三江南段景洪大勐龙地区基性高压变质岩岩石地球化学特征及其大地构造意义

Geochemical characteristics and tectonic significance of basic high-pressure metamorphic rocks in the Damenglong-Jinghong area,southern Sanjiang

本文以三江南段景洪大勐龙地区新发现的退变榴辉岩和蓝片岩为研究对象,对其进行了系统的地球化学、原岩性质及锆石U-Pb定年的综合研究。大勐龙地区退变榴辉岩呈透镜状产于白云钠长石英片岩、白云母片岩和斜长角闪(片)岩中,其原岩为亚碱性拉斑玄武岩,球粒陨石标准化稀土元素配分曲线呈轻稀土弱亏损、重稀土平坦型的分布特征,不具有Nb、Ta、Ti的亏损,与典型的N-MORB(正常型洋中脊玄武岩)地球化学特征一致,表明其原岩可能来源于亏损的地幔区,形成于洋中脊环境;变质锆石SHRIMP U-Pb年龄为230.3±1.7Ma,可能代表榴辉岩峰期的变质时代。蓝片岩的原岩为亚碱性拉斑玄武岩,具有较高Ti玄武岩的特征(TiO_(2)含量2.55%~2.88%,均大于2%),稀土元素总量变化范围小,比典型的N-MORB稀土总量偏高,轻稀土元素(LREE)亏损,无明显负Eu异常,地球化学性质与N-MORB类似。结合以往区域研究成果,进一步确定研究区退变榴辉岩和蓝片岩的原岩属于典型N-MORB型,该项研究不仅在昌宁-孟连缝合带的南段发现典型的特提斯洋壳残片,而且为进一步深入探讨滇西地区特提斯洋消减-闭合的动力学背景及其复杂的构造演化过程提供了重要的科学依据。

The retrograde eclogites and blueschists are newly discovered in the Damenglong-Jinghong area of southern Sanjiang,and have been investigated in detail in terms of their geochemical signatures,protolith natures,and zircon U-Pb chronological dating.The retrograde eclogites occur as lenses within the muscovite schist and plagioclase amphibolite.These rocks have subalkaline tholeiitic features,showing a geochemical affinity to N-MORB with light rare earth elements(LREE)depletions and the lack of negative anomalies in Nb,Ta,and Ti,which suggests that their basaltic protoliths are formed in a mid-ocean ridge setting with a depleted mantle source.The metamorphic zircons separated from the retrograde eclogites yielded SHRIMP U-Pb age of 230.3±1.7 Ma,which is interpreted to represent the timing of the peak eclogite-facies metamorphism.The mafic blueschist show bulk-rock compositions close to the subalkaline tholeiitic high-Ti basalts(with TiO_(2) contents of 2.55%~2.88%)and exhibit N-MORB-like REE patterns with negligible Eu negative anomaly.Combined with previous results,we proposed that the studied retrograde eclogites and blueschists were derived from N-MORB oceanic materials.Consequently,the typical remnants of Paleo-Tethys exposed in the southern part of the Changning-Menglian suture zone are newly identified in our study.These findings provide significant evidences for exploring the subduction-exhumation geodynamics and complex tectonic evolution of the Paleo-Tethys Ocean in the western Yunnan area.