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1.
Alkaline-ultrabasic mantle-derived magmas, their sources, and crystallization features: Data of melt inclusion studies 总被引：2，自引：0，他引：2
To find out the reasons responsible for the diversity of igneous rocks forming the alkaline-ultrabasic carbonatite Krestovskiy massif (the Maimecha–Kotui province, Russia) we have studied melt inclusions in clinopyroxene of trachydolerites, porphyric melanephelinites, and tholeiites. It was established that the homogenization temperatures of inclusions in these rocks are rather close: 1140–1180 °C, 1190–1230 °C, and 1150–1210 °C, respectively. Compositions of melt inclusions in clinopyroxenes from different rocks are significantly different. The chemical composition of clinopyroxene of trachydolerites corresponds to that of trachybasalts and their derivatives. The inclusions are enriched in Sr, Ba, P, and S and their total sum of alkalies (at K ≥ Na) is never less than 5–6 wt.%. Inclusions from the rims of clinopyroxene phenocrysts in porphyric melanephelinites are similar in composition also to inclusions in trachydolerites. But in the cores of clinopyroxene phenocrysts the composition of inclusions corresponds to nephelinite melt. The composition of some melt inclusions in the intermediate and cores zones of clinopyroxene from porphyric melanephelinite has high SiO2 (53–55 wt.%), MgO (8–9 wt.%) and a low (1–2 wt.%) total sum of alkalies (at Na ≥ K) and is depleted in Al2O3 (6–7 wt.%), which is similar to the composition of basaltic komatiites. The composition of inclusions in tholeiites is also basic, highly magnesian, and low-alkaline, Sr and Ba are rare to absent. Compared to the inclusions of basaltic komatiite composition, the inclusions in tholeiites are enriched in Al and depleted in Ca, Ti, and P. The melts trapped in clinopyroxenes from different rocks contain low (0.014–0.018 wt.%) water but they are enriched in F: from 0.37 wt.% in nephelinite melts to 0.1–0.06 wt.% in tholeiite and basaltic komatiite melts. Inclusions in all the rocks under study, host clinopyroxene, and the rocks themselves are significantly enriched in incompatible elements (1–2 orders of magnitude relative to the mantle norm). In tholeiites, the partitioning of these elements is rather uniform, while in trachydolerites and especially in melanephelinites it is contrasting with a drastic depletion in HREE relative to LREE, MREE, and HFSE. A conclusion is made that the Krestovskiy massif was formed by no less than three mantle-derived magmas: melanephelinite, tholeiite and basaltic komatiite. Magmas were generated in different magma sources at different depths with various degrees of enrichment in incompatible elements. These magmas were, most likely, dominated by melanephelinite magma. In intermediate chambers this magma differentiated to form derivative melts of nephelinite, trachydolerite–trachyandesite–trachyte compositions. Komatiite-basalt melts were, most likely, derivatives of primitive meimechite magmas. 相似文献
2.
