A palaeomagnetic study of the Charnian, Caldecote, and Uriconian volcanics and plutons, central England

Abstract

This palaeomagnetic study has investigated intrusive and extrusive calc-alkaline igneous rocks comprising the preOrdovician basement of central England and the Welsh Borderlands and dated ca. 560-530 Ma. Demagnetization studies resolve a range of magnetic components in both the older volcanic successions comprising the eastern Uriconian volcanics, and in the younger intrusive episodes comprising the Charnwood and South Leicestershire outcrops. PostOrdovician overprinting is largely absent and the remanence record dates from ca. 560-420 Ma. Within this time interval field tests are inadequate to confirm a primary origin and some components may be overprints acquired in immediate post-Uriconian (Cadomian) or during late Ordovician (early Caledonian) thermotectonic events. A first order continuity is recognized on a palaeomagnetic scale between the data from the Midlands-Welsh Borderlands and the terranes exposed in Anglesey and Southwest Wales. The British terranes moved through low latitudes during the interval represented by the Uriconian episode; they crossed the palaeoequator during the folding of these rocks and prior to emplacement of the later intrusive suite. Collectively they had rotated through cu. 170' and moved into intermediate latitudes by mid-Lower Cambrian times. Post-Lower Cambrian movements on the Malvernian lineament are a possible explanation for second order differences between the palaeomagnetic records of the English Midlands and the Welsh Borderlands; the largest possible regional rotation here is a ca. 90' movement of the Charnwood block with respect to the other outcrops. Correlation with the Gondwana APW path places the Anglo-Welsh basement in the vicinity of a comparable calc-alkaline volcanic province linked to a subducting plate margin in the Afro-Arabian area. Subsequent continental breakup led to termination of this volcanic activity and establishment of a passive marine shelf environment later in Lower Cambrian times.KEY WORDS Charnian Caldecote Uriconian Palaeomagnetism Cambrian 1. REGIONAL SETTING For the purposes of this paper the Late Precambrian-Lower Cambrian basement of England and Wales is regarded as that part of the geological column south of the Iapetus suture which predates deposition of the Lower Cambrian quartzites in a quiescent shelf environment during the latter part of Lower Cambrian times (ca. 535 Ma). This basement only appears as outcrop at a number of small and scattered locations. The exposed rock units fall into four broad divisions namely: (1) low grade metamorphic rocks, mostly of igneous derivation (the Malvernian, Stanner-Hanter Complex, Rushton schist, and Primrose Hill gneiss); (2) the Charnian sedimentary and volcanoclastic succession; (3) the Charnian, Caldecote, and Unconian volcanics and plutons; and (4) the Longmyndian sedimentary succession. Chemical and isotopic evidence imply a derivation of the igneous and metamorphic suites from crust with an antiquity no greater than ca. lo00 Ma (Thorpe et al. 1984). This terrane clearly contrasts with crust ...