地质年代学主要数据库现状分析与展望

地质年代学包括绝对地质年代学和相对地质年代学两大主要分支。历经百余年发展,地质年代学在理论体系、技术手段和分析方法等方面均取得了显著进步。因此,现代地质年代学在获取数据的质和量方面都有极大地提升。不同定年方法在适用温度范围内相互衔接、相互印证,所能揭示的地质演化历史也更为完整。如以高时间分辨率和高空间分辨率为代表的同位素年代学研究,分别提供了高质量的精确定年数据和海量的微区分析结果,搭建起了地质事件的精细时间框架,为理解地质事件的过程、速率以及地球系统协同演化奠定了基础。在地质年代学数据海量增长的同时,数据的储存和管理也日益得到重视。以数据集成、共享和互通为目标,一批数据库应运而生。文章尝试系统梳理地质年代学领域已有的主要数据库,并结合高时间分辨率定年和高空间分辨率分析领域的研究进展,探讨大数据时代进一步发展地质年代学的机遇与挑战。

The boring billion? - Lid tectonics, continental growth and environmental change associated with the Columbia supercontinent

The evolution of Earth＇s biosphere,atmosphere and hydrosphere is tied to the formation of continental crust and its subsequent movements on tectonic plates.The supercontinent cycle posits that the continental crust is periodically amalgamated into a single landmass,subsequently breaking up and dispersing into various continental fragments.Columbia is possibly the first true supercontinent,it amalgamated during the 2.0-1.7 Ga period,and collisional orogenesis resulting from its formation peaked at 1.95-1.85 Ga.Geological and palaeomagnetic evidence indicate that Columbia remained as a quasi-integral continental lid until at least 1.3 Ga.Numerous break-up attempts are evidenced by dyke swarms with a large temporal and spatial range; however,palaeomagnetic and geologic evidence suggest these attempts remained unsuccessful.Rather than dispersing into continental fragments,the Columbia supercontinent underwent only minor modifications to form the next supercontinent （Rodinia） at 1.1 -0.9 Ga; these included the transformation of external accretionary belts into the internal Grenville and equivalent collisional belts.Although Columbia provides evidence for a form of ‘lid tectonics’,modern style plate tectonics occurred on its periphery in the form of accretionary orogens.The detrital zircon and preserved geological record are compatible with an increase in the volume of continental crust during Columbia＇s lifespan; this is a consequence of the continuous accretionary processes along its margins.The quiescence in plate tectonic movements during Columbia＇s lifespan is correlative with a long period of stability in Earth＇s atmospheric and oceanic chemistry.Increased variability starting at 1.3 Ga in the environmental record coincides with the transformation of Columbia to Rodinia; thus,the link between plate tectonics and environmental change is strengthened with this interpretation of supercontinent history.

Deconvolving the pre-Himalayan Indian margin-Tales of crustal growth and destruction

The metamorphic core of the Himalaya is composed of Indian cratonic rocks with two distinct crustal affinities that are defined by radiogenic isotopic geochemistry and detrital zircon age spectra. One is derived predominantly from the Paleoproterozoic and Archean rocks of the Indian cratonic interior and is either represented as metamorphosed sedimentary rocks of the Lesser Himalayan Sequence(LHS) or as slices of the distal cratonic margin. The other is the Greater Himalayan Sequence(GHS) whose provenance is less clear and has an enigmatic affinity. Here we present new detrital zircon Hf analyses from LHS and GHS samples spanning over 1000 km along the orogen that respectively show a striking similarity in age spectra and Hf isotope ratios. Within the GHS, the zircon age populations at 2800-2500 Ma,1800 Ma, 1000 Ma and 500 Ma can be ascribed to various Gondwanan source regions; however, a pervasive and dominant Tonianage population(～860-800 Ma) with a variably enriched radiogenic Hf isotope signature(eHf = 10 to-20) has not been identified from Gondwana or peripheral accreted terranes. We suggest this detrital zircon age population was derived from a crustal province that was subsequently removed by tectonic erosion. Substantial geologic evidence exists from previous studies across the Himalaya supporting the Cambro-Ordovician Kurgiakh Orogeny. We propose the tectonic removal of Tonian lithosphere occurred prior to or during this Cambro-Ordovician episode of orogenesis in a similar scenario as is seen in the modern Andean and Indonesian orogenies, wherein tectonic processes have removed significant portions of the continental lithosphere in a relatively short amount of time. This model described herein of the pre-Himalayan northern margin of Greater India highlights the paucity of the geologic record associated with the growth of continental crust. Although the continental crust is the archive of Earth history, it is vital to recognize the ways in which preservation bias and destruction of continental crust informs geologic models.

