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1.
Explosive eruptions at Mauna Loa summit ejected coarse-grained blocks (free of lava coatings) from Moku'aweoweo caldera. Most are gabbronorites and gabbros that have 0–26 vol.% olivine and 1–29 vol.% oikocrystic orthopyroxene. Some blocks are ferrogabbros and diorites with micrographic matrices, and diorite veins (≤ 2 cm) cross-cut some gabbronorites and gabbros. One block is an open-textured dunite.

The MgO of the gabbronorites and gabbros ranges  7–21 wt.%. Those with MgO > 10 wt.% have some incompatible-element abundances (Zr, Y, REE; positive Eu anomalies) lower than those in Mauna Loa lavas of comparable MgO; gabbros (MgO < 10 wt.%) generally overlap lava compositions. Olivines range Fo83–58, clinopyroxenes have Mg#s  83–62, and orthopyroxene Mg#s are 84–63 — all evolved beyond the mineral-Mg#s of Mauna Loa lavas. Plagioclase is An75–50. Ferrogabbro and diorite blocks have  3–5 wt.% MgO (TiO2 3.2–5.4%; K2O 0.8–1.3%; La 16–27 ppm), and a diorite vein is the most evolved (SiO2 59%, K2O 1.5%, La 38 ppm). They have clinopyroxene Mg#s 67–46, and plagioclase An57–40. The open-textured dunite has olivine  Fo83.5. Seven isotope ratios are 87Sr/86Sr 0.70394–0.70374 and 143Nd/144Nd 0.51293–0.51286, and identify the suite as belonging to the Mauna Loa system.

Gabbronorites and gabbros originated in solidification zones of Moku'aweoweo lava lakes where they acquired orthocumulate textures and incompatible-element depletions. These features suggest deeper and slower cooling lakes than the lava lake paradigm, Kilauea Iki, which is basalt and picrite. Clinopyroxene geobarometry suggests crystallization at < 1 kbar P. Highly evolved mineral Mg#s, < 75, are largely explained by cumulus phases exposed to evolving intercumulus liquids causing compositional ‘shifts.’ Ferrogabbro and diorite represent segregation veins from differentiated intercumulus liquids filter pressed into rigid zones of cooling lakes. Clinopyroxene geobarometry suggests < 300 bar P. Open-textured dunite represents olivine-melt mush, precursor to vertical olivine-rich bodies (as in Kilauea Iki). Its Fo83.5 identifies the most primitive lake magma as  8.3 wt.% MgO. Mass balancing and MELTS show that such a magma could have yielded both ferrogabbro and diorite by ≥ 50% fractional crystallization, but under different fO2: < FMQ (250 bar) led to diorite, and FMQ (250 bar) yielded ferrogabbro. These segregation veins, documented as similar to those of Kilauea, testify to appreciable volumes of ‘rhyolitic’ liquid forming in oceanic environments. Namely, SiO2-rich veins are intrinsic to all shields that reached caldera stage to accommodate various-sized cooling, differentiating lava lakes.  相似文献   


2.
The Bellevue drillcore intersects ~3 km of Main and Upper Zone cumulates in the Northern Limb of the Bushveld Complex. Main Zone cumulates are predominately gabbronorites, with localized layers of pyroxenite and anorthosite. Some previous workers, using bulk rock major, trace and isotopic compositions, have suggested that the Main Zone crystallized predominantly from a single pulse of magma. However, density measurements throughout the Bellevue drillcore reveal intervals that show up-section increases in bulk rock density, which are difficult to explain by crystallization from a single batch of magma. Wavelet analysis of the density data suggests that these intervals occur on length-scales of ~40 to ~170 m, thus defining a scale of layering not previously described in the Bushveld Complex. Upward increases in density in the Main Zone correspond to upward increases in modal pyroxene, producing intervals that grade from a basal anorthosite (with 5% pyroxene) to gabbronorite (with 30–40% pyroxene). We examined the textures and mineral compositions of a ~40 m thick interval showing upwardly increasing density to establish how this type of layering formed. Plagioclase generally forms euhedral laths, while orthopyroxene is interstitial in texture and commonly envelops finer-grained and embayed plagioclase grains. Minor interstitial clinopyroxene was the final phase to crystallize from the magma. Plagioclase compositions show negligible change up-section (average An62), with local reverse zoning at the rims of cumulus laths (average increase of 2 mol%). In contrast, interstitial orthopyroxene compositions become more primitive up-section, from Mg# 57 to Mg# 63. Clinopyroxene similarly shows an up-section increase in Mg#. Pyroxene compositions record the primary magmatic signature of the melt at the time of crystallization and are not an artefact of the trapped liquid shift effect. Combined, the textures and decoupled mineral compositions indicate that the upward density increase is produced by the downward infiltration of noritic magma into a previously emplaced plagioclase-rich crystal mush. Fresh noritic magma soaked down into the crystallizing anorthositic mush, partially dissolving plagioclase laths and assimilating Fe-enriched pore melt. The presence of multiple cycles showing upward increases in density in the Bellevue drillcore suggests that downward magma infiltration occurred episodically during crystallization of the Main Zone.  相似文献   

