High-grade metamorphic rocks usually retain some records on metamorphic history, and ancient tectonic and geodynamic environments during their formation and evolution, for example, mineral compositional zoning, mineral inclusion, reaction texture, and rotating snowball (e.g., Vernon, 2004). Therefore, through the detailed petrologic study, the informations on formation and tectonic settings in which the rocks formed can be obtained, which help to reconstruct their complex geological processes (Liu et al., 2009). The petrology, metamorphic P-T conditions and geochronology of high-grade metamorphic rocks have been widely used to understand crustal evolution and geodynamic setting (e.g., Tang et al., 2016; Brown, 2014; Wei and Powell, 2004). Metamorphic evolutional processes of various metamorphic terrains often correspond to different tectonic settings and conditions. For example, near-isothermal decompression (ITD) along clockwise P-T paths is usually related to subduction or arc-continent collisional environment (Brown, 1993; Harley, 1989; England and Thompson 1984), whereas the counter-clockwise P-T paths of near-isobaric cooling (IBC) are generally associated with mantle magmatic underplating (Bohlen, 1991; Sandiford and Powell, 1986). The formation of continental crust has been ascribed to two distinct plate tectonic settings: active continental margin and intra-plate (Rudnick, 1995). Growth of continents in within-plate settings occurs in response to plume-related magmatism, whereas the continental crust currently grows at the plate boundaries through magmatic additions and arc-continent collisions. Furthermore, convergent continental margins are known to be major sites of lower crustal accretion through different mechanisms including basaltic under-plating at the base of the crust and subduction-accretion (Weber et al., 2002). As an important ring in the lithosphere, the lower crust is not only the important source region of various mafic magmas but also the bridge between the crust and the mantle. However, there are few reports of lower-crustal xenoliths from active continental margin (Weber et al., 2002; Kempton et al., 1997). Phase diagrams (P-T pseudosection) based on bulk composition, an internally consistent thermodynamic dataset and phase activity-composition (a-x) models have been widely applied to derive metamorphic P-T conditions (e.g., Palin et al., 2016; White et al., 2014).
In recent years, with further investigations on formation and evolution of the North China Craton (NCC) by many researchers, some important progress and achievements have been made in petrology, metamorphic process and isotopic geochronology, which mostly focused on the northern parts and the collisional belt between the eastern and western blocks or the central orogenic belt of the NCC. However, the study on formation and evolution of the Precambrian lower crust beneath the southeastern margin of the NCC is relatively weak. In addition, some key issues such as metamorphism stage division, peak-metamorphic conditions are still controversial (Liu Y C et al., 2015b). Recent studies show that the metamorphic rocks in the Wuhe metamorphic complex experienced a high-pressure granulite-facies metamorphism at 1.80-1.90 Ga (Wang et al., 2017; Liu Y C et al., 2013, 2009). The marble associated with metamorphic rocks in the area also experienced the 1.83-1.85 Ga granulite-facies metamorphism (Liu Y C et al., 2017). The xenoliths in the Xuzhou-Suzhou area were brought to the surface by the Mesozoic granitic and dioritic volcanics at the southeastern margin of the NCC provide direct windows into the present and/or fossil lower crust. Petrology, geochronology and geochemical studies of these xenoliths also indicate that most of them were formed at 2.4-2.5 Ga and underwent the 1.81-1.85 Ga granulite-facies metamorphism. A few Neoarchean lower-crustal xenoliths also experienced granulite-facies metamorphism at ~2.5 or ~2.1 Ga (Liu Y C et al., 2013, 2009; Wang et al., 2013; Guo and Li, 2009). Furthermore, the tectonic setting of the Paleoproterozoic high-pressure granulite-facies metamorphism in the NCC is still in debate with two models. The first involves a collisional environment created by the amalgamation process between the eastern and western blocks of the NCC (e.g., Guo et al., 2005, 2002; Zhao et al., 2005, 2000; Wilde et al., 2002), and the second proposed a plume-driven upwelling resulting in corresponding metamorphism during Late Neoarchean (e.g., Peng et al., 2005; Zhai and Liu, 2003; Zheng et al., 2003; Zhai et al., 2000). High-pressure granulites are generally characterized by peak mineral assemblages of garnet+clinopyroxene+plagioclase+ quartz in metabasite and garnet+kyanite+potash feldspar+quartz in metapelite and felsic rocks (Kotková and Harley, 2010; O'Brien and Rötzler, 2003; Pattison et al., 2003; Yardley, 1989). Formation pressure condition of exceed 1.4 GPa and above 800 ℃ for the garnet-bearing overprint requires a crustal thickening either by magma loading or collision, implying lower levels of thickened crust (or crust undergoing subduction) can be subjected to particularly high temperature (e.g., O'Brien and Rötzler, 2003). Besides, high-pressure granulites are occasionally associated with medium-temperature eclogite, as for instance in the Variscan belt (e.g., Carswell and O'Brien, 1993). High-pressure granulite in a given belt can provide insights into the evolution of the lower crust involved in continental collision and related processes, whereas direct petrological observation and geochemistry analysis of the lower crust xenoliths are crucial to understand continental growth in the early stage. The Precambrian metamorphic basement (Wuhe metamorphic complex) and the lower-crustal xenoliths exposed in the southeastern margin of NCC undoubtedly provided an excellent natural laboratory for this study. Previous studies only focused on the geochronology, petrology, and geochemistry without any phase equilibrium modeling on the metamorphic P-T conditions for the lower-crustal xenoliths in the region. Furthermore, although Nie et al. (2018) estimated the P-T conditions of peak and post-peak metamorphic stages by conventional geothermobarometry for the xenoliths in the area, however, in view of multistage metamorphic evolution and reworking (Liu Y C et al., 2013, 2009), the peak P-T estimations needs to be further checked and evaluated. In the meantime, some Fe-Mg partitioning thermometers fail to estimate the peak conditions of high-grade rocks, especially ultrahigh-temperature (UHT) metamorphic rocks, whose peak temperature is in excess of 900 ℃ (Harley, 2008, 1998; Kelsey, 2008). Therefore, the main objective of this study is to perform petrological and phase equilibrium modeling researches, and to better define precise peak-metamorphic P-T conditions and understand the metamorphic P-T evolution of the Precambrian lower crust of the southeastern margin of the NCC through an integrated investigation of mafic xenoliths from Jiagou.1 GEOLOGICAL SETTING
The NCC is one of the largest and oldest cratonic blocks in the world, as evidenced by the presence of > 3.6 Ga crustal remnants exposed at the surface or in lower crustal xenoliths (e.g., Zhang, 2012; Wu et al., 2008; Zheng et al., 2004; Liu D Y et al., 1992). It is adjacent to the Late Paleozoic Central Asian orogenic belt to the north, the Early Paleozoic to Early Mesozoic Qinling-Dabie orogenic belt to the south, the Early Paleozoic Kunlun-Qilian orogenic belt to the west, and the Sulu ultrahigh-pressure metamorphic belt to the east. Although considerable disputes exist on the amalgamation time and tectonic process (e.g., Zhai and Santosh, 2011; Santosh, 2010; Zhao et al., 2000), there have been increasing agreements recently that the metamorphic basement of the NCC can be divided into the western and eastern blocks which collided at 1.95 Ga to form the trans-North China Orogen (TNCO) (e.g., Santosh et al., 2013; Zhao and Zhai, 2013; Liu S W et al., 2012, 2006; Trap et al., 2012, 2009; Zhao et al., 2010, 2005, 2000; Zhang et al., 2009, 2006). Among them, the high-pressure granulites are mainly exposed in the northern part of TNCO and the southern part of the Jiao-Liao-Ji belt in the Early Proterozoic. They formed in the subduction-collision environment. The sapphire granulites in the western khondalite belt are believed to be the result of mantle magma underplating caused by post-collision extension, resulting in the product of UHT metamorphism in some areas (e.g., Jiao et al., 2011).
