Indirect dating of mafic intrusions by SHRIMP U–Pb analysis of monazite in contact metamorphosed shale: an example from the Palaeoproterozoic Capricorn Orogen, Western Australia

Abstract

In carbonaceous shales of the Palaeoproterozoic Maraloou Formation, Capricorn Orogen, Western Australia, the intrusion of subvolcanic dolerite sills related to interlayered basaltic lavas (Killara Formation) caused hydrothermal fluid circulation that resulted in the growth of sub-millimetre-sized monazite crystals. In situ U–Pb analysis of monazite from two shale samples about 50 m apart in a diamond drill-hole gives 207Pb/ 206Pb ages of 1843±14 Ma and 1841±22 Ma. Combined, the data yield an age of 1843±10 Ma (MSWD=1.11; n=19 of 22), which is interpreted as the age of intrusion by the sills. This date provides a minimum age for the Maraloou Formation, previously only constrained between 2200 Ma and 1800 Ma. The presence of peperites, along with the development of fine coke mosaics in intruded kerogenous shale indicates that the magma intruded wet, shallowly buried sediments, and thus, that the 1843 Ma date probably approximates the depositional age of the Maraloou Formation. Our results demonstrate the potential of dating mafic intrusive rocks by U–Pb analysis of low-temperature metamorphic monazite in contact aureoles.