内蒙古博克图晶洞花岗岩形成时代、特征及其地质意义

Formation Age,Characteristics and Geological Significance of Boketu Miarolitic Granite in Inner Mongolia

晶洞花岗岩成因类型和大地构造背景等方面的研究,目前颇具争议.先前的研究认为晶洞碱长、正长花岗岩属于A型花岗岩,形成于伸展环境,近年来的研究发现其成因类型具多样性.报道了博克图晶洞花岗岩的岩石学、岩石地球化学、年代学等方面的特征.SHRIMP锆石U-Pb定年结果为140.8±2.2Ma,指示其属于早白垩世的产物.且其具有高SiO_2,高ALK(Na_2O+K_2O)(7.99%～8.43%),K_2O/Na_2O=1.0～1.1,以及低的FeOT(1.15%～1.30%).微量元素富Th、U、Cs、Rb,贫Ba、Sr、P、Ti,10 000*Ga/Al、总稀土含量以及Zr+Nb+Y+Ce值均低于A型花岗岩最低平均值,由此指示其应属于Ⅰ型花岗岩,Hf同位素特征显示其成分为新生地壳来源.动力学机制上,博克图晶洞花岗岩主要受制于早白垩世时期蒙古-鄂霍次克洋闭合作用.在东北地区岩石圈由增厚到减薄转换事件中,早期的新生地壳在地幔物质的底侵作用下发生低程度的部分熔融作用.

Genetic types and tectonic backgrounds of miarolitic granite are still controversial.Previous studies have suggested that miarolitic alkali feldspar granites and miarolitic syenite granites belong to A-type granite formed in extensional environment.However,recent studies have found the diversity of the genetic types.In this paper,the petrology,geochemistry and geochronology of Boketu miarolite pluton are present.The zircon SHRIMP U-Pb dating results yield an age of 140.8±2.2 Ma,belonging to the Early Cretaceous.The pluton is characterized by high SiO2 content,and full high alkali content(7.99%-8.43%),with high K2 O/Na2 O(1.0-1.1);low total Fe content(1.15%-1.30%).On the primitive mantle-normalized spider diagrams,the granite is characterized by positive Th,U,Cs,Rb anomalies and negative Ba,Sr,P,Ti anomalies.10 000*Ga/Al ratio,the total rare earth content and Zr+Nb+Y+Ce values are lower than the minimum value of the A-type granites.It is indicated that it should belong toⅠ-type granite.Hf isotope compositions show the source of juvenile crust.It is mainly restricted by the closure of the Mongolian-Okhotsk ocean,during which the lithosphere transformation from thickening to thinning took place.The granites are formed by the low-degree partial melting of juvenile crust in the underplating of the mantle.