The Bengbu and Xuzhou-Suzhou areas lie in the eastern block along the southeastern margin of the NCC, about 100 km west of the Tan-Lu fault zone on the southwestern termination of the Sulu Orogen and about 300 km north of the Dabie Orogen and Hefei Basin (Fig. 1). The deformed Neoproterozoic to Late Paleozoic sedimentary cover and Late Archean to Paleoproterozoic metamorphic basement in the region were intruded by several small Mesozoic intrusions (e.g., Jiagou, Banjing and Liguo) (Fig. 1) mainly composed of dioritic to monzodioritic porphyry. The Precambrian metamorphic basement (Fig. 2a) in the studied area is mainly exposed at the surface in Bengbu, while no exposed basement occurs in the Xuzhou-Suzhou area whereas abundant deep-sourced xenoliths (Fig. 2b) occur in the Mesozoic intrusions (Liu Y C et al., 2015b, 2013, 2009; Guo and Li, 2009; Xu et al., 2009, 2006, 2002). Furthermore, the Precambrian metamorphic terrains and lower-crustal xenoliths in the region experienced a multistage metamorphic evolution (Liu Y C et al., 2017, 2013, 2009; Wang C C et al., 2017; Wang A D et al., 2013).
Most of the lower-crustal xenoliths in the studied area are mafic meta-igneous rocks, among which garnet-bearing are common. The main rocks include garnet amphibolite, garnet granulite and garnet-bearing mafic and felsic gneiss (Liu Y C et al., 2013, 2009). In addition, there are also mantle-derived xenoliths formed in the Paleozoic (393±7 Ma), e.g., spinel-bearing garnet clinopyroxenite, phlogopite clinopyroxenite and spinel-bearing pyroxenite, probably indicating that the North Qinling Orogen extends eastward beneath the southeastern margin of the NCC (at least to the Suzhou area of Anhui Province) (Liu Y C et al., 2013).2 PETROLOGY
The deep-sourced xenoliths in the area comprise abundant mafic rocks, less common feldspar-free ultramafic types and rare felsic gneisses. The xenoliths include various rock types such as garnet-bearing amphibolite, garnet hornblendite, garnet granulite, garnet-bearing hornblende-plagioclase gneiss, felsic gneiss, spinel-bearing garnet clinopyroxenite, phlogopite clinopyroxenite and spinel-bearing pyroxenite and so on. The lower-crustal origin is evident for mafic garnet-bearing granulite or amphibolite and gneiss xenoliths while the spinel-bearing garnet clinopyroxenites have different origins (Liu Y C et al., 2013). Four samples were collected from the southeastern margin of the NCC, which were exposed in the Mesozoic dioritic porphyry at Jiagou near Suzhou, the samples range in diameter from 2 to 20 cm. They can be divided into two groups including two garnet amphibolites (samples 07JG26-3 and 06JG2) and two garnet granulites (samples 07JG24 and 08JG31). Combined with the descriptions in an earlier paper (Nie et al., 2018), the salient aspects on the petrography for two types of mafic lower-crustal xenoliths are summarized as below. Except for special notes, other mineral abbreviations in figures and tables are after Whitney and Evans (2010).2.1 Garnet Amphibolites (Samples 07JG26-3 and 06JG2)
Garnet amphibolites are black and mainly consists of garnet (20%-25%), plagioclase (20%-25%), and amphibole (45%-50%) with minor clinopyroxene (20%-25%), rutile, quartz, titanite and apatite, and rare metal sulfide (Figs. 3a, 3b, 3c, 3d). Garnet is medium-to coarse-grained (0.4-1 mm), and mostly subhedral to anhedral. Garnet generally occurs as porphyroblast (some porphyroblasts have oriented rutile needles) (Figs. 3a, 3b), and sometimes contains minor inclusions of clinopyroxene, clinopyroxene+amphibole, amphibole+plagioclase+chlorite (Figs. 3a, 3cv, 3d, 3e). The inclusions are mainly concentrated in the core of garnet with less or no inclusions in the rim. Its edges and crevices are filled with retrograded amphibole and chlorite. Amphibole is subhedral to anhedral with particle size of around 0.4-0.8 mm. Amphiboles are divided into early brown and late green amphiboles (Fig. 3b). Brown amphiboles mostly occur as inclusions in garnet and a few of them are porphyroblast in the matrix, whereas the green amphiboles are generally intergrowths with plagioclase to form a symplectite. Some brown porphyroblast amphiboles contain early rutile inclusions, and most of the rutile are retrograded into titanite or ilmenite (Figs. 3a, 3b), whereas the cleavages are often filled with retrograde chlorite. Plagioclase is anhedral with particle size around 0.3-0.8 mm. Plagioclases have three types of occurrence, i.e., porphyroblasts, symplectites coexisted with clinopyroxene and/or amphibole (Figs. 3a, 3b) and inclusions in garnet (Fig. 3e). Clinopyroxene is subhedral to anhedral as inclusion in the porphyroblastic garnet or as porphyroblast in the matrix. Some of the clinopyroxenes have retrograde amphibole rim (Fig. 3). In the back-scattered images, amphiboles with high and low titanium content exhibit gray and dark, respectively (Figs. 3a, 3f), and there are amphibole and rutile needles in clinopyroxene or garnet (Figs. 3d, 3f).2.2 Garnet Granulites (Samples 07JG24 and 08JG31)
