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1.
In mafic granulites, garnet can form by reactions such as Opx + Pl = Cpx + Grt + Qtz; Opx + Pl = Grt + Qtz. As a result of isothermal decompression (ITD), garnet can then break down to a characteristic orthopyroxene-plagioclase symplectite. Mafic, iron-rich garnet-pyroxene granulite from the Guaxupé Massif has symplectite that formed by near-isothermal decompression, as a consequence of uplift of the granulite facies terrane. This symplectite was found to consist of vermicular clinopyroxene-orthopyroxene-plagioclase, with clinopyroxene clearly growing from the garnet that is breaking down, modal amounts of clinopyroxene being less than orthopyroxene. Electron probe analyses show clear differences between core (Cpx1), rim, and symplectite clinopyroxene (Cpx2). Considering also the presence of magnetite in the symplectite texture, garnet breakdown is thought to be better represented by a reaction such as Cpx1 + Grt + O2 = Cpx2 + Opx + Pl +Mt + Qtz.  相似文献   

2.
The Seiland Igneous Province of the North Norwegian Caledonides consists of a suite of deep-seated rift-related magmatic rocks emplaced into paragneisses during late Precambrian to Ordovician time. In the south-eastern part of the province, contact metamorphism of the paragneisses and later reworking of intrusives and associated contact aureoles have resulted in the development of three successive metamorphic stages. The contact metamorphic assemblage (M1) Opx + Grt + Qtz + Pl + Kfs + Hc + Ilm ± Crd is preserved in xenolithic rafts of paragneiss within metagabbro. Geothermobarometric calculations yield 930-960d? C and 5-6.5 kbar for the contact metamorphism. M1 was followed by cooling, accompanied by strong shearing, formation of the gneiss foliation and recrystallization at intermediate-P granulite facies conditions (M2). Stable M2 phases are Cpx + Opx + Pl +Ilm ± Hbl in metagabbro and Grt ± Sil ± Opx + Kfs + Qtz + Pl ± Bt + Ilm in host paragneiss. The M2 conditions are estimated to 700-750d? C and 5-7 kbar. A subsequent pressure increase is recorded in the M3 episode, which is associated with recrystallization in narrow ductile shear zones and secondary growth on M2 minerals. M3 is defined by the assemblages Grt + Cpx ± Opx + Pl + Ru + Qtz in metagabbro, and Grt ± Ky + Qtz + Pl ± Kfs + Bt + Ru in host paragneiss. M3 conditions are estimated to 650-700d? C and 8-10 kbar. The substantial pressure increase related to the M2 → M3 transition is interpreted to be a result of (early?) Caledonian overthrusting. Chemical zoning in cordierite and biotite suggest rapid cooling following the M3 event. The proposed P-T-t evolution implies that the tectonic evolution of the Seiland Igneous Province was long (at least 330 Ma) and complex and involved initial rifting and extension followed by crustal thickening and compression.  相似文献   

