哈尔滨东部海西期钾长花岗岩的锆石U-Pb年代学及其地质意义

Geochronology of the Hercynian potash feldspar granite and its geological significance in East Harbin,China

通过对哈尔滨东部地区海西期钾长花岗岩年代学、地球化学特征的研究,讨论兴蒙造山带的演化。研究区钾长花岗岩呈肉红色细粒,主要由钾长石、斜长石、石英和黑云母组成。岩石中SiO_2的质量分数为74.70%~75.70%,为酸性岩。Al_2O_3的质量分数为12.07%~13.18%;Na_2O+K_2O的质量分数为7.78%~8.09%,K_2O/Na_2O为0.93%~1.60%。里特曼指数为1.85~2.06。∑REE的质量分数为96.32×10^(-6)~172.83×10^(-6),δEu为0.21~0.77,为铕负异常;铝饱和指数为0.88~1.12,属于高钾钙碱性花岗岩,与造山带垮塌导致的大规模岩石圈伸展作用有关。该花岗岩LA-ICP-MS锆石U-Pb年龄为(298.54±0.86)Ma(MSWD=0.94),属于晚石炭世,形成于古亚洲洋闭合后兴安地块和松嫩地块碰撞拼合后的伸展作用。

The flesh pink fine-grained alkali-feldspar granites widely occur in East Harbin with unknown emplacement time. Study of chronology and geochemistry of these rocks are conducted in order to provide new evidence for discussion of the tectonic evolution of Xing＇an Mongolia Orogenic Belt or Paleo-Asian Ocean. Alkali-feldspar granite is mainly composed of perthite, plagioclase, quartz and biotite and they are e rich in SiO2 （the mass fraction is 74.70%-75.70%）, Al2O3 （12.07%-13.18%） and alkali （Na2O＋K2O=7.78%-8.09%）, with ratios of K20/Na20=0.93%-1. 60% and Litman index of 1.85-2.06. The concentrations of REE range from 96.32- 10% to 172.83 × 10 6 with remarkable negative Eu anomaly （δEu=0.21-0. 77） and A/CNK ratio of 0.88-1. 12, characteristic of high-K calc-alkaline granite. It indicates that the magma is originated from an extensional setting induced by post-orogenic collapse of the lithosphere. Zircon U-Pb dating yields age of 298.54±0. 86 Ma （MSWD=0. 94） and constrains the emplacement time of the granite to late Carboniferous period. It is considered that the alkali-feldspar granites generated under the extensional setting related to the collision between Xing＇an and Songnen Blocks after the closure of Paleo-Asian Ocean.