Characteristics by Provenance of Quartz Sandstone from the Neoproterozoic Changlongshan Formation in the Offshore Liaodong Bay and Its Geological Significance
[Objective]The quartz sandstone of the Changlongshan Formation in the Qingbaikou system from the Neoproterozoic was encountered during drilling in the southwestern Liaodong Bay,Bohai Sea.The in-depth study of its provenance characteristics is of great significance to the anatomy of major geological events from the Archean to Proterozoic in the North China Craton.[Methods]Five quartz sandstone samples collected from around 2482 meters in well BD4-1a were analyzed for whole-rock geochemistry and detrital zircon U-Pb geochronology.[Results]The quartz sandstones had high SiO2 content(88.53%-93.06%),the chemical index of alteration(CIA)value was 69.5～71.6,the index of compositional variability(ICV)value was 0.69-3.99,and the samples displayed high composition-al maturity,structural maturity,and moderate weathering.The Th/U ratio ranged from 3.46-4.92,the Cr/Zr ratio of 0.17～0.35.The sum of the rare earth elements(ΣREE)is(8.26-24.12)×10-6,the low/heavy rare earth element ratio(LREE/HREE)was 6.28～10.21,and the δEu was 0.77-1.04,which shows a weak negative anomaly.The U-Pb ages of detrital zircons ranged from 2744-1581 Ma;the main age peak was～1845 Ma,and the subordinate age peaks were～2225,～2340,～2600,～2725 Ma,respectively.[Conclusions]Whole-rock geochemistry indicates that the provenance mainly comes from felsic rocks of the upper crust.The 2.7-2.5 Ga detrital zircons record the growth and tectonothermal events of the continental crust in the North China Craton.The 2.2-1.8 Ga detrital zircons correspond to the crystalline basement formation during the amalgamation of the eastern and western blocks of the North China Craton,recording the geological events of the Columbia supercontinent's assembly.The 1.8-1.6 Ga detrital zircon ages likely record the breakup of the Columbia supercontinent.
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