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Article

Triassic Retrograde Metamorphism and Anatexis in the Sulu Orogenic Zone, Central China: Constraints from U–Pb Ages, Trace Elements, and Hf Isotopic Compositions of Zircon

by
Yongkang Ye
1,
Hengcong Lei
1,*,
Fei Xia
1,*,
Hui Zhang
1 and
Congjun Yu
2
1
School of Earth Sciences, East China University of Technology, Nanchang 330013, China
2
Geological Environment Monitoring Institute of Jiangxi Geological Survey and Exploration Institute, Nanchang 330000, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2025, 15(11), 6145; https://doi.org/10.3390/app15116145
Submission received: 25 April 2025 / Revised: 27 May 2025 / Accepted: 27 May 2025 / Published: 29 May 2025
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Abstract

:
We report information on the protolith, the Triassic retrograde metamorphism, and anatexis recorded in zircons extracted from granitic gneiss and biotite schist in the Sulu orogenic zone, central China. The schist is enclosed within the granitic gneiss in the form of a lens. Zircon grains from the schist sample indicate anatexis occurred at 214.6 ± 3.6 Ma (MSWD = 5.1), with εHf (t) values ranging from −22.6 to −18.3, corresponding to TDM C(Hf) ages between 2675 Ma and 2407 Ma. The granitic gneiss originated from magmatic rock formed at 774 ± 32 Ma (MSWD = 5.7) and subsequently underwent metamorphism at ~211 Ma. Three zircon cores from the granitic gneiss exhibit εHf (t) values ranging from −13.6 to −6.3, with TDM C(Hf) ages spanning 2487–2075 Ma. Six zircon rims from the gneiss yield εHf (t) values of −14.7 to −13.3, and TDM C(Hf) ages ranging from 2176 to 2092 Ma. We believe that the protolith of granitic gneiss is the Neoproterozoic magmatic rock, whose tectonic affinity is the northern margin of the Yangtze craton. The granitic gneiss experienced Triassic collisional orogeny-related metamorphism and subsequent retrograde metamorphism, with the timing of retrograde overprinting consistent with zircon-recorded anatexis in the schist. In addition, the protoliths of both the gneiss and schist exhibit close affinity to Archean-Paleoproterozoic crustal sources.

1. Introduction

The Dabie–Sulu orogenic belt, a prominent example of the largest metamorphic terranes characterized by ultrahigh-pressure (UHP) conditions globally, originated from the Triassic subduction of the Yangtze Craton (YC) beneath the North China Craton (NCC) [1,2,3,4,5,6]. As a result of subduction and subsequent exhumation during the Triassic, the YC underwent a sequence of metamorphic transformations, beginning with a prograde phase, reaching a peak stage of UHP metamorphism, and subsequently regressing to a retrograde phase [7,8]. With the increasing accuracy of geological dating and the comprehensive analysis of the internal structure, mineral inclusions, trace elements, and Hf-O isotopes of zircon and other dating accessory minerals, the age of each metamorphic stage of the Dabie–Sulu orogenic belt has been accurately defined [7,9,10,11,12]. Combined with a variety of geological thermobarometry, the P–T path of the Dabie–Sulu UHP metamorphic terrane has been well constrained, which can be divided into four metamorphic stages: (1) The metamorphic stage of HP quartz eclogite facies (246–244 Ma), with the metamorphic P–T conditions of 1.7–2.1 GPa and 570–690 °C, is characterized by the association of quartz eclogite facies inclusions preserved in zircons [13,14,15,16]; (2) The peak metamorphic stage of the UHP eclogite facies (240–225 Ma), with the metamorphic P–T conditions are 3.0–5.9 GPa and 750–900 °C, and the typical characteristics are the UHP metamorphic indicator minerals such as coesite or diamond preserved by the host minerals in the metamorphic rocks [6,9,10,15,16]; (3) The retrometamorphic stage of quartzite eclogite facies (225–215 Ma), with the metamorphic P–T conditions are 1.5–2.5 Gpa and 600–750 °C, and the representative mineral assemblage is garnet + omphacite + quartz, while the content of coesite obviously decreases or disappears [17,18,19]; (4) The amphibolite facies retrometamorphic stage (215–205 Ma), with the metamorphic P–T conditions of 06–1.0 GPa and 450–600 °C, are characterized by amphibolite facies retrometamorphic products (such as plagioclase + amphibolite) developed around such as garnet and omphacite [9,10,16,19].
The Sulu UHP metamorphic zone predominantly comprises orthogneisses, with minor amounts of eclogites, gneisses, granulites, and ultramafic rocks, which have Paleoproterozoic protolithic or metamorphic ages and appear as enclaves embedded within the orthogneisses [20,21,22,23,24]. The Northern Sulu orogenic zone, which delineates the northern extent of the YC, is central to studies of the Sulu zone and deciphering the tectonic evolutionary history of the YC. Decades of multidisciplinary research have systematically explored UHP metamorphism, unraveling the petrological and geodynamic controls on deep subduction and exhumation of continental material [5,14,18,19,25,26,27]. However, there have been fewer studies of retrograde metamorphism and anatexis. This greatly limits our complete understanding of the collisional orogeny process.
This study offers valuable insights that stem from the study of retrogressed biotite schist within the Sulu zone into the tectonic evolution of the Sulu orogenic belt and the northern margin of the YC. We investigated the biotite schist—an important but retrograde component of the Sulu zone and its surrounding rock, granitic gneiss, that have undergone retrograde metamorphism in northeast Sulu. This study presents an integrated analysis of petrology, geochronological data, and zircon Hf isotope compositions for both biotite schist and granitic gneiss samples. The results indicate that these metamorphic rocks underwent continental collision and subsequent retrograde metamorphism and anatexis during the Triassic. These rocks originate from the reworking of crustal materials dating back to the Paleoproterozoic.

