Sedimentary Geological Characteristics and Tectonic Environment of Luojiamen Formation in Northern Zhejiang, Eastern Section of Jiangnan Orogenic Belt

Abstract

This study advances our understanding of the Jiangnan Orogenic Belt by integrating high-precision geochronological data with interpretations of sedimentary and tectonic environments. Specifically, it addresses the controversy over the geological significance, origins, and tectonic significance of the Shengong Unconfomity: at the base of the Luojiamen Formation. This paper shows that the formation developed over four stages with the primary source of detritus lying in a volcanic arc to the south. The study also reassesses the “unconformity” between the Luojiamen Formation of the Heshangzhen Group and the Zhangcun Formation of the Shuangxiwu Group, concluding that it does not demarcate the end of the orogenic collision between the Cathaysia and Yangtze blocks.

1. Introduction

The Jiangnan Orogenic Belt formed during convergence between the Cathaysia and Yangtze blocks in the middle-to-late Precambrian collision and occupies the collision zone between these two formerly opposing crustal blocks. Stretching from Northern Guangxi to Northern Zhejiang [1,2,3,4,5,6], this belt includes a significant angular unconformity recognized by researchers like Ma et al. [7] as a marker of the “Shengong Event” [7,8,9]. The Luojiamen Formation located above the Shuangxiwu Group within this belt was the subject of numerous interpretations [10,11,12,13,14,15,16,17,18,19,20]. Some attribute the glacial characteristics of its basal conglomerates to the Chang’an Ice Age, arguing these features result from glaciation rather than tectonic activity [10,11]. Others suggest these same characteristics arise from rapid sediment deposition in a Precambrian foreland basin [12], formed by weathering denudation of the Shuangxiwu Group. Some studies show that the tectonic environment of Luojiamen Formation in Heshangzhen Group is a rift basin that began to form around 820 Ma [14,15,16,17,18]. Alternatively, some researchers propose an island arc setting, with the Luojiamen Formation deposited in a backarc basin associated with the Sibao (Jiangnan) orogeny during 830–820 Ma [19,20]. This paper focuses on the Jiangnan Orogenic Belt’s eastern segment, especially the Precambrian strata at the base of the Luojiamen Formation in the Fuyang area, Zhejiang Province. By analyzing sedimentary facies, cross-bedding, trough molds, and clastic rock framework grains, we assess the ancient tectonic events, providing a foundation for understanding the regional tectonic movements and evolutionary history of this segment.

2. Geological Setting

The oldest strata exposed in the study area are part of the Early Proterozoic Shuangxiwu Group, regionally divided into the Pingshui, Beiwo, Yanshan, and Zhangcun formations [21]. The overlying Heshangzhen Group is stratified from bottom to top into the Luojiamen, Hongchicun, and Shangshu Formations (Figure 1 and Figure 2). The Luojiamen Formation is bounded on both sides by unconformities, the lowermost of which makes a shallow angle with the underlying Zhangcun Formation, while the top of the Heshangzhen Group shows an unconformity with the Sinian Zhitang Formation [21].

The Luojiamen Formation begins with a base of grey-green blocky conglomerate. Overlying the conglomerate is a feldspar-bearing sandstone that transitions upward into rhythmically layered sandstone, siltstone, and mudstone interspersed with diabase intrusions in the middle-to-upper sections.

2.1. Regional Structural Characteristics

In the Zhangcun area of Fuyang, the Shuangxiwu Group is deformed into an anticlinal fold trending northeast–southwest. The strata on the southeast wing of the anticline are in positive sequence, while the northwest limb is steeply inclined or overturned. During the Precambrian, the study area was affected by the Shengong and Jinning orogenies. The Shengong orogeny is defined by the unconformity and conglomerates at the base of the Heshangzhen Group. The Jinning orogeny is represented by a northeast-plunging anticlinal structure, which was coaxially developed with the earlier anticlinal structure and its wings tilted outward. Post-Cambrian, the area underwent deformation during both the Caledonian and Indosinian orogenies [8]. The uplift, depression and fold of the Caledonian orogeny are basically consistent with the tectonic lines of the basement due to the control of the basement structure. The Indosinian orogeny developed superimposed folds with the Shuangxiwu Group at its core, particularly prominent in the Jiang–Shao fault zone, which experienced multiple phases of development under varying tectonic stresses [12].