3.
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.  相似文献   

4.
At Deobhog, migmatitic gneisses and granulites of the Eastern Ghats Belt are juxtaposed against a cratonic ensemble of banded augen gneiss, amphibolite and calcsilicate gneiss, intruded by late hornblende granite and dolerite. In the migmatitic gneiss unit, early isoclinal folds (syn‐D1M and D2M) are reoriented along N–S‐trending and E‐dipping shear planes (S3M), with (S1M–S3M) intersection lineations having steep to moderate plunges. The near‐peak P–T condition was syn‐D3M (≥900 °C, 9.5 kbar), as inferred from syn‐D3M Grt+Opx‐bearing leucosomes in mafic granulites, and from thermobarometry on Grt (corona)–Opx/Cpx–Pl–Qtz assemblages. The P–T values are consistent with the occurrence of Opx–Spr–Crd assemblages in spatially associated high‐Mg–Al pelites. A subsequent period of cooling followed by isothermal decompression (800–850 °C, c. 7 kbar) is documented by the formation of coronal garnet and its decomposition to Opx+Pl symplectites in mafic granulites. Hydrous fluid infiltration accompanying the retrograde changes is manifested in biotite replacing Opx in some lithologies. The cratonic banded gneiss–granite unit also documents two phases of isoclinal folding (D1B & D2B), with the L2B lineation girdle different from the lineation spread in the migmatitic gneiss unit. Calcsilicate gneiss (Hbl–Pl–Cpx–Scap–Cal) and amphibolite (Hbl–Pl±Grt±Cpx) within banded gneisses record syn‐D2B peak metamorphic conditions (c. 700 °C, 6.5 kbar), followed by cooling (to c. 500 °C) manifested in the stabilization of coronal clinozoisite–epidote. The D3B shear deformation post‐dates granite and dolerite intrusions and is characterized by top‐to‐the‐west movement along N–S‐trending, E‐dipping shear planes. Deformation mechanisms of quartz and feldspar in granites and banded gneisses and amphibole–plagioclase thermometry within shear bands in dolerites document an inverted syn‐D3B thermal gradient with temperature increasing from 350 to 550 °C in the west to ≥700 °C near the contact with the migmatitic gneiss unit. The thermal gradient is reflected in the stabilization of chlorite after hornblende in S3B shears to the west, and post‐D2B neosome segregation along D3B folds and shears to the east. The contrasting lithologies, early structures and peak metamorphic conditions in the two units indicate unconnected pre‐D3P–T –deformation histories. The shared D3 deformation in the two units, the syn‐D3 inverted thermal gradient preserved in the footwall cratonic rocks and the complementary cooling and hydration of the hanging wall granulites across the contact are attributed to westward thrusting of ‘hot’ Eastern Ghats granulites on ‘cool’ cratonic crust. It is suggested that the Eastern Ghats migmatitic gneiss unit is not a reworked part of the craton, but a para‐autochthonous/allochthonous unit emplaced on and amalgamated to the craton.  相似文献   

5.
Coronal reaction textures occur in metanorite and related intrusions in the Pan-African Dahomeyide orogen of West Africa; they apparently formed by retrograde subsolidus reactions during cooling of the intrusions from 900 to 700 °C at c. 9±1 kbar. The coronas that formed around orthopyroxene (Opx) consist of sequential layers of diopside (Dio), hornblende (Hbl), garnet (Grt) and plagioclase (Pl) with inclusions of kyanite (Ky) or sillimanite (Sil). Three well-organized mineral assemblage sequences have been identified and modelled using steady-state diffusion theory for closed and open systems. The mineral sequence Opx|Hbl+Qtz|Hbl|Pl+Ky, formed by diffusion-controlled reactions in a closed system as layer thicknesses are very sensitive to relative component mobilities defined by Onsager diffusion coefficient (Lii). Models of this corona (type i) that satisfy modes require LCaCa≥LMgMg≥LFeFe>LAlAl～LSiSi ; however, small open-system fluxes involving loss of Al and gain of Ca are required to obtain the best fit between model and observed mineral proportions. Under steady-state diffusion, the monomineralic hornblende layer grew by replacing plagioclase whereas the |Hbl+Qtz| grew by Opx replacement. The type ii corona, which consists of the sequence Opx|Dio|Grt+Dio|Pl+Sil, is also stable under steady-state diffusion in a closed system. Modelling results show that the diopside grew by replacing Opx whereas most of Grt+Dio grew by replacing plagioclase. Stable solutions to the closed-system diffusion model that approximate the mode are restricted to the L-ratio regions where Fe, Mg and Ca are more mobile than Si and Al but are unstable when LAlAl>100 LSiSi . However, type ii corona mineral proportions were only closely matched when open-system loss of Al and gain of small amounts of Fewere considered in the diffusion models and relative mobilities were LFeFe≥LCaCa≥LMgMg>LAlAl～LSiSi . The modelling results indicate that Ca and Mg were the most mobile elements in the formation of type i corona whereas Fe and Ca were the most mobile components in the growth of type ii coronas. A third corona type, consisting of the mineral sequence Act|Hbl|Grt|Pl+Sil, is only stable in open systems and requires large external fluxes involving gain of Fe, Al, Mg and Na and loss of Ca to obtain a solution of the diffusion model that approximates the estimated mineral proportions. Extensive recrystallization of plagioclase to produce sillimanite or kyanite inclusions accompanying corona formation may explain the open-system behaviour indicated by the diffusion models.  相似文献   

