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1.
Felsic intrusions in the Hannan region at the northwestern margin of the Yangtze Block mainly include the ca. 730 Ma adakitic Erliba and Wudumen plutons and the ca. 760 Ma calcic-alkali Xixiang and Tianpinghe bodies. These four intrusions were considered to have been formed by melting of the newly formed lower mafic crust. However, the two generations of granitoids have different lithologies and mineral compositions. Thermobarometry calculations reveal that the Erliba and Wudumen granitoids formed under approximately similar emplacement pressures (2.96–3.11 kbar) and temperatures (787–789°C). The Xixiang emplaced body was intruded at high pressure (?3.54 kbar) and low temperature (?676°C), whereas the Tianpinghe pluton solidified at low pressure (?2.00 kbar) and high temperature (～747°C). The four intrusions have similar oxygen fugacity ranges near the nickel-nickel oxide buffer, suggesting oxidized parental magmas. The Erliba and Wudumen are estimated to have been generated under pressures higher than 12 kbar, the Xixiang under a pressure of >10 kbar, and the Tianpinghe under a pressure of >6 kbar. Thus, the petrology and geochemical differences among these four felsic intrusions probably mainly resulted from variations of depth and degrees of partial melting. The whole-rock and mineral compositions have arc affinities, suggesting that they were formed in an active continental margin.  相似文献   

2.
The paper reports the first results of the petrological studies of magmatic melts that formed siliceous pyroclastic deposits related to voluminous eruptions on Iturup Island. The caldera-forming eruptions of the Lvinaya Past and the Vetrovoy Isthmus, having similar features, resulted from the evolution of silicic melts that originated from partial melting of metabasalts. According to the mineral thermometry results, the melt was crystallized at ~800°C. The phenocrysts from the Vetrovoy Isthmus pumices were crystallized at <1 kbar, while those from the Lvinaya Past were formed at higher pressures. The pyroclastic rock compositions in both calderas correspond to moderately aluminous dacite and rhyolitic dacite of the normal series, whose melts likely did not undergo significant crystallization differentiation before the eruptions. The main volatile components of the magma include H2O, CO2, S, F, and Cl. Degassing with emission of water–carbon-dioxide fluid accompanied the early crystallization of plagioclase in the Vetrovoy Isthmus pumice. Evidence of pre-eruption melt degassing in the Lvinaya Past were not found. Water release from the melts may be related to both the early magma degassing and the eruptions. The lack of data evidencing the deep differentiation and mixing of contrasting melts implies a relatively small time period between the acid melt appearance and eruptions.  相似文献   

3.
Crystallization experiments were performed on quartz diorite (~55 wt.% SiO2, 3.1–8.4 wt.% MgO) from the G?siniec Intrusion (Bohemian Massif, SW Poland) at 1?2 kbar, 750–850°C, various mole fractions of water and with fO2 buffered by the NNO buffer. The two natural quartz diorites (leucocratic poikilitic quartz diorite - ‘LPD’ and melanocratic quartz diorite - ‘MD’) differ in whole rock and mineral composition with MD being richer in MgO and poorer in CaO than LPD, probably due to accumulation of mafic minerals or melt removal in MD. LPD represents melt composition and is used to reconstruct crystallization conditions in the G?siniec Intrusion. The crystallization history of LPD magma, deduced from experimental and natural mineral compositions, includes a higher pressure stage probably followed by emplacement at ~2 kbar of partly crystallized magma at temperatures ~850?800°C and quick cooling. The mineral assemblage present in LPD requires water contents in the magma of at least 5 wt% and oxygen fugacity below that controlled by the NNO buffer. The compositions of mafic minerals in the MD composition were equilibrated at temperatures below 775°C and at subsolidus conditions. The equilibration was probably due to the reaction between water-rich, oxidizing residual melt and the cumulatic-restitic mineral assemblage. MD is characterized by occurrence of the euhedral cummingtonite and increasing anorthite content in the rims of plagioclase. A similar reaction was reproduced experimentally in both LPD and MD compositions indicating that cummingtonite may be a late magmatic phase in quartz dioritic systems, crystallizing very close to solidus and only from water saturated magma.  相似文献   

