Precambrian Crustal Evolution Research Articles

Lead Isotopes Constrain Precambrian Crustal Architecture, Thermal History and Lithospheric Foundering in Laurentia

ABSTRACTLaurentia (ancestral North America) records nearly 4 billion years of crustal evolution. Here, a newly compiled continental‐scale Pb isotopic database is used to evaluate the Precambrian crustal evolution of Laurentia. Pb model ages yield a 2.7 Ga peak, a 2.5–1.8 Ga minimum and 1.8–0.9 Ga continuum. Pb model ages yield thermochronometric data and track crustal growth via arc‐related magmatism and accretionary orogenesis. Model 232Th/204Pb and 238U/204Pb broadly correlate with mapped crustal domains. More homogeneous and less radiogenic 238U/204Pb and 232Th/238U after 2.7 Ga suggests a shift to more juvenile sources, loss of early isotopic reservoirs and greater crustal reworking. U and Th are fractionated from Pb in Proterozoic orogens with abundant ferroan and anorthosite–mangerite–charnockite–granite(AMCG)‐suite magmatism. This fractionation suggests the removal of Pb‐rich lower crust, supporting petrogenetic models involving lithospheric foundering and magmatic underplating. Lithospheric thinning and associated magmatism may have contributed to high middle Proterozoic geothermal gradients.

Constraints on the Growth and Evolution of Continental Crust in the Northeast Cathaysia Block: Insights from Detrital Zircon U-Pb Age and Hf-O Isotope Analysis in Tingjiang Sediments

Fluvial detrital zircons provide crucial insights into the early Precambrian crustal evolution. In this study, we utilize U-Pb geochronology and Hf-O isotopic compositions of detrital zircons from the Tingjiang River to comprehend the provenance and continental crustal evolution of the northeast Cathaysia Block. Zircon U-Pb ages display seven major populations at 105-177 Ma, 200-262 Ma, 376-520 Ma with a striking peak at ca. 422 Ma, 606-775 Ma, 836-1457 Ma, 1710-2042Ma, and 2300-2661Ma, corresponding to the documented tectonic-thermal events in the South China Block. Episodic magmatism was the response to the formation and breakup of the Columbia and Rodinia supercontinent. The episode of 422 Ma matches with the ages of widely spread early Paleozoic intraplate orogenic events in the South China Block. The isotopic signatures of Hf and O in the detrital zircons indicate that the majority of Precambrian zircons likely originated from the re-melted ancient continental crust, with contributions from juvenile mantle-derived magmas. Phanerozoic zircons, on the other hand, were formed through partial melting of recycled crustal material. Analysis of crustal growth rates, based on the two-stage Hf model ages, suggests a significant presence of juvenile crustal growth during the Paleoproterozoic era. Specifically, it is estimated that approximately 40% and 90% of the present crust within the northeastern Cathaysia Block were formed around 2.3 Ga and 1.6 Ga, respectively.

The continuing power of Nd isotope mapping in Precambrian orogenic belts: A demonstration from the Ontario Grenville Province

With the increasing availability of U-Pb-Hf isotope analysis of zircon suites, does whole-rock Nd isotope analysis still have a role to play in mapping Precambrian gneiss terranes? I answer this question by examining the Ontario Gneiss Belt of the Grenville Province, and show that Nd isotope mapping still has a vital role to play in studies of Precambrian crustal evolution. Although U-Pb-Hf isotope analysis of detrital zircons can provide large suites of crystallization ages and epsilon Hf values, the poor spatial control of these data means that precise geographical control of crustal structure from Nd isotope mapping is still important. I demonstrate this for several regions of the Ontario Gneiss Belt, including northern Algonquin Park, western Lake Nipissing and Georgian Bay. In all these regions, mapping complex geographical patterns of crustal formation ages allows detailed documentation of large-scale crustal structure. Only with such an accurate understanding of crustal structure can accurate models of tectonic evolution be constructed in a complex, deeply exhumed orogenic belt such as the Grenville Province.

Geochemistry, Petrogenesis and Tectonic Setting of the Mafic Dykes from the Amgaon and Khairagarh Regions, Bastar Craton, Central Indian Shield: Constraints on the Precambrian Crustal Evolution

The presence of dominantly dykes, dyke swarms and sills along with the basement rocks indicate that the study area has undergone deep erosion exposing the basement rocks and these magmatic bodies (dykes, dyke swarms and sills) actually represent the plumbing system for the mantle derived melts, which contributed significantly to the Precambrian crustal evolution processes in the Central Indian shield

Crustal evolution and tectonomagmatic history of the Indian Shield at the periphery of supercontinents

