were probably high-Mg liquids of compositions similar to komatiitic basalts which also occur in the greenstone belt. These high-Mg liquids are believed to be themselves the product of high pressure, OLIV+OPX fractional crystallization of more magnesian primary liquids of komatiitic composition. The higher La/Sm ratios of komatiitic basalts and tholeiites relative to komatiites in this belt, can be explained by small degrees of crustal assimilation. In the central part of the belt, late-stage, mafic igneous rocks have chemical compositions similar to Archean examples of contaminated volcanic rocks (e.g., Kambalda, Australia). These late-stage lavas consist of basalts and andesites with high-Mg, Ni and Cr abundances, LREE-enriched profiles and low Ti abundances. They are believed to be the products of crustal assimilation and crystallization of OPX-PLAG-CPX from high-Mg liquids of komatiitic affinity. The volcanic stratigraphy records the progressive effects of crustal contamination through time. A light sialic crust may have initially acted as a density barrier, preventing the eruption of primary high-Mg liquids and forcing fractionation at depth which produced more buoyant compositions. With subsequent thinning of the crust, the density barrier presumably failed, and primary liquids migrated directly toward the surface. Reaction of these liquids with tonalitic crust produced contaminated differentiates.  相似文献   

9.
Major, trace element and Sr isotopic compositions have been determined on 21 lava samples from Vico volcano, Roman Province, Central Italy. The rocks investigated range from leucite tephritic phonolites to leucite phonolites and trachytes. Trace element compositions are characterized by high enrichments of incompatible elements which display strong variations in rocks with a similar degree of evolution. Well-defined linear trends are observed between pairs of incompatible trace elements such as Th-Ta, Th-La, Th-Hf. A decrease of Large Ion Lithophile (LIL) elements abundance contemporaneously with the formation of a large central caldera is one of the most prominent characteristics of trace element distribution. Sr isotope ratios range from 0.71147 to 0.71037 in the pre-caldera lavas and decreases to values of 0.70974–0.70910 in the lavas erupted after the caldera collapse. Theoretical modelling of geochemical and Sr isotopic variations indicates that, while fractional crystallization was an important evolutionary process, AFC and mixing also played key roles during the evolution of Vico volcano. AFC appears to have dominated during the early stages of the volcanic history when evolved trachytes with the highest Sr isotope ratios were erupted. Mixing processes are particularly evident in volcanites emplaced during the late stages of Vico evolution. According to the model proposed, the evolution of potassic magmas emplaced in a shallow-level reservoir was dominated by crystal fractionation plus wall rock assimilation and mixing with ascending fresh mafic magma. This process generated a range of geochemical and isotopic compositions in the mafic magmas which evolved by both AFC and simple crystal liquid fractionation, producing evolved trachytes and phonolites with variable trace element and Sr isotopic compositions.  相似文献   

10.
New major and trace element analyses and Sr-isotope determinations of rocks from Mt. Somma–Vesuvius volcano produced from 25 ky BP to 1944 AD are part of an extensive database documenting the geochemical evolution of this classic region. Volcanic rocks include silica undersaturated, potassic and ultrapotassic lavas and tephras characterized by variable mineralogy and different crystal abundance, as well as by wide ranges of trace element contents and a wide span of initial Sr-isotopic compositions. Both the degree of undersaturation in silica and the crystal content increase through time, being higher in rocks produced after the eruption at 472 AD (Pollena eruption). Compositional variations have been generally thought to reflect contributions from diverse types of mantle and crust. Magma mixing is commonly invoked as a fundamental process affecting the magmas, in addition to crystal fractionation. Our assessment of geochemical and Sr-isotopic data indicates that compositional variability also reflects the influence of crustal contamination during magma evolution during upward migration to shallow crustal levels and/or by entrapment of crystal mush generated during previous magma storage in the crust. Using a variant of the assimilation fractional crystallization model (Energy Conservation–Assimilation Fractional Crystallization; [Spera and Bohrson, 2001. Energy-constrained open-system magmatic processes I: General model and energy-constrained assimilation and fractional crystallization (EC–AFC) formulation. J. Petrol. 999–1018]; [Bohrson, W.A. and Spera, F.J., 2001. Energy-constrained open-system magmatic process II: application of energy-constrained assimilation–fractional crystallization (EC–AFC) model to magmatic systems. J. Petrol. 1019–1041]) we estimated the contributions from the crust and suggest that contamination by carbonate rocks that underlie the volcano (2 km down to 9–10 km) is a fundamental process controlling magma compositions at Mt. Somma–Vesuvius in the last 8 ky BP. Contamination in the mid- to upper crust occurred repeatedly, after the magma chamber waxed with influx of new mantle- and crustal-derived magmas and fluids, and waned as a result of magma withdrawal and production of large and energetic plinian and subplinian eruptions.  相似文献   

