P-T Paths Derived from Garnet Growth Zoning in Danba Domal Metamorphic Terrain, Sichuan Province, West China

INTRODUCTION

In general, many continental orogens contain Barrovian sequence of moderate- to high-pressure metamorphic rocks as a result of continental collision and thickening on plate margins, which are usually characterized by inverted metamorphic isograds, that is to say, the high-grade metamorphic rocks occur structurally on the higher levels whereas the lowergrade rocks on the lower levels (Goscombe and Hand, 2000; Stephenson et al., 2000; Armstrong et al., 1992; Kohn et al., 1992; Spear and Hickmott, 1990). Associated with the moderate- or high-pressure Barrovian sequences, Buchan sequences of low-pressure metamorphic rocks are frequently exhumed in continental orogens, but the genetic relationship between the Barrovian and Buchan sequences is not entirely clear. For example, the Himalayan metamorphic belt is one of the largest inverted metamorphic isograds in the world, of which the inverted high-P/moderate-T Lesser Himalayan sequences are structurally overlain by high-T/moderate-P Greater Himalayan sequences, which are called the "paired metamorphic mountain belt" (Goscombe and Hand, 2000). Recent advances in studies of metamorphic P-T paths have been proven to be useful in interpreting the thermal and tectonic evolution of an orogen and in the detailed description of the interrelationships among deeply eroded terrains (Spear et al., 1995; Kohn et al., 1993; Spear, 1993). In this article, inverted Barrovian type metamorphic sequences associated with Buchan sequences exposed in the Danba area of the Songpan-Ganze orogenic belt (SGOB), located in western Sichuan, China, are described (Fig. 1). The SGOB occupies the northeastern portion of the Tibet plateau, which comprises the tectonically distinct Tethyan-Himalayan domain resulted from the closure of the paleo-Tethys subsequent to the India and Eurasia continent-continent collision and the convergence associated with the interaction between the Tibet, Indochina, South China and North China blocks (Xu et al., 1992; Dewey et al., 1988). Therefore, this orogen is critical in understanding the Mesozoic–Cenozoic tectonometamorphic evolution of the Tibet plateau.

Figure  1.  Geologic map showing the characteristics of western Sichuan Province, West China.

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Most previous work in the SGOB is focused on the analysis of orogenesis via microstructure, patterns of overprinting mineral generations, geochronology, and detailed description of the interrelationships among small-scale deformation, large-scale deformation, and thermal evolution at mid-crustal levels on the basis of studies of the deformational evolutions of deeply eroded terrains (Huang et al., 2003a, b, 2001; Mattauer et al., 1992; Xu et al., 1992). In contrast, metamorphic petrology of the SGOB has received quite little attention. They are the low-grade (lower greenshcist facies) metamorphic rocks, which form the main body of the SGOB. However, some metamorphic complexes up to upper amphibolite facies are distributed locally, and are associated with Mesoproterozoic basement-cored structural domes (Mattauer et al., 1992). For example, the Danba domal metamorphic terrain (DDMT) in the central SGOB is one of the largest exposures of the high-grade rocks in this orogen, and the associated inverted Barrovian sequences occur south of the area and Buchan sequences in the north. It is obvious that the deeply eroded DDMT is an important window through which information on the mid- or lower-crustal response to the orogenesis can be drawn. Huang et al.(2003a, 2001) provided qualitative P-T-t paths for the Barrovian sequences from the DDMT based on metamorphic phase relations resulting from the observed mineral parageneses and microtextures, and most P-T conditions for the major metamorphic zones are calculated from the metamorphic porphyroblast rim, e.g., garnet, and matrix mineral compositions using the computer program THERMOCALC (Powell et al., 1998). The peak P-T conditions, especially the temperature, may be gained through this approach, but both prograde and post-peak P-T histories are less thoroughly investigated. Cheng and Lai (2005) discussed the P-T paths of each metamorphic zone in Danba on the whole, while You et al. (2006) evaluated Danba domal metamorphic terrain.

This article focuses on the detailed reconstruction of the metamorphic history recorded by the metapelites from the Barrovian-type metamorphic sequence of the Danba metamorphic dome. A quantitative P-T path was reconstructed based on interpretation of reaction textures, chemical zoning of minerals, P-T data obtained by thermobarometry, and thermodynamic modeling of garnet zoning (Gibbs method). This P-T path provides important constraints on the crustal thickening processes by a way of nappe thrusting in the DDMT.

GEOLOGIC SETTING

The DDMT, located in the Central SGOB (Fig. 1), comprises detachment-thrust slices, resulting from the "bi-direction" collision between the Tibet block and the North China plate in N-S direction and the Tibet block and the Yangtze plate in E-W direction, respectively during Triassic–Tertiary orogency (Xu et al., 1992). The oldest rocks, exhumed at the core of the metamorphic domes, consist of Upper Proterozoic granites in the Gezong dome and migmatized granitic gneiss in Gongchai and Chunniuchang domes. The zircon U-Pb ages of 824±14 Ma for Gezong granites and 864±8 Ma for Gongchai migmatized granitic gneisses indicate that these basement granitic complexes formed in Neoproterozoic (Zhou et al., 2002). The Sinian (or Upper Proterozoic) metasediments, which dominantly comprise of pebble-bearing gneisses, quartzites, and marbles, overlie unconformably the Upper Proterozoic granitic basement. Silurian formation of pelitic schists, gneisses, and marbles with minor amphibolite interlayers is placed directly on top of the Sinian formation, and a series of thrust-detachment deformation zones or thrust shearing zones separate it beneath the Upper Proterozoic rocks. The Devonian is lithologically similar to the Silurian formation, but has a greater abundance of quartzites at the bottom. The Carboniferous consists of pyrite-bearing phyllites and limestones. Limestones occur in the lower part of Permian formation, whereas thick metabasalts in the Upper Triassic formation are composed of low-grade meta-sandstones, breccia, phyllites and meta-tuff, which are extensively distributed over the southeastern Songpan-Ganze orogenic belt, and has been interpreted as flysch-like affinity (Xu et al., 1992).

In the Danba area of the Songpan-Ganze belt, rocks exposed from deep crustal levels commonly occur as metamorphic complexes of regional scale with dome-like geometry. A continuous inverted Barrovian-type metamorphic sequence upward from the lowest structural levels including chlorite, biotite, garnet, staurolite, and kyanite zones to a low-pressure Buchan-type metamorphic sequence consisting of sillimanite zone and sillimanite+K-feldspar cordierite zone have been mapped in the Danba metamorphic complexes by Chinese scholars (Sichuan Bureau of Geology, 2000). The Barrovian-type metamorphic isograds are truncated by the Buchan-type and a metamorphic discontinuity (or gap) separates them from each other suggesting the superposition of Buchan type onto the Barrovian type.

