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411.
安徽女山麻粒岩包体的地球化学特征:下地壳组成及其构造属性初探 总被引:7,自引:2,他引:7
女山麻粒岩包体的Mg#值总体较低(28-51),在化学成分上主要表现为偏中性,没有明显的辉石、石榴子石或Fe-Ti氧化物的堆晶作用,但部分有斜长石的堆晶作用,略具Eu,Sr正异常,部分则有斜长石的分离结晶,略具Eu,Sr负异常。另外,女山麻粒岩包体亏损Rb,Th,Nb,Ta,富集Ba,LREE,Zr,Hf等,具较高的Nb/Ta,Zr/Hf比值,麻粒岩相变质作用过程造成了Rb的强烈亏损,但对Nb,Ta,U,Th的影响较弱。女山麻粒岩包体的形成过程中存在岩浆的结晶分异作用,通过分离结晶混染作用模型(AFC)模拟,表明华北克拉通太古宙基性火山岩和中必一麻粒岩分别为较可能的初始岩浆和混合源,初始岩浆经过10%-40%的AFC可以形成女山麻粒岩包体的微量元素组成。总体上,女山麻粒岩包体的主元素、微量元素特征均相似于华北克拉通太古宙中性麻粒岩、华北克拉通下地壳平均值,而与扬子克拉通下地壳平均值得明显差别。因此女山地区下地壳可能仍属华北克拉通的一部分,因而支持Li(1994)的碰撞拆离模式。 相似文献
412.
Mechanisms of magmatic gas loss along the Southeast Indian Ridge and the Amsterdam -St. Paul Plateau
P.G. BurnardK.A. Farley 《Earth and Planetary Science Letters》2002,203(1):131-148
New analyses of He, Ne, Ar and CO2 trapped in basaltic glasses from the Southeast Indian Ridge (Amsterdam-St. Paul (ASP) region) show that ridge magmas degas by a Rayleigh distillation process. As a result, the absolute and relative noble gas abundances are highly fractionated with 4He/40Ar* ratios as high as 620 compared to a production ratio of ∼3 (where 40Ar* is 40Ar corrected for atmospheric contamination). There is a good correlation between 4He/40Ar* and the MgO content of the basalt, suggesting that the amount of gas lost from a particular magma is related to the degree of crystallization. Fractional crystallization forces oversaturation of CO2 because CO2 is an incompatible element. Therefore, crystallization will increase the fraction of gas lost from the magma. The He-Ar-CO2-MgO-TiO2 compositions of the ASP basalts are modeled as a combined fractional crystallization-fractional degassing process using experimentally determined noble gas and CO2 solubilities and partition coefficients at reasonable magmatic pressures (2-4 kbar). The combined fractional crystallization-degassing model reproduces the basalt compositions well, although it is not possible to rule out depth of eruption as a potential additional control on the extent of degassing. The extent of degassing determines the relative noble gas abundances (4He/40Ar*) and the 40Ar*/CO2 ratio but it cannot account for large (>factor 50) variations in He/CO2, due to the similar solubilities of He and CO2 in basaltic magmas. Instead, variations in CO2/3He (≡C/3He) trapped in the vesicles must reflect similar variations in the primary magma. The controls on C/3He in mid-ocean ridge basalts (MORBs) are not known. There are no obvious correlated variations between C/3He and tracers of mantle heterogeneity (3He/4He, K/Ti etc.), implying that the variations in C/3He are not likely to be a feature of the mantle source to these basalts. Mixing between MORB-like sources and more enriched, high 3He/4He sources occurs on and near the ASP plateau, resulting in variable 3He/4He and K/Ti compositions (and many other tracers). Using 4He/40Ar* to track degassing, we demonstrate that mixing systematics involving He isotopes are determined in large part by the extent of degassing. Relatively undegassed lavas (with low 4He/40Ar*) are characterized by steep 3He/4He-K/Ti mixing curves, with high He/Ti ratios in the enriched magma (relative to He/Ti in the MORB magma). Degassed samples (high 4He/40Ar*) on the other hand have roughly equal He/Ti ratios in both end-members, resulting in linear mixing trajectories involving He isotopes. Some degassing of ASP magmas must occur at depth, prior to magma mixing. As a result of degassing prior to mixing, mixing systematics of oceanic basalts that involve noble gas-lithophile pairs (e.g. 3He/4He vs. 87Sr/86Sr or 40Ar/36Ar vs. 206Pb/204Pb) are unlikely to reflect the noble gas composition of the mantle source to the basalts. Instead, the mixing curve will reflect the extent of gas loss from the magmas, which is in turn buffered by the pressure of combined crystallization-degassing and the initial CO2 content. 相似文献
413.
