K-Rich and Carbonatic Melt Metasomatism in Depleted Upper Mantle: Geochemical Evidences from Peridotites in Pre-Pilot Hole of Chinese Continental Scientific Drilling Project
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摘要: 中国大陆科学钻探工程预先导孔(CCSD-PP1) 打在苏鲁超高压变质带芝麻坊超镁铁岩体上,钻孔穿透超镁铁岩体115m.超镁铁岩体由二辉橄榄岩、方辉橄榄岩和少量单辉橄榄岩和纯橄岩组成,与上下围岩接触的橄榄岩被强烈蛇纹石化.多数橄榄岩含有石榴石或其假象,普遍含有金云母和菱镁矿,少量样品中有钛斜硅镁石.在化学成分上,橄榄岩的Mg#指数变化于90.392.6之间,MgO含量(36.61%49.15%,平均45.17%) 与Na2O (0.01%0.25%)、Al2O3 (0.07%3.71%,多数 < 2.0%,平均1.46%)和CaO (0.12%2.53%,一个高达3.30%,平均1.00%) 呈负相关关系.与主量易熔元素相对亏损的特点相反,橄榄岩中显示了稀土元素富集、分馏和配分曲线显示近于平行和相似的特点,(La/Lu) N比值为3.1833.05;此外,多数样品具有高Ba (最高比原始地幔高100多倍) 含量,在蛛网图上显示Rb、Nb、Ta、Zr、Hf和Sr的负异常,Ti/Eu比值均低于1300.岩相学特征和难熔主量元素与不相容元素之间的无相关性表明橄榄岩至少受到了形成金云母和菱镁矿的2次交代作用.富含金云母的橄榄岩(例如C25-143-61,C32-149-71) 具有富钾趋势,并且显示K2O与Rb、Ba和Th等大离子亲石元素的正相关关系,未见K2O和稀土元素、Sr和Ca之间的相关关系.这些特征表明橄榄岩被含水、硅铝质碱性熔体交代,之后又被高Ba低Rb和高场强元素的镁质(菱镁矿) 碳酸岩熔体交代,并且强烈地改变了Ba的丰度和显示了特定地幔碳酸盐的稀土元素配分型式.全岩具有不均一的高放射性Sr (87Sr/86Sr=0.70840.7201)和低放射性Nd (εNd (t) =-1.14-8.55) 组成,结合已有的氧同位素研究资料,表明预先导孔PP1中的橄榄岩所代表的地幔可能在早期就遭受了来自深部的介质的交代作用.
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关键词:
- 石榴石橄榄岩 /
- 微量元素 /
- 地幔亏损 /
- 交代作用 /
- 中国大陆科学钻探工程
Abstract: The Chinese pre-pilot hole (PP1) is located at Zhimafang Village,Donghai County in the Sulu UHP terrane,east China. Ultrahigh-pressure peridotites of 115 m thick within gneiss recovered from the PP1 are composed of abundant lherzolite,harzburgite,and minor wehrlite and dunite. Peridotite near to the contacts with gneiss is strongly serpentinized. More than 90 vol% peridotites contain garnet and phlogopite; some contain magnesite and Ti-clinohumite. All peridotites contain lower "fertile elements" compared with primitive mantle,their Mg# numbers range from 90.3 to 92.6,and MgO content (36.61%-49.15%,averagely 45.17%) has negative correlation with Na2O (0.01%-0.25%),Al2O3 (0.07%-3.71%,most < 2.0%,averagely 1.46%) and CaO (0.12%-2.53%,one up to 3.30%,averagely 1.00%) contents. In contrast to major element depletion feature,the PP1 peridotites show light rare earth element-enriched and slight to moderately fractionated REE pattern of nearly parallel curves and roughly identical pattern with (La/Lu) N ratios of 3.18-33.05. Most of the peridotites contain high Ba (higher than 92 times of primitive mantle) and LREE and low HFSE,and are characterized by negative Rb,Nb,Ta,Zr,Hf and Sr anomalies (e.g.,C39-157-81) in spidergrams. Ti/Eu ratios are lower than ca. 1 300. The lack of correlation between refractory degree and enrichment of incompatible elements documents effect by metasomatism after mantle melting. Petrographic characteristics show multiple metasomatism of phlogopite