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Article

Thermochronology of the Laojunshan–Song Chai Granite Gneiss Massif (North Vietnam, South China)

by
Alexey Travin
1,2,*,
Nikolai Murzintsev
1 and
Nikolai Kruk
1
1
V.S. Sobolev Institute of Geology and Mineralogy SB RAS, 3 Akademika Koptyuga Avenue, 630090 Novosibirsk, Russia
2
Institute of Geology and Mineralogy, Novosibirsk State Technical University, Novosibirsk, K. Marx Ave., 20, 630087 Novosibirsk, Russia
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(3), 251; https://doi.org/10.3390/min14030251
Submission received: 19 December 2023 / Revised: 23 February 2024 / Accepted: 27 February 2024 / Published: 28 February 2024
(This article belongs to the Special Issue Petrogenesis, Geochronology, Mineralization and Geochemistry of Granite Rocks)
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Abstract

:
A reconstruction of the tectonothermal evolution of the Laojunshan–Song Chai granite gneiss massif (North Vietnam, South China) was carried out, based on summaries of the latest isotopic and fission-track dating results. The recorded wide range (420–465 Ma) of the age of granite gneiss rocks testifies to the long-term existence of a partially molten layer at a depth of 20–30 km for several tens of Ma. By the Devonian–early Carboniferous, a section of the excessively thickened crust was denudated, the massif was exhumated to the level of the upper crust, and isotope systems were “frozen”. The rate of uplift of the rocks of the massif is estimated to be about 0.2–0.5 mm/year. In the further history of the granite gneiss massif, episodes of repeated burial to a depth of about 13 km are recorded, associated with the Indosinian collision. The rocks have experienced metamorphism of the amphibolite-green schist facies, accompanied by tectonic transport in the form of a thrust sheet. Over the next 200 Ma, the uplift of the massif and the erosion of the overlying strata occurred in discrete pulses, during a sequence of active tectonic events. Thus, the thermochronological and P-T history of the Laojunshan–Song Chai massif is a kind of chronicle of regional tectonic–thermal events. In the history of the massif, traces of two orogenic cycles associated with the collision of the Cathaysia and Yangtze blocks in the Lower Paleozoic and the Indosinian collision in the Triassic are recorded.

