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1.
Investigations described from three Victorian damsites indicate that weathering of Ordovician bedrock is increased where it is overlain by basalt, even though the basalt is little weathered. Sub‐basaltic weathering may account for the fact that weathered rock beneath a Jurassic lava flow gives a weathering remagnetization age of Mid‐Cainozoic, consistent with palaeomagnetically determined ages of lateritization from elsewhere in Australia.  相似文献   

2.
New versions of the universal Jd-Di exchange clinopyroxene barometer for peridotites,pyroxenites and eclogites,and also garnet barometer for eclogites and peridotites were developed.They were checked using large experimental data sets for eciogitic(~530) and peridotitic systems(650).The precision of the universal Cpx barometer for peridotites based on Jd-Di exchange is close to Cr-Tschermalite method produced by Nimis and Taylor(2000).Cpx barometer was transformed by the substitution of major multiplier for K_D by the equations dependent from Al-Na-Fe.Obtained equation in combination with the thermometer of Nimis and Taylor(2000) allow to reconstruct position of the magma feeder systems of the alkali basaltic magma within the mantle diapirs in modern platforms like in Vitim plateau and other Southern Siberia localities and several localities worldwide showing good agreement of pressure ranges for black and green suites.These equations allow construct PTX diagrams for the kimberlite localities in Siberia and worldwide calculating simultaneously the PT parameters for different groups of mantle rocks.They give very good results for the concentrates from kimberlite lamproites and placers with mantle minerals.They are useful for PT estimates for diamond inclusions.The positions of eclogite groups in mantle sections are similar to those determined with new Gar—Cpx barometer produced by C.Beyer et al.(2015).The Fe rich eclogites commonly trace the boundary between the lower upper parts of subcontinental lithospheric mantle(SCLM) at 3-4 CPa marking pyroxenite eclogites layer.Ca-rich eclogites and especially grospydires in SCLM beneath Precambrian kimberlites occurs near pyroxenite layer but in younger mantle sections they became common in the lower parts.The diamondiferous Mg Cr-less group eclogites referring to the ancient island arc complexes are also common in the middle part of mantle sections and near 5-6 GPa.Commonly eclogites in lower apart of mantle sections are remelted and trace the high temperature convective branch.The Mg-and Fe-rich pyroxenites also show the extending in pressure trends which suggest the anatexic melting under the influence of volatiles or under the interaction with plums.  相似文献   

3.
We document the staircase of terraces of the River Tigris in the Diyarbakır area of SE Turkey, in the northern Arabian Platform, and improve control on the ages of these terrace deposits by dating of overlying basalt flows using the unspiked K–Ar technique. These fluvial terraces are formed of polymict gravel, including clasts derived from the Anatolian metamorphic terrane farther north as well as of local basalt. At least 9 Tigris terraces have been recognised so far, the highest of which, ∼200 m above present river level, marks the local transition from stacked deposition to fluvial incision, the timing of which is bounded between the mid Late Miocene and the Middle Pliocene. Our K–Ar dating indicates a hiatus in fluvial incision in the late Early Pleistocene, as basalts dated to 1.22 ± 0.02 and 1.07 ± 0.03 Ma overlie Tigris gravels at very similar levels, ∼60–70 m above the present river. The lower terraces record the subsequent entrenchment of the modern Tigris valley following an increase in incision rates in the early Middle Pleistocene, evident from the disposition of younger basalt, dated to 0.43 ± 0.02 Ma, capping fluvial gravel only ∼21–22 m above the present river level. Numerical modelling can account for the observed uplift history, as the response to coupling between surface processes and induced flow in the lower crust, with the mobile lower-crust thin (∼5–7 km thick), consistent with the known presence of a thick layer of mafic underplating at the base of the crust beneath the Arabian Platform. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

