Journal of Petrology Advance Access published online on July 2, 2004
Journal of Petrology, doi:10.1093/petrology/egh014
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1 GEOLOGICAL SURVEY OF WESTERN AUSTRALIA, 100 PLAIN STREET, EAST PERTH, WA, 6004, AUSTRALIA
* To whom correspondence should be addressed. E-mail: hugh.smithies{at}doir.wa.gov.au.
The c. 2·97-2·95 Ga magmatic history of the Mallina Basin, in the Pilbara Craton of NW Australia, includes what is perhaps the most lithologically diverse magmatism of any similar-sized Archaean terrain, and is unusual for similar-sized terrains of any age. The magmatism includes light rare earth element (LREE)-rich basaltic rocks, LREE-rich gabbros and rocks with boninite-like compositions (collectively the ‘Mallina mafic suite’), and high-Mg diorites (sanukitoids). The Mallina mafic suite is characterized by high primitive mantle normalized (La/Nb)PM (>3) and (La/
Accepted December 15, 2003
Article
Evidence for Early LREE-enriched Mantle Source Regions: Diverse Magmas from the c. 3·0 Ga Mallina Basin, Pilbara Craton, NW Australia
2 GEOSCIENCE AUSTRALIA, GPO BOX 378, CANBERRA, ACT, 2601, AUSTRALIA
Abstract 
b)PM (>2), and non-radiogenic Nd-isotopic compositions (
Nd(2·95 Ga) mostly <-1·0), suggesting that the magmas incorporated a crustal component. Despite having intruded through compositionally diverse continental crust, the magmatic rocks show a remarkably narrow range in La/Nb (
3·1), La/Sm (5·3) and La/Zr (0·15), and a small range of Nd(2·95 Ga) (-0·6 to -2·8) that is unlikely to be a result of assimilation of any single locally or regionally available crustal component. The Mallina mafic suite was probably derived from a mantle source that incorporated a homogeneous mix of ‘old Pilbara crust’ [i.e. >3·3 Ga, Nd(2·95 Ga) <-2·3, high La/Nb (Sm, Zr)] and crust that resembled the c. 3·12 Ga greenstones of the Whundo Group [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 Nd(2·95 Ga) (-0·4 to +1·2), suggesting a source that incorporated a greater proportion of the Whundo-like component. Evidence for enrichment of Archaean mantle source regions is typically extremely difficult to demonstrate and is primarily restricted to sequences that are c. 2·8 Ga or younger. The igneous rocks of the Mallina Basin, however, show that such sources existed by c. 3·0 Ga. Subduction of oceanic crust, including compositionally homogenized sediment, is the most obvious model for this mantle enrichment.
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