1. Introduction

The Laojunshan–Song Chai granite gneiss massif (Figure 1), formed during the Middle Paleozoic (460–430 Ma) Wuyi–Yunkai orogeny (North-East Vietnam, South China) [1,2], is characterized by a unique history among Phanerozoic granitoid massifs, a long history of its formation, transformations, and exhumation. This story has stretched over the entire Phanerozoic [3,4,5,6,7,8,9,10,11]. After its formation, the rocks of the massif in the Upper Silurian–Lower Devonian underwent metamorphism of the amphibolite facies, followed by a stage of exhumation and cooling. After a long quiet period, the massif experienced episodes of repeated burial and amphibolite facies metamorphism in the Late Permian– Early Triassic. The subsequent exhumation of the massif persisted until the present day. Apparently, such a long history is related to the tectonic position of the massif, located in the area of manifestation of the early Caledonian (Cathaysian), Hercynian, and Indosinian structures of Southeast Asia (Figure 1) [12,13,14,15]. Therefore, there is an understandable great interest in studying the massif using a wide range of petrological, structural, isotopic, and geochronological methods.
Despite the high degree of studying of the Laojunshan–Song Chai massif, there are still a number of problems that cause different interpretations among researchers. For example, there are conflicting data and ideas about the age and duration of the formation of the original granite massif. Some authors, based on the dating of individual samples of granitoids by zircon, mainly by the ion probe method and the LA-ICP method, take them as an estimate of the age of the formation of the massif as a whole—428 ± 5 [4], 426 ± 6 [5], ~430 [9]. Some authors, based on a series of U/Pb dating, believe that the formation of the massif occurred due to an extended period of magmatic activity from 444 to 420 Ma [8,10,11,16].
Various points of view are expressed on the mechanisms and thermal sources of intense tectonic–thermal effects on the rocks of the massif in the Late Permian–Early Triassic. Thus, based on the fact that the occurrence of gabbro–syenite, gabbro–monzodiorite, and basalt–rhyolite associations and high-Al granitoid rocks is interpreted as a result of the Emeishan mantle plume, a number of authors believe that the plume served as a source of thermal effects on the strata of the middle and lower crust of North Vietnam [17,18,19,20,21]. At the same time, a large number of researchers consider the collision of the South China Block with Indo-China to be the cause of Late Permian–Early Triassic thermotectonism ([5,6,11,22] and others).
In this study, we attempt to interpret the existing data set of the isotope dating of the rocks of the Laojunshan–Song Chai massif [4,5,6,7,8,9,10,11] based on the thermochronological approach, supplementing it with data published in the Russian-language segment of the literature [3,23]. This seems important, since during the joint Russian–Vietnamese studies, some of the samples were selected away from the routes of later expeditions (Figure 2). Most of the granitoids studied by the authors have a gneiss-like structure, although weakly gneiss-like and even massive rocks are noted in some areas [3].
In reconstructing the evolution of orogens, along with direct studies of sedimentary complexes, active faults, and landforms, it seems promising to study the thermal history of magmatic and metamorphic rocks formed in the root parts of these orogens using a set of methods characterized by different closure temperatures. For the correct interpretation of thermochronological data, it is important to refer the minerals used for dating to primary magmatic, superimposed syn-tectonic, and metamorphic parageneses.
This study includes the use of a series of geochronological methods characterized by the different closure temperatures of isotope systems of minerals: U/Pb dating of zircon (closure temperature Tc~940 °C), 40Ar/39Ar dating of amphibole (Tc~500 °C), 40Ar/39Ar dating of bioti-te (Tc~340 °C), feldspar (Tc~240 °C), and fission-track dating of apatite (Tc~110 °C) [24]. The Hodges summary shows the values of the closure temperatures of isotope systems calculated on the basis of kinetic parameters determined in laboratory experiments for a cooling rate of 5 °C Ma−1. A change in the cooling rate of the system over a sufficiently wide range leads to changes in the calculated closure temperature in the range of ±30 °C, while the relative position of isotopic systems of different minerals is preserved. Therefore, as a rule, the given values are used as an initial approximation. In cases where it is possible to use several isotope systems, as well as independent geological estimates, it is possible to refine both the cooling rate of the system and the actual values of the closing temperatures using several successive iterations. A comparison of the recorded values of the age of the isotope systems of minerals with the temperatures of their closure allows us to consistently estimate the depth of occurrence of rocks (taking into account the average temperature gradient of 25–30°/km) at various time intervals, starting from their formation and ending with their exhumation to the Earth’s surface as a result of tectonic events. This approach proved to be effective in reconstructing the stages of the tectonic–thermal evolution of Transbaikalia (Russia), performed on the basis of complex thermochronological studies of granitoids of the Angara–Vitim batholith, the largest in the Phanerozoic [25].
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Figure 1. A diagram showing the distribution of Early–Middle Paleozoic granites and metamorphic rocks in the eastern part of the South China Block (Adapted from Ref. [15]: Li, Z.-X., Li, X.-H., Warth, J.-A., Clark, C., Li, W.-X., Zhang, C.-L., and Bao, C., 2010, Magmatic and metamorphic events during the early Paleozoic Wuyi-Yunkai orogeny, southeastern South China: New age constraints and pressure-temperature conditions: Geological Society of America Bulletin, v. 122, p. 772–793, https://doi.org/10.1130/B30021.1. Copyright Geological Society of America. Used with permission). The small arrows show the direction of transport associated with the Indo-China–South China collision [26]. The large arrows show the position of the Laojunshan–Song Chai massif, and the Wuyi and Yunkai domains. The rectangle indicates the research area shown in Figure 2.
Figure 1. A diagram showing the distribution of Early–Middle Paleozoic granites and metamorphic rocks in the eastern part of the South China Block (Adapted from Ref. [15]: Li, Z.-X., Li, X.-H., Warth, J.-A., Clark, C., Li, W.-X., Zhang, C.-L., and Bao, C., 2010, Magmatic and metamorphic events during the early Paleozoic Wuyi-Yunkai orogeny, southeastern South China: New age constraints and pressure-temperature conditions: Geological Society of America Bulletin, v. 122, p. 772–793, https://doi.org/10.1130/B30021.1. Copyright Geological Society of America. Used with permission). The small arrows show the direction of transport associated with the Indo-China–South China collision [26]. The large arrows show the position of the Laojunshan–Song Chai massif, and the Wuyi and Yunkai domains. The rectangle indicates the research area shown in Figure 2.
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Figure 2. Geological diagram and points of geochronological testing of the Laojunshan–Song Chai massif (North Vietnam, South China) after [23], Vladimirov A.G., Travin A.V., Anh P.L., Murzintsev N.G., Annikova I.Yu., Mikheev E.I., Duong N.A., Man T.T., Lan T.T. THERMOCHRONOLOGY OF GRANITOID CATHOLITHS AND THEIR TRANSFORMATION INTO METAMORPHIC CORE COMPLEXES (EXAMPLE OF SONG-CHAI MASSIF, NORTHERN VIETNAM). Geodynamics & Tectonophysics. 2019;10(2):347-373. ((In Rus.) https://doi.org/10.5800/GT-2019-10-2-0418 with changes. Used with the permission of the editorial board of the journal Geodynamics and Tectonophysics. The sampling points are shown: 1—for [3], 2—for the remaining samples shown in Table 1.
Figure 2. Geological diagram and points of geochronological testing of the Laojunshan–Song Chai massif (North Vietnam, South China) after [23], Vladimirov A.G., Travin A.V., Anh P.L., Murzintsev N.G., Annikova I.Yu., Mikheev E.I., Duong N.A., Man T.T., Lan T.T. THERMOCHRONOLOGY OF GRANITOID CATHOLITHS AND THEIR TRANSFORMATION INTO METAMORPHIC CORE COMPLEXES (EXAMPLE OF SONG-CHAI MASSIF, NORTHERN VIETNAM). Geodynamics & Tectonophysics. 2019;10(2):347-373. ((In Rus.) https://doi.org/10.5800/GT-2019-10-2-0418 with changes. Used with the permission of the editorial board of the journal Geodynamics and Tectonophysics. The sampling points are shown: 1—for [3], 2—for the remaining samples shown in Table 1.
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Thus, the thermal history of magmatic and metamorphic rocks formed in the root parts of orogens can serve as an independent source of information about the tectonic evolution of folded regions.

