2.8 – 1.7 Ga history of the Jiao-Liao-Ji Belt of the North China Craton from the geochronology and geochemistry of ma ﬁ c Liaohe meta-igneous rocks

The assembly and long-term evolution of the Eastern Block of the North China Craton are poorly constrained. Here we use bulk rock geochronological and geochemical data from ma ﬁ c meta-igneous rocks (hornblendites, amphibolites and a metagabbro) of the Liaohe Group to reconstruct the Neoarchean to Paleoproterozoic history oftheJiao-Liao-JiBelt,locatedbetweentheLonggangandNangrimblocksthattogetherformtheEasternBlockof the North China Craton. The ma ﬁ c/ultrama ﬁ c meta-igneous rocks have intrusive or tectonic contacts with the Liaoji graniticrocks(~2.2 – 2.0 Ga), whichformthebasement of the Jiao-Liao-Ji Belt. The major and trace element data indicate that the protoliths had calc-alkaline composition and formed along an active continental margin subductionzone.Thema ﬁ crocksformawhole-rock 176 Lu/ 177 Hfisochronwithanageof2.25±0.31Ga,overlap-ping with U \\ Pb zircon ages for ma ﬁ c and granitic rocks from the Jiao-Liao-Ji Belt and consistent with being the emplacement age of the ma ﬁ c protoliths along the active continental margin. In contrast, the whole-rock 147 Sm/ 144 Nd isochron age of 2.83 ± 0.18 Ga is likely to re ﬂ ect the average age of the lithospheric mantle source from which the ma ﬁ c/ultrama ﬁ c protoliths were extracted. Together with geological evidence, we propose that the southwestern portion of the Longgang Block was an active continental margin since at least the early Paleoproteorozic. Literature age data from metamorphic zircons show that peak granulite metamorphism took place at ~1.96 – 1.88 Ga, resulting from the collisional event that fused the Longgang and Nangrim blocks into the Eastern Block of the North China Craton. Our bulk-rock 207 Pb/ 206 Pb age of 1824 ± 19 Ma and our 87 Rb/ 86 Sr age of 1671 ± 58 Ma re ﬂ ect retrograde (cooling) stages during the exhumation of the Jiao-Liao-Ji Belt after the orogenesis. ©2020TheAuthor(s).PublishedbyElsevierB.V.onbehalfofInternationalAssociationforGondwanaResearch.This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Here we present new whole rock geochemical and isotopic data from the mafic/ultramafic metamorphic rocks (amphibolites, hornblendites, anthophyllite-rich rock and metagabbro) in the North Liaohe Group (and one sample from the South Liaohe Group) of the JLJB on the Liaodong Peninsula (Figs. 1B and 2). Whole rock analysis potentially allow access to portions of the geologic record that are not accessible through zircons alone, since they effectively sample a larger chemical equilibration volume and are potentially robust on the centimeter to meter scale despite high grade metamorphism and metasomatism. Further, integrated whole rock analysis of mafic rocks across a terrane potentially allows insights into mantle (lithospheric and asthenospheric) processes operating on the regional scale. Application of whole rock data in the context of existing zircon geochronology therefore offers a powerful tool for resolving the time-integrated thermal evolution of the terrane and construction of geodynamic models.
Hence, through integration of previously published results from the entire JLJB with our new whole rock geochronological and geochemical data from mafic/ultramafic meta-igneous rocks, we present a new model for the geodynamic evolution of the JLJB from~2.8-1.7 Ga.

Regional geology
The JLJB is located in the northeastern part of the Eastern Block of the North China Craton (Fig. 1A). The northeast-southwest-trending belt is 50-300 km wide and extends for~1200 km from the eastern Jilin, via the eastern Liaoning, to the eastern Shandong province. Its central segment is situated between the Archaean Northern Liaoning-Northern Jilin Complex (Longgang Block) and the Archaean Southern Liaonan-Nangrim Complex (Nangrim Block). Its southern segment stretches across the Bohai Strait into the Archaean Eastern Shandong Complex on the Jiaodong Peninsula. The geology of the JLJB is summarized by , Li et al., 2011aLi et al., 1997a;Li et al., 1997b, Li and, Liu and Li (1996), Tam et al. (2011Tam et al. ( , 2012a and Xu and Liu (2019). The JLJB consists of deformed and metamorphosed (up to high pressure granulite facies; Zhou et al., 2004Zhou et al., , 2008a volcanic and sedimentary sequences, granitoid and gabbroic intrusives, and mafic dikes (Fig. 1B). The volcanic and sedimentary sequences are  Zhao et al., 2005) of the Eastern and Western blocks separated by the Trans-North China Orogen. The box shows the location of B) blow-up map (after Li et al., 2005aLi et al., , 2005b showing the Paleoproterozoic Jiao-Liao-Ji Belt (JLJB), which divides the Eastern Block into two microcontinental blocks: the Longgang (NW) and Nangrim (SE) blocks. Red box shows the location of Fig. 2. Blue circle shows South Liaohe sample location. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) referred to (going from north to south) as the Laoling and Ji'an groups in southern Jilin (Lu et al., 2004a(Lu et al., , b, 2005 and possibly the Macheonayeong Group in North Korea, the South and North Liaohe groups in eastern Liaoning, the Fenzishan and Jingshan groups, part of the Jiaobai Terrane in eastern Shandong on the Jiaodong Peninsula (Zhou et al., 2004(Zhou et al., , 2008a. The stratigrahic succession is transitional from a clastic-rich and bimodal volcanic sequence at the base through a middle carbonate-rich sequence to an upper pelitic sequence Li et al., 2005a). The JLJB can be subdivided into a northern sub-belt, including the Laoling, North Liaohe and Fenzishan groups, and a southern sub-belt, including the Ji'an, South Liaohe and Jingshan groups. Faults and ductile shear zones separate these two sub-belts (Li et al., 2005a). The Liaohe Group, in the central portion of the belt, has been subdivided (going from bottom to top) into the Langzishan, Li'eryu, Gaojiayu, Dashiqiao and Gaixian formations, with the lowermost Langzishan Formation only being found in the North Liaohe Group (e.g., Li et al., 2005a;Dong et al., 2019). The Ji'an Group from bottom to top is subdivided into the Mayihe, Huangchagou and Dadongcha formations. The Laoling Group can be divided from bottom to top into the Dataishan (Linjiagou), Zhenzhumen, Huashan, Linjiang and Dasuzi formations. In the south, the Jingshan Group is divided from bottom to top into the Lugezhuang, Yetou and Douya formaitons, while the Fenzishan Group is divided from bottom to top into the Xiaosong, Zhujiakuang, Zhanggezhuang, Jutun and Gangyu formations (Liu et al., 2015).
The Eastern Block of the North China Craton has undergone a complex evolution since the Archean. Its earliest crustal formation events are recorded in the Anshan Complex, extending back to~3.8 Ga (Liu et al., 1992;Song et al. 1996;Wan et al., 2012). Trondjemite-tonalitegranodiorite (TTG) gneisses provide evidence for a major phase of crustal growth at~2.7 Ga, followed by alkaline granite emplacement along the northern margin of the JLJB at~2.5 Ga. All of these units underwent dome-like deformation at~2.5 Ga, which was distinct from the post-Archean deformation (Jahn et al., 2008;Zhao et al., 2005). The~2.2-1.9 Ga NNE-trending Liaohe Group unconformably overlies the Neoarchean oval-shaped domes (Li et al., 2005a(Li et al., , 2005bLuo et al., 2004Luo et al., , 2008.

Description of rock types
Ten mafic (amphibolites, hornblendites and a metagabbro) and one ultramafic (anthophyllite-rich rock) meta-igneous rocks from the Liaohe Group were selected for geochemical analyses in this study (see Table 1 and Figs. 1, 2). They form layers, blocks or lenses in the Liaohe Group, which have intrusive or tectonic contacts with the Liaoji Granitoids (Li et al., 2005a). All but one sample are from the North Liaohe Group (Fig. 2) and the remaining sample (LH05-26-1) from the South Liaohe Group (Fig. 1B).

Analytical methods (major and trace element and isotope ratios)
Major and trace element data are shown in Table 1, Sr-Nd-Hf isotope data in Table 2, and Pb isotope data in Table 3. Major elements and selected trace elements (V, Zn, Ba, Sr and Zr) were determined on a Phillips X'Unique PW1480 X-ray fluorescence spectrometer (XRF) at GEOMAR using fused beads. H 2 O and CO 2 were determined by infrared photometry on a Rosemount CSA 5003. Values determined on reference samples JB-2, JB-3, JA-2 and JR-1, measured along with the samples, lie within 5% of Jochum et al. (2016) for JB-2 and JA-2 and Govindaraju (1994) for JB-3 and JR-1 for the major elements, except MnO in JB-2, and at several tenth weight percent of TiO 2 , MgO and CaO in JR-1. V, Zn, Ba, Sr and Zr generally deviate by b10% from reference values except V in JA-2. For details on XRF standard materials see Appendix A2.