Two garnet granulites are massive and black, and composed mainly of garnet (30%-35%), clinopyroxene (35%-40%), plagioclase (15%-20%) and amphibole (20%-25%) with minor rutile and quartz, and rare titanite and apatite (Figs. 4a, 4b, 4d), among them mostly clinopryoxene retrograde into amphibole. The porphyroblastic garnet is medium- to coarse-grained (0.4-1.2 mm) and mostly subhedral to anhedral. Among them, some have 3-4 groups of oriented rutile needles (Fig. 4c), while others contain mineral inclusions (Figs. 4a, 4d) of clinopyroxene, clinopyroxene+ amphibole, clinopyroxene+plagioclase+magnetite, clinopyroxene+ amphibole+plagioclase, amphibole+plagioclase, amphibole+ plagioclase+quartz, clinopyroxene and apatite. Garnet porphyroblasts mostly have cleavages filled with the late amphibole and chlorite. The clinopyroxenes mostly occur as inclusions in garnet (Figs. 4a, 4d) and a few occur as irregular porphyroblast in the matrix (Figs. 4e, 4f), most of them surround small amphiboles as its residues. Besides, some of clinopyroxenes have obvious retrograded rim, reflecting different degrees of replacement by amphibole±plagioclase (Figs. 4b, 4d, 4e). Plagioclases are mostly subhedral to anhedral with various sizes of 0.2-0.8 mm, and have three kinds of occurrence: as inclusions in garnet (Fig. 4a), intergrowths with green amphiboles or symplectites (Figs. 4e, 4f) and porphyroblast in the matrix. Amphibole was in the form of anhedral with grain size of 0.4-0.8 mm. The amphibole inclusions and the porphyroblast amphiboles surrounded by the retrograded chlorites are mostly brown, while later formed symplectite with plagioclase are usually green (Fig. 4b).
The above petrographic observations show that the two groups of mafic samples were originally basaltic with typical high-pressure granulite facies mineral assemblage of garnet+ plagioclase+clinopyroxene+quartz+rutile as suggested by Liu et al. (2009). The clinopyroxene, plagioclase, quartz and rutile exist in the garnet as mineral inclusions (Figs. 3a, 3d, 4a, 4d) indicating they also experienced peak high-pressure granulite-facies metamorphism. A large amount of decompression dissolution and multi-stage symplectites developed in all mafic samples. Among them, oriented rutile needles are in garnet (Fig. 4c) and some clinopyroxene with rutile and/or amphibole dissolution are observed (Figs. 3f, 4d) indicating that they experienced subsequent decompression. The retrograde texture as clinopyroxene+brown amphibole±plagioclase corona around garnet (Figs. 3a, 3b, 4a) represents the early granulite facies decompression. The amphibole+plagioclase corona around the garnet and the clinopyroxene transformation to a symplectite of amphibole+plagioclase and magnetite (Figs. 3a, 3b, 4f) indicate the metamorphism of late amphibolite facies. The different stages of symplectites and coronas correspond to the varied mineral inclusions in garnet. For instance, in back-scattered electron (BSE) images (Figs. 3c, 3d, 3f, 4d), the clinopyroxene inclusions in garnet have rutile needles, and their rims are surrounded by two stages of retrograded amphiboles. During the amphibolite facies retrogression, some of the clinopyroxenes were retrograded into amphiboles and/or replaced by green amphibole±plagioclase (Fig. 4e). Most of the rutiles were retrograded into titanite at this stage (Fig. 3b). Fractures within garnet are sometimes filled with amphiboles and chlorites representing a late stage of greenschist facies metamorphism (Figs. 3e, 4a), during which the titanite were transformed into ilmenite.3 ANALYTICAL METHODS
Mineral compositions were determined using a JXA-8230 electron microprobe at School of Resources and Environmental Engineering, Hefei University of Technology. The quantitative analysis was performed by using an accelerating voltage of 15 kV, a specimen current of 20 nA, and a beam diameter of 3 μm. The analytical precision was generally better than 2%.
