Circum-cratonic mantle archives the cumulative effects of plume and convergence events

Abstract

The cumulative effects of heating, various melts/fluids, and tectonic stress from plume and convergent events can produce complex geochemical signatures in circum-cratonic mantle domains. The lithospheric mantle beneath the northwestern Tarim Craton witnessed the Tarim plume and multi-stage convergent events, thus providing a good opportunity to investigate the depletion and enrichment of circum-craton mantle driven by their cumulative effects. Here, we present the in-situ mineral major/trace elements, clinopyroxene Sr isotopic and whole-rock analyses of platinum group elements and Re-Os isotopes for peridotite xenoliths from Piqiang in the northwestern Tarim Craton. Combined with peridotite xenoliths from Tuoyun (South Tianshan Belt) and Xikeer (inner Tarim), we established a lithospheric mantle profile to constrain the formation and subsequent evolution of the circum-Tarim mantle in response to the subduction of the archipelagic ocean and the impingement of the Tarim plume. The Piqiang peridotites have moderately refractory compositions (Mg# in Ol: 90.1–90.9; average values) and can be divided into Group A lherzolites (subgroups A1, A2 and A3) and Group B wehrlite. Group A1 lherzolites contain copper-poor sulfides and clinopyroxenes depleted in incompatible elements, and are interpreted as melting residua that experienced negligible metasomatism. Group A2 lherzolites have disequilibrium textures and are depleted in Nb and Ta with high Ti/Eu but low (La/Yb)N ratios in clinopyroxene, demonstrating recent volatile-rich melt metasomatism. Carbonatitic metasomatism from a mixed crust-mantle source is recorded in the Group A3 lherzolites, as their clinopyroxenes have highly variable 87Sr/86Sr ratios (0.7028–0.7057) with low Ti/Eu and high (La/Yb)N. Overall, Group A lherzolites display high initial 187Os/188Os ratios (0.1198–0.1271, excluding suprachondritic values), yielding the Neoproterozoic peak (∼650 Ma) TRD eruption model ages and the minimum estimate for the age of lithospheric mantle formation of 1.1 Ga. Their high-Na clinopyroxenes, low-Ca orthopyroxenes, and low melting pressures (<3 GPa) suggest that the initial melt extraction of these peridotites was the result of lateral mantle accretion triggered by multi-stage subduction of the Neoproterozoic Pan-Rodina Ocean. In this case, the greater fertility of the Tuoyun peridotites compared to those from Piqiang defines a trend of lateral accretion, while the low whole-rock Al2O3 but high 187Os/188Os ratios of the Xikeer peridotites point to a second melt extraction event. This melt extraction event is interpreted as reflecting low-flux plume melts that heated and melted the overlying lithospheric mantle at the edge of the early Permian Tarim plume. Meanwhile, the Piqiang peridotites, further from the center of the Tarim plume, underwent carbonatitic metasomatism originating from the lower-flux plume melts that involved the high-density components from recycled oceanic crustal materials. The mantle melt activity and faulting related to the India-Eurasia collision provided the reactive melts to form the Group B wehrlite and prepared migration pathways for volatile-rich melts to metasomatize Group A2 lherzolites. Collectively, the accretion, re-extraction, and metasomatic modification of circum-craton mantle reflected a response to the cumulative effects of the plume and convergence events. Our study reveals that the mechanical, thermal, and chemical effects from different dynamic events interacted with each another and worked together to affect the circum-craton mantle evolution.