Solution ICPMS analyses for trace elements were carried out on an Agilent 7500cs instrument at the Institute of Geosciences at Kiel University following the methods of Garbe-Schönberg (1993). Initial sample digestion was done in Parr© pressure digestion vessels in an oven at 120°C for 4 days. Reference material BHVO-2, BIR-1 and JGb-2 (two dissolutions) were prepared and measured along with the samples. See Appendix 3a for ICP-MS standard analyses. JGb-2 replicates within 1.7 ± 1.5% (1SD) except Zn (7.8%), Zr (21.9%) and Mo (35.2%). Sample 34-1 was replicated at 2.1 ± 1.6% (1SD) except Sc (12.7%), V (9.4%) Sr-Nd-Pb isotope analyses were undertaken at GEOMAR on TRITON (Sr\ \Nd) and MAT262 (Pb) thermal ionization mass spectrometers. Circa 100 mg of unleached sample powders were used for the isotope analyses. Chemical separation of Sr, Nd and Pb followed the chromatographic principles outlined in Jacques et al. (2013) and references therein. Isotope ratios were determined in static multi-collection mode with Sr and Nd being mass bias corrected to 86 Sr/ 88 Sr = 0.1194 and 146 Nd/ 144 Nd = 0.7219 for each integration, while Pb was externally corrected by applying 0.113‰ / amu to the measured Pb isotope ratios. The fractionation factor is based on repeat measurements of NBS981 and normalization to the values of Todt et al. (1996). The external 2SD errors of NBS981 (n = 195) are~200 ppm / amu and translate to 2SD errors of 0.0445%, 0.0607% and 0.0797% for 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb respectively. 87 Sr/ 86 Sr and 143 Nd/ 144 Nd sample ratios are reported relative to 87 Sr/ 86 Sr = 0.710250 ± 0.000006 (n = 5; 2SD) for NBS987 and 143 Nd/ 144 Nd = 0.511850 ± 0.000006 (n = 10; 2SD) for La Jolla. Hf chemistry followed the methods of Blichert-Toft et al., 1997, and 176 Hf/ 177 Hf was determined on a Nu-Plasma MC-ICPMS at GEOMAR. The long-term (2011-2018) standard bracketing normalized value of the in-house Hf SPEX CertiPrep ™ solution is 176 Hf/ 177 Hf = 0.282170 ± 0.000006 (2SD, n = 553) corresponding to 176 Hf/ 177 Hf = 0.282163 for JMC475. Sr-Nd-Hf-Pb replicate analysis of LH05-040-1 lie within 2SD of the standards (Tables 2 and 3). Age calculations were conducted using a root sum of squares approach to propagate the external 2SD of the isotope standards and a conservative 5% 2 sigma error from parent/daughter ratios. For regressions that returned MSWD values which were less than the appropriate value for the number of samples in the regression (typically MSWD = 1.89 for n = 10) no further error magnification was applied. For those above the target  MSWD, the error has been multiplied by the square root of the MSWD, highlighting additional sources of error beyond analytical uncertainty. See Table 4 for further discussion regarding the MSWD.
The Liaohe mafic rocks generally show relative enrichments in fluidmobile elements, such as Cs, Rb, Ba, U, K and Pb (Fig. 3). The only exception is the anthophyllite-rich rock, which has relative depletions in Rb, Ba, K and Sr. These elements are likely to have been removed during metamorphism. All samples show distinct relative depletion in Nb and have relatively flat heavy rare earth element (HREE) patterns with the exception of amphibolite LH05-014-7, which also has distinct major element composition compared to all other samples. LH05-014-7 has the highest incompatible element abundances and the highest immobile more to less incompatible element ratios, such as La/Yb, Nb/Yb and Th/Yb. Th\ \ 208 Pb (Appendix 4C), excluding the sample LH05 012-2 (anthophyllite-rich rock). The results are summarized in Table 4.
In summary, 147 Sm\ \ 144 Nd preserves a whole rock isochron age (Fig. 4A), which predates the age of the surrounding sedimentary package in which the mafic igneous rocks are hosted. The 10 mafic metaigneous Liaohe samples yield a bulk-rock Sm\ \Nd isochron age of 2.83 ± 0.18 Ga, an initial 143 Nd/ 144 Nd ratio of 0.50935 ± 0.00039 (MSWD of 1.2). εNd (2.83 Ga) ranges from +1.52 to +5.89 with one sample having a value of +6.38.
The 87 Rb\ \ 86 Sr system preserves a near isochronous relationship (MSWD = 2.3) at 1671 ± 58 Ma with an initial ratio of 0.70404 (Fig. 5). This initial ratio is relatively radiogenic and is consistent with derivation from a metamorphic or reset isochron.
An inverse 207 Pb\ \ 206 Pb isochron diagram yields a precise age of 1824 ± 19 Ma (MSWD =1; Fig. 6), whereas a conventional Pb\ \Pb isochron yields a far less precise, but overlapping age of 1.87 ± 0.60 Ga (MSWD = 44). Given the radiogenic nature of the measured ancient Pb isotopes and the correspondingly highly correlated errors and relatively low 204 Pb abundances, the inverse isochron approach is generally the favored for samples of this antiquity, and in the following discussion we consider this isochron as the preferred Pb\ \Pb isotope age in this sample suite.
Not surprisingly, U-Th-Pb isochron diagrams display varying degrees of open system behavior. Both U and Th are geochemically very different to Pb, and hence metamorphism and metasomatism will readily fractionate the parent element from the daughter. Nevertheless, all systems preserve broadly linear arrays with slopes corresponding tõ 1.70, 1.64 and 1.53 Ga for 232 Th/ 204 Pb versus 208 Pb/ 204 Pb, 235 U/ 204 Pb versus 207 Pb/ 204 Pb and 238 U/ 204 Pb versus 206 Pb/ 204 Pb respectively. The very high MSWD's for these regressions render propagation of errors on the ages of these regressions as geologically meaningless, however the simple errors generated by the spread of the data in the arrays (of the order of~0.20 Ga) allow for these ages to reflect either cooling of the terrane or a subsequent, very young event, which only partially reset the U-Th-Pb system.

Constraints on the age of formation and metamorphism of the Liaohe mafic meta-igneous rocks
The Liaohe mafic meta-igneous rocks form good positive linear correlations on the Sm\ \Nd, Lu\ \Hf, Rb\ \Sr and inverse 207 Pb/ 206 Pb isochron diagrams. Linear correlations on these isotope diagrams can either represent: 1) isochrons (or pseudo-isochrons) formed through radioactive decay of parent to daughter isotopes over extended time periods, or 2) two-component mixing in young samples or age-corrected older samples. Positive linear correlations of the measured isotopic composition on all isotope correlation diagrams in these presumed Paleoproterozoic rocks point to radiogenic ingrowth having formed these arrays, in particular since the linear arrays on all isotope correlation diagrams have positive slopes. Two-component mixing could also generate linear negative correlations. In addition, the Pb isotope ratios are more extreme than any Table 4 Summary of whole rock isochron data for the Liaohe mafic/ultramafic units. All regressions based on bomb dissolution including at least one duplicate (n = 11) with the exception of Sr (n = 10). Tremolite LH-05-012-2 not included in regressions. Isochron plots in Figs ⁎ for regressions of 11 samples, the target MSWD =1.89, as based upon the ratio of the minimization parameter (S) and the degrees of freedom in the system (2). Standard practice has been to multiply the error of the regression by the square root of the MSWD when the regressions return a MSWDNtarget, hence reflecting the greater uncertainty within the regression than implied by the assigned analytical uncertainties. We include here for reference the associated errors which have not had the a priori MSWD multiplication, since there may be non-geological sources of error and hence the non-corrected errors allow some sense of "errorchron" age ranges beyond the conventional statistical evaluation.
Phanerozoic rocks that we are aware of and thus are likely to reflect radiogenic ingrowth of U and Th rather than two-component mixing. As discussed below, zircons dated from other rocks in the Jiao-Liao-Ji     Belt have similar ages to those we report here for whole rocks. Alteration can affect parent-daughter ratios after emplacement, especially of Rb, Sr, U and Pb; however, Sm, Nd, Lu and Hf are generally fairly resistant to alteration and to metamorphism, through at least amphibolite metamorphic grade. Therefore, we interpret these linear arrays to form isochrons that provide age information about the origin and metamorphic history of the Liaohe mafic meta-igneous rocks.
