The Petrology,Geochemistry,Multiple Metamorphic Events And Tectonic Evolution Of The Exotic Rock Slices In The Sulu Ultrahigh-pressure Metamorphic Belt

The exploration of multiple metamorphic events and complex tectonic evolution of orogenic belt is one of the hottest topics of the metamorphic geology investigations in the world.Dabie-Sulu orogenic belt,as the largest high-ultrahigh pressure?HP-UHP?metamorphic belt worldwide,contains important informations of multiple transformations of mineral phases as well as multi-metamorphic evolution.It is therefore the target area for studying the complicated metamorphism of the continent-continent collisional orogenic events and the dynamics of tectonic evolution.Discoveries of intergranular coesite,diamond and coesite inclusions in metamorphic minerals indicate that the voluminous continental crustal materials were subducted deeply into the upper mantle depths and underwent UHP metamorphism.The protoliths of the UHP metamorphic rocks were mainly sourced from the northern margin of the Yangtze Block and underwent UHP metamorphism during Triassic.In recent years,abundant non-UHP Archean-Paleoproterozoic metamorphic rocks as exotic rock slices in the northern Sulu UHP belt were discovered continuously.Nonetheless,the geochemical characteristics,multiple metamorphic P-T-t paths,detailed geochronology,source materials and affinity of the exotic rock slices remain to be debated.In this paper,the exotic rock slices cropped out in the Weihai,Wuji,Haiyangsuo,Laixi and Wulian areas in the northern Sulu UHP belt were selected for detailed investigations.Integrated studies on petrology,geochemistry,mineral chemistry,metamorphism and geochronology were applied to establish the multiple metamorphic P-T-t paths and confirm the protolith and metamorphic ages of the exotic rock slices.On that basis,we rebuild a new tectonic evolution model of the Sulu orogeic belt.The exotic rock slices mainly consist of various rock types involving meta-mafic rocks,meta-granitic rocks and meta-sedimentary rocks.They always occur as tectonic slices within the Sulu UHP belt.Among these,the meta-mafic rocks mainly contain garnet-bearing mafic granulites with“red-eye socket”texture and?garnet-bearing?amphibolites.Whole-rock geochemical features show their protoliths are high-Fe,low-Ti subalkaline basalts with low Zr,P and high Nb contents and similar to island arc basalts.They may be derived from the deep depleted mantle and experienced the fractional crystallization proecess of clinopyroxene.These geochemical characteristics are distinguished from those of UHP eclogites.The protoliths of meta-granitic rocks are TTG and I/S-type granites,belonging to the low potassium-calciumalkaline series,which are essentially distinguishable from the A-type granite metamorphosed to UHP orthogneiss.The meta-sedimentary rocks consist mainly of paragneiss,marble,calcium-magnesium carbonate,quartzite and the felsic granular rock.Among them,the protoliths of the aluminum-rich meta-sedimentary rocks are mainly clay rock and greywacke,and some are aluminum-rich clay rock and acidic volcanic rock,similar to those of the Jingshan and Fenzishan groups in the Jiaobei terrane.The garnet-bearing mafic granulites with“red-eye socket”preserved various mineral reaction textures and two discrete episodes of granulite-facies metamorphic events have been recognized.The peak granulite-facies metamorphism of the first episode(M_1a)is characterized by relict coarse-grained mineral assemblage of garnet?high-Mg core?+clinopyroxene?high-Al core?+plagioclase?high-Ca core?±orthopyroxene?high-Al core?+quartz+ilmenite±magnetite with the P-T conditions of 730-900oC and 7.0-12.2 kbar.The second episodic granulite-facies metamorphism?M₂?is featured by“red-eye socket”textures by the following reaction of orthopyroxene+plagioclase+quartz?garnet+clinopyroxene+plagioclase+quartz,including a peak granulite-facies metamorphic stage(M_2a)and a l ate amphibolite-facies retrograde stage(M_2b).The peak stage(M_2a)is characterized by fine-grained mineral assemblage of garnet?high-Mg core?