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1.
N. Hald  C. Tegner   《Lithos》2000,54(3-4):207-233
The Paleozoic–Mesozoic Jameson Land Basin (East Greenland) is intruded by a sill complex and by a swarm of ESE trending dykes. Together with dykes of the inner Scoresby Sund fjord, they form a regional Early Tertiary intrusive complex located 200–400 km inland of the East Greenland rifted continental margin. Most of the intrusive rocks in the Jameson Land Basin are geochemically coherent and consist of evolved plagioclase–augite–olivine saturated, uncontaminated high-Ti basalt with 48.5–50.2 wt.% SiO2, 2.2–3.2 wt.% TiO2, 5.1–7.4 wt.% MgO, 9–17 ppm Nb and La/YbN=2.8–3.6. Minor tholeiitic rock types are: (a) low-Ti basalt (49.7 wt.% SiO2, 1.7 wt.% TiO2, 6.8 wt.% MgO, 2.6 ppm Nb and La/YbN=0.5) akin to oceanic basalts; (b) very-high-Ti basalt (48.6 wt.% SiO2, 4.1 wt.% TiO2, 5.1 wt.% MgO and 21 ppm Nb); and (c) plagioclase ultraphyric basalt. The tholeiitic dolerites are cut by alkali basalt (43.7–47.3 wt.% SiO2, 4.1–5.1 wt.% TiO2, 4.9–6.2 wt.% MgO, 29–46 ppm Nb and La/YbN=16–17) sills and dykes.Modelling of high-field-strength and rare-earth elements indicate that the high-Ti basalts formed from 6–10% melting of approximately equal proportions of garnet- and spinel-bearing mantle of slightly depleted composition beneath thick continental lithosphere. Conversely, dolerite intrusions and flood basalts of similar compositional kindred from adjacent but more rift-proximal occurrences in Northeast Greenland formed from shallower melting of dominantly spinel-bearing mantle beneath extended and thinned continental lithosphere. These variations in lithospheric thickness suggest the continent–ocean transition of the East Greenland rifted volcanic margin is sharp and narrow.40Ar–39Ar dating and paleomagnetism show that the high-Ti dolerites were emplaced at 53–52 Ma (most likely during C23r) and hence surprisingly postdate the main flood volcanism by 2–5 Ma and the inception of seafloor spreading between Greenland and Europe by 1–2 Ma. The formation of tholeiitic and alkaline magmas emplaced into the Jameson Land Basin corroborates to the importance of post-breakup magmatism along the East Greenland volcanic rifted margin. Upwelling of the ancestral Iceland mantle plume under central Greenland at 53–52 Ma (rather than under the active rift), perhaps accompanied by a failed attempt to shift the rift zone westward towards the plume axis, may have triggered post-breakup continental magmatism of the Jameson Land Basin and the inner Scoresby Sund region, along preexisting structural lineaments.  相似文献   

2.
Dykes of the East Greenland Tertiary dyke swarm can be dividedinto pre- and syn-break-up tholeiitic dykes, and post-break-uptransitional dykes. Of the pre- and syn-break-up dykes, themost abundant group (Tholeiitic Series; TS) has major elementcompositions similar to the main part of the East Greenlandflood basalts. A group of high-MgO tholeiitic dykes (Picrite–AnkaramiteSeries; PAS) are much less common, and are equivalent to someof the oldest lavas of the East Greenland flood basalts. Isotopiccompositions of the TS and PAS dykes partly overlap with thosefor Iceland, but Pb isotopic compositions extend to less radiogenicvalues than those seen in either Iceland or North Atlantic mid-oceanridge basalt (MORB). The isotopically depleted source requiredto account for this isotopic variation is interpreted as subcontinentallithospheric mantle with low 87Sr/86Sr and 206Pb/204Pb and high  相似文献   

