UC Santa Barbara

New geologic mapping, 40Ar/39Ar geochronology, and whole rock major and trace element and Sr-Nd isotopic data of pre-, syn-, and post-extensional volcanic rocks shed light on the nature of Miocene volcanism in the lower Colorado River Extensional Corridor, CA and AZ. Detailed geologic mapping of the western Whipple Mountains and eastern Mopah Range, where the thickest accumulation of Miocene volcanic rocks is exposed in steeply tilted (~76° SW) fault blocks, reveal a four-stage volcanic history spanning the onset of normal faulting.

The earliest volcanism in this area was effusive fissure and shield eruptions of basalt and basaltic andesite totaling ~25 km3 and producing a ~700 m thick composite surface of subdued topography. New geochronology from correlative exposures of these early lavas on the east and west sides of the corridor precisely date this early mafic volcanism from 21.1-20.0 Ma, improving existing constraints on the inception of volcanism in this part of the LCREC. The second stage consisted of primarily effusive silicic volcanism erupting ~56 km3 from ~20-19.5 Ma, and built a 1500-2000 m thick tablelands of overlapping rhyolite and dacite lava domes and tabular flows with subordinate pyroclastic surge and flow deposits and minor interfingering mafic lava flows. ~1.5 m.y after the onset of volcanism, three generations of normal faults formed across the field area resulting in rapid block tilting to the SSW and accumulation of syn-tectonic sediments and basaltic andesite lavas (third volcanic stage) of variable thickness in subsiding half grabens. Dips of syn-extensional strata fan upward from 67-14° SW. Total extension increased westward towards the Whipple Mountains and southwards, accommodated by rotational scissor faults. Onset of normal faulting and peak rates of associated tilting (~2.7x10-4 degrees/yr at ~19.5 Ma) immediately followed the period of peak eruption rate (3.9x10-4 km3/yr from 20.1-19.9 Ma and 4.2x10-5 km3/yr from 19.6-19.4 Ma) in the study area. The third stage of volcanism consisted of ~7.5 km3 of basaltic andesite eruptions in the study area beginning at 19.5 Ma and persisting until ~18.8 Ma, by which time extensional tilting had ceased. In the fourth and final volcanic stage, volcanic units were channelized in fault bounded paleo-valleys, beginning with emplacement of the 18.8 Ma Peach Springs Tuff followed shortly after by scattered eruptions of ~0.04 km3 of basalt and lastly of ~0.4 km3 of andesite between ~18.8-16.3 Ma. Our volume estimations for the study area suggest that nearly 490 km3 of total lava may have been erupted across the greater lower CREC in the vicinity of the Whipple Mountains.

78 Samples from the western Whipple Mountains and eastern Mopah Range (study area discussed above), Mohave Mountains, Aubrey Hills/Standard Wash, southern Whipple Mountains, and Turtle Mountains were analyzed to characterize the chemical evolution of Miocene volcanism. Pre-extensional samples are dominantly bimodal, ranging from 48.5 to 73.1 wt. % SiO2, with a gap from 54.3 to 67.1 wt %, while syn- and post-extensional lavas are dominantly mafic and intermediate, ranging from 50.0 to 60.2 wt. % SiO2 with minor post-extensional rhyolites (~70 wt. % SiO2). Many mafic and intermediate lavas are altered in the LCREC, but fresh lavas define linear trends on variation plots. 87Sr/86Sr and ɛNd ratios of 18 samples range from 0.706087 to 0.711366 and -1.23 to -12.37 respectively and correlate negatively with each other, indicating that assimilation of enriched crust played an important role in the evolution of Miocene lavas. The most isotopically primitive sample analyzed (a post-extensional basalt; 87Sr/86Sr = 0.706087, ɛNd = -1.23) is more primitive than the ancient enriched lithospheric source proposed by several authors as the likely mantle source of CREC lavas, suggesting that at least some CREC lavas represent partial melting of an OIB-like mantle source with modification by crustal assimilants affecting the most mafic lavas.