Jian-xin Zhao

Archaeology and age of a new hominin from Flores in eastern Indonesia

Excavations at Liang Bua, a large limestone cave on the island of Flores in eastern Indonesia, have yielded evidence for a population of tiny hominins, sufficiently distinct anatomically to be assigned to a new species, Homo floresiensis. The finds comprise the cranial and some post-cranial remains of one individual, as well as a premolar from another individual in older deposits. Here we describe their context, implications and the remaining archaeological uncertainties. Dating by radiocarbon (14C), luminescence, uranium-series and electron spin resonance (ESR) methods indicates that H. floresiensis existed from before 38,000 years ago (kyr) until at least 18 kyr. Associated deposits contain stone artefacts and animal remains, including Komodo dragon and an endemic, dwarfed species of Stegodon. H. floresiensis originated from an early dispersal of Homo erectus (including specimens referred to as Homo ergaster and Homo georgicus) that reached Flores, and then survived on this island refuge until relatively recently. It overlapped significantly in time with Homo sapiens in the region, but we do not know if or how the two species interacted.

Characterisation of a plume-related ∼ 800 Ma magmatic event and its implications for basin formation in central-southern Australia

Geochemical and Nd isotopic studies are reported for widespread Late Proterozoic (approximately 800 Ma) mafic dyke swarms and volcanics in central-southern Australia. These mafic suites, although occurring over a large area of > 1000 km, show remarkably uniform geochemical and isotopic features characterised by similar trace element distribution patterns, smooth LREE-enriched patterns, and a limited range Of epsilon(Nd)(800 Ma) values (+2.4 to +4.2), closely resembling the Hawaiian basalts and the high-Ti Karoo flood basalts. These features suggest that this mafic province was probably derived by decompressional melting of a large-scale, uniform asthenospheric mantle plume. Upwelling of the plume resulted in domal uplift of the continental lithosphere, aulacogen-type rifting and onset of flood basalt volcanism. Large-scale crustal extension and thinning followed by thermal subsidence as a result of the plume activity may have been responsible for the formation of the large sedimentary basins in central-southern Australia.

Increasing Australian–Indonesian monsoon rainfall linked to early Holocene sea-level rise

The Australian–Indonesian summer monsoon affects rainfall variability across the Indo–Pacific region. Reconstructions of monsoon strength from stalagmites show that precipitation increased from 11,000 to 7,000 years ago, as rising global sea level caused the flooding of the Indonesian continental shelf. The Australian–Indonesian summer monsoon affects rainfall variability and hence terrestrial productivity in the densely populated tropical Indo–Pacific region. It has been proposed that the main control of summer monsoon precipitation on millennial timescales is local insolation1,2,3, but unravelling the mechanisms that have influenced monsoon variability and teleconnections has proven difficult, owing to the lack of high-resolution records of past monsoon behaviour. Here we present a precisely dated reconstruction of monsoon rainfall over the past 12,000 years, based on oxygen isotope measurements from two stalagmites collected in southeast Indonesia. We show that the summer monsoon precipitation increased during the Younger Dryas cooling event, when Atlantic meridional overturning circulation was relatively weak4. Monsoon precipitation intensified even more rapidly from 11,000 to 7,000 years ago, when the Indonesian continental shelf was flooded by global sea-level rise5,6,7. We suggest that the intensification during the Younger Dryas cooling was caused by enhanced winter monsoon outflow from Asia and a related southward migration of the intertropical convergence zone8. However, the early Holocene intensification of monsoon precipitation was driven by sea-level rise, which increased the supply of moisture to the Indonesian archipelago.

Holocene marine 14C reservoir age variability: Evidence from 230Th-dated corals in the South China Sea