2.2. Geological Evolution

Previous studies have identified the Shuangxiwu Group as a calc-alkaline volcanic arc [22]. SHRIMP zircon ages for the tuff of the Pingshui Formation at the base of the Shuangxiwu Group in Zhejiang are 908 ± 6 Ma [16]. The andesite of the Beiwudang Formation within the same group has SHRIMP zircon ages of 901 ± 5 Ma and 926 ± 15 Ma [16,23]. The upper Zhangcun Formation and its andesitic breccia are dated to 899 ± 8 Ma and 891 ± 12 Ma [16,23], situating the Shuangxiwu Group between 980 and 890 Ma High-precision geochronological data indicate the Luojiamen Formation’s sedimentary strata were deposited between 850 and 820 Ma [18,19,23,24]. The Wannian Group in the Ganbei Dexing area, related to the Luojiamen Formation, has an age of 843.8 ± 5 Ma [25]. The Pingshui Group rhyolites in Zhejiang’s Pujiang area have SHRIMP zircon U-Pb ages of 825.3 ± 8.1 Ma and 830 ± 6 Ma [26], indicating contemporaneity among the Luojiamen Formation, Wannian Group, and Pingshui Group.

The study area’s evolution can be divided into the formation stage of the Jiangnan block during the Precambrian and its development post-Cambrian. Between 950 and 850 Ma, the oceanic crust attached to the Cathaysia block subducted beneath the Yangtze block, forming the magmatic arc represented by the Shuangxiwu Group. From 850 to 820 Ma, high-angle subduction of the Cathaysia oceanic lithosphere beneath the Yangtze block led to the formation of a southward-migrating magmatic arc, evidenced by the volcanic strata of the Pingshui Group in Pujiang and the Wannian Group in southern Ganbei Dexing. Concurrently, the southeastern margin of the Yangtze block experienced extensional tectonics, forming the Luojiamen Formation’s back-arc basin deposits. Between 820 and 815 Ma, the collision between the Cathaysia and Yangtze blocks formed the foreland basin deposits of the Shuangqiaoshan and Lengjiaxi groups. The final amalgamation of the Cathaysia and Yangtze blocks occurred after 815 Ma [25,27].

3. Materials and Methods

Fresh sedimentary samples, free of veins and contamination, were collected from the Luojiman Formation. A total of 72 rock samples were collected, with 30 being prepared into thin sections for polarizing and orthogonal microscope observation and skeletal particle analysis. Seven samples (Figure 1) were used for geochemical analysis. Thirty sandstone samples were selected, excluding those with a matrix volume fraction exceeding 25%, with SiO2 content ranging from 52.30% to 74.90%. The Dickinson–Gazzi point-counting method was used to analyze their framework grain composition. This method distinguishes lithic fragments containing mineral particles larger than 0.0625 mm. If mineral particles within a lithic fragment exceed this size, they are classified as the corresponding feldspar or quartz end members rather than lithic fragments. Conversely, smaller particles or matrix materials are counted as lithic fragments [28,29].

Analyzing paleocurrent directions within these basins offers insights into the inclinations of ancient slopes, sedimentary environments, sediment supply directions, and the geometric forms of sedimentary bodies [30]. In the Luojiamen Formation, cross-bedding is widely, albeit sporadically, exposed in both lower and middle-upper sections. Additional structures such as ripple marks, trough casts, and slump structures further indicate ancient water flow directions. Data from 20 sets of small-scale cross-bedding foresets and 3 sets of slump structure attitudes underwent corrective analysis.

