Well-preserved garnet growth zoning in granulite from the Dabie Mountains, central China

Ultra-high pressure eclogites and granulites both occur in the Dabie Mountains, central China. A garnet porphyroblast from felsic granulite in the Dabie Mountains has been analysed for compositional zoning by electron microprobe. Two segments of the porphyroblast have opposite compositional variations. Segment I (from centre outward 9  mm to analytical point 18) has decreasing X_Sps and increasing X_Pyr, while Segment II (from analytical point 18, 1  mm outward to the rim) has increasing X_Sps and X_Alm and decreasing X_Pyr and X_Grs. The compositional zoning in segment I is considered as growth zoning and that in Segment II as diffusive retrograde zoning. Garnet growth zoning records a P–T  path prior to the peak granulite metamorphism. The minimum P – T  conditions are estimated to be 1.35  GPa and 850  °C for peak metamorphism, based on the highest Mg/(Fe+Mg) composition in the garnet (analytical point 18) and matrix hypersthene, biotite and plagioclase. A symplectitic corona surrounds the porphyroblast and appears to have formed at 0.6  GPa and 700  °C. The well-preserved growth zoning in garnet suggests a short residence time for the granulite at peak metamorphism and thus rapid tectonic uplift history. The P–T  path is consistent with that of ultra-high-pressure eclogite in the area. Tectonic movements during a collisional event could have brought both the granulite and the eclogite to their present positions.     

Petrology and Petrogenesis of the Eclogite in Mt.Dabie Area,Central China

The occurrence,mineralogy and geochemistry of eclogites in the Mt.Dabie area show that they were subjected to a high-pressure metamorphism together with the country rocks,but their petrochemistry and REE geochemistry show some difference from those of the country rocks.The geochemical characteristics of the eclogites are similar to those of bot continental tholeiitic basalt and oceanic tholeiitic basalt.The rocks probably subducted to the upper mantle with the Dabie metamorphic complex.When elevated to the surface,they were subjected to different staes of retrogressive metamorphism.     

The duration of near‐peak metamorphism from diffusion modelling of garnet zoning

Garnet in a staurolite–kyanite zone sample from central Vermont displays a bell‐shaped Mn growth zoning with diffusional modification over the outer 100 μm. The diffusion is driven by the prograde net transfer reaction garnet + chlorite = kyanite + biotite as is evidenced by a well‐defined resorption zone on the rim. Analysis of the reaction history and resorbed garnet composition suggests that the peak temperature attained was 620–660 °C. Diffusional modelling of the rim diffusion provides an estimate of the duration of the metamorphic episode over which significant garnet diffusion occurs. The duration is a function of the assumed peak temperature and garnet diffusivities and range from a few hundred thousand years to a few million years. Such short durations require rapid tectonic burial and exhumation of relatively thin tectonic slices.     

最优化的矿物扩散地质速率计:北大别黄土岭麻粒岩缓慢冷却的证据

对北大别黄土岭麻粒岩中石榴子石进行了详细的电子探针工作，并根据一颗石榴子石的成分环带，建立了扩散模型并进行了数值模拟，结合最优化的方法，得到了极其缓慢的冷却速率0.1℃/a。从扩散动力学数值模拟的角度对大别山造山带的冷却速率进行了探索性研究。该结果表明，黄土岭麻粒岩在受麻粒岩相改造后经历的是一个极其缓慢、持续时间很长的冷却过程。     