Peralkaline acid liquids: A petrological study 总被引：1，自引：0，他引：1
Electron-microprobe analyses of the feldspars and associated ferromagnesian minerals in the peralkaline volcanics, comendites and pantellerites, are presented together with new data on the major and trace-elements of the rocks and residual glasses. The feldspar phenocrysts in the pantellerites span a narrower range (Or33–Or39) than those of the comendites (Or30–Or46); both sets show only limited increase in Or outwards, and the zoning is greatest in quartz-bearing assemblages. The feldspar microlites in the residual glasses are invariably more potassic (2–4% Or) than their associated phenocrysts. In pantellerites the feldspars become more potassic as the residual liquids become more sodic; thus the most potassic feldspar is found in the most sodic (and peralkaline) pantellerite.Of the ferromagnesian phenocrysts, aenigmatite is the most ubiquitous and is commonly associated with hedenbergite±fayalite, or ferrorichterite; in the later stages of crystallization (groundmass), it is associated with acmite, arfvedsonite and tuhualite. Aside from slight variation in Ti/Fe+Ti ratio, aenigmatite is virtually constant in composition. The pyroxenes from the different assemblages have zones which together almost span the range acmitehedenbergite. Both ferrorichterite and arfvedsonite incorporate F but not Cl, and are slightly potassic. Tuhualite exists as two varieties; one blue and potassic, the other violet and sodic; both varieties reject halogens. Using (estimated) free-energy data, a field in fo2, T space is postulated in which Fe-Ti oxides are absent; their place is taken by pyroxene and aenigmatite. The no-oxide field will be intercepted by a cooling liquid in which peralkalinity is increasing and in which fo2 is near but above the FMQ buffer.The characteristic pattern of trace-elements in peralkaline volcanics (e.g., high Nb, Ta, Zr, Mo, Zn, Cd, R.E, etc; low Sr, Ba, Mg) are considered to be as much evidence for the peralkaline (salic) condition as of the genetic process. Several lines of evidence suggest that at liquidus temperatures, peralkaline rhyolites are essentially anhydrous. 相似文献
3.
Petrology and CHIME geochronology of Pan-African high K and Sr/Y granitoids in the Nkambe area, Cameroon 总被引：1，自引：1，他引：0
The Central African Belt in the Nkambe area, northwestern Cameroon represents a collisional zone between the Saharan metacraton and the Congo craton during the Pan-African orogeny, and exposes a variety of granitoids including foliated and massive biotite monzogranites in syn- and post-kinematic settings. Foliated and massive biotite monzogranites have almost identical high-K calc-alkaline compositions, with 73–67 wt.% SiO2, 17–13 wt.% Al2O3, 2.1–0.9 wt.% CaO, 4.4–2.7 wt.% Na2O and 6.3–4.4 wt.% K2O. High concentrations of Rb (264–96 ppm), Sr (976–117 ppm), Ba (3680–490 ppm) and Zr (494–99 ppm), with low concentrations of Y (mostly< 20 ppm with a range 54–6) and Nb (up to 24 ppm) suggest that the monzogranites intruded in collisional and post-collisional settings. The Sr/Y ratio ranges from 25 to 89. K, Rb and Ba resided in a single major phase such as K-feldspar in the source. Garnet was present in the source and remained as restite at the site of magma generation. This high K2O and Sr/Y granitic magma was generated by partial melting of a granitic protolith under high-pressure and H2O undersaturated conditions where garnet coexists with K-feldspar, albitic plagioclase. CHIME (chemical Th–U-total Pb isochron method) dating of zircon yields ages of 569 ± 12–558 ± 24 Ma for the foliated biotite monzogranite and 533 ± 12–524 ± 28 Ma for the massive biotite monzogranite indicating that the collision forming the Central African Belt continued in to Ediacaran (ca 560 Ma). 相似文献
4.
Mica kimberlite and alkali picrite were identified in the northwestern Urik-Iya Graben of the eastern Sayan region. Typomorphism
of Cr-diopside and high-Cr (up to 55.22 wt % Cr2O3) spinel from kimberlite of the Bushkanai dike indicate that the melt was generated in the mantle, composed of spinel peridotite.
The high content of Cr-spinel (45–55 wt % Cr2O3) microlites in the groundmass of kimberlite and small amounts of ulvospinel and titanomagnetite in the absence of perovskite
testifies to the diamond potential of this kimberlite. Picroilmenite, manganoilmenite with an anomalously high MnO content
(11.37–17.78 wt %), and barium titanate with (wt %) 62.21 TiO2, 0.61 Cr2O3, 15.89 FeO, 4.05 MnO, 1.71 CaO, and 11.13 BaO close in composition to a new mineral species from the Murun pluton were identified
in the groundmass for the first time. Kimberlite from the Bushkanai dike belongs to the Zolotitsa low-Ti geochemical type
of kimberlites derived from the slightly enriched lithospheric mantle EM1. The distribution of trace elements, including REE,
in picrite from the same dike corresponds to the slightly depleted asthenospheric mantle. Different mantle sources of kimberlite
and picrite from the same dike indicate that these rocks are related to independent melts rather than to products of fractionation
of a common parental alkaline ultramafic magma. 相似文献
5.
