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This memoir highlights the scientific and academic contributions of Bor-ming Jahn who passed away in last December. He is best known for his ability to draw novel insights about the Earth’s tectonics, evolution, and geological processes from in-depth field and laboratory studies of isotopic and elemental geochemistry. He is recognized as a global leader in the applications of geochemical and isotope tracers (Sr-Nd-Hf-Pb-O) to regional tectonic evolution, and made outstanding contributions to the formation of dominant juvenile crust (>70%) for the Phanerozoic Central and Northeast Asian Orogenic Belts. He created the term “TTG” for tonalite-trondhjemite-granodiorite suites formed in ancient and present-day magmatic arc setting and is the first to date the oldest Chinese rocks and to constrain Triassic continental subduction for the Dabie-Sulu UHP terrane. He received numerous honors, and was the first Chinese Earth scientist to be the Editor-in-Chief of the celebrated Journal of Asian Earth Sciences , Elsevier. We are sad for his sudden passing; yet his scientific devotion and contributions will be remembered for decades to come. [ABSTRACT FROM AUTHOR]

Newly recognized occurrences of ultrahigh-pressure (UH P ) minerals including diamonds in ultrahigh-temperature (UH T ) felsic granulites of orogenic belts, in chromitites associated with ophiolitic complexes, and in mantle xenoliths suggest the recycling of crustal materials through deep subduction, mantle upwelling, and return to the Earth’s surface. This circulation process is supported by crust-derived mineral inclusions in deep-seated zircons, chromites, and diamonds from collision-type orogens, from eclogitic xenoliths in kimberlites, and from chromitities of several Alpine–Himalayan and Polar Ural ophiolites; some of these minerals contain low-atomic number elements typified by crustal isotopic signatures. Ophiolite-type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone. In addition to ringwoodite and inferred stishovite, a number of nanometric minerals have been identified as inclusions employing state-of-the-art analytical tools. Reconstitution of now-exsolved precursor UH P phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier UH P conditions. For example, Tibetan chromites containing exsolution lamellae of coesite + diopside suggest that the original chromitites formed at P > 9–10 GPa at depths of >250–300 km. The precursor phase most likely had a Ca-ferrite or a Ca-titanite structure; both are polymorphs of chromite and (at 2000 °C) would have formed at minimum pressures of P > 12.5 or 20 GPa respectively. Some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin, including zircon, feldspars, garnet, kyanite, andalusite, quartz, and rutile; the zircons possess much older U–Pb ages than the time of ophiolite formation. These UH P mineral-bearing chromitite hosts evidently had a deep-seated evolution prior to extensional mantle upwelling and partial melting at shallow depths to form the overlying ophiolite complexes. These new findings together with stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere, recycling of surface ‘organic’ carbon into the lower mantle, and ascent to the Earth’s surface through mantle upwelling. Intensified study of UH P granulite-facies lower crustal basement and ophiolitic chromitites should allow a better understanding of the geodynamics of subduction and crustal cycling. [ABSTRACT FROM AUTHOR]

Abstract: The Sanbagawa belt sensu lato in Japan is one of the world’s classical high-pressure (HP) metamorphic belts and has long been considered to be a coherent HP belt of the Cretaceous age. However, recent progress in detrital zircon geochronology together with the accumulation of abundant phengites K–Ar (Ar–Ar) ages revealed that the belt is comprised of two petrotectonic belts—Sanbagawa sensu stricto HP belt and Shimanto HP belt. These two belts have distinctly different ages of peak metamorphism and different P–T conditions of metamorphism. The Sanbagawa schists sensu stricto were metamorphosed in the conditions of the pumpellyite–actinolite facies through the epidote-blueschist to epidote–amphibolite facies and up to the eclogite facies during a prograde stage in a subduction zone at 120–115Ma. Phengite K–Ar geochronology revealed that the subsequent exhumation after the peak metamorphism took placed in a manner that the higher-grade rocks exhumed more rapidly in comparison with the lower-grade rocks; this is supported by a positive correlation between age and apparent metamorphic gradient that have formed during the post-metamorphic peak hydration/recrystallization. Moreover, the schists have experienced intense ductile deformation and long-term exhumation; it took longer than 31m.y. to reset the phengite K–Ar system. In contrast, the Shimanto HP schists were metamorphosed in the epidote-blueschist/greenschist transitional facies to the epidote–amphibolite facies and have experienced a short-term deformation for less than 13m.y. This short deformation formed a negative correlation between K–Ar age and apparent metamorphic gradient. These two contrasting age-temperature relationships suggest different exhumation processes between the two HP belts, most likely due to a change in subducting oceanic plates in the Cretaceous along the paleo-Japanese convergent margin. [Copyright &y& Elsevier]

