Yusheng Wan

Archean crustal evolution of the Jiaodong Peninsula, China, as revealed by zircon SHRIMP geochronology, elemental and Nd-isotope geochemistry

The Jiaodong Terrane in eastern Shandong Province is an important part of the Eastern Block of the North China Craton (NCC). In order to better understand the Precambrian crustal evolution of the NCC, we conducted a study of zircon geochronology, bulk-rock elemental and Nd-isotope geochemistry on gneisses and granodiorites from the Jiaodong Terrane. Zircon U-Pb SHRIMP analyses on biotite leptites and TTG gneisses yielded two groups of ages, one at ca. 2.90 Ga, and the other at 2.71 to 2.73 Ga. The new age results establish the existence of Mesoarchean and Neoarchean continental crust in the Jiaodong Terrane. The association of leptites, interpreted as metadacitic rocks, and TTG gneisses at 2.9 Ga was likely generated in an island-arc system, hence implying that plate tectonics, similar to the modern regime, was operative during the Mesoarchean. On the other hand, the results also indicate that the period of 2.71 to 2.73 Ga represents the most significant crust-forming episode in the Jiaodong Terrane. This is in contrast to the general understanding that the most important period of crustal growth and related metamorphism/deformation in the NCC took place in the terminal Archean (∼2.5 Ga). The geochemical and age constraints of the Neoarchean TTG rocks suggest that their formation was not related to subduction of oceanic crust, but to underplating and subsequent partial melting of lower crustal mafic rocks. Nd isotope data indicate that the Mesoarchean and Neoarchean rocks were mainly derived from juvenile sources with a limited amount of old crustal component. Like in other parts of the NCC, these rocks represent a juvenile addition to the late Archean continental crust. Finally, the formation of supracrustal rocks and most TTG gneisses in the period of 2.9 to 2.7 Ga distinguishes the Jiaodong Terrane from other tectonic units of the North China Craton.

NEW U-Pb AND Hf ISOTOPIC DATA CONFIRM ANSHAN AS THE OLDEST PRESERVED SEGMENT OF THE NORTH CHINA CRATON

The Anshan area in the northeastern part of the North China Craton contains the oldest known rocks in China. Rocks with ages of ∼3.8 Ga are present in three trondhjemite-dominated gneiss complexes, referred to as the Baijiafen, Dongshan and Shengoushi complexes. The oldest material identified in these complexes has weighted mean 207Pb/206Pb ages of 3800 ± 5 Ma, 3794 ± 4 Ma and 3777 ± 13 Ma, respectively. In addition, younger geological events at 3680 ± 19 Ma, 3620 ± 23 Ma, and 3573 ± 21 Ma have been identified, recording the emplacement of several generations of trondhjemite. A biotite schist of possible supracrustal origin contains a zircon population with a weighted mean 207Pb/206Pb age of 3723 ± 17 Ma and is cut by a 3620 Ma trondhjemite, implying formation prior to ∼3.62 Ga. TDM(Hf) model ages for the zircons range from 3256 to 4264 Ma, but are mostly ∼3.9 Ga, indicating that significantly older crust was not present in the source region of the trondhjemitic magmas.

Anatomy of Zircons from an Ultrahot Orogen: The Amalgamation of the North China Craton within the Supercontinent Columbia

We report SHRIMP U‐Pb, rare earth element, Hf isotope, and laser Raman spectroscopic data on zircons from ultrahigh‐temperature (UHT) granulites from the Jining Complex of the Khondalite Belt in the Western Block of the North China Craton (NCC). These UHT rocks form part of an ultrahot orogen that formed along the collisional margin of the NCC associated with the tectonics of assembly of the Paleoproterozoic supercontinent Columbia. Despite the core‐rim textures displayed by some of the UHT zircons, their age values sharply converge within error, yielding weighted mean 207Pb/206Pb ages of ca. 1.92 Ga, indicating growth by recrystallization under extreme thermal conditions. In general, the zircons show moderate heavy rare earth element enrichment with sharp positive Ce and negative Eu anomalies. The Hf isotope data from the UHT zircons also display a fairly uniform character, with the majority of them characterized by positive ϵHf values, without any indication of mixing between reworked crust and juvenile material. The majority of mineral inclusions in the UHT zircons, as revealed by laser Raman study, also support a magmatic source. Our results confirm extreme crustal metamorphism in the NCC and suggest asthenospheric upwelling, magmatic underplating, and plume activity in response to the collision‐extension events during the final assembly of the Paleoproterozoic supercontinent Columbia.

