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1.
High-calcium, nepheline-normative ankaramitic basalts (MgO > 10 wt.%, CaO/Al 2O 3 > 1) from Rinjani volcano, Lombok (Sunda arc, Indonesia) contain phenocrysts of clinopyroxene and olivine (Fo 85–92) with inclusions of spinel (Cr# 58–77) and crystallised melt. Olivine crystals have variable but on average low NiO (0.10–0.23 wt.%) and high CaO (0.22–0.35 wt.%) contents for their forsterite number. The CaO content of Fo 89–91 olivine is negatively correlated with the Al 2O 3 content of enclosed spinel (9–15 wt.%) and positively correlated with the CaO/Al 2O 3 ratios of melt inclusions (0.9–1.5). Major and trace element patterns of melt inclusions are similar to that of the host rock, indicating that the magma could have formed by accumulation of small batches of melt, with compositions similar to the melt inclusions. The liquidus temperature of the magma was 1275 °C, and its oxygen fugacity ≤ FMQ + 2.5. Correlations between K 2O, Zr, Th and LREE in the melt inclusions are interpreted to reflect variable degrees of melting of the source; correlations between Al 2O 3, Na 2O, Y and HREE are influenced by variations in the mineralogy of the source. The melts probably formed from a water-poor, clinopyroxene-rich mantle source. 相似文献
2.
At least 14 small (1–11 km across) 1.8 Ga Svecofennian post-collisional bimodal intrusions occur in southern Finland and Russian Karelia in a 600-km-long belt from the Åland Islands to the NW Lake Ladoga region. The rocks range from ultramafic, calc-alkaline, apatite-rich potassium lamprophyres to peraluminous HiBaSr granites, and form a shoshonitic series with K 2O+Na 2O>5%, K 2O/Na 2O>0.5, Al 2O 3>9% over a wide spectrum of SiO 2 (32–78%). Although strongly enriched in all rocks, the LILE Ba and Sr and the LREE generally define a decreasing trend with increasing SiO 2. Depletion is noted for HFSE Ti, Nb and Ta. Available isotopic data show overlapping values for lamprophyres and granites within separate intrusions and a cogenetic origin is thus not precluded. Initial magmas (Mg#>65) in this shoshonitic association are considered to be generated in an enriched lithospheric mantle during post-collisional uplift some 30 Ma after the regional Svecofennian metamorphic peak. However, prior to the melting episode, the lithospheric mantle was affected by carbonatite metasomatism; more extensively in the east than in the west. The melts generated in the more carbonate-rich mantle are extremely enriched in P 2O 54%, F12,000 ppm, LILE: Ba9000 ppm, Sr7000 ppm, LREE: La600 ppm and Ce1000 ppm. The parental magma underwent 55–60% fractionation of biotite+clinopyroxene+apatite+magnetite+sphene whereupon intermediate varieties were produced. After further fractionation, 60–80%, of K-feldspar+amphibole+plagioclase±(minor magnetite, sphene and apatite), leucosyenites and quartz-monzonites were formed. In the west, where the source was less affected by carbonatite metasomatism, calc-alkaline lamprophyres (vogesites, minettes and spessartites) and equivalent plutonic rocks (monzonites) were formed. Removal of about 50% of biotite, amphibole, plagioclase, magnetite, apatite and sphene produced peraluminous HiBaSr granites. The impact of crustal assimilation is considered to be low. At about 1.8 Ga, the post-collisional shoshonitic magmatism brought juvenile material, particularly enriched in alkalis, LILE, LREE and F, into the crust. Although areally restricted, the regional distribution of the post-collisional intrusions may indicate that larger volumes of 1.8 Ga juvenile material resides in unexposed parts of the crust. 相似文献
3.
