Complex sentence: Researchers believe this because the trace element geochemistry of the bentonite shows that its source was a felsic calc-alkalic magmatic source, which is characteristic of volcanism from a continental crust destructive plate margin setting.
Split into simpler sentences: Researchers believe this because the trace element geochemistry of the bentonite shows that its source was a felsic calc-alkalic magmatic source. This type of source is characteristic of volcanism from a continental crust destructive plate margin setting.