Petrogenesis of the Early Paleoproterozoic Felsic Metavolcanic Rocks from the Liaodong Peninsula, NE China: Implications for the Tectonic Evolution of the Jiao-Liao-Ji Belt, North China Craton

: The early Paleoproterozoic (ca. 2.2–2.1 Ga) tectonic evolution of the Jiao–Liao–Ji belt (JLJB) is a continuous hot topic and remains highly controversial. Two main tectonic regimes have been proposed for the JLJB, namely arc-related se tt ing and intra-continental rift. Abundant ca. 2.2–2.1 Ga volcanic rocks were formed in the JLJB, especially in the Liaodong Peninsula. These ca. 2.2–2.1 Ga volcanic rocks therefore could host critical information for the evolution of the JLJB. In this study, we report a suit of ca. 2.2–2.1 Ga felsic metavolcanic rocks in the Liaodong Peninsula of the JLJB to provide new insights into the above issue. Zircon U-Pb dating reveals that the felsic metavolcanic rocks were erupted at 2185–2167 Ma. They have variable ε Hf(t) values ( − 0.70 to +9.69), high SiO 2 (66.30–75.30 wt.%) and relatively low TiO 2 (0.03–0.78 wt.%), tFe 2 O 3 (0.55–5.03 wt.%), MgO (0.17–8.76 wt.%), Cr (9.16–67.30 ppm), Co (2.01–7.00 ppm) and Ni (3.90–25.70 ppm) contents with enrichments in light rare earth element (REE) and large ion lithophile element (LILE), and depletions in heavy REE and high ﬁ eld strength element (HFSE). Geochemical and isotopic results indicate that the felsic metavolcanic rocks were sourced from partial melting of ancient Archean TTG rocks and juvenile lower crustal materials. Combined with coeval A-type granites, bimodal volcanic rocks and the absence of typical arc magmatism, the most likely tectonic regime at ca. 2.2–2.1 Ga for the JLJB is an intra-continental rift.

Most recently, we implemented detailed geological investigations and identified a suit of felsic metavolcanic rocks in the Houxianyu area (Figure 2), where there is one of the best exposures in the northern Liaodong Peninsula, NE China.In this contribution, new petrology, zircon U-Pb geochronology, bull-rock major and trace elements and zircon Lu-Hf isotopic compositions are reported for the newly identified felsic metavolcanic rocks in the Liaodong Peninsula, with the aim of deciphering their timing of formation, petrogenesis, and tectonic setting, and thus further providing new constrains for the tectonic evolution of the JLJB during the Paleoproterozoic.
During the Late Paleoproterozoic, the JLJB underwent widespread and intense highgrade metamorphism [15][16][17]26,29,62].More and more mafic and pelitic granulites, characterized by clockwise P-T paths, have been identified in the Liaodong and Jiaodong peninsulas [15][16][17]20,26,29,62,63].Such high-grade metamorphism is interpreted as the result of the ca.1.90 Ga collision event between the Nangrim and Longgang blocks, which marks the final assembly of the EB in the eastern NCC [3].After the Late Paleoproterozoic orogenic event, the crystalline basement of the eastern NCC was overlain by Meso-to Neoproterozoic sedimentary rocks and experienced intense magmatism and structural deformation during the Mesozoic [64].

Petrography
A suit of felsic metavolcanic rocks of this study were newly identified in the Houxianyu area of the northern Liaodong Peninsula (Figure 2).These felsic rocks belong to metamorphosed volcano-sedimentary rock units of the Li'eryu Formation, Liaohe Group.The Paleoproterozoic and Mesozoic granitoids are in tectonic or intrusive contacts with these volcanic sequences.The felsic metavolcanic rocks of this study mainly consist of fine-grained meta-dacite and fine-grained meta-rhyolite, with layered or massive structures (Figure 3).The sample lithologies and locations and the methods applied to each rock sample are summarized in Table 1.The fine-grained meta-dacites are grey to white in color and occur as deformed layers (Figure 3a).They generally display weakly gneissic structures and fine-grained crystalloblastic textures, with a typical mineral assemblage of plagioclase (~37 vol.%), quartz (~35 vol.%), biotite (~18 vol.%),K-feldspar (~5 vol.%) and minor accessory minerals (~5 vol.%; e.g., tourmaline, zircon, apatite and some opaque minerals) (Figure 3b,c).The majority of these minerals are subhedral to anhedral, with grain sizes of 200-600 µm (Figure 3c).