4.
The paper reports data on the mineralogy, geochemistry, phase composition of comendites and pantellerites from Nemrut volcano, eastern Turkey; estimates of the crystallization conditions of minerals, composition of matrix glasses and melt inclusions in anorthoclase, fayalite, hedenbergite phenocrysts. LA-ICP-MS was applied to analyze the matrix glasses and phenocryst minerals. The distribution coefficients between phases and glass were calculated for P, B, Li, Rb, Cs, Ba, Sr, Zr, Hf, Ta, Nb, Sc, V, Cr, Ni, Cu, Pb, Th, U, Y, REE. Mass balance simulations of the comenditic and pantelleritic compositions, experimental data, data on melt inclusions are utilized to analyze the processes responsible for the derivation of the magmas, accumulation of components in them and to elucidate genetic links between the trachyte-comenditic, comenditic and pantelleritic melts. The origin of the residual comenditic and pantelleritic melts is explained by variations in the crystallization conditions of anorthoclase (dominant phase), hedenbergite, fayalite, Fe and Ti oxides in the parental trachyte-comenditic magma depending on the pressure and concentration of water dissolved in the melts. The accessory phases (REEand Sr-bearing fluorapatite and zircon) were likely involved in the fractionation of the melts. The following crystallization parameters were obtained by QUILF calculations for the hedenbergite, fayalite, and ilmenite phenocrysts (minimum values for quartz-free melts): 3 kbar, 763°C, ΔFMQ = ?1.27 for the Fe-comendites; 3.3–3.8 kbar, 715°C, ΔFMQ = ?1.8 for the pantellerites; 2.3 kbar, 748°C, ΔFMQ = ?1.16 for the low-Fe comendites. The equilibrium crystallization of anorthoclase phenocrysts in the comenditic melts proceeded at temperature ～750°C. Data on glasses of melt inclusions indicate that the comenditic and pantelleritic melts contained 1–3 wt % H2O. Analysis of literature data and estimates of the conditions under which the Nemrut magmas were derived suggest that the local chambers with H2O-undersaturated comenditic and pantelleritic melts could occur at centers of alkaline volcanism at depths within the range of 5 to 10–15 km (lithostatic pressure of 1–4 kbar), at temperatures <750°C and oxygen fugacity below the FMQ buffer.  相似文献   

5.
Mantle xenoliths (lherzolites, clinopyroxene dunites, wehrlites, and clinopyroxenites) in the Early Cretaceous volcanic rocks of Makhtesh Ramon (alkali olivine basalts, basanites, and nephelinites) represent metasomatized mantle, which served as a source of basaltic melts. The xenoliths bear signs of partial melting and previous metasomatic transformations. The latter include the replacement of orthopyroxene by clinopyroxene in the lherzolites and, respectively, the wide development of wehrlites and olivine clinopyoroxenites. Metasomatic alteration of the peridotites is accompanied by a sharp decrease in Mg, Cr, and Ni, and increase of Ti, Al, Ca contents and 3+Fe/2+Fe ratio, as well as the growth of trace V, Sc, Zr, Nb, and Y contents. The compositional features of the rocks such as the growth of 3+Fe/2+Fe and the wide development of Ti-magnetite in combination with the complete absence of sulfides indicate the high oxygen fugacity during metasomatism and the low sulfur concentration, which is a distinctive signature of fluid mode during formation of the Makhtesh Ramon alkali basaltic magma. Partial melting of peridotites and clinopyroxenites is accompanied by the formation of basanite or alkali basaltic melt. Clino- and orthopyroxenes are subjected to melting. The crystallization products of melt preserved in the mantle rock are localized in the interstices and consist mainly of fine-grained clinopyroxene, which together with Ti-magnetite, ilmenite, amphibole, rhenite, feldspar, and nepheline, is cemented by glass corresponding to quartz–orthopyroxene, olivine–orthopyroxene, quartz–feldspar, or nepheline–feldspar mixtures of the corresponding normative minerals. The mineral assemblages of xenoliths correspond to high temperatures. The high-Al and high-Ti clinopyroxene, calcium olivine, feldspar, and feldspathoids, amphibole, Ti-magnetite, and ilmenite are formed at 900–1000°. The study of melt and fluid inclusions in minerals from xenoliths indicate liquidus temperatures of 1200–1250°C, solidus temperatures of 1000–1100°C, and pressure of 5.9–9.5 kbar. Based on the amphibole–plagioclase barometer, amphibole and coexisting plagioclase were crystallized in clinopyroxenites at 6.5–7.0 kbar.  相似文献   

6.
罗雕  侯通  潘荣昊 《岩石学报》2020,36(7):2116-2126
本文报道了攀枝花钒钛磁铁矿含矿岩体边缘岩相带中的苦橄玢岩和岩体中淡色辉长岩的锆石微量元素特征。结果表明二者所含锆石都具有明显的Ce正异常和Eu负异常,以及轻稀土元素亏损和重稀土元素富集的特征,其Th/U比值为0.35～3.23,都属于典型的岩浆锆石。本次研究利用最新实验标定的锆石氧逸度计对苦橄玢岩和淡色辉长岩的氧逸度进行了估算。估算结果表明苦橄玢岩和淡色辉长岩均具较高的氧逸度,分别为QFM+0.3～QFM+2.5和QFM+0.7～QFM+3(QFM为石英-铁橄榄石-磁铁矿缓冲剂)。苦橄玢岩作为来自深部岩浆房侵入到攀枝花主岩体的富橄榄石"晶粥体",其高氧逸度的特征反映出攀枝花岩体的原生岩浆以及地幔源区是相对氧化的,而导致这一结果的原因很可能与古老俯冲事件导致的地幔交代作用有关。通过地幔柱-岩石圈相互作用,在较高氧逸度下发生部分熔融形成了铁质苦橄岩及其堆晶作用产物苦橄玢岩。此外,淡色辉长岩的氧逸度也显示出较高的特征,这说明这种氧化的特征很可能是贯穿了整个成岩过程的,对钒钛磁铁矿成矿,特别是导致铁钛氧化物早期结晶起到了不可忽视的作用。  相似文献   

