Qiong-Yan Yang

Late Palaeoproterozoic post-collisional magmatism in the North China Craton: geochemistry, zircon U–Pb geochronology, and Hf isotope of the pyroxenite–gabbro–diorite suite from Xinghe, Inner Mongolia

The North China Craton (NCC) witnessed a prolonged subduction–accretion history from the early to late Palaeoproterozoic, culminating with final collision at ca. 1.85 Ga and assembling the continental blocks into the cratonic framework. Subsequently, widespread post-collisional magmatism occurred, particularly along the Trans-North China Orogen (TNCO) that sutures the Eastern and Western blocks of the NCC. Here we present petrological, geochemical, and zircon U–Pb geochronological and Lu–Hf data from a pyroxenite (websterite)–gabbro–diorite suite at Xinghe in Inner Mongolia along the northern segment of the TNCO. The internal structures and high Th/U values of the zircons from the gabbro–diorite suite suggest magmatic crystallization. LA-ICP-MS U–Pb age data on three gabbros and one diorite from the suite yield emplacement ages of 1786.1 ± 4.8, 1783 ± 15 ,1754 ± 16 and 1767 ± 13 Ma, respectively. The εHf(t) shows mostly positive values (up to 5.8), with the lowest value at –4.2, suggesting that the magma was derived from dominantly juvenile sources. The generally low SiO2 and high MgO values, and other trace element features of the Xinghe suite are consistent with fractionation from a mantle-derived magma with a broadly E-MORB affinity, with no significant crustal contamination. Recent studies clearly establish that the major magmatic pulse associated with rifting of the NCC within the Columbia supercontinent occurred in the late Mesoproterozoic at ca. 1.3–1.2 Ga associated with mantle plume activity. This, together with the lack of robust geochemical imprints of rift-related magmatism in the Xinghe suite, prompts us to suggest a tectonic model that envisages magma genesis associated with post-collisional extension during slab break-off, following the westward subduction of the Eastern Block and its collision with the Western Block. The resulting asthenospheric upwelling and heat input might have triggered the magma generation from a heterogeneous, subduction-modified sub-lithospheric mantle source for the Xinghe rocks, as well as for similar late Palaeoproterozoic suites in the TNCO.

Proto-Japan and tectonic erosion: Evidence from zircon geochronology of blueschist and serpentinite

Blueschist facies rocks in the Shikoku island of southwest Japan were extruded from depth as blocks in the Kurosegawa serpentinite melange, where they occur in association with other tectonic blocks of tonalite-trondhjemite-granodiorite (TTG) rocks and calc-alkaline volcanics. Here we report magmatic zircons in the blueschist that yield 206Pb/238U mean ages in the range 505 ± 3 to 503 ± 3 Ma. These zircons crystallized at a mid-ocean ridge within basaltic rocks, and migrated to the continental margin of paleo-Asia. Subduction of the oceanic lithosphere into the mantle at a depth of ∼40 km resulted in metamorphism under jadeite-glaucophane facies conditions at 250 Ma. The blueschist facies rocks were then exhumed to the surface along a splay fault connected with the forearc region through tectonic extrusion. Zircons separated from serpentinite yielded a 206Pb/238U mean age of 152 ± 3 Ma, marking the timing of serpentine protrusion to the surface, consistent with geologic constraints. The serpentinites also contain minor ca. 500 Ma zircons, similar to those in the blueschist. The predominance of detrital ca. 150 Ma zircons in the serpentinite suggests extensive tectonic mixing of Jurassic trench turbidites with the mantle wedge. The Kurosegawa serpentinite melange belt extends along the strike of the orogenic belt for more than 800 km, parallel to the active trench. We propose that this belt marks the location of extensive tectonic erosion and provides an excellent case study for the destruction of continental crust along a convergent plate boundary.

Rapid oxygen diffusion during high temperature alteration of zircon

The mineral zircon through its isotopic and elemental signatures comprises the greatest archive recording the evolution of Earth’s continental crust. Recognising primary from secondary zircon compositional signatures is thus important for the accurate interpretation of this archive. We report two examples of metasedimentary rocks from high-grade shear zones within the Southern Granulite Belt of India, where anomalously high and homogeneous oxygen isotope signatures indicate disturbance of this isotopic system. Utilising the combined U-Pb-Hf-O and trace element signatures from these zircon grains, we postulate that fluid-assisted alteration has led to complete resetting of the oxygen isotope signatures. This case study presents a rarely observed natural example of potentially fast diffusion of oxygen under hydrous conditions. Given the pervasive nature of fluid interaction within high-grade and highly deformed rocks, we expect that such isotopic disturbance might be more common to nature than is currently reported. A lack of correlation between isotopic disturbance with cathodoluminescence or Th/U values, suggests that these altered zircon grains would not clearly be classified as metamorphic, in which case they would be expected to yield primary compositions. Caution is therefore advised when using detrital δ18O zircon compilations without a high level of scrutiny for primary versus secondary compositions.