Zircon U-Pb Dating
Three representative felsic metavolcanic rock samples were collected for the zircon U-Th-Pb isotope and trace element analyses (Table 1).Zircon grains were separated from crushed samples using combined techniques of magnetic separation and standard density methods.Typical and clear zircon grains without cracks were handpicked through the binocular microscope, and then fixed on an epoxy disk.After being ground and polished, the zircon grains were imaged by cathodoluminescence (CL) and reflected/transmitted light.Zircon U-Th-Pb isotopic compositions, as well as trace element concentrations, were measured using the instruments of laser ablation (LA) inductively coupled plasma (ICP)mass spectrometry (MS) at the Yanduzhongshi Geological Analysis Laboratories, Beijing, China.A total of 100 spots on zircon grains were analyzed, with a beam size of 30 µm in diameter.The data acquisition durations for each analysis spot included 20-30 s blank background acquisition and a sample measuring time of 50 s.Standard zircon 91,500 and NIST610 were utilized as external standards for the U-Th-Pb isotopes and trace element compositions of dated zircon grains.Quantitative calibrations for acquired original isotopic and trace element data were carried out using the ICPMSDataCal software [65].Sample weighted mean or intercept age calculations and Concordia diagrams were performed through the Isoplot program [66].

Bulk-Rock Geochemistry
A total of seven felsic metavolcanic rock samples were selected for bulk-rock major and trace element analyses (Table 1).Prior to geochemical analysis, fresh rock samples were crushed and powdered to < 200 mesh in a completely cleaned agate mill.Bulk-rock major element concentrations were measured using a Leeman Prodigy ICP-optical emission spectroscopy (OES) system at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, China.Loss on ignition (LOI) values were determined through placing 1 g of bulk-rock powders in the muffle furnace for two hours at a temperature condition of 1000 ℃, and then cooling and reweighing residual samples.The acquired results yielded analytical uncertainties better than 2% for most of the major elements.Bulk-rock trace and rare earth element concentration analyses were performed through an Agilent 7500a quadrupole ICP-MS instrument system.The acquired results yielded analytical accuracies generally better than 5% for most of the trace elements.

Zircon Lu-Hf Isotopic Analyses
The above three felsic metavolcanic rock samples, which had been dated by LA-ICP-MS, were used for in situ zircon Hf isotopic composition analyses at the Yanduzhongshi Geological Analysis Laboratories, Beijing, China.All analyses were conducted using the LA-MC-ICP-MS instrument system (a Neptune multi-collector (MC)-ICP-MS instrument equipped with a NewWave UP213 LA system).A total of 45 representative zircon Lu-Hf test spots were analyzed, similar or close to U-Pb dating domains.The beam size for each spot was ~40 µm in diameter.The test conditions included 16 J/cm 2 laser energy density and an 8 Hz laser repetition rate.More specific and detailed analytical procedures, instrument conditions and experimental processes were described by Wu et al. (2006) [67].

