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1.
Three texturally distinct symplectites occur in mafic granofels of the Arthur River Complex at MtDaniel, Fiordland, New Zealand. These include symplectic intergrowths of clinopyroxene and kyanite, described here for the first time. Pods of mafic granofels occur within the contact aureole of the Early Cretaceous Western Fiordland Orthogneiss batholith. The pods have cores formed entirely of garnet and clinopyroxene, and rims of pseudomorphous coarse‐grained symplectic intergrowths of hornblende and clinozoisite that reflect hydration at moderate to high‐P. These hornfelsic rocks are enveloped by a hornblende–clinozoisite gneissic foliation (S1). Narrow garnet reaction zones, in which hornblende and clinozoisite are replaced by garnet–clinopyroxene assemblages, developed adjacent to fractures and veins that cut S1. Fine‐grained symplectic intergrowths of (1) clinopyroxene and kyanite and (2) clinozoisite, quartz, kyanite and plagioclase form part of the garnet reaction zones and partially replace coarse‐grained S1 hornblende and clinozoisite. The development of the garnet reaction zones and symplectites was promoted by dehydration most probably following cooling of the contact aureole. Maps of oxide weight percent and cation proportions, calculated by performing matrix corrections on maps of X‐ray intensities, are used to study the microstructure of the symplectites.  相似文献   

2.
Early Palaeozoic kyanite–staurolite‐bearing epidote–amphibolites including foliated epidote–amphibolite (FEA), and nonfoliated leucocratic or melanocratic metagabbros (LMG, MMG), occur in the Fuko Pass metacumulate unit (FPM) of the Oeyama belt, SW Japan. Microtextural relationships and mineral chemistry define three metamorphic stages: relict granulite facies metamorphism (M1), high‐P (HP) epidote–amphibolite facies metamorphism (M2), and retrogression (M3). M1 is preserved as relict Al‐rich diopside (up to 8.5 wt.% Al2O3) and pseudomorphs after spinel and plagioclase in the MMG, suggesting a medium‐P granulite facies condition (0.8–1.3 GPa at > 850 °C). An unusually low‐variance M2 assemblage, Hbl + Czo + Ky ± St + Pg + Rt ± Ab ± Crn, occurs in the matrix of all rock types. The presence of relict plagioclase inclusions in M2 kyanite associated with clinozoisite indicates a hydration reaction to form the kyanite‐bearing M2 assemblage during cooling. The corundum‐bearing phase equilibria constrain a qualitative metamorphic P–T condition of 1.1–1.9 GPa at 550–800 °C for M2. The M2 minerals were locally replaced by M3 margarite, paragonite, plagioclase and/or chlorite. The breakdown of M2 kyanite to produce the M3 assemblage at < 0.5 GPa and 450–500 °C suggests a greenschist facies overprint during decompression. The P–T evolution of the FPM may represent subduction of an oceanic plateau with a granulite facies lower crust and subsequent exhumation in a Pacific‐type orogen.  相似文献   

3.
Granulite facies orthogneiss of the Arthur River Complex (ARC) at Milford Sound, western Fiordland records a complex Early Cretaceous magmatic and orogenic history for the Pacific Gondwana margin that culminated in the emplacement and burial of a dioritic batholith, the Western Fiordland Orthogneiss (WFO). Enstatite-bearing mafic to intermediate protoliths of the ARC and WFO intruded the middle to upper crust. The early deformation history of the ARC is preserved in the Pembroke Granulite, where two-pyroxene S1 assemblages that reflect P <8 kbar and T  >750 °C were only patchily recrystallized during later deformation. S1 is cut by garnet-bearing, leucogabbroic to dioritic veins, which are cut by distinctive D2 fractures involving anorthositic veins and garnet–diopside–plagioclase-bearing reaction zones. These zones are widespread in the ARC and WFO and record conditions of P ≈14 kbar and T  >750 °C. Garnet–clinopyroxene-bearing corona reaction textures that mantle enstatite in both the ARC and WFO reflect Early Cretaceous burial by approximately 25 km of continental crust. Most of the ARC is formed from the Milford and Harrison Gneisses, which contain steeply dipping S4 assemblages that envelop the Pembroke Granulite and involve garnet, hornblende, diopside, clinozoisite, rutile and plagioclase, with or without kyanite. The P–T history of rocks in western Fiordland reflects pronounced Early Cretaceous convergence-related tectonism and burial, possibly related to the collision and accretion of island arc material onto the Pacific Gondwana margin.  相似文献   

