The duration of granulite (G) and ultra–high temperature (UHT) conditions in regional metamorphism is critical to arguments regarding the tectonic settings of granulites and their relationships to Supercontinent assembly. Analysis of zircon geochronology integrated with trace element (REE) constraints on the timing of zircon growth or modification and evidence for metamorphic temperatures from Ti–in–zircon reveals that zircon can form episodically at or near the metamorphic peak in long–lived UHT granulites. This reflects the segregation, transfer and stagnation or trapping of melts which leads to local melt–rock interactions that promotes zircon crystallisation. In contrast, although rutile can retain UHT information in short duration (‘fast’) G–UHT terrains it is afflicted by Zr loss in long duration (‘slow’) terrains and yields temperatures significantly lower than those preserved by zircons formed in the same G–UHT events. An assessment of the age–duration evidence from several well–documented G–UHT terrains reveals that most are ‘slow’, having durations of UHT, Δt900, in the range 30–100 Myr. Some UHT terrains previously considered to be short–lived (Δt900 < 10 Myr) have longer durations of UHT in the light of recent geochronology and hence are also classed as ‘slow’. Of the models proposed to account for UHT metamorphism, the ‘large hot orogen’ (LHO) model for collisional orogeny provides the best setting for the formation of such ‘slow’ UHT granulites. LHO models can account not only for the P–T paths, which can range from UHT with near–isothermal decompression (UHT–ITD) through to decompression–cooling and UHT followed by near–isobaric cooling (UHT–IBC), but also for residence times under UHT conditions. The Napier Complex, the Earth’s premier UHT terrain, probably formed as trapped deep crust in the hot underbelly of a late–Archaean LHO. Shorter duration UHT and near–UHT granulites also exist, mostly with Δt900 less than 10 Myr. A number of these are likely to have formed as a consequence of severe lithospheric thinning and crustal extension accompanied by voluminous magmatism, which could occur in arc and back–arc settings affected by subduction roll–back or, as advocated by previous workers, continental arcs undergoing extension. However, attaining long–lived UHT conditions in these settings is unlikely unless the crust has inherently high radioactive heat production in addition to the transient heat added during extension and magmatism.
Granulites of the southern East African Orogen formed by continental collision during Gondwana assembly. The highest metamorphic gradients of 25–50 °C km−1 were attained at 0.58–0.53 Ga in a microcontinental block that was sandwiched between two collisional sutures and is now exposed in Madagascar and southern India. The 50 Myr duration of extreme pressure–temperature (P–T) conditions and lack of coeval mantle magmatism suggest that metamorphism was driven by radiogenic heat accumulation beneath a long–lived orogenic plateau. Bounding sutures most likely record transfer of this microcontinent across the Neoproterozoic Mozambique Ocean, analogous to Gondwanan terranes that crossed Tethys before final India–Asia collision and, like Tibet, these sutures mark the edges of a plateau that formed following terminal ocean closure and collision. Both sutures record moderate metamorphic gradients of 15–25 °C km−1 but with quite different ages. Metamorphism along the western suture at 0.65–0.61 Ga followed the end of magmatism in an adjacent 0.85–0.65 Ga ocean–arc terrane. It has an anti–clockwise P–T path that reflects preferential thickening of the hot arc during early stages of collision, and dates ocean closure at the western suture. Metamorphism along the eastern suture at 0.53–0.51 Ga has a clockwise P–T path and is widely assumed to date terminal collision in the East African Orogen. However, this event was coeval with rapid exhumation of granulites in the adjacent plateau and is more likely to reflect reactivation of a much older eastern suture during plateau collapse. Great care should be taken when using metamorphism to date ocean closure in ancient orogens. Rocks with hot metamorphic gradients give poor age constraints on initial collision because peak T is attained >50 Myr after ocean closure if radioactivity is a major part of the heat budget. Suture zone rocks with moderate metamorphic gradients can provide more reliable estimates for the time of ocean closure but are also prone to later reactivation in orogens with protracted histories.
Zircon and monazite in ultrahigh temperature (UHT) metamorphic rocks from the Rauer Islands of Prydz Bay in East Antarctica were investigated in terms of U–Th–Pb and rare earth elements (REE) chemistry along with textural context. All five analyzed samples, three from the Mather Paragneiss UHT unit and two from the host orthogneiss unit yield 522–517 Ma concordant zircon ages, with older protolith/inherited zircon ages of 3268 and 2800–2400 Ma along with highly discordant Mesoproterozoic to Neoproterozoic ages. Our data confirm the Archaean protolith age for the host orthogneiss surrounding the UHT Mather Paragneiss. The Archaean and Mesoproterzoic components of the Rauer Islands were not amalgamated in the Rauer Tectonic Event at 1030–990 Ma, and deposition of the Mather Paragneiss was considered at some time after the Rauer Tectonic Event. In contrast to the well–defined 520 Ma ages obtained from the zircons in the UHT rocks, monazite grains measured by electron microprobe show a distinct internal zonation, from 580–560 Ma dark–backscattered electron image (BSE) cores enriched in middle rare earth elements (MREE) and heavy rare earth elements (HREE) to 550–520 Ma mid–BSE mantles and 510–500 Ma bright–BSE rims. From the chemical and textural evidence we infer that the MREE–HREE–rich 580–560 Ma monazite cores may have formed through the decomposition of garnet during decompression just after the UHT event, whereas the MREE–HREE–depleted 550–500 Ma monazite grains/rims formed or recrystallized in reactions associated with subsequent extensive hydration during the upper–amphibolite to granulite–facies main Prydz Tectonic Event, which also caused marked recrystallization of zircon. The above data strongly support the interpretation that the UHT metamorphism occurred prior to 590–580 Ma.