On the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling

Abstract Finite difference models of Fe-Mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet-biotite Fe-Mg exchange thermometry. For rocks cooled from high grades where the garnet was initially homogeneous, the calculated temperature (T_calc) using garnet core and matrix biotite depends on the size of the garnet, the ratio of garnet to biotite in the rock (V_garnet/V_biotite) and the cooling rate. For garnets with radii of 1 mm and V_garnet/V_biotite<1, T_calc is 633, 700 and 777°C for cooling rates of 1, 10 and 100°C/Ma. For V_garnet/V_biotite= 1 and 4 and a cooling rate of 10° C/Ma, T_calc is approximately 660 and 610° C, respectively. Smaller and larger garnets have lower and higher T_calc, respectively. These results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below T_calc of the garnet core, provided that V_garnet/V_biotite is sufficiently small (<0.1) and that the composition of the biotite at the metamorphic peak has not been altered during cooling. Numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550–600° C generate a ‘well’in Fe/(Fe + Mg) near the rim during cooling. Maximum calculated temperatures for the assemblage garnet + chlorite + biotite + muscovite + plagioclase + quartz using the Fe/(Fe + Mg) at the bottom of the ‘well’with matrix biotite range from 23–43° C to 5–12° C below the peak metamorphic temperature for cooling rates of 1 and 100° C/Ma, respectively. Maximum calculated temperatures for the assemblage garnet + staurolite + biotite + muscovite + plagioclase + quartz are approximately 70° C below the peak metamorphic temperature and are not strongly dependent on cooling rate. The results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet-biotite Fe-Mg exchange thermometry on amphibolite facies rocks (T_max > 550° C) because the rim composition of the garnet, which is required to calculate the peak temperature, is that most easily destroyed by diffusion.     

Petrological significance of prograde homogenization of growth zoning in garnet: an example from the Llano Uplift'pa

In the Llano Uplift of central Texas (USA), prograde homogenization of garnet growth zoning took place during moderate- to high-pressure dynamothermal metamorphism over a narrow temperature range near the transition from the amphibolite to the granulite facies. This subtle record of early dynamothermal metamorphism survived subsequent static metamorphism at low pressures in the middle-amphibolite facies, despite the destruction of most high-pressure mineral assemblages that originated in the early metamorphic episode. Geographically systematic variations in the degree of homogenization indicate that the uplift as a whole underwent high-pressure metamorphism, in accord with emerging tectonic models for the mid-Proterozoic evolution of the southern margin of the North American continent.     

低温榴辉岩中石榴子石的成分分带:快速抬升的证据

对南大别黄镇冷榴辉岩中——颗石榴子石进行了详细的电子探针工作，对其特殊的成分分带，建立了扩散模型并进行数值模拟，结果得到4．5mm／a的快速抬升速率。该结果表明黄镇榴辉岩曾经历了一个快速的抬升过程。     

柴达木盆地北缘大柴旦和锡铁山榴辉岩中石榴子石环带对比及地质意义

大柴旦榴辉岩和锡铁山榴辉岩同为柴达木盆地北缘高压—超高压变质带的组成部分,但它们的岩相学特征、形成的PT条件及榴辉岩中石榴子石中保存的成分环带均具有明显的差异。大柴旦榴辉岩峰期变质的PT条件为T=620～730℃,P=2.3～2.8GPa,岩相学研究表明其经历了近等温降压或降温降压的后榴辉岩相历史,保存有进变质的矿物组合和石榴子石的生长环带及增生结构。相反,锡铁山榴辉岩的榴辉岩相的温度较高,岩相学研究显示其具有明显的后榴辉岩相的高温麻粒岩相的叠加,PT轨迹显示压力峰期早于温度峰期,具有略升温降压的后榴辉岩相变质历史,榴辉岩中石榴子石的成分环带表现为扩散环带,没有生长环带保留。大柴旦榴辉岩和锡铁山榴辉岩的不同的石榴子石环带特征反映了两者在形成过程中,特别是折返过程中不同的构造热历史。     

Coupled Lu–Hf and Sm–Nd geochronology constrains garnet growth in ultra-high-pressure eclogites from the Dabie orogen

