Zircon U-Pb Ages, Petrogenesis, and Tectono-Magmatic Evolution of Late Jurassic–Early Cretaceous S-Type Granitoid in Wujinshan Area of Northwestern Zhejiang, South China

Abstract

Northwest Zhejiang area (NWZJ) is one of the important parts of the large Qingzhou-Hangzhou mineralized belt in South China formed during the Late Jurassic–Early Cretaceous period. Through the study of zircon LA-ICPMS U-Pb dating, whole-rock geochemistry, and Sr-Nd isotopes for the Wujinshan granitoid in NWZJ, two distinct S-type granitic rocks of porphyry type granodiorite and granite were identified, and the two ages of 146.4 ± 1.5 Ma for granodiorite porphyry and 141.9 ± 1.4 Ma for granite porphyries were obtained. These rocks exhibited a geochemical affinity for S-type granitoid, and the two magmatic ages indicate that these rocks were intruded in two magmatic pulses. The Late Jurassic granodiorite porphyry showed moderate SiO₂ (64.38–67.89 wt.%) with higher K₂O + Na₂O (6.22–6.78 wt.%), lower K₂O/Na₂O (0.57–0.96), moderate Zr (170–215 ppm), high Sr (302–475 ppm), and low Mg^# (31–32) contents. The Early Cretaceous granite porphyries contained high SiO₂ (69.68–74.85 wt.%), variable K₂O + Na₂O (4.60–6.99), high K₂O/Na₂O (1.72–23.53), slightly higher Zr (160–255 ppm), variable Sr (25–412 ppm), and very low to intermediate Mg^# (13–44). The granodiorites had intermediate ∑REE (149–177 ppm), while granite samples showed moderate to high ∑REE content (147–271 ppm), and both rocks showed negative Eu anomalies (0.18–0.29). We propose that these two rocks were predominantly generated by the partial melting of Mesoproterozoic metamorphic basement and underwent variable degrees of fractionation and evolution. The Late Jurassic granodiorite porphyry was formed by the partial melting of Mesoproterozoic metamorphic basement with slab-derived melts or basaltic lower crust input following fractional crystallization, while the Early Cretaceous granite porphyries were generated by the partial melting of Mesoproterozoic metamorphic basement and crystal fractionation with variable magma mixing and assimilation in the upper crust. Tectonically, the Wujinshan granitoid formed in a volcanic arc setting largely affected by the subduction and slab rollback of the paleo-Pacific Plate.

1. Introduction

The geological process of magmatism provides valuable insights into the evolution of the lithosphere, the melting of the Earth’s crust, and deep-seated processes [1,2]. The influence of melts derived from slabs has significant implications for the origin and transport of magma, resulting in the formation of diverse rock varieties exhibiting heterogeneous mineralogical and geochemical characteristics [3,4].

Extensive Late Mesozoic (Yanshanian) magmatic activity gave rise to the huge volumes of granites and coeval volcanic rocks in the South China continental region [5,6]. The South China Block (SCB) is well known for the two prolific metallogenic belts, namely the Nanling metallogenic belt (NMB) and the Lower Yangtze River metallogenic belt (LYRB). The NMB is renowned for its extensive W, Sn, Mo, Bi, Be, Nb, and Ta polymetallic deposits, while the LYRB contains more than two hundred Cu, Fe, Au, Mo, Zn, Pb, and Ag polymetallic deposits [6,7]. The South China Block also contains numerous small-scale economically valuable polymetallic ore deposits that are closely associated with Late Mesozoic magmatic rocks, making them ideal geological fields for understanding the mineralization processes, crust-mantle interaction, and tectonic–magmatic evolution, particularly linked to the subduction of the paleo-Pacific Plate, which is believed to be the major trigger for widespread Late Mesozoic magmatic activity in the South China Block.

The region of Northwestern Zhejiang (NWZJ) is situated to the north of the regional NE-trending Jiangshan fault, close to the polymetallic Lower Yangtze River Metallogenic Belt (LYRB). NWZJ covers an area of about 10,000 square kilometers in the southeastern domain of the Yangtze Block. Stratigraphically, it is composed of a folded Proterozoic basement made up of siliceous mudstone, low-grade metamorphosed sandstone, spilite, and andesitic and rhyolitic tuff, and covered with Paleozoic to Mesozoic sedimentary strata comprising sandstone, siltstone, mudstone, and dolomitic and zebra limestone [8]. The geological features of NWZJ are primarily marked by a prevalent occurrence of Late Jurassic to Early Cretaceous (162–121 Ma) granites and their associated mafic rocks. These igneous rocks are found predominantly as stocks and sills, which are structurally controlled by regional faults in the basement that strike to the northeast. The Late Mesozoic granitoid, which are mostly granites, granodiorites, quartz monzogranites, and monzonites, made up to 90% of the total magmatic phases in the region. The granitoid rocks in this region host several economically important ore deposits.

In this study, we present zircon U-Pb ages, Sr-Nd isotopes, and major and trace elements of the Late Jurassic-Early Cretaceous S-type granitoid from the Wujinshan area in Northwest Zhejiang (NWZJ), and discuss two-stage magmatic activity, petrogenesis and tectonic–magmatic dynamics during Late Jurassic to Early Cretaceous (150–140 Ma) in Southeast China.

2. Regional Geological Background

The South China Block (SCB) is a significant tectonic block in East Asia with a complex geological history genetically linked with the merging and breaking up of the two supercontinents Rodinia and Gondwana [9,10,11,12]. SCB was formed by the Neoproterozoic amalgamation of two continental blocks, the northwestern Yangtze Block and the southeastern Cathaysia Block, along the Jiangshan–Shaoxing suture zone [8,10,13,14].

Positioned at the southern margin of Eurasia, SCB shares tectonic boundaries with the North China Craton in the north, the Indo-China Block to the southwest, the Philippine Sea Plate to the east, and the Tibetan Plateau to the west. Based on recent studies [8,15,16,17], the South China Block (SCB) can be categorized into three primary petro-tectonic units, namely the Yangtze block, the Cathaysia block, and the Jiangnan orogenic belt that lies between them (Figure 1a). During its evolutionary history, the SCB experienced multiple tectono-magmatic events: (1) the formation of Jiangnan orogen by the subduction of the Cathaysia block under the Yangtze block and its related magmatism during the early to middle Neoproterozoic [15,16]; (2) a tectonic shift from arc-continent collision to intra-continental regime together with intensive reworking in the early Paleozoic [9,18,19,20], resulting in the formation of large volumes of migmatites and granites [16]; (3) the Indosinian orogeny [5,13] involved the collisions of SCB with North China Craton to the north and the Indochina block to the south [8], giving rise to extensive granitic magmatism with small volumes of syenites [21,22], minor mafic intrusions, dykes, and rare enclaves [23,24,25]; (4) the intense Yanshanian (~200–80 Ma) tectono-magmatic event [26] in the late Mesozoic, caused by the westward subduction of the paleo-Pacific Plate under South China. The Late Mesozoic evolution of South China is predominantly characterized by the widespread (>200,000 km²) magmatic rocks mostly consisting of granitoid (>90%), felsic volcanic rocks, and associated subordinate mafic intrusive and volcanic rocks [5,27]. The intensive Mesozoic tectonic–magmatic events in South China resulted in massive granite emplacements and the formation of abundant associated ore deposits.

