Hornblende and Plagioclase Micro-Texture and Compositions: Evidence for Magma Mixing in High-Mg Adakitic Pluton, North China Craton

Abstract

In this study, we performed microtextural, major/minor element, and Sr-isotope analyses on hornblende and plagioclase (as phenocrysts, groundmass, or inclusions) from the Early Cretaceous Jiagou pluton (eastern North China Craton), to elucidate the magma source, possible magma mixing process, and the transition from low-Mg to high-Mg adakitic magmas. Petrographic study and electron microprobe (EMP) analyses reveal well-defined compositional zoning in hornblende and plagioclase phenocrysts. Outward from the core (first zone), the second and third zones show pronounced oscillatory zoning and significant variations in Mg# and An%, while the fourth zone is relatively homogeneous. A corroded albitic plagioclase core with sieve texture is enclosed in the first zone and locally intergrows with worm-like quartz streaks and fine hornblende inclusions, featuring Mg# = 81 (core) and 62 (rim). The new plagioclase infill has An% = 14–41. The corroded plagioclase has an initial ⁸⁷Sr/⁸⁶Sr = 0.7074, while that of zoned phenocrystic plagioclase ranges from 0.7068 to 0.7079, suggesting EMI and EMII mantle input. Inclusion hornblende is low in Ti and Cr, while phenocrystic hornblende shows higher Cr in the first zone and lower Cr in the outer zones. The newly discovered mafic microgranular enclaves (MMEs) and regional geochemical data suggest three major magma mixing events. The felsic parental magma was likely originated from a mixed EMI–EMII mantle source before mixing with a mafic magma derived from the partial melting of, successively, a low-Cr and a high-Cr peridotite. Our findings support a petrogenetic model of lower crustal delamination and highlight the critical role of repeated mafic injections in generating high-Mg adakitic magmas.

1. Introduction

Magma mixing/mingling is a fundamental igneous petrogenetic process. It produces a hybrid daughter magma with a composition intermediate of the parental magmas [1,2,3], as well as mafic microgranular enclaves (MMEs) in a more felsic host [2,3,4]. To reconstruct the parental magma composition, it is necessary to understand not only the daughter magma composition but also how many episodes of magma mixing/mingling have taken place.

Adakitic (or adakite-like) rocks exhibit similar geochemistry to the typical adakite from Adak, Aleutian Islands [5], except that the K₂O content is higher [6,7]. Adakitic rocks can be divided into low-Mg and high-Mg types, using the whole-rock MgO (threshold at 3 wt.%) or Mg# value [molar Mg/(Mg + Fe)] (threshold at 45) [6,8,9]. Understanding the petrogenesis of high-Mg adakitic rocks can provide important insights into crust–mantle interactions and continental crust growth [10,11], yet the formation of these rocks is still disputed to be through: (1) interaction between melts from the partial melting of young (<25 Myr, hot subducted oceanic slab) and mantle–wedge peridotite [6,12]; (2) interaction between melts (from the partial melting of the thickened, eclogite–facies mafic lower crust that foundered into the asthenospheric mantle) and peridotite in the subcontinental lithosphere mantle (SCLM) [10,13,14,15,16]; (3) the partial melting of highly hydrated mantle-wedge peridotite [8,17]; (4) the mixing of low-Mg adakitic melt with high-Mg basaltic melt derived from the metasomatized mantle [18,19,20,21,22]; (5) basaltic magma (from the partial melting of mantle peridotite) contaminated by deep crust-sourced felsic magma [23], particularly in continental arc settings.

Hornblende and plagioclase are major minerals in intermediate to felsic magmatic rocks and can occur as phenocrysts, groundmass, or inclusions. Compositions of the inclusion hornblende/plagioclase can record the magmatic physicochemical conditions (temperature, pressure and volatile activity) before the host mineral crystallization, whilst those of the groundmass hornblende and plagioclase can record the final crystallization stage. Meanwhile, compositions of the zoned phenocrysts can document potential fluctuations in the magmatic physicochemical conditions [20,24,25,26,27,28,29,30]. Therefore, in this paper, we report an integrated study of the micro-texture, major/minor element, and Sr-isotope compositions on the hornblende and plagioclase (in the form of inclusion, groundmass or phenocryst) from the Jiagou high-Mg adakitic intrusion in the eastern North China Craton, with the aim of revealing the origin of parental magmas and the subsequent multiphase magma mixing processes.

2. Regional and Local Geology

The North China Craton (NCC) is bounded to the south by the Yangtze Craton along the Qinling-Dabie-Sulu Orogen and by the Inner Mongolia-Daxinganling Orogen to the north (Figure 1). The NCC was finally cratonized through the amalgamation of the Western Block and Eastern Block at ~1.8 Ga, accompanied by high-grade regional metamorphism [31,32]. The NCC basement is largely composed of high-grade metamorphic rocks, with protoliths of tonalite–trondhjemite–granodiorite (TTG) and volcanic–sedimentary rocks (2.5–2.7 Ga, up to 3.8 Ga). The main magmatic and metamorphic events occurred at ca. 3.5–3.0 Ga, 2.6–2.45 Ga, and 2.2–1.8 Ga [31,33,34,35]. A prolonged, Mesoproterozoic–Mesozoic stable sedimentation occurred after the final cratonization, punctuated by several rift-related magmatic events (1.35–0.9 Ga) [31,33]. The reactivation of the NCC started in Late Carboniferous to Triassic, due to south-dipping (current orientation) subduction and collision, to form the Inner-Mongolia-Daxinganling Orogen [32,36,37,38]. These tectonic events have caused regional magmatism in Late Carboniferous, Late Triassic–Early Jurassic, and Late Jurassic–Early Cretaceous [34,39,40,41,42]. Along with lithospheric thinning and the decratonization of the NCC [42,43,44,45], the regional magmatism changed the Archean lithospheric mantle (recorded in the Paleozoic rocks) into the young Cenozoic lithospheric mantle characterized by EMI [46]. Among these magmatic rocks, the Early Cretaceous (~130 Ma) intermediate–felsic intrusions are adakitic and are widely exposed across the NCC, the Trans North China Orogen, and the Dabie Orogen [10,47,48,49,50,51,52]. During the Triassic Indosinian orogeny, the Yangtze Craton amalgamated with the NCC, involving the deep-subduction of the former [53]. This process led to (ultra) high-pressure metamorphism and the formation of eclogite (with/without coesite) at ~220 Ma [54,55,56,57].