6.
Abstract The widespread khondalite series of south-east Inner Mongolia consists largely of biotite–sillimanite–garnet gneiss and quartzo-feldspathic gneiss with some marble and mafic granulite layers. It has experienced two metamorphic events at c. 2500 and 1900–2000 Ma.
A pre-peak stage of the first metamorphism at T = 600–700°C and P > 6–7 kbar is recognized by the relict amphibolite facies assemblage Ky–Grt–Bt–Pl–Qtz and 'protected'inclusions of biotite, hornblende, sodic plagioclase and quartz in garnet or orthopyroxene. The peak stage, with T = c. 800 ± 50°C and P 8–10 kbar, is characterized by the widespread granulite facies assemblages Sil–Grt–Bt–Kfs–Pl–Qtz in gneiss and Opx–Cpx–Pl ± Hbl ± Grt in granulite. The P–T–t path suggests that the supracrustal sequence was buried in the lower crust by tectonic thickening during D1–D2.
The beginning of the second metamorphism is characterized by further temperature rise to 700°C or more at lower pressure. This stage is manifested by the appearance of cordierite after garnet, fibrolite (Sil2) after biotite in gneiss and transformation of Hbl1 into Opx2 and Cpx2 in granulite. Coronas of symplectitic Opx2 + Pl2 surrounding Grt1 and Cpx1 in mafic granulite are interpreted as products of near-isothermal decompression. The P–T–t path may be related tectonically to waning extension of the crust by the end of the early Proterozoic.  相似文献   

7.
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 VIA and VIB 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.  相似文献   

8.
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.  相似文献   

9.
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.  相似文献   

10.
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.  相似文献   

11.
Alpine‐type orogenic garnet‐bearing peridotites, associated with quartzo‐feldspathic gneisses of a 140–115 Ma high‐pressure/ultra‐high‐pressure metamorphic (HP‐UHPM) terrane, occur in two regions of the Indonesian island of Sulawesi. Both exposures are located within NW–SE‐trending strike–slip fault zones. Garnet lherzolite occurs as <10 m wide fault slices juxtaposed against Miocene granite in the left‐lateral Palu‐Koro (P‐K) fault valley, and as 10–30 m wide, fault‐bounded outcrops juxtaposed against gabbros and peridotites of the East Sulawesi ophiolite within the right‐lateral Ampana fault in the Bongka river (BR) valley. Six evolutionary stages of recrystallization can be recognized in the peridotites from both localities. Stage I, the precursor spinel lherzolite assemblage, is characterized by Ol+Cpx+Opx±Prg‐Amp ± Spl±Rt±Phl, as inclusions within garnet cores. Stage II, the main garnet lherzolite assemblage, consists of coarse‐grained Ol+Opx+Cpx+Grt; whereas finer‐grained, neoblastic Ol+Opx+Grt+Cpx±Spl±Prg‐Amp±Phl constitutes stage III. Stages IV and V are manifest as kelyphites of fibrous Opx+Cpx+Spl in inner coronas, and Opx+Spl+Prg‐Amp±Ep in outer coronas around garnet, respectively. The final (greenschist facies) retrogressive stage VI is accompanied by recrystallization of Serp+Chl±Mag±Tr±Ni sulphides±Tlc±Cal. P–T conditions of the hydrated precursor spinel lherzolite stage I were probably about 750 °C at 15–20 kbar. P–T determinations of the peak stage IIc (from core compositions) display considerable variation for samples derived from different outcrops, with clustering at 26–38 kbar, 1025–1210 °C (P‐K & BR); 19–21 kbar, 1070–1090 °C (P‐K), and 40–48 kbar, 1205–1290 °C (BR). Stage IIr (derived from rim compositions) generally records decompression of around 4–12 kbar accompanied by cooling of 50–240 °C from the IIc peak stage. Stage III, which post‐dates a phase of ductile deformation, yielded 22±2 kbar at 750±25 °C (P‐K) and 16±2 kbar at 730±40 °C (BR). The granulite–amphibolite–greenschist decompression sequence reflects uplift to upper crustal levels from conditions of 647–862 °C at P=15 kbar (stage IV), through 580–635 °C at P=10–12 kbar (stage V) to 350–400 °C at P=4–7 kbar (stage VI), respectively, and is identical to the sequence recorded in associated granulite, gneiss and eclogite. Sulawesi garnet peridotites are interpreted to represent minor components of the extensive HP‐UHP (peak P >28 kbar, peak T of c. 760 °C) metamorphic basement terrane, which was recrystallized and uplifted in a N‐dipping continental collision zone at the southern Sundaland margin in the mid‐Cretaceous. The low‐T , low‐P and metasomatized spinel lherzolite precursor to the garnet lherzolite probably represents mantle wedge rocks that were dragged down parallel to the slab–wedge interface in a subduction/collision zone by induced corner flow. Ductile tectonic incorporation into the underthrust continental crust from various depths along the interface probably occurred during the exhumation stage, and the garnet peridotites were subsequently uplifted within the HP‐UHPM nappe, suffering a similar decompression history to that experienced by the regional schists and gneisses. Final exhumation from upper crustal levels was clearly facilitated by entrainment in Neogene granitic plutons, and/or Oligocene trans‐tension in deep‐seated strike–slip fault zones.  相似文献   

