| Zhangbaling uplift is located in the eastern part of Anhui Province and in the southern part of Tanlu fault zone.It is mainly composed of Zhangbaling Group in the north and Feidong Group(or Fuchangshan Group)in the south.Its total area is about 4500 Km~2.The western boundary of the Zhangbaling Group is the Zhuding-Shimenshan fault,while the eastern boundary is limited by the Guhe-Sanbing fault.The length of the Zhangbaling Group is about 90 kilometer from South to north and the width is about 20 kilometer from east to west.The exposed area is about 1800 km ~2 and the thickness is about 3800 meter.Previous studies on Zhangbaling Group focused on geochemistry,tectonic geology,thermodynamics and chronology.The results of stratigraphic division,thermodynamics and chronology of the Zhangbaling Group are significantly different.As far as stratigraphic division is concerned,some scholars divide the Zhangbaling Group from bottom to top into Beijiangjun Group and Xileng Group from the perspective of sedimentology.Some scholars divide the Zhangbaling Group into four lithologic units from the bottom to the top in terms of petrography and tectonics,and put the Beijiangjun Group on top of the Xileng Group.In the study of metamorphic grade,it is believed that the Zhangbaling Group underwent greenschist facies metamorphism,blue schist facies metamorphism and amphibolite facies metamorphism.Chronological studies are relatively abundant,but their age span is large,ranging from 748 Ma to 975 Ma.Therefore,detailed field geological survey,regional metamorphic petrology,lithofacies,mineral chemistry,thermodynamics and zircon U-Pb chronology have been carried out in this study.Peak metamorphic conditions and Protolithic ages in this area have been accurately calibrated,and lithologic units have been thoroughly analyzed.The results are as follows:(1)Unit division:Based on the field observation,lithofacies,main metamorphic temperature evaluation results and chronology,and combined with previous research data,the Zhangbaling Group is divided into three units from bottom to top,it is:feldspar quartz schist unit(Unit-?),quartz muscovite schist unit(Unit-?),phyllite and other schist unit(Unit-?).(2)Evaluation of temperature conditions:Due to the lack of suitable pressure gauges,this study mainly evaluates the temperature conditions of Zhangbaling Group.Through petrographic observation,it is found that all the three units of Zhangbaling Group contain muscovite,so the Muscovite Ti thermometer is used in this temperature evaluation.The evaluation results are as follows:When the pressure is calibrated between 0.3 and 0.7GPa,the temperature of Unit-? is T=423-434?,and the metamorphic grade is epidote amphibolite facies.The temperature of Unit-? is T=355-364?,the metamorphic grade is greenschist facies.(3)Chronological study:Thirteen zircon samples of Zhangbaling Group were analyzed in this study.The cathodoluminescence electron image(CL),zircon histogram and age analysis result are indicated that:Unit-? and Unit-? are magmatic zircons,Unit-? is clastic zircon.The results show that:(1)The ages of five samples in Unit-? are basically the same,and all of them indicate the crystallization age of the original pulp,the range of variation is 748±12~752±17 Ma.(2)The chronological results of six samples from Unit-? are 741±15~733±9 Ma,indicating that the crystallization age of the protolith is Neoproterozoic.(3)The youngest zircon age of the two samples in Unit-? is 720±23~682±23 Ma,indicating the latest sedimentary age of the unit.Therefore,according to field observation,petrography and the results of metamorphic temperature evaluation,this study considers that it is reasonable to put the Beijiangjun Group on the Xileng Group. |
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