What problems are encountered in dating metamorphic rocks
A thick sequence of essentially flat-lying sandstones (the Middle Proterozoic Kombolgie Formation) was then deposited unconformably on the Archean-Lower Proterozoic basement and metasediments.
At Koongarra subsequent reverse faulting has juxtaposed the lower Cahill Formation schists and Kombolgie Formation sandstone.
Zheng documented the copious reporting of this problem in the literature where various names had been given to these anomalous isochrons, such as apparent isochron, mantle isochron and pseudoisochron, secondary isochron, source isochron, erupted isochron, mixing line, and mixing isochron.
Similar anomalous or false isochrons are commonly obtained from U-Th-Pb data, which is hardly surprising given the common open system behavior of the U-Th-Pb system.
Some of the lowermost overlying Lower Proterozoic metasediments were accreted to these domes during amphibolite grade regional metamorphism (estimated to represent conditions of 5-8kb and 550-630°C) at 1800-1870Ma.
Because no geologically meaningful results can be interpreted from the U-Th-Pb data at Koongarra (three uraninite grains even yield a Pb age of 0Ma), serious questions must be asked about the validity of the fundamental/foundational basis of the U-Th-Pb dating method.
This makes the task of creationists building their model for the geological record much easier, since claims of U-Th-Pb radiometric dating having proven the claimed great antiquity of the earth, its strata and fossils can be safely side-stepped.
Radiometric dating has now been used for almost 50 years to establish beyond doubt the earth's multi-billion year geological column.
Although this column and its age was firmly settled well before the advent of radiometric dating, the latter has been successfully used to help quantify the ages of the strata and the fossils in the column, so that in many people's minds today radiometric dating has proved the presumed antiquity of the earth.