Phanerozoic Orogenic Belts Research Articles

Three Dimensional Crustal Density Structure of Central Asia and its Geological Implications

This paper introduces the scale-depth law of multi-scale wavelet analysis for regional gravity data processing, and presents the results of its application to Central Asia for computation of the 3D crustal density structures. The wavelet analysis method is applied for characterizing 3D crustal density structure, producing five maps of density disturbance corresponding to different depths of equivalent layers in the crust. The results provide important evidence for the study of crustal structures and mass movement in Central Asia: (i) the small-scale and intensive linear density disturbances in the upper crust indicate Phanerozoic orogenic belts; (ii) there exists a horseshoe-shaped low-density belt in the middle crust coinciding with the Kazakhstan orocline; (iii) there is a very low density zone in the lower crust, extending from western Kunlun to Tianshan, probably indicating a lower-crust flow; (iv) there are a few lowdensity spots in the middle crust, which might be caused by low-density mass squeezing upward from the lower crust flows.

Uppermost mantle structure of the Australian continent from Pn traveltime tomography

AbstractPast studies of the seismic structure of the Australian continent have dominantly exploited surface wave tomography for the mantle, with seismic refraction, receiver functions, and ambient noise used for crustal structure. The 3‐D structure has been summarized in the Australian Seismological Reference Model (AuSREM), for which the zone immediately below the crust is the least well characterized. Pn traveltime tomography provides a way of improving structural information on the uppermost mantle across the continent. We have exploited waveforms from larger events across Australia recorded at both permanent and portable stations since 1993, supplemented by bulletin arrival times. To compensate for the large velocity contrasts, with much faster mantle wave speeds in the center and west, all events were relocated using the AuSREM model. After relocation, consistent patterns of traveltime residuals are obtained. We extract Pn as the first arrival in the distance range 1.8° to 15°. We use the FMTOMO approach to invert the travel‐time residuals to generate a P wave speed structure with a resolution of 3°×3°. There is strong heterogeneity in Pn wave speed in the uppermost mantle across the continent. The fastest Pn wave speed of 8.36 km/s beneath the Precambrian cratons of western and central Australia is 3.99% faster than the global ak135 model, and the slowest Pn wave speed 7.66 km/s on the eastern margin is 4.74% slower. The slowest velocities in the uppermost mantle are found along the eastern margin of the Australian continent beneath the Phanerozoic orogenic belts, with links to Neogene volcanism.

Cenozoic exhumation history of Sulu terrane: Implications from (U–Th)/He thermochrology

The Qinling–Dabie–Sulu orogen is the most prominent Phanerozoic orogenic belt in China. The discovery of ultra-high pressure (UHP) minerals in zircon inclusions suggests that the crust was subducted to deeper than 120km into the mantle and then exhumed to shallow crustal. Recently, low temperature thermochronology has been applied to constrain the final exhumation of Dabie Shan, while there are few studies describing the Cenozoic exhumation history of the Sulu belt. Here we report some (U–Th)/He ages for various lithologies from Sulu Orogenic belt and its northern part-Jiaobei terrane. The single grain He ages range between 18 and 154Ma, and most of the samples having large intra-sample age scattering. Several reasons such as invisible U/Th-rich inclusions, grain size effect, slow cooling rate, and zonation of parent nuclide or radiation damage effect may account for this dispersion. For all samples, the pattern of the single grain age data exhibits a peak at ~45Ma which is consistent with the borehole fission-track age pattern in adjacent Hefei Basin. Both (U–Th)/He and fission track ages of the Sulu area suggest an enhanced exhumation/cooling in Early-Middle Eocene in the southern part of Tan-Lu fault zone. This enhanced cooling event coincides with rapid subsidence of North China Basin and rapid uplift of its surrounding reliefs, which indicates basin-mountain coupling. This Eocene event is widespread in central China and could be far-field consequence of India–Asia collision. The convergence rate between Pacific Plate and Eurasia decreased substantially during early Tertiary and reached a minimum in Eocene (~30–40mm/yr) while at the same time the collision between India and Asia was completed. Therefore, the Cenozoic exhumation history of the Sulu Orogenic Belt was a combined result of far-field effect of India–Asia collision and declined subduction rate of the Pacific Plate under Eurasia.