7.
In this study, we investigate the metamorphic history of the Assynt and Gruinard blocks of the Archean Lewisian Complex, northwest Scotland, which are considered by some to represent discrete crustal terranes. For samples of mafic and intermediate rocks, phase diagrams were constructed in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) system using whole‐rock compositions. Our results indicate that all samples equilibrated at similar peak metamorphic conditions of ~8–10 kbar and ~900–1,000°C, consistent with field evidence for in situ partial melting and the classic interpretation of the central region of the Lewisian Complex as representing a single crustal block. Melt‐reintegration modelling was employed in order to estimate probable protolith compositions. Phase equilibria calculated for these modelled undepleted precursors match well with those determined for a subsolidus amphibolite from Gairloch in the southern region of the Lewisian Complex. Both subsolidus lithologies exhibit similar phase relations and potential melt fertility, with both expected to produce orthopyroxene‐bearing hornblende granulites, with or without garnet, at the conditions inferred for the Badcallian metamorphic peak. For fully hydrated protoliths, prograde melting is predicted to first occur at ~620°C and ~9.5 kbar, with up to 45% partial melt predicted to form at peak conditions in a closed‐system environment. Partial melts calculated for both compositions between 610 and 1,050°C are mostly trondhjemitic. Although the melt‐reintegrated granulite is predicted to produce more potassic (granitic) melts at ~700–900°C, the modelled melts are consistent with the measured compositions of felsic sheets from the central region Lewisian Complex.  相似文献   

8.
Feng  Yuannan  Lan  Tingguang  Pan  Lichuan  Liu  Tingting  Dong  Shaohua 《中国地球化学学报》2019,38(4):530-540

The Nanling Range in South China is well known for its rich granite-related W–Sn deposits. To elucidate the controls of different granite-related W–Sn metallogenesis in the region, we chose five representative ore-related granites (Yanbei, Mikengshan, Tieshanlong, Qianlishan, and Yaogangxian intrusions) in the Hunan–Jiangxi region, and studied their magmatic zircon ages and trace element geochemistry. Our new zircon data showed the differences in ages, temperatures and oxygen fugacity of the ore-forming magmas. Zircon U–Pb ages of the Yanbei and Mikengshan intrusions are characterized by 142.4 ± 2.4 and 143.0 ± 2.3 Ma, respectively, whereas the Tieshanlong and Qianlishan intrusions are 159.5 ± 2.3 and 153.2 ± 3.3 Ma, respectively. The Sn-related intrusions were younger than the W-related intrusions. The Ti-in-zircon thermometry showed that there was no systematic difference between the Sn-related Yanbei (680–744 °C) and Mikengshan (697–763 °C) intrusions and the W-related Tieshanlong (730–800 °C), Qianlishan (690–755 °C) and Yaogangxian (686–751 °C) intrusions. However, the zircon Ce4+/Ce3+ ratios of the Yanbei (averaged at 18.3) and Mikengshan (averaged at 18.8) intrusions are lower than those of the Tieshanlong (averaged at 36.9), Qianlishan (averaged at 38.4) and Yaogangxian (averaged at 37) intrusions, indicating that the Sn-related granitic magmas might have lower oxygen fugacities than those of the W-related. This can be explained by that, in more reduced magmas, Sn is more soluble than W and thus is more enriched in the residual melt to form Sn mineralization. The difference in source materials between the Sn-related and the W-related granites seems to have contributed to the different redox conditions of the melts.

  相似文献   

9.
Testing the Ballhaus–Berry–Green Ol–Opx–Sp oxybarometer (BBG) on independent experimental data indicates that it overestimates the oxygen fugacity by 0.6–1.3 log units under mildly reduced conditions (near the C–CO buffer) and by as much as 2–3 log units under reduced conditions (at the IW buffer and below it). A newly developed oxibarometer is suggested to minimize this effect and enhance the capabilities of redoxometry of low-pressure mineral associations, including magmatic melts undersaturated with respect to orthopyroxene (Opx). The new empirical equation of the oxybarometer is applicable to a wide range of mafic–ultramafic magmas of normal alkalinity, including terrestrial, lunar, and meteoritic systems under pressures of 0.001–25 kbar and oxygen fugacity ranging from IW–3 to NNO + 1. The derived regression fits the ΔQFM values of the calibration dataset (154 experiments) accurate to ~0.5 log units. The new oxybarometer eliminates systematic errors when redox parameters are evaluated for the reduced region (from IW–3 to C–CO) and for crystallization of magmas without Opx on the liquidus. The efficiency of the suggested model is demonstrated by its application to natural rocks: (1) low-Ti lunar basalts, (2) tholeiites from the Shatsky Rise, (3) Siberian flood basalts, (4) rocks of the layered series of the Yoko-Dovyren intrusion, and (5) mantle xenoliths collected in southern Siberia, Mongolia, China, and the southern Russian Far East. The values yielded by such oxybarometers for intrusive rocks, which underwent long-lasting cooling and postcumulus reequilibration, should be regarded with reserve.  相似文献   