3.
This whole rock and silicate mineral study focuses on the genesis of the Merensky reef sequence, as well as the footwall and hanging wall norites at an area of Rustenburg Platinum Mines in a demonstrably normal (undisturbed) environment. Continuous sampling provides major and trace element variations and mineral compositions and allows an evaluation of the post- liquidus processes which affected the sequence. Following the formation of liquidus phases three stages are envisaged to have modified the rocks. These are (a) migration of fluid during early compaction of cumulates, (b) circulation of fluids within the crystal mush, and (c) reaction and solidification of trapped liquid. Liquidus compositions are nowhere preserved in the sequence. A strong link is demonstrated between orthopyroxene compositions (e.g. Mg# and TiO2) and the incompatible trace element content of the whole rocks. The final amount of trapped liquid is shown to have been variable but never exceeded 10%. Calculated liquidus (pre-equilibration) orthopyroxene compositions show an up- sequence progression of evolving compositions from the footwall norite to the hanging wall norite. Initial Sr isotopic values do not support a simple magma mixing model by which radiogenic Main Zone magma mixes with that of the Critical Zone at the level of the Merensky reef. There is evidence that the hanging wall norite formed from a much more evolved magma. These conclusions have implications for the distribution and origin of the PGE-enriched Merensky reef package. Received: 7 October 1998 / Accepted: 5 March 1999  相似文献   

4.
长白山上新世以来玄武岩成分演变规律及其成因   总被引：4，自引：2，他引：2  
郭文峰  刘嘉麒  郭正府 《岩石学报》2014,30(12):3595-3611
火山岩成分的多样性是岩浆物理和化学过程在其产生、运移、存储和喷发过程中的综合反映。长白山火山区自上新世以来喷发了大量的玄武质火山岩,其成分变化范围较大(Mg O 3.2%~7.8%)。以往研究认为其成分的变化主要受地幔不均一、部分熔融程度和分离结晶的影响,没有明显地壳混染。本研究发现这些玄武岩经历了不同程度的上、下地壳的混染。同时,结合火山岩的年龄发现玄武岩地球化学成分和同位素比值随时间呈现脉动式的变化。根据87Sr/86Sr和Mg O的突变点可以分为3段:5~2Ma,2~1Ma,1~0Ma。通过定性和定量的模拟发现地幔不均一性和部分熔融程度差异造成玄武岩成分的变化有限,而分离结晶、地壳混染和岩浆补给的岩浆作用是形成玄武岩成分随时间脉动变化的主要原因。并结合能量约束-补给-混染-分离结晶算法(ECRAFC)模拟得出以下结论:天池和望天鹅喷发中心的玄武质岩浆最初都存储于同一下地壳岩浆房,可能由于上地壳构造差异导致岩浆迁移路径和存储区不同;长白山岩浆房迁移有从5~2Ma阶段由下地壳向上地壳逐渐变浅,2~1Ma阶段由上地壳向下地壳快速变深的规律,而1~0Ma阶段的玄武岩由岩浆从下地壳直接快速喷出地表形成;长白山玄武质岩浆的活动与本区的构造断裂活动密切的关系,5Ma以来,火山岩成分随时间的周期性波动可能与本区构造应力的周期性的强拉张-弱拉张过程有关。  相似文献   

5.
We have examined Re, Platinum-Group Element (PGE) and Os-isotope variations in suites of variably fractionated lavas from Kohala Volcano, Hawaii, in order to evaluate the effects of melt/crust interaction on the mantle isotopic signature of these lavas. This study reveals that the behavior of Os and other PGEs changes during magma differentiation. The concentrations of all PGEs strongly decrease with increasing fractionation for melts with MgO < 8 wt.%. Fractionation trends indicate significantly higher bulk partition coefficients for PGEs in lavas with less than 8 wt.% MgO (DPGE = 35–60) when compared to values for more primitive lavas with MgO > 8 wt.% (DPGE ≤ 6). This sudden change in PGE behavior most likely reflects the onset of sulfur saturation and sulfide fractionation in Hawaiian magmas at about 8 wt.% MgO.

