Ordavieian volcanicity By CHRISTOPHER J. STILLMAN Reconstructions of World palaeogeography through the Lower Palaeozoic period suggest extensive movement of lang-lived continental or micro-continental mas­ ses which produced relatively shortlived ocean basins. These basins opened by rifting, spread with the formation of new ocean crust and then closed by sub­ ductian of that crust with the attendant development of island arcs, marginal ba­ sins and active continental margins. It was in these situations that the great majo­ rity of Ordavieian volcanic rocks were erupted. Volcanics are found sporadically throughout Lower Palaeozoic strata and evidence of world-wide sustained cli­ maxes of volcanic activity are preserved commonly in late Cambrian to early Ordavieian and in middle to late Ordavieian sequences. There is undoubtedly a connection between geotectonic situation and the type and intensity of volcanism. Products of initial crustal rlfting are widely seen in the Cambrian and post-orogenie continental volcanism was common in the Devonian , but Ordavieian activity was dominantly subduction-related, pre- or syn-orogenic, occurring in submarine, island, coastal or eardilleran environ­ ments, often situated on crust whose instability was responsible for an abundan­ ce of distinctive features in the volcano-sedimentary record. The characteristics of this volcanism and the nature of its products is described, with reference to present day analogues. The chemistry of volcanic rocks is largely controlied by their teetona-magma­ tic environment and geochemical descriminant analysis has been widely used as a means of distinguishing between the environments. By such means Ordavieian volcanic rocks within, for instance, the Caledonide orogen have been recognised as the products of eruption above subductian zones bordering Iapetus; on contin­ ental plate margins that came tagether in collisions that largely destroyed the oceanic lithosphere , preserving i t only in small remnants in obducted slices. The substantial replacement of volcanism by plutanie activity in late Silurian to De­ vonian times is believed to be due to the suturing of the plates which common­ ly terminated subduction. Whilst adverting to the world-wide distribution of Ordavieian volcanism, emphasis in this paper is placed on the lapetus region. In particular the volca­ nie rocks of the British and Irish CaJedanides are described in some detail as an illustration of features which might be expected anywhere in similar teetona­ magmatic situations. C. J. Stillman, Department of Geology, Trinity College, Dublin, !reland. Introduction closed with extensive subductian and attendant development of island arcs and marginalba sins. Many of the problems which face those who The "docking" of continental masses with a see k to reconstruct the palaeogeography of the variety of relative motions ultimately created World in Ordavieian times derive from the diffi­ compressive or transpressive orogenie events, culty of positioning accurately the remarkably deforrning and dislocating the sequences of mobile continental masses of that time. Palaeo­ rocks which were formed in the Lower Palae­ magnetics and terrain analysis suggest the rnave­ ozoic at the margins of these continents; ment of relatively Jong-lived continental or rocks which include the great majority of Ordo­ microcontinental masses producing relatively vtctan volcanics. There is undoubtedly a short-Iived ocean basins which opened and connection between the geotectonic activity Bruton. (ed. /11 D. L. J, 19g4_ Aspects vf the Ordodeian Sysrem. nB- 194. Palaeonwlogical Conrributions from the University of Oslo, No. 295 . Universi­ 183 tctsforlaget. and the intensity, extent and type of volcan­ flows and hyaloclastites are deposited tagether icity; volcanic rocks are found sporadically with pelagic and hemipelagic sediment. The is­ throughout the Lower Palaeozoic, but wide­ land-are volcanoes, on the other hand, are gene­ spread and sustained periods of volcanic activi­ rated by calc-alkaline magma which is largely ty are common only in late Cambrian to early intermediate in composition, containing higher Ordavieian times and again in the middle to up­ concentrations of volatiles and producing pre­ per Ordovician, in both ofwhich periods world­ dominantly explosive eruptions which generate wide climaxes of activity have been recorded. pyraclastic ejecta. This, in a marine environmen t Almost everywhere these volcanics erupted devoid of terrigenous detritus, provides the vol­ in island arcs, marginal basins or volcanically canie sediment which is the principal infill to active continental margins. Their magmas be­ the adjacent. back-are basins. An example is longed to either or both of two magma "se­ the Granada Basin west of the Lesser Antilies ries"; l) the "Orogenic MagmaSeries ", a basalt­ are of the Carribean, where volcanic sediment andesite-dacite association with chemistry input has added 7 km of sediment in the 47Ma ranging from sub-alkaline island-are tholeiite since the inception of volcanism (Siggurdson