11.
《地学前缘(英文版)》2018,9(6):1711-1724
The Helanshan tectonic belt(HTB) is a major tectonic divide between the Alxa and Ordos blocks in the North China Craton. The geochronology and petrogenesis of the mafic dykes in the northern HTB are keys to understanding the tectonic evolution of this belt. The mafic dykes, intruded into the Neoarchean-Paleoproterozoic metamorphic basement, are mainly composed of diabase with a mineral assemblage of plagioclase(45%-60%), pyroxene(25%-35%), minor quartz and Fe-Ti oxides. The LA-ICPMS U-Pb analysis of zircon grains from representative dykes yield a weighted mean age of 206 ± 1.9 Ma, which represents the crystallization age of the dyke. The diabases show high contents of Fe_2 O_3~T(11.88-17.55 wt.%), low contents of SiO_2(45.65-50.95 wt.%) and MgO(3.31-5.50 wt.%) with low Mg#(=100×MgO/(MgO + FeO) atomic ration) of 33-44. They are characterized by enrichment of light rare earth elements(LREEs) and large ion lithophile elements(LILEs)(e.g., Rb, Ba and Pb), and slight depletion of high field strength elements(HFSEs). These features suggest that the magma has undergone extensive fractionation of olivine and pyroxene but only minor crustal contamination during its evolution. Their high Sm contents and La/Sm ratios, and low Sm/Yb ratios indicate that magma from which the dykes formed was derived from low degree(about 5%) partial melting of an enriched garnet + spinel lherzolite mantle source. Together with regional geology, these geochemical and geochronological data suggest that the mafic dykes in the HTB were formed in an intracontinental extensional setting during the late Triassic.  相似文献   

12.
Trace elements, including rare earth elements (REE), exhibit systematic variations in plutonic rocks from the Captains Bay pluton which is zoned from a narrow gabbroic rim to a core of quartz monzodiorite and granodiorite. The chemical variations parallel those in the associated Aleutian calcalkaline volcanic suite. Concentrations of Rb, Y, Zr and Ba increase as Sr and Ti decrease with progressive differentiation. Intermediate plutonic rocks are slightly enriched in light REE (La/Yb=3.45–9.22), and show increasing light REE fractionation and negative Eu anomalies (Eu/Eu*=1.03–0.584). Two border-zone gabbros have similar REE patterns but are relatively depleted in total REE and have positive Eu anomalies; indicative of their cumulate nature. Initial 87Sr/86Sr ratios in 8 samples (0.70299 to 0.70377) are comparable to those of volcanic rocks throughout the arc and suggest a mantle source for the magmas. Oxygen isotopic ratios indicate that many of the intermediate plutonic rocks have undergone oxygen isotopic exchange with large volumes of meteoric water during the late stages of crystallization; however no trace element or Sr isotopic alteration is evident.Major and trace element variations are consistent with a model of inward fractional crystallization of a parental high-alumina basaltic magma at low pressures (6 kb). Least-squares approximations and trace element fractionation calculations suggest that differentiation in the plutonic suite was initially controlled by the removal of calcic plagioclase, lesser pyroxene, olivine and Fe-Ti oxides but that with increasing differentiation and water fugacity the removal of sub-equal amounts of sodic plagioclase and hornblende with lesser Fe-Ti oxides effectively drove residual liquids toward dacitic compositions. Major and trace element compositions of aplites which intrude the pluton are not adequately explained by fractional crystallization. They may represent partial melts derived from the island arc crust. Similarities in Sr isotopes, chemical compositions and differentiation trends between the plutonic series and some Aleutian volcanic suites indicates that shallow-level fractional crystallization is a viable mechanism for generating the Aleutian calcalkaline rock series.LDGO Contribution no. 2964  相似文献   