An early Paleoproterozoic high-K intrusive complex in southwestern Tarim Block, NW China: Age, geochemistry, and tectonic implications 总被引：7，自引：2，他引：7
Chuan-Lin Zhang Zheng-Xiang Li Xian-Hua Li Hai-Feng Yu Hai-Ming Ye 《Gondwana Research》2007,12(1-2):101
Systematic geochronologic, geochemical, and Nd isotopic analyses were carried out for an early Paleoproterozoic high-K intrusive complex exposed in southwestern Tarim, NW China. The results provide a better understanding of the Paleoproterozoic tectonic evolution of the Tarim Block. Zircon U–Pb age dating indicates two Paleoproterozoic magmatic episodes occurring at ca. 2.41 Ga and ca. 2.34 Ga respectively, which were followed by a ca. 1.9 Ga metamorphic event. The 2.41 Ga granodiorite–adamellite suite shares characteristics of late to post-orogenic metaluminous A-type granites in its high alkalinity (Na2O + K2O = 7.6–9.3%), total REE (410–788 ppm), Zr (370–660 ppm), and Y (21.7–58.4 ppm) contents. εNd(t) values for the suite range from − 3.22 to − 4.71 and accordingly the Nd modal ages (T2DM) vary between 3.05 Ga and 3.17 Ga. Based on geochemical data, the 2.34 Ga suite can be subdivided into two sub-suites, namely A-type and S-type. However, both types have comparable Nd isotope compositions (εNd(t) ≈ − 0.41 to − 2.08) and similar narrow T2DM ranges (2.76–2.91 Ga).Geochemical and Nd isotopic data for the high-K intrusive complex, in conjunction with the regional geological setting, suggest that both the 2.41 Ga suite and the 2.34 Ga A-type sub-suite might have been produced by partial melting of the Archean mafic crust in a continental rift environment. The S-type sub-suite is thought to have formed by partial melting of felsic pelites and/or metagreywackes recycled from Archean crust (TTG?). Gabbro enclaves with positive εNd(t) value (2.15) have been found to be intermingling within the 2.34 Ga suite; ca. 2.34–2.36 Ga gabbroic dykes and adamellites have previously been documented in eastern Tarim. These observations indicate that the high-K intrusions may reflect the emergence of depleted mantle upwelling beneath the Tarim Block at that time. We suggest a three-stages model for the Precambrian crustal evolution in the Tarim Block: (1) the formation of proto-crust (TTG) by ca. 2.5 Ga, (2) episodes of felsic magmatism possibly occurring in continental rift environments at ca. 2.41 Ga and ca. 2.34–2.36 Ga, and (3) ca. 1.9 Ga metamorphism that may represent the solidification of the Precambrian basement of the Tarim Block. 相似文献
6.