River sand detrital zircons from several rivers flowing through the Peninsular Indian cratons were analyzed for U-Pb, Lu-Hf, and O isotopes to characterize the Precambrian crustal evolution of the Indian Shield and to constrain its role in the early supercontinent cycles. Analyzed river sand zircon samples exhibit a prominent age grouping at 2.7–2.4 Ga and additional peaks at 1.8–1.7 Ga, 1.0–0.8 Ga, and 0.6–0.5 Ga. The time-related Hf and O isotope trends of the Indian zircons display a slight offset from the global trends. The age peaks and distinct Hf-O isotopic compositions of the Indian zircons also lack coherence with the global patterns associated with supercontinent cycles, implying a discrete crustal evolutionary history for the Indian Shield. Their contrasting εHf-age trajectories indicate that the Indian Shield was accreted to the Columbia/Nuna supercontinental framework through a collision event that postdated the 2.1–1.8 Ga global-scale orogeny, and was part of a long-lived subduction system along the margin of Rodinia. This implies that the Indian Shield occupied a peripheral paleo-position during the assembly of the two Precambrian supercontinents. The ca. 2.6–2.4 Ga Indian river sand zircons have remarkably low δ18O values that are distinct from the global zircon O isotopic record. This period coincides with the fragmentation of the Archean supercraton and the flaring-up of a subaerial Large Igneous Province that facilitated the generation of 18O-depleted magmas. Depending on whether the 2.6–2.4 Ga zircons with low δ18O values (<4.7 ‰) are considered or not, approximately 2.77 ‰ or 1.78 ‰ rise in the zircon δ18O values occurred between ca. 2.4 Ga and ca. 1.87 Ga (decoupled from zircon Lu-Hf isotopes), further suggesting that the fine-grained sediments were enriched in 18O after the Great Oxidation Event. These high δ18O sediments were subsequently incorporated into the magmatic systems resulting in elevated δ18O in the zircons crystallizing from such melts. Nevertheless, a ca. 0.9 Ga peak of >10 ‰ in δ18O system was followed by a pronounced δ18O drop at ca. 0.759 Ga, suggesting that δ18O of recycled sedimentary reservoirs was not the only controlling factor for zircon δ18O characteristics. Variations in the Hf-O data for Indian and global zircons could be attributed to the initiation of the supercontinent cycle at ca. 2.0 Ga. A significant volume of the juvenile crust was added during the 3.0–2.7 Ga mafic magmatism in the Indian Shield, as underlined by the Hf model ages of all the zircon grains and those with mantle-like δ18O signatures. A gradual addition of continental crust during ca. 3.6 Ga to 3.1 Ga can also be deduced, with the oldest crust being derived from the mantle at ca. 4.4 Ga.

Chronological framework of Precambrian Dantazi Complex: Implications for the formation and evolution of the northern North China Craton

The Dantazi Complex in the North Hebei Terrane is located in the middle segment of the northern North China Craton, and plays a crucial role for understanding the early Precambrian crustal evolution of the craton. The Dantazi Complex is composed mainly of late Neoarchean tonalites-trondhjemites-granodiorites, potassic granites, sanukitoid granodiorites-monzodiorites and quartz syenites, which were emplaced by Paleoproterozoic gabbroic stocks. SHRIMP and LA–ICP–MS zircon dating analyses have revealed that the Neoarchean diversified granitoids of the Dantazi Complex were formed in ∼2.52–2.47 Ga, and preserved ∼2.60–2.53 Ga inherited/captured zircon ages. All these rocks experienced early metamorphism and deformation at ∼2.40 Ga, later granulite facies metamorphism at ∼1.80 Ga, and alterations at ∼1.73 Ga. Whole-rock chemical characteristics and Sm–Nd–Lu–Hf isotopes indicate that (1) The ∼2.52 Ga tonalites-trondhjemites-granodiorites were derived from partial melting of juvenile mafic crust; (2) the ∼2.51 Ga sanukitoid monzodiorite-granodiorite series came from slab-mantle interaction with different melt/rock ratios; (3) The ∼2.50–2.47 Ga potassic granites were generated by intracrustal recycling of metagreywacks that sourced from the mixture between juvenile and ancient crustal materials; and (4) The ∼2.50 Ga quartz syenites were originated from fractional crystallization of mantle-derived magmas. These petrogenetic processes reveal that the subduction-related crust-mantle interaction is the key reason for the diversity of late Neoarchean granitoids in the Dantazi Complex. Regional comparison indicates that the Precambrian North Hebei Terrane show distinct structure in the chronology and lithological assemblages from other basement terranes in the trans-North China Orogen and Western Block, implying that they were formed in different crust-mantle dynamic systems. Therefore, the Precambrian North Hebei Terrane was probably an independent micro-continental block at least until the early Paleoproterozoic. In the Archean crystalline basement range of the North Hebei–West Liaoning–East Hebei, from NW to SE, the magmatic crystallization ages gradually became older, and the granitoid assemblages transformed from sodium-rich to potassium-rich, the spatio-temporal evolution of chemical composition and lithological assemblages further indicates that the North Hebei Terrane was most likely developed in an arc environment with continuous retreat subduction caused by the slab roll-back.

