Origin of potassic postcollisional volcanic rocks in young, shallow, blueschist-rich lithosphere

Abstract

Potassium-rich volcanism occurring throughout the Alpine-Himalayan belt from Spain to Tibet is characterized by unusually high Th/La ratios, for which several hypotheses have brought no convincing solution. Here, we combine geochemical datasets from potassic postcollisional volcanic rocks and lawsonite blueschists to explain the high Th/La. Source regions of the volcanic melts consist of imbricated packages of blueschist facies mélanges and depleted peridotites, constituting a new mantle lithosphere formed only 20 to 50 million years earlier during the accretionary convergence of small continental blocks and oceans. This takes place entirely at shallow depths (<80 km) without any deep subduction of continental materials. High Th/La in potassic rocks may indicate shallow sources in accretionary settings even where later obscured by continental collision as in Tibet. This mechanism is consistent with a temporal trend in Th/La in potassic postcollisional magmas: The high Th/La signature first becomes prominent in the Phanerozoic, when blueschists became widespread.

Identification of High δ18O Adakite‐Like Granites in SE Tibet: Implication for Diapiric Relamination of Subducted Sediments

“…Such high Th/La ratios have been reported for lavas from the Alpine-Himalayan orogenic belt, which were interpreted to be inherited from metasedimentary lawsonite blueschist in the source (Y. Wang et al, 2021). Lawsonite can effectively fractionate Th from La, and survives under relatively high pressure from blueschist to eclogite facies (Tommasini et al, 2011).…”

“…NHRL (Northern Hemisphere Reference Line; Hart (1984)). Data source: subducted oceanic crust‐derived and subducted continental crust‐derived adakitic rocks (Q. Wang et al., 2008 and references therein), ASRR leucogranites (Zhang & Schärer, 1999); Jamaica adakite‐like lavas (Hastie et al., 2010); Alpin‐Himalayan lavas, OIB (oceanic island basalt), MORB (mid‐ocean‐ridge basalt) and arc magmas (compiled by Y. Wang et al., 2021); Paleogene Yunnan shoshonitic rocks (Huang et al., 2010; Lu et al., 2013; L. Xu et al., 2011; L.‐L. Xu et al., 2019) and thickened continental crust‐derived adakitic rocks (Lu et al., 2013; Zhou et al., 2019), Permian‐Triassic S‐type granites (J. Xu et al., 2021).…”

“…Furthermore, a succession of additional reactions at high pressures (ca. 2.6 GPa) would dilute the Th/La signatures (<0.5) of blueschist facies sedimentary melts (Y. Wang et al., 2021).…”

“…Lawsonite can effectively fractionate Th from La, and survives under relatively high pressure from blueschist to eclogite facies (Tommasini et al., 2011). Experimental modeling results suggest that under eclogite facies the metasediment‐derived melts generally have Th/La ratios <0.5 (Y. Wang et al., 2021). We thus infer that blueschist facies metamorphism may have occurred and played a significant role in redistributing trace elements in the source region of the ASRR adakite‐like granites.…”

“…Coeval mafic rocks are absent in the region, and the ASRR adakite-like granites do not show fractional crystallization of garnet or amphibole (Castillo, 2012) but rather have compositional trends (e.g., positive La/Sm vs. (Zhang & Schärer, 1999); Jamaica adakite-like lavas (Hastie et al, 2010); Alpin-Himalayan lavas, OIB (oceanic island basalt), MORB (mid-ocean-ridge basalt) and arc magmas (compiled by Y. Wang et al, 2021); Paleogene Yunnan shoshonitic rocks (Huang et al, 2010;Lu et al, 2013;L. Xu et al, 2011;L.-L. Xu et al, 2019) and thickened continental crust-derived adakitic rocks (Lu et al, 2013;Zhou et al, 2019), Permian-Triassic S-type granites (J.…”