壳-幔物质交换的岩浆岩石学研究

地质地球物理综合研究表明,岩石学地球化学意义上的壳幔边界与地球物理学确定的莫霍面并不等同,物质成分上的壳幔边界具有动态演化的特点,并具有两侧物质成分的双向交换。火成岩中的深源包体和出露的壳幔边界剖面研究揭示,岩浆底侵作用是地幔物质进入地壳的主要方式,并造成地壳垂向增生,而拆沉作用是地壳物质进入地幔的重要机制,上述两种过程的联合是大陆地壳内部地质演化的重要原因     

壳幔作用与花岗岩成因——以中国东南沿海为例

笔者在近年来的中国东南沿海花岗岩成因研究中,注意到下地壳之下的岩石圈地幔与下地壳之间必然有着十分密切的关系,认识到壳幔作用的重要形式是发生于壳－幔接口的玄武岩浆的底侵作用,它涉及地幔对地壳在“成分”和“热”两方面的贡献。研究表明,本区底侵作用十分发育,是中国东南大陆边缘陆壳演化的重要 过程。中国东南部中生代早期形成的花岗岩多为Ｓ－型花岗岩,它们主要是板块强烈挤压和导致地壳增厚,陆壳重熔形成的岩石。而且这期大规模花岗岩浆活动是与弧后拉张、岩石圈减薄 软流圈上涌作用直接有美．早期太平洋板块向欧亚大陆板块俯冲对大陆裂解起了诱导作用中国东南部晚中生代缝合带的年龄为100～11OMa,可能代表了晚中生代构造－岩浆作用由挤压,地壳增厚,陆壳重熔向扩张岩石圈减薄一双蜂式岩浆作用机制的转变年龄。     

暗色微粒包体是壳幔岩浆混合作用的证据吗?

暗色微粒包体常见于钙碱性花岗岩中,已普遍被认为是幔源基性岩浆与壳源酸性岩浆在地壳深部发生混合作用的产物。本文通过大量资料的分析研究,发现暗色微粒包体可以具有很大负值的全岩ε_Nd(t)值和锆石ε_Hf(t)值,及大于0.710的全岩[n(⁸⁷Sr)/n(⁸⁶Sr)]_i值,不存在幔源岩浆混合的痕迹;而且,大多数暗色微粒包体与寄主花岗岩在晶体化学、形成年龄、全岩和锆石同位素成分等方面显示出完全相似的特征,反映出两者在时空与物质上都具有紧密的成因联系。笔者认为,暗色微粒包体不应该是壳幔岩浆混合作用的产物。基于包体岩浆极小的体量和稍晚的侵位(相对于寄主花岗岩),笔者提出一种新的暗色微粒包体的形成方式：同造山花岗岩浆的主动上侵造成岩浆房内的“负压力”而导致岩浆房下部呈晶粥状态的闪长质层发生等温减压熔融作用,从而形成体量极小的包体岩浆;并即时“注入”地壳上部尚未固结的寄主花岗岩中,快速冷凝形成暗色微粒包体。因此,暗色微粒包体不能被视作为“壳幔岩浆混合作用”的证据。     

暗色微粒包体是壳幔岩浆混合作用的证据吗？

暗色微粒包体常见于钙碱性花岗岩中,已普遍被认为是幔源基性岩浆与壳源酸性岩浆在地壳深部发生混合作用的产物。本文通过大量资料的分析研究,发现暗色微粒包体可以具有很大负值的全岩εNd（t）值和锆石εHf（t）值,及大于0710的全岩\[n（87Sr）/n（86Sr）\]i值,不存在幔源岩浆混合的痕迹;而且,大多数暗色微粒包体与寄主花岗岩在晶体化学、形成年龄、全岩和锆石同位素成分等方面显示出完全相似的特征,反映出两者在时空与物质上都具有紧密的成因联系。本文认为,暗色微粒包体不应该是壳幔岩浆混合作用的产物。基于包体岩浆极小的体量和稍晚的侵位（相对于寄主花岗岩）,本文提出一种新的暗色微粒包体的形成方式：同造山花岗岩浆的主动上侵造成岩浆房内的 “负压力” 而导致岩浆房下部呈晶粥状态的闪长质层发生等温减压熔融作用,从而形成体量极小的包体岩浆;并即时 “注入” 地壳上部尚未固结的寄主花岗岩中,快速冷凝形成暗色微粒包体。因此,暗色微粒包体不能被视作为 “壳幔岩浆混合作用” 的证据。     