Data from two samples of whole-rock major element analysis was from Liu Y C et al. (2013). Whole-rock major element analysis was conducted by wet chemical methods at the Langfang Laboratory, Hebei Bureau of Geology and Mineral Resources. The trace elements were measured by an Elan DRCII ICP-MS at the CAS Key Laboratory of Crust-Mantle Materials and Environments, University of Science and Technology of China, Hefei. Analytical uncertainties range from ±1% to ±5% for major elements and ±5% to ±10% for trace elements. Detailed analytical procedures and instrument parameters for trace element analyses were reported by Hou and Wang (2007).4 RESULTS 4.1 Mineral Chemistry
Details of the mineral chemistry of the studied xenoliths were given in Nie et al. (2018) and only the salient features are summarize as below. In the meantime, some new data on electron microprobe analysis for the typical minerals are present to better define and understand the metamorphic evolution of the xenoliths.4.1.1 Garnet
Representative compositions of garnet are exhibited in Tables 1 and 2. They consist mainly of almandine (Alm), pyrope (Pyp) and grossularite (Grs). Garnet end member in the garnet amphibolite is Alm47-59Pyp14-27Grs22-33, and in the garnet granulites is Alm43-58Pyp16-34Grs19-27. From the core to the rim, most of the porphytoblastic garnets composition changes significantly. In the garnet amphibolites, the garnet is characterized by a large relatively homogeneous core domain with slightly higher XFe (0.47-0.49), XMg (0.25-0.32), and lower XCa (0.27-0.19) than that in the rim, and just XMn changes flatly. Garnet granulites display distinct compositional zoning from core to rim (XFe is 0.48-0.58 with gradually increases; XMg and XCa are gradually decreases with 0.25-0.19 and 0.28-0.17, respectively. XMn changes flatly). The above mentioned compositional changes from the core to the rim probably reflect the different formation temperatures (e.g., Zong et al., 2006; Chen et al., 2003). In general, calcium preserves the greatest degree of internal zonation for major elements. The decrease of calcium contents in both types of rocks may be related to the late decomposition of grossularite and its paragenetic anorthite, while the decrease of magnesium contents may be related to the exchange reaction between garnet and clinopyroxene in the matrix (e.g., Cooke, 2000). Among the four samples, some of garnet core compositions from garnet granulites are similar to the rim compositions of garnet from garnet amphibolites, suggesting a change in the temperature and pressure conditions during retrogression, which may be related to some factors such as the thermal duration, the size of the mineral grains, the degree of fracture development and the extent of fluid infiltration, and then affecting the diffusion of the constituents (e.g., Wang et al., 2011).
Clinopyroxenes are mainly diopside, and clinoenstatite and pigeonite occasionally occur in garnet amphibolite (Nie et al., 2018), suggesting the compositional difference between them (Tables 1 and 2). Clinopyroxene generally occurs as inclusions (Figs. 3a, 3c, 3f) in garnet amphibolites and has high Na2O and Al2O3 contents of 0.86 wt.%-0.97 wt.% and 3.33 wt.%-3.46 wt.%, respectively. Clinopyroxene cores in the matrix also have very high Na2O and Al2O3 contents (1.01 wt.%-1.08 wt.% and 3.40 wt.%-4.15 wt.%, respectively). Clinopyroxene margins in the matrix and relics of clinopyroxene porphyroblast replaced by amphibole and plagioclase have very low sodium and aluminum contents with Na2O ranges from 0.28 wt.% to 0.33 wt.% and Al2O3 from 1.39 wt.% to 1.78 wt.%. Variation of clinopyroxene composition in garnet granulite is similar to that of garnet amphibolite. Clinopyroxene exists as inclusions (Figs. 4a, 4d) with high contents of Na2O (0.83 wt.%-1.07 wt.%) and Al2O3(2.74 wt.%-4.91 wt.%), but clinopyroxenes occurred as assemblage inclusions with the amphibole and plagioclase have not such characters. The clinopyroxene porphyroblast core also exhibit extremely high sodium contents (1.07 wt.%-1.54 wt.%) and aluminum content (4.91 wt.%-7.01 wt.%). The clinopyroxene margins in the matrix remain around the smaller amphiboles and the clinopyroxene surrounded or replaced by amphibole and/or plagioclase displays very low sodium and aluminum with Na2O contents of 0.24 wt.%-0.38 wt.% and Al2O3 contents of 0.68 wt.%-2.67 wt.%, respectively.4.1.3 Plagioclase
The plagioclase in garnet amphibolite (Table 1) and garnet granulite (Table 2) are mainly andesine, and a few are albite. Plagioclase in the different samples occurs in various textural domains such as inclusions in garnet, as matrix grains and in symplectites or coronas with amphibole and/or clinopyroxene. In the garnet amphibolite, the plagioclase inclusions in garnet have high calcium contents with An27Ab70Or3, and often coexist with amphibole and clinopyroxene. The compositions of plagioclase porphyroblasts in matrix is An5-36Ab62-95Or0-2 which commonly associated with brown amphibolite or adjacent to clinopyroxene. The plagioclase which intergrowth with green amphibole formed into symplectite is lowly calcium content with composition of An6-48Ab50-94Or0-1. In the garnet granulite, the composition of plagioclase also changes greatly. Plagioclases with highly calcium content appear as inclusion assemblages with amphibole or clinopyroxene. These plagioclases are constituted of An29Ab69Or3. However, some clinopyroxene inclusions in garnet with rutile and amphibole dissolution or retrograded rim of amphibole that coexist with plagioclase tend to have lowly calcium content. In matrix, composition of plagioclase varies a widely range. Their cores have high calcium contents, and their rims often adjacent to or near the clinopyroxene with compositions of An5-51Ab48-94Or1-2. The symplectitic plagioclase with green amphibole contains more calcium (An32-79Ab21-68Or0-3) than the coarse-grained plagioclase in the matrix.4.1.4 Amphibole
Amphiboles in both types of rocks belong to the group of calcareous amphiboles. In the garnet amphibolite (Table 1), they are all pargasite and endenite while in the garnet granulite (Table 2) is dominantly pargasite and ferropargasite. The chemical composition of four mafic rocks is also similar. The amphibole inclusions and the grain amphibole in the matrix often display brown and exhibit higher contents of titanium with 0.53 wt.%-1.44 wt.% of TiO2. Depending on rock compositions, some of the clinopyroxene inclusions in garnet have amphibole needles with low Ti contents, and the retrograde amphibole rim of clinopyroxene in the matrix also contains low Ti contents. Furthermore, mostly green amphiboles have very low-titanium contents or even below detection limit (TiO2=0.03 wt.%-0.38 wt.%), in which several types of occurrence are identified as symplectite with plagioclase and fine grained amphiboles replaced early clinopyroxenes in garnet and in the matrix, respectively.