U–Pb zircon geochronology was applied to nine metasedimentary samples collected from Mt. Yuzhnaya, Condon Hills, and Mt. Lira in the inland region of the Rayner Complex of western Enderby Land, East Antarctica, in order to define the eastern limits of the western Rayner Complex that underwent the Pan–African metamorphism and to evaluate potential source areas of metasedimentary rocks. Condon Hills and Mt. Lira revealed metamorphic ages of ~ 894 and ~ 934 Ma, respectively, which are consistent with previously reported metamorphism in association with Rayner Structural Episode (RSE). Mt. Yuzhnaya samples affected by the RSE contain zircon grains rejuvenated during 590–570 Ma, which indicates that the Pan–African reworking can be extended up to Mt. Yuzhnaya. On the other hand, the Condon Hills samples include Archean detritus, and the age peaks from 3850 to 2491 Ma are the oldest components in the Rayner Complex of western Enderby Land. There is no evidence of reworked Napier Complex rocks in the studied Rayner samples.
The Sør Rondane Mountains in East Antarctica consist of the various metamorphic rocks and several plutonic rocks, which are related to formation of Gondwana supercontinent. In order to understand the detrital provenances, LA–ICP–MS zircon U–Pb dating were conducted from metamorphosed sedimentary rocks in southern Sør Rondane Mountains. Detrital cores with ~ 990–730 Ma ages were recognized from zircons in the pelitic gneisses and calc–silicate rocks from Menipa, Imingfjella, Mefjell and Arden, while the pelitic gneiss from Tvihøgda exhibited the Archean to Mesoproterozoic ages of ~ 2630–1060 Ma. The Neoproterozoic zircon ages are recognized not only from metamorphosed sedimentary rocks but also from metamorphosed igneous rocks distributed in the central and southern parts of the mountains. However, the Archean to Mesoproterozoic zircon ages are rare in these areas. The results imply the detrital zircons with the Neoproterozoic ages were probably derived from neighbouring igneous rocks (present metamorphosed igneous rocks), while those with Archean to Mesoproterozoic ages have clearly different provenances because of the lack in the Neoproterozoic components. This study suggests the possibility that southern Sør Rondane Mountains can be considered to be juvenile late Mesoproterozoic to Neoproterozoic terrane partly mixed with the Archean to Mesoproterozoic components.
Chlorine–rich (>0.3 wt%Cl) biotite inclusions in the core of garnet porphyroblasts in the garnet–sillimanite (Grt–Sil) gneiss from Skallevikshalsen, Lützow–Holm Complex (LHC), East Antarctica is estimated to be stable under >1.2 GPa, 820–850 °C, coexisted with granitic melt as suggested by the nanogranite/felsite inclusions. Rare occurrence of matrix biotite, in spite of the common occurrence of biotite as inclusions in garnet, suggests almost complete consumption of pre–existed matrix biotite during the prograde to peak metamorphism. Brine infiltration during prograde to peak metamorphism in Skallevikshalsen is supported by Cl–rich scapolite described in previous studies. Brine infiltration and progress of continuous biotite–consuming melting reactions were probably responsible for elevating the Cl content of biotite in the studied sample. In situ electron microprobe U–Th–Pb dating of monazite and the in situ laser ablation inductively coupled plasma mass spectrometry (LA–ICPMS) U–Pb dating of zircon in the Grt–Sil gneiss revealed that both monazite and zircon has the ‘older age population’ with ~ 650–580 Ma and the ‘younger age population’ with ~ 560–500 Ma. The REE and trace element pattern of one of the P–rich patches in the garnet core is different from the P–rich garnet rim. The isotope mapping of the same patch by LA–ICPMS revealed that the patch is also observed as a domain depleted in 51V, 89Y, 165Ho, 166Er, 169Tm, 172Yb, and 175Lu. Clear difference in 51V concentration between the patch and the garnet rim suggests that this patch is not a continuous part from the garnet rim, but is likely a relic of preexisted garnet. Kyanite included in the patch suggests that the precursor rock was presumably a medium– to high–pressure type metamorphic rock. Presence of the older age population (~ 650–580 Ma) monazites in Skallevikshalsen and Skallen also suggest that rocks in these areas experienced polymetamorphism, and resetting by the ~ 560–500 Ma metamorphic event was incomplete in these areas. Taking into account the presence of Cl–rich biotite inclusions in garnet, infiltration of brine accompanied by partial melting is one probable event that took place at ~ 560–500 Ma in the Skallevikshalsen area, and part of the monazite possibly recrystallized by this brine infiltration. Detailed microstructural observation using trace element mapping combined with detailed petrography especially focusing on the Cl–bearing minerals as a tracer of brines would become a powerful tool for better interpreting the results of monazite and zircon dating and for investigating the fluid–related crustal processes.
Vol.31 (1944) No. 5 and No.6 in the predecessor journal ″The Journal of the Japanese Association of
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