South Liaohe Group Research Articles

Tectonic setting of the South Liaohe Group in the Jiao‐Liao‐Ji Belt, North China Craton: Geochemical and geochronological constraints from metasedimentary rocks

The metasedimentary rocks of the South Liaohe Group are widely distributed along the Jiao‐Liao‐Ji Belt (JLJB) in eastern Liaoning Province. It is important to understanding tectonic setting and evolution of the JLJB by determining its sediment sources and properties of the provenances, sedimentary facies, metamorphic ages, and facies. After the detailed field geological surveys and comprehensive laboratory observation, many high‐grade metapelitic rocks are identified in the South Liaohe Group. The main rock type is sillimanite‐garnet‐cordierite‐biotite plagiogneiss. The zircon U–Pb geochronology of three samples shows that these high‐grade meta‐pelitic rocks are mainly derived from the Liaoji Granites and the Archean basement. The protolith formation ages are between 2.00 and 1.93 Ga. Subsequently, these protolithes underwent high‐pressure metamorphism resulted from collisional orogeny, with peak metamorphism at ca. 1.88 Ga. Then, at 1.88–1.84 Ga, these rocks experienced the retrograde metamorphism and exhumed to the surface during crustal uplift. Geochemical data show that the protoliths are alumina‐rich claystones and graywackes with low maturity, implying near‐source sedimentation. The chemical index of alteration (CIA) values of these samples range from 60 to 80, indicating that the protoliths underwent moderate weathering. The sediment source was mainly felsic components of the upper crust, possibly with the addition of ancient sediments. Various tectonic environment discrimination diagrams combined with previous results on metamorphism and isotope chronology of these metasedimentary rocks in the JLJB indicate that the rocks in the study area originated from an active continental margin with an evolving island arc.

Does the Liaoji granite represent basement in the Jiao–Liao–Ji Belt?

Lack of knowledge of the basement structure of the Paleoproterozoic Jiao–Liao–Ji Belt (JLJB) of the North China Craton has hindered understanding of the tectonic evolution of the belt and, to an extent, the entire craton. Previous studies of the JLJB were based mostly on petrological, structural, and geochemical analyses, resulting in a lack of understanding of the deep geological structure of the belt. In this study, we used a combination of geophysical and geological methods to interpret the deep geological structure of the JLJB on the basis of high-precision non-seismic geophysical data. We collected high-precision gravity, magnetic, and magnetotelluric sounding data. We also compiled data on rock physical parameters for important geological bodies, and obtained high-precision Bouguer gravity anomaly data from the China Geological Survey. We interpreted these data to visualize the subsurface distributions of geological bodies (including sedimentary and metamorphic units and intrusive rocks) in the Kuandian–Huairen area of the JLJB. We found that the Liaoji granites are developed on a large scale in the deep part of the JLJB, showing a weak gravity anomaly (−25 to −6 mGal) and high resistivity (103.5 to 106 Ω·m). These granites may have intruded along a series of normal faults at the time of the initial formation of the belt, and expanded over time. Subsequently, the low-resistivity Liaohe Group (10 to 102.5 Ω·m) was deposited on the Liaoji granite across a sedimentary contact. Therefore, the Liaoji granite forms the basement of the JLJB. The presence of extensional faults in the deep part of the JLJB suggests that the belt was also subjected to tectonic extension. The Yuejiadian–Xiaohuanggou fault to the east of Kuandian may be the boundary between the South Liaohe Group and the North Liaohe Group. The South Liaohe Group is relatively thin and tightly folded, whereas the North Liaohe Group is thicker and characterized by relatively open folds.

Carbon isotope and geochemical characteristics of the Paleoproterozoic graphite deposits in the Jiao-Liao-Ji belt, North China Craton: Implications for genesis and depositional environment

As the northern segment of the Jiao-Liao-Ji Paleoproterozoic graphite metallogenic belt, the Liaohe Group contains a significant amount of phaneritic graphite deposits, which are of great significance to the exploration of graphite resources in China. However, few studies have investigated the metallogeny of these deposits. To unravel the carbon source and metallogenic process of the graphite deposits in the Liaohe Group, we performed detailed petrological, zircon U-Pb dating, Raman spectroscopy, and carbon isotope studies for the graphite deposits in the North (MG-Magou graphite deposit) and South (WLG-Wuligou graphite deposit) Liaohe Groups. The results show that the graphite-bearing rocks of the two groups are located in the sandstone, shale, wacke, and arkose regions in the log (Fe2O3/K2O) – log (SiO2/Al2O3) sediment classification diagram. They have similar trace element characteristics and exhibit similar rare earth element features with post-Archean Australian average shales (PAAS). Geochemical data show that these samples are compositionally immature and underwent a relatively low degree of weathering, which are the products of rapid accumulation of sediments. The carbon isotopic values of graphites from the WLG and MG both show a wide range of variation (δ13CWLG = −13.69 to −17.82‰, δ13CMG = -16.49 to −25.72‰), which are caused by the demethanation of organic matter during metamorphism. Raman spectroscopy indicates that the peak metamorphic temperature of the graphite from WLG is higher than 650 ℃, which is higher than that of MG graphite deposits (T = 551–627 ℃) from the North Liaohe Group. Combined with previous geochronology data, we suggest that a large amount of organic matter was deposited in an active continental margin environment at 2130–2175 Ma. During the collisional orogeny between the Longgang and LiaonanNangrim blocks (1950 to 1800 Ma), organic matter gradually transformed into graphite through prograde metamorphism and accumulated to form the graphite deposits in the Liaohe Group.

