Geochemistry and zircon U-Pb-Hf isotopes of Paleoproterozoic granitic rocks in Wangjiapuzi area, eastern Liaoning Province, and their geological significance

Paleoproterozoic granites are widely distributed in the Wangjiapuzi area of eastern Liaoning Province. In the past, some granitic plutons were considered as granitic melange in this area. In this paper, the authors divided the granitic melange into two types, i.e., striate biotite monzogranites and gneissic biotite monzogranites, on the basis of comprehensive studies of field observation, petrography, geochemistry and isotope chronology. The results of geochemical analysis show that the two components are homogeneous, having uniform geochemical characteristics. They all show high SiO_2\, rich K₂O and poor Al₂O₃ features, with K₂O/Na₂O being 0.64~2.14. Their TiO₂ values are between 0.16% and 0.3%, with lower MnO, MgO, CaO and P₂O₅ content. The saturation index A/CNK is between 1.06 and 1.1, and A/NK is between 1.50 and 1.62, which suggests that the granodiorites should belong to the peraluminum calcium alkaline series. Some trace elements show that high field strength elements such as Nb, Ti, Ta are strongly depleted, with an obvious anomaly of negative Eu. All the geochemical characteristics suggest that they should belong to Atype granites.The most zircon grains of striate biotite monzogranites show clear oscillating zoning structures, and LA-ICP-MS zircon U-Pb weighted mean age is 2188±13Ma(MSWD=0.49), with the age of magma crystallization representing the age of the rock. The most zircons of gneissic biotite monzogranites show obvious metamorphic edge, zircons from the part of the nucleus show clear oscillating zoning structures. LA-ICP-MS nucleus zircon U-Pb weighted mean age is 2214±16Ma(MSWD=1.01), with the age of magma crystallization representing the age of the rock. LA-ICP-MS edge zircon U-Pb weighted mean age is 1905±13Ma(MSWD=4.5), representing the age of metamorphic rock. The model ages of these two types of granites obtained from Hf isotope are respectively 2387~2584Ma and 2474~2641Ma, the average of the crustal pattern is 2495~2808Ma and 2633~2868Ma, older than the age of rock formation. The source area of Paleoproterozoic granite in the study area was mainly Archean basement mixed with a small amount of Paleozoic Neoproterozoic crust. The formation environment of the dike granite diorite is reported by some geologists. It is shown that the early rising of the Jiao-Liao-Ji orogenic/activity belts experienced a tensile cracking process at about 2.2~2.15Ga, and then there was a process of subduction and compression around 2.0Ga.