3. Local Geological Features of the Wujinshan Area

The Wujinshan ore deposit area is located 30 km to the south of the Lin’an district of Hangzhou City in the NW Zhejiang (Figure 1b,c). The geological features in the study area are briefly described in the following sections.

3.1. Stratigraphy

The Wujinshan ore deposit (Figure 1d) is composed of Ordovician and Silurian strata, which predominantly consist of sandstone, limestone, mudstone, and shale. The Ordovician strata include the Wechang Group, Changwu Group, Huangnigang Group, Hule Group, Yanwashan Group, Ningguo Group, and Yinzhubu Group. The Silurian strata include the Xiaxiang Formation and Helixi Formation. The Yinzhubu and Ningguo groups consist of thick-layered light green–grey silt-stone, silty shale, and zebra limestone. The Huangnigang, Hule, and Yanwashan formations are composed of dark gray–black carbonaceous siliceous shale, dark gray thin-layered striated limestone, and grey mudstone. The Wechang, Xiaxiang, and Helixi formations mainly contain mudstone, silty shale, and sandstone.

3.2. Structure

The geological structure in the Wujinshan deposit area is characterized by NNE–SSW striking faults. The Wujinshan ore deposit area exhibits well-developed faults, which can be divided into three distinct groups: the NE Group, NW Group, and EW Group. The NE Group comprises a pair of faults that span approximately 2.5 km, with the northeastern end intersected by the NW Group. White tungsten mineralization and acidic vein fillings are observed along the fault zones. The NW fault system extends approximately 2 km, and it contains banded quartz, iron, and manganese mineralization. The EW Group is situated in the southern area of the Wujinshan magmatic complex with a length of approximately 2 km. Disseminated quartz veins are present within the fault zones. The emplacement of Wujinshan granitoid and the associated mineralization is significantly influenced by the NE striking faults. The region lacks a comprehensive folding configuration, with only a traction fold or local inversion that can be seen in proximity to fault regions. The geological structure in the region is predominantly characterized by a monocline formation that exhibits a northwestward inclination.

3.3. Igneous Rocks

The felsic magmatic intrusions in the Wujinshan magmatic complex consist of granodiorite, biotite granite, quartz monzonite, and syenite. The major igneous rock type present in the Wujinshan ore deposit region is the porphyritic Hengling granodiorite stock that intruded into Ordovician strata consisting of the Huangnigang, Hule, and Yanwashan formations. The granitic stock of significant size has a total exposed area of 2.24 square kilometers. The mineralization is primarily concentrated in the eastern and southeastern regions of the complex, which are structurally linked with NE-striking faults, strata, and magmatic intrusions. The mineral deposits predominantly exhibit a skarn-associated hydrothermal vein type and are disseminated throughout the complex. A granitic intrusion, exhibiting intense alteration signatures, with a small, elongated shape measuring 200 m in length and 70–85 m in width (equivalent to an area of 0.012 km²) resembling a pig-waist shape, is also found in the southeast of a larger granodioritic intrusion. There are several other small-scale felsic dikes and veins scattered in the complex.

3.4. Petrography of Wujinshan Granites

Ten samples were selected for petrography and geochemical analyses. Among these were four granodiorite porphyry specimens: coarse-grained porphyritic biotite-rich granitic rocks showing a fresh grey hue, transformed into a grey–yellow shade after alteration. The specimens exhibited mineral assemblages consisting of quartz (25%–30%), plagioclase (50%–60%; 0.2–4 mm), poly flake bicrystal partially altered to sericite, alkali feldspar (10%; 0.1–1 mm), biotite (10%–15%) partially altered to chlorite, and accessory muscovite (<2%). Secondary minerals included magnetite, pyrite, chalcopyrite, sphene, and apatite, among others, as depicted (Figure 2a–e).

The high SiO₂ granitic samples showed porphyritic texture with variable grain sizes. Most of the granitic samples were comprised of plagioclase (35%–45%), quartz (30%–40%), biotite (10%), muscovite (<5%) with accessory zircon, apatite, and magnetite. Some of the granite samples showed alteration signatures. The plagioclase phenocrysts were partially altered to sericite and saussurite (Figure 2f,g). The samples from granite dikes exhibited fine-grained to cryptocrystalline felsophyric texture with the occurrence of microstructures of alkali feldspar, plagioclase, and phenocrysts of quartz and biotite (altered to muscovite) (Figure 2h,i).

4. Materials and Methods

4.1. Zircon U-Pb Dating

Two relatively fresh samples of granite (14WJS24) and granodiorite (15WJS29) were selected for U-Pb isotope analyses. The process of zircon separation and preparation was carried out at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences Guiyang, China. The standard heavy-density and magnetic separation protocols were employed to isolate zircons from the bulk samples. The zircons were individually isolated and subsequently subjected to manual selection using a binocular. They were then embedded in epoxy and subjected to a polishing process that reduced their size by half. Cathodoluminescence (CL) imaging in combination with optical microscopy was used to select the least fractured rims of zircons for U-Pb spot analyses.

The zircon U-Pb isotope analyses of these two samples were conducted at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry (Guiyang, China), and the zircon U-Pb dating was performed on a Thermo Fisher Neptune Multicollector ICPMS. The detailed analytical procedures are as follows.