Diversity of zircon U-Pb geochronology of meta-sedimentary rocks from the Gaixian Formation, South Liaohe Group, Jiao-Liao-Ji belt: Implications for different provenance and crustal evolution

Comprehensive studies of depositional age and source material of the Liaohe Group are significant to the tectonic evolution of the Jiao-Liao-Ji Belt (JLJB). The Gaixian Formation is widely distributed and stratigraphically defined as the uppermost formation of the Liaohe Group. However, our new work reveals that not all meta-sedimentary rocks of the Liaohe Group deposited before the late Paleoproterozoic orogeny. Based on the geochronological results, these meta-sedimentary rocks can be subdivided into two categories. Detrital zircon from the category A meta-sedimentary rocks yield U-Pb age peaks at ca. 2.50 Ga, 2.18 Ga and 2.06 Ga. The youngest age peak of ca. 2.06 Ga, together with the metamorphic age of 1.95–1.84 Ga yielded by pervious studies, indicate their depositional ages of 2.06–1.95 Ga. Compared to the category A, detrital zircon from the category B yield similar 207Pb/206Pb age peaks at ca. 2.54 Ga, ca. 2.17 Ga and ca. 2.05 Ga, but importantly also at ca. 1.87 Ga which was never observed for the category A. The late Paleoproterozoic zircon (~1.87 Ga) include magmatic cores and weakly oscillatory zoned or unzoned rims. These rims yield two age peaks of ca. 1.88 Ga and ca. 1.80 Ga, which are interpreted as inherited partial melting ages. Therefore, the category B meta-sedimentary rocks deposited at some time after ca. 1.80 Ga. These new geochronological data reveal that the category B meta-sedimentary rocks are significantly different with those from the traditional Gaixian Formation. In combination with regional stratigraphy data, it is recommended that the category B meta-sedimentary rocks should be ascribed to the Changhai Group rather than the Liaohe Group.

The ~1.87 Ga granulite facies metamorphism of the South Liaohe Group in the Jiao-Liao-Ji Belt and its tectonic implications

To further understand the metamorphic evolution in the southern margin of the Jiao-Liao-Ji Belt in the Liaodong area, a detailed petrological and geochronological study was performed on representative metapelites from the South Liaohe Group. The peak metamorphic stage (M1) of the samples is represented by the mineral assemblage of garnet-biotite-plagioclase-sillimanite-quartz-magnetite in sample DD24-1 and garnet-biotite-plagioclase-ilmenite-quartz in sample DD24–2. The post-peak metamorphic stage (M2) is indicated by the appearance of cordierite with the mineral assemblage of biotite-cordierite-garnet-plagioclase-K feldspar-sillimanite-quartz-magnetite-ilmenite in sample DD24-1. The P-T calculations using pseudosections and conventional geothermobarometry imply that the M1, M2 and final retrograde stage (M3) metamorphisms occurred at 865–890 °C and 6.9–9.2 kbar, 840–860 °C and 5.3–6.2 kbar, and 570–670 °C and 3.3–4 kbar, respectively. The zircon SHRIMP U–Pb age data show that M1 metamorphism occurred 1867 ± 11 Ma and 1866 ± 8 Ma for samples DD24-1 and DD24–2, respectively. Together with previous studies, this study confirms that a low-P/T type granulite facies regional metamorphic event with a clockwise P-T path occurred in the South Liaohe and the Ji'an groups during 1.87–1.84 Ga. Together with the simultaneous postcollisional igneous activity in the South Liaohe and Ji'an groups, the low-P/T type metamorphism indicates that postcollisional activity occurred in the South Liaohe and Ji'an groups during 1.87–1.84 Ga due to slab break off after the collision. Combined with the prevailing postcollisional activity in the Nangrim Massif on the Korean Peninsula, this study suggests that the Longgang Block in the North China Craton subducted under the Nangrim Massif on the Korean Peninsula.

Detrital zircon U-Pb geochronology and Hf isotopes of the Liaohe Group, Jiao-Liao-Ji Belt: Implications for the Paleoproterozoic tectonic evolution

