摘要
与洋壳有关的酸性岩由于对了解幔源岩浆的演化以及判别古老蛇绿岩套及其构造位置的重要意义而倍受岩石学家和构造地质学家的关注。本文对发现于冲绳海槽玄武岩基质中的中酸性到酸性残余熔体进行了详细研究,它们提供了幔源玄武岩浆结晶分异形成酸性岩浆的直接证据。在细小的基质矿物间分布有一种玻璃质的残余熔体,其成分随距冷凝边距离(L)的增加而越来越酸性。在SiO2对Na2O+K2O图解上,残余熔体的投影点从玄武岩到英安岩均有分布,反映了一个连续的演化系列。在AFM图解上,残余熔体表现出与Thingmuli火山岩系列类似的拉斑玄武岩系列的演化趋势。我们的研究表明:残余熔体的演化受结晶分异作用控制。在早期结晶阶段,辉石的结晶起主导作用,结果造成残余熔体中SiO2、Al2O3、Na2O的迅速增加,FeO、MgO、CaO迅速降低。在晚期结晶阶段,斜长石成为主导结晶相,导致残余熔体中Al2O3、Na2O的迅速消减。Al2O3、Na2O从增加到降低的转变出现在SiO2=62%左右。在L=27.5mm处,85-90%的基质岩浆已发生了结晶作用,导致残余熔体中SiO2含量达到69-70%,而且此处还新出现了一种富FeTi的氧化物。该玄武岩中残余熔体和基质矿物的成分及演化特征分别与Thingmuli火山岩系列中酸性端元的组成相似,在也佐证了Thingmuli火山岩系列是幔源岩浆结晶分异的产物。
Oceanic-crust-related acidic rocks have received a considerable interest, as they are important for understanding the evolution of mantle-derived magmas, and for distinguishing ancient ophiolite suites and their tectonic settings. We present a detailed study of intermediate to acidic residual melts found in partially crystallized matrix of basalt lava from Okinawa Trough, which provides direct evidence for derivation of acidic magmas by crystal fractionation of mantle-derived basaltic parental magmas. We found a type of residual melts distributed within the interstitial space of tiny matrix minerals, whose compositions become increasingly acidic with increasing distance (L) away from the chilled margin. In SiO2 vs. Na2 O + K2 O diagram, analyses of such residual melts plot into basalt through to dacite fields, defining a complete and continuous evolution trend. In AFM diagram, the residual melts define a distinct tholeiitic trend, comparable with those of Thingmuli volcanic series. Our study shows that major element trends are controlled by crystal fractionation. During early-stage crystallization, augite fractionation played a dominant role, resulting in a rapid increase in SiO2, Al2O3 and Na2O, and abrupt decrease in FeO, MgO and CaO in the residual melts. During subsequent stages of crystallization, plagioclase kicked in and became a dominant fractionating phase, depleting Al2O3 and Na2O in the residual melts. The transition from an increase to a decrease in Al2 O3 and Na2 O is at SiO2 = 62%±. At L = 27. 5 mm, 85% -90% of the matrix magma has crystallized, leaving a residual melt with SiO2 = 69% -70%. During this stage of crystallization, a mixture of Fe-Ti-rich oxides becomes an additional precipitating phase. The compositional range and evolution trend of the residual melts and matrix minerals within the basaltic lava are comparable with those of Thingmuli volcanic series and phenocrysts in its acidic end members, respectively, suggesting that Thingmuli volcanic series was derived by crystal fractionation o
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2005年第1期158-168,共11页
Acta Petrologica Sinica
基金
国家自然科学基金项目(No.40221301和No.49972025)高等学校博士学科点专项科研基金项目(20020284033)资助课题
关键词
玄武岩
结晶分异
玻璃质残余熔体
酸性岩浆
电子探针
冲绳海槽
Basalt, Crystal fractionation, Glassy residual melt, Acidic magma, Electron microprobe, Okinawa Trough
