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1.
The Zealandia portion of the Pacific–Gondwana margin underwent widespread extension, fragmentation, separation and subsidence during the final stages in the breakup of Gondwana. Although these processes shaped the geology of New Zealand, their timing and the timing of subduction cessation in the region remain unclear. To investigate the timing of these processes, we used Lu–Hf garnet geochronology to date six samples of the Alpine Schist, which represents the metamorphic section of the former Zealandia margin. The garnet dates range from 97.3 ± 0.3 to 75.4 ± 1.3 Ma. Compositional zoning in garnet indicates that the spread in ages results from diachronous metamorphism in the upper plate at the Pacific–Gondwana margin, occurring concurrently with rifting of Zealandia from East Gondwana via opening of the Tasman Sea. Clear spatial trends in the timing of garnet growth throughout the Alpine Schist are absent, indicating that either regional age trends were offset by post‐metamorphic deformation, or that metamorphism did not result from a single regional heat source, and was instead driven by short‐duration, spatially dispersed processes such as episodic fluid‐fluxing or mechanical heating. Diachronous metamorphism of the Alpine Schist can be attributed to heat conduction from the rising upper mantle during widespread extension, progressive burial and heating of accretionary wedge sediments during ongoing horizontal shortening, or fluid‐fluxing sourced from a subducting and dehydrating Hikurangi Plateau. These results indicate that during separation of Zealandia from East Gondwana in the late Cretaceous, the crust at the Pacific–Gondwana margin remained hot, potentially facilitating the extensive thinning of the Zealandia lithosphere during this time.  相似文献   

2.
Sm–Nd, Lu–Hf, Rb–Sr and SIMS U–Pb data are presented for meta‐gabbroic eclogites from the eclogite type‐locality ( Haüy, 1822 ) Kupplerbrunn–Prickler Halt and other areas of the Saualpe (SE Austria) and Pohorje Mountains (Slovenia). Mg‐rich eclogites derived from early gabbroic cumulates are kyanite‐ and zoisite rich, whereas eclogites with lower Mg contents contain clinozoisite ± kyanite. Calculated P–T conditions at the final stages of high‐pressure metamorphism are 2.2 ± 0.2 GPa at 630–740 °C. Kyanite‐rich eclogites did not yield geologically meaningful Sm–Nd ages due to incomplete Nd isotope equilibration, whereas Sm–Nd multifraction garnet–omphacite regression for a low‐Mg eclogite from Kupplerbrunn yields an age of 91.1 ± 1.3 Ma. The Sm–Nd age of 94.1 ± 0.8 Ma obtained from the Fe‐rich core fraction of this garnet dates the initial stages of garnet growth. Zircon that also crystallized at eclogite facies conditions gives a weighted mean U–Pb SIMS age of 88.4 ± 8.1 Ma. Lu–Hf isotope analysis of a kyanite–eclogite from Kupplerbrunn yields 88.4 ± 4.7 Ma for the garnet–omphacite pair. Two low‐Mg eclogites from the Gertrusk locality of the Saualpe yield a multimineral Sm–Nd age of 90.6 ± 1.0 Ma. A low‐Mg eclogite from the Pohorje Mountains (70 km to the SE) gives a garnet–whole‐rock Lu–Hf age of 93.3 ± 2.8 Ma. These new age data and published Sm–Nd ages of metasedimentary host rocks constrain the final stages of the eo‐Alpine high‐pressure event in the Saualpe–Pohorje part of the south‐easternmost Austroalpine nappe system suggesting that garnet growth in the high‐pressure assemblages started at c. 95–94 Ma and ceased at c. 90–88 Ma, probably at the final pressure peak. Zircon and amphibole crystallization was still possible during incipient isothermal decompression. Rapid exhumation of the high‐pressure rocks was induced by collision of the northern Apulian plate with parts of the Austroalpine microplate, following Jurassic closure of the Permo‐Triassic Meliata back‐arc basin.  相似文献   

