雷州英峰岭玄武岩中单斜辉石巨晶的地球化学和地幔交代作用  

Geochemistry of clinopyroxene megacrysts from Yingfengling basalt and the metasomatism effect of their parental magma on mantle peridotites.

作　　者：于津海[1] O'Reilly Y S 周新民[1] 

机构地区：[1]南京大学地球科学系 [2]National Key Center of GEMOC,Department of Earth and Planetary Sciences,Macquarie University,North Ryde NSW 2109 Australia

出　　处：《岩石学报》2003年第4期637-649,共13页Acta Petrologica Sinica

基　　金：国家自然科学基金(批准号No.40273018;40132010)

摘　　要：雷州半岛英峰岭火山岩中单斜辉石巨晶以低Mg#值(0.62～0.47)、贫重稀土(HREE)和富Al2O3、TiO2、Na2O和中稀土(MREE)为特征,并具有明显的成分变化.随着Mg#降低,单斜辉石的Al2O3(11.58～6.97%)、TiO2(2.28～1.06%)和HREE逐渐降低.这种变化特征不同于世界上大多数辉石巨晶的变化规律.随着演化辉石巨晶的大多数不相容元素,如Nb、Sr、Zr、Hf和LREE-MREE逐渐增加.低的Mg#值和Ni、Co、Cr含量指示巨晶母岩浆是一种强烈演化的残余岩浆(Mg#=0.27～0.35).分离结晶模拟表明本研究最富镁的巨晶(Lz-54)的母岩浆很可能是由拉斑玄武质岩浆经70～80%以单斜辉石为主的结晶分异后形成.而本研究的其它辉石巨晶是该母岩浆在不同演化阶段(25～80%)分离结晶的产物.分离矿物中除了单斜辉石还包含了大量的石榴子石、斜长石和钛铁矿.温压计算表明英峰岭巨晶是高温岩浆在56～60km深的上地幔的结晶产物(T=1290℃～1130℃,P=1.73～1.83GPa),这一深度有利于石榴子石结晶.对比不同地区辉石巨晶的成分特征和形成条件,作者指出不同地区单斜辉石巨晶各种元素的不同变化规律主要受母岩浆成分和结晶条件控制.巨晶母岩浆在地幔结晶时对围岩橄榄岩进行了不同程度的交代作用.早期贫流体的岩浆对橄榄岩的交代作用较弱,只有Sr,La等强活动性元素被明显改变;晚期富流体(F,P,H2O)岩浆对橄榄岩的交代作用强烈,大多数元素被明显改变.因此,可以推断交代熔体中流体的含量是影响地幔橄榄岩交代作用的关键.The Yingfengling clinopyroxene megacrysts are characterized by low Mg-# (0.62similar to0.47) and heavy rare earth element (HREE), and high Al2O3, TiO2, Na2O and middle rare earth element (MREE) contents, with significant composition variation. As Mg of clinopyroxenes decreases, Al2O3 (11.58% to 6.97%), TiO2 (2.28% to 1.06%) and HREE decrease. These variation correlations are markedly different from those of clinopyroxene megacrysts from other localities worldwide. Most incompatible elements, such as Nb, Sr, Zr, Hf and LREE-MREE, increase with decreasing Mg-#, similar with other clinopyroxene megacrysts all over the world. Low Mg-# and Ni, Co and Cr contents indicate that the parental magma of these megacrysts is a strongly evolved residual melt (Mg-#=0.27similar to0.35). Fractional crystallization modeling suggests that the parental magma-of the Mg-richest megacryst (Lz-54) found in Yingfengling was derived probably from a primitive tholeiitic melt by about 70similar to80% fractional crystallization, and that other megacrysts were the products of this parental magma through 25similar to80% fractional crystallization of clinopyroxene, garnet, plagioclase and ilmenite. T-P calculations indicate that the Yingfengling megacrysts were crystallized in the upper mantle of 56similar to60 km deep (ca. 1.73 GPasimilar to1.83 GPa) and high temperature of 1290degreesCsimilar to1130degreesC, which is in favor of garnet crystallization. By comparing with the clinopyroxene megacrysts from other localities in compositions and formation conditions, it is suggested that the distinct composition variation exhibited by the clinopyroxene megacrysts from different volcanic rocks was principally controlled by both primary composition of parental magma and crystallization condition. The megacryst parental magma intruding into the upper mantle caused the different extent of metasomatism to wall-rock peridotites. Relatively early fluid-poor magma resulted in the weak metasomatism of peridotite, in which only highly active elements were gr

关 键 词：单斜辉石巨晶 地球化学 地慢交代作用 雷州半岛 

分 类 号：P588.14[天文地球—岩石学]