Geochemical analyses were conducted at the laboratory of the Hebei Provincial Institute of Regional Geological and Mineral Resource Survey. The initial step involved grinding the samples to a diameter of approximately 3 cm, using a porcelain crocodile crusher and a WC sample grinder to further pulverize them to 200 mesh. For the determination of major elements, 35 mg to 45 mg of the sample was placed into a Teflon vessel, with 0.5 mL each of HNO₃ and HF added, and dried on a 150 °C furnace. Subsequently, an additional 1 mL each of HNO₃ and HF was added and evaporated. After further addition of 2 mL of HNO₃ and 2 mL of 18 MΩ ultrapure water, the solution was dissolved in a 150 °C container, diluted a thousandfold, and 2 mL of this solution mixed with 2 mL of 10 ppb Rh for analysis using an Axios max X-ray fluorescence spectrometer (ICP-HEX-MS, Malvern Panalytical, Malvern, UK). Trace and rare earth elements were analyzed by acid dissolution followed by plasma mass spectrometry (Element II) and X-ray fluorescence spectroscopy. The precision for major elements was ±1%, with a detection limit below 20 μg/g, and the error margin for trace elements ranged from 1% to 5%.

4. Results

4.1. Sedimentary Geological Characteristics

4.1.1. Sedimentologic Analysis

The conglomerate at the bottom of Luojiamen Formation exhibits a massive, mixed texture, with clay and sand-grade matrix filling the gaps between clasts. The change of grain size in the conglomerate layer is not obvious, and the sedimentary structures such as banded lens can be seen occasionally, and the cross-bedding between the conglomerate and the sandstone layer is not obvious (Figure 3).

The lower formation also features wavy bedding (Figure 4b), parallel bedding (Figure 4c), and cross-bedding (Figure 4d). The rippling layers of the wavy bedding are not very undulating, with clay laminae 2–5 mm thick and sand laminae 5–25 mm thick, indicating that the bedding is generated under the hydrodynamic conditions of oscillation. The graded bedding shows a central coarser grain with finer grains above and below, suggesting alternating water flow speeds.

The middle-upper formation displays thinly interbedded siltstone and mudstone layers (Figure 4g), with identifiable Bouma sequences bc (Figure 4e). The b-division consists of moderately sorted medium sand, about 10 cm thick, with less distinct graded bedding. The c-division comprises siltstone, about 2 cm thick, with wavy bedding, transitioning continuously from the b-division to the d-division. Typical gravity flow structures, such as slump structures (Figure 4h) and graded bedding (Figure 4f), are observed in the middle-upper Luojiamen Formation. The absence of floating mud clasts and oriented structures suggests that the Bouma sequences in the Luojiamen Formation are likely formed by turbidity currents rather than sandy debris flows which have a slip movement at the bottom of the massive sandstone, the upper part of the massive sand body has floating mud gravel, grain sequence bedding is characterized by reverse grain sequence. Deformed clay particles in the siltstone, showing lenticular, flattened, and crescent shapes, indicate storm influence during deposition. The thickness of mudstone layers varies from a few millimeters to several tens of centimeters, increasing upward, indicating a deepening depositional environment.

The uppermost part of the Luojiamen Formation indicates that the mudstone layer has no evident structure, which indicates that the hydrodynamic conditions are very poor and scarce seawater supply during deposition (Figure 3).

4.1.2. Petrographic Analysis

Clasts in conglomerate at the base of the Luojiamen Formation consist mainly of brick-red granite, diabase, and andesite along with minor amounts of sedimentary rock. Granite clasts, brick-red and comprising quartz, orthoclase, and plagioclase with traces of mica, range from 0.1 to 10 cm in size and are sub-angular to sub-rounded (Figure 5a). The granite shows signs of dynamometamorphism, such as undulatory extinction in quartz and pressure twinning in plagioclase (Figure 5b). Diabase clasts are primarily gray-green, featuring diabase textures with phenocrysts of plagioclase and pyroxene; some of which are serpentinized. Andesite clasts are purple-red and display interlocking and spotted textures under the microscope, composed mostly of plagioclase (Figure 5c). Sedimentary rock clasts like siltstone and mudstone show good rounding and are mostly sericitized and chloritized. The bottom conglomerate layers are primarily composed of volcanic clasts, with occasional mudstone clasts, displaying poor sorting and small- to medium-sized conglomerate clasts (Figure 4a). Andesitic clasts are poorly rounded and angular, while mudstone and granite clasts exhibit better rounding, ranging from sub-angular to sub-rounded. The clasts in the basal clastic rocks have poor grain roundness, and the quartz grains have angular roundness (Figure 5g), high heterobasic content and heterobasic support (Figure 5f).