3.
Fe-rich tholeiitic liquids are preserved as chilled pillows and as the chilled base of a 27 meter thick macrorhythmic layer in the Pleasant Bay mafic-silicic layered intrusion. The compositions of olivine (Fo1) and plagioclase (An13−8) in these extremely fine grained rocks suggest that they represent nearly end stage liquids that formed by fractionation of tholeiitic basalt. Their major element compositions (∼17.5 wt% FeOT and 54 wt%SiO2) closely resemble highly evolved glasses in the Loch Ba ring dike and some recent estimates of end-stage liquids related to the Skaergaard layered intrusion, and are consistent with recent experimental studies of tholeiite fractionation. Their trace element compositions are consistent with extensive earlier fractionation of plagioclase, olivine, clinopyroxene, ilmenite, magnetite and apatite. The mineral assemblage of the chilled rocks (olivine, clinopyroxene, quartz, ilmenite and magnetite), apatite saturation temperatures, and very low Fe3+/Fe2+indicate conditions of crystallization at temperatures of about 950 °C and f O 2 about two log units below FMQ. Cumulates that lie about 3 meters above the chilled base of the macrorhythmic layer contain cumulus plagioclase, olivine, clinopyroxene, ilmenite, apatite and zircon. This mineral assemblage and the Fe-Mg ratio in clinopyroxene cores suggest that this cumulate was in equilibrium with a liquid having a composition identical to that of the chilled margin which lies directly beneath it. The high FeOT and low SiO2 concentrations of this cumulate (23.3 and 45.8 wt%, respectively) are comparable to those in late stage cumulates of the Skaergaard and Kiglapait intrusions. This association of a chilled liquid and cumulate in the Pleasant Bay intrusion suggests that late stage liquids in tholeiitic layered intrusions may have been more SiO2-rich than field-based models suggest and lends support to recent experimental studies of tholeiite fractionation at low f O2 which indicate that saturation of an Fe-Ti oxide phase should cause FeOT to decrease in the remaining liquid. Received: 17 January 1997 / Accepted: 10 June 1997  相似文献   

4.
拉脊山口蛇绿混杂岩是分布于中祁连和南祁连构造带之间蛇绿混杂带的重要组成部分。该混杂带中的岩石种类相对齐全,各岩性间为构造接触;其中辉绿岩以岩块和岩墙两种形式产出。辉绿岩块SiO2含量为49.80%~50.13%,MgO含量为5.43%~5.64%,FeOT为10.96%~11.52%,TiO2含量较高（2.38%~2.62%）;辉绿岩墙SiO2含量为43.41%~45.74%,MgO含量为9.04%~10.64%,FeOT为8.39%~9.96%,TiO2含量较低（0.89%~1.02%）,二者均属拉斑玄武岩系列。其中辉绿岩块ΣREE为135.4×10-6~150.9×10-6,（La/Yb）N=3.51~4.03,具有右倾型稀土配分模式,富集Rb、Ba、K、Sr等大离子亲石元素及Th、Nb、Ta、Zr、Hf、Ti等高场强元素,呈现洋岛拉斑玄武岩特征;辉绿岩墙ΣREE为36.10×10-6~43.72×10-6,（La/Yb）N=1.12~1.20,稀土配分曲线相对平坦,呈现出与洋中脊玄武岩相似的稀土和微量元素配分模式。这两类辉绿岩样品均缺乏Nb、Ta和Ti负异常,可能分别形成于洋岛/海山和洋中脊环境。SHRIMP锆石U-Pb测年结果显示,辉绿岩块形成时代为491.0±5.1Ma。这些不同构造属性的辉绿岩可能形成于原特提斯洋向北俯冲消减过程。  相似文献   

5.
Several recent publications suggest that the appearance of Fe-Ti oxides terminates iron enrichment and starts pronounced silica enrichment (the Bowen trend) during the differentiation of tholeiitic basalt. However, this does not appear to hold for the Skaergaard intrusion. New data from a ∼950 m long drill core (90–22) through its Upper Zone reveal that: (1) iron in plagioclase increases from ∼0.25 to ∼0.45 wt% FeOT with fractionation of evolved oxide ferrodiorites (An46-32) and (2) the evolving liquid, which is modelled by incremental bulk-rock summation, increased its iron content from 20.1 to 26.5 wt% FeOT and its silica content from 47.4 to 49.6 wt% SiO2 with fractional crystallisation (the Fenner trend). Positive correlation between modelled iron-content of the magmas, and measured iron-content of plagioclase, confirms that iron enrichment is petrologically feasible even with Fe-Ti oxides in the fractionating assemblage. As suggested by previous authors, fractional crystallisation closed to oxygen exchange is the likely reason why some layered intrusions diverge from the Bowen mechanism of differentiation. It is emphasised that both trends seem to exist in nature. Received:13 May 1996 / Accepted:5 January 1997  相似文献   