According to the above petrographic observations and the analysis of the electron probe composition of the minerals, it can be seen that the two types of rock samples in this study have similar mineral assemblages and only differ in mineral contents. According to the relationship between metamorphic structures and the chemical compositions of the minerals, the mafic lower-crustal xenoliths have undergone at least four stages of metamorphism: (Ⅰ) peak high-pressure granulite facies (garnet+ plagioclase+high-sodium clinopyroxene+quartz+rutile); (Ⅱ) granulite facies in the decompression phase (garnet+plagioclase+ low-sodium clinopyroxene (+rich-titanium amphibole)+quartz); (Ⅲ) retrogressed amphibolite facies (plagioclase+low-titanium amphibole+ilmenite+titanite); (Ⅳ) greenschist facies (chlorite+ calcite+magnetite).4.2 Phase Equilibria Modeling
For the mineral assemblages and compositions mentioned above, the model system NCFMASHTO (Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3) is chosen to know the corresponding mineral evolution process and calculate P-T pseudosection for samples 07JG14 (garnet amphibolite) and 08JG15 (garnet granulite) in this research. Assuming the fluid phase is pure water, it is in excess of the conditions below the solidus and K2O and MnO are not considered because they are too low in content. Pseudosection calculations were performed using Perplex 6.7.0, June 2014 updated version of Connolly (1990) and the internally consistent thermodynamic dataset from Holland and Powell (1998). The phases considered in the modeling and the corresponding activity-composition (a-x) models are garnet (Ganguly et al., 1996), amphibole (Dale et al., 2000), clinopyroxene (Holland and Powell, 1998), plagioclase (Fuhrman et al., 1988) and melt (Holland and Powell, 2001; White et al., 2001). Quartz, titanite and rutile are treated as pure end-member phases.
The effective bulk-rock compositions for pseudosection calculation (samples 07JG14 and 08JG15) were normalized into mole proportions in the model system, which were calculated on the basis of the whole-rock geochemical results (Table 3). According to the above petrological observations of the main minerals assemblages of the mafic lower-crustal xenoliths, which assuming MnO exists only in the spessartite and is deducted according to formula (MnO)3·Al2O3·(SiO2)3, CO2 and P2O5 are respectively calculated according to calcium carbonate CaO·CO2 and apatite (CaO)5·(P2O5)1.5·(H2O)0.5deducts the corresponding components. The iron content of the whole rock composition is calculated according to the reaction relationship of 2FeO+O=Fe2O3, in accordance with the ferric iron content being twice the oxygen content.
Figure 5 shows the P-T pseudosection of the garnet amphibolite sample 07JG14 in the NCFMASHTO system. The phase relationship of the main minerals is that there is no melt at high temperature and the solid phase is mainly controlled by temperature. The stable temperature and pressure range of the garnet is wider and the pressure is greater than 4 kbar; the rutile is stable under the pressure greater than 5-11 kbar; the phase boundary of rutile and garnet is mainly controlled by the pressure and disappears in the direction of low pressure, and mainly replaced by plagioclase and ilmenite. Amphiboles disappeared under high temperature and pressure conditions, and their disappearance temperature correlated with the H2O content in the system. Orthopyroxenes appear extensively and disappear toward low temperature and high pressure. Clinopyroxene is always present in the system. As shown in Fig. 5, the peak metamorphic mineral assemblages of Grt+Pl+Cpx+Qtz+Rt is stable in the region of more than 850 ℃ and more than 1.2 GPa in the sectional view. In an attempt to further constrain the peak field, mineral modes and compositional contours for garnet and plagioclase were calculated. Take this combination into consideration, the x(g) contour of the garnet is nearly parallel to the pressure axis in the phase diagram and is mainly controlled by the temperature and decreases with increasing temperature. The plagioclase of ca(pl) content in plagioclase is nearly parallel to the temperature axis and is strongly controlled by pressure and decreases with increasing pressure. In other samples, the x(g) content of the garnet was gradually increased from the core to the rim, showing a gradual cooling process. In other samples, the calcium content of plagioclase gradually increased from the core to the rim, showing a gradual decompression process. Combined use of the smallest ca(pl) contours of plagioclase and the smallest x(g) contours of garnet to determine the peak temperature and pressure conditions. The near-peak temperature and pressure conditions of sample 06JG2 were P=1.40 GPa, T=900 ℃, and they fell in the opx-pl-cpx-gt-q-ru-H2O region (green dot) in the phase diagram. They are basically consistent with the results of the observed mineral assemblages. The near-peak temperature and pressure conditions of sample 07JG26-3 were P=1.0 GPa, T=650 ℃, and they fell in the amp-cpx-gt-q-ru region (blue dot) in the phase diagram. The observed mineral assemblage is not well constrained by phase equilibria modeling as it occupies a large part of P-T space.
Figure 6 shows the P-T pseudosection of the garnet granulite sample 08JG15 in the NCFMASHTO system. This sample is roughly analogous to 07JG14 in the topological phase of the main mineral phase. The specific location of the phase boundary is somewhat different due to the different rock composition. Rutile, for example, is stable at higher pressures (> 8 kbar). The stable temperature of amphibolite is also relatively high and disappears under high temperature and high pressure conditions. The disappearance temperature has a certain correlation with the H2O content in the system. Peak metamorphic mineral assemblages of Grt+Pl+Cpx+Qtz+Rt are stable at the range of P=1.7-2.0 GPa and T=800-970 ℃ in this P-T pseudosection. Although the modelled peak assemblage P-T field is large, we still can improve the peak P-T estimates by mineral contours and measure mineral chemistry. Considering that in this combination, the garnet x(g) content has a negative slope in the phase diagram region, and is mainly controlled by temperature, which decreases with increasing temperature. However, content of ca(pl) in plagioclase has a positive slope and is strongly controlled by pressure and decreases with increasing pressure. Therefore, garnet and feldspar compositions can be used to provide an upper limit on the potential P-T conditions. The combination of the smallest ca(pl) contours of plagioclase and the smallest x(g) contours of garnet is used in this paper to determine the peak temperature and pressure conditions. The near-peak temperature and pressure conditions of the obtained sample 07JG24 were P=1.50 GPa, T=850 ℃, and it fell in the amp-pl-cpx-gt-q-ru-H2O region (yellow dot) in the phase diagram, deviating from the peak mineral assemblages observed by petrography. Sample 08JG31 was put into the opx-pl-cpx-gt-q-ru-H2O region with a temperature and pressure condition of P=1.40 GPa, T=835 ℃ (orange dot), and the deviation from the petrographic observation of the peak mineral assemblages.4.3 Conventional Geothermobarometry
The peak metamorphic P-T conditions on the mafic lower-crustal xenoliths in the region have been quantified by conventional geothermometry in addition to other metamorphic stages (Nie et al., 2018). Temperature of the peak granulite-facies metamorphic stage (Ⅰ) were calculated based on experimental and empirical calibrations of Fe-Mg fractionation for garnet-clinopyroxene pair (Ravana, 2000). The garnet- clinopyroxene-feldspar-quartz geobarometry based on experimental calibration (Eckert et al., 1991) was applied to constrain the peak metamorphic pressure. In garnet amphibolite, the estimated temperature and pressure ranges for sample 07JG26-3 are 665-763 ℃ and 1.3-2.3 GPa, and the temperature and pressure ranges of sample 06JG2 are 988-1 013 ℃ at 1.8 GPa. A similarity of P-T conditions were estimated in garnet granulite. Sample 07JG24 were calculated to be 710-865 ℃ and 1.3-2.2 GPa. In addition, core component of garnet and clinopyroxene were applied to calculate for sample 08JG31, the range of temperature and pressure are 786-924 ℃ and 1.3-1.7 GPa respectively (Nie et al., 2018). All of the calculated peak-metamorphic P-T results above are consistent with the present estimations by the phase equilibria modeling.
