Low-Al and high-Al trondhjemites in the Huai鈥檃n Complex, North China Craton: Geochemistry, zircon U-Pb and Hf isotopes, and implications for Neoarchean crustal growth and remelting

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• 作者：Hua-Feng Zhang^a ; ^{doctoria@sina.com.cn} ; Ming-Guo Zhai^b ; M. Santosh^c ; Sheng-Rong Li^a
• 关键词：Tonalite&ndash ; trondhjemite&ndash ; granodiorite ; Geochemistry ; Zircon geochronology ; Neoarchean ; Huai&rsquo ; an ; North China Craton
• 刊名：Journal of Asian Earth Sciences
• 出版年：2012
• 期刊代码：40_13679120
• 类别：ear
• 出版时间：30 April, 2012
• 卷：49
• 期：Complete
• 页码：203-213
• 文件大小：1658 K

摘要

Voluminous tonalite-trondhjemite-granodiorite (TTG) magmas were emplaced in the Huai鈥檃n Complex of North China Craton (NCC) during the Neoarchean. Here we present the geochemical characteristics of these rocks and identify low-Al (LAl) and high-Al (HAl) types of trondhjemites within the TTG suite. The LAl group displays high silica (SiO₂ = 77-80 wt.%), and low Al₂O₃ contents (11.06-11.89 wt.%), a strongly peraluminous (A/CNK = 1.16 and 1.19) composition and enrichment in HREE (La_N/Yb_N = 4 and 5, Gd_N/Yb_N = 0.35 and 0.9). Their low Rb (13 and 28 ppm), Th (0.3 and 7.0 ppm), relatively high Na₂O/K₂O values (3.6 and 4.1) and MgO (0.63 and 0.68 wt.%) with moderately high Mg# (36 and 53), suggest crystallization from a primitive source. The HAl group is characterized by relatively low SiO₂ (67.22-71.57 wt.%) and high Al₂O₃ contents (16.35-16.41 wt.%) with trace element contents and distribution patterns closely comparable to those of Archean high-Al TTGs. Our geochemical data do not favor a direct genetic link between the LAl and HAl trondhjemite groups. The zircon U-Pb data reveal that the LAl and HAl groups represent coeval intrusions at ca. 2.55-2.5 Ga. In situ analysis of Hf isotopes in the zircons (蔚Hf = +5.6 to +10, and T_DM = 2462-2595 Ma), together with the geochemical features of the low-Al trondhjemites suggest that they were generated under relatively low pressure (<8 kbar), through low degree partial melting of a newly accreted amphibolitic crust. Taking into consideration the contemporaneous (2.55-2.50 Ga) high-Al TTGs widely exposed in the Huai鈥檃n Complex, we suggest that the low-Al trondhjemites were formed by intraplating of mantle-derived magmas, and the coeval high-Al TTGs were produced by partial melting of a thickened lower crust, triggered by underplating. Our study attests to a major episode of Neoarchean crustal growth in the NCC, and remelting induced by underplating and intraplating of mantle-derived magmas.