2. Geological Setting and Sample Descriptions

2.1. Geological Setting

The Dabie–Sulu orogenic belt, resulting from the Triassic continental collision between the NCC and the YC (Figure 1), preserves one of the world’s largest concentrations of UHP metamorphic rocks [1,3,4,6]. Following the identification in eclogite of coesite and diamond, both serving as indicators of UHP conditions, in eclogites [28,29,30], this belt has emerged as a foremost site for studying UHP metamorphism associated with continental subduction. The Tanlu fault divides this orogenic belt into two distinct segments: the east-west-oriented Dabie orogen and the northeast-trending Sulu orogen (Figure 2).
The Sulu orogen, located east of the Tan–Lu fault, is bordered by the Wulian–Yantai Fault, delineating its boundary with the NCC [32] (Figure 2). At the heart of the northern Sulu orogen lies the Jiaodong terrane, which is mainly characterized by its high abundance of orthogneiss. This terrane also contains smaller quantities of amphibolites, eclogites, and garnet peridotite, as well as a minor proportion of metamorphic supracrustal rocks. The precursor rocks of these orthogneisses and amphibolites are Neoproterozoic bimodal magmatic rocks, which can be traced back to the rifting event that led to the disintegration of the supercontinent Rodinia [22,33,34]. Furthermore, a minor quantity of Archean to Paleoproterozoic rocks is present within the core of the orogenic belt [6,7,20,21,22,33,35].
Our samples were collected from the Weihai area, situated within the northern Sulu orogenic zone, characterized predominantly by orthogneiss, with smaller occurrences of eclogite interspersed among Archean to Paleoproterozoic rocks. The presence of coesite within zircon crystals sourced from orthogneiss suggests that it underwent a uniform UHP metamorphic event [9,10,36]. Additionally, within the UHP orthogneiss, there are embedded blocks of Archean to Paleoproterozoic rocks, which include ultramafic to mafic rocks.

2.2. Sample Descriptions

The biotite schist takes the form of a lens entirely encased in granite orthogneiss (Figure 3A). The biotite schist sample is grey-black in color and exhibits a crystalloblastic texture and lamellar structure. It is composed primarily of biotite (~60%), with quartz (~28%) and calcite (~6%) as secondary constituents, and contains minor quantities of apatite and sphene (Figure 3B,C). The adjacent wall rock, which is granitic gneiss, displays a grey-white coloration with a crystalloblastic texture and a distinct gneissic structure. The primary constituents of the sample are quartz (~45%), K-feldspar (~42%), and biotite (~8%), with smaller amounts of sphene and allanite also present (Figure 3D,E).

3. Analytical Methods

3.1. Zircon U-Pb Dating and Trace Element Analyses

Zircon U-Pb dating and trace element analysis were simultaneously performed using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) at the State Key Laboratory of Geological Processes and Mineral Resources (SKLGPMR), China University of Geosciences (Wuhan, China). The experimental setup employed a GeoLas 2005 laser ablation system (Coherent, Inc., Santa Clara, CA, USA) coupled with an Agilent 7500a ICP-MS (Agilent Technologies, Inc., Santa Clara, CA, USA). Analytical procedures followed the methodology described by Liu et al. (2008) [37], with helium serving as the primary carrier gas and argon as the make-up gas, mixed through a T-connector prior to plasma introduction. To enhance analytical performance, nitrogen was introduced into the central plasma gas flow (Ar + He) to improve sensitivity and resolution [38]. Each measurement cycle consisted of 20–30 s of background acquisition followed by 50 s of sample ablation. Signal acquisition and instrumental control were managed using Agilent Chemstation software (OpenLab CDS ChemStation Edition), while subsequent data processing, including background correction, signal integration, mass bias correction, and quantitative calibration for both trace elements and U-Pb isotopes, was conducted using ICPMSDataCal v.11.8 [39].
For U-Pb isotopic analysis, zircon 91500 served as the primary reference material, with duplicate measurements performed following every five unknown sample analyses. Instrumental mass fractionation and time-dependent drift were corrected through a linear interpolation of standard measurements following Liu et al. (2010) [39], utilizing the recommended isotopic ratios from Wiedenbeck et al. (1995) [40]. The associated uncertainties were systematically propagated to the final sample results. Data reduction included concordia diagram construction and weighted mean age calculation using Isoplot/Ex_ver3 [41]. Trace element quantification employed USGS reference glasses BCR-2G and BIR-1G for calibration, with values sourced from the GeoReM database, combined with internal standardization protocols [39].

3.2. Zircon Lu-Hf Isotopic Analyses

Following the completion of LA–ICP–MS U–Pb dating, in-situ zircon Hf isotope analyses were conducted using a Neptune Plus MC-ICP-MS system (Thermo Fisher Scientific Inc., Waltham, MA, USA), equipped with a GeoLas 2005 excimer ArF laser ablation system, located at the State Key Laboratory of Geological Processes and Mineral Resources (SKLGPMR) of China University of Geosciences, Wuhan. Equipped with a “wire” signal smoothing device, this laser ablation system is capable of generating smooth signals, even at laser repetition rates down to 1 Hz [42]. The study employed a laser ablation energy density of 5.3 J*cm−2. Within the ablation chamber, helium was used as the carrier gas and was mixed with argon (serving as a makeup gas) after the ablation process. A straightforward Y-junction was utilized downstream of the sample chamber to introduce a minor quantity of nitrogen (4 mL*min−1) into the argon makeup gas stream [38]. All data presented in this study were acquired by analyzing zircon in single-spot ablation mode, utilizing a spot size of 44 μm for the collection of all data points. Each measurement comprised a 20-s phase for background signal recording, followed immediately by a 50-s phase dedicated to capturing the ablation signal. The laser ablation system, MC-ICP-MS instrument, and analytical methodology used in this study adhered to the operational parameters outlined in Hu et al. (2012) [43].
The primary limitation in achieving precise in-situ zircon Hf isotope determination using LA–MC–ICP–MS stems from the substantial isobaric interference posed by 176Yb, with a lesser contribution from 176Lu, on the measurement of 176Hf. In this study, we utilized the real-time 176Yb value directly measured from the zircon sample. The ratios of 179Hf/177Hf and 173Yb/171Yb were employed to determine the mass bias corrections for Hf (βHf) and Yb (βYb), respectively. These corrections were normalized to the values of 179Hf/177Hf = 0.7325 and 173Yb/171Yb = 1.13017 [44], applying an exponential mass bias correction method. The interference of 176Yb on 176Hf was addressed by measuring the non-interfering 173Yb isotope and employing the ratio 176Yb/173Yb = 0.79381 [44] to compute the corrected value of 176Yb/177Hf. Analogously, the minor interference of 176Lu on 176Hf was corrected by measuring the intensity of the non-interfering 175Lu isotope and applying the recommended ratio 176Lu/175Lu = 0.02656 [45] to derive the corrected value of 176Lu/177Hf. Due to their comparable physicochemical properties, we utilized the mass bias of Yb (βYb) to determine the mass fractionation of Lu. The IC-PMSDataCal v.11.8 software was employed for the offline process of selecting and integrating analyte signals, along with executing mass bias calibrations [46].