Polished zircon grains were passed through a 193 nm Arfexcimer laser ablation system (GeoLasPro; Coherent, Santa Clara, CA, USA) with a 35–50 µm beam diameter, and the ablation process was conducted at a constant laser energy of 14 J/cm² at a frequency of 10 Hz. Subsequently, the treated zircons were introduced into the Neptune Multicollector system utilizing He as a carrier gas, and concurrent data acquisition was guaranteed through a static mode operation. Before the laser was connected, an assessment of the Neptune Multicollector system was conducted using standard analysis of Pb, Th, and U, and the parameters of the lens were optimized. The NIST SRM981 Pb standard was utilized to calibrate the voltage of the ion counter, and subsequently, the Faraday cup and ion counter were cross-corrected to ensure optimal counting efficiency within a range of 80%–90%. The ion counter’s linear stability was calibrated using a standard solution with a known ²⁰⁷Pb/²⁰⁶Pb ratio. TEMORA (²⁰⁶Pb/²³⁸U) age of 417 Ma [29] was employed as the external zircon age standard, while the sample content was ascertained using NIST 612 glass standard. The duration of the signal acquisition was 60 s, with a preceding blank determination of 20 s that was consistently conducted before each standard sample analysis. Ratios for instrument mass bias and isotope and elemental fraction were corrected by implying external standards. The weighted age calculations and Concordia diagram plotting were performed in Isoplot 4.15 software.

4.2. Major and Trace Element Analyses

Ten samples from the Wujinshan magmatic complex, including four granodiorites and six granites, were selected for the whole-rock geochemical analyses. The selected samples were carefully crushed in a jaw crusher and ground into powder in an agate mill. The powdered samples were then subjected to 200 µm sieves to ensure a uniform sample grain size distribution. The agate mills and sieves were cleaned with ethanol before and after the processing of each sample to prevent the mixing of impurities. The whole-rock major geochemical analyses were conducted at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry (Guiyang, China). The powdered samples were dried out in an electric oven at 100 °C for 3 h and kept in a dryer, and the burning loss was analyzed after 8 h. The dried powder of 1.3 to 1.5 g was placed in a ceramic crucible, and both the sample and crucible were weighed by a high-precision electric balance. The sample was placed in a high-temperature furnace and heated to 900 °C for 1 h, cooled for 3–4 h, and weighed. The XRF glass cake was made and placed in a plastic bottle with a 4.0 g Li₂B₄O₇ solution and mixed thoroughly, and a 0.4 g solution with 1% LiBr and 0.5% NH₄I was added. The sample was placed in a platinum crucible and burned at 1250 °C. The major element compositions were analyzed by X-ray fluorescence (XRF) spectrometer (Axios PW4400, PANalytical B.V., Almelo, The Netherlands).

The determination of trace elements, notably rare earth elements (REE), was executed utilizing inductively coupled plasma mass spectrometer (Agilent 7700x ICP-MS instrument, Santa Clara, CA, USA ). The specimens, comprising 50 mg rock powder, were dissolved in stainless steel bombs using a mixture of HF + HNO₃ and subjected to heat at ~190 °C for 48 h. The international standards BHVO-2, AGV-2, and BCR-2 were subjected to the analysis for data quality evaluation. Additionally, the internal Rh standard was employed to monitor signal drift. The precision of trace element data acquired via ICP-MS was validated to be within an accuracy of ±10% substantiated by duplicate analyses. The analytical procedures were the same as per expounded in the work of Qi et al. [30].

4.3. Sr-Nd Isotopic Analyses

Two granodiorite samples 14WJS17 and 15WJS29 were selected for Sr-Nd isotopic analyses. The whole-rock Sr-Nd isotopic analyses were performed at State Key Laboratory for Mineral Deposits Research, Nanjing University, China, using a Finnigan MAT 262 thermal ionization mass spectrometer (TIMS). The Sr and Nd fractions were standardized by the normalization values of ⁸⁶Sr/⁸⁸Sr = 0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219. A 5n repetition of analyses yielded ⁸⁷Sr/⁸⁶Sr ratio of 0.710148 ± 0.000013 (2σ) for the external standard NBS987 Sr and ¹⁴³Nd/¹⁴⁴Nd ratio of 0.512284 ± 0.000014 (2σ) for the JNdi-1 standard. The specific analytical procedures for the measured Sr-Nd isotopes were the same as described in Miyazaki and Shuto [31].

5. Results

5.1. Zircon U-Pb Ages

According to the zircon U-Pb dating results (

Table 1. Results of zircon U-Pb LA-ICP-MS age of the granitic rocks in the Wujinshan ore-deposit area.

