Discovery of garnet amphibolite in western part of the North Qinling orogenic belt and its inferred metamorphic process : Constraints from zircon geochronology and Hf-O isotopes

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. 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 480 similar to 500Ma 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. Now, the question is whether or not 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 X-Fe, CaO and Al2O, contents as those epidotes from eclogites. Meantime, garnet porphyroblast contains Amp + Cpx + Qtz inclusions. Such a mineral inclusion assemblage indicates that it could 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. 1Ma and Pb-206/U-238 weighted mean age of 443.2 +/- 2. 5Ma. Low Th and U abundances and extremely high Th/U ratios (0. 47 similar to 1. 66) in zircons illustrate that these zircons were not the prograde metamorphic zircons produced during the "dry" system, but should be formed during the intense fluid activity or metamorphic zircons completely reset during the retrograde metamorphism. 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 epsilon(Hf) (t) ( 2. 4 similar to 14. 7) and delta O-18 = 4. 25 parts per thousand 6. 39 parts per thousand) because of different degree affection during the retrograde metamorphism. The Hf isotopes yield roughly t(DM) ages of 0.48 similar to 0. 95Ga. Thus, 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 similar to 480Ma 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.