1、1、岩浆起源：熔融与部分熔融,熔 融 作 用（melting)：源岩全部发生熔融形成岩浆的过程富硅的矿物先熔融（熔融温度较低），后结晶；富铁镁的矿物后熔融，早结晶深熔作用（anatexis):指的是变质岩尤其是混合岩形成过程中地壳岩石发生分异和部分熔融的过程。,第八章 岩浆起源与演化,部分熔融（partial melting)： 岩石中仅部分矿物发生熔融在加热过程中，具有最低熔融温度的矿物首先熔融；在冷凝过程中具最低熔融温度的矿物最后结晶；部分熔融体是富硅（SiO2)的,未熔的残余是富FeO,MgO的；大洋（玄武岩）地壳是由贫硅的地幔岩部分熔融形成的；部分熔融形成的岩浆会聚积和迁移进入岩浆房（
2、分凝和上升作用）,二面角（ dihedral angle，）与熔体分凝能力,Illustration of the dihedral angle (q) of melt droplets that typically form at multiple grain junctions. After Hunter (1987) In I. Parsons (ed.), Origins of Igneous Layering. Reidel, Dordrecht, pp. 473-504.,-Increase T-Decrease P-Add Water,2、Why ：熔融原因？,（1）热的作用温度
3、随深度增加，俯冲板片被加热而发生熔融（火成岩矿物的熔融温度一般为700o1100oC)在伸展背景下地幔对流（软流圈上涌）玄武岩浆底侵作用例子：俯冲带,Crustal Geothermal Gradients,Crustal Rocks Melt!,convection in the mantle,models,observed heat flowwarm: near ridgescold: over cratons,from: http:/www.geo.lsa.umich.edu/crlb/COURSES/270,from: http:/www-personal.umich.edu/vdpl
4、uijm/gs205.html,Global Heat Flow,（2）压力的作用随压力增大，矿物的熔融温度增高；热的地幔岩石会由于压力降低而发生熔融例子：热点和洋中脊,（3）挥发份的作用：降低熔融温度,Plagioclase Water-saturated vs. Dry Solidi,MORB 减压熔融,3、Where:岩浆产生的构造背景举例,Mantle Plumes - “Hot” or “Wet” Spots?,Seismic Tomographic Image of Iceland Plume,From: ICEMELT Seismic Experiment - Wolfe et
5、al., 1997,Contour of -2.5%shear wavevelocity anomaly,Super Plumes?,From: www.seismo.berkeley.edu/gung/_Qplume/,Volcanic Hot Spots on Earths Surface (dots),Global shear wave velocity anomalies in deep mantle,Volatile Fluxing Mantle Wedge,Volatile Fluxing of Mantle Wedge,Downgoing Slab Crustal Melting
6、,Primitive Mantle Melts vs. Remelting of the Lower Crust,岩浆底侵作用,Island Arc Petrogenesis,A proposed model for subduction zone magmatism with particular reference to island arcs. Dehydration of slab crust causes hydration of the mantle (violet), which undergoes partial melting as amphibole (A) and phl
7、ogopite (B) dehydrate. From Tatsumi (1989), J. Geophys. Res., 94, 4697-4707 and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford.,4、不同源区的产生的岩浆类型,地 幔：玄武岩浆、碧玄岩浆、霞石岩浆、苦橄岩浆、 科马提岩浆、金伯利岩浆、碳酸岩岩浆；地 壳：花岗岩类；俯冲带（地幔、洋壳、陆壳）： a.俯冲洋壳：由于脱水，产生含Si高的流体，以产生“ 安山岩浆 ”, 埃达克岩(adakite) b.地幔楔性区：从靠近大洋的岛弧
8、拉斑玄武岩(TH)，向大陆K增高，变成碱性玄武岩浆 c.陆壳：钙碱性I型花岗岩和酸性火山岩,二、岩浆演化（岩浆分异）（Differentiation of Magmas),自然界岩浆岩类型多种多样，岩浆岩多样性原因？岩浆的不同起源原生岩浆的多样性岩浆的复杂演化 进化岩浆的多样性岩浆主要通过以下作用而发生成分改变：分异、同化混染、岩浆混合作用,火成岩成分的多样性,Analyses of a global sample of 41,000 igneous rocks of all ages,46.7% （1）分散在宽的范围内,53.3% 的样品聚积在中心带内（2，3，4）,分异作用的主要类型1、封
