AUGITE TRACKS CHANGES IN JURASSIC ARC MAGMAS, KLAMATH MOUNTAIN PROVINCE (KMP), CA AND OR

Jurassic magmatism in the KMP was dominated by arc activity recorded in plutons, cumulate sections of ophiolites, metavolcanic rocks, and volcanogenic meta-sandstone. Use of bulk compositions to interpret magmatic affinities is complicated by the cumulate nature of many plutons, hydration and metamorphism of volcanic rocks, and the mixed nature of volcanogenic sands. Because many of these rocks contain augite, identification of magmatic affinities and processes may be done using major and trace element data on the augite. Early Jurassic samples are from the Rattlesnake Creek terrane (RCt); Middle Jurassic samples are from the western Hayfork terrane (wHt) and Ironside Mountain batholith (IMB). Late Middle Jurassic magmatism involved the outboard metavolcanic Rogue Fm and plutonic Chetco Complex, Josephine ophiolite (JO) gabbro, and inboard plutons of the Wooley Creek suite.

Augite in the wHt is Mg- and Cr-rich, whereas in the IMB is Fe-rich and very low in Cr. REE patterns of wHt augite indicate adakitic and relatively primitive calc-alkaline magmatism, but REE in the IMB augite are enriched and display deep Eu anomalies, indicating evolved, mildly alkaline affinities. RCt augite indicates affinities transitional between adakitic and calc-alkaline magmas.

Rogue Fm augite varies widely in Mg# and completely overlaps with Mg# in Chetco augite. REE patterns in augite from both units indicate arc tholeiite and primitive calc-alkaline parental magmas. Augite in the JO is Mg- and Cr-rich and displays the smallest compositional range of any unit studied. JO augite has REE patterns typical of arc tholeiite parents.

Augite in Wooley Creek suite plutons varies widely in Mg# and Cr content, consistent with the wide range of rock types in these plutons. The REE abundances mirror this; however, REE patterns are typical of calc-alkaline magmas and display small or no Eu anomalies.

The data indicate that Early–Middle Jurassic arc magmatism was variably calc-alkaline to adakitic to mildly alkaline, whereas late Middle Jurassic magmatism was mainly arc tholeiitic outboard and calc-alkaline inboard. Moreover, early studies in the KMP interpreted the wHt/IMB and the Rogue/Chetco to be volcano-plutonic suites. The augite data show the former to be impossible and the latter to be likely.