The New England Batholith, eastern Australia: Geochemical variations in time and space

Abstract

The New England Batholith, Australia, is part of the Upper Paleozoic New England Fold Belt, with most plutons intruded into the deformed trench‐complex metasedimetary rocks in the southeast part of the Fold Belt. The Batholith was emplaced in two major periods of plutonism, the first during the Upper Carboniferous and the second during the Upper Permian and Triassic, with a major phase of metamorphism and deformation including westward overthrusting of the trench‐complex sedimentary rocks between the two periods. On the basis of petrography, geochemistry and isotopic characteristics, the granitoids of the Batholith are subdivided into five named intrusive suites and a group of leucoadamellites. The differences between the six groups are considered to reflect differences in their source‐rock types. The Carboniferous granitoids are peraluminous S‐type and are divided into the Bundarra Plutonic Suite, a belt of very coarse‐grained adamellites with cordierite ± garnet, and the Hillgrove Plutonic Suite, a belt of biotite‐rich ± garnet deformed adamellites and granodiorites. Both suites have δ 18O greater than 10, negative δ34S, 87Sr/86Sr initial ratios about 0.706, are ilmenite‐bearing and have low FeO3 ratios. The Bundarra Plutonic Suite, however, is consistently SiO2‐rich (greater than 70%), contains cordierite and has higher δ18O than the Hillgrove Plutonic Suite. The two S‐type suites are inferred to have formed by partial melting of the deepest parts of a wedge of trench‐complex sedimentary rocks oceanwards of an ‘Andean’ volcanic chain to the west, analagous to the lower Tertiary S‐type plutons of the Sanak‐Baranof plutonic belt of southern Alaska. Of the remaining four groups of plutons emplaced during the Upper Permian and Triassic, two are metaluminous I‐type granitoids, one the Clarence River Plutonic Suite (new name), a group of K‐poor granodiorites and tonalites with low 87Sr/86Sr initial ratios (average 0.7035), and the other the Moonbi Plutonic Suite, a group of K‐rich adamellites with low 87Sr/86Sr initial ratios (average 0.7045), δ18O less than 10 and δ34S positive. The Clarence River Plutonic Suite is inferred to have formed from the partial melting of a large‐ion lithophile element‐poor (gabbro?) source region and the Moonbi Plutonic Suite from a large‐ion lithophile element‐rich (shoshonite?) source region. A third group, the Uralla Plutonic Suite, is less metaluminous, has higher 87Sr/86Sr initial ratios (average 0.706), δ18O only just less than 10 and negative δ34S, and is considered to have formed in a region containing a physical mixture of metaluminous and peraluminous source rocks (gabbro and metasedimentary rocks?). Plutons of the fourth group are leucoadamellites, largely emplaced after the other suites, with low δ18O indicating an I‐type source. It is argued that this group has resulted from low degrees of ‘dry’ partial metling of an I‐type source region that had undergone an earlier melting event. These four groups of plutons are largely confined to a narrow belt parallel to, but 100 km west of, the present rifted margin of Australia. Their origin may be related to crustal thickening attendant on the overthrusting that occurred early in the Permian and/or a temperature increase in the lower crust due to the recovery of normal continental geotherms following the cessation of subduction.