9、闭系统的分异作用（1）结晶分异作用（晶体熔体之间的分异）：重力分异、流动分异、压滤作用、熔体对流分异（2）不混熔熔体的物理分异（3）熔体流体分离2、开放系统的分异作用（1）围岩混染物的同化作用（2）两种或多种不同岩浆的混合（3）AFC (同时发生结晶分异与同化混染）,（一）结晶分异作用,（晶体熔体之间的分异）：重力分异、流动分异、压滤作用、熔体对流分异,A. Crystal Settling重力分异,岩浆中晶体的重力沉降分异，需要克服岩浆对晶体的浮力和与岩浆之间的粘滞摩擦力以及岩浆上升速度。晶体能否从岩浆中沉降分异，取决于晶体与岩浆的密度差() 、晶体直径(B) 和岩浆的粘度()。当岩浆中晶体
10、的含量不高时，晶体在岩浆中的下沉速度可用斯托克斯定律来计算 Vs=gB2/18,Plagioclase Differentiation Mechanisms,Gabbro - Plane Polarized Light,Plagioclase zoning,Crystal Settling,zoned plag,Perthiticpyroxene,Magmatic Cross-Beds in Skaergaard Layered Intrusion,From: http:/www.uoregon.edu/dogsci/kays/313/plutonic.html,Crystal Fractio
11、nation of magmas as they rise,Tholeiite alkalineby FX at med to high PNot at low PThermal divideAl in pyroxenes at Hi PLow-P FX hi-Al shallow magmas (“hi-Al” basalt),Schematic representation of the fractional crystallization scheme of Green and Ringwood (1967) and Green (1969). After Wyllie (1971).
12、The Dynamic Earth: Textbook in Geosciences. John Wiley & Sons.,B. Flow segregation,The motion of the magma past the stationary walls of the country rock creates shear in the viscous liquidMagma must flow around phenocrysts, thereby exerting pressure on them at constrictions where phenocrysts are nea
13、r one another or the contact grain dispersive pressure, forcing the grains apart and away from the contactThis is probably a relatively minor effect,Langmuir ModelThermal gradient at wall and cap variation in % crystallizedCompositional convection evolved magmas from boundary layer to cap (or mix in
14、to interior),Formation of boundary layers along the walls and top of a magma chamber. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall,对流分异,Schematic section through a rhyolitic magma chamber undergoing convection-aided in-situ differentiation. After Hildreth (1
15、979). Geol. Soc. Amer. Special Paper, 180, 43-75.,The magma near the vertical contact becomes enriched in water from the wall rocks. Water-enriched boundary layer, although cooler, is less dense than the interior magma, and rises to concentrate near the top of the chamberThis is compositional convec
