Geochronological and Geochemical Constraints on the Petrogenesis of Lamprophyre from the Giant Weishan REE Deposit in China

: The Weishan REE deposit is located in the southwest of the Luxi Terrane of the North China Craton (NCC), where a large number of lamprophyre dikes are spatially exposed with the deposit. Here, we report petrology, geochemistry and zircon U-Pb geochronology data for the lamprophyre of the Weishan REE deposit in order to develop constraints for the determination of the petrogenesis, magma source and evolution of the lamprophyre and the tectonic environment. LA-LCP-MS zircon U-Pb dating shows that the crystallization age of the lamprophyre is 125 ± 0.86 Ma. The geochemical data suggest that these lamprophyres have high levels of Al 2 O 3 , K 2 O, MgO and alkalis, moderate level of Na 2 O and low levels of SiO 2 , Fe 2 O 3 and TiO 2 , and that they are enriched with LREEs (La, Ce) and LILEs (Rb, Ba) and depleted with regard to HREEs and HFSEs (Nb, Ta, Ti). They displayed negative ε Hf(t) values of − 14.98 to − 9.03, T DM1 ages of 1.1–1.4 Ga and T DM2 ages of 1.7–2.1 Ga, which suggest that the magma source originates from an enriched mantle. Low Rb/Sr and high Dy/Yb ratios suggest that the enriched mantle source was partially melted at the amphibole-bearing lherzolite garnet-facies. The high Ba/Th and Sr/Th ratios indicate that the enriched source was derived from subduction dehydration ﬂuids of the oceanic crust. We propose that the maﬁc dike intrusions are consistent with an Early Cretaceous alkaline magma emplacement in an extensional setting, in which the magma was not contaminated by crustal material during its emplacement.

Rare earth elements (REEs) are indispensable raw materials for modern science and technology and play significant roles in international resource strategy [10][11][12]. The Weishan REE deposit is located in Weishan Village, Weishan County, Zaozhuang City, Shandong Province. The deposit is a typical LREE deposit and is genetically related to the host alkaline complex. There are abundant lamprophyre dikes exposed in the mining area and its periphery. Extensive previous studies have been carried out on deposit geology,  [30]. (B) Geological sketch map of the Luxi Terrane [16].
The Weishan REE deposit is located in the southwestern Yicheng uplift of the western Luxi Terrane (Figure 1). The major strata include Neoarchean granodiorite and Quaternary sedimentary cover. The structure is dominated by NW and NE faults. The majority of the magmatic rock is comprised of Mesozoic quartz syenite, aegirine quartz syenite and alkali granite, which constitute the Weishan alkali complex. The alkaline magma intrudes into the Neoarchean granodiorites and takes the form of irregular branches. The proven resources in Weishan REE deposit amount to 44 t, with an average grade of 4.6% [31]. Most of the REE veins are distributed as NW trending, which is determined by the NW fault structure ( Figure 2). There are abundant lamprophyre dikes exposed in the Weishan REE deposit and its periphery. The lamprophyre takes the form of dikes intruded into the Neoarchean granodiorite, biotite plagioclase gneiss and Mesozoic alkaline complex. These lamprophyre dikes are mainly exposed as NE and NW trending and show size variations of 20 to 200 m in length and 2.0 to 5.0 m in width ( Figure 2). The rock is mostly characterized by porphyritic or non-porphyritic lamprophyre textures and massive structures. The main lithological types of lamprophyre are kersantite, spessartite and alkaline lamprophyre.

Whole-Rock Major and Trace Element Analyses
Geochemical data for the major and trace elements of the studied samples were obtained at the testing center of the Shandong Lunan Geological Engineering Investigation Institute, China. First, the samples were crushed and powdered to 200 mesh in an agate bowl. The ferrous oxide content was determined with the potassium dichromate volumetric method, and other major elements were analyzed with X-ray fluorescence spectrometry using a Rigaku RIX 2100 spectrometer (Rigaku Corporation, Tokyo, Japan)with uncertainties of less than 5%. Trace and rare earth elements were analyzed using a PEE 6000 ICP-MS instrument with uncertainties of less than 5%.