2. Geological Description of the Laojunshan–Song Chai Massif

The Laojunshan–Song Chai granite gneiss massif is located in the Northeast of Vietnam and in the South of China. Its Vietnamese part (S = 2500 km2) significantly exceeds in area all the known granitoid massifs of Vietnam. The massif forms an asymmetric metamorphic NE-SW antiform (Figure 2) ([3,4,6,8,11] and others). The overlying meta-sedimentary rocks of the Lower Paleozoic age observed in this area are host to the original granites. The central part of the massif mainly consists of granite gneisses formed from porphyry granites that were introduced in the Lower Paleozoic. Undeformed remnants of the original granite intrusion have been preserved in places. In the Middle Triassic, the massif underwent a transformation, which, based on thermo-barometric estimates, occurred at a temperature of 580 °C and a pressure of 5 kbar, which corresponds to a depth of 15 km [4]. The Upper Proterozoic and Lower Paleozoic sedimentary rocks hosting the massif underwent metamorphism close to orthogneiss, with the formation of sericite, chlorite, and quartz. The nature of the deformations is the same in the gneiss core and the meta-sedimentary cover. Kinetic indicators (shear bands, asymmetric porphyroclasts of K-feldspar) characterize the movement of the thrust with transport in a northern direction [11,22,26].
The composition of the rocks of the Laojunshan–Song Chai massif varies from quartz diorites to leucogranites [3,10,16,27]. All granitoids are peraluminous and mainly belong to the calc-alkaline series. The rocks are characterized by fractionated REE distribution spectra enriched with light lanthanides ((La/Yb)n = 7.13 − 16.85) and negative Eu anomalies (Eu/Eu* = 0.33–0.58); minimums in Ba, Nb, Sr, Zr, and Ti are distinguished on the multi-element spectra. As was noted earlier, the granite gneiss rocks of the Laojunshan–Song Chai massif are a southwestern continuation of the Early Paleozoic granitoids of the Wuyi–Yunkai orogen (Figure 1) and were formed under similar tectonic conditions, namely, granite magmatism 460–435 Ma ago and 435–400 Ma ago within the South China orogeny region, corresponding to syn-collisional orogen and post-collisional extension, respectively ([1,2,8,15,28,29,30] and others).
The Laojunshan–Song Chai granite gneiss massif is located in the outer zone of the Emeishan large igneous province [31,32]; it is also spatially related to the Song-Ma–Red River shear zone [33]. Thus, the tectonic–thermal history of the massif can bear the marks of many superimposed influences and serve as a kind of chronicle of regional geodynamic events.

