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1.
Phase equilibria modelling, laser‐ablation split‐stream (LASS)‐ICP‐MS petrochronology and garnet trace‐element geochemistry are integrated to constrain the P–T–t history of the footwall of the Priest River metamorphic core complex, northern Idaho. Metapelitic, migmatitic gneisses of the Hauser Lake Gneiss contain the peak assemblage garnet + sillimanite + biotite ± muscovite + plagioclase + K‐feldspar ± rutile ± ilmenite + quartz. Interpreted P–T paths predict maximum pressures and peak metamorphic temperatures of ~9.6–10.3 kbar and ~785–790 °C. Monazite and xenotime 208Pb/232Th dates from porphyroblast inclusions indicate that metamorphism occurred at c. 74–54 Ma. Dates from HREE‐depleted monazite formed during prograde growth constrain peak metamorphism at c. 64 Ma near the centre of the complex, while dates from HREE‐enriched monazite constrain the timing of garnet breakdown during near‐isothermal decompression at c. 60–57 Ma. Near‐isothermal decompression to ~5.0–4.4 kbar was followed by cooling and further decompression. The youngest, HREE‐enriched monazite records leucosome crystallization at mid‐crustal levels c. 54–44 Ma. The northernmost sample records regional metamorphism during the emplacement of the Selkirk igneous complex (c. 94–81 Ma), Cretaceous–Tertiary metamorphism and limited Eocene exhumation. Similarities between the Priest River complex and other complexes of the northern North American Cordillera suggest shared regional metamorphic and exhumation histories; however, in contrast to complexes to the north, the Priest River contains less partial melt and no evidence for diapiric exhumation. Improved constraints on metamorphism, deformation, anatexis and exhumation provide greater insight into the initiation and evolution of metamorphic core complexes in the northern Cordillera, and in similar tectonic settings elsewhere.  相似文献   

2.
In Rogaland, South Norway, a polycyclic granulite facies metamorphic domain surrounds the late‐Sveconorwegian anorthosite–mangerite–charnockite (AMC) plutonic complex. Integrated petrology, phase equilibria modelling, monazite microchemistry, Y‐in‐monazite thermometry, and monazite U–Th–Pb geochronology in eight samples, distributed across the apparent metamorphic field gradient, imply a sequence of two successive phases of ultrahigh temperature (UHT) metamorphism in the time window between 1,050 and 910 Ma. A first long‐lived metamorphic cycle (M1) between 1,045 ± 8 and 992 ± 11 Ma is recorded by monazite in all samples. This cycle is interpreted to represent prograde clockwise P–T path involving melt production in fertile protoliths and culminating in UHT conditions of ~6 kbar and 920°C. Y‐in‐monazite thermometry, in a residual garnet‐absent sapphirine–orthopyroxene granulite, provides critical evidence for average temperature of 931 and 917°C between 1,029 ± 9 and 1,006 ± 8 Ma. Metamorphism peaked after c. 20 Ma of crustal melting and melt extraction, probably supported by a protracted asthenospheric heat source following lithospheric mantle delamination. Between 990 and 940 Ma, slow conductive cooling to 750–800°C is characterized by monazite reactivity as opposed to silicate metastability. A second incursion (M2) to UHT conditions of ~3.5–5 kbar and 900–950°C, is recorded by Y‐rich monazite at 930 ± 6 Ma in an orthopyroxene–cordierite–hercynite gneiss and by an osumilite gneiss. This M2 metamorphism, typified by osumilite paragenesis, is related to the intrusion of the AMC plutonic complex at 931 ± 2 Ma. Thermal preconditioning of the crust during the first UHT metamorphism may explain the width of the aureole of contact metamorphism c. 75 Ma later, and also the rarity of osumilite‐bearing assemblages in general.  相似文献   

3.
Here, we present results of the first 40Ar/39Ar dating of osumilite, a high‐T mineral that occurs in some volcanic and high‐grade metamorphic rocks. The metamorphic osumilite studied here is from a metapelitic rock within the Rogaland–Vest Agder Sector, Norway, an area that experienced regional granulite facies metamorphism and subsequent contact metamorphism between 1,100 Ma and 850 Ma. The large grain size (~1 cm) of osumilite in the studied rock, which preserves a nominally anhydrous assemblage, increases the potential for large portions of individual grains to have remained essentially unaffected by the effects of diffusive argon loss, potentially preserving prograde ages. Step‐heating diffusion experiments yielded a maximum activation energy of ~461 kJ/mol and a pre‐exponential factor of ~8.34 × 108 cm2/s for Ar diffusion in osumilite. These parameters correspond to a relatively high closure temperature of ~620°C for a cooling rate of 10°C/Ma in an osumilite crystal with a 175 µm radius. Fragments of osumilite separated from the sample preserve a range of ages between c. 1,070 and 860 Ma. The oldest ages are inferred to date the growth of coarse‐grained osumilite during prograde granulite facies regional metamorphism, which pre‐date contact metamorphism that has historically been ascribed to the growth of osumilite in the region. The majority of fragments record ages between c. 920 and 860 Ma, inferred to reflect the growth of osumilite and/or diffusive argon loss during contact metamorphism. The retention of old 40Ar/39Ar dates was facilitated by the low diffusivity of Ar in osumilite (i.e. a closed system), large grain sizes, and anhydrous metamorphic conditions. The ability to date osumilite with the 40Ar/39Ar method provides a valuable new thermochronometer that may constrain the timing and duration of high‐T magmatic and metamorphic events.  相似文献   