The Jiao-Liao-Ji Belt (JLJB) is one of the most important Paleoproterozoic orogenic belts, which is intervening between the Longgang and Nangrim-Liaonan blocks. The Liaohe Group is an important research subject for understanding the Paleoproterozoic tectonic setting and evolution of the JLJB. However, the relationship of depositional spatial and chronological distributions between the North and South Liaohe groups are still under debates. In this paper, we present geochronological and zircon Lu-Hf isotopic results of meta-sedimentary rocks from the Liaohe Group to decipher their provenance depositional ages and environment, as well as their tectonic significance on the JLJB. The detrital zircons from the corresponding formations, including the Li’eryu, Gaojiayu and Dashiqiao formations of the North and South Liaohe groups, show similarities in age patterns and Hf isotopic compositions. The detrital zircons from the Li’eryu Formation of the two groups display unimodal 207Pb/206Pb age peaks at ca. 2.2–2.1 Ga. On the other hand, detrital zircons from the Gaojiayu and Dashiqiao formations of the two groups, yield two dominating age peaks at ca. 2.2–2.1 and ~2.5 Ga. In summary, all detrital zircons with age peaks at ca. 2.2–2.1 Ga, have εHf(t) value of −6.99 to +9.41 and two-stage Hf depleted mantle ages (TDM2 model ages) of 3143 to 2137 Ma. The ~2.5 Ga detrital zircons however, have εHf(t) value and TDM2 model ages which widely range from −9.37 to +10.13 and from 349 to 2367 Ma, respectively. Geochronological results indicate that ca. 2.2–2.1 Ga zircons were derived from the Liaoji granitoids. The adjacent basement rocks of the Longgang and Liaonan-Nangrim blocks provide the major sources to detrital zircons that yield an age peak at ~2.5 Ga. Combined with previous studies, the new geochronological data suggest that the protoliths of the corresponding formations of the North and South Liaohe groups deposited their detritus progressively in a back-arc basin. The Liaoji granitoids are the main sources providing detritus for the Li’eryu Formation. Subsequently, clastic sediments transported from the Longgang and Liaonan blocks, as well as from the Liaoji granitoids, were successively deposited and provided sources to the Gaojiayu and Dashiqiao formations.

Geochronology and geochemistry of Liaohe Group and Liaoji granitoid in the Jiao-Liao-Ji Belt, North China Craton: Implications for petrogenesis and tectonic evolution

Zircon U-Pb geochronological, whole-rock geochemical, and Hf isotopic studies were carried out on Liaohe Group and Liaoji granitoid within the central part of the Liaodong peninsula, NE China, to understand the Paleoproterozoic evolutionary history of Jiao-Liao-Ji Belt, North China Craton. Zircon U-Pb dating indicates the studied metavolcanic rocks from Dashiqiao Formation in South Liaohe Group had protolith ages of 2170 ± 29 Ma, 2170 ± 11 Ma, 2166 ± 17 Ma, and 2137 ± 9 Ma, with one showing a metamorphic age of 1866 ± 9 Ma. These metavolcanic rocks are dacite-andesite in compositions, belonging to calc-alkaline series. They are characterized by light rare earth elements (LREEs)-enriched patterns, and enriched in large ion lithophile elements (LILEs) relative to high field strength elements (HFSEs), with significant negative Nb-Ta-Ti-P anomalies, showing arc affinities. Combined with their εHf(t) values (–4.36 to +8.38), we suggest that the intermediate-acidic magma were derived from partial melting of Meso-Neoarchean juvenile lower continental crust, with a possible contribution of ancient materials. In addition, the studied monzonitic gneiss from Liaoji granitoid had a protolith age of 2167 ± 26 Ma, with a metamorphic age of 1883 ± 10 Ma. Based on previous studies of Liaohe Group and related meta-mafic rocks, as well as Liaoji granitoid, these Paleoproterozoic rocks not only share statistically indistinguishable Paleoproterozoic magmatic and metamorphic history for the Jiao-Liao-Ji Belt, but also collectively formed in an active continental margin or volcanic arc. These data, combined with previous studies within the Jiao-Liao-Ji Belt, suggest that an arc-continent collision model was favored for the Paleoproterozoic tectonic evolution of the Jiao-Liao-Ji Belt, and the significant arc-continental collision was probably at ca. 1.9 Ga.

Petrological and geochronological evidence for Paleoproterozoic granulite-facies metamorphism of the South Liaohe Group in the Jiao-Liao-Ji Belt, North China Craton

To better understand the formation of high-temperature metamorphic rocks, we present a detailed petrological and geochronological study of recently discovered cordierite-bearing granulites of the South Liaohe Group in the Jiao-Liao-Ji Belt, North China Craton. Petrographic observations indicate that four distinct mineral assemblages are present in the granulites: pre-peak amphibolite facies (M1), peak granulite facies (M2), post-peak decompression (M3), and late cooling retrogression (M4). M1 is preserved as fine-grained inclusions in the cores of garnet grains, represented by quartz + plagioclase + biotite + ilmenite. M2 is interpreted to have comprised garnet + sillimanite + plagioclase + quartz + biotite + ilmenite + melt. M3 was characterized by the formation of cordierite + sillimanite symplectites and cordierite + quartz coronas replacing garnet. M4 is indicated by the formation of staurolite, accompanied by the crystallization of melt. A combination of multi-equilibria geothermobarometers and pseudosection modeling constrains the P–T conditions of the M1, M2, M3, and M4 stages to P = 0.66–0.71 GPa and T = 620–650 °C, P = 0.96–1.10 GPa and T = 790–840 °C, P = 0.62–0.65 GPa and T = 725–785 °C, and P = 0.43–0.55 GPa and T = 595–625 °C, respectively. Zircon and monazite U–Pb dating yields three distinct and meaningful ages for the granulites: (1) 2200–2100 Ma for the granulite protoliths, (2) ca. 1945 Ma for the near-peak metamorphism, and (3) 1851–1839 Ma for the post-peak and late retrogressive metamorphism. Thus, a clockwise P–T–t path is determined for the cordierite-bearing granulites of the South Liaohe Group. Petrological and geochronological evidence from regional metamorphic rocks in the Jiao-Liao-Ji Belt suggests that the belt experienced a continuous orogenesis from 1950 to 1800 Ma.