The South China Sea (SCS) is well connected with the western Pacific and influenced by the East Asian monsoon. We have examined temporal variations in radiocarbon marine reservoir ages (R) and regional marine reservoir corrections (Delta R) of the SCS during the Holocene using paired measurements of AMS C-14 and TIMS Th-230 on 20 pristine corals. The results show large fluctuations in both R and Delta R values over the past 7500 years (yrs) with two distinct plateaus during 7.5-5.6 and 3.5-2.5 thousand calendar years before present (cal ka BP). The respective weighted mean Delta R values of these plateaus are 151 +/- 85 and 89 +/- 59 yrs, which are significantly higher than its modern value of -23 +/- 52 yrs. This suggests that using a constant modern Delta R value to calibrate C-14 dates of the SCS marine samples will introduce additional errors to the calibrated ages. Our results provide the first database for the Holocene R and Delta R values of the SCS for improved radiocarbon calibration of marine samples. We interpret the two Delta R plateaus as being related to two intervals with weakened El Nino - Southern Oscillation (ENSO) and intensified East Asian summer monsoon (EASM). This is because the C-14 content of the SCS surface water is controlled by both the C-14 concentration of the Pacific North Equatorial Current (NEC) which is in turn influenced by ENSO-induced upwelling along the Pacific equator and vertical upwelling within the SCS as a result of moisture transportation to midlatitude region to supply the EASM rainfall.

Thermal ionization mass spectrometry U-series dating of a hominid site near Nanjing, China

Mass spectrometric U-series dating of speleothems from Tangshan Cave, combined with ecological and paleoclimatic evidence, indicates that Nanjing Man, a typical Homo erectus morphologically correlated with Peking Man at Zhoukoudian, should be at least 580 k.y. old, or more likely lived during the glacial oxygen isotope stage 16 (similar to 620 ka). Such an age estimate, which is similar to 270 ka older than previous electron spin resonance and alpha counting U-series dates, has significant implications for the evolution of Asian H. erectus. Dentine and enamel samples from the coexisting fossil layer yield significantly younger apparent ages, that of the enamel sample being only less than one-fourth of the minimum age of Nanjing Man. This suggests that U uptake history is far more complex than existing models can handle. As a result, great care must be taken in the interpretation of electron spin resonance and U-series dates of fossil teeth.

Sm-Nd and U-Pb Zircon Isotopic Constraints On the Provenance of Sediments From the Amadeus Basin, Central Australia - Evidence for Ree Fractionation

The Amadeus Basin of central Australia is a Late Proterozoic to Late Palaeozoic ensialic depositional basin located between two Proterozoic basement blocks of different Nd crustal formation ages, the older Arunta Block (TDMNd = 2.0 to 2.2 Ga, U-Pb zircon ages = 1.5–1.9 Ga) to the north and the younger Musgrave Block (TDMNd = 1.7 to 1.9 Ga, U-Pb zircon ages = 1.0–1.7 Ga) to the south. Initial Nd isotopic compositions of the Amadeus Basin sediments generally fall into the region defined by the evolutionary trajectories of these two basement blocks, indicating that the sediments are dominated by essentially two source components: the older Arunta Block [147Sm/144Nd ≈ 0.114 and ϵNd(0) ≈ −20.4] and younger Musgrave Block [147Sm/144Nd ≈ 0.118 and ϵNd(0) ≈ −14.7] and/or their equivalents. Stratigraphically higher sediments plot more closely to the evolutionary trajectory of the Musgrave Block. This result, together with the average provenance ages, indicates the proportion of material from the Musgrave Block increases as the sediments become younger. The Sm-Nd isotopic data preclude a substantial involvement of Archaean sources. U-Pb zircon ion probe analyses of the Late Proterozoic Heavitree Quartzite and the late Cambrian Goyder Formation are consistent with the Sm-Nd isotopic constraints. Most of the zircons from the basal Heavitree Quartzite give concordant U-Pb zircon ages of 1500 to 1900 Ma, consistent with derivation from an Arunta-type source, whilst zircons from the younger Goyder sandstone give six discrete U-Pb zircon age groups of 511 ±20, 615 ± 15, 960 ± 109, 1190 ± 54, 1633 ±24 and 1878 ±48 Ma, indicative of a dominantly Musgrave-type source with a minor contribution from the Arunta Block or reworked preexisting Amadeus Basin sediments. The two youngest U-Pb zircon ages probably indicate some contribution from Late Proterozoic-Cambrian volcanics in central Australia or a previously unrecognised younger local source in the basement blocks. The 511 ±20 Ma constrains the maximum depositional age of the sediment. The Amadeus Basin sediments have a surprisingly wide range of 147Sm/144Nd ratios (0.077–0.136), irrespective of grain size and rock types. In addition, the calculated TDMNd values for samples from the same stratigraphic unit are well correlated with 147Sm/144Nd ratios, with samples of different stratigraphic units forming sub-parallel arrays. These phenomena are interpreted in terms of REE fractionation and preferential sorting of preexisting REE-rich phases during sedimentary recycling and deposition. This process can lead to either an increase or a decrease in 147Sm/144Nd ratio of a sediment relative to its source and therefore an erroneous estimate of its TDMNd provenance age. A theoretical model is developed which accounts for the observed correlation between TDMNd and 147Sm/144Nd ratios for suites of Stratigraphically related sediments and allows a reliable estimate to be made of the provenance age of individual stratigraphic units, assuming the average 147Sm/144Nd ratio of the source is known or can be estimated. The provenance age for each stratigraphic unit of the Amadeus Basin has been calculated using this method. The Sm-Nd isotopic systematics in the Amadeus Basin sediments suggest that 1. (1) REE fractionation during sedimentary recycling can in some cases be an important factor and needs to be considered; 2. (2) the provenance of the Amadeus Basin sediments was controlled by local source regions and cannot be used to infer large-scale continental averages. Hence, some caution must be evoked in using Sm-Nd isotopic constraints from restricted sampling for more generalised models of crustal evolution.