4.
In the Rogaland–Vest Agder terrain of the Sveconorwegian Province of SW Norway, two main Sveconorwegian metamorphic phases are reported: a phase of regional metamorphism linked to orogenic thickening (M1) and a phase of low-pressure thermal metamorphism associated with the intrusion of the 931 ± 2 Ma anorthosite-charnockite Rogaland igneous complex (M2). Phase M1 reached granulite facies to the west of the terrane and M2 culminated locally at 800–850 °C with the formation of dry osumilite-bearing mineral associations. Monazite and titanite U-Pb geochronology was conducted on 17 amphibolite- to granulite-facies orthogneiss samples, mainly from a suite of 1050 +2/−8 Ma calc-alkaline augen gneisses, the Feda suite. In these rocks, prograde negatively discordant monazite crystallized during breakdown of allanite and titanite in upper amphibolite facies at 1012–1006 Ma. In the Feda suite and other charnockitic gneisses, concordant to slightly discordant monazite at 1024–997 Ma probably reflects breakdown of biotite during granulite-facies M1 metamorphism. A spread of monazite ages down to 970 Ma in biotite ± hornblende samples possibly corresponds to the waning stage of this first event. In the Feda suite, a well defined monazite growth episode at 930–925 Ma in the amphibolite-facies domain corresponds to major clinopyroxene formation at the expense of hornblende during M2. Growth or resetting of monazite was extremely limited during this phase in the granulite-facies domain, up to the direct vicinity of the anorthosite complex. The M2 event was shortly followed by cooling through ca. 610 °C as indicated by tightly grouped U-Pb ages of accessory titanite and titanite relict inclusions at 918 ± 2 Ma over the entire region. A last generation of U-poor monazite formed during regional cooling below 610 °C, in hornblende-rich samples at 912–904 Ma. This study suggests: (1) that monazite formed during the prograde path of high-grade metamorphism may be preserved; (2) that monazite ages reflect primary or secondary growth of monazite linked to metamorphic reactions involving redistribution of REEs and Th, and/or fluid mobilisation; (3) that the U-Pb system in monazite is not affected by thermal events up to 800–850 °C, provided that conditions were dry during metamorphism. Received: 9 January 1997 / Accepted: 15 April 1998  相似文献   

5.
The Palaeo‐Mesoproterozoic metapelite granulites from northern Garo Hills, western Shillong‐Meghalaya Gneissic Complex (SMGC), northeast India, consist of resorbed garnet, cordierite and K‐feldspar porphyroblasts in a matrix comprising shape‐preferred aggregates of biotite±sillimanite+quartz that define the penetrative gneissic fabric. An earlier assemblage including biotite and sillimanite occurs as inclusions within the garnet and cordierite porphyroblasts. Staurolite within cordierite in samples without matrix sillimanite is interpreted to have formed by a reaction between the sillimanite inclusion and the host cordierite during retrogression. Accessory monazite occurs as inclusions within garnet as well as in the matrix, whereas accessory xenotime occurs only in the matrix. The monazite inclusions in garnet contain higher Ca, and lower Y and Th/U than the matrix monazite outside resorbed garnet rims. On the other hand, matrix monazite away from garnet contains low Ca and Y, and shows very high Th/U ratios. The low Th/U ratios (<10) of the Y‐poor garnet‐hosted monazite indicate subsolidus formation during an early stage of prograde metamorphism. A calculated P–T pseudosection in the MnCKFMASH‐PYCe system indicates that the garnet‐hosted monazite formed at <3 kbar/600 °C (Stage A). These P–T estimates extend backward the previously inferred prograde P–T path from peak anatectic conditions of 7–8 kbar/850 °C based on major mineral equilibria. Furthermore, the calculated P–T pseudosections indicate that cordierite–staurolite equilibrated at ~5.5 kbar/630 °C during retrograde metamorphism. Thus, the P–T path was counterclockwise. The Y‐rich matrix monazite outside garnet rims formed between ~3.2 kbar/650 °C and ~5 kbar/775 °C (Stage B) during prograde metamorphism. If the effect of bulk composition change due to open system behaviour during anatexis is considered, the P–T conditions may be lower for Stage A (<2 kbar/525 °C) and Stage B (~3 kbar/600 °C to ~3.5 kbar/660 °C). Prograde garnet growth occurred over the entire temperature range (550–850 °C), and Stage‐B monazite was perhaps initially entrapped in garnet. During post‐peak cooling, the Stage‐B monazite grains were released in the matrix by garnet dissolution. Furthermore, new matrix monazite (low Y and very high Th/U ≤80, ~8 kbar/850–800 °C, Stage C), some monazite outside garnet rims (high Y and intermediate Th/U ≤30, ~8 kbar/800–785 °C, Stage D), and matrix xenotime (<785 °C) formed through post‐peak crystallization of melt. Regardless of textural setting, all monazite populations show identical chemical ages (1630–1578 Ma, ±43 Ma). The lithological association (metapelite and mafic granulites), and metamorphic age and P–T path of the northern Garo Hills metapelites and those from the southern domain of the Central Indian Tectonic Zone (CITZ) are similar. The SMGC was initially aligned with the southern parts of CITZ and Chotanagpur Gneissic Complex of central/eastern India in an ENE direction, but was displaced ~350 km northward by sinistral movement along the north‐trending Eastern Indian Tectonic Zone in Neoproterozoic. The southern CITZ metapelites supposedly originated in a back‐arc associated with subducting oceanic lithosphere below the Southern Indian Block at c. 1.6 Ga during the initial stage of Indian shield assembly. It is inferred that the SMGC metapelites may also have originated contemporaneously with the southern CITZ metapelites in a similar back‐arc setting.  相似文献   