The Os-rich primitive lavas (≥ 8 wt.% MgO, > 0.1 ppb Os) display a narrow range of 187Os/188Os values (0.130–0.133), which are similar to values in high-MgO lavas from Mauna Kea and Haleakala Volcanoes and likely represent the mantle signature of Kohala lavas. However, Os-isotopic ratios become more radiogenic with decreasing MgO and Os content in evolved lavas, ranging from 0.130 to 0.196 in the shield-stage Pololu basalts and from 0.131 to 0.223 in the post-shield Hawi lavas. This reflects assimilation of local oceanic crust material during fractional crystallization of the magma at shallow level (AFC processes). AFC modeling suggests that assimilation of up to 10% upper oceanic crust could produce the most radiogenic Os-isotope ratios recorded in the Pololu lavas. This amount of upper crust assimilation has a negligible effect on the Sr and Nd-isotopic compositions of Kohala lavas. Thus, these isotopic compositions likely represent the composition of the mantle source of Kohala lavas.  相似文献   


6.
This article reports a study of chromitites from the LG-1 to the UG-2/3 from the Bushveld Complex. Chromite from massive chromitite follows two compositional trends on the basis of cation ratios: trend A—decreasing Mg/(Mg + Fe) with increasing Cr/(Cr + Al); trend B—decreasing Mg/(Mg + Fe) with decreasing Cr/(Cr + Al). The chromitites are divided into five stages on the basis of which trend they follow and the data of Eales et al. (Chemical Geology 88:261–278, 1990) on the behaviour of the Mg/Fe ratio of the pyroxene and whole rock Sr isotope composition of the environment in which they occur. Following Eales et al. (Chemical Geology 88:261–278, 1990), the different characteristics of the stages are attributed to the rate at which new magma entered the chamber and the effect of this on aAl2O3 and, in the case of stage 5, the appearance of cumulus plagioclase buffering the aAl2O3. The similarity of PGE profiles across the MG-3 and MG-4 chromitites that are separated laterally by up to 300 km and the variation in V in the UG-2 argue that the chromitites have largely developed in situ. Modelling using the programme MELTS shows that increase in pressure, mixing of primitive and fractionated magma, felsic contamination of primitive magma or addition of H2O do not promote crystallization of spinel before orthopyroxene (in general they hinder it) and that the Cr2O3 content of the magma was of the order of 0.25 wt.%. Less than 20% of the chromite in the magma is removed before orthopyroxene joins chromite, which implies a >13-km thickness of magma for the Critical Zone. It is suggested that the large excess of magma has escaped up marginal structures such as the Platreef. The PGE profile of chromitites depends on whether sulphide accumulated or not along with chromite. Modelling shows that contamination of Critical Zone magma with a felsic melt will induce sulphide immiscibility, although not chromite precipitation. The LG-1 to LG-4 chromitites developed without sulphide, whilst those from the LG-5 upwards had associated liquid sulphide. Much of the sulphide originally in the LG-5 and above has been destroyed as a result of reaction with the chromite.  相似文献   

7.
The evolution of major mineral compositions and trace element abundances during fractional crystallization of a model lunar magma ocean have been calculated. A lunar bulk composition consistent with petrological constraints has been selected. Major mineral compositions have been calculated using published studies of olivine-melt, plagioclase-melt, and pyroxene-olivine equilibria. Trace element abundances have been calculated using experimentally-determined partition coefficients where possible. In the absence of experimental determinations, published partition coefficients obtained by analyzing phase separates from porphyritic volcanic rocks have been used. Trace elements studied are La, Sm, Eu, Lu, Rb, Sr( Eu2+), Ni, Co, and Cr.The first mineral to crystallize is olivine, which varies in composition from Fo98 at the liquidus to Fo95 at 50% solidification. Orthopyroxene crystallizes from 50 to 60% solidification with a restricted composition range of En95-En93. Plagioclase and Ca-rich clinopyroxene (XWo arbitrarily set equal to 0.5) co-crystallize during the final 40% solidification. Plagioclase changes in composition from An97 to approximately An93, while clinopyroxene evolves from En46 to approximately En40. The concomitant evolution of major element abundances in the melt is also discussed.The concentration of Ni in the melt decreases rapidly because solid-melt partition coefficients are significantly greater than unity at all stages of crystallization. The concentration of Cr in the melt increases slowly during olivine crystallization, then drops precipitously during the crystallization of orthopyroxene and clinopyroxene. The concentration of Co in the melt decreases slowly during olivine and orthopyroxene crystallization, after which it returns slowly to its initial concentration. Rubidium and Sr are not fractionated relative to one another until the onset of plagioclase crystallization. Ratios of Rb/Sr, normalized to their initial concentrations in the magma, do not rise above 10 until 95% of the magma has solidified. The ratios of Eu/Sm and La/Lu, normalized to their initial concentrations in the magma, remain essentially unfractionated until the onset of crystallization of clinopyroxene plus plagioclase, at which point the normalized La/Lu ratio increases to approximately 1.3 at 100% solidification and the normalized Eu/Sm ratio decreases to approximately 0.2 at 100% solidification.The model calculations are used to place approximate constraints on the bulk composition of the primitive Moon. Consideration of the effect on plagioclase composition of the activities of NaO0.5 and SiO2 in the melt suggests that the primitive Moon contained less than 0.4 wt % NaO0.5 and approximately 42–43 wt % SiO2. Concentrations of the REE in model lunar anorthosites are consistent with the returned samples. Concentrations of the REE in several model ‘highland basalts’ (considered to be representative of the average lunar terrae) are too low when compared with returned samples. Several possible explanations of this discrepancy are considered. The possible role of spinel in a twostage geochemical evolution of mare basalt liquids is discussed.  相似文献   