4.
The Marguerite Amphibolite and associated rocks in northern Fiordland, New Zealand, contain evidence for retention of Carboniferous metamorphic assemblages through Cretaceous collision of an arc, emplacement of large volumes of mafic magma, high‐P metamorphism and then extensional exhumation. The amphibolite occurs as five dismembered aluminous meta‐gabbroic xenoliths up to 2 km wide that are enclosed within meta‐leucotonalite of the Lake Hankinson Complex. A first metamorphic event (M1) is manifest in the amphibolite as a pervasively lineated pargasite–anorthite–kyanite or corundum ± rutile assemblage, and as diffusion‐zoned garnet in pelitic schist xenoliths within the amphibolite. Thin zones of metasomatically Al‐enriched leucotonalite directly at the margins of each amphibolite xenolith indicate element redistribution during M1 and equilibration at 6.6 ± 0.8 kbar and 618 ± 25 °C. A second phase of recrystallization (M2) formed patchy and static margarite ± kyanite–staurolite–chlorite–plagioclase–epidote assemblages in the amphibolite, pseudomorphs of coronas in gabbronorite, and thin high‐grossular garnet rims in the pelitic schists. Conditions of M2, 8.8 ± 0.6 kbar and 643 ± 27 °C, are recorded from the rims of garnet in the pelitic schists. Cathodoluminescence imaging and simultaneous acquisition of U‐Th‐Pb isotopes and trace elements by depth‐profiling zircon grains from one pelitic schist reveals four stages of growth, two of which are metamorphic. The first metamorphic stage, dated as 340.2 ± 2.2 Ma, is correlated with M1 on the basis that the unusual zircon trace element compositions indicate growth from a metasomatic fluid derived from the surrounding amphibolite during penetrative deformation. A second phase of zircon overgrowth coupled with crosscutting relationships date M2 to between 119 and 117 Ma. The Early Carboniferous event has not previously been recognized in northern Fiordland, whereas the latter event, which has been identified in Early Cretaceous batholiths, their xenoliths, and rocks directly at batholith margins, is here shown to have also affected the country rock. However, the effects of M2 are fragmentary due to limited element mobility, lack of deformation, distance from a heat source and short residence time in the lower crust during peak P and T. It is possible that many parts of the Fiordland continental arc achieved high‐P conditions in the Early Cretaceous but retain earlier metamorphic or igneous assemblages.  相似文献   

5.
Permian‐aged metagabbros from the eclogite type‐locality in the eastern European Alps were partially to completely transformed to eclogite during Eoalpine intracontinental subduction. Microtextures developed along a preserved fluid infiltration and reaction front in the gabbros record the incipient gabbro‐to‐eclogite transition, allowing the details of the eclogitization process to be investigated. Original, anorthite‐rich igneous plagioclase is pervasively replaced by fine‐grained intergrowths of clinozoisite, kyanite and Na‐rich plagioclase. Where plagioclase was in contact with igneous orthopyroxene, 100–200 μm thick bimineralic coronae of symplectic kyanite and diopsidic clinopyroxene form along the edges of the grains. The rims of igneous orthopyroxene develop a complementary bimineralic corona of diopsidic clinopyroxene and garnet. Igneous clinopyroxene does not show any breakdown textures; however, jadeite content gradually increases towards the rims. In addition, exsolution lamellae inherited from the igneous clinopyroxene become progressively more jadeitic as eclogitization proceeds. Given that the igneous plagioclase is pervasively replaced by clinozoisite, kyanite and Na‐rich plagioclase, whereas kyanite–diopside symplectites are confined to narrow rim zones, we suggest that the development of these textures was controlled by the (im)mobility of different elements on different length scales. The presence of hydrous minerals in the core of anhydrous plagioclase indicates that H2O diffusivity occurred on a mm‐scale. By contrast, the size of the anhydrous diopside–kyanite and diopside–garnet symplectites indicate that Fe–Mg–Ca–Na diffusivity was limited to a 10s of μm scale. Chemical potential relations calculated in the idealized NCASH chemical system show that the clinozoisite–kyanite–albite intergrowths formed due to an increase of μH2O to plagioclase, whereas all other elements remained effectively immobile on the scale of this texture. Fluid conditions indicated by this texture span from virtually dry conditions (0.15) to H2O‐saturation, and therefore does not imply that the rocks were ever fluid‐saturated. Calculations in the CMAS and NCFMAS systems show that the gabbro‐to‐eclogite transition is characterized by the growth of garnet, diopsidic clinopyroxene and kyanite due to diffusion of Ca (+ Na) and Mg (+ Fe) along a μCaO (+ Na2O)–μMgO (+ FeO) chemical potential gradient developed between orthopyroxene and plagioclase compositional domains. The anhydrous nature of the textures indicate that the gabbro‐to‐eclogite transition is not driven by hydration; however, increased μH2O acts as a catalyst that increases diffusivity of all elements and rates of dissolution–precipitation, allowing the overstepped metamorphic reactions to occur. Our results show that crustal eclogite formation requires low H2O content, confirming that true eclogites are dry rocks.  相似文献   

