内蒙古西乌旗晚石炭世马尼塔埃达克岩的发现及其对古亚洲洋东段洋内俯冲的约束

Discovery of Late Carboniferous adakite in Manita,Inner Mongolia,and its constrains on intra-oceanic subduction in eastern Paleo-Asian Ocean

在内蒙古西乌旗迪彦庙SSZ型蛇绿岩北部新发现一套晚石炭世马尼塔埃达克岩。该埃达克岩位于兴蒙造山带东段贺根山缝合带内岩性为花岗闪长岩和斜长花岗岩。马尼塔埃达克岩SiO2含量为61.91%-75.16%,Al2O3含量为13.54%~17.42%MgO含量为0.33%~2.49%,富钠贫钾(Na2O=4.58%~5.48%,K2O=0.40%~2.08%,Na2O/K2O=2.35~12.96)属于低钾拉斑系列和钙碱性系列岩石;富Sr贫Y(Sr=309.55×10^-6~433.99×10^-6,Y=3.74×10^-6~14.66×10^-6)相对富集大离子亲石元素(LILE) K、Rb、Sr等,亏损高场强元素(HFSE) Nb、Ta、Zr、Ti、P等;稀土元素总量偏低(35.79×10^-6~70.10×10^-6),轻、重稀土元素分馏明显,(La/Yb)N值为1.59~10.11,无明显Eu异常(δEu=0.91~1.51),Yb含量为0.60×10^-6~1.86×10^-6,符合典型埃达克岩的地球化学特征。新获得的LA-ICP-MS锆石U-Pb年龄为315.76±0.94 Ma表明马尼塔埃达克岩的形成时代为晚石炭世。马尼塔埃达克岩与迪彦庙SSZ型蛇绿岩、前弧玄武岩、白音呼舒奥长花岗岩等组成洋内俯冲岩石组合,表明古亚洲洋东段在晚石炭世并未关闭,处于洋内俯冲消减过程。

The Late Carboniferous Manita adakite distributed along the Hegenshan suture zone in the eastern part of the Xingmeng Orogenic Belt was discovered to be outcropped in the northern section of Diyanmiao SSZ ophiolite in Xiwuqi,Inner Mongolia.It consists mainly of granodiorite and plagiogranite,which belongs to low-K tholeiitic and Calc alkaline series,chemically characterized by 61.91%~75.16% of SiO2,13.54%~17.42% of Al2 O3,and 0.33%~2.49% of MgO,with intensively rich in Na2 O(4.58%~5.48%),poor in K2 O(0.40%~2.08%) and high ratio of Na2 O/K2 O(2.35~12.96).The Manita adakite also manifests high Sr(309.55×10^-6~433.99×10^-6)and poor Y(3.74×10^-6~14.66×10^-6),with relatively enriched large ion lithophile elements(LILE) K,Rb,Sr,and depleted high field strength elements(HFSE) Nb,Ta,Zr,Ti and P,etc.In addition,it features low ∑REE(35.79×10^-6~70.10×10^-6) and clear REE fractionation((La/Yb)N=1.59~10.11),without obvious Eu anomaly(δEu=0.91~1.51),and contains Yb(0.60×10^-6~1.86×10^-6).All of the geochemical characteristics indicate that the Manita pluton belongs to typical adakite.The U-Pb age of LA-ICP-MS zircon is 315.76±0.94 Ma,indicating that the formation age of Manita adakite is Late Carboniferous.The Manita adakite,Diyanmiao SSZ ophiolite,fore arc basalt and Baiyinhushu trondhjemite constitute the intra-oceanic subduction rock assemblage,which indicates that the eastern part of the PaleoAsian Ocean was not closed in the Late Carboniferous and was in the process of subduction and extinction.