羌塘走構(gòu)油茶錯—納丁錯新生代火山巖巖石學(xué)、地球化學(xué)特征研究

1、羌塘走構(gòu)油茶錯納丁錯新生代火山巖巖石學(xué)、地球化學(xué)特征研究        【中文摘要】自印度大陸與歐亞大陸碰撞以來,躲北地區(qū)發(fā)育了多期火山活動。同位素測年資料表明,羌塘地區(qū)火山活動以鈉質(zhì)堿性玄武巖開始,時(shí)代為6044Ma,42Ma以來,北羌塘地區(qū)出現(xiàn)大量高M(jìn)g#高鉀鈣堿性火山活動,其峰值年齡為4237Ma。南羌塘高M(jìn)g#高鉀鈣堿性火山巖形成于35-31Ma,主要分布在納丁錯、走構(gòu)油茶錯一帶。魚鱗山-戈木錯堿性鉀質(zhì)-超鉀質(zhì)火山巖則噴發(fā)于31-24Ma。羌塘高M(jìn)g#高鉀鈣堿性系列火山巖的地球化學(xué)組成以具有埃達(dá)克巖的高Sr

2、低Y和HREE的強(qiáng)烈分餾為特征,有關(guān)該類巖漿的成因一直存在熟悉分歧,巖漿源區(qū)性質(zhì)和巖漿產(chǎn)生過程與巖石圈的增厚減薄關(guān)系成為青躲高原形成演化研究中的重要科學(xué)題目。通過對南羌塘新生代納丁錯、走構(gòu)油茶錯高M(jìn)g#高鉀鈣堿性火山巖的巖石學(xué)和地球化學(xué)特征研究,本文以為,納丁錯和走構(gòu)油茶錯火山巖為同源巖漿演化產(chǎn)物,時(shí)代為35-31Ma,巖石組合為橄欖玄武安粗巖、安粗巖、粗面巖;火山巖具有埃達(dá)克巖的重稀土強(qiáng)烈分餾和高sr/Y特征,原始巖漿為玄武質(zhì),火山巖的成分變化受巖漿AFC過程控制;原始巖漿起源于虧損的石榴石二輝橄欖巖地幔源區(qū),具有軟流圈地幔與巖石圈地幔的混源性質(zhì),巖漿產(chǎn)生與軟流圈的上涌作用有關(guān)。')

3、;【Abstract】 Since the collision between India continent and Eurasia continent , the multiphase volcanic activity are formed in northern Tibet . According to the isotopic dating, the volcanic activity of Qiangtang began from the sodium-rich alkali basalt , the age is 6044Ma, since 42Ma, the high Mg#

4、and K calc-alkaline volcanic activity has a plenty of appearance in north Qiangtang area whose peak age is 4237 Ma. the high Mg# and K calc-alkaline volcanic rock of south Qiangtang was formed in 35-31Ma, and it is mainly distributed in Zougouyouchacuo and Nadingcuo,yulinshan-gemucuo alkali potassic

5、-ultrapotassic volcanic rocks erupted at 31-24Ma .Geochemical characteristics of the high Mg# and K calc-alkaline volcanic rock of south Qiangtang is the intense fractionation of HREE, high Sr and low Y of adakites, and there are always some divergences on the genesis of it , the relationships betwe

6、en the nature of origin region and the generation process of magma and lithospheric thinning-after- thickening become important scientific issues in research on formation and evolution of Tibetan Plateau.This * takes the genesis study of the ougouyouchacuo and Nadingcuo high Mg# and K calc-alkaline

7、volcanic rock of south Qiangtang as study emphasis, and explores the relationships between the magmatic origin and lithospheric thinning-after- thickening.The main lithological associations of the Nadingcuo volcanic rock are basalt- latite and latite , while the one of Zougouyouchacuo volcanic rock

8、is mainly trachyte. The volcanic rock is mainly lava which is produced in the form of lava sheet and volcanic unaka and covers the pre-Jurassic strata.The chemical constituents of the Nadingcuo basalt- latite is of the characteristics of alkaline series , Na_2O/K_2O=2.53-2.06, while latite is of the

9、 blend feature of alkali-series and subalkali-series , Na_2O/K_2O=1.3-0.94 , the Zougouyouchacuo trachyte enters subalkali-series composition region, Na2O/K2O=1.06-0.85. In SiO_2-K2O diagram, volcanic rocks enter high K calc-alkaline-series composition region, partial latite enter shoshonite-series

10、composition region, which displays the magmatic property from basalt- latite to trachyte evolves from the sodium to the potassic ,and is of the blend feature of kohalaite-series and high K calc-alkaline-series.The results show that Zougouyouchacuo and Nadingcuo Cenozoic volcanic rocks is the product

11、 resulted from the evolution of a cognate magma. In the harker diagram, major oxidates presents regular transformation with the increasing of SiO_2 from the Nadingcuo basalt- latite to the Zougouyouchacuo trachyte, which displays the characteristics from the evolution of a cognate magma. Strong comp

12、atible elements, as Cr、Ni, vary in a large range, Cr varies between 1562×10(-6) from basalt- latite to latite, and indicates that magma composition change is under the control of fractional crystallization. But incompatible elementsLREE and Nb decrease with the increasing of SiO_2, which indica

13、tes entry of crust materials during the magma evolution. TheHf of rocks decreases with the increasing of SiO_2, which further proves entry of crust materials during the magma evolution.The Mg# of Zougouyouchacuo and Nadingcuo Cenozoic volcanic rocks varies between 57.7-43, and are of characteristics

14、 of high Mg# generally.They aslo have a strong enrichment in light rare earth element, relative HREE depletion and strong fractionation of light and heavy rare earth egative, andEu is from non-anomaly to weak positive anomaly, also high Sr/Y, the above geochemical characteristics of latite and trach

15、yte is consistent with adakites,and shows that garnet residues exists in magma source. But theLREE of the rocks decreasing with the increasing of SiO_2 and the existence of basic rocks show that their genetic interpretation is not the melting of crust eclogite.In the Th/La-Zr/Nb diagram, the basalt-

16、 latite enters low-Th/La and low-Zr/Nb mantle composition region constituted by depleted and OIB type mantle, while latite and trachyte vary towards active continental margin high K calc-alkaline composition region in turns, and differe from island arc volcanic rocks composition region. In the Ba/Nb

17、-La/Nb diagram, the trachyte locates in the edge of volcanic arc composition region, while the basalt- latite has a transition towards MORB and OIB mantle composition region, these characteristics indicate that the primitive basaltic magma is from asthenospheric mantle and lithosphere mantle formed

18、by paleosubdution which has a blend source. theHf from basalt- latite to trachyte varies between 5.5-2.2 which indicates source region is of the properties of weak depleted mantle and is consistent with low-Th/La and low-Zr/Nb.Based on the comprehensive comparison with the geochemical characteristics of the Cenozoic volcanic rocks of north Qian