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1.
http://www.sciencedirect.com/science/article/pii/S1674987113000327   总被引：1，自引：0，他引：1  
The Panzhihua intrusion in southwest China is part of the Emeishan large igneous province and host of a large Fe-Ti-V ore deposit.In previous interpretations it was considered to be a layered,differentiated sill with the ore deposits at its base.New structural and petrological data suggest instead that the intrusion has an open S-shape,with two near-concordant segments joined by a discordant dyke-like segment. During emplacement of the main intrusion,multiple generations of mafic dykes invaded carbonate wall rocks,producing a large contact aureole.In the central segment,magmatic layering is oriented oblique to the walls of the intrusion.This layering cannot have formed by crystal settling or in-situ growth on the floor of the intrusion;instead we propose that it resulted from inward solidification of multiple,individually operating,convection cells.Ore formation was triggered by interaction of magma with carbonate wall rocks.  相似文献   

2.
http://www.sciencedirect.com/science/article/pii/S1674987113000315   总被引：2，自引：0，他引：2  
The Panzhihua gabbroic intrusion,part of the plumbing system of the Emeishan large igneous province, intruded late-Proterozoic dolomites and marls about～263 Ma ago.The dolomites in the contact aureole were converted to brucite marbles and a diverse suite of forsterite,diopside and garnet skarns.The variation in mineralogy is explained in part by differences in the composition of the protolith,particularly the proportion of silica minerals and clay,and in part by transfer of elements from intruding magmas.The trace element compositions of most marbles and skarns are very similar to those of unmetamorphosed dolomites and marls,but some contain high Si,Ti,and Fe contents that are interpreted to have come from a magmatic source.Three brucite marbles sampled～10 m from the contact of the intrusion and named "enriched brucite marble" have trace element compositions very different from their dolomitic protolith:their rare earth elements are strongly enriched whereas levels of Nb-Ta,Zr-Hf and Ti are very low.These characteristics resemble those of carbonate liquid in equilibrium with silicate liquid or more probably with silicate minerals in the case of Panzhihua,a similarity we take to indicate that the sample underwent partial melting.Samples taken up to 300 m from the contact contain brucite indicating that high temperatures persisted well into the country rocks.However,other samples collected only tens of metres from the contact are only slightly recrystallized indicating that conditions in the aureole were highly variable.We suggest that temperatures within the aureole were controlled by conduction of heat from the main intrusion and by supply of additional heat from abundant small dykes within the aureole.Circulation of fluids derived from deeper levels in the aureole flushed the carbon dioxide from the dolomite,lowering temperature needed to partially melt carbonate to the temperatures attained near the intrusion.Irregular but extensive heating destabilized the carbonates of the aureole and decarbonation reactions associated with carbonate breakdown and melting emitted a large volume of CO2,with potential impact on global climate.  相似文献   

3.
Geologic, petrographic and petrochemical studies of the late Mesozoic K-rich melanocratic dykes, including lamprophyres, andesite porphyrites and dacite-porphyry in the gold field system in the Jiaodong Peninsula, China, have shown that these dykes are characterized by rich potassium and alkali but poor titanium. They belong to an ultra-high potassic, shoshonitic and high potassic calc-alkaline rock series. The parental magma has relatively high initial strontium ratios ((87Sr/86Sr)i=0.70895-0.71140) and low (143Nd/144Nd)i ratios (varying from 0.51135 to 0.51231); and its δ18OSMow. whole rock values vary from +5.8‰ to +10.6‰ with a mean of+7.1‰. These features suggest that the source region of the magma is an enriched mantle wedge transformed from a continental lithosphere mantle which has experienced metasomatism by mantle-derived fluids with H2O-dominated fluids that were provided during the underthrusting of an ocean crust. The initial magma was generated by low-degree partial melting of the enri  相似文献   