The Luojianmen Formation overlying the basal conglomerate mainly comprises feldspathic and lithic sandstones intruded locally by diabase dykes. Sandstone components include quartz, feldspar, and lithic fragments. Quartz shows poor rounding, with a content of approximately 10%. Feldspar content ranges from 40% to 65%, primarily plagioclase, exhibiting poor rounding and subhedral elongated shapes, with a grain size of 0.5–3 mm. Some feldspar grains are chloritized and sericitized (Figure 5d). The lithic fragment content in the lower part is relatively high, around 40%, with volcanic lithics comprising about 90% of the total (Figure 5e). The support type is matrix-supported, with the matrix mainly consisting of clay, accounting for up to 35% (Figure 5f).

The middle-upper section primarily consists of siltstone with tuff layers and diabase dikes. The quartz in this section shows better rounding than in the lower part, with its content increasing to about 20%. Feldspar content ranges from 30% to 70%, with a reduced proportion of plagioclase and better rounding. The lithic fragment content decreases in this section.

The uppermost part of the Luojiamen Formation consists of thick layers of dark gray mudstone. The mudstone can be seen in the microscope with an argillaceous structure, containing a small number of fine silty quartz feldspar particles with a strong chlorite.

4.1.3. Statistical Analysis of Cross-Bedding and Its Paleogeographical Significance

Sedimentary basins serve as key repositories for sediment accumulation, preserving valuable records of sedimentary processes. Data from 20 sets of small-scale cross-bedding foredeposits and three sets of slump structure attitudes underwent corrective analysis (

Table 1. Data of field measurement and horizontal correction of small-scale cross-bedding foredeposits and slump structures in the Luojiamen Formation.

Strata Surface	Cross-Bedding Surface		Strata Surface	Cross-Bedding Surface
	Field Measurement	Horizontal Correction		Field Measurement	Horizontal Correction
location 1: 29°53′47.6″ N, 120°02′28.10″ E			location 2: 29°53′52.38″ N, 120°02′35.26″ E
329°/46°	328°/61°	145.62°/15.02°	337°/58°	342°/74°	174.02°/16.63°
	324°/65°	135.27°/19.43°		350°/70°	204.29°/16.72°
	320°/66°	125.85°/21.31°		333°/68°	136.44°/10.61°
	317°/60°	110.80°/16.93°		343°/63°	204.76°/7.23°
	330°/59°	152.81°/13.02°		340°/61°	198.52°/3.96°
	326°/62°	139.45°/16.18°		341°/62°	198.91°/5.29°
	335°/65°	165.39°/19.62°		335°/65°	142.42°/7.22°
	327°/60°	141.87°/14.09°		341°/71°	173.43°/13.49°
	334°/58°	168.77°/12.62°		345°/70°	189.78°/13.98°
	325°/63°	136.94°/17.30°		339°/70°	165.98°/12.13°
location 3: 29°54′08.3″ N, 120°01′38.1″ E (Slump Structure)
339°/47°	341°/47°	180.34°/4.27°	339°/47°	324°/49°	74.08°/11.31°
	339°/60°	159.03°/13°

). The outcomes affirm that the dominant paleocurrents in the Fuyang section of the Luojiamen Formation flowed predominantly southeast and due south (Figure 6). These findings indicate that the sediment source was located to the south, shaping the depositional patterns and influencing the sedimentary tectonic environment.

4.1.4. Characteristics of Clastic Rock Framework Grains

For this study, 30 sandstone samples were selected (

Table 2. Statistical table of framework grains in sandstones from the Luojiamen Formation, Fuyang.

