Zircon u-pb dating and hf-o isotope characteristics of changchengian alkaline rocks from the yanliao rift in the north China craton

Abstract

Mafic alkaline rocks are geochemically enriched in alkali and incompatible elements and genetically related to extensional settings. They are derived from enriched mantle, and thus can decipher the mantle metasomatism and deep geodynamics. The Changchengian alkaline rocks, outcropped in the Yanliao Rift at northern margin of the North China Craton. include the potassium volcanic rocks of the Tuanshanzi and Dahongyu formations along with the sodium dike intruding into the Chuanlinggou Formation. SHRIMP zircon U-Pb isotope chronological dating of the volcanic rocks from Pinggu-Jixian area yield concordant ages of 1613 ± 11 Ma, 1634 ±18Ma (the Tuanshanzi Formation) and 1605 ± 19Ma, 1630 ± lOMa (the Dahongyu Formation), respectively. All these consistent ages within analytical error indicate the potassic volcanics were short-lived eruption, broadly coeval with the ca. 1625Ma sodic dikes. Tlie potassic volcanic rocks and sodic dikes display similar zircon Hf-0 isotope compositions, i. e. . £w( I) ranging from-2 to +4 with a Gaussian distribution pattern peaking at +0.6. while ranges from 4. 5%c to 7. l%c mostly higher than normal mantle zircon value. The potassic volcanic rocks and sodic dikes, composed of K-rich minerals such as hornblende or phlogopite. are originated from the common enrichment mantle which was previously metasomatized by elevated materials. The North China Craton underwent multi-rifting events during the Meso- A nd Neo-proterozoic. their zircon Hf isotopes abruptly shift from enrichment to depletion at ca. 1. 32Ga. suggesting that the lithospheric mantle underwent strong metasomatism. Tlie Mesoproterozoic mafic rocks in North China Craton can play a key role for testing the mechanism of supercontinental breakup driven by mantle plumes or/and deep plate subduction.