8.
Halogen-bearing minerals, especially apatite, are minor butubiquitous phases throughout the Bushveld Complex. Interstitialapatite is near end-member chlorapatite below the Merensky reef(Lower and Critical Zones) and has increasingly fluorian compositionswith increasing structural height above the reef (Main and UpperZones). Cl/F variations in biotite are more limited owing tocrystal-chemical controls on halogen substitution, but are alsoconsistent with a decrease in the Cl/F ratio with structuralheight in the complex. A detailed section of the upper LowerZone to the Critical Zone is characterized by an upward decreasein sulfide mode from 0·01–0·1% to trace–0·001%.Cu tends to correlate with other incompatible elements in mostsamples, whereas the platinum-group elements (PGE) can behaveindependently, particularly in the Critical Zone. The decreasein the Cl/F ratio of apatite in the Main Zone is associatedwith a shift to more radiogenic Sr isotopic signature, implyingthat the unusually Cl-rich Lower and Critical Zones are notdue to assimilation of crustal rocks. Nor is the Main Zone moreCl rich where it onlaps the country rocks of the floor, suggestinglittle if any Cl was introduced by infiltrating country rockfluids. Instead, the results are consistent with other studiesthat suggest Bushveld volatile components are largely magmatic.This is also supported by apatite–biotite geothermometry,which gives typical equilibrium temperatures of 750°C. Theincreasingly fluorian apatite with height in the Upper Zonecan be explained by volatile saturation and exsolved a Cl-richvolatile phase. The high Cl/F ratio inferred for the Lower andCritical Zone magma(s) and the evidence for volatile saturationduring crystallization of the Upper Zone indicate the Lowerand Critical Zones magma(s) were unusually volatile rich andcould easily have separated a Cl-rich fluid phase during solidificationof the interstitial liquid. The stratigraphic distribution ofS, Cu and the PGE in the Critical Zone cannot readily be explainedeither by precipitation of sulfide as a cotectic phase or asa function of trapped liquid abundance. Evidence from potholesand the PGE-rich Driekop pipe of the Bushveld Complex implythat migrating Cl-rich fluids mobilized the base and preciousmetal sulfides. We suggest that the distribution of sulfideminerals and the chalcophile elements in the Lower and CriticalZones reflects a general process of vapor refining and chromatographicseparation of these elements during the evolution and migrationof a metalliferous, Cl-rich fluid phase. KEY WORDS: Bushveld Complex; chlorine; platinum-group elements; layered intrusions  相似文献   

9.
The origin and evolution of the magma(s) involved in the formationof the olivine tholeiite cone-sheets which invade the 60-MaCuillin Igneous Complex, Isle of Skye, have been investigatedusing mineral chemistry data together with whole-rock major-and trace-element and isotope (Sr and Nd) geochemistry data.The most primitive compositions identified are almost identicalto those of the basalts being erupted at the present day alongthe spreading centre in Iceland. Rare examples of lavas fromthe slightly older lava pile of west-central and north Skye,together with a large number of dykes from the axial zone ofthe Skye Main Swarm, are of similar composition and are referredto as being of the Preshal More Basalt type (spelling is accordingto current Ordnance Survey of Great Britain maps). The intra-suitecompositional variation exhibited by the cone-sheets can beexplained in terms of relatively low-pressure fractionationof the three-phase assemblage olivine+clinopyroxene+plagioclasein their cotectic proportions of 10: 35: 55. Modelling of major-and compatible trace-element data indicates that the most evolvedcomposition may be derived by 60% crystallization of the leastevolved composition. Incompatible trace-element modelling impliesgreater degrees of crystal-liquid fractionation: Y and Zr indicate64 and 68% crystallization, respectively, whereas the rare earthelements (Eu, Yb, Gd, and Ce) indicate between 71 and 75% crystallization.This decoupled behaviour of compatible and incompatible elementsis attributed to the cone-sheet magma(s) evolving in a subjacentmagma chamber, before final emplacement in the overlying crust.Sr- and Nd-isotope data indicate that the cone-sheet magma(s)assimilated upper-crustal lithologies during fractional crystallization,most likely involving amphibolite facies gneiss of the LewisianGneiss Complex. This style of simultaneous assimilation andfractional crystallization (AFC) process in the upper crustfor the Skye cone-sheet magma(s) is in direct contrast to theprocesses identified for the magmas which produced the bulkof the lava field of west-central and north Skye, where assimilationoccurred after fractionation. The cone-sheet magmas were generatedby relatively large degrees of partial melting of a depletedmantle source associated with significant lithospheric stretching.The cause of this depletion, together with the temporal relationshipsbetween the cone-sheet magmas and the dominant transitionalmagmas of the Skye lava field and sill complex, are also discussed. * Reprint request to B. R. Bell  相似文献   

