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As the melting residue of the asthenosphere at ocean ridges, the oceanic mantle lithosphere is initially depleted, which might be re-enriched by various types of melts. However, the metasomatic enrichments in the sub-ridge mantle lithosphere are generally assumed to be limited, because these melts have low abundances of incompatible elements. In this study, we report a case of extreme metasomatic enrichment in abyssal peridotites from the Atlantis Bank oceanic core complex on the Southwest Indian Ridge. We identify two types of metasomatized peridotites (Type 1 and 2) and a clinopyroxenite, which show different degrees of interaction with evolved melts. Type 1 peridotite exhibits strong incompatible element zonation and negative Eu anomalies in clinopyroxene, indicating a short-term percolation of a modestly evolved mid-ocean ridge basalt (MORB). Type 2 peridotite shows textural and trace element features consistent with the impregnation of a highly evolved, oxide-saturated MORB magma. The clinopyroxenite shows remarkable enrichments in iron and incompatible trace elements and negative Eu anomalies in clinopyroxene and contains abundant zircon and apatite, which imply reaction with an extremely evolved Fe-Zr-P-rich melt. We attribute such extreme mantle metasomatic enrichments to the lateral infiltration of variably evolved melts from a local magmatic center, which is now exposed as the Atlantis Bank gabbroic massif. As slow- and ultraslow-spreading ridges exhibit similarly discontinuous lower crust, the intervening shallow mantle between local magmatic centers may have been pervasively metasomatized by the evolved melts, leading the way for substantial mantle enrichment., Author(s): Wei-Qi Zhang [sup.1], Chuan-Zhou Liu [sup.1] [sup.2] [sup.3], Henry J. B. Dick [sup.4], Ross N. Mitchell [sup.1] [sup.3], Bo-Da Liu [sup.1] Author Affiliations: (1) grid.9227.e, 0000000119573309, State Key Laboratory [...]

Metamorphic timescales harbor crucial information for quantifying lithospheric processes and identifying tectonic regimes. The Orosirian Period is known for global orogenesis during 2.1â1.8 Ga that possibly signifies the emergence of global-scale plate tectonics. While the metamorphic pressure-temperature (P-T) paths and T/P ratios have been extensively used to characterize 2.1-1.8 Ga orogens, their detailed metamorphic timescales remain underutilized. In this study, we use garnet diffusion modeling to explicitly explore the metamorphic timescales of dominant pelitic granulites in the Jiaobei terrane, the southwest part of the Jiao-Liao-Ji orogenic belt (JLJB), North China Craton. The pelitic granulites record high-pressure granulite-facies metamorphism of 13-14 kbar and 860-870 °C, followed by fast decompression (4â21 mm yr.sup.-1) with some degree of heating to medium-pressure granulite-facies retrogression of 7â8 kbar and 865â950 °C at c. 1.85 Ga. The subsequent cooling is nearly isobaric, but likely nonlinear with initially faster and later slower cooling (17â32 °C Myr.sup.-1, 950/865-700 °C; 5-7 °C Myr.sup.-1, 700-600 °C, respectively). The determined clockwise P-T-t path provides evidence for convergent plate boundary processes and the fast exhumation is likely related to an extensional setting. The relatively fast exhumation and cooling rates at high temperatures, as obtained via diffusion chronometry, represent the fastest rates yet reported from 2.1 to 1.8 Ga orogens and are several times to one order of magnitude slower than those observed in modern orogens. In the light of recent numerical modeling, we suggest that these slower metamorphic rates of the c. 2.1â1.8 Ga JLJB orogenic rocks are consistent with the features of orogenesis occurring at higher mantle temperatures, such as hotter orogenic thermal structures, less over-thickened orogenic crusts, and distributed strain patterns. Therefore, the timescale information from the JLJB indicates that Paleoproterozoic plate tectonics under higher temperatures might have caused subduction and collisional orogenesis to be distinct from their modern style. Notably, the detailed ancient metamorphic timescales achieved by diffusion modeling cannot be resolved geochronologically, highlighting the necessity of applying diffusion chronometry to quantify ancient tectonic processes., Author(s): Yi Zou [sup.1] [sup.2], Qiuli Li [sup.1] [sup.2], Xu Chu [sup.3], Mingguo Zhai [sup.1] [sup.4], Ross N. Mitchell [sup.1] [sup.5], Lei Zhao [sup.1], Ligang Zhou [sup.1], Yuquan Wang [sup.1], [...]

Calibrating heavy rare-earth elements (HREE) partitioning between monazite and garnet is a direct way to link datable accessory phases with major minerals, and is therefore vital for the interpretation of geochronological data and the establishment of P-T-t paths. However, HREE partitioning (D.sub.HREE) data for monazite and garnet are lacking for high-temperature-ultrahigh-temperature (HT-UHT: 850-1000 °C) conditions and published data under sub-solidus conditions (600-750 °C) show large variations. We empirically determine D.sub.HREE+Y between monazite and garnet that formed during near isobaric cooling along a P-T-t path from ~ 980 °C to 830 °C at ~ 9 kbar from 1.90 to 1.85 Ga from 6 UHT aluminous granulite samples in the Khondalite Belt of North China Craton. The calculated HREE + Y partitioning results overlap at the hand specimen to microdomain scale, implying bulk chemical equilibrium between monazite and garnet. The D.sub.Y (20 ± 9) and D.sub.Dy (50 ± 17) can be used to differentiate equilibrated and unequilibrated monazite-garnet pairs, while D.sub.Gd is indistinguishable and D.sub.Er has a large uncertainty. Our newly calibrated D.sub.Y and D.sub.Dy, in combination with a compilation of previously published results, indicate different monazite behaviour under sub-solidus and supra-solidus conditions, which might be related to the exhaustion of xenotime. The newly calculated HREE + Y partitioning under HT-UHT conditions can be used as an additional guide to decipher polymetamorphic evolution in high-grade terranes, and provides a basis for future experimental study., Author(s): Shujuan Jiao [sup.1] [sup.2], Noreen J. Evans [sup.3] [sup.4], Ross N. Mitchell [sup.1] [sup.2], Ian C. W. Fitzsimons [sup.3], Jinghui Guo [sup.1] [sup.2] [sup.5] Author Affiliations: (1) grid.9227.e, 0000000119573309, [...]

Two hypotheses have been proposed for the organizing pattern of successive supercontinents. 'Introversion' is the model whereby the relatively young, interior ocean stops spreading and closes such that a successor [...], Traditional models of the supercontinent cycle predict that the next supercontinent--'Amasia'--will form either where Pangaea rifted (the 'introversion'1 model) or on the opposite side of the world (the 'extroversion' (2-4) models). Here, by contrast, we develop an 'orthoversion' (5) model whereby a succeeding supercontinent forms 90° away, within the great circle of subduction encircling its relict predecessor. A supercontinent aggregates over a mantle downwelling but then influences global-scale mantle convection to create an upwelling under the landmass (6). We calculate the minimum moment of inertia about which oscillatory true polar wander occurs owing to the prolate shape of the non-hydrostatic Earth (5,7).By fitting great circles to each supercontinent's true polar wander legacy, we determine that the arc distances between successive supercontinent centres (the axes of the respective minimum moments of inertia) are 88° for Nuna to Rodinia and 87° for Rodinia to Pangaea--as predicted by the orthoversion model. Supercontinent centres can be located back into Precambrian time, providing fixed points for the calculation of absolute palaeolongitude over billion-year timescales. Palaeogeographic reconstructions additionally constrained in palaeolongitude will provide increasingly accurate estimates of ancient plate motions and palaeobiogeographic affinities.