6.
Flood basalt provinces may constitute some of the most catastrophic volcanic events in the Earth's history. A popular model to explain them involves adiabatic ascent of plumes of anomalously hot peridotite from a thermal boundary layer deep in the mantle, across the peridotite solidus. However, peridotitic plumes probably require unreasonably high potential temperatures to generate sufficient volumes of magma and high enough melting rates to produce flood volcanism. This lead to the suggestion that low melting eclogitic or pyroxenitic heterogeneities may be present in the source regions of the flood basalts. In order to constrain petrogenetic models for flood basalts generated in this way, an experimental investigation of the melting relations of homogeneous peridotite + oceanic basalt mixtures has been performed. Experiments were conducted at 3.5 GPa on a fertile peridotite (MPY90)–oceanic basalt (GA1) compositional join. The hybrid basalt + peridotite compositions crystallised garnet lherzolite at subsolidus temperatures plus quenched ne-normative picritic liquids at temperatures just above the solidus, over the compositional range MPY90 to GA150MPY9050. The solidus temperature decreased slightly from ∼1500 °C for MPY90 to ∼1450 °C for GA150MPY9050. Compositions similar to GA130MPY9070 have 100% melting compressed into a melting interval which is approximately 50–60% smaller than that for pure MPY90, due to a liquidus minimum. During adiabatic ascent of hybrid source material containing a few tens of percent basalt in peridotite, the lower solidus and compressed solidus–liquidus temperature interval may conspire to substantially enhance melt productivity. Mixtures of recycled oceanic crust and peridotite in mantle plumes may therefore provide a viable source for some flood volcanics. Evidence for this would include higher than normal Fe/Mg values in natural primary liquids, consistent with equilibration with more Fe-rich olivine than normal pyrolitic olivine (i.e. <Fo89–92). Modelling of fractionation trends in West Greenland picrites is presented to demonstrate that melts parental to the Greenland picrites were in equilibrium at mantle P–T conditions with olivine with Fo84–86, suggesting an Fe-enriched source compared with normal peridotite, and consistent with the presence of a basaltic component in the source. Received: 29 October 1999 / Accepted: 3 February 2000  相似文献   

7.
《地学前缘(英文版)》2018,9(6):1711-1724
The Helanshan tectonic belt(HTB) is a major tectonic divide between the Alxa and Ordos blocks in the North China Craton. The geochronology and petrogenesis of the mafic dykes in the northern HTB are keys to understanding the tectonic evolution of this belt. The mafic dykes, intruded into the Neoarchean-Paleoproterozoic metamorphic basement, are mainly composed of diabase with a mineral assemblage of plagioclase(45%-60%), pyroxene(25%-35%), minor quartz and Fe-Ti oxides. The LA-ICPMS U-Pb analysis of zircon grains from representative dykes yield a weighted mean age of 206 ± 1.9 Ma, which represents the crystallization age of the dyke. The diabases show high contents of Fe_2 O_3~T(11.88-17.55 wt.%), low contents of SiO_2(45.65-50.95 wt.%) and MgO(3.31-5.50 wt.%) with low Mg#(=100×MgO/(MgO + FeO) atomic ration) of 33-44. They are characterized by enrichment of light rare earth elements(LREEs) and large ion lithophile elements(LILEs)(e.g., Rb, Ba and Pb), and slight depletion of high field strength elements(HFSEs). These features suggest that the magma has undergone extensive fractionation of olivine and pyroxene but only minor crustal contamination during its evolution. Their high Sm contents and La/Sm ratios, and low Sm/Yb ratios indicate that magma from which the dykes formed was derived from low degree(about 5%) partial melting of an enriched garnet + spinel lherzolite mantle source. Together with regional geology, these geochemical and geochronological data suggest that the mafic dykes in the HTB were formed in an intracontinental extensional setting during the late Triassic.  相似文献   

8.
丹池成矿带是我国重要锡多金属成矿带,过去对丹池成矿带成岩成矿研究主要集中在大厂矿田及五圩矿田,而对丹池成矿带北部的芒场矿田岩浆作用时代、源区特征及其成矿性缺少分析。芒场矿田岩浆活动强烈,发育隐伏斑状花岗岩及有关的花岗斑岩脉和隐伏细粒花岗斑岩及有关的白云母花岗斑岩脉。本文分析花岗斑岩脉U-Pb年龄及花岗斑岩脉和白云母花岗斑岩脉主、微量元素组成,以探讨岩浆活动时代、岩浆源区特征及其成矿潜力。芒场矿田花岗斑岩脉锆石U-Pb年龄为89.1±0.9Ma (MSWD=0.9),和丹池成矿带内大厂矿田岩浆活动时代相近,表明丹池成矿带岩浆活动时代都发生于90Ma左右。白云母花岗斑岩脉具高A/CNK比值(2.69～2.88),含高铝硅酸盐矿物白云母及在Th-Rb图上沿S型花岗岩趋势线分布,和大厂矿田S型黑云母花岗岩的特征基本一致,表明其主要为S型花岗岩。花岗斑岩脉形成时代晚于白云母花岗斑岩脉,但其具有更低的SiO_2含量和更高的MgO、Fe_2O_3~T、CaO和TiO_2含量,且在SiO_2与TiO_2、Fe_2O_3~T、Al_2O_3和P_2O_5关系图中分布于不同区域,没有线性变化关系,显示两者不是同一岩浆结晶分异演化形成的,而为不同沉积变质岩部分熔融形成的。白云母花岗斑岩脉富Al_2O_3、K_2O、Rb、Cs、Sn、W、Nb和Ta,在Rb/Ba-Rb/Sr图上位于富粘土源区,为强风化作用形成的富粘土质富稀有金属源区部分熔融形成的产物。华南西部基底发育经强风化作用形成的富粘土质富稀有金属元素沉积变质源区,为华南西南缘大规模锡矿床的形成提供了物质基础。  相似文献   