6.
Collision‐related granitoid batholiths, like those of the Hercynian and Himalayan orogens, are mostly fed by magma derived from metasedimentary sources. However, in the late Neoproterozoic calcalkaline (CA) batholiths of the Arabian–Nubian Shield (ANS), which constitutes the northern half of the East African orogen, any sedimentary contribution is obscured by the juvenile character of the crust and the scarcity of migmatites. Here, we use paired in situ LASS‐ICP‐MS measurements of U–Th–Pb isotope ratios and REE contents of monazite and xenotime and SHRIMP‐RG analyses of separated zircon to demonstrate direct linkage between migmatites and granites in the northernmost ANS. Our results indicate a single prolonged period of monazite growth at 640–600 Ma, in metapelites, migmatites and peraluminous granites of three metamorphic suites: Abu‐Barqa (SW Jordan), Roded (S Israel) and Taba–Nuweiba (Sinai, Egypt). The distribution of monazite dates and age zoning in single monazite grains in migmatites suggest that peak thermal conditions, involving partial melting, prevailed for c. 10 Ma, from 620 to 610 Ma. REE abundances in monazite are well correlated with age, recording garnet growth and garnet breakdown in association with the prograde and retrograde stages of the melting reactions, respectively. Xenotime dates cluster at 600–580 Ma, recording retrogression to greenschist facies conditions as garnet continued to destabilize. Phase equilibrium modelling and mineral thermobarometry yield P–T conditions of ~650–680°C and 5–7 kbar, consistent with either water‐fluxed or muscovite‐breakdown melting. The expected melt production is 8–10 vol.%, allowing a melt connectivity network to form leading to melt segregation and extraction. U–Pb ages of zircon rims from leucosomes indicate crystallization of melt at 610 ± 10 Ma, coinciding with the emplacement of a vast volume of CA granites throughout the northern ANS, which were previously considered post‐collisional. Similar monazite ages (c. 620 Ma) retrieved from the amphibolite facies Elat schist indicate that migmatites are the result of widespread regional rather than local contact metamorphism, representing the climax of the East African orogenesis.  相似文献   

7.
Polymetamorphic garnet micaschists from the Austroalpine Saualpe Eclogite Unit (Kärnten, Austria, Eastern Alps) display complex microstructural and mineral–chemical relationships. Automated scanning electron microscopy routines with energy dispersive X‐ray (EDX) spectral mapping were applied for monazite detection and garnet mineral–chemical characterization. When the Fe, Mg, Mn and Ca element wt% compositions are used as generic labels for garnet EDX spectra, complex zonations and porphyroblast generations can be resolved in complete thin sections for selective electron‐microprobe analyses. Two garnet porphyroblast generations and diverse monazite age populations have been revealed in low‐Ca and high‐Al‐metapelites. Garnet 1 has decreasing Mn, constant Ca and significantly increasing Mg from cores to rims. Geothermobarometry of garnet 1 assemblages signals a crystallization along a M1 prograde metamorphism at ~650 °C/6–8 kbar. Sporadic monazite 1 crystallization started at c. 320 Ma. Subsequent pervasive 300–250 Ma high‐Y and high‐Gd monazite 1 formation during decompression coincided with the intrusion of Permian and Early Triassic pegmatites. Monazite 1 crystallized along the margin of garnet 1. Coronas of apatite and allanite around the large 320–250 Ma monazite signal a retrogressive stage. These microstructures suggest a Carboniferous‐to‐Early‐Permian age for the prograde M1 event with garnet 1. Such a M1 event at an intermediate‐P/T gradient has not yet been described from the Saualpe, and preceded a Permo‐Triassic low‐P stage. The M2 event with garnet 2 postdates the corona formation around Permian monazite. Garnet 2 displays first increasing XCa at decreasing XMg, then increasing XCa and XMg, and finally decreasing XCa with increasing XMg, always at high Ca and Mg, and low Mn. This records a P–T evolution which passed through eclogite facies conditions and reached maximum temperatures at ~750 °C/14 kbar during decompression‐heating. A monazite 2 population (94–86 Ma) with lower Y and Gd contents crystallized at decreasing pressure during the Cretaceous (Eo‐Alpine) metamorphism M2 at a high‐P/T gradient. The Saualpe Eclogite Unit underwent two distinct clockwise metamorphic cycles at different P–T conditions, related to continental collisions under different thermal regimes. This led to a characteristic distribution pattern of monazite ages in this unit which is different from other Austroalpine basement areas.  相似文献   