10.
The northern lobe of the Bushveld Complex is currently a highly active area for platinum-group element (PGE) exploration. This lobe hosts the Platreef, a 10–300-m thick package of PGE-rich pyroxenites and gabbros, that crops out along the base of the lobe to the north of Mokopane (formerly Potgietersrus) and is amenable to large-scale open pit mining along some portions of its strike. An early account of the geology of the deposit was produced by Percy Wagner where he suggested that the Platreef was an equivalent PGE-rich layer to the Merensky Reef that had already been traced throughout the eastern and western lobes of the Bushveld Complex. Wagner’s opinion remains widely held and is central to current orthodoxy on the stratigraphy of the northern lobe. This correlates the Platreef and an associated cumulate sequence that includes a chromitite layer—known as the Grasvally norite-pyroxenite-anorthosite (GNPA) member—directly with the sequence between the UG2 chromitite and the Merensky Reef as it is developed in the Upper Critical Zone of the eastern and western Bushveld. Implicit in this view of the magmatic stratigraphy is that similar Critical Zone magma was present in all three lobes prior to the development of the Merensky Reef and the Platreef. However, when this assumed correlation is examined in detail, it is obvious that there are significant differences in lithologies, mineral textures and chemistries (Mg# of orthopyroxene and olivine) and the geochemistry of both rare earth elements (REE) and PGE between the two sequences. This suggests that the prevailing interpretation of the stratigraphy of the northern lobe is not correct. The “Critical Zone” of the northern lobe cannot be correlated with the Critical Zone in the rest of the complex and the simplest explanation is that the GNPA-Platreef sequence formed from a separate magma, or mixture of magmas. Chilled margins of the GNPA member match the estimated initial composition of tholeiitic (Main Zone-type) magma rather than a Critical Zone magma composition. Where the GNPA member is developed over the ultramafic Lower Zone, hybrid rocks preserve evidence for mixing between new tholeiitic magma and existing ultramafic liquid. This style of interaction and the resulting rock sequences are unique to the northern lobe. The GNPA member contains at least seven sulphide-rich horizons with elevated PGE concentrations. Some of these are hosted by pyroxenites with similar mineralogy, crystallisation sequences and Pd-rich PGE signatures to the Platreef. Chill zones are preserved in the lowest Main Zone rocks above the GNPA member and the Platreef and this suggests that both units were terminated by a new influx of Main Zone magma. This opens the possibility that the Platreef and GNPA member merge laterally into one another and that both formed in a series of mixing/quenching events involving tholeiitic and ultramafic magmas, prior to the main influx of tholeiitic magma that formed the Main Zone.  相似文献   

11.
The Wangrah Suite granites (Lachlan Fold Belt, Australia) reflect different stages of differentiation in the magmatic history of an A-type plutonic suite. In this study we use experimentally determined phase equilibria of four natural A-type granitic compositions of the Wangrah Suite to constrain phases and phase compositions involved in fractionation processes. Each composition represents a distinct granite intrusion in the Wangrah Suite. The intrusions are the Danswell Creek (DCG), Wangrah (WG), Eastwood (EG) and Dunskeig Granite (DG), ordered from “most mafic” to “most felsic” by increasing SiO2 and decreasing FeOtotal.

Experimental investigation show that the initial water content in melts from DCG is between 2–3 wt. % H2O. If the DCG is viewed as the parental magma for the Wangrah Suite, then (1) fractionation of magnetite, orthopyroxene and plagioclase ( 20 wt. %) of the DCG composition, leads to compositions similar to that of the EG; (2) further fractionation of plagioclase, quartz, K-feldspar and biotite ( 40 wt. %) from the EG composition, leads to the DG composition. These fractionation steps can occur nearly isobarically and are confirmed by bulk rock Ba, Sr, Rb and Zr concentrations.

