The Mesozoic Tectonic Evolution Of Yanbian Area In The Eastern Segment Of Northern Margin Of The North China Block

Yanbian Area, which is at the eastern segment of the northern margin of the NCC, islocated between XingMeng Organic Belt and Longgang block. Because the final closuretime and location of Paleo-Asian Ocean and the subduction time of Paleo-Pacific areuncertain, the Mesozoic tectonic evolution of Yanbian Area is distuped all the time. Theproblems mentioned above limit the understanding of the Mesozoic tectonic evolution inYanbian Area. However, the study about Mesozoic igneous rocks and metamorphic rocksof Xindongcun formation in Qinglongcun group is done in this paper. All the work includein petrology, Geochronology and Geochemistry. Besides these, the characteristics ofductile shear zone are also analyzed. Combined with the distributed features of igneousrocks and Seluohe Group in the regional area, the Mesozoic tectonic evolution in YanbianArea is further discussed.The results of LA-ICP-MS Zircon dating indicate the magmatic activities in Helongarea of Yanbian are divided into three stages:(1) the early Triassic magmatic eventsincluding Zhixin pluton and Yongxin pluton;(2) the early-middle Jurassic magmatic eventsincluding Shiguoshuiku Pluton, Yueshandong Pluton, Shasongding Pluton and GaolingPluton;(3) the early Cretaceous magmatic events including Liudong Pluton and volcanicrocks in Quanshuicun Formation.The early Triassic magmatic events are composed of Zhixin pluton and Yongxinpluton and have characteristics of the similar bimodal igneous rocks. They are to thenortheast of Gudonghe Fault and spread in the north-eastern direction. The analyses ofZircons from Zhixin pluton yield weighted mean206Pb/238U age of251.05±0.95Ma whileThe analyses of Zircons from Yongxin pluton yield weighted mean206Pb/238U ages of238.5±2.8Ma and243.2±1.1Ma，which suggesting they are formed in the early Triassic.Geochemically, the amphibole gabbro of Zhixin pluton is low in SiO2and MgO, and theyare strongly enriched in large ion lithophile elements and depleted in high field strengthelements (e.g., Nb, Ta), the petrogeochemical features are consistent with the ones of basicigneous rocks formed in the subduction environment. Furthermore the rocks have low Isr and positive εNd, indicating that the magma source is from the mantle wedge influencedby the fluid. The granitic rocks in Yongxin pluton are composed of garnet monzograniteand biotite monzogranite, and the granitic rocks are deformed highly. Petrogeochemically,the granitic rocks are in high SiO2and Al2O3, and are also strongly enriched in large ionlithophile elements(e. g. Cs, Rb, K) and Zr (HFSE). They are depleted in high fieldstrength elements (Nb, Ta, P and Ti) and Ba, Sr (LILE). The petrogeochemical fetures arelikely to the granites whose magma sources are from crustal material. In the harkerdiagram, they have negative correlation between the major elements (Al2O3, CaO, MgO,P2O5, FeOt, TiO2) and SiO2, which indicate the magma experienced the fractionalcrystallization during the magma evolution. Compared to the basic rocks of Zhixin pluton,the granitic rocks have higher Isr and different εNd, the magma source of granitc rockswas also influenced by the fluid, and it was derived from partial melting of mafic rocks inlower crust. However, the similar bimodal igneous rocks in early Triassic were formed inan extensional environmentThe magmatic activities in the early to middle Jurassic are distributed widely,including granodiorite, monogranite, biotite diorite and peridotite gabbro. The analyses ofZircons from grandonite of Shiguoshuiku pluton yield weighted mean206Pb/238U age of193.06±0.8Ma. Petrogeochemical, the granitic rocks are in high SiO2and Al2O3, andhave low MgO contents. In the other hand, they are also strongly enriched in large ionlithophile elements(e.g. Cs, K, Th) and depleted in high field strength elements(Nb, Ta, Pand Ti). The characteristics of geochemistry are constant with igneous rocks formed in theactive continental margin. Besides these, the major contents (Al2O3, FeOt, MgO, TiO2,P2O5and CaO)are in negative linear relationship with SiO2, indicating the evolution ofmagma is dominated by the fractional crystallization. The Isr andεNdvalues of thesamples are similar to ones of crust, these features indicate the magma source were frompartial melting of mafic rocks in lower crust, and affected by the ancient crust materials.The Gaoling pluton is composed of porphyritic granodiorite and porphyriticmonogranite, The analyses of Zircons from porphyritic granodiorite yield weighted mean206Pb/238U age