Nine of the ten samples plotting on the Sm\ \Nd, Rb\ \Sr and 207 Pb/ 206 Pb isochrons come from the North Liaohe Group and therefore the age information strictly only applies to the North Liaohe Group. One sample from the South Liaohe Group, however, plots on all of the positive linear arrays formed by the North Liaohe samples, suggesting that the South Liaohe Group rocks, or at least some of them, were formed at a similar time and experienced a similar history to the North Liaohe Group rocks. Hence the bulk rock approach employed here explores processes that took place on lithospheric scales.
6.1.1. Sm\ \Nd and Lu\ \Hf isochrons: formation age of the mafic metaigneous Liaohe rocks and their mantle source Of the studied isotope systems, the Sm\ \Nd system is considered the most robust, due to the relative immobility of Sm and Nd (e.g., Schaefer, 2016). The excellent correlation of Zr with Sm (r 2 = 0.98) and Nd (r 2 = 94) and between Sm and Nd (r 2 = 0.96), excluding anomalous sample LH05-14-7, suggests that late-stage processes have not mobilized these elements. If post-emplacement mobilization due to alteration had taken place, it is unlikely that such a good correlation would have been preserved on the Sm\ \Nd isochron diagram. Therefore, we interpret the 2.83 ± 0.18 Ga Sm\ \Nd isochron age to reflect the age when Sm and Nd were last significantly fractionated from one another.
There are two possible interpretations for the Sm\ \Nd isochron: 1) The age is that of eruption/emplacement of the subsequently metamorphosed mafic rocks, or 2) the age reflects the stabilization/isolation of the mantle source from which these rocks were subsequently derived. On a whole rock scale, the latter scenario is plausible for large degrees of partial melting from sources that themselves contain uniform 143 Nd/ 144 Nd and Sm/Nd ratios. In the case of Precambrian rocks, these include previously depleted reservoirs, which have maintained closedsystem behavior from the rest of the convecting mantle for extended periods of time. Examples of whole rock Sm\ \Nd isochrons preserving ages that are hundreds of million years older than their emplacement have been reported in the literature for quite some time -significantly these include Archaean komatiites (Chauvel et al., 1985), Proterozoic mafic dikes (Schaefer, 1998) and metamorphosed mafic and felsic intrusive rocks (e.g., Theriault and Ross, 1991;Zhao and McCulloch, 1995). Indeed, such occurrences are possibly relatively common; however, they are overlooked in preference to calculated model ages from the data, or the isochrons are simply ignored as other geological constraints clearly rule out older isochron ages representing the time of emplacement.
In the Liaohe mafic meta-igneous rocks, field relationships show some intrusive contacts with the Liaoji Granitoids (dated between 2.2 and 2.0 Ga with a peak at~2.15 Ga; Zhou et al., 2008a, b;Meng et al., 2014;Wang et al., 2017;Zhang et al., 2018;Liu et al., 2019b) for some of the mafic outcrops, although others may be simply tectonically interleaved (Li et al., 2005a(Li et al., , 2005b. In any case, it is unlikely that the mafic protoliths were emplaced at~2.83 Ga, therefore a younger emplacement age of~2.2-2.1 Ga, similar to the major age range of the Liaoji Granitoids (e.g. Zhou et al., 2008a, b), seems to be the best age estimate for these rocks based on stratigraphic considerations. U\ \Pb dating of magmatic zircons from mafic meta-igneous rocks from the central Liaodong Peninsula (North Liaohe Group), similar to those studied here, yield two age groups: 1) 2547-2493 Ma, peak at 2503 Ma and 2) 2246-2135 Ma, peak at 2154 Ma with peak in T Hf model ages at 2.19 Ga (Meng et al., 2014). The older ages are interpreted to be inherited zircons derived from melting of underlying 2.5 Ga crust, providing direct evidence for Archean crust beneath this part of the JLJB. Therefore, it is likely that the Sm\ \Nd isochron reflects the time of formation of the mantle source from which these rocks were derived. Below we summarize literature studies providing evidence that parts of the crust beneath the JLJB separated from the mantle as much as 3.9 Ga ago with major crustal growth stages at 3.0-2.9 Ga, 2.8-2.7 Ga and~2.5 Ga, consistent with the 2.83 ± 0.18 Ga Sm\ \Nd whole rock isochron age reflecting lithospheric mantle stabilization betweeñ 3.0-2.6 Ga. The younger zircon age group is interpreted to reflect the emplacement age of the meta-igneous protoliths, derived by partial melting of depleted lithospheric mantle metasomatized by subduction-zone fluids/melts. Lu\ \Hf age of 2.25 Ga is within error of the peak of the younger zircon group (2.15 Ga). Thus, it is reasonable to suggest that whole-rock Lu\ \Hf age records the emplacement of the mafic protoliths into the crust and that the emplacement of the Liaohe mafic rocks appears to have been contemporaneous with the more voluminous felsic magmatism and most likely reflects the mafic endmember of this event. Hence the period~2.2-2.0 Ga represents significant addition of both mafic and felsic material to the crust (Lu et al., 2008;Li et al., 2006). The inverse Pb isochron provides a robust age of 1824 ± 19 Ma (MSWD = 1), and this represents the last time that the peak metamorphic mineral assemblage was open to Pb exchange. Since this age is wholly derived from Pb, it implies that Pb has been immobile over the subsequent~1.8 Ga. This becomes significant when considering the U\ \Pb and Th\ \Pb pseudo-isochrons, which show excess scatter due to open-system behavior between the parent elements (U and Th) and the daughter (Pb). Even though U\ \Pb and Th\ \Pb preserve ages which apparently coincide with the Rb\ \Sr age, significant ancient addition of U or Th to the Liaohe mafic rocks would have resulted in significant ingrowth in radiogenic Pb and hence disturbed the Pb\ \Pb isochron. Preservation of the Pb\ \Pb isochron indicates that this was not the case, and hence it is likely that any U or Th addition to the system had to occur relatively recently and the "ages" preserved by U-Th-Pb reflect closed system behavior between 1824 ± 19 Ma and a very young geologic event, very likely exposure and alteration of the rock units, which opened the isotopic systems.
Interestingly, metamorphic zircons or rims of older (2.5 or 2.2 Ga) zircons (n = 18) from the Liaohe mafic meta-igenous rocks yield a weighted average 207 Pb/ 206 Pb metamorphic age of 1896 ± 22 Ma (MSWD = 0.08; Meng et al., 2014) distinct from the whole-rock inverse Pb isochron age of 1824 ± 19 Ma from the same type of protoliths. Below we will show that the zircons and whole rocks most likely record distinct metamorphic events, peak prograde and post-peak retrograde amphibolite metamorphism respectively.
In contrast, the Rb\ \Sr system preserves a whole rock age of 1671 ± 58 Ma, which is significantly younger than the Pb\ \Pb age. Depending on the mineralogy present, whole rock Rb\ \Sr has long been recognized to have a significantly lower closure temperature than Pb\ \Pb (of the order of~500°C verses~600°C; Schaefer, 2016 and references therein) and hence it is reasonable to suggest that the Rb\ \Sr may simply reflect a cooling age. Whether these whole rock ages reflect uniform cooling from peak metamorphism, corresponding to b1°C per million years, or a subsequent, distinct thermal event which completely reset the Rb\ \Sr (but not the Pb\ \Pb system) at 1671 Ma cannot be resolved by this dataset alone. 40 Ar/ 39 Ar ages of 1830-1803 Ma were obtained from amphiboles in meta-volcanic rocks (amphibolite and mafic granulite) from Ji'an, South Liaohe and Jingshan Groups in the JLJB (Faure et al., 2004;Liu et al., 2015). Such high temperature amphiboles tend to have T c of~540 ± 40°C (Braun et al., 2006), slightly above that of Rb\ \Sr whole rock, and hence these data suggest the terrane cooled rapidly to~540°C by~1.8 Ga, but remained open to Sr at~500°C for the next~130 Ma. This would suggest that the terrane remained at midcrustal levels (25-35 km) until after~1671 Ma.
In conclusion, we interpret the younger Rb\ \Sr and 204 Pb/ 206 Pb-207 Pb/ 206 Pb ages compared to the Sm\ \Nd and Lu\ \Hf ages to reflect retrograde metamorphism of the Liaohe mafic metaigneous rocks. This interpretation implies that the isotopic composition of Sr and Pb was rehomogenized during metamorphism, but that the rocks remained largely closed systems until recently.