Deformation of amphibolites from the Paleoproterozoic Liaohe Group, Liaodong Peninsula, China: Implications to the crustal structure of the Jiao‐Liao‐Ji mobile belt in the eastern block, North China Craton

Structural evolution of the Paleoproterozoic Jiao‐Liao‐Ji orogenic belt in eastern North China Craton has been a subject of extensive study in the past decades. Studies of the characteristics of the crustal structure during orogenesis are essential to the understanding of the formation and evolution of the orogenic belt. In this paper, we present detailed studies of deformed amphibolites in the southern part of the orogenic belt to illustrate the changes of deformation conditions and variation of deformation mechanisms during the orogenic processes. Based on differences in structural characteristics of foliations, amphibolites from the Paleoproterozoic South Liaohe Group at Caohekou–Jiguanshan area are subdivided into four types (massive, weak‐gneissic, moderate‐gneissic, and strong‐gneissic). Our study shows that the different characteristics in microstructures, rock fabrics, and deformation mechanisms of the amphibolites were resulted from varying temperatures and pressures in relation to the tectonic deformation. The widely distributed massive amphibolites (P = 0.39 GPa, T = 639°C) display no or weak deformation, which is inferred to represent steady‐state crystallization. Weak‐gneissic amphibolites (P = 0.52 GPa, T = 753°C) were resulted from stress‐controlled grain growth. Amphiboles in such rocks show obvious (100) LPOs but minor evidence for crystal plastic deformation. Evidences for crystal plastic deformation of amphibole grains are common in moderate‐gneissic amphibolites (P = 0.61 GPa, T = 771°C). The amphibole grains have strong (100) and (001) LPOs. Dislocation slip is considered as the main deformation mechanism of the amphibole grains in the rocks. Both subgrains and mechanical twins of amphiboles from strong‐gneissic amphibolites (P = 0.77 GPa, T = 686°C) are long‐columnar shaped and show the same orientation as their host crystals. They are resulted from reactivation of (100)(001) slip system controlled by dynamic recrystallization, twinning, and twinning nucleation recrystallization. The evident pressure difference from 0.39 to 0.77 GPa during the formation of the various types of amphibolites may be related to the tectonic thickening during contractional orogenesis.

Geochemistry, zircon U-Pb dating and tectonic implications of the Palaeoproterozoic Ji’an and Laoling groups, northeastern Jiao-Liao-Ji Belt, North China Craton

The Jiao-Liao-Ji Belt is located between the Longgang and Langrim blocks in the Eastern Block of the North China Craton (NCC). The belt comprises Palaeoproterozoic volcanic–sedimentary successions that are divided into the Ji’an, South Liaohe, and Jingshan groups in the south and the Laoling, North Liaohe and Fenzishan groups in the north. The tectonic setting and depositional environments of the Liao-Ji area are debated. This paper presents geochemical and detrital zircon U-Pb data for the boron-bearing, graphite-bearing, aluminum-rich Ji’an Group and the clastic sediments and carbonates of the Laoling Group in southern Jilin Province, in order to constrain their depositional ages, provenance and tectonic setting. Zircon geochronology for both the Ji’an and Laoling groups yields age peaks of 2.57–2.41, 2.23–2.09, and 2.04–2.02 Ga, indicating that the sediments were derived from the Longgang and/or Liaonan block(s), the 2.2–2.1 Ga Liaoji granitoids and ∼2.0 Ga igneous rocks, respectively. On the basis of the youngest detrital zircons and the oldest metamorphic zircon peaks, depositional ages can be constrained to the period of 2.13–1.94, 2.10–1.94, and 1.95–1.90 Ga for the Mayihe, Huangchagou, and Dadongcha formations of the Ji’an Group, and 2.18–1.90, 2.08–1.91, 1.99–1.90, 1.96–1.90, and 1.94–1.90 Ga for the Dataishan, Zhenzhumen, Huashan, Linjiang, and Dalizi formations of the Laoling Group, respectively. The zircon age spectra of each formation indicate that deposition of the Ji’an and Laoling groups was related to the opening and closing of a back-arc during the period 2.2–1.9 Ga. The Dataishan Formation (Laoling Group) was deposited in the rift environment during opening of the back-arc. During the early stage, the Mayihe and Huangchagou formations (Ji’an Group) were deposited on the side of the magmatic arc, and the Zhenzhumen Formation (Laoling Group) was deposited on the passive-margin side. During the late stage, both Ji’an and Laoling groups were deposited in a pre-orogenic setting, including the Dadongcha, Huashan, Linjiang and Dalizi formations. Detrital zircon age spectra of the Ji’an Group are similar to those of the South Liaohe Group (with the exception of the Gaojiayu Formation), indicating that the two groups had a similar provenance.