Clinoenstatite is an uncommon mineral, and has been documented in Sulu ultrahigh-pressure (UHP) garnet pyroxenite. However, clinoenstatite (CLEN) was rarely reported in the previous studies of ophiolitic mantle peridotites. The studied ophiolitic peridotites are recovered from a drilling hole of the Luobusa ophiolitic massif, Tibet that is unique in occurrences of super reducing UHP minerals in both chromitite and peridotite. Our petrographic and TEM studies reveal abundant CLEN as lamellae associated with diopside exsolution in orthoenstatite (OREN) from the Luobusa mantle peridotite. In addition, termination of CLEN lamellae and Guinier-Preston zones (nanometer-scale precipitates with compositions between enstatite and diopside) are also observed in the OREN. Both CLEN lamellae and host OREN have nearly identical (Mg 0.9 Fe 0.1 ) 2 Si 2 O 6 compositions. The CLEN lamellae in host OREN from two core samples have even numbers of 9 Å (1 0 0) fringes without twins, and are oriented parallel to (1 0 0) of the host OREN. The crystallographic data of the CLEN are d 100 = 9.06 Å (R1), d 020 = 4.34 Å (R2), d 120 = 4.03 Å (R3), R1 ∠ R2 = 90°, R1 ∠ R3 = 64° and R2 ∠ R3 = 26°, whereas the host OREN have d 100 = 18.1 Å (R1), d 020 = 4.48 Å (R2), d 120 = 4.34 Å (R3), R1 ∠ R2 = 90°, R1 ∠ R3 = 77° and R2 ∠ R3 = 13°. No antiphase domain boundaries are observed in TEM images; it probably indicates that the clinoenstatite is the low- P polymorph of clinoenstatite (LCLEN) with P 2 1 / c symmetry. The exsolution of diopside and the presence of Guinier-Preston zones are attributed by cooling of primary high- T Ca-bearing (very minor) OREN. The transformation of LCLEN from OREN may have occurred mainly by cooling along the P-T boundaries between the OREN and LCLEN stability fields; shear stress only enhances the transition. The occurrence of CLEN lamellae in OREN and the existence of super reducing UHP minerals in peridotite provide an additional constraint for a very deep mantle origin of the Luobusa peridotite. [ABSTRACT FROM AUTHOR]

Abstract: Granitic leucosome and pegmatite are widely distributed within biotite-bearing orthogneiss in the northern part of the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. A combined study of mineral inclusions, cathodoluminescence (CL) images, U–Pb SHRIMP dates, and in situ trace element and Lu–Hf isotope analyses of zircons provided insight into the nature and timing of partial melting in these rocks. Zircon grains separated from biotite-bearing orthogneiss typically have three distinct domains: (1) pre-metamorphic (magmatic) cores with Qtz+Kfs+Pl+Ap inclusions, which record a Neoproterozoic protolith age of ∼790Ma, (2) mantles with Coe+Phe+Ap inclusions that record Triassic UHP age at 227±3Ma, and (3) narrow rims with quartz inclusions that record HP granulite-facies retrograde metamorphism at ∼210±3Ma. In contrast, zircons separated from granitic leucosome have only two distinct domains: (1) the central UHP areas with Coe+Phe+Ap inclusions record Triassic UHP age of 227±3Ma, and (2) outer magmatic areas with Qtz+Kfs+Ab+Ap inclusions that record partial melting time of 212±2Ma. Zircons separated from pegmatite contain mineral inclusions of Qtz+Kfs+Ap and show regular magmatic zoning from centre to edge. The centres record partial melting time of 212±2Ma in line with the outer domains of granitic leucosome, whereas the edges give a younger age of 201±2Ma related to Pb loss and partial recrystallization during late Triassic regional amphibolite-facies retrogression. These data indicate that partial melting in the north Sulu UHP gneissic rocks took place during post-UHP, retrograde HP granulite-facies metamorphism. Pre-metamorphic (magmatic) zircon cores from biotite-bearing orthogneiss give uniform 176Hf/177Hf of 0.28187±0.00003 (2SD; standard deviation) corresponding to εHf(790) and Hf model ages (T DM2) of about −16.3 and 2.41Ga, respectively. This is consistent with the generation of its protolith by reworking of Paleoproterozoic to late Archean crust. In contrast, UHP zircon domains from biotite-bearing orthogneiss and granitic leucosome are characterized by distinct trace element composition with low Lu/Hf (<0.006), low Th/U (<0.1) and considerably higher, 176Hf/177Hf (0.28233±0.00002; 2SD) than the pre-metamorphic cores. The uniform but significantly different Hf isotope composition between the UHP (εHf(227) =−14.6±0.8; 2SD) and pre-metamorphic (εHf(227) =−27.7) domains indicates equilibration of the Lu–Hf isotope system only within the UHP metamorphic mineral assemblage. The disequilibrium between whole rock and UHP zircon suggests that about two thirds of the whole rock Hf retained in the pre-metamorphic zircon domains. Zircon domains crystallized during partial melting at 212Ma in granitic leucosome and pegmatites have a Hf isotope composition indistinguishable from that of the UHP zircon domains. This suggests that only Hf (and Zr) equilibrated during UHP metamorphism was remobilized during partial melting while pre-metamorphic zircon remained stable or was not accessible. In contrast, the magmatic zircon edges from pegmatite have somewhat lower 176Hf/177Hf (∼0.28216) and εHf(t) (−17.6±1.2; 2SD) indicating some release of less radiogenic Hf for instance by dissolution of pre-metamorphic zircon during late regional amphibolite-facies retrogression. [Copyright &y& Elsevier]