河南嵩山地区新太古代TTG质片麻岩的成因及其地质意义：来自岩石学、地球化学及同位素年代学的制约
Petrological, geochemical and chronological constraints for the origin and geological significance of Neoarchean TTG gneiss in the Songshan area, North China Craton
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摘要: 河南嵩山地区位于华北克拉通南缘，是我国记录前寒武纪地质的典型地区之一。该区广泛出露新太古代TTG质片麻岩套，主要以英云闪长岩类为主，闪长岩类次之，测得的锆石SHRIMP年龄在2600~2500Ma。该套TTG质片麻岩富Na2O(3%~7%)、SiO2(>67%)、贫铁、镁，高的Na2O/K2O比值（多在1.5~5.2），Al2O3=13.72%~16.37%，A/CNK=0.97~1.21，属英云闪长岩-奥长花岗岩系列，显示新太古代富铝型TTG岩石特征。岩石富Sr (平均433×10-6)，Rb/Sr比值(N=27~150)，基本无Eu异常，低的Nb/Ta(14左右)、La/Nb (平均约为7)比值及其它微量元素特征表明其与岛弧或大陆边缘弧玄武质岩石特征相似。全岩Nd同位素和锆石Hf同位素数据显示岩石源区是来自亏损地幔的约2.66Ga的初生地壳；Mg#值变化较大反映存在地幔楔不同程度的混染。地球化学特征指示该岩浆是在较高温度和压力(约700~1000℃，>1.5Gpa)下由俯冲的含水玄武质洋壳部分熔融形成，残留相中有石榴石和角闪石而不含斜长石。嵩山地区TTG片麻岩的这种成因机制表明当时陆壳以水平方式增生，也说明在随后的2.5Ga左右微陆块碰撞拼合事件之前不同陆块之间很可能被一个古大洋所分隔。Abstract: Neoarchean TTG gneisses widely occurred in the Songshan area, Henan Province. They mainly consist of granitoid and minor diorite, with a crystallization age of 2600~2500Ma by zircon SHRIMP U-Pb technique. The TTG gneisses are rich in SiO2(>67%) and Na2O (3% to 7%), but have low K2O/Na2O ratios and low FeOT and MgO contents. Their A/NCK ratios range from 0.97 to 1.21, consistent with tonalities to trondhjemite trend. The Songshan TTG gneisses are geochemically similar to high-Al TTG assemblage. The Songshan TTG gneisses have low REE contents, with differentiated LREE/HREE patterns and no Eu anomalies on the chondrite-normalized REE patterns. The Songshan TTG have high Sr (433×10-6 on the average) and Sr/Y ratios (165 on the average) and low Rb/Sr ratios, and display negative Nb, Ta and Ti anomalies on the primitive-mantle normalized trace element patterns. Their Nb/Ta ratios are averaged to be about 14 and La/Nb ratios are 7. These indicate that the Songshan TTG may have formed in an island arc or a continent marginal arc. The whole rock Nd isotopic compositions and zircon Hf isotopic compositions indicate that the Songshan TTG may originated from a juvenile crust derived from a depleted mantle at 2.6Ga with minor assimilation of ancient crust, while the variable Mg# values of the Songshan TTG indicate different degrees of contamination of mantle wedge. Therefore, the Songshan TTG gneiss may have formed by partial melting of a subducted basaltic slab with garnet and amphibolite left in the residual magmas. The emplacement of the Songshan TTG indicates an ancient oceanic crust may have existed at 2.53 Ga before the collisions of different micro-continents in North China Craton.
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Key words:
- Neoarchean /
- TTG gneiss /
- Geochemistry /
- Geochronology /
- North China Craton
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