Integrated in situ zircon U–Pb age and Hf–O isotopes for the Helanshan khondalites in North China Craton: Juvenile crustal materials deposited in active or passive continental margin?
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The Khondalite Belt, one of the three major Paleoproterozoic mobile belts in the North China Craton, consists mainly of khondalite suite and S-type granites. It is thought to be a continental collision belt along which the Yinshan and Ordos Blocks amalgamated to form the Western Block of the North China Craton at ca. 1.95 Ga. While many resent studies revealed that the khondalite protoliths were sourced from juvenile crustal materials and the time interval between their deposition and subsequent metamorphism is less than 50 million years, the nature and depositional setting of the khondalite protoliths are still controversial. We carried out in this study an integrated analysis of U–Pb age and Hf–O isotopes for detrital zircons from the Helanshan Complex in the westernmost part of the Khondalite Belt. Precise SIMS U–Pb zircon results suggest that the timing of the deposition of the Helanshan khondalite protoliths can be constrained between ca. 2.00 and ca. 1.95 Ga by the minimum age of the detrital zircons from the khondalites and the maximum crystallization age of ca. 1.95 Ga for the intruded granites. The ca. 1.95 Ga granites, the oldest granites identified within the Khondalite Belt, were coeval with the regional high-grade metamorphism.The Helanshan khondalite protoliths were sourced mainly from a provenance with prolonged, episodic magmatism of ca. 2.18 Ga, 2.14 Ga, 2.09 Ga, 2.06 Ga, 2.03 Ga and 2.00 Ga, respectively. Detrital zircons of ca. 2.18–2.00 Ga from the khondalites have ɛHf(t) values ranging from +8.9 to −2.9 and Hf TDMC model ages between 2.8 and 2.1 Ga, with two major peaks at 2.6 Ga and 2.3 Ga. Their δ18O values show two major peaks at 6.6‰ and 8.2‰. Zircon Hf–O isotopic data indicate that both the juvenile and ancient crustal components were involved in their source rocks, and the Helanshan khondalite protoliths were most likely sourced from a ca. 2.18–2.00 Ga continental arc. The juvenile continental arc materials were uplifted, eroded, transported, deposited and metamorphosed within a short time interval of <50 million years. With the predominantly euhedral crystals of the detrital zircons and the immature clastic sediments for the Helanshan khondalite protoliths, we suggest that the khondalite protoliths were likely deposited in an active continental margin, rather than a passive margin as previously thought.
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