| 云南澜沧谦迈蛇绿混杂岩中斜长花岗岩成因及地质意义 |
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| 引用本文：郭翔宇,周听全,徐学章,徐定向,熊树斌,周坤,雷碧东,杨绍文.2025.云南澜沧谦迈蛇绿混杂岩中斜长花岗岩成因及地质意义[J].地球学报,46(6):1119-1134. |
| DOI：10.3975/cagsb.2025.021421 |
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| 基金项目:云南省地矿局“杨绍文专家工作室”(编号: YD202067); 云南省自然资源厅地质勘查基金项目“云南省1:5万下景张、新营盘、东岗、南北归4幅区域地质调查”(编号: D2017002) |
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| 中文摘要:云南澜沧地区谦迈蛇绿混杂岩是区域上勐库地区湾河蛇绿混杂的南延, 记录了原特提斯洋盆从扩张到俯冲消减的过程。本文以该蛇绿混杂岩带中斜长花岗岩为对象, 开展了岩相学、U-Pb年代学、Lu-Hf同位素及全岩地球化学研究。岩石地球化学显示该斜长花岗岩高硅、富铝和钠, 低镁和钾; 轻稀土富集, 具有Eu负异常。岩石富集Rb、Th、U、K等元素, 低Sr、高Y和Yb, 亏损Nb、Ta及Ti, 与岛弧岩浆的特征相吻合。斜长花岗岩样品中锆石均具有明显的振荡环带, Th/U比值分别为0.29~0.85、0.46~0.62; 均具有明显的Ce正异常和Eu负异常, 为典型岩浆锆石。通过LA-ICP-MS锆石U-Pb定年, 获得斜长花岗岩206Pb/238U加权平均年龄分别为(489±3) Ma、(464±4) Ma, 为早古生代岩浆活动的产物。锆石均具较低的εHf(t)值(–10.10 ~ –1.92), 模式年龄为1.60~2.07 Ga, 远大于其形成年龄。对斜长花岗岩的岩浆成因进行了探索, 推测岩浆来源于低温低压下洋壳运移剪切带中角闪岩相的部分熔融, 部分熔融的残留矿物组成为斜长石+角闪石±斜方辉石±钛铁矿(无石榴石)。结合区域资料认为, 原特提斯洋的岛弧作用至少始于晚寒武世或更早, 并于奥陶纪持续俯冲。 |
| 中文关键词:U-Pb年代学 地球化学 Lu-Hf同位素 斜长花岗岩 蛇绿混杂岩 云南澜沧 |
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| Petrogenesis and Geological Significance of Plagiogranite in Qianmai Ophiolitic Mélange, Lancang, Yunnan Province |
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| Abstract:The Qianmai ophiolitic mélange in the Lancang, southwestern Yunnan, is the southern extension of the Wanhe ophiolitic mélange in Mengku region, recording the process of the Proto-Tethys Ocean from expansion to subduction. In this paper, petrography，zircon U-Pb chronology, Lu-Hf isotopes, and whole-rock geochemistry are investigated in the plagiogranite of the ophiolitic mélange. The plagiogranite is characterized by relatively high SiO2, Na2O, and Al2O3, and low K2O and MgO contents. The geochemical data show that plagiogranite is enriched in light rare earth elements (REE) with negative Eu anomaly. The primitive mantle-normalized spider-grams are charac-terized enrichments in Rb, Th, U, and K, as well as high contents of Y and Yb, particularly negative anomalies in Sr and Nb-Ta-Ti, which are consistent with the characteristics of island arc magmas. Zircons from plagiogranite are typical magmatic zircon with negative Eu and positive Ce anomalies. Their ratios of Th/U are 0.29–0.85 and 0.46–0.62, which are typical features of magmatic zircons. LA-ICPMS zircon U-Pb dating showed that the weighted mean 206Pb/238U ages are (489±3) Ma and (464±4) Ma, suggesting that they are related to Early Paleozoic magma activity. All zircons have initial εHf(t) values ranging from –10.10 to –1.92, and model ages ranging from 1.60 to 2.07 Ga, which are older than the formation age. In conclusion, the plagiogranite was likely caused by partial melting of the amphibolite phase in the oceanic crust’s migration shear zone (residual mineral phase: Pl+Hb+Opx+Ilm (without Grt)) under low-temperature and low-pressure conditions. Combined with regional geological setting, our results suggest that the Proto-Tethys opened before the Late Cambrian and evolved during the Ordovician. |
| keywords:U-Pb chronology geochemistry Lu-Hf isotope plagiogranite ophiolitic mélange Lancang of Yunnan |
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