Constraints of mafic rocks on a Paleoproterozoic back-arc in the Jiao-Liao-Ji Belt, North China Craton

The Paleoproterozoic Jiao-Liao-Ji Belt (JLJB) in the North China Craton, a major terrane boundary, has remained controversial with respect to its tectonic history. Here we report LA–ICP–MS U–Pb zircon ages, and whole-rock elemental and isotopic compositions of mafic rocks intruding into the North and South Liaohe Groups, with the aim of evaluating their petrogenetic affinities and metamorphic history, and their implications for the tectonic evolution of the JLJB. The mafic rocks in the North Liaohe Group dominantly composed of meta-diabase and meta-gabbro, whereas mafic rocks in the South Liaohe Group are mainly (garnet-) amphibolite. LA–ICP–MS zircon U–Pb analyses yield crystallization and metamorphic ages of ca. 2130 Ma and ca. 1880 Ma, respectively, suggesting that they are coeval, and were metamorphosed at the same time. Whole-rock elemental and isotopic compositions suggest tholeiitic affinity for the protoliths of both of them, with variable SiO2 and MgO concentrations and Mg# and εNd(t) values. They both display enriched mid-oceanic-ridge basalt (E-MORB)-like trace element patterns with enrichment in light rare earth elements and depletion in some high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). The data suggest derivation of the melts from a depleted asthenospheric mantle, coupled with fractional crystallization and crustal contamination. Some of the mafic rocks in the South Liaohe Group have lower HFSE (e.g., Nb, Ta, Zr, Hf, and Ti) concentrations, and La/Sm and Th/La ratios, and higher U/Th ratios than their counterparts in the North Liaohe Group, possibly suggesting enhanced metasomatism by subduction-related fluids and/or melts. The geochemical features of the mafic rocks from JLJB contrast with those of typical intra-continental rifts and volcanic arcs, and may have formed in a back-arc basin. Our study supports the presence of a ca. 2.2–2.1 Ga back-arc basin that formed due to northward subduction and subsequent closure of an oceanic plate, resulting in arc–continent collision and formation of the JLJB at ca. 1.9 Ga.

A widespread Paleoproterozoic partial melting event within the Jiao-Liao-Ji Belt, North China Craton: Zircon U-Pb dating of granitic leucosomes within pelitic granulites and its tectonic implications

In the southern zone of the Paleoproterozoic Jiao-Liao-Ji Belt (JLJB), North China Craton (NCC), abundant granitic leucosomes are widespread within migmatites as irregular layers, lenses, and blocks. They are parallel to or as cross-cut foliations in the host rocks on various scales, and show distinct evidence of migmatization in the formation of these centimeter- to decimeter-scale segregations. The migmatites extend at least 1100 km in length, spreading over the whole JLJB, from the Ji’an Group in south Jilin, through the South Liaohe Group in Liaodong Peninsula, to the Jingshan Group in Jiaodong Peninsula, rather than as sporadic outcrops limited to the Jiaobei Terrane as reported in previous studies. The granitic leucosomes are comprised mainly of quartz, K-feldspar, and antiperthitic plagioclase (albite), with secondary minerals of garnet, biotite, zircon, etc. They are enriched in SiO2 (74.65–75.54 wt%), Al2O3 (13.25–14.87 wt%), and K2O + Na2O (8.19–8.95 wt%), but have a lower TiO2 + Fe2O3 + FeO + MgO + MnO content (0.42–1.34 wt%). The major element components are similar to those of granitic leucosomes of pelitic granulites in typical orogenic belts around the world. A combined study of whole-rock geochemistry, mineral inclusions, cathodoluminescence (CL) images, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dates of zircons revealed the nature and timing of partial melting of the host pelitic granulites. All dated anatectic zircons from various granitic leucosomes display magmatic zones with distinct cores and rims, and contain mineral inclusions of Qtz + Kfs + Ab ± Ap in both domains. Both the cores and rims exhibit unusually high U and Th contents, and anomalously low Th/U ratios (<0.10), indicative of an anatectic origin. The anatectic zircon cores and rims from eight granitic leucosome samples in the Ji’an and Liaohe groups exhibit two distinct weighted mean age groups ranging from 1870 ± 8 to 1865 ± 9 Ma and 1843 ± 13 to 1835 ± 11 Ma, with age peaks at ca. 1868 ± 3 Ma and 1841 ± 4 Ma, respectively. Similarly, abundant anatectic zircon cores and rims from granitic leucosomes in the Jingshan Group from the Jiaobei Terrane also showed two distinct groups with age peaks at 1862 ± 2 Ma and 1843 ± 3 Ma, respectively. These two distinct age groups can be interpreted as the initial timing of partial melting and crystallization end age of melts for the migmatites in the whole JLJB. The southern zone of the JLJB experienced a complicated granulite facies metamorphic evolution characterized by clockwise P-T-t paths, rather than anticlockwise P-T-t paths as reported in previous studies. The post-peak MP-LP granulite facies retrogression with a near isothermal decompression P-T path occurred at 1870–1840 Ma, which is completely consistent with the ages of partial melting. These age data provide powerful evidence that the widespread partial melting event within the southern zone of the JLJB occurred at the post-peak near-isothermal decompression stage, rather than at the peak granulite facies stage or the late amphibolite facies cooling stage. Thus, decompression melting is a major control factor on the generation of partial melting, and formation of the granitic leucosomes and migmatites within the southern zone of the JLJB. Under water-unsaturated conditions, hydrous minerals such as biotite underwent dehydration melting, and water released from the biotite results in felsic minerals occurring partial melting. These lines of evidence suggest that the pelitic granulites in the JLJB underwent intensive partial melting during the MP-LP granulite facies “hot” exhumation stage related to the extension and thinning tectonic setting. The resultant melts did not escape from the host rocks, leading to regionally extensive migmatization and subsequent retrogression.