岩浆混合作用研究综述

本文详细阐述了近年来国内外对岩浆混合作用的研究进展,集中在混合作用的验证及地幔物质参与对混合作用的影响。从能量、物质运移的角度认识岩浆混合作用,以揭示上地幔、地壳的信息,并为认识区域构造一岩浆演化提供约束。综合归纳了岩浆混合作用主要的研究方法,系统总结了混合作用发生的规律及主要影响因素,并应用已有理论和数据进行简单的数值模拟,合理解释岩脉的不混合特征。作为壳一幔间物质与能量交换的重要形式,阐述了岩浆混合与底侵作用的关系,并介绍了常见的岩浆混合作用岩石成因模型。最后说明了岩浆混合作用今后的发展趋势和存在问题。     

郭家岭岩体壳幔岩浆混合作用与侵位机制的动力学研究

暗色包体、岩石地球化学及Nd.Sr同位素的研究一致表明,胶东金矿区的郭家岭岩体是由壳幔岩浆混合形成的,混合比大体为3:2.该岩体岩性均一,缺少长英质端元和过渡性岩石类型正是壳幔岩浆混合程度高的表现。以胶东群地层150MPa.850℃条件下的部分熔融产物(熔体分数约35%)作为长英质端元,最偏基性的闪长质包体作镁铁质端元,按3:2的混合比较好地模拟了该岩体的成分,同时也满足了分层岩浆房发生整体对流的动力学条件。根据暗色包体计算了岩体的侵位速度,结果表明该岩体很可能是沿着现在的艾山岩体所占据的断裂通道侵位的。岩体呈EW向带状产出反映了基底构造——栖霞复背斜对该岩体形成的控制作用。     

区域成矿与壳幔作用初探

区域成矿研究从简单到复杂,从地表到地壳深部,现在已经向众多学科,纵深领域渗透.壳幔作用是现代地球动力学研究的一个关注点.本文对壳幔动力作用与区域成矿关系进行探讨,分别讨论了俯冲、底侵、拆沉和地幔柱对区域成矿作用的影响和作用.初步建立了深部动力作用与区域成矿之间的关系,为研究区域成矿的深部机理和规律提供依据.     

壳幔混合层的物质过程及其动力学意义

以评述壳幔混合层的物质过程为主,对壳幔混合层的概念、研究现状、形成机制及作用过程等作了简要综述。底侵作用、拆沉作用和部分熔融作用是壳幔混合层的主要物质过程。底侵和拆沉构成壳幔物质循环,部分熔融则为其提供物质和条件,这三者相互作用,影响和制约着壳幔物质交换,在大陆地壳变形中起着极其重要的作用。最后分析了壳幔混合层的研究在岩石圈动力学中的意义     

辽西中生代的壳幔过渡带的形成、破坏与改造

利用火成岩及其携带的捕虏体的岩石学、年代学等研究成果,反演辽西中生代时期的壳幔相互作用,重塑这时期的壳幔边界组成。对应于中生代活跃的壳幔相互作用,辽西义县．北票一带理论上应该存在一个发育的壳幔过渡带。然而现今的各种地球物理研究显示辽西的莫霍面是清晰的间断面,其上、下地震波速变化从6．5—6．7km／s截然变成8．0—8．1km／s。壳幔过渡带到那里去了？围绕这一问题,本文依据火山岩及其捕虏晶的研究,探讨了在软流圈底辟体上涌背景下,壳幔过渡带被破坏和改造的原因。最后从演化角度将辽西与张家口和西藏高原的壳．幔边界进行了比较。     

记载了早中生代壳幔演化的赤峰-凌源地质走廊

本文从内蒙赤峰至辽西凌源南北近200km的地质走廊中,选择早中生代(237～206 Ma)形成不同深度、不同产状的8种火成岩作为研究对象,探讨早中生代华北克拉通北缘的地质演化.这8种岩石包括:火山岩、岩墙群和闪长岩、碱性花岗岩、碱性杂岩、镁铁质麻粒岩、堆晶岩及其捕获的地幔岩包体.研究表明,在底侵作用的背景下,华北早中生代发生了强烈的壳幔相互作用和岩石圈地幔富集过程,伴随早中生代大陆地壳的增生和深部结构的调整,地表相应出现快速的差异抬升和剥蚀.     