6.
Petrology of high-pressure granulites from the eastern Himalayan syntaxis   总被引：36，自引：0，他引：36  
The eastern Himalayan syntaxis, situated at the eastern terminus of the Himalayas, is the least-known segment of the Himalayas. Recent research in this area has revealed that the syntaxis consists of the Gangdise, the Yarlung Zangbo, and the Himalayan units, each of which is bounded by faults. The Himalayan unit, the northernmost exposed part of the Indian plate, mainly contains amphibolite facies rocks, marked by the assemblages staurolite+kyanite+plagioclase+biotite+muscovite±sillimanite and garnet+amphibole+plagioclase, in the south; to the north, low- to medium-pressure granulite grade pelitic gneisses and marbles are present and are characterized by the assemblages garnet+sillimanite+K-feldspar+plagioclase or antiperthite+biotite+quartz±spinel±cordierite±orthopyroxene in gneisses, and anorthite+diopside±wollastonite and plagioclase+diopside+quartz+phlogopite+calcite in marbles. Within this unit, the Namula thrust system is a series of moderately north-dipping structures that displaced the granulite facies rocks southwards over the amphibolite facies rocks. High-pressure granulites occur as relics within these granulite facies rocks and contain garnet–kyanite granulite and garnet clinopyroxenite. The peak assemblage of the garnet–kyanite granulite includes garnet (core part)+kyanite+ternary feldspar+quartz+rutile. Sillimanite+garnet (rim part)+K-feldspar+ oligoclase+ilmenite+biotite and spinel+albite+biotite or spinel+cordierite±orthopyroxene, which are coronas around sillimanite and garnet, are retrograde products of this peak assemblage. Another peak assemblage includes very-high-Ca garnet (CaO 32–34 wt%, Alm10±Grs>80) and diopside (CaO 22–24 wt%), scapolite, meionite, quartz, and accessory Al-bearing titanite (Al2O3 4–4.5 wt%). The diopside has kink bands. Partial or complete breakdown of Ca-rich garnet during post-peak metamorphism produced pseudomorphs and coronas consisting of fine-grained symplectic intergrowths of hedenbergite and anorthite. Thermobarometric estimates in combination with reaction textures, mineral compositions, and recent experimental studies indicate that these peak assemblages were formed at P=c. 1.7–1.8 GPa, T =c. 890 °C, and the retrograde assemblages experienced near-isothermal decompression to P=0.5±0.1 GPa, T =850±50 °C. The whole-rock compositions indicate that marble and pelite are plausible candidates for the protoliths. These facts suggest the following (1) sedimentary rocks were transported to upper-mantle depths and equilibrated at those conditions to form these high-pressure granulites, which were then emplaced into the crust quickly. During the rapid exhumation of these rocks, the earlier high-pressure assemblages were overprinted by the later low- to medium-pressure assemblages, that is, the high-pressure granulite belt formed in the syntaxis. (2) The Namula thrust system is an important tectonic boundary in the syntaxis, or even in the Higher Himalaya more generally.  相似文献   

7.
A re‐evaluation of the P–T history of eclogite within the East Athabasca granulite terrane of the Snowbird tectonic zone, northern Saskatchewan, Canada was undertaken. Using calculated pseudosections in combination with new garnet–clinopyroxene and zircon and rutile trace element thermometry, peak metamorphic conditions are constrained to ～16 kbar and 750 °C, followed by near‐isothermal decompression to ～10 kbar. Associated with the eclogite are two types of occurrences of sapphirine‐bearing rocks preserving a rich variety of reaction textures that allow examination of the retrograde history below 10 kbar. The first occurs as a 1–2 m zone adjacent to the eclogite body with a peak assemblage of garnet–kyanite–quartz interpreted to have formed during the eclogite facies metamorphism. Rims of orthopyroxene and plagioclase developed around garnet, and sapphirine–plagioclase and spinel–plagioclase symplectites developed around kyanite. The second variety of sapphirine‐bearing rocks occurs in kyanite veins within the eclogite. The veins involve orthopyroxene, garnet and plagioclase layers spatially organized around a central kyanite layer that are interpreted to have formed following the eclogite facies metamorphism. The layering has itself been modified, with, in particular, kyanite being replaced by sapphirine–plagioclase, spinel–plagioclase and corundum–plagioclase symplectites, as well as the kyanite being replaced by sillimanite. Petrological modelling in the CFMAS system examining chemical potential gradients between kyanite and surrounding quartz indicates that these vein textures probably formed during further essentially isothermal decompression, ultimately reaching ～7 kbar and 750 °C. These results indicate that the final reaction in these rocks occurred at mid‐crustal levels at upper amphibolite facies conditions. Previous geochronological and thermochronological constraints bracket the time interval of decompression to <5–10 Myr, indicating that ～25 km of exhumation took place during this interval. This corresponds to minimum unroofing rates of ～2–5 mm year?1 following eclogite facies metamorphism, after which the rocks resided at mid‐crustal levels for 80–100 Myr.  相似文献   