Geochemical Implications for the Tectonic Setting in which the Liaohe Meta-igneous Rocks Originated
Based on the metamorphic mineral assemblages and the major element chemistry, the protoliths of the amphibolites and hornblendites were most likely mafic (basaltic to andesitic) rocks (lavas or dikes) and the protolith of the metagrabbro was a gabbroic rock, representing the intrusive equivalent of the basaltic protoliths. The Liaohe amphibolites and metagabbro samples have compositions similar to modern-day mafic calc-alkaline volcanic rocks from active continental margins, for example Central America (e.g. Sadofsky et al., 2009;Heydoloph et al., 2012), Kamchatka (Duggen et al., 2007;Portnyagin et al., 2007) and Chile (e.g. Jacques et al., 2013), although they extend to more mafic compositions. The more mafic compositions are likely to reflect greater degrees of melting in a hotter Neoarchaean to Palaeoproterzoic mantle. The distinct composition of amphibolite sample LH05-14-7 compared to the other amphibolites (having the highest SiO 2 , Al 2 O 3 and K 2 O and lowest MgO and CaO and second lowest FeO t and TiO 2 ) suggests that the protolith for this sample was more evolved and had a more K-rich (high-K) calc-alkaline type composition compared to the protoliths of the other samples (Table 1; Fig. 7).
The high-grade hornblendite samples have distinct major element contents compared to the medium-grade amphibolite and metagabbro samples. The positive correlation between MgO and SiO 2 is not consistent with a link between the amphibolites and hornblendites through differentiation. The lower SiO 2 and CaO but higher FeO t and TiO 2 in the hornblendites compared to the amphibolites are consistent with a greater abundance of hornblende (90% versus 50-60%) compared to plagioclase (10% versus 40-50%) ( Table 1). The lower MgO in the higher-grade metamorphosed hornblendites with greater hornblende to plagioclase ratio, however, is unexpected and either reflects less magnesium-rich hornblende in the hornblendites or may reflect a lower MgO content in the protoliths of the hornblendites than in the amphibolites and metagabbro (Table 1). Finally, the anthophyllite-rich rock must have had an ultramafic (possibly pyroxenitic) protolith, also consistent with the lower abundances of most incompatible elements.
The patterns on the incompatible multi-element diagram (Fig. 3) for the Liaohe mafic meta-igneous rocks point to a subduction zone origin for all of the samples, as reflected by pronounced negative troughs in Nb and Ta relative to neighboring elements and general enrichment in fluid-mobile elements (Cs, Rb, Ba, U, K, Pb and Sr) and Th (Fig. 3). Elements that are fluid mobile in subduction systems can also be mobilized by fluids in the crust, so we must be cautious in using these elements for petrogenetic interpretations. Nevertheless, the incompatible element patterns in general are very similar for the different samples. Only the absolute concentrations vary, reflecting concentration or dilution of the incompatible elements as a group as a result of differentiation or accumulation processes.
To minimize problems with post-emplacement mobilization of elements, we now look at fluid-immobile-element discrimination diagrams to distinguish the tectonic setting in which the protoliths formed. Elements that are highly resistant to alteration processes include lightly compatible transition trace elements, such as Co, and incompatible elements, in particular the middle (M) and heavy (H) rare earth elements (REE), e.g., Nd, Sm, Tb, Yb, and high field strength elements (HFSE), e.g., Nb, Ta, Zr, and Ti, and Y, and Th. Since SiO 2 and the alkalis (Na 2 O and K 2 O) are very mobile during alteration and metamorphism, we use the Nb/Y versus Zr/Ti diagram (Fig. 7A; e.g., Pearce, 1996) as an immobile proxy for the TAS (total SiO 2 vs. alkali) diagram, to assess the nature of the protoliths for the Liaohe mafic metaigneous rocks. All the samples plot within the basalt to basaltic andesite field, consistent with their major element compositions. In order to distinguish between a mid-ocean ridge and subduction-zone origin, we use the Nb/Yb versus Th/Yb diagram after Pearce (2008), which is capable of distinguishing MORB, intraplate or ocean island basalt (OIB) and volcanic arc basalts (Fig. 7B). All samples have elevated Th/Yb for their Nb/Yb ratios and plot within the range of volcanic arc samples. On an earlier version of this diagram, Ta/Yb versus Th/Yb, Pearce (1982) distinguishes between oceanic and active continental margin basalts and between tholeiitic, calc-alkaline, medium-and high-K calc-alkaline and shoshonitic arc rocks (Fig. 7C). Most of the Liaohe samples plot within the active continental margin calc-alkaline field, except sample LH05-14-7 with Th/Yb ratio of 7.1. This sample plots in the high-K calc-alkaline field, consistent with its high K 2 O content and enriched incompatible trace element composition (see Fig. 3). In order to further test the nature of the Liaohe mafic meta-igneous rocks, we use the Co versus Th diagram after Hastie et al. (2007), considered to be an immobile proxy for the SiO 2 vs K 2 O diagram (Fig. 7D), also used to discriminate between the tholeiitic, calc-alkaline and high-K calc-alkaline/ shoshonite series. Co is considered to be an even better proxy for SiO 2 than Zr/Ti (Hastie et al., 2007). The Liaohe samples plot within the calc-alkaline field except LH05-14-7, which again plots in the high-K calc-alkaline/shoshonite field, which agrees well with the Ta/Yb versus Th/Yb diagram and major element chemistry.
As noted by Pearce (2008), the classification diagrams need to be applied with caution to Archaean rocks and to rocks that have been metamorphosed above low-grade amphibolite facies. Nevertheless, we believe that the general consistency between the incompatible multielement patterns and immobile incompatible element discrimination diagrams provide us with an accurate picture of the origin of their protoliths, despite amphibolite facies metamorphism of the studied rocks. In summary, the Liaohe meta-mafic/ultramafic rocks have chemical characteristics consistent with being derived from primarily calcalkaline basaltic protoliths formed in an active continental margin setting.
Alternative hypotheses for explaining the incompatible trace element abundances in the basalts include large amounts of crustal assimilation by upper mantle E-MORB type basalts, for example during continental rifting, or by flood basalt (Large Igneous Province = LIP) melts with relatively flat incompatible element abundances. Trondjemite-tonalite-granodiorite (TTG) crustal rocks with ages of 2.9-2.7 Ga were present in the Jiaobei Terrane of the JLJB in eastern Shandong when the Liaohe mafic and ultramafic rocks formed (An, 1990;Wang and Yan, 1992;Tang et al., 2007;Zhou et al., 2008a, b;Liu et al., 2013a, b), providing further evidence for the presence of continental lithosphere. These TTG crustal rocks formed during stabilization of the lithospheric mantle as recorded by the Sm/Nd isochron of 2.83 ± 0.18 Ga. The TTG gneisses have very high SiO 2 but low MgO, FeO t , CaO and TiO 2 contents. As noted above, the Liaohe amphibolites and metagabbro samples have compositions similar to modern-day mafic calc-alkaline volcanic rocks from active continental margins, for example Central America (e.g. Sadofsky et al., 2009;Heydolph et al., 2012), Kamchatka (Duggen et al., 2007;Portnyagin et al., 2007) and Chile (e.g. Jacques et al., 2013). In the aforementioned studies, the authors conclude that the data allow only very minor crustal contamination. Large amounts of assimilation of crust, which would be required to generate the incompatible-element abundances, would have shifted the composition of the Liaohe mafic meta-igneous rocks towards andesitic to rhyolitic compositions and also have resulted in dramatically lower εNd (i) values than observed. Therefore, large amounts of crustal assimilation are not consistent with the mafic and ultramafic compositions of the studied samples.
The positive initial εNd values (1.5-6.4) calculated for all samples at 2.83 Ga indicate that the Liaohe samples were derived from a long-term depleted source, relative to the Chondritic Uniform Reservoir (CHUR). There is no correlation between initial εNd (2.83 Ga) or Sm/Nd and indicators of crystal fractionation (e.g. MgO, SiO 2 , Ni, Zr, Nb/Yb), which usually occurs in conjunction with assimilation (DePaolo, 1981). We also note a similar range in initial εNd for~2.7 Ga rocks from Abitibi, Canada (Blichert-Toft and Puchtel, 2010) and the Gadwal, India (Khanna et al., 2014), where assimilation of continental crust is believed to have played no more than a minor role. In summary, we do not find any evidence in support of significant crustal assimilation in the Liaohe mafic and ultramafic metamorphic rocks, implying that at least the immobile incompatible element ratios reflect the composition of the mantle melts and not major interaction between crust and mantle. The presence of older TTG rocks, together with the Sm\ \Nd lithospheric mantle stabilization age, provides further evidence that the Liaohe mafic meta-igneous rocks were formed in an active continental margin setting. In conclusion, we note that Faure et al. (2004) came to a similar conclusion based on the trace element geochemistry of what they describe as gabbro and pyroxenite samples from the North Liaohe Group.

