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

The western mélange belt (WMB) is a low PT assemblage of metasedimentary rocks, greenstones, metaintrusives, and rare ultramafic rocks. In our 7 quadrangle map area, the WMB consists of stratified metasedimentary and metaigneous rocks displaying low internal disruption. Matrix supported WMB mélange is uncommon in our area. WMB fossil ages range from Middle Jurassic to Lower Cretaceous, while zircon ages from metaintrusives rocks are Middle to Upper Jurassic; rare marble olistrosomes(?) have Permian fusulinids. The WMB metasandstones have been divided into three compositional facies: lithic; arkosic; and cherty. The lithic facies has a volcanic arc provenance and the arkosic facies has a granitic provenance.

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.