Alexander A. Nemchin

Sedimentary basin and detrital zircon record along East Laurentia and Baltica during assembly and breakup of Rodinia

Tectonic processes associated with supercontinent cycles result in a variety of basin types, and the isotopic dating of detrital minerals within sedimentary sequences assists palaeogeographical reconstructions. Basins located along the Laurentia–Baltica margin prior to assembly of Rodinia at 1.2–1.0 Ga are dominated by zircon detritus derived from contemporaneous magmatic arcs. Basins formed during assembly are also dominated by zircon detritus with ages similar to that of sediment accumulation, reflecting syn-collisional magmatism and rapid exhumation of the developing Grenville–Sveconorwegian orogen. Post-collision intracratonic basins lack input from syn-depositional magmatism, and are dominated by significantly older detritus derived from the mountain range as well as its foreland. Basins formed during late Neoproterozoic to Cambrian breakup of Rodinia are divisible into two types. Those within the Caledonides lie on the Grenville–Sveconorwegian foreland and incorporate Archaean and Palaeoproterozoic detritus derived from the cratonic interior and Mesoproterozoic detritus derived from the eroded remnants of the orogen. In the Appalachian orogen, such basins are dominated by Mesoproterozoic detritus with older detritus forming only a minor component, suggesting restricted input from the cratonic interior as a result of either the Grenville orogen still forming a drainage divide or the formation of rift shoulders.

Mesozoic crust-mantle interaction beneath the North China craton : A consequence of the dispersal of Gondwanaland and accretion of Asia

We present evidence from zircons entrained within lower-crustal xenoliths in the Cenozoic Hannuoba Basalt of multiple melting events beneath the North China craton in the late Mesozoic. Peak activity was between 180 and 80 Ma, the upper crustal signature of which was the generation of voluminous granitoids and related volcanic rocks, emplacement of dioritic and lamprophyric dikes, and widespread gold mineralization. The process involved partial loss of mantle lithosphere, accompanied by wholesale rising of asthenospheric mantle beneath eastern China. We correlate these events with lithospheric thinning resulting from the breakup and dispersal of Gondwanaland, accompanied by a major mantle overturn, fueled by the destruction of oceanic lithosphere and triggered by its sinking into the lower mantle during the subsequent accretion of Asia.

Single zircon ages from high-grade rocks of the Jianping Complex, Liaoning Province, NE China

Abstract The high-grade rocks of the Jianping Complex in Liaoning Provi nce, NE China, belong to the late Archaean to earliest Proterozoic granulite belt of the North China craton. Single zircon ages obtained by the Pb–Pb evaporation method and SHRIMP analyses document an evolutionary history that began with deposition of a cratonic supracrustal sequence some 2522–2551 Ma ago, followed by intrusion of granitoid rocks beginning at 2522 Ma and reaching a peak at about 2500 Ma. This was followed by high-grade metamorphism, transforming the existing rocks into granulites, charnockites and enderbites some 2485–2490 Ma ago. The intrusion of post-tectonic granites at 2472 Ma is associated with widespread metamorphic retrogression and ends the tectono–metamorphic evolution of this terrain. A similar evolutionary sequence has also been recorded in the granulite belt of Eastern Hebei Province. We speculate that the Jianping Complex was part of an active continental margin in the late Archaean that became involved in continental collision and crustal thickening shortly after its formation. There is a remarkable similarity between the ∼2500 Ma North China granulite belt and the equally old granulite belt of Southern India, suggesting that the two crustal domains could have been part of the same active plate margin in latest Archaean times.

Petrology and geochemistry of crustally contaminated komatiitic basalts from the Vetreny Belt, southeastern Baltic Shield: Evidence for an early Proterozoic mantle plume beneath rifted Archean continental lithosphere

New isotope and trace element data are presented for komatiitic basalts and a related peridotite Vinela Dike from the large Vetreny Belt in the southeastern Baltic Shield. The MgO contents of the erupted and intruded magmas are inferred to increase from 13 to 17% towards the center of the belt, which implies the respective increase in liquidus temperatures from 1370 to 1440°C. The elevated liquidus temperatures suggest that the source of the komatiite magmas had a substantially higher potential temperature (1630°C) than the ambient mantle (1480°C) and are regarded as evidence for the existence of a mantle plume underlying the region at ~2.45 Ga. Parental magmas to the lavas and the Vinela Dike were shown to have komatiite composition and were derived from a long-term LREE-depleted mantle source with [epsilon]Nd(T) of ca. +2.6. The evolution of these magmas en route to the surface was mainly controlled by 4-15% contamination with older felsic crustal rocks, which resulted in substantial changes in incompatible trace element and isotope ratios. The obtained Sm---Nd internal isochron ages of 2449 ± 35 and 2410 ± 34 Ma for the lavas, 2430 ± 174 Ma for the Vinela Dike, and a whole-rock Pb---Pb age of 2424 ± 178 Ma for the lavas together with a U---Pb zircon age of 2437 ± 3 Ma are identical to the reported U---Pb zircon and baddeleyite ages for numerous mafic-ultramafic layered intrusions in central and northern Karelia. From their chemical and isotope similarities, it is likely that these rocks had allied parental magmas. These magmas may have been emplaced in a continental rift setting during the interaction of a mantle plume and continental crust. Impinging of a plume head beneath the continental lithosphere resulted in its thinning, stretching, and rifting but failed to open a new ocean. This extensive magmatic event was responsible for a substantial contribution of early Proterozoic juvenile material to the Archean continental crust in the Baltic Shield.

