September 2023 LIP of the Month

The 1.24–1.21 Ga Licheng Large Igneous Province in the North China Craton

Wang Chong^1,*, Peng Peng^1,2

1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
2. University of Chinese Academy of Sciences, Beijing, China

*Corresponding e-mail address: wangchong@mail.iggcas.ac.cn

Extracted from:

Multiple episodes of Precambrian mafic magmatism have been reported from the North China Craton (NCC), including events at 1.78, 1.73, 1.70, 1.68, 1.62, 1.32, 1.23, 0.92–0.89, and 0.81 Ga (Peng et al., 2008, 2011, 2022; Wang et al., 2022). Among these, the 1.78, 1.32, and 0.92–0.89 Ga events have been most extensively studied (e.g., Peng 2015; Zhang et al., 2017). Here, we report new geochronological, geochemical, and paleomagnetic results of 1.24–1.21 Ga mafic dykes in the NCC. We suggest that the dyke swarms collectively form a LIP related to a plume event, which is named as the Licheng LIP after the 1.23 Ga Licheng dyke swarm first identified by Peng (2015).

Occurrences and petrographic features

Most of the studied dykes belonging to the Licheng LIP intrude Archean basement. These dykes show different trends in the different areas in which they have been recognized (Fig. 1). In the Jidong area (central-northern NCC), they trend N-S to NNE with width of 10–40 m. The dykes intrude Archean gneisses, sometimes exhibiting clear but weathered and fragmented chilled margins. The dyke interiors have a coarse granular texture. The studied rocks in the Jidong area mainly consist of plagioclase (50–55%) and pyroxene (35–40%) with minor amount of biotite, Fe-Ti oxides and zircon, and have an ophitic texture (Fig. 2).

In the central NCC, these dykes are >15-50 m wide, and are NW trending. Spheroidal weathering is ubiquitous. These rocks are composed of plagioclase (~50%) and pyroxene (~40%), with hornblende (2–5%), biotite (2–5%) and Fe-Ti oxides (~5%) (Fig. 2). The 1230 Ma Licheng dyke in this area is in the Licheng area in the middle of Shanxi Province (Peng, 2015). This dyke cuts the Mesoproterozoic strata and is covered by the Cambrian conglomerates. The rocks are gabbroic.

In the Luxi area (southeastern NCC), the dykes trend NNW and are between 10 and 30 m wide. Chilled margins are easily identifiable along contacts with the host granites. In general, the plagioclase grains in these dykes are over 1 mm in length, with various degrees of saussuritization. Clinopyroxene is fresh and small compared to plagioclase. Fe-Ti oxides are usually 0.1– 0.5 mm in size and exist as euhedral crystals within plagioclase and clinopyroxene groundmass (Fig. 2). The different trends of dykes in the different areas, define a fanning pattern which converges at southeastern NCC (around Luxi area), and can be inferred to define the centre of a mantle plume.

Ages

The 1.24–1.21 Ga mafic dykes have been widely identified in the northeastern (Jidong and Liaodong), southeastern (Luxi), and central NCC by zircon and baddeleyite U–Pb geochronology (Fig. 1). Recently, coeval mafic sills and basalt have been identified in the Proterozoic Fanhe Basin of Liaodong by zircon/baddeleyite U–Pb/Pb–Pb and whole-rock ⁴⁰Ar/³⁹Ar dating (Kong et al., 2022). Beside these, (near)coeval mafic dykes and monzonitic intrusions with zircon U–Pb ages of 1259 ± 12 Ma and 1222 ± 19 Ma also outcrop in the Dongman Island, central-western Korean Peninsula of the NCC (Kim et al., 2018).

Geochemistry and petrogenesis

The 1.24–1.21 Ga magmatism in the NCC mainly consists of mafic dykes, showing U–Pb ages of 1244–1207 Ma. We consider this as a single magmatic event based on overlapping age ranges, and similarities in geochemical characteristics. Most samples plot in subalkaline to alkaline basaltic fields in element classification diagrams (Fig. 3). To estimate the geochemical effect of the observed alteration on bulk rock chemical composition, we compared the LOI with major and trace elements. Most elements show no correlation with LOI. However, some elements such as Al, Ca and Rb displayed weak linear correlation with LOI, reflecting mobilization of these elements during the (plagioclase) alteration process. In addition, Zr shows a positive linear correlation with high field strength elements (HFSEs: Nb, Ta, Hf, Th, REE (La, Sm and Yb) and U), and no correlation with large-ion lithophile elements (LILEs: Rb, Sr and Ba). Thus, the HFSEs and REEs are considered more reliable in deciphering the petrogenesis than LILEs.