3. Thermochronology of the Laojunshan–Song Chai Massif

When reconstructing a complex, multi-stage history of geological rocks based on isotope dating methods, it is important to clearly understand the relationship between the closure temperature of the isotope system Tc and the formation temperature Tf of the dated mineral phase (Figure 3).
The thermochronological approach introduced by Dodson and Giletti [34,35] is based on the kinetic parameters of daughter isotopes measured in laboratory conditions [24] and assumes the immutability of the crystal structure of the mineral, while considering the case when the closure temperature is significantly lower than the temperature of the formation of the mineral—Tc < Tf. In the diagram in Figure 3, the dating of Da3 (40Ar/39Ar for amphibole) and Db4 (40Ar/39Ar for biotite) correspond to this case, if amphibole and biotite are really magmatic and their structure was not transformed during late events. In this case, the obtained dating corresponds to the closure time of the corresponding isotope system. In some situations, the ratio between the closure temperature of the isotope system Tc and the formation temperature Tf of the dated mineral phase is the opposite—Tc > Tf. In the diagram in Figure 3, the dating Dz1, Dz2 (U/Pb on zircon) correspond to this case. With this ratio, the obtained dating directly corresponds to the age of formation of the dated mineral phase, namely, zircon.
A summary of the dating of the rocks of the Laojunshan–Song Chai massif obtained by various methods is shown in Table 1 and in Figure 4. Each dating corresponds to the value of the closure temperature of the isotope system (Tc) or the formation (Tf) of the corresponding mineral phase.
Table 1. Summary of geochronology results of the Laojunshan–Song Chai massif (NE Vietnam, SW China).
Table 1. Summary of geochronology results of the Laojunshan–Song Chai massif (NE Vietnam, SW China).
SampleRock/Mineral *Metod **Age (Ma)Closure/
Formation
T (°C) ***		Reference
7 samplesgranite, granite gneiss/Whole rockRb/Sr465 ± 12760f[3]
SH-4/93granite, granite gneiss/zrnU/PbT464 ± 10760f[3]
10YN-22Dgranite gneiss/zrnU/PbL457 ± 5760f[9]
10YN-18Agranite gneiss/zrnU/PbL456 ± 12760f[9]
10YN-22Bgranite gneiss/zrnU/PbL453 ± 3760f[9]
DL-1, DL3granite gneiss/zrnU/PbI452 ± 5760f[8]
LJ1778-2granite gneiss/zrnU/PbL445.1 ± 2.4760f[11]
LJ1773-9A-1granite gneiss/zrnU/PbL441.3 ± 2.2760f[11]
14WS-17-2granite gneiss/zrnU/PbL436.3 ± 5.5760f[10]
DL-1, DL3granite gneiss/zrnU/PbI436 ± 6760f[8]
VN14-52-2granite gneiss/zrnU/PbL433.8 ± 3.2760f[10]
14WS-16granite gneiss/zrnU/PbL433.3 ± 2.5760f[10]
WS13-8granite gneiss/zrnU/PbL433.1 ± 2.7760f[10]
10YN-22Bgranite gneiss/zrnU/PbL430 ± 3760f[9]
10YN-18Agranite gneiss/zrnU/PbL430 ± 2760f[9]
15VN-58-1granite gneiss/zrnU/PbL429.5 ± 4.9760f[10]
15VN-58-1granite gneiss/zrnU/PbL429.5 ± 4.9760f[10]
WS13-11-1granite gneiss/zrnU/PbL429.1 ± 4.5760f[10]
WS13-7granite gneiss/zrnU/PbL429 ± 2.1760f[10]
WS13-24granite gneiss/zrnU/PbL429 ± 3.3760f[10]
10YN-13Bgranite gneiss/zrnU/PbL429 ± 3760f[9]
15VN-62-2granite gneiss/zrnU/PbL428.4 ± 4760f[10]
V159-3granite gneiss/zrnU/PbT428 ± 5760f[4]
LJ1829-2Agranite gneiss/zrnU/PbL427 ± 1.3760f[11]
15VN-63granite gneiss/zrnU/PbL426 ± 2.7760f[10]
V101-4granite gneiss/zrnU/PbI424 ± 6760f[5]
15VN-65granite gneiss/zrnU/PbL422.8 ± 2.7760f[10]
LJ1829-1Agranite gneiss/zrnU/PbL420.7 ± 1.0760f[11]
V160-3schist/mnz in grtTh/PbI419 ± 3550f[7]
DL-1-6granite gneiss/zrnU/PbI409 ± 6760f[8]
V160-3schist/mnz in grtTh/PbI380 ± 17550f[7]
V160-3schist/mnzTh/PbI255 ± 14575f[7]
V160-3schist/mnzTh/PbI246 ± 8575f[7]
LJ1773-9A-2amphiboliteU/PbL241.5 ± 1.4575f[11]
V160-3schist/mnzTh/PbI240 ± 3575f[7]
DL-5-6granite gneiss/ampAr/Ar237 ± 5500f[8]
VN-324-2granite gneiss/msAr/Ar236 ± 2366c[6]
DL-1-6granite gneiss/zrnU/PbS234 ± 10500f[8]
VN-322-2granite gneiss/msAr/Ar234 ± 2366c[6]
LJ1829-2Agranite gneiss/zrnU/PbL232.8 ± 1.8575f[11]
V160-3schist/mnzTh/PbI230 ± 5575f[7]
4-94-1granite gneiss/btK/Ar230 ± 2340c[3]
V160-3schist/mnzTh/PbI224 ± 7575f[7]
4-94-1granite gneiss/msK/Ar222 ± 2366c[3]
V160-3schist/mnzTh/PbI216 ± 40575f[7]
3-73a-1granite gneiss/msK/Ar212 ± 2366c[3]
VN-159-3granite gneiss/msAr/Ar210 ± 9366c[4]
V160-3granite gneiss/w.r.-msRb/Sr206 ± 5316c[4]
V160-3schist/mnzTh/PbI203 ± 5575[7]
VN-329-2granite gneiss/btAr/Ar201 ± 2340c[6]
VN-335-2granite gneiss/msAr/Ar198 ± 2366c[6]
4-89-1granite gneiss/btK/Ar192 ± 2340c [3]
VN-159-3granite gneiss/btAr/Ar190 ± 8340c[4]
VN-335-2granite gneiss/btAr/Ar176 ± 2340c[6]
V160-3granite gneiss/w.r.-btRb/Sr176 ± 3300c[4]
4-94-1granite gneiss/fspK/Ar175 ± 2230c[3]
VN-333-2granite gneiss/btAr/Ar166 ± 2340c[6]
VN-159-3granite gneiss/fspAr/Ar153 ± 3230c[4]
DL-2-6granite gneiss/msAr/Ar144 ± 2366c[8]
4-89-1granite gneiss/fspK/Ar144 ± 2230c[3]
DL-3-6granite gneiss/msAr/Ar140 ± 2366c[8]
DYK07-03Qz-Ms vein/msAr/Ar124.3 ± 0.7366c[36]
DYK06-08bt-pegmatite vein/btAr/Ar123.8 ± 0.7340c[36]
DL-4-6granite gneiss/btAr/Ar116 ± 3340c[8]
DYK06-13Granofels/btAr/Ar103.2 ± 0.5340c[36]
DL-1-6granite gneiss/btAr/Ar84 ± 1340c[8]
V159-3granite gneiss/zrnFT77.5 ± 4230c[4]
DYK06-09Granite massif/btAr/Ar64.4 ± 0.5340c[36]
V159-3granite gneiss/apFT33.6 ± 4110c[4]
9805-2granite gneiss/apFT24 ± 2110c[6]
9801-2granite gneiss/apFT23 ± 2110c[6]
9811-2granite gneiss/apFT20 ± 2110c[6]
9807-2granite gneiss/apFT20 ± 2110c[6]
9814-2granite gneiss/apFT19 ± 2110c[6]
9812-2granite gneiss/apFT19 ± 2110c[6]
* The following designations are used in the table: zrn—zircon, mnz—monazite, amp—amphibole, ms—muscovite, bt—biotite, fsp—feldspar, ap—apatite, w.r.—whole rock. ** U/Pb zircon dating and Th/Pb monazite dating were performed using the conventional thermal ionization method (U/PbT), the ion probe method using the SHRIMP II or Cameca mass spectrometer (U/PbI, Th/PbI), and the ICP laser ablation mass spectrometry method (U/PbL). *** Tc is the closure temperature of the corresponding isotope system; Tf is the temperature of formation of the corresponding mineral phase. The weighted average age values are highlighted, calculated using tabular data for three or more relatively ancient values from the mentioned studies.
In [23], the liquidus temperature of granite melts was estimated based on a Zr thermometer [37], according to which the ratio of zirconium in zircon and in the melt is a function of the temperature and chemical composition of this melt. The temperature range for the studied granite samples of the Laojunshan–Song Chai massif was 814–708 °C, which generally corresponds to 760 ± 54 °C. Based on these data, and also considering that all Lower Paleozoic U/Pb dates (Table 1) were obtained by ion probe (SHRIMP II) and LA-ICPMS methods for sections of zircon grains with oscillatory zonality, which is considered a sign of magmatic zircon, in the thermochronological diagram (Figure 4), the corresponding formation temperature is associated with the dating—760 ± 54 °C.
In the Ahrens–Weatherill diagram, almost all the ellipses of U/Pb zircon dating from the granites and granite gneisses of the Laojunshan–Song Chai massif obtained by LA-ICPMC or ion probe [5,8,9,10,11] are located in a continuous cloud along the concordia. In this situation, the authors calculate the age for the youngest cluster of dates, taking it as the age of granite formation, and consider older dates to be inherited [8,9]. Using tabular data from these studies, we calculated a weighted average of three or more relative to the older values given (Table 1, Figure 4). The calculated ages in the table are underlined. In this case, the dates for the magmatic zircon are in the range of 457 ± 5–409 ± 6 Ma.