Sample	Quartz	Qm	Qp	Lithic	Lv	Ls	Feldspar
1–1	162	150	12	118	116	2	238
1–2	170	152	18	120	116	4	220
1–3	200	180	20	119	119	0	195
1–4	289	270	19	54	50	4	40
1–5	148	137	11	100	97	3	120
2–1	140	120	20	91	91	0	93
2–2	181	175	6	180	176	4	176
3–1	206	199	7	98	96	2	116
3–2	160	145	15	115	110	5	100
3–3	200	160	40	177	160	17	150
3–4	163	149	14	99	89	10	217
3–5	185	160	25	120	120	0	123
3–6	181	158	23	179	169	10	169
3–7	112	101	11	117	110	7	101
3–8	158	129	29	105	101	4	98
3–9	138	116	22	128	128	0	161
3–10	282	222	60	187	179	8	260
3–11	185	151	34	87	81	6	101
3–12	266	219	47	148	144	4	200
3–13	324	306	18	189	189	0	288
4–1	237	192	45	106	104	2	157
4–2	249	205	44	117	103	14	172
4–3	303	234	69	123	120	3	235
4–4	147	116	34	149	144	5	198
4–5	224	184	40	146	140	6	234
4–6	171	151	20	112	109	3	193
4–7	189	171	18	162	158	4	159
4–8	208	184	24	124	120	4	244
4–9	280	246	34	211	207	4	310
4–10	197	168	29	170	163	7	268

Note: Qm—monocrystal quartz; Qp—polycrystal quartz; Lv—volcanic lithic; Ls—Sedimentary lithic.

), excluding any with a matrix volume fraction over 25%. Using the Dickinson–Gazzi point counting method, we analyzed the framework grain compositions, finding average quartz, feldspar, and lithic fragment contents of 39%, 26%, and 35%, respectively. Quartz grains were primarily monocrystalline, while feldspar was mostly plagioclase, and lithic fragments were largely volcanic. The majority of samples were categorized as feldspathic lithic sandstones, characterized by low maturity with the matrix filling the interstices and grains ranging from sub-rounded to rounded, predominantly supported by the matrix.

Analysis of the Qt–F–L diagram (Figure 7) shows that only 2 out of 30 samples fall entirely within the orogenic belt region, with the remainder located within or on the boundary of the volcanic arc. This indicates that the majority of the sandstone samples originate from a volcanic arc source area. In the Qm–F–L diagram, all but four samples are positioned within the volcanic arc genesis area. In the Qp–Lvm–Lsm diagram, fewer samples fall within the volcanic arc source area, and none into the accretionary wedge or orogenic belt regions. Dickinson’s ternary diagrams thus suggest that the Luojiamen Formation’s provenance is primarily volcanic arc. The high content of volcanic clasts in the Luojiamen Formation clastic systems indicates proximity to the volcanic source area.

4.2. Geochemical Characteristics of Rocks

4.2.1. Characteristics of Major Elements

Geochemical analyses of fine-to-silt-sized sandstone samples reveal SiO₂ content ranging from 52.30% to 74.90%, Al₂O₃ from 10.65% to 16.72%, K₂O from 0.93% to 4.34%, Na₂O from 0.86% to 5.28%, CaO from 0.75% to 2.57%, Fe₂O₃ from 0.43% to 3.41%, TiO₂ from 0.38% to 0.85%, MnO from 0.078% to 0.13%, and MgO from 0.71% to 3.30%. The moderate SiO₂ content and the Al₂O₃/SiO₂ ratios between 0.14 and 0.4, alongside low K₂O/Na₂O ratios (0.34 to 1.0). The CaO content in the study area is low, much less than 12%, which avoids the influence of biological origin to a certain extent. Generally, longer sediment transport distances correspond to higher maturity and lower Al₂O₃ content, resulting in lower A1₂O₃/SiO₂ ratios; shorter distances correlate with lower compositional maturity and lower K₂O/Na₂O ratios. The low K₂O/Na₂O values in the study area suggest that the source area was relatively close, with short transport distances of the sediments, thus aiding in the determination of the tectonic environment based on the source area.

