EXPLORING THE GEOCHEMICAL CHARACTER OF VOLCANISM IN THE WHIPPLE MOUNTAINS REGION, CA AND AZ; DID CRUSTAL MELTING PLAY A ROLE IN THE EXTENSIONAL COLLAPSE OF THE COLORADO RIVER EXTENSIONAL CORRIDOR?

Geochemical and isotopic data from pre-, syn-, and post-extensional lavas shed light on the Miocene magmatic and tectonic evolution of the Lower Colorado River Extensional Corridor (LCREC) in the vicinity of the Whipple Mountains (CA and AZ). The 20.5-16 Ma lavas are bimodal and range from basalt and trachybasalt to rhyolite. Bulk-rock major element data indicate that the suite follows a high-K calc-alkaline differentiation trend. Isotopic data indicate that crustal melting and assimilation play an important role in the evolution of these magmas.  ⁸⁷Sr/⁸⁶Sr_(i) values range from 0.706093 to 0.711527 while eNd_(i) values range from -1.23 to -12.37, and the two isotope ratios correlate negatively. ⁸⁷Sr/⁸⁶Sr_(i) increases and eNd_(i) decreases with increasing SiO₂. The importance of crustal mixing in the evolution of these magmas is further demonstrated by linear arrays on major oxide ratio-ratio mixing diagrams.

Eruption of a 1 km thick section of mafic lava flows and a 1-0.5 km thick section of silicic lavas flows and domes occurred over 1.5 million years. This eruptive period ended with a dramatic extensional event (19 Ma) that resulted in 40-60° of block rotation across the corridor. After this, volcanism was characterized by less frequent, smaller volume, primarily mafic eruptions. The fact that extensional block rotation followed so closely on the heels of the sustained, voluminous eruption of crustal-hybrid lavas suggests that the temporal link is more than just coincidence. We argue that the geochemical data is consistent with the hypothesis that thermal weakening of the crust drove extensional collapse within the LCREC.