Zircon U-Pb Geochronology
Zircon U-Pb isotopic data, CL images and Concordia diagrams for three felsic metavolcanic rock samples (18YK05-1, 18YK05-6 and 18YK06-1) are presented in Supplementary Table S1, Figure 4 and Figure 5, respectively.As shown in the CL images (Figure 4a), zircon crystals from Sample 18YK05-1 are generally subhedral to anhedral and grey to white in color, with grain lengths of approximately 50-100 µm and length/width ratios of 1.2-2.5.Most of the zircon grains exhibit clear and well-developed oscillatory zoning, or characteristic core-rim structures (Figure 4a).Combined with their relatively high Th/U ratios of 0.47-1.18(Supplementary Table S1), these zircon grains and cores are similar to those of felsic igneous rocks, indicating a magmatic origin.Nineteen U-Pb analysis spots on the above zircon domains yield apparent 207 Pb/ 206 Pb dates between 2188 ± 17 Ma and 2163 ± 24 Ma, with a weighted mean 207 Pb/ 206 Pb age of 2174 ± 10 Ma (n = 19, MSWD = 0.12), and all of them have concordances better than 96% and fall on or near the Concordia line (Figure 5a).Therefore, the age of 2174 ± 10 Ma represents the magma eruption age of Sample 18YK05-1.Six analyses on zircon core domains gave old apparent 207 Pb/ 206 Pb dates of 2529-2479 Ma, suggesting that these zircon cores could be inherited zircons derived from the Neoarchean basement rocks of the Longgang Block (Supplementary Table S1 and Figure 5a).In addition, two analyses on zircon grains or rims without obvious internal structures gave 207 Pb/ 206 Pb dates 1901 ± 30 Ma and 1898 ± 30 Ma, consistent with the Late Paleoproterozoic regional metamorphic event in the JLJB.
Zircon grains from Sample 18YK05-6 are euhedral to subhedral crystals and black to grey in color in the CL images (Figure 4b), with grain lengths ranging from 75 to 150 µm and aspect ratios of 1.1-2.0.The majority of zircon crystals have well-developed oscillatory growth zoning, and some of them display clear core-rim textures (Figure 4b).In addition, they show relatively high and variable Th/U ratios varying from 0.13 to 0.85 (Supplementary Table S1).All of these features indicate that these zircon grains or cores have a magmatic origin.Twenty-one U-Pb analysis spots with concordances better than 95% yield apparent 207 Pb/ 206 Pb dates between 2196 ± 21 Ma and 2163 ± 14 Ma (Supplementary Table S1).They define a discordant line and give an upper intercept age of 2167 ± 13 Ma (n = 21, MSWD = 0.19), which is interpreted as the magma eruption age of Sample 18YK05-6 (Figure 5b).
Zircon crystals from Sample 18YK06-1 are irregularly shaped, grey to white in color and subhedral to anhedral in CL images (Figure 4c), with grain lengths of approximately 70-100 µm and length/width ratios ranging from 1.0 to 2.0.Most of them exhibit obvious core-rim structures with well-developed oscillatory zoned cores surrounded by structureless and blurred rims (Figure 4c).In addition, some zircon grains with no obvious corerim structures are completely structureless or show clear oscillatory growth zoning throughout them.Thirty-five U-Pb analyses on zircon cores or domains with distinct oscillatory growth zoning, with high Th/U ratios of 0.30-1.08(mostly > 0.5), yield apparent 207 Pb/ 206 Pb dates between 2174 ± 12 Ma and 2065 ± 18 Ma (Supplementary Table S1).These analyses define a discordant line and give an upper intercept age of 2185 ± 14 Ma (n = 35, MSWD = 0.70), representing the magma eruption age of Sample 18YK06-1 (Figure 5c,d).Eight analyses yield old apparent 207 Pb/ 206 Pb dates of 3361-2288 Ma (Supplementary Table S1 and Figure 5c), which indicates that these zircon grains could be inherited in origin and derived from ancient surrounding rocks (e.g., Archean TTG rocks).In addition, seven analyses on zircon rims or domains without distinct internal structures have relatively low Th/U ratios of 0.06-0.14(Supplementary Table S1).They yield young apparent 207 Pb/ 206 Pb dates between 1956 ± 21 Ma and 1867 ± 14 Ma, with an upper intercept age of 1956 ± 66 Ma (n = 7, MSWD = 3.2) (Supplementary Table S1 and Figure 5c,d), which represents the metamorphic age of Sample 18YK06-1.
In summary, these felsic metavolcanic rocks were formed during the early Paleoproterozoic (2185-2167 Ma) and underwent the late Paleoproterozoic regional metamorphic event (1956-1898 Ma).