In situ zircon U-Pb dating and whole-rock geochemistry of metasedimentary rocks from South Liaohe Group, Jiao-Liao-Ji orogenic belt: Constraints on the depositional and metamorphic ages, and implications for tectonic setting

The Liaohe Group is an important Paleoproterozoic lithostratigraphic unit of the Jiao-Liao-Ji orogenic belt and traditionally subdivided into the North and South Liaohe Groups. However, their tectonic setting and evolution are still a matter of debate. The precise depositional and metamorphic data of the studied metasedimentary rocks from the South Liaohe Group provide important information on the controversial discussion of the tectonic setting and evolution. Based on cathodoluminescence (CL) imaging and connecting LA-ICP-MS U-Pb geochronology, zircon grains separated from metasedimentary rocks of different formations of the South Liaohe Group were subdivided into detrital and metamorphic zircons or zircon domains. Igneous zircon cores of detrital zircons of the Li’eryu, Gaojiayu, Dashiqiao and Gaixian Formations yield U-Pb age peaks at ∼2130 Ma, ∼2520 Ma, ∼2160 Ma/∼2500 Ma, and ∼2040 Ma, indicating that their sediments were transported from the Liaoji Granitoids, the Archean basement rocks of the adjacent Complex. In addition, deposition age of the sedimentary precursors was investigated which must have taken place at some time after ∼2.05 Ga, ∼2.07 Ga, ∼2.04 Ga and ∼1.92 Ga, respectively. These new geochronological data can be well explained by (1) a continent-arc collision model which suggests that the South Liaohe Group formed an intervening island arc and foreland basin sandwiched between the Longgang and Nangrim Complexes, and (2) a northward subduction of the Nangrim Complex source rocks of the South Liaohe Group; these source rocks were successively deposited along the margin of the two Complexes.

北辽河群浪子山组地层的重新厘定

古元古代北辽河群浪子山组变质沉积岩系主要分布在辽宁省鞍山市、海城市、辽阳市一带，其岩石组合以石英岩、片岩、千枚岩、变粒岩夹大理岩、透闪岩等为主，原岩恢复为砂岩、泥质砂岩、泥岩、泥灰岩等。前人将其置于北辽河群底部。通过对南北辽河群各组进行了详细的地质调查和分析对比工作，依据岩石组合特征和锆石年代学数据，笔者认为浪子山组应与南辽河群盖县组层位相当，故浪子山组不宜置于辽河群底部而应重新厘定为北辽河群顶部。The Paleoproterzoic clastic rocks are widely distributed in the southern portion of Liaoning prov-ince, grouped as the Langzishan Formation of the North Liaohe Group. The Langzishan Formation, previously defined as the bottom of the North Liaohe Group, mainly consists of sandstone, shaly sandstone, clay rock, siltstone interbedded with limestone, and argillaceous limestone. They were metamorphosed to be quartzite, schist, phyllite, marble, and tremolite rocks. Based on detailed field investigation, integrating with zircon U-Pb data, we proposed that the Langzishan Formation is equivalent to the Gaixian Formation (top of the South Liaohe Group). The Langzishan Formation should be grouped into the upper clastic sequence of the North Liaohe Group, but not the bottom as previous consideration.

Zircon U–Pb and Lu–Hf isotopic constraints on Archean crustal evolution in the Liaonan Complex of northeast China

The Changhai area is located in the southeastern part of the Liaonan Complex within the Eastern Block of the North China Craton (NCC) in China. Early Precambrian rocks in this area consist mainly of various types of gneisses and schists. We present cathodoluminescence images, U–Pb dates, and trace element and Hf isotope data for zircons from these rocks in order to constrain the formation and evolution of continental crust in the Liaonan Complex. Our results show that the magmatic precursors of the biotite monzonitic gneiss, granitic gneiss, garnet-bearing monzogranitic gneiss, and biotite granodioritic gneiss, that is, relatively sodium-rich gneisses in the area, were emplaced at ca. 2543Ma. Muscovite-bearing granitic gneiss, monzogranitic gneiss, and biotite granitic gneiss, that is, relatively potassium-rich gneisses in the area, were emplaced at ca. 2516Ma, whereas the sedimentary protolith(s) of the associated schists along the gneissose fabric of the gneisses were deposited after 2518Ma and metamorphosed at ca. 1885Ma. The latter age may represent the time of metamorphism of the South Liaohe Group in the Jiao–Liao–Ji Belt. Large numbers of inherited zircons with ages of 2596–2842Ma, coupled with the results of previous studies, suggest that multiple Meso- to Neoarchean magmatic events took place in the Liaonan Complex at ca. 2.84, 2.75, 2.67–2.61, 2.54, and 2.51Ga. Lu–Hf isotopic results for magmatic zircon grains with ages of 2.61–2.84Ga have positive εHf values (+2.4 to +11.3) and average continental crust model ages (TC DM) of 2.63–3.05Ga with a peak at 2.76–2.98Ga. Of the zircons with ages of ca. 2.54Ga and 2.51Ga, 94% have positive εHf values (+0.13 to +8.57, average=+3.1) and TC DM model ages of 2.53–3.06Ga, similar to those of zircons with ages of ca. 2.61–2.84Ga, suggesting that both sets of zircons were derived from partial melting of juvenile crust. However, the remaining zircons (6%) with ages of 2.54–2.51Ga and all metamorphic zircon grains with ages of ca. 1885Ma have negative εHf values of −0.12 to −3.00 and −8.8 to −15.9, respectively, and similar older TC DM model ages of 3.11–3.56Ga and 3.08–3.23Ga, respectively. These data, combined with those from previous studies of the NCC, indicate that the basement rocks of the Liaonan Complex underwent two significant juvenile crustal growth events at ca. 2.71–2.99Ga and 2.54–2.51Ga, a local reworking event at ca. 2.51Ga, and a major reworking event at 1.88Ga.

