| 粤西云开地区基底变质岩的组成和形成 |
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| 引用本文： | 周雪瑶,于津海,王丽娟,沈林伟,张春晖.粤西云开地区基底变质岩的组成和形成[J].岩石学报,2015,31(3):855-882. |
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| 作者姓名： | 周雪瑶 于津海 王丽娟 沈林伟 张春晖 |
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| 作者单位： | 内生金属矿床成矿机制研究国家重点实验室, 南京大学地球科学与工程学院, 南京 210093;内生金属矿床成矿机制研究国家重点实验室, 南京大学地球科学与工程学院, 南京 210093;内生金属矿床成矿机制研究国家重点实验室, 南京大学地球科学与工程学院, 南京 210093;内生金属矿床成矿机制研究国家重点实验室, 南京大学地球科学与工程学院, 南京 210093;内生金属矿床成矿机制研究国家重点实验室, 南京大学地球科学与工程学院, 南京 210093 |
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| 基金项目： | 本文受国家重点基础研究发展计划(2012CB416701)和国家自然科学基金项目(41330208、41272085)联合资助. |
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| 摘 要： | 云开地块被认为是华南西南部一个重要的前寒武纪变质基底出露区,但对其基底组成的认识仍存在较大争议。本文对云开地块内基底变质岩进行了岩石地球化学、锆石U-Pb-Hf同位素分析。分析结果表明云开地块的基底主要由于新元古代-早古生代的变质沉积岩组成。它们具有比上地壳平均成分更高的Si O2和相对更低的Al2O3、Ca O、Na2O,岩石成熟度中等。微量元素与PAAS相似,但Sr、Cr、Ni等强烈亏损,高场强元素Nb、Ta轻度亏损,而大多数样品的Zr、Hf、Th、U等轻度富集,说明源区更富集长英质组分而贫镁铁组分。地球化学特征和碎屑锆石组成指示这套沉积岩形成于被动大陆边缘环境,源区既有古老的再循环物质,也有大量未经明显改造的新元古代岩浆物质。综合本文和前人的锆石U-Pb定年数据,云开地块基底变质岩原岩可以分为两组:第一组样品形成较早(时代上限为850~522Ma),总体年龄谱特征显示出与华夏南岭地区的亲缘性。这组样品的源区主要有4次岩浆作用(2700~2400Ma、1800~1400Ma、1150~900Ma、850~700Ma),均涉及到古老地壳再循环以及新生地壳的加入,但以再循环的物质为主。最主要的新生地壳生长发生在新太古代和Grenville期。而第二组样品的沉积时代在517Ma之后,具有与扬子南缘新元古代沉积岩的相似性。这组亲扬子沉积物的源区具有不同的4次岩浆事件(2600~2350Ma、2000~1750Ma、1700~1500Ma、900~750Ma),新太古代晚期-古元古早期岩浆主要涉及古老基底再循环。古元古晚期-中元古早期岩浆大多起源于新生地壳物质,而新元古代是最重要的新生地壳生长期,同时也涉及大量古老地壳物质的再循环。不同时代沉积岩的碎屑物质组成变化表明大概在522~517Ma之间云开沉积盆地和物源区受到一定程度的构造运动影响,使得源区由华夏地块变成扬子地块。这期构造事件很可能是早古生代造山事件的初始阶段。根据本文资料和其他证据我们认为云开地块归属于华夏板块,扬子与华夏地块的分界线至少在云开地块以北,且很可能在平乐与平南之间。
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| 关 键 词： | 云开地块 基底变质沉积岩 晚新元古-早古生代 锆石U-Pb-Hf同位素 |
| 收稿时间： | 2014/8/13 0:00:00 |
| 修稿时间： | 2014/10/20 0:00:00 |
Compositions and formation of the basement metamorphic rocks in Yunkai terrane, western Guangdong Province, South China |
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| ZHOU XueYao,YU JinHai,WANG LiJuan,SHEN LinWei and ZHANG ChunHui.Compositions and formation of the basement metamorphic rocks in Yunkai terrane, western Guangdong Province, South China[J].Acta Petrologica Sinica,2015,31(3):855-882. |
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| Authors: | ZHOU XueYao YU JinHai WANG LiJuan SHEN LinWei and ZHANG ChunHui |
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| Institution: | State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China;State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China;State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China;State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China;State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China |
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| Abstract: | The Yunkai terrane had always been considered as an important part of the Precambrian basement outcropped in southwestern South China Block (SCB), but the understanding of basement compositions has been controversial. This study presents a suite of geochemical and zircon U-Pb-Hf isotopic data of the basement metamorphic rocks in the Yunkai terrane. Analytical results show that the Yunkai basement is mainly composed of meta-sedimentary rocks deposited in Late Neoporoterozoic-Early Paleozoic. These sedimentary rocks with moderate composition maturity show high SiO2 and low Al2O3, CaO, Na2O relative to the average upper crust composition. Their trace element characteristic are similar to those of post-Archean Australian shales (PAAS), with exception of strong depletion in Sr, Cr, Ni, slightly depletion in high field-strength element (e.g. Nb, Ta) and weakly enriched in Zr, Hf, Th, U, suggesting that provenances of these sedimentary rocks have more felsic components with minor mafic components. The geochemical characteristics and detrital zircon dating results indicate that these sedimentary rocks were deposited in a passive continental margin setting and derived from both the recycled old crust and some unmodified Neoproterozoic magma material. Integrating our data with previous data, it is suggested that the basement rocks in Yunkai terrane can be divided into two groups. First group samples were formed early (<850~522Ma), and show affinity with those in the Nanling area. Four major magmatic events (2600~2350Ma, 2000~1750Ma, 1700~1500Ma and 900~750Ma) occurred in their source, all of which involve both the reworking of older crust and the growth of juvenile crust. The most important juvenile crust generation took place in Neoarchean and Grenvillian. Second group samples were deposited after 517Ma and have the same components as those in the southern Yangtze Block, suggesting their affinity relation. Four different episodes of magmatism (2600~2350Ma, 2000~1750Ma, 1700~1500Ma, 900~750Ma) occurred in the source of second group samples. Late Neoarchean-Early Paleoproterozoic magmatism mainly involved the reworking of older crust. Minor juvenile crust was generated in Late Paleoproterozoic-Early Mesoproterozoic, and most intense juvenile crust generation was brought about in Neoproterozoic. The change of detritus in different episodes of sedimentary rocks manifests that the Yunkai terrane basin probably suffered the influence of a tectonic event at <522Ma to 517Ma, leading to provenance variation from the Cathaysia Block to the Yangtze Block. This event probably is initial stage of Early Paleozoic orogeny in the Cathaysia. According to our data and other lines of evidence, we think that the Yunkai terrane belongs to the Cathaysia Block, and the boundary between the Yangtze Block and the Cathaysia Block is likely located between Pingle and Pingnan in the north of Yunkai terrane. |
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| Keywords: | Yunkai Terrane Basement metamorphic rocks Late Neoproterozoic-Early Paleozoic Zircon U-Pb-Hf isotopic compositions |
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