New U-Pb perovskite ages reveal that diamondiferous ultramafic lamprophyre magmas erupted through the Archean crust of northern Labrador and Quebec (eastern Canada) between ca. 610 and 565 Ma, a period of strong rifting activity throughout contiguous Laurentia and Baltica. The observed Torngat carbonate-rich aillikite/carbonatite and carbonate-poor mela-aillikite dyke varieties show a large spread in Sr-Nd-Hf-Pb isotope ratios with pronounced correlations between isotope systems. An isotopically depleted component is identified solely within aillikites ( 87Sr/ 86Sr i = 0.70323-0.70377; εNd i = +1.2-+1.8; εHf i = +1.4-+3.5; 206Pb/ 204Pb i = 18.2-18.5), whereas some aillikites and all mela-aillikites range to more enriched isotope signatures ( 87Sr/ 86Sr i = 0.70388-0.70523; εNd i = −0.5 to −3.9; εHf i = −0.6 to −6.0; 206Pb/ 204Pb i = 17.8-18.2). These contrasting isotopic characteristics of aillikites/carbonatites and mela-aillikites, along with subtle differences in their modal carbonate, SiO 2, Al 2O 3, Na 2O, Cs-Rb, and Zr-Hf contents, are consistent with two distinctive metasomatic assemblages of different age in the mantle magma source region.Integration of petrologic, geochemical, and isotopic information leads us to propose that the isotopically enriched component originated from a reduced phlogopite-richterite-Ti-oxide dominated source assemblage that is reminiscent of MARID suite xenoliths. In contrast, the isotopically depleted component was derived from a more oxidized phlogopite-carbonate dominated source assemblage. We argue that low-degree CO 2-rich potassic silicate melts from the convective upper mantle were preferentially channelled into an older, pre-existing MARID-type vein network at the base of the North Atlantic craton lithosphere, where they froze to form new phlogopite-carbonate dominated veins. Continued stretching and thinning of the cratonic lithosphere during the Late Neoproterozoic remobilized the carbonate-rich vein material and induced volatile-fluxed fusion of the MARID-type veins and the cold peridotite substrate. Isotopic modelling suggests that only 5-12% trace element contribution from such geochemically extreme MARID-type material is required to produce the observed compositional shift from the isotopically most depleted aillikites/carbonatites towards enriched mela-aillikites.We conclude that cold cratonic mantle lithosphere can host several generations of contrasting vein assemblages, and that each may have formed during past tectonic and magmatic events under distinctively different physicochemical conditions. Although cratonic MARID-type and carbonate-bearing veins in peridotite can be the respective sources for lamproite and carbonatite magmas when present as the sole metasome, their concomitant fusion in a complex source region may give rise to a whole new variety of deep volatile-rich magmas and we suggest that orangeites (formerly Group 2 kimberlites), kamafugites, and certain types of ultramafic lamprophyre are formed in this manner. 相似文献
4.
风化壳的元素地球化学组成可以反映风化壳的发育类型及其气候特征,地史时期形成的古风化壳的元素地球化学组成可以揭示古风化壳的类型及其古气候特征.华北地区新元古代晚期（景儿峪组）至寒武纪（府君山组）期间的地层记录缺乏连续性,广泛发育一套典型的古风化壳,该古风化壳的发育过程可能记录了晚新元古代至早寒武纪期间的古气候特点.针对华北房山地区景儿峪组顶部保存的古风化壳中常量元素的组成、元素迁移富集特征和风化系数等分布规律开展了较系统分析,结果表明:（1）SiO 2、Al 2O 3、TFe 2O 3、CaO是景儿峪组顶部古风化壳的主要组分,Al 2O 3、TFe 2O 3、TiO 2、K 2O在风化壳的中上部相对富集,SiO 2则轻微亏损,CaO、Na 2O、MgO、P 2O 5被迁移淋失;（2）硅铝系数、硅铝铁系数、化学蚀变指数（CIA）、残积系数、风化淋溶系数（BA）等地球化学指标的垂向变化特征指示该古风化壳形成于温暖湿润条件下中等强度的化学风化作用,其风化过程可能经历了由较弱至较强再逐渐减弱的演变过程;（3）与现代发育于湖南、贵州、云南等地碳酸盐岩类基岩之上的风化壳元素地球化学特征对比,发现景儿峪组顶部古风化壳的Si淋失度和Fe、Al富集度均较低.综合研究区古风化壳的常量元素地球化学特征,同时结合新元古代晚期至寒武纪的华北板块古纬度迁移特征,认为房山地区景儿峪组顶部发育的古风化壳形成于温暖湿润的亚热带-热带气候,为脱硅富铝化程度较低的硅铝粘土型风化壳. 相似文献
5.
