扬子板块西缘早中生代“绿豆岩”成因及构造启示:锆石U-Pb年龄、微量元素及Hf同位素约束

doi:10.13745/j.esf.sf.2023.2.67

摘要/Abstract

摘要：

为了研究早中生代“绿豆岩”的火山灰来源,及其与古特提斯弧火山活动的时空联系,对滇东北乌蒙山地区两件绿豆岩样品进行了锆石U-Pb年代学及Hf同位素分析。结果显示:嘉陵江组绿豆岩谐和年龄为(250.0±2.8) Ma,ε_Hf(t)值为-18.1~-9.5,二阶模式年龄T_DM2为2 141~1 674 Ma;关岭组底部绿豆岩锆石谐和年龄为(247.0±1.2) Ma,ε_Hf(t)值为-16.4~-7.5,对应的T_DM2范围为2 050~1 559 Ma。两层绿豆岩相似的年代学数据、Hf同位素组成和模式年龄分布,表明两层绿豆岩具有相同的物质来源,均为古—中元古代地壳物质熔融产物。锆石微量元素具有酸性岩浆锆石特征,表明绿豆岩原岩为长英质岩石。通过与区域数据对比,确定“绿豆岩”火山灰来源于三江造山带,且有证据显示在早—中三叠世期间,沿西南三江—宋马—十万大山一带存在一条由古特提斯洋发生俯冲消减形成的岩浆弧,两层绿豆岩都是古特提斯洋俯冲背景下的火山活动记录。结合野外地质特征及项目研究成果,限定古特提斯弧火山作用至少于嘉陵江组绿豆岩形成前(254.9 Ma)启动,至关岭组底部绿豆岩形成时(247.1 Ma)仍在活动。

关键词: 绿豆岩, 锆石U-Pb年龄, Hf同位素, 古特提斯, 岩浆弧

Abstract:

In order to clarify the source of volcanic ash found in early Mesozoic “mung bean rocks” and the spatiotemporal relation between the volcanic ash and Paleotethyan arc volcanism, geochronology, trace elements and Hf isotope of zircons from samples collected from two mung bean rock layers, Wumengshan area, northeastern Yunnan, are analyzed. For rock sample from the Jialingjiang Formation, it yields zircon U-Pb age of (250.0 ± 2.8) Ma, ε_Hf(t) values from -18.1 to -9.5 and T_DM2 ages of 2141-1674 Ma, and from the Guanling Formation it yields zircon U-Pb age of (247.0 ± 1.2) Ma, ε_Hf(t) values from -16.4 to -7.5 and T_DM2 ages of 2050-1559 Ma. The similarity of results between samples indicates the mung bean rocks are derived from the same source, and are products of Paleo-Mesoproterozoic crustal materials. The trace element characteristics of zircons from mung bean rocks are consistent with data from acidic magmas, indicating protoliths of mung bean rocks are felsic. Combined with previous results, it is determined that volcanic ash found in mung bean rocks comes from the Sanjiang orogenic belt. Evidences have shown that there exists a magmatic arc along the Sanjiang-Songma-Shiwandashan area during the Early-Middle Triassic, arisen from the subduction of the Paleo-Tethys Ocean, and the mung bean rocks are records of volcanic activities caused by the subduction of the ancient Tethys oceans. Combined with geological data, it is considered the ancient Tethys arc volcanism starts before mung bean rock diagenesis in the Jialingjiang Formation (254.9 Ma) and lasts until sedimentation at the bottom of the Guanling Formation (247.1 Ma).

Key words: mung bean rocks, zircon U-Pb dating, Hf isotope, Paleo-Tethys, magmatic arc

中图分类号:

钏茂山, 胡乐, 蔺如喜, 毛崇祯, 李仕忠, 李锁明, 袁永盛. 扬子板块西缘早中生代“绿豆岩”成因及构造启示:锆石U-Pb年龄、微量元素及Hf同位素约束[J]. 地学前缘, 2024, 31(2): 204-223.

CHUAN Maoshan, HU Le, LIN Ruxi, MAO Chongzhen, LI Shizhong, LI Suoming, YUAN Yongsheng. Origin and tectonic implication of early Mesozoic “mung bean rock” in the western margin of the Yangtze Platform: Zircon U-Pb age, trace element and Hf isotope constraints[J]. Earth Science Frontiers, 2024, 31(2): 204-223.