9.
 The Aurora volcanic field, located along the northeastern margin of Mono Lake in the Western Great Basin, has erupted a diverse suite of high-K and shoshonitic lava types, with 48 to 76 wt% SiO2, over the last 3.6 million years. There is no correlation between the age and composition of the lavas. Three-quarters of the volcanic field consists of evolved (<4 wt% MgO) basaltic andesite and andesite lava cones and flows, the majority of which contain sparse, euhedral phenocrysts that are normally zoned; there is no evidence of mixed, hybrid magmas. The average eruption rate over this time period was ∼200 m3/km2/year, which is typical of continental arcs and an order of magnitude lower than that for the slow-spreading mid-Atlantic ridge. All of the Aurora lavas display a trace-element signature common to subduction-related magmas, as exemplified by Ba/Nb ratios between 52 and 151. Pre-eruptive water contents ranged from 1.5 wt% in plagioclase-rich two-pyroxene andesites to ∼6 wt% in a single hornblende lamprophyre and several biotite-hornblende andesites. Calculated oxygen fugacities fall within –0.4 and +2.4 log units of the Ni-NiO buffer. The Aurora potassic suite follows a classic, calc-alkaline trend in a plot of FeOT/MgO vs SiO2 and displays linear decreasing trends in FeOT and TiO2 with SiO2 content, suggesting a prominent role for Fe-Ti oxides during differentiation. However, development of the calc-alkaline trend through fractional crystallization of titanomagnetite would have caused the residual liquid to become so depleted in ferric iron that its oxygen fugacity would have fallen several log units below that of the Ni-NiO buffer. Nor can fractionation of hornblende be invoked, since it has the same effect as titanomagnetite in depleting the residual liquid in ferric iron, together with a thermal stability limit that is lower than the eruption temperatures of several andesites (∼1040–1080°C; derived from two-pyroxene thermometry). Unless some progressive oxidation process occurs, fractionation of titanomagnetite or hornblende cannot explain a calc-alkaline trend in which all erupted lavas have oxygen fugacites ≥ the Ni-NiO buffer. In contrast to fractional crystallization, closed-system equilibrium crystallization will produce residual liquids with an oxygen fugacity that is similar to that of the initial melt. However, the eruption of nearly aphryic lavas argues against tapping from a magma chamber during equilibrium crystallization, a process that requires crystals to remain in contact with the liquid. A preferred model involves the accumulation of basaltic magmas at the mantle-crust interface, which solidify and are later remelted during repeated intrusion of basalt. As an end-member case, closed-system equilibrium crystallization of a basalt, followed by equilibrium partial melting of the gabbro will produce a calc-alkaline evolved liquid (namely, high SiO2 and low FeOT/MgO) with a relative f O 2 (corrected for the effect of changing temperature) that is similar to that of the initial basalt. Differentiation of the Aurora magmas by repeated partial melting of previous underplates in the lower crust rather than by crystal fractionation in large, stable magma chambers is consistent with the low eruption rate at the Aurora volcanic field. Received: 7 July 1995 / Accepted: 19 April 1996  相似文献   