A geochemical and isotopic study was carried out for the Mesozoic Yangxin, Tieshan and Echeng granitoid batholiths in the southeastern Hubei Province, eastern China, in order to constrain their petrogenesis and tectonic setting. These granitoids dominantly consist of quartz diorite, monzonite and granite. They are characterized by SiO2 and Na2O compositions of between 54.6 and 76.6 wt.%, and 2.9 to 5.6 wt.%, respectively, enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILE), and relative depletion in Y (concentrations ranging from 5.17 to 29.3 ppm) and Yb (0.34–2.83 ppm), with the majority of the granitoids being geochemically similar to high-SiO2 adakites (HSA). Their initial Nd (εNd = − 12.5 to − 6.1) and Sr ((87Sr/86Sr)i = 0.7054–0.7085) isotopic compositions, however, distinguish them from adakites produced by partial melting of subducted slab and those produced by partial melting of the lower crust of the Yangtze Craton in the Late Mesozoic. The granitoid batholiths in the southeastern Hubei Province exhibit very low MgO ranging from 0.09 to 2.19 wt.% with an average of 0.96 wt.%, and large variations in negative to positive Eu anomalies (Eu/Eu = 0.22–1.4), especially the Tieshan granites and Yangxin granite porphyry (Eu/Eu = 0.22–0.73). Geochemical and Nd–Sr isotopic data demonstrate that these granitoids originated as partial melts of an enriched mantle source that experienced significant contamination of lower crust materials and fractional crystallization during magma ascent. Late Mesozoic granitoids in the southeastern Hubei Province of the Middle–Lower Yangtze River belt were dominantly emplaced in an extensional tectonic regime, in response to basaltic underplating, which was followed by lithospheric thinning during the early Cretaceous. 相似文献
7.
Mt. Shasta andesite and dacite lavas contain high MgO (3.5–5 wt.%), very low FeO*/MgO (1–1.5) and 60–66 wt.% SiO2. The range of major and trace element compositions of the Shasta lavas can be explained through fractional crystallization (~50–60 wt.%) with subsequent magma mixing of a parent magma that had the major element composition of an H2O-rich primitive magnesian andesite (PMA). Isotopic and trace element characteristics of the Mt. Shasta stratocone lavas are highly variable and span the same range of compositions that is found in the parental basaltic andesite and PMA lavas. This variability is inherited from compositional variations in the input contributed from melting of mantle wedge peridotite that was fluxed by a slab-derived, fluid-rich component. Evidence preserved in phenocryst assemblages indicates mixing of magmas that experienced variable amounts of fractional crystallization over a range of crustal depths from ~25 to ~4 km beneath Mt. Shasta. Major and trace element evidence is also consistent with magma mixing. Pre-eruptive crystallization extended from shallow crustal levels under degassed conditions (~4 wt.% H2O) to lower crustal depths with magmatic H2O contents of ~10–15 wt.%. Oxygen fugacity varied over 2 log units from one above to one below the Nickel-Nickel Oxide buffer. The input of buoyant H2O-rich magmas containing 10–15 wt.% H2O may have triggered magma mixing and facilitated eruption. Alternatively, vesiculation of oversaturated H2O-rich melts could also play an important role in mixing and eruption. 相似文献
8.
9.
The Permian–Jurassic Mahanadi and Pranhita–Godavari Rifts are part of a drainage system that radiated from the Gamburtsev Subglacial Mountains in central Antarctica. From 12 samples we analysed detrital zircons for U–Pb ages, Hf-isotopes, and trace elements to determine the age, rock type and source of the host magma, and TDM model age. Clusters, in decreasing order of abundance, are (1) 820–1000 Ma, host magmas felsic granitoids with alkaline rock, (2) 1500–1700 Ma felsic granitoids, (3) 500 to 700 Ma mafic granitoids with alkaline rock, (4) 2400–2550 Ma granitoids, and (5) 1000–1200 Ma felsic and mafic granitoids, mafic rock, and alkaline rock. TDM ranges from 1.5 to 3.5 Ga. Joint paleoslope measurements and zircon ages indicate that the Eastern Ghats Mobile Belt (EGMB) and lateral belts and conjugate Antarctica are potential provenances. Zircons from the Gondwana Rifts differ from those in other Gondwanaland sandstones in their predominant 820–1000 Ma and 1500–1700 Ma ages (from the EGMB and conjugate Rayner–MacRobertson Belt) that dilute the 500–700 Ma (Pan-Gondwanaland) ages. The 1000–1200 Ma zircons reflect the assembly of Rodinia, the 500–700 Ma ones that of Gondwanaland; the other ages reflect collisions in the region. 相似文献
10.
