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GEOLOGY, November 2007 97Geology, November 2007; v. 35; no. 11; p. 971974; doi: 10.1130/G4123A.1; 3 figures; Data Repository item 2007242. 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.

INTRODUCTION

Sandstone provenance helps to constrain thescale and pattern of ancient drainage, and it isa key tool in facies prediction and paleogeo-graphic reconstructions. A wide range of tech-niques can be used to assess the source of sandgrains, but not all yield definitive results. It canbe difficult to see through recycling and mix-

ing, particularly where the grains are robust andmake up a tiny fraction of the sand, as in the caseof zircon. In addition, the use of a trace mineralrequires detailed characterization of the sourcearea against which to compare the detritus.Denudation may have completely removed thesource rocks, and contemporaneity of magmaticevents in unrelated terranes can lead to ambigu-ity as grains of a given age may come from morethan one source area.

A new method, based on in situ Pb iso-topic analysis of single K-feldspar grains usinglaser ablationmulticollectorinductively coupled

plasmamass spectrometry (LA-MC-ICP-MS)(Tyrrell et al., 2006) offers some advantagesover other techniques. K-feldspar is a relativelycommon, generally first-cycle, frameworkmineral in sandstones. Importantly, K-feldsparcontains negligible U and Th; hence, its Pb iso-topic composition does not change significantlyover time. Furthermore, Pb in basement rocksshows broad regional variations (due to differ-ent ages and variations in U-Pb-Th fraction-ation) and is likely to be consistent between theupper and middle crust and, thus, insensitive to

erosion level. Hence, Pb isotopic mapping canbe used to identify important crustal bound-aries (Connelly and Thrane, 2005). Potentialsource areas can therefore be characterizedby a relatively small number of K-feldspar orgalena analyses. Two orientation studies haveshown that the Pb isotopic composition of feld-spar sand grains is relatively robust, and it can

survive weathering, transport, and diagenesis(Tyrrell et al., 2006). Targeted laser samplingwithin individual sand grains avoids internalheterogeneities (e.g., inclusions, altered regionswithin grains), avoiding some of the uncertain-ties inherent in multigrain or the single-grainleaching techniques previously employed todetermine Pb isotopes in detrital K-feldspar(e.g., Hemming et al., 1996), and MC-ICP-MS offers better precision than ion microprobetechniques (Clift et al., 2001).

The Pb provenance method is used here toexplore drainage evolution prior to and dur-

ing the breakup of Pangea, when opening ofthe North Atlantic stranded remnants of earlyrift basins on the conjugate passive margins.Here, we focus on basins offshore westernIreland, combining a new circum-Atlantic Pbdomain map (Fig. 1) with Pb isotopic datafrom K-feldspar in Triassic and Jurassic sand-stones. Together, these data (1) constrain thescale of the drainage, with implications for thedepositional setting and hinterland geology;(2) shed new light on the drainage orientationand source location; (3) demonstrate major

drainage reorganization driven by a change in

rift style; and (4) suggest minimal recycling oTriassic sand into Jurassic depocenters.

MESOZOIC BASINS WEST OF IRELAND

Pangean breakup west of Ireland involvedpolyphase rifting associated with collapse othe Variscan orogenic belt and protracted crustaextension along the Atlantic margin (Naylor andShannon, 2005). The Slyne, Erris, and DonegaBasins originally formed as part of a distributed network of Permian-Triassic depocenter(Dancer et al., 1999) as a consequence of wideextensional rifting (Praeg, 2004). Some of these

basins were internally drained, while others werefed by large rivers, such as those flowing northward from the trans-Pangean Variscan uplands(Audley-Charles, 1970). Sand-rich Triassic successions have been drilled in the basins west oIreland and have been identified seismically inthe Porcupine and Rockall Basins (Walsh et al.1999; Naylor and Shannon, 2005). In the SlyneBasin, Triassic sandstones, thought to be equivalent to the Sherwood Sandstone of NW Europehost the Corrib gas field and are composed ofine- to medium-grained arkosic fluvial and alluvial sandstones with subordinate sand-flat andplaya mudstone deposits (Dancer et al., 2005)

Previous interpretations based on dipmeter logspetrography, and whole-rock geochemistry havsuggested sand derivation from the Variscanuplands to the south, with additional input fromthe Irish Mainland (Dancer et al., 2005).

The Porcupine Basin, southwest of the SlyneBasin (Fig. 1), includes a Jurassic sequencedeposited during a phase of narrow extensional rifting (Croker and Shannon, 1987Naylor and Shannon, 2005). In the northern parof the basin, an Upper Jurassic (KimmeridgianTithonian) sequence of north-derived low-energyfluvial (meandering river) and marginal-marine

facies is replaced southward by shallow-marinesandstones and deep-water turbiditic fans (Butterworth et al., 1999; Williams et al., 1999). Petrography suggests a source including granites, basicintrusives, and metasedimentary rocks (Geraghty1999) of uncertain location.

