青藏高原中部色林错地区早白垩世物源分析及弧背前陆盆

青藏高原中部色林错地区早白垩世物源分析及弧背前陆盆地演化特征摘要：拉萨地块中部的色林错盆地由厚度超过3000m、向上变粗的下白垩统沉积地层组成。最下部的俄杀而补组主要由半深海－深海环境的黑色泥岩和浊积岩组成。多尼组整合覆盖于俄杀而补组之上，由底部的砂泥岩互层和顶部的砾岩、粗砂岩、红色泥岩、火山碎屑岩和流纹岩组成，其中，流纹岩给出了125.8±2.8Ma锆石U-Pb年龄。郎山组的圆笠虫灰岩平行不整合于多尼组之上，通过对灰岩中圆笠虫的分析鉴定得到郎山组在色林错地区的最早沉积时间为LateAptian-EarlyAlbian。在Q-F-L三角图中，大多数砂岩落在再旋回造山带和岩浆弧的范围内。前人的研究成果以及本次435个碎屑锆石的U-Pb年龄表明：冈底斯岛弧具有侏罗纪－早白垩世演化历史，色林错盆地早白垩世的物源主要来自南部的冈底斯岛弧和广泛分布于拉萨地块中部的石炭系砂岩。泥岩的主量元素、微量元素、稀土元素和Nd同位素等地球化学分析表明：色林错盆地物源区的大地构造背景属于类似安第斯造山带的活动大陆边缘，物源区可能主要是由古生代的沉积地层和上地壳部分熔融产生的火成岩组成，其地球化学成分具有平均上地壳的特征。铬尖晶石的成分、泥岩的部分主量元素和微量元素结果暗示了物源区也存在少部分基性－超基性岩。综合各种物源分析的结果并结合野外的地质调查，我们认为色林错盆地主要物源来自南部的冈底斯岛弧火成岩和古生代沉积地层，次要物源包括北部的BNS、羌塘地块和盆地东部的高地。早白垩世，色林错盆地是一个类似于安第斯造山带的弧背前陆盆地，盆地形成的起始时间为~130Ma。~140-90Ma期间，拉萨地块上发育了一套弧背褶皱冲断带和前陆盆地系统，从早到晚依次沉积了楚木龙组、俄杀而补组、多尼组、郎山组、塔克那组和竟柱山组。早白垩世褶皱冲断作用和弧背前陆盆地系统的发育暗示了拉萨地块发生地壳缩短增厚的时间可能从早白垩世就已经开始。Abstract：TheSelinCobasininthenorthernLhasaterraneincludesmorethan3000mofupward-coarseningLowerCretaceousstrata.Thelowermostpart,theEshaerbuFormation,consistsofdarkmudstoneandturbiditicsandstone.TheconformablyoverlyingDuoniFormationconsistsofalowermemberofinterbeddedlithicsandstone,siltstoneandmudstone，andanuppermemberofconglomerate,coarsesandstone,redmudstone,volcaniclasticrocks,andrhyolitictuff.AzirconsamplefromtherhyoliteyieldsaU-Pbageof125.8±2.8Ma.TheDuoniFormationisunconformablyoverlainbyrudist-bearingandorbitolinidlimestoneoftheLateAptian–EarlyAlbianLangshanFormation.IntheQ-F-Ldiagrams,mostofsandstonesfallintotherecycledorogenandmagmaticarc.435detritalzirconuranium-lead(U-Pb)agesdeterminedfromtheLowerCretaceousstratashowthatzircongrainswereprimarilyderivedfromtheGangdesemagmaticarcandtheCarboniferousstratatothesouth.Thegeochemicalanalyses,includingmajorelments,traceelements,rareearthelements(REE)andNdisotope,indicatethattheLowerCretaceousstrataintheSelinCobasinweremainlyderivedfromanAndean-typeactivecontinentalmarginwithitscompositionalcharacteristicsoftheUppercrust,whichmayconsistofPaleozoicstrataandigneousrocksandthelatterwereprobablyproducedbythemeltingofthethickenedUppercrust.Combinedwithpartofmajorandtraceelements,Cr-spinelanalysissuggeststhatbasic-ultrabasicrockscouldbeanothersubordinatematerialsource.Basedontheresultsofdifferentprovenanceanalyses,weconcludethatduringEarlyCretaceoustheSelinCobasinwasanAndean-typeretroarcforelandbasinwithitsprimaryprovenanceofGangdesemagmaticarcandPaleozoicstratatothesouthanditssecondaryprovenanceofBNSandQiangtangterranetothenorth.TheyoungestzirconagesinthelowermostexposureoftheEshaerbruFormationare~130MaandprovideamaximumdepositionalagefortheSelinCobasin.Duringtheperiodof~140-90Ma,aretroarcfoldandthrustbeltsandforelandbasinsystemhadbeendevelopedintheLhasaterraneanddepositedtheChumulongFormation,theEshaerbuFormation,theDuoniFormation,theLangshanFormation,theTakanaFromationandtheJingzhushanFormationorderly.ThepresencesofEarlyCretaceousthrustingandforelandbasinsystemsuggestthatcrustalthickeningintheLhasaterranelikelyinitiatedduringtheEarlyCretaceous.