10.
The solubility of sulphur in sulphide-saturated, H2O-bearing basaltic–andesitic and basaltic melts from Hekla volcano (Iceland) has been determined experimentally at 1,050°C, 300 and 200 MPa, and redox conditions with oxygen fugacity (logfO2) between QFM−1.2 and QFM+1.1 (QFM is a quartz–fayalite–magnetite oxygen buffer) in the systems containing various amounts of S and H2O. The S content of the H2O-rich glasses saturated with pyrrhotite decreases from 2,500 ppm in basalt to 1,500 ppm in basaltic andesite at the investigated conditions. Furthermore, the reduction of water content in the melt at pyrrhotite saturation and fixed T, P and redox conditions leads to a decrease in S concentration from 2,500 to 1,400 ppm for basaltic experiments (for H2O decrease from 7.8 to 1.4 wt%) and from 1,500 to 900 ppm (for H2O decrease from 6.7 to 1.7 wt%) for basaltic andesitic experiments. Our experimental data, combined with silicate melt inclusion investigations and the available models on sulphide saturation in mafic magmas, indicate that the parental basaltic melts of Hekla were not saturated with respect to sulphide. During magmatic differentiation, the S content in the residual melts increased and might have reached sulphide saturation with 2,500 ppm dissolved S. With further magma crystallization, the S concentration in the melt was controlled by the sulphide saturation of the magma, decreasing from ~2,500 to 900 ppm S.  相似文献   

11.
The Neoproterozoic Korab Kansi mafic-ultramafic intrusion is one of the largest (100 km2) intrusions in the Southern Eastern Desert of Egypt. The intrusion consists of Fe-Ti-bearing dunite layers, amphibole peridotites, pyroxenites, troctolites, olivine gabbros, gabbronorites, pyroxene gabbros and pyroxene-hornblende gabbros, and also hosts significant Fe-Ti deposits, mainly as titanomagnetite-ilmenite. These lithologies show rhythmic layers and intrusive contacts against the surrounding granites and ophiolitic-island arc assemblages. The wide ranges of olivine forsterite contents (Fo67.9-85.7), clinopyroxene Mg# (0.57–0.95), amphibole Mg# (0.47–0.88), and plagioclase compositions (An85.8-40.9) indicate the role of fractional crystallization in the evolution from ultramafic to mafic rock types. Clinopyroxene (Cpx) has high REE contents (2–30 times chondrite) with depleted LREE relative to HREE, like those crystallized from ferropicritic melts generated in an island-arc setting. Melts in equilibrium with Cpx also resemble ferropicrites crystallized from olivine-rich mantle melts. Cpx chemistry and its host rock compositions have affinities to tholeiitic and calc-alkaline magma types. Compositions of mafic-ultramafic rocks are depleted in HFSE (e.g. Nb, Ta, Zr, Th and U) relative to LILE (e.g. Li, Rb, Ba, Pb and Sr) due to the addition of subduction-related hydrous fluids (rich in LILE) to the mantle source, suggesting an island-arc setting. Fine-grained olivine gabbros may represent quenched melts approximating the primary magma compositions because they are typically similar in assemblage and chemistry as well as in whole-rock chemistry to ferropicrites. We suggest that the Korab Kansi intrusion crystallized at temperatures ranging from ~700 to 1100 °C from ferropicritic magma derived from melting of metasomatized mantle at <5 Kbar. These hydrous ferropicritic melts were generated in the deep mantle and evolved by fractional crystallization under high ƒO2 at relatively shallow depth. Fractionation formed calc-alkaline magmas during the maturation of an island arc system, reflecting the role of subduction-related fluids. The interaction of metasomatized lithosphere with upwelling asthenospheric melts produced the Fe and Ti-rich ferropicritic parental melts that are responsible for precipitating large quantities of Fe-Ti oxide layers in the Korab Kansi mafic-ultramafic intrusion. The other factors controlling these economic Fe-Ti deposits beside parental melts are high oxygen fugacity, water content and increasing degrees of mantle partial melting. The generation of Ti-rich melts and formation of Fe-Ti deposits in few layered intrusions in Egypt possibly reflect the Neoproterozoic mantle heterogeneity in the Nubian Shield. We suggest that Cryogenian-Tonian mafic intrusions in SE Egypt can be subdivided into Alaskan-type intrusions that are enriched in PGEs whereas Korab Kansi-type layered intrusions are enriched in Fe-Ti-V deposits.  相似文献   