through calc-alkaline suites to patash-rich sho­ et al. 1980). The bed-forms, nature of grading shonites; or: 2) a characteristically bimodal as­ and sarting depend both on the character of the sociation oftholeiitic basalt and calc-alkaline to source and the transport mechanism. Three of alkaline rhyolite, often with a "within plate" the major forms of transport described by Fish­ chemistry. By analogy with modern examples, er (197 1), are all important in volcanic sedi­ these Ordavieian volcanoes are believed to have ments; these are slides, sediment gravity flows erupted above subductian zones, and the dra­ and suspension fall-out. Individual volcanoes matic reduction of volcanicity and its replace­ provide point sources which may build up un­ ment by essentially plutanie activity in late Si­ stable piles of volcaniclastic material. These lurian to Devonian times is believed to be due may shed sediment gravity flows which initiate to suturing resulting from continental plate coll­ as debris or grain flows and extend down-slope isions which, in most cases, terminated the and across the basin floar as turbidites. The active subductian and initiated the variety of latter may demonstrate diagnostic fe atures such eratonising events which make up the end-Cale­ as the doubly graded sequence recognised by donian orogeny. Fiske & Matsuda (1964) in the submarine ash flows of the Tokiwa Formation in Japan. In some cases major deposits can be correlated di­ Characteristics of subduction-related rectly with individual ignimbrite eruptions volcanism which commonly provide the largest volume of Present-day analogues indicate two principal sediment; a fine example is the Minoan erup­ environments; firstly the volcanic island arcs, tion of Thera, in the Aegean Sea (Bond & often separated from the continental land mas­ Sparks 1976). lgnimbrites with their zones of ses by marginal back-are basins, such as are seen welding and reworked mudflows were former­ in the western Pacific and South East Asia, and ly regarded as diagnostic of subaerial eruption secondly the active continental marginssuch as but subaqueous exaroples have now been re­ the American Northwest, or the Andean region. cognised. It is believed that these flows rnaved In the first of these, the volcanism is of two dis­ within a carapace of steam which insulated tinct types: that of central volcanoes which them and permitted a more complete and uni­ build up arcs of oceanic islands, and that of sea­ form welding throughout the full thickness of floar rifts within extensional basins. The pro­ the sheet (Howeils et al. 1979; Francis & ducts of this latter type of activity closely re­ Howelis 1973). Accretionary lapilli remain one semble the new ocean floar produced by roid­ of the few unique indicators of subaerial erup­ ocean rifting except that, in many cases, the tion, but even these can commonly be re­ crust is sialic and the rifting is not complete. worked in aquagene deposits. Thus the result is a suite of tholeiitic basalt dy­ As the volcanoes build up from the sea bed, kes intruding a thinned continental basement, contemporaneous intrusion into the wet sedi­ on the surface of which submarine basaltic lava ment is not unusual; magma bodies reach a 184 ARC OPEN BACK-ARC VOLCANO OC EAN BASIN · Pelagic and hemipelagic Subaerial pyraclastics � y o can1c sand s r--1 � � 1 L____J sea1ments � Vo lcanic cong lamerates . Airfall ashlayer � P dlow lavas and h ya loc last1te � �lava s, debris flows � S ubaqueous_ o sh flo s Subaerial lava flows w Plutanies and dyk es f.!;-..,';-1 � and turb1d1tes � A. Island are volcano on subsiding crust (back- are basin subsiding faster than open ocean ) leanag enie sediments E::_:,;1 Subaerial lavas & tuffs � Va floar pillow lavas 17:77;1 Hyaloc lastite lava �Ocean floa r sediments c o re c=...Jde l t a s 6:::!=d B. Oeecnie isla nd volcano on rising ocean crust with rapid uprise of plutonic core Fig. l. Schematic cross seetian of is/and volcanoes, to portray th e distribution ofvolcanic sediments. A represents an istand are volcano, based on the diagramrnatic mode/ given by Siggurdson et al. (19 80). The cross seetian a/so illustrates the asymmetry produced by th e greater oversteepening of th e slopes de scending in to th e back are basin. B represents an oceanic is/and volcano, based on a diagramrnatic mode/ given by Stillman et al. ( 1982). The cross seetian illustrates the effe ct of th e rise at th e ocean floar which is particular/y rapid imme­ diately adjacent to the p/utonic care of the volcano. hydrastatic compensation leve! within the pile of several volcanoes may intedinger and the and spread laterally into the incoherent sedi­ volcaniclastic flows become intercalated with ment producing a range of effects both in the hemipelagic sediment fed from essentially air­ igneous material which pillows or disintegrates, fall volcanic dust, which provides the main and on the sediment which becomes fluidised source of sediment at distances beyond the dis­ thus destroying the normal bed forms (Koke­ tal limits of the turbidites.