8.
The growth and dissolution behaviour of accessory phases (and especially those of geochronological interest) in metamorphosed pelites depends on, among others, the bulk composition, the prograde metamorphic evolution and the cooling path. Monazite and zircon are arguably the most commonly used geochronometers for dating felsic metamorphic rocks, yet crystal growth mechanisms as a function of rock composition, pressure and temperature are still incompletely understood. Ages of different growth zones in zircon and monazite in a garnet‐bearing anatectic metapelite from the Greater Himalayan Sequence in NW Bhutan were investigated via a combination of thermodynamic modelling, microtextural data and interpretation of trace‐element chemical ‘fingerprint’ indicators in order to link them to the metamorphic stage at which they crystallized. Differences in the trace‐element composition (HREE, Y, EuN/Eu*N) of different phases were used to track the growth/dissolution of major (e.g. plagioclase, garnet) and accessory phases (e.g. monazite, zircon, xenotime, allanite). Taken together, these data constrain multiple pressure–temperature–time (P–T–t) points from low temperature (<550 °C) to upper amphibolite facies (partial melting, >700 °C) conditions. The results suggest that the metapelite experienced a cryptic early metamorphic stage at c. 38 Ma at <550 °C, ≥0.85 GPa during which plagioclase was probably absent. This was followed by a prolonged high‐T, medium‐pressure (~600 °C, 0.55 GPa) evolution at 35–29 Ma during which the garnet grew, and subsequent partial melting at >690 °C and >18 Ma. Our data confirm that both geochronometers can crystallize independently at different times along the same P–T path and that neither monazite nor zircon necessarily provides timing constraints on ‘peak’ metamorphism. Therefore, collecting monazite and zircon ages as well as major and trace‐element data from major and accessory phases in the same sample is essential for reconstructing the most coherent metamorphic P–T–t evolution and thus for robustly constraining the rates and timescales of metamorphic cycles.  相似文献   

9.
Abstract

Combined in situ monazite dating, mineral equilibria modelling and zircon U–Pb detrital zircon analysis provide insight into the pressure–temperature–time (P–T–t) evolution of the western Gawler Craton. In the Nawa Domain, pelitic and quartzo-feldspathic gneisses were deposited after ca 1760?Ma and record high-grade metamorphic conditions of ～7.5?kbar and 850?°C at ca 1730?Ma. Post-peak microstructures, including partial plagioclase coronae and late biotite around garnet, and subtle retrograde garnet compositional zoning, suggest that these rocks cooled along a shallow down-pressure trajectory across an elevated dry solidus. In the northwest Fowler Domain (Colona Block), monazite grains from pelitic gneisses record two stages of growth/recrystallisation interpreted to represent discrete parts of the P–T path: (1) ca 1710?Ma monazite growth during prograde to peak conditions, and (2) ca 1690?Ma Y-enriched monazite growth/recrystallisation during partial garnet breakdown and cooling towards the solidus. Relict prograde growth zoning in garnet suggests rocks underwent a steep up-P path to peak conditions of ～8?kbar at 800?°C. The new P–T–t results suggest basement rocks of the southwestern Nawa and northwestern Fowler were buried to depths of 20–25?km during the Kimban Orogeny, ca 10 Myrs after the sedimentary precursors were deposited. The P–T path for the Kimban Orogeny is broadly anti-clockwise, suggesting that at least the early phase of this event was associated with extension. Exhumation of rocks from both the southwestern Nawa and northwestern Fowler domains may have occurred during the waning stages of the Kimban Orogeny (<ca 1690?Ma). The limited low-grade overprint in these rocks may be explained by a mid-to-upper crustal position for these rocks during the subsequent Kararan Orogeny. Aluminous quartz-feldspathic gneiss of the Nundroo Block in the eastern Fowler Domain records peak conditions of ～7?kbar at 800?°C. Monazite grains from the Nundroo Block are dominated by an age peak at ca 1590?Ma, although the presence of some older ages up to ca 1690?Ma, possibly reflect partial resetting of older monazite domains. The P–T–t conditions suggest these rocks were buried to 20–25?km at ca 1590?Ma during the Kararan Orogeny. This high-grade metamorphism in the Nundroo Block is a mid-crustal expression of the same thermal anomaly that caused magmatism in the central-eastern Gawler Craton. Juxtaposition of rocks affected by the Kimban and Kararan orogenic events in the western Gawler Craton was controlled by lithospheric-scale shear zones, some of which have facilitated ～20 kilometres of exhumation.  相似文献   

