Petrogenetic simulation of the Archean trondhjemite from Eastern Hebei, China

• Published in: Science China. Earth sciences Vol. 60; no. 5; pp. 958 - 971
• Main Authors: Zhang, ShiWei, Wei, ChunJing, Duan, ZhanZhan
• Format: Journal Article
• Language: English
• Published: Beijing Science China Press 01-05-2017; Springer Nature B.V
• Subjects: Age; Computer simulation; Crystallization; Dating; Dehydration; Dikes; Earth and Environmental Science; Earth Sciences; Embankments; Fractionation; Garnet; Geochronometry; Gneiss; Lead; Magma; Melting; Melts; Minerals; Petrogenesis; Radiometric dating; Ratios; Researth Paper; Rock; Rocks; Trace elements; Zircon; 中国; 变质年龄; 太古宙; 奥长花岗岩; 成岩模拟; 河北; 花岗质岩石; 锆石U-Pb定年; Trondhjemite; Eastern Hebei; TTG gneiss; Phase equilibria modelling; Anatexis
• ISSN: 1674-7313; 1869-1897
• DOI: 10.1007/s11430-016-9025-9

It is generally believed that trondhjemitic rock, an important component of TTG rocks, is the anatectic product of mafic rocks. However, in many TTG gneiss terranes, for instance, the granulite facies terrane in Eastern Hebei, trondhjemites occur as small dikes, intrusions or leucosomes in tonalitic gneisses, suggesting their origin of in-situ partial melting. Based on the petrological analysis of a tonalitic gneiss sample from Eastern Hebei, in combination with zircon U-Pb dating, we investigated the petrogenesis of trondhjemite through simulating anatectic reactions and the major and trace element characteristics of the product melt at different pressures（0.7, 1.0 and 2.0 GPa）. The results indicate that hornblende dehydration melting in a tonalitic gneiss at 0.9–1.1 GPa and 800–850°C, corresponding to the high-T granulite facies, with melting degrees of 5–10wt.% and a residual assemblage containing 5–10wt.% garnet, can produce felsic melts with a great similarity, for instance of high La/Yb ratios and low Yb contents to the trondhjemitic rocks from Eastern Hebei. However, the modelled melts exhibit relatively higher K2 O, and lower CaO and Mg~ than those in the trondhjemitic dikes and leucosomes from Eastern Hebei, suggesting that the leucosomes may not only contain some residual minerals but also be subjected to the effect of crystal fractionation. The zircon U-Pb dating for the tonalitic and trondhjemitic rocks in the Eastern Hebei yields a protolith age of 2518±12 Ma and a metamorphic age of 2505±19 Ma for the tonalitic gneiss. The latter age is consistent with a crystallization age of 2506±6 Ma for the trondhjemitic rock, confirming a close petrogenetic relation between them.