浙江中部芙蓉山花岗斑岩及包体岩石地球化学研究

花岗岩的源区、温压条件及与其他岩石的共生组合的研究可以限定其形成构造背景,了解其形成的深部动力学过程。本文对浙江中部中生代芙蓉山花岗斑岩及其暗色包体开展了全岩主微量元素、锆石U-Pb年代学和Hf同位素、Ti温度计和全岩Sr-Nd同位素研究,探讨芙蓉山花岗斑岩的成因类型、源区特征及其与镁铁质包体之间的关系,并进一步限定其构造背景。锆石U-Pb定年结果显示芙蓉山花岗斑岩加权平均年龄为133.6±1.6 Ma (MSWD = 2.1,2σ),镁铁质包体加权平均年龄为130.3±4.2 Ma (MSWD = 5.8,2σ),其成岩年龄基本一致,形成于早白垩世。全岩地球化学表明芙蓉山花岗斑岩属于亚碱性系列,同时具有低的Ga/Al(<2.6),高的Na2O含量(2.75%-4.5%,平均3.78%),较高的全碱含量(Na2O + K2O = 7.93%-8.75%),以及较低的锆石Ti饱和温度特征（631-690℃）,这些特征显示芙蓉山花岗斑岩为I型花岗岩,而非A型花岗岩。芙蓉山花岗斑岩中锆石Hf同位素有比较大的范围[εHf(t)=-10.9~-1.1],全岩[n(87Sr)/ n(86Sr)]i = 0.7062-0.7078,相对富集的εNd(t)=-5.6 ~ -4.7值;而包体表现为均一的[n(87Sr)/ n(86Sr)]i=0.7071-0.7079和弱富集的εNd(t)=-3.8 ~-2.8特征。野外及岩相学和元素地球化学特征显示典型的岩浆混合特征。镁铁质包体源区可能来自于俯冲交代地幔,芙蓉山花岗斑岩则形成于古老富钾地壳熔体和交代地幔熔体混合后的结晶分异。混合模型计算表明混合比例为：~80%的地幔端元和~ 20%地壳端元。浙江中部芙蓉山富钾I型花岗斑岩与镁铁质包体共生可能指示其形成于古太平洋板块俯冲后撤初始弱伸展拉张的构造背景。     

浙江中部芙蓉山花岗斑岩及包体岩石地球化学研究

花岗岩的源区、温压条件及与其他岩石的共生组合的研究可以限定其形成构造背景,了解其形成的深部动力学过程.本文对浙江中部中生代芙蓉山花岗斑岩及其暗色包体开展了全岩主微量元素、锆石U-Pb年代学和Hf同位素、Ti温度计和全岩Sr—Nd同位素研究,探讨芙蓉山花岗斑岩的成因类型、源区特征及其与镁铁质包体之间的关系,并进一步限定其...     

华北大陆地壳—上地幔岩石学结构与演化

本文基于出露的前寒武纪变质岩系、中、新生代岩浆活动以及新生代玄武岩中上地幔包体的岩石学与地质压力计研究，结合地球物理测深资料与高温高压下岩石中地震波传播速度的实验成果，提出了华北大陆三个地区(河北平原、太行一五台、鄂尔多斯)的地壳一上地幔岩石学结构，讨论了界面性质及其演化。在强调Vp、 Vs、a结构与岩石学结构共同约束的基础上，有效地识别了不同地区硅铝质陆壳在物质组成上的差异和上地幔低速层或矿物相转变等特征。本文提出壳-幔岩石学结构及其演化，密切地与陆壳主要形成时期的太古一早元古构造岩浆事件相关，又与显生宙构造岩浆事件对它的改造程度有关，壳-幔岩石学结构是我们追索大陆的构造性质及其演化的一个重要记录和科学依据。     