3.
Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over‐thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of additional sources of heat and/or focusing mechanisms for heat transport, such as magmatic intrusions and/or advection by metamorphic fluids, may have contributed to the contemporaneous development of several different metamorphic zones. Here, we test the hypothesis of temporally focussed heating for the Wepawaug Schist, a Barrovian terrane in Connecticut, USA, using Sm–Nd ages of prograde garnet growth and U–Pb zircon crystallization ages of associated igneous rocks. Peak temperature in the biotite–garnet zone was dated (via Sm–Nd on garnet) at 378.9 ± 1.6 Ma (2σ), whereas peak temperature in the highest grade staurolite–kyanite zone was dated (via Sm–Nd on garnet rims) at 379.9 ± 6.8 Ma (2σ). These garnet ages suggest that peak metamorphism was pene‐contemporaneous (within error) across these metamorphic grades. Ion microprobe U–Pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn‐metamorphic peak igneous activity at 380.6 ± 4.7 Ma (2σ), indistinguishable from the peak ages recorded by garnet. A 388.6 ± 2.1 Ma (2σ) garnet core age from the staurolite–kyanite zone indicates an earlier episode of growth (coincident with ages from texturally early zircon and a previously published monazite age) along the prograde regional metamorphic T–t path. The timing of peak metamorphism and igneous activity, as well as the occurrence of extensive syn‐metamorphic quartz vein systems and pegmatites, best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes: the first at c. 389 Ma, and the second, corresponding to the regionally synchronous peak metamorphism, at c. 380 Ma.  相似文献   

4.
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   

5.
High-precision 40Ar/39Ar dating of lamprophyre dike swarms in the Western Province of New Zealand reveals that these dikes were emplaced into continental crust prior to, during and after opening of the Tasman Sea between Australia and New Zealand. Dike ages form distinct clusters concentrated in different areas. The oldest magmatism, 102–100 Ma, is concentrated in the South Westland region that represents the furthest inboard portion of New Zealand in a Gondwana setting. A later pulse of magmatism from ~ 92 Ma to ~ 84 Ma, concentrated in North Westland, ended when the first oceanic crust formed at the inception of opening of the Tasman Sea. Magmatic quiescence followed until ~ 72–68 Ma, when another swarm of dikes was emplaced. The composition of the dikes reveals a dramatic change in primary melt sources while continental extension and lithospheric thinning were ongoing. The 102–100 Ma South Westland dikes represent the last mafic calc-alkaline magmatism associated with a long-lived history of the area as Gondwana's active margin. The 92–84 Ma North and 72–68 Ma Central Westland dike swarms on the other hand have strongly alkaline compositions interpreted as melts from an intraplate source. These dikes represent the oldest Western Province representatives of alkaline magmatism in the greater New Zealand region that peaked in activity during the Cenozoic and has remained active up to the present day. Cretaceous alkaline dikes were emplaced parallel to predicted normal faults associated with dextral shear along the Alpine Fault. Furthermore, they temporally correspond to polyphase Cretaceous metamorphism of the once distal Alpine Schist. Dike emplacement and distal metamorphism could have been linked by a precursor to the Alpine Fault. Dike emplacement in the Western Province coupled to metamorphism of the Alpine Schist at 72–68 Ma indicates a period of possible reactivation of this proto Alpine Fault before it served as a zone of weakness during the opening of the oceanic Emerald Basin (at ~ 45 Ma) and eventually the formation of the present-day plate boundary (~ 25 Ma–recent).  相似文献   

6.
Sm–Nd (garnet), U–Pb (monazite) and Rb–Sr (biotite) ages from a composite migmatite sample (Damara orogen, Namibia) constrain the time of high‐grade regional metamorphism and the duration of regional metamorphic events. Sm–Nd garnet whole‐rock ages for a strongly restitic melanosome and an adjacent intrusive leucosome yield ages of 534±5, 528±11 and 539±8 Ma. These results provide substantial evidence for pre‐500 Ma Pan‐African regional metamorphism and melting for this segment of the orogen. Other parts of the migmatite yield younger Sm–Nd ages of 488±9 Ma for melanosome and 496±10, 492±5 and 511±16 Ma for the corresponding leucosomes. Garnet from one xenolith from the leucosomes yields an age of 497±2 Ma. Major element compostions of garnet are different in terms of absolute abundances of pyrope and spessartine components, but the flat shape of the elemental patterns suggests late‐stage retrograde equilibration. Rare earth element compositions of the garnet from the different layers are similar except for garnet from the intrusive leucosome suggesting that they grew in different environments. Monazite from the leucosomes is reversely discordant and records 207Pb/235U ages between 536 and 529 Ma, indicating that this monazite represents incorporated residual material from the first melting event. Monazite from the mesosome MES 2 and the melanosome MEL 3 gives 207Pb/235U ages of 523 and 526 Ma, and 529 and 531 Ma, respectively, which probably indicates another thermal event. Previously published 207Pb/235U monazite data give ages between 525 and 521 Ma for composite migmatites, and 521 and 518 Ma for monazite from neosomes. Monazite from granitic to granodioritic veins indicates another thermal event at 507–505 Ma. These ages are also recorded in 207Pb/235U monazite data of 508 Ma from the metasediment MET 1 from the migmatite and also in the Sm–Nd garnet ages obtained in this study. Taken together, these ages indicate that high‐grade metamorphism started at c. 535 Ma (or earlier) and was followed by thermal events at c. 520 Ma and c. 505 Ma. The latter event is probably connected with the intrusion of a large igneous body (Donkerhoek granite) for which so far only imprecise Rb–Sr whole‐rock data of 520±15 Ma are available. Rb–Sr biotite ages from the different layers of the migmatite are 488, 469 and 473 Ma. These different ages indicate late‐stage disturbance of the Rb–Sr isotopic system on the sub‐sample scale. Nevertheless, these ages are close to the youngest Sm–Nd garnet ages, indicating rapid cooling rates between 13 and 20°C Ma?1 and fast uplift of this segment of the crust. Similar Sm–Nd garnet and U–Pb monazite ages suggest that the closure temperatures for both isotopic systems are not very different in this case and are probably similar or higher than the previously estimated peak metamorphic temperatures of 730±30°C. The preservation of restitic monazite in leucosomes indicates that dissolution of monazite in felsic water‐undersaturated peraluminous melts can be sluggish. This study shows that geochronological data from migmatites can record polymetamorphic episodes in high‐grade terranes that often contain cryptic evidence for the nature and timing of early metamorphic events.  相似文献   