Paleoproterozoic crustal growth in the North China Craton: Evidence from the Lüliang Complex

The building of continental blocks and their assembly into supercontinents are linked to episodic crustal growth through Earth history, where both vertical and horizontal accretions in convergent plate margins play a significant role. Here we investigate the Paleoproterozoic crustal growth in the North China Craton (NCC) from a traverse covering continent-trench-ocean section in the Lüliang Complex within the western margin of the Trans-North China Orogen, a major accretionary belt that sutures the Eastern and Western Blocks in the NCC. We present petrological, geochemical and zircon U–Pb and Lu–Hf data from a suite of samples including meta granitoids and gabbroic diorite that built the continental arc, and metabasalts, felsic tuff, mafic and pelitic schists, and garnet amphibolites that form part of an imbricated package of oceanic and continental material that was accreted onto the eastern margin of the Western Block in the NCC. The salient geochemical features of the magmatic suite such as high Th/Ta and La/Nb ratios, negative Nb–Ta anomalies, and high concentration of LILE as compared to HFSE suggest magma derivation from sub-continental mantle source which has undergone different degree of enrichment with crustal components or material derived from subduction-related older components. In tectonic discrimination diagrams, the granitoids show VAG affinity and all the rocks straddle the field between IAB and MORB reflecting subduction-related arc signature. The majority of zircon grains in all the rock types from the Lüliang Complex show typical magmatic crystallization textures and high Th/U values. Zircons from the meta-granitoids and diorite enclave yield upper intercept or 207Pb/206Pb mean ages in the range of 2250±20 to 2084±69Ma. Those in the mafic schist show a mean age of 2234±9Ma. In the felsic schist group, the mean ages are in the range of 2298±15 to 1933±11Ma. A minor older population of zircons with ages of 2992±28Ma and 2610±80Ma also occurs in these rocks. In the metabasalt, zircon 207Pb/206Pb mean ages are between 2391±40 and 2022±39Ma with rare magmatic xenocrysts showing Mesoarchean age of 3294±21Ma. Zircons in the felsic tuff show mean ages of 2155±25 to 1897±24Ma, with xenocryst populations yielding mean ages of 2528±31Ma and 2710±20Ma. The youngest zircons with 207Pb/206Pb mean age of ca. 1.9Ga from the felsic schist (metasediment) and felsic tuff (volcanic suite) suggest deposition within the trench of an active convergent margin at this time. In the garnet-bearing amphibolite, the zircons yield a limited age range of 1996±17Ma to 1984±12Ma. The Lu–Hf data on zircons from all samples show dominantly positive ɛHf(t) values. In the granitoid–diorite suite, the ɛHf(t) values range up to 7.6 with crustal residence ages (TDMC) of 2371–2729Ma. Zircons in the mafic and felsic schists also possess positive ɛHf(t) values (up to 9.6) with TDMC in the range of 2366–2798Ma. In the metabasalt, both positive and negative ɛHf(t) values are displayed (3.8 to −3.2) with TDMC in the range of 2495–2830Ma. Zircons in the felsic tuff also show prominent positive ɛHf(t) value (up to 7.4) and crustal residence ages range from 2274 to 2607Ma. In the garnet-bearing amphibolite, the ɛHf(t) values lie between 3.3 and 4.8 and TDMC are in the range of 2321–2413Ma. The majority of ɛHf(t) values from all the rock types in the present study are compatible with juvenile magma sources, with limited input from crustal components. Their TDMC values indicate that both Neoarchean and Paleoproterozoic components were involved in the source, attesting to a continental arc setting. The Hf data also suggest the possible presence of Mesoarchean vestiges among the dominantly Neoarchean–Paleoproterozoic basement. Our data demonstrate active plate tectonics and major crustal growth during the Paleoproterozoic history of the NCC, broadly coeval with the making of the global supercontinent Columbia, and provide insights into continent building through subduction–accretion–collision tectonics analogous to those in Phanerozoic orogenic belts.

Rock magnetism of tiny exsolved magnetite in plagioclase from a Paleoarchean granitoid in the Pilbara craton

AbstractGranitoids are widespread in Precambrian terranes as well as the Phanerozoic orogenic belts, but they have garnered little attention in paleomagnetic studies, because granitoids often contain abundant coarse‐grained, magnetically unstable oxides. In this study, the first example of tiny, needle‐shaped, exsolved oxides in plagioclase in a Paleoarchean granitoid is reported. The magnetic properties of single plagioclase crystals with the exsolved oxide inclusions have been studied to determine their paleomagnetic recording fidelity. Demagnetization experiments and hysteresis parameters indicate that the oxide inclusions are near stoichiometric magnetite and magnetically very stable. First‐order reversal curve (FORC) diagrams reveal negligible magnetostatic interactions. Minimal interactions are also reflected by very efficient acquisition of anhysteretic remanent magnetization. Single plagioclase crystals exhibit strong magnetic remanence anisotropies, which require corrections to their paleodirectional and paleointensity data. Nonetheless, quantitative consideration of anisotropy tensors of the single plagioclase crystals indicates that the bias can be mitigated by properly averaging data from a few tens of single crystals. From the nonlinear thermoremanence acquisition of the plagioclase crystals, we estimate that the plagioclase crystals can reconstruct paleointensity up to 50 μT. Local metamorphic condition suggests that those magnetite may carry remanence of ∼3.2 to 3.3 Ga. We suggest that exsolved magnetite in granitoids is potentially a suitable target for the study of the early history of the geomagnetic field, and prompt detailed microscopic investigations as well as paleomagnetic tests to constrain the age of remanence.