Kimberlites, carbonatites and ultramafic, mafic and potassic lamprophyres have been produced in West Greenland in recurrent events since the Archaean. Five distinct age groups are recognised: Archaean (>2500 Ma). Early Proterozoic (1700–1900 Ma), Middle Proterozoic (Gardar, c. 1100–1300 Ma), Late Proterozoic (600 Ma) and Mesozoic-Tertiary (200-30 Ma) The rocks comprise two large and four small carbonatite occurrences, four kimberlite dyke swarms, one lamproite dyke swarm and one lamproite pipe, one dyke swarm of potassic lamprophyre (shonkinite) and some ten dyke swarms of ultramafic lamprophyre and monchiquite. Geochemical data for the various rock groups are presented. Some of the carbonatites may represent relatively unmodified mantle-derived melts. The kimberlites range from primitive to differentiated compositions, and there are regional differences between kimberlites within Archaean and Proterozoic basement. The ultrapotassic lamproites and shonkinites have strong negative Nb spikes in their trace element spectra. The ultramafic and monchiquitic lamprophyres encompass a large compositional variation; however, several of the dyke swarms have individual chemical characters. The rocks are very unevenly distributed in West Greenland, indicating a lithospheric control, probably by old weakness zones providing access to the surface. The kimberlites are considered to be mainly of asthenospheric derivation. The regional differences are interpreted in terms of melting with phlogopite as a residual phase, with smaller degrees of melting at deeper levels beneath the Archaean lithosphere than beneath the Proterozoic. The ultrapotassic lamproites and shonkinites occur almost exclusively within a continental collision zone with possible two-way subduction and they are interpreted as mainly of lithospheric derivation, with a contribution from a subducted slab. Data for the other rock types are equivocal. Except for the Archaean rocks, the age groups can be related to major geotectonic events. The Early Proterozoic group is related to continental collision at 1850 Ma and subsequent rifting; the Middle Proterozoic group is related to continental rifting (Gardar) and the Mesozoic group is likewise related to continental rifting prior to continental break-up in the Tertiary. The 600 Ma kimberlites and carbonatite are envisaged as cratonic, extra-rift activity in relation to continental break-up and formation of the Iapetus ocean further south, perhaps with a common cause in a broad, impinging mantle plume. 相似文献
6.
在苏鲁超高压变质带荣成蓝晶石榴辉岩中, 发现一具有特殊结构和地球化学特征的石榴辉石岩.该石榴辉石岩以包裹体的形式赋存于强烈退变的蓝晶石榴辉岩中.岩相学观测表明该岩石的主要结构为石榴石呈薄带、项链状相互连接的网状结构, 分割由辉石和石榴石、钛铁氧化物等出溶片晶组成的区域.全岩地球化学分析表明, 和邻近的蓝晶石榴辉岩相比, 该岩石具有(1) 相对低的SiO 2 (42.5%~43.1%), 异常高的CaO (21.4%~21.9%)和CaO/Al 2O 3比值(1.46~1.64); (2) 较高的TiO 2 (1.77%~1.89%)、V (359~419μg/g)、Nb (~8μg/g)、Y (17.7~23.1μg/g)和Zr (~150μg/g); (3) LREE富集和微弱的Eu负异常; (4) 较低的Cr、Ni和Co.上述地球化学特征表明荣成辉石岩的原岩形成于经过橄榄石分离结晶作用的超基性岩浆或经历了Na和K亏损过程的高钙基性岩浆.这2种过程都要求较高的温压条件, 压力 > 15×105kPa, 温度 > 1300 ℃, 这和荣成辉石岩的原岩的初始稳定条件的温压估算相一致.该研究结果表明在苏鲁超高压变质带中, 一些超高压变质岩的母岩形成于高温部分熔融或岩浆演化作用, 明显不同于大多数榴辉岩或片麻岩的原岩形成环境. 相似文献
7.
本文根据对世界上20多个国家和我国21个省碳酸盐岩石化学全分析数据的统计和换算结果,发现岩浆碳酸岩与沉积碳酸盐岩之间常量元素的一般含量无显著不同,但采用两组或逐步判别分析的方法,仍能将岩浆碳酸岩与沉积碳酸盐岩很好的区分开。判别效率可达90%以上。将这一方法应用到白云”鄂博矿区的“白云岩”和灰岩,90%以上的“白云岩”样品判为岩浆碳酸岩,灰岩的样品全部判为沉积碳酸盐岩,与它们的地质产状和地球化学特征一致。 相似文献
8.
