南海新生代堿性玄武巖的特征及其地球動力學意義

1、南海新生代堿性玄武巖的特征及其地球動力學意義        南海是西太平洋最大的邊緣海之一,位于歐亞板塊、印-澳板塊以及太平洋板塊之間。南海海底擴張停止(15.5Ma, Briais et al., 1993)后的板內(nèi)火山作用,影響著中國南部、中南半島、大洋基底和分裂的微大陸片段的廣大地區(qū)。對南海新生代玄武巖進行地球化學研究,不僅對于理解南海板內(nèi)火山作用的深部地幔過程有著重要意義,而且對南海形成演化及含油氣盆地形成的深部動力學機制有著深遠意義。南海新生代玄武巖K-Ar/Ar-Ar年齡為3.8-7.9Ma,表明為晚中

2、新世以來的巖漿活動產(chǎn)物,與周邊地區(qū)的堿性火山巖在年齡上的一致性。巖石學特征表明,南海新生代玄武巖的礦物組合為橄欖石、單斜輝石、斜長石,與特征的堿性玄武巖的礦物組合一致。由橄欖石所計算的南海底潛在地幔溫度(Tp)平均值為1661,暗示南海地區(qū)下的地?？赡艽嬖跓崃慨惓?為海南地區(qū)存在地幔柱的觀點提供了證據(jù)。單斜輝石富鈣、鈦,由單斜輝石-熔體平衡溫壓計計算的巖漿房深度分別為:堿玄巖巖漿房深度約49km(對應壓力為1.461.48 GPa);粗面玄武巖巖漿房約25km(對應壓力為0.76 GPa);玄武巖巖漿房約15km(對應壓力為0.44GPa)。由堿玄巖粗面玄武巖玄武巖,平衡溫度(K)依次降低:從

3、1535149814291369。由斜長石微晶所計算的巖漿噴出地表的溫度為989。主量元素特征表明,巖石類型主要為堿玄巖,有少量的粗面玄武巖和玄武巖,屬于堿性系列。微量元素方面,大離子親石元素(LILE)以及高場強元素(HFSE)特別是Nb、Ta、Ti、Y等元素均呈現(xiàn)富集現(xiàn)象,Yb、Sc、Sr以及K、U、Th等生熱元素相對虧損,微量元素及稀土元素分布巖石類似板內(nèi)OIB微量元素的全球平均值。同位素地球化學研究表明,源區(qū)存在兩個混合端員并具Dupal Pb異常:一個為DMM,位于軟流圈或巖石圈地幔中;另一個為EM2源區(qū),可能來自位于核-幔邊界處的海南地幔柱而非大陸底巖石圈地幔。研究表明,南半球Du

4、pal異常不存在全球范圍內(nèi)的地區(qū)專屬性,本區(qū)存在的Dupal異常與南半球Dupal異常可能不存在聯(lián)系。在南海新生代玄武巖的成因過程中,海南地幔柱在為巖石圈地幔的部分熔融作用提供所需的熱量同時,也在物質(zhì)上作出了貢獻。南海盆新生代堿性玄武巖由不同程度的部分熔融作用,以及巖漿在上升期間或者在高位巖漿房中的橄欖石等礦物分離結(jié)晶作用所形成,同時還可能發(fā)生了堆晶作用。構(gòu)造環(huán)境判別表明,玄武巖漿在上升到地表過程中幾乎未受到地殼混染。南海新生代玄武巖的地球化學研究表明,在玄武質(zhì)巖漿的深部地幔演化過程中,海南地幔柱可能起著重要的作用。通過引入海南島地幔柱這個概念,本文建立了一個新的有關南海形成演化的初步的概念性

5、模型:(1)50-32Ma,印度洋板塊-歐亞板塊碰撞及其所導致的太平洋板塊后退的綜合效應為南海地區(qū)提供了一個伸展環(huán)境,此伸展環(huán)境為地幔柱物質(zhì)的上升提供了通道;(2)32-21Ma,當?shù)蒯Ｖ^到達軟流圈時,通過側(cè)向物質(zhì)流開始同擴張中心發(fā)生相互作用,促進了南海的擴張,并在26-24Ma期間發(fā)生了洋脊重新就位事件,使擴張中心從原來的18°N附近(即現(xiàn)今西北海盆的中心)調(diào)整到15.5°N附近(即現(xiàn)今的東部亞盆);(3)21-15.5Ma,隨著地幔柱效應的逐漸增強,熱點-洋脊相互作用越來越強烈,在大約21Ma發(fā)生了洋脊的再次重新就位事件,誘發(fā)了西南海盆的擴張;(4)15.5-現(xiàn)在,

