云南中甸地区斑岩体锆石微量元素与Hf-O同位素特征及其对成岩成矿的启示

中甸铜多金属成矿带位于义敦岛弧南缘, 被分为东、西两个斑岩带。本文以东斑岩带的普朗、地苏嘎岩体, 西斑岩带的雪鸡坪岩体为研究对象, 通过锆石微量元素和Hf-O同位素特征研究, 结合所收集到的这些岩体的全岩地球化学数据, 来揭示两个斑岩带的岩浆来源与演化过程、构造背景, 并探讨造成其成矿差异的控制因素。结果表明, 3个岩体所含锆石都亏损轻稀土、富集重稀土, 具有明显的Ce正异常和Eu负异常, Th/U平均值分别为0.19、0.27、0.13, 均为岩浆锆石。普朗、地苏嘎和雪鸡坪岩体锆石ε_Hf(t)值分别为-0.31±0.6、0.0±0.6、1.71±0.7, 锆石δ¹⁸O值分别为6.2±0.3‰、6.2±0.4‰、6.1±0.4‰; 3个岩体的源区相差不大, 均为壳幔岩浆混合, 但普朗和地苏嘎岩体地壳物质贡献更多。全岩主微量元素变化特征与锆石Nb/Yb和Th/Nb与Eu/Eu^*的相关性显示东斑岩带的普朗和地苏嘎岩体比西斑岩带的雪鸡坪岩体的结晶分异程度更高。在锆石微量元素构造判别图解中, 这些岩体均落在火山弧或与弧相关的造山环境范围。结合全岩的Nb-Y和Rb-(Y+Nb)判别图解, 以及中甸地区成岩年龄的统计, 进一步判定东、西斑岩带岩体均形成于与晚三叠世甘孜-理塘洋壳西向俯冲相关的大陆弧背景。采用锆石氧逸度计估算的普朗、地苏嘎和雪鸡坪岩体的氧逸度分别为ΔFMQ+1.58、ΔFMQ+1.42、ΔFMQ+1.91(QFM为石英-铁橄榄石-磁铁矿缓冲剂)。结合Eu/Eu^*值和全岩V/Sc比值分析, 发现3个岩体均有较高氧逸度, 但普朗和地苏嘎的岩浆氧逸度略低于雪鸡坪, 说明在相对较高氧逸条件下, 氧逸度高低与成矿规模不成比例。3个岩体的锆石结晶温度分别为769±34℃、783±36℃、819±40℃。结合全岩Sr/Y比值, 发现普朗和地苏嘎岩浆含水量高于雪鸡坪。由此推断, 更高的水含量和岩浆分异度可能是东部斑岩带比西部斑岩带发育更多斑岩矿床的重要原因, 这也暗示东斑岩带具有更大的找矿潜力。

Zircon trace elements and Hf-O isotopes of porphyries in the Zhongdian area of Yunnan Province and their implications for petrogenesis and mineralization

The Zhongdian copper polymetallic mineralization belt is located on the southern margin of Yidun Island Arc, which can be subdivided into the eastern and the western porphyry belts, respectively. In this paper, combined with previous published whole-rock geochemical data of the ore-bearing porphyries, zircon trace element and Hf-O isotope analyses are conducted on the Pulang and Disuga ore-bearing porphyries from the eastern belt and the Xuejiping ore-bearing porphyry from the western belt to identify their origin, evolution process and tectonic setting, and further explore the differences of the control factors for their copper mineralization. The results show that zircons from the Pulang, Disuga and Xuejiping ore-bearing porphyries are characterized by depletion of light rare earth elements and enrichment of heavy rare earth elements, with the average Th/U values of 0.19, 0.27 and 0.13, respectively. The zircon εHf(t) and δ18O values of the three ore-bearing porphyries are 0.31±0.6, 0.0±0.6 and 1.71±0.7, and 6.2±0.3‰, 6.2%±0.4‰ and 6.1%±0.4‰, respectively, indicating that they are all derived from mixed crust-mantle materials. However, there is a greater contribution of crustal material on the Pulang and Disuga porphyries than that of the Xuejiping porphyry. Whole-rock major and trace element variations together with the relationships of zircon Nb/Yb vs. Eu/Eu* and Th/Nb vs. Eu/Eu* reveal that the Pulang and Disuga porphyries underwent a higher degree of fractional crystallization than the Xuejiping porphyry. On the tectonic-background discrimination diagram of zircon trace elements, all the porphyries fall into volcanic arcs or arc-related orogenic environments. Combined with the whole rock Nb-Y and Rb-(Y+Nb) discrimination diagrams and their dated ages, it is suggested that the porphyries both in the eastern and the western porphyry belts were formed in a continental arc setting associated with the Late Triassic westward subduction of the Ganzi-Litang oceanic crust. The oxygen fugacities of the Pulang, Disuga and Xuejiping porphyries are estimated to be ΔFMQ+1.58, ΔFMQ+1.42, ΔFMQ+1.91 respectively by using the zircon oxygen fugacity meter (QFM is a quartz-iron peridotite-magnetite buffer). Combined with zircon Eu/Eu* values and whole-rock V/Sc ratios, all the porphyries have a relatively high oxygen fugacity, however, the oxygen fugacities of the Pulang and Disuga plutons are lower than that of the Xuejiping, which indicates that the level of oxygen fugacity is not proportional to the scale of mineralization under a relatively high oxygen fugacity condition. The crystalline temperatures of the three ore-bearing porphyries are 769±34℃, 783±36℃, 819±40℃, respectively. Combined with the whole-rock Sr/Y ratios, it is found that the magmatic water contents of the Pulang and Disuga porphyries are higher than that of the Xuejiping porphyry. As a preliminary conclusion, higher water contents and higher differentiation degrees of magma may be the controlling factors for the development of more porphyry deposits in the eastern porphyry belt compared with the western porphyry belt, which suggests a greater mineralization prospecting potential for the eastern porphyry belt.