of180.47±0.74Ma, while The analyses of Zircons from porphyriticmonogranite yield weighted mean206Pb/238U age of72.25±0.97Ma and170.9±0.68Ma,respectively. Similar to geochemistrical characteristics of adakite rocks, all the sampleshave high SiO2and Al2O3contents, high Sr contents and low Y and Yb contents. They are also strongly enriched in light REE and depleted in heavy REE, while are enriched in largeion lithophile elements and depleted in high field strength elements. These rocks have lowIsr and negativeεNdvalues, the geochemical data indicate that the Gaoling pluton magmawas derived mainly from partial melting of mafic rocks in thickened lower crust whichlikely affected by Neoproterozoic accretion material and also was influenced by the ancientcrust.The analyses of Zircons from grabbro of Shasongding pluton and diorite ofYueshandong pluton yield weighted mean206Pb/238U age of175.6±1.1Ma and178.4±1.2Ma, respectively. Both of them are low in SiO2and high in MgO, the values of Rb/Srand Nb/Ta are consistent with the average values of mantle, indicating that their magmasource are from Mantle. Besides these, both of them are enriched in large ion lithophileelements and depleted in high field strength elements (Nb, Ta). With the low P2O5contentsand TiO2, the magma source of tholeiitic grabbro and cal-alkaline diorite are from themantle wedge affected by the fluid.The analyses of Zircons from monogranite of Liudong pluton yield weighted mean206Pb/238U age of118.58±0.87Ma and118.52±0.67Ma, respectively. All of the samplesare high in SiO2, Alk and Al2O3and low in TiO2and∑REE, They are also stronglyenriched in light REE and depleted in heavy REE, and are enriched in large ion lithophileelements (Ba, Rb, U, K) and depleted in high field strength elements (Nb, Ta, P, Ti). Thecharacteristics mentioned above are similar to the ones of crust. And the Isr andεNd(118Ma)values are also in the range of ones of low crust. All of the characteristics indicate themagma source of Liudong pluton is from partial melting of depleted low crust mafic rocks.The cal-alkaline volcanic rocks in Quanshuicun formation are formed in activecontinent margin. All the sample are high in SiO2and Al2O3and low in MgO, TiO2, Sc, Ni,V and Co. The samples are also enriched in large ion lithophile elements and depleted inhigh field strength elements. The characteristics of volcanic rocks indicate the primarymagma is from partial melting of low crust mafic rocks.The early Triassic igneous activities are related to the subduction of paleo-asianocean, and the igneous activities in early Jurassic to early Cretaceous are formed in thesubduction environment of Paleo-pacific ocean. The features of Sr-Nd in the study area aresimilar to the ones in the XingMeng Organic Belt, suggesting the existence ofNeoproterozoic crustal growth event. The metamorphic rocks in Xindongcun Group are composed of marble,muscovite-biotite schistis and small amount of volcanic rocks. Its rock association issimilar to the marine stratum, and the geochemistrical features are also similar to the onesof marine stratum formed in the neritic environment. The youngest detailed zircon age is242Ma, sugguesting the paleo-asian ocean is still being before242Ma in the study area.The deformation age of ductile shear zone is between240Ma and200Ma, and theresults of stress analysis indicate the formation of ductile shear zone is related to theNNE-direction compressive stress under the environment of paleo-asian ocean subduction.Based on the study above, we also analyze the characteristics of magmatic activitiesin Yanbian-Suifenhe area. And we find the magmatic activities are mainly distributed to thesouth Wangqing-Hunchun area, suggesting the paleo-asian ocean final suture is nearWangqing-Hunchun. And after~240Ma, the pale-asian ocean disappeared, and the tectonicsetting became the setting of continental collision and collapse which resulted analkaline-basic magmatic zone were formed in the Yanbian-suifenhe area. But during theJurassic period, the Yanbian-Suifenhe area is completely controlled by the subduction ofpale-pacific ocean, and magmatic rocks with charateristics of subduction were formed ineastern Jinlin and Heilongjiang area. During the150Ma-130Ma, the subduction ofpaleo-pacific ocean was weak and in the Late cretaceous, the subducion of paleo-pacificocean once began.Above all, the tectonic of Yanbian area was controlled by the subdution of paleo-asianocean before the middle Traissic, and the final sutre is located from Wangqing to Hunchun.The eastern of Jinlin and Heilongjiang area is affected by the paleo-pacific ocean from~200Ma.