16、tion,(二）同化混染作用,(1)玄武岩浆同化酸性岩(浆岩)：可以使岩浆成分变得更酸性， 并能促进玄武岩浆的分离结晶作用(降低了岩浆温度，并 且结晶加速)(2)同化泥质岩(含Al高)：以Al2O3和SiO2为主的岩石 透辉石Al2O3SiO2=AnPy 这样就可以形成苏长岩，因此有人认为苏长岩是同化混染作用形成的。,1、玄武岩浆的同化作用,2、花岗岩浆同化混染作用,主要是通过与固态高熔点的围岩或捕掳体发生反应，改造它们，而使得花岗岩浆成分发生改变 如同化混染了含Fe、Mg较高的围岩，可使岩浆基性程度增高，可以形成闪长岩或花岗闪长岩，同化碳酸盐岩，由于Ca增Si减，也可形成闪长岩或花岗闪长岩。
17、若花岗岩浆同化了泥质岩，岩浆中Al增加，可形成一些富Al矿物，如安徽某花岗岩普遍含红柱石。,Assimilation,Gradational border zones between homogeneous igneous rock (light) and country rock (dark). After Compton (1962), Manual of Field Geology. R. Compton.,Possible reactions,Mg2SiO4 + SiO2 = 2MgSiO3Olivine + Quartz =OrthopyroxeneCaMg2Si2O6 + Al2S
18、iO5 = CaAl2Si2O8 + MgSiO3Diopside + Sillimanite = Plagioclase + Orthopyroxene,Magmatic Differentiation: Assimilation,（三）岩浆混合作用,岩浆混合作用既是再造新生岩浆又是开放体系下岩浆演化的重要岩浆作用，因而它已成为岩浆多元性和火成岩多样性的重要因素。 混合岩浆的端元岩浆多为镁铁质和长英质成份，其中镁铁质岩浆多来自地幔，而长英质岩浆的源区则是地壳，而且它有时还是热的镁铁质岩浆作用于地壳部分熔融形成。,Basalt pillows accumulating at the botto
19、m of a in granitic magma chamber, Vinalhaven Island, Maine,Comingled basalt-Rhyolite Mt. McLoughlin, Oregon,Figure 11-8 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall,岩浆混合作用识别标志：1、在酸性岩中见基性端元的岩石团块、微粒包体；2、酸性端元的熔岩中见明显流变特征的基性端元熔岩条带；3、酸性端元中见基性端元的岩墙及其边缘的机械混合带和成 分过渡带
20、；4、矿物间明显的不平衡，如成分差别较大的斜长石共存、矿 物间的交代结构发育；5、化学成分符合混合趋势。,MagmaticDifferentiation:Magma Mixing,Mingling and mixing,Closepet granite (2.5 Ga), South India,Melt Inclusions in Quartz in Pantellerite,From: http:/wrgis.wr.usgs.gov/lowenstern/Mahood and Lowenstern, 1991,Harker diagramSmooth trendsModel with 3
21、assumptions:1 Rocks are related by FX2 Trends = liquid line of descent=path of crystallization3 The basalt is the parent magma from which the others are derived,Figure. Stacked variation diagrams of hypothetical components X and Y (either weight or mol %). P = parent, D = daughter, S = solid extract
22、, A, B, C = possible extracted solid phases. For explanation, see text. From Ragland (1989). Basic Analytical Petrology, Oxford Univ. Press.,变异图反映分异作用,(a)矿物X从液相L1中分离，液相变为L2(b)由矿物X和Y构成的C从液相L1中析出，使液相成分变为L2(c)由矿物X,Y和Z构成的C从液相L1中析出，使液相成分变为L2,Closepet granite (2.5 Ga), South India,Closepet granite (2.5 Ga), South India,岩石类型、岩浆过程与构造环境,大洋壳由玄武岩构成，而玄武岩来自软流圈上涌（降压）而产生的岩浆产物；与大陆壳相比，洋壳较薄而更年轻，这是由于洋壳在洋脊缓慢形成而在俯冲带循环回地幔的结果；大洋壳的平均年龄为75Ma (范围为0250Ma);,岩浆作用对地壳成因的意义,大陆壳，相当于花岗闪长岩成分，必须至少经历两阶段岩浆事件才能产生：（1）地幔物质熔融产生镁铁质岩浆；（2）镁铁质岩浆分异产生花岗岩留下更富铁镁的堆积岩，或镁铁质岩浆重熔产生花岗质熔体和更富铁镁的残余；（3）富铁镁的堆积岩或残余再循环返回到地幔拆沉作用或热剥蚀作用。,