Provenance record of a rift basin: U/Pb ages of detrital zircons from the Perth Basin, Western Australia

Abstract U/Pb dating of 588 detrital zircons by ion microprobe from Lower Triassic, Permian and lower Paleozoic sandstone samples from the Perth Basin yield ages ranging from Archean to early Paleozoic. The detrital age spectrum of Lower Triassic samples differ from underlying units in lacking Archean and post-Mesoproterozoic detritus. The Archean Yilgarn craton has previously been envisaged as the main source of detritus in the Perth basin. The bulk of the detritus, however, is of Mesoproterozoic age, which together with the overall broad spectrum of age ranges within individual samples indicates that the detritus was derived from multiple sources. Potential source regions recognized include the Yilgarn craton, the Mesoproterozoic Albany Fraser Orogen, the Meso- to Neoproterozoic Leeuwin complex of the Pinjarra orogen, and probably also the late Mesoproterozoic Northampton complex of the Pinjarra orogen. In addition, source terranes possibly associated with Greater India and East Antarctica, which originally formed the western and southern margins of the basin, may have also contributed detritus accounting for the input of material whose age cannot be related to the known age ranges in currently exposed bounding source blocks. The multi-sourced nature of the detritus suggests the Perth basin was not a simple half-graben structure down stepping to the west but contained a variety of uplifted blocks with an overall longitudinal supply of detritus from the south. The distribution of exposed source blocks changed dramatically at the start of the Triassic with the cessation of input from Archean and Neoproterozoic source regions. The differing age spectra between Triassic and Permian strata and between Late Permian and older sequences suggest any reworked detritus is of extra-basinal origin and not related to intra-basin erosion of underlying rock units. Potential sources for this reworked multicycle detritus are the metasedimentary sequences within the bounding source terranes.

Origin and age of the earliest Martian crust from meteorite NWA 7533

The ancient cratered terrain of the southern highlands of Mars is thought to hold clues to the planet’s early differentiation, but until now no meteoritic regolith breccias have been recovered from Mars. Here we show that the meteorite Northwest Africa (NWA) 7533 (paired with meteorite NWA 7034) is a polymict breccia consisting of a fine-grained interclast matrix containing clasts of igneous-textured rocks and fine-grained clast-laden impact melt rocks. High abundances of meteoritic siderophiles (for example nickel and iridium) found throughout the rock reach a level in the fine-grained portions equivalent to 5 per cent CI chondritic input, which is comparable to the highest levels found in lunar breccias. Furthermore, analyses of three leucocratic monzonite clasts show a correlation between nickel, iridium and magnesium consistent with differentiation from impact melts. Compositionally, all the fine-grained material is alkalic basalt, chemically identical (except for sulphur, chlorine and zinc) to soils from Gusev crater. Thus, we propose that NWA 7533 is a Martian regolith breccia. It contains zircons for which we measured an age of 4,428 ± 25 million years, which were later disturbed 1,712 ± 85 million years ago. This evidence for early crustal differentiation implies that the Martian crust, and its volatile inventory, formed in about the first 100 million years of Martian history, coeval with earliest crust formation on the Moon and the Earth. In addition, incompatible element abundances in clast-laden impact melt rocks and interclast matrix provide a geochemical estimate of the average thickness of the Martian crust (50 kilometres) comparable to that estimated geophysically.

Source of the Dalradian Supergroup constrained by U–Pb dating of detrital zircon and implications for the East Laurentian margin

Detrital zircons in siliciclastic units of the Dalradian Supergroup yield U–Pb ages ranging from 3.2 to 0.5 Ga. Detrital zircons from the sub-Grampian Group basement and the Grampian Group are predominantly of Palaeoproterozoic and Mesoproterozoic ages with Archaean grains absent or rare. In contrast, the overlying Appin, Argyll and Southern Highland groups contain a significant contribution of Archaean detrital zircon grains, some of which locally preserve evidence for late Palaeoproterozoic overgrowths dated at c. 1.8 Ga. In addition, on concordia plots Archaean grains are slightly discordant with a lower intercept at c. 1.8 Ga suggesting they were affected by a tectonothermal event at this time. Late Palaeoproterozoic and Mesoproterozoic grains also show evidence for overprinting by a tectonothermal event around 1.0–0.9 Ga. These tectonothermal events occurred in the source region before accumulation of the siliciclastic detritus. The analysed samples contained no definitive evidence for having been affected by a late Neoproterozoic (Knoydartian) event or of containing detritus derived from a source showing evidence for this event. The overall age range of detritus, combined with sparse palaeocurrent data, is consistent with derivation from the Laurentian foreland, especially the Labrador–Greenland region. Archaean detritus overlaps with that of Archaean cratons, notably the Superior, whereas Palaeoproterozoic detritus corresponds to the timing of suturing of Archaean cratons by a series of orogenic belts (Ketilidian–Makkovik, New Quebec, Nagssugtoqidian, Torngat belts). Mesoproterozoic detritus is consistent with derivation from the Grenville Orogen. The presence of a series of detrital age peaks in the late Palaeoproterozoic and early Mesoproterozoic (1.8–1.5 Ga), the paucity of mid-Mesoproterozoic detritus (1.4–1.2 Ga), and evidence for a tectonothermal event between 1.0 and 0.9 Ga is typical of the geological history of the Labrador–Greenland region of Laurentia.