4.
The Plio-Quaternary Balhaf–Bir Ali volcanic field (BBAVF) constitutes one of the largest volcanic fields in SE Yemen, covering some 500 km2. It comprises cinder cones complexes associated with vesicular lava flows and scoria–spatter cones. In many places, ultramafic xenoliths are encountered within these volcanics. The explosive volcanism is mainly of alkaline character including alkali olivine basalt, hawaiite and mugearite together with subordinate tuffaceous trachytes. Major, trace and REEs data from the basaltic rocks of the BBAVF are interpreted in terms of a mantle-lithospheric origin in which crustal contribution during the initial stage of rift magmatism has occurred. Magma genesis may have resulted from plume-derived melt introducing into the base of the lithosphere. A mantle plume source is proposed for the Balhaf–Bir Ali basaltic lavas that are here interpreted as having been generated by partial melting of garnet lherzolite in the uppermost part of asthenosphere. The magmatic evolution of Balhaf–Bir Ali volcanic field may be accounted for by the recent models developed for plume structure.  相似文献   

5.
《Gondwana Research》2011,19(4):596-610
There are large areas of Permian basaltic rocks in the Tarim basin (PBRT) in northwestern China. Precise Ar–Ar dating of these rocks revealed an eruption age span of 262 to 285 Ma. Most of the PBRT is composed of alkaline basaltic rocks with high TiO2 (2.43%–4.59%, weight percent), high Fe2O3 + FeO (12.63%–17.83%) and P2O5 (0.32%–1.38%) contents. Trace elements of these rocks have affinities with oceanic island basalts (OIB), as shown in chondrite normalized rare earth elements (REE) diagrams and primitive mantle normalized incompatible elements diagrams. The rocks show complex Sr–Nd isotopic character based on which they can be subdivided into two distinct groups: group 1 has relatively small initial (t = 280 Ma)87Sr/86Sr ratio (∼ 0.7048) and positive εNd(t) (3.42–4.66) values. Group 2 has relatively large initial 87Sr/86Sr ratio (0.7060–0.7083) and negative εNd(t) (from − 2.79 to − 2.16) values. Lead isotopes are even more complex with variations of (206Pb/204Pb)t, (207Pb/204Pb)t and (208Pb/204Pb)t ranging from 17.9265 to 18.5778, 15.4789 to 15.6067 and 37.2922 to 38.1437, respectively. Moreover, these two groups have different trace elements ratios such as Nb/La, Ba/Nb, Zr/Nb, Nb/Ta and Zr/Hf, implying different magmatic processes. Based on the geochemistry of basaltic rocks and an evaluation of the tectonics, deformation, and the compositions of crust and lithospheric mantle in Tarim, we conclude that these basaltic rocks resulted from plume–lithosphere interaction. Permian mantle plume caused an upwelling of the Tarim lithosphere leading to melting of the asthenospheric mantle by decompression. The magma ascended rapidly to the base of lower crust, where different degrees of assimilation of OIB-like materials and fractionation occurred. Group 1 rocks formed where the upwelling is most pronounced and the assimilation was negligible. In other places, different degrees of assimilation and fractionation account for the geochemical traits of group 2.  相似文献   

6.
《International Geology Review》2012,54(10):1163-1179
Native copper is widespread in the Lower Cretaceous Paraná basaltic province, southern Brazil, both as films in fractures and as massive balls in amygdules. The focus of this investigation is on the large concentration of occurrences (n = 85) that forms the Vista Alegre district in the border region of Rio Grande do Sul and Santa Catarina states. The high average of 220 ppm Cu content of the basalts resulted in ore of native copper, Cu oxides, abundant chrysocolla in the top of mineralizations, and minor malachite and azurite. Native copper is associated with dioctahedral and trioctahedral smectites, zeolites (heulandite and clinoptilonite), quartz, and calcite, typical of a low-T (100–150°C) hydrothermal alteration assembly. The PGE distribution shows enrichment in Pd in relation to Pt both in basalts and in native copper, supporting the hypothesis of hydrothermal origin of the mineralization. No evidence was found of direct precipitation of copper from the lava; based on field and petrographic evidence, integrated with BSE images, EPMA analyses, EGP contents of native copper, and bulk rock analyses, this is an epigenetic hydrothermal copper mineralization, followed by supergene enrichment.  相似文献   