6、由于印澳板塊前緣與巽他大陸碰撞,使得南海大約在15.5Ma停止擴張,并沿著南沙海槽及呂宋海溝向菲律賓島弧及巴拉望地塊之下俯沖,而南海熱點繼續(xù)活動,在地表處直到第四紀還有堿性玄武巖噴出The South China Sea is one of the largest marginal basins in the west Pacific, which lies at triple junction among three large plates, i.e., Eurasian plate, Indo- Australia plate and Pacific plate (or Philippi

7、ne plate ). A large scale intraplate volcanism after the cessation of seafloor spreading, affects a broad areas including the South China Sea itself, southeast China, Indochina peninsula and several rifted micro-continental segments in the South China Sea. A comprehensive study on Cenozoic alkali ba

8、salts is not only helpful to understand the deep mantle process of the intraplate volcanism occurred in the South China Sea, but also significant for understanding of the deep geodynamical regime for the formation and evolution of the South China Sea and a series of oil-bearing basins.The whole rock

9、 K-Ar/Ar-Ar ages for basaltic rock from the South China Sea range from 3.8 Ma to 7.9 Ma, which suggestS that they are products of magmatism since late Miocene, as is consistent with those of alkali basalt from around the South China Sea. Petrographic studies suggest that the mineral assemblage is co

10、mposed of olivine, clinopyroxene and plagioclase, which is consistent with common mineral assemblage of a characteristic alkali basalt. The average value for mantle potential temperatures (Tp) beneath the South China Sea, calculated from olivine-melt equilibrium, is 1661, which imply that there may

11、exist thermally anomaly in the mantle beneath the South China Sea, and provide a important evidence for the Hainan Plume. During the ascent form the magma origin to the surface, the temperatures and pressures of evolved magmas in different high level magma chambers calculating from clinopyroxene - m

12、elt equilibrium are as follows, (1) equilibrium temperatures (°K) for tephrite, trachybasalt and basalt are 15351498, 1429 and 1369, respectively; (2) equilibrium pressure for tephrite is 1.461.48 GPa , which corresponds to magma chamber at depth of 49km (the middle-lower part of oceanic lithos

13、phere); (3) equilibrium pressure for trachybasalt is 0.76 GPa , which corresponds to magma chamber at depth of 25 km (the middle-upper part of oceanic lithosphere); (4) equilibrium pressure for basalt is 0.44 GPa , which corresponds to magma chamber at depth of 15 km (the upper part of oceanic litho

14、sphere). The quenching temperature of magma erupted on the surface, calculated from plagioclase melt equilibrium, is 989.Major element compositions suggest that rock types are mainly tephrite, trachybasalt and basalt in subordination, and all rocks belong to alkali series. Large ion lithophile eleme

15、nts (LILE) and high field strength elements (HFSE), e.g., Nb, Ta, Ti and Y, are enriched, and Yb, Sc, Sr and heat-producing elements, e.g. K, Th, U, are relatively depleted. In a whole, the distributional patterns for trace elements and rare earth elements are similar to those of global intraplate O

16、IB average value. Sr-Nd-Pb isotopic data of these basaltic rocks strongly suggest there is a binary mixing model in origin with Dupal Pb anomaly, one is a depleted mantle end-member (DMM), and the other is EM2 component which may be not result from subcontinental lithospheric mantle, but the Hainan

17、plume originated from core-mantle boundary. Dupal anomaly is not only limited to South Hemisphere, and there have no relationship in Dupal anomaly between the South China Sea and South Hemisphere. The petrogenesis for basaltic rock described below, the Hainan plume contributes to partial melting of

18、source rock thermally and materially. During the ascent of magma from the origin to the surface, they experienced fractional crystallization and/or cumulation to different extent in different level magma chamber, and there have no obvious mixing inprint with continental lithospheric mantle and crust

19、.Hainan plume are introduced into a preliminary model about the formation and evolution of the South China Sea which this study newly builds. The model is as follows, (1), 50-32Ma. Integrated effects of the collision between Indian ocean plate and Euro-Asian plate and its resulting in the retrogress

20、ion of Pacific plate, created a extensional tectonic setting, which provided a channel for the ascent of mantle plume. (2), 32-21Ma. When the head of mantle plume arrived at asthenosphere, it immediately interacted with the spreading center of the South China Sea by lateral material flow, which enhanced spreading spead. During 26-24Ma, there took place ridge jump, which adjusted the spreading center from nearby 18°N(i.e., present-day center of NW sub-basin) to nearby 15.5°N(i.e., present-day center of East sub-ba