Four billion years of ophiolites reveal secular trends in oceanic crust formation

We combine a geological, geochemical and tectonic dataset from 118 ophiolite complexes of the major global Phanerozoic orogenic belts with similar datasets of ophiolites from 111 Precambrian greenstone belts to construct an overview of oceanic crust generation over 4 billion years. Geochemical discrimination systematics built on immobile trace elements reveal that the basaltic units of the Phanerozoic ophiolites are dominantly subduction-related (75%), linked to backarc processes and characterized by a strong MORB component, similar to ophiolites in Precambrian greenstone sequences (85%). The remaining 25% Phanerozoic subduction-unrelated ophiolites are mainly (74%) of Mid-Ocean-Ridge type (MORB type), in contrast to the equal proportion of Rift/Continental Margin, Plume, and MORB type ophiolites in the Precambrian greenstone belts. Throughout the Phanerozoic there are large geochemical variations in major and trace elements, but for average element values calculated in 5 bins of 100 million year intervals there are no obvious secular trends. By contrast, basaltic units in the ophiolites of the Precambrian greenstones (calculated in 12 bins of 250 million years intervals), starting in late Paleo- to early Mesoproterozoic (ca. 2.0–1.8 Ga), exhibit an apparent decrease in the average values of incompatible elements such as Ti, P, Zr, Y and Nb, and an increase in the compatible elements Ni and Cr with deeper time to the end of the Archean and into the Hadean. These changes can be attributed to decreasing degrees of partial melting of the upper mantle from Hadean/Archean to Present. The onset of geochemical changes coincide with the timing of detectible changes in the structural architecture of the ophiolites such as greater volumes of gabbro and more common sheeted dyke complexes, and lesser occurrences of ocelli (varioles) in the pillow lavas in ophiolites younger than 2 Ga. The global data from the Precambrian ophiolites, representative of nearly 50% of all known worldwide greenstone belts provide significant clues for the operation of plate tectonic processes in the Archean.

Geochemical characteristics and tectonic setting of the Tuerkubantao mafic-ultramafic intrusion in West Junggar, Xinjiang, China

Mineral chemistry, whole-rock major oxide, and trace element compositions have been determined for the Tuerkubantao mafic-ultramafic intrusion, in order to understand the early Paleozoic tectonic evolution of the West Junggar orogenic belt at the southern margin of the Central Asian orogenic belt. The Tuerkubantao mafic-ultramafic intrusion is a well-differentiated complex comprising peridotite, olivine pyroxenite, gabbro, and diorite. The ultramafic rocks are mostly seen in the central part of the intrusion and surrounded by mafic rocks. The Tuerkubantao intrusive rocks are characterized by enrichment of large ion lithophile elements and depleted high field strength elements relative to N-MORB. In addition, the Tuerkubantao intrusion displays relatively low Th/U and Nb/U (1.13–2.98 and 2.53–7.02, respectively) and high La/Nb and Ba/Nb (1.15–4.19 and 37.7–79.82, respectively). These features indicate that the primary magma of the intrusion was derived from partial melting of a previously metasomatized mantle source in a subduction setting. The trace element patterns of peridotites, gabbros, and diorite in the Tuerkubantao intrusion have sub-parallel trends, suggesting that the different rock types are related to each other by differentiation of the same primary magma. The intrusive contact between peridotite and gabbro clearly suggest that the Tuerkubantao is not a fragment of an ophiolite. However, the Tuerkubantao intrusion displays many similarities with Alaskan-type mafic-ultramafic intrusions along major sutures of Phanerozoic orogenic belts. Common features include their geodynamic setting, internal lithological zoning, and geochemistry. The striking similarities indicate that the middle Devonian Tuerkubantao intrusion likely formed in a subduction-related setting similar to that of the Alaskan-type intrusions. In combination with the Devonian magmatism and porphyry mineralization, we propose that subduction of the oceanic slab has widely existed in the expansive oceans during the Devonian around the Junggar block.