7.
Virtually all of the economic Ni–Cu–(platinum group element (PGE)) mineralization in the central part of the Cape Smith Belt of New Québec is hosted by thick olivine cumulate units in the Katinniq Member of the Raglan Formation at the base of the 1.9 Ga Chukotat Group. These units transgress underlying gabbros and pelitic metasediments, forming 50–200-m deep and 300–1,000-m wide V-shaped embayments and have been interpreted on the basis of surface geology, deep diamond core drilling, and magnetic inversion models to represent the remnants of one or more large, long (at least 20 km, possibly ≥50 km), sinuous, komatiitic basalt lava channels that formed by thermomechanical erosion of their substrates. We have used a mathematical model to test these hypotheses regarding komatiitic lava emplacement and erosion by lava. Our modeling predicts that an initially 10-m thick komatiitic basalt flow should have flowed turbulently near the vent and should have thermomechanically eroded unconsolidated pelitic sediment during emplacement to reach the observed degree of contamination of ≤10% at distances of ~30–60 km downstream from the source. Furthermore, our models predict that, at these distances downstream, a fully inflated 100-m thick komatiitic basalt flow would have had thermal erosion rates over consolidated gabbroic substrate of ~0.7–1.5 m/day, requiring ~70–140 days to incise a 100-m deep channel, depending on the initial temperature of the lava, the paleoslope, and the initial temperature and solidus temperature of the gabbro. These erosion rates would have been associated with volumetric flow rates of >105–106 m3/s and eruption volumes of >103–104 km3. Although these flow rates are orders of magnitude larger than those of most modern terrestrial basaltic flows, they are of the same order as those estimated for the largest terrestrial flood basalt flows and with those inferred for some of the largest extraterrestrial flows. Our predicted flow volumes are also of the same order as those of the largest terrestrial flood basalt units, consistent with the great thickness and widespread distribution of the Chukotat Group. Our modeling of thermomechanical erosion of gabbro by komatiitic basalt results in negligible contamination (<1%), and geochemical studies show that the spatially and petrogenetically related Chukotat basalts are uncontaminated, suggesting that the observed enrichments in U–Th–Light Rare Earth Elements (REE) > Middle REE–Heavy REE > Nb–Ta–Ti represent contamination by underlying Povungnituk semipelites. This result is consistent with present models for the genesis of the Ni–Cu–(PGE) mineralization in the Raglan Formation that involve thermomechanical erosion of unconsolidated, sulfidic semipelitic sediments, and decoupling of the miscible silicate and immiscible sulfide components.  相似文献   

8.
The Auckland volcanic field is a Quaternary monogenetic basaltic field of 50 volcanoes. Rangitoto is the most recent of these at ~500 year BP and may mark a change in the behaviour of the field as it is the largest by an order of magnitude and is unusual in that it erupted magmas of alkalic then subalkalic basaltic composition in discrete events separated by ≤50 years. Major and trace element geochemistry together with Sr–Nd and U-Th–Ra isotopes provides the basis for modelling the melting conditions that brought about the eruption of two chemically different lavas with very little spatial or temporal change. Sr–Nd isotopes suggest that the source for both eruptions is similar with a slight degree of heterogeneity. The basalts show high 230Th-excess compared with comparable continental volcanic fields. We show that the alkalic basalts give evidence for lower degrees of partial melting, higher amounts of residual garnet, a longer melting column and lower melting and upwelling rates compared with the subalkalic basalts. The low upwelling rates (0.1–1.5 cm/year) modelled for both magmas do not suggest a plume or major upwelling in the mantle region beneath Auckland; therefore, we suggest localised convection due to relict movement from the active subduction system situated 400 km to the southeast. A higher porosity for the initial alkalic basalt is based on 226Ra-excesses, suggesting movement of melt by two different porosities: the initial melt travelling in fast high porosity channels from greater depths preserving a high 230Th-excess and the subsequent subalkalic magma travelling from a shallower depth through lower porosity diffuse channels preserving a high 226Ra-excess; this creates a negative array in (226Ra/230Th) versus (230Th/238U) space previously only seen in mid ocean ridge Basalt data. This mechanism suggests the Auckland volcanic field may operate by the presence of discrete melt batches that are able to move at different depths and speeds giving the field its erratic spatial and temporal pattern of eruptions, a type of behaviour that may have implications for the evolution of other continental volcanic fields worldwide.  相似文献   

