Discovery of Mafic Granulites in the Muzhaerte Area, SW Tianshan, China

Abstract

Accretionary and collisional orogeny are often accompanied by the disturbance of the geothermal gradient, leading to high-temperature metamorphism. High-temperature metamorphic rocks are significant in their ability to help the reconstruction of the thermal histories of orogenic belts. The Tianshan Orogenic Belt, at the southwest margin of the Central Asian Orogenic Belt, is a record of the long-term subduction–collision–post-collision orogenic process that has taken place in the Phanerozoic Eon. Here, we report the discovery of mafic granulites in the Muzhaerte area, SW Tianshan. Petrographic observation reveals that the mafic granulites underwent two metamorphic stages. The peak mineral assemblage of the first stage is dominated by clinopyroxene + orthopyroxene + plagioclase + quartz + hornblende (hb¹) ± biotite, and the post-peak mineral assemblage of the second stage is dominated by clinopyroxene + plagioclase + quartz + hornblende (hb²) + biotite. The calculated results obtained from the two-pyroxene thermobarometers and the Al-in-hornblende barometer for the mafic granulites indicate that the metamorphic conditions of mafic granulites are 760–860 °C, <0.39–0.41 Gpa. The mafic granulites recorded a high-grade granulite facies thermal metamorphic event with the highest temperature limit currently recorded in the Central Tianshan Block.

1. Introduction

In plate tectonics, accretionary orogeny and collisional orogeny are often accompanied by the disturbance of the geothermal gradient, which leads to high-temperature metamorphism and the partial melting of crustal rocks [1,2,3]. High-temperature metamorphism and the partial melting of crustal rocks may lead to changes in the rock constitution, rock chemical composition, and rheological properties of the lithosphere [4,5,6]. High-temperature metamorphic rocks are significant in their ability to reconstruct the thermal histories of orogenic belts.

The South Tianshan Orogenic Belt (STOB) is a Paleozoic subduction–collision orogenic belt located in the southwest margin of the Central Asian Orogenic Belt (CAOB) [7,8]. As a key area of the Central Asian Orogenic Belt, the South Tianshan Orogenic Belt completely preserves the whole process from the regeneration of the oceanic crust to the continent–continent collision orogeny, and provides an excellent region for studying the tectonic evolution of the southwest Central Asian Orogenic Belt [7,8]. It was formed via the subduction of the South Tianshan Ocean (STO) into the northern Yili-Central Tianshan Block (YCTB), and the collision of the Tarim Craton with the YCTB after the closure of the oceanic crust [9,10]. Continuous subduction and collision resulted in a large number of ophiolite fragments, accretionary complexes, island arc migmatites and ultrahigh-high-pressure metamorphic rocks in the STOB and the YCTB [11,12,13,14,15,16]. In recent decades, in-depth research has been carried out on these rocks, and the tectonic evolutionary history of the STOB has been determined [17,18,19]. The STO opened in the Neoproterozoic era and began to subduct into the YCTB in the Early Paleozoic Era [16,20,21,22,23]. The closure of the STO occurred in the late Early Carboniferous period; then, syn- and post-collision magmatism occurred in the Tianshan Orogenic Belt [24,25,26,27]. The reported high-temperature metamorphic rocks in this orogenic belt consist of amphibolites, granitic and pelitic gneisses, with minor amounts of mafic granulites, which are assumed to be major constituents of the crystalline basement of Central Tianshan [7,28,29]. Their peak temperatures are ~700 °C, they are close to wet, and they possess the minimum melting conditions of supracrustal rocks, which are common during arc crust reworking [14,30,31]. It is unknown whether they represent the upper temperature limit during the evolution of the Central Tianshan arc. Herein, we report the discovery of orthopyroxene-bearing mafic granulites in the south of the Muzhaerte area, SW Tianshan Orogenic Belt. We conducted petrography observations and geothermobarometry calculations on mafic granulites and determined that the metamorphic conditions of granulites are 760–860 °C and <0.39–0.41 Gpa. The mafic granulites recorded a higher heat flow compared to previously reported metamorphic rocks in the Central Tianshan Block.

