Investigations On The Geological Feature And Tectonics Of The Intermediate-Acid Intrusive Rocks From Pagu Area,Nimu County,Tibet,China

The Pagu area,Nimu County is located in the south margin of the mid-Gangdese magmatic belt with extremely intense magmatic activities,which exhibited difference in scale,type and genesis during each evolutionary stage of the orogenic belt.This study identified the scale,rock types,and contact relationship between the intermidate-acid intrusive rocks in the Pagu area,and investigated their petrological,petrochemical,and chronological characteristics.The rock genesis,source area analysis and tectonic background of the intermediate-acid intrusive rocks in Pagu area were discussed comprehensively.The paper mainly achieved the following points:?1?The intermediate-acid intrusive rocks in the Pagu area are located in the Ladakh-Gangdese-Xiachayu magmatic arc belt,crossing two?-grade units:Cretaceous volcanic-magmatic arc belt??-1?and the Paleogene volcanic-magmatic arc belt??-2?.The intermediate-acid intrusive rock intrusion in the Pagu area was emplaced in the Tuff sandstone and sedimentary tuff of the Cretaceous Bima Formation,which is covered by the Quaternary strata.The type of dynamic metamorphic rock is mainly cataclasite,and the type of contact metamorphic rock is mainly hornfels.?2?The rock types of the intermediate-acid intrusive rocks in the Pagu area are:quartz monzodiorite,granodiorite,porphyritic monzonite granite,monzonite granite,and syenite granite.The Si O₂ content of quartz monzodiorite is between 56.82 and63.96%,the Al₂O₃ content is between 15.81 and 16.27%,the K₂O/Na₂O ratio is between 0.09 and 0.85,and the aluminum saturation index A/CNK is between 0.63and 0.87.The Rittmann Serial Index?is 1.91-2.17,which is ascribed to the quasi-aluminum calc-alkaline rock series;for granodiorite,Si O₂ content is62.47-66.75%,with Al₂O₃ content of 15.21-16.12%,K₂O/Na₂O ratio between 0.43and 1.26,the aluminum saturation index A/CNK between 0.86 and 1.01,and the Rittmann index?between 1.67 and 3.09,which is ascribed to the quasi-aluminum calc-alkaline rock series;the Si O₂ content of the porphyritic feldspar granite is between 69.85-76.28%,the content of Al₂O₃ is 12.44 to 14.37%,the ratio of K₂O/Na₂O is between 0.92 and 1.47,the aluminum saturation index A/CNK is between 1.01 and 1.09,and the Rittmann index?is 1.68 to 2.11,which is ascribed to peraluminum calc-alkaline rock series;for the syenite granite,its Si O₂ content is70.32-76.61%,with Al₂O₃ content of 12.43-14.64%,K₂O/Na₂O ratio of 1.13-1.44,aluminum saturation index A/CNK between 1.04-1.14,Littman index?of 1.78-2.35,which is ascribed to the series of peraluminum calc-alkaline rock;The Si O₂ content of the orthoclase granite is between 75.81-76.45%,with the Al₂O₃ content of12.51-12.67%,K₂O/Na₂O of 1.34-1.60,aluminum saturation index A/CNK between1.08 to 1.10,Rittman index from 1.83 to 2.00,which is ascribed to peraluminum calc-alkaline series.The intermediate-acid intrusive rocks in the Pagu area are characterized as high alkali,high potassium,and low iron content,and the degree of differentiation gradually increases from mesite to acid rocks.;the rock mainly exhibits a weak negative Eu anomaly?no obvious Ce anomaly?,and the magma might be in a relatively reducing environment during its evolution.The rock is enriched in large ion lithophile element?LILE?Rb and high field strength elements?HFSE?Th,U,Nd,Y,but depleted in Ba,Sr and high field strength elements P.The intermediate-acid intrusive rock in the Pagu area is an highly differentiated I type granite with comagmatic evolution and might have evolved with a trend to acid rock.It is noticed that both partial melting and mixing of the magma coexisted.The magmatic source of diorite and granodiorite is the basic magma of the upper mantle,whereas the magmatic source of the porphyritic monzonite granite,monzonite granite,and syenite granite is from the partial melting of the crust.?3?Quartz monzodiorite and granodiorite in the intermediate-acid intrusive rocks in the Pagu area were formed in a high-temperature and high-pressure environment,with deeper depths;the formation temperature of porphyritic monzonitic granites and monzonitic granites is higher and may form at a depth of 30km;the temperature and pressure of the syenite granite are relatively low,indicating that its formation depth may be lower.?4?The zircon U-Pb isotope dating indicates that the age of intermediate-acid intrusive rocks in the Pagu area is between 52.3 and 47 Ma.Based on the age and the field geological characteristics of the intrusive rocks,the emplacement time of intermediate-acid intrusive rocks in the Pagu area was considered to be Eocene.?5?Quartz monzodiorite and granodiorite exhibit the geochemical characteristics as island arc granite,and their tectonic regime could be in the northward subduction island arc environment of the Neo Tethys Ocean;The syenite granite is clearly in the tectonic setting of the collision,which was formed during the India-Asia continent collision.With the collision of the Indo-Eurasian plates,the subducted and inserted oceanic plates would be detached from the inserted continental plates due to the drag of gravity,resulting in the ascent of the asthenosphere,and the baking of huge heat could produce a large-scale magma source area.Part-melting process,and it will also cause the crustal to melt.During the formation process,due to the intrusion of the basic magma,the mixing of magma was triggered,forming a large area of Eocene intermediate-acid intrusive rocks distributed in the Pagu area.