Geochronological constraints on the metamorphic sole of the Semail ophiolite in the United Arab Emirates

The Semail ophiolite of Oman and the United Arab Emirates （UAE） provides the best preserved large slice of oceanic lithosphere exposed on the continental crust, and offers unique opportunities to study processes of ocean crust formation, subduction initiation and obduction. Metamorphic rocks exposed in the eastern UAE have traditionally been interpreted as a metamorphic sole to the Semail ophiolite. However, there has been some debate over the possibility that the exposures contain components of older Arabian continental crust. To help answer this question, presented here are new zircon and futile U-Pb geochronological data from various units of the metamorphic rocks. Zircon was absent in most samples. Those that yielded zircon and futile provide dominant single age populations that are 95-93 Ma, partially overlapping with the known age of oceanic crust formation （96.5-94.5 Ma）, and partially overlapping with cooling ages of the metamorphic rocks （95 90 Ma）. The data are interpreted as dating high-grade metamorphism during subduction burial of the sediments into hot mantle lithosphere, and rapid cooling during their subsequent exhumation. A few discordant zircon ages, interpreted as late Neoproterozoic and younger, represent minor detrital input from the continent. No evidence is found in favour of the existence of older Arabian continental crust within the metamorphic rocks of the UAE.

Lid tectonics-Preface

The idea that plate tectonics may not have operated deep in Earth’s Precambrian past has a long legacy. What predated plate tectonics is unknown, and advances in data - from geochemical,geological and tectonic, to paleomagnetic, as well as modelling approaches,and planetary science, have the potential to contribute significantly to the debate. To contrast with the activity of plate tectonics,in this issue we use the term ‘lid tectonics’ to encapsulate a variety of envisaged regimes e from stagnant, sluggish, plutonicsquishy,or heat pipe - which are characterized by comparatively subdued tectonic signatures.This special issue of Geoscience Frontiers aims to present a snapshot of current research in the field of lid-tectonics - from detailed field interpretations, to global conceptual models, data compilations,and insights from simulations, and provide unique insights into both the Precambrian Earth, and the dynamics of lid-tectonics.

Zircon U-Pb-Hf constraints from Gongga Shan granites on young crustal melting in eastern Tibet

The Gongga Shan batholith is a complex granitoid batholith on the eastern margin of the Tibetan Plateau with a long history of magmatism spanning from the Triassic to the Pliocene. Late Miocene-Pliocene units are the youngest exposed crustal melts within the entire Asian plate of the Tibetan Plateau.Here, we present in-situ zircon Hf isotope constraints on their magmatic source, to aid the understanding of how these young melts were formed and how they were exhumed to the surface. Hf isotope signatures of Eocene to Pliocene zircon rims(ε_(Hf)(t)=-4 to +4), interpreted to have grown during localised crustal melting, are indicative of melting of a Neoproterozoic source region, equivalent to the nearby exposed Kangding Complex. Therefore, we suggest that Neoproterozoic crust underlies this region of the Songpan-Ganze terrane, and sourced the intrusive granites that form the Gongga Shan batholith. Localised young melting of Neoproterozoic lower or middle crust requires localised melt-fertile lithologies. We suggest that such melts may be equivalent to seismic and magnetotelluric low-velocity and high-conductivity zones or "bright spots" imaged across much of the Tibetan Plateau. The lack of widespread exposed melts this age is due either to the lack of melt-fertile rocks in the middle crust, the very low erosion level of the Tibetan plateau, or to a lack of mechanism for exhuming such melts. For Gongga Shan, where some melting is younger than nearby thermochronological ages of low temperature cooling, the exact process and timing of exhumation remains enigmatic, but their location away from the Xianshuihe fault precludes the fault acting as a conduit for the young melts. We suggest that underthrusting of dry granulites of the lower Indian crust(Archaean shield) this far northeast is a plausible mechanism to explain the uplift and exhumation of the eastern Tibetan Plateau.