4.2.2. Characteristics of Trace and Rare Earth Elements

The Luojiamen Formation displays moderate total REE content, with a noticeable enrichment in light REEs. The chondrite-normalized REE distribution patterns (Figure 8) generally show a right-leaning inclination; δEu ranges from 0.80 to 1.17, averaging 0.9, and δCe from 0.82 to 0.93 (

Table 3. Trace element and rare earth element (REE) data of Luojiamen Formation (units are µg/g).

Sample	ΣREE	LREE	HREE	LREE/HREE	(La/Yb)N	δEu	δCe
1–2	215.03	197.07	17.96	10.97	12.83	1.17	0.92
2–2	103.40	89.23	14.17	6.30	6.20	0.96	0.84
3–3	126.37	109.41	16.96	6.45	6.44	0.80	0.82
3–9	207.27	184.36	22.91	8.05	9.37	0.82	0.89
4–1	173.80	156.05	17.75	8.79	9.23	0.87	0.85
4–6	146.44	130.17	16.27	8.00	8.14	0.82	0.86
4–9	158.54	140.25	18.29	7.67	7.47	0.88	0.93
average	161.55	143.79	17.76	8.03	8.53	0.90	0.87

Note: δEu = Eu_N/(Sm_N × Gd_N)^1/2 where N represents the normalized data; ΣREE = LREE + HREE; LREE = La + Ce + Pr + Nd + Sm + Eu; HREE = Gd + Tb + Dy + Ho + Er + Tm + Yb + Lu.

), indicating a weak negative anomaly. The primitive mantle-normalized trace element spider diagram for the Luojiamen Formation sandstones shows deficits in elements like Ti, P, Sr, Ta, Nb, and enrichments in La, K, Nd, Zr, Hf.

5. Discussion

5.1. Depositional Environment and Provenance Analysis

The conglomerate layer at the bottom of the Luojiamen Formation has poor roundness and sorting and is characterized by repeated gravity flow and traction flow. The conglomerate layer belongs to the fan root and fan subfacies of alluvial and diluvial fan facies, which is mainly the product of sediment transported by debris flow in the mountain valley and deposited in front of the mountain. It is more obvious that Luogen Formation does not develop the typical fan and fan-margin subfacies on land, instead of the underwater alluvial and diluvial fan facies in coastal shallow water. This indicates that the land mountain terrain at that time was close to the sea [15]. The lower section, comprised of feldspathic and lithic sandstones, shows wavy bedding, parallel lamination, and cross-lamination, typical of coastal facies influenced by strong hydrodynamic forces. The middle to upper sections display shallow marine authigenic minerals like glauconite and are marked by interbedded sandstone and mudstone turbidites. Previous studies by Gu et al. [15], using X-ray diffraction analysis of mudstone, confirmed the presence of glauconite, an authigenic mineral of tropical shallow marine environments, further supporting the shallow marine depositional environment for the middle-upper of the Luojiamen Formation [33]. The uppermost layers consist of unstratified mudstone, signaling a progression to deeper water environments, but no obvious sedimentary facies difference was found. A sedimentary environment with rising sea levels was formed due to the rapid fall of the crust and the insufficient supply of provenance, and the deposition rate was less than the fall rate.

According to statistical analysis of cross-bedding, the paleocurrent directions from south to north imply a topography that slopes from higher in the south to lower in the north. Combining the results of Characteristics of Clastic Rock Framework Grains, it is thus plausible that there was a magmatic arc to the south of the Luojiamen Formation, continuously supplying clastic sediments northward due to volcanic activity and the sloping terrain. The strata in the southern part of the Luogen Formation at the same time are the Wannian Group and the Pingshui Group, and the acidic volcanic rocks in the Wannian Group and the basalt of the Pingshui Group in Pujiang were both formed in the volcanic arc tectonic area [19,25,27]. It can be inferred that the prototype basin of the Luogen Formation is a back-arc basin matching the volcanic arc of the southern Wannian Group and the Pingshui Group in Pujiang.