Whole-Rock Geochemistry
The whole-rock major and trace element data and calculated parameters for the seven felsic metavolcanic rock samples are presented in Table 2.These samples are acid rocks with high SiO2 (66.30-75.30wt.%; average 69.51 wt.%) concentrations.They also contain relatively low TiO2 (0.03-0.78 wt.%; average 0.45 wt.%), total Fe2O3 (tFe2O3; 0.55-5.03wt.%; average 2.85 wt.%) and MnO (0.01-0.04 wt.%; average 0.02 wt.%) contents and show a large range of Al2O3 (10.57-17.23 wt.%), Na2O (0.99-9.01 wt.%) and K2O (0.04-7.15 wt.%).As shown in the protolith discrimination diagrams (Figure 6a,b) these felsic rock samples exhibit low TiO2, Ni contents and Zr/TiO2 ratios and mainly fall into the field of volcanic rocks rather than sedimentary rocks.In addition, most zircon grains from these samples are subhedral with the characteristics of magmatic oscillatory growth zonings, differing from detrital zircon grains from sedimentary rocks with relatively rounded shapes (Figure 4).Previous field and petrological studies also identified a suit of metavolcanic rocks (meta-rhyolites and meta-dacites) in the Li'eryu Formation, Liaohe Group [21,60,68,69].In the geochemical classification diagrams (Figure 6c,d), these samples also mainly plot in the fields of rhyolite and dacite.Therefore, the felsic rocks in this study are a suit of metavolcanic rocks and mainly comprise meta-rhyolites and meta-dacites.2 and Figure 7a).In the spider diagram of primitive mantle-(PM-) normalized trace elements (Figure 7b), most felsic metavolcanic rock samples are relatively enriched in large ion lithophile elements (LILEs; e.g., Rb, Ba and K), Th and U, and show negative anomalies of high field strength elements (HFSEs; e.g., Nb, Ta, Ti and P) and Sr.

