Characteristics And Tectonic Meanings Of The Caledonian Mafic Dikes In Gannan Area

The Gannan area is located in the eastern part of the Nanling east-west tectonic-magmatic belt,belonging to the Nanling rare-nonferrous metal metallogenic belt.The geological structure of Gannan area is complex—magmatic activity is very strong during not only Yanshanian but Caledonian as well.Most of the magmatic rocks in this area are acidic rocks and a small amount of mafic rocks are distributed sporadically.basic rock less sporadic distribution.The mafic dike that development is frequently related to the regional tectonic stretch originated in the deep(mantle or lower crust)of the earth,intrude into different crustal levels and is the product of shallow intrusion of deep seated magma source.The South China block is composed of the Yangtze block and the southeastern cathaysia block along the Jiangnan orogenic belt in the early Neoproterozoic,and undergoes the superposition of the late multi-period tectonic magmatic events,in which the Caledonian orogeny is one of the most important tectonic events.However,the current research on the Caledonian tectonic magmatic events mainly focused on granites,and rarely studied on acid volcanic rocks and basic rocks in the SCB.There are only 3 cases of Caledonian mafic rocks in southern China,which are currently distributed in Nanling area,Yunkai area and Wuyi area.The age of mafic rocks is mainly between 420-450 Ma.However,based on the related geochemical and isotopic studies,There is still a great controversy about the tectonic environment of Rock formation,and some people suggest that its was formed in intraplate environment,and some hold that formed in the island arc subduction environment(Yao et al.,2012;Wang et al.,2013).The mafic dikes,which located at Suichuan,Gannan,are studied as objects in this research.We have carried out detailed field observation and indoor petrology,geochemistry,zircons U-Pb age and Lu-Hf isotopic analysis research on the mafic dike,and discussed related problems about the petrological and geochemical characteristics,formation time,magma formation and tectonic environment.The analysis result of mafic dike LA-ICP-MS zircon U-Pb dating on mafic dikes show that Th/U is between 0.46 to 1.59,that suggest the zircon of typical magmatic origin.The dating result of Four samples(446±6.6 Ma,431.9 ±5.7 Ma,422.3 ± 7.3 Maand 426.6 ±6.9 Ma)indicate that the intrusion age is early Paleozoic silurian,and show that the mafic dike the product of extensional regime after caledonian fold orogeny.The result of Lu-Hf isotopic analysis On two samples—the variation range of initial 176 Hf / 177 Hf is small,and the corresponding ?Hf(t)values are between-4 ~-0.1—indicate that zircon is the product of Crust source rock remelting.zircon TDM2 values between 1424 ~ 1670 Ma,indicates that mafic crust Formed in proterozoic era may be the main source of the caledonian intrusive rock formation in early Paleozoic.Characteristics of the main elements and analysis results of trace elements show that the mafic dike fall into the tholeiite series and alkaline series,belong to the sodium-rich basite;TiO2 content is higher that three sample content is > 1%,show that the mafic dike belong to high titanium basite;A/NCK are less than 1,show the rock belongs to the aluminum basite;REE Distribution Curve Clearly tilted to the right(? LREE / ? HREE between 7.4 ~ 8.78)show that the characteristics of the sample is enrichment in LREE,and relative loss in the HREE.patterns of REE Distribution Curve is roughly similar to the mantle magmatic type of REE Distribution Curve,indicate that the souce of Gannan suichuan Mafic dike have close relationship with the mantle magma.It illustrate Paleozoic late caledonian magmatism is the result of combined action of mantle-derived magma and mafic crust formed In the proterozoic era;The mafic magmatism during late early Paleozoic is given priority to with crust source.Two classes of intermediate-acid rock mass(the caledonian synorogenic biotite monzonitic granite,granodiorite and after-orogenic biotite adamellite)are exposed in study area.caledonian acidic and mafic intrusive rocks' existence at the study area explains caledonian Invasive rock was from extruding acid intrusive rocks to stretching mafic rock,reflect the processes of the regional tectonic environment by extrusion gradually transform into stretching tension in the region.