An example of ultrahigh-temperature metamorphism: orthopyroxene–sillimanite–garnet, sapphirine–quartz and spinel–quartz parageneses in Al–Mg granulites from In Hihaou, In Ouzzal, Hoggar

Quartz Al–Mg granulites exposed at In Hihaou, In Ouzzal (NW Hoggar), preserve an unusual high-grade mineral association stable at temperatures up to 1050°C, involving the parageneses orthopyroxene–sillimanite–garnet–quartz, sapphirine–quartz and spinel–quartz. The phase relationships within the FMAS system show that a continuum exists between the earlier prograde reaction textures and those of the later decompressive event. The following mineral reactions involving sillimanite are deduced: (1) Grt+Qtz→Opx+Sil, (2) Opx+Sil→Grt+Spr+Qtz, (3) Grt+Sil+Qtz→Crd, (4) Grt+Sil→Crd+Spr, (5) Grt+Sil+Spr→Crd+Spl, (6) Grt+Sil→Crd+Spl, (7) Grt+Crd+Sil→Spl+Qtz and (8) Grt+Sil→Spl+Qtz. Minerals in quartz Al–Mg granulites display compositional variations consistent with the observed reactions. The Mg/(Mg+Fe²⁺) range of the main minerals is as follows: cordierite (0.81–0.97), sapphirine (0.77–0.88), orthopyroxene (0.65–0.81), garnet (0.33–0.64) and spinel (0.23–0.56). The reaction textures and the evolution of the mineral assemblages in the quartz Al–Mg granulites indicate a clockwise P–T trajectory characterized by peak conditions of at least 10 kbar and 1050°C, followed by decompression from 10 to 6 kbar at a temperature of at least 900°C.     

Regional-scale Grenvillian-age UHT metamorphism in the Mollendo–Camana block (basement of the Peruvian Andes)

The Mollendo–Camana Block (MCB) is a 50 × 150 km Precambrian inlier of the Andean belt that outcrops along the Pacific coast of southern Peru. It consists of stromatic migmatites of Paleoproterozoic heritage intensely metamorphosed during the Grenville event (c. 1 Ga; U‐Pb and U‐Th‐Pb ages on zircon and monazite). In the migmatites, aluminous mesosomes (FMAS) and quartzofeldspathic leucosomes (KFMASH), contain various amounts of K‐feldspar (Kfs), orthopyroxene (X_Mg Opx = 0.86), plagioclase (Pl), sillimanite (Sil; exceptionally kyanite, Ky) ilmenite (Ilm), magnetite (Mag), quartz (Qtz), and minor amounts of garnet (X_Mg Grt = 0.60), sapphirine (X_Mg Spr = 0.87), cordierite (X_Mg Crd = 0.92) and biotite (X_Mg Bt = 0.83). The ubiquitous peak mineral assemblage is Opx‐Sil‐Kfs‐Qtz‐(± Grt) in most of the MCB, which, together with the high Al content of orthopyroxene (10% Al₂O₃) and the local coexistence of sapphirine‐quartz, attest to regional UHT metamorphism (> 900 °C) at pressures in excess of 1.0 GPa. Fluid‐absent melting of biotite is responsible for the massive production of orthopyroxene that proceeded until exhaustion of biotite (and most of the garnet) in the southern part of the MCB (Mollendo‐Cocachacra areas). In this area, a first stage of decompression from 1.1–1.2 to 0.8–0.9 GPa at temperatures in excess of 950 °C, is marked by the breakdown of Sil‐Opx to Spr‐Opx‐Crd assemblages according to several bivariant FMAS reactions. High‐T decompression is also shown by Mg‐rich garnet being replaced by Crd‐Spr‐ and Crd‐Opx‐bearing symplectites, and reacting with quartz to produce low‐Al‐Opx‐Sil symplectites in quartz‐rich migmatites. Neither osumilite nor spinel‐quartz assemblages being formed, isobaric cooling at about 0.9 GPa probably followed the initial decompression and proceeded with massive precipitation of melts towards the (Os) invariant point, as demonstrated by Bt‐Qtz‐(± pl) symplectites in quartz‐rich migmatites (melt + Opx + Sil = Bt + Grt + Kfs + Qtz). Finally, Opx rims around secondary biotite attest to late fluid‐absent melting, compatible with a second stage of decompression below 900 °C. The two stages of decompression are interpreted as due to rapid tectonic denudation whereas the regional extent of UHT metamorphism in the area, probably results from large‐scale penetration of hot asthenospheric mantle at the base of an over‐thickened crust.     