10.
千里山-贺兰山地区分布着两组岩墙:一组北东走向,侵入古元古代变质基底岩系,见被晚前寒武系黄旗口组不整合截切,称为千里山岩墙群;一组北西走向,侵入基底岩系,见侵入黄旗口组,被石炭系不整合截切,称为贺兰山岩墙群。一条千里山岩墙分选出斜锆石,二次离子探针Pb-Pb定年获得813±7Ma的年龄(207Pb/206Pb平均年龄;MSWD=0.63,n=6),代表岩墙侵位时代。一条贺兰山岩墙分选出锆石,二次离子探针U-Pb定年获得最小一组年龄～370Ma(206Pb/238U年龄),近似代表岩墙侵位时代或者略大于侵位时代。千里山岩墙为拉斑玄武岩系列,以高TiO_2(2.7%～3.7%)和Fe_2OT_3(13.4%～17.0%)为特征;贺兰山岩墙为(弱)碱性系列,低TiO_2(1.0%～1.5%)和Fe_2OT_3(5.5%～12.4%)为特征。两者均显示轻稀土和大离子亲石元素富集,高场强元素相对亏损的特征;贺兰山岩墙群的富集和亏损特征均更为明显((La/Yb)N:贺兰山岩墙群2.0～5.5;千里山岩墙群1.9～2.4)。这些特征说明岩浆可能起源于交代的岩石圈地幔或者岩浆受到过地壳物质的混染。黄旗口组-王全口组-正目关组与上覆寒武系地层以及下伏千里山岩墙群的地质关系说明这些地层应该形成于新元古代晚期(810～541Ma)。千里山-贺兰山地区基底属于西华北克拉通的一部分,其以西是阿拉善地块;后者的构造归属长期存在争议。鉴于阿拉善地块发育同时期、岩浆性质基本相似的岩浆岩(狼山地区双峰式火山岩系;龙首山地区镁铁-超镁铁岩),考虑到两地的晚太古代-古元古代基底特征的相似性,我们认为阿拉善地块和千里山-贺兰山地块可能属于同一克拉通,同时经历新元古代中期伸展-裂谷事件。  相似文献   

11.
The Bancun diabase dyke and the Bali hornblende gabbro dyke in western Fujian Province were emplaced in the Early and Late Cretaceous periods, respectively; the former is designated to calc-alkaline series and the latter to K-high-calc-alkaline rock series. Both the dykes are characterized by such geochemical characteristics as high Al and Na2O>K2O. As for the Bancun dyke, Al2O3=16.32%–17.54% and K2O/Na2O=0.65–0.77; as for the Bali dyke, Al2O3=16.89%–17.81% and K2O/Na2O=0.93–0.99. Both the Bancun and Bali mafic dykes are relatively enriched in LILE and LREE, but depleted in HSFE, displaying the geochemical characteristics of continental marginal arc, with high initial Sr isotopic ratios and low εNd (t) values. The (87Sr/86Sr)i ratios of the Bancun diabase dyke are within the range of 0.708556–0.70903 and their εNd (t) values vary between -6.8 and -6.3; those of the Bali hornblende dyke are within the range of 0.710726–0.710746 and their εNd (t) values are -4.7– -4.9, showing the characteristics of enriched mantle EM II. The isotope and trace element data showed that the mafic dykes have not experienced obvious crustal contamination, and metasomatism caused by subduction fluids is the main factor leading to LILE and LREE enrichments. The enriched mantle is the source region for the mafic dykes, and mixing of subduction fluid metasomatized enriched mantle and EM II-type mantle constituted the mantle source region of both the Bancun and Bali mafic dykes. Upwelling of the asthenosphere mantle provided sufficient heat energy for the generation of magmas. In accordance with the discrimination diagram of their tectonic settings as well as their trace element geochemical characteristics, it is considered that the dykes both at Bancun and Bali possess the characteristics of continental marginal arc, revealing the tectonic environment of formation of the mafic dykes, the continental dynamic background as an intraplate tensional belt in which the mafic dykes were emplaced. Meanwhile, it is also indicated that the tensional tectonic stress mechanism is responsible for the formation of the mafic dykes in western Fujian Province.  相似文献   

12.
Field studies supplemented by petrographic analyses clearly reveal complete preservation of ophiolite suite from Port Blair (11°39′N: 92°45′E) to Chiriyatapu (11°30′24″N: 92°42′30″E) stretch of South Andaman. The ophiolite suite reveals serpentinite at the base which is overlain unconformably by cumulate ultramafic-mafic members with discernible cumulus texture and igneous layering. Basaltic dykes are found to cut across the cumulate ultramafic-mafic members. The succession is capped by well exposed pillow basalts interlayered with arkosic sediments. Olivine from the basal serpentinite unit are highly magnesian (Fo80.1–86.2). All clinopyroxene analyses from cumulate pyroxenite, cumulate gabbro and basaltic dyke are discriminated to be ‘Quad’ and are uniformly restricted to the diopside field. Composition of plagioclase in different lithomembers is systematically varying from calcic to sodic endmembers progressively from cumulate pyroxenite to pillow basalt through cumulate gabbro and basaltic dyke. Plagioclase phenocrysts from basaltic dyke are found to be distinctly zoned (An60.7-An35.3) whereas groundmass plagioclase are relatively sodic (An33-An23.5). Deduced thermobarometric data from different lithomembers clearly correspond to the observed preservation of complete ophiolite suite.  相似文献   

