PETROLOGY AND CHRONOLOGY OF A LATE CRETACEOUS ANATECTIC FRONT IN MONARCH CANYON IN THE FUNERAL MOUNTAINS, DEATH VALLEY NATIONAL PARK

The Funeral Mountains in Death Valley National Park expose Barrovian metamorphism in the footwall of the Tertiary Boundary Canyon detachment fault. In the highest grade part of the footwall, there is an apparent grade discordance across the Monarch Spring fault (MSF), with middle amphibolite facies rocks occurring above the MSF, and partially melted upper amphibolite facies rocks below. LASS-ICP-MS ages on monazite and xenotime date metamorphism at 96-70 Ma below the MSF. Detrital zircon data show that the MSF sits within the Neoproterozoic Horse Thief Fm while pressure-temperature (P-T) conditions determined from pseudosections require no pressure difference between upper and lower Monarch Canyon across the MSF. These observations suggest that there is little or no structural or stratigraphic offset across the MSF. A model in which the MSF represents a Late Cretaceous anatectic front may explain the discontinuity. The timing of Late Cretaceous metamorphism coincides with flat slab subduction. Water from the dewatering slab may have infiltrated the lower crust, causing anatexis and melt propagation. Thermodynamic modeling shows that migmatites in Monarch Canyon developed through both in-situ fluid-saturated partial melting and through the injection of externally derived melts, some of which may have been derived from within the subjacent Crystal Spring Fm. Trace element data from monazite and garnet indicate garnet growth in pelitic rocks continued through 78 Ma (possibly as late as 72 Ma). Three ages (164, 161, and 157 Ma) from a monazite inclusion in a garnet document that the rocks were first metamorphosed in the Jurassic, consistent with metamorphism at Chloride Cliff, to the south-east. A small population of ~130 Ma monazite ages (n=3) and a larger population of 95-70 Ma ages (n=191) document recrystallization during younger and higher temperature metamorphism. Recent improvements to thermodynamic activity models of amphibole have allowed us to develop pseudosections and P-T paths from garnet amphibolites. We made further improvements by calibrating thermodynamic properties for the Mn-tschermakite component of amphibole. Addition of Mn-tschermakite to the activity models results in accurate prediction of mineral assemblages and amphibole compositions at the estimated P-T conditions of the rocks.