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

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(SiO_2=75.78%~77.01%)、富钾(K_2O=4.58%~4.76%)、贫铝(Al2O3=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。

Zircon U-Pb ages，geochemical characteristics of alkali- feldspar granite in Xiaomoeke area in Great Xing’an Ｒange and their geological implications

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 K2O (4. 58% ～ 4. 76%)， and low Al 2 O 3 (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．