Monazite petrochronology dates Jurassic and Cretaceous cycles of prograde and retrograde metamorphism in the Funeral Mountains, California

Abstract

Pressure–temperature (P–T) modeling and U–Pb monazite petrochronology provide a detailed P–T-t history for the Funeral Mountains metamorphic core complex, revealing different aspects of the geologic history at different structural depths and enabling the dating of tectonic mode switching cycles in the southwestern US Cordillera. Monazite petrochronology and yttrium X-ray element maps reveal several generations of monazite that formed during the Jurassic to Late Cretaceous. In the Monarch Canyon study area, the staurolite-out isograd separates samples with predominantly Jurassic monazite from those with predominantly Cretaceous monazite. Monazite grains yielding Jurassic to Early Cretaceous dates are chemically distinct from those yielding mid- and Late Cretaceous dates. Jurassic monazite dates from the Funeral Mountains record both prograde and retrograde metamorphism, with the latter associated with garnet breakdown during decompression. Heavy rare earth elements and yttrium (HREE + Y) in a mid-Cretaceous 104 to 88 Ma monazite population link recrystallization to prograde garnet growth from staurolite breakdown, and in a Late Cretaceous 88 to 74 Ma population to retrograde garnet breakdown via a reversal of the staurolite breakdown reaction. Modeling and mineral textures indicate peak metamorphic conditions of 6–10 kbar at ca. 650–700 °C in the structurally deepest rocks in Monarch Canyon. In contrast, structurally shallower rocks experienced peak temperatures between 610 and 650 ºC during Jurassic metamorphism. Monazite petrochronology elucidates the progression of monazite dissolution-reprecipitation along this P–T path. Modeling reactions and mineral stability link specific reactions to changes to the HREE + Y concentrations in monazite, particularly related to garnet and staurolite reactions. This dataset, in conjunction with previous studies, enables the timing and duration of tectonic mode switching cycles in the Funeral Mountains to be quantified, improving our understanding of the complex geological evolution of this core complex.