12.
X-ray fluorescence, instrumental neutron activation, and particle-induced X-ray emission methods were used to determine the distribution of numerous trace elements among garnet (Grt), Ca-pyroxene (Cpx), hornblende (Hbl), biotite (Bt), plagioclase (Pl) and K-feldspar (Kf) in a high-grade metamorphic terrane within the Grenville Province of the Canadian Shield. Results are presented as distribution formulae, e.g. Sr: Kf 1.1 Pl 16 Hbl 2.2 Cpx 1.0 Bt 1.2 Grt Sc: Hbl 1.1 Cpx 1.0 Grt 7.8 Bt 22 Pl 2.6 Kf V: Hbl 1.15 Bt 2.07 Cpx 6.0 Grt (1.4% CaO)>1 (Pl, Kf) Zn: Bt 1.6 Hbl 1.62 Cpx 2.9 Grt 10 Pl Ga: Bt 1.2 Hbl 1.2 Pl 2.5 Cpx 1.3 Grt where numbers are distribution ratios, e.g. ppm Sr in Hbl/ppm Sr in Cpx=2.2. Examples of inter-element similarities and differences are (a) both Rb and Cs are concentrated in biotite relative to K-feldspar, but for Rb the ratio is 2.3 and for Cs it is 16, (b) the distribution formulae for seven lanthanides are similar except for the position of garnet, e.g. Ce: Hbl 2.7 Cpx 2.8 Pl 1.1 Bt 11 Kf 16 Grt Yb: Grt 2.8 Hbl 2.7 Cpx 9 Pl 1.0 Bt 7 Kf and (c) all of Sr, eight lanthanides, Zr, V and Cr are concentrated in hornblende relative to Ca-pyroxene by a factor that lies in the narrow range of 2.2–3.1. There is a larger variation (departure from the mean) in some distribution ratios than in others. Thus the mean ratios (Hbl/Cpx) for each of six elements and in parentheses the percentage relative standard deviation are Zn 1.62 (8.6), V 2.38 (12), Cr 2.42 (18), Sr 2.7 (28), Ba 2.9 (36) and Ni 1.66 (38). We suggest that variation of this kind is the result of differences from place to place in the magnitude of deformation and recrystallization (which facilitated the rearrangement of atoms), combined with rates of lattice and crystal-boundary diffusion that are unique for the various elements, thus permitting some trace elements to approach equilibrium more closely than others.  相似文献   