Petrogenesis
The early Paleoproterozoic felsic metavolcanic rocks in this study underwent lowgrade metamorphism as a result of a late Paleoproterozoic tectothermal event.Therefore, it is necessary to evaluate the metamorphism effects on their bulk-rock major and trace element data.All of the samples contain lower loss on ignition (LOI) contents (0.36-2.87 wt.%; average 1.27 wt.%) (Table 2), suggesting that the effects of low-grade metamorphism on geochemical compositions could be negligible.Such an explanation is also supported by the lack of obvious Ce anomalies (δCe = 1.04-1.13;average 1.08) in all samples (Table 2 and Figure 7a).Therefore, the geochemical data of felsic metavolcanic rocks in this study can be used to effectively discuss and trace their petrogenesis and tectonic implications.
The felsic metavolcanic rock samples show relatively variable tREE contents (50.97-181.74ppm), (La/Yb)N ratios (2.70-20.86),and negative Eu anomalies (δEu = 0.49-0.67)(Table 2), which implies that they could experience a fractional crystallization process.The negative correlation between SiO2 and Al2O3 suggests the felsic metavolcanic rocks experienced the plagioclase fractionation (Figure 10a), which is also consistent with their negative Eu anomalies in the REE pattern diagram (Figure 7a).These samples exhibit a negative evolutionary trend between Er and Dy (Figure 10b), which suggests the hornblende fractionation [90].Such an explanation is supported by the decrease in V contents with decreasing Cr concentrations [91] (Figure 10c).However, the lack of an obvious evolutionary trend between Th and V implies that the biotite fractionation process is negligible [90] (Figure 10d).In addition, their relatively variable tFe2O3 (0.55-5.03 wt.%) and TiO2 (0.03-0.78 wt.%) abundances are indicative of Fe-Ti oxide (e.g., magnetite and titanite) accumulation.Therefore, the felsic metavolcanic rocks in this study experienced a certain degree of fractional crystallization.The felsic metavolcanic rocks lack the mafic enclaves, which indicates that these samples may not have been contaminated by mantle-derived materials.Such an interpretation is also consistent with their low TiO2 (0.03-0.78 wt.%), tFe2O3 (0.55-5.03 wt.%), Cr (9.16-67.30ppm), Co (2.01-7.00ppm), and Ni (3.90-25.70ppm) (Table 2).The petrogenesis of felsic magmatism generally includes two main mechanisms: (1) fractional crystallization of mantle-derived magmas [91][92][93][94][95] and (2) partial melting of crustal materials [60,68,[96][97][98][99].As discussed above, although the primary magma of felsic metavolcanic rocks in this study inevitably experienced a certain degree of fractional crystallization during the later stage of magma eruption to the surface, their petrogenesis mechanism could not be the fractional crystallization of mantle-derived magmas.Firstly, the differentiation process of basaltic magmas will produce a complete set of magmatic evolutionary sequences, including basaltic, andesitic and rhyolitic magmatisms.However, the early Paleoproterozoic meta-volcanic sequences in the JLJB are characterized by bimodal volcanic rocks [21,40,77,78].Secondly, multiple positive correlations between La contents and La/Sm, La/Yb, La/Hf ratios strongly suggest that the petrogenesis of the felsic metavolcanic rocks is mainly controlled by the partial melting process rather than fractional crystallization (Figure 11a-c), which is also supported by the positive correlation between Ce contents and Ce/Zr ratios (Figure 11d).These rock samples contain relatively high SiO2 (66.30-75.30wt.%) and low tFe2O3 (0.55-5.03 wt.%), MgO (0.17  2), suggesting that they were mainly derived from partial melting of crustal rocks rather than mantle peridotite.
Thirdly, felsic magmatism generated by the fractional crystallization of mantle-derived magmas generally exhibits depleted and homogeneous zircon Hf compositions.Nevertheless, the zircon samples from the felsic metavolcanic rocks in the Liaodong Peninsula of the JLJB exhibit a large range of εHf(t) values, and some samples even fall below the CHUR line and show enriched zircon Hf compositions (Figure 8).Therefore, the fractional crystallization mechanism may not play a major role in geochemical compositions.In summary, the petrogenesis of the felsic metavolcanic rocks in this study are most likely the mechanism (2): partial melting of pre-existing crustal materials.Zircon CL images and U-Pb geochronological results reveal that the felsic metavolcanic rocks of this study contain many inherited magmatic zircon grains with ancient apparent 207 Pb/ 206 Pb ages ranging from 3361 ± 8 Ma to 2288 ± 12 Ma (concentrated at the Neoarchean) (Supplementary Table S1 and Figure 4a,c).Therefore, the Neoarchean TTG rocks, accounting for the majority of continental crust rocks in the Archean Longgang and Nangrim blocks [52,53,57], are potential protoliths for the felsic metavolcanic rocks.In this study, we collected zircon Hf isotopic data from reported early Paleoproterozoic felsic metavolcanic rocks in the Liaodong Peninsula and Neoarchean TTG rocks in the Longgang and Nangrim blocks.Zircon grains from the early Paleoproterozoic felsic metavolcanic rocks exhibit heterogeneous zircon Hf isotopic compositions with a broad range of εHf(t) values (approximately −4 to +9) (Figure 8).They can be divided into two groups based on their different εHf(t) values and TDM2 ages.Zircons of the Group-Ⅰ show similar ancient TDM2 ages of ca.3.0-2.5Ga and low εHf(t) values (about −4 to +4), which indicates that the felsic metavolcanic rocks could be derived from partial melting of ancient Archean TTG rocks.Such an explanation is also supported by their TDM2 ages and evolutionary trends being similar to those of Neoarchean TTG rocks (Figure 8).Differently, zircons of the Group-Ⅱ are close to the DM evolutionary line with higher εHf(t) values (about +4 to +9) and younger TDM2 ages (ca.2.5-2.2Ga), which suggests that juvenile mafic materials in the lower crust could be another source for the felsic metavolcanic rocks (Figure 8).In summary, the early Paleoproterozoic felsic metavolcanic rocks in the Liaodong Peninsula of the JLJB were most likely derived from partial melting of Archean TTG rocks and juvenile crustal materials.