13.
The picritic dykes occurring within fine-grained gabbro in the marginal zone and in the surrounding Proterozoic wall-rock marbles of the Panzhihua Fe–Ti oxide deposit closely correspond in bulk composition with the nearby Panzhihua intrusion. These dykes offer important constraints on the nature of the mantle source of the Panzhihua ore-bearing intrusion and its possible link to the Emeishan plume. U–Pb zircon dating of the picritic dyke yields a crystallization age of 261.4 ± 4.6 Ma, coeval with the timing of the main Panzhihua gabbroic intrusion and Late Permian Emeishan flood basalts. The Panzhihua picritic dykes contain 37.63–43.41 wt% SiO2, 1.15–1.56 wt% TiO2, 11.43–13.25 wt% TFe2O3, and 20.96–28.87 wt% MgO. Primitive-mantle-normalized patterns of the rocks are comparable to those of ocean island basalt. The rocks define a relatively small range of Os isotopic compositions and a low Os signature of ?0.13 to +2.76 for γOs (261 Ma). In combination with their Sr–Nd–Os isotopic compositions, we interpret that these rocks were derived from the Emeishan plume sources as well as the interactions of plume melts with the overlying lithosphere which had been extensively affected by eclogite-derived melts from the deep-subducted oceanic slab. Partial melting induced by an upwelling mantle plume that involved an eclogite or pyroxenite component in the lithospheric mantle could have produced the parental Fe-rich magma. Our study suggests that plume-lithosphere interaction might have played a key role in generating many world-class Fe–Ti oxide deposits clustered in the Panxi area.  相似文献   

14.
We present compositional data on a 1,250-m-thick sequence of sparsely porphyritic lavas that comprise the Geikie Plateau Formation, part of the ~55-Ma break-up-related flood basalts in East Greenland. Major element compositions are relatively restricted (6.3–7.6 wt% MgO; 2.2–2.4 wt% TiO2), with two excursions to more evolved compositions (2.4–3.4 wt% TiO2) that are similar to the inferred parental magma of the nearby Skaergaard Intrusion. Major and trace element calculations show that fractional crystallisation is the principal control on magma compositions, and the cyclical sequential variations imply regular magma chamber replenishment events. Isotopic data indicate minor crustal assimilation, but with different contaminants for the main group (amphibolitic gneiss) and evolved cycles (granulitic gneiss). Rifting episodes may have allowed more primitive magmas to ascend to shallow crustal levels and subsequently fractionate to more evolved compositions in a separate chamber, which was perhaps similar to the source of the Skaergaard Intrusion.Electronic Supplementary Material Supplementary material is available in the online version of this article at Editorial responsibility: I. Parsons  相似文献   

15.
Trace, rare earth elements (REE), Rb-Sr, Sm-Nd and O isotope studies have been carried out on ultramafic (harzburgite and lherzolite) dykes belonging to the newer dolerite dyke swarms of eastern Indian craton. The dyke swarms were earlier considered to be the youngest mafic magmatic activity in this region having ages not older than middle to late Proterozoic. The study indicates that the ultramafic members of these swarms are in fact of late Archaean age (Rb-Sr isochron age 2613 ± 177 Ma, Sri ∼ 0.702 ± 0.004) which attests that out of all the cratonic blocks of India, eastern Indian craton experienced earliest stabilization event. Primitive mantle normalized trace element plots of these dykes display enrichment in large ion lithophile elements (LILE), pronounced Ba, Nb and Sr depletions but very high concentrations of Cr and Ni. Chondrite normalised REE plots exhibit light REE (LREE) enrichment with nearly flat heavy REE (HREE; (ΣHREE)N ∼ 2–3 times chondrite, (Gd/Yb)N ∼ 1). The εNd(t) values vary from +1.23 to -3.27 whereas δ18O values vary from +3.16‰ to +5.29‰ (average +3.97‰±0.75‰) which is lighter than the average mantle value. Isotopic, trace and REE data together indicate that during 2.6 Ga the nearly primitive mantle below the eastern Indian Craton was metasomatised by the fluid (± silicate melt) coming out from the subducting early crust resulting in LILE and LREE enriched, Nb depleted, variable εNd, low Sri(0.702) and low δ18O bearing EMI type mantle. Magmatic blobs of this metasomatised mantle were subsequently emplaced in deeper levels of the granitic crust which possibly originated due to the same thermal pulse.  相似文献   

