Determination Of Permian Magmatic Intrusion Events In The Northwestern Margin Of Tarim Basin And Its Tectonic Significance

The Tarim Large Igeneous Province is one of the two important Permian Large Igeneous Provinces discovered in China,which not only has large-scale overflow basalt eruptions,but also has complex and diverse intrusive rocks and their assemblages.In this paper,the basic rocks and carbonatites which intruded into the Precambrian system in the Aksu area of the northwestern margin of Tarim Basin were investigated.Based on recognition of remote sensing images and geological survey of field sections,we investigated the petrology,zircon U-Pb chronology,whole rock geochemistry and SrNb-Pb isotopes of the basic rocks and carbonatites,determined the development characteristics,emplacement ages,magma source areas,magma evolution characteristics and petrogenesis of these two types of intrusive rocks,and further discussed the relationship between them and the Early Permian Tarim Large Igeneous Province,and explored their potential geodynamic implications.There are two types of basic rocks intruding into the Precambrian system in the Aksu area,i.e.,the basic dikes intruding into the Nanhua system and the basic sills intruding into the middle and lower Sinian system.These basic dikes and sills show typical intrusive contact relationship with wall rocks,which is supported by the quenched edge structure showing coarse and fine crystals respectively in the interior and edge of the sills,truncation of the bottom and top of the sills against the sandstone layers,and these basic intrusive rocks wrapping the wall rocks mass and so on.Under the microscope,the basic rocks are mainly characterized by typical diabasic structure and poikilitic structure.The study of zircon U-Pb geochronology shows that the formation ages of the upper and lower layers of basic sills in the Lower Sinian in Shenairike section are 290±6 Ma and 292±5 Ma,respectively,and the basic dikes in Yuermeinake section was formed at 287±2 Ma.These geochronological results show that the basic rocks developed in the Lower Sinian sandstone in Aksu area are not all Sinian basalts,but a considerable part of them are diabases which emplaced in Early Permian.And the emplacement age of these diabase is about 286-292 Ma.The wholerock geochemical characteristics indicate that the REE and trace element distribution patterns of these diabases are similar to the Oceanic Island Basalt(OIB),enriched in large ion lithophile elements such as Ba,U and LREE,slightly depleted in Nb and Ta,and obviously depleted in Rb,K,Pb and Sr.They have high Nb/Ta,Ti/Zr,Ta/Th,Zr/HF and La/Nb ratios and low Mg# values.Combing with the Sr-Nd-Pb isotope test results,we suggest that these diabases were derived from the partial melting of the carbonatational enriched lithospheric mantle.The carbonatites in this study were mainly exposed in the Yuermeinake section and intruded into the Nanhua system in dyke or dike forms.The zircon U-Pb geochronology dating of these carbonatites yield multiple peak ages,suggesting that these zircons were captured by magma of carbonatites.The concordant ages of the three youngest zircons were 272±4 Ma,combined with the regional geological analysis,suggesting that the carbonatites were probably formed in the Early Permian(~270Ma).Petrological observations and electron probe test results show that the carbonatites are mainly calcite(>90%)and contains dolomite,barite,celestite,hematite and other minerals,and the carbonatites have obvious exsolution structure between calcite and dolomite,and its calcite groundmass wrapped the wall rocks.Element geochemistry results show that carbonatites have high Ca O(44.40%~50.40%),low Si O2(1.83%~7.97%),extremely low total rare earth elements(REE)(21.7×10-6~91.3×10-6),and relatively enriched in the large ion lithophile elements like Sr,Rb,Ba,relatively depleted in the high field-strength elements such as Nb,Ta,Ti,high(87Sr/86Sr)i values(0.710106~0.710558),low ?Nd(t)values(-8.46~-12.80),and high stable isotope of?13CV-PDB(-2.69‰~-2.93‰)and ?18OV-SMOW(17.33‰~17.86‰).After comprehensive analysis,we suggest that these carbonatites are very similar to the typical crustal carbonatites.Combined with previous results,we propose that the intrusion events of basic rocks and carbonatites in Aksu area of the northwestern margin of Tarim should be a part of the Permian Tarim Large Igneous Province.Early Permian diabase dikes and sills are very similar to the Keping basalts in the early stage of the Tarim Large Igneous Province in their formation age,magma source area,magma evolution characteristics and tectonic setting,and they are adjacent in spatial development.The crustal carbonatites intruding into the Nanhua system are probably the product of the late magmatic event in the Tarim Large Igneous Province.When the mantle plume in the northwestern margin of Tarim was decompressed,it would form high-temperature basic magma and emplaced upward.These basic magma would heated the carbonate reservoir in the lower crust and partially melted it to formed carbonatites magma,which was rapidly emplaced upward along the extension fault.This study not only enriched the rock types of the Permian Tarim Large Igneous Province,but also improved the understanding of the complex crust-mantle interaction processes in the deep area of Early Permian Tarim basin.