高校地质学报
高校地質學報
고교지질학보
GEOLOGICAL JOURNAL OF CHINA UNIVERSITIES
2005年
1期
9-46
,共38页
地幔杆栏岩%地幔分熔%玄武岩浆的成因与演化%中国东部%岩石圈减薄%中-新生代基性火山作用%板块构造的特例
地幔桿欄巖%地幔分鎔%玄武巖漿的成因與縯化%中國東部%巖石圈減薄%中-新生代基性火山作用%闆塊構造的特例
지만간란암%지만분용%현무암장적성인여연화%중국동부%암석권감박%중-신생대기성화산작용%판괴구조적특례
以全球大地构造为背景讨论了玄武岩浆起源和演化的一些基本概念.这些概念的正确理解有助于合理解释各种环境中火成岩的形成机制,也有助于依据野外岩石组合来判别古构造环境.在此基础上结合已有资料和观察,对中国东部中生代岩石圈减薄及中-新生代基性火山岩成因提出了一些新解释.这些解释与地质观察相吻合,且符合基本的物理学原理.虽然中国东部基性火山活动可称为"板内"火山活动,但它实际上是板块构造的特殊产物.中国东部中生代岩石圈减薄是其下部被改造为软流层的缘故.这种改造是加水"软化"所致.水则源于中国东部地幔过渡带(410～660 km)内古太平洋(或其前身)俯冲板块脱水作用.其将岩石圈底部改造为软流层的过程,实际上就是岩石圈减薄的过程.因为软流层是地幔对流的重要部分,而大陆岩石圈则不直接参与地幔对流.中生代玄武岩具有εNd＜0的特征,说明其源于新近改造而成的软流层,亦即原古老岩石圈之底部.中国大陆北北东-南南西向的海拔梯度突变界线与东-西部重力异常,陆壳厚度变化,以及地幔地震波速变化梯度吻合.因此可将北北东-南南西向梯度线称为"东-西梯度界".该界东-西海拔高差(西部高原与东部丘陵平原),陆壳厚度差异(西部厚而东部薄)和100～150 km的深度范围地幔地震波速差异(西部快而东部慢),均受控于上地幔重力均衡原理.这表明西部高原岩石圈厚度＞150～200 km,而东部丘陵平原岩石圈厚度＜80km."遥远"的西太平洋俯冲带具有自然的地幔楔吸引作用.此吸引作用可引起中国东部"新生"软流层东流.软流层东流必将引起西部高原底部软流层的东向补给(流动).这一过程必然导致东移软流层的减压,即从西部的深源(岩石圈深度＞150～200 km处)到东部的浅源(岩石圈深度～80km处).东移软流层的减压分熔可合理解释具有软流圈地球化学特征(εNd＞0)的新生代中国东部基性火山活动及玄武岩的成因.这些对中国东部中-新生代地质过程的解释,将为更加细致的,以岩石学和地球化学为主的讨论所验证.
以全毬大地構造為揹景討論瞭玄武巖漿起源和縯化的一些基本概唸.這些概唸的正確理解有助于閤理解釋各種環境中火成巖的形成機製,也有助于依據野外巖石組閤來判彆古構造環境.在此基礎上結閤已有資料和觀察,對中國東部中生代巖石圈減薄及中-新生代基性火山巖成因提齣瞭一些新解釋.這些解釋與地質觀察相吻閤,且符閤基本的物理學原理.雖然中國東部基性火山活動可稱為"闆內"火山活動,但它實際上是闆塊構造的特殊產物.中國東部中生代巖石圈減薄是其下部被改造為軟流層的緣故.這種改造是加水"軟化"所緻.水則源于中國東部地幔過渡帶(410～660 km)內古太平洋(或其前身)俯遲闆塊脫水作用.其將巖石圈底部改造為軟流層的過程,實際上就是巖石圈減薄的過程.因為軟流層是地幔對流的重要部分,而大陸巖石圈則不直接參與地幔對流.中生代玄武巖具有εNd＜0的特徵,說明其源于新近改造而成的軟流層,亦即原古老巖石圈之底部.中國大陸北北東-南南西嚮的海拔梯度突變界線與東-西部重力異常,陸殼厚度變化,以及地幔地震波速變化梯度吻閤.因此可將北北東-南南西嚮梯度線稱為"東-西梯度界".該界東-西海拔高差(西部高原與東部丘陵平原),陸殼厚度差異(西部厚而東部薄)和100～150 km的深度範圍地幔地震波速差異(西部快而東部慢),均受控于上地幔重力均衡原理.這錶明西部高原巖石圈厚度＞150～200 km,而東部丘陵平原巖石圈厚度＜80km."遙遠"的西太平洋俯遲帶具有自然的地幔楔吸引作用.此吸引作用可引起中國東部"新生"軟流層東流.軟流層東流必將引起西部高原底部軟流層的東嚮補給(流動).這一過程必然導緻東移軟流層的減壓,即從西部的深源(巖石圈深度＞150～200 km處)到東部的淺源(巖石圈深度～80km處).東移軟流層的減壓分鎔可閤理解釋具有軟流圈地毬化學特徵(εNd＞0)的新生代中國東部基性火山活動及玄武巖的成因.這些對中國東部中-新生代地質過程的解釋,將為更加細緻的,以巖石學和地毬化學為主的討論所驗證.