Paleogeographic development of the east Laurentian margin: Constraints from U-Pb dating of detrital zircons in the Newfoundland Appalachians

An upper Neoproterozoic to Ordovician Laurentian margin sedimentary sequence in the Newfoundland Appalachians records a cycle of ocean opening and closing. U-Pb dating by ion microprobe of 341 detrital- zircon grains from six samples from this sequence yielded ages from 3592 ± 5 Ma to 572 ± 14 Ma. Three of the analyzed samples are from rift-related formations, two are from drift-related formations, and one is from a foreland-basin unit. The U-Pb data allow grouping of the zircons into four major age components: (1) Archean grains with a maximum age frequency between 2850 and 2600 Ma, (2) Paleoproterozoic grains ranging from 1950 to 1750 Ma, (3) Mesoproterozoic to early Neoproterozoic grains ranging in age between 1450 and 950 Ma, and (4) Neoproterozoic grains ranging between 760 and 570 Ma. The relative proportions of these four age components vary among samples. Mesoproterozoic detritus is present in all samples, whereas Neoproterozoic detritus is restricted to two of the three rift-related samples. Two samples— one from a rift-related formation and one from a drift-related formation, but both lying at the base of their stratigraphic column—lack Paleoproterozoic and Archean detrital zircons. Potential source regions for the detrital zircons occur within the Laurentian hinterland: zircons of Archean age correspond to the age of rock units formed during major magmatic and tectonothermal pulses in the Superior craton, zircons with Paleoproterozoic ages correspond to the ages of orogenic belts marginal to the craton (e.g., Ungava, New Quebec, and Torngat), Mesoproterozoic zircons correspond to the ages of lithologies from the Grenville orogen, and Neoproterozoic zircons are the age of rift-related igneous activity along the Laurentian margin that is now preserved within the Appalachian orogen. The youngest grains in the rift-related sedimentary rocks yield ages of 580–570 Ma and provide a maximum age for accumulation of the Newfoundland sedimentary sequence.

Linking source and sedimentary basin: Detrital zircon record of sediment flux along a modern river system and implications for provenance studies ☆

Abstract U/Pb analysis of detrital zircons from sands along the Frankland River in SW Australia reveal marked downstream changes in their age signatures that correlate with changing basement character. The Archean Yilgarn craton underlies the upper 70% of the river catchment (by area) and detrital zircons from sands in this region are characterized by a unimodal ∼2.7 Ga age signature. The lower part of the river catchment flows over the Mesoproterozoic Albany–Fraser orogen and input from this source results in progressive downstream masking of Yilgarn detritus so that in the most downstream samples this source constitutes only 25% of the detrital zircon age spectrum. The Frankland River age data indicate that the lower portions of the drainage system did not form a non-erosive pathway for sediment flux but rather basement lithologies were a continuing source of sediment yield. This probably reflects rejuvenation and incision of the lower reaches of the river system in response to a drop in baseline and possible sequestering of Yilgarn detritus in upstream reservoirs along the broad open mid-region valleys of the river system. These data emphasize that in ancient orogens, where the link between source and sample site is often not preserved, care needs to be exercised in relating detrital zircon age signature to interpretations of source distribution and paleogeographic reconstructions.

Determining Precambrian crustal evolution in China: a case-study from Wutaishan, Shanxi Province, demonstrating the application of precise SHRIMP U-Pb geochronology

Abstract SHRIMP U-Pb zircon analyses from eight samples of metamorphosed intermediate to felsic volcanic rocks from the lower, middle and upper ‘subgroups’ of the Wutai sequence in the North China Craton define a weighted mean 207Pb/206Pb age of 2523 ± 3 Ma. Although individual rock ages range from 2533 ± 8 Ma to 2513 ± 8 Ma, all overlap within the error of the mean and do not support a stratigraphic interpretation for the sequence, since variations within individual previously assigned ‘formations’ in the sequence match the total age range. Contrary to previous interpretations, there is no correlation in age with metamorphic grade. These features highlight the need to reformulate stratigraphic schemes when defining the Precambrian geology of the North China Craton. The similarity in age between volcanic rocks of the Wutai Complex and higher-grade gneisses of the adjacent Fuping and Hengshan complexes supports the view that all three complexes represent portions of a Late Archaean arc complex that was tectonically dismembered and then re-assembled. There is no Fuping or Wutai orogeny in this, its type area: all three complexes were deformed and metamorphosed during collision of the eastern and western blocks of the North China Craton in the Lüliang orogeny c. 1.8 Ga ago.