北山造山带南部黄草滩岩体年代学、地球化学及地质意义

Geochronology,Geochemistry and Geological Significance of the Huangcaotan Pluton in the Southern Beishan Orogenic Belt

北山位于中亚造山带南缘中部东西段的交接地区,是研究中亚造山带东西段交界关系的关键地区,其中的花岗质岩石记录了造山过程中深部岩浆的丰富信息。北山南部黄草滩岩体是其代表,主要由闪长岩和花岗闪长岩组成,锆石U-Pb定年表明其年龄分别为（402±3）Ma、（394±7）Ma。岩石SiO_2含量相对较高（59.24%~71.54%）,富Na（Na_2O=3.76%~4.09%）,总体显示准铝质-弱过铝质（A/CNK=0.94~1.11）的典型I型花岗岩特征。此外,∑REE相对较低、LREE富集、轻重稀土分馏明显以及具弱负Eu异常,LILE中Rb、Th、U和K等高度富集,HFSE中Nb、P、Hf和Ti等强烈亏损,显示出与俯冲相关的地球化学特征,其形成可能与幔源岩浆底侵使中下地壳变玄武质岩石部分熔融有关,并伴有幔源组分的混染。结合区域地质研究成果,认为岩体形成于活动大陆边缘火山弧环境,可能是北山柳园地区古亚洲洋向北俯冲消减作用下岩浆活动的产物,代表早泥盆世在北山南部可能发生过与大洋俯冲有关的岩浆事件。

The Beishan orogenic belt is the transition zone located between the east and west part of southern margin of the Central Asian Orogenic Belt. It is the key area for the study of contact relationship between west and east of the Central Asian Orogenic Belt. The magma information during the orogenetic process was recorded in the granites,as a typical represent of the Huangcaotan pluton in the southern of Beishan,which consists mainly of diorite and granodiorite. Zircon U-Pb dating yields the crystallization ages of（ 402 ± 3） Ma for diorite and（ 394 ± 7） Ma for granodiorite,respectively. The Huangcaotan pluton displays typical geochemical characteristics of I-type granite： high SiO_2contents（ 59. 24%- 71. 54%）,high Na_2O contents（ 3. 76%- 4. 09%）,and aluminum-weakly peraluminous affinity（ A/CNK = 0. 94- 1. 11）. The relatively lower level of ∑REE,enriched LREE,obvious fractionation of heavy and light rare earth,slightly negative Eu anomaly,together with the highly enriched Rb,Th,U and K in LILE and depleted Nb,P,Hf and Ti in HFSE,indicate subductionrelated geochemical features. The granite may be derived from partial melting of meta-basaltic rocks in the middle-lower crust caused by underplating of mantle-derived magma and contaminated by the mantle-derived materi-al. Together with regional geology,it is proposed that the Huangcaotan pluton was formed in volcanic arc on an active continental margin and may be the product of magmation related to the northward subduction of Paleo-Asian Ocean in Liuyuan area,representing an important magmatism related to oceanic slab subduction from the Early Devonian in the southern of Beishan orogenic belt.