Discovery of a Mesoproterozoic granite in the northern Alxa Block and its tectonic implication

A Mesoproterozoic granite covering about 5 km2 is firstly recognized in the Zhusileng area, which is located between the northern Alxa Block and the South Gobi microcontinent. New geochronological data from this area provide new insight for understanding the Precambrian crustal evolution of the middle segment of the southern Central Asian Orogenic Belt (CAOB). Based on the crystal shapes, internal textures, and high Th/U ratios, the zircons from this Mesoproterozoic granite are interpreted as magmatic origin. Meanwhile, LA‐ICP‐MS zircon U–Pb dating shows that it was emplaced at 1458 ± 7.5 Ma. Published geochronological data and Sr–Nd isotopic compositions of igneous rocks from the Zhusileng area and its vicinity are also compiled in this study. The Late Palaeozoic granitoids from the Zhusileng area have strongly negative εNd(t) values (−0.1 to −10.8), high (87Sr/86Sr)i (0.69657–0.73423) and old Nd model ages (0.9–1.8 Ga), suggesting the existence of Precambrian microcontinent in the Zhusileng area. Furthermore, the statistical analysis (or isotopic mapping) of Sr–Nd isotopic compositions of Palaeozoic granitoids from the middle segment of the southern CAOB also displays similar εNd(t) values and relatively old Nd model ages along the Hanshan Block, Zhusileng, Tsaggan Uul terrane, and Hutag Uul terrane. Therefore, we propose that those units may share a common basement during Mesoproterozoic to Early Neoproterozoic.

Archean to Paleoproterozoic crustal evolution of the southern Yangtze Block (South China): U–Pb age and Hf-isotope of zircon xenocrysts from the Paleozoic diamondiferous kimberlites

Crustal zircon xenocrysts from mantle-derived magmatic rocks have the potential to probe the deep crust. Here we present integrated U–Pb dating and Hf-isotope analyses of zircons from a Paleozoic diamondiferous kimberlite dike in the Maping area of Zhenyuan County (southeastern Guizhou Province), with implications for the tectonothermal evolution of unexposed continental crust beneath the southern Yangtze Block, South China. All zircons (n = 236) show a wide range of U–Pb ages between Mesoarchean and middle Carboniferous. Among them, 96 zircons with 90–110% concordance yield concordant ages from 2942 ± 8 Ma to 342 ± 2 Ma, and form major age peaks at ∼2.9 Ga, ∼2.6 Ga and ∼2.0 Ga. The overwhelming majority of zircons are dominated by Mesoarchean–Paleoproterozoic U–Pb ages regardless of their concordance degrees. The zircon populations mainly consist of magmatic zircons, with minor metamorphic grains (Th/U < 0.10). The youngest magmatic zircon (Th/U = 0.43) with a well concordant 206Pb/238U age of 342 ± 2 Ma (M1-16) is interpreted as the maximum emplacement age of the Maping diamondiferous kimberlites. Most zircons with pre-eruption ages are considered to be xenocrysts, and they may be derived from the deep-seated continental crust through which kimberlite host magmas have passed. Their U–Pb ages and Hf isotopic compositions suggest the possible existence of a highly evolved Archean basement beneath the southern Yangtze Block, South China, which is much older than its known surface rocks. A lot of magmatic zircon xenocrysts reveal complex Precambrian crustal evolution in the southern Yangtze Block. These processes involved the important growth of continental crust at 2.6–2.5 Ga, and in the meantime, crustal reworking could be intermittently proceeding at 3.0–2.6 Ga. In addition, a group of xenocrystic zircons are identified to be of metamorphic origin, indicating that the proposed Archean basement beneath the southern Yangtze Block likely experienced a metamorphic event around 2.0 Ga. This geologically significant episode is consistent with the well-developed coeval metamorphism in other places of the Yangtze Block (e.g., Kongling Terrane), which has been considered to link to the assembly of the Paleoproterozoic Nuna/Columbia supercontinent. Our zircon data implies that the unexposed Archean basement beneath the southern Yangtze Block was affected by multiple thermal activities.

