7第七章 岩浆起源、上升与侵位.ppt

1、岩浆的形成和分异过程（据 周新民）,第七章 岩浆起源、 上升与侵位,一、岩浆起源,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。,1、熔融与部分熔融,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

2、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（分凝和上升作用）,The ability to form an interconnected film is dependent upon the dihedral angle () a property of the melt,Figure 11-1 Illustration of the dihedral angle (q) of melt droplets that typically form at multiple grain junctions. After Hunter (1987) In I. Parsons (e

3、d.), 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!,（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

4、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?,Volatile Fluxing Mantle Wedge,Downgoing Slab Crustal Melting,Primitive Mantle Melts vs. Remelting of the Lower Crust,岩浆底侵作用,Island Arc Petrogenesis,Figure 16-11b. A p

5、roposed 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 phlogopite (B) dehydrate. From Tatsumi (1989), J. Geophys. Res., 94, 4697-4707 and Tatsumi and

6、 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,幔源原生岩浆（Primitive magma）的识别 根据Basaltic Volca

7、nism on the Terrestrial Planets(p409)和Igneous Petrogenesis（p.22),上地幔产生的原生岩浆，有比较高的Mg值和Ca/(Ca+Na)比值，高的Ni, Cr含量和低的不相容元素含量（由于橄榄石、辉石等的分异，使残余岩浆中Mg,Ni,Cr含量降低，而不相容元素增高）：,Mg-number SiO2/Al2O3图 （据Kempton et al., 1997, J. Petrol),(1) 11%的MgO (Mg=Mg/(Mg+Fe2+)=0.68-0.75） (2)高Ni(400-500ppm),高Cr(1000 ppm),SiO250%

8、(3) 含超镁铁质岩包体：主要见于碱性玄武岩中，部分拉斑玄武岩中也有，包体的存在说明岩浆上升速度至少在15cm/s内，岩浆上升过程中不可能发生分异。 (4) 大规模喷发序列的平均值,岛弧安山岩浆成因（洋陆 和 洋洋 俯冲带）,Conceptual framework of subduction factory， after Yoshiyuki, 2006, JMPS,原料： 大洋地壳， 大陆地壳 岩石圈地幔， 地幔楔 产物： 1) 钙碱性岩系中的安山岩 俯冲洋壳的脱水导致地幔楔的部分熔融 2) 高Mg安山岩 幔源岩浆分异 or 洋壳熔融并受到地幔楔交代 3) 埃达克岩 (adakite) 洋壳

9、深俯冲后自身的部分熔融 残留： 改造后的洋壳 掉落到地幔深部,花岗岩类的源岩判别（最好用冷凝边成分）,（原图据Alther r R 等，2000，Lithos；数据据徐夕生等2007）,二、岩浆上升、侵位机制与岩体构造,magma Ascent and Emplacement of Magmas and Igneous Structures,岩浆是如何在地壳中上侵的 How does magma rise through crust? 岩浆是如何开辟占位空间的 How is room made for the magma?,Controls on magmas rising or sinkin

10、g; Temp Pressure Composition/mineralogy,中性浮力理论（Neutral buoyancy),Mafic and ultramafic magmas are less dense than peridotite (magma source) Magmas are positively buoyant and rise,白榴石霞石,石英,花岗岩,斜长石,碱性长石,金云母-黑云母,玄武岩,橄榄岩,角闪石,斜方辉石,单斜辉石,橄榄石,铁钛氧化物,流纹质熔体,碱性玄武质熔体,拉班玄武质熔体,安山质熔体,Mantle is peridotite Melt this t

11、o make basalt Continental crust is a granodiorite Melt this for rhyolite Ocean crust is basalt What melts reach surface easiest through cont. and ocean crust? What composition volcanic rocks are observed most?,Crustal rocks: feldspar rich, less dense than mantle Hard for mantle melts to rise through

12、 cont. crust.,白榴石霞石,石英,花岗岩,斜长石,碱性长石,金云母-黑云母,玄武岩,橄榄岩,角闪石,斜方辉石,单斜辉石,橄榄石,铁钛氧化物,流纹质熔体,碱性玄武质熔体,拉班玄武质熔体,安山质熔体,Fig. 12. Cross section of the The YellowstoneSnake River Plain volcanic system (YSRP) density model . 括号外的数字代表岩石密度（kg/m3),括号内的数字代表 P 波速 (km/s） （Journal of Volcanology and Geothermal Research 188 ，

