Adakite‐Like Potassic Magmatism and Crust‐Mantle Interaction in a Postcollisional Setting: An Experimental Study of Melting Beneath the Tibetan Plateau

Abstract

Adakite‐like potassic rocks are widespread in postcollisional settings where they provide potential insights into deep crustal processes that include partial melting, lower crustal flow and thickening, plateau uplift, and the creation of porphyry metal deposits. Although substantial progress has been made in characterizing the geochemical and geophysical features of postcollisional adakite‐like potassic rocks, their petrogenesis remains controversial. Here we report direct experimental evidence for the origins of these rocks with partial melting experiments on (1) garnet amphibolite and (2) the same garnet amphibolite mixed with 20 wt. % of a primitive Tibetan shoshonite. The experiments were conducted at 1.5–2.0 GPa and 800–1,000 °C. The partial melts of garnet amphibolite have typical adakitic signatures and are calc‐alkalic but lack the enrichment in potassium and other strongly incompatible elements (Rb, Ba, Th, U) that are characteristic of Tibetan adakite‐like rocks. In contrast, all characteristic features of the natural adakite‐like rocks are convincingly reproduced by the hybrid experiments. This includes negative inflections for Nb, Ta, and Ti on mantle‐normalized plots which are inherited from source materials rather than the effects of rutile in melting residues. The input of mantle‐derived shoshonitic mafic melts to a crustal source can be argued to provide not only the high concentrations of incompatible elements characteristic of adakite‐like potassic magmas but also the heat necessary for crustal melting. Our experimental results demonstrate that, in the case of the Tibetan Plateau at least, the production of adakite‐like potassic rocks in postcollisional settings can be best explained by such a model.

Petrofabrics and Seismic Properties of Himalayan Amphibolites: Implications for a Thick Anisotropic Deep Crust Beneath Southern Tibet

“…Partial melting of garnet amphibolites alone can produce melts with some but not all of the characteristics of adakite‐like potassic rocks from the Lasha Block. Specifically, the partial melts of garnet amphibolite are depleted in potassium and large ion lithophile elements (LILE) when compared to natural adakite‐like rocks (X. Wang et al, 2019). However, melting experiments at 1.5–2.0 GPa and 800–1000°C on a mixed garnet amphibolite (80 wt.…”

“…%) plus shoshonite (20 wt. %) source can result in melts compositionally similar to the Tibetan adakite‐like potassic rocks (X. Wang et al, 2019). In this case, mantle‐derived magmas provide not only a necessary heat source for the melting of thickened lower crust but also a metasomatic agent for the high concentrations of potassium and strongly incompatible elements needed to form postcollisional adakite‐like potassic rocks.…”

“…Their genesis has been ascribed to partial melting of an overthickened hydrated lower crust (Long et al, 2015), subducted Neo-Tethyan oceanic slab at mantle wedge (Liu et al, 2014), or the dehydration partial melting of amphibolite stimulated by underplating of mantle-derived magma (X. Wang et al, 2019). Partial melting of garnet amphibolites alone can produce melts with some but not all of the characteristics of adakite-like potassic rocks from the Lasha Block.…”

“…Specifically, the partial melts of garnet amphibolite are depleted in potassium and large ion lithophile elements (LILE) when compared to natural adakite-like rocks (X. Wang et al, 2019). However, melting experiments at 1.5-2.0 GPa and 800-1000°C on a mixed garnet amphibolite (80 wt.…”

“…In contrast, recent amphibolite dehydration melting experiments have demonstrated that the genesis of adakite-like potassic magmas from Himalayan amphibolites requires the input of mantle-derived shoshonitic mafic melts to provide not only the high concentrations of incompatible elements but also the heat necessary for crustal melting (X. Wang et al, 2019).…”