9.
Basaltic porphyries from the northeast North China craton (NCC) provide an excellent opportunity to examine the nature of their mantle source and the secular evolution of the underlying mantle lithosphere. In addition, the study helps to constrain the age and the mechanism of NCC lithospheric destruction. In this paper, we report geochronological, geochemical, and Sr–Nd isotopic analyses of a suite of mafic lavas. Detailed laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb dating yielded an age of 223.3 ± 1.1 million years, which we regard as representing the crystallization age of the basaltic porphyries. The bulk-rock analysed samples are enriched in both large ion lithophile elements (LILEs) (i.e. Ba, Sr, and Pb) and light rare earth elements (LREEs), but depleted in high field strong elements (HFSEs) (i.e. Nb, Ta, Zr, Hf, and Ti) and heavy rare earth elements (HREEs), without significant Eu anomalies (Eu/Eu*?= 089–0.98). The basaltic porphyries have undergone low degrees (～5%) of partial melting of a garnet-bearing lherzolite mantle. The rocks display very uniform (87Sr/86Sr) i (0.70557–0.70583) and negative ?Nd (t) values (–11.9 to –10.1). These features indicate that the western Liaoning basaltic porphyries were derived from a common enriched lithosphere mantle that had previously been metasomatized by fluids related to subduction of Palaeo-Asian sedimentary units. However, the mafic melts were not affected to a significant degree by crustal contamination. Based on earlier studies, these findings provide new evidence that the northeast margin of the NCC had undergone a phase of post-orogenic extensional tectonics during the Middle Triassic. Furthermore, lithospheric thinning occurring across the northern NCC might have been initiated during Early Triassic times and was likely controlled by the final closure of the Palaeo-Asian Ocean, as well as the collision of Mongolian arc terrenes with the NCC.  相似文献   

10.
Zircon-bearing xenoliths in continental basalts are often interpreted as witnesses of the continental basement uplifted during volcanic eruptions. Nevertheless, their origin is still debated. The Devès basaltic plateau belongs to the alkaline volcanic province of the French Massif Central. In few outcrops, zircon-bearing nepheline syenite xenoliths were preserved. U–Pb dating of the zircon crystals define an age of 956 ± 11 kyr constraining the crystallisation time of the zircons and consequently of the host xenoliths. This age, together with mineral chemistry arguments lead us to conclude that these minerals do not derive from a continental protolith. Rather, they likely result from the crystallisation of a liquid characterised by a nepheline–felspar composition and produced by the differentiation of a basaltic magma or, alternatively, by the low degree partial melting of a metasomatised lithospheric mantle. Such alkaline sialic rock and xenoliths may occur in large volumes at depth and generate the large amounts of zircon megacrysts discovered worldwide in secondary deposits within continental basaltic provinces.  相似文献   

11.
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12.
The paper presents results of testing currently used models proposed to describe Cr-spinel–melt equilibrium: models of the MELTS family by M.S. Ghiorso with colleagues, the SPINMELT program by A.A. Ariskin and G.S. Nikolaev, and the “MELT–CHROMITE spinel calculator” by А.А. Pustovetov and R.L. Roeder. The new calibration of the SPINMELT model presented in this publication enables calculating a sixcomponent (Mg, Fe2+, Cr, Al, Fe3+, and Ti) composition of Cr-spinel and the \(T - {f_{{o_2}}}\) parameters of its stability on the liquidus of basaltic melts under pressures up to 15 kbar. The model is based on results of 392 runs from 43 experimental studies, including systems of normal alkalinity at \({f_{{o_2}}}\) ≤ QFM + 2. The experimental dataset (which was extended compared to that used for the previous calibration) allowed us not only to estimate the pressure effect, but also apply the model to aluminous and hydrous systems. Tests of the SPINMELT-2.0 model show that the errors of the calculated temperature of the spinel–melt equilibrium increase with pressure from 16°C at 1 atm to 50°C at 15 kbar. Experimental spinel compositions are reproduced by the model accurate to < 3 at % Al and Cr, and no worse 1 at % for the other cations.  相似文献   