Zircon U–Pb and Lu–Hf isotopic and whole-rock geochemical constraints on the protolith and tectonic history of the Changhai metamorphic supracrustal sequence in the Jiao–Liao–Ji Belt, southeast Liaoning Province, northeast China

The Changhai metamorphic supracrustal rocks, located in the eastern–central part of the Jiao–Li–Ji Belt in the North China Craton (NCC), are composed mainly of various garnet–mica schists, along with minor quartzites and marbles. This study presents whole-rock major and trace element data, zircon U–Pb dates and Hf isotope data for these rocks in order to constrain their protolith age and provenance, and discuss the tectonic implications. Geochemical results indicate that the source rocks were mainly granitoids with a possible minor contribution from clastic sediments with an active continental margin signature. Detrital zircons have U–Pb age peaks at approximately 1887, 2174, 2552, 2765, and 3212Ma, ɛHf values of −11.1 to +13.0, and three major time windows of average continent crustal model ages (TDMC) of 2.04–2.33, 2.48–2.56, and 2.72–2.93Ga. Besides, these units also contain significant numbers of concordant metamorphic zircons that yielded a peak age of ca. 248Ma, indicating that the region was modified by an early Triassic tectono-thermal event. These results suggest that sediments of the Changhai metamorphic supracrustal rocks were mainly sourced from nearby basement granitoid rocks and, to a lesser extent, from Paleoproterozoic metamorphosed strata such as the North and South Liaohe groups. Furthermore, the source rocks of the magmatic zircons analyzed in this study appear to have originated from interaction between old continental crust and juvenile material. The youngest concordant zircon age peak at 1879Ma coincides with the timing of formation of regionally widespread granitoids, mafic igneous rocks, and metamorphism of the South Liaohe and Ji’an groups in the Jiao–Liao–Ji Belt, and suggest that the sedimentary protoliths of the Changhai metamorphic supracrustal rocks were deposited after this time. The results indicate that the Archean Liaobei–Jinan Complex in the north and the Liaonan–Nangrim Complex in the south were already a single continental block by 1887Ma, and that the Changhai metamorphic supracrustal rocks were deposited at an active continental margin.

PbSL dating of garnet and staurolite: Constraints on the Paleoproterozoic crustal evolution of the Eastern Block, North China Craton

It is well accepted that the North China Craton formed by assembly of the Eastern and Western blocks along the Trans-North China Orogen, with cratonization in the Paleoproterozoic (∼1.85 Ma). However, there is controversy about the formation of the Eastern and Western blocks themselves, especially the timing of cratonization of the Eastern Block. The Jiao-Liao-Ji orogenic belt is the most important Paleoproterozoic Orogenic belt in the Eastern Block and consists mainly of the North and South Liaohe Groups and the Liaoji Granitoids. The timing of metamorphism of the Liaohe Group is the key to understanding the evolution of the Eastern Block of the North China Craton. Because the isotope systematics of metamorphic minerals can be used to date parts of the prograde thermal histories of metamorphic belts, dating of minerals, such as garnet and staurolite has the distinct advantage that time information can be related directly to the pressure–temperature-deformation history recorded by the same mineral. In this paper we present results of precise 207Pb/ 206Pb stepwise-leaching (PbSL) dating of garnet and staurolite from the metasedimentary rocks of the south Liaohe Group. The primitive Pb and radiogenic Pb are selectively extracted from garnet and straurolite by leach steps and determined by multiple collector (MC-) ICP-MS. The garnet and staurolite record two groups of 207Pb/ 206Pb ages, at 1.93–1.91 Ga and ∼1.86 Ga, respectively. These constrain the time of peak metamorphism of the Liaohe Group during collision between the Longgang Block in the north and the Liaonan (Nangrim) Block in the south and a subsequent post-collisional event, respectively. Our data for the South Liaohe Group and previously published data for the Liaoji Granite, combined with key lithological, structural and metamorphic information, indicate that the cratonization of the Eastern Block of the North China Craton occurred at 1.93–1.91 Ga.

Timing of metamorphism in the Paleoproterozoic Jiao-Liao-Ji Belt: New SHRIMP U–Pb zircon dating of granulites, gneisses and marbles of the Jiaobei massif in the North China Craton

