Search

Abstract: This study presents Lu–Hf geochronology of zoned garnet in high‐P eclogites from the North Qilian orogenic belt. Selected samples have ~mm‐sized garnet grains that have been sampled with a micro‐drill and analysed for dating. The Lu–Hf dates of bulk garnet separates, micro‐drilled garnet cores and the remnant, rim‐enriched garnet were determined by two‐point isochrons, with cores being consistently older than the bulk‐ and rim‐enriched garnet. The bulk garnet separates of each sample define identical garnet–whole rock isochron date of c. 457 Ma. Consistent U–Pb zircon dates of 455 ± 8 Ma were obtained from the eclogite. The Lu–Hf dates of the drilled cores and rim‐rich separates suggest a minimum garnet growth interval of 468.9 ± 2.4 and 452.1 ± 1.6 Ma. Major and Lu element profiles in the majority of garnet grains show well‐preserved Rayleigh‐style fractionated bell‐shaped Mn and Lu zoning profiles, and increasing Mg from core to rim. Pseudosection modelling indicates that garnet grew along a P–T path from ~470–525°C and ~2.4–2.6 GPa. The exceptional high‐Mn garnet core in one sample indicates an early growth during epidote–blueschist facies metamorphism at <460°C and <0.8 GPa. Therefore, the Lu–Hf dates of drilled cores record the early prograde garnet growth, whereas the Lu–Hf dates of rim‐rich fractions provide a maximum age for the end of garnet growth. The microsampling approach applied in this study can be broadly used in garnet‐bearing rocks, even those without extremely large garnet crystals, in an attempt to retrieve the early metamorphic timing recorded in older garnet cores. Given a proper selection of the drill bit size and a detailed crystal size distribution analysis, the cores of the mm‐sized garnet in most metamorphic rocks can be dated to yield critical constraints on the early timing of metamorphism. This study provides new crucial constraints on the timing of the initial subduction (before c. 469 Ma) and the ultimate closure (earlier than c. 452 Ma) of the fossil Qilian oceanic basin. [ABSTRACT FROM AUTHOR]

Garnet in metapelites from the Wölz and Rappold Complexes of the Austroalpine basement east of the Tauern Window typically shows two distinct growth zones. A first garnet generation usually forms the cores of garnet porphyroblasts and is separated by a prominent microstructural and chemical discontinuity from a second garnet generation, which forms rims of variable width. Whereas the rims were formed during the Eo-Alpine metamorphic overprint, the garnet cores represent remnants of at least two pre-Eo-Alpine metamorphic events. The pressure and temperature estimates obtained from garnet isopleth thermobarometry applied to the first growth increments of the pre-Eo-Alpine garnet cores from the Wölz and Rappold Complexes cluster into two distinct domains: (i) in the Wölz Complex, incipient growth of the first-generation garnet occurred at 4 ± 0.5 kbar and 535 ± 20 °C, (ii) in the Rappold Complex, incipient growth of the oldest garnet cores took place at 5.3 ± 0.3 kbar and 525 ± 15 °C. The Eo-Alpine garnet generation started to grow at 6.5 ± 0.5 kbar and 540 ± 10 °C. According to radiometric dating, the low-pressure garnet from the Wölz complex was formed during a Permian metamorphic event. The first-generation garnet of the Rappold Complex is probably of Variscan age. [ABSTRACT FROM AUTHOR]

New results on the pressure–temperature–time evolution, deduced from conventional geothermobarometry and in situ U-Th-total Pb dating of monazite, are presented for the Bemarivo Belt in northern Madagascar. The belt is subdivided into a northern part consisting of low-grade metamorphic epicontinental series and a southern part made up of granulite facies metapelites. The prograde metamorphic stage of the latter unit is preserved by kyanite inclusions in garnet, which is in agreement with results of the garnet (core)-alumosilicate-quartz-plagioclase (inclusions in garnet; GASP) equilibrium. The peak metamorphic stage is characterized by ultrahigh temperatures of ∼900–950 °C and pressures of ∼9 kbar, deduced from GASP equilibria and feldspar thermometry. In proximity to charnockite bodies, garnet-sillimanite-bearing metapelites contain aluminous orthopyroxene (max. 8.0 wt% Al2O3) pointing to even higher temperatures of ∼970 °C. Peak metamorphism is followed by near-isothermal decompression to pressures of 5–7 kbar and subsequent near-isobaric cooling, which is demonstrated by the extensive late-stage formation of cordierite around garnet. Internal textures and differences in chemistry of metapelitic monazite point to a polyphasic growth history. Monazite with magmatically zoned cores is rarely preserved, and gives an age of c. 737 ± 19 Ma, interpreted as the maximum age of sedimentation. Two metamorphic stages are dated: M1 monazite cores range from 563 ± 28 Ma to 532 ± 23 Ma, representing the collisional event, and M2 monazite rims (521 ± 25 Ma to 513 ± 14 Ma), interpreted as grown during peak metamorphic temperatures. These are among the youngest ages reported for high-grade metamorphism in Madagascar, and are supposed to reflect the Pan-African attachment of the Bemarivo Belt to the Gondwana supercontinent during its final amalgamation stage. In the course of this, the southern Bemarivo Belt was buried to a depth of >25 km. Approximately 25–30 Myr later, the rocks underwent heating, interpreted to be due to magmatic underplating, and uplift. Presumably, the northern part of the belt was also affected by this tectonism, but buried to a lower depth, and therefore metamorphosed to lower grades. [ABSTRACT FROM AUTHOR]