10.
The Ross orogen of Antarctica is an extensive (>3000 km‐long) belt of deformed and metamorphosed sedimentary rocks and granitoid batholiths, which formed during convergence and subduction of palaeo‐Pacific lithosphere beneath East Gondwana in the Neoproterozoic–early Palaeozoic. Despite its prominent role in Gondwanan convergent tectonics, and a well‐established magmatic record, relatively little is known about the metamorphic rocks in the Ross orogen. A combination of garnet Lu–Hf and monazite U–Pb (measured by laser‐ablation split‐stream ICP‐MS) geochronology reveals a protracted metamorphic history of metapelites and garnet amphibolites from a major segment of the orogen. Additionally, direct dating of a common rock‐forming mineral (garnet) and accessory mineral (monazite) allows us to test assumptions that are commonly used when linking accessory mineral geochronology to rock‐forming mineral reactions. Petrography, mineral zoning, thermobarometry and pseudosection modelling reveal a Barrovian‐style prograde path, reaching temperatures of ~610–680 °C. Despite near‐complete diffusional resetting of garnet major element zoning, the garnet retains strong rare earth element zoning and preserves Lu–Hf dates that range from c. 616–572 Ma. Conversely, monazite in the rocks was extensively recrystallized, with concordant dates that span from c. 610–500 Ma, and retain only vestigial cores. Monazite cores yield dates that overlap with the garnet Lu–Hf dates and typically have low‐Y and heavy rare earth element (HREE) concentrations, corroborating interpretations of low‐Y and low‐HREE monazite domains as records of synchronous garnet growth. However, ratios of REE concentrations in garnet and monazite do not consistently match previously reported partition coefficients for the REE between these two minerals. High‐Y monazite inclusions within pristine, crack‐free garnet yield U–Pb dates significantly younger than the Lu–Hf dates for the same samples, indicating recrystallization of monazite within garnet. The recrystallization of high‐Y and high‐HREE monazite domains over >50 Ma likely records either punctuated thermal pulses or prolonged residence at relatively high temperatures (up to ~610–680 °C) driving monazite recrystallization. One c. 616 Ma garnet Lu–Hf date and several c. 610–600 Ma monazite U–Pb dates are tentatively interpreted as records of the onset of tectonism metamorphism in the Ross orogeny, with a more robust constraint from the other Lu–Hf dates (c. 588–572 Ma) and numerous c. 590–570 Ma monazite U–Pb dates. The data are consistent with a tectonic model that involves shortening and thickening prior to widespread magmatism in the vicinity of the study area. The early tectonic history of the Ross orogen, recorded in metamorphic rocks, was broadly synchronous with Gondwana‐wide collisional Pan‐African orogenies.  相似文献   

11.
Monazite is a key accessory mineral for metamorphic geochronology, but interpretation of its complex chemical and age zoning acquired during high-temperature metamorphism and anatexis remains a challenge. We investigate the petrology, pressure–temperature and timing of metamorphism in pelitic and psammitic granulites that contain monazite from the Greater Himalayan Crystalline Complex (GHC) in Dinggye, southern Tibet. These rocks underwent isothermal decompression from pressure of >10 kbar to ~5 kbar at temperatures of 750–830 °C, and recorded three metamorphic stages at kyanite (M1), sillimanite (M2) and cordierite-spinel grade (M3). Monazite and zircon crystals were dated by microbeam techniques either as grain separates or in thin sections. U–Th–Pb ages are linked to specific conditions of mineral growth on the basis of zoning patterns, trace element signatures, index mineral inclusions (melt inclusions, sillimanite and K-feldspar) in dated domains and textural relationships with co-existing minerals. The results show that inherited domains (500–400 Ma) are preserved in monazite even at granulite-facies conditions. Few monazites or zircon yield ages related to the M1-stage (~30–29 Ma), possibly corresponding to prograde melting by muscovite dehydration. During the early stage of isothermal decompression, inherited or prograde monazites in most samples were dissolved in the melt produced by biotite dehydration-melting. Most monazite grains crystallized from melt toward the end of decompression (M3-stage, 21–19 Ma) and are chemically related to garnet breakdown reactions. Another peak of monazite growth occurred at final melt crystallization (~15 Ma), and these monazite grains are unzoned and are homogeneous in composition. In a regional context, our pressure–temperature–time data constrains peak high-pressure metamorphism within the GHC to ~30–29 Ma in Dinggye Himalaya. Our results are in line with a melt-assisted exhumation of the GHC rocks.  相似文献   

12.
The Fosdick migmatite–granite complex in West Antarctica records evidence for two high‐temperature metamorphic events, the first during the Devonian–Carboniferous and the second during the Cretaceous. The conditions of each high‐temperature metamorphic event, both of which involved melting and multiple melt‐loss events, are investigated using phase equilibria modelling during successive melt‐loss events, microstructural observations and mineral chemistry. In situ SHRIMP monazite and TIMS Sm–Nd garnet ages are integrated with these results to constrain the timing of the two events. In areas that preferentially preserve the Devonian–Carboniferous (M1) event, monazite grains in leucosomes and core domains of monazite inclusions in Cretaceous cordierite yield an age of c. 346 Ma, which is interpreted to record the timing of monazite growth during peak M1 metamorphism (～820–870 °C, 7.5–11.5 kbar) and the formation of garnet–sillimanite–biotite–melt‐bearing assemblages. Slightly younger monazite spot ages between c. 331 and 314 Ma are identified from grains located in fractured garnet porphyroblasts, and from inclusions in plagioclase that surround relict garnet and in matrix biotite. These ages record the growth of monazite during garnet breakdown associated with cooling from peak M1 conditions. The Cretaceous (M2) overprint is recorded in compositionally homogeneous monazite grains and rim domains in zoned monazite grains. This monazite yields a protracted range of spot ages with a dominant population between c. 111 and 96 Ma. Rim domains of monazite inclusions in cordierite surrounding garnet and in coarse‐grained poikiloblasts of cordierite yield a weighted mean age of c. 102 Ma, interpreted to constrain the age of cordierite growth. TIMS Sm–Nd ages for garnet are similar at 102–99 Ma. Mineral equilibria modelling of the residual protolith composition after Carboniferous melt loss and removal of inert M1 garnet constrains M2 conditions to ～830–870 °C and ～6–7.5 kbar. The modelling results suggest that there was growth and resorption of garnet during the M2 event, which would facilitate overprinting of M1 compositions during the M2 prograde metamorphism. Measured garnet compositions and Sm–Nd diffusion modelling of garnet in the migmatitic gneisses suggest resetting of major elements and the Sm–Nd system during the Cretaceous M1 overprint. The c. 102–99 Ma garnet Sm–Nd ‘closure’ ages correspond to cooling below 700 °C during the rapid exhumation of the Fosdick migmatite–granite complex.  相似文献   