8.
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   

9.
Garnet granulite facies mid‐to lower crust in Fiordland, New Zealand, provides evidence for pulsed intrusion and deformation occurring in the mid‐to lower crust of magmatic arcs. 238U‐206Pb zircon ages constrain emplacement of the ～595 km2 Malaspina Pluton to 116–114 Ma. Nine Sm‐Nd garnet ages (multi‐point garnet‐rock isochrons) ranging from 115.6 ± 2.6 to 110.6 ± 2.0 Ma indicate that garnet granulite facies metamorphism was synchronous or near synchronous throughout the pluton. Hence, partial melting and garnet granulite facies metamorphism lasted <5 Ma and began within 5 Ma of pluton emplacement. Garnet granulite facies L‐S tectonites in the eastern part of the Malaspina Pluton record the onset of extensional strain and arc collapse. An Sm‐Nd garnet age and thermobarometric results for these rocks directly below the amphibolite facies Doubtful Sound shear zone provide the oldest known age for extension in Fiordland at ≥112.8 ± 2.2 Ma at ～920 °C and 14–15 kbar. Narrow high Ca rims in garnet from some of these suprasolidus rocks could reflect a ≤ 1.5 kbar pressure increase, but may be largely a result of temperature decrease based on the Ca content of garnet predicted from pseudosections. At peak metamorphic conditions >900 °C, garnet contained ～4000 ppm Ti; subsequently, rutile inclusions grew during declining temperature with limited pressure change. Garnet granulite metamorphism of the Malaspina Pluton is c. 10 Ma younger than similar metamorphism of the Pembroke Granulite in northern Fiordland; therefore, high‐P metamorphism and partial melting must have been diachronous for this >3000 km² area of mid‐to‐lower crust. Thus, two or more pulses of intrusion shortly followed by garnet granulite metamorphism and extensional strain occurred from north to south along the axis of the lower crustal root of the Cretaceous Gondwana arc.  相似文献   

10.
Detailed X‐ray compositional mapping and microtomography have revealed the complex zoning and growth history of garnet in a kyanite‐bearing eclogite. The garnet occurs as clusters of coalesced grains with cores revealing slightly higher Ca and lower Mg than the rims forming the coalescence zones between the grains. Core regions of the garnet host inclusions of omphacite with the highest jadeite, and phengite with the highest Si, similar to values in the cores of omphacite and phengite located in the matrix. Therefore, the core compositions of garnet, omphacite, and phengite have been chosen for the peak pressure estimate. Coupled conventional thermobarometry, average P–T, and phase equilibrium modelling in the NCKFMMnASHT system yields P–T conditions of 26–30 kbar at 800–930°C. Although coesite is not preserved, these P–T conditions partially overlap the coesite stability field, suggesting near ultra‐high–pressure (UHP) conditions during the formation of this eclogite. Therefore, the peak pressure assemblage is suggested to have been garnet–omphacite–kyanite–phengite–coesite/quartz–rutile. Additional lines of evidence for the possible UHP origin of the Mi?dzygórze eclogite are the presence of rod‐shaped inclusions of quartz parallel to the c‐axis in omphacite as well as relatively high values of Ca‐Tschermak and Ca‐Eskola components. Late zoisite, rare diopside–plagioclase symplectites rimming omphacite, and minor phlogopite–plagioclase symplectites replacing phengite formed during retrogression together with later amphibole. These retrograde assemblages lack minerals typical of granulite facies, which suggests simultaneous decompression and cooling during exhumation before the crustal‐scale folding that was responsible for final exhumation of the eclogite.  相似文献   