Early Precambrian Crustal Evolution in the Irkut Block (Sharyzhalgai Uplift, Southwestern Siberian Craton): Synthesis of U–Pb, Lu–Hf and Sm–Nd Isotope Data

Abstract —The paper presents a synthesis of zircon U–Pb and Lu–Hf and whole-rock Sm–Nd isotope data from main early Precambrian (3.40 to 1.85 Ga) metamorphic and magmatic units of the Irkut block (Sharyzhalgai uplift, southwestern Siberian craton). The Archean complexes consist of relict Paleoarchean (3.4 Ga) melanocratic granulites and predominant Neoarchean mafic and felsic granulites (2.70–2.66 Ga), paragneisses (≤2.75 Ga), and gneissic granites (2.54 Ga). The Paleoproterozoic complexes include paragneisses (1.95–1.85 Ga), granitoids and charnockites (1.86–1.84 Ga), as well as mafic intrusions and dikes (1.86 Ga). Few detrital zircons with Hf model ages of ≥3.6 Ga mark the Eoarchean onset of crustal growth in the Irkut block. Isotopic data record two major stages of crustal growth in early Precambrian evolution of the Irkut block: Paleoarchean (3.6–3.4 Ga) and Neoarchean (~2.7–2.66 Ga). The Paleoarchean crustal growth was most likely associated with plume magmatism fed from depleted and primitive mantle sources. The spatial distribution of Paleoarchean crust is traceable in isotopic signatures of magmatic and detrital zircons from most of Mesoarchean to Paleoproterozoic units. The Neoarchean crustal growth from a depleted mantle source was due to subduction magmatism. Moderate crustal growth occurred in the Paleoproterozoic from 2.30 to 1.85 Ga. At the turn of 1.86–1.85 Ga, mafic magmas and products of their fractionation formed from both depleted and enriched sources under postcollisional extension; the latter sources were the subcontinental lithospheric mantle formed during Neoarchean subduction. Three major stages of crustal recycling have been established: Mesoarchean (~3.0 Ga), Neoarchean (~2.55 Ga), and Paleoproterozoic (1.86–1.85 Ga), which are characterized by near-coeval intracrustal melting and metamorphism. The recycling during the ~2.55 Ga and 1.86–1.85 Ga events apparently occurred in a collisional setting. The 2.7 Ga subduction-related felsic magmas also formed through the recycling of the Paleo-Mesoarchean crust. The hypothesized scenario for the geological evolution of the Irkut block is the dominant vertical growth and crustal recycling for about two billion years. Available isotope data record similar major crustal growth in the Paleoarchean and growth combined with recycling during the Neoarchean and Paleoproterozoic events in both the southwestern and northern and central parts of the Siberian craton. The Irkut block in the southwest differed in a long and continuous recycling during the Mesoarchean and pronounced Neoarchean crustal growth.

Crustal shear wave velocity and radial anisotropy beneath Southern Africa from ambient noise tomography

Southern Africa is an amalgamation of a distinct Archean core, several Proterozoic mobile belts, and large igneous provinces, making it an ideal natural laboratory for studying Precambrian crustal evolution and deformation. Here we present a new radially anisotropic shear-velocity model of the crust and uppermost mantle beneath Southern Africa using Rayleigh- and Love-wave ambient noise tomography from multiple seismic networks. In the upper and middle crust, the distribution of the isotropic shear velocity correlates well with known surface geology. However, in the lower crust there is no systematic lateral variation of shear velocity between the Archean and Proterozoic terranes, which is likely due to the intensive modifications of Precambrian crust by several later thermo-tectonic events through processes such as lower crustal flow. The strong positive radial anisotropy (VSH > VSV) in the lower crust, when combined with weak azimuthal anisotropy imaged previously, suggests a complex flow pattern, the directions of which likely varied on local-scale due to inhomogeneous shortening or extension during the multiple post-cratonization thermo-tectonic events.

Early Neoproterozoic tectonic evolution of northern Yangtze Block: Insights from sedimentary sequences from the Dahongshan area