So far, the metamorphic peak conditions of the Danba domal terrain have been relatively well defined (Huang et al., 2003a, b, 2001), whereas the metamorphic conditions of both prograde and post-peak histories are less thoroughly investigated. Undoubtedly, an accurate analysis of metamorphic reaction histories and the P-T paths is of crucial importance, especially in testing the viability of different tectonic models and their application to the Danba domal metamorphic terrain in the Songpan-Ganze orogenic belt.

Xu et al. (1992) interpreted the general structure in Danba metamorphic domes as detachment-thrust imbricated slices, and three structural units can be recognized on the basis of metamorphic grades and deformation styles across the nappe slices. The lowermost structural unit is a number of the Precambrain high-grade metamorphic core complexes that dominantly consist of migmatized granitic gneisses at the Gongchai and the Chunniuchang domes, and a lowgrade metamorphic core of Upper Proterozoic granites weakly deformed and metamorphosed at the Gezong dome. The middle unit comprises the medium structural level and consists of the sequence of Silurian–Permian meta-sediments and volcanic rocks. Amphibolite facies metamorphic mineral assemblages and strongly sheared bedding rheologic deformation styles are preserved in the lower part of this unit, and the intensity of deformation and metamorphic grade is very low in the upper part, that is within the Carboniferous and the Permian formations. The highest structural unit is composed of only Triassic sequence of anchimetamorphic flyschlike sediment that is characterized by large-scale folds and brittle fault deformation styles.

The disposition of regional metamorphic zones is shown in Fig. 2. The series of metamorphic zones are relatively perfect in the south compared to the north. Metamorphic grade increases gradually northwards from chlorite zone, through garnet, staurolite, kyanite, sillimanite plus muscovite, sillimanite plus K-feldspar, to migmatite zone in the south; however, the north chlorite zone is in direct contact with the sillimanite zone and other zones of middle metamorphic grade, which is not present in Fig. 2. It is reported by the Bureau of Geological Mineral Resources of Sichuan Province, China (1994) that like the metamorphic zones exposed in eastern Scottish Dalradian (Harte and Hudson, 1979) in Danba region Buchan-type metamorphism occurred in the north as well, while the Barrovian type in the south. However, based on the well-developed thrust nappe structures mentioned above (Fig. 1, Xu et al., 1992), the absence of midmetamorphic grade zone in the north might be related to the thrust tectonics syn- or post-metamorphism, since the discontinuities between the metamorphic zones are almost parallel to the faults or thrusts.

Figure  2.  Geologic map showing the disposition of regional metamorphic zones in the study area. Grt. garnet; St. staurolite; Sil. sillimanite; Bt. biotite; Kfs. K-feldspar; Ky. Kyanite; Ms. muscovite.

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METAMORPHIC ZONES

Metamorphic zones are determined dominantly on the basis of index minerals or mineral assemblages of pelitic rocks. Muscovite and quartz are common minerals, and plagioclase exists almost in all rocks. Accessory minerals mainly consist of ilmenite, magnetite, apatite, zircon, monazite, graphite and epidote. The typical mineral assemblages in the main metamorphic zones are shown as follows.

Biotite zone is mostly distributed in Devonian–Permian rocks, and the typical mineral assemblage is: Chl+Bt. Biotite is present as coarse-grained porphyroblastic crystals in matrix consisting of white mica, quartz and chlorite, with lots of quartz and graphite inclusions.

Garnet zone: garnet is the most common mineral besides quartz and muscovite in the rocks from the area, and it coexists with other ferromagnesian minerals from this zone, through the staurolite, kyanite, sillimanite zones and finally to migmatite zone. In fact, garnet zone is distributed in a very strictly narrow belt that is on top of the Upper Silurian pelitic rocks and parallel to the boundary of strata sequences. The typical mineral assemblage is Grt+Bt±Chl without staurolite. Euhedral crystals of garnet contain lots of fine-grained quartz inclusions in their cores and mantles and are replaced by retrograde chlorite, which means that reaction Grt+Bt+H₂O=Chl+Ms+Qtz happened during the retrograde metamorphism.

Staurolite zone: staurolite first appears in pelitic rocks when chlorite is completely consumed during the growth of garnet and staurolite. This zone is exposed mainly to Silurian in the south just like garnet zone occurred only in Silurian strata. The mineral assemblage is characterized by St+Grt+Bt±Ky. As shown in Fig. 3, quartz-enriched bands occurred as inclusions throughout the garnet except for the 600 μm wide zone at the garnet rim. The banded inclusions define a primary compositional layering (S₀), which crosses the schitosity S₁ in the matrix at a large angle. Initially, S₁ might be parallel to the S₀ and then was isoclinally crenulated, giving to S₂, which as well crosses the matrix fabrics defined by biotite, muscovite, kyanite and staurolite at a large angle. The fabrics within the garnet are truncated by the inclusion-free zone at the garnet rim. It is obvious that the garnet grew in different mineral assemblages from its core to the rim, and most of the garnet grew with deformation (S₂). The schistosity in the matrix defined by muscovite, bitotite, kyanite and quartz only represents the last stage of metamorphism. Meanwhile, the garnet was consumed during the retrograde metamorphism. For example, most of the garnet rims are dissolved and replaced by chlorite and muscovite.

Figure  3.  Microphotographs showing the staurolite zone mineral assemblage and deformation. Ilm. Ilminite.

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Kyanite+sillimanite zone: staurolite disappears from the mineral assemblages, and sillimanite becomes stable in this zone. It is distributed in the south of this area along with the staurolite zone and is formed from the Silurian sedimentary rocks. The characteristic mineral assemblage is kyanite+ sillimanite+garnet+biotite. Within the garnet grains, except for its rim, occur various coarse-grained inclusions such as biotite, plagioclase, quartz, illuminate and a few epidotes. Most garnets are consumed along their margins. Moreover, kyanite was replaced gradually by sillimanite by the reaction: kyanite=sillimanite. Sample (G98686) from this zone will be discussed in detail later, out of which a P-T path is extracted on the basis of Gibbs method. As shown above, the last three metamorphic zones, garnet zonestaurolite zone-kyanite+sillimanite zone, are only exposed in the south of Danba domain; as a result, such distribution of metamorphic grades may be related to the thrust activation during or post the formation of the Danba Barrovian-type metamorphism. These three zones in the north were probably truncated by later faults or deeply buried by the tectonic slice overlain them.