The Precambrian Khondalite Belt in the Daqingshan area, North China Craton: evidence for multiple metamorphic events in the Palaeoproterozoic era

Abstract High-grade pelitic metasedimentary rocks (khondalites) are widely distributed in the northwestern part of the North China Craton and were named the ‘Khondalite Belt’. Prior to the application of zircon geochronology, a stratigraphic division of the supracrustal rocks into several groups was established using interpretative field geology. We report here SHRIMP U–Pb zircon ages and Hf-isotope data on metamorphosed sedimentary and magmatic rocks at Daqingshan, a typical area of the Khondalite Belt. The main conclusions are as follows: (1) The early Precambrian supracrustal rocks belong to three sequences: a 2.56–2.51 Ga supracrustal unit (the previous Sanggan ‘group’), a 2.51–2.45 Ga supracrustal unit (a portion of the previous upper Wulashan ‘group’) and a 2.0–1.95 Ga supracrustal unit (including the previous lower Wulashan ‘group’, a portion of original upper Wulashan ‘group’ and the original Meidaizhao ‘group’) the units thus do not represent a true stratigraphy; (2) Strong tectono-thermal events occurred during the late Neoarchaean to late Palaeoproterozoic, with four episodes recognized: 2.6–2.5, 2.45–2.37, 2.3–2.0 and 1.95–1.85 Ga, with the latest event being consistent with the assembly of the Palaeoproterozoic supercontinent Columbia; (3) During the late Neoarchaean to late Palaeoproterozoic (2.55–2.5, 2.37 and 2.06 Ga) juvenile, mantle-derived material was added to the crust.

Juvenile magmatism and crustal recycling at the end of the Neoarchean in Western Shandong Province, North China Craton: Evidence from SHRIMP zircon dating

Western Shangdong Province experienced major crustal growth as a result of 2.75 to 2.50 Ga tectonothermal events, different from other Archean areas in the North China Craton. Besides early Neoarchean tonalite-trondhjemite-granodiorite (TTG) and supracrustal assemblages, there are large volumes of late Neoarchean gabbroic, dioritic and granitic rocks in western Shandong. SHRIMP zircon dating of thirty-one samples of different rock types from late Neoarchean rocks yielded a narrow age range from 2560 to 2490 Ma. Based on these data and previously published results, the following conclusions can be drawn: 1) the Archean basement can be divided into three belts: a Late Neoarchean (2525-2490 Ma) crustally-derived granite belt in the northeast, an Early Neoarchean (2.75-2.60 Ga) rock belt in the center, and a Late Neoarchean (2550-2500 Ma) belt of juvenile rocks in the southwest; 2) the tectonic regime in western Shandong Province probably changed from compressional to extensional at around 2525 Ma (between 2530 and 2520 Ma); 3) crustal recycling with addition from the mantle occurred at the end of the Neoarchean; 4) the late Archean magmatic rocks were probably formed in an arc environment.

Geochronology and geodynamics of Scottish granitoids from the late Neoproterozoic break-up of Rodinia to Palaeozoic collision

Thirty-seven granitoids from Scotland have been dated using the sensitive high-resolution ion microprobe zircon method. Granitoids were intruded during: (1) crustal stretching at c. 600 Ma after Rodinia broke up (A-types); (2) the Grampian event of crustal thickening when the Midland Valley Arc terrane collided with Laurentia at c. 470 Ma (S-types); (3) erosion and decompression of the over-thickened Laurentian margin at c. 455 Ma (S-types); (4) subduction of Iapetus Ocean lithosphere under Laurentia starting at 430 Ma (I-types); (5) roll-back beginning at 420 Ma (I-types); (6) bilateral slab break-off and lithospheric delamination at 410 Ma (I- and S-type granites) when Baltica hard-docked against the Northern Highland terrane and Avalonia soft-docked against the Grampian Highland terrane. Far-field Acadian events at 390 Ma were recorded by I-type granites intruded along active sinistrally transpressive faults. I-types formed in lower crustal hot zones above subduction zones, whereas S-types formed in lower crustal hot zones above lithospheric windows through which hot asthenosphere had risen.