In contrast, the generation of the most abundant WG composition cannot be explained by fractional crystallisation from the DCG at isobaric conditions because of the high K2O content of this granite. Magma Mixing could be the process to explain the chemical distinctiveness of the Wangrah Granite from all the other granites of the Wangrah Suite.  相似文献   


12.
新疆哈密黄山东铜镍硫化物矿床成岩成矿作用   总被引：7，自引：1，他引：6  
黄山东铜镍硫化物矿床赋存于橄榄岩、苏长岩、辉长岩和闪长岩组成的镁铁-超镁铁质杂岩体中,赋矿岩体包含至少4套岩石组合。不同类型岩石微量元素和稀土元素原始地幔标准化配分模式指示,该矿床明显亏损Nb、Ta、Zr、Hf等高场强元素和Cr元素,富集Sr及大离子亲石元素;(La/Yb)N=1.08～2.70,δEu=0.50～2.57;含矿岩石Cu/Pd比值和Ti/Pd比值大于原始地幔值,表明不同类型岩石是高镁玄武质岩浆在深部分异结晶演化的产物。根据橄榄石和全岩化学组成可估算出母岩浆MgO含量约为12%。成矿岩浆深部演化过程中,富硅的地壳混染组分和外来流体的加入可能促成了岩浆中的硫饱和;深部熔离的不混溶硫化物珠滴被上升岩浆携带,富集在橄榄岩和苏长岩的底部。  相似文献   

13.
We report the occurrence of orthopyroxene gabbro from the Phenai Mata Igneous Complex (containing thoeliitic and alkaline rocks) that occur within Deccan Traps. The P-T calculations based on two pyroxene thermometry vary from 8.5±1.0 kbar and 963±39 °C. These gabbroic rocks exhibit high Mg# (0.67 to 0.71). But their primary magma signature can be negated due to their high SiO2 (> 50 wt %), low Ni (32–35 ppm) and Cr (105–182 ppm) contents. Further, simple fractional crystallization was not responsible for the modification of the magma. Modeling carried out using trace element concentrations revealed that concurrent assimilation and fractional crystallization (AFC) was responsible for the genesis of these rocks. Small pods of magma could have accumulated in the crustal portions and concurrent assimilation and fractional crystallization have taken place in the generation of gabbro and orthopyroxene gabbro in the present study area.  相似文献   

14.
The caldera-forming 26·5 ka Oruanui eruption (Taupo,New Zealand) erupted 530 km3 of magma, >99% rhyolitic, <1%mafic. The rhyolite varies from 71·8 to 76·7 wt% SiO2 and 76 to 112 ppm Rb but is dominantly 74–76 wt% SiO2. Average rhyolite compositions at each stratigraphiclevel do not change significantly through the eruption sequence.Oxide geothermometry, phase equilibria and volatile contentsimply magma storage at 830–760°C, and 100–200MPa. Most rhyolite compositional variations are explicable by28% crystal fractionation involving the phenocryst and accessoryphases (plagioclase, orthopyroxene, hornblende, quartz, magnetite,ilmenite, apatite and zircon). However, scatter in some elementconcentrations and 87Sr/86Sr ratios, and the presence of non-equilibriumcrystal compositions imply that mixing of liquids, phenocrystsand inherited crystals was also important in assembling thecompositional spectrum of rhyolite. Mafic compositions comprisea tholeiitic group (52·3–63·3 wt % SiO2)formed by fractionation and crustal contamination of a contaminatedtholeiitic basalt, and a calc-alkaline group (56·7–60·5wt % SiO2) formed by mixing of a primitive olivine–plagioclasebasalt with rhyolitic and tholeiitic mafic magmas. Both maficgroups are distinct from other Taupo Volcanic Zone eruptivesof comparable SiO2 content. Development and destruction by eruptionof the Oruanui magma body occurred within 40 kyr and Oruanuicompositions have not been replicated in vigorous younger activity.The Oruanui rhyolite did not form in a single stage of evolutionfrom a more primitive forerunner but by rapid rejuvenation ofa longer-lived polygenetic, multi-age ‘stockpile’of silicic plutonic components in the Taupo magmatic system. KEY WORDS: Taupo Volcanic Zone; Taupo volcano; Oruanui eruption; rhyolite, zoned magma chamber; juvenile mafic compositions; eruption withdrawal systematics  相似文献   