13.
We present the geochemistry and intrusion pressures of granitoids from the Kohistan batholith, which represents, together with the intruded volcanic and sedimentary units, the middle and upper arc crust of the Kohistan paleo-island arc. Based on Al-in-hornblende barometry, the batholith records intrusion pressures from ~0.2 GPa in the north (where the volcano-sedimentary cover is intruded) to max. ~0.9 GPa in the southeast. The Al-in-hornblende barometry demonstrates that the Kohistan batholith represents a complete cross section across an arc batholith, reaching from the top at ~8–9 km depth (north) to its bottom at 25–35 km (south-central to southeast). Despite the complete outcropping and accessibility of the entire batholith, there is no observable compositional stratification across the batholith. The geochemical characteristics of the granitoids define three groups. Group 1 is characterized by strongly enriched incompatible elements and unfractionated middle rare earth elements (MREE)/heavy rare earth element patterns (HREE); Group 2 has enriched incompatible element concentrations similar to Group 1 but strongly fractionated MREE/HREE. Group 3 is characterized by only a limited incompatible element enrichment and unfractionated MREE/HREE. The origin of the different groups can be modeled through a relatively hydrous (Group 1 and 2) and of a less hydrous (Group 3) fractional crystallization line from a primitive basaltic parent at different pressures. Appropriate mafic/ultramafic cumulates that explain the chemical characteristics of each group are preserved at the base of the arc. The Kohistan batholith strengthens the conclusion that hydrous fractionation is the most important mechanism to form volumetrically significant amounts of granitoids in arcs. The Kohistan Group 2 granitoids have essentially identical trace element characteristics as Archean tonalite–trondhjemite–granodiorite (TTG) suites. Based on these observations, it is most likely that similar to the Group 2 rocks in the Kohistan arc, TTG gneisses were to a large part formed by hydrous high-pressure differentiation of primitive arc magmas in subduction zones.  相似文献   

14.
A number of trace elements have been determined spectrographically in the rocks and minerals of the Skaergaard intrusion, East Greenland. The original basic magma from which the varied rocks of the complex were developed is shown to have had a normal trace element composition. The sorting out of the trace elements into the various mineral series produced by strong fractional crystallization of the original basic magma is traced in detail by means of analyses of the separated minerals. Certain of the trace elements (Cr, Ni) are shown to be strongly concentrated in the early rocks so that later fractions have little or none of them; other elements (P, V, Cu, Sc, Mn, S) reach maximum values in the middle, or late middle stages represented by certain olivine-free gabbros and ferrogabbros; other elements (Li, Zr, Y, La, Ba, Rb) tend to remain in the residual liquid during fractionation and are thus abundant in the latest granite fraction. Still other trace elements (Co, Sr, Ga, Mo) show only small changes in amount throughout the series. Of these Co is a little more abundant in the early and middle stages, Sr in the middle stages, Ga in the later stages and Mo in the early and later but not in the middle stages. The distribution of the trace elements in the rocks is considered in relation to the varying composition of the minerals produced by the fractional crystallization processes and an attempt is made to discuss the mineral compositions in terms of crystal chemical concepts.The Skaergaard sequence of differentiation from gabbros, through ferrogabbros, to granite is considered to be a common trend of fractionation of basic magma at high levels in the crust, and the observed changes in trace element composition are therefore regarded as having wide geochemical significance. The trace element composition of the intermediate Skaergaard differentiates is significantly different from that of diorites reported by other workers and suggests that diorites have had some other origin than by fractionation of basic magma. On the other hand the trace element composition of many granites resembles that of the granite fraction produced in the Skaergaard intrusion.  相似文献   

15.
黑龙江穆棱砍椽沟钼铜矿床地质与地球化学特征   总被引：1，自引：0，他引：1  
砍椽沟钼铜矿床位于佳木斯—牡丹江多金属成矿带的南东段,钼铜矿化产于花岗闪长岩和花岗斑岩岩体中。矿床主量元素分析结果表明,成矿岩体具高Sr、Na、K和低Y、Mg,显示出钙碱质岩浆演化特点,应属C型埃达克岩;微量元素测试结果显示本区花岗质岩石亏损Nb、Nd等高场强元素,富集Rb、Ba等大离子亲石元素,同时表明矿区的花岗质岩石具有埃达克岩属性;稀土元素组成具有轻微负Eu异常的特点,表明斜长石发生一定程度的分离结晶作用,或曾受到斜长石分离结晶作用的影响,且岩浆为地壳来源,在岩浆岩源区存在着一定数量的石榴石残留。矿床属斑岩型--热液脉型成因,成矿岩体的形成与太平洋板块俯冲作用关系密切。  相似文献   