The Mohorovičić discontinuity beneath the continental crust: An overview of seismic constraints

Abstract The seismic signature of the Moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image. Seismic data of different types, passive source (earthquake) broad-band recordings, and controlled source seismic refraction, densely recorded wide-angle deep seismic reflection, and normal incidence reflection (using VibroseisTM, explosives, or airguns), have contributed to the description of the Moho as a relatively complex transition zone. Of critical importance for the quality and resolution of the seismic image are the acquisition parameters, used in the imaging experiments. A variety of signatures have been obtained for the Moho at different scales generally dependent upon bandwidth of the seismic source. This variety prevents the development of a single universally applicable interpretation. In this way source frequency content, and source and sensor spacing determine the vertical and lateral resolution of the images, respectively. In most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the Moho, and its relationship to a petrologic crust–mantle transition. In regional seismic studies carried out using passive source recordings the Moho is a relatively well defined structure with marked lateral continuity. The characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area. Refraction and wide-angle studies suggest the Moho to be often a relatively sharp velocity contrast, whereas the Moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity. The origin of the Moho and its relation to the crust–mantle boundary is probably better constrained by careful analysis of its internal details, which can be complex and geographically varied. Unlike the oceanic Moho which is formed in a relatively simple, well understood process, the continental Moho can be subject to an extensive variety of tectonic processes, making overarching conclusions about the continental Moho difficult. Speaking very broadly: 1) In orogenic belts still undergoing compression and active continental volcanic arcs, the Moho evolves with the mountain belt, 2) In collapsed Phanerozoic orogenic belts the Moho under the collapse structure was formed during the collapse, often by a combination of processes. 3) In regions having experienced widespread basaltic volcanism, the Moho can result from underplated basalt and basaltic residuum. In Precambrian terranes the Moho may be as ancient as the formation of the crust, in others Precambrian tectonic and magmatic processes have reset it. We note that seismic reflection data in Phanerosoic orogens as well as from Precambrian cratonic terranes often show thrust type structures extending as deep as the Moho, and suggest that even where crust and mantle xenoliths provide similar age of formation dates, the crust may be semi-allochothonous.

Late Paleozoic mafic–ultramafic intrusions in southern Central Asian Orogenic Belt (NW China): Insight into magmatic Ni–Cu sulfide mineralization in orogenic setting

The mafic–ultramafic intrusions hosting magmatic Ni–Cu sulfide ore deposits, usually present within the Precambrian cratons and on their margin, have been extensively studied, whereas those occurring within the Phanerozoic orogenic belts have received little attention. The southern margin of the Central Asian Orogenic Belt is a region of late Paleozoic mafic–ultramafic intrusions emplaced during post-orogenic stage. In order to recognize the mineralization features, this study compares barren or weakly mineralized mafic–ultramafic intrusions in the Beishan with those hosting Ni–Cu sulfide ore deposit in the Altay and Eastern Tianshan in petrology, mineralogy and geochemistry aspects. Several mineralization features can be recognized with respects to petrology, mineralogy and geochemistry from comprehensive comparisons. Among the mineralization indicators, we highlight strong alteration accompanying sulfide emplacement, post-erosion exposure of magma conduit, abundant primary hydrous minerals and potential transportation mechanism of ore-forming elements. Other indicators are the presence of orthopyroxene and pyroxenite for potential sulfide formation through crystal fractionation, restricted Fo and Ni ranges in olivines for sulfide segregation, and some other special geochemical characteristics related to crustal contamination. The variation in mineralization could be ascribed to different degrees of partial melting, crustal assimilation and subduction-related modification of a mantle source. We suggest that the Ni–Cu sulfide mineralization of mafic–ultramafic intrusions within orogenic belts is remarkably different from that of cratonic and continental marginal settings in respects of petrogenesis and mineralizing mechanism. Magmatic conduit-type Ni–Cu sulfide ore deposit is a target for future prospecting and exploration for mafic–ultramafic intrusions in the orogenic belts.

Precambrian geology of China

Precambrian geology of China

An intrusion-related gold deposit (IRGD) in the NW of Spain, the Linares deposit: Igneous rocks, veins and related alterations, ore features and fluids involved