4. Results

4.1. Zircon Morphology

Zircons sourced from biotite schist exhibit transparency and can be colorless or brown, with granular or irregular shapes. In CL images, the majority of zircon grains display weak luminescence and either planar zoning or an absence of zoning, aligning with the features of metamorphic zircons or anatectic zircons (Figure 4). Zircon grains derived from granitic gneiss samples are transparent and can be colorless or brown, with a columnar or oval shape. CL images reveal a relatively intricate formation pattern for these zircon grains, with the majority of grains exhibiting core-rim structures (Figure 4). In CL images, the cores of the zircons appear gray and exhibit oscillation zoning, resembling magmatic zircons due to their pronounced zoning patterns. Conversely, the rims display planar or faint zoning, which is characteristic of metamorphic zircons (Figure 4).

4.2. Zircon U–Pb Ages and Trace Elements

Twelve zircon grains from the biotite schist sample were analyzed for LA-ICP-MS U-Pb dating and trace element analysis (Table 1 and Table 2). These spots have relatively medium Th (129–2018 ppm) and U (391–1133 ppm) contents with high Th/U ratios of 0.21–1.78, and most of the ratios are greater than 1 (Figure 5). Twelve spot analyses conducted on the zircons resulted in nearly concordant 206Pb/238U ages, spanning from 206 Ma to 225 Ma, with a weighted mean age of 214.6 ± 3.6 Ma (MSWD = 5.1). When compared to chondrite patterns, most of the spots display flat heavy rare earth element (HREE) patterns, marked by a positive Ce anomaly and an absence of a notable negative Eu anomaly. In contrast, a single spot has enrichment in HREE (Figure 6). Therefore, the crystallization age of the anatectic zircon may be indicated by 215 Ma.
The U-Pb dating results and trace element data for ten zircon spots, derived from granitic gneiss and analyzed using LA–ICP–MS, are presented in Table 3 and Table 4, respectively. Four-spot analyses conducted on Type 1 samples showed moderate concentrations of Th, ranging between 64 and 203 ppm, low levels of U, varying from 115 to 208 ppm, and high Th/U ratios, which were between 0.56 and 1.45 (Figure 5). The spot analyses yielded a concordant range of 206Pb/238U ages, spanning from 746 to 796 Ma, with a weighted mean age of 774 ± 32 Ma (MSWD = 5.7) (Figure 6). When compared to chondrite patterns, the zircons display enrichment in HREE, accompanied by a notable negative Eu anomaly and a positive Ce anomaly, both indicative of an igneous origin (Figure 6) [47,48]. The analysis of six spots from zircon type 2 showed moderate concentrations of Th, ranging from 66 to 123 ppm, and significantly higher concentrations of U, spanning from 1633 to 3463 ppm, resulting in very low Th/U ratios of 0.03 to 0.05 (Figure 5). The six LA-ICP-MS U-Pb spot analyses performed on zircon type 1 produced a concordant range of 206Pb/238U ages, spanning from 209 to 216 Ma, with a weighted mean age of 211.1 ± 2.3 Ma (MSWD = 1.3) (Figure 6). These spots exhibit low contents of light rare earth elements (LREE) but elevated levels of HREE, accompanied by a distinct negative Eu anomaly and a positive Ce anomaly (Figure 6).

4.3. Zircon Hf Isotope

The initial Hf ratios for all analytical spots were calculated using their respective U–Pb ages. The analysis of ten zircon grains from the biotite schist sample revealed a range of 176Yb/177Hf ratios from 0.005020 to 0.079955, and 176Lu/177Hf ratios varying from 0.000127 to 0.002027. The initial 176Hf/177Hf ratios fell between 0.282002 and 0.282129. The εHf(t) values were calculated to be between −22.6 and −18.3, with a mean value of −20.7 ± 0.4 (n = 10) (Table 5, Figure 7). Furthermore, the TDM C(Hf) ages ranged from 2675 Ma to 2407 Ma.
Three zircon cores extracted from the granitic gneiss sample exhibited 176Yb/177Hf ratios ranging from 0.038686 to 0.092186, with 176Lu/177Hf ratios varying between 0.001131 and 0.002455. The initial 176Hf/177Hf ratios were found to be between 0.281966 and 0.282128, with εHf(t) values ranging from −13.6 to −6.3. The mean εHf(t) value was calculated as −7.8 ± 0.7 (n = 3) (Figure 7). Furthermore, the TDM C(Hf) ages for these zircon cores ranged from 2487 Ma to 2075 Ma. Six zircon rims from the same granitic gneiss sample showed 176Yb/177Hf ratios ranging from 0.025094 to 0.042182, and 176Lu/177Hf ratios varying from 0.000973 to 0.001768. The initial 176Hf/177Hf ratios were between 0.282230 and 0.282270, with εHf(t) values ranging from −14.7 to −13.3. The mean εHf(t) value for these zircon rims was −14.0 ± 0.4 (n = 6) (Figure 7). Additionally, the TDM C(Hf) ages for these zircon rims fell within the range of 2176 Ma to 2092 Ma. All the Lu-Hf isotope analysis results obtained from both zircon cores and rims are summarized in Table 6, with the reported errors representing the 2σ of the mean.