The Jiagou pluton (the study area) is situated south of Xuzhou, approximately 100 km west of the Tan-Lu regional fault zone (Figure 1b). The intrusion (outcrop area: ~1.8 km²) intruded the folded Neoproterozoic–Ordovician sequence (Figure 2b), and its emplacement was controlled by NW-trending normal faults (Figure 1b). The pluton comprises mainly dioritic to monzodioritic porphyry (Figure 2a,b) and minor quartz diorite porphyry and is intruded by several quartz monzonite porphyry dykes (Figure 2c). Zircon U-Pb dating indicates that the pluton was emplaced at ~130 Ma [51,58,59]. Previous studies suggest that the Jiagou pluton is characterized by SiO₂ contents of 61.2–62.5 wt%, high Al₂O₃ (15.1–17.6 wt%), moderate to high MgO (3.0–4.7 wt%), and elevated high Mg# values (50–64). They display medium K₂O contents (2.3–2.5 wt%), high Sr (947–1180 ppm), low Y (10.8–16.1 ppm), and high Sr/Y ratios (36–81) signature [51,58], typical of adakitic rocks [5,6,7]. This indicates I-type affinity with distinctive high-Mg adakitic features [6,8,9,51,58]. The pluton contains diverse types of enclaves. Some were probably derived from a deeper source (eclogite, pyroxenite, and garnet-amphibolite/-granulite) with resorbed/corroded margins, whereas others may have had a shallower source (metagabbro and granite) with sharp intrusive contact. The recently discovered, centimeter-sized mafic microgranular enclaves (MMEs) are featured by corroded margins (Figure 2d).

Figure 1. (a) Tectonic sketch map of the southeastern margin of the North China Craton (NCC). The inset shows the location of the NCC in China; (b) Geological map of the study area, with the red stars denoting the sample location in this study (modified after [59,60,61]).

Figure 1. (a) Tectonic sketch map of the southeastern margin of the North China Craton (NCC). The inset shows the location of the NCC in China; (b) Geological map of the study area, with the red stars denoting the sample location in this study (modified after [59,60,61]).

3. Petrography and Mineralogy

The dioritic porphyry (Figure 2b,d and Figure 3a–f) contains mainly plagioclase (~60 vol.%) and hornblende (~30 vol.%), subordinate quartz and K-feldspar (<10 vol.%), and accessory (<5 vol.%) garnet, rutile, titanite, zircon, and pyrite. The hornblende occurs as phenocryst, groundmass, or inclusion, whilst the plagioclase occurs only as phenocryst or groundmass. Garnet is mostly present as xenocrysts. It is anhedral granular forms with distinct embayed resorption margins and occurs sporadically within the pluton.

Hornblende phenocrysts (Hb1) in sample 18JG-1 display complex zonation patterns and have up to four zones, designated as Hb1a, Hb1b, Hb1c, and Hb1d from core to rim. In many hornblende phenocrysts (Figure 3a), there is a relatively broad and bright greenish yellow inner sub-zone (Hb1a-1), which transitions to a narrower, darker yellow-brownish outer sub-zone (Hb1a-2). Each of the Hb1b, Hb1c, and Hb1d zones has a similar inner and outer sub-zone and color transition to that of Hb1a, although each sub-zone is much thinner. Hornblende inclusions occur in various locations, including in zone Pl0 (Hb0) (Figure 3e,f), Pl1b (Hbib), and Pl1c (Hbic), as well as in the Pl1b–Pl1c contact (Hbib/c) (Figure 3g).

Plagioclase phenocrysts (Pl1) in sample 18JG-7 also exhibit up to four zones, designated as Pl1a, Pl1b, Pl1c, and Pl1d from core to rim (Figure 3b,c). The core exhibits a sieve texture, featured by a subrounded core with a chaotic intergrowth of the earliest-formed plagioclase (Pl0) and worm-like quartz with interstitial later-stage plagioclase (Pl0m) (Figure 3c,d). There is no clear boundary between Pl0 and Pl0m. Zone Pl1a shows clean and well-defined bands and encapsulates the sieve-textured core with no distinct boundary. Zones Pl1b and Pl1c display clear contact, as well as well-defined, oscillatory zoning, with their outer sub-zone being much brighter (higher SEM reflectance) than the inner one (Figure 3d). Meanwhile, groundmass hornblende and plagioclase are microcrystalline and developed only a single zone or have no zonation.

The MMEs in the Jiagou pluton are composed predominantly of hornblende and plagioclase. Some hornblende crystals occur as zoned phenocrysts or as acicular radial aggregates in the groundmass (Figure 3h).

4. Analytical Methods

In-situ major and minor elements analyses of minerals were performed on a JEOL JXA-8230 electron probe microanalyzer (EPMA) at the Wuhan Sample Solution Analytical Technology Co., Ltd. (Wuhan, China). Operating conditions include a 15 kV accelerating voltage, a 10 nA beam current, and a 3 μm beam diameter. Major and minor element concentrations were determined by EPMA. The analytical accuracy for major elements is better than 2% relative, and the precision is generally better than 1% (1σ). For minor elements, accuracy and precision are within 5–10% and 2–5% relative, respectively. Multiple spots on each homogeneous mineral grain were analyzed to ensure data representativeness. The calibration standards for the plagioclase analysis include diopside for Ca, rutile for Ti, pyrope for Al, quartz for Si, pyroxmangite for Mn, hematite for Fe, albite for Na, olivine for Mg, orthoclase for K, fluorapatite for P, and Sr- sulfate for Sr, while those for the hornblende analysis are similar, in addition to tugtupite for Cl and two metal (Cr, Ni) standards provided by SPI. Data correction used the JEOL ZAF correction method. Calculation for the hornblende and plagioclase formula followed the method of [62] and of [63], respectively.

