Mantle Upwelling or Plume Activity on the Periphery of the Warakurna LIP: Evidence from the Geochemistry and Petrogenesis of the Alcurra Dolerite in the Eastern Musgrave Province

摘要

The Alcurra Dolerite in the Musgrave Province, central Australia, is part of the c. 1075Ma Warakurna large igneous province (LIP), which is interpreted to have been emplaced contemporaneously with intracontinental rifting. The Alcurra Dolerite in the eastern Musgrave Province comprises two types of continental tholeiite. Dominant Group 1 dolerites are mainly magnesian tholeiites, whereas subordinate Group 2 dolerites are normal tholeiites. These two groups have distinct chondrite-normalized rare earth element (REE) patterns, primordial mantle-normalized trace element patterns, and incompatible element ratios (e.g. Th/Yb, Nb/Yb, La/Yb and Zr/Y). Both groups are low-Ti and low-Zr, but Group 1 dolerites have higher Ti/Zr (>60) and Ti/Y (>310) values than those of Group 2. Group 1 mainly comprises olivine-microgabbros, and is volumetrically dominant, whereas Group 2 is proportionally minor and includes noritic-microgabbros. Dolerites from both groups plot in discrete fields in Nd-143/Nd-144 versus Sm-147/Nd-144 space, along a mixing trend between chondritic or depleted mantle and the c. 159Ga Musgravian basement. Group 1 dolerites have low Rb, Th and K2O contents that preclude their contamination by Musgravian continental crust. They also have incompatible element ratios that overlap with mid-ocean ridge basalt or primitive mantle. Group 2 dolerites have more negative Nd values and are also the more incompatible element enriched group; they may have, to some extent, been directly contaminated by Musgravian basement. Low concentrations of light REE (LREE) in Group 1 dolerites indicate a moderate to high percentage of partial melting of a relatively depleted source. Mixing this partial melt with a small proportion of melt derived by a moderate percentage of partial melting of a more incompatible element enriched mantle source could have resulted in the low (La/Sm)(N) ratios and low absolute La contents seen in these dolerites. Group 1 is divided into two subgroups (1A and 1B) on the basis of LREE abundances and Nb/Yb ratios. These two subgroups may reflect different percentages of partial melting of the depleted source. Petrogenetic interpretations for the Alcurra Dolerite of the eastern Musgrave Province must address published geophysical evidence for asthenospheric mantle upwelling and underplating in the Musgrave Province, and for contemporaneous low- and high-Ti Alcurra Dolerites indicative of a plume to the west where they are associated with Giles Suite layered intrusions. The large proportion of probable asthenosphere-derived melt in the petrogenesis of the Group 1 dolerites from the eastern Musgrave Province indicates either substantial heat transfer, or significant decompression in the mantle. The Warakurna LIP could result from a complex interaction between a plume and mantle upwelling.