5. Discussion

5.1. Tectonic Affinity of the Schist and Gneiss

The Yangtze and North China Cratons are acknowledged as the two largest Precambrian tectonic units in China. The NCC is distinguished by magmatism at around 2.5 Ga and metamorphism at approximately 1.8 Ga [56,57,58] without Neoproterozoic magmatic rocks. Conversely, a prominent feature of the YC is the widespread presence of Neoproterozoic igneous rocks, dating to approximately 0.8–0.7 Ga [59,60]. Hence, the occurrence of Neoproterozoic igneous rocks provides a dependable marker for distinguishing the tectonic characteristics of the YC from those of the NCC [13,61,62]. As described above, the magmatic core of zircons from the studied granitic gneiss yielded a concordant 206Pb/238U age of 774 ± 32 Ma. Obviously, the origin of the protolith for the granitic gneiss is hypothesized to be the northern margin of the YC. The sample biotite schist is completely encased by granitic gneiss as a lens. Therefore, the biotite schist’s protolith is also considered part of the ancient basement of the YC.

5.2. Retrograde Metamorphism and Anatexis During the Late Triassic

For an extended period, the investigation of UHP metamorphism and anatexis within the Sulu orogenic belt has garnered considerable interest. Previous studies of the China Continental Scientific Drilling Project (CCSD) eclogite and granitic gneiss have found petrology and zircon evidence of partial melting, such as mineral intergranular melt, garnet, and accessory minerals (zircon) appeared in felsic multiphase solid inclusions [63]. Substantial microscopic evidence of partial melting, including petrological features (e.g., intergranular melt, felsic multiphase solid inclusions in garnet) and zircon characteristics, has been identified in both eclogites and granitic gneisses [64,65,66]. The origin of the late Triassic ultra-alkaline intrusion rocks emerging from the Shidao area in the northeast Sulu orogenic zone is also related to the dehydration and melting of the UHP rocks [67]. The magmatic activity that occurred synchronously with exhumation, exemplified by these exposed potassium-rich granites, signifies the dehydration and melting processes experienced by the UHP metamorphic rocks during their exhumation [15,68]. The syn-exhumation granites (218–216 Ma) emerging from Mibai Mountain in the Lanshan area in the middle of the Sulu orogenic zone are also thought to be the product of decompression and melting of subduction continental crust during the exhumation stage [69]. Both experimental petrological evidence and micro- to macro-scale observations of partial melting in natural samples affirm that extensive anatexis has occurred in the metamorphic rocks of the Sulu orogenic zone. This phenomenon offers an exemplary opportunity to investigate melt/fluid dynamics during the process of continental collision orogeny.
Chronostratigraphic analyses further indicate that the UHP metamorphic rocks within the Sulu orogenic zone underwent multiple episodes of anatexis during both the exhumation phase and the post-collisional period. Li et al. (2016) [64] studied the migmatite in the Weihai area and identified that the anatexis of the UHP metasdepositional rocks occurred at 230–227 Ma, while the anatexis of the underlying UHP metamorphic granite occurred at 218–214 Ma, both of which were caused by the dehydration and decomposition of water-bearing minerals such as phengite. Xu et al. (2013) [70] further studied the Weihai migmatite and concluded that the plagioclase-rich leucosome (228 ± 2 Ma) was formed in the early initial melt separation crystallization, while the potassium feldspar-rich leucosome (219 ± 2 Ma) was formed in the later evolution of the melt. Liu et al. (2010) [34] identified the UHP metamorphic zircon (227 ± 3 Ma, containing coesite inclusions) and the anatectic zircon (212 ± 2 Ma) at the leucosome of the Weihai migmatite, which is believed to have occurred in the HP granulite phase during the exhumation process of the UHP eclogitic metamorphism after the peak metamorphism. To sum up, there are two main stages of anatexis during the Triassic. The first stage of anatexis occurred at ~222 Ma, probably near the peak stage of the UHP metamorphism (~230 Ma), which belongs to the partial melting record of the initial exhumation under UHP conditions [71,72]; The second stage of the anatectic event occurred at ~215 Ma, which represents the dehydration and melting event of phengite when the subducting slab further exhumated to the depth of the lower crust [34,73,74].
The zircon rims of metamorphic origin in the granitic gneiss provided an age of 211.1 ± 2.3 Ma, suggesting that the granitic gneiss underwent retrograde metamorphism subsequent to UHP metamorphism during the Triassic period of continental collisional orogenesis. Anatectic zircons of the biotite schist yielded nearly concordant 206Pb/238U ages ranging from 206 Ma to 225 Ma with a weighted mean of 214.6 ± 3.6 Ma. It is evident that the Triassic period witnessed simultaneous occurrences of retrograde metamorphism and anatexis. Based on the above research combined with previous studies, we propose a hypothetical model for the metamorphism and anatexis experienced by the biotite schist and granitic gneiss. The model suggests that (1) during the Triassic Subduction–collision stage (~245–215 Ma), the Yangtze Craton continental crust was subducted beneath the North China Craton continental crust (Figure 8A); (2) after break-off of the deeply subducted oceanic slab (~215 Ma), the Yangtze Craton continental crust reached its maximum depth and underwent UHP metamorphism (Figure 8B); (3) subsequently, the deeply subducted and UHP metamorphosed Yangtze Craton continental crust was exhumed (~215–205 Ma), during which it experienced retrograde metamorphism and anatexis, as recorded by zircons in the granitic gneiss and biotite schist (Figure 8C). These multistage metamorphic and anatectic events during the Triassic may indicate that deep subducted continental crust materials underwent significant reconstruction during the continental collision stage and post-collision tectonic evolution.