Ultra-high-pressure eclogites from the Dabie orogen that formed over a range in temperatures (∼600 to > 700 °C) have been investigated with combined Lu–Hf and Sm–Nd geochronology. Three eclogites, sampled from Zhujiachong, Huangzhen and Shima, yield Lu–Hf ages of 240.0 ± 5.0, 224.4 ± 1.9 and 230.8 ± 5.0 Ma and corresponding Sm–Nd ages of 222.5 ± 5.0, 217.6 ± 6.1 and 224.2 ± 2.1 Ma respectively. Well-preserved prograde major- and trace-element zoning in garnet in the Zhujiachong eclogite suggests that the Lu–Hf age mostly reflects an early phase of garnet growth that continued over a time interval of c. 17.5 Myr. For the Huangzhen eclogite, despite preserved elemental growth zoning in garnet, textural study reveals that the Lu–Hf age is biased towards a later garnet growth episode rather than representing early growth. The narrow time interval of <6.6 Myr defined by the difference between Lu–Hf and Sm–Nd ages indicates a short final garnet growth episode and suggests a rapid cooling stage. By contrast, the rather flat element zoning in garnet in the Shima eclogite suggests that Lu–Hf and Sm–Nd ages for this sample have been reset by diffusion and are cooling ages. The new Lu–Hf ages point to an initiation of prograde metamorphism prior to c . 240 Ma for the Dabie orogen, while the exact peak metamorphic timing experienced by specific samples ranges between c . 230 to c. 220 Ma.     

Peak metamorphic temperatures from cation diffusion zoning in garnet

A model that relates the characteristic diffusion length and average cooling rate to peak temperature was developed for chemical diffusion in spherical geometries on the basis of geospeedometry principles and diffusion theory. The model is quantitatively evaluated for cation diffusion profiles in garnet. Important model parameters were calibrated empirically using diffusion zoning of Ca in garnet from the Pikwitonei Granulite Domain, a terrane for which the thermal history has been well characterized. The results are used: (i) to empirically test diffusion parameters for Mg and Fe(II) and (ii) to develop a tool that uses the diffusion zoning of these cations in garnet to constrain peak temperature conditions for garnet‐bearing rocks. The thermometric approach was externally tested by applying it to garnet crystals from various metamorphic terranes worldwide and comparing the results to published peak temperature estimates. The results overlap within uncertainties in all cases, but result that are based on Fe(II) and Mg chemical‐diffusion profiles are up to three times more precise than those acquired by conventional methods. The remarkable consistency of the results implies that the model is robust and provides a reliable means of estimating peak temperatures for different types of high‐grade metamorphic rock. The tool could be of particular advantage in rocks where critical assemblages for conventional thermometry do not occur or have been replaced during retrogression.     

Decoding the complex internal chemical structure of garnet porphyroblasts from the Zermatt area,Western Alps

Garnet is a prototypical mineral in metamorphic rocks because it commonly preserves chemical and textural features that can be used for untangling its metamorphic development. Large garnet porphyroblasts may show extremely complex internal structures as a result of a polycyclic growth history, deformation, and modification of growth structures by intra‐ and intercrystalline diffusion. The complex internal structure of garnet porphyroblasts from garnet–phengite schists (GPS) of the Zermatt area (Western Alps) has been successfully decoded. The centimetre‐sized garnet porphyroblasts are composed of granulite facies garnet fragments overgrown by a younger generation of grossular‐rich eclogite facies garnet. The early granulite facies garnet (G‐Grt) formed from low‐P, high‐T metamorphism during a pre‐Alpine orogenic event. The late garnet (E‐Grt) is typical of high‐pressure, low‐temperature (HPLT) metamorphism and can be related to Alpine subduction of the schists. Thus, the garnet of the GPS are polycyclic (polymetamorphic). G‐Grt formation occurred at ~670 MPa and 780°C, E‐Grt formed at ~1.7 GPa and 530°C. The G‐Grt is relatively rich in Prp and poor in Grs, while E‐Grt is rich in Grs and poor in Prp. The Alm content (mol.%) of G‐Grt is 68 of E‐Grt 55. After formation of E‐Grt between and around fragmented G‐Grt at 530°C, the GPS have been further subducted and reached a maximum temperature of 580°C before exhumation started. Garnet composition profiles indicate that the initially very sharp contacts between the granulite facies fragments of G‐Grt and fracture seals of HPLT garnet (E‐Grt) have been modified by cation diffusion. The profiles suggest that Ca did not exchange at the scale of 1 µm, whereas Fe and Mg did efficiently diffuse at the derived maximum temperature of 580°C for the GPS at the scale of 7–8 µm. The Grt–Grt diffusion profiles resulted from spending c. 10 Ma at 530–580°C along the P–T–t path. The measured Grt composition profiles are consistent with diffusivities of log D_MgFe = ?25.8 m²/s from modelled diffusion profiles. Mg loss by diffusion from G‐Grt is compensated by Fe gain by diffusion from E‐Grt to maintain charge balance. This leads to a distinctive Fe concentration profile typical of uphill diffusion.     