Sillimanite+muscovite zone: it is characterized by the disappearance of kyanite and the coexistence of sillimanite and muscovite in the Silurian pelitic rocks. It is the most extensive and widespread metamorphic zone in Danba domain and is distributed around the domain centers (Fig. 2). The mineral assemblage is garnet+sillimanite+biotite in most metapelitic rocks. Most fibrolites intergrow with biotite along the margin of the biotite. The inclusions within garnet such as quartz, biotite, plagioclase, become evidently larger than those in lower grade phases mentioned above. Retrograde chlorite occurs at the rim of garnet from the rock that lies close to faults or shear zones.

Sillimanite+K-feldspar zone: it is basically restricted to the Lower Silurian sequence of the neighborhood of Chunniuchang granitic intrusion. Garnet+K-feldspar+biotite is the common assemblage in the zone. Migmatization occurs extensively in this zone and formed lots of granitic pegmatite lenses and dykes bearing coarse-grained sillimanite and garnet crystals and trending parallel to the regional foliation. Randomly arranged coarse-grained muscovite occurs in the leucosome of the migmatized rocks and contain inclusion of sillimanite and biotite that has continuity in fabric with the matrix (Fig. 4). Furthermore, myrmekitic textures are well developed as well, some newly formed plagioclases crystallized and overgrew around the older one (Fig. 5). These microstructures can be interpreted by the dehydration melting reaction: sillimanite+K-feldspar+liquid=muscovite+plagioclase (Spear et al., 1999b). The muscovite and the new plagioclase rim obviously resulted from the retrograde metamorphism. It is indicated that the rocks from this zone suffered partial melting and the granitic liquid occurred at the peak stage of metamorphism, and then cooled down at the pressures above the invariant point IP1, i.e., about 650 ℃, 4×10⁸ Pa in the system of NaKFMASH (Spear et al., 1999b).

Figure  4.  Microphotograph showing that the internal fabric of sillimanite and biotite inclusions within the muscovite porphyroblast is continuous with the external foliation.

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Figure  5.  Microphotograph showing new plagioclase crystals overgrow on the old one by: sillianite+K-feldspar+melt=plagioclase+muscovite. Pl. plagioclase.

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Migmatite zone occurs dominantly in the pre-Cambrian granitic intrusions located at the center of the Danba metamorphic complex, especially in the Chunniuchang intrusion (Fig. 2). In addition, the protolith of the migmatites from this zone represents the oldest rocks in this area, and a stratigraphical unconformity between it and the above Silurian was well preserved in the related metamorphic rocks. That is to say, on one hand, the migmatites and related intrusions are the exposure of basement rocks that are almost the same as those distributed along the boundary of the Sichuan foreland basin and Yajiang-Maerkang fold zone to the south and east of the Danba domain (Fig. 1). On the other hand, they were probably located in the heat center during Barrovian-type metamorphism in Danba.

According to the distribution of metamorphic zones in the Danba domain, we can find that there is a relationship between metamorphic grade and timing of stratigraphical sequence. That is, the higher the metamorphic grade is, the older the stratigraphical time. The so-called "intrusions" at the center of the domal structure are only the exposures of the ancient crystallized basement over this region, which were formed in Early–Middle Proterozoic that is much older than the formation time of the Danba metamorphic domain. It means that the heat causing the Barrovian-type metamorphism at Danba is not from these "intrusions"; in contrast, they suffered the later metamorphism along the strata over them.

Xu et al. (1992) suggested that the dominant metamorphism in Danba domain occurred at about 150±3 Ma on the basis of Rb-Sr isochron of minerals from staurolite zone, biotite zone and kyanite zone, respectively. So far we have dated some minerals from these rocks and obtained the ages that are related to metamorphism (these data will be published in another article). Dating of a monazite from sillimanite-K-feldspar gneiss near the center of the Domain shows U-Pb concordant age of 175.3 Ma. Meanwhile the Ar/Ar age of an amphibole from garnet-amphibolite in the staurolite zone is 175.2±0.4 Ma. In addition, dating of biotite out of staurolite-kyanite schist (STW05) gives an Ar/Ar age of 155.4±0.5 Ma, which is very close to Xu et al. (1992). However, most biotites dis-tributed in staurolite through kyanite to sillimanite+Kfeldspar zone indicate the age of 63–65 Ma based on the results of Ar/Ar isotope dating. One exception is that a biotite from migmatized gneiss on the margin of Gongchai intrusion shows the Ar/Ar age of only 20 Ma (Xu et al., 1992). This age may be related to the formation and uplift of Tibet plateau, not to the forma tion of the metamorphic complex domain in this area. In fact, one of the Ar/Ar isotope datings of biotite from the shearing fault cross-cutting the center of Danba domain yields an age of 59.5±0.3 Ma. It might represent the time of the retrograde metamorphism or cooling down of the metamorphic terrain. Therefore, it is reasonable to consider that the Barrovian-type metamorphism of Danba domain occurred during 175–150 Ma, meanwhile the S₁ and S₂ in garnet were formed. Then the peak metamorphism might be approached at about 64 Ma, in which the main mineral assemblages in this area and S₃ foliation in the mineral matrix were formed.

REACTION TEXTURES AND MINERAL CHEMISTRY OF SAMPLE G98686 FROM KYANITE ZONE

In order to extract the P-T trajectory of Danba metamorphic complexity domain, we select one sample G98686 out of the kyanite-sillimanite zone near the center of the domain mainly because the porphyroblastic garnet in it kept fine growth zoning though the sample underwent later alteration at the rim. In addition, there are many mineral inclusions as biotite and plagioclase, which help to gain peak metamorphic temperature. Although this sample cannot represent the whole domain, it, at least, provides some important constraints for the establishment of geodynamic models for this area.

Sample G98686 is sillimanite-bearing kyanite schist. The matrix minerals include sillimanite, kyanite, biotite, muscovite, plagioclase, quartz, a little staurolite and accessory minerals such as ilmenite, apatite, magnetite, zircon, and monazite. Garnet occurs as porphyroblasts with lots of inclusions consisting of biotite, plagioclase, quartz and ilmenite distrib-uted throughout the garnet porphyroblast except for the 250 μm wide zone at its rim (Fig. 6). The garnet occurs only as irregular shape because of garnetconsuming reactions at the rim during the metamorphism.