15.
Experimental studies, mainly under 3 kbars pressure, have been undertaken on representative samples to determine if any of these compositions could be parental magma to the Bushveld Complex. One such composition, with 12.5% MgO, Mg/(Mg + Fe) of 0.72 and quartz-normative, crystallizes olivine, Fo88, as liquidus mineral, at about 1,300° C, followed at only slightly lower temperature by orthopyroxene at 3 kbars pressure. There is a temperature drop of over 100° C before the appearance of plagioclase and finally clinopyroxene. This crystallization sequence is in excellent agreement with the observed sequence in the lower part of the Bushveld Complex.Results at higher pressures show that this composition cannot be a partial melt from mantle peridotite because olivine is replaced by orthopyroxene as the liquidus mineral at lower crustal pressures. A combination of olivine fractionation and contamination was probably involved in the early evolution of this magma.Experimental data on the other compositions show that they are not suitable as parental magma to the lowest portion of the complex. However, the data are used to construct phase diagrams within the basalt tetrahedron at 3 kbars pressure, which are of relevance to the crystallization of basic magmas in the upper crust.Research undertaken at the Grant Institute of Geology, University of Edinburgh, Scotland  相似文献   

16.
Orthopyroxene is a normal if rare, constituent of some minettes and lamproites. It is sometimes partly xenocrystic, but mostly authigenic, i.e. separated directly from lamprophyric magmas, as can be shown when orthopyroxene lamprophyres are compared with the experimental phase diagrams of appropriate systems. The orthopyroxene-bearing lamprophyres represent modified mantle-derived magmas characterized by a high MgO, high SiO2 speciation which was acquired in different ways in individual regions. The crystallization of orthopyroxene lamprophyres was not controlled by particular pressure — temperature conditions. Compared with clinopyroxene potassic lamprophyres the orthopyroxene lamprophyres represent more primitive magmas. Orthopyroxene minettes can also be altered by a post-magmatic hydrothermal inflow, which occasionally increases their CaO content to the level normal in clinopyroxene minettes. If this had happened before magma consolidation, clinopyroxene instead of orthopyroxene would have crystallized. Orthopyroxene minettes are well defined mineralogically and chemically and may be related to other types in the minette series.  相似文献   

17.
Melt inclusions and fluid inclusions in the Fangcheng basalt were investigated to understand the magma evolution and fluid/melt-peridotite interaction. Primary silicate melt inclusions were trapped in clinopyroxene and orthopyroxene phenocrysts in the Fangcheng basalt. Three types of melt inclusions (silicate, carbonate, and sulfide) coexisting with fluid inclusions occur in clinopyroxene xenocrysts and clinopyroxene in clinopyroxenite xenoliths. In situ laser-ablation ICP-MS analyses of major and trace element compositions on individual melt inclusions suggest that the silicate melt inclusions in clinopyroxene and orthopyroxene phenocrysts were trapped from the same basaltic magma. The decoupling of major and trace elements in the melt inclusions indicates that the magma evolution was controlled by melt crystallization and contamination from entrapped ultramafic xenoliths. Trace element patterns of melt inclusions are similar to those of the average crust of North China Craton and Yangtze Craton, suggesting a considerable crustal contribution to the magma source. Calculated parental melt of the Fangcheng basalt has features of low MgO (5.96 wt%), high Al2O3 (16.81 wt%), Sr (1,670 ppm), Y (>35 ppm), and high Sr/Y (>40), implying that subducted crustal material was involved in the genesis of the Fangcheng basalt. The coexisting fluid and melt inclusions in clinopyroxene xenocrysts and in clinopyroxene of xenoliths record a rare melt-peridotite reaction, that is olivine + carbonatitic melt1 (rich in Ca) = clinopyroxene + melt2 ± CO2. The produced melt2 is enriched in LREE and CO2 and may fertilize the mantle significantly, which we consider to be the cause for the rapid replacement of lithospheric mantle during the Mesozoic in the region.  相似文献   

18.
Trace-element data are presented for the first time for any coal seam in India, across a full working section, based on systematically collected channel samples of coal, together with their maceral composition. The trace-element variation curves along the seam profile are presented together with group maceral compositions of Kargali Bottom, Kargali Top, Kargali, Kathara, Uchitdih, Jarangdih Bottom, Jarangdih, and Jarangdih Top seams, East Bokaro coalfield. The Kathara and Uchitdih seams have also been sampled at two other localities and lateral variation in data in their trace-element and maceral compositions is also evaluated.The East Bokaro coals have: Ba and Sr > 1000 ppm; Mn < 450 ppm; Zr < 400 ppm; Ni and V < 250 ppm; Cr < 185 ppm; La < 165 ppm; Cu, Nb, and B < 125 ppm; Pb, Co and Y < 75 ppm; Ga, Sn, Mo, In and Yb < 15 ppm; Ag 2 ppm; and Ge 7 ppm. Petrographically, the coals are dominant in vitrinite (33–97%), rare in exinite (<15%), and semifusinite (0.8–49%) is the dominant inertinite maceral, with variable mineral and shaly matter (11–30%), graphic representation of trace elements versus vitrinite, inertinite, and coal ash indicates the affinity of (a) vitrinite with Cu, Ni, Co, V, Ga and B; (b) inertinite with Nb and B; and (c) coal ash (mineral matter) with Pb, Cu, Ni, La, Mn and Y; Ba, Cr, Sr, Zr, Cu and Ni are of organic as well as inorganic origins.The trend of the variation patterns and average compositions of the different seams are shown to be distinct and different. The variation along the same profile is inferred to be different for different seams of the coalfield.Trace-element data for certain coals of seams from different coalfields in the Gondwana basins of India are presented. There is a wide difference for each of these basins with respect to certain elements. This is suggestive of the proportions of Cu, Ni, V, Y, Ba, Sr, Cr, B, Zr and Ag, characterizing the different Gondwana Basins.  相似文献   