7.
Although the U–Pb zircon chronometer has been widely used for dating metamorphism in moderate‐ to high‐grade rocks, it is generally difficult to link the U–Pb age of zircon to specific metamorphic reactions. In this study, the initial Hf isotopic composition of secondary zircon is compared with the evolution of Hf isotopic composition of the bulk sample, back‐projected from the measured value through time. This approach may enhance the interpretation of radiometric ages performed on metamorphic mineral assemblages. Here, U–Pb, Sm–Nd and Lu–Hf geochronology and thermobarometry have been integrated and applied to two metamorphosed diabase dykes in the Sveconorwegian orogen, SW Sweden. The dykes are located ～5 km east of the NNE‐trending Göta Älv deformation zone in the Idefjorden terrane, and trend parallel to this zone. The Lunden dyke is recrystallized into a coronitic, granulite facies assemblage. U–Pb isotopic analyses of baddeleyite in this dyke indicate an emplacement age of c. 1300 Ma. Thermobarometric techniques applied to garnet and omphacitic clinopyroxene coronas indicate high‐pressure metamorphism at ～15 kbar and ～740 °C. The growth of polycrystalline zircon at the expense of baddeleyite occurred at 1046 ± 6 Ma. The identical Hf isotopic composition of polycrystalline zircon and baddeleyite shows that the baddeleyite‐to‐zircon transition took place before Hf equilibration among the other metamorphic minerals and, hence the c. 1046 Ma age of polycrystalline zircon sets an upper age limit of metamorphism of this sample. The Haregården dyke is recrystallized into a granoblastic transitional upper amphibolite to granulite facies assemblage. The estimated P–T conditions are ～10 kbar and ～700 °C. Analyses of small (～30 μm), clear and round zircon in this sample yield a Concordia U–Pb age of 1026 ± 4 Ma, which is indistinguishable from the Lu‐Hf and Sm‐Nd mineral isochron ages of 1027 ± 9 and 1022 ± 34 Ma, respectively. This type of secondary zircon plots at the lower end of the Lu‐Hf isochron and indicates simultaneous growth with garnet at c. 1026 Ma, a time when Hf isotopic equilibrium among minerals must have been reached.  相似文献   