Strontium isotope ratios of plagioclase were measured on a Thermo Fisher Scientific Neptune Plus MC-ICP-MS (Bremen, Germany), in combination with a Coherent GeoLas HD excimer ArF laser ablation system (Göttingen, Germany) at the Wuhan Sample Solution Analytical Technology Co., Ltd. The Neptune Plus was equipped with nine Faraday cups fitted with 1011 Ω resistors. The Faraday collector configuration of the mass system was composed of an array from L4 to H3 to monitor Kr, Rb, Er, Yb, and Sr. The combination of the high-sensitivity X-skimmer cone and Jet-sample cone was employed. In the laser ablation system, helium was used as the carrier gas for the ablation cell. The spot diameter was 90 μm for a single laser spot ablation, depending on the Sr signal intensity. The pulse frequency was 8 Hz, and the laser fluence was kept constant at ~10 J/cm². A new signal smoothing device was used downstream from the sample cell to eliminate the short-term signal fluctuation [64]. All the Sr isotope data reduction was conducted using the Iso-Compass software [65]. The interference correction strategy was the same as the one described by [66,67]. The user-specified ⁸⁷Rb/⁸⁵Rb ratio was calculated by measuring the USGS glass standard BCR-2G, which has a known ⁸⁷Sr/⁸⁶Sr ratio. Following the interference corrections, Sr isotope mass fractionation was corrected by assuming ⁸⁸Sr/⁸⁶Sr = 8.375209 [66,67] and applying the exponential law. Two natural feldspar megacryst standards, YG0440 (albite) and YG4301 (anorthite), were measured as unknowns to assess the accuracy of the calibration method [67].

5. Results

Representative phenocrystic/groundmass hornblende and plagioclase and hornblende inclusions were selected for qualitative wavelength-dispersive spectrometer (WDS) compositional mapping and quantitative analysis by EPMA. In situ Sr-isotope analysis was carried out on the plagioclase phenocrysts. The WDS maps and corresponding compositional characteristics are shown in Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, Figure 13, Figure 14 and Figure 15, whilst the data are listed in Supplementary Tables S1–S5 and

Table 1. Representative EPMA data (wt.%) of phenocrystic hornblende from the Jiagou pluton (sample 18JG-1).

Spot No.	1	3	13	15	24	29	35	53	61
Zone	Hb1d	Hb1d/Hb1c	Hb1c	Hb1c/Hb1b	Hb1b	Hb1b/Hb1a	Hb1a	Hb1a	Hb1a
SiO2	46.70	43.32	45.53	43.00	45.24	42.97	44.72	45.18	44.98
Al2O3	7.75	10.79	10.30	12.17	10.87	12.33	10.91	10.71	11.46
TiO2	1.09	1.50	1.32	1.76	1.32	1.66	1.41	1.39	1.28
Cr2O3	0.07	0.02	0.06	0.02	0.37	0.00	0.32	0.27	0.19
FeO	15.96	15.85	10.86	13.06	9.10	11.97	9.73	9.81	8.72
MnO	0.44	0.31	0.17	0.21	0.10	0.14	0.14	0.12	0.07
MgO	11.67	10.61	15.15	12.95	16.20	13.73	16.26	16.03	16.37
CaO	11.58	11.28	10.94	10.94	11.05	10.52	10.96	10.79	11.01
Na2O	1.30	1.66	2.01	2.21	2.13	2.21	2.02	2.02	2.17
K2O	0.71	0.82	0.73	0.77	0.86	0.71	0.83	0.71	0.94
NiO	0.00	0.00	0.00	0.00	0.03	0.01	0.06	0.03	0.00
Cl	0.01	0.01	0.00	0.00	0.00	0.00	0.01	0.01	0.01
P2O5	0.00	0.02	0.00	0.00	0.00	0.01	0.01	0.00	0.00
Total	97.27	96.19	97.06	97.09	97.27	96.25	97.37	97.07	97.20
Structural formulae based on 15 cations
Si	6.94	6.54	6.60	6.33	6.51	6.32	6.43	6.51	6.46
AlIV	1.06	1.46	1.40	1.67	1.49	1.68	1.57	1.49	1.54
SumT =	8.00	8.00	8.00	8.00	8.00	8.00	8.00	8.00	8.00
AlVI	0.30	0.46	0.36	0.44	0.35	0.46	0.28	0.33	0.40
Fe3+	0.20	0.23	0.41	0.39	0.41	0.49	0.56	0.51	0.43
Ti	0.12	0.17	0.14	0.19	0.14	0.18	0.15	0.15	0.14
Cr	0.01	0.00	0.01	0.00	0.04	0.00	0.04	0.03	0.02
Mg	2.59	2.39	3.27	2.84	3.48	3.01	3.49	3.44	3.51
Fe2+	1.79	1.75	0.80	1.13	0.57	0.85	0.48	0.54	0.50
Mn	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
SumC =	5.00	5.00	5.00	5.00	5.00	5.00	5.00	5.00	5.00
Mg	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Fe2+	0.00	0.03	0.10	0.09	0.11	0.12	0.13	0.13	0.11
Mn	0.05	0.04	0.02	0.03	0.01	0.02	0.02	0.02	0.01
Ca	1.84	1.83	1.70	1.72	1.70	1.66	1.69	1.67	1.69
Na	0.10	0.11	0.18	0.16	0.18	0.20	0.17	0.19	0.18
SumB =	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00
Na	0.27	0.38	0.38	0.47	0.42	0.43	0.40	0.38	0.42
K	0.13	0.16	0.13	0.14	0.16	0.13	0.15	0.13	0.17
SumA =	0.41	0.54	0.52	0.61	0.57	0.56	0.55	0.51	0.59
Mg#	59	57	78	70	84	75	85	84	85

Note: Al^IV and Al^VI denote the numbers of aluminum ions in tetrahedral and octahedral coordination, respectively.

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Table 2. EPMA data (wt.%) of hornblende (as inclusions and in groundmass) from the Jiagou pluton.