Figure  6.  Back scatter electronic microphotograph showing the inclusions in garnet from sample G98686.

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Garnet-Producing Reactions and Mineral Chemis-try

In general, mineral assemblages in a sample, especially the matrix minerals, represent the equilibrium paragenetic minerals at the peak metamorphism. However, some inclusion minerals preserved within some porphyroblast, such as garnet, might record the relict equilibrium and some other minerals in reaction rims such as corona and symplectites represent the retrograde metamorphic alteration or overprint mineral assemblages. In particular, garnet is so stable and is a refractory mineral that element diffusion within it is much slower than in mica in low- to high-grade metamorphism (Spear et al., 1999a; Spear, 1988b). Therefore, the inclusions and chemical zoning of garnet can record the reaction history of the whole rock to a great extent. As shown in Fig. 6, through the core to the mantle of garnet exist lots of inclusions, such as biotite, plagioclase and quartz, but they disappear sharply in the rim. It is reasonable to assume that garnet core and rim grew in completely different equilibrium environments, i.e., in different mineral assemblages, respectively. The growth of garnet core and mantle is obviously related to plagioclase, biotite and quartz included within it. Although there is no chlorite in the sample, it is a very common rock-forming mineral in the area, especially in low grade such as chlorite zone and garnet zone. Meanwhile, there is no chloritoid in both the sample and the study area, which means that chloritoid was not involved in the reactions in the pelitic rocks from this area. On the basis of Spear and Cheny's (personal communication) petrogenetic grid for KFMASH system, the garnet core and mantle were produced by the following reaction with the consumption of chlorite

As a result, chlorite might be consumed completely by either the above continuous reaction or other reactions that involve kyanite or/and staurolite. For example

and/or

Reaction (2) requires consumption of garnet and produces staurolite. In fact, the amount of staurolite in the sample is so small that the effects caused by this reaction can be ignored. Spear et al. (1999a) suggested that staurolite stability shrunk greatly with extracomponents such as Mn and Ca addition into the KFMASH system, so that staurolite made little constraint on the disappearance from the sample. Reaction (3) cannot only lead to consumption of chlorite completely in the sample, but also garnet. However, the continuous decrease of Mn from the garnet core to the rim in content (Fig. 7) shows that the garnet core and mantle rarely suffered consumption prior to the growth of garnet rim. This is because garnet is almost the only mineral that contains Mn in the rock. If it were consumed before the crystallization of the rim, there would be an Mn-enriched zone between the rim and the mantle of the garnet. Therefore, we can conclude that the rim of the garnet overgrew on the early garnet's rim, and the reaction (3) made little contribution on the growth of the garnet.

Figure  7.  X-ray mapping photographs showing the composition change of the garnet in sample G98686.

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The inclusion-free rim of the garnet has completely different compositions from the core and the mantle. X-ray mappings (Fig. 7) of the garnet clearly show the distributions of Ca, Fe, Mg and Mn within the garnet. The core and the inclusion-free rim of the garnet are enriched with Ca. The mole fraction of grossular in the garnet decreases from 0.076 at the core, through 0.070 in the mantle, to 0.055 near the rim, but increases rapidly up to 0.091 at the rim. It can be observed that the boundary between the high-Ca rim and low-Ca mantle of the garnet is so sharp that it can be easily accounted for by very slow diffusion rate of element Ca in the garnet crystal under the retrogressive metamorphic conditions. Therefore, the X-ray mapping of Ca well records the history of garnet growth. Most of the Mn of the garnet is concentrated in its core and the mole fraction of spessartine drops down outwards except for the outmost rim where the Mn increases slightly, which are related to the consumption of garnet during later metamorphism. The mole fraction of spessartine of the garnet ranges from 0.085 at the core, through 0.072, 0.058, 0.040 in the mantle, to the 0.021 on the rim. Spear et al. (1990b) suggested that Mn isopleth of garnet in pelitic rocks represents the real-time of growth of garnet on the basis that Mn always combines into the lattice of garnet more easily than Fe, Mg and Ca due to increase of either pressure or temperature during metamorphism. That is, contours of spessartine of the garnet record the shape-changes of the garnet during its crystallization. Thus, the distribution of Mn in the garnet is a good indicator for us to determine processes of the nucleation and growth of the garnet. By contrast, the almandine increases gradually from 0.720 at the garnet core to 0.786 on the rim. The patterns of the distribution of Mg in the garnet behave irregularly. First, it increases from 0.120 at the core to 0.137 near the rim, and then drops to 0.101 on the outer margin. Like the distribution of Ca and Mg in the garnet, it is characterized by decrease of the ratio of Fe/(Fe+Mg) from 0.857 at the core to 0.845 near the rim, and then a sharp increase to 0.886 on the rim. Therefore, Mg, Ca and Fe/(Fe+Mg) obviously indicate the processes of two-stage growth of the garnet that coexisted with completely different mineral assemblages, respectively. It is the discontinuity of the compositions of Ca and Mg that records the important reaction history in the rock.

The plagioclases included in or out the garnet show systematic variation in the compositions. The molal amounts of anorthite of the plagioclase inclusions within the garnet decrease exponentially from 0.41 at the core to 0.26 near the rim with decrease of grossular of the garnet surrounding it (Fig. 8, Table 1). It should be noted that the mole fraction of plagioclase at the core is estimated from trend regression analysis on the basis of exponential function, since there is not any plagioclase inclusion at the core of the garnet. In addition, the plagioclase in the matrix is characterized by discontinuous or patched chemical composition zoning (Fig. 9). Within the same crystal of plagioclase, high-albite with X_An=0.25, occurs near its center, but high-anorthite with X_An=0.31 on the margin. Like the garnet, all these compositions of the discontinuously zoned plagioclase record the history of the reaction. The growth of the garnet via reactions (1), (2) and (3) leads to consumption of plagioclase, which would result in the transition of the plagioclase from Ca-enriched to Na-enriched components, since some Ca was incorporated into the crystal of the garnet during the metamorphism.

Figure  8.  Compositional zoning of the plagioclase inclusions in garnet in sample G98686. Qtz. Quartz.

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Table  1.  Temperatures and pressures result from Gibbs method for the garnet in sample S98686 from the Barrovian

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Figure  9.  Sketch showing the compositional zoning of plagioclase in the matrix of sample G98686.

