摘要
中国大陆科学钻探工程第三先导孔(PP3钻孔),位于秦岭-大别-苏鲁超高压造山带的东部,赣榆岗上超镁铁岩体中。钻孔中超镁铁岩的岩性包括纯橄岩和石榴超镁铁岩等,该超镁铁岩的SiO_2含量均值在43.68%,变化范围为41.9%~47.11%;MgO均值在44.71%,变化范围为47.12%~48.62%;Cr_2O_3均值在0.39%,变化范围为0.30%~0.47%;CaO均值0.12%(变化范围为0.006%~0.34%);Na2O均值在0.05%(变化范围为0.005%~0.32%)。超镁铁质岩的Mg#(Mg/(Mg Fe)×100)稳定在91.9~93.0之间,Cr#(Cr/(Cr Fe)×100)均值在38,较高;变化在30~45之间,变化小。其中闪石化金云母超镁铁岩具有最低的值(Cr#=19)。Ni含量在2100×10~(-6)~2500×10~(-6),CaO含量均值为0.13%,Al_2O_3含量均值为0.41%。岩体成分均一,表现为高镁,低钙和低铝的特征。超镁铁岩的不相容元素和稀土元素总量很低,稀土元素总量均值在0.60×10~(-6)。(La/Yb)N比值在6.9~51.2,均值在16.1,重稀土元素严重亏损,是中国东部最亏损的地幔岩之一。超镁铁岩中橄榄石成分(Fo在88.7~93.1之间,Fa在6.8~11.1之间),从早期到晚期,岩石Fo值从93→91~92.4→88.7~89.1。铬尖晶石Cr#值从51到89变化,TiO_2和MnO_2值分别低于0.26%和0.46%,晚期铬尖晶石Cr#值增大,Ti含量减小。单斜辉石由透辉石(Wo_(45.9)8En_(47.89)Fs_(2.73)Ac_(3.39))和顽透辉石(Wo_(27.61)En_(68.78)Fs_(2.27)Ac_(1.34))两种,透辉石(Cpx_Ⅰ)在顽透辉石(Cpx_Ⅱ)中呈被交代的残余粒状。角闪石和金云母呈明显的条带状和脉状,局部发育。岩石具有弱的Na和K交代作用,较高的Sr和Ba等元素,亏损高场强元素(HSFE),以及特征的稀土元素和微量元素配分曲线;岩石中主要组成矿物橄榄石从早期到晚期,矿物Mg#指数下降(Mg#从93→88);单斜辉石中可看到透辉石被顽透辉石交代现象;次生角闪石和金云母的形成等特征都显示超镁铁岩经历了碳酸盐交代作用。在超镁铁岩矿物橄榄石中发现细小白云石和菱镁矿等碳酸盐矿物更有力证明了苏鲁超高压变质带超镁铁岩经历了深部碳酸盐交代作用。PP3钻孔超镁铁岩属于强烈亏损地幔岩区域,单斜辉石和角闪石等含水矿物在超镁铁岩中含量低,交代作用的范围和规模有限,且交代作用在动力学上是快速的和不平衡的过程。
The ultramafic body sampled in Hole PP3 is located in the eastern part of the Dabie-Sulu UHP metamorphic belt near Donghai County.It consists chiefly of garnet peridotite,dunite and serpentinite.MgO content of the ultramafic body varies from 46.2wt%to 48.6wt%and CaO from 0.005%to 0.34%,and Al_2O_3 varies from 0.23wt%to 1.19wt% with the average of 0.41wt%;Mg#is generally high(92to 93)and Cr#is also high(30to 45),and variable in a small scale.Garnet-bearing peridotites have the same Mg# and Mg/Si(~1.6)as garnet-free peridotites,which shows enrichment of Mg,depleted of Ca and Al,peridotites are extremely depleted in the east of China.The principal minerals include olivine,chromium spinel,diopside,augite(and/or garnet),orthopyroxene,amphibole and phlogopite.Individual olivine grains are compositionally uniform and have forsterite contents ranging from 88.7to 94.5,with up to 0.79% NiO and 0.95% Cr_2O_3.olivine grains within a given sample are uniform in composition but olivine shows multistage,original olivines are typically more magnesian than later grains.Olivine in a sample of amphibolitized phlogopite peridotite has the lowest forsterite contents(88.7).Composition of clinopyroxenes is attributed to two generations.The later generation which is called as endiopside(Wo_(27.61)En_(68.78)Fs_(2.27)Ac_(1.34))with higher MgO often rims the early diopside(Wo_(45.98)En_(47.89)Fs_(2.73)Ac_(3.39)).Chromite in PP3 ultramafic rocks is compositionally variable,Cr#(molar100Cr/(Cr Al))varies from 51 to 89,and TiO_2 and MnO_2are lower than 0.26wt% and 0.46wt% respectively.Both the olivine and orthopyroxene are highly magnesian.Olivines,chromites,and clinopyroxenes show multistage.PP3 ultramafic body is extremely depleted,but geochemistry shows that the rocks have high Sr and Ba content and depleted HSFE with a characteristic REE pattern,all show the ultramafic body was metasomatised by carbonate in mantle and indeed it is found that diopside was metasomatised by endiopside and amphibole and phlogopite were formed in part and even magnesite and dolomite was formed in olivines,but the metasomatism is limited and fast and out of dynamic balance.