HIGH-PRESSURE METAMORPHIC EVENT RECORDED IN AMPHIBOLITE IN THE JARBO GAP OPHIOLITE, CENTRAL BELT, SIERRA NEVADA, CALIFORNIA

The Central Belt in the northern Sierra Nevada consists of Triassic-Jurassic arc and subduction complex rocks. The Jarbo Gap ophiolite, which is the northernmost exposure of the Central Belt, consists of an ophiolitic mélange of mafic-to-intermediate volcanic rocks, ultramafic rocks, metagabbro, metaclastic rocks, and chert. Cross-cutting intrusions have been dated to 206±43 Ma and 204±2 Ma using Sm/Nd and U/Pb zircon geochronology, respectively. Large parts of the Jarbo Gap ophiolite have been metamorphosed to amphibolite facies, with a report of mafic garnet amphibolite. The origin of the high-temperature metamorphism is unknown. While high-pressure minerals have been reported in other parts of the Central Belt leading to speculation that the Jarbo Gap ophiolite preserves a metamorphic sole, high-pressure mineral assemblages have not been described from these rocks. Here we present petrographic evidence of high-pressure metamorphism in the amphibolite-grade rocks at Jarbo Gap.

Mafic amphibolite is exposed in a continuous broad band in the Jarbo Gap ophiolite bounded by ultramafic-dominated units. It consists of moderately foliated green to green-blue hornblende, clinozoisite, epidote, titanite, and plagioclase. A greenschist mineral assemblage of actinolite and chlorite overprints the rock. Locally, rutile is present in the core of titanite, indicating the high-pressure origin of these rocks. These high-pressure rocks stand in contrast to rutile-free amphibolite which may represent low-pressure rocks of the unsubducted upper plate.

The amphibolite body at Jarbo Gap has been proposed to be a metamorphic sole, however, the 7-km map width of the unit is inconsistent with structurally thin soles present in other ophiolites. At this point the extent of the rutile-bearing amphibolite is not clear. Rutile has so far been found along opposite sides of the amphibolite body adjacent to the ultramafic-dominated units. Additional work will be done to determine whether the high-pressure amphibolite extends across the entire body or is only present adjacent to ultramafic rocks. This rutile-bearing amphibolite thus represents the northern along-strike extent of high-pressure, high-temperature rocks in the Central Belt.