含榴花岗片麻岩 Lu-Hf 年代学初探

对西大别四道河含石榴子石花岗片麻岩进行了锆石 U-Pb 和石榴子石 Lu-Hf 年代学测试,锆石 U-Pb谐和年龄为(223±1) Ma,石榴子石-全岩 Lu-Hf 等时线年龄为(212.2±0.7) Ma.石榴子石具有极高的母子体同位素比值(176Lu/177Hf =~300).结合锆石和石榴子石的微量元素特征,该锆石 U-Pb 年龄代表的可能是超高压/高压变质时间,石榴子石 Lu-Hf 年龄代表的是石榴子石重结晶时间,可能指示了后期退变质作用流体活动     

Early Cooling History of Eciogites from the Dabie-Sulu Orogen： Constraints from Diffusion Kinetics of Garnet

Abstract For the first time, we apply different geospeedometric models to garnet zoning patterns that were obtained in this study from detailed EMP analyses for garnets from eclogites and granulite in the Dabie‐Sulu orogen. Various zonings of cation diffusion were preserved in the garnets, enabling the acquirement of average cooling rates for the high‐to ultrahigh‐pressure rocks without using geochronological approaches. The coesite‐bearing hot eclogites yield fast cooling rates of about 20 to 30°C/Ma subsequent to peak metamorphic temperatures, whereas the cold eclogite gives a relatively slow cooling rate of 8°C/Ma at its initial exhumation. A very slow cooling rate of <0.3°C/Ma is obtained for the granulite at Huangtuling, suggesting that the granulite may not be involved in the continental deep subduction.     

Oscillatory zoning in eclogitic garnet and amphibole,Northern Serpentinite Melange,Cuba: a record of tectonic instability during subduction?

Exotic blocks of eclogite from distant localities along the Northern Serpentinite Melange of Cuba have comparable P–T histories that include high‐pressure prograde sections (450–600 °C, >15 kbar) associated with subduction of oceanic lithosphere, and retrograde sections within the albite–epidote amphibolite facies (<500 °C, <10 kbar) related to melange uplift. ⁴⁰Ar/³⁹Ar and Rb/Sr cooling ages (118–103 Ma) of one of the blocks indicate pre‐Aptian subduction and Aptian–Albian uplift. Detailed X‐ray imaging and profiling further reveals that minerals in these eclogite blocks (notably garnet and amphibole) display subtle but well defined oscillatory zoning that developed along the prograde trajectory of the rocks, previous to attainment of peak eclogitic conditions. The chemistry (e.g. coupled changes of Mg# and Mn in garnet, and of Si, Ti, Al and Na in amphibole) and geometry (euhedral to anhedral shapes) of the oscillations can be interpreted in terms of subtle fluctuations in P–T during the general prograde subduction‐related metamorphic path. A (near‐) equilibrium model is presented for the formation of oscillations at near peak conditions by means of recurrent dissolution‐growth reaction processes. This model for near‐peak conditions, and the chemical signatures of earlier oscillations (notably in amphibole), suggest that episodes of retrogression (upward movement?) affected parts of the subducting slab. It is proposed that these retrograde episodes record the tectonic rupture of the subducting slab and, probably, of the upper plate mantle, either due to the intrinsic dynamic behaviour of subduction systems or to the effects of the plate‐tectonic rearrangement of the Caribbean region during the Early Cretaceous.     

