Brigantian and Middle Triassic remagnetizations in Lower Carboniferous carbonates,Northern Ireland:Role of diagenesis and fluid flow

摘要

Paleomagnetism(18 sites,231 specimens)of Lower Carboniferous carbonates in Northern Ireland reveals three characteristic remanent magnetization(ChRM)components.Six sites from Brigantian limestones have a Middle Triassic(239 +-7 Ma)secondary chemical remanent magnetization(CRM)in hematite,likely from alteration of the limestones by oxidizing meteoric fluids when continental red beds were deposited immediately above.Twelve sites from early Asbian limestones retain ChRM directions residing in pyrrhotite and magnetite.Their paleopoles are statistically indistinct,but suggest that the pyrrhotite remanence(326 +-4 Ma)is about a million years younger than the magnetite remanence(327 +-3 Ma).More importantly,the primary ChRM in these limestones was reset approx 3 or 4Ma after deposition,probably by fluids involved in their diagenesis,giving secondary CRMs that are approx 8Ma younger than those,observed in the Lower Carboniferous carbonates that host the Navan Zn-Pb deposit in the Irish Midlands,suggesting two unrelated fluid histories.