Precambrian sedimentary strata are widely distributed along the periphery of the Yangtze Block. However, the age, provenance and tectonic setting of the sedimentary successions on the northern margin still remain unclear, which exerted considerable influence on the understanding of Precambrian crustal evolution of the Yangtze Block. Here we report integrated detrital zircon U-Pb dates and Lu-Hf isotopes from the Dahongshan area to the northeast of the nuclei (i.e. the Kongling Complex) of the Yangtze Block. The youngest U-Pb dates of 995 Ma and 810 Ma of detrital zircon grains place constraints on the maximum depositional ages of the underlying Dagushi Group and the overlying Huashan Group, respectively. Both groups show pre-Mesoproterozoic age populations at ca. 2050 Ma and ca. 2700 Ma and corresponding εHf(t) values varying from –24.1 to + 2.3, consistent with counterparts of the Kongling Complex and Yangpo Complex in northern Yangtze Block. The εHf(t) values of Neoproterozoic (1000–800 Ma) detrital zircon grains in the Huashan Group show a decrease after ca. 880 Ma (from + 13.5 to –18.5), possibly indicating a transition from arc-continent collision to continental arc setting on the northern margin of the Yangtze Block. In combination with sedimentary characteristics and the new isotopic results, the Dagushi Group probably formed in a shallow marine basin setting and received dominant detritus from the Kongling and Yangpo complexes. In contrast, the Huashan Group received sediments recycled from the underlying Dagushi Group and contemporary igneous rocks in the Dahongshan and Kongling areas. The changes in provenance and tectonic setting recorded in the two sedimentary sequences suggest a tectonic transition from late Mesoproterozoic passive continental margin to Neoproterozoic subduction and accretion in response to the amalgamation of Rodinia supercontinent. On the basis of distinct location and history, Pre-Nanhua Neoproterozoic sedimentary sequences distributed in the periphery of Yangtze Block are dispersed into four separate domains. Through bivariate kernel density estimates and bootstrap resampling method, we have identified that these domains were characterized by asynchronous igneous impulse and heterogeneous character of igneous activity in early Neoproterozoic along with varying degrees of integration of crustal growth and reworking.

Amphibolite–granulite facies mid-crustal basement in Deccan Large Igneous Province and its implication on Precambrian crustal evolution: evidence from Killari borehole studies

Amphibolite–granulite facies mid-crustal basement in Deccan Large Igneous Province and its implication on Precambrian crustal evolution: evidence from Killari borehole studies

Precambrian crustal evolution of the Tethyan Yunnan, Southwest China: Records in detrital zircons from Paleozoic sedimentary rocks of the Baoshan block

The Baoshan Block in western Yunnan is one of the major micro-continents in the eastern Tethyan orogenic belt, but its provenance and crustal evolution remain enigmatic. Sedimentary rocks prior to an amalgamation of continental blocks recorded key information about crustal evolution and the origin of terranes and can provide vital data for understanding orogenic processes. Here, we present U–Pb ages and Hf isotopic data on detrital zircon and Nd isotopic data on whole rock for Paleozoic sedimentary rocks from the Baoshan Block to constrain their provenance and tectonic history. Crystallization ages of detrital zircon grains illustrate that the magmatic activities of the source areas occurred at ~2.5 Ga, ~1.9 Ga, and ~1.0–0.5 Ga, with a major peak occurring around 0.95 Ga. Several old zircon grains from >3.0 Ga up to ~3.8–3.7 Ga were observed. Zircon Hf isotopic data from one sample suggest that the major crustal growth occurred at ~2.1 Ga. Cambrian to Carboniferous sedimentary rocks showed similar Nd isotopic features with two–stage Nd model ages of approximately 2.1 Ga. The age distribution patterns and isotopic data suggest that detritus material in the Paleozoic sedimentary rocks was mainly derived from continental terrane similar to the Indian craton, implying that the Baoshan Block was likely adjacent to Northwest India before the collision of the Baoshan and Simao blocks. No significant inputs of the juvenile crust were documented in the Nd–Hf isotopic data, appearing to be incompatible with a magmatic arc setting for the Baoshan Block during the Paleozoic. This block might have been situated at the interior of Gondwana before rifting and drifting northwards to the Eurasia continent.

Detrital zircon in the Huashan Group, northern Yangtze Block: Implications for the nature of Neoproterozoic sedimentary basins and Precambrian crustal evolution

The Huashan Group preserves Neoproterozoic volcanic‐sedimentary sequences in the northern Yangtze Block and is an ideal target for determining the provenance and characteristics of Neoproterozoic sedimentary basins and the Precambrian crustal evolution of the Yangtze Block. Here, we report LA‐ICP‐MS detrital zircon U–Pb–Hf isotopes from four metamorphic clastic rock samples in the Huashan Group. Detrital zircons ages yield four major peaks at ca. 2,670, 2,040, 940, and 840 Ma and five subordinate peaks at greater than or equal to ca. 2,850, 2,490, 1,840, 1,600, and 1,240 Ma. The provenance of the Huashan Group includes Archean to mid‐Neoproterozoic magmatic rocks, and reworked sedimentary rocks from the northern Yangtze Block. The Huashan Group was deposited from ca. 830 to 800 Ma and represents sedimentation accompanying the development of a mid‐Neoproterozoic continental rift basin in Yangtze Block associated with the breakup of the Rodinia supercontinent. Hafnium isotope analyses of detrital zircon from the Huashan Group suggest that most crustal growth in the northern Yangtze Block occurred during the Late‐Mesoproterozoic and early to Middle Neoproterozoic and crustal reworking occurred from the Archean to the Neoproterozoic.

