Early Paleozoic crustal anatexis in the intraplate Wuyi-Yunkai orogen, South China

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• 作者：Dan Wang ; Jianping Zheng ; Qiang Ma ; William L. Griffin ; Huan Zhao ; Jean Wong
• 关键词：Charnockite ; Crustal anatexis ; Early Paleozoic ; South China
• 刊名：Lithos
• 出版年：2013
• 出版时间：15 August, 2013
• 年：2013
• 卷：175-176
• 期：Complete
• 页码：124-145
• 全文大小：3031 K

文摘

Early Paleozoic amphibolite- to granulite-facies metamorphism, crustal anatexis and coeval magmatism are extensively developed in the Wuyi-Yunkai intraplate orogen in the South China block. However, the exact timing of granulite-facies partial melting and its link with orogenesis have not been well constrained. In this study, the charnockites, gneissic migmatites and Al-rich gneisses (Grt-Sil-Bt gneiss and Bt-Pl gneiss) from the Gaozhou Complex of the Yunkai uplift in the Cathaysia block were selected for the analysis of whole-rock major elements and zircon U-Pb dating, trace elements and Lu-Hf isotopes. The Gaozhou Complex experienced early Paleozoic regional high-temperature (up to 850 ¡ãC), low- to medium-pressure (4-7 kbar) metamorphism accompanied by crustal anatexis. The melts were produced through the dehydration of mica, such as biotite + quartz + plagioclase = orthopyroxene + K-feldspar + melt and biotite + quartz + plagioclase + sillimanite = garnet + K-feldspar + melt in the charnockites, and muscovite + quartz + plagioclase = sillimanite + K-feldspar + melt in the Grt-Sil-Bt gneisses. The charnockites, gneissic migmatites and gneisses are felsic with SiO₂ > 64 % and peraluminous with A/CNK > 1.0, reflecting protoliths with affinities to sedimentary rocks. Zircons from these rocks partly show clear core-rim structure and yield concordant ages mainly around 440-425 Ma, with minor groups at 2.8-2.4 Ga, 1.5-1.25 Ga, 1.2-0.9 Ga, 850-540 Ma and 460-450 Ma. The 440-425 Ma grains are euhedral, oscillatorily-zoned and have steep slopes from the LREE to the HREE with a positive Ce anomaly and clear negative Eu anomaly, suggesting they (re-) crystallized in the melts. These early Paleozoic zircons have negative ¦Å_Hf (t) (? 34.1 to ? 1.5) and much older T_CRUST (3.6-1.5 Ga), demonstrating they were formed by re-melting of old crustal materials (> 1.5 Ga). The zircons with ages of 2.8-2.4 Ga, 1.6-1.2 Ga and 1.2-0.9 Ga have relatively high ¦Å_Hf (t) values (up to + 10.2-+ 15.2). The 850-540 Ma zircons show variable ¦Å_Hf (t) values of + 9.0 to ? 24.0 with T_DM (depleted mantle Hf model ages) = 2.2-1.0 Ga and T_CRUST (crustal Hf model ages) = 3.1-1.1 Ga. Combined with the published data, we suggest that the Cathaysia block contains Archean materials as old as 3.6 Ga and has had a complex evolution, including the addition of juvenile materials at ca 2.7 Ga, 1.6-1.2 Ga and 1.2-0.9 Ga. Reworking of old crustal components dominated at ca 850-750 Ma, 750-540 Ma, 460-450 Ma and more intensively at ca 440-425 Ma. Synthesizing the obtained results, we argue that the Yunkai charnockites, gneissic migmatites and Bt-Pl gneisses were formed due to the early Paleozoic high-T crustal anatexis, which may have been triggered by crustal shortening and thickening during the intraplate Wuyi-Yunkai orogeny in the South China block.