长白山新生代火山岩之下新太古代基底年代学及地球化学特征

对长白山第四系玄武岩覆盖区零星出露的花岗质岩石和片麻岩进行LA-ICPMS锆石U-Pb年龄测定及地球化学分析,结果显示花岗质岩石的年龄为2 509~2 558 Ma;斜长角闪片麻岩的年龄为2 475~2 553 Ma。地球化学特征上,这些岩石均属奥长花岗岩-英云闪长岩-石英闪长岩系列(TTG),稀土元素配分模式图呈右倾型,轻重稀土元素分馏明显(LREE/HREE=3.85~34.48),白云母二长花岗岩具有铕的负异常(δEu=0.41~0.54),富集Rb、K、Th等元素,亏损Nb、Sr、P、Ti等元素;黑云斜长角闪石片麻岩具有铕负异常(δEu=0.67~0.76),富集Rb、K、La、Nd等元素,亏损Th、Nb、Sr、P、Yb等元素;花岗质片麻岩和黑云斜长片麻岩具有铕的弱负-正异常(δEu=0.87~3.3),富集Rb、K、Sr等元素,亏损Nb、Th、P等元素。年代学和地球化学特征与白山地块和和龙地块中的同类岩石基本一致,表明白山地块与和龙地块在晚太古代是同一个陆块(龙岗陆块),二者目前的分布格局是后期北东向左行断裂构造改造的结果。

Geochronological and geochemical characteristics of Neoarchean basement under Conozoic basalt in Changbai Mountain

The LA-ICPMS zircon U-Pb geochronological and geochemical analysis for granitic rocks and amphibolite gneiss exposed sporadically in the Quaternary basalt coverage area of Changbai Mountain show that the zircon U-Pb age of granitic rocks and amphibolite gneiss are 2 509~2 558 Ma and 2 475~2 553 Ma, respectively. Geochemically, both granitic rocks and amphibolite gneiss belong to tonalite-trondhjemite-granodiorite series (TTG). The REE pattern exhibits rightward sloping with significant fractionation of light rare earth elements (LREE) and heavy rare earth elements (HREE) (LREE/HREE=3.85~34.48). Muscovite monzonitic granite displays slightly negative Eu anomalies (δEu=0.41~0.54), enrichment of Rb, K, Th and strongly negative Nb, Sr, P, Ti anomalies. Biotite hornblende plagiogneiss shows negative Eu anomalies (δEu=0.67~0.76), enrichment of Rb, K, La, Nd and depletion of Th, Nb, Sr, P, Yb. Granitic gneiss and biotite plagiogneiss display slightly negative to positive Eu anomalies (δEu=0.87~3.3), enrichment of Rb, K, Sr and depletion of Nb, Th and P. Their geochronological and geochemical characteristics are similar to relevant rocks of the Baishan and Helong massifs, indicating the Baishan Block and Helong Block are the same block (Longgang block) during Late Archaean. Current distribution patterns of Baishan and Helong blocks are the results of reworking by the NE-trending left-lateral faults afterwards.