Some conclude that it formed at an ancient ocean-ocean convergent zone, where subduction-caused partial melting and metasomatism of the mantle as well as the intrusion of tonalites partially melted the overlying supracrustal rocks and created the first continental crust.

The same rock formation also contains a type of trilobite that was known to live 415 to 425 million years ago.

Since the rock formation contains both types of fossils the ago of the rock formation must be in the overlapping date range of 415 to 420 million years.

These isotopes break down at a constant rate over time through radioactive decay.

By measuring the ratio of the amount of the original (parent) isotope to the amount of the (daughter) isotopes that it breaks down into an age can be determined.

The belt contains variably metamorphosed mafic volcanic and sedimentary rocks.

The occurrence of boninitic geochemical signatures, characterized by extreme depletion in trace elements that are not fluid mobile, offers evidence that plate tectonic processes in which lithic crust is melted may have been responsible for the creation of the belt.Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites work best as index fossils.If the fossil you are trying to date occurs alongside one of these index fossils, then the fossil you are dating must fall into the age range of the index fossil. In a hypothetical example, a rock formation contains fossils of a type of brachiopod known to occur between 410 and 420 million years.We define the rate of this radioactive decay in half-lives.If a radioactive isotope is said to have a half-life of 5,000 years that means after 5,000 years exactly half of it will have decayed from the parent isotope into the daughter isotopes.Scientists can use certain types of fossils referred to as index fossils to assist in relative dating via correlation.