Search

ABSTRACT. We present an integrated study of zircon U-Pb dating and O-Hf isotopic analyses combined with whole-rock geochemistry and Nd isotopic systematics on widespread late Neoarchean syenogranites (Qidashan Pluton) and other granitoids (granodiorite, quartz monzonite, monzogranite) in the Anshan-Benxi area, eastern North China Craton. All these rocks were emplaced at ca. 2.5 Ga according to SHRIMP zircon U-Pb dating and other indirect methods. The syenogranites are characterized by high SiO2 and K2O /Na2O and low CaO, FeOt, MgO, TiO2 and P2O5. However, they differ in trace element and REE compositions and can be roughly subdivided into two types. Type 1 syenogranite has strongly negative Eu anomalies and Ba depletion, with large variations of LREE to HREE differentiation; type 2 syenogranite has no negative Eu anomaly and Ba depletion. Other granitoids are also rich in K2O and show similar trace element and REE compositional features as the type 2 syenogranite. Most syenogranite samples have whole-rock εNd(t) values of -10.5 to -2.7 and tDM(Nd) ages of 2.96 to 3.90 Ga, whereas the other granitoids have εNd(t) values of -5.2 to -2.2 and tDM(Nd) ages of 2.95 to 3.19 Ga. Magmatic zircon from the syenogranites has εHf(t), values and Hf crustal model ages ranging mainly from -11.0 to +4.4 and 2.70 to 3.46 Ga, respectively; those from other granitoids have εHf(t) values and Hf crustal model ages of -16.0 to + 2.4 and 2.81 to 3.72 Ga, respectively. All these rocks contain Meso- to Paleoarchean xenocrystic zircon grains with εHf(t) values and Hf crustal model ages ranging from -14.3 to +8.3 and 2.79 to 3.93 Ga, respectively. Decoupling of wholerock Nd and Hf-in-zircon isotopes occurs in some samples. Most low-U magmatic zircon (U<200 ppm) from all granitoids has δ18O values in the range of +4 to +7.2 permil, whereas high-U magmatic zircon (U≥200 ppm) shows a larger variation with the lowest δ18O value being 1.6. The xenocrystic zircon grains have δ18O values in the normal range of Archean magmatic zircon. Combined with previous studies, we conclude that the 2.5 Ga granitoids formed by recycling of mature, old continental crust, including metasedimentary sources, probably in an extensional tectonic environment. [ABSTRACT FROM AUTHOR]

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 Paleopro-terozoic 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 unonented 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 α18O(‰)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. [ABSTRACT FROM AUTHOR]

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 Meso-archean 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Assegaai, De Kraalen and Witrivier greenstone belts in the southeast part of the Kaapvaal craton contain strongly deformed volcano-sedimentary successions affected by amphibolite- to granulite-facies metamorphism at ~3.22 to 3.20 Ga. These greenstone belts consist predominantly of mafic-ultramafic volcanic successions intercalated with chert and BIF. Single zircon SHRIMP ages of 3222±8 and 3193±5 Ma for two granitoids intrusive into the Assegaai greenstone belt provide a minimum age for the succession. Calc-silicates and amphibolites of the De Kraalen and Witrivier greenstone belts record a high-pressure metamorphic event (M1) at ~12 to 15 kbar and ~600 °C and 800 °C, respectively. This high-pressure event was overprinted by a medium-pressure (7-8 kbar) amphibolite-facies event (M2A) that was recorded in all three greenstone belts, suggesting that the De Kraalen and Witrivier supracrustal rocks were buried to a deeper crustal level than the Assegaai rocks during crustal thickening. Decompression of the rocks along clockwise P-T paths possibly occurred during subsequent crustal extension at ~3.22 to 3.20 Ga. The low geothermal gradient of this area (15-20 °C/km) contrasts with higher geothermal gradients reported for other Archean terrains worldwide and may support the view that crustal thickening processes related to convergence in a setting similar to subduction zones were operating in Paleoarchean times. [ABSTRACT FROM AUTHOR]

Sensitive high resolution secondary ion mass spectrometry (SHRIMP II), laser ablation inductively coupled plasma mass spectrometry (ICP-MS) and laser Raman spectroscopy have been used to determine the ages in zircons from carbonatite dikes associated with the Bayan Obo giant REE-Nb-Fe deposit, Inner Mongolia, China. Analyses of small amounts of zircon extracted from large samples of three carbonatite dikes yield late Palaeoproterozoic 207Pb/206Pb ages of 1894 ± 27, 1944 ± 20 and 1956 ± 9 Ma. One sample also contained inherited grains with ages up to ~2.55 Ga. Zircon grains, even from a single rock, have a wide range of REE patterns, suggesting that they are probably inherited zircons trapped during magma genesis, not zircons crystallized from a single magma. None of the zircon grains has the extremely high Th/U characteristic of the Bayan Obo ore deposit or of the associated carbonatites. Further, mineral inclusions in the zircon identified by Raman spectroscopy are all silicate minerals (quartz and feldspars), consistent with crystallization of the zircon from silicate, not from carbonate, magmas. The Palaeoproterozoic zircons in the Bayan Obo carbonatite dikes did not crystallize from carbonatite magma at the time of dike emplacement, but were derived from wall rock contamination. The ages obtained from the zircons are consistent with ages measured on basement igneous rocks elsewhere in the Bayan Obo region. [ABSTRACT FROM AUTHOR]