4.
Recent work on the Panzhihua intrusion has produced two separate models for the crystallisation of the intrusion:（1） low-Ti,high CaO and low H2O（0.5 wt.%） parent magma（equivalent to Emeishan low-Ti basalt） at FMQ;and（2） high-Ti,low CaO and higher H2O（>1.5 wt.%） parent magma（equivalent to Emeishan high-Ti basalt） at FMQ + 1.5.Modelling of these parent magma compositions produces significantly different results. We present here detailed f（O2） and H2O modelling for average compositions of both Emeishan high-Ti and low-Ti ferrobasalts in order to constrain the effects on crystallisation sequences for Emeishan ultra-mafic -mafic layered intrusions.Modelling is consistent with numerous experimental studies on ferro-basaltic magmas from other localities（e.g.Skaergaard intrusion）.Modelling is compared with the geology of the Panzhihua intrusion in order to constrain the crystallisation of the gabbroic rocks and the Fe-Ti oxides ore layers.We suggest that the gabbroic rocks at the Panzhihua intrusion can be best explained by crystallisation from a parent magma similar to that of the high-Ti Emeishan basalt at moderate H2O contents（0.5-1 wt.%） but at the lower end of TiO2 content for typical high-Ti basalts（2.5 wt.%TiO2）. Distinct silicate disequilibrium textures in the Fe-Ti oxide ore layers suggest that an influx of H2O may be responsible for changing the crystallisation path.An increase in H2O during crystallisation of gabbroic rocks will result in the depression of silicate liquidus temperatures and resultant disequilibrium with the liquid.Continued cooling of the magma with high H2O then results in precipitation of Mt-Uv alone. The H2O content of parent magmas for mafic layered intrusions associated with the ELIP is an important variable.H2O alters the crystallisation sequence of the basaltic magmas so that at high H2O and f（O2） Mt -Uv crystallises earlier than plagioclase and clinopyroxene.Furthermore,the addition of H2O to an anhydrous magma can explain silicate disequilibrium texture observed in the Fe-Ti oxide ore layers.  相似文献   

5.
The granitioids in question are located in the geosynclinal fold belt of the Northern Tianshan Mountains.The magmas are intruded into a Carboniferous marine volcanic-sedimentary rock series. forming a contempo-raneous. intermediate-basic to acid. composite intrusion. With a whole-rock Rb-Sr isochron age of 315.7 Ma. itis considered to be of middle Heicynian age. The granitoids have a SiO_2 content of 48-78%, a calc-alkali indexof 60 and silica-alkali indices ρ=2.2-2.4 and σ=1.5-2. From basic to acid compositions, the total REE con-tent increases from 38 to 143 ppm. δEu ranges from 0.47 to 0.86 and the Eu anomalies are negative. Thenormalized curves of REE of various rock types are very similar. These coupled with the characteristics of traceelement geochemistry show that the intrusion is an island arc-continental margin, calc-alkaline one. Theδ~(18)O values range from 5.8 to 8.9‰ and the ~(87)Sr/~(86)Sr initial ratio is 0.705. The materials of the intrusion arecomposed of a mixture of the magmas at the top of the upper mantle and in the lower crust, This indicates thatthe rocks might be consanguineous. differentiated and evolved products and possess the characteristics of Ⅰtype granites.  相似文献   

6.
The Mazaertag layered intrusion is located in the northwestern part of the Tarim large igneous province where several early Permian layered mafic-ultramafic intrusions host important Fe-Ti oxide deposits. The intrusion covers an area of ～0.13 km~2 and has a vertical stratigraphic thickness of at least300 m. It consists chiefly of olivine clinopyroxenite, and is cut through by the nearby mafic-ultramafic dykes. In this paper, we report new mineral chemistry data and whole-rock chemical and isotopic compositions for the Mazaertag intrusion along with whole-rock isotopic compositions for the nearby mafic dykes. The averaged compositions of cumulus olivine, clinopyroxene and intercumulus plagioclase within individual samples range from Fo_(71-73),Mg~# = 76 to 79 and An_(65-75) but they do not define sustained reversals. The observed mineral compositions are consistent with the differentiation of a single batch of magma in a closed system. Rocks of the Mazaertag intrusion are characterized by enrichment in light REE relative to heavy REE, positive Nb and Ta anomalies and a small range of age-corrected ε_(Nd)(t)(-0.1 to +0.9) and initial ~(87)Sr/~(86)Sr values(0.7044 to 0.7068). The slightly lower ε_(Nd)(t), initial ~(206)Pb/~(204)Pb and higher initial ~(87)Sr/~(86)Sr values of the intrusion compared to those of the least contaminated dykes[ε_(Nd)(t) =+2.8 to +3.4;(~(206)Pb/~(204)Pb)_i = 18.516-18.521;(~(87)Sr/~(86)Sr)_i = 0.7038-0.7041] imply that the Mazaertag magma was subjected to small to modest degrees of contamination by the upper crust. The Sr-Nd isotopic compositions of the least contaminated dykes are consistent with derivation from a FOZO-like mantle source. The parental magma of the Mazaertag intrusion, estimated from clinopyroxene compositions using mineral-melt partition coefficients, has trace element compositions similar to some of the most primitive mafic dykes in the same area. This suggests that the Mazaertag intrusion and mafic dykes shared a similar mantle source. Therefore, the parental magma of the Mazaertag intrusion was interpreted to have originated from a mantle plume. Based on the Cr_2 O_3 contents in titanomagnetite and less-evolved characteristics of the Mazaertag intrusion compared to the Wajilitag Fe-Ti oxide deposit in Bachu, it is speculated that there might not be a potential to find economic Fe-Ti oxide mineralization in the intrusion.  相似文献   