In order to analyze the entire set of concordant Lower Paleozoic point U/Pb dating of zircon grains from the granites and granite gneisses of the massif [8,9,10,11,16], we plotted them on diagrams of the dependence of Th/U ratio, U content, and ɛHf(t) [9,10] on the measured age (Figure 5).
The age probability density curve shows two weakly separated peaks, with ages of 442 and 430 Ma. The mass formation of zircon grains in partial melting processes already in the Ordovician is supported by the fact that 63 points fall into this period. If we analyze the data on such a geochemical parameter as the Th/U ratio, we can see that there is no fundamental difference between the points falling into the Ordovician or Silurian. In both cases, the value varies from n × 10−2 to 1 or more, while the peak with an age of 430 Ma is characterized by a greater range of values. A range of values is observed for Ordovician and Silurian points and for uranium content from n × 10 to 10,000 ppm, while for a peak with an age of 442 Ma, about 10 points are characterized by values exceeding 5000 ppm. There is no fundamental difference in the values of ɛHf(t). Both Ordovician and Silurian points mostly fall in the range from −5 to 4, which corresponds to a model age of 1.2–1.7 Ga. [9,10]. Apparently, such a high variability in the geochemical parameters of zircon is due to the heterogeneity of the substrate composition and conditions in the local areas of partial melting.
Ponomareva and co-authors [3] performed dating using the classical TIMS U/Pb method for zircon from the least gneiss-like, massive granitoid samples. According to the results obtained for dimensional fractions of zircon a discordia with an upper intersection of T = 458, ± 10 Ma was obtained. In the same work, Rb/Sr isochronous dating on five whole-rock samples of the granite gave an age of 464 ± 12 Ma [3]. The closeness of the Rb/Sr isotope system during superimposed tectonic–thermal events in relation to isotope exchange and, accordingly, its stability, are determined not only and not so much by the value of the heating temperature but by the presence of fluids that ensure the migration mobility of cations [38,39]. Therefore, the isotopic Rb/Sr system at the level of whole rocks is quite conservative and, even with high degrees of metamorphic deformation transformations, can preserve information about the age of primary magmatic events [40]. Two estimates of age by the U/Pb method for zircon (upper intersection of discordia) and by the Rb/Sr isochronous method for whole-rock samples [3] agree with each other and do not contradict the assumption made above on the basis of data from point U/Pb dating on the manifestation of partial melting in the Ordovician. With that said, the two named dates on the thermochronological diagram (Figure 4) correspond to a formation temperature of 760 ± 54 °C.
For monazite inclusions preserved in garnet from garnet–mica schist Th-Pb, dating with an age of 419 ± 3, 380 ± 17 Ma was obtained by the ion microprobe method [7]. The first of them, as much more accurate, can be taken as the age of the metamorphic event. Using garnet–biotite and garnet–biotite–plagioclase thermo-barometers, the conditions of metamorphism T~550 °C, P~6 kbar were calculated from inclusions of biotite and plagioclase in garnet from the same sample [4]. This pressure estimate corresponds to a depth of ~20 km. Since the closure temperature of the U/Pb isotope system in monazite is more than 900 °C [24], on the thermochronological diagram, the value of the temperature of formation of metamorphic monazite T~550 °C corresponds to these two dates (Table 1, Figure 4).
The totality of the data observed for the Early Paleozoic stage of the history of the Laojunshan–Song Chai massif is consistent with the model of Wuyi–Yunkai orogeny adopted by a large number of researchers ([2,15,29,30,41,42,43] and others). As a result of Early Paleozoic compression and intense thrust between the Cathaysia and Yangtze blocks, the thickness of the crust doubled. The culminating stage of orogeny, limited to about the age of 460–440 Ma, the stage of syn/post-orogenic magmatism and orogen collapse (450–430 Ma), and the final stage of post-orogenic denudation and cooling after 420 Ma are distinguished.
A large amount of Ordovician U/Pb zircon dating from the granitoids of the massif, as well as Rb/Sr isochronous dating for whole-rock samples of granite, suggest that the migmatization, formation, and consolidation of early portions of the granite melt of the Laojunshan–Song Chai massif, at a depth equal to or greater than 20 km, began to occur 460 ± 8 Ma ago back in the process of collisional thickening of the crust. Apparently, the existence of a migmatized, partially molten layer at these depths could last for a long time, tens of Ma, as evidenced by the “sliding” on the concordia of ellipses corresponding to the LA-ICPMS and SHRIMP dating methods [8,9,10,11,16], as well as the high probability density of the distribution curve of the measured age during the Ordovician and Silurian (Figure 5a).
The event recorded by the Th-Pb monazite inclusion method, with an age of 419 ± 3 Ma, probably corresponds to the retrograde metamorphism of the final stage of orogeny, after or during which, the denudation of the overlying strata took place under extension conditions and, accordingly, the cooling and lifting of the rocks of the massif to the surface.
Estimates of the P-T conditions of metamorphism obtained on the basis of plagioclase–amphibole geothermobarometry in amphibolites of the massif—685–596 °C and 0.98–0.8 GPa [44] and 709–624 °C, 0.72–0.63 GPa [45]—correspond to the early Ordovician stage. The totality of the available estimates of P-T conditions enables us to reconstruct the Lower Paleozoic evolution of the Laojunshan–Song Chai metamorphic complex in the P-T diagram (Figure 6).
The presented pressure values correspond to a depth range of 20–30 km. The hypothesis of the long-term existence of a deep partially molten layer is supported by the fact that, within the entire Wuyi–Yunkai orogen, the process of granite formation is extended over 50 Ma, while the maximum intensity of the process falls within the age range of 440–450 Ma (summary histogram in the inset Figure 4) and corresponds to the formation of the granite gneisses and granites of the syn/post-orogenic stage [2].
By the Devonian–Early Carboniferous, post-orogenic collapse processes in the territory of northeastern Vietnam and southern China were finally completed, marine transgression occurred, a stable tectonic regime was established, and a stable carbonate platform was formed in the Early Carboniferous (review in [46]). This means that the entire section of the excessively thickened crust had been denudated. Isotope systems of granite gneisses of the Laojunshan–Song Chai massif do not record any active events during this period. It is logical to assume that, by this time, the massif was uplifted to the level of the upper crust, and the isotope systems were “frozen”.
In the sedimentary chronicle of the Early and Middle Devonian Van Canh and Van Huong silicastic formations (Northeastern Vietnam), potential traces of erosion of Lower Paleozoic igneous rocks are recorded using U/Pb geochronology of clastic zircon [47]. In the age spectra of the samples, a significant contribution (23% and 41%, respectively) is the peak of 419–485 Ma, consistent with the age range of the granitoids of the Laojunshan–Song Chai massif. If we assume that this peak really corresponds to the exit of the roof of the massif to the surface, then the amplitude of denudation should be about 20 km. On the other hand, rocks of the volcano–plutonic belt, occurring from the western Kon Tum terrane to northern Laos, could be the source of magmatic zircon of a close age (review in [46]).
