Using Zircon Isotope Compositions to Constrain Crustal Structure and Pluton Evolution: the Iapetus Suture Zone Granites in Northern Britain

Abstract

The Trans-Suture Suite (TSS) of calc-alkaline granite plutons straddle both sides of the Iapetus Suture in Northern Britain. Their emplacement during the early Devonian post-dates subduction of the Iapetus Ocean and their origin and tectonic relations have proved difficult to reconcile with tectonic evidence for orogenic convergence and uplift. We report the first in situ study of zircon U^Pb, O and Hf isotopes from magmatic zircons from three TSS granites. Ages of 410 � 6 Ma for the Criffell pluton, 416 � 5M a for the Shap pluton and 410 � 3 Ma for the outer zone of the Fleet pluton are coincident with the intrusion of regionally prolific lamprophyre dykes within transtensional tectonic environments. Resolvable age differences between the outer and inner two zones of the Fleet pluton (387 � 5 Ma) suggest two distinct stages of emplacement that bracket an � 10 Myr phase of transpression recognized from geological evidence. Mean zircon oxygen isotope compositions (d 18 O) range from � 5·0o to � 9·0o and generally increase in tandem with inter-grain isotope heterogeneity in more silicic magmas, providing evidence for increased additions from sedimentary sources in addition to the involvement of more mafic magmas. Magmatic zircons from dioritic enclaves from the Criffell granodiorites have U^Pb ages up to � 9 Myr older than their host rocks and have distinct oxygen isotope population distributions. It is suggested that these may represent entrained, cognate material derived from deeper crustal hot zones. Initial eHf values from the three plutons are distinct from each other, show little or no variation within plutons and differ substantially from mantle values, requiring significant crustal reworking. Zircon Hf model ages (0·9^1·0 Ga) indicate that most reworked crust was of Avalonian origin, consistent with geophysical evidence for underlying Avalonian crust beneath the Iapetus Suture.