Ultra-high-temperature metamorphism and multistage decompressional evolution of sapphirine granulites from the Palni Hill Ranges, southern India

Sapphirine granulites from a new locality in the Palni Hill Ranges, southern India, occur in a small enclave of migmatitic, highly magnesian metapelites (mg=85–72) within massive enderbitic orthogneiss. They show a variety of multiphase reaction textures that partially overprint a coarse-grained high-pressure assemblage of Bt+Opx+Ky+Grt+Pl+Qtz. The sequence of reactions as deduced from the corona and symplectite assemblages, together with petrogenetic grid considerations, records a clockwise P–T evolution with four distinct stages. (1) Equilibration of the initial high-P assemblage in deep overthickened crust (12 kbar/800–900 °C) was followed by a stage of near-isobaric heating, presumably as a consequence of input of extra heat provided by the voluminous enderbitic intrusives. During heating, kyanite was converted to sillimanite, and biotite was involved in a series of vapour-phase-absent melting reactions, which resulted in the ultra-high-temperature assemblage Opx+Crd+Kfs+Spr±Sil, Grt, Qtz, Bt, coexisting with melt (equilibration at c. 950–1000° C/11–10 kbar). (2) Subsequently, as a result of decompression of the order of 4 kbar at ultra-high temperature, a sequence of symplectite assemblages (Opx+Sil+Spr/Spr+Crd→Opx+Spr+Crd→Opx+Crd→Opx+Crd+Spl/Crd+Spl) developed at the expense of garnet, orthopyroxene and sillimanite. This stage of near-isothermal decompression implies rapid ascent of the granulites into mid-crustal levels, possibly due to extensional collapse and erosion of the overthickened crust. (3) Development of late biotite through back-reaction of melt with residual garnet indicates a stage of near-isobaric cooling to c. 875 °C at 7–8 kbar, i.e. relaxation of the rapidly ascended crust to the stable geotherm. (4) A second period of near-isothermal exhumation up to c. 6–5 kbar/850 °C is indicated by the partial breakdown of late biotite through volatile phase-absent melting reactions. Available isotope data suggest that the early part of the evolutionary history (stages 1–3) is presumably coeval with the early Proterozoic metamorphism in the extended granulite terrane of the Nilgiri, Biligirirangan and Shevaroy Hills to the north, while the exhumation of the granulites from mid-crustal levels (stage 4) occurred only during the Pan-African thermotectonic event, which led to the accretion of the Kerala Khondalite Belt to the south.     

Melt-producing and melt-consuming reactions in the Achankovil cordierite gneisses, South India

Migmatitic cordierite gneisses within the Achankovil Zone (AZ) of southern Pan‐African India record melt‐producing and subsequent melt‐consuming mineral reactions. Early mineral assemblages Bt‐Sil‐Qtz and Bt‐Sil‐Spl, deduced from inclusion textures in garnet prophyroblasts, break down via successive dehydration melting reactions to high‐T phase assemblages (e.g. Grt‐Crd‐Liq, Opx‐Liq, Spl‐Crd‐Liq). Later back reactions between the restite and the in situ crystallizing melt resulted in thin cordierite coronas separating garnet from the leucosome, and partial resorption of garnet to Opx‐Crd or Crd‐Bt‐Qtz symplectites. Leucosomes generally display a moderate (low‐strain gneisses) to strong (high‐strain gneisses) depletion of alkali feldspar attributed to mineral‐melt back reactions partly controlled by the degree of melt segregation. Using a KFMASH partial petrogenetic grid that includes a melt phase, and qualitative pseudosections for microdomains of high and low Al/Si ratios, the successive phase assemblages and reaction textures are interpreted in terms of a clockwise P–T path culminating at about 6–7 kbar and 900–950 °C. This P–T path is consistent with, but more detailed than published results, which suggests that taking a melt phase into account is not only a valid, but also a useful approach. Comparing P–T data and lithological and isotopic data for the AZ with adjacent East Gondwana fragments, suggests the presence of a coherent metasedimentary unit exposed from southern Madagascar via South India (AZ) and Sri Lanka (Wanni Complex) to the Lützow–Holm Bay in Eastern Antarctica.     

超高温变质作用：以华北内蒙古土贵乌拉地区为例

超高温麻粒岩(富Mg-Al)是指温度高于900℃、压力为0.7~1.3GPa条件下形成的麻粒岩相变质岩,它记录了下地壳超高温极端变质作用的地质信息。富Mg-Al典型超高温矿物组合有:假蓝宝石+石英,尖晶石+石英,大隅石+石榴石,斜方辉石+夕线石+石英,高氟黑云母和钙镁闪石,刚玉+石英。目前世界上发现的超高温麻粒岩带(块)地区有非洲阿尔及利亚、南非、东南极、巴西中部、澳大利亚中部、印度南部和东南部等地。我们在华北克拉通北部内蒙古中南部地区孔兹岩区中发现了超高温麻粒岩,岩性主要为灰黑色条带状夕线石榴黑云片麻岩,其中含有尖晶石+石英、假蓝宝石+石英、斜方辉石+夕线石+石英以及刚玉+尖晶石+石榴石等超高温矿物组合,指示温度达1000℃,压力超过1.0GPa的变质作用。独居石定年获得了(1927±11)Ma以及(1819±11)Ma两个峰期年龄,代表变质年龄。华北克拉通北部超高温麻粒岩的发现对研究华北克拉通与哥伦比亚超大陆的演化关系有重要意义。     

Experimental study of phase relations involving osumilite in the system K2O-FeO-MgO-Al2O3-SiO2-H2O at high pressure and temperature