5.2. Analysis of Geochemical Characteristics of Rocks

In general, the tectonic framework is divided into active continental margins, continental arcs, oceanic island arcs, and passive continental margins, each influencing the sedimentary characteristics of basin fill within their respective settings. This study identifies the tectonic environment of the Luojiamen Formation through an analysis of its major, trace, and rare earth elements.

5.2.1. Major Elements Environment Discrimination

Geochemical analyses of sediment compositions provide insights into sedimentary basins’ tectonic backgrounds. Utilizing the K₂O/Na₂O-SiO₂ diagram [32] (Figure 9a), most samples from the Luojiamen Formation align with the characteristics of island arcs (ARC) and active continental margins (ACM), with one sample indicating a passive margin setting. The K₂O/Na₂O-SiO₂/Al₂O₃ diagram (Figure 9b) places samples primarily within an island arc context, which is consistent with the sedimentary rock data in the basin showing lower Na₂O and CaO levels compared to their source rocks while having higher SiO₂ content. This disparity suggests that reliance solely on major elements for determining sedimentary backgrounds might be insufficient.

5.2.2. Trace Elements Environment Discrimination

To further determine the tectonic background of the source area, less reactive trace elements such as La, Ce, Nd, Y, Th, Zr, Hf, Nb, Ti, and Sc were used as discriminants. Using the sandstone provenance discrimination diagrams proposed by Bhatia M R [30], the trace elements of the samples primarily fall within the continental arc, with a few in the active continental margin and its vicinity, as seen in the Sc/Cr–La/Y diagram (Figure 10a) and the Th–Co–Zr/10 diagram (Figure 10b). These indicate that the main tectonic environment of the Luojiamen Formation is a continental arc, potentially transitioning to an active continental margin.

5.2.3. Rare Earth Elements (REE) Environment Discrimination

Bhatia [36] delineated how REEs vary across different tectonic settings—from oceanic island arcs to continental arcs, and from active continental margins to passive continental margins. In these settings, La and Ce concentrations increase in sandstones, as do ∑REE, ∑LREE/∑HREE, and La/Yb ratios, while δEu decreases. Given that continental crust typically has higher REE and HREE levels than the mantle, the ∑REE and ∑LREE/∑HREE ratios are elevated in settings influenced by continental materials. This aligns with the observed REE characteristics in the Luojiamen Formation’s sandstones, which display the ΣREE average of 161.55 × 10⁻⁶, and normalized (La/Yb)_N values averaging 8.53, with δEu averaging 0.9. These REE profiles are indicative of a continental arc environment (

Table 4. REE characteristics of gritstone in different sedimentary basin tectonic settings (date source: [36]).

Tectonic Settings	La	Ce	∑REE	La/Yb	(La/Yb)N	Eu/Eu *
oceanic island arcs	8 ± 1.7	19 ± 3.7	58 ± 10	4.2 ± 1.3	2.8 ± 0.9	1.04 ± 0.11
continental arcs	27 ± 4.5	59 ± 8.2	146 ± 20	11 ± 3.6	7.5 ± 2.5	0.79 ± 0.13
active margins	37	78	186	12.5	8.5	0.6
passive margins	39	85	210	15.9	10.8	0.56

Note: except for the ratio, units are µg/g; Eu/Eu * = (Eu/0.087)/{[(Sm/0.231) + (Gd/0.306)]/2}.

).

A comprehensive analysis of the geochemical characteristics of the Luojiamen Formation shows that the identification of major elements cannot accurately judge its tectonic background. Trace elements can be inferred as the transition from a continental island arc to an active continental margin, while the characteristics of rare earth elements are indicated as a continental island arc. Therefore, it can be inferred that the tectonic environment of the Luojiamen Formation is mainly a continental island arc environment, which may transition to an active continental margin.

