Asthenosphere-induced melting of diverse source regions for East Carpathian post-collisional volcanism

Antoine J J Bracco Gartner,Ioan Seghedi,Igor K Nikogosian,Paul R D Mason

doi:10.1007/s00410-020-01690-4

Abstract

The occurrence of post-subduction magmatism in continental collision zones is a ubiquitous feature of plate tectonics, but its relation with geodynamic processes remains enigmatic. The nature of mantle sources in these settings, and their interaction with subduction-related components, are difficult to constrain using bulk rocks when magmas are subject to mixing and assimilation within the crust. Here we examine post-collisional magma sources in space and time through the chemistry of olivine-hosted melt inclusions and early-formed minerals (spinel, olivine and clinopyroxene) in primitive volcanic rocks from the Neogene–Quaternary East Carpathian volcanic range in Călimani (calc-alkaline; 10.1–6.7 Ma), Southern Harghita (calc-alkaline to shoshonitic; 5.3–0.03 Ma) and the Perșani Mountains (alkali basaltic; 1.2–0.6 Ma). Călimani calc-alkaline parental magma compositions indicate a lithospheric mantle source metasomatised by ~ 2% sediment-derived melts, and are best reproduced by ~ 2–12% melting. Mafic K-alkaline melts in Southern Harghita originate from a melt- and fluid-metasomatised lithospheric mantle source containing amphibole (± phlogopite), by ~ 5% melting. Intraplate Na-alkaline basalts from Racoș (Perșani) reflect small-degree (1–2%) asthenosphere-derived parental melts which experienced minor interaction with metasomatic components in the lithosphere. An important feature of the East Carpathian post-collisional volcanism is that the lithospheric source regions are located in the lower plate (distal Europe-Moesia), rather than the overriding plate (Tisza-Dacia). The volcanism appears to have been caused by (1) asthenospheric uprise following slab sinking and possibly south-eastward propagating delamination and breakoff, which induced melting of the subduction-modified lithospheric mantle (Călimani to Southern Harghita); and (2) decompression melting as a consequence of minor asthenospheric upwelling (Perșani).

Highlights

A striking feature of plate tectonics is the occurrence of volcanism near continental plate boundaries after collision
A relatively recent occurrence of post-collisional volcanism is found in the Neogene–Quaternary East Carpathian volcanic range (Fig. 1), where it postdates the westward subduction of a small ocean basin beneath the Tisza-Dacia microplate
We infer that metasomatic enrichment of the mantle by hydrous fluids dehydrated from the subducted slab (e.g. Castillo 2008), which produces a LILE-enriched mantle mineralogy including amphibole (± phlogopite), is the principal cause of the anomalously potassic, adakite-like signatures in Southern Harghita

Summary

Introduction

A striking feature of plate tectonics is the occurrence of volcanism near continental plate boundaries after collision. Such post-collisional volcanic activity is often ascribed to thermal perturbation or decompression as a consequence of asthenospheric upwelling following slab breakoff (von Blanckenburg and Davies 1995), delamination (Bird 1979) or slab tearing Whilst post-collisional volcanism cryptically reflects the physical process by which it was induced and, to some extent, the regional. A relatively recent occurrence of post-collisional volcanism is found in the Neogene–Quaternary East Carpathian volcanic range (Fig. 1), where it postdates the westward subduction of a small ocean basin beneath the Tisza-Dacia microplate. Magmatic activity gradually shifted from northwest (~ 10 Ma) to southeast (< 0.03 Ma) with waning intensity (Peltz et al 1987; Pécskay et al 1995, 2006; Szakács et al 1997, 2018), and has been linked to progressive along-strike slab breakoff (Mason et al 1998; Seghedi et al 1998) and/or lithospheric removal (e.g. Gîrbacea and Frisch 1998; Houseman and Gemmer 2007)

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Conclusion