13.
The South Tibetan detachment system (STDS) in the Himalayan orogen is an example of normal‐sense displacement on an orogen‐parallel shear zone during lithospheric contraction. Here, in situ monazite U(–Th)–Pb geochronology is combined with metamorphic pressure and temperature estimates to constrain pressure–temperature–time (P–T–t) paths for both the hangingwall and footwall rocks of a Miocene ductile component of the STDS (outer STDS) now exposed in the eastern Himalaya. The outer STDS is located south of a younger, ductile/brittle component of the STDS (inner STDS), and is characterized by structurally upward decreasing metamorphic grade corresponding to a transition from sillimanite‐bearing Greater Himalayan sequence rocks in the footwall with garnet that preserves diffusive chemical zoning to staurolite‐bearing Chekha Group rocks in the hangingwall, with garnet that records prograde chemical zoning. Monazite ages indicate that prograde garnet growth in the footwall occurred prior to partial melting at 22.6 ± 0.4 Ma, and that peak temperatures were reached following c. 20.5 Ma. In contrast, peak temperatures were reached in the Chekha Group hangingwall by c. 22 Ma. Normal‐sense (top‐to‐the‐north) shearing in both the hangingwall and footwall followed peak metamorphism from c. 23 Ma until at least c. 16 Ma. Retrograde P–T–t paths are compatible with modelled P–T–t paths for an outer STDS analogue that is isolated from the inner STDS by intervening extrusion of a dome of mid‐crustal material.  相似文献   

14.
《Gondwana Research》2009,15(4):624-643
The Higo Complex of west-central Kyushu, western Japan is a 25 km long body of metasedimentary and metabasic lithologies that increase in metamorphic grade from schist in the north to migmatitic granulite in the south, where granitoids are emplaced along the southern margin. The timing of granulite metamorphism has been extensively investigated and debated. Previously published Sm–Nd mineral isochrons for garnet-bearing metapelite yielded ca.220–280 Ma ages, suggesting high-grade equilibration older than the lower grade schist to the north, which yielded ca.180 Ma K–Ar muscovite ages. Ion and electron microprobe analyses on zircon have yielded detrital grains with rim ages of ca.250 Ma and ca.110 Ma. Electron microprobe ages from monazite and xenotime are consistently 110–130 Ma. Two models have been proposed: 1) high-grade metamorphism and tectonism at ca.115 Ma, with older ages attributed to inheritance; and 2) high-grade metamorphism at ca.250 Ma, with resetting of isotopic systems by contact metamorphism at ca.105 Ma during the intrusion of granodiorite. These models are evaluated through petrographic investigation and electron microprobe Th–U–total Pb dating of monazite in metapelitic migmatites and associated lithologies. In-situ investigation of monazite reveals growth and dissolution features associated with prograde and retrograde stages of progressive metamorphism and deformation. Monazite Th–U–Pb isochrons from metapelite, diatexite and late-deformational felsic dykes consistently yield ca.110–120 Ma ages. Earlier and later stages of monazite growth cannot be temporally resolved. The preservation of petrogenetic relationships, coupled with the low diffusion rate of Pb at < 900 °C in monazite, is strong evidence for timing high-temperature metamorphism and deformation at ca.115 Ma. Older ages from a variety of chronometers are attributed to isotopic disequilibrium between mineral phases and the preservation of inherited and detrital age components. Tentative support is given to tectonic models that correlate the Higo terrane with exotic terranes between the Inner and Outer tectonic Zones of southwest Japan, possibly derived from the active continental margin of the South China Block. These terranes were dismembered and translated northeastwards by transcurrent shearing and faulting from the beginning to the end of the Cretaceous Period.  相似文献   

15.
The Red River shear zone (RRSZ) is a major left‐lateral strike‐slip shear zone, containing a ductilely deformed metamorphic core bounded by brittle strike‐slip and normal faults, which stretches for >1000 km from Tibet through Yunnan and North Vietnam to the South China Sea. The RRSZ exposes four high‐grade metamorphic core complexes along its length. Various lithologies from the southernmost core complex, the Day Nui Con Voi (DNCV), North Vietnam, provide new constraints on the tectonic and metamorphic evolution of this region prior to and following the initial India–Asia collision. Analysis of a weakly deformed anatectic paragneiss using P–T pseudosections constructed in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (MnNCKFMASHTO) system provides prograde, peak and retrograde metamorphic conditions, and in situ U–Th–Pb geochronology of metamorphic monazite yields texturally controlled age constraints. Tertiary metamorphism and deformation, overprinting earlier Triassic metamorphism associated with the Indosinian orogeny and possible Cretaceous metamorphism, are characterized by peak metamorphic conditions of ~805 °C and ~8.5 kbar between c. 38 and 34 Ma. Exhumation occurred along a steep retrograde P–T path with final melt crystallizing at the solidus at ≥~5.5 kbar at ~790 °C. Further exhumation at ~640–700 °C and ~4–5 kbar at c. 31 Ma occurred at subsolidus conditions. U–Pb geochronological analysis of monazite from a strongly deformed pre‐kinematic granite dyke from the flank of the DNCV provides further evidence for exhumation at this time. Magmatic grains suggest initial emplacement at 66.0 ± 1.0 Ma prior to the India–Asia collision, whereas grains with metamorphic characteristics indicate later growth at 30.6 ± 0.4 Ma. Monazite grains from a cross‐cutting post‐kinematic dyke within the core of the DNCV antiform provide a minimum age constraint of 25.2 ± 1.4 Ma for the termination of fabric development. A separate and significant episode of monazite growth at c. 83–69 Ma is suggested to be the result of fluid‐assisted recrystallization following the emplacement of magmatic units.  相似文献   