湖南桂东小江花岗岩体:一个潜在Rb-Nb-Y矿床的岩石化学特征及其成矿远景

铷是一种具有广泛用途的稀有金属。湖南桂东县小江地区曾于1958年地质预查确定为砂锡矿,但未见进一步工作的报道。本次工作系统采集了矿区的细粒二云母花岗岩、黑云母斑状花岗岩、粗粒黑云母花岗岩的岩石样品。精细测试表明,三者是同期花岗岩浆多阶段演化的产物,随着演化程度的增高,岩体内的稀有金属含量逐渐增高。细粒二云母花岗岩属于重稀土富集型花岗岩,稀土配分模式表现出岩浆晚期富挥发份组成而造成的四分组效应,其基岩中的Rb2O含量超过0.1%,达到花岗岩型铷矿床的工业品位;Nb的含量达到伴生矿的要求,具有全岩矿化的特征;风化壳中Y的含量达到许多离子吸附型矿床的钇含量。Rb元素主要赋存在二云母花岗岩的钾长石和黑云母中,Y主要赋存在萤石中。按照二云母花岗岩的厚度50m计算,Rb2O储量约为16万吨。除此之外,矿区还应该注意寻找伟晶岩型稀有金属矿脉,并重视矿区风化壳发生离子吸附型重稀土矿化的可能性。     

The Generation of Granitic Magmas by Intrusion of Basalt into Continental Crust

When basalt magmas are emplaced into continental crust, meltingand generation of silicic magma can be expected. The fluid dynamicaland heat transfer processes at the roof of a basaltic sill inwhich the wall rock melts are investigated theoretically andalso experimentally using waxes and aqueous solutions. At theroof, the low density melt forms a stable melt layer with negligiblemixing with the underlying hot liquid. A quantitative theoryfor the roof melting case has been developed. When applied tobasalt sills in hot crust, the theory predicts that basalt sillsof thicknesses from 10 to 1500 m require only 1 to 270 y tosolidify and would form voluminous overlying layers of convectingsilicic magma. For example, for a 500 m sill with a crustalmelting temperature of 850 ?C, the thickness of the silicicmagma layer generated ranges from 300 to 1000 m for countryrock temperatures from 500 to 850?C. The temperatures of thecrustal melt layers at the time that the basalt solidifies arehigh (900–950?C) so that the process can produce magmasrepresenting large degrees of partial fusion of the crust. Meltingoccurs in the solid roof and the adjacent thermal boundary layer,while at the same time there is crystallization in the convectinginterior. Thus the magmas formed can be highly porphyritic.Our calculations also indicate that such magmas can containsignificant proportions of restite crystals. Much of the refractorycomponents of the crust are dissolved and then re-precipitatedto form genuine igneous phenocrysts. Normally zoned plagioclasefeldspar phenocrysts with discrete calcic cores are commonlyobserved in many granitoids and silicic volcanic rocks. Suchpatterns would be expected in crustal melting, where simultaneouscrystallization is an inevitable consequence of the fluid dynamics. The time-scales for melting and crystallization in basalt-inducedcrustal melting (10²–10³ y) are very short compared tothe lifetimes of large silicic magma systems (>10⁶ y) orto the time-scale for thermal relaxation of the continentalcrust (> l0⁷ y). Several of the features of silicic igneoussystems can be explained without requiring large, high-level,long-lived magma chambers. Cycles of mafic to increasingly largevolumes of silicic magma with time are commonly observed inmany systems. These can be interpreted as progressive heatingof the crust until the source region is partially molten andbasalt can no longer penetrate. Every input of basalt triggersrapid formation of silicic magma in the source region. Thismagma will freeze again in time-scales of order l0²–10³y unless it ascends to higher levels. Crystallization can occurin the source region during melting, and eruption of porphyriticmagmas does not require a shallow magma chamber, although suchchambers may develop as magma is intruded into high levels inthe crust. For typical compositions of upper crustal rocks,the model predicts that dacitic volcanic rocks and granodiorite/tonaliteplutons would be the dominant rock types and that these wouldascend-from the source region and form magmas ranging from thosewith high temperature and low crystal content to those withhigh crystal content and a significant proportion of restite.     