5.3. Paleoproterozoic Tectonothermal Event

Zircon, a ubiquitous accessory mineral in various rock types, is highly valued for its exceptional resistance to alteration, significant U-Th content, and remarkably slow Pb diffusion rates [76,77]. These characteristics make it an ideal geochronometer for preserving precise records of geological events [78]. Even when subjected to multiple magmatic and high-grade metamorphic events, zircon maintains its primary geochemical and isotopic signatures, rendering it particularly valuable for investigating ancient tectonic evolution [79,80,81,82,83]. The mineral’s extremely low 176Lu/177Hf ratio (resulting from high Hf but negligible Lu content) ensures its measured 176Hf/177Hf ratio accurately reflects the initial Hf isotopic composition at formation [84]. Modern MORB (representing asthenospheric mantle) has εHf = +13.9 [85]. During mantle differentiation, crustal materials develop lower Lu/Hf ratios than depleted mantle, causing their 176Hf/177Hf to evolve more slowly. Consequently, crustal εHf(t) values become increasingly negative over time, while mantle values trend positive. Positive zircon εHf(t) values suggest protolith formation involved mantle-derived or ancient crustal components, whereas negative values indicate predominantly crustal sources [86,87].
Metamorphic zircon can originate through substitution reactions that take place under subsolidus conditions, which Hoskin and Black (2000) [88] termed “solid-state recrystallization”, or alternatively, through crystallization from melts or fluids. Crystallization of zircon has been documented to occur during prograde [89,90], peak [88,91], as well as retrograde [92,93] P–T conditions. In metamorphic rocks, various patterns of Hf isotope homogenization have been documented, ranging from partial to complete, which may result from the dissolution of relic grains and the subsequent formation of new zircon grains [94,95]. Alternatively, significant variability in Hf isotope compositions has also been observed [96,97]. This becomes especially significant in cases where other minerals have undergone extensive retrograde metamorphic overprinting, resulting in the loss of their primary information.
The ancient basement of the YC was established during the Archean era, with rock exposures dating back to 3.3–2.5 Ga sporadically distributed across the region [3,98]. Multiple areas within the YC preserve evidence of tectonothermal events at approximately 2.5 Ga. For instance, zircon U-Pb ages of 2493 ± 19 Ma have been obtained from TTG gneisses of the Zhangbaling Group in the Nanhuang area of Jiashan [99]. In the Dabie orogenic belt, zircon analyses of Shuanghe gneisses yield an upper intercept age of 2458 ± 76 Ma [100]. Chen et al. (2002) [101] similarly reported an upper intercept age of 2489 ± 25 Ma from eclogite samples collected from the same region. Detrital zircon age spectra from the northern YC display a significant peak at ~2.49 Ga. As demonstrated by Hu et al. (2013) [102], gneisses from the Douling Complex in the South Qinling orogen show protolith ages of ~2.5 Ga, with associated metasedimentary rocks recording metamorphic ages of ~2.48 Ga, leading to their proposal of a magmatic event between 2.51 and 2.47 Ga in the northern YC. Comparable Archean geological records are well documented in the Dabie–Sulu orogenic belt [6,20,21]. Furthermore, the YC exhibits widespread Paleoproterozoic tectonothermal activity spanning 2.1–1.8 Ga [33,35,103,104,105].
Zircon Hf isotopic studies have documented multiple episodes of continental crustal growth along the northern margin of the YC during Archean, Paleoproterozoic, and Neoproterozoic times, with the Neoproterozoic crustal formation involving remelting of pre-existing Paleoproterozoic crustal materials [23,49,50,51,52,53,54]. In our investigation, the εHf(t) values obtained from both magmatic and metamorphic zircons in the granitic gneiss can be projected back along the normal crustal evolution line to Paleoproterozoic crustal sources of ~2.5 Ga age (Figure 7). The metamorphic zircon rims separated from the gneiss display εHf(t) values between −14.7 and −13.3, with corresponding TDM C(Hf) model ages ranging from 2176 Ma to 2092 Ma. These isotopic characteristics strongly suggest that the protolith formed through reworking of ancient crustal sources, incorporating components derived from Paleoproterozoic crust aged between 2.5 and 2.0 Ga.
Zircons from the biotite schist exhibit εHf(t) values ranging from −22.6 to −18.3 (Figure 7), with corresponding TDM C(Hf) model ages between 2675 Ma and 2407 Ma. The zircon cores from the granitic gneiss show εHf(t) values of −13.6 to −6.3 and TDM C(Hf) ages from 2487 Ma to 2075 Ma. Significantly, the εHf(t) values of these metamorphic zircons follow the normal crustal evolution trend defined by Neoproterozoic magmatic zircons. Based on the combined geochronological and Hf isotopic evidence from the granitic gneiss, we conclude that its protolith represents Neoproterozoic magmatic rocks derived from reworking of 2.5–2.0 Ga crustal materials. During the Triassic continental collision orogeny, these Neoproterozoic magmatic rocks experienced progressive metamorphic evolution to form the current metamorphic assemblages. The synchronous timing of retrograde metamorphism and anatexis suggests that following slab break-off, the deeply subducted Yangtze Craton crust underwent exhumation accompanied by retrograde metamorphism. Simultaneously, in this extensional regime, decompression during exhumation triggered and progressively intensified crustal anatexis.