16.
Granulite facies cordierite–garnet–biotite gneisses from the southeastern Reynolds Range, central Australia, contain both orthopyroxene‐bearing and orthopyroxene‐free quartzofeldspathic leucosomes. Mineral reaction microstructures at the interface of gneiss and leucosome observed in outcrop and petrographically, reflect melt‐rock interaction during crystallization. Accessory monazite, susceptible to fluid alteration, dissolution and recrystallization at high temperature, is tested for its applicability to constrain the chemical and P–T–time evolution of melt‐rock reactions during crystallization upon cooling. Bulk rock geochemistry and phase equilibria modelling constrain peak pressure and temperature conditions to 6.5–7.5 kbar and ~850°C, and U–Pb geochronology constrains the timing of monazite crystallization to 1.55 Ga, coeval with the Chewings Orogeny. Modelling predicts the presence of up to 15 vol.% melt at peak metamorphic conditions. Upon cooling below 800°C, melt extraction and in situ crystallization of melt decrease the melt volume to less than 7%, at which time it becomes entrapped and melt pockets induce replacement reactions in the adjacent host rock. Replacement reactions of garnet, orthopyroxene and K‐feldspar liberate Y, REE, Eu and U in addition to Mg, Fe, Al, Si and K. We demonstrate that distinguishing between monazite varieties solely on the basis of U–Pb ages cannot solve the chronological order of events in this study, nor does it tie monazite to the evolution of melt or stability of rock‐forming minerals. Rather, we argue that analyses of various internal monazite textures, their composition and overprinting relations allow us to identify the chronology of events following the metamorphic peak. We infer that retrograde reactions involving garnet, orthopyroxene and K‐feldspar can be attributed to melt‐rock interaction subsequent to partial melting, which is reflected in the development of compositionally distinct monazite textural domains. Internal monazite textures and their composition are consistent with dissolution and precipitation reactions induced by a high‐T melt. Monazite rims enriched in Y, HREE, Eu and U indicate an increased availability of these elements, consistent with the breakdown of orthopyroxene, garnet and K‐feldspar observed petrographically. Our study indicates that compositional and textural analysis of monazite in relation to major rock‐forming minerals can be used to infer the post‐peak chemical evolution of partial melts during high‐ to ultrahigh‐temperature metamorphism.  相似文献   

17.
Amphibolite facies metasedimentary schists within the Yukon‐Tanana terrane in the northern Canadian Cordillera reveal a two‐stage, polymetamorphic garnet growth history. In situ U‐Th‐Pb Sensitive High Resolution Ion Microprobe dating of monazite provide timing constraints for the late stages of garnet growth, deformation and subsequent decompression. Distinct textural and chemical growth zoning domains, separated by a large chemical discontinuity, reveal two stages of garnet growth characterized in part by: (i) a syn‐kinematic, inclusion‐rich stage‐1 garnet core; and (ii) an inclusion‐poor, stage‐2 garnet rim that crystallized with syn‐ to post‐kinematic staurolite and kyanite. Phase equilibria modelling of garnet molar and compositional isopleths suggest stage‐1 garnet growth initiated at ~600 °C, 8 kbar along a clockwise P–T path. Growth of the compositionally distinct, grossular‐rich, pyrope‐poor inner portion of the stage‐2 overgrowth is interpreted to have initiated at higher pressure and/or lower temperature than the stage‐1 core along a separate P–T loop, culminating at peak P–T conditions of ~650–680 °C and 9 kbar. Stage‐2 metamorphism and the waning development of a composite transposition foliation (ST) are dated at c. 118 Ma from monazite aligned parallel to ST, and inclusions in syn‐ to post‐ST staurolite and kyanite. Slightly younger ages (c. 112 Ma) are obtained from Y‐rich monazite that occurs within resorbed areas of both stage‐1 and stage‐2 garnet, together with retrograde staurolite and plagioclase. The younger ages obtained from these texturally and chemically distinct grains are interpreted, with the aid of phase equilibria calculations, to date the growth of monazite from the breakdown of garnet during decompression at c. 112 Ma. Evidence for continued near‐isothermal decompression is provided by the presence of retrograde sillimanite, and cordierite after staurolite, which indicates decompression below ~4–5 kbar prior to cooling below ~550 °C. As most other parts of the Yukon‐Tanana terrane were exhumed to upper crustal levels in the Early Jurassic, these data suggest this domain represents a tectonic window revealing a much younger, high‐grade tectono‐metamorphic core (infrastructure) within the northern Cordilleran orogen. This window may be akin to extensional core complexes identified in east‐central Alaska and in the southeastern Canadian Cordillera.  相似文献   