8.
We report SHRIMP U–Th–Pb monazite, conventional U–Pb titanite, Sm–Nd garnet and Rb–Sr muscovite and biotite ages for metamorphic rocks from the Danba Domal Metamorphic Terrane in the eastern Songpan‐Garzê Orogenic Belt (eastern Tibet Plateau). These ages are used to determine the timing of polyphase metamorphic events and the subsequent cooling history. The oldest U–Th–Pb monazite and Sm–Nd garnet ages constrain an early Barrovian metamorphism (M1) in the interval c. 204–190 Ma, coincident with extensive Indosinian granitic magmatism throughout the Songpan‐Garzê Orogenic Belt. A second, higher‐grade sillimanite‐grade metamorphic event (M2), recorded only in the northern part of the Danba terrane, was dated at c. 168–158 Ma by a combination of U–Th–Pb monazite and titanite and Sm–Nd garnet ages. It is suggested that M1 was a thermal event that affected the entire orogenic belt while M2 may represent a local thermal perturbation. Rb–Sr muscovite ages range from c. 138–100 Ma, whereas Rb–Sr biotite ages cluster at c. 34–24 Ma. These ages document regional cooling at rates of c. 2–3 °C Myr?1 following the M1 peak for most of the terrane. However, those parts of the terrane affected by the higher‐temperature M2 event (e.g. the migmatite zone) experienced initially more rapid (c. 8 °C Myr?1) cooling after peak M2 before joining the regional slow cooling path defined by the rest of the terrane at c. 138 Ma. Regional slow cooling between c. 138 and c. 30 Ma is thought to be the result of post‐tectonic isostatic uplift after extensive crustal thickening caused by collision of the South and North China Blocks. The clustering of biotite Rb–Sr ages marks the onset of rapid uplift across the entire terrane commencing at c. 30–20 Ma. This cooling history is shared with many other regions of the Tibet Plateau, suggesting that uplift of the Tibet Plateau (including the Songpan‐Garzê Orogenic Belt) occurred predominantly in the last c. 30 Myr as a response to the continuing northwards collision of India with Eurasia.  相似文献   

9.
Migmatites comprise a minor volume of the high‐grade part of the Damara orogen of Namibia that is dominated by granite complexes and intercalated metasedimentary units. Migmatites of the Southern Central Zone of the Damara orogen consist of melanosomes with garnet+cordierite+biotite+K‐feldspar, and leucosomes, which are sometimes garnet‐ and cordierite‐bearing. Field evidence, petrographic observations, and pseudosection modelling suggest that, in contrast to other areas where intrusion of granitic magmas is more important, in situ partial melting of metasedimentary units was the main migmatite generation processes. Pseudosection modelling and thermobarometric calculations consistently indicate that the peak‐metamorphic grade throughout the area is in the granulite facies (~5 kbar at ~800°C). Cordierite coronas around garnet suggest some decompression from peak‐metamorphic conditions and rare andalusite records late, near‐isobaric cooling to <650°C at low pressures of ~3 kbar. The inferred clockwise P–T path is consistent with minor crustal thickening through continent–continent collision followed by limited post‐collisional exhumation and suggests that the granulite facies terrane of the Southern Central Zone of the Damara orogen formed initially in a metamorphic field gradient of ~35–40°C/km at medium pressures. New high‐precision Lu–Hf garnet‐whole rock dates are 530 ± 13 Ma, 522.0 ± 0.8 Ma, 520.8 ± 3.6 Ma, and 500.3 ± 4.3 Ma for the migmatites that record temperatures of ~800°C. This indicates that high‐grade metamorphism lasted for c. 20–30 Ma, which is compatible with previous estimates using Sm–Nd garnet‐whole rock systematics. In previous studies on Damara orogen migmatites where both Sm–Nd and Lu–Hf chronometers have been applied, the dates (c. 520–510 Ma) agree within their small uncertainties (0.6–0.8% for Sm–Nd and 0.1–0.2% for Lu–Hf). This implies rapid cooling after high‐grade conditions and, by implication, rapid exhumation at that time. The cause of the high geothermal gradient inferred from the metamorphic conditions is unknown but likely requires some extra heat that was probably added by intrusion of magmas from the lithospheric mantle, i.e., syenites that have been recently re‐dated at c. 545 Ma. Some granites derived from the lower crust at c. 545 Ma are the outcome rather than the cause of high‐T metamorphism. In addition, high contents of heat‐producing elements K, Th, and U may have raised peak temperatures by 150–200°C at the base of the crust, resulting in the widespread melting of fertile crustal rocks. The continuous gradation from centimetre‐scale leucosomes to decametre‐scale leucogranite sheets within the high‐grade metamorphic zone suggests that leucosome lenses coalesced to form larger bodies of anatectic leucogranites, thereby documenting a link between high‐grade regional metamorphism and Pan‐African magmatism. In view of the close association of the studied high‐T migmatites with hundreds of synmetamorphic high‐T granites that invaded the terrane as metre‐ to decametre‐wide sills and dykes, we postulate that crystallization of felsic lower crustal magma is, at least partly, responsible for heat supply. Late‐stage isobaric cooling of these granites may explain the occurrence of andalusite in some samples.  相似文献   