Zircon LA-ICP-MS U-Pb Dating
Zircon separation was completed at the Langfang Geological Survey, Hebei Province, China. The samples were crushed to 40-60 mesh, and the zircon grains were separated through coarse and fine crushing, panning, magnetic separation and other methods. Transmission, reflection and cathodoluminescence (CL) imaging were completed at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The relatively complete and transparent zircon crystals were carefully handpicked under a binocular microscope and mounted in the epoxy, polished to nearly half-section to expose the internal structure and cleaned in an ultrasonic cleaner containing a 5% HNO 3 solution. Zircon grains were carefully examined under microscopes and electron microscopes to identify internal structures and textures.
LA-ICP-MS Zircon U-Pb isotope analysis was completed at the Isotopic Laboratory, Tianjin Center, China Geological Survey. The laser analysis was performed using a Neptune double-focusing multiple-collector ICP-MS (Thermo Fisher Scientific, Waltham, MA, USA) attached to a NEW WAVE 193 nm-FX ArF Excimer laser ablation system (Express Scripts Inc., Saint Louis, MI, USA). All zircon analysis was completed with a beam diameter of 35 µm, 8 Hz repetition rate and an energy density of 11 J/cm 2 . Zircon 91,500 is used as an internal standard for U-Pb chronology analysis. In order to ensure the accuracy of the test, the zircon 91,500 standard sample was tested twice, before and after every eight samples were tested. An SRM 610 glass standard sample was used as an external standard to calculate the U, Th and Pb concentrations of zircon. Data were calculated using ICPMS DataCal 8.4 at the China University of Geosciences, Wuhan, China [32], and zircon age concordant plots were obtained using the Isoplot 4.0 program [33]. Common Pb corrections were undertaken using the method described by Anderson [34].

Zircon Lu-Hf Isotope
The zircon Lu-Hf isotope analysis was completed in the Isotopic Laboratory at Tianjin Center, China Geological Survey, using a Neptune MC-ICP-MS (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a NEW WAVE 193 nm-FX ArF Excimer laser ablation system (Express Scripts Inc., Saint Louis, MI, USA). The laser ablation beam diameter was 42 µm, the energy density was 14 J/cm 2 and the repetition rate was 10 Hz. A GJ-1 standard sample was used as an external standard for in situ zircon Hf isotopic analyses.

Zircon U-Pb Geochronology
Most of the zircons from the lamprophyre (20CS15) are colorless, transparent, subhedral to anhedral and had rounded or irregular morphology. The zircon grains show grain sizes of 50-300 µm and aspect ratios of 1:1-2.5:1. In the cathodoluminescence (CL) image, the zircons show obvious patchy, banded and sector zoning ( Figure 8A), indicating a magmatic origin [40].
Seventeen zircons from the lamprophyre (20CS15) were analyzed ( Table 2). To obtain an accurate and reliable age, most of the analyzed spots were selected from the large grains of the magmatic oscillatory zoning domain. The measured concentrations of U and Th in these zircons are 545-1999 ppm and 14-3290 ppm, respectively. Th/U ratios ranged from 0.38 to 0.61, similar to magmatic zircon (Th/U > 0.1) [41]. The sample yield a weighted mean age of 125 ± 0.86 Ma (2σ, n = 17, MSWD = 0.60) ( Figure 8B), representing the crystallization age of lamprophyre.

