甘肃阳山金矿床花岗岩成因:来自地球化学、Sr-Nd-Pb同位素和年代学的证据

甘肃文县阳山金矿为西秦岭近十年来发现的超大型卡林–类卡林型金矿床,地处扬子板块西北部碧口地体北缘。本文选取阳山矿区与矿体在空间上密切相关的花岗岩样品,进行系统的元素地球化学特征分析,结合前人的年代学和Sr-Nd-Pb同位素数据来探讨其成因。阳山花岗岩SiO_2含量为60.64%~80.77%、Al_2O_3为13.68%~23.71%,K_2O为0.74%~4.32%,A/CNK为1.34~6.02,富集Cs、U、K、Pb等元素,亏损Ba、Nb、La、Ce、Sr、Ti等元素,ΣREE=14.45×10–6~143.14×10–6,平均70.31×10–6,LREE/HREE=1.39~19.52,平均10.12,具有轻稀土富集,重稀土亏损的特征,(La/Yb)N=1.48~37.26,平均17.08,球粒陨石标准化配分模式显示弱负Eu异常(δEu=0.50~1.21,平均0.77),为过铝质钙碱性花岗岩。结合前人对花岗岩、碧口群和泥盆系三河口组的元素地球化学、年代学和Sr-Nd-Pb同位素等测试数据以及区域构造演化历史,认为阳山花岗岩来源于碰撞增厚的地壳物质——碧口群的部分熔融,在上升的过程中与泥盆系发生混染。

Petrogenesis of Granites in Yangshan Gold Deposit,Gansu Province：Evidence from Geochemical Characteristics,Sr-Nd-Pb Isotope and Geochronology

The Yangshan gold deposit, a super-large Carlin-type or Carlin-like gold deposit, was discovered in the western Qinling, Gansu province decade ago. It is located in the north margin of the Bikou Terrane, Northwest Yangtze craton. This paper reports the geochemical characteristics of the granites, which are spatially associated with the orebodies, and discusses the petrogenesis and source of the granites in the Yangshan gold deposit. The SiO2, K2O and Al2O3 of the Yangshan granites are in the ranges 60.64%~80.77%, 2.27%~4.32%, and 11.83%~23.71% respectively. The A/CNK ratios vary from 1.24 to 4.29. The Yangshan granites are also enriched in Cs, U, K, Pb, and depleted in Ba, Nb, La, Ce, Sr, Ti. Rare earth element (REE) contents of the granites range from 14.45×10?6 to 143.14×10?6(average 70.31×10?6), and significantly enriched in LREE with (La/Yb)N ratios of 1.48~37.26 (average 17.08). The REE patterns are characterized by slightly negative Eu anomalies, withδEu=0.50~1.21 (average 0.77). Geochemical characteristics of major element and trace element show that these granites belong to peraluminous and calc-alkaline series. Compariing with the major and trace elements, geochronology, and Sm/Nd, Rb/Sr and Pb isotopes of the Yangshan granites, Bikou terrane, and Devonian Sanhekou section of the previous works, we propose that the granites were derived from partial melting of the Bikou group as a result of collision and crustal thickening, and were contaminated by the Devonian sedimentary rocks during ascending.