13.
Barrington shield volcano was active for 55 million years, based on basalt K–Ar and zircon fission track dating. Activity in the northeast, at 59 Ma, preceded more substantial activity between 55 and 51 Ma and more limited activity on western and southern flanks after 45 Ma. Eruptions brought up megacrystic gemstones (ruby, sapphire and zircon) throughout the volcanism, particularly during quieter eruptive periods. Zircon fission track dating (thermal reset ages) indicates gem‐bearing eruptions at 57, 43, 38, 28 and 4–5 Ma, while U–Pb isotope SHRIMP dating suggests two main periods of zircon crystallisation between 60 and 50 Ma and 46–45 Ma. Zircons show growth and sector twinning typical of magmatic crystallisation and include low‐U, moderate‐U and high‐U types. The 46 Ma high‐U zircons exhibit trace and rare‐earth element patterns that approach those of zircon inclusions in sapphires and may mark a sapphire formation time at Barrington. Two Barrington basaltic episodes include primary lavas with trace‐element signatures suggesting amphibole/apatite‐enriched lithospheric mantle sources. Other basalts less‐enriched in Th, Sr, P and light rare‐earth elements have trace‐element ratios that overlap those of HIMU‐related South Tasman basalts. Zircon and sapphire formation is attributed to crystallisation from minor felsic melts derived by incipient melting of amphibole‐enriched mantle during lesser thermal activity. Ruby from Barrington volcano is a metamorphic type, and a metamorphic/metasomatic origin associated with basement ultramafic bodies is favoured. Migratory plate/plume paths constructed through Barrington basaltic episodes intersect approximately 80% of dated Palaeogene basaltic activity (65–30 Ma) along the Tasman margin (27–37°S) supporting a migratory plume‐linked origin. Neogene Barrington activity dwindled to sporadic gem‐bearing eruptions, the last possibly marking a minor plume trace. The present subdued thermal profile in northeastern New South Wales mantle suggests future Barrington activity will be minimal.  相似文献   

14.
Olivine-hosted melt inclusions have been analyzed from the young (4,150 ± 300 ybp) Dotsero basaltic (48.2 wt% SiO2) lava flow in Northwest Colorado, USA. Silicate melt-inclusion compositions have a bimodal distribution (41–46 wt% SiO2 and 47–50 wt% SiO2). Low-Si melt inclusions record high pre-eruptive sulfur concentrations (>1,000 ppm S) and variations in their major- and trace-element compositions appears to be related to shallow assimilation of local basement sandstone. Whole-rock compositions are modeled as a contamination of low-Si inclusion compositions with ~10 wt% sandstone. Host olivine crystallization may have accompanied magma injection into a shallow storage chamber. In contrast to the low-Si melt inclusions, the high-Si population is relatively degassed and records late-stage rapid crystallization either during or post-eruption. Hopper or skeletal olivine grains in conjunction with the bimodal inclusion compositions suggest relatively rapid cooling rates at the time of eruption and inclusion entrapment. Inclusion compositions, in conjunction with mineral textures, therefore provide a more complete picture of shallow magma processes, coupling the relative timing of undercooling and crystallization, assimilation and melt compositional evolution. Most of the inclusion and host textural and compositional data indicates late and very shallow petrogenetic processes and does not appear to record deeper (mid-, lower-crustal) processes.  相似文献   

15.
To better understand the formative mechanism of the Cretaceous Gyeongsang Basin in South Korea, we determined the geochemical compositions of Early Cretaceous syntectonic basaltic rocks intercalated with basin sedimentary assemblages. Two distinct compositional groups appeared: tholeiitic to calc-alkaline basalts from the Yeongyang sub-basin and high-K to shoshonitic basaltic trachyandesites from the Jinju and Uiseong sub-basins. All collected samples exhibit patterns of light rare earth element enrichment and chondrite-normalized (La/Yb)N ratios ranging from 2.4 to 23.6. In a primitive-mantle-normalized spidergram, the samples show distinctive negative anomalies in Nb, Ta, and Ti and a positive anomaly in Pb. The basalts exhibit no or a weak positive U anomaly in a spidergram, but the basaltic trachyandesites show a negative U anomaly. The basalts have highly radiogenic Sr [(87Sr/86Sr)i = 0.70722–0.71145], slightly negative εNd, positive εHf [(εNd)i = −2.7 to 0.0; (εHf)i = +2.9 to +6.4], and radiogenic Pb isotopic compositions [(206Pb/204Pb)i = 18.20–19.19; (207Pb/204Pb)i = 15.60–15.77; (208Pb/204Pb)i = 38.38–39.11]. The basaltic trachyandesites are characterized by radiogenic Sr [(87Sr/86Sr)i = 0.70576–0.71119] and unradiogenic Nd, Hf, and Pb isotopic compositions [(εNd)i = −14.0 to −1.4; (εHf)i = −17.9 to +3.7; (206Pb/204Pb)i = 17.83–18.25; (207Pb/204Pb)i = 15.57–15.63; (208Pb/204Pb)i = 38.20–38.70]. The “crust-like” signatures, such as negative Nb–Ta anomalies, elevated Sr isotopic compositions, and negative εNd(t) and εHf(t) values, of the basaltic trachyandesites resemble the geochemistry of Early Cretaceous mafic volcanic rocks from the southern portion of the eastern North China Craton. Considering the lower-crust-like low U/Pb and high Th/U ratios and the unradiogenic Pb isotopic compositions, the basaltic trachyandesites are considered to be derived from lithospheric mantle modified by interaction with melts that originated from foundered eclogite. Basaltic volcanism in the Yeongyang sub-basin is coeval with the basaltic trachyandesite magmatism, but it exhibits an elevated 87Sr/86Sr ratio at a given 143Nd/144Nd and highly radiogenic Pb isotopic compositions, which imply an origin from an enriched but heterogeneous lithospheric mantle source. Melts from subducted altered oceanic basalt and pelagic sediments are considered to be the most likely source for the metasomatism. An extensional tectonic regime induced by highly oblique subduction of the Izanagi Plate beneath the eastern Asian margin during the Early Cretaceous might have triggered the opening of the Gyeongsang Basin. Lithospheric thinning and the resultant thermal effect of asthenospheric upwelling could have caused melting of the metasomatized lithospheric mantle, producing the Early Cretaceous basaltic volcanism in the Gyeongsang Basin.  相似文献   

