Early Paleozoic Metamorphism And Magmatism Of The North Qinling Orogen In The Baiji Areas

The Qinling orogenic belt is a complicated orogenic belt which was resulted from multiple plate subduction-collision orogeny and subsequent several-stages mountain-building uplifting. The petrological geochemical and isotopic studies have already demonstrated that the North Qinling belt is a mini-continent on an oceanic island basis. A breakthrough progress in recent years is the discovery of occurrence of abundant retrograde eclogites and high-pressure granulites in high-grade metamorphic rocks of the Qinling Group in the eastern part of the North Qinling belt, i.e. the Shaanxi-Henan border area and the Tongbai region, in which zircons from parts of retrograde eclogites contain coesite inclusion. These discoveries demonstrated that deep subduction during 500-480 Ma indeed happened in the eastern part of the North Qinling belt and the Tongbai region,subsequent uplifting of the orogenic belt produced the retrograde eclogite. Meanwhile these high-grade metamorphic rocks displayed a planar distribution in places like Guanpo,Songshugou, Fushui, and Zhaigen. But these rocks only were found in the eastern of North Qinling belt. Now, is there the similar deep subduction happened in the western part of the North Qinling belt? This paper first found the body of the garnet amphibolite in the western part of the North Qinling belt in Meixian, Baoji city. The garnet amphibolite is dominantly composed of garnet and amphibole with minor quartz and diopside. Garnet porphyroblasts show obviously compositional zoning yielded by the diffusion during the retrograde metamorphism. The epidote porphyroblasts show similar compositions in XFe,CaO and Al2O3 contents as those epidotes from eclogites. Meantime, garnet porphyroblast contains Amp+Cpx+Qtz inclusions. Such a mineral inclusion assemblage indicates that it should be eclogite prior to the retrograde metamorphism and the garnet amphibolite was formed with the fluid activity during the retrograde metamorphism. Zircon SIMS geochronology yielded an excellent U-Pb concordant age of 443.9±1.1 Ma and 206Pb/238U weighted mean age of 443.2±2.5 Ma.. Therefore, the 443 Ma is interpreted as the retrograde metamorphic age of the garnet amphibolite, i.e. it records the event of the retrograde metamorphism. These zircons display a depleted Hf isotopic feature and normal mantle-like oxygen isotopic compositions although they have a wide range in?Hf(t)(2.4-14.7) and ?18O(4.25-6.39‰) because of different degree affection during the retrograde metamorphism. Zircon Hf-O isotopic features point out that the protolith of the garnet amphibolite were Neoproterozoic basaltic rocks from the depleted sources.Compared with the time interval between the prograde and retrograde of the eclogites in the eastern part of the North Qinling orogenic belt, we can speculate that the peak metamorphism of the garnet amphibolite was roughly 470-480 Ma in the western part of the North Qinling orogenic belt. In summary, the time difference occurred in the early Paleozoic subduction-collision events between the eastern and western parts of the North Qinling orogenic belt, namely Early Paleozoic subduction-collision events in the North Qinling orogenic belt along the Shangnan - Baoji line may occur from East to west. For the integrated study of metamorphism, I attempted to understand the genesis of igneous rocks.There are six magmatic rock samples collected in the area of Baoji-Tianshui. The CL images of representative zircons reveal a relatively obvious oscillatory zoning of magmatic crystallization. Based on the U-Pb age, there are two stages magmatism: -450 Ma and-430 Ma. The isotope characteristics of the Hf and O from the zircons indicated they derived from the juvenile depleted mantle. Combined with the results of the early-Paleozoic metamorphism of the North Qinling orogen in the Baoji-Tianshui areas,the two episodes magmatism may form in an exhumation process due to the decompressional melting of the subduction plate.