摘要
秦岭造山带南缘存在大片的早古生代基性火山岩,荞麦冲金矿的正长斑岩脉体呈北西向侵入基性火山岩地层中。本文通过对该正长斑岩脉体中的岩浆岩锆石的LA-ICP-MS U-Pb定年、元素地球化学以及Hf同位素研究,探讨正长斑岩脉体的侵入时代、岩石成因和地球动力学背景。锆石U-Pb定年结果：正长斑岩脉体的侵入时代可以分为2期,相对应的2件正长斑岩锆石206Pb/238U年龄加权平均值分别为（415±7）Ma、（477±6）Ma,前者属于晚志留世时期,后者为寒武纪到早奥陶世时期。地球化学特征上显示具较高的Si O2含量（〉75%）,里特曼指数δ=1.59,高Al低Ti的特点,Eu和Sr具有明显负异常,为钙碱性脉体。正长斑岩的Hf同位素检测结果得到εHf（t）全部为正值,其变化范围较大（3.4~11.8）,说明岩浆主要来自亏损地幔,个别εHf（t）异常,推断岩浆上侵过程中混入少量地壳物质,从而显示壳幔混合特征。在基性岩的地球化学表现为稀土总量很高（∑REE=149.62×10-6~321.55×10-6）,轻稀土强烈富集型,呈现出高度分异的特点。研究表明,在早古生代时期,区域上为板内裂解的构造环境,使软流圈地幔上涌冷凝形成基性岩地层,正长斑岩脉体在侵入基性岩形成过程,从地壳深部的Au带入地壳表层,为地层Au的预富集,后期剪切成矿作用提供了Au的物质来源;正长斑岩脉体形成时代的厘定,确定了该区金源层形成时代,对后期金的富集具有重要意义。
On the southern margin of the Qinling orogenic belt, there exists a multitude of Early Paleozoic mafic volcanic rocks, and the syenite porphyry veins in the Qiaomaichong gold deposit were intruded in NW direction into mafic volcanic rocks in the study area. By means of studying new zircon LA-ICP-MS U–Pb age and Hf-isotope, whole-rock major and trace elements of syenite porphyry veins in the Qiaomaichong gold deposit and the mafic country rock, the authors tried to constrain the age and petrogenesis and geodynamic background of the siliceous veins. It is revealed that the intrusion of the syenite porphyry can be divided into two phases, and the corresponding two zircon samples of syenite porphyry have206 Pb /238 U weighted average ages of（415±7） Ma and（477±6） Ma respectively. The former age belongs to the Late Silurian, whereas the latter to the Early Ordovician. Veins have a high Si O2（75%） content, σ=1.59,and A/CNK=1.28, which suggests peraluminous alkaline siliceous shear veins. The εHf（t） values are all positive, and vary in a large range（ 3.4 to 11.8）, which suggests that almost all the matter came from the depleted mantle source,exhibiting a crust-mantle mixing feature. The mafic rocks have a high amount of REE（149.62×10-6-321.55×10-6）, and are strongly enriched in light rare earth elements, showing a high degree of differentiation. Research shows that this area was within plate tectonic cleavage in the early Paleozoic, so the mantle asthenosphere upwelling of magma formed mafic rocks. Then the veins brought the Au from deep crust into the surface crust in the invasion process, thus providing material of Au sources for preconcentration of Au and later shear mineralization. The determination of the age of the formation of veins has a great significance for the study of late stage gold enrichment.
出处
《中国地质》
CAS
CSCD
北大核心
2015年第5期1303-1323,共21页
Geology in China
基金
国家自然科学重点基金项目(41030423)
中国地质调查局地质调查工作项目(1212011120354)联合资助
关键词
秦岭南缘
荞麦冲金矿
锆石U-PB
HF同位素
地球化学
正长斑岩
southern margin of Qinling orogenic belt
Qiaomaichong gold deposit
zircon U-Pb geochronology
zircon Hf isotope
geochemistry
syenite porphyry