19.
The regional distribution and chemical composition of massive and disseminated chromitites through a Platreef sequence and along a strike distance of over ∼20 km were investigated to correlate them both within the framework of the northern limb and to the eastern and western limbs of the Bushveld Complex. The chromitite layers and seams of the Platreef form two main chromite-bearing zones: the Upper Chromitite that occurs as two to three discontinuous seams in feldspathic pyroxenite at approximately 20 m below the Platreef top contact and the Lower Chromitite that is composed of multiple seams in feldspathic harzburgite, pyroxenite and norite close to the bottom contact of the Platreef with footwall. Electron micro-probe analyses reveal that the chemical composition of chromite depends on the host rock type. Norite and pyroxenite host chromite with the highest Cr2O3 content while harzburgite-hosted chromites are Cr and Mg poor. The wide range in chromite compositions is explained by the influence of late-magmatic processes including post-cumulus growth and re-equilibration, interaction with fluid- and sulphide-saturated magmatic liquid and contact metamorphism. Each of these processes is characterised by its own distinct geochemical signature, but generally they lead to a decrease in Mg and Al and an increase in both di- and tri-valent Fe in the chromite. The occurrence of chromitite locally on the different distance from the contact between the upper Platreef sills and the overlying Main Zone magma suggests erosion of the upper Platreef by the Main Zone as it was emplaced. The localisation of chromitites supports an independent development of the northern limb during the Critical Zone emplacement although the chemical composition of chromite and co-existing silicates from ultramafic rocks suggest a Critical Zone affinity with the eastern and western limbs of the Bushveld Complex.  相似文献   

20.
The Lower Zone of the Eastern Bushveld Complex in the Olifants River Trough   总被引：4，自引：4，他引：4  
The Lower Zone of the Eastern Bushveld Complex in the OlifantsRiver Trough reaches 1584 m in thickness and is divisible intoBasal subzone, Lower Bronzitite, Harzburgite subzone, and UpperBronzitite. The Lower Zone is directly and conformably overlainby the Critical Zone; there is no break between the two. The principal cumulus minerals in the Lower Zone are bronziteand olivine. Chromite is an accessory cumulus mineral in peridotites,especially in the Harzburgite subzone, and cumulus plagioclaseoccurs in two thin units in the Basal subzone. Elsewhere plagioclase,with or without chromian augite, is postcumulus in origin. Electron microprobe analyses show that the range in En and Focontents of bronzite and olivine, respectively, is only a fewper cent over the entire rock sequence. Highest values of bothare found in the Harzburgite subzone. From modal and mineralanalyses the bulk composition of the Lower Zone (wt. per cent)is calculated as SiO2—53.94, TiO2—0.08, Cr2O3—0.55,V2O3—0.01, Al2O3—2.64, NiO—0.09, FeO (totalFe as FeO)—9.62, MnO—0.20, MgO—31.72, CaO—1.48,K2O—0.1, Na2O—0.13. This composition is unlike thatof any magma type, indicating that the Lower Zone is indeeda pile of crystal cumulates. From the data for the Lower Zone, together with available datafor the Critical, Main, and Upper Zones, the average MgO contentof the Eastern Bushveld Complex is calculated as about 13 percent, the Cr content as in excess of 1000 ppm. Even if the Complexformed from a single body of magma, the magma cannot have beentholeiitic, but rather olivine tholeiitic or picritic. An hypothesis of evolution of the Lower Zone is presented. Shiftsin total pressure are inferred to have been a major factor inproducing the succession of rock types and in producing theextraordinarily persistent chromitites of the overlying CriticalZone. It is suggested that the extraordinary richness in chromiteof the Bushveld is related to its formation not from tholeiiticmagma, but from more Mg-rich, chromium-rich magma drawn froma deeper level of the mantle than that which has yielded thetholeiitic basalts.  相似文献   

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