Multi-metamorphism Of Amphibolites In Qinling Complex.and Its Tectonic Implications

Over the past 10 years,with the constantly discovery of the various types of high pressure/Ultra-high pressure (HP/UHP) metamorphic rocks in the North Qinling complex, and the in-depth study of the continental subduction/deep subduction during the Early Paleozoic, new approach to understand the tectonic evolution of the Qinling orogenic belt in Early Paleozoic was established. As one of the main components in Qinling complex, amphibolites can provide key constrains for further understanding of the nature, scale and structure attributes of Early Paleozoic continental crust subduction/deep subduction in the North Qinling belt. Especially the relationship between the amphibolites and the HP/UHP metamorphic rocks,and the study whether the amphibolites suffered HP/UHP metamorphism. Thus,in this paper,three amphibolites samples from Qingyouhe, North Xixia and Gengjiadian areas in Qinling complex were selected. Combination with systematic petrographic,geochemical and geochronological study,the main results are obtained as follows:1. The protolith age of Qingyouhe amphibolite is 774±13Ma,and has undergone 493 ±5Ma eclogitic metamorphism and 448±7Ma amphibolitic metamorphism; North Xixia amphibolites experienced -500Ma,-450Ma,-420Ma and -400Ma multi-metamorphism; Gengjiadian village amphibolite was formed during 886-613Ma, two different period of metamorphic ages are -495±3Ma and -452±6Ma. All these metamorphic ages of the three different amphibolites are consistent with the HP-UHP rocks in this region (peak metamorphism age is -500Ma and the retrograde metamorphic age are during -450Ma and-420Ma),all the evidence above implied that these amphibolites in this study may have experienced the same metamorphic evolution process with the HP-UHP rocks in this region, they should be a part of the Early Paleozoic north Qinling HP-UHP metamorphic belt.2. The protolith age(774±13Ma),eclogite-facies age(493±5Ma), and amphibolite-facies retrogression age(448±4Ma) from amphibolite in Qingyouhe area are coincide with other HP-UHP rocks of north Qinling complex. Additionally,three stages of metamorphic ages observed in individual samples or individual zircon grain, suggesting that the amphibolites records multiple period of zircon grows and is interpreted as a tectonic process from subduction to exhumations. Thus,we infer that some low-grade metamorphic facies rocks in the Qinling complex may also experienced HP-UHP metamorphic in early time, but it is difficult to identify due to the succedently strong overprinted retrogress metamorphism. The distribution of the HP/UHP rocks in Qinling complex may be far more than the current observation.3. Whole rock Sr-Nd isotopic sNd (t) of the amphibolites from Qingyouhe and Gengjiadian village are 3.28-4.29 and 3.46-6.46, respectively, zircon sHf (t) (5.93-11.10) are obtained for Qingyouhe amphibolite; in Zr-Zr/Y, Zr-Ti and Hf-Th-Ta diagram, the point of amphibolite rocks fall within the area of E-MORB and within-plat basalt, while the area of tholeiite in Ta/Yb-Th/Yb diagram. Combined with field geological occurrence, regional geological context, particularly its formation age (700-843Ma) of protolith far earlier than the peak metamorphic age (-500Ma) for comprehensive analysis, these amphibolites are first formed as basic rocks which were in placed in the continental crust and then undergo subduction/deep subduction and then exhumation accompany with the continental crust. sNd (t) of north Xixia amphibolite is -4.66-+2.54, indicating its protolith maybe tuffaceous rock, is typical parametamorphic amphibolite. All results above coupled with the HP/UHP eclogites, which protolith is continental tholeiitic basalts, and the typical continental formation UHP rocks in Songshugou in this region, make a further sign of that the North Qinling HP/UHP metamorphic belt is the product of continental subduction/deep subduction.4. In this study, we have obtained ages of 774±13Ma and 886-613Ma for the plagioclase amphibolite from Qinyouhe and Gengjiadiancun areas, respectively, which were interpreted as the formation ages of the protoliths. This results are consistent with previous studies for the formation ages of HP/UHP metamorphic rocks protoliths and indicate that the HP/UHP metamorphic rock in the North Qinling belt may have similar magmatic protolith ages of 700-843Ma. Moreover, the plagioclase amphibolites and other orthometamorphic rocks show the similar Sr-Nd isotopic features with Neoproterozoic basalt dykes in the south Qinling, This, therefore, suggest that continental origin of the HP/UHP rocks precursors in the North Qinling belt probably derived from the Neoproterozoic continental tectonic body in the South Qinling belt.5. The magmatic zircons sampled from plagioclase amphibolite in the southern Gengjiadian village yield an age of 886±5 Ma. This plagioclase amphibolite have basaltic bulk compositions (SO2 content of 48.48-50.17%), however, in the CL image, zircon grains display distinct oscillatory zoning rather than broad zoning, suggesting their acidic magmatic origin. Additionally, the amphibolite have high positive whole-rock sNd(t) (3.46-6.46), but have negative zircon εHf(t) (-9.76±2.84), indicating the geochemical features between whole-rock and zircon are quite different. Consequently, the age of 886±5 Ma can not be interpreted as the formation age of protolith, these zircons may capture from other magmatic rocks during protolith emplacement. As an important revelation, U-Pb age determinations on magmatic zircons cores in high grade metamorphic rocks can not represent the formation ages of protolith. This study revealed that consistency between CL image, rock property, rock geochemical and zircon isotopic compositions will help us to define the age of protoliths.