Timing and Mechanism of Calcites in Fractures of Middle Ordovician of Northern Tarim Basin, North West China

Abstract

Absolute radiometric dating has huge potential to better understand diagenetic processes and evolution of carbonate platforms and has been applied to young strata successfully. Furthermore, recent development of U-Pb dating by laser ablation applied directly on thin sections makes it possible to date older carbonate.Middle Ordovician in Northern Tarim basin experienced multiple stages of tectonic movement and related karstification, and six types of vugs and fractures were recognized, including vugs, horizontal fracture, tectonic fracture, dissolved fracture, irregular fracture, and fault, filled with calcite partially or completely. Based on the analysis of petrographic sequences of the calcites, laser-based U-Pb dating was conducted, and additionally C, O, and Sr isotopes and REE were analyzed to verify the results of dating and corresponding diagenetic environment. U-Pb datings revealed that calcite in vugs formed at 344±20 Ma during early Hercynian; magacalcite in fault precipitated during 316.8 to 251.8 Ma of late Hercynian; calcite in irregular fracture formed at 118±26 Ma of Yanshanian, perfectly corresponding to the three tectonic episodes exerted important influences on the region. The diagenetic environments revealed by conventional C, O, Sr and REE analysis are consistent with the corresponding geological settings. Calcite in vugs, showing relatively positive C and negative O, similar 87Sr/86Sr to host rock, and obviously negative Ce anomalies, interpreted as the products of meteoric karstification related to the uplift of Tabei uplift during early Hercynian. The calcites in faults and tectonic fractures (including irregular fractures) showing obviously negative O, 87Sr/86Sr similar to seawater value of Cambrian, and REE pattern different from either meteoric or hydrothermal environments, revealing the features of burial formation fluids probably related to the activities of strike-slip faults induced by late Hercynian and Yanshanian tectonic movements. The consistence of petrographic sequence, dating results, diagenetic environments, and fluids properties revealed the reliability of laser-based U-Pb dating on the Paleozoic calcite. Furthermore, considering the close relationship of megacalcite and tectonic fractures to reservoirs, it is proposed that the main reservoirs were formed by strike-slip faults during late Hercynian, probably are just empty spaces in the fault and fractures, instead of previously thought karstification related to unconformity.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019