13.
Widespread evidence for ultrahigh‐pressure (UHP) metamorphism is reported in the Dulan eclogite‐bearing terrane, the North Qaidam–Altun HP–UHP belt, northern Tibet. This includes: (1) coesite and associated UHP mineral inclusions in zircon separates from paragneiss and eclogite (identified by laser Raman spectroscopy); (2) inclusions of quartz pseudomorphs after coesite and polycrystalline K‐feldspar + quartz in eclogitic garnet and omphacite; and (3) densely oriented SiO2 lamellae in omphacitic clinopyroxene. These lines of evidence demonstrate that the Dulan region is a UHP metamorphic terrane. In the North Dulan Belt (NDB), eclogites are characterized by the peak assemblage Grt + Omp + Rt + Phn + Coe (pseudomorph) and retrograde symplectites of Cpx + Ab and Hbl + Pl. The peak conditions of the NDB eclogites are P = 2.9–3.2 GPa, and T = 631–687 °C; the eclogite shows a near‐isothermal decompression P–T path suggesting a fast exhumation. In the South Dulan Belt (SDB), three metamorphic stages are recognized in eclogites: (1) a peak eclogite facies stage with the assemblage Grt + Omp + Ky + Rt + Phn at P = 2.9–3.3 GPa and T = 729–746 °C; (2) a high‐pressure granulite facies stage with Grt + Cpx (Jd < 30) + Pl (An24–29) + Scp at P = 1.9–2.0 GPa, T = 873–948 °C; and (3) an amphibolite facies stage with the assemblage Hbl + Pl + Ep/Czo at P = 0.7–0.9 GPa and T = 660–695 °C. The clockwise P–T path of the SDB eclogites is different from the near‐isothermal decompression P–T path from the NDB eclogites, which suggests that the SDB was exhumed to a stable crustal depth at a slower rate. In essence these two sub‐belts formed in different tectonic settings; they both subducted to mantle depths of around 100 km, but were exhumed to the Earth's surface separately along different paths. This UHP terrane plays an important role in understanding continental collision in north‐western China.  相似文献   

14.
Eclogites in the Tromsø area, northern Norway, are intimately associated with meta-supracrustals within the Uppermost Allochthon of the Scandinavian Caledonides (the Tromsø Nappe Complex). The whole sequence, which includes pelitic to semipelitic schists and gneisses, marbles and calc-silicate rocks, quartzofeldspathic gneisses, metabasites and ultramafites, has undergone three main deformational/metamorphic events (D1/M1, D2/M2 and D3/M3). Detailed structural, microtextural and mineral chemical studies have made it possible to construct separate P–T paths for these three events. Chemically zoned late syn- to post-D1 garnets with inclusions of Bt, Pl and Qtz in Ky-bearing metapelites indicate a prograde evolution from 636°C, 12.48 kbar to c. 720°C, 14–15 kbar. This latter result is in agreement with Grt–Cpx geothermometry and Grt–Cpx–Pl–Qtz geobarometry on eclogites and trondhjemitic to dioritic gneisses. Maximum pressures at c. 675°C probably reached 17–18 kbar based on Cpx–Pl–Qtz inclusions in eclogitic garnets, and Grt–Ky–Pl–Qtz and Jd–Ab–Qtz in trondhjemitic gneisses. Post-D1/pre-D2 decompressional breakdown of the high-P assemblages indicates a substantial drop in pressure at this stage. Inclusions and chemical zoning in syn- to post-D2 garnets from metapelites record a second episode of prograde metamorphism, from 552°C, 7.95 kbar, passing through a maximum pressure of 10.64 kbar at 644°C, with final equilibration at c. 665°C, 9–10 kbar. The corresponding apparently co-facial paragenesis Grt + Cpx + Pl + Qtz in metabasites yields c. 635°C, 8–10 kbar. In the metapelites post-D3, Grt in apparent equilibrium with Bt, Phe and Pl yield c. 630°C, 9 kbar. The D1/M1 and D2/M2 episodes are exclusively recorded in the Tromsø Nappe Complex and must thus pre-date the emplacement of this allochthonous unit on top of the underlying Lyngen Nappe, while the D3/M3 episode is common for the two units. A previously published Sm–Nd mineral isochron (Grt–Cpx–Am) on a partly retrograded and recrystallized ecologite of 598 ± 107 Ma represents either the timing of formation of the eclogites or the post-eclogite/pre-D2 decompression stage, while a Rb–Sr whole rock isochron of an apparently post-D1/pre-D2 granite of 433 ± 11 Ma is consistent with a K–Ar age of post-D1/pre-D2 amphiboles from a retrograded eclogite of 437 ± 16 Ma which most likely record cooling below the 475–500°C isotherm after the M3 metamorphism.  相似文献   

