Lingmin Zhang

Jurassic zircons from the Southwest Indian Ridge

The existence of ancient rocks in present mid-ocean ridges have long been observed but received less attention. Here we report the discovery of zircons with both reasonably young ages of about 5 Ma and abnormally old ages of approximate 180 Ma from two evolved gabbroic rocks that were dredged from the Southwest Indian Ridge (SWIR) in the Gallieni fracture zone. U–Pb and Lu–Hf isotope analyses of zircons were made using ion probe and conventional laser abrasion directly in petrographic thin sections. Young zircons and their host oxide gabbro have positive Hf isotope compositions (εHf = +15.7–+12.4), suggesting a highly depleted mantle beneath the SWIR. The spread εHf values (from−2.3 to−4.5) of abnormally old zircons, together with the unradiogenic Nd-Hf isotope of the host quartz diorite, appears to suggest an ancient juvenile magmatism along the rifting margin of the southern Gondwana prior to the opening of the Indian Ocean. A convincing explanation for the origin of the unusually old zircons is yet to surface, however, an update of the theory of plate tectonics would be expected with continuing discovery of ancient rocks in the mid-oceanic ridges and abyssal ocean basins.

Pseudosection modelling and garnet Lu–Hf geochronology of HP amphibole schists constrain the closure of an ocean basin between the northern and southern Lhasa blocks, central Tibet

The P–T-t path of high-pressure metamorphic rocks in subduction zones may reveal valuable information regarding the tectonic processes along convergent plate boundaries. Herein, we present a detailed petrological, pseudosection modelling and radiometric dating study of several amphibole schists of oceanic affinity from the Lhasa Block, Tibet. The amphibole schists experienced an overall clockwise P–T path that was marked by post-Pmax heating–decompression and subsequent isothermal decompression following the attainment of peak high-pressure and low-temperature conditions (~490 °C and 1.6 GPa). Pseudosection modelling shows that the amphibole schists underwent water-unsaturated conditions during prograde metamorphism, and the stability field of the assemblage extends to lower temperatures and higher pressures within the water-unsaturated condition relative to water-saturated model along the prograde path. The high-pressure amphibole schists were highly reduced during retrograde metamorphism. Precise evaluation of the ferric iron conditions determined from the different compositions of epidote inclusions in garnet and matrix epidote, is crucial for a true P–T estimate by garnet isopleth thermobarometry. Lu–Hf isotope analyses on garnet size separates from a garnet-bearing amphibole schist yield four two-point garnet–whole rock isochron ages from 228.2 ± 1.2 Ma to 224.3 ± 1.2 Ma. These Lu–Hf dates are interpreted to constrain the period of garnet growth and approximate the timing of prograde metamorphism because of the low peak metamorphic temperature of the rock and the well-preserved Mn/Lu growth zoning in garnet. The majority of zircon U–Pb dates provide no constraints on the timing of metamorphism, however, two concordant U–Pb dates of 222.4 ± 3.9 Ma and 223.3 ± 4.2 Ma were obtained from narrow and uncommon metamorphic rims. Coexistence of zircon and sphene in the samples implies that the metamorphic zircon growth was likely assisted by retrogression of rutile to sphene during exhumation. The near coincident radiometric dates of zircon U–Pb and garnet Lu–Hf indicate rapid burial and exhumation of the amphibole schists, suggesting a closure time of c. 224–223 Ma for the fossil ocean basin between the northern and southern Lhasa blocks. This article is protected by copyright. All rights reserved.

Unusual replacement of Fe-Ti oxides by rutile during retrogression in amphibolite-hosted veins (Dabie UHP terrane): A mineralogical record of fluid-induced oxidation processes in exhumed UHP slabs

Abstract The replacement of rutile by Fe-Ti oxides is a common phenomenon during the retrogression of eclogites. Here, we report an unusual case regarding the replacement of Fe-Ti oxides by rutile during greenschist-facies metamorphic overprinting of veins in amphibolites (retrograded eclogites) from the Dabie ultrahigh-pressure (UHP) terrane, eastern China. The veins mainly consist of plagioclase, Fe-Ti oxides, and quartz, which crystallized from a Ti-rich amphibolite-facies fluid that formed during exhumation of the eclogites. Two types of textures involving the replacement of Fe-Ti oxides by rutile are recognized in the veins: (1) the first type is characterized by the development of rutile coronas (Rt-C) and other silicates (high-Fe epidote, muscovite, and chlorite) around the external boundaries of the Fe-Ti oxide grains, and (2) the second type is characterized by the formation of symplectitic intergrowths of rutile (Rt-S) and magnetite after exsolved hemo-ilmenite (H-Ilm) lamellae in the Fe-Ti oxides. The micro-textures, mineral assemblages, and Zr-in-rutile thermometry indicate that both replacement reactions involved mineral re-equilibration processes in the presence of an infiltrating fluid phase at ~476–515 °C, taking place by an interface-coupled dissolution-precipitation mechanism. Thermodynamic modeling reveals that both replacement reactions occurred during oxidation processes under relatively high-oxygen fugacity (fo2) conditions, approximately 2.5–4.5 logfo2 units higher than the fayalite-magnetite-quartz (FMQ) reference buffer. In situ Sr isotopic analyses of epidote (Ep-C) coexisting with the Rt-C suggest that the infiltrating fluid involved in the greenschist-facies replacement reactions was externally derived from the surrounding granitic gneisses (the wall rocks of the amphibolites). Compared with the rutile in the UHP eclogites (Rt-E) and amphibolites (Rt-A), the Rt-C is characterized by distinctly lower contents of Nb (<10 ppm) and Ta (<2 ppm) and Nb/Ta ratios (<10) and higher contents of Cr (>340 ppm) and V (>1580 ppm). These results provide a geochemical fingerprint for distinguishing the low-pressure (LP) rutile from relic high-grade phases in retrograded HP-UHP rocks. Our results reveal that rutile can form during LP retrograde stage in UHP rocks by high-fo2 fluid-induced replacement reactions. The unusual replacements of Fe-Ti oxides by rutile-bearing assemblages during retrogression provide important constraints on fluid-mineral reactions and fo2 variations in exhumed UHP slabs.

Increasing Chlorinity in Fluids Along the Prograde Metamorphic Path: Evidence of Apatite from Yangkou Eclogite, Sulu, China

The apatites in eclogitized rocks at Yangkou from the Sulu ultrahigh-pressure metamorphic terrane are studied in this chapter. Along the prograde metamorphic path, three types of rocks were formed: coronal metagabbro, coronal eclogite, and coesite-bearing eclogite. The apatites in coronal metagabbro are preexisting gabbroic minerals with low Cl content. The apatites coexisting with eclogitic minerals in coronal eclogite shows the same compositions as those in coronal metagabbro. However, some apatites in coesite-bearing eclogite, with increasing Cl zoning from core to rim, show evidence for enrichment of Cl and OH at the expense of F at first, then further enrichment of Cl at the expense of F and OH at the ultrahigh-pressure stage. We infer that these high Cl apatites were formed either through reequilibration with brines, or precipitation from such brines during ultrahigh-pressure metamorphic process, and that the brines had higher chlorinity than those at the high-pressure stage. The increasing chlorinity is thought to reflect Cl enrichment in fluids during prograde metamorphic evolution. Loss of water by growth of phengite in gneissic rocks can account for the increasing salinity of the fluid during the subduction process of continental crust.

Microsampling Lu-Hf geochronology on mm-sized garnet in eclogites constrains early garnet growth and timing of tectonometamorphism in the North Qilian orogenic belt

Handling Editor: Doug Robinson 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 bulkand 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 garnetbearing 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.