Geochemistry and zircon U-Pb geochronology of Miocene plutons in the Urumieh-Dokhtar magmatic arc,east Tafresh,Central Iran

ABSTRACT

The Tafresh plutons that include Ahmadabab diorite, Vasfonjerd monzonite, Mehrezamin diorite and Chahak diorite, located to the east of Tafresh city, north-central Iran, are part of Urumieh-Dokhtar magmatic arc. U-Pb dating of zircon grains provides emplacement ages of 22.3 ± 1 Ma for the Ahmadabad diorite, and tightly clustered ages of 22.2 ± 0.2 Ma, 21.3 ± 0.2 Ma, and 21.7 ± 0.4 Ma for Vasfonjerd monzodiorite, Mehrezamin diorite-monzonite, and Chahak diorite-monzonite plutons, respectively. These rocks are metaluminous to weakly peraluminous, calc-alkaline, and characterized by enrichment in light rare earth elements, Nb-Ta negative anomalies, and high LILE/HFSE ratios. Tafresh plutonic rocks originated from a parental magma source and experienced different degrees of partial melting. Geochemical signatures of Tafresh plutonic rocks, such as a wide range of Y/Nb (2.7–8.4) and low Zr/Nb (19.5–35.) ratios, Nb/Ta (11.46–18.15), argue for mantle–crust interaction during generation of Tafresh magmas. Relatively low Nb/La ratios further indicate that the lithospheric mantle played a significant role in melt generation. HREE signatures (i.e. decrease Dy/Yb with increasing SiO₂) preclude substantial involvement of garnet either in the residue, both during partial melting and fractionation of the magma. The plutons are a product of final stages of subduction-related magmatism prior to the collision between the Arabian and Eurasian tectonic plates.     

Petrogenesis of Dashenshan I-type granodiorite: implications for Triassic crust–mantle interaction,South China

Triassic granodiorites in South China (SC) provide an opportunity to examine crust–mantle interactions that may have been caused by a mantle plume. Here we present a combined study of chronological, geochemical, and Sr–Nd–Hf isotopic compositions for Dashenshan granodiorites. These are high-K, calc-alkaline, I-type granodiorites that yield a U–Pb zircon age of 211 ± 3 Ma. They are metaluminous to weakly peraluminous (A/CNK < 1.1), with 3.04–3.89 wt.% Na₂O and 3.24–3.86 wt.% K₂O, and Na₂O/K₂O ratio ranging from 0.79 to 1.11. These granodiorites contain 67.7–72.6 wt.% SiO₂ but show moderate Mg^# values (44.2–57.8) and variable contents of Ni (3.6–29.9 ppm) and Cr (7.6–53.5 ppm). They exhibit light rare earth element (REE) enrichment and flat, heavy REE patterns with negative Eu anomalies (Eu/Eu* = 0.52–0.87). They also display strongly negative Ba, Sr, Nb, Ta, P, and Ti anomalies and positive Rb, Th, K, and Pb anomalies. Dashenshan granodiorites have high whole-rock initial ⁸⁷Sr/⁸⁶Sr ratios (0.7121–0.7172), negative ε_Nd (t) values (–8.8 to –6.8), and negative zircon ε_Hf (t) values (–6.6 to –3.3). These results suggest that the Dashenshan granodiorites were generated by a mixing between crustal melt and mantle-derived magma in an extensional setting. We conclude that generation of the Dashenshan pluton may reflect an interaction between a mantle plume and the overlying SC crust.     