Samples	Contents (ppm)			Th/U	Isotopic Ratios						Age (Ma)
	Pb	Th	U		206Pb/238U	1σ	207Pb/235U	1σ	207Pb/206Pb	1σ	206Pb/238U	1σ	207Pb/235U	1σ	207Pb/206Pb	1σ
15WJS29
3	89.0	384	2402	0.16	0.0227	0.0003	0.1538	0.0079	0.0496	0.0027	145	2.2	145	6.9	176	128
4	52.4	332	732	0.45	0.0227	0.0005	0.1604	0.0117	0.0552	0.0050	145	3.1	151	10.3	417	199
5	36.7	279	640	0.44	0.0229	0.0005	0.1623	0.0166	0.0504	0.0056	146	3.2	153	14.5	217	237
6	78	499	867	0.57	0.0231	0.0005	0.1616	0.0154	0.0534	0.0057	147	3.2	152	13.5	343	246
7	49.9	323	596	0.54	0.0234	0.0005	0.1558	0.0126	0.0522	0.0050	149	3.3	147	11.1	300	220
8	62.0	395	898	0.44	0.0234	0.0006	0.1528	0.0166	0.0507	0.0061	149	3.6	144	14.6	228	256
9	78	524	1003	0.52	0.0234	0.0004	0.1567	0.0105	0.0508	0.0040	149	2.8	148	9.2	232	216
10	67.5	418	835	0.50	0.0240	0.0005	0.1625	0.0113	0.0514	0.0045	153	2.9	153	9.9	257	200
11	52.9	305	690	0.44	0.0228	0.0005	0.1453	0.0123	0.0479	0.0048	145	3.2	138	10.9	100	222
12	39.5	236	609	0.39	0.0242	0.0006	0.1713	0.0140	0.0562	0.0063	154	3.8	161	12.1	461	255
13	49.4	307	647	0.47	0.0238	0.0005	0.1634	0.0113	0.0527	0.0048	151	3.3	154	9.9	317	212
16	82	518	865	0.60	0.0233	0.0005	0.1639	0.0118	0.0534	0.0044	149	3.0	154	10.3	346	189
17	76	486	893	0.54	0.0232	0.0005	0.1626	0.0119	0.0524	0.0043	148	2.9	153	10.4	302	191
18	80	531	932	0.57	0.0230	0.0005	0.1453	0.0114	0.0474	0.0040	146	2.9	138	10.1	77.9	180
19	62.7	394	916	0.43	0.0223	0.0004	0.1589	0.0113	0.0524	0.0041	142	2.7	150	9.9	302	184
20	52.7	273	437	0.63	0.0248	0.0007	0.2954	0.0300	0.0926	0.0096	158	4.6	263	23.5	1480	194
Inherited zircons
1	509	708	1160	0.61	0.0811	0.0014	0.6523	0.0331	0.0577	0.0031	502	8.4	510	20.4	517	117
2	467	692	475	1.46	0.0855	0.0017	0.8211	0.0537	0.0681	0.0047	529	9.9	609	29.9	872	144
14	160	330	663	0.50	0.0622	0.0014	0.5011	0.0290	0.0577	0.0034	389	8.4	412	19.6	520	130
15	254	276	624	0.44	0.1300	0.0021	1.0602	0.0476	0.0592	0.0028	788	12.1	734	23.5	576	99.1
14WJS24
1	11.3	196.1	403.3	0.49	0.0222	0.0003	0.1483	0.0046	0.0491	0.0016	142	1.7	140	4.0	154	79.6
2	5.7	102.6	223.4	0.46	0.0220	0.0003	0.1681	0.0070	0.0564	0.0025	140	2.0	158	6.1	478	100
4	5.4	82.6	222.1	0.37	0.0218	0.0005	0.1573	0.0092	0.0539	0.0032	139	3.4	148	8.0	369	132.4
5	7.2	109.9	290.5	0.38	0.0222	0.0004	0.1424	0.0060	0.0473	0.0022	142	2.4	135	5.4	65	107.4
6	4.3	66.9	168.7	0.40	0.0226	0.0004	0.1440	0.0091	0.0476	0.0034	144	2.7	137	8.1	80	159.2
7	7.4	91.0	268.8	0.34	0.0218	0.0005	0.1882	0.0142	0.0630	0.0045	139	3.2	175	12.1	709	153.7
9	11.0	205.9	418.9	0.49	0.0222	0.0003	0.1509	0.0056	0.0500	0.0019	142	2.1	143	4.9	195	88.9
10	9.2	183.6	360.5	0.51	0.0221	0.0004	0.1515	0.0060	0.0507	0.0021	141	2.2	143	5.3	233	94.4
11	8.2	79.2	226.1	0.35	0.0217	0.0005	0.3015	0.0254	0.0972	0.0071	138	3.2	268	19.8	1572	136.9
12	6.7	82.6	235.2	0.35	0.0227	0.0004	0.1579	0.0083	0.0496	0.0029	145	2.8	149	7.2	176	137
Inherited zircon
3	7.7	62.4	54.2	1.15	0.0755	0.0017	0.6779	0.0342	0.0654	0.0031	469	10.4	525	20.7	789	100

), Wujinshan magmatic complex represents two-stage granitic magmatic activity. The granodiorite intrusion was emplaced during the Late Jurassic (146.4 ± 1.5 Ma), while the high SiO₂ granite shows the emplacement age of the Early Cretaceous (141.9 ± 1.4 Ma). The first magmatic pulse, which is predominantly a biotite-rich granodiorite porphyry, was intruded in the Paleozoic strata and formed a stock, while the later muscovite-bearing granite porphyries were intruded in the form of sills and dykes.

Most of the zircons were euhedral, colorless, transparent, and short prismatic grains, having a maximum length of 100 µm with a length-to-width ratio of 3:1. In the Cathodoluminescence images, the dated zircons showed distinct oscillatory zoning, and most of the zircons possessed inherited cores, indicating a composite magmatic origin. The calculated Th/U ratios of the dated zircons showed a range of 0.16 and 1.46, with most of the ratios concentrated between 0.30 and 0.50, indicating a magmatic origin. The samples yielded a wide range of ²⁰⁸Pb/²³⁸U ages. A significantly large number of zircons also showed greater ages than the calculated weighted mean ages. In this study, only concordant or nearly concordant (below 150 Ma) values were used to calculate and plot the weighted mean ages.

Twenty zircon spot analyses were performed on granodiorite sample 15WJS29. Twelve zircons yielded a concordant weighted average age of 146.4 ± 1.5 Ma (MSWD = 2.6, Figure 3a). The weighted mean age was calculated using only the concordant or nearly concordant values. Five inherited zircons showed ²⁰⁶Pb/²³⁸U ages of 788 Ma, 529 Ma, 502 Ma, 469 Ma, and 389 Ma.

For the granitic sample 14WJS24, eleven zircon grains were spotted and targeted for LA-ICPMS U-Pb analyses (Table 1). Seven zircons show a close cluster near the concordia line, yielding a ²⁰⁶Pb/²³⁸U weighted average age of 141.9 ± 1.8 Ma (MSWD = 1.8, Figure 3b). One zircon shows an inherited ²⁰⁶Pb/²³⁸U age of 469 Ma.

5.2. Major and Trace Elements

The result data for major and trace element analyses of Wujinshan granitoid are presented in

Table 2. Representative major, trace elements, and Sr-Nd isotopes of Wujinshan granitoids.