Abstract Phase relations and mineral chemistry for garnet (Grt), orthopyroxene (Opx), sapphirine (Spr), water-undersaturated cordierite (Crd), osumilite (Osu), sillimanite (Sil), K-feldspar (Kfs), quartz (Qtz) and a water-undersaturated liquid (Liq) have been determined experimentally in the system KFMASH (K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O) under low P_H2O and f_O2 conditions. Four compositions have been studied with 100 [Mg/(Mg + Fe)] ranging from 65.6 to 89.7. Based on our experimental data, a P-T grid is derived for the KFMASH system in the presence of quartz, orthopyroxene and liquid. Osumilite has been found in various mineral assemblages from 950 to 1100°C and 7.5 to 11 kbar. In the temperature range 1000-1100°C, the pair Os-Grt is stable over a pressure range of about 3kbar. The divariant reaction Os + Opx = Grt + Kfs + Qtz runs to the right with increasing pressure. Because osumilite is the most magnesian phase it is restricted to Mg-rich compositions at high pressure. The reaction defining the upper pressure stability limit of Os-Grt is located around 11 kbar with a nearly flat dP/dT slope over the temperature range 950–100°C. Over the entire temperature range investigated osumilite is not stable beyond 12 kbar. The data imply a restricted pressure range between 11 and 12 kbar for the stability of the assemblage Os-Opx-Sil-Kfs-Qtz. At 1050°C and above, osumilite occurs in various mineral assemblages together with the high-T pair Spr-Qtz. When coexisting with garnet, orthopyroxene or sapphirine, osumilite is always the most magnesian phase. At 1050 and 1100°C, liquid is invariably the most Fe-rich phase in the run product. Our data support a theoretical P-T grid for the KFMAS system in which osumilite is stable outside the field of the high-T assemblage Spr-Qtz. Moreover, our grid indicates that Os-Opx-Sil-Kfs-Qtz has a more restricted pressure and compositional stability domain than Os-Grt, in agreement with natural occurrences. Osumilite is stable over a large pressure range, such that in Mg-rich rocks, and at high temperature, it can occur at any depth in normal thickness continental crust.     

假蓝宝石(Sapphirine)的矿物学特征及其在超高温变质作用研究中的应用

假蓝宝石是Mg-Al质麻粒岩中一种特殊的高温矿物,对超高温变质作用的研究有重要的意义。本文通过对全球66个超高温麻粒岩中47个含假蓝宝石麻粒岩地区的文献调研,总结了几种最常见的含假蓝宝石矿物组合产出的结构位置和变质反应关系,以及假蓝宝石的矿物化学特征。假蓝宝石的化学成分一般位于7∶9∶3端元左右,X_(Mg)大于0. 7,XFe_(3+)变化范围很宽,为0～0. 7。含假蓝宝石矿物组合的形成和演化指示了岩石经历的P-T轨迹。岩石中保留的假蓝宝石取代尖晶石、Grt/Opx+Sil取代Spr+Qz组合,以及随后的Spr+Crd±Opx后成合晶取代Grt/Opx+Sil组合的结构,一般可能指示了逆时针P-T轨迹中冷却和随后减压的部分;岩石中Grt/Opx+Sil/Ky或富Mg十字石反应形成Spr+Qz组合的结构可能指示了顺时针P-T轨迹中减压升温的部分。超高温变质岩不同的P-T轨迹暗示着它们的成因机制并不单一,前者可能是幔源基性岩浆底侵或增生作用的结果,后者可能与长期的热造山作用相关。     

Continuous reactions with amphibole, garnet and plagioclase

Abstract End-member, continuous and degenerate reactions are derived for the multisystem with the six components Na₂O, CaO, (Mg/Fe)O, Al₂O₃, SiO₂, H₂O among the phases plagioclase_ss, garnet_ss, amphibole_ss, cpx, opx, olivine, spinel, quartz and an aqueous fluid. The chemography of this system is degenerate due to the co-linearity 2Opx = Ol + Qtz. This co-linearity has its implications both on reaction space and phase equilibria. From a total of 28 reaction systems, reaction space is derived for nine subsystems (phases in parentheses are absent): Case A₁: (Cpx,Ol) (Cpx,Opx) and (Cpx,Qtz), Case A₂: (Spl,Ol) (Spl,Opx) and (Spl,Qtz), Case B: (Ol,Opx) (Ol,Qtz) and (Opx,Qtz). In the absence of either cpx or spl (case A), three reactions form an invariant point, either [Cpx] or [Spl], where the co-linear phases olivine, opx and quartz coexist on the transformation line 2Opx = Ol + Qtz. Changing mineral compositions force invariant points to move along the line with the different reaction curves changing their relative position according to Schreinemakers’rules. Zero contours, i.e. the location where (a) phase(s) disappear(s) in reaction space correspond to singular points in phase diagrams. Two types are distinguished; singular points of indispensable and of substitutable phases. In the first case the phase disappears from the entire bundle while in the second it disappears from a single reaction. In the specific case where the substitutable phases are also the co-linear ones, two of the three co-linear phases disappear simultaneously. Two of the three reaction curves coincide. In the system including Cpx and Spl (Case B) three reactions, (Ol,Opx) (Ol,Qtz) and (Opx,Qtz), oppose three invariant points, [Ol], [Opx] and [Qtz]. Invariant points no longer move along the line 2Opx = Ol + Qtz. The coincidence of the zero contours of all three co-linear phases in reaction space-the result of the chemographic degeneracy-causes the respective singular points to coincide in the phase diagrams. This is the location where curves must be rearranged in a bundle to conform Schreinemakers’rules. The reaction Grs₁Prp₂= 2 Ol + An is fourth order degenerate and part of all nine subsystems (cases A and B). It can be used to relate the different phase diagrams to one another.     

