安徽宁芜地区铁氧化物-磷灰石矿床中金云母Ar-Ar定年及其地球动力学意义

Timing of mineralization and geodynamic framework of iron-oxide-apatite deposits in Ningwu Cretaceous basin in the Middle-Lower Reaches of the Yangtze River,China:Constraints from Ar-Ar dating on phlogopites

铁氧化物-磷灰石型铁矿床是长江中下游成矿带的重要组成部分,对其成岩成矿时限的精确厘定,是理解长江中下游地区中生代大规模成矿作用的动力学演化的关键。本文分别对宁芜盆地南部白象山、和睦山及中部的陶村三个大-中型铁矿床中与磁铁矿密切共生的金云母单矿物进行了Ar-Ar同位素测年,获得白象山矿床金云母Ar-Ar坪年龄为134.9±1.1Ma,对应的反等时线年龄为133±2Ma;和睦山矿床金云母Ar-Ar坪年龄为132.9±1.1Ma,对应的反等时线年龄为133.9±2.4Ma;陶村矿床金云母的反等时线年龄为128±14Ma。该区铁氧化物-磷灰石型矿床与该区火山-次火山岩具有密切的时间联系,成岩成矿时限主要集中于135～126Ma。综合研究表明,长江中下游第二期成岩成矿作用的时限为135～126Ma,对应于岩石圈的大规模快速减薄的地球动力学背景。

The iron-oxide-apatite deposits located in Ningwu area are important components of the Middle-Lower Yangtze River metallogenic belt. Constraining the timing of petrogenesis and mineralization in this area is crucial to better understand the deep geodynamic processes which controlled the Mesozoic large-scale mineralization of this area. We here present the Ar-Ar dating results for iron-oxide-apatite mineralization conducted on three phlogopites collected from the Baixiangshan,Hemushan,and Taocun iron deposits. Phlogopite from the large-sized Baixiangshan iron deposit yielded a well-defined 40Ar/39Ar plateau age of 134.9±1.1Ma and an inverse isochron age of 133±2Ma. Phlogopite from the medium-sized Hemushan iron deposit yielded a well-defined 40Ar/39Ar plateau age of 132.9±1.1Ma and an inverse isochron age of 133.9±2.4Ma. Phlogopite from the large-sized Taocun iron deposit defined an inverse isochron age of 128±14Ma. These ages are consistent with the published U-Pb ages for zircons separated from volcanic-subvolcanic rocks in this area,suggesting that iron-oxide-apatite deposits are temporally associated with volcanic-subvolcanic rocks in Ningwu area,and constraining that its petrogenesis and metallogenis predominately took place during 135 to 126Ma. A summary of regional geochronological data suggests that the second pulse of petrogenesis and mineralization in the Middle-Lower Yangtze River area mainly concentrated on 135~126Ma,corresponding to a geodynamic setting of rapid thinning of lithosphere.