5.3. Tectonic Environment Discussion

(1) 950–850 Ma: This period is characterized by the subduction of the oceanic lithosphere of the Cathaysia Block beneath the Yangtze Block. This process led to the formation of a magmatic arc represented by the Shuangxiwu Group volcanic rock series and the Xiqiu granite, an accretionary wedge represented by the Zhangshudun ophiolite, and a back-arc basin represented by the Likou Group Huan Sandstone Formation (Figure 11a) [25,27].

(2) 850–825 Ma: During this period, the oceanic lithosphere of the Cathaysia Block underwent high-angle subduction beneath the Yangtze Block. The magmatic arc initially represented by the Shuangxiwu Group volcanic rock series migrated southwards, culminating in the formation of the Wannian Group volcanic arc at approximately 843.8 ± 5 Ma. Concurrently, the southeastern margin of the Yangtze Block experienced extensional forces due to the subduction of the Cathaysia Block, leading to crustal thinning and eventual rifting. This tectonic activity resulted in the formation of the Fuchuan ophiolite suite (840–820 Ma) in southern Anhui. In the study area, this extensional regime facilitated the deposition of a back-arc basin sedimentary system represented by the Luojiamen Formation. This stage highlights significant magmatic and sedimentary processes influenced by the tectonic evolution of the region, shaping the geological characteristics of the Luojiamen Formation (Figure 11b) [25,27].

Some scholars view that the angular unconformity between the Shuangxiwu Group’s Zhangcun Formation and the Heshangzhen Group’s Luojiamen Formation is the result of the collision orogeny between the Cathaysia and Yangtze blocks, which is called the Shennong Event, marking the end of the collision orogeny and the beginning of the rift movement [14,15,16,17,18,37].We through the field observations of the Luojiamen section in Qingong Village, as well as laboratory research and analysis of the predecessors’ chronology information, for the presence of saint movement and angular unconformity and its significance. The age of the Luojiamen Formation is around 830 Ma, which predates the rifting of the Jiangnan Orogenic Belt (which began around 800 Ma). The “unconformity” between the Zhangcun Formation at the top of the Shuangxiwu Group and the base of the Heshangzhen Group’s Luojiamen Formation may simply represent a local transition from a volcanic arc environment to a back-arc basin and does not represent the end of the collision orogeny between the ancient Cathaysia and Yangtze blocks.

6. Conclusions

(1)

Stratigraphic Sequence and Sedimentary Characteristics:

Through a combination of field investigations combined with an analysis of the Luojiamen Formation, we described its lithology, structure, sedimentary sequences, and geochemical characteristics. The Luojiamen Formation predominantly represents an upward-deepening sedimentary sequence.

(2)

Paleocurrent Directions:

Small-scale cross-bedding in the middle-upper parts of the Luojiamen Formation, corrected for horizontal orientation, indicates paleocurrents flowing towards the northwest and due north, suggesting a south-to-north topographic gradient. This implies that the sediment source was primarily from the southeast and south.

(3)

Provenance and Basin Analysis:

The statistical analysis of detrital framework grains and the geochemical characteristics of the clastic rocks indicate that the provenance of the Luojiamen Formation in Fuyang is mainly from a volcanic arc, with the original basin being a back-arc basin.

(4)

Source Matching:

The provenance of the Luojiamen Formation appears to be from the south, with the original basin likely being a back-arc basin. It is inferred that the Luojiamen Formation could correspond to the contemporaneous “Pingshui Group” and Wan’an Group, forming a matching arc–basin system.

(5)

Re-evaluation of the Angular Unconformity:

The angular unconformity between the Luojiamen Formation and the Zhangcun Formation of the Shuangxiwu Group is unlikely to represent the end of the collisional orogeny between the Cathaysia and Yangtze blocks. Instead, it may reflect a local transition from a volcanic arc to a back-arc basin environment.

These conclusions underscore the dynamic and complex geological evolution of the Jiangnan Orogenic Belt and provide critical insights into the regional tectonic activities that shaped this area. Further studies, particularly in the fields of geochronology and petrology, are encouraged to refine these interpretations and enhance our understanding of the area’s geological history.