2. Geological Setting

The Tianshan Orogenic Belt, sandwiched between the Yili-Kazakhstan Plate and the Tarim Craton, is an important part of the CAOB, which has experienced a long and complex process of accretion and orogeny [8,20,32]. From north to south, the Tianshan Orogenic Belt in China can be divided into the North Tianshan Orogenic Belt, the YCTB and the STOB [25,33]. Mid-ocean ridge basalts representing the Terskey Ocean crust have been found in the Nalati northern margin fault of the YCTB, so the YCTB is further divided into the Yili Block (YB) and the Central Tianshan Block (CTB) [34].

The formation of the North Tianshan Orogenic Belt (NTOB) is related to the southward subduction of the North Tianshan Ocean (NTO) into the YB [7,35]. A large number of ophiolite and arc magmatic rocks associated with subduction are exposed in the NTOB and the northern margin of the YB [36,37]. The NTOB and the YB are separated by the North Tianshan Fault. The boundary between the YB and the CTB is the Nalati north margin fault, which is considered to be connected with the Nikolayev line in Kyrgyzstan [20,34,38]. The formation of the STOB is related to the northward subduction of the STO into the CTB [32,39,40,41]. A large number of ophiolites, arc magmatic rocks and ultrahigh-high-pressure metamorphic rocks related to subduction are exposed in the STOB, the CTB and the southern margin of the YB [12,42,43,44]. The STOB and the YB are separated by the South Central Tianshan Fault.

The Muzhaerte area (near the Muzhaerte River) is located at the south margin of the CTB and the north edge of the South Central Tianshan Fault (Figure 1). The CTB is a long and narrow terrain with a Precambrian crystalline basement between the YB and the STOB [45]. The Precambrian crystalline basement includes Neoarchean–Neoproterozoic metamorphic granitoid and mafic rock, amphibolite, migmatite, biotite plagioclase gneiss, metamorphic clastic rock and carbonate rock [46,47]. There were many magmatic rocks of island arc produced by the subduction of the STO, as well as many high-temperature metamorphic rocks related to the arc magmatism of the Early Paleozoic Era [48,49,50]. The pre-Carboniferous magmatic rocks and strata were subjected to deformation and greenschist–amphibolite facies metamorphism grades [51,52]. After the collision between the Tarim Craton and the YCTB, some Permian magmatic rocks also appeared in the CTB under the tense post-collision environment [26,48]. The Muzhaerte area on the north side of the South Central Tianshan Fault is considered as a high-temperature metamorphic belt, while the area on the south side of the fault is considered as an ultrahigh/high-pressure metamorphic belt, forming a contrasting paired metamorphic belt [43]. The peak metamorphic conditions obtained from the previous studies of pelitic granulites in this area are 630–705 °C, 0.47–0.58 GPa [30,31]. The main lithology of the ultrahigh/high-pressure metamorphic belt is interbedded and lenticular muscovite schist, blueschist and eclogite [28,53]. According to the summary of the P-T path of published ultrahigh/high-pressure metamorphic rock, the peak metamorphic conditions of metamorphic rock have been estimated to be 430–510 °C, 2.2–3.3 Gpa [13,53,54]. The U-Pb ages of zircons from eclogite and muscovite schist containing coesite are ~320 Ma, which represents the final time of collision between Tarim Craton and the CTB [20,21,43]. The metamorphic rocks in two metamorphic belts show disparate geothermal gradients.

