Open And Closure Of The Southern Paleo-Asian Ocean ——Constraints From Neoproterozoic And Late Paleozoic Igneous Rocks Of Niujuanzi,Beishan Region,Northwestern China

The open and closure of ocean is the main feature for global plate tectonics.The Paleo-Asian Ocean(PAO)from the breakup of Rodinia supercontinent during the Neoproterozoic,and formed the Central Asian Orogenic Belt(CAOB)by the long-term subduction and collision during Paleozoic,which is a very important stage in the world.The Beishan region is located in the southernmost of CAOB and is a key area for studying the open and closure of PAO.However,no evidence of igneous rocks related to the breakup of PAO has been found,and the closure time of the ocean is also controversial in the Beishan region.In this paper,zircon U-Pb chronology,geochemical and Sr-Nd isotope methods are used to first infer the Neoproterozoic gabbro and Late Devonian granite that related to the open and closure of PAO in the Niujuanzi area of central Beishan region.These are significant for tectonic evolution of the Beishan region and perfect open and closure the PAO.The main conclusions are as follows:1.First discovered the gabbro formed in continental rift setting in the Beishan region.The LA-ICP-MS baddeleyite U-Pb age of gabbro is 767.9±9.2 Ma,formed during the Neoproterozoic.The gabbro has low contents of Si O₂(47.22-50.24 wt%),K₂O(0.19-0.99 wt%)and Na₂O(1.43-2.70 wt%),high Ca O(11.57-12.28 wt%)and Mg O(8.06-8.31 wt%),Mg^#value of 55,Al₂O₃and P₂O₅ are 12.80–13.14 wt%and 0.26–0.37 wt%.It is belonging to tholeiite series.The gabbro shows enrichment of Th,U and depletion of Rb,Ni,Ta,Ti,with Eu slightly negative anomalies,?_Nd(t)value of 2.55-3.19.Their La/Nb ratios of 1.59-1.70,Nb/Ta ratios of 14.06-14.95,Th/Nb ratios of 0.29-0.31,Ce/Pb ratios of 10.71-11.61.Their contents of Cr(89.40-97.36 ppm)and Ni(104.44-116.83 ppm)are lower than basaltic magma.Moreover,all gabbro samples are fall into the field of within plate(continent)rift in the tectonic discriminant diagram of basalt These features indicate that gabbro generated from the depleted mantle and underwent crustal mixing during magma evolution,with formed in the continental rift setting.2.The LA-ICP-MS zircon U-Pb age of syenogranite is 376±2.1 Ma,formed during the Late Devonian.They have high Si O₂(68.29-71.65 wt%)?K₂O(6.55-7.13 wt%)and Na₂O(4.59-5.47 wt%),and low Ca O(0.91–1.50 wt%),Al₂O₃content of 14.17–14.91 wt%,K₂O/Na₂O ratios of 1.07-1.60,A/CNK values of 0.75--0.86,Mg^#values of 5.18-25.71,10000Ga/Al values of 4.13-4.95 and(Zr+Nb+Ce+Y)values of 410-543 ppm.Their temperature is 903-950?,with show enriched in Th,U,P,K and depleted in Ba,Sr,P,Ti,with significant negative Eu anomalies.The syenogranite show high ratios of Y/Nb(3.19--4.60)and Rb/Nb(8.49-11.96),and low ratios of Zr/Hf(15.84-18.44)and Nb/Ta(6.72-7.53).Furthermore,the syenogranite have positive?_Nd(t)values(+1.87-+2.20).They belong to A2 type granite,which from partial melting of the lower crust and formed in post-collision extensional tectonic setting.3.The LA-ICP-MS zircon U-Pb age of the monzogranite is 374.4±2.4 Ma,formed during the Late Devonian.The monzogranite show high contents of Si O₂(70.38-73.31 wt%),K₂O(7.03-7.65 wt%)and Na₂O(4.52-5.36 wt%),low Ca O(0.57-0.86 wt%)?Mg O(0.04–0.23 wt%)and Mg^#(5.08-21.78).The contents of Al₂O₃ are 13.25-14.27 wt.%,K₂O/Na₂O ratios of 1.31-1.56,A/CNK value of 0.77-0.84,10000Ga/Al value of 3.75-4.43 and(Zr+Nb+Ce+Y)value of 354-491ppm.The temperature of syenogranite magma is 877-931?.The monzogranite is enriched in Th and U,depleted in Ba,Sr,P,Ti and strongly negative Eu anomaly,with have high ratios of Y/Nb(3.68-5.31)and Rb/Nb(11.51-14.09)and low ratios of Zr/Hf(16.39-17.92)and Nb/Ta(7.06-8.01).These features indicate that monzogranite belongs to A2 type granite,which originated from partial melting of the lower crust and formed in post-collisional extensional tectonic setting.4.Based on the evolution of regional tectonic and combine with previous studies,the Neoproterozoic gabbro was formed in a within plate(continent)rift environment in response to the breakup of Rodinia supercontinent,indicating the open of the southern PAO.The Late Devonian syenogranite and monzogranite were successively intrusions of same period of magmatic activity and formed in post-collisional extensional tectonic setting,indicating closed of branch ocean that represented by Hongliuhe-Niujuanzi-Xichangjing ophiolite belt.This indicates that the study area has entered the stage of post-orogenic tectonic evolution.Therefore,these results suggest that the PAO was opened during the Neoproterozoic(?767.9 Ma)in the Beishan region,and the branch ocean had closed during before the Late Devonian(376-374 Ma).