Liquid compositions from low-pressure experimental melting of pelitic rock from Morton Pass, Wyoming, USA

Low‐pressure crystal‐liquid equilibria in pelitic compositions are important in the formation of low‐pressure, high‐temperature migmatites and in the crystallization of peraluminous leucogranites and S‐type granites and their volcanic equivalents. This paper provides data from vapour‐present melting of cordierite‐bearing pelitic assemblages and augments published data from vapour‐present and vapour‐absent melting of peraluminous compositions, much of which is at higher pressures. Starting material for the experiments was a pelitic rock from Morton Pass, Wyoming, with the major assemblage quartz‐K feldspar‐biotite‐cordierite, approximately in the system KFMASH. A greater range in starting materials was obtained by addition of quartz and sillimanite to aliquots of this rock. Sixty‐one experiments were carried out in cold‐seal apparatus at pressures of 1–3.5 kbar (particularly 2 kbar) and temperatures from 700 to 840 °C, with and without the addition of water. In the vapour‐present liquidus relations at 2 kbar near the beginning of melting, the sequence of reactions with increasing temperature is: Qtz + Kfs + Crd + Sil + Spl + V = L; Qtz + Kfs + Crd + Spl + Ilm + V = Bt + L; and Qtz + Bt + V = Crd + Opx + Ilm + L. Vapour‐absent melting starts at about 800 °C with a reaction of the form Qtz + Bt = Kfs + Crd + Opx + Ilm + L. Between approximately 1–3 kbar the congruent melting reaction is biotite‐absent, and biotite is produced by incongruent melting, in contrast to higher‐pressure equilibria. Low pressure melts from pelitic compositions are dominated by Qtz‐Kfs‐Crd. Glasses at 820–840 °C have calculated modes of approximately Qtz₄₂Kfs₄₆Crd₁₂. Granites or granitic leucosomes with more than 10–15% cordierite should be suspected of containing residual cordierite. The low‐pressure glasses are quite similar to the higher‐pressure glasses from the literature. However, X_Mg increases from about 0.1–0.3 with increasing pressure from 1 to 10 kbar, and the low‐temperature low‐pressure glasses are the most Fe‐rich of all the experimental glasses from pelitic compositions.     

青藏高原东南缘哀牢山构造带泥质高压麻粒岩的发现及其构造意义

哀牢山构造带泥质高压麻粒岩主要由石榴石、夕线石、钾长石和斜长石变斑晶及尖晶石、铁假蓝宝石、蓝晶石、石英、金红石和钛铁矿包裹体组成,为确定印支地块和华南地块的边界提供了关键性标志。石榴石-黑云母-斜长石-石英地质温压计(GBPQ)计算结果及标志性高温矿物组合(Spl+Qz)表明泥质高压麻粒岩的形成和演化经历了高压/高温进变质到中温/低压退变质的顺时针P-T演化过程。其中:1)高压/高温进变质阶段的矿物组合为Ky+Sil+Grt1+Kf1+Pl1+Spr+Ter(Kf+Pl)+Bt1+Spl+Qtz+Ilm1+Rut1,形成于850～919℃,≥10.4kbar;2)中温/低压退变质阶段的矿物组合为Grt2+Bt2+Pl2+Ms+Qtz+Ilm2+Rut2,早期和晚期的温压条件分别为664～754℃,4.9～6.5kbar和572～576℃,3.5～3.9kbar。反映陆壳物质在碰撞过程中俯冲到地下深处(≥30km)经高压高温变质后快速折返到中上地壳的动力学演变轨迹。     

Petrology and geothermobarometry of granulite facies metapelites from the Hisøy-Torungen area, south Norway: new data on the Sveconorvegian P–T–t path of the Bamble sector

ABSTRACT The high-grade rocks (metapelite, quartzite, metagabbro) of the Hisøy-Torungen area represent the south-westernmost exposures of granulites in the Proterozoic Bamble sector, south Norway. The area is isoclinally folded and a metamorphic P–T–t path through four successive stages (M1-M4) is recognized. Petrological evidence for a prograde metamorphic event (M1) is obtained from relict staurolite + chlorite + albite, staurolite + hercynite + ilmenite, cordierite + sillimanite, fine-grained felsic material + quartz and hercynite + biotite ± sillimanite within metapelitic garnet. The phase relations are consistent with a pressure of 3.6 ± 0.5 kbar and temperatures up to 750–850°C. M1 is connected to the thermal effect of the gabbroic intrusions prior to the main (M2) Sveconorwegian granulite facies metamorphism. The main M2 granulite facies mineral assemblages (quartz+ plagioclase + K-feldspar + garnet + biotite ± sillimanite) are best preserved in the several-metre-wide Al-rich metapelites, which represent conditions of 5.9–9.1 kbar and 790–884°C. These P–T conditions are consistent with a temperature increase of 80–100°C relative to the adjacent amphibolite facies terranes. No accompanying pressure variations are recorded. Up to 1-mm-wide fine-grained felsic veinlets appear in several units and represent remnants of a former melt formed by the reaction: Bt + Sil + Qtz→Grt + lq. This dehydration reaction, together with the absence of large-scale migmatites in the area, suggests a very reduced water activity in the rocks and XH₂O = 0.25 in the C–O–H fluid system was calculated for a metapelitic unit. A low but variable water activity can best explain the presence or absence of fine-grained felsic material representing a former melt in the different granulitic metapelites. The strongly peraluminous composition of the felsic veinlets is due to the reaction: Grt +former melt ± Sil→Crd + Bt ± Qtz + H₂O, which has given poorly crystalline cordierite aggregates intergrown with well-crystalline biotite. The cordierite- and biotite-producing reaction constrains a steep first-stage retrograde (relative to M2) uplift path. Decimetre- to metre-wide, strongly banded metapelites (quartz + plagioclase + biotite + garnet ± sillimanite) inter-layered with quartzites are retrograded to (M3) amphibolite facies assemblages. A P–T estimate of 1.7–5.6 kbar, 516–581°C is obtained from geothermobarometry based on rim-rim analyses of garnet–biotite–plagioclase–sillimanite–quartz assemblages, and can be related to the isoclinal folding of the rocks. M4 greenschist facies conditions are most extensively developed in millimetre-wide chlorite-rich, calcite-bearing veins cutting the foliation.     