Temporal and geochemical evolution of the Jiao-Liao-Ji Belt
The lithospheric stabilization, emplacement and metamorphic ages that we have determined on the Liaohe mafic meta-igenous rocks are consistent with other geochemical and age data from the JLJB. Below we review the Neoarchean through Paleoproterozoic history of the JLJB (N2.8-1.7 Ga) using literature data combined with our new data.
As mentioned above, the JLJB can be subdivided into a northern belt, including the Fenzishan, North Liaohe and Laoling groups, and a southern belt, consisting of the Wuhe, Jingshan, South Liaohe and Ji'an groups (going from southwest to northeast). Detrital zircons in metasediments from the Jiaobei Terrane (southwestern JLJB), the South Liaohe Group (central JLJB) and the Ji'an and Laoling groups (northeastern JLJB) record nearly continuous magmatism (on a scale of b100 Ma) from 3.6 to 2.0 Ga Wan et al., 2006;Zhou et al., 2008a, b;Liu et al., 2013a, b;Wang et al., 2017;Zhang et al., 2018;Liu et al., 2015Liu et al., , 2019b. Such detrital zircon records are heavily weighted towards intermediate to felsic magmatism as mafic and ultramafic lithologies produce a paucity of zircons available to be eroded.
Considerable age and geochemical data are available from the Jiaobei (Jiaodong) Terrane from magmatic and metamorphic zircons. Magmatic zircons are characterized by low luminescence, often show zoning, and high Th/U ratios, whereas metamorphic zircons and zircon rims show nebulous zoning or are structureless, have high luminescence and relatively low Th/U ratios. SHRIMP and LA-ICP-MS zircon U\ \Pb analyses from supracrustal rocks and granitoid gneisses in the Jiaobei Terrane record three magmatic events between 2.9 and 2.5 Ga, taking place at 2.9,~2.8-2.7 and~2.55 Ga (Tang et al., 2007;Jahn et al., 2008;Zhou et al., 2008b;Liu et al., 2013a, b;Wu et al., 2014).
Mafic meta-igneous lenses, enclaves and blocks, probably representing parts of stretched and thinned dikes, in the Archean TTG gneisses of the Jiaobei Massif produce similar and younger ages than the TTG gneisses. A mafic granulite sample yielded SHRIMP 207 Pb/ 206 Pb zircon weighted mean ages of 2638 ± 22 Ma, interpreted as the crystallization age of the mafic igneous protolith of the granulite sample, and 2703 ± 12 Ma, interpreted as the crystallization age of xenocrystic zircons in the protolith derived from the underlying basement (Tam et al., 2011). Magmatic zircons from the supracrustal amphibolites in the Jiaobei Terrane yielded weighted mean concordant U\ \Pb zircon ages of 2.59-2.50 Ga, interpreted as the crystallization ages of the mafic magmatic protoliths (Zhang et al., 2003;Tang et al., 2007;Wu et al., 2014). Xenocrystic U\ \Pb zircon ages in mafic metaigneous rocks from the Liaohe Group range from 2.55 to 2.46 Ma and largely overlap ages of the younger amphibolites in the Jiaobei Massif (Meng et al., 2014;Xu et al., 2020). Finally, a mafic granulite from the Jiaobei Terrane yielded a LA-ICP-MS weighted mean 207 Pb/ 206 Pb zircon crystallization age of 2379 ± 54 Ma (Tang et al., 2007). In conclusion, the magmatic zircon age data provide evidence that mafic magmatism took place at least from~2.7-2.5 Ga (Tang et al., 2007;Meng et al. 2014), overlapping with the younger end of TTG formation in the JLJB (Lu et al., 2008).
Geochemical data is sparse from these mafic meta-igneous rocks. An amphibolite (03SD06) in the Jiaobei TTG gneisses, which produced a weighted mean 207 Pb/ 206 Pb zircon age of 2506 ± 18 Ma interpreted to be the crystallization age of a mantle-derived mafic dike, however, has subduction-type geochemical characteristics, e.g. Nb depletion relative to Th and La (Tang et al., 2007). Hf model ages from the magmatic zircons in the Jiaobei Terrane range from 3.9 to 2.6 Ga with a major peak at 3.4-3.1 Ga and a subordinate peak at 2.8-2.7 Ga (Wu et al., 2014). The Hf isotopes point to major juvenile crustal growth with substantial additions of older crust between 3.4 and 3.1 Ga and 2.8-2.7 Ga and crustal reworking with minor juvenal addition at~2.55 Ga (Wu et al., 2014). The younger~2.55 Ga episode of granitoid formation presumably resulted primarily from remelting of the~2.8-2.7 Ga juvenile crust. The sparse geochemical data available for these older mafic meta-igneous rocks are consistent with formation in a subduction zone.
Our 2.83 ± 0.18 Ga lithospheric stabilization (Sm\ \Nd whole rock) isochron for Liaohe mafic meta-igenous rocks provides evidence of the complementary mantle contribution to the 2.8-2.7 Ga crustal evolution in the Jiaodong Terrane (e.g. Liu et al., 2013a, b), and reflects the timing of final melt extraction and possibly ultimate cratonization of the SCLM beneath this terrane. Rocks with ages N3 Ga are rare in the Eastern Block of the North China Craton but are found in the Anshan Domain north of the Liaohe Group (~3.8-2.5 Ga; Song et al., 1996;Wan et al., 2005;Lu et al., 2006;Wu et al., 2014), suggesting that the studied igneous rocks were formed on basement of the former Longgang Block (northwestern part of the present Eastern Block).
Abundant evidence exists that mafic and granitic magmatism took place between~2.25-2.00 Ga in the central JLJB. In one of the earliest studies, zircons from a granite in the South Liaohe Group (also called the Kuandian Complex) in eastern Liaoning Province produced a minimum upper intercept U\ \Pb age of 2.14 ± 0.05 Ga, whereas whole rock Nd isotope data point to an age between 2.4 and 2.3 Ga for granite and amphibolite rocks (Sun et al., 1993). The authors interpreted the granites to be derived from the amphibolite protoliths (basalts) through fractional crystallization with little to no crustal assimilation based on the Nd isotope ratios and REE abundances. In the absence of crustal assimilation, the MORB normalized (Th, Ce)/(Nb, Ta) ratios N1 in the amphibolites (estimated from Fig. 4 in Sun et al., 1993) point to a subduction zone origin rather than through intraplate (continental flood basalt) volcanism as proposed by the authors. A subductionzone origin is also consistent with the high fluid-mobile-element (Sr, K, Rb, Ba) and Th contents.
A compilation of U\ \Pb zircon ages (267) from 17 Liaohe mafic samples from the Liaodong Peninsula give an age range of~2.25-2.02 Ga with a peak of activity at 2.12-5 Ga (Xu et al., 2020). SHRIMP and LA-ICP-MS U\ \Pb zircon ages for the Liaoji Granitoids (North and South Liaohe Groups), monzogranitic geneisses from the Jiaobei Terrane, and syenogranites from the Jilin Province yield a very similar range in crystallization ages of~2.25-2.00 Ga Lu et al. 2004aLu et al. , b, 2006Luo et al. 2004Luo et al. , 2008Wan et al., 2006;Li and Zhao, 2007;Liu et al., 2013a, b;Meng et al., 2014;Xu et al., 2018a, b). U\ \Pb zircon ages (88) from four felsic tuffs from the Liaodong Peninsula primarily fall within the range of 2.3-2.1 Ga with a peak at 2.17 Ga (Xu et al., 2020). It is therefore reasonable that the mafic meta-igneous rocks investigated here were part of this magmatic event, and indeed represent the mafic end member of subduction-zone magmatism active during this time. In conclusion, we interpret the mafic rocks to have been derived from a lithospheric mantle source, which was isolated at 2.83 ± 0.18 Ga (Sm\ \Nd isochron), which was part of a convergent margin when the mafic magmas formed at 2.25 ± 0.31 Ga (Lu\ \Hf isochron). Now we will review the geochemical data for the Jiaobei Terrrane and Liaohe Group mafic meta-igneous rocks. Liu et al. (2013a, b) suggest that the crustal reworking at~2.5 Ga resulted from magma underplating by upwelling plumes. Granitoid formation as a result of plume-related underplating applies well to the Taishan area in western Shandong Province, where~2.7 Ga greenschist to amphibolite facies komatiitic and tholeiitic basalts are associated with a~2.5-2.7 Ga TTG and supracrustal sequence (Wang et al., 2013). When comparing the Jiaobei (Tang et al., 2007) and Liaohe mafic meta-igenous rocks with similaraged komatiitic and tholeiitic basalts from the Taishan area in western Shandong Province (Wang et al., 2013), the difference in origin can be demonstrated in highly immobile incompatible element ratios, such as Nb/La (0.71-1.61 in the Taishan mafic volcanics versus 0.04-0.98 in the Jiaobei and Liaohe mafic meta-igneous rocks), Nb/Th (7.25-23.75 versus 0.22-4.56, respectively) and Th/Yb (0.05-0.60 versus 0.40-7.12, respectively). Low Nb/La, Nb/Th and high Th/Yb are source characteristics transferred into the resulting subduction zone mafic magmatism. The compositions of the mafic Liaohe rocks in the JLJB overlap with modern-day mafic arc igneous rocks, providing strong support that they formed in a subduction-zone environment. Excluding one sample, the initial εNd (i) of the Jiaobei amphibolites show depleted (+3.4 − +5.7) mantle source compositions (Tang et al., 2007), similar to the Liaohe mafic/ultramafic meta-igneous rocks (εNd = +1.5 − +6.4). The zircons also yield normal mantle O isotope compositions for these samples (Tang et al., 2007). In conclusion, there is no evidence for plume-related magmatism between~2.6-2.0 Ga in the JLJB, but rather for subduction-related magmatism.