The temperature and pressure estimation of granulite facies (Ⅱ) in the decompression stage are limited by two different methods. The first method is to combine Fe-Mg fractionation for garnet-clinopyroxene pair (Ravana, 2000) with garnet- clinopyroxene-feldspar-quartz geobarometry (Eckert et al., 1991) to constrain. The P-T conditions of garnet amphibolites are consistent within the error range. The estimated results of sample 07JG26-3 are 627-692 ℃ and 1.2-1.4 GPa, sample 06JG2 are 654 ℃ and 0.6 GPa, respectively. In garnet granulite, the temperature and pressure ranges are 734-760 ℃ and 1.0-1.3 GPa respectively for sample 08JG31. The second method uses the amphibole-plagioclase thermometer (Holland and Blundy, 1994) and the amphibole-plagioclase-quartz geobarometry (Bhadra and Bhattacharya, 2007) to solve the problem. Computation based on this method shows temperature and pressure range of 676-702 ℃ and 0.9-1.1 GPa for sample 07JG26-3 and 636-765 ℃ and 0.6-1.3 GPa for sample 07JG24 respectively. In garnet granulite of sample 08JG31, the temperature range of 662-734 ℃ at 1.1 GPa (Nie et al., 2018).
Amphibole-plagioclase thermometer (Holland and Blundy, 1994) and amphibole-plagioclase-quartz geobarometry (Bhadra and Bhattacharya, 2007) were joint applied to estimate the retrograde P-T conditions of amphibolite-metamorphic stage (Ⅲ). The temperature and pressure ranges for 07JG26-3 and 06JG2 were estimated to be 569-647 ℃/0.3-0.9 GPa and 600-623 ℃/0.57-1.0 GPa, respectively. In garnet granulites, sample 07JG24 gives temperature range of 630-670 ℃, and pressure range of 0.6-1.0 GPa. Sample 08JG31 calculated temperature ranges of 639-642 ℃ and pressure range of 0.7-0.8 GPa. We also obtained the metamorphic pressure results of Al content in amphibole geobarometry (Schmidt, 1992; Johnson and Rutherford, 1989; Hollister et al., 1987; Hammarstrom and Zen, 1986), and the calculation results of the four samples agree with those of the above mineral pairs (Nie et al., 2018).5 DISCUSSION 5.1 Peak Metamorphic P-T Conditions and P-T-t Path
Partial melting and multistage metamorphic evolution and reworking commonly result in Fe-Mg exchange or replacement between minerals (Liu Y C et al., 2015a, b; Pattison et al., 2003; Frost and Chacko, 1989), and frequently the closure temperature is lower than granulite facies metamorphic temperature because of the Fe-Mg exchange between minerals (Pattison et al., 2003; Spear and Florence, 1992; Frost and Chacko, 1989; Harley, 1989). This poses great difficulties and challenges for the limitation of the temperature and pressure of the Precambrian lower-crustal metamorphic rocks in the study area, especially when determining the conditions of metamorphic temperature and pressure in the peak period, the results are often lower than the actual temperature and pressure conditions. Such difficulties and challenges still exist at the time of the temperature-pressure limitation of the peak granulite facies metamorphism in this paper, which shows that a large range of temperature and pressure results have been obtained using the traditional geothermometry (Fig. 7). However, the use of the Perplex program to quantify the symbiotic composition of minerals and the quantitative calculation using the mineral composition contours make up for the deficiencies of traditional methods. When the traditional geothermobarometry was used for temperature and pressure limitation, a very low metamorphic temperature and pressure are calculated using the clinopyroxene inclusions in garnet and its adjacent core components, which are much lower than that of the high-pressure granulites. The calculate results may reflect post growth re-equilibration of garnet and clinopyroxene and represent the conditions of temperature and pressure in the retrograde metamorphic stage. The quantitative calculation of the garnet amphibolite for sample 07JG26-3 on the P-T pseudosection defines lower temperature and pressure conditions, which falls in the amphibolite facies region. However, the higher temperature and pressure range was calculated for the core component with high sodium content of clinopyroxene in the matrix and the adjacent garnet component with high Ca/Mg ratio. Therefore, when determining the conditions of temperature and pressure in the metamorphic stage of rocks, it cannot be limited to the occurrence of mineral pairs, but must also be combined with the corresponding mineral composition (minimum use of minerals that are later modified) for analysis and calculation. It is generally believed that the higher temperature and pressure conditions in the calculation result are closer to the actual peak-peak metamorphic conditions, and it is more appropriate to take the average of the higher temperature-pressure range as the temperature and pressure conditions during the metamorphic stage of the peak granulite phase. The garnet amphibolites are 838-862 ℃ and 1.5-2.0 GPa and the garnet granulite is 817-826 ℃ and 1.5-1.9 GPa. The temperature-pressure conditions estimated by the two methods are consistent within error. Based on the minimum value of the contours of the garnet x(g) and the minimum value of the contours of the plagioclase ca(pl), the peak metamorphic temperature and pressure conditions of two types of rock samples are accurately defined. The calculated results (P=1.40 GPa, T=900 ℃) of the garnet amphibolite, sample 06JG2 fell in the peak granulite facies metamorphic region, in agreement with the petrographic observation. However, the calculated results of the two garnet granulite samples deviate from the real peak mineral assemblage observed by petrographic analysis. In this regard, they may be derived from the late metamorphic overprints, resulting in the lower temperature and pressure values defined by traditional geothermometry than the actual ones. Therefore, combining with the traditional mineral-pairs method and the quantitative calculation of phase equilibrium modeling, the peak granulite facies metamorphic conditions for the studied samples are limited to P=1.4-1.6 GPa, T=850-900 ℃.