6. Conclusions

  • The biotite schist records a significant anatectic event at ~215 Ma (Triassic). This thermal event corresponds to the late-stage collision in the regional orogenic cycle.
  • The granitic gneiss reveals a complex polyphase history. Protolith formation during Neoproterozoic (~774 Ma) magmatism, potentially related to Rodinia breakup, as indicated by inherited zircon cores with magmatic zoning Subsequent Triassic metamorphic overprinting at ~211 Ma, documented by a) Metamorphic zircon rims with low Th/U ratios; b) Development of gneissic foliation with syn-kinematic mineral growth; c) Zircons Hf isotopic signatures suggest derivation from reworked Paleoproterozoic (~2.5–2.0 Ga) crustal sources.
  • Critical unresolved questions requiring further investigation, namely (a) Detailed geochemical comparison between Neoproterozoic and Triassic granitoids; (b) Zircon trace element analysis to discriminate source characteristics; (c) High-precision dating of Triassic magmatism to constrain its tectonic setting.

Author Contributions

Y.Y.: investigation, data curation, writing—original draft, writing—reviewing and editing; H.L.: conceptualization, methodology, investigation, writing—original draft, writing—reviewing and editing, visualization, supervision, project administration; funding acquisition; F.X.: methodology, investigation, writing—original draft, writing—reviewing and editing, supervision; funding acquisition; H.Z.: investigation, writing—original draft; C.Y.: investigation. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the National Natural Science Foundation of China (Nos. 42202056 and 42172098), Jiangxi Provincial Natural Science Foundation (No. 20202BABL213031).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data will be made available on request to the corresponding author.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Schematic geological map of the Dabie–Sulu orogenic belt (modified after Lei, 2015 [31]). NCC and YC in the insert denote the North China Craton and the Yangtze Craton, respectively. CCSD, the drill site of the Chinese Continental Scientific Drilling. HP—High pressure, UHP—Ultrahigh pressure.
Figure 1. Schematic geological map of the Dabie–Sulu orogenic belt (modified after Lei, 2015 [31]). NCC and YC in the insert denote the North China Craton and the Yangtze Craton, respectively. CCSD, the drill site of the Chinese Continental Scientific Drilling. HP—High pressure, UHP—Ultrahigh pressure.
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Figure 2. Regional tectonic map of the Sulu orogen in Central China, showing sample location (modified after Xiang et al., 2014) [32].
Figure 2. Regional tectonic map of the Sulu orogen in Central China, showing sample location (modified after Xiang et al., 2014) [32].
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Figure 3. (A) Field photograph of biotite schist and granitic gneiss from the Sulu orogenic belt, with yellow five-pointed stars indicating sampling locations; (B,C) Photomicrographs of biotite schist, biotite + quartz + calcite; (D,E) Photomicrographs of granitic gneiss, quartz + K-feldspar + biotite. Abbreviations: Qz (quartz), Bt (biotite), Cal (calcite), Ap (apatite), Kfs (K-sfeldspar), Sph (sphene), Alm (allanite).
Figure 3. (A) Field photograph of biotite schist and granitic gneiss from the Sulu orogenic belt, with yellow five-pointed stars indicating sampling locations; (B,C) Photomicrographs of biotite schist, biotite + quartz + calcite; (D,E) Photomicrographs of granitic gneiss, quartz + K-feldspar + biotite. Abbreviations: Qz (quartz), Bt (biotite), Cal (calcite), Ap (apatite), Kfs (K-sfeldspar), Sph (sphene), Alm (allanite).
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Figure 4. Cathodoluminescence (CL) images of zircons from biotite schist and granitic gneiss in the Sulu orogenic belt, showing the inner structures of zircons. The red and yellow circles indicate LA–ICP–MS dating spots and corresponding U–Pb ages and Lu-Hf isotopes.
Figure 4. Cathodoluminescence (CL) images of zircons from biotite schist and granitic gneiss in the Sulu orogenic belt, showing the inner structures of zircons. The red and yellow circles indicate LA–ICP–MS dating spots and corresponding U–Pb ages and Lu-Hf isotopes.
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Figure 5. Th/U values versus U–Pb ages of analyzed zircons from biotite schist and granitic gneiss in the Sulu orogenic belt.
Figure 5. Th/U values versus U–Pb ages of analyzed zircons from biotite schist and granitic gneiss in the Sulu orogenic belt.
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Figure 6. (A,B) Zircon U-Pb Concordia diagrams and Chondrite-normalized REE patterns of biotite schist from the Sulu orogenic belt; (C,D) Zircon U-Pb Concordia diagrams and Chondrite-normalized REE patterns of granitic gneiss from the Sulu orogenic belt.
Figure 6. (A,B) Zircon U-Pb Concordia diagrams and Chondrite-normalized REE patterns of biotite schist from the Sulu orogenic belt; (C,D) Zircon U-Pb Concordia diagrams and Chondrite-normalized REE patterns of granitic gneiss from the Sulu orogenic belt.