18.
Monazite in granulite facies metatexite migmatites (Christie Gneiss) hosting the Challenger Au deposit, South Australia, records a series of growth and resorption stages over a c. 60 Myr period between 2470 and 2410 Ma. A combination of electron microprobe X‐ray mapping and in situ ion‐microprobe dating was used to delineate and date five compositional domains. The oldest prograde metamorphic components are preserved in granoblastic gneisses surrounding the deposit, and as small high‐Y cores in large monazite grains in Au‐bearing migmatites. In metatexite leucosomes, these cores were partially resorbed prior to the growth of large high‐Th monazite domains that crystallized during partial melting and stromatic migmatite development at c. 2443 Ma. Subsequent heating to biotite dehydration conditions (c. 850 °C at 7 kbar) caused further partial melting roughly 10–15 Myr later, giving rise to c. 2428 Ma domains surrounding partly resorbed 2443 Ma grains that were entrained in the higher‐temperature melts. This period of partial melting coincided with isoclinal folding culminating in dextral transpression and represents the most likely window for remobilization of Au‐bearing polymetallic sulphide melts into low‐strain domains. Localized reaction of residual melt with the granulite facies assemblage during cooling gave rise to narrow high‐Y rims dated at 2414 ± 7 Ma. Although monazite from unmineralized granoblastic gneisses and migmatitic ore zones display the same range of U‐Pb dates, monazite in migmatites displays a higher overall Ca + Th + U content, indicating that compositional heterogeneities between ore zones and host rocks developed prior to 2470 Ma, perhaps a consequence of the hydrothermal alteration inferred to have accompanied gold mineralization.  相似文献   

19.
Several petrographic studies have linked accessory monazite growth in pelitic schist to metamorphic reactions involving major rock‐forming minerals, but little attention has been paid to the control that bulk composition might have on these reactions. In this study we use chemographic projections and pseudosections to argue that discrepant monazite ages from the Mount Barren Group of the Albany–Fraser Orogen, Western Australia, reflect differing bulk compositions. A new Sensitive High‐mass Resolution Ion Microprobe (SHRIMP) U–Pb monazite age of 1027 ± 8 Ma for pelitic schist from the Mount Barren Group contrasts markedly with previously published SHRIMP U–Pb monazite and xenotime ages of c. 1200 Ma for the same area. All dated samples experienced identical metamorphic conditions, but preserve different mineral assemblages due to variable bulk composition. Monazite grains dated at c. 1200 Ma are from relatively magnesian rocks dominated by biotite, kyanite and/or staurolite, whilst c. 1027 Ma grains are from a ferroan rock dominated by garnet and staurolite. The latter monazite population is likely to have grown when staurolite was produced at the expense of garnet and chlorite, but this reaction was not intersected by more magnesian compositions, which are instead dominated by monazite that grew during an earlier, greenschist facies metamorphic event. These results imply that monazite ages from pelitic schist can vary depending on the bulk composition of the host rock. Samples containing both garnet and staurolite are the most likely to yield monazite ages that approximate the timing of peak metamorphism in amphibolite facies terranes. Samples too magnesian to ever grow garnet, or too iron‐rich to undergo garnet breakdown, are likely to yield older monazite, and the age difference can be significant in terranes with a polymetamorphic history.  相似文献   

20.
Monazite is a common accessory phase in felsic granulite ribbon mylonites exposed in the Upper Deck domain of the Athabasca granulite terrane, western Canadian Shield. Field relationships, bulk rock geochemistry and phase equilibria modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 system are consistent with the garnet‐rich rocks representing the residual products of ultrahigh temperature melting of biotite‐bearing paragneisses driven by intraplating of mafic magma in continental lower crust. The c. 2.64–2.61 Ga Y‐rich resorbed monazite cores included in garnet are interpreted as relicts of detrital grains deposited on the Earth's surface after c. 2.61 Ga. Yttrium‐poor monazite domains in garnet are depleted in Sm and Gd and linked to fluid‐absent melting of biotite + plagioclase + quartz ± sillimanite during a prograde loading path from ≤0.8 to ≥1.4 GPa. The c. 2.61–2.55 Ga Y‐depleted, Th‐rich monazite domains crystallized in the presence of garnet + ternary feldspar ± orthopyroxene + peraluminous melt. The c. 2.58–2.52 Ga monazite rims depleted in Th + Ca and enriched in Eu are linked to localized melt extraction synchronous with growth of high‐pressure (HP) grossular‐rich garnet at the expense of plagioclase during crustal thickening, culminating at >950 °C. Re‐heating and dextral transpressive lower crustal reactivation at c. 1.9 Ga resulted in syn‐kinematic growth of (La + Ce)‐enriched monazite and a second generation of garnet, concurrent with recrystallization of feldspar and orthopyroxene at 1.0–1.2 GPa and 600–700 °C. Monazite grains in this study are marked by positive Eu‐anomalies relative to chondrite. A direct link is implied between Y, Sm, Eu and Gd in monazite and two major phases in continental lower crust: garnet and plagioclase. Positive Eu‐anomalies in lower crustal monazite associated with modally abundant garnet appear to be directly related to Eu‐enrichment and depletions of Y, Sm and Gd that are consequences of garnet growth and plagioclase breakdown during HP melting of peraluminous bulk compositions.  相似文献   

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