Stability of sapphirine + quartz in the oxidized rocks of the Wilson Lake terrane,Labrador: calculated equilibria in NCKFMASHTO

The equilibrium coexistence of sapphirine + quartz is inferred to record temperatures in excess of 980 °C, based on the stability of this assemblage in the simplified chemical system FeO–MgO–Al₂O₃–SiO₂ (FMAS) system. However, the potential for sapphirine to contain significant Fe³⁺ suggests that the stability of sapphirine + quartz could extend to lower temperatures than those constrained in this ideal system. The Wilson Lake terrane in the Grenville Province of central Labrador preserves sapphirine + quartz‐bearing assemblages in highly oxidized bulk compositions, and provides an opportunity to explore the stability of sapphirine + quartz in such rock compositions within the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O (NCKFMASHTO) chemical system. Starting with the phase equilibria in FeO–MgO–Al₂O₃–SiO₂–TiO₂–O (FMASTO), expansion into K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O (KFMASHTO) allows the effect of the stability of the additional phases, biotite, K‐feldspar and melt, on the stability of sapphirine + quartz to be assessed. These phase relations are evaluated generally using P–T projections, and the ultimate extension into NCKFMASHTO is done with pseudosections. Conditions of peak metamorphism in the Wilson Lake terrane are constrained using P–T pseudosections, and the appropriate H₂O and O contents to use in the modelled compositions are investigated using T–M_H2O and T–M_O pseudosections. The peak P–T estimates from a sapphirine + quartz‐bearing sample are ～960 to 935 °C at ～10 to 8.6 kbar, similar to estimates from orthopyroxene + sillimanite + quartz ± garnet‐bearing samples. Whereas the sapphirine + quartz‐bearing sample is more Fe‐rich than the orthopyroxene + sillimanite‐bearing sample on an all‐Fe‐as‐FeO basis, once the oxidation state is taken into account, the former is effectively more magnesian than the latter, accounting for the sapphirine occurrence.     

Petrogenesis of garnet-bearing ultramafic rocks and associated eclogites in the Su-Lu ultrahigh-P metamorphic terrane, eastern China

Ultramafic blocks that themselves contain eclogite lenses in the Triassic Su-Lu ultrahigh-P terrane of eastern China range in size from hundreds of metres to kilometres. The ultramafic blocks are enclosed in quartzofeldspathic gneiss of early Proterozoic age. Ultramafic rocks include garnetiferous lherzolite, wehrlite, pyroxenite, and hornblende peridotite. Garnet lherzolites are relatively depleted in Al₂O₃ (<3.8wt%), CaO (<3.2%) and TiO₂ (<0.11 wt%), and are low in total REE contents (several p.p.m.), suggesting that the rocks are residual mantle material that was subjected to low degrees of partial melting. The eclogite lenses or layers within the ultramafic rocks are characterized by higher MgO and CaO, lower Al₂O₃ and TiO₂ contents, and a higher CaO/Al₂O₃ ratio compared to eclogites enclosed in the quartzofeldspathic gneiss. Scatter in the plots of major and trace elements vs. MgO, REE patterns and La, Sm and Lu contents suggest that some eclogites were derived from melts formed by various degrees (0.05–0.20) of partial melting of peridotite, and that other eclogites formed by accumulation of garnet and clinopyroxene ± trapped melt in the upper mantle. Both ultramafic and eclogitic rocks have experienced a complex metamorphic history. At least six stages of recrystallization occurred in the ultramafic rocks based on an analysis of reaction textures and mineral compositions. Stage I is a high temperature protolith assemblage of Ol + Opx + Cpx + Spl. Stage II consists of the ultrahigh-pressure assemblage Ol + Cpx + Opx + Grt. Stage III is manifested by the appearance of fine-grained garnet after coarse-grained garnet. Stage IV is characterized by formation of kelyphitic rims of fibrous Opx and Cpx around garnet, and replacement of garnet by spinel and pargasitic-hornblende. Stage V is represented by the assemblage Ol + Opx + Prg-Hbl + Spl. The mineral assemblages of stages VI_A and VI_B are Ol + Tr-Amp + Chl and Serp + Chl ± talc, respectively. Garnet and orthopyroxene all show a decrease in MgO with retrogressive recrystallization and Na₂O in clinopyroxene also decreases throughout this history. Eclogites enclosed within ultramafic blocks consist of Grt + Omp + Rt ± Qtz ± Phn. A few quartz-bearing eclogites contain rounded and oval inclusion of polycrystalline quartz aggregates after coesite in garnet and omphacite. Minor retrograde features include thin symplectic rims or secondary amphiboles after Cpx, and ilmenite after rutile. P-T estimates indicate that the ultrahigh-metamorphism (stage II) of ultramafic rocks occurred at 820-900d? C and 36-41 kbar and that peak metamorphism of eclogites occurred at 730-900d? C and >28 kbar. Consonant with earlier plate tectonic models, we suggest that these rocks were underplated at the base of the continental crust. The rocks then underwent ultrahigh-pressure metamorphism and were tectonically emplaced into thickened continental crust during the Triassic collision between the Sino-Korean and Yangtze cratons.     