16.
High-Ti melanephelinite (3.8–5.9 wt% TiO2), medium-Ti (phono)tephrite (2.7–3.1 wt% TiO2), and low-Ti olivine melanephelinite/basanite (1.9–2.3 wt.% TiO2) are subordinate rock types in the central European Cenozoic Volcanic Province. A contrasting melanephelinite to (phono)tephrite series occurs in the Lou?ná–Oberwiesenthal Volcanic Centre (37–28 Ma) and also as satellite volcanic bodies (26–12 Ma) together with olivine melanephelinite/basanite (17–20 Ma) on the southwestern periphery of the Kru?né hory mountains (Erzgebirge). The volcanic rocks intrude the Variscan basement of the uplifted shoulder of the Oh?e/Eger Rift in the Kru?né hory mountains of the Bohemian Massif. Low Mg# (44–59) and Cr, Ni contents and enrichment of LILE, Zr, Hf, Nb, Ta, U, Th and LREE in the high-Ti melanephelinites contrast with the composition of primitive low-Ti olivine melanephelinites/basanites displaying high Mg# (63–74) and high contents of compatible elements. The high-Ti melanephelinites reveal a wide range in initial 87Sr/86Sr of ca. 0.7034–0.7038 and εNd of 2.4–4.9. The low-Ti melanephelinites show an overlapping range of initial 87Sr/86Sr of ca. 0.7035–0.7036 and εNd of 4.3–5.5. The large variation in initial 87Sr/86Sr ratios at similar εNd values in those rock types is interpreted as evidence for melting of metasomatized lithospheric mantle sources comprising K-bearing phases with radiogenic Sr. Modification of the olivine-free alkali basaltic magmas by differentiation or crustal contamination could give rise to the medium-Ti (phono) tephrites. The initial isotope ratios of all samples are consistent with HIMU-mantle sources and contributions from lithospheric mantle. The olivine-free melanephelinitic rocks often contain alkali pyroxenite–ijolite xenoliths with initial 87Sr/86Sr ratios of ca. 0.7036 and εNd of 3.0. We interpret these xenoliths as samples of an intra-crustal alkali complex derived from similar mantle sources as those for the basaltic volcanic rocks.  相似文献   