图/表 13

图1 研究区大地构造简图(a,b)及区域地质图(c)(a,b据文献[12]修改)

Fig.1 Tectonic outline(a,b) and geological map (c) of the study area。Modified after[12].

图2 乌蒙山地区关岭组、嘉陵江组绿豆岩野外特征(a,c)及显微结构(b,d) a—关岭组底部绿豆岩野外照片;b— PM054-22-1样品镜下照片;c—嘉陵江组一、二段界线处绿豆岩野外照片;d— PM054-6-1 样品镜下照片。VRF—火山岩屑;Fs—长石;Q—石英;Hm—水白云母。

Fig.2 The field (a, c) and microscopic pictures (b, d) of the mung bean rocks from the Jialingjiang and Guanling Formation in the Wumeng mountain area

表1 乌蒙山地区绿豆岩样品锆石微量元素分析结果

Table 1 Zircon trace elements analysis for the mung bean rocks in the Wumeng mountain area

表2 乌蒙山地区绿豆岩样品锆石样品LA-ICP-MS锆石U-Pb同位素测试结果

Table 2 Zircon U-Pb isotopic dating results for the mung bean rock samples from the Wumeng mountain area

表3 PM054-6-1和PM054-22-1锆石Hf同位素分析结果

Table 3 Hf isotopic analysis for zircons from PM054-6-1 and PM054-22-1 samples

图3 乌蒙山地区绿豆岩锆石阴极发光(CL)图像及年龄

Fig.3 Zircon cathodoluminescence (CL) images and ages of the mung bean rocks from the Wumeng mountain area

图4 锆石球粒陨石标准化稀土配分曲线(球粒陨石标准化值据文献[16]) a,b —本次研究绿豆岩锆石;c—三江古特提斯酸性岩浆锆石,据文献[17];d—P-T界线煤山剖面凝灰岩锆石,据文献[18];e— 峨眉山玄武岩顶部凝灰岩锆石,据文献[19]。

Fig.4 Chondrite-normalized REE patterns of the zircons. Adapted from [16].

图5 乌蒙山地区绿豆岩U-Pb年龄谐和图和年龄谱图 a,b—PM054-6-1谐和年龄及加权平均年龄谱图;c,d—PM054-22-1谐和年龄及加权平均年龄谱图。

Fig.5 U-Pb concordia age and relative probability plots of the zircons from the mung bean rocks in the Wumeng mountain area

图6 锆石εHf(t)与侵位年龄的关系(据文献[22]) 1—西伯利亚大火成岩省(270~229 Ma),据文献[23];2—四川龙门洞剖面(247.49±0.68)Ma,据文献[24];3—德钦剖面(247~245 Ma),据文献[25];4—白马雪山岩体(253.6和254.5 Ma),据文献[26]。

Fig.6 Zircon εHf(t) versus the emplacement ages. Adapted from [22].

图7 锆石微量元素判别火山岩岩性 Nb/Ta-Y图解(底图据文献[28])

Fig.7 Discrimination of volcanic rock lithology by zircon trace elements Nb/Ta-Y diagram. Adapted from [28].

图8 锆石微量元素判别绿豆岩构造背景图 a—Nb/Hf-Th/U图解(据文献[30]);b—Hf/Th-Th/Nb 图解(据文献[30]);c—Th-Pb 图解(据文献[31]); d—U/Yb-Nb/Yb 图解(据文献[28])。

Fig.8 Discrimination of tectonic setting by mung bean rock zircon trace elements

图9 锆石微量元素εHf(t)值分布特征及构造背景 (底图据文献[60];猫耳山、朝天数据据文献[60];上司数据据文献[61];煤山数据据文献[62];峨眉山大火成岩省数据据文献[37];新安寨岩体、通天阁岩体数据据文献[47])

Fig.9 Distribution characteristics of εHf(t) values of zircons trace element and their tectonic setting

图10 西南三江—宋马—十万大山造山带锆石年龄及Hf同位素对比(底图和部分数据来源于文献[19,25-26,45,48-49,52-53])

Fig.10 Zircon age and Hf isotope correlation of Sanjiang-SongMa-Shiwandashan Orogen (Background image and part of data are from [19,25-26,45,48-49,52-53])

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