Zircon petrochronology in large igneous provinces reveals upper crustal contamination processes: new U–Pb ages, Hf and O isotopes, and trace elements from the Central Atlantic magmatic province (CAMP)

Davies, J. H. F. L.; Marzoli, A.; Bertrand, H.; Youbi, N.; Ernesto, M.; Greber, N. D.; Ackerson, M.; Simpson, G.; Bouvier, A.-S.; Baumgartner, L.; Pettke, T.; Farina, F.; Ahrenstedt, H. V.; Schaltegger, U. (2021). Zircon petrochronology in large igneous provinces reveals upper crustal contamination processes: new U–Pb ages, Hf and O isotopes, and trace elements from the Central Atlantic magmatic province (CAMP). Contributions to mineralogy and petrology, 176(1) Springer-Verlag 10.1007/s00410-020-01765-2

[img]
Preview
Text
Zircon_petrochronology_in_large_igneous_provinces_.pdf - Published Version
Available under License Creative Commons: Attribution (CC-BY).

Download (4MB) | Preview

Zircon occasionally crystallizes in evolved melt pockets in mafic large igneous province (LIP) magmas, and in these cases, it is used to provide high-precision age constraints on LIP events. The precision and accuracy of high-precision ages from LIPs are crucially important, because they may be implicated in mass extinctions. However, why zircon crystallizes in these magmas is not clearly understood, since their mafic compositions should limit zircon saturation. Here, we investigate the occurrence of zircon (and baddeleyite) in intrusive and extrusive mafic rocks from Central Atlantic Magmatic Province (CAMP) using petrography, trace-element analysis, Ti temperatures, Hf and oxygen isotopes, and high-precision U–Pb geochronology, along with petrological and thermal modeling. We provide new ages for CAMP sills that intruded into Paleozoic sediments in Brazil, indicating that the high and low Ti magmatism in this area occurred synchronously over 264 ± 57 ka. We show that upper crustal assimilation, especially of shales, during the emplacement of the CAMP likely led to zircon saturation. Assimilation of upper crustal sediments is also supported by high δ18O values and some rare negative εHf values in the zircon crystals. The only extrusive sample analyzed was the North Mountain basalt in Nova Scotia, Canada. This sample contains a large age variation in its zircon crystals (up to 4 Ma), and the older crystals have slightly more negative εHf values suggesting the presence of small (micron scale) xenocrystic cores associated with very late-stage sediment assimilation. However, the CAMP dataset as a whole suggests that the presence of xenocrystic cores is rare. Assuming no xenocrystic cores, and considering the zircon undersaturated nature of LIP mafic melts, the oldest zircon age clusters in a population should record the magma emplacement (or time when assimilation occurred), and the younger ages in a population are more likely to reflect Pb loss, especially given the high U concentrations of LIP zircon. Our identification of heterogeneous isotopic and elemental compositions in LIP zircon indicates that zircon in these magmas saturate in isolated minute melt pockets just before the system cools below its solidus.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences

UniBE Contributor:

Greber, Nicolas and Pettke, Thomas

Subjects:

500 Science > 550 Earth sciences & geology

ISSN:

0010-7999

Publisher:

Springer-Verlag

Language:

English

Submitter:

Nicolas Greber

Date Deposited:

02 Feb 2021 11:53

Last Modified:

07 Feb 2021 03:02

Publisher DOI:

10.1007/s00410-020-01765-2

BORIS DOI:

10.48350/151164

URI:

https://boris.unibe.ch/id/eprint/151164

Actions (login required)

Edit item Edit item
Provide Feedback