Sample	14WJS16	14WJS17	15WJS04	15WJS29	14WJS27	15WJS26	14WJS22	14WJS24	15WJS14	15WJS15
	Granodiorite Porphyry				Granite Porphyries
SiO2 (wt.%)	64.70	64.38	67.89	67.40	73.22	69.68	70.23	72.60	74.85	70.25
TiO2	0.42	0.47	0.38	0.40	0.34	0.21	0.40	0.27	0.18	0.46
Al2O3	15.25	16.09	15.63	15.46	16.17	14.15	14.74	15.52	14.79	15.76
Fe2O3	4.22	4.95	3.96	4.21	2.17	2.17	3.33	1.54	1.27	3.57
FeOT	3.80	4.45	3.57	3.78	1.95	1.95	2.99	1.39	1.14	3.21
MnO	0.06	0.06	0.05	0.07	0.01	0.03	0.08	0.02	0.02	0.08
MgO	0.99	1.10	0.92	0.93	0.57	0.65	0.33	0.60	0.46	0.28
CaO	3.48	4.07	2.86	3.49	0.13	2.75	0.32	0.27	0.12	0.19
Na2O	3.18	4.24	3.71	3.58	0.36	2.25	2.46	1.88	0.18	2.57
K2O	3.05	2.40	3.07	3.14	4.24	3.57	4.23	3.89	4.33	4.42
P2O5	0.17	0.21	0.16	0.17	0.09	0.12	0.17	0.14	0.02	0.17
LOI	3.65	1.64	1.25	1.20	2.84	3.88	2.34	2.06	2.38	2.07
Total	100.6	99.92	101.2	101.1	100.2	100.6	99.71	98.86	98.67	99.85
Ga (ppm)	20.0	23.2	21.0	21.0	23.1	20.0	19.5	18.4	19.9	16.1
Rb	127	76.6	72.0	116	236	92.8	159	168	170	164
Sr	303	476	457	379	25.8	413	126	100	38.9	197
Y	18.8	19.1	19.2	20.1	9.61	19.6	43.6	26.9	25.4	20.9
Zr	170	215	198	210	193	178	223	206	255	160
Nb	11.2	9.76	10.3	12.4	12.0	11.3	15.7	12.5	16.0	10.7
Cs	6.21	3.84	5.18	4.03	17.26	3.93	7.14	14.84	8.84	16.18
Ba	582	618	683	713	327	654	7111	505	285	555
La	25.9	26.0	34.0	31.5	36.5	25.4	49.1	42.8	46.6	28.1
Ce	53.3	51.6	65.1	61.8	62.7	51.2	86.5	82.7	90.6	56.8
Pr	6.67	6.78	8.12	7.76	8.65	6.49	11.7	10.3	10.9	6.76
Nd	25.2	25.4	29.6	28.0	30.1	24.5	42.2	37.0	38.0	23.9
Sm	4.97	4.97	5.43	5.31	5.12	4.96	8.43	6.35	6.92	4.30
Eu	1.24	1.44	1.45	1.32	1.32	1.27	2.26	1.11	1.29	0.96
Gd	4.65	4.73	5.15	5.10	4.12	4.74	8.54	5.87	6.36	3.95
Tb	0.73	0.74	0.82	0.78	0.63	0.76	1.36	0.91	0.94	0.64
Dy	3.47	3.60	3.74	3.76	2.20	3.67	7.07	4.47	4.78	3.35
Ho	0.72	0.73	0.81	0.77	0.53	0.77	1.51	1.02	0.98	0.77
Er	1.70	1.78	1.77	1.84	1.01	1.79	3.87	2.58	2.56	2.07
Yb	1.56	1.62	1.62	1.74	0.91	1.65	3.69	2.82	2.50	2.33
Lu	0.30	0.31	0.35	0.33	0.26	0.32	0.67	0.54	0.44	0.43
Hf	4.84	5.73	5.20	5.71	5.28	4.98	6.07	5.51	6.68	4.53
Ta	0.94	0.80	0.79	1.06	1.05	0.95	1.42	1.00	1.33	0.89
Th	9.07	8.20	8.66	10.4	11.1	8.89	18.1	9.97	17.3	8.58
U	2.85	2.45	2.20	2.90	2.82	2.65	4.44	2.71	4.28	2.73
A/CNK	1.30	1.21	1.30	1.24	3.05	1.40	1.65	2.06	2.89	1.72
σ	1.79	2.06	1.85	1.85	0.70	1.27	1.64	1.12	0.64	1.79
Mg #	0.32	0.31	0.32	0.31	0.34	0.37	0.16	0.44	0.42	0.13
ΣREE	150	149	177	171	164	147	271	226	239	156
Eu/Eu*	4.81	4.85	5.28	5.21	4.59	4.85	8.48	6.11	6.64	4.12
Zr+Nb+Ce+Y	179	226	207	219	197	184	227	209	258	164
87Rb/86Sr		0.465900		0.886500
87Sr/86Sr		0.708487		0.710408
±2σ		0.000006		0.000006
(87Sr/86Sr)i		0.707500		0.708600
147Sm/144Nd		0.009056		0.007136
143Nd/144Nd		0.512300		0.512300
±2σ		0.000005		0.000005
εNd(t)		−3.5		−2.9
TDM1 (Ga)		0.58		0.58
TDM2 (Ga)		1.21		1.17

Notes: Fe₂O₃^T as total iron, LOI for loss on ignition; Eu/Eu* = Eu_N/$\surd$(Sm_N * Gd_N), chondrite and primitive mantle normalization values from [34]; Mg^# = (MgO/40.31)/(MgO/40.31 + Fe₂O₃^T/79.85); A/CNK = molecular Al₂O₃/(CaO + Na₂O + K₂O); σ = (Na₂O + K₂O)/(SiO₂ − 43).

. The samples from Wujinshan magmatic complex show SiO₂ contents ranging from 64.3–74.8 wt.%, being plotted in the granodiorite and granite fields of the TAS diagram (Figure 4a). Most of the samples were enriched in K₂O and plotted within the high-K calc-alkaline series on a SiO₂ vs. K₂O diagram (Figure 4b). Significant geochemical differences were identified between the two granitic rocks. The granodiorite porphyry samples had higher values of Fe₂O₃^T (3.9–4.9 wt.%), MgO (0.92–1.10 wt.%), CaO (2.87–4.07 wt.%), and Na₂O (3.18–4.24 wt.%), and lower A/CNK ratios (1.21–1.30), while the silica-rich granite porphyries possessed comparatively low contents of Fe₂O₃^T (1.27–3.57 wt.%), MgO (0.28–0.65 wt.%), CaO (0.12–2.78 wt.%), and Na₂O (0.18–2.57 wt.%), with relatively high A/CNK (1.4–3.05) compositions.

In the Harker variation diagrams (Figure 5a–f), both rocks show significant compositional variations and a certain degree of fractionation. The granodiorite rocks are moderately peraluminous with A/CNK ratios (1.21–1.30), while the granites are moderately to strongly peraluminous A/CNK (1.4–3.05) and ASI (1.42–3.11) (Figure 6a,b).

Both the Late Jurassic granodiorite and Early Cretaceous granite samples show high ∑REE (147–271 ppm), low to intermediate Mg^# (16–44), and moderate negative Eu-anomalies (Eu/Eu* = 4.58–8.48) in chondrite-normalized REE patterns (Figure 7a). In the primitive mantle-normalized spidergram (Figure 7b), these rocks show significant depletion in high field strength elements (e.g., Nb, Ta) except Zr, which shows no significant depletion. The Early Cretaceous granite porphyry shows moderate Ba and strongly negative Sr anomalies.

Both rocks show positive anomalies in large ion lithophile elements (e.g., Rb, Th, U, and LREE). These typical chondrite-normalized REE and primitive mantle-normalized trace element patterns are indicative of their geochemical resemblance with volcanic arc granites [37,38].