The Linares deposit is an intrusion-hosted gold deposit located in the NW of the Iberian Variscan Belt. The igneous rocks hosting the ore are the Linares adamellite and the Arganzúa leucogranite. They are high-K, calc-alkaline and slightly peraluminous post-collisional igneous rocks, a mixture of I- and S-types, the Arganzúa leucogranite representing a late-stage fractionated phase. The intrusions have a reduced to intermediate oxidation state which spans the boundary between the ilmenite and magnetite series. The gold mineralization occurs disseminated or along a network of microfractures and veins of variable size, commonly in sheeted vein array. The alteration observed includes both potassic, with K-feldspar and secondary biotite developed mainly in the Arganzúa leucogranite, and sericitic and propylitic alterations. A sericite–chlorite–carbonate alteration, related to hairline fractures, is broadly distributed but rarely pervasive. Wolfram-bearing mineralization is followed by sulphide precipitation, mainly pyrrhotite, chalcopyrite, pyrite, arsenopyrite, molybdenite and galena, with minor löllingite and sphalerite. Gold is commonly associated with Bi–Te-bearing minerals with Bi/Te(S+Se)≥1, consistent with a pyrrhotite-buffered environment. This is reflected in the strong correlation observed between Au and Bi. The fluids involved in the sulphide precipitation are aqueous-carbonic (CO2 and ±CH4), with low salinity (0.5–6.3wt.% NaCl eq.) and variable amounts of other volatiles (N2 and H2S). The fluid inclusion study registered an adiabatic drop from a lithostatic pressure of 1.8–2.6kbar to a hydrostatic pressure of 0.3–0.9kbar, at temperatures of 300–400°C. The fault-valve phenomena could explain the pressure drop producing a volatile release from an early CO2-rich fluid. A later trapped fluid with a wide-ranging CO2/CH4 ratio suggests an isothermal mixture with a CH4(±N2)-rich fluid external to the granitoid (metamorphic). The effervescence and the fluid-mixing could be the possible mechanisms of ore precipitation in this system. The Linares deposit presents a number of features in common with intrusion-related gold deposits elsewhere in Phanerozoic orogenic belts. The existence of intrusion-related gold systems would have potentially important implications for exploration in the NW of the Iberian Variscides.

Precambrian key tectonic events and evolution of the North China craton

Abstract The North China craton (NCC) is one of oldest cratons in the world, with crust up to c . 3.8 Ga old, and has a complicated evolution. The main Early Precambrian geological events and key tectonic issues are as follows. (1) Old continental nuclei have been recognized in the NCC, and the oldest remnants of granitic gneiss and supracrustal rocks are 3.8 Ga old. The main crustal growth in the NCC took place at 2.9–2.7 Ga. The NCC can be divided into several microblocks, which are separated by Archaean greenstone belts that represent continental accretion surrounding the old continental nuclei. (2) By 2.5 Ga, the microblocks amalgamated to form a coherent craton by continent–continent, arc–continent or arc–arc collisions. The tectonic processes in Neoarchaean and modern times appear to differ more in degree than in principle. Extensive intrusion of K-granite sills and mafic dykes and regional upper amphibolite- to granulite-facies metamorphism occurred, and marked the beginning of cratonization in the NCC. Coeval ultramafic–mafic and syenitic dykes of c . 2500 Ma in Eastern Hebei indicate that the NCC became a stable, thick and huge continent at the end of the Archaean, and probably was a part of the Neoarchaean supercontinent that has been suggested by previous studies. (3) In the period between 2500 and 2350 Ma, the NCC was tectonically inactive, but the development of a Palaeoproterozoic volcanic and granitic rocks occurred between 2300 and 1950 Ma. The volcanic–sedimentary rocks are termed Palaeoproterozoic mobile belts; these have a linear distribution, and were affected by strong folding and metamorphism at 1900–1850 Ma, and intruded by granites and pegmatites at 1850–1800 Ma. The Palaeoproterozoic mobile belts formed and evolved within the craton or continental margin (epicontinental geosyncline). Some 2.30–1.95 Ga rift-margin, passive continental margin deposits, analogous arc or back-arc assemblages, as well as HP and HT–UHT metamorphic complexes seem to be comparable with many in the late Phanerozoic orogenic belts. Regarding Palaeoproterozoic orogeny in other cratons, it is possible that a global Palaeoproterozoic orogenic event occurred, existed and resulted in the formation of a pre-Rodinian supercontinent at c . 2.0–1.85 Ga. (4) In contrast, the c . 1800 Ma event is an extension–migmatization event, which includes uplift of the lower crust of the NCC as a whole, the emplacement of mafic dyke swarms, continental rifting, and intrusion of an orogenic magmatic association. This event has been considered to be related to the break-up of the pre-Rodinian supercontinent at 1.8 Ga, attributed to a Palaeoproterozoic plume. (5) As HP and HT–UHT metamorphic rocks occur widely in the NCC, their high pressure of 10–14 kbar has attracted attention from researchers, and several continental collisional models have been proposed. However, it is argued that these rocks have much higher geothermal gradient and much slower uplift rate than those in Phanerozoic orogenic belts. Moreover, HP and HT–UHT rocks commonly occur together and are not distributed in linear zones, suggesting that the geological and tectonic implications of these data should be reassessed.

