Evidence for Early LREE-enriched Mantle Source Regions: Diverse Magmas from the c. 3{middle dot}0 Ga Mallina Basin, Pilbara Craton, NW Australia

doi:10.1093/petrology/egh014

Journal of Petrology Advance Access originally published online on July 2, 2004
Journal of Petrology 2004 45(8):1515-1537; doi:10.1093/petrology/egh014

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Evidence for Early LREE-enriched Mantle Source Regions: Diverse Magmas from the c. 3·0 Ga Mallina Basin, Pilbara Craton, NW Australia

R. H. SMITHIES¹^,*, D. C. CHAMPION² and S-.S. SUN²

¹ GEOLOGICAL SURVEY OF WESTERN AUSTRALIA, 100 PLAIN STREET, EAST PERTH, WA, 6004, AUSTRALIA
² GEOSCIENCE AUSTRALIA, GPO BOX 378, CANBERRA, ACT, 2601, AUSTRALIA

^* Corresponding author. E-mail: hugh.smithies{at}doir.wa.gov.au

The c. 2·97–2·95 Ga magmatic history ofthe Mallina Basin, in the Pilbara Craton of NW Australia, includeswhat is perhaps the most lithologically diverse magmatism ofany similar-sized Archaean terrain, and is unusual for similar-sizedterrains of any age. The magmatism includes light rare earthelement (LREE)-rich basaltic rocks, LREE-rich gabbros and rockswith boninite-like compositions (collectively the ‘Mallinamafic suite’), and high-Mg diorites (sanukitoids). TheMallina mafic suite is characterized by high primitive mantlenormalized (La/Nb)_PM (>3) and (La/Yb)_PM (>2), and non-radiogenicNd-isotopic compositions ( ${varepsilon}$ _{Nd(2·95 Ga)} mostly <–1·0),suggesting that the magmas incorporated a crustal component.Despite having intruded through compositionally diverse continentalcrust, the magmatic rocks show a remarkably narrow range inLa/Nb ( $~$ 3·1), La/Sm ( $~$ 5·3) and La/Zr ( $~$ 0·15),and a small range of ${varepsilon}$ _{Nd(2·95 Ga)} (–0·6 to–2·8) that is unlikely to be a result of assimilationof any single locally or regionally available crustal component.The Mallina mafic suite was probably derived from a mantle sourcethat incorporated a homogeneous mix of ‘old Pilbara crust’[i.e. >3·3 Ga, ${varepsilon}$ _{Nd(2·95 Ga)} <–2·3,high La/Nb (Sm, Zr)] and crust that resembled the c. 3·12Ga greenstones of the Whundo Group [ ${varepsilon}$ _{Nd(2·95 Ga)} >–0·4,low La/Nb (Sm, Zr)], which crop out to the NW of the basin.Compared with the Mallina mafic suite, the high-Mg diorites(sanukitoids) have higher ${varepsilon}$ _{Nd(2·95 Ga)} (–0·4to +1·2), suggesting a source that incorporated a greaterproportion of the Whundo-like component. Evidence for enrichmentof Archaean mantle source regions is typically extremely difficultto demonstrate and is primarily restricted to sequences thatare c. 2·8 Ga or younger. The igneous rocks of the MallinaBasin, however, show that such sources existed by c. 3·0Ga. Subduction of oceanic crust, including compositionally homogenizedsediment, is the most obvious model for this mantle enrichment.

KEY WORDS: Archaean; crustal evolution; enriched mantle; mafic magmas; boninite; sanukitoid; subduction

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