Mesozoic Magmatic Events In Northern Liaoning And Central Jilin Provinces:Constraints On The Regional Tectonic Evolution

This thesis studies the Mesozoic igneous rocks in the northern Liaoning and central Jilin provinces,NE China.Field geological observations,petrographic study,and zircon LA-ICP-MS U-Pb dating results indicate that the Mesozoic magmatic activity in the northern Liaoning and central Jilin provinces can be subdivided into three stages at220~205 Ma,195~175 Ma,and 170~163 Ma.Based on the whole-rock major and trace element data as well as zircon Hf isotope data,together with their rock association,spatial-temporal extent,and regional tectonic events,this thesis discusses the petrogenesis and tectonic settings of different stages(T3-J2)of the Mesozoic igneous rocks as well as the Mesozoic tectonic evolution of the northern Liaoning and central Jilin provinces.Additionally,this thesis also discusses the spatial extent of the influence of the Paleo-pacific and Mongol-Okhotsk tectonic regimes as well as their constraints on regional metallization.1.Late Triassic(220~205 Ma)— a post-orogenic extension after the closure of the Paleo-Asian OceanThe Late Triassic(220~205 Ma)intrusive rocks in the northern Liaoning and central Jilin provinces consist of a typical bimodal igneous rock associations including gabbros,granodiorites and syengranites.The gabbros have low Mg#(Mg# = 0.62–0.63)and Cr,Co,Ni contents(Cr = 113–146 ppm,Co = 39.4–42.0 ppm,Ni = 54.8–58.9 ppm),and enriched in LREEs and LILEs and depleted in HREEs and HFSEs.Magamtic zircons have ?Hf(t)values of +7.3 to +13.3 and Hf single-stage model ages(TDM1)of 598~344 Ma,suggestingthat their primary magma was derived from partial melting of a depleted lithospheric mantle modified by subducted slab-derived fluids.The Late Triassic granitoids have high Si O2,K2 O,low Mg O,TFe2O3,Ca O,and enriched in LREEs and LILEs and depleted in HREEs and HFSEs(such as Nb?Ta?P and Ti).Their magmatic zircons have ?Hf(t)values of+5.2 to +12.6 and Hf two-stage model ages(TDM2)of 1003~537 Ma,suggesting that their primary magma was mainly derived from partial melting of the lower crust accreted in Neoproterozoic.The Late Triassic(220~205 Ma)bimodal intrusive rocks formed in an extensional environment related to the post-collisional extensional environment after the closure of the Paleo-Asian Ocean.2.Early Jurassic(195~175 Ma)— an active continental margin setting related to the subduction of the Paleo-Pacific plateThe 195~175 Ma igneous rocks are distributed in two areas,igneous rocks consist of andesites,dacites,rhyolites,rhyolitictuffs,syengranite and alkali-feldspar granite from the Nanloushan,Sihetun,Xiaofengmidingzi formations and granitic plutons in the central Jilin province.The coeval intrusive rocks consist of gabbros,quartz monzonites,granodiorite,a typical bimodal igneous rock association in the northern Liaoning province.Early Jurassic intermediate-acidic igneous rocks in the central Jilin province display good linear correlation between Si O2 and other major oxides,indicating that mineral fractional crystallization can be responsible for these compositional variations.Andesite,as the relatively basic end member of this suite of rocks,has the lowest degree of evolution,which reflects the nature of magma source.Andesites have high Cr(42.1-45.6ppm)and Ni(23.5-24.9 ppm),their Nb/Ta ratios range from 13.4 to 13.6,being lower than the mantle average of 17.5,much higher than the crustal value of 11,suggesting that their primary magma was derived from partial melting of a lithospheric mantle.They are enriched in LREEs and LILEs and depleted in HREEs and HFSEs,suggesting that they were formed in an active continental margin setting.Combine with their positive zircon?Hf(t)values(+1.3~+7.4),suggesting that their primary magma was derived from partial melting of a depleted lithospheric mantle modified by subducted slab-derived fluids.Differently,the coeval Dahechuan alkali-feldspar granites and rhyolites fromXiaofengmidingzi formation in Changchun have high Si O2,low Mg O,TFe2O3,Ca O,Cr and Ni(Cr = 0.23–1.23 ppm,Ni = 0.16–1.03 ppm)contents.They have low total rare earth elements content(?REE=125–167 ppm),and are enriched in LREEs and LILEs and depleted in HREEs and HFSEs(such as Nb ? Ta ? P and Ti).Combined with their positive zircon ?Hf(t)values(+5.2~+9.7)and with Hf two-stage model ages(TDM2)of1091~640 Ma,suggesting that their primary magma was mainly derived from partial melting of the lower crust accreted in Neoproterozoic.Early Jurassic gabbros in northern Liaoning province exhibit a typical cumulate texture,relatively flat and convex REE patterns [(La/Yb)N = 1.7-3.6],with low total REE abundances and light REE(? REE = 28-35 ppm,LREE=22-25 ppm),different from the Late Triassic gabbros.Early Jurassic(~175 Ma)gabbros have geochemical properties of arc–type igneous rocks such as enrichment in light rare-earth elements(LREEs)and large ion lithophile elements(LILEs),and depletion in heavy rare-earth elements(HREEs)and high field-strength elements(HFSEs).The magmatic zircons have ?Hf(t)values of +4.3 to+10.3 and Hf single-stage model ages(TDM1)of 671~418 Ma,suggesting that their primary magma was derived from partial melting of a depleted lithospheric mantle modified by subducted slab-derived fluids.The Early Jurassic granodiorites exhibit Middle-K calc-alkaline series and have an affinity to adakites.Combined with their