Crustal thickness and adakite occurrence in the Philippines: Is there a relationship?

Abstract

Adakites are increasingly being recognized worldwide in a variety of tectonic settings. Models on the formation of this geochemically distinct class of volcanic rocks have evolved from partial melting of subducted young, hot oceanic slabs to magmatism resulting from oblique subduction, low-angle or flat subduction, or even slab-tearing. Some workers have also pointed to the partial melting of thickened crust to explain the generation of adakitic melts. Rare earth element ratios from adakites and adakitic rocks in the Philippines were used in this study to obtain approximations of the levels where they were generated. These were tied to available geophysical data that defines the crustal thickness of the areas where the samples were collected. High Sm/Yb and La/Yb ratios denote the involvement of amphiboles, and in some cases garnet, in the generation of adakites and adakitic magmas. The presence of amphibole and garnet as residual phases suggests high pressures corresponding to thicker crust (~30 to 45 km). Adakites and adakitic rocks formed through processes other than melting of subducted young oceanic crust would need Ն30 km to account for the heavy rare earth element signatures. If mantle fractionation is not the process involved, crustal thickness is critical to generate adakites and adakitic rocks.

Western Northern Luzon Isotopic Evidence of Transition From Proto‐South China Sea to South China Sea Fossil Ridge Subduction

“…We therefore argue that the Monglo dacites are derived from mantle‐derived basaltic magmas coupled with garnet‐bearing assemblage differentiation. Fractionation of a garnet‐bearing assemblage is plausible for the Monglo dacites as geophysical and geochemical data show that crustal thickness in western Northern Luzon is ~30 km (Dimalanta & Yumul, ), which critically meets the requirement of garnet‐bearing assemblage fractionation.…”

Slab partial melts from the metasomatizing agent to adakite, Tafresh Eocene volcanic rocks, Iran

“…In a subduction framework, slab melting is realized in the amphibolite–eclogite transition zone that normally corresponds to the 70–80 km depth range (Defant & Drummond 1990). Slabs subducting island arcs do not usually attain such crustal thickness (Dimalanta & Yumul 2008); that should be the major reason for the scarcity of adakitic rocks in the Eocene volcanic succession of the UDMA. Thus, the confinement of slab–melt fingerprints to the Tafresh Eocene volcanic rocks is likely to be a consequence of the particular conditions in the study area.…”

Ruby Xenocrystals in Dacite from Central Mexico

“…La/Yb ratio varies between 13.53 15.31 falling in the range of modern continental margin adakites (La/ Yb<50; Sun et al, 2012) but it is lower than silica rich adakites (La/Yb~20; Moyen, 2009). Sm/Yb=2.31-2.46 and La/Sm=5.80 6.30 fall in the range of adakites (Sm/Yb=3 7 and La/Sm=~2 10; Dimalanta & Yumul, 2008).…”

“…Based on the geochemistry of some trace elements, mohometric methods were applied to establish the crustal depth of formation of the dacite such as the La/Yb ratio. This is because Dimalanta & Yumul (2008) proposed that the La/Yb ratios in adakitic rocks can be applied as an approach to investigate crustal thicknesses, so that a La/Yb=10 ratio represents 30-35 km thickness and a La/Yb=10 20 ratio a crustal thickness greater than 42 km. In our study, La/Yb ratio is in the 13.80 15.30 range, consequently a crust thickness greater than 42 km is considered.…”