12.
An intrusive granitoid pluton into TTG-Dharwar Supergroup greenstone sequence is being reported for the first time from the Dharwar Foreland region. Based on field and petrographic characteristics, these granitoids are classified as - quartz-monzodiorites and granites. Occasional mafic bodies of dioritic-granodioritic composition with size ranging from small microgranular magmatic enclaves to bodies of several centimeters are common in these granitoids.The granitoids are devoid of any crystal-plastic fabric as well as high-strain characteristics. The textural (CSD) studies indicate that the quartz-monzodiorites are derived from magma mixing whereas the granites are derived from equilibrium crystallization of the magma derived from the reworking of quartz-monzodiorites. The P-T estimates indicate that the quartz-monzodiorites were crystallized at higher temperature (>950 °C) and pressure (3.09–4.36 kbar) conditions in a reducing environment at mid-crustal levels. However, the granites indicate lower temperature (<750 °C) and pressure (0.89–1.88 kbar) conditions of crystallization in an oxidizing environment at shallow-crustal levels. The bulk rock chemical characteristics indicate that the quartz-monzodiorites were derived from the melt generated by the mixing of two melts - a melt derived from the differentiation of sanukitoids senso lato (s.l.) and a melt derived from the partial melting of TTG. On the other hand, reworking of the hot crystallizing quartz-monzodiorite due to its rapid upliftment to shallow crustal levels resulted in a decompression melting which gave rise to granitic melts.The relative age of the Dharwad granitoids is estimated to be ∼2580–2560 Ma and unlike the other older granitoids (> 2.61 Ga) reported from the northern part of the Shimoga greenstone belt, the studied granitoids marks the final stage of cratonization in the Foreland region.  相似文献   

13.
The compositions of parental melts of Tolbachinsky Dol (Kamchatka) basalts were estimated from the compositions of olivine-hosted (Fo90.5-83.1) primitive melt inclusions in the rocks of the Northern breakthrough of the Great Tolbachik Fissure Eruption (1975 A.C.) and of the late-Holocene cone “1004”. The parental melts contain 100–150 ppm Cu and 0.16–0.30 wt % S. These concentrations are much higher than those determined for the initial magmas of mid-ocean ridge basalts (MORB), for example of the Juan de Fuca ridge (Cu = 55–105 ppm, S=0.09–0.12 wt %). Modeling of mantle melting under variable redox conditions demonstrated that the high Cu and S contents in the Tolbachinsky Dol melts can be obtained by 6–12% melting of DMM-like source under oxidized conditions (ΔQFM = +1.2 ± 0.1) and do not require a significant (>30–35% for S) subduction-related influx of these elements to the mantle source. The high contents of Cu and S in the Tolbachinsky Dol melts are largely explained by the increase of sulfide solubility in a silicate melt under oxidized conditions. In contrast, relatively reduced (ΔQFM ～ 0) conditions of MORB generation result in low contents of Cu and S in their initial magmas. The estimated ΔQFM values agree well with the data obtained using the Cr-spinel–olivine oxybarometer. The high oxygen potential of Tolbachinsky Dol primary magmas is inherited by more evolved magmas, thus favouring Cu enrichment up to 270 ppm during magma fractionation, approaching maximum copper contents in the global systematics of island-arc rocks.  相似文献   