15.
Although ophiolitic rocks are abundant in Anatolia (Turkey), only in rare cases have they experienced high‐grade metamorphism. Even more uncommon, in Anatolia and elsewhere are high‐grade meta‐ophiolites that retain an oceanic lithosphere stratigraphy from upper crustal mafic volcanic rocks through lower crustal gabbro to mantle peridotite. The Berit meta‐ophiolite of SE Turkey exhibits both features: from structurally higher to lower levels, it consists of garnet amphibolite (metabasalt), granulite facies metagabbro (as lenses in amphibolite inferred to be retrogressed granulite) and metaperidotite (locally with metapyroxenite layers). Whole‐rock major and trace‐element data indicate a tholeiitic protolith that formed in a suprasubduction setting. This paper presents new results for the metamorphic P–T conditions and path of oceanic lower crustal rocks in the Berit meta‐ophiolite, and an evaluation of the tectonic processes that may drive granulite facies metamorphism of ophiolite gabbro. In the Do?an?ehir (Malatya, Turkey) region, granulite facies gabbroic rocks contain garnet (Grt)+clinopyroxene (Cpx)+plagioclase (Pl)+corundum (Crn)±orthopyroxene (Opx)±kyanite (Ky)±sapphirine (Spr)±rutile. Some exhibit symplectites consisting of Crn+Cpx, Ky+Cpx and/or coronas of garnet (outer shell) around a polygonal aggregate of clinopyroxene that in some cases surrounds a polygonal aggregate of orthopyroxene. Coronitic and non‐coronitic textures occur in proximity in mm‐ to cm‐scale layers; corona structures typically occur in plagioclase‐rich layers. Their formation is therefore related primarily to protolith type (troctolite v. gabbro) rather than P–T path. Phase diagrams calculated for a kyanite‐rich granulite, a plagioclase‐rich non‐coronitic granulite, and a plagioclase‐rich coronitic granulite (taking into account changes in effective bulk composition during texture development) predict peak conditions of ~800°C, 1.1–1.5 GPa; these conditions do not require invoking an unusually high geothermal gradient. In the coronitic metagabbro, reaction textures formed along the prograde path: Crn–Cpx symplectites grew at the expense of garnet, sapphirine and plagioclase. Peak conditions were followed by isobaric cooling of ~150°C. Hornblende–plagioclase thermometry results for host amphibolite (Hbl+Pl±Crn±Grt±relict Cpx) indicate retrograde conditions of 620–675°C and 0.5–0.8 GPa accompanied by infiltration of H2O‐rich fluid. This anticlockwise P–T path differs from an isothermal decompression path previously proposed for these rocks based on the presence of symplectite. Metamorphism of the ophiolitic rocks was driven by closing of the southern Neotethys Ocean, as oceanic lithosphere was obducted (most SE Anatolian ophiolites) or underthrust (Berit meta‐ophiolite). This was followed by subduction of a continental margin, driving cooling of the Berit granulite after the thermal peak at depths of ~40 km.  相似文献   