With this in mind, for rocks located on the modern erosive section of the Laojunshan–Song Chai massif, the generalized curve of thermal evolution (Figure 4) shows cooling to temperatures of 100–200 °C in the Devonian, which at a thermal gradient of 25 °C/km, corresponds to depths of 4–8 km. In the P-T diagram, the curve corresponding to the P-T evolution of the massif during the Wuyi–Yunkai orogeny (Figure 6, red) can be continued through the point corresponding to the metamorphic event with an age of 419 ± 3 Ma, towards the origin of the coordinates. On this basis, the average exhumation rate of the Laojunshan–Song Chai massif, which is about 0.2–0.5 mm/year, can be estimated. Such a rate of exhumation could be associated with tectonic processes and parageneses corresponding to early high-temperature plastic deformations which are recorded within the massif [11,22]. The deformations of the later stages are superimposed on these deformations.
The further geological evolution of the region took place under stable platform conditions, as evidenced by the accumulation of Permian carbonate strata of the Nanpanjiang basin [48]. The formation of turbidite strata of the Middle Triassic is considered to be evidence of Indosinian orogeny and corresponds to the formation of a synorogenic foreland basin [49,50].
After a long break (~130 Ma) in the geological history of the Laojunshan–Song Chai massif, based on a set of dates obtained by the Th/Pb ion probe method for monazite from a matrix of garnet–mica schist [7], the U/Pb method for metamorphic zircon rims [8,11], K/Ar, 40Ar/39Ar methods for amphibole, muscovite, and biotite from granite gneisses, as well as by the Rb/Sr method for pairs “whole rock—biotite”, “ whole rock—muscovite” [3,4,6], the following active event is recorded. The dates obtained by the listed methods are in the range of 190–246 Ma, forming at least two clusters with the age of the maxima 231 ± 2 and 198 ± 2 Ma (Figure 4).
Using garnet–biotite and garnet–biotite–plagioclase thermometers for biotite and plagioclase in the matrix of garnet–mica schist, the conditions of metamorphism T~550 °C, P~3.8 kbar were calculated [4], which correspond to a depth of ~13 km. This means that, at a time of 237 ± 2 Ma ago, the rocks of the massif were re-immersed to a depth of about 13 km. This event is significantly (by 20 Ma) late, relative to the age of formation of igneous rocks of the Emeishan large igneous province (255–259 Ma, [32]); therefore, the Emeishan plume cannot be considered as a source of heating under this metamorphism.
Based on a complex of structural, kinematic, and geochronological data, it is shown that the orthogneisic rocks of the Laojunshan–Song Chai massif and its Lower Paleozoic sedimentary rocks represent a cover plate, which in the Triassic, during the collision of Indo-China with the South China Block, experienced tectonic displacement in a northeasterly direction ([11,21,22] and others). Considering that the dates obtained by the Th/Pb monazite ion probe method from the garnet–mica schist matrix [7] are in the range of 203–246 Ma (Figure 4), it can be assumed that tectonic transport processes occurred in this time range, simultaneously with the metamorphism of the lower level of the amphibolite and of the green schist facies.
At the same time, almost immediately after the beginning of the described events, the closure of the isotope system of syn-deformation muscovite, biotite, and feldspar begins to be recorded (see Figure 4, Table 1). The dating of these minerals from the rocks of the massif extends in a band up to the values of about 80 Ma. It is logical to assume that, immediately after the early metamorphic event, the rocks of the massif should have been exhumed during tectonic transportation to a depth of about 10 km and for a long time were located within the transition zone of 5–10 km. The closure of the isotope system of muscovite, biotite, and feldspar in various parts of the massif, which lasted for 150 Ma, may be due to the fact that, against the background of a gradual rise in the massif due to the erosion of surface sedimentary rocks, there was at different times an outcrop of rocks from the zone of partial rejuvenation of the corresponding isotope systems. At the same time, the occurrence of local active thermal events, such as the introduction of the Laojunshan Cretaceous granite massif (age 87 Ma, [51]), could result in the local rejuvenation of isotope systems in closely located host rocks.
A reference point estimating the position of the massif in time–depth coordinates can be obtained, based on the results of a study of the largest-in-China Dyakou emerald deposit located within the massif [36]. Emerald mineralization is represented by pegmatite and associated quartz veins that penetrate into deformed rocks of the Laojunshan–Song Chai massif [36]. The results of the determination of hydrogen and oxygen isotopes in the waters of the emerald channel and emerald, respectively, correspond to the source of the magmatic fluid. The fractionation value δ18O between emerald and quartz allows us to estimate the temperature of veins from 365° to 420 °C. Fluid inclusions indicate that the emerald was deposited from saline solutions in the range from almost pure water to 10.5 wt.% NaCl. The isochores of fluid inclusions, intersected with the δ18O data, give pressures varying along the geothermal gradient from 1.5 to 3.3 kbar. 40Ar/39Ar geochronology of biotite and muscovite allows us to estimate the age of formation of the deposit 124 ± 1 Ma [36]. Thus, by this time, the Laojunshan–Song Chai massif should have been located at a depth of 8 ± 3 km (Figure 4). Accordingly, the line of evolution of the massif can be indicated on the P-T diagram (blue, Figure 6), starting with the Indo-China–South China collision metamorphism, with an age of 237 ± 2 Ma (maximum Th/Pb dating by syn-metamorphic monazite from a matrix of garnet–mica schist [7]).
The roof of the Laojunshan granite massif, which was formed 87 Ma ago, is observed on the modern erosion section (Feng et al., 2013 [51]). According to borehole drilling data, the roof of the Cretaceous granite massif is fixed at a depth of ~100 m within the Sn–polymetallic Dulong deposit (Xu et al., 2015 [52]). In addition, the absence of a significant contact halo in the gneisses, granitogneisses, and Proterozoic metamorphic rocks enclosing the Cretaceous granite massif and, at the same time, the widespread development of hydrothermal Sn–polymetallic deposits associated with Cretaceous granites (Liu et al., 2007 [53], Xu et al., 2015 [52], Cheng et al., 2016 [54], and others) suggests that the Laojunshan–Song Chai granite gneiss massif, by this time, was in the conditions of a subvolcanic facies depth of about 2–6 km. Thus, the following reference point can be indicated on the time–depth diagram (Figure 4).
The last age limit (20 ± 1 Ma, [6]) in the history of the Laojunshan–Song Chai massif is fixed by the results of fission-track-method dating for apatite (Table 1, Figure 4). This event is associated with the cooling of the rocks of the massif below 100 °C and, accordingly, exhumation to a depth of less than 3 km during the Cenozoic activation of the Song Ma–Red River shear zone.