Thousands of metamorphic zircons or zircon rims have been dated from the JLJB. An older metamorphic episode has been recorded in mafic meta-igneous rocks from the Jiaobei Terrane following the 2.55 Ga magmatic event. Wu et al. (2014), for example, report mean weighted 207 Pb/ 206 Pb metamorphic ages of 2.52-2.46 Ga in amphibolites and granitoids and an apparent 207 Pb/ 206 Pb age of 2.4 Ga in a biotite-plagioclase gneiss. Most U\ \Pb ages from metamorphic zircons and rims from the JLJB, however, range from 1.97-1.73 Ga (e.g. Zhao et al., 2012;Wu et al., 2014;Liu et al., 2015Liu et al., , 2019b. The granulite-facies metamorphic evolution of the northern (including Fenzishan, North Liaohe and Laoling groups) and southern (including Jingshan, South Liaohe and Ji'an groups) zones of the JLJB were believed to be distinct with the northern zone undergoing a clockwise P-T-t (pressure-temperature-time) path and the southern zone an anti-clockwise path (Lu, 1996;Lu et al., 1996;He and Ye, 1998;Li et al., 2001b;Zhao et al., 2012), but recently it has been demonstrated that the southern, like the northern, zone also underwent a clockwise P-T-t path (see Liu et al., 2019a, b and references therein). Mineral inclusions in dated zircons indicate that peak granulite-facies metamorphism took place at 1.94-1.89 Ga along the entire JLJB (including the North and South Liaohe Group rocks and the Liaoji Granitoids in the central JLJB, the Jingshan Group in the southern JLJB, and the Ji'an and Laoling Groups in the northeastern JLJB), which is interpreted to reflect collision of the Nangrim with the Longgang Block (Luo et al. , 2008Li et al., 2005aLi et al., , 2005bLu et al., 2006;Wan et al., 2006;Li and Zhao, 2007;Tam et al., 2011Tam et al., , 2012aWu et al., 2014;Wang et al., 2017;Zhang et al., 2018;Liu et al., 2019a, b). Specifically, the metamorphic zircons and rims from the Liaohe mafic meta-igneous rocks (with an age range of 1.92-1.87 Ga and a peak at 1.90 Ma; Meng et al., 2014) demonstrate that these rocks underwent this metamorphic event, recorded along the entire JLJB.
Inclusions in dated metamorphic zircons from meta-sedimentary and meta-mafic/ultramafic rocks throughout the JLJB indicate that retrograde metamorphism took place from~1.88 to b1.73 Ga (Lu et al., 2006;Zhao et al., 2006b;Zhou et al., 2008a;Tam et al., 2011;Wu et al., 2014;Liu et al., 2019a, b). Felsic magmatism, including intrusion of porphyritic monzogranites, granites and syenogranites, also took place throughout the JLJB between 1.88 and 1.80 Ga in an anorogenic or post-tectonic extensional setting (Cai et al., 2002;Li et al., 2004;Lu et al., 2004aLu et al., ,b, 2006Zhai et al., 2005;Li and Zhao, 2007;Tam et al., 2011;Liu et al., 2013a, b). Porphyritic monzogranites with similar ages (1.87-1.84 Ga) occur in North and South Korea within both the Nangrim Block and the Imjingang Belt, suggesting that this was a large-scale event that occurred after the collision of the Nangrim Block with the Longgang Block (Zhao et al., 2006a, b;Kim et al., 1999;Zhai et al., 2005;Kim and Cho, 2003;Lee et al., 2005). Migmatites cover N1100 km of the southern zone of the JLJB, extending from the Jingshan Group on the Jiaodong Peninsula in the south, through the South Liaohe Group on the Liaodong Peninsula to the Ji'an Group in south Jilin (Liu et al., 2019b). Anatectic zircons in granitic leucosomes, widespread in the migmatites, show that extensive partial melting took place during a similar time interval of~1.88-1.80 Ga (Liu et al., 2013b(Liu et al., , 2019a. Together the felsic magmatism and partial melting to form the migmatites and granitic leucosomes resulted from exhumation, due to extension and thinning, of the JLJB related to the post-peak MP-LP granulite facies retrograde metamorphism with a near isothermal decompression P-T path occurring at 1.87-1.84 Ga along the JLJB and between 1.85 and 1.84 Ga in the South Liaohe Group (Liu et al., 2015(Liu et al., , 2019b. Thereafter an amphibolite facies retrogression took place between 1.83 and 1.80 Ga (e.g. Liu et al., 2015Liu et al., , 2019b, for example recorded in the four amphibolite samples investigated by Wu et al. (2014), which yielded 207 Pb/ 206 Pb metamorphic ages of 1854 ± 12, 1838 ± 25, 1836 ± 73 and 1823 ± 41 and 1808 ± 27 Ma. These ages are within error of our whole rock 207 Pb/ 206 Pb age of 1824 ± 19 Ma for the Liaohe mafic (amphibole-bearing) meta-igneous rocks (amphibolites, hornblendites and metagabbro). Therefore, it is apparent that the terrane was subject to an elevated geotherm for an extended period of time after peak orogenesis, cooling through the Pb\ \Pb closure temperature of~600°C by 1824 Ma and soon after through the Ar\ \Ar hornblende closure temperature of 540°C at 1800 ± 10 (Faure et al., 2004). The terrane did however remain at mid-crustal depths above the~500°C closure temperature of Rb\ \Sr until~1671 ± 58 Ma.
In summary, dating of detrital zircons shows that crustal formation in the JLJB extends back to ≥3.6 Ga, whereas direct dating of metaigneous rocks shows that the lithospheric mantle became isolated and presumably stabilized between~3.0-2.6 Ga based on our whole-rock mafic Lioahe meta-igneous Sm\ \Nd age (2.83 ± 0.18 Ga). Felsic magmatism at~2.55 Ga, was predominantly generated by remelting of 2.8-2.7 Ga TTG (Jahn et al., 2008) and supracrustal rocks, but some juvenile mafic magmas with calc-alkaline (subduction-related) compositions were also produced during this event (Tang et al., 2007). Subsequent mafic volcanism at~2.25-2.02 Ga in the Liaohe Group also shows subduction-related geochemical characteristics (Lu et al., 2006;Li and Zhao, 2007). This complete package, including supracrustal sedimentary sequences, experienced prograde collision-related metamorphism between~1.96-1.88 Ga, whereas retrograde post-tectonic metamorphism (extension, cooling and exhumation) and anorogenic magmatism commenced between~1.88-1.80 Ga, passing through Pb-Pb whole-rock, Ar-Ar hornblende and Rb-Sr whole-rock closure temperatures at 1824 ± 19, 1800 ± 10 and 1671 ± 58 Ma respectively. Results of this study, in conjunction with those from previously published lithological, structural, geochemical and geochronological studies, enable us to place constraints on the evolution of the JLJB. We will begin by reviewing existing models and then will propose a new model integrating our new whole rock data with published whole rock and zircon data. The existing models for the origin and evolution of the JLJB can be divided into two endmember groups (e.g. see summary by Zhao and Zhai, 2013): 1) opening and closing of an intracontinental rift (e.g., Peng and Palmer, 1995;Li et al., 2004Li et al., , 2005aLi et al., , 2005bLi et al., , 2006Luo et al., 2004Luo et al., , 2008Li and Zhao, 2007) and 2) arc (island arc or active continental margin) -continent collision (e.g., Bai, 1993;Bai and Dai, 1998;He and Ye, 1998;Faure et al., 2004;Lu et al., 2006;Xu et al., 2018a, b).