10.
Sm–Nd garnet‐whole rock geochronology, phase equilibria, and thermobarometry results from Garnet Ledge, south‐eastern Alaska, provide the first precisely constrained P–T–t path for garnet zone contact metamorphism. Garnet cores from two crystals and associated whole rocks yield a four point isochron age for initial garnet growth of 89.9 ± 3.6 Ma. Garnet rims and matrix minerals from the same samples yield a five point isochron age for final garnet growth of 89 ± 1 Ma. Six size fractions of zircon from the adjacent pluton yield a concordant U–Pb age of 91.6 ± 0.5 Ma. The garnet core and rim, and zircon ages are compatible with single‐stage garnet growth during and/or after pluton emplacement. All garnet core–whole rock and garnet rim‐matrix data from the two samples constrain garnet growth duration to ≤5.5 my. A garnet mid‐point and the associated matrix from one of the two garnet crystals yield an age of 90.0 ± 1.0 Ma. This mid‐point result is logically younger than the 90.7 ± 5.6 Ma core–whole rock age and older than the 88.4 ± 2.5 Ma rim‐matrix age for this sample. A MnNaCaKFMASH phase diagram (P–T pseudosection) and the garnet core composition are used to predict that cores of garnet crystals grew at 610 ± 20 °C and 5 ± 1 kbar. This exceeds the temperature of the garnet‐in reaction by c. 50 °C and is compatible with overstepping of the garnet growth reaction during contact metamorphism. Intersection of three reactions involving garnet‐biotite‐sillimanite‐plagioclase‐quartz calculated by THERMOCALC in average P–T mode, and exchange thermobarometry were used to estimate peak metamorphic conditions of 678 ± 58 °C at 6.1 ± 0.9 kbar and 685 ± 50 °C at 6.3 ± 1 kbar, respectively. Integration of pressure, temperature, and age estimates yields a pressure‐temperature‐time path compatible with near isobaric garnet growth over an interval of c. 70 °C and c. 2.3 my.  相似文献   

11.
The Scandinavian Caledonides contain several non-cogenetic eclogite-bearing terranes that were metamorphosed before the main orogenic stage in Scandian time (430–395 Ma). Although petrological and geochronological data from these terranes have provided essential information on the geodynamic history of the Caledonian orogenic cycle, the general picture is still patchy. To refine existing geodynamic models, we have dated the eclogite occurrence in the Jæren nappe, SW Norway, by Lu–Hf and Sm–Nd geochronology. Five out of the six studied samples provide a weighted mean Lu–Hf age of 469.9 ± 1.2 Ma (±2σ). One sample provided a significantly younger age of 457.9 ± 2.4 Ma. Garnet from the younger sample grew exclusively at eclogite facies conditions. In contrast, garnet from the other samples comprises prograde cores and peak metamorphic rims. Age estimates that take Lu-contributions of each zone into account provide an age of 471.0 ± 0.9 Ma for the cores and suggest a ca. 455 Ma age for the rims, which is identical to the bulk-garnet age of the younger sample. The same pattern is indicated by Sm–Nd ages, although these are relatively imprecise and reflect isotopic disturbance during thermal overprinting upon exhumation. The data define a new high-pressure age population for the Scandinavian Caledonides, which allows more detailed insight into the subduction history that affected the Baltoscandian margin before Scandian continental collision. Furthermore, this study highlights the potential complexities involved in garnet geochronology and shows the strength of Lu–Hf dating for unraveling the geochronological record of HP rocks.  相似文献   