A fluid factory in solid Earth

Global material circulation in our planet from the surface to the bottom of the mantle is controlled by a combination of plate, plume, and “anti–plate” tectonics, where fluids and melts play an active role. Whereas crustal fluids are dominated by CO 2 and H 2 O, with subordinate CH 4 and N 2 , the volatiles in the lower mantle are speculated to be dominantly CO 2 . The process of Archean subduction aided in the sequestration of CO 2 from the ocean-atmosphere system, with the ultimate probable destination being the mantle. When a rising “superplume” hits the tectosphere, the predicted carbonated continental keel would become enriched in CO 2 . During the Phanerozoic, the carbonated upper mantle was drastically reduced in size, as speculated from the scarcity of dry, “ultrahot” orogens. Free fluid circulation within Earth is present only in restricted zones, mostly along the plate boundaries and intracontinental rifts, and particularly along subduction zones, where the fluid is mostly water dominated. The propagating water front in the deep mantle in modern Earth may correspond to the apparent increase in pressure through geologic time, which might be one of the reasons for a general lack of ultrahigh-pressure metamorphic belts in the Archean and their prevalence in the Phanerozoic orogenic belts.

Ancient orogens and modern analogues: an introduction

Abstract Plate-tectonics principles have been routinely applied to the study of Phanerozoic orogenic belts and, more controversially, to Precambrian orogens as far back as the Early Archaean. Recent advances in a variety of fields have vastly improved our understanding of ancient orogenic belts, so that realistic modern analogues can be entertained. This volume presents up-to-date syntheses of some classic modern and ancient orogenic belts as well as examples of some of the processes responsible for their evolution.

The Quetico Intrusions of Western Superior Province: Neo-Archean examples of Alaskan/Ural-type mafic–ultramafic intrusions

The Quetico Intrusions are a series of ultramafic–mafic igneous rocks that occur along the northern Quetico subprovince boundary. These intrusions show lithological zoning from cores of olivine-bearing ultramafic rocks to margins of hornblendite and gabbroic rocks, and they are commonly accompanied by Cu–Ni–platinum group element (PGE) sulphide mineralization. We identified three additional Quetico Intrusions in the centre and southern part of the Quetico subprovince, bringing the entire Quetico Intrusion array to be over 125 km in length across the subprovince. The Samuels Lake intrusion in the centre of the subprovince has an U–Pb age of 2688 + 6/−5 Ma, which is similar to the age of ∼2690 Ma for the Quetico Intrusions along the northern subprovince boundary, suggesting that all the Quetico Intrusions are likely to be contemporaneous. The parental magmas of the intrusions were Mg-rich based on the high Mg of olivine and clinopyroxene, and they were hydrous as evidenced by abundant hornblende, early crystallization of clinopyroxene, and the common occurrence of mafic pegmatite. The bulk rocks show arc geochemical signatures, including a high concentration of large ion-lithophile elements combined with a low concentration of high field-strength elements. The mineral compositions of pyroxene and hornblende also plot in the field of arc igneous rocks. The data suggest the derivation of the parental magmas from a mantle wedge or a refractory mantle that was previously metasomatized in a subduction setting. The Quetico Intrusions display many similarities with Alaskan/Ural-type zoned mafic–ultramafic intrusions along major sutures of Phanerozoic orogenic belts. Common features include their geodynamic settings, internal lithological zoning, mineralogy and mineral chemistry, and common association with PGE mineralization. The striking similarities between the two indicate that the late Archean Quetico Intrusions likely formed in a tectonic setting similar to that of the Alaskan/Ural-type intrusions. We propose that the parental magmas for the Quetico Intrusions formed during the very early stage of accretion of the Wawa arc to the Wabigoon subprovince. Oblique accretion of the Wawa arc likely steepened the subduction angle, which allowed upwelling of hot asthenospheric mantle. This resulted in high degrees of partial melting in the mantle wedge to produce the Quetico Intrusions.

Speculations on evolution of the terrestrial lithosphere–asthenosphere system—Plumes and plates