5.3. Bulk-Rock Sr-Nd Isotopes

Sr-Nd isotope analytic results are presented in Table 2. The εNd(t) values for samples were calculated using their representative ²⁰⁶Pb/²³⁸U ages. The samples 14WJS17 and 15WJS29 had ⁸⁷Sr/⁸⁶Sr ratios of 0.708487 and 0.710408 yielding Isr ratios of 0.707526 and 0.708551with lower εNd(t) values of −3.54 and −2.95, respectively. These samples have T_DM1 ages of 643 Ma and 620 Ma, and T_DM2 ages of 1212 Ma and 1168 Ma.

6. Discussion

6.1. Timing of Magmatism

In the NW Zhejiang, most of the small individual granitic stocks that were intruded during the Late Mesozoic are genetically associated with ore deposits. The genesis of these granitic rocks and the associated ore deposits has been well-documented by some researchers [39,40,41,42]. The occurrence of well-preserved granitic rocks from the Jurassic–Cretaceous period makes the NW Zhejiang area an ideal geological field to study the tectonic–magmatic dynamics and genesis of the ore-forming process during Late Mesozoic in the region.

The magmatism during the Late Jurassic and Early Cretaceous was predominant in the NW Zhejiang and the neighboring areas of Jiangxi and Anhui provinces [26], which largely generated S- and I-type granites [43]. In the Zhejiang area, the early Yanshanian magmatism is more prevalent in the northwestern region, while the southeastern part is characterized as the Late Yanshanian volcanic–intrusive domain. During the early Yanshanian (165–135 Ma) period, the magmatism in the Zhejiang province was limited to the small volumes of igneous rocks that mainly occurred in the form of stocks and sills structurally affected by the NE-striking regional basement faults. The spatial–temporal distribution patterns of Late Mesozoic granitic and volcanic rocks and associated ore deposits are presented by researchers [44,45,46].

The zircon LA-ICP-MS U-Pb isotope dating of the analyzed samples of Wujinshan granitoid indicate two emplacement ages: the Late Jurassic (146.4 ± 1.5 Ma) for granodiorite porphyry and Early Cretaceous (141.9 ± 1.4 Ma) for granite porphyries.

The two magmatic ages represent two-stage magmatism in the Wujinshan area. These granitic stocks intruded into the Ordovician strata in two magmatic pulses. The first magmatic pulse, which predominantly has a granodioritic composition, was emplaced at 146.4 ± 1.5 Ma, while the second magmatic pulse, which predominantly contains small intrusions and dykes of high SiO₂ granite porphyries, was emplaced in the Early Cretaceous (141.9 ± 1.4 Ma). The small granitic intrusion in the Wujinshan magmatic complex show geochemical likeness to the Early Cretaceous granitic dike, although there have been no geochronological data till now. We propose that the granitic dikes and small intrusion are related to the same Early Cretaceous magmatic pulse and both the magmatic bodies of Wujinshan S-type granitoid were emplaced in the Early Yanshanian magmatic stage [47]. Although separated by a 4 Ma of geological time, considering the timing of emplacement of worldwide stocks and plutons [48], it is likely possible that these two magmatic pulses contemporaneously belong to a single magmatic event.

The presence of inherited zircons signifies that the Wujinshan magmatic complex might offer valuable insights into the chronological sequence of geological events that contributed to the evolution of the South China Block. Some researchers have previously reported inherited zircons with ages ranging from 776.0–772.0 Ma and 797.0–662.0 Ma in the Jiaokengkou and Tongcun granite porphyries located in Northwestern Zhejiang [26,49]. The present study reveals that one out of five inherited zircons, with a U-Pb age of 788 Ma, show a likely genetic link with the amalgamation and breaking up of the Rodinia supercontinent, followed by the Neoproterozoic amalgamation of the Yangtze and Cathaysia continental blocks to form the South China craton [50]. Two inherited zircons with U-Pb isotope ages of 469 and 389 Ma probably represent the Caledonian intracontinental collision-related magmatism, crustal thickening, and metamorphic events [19,51]. The temporal–spatial distribution patterns indicate that Wujinshan granites have a likely genetic relationship with the other Early Yanshanian granitoid and the associated felsic rocks in the Northwestern Zhejiang and other regions of the South China Block.

6.2. Petrogenesis

6.2.1. Petrogenetic Classification of Wujinshan Granite

Granite is one of the most abundant igneous rocks present in the continental crust and a major contributing rock to continental growth and reworking. The two most common I-type and S-type granites were first defined by Chappell and White [52], and the classification is merely based on their origin/source rock. The two granitic types originated from different sources, namely meta-igneous and meta-sedimentary sources. I-type granites originate from the melting of meta-igneous rocks, primarily due to the breakdown of hornblende at high temperatures, while the S-type granites form from the melting of metasedimentary source materials, primarily as a result of dehydration reactions of biotite or muscovite [53]. Various petrological and geochemical parameters can be used to differentiate these two types of granites [54].

The analyzed Wujinshan granodiorite and granite samples were peraluminous, with higher A/CNK ratios (1.0–3.0) (Figure 6a) and K₂O/Na₂O values of (4.5–10.4), much higher than typical peraluminous or strongly peraluminous I-type granites (Figure 6a,b), which commonly have low K₂O/Na₂O ratios (<1.0), unlike conventional S-type granites that usually contain high K₂O/Na₂O ratios (>1.0) [53,55,56,57]. The A/CNK values in combination with K₂O/Na₂O ratios can effectively be used for I- and S-type granite distinction [57]. In the Nb versus 10,000 × Ga/Al diagram (Figure 8a), most of the samples are plotted in I- and S-type granite fields. In the I- and S-type granite discrimination ternary plot (Al₂O₃ − (K₂O + Na₂O) vs. CaO vs. (FeO^T + MgO), all the samples are plotted in an S-type granite field (Figure 8b).

The S-type granitic rocks were eminent for containing higher concentrations of Al than Na, Ca, and K in the rock, which is a condition required to form feldspars, primarily because of their origin from the previously altered source [59]. These rocks were also characterized by high molar Al₂O₃/(Na₂O + K₂O + CaO) (>1.1), CIPW normative corundum (>1%), and irregular variation diagrams [60].

The Wujinshan granitoid rocks contained more Al, ranging from 0.24 to 0.31 with an average of 0.30, while Na, Ca, and K possessed average values of 0.07, 0.06, and 0.03, respectively. These rocks contained molar Al₂O₃/(Na₂O + K₂O + CaO) > 1.1%, CIPW normative corundum > 1%, irregular variation diagrams, and inherited zircons, establishing their affinity to the S-type granites. All the samples are plotted in the S-type granitic field in the modified alkali lime index vs. SiO₂ diagram (Figure 6d) [36]. Given the presence of primary muscovite and some inherited zircons in combination with the above-mentioned geochemical features, we propose that the Wujinshan granitoid suite is predominantly composed of S-type granites.

