内蒙古正镶白旗地区早古生代O型埃达克岩的厘定及其意义

The identification of Early Paleozoic O-type adakitic rocks in Zhengxiangbaiqi Area,Inner Mongolia and its significance

内蒙古正镶白旗地区新民岩体主要岩石类型为二长花岗岩。两个二长花岗岩样品给出（457±11）Ma和（423．1±9．9）Ma的LA-ICP-MS锆石U-Pb年龄值，两年龄样品中均存在〉1000Ma的年龄值，说明该区存在古老的结晶基底或微陆块。岩石具有高Si（w（SiO2）=73．52％～74．68％〉56％）、富Al（w（Al2O03）=14．31％-15．26％）、富Na（w（Na2O）=3．82％～5．25％，Na2O/K2O值为1．26～2．5）、低Mg（侧（MgO）=0．29％～0．39％〈3％）的特点。较高的Sr含量（（435．4～719．7）×10^-6〉400×10^-6），较低的Y（（3．46-4．81）×10^-6〈18×10^-6）和Yb（（0．38～O．48）×10^-6〈1．9×10^-6）含量；较大的Sr／Y值（103．544177．09〉40）。稀土元素总量较低（∑REE：（20．98-40．79）×10^-6），轻重稀土分馏明显（（La／Yb）N：8．32～16．90）。具有正铕异常（δEu：1．18～2．20）。在原始地幔标准化蛛网图中．富集LILE（Ba、K、Sr、Rb），亏损HFSE（Th、U、Nb、Ta、P、Ti），为O型埃达克岩。结合区域地质背景，新民岩体的形成指示古亚洲洋向华北板块俯冲消减事件。

The Xinmin pluton is mainly composed of adamellite in Zhengxiangbaiqi area, Inner Mongolia. The LA-ICP-MS zircon UPb ages of two adamellite samples are 452 ±11 Ma and 423. 1± 9. 9 Ma respectively. Two age samples both have the age of more than 1000 Ma, suggesting that this area have ancient crystallization basement or micro-massif. Geochemical studies indicate that the Xinmin pluton is rich in SiO2 （23. 52%- 74. 68%）, Al2O3 （14. 31%- 15.26%）, Na2O（Na2O contents vary from 3.82% to 5. 25% and the ratios of N ae O/K2 O are between 1.26 and 2.5） and St（ （435.4- 719.7）×10^-6）, but low in MgO（0. 29 %- 0.39% ）, Y （ （ 3.46 - 4. 81 ） × 10^-6 ） and Yb （ （ 0. 38 - 0. 48 ） × 10^-6 ）, with high Sr/Y ratios ranging from 103. 54 to 122. 09. The Xinmin granitoid has low REE contents （∑ REE： （20. 98 - 40. 79） × 10^-6, with strong fractionations between I.REE and HREE（（La/Yb）N ：8.32 - 16.90） and Eu positive anomalies（SEu： 1.18 - 2.2）. The spider diagram of primitive mantle normalized trace elements show the enrichment in LILE such as Ba, K, Sr, and Rb, and the depletion in HFSE such as Th, U, Nh, Ta, P, and Ti. All of the above features show that Xinmin plutons have the characteristics of O-type adakites. Combined with regional geological background, the Xinmin granitoid reflects the subduction event ofthe Paleo-Asian Ocean to the North China Plate.