Is the Ordos Block Archean or Paleoproterozoic in age? Implications for the Precambrian evolution of the North China Craton

The Ordos Block is a large but poorly exposed crustal segment in the western part of the North China Craton. Little is known about its Precambrian basement rocks and their evolution because of an extensive cover by younger sedimentary rocks. We report SHRIMP zircon ages and Hf-in-zircon isotopic compositions for five samples recovered from drill holes that penetrated into the Ordos basement. Based on these data, two age groups of rocks can be distinguished: an early Paleoproterozoic group that is represented by one sample from the northern margin of the Ordos Block and a late (or middle-late) Paleoproterozoic group consisting of the remaining four samples. The early Paleoproterozoic sample contains zircon cores and metamorphic rims with ages of >2.4 Ga and >2.28 Ga, respectively. The cores have εHf(t) values of −8.8 to 6.2 and Hf crustal model ages of 2606 to 3221 Ma. The remaining rocks of late (or middle-late) Paleoproterozoic age contain zircons that may also show a core-rim structure, but with the cores having ages of ∼2.08 Ga and the metamorphic rims recording ages of ∼1.9 Ga. The zircon cores have εHf(t) values of −9.1 to 10.1, and Hf crustal model ages of 2039 to 3068 Ma. Combined with data from earlier studies, we draw the conclusion that late Paleoproterozoic metasedimentary rocks are widespread in the Ordos basement. There is no evidence from our samples of an Archean component, and this calls into question the prevailing view that the Ordos basement is an Archean cratonic block. The fact that the Ordos basement was involved in a widespread late Paleoproterozoic tectono-thermal event indicates that most current models for the tectonic evolution of the western portion of the North China Craton need to be revised.

SHRIMP zircon U-Pb dating of late Paleoproterozoic kondalites in the Daqing Mountains area on the North China Craton

The Daqing Mountains area comprises a typical occurrence of the Khondalite Belt in the Western Block of the North China Craton (NCC). In this area, both early and late Paleoproterozoic metasedimentary rocks have been identified in what was originally called the Upper Wula Mountains “Subgroup”. Six metasedimentary rock samples yielded SHRIMP U-Pb zircon ages of 2.56–2.04 Ga for detrital and 1.96–1.83 Ga for metamorphic zircons. Based on these data and previously published results, the following conclusions can be drawn: 1) The source region for the late Paleoproterozoic detrital sedimentary rocks is mainly 2.55–2.4 and 2.2–2.04 Ga in age, consistent with the early Precambrian geological history identified widely in the basement of the NCC. 2) The majority of sedimentary rocks of the khondalite series were deposited between 2.04 and 1.95 Ga, and then in a protracted period (1.96 and 1.83 Ga) underwent a complex history of amphibolite to granulite-facies metamorphism.