12.
《Gondwana Research》2016,29(4):1482-1499
The Lhasa terrane, the main tectonic component of the Himalayan–Tibetan orogen, has received much attention as it records the entire history of the orogeny. The occurrence of Permian to Triassic high-pressure eclogites has a significant bearing on the understanding of the Paleo-Tethys subduction and plate suturing processes in this area. An eclogite from the Bailang, eastern Lhasa terrane, was investigated with a combined metamorphic P–T and U–Pb, Lu–Hf, Sm–Nd and Ar–Ar multichronometric approach. Pseudosection modeling combined with thermobarometric calculations indicate that the Bailang eclogite equilibrated at peak P–T conditions of ~ 2.6 GPa and 465–503 °C, which is much lower than those of Sumdo and Jilang eclogites in this area. Garnet–whole rock–omphacite Lu–Hf and Sm–Nd ages of 238.1 ± 3.6 Ma and 230.0 ± 4.7 Ma were obtained on the same sample, which are largely consistent with the corresponding U–Pb age of 227.4 ± 6.4 Ma for the metamorphic zircons within uncertainty. The peak metamorphic temperature of the sample is lower than the Lu–Hf and Sm–Nd closure temperatures in garnet. This, combined with the core-to-rim decrease in Mn and HREE concentrations, the slightly U-shaped Sm zonation across garnet and the exclusive occurrence of omphacite inclusion in garnet rim, are consistent with the Lu–Hf system skewing to the age of the garnet core and the Sm–Nd system favoring the rim age. The Sm–Nd age was thus interpreted as the age of eclogite-facies metamorphism and the Lu–Hf age likely pre-dated the eclogite-facies metamorphism. 40Ar/39Ar dating of hornblende from the eclogite yielded ages about 200 Ma, which is interpreted as a cooling age and is probably indicative of the time of exhumation to the middle crust. The difference of peak eclogite-facies metamorphic conditions and the distinct metamorphic ages for the Bailang eclogite (~ 2.6 GPa and ~ 480 °C; ca. 230 Ma), the Sumdo eclogite (~ 3.4 GPa and ~ 650 °C; ca. 262 Ma) and Jiang eclogite (~ 3.6 GPa and ~ 750 °C; ca. 261 Ma) in the same (ultra)-high-pressure belt indicate that this region likely comprises different slices that had distinct P–T histories and underwent (U)HP metamorphism at different times. The initiation of the opening the Paleo-Tethys Ocean in the Lhasa terrane could trace back to the early Permian. The ultimate closure of the Paleo-Tethys Ocean in the Lhasa terrane was no earlier than ca. 230 Ma.  相似文献   

13.
A typical HP/MT (high pressure/medium temperature) eclogite from Xiongdian, northwestern Dabie Mountains, has been geochronologically studied using the single-zircon U-Pb, 40Ar-39Ar and Sm-Nd methods. Prismatic zircons occurring as inclusions within garnets define a minimum crystallization age of 399.5±1.6 Ma. 40Ar-39Ar dating on amphibole gives a plateau age.of 399.2 ± 4 Ma, which is interpreted as a retrogression age of amphibolite facies. This integrated study enables us to conclude that the age of high-pressure metamorphism is older than 399.5 ± 1.6 Ma, suggesting Caledonian collision between the North China and Yangtze plates. Round zircon within the aggregate of quartz and muscovite gives a concordant age of 301± 2 Ma, reflecting a later retrogression event. An age profile of post-eclogite metamorphism is documented, including amphibolite facies metamorphism at 399.2 Ma shortly after eclogitization and later retrogressive metamorphism at 301 Ma. Sm-Nd mineral isochron of garnet+omphacite gives  相似文献   

14.
The Qilian–Qaidam orogenic belt at the northern edge of the Tibetan Plateau has received increasing attention as it recorded a complete history from continental breakup to opening and closure of ocean basin, and to the ultimate continental collision in the time period from the Neoproterozoic to the Paleozoic. Determining a geochronological framework of the initiation and termination of the fossil Qilian Ocean subduction in the North Qilian orogenic belt plays an essential role in understanding the whole tectonic process. Dating the high-pressure metamorphic rocks in the North Qilian orogenic belt, such as blueschist and eclogite, is the key in this respect. A blueschist from the southern North Qilian orogenic belt was investigated with a combined metamorphic P–T and U–Pb, Lu–Hf, and Sm–Nd multichronometric approaches. Pseudosection modeling indicates that the blueschist was metamorphosed under peak P–T conditions of 1.4–1.6 GPa and 530–550 °C. Zircon U–Pb ages show no constraints on the metamorphism due to the lack of metamorphic growth of zircon. Lu–Hf and Sm–Nd ages of 466.3 ± 2.0 Ma and 462.2 ± 5.6 Ma were obtained for the blueschist, which is generally consistent with the U–Pb zircon ages of 467–489 Ma for adjacent eclogites. Lutetium and Sm zoning profiles in garnet indicate that the Lu–Hf and Sm–Nd ages are biased toward the formation of the garnet inner rim. The ages are thus interpreted to reflect the time of blueschist-facies metamorphism. Previous 40Ar/39Ar ages of phengitic muscovite from blueschist/eclogite in this area likely represent a cooling age due to the higher peak metamorphic temperature than the argon retention temperature. The differences of peak metamorphic conditions and metamorphic ages between the eclogites and adjacent blueschists indicate that this region likely comprises different tectonic slices, which had distinct P–T histories and underwent high-pressure metamorphism at different times. The initial opening of the Qilian Ocean could trace back to the early Paleozoic, and the ultimate closure of the Qilian Ocean was no earlier than c. 466 Ma.  相似文献   