The Meatiq dome (Eastern Desert, Egypt) a Precambrian metamorphic core complex: petrological and geological evidence

The Meatiq basement, which is exposed beneath late Proterozoic nappes of supracrustal rocks in the Central Eastern Desert of Egypt, was affected by three metamorphic events. The ophiolite cover nappes show only the last metamorphic overprint. The M1 metamorphic event (T ≥750 °C) is restricted to migmatized amphibolite xenoliths within the Um Ba′anib orthogneiss in the structurally lowest parts of the basement. Typical upper amphibolite facies M2 mineral assemblages include Grt–Zn-rich Spl–Qtz±Bt, Grt–Zn-rich Spl–Ms–Kfs–Bt–Sil–Qtz and locally kyanite in metasedimentary rocks. The mineral assemblages Ms–Qtz–Kfs–Sil in the matrix and Sil–Grt in garnet cores indicate that peak M2 P–T conditions exceeded muscovite and staurolite stabilities. Diffusional equilibration at M2 peak temperature conditions caused homogeneous chemical profiles across M2 garnets. Abundant staurolite in garnet rims and the matrix indicates a thorough equilibration during M2 at decreasing temperature conditions. M2 P–T conditions ranged from 610 to 690 °C at 6–8 kbar for the metamorphic peak and 530–600 °C at about 5.8 kbar for the retrograde stage. However, relic kyanite indicates pressures above 8 kbar, preceeding the temperature peak. A clockwise P–T path is indicated by abundant M2 sillimanite after relic kyanite and by andalusite after sillimanite. M2 fluid inclusions, trapped in quartz within garnet and in the quartz matrix show an array of isochores. Steepest isochores (water-rich H₂O-CO₂±CH₄/N₂ inclusions) pass through peak M2 P–T conditions and flatter isochores (CO₂-rich H₂O-CO₂±CH₄/N₂ inclusions) are interpreted to represent retrograde fluids which is consistent with a clockwise P–T path for M2. The M3 assemblage Grt–Chl in the uppermost metasedimentary sequence of the basement limits temperature to 460 to 550 °C. M3 temperature conditions within the ophiolite cover nappes are limited by the assemblage Atg–Trem–Tlc to<540 °C and the absence of crysotile to >350 °C. The polymetamorphic evolution in the basement contrasts with the monometamorphic ophiolite nappes. The M1 metamorphic event in the basement occurred prior to the intrusion of the Um Ba′anib granitoid at about 780 Ma. The prograde phase of the M2 metamorphic event took place during the collision of an island arc with a continent. The break-off of the subducting slab increased the temperature and resulted in the peak M2 mineral assemblages. During the rise of the basement domain retrograde M2 mineral assemblages were formed. The final M3 metamorphic event is associated with the updoming of the basement domain at about 580 Ma along low-angle normal faults.     

Calculated phase equilibria in K2O-FeO-MgO-Al2O3-SiO2-H2O for silica-undersaturated sapphirine-bearing mineral assemblages

Silica‐undersaturated, sapphirine‐bearing granulites occur in a large number of localities worldwide. Such rocks have historically been under‐utilized for estimating P–T evolution histories because of limited experimental work, and a consequent poor understanding of the topology and P–T location of silica‐undersaturated mineral equilibria. Here, a calculated P–T projection for sapphirine‐bearing, silica‐undersaturated metapelitic rock compositions is constructed using THERMOCALC for the FeO‐MgO‐Al₂O₃‐SiO₂ (FMAS) and KFMASH (+K₂O + H₂O) chemical systems, allowing quantitative analysis of silica‐undersaturated mineral assemblages. This study builds on that for KFMASH sapphirine + quartz equilibria [Kelsey et al. (2004) Journal of Metamorphic Geology, vol. 22, pp. 559–578]. FMAS equilibria are significantly displaced in P–T space from silicate melt‐bearing KFMASH equilibria. The large number of univariant silica‐undersaturated KFMASH equilibria result in a P–T projection that is topologically more complex than could be established on the basis of experiments and/or natural assemblages. Coexisting sapphirine and silicate melt (with or without corundum) occur down to c. 900 °C in KFMASH, some 100 °C lower than in silica‐saturated compositions, and from pressures of c.≤1 to ≥12 kbar. Mineral compositions and composition ranges for the calculated phases are consistent with natural examples. Bulk silica has a significant effect on the stability of sapphirine‐bearing assemblages at a given P–T, resulting in a wide variety of possible granulite facies assemblages in silica‐undersaturated metapelites. Calculated pseudosections are able to reproduce many naturally occurring silica‐undersaturated assemblages, either within a single assemblage field or as the product of a P–T trajectory crossing several fields. With an understanding of the importance of bulk composition on sapphirine stability and textural development, silica‐undersaturated assemblages may be utilized in a quantitative manner in the detailed metamorphic investigation of high‐grade terranes.     

