What caused the Early Silurian change from mafic to silicic (S-type) magmatism in the eastern Lachlan Fold Belt?

One of the most significant, but poorly understood, tectonic events in the east Lachlan Fold Belt is that which caused the shift from mafic, mantle-de rived calc-alkaline/shoshonitic volcanism in the Late Ordovician to silicic (S-type) plutonism and volcanism in the late Early Silurian. We suggest th at this chemical/isotopic shift required major changes in crustal architect ure, but not tectonic setting. and simply involved ongoing subduction-relat ed magmatism following burial of the pre-existing, active intraoceanic are by overthrusting Ordovician sediments during Late Ordovician - Early Siluri an (pre-Benambran) deformation, associated with regional northeast-southwes t shortening. A review of 'type' Benambran deformation from the type area ( central Lachlan Fold Belt) shows that it is constrained to a north-northwes t-trending belt at cc 430 Ma (late Early Silurian), associated with high-gr ade metamorphism and S-type granite generation. Similar features were assoc iated with ca 430 Ma deformation in east Lachlan Fold Belt, highlighted by the Cooma Complex. and formed within a separate north-trending belt that in cluded the S-type Kosciuszko, Murrumbidgee, Young and Wyangala Batholiths. As Ordovician turbidites were partially melted at ca430 Ma, they must have been buried already to similar to 20 km before the 'type' Benambran deforma tion. We suggest that this burial occurred during earlier northeast-southwe st shortening associated with regional oblique folds and thrusts, loosely r eferred to previously as latitudinal or east-west structures. This event al so caused the earliest Silurian uplift in the central Lachlan Fold Belt (Be nambran highlands), which pre-dated the 'type' Benambran deformation and is constrained as latest Ordovician - earliest Silurian (ca 450-440 Ma) in ag e. The south- to southwest-verging, earliest Silurian folds and thrusts in the Tabberabbera Zone are considered to be associated with these early obli que structures, although similar deformation in that zone probably continue d into the Devonian. We term these 'pre'- and 'type'-Benambran events as 'e arly' and 'late' for historical reasons, although we do not consider that t hey are necessarily related. Heat-flow modelling suggests that burial of 'a verage' Ordovician turbidites during early Benambran deformation at 450-440 Ma, to form a 30 km-thick crustal pile, cannot provide sufficient heat to induce mid-crustal melting at ca430 Ma by internal heat generation alone. A n external, mantle heat source is required, best illustrated by the mafic c c 430 Ma, Micalong Swamp Igneous Complex in the S-type Young Batholith. Mod ern heat-flow constraints also indicate that the lower crust cannot be fels ic and, along with petrological evidence, appears to preclude older contine ntal 'basement terranes' as sources for the S-type granites. Restriction of the S-type batholiths into two discrete, oblique, linear belts in the cent ral and east Lachlan Fold Belt supports a model of separate magmatic arc/su bduction zone complexes, consistent with the existence of adjacent, structu rally imbricated turbidite zones with opposite tectonic vergence, inferred by other workers to be independent accretionary prisms. Are magmas associat ed with this 'double convergent' subduction system in the east Lachlan Fold Belt were heavily contaminated by Ordovician sediment, recently buried dur ing the early Benambran deformation, causing the shift from mafic to silici c (S-type) magmatism. in contrast, the central Lachlan Fold Belt magmatic a re, represented by the Wagga-Omeo Zone, only began in the Early Silurian in response to subduction associated with the early Benambran northeast-south west shortening. The model requires that the S-type and subsequent I-type (Late Silurian - D evonian) granites of the Lachlan Fold Belt were associated with ongoing, su bduction-related tectonic activity.