Rift-related models propose that the Longgang and Nangrim (= Langrim = Langling) Blocks originally formed a single continental block that was rifted apart in the early Paleoproterozoic (2.2-1.9 Ga), accompanied by deposition of sedimentary and volcanic rocks into the rift basin and intrusion of mafic and granitoid rocks along rift-related faults. Closure of the rift in the late Paleoproterozoic resulted in the formation of the Jiao-Liao-Ji Belt Li et al., 2001aLi et al., , 2001bLi et al., , 2004Li et al., , 2005aLi et al., , 2006Li et al., , 2012. Zhao and Zhai (2013) argue that the following lines of evidence support this model: (1) bimodal volcanic suites in the JLJB, including meta-mafic volcanic rocks (greenschists and amphibolites) and meta-rhyolites Sun et al., 1993;Peng and Palmer, 1995); (2) large volumes of A-type granites in the JLJB (e.g. Li and Zhao, 2007); (3) low-pressure, anticlockwise, P-T paths of the Ji'an, South Liaohe and Jingshan Groups (Lu, 1996;Lu et al., 1996;He and Ye, 1998b;Li et al., 2001b), which are not consistent with arc-or continent-continent collision, and (4) non-marine borate deposits in the JLJB with similarities to borate-bearing successions in other Proterozoic rifting environments (Jiang, 1987;Peng et al., 1998). Problems with the rift model are explaining 1) what triggered its closing and the deformation event at~1.96-1.88 Ga, 2) the high-pressure, clockwise P-T-t paths found in both the northern (Fenzishan, North Liaohe and Laoling) and more recently also in the southern zone of the JLJB (e.g. Jingshan, South Liaohe and Ji'an groups; Liu et al., 2019a, b), showing that the anti-clockwise paths were incorrect, and 3) the origin of highpressure pelitic rocks that require subduction or continental collision to get them to sufficient depths to undergo high pressure metamorphism (e.g. Zhao and Zhai, 2013;Liu et al., 2019a), and 4) the presence of mafic meta-igneous rocks with subduction-related geochemistry (Faure et al., 2004;Meng et al., 2014;Xu et al., 2018aXu et al., ,b, 2020Xu et al., 2020; this study).
The second group of models argue for arc (island arc or active continental margin) -continent collision (e.g. Bai, 1993;Bai and Dai, 1998;He and Ye, 1998;Faure et al., 2004;Lu et al., 2006;. Bai (1993), the first to argue that arc-continent collision formed the JLJB, proposed that the Liaonan-Nangrim Block was an island arc and the Liaohe Group a back-arc basin. Collision of this arc system with the Archean Longgang Block caused the closure of the back-arc basin and formation of the JLJB in the Paleoproterozoic. Faure et al. (2004) showed that the mafic/ultramafic meta-igenous rocks have incompatibleelement characteristics similar to continental arcs and proposed that subduction was beneath the southern Palaeoproterozoic (Nangrim) continental block and that it was thrust upon the Anshan Block when the two blocks collided. Other variations of the arc-continent collisional model have also been proposed (e.g. Bai and Dai, 1998;He and Ye, 1998;Lu et al., 2006) with the most recent models favoring subduction of the Nangrim Block northwestwards beneath the Longgang active continental margin Block (e.g. Wang et al., 2017;Xu et al., 2018b;Zhang et al., 2018). Meta-sediments in the JLJB have been interpreted as having formed in the forearc and passive margin of the Nangrim Block (e.g. South Liaohe Group; Wang et al., 2017) or in a backarc basin (e.g. North Liaohe group; Wang et al., 2017;Ji'an and Laoling groups;Zhang et al., 2018). The mafic Liaohe meta-igneous rocks have been interpreted by some to have EMORB type compositions and thus it has been argued that they formed in a backarc basin floored by oceanic crust (Tang et al., 2007;Meng et al., 2014). The absence of an ophiolite associated with the JLJB, however, is not consistent with a backarc basin floored by ocean crust, since closure of back-arc basins floored by ocean crust generally result in obduction (rather than subduction) of the seafloor. We note that non-marine borate deposits could have formed in a backarc rift setting that had not progressed to becoming a marine backarc basin. Finally, the geochemical data from mafic metaigneous rocks in the JLJB point to them having been formed as part of a magmatic arc (active continental margin; e.g. Faure et al., 2004 and this study) rather than in a back-arc basin setting.
We now review our whole rock data combined with published results from whole rocks and zircons to present an integrated model for the evolution of the JLJB from~2.8-1.7 Ga. As summarized above, U\ \Pb zircon age data from detrital zircons in supracrustal rocks (3.6-2.0 Ga), TTG gneisses (2.9-2.5 Ga) and mafic meta-igneous rocks from the Liaohe and the Jiaobei Terranes (2.7-2.5 Ga) provide direct evidence that Neoarchaean crust exists beneath the Liaohe Group and Liaoje granitic rocks and/or that the JLJB crust was located adjacent to crust with these ages (Tang et al., 2007;Jahn et al., 2008;Zhou et al., 2008b;Liu et al., 2013a, b;Wu et al., 2014;Zhang et al., 2018;Liu et al., 2015Liu et al., , 2019b). The Hf model ages from the zircons in the Jiaobei Terrane extend as far back as 3.9 Ga and show major juvenile crustal growth between 3.4 and 3.1 Ga and 2.8-2.7 Ga and crustal reworking with minor juvenal addition at~2.5 Ga (Wu et al., 2014). Our Sm\ \Nd lithospheric mantle age of 2.8 ± 0.2 Ga from the mafic meta-igneous rocks from the Liaohe Group points to lithospheric mantle stabilization (cratonization) between~3.0-2.6 Ga (Fig. 8A). Unfortunately, there is little geochemical data available from the mafic samples with ages ≥2.3 Ga, which provides information about the petrogenesis of these rocks. An amphibolite sample dated at 2.50 Ga, however, shows a subduction-type geochemical signature, suggesting that subduction took place at this time (Tang et al., 2007) and may have caused the crustal reworking. Lithospheric stabilization taking place at 2.8 ± 0.2 Ga argues against a significant role for mantle plumes and lithospheric drips (Nebel et al., 2018) in causing crustal growth between 2.9 and 2.7 Ga, because plumes and lithospheric drips would cause lithospheric mantle thinning rather than stabilization. Therefore, although speculative, we favor formation of the~3.0-2.5 Ga crust and lithospheric mantle forming the JLJB basement through subduction.
The major phase of magmatism in the JLJB took place between 2.3 and 2.0 Ga and was subduction-related (Fig. 8B). The Liaohe mafic meta-igneous rocks were emplaced at 2.25 ± 0.31 Ga (whole rock isochron) and~2.25-2.02 Ga with peak age at 2.15-2 Ga (U\ \Pb zircon ages; Xu et al., 2020), overlapping with the Liaoje granitoid rocks emplaced between~2.2-2.0 Ga (e.g. Li and Zhao, 2007;Zhou et al., 2008a, b). The geochemistry of the Liaohe mafic/ultramafic metaigneous rocks is consistent with mafic arc magmatism having formed along an active continental margin (e.g. Bai and Dai, 1998;Faure et al., 2004). An active continental margin setting is further supported by the presence of Neoarchaean supracrustal rocks and TTG gneisses in the JLJB covering the age range of 2.9-2.5 Ga (Tang et al., 2007;Jahn et al., 2008;Zhou et al., 2008a, b;Liu et al., 2013a, b;Wu et al., 2014). In addition, U\ \Pb ages of detrital zircons from meta-sediments along the entire JLJB record nearly continuous (on a scale of b100 Ma) intermediate to felsic magmatism and/or metamorphic events from 3.6 to 2.0 Ga Wan et al., 2006;Zhou et al., 2008a, b;Liu et al., 2013a, b;Wang et al., 2017;Zhang et al., 2018;Liu et al., 2019b), and Hf model ages indicate crustal growth between 3.9 and 2.5 Ga (Wu et al., 2014). Crustal rocks with such ages have been identified in the Archaean Anshan Sequence in the Liaoning Province on the Longgang Block (3.8-2.5 Ga), which consists primarily of granitic rocks that experienced greenschist-to granulite-facies metamorphism, but not on the Nangrim Block (2.55-2.45 Ga), which consists primarily of quartz diorites and amphibolites that were exposed to amphibolitefacies metamorphism (Lu et al., 2006;Dong et al., 2017;Liu et al., 2017c;Wang et al., 2017;Liu et al., 2019a). Considering the overlap in magmatic evolution of the crust beneath the JLJB and on the Longgang Block, we place the active Paleoproterozoic (~2.3-2.0 Ga) continental margin on the southeast side of the Longgang Block rather than on northwest side of the Nangrim Block (Fig. 8B). Although it is likely that the protoliths of some of the meta-sedimentary rocks were deposited in a back-arc (as well as forearc) basin (e.g., Wang et al., 2017;Zhang et al., 2018), there is no direct evidence in the form of an ophiolite for this basin having been floored by oceanic crust formed by back-arc spreading.