4. Conclusions

The U/Pb obtained by the dating method for magmatic zircon from the granites and granitogneisses of the Laojunshan–Song Chai massif are in the range of 420–465 Ma, while the geochemical parameters Th/U, U, ɛHF(t), with rare exceptions, do not give grounds to consider older zircons inherited or xenolith. Apparently, the collision-thickening of the crust during the Wuyi–Yunkai orogeny led to the formation of a partially molten layer that existed continuously for about 40 Ma.
By the Devonian–Early Carboniferous, the massif had been exhumed to the level of the upper crust, and there was a “freeze” of isotope systems. The average exhumation rate of the Laojunshan–Song Chai massif can be estimated as 0.2–0.5 mm/year.
Based on a complex of structural, kinematic, and geochronological data, the orthogneisic rocks of the Laojunshan–Song Chai massif and its Lower Paleozoic sedimentary rocks experienced tectonic displacement in a northeasterly direction and metamorphism of amphibolite and green schist facies in the Triassic (203–246 Ma), during the collision of Indo-China with the South China Block. The Emeishan large igneous province (255–259 Ma, [32]) cannot be considered as a source of heating under this metamorphism.
In the Middle and Late Mesozoic, and in the Cenozoic, the history of exhumation of the Laojunshan–Song Chai massif to the surface consists of discrete pulses associated with active tectonic events.

