ALBERT

Description: In our complementary geochemical study (Part 1), the Malaysian granitoids of the Southeast Asian tin belt were divided into a Middle Permian to Late Triassic I-type–dominated Eastern province (Indochina terrane) and a Triassic to Early Jurassic transitional I/S-type Main Range province (Sibumasu terrane), separated by the Bentong-Raub suture zone which closed in the Late Triassic. Previous geochronology has relied on only a few U-Pb zircon ages together with K-Ar and whole rock Rb-Sr ages that may not accurately record true magmatic ages. We present 39 new high-precision U-Pb zircon ion microprobe ages from granitoids and volcanics across the Malay Peninsula. Our results show that ages from the Eastern province granitoids span 289–220 Ma, with those from the Main Range province granitoids being entirely Late Triassic, spanning 227–201 Ma. A general westerly younging magmatic trend across the Malay Peninsula is considered to reflect steepening and roll-back of the Bentong-Raub subduction zone during progressive closure of Paleo-Tethys. The youngest ages of subduction-related granites in the Eastern province roughly coincide with the youngest ages of marine sedimentary rocks along the Paleo-Tethyan suture zone. Our petrogenetic and U-Pb zircon age data support models that relate the Eastern province granites to pre-collisional Andean-type magmatism and the western Main Range province granites to syn- and post-collisional crustal melting of Sibumasu crust during the Late Triassic. Tin mineralization was mainly associated with the latter phase of magmatism. Two alternative tectonic models are discussed to explain the Triassic evolution of the Malay Peninsula. The first involves a second Late Triassic to Jurassic or Early Cretaceous east-dipping subduction zone west of Sibumasu where subduction-related hornblende and biotite–bearing granites along Sibumasu are paired with Main Range crustal-melt tin-bearing granites, analogous to the Bolivia Cordilleran tin-bearing granite belt. The second model involves westward underthrusting of Indochina beneath the West Malaya Main Range province, resulting in crustal thickening and formation of tin-bearing granites of the Main Ranges. Cretaceous granitoids are also present locally in Singapore (Ubin diorite), on Tioman Island, in the Noring pluton, of the Stong complex (Eastern Province), and along the Sibumasu terrane in southwest Thailand and Burma (Myanmar), reflecting localized crustal melting.

Description: The Malaysian granitoids of the Southeast Asian tin belt have been traditionally divided into a Permian to Late Triassic "I-type"–dominated arc-related Eastern province (Indochina terrane) and a Late Triassic "S-type"–dominated collision-related Main Range province (Sibumasu terrane), separated by the Bentong-Raub Paleo-Tethyan suture that closed in the Late Triassic. The present study, however, shows that this model is oversimplified and that the direct application of Chappell and White’s (1974) I- and S-type classification cannot account for many of the characteristics shared by Malaysian granitoids. Despite being commonly hornblende bearing, as is typical for I-type granites, the roof zones of the Eastern province granites are hornblende free. In addition, the Main Range province granitoids contain insignificant primary muscovite, and are dominated by biotite granites, mineralogically similar to many of the plutons of the Eastern province. In general, the Malaysian granitoids from both provinces are more enriched in high field strength elements than typical Cordilleran I- and S-type granitoids. The mineralogy and geochemistry of the Eastern province granitoids, and their relationship with contemporaneous volcanics, confirm their I-type nature. The bulk liquid lines of descent of both granitic provinces largely overlap with one another. Sr-Nd isotopic data further demonstrate that the Malaysian granitoids, especially those of the Main Range, were hybridized melts derived from two "end-member" source regions, one of which is isotopically similar to the Kontum orthoamphibolites and the other akin to the Kontum paragneisses of the Indochina block. However, there are differences in the source rocks for the two provinces, and it is suggested in this paper that these are related to differing proportions of igneous and sedimentary protoliths. The incorporation of sedimentary-sourced melts in the Eastern province is insignificant, which allowed the granites in this belt to maintain their I-type nature. The presence of minor primary tin mineralization in the Eastern province compared to the much more significant tin endowment in the Main Range is considered to reflect the incorporation of a smaller proportion of sedimentary protolith in the melt products of the former.

Description: Zircon is arguably the most commonly used geochronometer, but the reliability of ages obtained requires a full understanding of processes that might compromise the integrity of its U-Pb systematics. Here we present the results of a multifaceted ion microprobe study of zircon grains from the Napier Complex, East Antarctica, a region affected by pervasive high-temperature metamorphism at 2.5 Ga, and from which previous zircon geochronological interpretations have been problematic. Both U-Pb spot analysis (~15 μm) and high spatial resolution (~2 μm) scanning ion imaging of Pb isotopes have been applied in an attempt to quantify the effects of metamorphism. Spot analyses spread along concordia yielding 207 Pb/ 206 Pb ages from 2.5 Ga to 3.9 Ga, with the oldest grains reversely discordant. Ion images of uranogenic Pb reveal a surprising micrometer-scale patchy distribution that is unrelated to crystal morphology or damage. The 207 Pb/ 206 Pb ratios within these subdomains correspond to apparent zircon ages as old as 4.2 Ga. These are interpreted as artifacts of ancient redistribution of radiogenic Pb, a process that can generate meaningless ages, and are not relicts of ancient (including Hadean) zircon. Scanning ion imaging thus facilitates identification of unsupported radiogenic Pb and enables testing of the validity of old ages from zircon known to have a long and complicated history.

