EVIDENCE FOR LATE ADAKITE MAGMATISM IN THE TRIASSIC-JURASSIC ARC, SIERRA NEVADA CENTRAL BELT, CALIFORNIA

The Central Belt of the Sierra Nevada, California is composed of ophiolitic, arc, and subduction complex rocks assembled during Triassic-Jurassic subduction along the Cordilleran margin. Rocks near Jarbo Gap, north of Lake Oroville, are the northernmost exposures of the Central Belt. Recent work shows that these rocks consist of an upper-plate ophiolite and lower-plate subduction complex. The ophiolite is comprised of ultramafic rocks overlain by a relatively thin sequence of mafic and pelagic sedimentary rocks. The subduction complex is made up of imbricated and folded rutile- and garnet-bearing mafic amphibolite and metachert. Rocks of the subduction complex have partial melt textures and leucosomes that indicate a late high-temperature partial melt event.

Mafic-to-intermediate dikes cross-cut the ultramafic and overlying sedimentary sections of the ophiolite. The dikes generally contain hornblende and plagioclase phenocrysts. Based on high MgO, Ni, Cr, Sr, and La/Yb from a small number of these dikes at a single locality, previous studies interpreted these rocks as boninites formed at the initiation of subduction. However, a reassessment of the data shows that the geochemistry does not match modern criteria for boninites, especially the high La/Yb, which is more consistent with adakitic magmatism.

Preliminary whole-rock geochemistry on a larger number of these dikes from multiple localities at Jarbo Gap, including several that cross-cut the sedimentary cover of the ophiolite, show these dikes to have mean values of SiO₂=55%, Al₂O₃=15%, MgO=6%, Sr/Y=38, La/Yb=12, Sr=530 ppm, Ni=130 ppm, and Cr=240 ppm. Based on the high values of Sr/Y, La/Yb, MgO, Ni, and Cr; depleted HREE; and low SiO₂ we interpret these rocks as low-silica adakites (LSA). We note however that they have lower Sr than is usually reported in LSAs.

LSAs are thought to be generated by the interaction of melt from a garnet-bearing slab with a thin mantle wedge. This has been proposed to occur during subduction of a ridge or melting of the edge of a slab window. Whereas the LSAs at Jarbo Gap could have been caused by these processes, we instead propose that they may have been generated by melting of the high-pressure subduction complex that underlies the ophiolite.