Metamorphic history of high-pressure granulites in Payer Land, Greenland Caledonides

A high‐P granulite facies gneiss complex occurs in north‐west Payer Land (74°28′?74°47′N) in the central part of the East Greenland Caledonian (Ordovician–Devonian) orogen. High‐P metamorphism of the Payer Land gneiss complex resulted in formation of the assemblages Grt + Cpx + Amp + Qtz + Ru ± Pl in mafic rocks, and Grt + Ol + Cpx + Opx + Spl in rare ultramafic pods. Associated metapelites experienced anatexis in the kyanite stability field. Peak metamorphic assemblages formed around 800–850 °C at pressures of c. 1.4–1.7 GPa, corresponding to crustal depths of c. 50 km. Mafic granulites contain abundant reaction textures, including the replacement of garnet by symplectites of Opx + Spl + Pl, indicating that the high‐P event was followed by decompression while the granulites remained at elevated temperatures. Charnockitic gneisses from Payer Land show evidence of late Archean (c. 2.8–2.4 Ga) crustal growth and subsequent Palaeoproterozoic (c. 1.85 Ga) metamorphism. The gneiss complex experienced intense reworking during the Caledonian continental collision. On the basis of Caledonian monazite ages recorded from the high‐P anatectic metapelites, the clockwise P–T evolution and formation of the high‐P granulite facies assemblages is related to Caledonian crustal thickening, which resulted in formation of eclogites approximately 300 km north of Payer Land. The Payer Land granulites comprise a metamorphic core complex, which is separated from the overlying low‐grade supracrustal rocks (the Neoproterozoic Eleonore Bay Supergroup) by a late Caledonian extensional fault zone, the Payer Land Detachment. The steep, nearly isothermal, unloading P–T path recorded by the granulites can be explained by erosional and tectonic unroofing along the Payer Land Detachment.     

Metamorphic evolution of the Río Santa Rosa Granulites,northern Sierra de Comechingones,Argentina

Highly anhydrous granulites from Río Santa Rosa in the eastern Sierras Pampeanas of Argentina occur as a thick lens surrounded by melt-depleted migmatites. Grt–Crd granulite composed of Qtz+Pl+Grt+Crd+Ilm±Spl±Ath±Phl is the dominant rock, whereas Opx–Grt granulite appears as discontinuous lenses in the center of the granulite body. Grt–Crd granulite includes blocks of metabasite that are relics of refractory lithologic beds interlayered in the supracrustal sequence. A distinct assemblage composed of Qtz, Pl, Grt, Crd, Opx, Spl, Crn, Sil, Bt, Phl, Ath, and Fe–Ti oxides in different combinations was generated in a reaction zone between Grt–Crd granulites and metabasites at peak metamorphism (850–900 °C and 7.6±0.5 kbar). The P–T trajectory of Grt–Crd granulites suggests an early prograde garnet-forming stage followed by nearly isothermal decompression that caused garnet breakdown. Melting and melt draining accompanying garnet growth was active during heating (to 900 °C) at intermediate pressures (∼7.6 kbar). Peak P–T estimates for Opx–Grt granulites are similar to those obtained with Grt–Crd granulites, which indicates that both granulites passed through the highest thermal stage. These results constrain the late evolution of Opx–Grt granulite to a garnet-consuming stage. Furthermore, they imply that garnet formation in Opx–Grt granulite happened at an early prograde P–T trajectory. Garnet growth in Opx–Grt granulite cannot result from heating at high pressure, which would lead to an apparent contradiction in the prograde P–T paths of the two granulites. This discrepancy may be solved by demonstrating that Opx–Grt granulite is the product of synmetamorphic mafic magmatism that was contaminated while cooling. The Río Santa Rosa granulites are inferred to have formed in a thickened crust in which mafic magmatic activity providing a local heat input.     

Upper‐amphibolite facies partial melting of paragneisses from the Epupa Complex,NW Namibia,and relations to Mesoproterozoic anorthosite magmatism