Lamprophyres consisting mainly of diopside, phlogopite and K-feldspar formed in the early Tertiary around 60 Ma in the Beiya area and are characterized by low SiO 2 ± 46–50 wt.%), Rb (31–45 ppm) and Sr (225–262 ppm), high Al 2O 3, (11.2–13.1 wt.%), CaO (8.0–8.7 wt.%), MgO (11.5–12.1 wt.%), K 2O(4.9–5.5 wt.%), TiO 2 (2.9–3.3 wt.%) and REE (174–177 ppm), and compatible elements (e.g. Sc, Cr and Ni) and HSF elements (e.g. Th, U, Zr, Nb, Ta, Ti and Y), and low 143Nd/ 144Nd 0.512372–0.512536, middle 87Sr/ 86Sr 0.707322–0.707395, middle 206Pb/ 204Pb 18.50–18.59, 207Pb/ 204Pb 15.60–15.65 and 208Pb/ 204Pb 38.75–38.8. These rocks developed peculiar quartz megacrysts with poly-layer reaction zones, melt inclusions, and partial melted K-feldspar and plagioclase inclusions, and plastic shapes. Important features of these rocks include: (1) hybrid composition of elements, (2) abrupt increase of SiO 2 content of the melt, recorded by zoned diopside, (3) development of sanidine and aegirine-augite reaction zones, (4) alkaline melt and partial melted K-feldspar and plagioclase inclusions, (5) deformed quartz inclusions associated with quartz megacrysts, (6) the presence of quartz megacrysts in plastic shape with their parent melts, (7) the occurrence of olivine, high-MgO ilmenite and spinel inclusions within earlier formed diopside, phlogopite and magnetite. Median 87Sr/ 86Sr values between Tertiary alkaline porphyries in the Beiya area and the western Yunnan and Tertiary basalt in the western Yunnan indicate that the Beiya lamprophyre melts were derivative and resulted from the mixing between basic melts that were related to the partial melting of phenocrysts of spinel iherzolite from a mantle source. The alkaline melts originated from partial melting along the Jinshajiang subduction ductile shear zone at the contact between the buried Palaeo-Tethyan oceanic lithosphere and the upper mantle lithosphere. The alkaline melts are composed of 65% sanidine (Or 70Ab 28An 2) and 35% SiO 2. The melt mixing occurred in magma chambers in the middle-shallow crust at 8–10 km before the derivative lamprophyre melts intruded into the shallow cover in Beiya area. This mixing of basic and alkaline melts might represent a general process for the formation of lamprophyre in the western Yunnan. 相似文献
9.
The megacryst suite of the Grib kimberlite pipe (Arkhangelsk province, Russia) comprises garnet, clinopyroxene, magnesian ilmenite, phlogopite and garnet-clinopyroxene intergrowths. Crystalline inclusions, mainly of clinopyroxene and picroilmenite, occur in garnet megacrysts. Ilmenite is characterized by a wide range in the contents of MgO (10.6–15.5 wt.%) and Cr 2O 3 (0.7–8.3 wt.%). Megacryst garnets show wide variations in Cr 2O 3 (1.3–9.6 wt.%) and CaO (3.6–11.0 wt.%) but relatively constant MgO (15.4–22.3 wt.%) and FeO (5.2–9.9 wt.%). The pyroxenes also show wide variations in such oxides as Cr 2O 3, Al 2O 3 and Na 2O (0.56–2.95; 0.86–3.25; 1.3–3.0 wt.%, respectively). The high magnesium and chromium content of all these minerals puts them together in one paragenetic group. This conclusion was confirmed by studies of the crystalline inclusions in megacrysts, which demonstrate similar variations in composition. Low concentration of hematite in ilmenite suggests reducing conditions during crystallization. P– T estimates based on the clinopyroxene geothermobarometer (Contrib. Mineral. Petrol. 139 (2000) 541) show wide variations (624–1208 °C and 28.8–68.0 kbars), corresponding to a 40–45 mW/m 2 conductive geotherm. The majority of Gar-Cpx intergrowths differ from the corresponding monomineralic megacrysts in having higher Mg contents and relatively low TiO 2. The minerals from the megacryst association, as a rule, differ from the minerals of mantle xenoliths, but garnets in ilmenite-bearing peridotite xenoliths are compositionally similar to garnet megacrysts. The common features of trace element composition of megacryst minerals and kimberlite (they are poor in Zr group elements) suggest a genetic relationship. The origin of the megacrysts is proposed to be genetically connected with kimberlite magma-chamber evolution on the one hand and with associated mantle metasomatism on the other. We suggest that, depending on the primary melt composition, different paragenetic associations of macro/megacrysts can be crystallized in kimberlites. They include: (1) Fe–Ti (Mir, Udachnaya pipes); (2) high-Mg, Cr (Zagadochna, Kusova pipes); (3) high-Mg, Cr, Ti (Grib pipe). 相似文献
10.
The Columbia seamount 825 km offshore from Brazil at 20°S lies on the east–west ‘trace’ of the Trindade hotspot. Continental and oceanic magmatism believed to have originated with this hotspot is alkalic and SiO 2-undersaturated, and dates from 85 Ma in southern Brazil to <3 Ma on the islands of Trindade and Martin Vaz 1100 km offshore. An ankaramite (clinopyroxene 16 vol%) dredged from Columbia seamount (est. 10 Ma) conforms to this geochemistry with SiO 2-undersaturated Al-rich clinopyroxene (8–13 wt.% Al 2O 3) and rhönite. Clinopyroxene isotopic compositions are 87Sr/ 86Sr=0.703900, 143Nd/ 144Nd=0.512786, 206Pb/ 204Pb=19.190, 207Pb/ 204Pb=15.045, and 208Pb/ 204Pb=39.242 — resembling those for Trindade, except for slightly higher 207Pb/ 204Pb. The isotopic composition and abundance ratios among weathering-resistant Nb, La, and Yb suggest that Columbia seamount magmatism represents the present-day Trindade plume, but 10 million years earlier and perhaps when the plume manifested a signature of ‘contamination’ from subducted sediments. The Columbia seamount analyses provide the first quantitative assessment for the Trindade hotspot trace existing between the Brazil margin and Trindade, strengthening the case for a continuum of magmatism extending from the 85 Ma Brazilian igneous provinces of Poxoréu and Alto Paranaiba. 相似文献
11.