7.
There are two types of temporally and spatially associated intrusions within the Emeishan large igneous province (LIP); namely, small ultramafic subvolcanic sills that host magmatic Cu-Ni-Platinum Group Element (PGE)-bearing sulfide deposits and large mafic layered intrusions that host giant Ti-V magnetite deposits in the Panxi region. However, except for their coeval ages, the genetic relations between the ore-bearing intrusions and extrusive rocks are poorly understood. Phase equilibria analysis (Q-Pl-Ol-Opx-Cpx system) has been carried out to elucidate whether ore-bearing Panzhihua, Xinjie and Limahe intrusions are co-magmatic with the picrites and flood basalts (including high-Ti, low-Ti and alkali basalts), respectively. In this system, the parental magma can be classified as silica-undersaturated olivine basalt and silica-saturated tholeiite. The equivalents of the parental magma of the Xinjie and Limahe peridotites and picrites and low-Ti basalts are silica-undersaturated, whereas the Limahe gabbro-diorites and high-Ti basalts are silica-saturated. In contrast, the Panzhihua intrusion appears to be alkali character. Phase equilibria relations clearly show that the magmas that formed the Panzhihua intrusion and high-Ti basalts cannot be co-magmatic as there is no way to derive one liquid from another by fractional crystallization. On the other hand, the Panzhihua intrusion appears to be related to Permian alkali intrusions in the region, but does not appear to be related to the alkali basalts recognized in the Longzhoushan lava stratigraphy. Comparably, the Limahe intrusion appears to be a genetic relation to the picrites, whereas the Xinjie intrusion may be genetically related to be low-Ti basalts. Additionally, the gabbro-diorites and peridotites of the Limahe intrusion are not co-magmatic, and the former appears to be derived liquid from high-Ti basalts.  相似文献   

8.
The Panzhihua mafic intrusion, which hosts a world-class Fe-Ti-V ore deposit, is in the western Emeishan region, SW China. The formation age(～260 Ma), and Sr and Nd isotopes indicate that the Panzhihua intrusion is part of the Emeishan large igneous province and has little crustal contamination. To assess ore genesis of the Panzhihua Fe-Ti-V ore deposit, two different models have been provided to explain the formation, namely silicate immiscibility and normal fractional crystallization. Silicate...  相似文献   