Mafic intrusions can be contaminated by the host crustal rocks during emplacement. Some trace elements (e.g. Nb, La and Th) and their ratios are sensitive to this process because of their distinct differentiation between crust and mantle (e.g., Pearce, 2008). The Nb/La ratios of N-MORB, E-MORB, OIB and average crust are 0.93, 1.32, 1.30 and 0.67, while the Th/Nb ratios are 0.05, 0.07, 0.08 and 0.48, respectively. Some samples of this magmatism present low Th/Nb ratios (~0.05 to 0.09) and high Nb/La ratios (~0.8 to 1.3), indicating a low degree of crustal contamination.

Multiple geochemical proxies, especially the continuous variations in elements (Figs. 3, 4), parallel REE and spider patterns (Fig. 5), indicate that these dykes likely shared the same source even if the event lasted for about 30 Ma. None of the studied samples were derived from primary magma since all Mg# values are < 60. There is no obvious correlation between MgO and Al₂O₃, tFe₂O₃, TiO₂, Cr and Ni (Fig. 4c-g), so fractional crystallization seems to be a minor influence, which is also supported by the La/Sm versus La diagram (Fig. 6c). In early magmatic evolution, the magma mainly experienced partial melting showing oblique linear tendency between La/Sm and La (Fig. 6c). It gave way to weak crystallization-influenced differentiation when La reached a level over ~30 ppm (Fig. 6c). Some samples show a weakly to significantly positive Eu- anomaly (Fig. 5a), reflecting only a minor influence of plagioclase extraction.

Ratios of La/Yb_N generally range from 6.8 to 13.9, with values up to 18.6 (Fig. 4l), reflecting the variation of degree of partial melting. All the samples have high REE contents (96–308 ppm), high La/Yb_N (6.8–18.6), which distinguish them from MORB. The Th-Nb proxy indicates an OIB affinity, but with slight interaction with E-MORB (Fig. 6e). Their high TiO₂/Yb ratios (Fig. 6f) are indicative of garnet residues, and the rocks likely originated from melting beneath a thick lithosphere (Pearce, 2008). The diagonal trends from the OIB to the MORB fields reflect hot mantle flow towards thinner lithosphere (Fig. 6e-f) (Pearce, 2008). Some samples show ɛHf (t = 1230 Ma) of 1.8–10 (Wang et al., 2015) and ɛNd (t = 1230 Ma) of 0–1.6 (Peng et al., 2013; Wang et al., 2016), which is suggestive of their originating from a depleted asthenosphere. Additionally, these dykes have high Ti content (usually >1.5 wt.%), indicating possibly a similar plume-related source as the Emeishan flood basalts (Xu et al., 2001).

Paleomagnetism

~1.24 Ga dykes

This E-SE moderate to shallow downward stable characteristic remanence magnetization (ChRM) has been isolated after both thermal and AF demagnetization from nine dykes of the ca. 1.24 Ga group. (Fig. 7) Baked contact tests have been performed on dykes with positive result, where country rocks are Archean gneisses (Fig. 8). The mean paleomagnetic pole for this group of dykes is at 2.0°N, 165.1°E (A₉₅ = 11.0°), which is close to the ca. 1.22 Ga NCC paleopole reported by Ding et al. (2020).

~1.21 Ga dyke

The 1206.7 ± 1.7 Ma dyke is located in the Miyun District, Beijing. This dyke is about 30 Ma younger than the other studied dykes and gives a different remanent direction: SW moderate downward. Both thermal and AF demagnetization isolated a stable ChRM with 540–580°C unblocking temperatures and > 40 mT coercivity (Fig. 7). The mean paleomagnetic direction is D = 204.5°, I = 39.5°, α₉₅ = 6.5°, and the corresponding paleopole is 23.0°S, 92.5°E, dp/dm = 4.7°/7.8°. This pole should be considered a virtual geomagnetic pole (VGP) because it is based on the paleomagnetic direction from just one dyke that does not average out secular variations.

Tectonic implications

A LIP-generating event refers to mantle plume-induced, high-volume magmatic activity during a period of up to tens of million years, which are sometimes associated with continental breakup (Ernst, 2014). A LIP event can be characterised by its volume (> 0.1 Mkm³), area (> 0.1 Mkm²), and duration (< 50 Ma), and commonly represents a pulsed sequence of magmatism in an intraplate tectonic setting (Ernst, 2014 and references therein). Several of these characteristics are identifiable from the 1.24–1.21 Ga magmatism in the NCC.