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Figure 7. Epsilon Hf values versus U–Pb ages of analyzed zircons from biotite schist and granitic gneiss in the Sulu orogenic belt. εHf(t) values of metamorphic zircons in biotite schist are distributed from −22.6 to −18.3. εHf(t) values of magmatic zircons in granitic gneiss are distributed from −13.6 to −6.3. εHf(t) values of anatectic zircons in granitic gneiss are distributed from −14.7 to −13.3. The εHf(t) values of zircons from the northern margin of the YC are from Liu et al. (2008, 2009); Peng et al. (2009, 2012); Kong et al. (2009); Xiong et al. (2009); Gao et al. (2011); Yang et al. (2016) [23,49,50,51,52,53,54,55].
Figure 7. Epsilon Hf values versus U–Pb ages of analyzed zircons from biotite schist and granitic gneiss in the Sulu orogenic belt. εHf(t) values of metamorphic zircons in biotite schist are distributed from −22.6 to −18.3. εHf(t) values of magmatic zircons in granitic gneiss are distributed from −13.6 to −6.3. εHf(t) values of anatectic zircons in granitic gneiss are distributed from −14.7 to −13.3. The εHf(t) values of zircons from the northern margin of the YC are from Liu et al. (2008, 2009); Peng et al. (2009, 2012); Kong et al. (2009); Xiong et al. (2009); Gao et al. (2011); Yang et al. (2016) [23,49,50,51,52,53,54,55].
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Figure 8. Hypothetical model for the genetic mechanism of the metamorphism and anatexis of biotite schists and granitic gneisses in the Sulu orogenic belt (modified after Lei et al., 2024) [75]. (A) Subduction–collision stage (~245–215 Ma): Subduction of the Yangtze Craton continental crust beneath the North China Craton; (B) Slab break-off stage (~215 Ma): The Yangtze Craton crust reaches maximum depth, undergoing ultrahigh-pressure (UHP) metamorphism; (C) Post-collision stage (~215–205 Ma): Exhumation of the deeply subducted and UHP-metamorphosed Yangtze Craton crust, accompanied by retrograde metamorphism and anatexis.
Figure 8. Hypothetical model for the genetic mechanism of the metamorphism and anatexis of biotite schists and granitic gneisses in the Sulu orogenic belt (modified after Lei et al., 2024) [75]. (A) Subduction–collision stage (~245–215 Ma): Subduction of the Yangtze Craton continental crust beneath the North China Craton; (B) Slab break-off stage (~215 Ma): The Yangtze Craton crust reaches maximum depth, undergoing ultrahigh-pressure (UHP) metamorphism; (C) Post-collision stage (~215–205 Ma): Exhumation of the deeply subducted and UHP-metamorphosed Yangtze Craton crust, accompanied by retrograde metamorphism and anatexis.
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Table 1. LA–ICP–MS zircon U-Pb isotopic data for the biotite schist.
Table 1. LA–ICP–MS zircon U-Pb isotopic data for the biotite schist.
SpotElement (ppm)Th/U207Pb/206Pb207Pb/235U206Pb/238U207Pb/206Pb207Pb/235U206Pb/238U
PbThURatio±1σRatio±1σRatio±1σAge (Ma)±1σAge (Ma)±1σAge (Ma)±1σ
1279 2018 1133 1.780.048470.00140.228600.00650.03410.0003120.570.4209.05.42162.1
237 129 611 0.210.049300.00170.221080.00790.03240.0003161.283.3202.86.62062.0
3192 1408 921 1.530.048270.00140.224180.00640.03350.0004122.373.1205.45.32132.3
4103 668 665 1.000.049020.00160.233390.00780.03440.0004150.177.8213.06.52182.3
574 497 524 0.950.049780.00200.232940.00920.03390.0004183.486.1212.67.62152.8
692 695 529 1.310.047880.00190.227300.00890.03450.0005100.183.3208.07.32193.1
776 534 587 0.910.049860.00180.231670.00810.03350.0004187.1113.9211.66.72132.4
886 616 547 1.120.047640.00170.216050.00750.03280.000479.785.2198.66.32082.3
951 340 391 0.870.053290.00330.258940.01590.03490.0004342.7140.7233.812.82212.6
1073 439 417 1.050.048160.00200.232480.00970.03490.0005105.6−94.4212.28.02212.9
1179 540 466 1.160.047720.00210.221990.00960.03380.000487.1100.0203.67.92142.6
12240 1681 984 1.710.048110.00160.236440.00810.03560.0005105.677.8215.56.62253.3
Table 2. LA–ICP–MS trace element analyses of zircons for the biotite schist.
Table 2. LA–ICP–MS trace element analyses of zircons for the biotite schist.
SpotLaCePrNdSmEuGdTbDyHoErTmYbLu
14.95236.6412.51122.3188.8339.38152.7027.62207.5640.73131.7122.04218.4524.23
2 7.320.041.121.430.996.211.3613.633.6117.803.9148.747.28
35.33155.549.8994.6964.7231.28129.5229.12263.1864.76261.0550.61543.8364.26
41.2496.843.7540.5538.2420.36100.7725.41255.1867.79283.7056.08615.6777.01
50.3430.840.819.6810.195.3127.726.7071.3019.2378.8614.94169.0521.49
60.1332.870.739.698.834.9324.234.4034.097.1924.794.3048.126.26
70.8334.361.8619.0713.685.3924.164.3334.927.4731.166.1773.9311.17
81.6778.253.9641.9137.6619.1589.4123.02224.3059.54253.9450.44553.6369.30
91.1948.062.7028.0523.8111.4452.3812.57121.9631.61131.9026.04282.2836.22
100.0712.100.222.322.871.679.412.2926.047.8036.987.6290.2613.18
111.1565.393.7737.9532.8215.2661.8512.7199.0920.8076.8014.03149.8518.72
125.66141.079.5786.2056.6826.0398.3419.56160.6935.22129.3722.91232.4026.60
Table 3. LA–ICP–MS zircon U-Pb isotopic data for the granitic gneiss.
Table 3. LA–ICP–MS zircon U-Pb isotopic data for the granitic gneiss.