16.
Granitoids from the central Mawson Escarpment (southern Prince Charles Mountains, East Antarctica) range in age from Archaean to Early Ordovician. U–Pb dating of zircon from these rocks indicates that they were emplaced in three distinct pulses: at 3,519 ± 20, 2,123 ± 12 Ma and between 530 and 490 Ma. The Archaean rocks form a layer-parallel sheet of limited extent observed in the vicinity of Harbour Bluff. This granitoid is of tonalitic-trondhjemitic composition and has a Sr-undepleted, Y-depleted character typical of Archaean TTG suites. εNd and TDM values for these rocks are −2.1 and 3.8 Ga, respectively. Subsequent Palaeoproterozoic intrusions are of granitic composition (senso stricto) with pronounced negative Sr anomalies. These rocks have εNd and TDM values of −4.8 and 2.87 Ga, indicating that these rocks were probably melted from an appreciably younger source than that tapped by the Early Archaean orthogneiss. The remaining intrusions are of Early Cambrian to Ordovician age and were emplaced coincident with the major orogenic event observed in this region. Cambro–Ordovician intrusive activity included the emplacement of layer-parallel pre-deformational granite sheets at approximately 530 Ma, and the intrusion of cross cutting post-tectonic granitic and pegmatitic dykes at ca. 490 Ma. These intrusive events bracket middle- to upper-amphibolite facies deformation and metamorphism, the age of which is constrained to ca. 510 Ma—the age obtained from a syn-tectonic leucogneiss. Nd–Sr isotope data from the more felsic Cambro–Ordovican intrusions (SiO2 > 70 wt%), represented by the post-tectonic granite and pegmatite dykes, suggest these rocks were derived from Late Archaean or Palaeoproterozoic continental crust (TDM ∼ 3.5–2.3 Ga, εNd ∼ −21.8 to −25.9) not dissimilar to that tapped by the Early Proterozoic intrusions. In contrast, the compositionally more intermediate rocks (SiO2 < 65 wt%), represented by the metaluminous pre-tectonic Turk orthogneiss, appear to have melted from a notably younger lithospheric or depleted mantle source (TDM = 1.91 Ga, εNd ∼ −14.5). The Turk orthogneiss additionally shows isotopic (low 143Nd/144Nd and low 87Sr/86Sr) and geochemical (high Sr/Y) similarities to magmas generated at modern plate boundaries—the first time such a signature has been identified for Cambrian intrusive rocks in this sector of East Antarctica. These data demonstrate that: (1) the intrusive history of the Lambert Complex differs from that observed in the adjacent tectonic provinces exposed to the north and the south and (2) the geochemical characteristics of the most mafic of the known Cambrian intrusions are supportive of the notion that Cambrian orogenesis occurred at a plate boundary. This leads to the conclusion that the discrete tectonic provinces observed in the southern Prince Charles Mountains were likely juxtaposed as a result of Early Cambrian tectonism.  相似文献   

17.
Mafic intrusives emplaced within the mélange zone of the Manipur Ophiolitic Complex are subalkalinetholeiitic affinity with Fe-enrichment. Based on the field occurrences, textures-mineralogy and whole-rock compositions, these mafic intrusives can be identified as type-I (gabbro intrusives) and type-II (basalt-dolerite dykes). The type-I resembling enriched-type mid-ocean ridge basalt (E-MORB) shows moderate LREE enrichment (LaN/SmN = 2.5–2.6), slightly enriched MORB normalized HFSE patterns possibly represent melts derived from enriched MORB sub-oceanic mantle sources by small degree of partial melting. The other type-II has normal-type mid-ocean ridge basalt (N-MORB) geochemical features, as it exhibits nearly flat to depleted LREE (LaN/SmN = 1.0–0.6), flat MORB normalized HFSE patterns with slight LREE/HREE depletion (CeN/YbN = 1.37–0.46). It might have been derived from depleted MORB type sub-oceanic mantle source. The MORB signature displayed by these mafic intrusives indicates that they are dismembered fragments of oceanic crust generated at mid-ocean spreading ridge system and support the hypothesis that the Manipur ophiolites was initially formed in the divergent plate margin.  相似文献   

18.
《Lithos》2004,72(1-2):1-18
The northern margin of the Nagssugtoqidian mobile belt in Southeast Greenland exposes a suite of moderately fractionated Fe-rich tholeiitic dykes of Paleo-proterozoic age. The dykes were intruded during extension of the crust prior to the development of the Nagssugtoqidian mobile belt. Although the dykes recrystallized under amphibolite facies conditions during Proterozoic orogenesis, they suffered little deformation. Excluding two very evolved samples, the compositions range between: 7.8–4.6 wt.% MgO, 44.5–52.9 wt.% SiO2 and 1.8–6.9 wt.% total alkalis. One group of mafic dykes shows distinct mantle-normalized trace element patterns with high abundance of low field strength elements and light rare earth elements and low abundances of high field strength elements. These characteristics are consistent with a process of fractional crystallization coupled with assimilation of the regional granulitic crust. Relatively high rates of assimilation to fractional crystallization (0.7) are required to generate the level of incompatible trace elements. This points to lower crustal conditions, and the assimilation is believed to have taken place at the base of the continental crust.Trace element variations indicate fractionation at high pressure involving clinopyroxene as the main extracted phase. We evaluate two fractionation models corresponding to a pressure of 0.9 and 1.5 GPa, respectively, and show that the trace element variations require polybaric fractionation at pressures from a maximum of 1.5 GPa to a minimum of 0.9 GPa.  相似文献   

