LAURENTIA INTRACONTINENTAL BREAKUP STRUCTURES AND PALEOGEOGRAPHY, ILLUMINATED BY DETRITAL ZIRCON AGE SPECTRA FROM NEOPROTEROZOIC-CAMBRIAN SANDSTONES OF REGIONAL EXTENT IN COLORADO, USA

Detrital zircon (DZ) U-Pb ages aid the investigation of the sedimentary provenance of basal Cambrian Sawatch Sandstone and Neoproterozoic remobilized siliciclastic rocks in central Colorado, with bearing on intracontinental paleogeography and tectonics of Laurentia. Eighteen DZ samples were obtained from 12 sites of clastic-crystalline contacts, separated by up to 120 km, and analyzed by LA-ICPMS at the Arizona Laserchron Center. Cambrian Sawatch Sandstone, sampled from quartz arenite beds overlying the Great Unconformity at three locations, lack 1.31 to 0.93 Ga zircons. U-Pb ages define sharp peaks at ca. 1.44 Ga (51% of analyses) and ca. 1.70 Ga (45%) on normalized probability plots. Five tightly clustered youngest grains in 1 sample produce a weighted mean age of 520 ± 11 Ma, for a maximum age of Sawatch deposition.

Neoproterozoic Tava sandstone, composed of unsorted fine-grained quartz arenite supporting isolated rounded pebbles, yields prevalent ages 1.31 to 0.93 Ga. The broad age peak in normalized relative age probability diagrams for all Tava samples correlates to the widespread U Pb age signature of Grenville orogen detritus, that is pronounced in DZ age distributions for other Neoproterozoic strata in Utah, Arizona and California. Kolmogorov-Smirnov (K-S) statistics show a high probability of correlation among the 15 Tava samples, broadening the Tava sandstone’s known areal extent to 25000 km², and moderately high correlation factors to other Neoprot. strata. This suggests that the Colorado siliciclastics initiated as part of the continental system carrying well-mixed detritus away from Grenville mountains to sites of deposition across a broad, low relief, intercontinental region. Four Tava sandstone injectite samples, however, display sharp narrow age peaks at ca. 1.45 and ca. 1.72 Ga that are attributable to local sources in the Routt and Berthod plutonic suites. The presence of locally derived, 1.7 and 1.4 Ga DZ in forcefully injected Tava sandstone requires erosion and exhumation of Proterozoic crystalline basement, well in advance of development of the Great Unconformity. There is a possibility of initation of fault-controlled depocenters. During Sawatch deposition, Grenville detritus was no longer available and DZs derived from local Proterozoic sources.