이전구대지구조위배경토론료현무암장기원화연화적일사기본개념.저사개념적정학리해유조우합리해석각충배경중화성암적형성궤제,야유조우의거야외암석조합래판별고구조배경.재차기출상결합이유자료화관찰,대중국동부중생대암석권감박급중-신생대기성화산암성인제출료일사신해석.저사해석여지질관찰상문합,차부합기본적물이학원리.수연중국동부기성화산활동가칭위"판내"화산활동,단타실제상시판괴구조적특수산물.중국동부중생대암석권감박시기하부피개조위연류층적연고.저충개조시가수"연화"소치.수칙원우중국동부지만과도대(410～660 km)내고태평양(혹기전신)부충판괴탈수작용.기장암석권저부개조위연류층적과정,실제상취시암석권감박적과정.인위연류층시지만대류적중요부분,이대륙암석권칙불직접삼여지만대류.중생대현무암구유εNd＜0적특정,설명기원우신근개조이성적연류층,역즉원고로암석권지저부.중국대륙북북동-남남서향적해발제도돌변계선여동-서부중력이상,륙각후도변화,이급지만지진파속변화제도문합.인차가장북북동-남남서향제도선칭위"동-서제도계".해계동-서해발고차(서부고원여동부구릉평원),륙각후도차이(서부후이동부박)화100～150 km적심도범위지만지진파속차이(서부쾌이동부만),균수공우상지만중력균형원리.저표명서부고원암석권후도＞150～200 km,이동부구릉평원암석권후도＜80km."요원"적서태평양부충대구유자연적지만설흡인작용.차흡인작용가인기중국동부"신생"연류층동류.연류층동류필장인기서부고원저부연류층적동향보급(류동).저일과정필연도치동이연류층적감압,즉종서부적심원(암석권심도＞150～200 km처)도동부적천원(암석권심도～80km처).동이연류층적감압분용가합리해석구유연류권지구화학특정(εNd＞0)적신생대중국동부기성화산활동급현무암적성인.저사대중국동부중-신생대지질과정적해석,장위경가세치적,이암석학화지구화학위주적토론소험증.
Some basic concepts of basaltic magma generation and evolution are discussed in the context of global tectonics. These concepts need better understanding before invoking elusive possibihlities in igneous petrogenesis on all scales and in all tectonic environments. A hypothesis for the Mesozoic lithosphere thinning and Mesozoic-Cenozoic basaltic volcanism in eastern China is presented. This hypothesis is consistent with observations and complies with basic physics. While the eastern China volcanism can bedefined as "intra-plate" volcanism, it is in fact a special consequence of plate tectonics. The Mesozoic lithosphere thinning in eastern China is best explained by a process that “transformed” the deep portion of the lithosphere into convective asthenosphere by hydration. The water that did so may come from dehydration of subducted Pacific (or predecessor) oceanic lithosphere that is presently lying horizontally in the transition zone beneath eastern Chinese continent as detected by seismic tomographic models. The Mesozoic volcanism may be genetically associated with the lithospheric thinning because the basaltic source is ancient isotopically enriched (εNd ＜ 0 ) lithosphere--being converted to the asthenosphere. The NNE-SSW Great Gradient Line (GGL) marked by the sharp altitude, gravity anonaly, crustal thickness, and mantle seismic velocity changes from the plateau west to the hilly plains of eastern China is an expression of variation in lithospheric thickness from probably ＞ 150 ～ 200 km beneath the plateaus in the west to the thin, probably ＜ 80 km, beneath eastern China. The “remote” western Pacific subduction systems induce asthenospheric flow from beneath eastern China towards the subduction zones, which, in turn, requires asthenospheric material replenishment from beneath the plateaus to the eastern China. As a result, such eastward asthenospheric flow experiences an upwelling and decompression (from beneath thickened to thinned lithosphere ), which causes the flowing asthenosphere (e. G.,εNa ＞ 0 ) to partially melt and produce Cenozoic eastern China basaltic volcanism. Such volcanism may have actually begun at the end of the Mesozoic lithosphere thinning in the late Cretaceous. This simplistic concept is currently being substantiated with detailed petrologic and geochemical data.