13、2009， 108127）,Andesite melts rise easily in any crust Basalts not expected to rise through cont. crust,How do basalt erupt?,Magma density reduced by exsolution and expansion of volatiles Coke bottle,How do basalt erupt?,Magma overpressure(hydraulic head,液压压头) Magma pushed out of ground by weight of

14、overlying more dense rock Deeper the magma column, more overpressure,Where do basalts go?,Basalt magmas “underplate” cont. crust Or intrude it Magmas can stall out with neutral buoyancy Crystallize more, become less dense, rise.,How magmas ascend,Diapirs Buoyant bodies of magma pushing slowly throug

15、h surrounding ductile country rock In lower crust and mantle,How magmas ascend,Diapirs Common for granitic rocks Need ductile/plastic country rock Ascent velocity: 0.1-50 m/y,How magmas ascend,Dikes Magmas rises quickly through cracks and fractures Common for basalt Need brittle country rock Ascent

16、velocity 0.1-1 m/s,Dike swarms, Greenland,Radial dike swarm, Spanish PeaksandShip Rock, New Mexico,Dikes,Triassic and Jurassic basalt dikes Why are most dikes basalt?,Intrusion style with tectonic environment,Compressional Greatest stress horizontal Magma cant fracture crust Can spread horizontally

17、(sills) by lifting overlying crust Crystallize, become less dense, rise more,Intrusion style with tectonic environment,Extensional Greatest stress vertical Minimum stress (horizontal) less than magma pressure Magma can vertically dike through crust Overpressure creates fractures,Volcanic rocks with

18、tectonic environment,Extension (rifts) Continents: basalt common Ocean: basalt only rock type,Compression (subduction zone) Basalt less common than andesite Why? Basalt stalled in lower-middle crust Then crystallizes to form andesite,Diapir intrusions,Most granites emplaced as diapirs Lower density

19、than country rock When equal density, they stall out,Diapir intrusions,Small to moderate intrusions: pluton or stock Few are homogeneous in composition,Diapir intrusions,Batholith: large granitic intrusion Multiple plutons,Diapir intrusions,How is room made for plutons?,Emplacement,Assimilation of w

20、all rock #2 Partial melting into magma,Emplacement,Stoping: pieces of country rock incorporated (digested) into magma #3 Evidence with xenoliths (pieces of foreign country rock),Emplacement,Stoping: pieces of country rock incorporated (digested) into magma #3 Evidence with xenoliths (pieces of forei

21、gn country rock),Emplacement,Ring-fracture stoping Fault-bounded roof slab detaches from roof and falls into magma chamber,Emplacement,Doming: magma makes room for itself Related fault-block uplift,Emplacement,Doming Lacolith: flat-floored intrusion with dome on top Magmas forcefully inserted shallo

22、wly Begin as sills, inflate as more magma injected,Emplacement,Tectonically created room Emplacement in fault zones Emplacement into extension zones,Intrusion-host interface,Emplacement recorded in contact Sharp contact Intrusion of magma against cooler, shallow rock Rapid cooling No interaction,Int

23、rusion-host interface,Emplacement recorded in contact Sharp contact,Gradational contact,Intrusion-host interface,Emplacement recorded in contact Gradational contact Thermal, chemical, physical interaction Hot country rock skarns,Magma emplacement & deformation,Pretectonic? Syntectonic? Posttectonic?

24、 Is pluton deformed as wallrock? Pluton foliation parallel to country rock foliation,Magma emplacement & deformation,Pretectonic? Syntectonic? Posttectonic? Does pluton cross-cut wallrock foliation? Syntectonic can be tough to tell,Magma emplacement,Depth of emplacement Shallow emplacement: sharper contacts with country rock Deeper emplacement: more ductile with country rock,侵入体与构造关系火成岩侵位与区域构造的关系：协调侵入与不协调侵入（马昌前等，1994）。与区域构造线间的协调关系，与围岩产状是否一致。,Fig. 9. A ma