Geochronological Constraints On The Tectonic Evolution Of The Liao-Ji Paleoproterozoic Rift Zone

Liao-Ji Proterozoic rift zone distributes in the area of the east of Liaoningprovince and the south of Jilin province，which is located in the east of Tan-Lufault．It’s a northward prominent arc structural belt which is nearly east-west andsandwich between the northern Longgang Archaeozoic massifs、southern LiaoNanblock and Langlin block of North Korea．The rift zone developed a set of Paleoproterozoic metamorphic rocks：In the partof Jilin province of the rift zone， there is Ji’an group(Mayihe Formation，Huangchagou Formation and Dadongcha Formation from bottom tu up) and LaolingGroup(Xinnongcun Formation，Banfanggou Formation，Zhenzhumen Formation，Huashan Formation and Linjiang Formation from bottom tu up)；In the part of Jilinprovince of the rift zone，there is Liaohe group(Langzishan Formation，LieryuFormation，Gaojiayu Formation，Dashiqiao Formation and Gaixian Formation frombottom to up)．At the same time，there is also existing a set of rifting period，orogenicperiod and post-orogenic period Paleoproterozoic granite on the rift zone．The protolith of each formation of Ji’an group，Laoling group and Liaohe groupof Liao-Ji rift zone have been restorated．The results of protolith restores indicate thatMayihe Formation、Huangchagou Formation of Ji’an group and Lieryu Formation ofLiaohe group were mainly consist of meta-volcanic rocks and a small amount ofmeta-sedimentary rocks；the other formations of rift zone are meta-sedimentaryrocks．The meta-volcanic rocks have the characteristics of bimodal volcanics：thebasic components are alkaline series basalts and tholeiite basalts；acidic componentsare alkaline and calc-alkaline rhyolite．Through the technology of LA-ICP-MS and SHRIMP， Researching thegeochronology of the rift zone basement、meta-volcanics、meta-sedimentary、graniteof the Liao-Ji Proterozoic rift zone and Yushulazi Formation which is overlapped theProterozoic strata． Five meta-volcanic rocks samples of rift zone have been analysised and showfour age groups：the first group(2733±50) Ma) and second group((2457~2494) Ma)are representative of the magmatic zircons ages，representing the two periods ofmagmatic event age of the rift zone basement；the third group((2165~2179) Ma) isalso magmatic zircons，representing the protolith age of volcanic rocks of riftingperiod；the fourth group((1875~1879) Ma) is metamorphic zircon，representing theage of regional metamorphism．Seven meta-sedementary rocks age samples of rift zone are analysised anddisplayed four group of age values．The first(2650Ma±) and second group((2456~2520) Ma) are magmatic detrital zircons，representing the two periods of magmaticevent age of the rift zone basement；the third group((2165~2179) Ma) is alsomagmatic detrital zircons，representing the age of magmatic event of rifting periods ofthe rift zone；the forth group((1860~1883) Ma) is metamorphic zircon，representingthe age of regional metamorphism． In addition， detrital zircons from themetasediments also contains the age of3000Ma±and3283Ma±，which areinherited from the Archean basement．Magmatic rocks of the Liao-Ji Proterozoic rift zone include mafic dyke swarmsand granitiods．(1874±31) Ma metamorphism age and (2163±22) Ma magmatic ageare gived by mafic dykes，representing regional metamorphism age and magmaticevent age of rifting periods respectively．The age of the rift zone basement granite(Lianshanguan granite) is (2533±22) Ma；the rifting periods granite(Qianzhuogoupluton and Hupiyu pluton) are A-type granite，which age is (2168~2173) Ma andconsistent with the age of rifting periods of volcanic rocks； Orogenic periodgranite(Shuangcha pluton and Housongshugou pluton) are syn-collisional granites，which age is (1865~1877) Ma and consistent with the regional metamorphism age ofrift zone．The youngest detrital zircon ages of Yushulazi formation which unconformity onthe top of Liaohe group was (1728±66) Ma，indicating the end of orogeny before the(1728±66) Ma．At the same time，Yushulazi formation contain the magmatic detritalzircon age of (2480±22) Ma、the magmatic detrital zircon age of (2179±27) Ma、magmatic and metamorphic detaital zircon age of (1877±26) Ma，which consistentwith the latest magmatic event age of basement、volcanic rock and granite ages ofrifting period、metamorphism and synorogenic granite age respectively．In shortly，Liao-Ji Proterozoic rift zone basement formed before (2456~2533)Ma；rifting periods was (2150~2189) Ma；volcanic-sedimentation was between2189 Ma and1883Ma；The closure effect of the rift zone occurred in (1860~1883) Maand ended before (1728±66) Ma．The latter age is youngest detrital zircon age ofYushulazi Formation．...