SpotElement (ppm)Th/U207Pb/206Pb207Pb/235U206Pb/238U207Pb/206Pb207Pb/235U206Pb/238U
PbThURatio±1σRatio±1σRatio±1σAge (Ma)±1σAge (Ma)±1σAge (Ma)±1σ
Magmatic zircon
11101881291.450.07220.00251.22280.04410.12280.0016990.771.5811.120.1746.59.2
246641150.560.06760.00261.18700.04520.12700.0016857.4113.1794.621.0770.99.1
31302032080.980.06260.00201.14030.03600.13140.0013694.564.7772.617.1795.67.7
4971581620.970.06370.00211.13080.03760.12780.0013731.569.3768.117.9775.27.7
Metamorphic zircon
1717922740.030.04960.00110.22990.00530.03330.0003176.051.8210.14.4211.42.0
2496616330.040.05000.00130.22970.00610.03310.0003194.528.7209.95.0209.81.8
310110733810.030.05030.00120.23360.00580.03330.0003209.389.8213.24.8211.52.1
410212334630.040.04940.00120.23470.00610.03400.0004168.662.0214.15.0215.82.2
5728523190.040.05090.00130.23640.00640.03320.0003239.052.8215.55.2210.82.0
6558117680.050.04930.00120.22570.00530.03300.0003161.253.7206.74.4209.11.8
Table 4. LA–ICP–MS trace element analyses of zircons for the granitic gneiss.
Table 4. LA–ICP–MS trace element analyses of zircons for the granitic gneiss.
SpotLaCePrNdSmEuGdTbDyHoErTmYbLu
Magmatic
zircon
10.01128.840.103.476.572.8638.3513.55175.9866.37330.8267.66748.39111.77
20.0124.890.030.601.560.4210.533.9861.7124.68140.1133.19412.0065.20
30.5489.180.253.065.121.3726.989.69139.5555.66295.9666.02793.78124.87
40.1647.220.152.274.571.3621.787.34104.7339.40211.1748.25588.2190.93
Metamorphic zircon
10.069.120.121.070.890.315.041.8528.7613.7495.9428.31462.52104.48
2 7.110.02 0.570.203.361.5023.3711.6481.1224.46399.9892.20
30.1611.130.433.872.910.806.761.9428.8912.7091.2626.46440.01103.26
40.109.580.191.301.090.404.862.0932.8515.59110.7333.00560.32128.99
50.3610.880.482.671.510.494.191.4222.2710.7770.6921.28352.9981.18
60.058.540.081.080.940.343.861.7627.5113.0994.8729.27490.61111.76
Table 5. LA–MC–ICP–MS zircon Lu-Hf isotopic data of the biotite schist.
Table 5. LA–MC–ICP–MS zircon Lu-Hf isotopic data of the biotite schist.
SpotAge (Ma)176Hf/177Hf±(2σ)176Lu/177Hf±(2σ)176Yb/177Hf176Hf/177Hf (t)εHf(0)εHf(t)±(2σ)TDMTDM(Hf2)2σTDM(Hf)C
12160.2820940.0000160.0005530.0000020.0260180.282092−24.0−19.30.616123420222473
22060.2820420.0000160.0001270.0000010.0055970.282041−25.8−21.30.616653593212592
32130.2820780.0000210.0011870.0000430.0550770.282073−24.6−20.10.716613482292516
42180.2821290.0000210.0020270.0000990.0799550.282121−22.7−18.30.716263324292407
52150.2820800.0000160.0003580.0000080.0148050.282078−24.5−19.80.616233464212504
62190.2820560.0000150.0001350.0000020.0050200.282055−25.3−20.60.516463532212553
72130.2820320.0000160.0002280.0000050.0091810.282031−26.2−21.60.616833616222611
82210.2820020.0000180.0003660.0000110.0136020.282001−27.2−22.40.617293699252672
92140.2820030.0000160.0003210.0000070.0130770.282001−27.2−22.60.617263706212675
102250.2820640.0000190.0006700.0000110.0284620.282061−25.0−20.20.716573502272534
Table 6. LA–MC–ICP–MS zircon Lu-Hf isotopic data of the granitic gneiss.
Table 6. LA–MC–ICP–MS zircon Lu-Hf isotopic data of the granitic gneiss.
SpotAge (Ma)176Hf/177Hf±(2σ)176Lu/177Hf±(2σ)176Yb/177Hf176Hf/177Hf (t)εHf(0)εHf(t)±(2σ)TDMTDM
(Hf2)		2σTDM
(Hf)C
Magmatic zircon
17460.281970.000020.002450.000050.092190.28193−28.5−13.30.818793216312487
27710.282130.000010.001130.000000.038690.28211−22.8−6.40.515902622202075
37960.282130.000020.002090.000010.071210.28210−22.8−6.30.616312635242091
Metamorphic zircon
12110.282250.000010.001570.000010.039290.28224−18.6−14.20.414392954182144
22100.282240.000010.001210.000010.029830.28224−18.8−14.30.414332968172153
32110.282250.000010.001480.000010.035530.28225−18.4−14.00.414292937162132
42160.282270.000010.001770.000010.042180.28226−17.8−13.30.414142878172092
52110.282230.000010.000970.000000.025090.28223−19.2−14.70.514403000192176
62090.282270.000020.001410.000010.036130.28226−17.9−13.50.614052893222100
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Ye, Y.; Lei, H.; Xia, F.; Zhang, H.; Yu, C. Triassic Retrograde Metamorphism and Anatexis in the Sulu Orogenic Zone, Central China: Constraints from U–Pb Ages, Trace Elements, and Hf Isotopic Compositions of Zircon. Appl. Sci. 2025, 15, 6145. https://doi.org/10.3390/app15116145

AMA Style

Ye Y, Lei H, Xia F, Zhang H, Yu C. Triassic Retrograde Metamorphism and Anatexis in the Sulu Orogenic Zone, Central China: Constraints from U–Pb Ages, Trace Elements, and Hf Isotopic Compositions of Zircon. Applied Sciences. 2025; 15(11):6145. https://doi.org/10.3390/app15116145

Chicago/Turabian Style

Ye, Yongkang, Hengcong Lei, Fei Xia, Hui Zhang, and Congjun Yu. 2025. "Triassic Retrograde Metamorphism and Anatexis in the Sulu Orogenic Zone, Central China: Constraints from U–Pb Ages, Trace Elements, and Hf Isotopic Compositions of Zircon" Applied Sciences 15, no. 11: 6145. https://doi.org/10.3390/app15116145

APA Style

Ye, Y., Lei, H., Xia, F., Zhang, H., & Yu, C. (2025). Triassic Retrograde Metamorphism and Anatexis in the Sulu Orogenic Zone, Central China: Constraints from U–Pb Ages, Trace Elements, and Hf Isotopic Compositions of Zircon. Applied Sciences, 15(11), 6145. https://doi.org/10.3390/app15116145

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