16.
The mafic dykes in Wadi Mandar-Wadi Um Adawi area are as follows: (1) calc-alkaline lamprophyre (i.e., kersantite and spessartite), (2) diabase, and (3) alkaline lamprophyre (i.e., camptonite). The field relations reveal that the emplacement of calc-alkaline lamprophyres preceded the diabase dykes, while alkaline lamprophyres emplaced later than the diabase dykes. Calc-alkaline are basaltic andesite, basaltic trachyandesite to basalt, while the diabase dykes and alkaline lamprophyres are basaltic in composition. These dykes are characterized by metaluminous character. Calc-alkaline lamprophyres and diabase dykes show transitional affinity from calc-alkaline to alkaline, while the alkaline lamprophyres exhibit more strong alkaline character. The mafic dykes were crystallized under temperature 1100–1150 °C and pressure 3–5 kbars in a high oxygen fugacity conditions. Fe-Ti oxides in the dykes are represented by ilmenite and Ti-magnetite. The chemistry of the sulfides hosted in those mafic dykes suggests a magmatic-hydrothermal origin for these minerals. The geochemical behavior of high field strength elements and large ion lithophile elements in these dykes excludes the derivation of diabase or alkaline lamprophyre either by partial melting or fractional crystallization from calc-alkaline lamprophyre. The parental magmatic sources of the studied dykes were generated from crustal material with addition of mantle-derived melt during the post-collisional stage. The mafic dykes in Wadi Mandar-Wadi Um Adawi area were generated from different magmatic sources by partial melting and subsequent fractional crystallization. In addition, the crustal contamination/assimilation process has a prominent role in the magmatic evolution of diabase and alkaline lamprophyre dykes.  相似文献   

17.
The Tormes dome consists of S-type granites that intruded into Ordovician augen gneisses and Neoproterozoic–Lower Cambrian metapelites/metagreywackes at different extents of migmatization. S-type granites are mainly equigranular two-mica granites, occurring as: (1) enclave-laden subvertical feeder dykes, (2) small external sill-like bodies with size and shape relations indicative for self-similar pluton growth, and (3) as large pluton bodies, emplaced at higher levels than the external ones. These magmas were highly mobile as it is inferred from the high contents of fluxing components, the disintegration and alignment of pelitic xenoliths in feeder dykes and at the bottom of some sill-like bodies. Field relations relate this 311?Ma magmatism (U–Pb monazite) to the regional shearing of the D3 Variscan event. Partial melting modeling and the relatively high estimated liquidus temperatures indicate biotite-dehydration partial melting (800–840°C and 400–650?MPa) rather than water-fluxed melting, implying that there was no partial melting triggered by externally derived fluids in the shear zones. Instead, the subvertical shear zones favored extraction of melts that formed during the regional migmatization event around 320?Ma. Nd isotope variation among the granites might reflect disequilibrium partial melting or different protoliths. Mass-balance and trace element partial melting modeling strongly suggest two kinds of fertile crustal protoliths: augen gneisses and metapelites. Slight compositional variation among the leucogranites does not reflect different extent of protolith melting but is related to a small amount of fractional crystallization (<13% for the equigranular granites), which is generally more pronounced in shallower batholitic leucogranites than in the small and homogeneous sill-like bodies. The lower extent of fractional crystallization and the higher-pressure emplacement conditions of the sill-like bodies support a more restricted movement through the crust than for batholitic leucogranites.  相似文献   