15.
New 40Ar/39Ar ages of igneous rocks clarify the nature, timing and rates of movement of the oceanic Pacific, Phoenix, Farallon and Hikurangi plates against Gondwana and Zealandia in the Late Cretaceous. With some qualifications, cessation of spreading at the Osbourn Trough is dated c. 79 Ma, i.e. 30–20 m.y. later than 110–100 Ma Hikurangi Plateau-Gondwana collision. Oceanic crust of pre-84 Ma is confirmed to be present at the eastern end of the Chatham Rise, and a 99–78 Ma intraplate lava province erupted across juxtaposed Zealandia, Hikurangi Plateau and oceanic crust. We propose a new regional tectonic model in which a mechanically jammed Hikurangi Plateau resulted in the dynamic propagation of small, kinematically misaligned short-length 110–84 Ma spreading centres and long-offset fracture zones. It is only from c. 84 Ma that geometrically stable spreading became localized at what is now the Pacific-Antarctic Ridge, as Zealandia started to split from Gondwana.  相似文献   

16.
A regional petrographic reconnaissance of psammitic and pelitic rocks in the Otago Schist, New Zealand, has revealed the presence of garnet (“grossalspite” with typical rim composition almandine41, spessartine25, grossular33, pyrope1) and biotite in 37 new samples, more than doubling the previously known number. A new garnet–biotite–albite zone can now be defined in the greenschist facies Otago Schist that is distinct from the better-known biotite, garnet and oligoclase zones in the along-strike Alpine Schist. The garnet–biotite–albite zone is in part metamorphically discontinuous with adjacent schists and does not support models of simple, continuous, progressive Jurassic regional metamorphism in Otago. The structurally higher (lower grade) boundary of the zone coincides in at least three places with previously mapped regional shear zones. The structurally lower (expected higher grade) boundary of the zone appears to be obliterated by a chlorite zone overprint which can be spatially related to Alpine Schist recrystallisation of ?Cretaceous age. The Otago situation serves as an example of the subtle metamorphic discontinuities that probably pervade many orogenic belts.  相似文献   

17.
Dating ultra‐high–pressure (UHP) metamorphic rocks provides important timing constraints on deep subduction zone processes. Eclogites, deeply subducted rocks now exposed at the surface, undergo a wide range of metamorphic conditions (i.e. deep subduction and exhumation) and their mineralogy can preserve a detailed record of chronologic information of these dynamic processes. Here, we present an approach that integrates multiple radiogenic isotope systems in the same sample to provide a more complete timeline for the subduction–collision–exhumation processes, based on eclogites from the Dabie–Sulu orogenic belt in eastern China, one of the largest UHP terranes on Earth. In this study, we integrate garnet Lu–Hf and Sm–Nd ages with zircon and titanite U–Pb ages for three eclogite samples from the Sulu UHP terrane. We combine this age information with Zr‐in‐rutile temperature estimates, and relate these multiple chronometers to different P–T conditions. Two types of rutile, one present as inclusions in garnet and the other in the matrix, record the temperatures of UHP conditions and a hotter stage, subsequent to the peak pressure (‘hot exhumation') respectively. Garnet Lu–Hf ages (c. 238–235 Ma) record the initial prograde growth of garnet, while coupled Sm–Nd ages (c. 219–213 Ma) reflect cooling following hot exhumation. The maximum duration of UHP conditions is constrained by the age difference of these two systems in garnet (c. 235–220 Ma). Complementary zircon and titanite U–Pb ages of c. 235–230 Ma and c. 216–206 Ma provide further constraints on the timing of prograde metamorphism and the ‘cold exhumation' respectively. We demonstrate that timing of various metamorphic stages can thus be determined by employing complementary chronometers from the same samples. These age results, combined with published data from adjacent areas, show lateral diachroneity in the Dabie–Sulu orogeny. Three sub‐blocks are thus defined by progressively younger garnet ages: western Dabie (243–238 Ma), eastern Dabie–northern Sulu (238–235 Ma) and southern Sulu terranes (225–220 Ma), which possibly correlate to different crustal slices in the recently proposed subduction channel model. These observed lateral chronologic variations in a large UHP terrane can possibly be extended to other suture zones.  相似文献   