Numerous Neoproterozoic felsic and mafic–ultramafic intrusions occur in the Hannan region at the northern margin of the Yangtze Block. Among these, the Wudumen and Erliba plutons consist of granodiorites and have SHRIMP zircon U–Pb ages of 735 Ma. The rocks have high K2O (0.8–3.6 wt.%) and Na2O (4.4–6.4 wt.%) and low MgO (0.4–1.7 wt.%). They also have high Sr/Y (32–209) and (La/Yb)n ratios (4.4–38.6). Their εNd values range from − 0.41 to − 0.92 and zircon initial 176Hf/177Hf ratios from 0.282353 to 0.282581. These geochemical features are similar to those of adakitic rocks produced by partial melting of a thickened lower crust. Our new analytical results, combined with the occurrence of voluminous arc-related mafic–ultramafic intrusions emplaced before 740 Ma, lead us to propose that the crustal evolution in the northern margin of the Yangtze Block during Neoproterozoic involved: (1) rapid crustal growth and thickening by underplating of mafic magmas from the mantle which was modified by materials coming from the subducting oceanic slab from 1.0 to 0.74 Ga, and (2) partial melting of the thickened lower crust due to a thermal anomaly induced by upwelling of asthenosphere through an oceanic slab window, producing the 735 Ma adakitic Wudumen and Erliba plutons. Our model suggests that the crustal thickness was more than 50 km at the northern margin of the Yangtze Block at 735 Ma, and rule out the possibility of a mantle plume impact causing the > 735 Ma magmatism in the region. 相似文献
11.
M. L. Tolstykh V. B. Naumov M. G. Gavrilenko A. Yu. Ozerov N. N. Kononkova 《Geochemistry International》2012,50(6):522-550
Melt inclusions in olivine and plagioclase phenocrysts from rocks (magnesian basalt, basaltic andesite, andesite, ignimbrite,
and dacite) of various age from the Gorely volcanic center, southern Kamchatka, were studying by means of their homogenization
and by analyzing the glasses in 100 melt inclusions on an electron microprobe and 24 inclusions on an ion probe. The SiO2 concentrations of the melts vary within a broad range of 45–74 wt %, as also are the concentrations of other major components.
According to their SiO2, Na2O, K2O, TiO2, and P2O5 concentrations, the melts are classified into seven groups. The mafic melts (45–53 wt % SiO2) comprise the following varieties: potassic (on average 4.2 wt % K2O, 1.7 wt % Na2O, 1.0 wt % TiO2, and 0.20 wt % P2O5), sodic (3.2% Na2O, 1.1% K2O, 1.1% TiO2, and 0.40% P2O5), and titaniferous with high P2O5 concentrations (2.2% TiO2, 1.1% P2O5, 3.8% Na2O, and 3.0% K2O). The melts of intermediate composition (53–64% SiO2) also include potassic (5.6% K2O, 3.4% Na2O, 1.0% TiO2, and 0.4% P2O5) and sodic (4.3% Na2O, 2.8% K2O, 1.3% TiO2, and 0.4% P2O5) varieties. The acid melts (64–74% SiO2) are either potassic (4.5% K2O, 3.6% Na2O, 0.7% TiO2, and 0.15% P2O5) or sodic (4.5% Na2O, 3.1% K2O, 0.7% TiO2, and 0.13% P2O5). A distinctive feature of the Gorely volcanic center is the pervasive occurrence of K-rich compositions throughout the whole
compositional range (silicity) of the melts. Melt inclusions of various types were sometimes found not only in a single sample
but also in the same phenocrysts. The sodic and potassic types of the melts contain different Cl and F concentrations: the
sodic melts are richer in Cl, whereas the potassic melts are enriched in F. We are the first to discover potassic melts with
very high F concentrations (up to 2.7 wt %, 1.19 wt % on average, 17 analyses) in the Kuriles and Kamchatka. The average F
concentration in the sodic melts is 0.16 wt % (37 analyses). The melts are distinguished for their richness in various groups
of trace elements: LILE, REE (particularly HREE), and HFSE (except Nb). All of the melts share certain geochemical features.