17.
The Emeishan continental flood basalt, which is widespread in Yunnan, Guizhou and Sichuan provinces of Southwest China, is the volcanic product of a Permian mantle plume, and native copper-chalcocite mineralization associated with the basalt is very common in the border area of Yunnan and Guizhou provinces. The mineralization occurred in the tuff intercalation and terrestrial sedimentary rock intercalation which were formed during the main period of basalt eruption. The orebodies are controlled by the stratigraphic position and faults. Metal ore minerals in the ores are mainly native copper, chalcocite and tenorite, with small amounts of chalcopyrite, bomite, pyrite and malachite, and sometimes with large amounts of bitumen, carbon and plant debris. Several decades of ore deposits are distributed in the neighboring areas of the two provinces, while most of them are small-scale deposits or only ore occurrences. By comparing the lead isotopic composition of the ores with that of the wall-rocks, cover and basement rocks of various periods, the source of copper in this type of ore deposits was studied in this paper. The results showed that: (1) The Pb isotopic composition of the ores from ten deposits is absolutely different from that of sili-ceous-argillaceus rocks of the Upper Permian Xuanwei Formation, limestones of the Lower Permian Series and Carboniferous, Cambrian sandstone-shale and recta-sedimentary rock and dolomite from the upper part of the Meso-Proterozoic Kunyang Group, This indicates that ore lead was derived neither from the cover rock nor from the basement rocks; (2) Although the Neo-Proterozoic Siman dolomite and silicalite, and dolomite in the lower part of the Kunyang Group are similar in Pb isotopic composition to the ores, lead and copper contents in these rocks are very low and they have not made great contributions to copper mineralization; (3) The ores have the same Pb iso-topic composition as the basalt, the latter being enriched in copper. These facts indicate that lead and copper were derived from the basalt. According to the regional geological data and the geological-geochemical characteristics of the ore deposits, it is suggested that ore-forming materials were leached out from the basalt. The thickness and buried depth of the basalt and regional tectonic dynamics can affect the formation of large-scale copper deposits. Therefore, exploration for this type of ore deposits should be conducted in the areas from western Yunnan to western Sichuan, where there are developed basalts of great thickness, with extensive tectonic movement and magmatic activity.  相似文献   

18.
We conducted melting experiments on a low MgO (3.29 wt.%) basaltic andesite (54.63 wt.% SiO2) from Westdahl volcano, Alaska, at XH2O = 0.7–1 and fO2 ~ Ni–NiO, at pressures = 0.1–180 MPa and temperatures = 900–1,200 °C. We examine the evolution of the melt along a liquid line of descent during equilibrium crystallization at high H2O and fO2 conditions, starting from a high FeOt/MgO, low MgO basaltic andesite. Ti-magnetite formed on the liquidus regardless of XH2O, followed by clinopyroxene, plagioclase, amphibole, and orthopyroxene. We observe slight but significant differences in the phase stability curves between the XH2O = 1 and 0.7 experiments. Early crystallization of Ti-magnetite and suppression of plagioclase at higher pressures and temperatures resulted in strongly decreasing melt FeOt/MgO with increasing SiO2, consistent with a “calc-alkaline” compositional trend, in agreement with prior phase equilibria studies on basalt at similar H2O and fO2. Our study helps quantify the impact of small amounts of CO2 and high fO2 on the evolution of melts formed during crystallization of a low MgO basaltic andesite magma stored at mid- to shallow crustal conditions. Like the prior studies, we conclude that H2O strongly influences melt evolution trends, through stabilization of Ti-magnetite on the liquidus and suppression of plagioclase at high P–T conditions.  相似文献   

19.
《Chemical Geology》1999,153(1-4):259-279
The corrosion of basaltic dykes from central Germany which intruded during the Tertiary into rock and potash salts of Upper Permian age (Zechstein) has been studied. The corrosion behavior of these glassy basalts can serve as a natural analogue for the long-term corrosion expected for nuclear waste glasses in a salt repository. Our data demonstrate mobility and fractionation of the rare earth elements (REE) during a postintrusive circulation of salt brines. The processes controlling this behavior of the REE were dissolution and reprecipitation of phosphates. The K–Ar data document the synintrusive assimilation of a salt phase followed by a postintrusive fluid circulation in the peripheral parts. This circulation removed the assimilated salt and imported highly radiogenic salt Ar which was integrated into newly formed sheet silicates. The central part of the dyke was not affected by this circulation and could retain a mantle gas phase. The Sr isotope data document a synintrusive assimilation of salt Sr and a strong exchange with salt fluids during the postintrusive corrosion which was strongest in a zone next to the chilled dyke margins. This locally stronger alteration is probably due to different cooling and shrinking rates of the basalt at the margins and in the core of the dyke which led to the formation of a highly permeable fractured zone.  相似文献   

20.
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