6.2.2. Potential Magma Source for Wujinshan S-Type Granites

The distinction between I- and S-type granites is indicative of their derivation either from meta-igneous (I-type) or metasedimentary (S-type) source rocks [52]. The analyzed Wujinshan granitic samples possessed moderately high Ga/Al ratios (1.93–2.72) and refractory element (Zr + Nb + Ce + Y) contents of 163–257 ppm. However, they contained moderately low FeO^T/MgO ratios (2.30–4.05), except for two samples, 14WJS22 and 15WJS15, which had high FeO^T/MgO ratios of 9.09 and 11.67, respectively. It is suggested that the source rocks for the Wujinshan S-type granitoid probably underwent a prior alteration at the surface or near the surface of the Earth. Experimental studies have provided evidence for variable melt generation through the dehydration melting of heterogeneous sedimentary protoliths [61]. Based on sedimentary protolith, we can subdivide the analyzed Wujinshan S-type granitoid samples into two types. The Late Jurassic granodiorite porphyry (146.4 Ma) had low Al₂O₃/TiO₂ (34.53–41.02) and Rb/Sr (0.15–0.41), and high CaO/Na₂O (0.77–1.09) ratios, resembling the peraluminous granites in the Lachlan Fold Belt [52], and these particular geochemical characteristics suggested a dominant meta-greywacke source with possible basaltic input from the slab-derived melts (Figure 9a,b) at a high formation temperature (Figure 10a).

Hence, we propose that the Late Jurassic S-type porphyritic granodiorite was derived from the partial melting of the Mesoproterozoic metamorphic basement, with slab-derived melt input following subsequent fractional crystallization (Figure 10b).

The Early Cretaceous granite (141.9 Ma) had high Al₂O₃/TiO₂ ratios (34.20–82.14), a wide-range Rb/Sr (0.22–9.13), and low CaO/Na₂O (0.07–0.60), except one sample, 15WJS26, which had a higher CaO/Na₂O ratio (1.12). These geochemical features suggest that the Early Cretaceous high SiO₂ granite porphyries were predominantly formed from meta-pelite sources with a certain meta-greywacke contribution (Figure 10a,b). The geochemical variations in granite samples imply that the source was previously metamorphosed. A wide range of Mg^# (0.13–0.44) values suggest that these rocks are more evolved. The presence of moderately negative Eu and Ba and strongly negative Sr anomalies indicates plagioclase and K-feldspar fractionation [64]. This observation leads us to propose that the Early Cretaceous granitic porphyries were formed by various degrees of fractionation. Hence, the formation of Early Cretaceous porphyries was mainly controlled by dominant plagioclase and minor alkali feldspar fractionation with upper crust contamination at variable formation temperatures. (Figure 10a,b).

The melts generated through the basaltic lower continental crust commonly have low Mg^# values (<40), but those with higher Mg^# values (>40) are necessarily generated through mantle input [65]. Both the granodiorite and granite show relatively low Mg^# values, implying that they are predominantly derived from crustal melts.

The ⁸⁷Sr/⁸⁶Sr ratios of Early Yanshanian (165–135 Ma) magmatic rocks in Zhejiang province suggest a continental crust dominance in the origin of parental magma [47], and the calculated positive and negative εNd(t) values indicate crust–mantle interaction or heterogeneous magma source [66]. From the geochemical characteristics in combination with isotopic evidence, it is speculated that the early Yanshanian magmatic rocks, which are mostly granodiorites and porphyry granites, are derived from continental crust together with depleted mantle input [47].

The average initial ⁸⁷Sr/⁸⁶Sr ratio of (0.7033) for the mantle is relatively low as compared to the higher Isr value for the crust [67]. The Late Jurassic granodiorite porphyry had relatively high (⁸⁷Sr/⁸⁶Sr)i ratios (0.7075–0.7086), corresponding to the εNd(t) values (−3.48~−2.94) and T_DM2 model ages (1.2~1.16 Ga). These isotope features indicate a crustal source in the generation of the granodiorite porphyry. Hence, in the (⁸⁷Sr/⁸⁶Sr)_i versus εNd(t) diagram (Figure 11), these samples are plotted in a transitional zone between crust-derived melts and mantle array, suggesting that the magma was affected by slab-derived meta-basaltic materials.

6.3. Tectono-Magmatic Dynamics for Wujinshan S-Type Granitoid

S-type granites are frequently associated with continental collisional and intraplate orogeny [53,68], and are also considered to be formed as a result of tectonic transition from subduction to post-subduction. Furthermore, the presence of a meta-sedimentary source within the lower crust is crucial for S-type granite formation [69].

The Wujinshan S-type granitoids fall in the high-K calc-alkaline series (Figure 4b), and all the samples are placed in the volcanic arc field in the tectonic discrimination plots (Figure 12a,b). These typical features suggest that these granitic porphyries are formed under subduction-related tectonic settings. The early Yanshanian magmatic rocks were formed under the subduction-induced island arc environment caused by paleo-Pacific Plate subduction, with the significant input of oceanic crust melts.

The high-K calc-alkaline rocks in convergent plate margins are mostly formed by two processes: the mixing of crustal material with the slab-derived fluid-enriched mantle magma [72,73], and the melting of crustal rocks through the decompression caused by lithospheric delamination and slab break-off in the syn-collisional or post-collisional settings [74,75]. The I-type granites are produced by the mixing of crust and mantle-derived magmas, and generally represent a crust–mantle interaction, while the S-type granites are mostly formed from the crustal melts generated through the compressional heat in combination with the fluids derived from the dehydration reactions, mostly linked to the collisional orogeny [47]. During the Late Jurassic period, the volumetric granitic magma in the volcanic arc setting predominantly has a crustal source, possibly related to the lithosphere underplating and crustal remelting.

The magmatism and tectonic transition of South China during the Late Mesozoic is a subject of extensive debate, and various tectonic models have been presented. The popular tectonic models for Late Mesozoic magmatism in South China are: the formation of Andean-type active continental margin by the northwestward paleo-Pacific Plate subduction [5,28,76], paleo-Pacific Plate subduction accompanied by crustal remelting [77], back-arc extension or crustal thinning as a result of oblique subduction of paleo-Pacific Plate [21], rapid slab rollback [78], intracontinental magmatism triggered by mantle upwelling in extensional tectonic settings [79], intracontinental subduction orogeny [80], and multi-plate convergence [22,79].