In the early Earth, accretionary impact heating, including collision with a large, Mars-sized object, decay of short-lived radioisotopes, and (after an initial thermal run-up) continuous segregation of the liquid Fe–Ni core resulted in extensive the melting of the silicate mantle and in the formation of a near-surface magma mush ocean. Progressive, continuous degassing and chemical–gravitational differentiation of the crust–mantle system accompanied this Hadean stage, and has gradually lessened during the subsequent cooling of the planet. Mantle and core overturn was vigorous in the Hadean Earth, reflecting deep-seated chemical heterogeneities and concentrations of primordial heat. Hot, bottom-up mantle convection, including voluminous plume ascent, efficiently rid the planet of much thermal energy, but gradually decreased in importance with the passage of time. Formation of lithospheric scum began when planetary surface temperatures fell below those of basalt and peridotite solidi. Thickening and broadening of lithospheric plates are inferred from the post-Hadean rock record. Developmental stages of mantle circulation included: (a) 4.5–4.4 Ga, early, chaotic magma ocean circulation involving an incipient or pre-plate regime; (b) 4.4–2.7 Ga, growth of small micro-oceanic and microcontinental platelets, all returned to the mantle prior to 4.0 Ga, but increasing in size and progressively suturing sialic crust-capped lithospheric amalgams at and near the surface over time; (c) 2.7–1.0 Ga, assembly of cratons surmounting larger, supercontinental plates; and (d) 1.0 Ga–present, modern, laminar-flowing asthenospheric cells capped by gigantic, Wilson-cycle lithospheric plates. Restriction of komatiitic lavas to the Archean, and of ophiolite complexes ± alkaline igneous rocks, high-pressure and ultrahigh-pressure metamorphic terranes to progressively younger Proterozoic–Phanerozoic orogenic belts supports the idea that planetary thermal relaxation promoted the increasingly negative buoyancy of cooler oceanic lithosphere. The Thickening of oceanic plates enhanced the gravitational instability and the consequent overturn of the outer Earth as cold, top-down oceanic mantle convection. The scales and dynamics of deep-seated asthenospheric circulation, and of lithospheric foundering + shallow asthenospheric return flow evidently have evolved gradually over geologic time in response to the progressive cooling of the Earth.

POSSIBLE INTRUSION-RELATED GOLD SYSTEMS INTHE WESTERN LACHLAN OROGEN, SOUTHEAST AUSTRALIA

Several gold deposits occurring in the western Lachlan orogen have geological, geochemical, and geochronological characteristics that distinguish them from typical vein-hosted orogenic gold deposits of the central Victorian gold province. The latter are responsible for more than 90 percent, of primary (hard-rock) gold production from this region and are generally considered to represent the only economically significant type of gold deposit in the western Lachlan orogen. Atypical gold occurrences at Malmsbury, Myrtle Creek, Mount Piper, and the Wonga deposit in the Stawell goldfield are characterized by a close spatial and temporal association with posttectonic felsic intrusions, disseminated to stockwork-style mineralization, alteration dominated by sericitization, sulfidation, silicification, carbonatization and tourmalinization, and associated complex Au ± Mo-W-Bi-Te-Cu. The deposits have a number of features in common with intrusion-related gold deposits elsewhere in Phanerozoic orogenic belts. Although production from this type of gold mineralization in the western Lachlan orogen has been small compared to orogenic gold deposits, the possible existence of intrusion-related gold deposits has potentially important implications for exploration in this region and also provides significant clues to the tectonic framework and Paleozoic metallogeny of eastern Australia.

Global tectonic significance of the Solomon Islands and Ontong Java Plateau convergent zone

Oceanic plateaus, areas of anomalously thick oceanic crust, cover about 3% of the Earth's seafloor and are thought to mark the surface location of mantle plume “heads”. Hotspot tracks represent continuing magmatism associated with the remaining plume conduit or “tail”. It is presently controversial whether voluminous and mafic oceanic plateau lithosphere is eventually accreted at subduction zones, and, therefore: (1) influences the eventual composition of continental crust and; (2) is responsible for significantly higher rates of continental growth than growth only by accretion of island arcs. The Ontong Java Plateau (OJP) of the southwestern Pacific Ocean is the largest and thickest oceanic plateau on Earth and the largest plateau currently converging on an island arc (Solomon Islands). For this reason, this convergent zone is a key area for understanding the fate of large and thick plateaus on reaching subduction zones. This volume consists of a series of four papers that summarize the results of joint US–Japan marine geophysical studies in 1995 and 1998 of the Solomon Islands–Ontong Java Plateau convergent zone. Marine geophysical data include single and multi-channel seismic reflection, ocean-bottom seismometer (OBS) refraction, gravity, magnetic, sidescan sonar, and earthquake studies. Objectives of this introductory paper include: (1) review of the significance of oceanic plateaus as potential contributors to continental crust; (2) review of the current theories on the fate of oceanic plateaus at subduction zones; (3) establish the present-day and Neogene tectonic setting of the Solomon Islands–Ontong Java Plateau convergent zone; (4) discuss the controversial sequence and timing of tectonic events surrounding Ontong Java Plateau–Solomon arc convergence; (5) present a series of tectonic reconstructions for the period 20 Ma (early Miocene) to the present-day in support of our proposed timing of major tectonic events affecting the Ontong Java Plateau–Solomon Islands convergent zone; and (6) compare the structural and deformational pattern observed in the Solomon Islands to ancient oceanic plateaus preserved in Precambrian and Phanerozoic orogenic belts. Our main conclusion of this study is that 80% of the crustal thickness of the Ontong Java Plateau is subducted beneath the Solomon island arc; only the uppermost basaltic and sedimentary part of the crust (∼7 km) is preserved on the overriding plate by subduction–accretion processes. This observation is consistent with the observed imbricate structural style of plateaus and seamount chains preserved in both Precambrian and Phanerozoic orogenic belts.