and magnesite. Phl-rich peridotites (such as samples C25-143-61,C32-149-71) have the K2O enrichment trend and good correlations between K2O and some LILE such as Rb,Ba and Th. No positive correlations between K2O and REE and between Sr and Ca are seen. These signatures show that peridotites were metasomatised by hydrous,silicic,aluminous and alkaline melts containing some LILE,and then severely overprinted by metasomatism of magnesite melt containing high Ba and low Rb and HFSE which modified Ba content drastically and endued REE patterns of mantle carbonatite melt. Whole rock has heterogeneously high radiogenic Sr (87 Sr/86 Sr=0.708 4-0.720 1) and low radiogenic Nd (ε_ Nd (t) =-1.14 to-8.55),which indicates the peridotites from PP1 hole was probably derived from long-term enriched mantle by agents from depth,especially combined with oxygen isotope compositions of anhydrous and hydrous minerals reported by previous studies. -
图 2 PP1钻孔中含金云母和菱镁矿橄榄岩的显微照片
a.含菱镁矿金云母二辉橄榄岩.菱镁矿产出于橄榄石(Olv)、金云母(Phl) 粒间的菱镁矿(Mag), 还见金云母被菱镁矿消蚀; b.含菱镁矿金云母单辉橄榄岩, 显示了绿泥石化(Chl)和定向的富金云母集合体及其附近的菱镁矿; c.含菱镁矿金云母单辉橄榄岩, 金云母定向分布, 菱镁矿消蚀了金云母.照片a—c为正交偏光照相; d.边部具有含水相钛斜硅镁石(Ti-Chu)和金云母的细脉, 其中部为细粒滑石、绿泥石、钛斜硅镁石和角闪石(照片左上角).钛斜硅镁石边部部分分解橄榄石+钛铁矿(Olv+Ilm) (单偏光)
Fig. 2. Photomicrographs of phlogopite and magnesite-bearing peridotite from PP1 borehole
图 3 PP1 钻孔橄榄岩的MgO与其他主量元素及Cr、Ni的变异图
(克拉通和非克拉通尖晶石橄榄岩区域引自Downes et al, 2004及其参考文献)
Fig. 3. MgO contents vs. major element variation of peridotites in PP1 borehole
图 8 PP1钻孔橄榄岩的Ti/Eu比值对地幔标准化的(La/Yb)N比值投点图, 还标出了碳酸盐和硅酸盐交代趋势(Rudnick et al., 1993;Coltori et al., 1999修改)
Fig. 8. Ti/Eu versus normalized (La/Yb) N ratio for peridotites in PP1 borehole. Also shown are the carbonatic and silicate metasomatic trends
图 9 PP1钻孔橄榄岩全岩的εNd对(Ce/Yb) N投点图, 还标出了富集趋势.五角星代表了现代MORB源区的地幔成分(Yoshikawa and Nakamura, 2000)
Fig. 9. εNd versus (Ce/Yb) N for whole rocks from PP1 borehole, showing consistency of enrichment trend. Star represents present-day MORB source mantle
表 1 PP1钻孔橄榄岩的全岩主量元素(%)和微量元素(10-6) 数据
Table 1. Whole-rock major (%) and trace (10-6) elements of PP1 peridotites
表 2 PP1钻孔橄榄岩不同岩石的主量元素变化范围、平均值和标准差(%)
Table 2. Major element variation ranges and averages standard deviation (%) of different rocks from PP1
表 3 PP1钻孔橄榄岩石榴石和无石榴石橄榄岩的主量元素变化范围、平均值和标准差(%)
Table 3. Major element variation ranges and averages standard deviation(%) of Grt and Grt-free rocks from PP1
表 4 PP1钻孔橄榄岩不同岩石的∑REE变化范围、平均值和标准差(10-6) 以及(La/Lu) N比值
Table 4. ∑REE ranges and averages standard deviation (10-6) and (La/Lu) N of different rocks from PP1
表 5 PP1钻孔橄榄岩的Sr、Nd同位素组成
Table 5. Sr and Nd isotope analyses of peridotites from PP1
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