It is worth noting that the Jurassic (Early Yanshanian) granites and volcanic rocks are widely distributed in the southwestern and central parts of the South China Block, but the Cretaceous and Late Cretaceous magmatism is concentrated along the northern and southeastern parts of the South China Block. This magmatic shift is linked with a tectonic transition from a compressional to an extensional regime caused by a slab rollback of the paleo-Pacific Plate. The exact age of this tectonic transition and the onset of slab rollback is debatable. However, it is believed that the Late Jurassic (170–145 Ma) was a period of inland compression coupled with paleo-Pacific Plate subduction, which predominantly generated I- and S-type granites, while the Early Cretaceous (145–120 Ma) is generally characterized by large-scale A-type granite magmatism in a continental extension environment due to lithospheric thinning accompanied by slab rollback and continental collisions [26]. Recent studies suggested that the NW Zhejiang was part of an active continental magmatic arc during 165–150 Ma, and the tectonic transition from an active continental margin to a back-arc extension occurred during 150–135 Ma due to slab rollback of the paleo-Pacific Plate [46,47]. Different geological age ranges have been proposed for the tectonic transition and onset of slab rollback. Some researchers propose that this tectonic transition and slab rollback occurred during the Late Jurassic to Early Cretaceous [26,46,47], while others think it started in the Early Cretaceous after 145 Ma [28,45,81]. We propose that the Wujinshan granitoid porphyries were formed in a volcanic arc tectonic setting largely affected by slab rollback of the subducting paleo-Pacific Plate (Figure 13).

6.4. Hydrothermal Alterations

The intensive chemical weathering and hydrothermal alterations made it extremely difficult to determine the lithologies and the field geological features at most of the places in the study area. However, by combining the lithological characteristics of the less altered sections with drilled samples, we can roughly divide the study area into two types of granitoid bodies of the porphyritic granodioritic stock (Hangling granodiorite) and porphyry type biotite granite. Considering the observed patterns of alterations, it is crucial to discuss the degree and role of chemical weathering and alterations to understand the granite interaction with the wall rocks and the hydrothermal fluids, ore-forming conditions, and processes.

Magmatic fluids with high temperatures (>350 °C) in the earlier magmatic stage react with the host rocks, resulting in the forming of sodic–calcic and potassic alteration zones in the core of porphyry systems [82,83]. During the advanced phases of magmatism, the magmatic fluids intermix with meteoric waters and interact with the wall rocks, causing sericite alteration at reduced temperatures [84]. The alteration zones are formed when the primary magmatic minerals undergo hydrothermal alteration and are subsequently replaced by the secondary alteration minerals, such as biotite, chlorite, sericite, calcite, actinolite, epidote, kaolinite, and magnetite [83]. Numerous geochemical methods have been proposed in previous studies to understand and evaluate the extent of chemical alteration in various rock types [85,86,87,88].

The hydrothermal alteration generally occurs where the ore bodies come in contact with the wall rocks, resulting in the development of massive veinlet-disseminated stockwork mineralization in these contact zones. By using molar ratios, it becomes possible to compare the compositions of altered rocks with hydrothermal minerals, allowing the evaluation of geochemical trends with the alteration mineralogy to evaluate mineral zonation [87]. The (2Ca + Na + K)/Al vs. K/Al molar ratios plot (Figure 14a) provides the graphical means to assess the degree of K-metasomatism, K, Ca, and Na reduction and Ca-metasomatism that affects the altered rocks [87,89]. The Late Jurassic granodiorites are plotted between 0.8–1.0 molar 2(Ca + Na + K/Al) in the x-axis and 0.1–0.2 K/Al field in the x-axis close to the fresh-rock reference line, thus showing insignificant alteration and metasomatism. The Early Cretaceous granites are placed above and to the left side of fresh-rock compositions, representing the K gain and the Na and Ca loss. These geochemical trends are attributed to the destruction of primary plagioclase and its replacement by K-bearing phases as a result of K-metasomatism [87]. The Early Cretaceous granites showed significant plagioclase replacement with the K-bearing secondary minerals, such as K-mica (illite) and mixed-layer illite-smectite.

To further determine the degree of alteration in the Wujinshan granites, we use the statistical empirical index (MFW diagram, Figure 14b) [86]. The MFW diagram is based on eight major oxides, and it can effectively evaluate the degree of alterations. The M and F apices in the MFW ternary plot indicate the fresh, unaltered mafic and felsic rocks, while the W is the weathered end product of these rocks. The formation of secondary minerals and the loss of primary minerals are both represented by an increasing W value. The calculated W values (0.5–5.8) for Wujinshan granite samples indicate their intensive degree of alteration/weathering, and these samples fall in extensively weathered soil fields in the plot. On the other hand, the granodiorite samples show a low degree of alterations with W values of (0.45–0.52).

Minor alteration patterns in the granodiorite porphyry samples were observed. The plagioclase phenocrysts showed partial sericite alteration in the core and the outer edges, and the biotite was partially altered to chlorite, whereas in the high SiO₂ granite porphyries, some samples showed significant plagioclase alteration to sericite, saussurite, and K-bearing secondary minerals, and the biotite was altered to muscovite.

7. Conclusions

Based on the above study, the conclusions are as follows.

The zircon U-Pb dating of granitic rocks from the Wujnishan magmatic complex indicates two emplacement ages, the Late Jurassic (146.4 ± 1.5 Ma) age for granodiorite porphyry and the Early Cretaceous (141.9 ± 1.4 Ma) for high SiO₂ granite porphyries. These two magmatic pulses are coeval and related to a single composite magmatic event.

The two granitoid porphyries are predominantly S-type granites generated primarily from Mesoproterozoic basement with variable degrees of evolution in upper crust during magma ascent. The Late Jurassic granodiorite was formed by the partial melting of the Mesoproterozoic metamorphic basement with a certain slab-derived melt input following subsequent fractional crystallization. The Early Cretaceous granite was predominantly generated from the Mesoproterozoic metamorphic basement through partial melting, with variable degrees of fractionation and magma mixing in the upper crust.

The Wujinshan granitoid porphyries were formed in a volcanic arc-type tectonic setting related to the paleo-Pacific Plate subduction, and their distinct geochemical characteristics and variable degrees of evolution indicate that they are genetically linked with the Late Jurassic to Early Cretaceous (150–140 Ma) tectonic transition and slab rollback of the paleo-Pacific Plate.

The Late Jurassic granodiorite porphyry was less altered, showing insignificant post-deposition alterations, while the Early Cretaceous granite porphyries showed variable degrees of late-stage secondary mineralization by the K-metasomatic process and post-deposition alteration.