Sample	18JG-2	18JG-2	18JG-7	18JG-1	18JG-1	18JG-1	18JG-1	18JG-4	18JG-4	18JG-7
Zone	Hb0c	Hb0r	Hbib	Hbib	Hbib/c	Hbib/c	Hbic	Hbic	Groundmass	Groundmass
SiO2	45.08	42.75	41.99	40.54	43.16	41.49	41.99	43.03	42.14	43.79
Al2O3	10.76	11.75	13.42	13.84	11.75	12.06	12.38	11.35	10.86	10.99
TiO2	1.176	1.283	1.195	1.550	1.481	1.682	1.274	1.258	1.381	1.249
Cr2O3	0.10	0.06	0.51	0.03	0.10	0.01	0.16	0.02	0.18	0.03
FeO	10.95	15.78	15.49	18.17	13.70	17.42	18.26	13.67	18.74	17.03
MnO	0.175	0.304	0.292	0.262	0.168	0.316	0.269	0.199	0.417	0.260
MgO	15.35	11.66	10.46	9.349	12.90	9.680	9.260	13.05	9.760	10.66
CaO	10.45	11.37	10.74	11.22	11.41	11.28	10.86	11.38	10.77	11.22
Na2O	2.076	1.897	1.723	1.932	1.955	1.664	1.720	1.998	1.673	1.794
K2O	1.033	1.133	1.440	1.292	0.930	1.062	1.322	0.775	1.134	0.904
NiO	0.030	0.000	0.005	0.002	0.000	0.075	0.021	0.021	0.023	0.008
P2O5	0.039	0.034	0.002	0.007	0.034	0.022	0.005	0.031	0.012	0.017
Cl	0.000	0.004	0.073	0.013	0.003	0.000	0.007	0.026	0.031	0.004
Total	97.22	98.02	97.32	98.20	97.58	96.76	97.52	96.81	97.11	97.95
Structural formulae based on 15 cations
Si4+	6.52	6.34	6.26	6.08	6.35	6.30	6.33	6.38	6.39	6.51
AlIV	1.48	1.66	1.74	1.92	1.65	1.70	1.67	1.62	1.61	1.49
SumT =	8.00	8.00	8.00	8.00	8.00	8.00	8.00	8.00	8.00	8.00
AlVI	0.36	0.39	0.62	0.53	0.39	0.45	0.53	0.37	0.32	0.44
Fe3+	0.53	0.39	0.37	0.39	0.37	0.32	0.37	0.40	0.47	0.32
Ti4+	0.13	0.14	0.13	0.17	0.16	0.19	0.14	0.14	0.16	0.14
Cr4+	0.01	0.01	0.06	0.00	0.01	0.00	0.02	0.00	0.02	0.00
Mg2+	3.31	2.58	2.32	2.09	2.83	2.19	2.08	2.89	2.20	2.36
Fe2+	0.66	1.50	1.49	1.81	1.23	1.85	1.86	1.20	1.82	1.73
Mn2+	0	0	0	0	0	0	0	0	0	0
SumC =	5.00	5.00	5.00	5.00	5.00	5.00	5.00	5.00	5.00	5.00
Mg2+	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Fe2+	0.13	0.07	0.07	0.08	0.09	0.05	0.07	0.09	0.08	0.06
Mn2+	0.02	0.04	0.04	0.03	0.02	0.04	0.03	0.02	0.05	0.03
Ca2+	1.62	1.81	1.72	1.80	1.80	1.84	1.75	1.81	1.75	1.79
Na+	0.23	0.08	0.18	0.08	0.09	0.07	0.14	0.08	0.12	0.12
SumB =	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00
Na+	0.36	0.46	0.32	0.48	0.46	0.42	0.36	0.50	0.38	0.40
K+	0.19	0.21	0.27	0.25	0.17	0.21	0.25	0.15	0.22	0.17
SumA =	0.55	0.68	0.59	0.73	0.64	0.62	0.61	0.64	0.60	0.57
Mg#	80.7	62.2	59.9	52.6	68.3	53.6	51.8	69.0	53.7	56.9

Note: The Hb0^c and Hb0^r represent the core and rim of the Hb0, respectively. Al^IV and Al^VI denote the numbers of aluminum ions in tetrahedral and octahedral coordination, respectively.

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Table 3. Representative EPMA data (wt.%) of phenocrystic and groundmass plagioclase from the Jiagou pluton (sample 18JG-7).

Spot No.	1	3	5	18	22	29	36	38	39	40	41	44	51	57	58	62	63	15	09
Zone	pl1d	pl1d	pl1d/ Pl1c	Pl1c	Pl1c/ Pl1b	Pl1b	Pl1b/Pl1a	Pl1a	Pl0	Pl0	Pl0m	Pl0	Pl0	Pl0m	Pl0m	Pl0m	Pl10	Groundmass	Groundmass
SiO2	65.84	65.47	66.58	54.94	62.64	57.06	60.01	59.77	65.19	65.43	59.83	65.22	66.00	64.46	57.09	62.43	64.76	60.22	63.30
TiO2	0.02	0.00	0.05	0.02	0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.00	0.000	0.000
Al2O3	21.04	21.27	20.80	27.62	22.71	26.98	25.10	25.21	22.15	21.93	25.16	21.52	20.98	22.11	26.64	22.81	21.67	24.73	23.27
FeO	0.06	0.07	0.06	0.14	0.09	0.20	0.10	0.11	0.00	0.00	0.14	0.04	0.00	0.00	0.18	0.13	0.05	0.054	0.026
MnO	0.00	0.00	0.00	0.03	0.00	0.00	0.01	0.00	0.03	0.00	0.03	0.03	0.00	0.00	0.04	0.00	0.00	0.000	0.000
MgO	0.02	0.03	0.01	0.00	0.00	0.00	0.01	0.00	0.01	0.00	0.02	0.00	0.00	0.01	0.00	0.00	0.00	0.025	0.000
CaO	1.68	2.27	1.28	9.24	4.14	8.48	5.85	6.21	2.61	2.33	6.60	2.33	1.40	2.77	8.11	3.95	2.50	5.994	3.823
Na2O	9.84	9.84	10.71	5.61	8.53	6.23	7.52	7.52	9.92	9.94	7.51	9.77	10.53	9.66	6.31	9.07	10.01	7.561	8.429
K2O	0.92	0.11	0.09	0.23	0.41	0.25	0.43	0.39	0.19	0.17	0.39	0.20	0.03	0.07	0.24	0.54	0.17	0.570	0.628
P2O5	0.00	0.03	0.01	0.04	0.00	0.05	0.04	0.00	0.02	0.00	0.00	0.00	0.01	0.03	0.00	0.00	0.00	0.000	0.028
SrO	0.17	0.20	0.16	0.20	0.00	0.30	0.22	0.16	0.00	0.04	0.19	0.00	0.20	0.07	0.16	0.06	0.00	n.a.	n.a.
Total	99.58	99.28	99.75	98.06	98.52	99.54	99.28	99.39	100.1	99.83	99.88	99.10	99.17	99.18	98.78	99.00	99.15	99.15	99.50
Structural formulae based on 8 oxygen
Si	2.92	2.92	2.93	2.53	2.82	2.58	2.70	2.69	2.87	2.89	2.68	2.90	2.93	2.87	2.60	2.79	2.88	2.71	2.83
Ti	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Al	1.10	1.12	1.08	1.50	1.21	1.44	1.33	1.33	1.15	1.14	1.33	1.13	1.10	1.16	1.43	1.20	1.13	1.31	1.22
Fe	0.00	0.00	0.00	0.01	0.00	0.01	0.00	0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.01	0.01	0.00	0.00	0.00
Mn	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Mg	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Ca	0.08	0.11	0.06	0.46	0.20	0.41	0.28	0.30	0.12	0.11	0.32	0.11	0.07	0.13	0.40	0.19	0.12	0.29	0.18
Na	0.85	0.85	0.92	0.50	0.75	0.55	0.66	0.66	0.85	0.85	0.65	0.84	0.91	0.83	0.56	0.79	0.86	0.66	0.73
K	0.05	0.01	0.01	0.01	0.02	0.02	0.03	0.02	0.01	0.01	0.02	0.01	0.00	0.00	0.01	0.03	0.01	0.03	0.04
P	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
An%	8	11	6	47	21	42	29	31	13	11	32	11	7	14	41	19	12	29	19
Ab%	87	88	93	52	77	56	68	67	86	88	66	87	93	86	58	78	87	67	77
Or%	5	1	1	1	2	2	3	2	1	1	2	2	0	0	1	3	1	4	4