对东天山大白石头南片麻状花岗岩的研究,可以为新元古代早期Rodinia超大陆汇聚事件提供约束.在野外调查和岩相学研究的基础上,对该片麻状花岗岩开展了LA-ICP-MS锆石U-Pb定年、全岩地球化学和锆石原位Hf同位素分析.研究显示,片麻状花岗岩LA-ICP-MS锆石U-Pb年龄为922.7±7.9 Ma.岩石富SiO 2（70.04%~71.60%）、碱（Na 2O+K 2O=5.93%~6.58%）、Al 2O 3（13.88%~14.91%）和低MgO（1.13%~1.29%）.岩石Al 2O 3/TiO 2（25~27）小于100,CaO/Na 2O（0.7）大于0.3,K 2O/Na 2O（1.6~2.2）大于1.0,富集大离子亲石元素Rb、Th、K及La等,亏损Ba、Ta、Nb和Sr等,（La/Yb） N（7.29~8.11）小于10,δEu小于0.5,均显示出S型花岗岩特征.锆石ε Hf（t）均为正值（3.226 78~13.727 46）,二阶段模式年龄为920~1 598 Ma,表明新元古代花岗岩形成于大陆边缘构造环境.综合已有研究结果,可以推断天山地区出露的该期构造岩浆事件可能对应于新元古代的罗迪尼亚（Rodinia）超大陆汇聚事件. 相似文献
12.
Weathered and fresh samples of metamorphic rocks from Sri Lanka were collected from various localities and analysed for major elements by XRF method (RIGAKU, KG-X System, Japan). The content of water was determined by the ignition method. The XRF results, obtained from these samples form the basis of a new index of chemical weathering, particularly for Sri Lanka, which is named the Silica-Titania Index, and is calculated as follows: Silica-Titania Index = {(SiO2/TiO2)/[(SiO2/TiO2) × (Al2O3/TiO 2) × (SiO2/Al2O3)]} × 100(molecular proportions). The index can be used to determine the degree of weathering in chemically weathered silicate rocks of other countries in tropical regions. A triangular diagram plots the position of this index. The point load strengths of fresh rocksand weathered rocks with different degrees of weathering were correlated with the values of this new chemical index. The relative variation in strengths of fresh rocks and weathered rocks clearly indicates its suitability and usefulness for engineering geologists. 相似文献
13.
Ultrapotassic rocks in the Bucak area of Isparta Angle, SW Turkey, show unusually low SiO 2 (46.8–49.2 wt.%) and high MgO (10.4–11.6 wt.%) contents, and lamproitic affinity (K/Na, > 2.5; Mg#, 73–75; Al 2O 3, 9.2–11 wt.%, CaO 7.4–10.6 wt.%, Cr, 525–675 ppm; Ni, 442–615 ppm). They are made up by phlogopite (30–40 vol.%), leucite (25–30 vol.%), olivine (5–20 vol.%), which rarely contain Cr-spinel, clinopyroxene (5–10 vol.%), sanidine (5 vol.%) and richterite, with accessory apatite, magnetite and ilmenite. One sample also include negligible sodalite in groundmass, which is unusual mineral in lamproites. Mineral phase variation and textures record discrete phases of pre-eruptive crystallization: (1) early appearance of (Cr-spinel-bearing) olivine, Ti poor phlogopite ± apatite at pressures of ca. 1.0–2.0 GPa, at or close to the lithospheric Mechanical Boundary Layer (MBL), and (2) later appearance of Ti rich phlogopite, clinopyroxene, richterite, leucite, sanidine, and other minor phases, at pressures of ca. 0.1–1.0 GPa, indicating discrete, pressure-specific fractionation events. The Bucak silica poor ‘leucite’ lamproites were probably generated by partial melting of phlogopite-bearing, refractory peridotite at pressures of ca. 1.5–2 GPa, higher than those proposed for SiO 2-saturated ‘phlogopite’ lamproites (ca. 1–1.5 GPa) from Afyon, to the North. The depth (total pressure) of melt segregation probably dominates over volatile partial pressures (e.g. of CO 2, F, H 2O) in determining the SiO 2-undersaturated character of Bucak magmas. 相似文献
14.
