甘肃花牛山东钾长花岗岩40Ar/39Ar同位素年龄及其地质意义

对甘肃花牛山东复式花岗岩体中钾长花岗岩的钾长石进行了详细的⁴⁰Ar/³⁹Ar同位素年龄测定,11个加热阶段所获数据构成-条相关关系非常好的直线,其对应的等时线年龄为194.25±1.96 Ma(2σ).⁴⁰Ar/³⁶Ar初始值为288.87±2.17(2σ),接近于尼尔值(295.5).鉴于该岩体形成之后未受到明显的构造-岩浆活动或其它热事件的影响,因此,194 Ma代表了钾长花岗岩钾长石的形成时代.花牛山东钾长花岗岩是燕山早期构造-岩浆活动的产物.由此推断,花牛山地区构造-岩浆活动时间不是印支期,更不是海西期,而是中生代燕山早期.

40Ar/39Ar ISOTOPIC AGE DATING ON K-FELDSPAR SEPARATES FROM EASTERN HUANIUSHAN GRANITE,GANSU PROVINCE, AND ITS GEOLOGICAL SIGNIFICANCE

Located at southeastern margin of the Kazakhstan plate, Huaniushan was one of the most important Mo, Pb-Zn-Ag and Au metallogenic district, and was considered as one of the key parts of the Beishan orogenic belt. The Precambrian and Phanerozoic granitoid intrusives and the related Mo, Pb-Zn-Ag and Au deposits(prospects)were widely distributed. Geochronological study of these granitoids attracted geologists’ attention both at home and abroad. The ⁴⁰Ar/³⁹Ar isotope data for K-feldspar separates from the potassium granite of the eastern Huaniushan granitoid intrusive, northwestern Gansu province, were presented in this paper. Eleven stepwise heating Ar data of the K-feldspar separates gave an ⁴⁰Ar/³⁹Ar isochron age of 194.25±1.96 Ma(2σ)and ⁴⁰Ar/³⁶Ar initial value of 288.87±2.17(2σ), which was obviously close to Nier’s value. This age might represent the crystallization time of the K-feldspar of the potassium granite from the eastern Huaniushan granitoid intrusive, and was considered as the youngest age of tectono magmatic events occurring within the Huaniushan district. Although no clear correlation existed in between the ⁴⁰Ar/³⁹Ar age of the K-feldspar from the potassium granite and lead isotopic model age of the sulfide separates from the Mo, Pb Zn Ag and Au ore veins, some of these galena separates had modal age of 126～180 Ma, which was lower than that of the ⁴⁰Ar/³⁹Ar isochron age of the K-feldspar. The molybdenum, lead zinc silver and gold deposits(prospects)showed a clear temporal relation to the Yanshanian high K granite. The isotopic age data and field petrological observation indicated that the latest emplacement event of the granitoid magma occurred during the Mesozoic Yanshanian orogeny, and was genetically associated with Mo, Pb Zn Ag and Au mineralization.