zircon?Hf(t)values(+2.9~+10.3)and Hf two-stage model ages(TDM2)of 1393-732 Ma,we conclude that the primary magma was derived from partial melting of a thickened lower crust.In contrast,the coeval quartz monzonites collected from the northern margin of the NCC have relatively high HREE and total REE abundances(? REE = 195-201 ppm).Their zircon ?Hf(t)values and Hf two-stage model ages(TDM2)vary from –8.4 to –11.9 and from 2720 to 2397 Ma,respectively,indicating that its primary magmas could be derived from partial melting of ancient lower continental crust.The polar variation trend of K2 O contents of the Early Jurassic volcanic rocks from the continental margin to intracontinent such as northern Liaoning and central Jilin provinces reveals the beginning of subduction of the Paleo-Pacific plate beneath the Eurasian continent,whereas the early Jurassic bimodal intrusive rocks in northern Liaoning and central Jilin provinces formed in anextensional environment similar to back-arc setting.3.Middle Jurassic(170~ 163 Ma)— an extensional environment related to the collapse of thickened crust after the closure of the Mongol-Okhotsk Ocean.The Middle Jurassic(170~163 Ma)intrusive rocks consist of granodiorites,quartz monzonites and minor late Middle Jurassic(~163 Ma)gabbros.The late Middle Jurassic gabbros and coeval granitoids represent a typical bimodal igneous rock association.These gabbros exhibit a cumulate texture and have similar geochemical characteristics to Early Jurassic(~175 Ma)gabbros,but are more depleted in HREEs and HFSEs.The magmatic zircons have ?Hf(t)values of +5.7 ~+10.0 and Hf single-stage model ages(TDM1)of587~425 Ma,suggesting that the primary magma was derived from partial melting of depleted lithospheric mantle that was previously metasomatized by subducted slab-derived fluids.The Middle Jurassic granitoids have low Si O2,and low Co,Ni,Cr contents as well as low Mg# values(0.37-0.43),similar geochemically to low-Mg adakitic rocks,suggesting that its primary magma was derived from partial melting of the thickened crust.The Middle Jurassic granitoids in the Xing'an–Mongolian Orogenic Belt have positive zircon ?Hf(t)values(+0.8 ~+6.4).In contrast,the coeval granitoids in northern margin of the NCC have negative zircon ?Hf(t)values(-16.8 ~-1.5).These features suggest that the former was derived from partial melting of a juvenile lower crust,whereas the latter was derived from partial melting of ancient lower crust.Combined with the distribution of the Middle Jurassic igneous rocks,the presence of the Middle Jurassic large scale thrust–nappe structures in the northern margin of the NCC,and the Late Jurassic alkaline volcanic rock assemblage related to the collapse or delamination of a thickened crust,we conclude that the closure of the Mongol-Okhotsk Ocean happened in the Middle Jurassic.The ~163 Ma the bimodal igneous rocks were formed in an extensional environment related to the collapse or delamination of a thickened crust after the closure of the Mongol-Okhotsk Ocean.4.The spatial-temporal extent of influence of the Paleo-Pacific and Mongol-Okhotsk tectonic regimes on NE China and their metallogenic implicationsThe Late Triassic bimodal igneous rock association indicates a regional extensional environment,consistent with the post-orogenic extension after the closure of the PaleoAsian Ocean.The Late Triassic magmatic events are mainly distributed in both sides along the Solonker–Xra Moron–Changchun–Yanji suture belt.The east-west trending Late Triassic bimodal igneous rock zone occur in the continental margin accretionary complex belt to the northern margin of the NCC,including Hongqiling in Panshi City,Piaohechuan in Jilin City,Changren-Zhangxiang in Yanji,Taiping granodiorite and Maanling syengranite.In the northern margin of the NCC,an east-west trending early Mesozoic alkaline rocks zone are formed.The Cu-Ni sulfide deposits are associated with the Late Triassic ultramafic-mafic magmatism.The Early Jurassic calc-alkaline volcanic rocks and the polar change of K2 O contents indicate the beginning of subduction of the Paleo-Pacific plate beneath the Eurasian continent,whereas the Early Jurassic bimodal intrusive rocks in northern Liaoning and central Jilin provinces formed in an extensional environment similar to back–arc setting.It can be concluded that the spatial extent of influence of subduction of the Paleo-Pacific plate was involved to the east of the Songliao Basin,Lesser Xing'an–Zhangguangcai ranges and the northern margin of the NCC.The formation of a large number of porphyry–type of ore deposits in the eastern Heilongjiang–Jilin provinces are closely associated with the Early Jurassic magmatic events.The Middle Jurassic adakitic magmatism reveals the closure of the Mongolia Okhotsk Ocean.The Middle Jurassic adakitic rocks and late Middle Jurassic bimodal intrusive rocks in northern Liaoning and central Jilin provinces could correspond to the closure of the Mongol-Okhotsk Ocean and post-orogenic extension,respectively.They are the products of far-away control by the Mongol–Okhotsk tectonic regime.It can be concluded that the spatial extent of influence of the Mongol–Okhotsk tectonic regime is involved to the west of the Songliao Basin and the northern margin of the NCC.The Middle Jurassic high sulfidation-type copper deposits are related to this stage of magmatic event,which is associated with the extension after the closure of the Mongol-Okhotsk Ocean.