Dating of zircons

In the Geo Analytical Lab at Washington State University, U-Pb dating of zircon is carried out with a New Wave UP-213 (Nd-YAG 213 nm) laser system coupled with a Thermo Finnigan Element2 magnetic sector ICP-MS.

Prior to analysis, zircons are mounted with standards in and polished.

The dating results are as follows: (1) The granitoid intrusions occurred in the Late Triassic and Early Jurassic; (2) The earl-stage magmatism was nearly simultaneous with the folding of Triassic strata, suggesting that the magmatism might have related with a large detachment under the Triassic strata; (3) Some of the crystal zircons contain rounded or unregular cores in shape, and it is thought that the rounded zircons were captured from the wall rocks during the upwelling of the granitic magma or detrital zircons from partial melting of the lower crust.

Based on the SHRIMP dating, the core zircons yield ages of the Permian, early Paleozoic, late Paleozoic, and early Proterozoic, respectively.

The present of Proterozoic detrital zircons indicate that the Songpan-Ganzi terrane is presumably floored with Precambrian basement rocks.

In the case of the Kamchatkan, and possibly Olympic, turbidites, zircon He ages are partially reset. Typically these ages correspond to crystallization and exhumation or eruption ages, and their combination can be used to more confidently resolve candidate source terrains, establish maximum depositional ages, and constrain the thermal histories of orogenic source regions.Laser induced time-dependant elemental fractionation is corrected off-line using the regression line method (Sylvester and Ghaderi, 1997; Horn et al., 2000; Kosler et al., 2002).Geochronology and thermochronology on detrital material provides unique constraints on sedimentary provenance, depositional ages, and orogenic evolution of source terrains. We present examples from Mesozoic aeolian sandstones, both modern and Paleogene fluvial sediments, and active margin turbidite assemblages from the Cascadia and Kamchatka margins.In this paper we describe a method and case-studies of measurement of both U/Pb and (U-Th)/He ages on single crystals of zircon that improves the robustness of constraints in each of these areas by establishing both formation and cooling ages of single detrital grains. Important results include the fact that detritus from ancient orogens may dominate sediments thousands of kilometers away, crustal melting and exhumation appear to be spatially-temporally decoupled in at least two orogens, and first-cycle volcanic zircons older than depositional age are surprisingly rare in most settings except in the continental interior.Typically these ages correspond to crystallization and exhumation or eruption ages, and their combination can be used to more confidently resolve candidate source terrains, establish maximum depositional ages, and constrain the thermal histories of orogenic source regions. In the case of the Kamchatkan, and possibly Olympic, turbidites, zircon He ages are partially reset.