+clinopyroxene?high-Al core?+plagioclase?high-Na core?+quartz+ilmenite surrounding the relict coarse-grained granulite-facies minerals of M_1a.They were formed under HP granulite-facies P-T conditions of 730-850oC and 11.0-15.5 kbar.Both of the granulite-facies metamorphic events(M_1a and M_2a)were overprinted by a l ate amphibolite-facies retrogression(M_2b)of symplectites of green amphibole+plagioclase+quartz or green amphibole corona formed under the P-T conditions of 610-760oC and 5.5-9.3 kbar.It is suggested that the second episodic metamorphism exhibits a clockwise P-T path including near-isothermal decompression?ITD?to decompression cooling evolutional process.Abundant magmatic zircon U-Pb dating results reveal that the meta-mafic rocks exhibit two magmatic events involving ca.2.55 Ga and ca.1.85 Ga and TTG-granitic rocks show four magmatic events of ca.2.7 Ga,ca.2.55 Ga,ca.2.45 Ga and ca.1.85 Ga.These protolith ages are completely consistent with the time sequences of the significant magmatic events within the Jiaobei terrane.The detrital zircons of the meta-sedimentary rocks show two peaks of ca.2.5 Ga and ca.2.07 Ga which are similar to those of the meta-sedimentary rocks of the Jingshan and Fenzishan groups in the Jiaobei terrane.The late Paleoproterozoic metamorphic ages of 1.95-1.80 Ga recorded by metamorphic zircons of the exotic rock slices are also similar to those of the metamorphic rocks in the Jiaobei terrane.Combined with the investigations of metamorphic evolution,these ages represent the peak metamorphic timing of the first granulite-facies metamorphic events of the exotic rock slices.In addition,metamorphic zircon and monazite U-Pb dating as well as muscovite and amphibole Ar-Ar dating show that the second episodic metamorphism occurred during 252-205 Ma?Triassic?.These ages can be subdivided into three groups of 252-241 Ma,ca.235 Ma and 220-205 Ma,corresponding to the pre-peak prograde,peak and post-peak retrograde metamorphic age of the second metamorphic event,respectively.In summary,the geochemical characteristics and the Paleoproterozoic metamorphic ages of the non-UHP meta-mafic rocks,meta-granitic rocks and the meta-sedimentary rocks as exotic rock slices are uniformly consistent with the similar rocks in the Jiaobei terrane,but distinct from the UHP rocks in the Sulu UHP belt.It is elucidated that the exotic rock slices in the Sulu UHP belt have the affinity to the Jiaobei terrane.More importantly,the identification of the Triassic metamorphic event with a clockwise collisional orogenic P-T-t path demonstrated that the exotic rock slices derived from the southeastern margin of the North China Block were involved into the Triassic subduction-collisional orogenic evolution between the North China Block and the Yangtze Block.In lines of these evidences,the northern Sulu UHP belt should be composed of both the non-UHP exotic rock slices derived from the Jiaobei terrane and the UHP rocks derived from the Yangtze Block.Accordingly,a new Triassic subduction-collisional orogrenic model between the North China Block and the Yangtze Block was established.At early Triassic?252-241 Ma?,the Yangtze Block began to move northwards and subducted beneath the North China Block,and the Paleoproterozoic metamorphic basement in the southeastern margin of the North China Block were dragged by the subducting Yangtze Block simultaneously.At middle Triassic?ca.235 Ma?,the materials from the northern margin of the Yangtze Block were subducted into the upper mantle depths and underwent UHP metamorphism.Meanwhile,the meta-mafic rocks,the meta-granitic rocks and the meta-sedimentary rocks from the northeastern margin of the North China Block were subducted into the depth of 35-45 km and experienced HP granulite-facies metamorphism.At late Triassic?220-205 Ma?,the non-UHP exotic rock slices were uplifted rapidly to the earth's surface together with the UHP rocks,and formed a mélange zone in the northern Sulu area.