The clockwise P-T-t path for the northern and southern zones of the JLJB (e.g. Liu et al., 2019a) are consistent with a collisional event having taken place between~1.96-1.88 Ga, as seen in metamorphic zircons and zircon rims from both felsic and mafic meta-igneous rocks throughout the JLJB. Mineral assemblages in the zircons indicate that peak granulite-facies metamorphism occurred at~1.94-1.89 Ga. This prograde metamorphic event no doubt represents collision and orogenesis between the Longgang Block and the Nangrim (micro-continental) Block to form the Eastern Block of the North China Craton (Fig. 8C). Inclusions in U\ \Pb dated zircons indicate that retrograde metamorphism took place between~1.88-1.73 Ga throughout the JLJB (e.g. Liu et al., 2019a, b). Felsic magmatism and migmatites with granitic leucosomes, representing partial melting of felsic minerals (quartz and feldspars), record post-peak MP-LP granulite facies retrograde metamorphism with near isothermal decompression between 18.7 and 18.4 Ga along the JLJB, followed by an amphibolite facies retrogression between 1.83 and 1.80 Ga (e.g. Liu et al., 2015Liu et al., , 2019b. Our whole rock inverse Pb\ \Pb isochron age of 1.82 ± 0.2 Ga for the Liaohe mafic (amphibole-bearing) meta-igneous rocks records this event. This post-orogenic period is associated with regional cooling, but temperatures remained above 500°C (at mid-crustal levels of~25-35 km) until~1.67 ± 0.6 Ga (Liaohe mafic meta-igneous whole-rock Rb\ \Sr isochron). We propose that overthickening of the lithosphere during the collisional event is likely to have resulted in lithospheric destabilization, resulting in lithospheric mantle detachment/delamination . Lithospheric mantle removal triggered orogenic collapse, causing extension and thinning, coupled with exhumation and anatexis (Fig. 8D).
Assembly of the North China Craton appears to have taken place through amalgamation of at least four micro-continental blocks via three collisional events. The Nangrim-Longgang collision described here formed the Eastern Block (Fig. 8), whereas amalgamation of the Yinshan and Ordos micro-continental blocks formed the Western Block. Assembly of these four blocks occurred contemporaneously but We favor formation of crust and lithosphere through subduction processes, since mantle plumes and lithospheric dripping are likely to cause thinning of the lithospheric mantle rather than its stabilization. We show subduction but the direction of subduction is assumed. Subduction is likely to have occurred intermittently until~2.3 Ga. (B)~2.3-2.0 Ga. Subduction of oceanic crust attached to the Nangrim Block northwestwards beneath the active southeast continental margin on the Longgang Block. Mafic melts in the subduction zone are derived from the~2.8 Ga lithospheric mantle. Melting results from addition of hydrous fluids/melts from the subducting oceanic crust to the lithospheric mantle beneath the southeast edge of the Longgang Block . Granites are formed coevally and primarily by differentiation of mafic melts due to mafic magma underplating (Li et al., 2001Li and Zhao, 2007). (C)~1.95-1.88 Ga. Collision of the active Longgang continental margin with the passive Nangrim continental margin caused closure of the ocean basin, crustal thickening, orogenesis, and peak metamorphism up to granulite grade. (D)~1.88-1.67 Ga. Destabilization of the thickened lithosphere resulted in delamination/ detachment of the lithospheric mantle and possibly lower crust, causing extension and thinning of the lithosphere in the JLJB and orogenic collapse. Exhumation resulted in retrograde metamorphism and crustal anatexis that generated post-tectonic anorogenic granites and migmatites. MP-LP retrograde metamorphism was isothermal between~1.87-1.84 Ga and went through the amphibolite facies between~1.83-1.80 Ga (Liu et al., 2019a, b), which is recorded in the whole-rock inverse Pb\ \Pb isochron of the Liaohe mafic meta-igneous rocks. Rocks cooled from~600 to~500°C between 1.82 Ga (Pb\ \Pb inverse isochron) and 1.67 Ga (Rb\ \Sr isochron). independently at~1.95 Ga. Collision between the Western and Eastern Blocks took place at~1.85 Ga during the time that the JLJB rocks were undergoing isothermal retrograde metamorphism. As continental blocks became larger, thicker and more abundant in the Paleoproterozoic, collisional tectonics may have resulted in the amalgamation of many smaller blocks into cratons , such as the North China Craton, that later were fused together through further collisions to form supercontinents such as the Meso-Paleoproterozoic Supercontinent Columbia (Zhao et al., 2002a, b;Li et al., 2019), which at some stage became unstable and broke apart only to reassemble again at a later stage .

Conclusions
Our study of whole rock geochronology and geochemistry of mafic/ ultramafic meta-igneous rocks from the Liaohe Group of the Eastern Block of the North China Craton, when interpreted in conjunction with the U\ \Pb and Lu\ \Hf zircon record, allows us to add key constraints to the evolution of Neoarchaean-Paleoproterozoic tectonics of the North China Craton. These include: 1) Mantle lithospheric stabilization took place at~2.8 Ga beneath the Jiao-Liao-Ji Belt. This lithospheric mantle was subsequently sampled by the Liaohe mafic and ultramafic subduction-related magmas. Due to high degrees of melting, the mafic magmas preserved the Sm\ \Nd age of their lithospheric mantle source.
2) The geochemistry of the Liaohe mafic magmatism points to an origin along an active continental margin. Available published data from other parts of the Jiao-Liao-Ji Belt also point to a subduction origin and display similar emplacement and metamorphic ages as the Liaohe mafic meta-igneous rocks. 3) Emplacement of the Liaohe mafic and ultramafic rocks, as preserved in a whole-rock Lu\ \Hf isochron (2.25 ± 0.31 Ga), was likely synchronous with the formation of the Liaoje granitoids intruded between 2.2 and 2.0 Ga in an active continental margin subductionzone setting. We place the active continental margin on the southeastern side of the Longgang Block, since Archaean supracrustal rocks and TTG gneisses from the JLJB have zircon U\ \Pb ages and Hf model ages (3.9-2.5 Ga), similar to ages from the Archean Anshan sequence (3.8-2.5 Ga) in the northern Liaoning Province on the Longgang Block but much older than ages found on the Nangrim Block (2.55-2.45 Ga) thus far. 4) Collision of the active Longgang continental margin with the passive Nangrim continental margin at~1.96-1.88 Ga, as recorded in metamorphic zircons (by U\ \Pb age dating) from the Liaohe mafic meta-igneous rocks and felsic rocks along the entire Jiao-Liao-Ji Belt, caused orogensis and granulite-grade metamorphism. 5) Crustal thickening related to the collisional event triggered lithospheric destabilization and detachment/delamination, resulting in exhumation and retrograde metamorphism, crustal anatexis and generation of a post-tectonic anorogenic granites at~1.88-1.80 Ga, recorded in the mafic meta-igneous rocks (whole-rock Pb\ \Pb age of 1.82 ± 0.02 Ga) and metamorphic zircons (U\ \Pb) throughout the Jiao-Liao-Ji Belt. Cooling due to exhumation reached a temperature of ≤500°C at~1.67 ± 0.06 Ga, based on the Rb\ \Sr whole rock isochron.
In conclusion, whole-rock isotopic analysis of mafic lithologies enables extension of the zircon geochronological record to include the age of lithospheric mantle stabilization (Sm\ \Nd), the timing of mafic magmatism (Lu\ \Hf), consistent with zircon age data from similar mafic meta-igneous Liaohe rocks, exhumation and retrograde amphibolite metamorphism (Pb\ \Pb), and constraints on post-orogenic cooling (Rb\ \Sr, U\ \Pb and Th\ \Pb).

Credit author statement
KH wrote the manuscript with significant input from BS and SL. SL carried out field work to collect samples. FH carried out the isotope analyses and contributed to the manuscript text. XL contributed to the interpretations in the manuscript. DG-S carried out the ICP-MS analyses. RZ contributed to the interpretations in the manuscript. YL contributed to development of the model and drafted the model figure.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.