Author Contributions

Writing the article, preparation of drawings, literary review, A.T.; interpretation of the data in terms of thermal history, participation in the preparation of the article and drawings, N.M.; participation in the preparation of the article, interpretation of the data, participation in the discussion of the results obtained and in writing the article, N.K. All authors have read and agreed to the published version of the manuscript.

Funding

The study was supported by the Russian Science Foundation (grant No. 22-17-00038, thermochronology) and was carried out on government assignment to the V.S. Sobolev Institute of Geology and Mineralogy (Project No. 122041400171-5).

Data Availability Statement

The data presented in this study are available on request from the corresponding author (Dr. Alexey Travin, Email: [email protected]).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 3. A diagram demonstrating the variants of the relationship between the closure temperature of the isotope system of a mineral and the temperature of its formation. The blue line is the thermal history of the system. The yellow color corresponds to the period in the evolution of the system when recrystallization processes occur; the red color corresponds to melting processes. Dz1, Dz2—U/Pb-method dating by zircon (Tc~940 °C, Tf~750 °C); Da3, Db4—40Ar/39Ar-method dating by amphibole and biotite (Tc~500 °C and Tc~340 °C, Tf~750 °C), respectively.
Figure 3. A diagram demonstrating the variants of the relationship between the closure temperature of the isotope system of a mineral and the temperature of its formation. The blue line is the thermal history of the system. The yellow color corresponds to the period in the evolution of the system when recrystallization processes occur; the red color corresponds to melting processes. Dz1, Dz2—U/Pb-method dating by zircon (Tc~940 °C, Tf~750 °C); Da3, Db4—40Ar/39Ar-method dating by amphibole and biotite (Tc~500 °C and Tc~340 °C, Tf~750 °C), respectively.
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Figure 4. (Below). Thermochronograms of the Laojunshan–Song Chai granite gneiss massif, constructed on the basis of a summary of geochronology results (Table 1). The blue rectangles show estimates of metamorphism stages conditions according to [4]. The black solid line above shows the probability density curve for Th/Pb dating by monazite inclusions from mica schist [7]. The red stripe for comparison shows the age of the Emeishan large igneous province [31,32]. The experimental points corresponding to the data obtained in this work are highlighted in green. (Above). A depth evolution curve based on reference points for rocks of the Laojunshan–Song Chai granite gneiss massif is presented. Details are in the text.
Figure 4. (Below). Thermochronograms of the Laojunshan–Song Chai granite gneiss massif, constructed on the basis of a summary of geochronology results (Table 1). The blue rectangles show estimates of metamorphism stages conditions according to [4]. The black solid line above shows the probability density curve for Th/Pb dating by monazite inclusions from mica schist [7]. The red stripe for comparison shows the age of the Emeishan large igneous province [31,32]. The experimental points corresponding to the data obtained in this work are highlighted in green. (Above). A depth evolution curve based on reference points for rocks of the Laojunshan–Song Chai granite gneiss massif is presented. Details are in the text.
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Figure 5. The dependence of the Th/U ratio (a), the content of U (b), and ɛHf(t) (c) on the measured Lower Paleozoic age for point measurements (429 measurements in total) according to [8,9,10,11,16]. The dependence of ɛHf(t) is based on data (234 measurements) [9,10]. The thick black line in the upper diagram shows the integral age probability density curve.
Figure 5. The dependence of the Th/U ratio (a), the content of U (b), and ɛHf(t) (c) on the measured Lower Paleozoic age for point measurements (429 measurements in total) according to [8,9,10,11,16]. The dependence of ɛHf(t) is based on data (234 measurements) [9,10]. The thick black line in the upper diagram shows the integral age probability density curve.
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Figure 6. Petrogenetic P-T diagram showing P-T trajectories reconstructed based on the published data. The Lower Paleozoic part of the P-T evolution of the massif is highlighted in red, and the Mesozoic–Cenozoic part is highlighted in blue.
Figure 6. Petrogenetic P-T diagram showing P-T trajectories reconstructed based on the published data. The Lower Paleozoic part of the P-T evolution of the massif is highlighted in red, and the Mesozoic–Cenozoic part is highlighted in blue.
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Travin, A.; Murzintsev, N.; Kruk, N. Thermochronology of the Laojunshan–Song Chai Granite Gneiss Massif (North Vietnam, South China). Minerals 2024, 14, 251. https://doi.org/10.3390/min14030251

AMA Style

Travin A, Murzintsev N, Kruk N. Thermochronology of the Laojunshan–Song Chai Granite Gneiss Massif (North Vietnam, South China). Minerals. 2024; 14(3):251. https://doi.org/10.3390/min14030251

Chicago/Turabian Style

Travin, Alexey, Nikolai Murzintsev, and Nikolai Kruk. 2024. "Thermochronology of the Laojunshan–Song Chai Granite Gneiss Massif (North Vietnam, South China)" Minerals 14, no. 3: 251. https://doi.org/10.3390/min14030251

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