Silicate and oxide mineral inclusions in diamonds from the geologically and historically important De Beers Pool kimberlites in Kimberley, South Africa, are characterised by harzburgitic compositions (>90%), with lesser abundances from eclogitic and websteritic parageneses. The De Beers Pool diamonds contain unusually high numbers of inclusion intergrowths, with garnet+orthopyroxene±chromite±olivine and chromite+olivine assemblages dominant. More unusual intergrowths include garnet+olivine+magnesite and an eclogitic assemblage comprising garnet+clinopyroxene+rutile. The mineral chemistry of the De Beers Pool inclusions overlaps that of most worldwide localities. Peridotitic garnet inclusions exhibit variable CaO (<5.8 wt.%) and Cr 2O 3 contents (3.0–15.0 wt.%), although the majority are harzburgitic with very low calcium concentrations (<2 wt.% CaO). Eclogitic garnet inclusions are characterised by a wide range in CaO (3.3–21.1 wt.%) with low Cr 2O 3 (<1 wt.%). Websteritic garnets exhibit intermediate compositions. Most chromite inclusions contain 63–67 wt.% Cr 2O 3 and <0.5 wt.% TiO 2. Olivine and orthopyroxene inclusions are magnesium-rich with Mg-numbers of 93–97. Olivine inclusions in chromite exhibit the highest Mg-numbers and also contain elevated Cr 2O 3 contents up to 1.0 wt.%. Peridotitic clinopyroxene inclusions are Cr-diopsides with up to 0.8 wt.% K 2O. Eclogitic and websteritic clinopyroxene inclusions exhibit overlapping compositions with a wide range in Mg-numbers (66–86). Calculated temperatures for non-touching inclusion pairs from individual diamonds range from 1082 to 1320 °C (average=1197 °C), whereas pressures vary from 4.6 to 7.7 GPa (average=6.3 GPa). Touching inclusion assemblages are characterised by equilibration temperatures of 995 to 1182 °C (average=1079 °C) and pressures of 4.2–6.8 GPa (average=5.4 GPa). Provided that the non-touching inclusions represent equilibrium assemblages, it is suggested that these inclusions record the conditions at the time of diamond crystallisation (1200 °C; 3.0 Ga). The lower average temperatures for touching inclusions are attributed to re-equilibration in a cooling mantle (1050 °C) prior to kimberlite eruption at 85 Ma. Pressure estimates for touching garnet–orthopyroxene inclusions are also skewed towards lower values than most non-touching inclusions. This apparent difference may be an artefact of the Al-exchange geobarometer and/or the result of sampling bias, due to limited numbers of non-touching garnet–orthopyroxene inclusions. Alternatively pressure differences could be caused by differential uplift in the mantle or possibly variations in thermal compressibility between diamond and silicate inclusions. However, thermodynamic modelling suggests that thermal compressibility differences would cause only minor changes in internal inclusion pressures (<0.2 GPa/100 °C). 相似文献
15.
Primitive olivine-mica-K-feldspar lamprophyre dykes, dated at 1831 ± 6 Ma, intrude lower greenschist facies rocks of the Early Proterozoic Pine Creek Inlier, of northern Australia. They are spatially, temporally and probably genetically associated with a post-tectonic composite granite-syenite pluton (Mt. Bundey pluton). The dykes have unusually high contents of large-ion-lithophile (LILE) and LREE elements (e.g. Ba up to 10,000 ppm, Ce up to 550 ppm, K 2O up to 7.5 wt. %) that resemble the concentrations found in the West Kimberley olivine and leucite lamproites. However, mineralogically the Mt. Bundey lamprophyres resemble shoshonitic lamprophyres and lack any minerals diagnostic of lamproites; leucite or leucite-pseudomorphs are absent. Mineral compositions are also unlike those in lamproites: micas contain higher Al 2O 3 than lamproitic mica; amphiboles are secondary actinolites after diopside; and oxides consist of zincian-chromian magnetite and groundmass magnetite. Heavy mineral concentrates contain mantle-derived xenocrysts of magnesiochromite, pyrope, Cr-diopside and rutile indicating a depth of sampling > 70 km. The Mt. Bundey lamprophyres are non-peralkaline to borderline peralkaline (molar (K + Na)/Al = 0.8 − 1.0) and potassic rather than ultrapotassic (molar K/Na < 2.5). They have distinctive major element compositions (≈46−49 wt. % SiO 2, ≈1.5−2 wt. % MgO, ≈7 wt. % CaO), and element ratios (e.g. molar Al/Ti ≈10, K/Na ≈2) that indicate they are best classified amongst transitional lamproites, i.e. potassic rocks such as cocites, jumillites and Navajominettes, that have geochemical characteristics transitional between Groups I and III. (Foley et al., 1987). The Mt. Bundey lamprophyres have LILE enrichment patterns that resemble the W. Kimberley pamproites but have moderate negative Ta---Nb---Ti anomalies and HREE abundances that are closely similar to the jumillites of southeastern Spain and Mediterranean-type lamproites. Single-stage modelling of Rb---Sr data is consistent with enrichment of the source-region of the Mt. Bundey lamprophyres ≈ 120–170 Ma before partial melting; i.e. at 1.95–2.10 Ga. Source enrichment does not appear to be associated with subduction processes, but may instead relate to incipient rifting of the Archaean basement. Negative Ta---Nb---Ti anomalies in the Mt. Bundey dykes may, therefore, relate to stability of residual titanate minerals in an oxidized subcontinental mantle source. This view is supported by high Fe 3+/ΣFe ratios of mantle-derived magnesiochromite xenocrysts which indicate oxidized mantle conditions (ƒ o2 ≈ FMQ + 1 long units), and by the presence of xenocrystic Cr-bearing rutile. Although the Mt. Bundey dykes have sampled upper mantle material, the oxidized nature of the magma source-region, and of the magma itself, suggests that conditions may not be favourable for diamond survival at depth nor for diamond transport in transitional lamproite magmas of this kind. 相似文献
16.
