科研之友
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摘要
As the northern segment of Jiao-Liao-Ji Belt of North China Craton, the Liaoji region preserves good exposure of Paleoproterozoic sedimentation and magmatism. In this contribution, we conducted petrologic, geochemical and petrogenesis study on two Paleoproterozoic monzogranitic gneisses (Jiguanshan pluton and Laoheishan pluton) in the Liaoji region. The study shows that the monzogranites have relatively higher K2O/Na2O, FeOT/MgO ratios and Ga, Zr, Y contents, while low contents of CaO, MgO, Sr, Cr, Co and Ni. Concentration of the total alkali (K2O + Na2O) is 7.97% similar to 9.08%. In the diagram of SiO2-K2O, monzogranites mainly lie in the high-K calc-alkaline series. A/CNK values of the monzogranites are 0.84 similar to 1.03, while their A/NK values are 1.11 similar to 1.17. 10000Ga/A1 values of all the samples are higher than 2.6, thus, these monzogranites are classified as aluminous A-type granites with high-K calc-alkaline affinity. E-Nd(t) values of six samples from the monzogranites are -3.3 similar to -0.9, corresponding to t(Dm2) of 2860 2669Ma. Thus, the monzogranites originated from partial melting of Archean crust. The monzogranites are characterized by enrichment in large ion lithophile elements (such as Rb, U, K) and light rare earth elements, and depletion in high field strength elements (such as Nb, Ta, Ti), showing evident signature of subduction-zone magmatism. The monzogranites plot in the region of volcanic arc in the Rb/30-Hf-Ta x 3 diagram. In the diagrams of Y/Nb-Rb/Nb and Nb-Y-Ce, the monzogranites are located in the region of A(2)-type granites due to theirs relatively higher Y than Nb, which is similar to island-arc basalts in terms of Y/Nb ratios. The Liaoji region was a continental back-arc basin in the Early Paleoproterozoic, and strong extension generated the monzogranites in this back-arc basin. The monzogranites were later deformed and pushed together with their surrounding rocks in later processes of terrane amalgamation.