9.
http://www.sciencedirect.com/science/article/pii/S1674987113000595   总被引：11，自引：0，他引：11  
Magmatic oxide deposits in the～260 Ma Emeishan Large Igneous Province(ELIP),SW China and northern Vietnam,are important sources of Fe,Ti and V.Some giant magmatic Fe-Ti-V oxide deposits, such as the Panzhihua,Hongge,and Baima deposits,are well described in the literature and are hosted in layered mafic-ultramafic intrusions in the Panxi region,the central ELIP.The same type of ELIP- related deposits also occur far to the south and include the Anyi deposit,about 130 km south of Panzhihua,and the Mianhuadi deposit in the Red River fault zone.The Anyi deposit is relatively small but is similarly hosted in a layered mafic intrusion.The Mianhuadi deposit has a zircon U-Pb age of～260 Ma and is thus contemporaneous with the ELIP.This deposit was variably metamorphosed during the Indosinian orogeny and Red River faulting.Compositionally,magnetite of the Mianhuadi deposit contains smaller amounts of Ti and V than that of the other deposits,possibly attributable to the later metamorphism.The distribution of the oxide ore deposits is not related to the domal structure of the ELIP.One major feature of all the oxide deposits in the ELIP is the spatial association of oxide-bearing gabbroic intrusions,syenitic plutons and high-Ti flood basalts.Thus,we propose that magmas from a mantle plume were emplaced into a shallow magma chamber where they were evolved into a field of liquid immiscibility to form two silicate liquids,one with an extremely Fe-Ti-rich gabbroic composition and the other syenitic.An immiscible Fe-Ti-(P) oxide melt may then separate from the mafic magmas to form oxide deposits.The parental magmas from which these deposits formed were likely Fe-Ti-rich picritic in composition and were derived from enriched asthenospheric mantle at a greater depth than the magmas that produced sulfide-bearing intrusions of the ELIP.  相似文献   

10.
A series of Cretaceous plutons is present in the eastern Pontides of northeastern Turkey.The Turnagl intrusion is the least studied and,thus,the least understood plutons in the orogen.This intrusion consists of hornblende-biotite granodiorites emplaced at 78 Ma based on LA-ICP-MS U-Pb zircon dating.It is of sub- alkaline affinity,belongs to the medium- to high-K calc-alkaline series,and displays features typical of Itype granites.The rocks of the intrusion are enriched in large-ion lithophile elements and light rare earthelements with negative Eu anomalies（Eu/Eu*= 0.69-0.82）,but are deficient in high-field-strength elements.They have a small range of (87Sr/86Sr)i（0.7060-0.7063）,εNdi（-2.6 to -3.1）,and δ18O（+8.1 to +9.1） values.Their Pb isotopic ratios are 206Pb/204Pb = 18.63-18.65,207Pb/204Pb = 15.62-15.63,and 208Pb/204Pb = 38.53-38.55.The fractionation of plagioclase,hornblende,and Fe-Ti oxides had key functions in the evolution of the Tumagl intrusion.The crystallization temperatures of the melts ranged from 758 to 885℃ as determined by zircon and apatite saturation thermometry.All these characteristics,combined with the low values of K2O/Na2O and（Na2O + K2O）/（FeOt+MgO + TiO2）,as well as the high values of （CaO + FeOt + MgO + TiO2）.suggest an origin by dehydration melting from a metabasaltic lowercrustal source.  相似文献   

11.
Peralkaline syenite and granite dykes cut the Straumsvola nepheline syenite pluton in Western Dronning Maud Land, Antarctica. The average peralkalinity index (PI?=?molecular Al/[Na?+?K]) of the dykes is 1.20 (n?=?29) and manifests itself in the presence of the Zr silicates eudialyte, dalyite and vlasovite, and the Na–Ti silicate, narsarsukite. The dykes appear to have intruded during slow cooling of the nepheline syenite pluton, and the petrogenetic relationship of the dykes and the pluton cannot be related to closed-system processes at low pressure, given the thermal divide that exists between silica-undersaturated and oversaturated magmas. Major and trace element variations in the dykes are consistent with a combination of fractional crystallization of parental peralkaline magma of quartz trachyte composition, and internal mineral segregation prior to final solidification. The distribution of accessory minerals is consistent with late-stage crystallization of isolated melt pockets. The dykes give an Rb–Sr isochron age of 171?±?4.4 Ma, with variable initial 87Sr/86Sr ratio (0.7075?±?0.0032), and have an average ε Nd of ? 12.0. Quartz phenocrysts have δ18O values of 8.4–9.2‰, which are generally in O-isotope equilibrium with bulk rock. Differences in the δ18O values of quartz and aegirine (average Δquartz?aegirine = 3.5‰) suggest aegirine formation temperatures around 500 °C, lower than expected for a felsic magma, but consistent with poikilitic aegirine that indicates subsolidus growth. The negative ε Nd (< ? 10) and magma δ18O values averaging 8.6‰ (assuming Δquartz?magma = 0.6‰) are inconsistent with a magma produced by closed-system fractional crystallization of a mantle-derived magma. By contrast, the nepheline syenite magma had mantle-like δ18O values and much less negative ε Nd (average ??3.1, n?=?3). The country rock has similar δ18O values to the granite dykes (average 8.0‰, n?=?108); this means that models for the petrogenesis of the granites by assimilation are unfeasible, unless an unexposed high-δ18O contaminant is invoked. Instead, it is proposed that the peralkaline syenite and granite dykes formed by partial melting of alkali-metasomatised gneiss that surrounds the nepheline syenite, followed by fractional crystallization.  相似文献   