Zircon Lu-Hf Isotope
The Lu-Hf isotopic analysis of zircons from the lamprophyre (20CS15) ( Table 3) show that the 176 Lu/ 177 Hf ratios for all measured results ranged from 0.0011 to 0.0015. The 176 Hf/ 177 Hf ratios of 17 zircons ranged from 0.282274 to 0.282442, with an average of 0.282359. Zircon εHf(t) values were all in the negative range, from −14.98 to −9.03 ( Figure 9A) with an average of −11.98, and they were plotted between the lower crust and paleo-lower crust reconstruction lines ( Figure 10). The single-stage-model and two-stagemodel ages were 1.1-1.4 Ga and 1.7-2.1 Ga, respectively ( Figure 9B).

Early Cretaceous Intrusion of Lamprophyre Dike
Zircons from the lamprophyre of the Weishan area show obvious patchy, banded and sector zoning and high Th/U ratios of 0.38-0.61, indicating magmatic zircon. The analyzed spots yielded a weighted mean age of 125 Ma, which can be interpreted as the crystallization age of the lamprophyre. Qiu et al. [43] reported a K-Ar age of 120 Ma from the phlogopite of the lamprophyre in the Jingziyu area. Yang et al. [42] reported a zircon LA-ICP-MS U-Pb age of 126-132 Ma from the lamprophyre in the Xiaya and Jingziyu areas. The magma source of the lamprophyre from the Weishan, Jingziyu and Xiaya areas of the Luxi Terrane was an emplaced simultaneous source. Therefore, the mafic dike intrusions in the Luxi area were formed from 132-120 Ma, which is consistent with peak Early Cretaceous magmatic events in the eastern NCC.
Liang et al. [21] discussed the zircon LA-ICP-MS U-Pb age of Weishan alkaline rocks and showed that the crystallization age of the quartz syenite was 122 Ma and that of the aegirine quartz syenite porphyry was 130 Ma. Wei et al. [25] obtained zircon U-Pb ages of 127-126 Ma for the quartz syenite and alkali granite. Therefore, the alkaline and mafic magma were emplaced simultaneously during the Early Cretaceous.

Crustal Contamination
The  [44]. Although crustal contamination shows only a limited influence on rapid upwelling and low viscosity magma [45,46], it is necessary to evaluate the effects of crustal contamination.
The crust-like trace element features included enrichment in LILEs (Rb, Ba) and LREEs, depleted HREEs and HFSEs (Nb, Ta, Ti, Zr), negative Nb-Ta and Ti anomalies, which may indicate continental crust material contamination from assimilation or a fractional crystallization process during the magma intrusion [47][48][49]. However, the lamprophyres show higher concentrations of Ba (1651-6457 ppm) and Sr (1037-9809 ppm) than the average continental crust (390 ppm Ba; 325 ppm Sr) [50], implying that the crust contamination had limited influence on the trace elements during the magma emplacement [51]. In addition, the low Lu/Yb ratios of 0.09-0.13 were close to the mantle-derived magmas (Lu/Yb, 0.14-0.15) rather than the continental crust (Lu/Yb, 0.16-0.18) [39], indicating the absence of crustal contamination. The geochemical evidence was consistent with the absence of inherited zircons within the lamprophyre. Previous studies have also suggested that crustal contamination has limited influence during mafic dike intrusion [52][53][54][55]. Therefore, the crust-like trace element features are not the result of crustal contamination from when the magma was ascending.