11.
Polymetamorphic metapelites and embedded eclogites share a complex, episodic interplay of dehydration and fluid infiltration at the eclogite type‐locality (Saualpe–Koralpe, Eastern Alps, Austria). The metapelites inherited a fluid content (i.e. mineral‐bound OH expressed in terms of mol.% H2O) of ～6–7 mol.% H2O from high‐T–low‐P metamorphism experienced during the Permian. At or near Pmax of the subsequent Eoalpine event (～20 kbar and 680°C), the breakdown of paragonite to Na‐rich clinopyroxene and kyanite in metapelites released a discrete pulse of hydrous fluid. Prior to the dehydration event, the rocks were largely fluid absent, allowing only limited re‐equilibration during the prograde Eoalpine evolution. Similarly, Permian‐aged gabbros have persisted metastably due to the absence of a catalyst prior to fluid‐induced re‐equilibration. The fluid triggered partial to complete eclogitization along a fluid infiltration front partially preserved in metagabbro. Near‐isothermal decompression to ～7.5–10 kbar and 670–690°C took place under fluid‐absent conditions. After decompression, a second breakdown of phengitic white mica and garnet produced muscovite, biotite, plagioclase and ～0.1–0.7 mol.% H2O that enhanced extensive fluid‐aided re‐equilibration of the metapelites. Potential relicts of high‐P assemblages were largely obliterated and replaced by the recurrent amphibolite facies assemblage garnet+biotite+staurolite+kyanite+muscovite+plagioclase+ilmenite+quartz. The hydrous fluid originating from the metapelites infiltrated the embedded eclogites at these P–T conditions and induced the local breakdown of the peak assemblage omphacite and garnet to fine‐grained symplectites of diopside and plagioclase. Further fluid infiltration led to the formation of hornblende–quartz poikiloblasts at the expense of the symplectites. The metapelites re‐equilibrated until the growth of retrograde staurolite consumed any remaining free fluid, thereby terminating the process. Further re‐equilibration is inhibited by both the lack of a catalytic fluid and H2O as a reactant essential for rehydration reactions. The interplay between fluid sources and fluid sinks describes a closed cycle for the rocks at the eclogite type‐locality. Final, near‐isobaric cooling is indicated by a slight increase of XFe in garnet rims. Post‐decompression dehydration and fluid‐aided re‐equilibration arrested by the introduction of staurolite might explain the apparently homogeneous retrogression conditions as well as the notorious absence of diagnostic high‐P assemblages in metapelites at the eclogite type‐locality.  相似文献   

12.
Migmatites with sub‐horizontal fabrics at the eastern margin of the Variscan orogenic root in the Bohemian Massif host lenses of eclogite, kyanite‐K‐feldspar granulite and marble within a matrix of migmatitic paragneiss and amphibolite. Petrological study and pseudosection modelling have been used to establish whether the whole area experienced terrane‐wide exhumation of lower orogenic crust, or whether smaller portions of higher‐pressure lower crust were combined with a lower‐pressure matrix. Kyanite‐K‐feldspar granulite shows peak conditions of 16.5 kbar and 850 °C with no clear indications of prograde path, whereas in the eclogite the prograde path indicates burial from 10 kbar and 700 °C to a peak of 18 kbar and 800 °C. Two contrasting prograde paths are identified within the host migmatitic paragneiss. The first path is inferred from the presence of staurolite and kyanite inclusions in garnet that contains preserved prograde zoning that indicates burial with simultaneous heating to 11 kbar and 800 °C. The second path is inferred from garnet overgrowths of a flat foliation defined by sillimanite and biotite. Garnet growth in such an assemblage is possible only if the sample is heated at 7–8 kbar to around 700–840 °C. Decompression is associated with strong structural reworking in the flat fabric that involves growth of sillimanite in paragneiss and kyanite‐K‐feldspar granulite at 7–10 kbar and 750–850 °C. The contrasting prograde metamorphic histories indicate that kilometre‐scale portions of high‐pressure lower orogenic crust were exhumed to middle crustal levels, dismembered and mixed with a middle crustal migmatite matrix, with the simultaneous development of a flat foliation. The contrasting P–T paths with different pressure peaks show that tectonic models explaining high‐pressure boudins in such a fabric cannot be the result of heterogeneous retrogression during ductile rebound of the whole orogenic root. The P–T paths are compatible with a model of heterogeneous vertical extrusion of lower crust into middle crust, followed by sub‐horizontal flow.  相似文献   

