内蒙古乌拉山地区沙德盖岩体年代学、地球化学特征及成因探讨

Geochronology and Geochemistry of Shadegai Granites in Wulashan Area,Inner Mongolia and Its Geological Significance

沙德盖岩体位于华北克拉通北缘中段、哈达门沟大型金(钼)矿田范围内。首次利用锆石SHRIMP U-Pb法对其定年,获得15个锆石颗粒206 Pb/238 U年龄的加权平均值(221.6±2.1)Ma(MSWD=1.6),表明岩体侵位于印支中期。岩石地球化学特征表现为高硅(SiO2质量分数为71.21%～73.67%)、富钾(K2O/Na2O为1.01～1.37)、富碱(K2O+Na2O为8.23%～9.96%)、弱过铝质(Al2O3为13.11%～14.31%),里特曼指数σ=2.43～3.52,钙碱性-碱性;富集轻稀土(LREE/HREE为17.68～14.92,(La/Yb)N为22.85～16.58)和Eu略亏损(δEu=0.95～0.93);微量元素亏损Nb、Ta、P、Ti、Sr,富集K、La、Ce、Hf等元素,具有A型花岗岩特征;(87 Sr/86 Sr)i=0.705 31～0.702 29,(143 Nd/144 Nd)i=0.511 682～0.511 620,εNd(t)=-13.1～-14.3,具有壳源特点;钕两阶段模式年龄T2DM=2 061～2 160Ma,在铅构造模式图上样品投点于地幔与下地壳之间。综合分析认为其形成于同碰撞向后碰撞构造体制转换,伸展构造和幔源基性岩浆的底侵导致早期古老基底地壳部分熔融,很可能是形成沙德盖岩体的主要动力机制。华北克拉通北缘印支期构造岩浆活动及成矿作用是普遍存在的。

The Shadegai granite is located in Hadamengou large gold-molybdemum orefield,in the north margin of North China craton.The SHRIMP U-Pb weighted average age of 15 zircon grains from the Shadegai granites is（221.6±2.1） Ma（MSWD=1.6）,which indicates the granite emplaced in the medium Indosinian epoch.Geochemical characteristics show that the granite is characterized by high silicon（SiO2=71.21%Northern margin of North China craton;Shadegai granite;geochronology;geochemistry;Triassic 73.67%）,potassium-rich（K2O/Na2O=1.01-1.37）,rich alkaline（K2O＋Na2O=8.23%-9.96%）,weakly peraluminous（Al2O3=13.11%-14.31%）.The Rittmann index（σ） of granite varies from 2.43 to 3.52,which means that the rock samples are calc-alkaline to alkaline.The ∑REE varies from 185.70×10-6 to 96×10-6,the ratios of LREE/HREE and LaN/YbN are 17.68-14.92 and 22.85-16.58,respectively,and δEu is weakly negative anomalies.Trace elements composition exhibit such characteristics of A-type granite,as depletion of Nb,Ta,P,Ti,Sr,and enrichment of K,La,Ce,Hf.The values of（87Sr/86Sr）i is from 0.705 31 to 0.702 29,（143Nd/144Nd）i from 0.511 682 to 0.511 620,and εNd（t） from-13.1 to-14.3,indicating that the magma source of Shadegai granite should have been mainly of crust-derivation.Nd isotope two-stage model ages（T2DM） are 2 061-2 160 Ma.In the tectonic model diagram of lead isotopic compositions,the granite samples are plotted between the mantle and lower crust curves.The comprehensive analysis suggests that the Shadegai granite formed under the tectonic setting of transformation from syn-collision to post-collision.Statistics indicate that the magmatism and mineralization took place extensively in the northern margin of North China craton during the Indosinian epoch.