3. Materials and Methods

The mafic granulites were collected from the east of the Muzhaerte River, in the high-temperature metamorphic belt of the Central Tianshan Block. The studied granulites consist of plagioclase, clinopyroxene, orthopyroxene, hornblende, quartz and biotite, with accessory apatite and ilmenite. They are massive, showing typical granoblastic textures (Figure 2a). Both clinopyroxene and orthopyroxene are anhedral, with grain sizes ranging from 0.1 to 0.5 mm (Figure 2b), and are mostly surrounded by hornblende (Figure 2c). Most of the clinopyroxene occurs as porphyroblasts, containing biotite, hornblende inclusions (Figure 2d). Some of the clinopyroxene develops oriented orthopyroxene rods along the c-axis (Figure 2e). Orthopyroxene occurs as relict individual grains or as intergrowth with clinopyroxene. Hornblende can be divided into two sub-types based on morphology. In the first type, the hornblende (hb¹) occurs as inclusions in the pyroxenes or shows granoblastic texture with pyroxene. This indicates that this type of hornblende is intergrown with pyroxene. Some hornblende grains are small and round, and occur as residues of prograde dehydration melting during progressive metamorphism. Part of these grains have cuspate boundaries, which represent the growth during the later cooling process. The other type of hornblende (hb²) is formed at the rim of pyroxene, or in the form of huge grains that contain some anhydrous inclusions (pyroxene and plagioclase), indicating that it was formed in the retrogressive metamorphic process (Figure 2f). Plagioclase and quartz are subhedral to anhedral and have grain sizes of 0.2–0.4 mm. Biotite is mostly distributed around anhydrous pyroxene or amphibole.

The textures show that the Muzhaerte mafic granulites underwent at least two metamorphic stages of peak and post-peak metamorphism (Figure 3). During prograde metamorphism, hornblende is decomposed to form clinopyroxene, orthopyroxene and plagioclase, whereas the other hornblende maintains stability, shown by its rounded occurrences in the matrix or as inclusions in pyroxene. Thus, the peak mineral assemblage is clinopyroxene + orthopyroxene + plagioclase + quartz + hornblende (hb¹) ± biotite (Phase I). During cooling, the unmigrated melt reacts with clinopyroxene, orthopyroxene and plagioclase or pyroxene hydrates in an aqueous fluid phase to form the hydrous mineral amphibole. Therefore, amphibole grows along the rim of pyroxene or early-stage amphibole. The mineral assemblage of the post-peak stage is clinopyroxene + plagioclase + quartz + hornblende (hb²) + biotite (Phase II). The absence of garnet and rutile indicates that the Muzhaerte mafic granulites were formed at medium to low pressures.

The determination of the metamorphic conditions of mafic granulites has generally relied on conventional geothermobarometry. Two-pyroxene thermobarometers are often used to calculate the metamorphic temperature conditions of mafic granulites. The principle of thermometers is based on the molar ratio relationship between the Fe and Mg ions in the exchange reaction between clinopyroxene and orthopyroxene, in order to restore the temperature at which the mineral combination is stable [55,56,57]. The typical thermobarometers used currently include three versions established by Wood [57], Wells [56] and Brey [55]. Wood [40] proposed an empirical formula for calculating the equilibrium temperature of two-pyroxene assemblages by considering Fe²⁺ in the miscibility gap between two pyroxenes [57]. Wells [39] used most of the available experimental data for multi-component pyroxene to calibrate a two-pyroxene thermobarometer [56]. Brey [38] evaluated the original thermobarometer based on the experimental data of the four-phase Lherzolite and developed a new version of the thermobarometer, which is relatively more suitable for higher-temperature and -pressure conditions [55].

Figure 3. Conjectural metamorphic P-T trajectory of the mafic granulites in the Muzhaerte area (modified after [58]). The mafic granulites underwent at least two metamorphic stages of peak and post-peak metamorphism. Metamorphic facies code: GR—granulite facies; HAM: high-amphibolite facies.

Figure 3. Conjectural metamorphic P-T trajectory of the mafic granulites in the Muzhaerte area (modified after [58]). The mafic granulites underwent at least two metamorphic stages of peak and post-peak metamorphism. Metamorphic facies code: GR—granulite facies; HAM: high-amphibolite facies.