The evolution of the mineral assemblages and P–T conditions during partial melting of upper‐amphibolite facies paragneisses in the Orue Unit, Epupa Complex, NW Namibia, is modelled with calculated P–T–X phase diagrams in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O system. The close concordance of predictions from the phase diagrams to petrographic observations and thermobarometric results documents that quantitative phase diagrams are suitable to explain the phase relationships in migmatitic upper‐amphibolite facies low‐ and medium‐pressure metapelites, which occur in many high‐grade metamorphic terranes worldwide. Different mineral assemblages in the migmatitic metapelites of the Orue Unit reflect regional discrepancies in the metamorphic grade: in a Northern Zone, early biotite–sillimanite–quartz assemblages were replaced via melt‐producing reactions by cordierite‐bearing assemblages. In a Southern Zone, they were replaced via melt‐producing reactions by garnet‐bearing assemblages while cordierite is restricted to rare metapelitic granofelses, which preserve Grt–Sil–Crd–Bt peak assemblages. Peak‐metamorphic conditions of 700–750 °C at 5.5–6.7 kbar in the Southern Zone and of ～750 °C at 4.5 kbar in the Northern Zone are estimated by integrating thermobarometric calculations with data from calculated mineral composition isopleths. Retrograde back‐reactions between restite and crystallizing melt are recorded by the replacement of garnet by biotite–sillimanite and/or biotite–muscovite intergrowths. Upper‐amphibolite facies metamorphism and partial melting (c. 1340–1320 Ma) in the rocks of the Southern Zone of the Orue Unit, which underwent probably near‐isobaric heating–cooling paths, are attributed to contact metamorphism induced by the coeval (c. 1385–1319 Ma) emplacement of the Kunene Intrusive Complex, a huge massif‐type anorthosite body. The lower‐pressure metapelites of the Northern Zone are interpreted to record contact metamorphism at an upper crustal level.     

Sapphirine parageneses from the Caraiba complex, Bahia, Brazil: the influence of Fe2+–Fe3+ distribution on the stability of sapphirine in natural assemblages

Abstract Sapphirine-bearing rocks occur in three conformable, metre-size lenses in intrusive quartzo-feldspathic orthogneisses in the Curaçà valley of the Archaean Caraiba complex of Brazil. In the lenses there are six different sapphirine-bearing rock types, which have the following phases (each containing phlogopite in addition): A: Sapphirine, orthopyroxene; B: Sapphirine, cordierite, orthopyroxene, spinel; C: Sapphirine, cordierite; D: Sapphirine, cordierite, orthopyroxene, quartz; E: Sapphirine, cordierite, orthopyroxene, sillimanite, quartz; F: Sapphirine, cordierite, K-feldspar, quartz. Neither sapphirine and quartz nor orthopyroxene and sillimanite have been found in contact, however. During mylonitization, introduction of silica into the three quartz-free rocks (which represent relict protolith material) gave rise to the three cordierite and quartz-bearing rocks. Stable parageneses in the more magnesian rocks were sapphirine–orthopyroxene and sapphirine–cordierite. In more iron-rich rocks, sapphirine–cordierite, sapphirine-cordierite–sillimanite, cordierite–sillimanite, sapphirine–cordierite–spinel–magnetite and quartz–cordierite–orthopyroxene were stable. The iron oxide content in sapphirine of the six rocks increases from an average of 2.0 to 10.5 wt % (total Fe as FeO) in the order: C,F–A,D–B,E. With increase in Fe there is an increase in recalculated Fe₂O₃ in sapphirine. The four rock types associated with the sapphirine-bearing lenses are: I: Orthopyroxene, cordierite, biotite, quartz, feldspar tonalitic to grandioritic gneiss; II: Biotite, quartz, feldspar gneiss; III: Orthopyroxene, clinopyroxene, hornblende, plagioclase meta-norite; IV: Biotite, orthopyroxene, quartz, feldspar, garnet, cordierite, sillimanite granulite gneiss. The stable parageneses in type IV are orthopyroxene–cordierite–quartz, garnet–sillimanite–quartz and garnet–cordierite–sillimanite. Geothermobarometry suggests that the associated host rocks equilibrated at 720–750°C and 5.5–6.5 kbar. Petrogenetic grids for the FMASH and FMAFSH (FeO–MgO–Al₂O₃–Fe₂O₃–SiO₂–H₂O) model systems indicate that sapphirine-bearing assemblages without garnet were stabilized by a high Fe³⁺ content and a high X_Mg= (Mg/ (Mg+Fe²⁺)) under these P–T conditions.     

内蒙古中南部古元古代高温型双变质带及其构造意义

内蒙古中南部集宁-丰镇-土贵乌拉地区发育了一条超高温变质岩带.通过对土贵乌拉地区超高温变质岩的矿物组合研究,发现其中包含了尖晶石+石英、假蓝宝石+石英、斜方辉石+夕线石+石英的超高温标型矿物组合,并且在超高温变质作用区域内,经历了初始近等压冷却,随后近等温降压的逆时针退变质过程.已有的年代学数据表明,超高温变质作用峰期变质时代为1 920 Ma左右,降压退变质时代为1 850 Ma左右.在超高温变质岩带的东南地区,包括怀安-宣化一带,还出露了高压麻粒岩带.高压麻粒岩经历了近等温降压的顺时针退变质过程.年龄数据表明高压麻粒岩峰期变质时代为～1 900 Ma,退变质时代为1 850 Ma左右.内蒙古中南部超高温变质岩带和高压麻粒岩带相邻分布、产状相似且变质时代接近,共同构成了类似都城秋穗提出的双变质带;由于其温度偏高,故称为高温型双变质带,为世界上最古老的双变质带之一.该高温型双变质带可能反映了内蒙古中南部不同层次地壳物质在早前寒武纪碰撞作用中经历了P-T演化截然不同的变质作用,两条变质带在经历了各自的峰期变质作用后,作为一个共同的地体抬升.对内蒙古中南部古元古代高温型双变质带开展进一步研究,为认识华北克拉通早期构造背景、地壳热演化,特别是板块构造、不同地壳层次的构造作用提供了难得的科学机遇.