First, areal extent. The studied dykes are discontinuous in the field, and each can only be traced over several hundred metres, or a few kilometres, making it difficult to evaluate the total areal extent and volume. These dykes are in general over 10 m in width, but can be up to 80 m wide, a scale commonly associated with LIPs (Ernst, 2014). After restoring the effect of the Tan-Lu fault, this event is estimated to cover an area of > 0.1 × 10⁶ km² in the central and eastern NCC (Fig. 1). If the small dykes in the Bayan Obo rift (1227 ± 60 Ma, Sm-Nd isochron; Yang et al., 2011) are included, and if the mafic dykes and intrusions in the central-western Korean Peninsula (Kim et al., 2018) also belong to the same generation, the areal extent is even larger.

Second, the age spread. Constrained by the age data, this magmatic event lasted from ca. 1244 Ma to 1207 Ma (U–Pb ages), with a total duration of ~30 Ma. Considering the age uncertainties, and their consistent geochemical characteristics (Figs. 3–6), we regard these dykes as belonging to the same LIP event. Ernst (2014) concluded that LIPs with > 20 Ma age span were emplaced in several shorter duration pulses rather than as a single continuous episode. The resolution of our ages and limited sampling precludes a confident identification of discrete pulses of dyke emplacement; the three age populations, however — ~1230 Ma, ~1220 Ma and ~1207–1210 Ma — suggests multiple pules of magmatism.

Third, an intraplate origin. These intrusions were emplaced within the NCC, and have intraplate geochemical characteristics (Fig. 6d). Moreover, this dyke swarm present a fan-like radiating geometry in the field (Fig. 1). Considering all, we interpret that the studied rocks originated from a mantle plume source. In summary, we suggest that the dyke swarms collectively form a 1.24–1.21 Ga LIP related to a plume event in the NCC, with plume centre at the locus of the fanning swarm (southeastern NCC).

In this study, we obtain a ~1.24 Ga paleopole (2.0°N, 165.1°E, A₉₅ = 11.0°) with a positive baked contact test (Fig. 8). It thus represents a primary key paleopole for the NCC. Selected paleomagnetic poles are used for assessing regional paleogeography. Here we discuss the reconstruction and relationships among the NCC, proto-Australia and Laurentia from ~1.4 to ~1.2 Ga. A long-lasting connection between the NCC and the NAC in the Mesoproterozoic has been proposed based on the similarity between their APWPs and their comparable tectonostratigraphic records, including coeval magmatism, ore deposits, fossils, and comparable hydrocarbon-bearing potential layers (Wang et al., 2019). Paleomagnetic reconstruction based on the updated 1.24 Ga paleomagnetic poles indicates that the two continents likely have started to pull away from each other, or at least the connection of northeastern NCC and northern NAC was broken by that time (Fig. 9). This is consistent with the previous suggestion of a breakup of the NCC from the NAC at ca. 1.32 Ga based on the presence of unconformities within the NCC (between the Xiamaling Formation and the overlying Changlongshan Formation) and the NAC (between the Roper Group and the overlying Cambrian volcanics) (Ahmad et al., 2013; Zhang et al., 2017).

Considering that the likely plume-triggered 1.32 Ga mafic magmatism occurred after the deposition of the Xiamaling Formation and the Roper Group, the two unconformities might represent the breakup event or plume-related uplift and erosion (Zhang et al., 2017). We notice that the Licheng LIP of NCC may potentially be linked to the same-aged Marnda Moorn LIP of the Western Australian Craton (WAC) at 1.24–1.21 Ga (dotted optional position for the NCC in Fig. 9b). In summary, our paleogeographic reconstruction for 1.24–1.21 Ga implies a dispersed Laurentia, NCC and proto-Australia, indicating that the component cratons of the Columbia supercontinent were separated from each other by that time.

In conclusion, a combination of field investigation with geochronological and geochemical studies allowed us to define a > 0.1 × 10⁶ km² Licheng LIP in the NCC. This intraplate magmatic event continued for at least ~1244 Ma to ~1207 Ma. The studied mafic dykes have OIB-like features in trace-element ratios and patterns, and show radial geometry, indicating a plume origin that could be from the southeastern NCC. Nine dykes yield a ~1.24 Ga paleomagnetic pole at 2.0°N, 165.1°E, A₉₅ = 11.0°, supported by positive baked contact test. The paleomagnetic analyses show that the NCC separated from proto-Australia after 1.32 Ga, and remained unconnected to Laurentia, indicating the break-up of the supercontinent Columbia occurred by this time.

References