Late Archean high-Mg granitoids (sanukitoids) in the southern Karelian domain, eastern Finland: Pb and Nd isotopic constraints on crust−mantle interactions
Late Archean high-Mg granitoids (sanukitoids) in the southern Karelian domain, eastern Finland: Pb and Nd isotopic constraints on crust−mantle interactions
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DOI：
10.1016/j.lithos.2004.05.007
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发表时间：
2005
期刊：
影响因子：
3.5
通讯作者：
Jaana Halla
中科院分区：
文献类型：
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作者：
Jaana Halla
芬兰东部Nilsiä和Lieksa地区晚太古代(约2.73Ga)斑岩型高K高镁花岗岩具有与加拿大Fennoscandian(波罗的海)地盾Karelian域其他部分和加拿大太古宙Sanukitone套的高镁Sanukitone系列相似的地球化学特征。尼尔西和利克萨花岗岩具有低的二氧化硅含量(62.7−67.0)和高的镁值(0.45−0.52)。富K_2O(2.40−4.73wt.%)、P_2O_5(0.25ppm−0.42wt.%)、锶(610−850ppm)、Ba(1200ppm−2300ppm)、铬(40ppm−80ppm)和轻稀土[(La/Yb)N=19-65]。它们具有负Eu异常(Eu/Eu*0.5 3−0.81)，U强烈亏损，高Th/U比值，低U/Pb值，源区μ2值高(>；9)，正ɛND(2.7Ga)值0.3−1.4，亏损地幔模式年龄2.75−2.86Ga。高的μ2值表明地壳源区明显较老，显然与支持地幔起源的正ɛNd值(2.7Ga)相矛盾。Nilsiä和Lieksa花岗岩的地球化学和同位素数据表明，Nilsiä和Lieksa花岗岩起源于一个富含LILE、U、Th和Pb的地幔楔形源区，在熔融前不久(高达200 Ma)与俯冲有关的板片脱水过程中大陆物质的再循环。相当大数量的地壳铅来自俯冲沉积进入上覆地幔楔体。因此，地幔楔铅的同位素组成被地壳铅的同位素特征所覆盖。认为俯冲带过程中的地壳再循环对太古宙地幔楔体的富集化和高μ花岗岩类的形成具有重要作用。
Late Archean (ca. 2.73 Ga) porphyritic high-K and high-Mg granitoids in the Nilsiä and Lieksa areas, eastern Finland, have geochemical signatures similar to those of high-Mg sanukitoid series from other parts of the Karelian domain of the Fennoscandian (Baltic) Shield and the Archean sanukitoid suites from Canada. The Nilsiä and Lieksa granitoids have low SiO2contents (62.7−67.0) and high Mg numbers (0.45−0.52). They are enriched in K2O (2.40−4.73 wt.%), P2O5(0.25−0.42 wt.%), Sr (610−850 ppm), Ba (1200−2300 ppm), Cr (40−80 ppm) and LREE [(La/Yb)N=19–65]. They are characterized by a negative Eu anomaly (Eu/Eu* 0.53−0.81) and show strong depletion in U, high Th/U ratio, low U/Pb ratio, high μ2values for the source (>9), positive ɛNd(2.7 Ga) values of 0.3−1.4 and Nd depleted mantle model ages 2.75−2.86 Ga. The high μ2values pointing to a significantly older crustal source apparently contradicts the positive ɛNd(2.7 Ga) values supporting a mantle origin. The geochemical and isotopic data suggest that the Nilsiä and Lieksa granitoids originated at ca. 2.73 Ga from a mantle-wedge source enriched in LILE, U, Th and Pb by recycling of continental material in subduction-related slab dehydration processes shortly (up to 200 Ma) before melting. A considerably large amount of crustal lead was contributed from subducting sediments into the overlying mantle wedge. Consequently, the isotopic composition of mantle-wedge lead was overprinted by crustal lead isotope signatures. It is suggested that crustal recycling through subduction zone processes played an important role for the enrichment of the mantle wedge and generation of mantle-derived high-μ granitoids in the Archean.