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The amount of component phengite is more sensitive to the changes of metamorphic pressure than temperature. The muscovite in the sample occurs only in the matrix, but those near the garnet show obvious chemical zoning in the amount of phengite. They contain higher phengite, 0.066 mole fraction, at the margin compared to 0.052 inside. It indicates a late high-pressure metamorphism made at the muscovite to adjust its composition, having more phengite component at the margin. So the crystallization of the garnet margin that contains more grossular and less pyrope component was completed in the condition of higher pressure in the later metamorphism.

On the basis of the mineral chemistry and textures, the inclusion-free rim of the garnet was stable with the matrix mineral assemblage plagioclase, quartz, biotite, kyanite and muscovite and it grew in this assemblage, which were supplied with or absorbed essential materials for the growth of the garnet rim. The reaction concerning the formation of the garnet rim might be

This reaction is the boundary reaction that separates the inclusion-free rim of the garnet from its core and mantle. So the composition discontinuity occurs along this boundary. Moreover, kyanite appeared in the final matrix instead of chlorite, muscovite became much more phengite-enriched, and plagioclase more sodic.

Garnet-Consumption Reactions and Mineral Chemistry

As a matter of fact, the garnets appear in this sample only as residue that was partly preserved during the metamorphism, and most of them were dissolved by the garnet-consumption reactions after the growth. Only at the site in contact with quartz, for example, the probe analysis spot 10 in Fig. 8 shows that a relatively perfect profile of the garnet growth might be preserved. At the site where garnet was highly dissolved, sillimanite and biotite occur, which means that the consumption of the garnet took place during the transition from the stable field of kyanite to sillimanite. The texture of the sample shows the obvious transition reaction

which progressed through two simple reactions occurring in different textural domains. As indicated in Fig. 6, the rim of kyanite is replaced by muscovite. The reaction concerning this replacement should be

And within the same thin section, sillimanite replaced muscovite and garnet (Fig. 5), which demonstrates that the reaction is

Because of the strong replacement of the biotite, sillimanite and plagioclase for the garnet rim in the sample (Fig. 6), it is reasonable to consider that reaction (4) progressed from the right to the left and reaction (5) played an important role in the consumption of the garnet. When the garnet was consumed during the later metamorphism, plagioclase would grow and its composition would become more and more calcic due to the release of Ca out of garnet. So it is observed that some sodic plagioclase core (X_An=0.25) in the matrix was completely surrounded by relatively calcic plagioclase rim (X_An=0.31) (Fig. 9). It is easy to find that the ratios of Fe/(Fe+Mg) of the garnet and biotite increase towards the boundary between the two minerals. A zone of the high-ratio of Fe/(Fe+Mg) both in the rims of the garnet occurs along the outline of the garnet. As it is known that the atom or ion of Mn, Fe and Mg is more easily diffused during the later metamorphism at relatively high temperatures than Ca, we can consider that the high ratio of Fe/(Fe+Mg) results from those reactions concerning the consumption of the garnet. And they are net-transfer continuous reactions taking place between the garnet, biotite and sillimanite, since both the ratios of Fe/(Fe+Mg) of the biotite and garnet increase simultaneously in the same way at their rims. On the outermost rim of the garnet, Mn increases slightly (Fig. 7). This indicates the dissolution of the garnet, releasing Ca, Fe and Mg into other minerals such as plagioclase and biotite and left Mn at its rim.

GEOTHERMOBAROMETRY

Determination of metamorphic temperatures and pressures depends on mineral assemblages, metamorphic reactions, compositions of minerals in equilibrium condition, and calibration of geothermobarometers. Based on the crystallization processes of the garnet mentioned above, two kinds of mineral assemblages, garnet+biotite+chlorite+muscovite+ plagioclase+quartz and garnet+biotite+kyanite/sillimanite+muscovite+plagioclase+quartz occurred during the metamorphism. Because garnet and biotite coexist through the whole metamorphism, an exchange reaction, FeMg_-1, between them is a useful thermometer, which has been calibrated by many authors so far. It is the only geothermometer for the sample we studied. Application of the calibration of Hodges and Spear (1982), Berman (1990), Patino Douce (1998), Reinhardt and Kleemann (1994), Ganguly and Saxena (1984) gives similar results that span about 100 ℃ (i.e., 50 ℃). Geobarometers applicable to the mineral assemblage garnet+biotite+chlorite+ muscovite+plagioclase+quartz include garnet-biotite-muscovite-quartz (GBMP), which were calibrated by Ghent and Stout (1981), Hodges and Crowley (1985), Powell and Holland (1988) and Hoisch (1991) and garnet-muscovite-plagioclase-quartz (GMP), which was calibrated by Hoisch (1987). And the geobarometers applicable to the mineral assemblage garnet+ biotite+kyanite/sillimanite+muscovite+plagioclase+ quartz include both the above geobarometers and garnet-plagioclase-kyanite-quartz (GASP) that was calibrated by Newton and Wood (1979), Hodges and Spear (1982), Ganguly and Saxena (1984), Hodges and Crowley (1985) and Kozoil (1989). All these geobarometers give similar results that span about 1.0×10⁸ Pa (i.e., 0.5×10⁸ Pa).

All mineral composition analyses were measured on the JEOL733 electron microprobe at Rensselaer Polytechnic Institute (RPI), USA. The operating conditions are the same as that described by Kohn et al. (1992). Figure 10 shows the distribution of the analyzed spots of the sample G98686 that was used to estimate the metamorphic temperatures and pressures.

Figure  10.  Diagram showing the P-T path for the sample G98686. Aluminum silicate phase from Holdaway (1971).