Tectonic Implications
The early Paleoproterozoic tectonic evolution of the JLJB has long been controversial [13,14,24,32,33].Therefore, contemporaneous magmatic rocks can provide valuable insights into this issue.There is a view that ca.2.2-2.1 Ga metavolcanic sequences, mafic and granitic intrusions with arc-type geochemical characteristics (enrichment in LILE and depletion in HFSE) were considered to be associated with magmatic arc-related environments [27,33,38,46,60,61,68].However, many studies revealed that the JLJB went through an intra-continental rift system during the early Paleoproterozoic.Firstly, the Longgang and Nangrim blocks on both sides of the JLJB have similar Archean basement rocks geochemically and geochronologically [3,40], which indicates that the two blocks previously belonged to a unified continent.Secondly, the existence of regional large-scale mafic dykes generally represents a crustal extension tectonic regime.In the whole JLJB, abundant ca.2.2-2.1 Ga mafic intrusions were identified in southern Jilin Province and on the Liaodong Peninsula [31,38,44,46,79].Thirdly, the metavolcanic sequences in the JLJB are mainly composed of bimodal volcanic rocks [21,40,77,78], which indicates that the JLJB experienced an intra-continental rift rather than magmatic arc during the early Paleoproterozoic.Fourthly, a large number of Liaoji granites with an affinity of A-type granite also suggest an intra-continental rift setting [21,22,43,45,80].Finally, the absence of coeval typical arc magmatism in the JLJB could be in disagreement with an arc-related setting [3,18,21,22].Therefore, these lines of evidence indicate that the JLJB most likely went through an intracontinental rift during the early Paleoproterozoic.In this study, the ca.2185-2167 Ma felsic metavolcanic rocks are closely related to A-type granite and metavolcanic rocks in the spatial and temporal distribution.It can be inferred that they were also generated in an intra-continental rift setting.
Liu et al. (2020) [22] identified a suit of synchronous A-type and adakitic granites, which were formed in an intra-continental rift trigged by lithospheric delamination at ca. 2.2 Ga.Generally, the lithospheric delamination could result in the decompression melting and upwelling of asthenosphere mantle and generate the mafic volcanic and intrusive rocks in the JLJB.Meanwhile, the upwelling of hyperthermal mantle-derived magmas will further induce the appearance of initial rift and partial melting of crustal rocks and produce the numerous A-type Liaoji granites and felsic volcanic rocks of this study.Such a delamination-rift model can effectively explain the special early Paleoproterozoic lithological assemblages in the JLJB.
A further extension of continental rift would have resulted in an initial ocean basin and separated unified Eastern Block into two continental blocks (i.e., the Longgang and Nangrim blocks).A subsequent regional high-grade tectothermal event at ca. 1.95-1.85Ga resulted in the collision between the Longgang and Nangrim blocks [15][16][17]26,29,63].Correspondingly, the felsic metavolcanic rocks in this study also recorded metamorphic ages of 1956-1898 Ma (Figure 5).Notably, increasing studies from metamorphic geology, especially the discovery of a high-pressure mafic/pelitic granulite with a clockwise P-T-t path, strongly revealed that the JLJB experienced oceanic subduction before the final collision [16,17,26,63].Therefore, the most likely tectonic scenario is that the JLJB went through an intra-continental rift system during the early Paleoproterozoic (ca.2.2-2.1 Ga), followed by a transition phase from rift to oceanic subduction, and later a continent-continent collision at ca. 1.95-1.85Ga [3,21,22,31].

Figure 2 .
Figure 2. Simplified geological map of the Houxianyu area showing the sampling location.

Figure 8 .
Figure 8.The εHf(t) versus age diagram of the felsic metavolcanic rocks.Reported Hf isotopic data of the felsic metavolcanic rocks are from[60,68].Reported Hf isotopic data of the TTG rocks in the Longgang Block are from[52,53,75,76].Reported Hf isotopic data of the TTG rocks in the Nangrim Block are from[57].

Table 1 .
The simplified list of the felsic metavolcanic rock samples in this study.

Table 2 .
Bulk-rock major (wt.%) and trace (ppm) element data of the felsic metavolcanic rocks in the Liaodong Peninsula of the Jiao-Liao-Ji Belt.

Table 3 .
Zircon Hf isotopic data of the felsic metavolcanic rocks in the Liaodong Peninsula of the Jiao-Liao-Ji Belt.The present-day 176 Hf/ 177 Hf and176Lu/ 177 Hf ratios are 0.282772 and 0.0332 for the chondrite, and 0.28325 and 0.0384 for depleted mantle.The 176 Lu/ 177 Hf ratio of the average continental crust is 0.015.The decay constant of176Lu is 6.54 × 10 −12 a −1 .