18.
The Myggbukta caldera complex and a swarm of basic dykes constitute the latest Tertiary magmatism in the Hold with Hope region, East Greenland. The Sr and Nd isotope ratios of these rocks show coherent variations which extend to high 87Sr/86Sr and low 143Nd/144Nd values and require a contribution from continental lithosphere. Broad correlations with major element differentiation indices suggest that the continental component was incorporated during magmatic differentiation thereby favouring a crustal contamination process. Trace element concentrations are strongly correlated with isotopic compositions but display ranges for many incompatible elements which extend beyond likely crustal contaminant compositions. This is readily modelled by AFC processes in which the dominant cause of trace element enrichment is the concentration effect of fractional crystallisation rather than the composition of the contaminant. The simplest such models still require unrealistically high degrees of fractional crystallisation to explain the ten-fold enrichment of some trace elements. This can be overcome if the primary magmas entering the crust already had highly variable trace element compositions. Such variability is readily achieved if melts from different parts of the melting column escape without thorough homogenization. An AFC model which incorporates variability in parental magma composition is then able to simulate the range of compositions observed at Hold with Hope. This carries the implication that the variations observed are more readily attributed to changes in uncontaminated parental magma than to variations in the composition or amount of contaminant. Received: 5 March 1998 / Accepted: 16 June 1998  相似文献   

19.
Early Cretaceous tholeiitic picrite-to-rhyolite dykes aroundSpitzkoppe, western Namibia, are part of the extensive HentiesBay–Outjo swarm, penecontemporaneous with 132 Ma Etendekalavas 100 km to the NW. Although only intermediate to rhyoliticdykes contain clinopyroxene phenocrysts, the behaviour of Ca,Al and Sc in the dyke suite shows that liquidus clinopyroxene—togetherwith olivine—was a fractionating phase when MgO fell to9 wt %. Both a plot of CIPW normative di–hy–ol–ne–Qand modelling using (p)MELTS show that a mid-crustal pressureof 0·6 GPa is consistent with this early clinopyroxenesaturation. Sr, Nd, Hf and Pb isotope variations all show trendsconsistent with AFC contamination (assimilation linked to fractionalcrystallization), involving Pan-African Damara belt continentalcrust. The geochemical variation, including isenthalpic AFCmodelling using (p)MELTS, suggests that the picrites (olivine-richcumulate suspensions) were interacting with granulite-faciesmetamorphic lower crust, the intermediate compositions withamphibolite-facies middle crust, and the rhyolitic dykes (anda few of the basalts) with the Pan-African granites of the uppercrust. The calculated densities of the magmas fall systematicallyfrom picrite to rhyolite and suggest a magmatic system resemblinga stack of sills throughout the crust beneath Spitzkoppe, withthe storage and fractionation depth of each magma fraction controlledby its density. Elemental and isotopic features of the 20 wt% MgO picrites (including Os isotopes) suggest that their parentalmelts probably originated by fusion of mid-ocean ridge basalt(MORB) source convecting mantle, followed by limited reactionwith sub-continental lithospheric mantle metasomatized justprior to the formation of the parental magmas. Many of the distinctivefeatures of large-volume picritic–basaltic magmas maynot be derived from their ultimate mantle sources, but may insteadbe the results of complex polybaric fractional crystallizationand multi-component crustal contamination. KEY WORDS: flood basalts; Spitzkoppe; picrite; trace elements; hafnium isotopes; Etendeka  相似文献   

20.
Major and trace element data for ultrabasic to silicic metavolcanics from five Archaean volcanic-sedimentary belts in South India are summarized. Their total alteration to greenschist and amphibolite facies assemblages poses problems for petrogenetic interpretation of these data. However, in the absence of unmetamorphosed lavas, some interpretation is essential.The data suggest that volcanism in each belt was petrogenetically similar. The presence of both light rare-earth element-enriched and -depleted lavas in individual volcanic sequences, together with variations in ZrY ratios is interpreted as a consequence of fractionation within ultramatic mantle diapirs, induced by upward vapour migration. Other chemical variations probably resulted from fractional crystallization of mafic phases from parental high MgO, high Cr magmas, the influence of plagioclase fractionation only being noticeable in late andesites and dacites. The consistent depletion of all basaltic samples in the high field strength elements Mb and Ta relative to the low field strength element Th and to La further emphasises the importance of a vapour phase in their petrogenesis. In this regard they resemble modern orogenic basalts. The favoured tectonic setting is that of ensialic, multiple back-arc basins above a thinner lithosphere than in modern back-arc environments, which rode over a shallow-angled subduction zone.  相似文献   

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