The Paleoproterozoic Jiao-Liao-Ji Belt lies in the Eastern Block of the North China Craton, with its southern segment extending across the Bohai Sea into the Jiaobei massif. High-pressure pelitic and mafic granulites have been recently recognized in the Paleoproterozoic Jingshan Group (Jiaobei massif). New SHRIMP U–Th–Pb geochronology combined with cathodoluminescence (CL) imaging of zircon has been applied to the determination of the timing of the metamorphism of the high-temperature and high-pressure granulites and associated gneisses and marbles. Metamorphic zircons in these high-pressure granulites, gneisses and marbles occur as either single grains or overgrowth (or recrystallization) rims surrounding and truncating oscillatory-zoned magmatic zircon cores. Metamorphic zircons are all characterized by nebulous zoning or being structureless, with high luminescence and relatively low Th/U values. Metamorphic zircons from two high-pressure mafic granulites yielded 207Pb/ 206Pb ages of 1956 ± 41 Ma and 1884 ± 24 Ma. One metamorphic zircon from a garnet–sillimanite gneiss also gave an apparent 207Pb/ 206Pb age of 1939 ± 15 Ma. These results are consistent with interval of ages of c. 1.93–1.90 Ga already obtained by previous studies for the North and South Liaohe Groups and the Laoling Group in the northern segment of the Jiao-Liao-Ji Belt. Metamorphic zircons from a high-pressure pelitic granulite and two pelitic gneisses yielded weighted mean 207Pb/ 206Pb ages of 1837 ± 8 Ma, 1821 ± 8 Ma and 1836 ± 8 Ma respectively. Two diopside–olivine–phlogopite marbles yielded weighted mean 207Pb/ 206Pb ages of 1817 ± 9 Ma and 1790 ± 6 Ma. These Paleoproterozoic metamorphic ages are largely in accordance with metamorphic ages of c. 1.85 Ga produced from the Ji'an Group in the northern segment of the Jiao-Liao-Ji Belt and c. 1.86–1.80 Ga obtained for the high-pressure pelitic granulites from the Jingshan Group in the southern segment. As this metamorphic event was coeval with the emplacement of A-type granites in the Jiao-Liao-Ji Belt and its adjacent areas, it is interpreted as having resulted from a post-orogenic or anorogenic extensional event.

A comparison of U–Pb and Hf isotopic compositions of detrital zircons from the North and South Liaohe Groups: Constraints on the evolution of the Jiao-Liao-Ji Belt, North China Craton

The Jiao-Liao-Ji orogenic belt is one of the important Paleoproterozoic mobile belts in the North China Craton (NCC) and its central segment consists of the North and South Liaohe Groups and the Liaoji Granitoids. The North and South Liaohe Groups consist of sedimentary and volcanic successions metamorphosed from greenschist to lower amphibolite facies, and the Liaoji granitoids are divisible into 2.2–2.1 Ga (pre-tectonic) monzogranitic gneisses and 1.88–1.85 Ga (post-tectonic) porphyritic monzogranites and alkaline syenites. Combined with cathodoluminescence (CL) imaging, we report LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) U–Pb and Hf isotopic data for detrital zircons from the North and South Liaohe Groups. Zircons grains from both groups have igneous zircon cores that yielded two U–Pb age populations at 2.0–2.2 Ga (major) and ∼2.5 Ga (minor), indicating that their provenances were similarly dominated by Paleoproterozoic Liaoji Granitoids and late Archean basement rocks. Some structureless zircon rims in both groups yield concordant or upper intercept ages ∼1.9 Ga, interpreted as the peak metamorphic age of the two groups. These results suggest that both groups were deposited in the period 2.0–1.9 Ga, shortly after the emplacement of the Liaoji Granitoids. Moreover, zircons from both groups have similar Hf model age peaks ( T DM C peaks at 2.66–2.75 Ga and 2.80–3.00 Ga) and ɛHf values (−5 to +9 and −7 to +5). Based on these striking similarities, we propose that the protoliths of the North and South Groups may have formed simultaneously, and that the basement rocks underneath the two groups belong to the same Archean continental block rather than two different blocks as previously considered. The Archean basement underwent Paleoproterozoic rifting, accompanied by the emplacement of the Liaoji granitoids at 2.2–2.1 Ga and the deposition of the North and South Liaohe Groups at 2.1–1.9 Ga, and the rift was subsequently closed by ∼1.9 Ga.

Are the South and North Liaohe Groups of North China Craton different exotic terranes? Nd isotope constraints

The Liaohe Group is an important Paleoproterozoic stratigraphic unit in the northeastern part of the North China Craton and is traditionally subdivided into the North and South Liaohe Groups. Associated with both the North and South Liaohe Groups are voluminous Paleoproterozoic granitoid rocks, named the Liaoji granitoids. Different tectonic models, including terrane amalgamation, continent–arc collision and rift closure, have been proposed to interpret the tectonic setting and evolution of the North and South Liaohe Groups and associated Liaoji granitoids. At the centre of the controversy between these models is whether or not the North and South Liaohe Groups developed on the same Archean basement. Nd isotopic geochemistry of the Liaoji granitoids provides important constraints on this controversial issue. The Liaoji granitoids associated with the North and South Liaohe Groups display similar ε Nd values, restricted to a narrow range from 0 to 2, implying that these granitoid rocks were derived from the same or a similar magma source. Moreover, the Liaoji granitoids associated with the North and South Liaohe Groups have similar Nd model ages ( T DM), ranging from 2.4 to 2.6 Ga, suggesting that the protoliths of the Liaoji granitoids associated with both groups may have formed simultaneously, and that the basement rocks underneath the Liaoji granitoids and associated North and South Liaohe Groups belong to the same continental block rather than two different blocks. Combining lithological, structural and geochronological considerations, we interpret the North and South Liaohe Groups as having developed on a single late Archean basement that underwent Paleoproterozoic rifting associated with the intrusion of the Liaoji granitoids and the formation of the Liaohe Group, and closed upon itself in the Paleoproterozoic.