准噶尔盆地玛湖凹陷早二叠世风城组沉积时期为典型的碳酸盐型咸化湖盆,湖盆咸化过程中析出大量的碱矿(钠碳酸盐类矿物)。通过含碱层段主量元素、碳氧同位素分析,结合岩心宏观及微观矿物岩石特征,探讨了含碱层段韵律特征及形成机理。研究层段碱矿与沉凝灰岩互层,形成明暗相间的咸化韵律。浅色碱矿层质地较纯,主要为苏打和小苏打,主量元素表现为 Na 2O含量高,CaO、MgO、SiO 2和Al 2O 3含量低;暗色层段主要为含斑点状碳钠钙石的沉凝灰岩,主量元素表现为Na 2O含量低,CaO、MgO、SiO 2和Al 2O 3含量高。浅色碱矿层和暗色沉凝灰岩段碳氧同位素值均为正,表明其沉积水体具高盐度特征。浅色含碱层段具有更高的 δ 18O 值,反映了碱矿层沉积时期强烈蒸发的气候条件。根据δ 18O/δ 13C值的波动将研究层段划分为炎热干旱和相对湿润的气候条件,并据此解释不同韵律层盐类矿物形成机理。暗色的沉凝灰岩沉积代表地表流水注入量大于蒸发量,沉凝灰岩中盐类矿物含量少或者呈斑点状从颗粒间的卤水中析出;随着蒸发量的增高以及古气候由温暖潮湿转变为干旱炎热,地表流水注入量急剧减少,强烈蒸发控制了碱矿的沉积,形成浅色质地较纯的碱矿层。气候的频繁交替形成了风二段碱矿与沉凝灰岩频繁互层的特征。 相似文献
17.
The Komati Formation of the Barberton greenstone belt (BGB), South Africa, is composed of both Al-undepleted and -depleted komatiites. The Al-undepleted komatiites are characterised by Al 2O 3/TiO 2 and CaO/Al 2O 3 ratios of 15–18 and 1.1–1.5, respectively, and exhibit chondritic trace element contents and (Gd/Yb) N ratios. In contrast, the Al-depleted komatiites show significantly lower Al 2O 3/TiO 2 ratios of 8–12, highly variable CaO/Al 2O 3 (0.19–2.81) ratios combined with (Gd/Yb) N ratios varying from 1.08 to 1.56. A Sm–Nd whole rock isochron for komatiites of the Komati Formation gives an age of 3657±170 Ma. 147Sm/ 144Nd ratios (0.1704 and 0.1964) are all lower than the chondritic value of 0.1967. The komatiite i,Nd(3.45) values cluster at +1.9±0.7. Trace element distribution indicates that most of the primary geochemical and isotopic features of the komatiites were preserved in line with the conservation of the primary chemical composition of clinopyroxene. High field strength element and rare earth element abundances indicate that crustal contamination and post-crystallisation processes did not disturb the primary features of komatiites. The Sm/Nd and Nb/U ratios of komatiites indicate that the Barberton greenstone belt mantle source has undergone melt extraction prior to komatiite formation. Variations of Al2O3/TiO2, (Gd/Yb)N, Zr/Sm and Sm/Nd ratios of komatiites indicate that a batch melting of slightly depleted mantle source during with garnet and/or clinopyroxene remained in the residue can produce the geochemical isotopic feature of the Barberton greenstone belt komatiites. Typical geochemical fingerprints of subduction-related processes (LILE enrichment, HFSE depletion compared to REE), as known from modern subduction zones, are not observed. Komatiites exhibit Ti/Zr, La/Nb, Nb/U, Sr/Nd and Ba/La ratios comparable to those of oceanic island basalt and mid-ocean ridge basalt. (La/Nb)PMN, (Sm/Yb)PMN, positive δNb values and flat or slightly enriched REE patterns suggest that BGB komatiites are part of an oceanic plateau rather than an oceanic island such as Iceland. Therefore, an oceanic plateau or mid-ocean ridge, in connection with an oceanic plateau, such as Ontong Java plateau or Caribbean–Colombian oceanic plateau, is a suitable tectonic setting for the formation of the BGB komatiites. 相似文献
18.