Nature of the Magma Source
The abundance of K 2 O, alkali enrichment and low εHf(t) values (−14.98 to −8.69) suggest that the magma source of the lamprophyres was derived from an enriched mantle [56]. In the Th/Yb vs. Nb/Yb diagram, the lamprophyres also fell into the enriched mantle field ( Figure 11A). The high LREE/HREE ratios of those alkaline lamprophyres imply that the enriched source was the lithospheric mantle [57]. Moreover, in the La/Sm vs. La diagram, the lamprophyres show an obvious positive correlation between La/Sm and La, suggesting that the main petrogenetic mechanism was controlled by partial melting rather than fractional crystallization ( Figure 11B). The lamprophyres exhibited high concentrations of K 2 O and were enriched with LILEs and LREEs, which consistently indicate an enriched mantle source. Experimental petrology studies suggest that the volatile-rich minerals phlogopite and amphibole are the major host phases for a LILE-rich lithospheric mantle [60,61]. The compatible elements Rb, Sr and Ba are useful proxies for distinguishing phlogopite from amphibole [62]. The MELTS equilibria with phlogopite show higher Rb/Sr (>0.1) and lower Ba/Rb (<20) ratios. Comparatively, the MELTS equilibria with amphibole show a high Ba concentration and Ba/Rb (>20) ratio [63]. Therefore, there were low Rb/Sr (0.01-0.14) and high Ba/Rb (7-69) ratios in the lamprophyres from the Weishan area ( Figure 12A), indicating that the magma source for the lamprophyre was formed by the partial melting of an amphibole-bearing lherzolite mantle. The Dy/Yb vs. K/(Yb*1000) diagram ( Figure 12B) is a useful plot to distinguish the spinel-facies (low Dy/Yb ratio; <1.5) and garnet-facies (high Dy/Yb ratio; >2.5) of partial melting [64]. The lamprophyres show high Dy/Yb (3.00-4.72) ratios and K/(Yb*1000) (12.3-46.7) values and were plotted along the garnet-facies and garnet-facies amphibole-bearing lherzolite curve, which implies a low degree of partial melting (0.1-2%) in the garnet-facies. Experimental studies have suggested that the depth of the garnet transition zone is~85 km [65]. In summary, the magma source of the lamprophyre from Weishan area originated in an enriched mantle through low-degree partial melting of an amphibole-bearing lherzolite mantle at garnet-facies of 85 km depth.

Modification of the Mantle Source by Metasomatism
The lamprophyres exhibited higher Th/Yb ratios than MORBs and OIBs [66] (Figure 11A), implying that the mantle magma was enriched. High La/Ta ratios (>30) indicate that the lithospheric mantle enrichment is related to subduction [67]. The Nb/U ratio (3.38) of lamprophyre was lower than the typical MORB and OIB compositions (Nb/U = 47 [68]; lower (Nb/U = 25) and upper (Nb/U = 4.5) crust [69]), which, together with the high Ba/Th (33.20-658.29) values, suggest that the enriched source was derived from slab dehydration fluids [70,71]. Previous studies have proposed that Early Cretaceous mafic rocks are characterized by high concentrations of H 2 O (2-4 wt.%) [72] and suggest that the oceanic crust is the primary source [73]. In the Ba/Th vs. Th/Nb and Sr/Th vs. Th/Ce diagrams, the lamprophyre show large variations in the Ba/Th and Sr/Th ratios, indicating that the mantle source was metasomatized by a fluid source rather than melting [53] (Figure 13A,B). In addition, the new geochemical data from this study show a similar composition as that found for the Xiaya and Jingziyu, indicating that the magma source of the mafic dike in the Luxi Terrane was derived from an enriched mantle source. We therefore propose that the magma emplacement of the 125 Ma lamprophyres from the Weishan area is consistent with Early Cretaceous (120-132 Ma) mafic dike intrusion in the Luxi Terrane [19,75]. The 120-110 Ma compression to extension tectonic transition regime in the eastern NCC triggered large-scale extension and lithosphere thinning [75][76][77][78][79] ( Figure 14). The lithospheric mantle was metasomatized by dehydration fluids during the subduction process, and the magma source of the lamprophyre was emplaced during the subsequent extension process in the Early Cretaceous.

Conclusions
(1) The parent magma of the lamprophyres in the Weishan area was formed by lowdegree partial melting of the amphibole-bearing lherzolite mantle at garnet-facies without crustal assimilation during its emplacement. The enriched mantle source of the parent magma was derived from the subducted slab dehydration fluids.
(2) The lamprophyre from the Weishan area was formed in an extensional tectonic setting and the mantle source was metasomatized during the subduction process in the Early Cretaceous.