Geochemistry of Adakites from the Philippines: Constraints on Their Origins

Abstract

Abstract. We have identified in the Philippine Archipelago 230 samples of Late Miocene to Quaternary intermediate and evolved magmatic rocks or glasses, the compositions of which plot within the adakitic field defined by Defant and Drum‐mond (1990) using Sr/Y ratios versus Y contents. These rocks belong to four different subductions, along the Manila Trench (Batan, Northern Luzon, Central Luzon), the Negros and Sulu Trenches (Negros and Western Mindano), the Cotobato Trench (Southern Mindanao) and the Philippine Trench (Eastern Mindanao). Lavas from Central Mindanao overlie the deep remnants of the Molucca Sea Plate, and were emplaced in a post‐collision setting. All these samples show a significant depletion in Y and HREE with respect to their “normal” calc‐alkaline equivalents, suggesting that garnet was either a residual phase during partial melting or a fractionating mineral during differentiation or assimilation coupled with fractional crystallisation (AFC). However, only 19 samples out of our set (i.e., 8 %) display very high Sr/Y ratios (100–250). Our preferred model for the genesis of these “typical adakites” is ca. 20 % partial melting of subducted altered oceanic metabasalts converted to eclogite. This melting process could have been triggered by water from the underlying serpentinites. Most of the samples, termed “intermediate adakites”, display major and trace element chemical features intermediate between those of the former group and those of normal calc‐alkaline lavas. We show that magma mixing between slab‐derived adakitic magmas and mafic mantle‐derived melts accounts for most of the trends linking typical and intermediate adakites, although an additional contribution of mantle is required in some cases.

Is gold solubility subject to pressure variations in ascending arc magmas?

“…Au-Cu-Mo deposits represent major metal resources known to be spatially and temporally associated with intrusive, subduction-linked calc-alkaline magmatism and, according to some studies, notably with particular arc magmas whose petrogenesis may involve garnet fractionation (i.e., adakitic melts), either by slab partial melting in the upper mantle or by deep crustal melting/fractionation (e.g., Jégo et al, 2005 and references therein; Zellmer et al, 2012; see also Richards and Kerrich, 2007 for a synthesis). Although the bulk of the ore metals seems to originate from the magmas, there is as yet no consensus on processes responsible for their concentration.…”

Metasomatic interactions between slab-derived melts and depleted mantle: Insights from xenoliths within Monglo adakite (Luzon arc, Philippines)

“…A quarry along the Naguilian road near Monglo village exposes a ca. 50 m thick hornblende-bearing dacitic intrusive sheet dated at 8.65 ± 0.24 Ma (Jego et al, 2005). It crosscuts the Klondyke Formation (Fig.…”

“…It displays very low Y and Yb values approximating those of the primitive mantle (Sun and McDonough, 1989), highly fractionated rare earth element (REE) patterns and Sr/Y ratio of 130-150, a diagnostic feature for adakites (Defant and Drummond, 1990). Like many adakites, it contains relatively high amounts of Ni (50-53 ppm), Co (14-17 ppm) and Cr (62-84 ppm), which might indicate interactions with its ultramafic xenoliths (Jego et al, 2005).…”

“…1), a Middle to Late Miocene sequence of coarse clastic rocks, polymictic conglomerates, sandstones and vitric tuffs, with minor shales and siltstones (Tamesis et al, 1982). The geochemistry of this dacite has been discussed by Jego et al (2005), who consider it as a "typical" or "genuine" adakite derived from ca. 20% hydrous partial melting of altered oceanic metabasalts converted to eclogite.…”

“…The Monglo adakite ( [Jego et al, 2005] and ) is a small intrusive body located 16 km northwest of Baguio City in Northern Luzon (N16°28′52; E120°29′55, altitude 720 m). A quarry along the Naguilian road near Monglo village exposes a ca.…”