19.
Mafic dykes of Almora region intrude the Precambrian crystalline rocks of Kumaun Lesser Himalaya. Mafic dykes exhibit fine grained margin and medium to coarse grained core, melanocratic, low to highly ferromagnetic (MS=0.85?38.58×10?3SI) in nature commonly showing subophitic to ophitic textures with ol-pl-cpx-hbl-bt-mt-ap-sp assemblage, and modally correspond to leucogabbro and olivinegabbro (sensu stricto). Olivine (Fo61-Fo33), clinopyroxene (Wo46-En42-Fs22 to Wo40-En36-Fs15) and plagioclase (An58-An12) have crystallized in the temperature range of ca1400–980°C at pressure <2 kbar in an olivine tholeiitic basalt parent. Low acmite (Napfu=0.033?0.025), (Mg#=0.64–0.82), Ti-Al contents of clinopyroxenes and their evolution along enstatite-ferrosilite join (i.e. Mg?Fe substitution) strongly suggest tholeiitic nature of mafic dyke melt with changing activities of alumina and silica. Clinopyroxene compositions of mafic dykes differ markedly as compared to those observed for adjoining Bhimtal volcanics but closely resemble to that crystallized in tholeiitic melts of Deccan province. Observed Cr vs Mg# variation, enriched LILE (Sr, Ba)-LREE and positive Eu-anomaly of the studied mafic dykes are indicative of fractional crystallization of olivine-clinopyroxene -plagioclase from a crustally-contaminated tholeiitic basalt magma derived from enriched mantle source. The mafic dykes of Almora are geochemically identical to mafic dykes of Nainital, but are unrelated to Precambrian mafic volcanic flow and dykes of NW Himalaya and dykes of Salma and Rajmahal regions.  相似文献   

20.
Mt. Baker is a dominantly andesitic stratovolcano in the northern Cascade arc. In this study, we show that the andesites are not all derived from similar sources, and that open-system processes were dominant during their petrogenesis. To this end, we discuss petrographic observations, mineral chemistry, and whole rock major and trace element chemistry for three of Mt. Baker’s late Pleistocene to Holocene lava flow units. These include the basalt and basaltic andesite of Sulphur Creek (SC) (51.4–55.8 wt% SiO2, Mg# 57–58), the Mg-rich andesite of Glacier Creek (GC) (58.3–58.7 wt% SiO2, Mg# 63–64), and the andesite and dacite of Boulder Glacier (BG) (60.2–64.2 wt% SiO2, Mg# 50–57). Phenocryst populations in all units display varying degrees of reaction and disequilibrium textures along with complicated zoning patterns indicative of open-system processes. All lavas are medium-K and calc-alkaline, but each unit displays distinctive trace element and REE characteristics that do not correlate with the average SiO2 content of the unit. The mafic lavas of SC have relatively elevated REE abundances with the lowest (La/Yb)N (~4.5). The intermediate GC andesites (Mg- and Ni-rich) have the lowest REE abundances and the highest (La/Yb)N (~6.7) with strongly depleted HREE. The more felsic BG lavas have intermediate REE abundances and (La/Yb)N (~6.4). The high-Mg character of the GC andesites can be explained by addition of 4% of a xenocrystic olivine component. However, their depleted REE patterns are similar to other high-Mg andesites reported from Mt. Baker and require a distinct mantle source. The two dominantly andesitic units (GC and BG) are different enough from each other that they could not have been derived from the same parent basalt. Nor could either of them have been derived from the SC basalt by crystal fractionation processes. Crystal fractionation also cannot explain the compositional diversity within each unit. Compositional diversity within the SC unit (basalt to basaltic andesite) can, however, be successfully modeled by mixing of basalt with compositions similar to the dacites in the BG unit. Given that the BG dacites erupted at ~80–90 ka, and a similar composition was mixed with the SC lavas at 9.8 ka, the process that produced this felsic end-member must have been repeatedly active for at least 70 ka.  相似文献   

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