大别山区榴辉岩带中浅变质火山－碎屑岩层组合的发现及其地质意义

大别山区榴辉岩带中浅变质火山一碎屑岩层组合，及其中相当于震旦晚期孢粉组合的发现，为确定大别山区表壳岩的时代，及其与邻区的地层对比提供了重要依据；通过对区内标志岩层组合的系统追索，所确定的构造变形特征，对正确认识该造山带的形成演化具有重要地质意义。     

On the mechanism of resorption zoning in metamorphic garnet

Abstract An analytical electron microscope study of almandine garnet from a metamorphosed Al–Fe‐rich rock revealed detailed composition profiles and defect microstructures of resorption zoning along fluid‐infiltrated veins and even into the garnet/ilmenite (inclusion) interface. This indicates a limited volume diffusion for the cations in substitution (mainly Ca and Fe) and an interface‐controlled partition for the extension of a composition‐invariant margin. A corrugated interface between the Ca‐rich margin/zone and the almandine garnet core is characterized by dislocation arrays and recovery texture further suggesting a resorption process facilitated by diffusion‐induced recrystallization, diffusion‐induced dislocation migration and diffusion–induced grain boundary migration. Integrated microstructural and chemical studies are essential for understanding the underlying mechanisms of processes such as garnet zoning and its modification. Without this understanding, it will not be possible to reliably use garnet compositions for thermobarometry and other applications that rely on garnet chemical information.     

Constraining the P–T path of a MORB-type eclogite using pseudosections, garnet zoning and garnet-clinopyroxene thermometry: an example from the Bohemian Massif

A mid‐ocean ridge basalt (MORB)‐type eclogite from the Moldanubian domain in the Bohemian Massif retains evidence of its prograde path in the form of inclusions of hornblende, plagioclase, clinopyroxene, titanite, ilmenite and rutile preserved in zoned garnet. Prograde zoning involves a flat grossular core followed by a grossular spike and decrease at the rim, whereas Fe/(Fe + Mg) is also flat in the core and then decreases at the rim. In a pseudosection for H₂O‐saturated conditions, garnet with such a zoning grows along an isothermal burial path at c. 750 °C from 10 kbar in the assemblage plagioclase‐hornblende‐diopsidic clinopyroxene‐quartz, then in hornblende‐diopsidic clinopyroxene‐quartz, and ends its growth at 17–18 kbar. From this point, there is no pseudosection‐based information on further increase in pressure or temperature. Then, with garnet‐clinopyroxene thermometry, the focus is on the dependence on, and the uncertainties stemming from the unknown Fe³⁺ content in clinopyroxene. Assuming no Fe³⁺ in the clinopyroxene gives a serious and unwarranted upward bias to calculated temperatures. A Fe³⁺‐contributed uncertainty of ±40 °C combined with a calibration and other uncertainties gives a peak temperature of 760 ± 90 °C at 18 kbar, consistent with no further heating following burial to eclogite facies conditions. Further pseudosection modelling suggests that decompression to c. 12 kbar occurred essentially isothermally from the metamorphic peak under H₂O‐undersaturated conditions (c. 1.3 mol.% H₂O) that allowed the preservation of the majority of garnet with symplectitic as well as relict clinopyroxene. The modelling also shows that a MORB‐type eclogite decompressed to c. 8 kbar ends as an amphibolite if it is H₂O saturated, but if it is H₂O‐undersaturated it contains assemblages with orthopyroxene. Increasing H₂O undersaturation causes an earlier transition to SiO₂ undersaturation on decompression, leading to the appearance of spinel‐bearing assemblages. Granulite facies‐looking overprints of eclogites may develop at amphibolite facies conditions.     