SAMPLING AND METHODOLOGY

Medium-grained sandstones were sampledfrom cored Triassic intervals in two SlyneBasin wells (18/251 and 18202z; Fig. 1and from Upper Jurassic intervals in two well

Drainage reorganization during breakup of Pangea revealed by

in-situ Pb isotopic analysis of detrital K-feldspar

S. TyrrellP.D.W. Haughton UCD School of Geological Sciences, University College Dublin, Belfield, Dublin 4, IrelandJ.S. Daly

ABSTRACT

Pb isotopes in detrital K-feldspar grains provide a powerful provenance tracer for feld-

spathic sandstones. Common Pb isotopic compositions show broad (hundred-kilometer scale)

regional variation, and this signature can survive weathering, transport, and diagenesis. The

feldspar Pb signature can be measured rapidly using laser ablationmulticollectorinductively

coupled plasmamass spectrometry (LA-MC-ICP-MS), and careful targeting can avoid inclu-

sions and altered regions within grains. Here, we combine a new Pb domain map for the

circumNorth Atlantic with detrital K-feldspar Pb isotopic data from Triassic and Jurassic

sandstones from basins on the Irish Atlantic margin. The Pb isotopic compositions reveal

otherwise cryptic feldspar populations that constrain the evolving drainage pattern. Triassic

sandstones originated from distant Archean and Paleoproterozoic rocks, probably in Green-

land, Labrador, and the Rockall Bank to the NW, implying long (>500 km) transport across

a nascent rift system. Later, Jurassic sandstones had a composite Paleo- and Mesoproterozoic

source in more proximal sources to the north (

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from the northern Porcupine Basin (26/281and 35/82; Fig. 1).

The Pb isotopic composition of sand-sizedK-feldspar grains was analyzed using LA-MC-ICP-MS at the Geological Institute, Copenhagen,following Tyrrell et al. (2006). Prior to analysis,grains were imaged using backscattered-electronmicroscopy (BSE) and cathodoluminescence(CL) to avoid intragrain heterogeneities, whichmight have compromised the Pb signal. Pol-ished K-feldspar surfaces were ablated alongpredetermined 300700 m tracks, guided bythe BSE and CL imaging. Typical 2 errors on206

Pb/204

Pb were

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GEOLOGY, November 2007 973

those from 83/20-sb01 in the south dominantlyshow Proterozoic II and Avalonian affinities(Figs. 2A and2B; GSA Data Repository).

SAND PROVENANCE ANDIMPLICATIONS FOR PALEODRAINAGE

The two isotopically distinct K-feldspargroups in Triassic sandstones from the SlyneBasin correspond to a combined Archean(Triassic group 1; Fig. 2A) and Proterozoic Isource (Triassic group 2; Fig. 2A). There isno significant K-feldspar component originat-ing from the Irish mainland (Proterozoic II) orfrom a more southerly (Avalonian or Variscan)source. This would appear to exclude derivationof sand from the south and east, as previouslysuggested (Dancer et al., 2005). Derivation ofsand from the north and west is consistent with

the K-feldspar Pb populations, where Archeangrains come from Labrador or Greenland andProterozoic I grains come from south Green-land, south Labrador, and/or from Rockall Bank

(Fig. 3A). These data imply grain transport inexcess of 500 km. The NW-SE orientation ofthe paleodrainage corresponds well with theorientation of the protoLabrador Sea on Tri-assic paleogeographic reconstructions (Eide,2002; Fig. 3A). The sand delivery system is ona similar scale to that envisaged to have oper-ated elsewhere during the Triassic, such as theBudleighensis river system, which drainednorthward from the uplifted Variscides to feedbasins in the East Irish Sea and farther north(Audley-Charles, 1970; Warrington and Ivimey-Cook, 1992). The subdued physiography ofPangea during the onset of wide extensional

rifting was probably important in allowing theoperation of large-scale drainage systems.

The two groups of isotopically distincK-feldspar from Upper Jurassic sandstones inthe northern Porcupine Basin correspond to acombined Proterozoic I (Jurassic group 1) andProterozoic II source (Jurassic group 2). Thereare no significant Archean, Avalonian, or Variscancontributions, ruling out a far-northerly source oany input from the south. Significantly, there are

no indications that K-feldspar grains have beenrecycled from inverted Triassic sandstones. Thesedata are consistent with existing paleogeographicmodels (Butterworth et al., 1999) that envisagedrainage from north to south with grain transpordistances

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are acknowledged for providing samples from onshorewestern Ireland. We acknowledge Cormac OConnellfor help with scanning-electron microscope (SEM)imaging. Tom McKie (Shell International Petro-leum), Mick Hanrahan, and Barbara Murray (PAD)are acknowledged for facilitating core sampling. Wethank Martin Bizzaro, David Ulfbeck, and Tod Waight(Geological Institute, Copenhagen) for assistance withLA-MC-ICP-MS, and Sidney Hemming, Martin Lee,and an anonymous reviewer for detailed reviews thatgreatly improved the manuscript.

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Manuscript received 7 February 2007Revised manuscript received 15 June 2007Manuscript accepted 21 June 2007

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