and

Table 4. Sr isotopic ratios for zonal plagioclase from the Jiagou pluton.

	Sample	18JG-7 Four-Zoned Plagioclase	18JG-2 Four-Zoned Plagioclase	18JG-4 Plagioclase A	18JG-4 Plagioclase B	18JG-4 Plagioclase C	18JG-7 Plagioclase A	18JG-7 Plagioclase B	18JG-7 Plagioclase C
Occurrence
Pl0		-	0.707447	-	-	-	-	-	-
Pl0 + Pl0m		0.707019–0.707302	-	0.707320–0.707409	-	-	0.707370–0.707510	-	-
Pl1a		0.707059	0.707080–0.707198	0.707296	-	-	0.707255	0.706958–0.707005	-
Pl1b		0.706821	0.706980–0.706891	0.707365	0.706824–0.706905	0.706824–0.706847	0.706828–0.707138	0.706894–0.706925	0.707279–0.707369
Pl1c		0.707338	0.707741	0.707718	0.707091–0.707369	0.707361–0.707648	0.707322	0.707406	0.707313–0.707425
Pl1d		0.707473	0.707892	–	–	–	–	–	–

Note: Four-zoned plagioclase (from sample 18JG-4 and 18JG-2) represent the zonal plagioclase in Figure 10 and Figure 13a and Figure 11 and Figure 13b, respectively. Plagioclase A, B, C (from samples 18JG-4 and 18JG-7) represent the zonal plagioclase (a), (b), and (c) in Figure 14 and Figure 15, respectively.

.

5.1. Hornblende

WDS mapping on the four-zoned hornblende phenocryst (sample 18JG-1) reveals oscillatory zoning of Mg, Fe, Al, Ca, Ti, and Cr (Figure 4). In zones Hb1a, Hb1b, and Hb1c, Mg content is higher in the inner sub-zones and lower in the outer ones, and the opposite is true for the Fe, Al, and Ti contents. The inner high-Mg, low-Fe sub-zone is wider than the outer low-Mg, high-Fe sub-zone. A distinct low-Ti-Cr and high-Mg domain is present in the core (Hb1a), which likely represents the residue of an early-crystallized hornblende (Hb0). Furthermore, a very thin band (Hb1d) is locally developed on the Hb1c margin. High Cr concentration is only distributed within Hb1a, and the Cr content is generally below the detection limit in zones Hb1b, Hb1c, and Hb1d.

WDS mapping on a three-zoned hornblende phenocryst (sample 18JG-4) exhibits distinct zoning for Mg, Fe, Al, and Ti, weak zoning for Ca, and no zoning for Cr (Figure 5). The Ti map shows a corrosion remnant, probably a low-Ti hornblende inclusion with embayed (resorbed) grain margin. Hb1b has higher Ti content and rimed around the inclusion, while Hb1c has a wide inner sub-zone and a discontinuous thin outer sub-zone. Hb1d developed a rim (similar width as Hb1c) and has an inner and outer sub-zone with largely the same width. Both Hb1c and Hb1d developed oscillatory compositional zoning.

Analysis results for the core-rim transect of the four-zoned phenocrystic hornblende (sample 18JG-1) are consistent with the presence of four compositional zones (Hb1a to Hb1d) (Figure 4 and Figure 6; Table 1 and Table S1). The MgO, FeO, and Al₂O₃ contents exhibit clear oscillatory variations across Hb1b to Hb1d. In zone Hb1a, the MgO content remains stable (~16.4 wt.%) in Hb1a-1 near Hb0 and decreases (16.0–16.3 wt.%) away from Hb0 to reach 13.7 wt.% in Hb1a-2. Zone Hb1b (38 µm wide) shows a sharp MgO rise from 13.7 to 16.3 wt.%, with a mean VRMD (Variation Ratio of Mg# to Distance) of 9/10 µm for Hb1b-1, followed by a gradual MgO drop to 12.9 wt.% (mean VRMD = 13/28 µm for Hb1b-2). In zone Hb1c (~25 µm wide), MgO content increases sharply from 12.9 to 15.1wt.% (mean VRMD = 7/5 µm for Hb1c-1) before gradually decreasing to 10.1 wt.% (mean VRMD = 22/20 µm for Hb1c-2). The outermost zone Hb1d (~2.5 µm wide) shows a MgO rise from 10.1 to 11.7 wt.%, with a mean VRMD of 3/2.5 µm.

The FeO content generally shows an opposite trend to that of MgO, whilst the Al₂O₃ content exhibits a complex zoning pattern. It has a parallel trend to MgO in zone Hb1a-1 and is then parallel to FeO for the rest of Hb1a-2 and across Hb1b to Hb1d (Figure 6b).