Stephen H. Watts 《地球表面变化过程与地形》1985,10(2):161-172
Surface specimens obtained from highly-weathered upland plateau outcrops at three localities in high Arctic Canada have been studied using the scanning electron microscope. Observations are given on factors influencing bedrock surface microfracturing processes. Evidence for the concentration of both salts and organic material in surface cracks which could enhance microfracturing is presented. The importance of lithological parameters including: mineralogy, texture, and structures present in influencing the actual processes of disintegration is reiterated. Under cold, dry high Arctic conditions the combined and apparently inseparable affects of frost action, salt crystallization, and organic activity may have contributed to microfracturing largely responsible for widely scattered examples of highly weathered bedrock terrain. 相似文献
414.
本文总结了广东韶关地区某些有矿、矿化和无矿花岗岩在造岩组分、成矿元素钨、微迹元素及其成岩时物理化学条件等方面存在的差异。提出K/Na比值、全岩和黑云母的H_2O~+含量、岩体钨丰度及钨随岩浆结晶分异过程的富集程度、黑云母WO_3含量、微迹元素(Mo、Cu、Ag、Bi)的丰度和统计参数、岩体形成过程中压力、氧化条件的变化等特征均可作为评价该区燕山期花岗岩类含矿性的地球化学指标。 相似文献
415.
四川攀西裂谷带峨眉玄武岩的岩石学及其成因研究 总被引:1,自引:0,他引:1
Li Changnian 《地球科学》1986,(6)
峨眉玄武岩属典型的大陆拉斑玄武岩。该玄武岩是在特定的温度、压力、氧逸度条件及裂谷的构造背景下形成的。它同时遭受分离结晶作用和地壳的同化混染作用。且可以用Depaolo AFC模型描述之。通过计算所获得的图解表现了C_L~i/C_(o,L)~i,F和r之间的相互关系(i为不相容的微量元素)。用B_(13)层玄武岩数据计算的C_(o,L)~(La)值为6.15ppm,F为0.75,r为0.75。 相似文献
416.
岩浆长期分离结晶对岩石锶同位素初始比值的影响 总被引:6,自引:0,他引:6
讨论了在岩浆长期分离结晶作用的模式,由于晚期残留熔体相对于早期结晶相其Rb/Sr比增高,当有一定滞留时间条件下,将导致晚期结晶相~(87)Sr/~(86)Sr初始比增高,因而在一个复式岩基内会形成具有不同锶同位素初始比的多个岩石单元。文中以安徽地洼阶段的黄山一狮子岭岩体为例,通过模拟计算,得到其初始岩浆(?)=0.7122,(Rb/Sr)0=3,该岩浆的80%分离结晶形成黄山岩体,其(?)=0.7143(t=130Ma),其余20%的残余熔体结晶后形成狮子岭岩体,其(?)=0.7180(t=123.1Ma)。 相似文献
417.
418.
我国东部苏鲁皖地区新生代碱性玄武岩中,除了含有大量地幔橄榄岩类捕虏体以外,尚含有一定数量的石榴石、普通辉石和歪长石巨晶。这些巨晶是在地幔不同深度上从玄武岩中晶出的。巨晶组合的分离结晶作用对熔体稀土元素含量有很大影响。赋存巨晶的碱性玄武岩所具有的LREE富集、HREE亏损的稀土元素分配型式是由地幔橄榄岩类部分熔融程度、石榴石巨晶和普通辉石巨晶的早期高压熔离和玄武岩的结晶分异作用等综合因素造成的。 相似文献
419.
420.