GEOCHEMISTRY AND AGE OF THE WESTERN MÉLANGE BELT IN THE CASCADE FOOTHILLS AND EASTERN PUGET LOWLANDS OF WASHINGTON STATE: A RECORD OF MULTIPLE MESOZOIC ARC TERRANE ACCRETIONS
We utilized geochemistry of metasedimentary and metaigneous rocks and detrital U-Pb dates from metasandstones to constrain the formation of the WMB. U-Pb detrital zircon dates from three arkosic sandstones have youngest age distributions of 74, 87, and 96 Ma. The two younger samples have Precambrian detrital zircons. U-Pb detrital zircon ages from three lithic sandstones have youngest age distributions of 110, 159, and 166 Ma. Only the youngest lithic sample has Precambrian detrital zircons. Whole-rock geochemistry of metasedimentary samples have trace element values that are similar to modern arc derived sediments. One group of sediments, including lithic sandstones, have intermediate to mafic provenance affinities; while another group, including the arkosic sandstones, have intermediate to slightly felsic types. Whole-rock geochemistry of metaigneous samples are mostly tholeiitic with minor calc-alkaline affinities—most samples have arc affinities, with 2 MORBs and 1 OIB.
The WMB consists of Jurassic volcanic island arc terrane that has been thrust over latest Cretaceous continental arc with a granitic provenance. This is supported by our mapping of the internal WMB Sultan River and Lake Chaplain thrusts that places 74-96 Ma arkosic sediments structurally below both 159-155 Ma lithic metasediments and the metagabbro. Our data suggests that final WMB thrusting occurred after 74 Ma and after ~85-90 Ma thrusting of the mostly high-P, Northwest Cascade System nappes mapped to the north.