Zones Hb1a, Hb1b, and Hb1c comprise mainly magnesio-hastingsite and minor pargasite and edenite, while Hb1d comprises mainly edenite (Figure 7) [68]. No clear correlation is found in (Na + K) vs. ^VIAl, Ti vs. ^IVAl, ^VIAl vs. ^IVAl, Ca vs. Al_tot, and Mg vs. Al_tot (Figure 8).

Quantitative compositional analysis of the three-zoned phenocrystic hornblende indicates high contents of MgO (10.9–14.6 wt.%) and FeO (11.4–17.1 wt.%). The MgO content is the lowest for the hornblende inclusions in the core (10.9wt.%) and increases through Hb1b to reach a maximum in the Hb1c-1 (14.9wt.%), then decrease through Hb1c-2 to the hornblende inclusion (~10 wt.%) (Figure 9, Table S5). Such variation is broadly comparable to that in the four-zoned hornblende phenocryst from sample 18JG-1.

Hornblende in the groundmass (zoned or un-zoned) has a broad Mg# value (54–80) (MgO = 9.76–13.92 wt.%) (Table 3, Table S5). The inclusion hornblende is predominantly pargasitic or edenitic (Figure 7b) and has a wide Mg# range (81–52) (MgO = 15.35–9.26 wt.%). Hb0 in the earliest-formed Pl0 from sample 18JG-2 has the highest Mg# (81) (MgO = 15.35 wt.%) in the core and lower Mg# (62) (MgO = 11.66 wt.%) in the rim, displaying zoned composition. In samples 18JG-7 and 18JG-1, some hornblende inclusions (in plagioclase phenocryst) also show Mg# (and MgO) zonal pattern: Mg# = 60 (core) (MgO = 10.46 wt.%) and 53 (rim) (MgO = 9.35 wt.%) of Hbib in zone Pl1b; Mg# = 68 and 54 (MgO = 12.90 and 9.68 wt.%) for Hbib/c on the Pl1b-Pl1c boundary; Mg# = 52 (MgO = 9.26 wt.%) for Hbic inside Pl1c. Sample 18JG-4 has an un-zoned Hbib inside Pl1b, with notably higher Mg# (69) (MgO = 13.05 wt.%) than that in groundmass hornblende (Mg# = 54) (MgO = 9.76 wt.%) (Table 2). In general, the phenocrystic, groundmass, and inclusion hornblende have almost the same Mg# range of ~50 to ~80.

5.2. Plagioclase

WDS mapping on the four-zoned plagioclase phenocryst from sample 18JG-7 further reveals the internal relationship between the corroded plagioclase core (Pl0 + Pl0m) and zoned plagioclase (Pl1a to Pl1d) (Figure 10). On the calcium map, Pl0 is shown as the dark blue (low-Ca) background with local bluish-green to green (medium-Ca) Pl0m occurrence. In the sodium map, Pl0 contains many worm-like red to green (high- to medium-Na) streaks. Considering the Ca, Na, and K maps, Pl0 likely comprises sodic plagioclase (e.g., albite) and hosts calcic plagioclase patches and worm-like quartz. Pl1a occurs as intermittent thin bands around the embayed Pl0 + Pl0m grain margin. Pl1b and Pl1c show a distinct boundary between them, and both have oscillatory zoning with Ca increasing in the inner sub-zone and decreasing in the outer one. Similar zoning patterns are present in a euhedral plagioclase from sample 18JG-2, which consists of an intergrowing Pl0 + Pl0m core (with hornblende inclusions), surrounded by successively Pl1a to Pl1d. Pl1a to Pl1c display oscillatory zoning with MgO increasing in the inner sub-zone and decreasing in the outer one (Figure 11).

Quantitative core-rim composition analysis on the plagioclase phenocryst (18JG-7) confirms the presence of an outermost Pl1d around the corroded core and zone Pl1a to Pl1d (Figure 12, Table 3 and Table S2). Pl0 features relatively stable An content (An7 to An13, avg. An10 in a 1060 µm-long profile), while Pl0m comprises sodic- to calcic-plagioclase (An14–39). Away from the core, Pl1a shows a slight An content drop (from An31 to An29) across a length of ~80 µm, and the variation ratio of An content to distance (VRAD) is 2/80 µm. Pl1b shows increasing An content (from An29 to An42) across a length of 117 µm (mean VRAD = 13/117 µm), followed by a gentle then abrupt drop to An21 (mean VRAD = 21/99 µm). Pl1c is characterized by a sharp An increase (from An21 to An47) across a distance of 23 µm (mean VRAD = 26/33 µm), followed by an abrupt drop to An6 across a distance of 117 µm (mean VRAD = 41/117 µm). The outermost Pl1d shows a rapid rise again (from An6 to An11), followed by a drop to An6 across a length of 34 µm. Meanwhile, the groundmass plagioclase shows distinct An range (An14 to An43), and the groundmass K-feldspar has a higher albite component (Or84Ab15An7).

5.3. Sr Isotopic Compositions of Plagioclase Phenocrysts

Forty-eight Sr isotopic analyses have been performed on eight zoned plagioclase phenocryst grains from sample 18JG-2, 18JG-4, and 18JG-7. The results are illustrated in Figure 13, Figure 14 and Figure 15, summarized in Table 4, and listed in Table S3.

Pl0 from sample 18JG-2 yielded an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.707447. Ten analyses on Pl0 + Pl0m from the three samples yielded initial ⁸⁷Sr/⁸⁶Sr = 0.707019 to 0.707510. Six spot analyses on Pl1a, 17 spots on Pl1b and 11 spots on Pl1c yielded initial ⁸⁷Sr/⁸⁶Sr = 0.706958–0.707296, 0.706821–0.707369, 0.707091–0.707741, respectively. Two analyses on Pl1d from samples 18JG-7 and 18JG-2 yielded initial ⁸⁷Sr/⁸⁶Sr = 0.707473 and 0.707892, respectively. The ⁸⁷Sr/⁸⁶Sr ratios generally show positive correlations with the An content at/around the analysis spot location (Figure 13, Figure 14 and Figure 15). Our new data indicate that the single initial ⁸⁷Sr/⁸⁶Sr = 0.707447 for Pl0 is slightly lower than that of the bulk-rock ⁸⁷Sr/⁸⁶Sr (0.7075 [69]). Comparatively, the zoned plagioclase phenocrysts have an initial ⁸⁷Sr/⁸⁶Sr range of 0.706821–0.707892, encompassing the bulk-rock initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7075.