西南天山二叠纪中酸性侵入岩的地质学和地球化学：岩石成因和构造背景

选择3个典型岩体,即位于西南天山东段的拜城县英买来岩体和位于西段阔克萨岭区的川乌鲁岩体、巴雷公岩体（为了对比,也选择了位于塔里木盆地西北缘的麻扎山岩体）,进行了岩石学和地球化学研究。结果表明,这些岩体具有不同的特点。英买来岩体为黑云母花岗岩和二云母花岗岩,具有高的SiO2含量,弱过铝,高的Sr同位素初始值（约0.710）和负的εNd(t)值（-4～-6）,属于S—A型之间的过渡类型。麻扎山岩体由正长岩组成,属于碱性岩,微量元素标准化图解和其他岩体明显不同的是没有明显的Nb和Ta的负异常。川乌鲁岩体是一个由3个不同期次岩石组成的杂岩体,主体为正长岩-二长岩,地球化学特征显示是由基性岩浆和酸性岩浆不同程度混合形成的。位于同一构造区的巴雷公岩体则与川乌鲁岩体中的花岗斑岩的地球化学特征相似。综合岩石学和地球化学特征推测,南天山东段的英买来岩体是地壳熔融的结果,没有任何地幔物质加入的地球化学信息,西段的阔克萨岭地区酸性岩浆的形成则可能是来自于幔源底侵的基性岩浆导致薄的地壳发生熔融的结果。麻扎山岩体则完全是不同构造背景的产物,有可能与发生在塔里木盆地的二叠纪大规模的岩浆活动有关。因此,二叠纪岩浆活动的性质主要受地壳成分和结构的控制。     

Genesis and Mixing/Mingling of Mafic and Felsic Magmas of Back-Arc Granite: Miocene Tsushima Pluton, Southwest Japan

The Middle Miocene Tsushima granite pluton is composed of leucocratic granites, gray granites and numerous mafic microgranular enclaves (MME). The granites have a metaluminous to slightly peraluminous composition and belong to the calc‐alkaline series, as do many other coeval granites of southwestern Japan, all of which formed in relation to the opening of the Sea of Japan. The Tsushima granites are unique in that they occur in the back‐arc area of the innermost Inner Zone of Southwest Japan, contain numerous miarolitic cavities, and show shallow crystallization (2–6 km deep), based on hornblende geobarometry. The leucocratic granite has higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.7065–0.7085) and lower ε_Nd(t) (?7.70 to ?4.35) than the MME of basaltic–dacitic composition (0.7044–0.7061 and ?0.53 to ?5.24), whereas most gray granites have intermediate chemical and Sr–Nd isotopic compositions (0.7061–0.7072 and ?3.75 to ?6.17). Field, petrological, and geochemical data demonstrate that the Tsushima granites formed by the mingling and mixing of mafic and felsic magmas. The Sr–Nd–Pb isotope data strongly suggest that the mafic magma was derived from two mantle components with depleted mantle material and enriched mantle I (EMI) compositions, whereas the felsic magma formed by mixing of upper mantle magma of EMI composition with metabasic rocks in the overlying lower crust. Element data points deviating from the simple mixing line of the two magmas may indicate fractional crystallization of the felsic magma or chemical modification by hydrothermal fluid. The miarolitic cavities and enrichment of alkali elements in the MME suggest rapid cooling of the mingled magma accompanied by elemental transport by hydrothermal fluid. The inferred genesis of this magma–fluid system is as follows: (i) the mafic and felsic magmas were generated in the mantle and lower crust, respectively, by a large heat supply and pressure decrease under back‐arc conditions induced by mantle upwelling and crustal thinning; (ii) they mingled and crystallized rapidly at shallow depths in the upper crust without interaction during the ascent of the magmas from the middle to the upper crust, which (iii) led to fluid generation in the shallow crust. The upper mantle in southwest Japan thus has an EMI‐like composition, which plays an important role in the genesis of igneous rocks there.     

大兴安岭北端阿乌尼地区中生代杂岩体的岩浆混合作用

通过外野地质特征、岩相学、岩石地球化学等方面的研究，从不同的角度论证了大兴安岭阿乌尼地区出露的岩浆杂岩体在成因上具有岩浆混合作用的特点。同位素上具低的Sr初始比值和正的εNd（t）值，明显具有幔源特点，而微量元素相对富集大离子亲石元素和高场强元素又具有壳源特征，表明阿乌尼地区的岩浆岩可能是壳幔相互作用的产物。