Geochemical and SmNd isotopic study of Neoproterozoic ophiolites from southeastern China: petrogenesis and tectonic implications

Geochemical and SmNd isotopic data are presented for two Neoproterozoic ophiolite suites (∼970 Ma), which were tectonically emplaced into the Banxi Group and equivalent sequences of the Yangtze Block in the southern Anhui and northeastern Jiangxi Provinces, southeast China during the Neoproterozoic (0.9-0.8 Ga). The two ophiolite suites are similar to each other in terms of geochemical feeatures, with volcanics showing affinity to basalts forming in subduction-related environments, typical of the ‘supra-subduction zone (SSZ)’ ophiolites. However, the two suites differ significantly in their initial Nd isotopic compositions. The Jiangxi suite displays uniform initial ϵNd values of +5.5 ± 1.2, implying derivation from a depleted upper mantle source without significant contamination by evolved continental materials. In contrast, the Anhui suite shows a large range of initial ϵNd values (+4.5 to −1.0) which are strongly correlated with SmNd, Nd, MgO and SiO2, consistent with binary mixing between a depleted mantle component (initial ϵNd ∼ +5.5) and an evolved crustal component (initial ϵNd ∼ −1), accompanied by crystal fractionation at different stages. The geochemical and Nd isotopic signatures of the two distinct ophiolite suites, as well as the spatial and temporal distribution of different types of syntectonic granitoids and volcanic series in the region, can be explained in terms of continent-arc-continent collision between the Yangtze and Huanan Blocks during the Neoproterozoic. In this model, it is proposed that the Yangtze and Huanan Blocks were separated by a multi-arc oceanic basin during the early Neoproterozoic (∼1.0 Ga), with both oceanic and continental margin island arcs being developed to the south of the Yangtze Block. The Jiangxi and Anhui ophiolite suites are considered as tectonically emplaced fragments of oceanic crust forming in the inter-arc and continental-margin basins, respectively.

Geochemical and Nd Isotopic Systematics of Granites From the Arunta-Inlier, Central Australia - Implications for Proterozoic Crustal Evolution