The concentrations of elements systematically increase from the mafic to acid melts (except only for the Sr and Eu concentrations,
because of active plagioclase fractionation, and Ti, an element contained in ore minerals). The paper presents a review of
literature data on volcanic rocks in the Kurile-Kamchatka area in which melt inclusions with high K2O concentrations (K2O/Na2O > 1) were found. K-rich melts are proved to be extremely widespread in the area and were found on such volcanoes as Avachinskii,
Bezymyannyi, Bol’shoi Semyachek, Dikii Greben’, Karymskii, Kekuknaiskii, Kudryavyi, and Shiveluch and in the Valaginskii and
Tumrok Ranges. 相似文献
12.
Origin of glass in upper mantle xenoliths from the quaternary volcanics of Gees,West Eifel,Germany 总被引：6，自引：0，他引：6
A. D. Edgar F. E. Lloyd D. M. Forsyth R. L. Barnett 《Contributions to Mineralogy and Petrology》1989,103(3):277-286
Glasses have been analysed from six mantle-derived xenoliths (5 orthopyroxene and/or olivine-rich, 1 clinopyroxene-rich) from the Quaternary volcanics S.E. of Gees, West Eifel, Germany. The glasses in these xenoliths occur as pools surrounding and embaying spinels, as inclusions in spinels, as veins and stringers within phlogopiterich veins, and as jackets partially surrounding some of the xenoliths. Glasses analysed are felsic and characterised by low to intermediate SiO2 (40–62 wt.%), variable CaO (1–11 wt.%) and MgO (1–4 wt.%), high Al2O3 (14–21 wt.%), and up to 11 wt.% Na2O + K2O. The jacket glasses have the lowest SiO2, highest CaO and MgO. Variations in all of the glass compositions are similar and imply a unifying factor or process in their formation. Glass as pools and stringers within veins of phlogopite forms part of the same trends shown by all the glasses when plotted on bivariate (oxide vs SiO2) diagrams but can be distinguished from glass surrounding and enclosed by spinels. Glasses occurring as jackets are similar in composition to those in pools and veinlets associated with phlogopite but are of quite different composition to the glasses found within the xenoliths that they partially enclose. The occurrence and chemistry of the glasses do not support such glasses as representing original or differentiated magma trapped during formation of the xenolithic assemblages. The chemistry of the glasses also makes it unlikely that they were produced by dissociation of phlogopite during ascent of the xenoliths. The most likely origin for the glasses is that they represent volatile-rich melts which migrated through upper mantle material. These melts are likely to be responsible for the heterogeneous nature of the mantle underlying this part of the West Eifel region. 相似文献
13.
Water diffusion in silicate melts is important for understanding bubble growth in magma, magma degassing and eruption dynamics of volcanos. Previous studies have made significant progress on water diffusion in silicate melts, especially rhyolitic melt. However, the pressure dependence of H2O diffusion is not constrained satisfactorily. We investigated H2O diffusion in rhyolitic melt at 0.95–1.9 GPa and 407–1629 °C, and 0.2–5.2 wt.% total water (H2Ot) content with the diffusion-couple method in a piston-cylinder apparatus. Compared to previous data at 0.1–500 MPa, H2O diffusivity is smaller at higher pressures, indicating a negative pressure effect. This pressure effect is more pronounced at low temperatures. Assuming H2O diffusion in rhyolitic melt is controlled by the mobility of molecular H2O (H2Om), the diffusivity of H2Om (DH2Om) at H2Ot ≤ 7.7 wt.%, 403–1629 °C, and ≤ 1.9 GPa is given by
DH2Om=D0exp(aX),