Xenocrystic/inherited Precambrian zircons entrained within igneous rocks from eastern South China: Tracking unexposed ancient crust and implications for late Paleoproterozoic orogenesis

South China is characterized by widespread igneous rocks with varied ages and nature in its eastern part, which contain abundant Precambrian xenocrystic/inherited zircons that bear important information of the composition and evolution of the underlying ancient crust. This paper for the first time presents a compiled U-Pb age database of 1416 Precambrian xenocrystic/inherited zircons from igneous rocks in eastern South China, and attempts to provide a constraint on the Precambrian crustal evolution of the constituent Yangtze and Cathaysia blocks. These xenocrystic/inherited zircons, as a conceivable proxy of the unexposed continental crust, document three major tectonothermal events related to continental accretion and subsequent modification that possibly built the continental crust of the eastern Yangtze Block (EYB) at 2.70–2.40 Ga, 2.10–1.55 Ga, and 0.95–0.70 Ga, and the Cathaysia Block (CAB) at 2.70–2.40 Ga, 2.05–1.75 Ga, and 1.10–0.70 Ga, pointing to a complex Precambrian evolutionary history for South China. The EYB zircons are unexpectedly dominated by a 2.10–1.55 Ga age population that shows a multimodal distribution with peaks at 2.05 Ga, 2.0 Ga, 1.90 Ga, 1.85 Ga, and 1.58 Ga, and the CAB zircons are characterized by a unimodal 2.0–1.75 Ga age population that conspicuously peaked at 1.85 Ga, both of which overlap with the tenure of the Nuna supercontinent. These xenocrystic/inherited zircons from both blocks generally have negative εHf(t) values, which in combination with coeval regional magmatic and metamorphic records can assist to trace a possible prolonged (2.05–1.75 Ga) orogenic process in the EYB and a short-lived (1.9–1.8 Ga) orogeny in the CAB. Such orogeneses are proposed to be correlated with the assembly of the Nuna supercontinent.

Geochronological and Lu-Hf isotopic study on detrital zircons of the Jianshan Formation, Bayan Obo Group in Shangdu area, Inner Mongolia: Constraints on Precambrian crustal evolution of the Western Block, North China Craton

华北克拉通前寒武纪地壳生长过程是地质学的研究热点。本文对华北克拉通西部陆块北缘的白云鄂博群尖山组进行了碎屑锆石U-Pb定年和Hf同位素分析，并在此基础上对尖山组的沉积时限、物源及西部陆块北缘的前寒武纪地壳生长过程进行了探讨。样品TM33中碎屑锆石年龄主要集中在1872~2100Ma（n=64），并有~1960Ma的主年龄峰值和~2000Ma的次年龄峰值，Hf同位素分析结果显示锆石的ε_Hf（t）值在-2.6~7.9之间（除1个测点为-17.6），Hf同位素二阶段模式年龄（t_DM2）变化范围在2240~4386Ma之间。样品Z1724中碎屑锆石年龄分布于1784~2200Ma（n=44），并有~1965Ma的主年龄峰值和~2080Ma次年龄峰值。谐和线上最年轻一组锆石的加权平均年龄为1794±73Ma（n=2），ε_Hf（t）在-5.4~3.2之间，Hf同位素二阶段模式年龄（t_DM2）变化范围在2584~3203Ma之间。基于以上数据，结合侵入白云鄂博群底部1670Ma的辉长岩以及白云鄂博群底部全岩²⁰⁷Pb-²⁰⁶Pb等时线年龄为1649Ma的沉积碳酸盐岩，尖山组的沉积时间被限定在1800~1650Ma。尖山组的物源主要来自固阳、西乌兰不浪地区新太古代基底以及大青山和卓资地区古元古代晚期孔兹岩带。综合前人对研究区及邻区中新元古代沉积地层中碎屑锆石的Hf同位素研究结果，认为华北克拉通西部陆块北缘可能存在2600~2750Ma、~2500Ma和2000~2100Ma三期主要的地壳生长事件，且最后一次规模可能小于前两次。;The Precambrian crustal growth of North China Craton (NCC) has been a hot topic for geologists for a long time. This paper presents LA-ICP-MS U-Pb dating and in-suit Lu-Hf isotopic analysis results of detrital zircons from Jianshan Formation, Bayan Obo Group in the northern western block of North China Craton to illustrate its depositional time and provenance as well as the Precambrian crustal growth in this area. The ages of detrital zircons from Sample TM33 cluster at 1872~2100Ma (n=64), with a major age peak at ~1960Ma and a minor peak at ~2000Ma. The in-suit Lu-Hf isotopic analysis of the zircons indicate their ε_Hf(t) values vary from -2.6 to 7.9 (except one of -17.6), with two-stage Hf model ages (t_DM2) varying between 2240Ma and 4386Ma (averaged at 2749Ma). The ages of detrital zircons from Sample Z1724 are mainly between 1784~2200Ma (n=44), with a major age peak at ~1965Ma and a minor peak at ~2080Ma. The weighted average age of the youngest concordia ages is 1794±73Ma (n=2). In-suit zircon Lu-Hf isotopic analysis indicate their ε_Hf(t) values vary from -5.4 to 3.2, with two-stage Hf model ages (t_DM2) between 2584~3203Ma. Based on the above data and combined with the 1670Ma gabbro intruding into Bayan Obo Group and the carbonatite with whole rock ²⁰⁷Pb-²⁰⁶Pb age at 1649Ma, the depositional age for Jianshan Formation is between 1800Ma and 1650Ma. The main provenances for Jianshan Formation are the Neo-Archean basement in Guyang, Xiwulanbulang and Kondalite Belt in Daqingshan, Zhuozi. There might exist three crustal growth events in the northern western block of NCC at 2600~2750Ma、~2500Ma and 2000~2100Ma respectively, and the scale of the last one might be much smaller than the other two.

