Evaluation of petrogenetic models for Lachlan Fold Belt granitoids: Implications for crustal architecture and tectonic models

Abstract

The restite (one source‐component) model suggests that granitoids are derived from contrasting source rocks and that the typical linear chemical variation of Lachlan Fold Belt granitoids is produced by restite separation. However, it cannot explain the general chemical and isotopic similarity of S‐ and I‐type granitoids in the eastern Lachlan Fold Belt, the similarity of zircon inheritance patterns between the two granite types, nor their apparently simple ϵNd‐Sr isotopic array. A two source‐component mixing model, based on the ϵNd‐Sr isotopic array, suggests that linear chemical variation of the granitoids reflects variable incorporation of deeply buried Ordovician sedimentary rocks and basaltic magmas. However, isotopically defined mixes do not match the predicted chemical mixes. Also systematic and sympathetic isotopic and chemical variations are observed for both the felsic and mafic granites within suites across the Bega Batholith. For a simple two‐component model to apply, both the crustal and mantle end‐members would have to change composition in the same way, which is unrealistic. A three source‐component mixing model incorporates aspects of the other two models. It suggests that I‐type magmas, formed in the lower crust by mixing of basaltic magmas with partial melts derived from the greenstone succession, are variably contaminated in the deep crust by migmatised Ordovician metasedimentary rocks. If sufficiently contaminated, the I‐type granites become peraluminous, S‐type granites and a restitic component may be retained. The three‐component mixing model also places several important constraints on eastern Lachlan Fold Belt tectonic evolution: (i) no Proterozoic continental basement, nor Proterozoic basement terranes, need exist beneath the Lachlan Fold Belt; (ii) the Neoproterozoic‐Cambrian greenstone succession and Ordovician sedimentary crust was considerably thickened before generation of the oldest S‐type granites at ca 430 Ma; (iii) the greenstone succession dominated the lower crust after the crustal thickening, extending to depths between 20 to 30 km, whereas Ordovician metasediment dominated the higher crustal levels (0–20 km); (iv) the mid‐crust may have been subjected to high heat‐flux before generation of the I‐type magmas in the lower crust, possibly associated with thermal recovery following end‐Ordovician crustal thickening; (v) the remarkably homogeneous, high μ Pb crustal reservoir of the Lachlan Fold Belt is probably Ordovician sediment; (vi) the I‐S line is not a major tectonic boundary, but reflects the eastern limit of Ordovician sedimentary rocks that were substantially melted; (vii) Bega Batholith granitoids show a systematic eastward decrease in degree of contamination by Ordovician sedimentary rocks, reflecting an eastward tapering Late Ordovician accretionary prism into which they intruded; (viii) the inferred west‐dipping décollement surface between Ordovician metasedimentary rocks and Cambrian greenstone succession could reflect a Late Ordovician fossil subduction zone; and (ix) Siluro‐Devonian Bega Batholith granitoids are probably all subduction‐related, which might apply to silicic magmatism throughout the entire eastern Lachlan Fold Belt.