14.
The Wuhe Complex in the Bengbu area of the Jiao–Liao–Ji Belt, southeast North China Craton, contains garnet-bearing mafic granulites that have undergone high-pressure (HP) and ultrahigh-temperature (UHT) metamorphism. These granulites also experienced partial melting and occur as lenses within marbles. Petrographic observations and quantitative phase equilibria modeling reveal clockwise P–T paths, involving an inferred HP stage followed by decompressional, medium-pressure, granulite-facies metamorphism and subsequent cooling. The HP assemblage of garnet + clinopyroxene + plagioclase + K-feldspar ± amphibole ± quartz ± rutile indicates P–T conditions of 840–980 °C and 12–17 kbar. This was followed by post-peak, near-isothermal decompression with the development of orthopyroxene + clinopyroxene + plagioclase + K-feldspar + garnet + amphibole + ilmenite at 850–960 °C and 7–10 kbar, resulting in the development of orthopyroxene rims on resorbed garnet. Pyroxene and ternary feldspar thermometry yielded high temperatures of ~1150 °C and 1055–1087 °C at 10 kbar, respectively, which constrain the minimum crystallization temperatures of the igneous protoliths. The host and lamellae of the pyroxene and ternary feldspar are relict magmatic minerals/textures that survived metamorphism due to the silica-undersaturated bulk-rock conditions. Zr-in-rutile thermometry yielded temperatures of ~935 °C and 800 °C, with the former being consistent with the predicted peak metamorphic temperatures. Small amounts of melts (up to 5%) were generated during decompression of the Bengbu mafic granulites. The generated partial melts were mainly (quartz) monzonite at 900–920 °C, and the silica contents of the melts were controlled by the quartz stability field in P–T pseudosections. The partial melts were enriched in Na and strongly depleted in Fe–Mg at the peak pressure of ~14 kbar and 920 °C, and later evolved to Fe–Mg-rich and high-K compositions during decompression. The melt compositions in the studied rocks are similar when the pressures reached ~9 kbar. The modal proportion of amphibole increased as the melt H2O content decreased at lower pressures, indicating that the limited H2O remaining in the host rocks was consumed to produce amphibole. U–Pb geochronology of zircon containing inclusions of clinopyroxene, plagioclase, and apatite constrains the timing of metamorphism to 1930–1840 Ma, as is the case for HP granulites from Shandong, Liaoning, and southern Jilin in the central and northeastern Jiao–Liao–Ji Belt. The Wuhe HP–UHT mafic granulites were ultimately sourced from upwelling asthenosphere-derived magma at ~2.1 Ga, which intruded and crystallized at shallower depths. The igneous protoliths were then buried to middle–lower crustal levels and experienced HP–UHT granulite-facies metamorphism and partial melting at 1.95–1.90 Ga related to continental subduction and overthickening. The HP–UHT mafic granulites were rapidly exhumed at ~1.85 Ga and generated small volumes of (quartz) monzonite during decompression. The newly discovered Paleoproterozoic HP–UHT mafic granulites associated with partial melting suggest that the continent materials were deeply subducted to the lower crustal levels and that additional heating was not involved. The finding of the HP–UHT granulites, together with the widespread distributions of the granulite-facies metamorphic rocks and the determination of the clockwise P–T–t paths, reveal that the Paleoproterozoic Jiao–Liao–Ji orogenic belt extends at least 1000 km, starting from southern Jilin, passing through the southeastern Liaoning and Jiaobei terranes, and elongating to the Bengbu area in Anhui.  相似文献   

15.
In this paper, we discuss the formation conditions of rhyolites and results of their interaction with later portions of basic magmas on the basis of the investigation of melt and fluid inclusions in minerals from a rhyolite xenolith and host neovolcanic basalts of the Cleft segment of the Juan de Fuca Ridge. In terms of bulk chemistry and the compositions of melt inclusions in pyroxene and olivine phenocrysts, the basic rocks of the southern part of this segment are typical MOR basalts. Their olivine, clinopyroxene, and plagioclase crystallized at temperatures of 1160–1280°C and a pressure range between 20 and 100 MPa. The xenolith is a leucocratic rock with negligible amounts of mafic minerals, which clearly distinguishes it from the known occurrences of silicic rocks in the rift valleys of MOR. The rhyolite melt crystallized at temperatures of 900–880°C. The final stages of rhyolite melt crystallization at temperatures of 780–800°C were accompanied by the release of a saline aqueous fluid with high chloride contents. Based on the geochemical characteristics of melt inclusions and melting products, it can be suggested that the magmatic melt was produced by melting of metamorphosed oceanic crust within the Cleft segment under the influence sof saline aqueous fluid trapped in the pores and interstices of the rock. The rock represented by the xenolith is a late differentiation product of such melts. The ultimate products of silicic melt fractionation show high volatile contents: H2O > 3.0 wt %, Cl ～ 2.0 wt %, and F ～ 0.1 wt %. The interaction of the xenolith with the host basaltic melt occurred at temperatures equal or slightly higher than those of ferrobasalt melts (1190–1180°C). During ascent the xenolith occurred for a few tens of hours in high-temperature basic magma, and diffusion exchange between the basaltic and silicic melts was very minor.  相似文献   