The P-T conditions of the granulite facies metamorphism during decompression was estimated by two methods. The temperature calculated by the garnet-clinopyroxene thermometer (Ravana, 2000) was lower than the amphibole-plagioclase thermometers (Holland and Blundy, 1994). While the pressure results obtained using the garnet-clinopyroxene-feldspar-quartz barometer (Eckert et al., 1991) are generally higher than those of amphible-plagioclase-quartz barometer (Bhadra and Bhattacharya, 2007). We speculate that this may be due to the application of different geothermobarometry. The results of the two methods are consistent within the error range. In the P-T pesudosection, the calculated results are mostly in the range of mid-low pressure granulite facies. However, only a few of the calculated P-T results obtained from conventional method are located in the field of high-pressure granulite and low-pressure granulite facies, and high-amphibolite and amphibolite facies transition (Fig. 7), the temperature and pressure range is 667-730 ℃/1.0-1.3 GPa. But, the orthopyroxene widely predicated in the P-T pseudosection is not observed by petrographic observations. It appears that the probable cause is the original small amount of orthopyroxene in the system was consumed in the retrograde stage, thus leaving no evidence of typical low-pressure granulite facies mineral assemblages. Besides, the assemblages of amp-pl-cpx-gt-q observed by petrological observation in pseudosection have much higher P-T conditions (Figs. 5, 6). The reason for this difference is that we take consideration of more highly titanium content in NCFMASHTO system, or post growth re-equilibration of minerals occurred because of the late metamorphic overprints. Temperature range of amphibolite retrograde metamorphism is 609-646 ℃ and the pressure calculated using mineral assemblage barometer and single mineral barometer are consistent within the error range (Fig. 7). The range of pressure calculation in garnet amphibolite is 0.4-0.8 GPa, and the pressure range in the garnet granulite calculation is 0.6-0.8 GPa. Therefore, it is appropriate to define the pressure in the metamorphic stage of the study area to 0.4-0.8 GPa. The P-T paths of four samples derived from the published data by conventional method (Nie et al., 2018) are plotted in Fig. 7, while the new P-T path is exhibited in Fig. 8 which adds the present estimations by pseudosection for two groups of mafic samples and combines with the previously published zircon U-Pb dating data (Liu et al., 2016). The combination of the amphibolite facies minerals present in the P-T pseudosection is also consistent with the petrographic observations. The above results are consistent with the existing zircon researches of the Precambrian basement rocks (Liu Y C et al., 2013, 2009; Wang et al., 2013, 2012). The dated zircons are mainly metamorphic and rarely magmatic. In addition, high-temperature (particularly > 900 ℃) partial melting may completely dissolve refractory mineral phases such as zircon. As a result, high-grade metamorphic rocks such as granulite in continental collisional orogen and granulite terrains have rarely retained early magmatic zircons, whereas most of the zircons are metamorphic and near rounded (Klaver et al., 2015; Liu Y C et al., 2015b; Hermann and Rubatto, 2003).5.2 Ultra-High Temperature Metamorphism
Ultra-high temperature (UHT) metamorphism is the most extreme case formed by regional crustal metamorphism commonly occurring in the deep crust, with temperature conditions of T=900-1 100 ℃ and pressure conditions of P=0.7-1.3 GPa (e.g., Kelsey and Hand, 2015; Harley, 2008, 1998). The UHT crustal metamorphism has been recognized in more than 50 localities globally in the metamorphic rock record and is accepted as 'normal' in the spectrum of regional crustal processes. The UHT metamorphism is typically identified on the basis of diagnostic mineral assemblages such as sapphirine+quartz, orthopyroxene+sillimanite+quartz and osumilite in Mg-Al-rich rock composition (e.g., Harley, 2008; Kelsey, 2008). The garnets in rocks formed under extreme temperature and pressure conditions often contain oriented rutile needles, such as high-pressure eclogites (e.g., Liu Y C et al., 2015a, 2011; Ye et al., 2000), mantle-derived rocks (e.g., Liu Y C et al., 2013; Song et al., 2004), and granulite facies metamorphic rocks with temperatures exceeding 900 ℃ (e.g., O'Brien et al., 1997). Recent studies have shown that oriented rutile needles in garnet are directly formed from high-titanium garnet during decompression and cooling process (e.g., Proyer et al., 2013; Ague and Eckert, 2012). It indicates that the host rocks have been suffered from UHT or ultrahigh pressure (UHP) metamorphism (e.g., Ague et al., 2013; Ague and Eckert, 2012; Liu et al., 2011; Zhang et al., 2003; Ye et al., 2000). Experimental studies have shown that the orientation of the rutile needles may be controlled by the host garnet lattice (e.g., Proyer et al., 2013; Ague and Eckert, 2012), and that the observed 3-4 groups oriented parallel to each other under the microscope are also one of the most important evidences for its dissolution mechanism (e.g., Proyer et al., 2013). In this study, it was observed that most of the garnets in the mafic lower-crustal xenoliths of the study area had 3-4 groups of oriented rutile needles paralleled to each other (Fig. 4c). Garnet also contains larger particles of rutile (Figs. 3a, 3b, 3e), suggesting that their host rock is formed under extreme conditions of temperature and pressure, and that the garnet is rich-titanium on early and experienced uplift and cooling processes after the peak metamorphism. As products of the above progress, the oriented rutile needles were formed. In addition, clinopyroxene often also has 2-3 groups oriented rutile needles that parallel to each other (Fig. 4c). Amphibole also commonly contains rutile grains, and rutiles are mostly retrograded into titanite or ilmenite (Figs. 3a, 3b), suggesting that the rocks in this area were rich in titanium in the early stages and experienced process of uplifting and decompression in the later period. When the temperature and pressure conditions in the metamorphic stage are limited, the testing of the mineral components is performed in the same laboratory and a large number of analyses are