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Choosing spot analyses that represent equilibrium compositions of phases is very important for the correct determination of metamorphic conditions. Asshown in Fig. 8, Mn of the garnet decreases along the traverse line from point 2, through points 4, 15 and 8, to 10, which indicates that growth of the garnet starts near point 1 and ends up at points 10. Because Mn prefers to incorporate into crystals of garnet, its content in the garnet may be regarded as an indicator of the process of garnet growth. That is, isopleth of spessartine within the garnet represents growth lines of the garnet. Plagioclase 15 next to garnet point 5 and the biotite inclusion 24 are almost located on the same isopleth of spessartine of the garnet. That is, garnet 5, plagioclase 15 and biotite 24 were crystallized at the same time during the growth of the garnet. In addition, the size of the biotite inclusion 24 is about 385 m long and 128 m wide that are large enough to prevent its composition from complete diffusion with the nearby garnet during retrograde metamorphism. It is obvious from Fig. 7 that there is no such complete diffusion with the plagioclase inclusion and its host crystal the garnet. In order to apply geobarometers GBMP and GMP to the sample, we need to know the composition of muscovite coexisting with the above three minerals in the equilibrium status. There are two kinds of muscovite in the matrix, one is high-phengite (point 35) and the other is low-phengite (point 32). We consider that the low-phengite muscovite (point 32) was stable with garnet 5, biotite 24 and plagioclase 15. This is because the high-phengite muscovite only occurs on the margin of the low-phengite muscovite. Moreover, composition of muscovite has small effect on the computation of the geothermobarometries than those of garnet, biotite and plagioclase. Based on the mineral composition, the geothermometers and the geo-barometers give the average P, T condition, (543±30) ℃, (4.9±0.3)×10⁸ Pa, and shown in Fig. 10. Although the temperatures arising from different geothermometers span a wide range of about 100 ℃, the calibration of Hodges and Spear (1982), Berman (1990), Patino Douce and Harris (1998), Reinhardt and Kleemann (1994) gives almost the same temperature, 543 ℃. Therefore, we think that the temperature and pressure represent a reasonable mark point on the P-T path of the garnet growth.

The rim of the garnet is surrounded directly by the matrix minerals plagioclase, biotite, kyanite and muscovite; hence, we consider that the exchange reactions between them had approached equilibrium during growth of the garnet rim. The combination of spot analysis of garnet 10, biotite 27, plagioclase 18, muscovite 35 and kyanite can be used to estimate pressure and temperature of the equilibrium based on the geothermobarometries GASP, GMP, GBMP and garnetbiotite. In order to avoid the effect of Fe-Mg diffusion on the estimation, we selected spot 27 of biotite that is far from garnet 10. However, the diffusion substitution of NaSiCa_-1Al_-1 in plagioclase and Al₂Si_-1(Mg, Fe)_-1 in muscovite is not so strong, so we adopt spot 35 at the rim of the muscovite and spot 18 in plagioclase next to the garnet in our P, T calculation. The results are 534±29 ℃, (5.8±0.3)×10⁸ Pa and shown in Fig. 12. Using the same calibration of geothermobarometries as the previous one, we find that the standard deviation of temperatures is ±29 ℃, which is almost equal to the previous. Moreover, the three geobarometries GASP, GMP and GBMP give a very close pressure, (5.8±0.3)×10⁸ Pa, at temperature 534 ℃. In comparison with the interior of the garnet (spot 5), we find that the rim (spot 10) was formed at lower temperature but higher pressure.

P-T PATH

The P-T paths for various Barrovian and Buchan type zones can be constructed using garnet-biotite geothermometer proposed by Hodges and Spear (1982), Indares and Martingole (1985), and Grt-Pl-Ms-Bt geobarometer by Hodges and Crowley (1985), Hoisch (1991). On the basis of these studies, Cheng and Lai (2005) presented an overall metamorphic P-T condition for each zone developed in the DDMT, strengthening the clockwise pattern of the P-T paths. The P-T paths obtained reflect a kinematic regime of a continent-continent collision geodynamics.

However, the prograde and post-peak history of these zones can only be revealed by a careful study on samples, which are representative of the Barrovian metamorphic event.

P-T paths for the garnet growth in sample S986S8 were computed using the program Gibbs coded and modified by Spear since 1988 on the basis of Gibbs method (Spear, 1988a).

As indicated previously, the garnet from sample S986S8 arose from two stages of growth, so the modeling of the P-T paths is computed in the two stages, respectively. The first calculation is performed in the mineral assemblage garnet+biotite+chlorite+ plagioclase+muscovite+quartz+H₂O and the second in the assemblage garnet+biotite+kyanite+plagioclase+ muscovite+quartz+H₂O, both of which in the system SiO₂-Al₂O₃-MgO-FeO-MnO-CaO-Na₂O-K₂O-H₂O (NCMnKFMASH).

The first calculation of P-T paths for the former assemblage requires the composition of chlorite. Unfortunately, the chlorite coexisting with the garnet core and mantle or near the rim in the period of their formation was consumed completely. Thus, the composition of chlorite has to be obtained from other ways. Spear and Hickmott (1990) presented a similar model with the problem considered in this article for garnet and plagioclase growth. We can deduce the chlorite composition and pressure and temperature for point 5 within the garnet from the Spear and Hickmott's (1990) reference point at 6×10⁸ Pa and 500 ℃ (Table 2). The other minerals assumed to be in equilibrium with garnet 5 are plagioclase 15, biotite 24 and muscovite 32.

Table  2.  Components of minerals at reference conditions

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In this NCMnKFMASH system, consisting of the above 7 mineral phases, there are 19 total variables, two of which are pressure and temperature, and 15 constraint equations, eight (i.e., 17 component variables–9 independent variables) of which refer to dependent equations between the phase components, and 7 (total phase numbers) of which refer to conservation of mass for each phase (i.e., ∑x_i=1 for each phase) without consideration of mass balance equations. The degree of freedom of the system is 4 (i.e., 19–15). Here we select the four variables X_Alm, X_Sps, X_Grs of garnet and X_An of plagioclase as monitor variables to perform the calculation. That is, once given the values of the four variables, the other intensive variables like mole fraction of component of any phase and pressure and temperature can be determined using Gibbs. In general, the choice of the eight dependent constraint equations of the system is arbitrary. On the basis of the geothermobarometry used in this article, we consider that the following eight equations might control the compositions of minerals and the evolutionary history of pressure and temperature. Five of the eight equations are exchange reactions such as

Three of the eight equations are net transfer reactions

As a result of the calculation, the temperature and pressure are 574 ℃ and 5.1×10⁸ Pa (Table 2), which is basically consistent with the values given using the geothermobarometers, 543±30 ℃ and (4.9±0.3)×10⁸ Pa. Thus, the composition of chlorite that arose from the calculation is close to the real values and is reliable. Based on the composition of chlorite and metamorphic conditions recorded by garnet 5, we may consequently choose this point (garnet 5) as a reference point to compute pressures and temperatures of those points in the garnet core and rim. In addition, the monitor variables are X_Alm, X_Sps, X_Grs of garnet and X_An of plagioclase. The results are listed in Table 1.