16.
Dehydration melting of tonalites. Part II. Composition of melts and solids   总被引：6，自引：0，他引：6  
 Dehydration melting of tonalitic compositions (phlogopite or biotite-plagioclase-quartz assemblages) is investigated within a temperature range of 700–1000°C and pressure range of 2–15 kbar. The solid reaction products in the case of the phlogopite-plagioclase(An45)-quartz starting material are enstatite, clinopyroxene and potassium feldspar, with amphiboles occurring occasionally. At 12 kbar, zoisite is observed below 800°C, and garnet at 900°C. The reaction products of dehydration melting of the biotite (Ann50)-plagioclase (An45)-quartz assemblage are melt, orthopyroxene, clinopyroxene, amphibole and potassium feldspar. At pressures > 8 kbar and temperatures below 800°C, epidote is also formed. Almandine-rich garnet appears above 10 kbar at temperatures ≥ 750°C. The composition of melts is granitic to granodioritic, hence showing the importance of dehydration melting of tonalites for the formation of granitic melts and granulitic restites at pressure-temperature conditions within the continental crust. The melt compositions plot close to the cotectic line dividing the liquidus surfaces between quartz and potassium feldspar in the haplogranite system at 5 kbar and a H 2O = 1. The composition of the melts changes with the composition of the starting material, temperature and pressure. With increasing temperature, the melt becomes enriched in Al2O3 and FeO+MgO. Potash in the melt is highest just when biotite disappears. The amount of CaO decreases up to 900°C at 5 kbar whereas at higher temperatures it increases as amphibole, clinopyroxene and more An-component dissolve in the melt. The Na2O content of the melt increases slightly with increase in temperature. The composition of the melt at temperatures > 900°C approaches that of the starting assemblage. The melt fraction varies with composition and proportion of hydrous phases in the starting composition as well as temperature and pressure. With increasing modal biotite from 20 to 30 wt%, the melt proportion increases from 19.8 to 22.3 vol.% (850°C and 5 kbar). With increasing temperature from 800 to 950°C (at 5 kbar), the increase in melt fraction is from 11 to 25.8 vol.%. The effect of pressure on the melt fraction is observed to be relatively small and the melt proportion in the same assemblage decreases at 850°C from 19.8 vol.% at 5 kbar to 15.3 vol.% at 15 kbar. Selected experiments were reversed at 2 and 5 kbar to demonstrate that near equilibrium compositions were obtained in runs of longer duration. Received: 27 December 1995 / Accepted: 7 May 1996  相似文献   

17.
We performed detailed field and drill core mapping of partial melting features and felsic rocks (footwall granophyres, FWGRs) representing segregated and crystallized partial melts within the contact aureole of the Sudbury Igneous Complex (SIC) in the 1.85 Ga Sudbury impact structure. Our results, derived from mapping within the North (Windy Lake, Foy, Wisner areas) and East Ranges (Skynner, Frost areas) of the structure, reveal that partial melting was widespread in both felsic and mafic footwall units up to distances of 500 m from the basal contact of the SIC. Texturally and mineralogically, significant differences exist between rocks formed by partial melting within and between localities. In general, however, melt bodies are dominated by different quartz-feldspar intergrowths (e.g. granophyric, graphic) and miarolitic cavities up to 5 cm in diameter. Major and trace element compositions of Wisner and Frost FWGRs imply that they crystallized from melts dominantly derived from partial melting of felsic Levack Gneiss and Cartier granitoid rocks, as well as from gabbroic rocks only at Frost. These results accord with our observations on in situ partial melting features and crystallized melt of microscopic scale in both felsic and mafic rocks. We conclude that partial melting occurred at a pressure of 1.5 ± 0.5 kbar and at temperatures up to 750°C in the Wisner area and up to 900°C in the Frost and Windy Lake areas. Segregations of partial melt into veins and dikes are present in all localities, and were promoted by deformation of the Sudbury structure in the Penokean orogeny as indicated by dominant strike directions. Whereas veins and dikes reflect brittle conditions during melt migration, sheared melt pods in the Sudbury breccia matrix indicate ductile conditions during their crystallization. Our results suggest a close genetic association of partial melting, melt segregation, and hydrothermal processes responsible for remobilization of Cu–Ni–PGE sulphides into and within the SIC footwall.  相似文献   

18.
The study of melt inclusions in Cr-spinels from melanocratic troctolites provided the first direct information on the physicochemical parameters of enriched magmatic systems that produced high-Fe and high-Ti intrusive complexes in the Sierra-Leone region (Central Atlantic, 6°N). These complexes are made up of predominating hornblende Fe-Ti oxide gabbronorites and gabbrodiorites with subordinate amount of ultramafics, diorites, quartz diorites, and trondhjemites. The study of melt inclusions and rocks showed that the majority of gabbroids of the Central Atlantic (Sierra Leone area and 15°20′ Fracture Zone) were derived from N-MORB-type melts, whereas differentiated Fe-Ti-oxide rocks were crystallized from other melts, which were preserved as inclusions in the Cr-spinels from the melanocratic troctolites of the Sierra Leone region. The ion-microprobe study of these inclusions yield direct evidence on the elevated water content (up to 1.24–1.77 wt %) in the parental melts of Fe-Ti oxide rocks. Data on trace and rare-earth element distribution together with high (La/Sm)N and (Ce/Yb)N ratios in the inclusions indicate the possible influence of deep plume source on the generation of these magmas. Simulation based on melt inclusion data testifies that high-Fe intrusions of the Sierra Leone area were crystallized from the water-saturated magmas at relatively low temperatures (1020–1240°C). It was shown that the geochemically enriched Fe-Ti melts were presumably formed regardless of N-MORB-type magmatism predominant in Central Atlantic, under the influence of new mantle plume that caused melting of hydrated oceanic lithosphere.  相似文献   