6. Discussion

6.1. Genesis of Plagioclase Sieve-Textured Core and Magma Mixing

The sieve texture of magmatic phenocrysts (e.g., plagioclase) can be formed via magma mixing and decompression processes [70,71,72], with the former being controlled by the temperature of mafic magma injection [70]. If the temperature reaches the plagioclase liquidus, plagioclase will start to dissolve and produce rounded edges. When the An% of the phenocrystic plagioclase is much lower than the plagioclase in equilibrium with the melt, the former will be further corroded to form a porous, sieve-textured core. The corroded plagioclase host can react with interstitial melt and becomes more calcic away from the contact. In contrast, if the An% of the plagioclase phenocryst is not much lower than the plagioclase in equilibrium with the replenished mafic magma, the phenocryst will react to produce a sieve texture and more calcic rims and continue to grow to give a euhedral shape. Meanwhile, if the An% of the plagioclase phenocryst is similar to or higher than the plagioclase in equilibrium with the replenished mafic magma, the phenocryst will continue to grow to give a euhedral shape.

Previous works suggested that the sieve-textured phenocrysts formed via decompression differ fundamentally from those formed by magma mixing [70,73]. During isothermal decompression in a magma chamber, the decompression-induced dissolution of the plagioclase phenocrysts would generate the sieve texture, which in turn promoted mixing between the melt (from the dissolved plagioclase phenocrysts) and the residual magma, which is commonly more sodic [73]. When this hybrid magma infiltrates and crystallizes in the cavities, it cannot generate more calcic plagioclase inside the cavities of the existing plagioclase.

Our analysis on the sieve-textured Pl0m patches (likely formed from the hybrid magma) reveal a broad An range (14 to 41% An). The maximum An content (41%) is much higher than the average An content (10%) of P10, which crystallized from the earliest felsic magma. Such substantial An difference between Pl0m and Pl0 suggests magma mixing, rather than decompression process, to be the key mechanism in the plagioclase formation [24].

Other lines of evidence that support magma mixing include: (1) MMEs occur throughout the Jiagou pluton (Figure 2d and Figure 3h); (2) deep-sourced (ultra)mafic enclaves, including eclogite, garnet pyroxenite/amphibolite/granulite implies that the mafic magma was sourced from the lower crust or upper mantle [16,40,60,74,75,76,77,78,79]; (3) oscillatory zoning of the phenocrystic plagioclase and hornblende is also indicative of magma mixing (to be discussed in next section); (4) the initial ⁸⁷Sr/⁸⁶Sr_(t) ratio of the zoned phenocrystic plagioclase defines a wide range (0.70682–0.70789 (Figure 13, Figure 14 and Figure 15), suggesting a mixed EMI and EMII source region. This is also supported by the reported initial ¹⁴⁴Nd/¹⁴³Nd (0.511889–0.512349) and εNd_(t) (−4.43 to −13.14) value [69].

6.2. Oscillatory Zoning and Multiple Magma Mixing

Oscillatory composition zoning in hornblende and plagioclase phenocrysts can be attributed to multiple episodes of magma recharge and mixing process [24,29,30,72,80], local disequilibrium crystallization [81], or a combination of both [82].

According to [24], magma mixing-induced oscillatory zoning is characterized by sharp compositional boundaries between reverse and normal zones, with significant repeatedly high An% variation (typically 10–30%) on the discontinued boundary (Figure 16c). This is because the mixing of mafic magma with felsic magma would significantly increase the Mg-Fe-Ca contents in the magma. In contrast, oscillatory zoning caused by local disequilibrium crystallization would display more gradual and subtle An% variation (usually < 10). Reverse zoning is developed under conditions of decreasing undercooling or increasing residual components adjacent to the crystal face, and vice versa for normal zoning (Figure 16b) [81]. When both magma mixing and disequilibrium crystallization occur, the resulting An% profile would display a combination of abrupt and gradual transitions with moderate compositional variations at the contact between the reverse and normal zoning (Figure 16a) [82].

In this study, Pl1b, Pl1c, and Pl1b exhibit distinct oscillatory zoning patterns (Figure 10, Figure 11 and Figure 12). The plagioclase phenocryst transect (sample 18JG-7) reveals multiple stages of magmatic process (Figure 12): The initial Pl1a shows a subtle An drop (from 31 to 29%), reflecting the brief late-stage fractionation of the hybrid magma. This magma was likely formed through the mixing of the hot replenished mafic magma with the residual felsic magma. The modification of Pl0 to form Pl0m probably occurred after the crystallization of the Pl0 + Qtz + Hb0 assemblage. Zones Pl1b and Pl1c are characterized by a gradual then rapid An rise in the reverse-zoning part, followed by a gradual then rapid An drop in the normal-zoning part. The wide An range (13 to 41) suggests episodic magma mixing and fractional crystallization. The final Pl1d shows insignificant compositional variation, which probably reflects either minor magma mixing or disequilibrium crystallization.

The Mg# profile of the phenocrystic hornblende (sample 18JG-1) shows comparable variation patterns (Figure 6): Hb1a shows steady Mg# oscillation (between 86 and 83) for a considerable distance, interpreted to be stable crystallization process until normal zoning occur (Mg# drops to ~76). Both Hb1b and Hb1c show similar reverse and normal zoning, which suggests magma cooling and fractional crystallization separated by discrete magma mixing events. Hb1d developed only reverse zoning with a small Mg# increase (from 56 to 59), which may have caused by disequilibrium crystallization.

6.3. Petrogenesis of the Felsic and Mafic Magma End-Member

Oscillatory composition zoning in hornblende and plagioclase phenocrysts can be attributed to multiple episodes of the magma recharge and mixing process [24,29,30,72], local disequilibrium crystallization [81], or a combination of both [82].

The Pl0 + quartz + Hb0 assemblage in the Pl1a core is characterized by the lack of K-feldspar. The Pl0 was strongly modified by the first batch of mafic magma, resulting in a corroded margin with high An content (up to 41%) in the resorbed zone. Pl0 is generally sodic (avg. An₁₀), corresponding to albite-albitic oligoclase. The quartz occurs as worm-like streaks, and it co-crystallized with the albite-albitic oligoclase. Hb0 occurs as fine inclusions in Pl0, suggesting it was formed before Pl0. The mineral assemblage and compositions indicate that the rock is adakitic. Additionally, Hb0 displays compositional zoning, with low Ti-Cr contents and Mg# = 81 (core) and 62 (rim) (Figure 4). Since Hb0 is the sole mafic mineral in the assemblage, its Mg# (62–81, avg. 71), is considered representative of the whole-rock Mg# value. This suggests that the adakitic magma was already high-Mg before the magma mixing occurred.