13.
The Arthur River Complex is a suite of gabbroic to dioritic orthogneisses in northern Fiordland, New Zealand. The Arthur River Complex separates rocks of the Median Tectonic Zone, a Mesozoic island arc complex, from Palaeozoic rocks of the palaeo‐Pacific Gondwana margin, and is itself intruded by the Western Fiordland Orthogneiss. New SHRIMP U/Pb single zircon data are presented for magmatic, metamorphic and deformation events in the Arthur River Complex and adjacent rocks from northern Fiordland. The Arthur River Complex orthogneisses and dykes are dominated by magmatic zircon dated at 136–129 Ma. A dioritic orthogneiss that occurs along the eastern margin of the Complex is dated at 154.4 ± 3.6 Ma and predates adjacent plutons of the Median Tectonic Zone. Rims on zircon cores from this sample record a thermal event at c. 120 Ma, attributed to the emplacement of the Western Fiordland Orthogneiss. Migmatitic Palaeozoic orthogneiss from the Arthur River Complex (346 ± 6 Ma) is interpreted as deformed wall rock. Very fine rims (5–20 µm) also indicate a metamorphic age of c. 120–110 Ma. A post‐tectonic pegmatite (81.8 ± 1.8 Ma) may be related to phases of crustal extension associated with the opening of the Tasman Sea. The Arthur River Complex is interpreted as a batholith, emplaced at mid‐crustal levels and then buried to deep crustal levels due to convergence of the Median Tectonic Zone arc and the continental margin.  相似文献   

14.
High‐pressure (HP) granulites and eclogitized metagabbro are exposed along an orogen‐parallel high‐P belt that was developed at c. 1050–1020 Ma in the NE Grenville Province. Among these rocks, mafic granulites derived from a Labradorian anorthosite suite of the Lelukuau terrane contain garnet, Al‐Na diopside, and, depending on bulk composition, plagioclase and kyanite. Moreover, the distribution of phases is influenced by the original igneous texture. For instance, in high XMgO leucocratic varieties, garnet porphyroblasts nucleated together with kyanite in An‐rich cores of plagioclase domains whereas in low XMgO rocks garnet occurs together with clinopyroxene within formerly igneous ferromagnesian domains and kyanite is missing. In contrast, garnet pseudomorphs after igneous plagioclase in melanocratic varieties display evidence of earlier corona development. Metamorphic textures are consistent with a two stage evolution: (a) development of garnet and Al‐Na‐diopside (Cpx1) under high‐P metamorphic conditions, concomitant with elimination of plagioclase in the mesocratic to melanocratic varieties; and (b) partial loss of Al‐Na from Cpx1 resulting in production of new andesitic plagioclase, and growth of new clinopyroxene (Cpx2) after garnet and quartz in leucocratic to mesocratic rocks consistent with decompression. Widespread equilibrium textures between garnet‐Pl2‐Cpx2 and/or reset Cpx1 are consistent with development at the thermal peak. Estimated P–T conditions for the presumed thermal peak fall in the range 1500–1800 MPa and 800–900 °C and are comparable to those recorded by eclogitized gabbros from other parts of the high‐P belt of the NE Grenville province. Low jadeite content of clinopyroxene from the HP granulites is attributed to the low bulk Na2O/(Na2O + CaO) of these rocks relative to common basaltic compositions. Scarcity of apparent retrograde textural overprint in both the HP granulites and the eclogites suggests fast subsequent cooling, consistent with extrusion of the high‐P belt towards the foreland shortly after the metamorphic peak.  相似文献   

15.
Metamorphic terranes comprised of blueschist facies and regional metamorphic (Barrovian) rocks in apparent structural continuity may represent subduction complexes that were partially overprinted during syn‐ to post‐subduction heating or may be comprised of unrelated tectonic slices. An excellent example of a composite blueschist‐to‐Barrovian terrane is the southern Sivrihisar Massif, Turkey. Late Cretaceous blueschist facies rocks are dominated by marble characterized by rod‐shaped calcite pseudomorphs after aragonite and interlayered with blueschist that contains eclogite and quartzite pods. Barrovian rocks, which have 40Ar/39Ar white mica ages that are >20 Myr younger than those of the blueschists, are also dominated by marble, but rod‐shaped calcite has been progressively recrystallized into massive marble within a ～200‐m transition zone. Barrovian marble is interlayered with quartzite and schist in which isograds are closely spaced and metamorphic conditions range from chlorite to sillimanite zone over ～1 km present‐day structural thickness. Andalusite, kyanite and prismatic sillimanite are present in muscovite‐rich quartzite; in one location, all three are in the same rock. Andalusite pre‐dates Barrovian metamorphism, kyanite is both pre‐ and syn‐Barrovian and sillimanite is entirely Barrovian. Muscovite with phengitic cores and relict kyanite in quartzite below the staurolite‐in isograd are evidence for pre‐Barrovian subduction metamorphism preserved at the low‐T end of the Barrovian domain; above the staurolite isograd, all evidence for subduction metamorphism has been erased. Some regional metamorphism may have occurred during exhumation, as indicated by syn‐kinematic high‐T minerals defining the fabric of L‐tectonite. Quartz microstructures in lineated quartzite reveal a strong constrictional fabric that may have formed in a transtensional bend in the plate boundary. Transtension accounts for the closely spaced isograds and development of a strong constrictional fabric during exhumation.  相似文献   

