Abstract Situated within the southern segment of the South China Block, the Ganzhou Basin formed owing to subduction of the palaeo-Pacific plate beneath the South China Block. Late Cretaceous successions in this basin consist of fluvial and lacustrine facies red beds hosting abundant dinosaur and dinosaur egg fossils. This study reports detrital zircon geochronological data from a crystallized tuff and four sandstones found in the Late Cretaceous Ganzhou Group of the Ganzhou Basin. Age distributions included four major age subpopulations of predominantly Triassic, Devonian–Ordovician, Neoproterozoic and Paleoproterozoic ages. These indicate source material derived from Yanshanian and Triassic granitoids as well as from Kwangsian and Jiangnan orogens. Age signatures generally resemble those recorded in the adjacent Nanxiong Basin but also include distinctive features. Provenance signatures from successive units indicate a tectonic transition from intracontinental extension at c. 120 Ma to compression near the Cretaceous/Paleogene boundary. This tectonic transition was probably driven by continent–continent collision between the Indian and Eurasian plates, as well as by a shift in the subduction direction of the palaeo-Pacific plate beneath the Eurasian plate.

Abstract Ion adsorption-type rare earth element (REE) deposits are the predominant source of heavy REE (HREEs) and yttrium in the world. Economic examples of the deposits are confined almost exclusively to areas underlain by granitic rocks in southern China. These deposits are termed “ion adsorption-type” because the weathered granites contain more than ~50% ion-exchangeable REY (REE + Y), relative to whole-rock REY. The ore grades range from 140 to 6,500 ppm (typically ~800 ppm) REY, and some of the deposits are remarkably enriched in HREEs. The Yanshanian (Jurassic-Cretaceous) granites that weather to form the deposits are products of subduction-related or extensional intraplate magmatism. These parent granites for the REE deposits are biotite- and/or muscovite-bearing granites and are characterized by >70% SiO 2 , <0.08% P 2 O 5 , and metaluminous to weakly peraluminous (ASI < 1.1) compositions. The highly differentiated (SiO2 >~75%) muscovite granites are HREE enriched relative to the biotite granites and are notably characterized by occurrences of fluorite and hydrothermal REE-bearing minerals, particularly REE fluorocarbonates that formed in a deuteric alteration event. Magmatic allanite and titanite are either altered to form hydrothermal REE-bearing minerals or almost completely broken down during weathering. The weatherable REE-bearing minerals, including fluorocarbonates, allanite, and titanite, are the source minerals for the ion adsorption ores. The HREE grades of the ion adsorption ores are strongly influenced by the relative abundances and weathering susceptibilities of these REE-bearing minerals in the parent granites. The presence of easily weathered HREE minerals in the underlying granites appears to be the primary control of the HREE-rich deposits, although solution and solid phase chemistry during development of the weathering profile may influence REE fractionation. Monazite, zircon, and xenotime are also present in the granites, but because they are more resistant to chemical weathering, they are typically not a source of REEs in the weathered materials. The REE-bearing minerals are decomposed by acidic soil water at shallow levels in the weathering profile, and the REE 3 + ions move downward in the profile. The REEs are complexed with humic substances, with carbonate and bicarbonate ions, or carried as REE 3+ ions in soil and ground water at a near-neutral pH of 5 to 9. The REE 3 + ions are removed from solution by adsorption onto or incorporated into secondary minerals. The removal from the aqueous phase is due to a pH increase, which results from either water-rock interaction or mixing with a higher pH ground water. The REEs commonly adsorb on the surfaces of kaolinite and halloysite, to form the ion adsorption ores, due to their abundances and points of zero charge. In addition, some REEs are immobilized in secondary minerals consisting mainly of REE-bearing phosphates (e.g., rhabdophane and florencite). In contrast to the other REEs that move downward in the weathering profile, Ce is less mobile and is incorporated into the Mn oxides and cerianite (CeO 2) as Ce 4 + under near-surface, oxidizing conditions. As a result, the weathering profile of the deposits can be divided into a REE-leached zone in the upper part of the profile, with a positive Ce anomaly, and a REE accumulation zone with the ion adsorption ores in the lower part of the profile that is characterized by a negative Ce anomaly. The thickness of the weathering profiles generally ranges from 6 to 10 m but can be as much as 30 m and rarely up to 60 m. The negative Ce anomaly in weathered granite terrane is thus a good exploration indicator for ion adsorption ores. A temperate or tropical climate, with moderate to high temperatures and precipitation rates, is essential for chemical weathering and ion adsorption REE ore formation. Low to moderate denudation, characteristic of such a climate in areas of low relief, are favorable for the preservation of thick weathering profiles with the REE orebodies.

The granites of Hong Kong comprise a variety of assemblages dominated by chemically evolved compositions. They are divided into two suites based on petrographic, geochemical, and age criteria. The oldest and most primitive intrusive units are deformed biotite-hornblende granodiorites and monzogranites of the Lamma Suite. These rocks are characterised by high CaO (1.4–2.7%), and low Nb and Y contents. The Lion Rock Suite (LRS) is dominated by relatively undeformed monzogranite with subordinate quartz syenite and comprises three subgroups. Granites of subgroup I are separated into coarse- and fine- to medium-grained lithologies. The fine- to medium-grained granites are predominantly fluorite-bearing with silica contents ranging from 75.5–78%. They are characterised by high total REE, Ga, F, Rb, Nb, and Y contents and yield a Rb–Sr whole-rock isochron age of 155 ± 6 Ma with an initial 87 Sr/ 86 Sr ratio of 0.7101 ± 0.0060 (MSWD = 4.6). Granites of subgroup II comprise a diverse range of compositions (SiO 2 = 63–77%) and are characterised by highly variable trace element abundances. Coarse-grained granites yield an age of 148 ± 9 Ma with an initial 87 Sr/ 86 Sr ratio of 0.7060 ± 0.0006 (MSWD = 0.1). Granites of subgroup III are moderately to highly evolved (SiO 2 = 72.5–77.9%) and the silica-rich compositions are marked by enrichment in Y, Nb, Rb and depletion in Ba and Sr. Rb–Sr whole-rock isochron ages for individual plutons vary from 138 ± 1 to 136 ± 1 and corresponding initial 87 Sr/ 86 Sr ratios are 0.7080 ± 0.0002 (MSWD = 1.2) and 0.7092 ± 0.0006 (MSWD = 0.4). Granites of the Lamma Suite and coarse-grained granites of LRS subgroup I are interpreted as synorogenic I-types, whereas those of LRS subgroups II and III are interpreted as late-orogenic to postorogenic, fractionated I-types. Fine-to medium-grained granites of LRS subgroup I have distinctive A-type affinities and together with their association with quartz syenite indicate a transition from compressional to tensional tectonics.