A PETROLOGIC PARADOX IN CENTRAL DEATH VALLEY, CALIFORNIA

A quartz monzonite stock in the central Death Valley volcanic field, in the vicinity of latitude 36^o00'00"N and longitude 116^o22'30"W, displays features seemingly incongruous with reason. A veneer of volcanic rock and sparse sedimentary breccia rests nonconformably on the stock, and has been deformed into a dome about it, apparently by upward movement of the quartz monzonite along nearly vertical faults. As there is no regional stress field that can account for this deformation, we conclude the mechanical energy for it was igneous in origin. A biotite Ar-Ar age of the stock (9.76 ± 0.25 Ma) and a biotite K-Ar age of the Shoshone volcanics, which comprises a part of the veneer (9.72 ± 0.20 Ma, mean of two measurements), are indistinguishable. Nevertheless, for the following reasons we believe these to be reliable emplacement ages. Quartz monzonite stock -- High biotite TiO₂ (5.49 wt. %), low K-spar obliquity (0.2), and magnetite-ilmenite geobarometry (emplacement <3km) indicate biotite cooled rapidly from a high temperature through its closure temperature at a shallow level. Microprobe analyses and the Ar-Ar degassing curve show no evidence of post-crystallization alteration or reheating of biotite. Shoshone volcanics (rhyolitic lava flows and tuffs) -- Biotite phenocrysts appear to be primary. Although the rocks have been hydrothermally altered, biotite is resistant to such effect. Even so, extreme alteration (relative Ar loss) produces anomalously young ages, as does re-heating. Because the observed biotite age is as old as it can be without being impossibly older than the quartz monzonite, we conclude that the biotite age is not anomalously young. Nor is biotite anomalously old due to extraneous Ar. Because this has a large disproportional effect on low-potassium minerals, coexisting plagioclase should appear older than biotite, but it does not. As implausible as it may seem, evidence indicates (a) the quartz monzonite stock and the nonconformably overlying veneer were emplaced at virtually the same time; (b) the stock continued to rise, propelled by forces of igneous origin, to deform the overlying veneer. Any attempt at understanding the origin of this stock must account for this paradoxical evidence. Acknowledgment: Lauren A. Wright kindly supplied unpublished K-Ar ages. Syed Almashoor and Gary Novak did the geochemical studies of the quartz monzonite. Anthony Schuetze assissted in the field work.