In the Kakkaponnu area within the Achankovil Shear Zone (ACSZ), southern India, an undeformed ultramafic body occurs within intensely deformed granulite facies metamorphic rocks of Pan-African age. The Kakkaponnu ultramafic body is composed of spinel-dunite, phlogopite-dunite, glimmerite, graphite-spinel-glimmerite, and phlogopite-graphite-spinellite. The spinel-dunite is a fine- to medium-grained rock composed mainly of olivine and aluminous spinel and is characterized by relatively high MgO (50.39–50.90 wt.%), (Mg/ (Mg+Fe) = 0.95), Al 2O 3 (7.8–8.98 wt.%), and low Ni (10–14 ppm). The phlogopite-dunite comprises serpentinized olivine, phlogopite and subordinate amounts of dolomite and is high in MgO (36.5 wt.%), Mg# [(Mg/(Mg+Fe) = 0.97], and K 2O (%%5.5 wt.%). Olivine in the spinel-dunite is marked by unusually high MgO (Mg# = 0.96) and extremely low NiO (<0.14 wt.%). Spinels in all rock variants are highly aluminous with low Cr# [Cr/(Al+Cr)] ratio (<0.01). Magnesian ilmenite [Mg# = 59], rutile, zirconolite and baddeleyite are main accessory phases. No significant compositional variation is noted between large grains and small inclusions for all minerals. Abundant graphite, magnesite, melt and ubiquitous CO 2 fluid inclusions are identified in the olivine and spinel grains. The data imply that the Kakkaponnu ultramafic body was formed by progressive crystallization of highly potassic CO 2-rich melts injected into lower crustal levels. K-Ar ages of 470.5±9.3 and 464.5±9.2 Ma are obtained for phlogopite separates from glimmerite and phlogopite-dunite respectively. These ages are comparable to the phlogopite K-Ar ages reported from lithospheric shear zones in southern Madagascar, which was once conjugated to the Southern Peninsular India prior to the Gondwana breakup. This implies widespread highly potassic CO 2-rich fluid/melt influx along shear zones in this part of East Gondwana continent. 相似文献
19.
SHRIMP zircon U–Pb ages and geochemical and Sr–Nd–Pb isotopic data are presented for the gabbroic intrusive from the southern Taihang Mountains to characterize the nature of the Mesozoic lithospheric mantle beneath the central North China Craton (NCC). The gabbroic rocks emplaced at 125 Ma and are composed of plagioclase (40–50%), amphibole (20–30%), clinopyroxene (10–15%), olivine (5–10%) and biotite (5–7%). Olivines have high MgO (Fo = 78–85) and NiO content. Clinopyroxenes are high in MgO and CaO with the dominant ones having the formula of En 42–46Wo 41–50Fs 8–13. Plagioclases are dominantly andesine–labradorite (An = 46–78%) and have normal zonation from bytownite in the core to andesine in the rim. Amphiboles are mainly magnesio and actinolitic hornblende, distinct from those in the Precambrian high-pressure granulites of the NCC. These gabbroic rocks are characterized by high MgO (9.0–11.04%) and SiO 2 (52.66–55.52%), and low Al 2O 3, FeOt and TiO 2, and could be classified as high-mg basaltic andesites. They are enriched in LILEs and LREEs, depleted in HFSEs and HREEs, and exhibit ( 87Sr/ 86Sr) i = 0.70492–0.70539, εNd( t) = − 12.47–15.07, ( 206Pb/ 204Pb) i = 16.63–17.10, Δ8/4 = 70.1–107.2 and Δ7/4 = − 2.1 to − 9.4, i.e., an EMI-like isotopic signatures. Such geochemical features indicate that these early Cretaceous gabbroic rocks were originated from a refractory pyroxenitic veined-plus-peridotite source previously modified by an SiO 2-rich melt that may have been derived from Paleoproterozoic subducted crustal materials. Late Mesozoic lithospheric extension might have induced the melting of the metasomatised lithospheric mantle in response to the upwelling of the asthenosphere to generate these gabbroic rocks in the southern Taihang Mountains. 相似文献
20.
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
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