16.
Granulites from Huangtuling in the North Dabie metamorphic core complex in eastern China preserve rare mineralogical and mineral chemical evidence for multistage metamorphism related to Palaeoproterozoic metamorphic processes, Triassic continental subduction‐collision and Cretaceous collapse of the Dabie Orogen. Six stages of metamorphism are resolved, based on detailed mineralogical and petrological studies: (I) amphibolite facies (6.3–7.0 kbar, 520–550 °C); (II) high‐pressure/high‐temperature granulite facies (12–15.5 kbar, 920–980 °C); (III) cooling and decompression (4.8–6.0 kbar, 630–700 °C); (IV) medium‐pressure granulite facies (7.7–9.0 kbar, 690–790 °C); (V) low‐pressure/high‐temperature granulite facies (4.0–4.7 kbar, 860–920 °C); (VI) retrograde greenschist facies overprint (1–2 kbar, 340–370 °C). The P–T history derived in this study and existing geochronological data indicate that the Huangtuling granulite records two cycles of orogenic crustal thickening events. The earlier three stages of metamorphism define a clockwise P–T path, implying crustal thickening and thinning events, possibly related to the assembly and breakup of the Columbia Supercontinent at c. 2000 Ma. Stage IV metamorphism indicates another crustal thickening event, which is attributed to Triassic subduction/collision between the Yangtze and Sino‐Korean Cratons. The dry lower crustal granulite persisted metastably during the Triassic subduction/collision because of the lack of hydrous fluid and deformation. Stage V metamorphism records the Cretaceous collapse of the Dabie Orogen, possibly due to asthenosphere upwelling or removal of the lithospheric mantle resulting in heating of the granulite and partial melting of the North Dabie metamorphic core complex. Comparison of the Huangtuling granulite in North Dabie and the high‐pressure–ultrahigh‐pressure metamorphic rocks in South Dabie indicates that the subducted upper (South Dabie) and lower (North Dabie) continental crusts underwent contrasting tectonometamorphic evolution during continental subduction‐collision and orogenic collapse.  相似文献   

17.
High‐pressure kyanite‐bearing felsic granulites in the Bashiwake area of the south Altyn Tagh (SAT) subduction–collision complex enclose mafic granulites and garnet peridotite‐hosted sapphirine‐bearing metabasites. The predominant felsic granulites are garnet + quartz + ternary feldspar (now perthite) rocks containing kyanite, plagioclase, biotite, rutile, spinel, corundum, and minor zircon and apatite. The quartz‐bearing mafic granulites contain a peak pressure assemblage of garnet + clinopyroxene + ternary feldspar (now mesoperthite) + quartz + rutile. The sapphirine‐bearing metabasites occur as mafic layers in garnet peridotite. Petrographical data suggest a peak assemblage of garnet + clinopyroxene + kyanite + rutile. Early kyanite is inferred from a symplectite of sapphirine + corundum + plagioclase ± spinel, interpreted to have formed during decompression. Garnet peridotite contains an assemblage of garnet + olivine + orthopyroxene + clinopyroxene. Thermobarometry indicates that all rock types experienced peak P–T conditions of 18.5–27.3 kbar and 870–1050 °C. A medium–high pressure granulite facies overprint (780–820 °C, 9.5–12 kbar) is defined by the formation of secondary clinopyroxene ± orthopyroxene + plagioclase at the expense of garnet and early clinopyroxene in the mafic granulites, as well as by growth of spinel and plagioclase at the expense of garnet and kyanite in the felsic granulite. SHRIMP II zircon U‐Pb geochronology yields ages of 493 ± 7 Ma (mean of 11) from the felsic granulite, 497 ± 11 Ma (mean of 11) from sapphirine‐bearing metabasite and 501 ± 16 Ma (mean of 10) from garnet peridotite. Rounded zircon morphology, cathodoluminescence (CL) sector zoning, and inclusions of peak metamorphic minerals indicate these ages reflect HP/HT metamorphism. Similar ages determined for eclogites from the western segment of the SAT suggest that the same continental subduction/collision event may be responsible for HP metamorphism in both areas.  相似文献   