Description: Constraining the origin and history of very ancient detrital zircons has unique potential for furthering our knowledge of Earth’s very early crust and Hadean geodynamics. Previous applications of the Ti-in-zircon thermometer to 〉4 Ga zircons have identified a population with relatively low crystallization temperatures ( ) of ~685 °C. This could possibly indicate wet minimum-melting conditions producing granitic melts, implying very different Hadean terrestrial geology from that of other rocky planets. Here we report the first comprehensive ion microprobe study of zircons from a transect through the differentiated Sudbury impact melt sheet (Ontario, Canada). The new zircon Ti results and corresponding fully overlap with those of the Hadean zircon population. Previous studies that measured Ti in impact melt sheet zircons did not find this wide range because they analyzed samples only from a restricted portion of the melt sheet and because they used laser ablation analyses that can overestimate true Ti content. It is important to note that internal differentiation of the impact melt is likely a prerequisite for the observed low in zircons from the most evolved rocks. On Earth, melt sheet differentiation is strongest in subaqueous impact basins. Thus, not all Hadean detrital zircon with low Ti necessarily formed during melting at plate boundaries, but at least some could also have crystallized in melt sheets caused by intense meteorite bombardment of the early, hydrosphere-covered protocrust.

Description: Mapping the age and trace element and Sm-Nd isotope compositions of monazite grains from a peraluminous Cretaceous granite using laser ablation–split stream analysis reveals a wide range in Nd isotope and rare earth element (REE) compositions within and between single grains. These data corroborate isotopic variability indicated by Hf isotope analysis of zircon in the same granite sample. The REE variations indicate that monazite grew during fractional crystallization. Hf and Nd isotopes indicate that the granitic magma was generated from at least two distinct Proterozoic sources of approximately the same age: one component that had highly radiogenic initial 176 Hf/ 177 Hf and 143 Nd/ 144 Nd and a second component that was notably less radiogenic. This study highlights the utility of in situ REE and Sm-Nd isotope data in monazite in magmatic systems. Further, it refines the zircon-based constraints on magmatic processes because of sensitivity of light REEs to fractional crystallization, lower probability of complications owing to inheritance, and smaller analytical volumes required.

Description: Zircon grains from the Kiruna iron oxide–apatite (IOA) ore bodies in northern Sweden are distinct in their hafnium and oxygen isotopic ratios compared to zircon grains from adjacent metavolcanic host rocks and related intrusions. Here, we combine these two isotopic systems on previously dated zircon grains to improve our understanding of these ore deposits with a long-debated origin. Contrasting theories for the formation of the Kiruna iron ores suggest either (1) emplacement through immiscible silicate–iron oxide melts or (2) transportation and deposition of iron by hydrothermal fluids. Zircon from the metavolcanic host rocks and intrusions have oxygen isotopic ratios ( 18 O ~3) that lie below typical magmatic compositions, which is evidence that roof rocks altered by meteoric water were digested into the magma. In contrast, the ores show an influence of a fluid that is higher in 18 O (~7). Based on these findings, we propose the involvement of episodic magmatic-hydrothermal fluids in the ore genesis of the Kiruna iron ore deposits: (1) the first episode related to a deep-seated magmatism and to regional-scale metasomatic alteration, and (2) a later fluid event related to shallow intrusions and responsible for the ore formation. Distinct differences in the Hf isotopic ratios for host rocks and intrusions ( Hfi = –6 to –10, Archean crust) and ore ( Hfi = –5 to +3, depleted mantle) further allow us to screen possible fluid sources for their involvement in the ore process.

Description: We present 54 Fe and 56 Fe data on pyrite from burrow-like and irregularly shaped pyrite concretions from Holocene postglacial lacustrine clays in the northern Baltic Sea collected using a secondary ion mass spectrometry (SIMS) microanalysis technique. The burrow-like concretions were formed in reducing, mucus-coated worm burrows in oxic surface sediments. Framboidal pyrite in the cores of the burrow-like concretions shows extensively fractionated 56 Fe values down to –3.1. The framboids are cemented by poorly crystalline FeS 2 with 56 Fe values between –2.1 and +1.4. The irregularly shaped concretions with microcrystalline textures were formed in organic-poor sediment pore spaces, and display a wide spread of 56 Fe values up to +4.1. The measured 56 Fe values reflect the preferential capture of 54 Fe to pyrite in the diagenetic sequence and the 56 Fe enrichment of remaining pore water. The diagenetic sequence of the pyrite materials is supported by previous petrographical study and 34 S/ 32 S microanalysis of the same samples. Our results demonstrate substantial early-diagenetic 56 Fe and 34 S heterogeneity within individual pyrite grains, underlining the necessity of high spatial resolution measurements in studying biological and abiological isotopic signatures.