19.
We present the results of electrochemical measurements of the intrinsic oxygen fugacity of olivine separates representing seven rock types from the central part and southwestern termination of the Yoko–Dovyren mafic—ultramafic massif. The \({f_{{O_2}}}\) values were determined using a high-temperature solid-electrolyte double-cell assembly developed at the Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. A total of 59 experiments were performed (from 7 to 16 for each sample) at the atmospheric pressure and within the temperature range of 800–1050°C at the 30–50°C increment. The results were interpreted using the calculated log \({f_{{O_2}}}\) – 1/T(K) and log \({f_{{O_2}}}\) – T(°C) dependences. It was shown that the subsolidus temperature range of the rocks (below ~1050°C) is characterized by lowest intrinsic oxygen fugacity of olivine, which is 1–4 log units below the QFM buffer. For the solidus temperatures of ~1100°C, the more oxidized conditions ranging approximately from QFM to ~QFM-3 were measured. Extrapolating the log \({f_{{O_2}}}\) – T°C dependences to the temperatures of the original magmas (~1200–1300°C) produces the maximum scatter in oxygen fugacities from ~QFM+2.5 to QFM-1. The estimated range of redox conditions for the Dovyren magma crystallization lies between the QFM and ~QFM-2.5 buffer equilibria. This is consistent with the complete absence of primary magmatic titanomagnetite and the presence of ilmenite in the Dovyren rocks.  相似文献   

20.
Tertiary volcanics in the northern zone of the Eastern Pontides are characterized by subaerial and shallow-subaqueous facieses, and are divided into three volcanic suites: Eocene aged (1) basalt-trachybasalt-basaltic trachyandesite (BTB) and (2) trachyte-trachyandesite (TT), and Miocene aged (3) basanite-tephrite (BT) suites. Clinopyroxene is a common phase in all three volcanic suites, and has different compositions with Mg# varying from 0.57 to 0.91 in BTB suite and 0.57–0.84 in TT suite to 0.65–0.90 in BT suite. Feldspars in all suites generally exhibit wide range of compositions from sanidine to albite or anorthite and have weak normal and reverse compositional zoning. Olivines in BTB and BT suites have Fo60–92. Hornblendes in BTB, TT and BT suites are commonly magnesio-hastingsite and rare pargasite in composition (Mg#: 0.67–0.90). Brown mica is mainly phlogopite with Mg# ranging from 0.56 to 0.92 in the BTB suite, 0.59–0.84 in the TT suite, and 0.75–0.93 in the BT suite. Analcime is present only in the BT suite rocks. Fe–Ti oxides in all suites are mainly composed of magnetite and titanomagnetite. Textural petrographic and mineral chemical data suggest that magmas had undergone hydrous and anhydrous crystallizations in deep-, mid-, and shallow-crustal magma chambers. Clinopyroxene thermobarometric calculations show that Eocene magma chambers were characterized by temperature ranging from 1,100 to 1,244 °C and pressure ranging from 1.84 to 5.39 kbar. Similarly, the Miocene magma chambers were characterized by temperature ranging from 1,119 to 1,146 °C and pressure ranging from 4.23 to 4.93 kbar. Hornblende thermobarometry, oxygen fugacity, and hygrometer reveal that the crystallization temperature of Eocene volcanics range from 956 to 959 °C at pressure ranging from 6.49 to 6.52 kbar. Eocene volcanics were characterized by water content ranging from 7.83 to 8.57 wt.% and oxygen fugacity of 10?9.36 to 10?9.46 (ΔNNO+2). Miocene volcanics had crystallization temperature ranging from 970 to 978 °C at pressure ranging from 8.70 to 9.00 kbar with water content ranging from 8.04 to 8.64 wt.% and oxygen fugacity ranging from 10?8.75 to 10?8.87 (ΔNNO+2). Brown mica thermobarometric data show that Eocene volcanics were characterized by relatively high oxygen fugacity varying from 10-10.32 to 10-12.37 (HM) at temperature ranging from 858 to 953 °C and pressure ranging from 1.08 to 1.41 kbar. Miocene volcanics were crystallized at highly oxidized conditions, which are characterized by high oxygen fugacity of 10?12.0 (HM) at temperature of 875 °C and pressure of 2.09 kbar. The wide range of obtained temperatures for clinopyroxenes of the suites denotes that the equilibration of clinopyroxene crystals initiates from depth until close to the surface before magma eruption. The compositional variations, resorbed core and reverse zoning patterns in clinopyroxene phenocrysts, as well as variable pressures of crystallization, further indicate that the magmas that formed the suites were polybaric in origins and were composite products of more than one petrogenetic stage. The observed range of phenocryst assemblage and different compositional trends possibly originated from fractionation of magmas with different initial water contents under variable pressures of crystallization. The repeated occurrence of magmas from different suites during a single period of activity suggests that the magmatic system consists of several conduit systems and that magma reservoirs are dispersed at different levels of crustal magma chambers.  相似文献   

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