Temperature–time path for the low-pressure Ryoke metamorphism, Japan, based on chemical zoning in garnet

Garnet crystals from low-pressure/high-temperature (LPHT) Ryoke metamorphic rocks in the Yanai district, south-western Japan, show several kinds of chemical zoning patterns that systematically vary with grain radius between c . 0.1 and 0.5  mm. Large grains (> c . 0.4  mm) show normal zoning and small grains (< c . 0.4  mm) show unzoned or reversely zoned cores. Observations of the chemical zoning and of the spatial and size distributions of the garnet grains between c . 0.1 and 0.5  mm in radius suggest that they were formed by continuous nucleation and diffusion-controlled growth.
A previously estimated temperature–time path ( T  – t path) for the Ryoke metamorphism, using 1-D numerical simulation, is characterized by a rapid increase in temperature, 0.0017  °C yr⁻¹ on average, and a period of high temperature (>600  °C) shorter than 0.5 Myr, which was presumably caused by the intrusion of a granodiorite sheet. Chemical zoning of garnet grains with different radii simulated for the T  – t path using a numerical model of continuous nucleation and diffusion-controlled growth, in combination with intracrystalline diffusion, compares well with the observed zoning patterns in garnet grains with different radii. This is in spite of the fact that the simulated zoning patterns vary greatly, depending on subtle differences in the T–t history. Therefore, they suggest that the T–t path gives a good explanation for the LPHT Ryoke metamorphism. Although this study only refers to the Ryoke metamorphism, the technique may be applicable to thermal modelling of other metamorphic terranes.     

Pressure-temperature path of Sanbagawa prograde metamorphism deduced from grossular zoning of garnet

The zonal structure of prograde garnet in pelitic schists from the medium-grade garnet zone and the higher-grade albite-biotite zone was examined to investigate the evolution of prograde P–T paths of the Sanbagawa metamorphism. The garnet studied shows a bell-shaped chemical zoning of the spessartine component, which decreases in abundance from the core towards the rim. Almandine and pyrope contents and X_Mg [=Mg/(Mg+Fe²⁺)] increase monotonously outwards. The general scheme of the zonal structure for grossular content [X_Grs=Ca/(Fe²⁺+Mn+Mg+Ca)] can be summarized as: (1) X_Grs increases outwards (inner segment) and reaches a maximum at an intermediate position between the crystal core and the rim, then decreases towards the outermost rim (outer segment) (2) the inner segment of garnet in the garnet zone samples tends to have a higher X_Grs/X_Sps values for a given X_Sps than those in the albite–biotite zone samples (3) average X_Sps at the maximum X_Grs position in the albite–biotite zone samples ranges from 0.02 to 0.12 and is lower than that in the garnet zone samples (0.13–0.32) (4) the maximum X_Grs in the albite–biotite zone samples (0.34–0.39 on average) tends to be higher than that in the garnet zone samples (0.26–0.36), and (5) differences of X_Grs between the maximum and rim in the albite–biotite zone samples are between 0.10 and 0.14 and higher than those in the garnet zone samples (< 0.11). These facts imply that albite–biotite zone materials (a) were recrystallized under lower dP/dT conditions at an early stage of the prograde metamorphism (b) began their exhumation under higher P–T conditions and (c) have been continuously heated during exhumation for a longer duration than the garnet zone materials. The systematic changes of prograde P–T paths can be interpreted as documenting the evolution of the Sanbagawa subduction zone.     

皖大别山区滑坡的形成机理研究

大别山地区是滑坡频发的地区,通过对大别山区滑坡地质资料的广泛收集和野外实地调查,分析研究了大别山区滑坡的基本类型和形成条件,着重调查了诱发滑坡的主要因素,并从点、面、体等方面深入剖析了大别山区滑坡的机理。