At present, it is difficult to accurately constrain the pressure of the mafic granulites, so an Al-in-hornblende barometer is selected to roughly evaluate the pressure condition. The Al-in-hornblende barometer calculates the pressure based on the Al content in hornblende [59,60,61]. Although the barometer has been proposed for using in magmatic rocks, it is also suitable for metamorphic rocks when the pressure range is 0.2–1.3 Gpa and the rock contains mineral combinations of quartz/alkali-feldspar, plagioclase, hornblende, biotite and Fe-Ti-oxide [50]. However, the correlation between the pressure and temperature of the system implies that the temperature will also affect the calculation of the pressure. Therefore, we selected Anderson’s [50] version of the Al-in-hornblende barometer with temperature correction, and Hammarstrom’s [51] and Schmidt’s [52] version without temperature correction for the pressure calculation.

4. Results and Discussion

The calculated results for the two-pyroxene thermobarometers and the Al-in-hornblende barometers are listed in Supplementary Tables S1 and S2. The temperature range calculated according to the Wood [40] thermobarometer is 762–811 °C, with an average temperature of 789 °C. The temperature range calculated according to the Wells [39] thermobarometer is 785–860 °C, with an average temperature of 829 °C. The temperature range calculated according to the Brey [38] thermobarometer is 533–632 °C, with an average temperature of 590 °C. The pressure ranges calculated based on the Anderson [42] barometer are 0.35–0.37 Gpa and 0.39–0.41 Gpa at the given temperatures of 780 °C and 760 °C. As the given temperature increases, the calculated pressure result will decrease. The pressure range calculated based on the Hammarstrom [43] barometer is 0.49–0.51 Gpa. The pressure range calculated based on the Schmidt [44] barometer is 0.53–0.55 Gpa.

The calculation results of the Brey thermobarometer are significantly lower than those of the other two thermobarometers. This may be because the Brey thermobarometer results are obtained using lherzolite samples under higher temperatures and pressures, so it is not suitable for analyzing mafic granulites. The calculation results of the Anderson barometer are significantly lower than those of the other two versions, indicating that neglecting the influence of temperature will result in an overestimation of the calculation result for pressure. Therefore, we preliminarily believe that the metamorphic conditions of mafic granulites are 760–860 °C and <0.39–0.41 Gpa.

The discovered Muzhaerte mafic granulites record the high-temperature granulite facies metamorphism of the CTB. The associated meta-sedimentary rocks and pelitic gneiss (Cordierite garnet sillimanite gneiss) in the adjacent area also record high-temperature metamorphism [14,31,48]. Phase equilibrium modeling shows that the peak metamorphic conditions of the meta-sedimentary rocks and pelitic gneiss are 728 °C and 0.72 Gpa, and 681–705 °C and 0.54–0.58 Gpa, respectively. This suggests that the mafic granulites examined in the study are able to record a higher heat flow and the highest peak temperature in the CTB found so far. The mafic granulites may represent the Precambrian basement of the CTB or form in the continental arc environment during oceanic subduction or the post-collisional extensional environment. At present, there are no geochronology data available with which to judge the tectonic background of granulite formation, but their higher peak temperature and lower peak pressure may suggest that they may have been formed in a shallower tensile environment and unrelated to continental arc magmatism. Considering that the Tianshan Orogenic Belt was in a post-collision environment with intraplate magmatic rocks in the Permian period, the granulite facies metamorphism may have occurred in a post-collision environment presumably, under the condition that the thinning of the crust and asthenosphere upwelling provided heat [25].

5. Conclusions

• The Muzhaerte mafic granulites underwent two metamorphic stages, the first being the peak mineral assemblage of clinopyroxene + orthopyroxene + plagioclase + quartz + hornblende (hb¹) ± biotite, and the second being the post-peak mineral assemblage of clinopyroxene + plagioclase + quartz + hornblende (hb²) + biotite.
• Using the two-pyroxene thermobarometers and the Al-in-hornblende barometer, the metamorphic conditions of the mafic granulites were determined to be 760–860 °C, and <0.39–0.41 Gpa. This recorded a high-grade granulite facies thermal metamorphic event with the highest temperature limit currently recorded in the CTB.