U–Pb age and Hf isotope record of detrital zircon grains from the North Delhi Supergroup, NW India: implications for provenance and stratigraphic correlations

The depositional ages of two Proterozoic supracrustal belts, the older Aravalli Supergroup and the younger Delhi Supergroup, overlying the Aravalli Banded Gneissic Complex in NW India are still unresolved. This study presents new combined detrital zircon U–Pb and Lu–Hf isotope data of two quartzite successions from the Alwar complex of North Delhi Supergroup to constrain the depositional ages and Precambrian crustal evolution. The new results show about 90% of detrital zircon population at ca. 1850 Ma, with minor age peaks at ca. 2900, 2700, 2500 and 2200 Ma and older Eo- to Paleoarchean zircon grains at 3655 ± 20 and 3222 ± 27 Ma. The age peaks of 1850 and 2500 Ma correspond to the known magmatic events, while the detritus for others were derived from a so-far unknown crustal source(s) in the region. The data also suggest two metamorphic overprints at 1470–1420 Ma and ca. 1650 Ma, which are consistent with known events of metamorphism in the region. The zircon U–Pb ages, Hf isotopes and field relations indicate that the sedimentary protoliths of the studied quartzite were deposited at ca. 1750 Ma in a collisional setting after the emplacement of subduction-related ca. 1850 Ma granitoids, but prior to the intrusion of 1730–1700 Ma rift-related A-type granites. The regional correlations suggest that the quartzite successions are not part of the Alwar Group as currently considered, but that they apparently belong to the older Aravalli Supergroup. Largely subchondritic eHf values for most of the zircon population signify predominance of crustal reworking during Neoarchean and Paleoproterozoic. The Hf model ages of the Eoarchean zircon indicate the existence of an ancient crust at ca. 4000 Ma in the Aravalli orogen and/or its hinterland.

Precambrian crustal evolution of the southwestern Tarim Craton, NW China: Constraints from new detrital zircon ages and Hf isotopic data of the Neoproterozoic metasedimentary rocks

Neoproterozoic metavolcanic-sedimentary sequences are well preserved along the southwestern margin of Tarim Craton, NW China. In order to constrain the depositional timing of the metasedimentary sequences and better understand the crustal evolution, we present new detrital zircon U–Pb ages and Hf isotopic data for the Neoproterozoic metasediments collected from the eastern Aliankate Group in the southwestern Tarim Craton (STC). The detrital zircons are characterized by high Th/U ratios, clear concentric zoning and euhedral shapes, which imply that the metasediments of the Aliankate Group were mainly derived from a proximal magmatic source. The U–Pb dating of the detrital zircons from three samples shows similar age spectra with an overwhelming population of early to middle Neoproterozoic ages. Zircons from this Neoproterozoic population of the three samples yielded uniform youngest peak age at 790–808 Ma, respectively, indicating the maximum depositional age of the eastern Aliankate Group. Based on the published age data of detrital zircons, we suggest that the Aliankate Group most likely deposited at 776–790 Ma. Because of the slightly different main age peaks, we also suggest that the Neoproterozoic source materials for the eastern and western of this group may be different although they were both derived from a Neoproterozoic source. The crustal Hf model ages of the Aliankate detrital zircons yield three distinct populations with peaks at 2.13–2.20 Ga, ~2.81 Ga, and ~3.03 Ga, respectively. In combination with the previous published data, we suggest that the southwestern Tarim Craton experienced similar crustal evolution to the northern in the Archean, especially for the earliest three crustal growth events at ~3.6 Ga, ~3.0 Ga, and ~2.8 Ga, but much different since the early Paleoproterozoic. Therefore, we suggest that the southwestern and northern Tarim Craton have uniform Archean basement and likely separated from each other in the early Paleoproterozoic.

