DETRITAL ZIRCON-BASED RECONSTRUCTION OF THE RIFTED LAURENTIAN MARGIN IN THE SOUTHWESTERN SIERRA NEVADA MOUNTAINS: FROM PROTEROZOIC BASEMENT, THROUGH NEOPROTEROZOIC RIFT ZONE, TO EXOTIC EARLY MESOZOIC TURBIDITES

Detrital zircon U-Pb dates from 13 metamorphic pendants in the Greenhorn Mountains (SW Sierra Nevada) explicate the constitution of Laurentia’s southwest margin. Discordance modeling and filtering removes effects of Cretaceous lead-loss to reveal four suites comprising this region’s pre-Mesozoic arc framework: (1) Two gneissic pendants adjacent to the 0.706 isopleth are interpreted as derived from the Mojave Province and its Mesoproterozoic cover, being characterized by a large ca. 1.7 Ga age peak, a small 1.4 Ga peak, and a few Archean- Paleoproterozoic zircon. (2) Two pendants composed of ribbon cherts, carbonates and basalt contain a similar zircon age distribution intermixed with rare (n=1–5) early rift-aged zircons (693 – 660 Ma). (3) Schists and quartzites from four pendants contain the ca. 1.75 – 1.4 – 1.1 Ga zircon age triad characteristic of Laurentia-derived strata. These rocks also contain rare, late rift-age zircon (650 – 550 Ma), analogous to passive margin strata exposed east of the Sierra Nevada Mountains. (4) Two pendants include quartz-rich strata of low metamorphic grade yielding 1.75 – 1.4 – 1.1 Ga zircon and abundant, early rift-aged zircon (ca. 683 Ma, n=25/307; ca. 660 Ma, n=40/311). These rocks may record deposition proximal to a branch of the expanding rift zone. The framework of the southwestern Cretaceous arc also contains two early Mesozoic overlap sequences: the Kings Sequence, interpreted as a Jurassic arc volcanic-volcaniclastic blanket; and chert-argillites of the Calaveras Complex. As expected, two Kings Sequence turbidites contain an arc-plus-Laurentian zircon assemblage that provide Early Jurassic maximum deposition age (MDA) estimates (ca. 182 Ma). In contrast, turbidites associated with the Calaveras Complex yield older MDA estimates (ca. 194, 253, 300 Ma) and contain zircons like those in the exotic Sierra City mélange (Paleozoic Shoo Fly Complex): a dominant 1.2 – 1.0 Ma age peak accompanied by ample 625 – 580 Ma and 420 – 365 Ma age peaks. Zircon eHf data distinguish the 700 – 500 Ma zircon in the Kings Sequence from those in the Calaveras Complex. We propose that late Paleozoic-early Mesozoic turbidites were shed eastward into pelagic Calaveras basins from an offshore exotic terrane. Sample distribution suggests minimal translational disruption within the Greenhorn Mountains.