12.
Sulfide mineralization in the Voisey’s Bay Intrusion, Labrador, Canada, is closely associated with country rock xenoliths that have extensively reacted with basaltic magma. In order to better understand the processes that control the assimilation of country rocks by mafic magma, a detailed study of oxygen isotope systematics related to magma-country rock interaction in the Voisey’s Bay area was undertaken. Protracted interaction of the xenoliths with magma produced refractory mineral assemblages in the xenoliths (2-10 cm in diameter) composed of Ca-rich plagioclase, corundum, hercynite, and minor magnetite. Overgrowth rims of plagioclase and biotite that surround most xenoliths separate the restites from the enclosing igneous matrix. The δ18O values of minerals from regionally metamorphosed pelitic and quartzofeldspathic protoliths are: plagioclase (8.7-12.3‰), orthoclase (9.5-9.8‰), biotite (5.2-8.7‰), garnet (8.3-10.8‰), pyroxene (8.0-10.1‰), and quartz (9.6-14.0). The δ18O values of minerals from the hornfels in the contact aureole of the intrusion are consistent with modeling which indicates that as a result of essentially closed system contact metamorphism oxygen isotope values should differ only slightly from those of the protoliths. Hercynite, plagioclase, and corundum separates from the xenoliths have δ18O values that vary from 2.9‰ to 10.5‰, 5.6‰ to 10.9‰, and 2.0‰ to 6.8‰, respectively. Although a siliceous 18O-enriched melt has been lost from the xenoliths, corundum, and feldspar δ18O values are significantly lower than expected through melt loss alone. The relatively low δ18O values of minerals from the xenoliths may be a function of incomplete isotopic exchange with surrounding mafic magma which had a δ18O value of ∼5.5‰ to 6.0‰. The high-18O melt that was released from the xenoliths is partially recorded in the plagioclase overgrowth on the margin of the xenoliths (δ18O values from 6.2‰ to 10.7‰), and in hercynite that replaced corundum. However, mass balance calculations indicate that a portion of the partial melt must have been transferred to magma that was moving through the conduit system. δ18O and δD values of biotite surrounding the plagioclase overgrowth range from 5.0‰ to 6.2‰ and −58‰ to −80‰, respectively. These data suggest that the outermost rim associated with many xenoliths has closely approached isotopic equilibrium with uncontaminated mafic magma. The current gabbroic to troctolitic matrix of the xenoliths shows no evidence for contamination by the high-18O partial melt from the xenoliths. The feldspar and biotite overgrowths on the xenoliths that formed after the motion of the xenoliths relative to the magma had stopped prevented further isotopic exchange between the xenoliths and final magma. The minerals within the xenoliths are not in oxygen isotopic equilibrium with each other, due in part to rapid thermal equilibration, partial melting, and partial exchange with flow through magma.  相似文献   

