摘要:秦岭是一个多阶段演化的复杂大陆碰撞造山带，经历了多期的洋—陆俯冲聚合过程，目前对于南秦岭勉略洋盆的闭合时间仍旧存在不同认识。本文对南秦岭佛坪西岔河角闪黑云片岩中的锆石开展了内部结构分析、原位微量元素组成分析和LA-ICP-MS微区U-Pb定年。CL图像揭示锆石内部无环带，或发育扇形分区，显示典型的变质成因锆石的内部结构特征。这些锆石在稀土配分图中显示LREE亏损，HREE富集，具有明显的Ce正异常和微弱的Eu负异常，与变质重结晶稀土配分图类似。此外，它们的Hf同位素组成均一，176Hf/177Hf和176Lu/177Hf的比值分别介于0.282 436～0.282 570和0.000 262～0.001 138，其εHf（t）值介于-7～-2，两阶段Hf模式年龄（TDM2）为1 707～1 409 Ma，晚于前人对长角坝岩群沉积时代的认识，可能与变质作用过程中流体的加入有关，使得176Hf/177Hf比值升高。这些变质成因锆石206Pb/238U加权平均年龄为220±1 Ma，该变质年龄及其锆石Ti温度计算结果（平均值约为716 ℃）均与佛坪热穹隆变质时代及变质温度一致，揭示这套角闪黑云片岩形成于高温伸展环境。由于典型的热穹隆常发育于伸展的构造环境中，因此佛坪热穹隆变质时代的确定揭示南秦岭勉略洋盆的闭合可能发生在晚三叠世之前。
关键词: 秦岭/
变质锆石/
U-Pb定年/
锆石Ti温度/
构造演化
Abstract:Qinling Mountain, as a very complex continental collision orogeny, was formed by multistage subduction-accretion processes, which resulted in the closure of different oceans and the collision of different continental blocks. The Mianlüe Ocean occurred in the southern Qinling Mountain and its closure time is still poorly constrained. In this paper, internal structure analysis, in situ trace element analysis and U-Pb dating were carried out for zircons of a hornblende biotite schist collected from the Foping area, South Qinling. Most of these zircons display section zoning in CL images, whereas others do not show section zoning or any clear oscillatory zoning, indicating typical internal structure of metamorphic zircon. The studied zircon grains are depleted in LREE, but enriched in HREE with positive Ce anomalies and negative Eu anomalies. This kind of REE patterns is similar to that of zircons formed by metamorphic recrystallization. In addition, they have uniform Lu-Hf isotopic compositions. Their 176Hf/177Hf and 176Lu/177Hf ratios vary from 0.282 436 to 0.282 570 and from 0.000 262 to 0.001 138, respectively. The zircons have εHf（t）values between -7 and -2 and two stage model ages（TDM2）at 1 707～1 409 Ma. The TDM2 ages are younger than the deposition age of the Changjiaoba Group, which is likely because the addition of fluids during metamorphism increased the 176Hf/177Hf ratios of zircons. Our LA-ICP-MS U-Pb dating of the metamorphic zircons yielded a weighted 206Pb/238U mean age of 220±1 Ma, which is interpreted as the metamorphic age of the hornblende biotite schist. This metamorphic age and the calculated zircon Ti temperatures（mean = ～716 ℃）are in a good agreement with the metamorphic age and temperature of the Foping thermal dome, which suggest that the hornblende biotite schist was formed in an extension environment with high temperatures. Because typical Thermal Dome always developed in an extension environment, our constraints on the metamorphic age of the Foping thermal dome indicate that the closure of the Mianlüe oceanic basin likely happened prior to the Late Triassic.
Key words:Qinling/
Metamorphic zircon/
U-Pb dating/
Zicon Ti temperature/
Tectonic evolution
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