Regional remagnetization of Irish Carboniferous carbonates dates Variscan orogenesis, not Zn-Pb mineralization
2017
Paleomagnetic methods have been used in economic geology to
date mineralization in sediment-hosted ore deposits and thereby help
to develop ore deposit models and understand the geodynamic settings
in which mineralization can occur. However, paleomagnetic
ages are sometimes inconsistent with other geochronological techniques
and with geological observations. Here we test the veracity of
paleomagnetic ages for sediment-hosted ores through a study of the
Irish Midlands ore field. We find that unaltered rocks distal to mineralization
that are of equivalent age to the ore host sequence have
comparable characteristic remanent magnetic directions to those
previously derived from the ores. This indicates that remagnetization
of the rocks was probably independent of the ore-forming process.
Comparison with the apparent polar wander path for Europe suggests
an age of ca. 310 Ma for this event, consistent with the timing of the
Variscan orogeny. Fold test results support this, indicating the signal
was acquired after tilting and/or folding of the host rocks. Petrology
and magnetic data suggest that nanometric magnetite particles are
the remanence carrier. Based on independent geochronological and
geological constraints, we conclude that mineralization formed in
Ireland in the early Carboniferous coincident with basin development
and that paleomagnetic dates were reset during the later orogenic
overprint. Caution is therefore warranted in the interpretation of
paleomagnetic dates for ore systems, and geodynamic models for
mineral systems based on these may be erroneous.
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