13.
《Chemical Geology》2003,193(3-4):181-193
Xenolith samples of marine terrigenous sediments and altered Jurassic MORB from Gran Canaria (Canary Islands) represent samples of sub-island oceanic crust. These samples are postulated to define end-members for crustal contamination of basaltic and felsic ocean island magmas. The meta-igneous rocks show great heterogeneity in oxygen isotope compositions (δ18O 3.3–8.6‰), broadly correlating with their stratigraphic position in the oceanic crust. Gabbros interpreted as fragments of oceanic crust layer 3 have δ18O values of 3.3–5.1‰, which is lower than MORB (5.7–6.0‰). Layer 2 lavas and dykes show a broader range of δ18O of 4.1–8.6‰. Therefore, high-temperature metamorphism seems to have been the dominant process in layer 3, while both high- and low-temperature alteration have variably affected layer 2 rocks. Siliciclastic sediments have high δ18O values (14.1–16.4‰), indicating diagenesis and low-temperature interaction with seawater. The oxygen isotope stratigraphy of the crust beneath Gran Canaria is typical for old oceanic crust and resembles that in ophiolites. The lithologic boundary between older oceanic crust and the igneous core complex at 8–10 km depth—as postulated from geophysical data—probably coincides with a main magma stagnation level. There, the Miocene shield phase magmas interacted with preexisting oceanic crust. We suggest that the range in δ18O values (5.2–6.8‰) [Chem. Geol. 135 (1997) 233] found for shield basalts on Gran Canaria, and those in some Miocene felsic units (6.0–8.5‰), are best explained by assimilation of various amounts and combinations of oceanic and island crustal rocks and do not necessarily reflect mantle source characteristics.  相似文献   

14.
Migmatites produced by low-pressure anatexis of basic dykes are found in a contact metamorphic aureole around a pyroxenite–gabbro intrusion (PX2), on Fuerteventura. Dykes outside and inside the aureole record interaction with meteoric water, with low or negative δ18O whole-rock values (+0.2 to −3.4‰), decreasing towards the contact. Recrystallised plagioclase, diopside, biotite and oxides, from within the aureole, show a similar evolution with lowest δ18O values (−2.8, −4.2, −4.4 and −7.6‰, respectively) in the migmatite zone, close to the intrusion. Relict clinopyroxene phenocrysts preserved in all dykes, retain typically magmatic δ18O values up to the anatectic zone, where the values are lower and more heterogeneous. Low δ18O values, decreasing towards the intrusion, can be ascribed to the advection of meteoric water during magma emplacement, with increasing fluid/rock ratios (higher dyke intensities towards the intrusion acting as fluid-pathways) and higher temperatures promoting increasing exchange during recrystallisation. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

15.
This paper investigates the origin of low-δ18O quartz porphyry dykes associated with the 144–133 Ma Koegel Fontein Igneous Complex, which was intruded during the initial phase of breakup of Africa and South America. The 25-km diameter Rietpoort Granite is the largest and youngest phase of activity, and is roofed by a 10-km diameter pendant of gneiss. Quartz porphyry (QP) dykes, up to 15 m in width, strike NW–SE across the complex. The QP dykes that intruded outside the granite have similar quartz phenocryst δ18O values (average 8.0‰, ± 0.7, n?=?33) to the granite (average 8.3?±?1.0, n?=?7). The QP dykes that intruded the roof pendant have quartz phenocrysts with more variable δ18O values (average 1.6‰, ± 2.1, n?=?55). In some cases quartz phenocrysts have δ18O values as low as ? 2.5‰. The variation in δ18O value within the quartz crystal population of individual dykes is small relative to the overall range, and core and rim material from individual quartz phenocrysts in three samples are identical within error. There is no evidence that quartz phenocryst δ18O values have been affected by fluid–rock interaction. Based on a ?quartz?magma value of 0.6‰, magma δ18O values must have been as low as ? 3.1‰. Samples collected along the length of the two main QP dykes that traverse the roof pendant have quartz phenocryst δ18O values that range from +?1.1 to +?4.6‰, and ? 2.3 to +?5.6‰, respectively. These δ18O values correlate negatively (r = ? 0.96) with initial 87Sr/86Sr, which can be explained by the event that lowered δ18O values of the source being older than the dykes. We suggest that the QP dykes were fed by magma produced by partial melting of gneiss, which had been variably altered at high temperature by 18O-depleted meteoric water during global glaciation at ~?550 Ma. The early melts had variable δ18O value but as melt pockets interconnected during melting, the δ18O values approached that of average gneiss. Variable quartz phenocryst δ18O values in the same dyke can be explained by vertical emplacement, at variable rates of ascent along the dyke. The lateral variation in quartz, and hence magma δ18O value at a particular point along a single dyke would depend on the rate of ascent of magma at that point along the dyke, and the ‘age’ of the particular magma batch.  相似文献   