16.
Migmatitic, granulite-grade mafic gneisses make up a significant part of the Kapuskasing Structural Zone (KSZ), Ontario. Although they contain a common mineral assemblage [hornblende (Hbl)+plagioclase (Pl)+diopside (Di)±garnet (Grt)+quartz (Qtz)±titanite (Ttn)], the mafic gneisses show wide variations in modal mineralogy from hornblende-rich to diopside+garnet-rich varieties and all gradations between. Up to 25 vol.% segregated plagioclase+quartz-rich (trondhjemitic) leucosome (Tdh) is intimately associated with the mafic gneiss, occurring in a continuum of patches, veins and transecting dykes at scales ranging from decimetres to micrometres. The texture and composition of the leucosome, combined with P-T estimates for the host rocks above the solidus, suggest it represents crystallized trondhjemitic melt. Quartz is mainly restricted to the segregated leucosomes but more rarely occurs in a variety of interstitial textures in the mafic gneiss, suggesting that it crystallized from a melt phase rather than having been present as a solid phase at peak metamorphic conditions. Modal and textural data indicate a reaction relationship of the form: Hbl+Pl(+Qtz?)=Grt+Di+Ttn+leucosome (Tdh), implying that the granulite-forming process involved dehydration melting of an amphibolite protolith. Pressure-temperature estimates from Grt+Di+Pl+Qtz geothermobarometry are 9 kbar and 685-735 °C; however, based on experimental studies of dehydration melting of amphibolite, we estimate that peak conditions were closer to 11 kbar, 850 °C. Mass balance analysis, using the technique of singular value decomposition, and reaction space analysis were used to quantify the reaction and to determine the controls on reaction progress. The following mass balance provides a model for the natural reaction:1.00 Hbl+0.92 Pl+3.76 Qtz=1.14 Grt+1.54 Di+0.21 Ttn+1.49 Tdh+0.14 ‘pg’+0.39 Fe?1Mg+0.33 NaSiCa?1Al?1where ‘pg’ is a pargasite-like exchange. In all model mass balances tested, quartz is a reactant with a large coefficient. We argue that the abundance of quartz in the amphibolite protolith was the primary control on the differing extents of reaction observed. Mineral compositional variation exerted a secondary control on reaction progress, with Fe-richer layers containing An-richer plagioclase and more actinolitic amphibole reacting earliest (i.e. at lowest temperatures). Comparison of the calculated amount of melt produced in the gneisses with that now observed implies expulsion of 5–30% of the melt. These volumes are similar to those predicted from REE modelling of Archaean tonalities and trondhjemites from a garnet amphibolite source, suggesting that the KSZ mafic gneisses may be representative of partially depleted source rocks for trondhjemite-tonalite generation.  相似文献   

17.
帕米尔高原从西到东展布的8个新生代变质穹窿构成帕米尔高原变质地壳的主体,沙克达拉穹窿是其中最大的一个。沙克达拉穹窿变质杂岩中石榴矽线石片麻岩峰期组合(Grt+Ky+Bi+Rt+Pl+Qz)变质作用温压条件为T约810 ℃/P约10 kbar, 石榴石单斜辉石基性麻粒岩峰期组合(Grt+Cpx+Rt+Pl+Qz)变质作用温压条件为T约824 ℃/P约16.3 kbar, 榴闪岩退变较强,其残留峰期组合(Grt+Pl+Hbl+ilm+Qz)变质作用温压条件为T约683 ℃～873 ℃/P约8.6～11.7 kbar。基性麻粒岩变质锆石的U-Pb年龄为19～35 Ma,反映了从晚始新世到早中新世帕米尔高原下地壳加热加厚过程。帕米尔穹窿的变质作用可以与高喜马拉雅结晶岩系类比,在新生代印度亚洲大陆碰撞过程中,帕米尔陆内各地体沿前新生代缝合带的陆内俯冲可能是帕米尔下地壳加厚的主要动因。  相似文献   