18.
High‐pressure granulites are characterised by the key associations garnet‐clinopyroxene‐plagioclase‐quartz (in basic rocks) and kyanite‐K‐feldspar (metapelites and felsic rocks) and are typically orthopyroxene‐free in both basic and felsic bulk compositions. In regional metamorphic areas, two essential varieties exist: a high‐ to ultrahigh‐temperature group and a group representing overprinted eclogites. The high‐ to ultrahigh‐temperature type formerly contained high‐temperature ternary feldspar (now mesoperthite) coexisting with kyanite, is associated with garnet peridotites, and formed at conditions above 900 °C and 1.5 GPa. Clinopyroxene in subordinate basic rocks is Al‐rich and textural evidence points to a high‐pressure–high‐temperature melting history. The second variety contains symplectite‐like or poikilitic clinopyroxene‐plagioclase intergrowths indicating former plagioclase‐free, i.e. eclogite facies assemblages. This type of rock formed at conditions straddling the high‐pressure amphibolite/high‐pressure granulite field at around 700–850 °C, 1.0–1.4 GPa. Importantly, in the majority of high‐pressure granulites, orthopyroxene is secondary and is a product of reactions at pressures lower than the peak recorded pressure. In contrast to low‐ and medium‐pressure granulites, which form at conditions attainable in the mid to lower levels of normal continental crust, high‐pressure granulites (of nonxenolith origin) mostly represent rocks formed as a result of short‐lived tectonic events that led to crustal thickening or subduction of the crust into the mantle. Short times at high‐temperature conditions are reflected in the preservation of prograde zoning in garnet and pyroxene. High‐pressure granulites of both regional types, although rare, are known from both old and young metamorphic terranes (e.g. c. 45 Ma, Namche Barwa, E Himalaya; 400–340 Ma, European Variscides; 1.8 Ga Hengshan, China; 1.9 Ga, Snowbird, Saskatchewan and 2.5 Ga Jianping, China). This spread of ages supports proposals suggesting that thermal and tectonic processes in the lithosphere have not changed significantly since at least the end of the Archean.  相似文献   

19.
Summary Lenses and pods of mafic rocks from the Monotonous Unit near Svetlik are characterized by eclogite facies mineral assemblages; however some inclusion patterns (oriented quartz rods in clinopyroxene and cuboids of disordered graphite in garnet) that are usually known from ultra-high pressure rocks were also observed in one sample. Conventional thermobarometry yielded maximum P–T conditions of 2.0–2.5 GPa and 750 °C. Decompression and heating at amphibolite and granulite facies conditions resulted in the formation of at least five distinct types of symplectites. These include symplectitic intergrowth of ilmenite and clinopyroxene after titanite, described here for the first time from the Moldanubian Zone. In addition, symplectites of plagioclase and biotite with accessory amounts of spinel after tabular pseudomorphs (after phengite?) are also reported here. Mass balance relations indicate that symplectites of diopside + plagioclase after omphacite and plagioclase + spinel (sapphirine) after kyanite + garnet, formed by nearly isochemical reactions. All other symplectite-forming reactions were allochemical and were accelerated by the presence of fluid in the primary phases. Preserved zoning pattern in garnet with high compositional gradient in some samples suggests that the rocks were affected briefly by granulite facies overprint.  相似文献   

20.
Abstract The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet–clinopyroxene–rutile–hornblende–plagioclase–quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite = grossular + rutile + quartz + H2O) yield pressures (6–10 kbar, 400–600 °C and 8–12 kbar 450–680 °C) at or below the minimum peak conditions from matrix phases (10–13 kbar at 600–800 °C). Inclusions of hornblende, biotite and quartz give higher pressures (13–16 kbar and 630–660 °C). Early matrix pyroxene is partially or fully broken down to a diopside–plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two‐pyroxene thermometry of this assemblage gives a temperature of c. 750 °C. Pairing the most Mg‐rich garnet composition with the assemblage plagioclase–diopside–hypersthene–quartz gives pressures of 14–16 kbar at this temperature. The hornblende–plagioclase–garnet rim–quartz assemblage yields 9–12 kbar and 500–550 °C. The combined P–T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P–T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic.  相似文献   

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