Numerous high-Mg adakitic intrusions and volcanic units are present in the Eastern NCC. The partial melting of eclogite and other high-pressure garnet-bearing metamorphic rocks cannot produce such low-Mg magmas (Mg# < 45) [83,84,85,86]. To achieve high-Mg, these low-Mg melts would need to interact with high-Mg rocks, with three potential mechanisms: (1) interaction between melts derived from partially melting young (<25 Myr), hot subducted oceanic slab and mantle-wedge peridotite [6,12,83]; (2) interaction between melts (from a partial melting of the thickened, eclogite-facies mafic lower crust foundered into the asthenosphere) and peridotite in the subcontinental lithosphere mantle (SCLM) [10,13,14,15,69,87,88,89,90]; (3) magma mixing [18,20,27,80,91,92]. As the Jiagou pluton is already high-Mg adakitic, magma mixing is unlikely a viable mechanism.

Published experimental studies and field observation support the first mechanism [85,90,91,93]. Geochemical evidence, such as a high content of radiogenic Pb isotopes and a MORB-type Nd isotopic signature, indicates an oceanic crustal origin for the eclogite [46]. Mechanism (2) is also supported by: (a) the presence of deep-sourced enclaves of eclogite, garnet-pyroxenite (2.5–1.9 Ga), and dunite in the pluton, suggesting interactions with the NCC lower crust-SCLM; (b) the existence of thickened lower crust in the Mesozoic North China; (c) mineralogy, geochemistry, and high-P-T experimental data show that the dunite enclaves were formed through interaction between eclogite-derived melts and the mantle peridotite; (d) mineralogical and Nd-Os isotopes of two Early Cretaceous primitive basaltic units from the NCC suggest that adakitic melts were generated before the basaltic melts, and that the clinopyroxene phenocrysts in these basalts were crystallized from eclogite-derived melts [46].

The high-Mg adakitic rocks in this study exhibit a simple mineral assemblage and composition. On the εNd_(t) vs. (⁸⁷Sr/⁸⁶Sr) _(t) diagram, the Pl0 data plot within the fields of Xuhuai intrusion and NCC adakites, close to the North Dabie mafic rocks, and straddle across the enriched mantle EMI and EMII source region. The whole-rock data do not display clear oceanic crust signature, suggesting that the primary adakitic melt was generated by the partial melting of eclogite derived from the subducted oceanic slab [46]. Meanwhile, the eclogite xenoliths contain complex zircon age populations of 2.5–2.4 Ga, 1.85–1.75 Ga, 0.76 Ga, and 210–230 Ma [69,75], among which the 0.76 Ga population serves as a key marker for distinguishing the Yangtze Craton from the NCC. Therefore, we suggest that the earliest high-Mg adakitic melt of the Jiagou pluton was generated by the partial melting of eclogite from the deeply-subducted Yangtze Craton slab (which foundered into the asthenospheric mantle). Such melt may have subsequently ascended and reacted with a mixed source of EMI (NCC lithospheric mantle) and EMII (Yangtze Craton mantle) components, resulting in the high-Mg character.

The replenished magma batches causing the Hb1a, Hb1b, and Hb1c formation were likely derived from two source regions: The source for Hb1a shows relatively higher Cr content, while that for Hb1b and Hb1c has comparatively lower Cr content (Figure 4). The high-Cr source region may have been Cr-rich clinopyroxene-bearing peridotite, whilst the low-Cr source region may have been dominated by peridotite with Cr-poor clinopyroxene (formed from olivine). Initial ⁸⁷Sr/⁸⁶Sr ratio from Pl1a, Pl1b, Pl1c, and Pl1d plot within the same region as Pl0, but it covers a broader range (Figure 17). Such isotope variation is consistent with published whole-rock isotope studies on the Jiagou pluton and other plutons around it, which yielded initial Nd isotope ratio (¹⁴³Nd/¹⁴⁴Nd(t) = 0.511889−0.512349, and εNd(t) = −4.43 to −13.14) and ⁸⁷Sr/⁸⁶Sr(t) = 0.706000−0.707738 [69,94]. This suggests that the replenished mafic magma batches were generated from a mixed EMI and EMII mantle source. The replenished magma batch that formed Pl1a was produced by partial melting of a Cr-rich clinopyroxene peridotite source, while those that formed Pl1b, Pl1c, and Pl1d were derived from a Cr-poor clinopyroxene peridotite source.

6.4. Implication of Multiple Magma Mixing for High-Mg# Adakite Petrogenesis

Two criteria are commonly used to define high-Mg adakitic rocks: (1) MgO ≥ 3 wt.% and (2) Mg# ≥ 45 for whole-rock compositions [8,9]. As Mg# describes the atomic proportion of Mg relative to the sum of Mg and Fe, it could remain unchanged even if magma mixing dilutes both the MgO and FeO_T contents of the resultant hybrid magma. Consequently, the repeated injection and mixing of mafic magma is essential to effectively increase/maintain both the MgO content and Mg# in high-Mg adakitic rocks. We propose that the successive injection of three batches of high-Mg mafic magmas have led to progressive enrichment in Mg, Fe, Ca, and Na. This process may have caused the observed increase in the proportions of calcic-plagioclase and mafic minerals in the Jiagou pluton.

7. Conclusions

• Three distinct magma mixing events may have occurred during the formation of the Early Cretaceous Jiagou pluton, in the southeastern NCC margin.
• The initial felsic magma end-member is adakitic with low Ti and Cr contents and high Mg#. This initial magma was generated from the partial melting of eclogite-bearing subducted slab that was foundered into the asthenospheric mantle, where it reacted with a mixed enriched EMI and EMII mantle component.
• The three replenished mafic magma batches may have been derived from a Cr-rich clinopyroxene peridotite mantle source and a Cr-poor clinopyroxene (formed from olivine) peridotite mantle source. We suggest that repeated mafic magma injection and mixing, especially high-Mg ones, is a key mechanism for generating high-Mg adakitic magma from initial low-Mg adakitic melts.