ELSVIER Earth and Planetary Science Letters 144 ( 1996) 43 I-433 EPSL U-Pb, Pb-Pb and Sm-Nd dating of authigenic monazite: implications for the diagenetic evolution of the Welsh Basin Jane Evans a,*, Jan Zalasiewicz b*’ zyxwvutsrqponmlkjihgfedcbaZYXWVUT ’ NERC zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Isotope Geosciences Laboratory. Keyworth. Nottingham NC12 5GG. UK ’ Department of Geology. Unirer.si& of Leiceste,: Unic,er.~ity Road, Leicester LEI 7RH. UK Received 4 January 1996; accepted 6 September 1996 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR Abstract Authigenic monazite nodules from Silurian turbiditic rocks in the central Welsh Basin are zoned with respect to Nd isotope composition. The cores are relatively radiogenic, and the rim-to-core trend projects toward the composition of associated, very early authigenic, apatite concretions. These authigenic phases therefore equilibrated with changing fluid composition as they formed. The apatites and the monazite nodule cores may have included significant Nd from organic matter or seawater, whereas the REEs in the outer parts of the monazite nodules were derived from elastic detritus. The apparent Sm-Nd ‘age’ obtained from these phosphates is a mixing line, which gives an Ordovician age for these Silurian rocks. The U-Pb isotope system does not show similar trends, and shows promise as an alternative method for dating such phosphates: a date of 417 f 11 Ma 20 (MSWD = 107) agrees well with recent estimates of the time of the diagenetic, smectite-illite transition in the rocks of this study. A PbbPb isochron gave an age of 422 + 24 Ma 2a (MSWD = 0.9) this shows that the Pb is in isotopic equilibrium and is undisturbed, and that the elevated MSWD of the U-Pb regression is caused by slight subsequent mobility in the U-Pb system. The event dated by the monazite is probably compactional dewatering during accelerated mid-Silurian sediment loading in the central Welsh Basin. Krywordx: U/Pb; diagenesis: monazite: mudatone: Welsh Basin: Sm/Nd 1. Introduction The Welsh marine sedimentary basin existed be- tween Cambrian and Silurian times and comprises a sequence of Palaeozoic sedimentary and volcanic rocks. The basin was founded on the continental crust of a microcontinent called Eastern Avalonia. which rifted away from Gondwana in the early Or- * Corresponding author. Fax: +44 115 9363200. E-mail: j.evans@nigl.nerc.ac.uk ’ Fax: + 44 116 2523918. E-mail: jazl @le.ac.uk dovician and moved northward to impinge upon Laurentia in the early Silurian [l]. Final collision with Gondwana with the assembled parts of Laura- sia. Baltica and Avalonia resulted in the Acadian Orogeny of early to mid Devonian age. This colli- sion caused shortening and deformation of the pre- Acadian rocks. Sedimentation rates during the Or- dovician were moderate (an estimated maximum thickness of 1000 m for the combined Caradoc and Ashgill strata [2]) but increased considerably during the Silurian, when up to 4200 m of tutbidite and mudrocks were deposited [2,3]. The changing basin 0012-821X/96/$12.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PiI SOOl2-821X(96)00177-X