16.
胶东金矿区矿田体系产出的脉岩是一套富含碳酸盐矿物相的中基性-中酸性脉岩，岩石类型主要有煌斑岩、安山玢岩类。对部分矿田体系产出脉岩的碳酸盐(方解石)碳、氧同位素及全岩氧同位素的研究表明该套脉岩的δ18OSMOW，方解石、δ13CPDB，方解石和δ  相似文献   

17.
Numerous intrusive bodies of mafic–ultramafic to felsic compositions are exposed in association with volcanic rocks in the Late Permian Emeishan large igneous province (ELIP), southwestern China. Most of the granitic rocks in the ELIP were derived by differentiation of basaltic magmas with a mantle connection, and crustal magmas have rarely been studied. Here we investigate a suite of mafic dykes and I-type granites that yield zircon U-Pb emplacement ages of 259.9 ± 1.2 Ma and 259.3 ± 1.3 Ma, respectively. The εHf(t) values of zircon from the DZ mafic dyke are –0.3 to 9.4, and their corresponding TDM1 values are in the range of 919–523 Ma. The εHf(t) values of zircon from the DSC I-type granite are between –1 and 3, with TDM1 values showing a range of 938–782 Ma. We also present zircon O isotope data on crust-derived felsic intrusions from the ELIP for the first time. The δ18O values of zircon from the DSC I-type granite ranges from 4.87‰ to 7.5‰. The field, petrologic, geochemical and isotopic data from our study lead to the following salient findings. (i) The geochronological study of mafic and felsic intrusive rocks in the ELIP shows that the ages of mafic and felsic magmatism are similar. (ii) The DZ mafic dyke and high-Ti basalts have the same source, i.e., the Emeishan mantle plume. The mafic dyke formed from magmas sourced at the transitional depth between from garnet-lherzolite and spinel-lherzolite, with low degree partial melting (<10%). (iii) The Hf-O isotope data suggest that the DSC I-type granite was formed by partial melting of Neoproterozoic juvenile crust and was contaminated by minor volumes of chemically weathered ancient crustal material. (iv) The heat source leading to the formation of the crust-derived felsic rocks in of the ELIP is considered to be mafic–ultramafic magmas generated by a mantle plume, which partially melted the overlying crust, generating the felsic magma.  相似文献   

18.
Zircons from 71 diverse rocks from the Qinling-Tongbai-Dabie-Sulu orogenic belt in east-central China and, for comparison, eight from adjoining areas in the South China and North China blocks, have been analyzed for in situ 18O/16O ratio and/or U–Pb age to further constrain the spatial distribution and genesis of Neoproterozoic low-δ 18O magmas, that is, δ 18O(zircon) ≤4 ‰ VSMOW. In many metaigneous rock samples from Tongbai-Dabie-Sulu, including high-pressure and ultrahigh-pressure eclogites and associated granitic orthogneisses, average δ 18O values for Neoproterozoic “igneous” zircon cores (i.e., 800–600 Ma) vary from ?0.9 to 6.9 ‰, and from ?9.9 to 6.8 ‰ for Triassic metamorphic rims (i.e., 245–200 Ma). The former extend to values lower than zircons in primitive magmas from the Earth’s mantle (ca. 5–6 ‰). The average Δ 18O (metamorphic zircon ? “igneous” zircon) values vary from ?11.6 to 0.9 ‰. The large volume of Neoproterozoic low-δ 18O igneous protoliths at Tongbai-Dabie-Sulu is matched only by the felsic volcanic rocks of the Snake River Plain hotspot track, which terminates at the Yellowstone Plateau. Hence, the low-δ 18O values at Tongbai-Dabie-Sulu are proposed to result from shallow subcaldera processes by comparison with Yellowstone, where repeated caldera-forming magmatism and hydrothermal alteration created similar low-δ 18O magmas. However, the possibility of involvement of meltwaters from local continental glaciations, rather than global Neoproterozoic glaciations, cannot be precluded. Our data indicate that Neoproterozoic low-δ 18O magmas that are either subduction- or rift-related are present locally along the western margin of the South China Block (e.g., Baoxing Complex). It appears that Neoproterozoic 18O-depletion events in the South China Block as the result of hydrothermal alteration and magmatism affected a much larger area than was previously recognized.  相似文献   

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