The second calculation of the P-T path is performed with the appearance of kyanite instead and the chlorite in the former assemblage was consumed during growth of the garnet. Here we assume that the growth of kyanite is not related to chlorite. Similar to the first calculation, the number of constraint equations becomes 13, six of which are dependent equations, and the degree of freedom of this new system isstill four. Because of the absence of chlorite, the dependent equations related to chlorite (2'), (4'), and (6') are not involved in this system. However, one net transfer equation accounting for the appearance of kyanite must be activated in the second assemblage. That is the following reaction

which will occur instead of equation (6). And then the 6 dependent equations for the second calculation comprise (1'), (3'), (5'), (7'), (8'), and (9'). We choose the point 106 in the garnet as the reference point (or start point) to calculate the temperature and pressure of point 10 of the garnet. The results are 553 ℃, 5.6×10⁸ Pa, which is very close to the values from geothermobarometries, 534±29 ℃, (5.8±0.3)×10⁸ Pa (Table 2).

The results of P-T path calculations on the traverse through the garnet crystal from sample G98686 are shown in Fig. 10. The P-T path shows a period of nearly isothermal compression from the core to the mantle (G2-G106), which is located in the stability field of sillmanite, followed by nearly isobaric cooling towards the rim, which is in the field of kyanite.

The P-T path computed for the staurolite-kyanite zone of Barrovian type metamorphism in western Sichuan of West China may be used to constrain a thermo-tectonic model for the evolution of the nappe complex. Compared to P-T paths for subduction complex and metamorphic complex that show clockwise P-T trajectories, this P-T path is very close to those for Fall Mountain nappe complex of New Hampshire, eastern USA (Spear, 1993, 1989). As shown in Fig. 1, Danba metamorphic dome is located in a series of nappe slices that thrust from the north to the south (Hou et al., 1994; Xu et al., 1992). At least three structure levels can be recognized on the basis of metamorphic grades and deformation styles across the thrust-decollement zones (Fig. 11). The lowermost is Neoproterozoic granitic basement complex, which is slightly deformed and suffered migmatization at Gongchai domal complex in the sillimanite+ K-feldspar zone. Proterozoic–Silurian metasedimentary rocks of amphibolite facies were thrust southward on top of the Proterzoic granitic basement, which preserved a typical Barrovian metamorphic zonation, i.e., from biotite zone to sillimanite+K-feldspar zone. The types of structures in Silurian gneisses and schists on top of this structure level indicate a strongly sheared bedding rheologic deformation during the thrusting decollement parallel to the strata. The studied sample used to calculate the P-T path is collected from the staurolite-kyanite zone in this structure level (black dot on Fig. 11). The structure layer lying on Proterozoic–Silurian is composed of Devonian through Carboniferous metasedimentary rocks to Permian metabasalts, of which metamorphic grades, in general, lower than the garnet zone, are obviously lower than those in the underlying structure layer and deformation is also less intensive. The deformation styles in this structure level are dominantly characterized by bedding cleavage and local bedding folding. The top structure level consists of Triassic sedimentary clastic rocks, on which only very low metamorphism, never higher than the biotite zone, was overprinted, and within which those primary sedimentary structures and textures are still well-preserved. Large-scale folds and brittle faults characterize the deformation styles within this structure layer. It is obvious that the decollement-thrusting had resulted in crustal thickening in this area. The P-T path for sample S986S8 from the Proterozoic–Silurian structure layer recorded this thickening event. The metamorphic conditions recorded by the core of the garnet indicate that initial crystallization of the garnet is under low pressure garnet-chlorite zone, which corresponds to a depth of about 19 km based on overlying rock density of 2.65 g/cm³ (Fig. 11a). The initial metamorphism resulting in crystallization of garnet and biotite in the sample may be related to upward thermal flow from the bottom of the crust or magma intrusion within the lower crust. Following the initial growth of garnet, the Triassic structure layers along with the Devonian– Permian layer were thrust over the underlying slice caused by thickening of the crust and metamorphic pressure increased by 1×10⁸ Pa that correspondes to the increase in depth of about 3 km. That is to say that the studied sample was buried to a depth of 22 km (Fig. 11b) when the garnet rim grew along with the appearance of kyanite and disappearance of chlorite. It is very interesting that the P-T path follows counterclockwise, nearly isobaric cooling trajectory, with only a slight rise in pressure after the maximum temperature is reached. This may be ascribed to the thrust thickening of the crust by cold Triassic sedimentary rocks. The cold upper plate absorbed heat out of the lower plate so that it resulted in the decrease of tem-perature during the crustal thickening event. Lastly, the metamorphic domal complex was exhumed to surface (Fig. 11c) by uplift of crust during late or post orogenesis.

Figure  11.  Simplified tectonic models on the basis of P-T path for the kyanite zone.

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CONSTRAINTS ON P-T PATH FROM P-T-X-M PHASE RELATIONS

The previous P-T path calculation is based on the P-T-X relations of the corresponding mineral assemblages without consideration of constraints of mass balance. If this P-T path records the real metamorphic processes, changes of not only chemical compositions but also modes of the rock-forming minerals would be consistent with the P-T trajectory. In order to confirm the P-T path and determine the phase relations in the assemblages, it is necessary for us to do the P-T-X-M modeling. That is to contour the P-T space by isopleth of mineral chemical compositions and abundance. The P-T-X-M modeling is performed on the assumption that crystallization occurred in the state of complete equilibrium for the specified assemblages. That is to say, fractional crystallization of minerals didn't take place. Although this is not true, it approaches nearly the real world and indicates the trends of the metamorphism. With the addition of mass balance constraints, the system becomes closed and the variance will be reduced to two (Menard and Spear, 1994). The new additional variables are modes or moles of mineral phases, M phase. In this article, the P-T-X-M calculation is performed for the two assemblages discussed previously, one of which is quartz+muscovite+ plagioclase+garnet+biotite+chlorite. The other contains kyanite instead of chlorite in the former assemblage.