18.
Garnet Sm–Nd and zircon U–Pb ages, and pressure–temperature–time paths elucidate Late Cretaceous crustal thickening which occurred within magmatic arc rocks of the Insular Superterrane. Voluminous tonalitic magma of the Mount Stuart batholith intruded at <3 kbar into upper crustal sedimentary rocks between 96 and 91 Ma, with initial intrusion prior to garnet growth in the metasedimentary rocks. Arc plutonism then shifted northward as crustal thickening commenced. Initial garnet growth, locally with kyanite and staurolite replacing andalusite, at c. 91 Ma was directly associated with intrusion of granodiorite to tonalite sheets at 7 kbar, north of the Mount Stuart batholith, within the Nason Ridge Migmatitic Gneiss. Subsequent heating and garnet growth, which postdates emplacement of large plutons, occurred between 88 and 86 Ma. This late garnet growth occurred at pressures of 6–8 kbar. The history of garnet growth and intrusion indicates that initial garnet zone and higher temperature metamorphism was restricted to contact aureoles. However, later widespread garnet growth at higher pressure probably resulted from heating as the orogenic wedge approached thermal equilibrium after crustal thickening. We conclude that metasedimentary rocks outside narrow contact aureoles remained at temperatures significantly below those of garnet growth and that the growth of garnet lasted <6 Myr. Heating to temperatures that stabilized garnet after pluton emplacement is compatible with intrusion of arc plutons into an accretionary wedge (Chiwaukum Schist) which was tectonically thickened and/or overthrust causing loading and thermal relaxation.  相似文献   

19.
Pressure–temperature pseudosections for ‘greyschist’ (metamorphosed greywacke and argillite) from the Alpine Schist (Haast Schist group) near Hokitika (Southern Alps, New Zealand) are used to gain new insights into its metamorphic history. The rocks were metamorphosed at relatively low‐grade conditions associated with the first appearance and initial growth of garnet in the stability field of albite. The measured and predicted garnet compositional zoning data are used to construct an overall P–T path by combining P–T path results from nearby rocks that have a range of MnO contents. The P–T path obtained is steep from ～380 °C/2.5 kbar up to ～490 °C/8.5 kbar, then recurves sharply with garnet growth continuing during early decompression to ～500 °C/6.5 kbar. Most garnet growth in the study area took place in the stability field of albite, with oligoclase appearing only during decompression, when the peristerite gap was entered. On appearance of oligoclase, there is a marked decrease in the CaO content of garnet. The preservation of mineral assemblages from near‐peak temperature conditions can be understood in terms of the P–T path subsequently becoming tangential to water content contours, during cooling with further decompression.  相似文献   

20.
Eclogite facies mineral assemblages are variably preserved in mafic and ultramafic rocks within the Western Gneiss Region (WGR) of Norway. Mineralogical and microstructural data indicate that some Mg–Cr-rich, Alpine-type peridotites have had a complex metamorphic history. The metamorphic evolution of these rocks has been described in terms of a seven-stage evolutionary model; each stage is characterized by a specific mineral assemblage. Stages II and III both comprise garnet-bearing mineral assemblages. Garnet-bearing assemblages are also present in Fe–Ti-rich peridotites which commonly occur as layers in mafic complexes. Sm–Nd isotopic results are reported for mineral and whole rock samples from both of these types of peridotites and related rocks. The partitioning of Sm and Nd between coexisting garnet and clinopyroxene is used to assess chemical equilibrium. One sample of Mg–Cr-type peridotite shows non-disturbed partitioning of Sm and Nd between Stage II garnet and clinopyroxene pairs and yields a garnet–clinopyroxene–whole-rock date of 1703 ± 29 Ma (I= 0.51069, MSWD = 0.04). This is the best estimate for the age of the Stage II high-P assemblage. Other Stage II garnet–clinopyroxene pairs reflect later disturbance of the Sm–Nd system and yield dates in the range 1303 to 1040 Ma. These dates may not have any geological significance. Stage III garnet–clinopyroxene pairs typically have equilibrated Sm–Nd partitioning and two samples yield dates of 437 ± 58 and 511 ± 18 Ma. This suggests that equilibration of the Stage III high-P assemblage is related to the Caledonian orogeny and is more or less contemporaneous with high-P metamorphism of ‘country-rock’eclogites in the surrounding gneisses. The Sm–Nd mineral data for the Fe–Ti-rich garnet peridotites and for a superferrian eclogite, which occurs as a dyke within the Gurskebotn Mg–Cr-type peridotite, are consistent with a Palaeozoic high-P metamorphism. Finally a synoptic P–T–t path is proposed for the Mg–Cr-type peridotites which is consistent with the petrological and geochronological data.  相似文献   

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