Kenshi Maki

Tectonic erosion in a Pacific-type orogen: Detrital zircon response to Cretaceous tectonics in Japan

U-Pb dating of detrital zircons from the Lower Cretaceous Sanbagawa and the recently recognized Upper Cretaceous Shimanto high-pressure (HP) metamorphic rocks in southwestern Japan has revealed the presence of abundant Proterozoic (ca. 1500–2000 Ma) detrital grains. In contrast, coeval non- to weakly metamorphosed accretionary complex (AC) and forearc basin sediments in southwestern Japan lack these older signatures. The only possible source of the Proterozoic detrital grains is the Jurassic AC in southwestern Japan, which structurally overlies the Cretaceous HP units. The Proterozoic grains were incorporated into the protoliths of HP-ACs, without polluting coeval forearc basin to trench sediments, likely by tectonic erosion in the forearc domain. Along the Cretaceous Wadati-Benioff plane, the tectonic erosion peeled off the sole part of the pre-existing forearc crust and mixed it with the subducting trench sediments prior to the peak HP metamorphism. In the Cretaceous subduction-related margin around Japan, the tectonic erosion likely occurred twice.

Hf isotope and REE compositions of zircon from jadeitite (Tone, Japan and north of the Motagua fault, Guatemala): implications on jadeitite genesis and possible protoliths

Zircon separates from one jadeitite sample (JJ) from Tone, Japan and one from Guatemala (GJ) were studied for mineral inclusions, age dating, trace-element determination and Hf isotope analysis. These zircons can be categorized into two types. Type I (igneous) zircons are characterized by the presence of mineral inclusions, among others K-feldspar, which is not present in jadeitite matrix. They also show higher Th/U ratios, larger Ce anomalies and higher 176 Lu/ 177 Hf ratios. Type II (metasomatic/solution-precipitate) zircons contain omphacite/jadeite inclusions and exhibit lower Th/U ratios, smaller Ce anomalies and lower 176 Lu/ 177 Hf ratios. Both types of zircons display high eHf( t ) values, slightly lower than the depleted mantle evolution line. The JJ sample contains both type I and II zircons. SHRIMP and geochemical data indicate that this jadeitite sample was formed through the mechanism of whole-sale metasomatic replacement at ~80 Ma from an igneous protolith of juvenile origin with an age of 136 ± 2 Ma. The GJ sample contains only type II zircons and may have formed through a mechanism of, or close to, vein precipitation at 98 ± 2 Ma. The two samples therefore testify that both mechanisms may have been in operation during jadeitite formation. Based on Hf isotope composition of type I zircons and the back-calculated REE pattern of the presumed protolith, the geochemical characteristics of the protolith of the Tone jadeitite were shown to be similar to those of oceanic plagiogranites derived from partial melting of cumulate gabbros or subduction-zone adakitic granites originated from partial melting of subducted oceanic crust. The latter, however, is a more probable candidate because the former is known to be poor in K 2 O, which, in contrast, is a notable chemical component in Tone jadeitite. On the basis of the available data, it is also suggested that the protolith, the physicochemical conditions and the extent of jadeitization may all play a role in dictating the chemical variations of jadeitites.

Origin and Metamorphic Evolution of Garnet Clinopyroxenite from the Sulu UHP Terrane, China: Evidence from Mineral Chemistry and Microstructures

Publisher Summary Garnet clinopyroxenite lenses or layers are common in orogenic peridotites, and also found as xenoliths in volcanic rocks and kimberlites. Pyroxenites have multiple origins, such as crystal liquid accumulation, refertilization of peridotite due to melt infiltration, subduction of oceanic crust and recycling of lower crystal cumulate and restites, and metasomatic process. In the DabieSulu ultrahigh-pressure (UHP) terrane, garnet pyroxenite can be divided into two types: as lenses or thin layers in mantle-derived peridotite, such as in the Xugou and Yangkou garnet peridotites and as a member of crust-hosted mafic and ultramafic cumulate complex, such as Bixiling body. Type (1) pyroxenite has a mantle origin. Type (2) pyroxenite is a cumulate resulting from differentiation of a basic magma. In addition, some pyroxene layers have been attributed to metasomatism. The Hujialing garnet clinopyroxenite occurs as lenses within a dunite body that is located in the southeastern Rizhao city (RZ) of Shandong Province of eastern China. The unique Hujialing Grt clinopyroxenite provides an opportunity to study origin of lamellar phases, mineral behavior, and rock textural and structural readjustment during subduction-zone metamorphism; this in turn provides insights into the juxtaposition processes of mantle and crust during the collision of the Yangtze and Sino-Korean cratons.