喜马拉雅造山带纳木那尼穹隆古元古代岩石单元深熔作用

摘要

纳木那尼穹窿位于特提斯喜马拉雅带西段,属变质杂岩体,由黑云母片麻岩、花岗片麻岩、糜棱岩、混合岩、变杂砂岩、角闪岩、大理岩及后期侵位的电气石花岗岩和二云母花岗岩组成.本次研究对穹隆核部出露的混合岩、花岗片麻岩、电气石花岗岩及边缘出露的二云母花岗岩和黑云母片麻岩进行了岩相学、锆石U-Pb定年及地球化学研究,结果表明:(1)混合岩(T0768-4A-4C)锆石206 pb/238U谐和图上交点年龄为1873±28Ma,207 pb/206 Pb加权平均年龄为1877±21Ma.混合岩Sr同位素比值(1.25018～1.44452)和εNd(t)值(-28.8～-28.5)指示其具其有低喜马拉雅岩石单元的地球化学属性;(2)花岗片麻岩锆石核部206 pb/238U谐和图上交点年龄为1878±9Ma,下交点年龄为10.9±0.5Ma.个别震荡环带边记录有13.1±0.3Ma的年龄数据,表明古元古代花岗片麻岩可能经历了～l0Ma左右的熔融事件;(3)侵位于古元古代混合岩和花岗片麻岩之中的电气石花岗岩(T0768-LG)具有与深熔事件相一致的年龄,其206 pb./238U谐和年龄为9.0±0.2Ma;(4)穹隆核部电气石花岗岩εNd(t)值集中在(-18.9～-16.1),显著低于穹隆边缘的二云母花岗岩(εNd(t)=-14.4～-10.3),指示电气石花岗岩部分熔融源区有更多成熟地壳物质的加入;(5)个别电气石花岗岩εNd(t)值为-12.6,可能是岩浆上升过程中受到变泥质岩的混染所致.本次在纳木那尼穹隆的研究结果支持19～ 13Ma左右喜马拉雅造山带发生构造转换的模型(Zhang et al.,2012),并表明这种构造转化可能进一步引发了淡色花岗岩部分熔融源区的变化.南北伸展阶段为深度相对较浅的高喜马拉雅变泥质岩和杂砂岩等发生部分熔融,形成穹隆边缘的二云母花岗岩(～ 16Ma);进入东西向伸展阶段后,深熔作用导致深部主中央逆冲断层(MCT)附近的古元古代岩石单元和变泥质岩混合源区发生部分熔融,岩浆沿着南北向断裂带上升,形成电气石花岗岩体(～9Ma).%The Gurla Mandhata dome is located in the western Tethyan Himalaya.It consists of biotite gneiss,granitic gneiss,mylonite,migmatite,meta-sandstone,amphibolite and marble intruded by tourmaline granite and two-mica granite.An integrated petrographic,zircon U-Pb dating and geochemical study has been carried out on the migmatite,granitic gneiss,tourmaline granite and two-mica granite in Gurla Mandhata dome.The data results show that:(1) the upper intercept 206Pb/238 U age of the migmatite (T0768-4A-4C) is 1873 ±28Ma,with weighted 207pb/20rpb age at 1877 ±21Ma.Meanwhile,Sr (87Sr/86Sr(i) =1.25018 ～ 1.44452)and Nd (εNd (t) =-28.8 ～-28.5) isotopic compositions of the migmatite indicate that it has the same geochemical properties as those of the Lower Himalayan Sequence.(2) The upper and lower intercept 206Pb/238U ages of the granitic gneiss (TG-LE-11) are 1878 ±9Ma and 10.9 ±0.5Ma,respectively.A 13.1 ±0.3Ma magmatic crystallized zircon age was obtained in the granitic gneiss,indicated an anatexis event happened in the Mandhata Massif around 10Ma;(3) the concordant age of tourmaline granites that intruded into the Paleoproterozoic migmatite is 9.0 ± O.2Ma,in accordance with the anatexis age of the granitic gniess;(4) εNd (t) values (-18.9 ～-16.1) of tourmaline granite are lower than that of the two-mica granite (εNd (t) =-14.4 ～-10.3),indicating the contribution of mature crust materials in the source melting region of the tourmaline granitic pluton;(5) the relative lower εNd (t) values (-12.6) of the individual tourmaline granite may be resulted from partly dissemination of the magma druing the rising stage.This study supports the model proposed by Zhang et al.(2012) that the tectonic regime of Himalayan orogen was in a period of transition and adjustment from N-S extension to E-W extension during 19 ～ 13Ma,and further induced the transition of the melting source of the Himalayan leucogranite.During the period of the N-S extension stage,melting of the meta-pelite and meta-sandstone formed the two-mica granite (～ 16Ma);while during the stage of the E-W extension happened along the southern Tibetan rift system,the mixing source of metapelites and Paleoproterozoic crust materials adjacent to the Main Central Thrust (MCT) began to melt,giving rise to the drawing up of the magmas along the S-N normal fault and finally forming the tourmaline-bearing leucogranitic plutons.