北山造山带中部早古生代伸展构造体制:来自石板井辉长岩的年代学及地球化学证据

Early Paleozoic Crustal Extensional Tectonic Regime in the Central Part of Beishan Orogenic Belt:New Evidence from Geochronology and Geochemistry of Gabbro in Shibanjing

北山造山带位于华北板块、塔里木板块和哈萨克斯坦板块交汇部位。为了研究北山地区早古生代大地构造属性,基于1∶5万区域地质矿产调查,采用LA-ICP-MS锆石U-Pb同位素定年法对分布于石板井地区的辉长岩体进行了年龄测定和岩石地球化学测试。结果表明:辉长岩体锆石U-Pb年龄为498.9±2.4Ma,属晚寒武世。地球化学数据显示:岩石具有较低的SiO_2(40.39%~53.62%)和较高的Al_2O_3(12.56%~19.26%)、CaO(8.09%~10.93%)含量和Mg#值。稀土总量较高(∑REE=62.69×10^(-6)~250.62×10^(-6)),轻重稀土分异明显[(La/Yb)N=4.89~16.94],无明显Eu异常,δEu=0.82~1.28。岩石富集大离子亲石元素Rb、Ba、K等,亏损高场强元素Nb、Ta、P、Zr、Ti等。Rb/Nb-Rb/Zr、(Ce/Yb)N-(Yb)N、Zr-Y和Zr-Nb图解显示该岩体岩浆起源于过渡型岩石圈地幔的部分熔融,并且在上升侵位过程中发生明显的地壳混染。辉长岩体具有相对高的TiO2(0.65%~1.98%)、Nb(4.47×10^(-6)~18.56×10^(-6))、Ta(0.44×10^(-6)~1.86×10^(-6))、Zr(48.07×10^(-6)~386.86×10^(-6))、Hf(2.17×10^(-6)~21.23×10^(-6))含量,相近于板内玄武岩。此外,在辉长岩体南侧发育同期二长花岗岩,二者构成双峰式岩浆岩组合。综合上述特征,辉长岩产出构造背景为板内伸展构造体制,并推测在晚寒武世古亚洲洋未发生俯冲,仍处于扩张期。

Abstract：The Beishan orogenic belt is situated in the juncture area of Paleozoic Kazakstan Plate, Tarim Plate and North China Plate. On the basis of survey result of 1∶50000 regional geological mineral resources, this study conducted analysis of the gabbro distributed in the Baijing area using LA-ICP-MS zircon U-Pb dating and geochemical analysis in order to understand Early Paleozoic tectonic attribute of the Baishan area. The results show that these grabbros have U-Pb ages of 498.9±24Ma and should be in Late Cambrian with detail ages of （498.9±2.4） Ma. Geochemical data indicate that the gabbros are low in SiO2（40.39%~53.62%）, high in Al2O3（12.56%~19.26%）, CaO （8.09%~10.93%） and Mg# value, and have high REE（6269×10-6～250.62×10-6）, slightly Eu anomalies （δEu=0.82~1.28）, enrichment in LREE and relatively depletion in HREE ／[（La/Yb）N=4.89~16.94／]. Those rocks are enriched in LILE（Rb, Ba, K）, and depleted in HFSE（Th, U, Nb, Ta）, with the low Lu/Y（0.01～0.02）, high La/Nb（1.04~3.39） and La/Ta（11.73~55.32）. Geochemical diagrams show that the magma might originate from partial melting of the transitional lithospheric mantle, with significant crustal contamination during ascending process. Gabbro is characterized by relatively high TiO2（0.65%~1.98%）, Nb（4.47×10-6~18.56×10-6）, Ta（0.44×10-6~1.86×10-6）, Zr（48.07×10-6~386.86×10-6）, Hf（2.17×10-6~21.23×10-6）, similar to those of the intraplate basalts. In addition,synchronous monzonite granite developed in the south of gabbro body, along with gabbro, contributes bimodal magmatic rock assemblage. All the characteristics above show that the gabbro formed in an intra-continent extensional setting. It can be future presumed that the late Cambrian witnessed no Paleo-Asian Ocean subduction but the expansion stage.