大兴安岭小莫尔可地区碱长花岗岩的锆石 U--Pb 年龄、地球化学特征及其地质意义

摘要

The authors studied the zircon LA--ICP--MS U--Pb dating and geochemical characteristics of alkali-feldspar granites in Xiaomoeke area,and discussed their petrogenesis and tectonic significance.Most zircons of the selected samples are subhedral in shape,display distinctly oscillatory zoning structures with high values of Th /U (0.38 ~0.68),implying a magmatic origin.The dating results show that the alkali-feldspar granites are formed in the Early Carboniferous ((348.9 ±2.1)Ma).The geochemical data indicate that the major elements of alkali-feldspar granites are characterized by high SiO2 (75.78% ~77.01%)and K2 O (4.58% ~4.76%), and low Al2 O3 (11.80% ~12.37%),CaO (0.28% ~0.31%)and MgO (0.05% ~0.09%).The trace ele-ments are characterized by depletion of Ba,Sr,P,Ti and enrichment of LILE such as Rb,Th,U,K.The REE are characterized by the typical sea-gull lines with obvious negative Eu and enrichment of LREE.Geochemis-try shows that the alkali-feldspar granites are quite similar to the A-type granite.On the base of the classification diagrams proposed by Eby,the alkali-feldspar granites can be classified into A2,and were formed under a trans-forming tectonic setting from collision to extension.Combining with the regional research data,the authors consid-ered that it is probably related to the system of collision and merging of Xing’an terrane and Songnen terrane.%笔者研究了小莫尔可地区碱长花岗岩的锆石 LA-ICP-MS U-Pb 测年结果及地球化学特征，并对其成因及构造意义进行了探讨。样品中的锆石多呈半自形晶，具有振荡生长环带和较高的 Th ／U 比值（0．38～0．68），揭示其为岩浆成因。测年结果显示，该区碱长花岗岩形成于早石炭世（（348．9±2．1）Ma）。岩石地球化学分析表明，岩体主量元素具有高硅（SiO2＝75．78％～77．01％）、富钾（K2 O ＝4．58％～4．76％）、贫铝（Al2 O3＝11．80％～12．37％）、贫钙（CaO ＝0．28％～0．31％）、低镁（MgO ＝0．05％～0．09％）的特征；微量元素具有亏损 Ba、Sr、P、Ti 等元素，富集 Rb、Th、U、K 等大离子亲石元素的特征；稀土元素呈现 Eu 负异常明显、轻稀土元素轻度富集的“海鸥”式分配，具有典型的 A 型花岗岩特征。通过 Eby 的构造环境判别图解，进一步判别其为 A2型花岗岩，代表其形成于碰撞伸展的转换过程之中。结合区域研究资料，认为其与兴安地块与松嫩地块的碰撞拼合体系有关。