Petrogenesis and geochronology of Paleoproterozoic magmatic rocks in the Kongling complex: Evidence for a collisional orogenic event in the Yangtze craton

The early Precambrian crustal evolution of the Yangtze craton, especially the Paleoproterozoic orogenic event, has drawn great attention. The early Precambrian magmatism in the Archean-Palaeoproterozoic basement Kongling complex is critical for understanding the early Precambrian tectonic evolution of the Yangtze craton. We report the presence of two distinct groups of Paleoproterozoic felsic rocks in the Kongling complex, providing new insights into the Paleoproterozoic orogenic event in the northern Yangtze craton. Group 1 is composed mainly of deformed or weakly-deformed monzogranite, syenite, and quartz monzonite, and formed at 2.0–1.95 Ga. They have high Sr/Y (60–71) and La/Ybcn (32–464) ratios and are inferred to have equilibrated with garnet-rich residues, suggesting that they have been generated by partial melting of thickened continental crust. Group 2 is represented mainly by undeformed to weakly-deformed K-feldspar granitic rocks, with minor rhyolites, and was emplaced at 1.87–1.84 Ga. In contrast to Group 1, Group 2 rocks display flat HREE patterns and large negative Eu, Sr and Ti anomalies, with low Sr/Y (<10) and La/Ybcn (16–21) ratios. Group 2 rocks have a typical A-type affinity with high Ga/Al ratios (2.83–3.82), and high crystallization temperatures (850–950 °C). Group 2 rocks are interpreted to have been derived from relatively shallow partial melting of thin continental crust during post-orogenic collapse, which marked lithospheric extension of the Yangtze craton in the late Paleoproterozoic (ca. 1.85 Ga).Zircons from Group 1 rocks have εHf (t) values ranging from −9.4 to −14.9, whereas zircons from Group 2 rocks have lower εHf (t) values ranging from −14.2 to −20.6. Both groups have similar two-stage Hf model ages ranging between 3.2 and 3.6 Ga. The production of the two groups of granitoids in the Huangling dome is ascribed to the reworking (dehydration melting) of Archean rocks in the Paleoproterozoic. Widespread Paleoproterozoic magmatism and metamorphism indicate that the Yangtze craton experienced an orogenic event at ca. 2.0 Ga and subsequent orogenic collapse caused by lithospheric extension at ca. 1.85 Ga.

Implication of Mesoproterozoic (∼1.4 Ga) magmatism within microcontinents along the southern Central Asian Orogenic Belt

The southern Central Asian Orogenic Belt contains numerous microcontinents including the Kazakhstan-Yili block, Central Tianshan Arc and Junggar block in the west, the Erguna, Xing'an, Songliao and Jiamusi-Khanka blocks in the east, bridging by the Tuva-Mongolia block, Beishan Orogenic Belt and Alax block in the central region. New geochemical and geochronological data from the microcontinental blocks within the southern Beishan Orogenic Belt provide new insight into the Precambrian crustal evolution of this region. Five granitoids from the Jiujing region yielded igneous zircon 206Pb/238U ages of 1450–1401 Ma, and zircon xenocrysts from late Paleozoic plutons in the Jiujing, Qiaowan, and Shibandun regions have 206Pb/238U ages that also cluster at ∼1.4 Ga. Those data suggest that crust of this age is widespread beneath the southern Beishan Orogenic Belt. The ∼1.4 Ga zircons display depleted Hf isotopic signatures (εHf(t) = 2.0–11.1) and show relatively young two-stage Hf model ages (1.9–1.4 Ga), indicating rapid reworking of juvenile mantle-derived materials. Integrating these results with similar geochronological data from the Kyrgyz North Tianshan, Yili block, Central Tianshan Arc, northern Alxa block and Xilinhot block, we propose that the CAOB incorporates an extensive assemblage of Mesoproterozoic (1450–1360 Ma) magmatism that can be traced over ca. 3000-km long a west-east trending belt. These Mesoproterozoic granitoids display distinct geochemical signatures (e.g., Ga/Al*10,000 ranging from 1.46 to 3.88 and Zr + Nb + Ce + Y varying largely from 101 to 1066 ppm) and show a temporal progression from orogenic to anorogenic over a relatively short period. This suggests a tectonic setting that likely evolved from compressional to extensional environments during 1450–1360 Ma, perhaps resulting from slab roll back. We suggest that the 1450–1360 Ma magmatism along the southern CAOB corresponds with a transition from a compressional to extensional environment on the margin of the Nuna supercontinent.