西藏斯弄多地区早侏罗世二长花岗岩地质意义

GEOLOGICAL SIGNIFICANCE OF THE EARLY JURASSIC MONZOGRANITE IN THE SINONGDUO AREA,CENTRAL LHASA BLOCK

拉萨地块中南部冈底斯带经历了多幕式构造岩浆事件,而学界对其早中生代岩浆岩成因和构造背景争议较大。为此,本次研究针对冈底斯带中段斯弄多地区粗粒二长花岗岩开展了野外地质与岩相学研究,辅以全岩地球化学分析、锆石U-Pb定年与Lu-Hf同位素分析,进而探讨其成因与地质意义。该岩体侵入于拉嘎组变质碎屑岩中,与上覆林子宗群火山岩呈异岩不整合接触。LA-ICPMS锆石U-Pb定年表明该岩体侵位于早侏罗世(199.8±2.1 Ma),且含有大量晚三叠世年龄(200~220 Ma)的岩浆锆石以及一颗2.5 Ga左右的继承或捕获锆石。岩体为弱过铝质(A/NCK:1.06~1.17),高钾钙碱性(K2O/Na2O:1.84~2.26),Mg#值为37~42,轻稀土富集((La/Yb)CN:15.12~20.31),重稀土平坦,富集大离子亲石元素而亏损Nb-Ta-Ti等高场强元素,其锆饱和温度处于771~783℃。锆石εHf(t)值为-2.74~-7.27,TDM2为1.4~1.7 Ga。结合背景资料,本次研究认为(1)斯弄多晚三叠-早侏罗岩体侵位于217~199 Ma;(2)该岩体具有俯冲带上盘火成岩的地球化学特征,形成于有流体参与的大陆地壳部分熔融,可能伴有新生幔源物质加入;(3)斯弄多地区晚三叠世-早侏罗世应处于雅江洋北向俯冲形成的活动大陆边缘。

The Gangdise belt in the central and southern part of the Lhasa Block has undergone several episodic tectono-magmatic events and formed some Mesozoic magmatic rocks.However,the genesis and tectonic setting of these magmatic rocks in the academic community are highly controversial.Therefore,the field geological and petrographic studies,combined with whole-rock geochemical analysis,zircon U-Pb dating and Lu-Hf isotope analysis,have been carried out on the coarse monzonitic granites in the Sinongduo area in the middle Gangdise Belt in this paper,so as to explore their genesis and geological significance.The study shows that the Sinongduo monzonitic granite intruded into the metamorphic clastic rocks of Laga Formation and is in discordant contact with the volcanic rocks of the overlying Linzizong Group.LA-ICP MS zircon U-Pb dating indicates that the intrusion occurred in the Early Jurassic(i99.82.1 Ma),and the rock also contains a large number of Late Triassic magmatic zircons(200 to 220 Ma)and one inherited or captured zircon(about 2.5 Ga).Geochemical analysis shows that the rock mass has weak peraluminous(A/NCK=1.06-1.17),high potassium calc alkaline(K_(2)O/Na_(2)O=1.84-2.26)with Mg value of 0.37-0.41.Light rare earth is enriched((La/Yb)cn=15.12-20.31),heavy rare earth is flat,large ion lithophile elements are enriched while high field strength elements such as Nb-Ta-Ti are depleted,and the saturation temperature of zircon is 771 to 783℃C.Zircon eHf(t)values range from-2.74 to-7.27,and Tpm"ranges from 1.4 to 1.7 Ga.Based on our study and regional background data,it is considered that(1)the late Triassic and early Jurassic intrusion of Sinongduo monzonitic granite occurred at 217 to 199 Ma.(2)The rock mass has the geochemical characteristics of the hanging wall igneous rock in the subduction zone,formed in the partial melting of the continental crust with the participation of fluids,and may be accompanied by the addition of new mantly-derived materials.(3)In the Late Triassic and Early Jurassic,the Sinongduo area was located in the active continental margin during the northward subduction of the Yarlung Zangpo Ocean.