Magnesian dyke suites of the 2.7 Ga Kambalda Sequence, Western Australia: Evidence for coeval melting of plume asthenosphere and metasomatised lithospheric mantle

摘要

Three premetamorphic magnesian dyke suites are prominent in the Woolyeneer basalt Formation of the ～2.7 Ga Kambalda Sequence, Kalgoorlie Terrane, Yilgarn craton, Western Australia: the Butterfly (Mg# 76-70; Ni 640-370 ppm), the Norseman (Mg# 72-66, Ni 1105-1030 ppm), and the Crown (Mg# 73-72, Ni 1240-1170 ppm) Dyke suites. Butterfly Dykes have flat HREE at 6-8 times chondrite, mildly depleted LREE, and Nb/Th > 8; compositionally they resemble high-Mg basalts associated with komatiites and tholeiitic basalts in many Archean greenstone terranes erupted from mantle plumes. Crustally contaminated counterparts have the conjunction of greater (La/Sm)_N with Nb/Th < 8. Norseman and Crown suites have composite compositional characteristics: (1) high MgO contents, low Al_2O_3 and Al_2O_3/TiO_2 ratios, enriched LREE, but fractionated HREE; and (2) positive Nb anomalies, fractionated REE, deep negative P anomalies, and downturn from Pr to Th on primitive mantle normalised diagrams. The former characteristics are of norites, abundant in Neoarchean to Proterozoic intrusions, and interpreted to be melts of low-Al metasomatised continental lithospheric mantle (CLM), whereas the latter characteristics are typical of high-μ ocean island basalts (HIMU-OIB) where the concensus is of ocean crust processed through a subduction zone and recycled into the mantle plume source of HIMU-OIB. Epsilon Nd values of +2.4 to +3.6 are in keeping with a depleted mantle source(s). Accordingly these dyke suites are consistent with melting of depleted continental lithospheric mantle metasomatised by low degree melts from HIMU-OIB. On the Th/Yb versus Nb/Yb discrimination diagram, Butterfly Dykes plot near depleted mantle, and Noresmam and Crown suites within the MORB-OIB array towards the HIMU endmember; consequently, the MORB-OIB array was established in the Archean mantle. Taken with published trace element and Sm-Nd isotope data, the most straightforward geodynamic-magmatic scenario is that the Butterfly Dykes are intrusive equivalents of high-Mg flows in the Kambalda Sequence, erupted from a mantle plume melting at the base of the lithosphere, whereas Norseman and Crown Dyke suites represent melts of metasomatised mantle lithosphere induced by some combination of rifting and thermal input from the plume.