Pb isotope systematics and time-integrated Th/U of SE-Asian continental crust recorded by single K-feldspar grains in large rivers

To elucidate time-integrated Th/U and Pb isotope characteristics of the SE-Asian continent, 159 individual K-feldspar grains from the rivers Mekong, Salween, Irrawaddy and Red River were analyzed. Earlier U–Pb dating and Hf isotope results of zircon and baddeleyite from the same river sands have established its crustal growth and recycling history, also showing that these heavy minerals are sufficiently representative for the evolution of large continental regions [Geochim. Cosmochim. Acta 64 (2000) 2067]. We therefore consider K-feldspar populations in these sands to be the best possible way to investigate Pb isotope systematics on the continental scale, and analyzing individual grains overcomes the averaging effect of multi-grain measurements. To test the basic condition that U/Pb-ratios are very low in K-feldspar, Pb concentration were measured in all grains, and U in about 10% of them, corroborating Pb concentrations between 4 and 292 ppm with a mean value of 80±40 ppm, and U and Th well below 0.1 and 0.4 ppm, respectively. In situ decay of U can therefore be neglected in all grains and measured Pb isotope ratios are those acquired by the magmatic or metamorphic host material at the time of K-feldspar crystallization. Relative to the model evolution of 206Pb/ 204Pb in upper continental crust, model ages range between 0.5 and 0.2 Ga, whereas many 207Pb/ 204Pb are significantly more radiogenic than any type of Phanerozoic crust. This confirms important recycling of Precambrian crust into the K-feldspar source material such as already observed by Hf signatures for the heavy mineral source lithologies. An important observation is that none of the K-feldspars has preserved Precambrian Pb isotope signature and in consequence, detrital sediments covering the SE-Asian continent must essentially originate from Phanerozoic orogenic belts, where K-feldspars have been re-equilibrated by metamorphic reactions or crystallization in newly formed melts. To dominate delivery of detrital material to such a large extent on the continental scale, these Phanerozoic orogenies must have generated important high mountains ranges, to be subsequently flattened by erosion; today topographic highs are exclusively due to uplift in relation to the India–Asia collision. Concerning 232Th/ 208Pb and time-integrated Th/U ratios, only 4 out of 159 grains plot on the model curves for continental crust, with all other ratios being significantly more radiogenic in 208Pb/ 204Pb, indicating high Th/U source material. If these time-integrated Th/U ratios are modeled in the frame of the known crust formation events, Th/U ratios lie at 4.15–4.22 plotting significantly higher than Th/U deduced for fine-grained sediments or predicted by classical models for average or upper continental crust.

The nature of basement beneath the Songliao Basin in NE China: geochemical and isotopic constraints

The Songliao Basin, located in northeastern China with an area of about 260,000 km2 and having the Daqing Oil Field as the most important center of the petroleum industry in China, is much debated about the basement nature. According to the drill holes, its basement is mainly composed of granites and the Paleozoic strata, some of which have undergone weak metamorphism and somewhat deformation. Gneiss has also been identified, but the petrographical studies indicate that the gneiss is in fact deformed granitic intrusions. Therefore, the granites are the major rock types beneath the Songliao Basin. Geochemical research indicates that these granites are peraluminous, and generated mainly by partial melting of juvenile crust. Geochronological studies show that one undeformed granitic sample gives an U-Pb age of 305 ± 2 Ma, and the mylonitic granite yields an U-Pb age of 165 ± 3 Ma; neither sample contains inherited zircon, which suggests that the basement of the Songliao Basin is not Precambrian, but part of Phanerozoic orogenic belt. With respect to Sr-Nd isotopes, these basement granites are characterized by low ISr (≈0.705), high εNd(t) (−2.2–+2.4) and young Nd model ages (690 to 1160 Ma with most less than 1000 Ma) which is the same as that of the granites in the surrounding orogenic belts such as the Zhangguangcai, Great Xing'an and Lessor Xing'an Ranges, but distinctly different from granites emplaced in the adjacent Precambrian Jiamusi Massif. The simple mass balance calculation indicates that the mixing between the depleted mantle and ancient crust could not reasonably explain the Sr-Nd isotopic features, but the mixing modeling between the juvenile and pre-exited ancient crust suggests that the granites would have about 90% of the juvenile crustal component. Therefore, the Sr-Nd isotopic studies suggest that the basement of the Songliao Basin is juvenile crust, identical to the surrounding orogenic belts, and that there is no Precambrian basement to the Songliao Basin.