Formation and Evolution of Archean Continental Crust of the North China Craton

The North China Craton (NCC) has had a long geological history back to ca. 3.8 Ga ago. In the Anshan area, northeastern part of the craton, three distinct complexes with ages of 3.8–3.1 Ga (Baijiafen, Dongshan, and Shengousi) have been identified, along with widespread 3.1–2.5 Ga rocks of different origins and ages. In eastern Hebei Province, abundant 3.88–3.4 Ga detrital zircons were obtained from metasedimentary rocks of the Caozhuang Complex, and the oldest rock identified is a 3.4 Ga gneissic quartz diorite. The oldest zircons that may originally have been derived from the NCC are 4.1–3.9 Ga grains in Paleozoic volcano-sedimentary rocks in the northern Qinling Orogenic Belt bordering the NCC in the south. 3.0–2.8 Ga rocks occur in Anshan, eastern Hebei, eastern Shandong, and Lushan. ca. 2.7 Ga rocks of igneous origin are exposed in eight areas of the NCC, but ~2.7 Ga supracrustal rocks have so far only been identified in western Shandong. ca. 2.5 Ga intrusive and supracrustal rocks and associated regional metamorphism occur in almost all Archean areas of the NCC. Banded iron formations contain the most important ore deposit of the Archean in the NCC and mainly formed during the late Neoarchean. Ancient crustal records obtained from deep crust beneath the NCC are similar to those in the exposed areas, with the oldest ca. 3.6 Ga rock enclaves occurring in Xinyang near the southern margin of the NCC. This synthesis is based on the compilation of a large database of zircon ages as well as whole-rock Nd isotopic and Hf-in-zircon isotopic data in order to understand the formation and evolution of the early Precambrian basement of the NCC. Considering the craton as an entity, there is a continuous age record from 3.8 to 1.8 Ga, and two tectono-thermal events are most significant in the late Neoarchean to the earliest Paleoproterozoic and late Paleoproterozoic history, with age peaks at ~2.52 and ~1.85 Ga, respectively. Whole-rock Nd and Hf-in-zircon isotopic data show similar features, documenting the addition of juvenile material to the continental crust at 3.8–3.55, 3.45, 3.35–3.3, 2.9, and 2.85–2.5 Ga with the late Mesoarchean to early Neoarchean being the most important period. Crustal recycling began as early as 3.8 Ga and continued until 3.25 Ga and appears to have played a more important role than juvenile additions between 3.25 and 2.90 Ga. After outlining the general geological history of the NNC basement, we discuss several issues relating to Archean crust formation and evolution and arrive at the following major conclusions: (1) Similar to several other cratons, the late Mesoarchean to early Neoarchean was the most important period for rapid production of continental crust, and the most intensive and widespread tectono-thermal event occurred at the end of the Neoarchean. (2) In our new tectonic model, we define and outline three ancient terranes containing abundant 3.8–2.6 Ga rocks, namely the Eastern Ancient Terrane, Southern Ancient Terrane, and Central Ancient Terrane. (3) Vertical magmatic growth is seen as the main mechanism of crust formation prior to the Mesoarchean. We favor a multi-island arc model related to subduction/collision and amalgamation of different ancient terranes in the late Neoarchean. (4) The NCC may already have been a large crustal unit as a result of cratonic stabilization at the end of the late Neoarchean, probably due to magmatic underplating.

Paleoproterozoic crustally derived carbonate-rich magmatic rocks from the Daqinshan area, North China Craton: Geological, petrographical, geochronological and geochemical (Hf, Nd, O and C) evidence

Most carbonate-rich magmatic rocks are mantle-derived, namely carbonatite(s), with a minority of them being contaminated by crustal rocks. It is debated whether there are also carbonate-rich magmatic rocks derived solely from crustal sources. In this contribution, we report crustally derived carbonate-rich magmatic rocks, named here crustal carbonatite(s), in the Daqingshan area, Western Block of the North China Craton. The Daqingshan crustal carbonatites were previously considered to be metasedimentary marbles. However, they cut adjacent rocks and contain some enclaves that are irregular in shape and show unoriented distribution of lithologies that cannot be found in the local wall rocks. Zircons from the crustal carbonatites show clear oscillatory zoning, and contain calcite, quartz, feldspar, diopside and CO2 inclusions. The zircons have correlated U and Th abundances and give a U/Pb age of 1951 ± 5 Ma, being the same age as metamorphic zircon cores and rims from a diopsidite enclave (1954 ± 27 Ma of core and 1944 ± 40 Ma of rim). The zircon from the crustal carbonatite has tDM(Hf) and εHf(t) of 2353 to 2457 Ma and –3.2 to 0.7, whereas the tDM(Hf) and εHf(t) of the core and rim zircons from diopsidite range from 2228 to 2160 Ma and 3.0 to 4.8 and 2219 to 2057 Ma and 3.2 to 7.4, respectively. The δ18O (‰)Zircon-V-PDB of zircon from the crustal carbonatite ranges from –21.5 to –19.6, with O isotope equilibrium temperature being 555°C to 635 °C. The crustal carbonatite shows a large variation in chemical composition, with SiO2=10.2–37.3% and Total REE=48–267 ppm. The tDM(Nd) age and εNd(t) are ∼2.5 Ga and –2.9 to –2.4. The δ18O(‰)Rock-V-PDB and δ13C(‰) Rock-V -PDB vary from –19.5 to –15.2 and from –5.2 to –2.4, being distributed between primary carbonatite field and the field of dolomitic marble from the study area in δ18O(‰) vs. δ13C(‰) diagram. Combined with previous studies, we drew the conclusion that some of the carbonate-rich rocks in the study area are magmatic in origin, by anatexis of impure marble plus common contamination by calc-silicates and other materials. This is consistent with the high-P-T experiments of CaO-CO2-H2O system (Wyllie and Tuttle, 1960) and MgO-CaO-CO2-H2O system (Fanelli and others, 1986), which indicate that partial melting of limestone will happen when temperature is > 700°C and when water is also present.