Generation and preservation of continental crust in the Grenville Orogeny

Detrital zircons from modern sediments display an episodic temporal distribution of U-Pb crystallization ages forming a series of ＇peaks＇ and ＇troughs＇. The peaks are interpreted to represent either periods of enhanced generation of granitic magma perhaps associated with mantle overturn and superplume events, or preferential preservation of continental crust during global collisional orogenesis. The close association of those peaks with the assembly of supercontinents implies a causal relationship between collisional orogenesis and the presence of zircon age peaks. Here these two end-member models （episodic periodicity of increased magmatism versus selective preservation during collisional orogenesis） are assessed using U-Pb, Hf, and 0 analysis of detrital zircons from sedimentary successions deposited during the - 1.3-1.1 Ga accretionary, -1.1-0.9 Ga collisional, and 〈 0.9 Ga extensional collapse phases of the Grenville orogenic cycle in Labrador and Scotland. The pre-collisional, accretionary stage provides a baseline of continental crust present prior to orogenesis and is dominated by Archean and Paleoproterozoic age peaks associated with pre-1300 Ma Laurentian geology. Strata deposited during the Grenville Orogeny display similar Archean and Paleoproterozoic detrital populations along with a series of broad muted peaks from - 1500 to 1100 Ma. However, post-collisional sedimentary successions display a dominant age peak between 1085 and 985 Ma, similar to that observed in modern North American river sediments. Zircons within the post-orogenic sedimentary successions have progressively lower EHf and higher -lSO values from - 1800 to - 1200 Ma whereupon they have higher EHf and -3180 within the dominant 1085-985 Ma age peak. Furthermore, the Lu-Hf isotopic profile of the Grenville-related age peak is consistent with significant assimilation and contamination by older crustal material, The timing of this dominant age peak coincides with the peak of metamorphism and magmatism associated with the Grenville Orogeny, which is a typical collisional orogenic belt. The change from broad muted age peaks in the syn-orogenic strata to a single peak in the post-orogenic sedimentary successions and in the modern river sediments implies a significant shift in provenance following continental collision. This temporal change in provenance highlights that the source（s）, from which detrital zircons within syn-orogenic strata were derived, was no longer available during the later stages of the accretionary and collisional stages of the orogenic cycle. This may reflect some combination of tectonic burial, erosion, or possibly recycling into the mantle by tectonic erosion of the source（s）. During continental collision, the incorporated continental crust is isolated from crustal recycling processes operative at subduction margins. This tectonic isolation combined with sedimentary recycling likely controls the presence of the isotopic signature associated with the Grenville Orogeny in the modern Mississippi and Appalachian river sed- iments. These results imply that zircon age peaks, which developed in conjunction with supercontinents, are the product of selective crustal preservation resulting from collisional orogenesis.

"Miles wide and miles deep"-Exploring the depth and breadth of geoscience during the first ten years of Geoscience Frontiers

When early explorers first crossed the Platte River in what is now Nebraska (USA), it was said the river was “a mile wide and an inch deep”(Mokler, 1923;Smith, 1971). This phrase was used to describe not only the difficulty in crossing the river but also in navigating its length. The trouble with a river being too wide is the risk that it won’t provide the depth necessary to be useful. The same thing can be said of multidisciplinary scientific journals. While a journal can claim to be multidisciplinary, there is a risk of it being so broad that its articles can only engage the reader at a superficial level. Nothing could be further from the truth with Geoscience Frontiers. Over the past ten years, this journal has successfully navigated the wide breadth of geoscience while providing a level of depth and detail that rivals discipline-specific journals.

Deep seated magmas and their mantle roots:Introduction

In the last decade there has been a considerable effort to better understand the joint evolution of mafic and ultramafic magmatic systems and their deep mantle roots,through integrated petrological and thermo-barometric studies.Magma generation is regarded as the result of complex processes including melting,creation of channels for melt transfer,and interaction with the wall-rocks.

北喜马拉雅萨迦穹窿中苦堆和萨迦淡色花岗岩的U-Pb年龄及其地质意义

利用TIMS方法，对产于北喜马拉雅萨迦穹窿中的苦堆和萨迦淡色花岗岩锆石、独居石和磷钇矿进行了U-Pb定年．结果表明，苦堆淡色花岗岩的岩浆结晶年龄为27．5±0．5Ma，而萨迦淡色花岗岩的岩浆结晶年龄为14．4±0．2Ma，因此将北喜马拉雅淡色花岗岩形成的时间跨度从原有的15～10Ma扩展到27．5～10Ma，并据此对北喜马拉雅淡色花岗岩的成因机制进行了年代学制约，指出北喜马拉雅淡色花岗岩有着多样的成因机制．