For the first assemblage condition for the garnet core, probe analysis number G2 (561 ℃, 4.421×10⁸ Pa) (Table 3) was chosen as a reference point to create contour diagrams (Fig. 12). Bulk compositions (listed in Table 3) for the reference point was obtained from modification of natural rock (mica schist, sample V152, Bureau of Geological Mineral Resources of Sichuan Province, China, 1994) collected from biotite zone, which corresponds to the same stratigraphic unit as sample S986S8. The compositions of muscovite, biotite, plagioclase and chlorite associated with garnet G2 were inferred previously. The points G4, G5 and G106 within the garnet on the traverse towards the line from the core to the rim were drawn in Fig. 12 so as to be compared with the P-T-X-M isopleths. As shown in Fig. 12, the P-T-X-M prediction modeling is very close to the natural cases, although the contours are constructed assuming equilibrium crystallization of minerals. That means all minerals in the closed system are supposed to be homogeneous without any chemical zoning. By contrast, both garnet and plagioclase in most natural rocks, in general, preserved chemical zoning that results from either fractional crystallization or diffusion. Each chemical zoning of a mineral, like a snapshot, records the composition of the mineral equilibrated at that time. The measured values of grossular, almandine and spessartine of garnet and anorthite of plagioclase are so similar to the estimated values by the P-T-X-M modeling that the points G2 at the core, G4 in the mantle, and G5 near the rim within the garnet crystal are almost located on the corresponding isopleths (Fig. 12). However, mole fraction of pyrope and ratio of Fe/(Fe+Mg) of the garnet deviated far from the contours. This may be attributed to diffusion of Mg and Fe within the garnet during retrograde metamorphism and/or fractional crystallization of garnet. Accordingly, we believe that this P-T-X-M modeling can be used to interpret qualitatively early heating process producing garnet and consuming chlorite as indicated by reaction (1). According to Fig. 12, the variations of mole fraction of pyrope, X_Pyr, and the ratio Fe/(Fe+Mg) of garnet are sensitive to temperature. In general, X_Pyr monotonically increases with temperature, whereas ratio Fe/(Fe+Mg) decreases. A maximum of almandine mole fractions, X_Alm, appears at around 560 ℃, which is obviously different from the behaviors of other end members of garnet. Menard and Spear (1994) and Spear et al. (1991) suggested that it is related to the thermal stabilities of the Mn, Fe and Mg end member transition reaction from chlorite+ quartz to garnet. All the points from the garnet core towards the rim are located on the low temperature side of the maximum of X_Alm indicating that the garnet growth was at temperatures lower than that of the end member reaction daphnite+4quartz=3almandine+ 8H₂O that took place. This is the reason why only a monotonically increasing trend of X_Alm appeared from the garnet core towards the rim without any maximum. Contours for spessartine mole fractions and garnet moles are very similar in shape that shows a negative slope at high pressures. It indicates that the spessartine content and garnet abundance depend on not only temperature to a great extent, but also slightly on pressure. Mn can be more easily incorporated in garnet lattice than other elements so that spessartine content decreases monotonically with garnet growth during heating. By contrast, variation of chlorite abundance decreases with temperature. The behavior of grossular is similar to spessartine in dependence on temperature and pressure. With the increase of garnet abundance, Ca incorporated in garnet lattice was diluted with other elements so that grossular content decreases monotonically with temperature. As shown in Fig. 12, only anorthite content of plagioclase depends dominantly on metamorphic pressure. Therefore, anorthite of plagioclase can be used as a pressure monitor in this P-T-X-M modeling. The examined plagioclases from the garnet core towards the rim show a decreasing trend of anorthite content that recorded a rise of pressure or loading process during heating of the early metamorphism.

Table  3.  Bulk compositions for the reference point

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Figure  12.  Diagrams showing the P-T-X-M relations for the core and the mantle of the garnet from Ky zone.

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For the second mineral assemblage that kyanite was involved in without chlorite, a similar method as the first assemblage was applied to predict the compositions and abundance of the coexisting minerals garnet and plagioclase. The reference point for this assemblage was assigned to the condition under which the garnet rim (point 10) and the matrix minerals were stable. The P-T condition resulting from the geobarothermometries was described previously. That is, 554 ℃ and 5.592×10⁸ Pa (Fig. 13). The P-T path from the garnet core (G2) through the mantle (G4-G106) to the rim (G10) was drawn in Fig. 13 so that it was compared with the counters of the second P-T-X-M modeling. As shown in Fig. 13, the P-T trajectory of nearly isobaric cooling from the garnet mantle (G106) to the rim (G10) can be used to interpret the variation tendency of compositions of garnet and plagioclase. Indeed, pyrope and grossular content and Fe/(Fe+Mg) of garnet increase obviously along the traverse from the mantle to the rim (Figs. 6–9) that is consistent with the prediction results of the P-T-X-M modeling in Fig. 13. Spessartine of the garnet decreaseds, whereas almandine increases slightly from the mantle to the rim (Figs. 6–9), which was characterized by the nearly isobaric cooling P-T path nearly parallel to their counters in Fig. 13. Therefore, the counters of X_Alm and X_Sps of the garnet are important constraints for determination of the P-T trajectory during the late metamorphism. With the slight rise in pressure, anorthite content reduced rapidly. Unlike garnets, the plagioclase in the matrix shows irregular chemical zoning patterns.

Figure  13.  Diagrams showing the P-T-X-M relations for the rim and the mantle of the garnet from kynite zone.

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CONCLUSIONS

The most prominent metamorphic pattern in DDMT is the juxtaposition of the Barrovian type and Buchan type of metamorphism. The former includes chlorite, biotite, garnet, staurolite, kyanite and sillimanite zones, while the latter only developes silimanite+muscovite and sillimanite+K-feldspar zones. As the Barrovian zones reflect much greater P/T than Buchan zones, we emphasize on the former, especially higher grade St-Ky and Ky-Sil zones, to trace the peak metamorphic condition during the crustal events.

Petrological textures in the thin section show that there are two kinds of metamorphic reactions: one is related to garnet production and the other needs garnet consumption with a sample in Ky-Sil (sample).

According to the two mineral assemblages of sample G98686 from kyanite zone during the growth of garnet, we get an average P, T condition of 543±30 ℃, (4.9±0.3)×10⁸ Pa for the first growth stage and 534±29 ℃, (5.8±0.3)×10⁸ Pa for the second stage of garnet growth by geothermobarometry.

Counterclockwise P-T paths were drawn with Gibbs method by NCMnKFMASH system for two typical mineral assemblages in sample G98686. To approach the real thermal history, we take into account the mass while modeling P-T. It is suggested that P-T-X-M modeling is useful to interpret qualitatively early heating process though the composition of mineral changes during different stages of metamorphism.

A thermal tectonic model is presented where there are at least three structure levels across the thrust-decollement zones according to the P-T paths, metamorphic grades and deformation styles for the staurolite-kyanite zone of the Barrovian type metamorphism. It suggests that the special mineral assemblages and deformation are related to the crustal uplift during late or post orogenesis in the regional thrusting tectonic setting.