Geochemical and SmNd isotopic results are reported for granites from the Proterozoic Arunta Inlier of central Australia. These, combined with new geochronological data for the granites (Zhao and Bennett, 1995), allow important constraints to be placed on Proterozoic tectonic and crustal evolution in central Australia. Granites from the Arunta Inlier can be divided into three major geochemical groups, a Calcalkaline-trondhjemitic Group (CAT), a High-heat-production Group (HHP), and a volumetrically significant Main Group. The CAT Group, which occurs only in the southern margin of the inlier, is characterised by high Na2O, NaK, Sr, KRb and SrY, and relatively low K2O, Rb, RbSr, Th, U, REE, Nb and Y, analogous to calc-alkaline suites occurring in modern convergent plate margins. The HHP Group, which occurs mainly in the interior of the Arunta Inlier and is spatially associated with the Main Group, is characterised by high K, Rb, Th, U, RbSr and RbZr, and relatively low Sr, Ba, NaK, KRb, BaRb, MgO, Cr and Ni. The Main Group, which occurs throughout the Arunta Inlier, is geochemically intermediate between the CAT and HHP groups. It is geochemically analogous to the 1880-1850 Ma old Barramundi Igneous Association recognised in other Proterozoic terrains of central Australia, and can be further subdivided into four age subgroups, 1820, 1780-1750, 1650 and 1615-1600 Ma, respectively. All three groups show similar negative Nb anomalies on the trace-element-normalized diagrams. Despite the geochemical diversity of the Arunta granites, no correlations between the Nd isotopic and geochemical signatures are observed. The ranges of initial ϵNd values and Nd-depleted mantle model ages (TNdDM)) for the three groups of granites overlap with each other. Overall, there are two groups of TNdDM ages, the most common ranging from 2.33 to 1.96 Ga, and the other from 1.83 to 1.72 Ga. Although initial ϵNd values of the granites show large variations, there is a general trend for the ϵNd values to increase (i.e., become less negative) with decreasing crystallization ages. The large geochemical and isotopic variations of the Arunta granites reflect considerable heterogeneity in the sources of the granites and do not support a uniform, 2.3-2.1 Ga old Barramundi-type underplate source for the origin of the granites. The combined geochemical and Nd isotopic data suggest the sources of the granites contain at least two main components, an Palaeoproterozoic mantle-derived component and an older crustal component. The older crustal component was incorporated into the source region of the granites either through magma assimilation of crustal materials at lower crustal levels or more probably via sediment subduction and mantle wedge metasomatism. The increasing ϵNd values with decreasing crystallization ages may be a result of progressively increasing proportions of newly accreted island-arc materials being incorporated in the subducted sediments. Alternatively, a juvenile mantle-derived component may have been added to the source region of the younger granites. It is apparent that the Nd isotopic signatures in the granites has resulted from complex processes and neither simple mixing nor simple two-stage protolith models can as yet satisfactorily explain the observations. The petrogenesis of the three geochemical groups of granites can be interpreted in a plate-tectonics scenario. It is considered that the CAT Group was formed by fractionation of arc-type magmas and/or partial melting of arc-related intrusions or underplates in a subduction-related continental margin setting, whilst the formation of the Main Group may involve partial melting of fractionated and/or modified arc-related underplates, but not necessarily in a subduction environment. The younger HHP Group was probably generated by remelting of the coexisting Main Group during subsequent tectonothermal events.

Geochemical and SrNd isotopic study of charnockites and related rocks in the northern Prince Charles Mountains, East Antarctica: implications for charnockite petrogenesis and proterozoic crustal evolution

