THERE AND BACK AGAIN: PUTTING ABSOLUTE AGE CONSTRAINTS ON A FULL, DIACHRONOUS, OROGENIC CYCLE (Invited Presentation)

Metamorphic belts record juxtaposition of rock that achieved different P-T-D conditions at different times. The final geometry of variably metamorphosed rocks is a function of prograde burial, heating, and imbrication overprinted by uplift and exhumation. Reconstructing this complex time-space evolution requires focused in situ geochronology with specific chronometric minerals and decay schemes targeted to record different stages of the cycle. This talk will highlight examples of how this approach is applied to metasedimentary rocks ranging from greenschists to granulites. This work draws on in situ laser ablation geochronology, MS/MS mass spectrometry, and a growing list of unconventional geochronometers.

Low-grade metasedimentary schists pose unique challenges for geochronology, most notably the persistence of detrital accessory minerals such as zircon, apatite, and micas. This is compounded by small grain sizes of neoformed metamorphic accessory minerals necessitating beam sizes <10µm. Nonetheless epitaxial overgrowths, micro-zircon inclusions, and new growth of Fe(Ti)-oxides can be used for U-Pb dating. Zoning in lepidoblastic micas might also reveal new generations of growth suitable for Rb-Sr dating. Amphibolite facies metasediments generate abundant conventional chronometric minerals including garnet that can be targeted for in situ MS/Ms Lu-Hf dating; new generations of biotite suitable for Rb-Sr dating are also produced. The challenge in these rocks is to relate dates to P-T-D conditions preserved by chemical zoning and textures. Migmatite and granulite normally contain large abundant zircon, monazite, and apatite but the question of whether these record prograde or retrograde conditions requires critical testing. In deeper parts of an orogen where temperatures exceed 650°C for (tens of) millions of years apatite is uniquely suited to assess rates of exhumation by examining diachroneity between U-Pb and Lu-Hf ages. Fabrics superimposed during exhumation can also be dated using texturally controlled mica Rb-Sr dating and cross-cutting aplite or pegmatite intrusions dated using apatite or complex oxides.

The value in this approach lies in the discovery of cryptic diachroneity and an appreciation of the richness of processes affecting regionally metamorphosed rocks.