performed to reasonably select the geothermobarometry and use the appropriate position and composition of the mineral pairs to calculate in order to ensure the accuracy of the data and the reliability of the calculation results. About the peak granulite facies metamorphism, sample 06JG2 was defined the high temperature range of 988-1 013 ℃ using the traditional geothermometer (Nie et al., 2018) and the quantitative calculation of the mineral composition contours in the P-T pseudosection in this study was performed to obtain the similar high temperature of T=900 ℃ as mentioned above. Furthermore, orthopyroxene as the UHT marker mineral has widely appeared in both types of rock systems. Combined with the petrographic observation and the results of temperature and pressure estimations, it was concluded that the Precambrian lower-crustal rocks in the area are likely to undergo UHT metamorphism. However, because most of the lower-crust rocks that have undergone complex metamorphic evolution and especially have been subjected to strong retrograde metamorphism and mylonitization, the minerals (assemblages) and related evidence formed at early UHT conditions have disappeared and thus not easy to identify (e.g., Roever et al., 2003). Anyway, the UHT metamorphism in the region needs to be further constrained by more evidences.5.3 Tectonic Significance
Previous studies on petrology and zircon U-Pb geochronology of the massive lower-crustal and mantle xenoliths in the Mesozoic diorite porphyry of the Jiagou have revealed that the precursors of Precambrian lower-crust xenoliths included at least two types that one formed in 2.5-2.6 Ga and the other formed in about 2.1 Ga. They experienced 2.5-2.6 and 2.1 Ga magmatic-thermal events, 1.8-1.9 Ga granulite facies metamorphism (Liu Y C et al., 2009), and a few Late Archean lower-crustal xenoliths also experienced ~2.5 Ga (Wang et al., 2013) or ~2.1 Ga (Liu Y C et al., 2013) granulite facies, and 390 and 176 Ma metamorphic overprinting (Liu Y C et al., 2009). The 2.5-2.6 Ga xenoliths showed certain difference in metamorphic evolution process due to the different crust depths. The rocks located in the lower part of the lower crust experienced 2.5, 2.1, and/or 1.8-1.9 Ga granulite facies metamorphism, and the 390 and 176 Ma retrograde metamorphism. Furthermore, the whole-rock Pb-Pb dating for the marbles and various rocks such as mafic granulite from the Wuhe complex yielded isochron ages of 1 911±64 and 1 906±61 Ma, respectively, in agreement with the above mentioned HP granulite facies ages of 1 901±8 Ga defined by zircon U-Pb dating (Liu Y C et al., 2016). That is to say, the ~1.91 Ga is interpreted as the peak metamorphic time during which Pb isotopes. In addition, some researchers believe that the 1.80-1.91 Ga granulite facies metamorphism may be due to the large-scale crust heating and thickening caused by undergrowth of ~1.9 Ga mantle-derived magma at the bottom of the lower crust, which were correspond to extension, rifting and related mafic magmatic emplacement of North China Craton during this period (e.g., Liu Y C et al., 2013, 2009; Hou et al., 2008, 2006; Peng et al., 2005; Zhai et al., 2000). Recently, based on an integrated petrological and zircon geochronological investigation, Liu Y C et al. (2016) suggested that the Precambrian basement rocks at the southeastern margin of North China Craton experienced ~1.91 Ga orogenesis, ~1.83 Ga granulite-facies metamorphism and ~1.75 Ga amphibolite-facies retrogression. In this regard, combined with previously published data and this study, it is indicated that the mafic lower-crust xenoliths here underwent peak high-pressure granulite facies metamorphism at ~1.91 Ga high-pressure granulite-facies metamorphism, subsequently ~1.83 Ga low-pressure decompression stage, ~1.75 Ga amphibolite facies retrograde metamorphism and recent greenschist facies metamorphism. Recently, Wang et al. (2018) also provided a similar clockwise P-T-t path derived from the Precambrian metamorphic terrane (Wuhe complex/Group) at the southeastern margin of the North China Craton. What's more, a variety of rocks such as basic granulites, pelitic granulites and related rocks in the Jiaobei terrain have been documented to suffer from the Paleoproterozoic high-pressure granulite facies and subsequently near-isothermal decompression and near-isobaric cooling with a clockwise P-T path (Liu P H et al., 2015, 2013; Li S Z et al., 2012; Tam et al., 2012). In view of the near-isothermal decompressional clockwise P-T path constructed in this study (Fig. 8), the Precambrian lower-crustal rocks underwent a complex evolutional process related to the subduction or arc-continent collision in the Paleoproterozoic as previously suggested by Wang et al. (2017). Therefore, the mafic granulite xenoliths from Jiagou has a clockwise metamorphic P-T path, similar to those of the Precambrian metamorphic basement from the Wuhe complex and Jiaobei terrain. As a result, the studied area is further documented to belong to the westward extension of the Jiao-Liao-Ji belt, both constituting the Paleoproterozoic collisional orogenic belt in the eastern block of the NCC as a whole as previously proposed by Liu et al. (2017) and Wang et al. (2017).6 CONCLUSIONS
(1) Different types of the mafic lower-crustal xenoliths from Jiagou at the southeastern margin of the North China Craton have similar peak-metamorphic mineral assemblages of high-pressure granulite facies.
(2) The new phase equilibrium modeling results suggest that the peak metamorphic P-T conditions for the mafic lower-crustal xenoliths in the area are 850-900 ℃/1.4-1.6 GPa. In addition, some of the xenoliths most likely experienced ultra-high temperature metamorphism.
(3) The Precambrian lower-crustal rocks in the study area are further documented to undergo a clockwise P-T path with the near-isothermal decompression, reflecting the Paleoproterozoic subduction or arc-continent collision setting. In addition, because of different depths or localities within the subducting plates, their P-T conditions in the same metamorphic stage might be varied.ACKNOWLEDGMENTS
This research was financially supported by the National Natural Science Foundation of China (No. 41773020), the National Basic Research Program of China (No. 2015C
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