U-Pb age and origin of gem zircon from the New England sapphire fields, New South Wales, Australia

Chemical compositions and geochronological data utilising the laser ablation ICP-MS technique are presented for zircon megacrysts found in alluvial gem corundum deposits associated with Upper Cretaceous-Cenozoic alkali basalts in the Inverell district-New England field, New South Wales, eastern Australia. Three localities, Kings Plains, Swan Brook and Mary Anne Gully, produce gem-quality transparent dark brown and yellow zircon megacrysts, mostly under 10 mm in size. Although brown zircon shows relative enrichment in Hf and REE, there are no differences in relative transition metal concentrations between the colours. Chemical homogeneity within a single crystal indicates stable crystallisation conditions. The 206Pb/ 238U age of zircon megacrysts from these three localities define older and younger groups of 216-174 Ma and 45-37.7 Ma, respectively. The ε{lunate} Hf values of zircon megacrysts from Kings Plains show +7.51±0.34 in the older group and +10.72±0.31 in the younger group. Swan Brook zircons give +11.54±0.47 and +8.32±0.58, and Mary Anne Gully zircons are +13.67±0.63 and +8.50±0.48, respectively. These zircons from New England alluvial gem deposits have two main formational events around Upper Triassic-Lower Jurassic and Eocene episodes. Most originated from lithospheric mantle and all were brought-up by later host basaltic magmas

Corundum (sapphire) and zircon relationships, Lava Plains gem fields, NE Australia: Integrated mineralogy, geochemistry, age determination, genesis and geographical typing

International audienceGem minerals at Lava Plains, northeast Queensland, offer further insights into mantle-crustal gemformation under young basalt fields. Combined mineralogy, U-Pb age determination, oxygen isotope and petrological data on megacrysts and meta-aluminosilicate xenoliths establish a geochemical evolution in sapphire, zircon formation between 5 to 2 Ma. Sapphire megacrysts with magmatic signatures (Fe/Mg ∼100–1000, Ga/Mg 3–18) grew with ∼3 Ma micro-zircons of both mantle (δ18O 4.5–5.6%) and crustal (δ18O 9.5–10.1‰) affinities. Zircon megacrysts (3±1 Ma) show mantle and crustal characteristics, but most grew at crustal temperatures (600–800°C). Xenolith studies suggest hydrous silicate melts and fluids initiated from amphibolized mantle infiltrated into kyanite+sapphire granulitic crust (800°C, 0.7 GPa). This metasomatized the sapphire (Fe/Mg ∼50–120, Ga/Mg ∼3–11), left relict metastable sillimanite-corundum-quartz and produced minerals enriched in high field strength, large ion lithophile and rare earth elements. The gem suite suggests a syenitic parentage before its basaltic transport. Geographical trace-element typing of the sapphire megacrysts against other eastern Australian sapphires suggests a phonolitic involvement

The botanical provenance and taphonomy of Late Cretaceous Chatham amber, Chatham Islands, New Zealand

Fossil resin (amber) has been recently reported as common, but small, sedimentary components throughout thelower Upper Cretaceous (Cenomanian; 99–94 Ma) strata of the Tupuangi Formation, Chatham Islands, easternZealandia. From these deposits, resin has also been identified and obtained from well-preserved, coalified specimensof the conifer fossil Protodammara reimatamorioriMays and Cantrill, 2018. Here, we employed attenuatedtotal reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) to both dispersed and in situ amber specimens.These resulted in very similar chemical signatures, indicating that these fossils are likely from the same orclosely-related botanical sources. The FTIR data are typical of a conifer source within the ‘cupressaceous resins’category of Tappert et al. (2011). Carbon-13 nuclear magnetic resonance spectroscopy (13C NMR) facilitatedthe probable identification of these ambers as ‘Class Ib' (sensu Anderson et al. 1992). Based on these spectraldata sets, the likely botanical sources of the amber were either Araucariaceae or Cupressaceae; both of these coniferfamilies were common and widespread in the Southern Hemisphere during the Cretaceous. However, themorphology and anatomy of P. reimatamoriori support an affinity to the latter family, thus indicating that the Cretaceousamber of the Chatham Islands was generally produced by members of the Cupressaceae. Comparing theFTIR data to the published spectra of modern resins, we also identify a band ratio which may aid in distinguishingbetween the FTIR spectra of Araucariaceae and Cupressaceae, and outline the limitations to this approach. A highconcentration of ester bonds in Chatham amber specimens, which exceeds typical Cupressaceae resins, is probablycaused by taphonomic alteration via thermal maturation. The source of thermal alteration was likely preburialwildfires,conditions forwhich P. reimatamoriori was adapted to as part of its life cycle. A comparison of ambersof the Chatham Islands with modern resins and amber from various localities in Australasia reveals that,taphonomic influences aside, Chatham amber has a unique signature, suggesting that members of the basalCupressaceae (e.g., Protodammara) were not major contributors to other documented Australasian amber deposits.The closest analogy to Chatham amber deposits appears to be the Upper Cretaceous Raritan Formation,USA, which is characterised by its rich amber, charcoal and Cupressaceae fossil assemblages. This study furthersupports the hypotheses that the early Late Cretaceous south polar forests were dominated by Cupressaceae,and regularly disturbed by wildfires.Fieldwork and research supported by National Geographic Society (grant 9761-15) awarded to C.M.; additional financial support provided by the Paleontological Society and Monash University.</p