U-PB AGES OF DETRITAL ZIRCONS FROM PELAGIC LIMESTONES: APENNINE SAMPLES AND PROVENANCE ANALYSIS

We present what we believe is the first dating study of detrital zircons from slowly-deposited marine pelagic limestones. The samples came from the (1) Massignano (volcanic ash layer in Upper Eocene, Scaglia Cinerea fm.), (2) Gubbio Bottaccione (basal Paleocene, Scaglia Rossa fm.), and (3) Monte Acuto (Lower Cretaceous, Maiolica fm.) sections in the Umbria-Marche sequence in the Italian Apennines, each acquired by acid dissolution of several hundred kg of limestone. U-Pb age determination of individual zircons was done with Mark Pecha and Dominique Giesler at the Arizona LaserChron Center, using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS).

(1) The mostly euhedral prismatic zircon grains with internal oscillatory zoning patterns from the 6.15 meter level of the Massignano section yielded the expected mean age of 35.43 ± 0.40 (2σ) Ma. These zircons were probably carried by paleowinds as volcanic ash from the then-active island of Sardinia and deposited in an oceanic environment.

(2) The Bottaccione zircons, from a 20 cm layer immediately above the K-Pg boundary, exhibit rounded to subrounded shapes and a wide range of internal zoning patterns suggestive of metamorphic and resorption processes. The ages of these zircons range from 63.6 Ma to 947.6 Ma, with peak ages centering around 650 Ma, 463 Ma, and 336 Ma, falling within the range of the Pan-African, Caledonian and Variscan/Hercynian orogenies respectively. Central Asia (around Mongolia) and northwest Africa have previously been suggested as possible sources for dust in the basal Paleocene of the Scaglia Rossa. Our age data suggest Morocco, rather than Mongolia, as the probable the source for the dust, including zircon grains, found in the Bottaccione section.

(3) The Monte Acuto-Maiolica zircons yielded ages between 32 Ma and 2590 Ma and will require further study to interpret.

In this study, we demonstrate that valuable information can be obtained from detrital zircon grains deposited in open ocean environments as pelagic limestones. Future research, including closer examination of the geology of Sardinia, Morocco and Mongolia, along with more U-Pb geochronologic analyses of available zircons must be conducted to further validate and constrain the provenance of each of these samples.