18.
An ultra-high-pressure (UHP) metamorphic slab at Yangkou Beach near Qingdao in the Sulu region of China consists of blocks of eclogite facies metagabbro, metagranitoid, ultramafic rock and mylonitic orthogneisses enclosed in granitic gneiss. A gradational sequence from incipiently metamorphosed gabbro to completely recrystallized coesite eclogite formed at ultra-high-pressures was identified in a single 30 m block; metagabbro is preserved in the core whereas coesite eclogite occurs along the block margins. The metagabbro contains an igneous assemblage of Pl+Aug+Opx+Qtz+Bt+Ilm/Ti-Mag; it shows relict magmatic textures and reaction coronas. Fine-grained garnet developed along boundaries between plagioclase and other phases; primary plagioclase broke down to Ab+Ky+Ms+Zo±Grt±Amp. Augite is rimmed by sodic augite or omphacite, whereas orthopyroxene is rimmed by a corona of Cum±Act and Omp+Qtz layers or only Omp+Qtz. In transitional rocks, augite and orthopyroxene are totally replaced by omphacite, and the lower-pressure assemblage Ab+Ky+Phn+Zo+Grt coexists with domains of Omp (Jd70–73)+Ky±Phn in pseudomorphs after plagioclase. Both massive and weakly deformed coesite-bearing eclogites contain Omp+Ky+Grt+Phn+Coe/Qtz+Rt, and preserve a faint gabbroic texture. Coesite inclusions in garnet and omphacite exhibit limited conversion to palisade quartz; some intergranular coesite and quartz pseudomorphs after coesite also occur. Assemblages of the coronal stage, transitional and UHP peak occurred at about 540±50 °C at c. 13 kbar, 600–800 °C at ≥15–25 kbar and 800–850 °C at >30 kbar, respectively. Garnet from the coronal- through the transitional- to the eclogite-stage rocks show a decrease in almandine and an increase in grossular±pyrope components; garnet in low-grade rocks contains higher MnO and lower pyrope components. The growth textures of garnet within pseudomorphs after plagioclase or along grain boundaries between plagioclase and other phases are complex; the application of garnet zoning to estimate P–T should be carried out with caution. Some garnet enclosing quartz aggregates as inclusions shows radial growth boundaries; these quartz aggregates, as well as other primary and low-P phases, persisted metastably at UHP conditions due to sluggish reactions resulting from the lack of fluid during prograde and retrograde P–T evolution.  相似文献   

19.
在高喜马拉雅带的定日县曲当—扎乡一带出露的高喜马拉雅结晶岩系中, 发现了高压变质的石榴辉石岩及其降压变质的镁铁质麻粒岩组合, 早期高压条件下形成的石榴辉石岩矿物组合为Grt+Cpx (富铝) +Ru+Q, 斜长石已完全消失, 形成温度为845~896℃, 压力大于1.2GPa, 已达到榴辉岩相的压力条件.中期的麻粒岩相组合为Opx+Pl±Cpx±Ga, 其中Opx、Cpx和Pl为石榴石的后成合晶, 形成温度为993~776℃, 压力为0.90~1.21GPa, 为中压麻粒岩相产物, 晚期矿物仅见普通角闪石、斜长石和石英, 是角闪岩相退变质的产物, 表明HHC经历了降压升温-降压降温的快速抬升过程, 证明其抬升作用与地幔热源的参与有关.   相似文献   

20.
The Xolapa Complex (XC) is the largest plutonic and metamorphic mid‐crustal basement unit in Mexico and represents an ancient continental magmatic‐arc. A complete range from metatexite to diatexite migmatitic structures has been produced during a single high‐grade metamorphic event. However, structural relics reveal the existence of early Cpx + Pl + Qtz ± Opx and Grt + Opx + Pl + Qtz ± Cpx pre‐migmatitic metamorphic assemblages. Field relationships and microstructural observations allow us to constrain five pre‐, syn‐ and post‐migmatitic deformational phases. It is argued that migmatitic structures and minor anatectic granites were developed during ductile recumbent folding and shear structures related to the D2–D3 phases. Late post‐migmatitic ductile‐brittle deformation is evidenced by the development of NNE trending transpressional thrusting (D4), and E–W left‐lateral mylonitic shear zones (D5). Biotite‐breakdown melting in felsic rocks and amphibole‐breakdown melting in mafic rocks, as well as geothermobarometric results, indicate that metamorphism took place at temperatures from 830 to 900 °C and pressures ranging from ≥6.3 to 9.5 kbar. Late migmatitic assemblages equilibrated in the highest temperature range along a clockwise P–T path. The relationships between the large diversity of migmatitic structures and the progressive production of melt suggest that feedback relations prevailed as a time‐marker during a contractional regime. Deformation, metamorphism, and plutonism of the XC show that this terrane evolved as a north‐east‐verging thrust system with synkinematic metamorphism and partial melting, during the Late Cretaceous – Palaeogene. The tectonothermal history of XC is analogous to a Cordilleran metamorphic magmatic‐arc formed in an accretionary tectonic framework. This new model provides constraints on the exhumation mechanism and thermal evolution of southern Mexico.  相似文献   

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