Charnockite plutons were intruded into Meso-Neoproterozoic (∼1000 Ma) high-grade metamorphic zone in the northern Prince Charles Mountains (PCM), East Antarctica, immediately after peak granulite metamorphism in the region. Detailed geochemical and SrNd isotopic studies were carried out on these plutons and related rocks, which enables important constraints to be placed on the regional tectonic setting as well as the origin of igneous charnockites. The PCM charnockites are geochemically distinctive, characterised by having much higher TiO2, P2O5, K2O, K2ONa2O, Zr, Nb, Y, Pb, La, Ce, and Ba, and lower MgO, CaO, Mg#, Th, U, SrBa, and RbBa, than, for example, I-type granites from the Lachlan Foldbelt. The decrease in Zr, Nb, Y and Ce with increasing SiO2, sharply contrast with those of the I-type granites. Isotopically, the PCM charnockites are relatively uniform and evolved, characterised by limited ranges of initial 87Sr86Sr ratios (0.7063 to 0.7100), initial ϵNd values (mainly −4.0 to −5.9), and Nd depleted mantle model ages (1.60 to 1.98 Ga), implying derivation from pre-existing crustal sources. The geochemical and isotopic features are most consistent with crystal fractionation of dry hot magmas, with pyroxene, K-feldspar, plagioclase, apatite, zircon, ilmenite and magnetite as early-crystallizing phases. Although the involvement of a mantle-derived magma via AFC process cannot be ruled out, we consider that the charnockitic magmas were mainly derived from pre-existing subduction-related crustal sources, geochemically and isotopically similar to those of the I-type granites. The partial melting probably occurred under dry granulitic conditions at elevated temperatures (950–1050°C), with orthopyroxene, plagioclase and magnetite being residual phases. Under such conditions, elements including K, Rb, Ba, La, Ce, Nd, Zr, Nb and Y will be strongly incompatible and partition into the melt. The relatively low U and Th values in the charnockites are probably due to U-, Th-depletion in the sources, which may have been caused by dehydration and U-, Th-removal during amphibolite-to-granulite transition of the sources. We consider that the PCM charnockites and related regional metamorphism resulted from Meso- to Neoproterozoic continental collision between Archaean and Palaeoproterozoic cratonic blocks in East Antarctica. The Meso-Neoproterozoic collision was probably a global event, possibly related to construction of the Rodinia Supercontinent. During this collisional period or earlier orogenic events in the region (e.g. arc-continent collision in the Palaeo-Mesoproterozoic), calcareous sediments formed at plate margins or back-arc basins would have been tectonically transported to depth. Release of CO2-rich fluids upon tectonic burial may have been responsible for amphibolite-to-granulite transition without causing dehydration melting to generate I-type granites. Instead, subsequent uplift of the dehydrated, but fertile, granulite crust during a period of crustal rebound may have facilitated decompression melting to produce high-temperature, water-deficient, charnockitic melts. Syn- to post-collision lithosphere delamination, asthenosphere upwelling and magma underplating may have also aided in heating the lower crust above its water-deficient solidus temperature.

Geochemical and Isotopic Studies of Syenites From the Yamato Mountains, East Antarctica - Implications for the Origin of Syenitic Magmas

Voluminous syenites were intruded during the waning stage of the granulite facies metamorphism in the Yamato Mountains of East Antarctica. The area has been interpreted as part of a Cambrian continental collision zone with regional upper amphibolite to granulite facies metamorphism occurring during ca. 500–660 Ma period. Regardless of minor geochemical variations between different groups, all syenites are characterised by high K20 + Na20 (8–12%), K20/Na20 (∼2), Sr (800–3500 ppm), Ba (2000–8500 ppm), and comparatively high TiO2, P205, Zr, and light REES relative to I-type granites. They are significantly higher in Mg number (50–75) compared with typical calc-alkaline suites, igneous charnockites, or A-type granites and define a distinctive trend on an AFM (alkali-FeOtot-MgO) diagram. Their trace element distribution diagrams are characterised by pronounced enrichment in LIL and REES, large negative Nb and Ti anomalies, and no depletion in Sr or Ba relative to the neighbouring elements. In this regard, they closely resemble the ∼500 Ma post-tectonic mela-syenite to alkali basalt dikes widely occurring in East Antarctica. Such geochemical features are distinct from rift- or hotspot-related syenites, which are usually characterised by low K/Na ratios, negative Ba and Sr anomalies, and a lack of negative Nb anomalies. Initial isotopic compositions of the syenites are characterised by relatively low initial eNd values (−2.6 to −5.5) and high Sri ratios (0.7057–0.7088). Since the syenites are extremely enriched in Sr and Nd, such isotopic signatures are interpreted as reflecting the nature of the mantle source, rather than significant crystal contamination. Such isotopic signatures are also distinct from those of the rift- or hotspot-related syenites which are thought to be derived from depleted asthenospheric mantle. Considering the distinctive geochemical signatures of the Yamato syenites and their analogy to posttectonic alkaline mafic dikes in Antarctica, it is proposed that the syenites were generated by fractionation and magma mixing (with a crystal melt) of a Si-undersaturated alkali basaltic magma in a lower-crust magma chamber, followed by further crystal fractionation/cumulate-melt unmixing at middle to upper crustal levels (<5 kbars). Tectonically, it is proposed that the syenites were probably formed within the hinterland of the proposed Cambrian continental collision zone, with the parental magma being derived ultimately by partial melting of the metasomatised mantle wedge above the deepest part of the subduction zone. Similar models may also apply to the origin of some post-tectonic alkaline dikes in East Antarctica, with their sources being the continental lithospheric mantle previously modified by subduction-related processes.