新疆东准噶尔北缘顿巴斯套地区晚泥盆世石英闪长玢岩地球化学特征及构造意义

Geochemical Characteristics and Tectonic Implications of Late Devonian Quartz Diorite Porphyry in Dunbasitao Area,Northern Margin of East Junggar Basin,Xinjiang

新疆顿巴斯套矿区石英闪长玢岩的研究对厘清古亚洲洋晚泥盆世的演化具有重要意义.新疆顿巴斯套矿区石英闪长玢岩位于东准噶尔盆地北缘,为确定其岩浆侵入活动时限和岩石成因,讨论准噶尔地块北侧的古亚洲洋在晚古生代早期的俯冲作用,开展锆石LA-ICP-MS U-Pb年代学、Hf同位素地球化学,全岩主微量元素地球化学、Sr-Nd同位素地球化学研究.矿区内石英闪长玢岩单颗粒锆石LA-ICP-MS U-Pb定年结果为378±2Ma,指示晚泥盆世岩浆活动.石英闪长玢岩全岩SiO_2含量为54.94%～68.64%,富Na_2O(平均含量为4.34%),Na_2O/K_2O为1.19～2.72,里特曼指数(σ)大多小于3.3,Fe_2O_3T含量为2.23%～5.65%、MgO含量为0.81%～2.77%、Al_2O_3含量为14.31%～16.99%,TiO_2含量为0.32%～1.02%,CaO含量为2.58%～4.97%,Mg~#变化于38.1～51.2,A/CNK为0.83～1.08,属准铝质钙碱性-高钾钙碱性系列.岩石富集大离子亲石元素(LILE)和轻稀土元素(LREE),亏损高场强元素(HFSE,如Nb、Ta、P、Ti),稀土元素配分曲线右倾,具有明显的轻重稀土分异,基本无Eu异常.全岩有较低的(^(87)Sr/^(86)Sr)_i(0.703 783～0.703 901),较高的ε_(Nd)(378Ma)(+6.2～+6.7),年轻的TDM(560～608Ma),锆石ε_(Hf)(t)相对较高,变化于+12.1～+15.1,对应的一阶段模式年龄TDM1(Hf)为403～516Ma,显示亏损地幔源区特征.综合上述地球化学特征认为石英闪长玢岩为俯冲环境下洋壳部分熔融的产物,俯冲洋壳脱水产生的流体、熔体在上升过程中与地幔楔发生了交代反应.结合新疆北部蛇绿岩及东准噶尔北缘、阿尔泰南缘相关岛弧花岗岩、火山岩等综合对比,认为区内中-晚泥盆世存在古亚洲洋的南北双向俯冲.

Research on the quartz diorite porphyry in Dunbasitao gold deposit,Xinjiang,will throw light on the evolution of the paleo-Asian ocean in Late Devonian.The quartz diorite porphyry in Dunbasitao gold deposit is located in the northern margin of the East Junggar basin,Xinjiang Uygur Autonomous Region.LA-ICP-MS zircon U-Pb dating yields a weighted mean age of378±2 Ma for the intrusion,indicating Late Devonian magmatism.The quartz diorite porphyry shows variable SiO2 contents(54.94%-68.64%),with high Na2 O(average 4.34%)and Na2 O/K2 O(1.19%-2.72%),and it has medium Fe2 O3 T(2.23%-5.65%),MgO(0.81%-2.77%),Al2O3(14.31%-16.99%),low TiO2(0.32%-1.02%)with Rittman index(σ)<3.3.Mg#values of the rocks range from 38.1 to 51.2 with CaO content of 2.58%-4.97%,A/CNK of 0.81 to 1.08.The quartz diorite porphyry is metaluminous calc-alkaline-high-K calc-alkaline.It is enriched in LILE(Rb,Ba,Tu,K,La,Sr)and LREE,and depleted in HFSE(Nb,Ta,P,Ti)relative to the primitive mantle;the REE exhibits right-dipping patterns having significant LREE and HREE differentiation.All the samples have low(87Sr/86Sr)i(0.703 783-0.703 938),highεNd(t)(+6.2 to+6.7)and zirconεHf(t)=+12.1 to+15.1,as well as young Nd isotopic model ages(TDMranging in 546-608 Ma)and zircon Hf isotopic model age(TDM1 ranging in 403-516 Ma),indicating a depleted mantle source.The rock was sourced from subducting slab during the subduction of the paleo-Asian ocean in Middle-Late Devonian.Trace element discrimination diagrams indicate that rock formed in volcanic arc environment,which implies bidirectional subduction(north and south)existing in early Late Paleozoic,combined with spatial distribution of ophiolite,granodiorites and volcanic rocks in the northern Xinjiang and northern margin of East Junggar.